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Petrologic and geochronologic study of Grenville-Age granulites and post-Granulite plutons from the La Mixtequita area, state of Oaxaca in southern Mexico, and their tectonic significance

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Petrologic and geochronologic study of Grenville-Age granulites and post-Granulite plutons from the La Mixtequita area, state of Oaxaca in southern Mexico, and their tectonic significance

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Content PETROLOGIC AND GEOCHRONOLOGIC STUDY OF GRENVILLE-AGE GRANULITES AND POST-GRANULITE PLUTONS FROM THE LA MIXTEQUITA AREA, STATE OF OAXACA IN SOUTHERN MEXICO, AND THEIR TECTONIC SIGNIFICANCE by Gustavo Murillo-Muneton A Thesis Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE (Geological Sciences) December 1994 Copyright 1994 Gustavo Murillo-Muneton Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. U N IV E R S IT Y O F S O U T H E R N C A L IF O R N IA T H E G RA D U A TE S C H O O L U N IV E R S IT Y PARK LO S A N G E L E S , C A L IFO R N IA 9 0 0 0 7 This thesis, written by Gustavo Murillo-Muneton under the direction of h.3-.?.....Thesis Committee, and approved by all its members, has been pre sented to and accepted by the Dean of The Graduate School, in partial fulfillment of the requirements for the degree of Master of Science Dean .2 3 > 1994 T H E S IS C O M M IT T E E Chairman Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I would like to dedicate this work to my wife Lety and my kids Gustavo and Melissa. Thanks for their love, patience, and comprehension. This study is also dedicated to my parents, especially to my mother. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS There are many people that I would like to thank because they supported and provided my education at the University of Southern California. First, I want to express my sincere thanks to M. S. Baldomero Carrasco Velazquez from the Instituto M exicano del Petroleo. Thanks to him I was supported financially by the Instituto Mexicano del Petroleo during my stay at USC for two and a half years. He also encouraged me during the most difficult times when I was at USC. I want to specially thank my advisor, Dr. J. Lawford Anderson . Dr. Anderson provided me academic guidance, encouragement, friendship, and introduced m e to the exciting field of the geothermobarometry. I also wish to thank the other committee members, Dr. Jean Morrison and Dr. Gregory A. Davis for their discussions and contributions to improve the academic quality of this study. This project was supported by NSF grant EAR-9219347 (to Dr. Anderson), the Foss Foundation for Mineralogic Research and the Graduate Student Research Fund of the Department of Geological Sciences at USC. Logistic support for field work was provided by the Instituto M exicano del Petroleo. Special thanks also are due to Dr. Richard M. Tosdal from the U.S. Geological Survey at Menlo Park, CA. for his contribution to this project with the U-Pb geochronologic analyses. Thanks to Octavio Navarrete Rivera and Agustin Fuziwara for their assistance during field work. I am also appreciative to my friends at USC. First of all, I want to thank Eric Hovanitz and Dave Mayo for their sincere friendship. This thesis would not had been written without the assistance of Eric Hovanitz. Eric spent many hours revising the grammar of the first draft. Dave Mayo provided very valuable discussions concerning my project and also helped me perform the X-ray analyses at USC. I also had interesting discussions about thermobarometry with John Bendixen. I would like to express my sincere thanks to my sister Guille, she also encouraged during the hardest times. Lastly, thanks to Steve Bachtel from Texas A&M University for the final corrections. iii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS Page D E D IC A T IO N ................................................................................................................................. ii A C K N O W L E D G M E N T S ...........................................................................................................iii L IS T O F T A B L E S .....................................................................................................................vi L IS T O F F IG U R E S ................................................................................................................ vii A B S T R A C T ........................................................................................................................................x IN T R O D U C T IO N ............................................................................................................................ 1 O b je c tiv e s ..............................................................................................................................3 P re v io u s W o rk ................................................................................................................. 4 W O R K M E T H O D S ......................................................................................................................6 F ie ld W o rk ..........................................................................................................................6 P e tro g ra p h ic T e c h n iq u e s...............................................................................................6 E lectron M icroprobe A n aly sis................................................................................... 7 S tab le Iso to p e A n aly sis...............................................................................................7 M ajor and T race Elem ent A nalysis..........................................................................8 G e o c h ro n o lo g ic A n a ly sis............................................................................................. 9 R E G IO N A L G E O L O G Y .......................................................................................................... 10 PETR O LO G Y O F TH E GU ICH ICO V I C O M PLEX .......................................................21 P e tro g ra p h y ............................................................. 21 T h e rm o b a ro m e try ........................................................................................................... 27 M in eral C h em istry ........................................................................................... 27 G a rn e t........................................................................................................ 28 P y ro x e n e ................................................................................................. 31 P la g io c la s e ............................................................................................. 33 H o rn b le n d e ..............................................................................................33 B io tite ....................................................................................................... 33 B a ro m e try .............................................................................................................36 D iscussion of the B arom etry......................................................... 43 T h e rm o m e try ......................................................................................................45 T w o-pyroxene T h erm o m etry ..........................................................45 G arnet-orthopyroxene T herm om etry............................................ 51 G arnet-clinopyroxene T herm om etry............................................. 58 G arnet-biotite T herm om etry.......................................................... 66 G arnet-hornblende T herm om etry..................................................76 D iscussion of the T herm om etry................................................... 78 G e o c h ro n o lo g y .................................................................................................................84 U -P b........G eo ch ro n o lo g y ..................................................................................84 K -A r....... G eo ch ro n o lo g y ..................................................................................86 PETROLOGY O F THE LA M IXTEQUITA BATHOLITH........................................... 91 P e tro g ra p h y .......................................................................................................................91 G e o c h ro n o lo g y .................................................................................................................95 U -P b G e o c h ro n o lo g y ................................................................................. 95 K -A r G e o c h ro n o lo g y .................................................................................99 Page iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. G e o c h e m is try .................................................................................................................. 103 M ajor Oxide and Trace Elem ent G eochem istry...................................... 103 R are Earth Elem ent G eochem istry.......................................................... 116 D iscussion o f the G eochem istry..............................................................118 TECTONIC IMPLICATIONS OF THE GUICHICOVI COMPLEX AND TH E LA M IX TEQ U ITA B A TH O LITH ............................................................. 123 C O N C L U S IO N S ......................................................................................................................... 131 R E F E R E N C E S ............................................................................................................................ 134 A P P E N D IX E S ..............................................................................................................................143 E lectron M icroprobe D ata.......................................................................................143 I. G a rn e t........................................................................................................... 144 II. C lin o p y ro x e n e .......................................................................................... 150 III. O rth o p y ro x e n e ....................................................................................... 156 IV. P la g io c la s e .............................................................................................. 159 V. H o rn b le n d e ..............................................................................................161 VI. B io tite ........................................................................................................163 v Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES Page Table 1. M ineralogy o f the Guichicovi com plex.............................................................22 Table 2. Stable isotope analysis of graphite from the Guichicovi com plex......................26 Table 3. Gamet-orthopyroxene-clinopyroxene-plagioclase-quartz barometry of the G uichicovi co m p lex ............................................................................................ 41 Table 4. Clinopyroxene-orthopyroxene thermometry of the Guichicovi com plex............48 Table 5. Garnet-orthopyroxene thermometry of the Guichicovi com plex.......................54 Table 6. Garnet-clinopyroxene thermometry of the Guichicovi com plex........................ 62 Table 7. G arnet-biotite thermometry of the Guichicovi com plex....................................67 Table 8. G arnet-hom blende thermometry of the Guichicovi com plex............................77 Table 9. U-Pb geochronologic data of the Guichicovi com plex.....................................87 Table 10. K-Ar geochronologic data of the Guichicovi com plex....................................89 Table 11. M ineralogy o f the La M ixtequita batholith..................................................... 92 Table 12. U-Pb geochronologic data of the La M ixtequita batholith.............................. 96 Table 13. K-Ar geochronologic data of the La M ixtequita batholith............................. 100 Table 14. Geochemistry of the Permian suite of the La Mixtequita batholith................. 104 Table 15. Geochemistry of the Early Jurassic suite of the La M ixtequita batholith 105 Table 16. Geochemistry of the post-Early Jurassic dikes of the La Mixtequita b a th o lith ............................................................................................................................106 v i Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES Page Figure 1. Tectono-stratigraphic terranes in Mexico with distribution of Grenville-age m etam o rp h ic ro c k s ..................................................................................... 2 Figure 2. Location o f the study area.................................................,............................. 11 Figure 3. Geologic map of the La Mixtequita area with location of samples for th is s tu d y ..........................................................................................................................13 Figure 4. Exposure of the Guichicovi complex along the dirt road Nuevo Centro- M onte A guila, O ax ...................................................................... ,.............................14 Figure 5. Exposure of the intrusive contact between the Guichicovi complex and Permian plutons o f the La M ixtequita batholith in Monte Aguilq, Oax................. 14 Figure 6. Exposure of Permian granitoids of the La M ixtequita batholith qlong the Aguacatengo River in the vicinities of Santiago Tutla Nuevo, Qax....l............... 17 Figure 7. Exposure of Early Jurassic granitoids of the La Mixtequita batholith along the Victor (San Juan) River in the vicinities of Villanueva Seguiido, O ax.............17 Figure 8. Photomicrograph of a quartz-bearing garnet two-pyroxene mafic granulite (sam ple M IX TE-62) o f the Guichicovi com plex................................................23 Figure 9. Photomicrograph of a gamet-homblende-biotite two-pyroxene mafic granulite (sam ple M IXTE-64) of the Guichicovi com plex...., ................ 23 Figure 10. (Almandine+spessartine)-grossular-pyrope ternary plot of garnets from the G uichicovi c o m p lex ...............................................................,............................29 Figure 11. Compositional variation of plagioclase from the Guichicovi com plex............ 34 Figure 12. Classification of amphiboles from the Guichicovi complex according to the L eake's (1978) schem e......................................................... 35 Figure 13. Gamet-orthopyroxene-clinopyroxene-plagioclase-quartz barorpetry applied to a single mineral association from the Guichicovi complex..................42 Figure 14. Gamet-orthopyroxene-plagioclase-quartz barometry of the Guichicovi c o m p le x ...............................................................................................................................42 Figure 15. Comparison of the clinopyroxene-orthopyroxene thermometric calibrations of W ood and Banno (1973) and Wells (1977) for the Guichicovi com plex..........49 Figure 16. Projection of the averaged compositions of coexisting clino- arid orthopyroxenes in the Lindsley's (1983) pyroxene quadrilateral for the G u ich ico v i c o m p le x .................................................................................... 50 v ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Page Figure 17. Gamet-orthopyroxene thermometric calibrations applied to a single m ineral pair for the G uichicovi com plex............................................................. 55 Figure 18. Gamet-orthopyroxene thermometry for the Guichicovi complex using the calibration of Lee and Ganguly (1988).........................................................55 Figure 19. (Fe2+/Mg)oar versus (Fe2+/Mg)opX for mineral pairs used for gamet- orthopyroxene thermometry for the Guichicovi com plex....................................57 Figure 20. Gamet-clinopyroxene thermometric calibrations applied to a single mineral pair for the G uichicovi com plex...........................................................................63 Figure 21. Discrepancy in gamet-clinopyroxene thermometry according to the grossular model adopted (total X ca versus true grossular)................................................. 63 Figure 22. Gamet-clinopyroxene thermometry for the Guichicovi complex using the calibration of Ellis and Green (1979)............................................................65 Figure 23. (Mg/Fe2+)oar versus (Mg/Fe2+)cp x for mineral pairs used for gamet- clinopyroxene thermometry for the Guichicovi com plex....................................65 Figure 24. Gamet-biotite thermometric calibrations applied to a single mineral pair for the G uichicovi com plex................................................................................... 72 Figure 25. Gamet-biotite thermometry for the Guichicovi complex using the calibration of Hodges and Royden (1984)....................................................72 Figure 26. (Mg/Fe2+)car versus (Mg/Fe2+)Bi0 for mineral pairs used for gam et-biotite thermometry for the Guichicovi com plex..................................... 75 Figure 27. Kd versus T(°C) for gamet-homblende thermometry using the calibration of Graham and Powell (1984) for the Guichicovi com plex................................79 Figure 28. (Mg/Fe2+)car versus (Mg/Fe2+)nbi for mineral pairs used for garnet-hornblende thermometry for the Guichicovi com plex............................. 79 Figure 29. Geologic map of the La Mixtequita area with thermometric results and structural attitudes for the Guichicovi com plex........................................... 81 Figure 30. Geologic map of the La Mixtequita area with geochronologic data of the Guichicovi com plex and La M ixtequita batholith................................................85 Figure 31. U-Pb concordia diagram for samples from the Guichicovi com plex.............. 88 Figure 32. U-Pb concordia diagrams for samples from the La M ixtequita batholith 97 Figure 33. Harker-type diagrams showing chemical variations for the La M ixtequita b a th o lith ........................................................................................................................... 107 viii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Page Figure 34. SiC>2 versus molecular [Al2 0 3 /(Ca0 +Na2 0 +K 2 0 )j for the La Mixtequita b a th o lith ............................................................................................................................ 111 Figure 35. Molecular Al2 0 3 -Ca0 -(Na2 0 +K 2 0 ) ternary plot for the La Mixtequita b a th o lith ............................................................................................................................ 111 Figure 36. Classification of the La Mixtequita batholith according to the Peacock's (1931) sc h e m e .............................................................................................................. 112 Figure 37. Si02 versus K20+N a20 (Irvine and Baragar, 1971) for the La M ixtequita b a th o lith ............................................................................................................................ 113 Figure 38. (K 20+N a20)-Fe0*-M g0 diagram (Irvine and Baragar, 1971) for the La M ix teq u ita b a th o lith ........................................................................................... 113 Figure 39. FeO*/MgO ratio versus SiC>2 (Miyashiro, 1974) for the La Mixtequita b a th o lith ............................................................................................................................ 114 Figure 40. SiC>2 versus K 2 O for the La M ixtequita batholith........................................ 114 Figure 41. Tectonic discrimination diagrams for granitoids (Pearce et al., 1984) of the La M ixtequita b ath o lith ....................................................................................115 Figure 42. Chondrite-normalized REE patterns for the La Mixtequita batholith.............117 Figure 43. Rb versus Sr plot for the La M ixtequita batholith.......................................119 Figure 44. Tectonic models for the translation of the Guichicovi com plex....................127 ix Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A B S T R A C T The Guichicovi complex of Oaxaca, southern Mexico represents the southeastemmost exposure of Grenville-age granulites in North America. Granulite P-T conditions were calculated by cation-exchange and multi-phase reaction equilibria. OPX-GAR-PLA-QTZ barometry yielded 7.4 ±0.3 kbar at 837 ±59 °C based on TW O-PX thermometry. At 7 kbar, cation-exchange thermometers yielded 755 ±25 °C (GAR-OPX), 717 ±21 °C (GAR-CPX), 649 ±11 °C (GAR-BIO), and 631 ±18 °C (GAR- HBL) suggesting diffusion-controlled cation re-equilibration during rapid cooling. U-Pb data for a gamet-homblende granitic gneiss are concordant at 986 ±4 Ma. Four fractions from a gamet-pyroxene granulite define an age of 980 to 990 Ma. K-Ar hornblende ages include 911 ±46 Ma and younger K-Ar dates relate to the La Mixtequita batholith. Discordant zircons from the batholith show at least two stages of plutonism at 254 ±7 and 189 ±3 Ma. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION Southern Mexico is characterized by complex pre-Mesozoic geology. Several tectono-stratigraphic terranes (and/or igneous-metamorphic belts) have been recognized in this region of Mexico (Ortega-Gutierrez, 1981a; Carfantan, 1983; Campa and Coney, 1983). Included are the Oaxacan, Mixteco, Maya, Juarez, and Xolapa terranes (Figure 1). Whereas the Oaxacan is Late Precambrian (Grenville), the Mixteco and the Maya are Paleozoic in age, and the Juarez and Xolapa are Mesozoic (Ortega-Gutierrez, 1981a; Campa and Coney, 1983). The Precambrian and Paleozoic basement terranes were subsequently affected by a Permo-Triassic plutonic event (Torres et al., in progress) that further complicates deciphering the tectonic evolution of southern Mexico. The basement of the Maya terrane is probably the least known. A crystalline complex exposed in the La Mixtequita area of the eastern area of the state of Oaxaca has been considered as part of the basement o f the Maya terrane (Campa and Coney, 1983; Ortega-Gutierrez et al., 1990). Previous unpublished work conducted by geologists from Petroleos Mexicanos (PEMEX) and the Instituto Mexicano del Petroleo (IMP), interpreted the crystalline rocks from the La Mixtequita area as a Paleozoic granitic complex, termed La Mixtequita batholith, grading locally to gneisses (Gonzalez, 1967 and 1969; Ruiz , 1978; Lopez and Rodriguez, 1986). The age of the gneisses was tentatively considered to be Precambrian (Quezada, 1978). Although Gonzalez (1967 and 1969) assigned the gneisses to the "catazone" facies, it was not until later that granulite metamorphic facies rocks were recognized in a restricted area (Araujo, 1983; Zaldlvar and Ortiz, 1984; Torres, 1986). In general, the basement of the La Mixtequita area was considered mainly as a batholith with scarce high-grade metamorphic rocks. This study has revealed that the La Mixtequita batholith is bordered by extensive Late Precambrian granulite metamorphic facies rocks. This high-grade metamorphic terrain 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. > z ;CHIHUAHUA» t£%r%w5>&r; y.y.y:v?ffe 'Y 'Y 'yCOA HU ILA * \ N \ n \ /■///////V ///:// . . . . / / / / / / / / //////✓ // // / / / /////// ////// / / / / / / / / // / / / / / / / / / / / / / 400 kms MAYA Grenville-age metamorphic rocks: 1. Sierra del Cuervo 2. El Carrizalillo 3. Novillo gneiss 4. Huiznopala gneiss 5. Oaxacan complex (Oaxaca Terrane) 6. Guichicovi complex Figure. 1. Tectono-stratigraphic terranes in Mexico (Campa and Coney, 1983) with distribution of the Grenville-age metamorphic rocks. The terrane names are given in the map and include: A= Alisitos, V= Vizcaino, CA= Caborca, SM= Sierra Madre, GU= Guerrero, MI= Mixteco, XO= Xolapa, and J= Juarez. Others abreviations are: M-S M= Mojave- Sonora Megashear. For references of the Grenville-age metamorphic rocks see text. 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. is named the Guichicovi complex, representing the southeastemmost occurrence of Grenville age granulites in North America. Objectives The present work is a petrologic and geochronologic study of crystalline rocks from the La M ixtequita area in southern Mexico. Based on petrography and microprobe analyses, the metamorphic conditions of the Guichicovi gneiss were calculated using a range of well calibrated thermobarometers (Murillo-Muneton and Anderson, 1994; Murillo- Muneton et al., 1994). This study represents the first quantitative work in this region of Mexico to define the peak metamorphic conditions of the Guichicovi gneiss. Also in this study, the name Guichicovi gneiss (Murillo and Torres, 1990; Murillo and Navarrete, 1992; Murillo et al., 1992; Murillo-Muneton and Anderson, 1994) is changed to Guichicovi complex because of extensive variations in lithology within the terrane. The common presence in the Guichicovi complex of garnet with clinopyroxene, orthopyroxene, biotite, quartz and plagioclase allows the application of several thermobarometric calibrations. Pressure calculations are based on phase equilibria barometry (gamet- pyroxene-plagioclase-quartz) while the temperature estimates are based on solvus pyroxene thermometry and Mg-Fe cation exchange thermometry (gamet-orthopyroxene, gamet- clinopyroxene, gamet-biotite, gamet-homblende). An evaluation of the thermobarometric formulations applied to the Guichicovi complex is discussed in order to constrain the P-T metamorphic conditions of this terrane. The La Mixtequita batholith is characterized according to its petrography, major and trace element and rare earth element geochemistry. Timing of the granulite metamorphism of the Guichicovi complex and the magmatic activity related to the La Mixtequita batholith was determined by U-Pb and K-Ar geochronology (Murillo-Muneton et al., 1994). This thesis includes the first U-Pb geochronology 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. conducted in crystalline rocks of the La Mixtequita area. The tectonic significance of both the Guichicovi complex and La Mixtequita batholith is discussed. Previous Work The La Mixtequita area in eastern Oaxaca has been the subject of several different geological studies. Most of the previous work is unpublished and was carried out by geologists from Petroleos Mexicanos (PEMEX). Stratigraphic and structural studies with emphasis on oil interests were done by Gonzalez (1967 and 1969), Quezada (1978), Araujo (1983), Lopez and Rodriguez (1986) and Torres (1986). During these studies, the stratigraphy and main structures of La Mixtequita area were defined. Gonzalez (1967 and 1969) introduced the name La Mixtequita batholith for the magmatic rocks in this region. Although, the lithology of the magmatic rocks was fairly well known, the existence of Precambrian granulites was inferred. The main goal of PEMEX studies was the identification of the sedimentary cover and structural geology in this region of Mexico. The La Mixtequita area has also been the subject of few regional tectonic studies. The southern portion has been included in regional unpublished tectonic works carried out by M artinez et al. (1987) and Zaldivar and Ortiz (1984), geologists from the IMP, and by Carfantan (1983). These works helped by identifying the presence of Precambrian gneisses on the southern edge of the La Mixtequita batholith. Early geochronologic determinations on crystalline rocks of the La Mixtequita region were done using K-Ar. All geochronologic data in this thesis are referred to the geologic time scale of Palmer (1983). C. Schlaefer (pers. comm., 1972), from the IMP, dated a biotite pegmatite from the Guichicovi, Oaxaca area and obtained an age of 866 ± 29 Ma (Precambrian); this age was never formally published. Ruiz (1978), also from the IMP, and Quezada (1978) from PEMEX reported Mesozoic K-Ar ages from the La Mixtequita batholith. Subsequently, Damon et al. (1981) published two Early Jurassic K-Ar ages from 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. plutonic phases of the batholith. A more detailed analysis of the La Mixtequita region was performed by Murillo and Torres (1990) and Murillo and Navarrete (1992) from the IMP, including preliminary geochemistry of the batholith and K-Ar dating of the high-grade metamorphic rocks and plutons. This data helped to redefine the basement of the La M ixtequita area and subsequently to correctly constrain the presence of Precambrian granulite facies rocks in this region of southeastern Mexico (Murillo et al., 1992). 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. WORK METHODS Field Work Field work was performed over a period of four weeks. Two weeks of field work were spent by the author in 1991, during a project developed by the IMP. Another two weeks were spent during the Summer, 1992. Five topographic maps (1: 50 000) published by the Instituto Nacional de Estadfstica, Geografia e Informatica were employed to locate samples and for regional geological mapping. These maps include San Felipe Cihualtepec, San Juan Mazatlan, Jesus Carranza, Donaji and Matfas Romero (respectively E15C32, E15C42, E15C33, E15C43, and E15C53). The geologic map of the thesis area was built from two topographic maps at a scale of 1: 250 000, the Minatitlan (E l5-7) and Juchitan (E15-10, D15-1) charts. Mapping of the Jurassic to Cenozoic sedimentary cover overlying the Guichicovi complex and the La Mixtequita batholith was taken from previous studies by geologists from PEMEX (Gonzalez 1967 and 1969; Quezada, 1978; Araujo, 1983; Lopez and Rodriguez, 1986; Torres, 1986). In addition, location of major faults (Valle Nacional and Paraiso) was based on the work of Gonzalez (1967 and 1969), Quezada (1978), Araujo (1983), Torres (1986) and Lopez and Rodriguez (1986). In total, 47 samples of high-grade metamorphic rocks from the Precambrian Guichicovi complex and 33 samples of igneous rocks from the La Mixtequita batholith were collected during field work . Petrographic Techniques A total of 70 thin sections were studied. Thin sections of metamorphic rocks were cut parallel to mineral lineation and perpendicular to foliation. Thin sections of igneous rocks showing foliated fabrics were made following the same procedure as that in metamorphic rocks. Petrographic analyses of the thin sections was done using a Nikon petrographic microscope. Modal determinations of minerals from igneous rocks were done 6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. by visual estimation. Nomenclature of igneous rocks was assigned according to Streckeisen (1976). Electron Microprobe Analysis Garnet, clinopyroxene, orthopyroxene, plagioclase, hornblende and biotite from nine samples were analyzed by electron microprobe. The samples include: one graphite- bearing two-pyroxene granulite (ISTEH-33), one gamet-clinopyroxene quartzo-feldspathic gneiss (M IXTE-21), one gamet-clinopyroxene-homblende quartzo-feldspathic gneiss (MIXTE-22), one hornblende quartzo-feldspathic granulite (MIXTE-26), two hornblende two-pyroxene mafic granulites (ISTEH-33 and MIXTE-15), two gamet-homblende two- pyroxene mafic granulites (MIXTE-62 and -64), and one garnet two-pyroxene mafic granulite (MIXTE-63). Samples were analyzed using the CAM ECA electron microprobe of the Department of Geology at the Texas A&M University in College Station, Texas. The microprobe was operated at 15 KV with a 10 nA sample current for garnet, hornblende and plagioclase, and a 20 nA sample current for pyroxene. Beam width was 1, 5 and 10 (i for garnet and pyroxene, hornblende and biotite, and plagioclase, respectively. Standardization was based on natural silicates and oxides. Cores and rims o f coexisting minerals were analyzed. A minimum of two spots were analyzed in cores of garnet, pyroxene, and hornblende. Stable Isotope Analysis Carbon isotopic composition of three samples of graphite were determined at USC. The samples include a graphite two-pyroxene quartzo-feldspathic granulite (ISTEH-33), a gamet-graphite quartzo-feldspathic paragneiss (MIXTE-61) and a graphite quartzite (ISTEH-81). Graphite was separated by hand picking from crushed samples and then immersed in dilute HC1 to eliminate any possible calcite (carbon) contamination. Graphite 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. was mixed with an excess stoichiometric quantity of CuO and thoroughly mixed together in an agate mortar. The mixture of graphite and CuO was then placed into Ni-cups. Oxidation of graphite was performed in a tube-fumace at 1100 °C and the resulting CO 2 was cryogenically collected and purified. The 1 3C /1 2 C isotopic ratios of the extracted CO 2 were measured utilizing a VG PRISM mass spectrometer using R7905 reference gas. NBS-21 graphite standard was used for laboratory standardization. Major and Trace Element Analysis Thirty igneous samples were chemically analyzed. Two to four kg samples were crushed in a steel-jaw crusher. Weathered rock chips were removed from coarse crushed samples in order to use only the freshest portions for geochemical analysis. The finest fraction and powder were excluded by sieving to avoid contamination from the steel jaw crusher. The remaining coarse fraction was then recrushed in a tungsten-carbide-lined jaw crusher. Next, the samples were powdered in a tungsten-carbide ball mill for 30 minutes. M ajor element chemistry was performed by X-ray fluorescence. Sample powders were dried in an oven at 110 °C overnight to ensure the loss o f atmospheric water. Then, the dried powder samples were fused with previously ignited flux at 1000 °C. During this analytical step, ~ 0.785 g of sample were employed with 4 g of flux and lanthanum oxide. After this procedure the loss on ignition of samples was calculated. The resulting glass discs were analyzed on a RIGAKU X-ray fluorescence spectrometer at USC for weight % of major oxides with total Fe as FeO*. Pressed powder pellets of samples were used for trace element analysis. Approximately 4 g of dried powder sample was mixed with ~ 0.80 g of cellulose during 30 minutes in a tungsten-carbide ball mill. This mixture was then poured into a steel mold, covered with additional cellulose and pressed at 7000 psi. The resulting pressed powder 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. pellets were analyzed on the RIGAKU X-ray fluorescence spectrometer at USC. Determination of trace element compositions was given in part per million (ppm). Geochronologic Analysis A garnet-homblende quartzo-feldspathic gneiss, a clinopyroxene-gamet quartzo- feldspathic gneiss, a homblende-biotite quartz monzonite, a homblende-pyroxene-biotite quartz diorite and a leucogranite were dated by the U-Pb method in the U.S. Geological Survey, Menlo Park, CA. The author separated the zircons using the Wilfley table, heavy liquids and the Franz Isodynamic separator. Dr. Richard Tosdal analyzed the zircon fractions using a FTNIGAN-MAT MAT-262 multiple collector mass spectrometer. In addition, previously unreported K-Ar ages of seven samples dated by an IMP contract with the Department of Geological Science at the University of Arizona are also included. Samples analyzed by the K-Ar method include an amphibolite, a clinopyroxene-homblende quartzo-feldspathic gneiss, a gamet-clinopyroxene quartzo-feldspathic gneiss (also dated by U-Pb), a biotite granite, a biotite-homblende quartz monzonite, a biotite-homblende quartz monzodiorite, and a leucogranite (also dated by U-Pb). 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REGIONAL GEOLOGY The study area lies within the La Mixtequita region in eastern Oaxaca, southern Mexico (Figure 2). Gonzalez (1967 and 1969) first informally named the plutonic rocks of the La Mixtequita area (a small village) as the La Mixtequita batholith. He described the batholith as comprised of granitic intrusive rocks that grade locally to "catazone" metamorphic rocks. In addition, he observed that the metamorphic rocks consist of biotite- sillimanite gneiss, granitic gneiss and migmatite. This author inferred two periods of magmatic activity for the La Mixtequita batholith in the Paleozoic and Late Cretaceous. C. Schlaefer (pers. comm., 1972) obtained an K-Ar age of a biotite pegmatite from Guichicovi, Oaxaca, in the southeastern margin of the La Mixtequita batholith. Based on Schlaefer's K-Ar date, Quezada (1978) suggested that the La Mixtequita batholith must correspond to a Precambrian basement with magmatic reactivation during Permian to Triassic time. According to Ruiz (1978), based on K-Ar dating, the granitic gneisses o f the La M ixtequita batholith were generated by Mesozoic deformation of the plutons. The existence of granulites in the La Mixtequita region was recognized in the 1980's. The first report of granulites in the southeastern margin of the La Mixtequita batholith was done by Araujo (1983). He identified gneissic "granite", granoblastite, granulite and hornblende gneiss. However, this author did not map those metamorphic rocks but inferred the probable presence of Precambrian rocks in the La Mixtequita batholith. Zaldivar and Ortfz (1984) recognized scarce outcroppings of granulite facies gneisses in the southern margin of the La Mixtequita batholith. Those authors correlated the granulite facies rocks with the Oaxacan Complex (Oaxacan tectono-stratigraphic terrane, Figure 1). Mapping of Precambrian gneisses in the same area as Zaldivar and Ortiz was done by Torres (1986), however, he did not make any comment on them. L6pez and Rodriguez (1986) interpreted the exposed gneisses in the southern La Mixtequita batholith 1 0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Tijuana G ulf o f Mexico Guadalajara Isthmus of Tehuantepec Mexico City 300 km Scale STATE OF OAXACA Figure 2. Location of the study area, shown in stippled pattern enclosing the La Mixtequita region in the State of Oaxaca. 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. to be the product of extreme dynamometamorphism associated with the Valle Nacional strike-slip fault (discussed later). During the 1990's it was recognized that what was considered the La Mixtequita batholith consisted of a vast region of Precambrian granulite facies metamorphic rocks. Murillo and Torres (1990) informally reassigned the basement rocks of the La Mixtequita area into three distinctive crystalline units. They applied the name Guichicovi gneiss to the Precambrian granulite facies rocks in the southern margin of the La M ixtequita batholith. These authors kept the name La Mixtequita batholith for the post-granulite plutons. Murillo and Torres (1990) applied the name Mazatlan metamorphic complex to a series of Lower(?) Paleozoic greenschist facies pelitic schists that are in tectonic contact with the granulites and plutons along their western boundaries. These Paleozoic metapelites had been recognized previously by Ruiz (1978), Araujo (1983), Torres (1986), Zaldfvar and Ortiz (1984) and Lopez and Rodriguez (1986). Subsequently, Murillo and Navarrete (1992) and Murillo et al. (1992) recognized on the basis of field work and K-Ar dating, that the Guichicovi gneiss comprises more than half of what had been originally considered as the La Mixtequita batholith. They assigned a Grenville age to the Guichicovi gneiss and a Middle Triassic-Early Jurassic age to the La Mixtequita batholith. The regional geology of the La Mixtequita area is illustrated in Figure 3. The Guichicovi gneiss represents the oldest terrane in the La Mixtequita area. In the present study, the term Guichicovi gneiss is abandoned and the name Guichicovi complex is used because many granulite lithologies are present in addition to gneiss. The terrane constitutes the southeastern portion of the Mixtequita area (Figure 3). The longest axis of this terrane trends roughly NW30°SE and is 30 km long, while its width ranges from 19 to 23 km. The total exposure area is ~ 590 km2. In general, the Guichicovi complex consists of layers of mafic and felsic granulites, amphibolites, and quartzo-feldspathic gneisses (Figure 4) with or without clinopyroxene, garnet, hornblende, biotite and graphite. These 1 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 00 o£6 m 3> ■ * ■ *» i!i!i !i!i!i , i!i !i , i i.-f * :::: k ' A ' V to '- T V '/ 'g V y V ' I ' i N/ . V * * ^ \ - R \ \ \ ^ v< s ✓ /jA * / / / W / f t o -p \ \ % s ^ r s > 5 f8 ,k' / ^ / /, < % • ? ■ < ? ttN N N N k . . . ti p i» 3 ' / s s /. i \ \ \ V H P 0 \ \ \ S 51 © 56 1/ C O .JJ / ✓ N * „ n \ \ \ \ \ N \ ^ « .v -' / / / / / / « W I ? ,0C oS6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. Monte A g u ila -w P alom ares la^K - A A M K r B - 1 6 - M X Ie I3 & A - * ■ * Vflxre ,3 & A Jm m - is MXXTS'23Aj San Juaiuto B Oia. L Sarabia >-MDCre-214^A B T H B O tt- g j : - : ^ S T H B . 3 3 : ^ k . Juan Maza E. Colorado 17° 00’ MJXTE-64A M I X T B - 6 5 - : ^ & M I X T E - « * « m d it w T A Platamilo w x t e w . * '- S. J. Guich i coviV; Ocotaf Zacatal MATIAS ROMERO 16s 52- 16 52 EXPLANATION N A Cenozoic sediments Marine Upper Jurassic-Upper Cretaceous sediments ^V/-V/-~-V~] Middle Jurassic continental red beds ■ W W 'J Permian and Eariy Jurassic plutons / V / V J and post-Eariy Jurassic dikes | Paleozoic greenschist metamorphic facies rocks Middle Proterozoic granulite metamorphic facies rocks High-grade metamorphic sample Igneous sample Scale 1 :250 000 2 4 6 8 10 km 1 Dextral strike-slip fault Dextral strike-slip(?) fault Geologic contact ' Highway o r paved road City ------------ Dirt road Small town Figure 3. Geologic map of the La Mixtequita area in the state of Oaxaca with distribution of samples for this study. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 4. Exposure of the Guichicovi complex along the dirt road Nuevo Centro-Monte Aguila, Oax. The Guichicovi complex in this location consists of granitic gneiss, note the well-defined gneissic texture, with intercalations of amphibolite. Hammer is for scale. Figure 5. Exposure of the intrusive contact between the Guichicovi complex and Permian rocks of the La Mixtequita batholith in Monte Aguila, Oax. Note the angular dark xenoliths of the gneiss in the granitic rocks. Compass is for scale. 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. metamorphic layers were probably inherited from former sedimentary strata. The southern portion of this complex is extensively exposed along the San Juan Guichicovi-Ocotal- Platanillo dirt road. A granulitic hornblende diorite orthogneiss (meta-jotunite) was recognized south of Santiago Tutla Nuevo along the Aguacatenango River. This dioritic pluton is pre-kinematic to the granulite metamorphism. It lies structurally below layered quartzo-feldspathic gneiss, it lacks a metamorphic contact aureole, and it too underwent granulite metamorphism. Rare massive marbles with subangular inclusions ("xenoliths") of calc-silicate gneisses are exposed south of Colonia Sacrificio in the Los Jabalines ranch. A small body of Fe-sulfide-rich clinopyroxenite intruding the layered granulitic rocks was recognized northeast of Encinal Colorado (Murillo and Navarrete, 1992). The Guichicovi complex is intruded by the La Mixtequita batholith. The northern boundary of this Precambrian terrane corresponds to a E-W trending intrusive contact with the southern Permian segment of the batholith (Figure 3). This contact is exposed along the dirt road that connects the villages of San Pedro and Monte Aguila. Small-scale metamorphic roof pendants occur in the Permian plutonic rocks in this vicinity and near Monte Aguila (Figure 5). Another, larger roof-pendant in Early Jurassic plutonic rocks of the La Mixtequita batholith is observed in the vicinity of Brena Torres Nuevo. Here, homblende-clinopyroxene quartzo-feldspathic gneisses occur as remnants of the eroded Guichicovi complex. In addition to these field relationships, an amphibolite and a gneiss from the area of Monte Aguila yielded two anomalous K-Ar ages (Upper Paleozoic) due to reheating during the emplacement of Permian plutons. Moreover, upper intercept U-Pb ages of those plutonic rocks are Late Precambrian which indicates inheritance from lithologies similar to or equivalent to the Guichicovi complex. This is discussed later in the section of geochronology. The La Mixtequita batholith constitutes the northern portion o f the La Mixtequita region (Figure 3). Based mainly on geochronology, spatial relationships, geochemistry and 1 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. field work, the La Mixtequita batholith is a composite assemblage that can be divided into three distinct lithologic groups. These magmatic groups include Permian granitoids, Early Jurassic plutons, and undated post-Early Jurassic dikes. The first two units comprise the main framework of the plutonic complex, whereas the former is a minor component. Permian magmatic granitoids constitute the southern portion of the La Mixtequita batholith. Good exposures of the Permian plutons are observed near the village of La Mixtequita along the MEX-147 Highway. However, the best outcrops of these plutons are observed along the dirt road that connects Santiago Tutla Nuevo with the MEX-147 Highway, In addition, fresh exposures can be observed along the Aguacatengo river near Santiago Tutla Nuevo (Figure 6). In general, the Permian rocks consist of quartz monzonite, typically containing hornblende and biotite as major accessory minerals, grading locally to granite, quartz diorite and quartz monzodiorite with similar accessory mineral assemblages. These rocks are generally undeformed, but cataclastic foliation and magmatic banding are locally observed. In the vicinity of Santiago Tutla Nuevo, pink K- feldspar phenocrysts are typical. Sporadic latitic and andesitic dikes of unknown age crosscut the Permian plutonic rocks. As discussed above, this portion of the La Mixtequita batholith intrudes the Guichicovi complex. Its contact can be observed along the San Pedro-Monte Aguila dirt road. The Early Jurassic magmatic suite comprises the central and northern areas of the La Mixtequita Batholith. The best exposure of these rocks can be seen along the MEX-147 Highway, between La Mixtequita and Villanueva Segundo towns (Figure 7). In contrast to the Permian plutons, the Early Jurassic plutons show a more varied lithology. Rock types include gabbro, monzodiorite, quartz diorite. quartz monzodiorite and pink leucogranite. Accessory minerals of these rocks include hornblende, biotite and pyroxene. Muscovite is a common accessory mineral in the leucogranite. Undeformed gabbro occurs north of Villanueva Segundo. Undeformed pink leucogranites phases are widespread and often are 1 6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6. Exposure of Permian granitoids of La Mixtequita batholith along the Aguacatengo River in the vicinities of Santiago Tutla Nuevo, Oax. Figure 7. Exposure of Early Jurassic granitoids of La Mixtequita batholith along the Victor (San Juan) River in the vicinities of Villanueva Segundo, Oax. 1 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. associated with pegmatites. These plutons crosscut the other lithologies and represent the latest phases of the plutonic activity of the La Mixtequita batholith. Intermediate plutonic rocks exhibit a local and weak foliation or are undeformed. The boundary between the Permian and the Early Jurassic magmatic suites is obscure because the rocks are physically similar in lithology and structure. The Guichicovi complex underlies the sharpest and highest hills (200-1300 m above sea level) in the area, whereas both members of the La Mixtequita batholith comprise smooth hills that lie less than 200 m above sea level. Because there is no contrasting geomorphic difference between the two main magmatic units, mapping of the contact between them was not possible. The latest magmatic activity in the La Mixtequita batholith is represented by sporadic dikes that crosscut the two plutonic phases. These dikes consist of fine-grained latite, andesite, monzonite, diorite, and minor spessartite. In the vicinity of Brena Torres Nuevo and along a walking road going from that small town to the village of Arroyo Lirio, the best exposure of these dikes are observed. Here the dikes trend approximately E-W. The age of these rocks is largely unknown. A Late Cretaceous K-Ar age was reported by Ruiz (1978) for an andesite that probably corresponds to these dikes. However, this date could be anomalous according to field relationships discussed below. Thus, in this work only a post-Early Jurassic age is inferred to these dikes. The Guichicovi complex and the La Mixtequita batholith, along their western boundaries, are in tectonic contact with Lower (?) Paleozoic greenschist facies pelitic schists. The regional strike-slip Valle Nacional fault, representing the main tectonic feature in this region, separates the Guichicovi complex and the plutons from the schist (Figure 3). This regional strike-slip fault is considered to be dextral (Ldpez and Rodriguez, 1986; Torres, 1986) and was active during Late Cretaceous to Early Tertiary time (Araujo, 1983; Lopez and Rodriguez, 1986). It offsets the overlying Mesozoic sedimentary column (see below). 1 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The crystalline rocks of the La Mixtequita region are nonconformably overlain by a succession of Middle Jurassic to Tertiary sedimentary rocks (Figure 3). Included is the Todos Santos Formation, a sequence of continental red beds whose age has been the subject of controversy. Ages assigned to the red beds include Triassic to Jurassic (Gonzalez, 1967 and 1969), Upper Triassic to Middle Jurassic (Torres, 1986), Middle Jurassic (Quezada, 1978; Murillo and Torres, 1990) and Middle to Upper Jurassic (Lopez and Rodriguez, 1986). The unconformity between the crystalline basement rocks and unmetamorphosed red beds is observed along the northern and eastern margins of the La M ixtequita batholith on the MEX-147 Highway, between the towns of Palomares-La Mixtequita and Villanueva Segundo-Felipe Angeles. The absence of a contact metamorphic aureole across the red beds strongly indicates a depositional contact. In the San Juan Guichicovi area, the Todos Santos red beds lie unconformably above the Guichicovi complex. The red beds are, in tum, transitionally overlain by or in fault contact with marine sediments that constitute the Victoria Limestone and the La Zacatera Group (Quezada, 1978; Araujo, 1983). The age o f the marine sediments, based on their fossil assemblages, is Callovian (uppermost Middle Jurassic) to Lower Cretaceous (Quezada, 1978; Araujo, 1983). Therefore, since the Todos Santos red beds lay between the Early Jurassic plutons and the La Victoria Limestone and the La Zacatera Group, they must be Middle Jurassic in age. Likewise, as the post-Early Jurassic dikes apparently do not intrude the Todos Santos red beds, then the age of the dikes is probable pre-Middle Jurassic. However, because the precise age of the red beds is unknown, the dikes could be either Early Jurassic (uppermost) or Middle Jurassic (lowermost). The Middle and Upper Cretaceous rocks of the study area are represented by marine Sierra Madre (platform carbonates) and Alaska formations (weakly metamorphosed thin bedded clastic rocks), respectively (Gonzalez, 1967 and 1969; Quezada, 1978; Araujo, 1983; Torres, 1986; Murillo and Navarrete, 1992). Finally, the Cenozoic in this region consists of marine and continental siliciclastic 1 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. deposits and alluvium (Gonzalez, 1967 and 1969; Quezada, 1978; Araujo, 1983; Torres, 1986). Another important structure in the study area is the Paraiso fault (Araujo, 1983) which lies along the eastern margin of the crystalline rocks of La Mixtequita area (Figure 3). The Paraiso fault parallels the regional dextral Valle Nacional Fault. The classification of the Paraiso fault has been the subject of controversy. Gonzalez (1967) and Quezada (1978) consider it to be a normal fault. To the contrary, Araujo (1983) suggested that this fault is a secondary fault of the Valle Nacional fault with both sinistral and reverse components of slip. The age of Paraiso fault is poorly constrained. Quezada (1978) assigned a Tertiary age to it, whereas Araujo (1983) suggested a Late Cretaceous age. Finally, what is clear is that both the Valle Nacional and Paraiso faults imposed the present NW30°SE orientation on the Guichicovi complex and the La Mixtequita batholith. In Figure 3, a thin selvage of the Todos Santos red beds is caught along the Valle Nacional fault zone between the Lower(?) Paleozoic greenschist metapelites and the high-grade gneisses and magmatic rocks. The red beds in this zone show foliation and low-grade recrystallization. In contrast, red beds along the Paraiso fault do not show advanced mineral recrystallization, but do exhibit rather strong cataclastic deformation. These observations suggests that the Valle Nacional fault may correspond to deeper crustal levels than the Paraiso fault. That the Paraiso fault could be a strike-slip fault (Araujo, 1983) associated with Late Cretaceous-Tertiary tectonics of this region of southern Mexico is consistent with observations made in this study. 20 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PETROLOGY OF THE GUICHICOVI COMPLEX Petrography In order to define the lithologic characteristics of the Guichicovi complex, 47 samples were analyzed using the petrographic microscope. Samples were collected from the northern, eastern, and southern regions of the metamorphic complex. Distribution of samples is presented in Figure 3. The central and western areas of the terrane were not studied due to lack of access. The Guichicovi complex is characterized by an extensive variety of high-grade metamorphic lithologies. In general, the types of rocks found in the Guichicovi complex include: granulite, para- and orthogneiss, amphibolite, and scarce marble, calc-silicate, and quartzite. The mineral assemblages of this metamorphic complex are presented in Table 1 . Granulites in the Guichicovi gneiss are mafic to felsic in composition. In this study, the terms felsic, granitic and quartzo-feldspathic are used as synonyms and indicate the presence of K-feldspar and quartz, with or without coexisting plagioclase. Granulites usually show gneissic, banded and occasionally massive textures and, under the petrographic microscope, exhibit granoblastic and gneissic textures. Major rock types include two-pyroxene, gamet-homblende and graphite-bearing two-pyroxene granulites (Figures 8 and 9). Granitic (chamockite) granulites and a dioritic (jotunite) orthogneiss containing hornblende and hypersthene also were identified. Felsic granulite assemblages include: orthopyroxene±homblende±clinopyroxene and orthopyroxene+clinopyroxene+graphite. Graphite occurs as both grain-boundary crystals and flakes following the rock foliation. Mafic granulites have the following mineral assemblages: plagioclase+orthopyroxene+clinopyroxene±quartz, plagioclase+gamet+orthopyroxene+clinopyroxene+homblende+quartz, plagioclase+garnet+orthopyroxene+clinopyroxene+quartz, and 2 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. Table 1, Mineralogy of the Guichicovi complex SAMPLE CLASSIFICATION LOCATION COORDINATES M N E R A L O G Y U l (N) Long. (W) 9 “ K-Fel Pla Bio Cpx Ops Gar Hbl Cc Sep Graph Sph Rut Zr OM Act <rm) Hp* Mus* MIXTH-9 Granitic gneiss Nuevo Centro-Mante Aguila 95° 15' 32" 17° 09*41" x m X(o) X * MIXTE-10 Px-HH feldspathic gneas Nuevo Ccntro-Mcnt Aguila 95° 15'32" 17° 09*41" X(o.p) X X ♦ ♦ - MIXTE-U Granitic gneiss Nuevo Centro-Mame Aguila 95° 15* 12" 17® 08* Se X(@) X (p .o ^) X(ap) -(nn) MDCTE-12 Amphibolite Mome Aguila 95° 14' 43" t t 0 07* 37“ X X X + X M1XTE-13 Amphibolic Mccac Aguila 95° 14' 14" 17® 07*27" X X - + X MDCTE-14 Granitic gneiss Mctse Aguila 95° 13*50" 17° 07*13" X(@) X{p,o) X(mk) - - ♦ - MDOE-I5 Hbl Two-Pz felsic granulite C d. Saoificio 95° 13'11" 17° 07* 39" X X X X ♦ X MSXTE-16 Retrograded Gar grauolic C d. Saoificio 95° 12* 12" 17° 07* 40" xt@ ) X X X X ♦ MDCtE-17 Diop-Scp-Grapfa marble C d . Saoificio 95° 11'42" 17° 08* 13" x ■H p) ♦ X X X + + MDCTE-18 Calc-sOicate gneiss C d . Saoificio 95° 11'42" 17° 08*13" X Xfp.omc) X(mk) + X X + X MIXTE-19 Cps-Sph-Pla gneiss C d . Saoificio 95° IL 19" 17° 08* 33" X X ♦ X MIXTE-20 Hbl granitic gneiss C d . Saoificio 95° 10*37" 17° 08*41" Xl@) X(ojnc) X(ap) + ♦ MDOE-21 Gar-Px quartzo feldspathic gneiss San Juanito 95° 10' 14" 17® 04*09" X Xfpznc) X X X + MIXTE-22 Gar-Px-Hbl quartzo fclcbpathic gneiss San Juanita 95° 10* 14" 17° 04* 09" X X(P) Xfmk, ap) ♦ X ♦ ♦ MDCIE-23 Retiogradrd Hbl-Bio diocxbc gneis* San Juanho-Cd. Saoificio 95° 09* 50* 17° 06* 31" ♦ X X * ■ MDCTE-24 Cpx quartzo-feldspathic gneiss Zacatal-El Ocotal 95° 11*09" 16° 56* 14" X X(o) X X + ♦ MIXTE-25 A m phidde Zacatal-Hl Ocotal 95° 10*55" 16® 56’ 28" X X - X MDOE-26 Hbl Tw oPx felsic gramlite Zacatal-El Ocotal 95° 10*06" 16° 56* 38" X Xto.p) X ♦ X ♦ - X MDCTC-28 Hbl quartzo-feldspathic gneiss Zacatal-El Occtal 95° 10* 34" 16° 56* 31" X X(p) X ♦ X ♦ MIXTE-29 Qtz-Mgt gneiss Zacatal-El Ocotal 95° 09*02" 16° 56* 43" X X MIXTE-30 Gar granitic gneiss El Occtal-San Juan Guichicovi 95° 07' 17" 16° 56*44" X X(P) X + - - ♦ MIXTE-31 Granitic gneiss El OcouJ-San Juan Guichicovi 95° 07* 03" 16° 56* 32" X X(p) X - MIXTE-37 Hbl-Cpx quartzo-feldspathic gneiss Bretu Tones Nuevo 95°14* 35" 17°14*39" X X X X - X MIXTE-M. Retrograded dwntK grwuliU. crthogneiss Santiago Tuda Nuevo 95° 21* 20" 17° 08*45“ X -<nn) X ♦ ♦ ♦ X M1XTE-47 Hbl dxmiK artbogneua Santiago Tuda Nuevo 95'* 21* 20* 17° 08' 45“ ♦ X(ap) Hnn) X + X MIXTE-V8 GrarulK. gneiss San&ago Tuda Nuevo 95° 21*20" 17° 08*45" Xl@) Ho) X Hnn) •Hnn) - MIXTE-19 Gar granitic gneiss Santiago Tuda Nuevo 95° 21* 20" 17® 08*45" X <@ > X ■Hnn) ♦ ♦ MIXTE-50 Gar granitic gneas Santiago Tuda Nuevo 95° 21*20" 17° 08*45" X(@) X(p) X(@) •Hnn) X - - MIXTE-51 Cpx quartzo-feldspathic gneiss Santiago Tuda Nuevo 95° 21* 21" 17® 08*48" X X X X ♦ X MIXTE-52 Hydotherraalized Cpx feldspathic gneiss Santiago Tuda Nuevo 95° 21*21" 17° 08* 48" X(o) X X ♦ ♦ MIXTE-60 Cpx granitic gneiss El Ocotal 95® 08' 11" 16° 57*26" X X(p) X ♦ MDCTE-61 Gar-Graph qcaxtzo-fcldspithic paragneiss El Ocotal 95° 08* 00“ 16° 56* 37" x<@> X(p) X X X - - MDCIE-62 Gar-Hbl TwoPx mafic granulite El Ocotal 95° 08*01" 16° 58* 01" x<@) X - X X X X . X MDCTE-63 Hy<feothennaIi2ed Grt baste granulite El Ocotal 95° 08* 01“ 16° 58*01" + 4mc) X X X + X MIXTE-64 Gar-Hbl mafic grazulite El Ocotal 95° 09* 16" 16° 58* 21" X X X X + MIXTE-65 Granitic granulite (charnockite?) El Ocotal 95° 08*57" 16° 58* 04" xi@ ) X(p) X X ♦ MIXTE-67 Retrograded HU quartzo-feldspaihx: gneiss Brcna Tones Nuevo 95° 12*41" 17° 14* 32" ♦ X - X X X CHOX-58 Hydrothennalixed Gar-Hbl granitic gneiss Platanilo 95° 14* 28" 16° 57*04" X X(p) X(mk) X X - ♦ X ISTEH-33 Graph Two-Px felsic granulite Eneiml Colorado 95° 08*20" 17° 01* 47" X(@) X(p,@> ♦ - X X ♦ - - ISTEH-34 HU Two-Px mafic grarxtlite Enciml Colorado 95° 07* 27" 17°01*21" X X X X X ISTEH-35 Cpx-Ha gneiss Frvm^l Cdcrado 95° 07* 10" 17°01* 11" X X * 1STEH-36 Cataclastic granitic gneiss Enchol Cdcrado 95® 07* 03“ 17° 00* 56" X X(mc,p) X - IS TEH-42 Hbl quartzo-feldspathic gneiss C d . Saoificio 95° 10* 12" 17° 09* 13" x<@) X - X ISTEH-79 Cpx-Kbl quartzo-feldspathic gneiss Nuevo Ccrsro-Mcnfc Aguila 95° 15*34" 17° 09*57" X X X X X + - ISTEH-8I Graph quaitxhe Nuevo Centn>Mocfc Aguila 95® 15'02" 17° 08* 38" X(@l X(rm) ♦ - X IS TEH-8 2 Am rhialite Nuevo Centro-Monte Aguila 95° 15*04" 17° 08* 26" X X X SYMBOLS: Q c= quartz, K-Fe!» alkali feldspar (p= perthiie, mc= rxucrodme, o s arthodase), Pl»= ptsgioclase (mk= mynnekilc, ap= antiperthite), Bios bionic, Px= pyroxene, Opx= onbopyroxene, Cpx= clnopyroxene, Gar* garret, Hbl* hornblende. Acts actmdite, Cc= calcitc, Scp= acapdite, Graphs graphite, Spb= aphene, Ap= apatite. Ruts ruble, Mus= muscovite, Zr= zircon, OM= opaque minerals, Mgt= magnetite, Ep= epidote. OTHERS: @= acicular inclusions (runic). mj= retrograded metamorpiuc origin. X= >5%, ♦* 5-1%,-s traces; in modal quamitics baaed on visual cstanatca.(A ) secondary origin. Figure 8. Photomicrograph of a quartz-bearing garnet two-pyroxene mafic granulite (sample MIXTE-62) of the Guichicovi complex. Biotite appears as a minor component in the lower part. Length of the photo is approximately 2.5 mm long and 1.5 mm wide. Figure 9. Photomicrograph of a gamet-homblcnde-biotite two-pyroxene mafic granulite (sample MIXTE-64) of the Middle Protcrozoic Guichicovi complex. Note the strong red color of the biotite and the brown color of hornblende. Length of the photo is approximately 2.5 mm long and 1.5 mm wide. 23 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. plagioclase+orthopyroxene+clinopyroxene+homblende. The dioritic orthogneiss has the mineral paragenesis plagioclase+orthopyroxene+homblende±biotite. The K-feldspar in granulites is typically perthitic and pyroxene is usually not exsolved. Some granulites show evidence of retrograde metamorphism in the form of fibrous and prismatic green pale amphibole (tremolite-actinolite) replacing the orthopyroxene and scarce clinopyroxene. The ubiquitous presence of orthopyroxene in the Guichicovi complex indicates that the crystalline terrane underwent uniform granulite facies regional metamorphism. Gneisses present a diverse mineral paragenesis. Under the petrographic microscope, gneisses have gneissic, banded, and granoblastic textures. Granitic gneiss is commonly composed of quartz (occasionally with fine acicular inclusions of probable rutile), K-feldspar (perthite, orthoclase and a little microcline) and plagioclase (sometimes myrmekitic and antiperthitic), ± garnet, hornblende and/or biotite. K-feldspar-free rocks include homblende-clinopyroxene quartzo-feldspathic gneiss and retrograded homblende- biotite dioritic gneiss. Gamet-graphite quartzo-feldspathic paragneiss was also identified. Intermediate gneisses include clinopyroxene-plagioclase gneiss with or without hornblende. Sphene, zircon, apatite and opaque minerals occur in most of the rocks of the Guichicovi complex. Local regressive metamorphism and hydrothermal alteration was recognized in some of the gneisses described above. Retrograde metamorphic features include: partial and total replacement of pyroxenes and hornblende by pale green amphibole (tremolite- actinolite), and aggregates of fine biotite after garnet and opaque minerals. Hydrothermal alteration includes strong alteration of plagioclase to sericite and epidote, and the presence o f chlorite as veins or replacing former biotite or other mafic minerals. Amphibolite commonly occurs in the Guichicovi complex. Its texture is gneissic in both hand sample and under the petrographic microscope. These rocks are made up of 2 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. plagioclase and hornblende, sometimes with significant amounts of sphene. Some samples show the same effects of retrograde metamorphism and hydrothermalism. Other lithologies observed in the Guichicovi complex in minor proportions are quartz-magnetite gneiss, graphite-bearing quartzite and calcareous metasediments. The quartz-magnetite gneiss is exclusively composed of well-developed bands of quartz and magnetite. Graphite-bearing quartzite consists of ~ 80% quartz with subordinate amounts of graphite and minor clinopyroxene and secondary fine-grained biotite and actinolite. The calcareous metasediments include massive diopside-scapolite-graphite marble and calc- silicate gneiss. They are distinguished by the high content of Ca-rich minerals like calcite, scapolite and diopside; the later sometimes exhibits rims of pale amphibole (tremolite- actinolite) due to regressive metamorphism. Other mineral phases in the calc-silicate gneiss include: quartz, orthoclase, microcline, plagioclase, and sphene. The lithologic variation of the Guichicovi complex suggests that the protoliths included a sedimentary sequence intruded by intermediate and probably mafic intrusions. The quartzo-feldspathic granulites and gneisses are interpreted to be derived from arkosic sediments. This interpretation is supported by the presence of abundant flakes of graphite laying parallel to the gneissosity in some rocks. Stable isotope analyses conducted by the author of some foliated graphites gave 5C 13= -22.87 and -22.35 %o (Table 2), consistent with an organic origin (Barker and Friedman, 1969; Hoefs, 1980). In contrast, the grain- boundary graphite was found to have a 8 C I3= -14.46 %c (Table 2). Moreover, "metamorphic layering" probably was inherited from the former layer surfaces. Other evidence of the sedimentary heritage of this metamorphic complex is the presence of scarce intercalations of massive marbles and calc-silicates which represent original impure calcareous sediments. The amphibolites might have been derived from marls or mafic lavas. Evidence of pre-granulite metamorphic plutonism is represented by the dioritic orthogneiss (meta-jotunite). 2 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. i I i i Table 2. Carbon isotopic composition of graphite from the Guichicovi complex SAMPLE CLASSIFICATION LOCATION 313C (%«) REMARKS M1XTE-61 Gamet-graphite quartzo-feldspathic paragneiss El Ocotal-E! Tejon -22.35 1STEH-33 Graphite two-pyroxene felsic granulite Encinal Colorado -14.46 1STEH-81 Graphite quartzite Nuevo Centro-Monte Aguila -22.87 TRAP-3 <•> Graphite biotite quartzo-feldspathic paragneiss El Trapiche, Oax. -22.90 91-MEX-l <•) Wollauunitc quartz-graphite-gamel paragneiss -9.49 Boundary between quartz-rich zone and wollastonile-rich zone * -7.44 Quaru-rich zone of sample " .. -6.64 Quartz-rich zone of sample NOTE: (*) Samples correspond to the Grenville-age Oaxacan complex from the location reported by Goel et al. (1991). to On Thermobarometry MINERAL CHEMISTRY Garnet, pyroxene, plagioclase, hornblende and biotite compositions were determined by microprobe analysis in order to estimate metamorphic conditions (pressure and temperature) of the Guichicovi complex. A description of these mineral phases is given below. In addition, a list of thermodynamic and mineral abbreviations used in this study is also provided: T= temperature in °C or K P= pressure kbar= kilobars K= equilibrium constant Kq= distribution coefficient W = Margules parameters a= activity y= activity coefficient R= gas constant AG°= Standard Gibbs free energy change a.p.f.u.= atoms per formula unit Gar= garnet Alm= almandine Gros= grossular Pyr= pyrope Sp= spessartine And= andradite Px= pyroxene Cpx= clinopyroxene Opx= orthopyroxene Wo= wollastonite En= enstatite Fs= ferrosilite Hd= hedenbergite Qtz= quartz Bio= biotite Ann= annite Phlo= phlogopite Hbl= hornblende Pla= plagioclase Ab= albite An= anorthite 27 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GARNET Garnet is a common phase in the Guichicovi complex. Garnet was analyzed from two gamet-pyroxene quartzo-feldspathic gneisses (samples MIXTE-21 and -22) and three mafic granulites (MIXTE-62, -63 and -64). Garnet from the felsic rocks occurs as euhedral grains and also forms symplectic arrays with quartz and feldspar. Garnet from mafic granulites is found as euhedral crystals coexisting with pyroxene, hornblende and biotite (Figures 8 and 9). Garnet formulas were normalized based on 8 cations and Fe3+ was calculated from stoichiometry and charge balance. Garnet is typically poor in Mn (<3% of spessartine) and Ca-rich (grossular + andradite ~ 20 mole%). In terms of Fe2+, M g and Ca, there are clear chemical differences between garnet in felsic gneisses and that in the mafic granulites (Figure 10). Garnet from felsic rocks has greater almandine component. Garnet of sample MIXTE-21 can be described as Alm 6 7.6 8Pyr4.7 G ros 1g.2 1And4 .6Sp2 whereas garnet in sample MIXTE-22 is Alm 6 6-6 8Pyr9-iiG rosi4_igAnd3.8Sp2-3 . On the other hand, garnet in the mafic rocks is richer in pyrope and poorer in almandine components than garnet from the felsic gneisses. The garnet composition among the mafic rocks is also more homogeneous: Alm54.59Pyr21.2 6Gros 15.2oAndo-4Sp 1.2 (M IXTE-62), Alm55_59Pyr2o-24Gro si2-i9Ando.6Sp2-3 (M IXTE-63), and Alm56-61 Pyr 18 -2 4G r°s]3. [8Ando^Sp 1.2 (M IX TE-64). A significant systematic zoning in garnets is not evident, but slight differences between core and rim compositions are recognized. Ca is homogeneous throughout the grains in most samples. However, garnet rims in all mafic granulites and one felsic gneiss (MIXTE-22) show lower content in Mg than cores. Most garnet rims are richer in Fe2+ than are cores. These compositional variations are attributed to chemical re-equilibration 2 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ^ A Im + ^ S p •21 MKTE-62 X, (b) M IX TE-63 50 Figure 10. (Almandine+spessartine)-grossular-pyrope ternary plot of gamets, including cores and rims, from the Guichicovi complex, (a) Open circles= sample MIXTE-21, open triangles= sample MIXTE-22, and open squares= sample MIXTE-23. (b) Open diamonds= sample MIXTE-63. (c) Minus symbols= sample MIXTE-64. 29 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. X A lm + X Sp MIXTE-64 Continuation of Figure 10. 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. between garnet and the coexisting phases (pyroxenes, hornblende and/or biotite) during retrograde cooling. PYROXENE Pyroxene is a common mineral phase in the Guichicovi complex. It occurs in both felsic and mafic rocks. Coexisting orthopyroxene and clinopyroxene is a common feature of these rocks (e.g. in samples MIXTE-15, -62, -63, -64, ISTEH-33 and -34). Clinopyroxene is both subhedral and euhedral, and pale green color in plane light (Figures 8 and 9). Clinopyroxene is typically fresh and shows no exsolution. Orthopyroxene is both subhedral and euhedral and shows neutral color in plane light. Some grains (sample ISTEH-34) exhibit the distinctive pleocroism of hypersthene, that is, a change from neutral to reddish color in plane light. Orthopyroxene also shows no exsolution, however, it has a uralitic rim alteration composed of actinolitic amphibole and/or chlorite. In general, clinopyroxene (augite) compositions are independent of the bulk rock composition. Pyroxene formulas were normalized based on 4 cations and Fe3+ was calculated from stoichiometry and charge balance. In terms of the major components (wollastonite, enstatite and ferrosilite), clinopyroxene from felsic samples shows significant chemical variations, particularly in terms of Fe/Mg ratio or enstatite and ferrosilite components. Lindsley’ s (1983) clinopyroxene normalization scheme was used to calculate the wollastonite, enstatite and ferrosilite components. Clinopyroxene compositions from felsic samples are W 0 4 3 .4 5En4 6 .4 7 Fs9.1 1 (ISTEH-33), W 0 4 1 .4 3 En 2 0-2 4Fs3 3 .3 9 (M IXTE-21), W 0 4 0 .4 3En 2 8 -3 iFs2 7 .3 0 (M IXTE-22), and W 0 4 3 .4 5En3 8F si8 .2 0 (ISTEH-33). Clinopyroxene components from mafic rocks have higher enstatite proportions and are fairly homogeneous in composition. Average com positions are: W 0 4 1 .4 2 En4 3 .4 6 F s 13.15 (ISTEH-34), W 0 3 9 .4 0En4 1.4 2 F si 8 .2 0 3 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (M IXTE-15), W041.44En41.44Fs 13.i6 (MIXTE-62), W038.45En39.44Fs 13.is (MIXTE-63), and W041.43En41.43Fsj5.i8 (MIXTE-64). Slight compositional variations are distinguished between cores and rims in clinopyroxene of some samples. Samples MIXTE-21 and -22 (felsic gneisses), -15 and -62 (mafic granulites) have rim compositions that are richer in Mg than their corresponding cores. This weak chemical variation is also observed in samples MIXTE-63 and -64 (mafic granulites). In samples ISTEH-33 (felsic granulite) and ISTEH-34 and MIXTE-15 (mafic granulites) the clinopyroxene cores are richer in Fe2+ than rims. Likewise, Ca in samples ISTEH-33, -34 and MIXTE-15 is more concentrated in rims than cores. Conversely, in samples MIXTE-21, -22, -15, -64, and ISTEH-34 Na is depleted in the rims versus the cores. Orthopyroxene is generally homogenous within a sample, but shows chemical variations among the different samples. Although Ca is a minor component, it is found in slightly.higher amounts in orthopyroxene of mafic granulites (0.025-0.045 a.p.f.u.) than in the orthopyroxene of felsic granulites (0.022-0.026 a.p.f.u., samples ISTEH-33 and MIXTE-26). Mg and Fe2+ zoning was not recognized and Mg and Fe proportions do not relate to rock bulk composition. Mg is not zoned, except in sample MIXTE-64 (mafic granulite) in which rims have a higher content (1.084-1.104 a.p.f.u.) than cores (1.097- 1.163 a.p.f.u.). M g# (Mg/Mg+Fe2+) and wollastonite, enstatite and ferrosilite components show no systematic distribution with orthopyroxene composition. Mg# of orthopyroxene ranges from 0.52-0.62 in mafic granulites and 0.49 to 0.98 in felsic granulites. Orthopyroxene compositions of the felsic rocks are: W 0 1 -En6 s.6 9-Fs3 0 .3 1 (ISTEH-33) and W 0 2 En 4 7.4 8Fs4 3 .4 5 (MIXTE-26) and those of mafic granulites are: W 0 1 .2En5 8-6 0Fs3 9 .4 0 (ISTEH-34), W o,-2En 5 2.5 3Fs4 5 .4 7 (MIXTE-15), W 0 1 .2 En 5 5.6 0Fs3 9 .4 2 (M IXTE-62), and W 0 1 . 3 E n 5 6 F s 4 0 . 4 2 (MIXTE-64). 3 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PLAGIOCLASE Plagioclase composition varies from sample to sample, but within any one sample, variation in the form of zonation is slight. Plagioclase from the felsic gneisses is subhedral, fresh and generally shows no twining. Plagioclase from the mafic granulites is both subhedral and euhedral, showing commonly albite-type twining and alteration to sericite and epidote. Plagioclase formulas were normalized based on 8 oxygens per formula unit. Plagioclase from a felsic gneiss (MEXTE-22) is An2 4 .2 8 whereas plagioclase from a mafic granulite (MIXTE-63) is AJI4 6 .4 8 and in another mafic granulite (MIXTE-62), the plagioclase is An 74_77 (Figure 11). In general, the orthoclase component is <1.7 % in all samples. HORNBLENDE Two different types of hornblende from the Guichicovi complex were analyzed by microprobe. Hornblende from a felsic gneiss (MIXTE-21) is typically green in color. This hornblende is subhedral and occasionally forms symplectic-type intergrowths with garnet. Hornblende from the mafic granulites (MIXTE-62 and -64) shows a distinctive brown color and is both subhedral and euhedral (Figure 9). The two different hornblende types have different chemistries. Hornblende formulas were normalized to 13 cations. According to the amphibole classification scheme of Leake (1978), green hornblende is magnesian hastingsite and brown hornblende is ferroan pargasitic hornblende and ferroan pargasite (Figure 12). The latter is richer in Ti and F and poorer in Cl. In general, hornblende is not zoned. BIOTITE Biotite from two mafic granulites (MIXTE-62 and -64) was analyzed. Biotite is a minor mineral phase in those samples and occurs as both large and small grains. Largest 3 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.80 (a) < Filled symbols* interiors Open symbols* rims 0.78- 0.76- ° MIXTE 21 0.74- 0.72- MIXTE f e 22 0.70- ---------- ---------- (------------1 ----------- 1 ----------- (b) S 0.20 0.60- 0.58 -. 0.56 • ■ 0.54 ■ ■ 0.52 0.50 (C ) 3 0.22 + 0.40 0.42 0.24 0.26 An 0.28 0.30 Filled symbols* interiors Open symbols* rims MIXTE 63 — I ---------- 1 ---------- h - 0.44 0.46 0.48 An 0.50 U .J U • Filled symbols* interiors Open symbols* rims 0.28 • 0.26- O /> MIXTE 0.24 - 1 62 > ♦ 0.22 - 0.20- ----------------- 1 ------------------j------------------ ^---------- 1 ---------- 0.70 0.72 0.78 0.80 0.74 0.76 An Figure 11. Compositional variation of plagioclasc from the Guichicovi complex. Symbols: filled= interiors and open= rims. Samples as follows: (a) MIXTE-21= squares and MIXTE-22= circles, (b) MIXTE-63= triangles and (c) MIXTE-62= diamonds. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.0 - Silicic edenitc (a) 4 * > U h 4* 00 s ’ S s 0 .5 ------------------ Silicic ferro-edenite ,3 + ^ a iVI (Na+K)AS0.50; Ti<0.50; F e ^ A l I Magnesio- hastingsite Edcnite L____________ | |Magnesio-| I | hastingsitic a l Edenitic |homblende| ^omHcndej-------j ----------- I I I Magnesian- |_________|Magnesianj hastingsite • .hastingsitic. 'hornblende1 Feno- I /•: : : m ix te - 2 1 Ferro-edcnite ■ edenitic . hornblende' i-— 4 (Hastingsitic| Hastingsite hornblende i (b) 1.0 - 4 * 4 > lu 1 ° - 5 " 5 a 2 0.0 8.0 Silicic edcnite Silicic ferro-edenite 7.5 (Na+K)A>0.50; Ti<0.50; Fe3+<A1VI Edcnite I . JL. I I I I I | Patgasitic | p i(e |homblende| Edenitic | MIXfTE-62 jhomblendei *P I I |_________ | Feiroan- MIXTE-64 | patgasitic | Ferroan-pargasite |homblende| Ferro- |________ _ J ___________________ Ferro-edenite | edenitic . . ; hornblende Feno- I pargasilic I Ferro-pargasite |hornblende| I L _ — h 7.0 6.5 —F- 6.0 Si Figure 12. Classification of amphiboles from the Guichicovi complex according to Leake (1978). (a) Sample MIXTE-21= filled circles, (b) Open diamonds= sample MIXTE-62 and open circles= sample MIXTE-64. 3 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. crystals appear to be in textural equilibrium with coexisting mafic mineral phases indicating an origin close to the peak metamorphism (Figure 9). Biotite is typically Ti-rich (up to 5.18 wt%), unzoned and exhibits a strong reddish color in plane light (Figure 9). In order to keep the well-known vacancy in the octahedral site of biotite (Guidotti, 1984), the biotite was normalized to 11 oxygens per formula unit. F content in biotite of the Guichicovi complex is consistent with the observation by many authors (Guidotti, 1984) that F in biotite from granulite-grade rocks generally exceeds 1.0 wt% and in the Guichicovi biotites, F ranged 1.1 to 3.4 wt%. Two small grains from sample MLXTE-62 had higher Mg and lower Fe* and Ti. The contrasting composition of the large and small biotites and the textural appearance suggest that the small biotites are late (post-peak metamorphism) in origin. BAROMETRY The barometric conditions of the Guichicovi complex were estimated using multi phase mineral equilibria. The presence of the gamet-orthopyroxene-clinopyroxene- plagioclase-quartz allows the application of two barometric calibrations (Newton and Perkins, 1982; Perkins and Chipera, 1985a). These two calibrations are discussed below. Newton and Perkins (1982) calibrated two geobarometers using determined thermodynamic data, principally enthalpy of the solution and heat capacity measurements. Their barometers are based on the equilibria of the mineral association gamet- orthopyroxene-plagioclase-quartz. The essential principle of these barometers is the strong pressure dependence of the Ca exchange between coexisting garnet and plagioclase (Spear, 1989). One calibration involves orthopyroxene and the other clinopyroxene. The orthopyroxene-reaction is: CaAl2Si208 + M g2Si20 6 = l/3Ca3Al2Si30)2 + 2/3Mg3Al2Si30)2 + Si02 Anorthitc Enstatite Grossular Pyropc Quartz 3 6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The clinopyroxene-reaction is: CaAl2Si20 8 + CaMgSi20 6 = 2<3Ca3Al2Si3O i2 + l/3M g3Al2Si30 i 2 + S i0 2 Anorthite Diopside Grossular Pyrope Quartz The barometric equations for these reactions are: P(O px-reaction) (bars) = 3944 + 13.070T(K) + 3.5038T(K) In KA P(C px-reaction) (bars) = 675 + 17.179T(K) + 3.5962T(K) In KB ( 1) (2) where the equilibrium constants (K . and K„) are: 3 Gar-Gar2 Ka = M g for the (Opx-reaction) and S-A n^-En _.Gar2_Gar Kb = _ 9 l Mi for the (Cpx-reaction) ^•An^Dio Activity models for garnet components are: (3300 - 1.5T)(Xgf + X fip ffi) RT a M g = 7MgX Mg. where y g = exp where, R = 1.987 cal/mol K (3300- 1.5T)(x£f2 + x £ n a r Xgf) RT Activity model for anorthite is: ^■ A n- Y A h XA n(l + XA n )2 where yAn= exp XA b 2 [2025 + 2XA n (6746 - 2025)] RT 37 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The activity models for the pyroxene components are given by: a En = XMgXMg in orthopyroxene and aoi0 = X ^ X ^ g in clinopyroxene These activity models were adopted from previous studies including: Ganguly and Kennedy (1974) for garnet (ternary solution model), Newton et al. (1980) for plagioclase anorthite and W ood and Banno (1973) for enstatite and diopside ("ideal two-site"). The stated uncertainties for these geobarometers are 1500 bar for the orthopyroxene-barometer and 1600 for the clinopyroxene-barometer (Newton and Perkins, 1982). Perkins and Chipera (1985a) also calibrated the orthopyroxene reaction barometer and its equivalent Fe-reaction using theoretical, experimental, and thermodynamic data. The corresponding two calibrated reactions are: Mg-reaction: C a 3 A l2 S i3 0 j2 + 2 M g 3 A l2 S i3 0 j2 + 3 S i0 2 = 3 M g 2 S i2 0 g + 3 C a A l2 S i2 0 s Grossular Pyrope Quartz Enstatite Anorthite Fe-reaction: C a 3 A l2 S i3 0 i2 + 2 F e 3 A l2 S i3 0 i2 + 3 S i0 2 = 3Fe2Si2C>6 + 3 C a A l2 S i2 0 s Grossular Almandine Quartz Ferrosilite Anorthite Note that the Mg-reaction is the same as the Opx-reaction of Newton and Perkins (1982) but written in the opposite direction. The geobarometric equations are: P(M g-reaction) = 6.1346 - 0.3471 In Ki + 0.0136T(°C) - 0.001140T(°C) In Kj (3) P ( F e - r e a c t i o n ) = 0.0630 - 0.3482 In K2 + 0.0143T(°C) - 0.000997T(°C) In K2 (4) 3 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. where the equilibrium constants (Kj and K L j) are: Ki = a En a A n for the Mg-reaction and S-GrosS-Pyr K2 = a Fs a A n . for (he Fe-reaction. S-GrosS-Alm^ The activity models for these calibrations were also adopted from other studies. Garnet is treated as four-component solutions and their activities are calculated according to the model of Ganguly and Saxena (1984). Activity of anorthite is estimated using Newton's (1983) model. Activities of the pyroxene components are calculated according to the ideal two-site mixing model of Wood and Banno (1973). The resulting expressions for the activity models are (Perkins and Chipera, 1985a and 1985b): for garnets, the activity coefficients are: y g f = ex p {x £ a(1.52 - 5.17XFe) + x £ ,g(0.10 + 2.26XFe) + XCaXMg(0.40 - 1.45XFc + 1.50XCa- 1.50XMg) + XcaXvm(0.23 - 2.58XFe) + X MgXMn(0.66 + 1.13XFe)} -ygf = exp{X^jg( 1.24 - 3.00XCa) + X(le(- 1.07 + 5.16XCa) + X MgX Fe(0.05 + 1.08XCa + 1.13Xvig- 1.13XFe) + XMgXMn(- 0.99 - 1.50XCa) + XFeXMn(0.23 + 2.58X ca)} y g = exp{XFe( 1.23 - 2.26XMg) + x £ a(- 0.26 + 3.00XMg) + XFeXCa(0.92 + 0.37XM g + 2.53XFe-2.53Xca) + X FeX Mn(2.14- 1.13XMg) + 1.48Xj*,n + XcaXMll(1.96 + 1.50XMg)} and the garnet component activities are: 9-Alm= (TF^XpgO , aGros= (yCa^Ca^ ■ and apyr= (TM g XMg) for anorthite: 3 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. where yAn= exp x j b (1032 + 4727XA n ) T(K) for pyroxene components are: a En = X^gXMg and a Fs= X ^ X & ? * Sample MIXTE-62, a mafic granulite with quartz as a subordinate phase, was used for barometric purposes of the Guichicovi complex. Core compositions were utilized for geobarometry. When the four different barometric equations are applied to this sample, the resulting pressures are markedly discordant (Table 3). Figure 13 shows the discrepancy among the different barometers for a single mineral assemblage (gamet.a3, orthopyroxene.al, clinopyroxene.a2, plagioclase.al). For thermometric reference, the average of two-pyroxene thermometry (837 °C; Lindsley, 1983) and gamet-clinopyroxene thermometry (717 °C; Ellis and Green, 1979) calculated for the Guichicovi complex (discussed later) were considered. At 837 °C, for the mineral assemblage considered, the results were as follows: 7.4 and 4.8 kbar for the Opx- and Cpx-reactions of Newton and Perkins (1982), respectively and 8.5 and 9.7 kbar for the Mg- and Fe-barometers of Perkins and Chipera (1985a), respectively. At 717 °C, for the Newton and Perkins (1982) calibration, 7.3 and 4.6 kbar were estimated for the Opx- and Cpx-reactions, respectively. Using 717 °C, the Perkins and Chipera (1985a) calibration yielded 7.8 and 8.2 kbar for the Mg- and Fe-barometers, respectively. 40 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. Table 3. Garnet-orthopyroxene-clinopyroxene-plagioclase-quartz barometry for the Guichicovi complex (sample MIXTE-62, cores) GARNET a3 a3 a3 a3 f3 f3 f4 f4 i4 i4 i5 i5 ORTHOPYROXENE a l a l a2 a2 fl fl f2 f2 il il i2 i2 CLINOPYROXENE a2 a2 a3 a3 f3 f3 f4 f4 i4 i4 i3 B PLAGIOCLASE al al a2 a2 fl fl f2 f2 il il i2 i2 XAlm.Gar 0.561 0.561 0361 0.561 0.559 0.559 0.555 0355 0.554 0.554 0.557 0.557 XPy.Gar 0.237 0.237 0337 0.237 0350 0.250 0.251 0.251 0.252 0.252 0.253 0.253 XGros,Gar 0.180 0.180 0.180 0.180 0.167 0.167 0.158 0.158 0.155 0.155 0.177 0.177 XSp.Gar 0.014 0.014 0.014 0.014 0.011 0.011 0.014 0.014 0.014 0.014 0.013 0.013 XFe.M 1 .Opx 0.384 0.384 0.374 0.374 0.372 0.372 0.378 0.378 0.382 0.382 0.372 0.372 XFe,M2,Opx 0.375 0375 0.362 0.362 0.362 0.362 0.369 0.369 0.378 0.378 0.363 0.363 XMgJvfl.Opx 0.560 0.560 0.560 0.560 0.579 0.579 0.574 0.574 0.576 0.576 0.570 0.570 XMg.M2.Opx 0.574 0.574 0378 0.578 0395 0.595 0.587 0.587 0382 0.582 0.585 0.585 XFe,Ml,Cpx 0.246 0.246 0327 0.227 0.255 0.255 0.242 0.242 0.229 0.229 0.233 0.233 XCa.Cpx 0.865 0.865 0.875 0.875 0.865 0.865 0.874 0.874 0.881 0.881 0.877 0.877 XMg.Cpx 0.735 0.735 0.722 0.722 0.726 0.726 0.742 0.742 0.750 0.750 0.736 0.736 XAn 0.763 0.763 0.762 0.762 0.739 0.739 0.763 0.763 0.758 0.758 0.766 0.766 XAb 0.235 0.235 0.237 0.237 0.260 0.260 0.236 0.236 0.241 0.241 0.232 0.232 T (°C ) 717 837 717 837 717 837 717 837 717 837 717 837 Gar-O px-Cpx-Pla-Qlz (Newton & Perkins, 1982) K (Opx-reaction) 0.062 0.059 0.062 0.059 0.060 0.057 0.057 0.054 0.056 0.054 0.068 0.064 K (Cpx-reaction) 0.025 0.024 0.025 0.024 0.024 0.022 0.020 0.019 0.019 0.018 0.026 0.025 P(kbar) for Opx-reaction 7 J 7.4 7.2 7.4 7.1 7 3 6.9 7.1 6.9 7.1 7.6 7.8 P(kbar) for Cpx-reaction 4.6 4.8 4.6 4 3 4 3 4.5 3.8 3.9 3.6 3.7 4.7 5.0 Average P(kbar) for O px-reaction 7.2 7.4 ± 0.2 0.3 Average P(kbar) for Cpx-reaction 4.3 4.4 ± 0.5 0.5 G ar-O px-Pla-Q tz (Perkins & C hipera, 1985a) K(Mg-reaction) 1000.49 1000.49 1018.37 1018.37 1347.46 1347.46 1705.97 1705.97 1764.31 1764.31 877.65 877.65 K(Fe-reaction) 7.177 7.177 5.931 5.931 6.972 6.972 10.087 10.087 11.652 11.652 6.071 6.071 P(kbar) for Mg-reaction 7.8 8.5 7.8 8 3 7 3 8.1 7.2 7 3 7 3 7.8 8.0 8.7 P(kbar) for Fe-reaction 8.2 9.7 8.4 9 3 8 3 9.7 7.9 9 3 7.7 9.1 8.4 9.9 Average P(kbar) for M g-reaction 7.6 8 3 ± 0.3 0.4 Average P(kbar) for Fe-reaction 8.1 9.6 ± 0.3 0.3 NOTE; As T reference, the average two-pyroxene T (837 °C) and the gamel-clinopyroxene T (717 °C). a, f and i represent mineral spots analyzed by microprobe. 11 10 9 8 7 6 + 5 4 o 3 X -a AT, k r z i (Gar-Cpx)' _ _ -petV an s = 837 °C <f (Opx-Cpx)~ & Ch'peta (\9% 5a) (Fe) _______________ Perkins & Chipera (1985a) (M g )' — -N ew ton & Perkins (1982) (Opx)- I —O — Newton & Perkins (1982) (Cpx) • H ----------------------- 1 ------------ i 700 750 800 T(°C) 850 900 Figure 13. Orthopyroxene-clinopyroxene-plagioclase-gamet-quartz barometry applied to a single mineral association of sample MIXTE-62 of the Guichicovi complex. The Newton and Perkins calibration (1982) includes the Opx-reaction and Cpx-reaction. The calibration of Perkins and Chipera (1985a) includes the Mg-reaction and Fe-reaction. As reference the averages of two-pyroxene thermometry (open diamonds) and gamet-clinopyroxene thermonetry (open triangles) are shown. 9.5 ’(O p x -C p x f8 3 7 ^ ,= 717 °C (Gar-Cpx)' 9.0 ” 4 1 8.5 " , a 985a) *:X p = 8.3 ± 0 .4 kb [...„„==»=== lar 8.0 P= 7.6 ± 0.3 kbar- C L , 7.5 iNewton & Perkins (1982) P= 7.4 ± 0 .3 kbar P= 7.2 ± 0 .2 7 .0 ” 6.5 700 750 800 850 900 T(°C) Figure 14. Orthopyroxene-plagioclase-gamet-quaru barometry for sample MIXTE-62 of the Guichicovi complex. Each line represents a single mineral association used for barometric calculations. Solid lines= Newton and Perkins calibration (1982), broken lines= Perkins and Chipera (1985a) for the Mg-reaction, and thickest Iines= averages. For reference, the average of two-pyroxene thermometry (open diamonds) and gamet-clinopyroxene thermometry (open triangles) are included. 4 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DISCUSSION OF THE BAROMETRY In part, the differences among the four geobarometers are explained in terms of the different activities models. At 837 °C (average two-pyroxene thermometry) and 717 °C (average gamet-clinopyroxene thermometry), the respective overall averaged pressures are: 7.4 ± 0.3 and 7.2 ± 0.2 kbar (Opx-reaction), 4.4 ± 0.5 and 4.3 ± 0.5 kbar (Cpx-reaction), 8.3 ± 0.4 and 7.6 ± 0.3 kbar (Mg-reaction) and 9.6 ± 0.3 and 8.1 ± 0.3 kbar (Fe-reaction). As one can see in Figures 13 and 14, the Newton and Perkins (1982) calibration has a smaller temperature-dependence than the Perkins and Chipera (1985a) calibration. The Perkins and Chipera calibration yields higher pressures than the Newton and Perkins calibration for the same reaction (Opx- or Mg-reactions) and the difference between the two formations are 0.9 kbar at 836 °C and 0.4 at 717 °C (Figure 14). This difference is influenced by the different activity models for garnet and anorthite components. The differences observed in the calculated pressures are also a current problem for those calibrations. Newton and Perkins (1982) noted that the clinopyroxene barometer yields pressures lower than the equivalent orthopyroxene barometer on the order of 1 to 3 kbar. Such difference is observed also for the Guichicovi complex, i.e. at 837 °C, the average orthopyroxene-barometer pressure is higher by 3 kbar than the average clinopyroxene-barometer pressure. According to Newton and Perkins (1982), the cause of this discrepancy is not completely known, but it could be attributed to the uncertainties in the AG° of the reactions, the validity of the activity models or the preferential re equilibration of clinopyroxene during uplifting. Perkins and Chipera (1985a) also observed that their two barometers give discordant pressures. When the Fe2+/(Fe2++Mg) ratio of orthopyroxene is larger than 0.5, the M g-barometer yields higher pressures than the Fe-barometer by 1-5 kbar. When the Fe2+/(Fe2++Mg) ratio of orthopyroxene is smaller than 0.5, the Mg-barometer provides lower pressures than the Fe-barometer by 1 -2 kbar. In the case of the Guichicovi complex, 4 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the P(Fe-reactionp’P(M g-reaction)- For example, at 837 °C the average P(Fe-reaction) is 9.6 kbar and the average P(M g-reaction) is 8.3 kbar, a difference of 1.3 kbar. Considering that the Fe2+/(Fe2++Mg) of orthopyroxene ranges from 0.38 to 0.43, the resulting pressures are consistent with the discrepancies noticed by Perkins and Chipera (1985a). However, at lower temperatures, the discrepancy between the two barometers decreases markedly, such that at 718 °C the difference is only 0.5 kbar. The origin of the discrepancy between these two barometers still remains unknown, but could be related to errors in the pyrope or enstatite activity models (Perkins and Chipera, 1985a). The effect on pressure of the assumed model for the grossular component of garnets was evaluated. In the previous calculations, the grossular component was considered as true grossular [(Ca - 1.5 *Fe3+)/(Mg+Ca+Fe2++Mn)]. However, if the grossular component is assumed to be Xca (molar fraction of Ca) in garnet, the average pressures at 837 °C increases as follows: 7.7 ± 0.1 kbar for the Opx-barometer, 5.1 ± 0.1 kbar for the Cpx-barometer, 8.7 ± 0.1 kbar for the Mg-barometer and 9.9 ± 0.1 kbar for the Fe-barometer. The differences are 0.3 (Opx- and Fe-reactions), 0.4 (Mg-reaction) and 0.7 kbar (Cpx-reaction). Because these differences are not large, the use of true grossular component in the calculations is considered appropriate. The Opx-barometer of Newton and Perkins (1982) provides the best results for the Guichicovi Complex. Support for this conclusion includes: (1) Newton and Perkins (1982) consider the Opx-barometer to be of superior accuracy compared to the Cpx-barometer, and (2) the Opx-barometer is significantly less-temperature dependent than the formulation of Perkins and Chipera (1985a) for the same reaction. Similarly, Mora and Valley (1985) and Mora et al. (1986) calculated peak metamorphic pressures using the Opx-barometer of Newton and Perkins (1982) for the Oaxaca complex, which is an equivalent terrane to the Guichicovi complex (see discussion later). These authors report a peak pressure of 7.5 ± 1 kbar, which is consistent with the results calculated here. 4 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In summary, taking as reference the two-pyroxene thermometry results (837 °C), the peak metamorphic pressure for the Guichicovi complex is 7.4 ± 0.3 kbar. Considering analytical uncertainties for the barometric calibration the Guichicovi complex underwent granulite facies metamorphism at 7-8 kbar. THERMOMETRY Calculation of the metamorphic temperatures for the Guichicovi complex was based on two-pyroxene and Fe2+-Mg cation exchange thermometry. Fe2+-Mg cation exchange thermometry utilized the following systems: gamet-orthopyroxene, gamet-clinopyroxene, gamet-biotite, and gamet-homblende. A description of the basis, thermometric expressions and results of the different geothermometers is given below. TWO-PYROXENE THERMOMETRY The orthopyroxene-clinopyroxene geothermometer is based on the solubility of enstatite in diopside coexisting with orthopyroxene (Wood and Banno, 1973). The miscibility gap between diopside and enstatite is represented by the following reaction: Three different thermometric formulations of the two-pyroxene thermometry were applied to two-pyroxene bearing granulites of the Guichicovi complex. W ood and Banno (1973) developed an empirical calibration assuming that both ortho- and clinopyroxene phases behave as ideal two-solutions of CaM gSi206 and Mg2Si20g components. The (M g 2S i2 0 6)opx (M g 2Si2 0 6 )ci v lvI6 2 ';,12 '“'6 /C p x Diopside-enstatite solid solution Enstatite-diopside solid solution 4 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. uncertainty of this calibration is 60 °C. The thermometric expression and activity models are as follows: T (K)= -10202 (5) Cpx ln( MBSfaft -O p x M giSij06 ■ ) - 7.65X °px + 3.88 (X ° px)2 - 4.6 where the activity models are given by: a M P g2Si2 0 6- ^ M g ^ M g ) c p x . and a M P g2S i;0 6- ( ^ M g ^ M g )O p x Other two-pyroxene geothermometer used in this study is an updated formulation of the W ood and Banno (1973) thermometer. Wells (1977) used more recent available experimental data and derived a new thermometric equation. The Wells (1977) activity models for M g2Si2C >6 in ortho- and clinopyroxenes use the two ideal two-site solution model from Wood and Banno (1973). The Wells (1977) geothermometer has a temperature range of 785-1500 °C and an uncertainty of 70 °C. This formulation is also applicable to M g-rich compositions (Xpe= 0.0 to 1.0). The mathematical formulation is: W ood and Banno (1973) calibration. Both of these formulations of the two-pyroxene thermometer are pressure independent and were applied to 34 mineral pairs from two felsic granulites (ISTEH-33 and 4 6 T (K)= 7341 (6) 3.355 + 2.44X ?fx - ln(— — ^ Fe Opx Mg:Si:On where Xpe px = Fe2+ /(Fe2++Mg) and the activity models are the same as those from the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. M IXTE-26) and four mafic granulites (ISTEH-34, M IX TE-15, -62, and -64). Results of these calculations are shown in Table 4. The Wood and Banno (1973) calibration yielded an temperature average for all samples of 851 ± 25 °C for cores and 839 ± 29 °C for rims. W ells' (1977) formulation gave higher temperatures than the Wood and Banno (1973) expression (Figure 15) with an overall average of 887 ± 19 °C for cores and 865 ± 27 °C for rims. Although in some samples orthopyroxene shows a thin rim of alteration, chemical equilibrium is assumed for the coexisting pyroxenes. Therefore, the differences in the resulting core and rim temperatures are interpreted as being the effect of post-peak chemical re-equilibration. Lindsley's (1983) graphical pyroxene solvus geothermometer was also applied to the Guichicovi complex. This calibration accounts for minor components such as Fe3+, Na, Cr, Ti, Al in the pyroxene and projects the wollastonite, ferrosilite, and enstatite components in the pyroxene quadrilateral. Projection of average sample compositions are presented in Figure 16. Clinopyroxene core compositions plot between the 700 and 900 °C isotherms with a mean temperature of 837 ± 59 °C. Orthopyroxene core compositions plot between the 600 and 700 °C isotherms with a mean temperature of 658 ± 79 °C. Similarly, clinopyroxene rims plot between the 700 and 900 °C isotherms (mean temperature = 805 ± 59 °C) and orthopyroxene rims lay between the 600 and 700 °C isotherms (mean temperature = 630 ± 39 °C). The difference between core and rim temperatures are 31 °C for the clinopyroxene limb and 28 °C for the orthopyroxene limb. The orthopyroxene temperatures are generally regarded as inaccurate due to the steepness of the solvus on the orthopyroxene limb. Thus, those temperatures are discounted. The core to rim discrepancies are attributed to post-peak metamorphism resetting. 4 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 4. Cllnopyroxcnc-orthopyroiene therm om etry for Ihe Guichicovi co m p lq TEMPERATURES (°C) CLINOPYRGXENB O R T H O P Y R O X E N B ForU»c*libmioa of Liadilay (1983) SAMPLE CPX OPX W&B Well* Liodiky (1983) Mg Fc2* XMg XMg Mg M+ XMg XMg Ft* KD ORTHOPYROXENB CLINOPYROXENE ISTEll-33 k2 con *2 eon 909 921 0629 0.183 0.730 0079 1344 0604 0664 0663 OJIO 0131 1.10 68.26 30 64 42.64 46.89 10.46 1STEH-33 b2 cor* b2*b3con 903 919 0836 0.163 0.739 0077 1333 0613 0676 0658 0313 0.131 1.17 67.68 31.15 4348 4730 9.22 Aver (com) ♦97 924 644 424 1.14 47.97 39.99 4196 47.19 9.84 ISIEH-33 •1 i n *1 r n 902 912 0.837 0.172 0.763 0074 1347 0603 0683 0664 0309 0.124 1.17 6836 3037 4348 46.92 9.60 ISTEll-33 b l r n bl rim 660 834 0 824 0.133 0.770 0034 IJ33 0.618 0674 0659 0317 0093 063 67.74 3141 44.99 4639 861 ISTEll-33 c3 tin c3 r n 664 664 0830 0.163 0.767 0064 1334 0396 0666 0.668 0303 0.106 1.12 68.67 3030 4401 46.69 930 Aver (riat) U ) 664 714 444 1.95 68.23 39.73 44.16 46.47 9.17 t 2 1 28 ISTEH-34 *2**4 can 43 con 837 661 0.678 0211 0617 0061 1.122 0 761 0JJ7 0366 0.404 0119 144 38.74 3982 4039 4534 1408 ISTEH-34 b3 cor* b3 cor* 640 667 0673 0239 0610 0063 1 112 0168 0348 0363 0409 0.124 134 3834 4032 4109 4348 1543 ISTEH-34 cl«c2con cl*<2 con 643 863 0683 0.237 0624 0061 1 133 0733 0361 0373 0393 0.118 1.67 3966 38.66 41.74 4330 1496 Aver (com) 646 864 664 424 148 38.91 39.44 41.14 44.94 1441 ISTEH-34 ■ 1 ria t t l m 3 612 827 0 693 0203 0646 00*9 1 129 0784 0362 0366 0410 0096 1.28 38.26 4046 4231 4437 13.12 ISTEH-34 *3 rim *2 ria 637 639 0.701 0.210 0643 0038 1.134 0733 0361 0373 OJ99 0.116 1.22 3936 3941 4139 44.94 1347 ISTEH-34 bl c«n b2 rim 629 646 0682 0216 0626 0035 1.126 0758 0352 0374 0402 0110 1.13 3908 39.79 4133 4443 14.20 ISTEII-34 b2 r n b l r n 810 824 0683 0.206 0638 0047 1.117 0771 0.332 0363 0408 0096 1.10 5832 4038 4206 4447 13.48 Aver (riai) 822 639 644 444 1.18 3841 49.91 4143 44.41 1157 t 13 17 MDOE-13 *2 can *3**4c on 640 683 0634 0298 0366 0066 0993 0638 0492 0302 0.463 0136 1.64 5268 45.48 3939 41.10 1931 MDOE-15 43**4 con *2 r n 631 681 0644 0.298 0J77 0068 1014 0663 0309 0306 0460 0.132 163 33.15 45.22 3980 41.16 1901 MDOE-13 b2+b3cen b3+b4con 643 689 0633 0-303 0364 0070 0999 0134 0494 0303 0.461 0.139 1.69 3301 45 30 3937 41.07 1936 MDOE-13 c3k 4 cor* c>+c4ccn 631 90* 0 633 0121 0336 0076 0990 0680 0492 0.496 0470 0.176 139 32.11 4630 3907 4043 2030 Aver (com) 643 894 944 424 1.49 5174 4538 3944 49.94 19.49 i 6 10 MDOE-13 *1 riai •1 i n 810 643 0646 0773 0394 0033 0996 0896 0498 0499 0473 0126 1.28 31.99 46.73 41.25 41.21 1734 MDOE-13 b l r n b2 r n 813 647 0649 0280 0396 0033 0996 0880 0493 0302 0469 0.121 143 3233 46.23 41.24 4103 17.71 MDOE-13 b4rim bl ria 824 662 0649 0283 0390 0060 1009 0876 0301 0309 0465 0.138 133 32.81 45.13 40.72 41.23 1106 MDOE-13 d i i a c2 ria 112 644 0.639 0281 0383 0034 1001 0676 0300 0302 0467 0.126 149 5234 43.97 4099 40.99 1802 Aver (rim*) t 613 6 649 9 649 424 139 5142 46.29 41.95 41.12 17.83 MDOE-26 62 core dl+d2cen 823 876 0669 0347 0609 0060 0926 0 989 0469 0457 0316 0169 1.91 47.43 3066 42.84 37.64 1931 MDOE-26 b2 tin b2*b3 con 822 870 0 633 0332 0396 0039 0931 0975 0472 0459 0312 0162 2.10 4731 3009 4233 38.15 1931 Aver (com) 123 V i 741 (34 191 4742 5938 4149 J7.99 1941 MDOE-26 b2 r n bl rim 2 821 868 0633 0332 0396 0039 0942 0968 0476 0466 0307 0159 1.78 4843 49.79 4233 38.15 19.31 MDOE-26 dl r n d3 ria 801 640 0687 0331 0641 00*6 0923 0916 0467 0456 0317 0.138 1.96 4740 50 64 4431 37 46 1803 Aver (rial) 811 694 744 464 1J7 47.91 59.21 4132 37.81 11.47 t 14 20 MDOE-62 b3*b4cor< b3*b4cor« 631 692 0.717 0.212 0648 0070 1 122 0180 0355 0367 0410 0143 169 3800 4031 41.12 41.26 1602 MDOE-62 *4 con el con 833 666 0.703 0273 0646 0057 1037 0809 0319 0337 0433 0131 245 53.26 42.29 43.10 40.89 1601 Aver (com) 649 819 844 724 2.97 56.43 4139 4241 4138 14.91 t 17 19 MSOE-62 bl i n bl ria 639 894 0113 0236 0669 0061 1 116 0765 0353 0343 0406 0143 215 5110 3915 42.11 42.94 1495 MDOE-62 b 2 rn b2 i n 632 666 0.762 0.238 0.700 0062 t 112 0789 0331 0361 0415 0140 135 57 39 40.86 4307 4341 1332 MIXTC-62 • In 2 r n •2 rim 642 677 0.721 0.263 0660 0061 1081 0811 0332 0349 0429 0138 166 36.19 42.13 4263 41.97 15.41 MDOE-62 (1 rim (2 rim 672 907 0.733 0732 0680 0073 1 162 0 747 0374 0387 0391 0148 143 59.99 3839 41.83 4337 1437 Aver (rimi) 134 891 819 469 1.78 57.97 4939 4242 4197 14.61 i 12 12 MDOE-64 |3 cor* |2*t3con 639 903 0701 0301 0623 0076 1092 0824 0340 0332 0430 0.159 1.46 36 17 4238 41.17 41.16 17.63 MDOE-64 h3 con M eon 838 894 0.726 0261 0661 0063 1086 0 760 0340 0346 0412 0143 341 36.81 39.78 4233 42.43 13.23 MDOE-64 1 3 con Hcon 861 906 0 699 0281 0623 0076 1084 0816 0334 0350 0429 0.161 133 56.19 42.28 40.46 42.46 1707 Ava (com) 639 941 664 724 2.13 5639 4148 4132 4192 16.45 MDOE-64 (1 ria t l i r a 2 IS4 693 644 444 0724 0 264 0634 0069 1097 0113 0543 0334 0.426 0131 131 56.41 4197 41.73 4179 15.57 Ccr* i r i n f i 151 I T 837 436 t 23 19 39 49 R ia a****** 139 643 645 434 t 29 22 39 39 AllDKliVlAYIONS: WAD- Wood md Bimo, WoVwalLaionM, Erv> cntutw, Kt» (c t t m Ju 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 950 Cores 900-■ £ $ 850 Rims Cores Rims W&B= 851 °C ± 25 839 °C ± 29 Wells= 887 °C ± 19 865 °C ± 27 800 800 850 900 950 T(°C) Wood & Banno (1973) Figure 15. Comparison of the two-pyroxene thermometric calibrations of Wood and Banno (1973) and Wells (1975) for the Guichicovi complex. Filled symbols, pluses and asterisks represent cores. Open symbols, crosses and minus represent rims. Samples are as follows: ISTEH-33= squares, ISTEH-34= diamonds, MIXTE-15=triangles, MIXTE-26= circles, MIXTE-62= asterisks and crosses, MIXTE-64= pluses and minus. 4 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Averages (Cores): T (Cpx) = 837 ± 5 9 °C T (Opx) = 658 ± 49 °C Dio Hd 5 kbar -5 0 0 °C — 600 °C. 700 °C- "800 ° C - " 9 0 0 °c— iooo 1200°C 1100 .900 °C 800 °C - jqq oC 25 50 En 75 Averages (Rims): T (Cpx) = 805 ± 5 9 °C T (Opx) = 630 ± 3 9 °C Dio Hd — 500 °C — 600 °C- ~~700 °C. — 800 °C- "9 0 0 ° c ~ ~'000or 5 kbar 1100 °c, 1200°c 900 °C 800 °C 600 °c son °c En Figure 16. Projection of the average clinopyroxene and orthopyroxene compositions of of the Guichicovi complex in the pyroxene quadrilateral of Lindsley (1983) used for two-pyroxene thermometry, (a) Core averages, (b) Rim averages. Samples are as follows: 1= ISTEH-33,2= ISTEH-34, 3= MIXTE-15,4= MIXTE-26,5= MIXTE-62. and 6= MIXTE-64. 50 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GARNET-ORTHOPYROXENE THERMOMETRY Gamet-orthopyroxene thermometry was applied to two mafic granulites. The gamet-orthopyroxene geothermometry is based on the temperature-dependent partitioning of Mg and Fe2+ between coexisting garnet and orthopyroxene. This cation exchange is represented by the following reaction (Harley, 1984): l/ 3 Mg3Al2Si3 0 i2 + l/2Fe 2Si20 6 = l/3Fe 3Al2Si30 I2 + l/2M g 2Si20 6 Pyrope Ferrosilite Almandine Enstatite Four different calibrations were used for gamet-orthopyroxene thermometry (Mori and Green, 1978; Harley, 1984; Sen and Bhattacharya, 1984, and Lee and Ganguly, 1988). The Mori and Green (1978) calibration is based on high pressure experiments using synthetic and natural ultramafic compositions at 30-40 kbar and 950-1500 °C. The authors recommended the thermometer for garnet lherzolites, however, it was tested for the granulites of the Guichicovi complex. The equation for the Mori and Green (1978) calibration i s : ln K C a r/o Px= j m . . o 1 2 ( 7 ) where KGar/0 Px= = KD (distribution coefficient) (Fe/Mg)0px Harley (1984) also developed an experimental calibration for gamet-orthopyroxene thermometry. The fractionation of Fe2+ and Mg between garnet and orthopyroxene was investigated in a pressure range of 5-30 kbar and a temperature range of 800-1200 °C. The effects of Ca in garnet on the Mg-Fe distribution was taken in account leading to the following equation: 5 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 740+ 1 4 0 0 X & + 22.86 P(kbar) T( C)--------------- R In KD + 1.96-----------------2 7 3 (8) where Kd is the same as in Mori and Green (1978), R= 1.9872 cal/mol K, and X q^ s = m olar fraction of the grossular component in garnet. Sen and Bhattacharya (1984) formulated a gamet-orthopyroxene thermometer applicable to felsic and mafic granulite facies lithologies. These authors assume that the orthopyroxene solution is ideal in the temperature range for granulites and that since garnets o f granulite metamorphic facies rocks are typically poor in Mn, a ternary symmetric solution model was adopted for garnet. The following equation was defined: 2713 + 0.022 P(bars) + 3300X gf + 195(X&ar - Xjjg) _ , „ T (°C )=--------------------- —----------— ------ ^ - 273.15 (9) -1.9872 In Kd + 0.787 + 1.5 x £ “ v (Fe/Mg)0px where, KD= H (Fe/Mg)car Lee and Ganguly (1988) experimentally investigated the equilibrium fractionation of Fe and Mg between coexisting garnet and orthopyroxene as a function of temperature and the Fe/Mg ratio in the system Fe0 -Mg0 -Al2 0 3 -Si0 2 . These experiments were carried at 20-45 kbar and 975-1400 °C. For this calibration the effects of Ca and Mn in garnet on Kd are included. The thermometric expression is: 1 AW caXgf T(K) R /■Gar R - 12.067 (10) - 1.574 In T(K) = In KD 5 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. where, Kd is the same as in Mori and Green (1978), AW ca = AW m„ = 3000 (±500) cal/mol for X g f < 0.30 and X $£ < 0.30, R= 1.9872 cal/mol K. These four different calibrations were applied to two mafic granulites (MIXTE-62 and -64) of the Guichicovi complex and results are shown in Table 5. The temperatures vary according to the calibration used in the following order: T(Lee and Ganguly, 1988) > T(Sen and Bhattacharya, 1984) > T(Mori and Green, 1978) > T(Harley, 1984)- Figure 17 presents the application of the four calibration to a single mineral rim pair for sample MIXTE-64 (gamet.c3 and orthopyroxene.cl). At 7 kbar, the calculated temperatures are 6 6 8 °C (Lee and Ganguly, 1988), 660 °C (Sen and Bhattacharya, 1984), 613 °C (Harley, 1984) and 603 °C (Mori and Green, 1978). The overall temperature ranges for the different formulations at 7 kbar are as follows. For cores: 723-783 °C (mean 755 ± 25 °C), Lee and Ganguly (1988) (Figure 18), 705-764 °C (mean 737 ± 23 °C), Sen and Bhattacharya (1984), 655-738 °C (mean 703 ± 32 °C), Mori and Green (1978), and 648-693 °C (mean 673 C ± 17 °C), Harley (1984). For rims: 668-721 °C (mean 692 ± 23 °C), Lee and Ganguly (1988) (Figure 18), 660-679 °C (mean 683 ± 20 °C), Sen and Bhattacharya (1984), 603-673 °C (mean 636 ± 26 °C), Mori and Green (1978), and 613-653 °C (mean 632 ± 16 °C), Harley (1984). Considering that the calibration of Lee and Ganguly (1988) is an experimental calibration accounting for the effect of Ca and Mn of garnet on the Kq and that it is a more 5 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. Table 5. Gamet-orthopyroxene thermometry at 7 kbar for the Guichicovi complex SAMPLE GARNET OPX TEMPERATURES (°Q G A R N E T M&G S&B H L&G Aim Pyr Sp Gros ORTHOPYROXENE Mg Fe2+ Fs En KD’ s (S&B) (H. L&G.) MIXTE-62 a3 core al+a2 core 655 705 648 723 MIXTE-62 i4+i5 core il+ i2 core 692 728 667 744 Aver 673 716 658 733 ± 27 16 13 15 MIXTE-62 f2+fl rim fl+f2 rim 648 6% 643 712 M KTB-62 13+14 rnre fl+12 rim 672 709 653 72} MIXTE-62 il+ i3 rim il+ i2 core 638 679 631 685 Aver 653 695 642 706 ± 18 15 11 18 M1XTF. 64 c4-*c5 core c2 core 723 755 686 777 m ix t f : 64 g ' tore g2+g3 core 738 764 693 783 MIXTfc-64 h6rh7 core h3+h4 core 705 735 672 750 Aver 722 751 684 770 ± 17 15 11 18 MIXTE-64 c3 rim cl rim 603 660 613 668 MIXTE-64 h3+h4 rim hl+h2 rim 620 668 621 674 Aver 612 664 617 671 ± 12 6 5 4 Core average 703 737 673 755 ± 32 23 17 25 Rhn average 636 683 632 692 ± 26 20 16 23 0.561 0.556 0.237 0.252 0.014 0.013 0.180 0.168 1.136 1.156 0.747 0.747 0.388 0.386 0.590 0.597 0.278 0.293 3.601 3.410 0.561 0.557 0.568 0.243 0.250 0.238 0.014 0 012 0.014 0.174 0.162 0.156 1.168 1.168 1.156 0.740 rv * 7 ,4ft U./W 0.747 0381 ft 70S U J Oi 0386 0.602 0.602 0.597 0.275 0.285 0.271 3.635 3.508 3.696 0.563 0.238 0.017 0.170 1.093 0.792 0.411 0.568 0.306 3.269 0.565 0.233 0.017 0.164 1.092 0.824 0.423 0.561 0.312 3.207 0.565 0.239 0.015 0.160 1.095 0.774 0.405 0.573 0.298 3.351 0.587 0.216 0.017 0.170 1.119 0.779 0.403 0.579 0.256 3.909 0.573 0.223 0.015 0.161 1.141 0.770 0.397 0.588 0.263 3.803 NOTE: M&G= Mori & Green (1978) is a pressure-independent calibration. S&B= Sen & Bhattacharya (1984), H= Harley (1984), L&G= Lee & Ganguly (1988). L ri P(kbar) c 3 s Mon & Sen & Bhattacharya ^wiguly Green F .P&Ch Figure 17. Gamet-orthopyroxene thermometry calibrations applied to a single rim-rim pair (gamet.c3-orthopyroxene.cl) of sample MIXTE-64 from the Guichicovi complex. Barometric calibrations for the assemblage Qtz-Opx-Pla-Gar are: N&P= Newton and Perkins (1982) and P&Ch= Perkins and Chipera (1985a) using the Mg-reaction. Aver. (@ 7 kbar)= T (°C)= 692 ± 23 -------------------- Rim s----------- T (°C)= 755 ± 2 5 Cores — 9 -- 8 -- P & C h 660 680 700 720 740 760 780 800 T(°C) Figure 18. Gamet-orthopyroxene thermometry for the Guichicovi complex using the calibration of Lee and Ganguly (1988), including samples MIXTE-62 and MIXTE-64. Filled circlcs= cores and open squares= rims. Barometric calibrations for the assemblage Qtz-Opx-Pla-Gar are: N&P= Newton and Perkins (1982) and P&Ch= Perkins and Chipera ( 1985a) using the Mg-reaction. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. recent formulation, it is reasonable to assume that it provides the best results. Hence, the cores of the gamet-orthopyroxene system record a temperature range of 723-783 °C (average 755 ± 25 °C). The Sen and Bhattacharya (1984) thermometer also yielded reasonable results, the core temperature average is just 18 °C lower than the core temperature average calculated with the Lee and Ganguly calibration. The difference between the temperatures for core and rim compositions is attributed to chemical re-equilibration during cooling. Considering the results from the Lee and Ganguly (1988) calibration, the mean temperature for rims is 63 °C lower than the mean temperature for cores. Evidence of this cation re-equilibration is shown in Figure 19. For sample MIXTE-64, it is remarkable that garnet cores are richer in Mg than garnet rims. On the other hand, for sample MIXTE-62, orthopyroxene cores are clearly richer in Fe2+ and poorer in M g than their respective rims. This statement is consistent with the fact that Kd decreases with increasing temperature. Therefore, at lower temperatures, the Fe2+/M g ratio of garnet increases whereas the Fe2+/M g ratio of orthopyroxene decreases. The effect of the adopted formulation for the grossular component on the calculations was evaluated. In the discussions above, the X ca was considered as true grossular, that is (Ca-1.5*Fe3+)/(Ca+Fe2++Mg+Mn). However, using the results from the Lee and Ganguly (1988) calibration at 7 kbar, if the grossular component is considered as Xca in garnet, the calculated temperatures increase. For cores compositions, the average temperature increases from 755 °C to 769 °C, an increase of 14 °C. For rims compositions, the average temperature increases from 692 °C to 708 °C, an increment of 16 °C. Because these differences using the two distinct grossular models are not large, it is assumed in this study that the thermometric estimations described above, using the true grossular as X ca in garnets are reasonable. 56 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.80 Open symbols= RIMS Closed symbols= CORES 0.75 ■ • K S O H. 0.70 ■ ■ fe MIXTE-64. 0.65 - - MIXTE-62 ( E 0.60 2.0 2.2 2.4 2.6 2.8 (Fe2+/Mg)Gar Figure 19. (Fe2+/Mg)Gar versus (Fe2+/Mg)GpX for mineral pairs used for gamet-orthopyroxene thermometry of Guichicovi complex. Filled symbols= cores and open symbols= rims. Sample MIXTE-62= squares and MIXTE-64= circles. 57 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GARNET-CLINOPYROXENE THERMOMETRY Gamet-clinopyroxene geothermometry was applied to felsic and mafic granulites. The reaction that governs the distribution of Fe2+ and Mg between coexisting garnet and clinopyroxene mineral phases is (R&heim and Green, 1974): l/3M g3Al2Si30i2 + CaFeSi206 = l/3Fe3Al2Si30i2 + CaMgSi206 Pyrope Hedenbergite Almandine Diopside Six thermometric formulations for the gamet-clinopyroxene system were applied to the Guichicovi complex. A brief summary of the basis of these formulations is given below. RSheim and Green (1974) experimentally calibrated the Kd of the reaction as a function of pressure and temperature for natural basaltic and eclogitic compositions. Their experiments were carried out at 20-40 kbar and 600-1400 °C and yielded the following thermometric equation: ^ 3686 + 28.35 P(kbar) T ( K ) = In KD + 2.33------ (U ) where Kd= ^ S = = l. (Fe/M g)Cpx Ellis and Green (1979) experimentally calibrated the Kd as a function o f pressure, temperature and X ca in garnet (grossular). These authors used a series of basaltic compositions within the system Ca0 -M g0 -Fe0 -Al2 0 3 -Si0 2 in the range of 24-30 kbar and 750-1300 °C. They observed that Kd is independent of the M g# of the involved mineral phases. The mathematical expression of this calibration is: 3 1 0 4 X ^ + 3030+ 10.86 P(kbar) U 3 In KD + 1.9034 U ) where, Kd is the same as in R&heim and Green (1974). 5 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ganguly (1979) calibrated the Kd as a function of pressure, temperature and composition using thermodynamic mixing data o f garnet and clinopyroxene and thermochemical and selected experimental data. The effect of Xm h in garnet on Kd is integrated in this thermometer. The thermometric equation is: 4801 + 11.07 P(kbar)+ 1586X S f+ 1308XS? T (K )------------------------ in Kd + 2.930---------------------- <13) where Kd is the same as in Raheim and Green (1974). Powell (1985) evaluated the experimental data of Ellis and Green (1979) using a combination of regression diagnostics and robust regression. This is a mathematical procedure that provides a good way of analyzing a dataset to find a reliable fit of the data. The computing methods apparently are a more precise approach than the application of least squares in defining the values of the parameters in the resulting equation. Powell's (1985) thermometric expression is: T (K)= 2790 + 10 P(kbar) + 3 1 4 0 X ^ (W) 1.735 + In Kd where Kd is the same as in R&heim and Green (1974). Krogh (1988) reformulated the gamet-clinopyroxene thermometer using pre existing experimental data. This calibration is essentially a reinterpretation of the experimental data from R&heim and Green (1974), Mori and Green (1978), and Ellis and Green (1979). Krogh (1988) observed a curvilinear relationship between Kd and Xca in garnet and determined the following thermometric equation: T - 6173(Xga ar)2 + 673!X £a af + 1879 + 10 P(kbar) ... . 1 } In KD + 1.393 ( } 59 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. where Kq is the same as in R&heim and Green (1974). Sengupta et al. (1989) published a variation of the Ellis and Green (1979) calibration. These authors included the interaction parameters (W) for non-ideal solutions in garnets. They define the W^g.Mn (1600+500 cal/mol), which is the largest uncertainty for the formulation of the activity model for garnet components. In this formulation, garnet is treated as quaternary solid solutions and the activity coefficients for Fe and Mg in garnets were defined following the method of Ganguly and Saxena (1984). According to Sengupta et al. (1989), this expression is appropriate for Mn-rich garnets, and also works well for normal (Mn-poor) granulites such as those of the Guichicovi complex (all garnets are poor in Mn with X sp <0.01). This geothermometer has been suggested to yield good results where Xca in garnet >0.30, but can give anomalous temperatures where XFe in garnet >0.65 and where clinopyroxene has appreciable Na, Al and/or Fe (Sengupta et al., 1989). The mathematical expression of this thermometer is: T 3 0 3 0 + 1 ° - 8 6 P(kbar) In KD + 1.9034 + In - In where Kq is the same as in R&heim and Green (1974) and the activity coefficients are defined as follows: In y g f = X qj(1.52 - 5.17XFe) + x £ ,g(0.10 + 2.26XFe) + X caX M g(3.01 - 6.67XFe + 1.50XCa - 1.50XMg) + XCaX Mn(0.98 - 4.08XFe) + XMgXMn(0.02 + 3.71XFe) In 7M g = XFe(1.23 - 2.26XMg) + x £ a(- 0.26 + 3.00XMg) + XFeXCa(3.53 - 4.85X M g + 2.58XPe - 2.58XCa) + XFeX Mn(1.3 - 2.75XMg) + 0.78X ^n + XCaX Mn(1.27 + 3.00X[vjg) 6 0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Gamet-clinopyroxene thermometry of the Guichicovi complex was based on the analyses of 54 mineral pairs. Samples MIXTE-21, -22, 62,63, and 64 were used for gamet-clinopyroxene thermometry. Using a pressure of 7 kbar as reference, results are shown in Table 6 . As an example of calibration precision, the six thermometric formulations are presented in a single rim-rim mineral pair (sample MEXTE-63, gamet.a5- clinopyroxene.a5) in Figure 20. A marked discrepancy among the thermometers is observed. For example, the calculated temperatures are: 629 °C (Krogh, 1988), 650 °C (Raheim and Green, 1974), 673 °C (Powell, 1985), 694 °C (Ellis and Green, 1979), 710 °C (Sengupta et al., 1989) and 807 °C (Ganguly, 1979). The Ganguly (1979) calibration yields the highest temperatures and the Krogh (1988) formulation provides the lowest values. Previous studies of gamet-clinopyroxene thermometry suggest that the Ellis and Green (1979) calibration works well for granulites. Johnson et al. (1983) evaluated the calibrations available at that time, including among others the Ellis and Green (1979) and Ganguly (1979) formulations. They concluded that the Ellis and Green (1979) thermometer was more accurate for granulites. Percival (1983) determined that the Ellis and Green (1979) thermometer was consistent with the Ferry and Spear (1978) gamet-biotite thermometer for high-grade metamorphic rocks from the central Superior Province of the Canadian Shield. Graham and Powell (1984) recognized that this geothermometer gave good results and they used it to formulate an empirical Fe2+-Mg cation exchange geothermometer for coexisting gamet-homblende. Barth and May (1992) applied the Ellis and Green (1979) formulation to Mesozoic granulites from Cucamonga terrane in southern California and obtained consistent results with two-pyroxene, gamet-orthopyroxene, and gamet-biotite geothermometers. Grant (1989) employed the Ellis and Green (1979) thermometer to the Central Complex Belt of the southwest Grenville Province in Canada. He concluded that the Ellis and Green (1979) calibration gave temperature estimates in 6 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 6. Garncl«cl(nopyroxenc thermometry al 7 k bar for the Gulchlcovl complex_____________________ SaMpLB GARNET CPX TEMPERATURES (°C) G A R N E T CL1NOPYROXENB W RAGBAG G P U 5 .eld. Aim Py XCa XMg XFa2»_______ MIXTE-21 a3«a4+a5 coni a2 core 628 719 801 700 679 0.672 0.061 0.241 0-374 o.5a 7.249 MIXTE-21 dl*d2 cere d3*d4 core 619 711 795 692 671 0.679 0.056 0.245 0.359 0569 7.580 MIXTE-21 e2ccre e2+«4core 6IS 720 798 702 684 0.666 0.050 0.262 0.344 0.601 7.701 MDCTE-21 f3*f4core f2*f3 core 660 770 841 753 739 0.665 0.062 0.256 0342 0.589 6.260 Aver 630 730 809 712 6*3 ± 20 27 21 28 31 MIXTB-21 at rim al rim 599 687 774 667 6 a 0.676 0.059 0.245 0598 0345 8.380 MIXTE-21 b t lymplecbc b2 rim 592 683 769 663 641 0.675 0.054 0.250 0.390 0.561 8.659 MDCTE-21 < 1 3 rim dl rim 583 679 762 659 639 0.671 0.048 0.261 0.383 0586 9.109 MIXTB-21 < 1 4 core d2rim 607 696 783 677 654 0.679 0.057 0.243 0379 0.566 8.012 MIXTB-21 • 1 rim el rim 565 654 741 634 611 0.681 0.042 0.257 0581 0.609 10.033 MIXTB-21 ft rim fl rim 627 736 809 718 702 0.658 0.061 0.264 0.375 0559 7.277 Aver 395 609 773 670 649 ± 21 27 23 28 30 MIXTE-22 a2+a3core a3*a4 core 656 721 821 701 668 0.671 0.103 0.201 0.481 0.490 6.365 MIXTE-22 bl*b2core b3+b4care 651 723 818 703 675 0.666 0.099 0.212 0.482 0.495 6.522 Aver 653 722 819 702 671 ± 4 2 2 2 5 MIXTE-22 a4rim al rim 597 656 761 635 601 0.679 0.090 0.208 0516 0.462 8.462 MIXTE-22 b3 rim b2rim 596 655 761 634 600 0.674 0.091 0.209 0.523 0.454 8.511 MIXTE-22 b4 rim bi nm 608 673 775 652 621 0.675 0.088 0.213 0509 0.487 7.991 MIXTE-22 c l nm c l rim 606 671 773 650 619 0.673 0.088 0.213 0.482 0.456 8.064 MIXTE-22 c2rim c2rim 645 726 817 707 682 0.656 0.096 0.224 0.473 0.484 6.691 Aver 610 676 777 656 624 ± 20 29 23 30 34 MIXTE-62 a3ccre a2core 657 711 815 690 652 714 0561 0.237 0.188 0.735 0.275 6.332 MIXTE-62 c4+c5core c3core 634 682 790 660 619 674 0.557 0.244 0.186 0, 7a 0.241 7.048 MIXTE-62 d2*cD core d2+d3 cere 661 711 817 690 649 703 0.553 0.251 0.182 0.729 0.258 6.212 MIXTE-62 f3*f4 core (3*f4coie 673 723 828 703 662 721 0.557 0.250 0.180 0.734 0.277 5.900 MIXTE-62 h5*h6ccr* h2 core 655 706 812 685 645 686 0341 0.260 0.185 0.759 0.247 6.390 MIXTE-62 »4«i3 core i3«a4 core 654 699 808 678 635 692 0.356 0.252 0.179 0.743 0.255 6.432 Aver 656 705 811 685 6 a 696 ± 13 14 13 14 15 18 MIXTE-62 a l rim al rim 601 643 756 621 577 634 0563 0.240 0.185 0.773 0.219 8.274 MDCTE-62 a2 rim a4 rim 620 661 774 639 594 659 0.568 0.239 0.179 0.763 0.241 7.518 MIXTE-62 c2rim c2rim 612 652 765 630 585 658 0.577 0.230 0.180 0.753 0.241 7.837 MIXTE-62 c3 rim cl rim 616 656 770 634 590 642 0.556 0.251 0.180 0.783 0.225 7.685 MIXTE-62 d t nm d l rim 651 697 805 676 633 692 0.559 0.248 0.180 0.745 0.258 6.517 MDCTE-62 12 rim fl rim 635 679 790 658 614 678 0.561 0.240 0.181 0.753 0.252 7.003 MIXTE-62 il rim il rim 651 696 805 675 631 689 0.558 0.252 0.178 0.742 0.253 6.500 MIXTE-62 12 rim i2rim 661 71! 818 690 647 695 0.547 0.261 0.179 0.770 0.262 6.151 Aver 631 674 785 653 60* 668 ± 22' 25 22 25 26 24 MDCTB-63 e4+a5core a2ccre 675 728 834 707 667 746 0.567 0.227 0.182 0.705 0501 5.847 MDCTE43 c l k 2 core c l+c 2 core 657 710 816 689 650 720 0563 0.227 0.187 0.715 0.280 6J48 MIXTE453 d l *< 12 core d3core 647 697 805 676 636 695 0556 0.238 0.186 0. 7a 0.262 6.639 MDOE-63 g3*g4core g3+g4core 641 688 798 667 625 698 0.570 0.228 0.183 0.737 0.270 6819 Aver 655 705 813 685 6 a 715 ± 15 17 16 18 19 24 M DOE43 *1 rim a3 rim 650 694 807 673 629 714 0.573 0.221 0.178 0.732 0.290 6.551 MIXTB-63 a2rim al rim 647 692 80! 670 626 713 0.577 0.220 0.178 0.766 0501 6.617 MIXTB-63 d3 rim d2rim 608 650 765 628 585 646 0.561 0.232 0.184 0.760 0.230 7.981 MIXTE-63 d4 rim dl rim 674 724 831 701 663 738 0.567 0.233 0.180 0.749 0.310 5.875 MIXTE63 g l rim gl rim 646 748 825 730 712 724 0.518 0 214 0.248 0.763 0.278 6647 MIXTE-63 |2 rim g2rim 636 687 795 666 626 689 0.561 0.230 0.189 0.737 0.258 6.978 Aver 6 a 69* 80S 67* 640 701 ± 21 33 23 35 43 33 MIXTE-64 a5*a6core a3*a4core 688 740 844 720 679 750 0.563 0.240 0.179 0.693 0.293 5.546 MDCTE-64 b l* b 2 c « e b3core 687 743 845 723 685 752 0.559 0.239 0.184 0.701 0.296 5.551 MIXTE-64 g3 core g3 core 684 740 841 720 682 754 0.565 0.233 0.185 0.701 0.301 5.640 MIXTE-64 h6+h7 core h3ccre 648 695 N O ! 671 611 698 0565 0.239 0.181 0.726 0.261 6.592 Aver 677 72* 813 70* 66* 738 ± 19 23 20 24 26 27 MIXTE-64 al rim a2rim 659 706 814 685 641 715 0568 0.236 0.180 0.725 0.278 6.283 MDCTE-64 a2rim al rim 654 700 809 679 636 706 0.566 0.237 0.180 0.725 0.269 6.424 M1XTB-64 b3rim b2 nm 671 725 829 705 667 736 0.56! 0.232 0186 0.713 0.291 5.960 MDCTE-64 b4 rim b l rim 660 710 816 689 649 722 0.569 0.231 0.183 0.719 0.283 6.260 MDCTE-64 g2 rim g2rim 629 677 787 656 615 687 0571 0.224 0.187 0.729 0.257 7.211 MDCTE-64 M rim h2rim 634 683 790 662 621 691 0570 0.228 0.187 0.733 0.260 7.058 MIXTE-64 h5 rim hi rim 642 689 798 667 625 692 0.566 0.236 0.182 0.733 0.258 6.787 Aver 650 69* 006 678 6J7 707 t 15 17 ts 17 18 19 Core average 654 717 817 6*7 662 714 t 21 21 17 22 28 27 Rim average 628 687 7*0 667 631 6*1 t 27 26 24 27 11 10 A BBR E V IA T IO N S K A O - K U tom A O rccn (1974), E A O - PUia A O fte n (197V), 0 > O tn fu iy (1979), P - (1 9 (3 ), K - K r m « h ( l 9 U ) . S. el a l= Sengupta e l al. (19891 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. P (kbar) RShcim & Krough Green P ( 1988) ( 1974) ( ? f •• : i 1 — A - " : Ellis & Sengupra awell Green etal. Ganguly 1985) (1979) (1989) ( 1979) T T T / 1 — " - f f ' - ' T l l l --------------------. ci— a 1 > ' .. i 1 1 --------------------1 --------- — i r - 9 i 1 1 1 --------- 1 ------------------ 1 -------------------1 ------------------ 600 650 700 750 800 850 T(°C) Figure 20. Different gamet-clinopyroxene thermometry calibrations applied to a single rim-rim pair of sample MIXTE-63 from the Guichicovi complex. Barometric calibrations for the assemblage Qtz-Opx-Pla-Gar are: N&P= Newton and Perkins (1982) and P&Ch= Perkins and Chipera (1985a) using the Opx- and Mg-reactions, respectively. 800 750 -- & 0 1 700 - 'k Cores Rims 600 600 650 750 800 700 T(°C)[XCa= X c a in Gar] Figure 21. Differences in gamet-clinopyroxene thermometry (Ellis and Green calibration) for the Guichicovi complex according to the grossular model adopted. Filled symbols and asterisks= cores. Open symbols and pluses= rims. Samples are as follows: squares= MIXTE-21, diamonds= MIXTE-22, triangles= MIXTE-62, circles= MIXTE-63, and asterisks and pluses= MIXTE-64. 6 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. good agreement with the two-pyroxene thermometer of Lindsley (1983). Alternatively, Green and Adam (1991) recently suggested, based on purely experimental work, that the Ellis and Green (1979) thermometer overestimates the temperatures at lower pressures due to extrapolation from the calibration pressures of 24-30 kbar used in the initial experimental work. Nevertheless, following the general agreement of numerous authors, the present work uses the formulation of Ellis and Green (1979) to determine the most reliable temperatures recorded by the gamet-clinopyroxene system. The effect on calculated temperature of the grossular model utilized for gamet- clinopyroxene thermometry was evaluated. A substantial reduction in the estimated temperature occurs when the andradite component of the garnet is subtracted from the garnet. For example, at 7 kbar, the average core temperature is 717 + 21 °C assuming the grossular component is total Xca in garnets. At the same pressure, but assuming that the grossular component is true grossular, the average temperature for cores is 694 ± 18 °C. The average temperature for cores decreases 23 °C. Also at the same pressure, the average rim temperature decreases from 687 ± 28 °C to 658 ± 29 °C, an average reduction of 29 °C. In general, this discrepancy ranges from 0-66 °C (Figure 21). Graphite is a common mineral in the Guichicovi complex that may suggest relatively reducing conditions (low oxygen fugacities). Oxygen fugacity can vary within the same metamorphic unit and is governed by the rock mineral assemblages (Frost, 1991). Because of the uncertainty in the oxygen fugacity conditions and subsequently the actual ferric iron content in garnets, and the large discrepancies in the calculated temperatures, Xca in garnet was employed as the grossular component for gamet-clinopyroxene thermobarometry in this study. Core compositions record higher temperatures than rim compositions for gamet- clinopyroxene thermometry. The average temperatures are 717 ± 21 °C and 687 ± 28 °C for core and rim compositions, respectively (Figure 22). The difference in the average 6 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (Mg/Fe2+)c For RIMS Aver.= 687 ± 28 °C @ 7kbar For CORES . Avcr.= 717 ± 21 °C @ 7 k b ar 640 660 680 700 T(°C) 720 740 760 780 Figure 22. Gamet-clinopyroxene thermometry calculated at 7 kbar for the Guichicovi complex using the calibration of Ellis and Green (1979). Samples employed are MIXTE-21, -22, -62, -63 and -64. Stippled bars= rims and white bars= cores. 4.0 Open symbols & pluses= rims Filled symbols & asterisks= cores MIXTE-62, 3.0 -- 2.0 - - MIXTE-22 1.0 - - MIXTE-21 CB 0.0 0.1 0.2 0.3 0.4 0.0 0.5 (Mg/Fe2+)Gar Figure 23. Mg/Fe2+ ratios for garnets and clinopyroxenes used in gamet-clinopyroxene thermometry for the Guichicovi complex. Filled symbols and asterisks= cores. Open symbols and pluses= rims. Samples are as follows: squares= MIXTE-21, circles=MIXTE-22, diamonds= MIXTE-62, triangles= MIXTE-63 and asterisks and pluses= MIXTE-64. 65 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. temperatures is 30 °C. This difference is attributed to chemical resetting of rims during cooling. This interpretation is shown in Figure 23, where it is clear that cores of garnets from samples MIXTE-22, -62, -63, and -64 have higher Mg/Fe2+ ratios than rims. W hile for clinopyroxene, cores have slightly lower Mg/Fe2+ ratios than rims. This chemical behavior of Mg and Fe2+ in coexisting garnet and clinopyroxene is consistent with the fact that Kd decreases with increasing temperature. The consequence is that the Fe2+/Mg ratio of garnet decreases as garnet becomes richer in Mg. Whereas, an increase in temperature will cause an enrichment in Fe2+ in clinopyroxenes and a subsequent increase of its Fe2+/M g ratio. In summary, the gamet-clinopyroxene geothermometer records for the Guichicovi complex a temperature range of 682-770 °C (mean 717 ± 21°C) and 634-748 °C (mean 687 ± 28°C) for core and rim compositions, respectively. GARNET-BIOTITE THERMOMETRY Different formulations have been proposed for the Fe2+-Mg exchange gamet-biotite geothermometer. In this study, five of those formulations (Ferry and Spear, 1978; Ganguly and Saxena, 1984; Indares and Martignole, 1985; Hodges and Royden, 1984; Williams and Grambling, 1990) were applied to two samples of the Guichicovi complex. Thermometric results are illustrated in Table 7. A short description of the geothermometers is given below, including the basis and mathematical expressions, and afterwards, the results for the Guichicovi complex are discussed. The exchange reaction between garnet and biotite solid solutions is (Ferry and Spear, 1978): Fc3Al2Si30i2 + KMg3AlSi30io(OH)2 = Mg3Al2Si30)2 + KFe3AlSi30]o(OH)2 A lm andine P h lo g o p ite Pyrope A nnite 66 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. Table 7. Garnel-biotile therm om etry at 7 k b a r for the Guichicovi complex_______________________________________________________ SAMPLE GARNET BIOTITE TEMPERATURES(°C) G A R N E T B IO TI TE KD's F&S G&S l&M H&R W&G Aim Gros Py Sp XFe XMg XTi XAlvi IF&S. l&M, W&G) (G&S) (H&R) M IXTE-62 c4+c5 core c2+c3 int 574 507 550 639 609 0.557 0.168 0.244 0.013 0.256 0.655 0.089 0.000 0.171 5.838 0.009 M IXTE-62 c l rim c l rim 486 435 461 547 522 0.582 0.162 0.212 0.013 0.237 0.673 0.089 0.000 0.128 7.818 0.004 MDCTE-62 h i rim h i rim 416 363 447 478 450 0.555 0.173 0.246 0.012 0.169 0.781 0.049 0.000 0.0% 10.430 0.002 A ver (rim s) 423 399 454 513 486 ± 50 51 10 48 51 M IXTE-64 d3+d4 core d2+d3 int 5 % 532 558 661 .633 0.565 0.165 0.235 0.015 0.276 0.626 0.098 0.000 0.183 5.463 0.010 M IXTE-64 e4core e3+e4 int 588 520 554 646 617 0.575 0.149 0.227 0.018 0.283 0.626 0.090 0.000 0.179 5.594 0.009 A ver 592 526 556 654 625 ± 6 8 3 11 11 MDCTE-64 c 6 n m cl+ c 2 rim 463 397 439 512 484 0.572 0.137 0.228 0.018 0.208 0.711 0.081 0.000 0.117 8.575 0.003 MDCTE-64 d2 nm dl rim 518 486 500 588 569 0.598 0.180 0.197 0.019 0.275 0.631 0.093 0.000 0.143 6.980 0.006 MDCTE-64 d l nm d4 rim 520 492 507 596 579 0.583 0.199 0.192 0.016 0.275 0.629 0.095 0.000 0.144 6.939 0.006 MDCTE-64 e6 rim el rim 503 472 482 568 551 0.607 0.172 0.185 0.018 0.281 0.627 0.092 0.000 0.136 7.348 0.005 MDCTE-64 e5 rim e2rim 568 512 522 629 604 0.584 0.157 0.212 0.018 0.285 0.613 0.102 0.000 0.168 5.937 0.008 A ver 514 472 490 579 557 ± 38 44 32 43 45 C o re av erag e 586 519 554 649 620 ± 11 12 4 11 12 R im average 496 451 480 560 537 ± 48 55 32 52 55 ABBREVIATIONS: F&S=Feny & Spau- (1978), G&S= Ganguly & Saxena (1984). 1&M= Indarea & Martignole (1985), H&R= Hodgea and Royden (1984), W&G= Williams & Grambling (1990). O v Ferry and Spear (1978) experimentally calibrated the gamet-biotite geothermometer in the temperature range 550-800 °C at 2.07 kbar. They considered that Fe and M g ideally mix in biotite and garnet in the composition range 0.80 <Fe/(Fe+Mg)<1.00. The effect of Ca in garnet and Ti and Alv i in biotite on the Kq of the reaction was not estimated in this calibration. However, these authors suggest that the geothermometer is useful where (Ca+Mn)/(Ca+Mn+Fe+Mg) in garnet is up to -0.20 and (Alvi+Ti)/(Alvi+Ti+Fe+Mg) in biotite is up to -0.15. The thermometric equation for this calibration is: 12454 + 0.057 P(bars) T(K )- 4.662 - 3R In KD ( ' 7) where, Kd= 7rr^r= “r ^ and R= 1.987 cai/mol K (Mg/Fe)B io Ganguly and Saxena (1984) published a new formulation for the Ferry and Spear (1978) calibration. These authors refined the experimental calibration of Ferry and Spear (1978) using thermodynamic data and calculation of Margules parameters for the garnet components. For this thermometer, garnets are treated as quaternary solid solutions, the Mg-Fe join for garnet is assumed to be asymmetrical, and the molar fractions of grossular and spessartine are considered in the formulation. The thermometric equation for this thermometer is: A + [WF eMg(XFe - XMg) + 3000Xca + 3000XM n]G a7R T(K )" --------------------------- In KD + 0.782---------------------------- (18) W here, KD= ^ ^ - g - - Gar’ A= 2089 - ° '8Q^ cMg + 9.45 P(kbar), (Fe/Mg)Bj0 R WpcMg~ 200 Mg (Fe2++Mg) Gar + 2500 Fe2+ (Fe2++Mg) Gar , and R= 1.987 cal/mol K. 68 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Hodges and Roy den (1984) also made some corrections to the calibration of Ferry and Spear (1978) and presented a different formulation, based on garnet solution models adopted from Hodges and Spear (1982). In such models, garnet is treated as a quaternary solid solution and is defined such that only the Mg-Ca mixing is significantly non-ideal. For this formulation the activities for almandine and pyrope components are temperature- dependent. The equations involved in this thermometer are: T(K)= 12454 + 0.057 [P(bars) - 1] 4.662 - 1.987 In K, (19) W here the equilibrium constant, Ki = apyra~ A nn ^■A lm & Phlo and the activity models for the involved components are: a A nn=(xPi0 )3, a P h l0= ( X g ) 3, a A in r^ X ^ f exp [1.5T(K) - 3300] x g x g f RT(K) and apyi— (x ^ g e x p [ 3300 - 1.5T(K)] [ x & a r 2 + Xg?fXga ar + X g f X f ij RT(K) Indares and Martignole (1985) also refined the earlier calibration of Ferry and Spear (1978). In this formulation, available thermodynamic and empirical data is employed to correct the Ferry and Spear (1978) calibration for the interaction of Ca on the Fe-Mg join in garnet. Because Mn is a minor component in granulite facies garnets, it is assumed that Mn does not significantly effect the Fe-Mg join in garnet. This formulation takes into account the effects of Ti and Alv i in the biotite on temperature. The mathematical expression of this geothermometer is: 6 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. T (K )= 12454 + 0.057 P(bars) + 3(- 1590X^8 - 745IX ? '0) - 3[-3000(Xga ar + X ^ ) ] 4.662 -5.9616 In KD (20) Where, Kq= (Fe/Mg)B jo (Fe/Mg)car Williams and Grambling (1990) proposed a modification of the calibration of Ferry and Spear (1978) for rocks with Mn-rich garnets. This thermometer is based on empirical analysis of natural gamet-biotite mineral pairs from Proterozoic rocks of north-central New Mexico. In this formulation, Ca is considered to have significantly nonideal mixing in Fe- Mg garnet even at low concentrations. For this formulation the effects of Ti, Al, Fe3+, and Mn in biotite on calculated temperature are considered to be minor because of the small quantities of these elements in the biotite used. Margules interaction parameters (W) for Mn and Ca components in garnet were estimated from statistical analysis and included into the thermometric expression of Ferry and Spear (1978). Garnet used for the conception of this thermometer are Mn-rich (spessartine up to 0.75) and Ca-poor (grossular <0.10). The biotite is low in Ti (<2.0 wt%). Hence, this geothermometer is restricted to garnet with spessartine component <0.50 and grossular component <0.10. Biotite must have similar compositions as those utilized in its formulation. Although the garnet from the Guichicovi complex are high in Ca and low in Mn and the biotites are high in Ti, this thermometer was tested in this work. The thermometric equation is: T (K )= -15796 + 79.50 P(bars) +W M gFe (XA|m - XP yr) - 12550XG ros - 8230XSp R[ln KD + In (1 - F e ^ 0) - 0.782 - In (0.90)] (2 1 ) 7 0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. where, Kd is the same as in Ferry and Spear (1978), R= 8.3147 joules/mol K, Feeto= 0.15 (and is constant for all the biotites used), mdw agsM g^ + m sF ^ (Mg + Fe2*)oar The application of the five different gamet-biotite formulations to the Guichicovi complex yielded remarkably discordant results. Figure 24 shows the variation among the different thermometers applied to a single rim mineral pair for sample MIXTE-64 (gamet.e6 -biotite.el). At 7 kbar, calculated temperatures are: 472 °C (Ganguly and Saxena, 1984), 482 °C (Indares and Martignole, 1985), 503 °C (Ferry and Spear, 1978), 551 °C (Williams and Grambling, 1990), and 568 °C (Hodges and Royden, 1984). The overall temperature range for the five thermometers at 7 kbar, in the same increasing order, is as follows. For core compositions: 507-532 °C (mean 519 + 12 °C), Ganguly and Saxena (1984), 550-558 °C (mean 554 + 4 °C), Indares and Martignole (1985), 574-588 °C (mean 586 ± 11 °C), Ferry and Spear (1978), 609-633 °C (mean 620 ± 12 °C), Williams and Grambling (1990), and 639-661 °C (mean 649 ± 11 °C), Hodges and Royden (1984). For rim compositions: 363-512 °C (mean 451 + 55 °C), Ganguly and Saxena (1984), 447-522 °C (mean 560 ± 52 °C), Indares and Martignole (1985), 416-568 °C (mean 496 ± 4 8 °C), Ferry and Spear (1978), 450-604 °C (mean 537 ± 55 °C), Williams and Grambling (1990), and 478-529 °C (mean 560 ± 52 °C), Hodges and Royden (1984). 7 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. P(kbar) Ganguly & Saxena (1984) Ferry & Spear (1978) Williams & Hodges & Grambling Royden (1990) (1984) 9 .. 8 • ■ 7 * ■ D -- Martignole (1985) 400 450 500 550 600 T(°C) Figure 24. Gamet-biotite thermometry calibrations applied to a single rim-rim pair (gamet.e6-biotite.el) for sample MIXTE-64 of the Guichicovi complex. Qtz-Opx-Pla-Gar barometric calibrations are: N&P= Newton and Perkins (1982) (Opx-reaction) and P&Ch = Perkins and Chipera (1985a) (Mg-reaction). 10 560 ± 5 2 ° C — Rims — Aver (@ 7 kbar)= 6 4 9 ± 11°C ■> < -C o res-> 9 8 7 •N &P L — P& Ch ; 66 5 450 500 550 600 650 700 T(°C) Figure 25. Gamet-biotite thermometry for the Guichicovi complex using the calibration of Hodges and Royden (1984), including samples MIXTE-62 and MIXTE-64. Filled circles= cores. Open circles= rims. Qtz-Opx-Pla-Gar barometric calibrations are: N&P= Newton and Perkins (1982) (Opx-reaction) and P&Ch = Perkins and Chipera (1985a) (Mg-reaction). 7 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Several criteria were evaluated in order to choose the most suitable gamet-biotite geothermometer for this study. For evaluation of the formulations, a single gamet-biotite pair was selected in which the biotite is large and does not occur as a rim around garnet or other mafic mineral phase suggesting retrograde origin. It is clear that all the discussed gamet-biotite formulations yield significantly lower values when compared to the gamet- pyroxene and other thermometers. The Ferry and Spear (1978) thermometer gave a low temperature (503 °C) probably because it does not account for the effect of Ca in garnet on Kd- Another reason could be that garnet of the Guichicovi complex is slightly higher in Ca than the range considered valid for such calibration. The Ganguly and Saxena (1984) geothermometer, even though it accounts for the Ca, Mn and the interaction parameters in the formulation, gives the lowest result (472 °C). The W illiams and Grambling (1990) thermometer, which gives a higher temperature (551 °C) than the previous formulations, is not considered suitable for the Guichicovi complex because the garnet used in formulation are rich in Ca and poor in Mn. The Indares and Martignole (1985) and Hodges and Royden (1984) thermometers seem to be the most appropriate to the Guichicovi complex. Unfortunately, these two formulations gave contrasting results. For the evaluated gamet-biotite mineral pair, the Indares and Martignole (1985) thermometer gave 482 °C while the Hodges and Royden (1984) thermometer yielded 568 °C, a difference of 8 6 °C. The Indares and Martignole (1985) thermometer is suitable for this study because it accounts for the effect of Ca in garnet and Ti in biotite on Kd and those elements occur in important amounts in the minerals used in this study. The Hodges and Royden (1984) thermometer is also appropriate for this study because it introduces the activity models of Fe2+ and Mg and the effect of Ca in garnet on the thermometric formulation. The Hodges and Royden (1984) thermometer is probably the most suitable formulation for the Guichicovi complex based on the fact that its temperatures are similar to that derived from gamet-pyroxene thermometry. 7 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Like the previous geothermometers, core compositions record higher temperatures than rim compositions for the gamet-biotite thermometer. At 7 kbar, the Hodges and Royden (1984) thermometer yielded 639-661 °C (mean 649 ± 11 °C) for cores and 478-526 °C (mean 560 ± 52 °C) for rims (Figure 25). The difference between these averages is attributed to cation re-equilibration during cooling. In Figure 26, it is clear that for garnet from sample MEXTE-64, the core compositions have a higher Mg/Fe2+ ratio than the rim compositions. Garnet in sample MIXTE-62 follows essentially the same pattern as garnets o f sample MIXTE-64 do, however, biotite in one rim mineral pair has an anomalous M g/Fe2+ ratio (>4.5). This biotite is very small and could be late (post-peak metamorphism) in origin. This biotite (M IXTE-62.hl.rim) also gives the lowest calculated temperatures. Cation re-equilibration due to cooling is consistent with the observation that Kd decreases with decreases temperature. According to this relationship, at lower temperatures, the Mg/Fe2+ ratio of garnet becomes lower. The effect of using X ca as true grossular in the calculations is minor. All the discussed results above were calculated using true grossular. If X ca in garnet is assumed to be the grossular component, the calculations vary just slightly. For example, using the Hodges and Royden (1984) thermometer, the mean core temperature increases only 10 °C (659 ± 11 °C) while the mean rim temperature increases just 9 °C (569 ± 5 1 °C). The reason for this minor effect is that the analyzed garnet has only a small amount o f andradite com ponent (0-4%). In summary, the gamet-biotite geothermometry records a temperature range of 639- 661 °C (mean 649 ± 11 °C) for cores and 478-526 °C (mean 560 ± 52 °C) for rims for the Guichicovi com plex. 7 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5.0 Squares= MIXTE-62 Circles= MIXTE-64 Rims Cores 0.30 0.35 0.40 0.45 (Mg/Fe“+)Gar Figure 26. (Mg/Fe2+)Gar versus (Mg/Fe*)g;0 for mineral pairs used for gamet-biotite thermometry of the Guichicovi complex. Filled symbols= cores. Open symbols= rims. Samples: MIXTE-62= squares and MIXTE-64= circles. Fe*= total Fe. 7 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GARNET-HORNBLENDE THERMOMETRY Graham and Powell (1984) calibrated an empirical gamet-homblende geothermometer. The authors calibrated the gamet-homblende Fe2+-Mg exchange geothermometer against the gamet-clinopyroxene geothermometer of Ellis and Green (1979) using data on coexisting gamet+homblende+clinopyroxene in amphibolite and granulite facies rocks. This thermometer is based on the following Fe2+-Mg exchange reaction (Graham and Powell, 1984): l/4 N a C a 2 F e4 A l3 S i6 0 2 2 (0 H )2 + l/3 M g 3 A l2 S i3 0 j2 = Ferro-pargasite Pyrope l/4 N aC a2M g4A l3Si6022 (0 H )2 + l/3 F e 3A l2Si3O i2 Pargasite Almandine The thermometric equation for this calibration is: 2880 + 3280X £f ^ ’ In KD + 2.426 1 ’ where, K0 = ® ^ * (Fe/Mg)H b i The Graham and Powell (1984) geothermometer was applied to three samples of the Guichicovi complex. These samples include a gamet-homblende-clinopyroxene quartzo-feldspathic gneiss (MIXTE-21) and two mafic granulites (MIXTE-62 and -64). Table 8 presents the calculated temperatures using this geothermometer. For these computations, the X ca in garnet was considered to be "true grossular". The use of this model for the Ca component in garnet has a strong effect on the calculated temperatures for the felsic gneiss. Fe3+, calculated from stoichiometry and charge balance neutrality, is relatively high in garnet and hornblende of the felsic gneiss. Therefore, gamet-homblende 7 6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 8. Gar-Hbl thermometry (Graham and Powell, 1984) for the Guichicovi complex SAMPLE GARNET HORNBLENDE T(°C ) G A R Gros N E T (Mc/Fe2+) HORNBLENDE (Me/Fe2+) KD MIXTE-21 c3 rim e3 core S70 0.185 0.074 0.411 5.526 MIXTE-21 cl symplcc c2 rim 575 0.186 0.084 0.455 5.431 MIXTE-21 c2 int e l rim 554 0.196 0.074 0.464 6.233 Aver 565 ± 14 MIXTE-62 c4+c5 core c3+c4 core 634 0.168 0.439 1.703 3.881 MIXTE-62 d2+d3 core d4 core 601 0.157 0.455 1.958 4.308 MIXTE-62 a5 rim a2 core 627 0.197 0.444 1.977 4.453 Aver (cores) 621 ± 18 MIXTE-62 c l rim c l rim 570 0.162 0.364 1.843 5.063 MIXTE-62 c2 rim c2 rim 591 0.180 0.399 1.960 4.910 MIXTE-62 d4 rim d3 core 552 0.168 0.413 2.328 5.637 MIXTE-62 d5 rim d l rim 548 0.165 0.419 2.395 5.711 Aver (rims) 565 ± 20 MIXTE-64 a5+a6 core a3+a4 core 622 0.165 0.427 1.725 4.043 MIXTE-64 c4+c5 core c2 core 659 0.170 0.422 1.493 3.535 MIXTE-64 c3+e4 core e3+e4 core 640 0.165 0.394 1.475 3.744 MIXTE-64 h6+h7 core h2 core 633 0.160 0.422 1.601 3.791 Aver (cores) 638 ± 16 MIXTE-64 a3 rim a l rim 574 0.171 0.371 1.906 5.138 MIXTE-64 a4 rim a2 rim 578 0.169 0.391 1.954 4.995 MIXTE-64 c7 rim c l rim 579 0.173 0.376 1.902 5.057 MIXTE-64 c l rim e l rim 609 0.164 0.369 1.566 4.248 MIXTE-64 c2 rim e2 rim 600 0.184 0.357 1.703 4.765 MIXTE-64 h2 rim h i rim 562 0.156 0.371 1.908 5.149 Aver (rims) 584 ± 18 C ore average (°C) 631 ± 18 Rim average (°C) 574 ± 19 NOTE: the overall core average docs nol include the Mixte21.c3 (grt.rim-hbl.core) 77 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. temperatures for sample MIXTE-21 in Table 8 are 51-62 °C lower than those estimated assuming X ca as grossular in garnets. For the two mafic granulites, the use of X ca= true grossular decreases the temperatures from 0 to 25 °C (mean 12 °C). Therefore, it is reasonable not to include the calculated T (570 °C) for the mineral pair garnet (e3.rim)- homblende (e3.core) from sample MIXTE-21 into the overall average temperature for cores. The reason is because it is 61 °C lower than the overall average (631 ± 1 8 °C). Core compositions record higher temperatures than rim compositions for gamet- homblende thermometry. The overall average temperature using cores is 631 ± 18 °C, whereas overall average temperature using rims is 574 ± 19 °C (Figure 27). The difference between these results is 57 °C. This thermometric difference is interpreted as chemical re equilibration for rim compositions during cooling. Evidence of this phenomenon is illustrated in Figure 28. Clearly, cores of garnet from the mafic granulites (MIXTE-62 and -64) have higher Mg/Fe2+ ratios than rims. This effect is not recognized for garnets in the felsic gneiss (MIXTE-21). For the felsic gneiss, rims of hornblende have higher M g/Fe2+ ratios than the core analyzed. These differences are consistent with the fact that Kd decreases with increasing temperature. At higher temperatures, cores of garnet become richer in Mg and cores of hornblende becomes richer in Fe2+. Whereas at lower temperatures, rims of garnet become richer in Fe2+ and rims of hornblende becomes richer in Mg. In summary, the gamet-homblende geothermometry records a temperature range of 601-659 °C (mean 631 ± 18 °C) for core compositions and 554-609 °C (mean 574 ± 19 °C) for rim compositions for the Guichicovi complex. DISCUSSION OF THE THERMOMETRY By far, the most important observation in the geothermometric results is the large variation from the different thermometers. For mineral core compositions the following 78 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 0 0 Aver. (Cores)= 631 ± 18°C ; /ns Core * . . - > \ j M ' r ' - s i b B \ rt — * ■ - _ * C 600 - ■ Green Hbl Brown Hbl 550 •• Aver. (Rims) = 574 ± 19 °C 500 3.5 4.0 4.5 5.0 5.5 6.0 6.5 (Fe2+/Mg)Gar/(Fe2+/Mg)Hbj Figure 27. K p versus T (°C) for the Guichicovi complex using the gamet-homblende thermometry calibration of Graham and Powell (1984). Filled symbols= cores. Open symbols= rims. Samples as follows: squares= MEXTE-21, diamonds= MIXTE-62 and circles= MIXTE-64. 2.5 / T g a r - o p x > T g a r - c p x > T g a r - b i o > T g a r - h b l - The average temperatures calculated for each individual sample using the different thermometers are illustrated in Figure 29. An important question is what thermometric result most closely represents the peak metamorphic temperature for the Guichicovi complex. There are two possible alternatives: the two-pyroxene and the gamet-pyroxene thermometers. Using core compositions, the pyroxene solvus thermometer (clinopyroxene limb) of Lindsley (1983) yields a somewhat large temperature range of 740-900 °C while the two-pyroxene calibration of Wood and Banno (1983) provides a narrower temperature range, that is 822-909 °C. In general, the overall temperature averages are fairly similar: 837 ± 59 °C (Lindsley, 1983) and 851 ± 21 °C (Wood and Banno, 1983). This implies a relative consistency in these thermometers when applied to the Guichicovi complex. In addition, there are several papers in which the application o f the solvus thermometer of Lindsley (1983) to other granulite terranes has been successful when compared to other independent geothermometers (e.g. Lamb et al., 1986; Stiiwe and Powell, 1989a and 1989b; Grant, 1989; Barth and May, 1992). These observations indicate that the calculated temperatures using the experimental calibration of Lindsley (1983) correspond to the peak metamorphic temperatures. However, several workers consider that the two-pyroxene thermometry does not work well in granulite terranes. For instance. Bohlen and Essene (1979) evaluated the Wood and Banno (1973) and Wells (1977) formulations on granulites from the Adirondacks. Bohlen and Essene (1979) compared the two-pyroxene thermometry to both 80 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. , 0 0 o £6 jfr * y / ✓ 2 / ✓ i^ k \ \ y \ \ * a s v / / /»/ / 3 / 4 s \ \ \ \ \ • < \ I • ' ' V ' / ■ • s ' / / o ✓ ✓ s / / / / >/ /J * i / w w m SI oS6 \ \ \ S \ \ S \ * 1 N . ^ \ ///// /// / w / A A A A A A A A A ¥ A X , 0E o £6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Reproduced w ith permission of th e copyright owner. Further reproduction prohibited without perm ission. ;San Juanitoi Sarabia Juan Mazal Colorado! I860 (O) 770(H ) 729 (A) 654(B ) 638 Q ) :Platanillo: f S. J . Guichicovi' Ocotal M A TIA S R O M E R O EXPLANATION N A Cenozoic sediments Marine Upper Jurassic-Upper Cretaceous sediments Middle Jurassic continental red beds Permian and Early Jurassic plutcns and post-Early Jurassic dikes Paleozoic greensdtist metamorphic facies rocks Middle Proterozoic granulite metamorphic facies rocks GEOTHERMOMETRIC SYMBOLS (Numbers in °C): (£2) Two-pyroxene thermometry (Lindsley, 1983) (p.) Gamet-orthopyroxene thermometry (Lee and Ganguly, 1988) (A) Gamet-clinopyroxene thermometry (Ellis and Green, 1979) (B) Gamet-biotite thermometry (Hodges and Royden, 1984) (3) Gam et-homblende thermometry (Graham and Powell, 1984) Scale 1: 250 000 2 4 6 8 10 km Dextral strike-slip fault Dextral strike-slip(?) fault Geologic contact Highway or paved road City -------------Dirt road Small town X Foliation dip and strike Vertical foliation and strike Figure 29. Geologic map of the La Mixtequita area with thermometric data of the Guichicovi complex. Numbers represent average temperarures for core compositions calculated at 7 kbar, excluding the Two-pyroxene system. For details see text. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. oxide and feldspar thermometry and concluded that both two-pyroxene formulations were imprecise and inaccurate in the Adirondack granulites. These authors based their conclusion on the lack o f consistency of the calculated temperatures, the significantly higher resulting temperatures (50-150 °C) higher than both oxide and feldspar thermometry and also observed a substantial compositional effect on the temperature. Lindsley (1983) applied his graphical calibration to the Adirondack granulites and observed that his clinopyroxene thermometer also yielded scattered and inconsistent temperatures (500- 880 °C) compared with temperatures determined by Bohlen and Essene (1977) using oxide and feldspar thermometry for the same rocks. Indeed, the orthopyroxene limb provided quite low temperatures (500-620 °C) suggesting resetting of the orthopyroxene upon cooling or an inappropriate calibration for such limb (Lindsley, 1983). The calculation of pyroxene ferric iron from microprobe analysis is also a cause of the observed variation in the graphical solvus thermometer (Lindsley, 1983). The problems outlined by Lindsley (1983) on the application of his graphical solvus thermometer to the Adirondack granulites were later corroborated by Bohlen et al. (1985). Stephenson (1984) evaluated different two-pyroxene calibrations on Precambrian granulites from western Australia and although he obtained acceptable results with the clinopyroxene limb of Lindsley (1983), he concluded that this thermometer provides relatively poor precise results and also noted an apparent dependence of Mg/Fe2+ on calculated temperature. Alternatively, the temperatures calculated for the Guichicovi complex using both gamet-orthopyroxene and gamet-clinopyroxene thermometry could correspond to peak metamorphic temperatures. Using core compositions, these thermometers provided reasonably consistent results, 723-783 °C for the gamet-orthopyroxene formulation (Lee and Ganguly, 1988) and 682-770 °C for the gamet-clinopyroxene calibration (Ellis and Green, 1979). It is important to note that these calculated temperatures are similar to the feldspar thermometric temperatures (730 ± 50 °C) estimated by Mora and Valley (1985) for 8 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the Grenville Oaxacan Complex, a granulite terrane in southern Mexico which correlates well with the Guichicovi complex (see discussion later). Consequently, this second alternative would be similar to the peak temperatures defined by Bohlen et al. (1980 and 1985) for the Adirondack granulites. However, due to the variation among the different thermometers applied to the Guichicovi complex, I considered that the peak metamorphic temperature is that determined by the pyroxene graphical solvus thermometer (Lindsley, 1983) which, as was discussed above, has been successfully applied in several granulite terranes. Another important question about the geothermometry of the Guichicovi complex is the significance of the observed systematic thermometric trend ( T t w o - p x > T g r t - o p x > T g r t - c p x > T g r t - b i o > T g r t - h b l ) - T w o reasonable explanations may be outlined: ( 1 ) cation resetting upon cooling and (2 ) incomplete cation equilibration associated with very rapid cooling. Chemical evidence of cation re-equilibration was recognized in all the geothermometers utilized in this study as was previously discussed, this effect would favor the first interpretation. Indeed, Frost and Chacko (1989) recently proposed a granulite uncertainty principle implying that at least three commonly used geothermometers (two- pyroxene, two-oxide and gamet-clinopyroxene) have apparent closure temperatures below those characteristic of the granulite facies. This principle also supports the first explanation. The U-Pb zircon geochronology of the Guichicovi complex suggests a metamorphic age of 980-990 Ma (see next section on the geochronology of this complex). Two K-Ar ages of 911 ± 46 Ma on hornblende and 8 6 6 ± 29 Ma on mica indicate a rapid cooling event for this complex that favors the second explanation. In addition, the observed thermometric trend could be associated with different cation-diffusion rates among garnet and the various coexisting mineral phases (clino- and orthopyroxene, hornblende and biotite) during cooling times. Spear and Florence (1992) recently evaluated gamet-biotite thermometry in terms of cation diffusion. Spear and Florence (1992) observed that the gametcore- 8 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. biotitem atriX temperatures are always lower than the peak metamorphic temperatures by a quantity that depends on the size of the garnet, the cooling rate and the gamet/biotite volumetric ratio. This means that to quantitatively interpret the observed thermometric trend it is necessary to account for all the mentioned parameters and also include all the coexisting Mg-Fe-rich phases in the sample. However, the lack of documentation about these parameters and their effects in other systems (for example gamet-clinopyroxene, gamet- orthopyroxene, etc.) impedes a quantitative evaluation of cation diffusion. Nevertheless, the abundant chemical evidences of cation re-equilibration in the different systems clearly indicates that this phenomenon played an important role in this complex. In summary, peak metamorphic temperatures for the Guichicovi complex are thought to be 837 ± 59 °C (solvus pyroxene thermometry; Lindsley, 1983), 755 ± 25 °C (gamet-orthopyroxene thermometry; Lee and Ganguly, 1988), or 717 ± 2 1 °C (gamet- clinopyroxene thermometry, Ellis and Green, 1979). The preferred alternative is the first for it is based on fairly consistent estimated temperatures, the successful application of the solvus pyroxene thermometer in other studies, and the extensive variation among the diverse geothermometers employed. Indeed, such observed variation is also largely favored by chemical evidence of cation re-equilibration during cooling. Geochronology U-Pb GEOCHRONOLOGY Zircon fractions of two samples from the Guichicovi complex were dated by U-Pb. Those two samples include a gamet-homblende granitic gneiss (CHOX-52) and a gamet- clinopyroxene-homblende quartzo-feldspathie gneiss (MEXTE-22). The former is from the Platanillo area and the later is from south of San Juanito, located in the southwestern edge and the central-eastern margin of this terrane. respectively (Figure 30). 8 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ,00 o£6 SI 0S6 m m s q ' ' m -H ' , 0£ o £6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. ;San Juanita Sarabia :980*990 (Zr) Juan M azatlan: tE. Colorado! iPIatanillo: CHOX-58 s t986 ±4(Zr>:: 146* 3 (PO) f S. J . Guichicovi: Ocotal 'E l Zacatal: M A TIA S R O M E R O EXPLANATION N A / / / / . \ s \ \ \ •rtf*. Cenozoic sediments Marine Upper Jurassic-Upper Cretaceous sediments Middle Jurassic continental red beds Permian and Early Jurassic plutons and post-Early Jurassic dikes Paleozoic greensdiist metamorphic facies rocks Middle Proterozoic granulite metamorphic facies rocks Scale 1:250 000 2 4 6 8 10 km Dextral strike-slip fault Dextral strike-slip(?) fault Geologic contact Highway or paved road Dirt road GEOCHRONOLOGIC SYMBOLS (ages in Ma) RS K-A r dated sample (H= hornblende, B= biotite, 0 = orthoclase) ( 3 U -Pb dated sample (Zr= zircon) U-Pb and K-Ar dated sample (Zr= zircon, 0 = orthoclase, OP = perthitic orthoclase, B= biotite) City H Small town Figure 30. Geologic map of the La Mixtequita area with geochronologic data of the Guichicovi complex and La Mixtequita batholith. CO U 1 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. A total of seven zircon fractions from the two samples were analyzed. Analytical data is illustrated in Table 9. Plotting of these zircon fractions on the concordia diagram is presented in Figure 31. Two of three zircon fractions of sample CHOX-52 lay on the concordia and indicate an age of 986 ± 4 Ma. The poorer quality data for sample MIXTE- 22 suggest an age of 980-990 Ma. These analyses show that the granulite facies metamorphism of the Guichicovi complex occurred from 980 to 990 Ma ago. K-Ar GEOCHRONOLOGY Six samples from the Guichicovi complex have been dated by K-Ar. Location of the dated samples is illustrated in Figure 30 and the analytical data is presented in Table 10. Previous K-Ar ages of the Guichicovi complex include: 8 6 6 ± 29 Ma for a biotite pegmatite (SC-49-72) from San Juan Guichicovi (C. Schlaefer, pers. comm. 1972), 223 ± 11 M a (perthite) for a granulitic gneiss (MVH-58-87) from the Ocotal area (Martfnez et al., 1987), a hornblende age of 911 ± 46 Ma for a hornblende two-pyroxene granulite (ISTEH- 34) from the Encinal Colorado area (Murillo and Navarrete, 1992) and another hornblende age of 279 ± 22 Ma for a clinopyroxene-homblende quartzo-feldspathic gneiss (ISTEH-79) near the contact with the Permian plutons (Murillo and Navarrete, 1992). The first two dates correspond to samples from the southern margin o f the terrane, the third to the eastern side and the last age from the northern margin of this complex (Figure 30). Three new K-Ar ages are included in this study. Sample ISTEH-79, a clinopyroxene-homblende quartzo-feldspathic gneiss was reanalyzed and yielded a hornblende age of 263 ± 6 M a . Perthitic orthoclase of the gamet-homblende granitic gneiss (CHOX-58), that was dated at 986 ± 4 Ma by U-Pb (zircon) in this work, gave 146 ± 3 Ma. Finally, the hornblende of an amphibolite from Monte Aguila yielded 309 ± 7 Ma. The K-Ar data of the Guichicovi complex can be interpreted as cooling and/or anomalous ages due to partial loss of radiogenic Ar40. The oldest date, 911 ± 46 86 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. i Table 9. U-Pb geochronologic data of the Guichicovi complex FRACTION O) Weight (mg) 206Pb* (ppm) 238U (ppm) OBSERVED RATIOS(p) 206Pb 207Pb 208Pb 204Pb 206Pb 206Pb ATOMIC RATIOS(n,A) 206Pb* 207Pb* 207Pb* 238U 235U 206Fb* AGES (Ma) 206Pb* 207Pb* 238U 235U 207Pb* 206Pb* Sample CHOX-58 (Garnet-bornblende granitic gneiss) M>163 2.4 30.0 206.9 3086 0.07659 0.0858 0.16553(0.1) 1.6449(0.7) 0.07207(0.6) 987.4 987.6 988 ±13 N>163 2.0 29.3 203.7 2232 0.07830 0.0912 0.16523(0.2) 1.6407(0.4) 0.07202(0.3) 985.8 986.0 986 ± 7 N<80 2.5 28.0 196.5 5181 0.07474 0.0872 0.16410(0.1) 1.6309(0.3) 0.07208(0.3) 979.5 982.2 988 ± 6 Sample MIXTE-22 (Garnet-pyroxene quartzo-feldspathic gneiss) N>163 4.4 8.9 62.5 6369 0.07397 0.0996 0.16484(0.1) 1.6325(0.7) 0.07183(0.7) 983.6 982.8 981 ± 13 N<80 3.4 10.4 73.6 7042 0.07408 0.1054 0.16387(0.2) 1.6301(1.0) 0.07215(1.0) 978.2 981.9 990 ± 20 M>163 5.3 10.0 70.0 4184 0.07554 0.1056 0.16459(0.2) 1.6392(1.7) 0.07223(1.5) 982.2 985.4 992 ±31 M<80 2.0 10.3 72.8 3546 0.07583 0.1091 0.16361(0.2) 1.6223(1.4) 0.07192(1.3) 976.8 978.9 984 ±26 (*) Denotes radiogenic Pb. Sample dissolution and ion exchange chemistry modified from Krogh (1973) and Mattison (1987). (3) N-Nonmagnetic and M-Magnetic at 1.8° and 0.5° side slope on a Franz Isodynamic separator. Sizes are in microns. (p) Observed ratios collected on Farrady cups on Fmigan-M at MAT 262 multiple collector mass spectrometer at the U.S. Geological Survey in Menlo Park, CA. Uncertainties in the 208Pb/206Pb and 207Pb/206Pb ratios are <0.1% and the uncertainty in the 206Pb/204Pb is <20%. (7t) Observed ratios were corrected for 0.125% per unit mass fractionation based on replicate analyses o f NBS 981 and 983, for laboratory blank that has averaged <0.2 ng Pb, and for common Pb ratios based upon 1000 M a (CHOX-58 and CHOX-22) ages using average crustal growth curve o f Stacey and Kramers (1975). Atomic ratios calculated using the following constants: 238U/235U= 137.88,235U= 0.98485 X 10 E-9 yr-1; 238U= 0.155125 X 10 E-9 yr-1, and corrected. (A) Errors in percent are shown in parentheses. oo 0 .1 6 8 □ Grt-Px-Hbl qturtzo-fcldspatiiic gneki (986 ± 4 M») H G rl-Px quxrtzo-feM spxthlc gneiss (980-990 M») 0 .167 0 .1 6 6 990, 0 .1 6 5 -9B0. 0 .164 0.163 970. 0.1 6 2 1.61 1.63 1.65 1.67 Figure 31. U-Pb concordia diagram of two samples from the Guichicovi complex. Ellipses represent error in the analytical data. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. Table 10. K-A r geochronologic data of the Guichicovi complex 40Ar» Av. 40Ar* < * > 40Ar* % 40Ar# Av. %4QAr# %K Av. %K 40K Age (Ma.) REF. 0.6296 0.6388 8.090 8.143 9.935 866 ±29 (D 0.6479 8.197 0.16904 99.0 1.0 J038 12.27 223 ± 11 (ID 33300E-09 99.0 1.62 4.8300E-08 911 ±46 (I1D 7.1130E-10 47.0 139 4.1484E-08 279 ±22 (IID 574.4 574.4 2.0 2.1 1.183 1.171 263 ±6 (IV) 573.9 2.4 1.180 576.4 1.6 1.170 574.1 23 1.147 573.3 23 1.176 558.1 552.6 0.6 1.7 0.938 0.945 309 ±7 (IV) 549.9 2.1 0.941 550.7 2.0 0.937 554.4 1.1 0.947 549.7 2.9 0.964 2205 2222 03 0.6 8.495 8.437 146 ±3 (IV) 2235 03 8.421 2237 13 8335 2232 1.1 8332 2217 -0.1 8318 2207 03 8320 SAMPLE CLASSIFICATION LOCATION M INERAL ANALYZED SC-49-72 Biotite or phlogopite pegmatite MVH-58-87 Granulitic gneiss ISTEH-34 ISTEH-79 Hornblende two-pyroxene mafic granulite Hocnblende-pyroxene quartz o- feldspathic gneiss IS TEH -82 Amphibolite CHOX-58 Ocotal Encinal Colorado Monte Aguila Perthite Hornblende Hornblende Hydrothermalized Garoet-horn blende granitic gneiss Monte Aguila PJataniUo Hornblende Pcrthitic orthoclase NOTE: (*) denotes radiogenic Ar. (#) denotes atmospheric Ar. Analysis of 40Ar* and 40K are expressed in ppm for samples SC-49-72, MVH-58-87. Analysis of 40Ar* are expressed in 10 E-12 mole/gram for samples ISTEH-79 (this study), ISTEH-82 and CHOX-58. Analysis of 40Ar* and 40K are expressed in mole/gram for samples ISTEH-34 and ISTEH-79 (Murillo and Navarrete, 1992). REFERENCES: (I)= C. Schlaefer (pers. comm., 1972), (H)= Martinez et al. (1987), (IH)= Murillo and Navarrete (1992) and (IV)= this study. 00 vO (hornblende) and 8 6 6 ± 29 M a (mica) are considered to represent cooling ages probably associated with very rapid cooling of the Guichicovi complex. This interpretation is based on the known closure or blocking temperatures of these minerals. Harrison (1981) observed that hornblende is extremely retentive of radiogenic Ar4 0 and defined a closure temperature range from 578 °C (rapidly cooled) to 490 °C (slowly cooled). Biotite has a closure temperature range from 300 °C to 350 °C (McDougall and Harrison, 1988). On the other hand, the 309 ± 7, 279 ± 22, and 263 ± 6 Ma ages, all measured on hornblende, may be anomalous ages and can be explained in terms of advanced loss of radiogenic Ar40. These ages correspond to samples near Permian plutons (Figure 30). The loss of radiogenic Ar4 0 is attributed to the magmatic activity of the La Mixtequita batholith. Therefore, the Paleozoic ages are interpreted to represent reheating effects on the Guichicovi complex. The Mesozoic ages (223 ± 11 and 146 ± 3 Ma) obtained from perthitic alkali feldspar presumably also represent reheating ages caused by the plutonism in this region. It should be noted that these Mesozoic ages are significantly younger than the Paleozoic ages. This large difference is attributed to the remarkably low closure or blocking temperature range of the exsolved alkali feldspars, that is, 150 to 132 °C (Foland, 1974; Harrison and McDougall, 1982). 90 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PETROLOGY OF THE LA MIXTEQUITA BATHOLITH Petrography Thirty three hand samples were analyzed by petrography in order to define the lithologic characteristics of the La Mixtequita batholith. Samples were collected from the southern, central and northeastern portions of this plutonic complex (Figure 3). Due to the lack of access to the northwestern segment of the batholith, only one sample was collected from this region. As was discussed on the section of the regional geology, the La Mixtequita batholith, a composite batholith, is divided into three distinctive magmatic groups or suites. There suites include: (1) Permian granitoids, (2) Early Jurassic plutons, and (3) post-Early Jurassic dikes. The lithology of these groups is described below and their mineralogy is listed in Table 11. The Permian plutons of the La Mixtequita batholith consist mostly of quartz monzonite and quartz monzodiorite with minor amounts of quartz diorite and granite. In hand sample, most o f these rocks exhibit a gray color resulting from the mixture of quartz, white feldspar, hornblende and biotite. They show medium- to coarse-grained phaneritic texture and generally massive to locally foliated structures. In thin section, the Permian granitoids typically present subidiomorphic granular texture grading locally to banded and cataclastic textures. Quartz monzonite is comprised of quartz, sodic plagioclase (sometimes with myrmekitic intergrowths) and alkali feldspar (orthoclase and minor microcline)with hornblende and biotite. Traces of clinopyroxene in the cores of hornblende were identified in two samples. Other minor phases include sphene, apatite, zircon and opaque minerals. In a few samples, plagioclase is partially altered to sericite and biotite is partially to completely transformed to chlorite. In addition, veins of epidote-chlorite occur as evidence of hydrothermal alteration. Quartz monzodiorite contains the same mineralogy as the quartz monzonite but has no trace of clinopyroxene and show higher contents of quartz and plagioclase. Quartz diorite and granite have the similar mineralogical composition to the 9 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 „ | < 3 S Ji K u j sj to u T S O * A O m 9 o s “ s U J M * ? — (J S I E * 6 ~ |K X " s U J i§ < s r 8 ! 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C o 5 t g 5 t r n c 2 » r 0 ^ r ^ O k 3 o r ^ o o o o o a o o o a o o o o o o o o F>» t-r -t'»r-r-t'»r««'r-r-r-r»r»r*»r-r- ^ M v b s O ' t g ’n f'k r' M V' tv o sD ce so fH 0. 0 O 0 (X A « 5 0 O O ^ J i l J j i l l J U i j f I l l i l S I J I ! i l l j l ! 55 X X X X X X X X X X X X X X XXX X X X X X h z £ a & § z z i l l u u m £ T 3 W 92 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. quartz monzonites, however, the quartz diorite has no K-feldspar and a low quartz content while the granite shows a higher quartz content. The opaque mineral in the Permian rocks is generally magnetite. The Early Jurassic plutons of the La Mixtequita batholith range from mafic to felsic in composition. The less evolved rock correspond to gabbro from the vicinity of Villanueva Segundo. Gabbro is massive and shows dark greenish gray color and medium- to coarse grained phaneritic texture. The gabbro displays idiomorphic granular and trachytoid (probably cumulate) textures grading locally to intergranular and intersertal. The essential minerals in the mafic rocks are calcic plagioclase, orthopyroxene (hypersthene), clinopyroxene, and brown hornblende with minor biotite. Orthopyroxene shows an advanced degree of subsolidus uralitic alteration to columnar actinolitic amphibole. Other minor accessory minerals include apatite and opaque minerals. Monzodiorite and quartz diorite are gray in color, medium- to coarse-grained phaneritic. They are usually unfoliated, but occasionally exhibit mylonitic fabrics. These rocks display subidiomorphic granular and occasionally mylonitic textures. Monzodiorite contains plagioclase, alkali feldspar (orthoclase), clinopyroxene and biotite as essential minerals; minor phases include quartz, green hornblende, sphene, opaque minerals and minor interstitial, secondary, calcite. A few casts of former orthopyroxene(?) are totally transformed to acicular actinolitic amphibole. Quartz diorite presents the same mineralogy as the monzodiorite except for higher amounts of quartz and green hornblende and contains other accessory minerals like orthopyroxene (partially altered to columnar actinolitic amphibole), apatite, and zircon. One cataclastic quartz diorite (MIXTE-69) contains approximately 30 % secondary hydrothermal epidote. Finally, the more evolved lithofacies is pink leucogranite that also represent the latest stages of the Jurassic plutonic activity in the La Mixtequita batholith. Granite displays massive structure and medium- to coarse phaneritic texture although pegmatitic texture is not uncommon. Granite shows generally xenomorphic granular 9 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. texture. The essential minerals include quartz, K-feldspar (orthoclase and minor microcline), sodic plagioclase (often with myrmekitic and scarce antiperthitic intergrowths), hornblende and biotite. Muscovite occurs in some samples with coexisting biotite. Accessory minerals include sphene, zircon, apatite, opaque minerals, and tourmaline. The opaque mineral in the Early Jurassic rocks is generally magnetite. The final igneous activity in the La Mixtequita batholith corresponds to emplacement of dikes consisting of diorite, monzodiorite, andesite and latite. It is unclear if these rocks are co-magmatic with the batholith or if they are genetically related to the Early Jurassic plutonism. For practical purposes they are grouped as post-Early Jurassic dikes. The most mafic dikes correspond to a coarse-grained hydrothermally altered homblende- clinopyroxene monzonite from Brena Torres Viejo. Its porphyritic nature (hornblende and clinopyroxene occurring as phenocrysts) might indicate that it may be a lamprophyre, specifically the variety spessartite. This dike rock is comprised mainly of green and brown hornblende, plagioclase, alkali feldspar (orthoclase, microcline and a few graphic intergrowths), clinopyroxene and minor quartz, sphene, apatite and opaque oxides (magnetite). Its texture is subidiomorphic granular. Evidence of hydrothermal alteration includes total replacement of former pyroxene(?) by actinolitic amphibole and strong alteration of plagioclase to sericite and epidote. Diorite dikes are massive and very compact, displaying dark green color and fine-grained phaneritic texture. These rocks show fine grained subidiomorphic granular, intergranular and intersertal textures. They contain plagioclase, clinopyroxene, and green hornblende as essential minerals; accessory include opaque minerals, sphene and apatite. Evidence of hydrothermal alteration in diorites include advanced transformation of plagioclase to sericite and epidote, chloritization of pyroxene and the occurrence of epidote-calcite veins. Monzodiorite dikes have textural and mineralogical characteristics similar to the diorites and also contain perthitic orthoclase, biotite and traces of quartz and sphene. Porphyritic hornblende andesite dikes are massive, 9 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. very compact, and display porphyritic texture with a dark greenish gray color. They are composed of phenocrysts of plagioclase and green hornblende immersed in an unoriented microlithic groundmass consisting of plagioclase, chlorite, epidote, opaque minerals, and patches of calcite. Finally, the porphyritic biotite latite is massive and shows white phenocrysts of plagioclase in a light green aphanitic matrix. This rock consists of zoned phenocrysts of plagioclase (partially altered to sericite and epidote) included in an unoriented groundmass of plagioclase, alkali feldspar, sericite, and minor strongly chloritized biotite and quartz. Geochronology U-Pb GEOCHRONOLOGY Three plutonic samples were dated by the U-Pb method (zircon). These samples include a homblende-biotite quartz monzonite (CHOX-72), a leucogranite (CHOX-129), and a homblende-pyroxene-biotite quartz diorite (ISTEH-38). The first sample is from the southwestern margin of the batholith and the last two samples are from the northern region of the batholith (Figure 30). The analytical data is given in Table 12. Four zircon fractions of sample CHOX-72 form a well defined discordia plot with intercept ages of 254 ± 7 Ma and 1079 ± 15 Ma (Figure 32). Four zircon fractions of sample CHOX-129 also form a good regression with intercepts of 145 ± 137 Ma and 1014 ± 90 Ma (Figure 32). The large uncertainties for sample CHOX-129 are the result of the clustering of the data and their position with respect to the concordia line. The four zircon fractions analyzed of sample ISTEH-38 did not form a good chord, however, a zircon fraction intercepts the concordia line at 189 ± 3 Ma (Figure 32). The contact between the quartz monzonite (CHOX-72) with the quartz diorite (ISTEH-38) and the leucogranite (CHOX-129) was not precisely defined. However, it is clear that the leucogranite intrudes the quartz diorite and that all of these intrude the 9 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. Table 12. U-Pb geochronologic data of La Mixtequita batholith FR A C T IO N (3) W eight (m g) 206Pb* (ppm ) 238U (ppm ) O B S E R V E D R A T IO S (p) 206Pb 207Pb 208Pb 204Pb 206Pb 206Pb A T O M IC R A T IO S (re, A) 206Pb* 207Pb* 207Pb* 238U 235 U 206Pb* A G ES (M a) 206Pb* 207Pb* 238U 235 U 207Pb* 206Pb* S a m p le C H O X -7 2 (H o rn b le n d e -b io tite q u a r tz m o n zo n ite) N >163 9.1 10.9 118.0 2375 0.07645 0.1064 0.10649(0.1) 1.0357(0.6) 0.07054(0.5) 652.3 721.8 944 ± 10 M > 130 8.8 8.6 109.9 463 0 0.07154 0.0931 0.09055(0.2) 0.85572(1.4) 0.06854(1.2) 558.8 627.8 885 ± 2 5 M < 80 5.8 6.3 146.7 3413 0.06108 0.1490 0.04913(0.6) 0.38544(1.8) 0.05691(1.6) 309.2 331.0 488 ± 34 N < 80 5.8 4.5 122.4 3374 0.05710 0.1673 0.04226(1.0) 0.30798(1.8) 0.05285(1.5) 266.9 272.6 322 ± 33 S a m p le C H O X -1 2 9 (leu c o g ran ite) N < 130 2.8 29.8 314.9 1129 0.0 8 3 3 6 0 .1119 0.10774(0.1) 1.0522(1.0) 0.07083(1.0) 659.6 730.0 953 ± 19 N>163 6.1 19.9 221.4 984 0.08499 0.1187 0.10206(0.1) 0.99341(0.8) 0.07059(0.7) 626.4 700.5 946 ± 14 M >163 6.6 27.3 301.4 608 0.09378 0.1347 0.10170(0.1) 0.98736(1.3) 0.07042(1.2) 624.3 697.4 941 ± 2 4 M < I6 3 4.1 34.6 410.1 583 0.09439 0.1268 0.09467(0.1) 0.91418(1.0) 0.07004(0.9) 583.1 659.3 929 ± 19 S a m p le IS T E H -3 8 (H o rn b le n d e -p y ro x e n e -b io tite q u a r tz d io rite ) N>163 10.2 2.0 78.0 1531 0.06058 0.2125 0.02970(0.5) 0.20932(6.9) 0.05112(6.5) 188.7 193.0 241 ± 1 5 0 N>163 18.6 3.7 140.0 1477 0.05972 0.2032 0.02985(0.1) 0.20533(1.6) 0.04989(1.5) 189.6 189.6 190 ± 3 6 M <100 19.0 3.1 120.2 4464 0.05477 0.1864 0.02934(0.3) 0.20863(4.1) 0.05157(3.9) 186.4 192.4 266 ± 89 M >163 19.5 2.6 99.8 1475 0.05990 0.2041 0.03002(0.3) 0.20720(4.1) 0.05005(3.9) 190.7 191.2 197 ± 9 0 (*) Denotes radiogenic Pb. Sample dissolution and ion exchange chemistry modified from Krogh (1973) and Mattison (1987). (3) N-Nonmagnetic and M-Magnetic at 1.8° and 0.5° side slope on a Franz lsodynamic separator. Sizes are in microns. (p) Observed ratios collected on Farrady cups on Finigan-Mat MAT 262 multiple collector mass spectrometer at the U.S. Geological Survey in Menlo Park, CA. Uncertainties in the 208Pb/206Pb and 207Pb/206Pb ratios are <0.1% and the uncertainty in the 206Pb/204Pb is <20%. (it) Observed ratios were corrected for 0.125% per unit mass fractionation based on replicate analyses o f NBS 981 and 983, for laboratory blank that has averaged <0.2 ng Pb, and based upon the Pb isotopic compositions 208:207:206:204 of the feldpars: CHOX-72= 37.512:17.999:15.580:1; CHOX-129= 37.368:15.563:17.910:1; ISTEH-38= 37.964:15.587:18.337:1. Atomic ratios calculated using the following constants: 238U/235U= 137.88, 235U= 0.98485 X 10 E-9 yr-1; 238U= 0.155125 X 10E-9 yr-1, and corrected. (A) Errors in percent are shown in parentheses. V O o\ 800 0.13 700 0.11 600, 3 0.09 m % I 0.07 500, 400, Sample CHOX-72 (homblende-biotite quartz monzonite) Intercepts al 1079± 15 Ma and 254 ± 7 Ma. ___________ (MSMD= 0.013) 0.05 300, 0.03 0.4 0.2 0.6 0.8 1.0 1.2 207Pb/235U 0.14 800 700 0.12 600 0.10 500 D 0.08 00 m % ^ 0.06 C l 400 300 0.04 200 0.02 Sample CHOX-129 (leucogranite) Intercepts at 1014 ± 90 Ma and 145 ± 137 Ma. (MSMD= 0.7) 100 0.00 0.1 0.3 0.5 0.7 0.9 1.1 201pb /235v Figure 32. U-Pb concordia diagrams for samples from the La Mixtequita batholith. (a) Sample CHOX-72, a homblende-biotite quartz monzonite. (b) Sample CHOX-129, a leucogranite. (c) Sample ISTEH-38, a homblende-pyroxenc quartz diorite. 97 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 .0 3 2 200 Hbl-Px-Bio quartz diorite (189 ± 3 Ma) 0.031 1 9 6 192 238 0 .0 3 0 184 0 .0 2 9 180 0 .0 2 8 0 .1 9 0 0 .1 9 4 0.1 9 8 0.202 0 .2 0 6 0 .2 1 0 0 .2 1 4 0.218 207P b /235U Continuation of Figure 32. Ellipses represent error in the analytical data. 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Proterozoic granulites and are largely undeformed. It is concluded that the lower intercepts date emplacement. This requires the age for the quartz monzonite to be 254 ± 7 Ma (Permian) and the age of the quartz diorite to be 189 ± 3 Ma (Early Jurassic). The age of the leucogranite probably is also Early Jurassic (see the K-Ar geochronology section below). These ages, the first U-Pb data for the La Mixtequita batholith, suggest that the southern segment o f this batholith is Permian in age while the northern portion is Early Jurassic. This conclusion is also supported by geochemistry of these and other samples. The Proterozoic upper intercept ages support the observed stratigraphic relationships that the plutons were emplaced into the Middle Proterozoic Guichicovi complex. K-Ar GEOCHRONOLOGY Numerous samples from the La Mixtequita batholith have been dated by K -A r. Table 13 represents a compilation of previous K-Ar data for this plutonic complex. It also includes five new K-Ar ages reported in this work. Figure 30 shows the age distribution of most of the samples analyzed by K-Ar. Although the K-Ar geochronology o f this batholith suggests a complex thermal history, a rough systematic age distribution can be recognized. This age distribution follows the same pattern o f the U-Pb ages. Permian-Triassic K-Ar ages concentrate in the southern segment o f the plutonic complex. A K-Ar age of 235 ± 6 Ma (hornblende) for a hornblende granodiorite (QU- 8399) from the Junapa river was reported by Quezada (1978). This river cuts across the southeastern edge of the batholith, southeast of the village of La Mixtequita. Rufz (1978) obtained 216 ± 6 M a and 211 ± 6 Ma ages on hornblendes from two biotite-homblende granitic gneisses (RS-313 and -312) also in proximity to the La Mixtequita village and in the Arroyo Lirio area, respectively. In the present study, high-grade metamorphic rocks were not recognized in the vicinities of these villages. These samples reportedly correspond to the granitoids affected by dynamic metamorphism (Ruiz, 1978). The same author also 9 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. T able 13. K -A r geochronologic data of the L a M ixtequita batholith SAMPLE CLASSIFICATION LOCATION MINERAL ANALYZED 40Ar* Av. 40AT* % 40Ar* % 40 Ar# Av * 4 0 Ar# Av. %K 40K Age (Ms.) REFERENCE QU-8399 Hcrnbtende granodiorite Juflsps River Hornblende 000937 0.0094 0 0526 00523 0.05245 0 6399 235 ± 6 Qoezsds(1978) QU-8533 Biotito-fcocnbleade tonshtc Victor Ssn Ja x i River Bioale 0 03166 0 0317 2081 2.175 2.128 2.596 1981 7 Qnezsds(1978) RS-307 Biotito-hcrabieade grenodioriie Aguacalengo River sres Hornblende 0 0043 00043 0.7521 07521 0.7521 0.9175 79 + 2 Ruiz (1978) RS-310 Biotite-bcrnbiendc gnaodiorike La Mixtequiia v e a Hornblende 0.01451 00143 0818 0.8299 08239 1005 2 3 2 1 5 Ruiz (1978) RS-311 Andesite Arroyo Lirio sres Whole rock 0 0064 0.0064 1.14404 13804 14104 1.7206 67 ± 2 Ruiz (1978) RS-312 Biotito-Hornbfcnde granitic gneiss Arroyo Lino area Hornblende 0.01138 0.0114 0.672 0.6975 0.6847 0 8654 2 1 1 1 6 Rtdz(1978) RS-313 Biocto-Harnbieade granitic p e a t La Mixtequita sres Hornblende 0.0143 00143 0 9331 0.8214 0 8773 1.0703 2 1 6 1 6 Ruiz (1978) RS-318 Bioake-barnblcade grsnodiORie S of Jalicpec Biotite 0.08858 00886 6139 61645 6.1135 7489 192 ± 5 Ruiz 0978) UAKA-S0-01 Diorifc Felipe A n g e la ares Bionte 2420 2417 2.7 2.8 6.926 6.933 1 9 1 1 4 D tm oaetaL (1981) 2408 3.0 6937 2408 3.0 6.935 2431 2.4 UAKA-80-02 Quartz raamonitic porphyry Felipe Angeles are* Biotite 1367 1365 5.6 5.6 3.898 3.898 1 9 1 1 4 D*m anetsL(1981) 1364 5 8 3.896 1364 5.6 3.899 1364 5.6 ISTEH-38 Homblende-pyroxcne-btobtt quartz diortke Villanueva Segundo Bioaie 2.6307E-09 81 0 6 6 0 1.9697E-07 2 1 0 + 1 7 Murillo snd Navarrete (1992) IS T H I » V B ios* g r« sv Villanueva Scgundo arei Orthoclase 1727 1721 2 6 5 0 8866 9009 107 ± 2 Present «mdy 1723 3 0 9008 1717 3 2 8.997 1716 110 9 040 9030 ISTEH-41 Hornblende-bionic qiurtz monzomie Nuevo Centro ares Hornblende 472 6 4793 3.2 2 3 0931 092 6 277 ± 6 Present mxty 481.2 1 9 0.946 481.1 1.7 0.925 478.0 2.2 0.93 482.6 2.4 0.929 481.4 2.3 0.908 0.906 0.929 ISTEH-41 HcrnMcnde-biotile quartz m oazodie Nuevo Centro area Biotite 244 4 2E-09 76.0 4 35 1J579E-07 286 ±23 Present Kudy CHOX-128 HotnMcnidvtaotiie qaanz monxodiorile S of Jalicpec Hornblende 95 3 95.7 103 12.2 0369 0366 196 ± 4 Present iludy 95.9 14.1 0363 97.8 10.7 0.266 96.4 12.7 0.263 93.6 12.0 0.269 95.0 13.4 0.268 CHOX-129 Leocognaite ElTcstngnero Orthoclase 2250 2250 1.8 1.4 6.963 6.842 1 8 0 1 4 Preaenl ttudy 2253 1.4 6.692 2252 1.1 6890 2247 1.4 6886 2250 1.4 6.781 2247 1 4 NOTE: (*) denote* radiogenic A r,(<) denotes ionospheric Ar. Analyris cf40A r* snd 40K are expremed in ppm for s ta p le s QU-8399. QIL8533, RS-307, RS-310, RS-311, RS-312, RS-313. RS-318. Analysis a t 40Ar* s it expressed in 10 E-12 mole/gram for aunplea U A K A -804I. UAKA-8<W2. ISTEH-39. ISTEH-41 (hornblende), CHOX-128 snd CHOX-129. Analysis of 40Ar+ snd 40K are expressed in tnoie/gmn for samples ISTEH-38 snd ISTEH-4 1 (biotite) dated a biotite-homblende granodiorite (RS-310) from the La Mixtequita village and obtained an age of 232 ± 5 Ma for the amphibole. For the present study, a biotite- homblende quartz monzonite (ISTEH-41) from Nuevo Centro, in the southern region of the batholith, provided ages of 277 ± 6 M a and 286 ± 23 Ma on hornblende and biotite, respectively. Based on the Permian U-Pb age (254 ± 7 Ma) for the southern margin of the batholith, a Permian age is assumed for all of these rocks. Therefore, the 277 ± 6 Ma and 286 ± 23 Ma K-Ar ages are interpreted as slightly anomalous old ages. Anomalous old K- Ar ages can be obtained in some cases when the mineral analyzed has incorporated or trapped radiogenic Ar from the environment (McDougall and Harrison, 1988). The Triassic dates (211-235 Ma), however, are considered to represent either cooling ages of the granitoids or reheating associated with Early Jurassic plutonism. The Early Jurassic K-Ar ages concentrate in the central and northern segments of the La M ixtequita batholith (Figure 30). Quezada (1978) reported a K-Ar age of 198 ± 7 M a (biotite) for a biotite-homblende tonalite (QU-8533) from the northern boundary of this batholith. An age of 192 ± 5 M a (biotite) for a biotite-homblende granodiorite (RS-318) from the northwestern region of the batholith was obtained by Ruiz (1978). Biotite from a diorite (UAKA-80-01) and a quartz monzonitic porphyry (UAKA-80-02) from the central and central-eastern area of the batholith yielded 191 ± 4 Ma ages for both samples (Damon et al., 1981). A biotite age of 210 ± 17 M a from sample ISTEH-38, the tonalite dated by U-Pb for the present study as 189 ± 3 Ma, was obtained by Murillo and Navarrete (1992). In addition, in this study two new Early Jurassic K-Ar ages are reported. A biotite- homblende granodiorite (CHOX-128), from the same northwestern portion of the batholith yielded a hornblende age of 196 ± 4 Ma similar to the Early Jurassic age reported by Ruiz (1978). The leucogranite (CHOX-128), dated by U-Pb as 145 ± 137 M a for this study, provided an orthoclase age of 181 ± 4 Ma. There is a clear consistency in the Early Jurassic K-Ar ages. It is likely that K-Ar ages slightly older than corresponding U-Pb ages are due 101 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. to incorporation of inherited radiogenic Ar from the environment. The 180 ± 4 M a K-Ar age for the leucogranite is considered as a cooling age. Furthermore, this cooling age supports the interpretation that the U-Pb age (145 ± 137 Ma) for the leucogranite is probably an Early Jurassic age. Only three K-Ar ages are Cretaceous (Table 13). Ruiz (1978) reported a K-Ar hornblende age of 79 ± 2 M a for a biotite-homblende granodiorite from the Aguacatengo river, from the southwestern margin of the batholith. The same author also obtained a K-Ar whole rock age of 67 ± 2 M a for an andesite from the Arroyo Lirio area, in the south- central region of the batholith. In this study, a biotite granite from the east of Villanueva Segundo was dated by K-Ar using orthoclase and yielded an age of 107 ± 2 Ma. These ages are considered anomalous because the Middle Jurassic sedimentary cover overlying the batholith is unaffected by contact metamorphism. Instead, there is an unconformity between the plutons and the Middle Jurassic sediments. Also, field observations and geochemical data show that the biotite granite belongs to the same plutonic event as the Early Jurassic leucogranite (CHOX-128). Therefore, the Cretaceous K-Ar ages are concluded to represent cooling ages. The K-Ar age from the andesite may corresponds to the emplacement of dikes representing the final magmatic activity in the La Mixtequita batholith. Because dikes were not seen affecting the overlying Middle Jurassic to Tertiary sedimentary cover, they are considered as post-Early Jurassic age (probably early Middle Jurassic). Although the possibility remains that a Cretaceous phase of pluton or dike emplacement exists within the batholith, no evidence of such plutonism was found in this study. 102 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Geochemistry MAJOR AND TRACE ELEMENT GEOCHEMISTRY In order to geochemically characterize the three magmatic suites recognized in the La Mixtequita batholith, major and trace elements were analyzed for 30 samples. Tables 14, 15 and 16 present the major and trace element analysis for the Permian granitoids, the Early Jurassic plutons and the post-Early Jurassic dikes, respectively. Harker type diagrams were constructed in order to distinguish geochemical trends in the three magmatic suites. A geochemical lineage is clear for the Permian plutons, even though it has a relatively narrow compositional range (58.93 to 65.75 wt% of Si0 2 ) (Figure 33). W ith increasing silica, K2O, Ba and La increase whereas decreasing trends occur for CaO, AI2O 3 , MgO, FeO*, MnO, TiC>2 , P 2O 5 , Z r and Y. No changes were observed for Na2 0 , Sr, Rb, and Cr. A very well-defined geochemical trend is also evident for the Early Jurassic suite which ranges from mafic (49.86 wt% o f SiC>2) to felsic (76.35 wt% of SiC> 2 ) (Figure 33). K2O, Rb and Ba increase with silica whereas CaO, AI2O 3 , MgO, FeO*(total iron), MnO, Ti0 2 , P 2O 5 , Cr and Y all decrease. No correlation is seen between Si0 2 and Na2 0 , Sr, Zr and La. The post-Early Jurassic dikes, the youngest magmatic suite in the batholith, lack the trend of a comagmatic series (Figure 33). Although only six samples were analyzed, the data are scattered relative to that of the Permian and Jurassic plutons. The suite ranges in composition from 50.28 % of Si0 2 (spessartite) to 71.13 % of Si0 2 (latite). Only one sample falls on trend with the Jurassic portion of the batholith, specifically sample MIXTE- 53 (at 54.21 % Si0 2 ). The spessartite (MIXTE-6 8 ) has much lower AI2O 3 and very high P20 5, La, and Cr. Samples MIXTE-55 (52.13 % S i0 2) and M IXTE-56 (57.47 % S i0 2) have high Zr, Y, and Ti, and Zr and Cr, respectively. The two high silica dike samples (MIXTE-39 and -43) have low K and Ba and high Ca relative to the remainder of the batholith. 1 0 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 14. Geochemistry of the Permian suite of the La Mixtequita batholith SAMPLE MIXTE CHOX 44 75 ISTEH 41 MIXTE MIXTE MIXTE MIXTE MIXTE CHOX MIXTE MIXTE 40 6 6 70 41 42 72 59 45 Major Oxides (wt %) Si02 58.93 59.76 60.46 61.32 62.48 62.62 63.61 64.19 65.40 65.75 67.60 A1203 17.42 18.17 17.49 16.26 18.79 18.82 15.79 17.58 16.60 16.42 15.83 Ti02 0.91 0.61 0.67 0.72 0.39 0.41 0.69 0.48 0.45 0.48 0.42 FeO* 6.34 4.64 5.12 5.26 3.15 3.13 4.52 3.56 3.09 3.54 2.92 MgO 2.89 2.53 2.61 3.24 2.05 1.92 2.50 1.82 1 .6 6 1.74 1.54 MnO 0.129 0.130 0.107 0.109 0.094 0.095 0.086 0.082 0.076 0.082 0.067 CaO 5.79 4.80 5.33 5.44 2.82 3.58 4.29 4.81 4.20 3.97 3.80 Na20 4.23 5.32 4.30 3.80 4.92 6.08 3.72 4.24 4.35 4.07 3.69 K2 0 1.99 1.77 1.95 2.72 3.72 2.06 3.67 2.61 2.32 3.22 2.94 P205 0.32 0.27 0.25 0.25 0.19 0.19 0 .21 0 .2 0 0.17 0.19 0.15 LOI 0.63 L24 1.38 0.95 2 .1 0 1 .66 1.40 1.28 1.0 0 1.37 0.75 Sum 99.58 99.74 99.67 100.08 100.69 100.56 100.49 100.84 99.32 100.82 99.72 Trace Elements (ppm) Ba 834 312 1150 823 4051 583 918 1336 724 1231 1367 Cr 6 .8 96.5 15.7 43.2 6.4 6.1 46.2 1.3 9.6 2 .8 1 2 .6 Cu 8.5 56.5 6 .8 5.6 4.7 5.2 62.0 7.3 3.3 7.0 5.2 La 61 13 10 18 17 16 15 19 23 23 2 2 Nb 8.4 0 .0 4.9 3.8 0 .2 3.8 10.1 5.0 9.0 11.3 11.5 Ni 0 .0 41.0 1.0 13.8 0 .0 0 .0 15.3 0 .0 0 .0 0 .0 0 .0 Pb 8.5 8 .2 8.3 9.0 7.4 6 .0 12.3 9.3 11.7 1 1 .6 10.1 Rb 33.2 30.8 30.7 48.4 55.9 30.4 83.1 32.6 30.5 45.1 43.5 Sr 745 352 797 573 878 827 494 809 745 694 695 Th 4.5 2.3 0 .0 1.3 0 .0 0.9 0.7 1.1 1.0 1.3 0.9 U 1.3 1.2 1.2 1.8 2 .0 1.2 3.0 1.3 1.2 1.7 1.6 Y 26.1 29.1 16.8 25.0 1 1 .1 13.2 2 2 .8 14.4 17.5 15.6 13.6 Zn 101 80 89 83 63 110 77 80 57 64 56 Zr 208 147 186 185 110 96 314 140 136 137 131 S 32 637 37 48 29 45 49 34 21 17 2 2 Cl 396 178 269 385 161 347 704 140 101 99 72 Ga 2 1 .2 16.6 20.5 19.6 19.7 19.3 17.8 17.8 17.7 17.8 17.6 Rare Earth Elements (ppm) La 11.9 Ce 25.0 Nd 13.0 Sm 2.7 Eu 0.96 Tb 0.400 Yb 1.34 Lu 0.200 Important ratios Na20+K20 6.22 7.09 18.9 38.0 19.0 3.84 1.73 0.400 1.33 0.19 6.25 6.52 8.64 8.15 7.39 6.85 6.67 7.28 6.64 A/CNK 0.887 0.937 0.926 0.852 1.090 1.004 0.883 0.948 0.960 0.944 0.979 (La/Lu )n Eu/Eu* Mg# 0.313 6.4 1.097 0.353 10.8 1.564 0.337 0.381 0.395 0 380 0.356 0.339 0.349 0.330 0.345 Analyses of major oxides and trace element were determined by XRF at USC. Analyses of rare earth elements from Murillo and Navarrete (1992) FeO* = as total iron. A/CNK= molecular AI203/(Ca0 + Na20 + K20) Mg#= MgO/(MgO + FeO*) Eu/Eu*= ((Sm/Tb)A (2/3)*Tb) using chondrite-normalized values. (La/Lu)n= La/Lu ratio using chondrite-normalized values. NOTE: REE abundances were normalized tocondritic values of Taylor (1980). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table IS. Geochemistry of the Early Jurassic suite of the La Mixtequita batholith SAMPLE ISTEH MIXTE MIXTE CHOX ISTEH CHOX ISTEH MIXTE MIXTE CHOX MIXTE MIXTE 37 35 33 74 38 128 39 57 38 129 32 34 Major Oxides (wt %) Si02 49.86 50.69 51.15 54.31 56.45 63.41 70.39 71.80 73.42 74.07 76.02 76.35 A1203 19.42 18.54 19.19 16.50 17.23 16.76 15.21 15.99 14.69 15.24 12.53 13.69 T i02 1.00 1.11 0.94 1.00 1.00 0.58 0.29 0.12 0.21 0.12 0.08 0.08 FeO* 8.73 9.27 • 8.21 7.60 7.51 4.99 1.89 0.61 1.27 0.18 0.37 0.42 MgO 6.25 5.43 5.43 5.10 4.10 2.45 0.73 0.29 0.14 0.08 0.06 0.12 MnO 0.169 0.196 0.167 0.148 0.145 0.127 0.038 0.027 0.010 0.011 0.011 0.011 CaO 10.80 8.06 9.64 8.12 7.82 5.06 1.74 1.47 0.34 1.21 0.63 0.34 N a20 2.44 2.56 3.36 2.89 3.27 3.88 4.42 2.65 5.51 4.88 2.34 3.77 K 20 0.30 1.32 0.34 1.32 1.23 1.64 3.56 6.31 3.58 3.63 6.40 4.45 P205 0.13 0.12 0.18 0.14 0.20 0.15 0.07 0.04 0.05 0.00 0.00 0.01 LOI 0.68 2.51 1.32 2.55 0.79 0.63 1.34 1.76 0.54 0.21 1.49 1.50 Sum 99.78 99.80 99.93 99.68 99.75 99.70 99.68 101.07 99.75 99.65 99.93 100.74 Trace Elements (ppm) Ba 99 363 203 498 344 589 1510 2075 453 1105 896 850 Cr 74 56 58 13 51 9 0 0 0 0 0 0 Cu 13 18 28 48 51 7 5 10 2 4 2 4 La 19 1 15 16 7 21 13 18 12 10 11 1 Nb 1 0 0 4 3 9 9 3 8 7 14 13 Ni 42 21 34 2 21 0 0 0 0 0 0 0 Pb 5 8 5 11 8 7 13 14 3 12 12 8 Rb 8 34 9 35 35 32 42 70 41 55 85 59 Sr 328 396 295 680 247 298 546 487 282 586 271 464 Th 2.0 2.1 0.0 0.2 1.8 0.9 0.5 1.1 0.6 0.7 5.0 0.0 U 0.4 1.3 0.4 1.3 1.3 1.2 1.6 2.5 1.5 2.0 3.0 2.2 Y 14 26 23 20 33 23 10 13 11 8 9 6 Zn 79 114 67 95 83 70 44 28 6 19 29 13 Zr 25 82 29 147 150 142 126 87 111 106 53 39 S 59 581 45 51 381 63 19 55 11 94 40 7 Cl 608 227 585 400 481 191 210 68 102 0 30 46 Ga 18 17 17 21 18 17 16 15 11 16 12 12 Rare Earth Elements (ppm) La 4.4 Ce 10.0 Nd 7.0 Sm 1.71 Eu 1.0 Tb 0.3 Yb 1.25 Lu 0.19 9.7 21.0 12.0 2.79 0.89 0.4 2.25 0.37 11.3 26.0 15.0 3.97 1.25 0.8 2.88 0.44 21.4 40.0 16.0 2.66 1.01 0.3 0.81 0.11 Important ratios N a20+K 20 2.74 3.88 3.70 4.21 4.50 5.52 799 8.96 9.09 8.52 8.74 8.21 A/CNK 0.810 0.913 0.819 0.788 0.823 0.966 1 064 1.154 1.084 1.076 1.051 1.177 (La/Lu)n Eu/Eu* Mg# 2.5 1.706 0.417 0.369 0.398 2.8 0.995 0.402 2.8 0.877 0.353 0.329 210 1 283 0.279 0.317 0.100 0.311 0.130 0.215 Analyses of major oxides and trace element were determined by XRF at USC. Analyses of rare earth elements from Murillo and Navarrete (1992) FeO* = as total iron. A/CNK= molecular A1203/(Ca0 + Na20 +K20) Mg#= MgO/(MgO + FeO*) Eu/Eu*= ((Sm/Tb)A (2/3)*Tb) using chondrile-notmalized values. (La/Lu)n= La/Lu ratio using chondrite-normalized values. NOTE: REE abundances were normalized to condritic values of T aylor (1980). [ Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 16. Geochemistry of the post-Early Jurassic dikes of the La Mixtequita batholith SAMPLE ROCK TYPE MIXTE 39 Latite MIXTE 43 Andesite MIXTE 56 Diorite MIXTE 53 Monzodiorite MEXTE 55 Diorite MIXTE 68 Spessartite Major Oxides (wt %) Si02 71.13 69.85 57.47 54.21 52.13 50.28 A1203 16.79 16.75 17.93 17.36 16.88 11.65 Ti02 0.14 0.39 0.95 0.96 1.54 1.20 FeO* 1.00 1.95 7.35 7.35 9.80 8.48 MgO 0.92 1.23 3.86 4.99 4.60 12.49 MnO 0.020 0.023 0.146 0.152 0.181 0.150 CaO 3.08 3.83 6.61 8.09 8.65 9.93 Na20 4.82 5.04 3.10 2.63 2.22 2.26 K20 1.03 0.97 2.13 1.61 1.20 1.15 P205 0.06 0.13 0.20 0.14 0.27 0.36 LOI 1 2 1 0.60 1.84 m . 2.15 1.82 Sum 100.22 100.77 101.58 100.58 99.64 99.76 Trace Elements (ppm) Ba 162 258 655 375 314 536 Cr 0.0 3.7 20.2 95.1 39.0 491.3 Cu 2.2 3.6 42.5 54.4 88.8 45.5 La 0 7 19 11 19 31 Nb 5.3 0.6 1.8 0.0 0.0 4.2 Ni 0.0 0.1 8.4 40.5 27.0 219.9 Pb 4.6 4.3 9.0 7.7 9.5 6.0 Rb 26.8 15.6 54.5 37.6 34.5 17.1 Sr 474 602 362 312 342 572 Th 0.5 0.4 1.6 1.8 0.9 2.9 U 1.1 0.7 2.0 1.4 1.3 0.7 Y 4.9 7.6 35.2 29.4 49.3 22.1 Zn 17 20 81 77 118 98 Zr 111 169 191 136 203 141 S 6 16 340 751 1205 719 Cl 0 36 227 274 295 235 Ga 15.3 18.9 16.4 16.9 18.5 15.0 Important ratios Na20+K20 5.84 6.00 5.23 4.25 3.42 3.41 A/CNK 1.15 1.03 0.92 0.83 0.82 0.51 Me# 0.48 0.39 0.34 0.40 0.32 0.60 Analyses of major oxides and trace element were determined by XRF at USC. FeO* = as total iron. A/CNK= molecular A1203/(Ca0 + Na20 + K20) Mg#= MgO/(MgO + FeO*) 106 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Permian 18 - - Early Jurassic Early Jurassic 4 - - Permian 2 - - ° o S i0 2 (wt%) Early Jurassic- 5 - - Early Jurassic S' % O © < ■ O O 85 45 55 65 75 85 8 - - Early Jurassic Permian •2 - ■ riff 75 45 55 65 85 S i02 (wl%) 6 -- 5J I44 9 r3 4- z i 4- o Permian Early Jurassic 45 55 + 65 S i0 2 (wt%) 75 85 Figure 33. Harker diagrams showing chemical variations for the La Mixtequita batholith. Symbols are as follows: filled circles and squares= Permian rocks; circles= undated samples and squares= dated samples. Open circles and squares= Early Jurassic rocks; circles= undated samples and squares= dated samples. Crosses= post-Early Jurassic dikes (undated). 107 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.24' 1.6 •O'. X Oo .. x..a 1.2 -- Early Jurassic 0.16 tS % Early Jurassic t 0.8 - - O C S 0.08- - Permian- Permian 0.4 36 □ 0 .00- 0.0 45 55 65 SiOj (wt%) 85 S i0 2(wt% ) 0.4 1000 •Permian 80 0 .. 0.3 -- 0 0 , Permian 0.2 -• 0.1 2 0 0-. Early Jurassic Early Jurassic 0.0 S i0 2(wt%) 100 400 80-- 300-- Permian 5 60 Permian O i V '2 < / ••-.O ^ ; Early Jurassic----- 100' 2 0 - - ly Jurassic oo. 45 55 45 65 75 55 65 75 85 85 S i0 2 (wt%) S i0 2 (wt%) Continuation of Figure 33 (b). 108 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2500 35 2000- /‘o”; 30 - X Early Jurassic Early Jurassic----- - 25 - ___ B a (ppm) © Ln 8 8 f i B *V----- Permian ______ _ • Permian-----V® / \ p .. $ * 1=20 ■ o , & j l 5 . 10 ■ a ' K O ° X • - / □ • - - 7 ... W - . . i » : 500- ■ X o ...... o p X 0- l & z s k r ^ S - i--------------1 — 5 — i------------- 5 - 0 • - O y | 0 1 1 45 55 65 75 85 45 55 65 75 85 S i02 (wt%) S i0 2(wt%) 1 20 Early Jurassic 1 0 0 - - 40 ■■ 8 0 - - | 3°" a . >* 20 - - 6 0 - - Early Jurassic 4 0 - - Permian li ’®) 65 75 Permian 2 0- - 45 45 65 55 55 75 85 85 S i0 2 (wt%) SiO j (wt%) Continuation of Figure 33 (c). 109 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In addition, several discrimination diagrams were employed to support the geochemical characterization of the magmatic suites of the La Mixtequita batholith. Most of the samples corresponding to Permian plutons are metaluminous (Figures 34 and 35). Two samples that are peraluminous (MIXTE- 6 6 and -70) display cataclastic effects and their plagioclase and biotite are strongly serictized and chloritized, respectively. These two samples may not have their pristine major oxide chemistry. The Early Jurassic suite ranges from metaluminous and becomes peraluminous at SiC>2 >65 wt% (Figures 34 and 35). According to the Peacock's (1931) alkali-lime index, the Permian granitoids are calcic (alkali-lime index= 61.2) while the Early Jurassic plutons are calc-alkalic (alkali-lime index= 58.2)(Figure 36). The various components of the La Mixtequita batholith lack iron enrichment and are in general calc-alkaline. Figure 37 shows that most samples from the three magmatic suites fall into the subalkaline field in a SiC>2 versus total alkalis plot (Irvine and Baragar, 1971). Only two samples from the Permian granitoids plot in the alkaline field, however, these two rocks correspond to the cataclastic and altered samples (MIXTE- 6 6 and -70) discussed above and therefore are considered abnormally alkaline. The AFM ternary plot (Irvine and Baragar, 1971) and the FeO*/MgO ratio versus Si0 2 plot (Miyashiro, 1974) indicates that the entire Permian suite is calc-alkaline (Figures 38 and 39). The Early Jurassic suite is also mainly calc-alkaline (Figure 38) with some mafic and felsic tholeiitic portions (Figure 39). The post-Early Jurassic dikes are in general calc-alkaline although the two more mafic samples are marginally tholeiitic (Figures 38 and 39). In terms of the K 2O, the Permian granitoids span the range from middle- to high-K while the Early Jurassic plutons cover the entire spectrum from low- to high-K (Figure 40). The mafic and intermediate dikes have middle- to high-K characteristics and the more felsic dikes show low-K character (Figure 40). The tectonic discrimination diagrams of Pearce et al., (1984) (Figure 41) indicate that 1 1 0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. o. PERALUMINOUS 1 .0 ■“ Permian OX X □ U 0.8 < ! Early Jurassic METALUMINOUS+SUB ALUMINOUS +PERALKALINE 0.6 " 0.4 45 55 65 75 85 SiO, (wt%) Figure 34. Si02 versus molecular [ALOj/fCaO+NajO+K^O)] for rocks of the La Mixtequita batholith. Symbols as in Figure 33. AU ° 3 / PERALUMINOUS METALUMINOUS PERALKALINE CaO Figure 35. Molecular alumina-alkali-lime ternary plot for rocks of the La Mixtequita batholith. Symbols are as follows: filled circles= Permian rocks, open circles= Early Jurassic rocks and crosses= post-Early Jurassic dikes. 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 12 10 % • w 0 a U 1 I 8 - - 4 - - 2 .. 45 AC "O. A A A CA \ ♦ (K 2 O+NB2 O) (Early Jurassic) C ^O + N ajO ) r-A -i > o A / (Permian) o c w f ^ \ CaO (Early Jurassic) \ A 5 5 65 SiO, (wt%) + 75 85 Figure 36. Classification of the La Mixtequita batholith according to the alkali-lime index of Peacock (1931). For Permian plutons: open diamonds= Si02 versus CaO and filled diamonds= S i0 2 versus (KjO +Na2 0). For Early Jurassic plutons: open triangles= SiO, versus CaO and filled triangles= S i0 2 versus (KjO + N ap). Geochemical boundaries: C= calcic, CA= calc-alkalic, and AC= alkali-calcic. 1 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14 1 2 " ^ 10 - t I 8+ \ 6 £ 4 - 2 " 0 Permian o o ALKALINE SUB ALKALINE Early Jurassic / 4- 40 50 -------------h- 60 70 Si02(wt%) 80 90 Figure 37. S i02 versus K2 0 + N a p for the La Mixtequita batholith. Alkaline and subalkaline division from Irvine and Baragar (1971). Symbols as in Figure 33. FeO* THOLEIITIC — X CALC-ALKALINE Kp+Nap MgO Figure 38. Fe0*-(K2 0+Na2 0)-M g0 ternary plot for the La Mixtequita batholith. Calc-alkaline and tholeiitic division from Irvine and Baragar (1971). All Fe as FeO*. Symbols as in Figure 35. 1 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10 o O il # o ,<o u, 8 - 6 - - 4 - 2 - 45 • O , Jf' THOLEIITIC / . O ; CALC-ALKALINE .. o U, Sj-x ........ Permian Early Jurassic 55 1 ------- 65 SiO, (wt%) - + - 75 85 Figure 39. Si02 versus FeO*/MgO ratio for the La Mixtequita batholith. Calc-alkaline and tholeiitic boundary from Miyashiro (1974). All Fe as FeO*. Symbols as in Fig. 33. O . Early Jurassic 5 - • MGH-K Permian MIDDLE-K XX LOW-K 45 65 55 75 85 SiO, (wt%) Figure 40. Si02 versus K2 0 for the La Mixtequita Batholith. High-, middle- and low-K divisions from Gill (1981). Symbols as in Figure 33. 114 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10000 0 Permian rocks O Early Jurassic rocks X Post-Early Jurassic dikes 1000 -- \ e j | 100-- WPG VAG + syn-COLG 1 0-- ORG 1 10 100 1000 Y (ppm) 10000 1000 - - (b) B j | 100 £ 10 0 Permian rocks O Early Jurassic rocks X Post-Early Jurassic dikes / / / syn-COLG / / / / WPG / / / / / X • / / o O p ORG VAG ------------------------,---------------!— f— ----------- 1 --------------- 10 100 Y+Nb (ppm) 1000 10000 Figure 41. Tectonic discriminant diagrams for granitoids (Pearce et al., 1984) applied to the La Mixtequita batholith. (a) Y-Nb and (b) (Y+Nb)-Rb. Symbols as in Figure 35. VAG= volcanic arc granites, syn-COLG= collision granites, ORG= ocean ridge granites and WPG= within plate granites. 1 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the entire La Mixtequita magmatic complex has Nb, Y, and Rb abundances typical of magmatic a rc s. RARE EARTH ELEMENT (REE) GEOCHEMISTRY REE chemistry of six samples from the La Mixtequita batholith was reported by Murillo and Navarrete (1992). Two samples from the Permian suite include a homblende- biotite quartz diorite (CHOX-75) and a homblende-biotite quartz monzodiorite (ISTEH- 42). Four samples from the Early Jurassic suite span the whole compositional range including a homblende-biotite-hypersthene gabbro (ISTEH-37), a clinopyroxene-biotite monzodiorite (CHOX-74), a homblende-pyroxene-biotite quartz diorite (ISTEH-38) and a biotite granite (ISTEH-39). REE abundances were normalized to chondritic values of Taylor (1980). REE fractionation is evident in the Permian samples. Both samples show similar REE patterns (Figure 42). A moderate depletion in heavy REE (HREE) is noticeable, (La/Lu)N= 10.8 for the quartz monzodiorite and (La/Lu)N= 6.4 for the quartz diorite. A distinctive characteristic of these rocks is the presence of a positive Eu anomaly that is well pronounced for the quartz monzodiorite and discretely observed in the quartz diorite. The fact that silica in these two rocks is similar (59.76 and 60.46 % of SiC>2), the different REE patterns is surprising. The positive Eu anomaly in ISTEH-41 accompanies other elemental features including high CaO, Ba, and K2O contents supportive of the conclusion that the anomaly is due to plagioclase and K-feldspar accumulation. The light REE (LREE) are higher for the more silica rich rock which would be anticipated if the accumulation also involves a mineral phase that concentrate LREE, such as hornblende or allanite. REE distribution in the Early Jurassic suite is more complex (Figure 42). The gabbro (49.86 wt% Si0 2 ) shows a relatively flat REE distribution, with La= 12 x chondrite, Lu= 5 x chondrite and (La/Lu)N= 2.5. Moreover, the gabbro shows a clear positive Eu anomaly. 1 1 6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 0 0 r t S lO jtw tS b) '^-50.46 (a) 5 1 0 -* PERMIAN SAMPLES a CHOX-75 a — ISTEH-41 H 1 -----1 ---- 1 ---- 1 -----1 -----1 ----- 1 — H ----1 -----1 -----1 -----h La Ce Nd Sm Eu Tb Yb Lu 100 EARLY JURASSIC SAMPLES « CHOX-74 0-------- ISTEH-37 • ISTEH-38 -O ISTEH-39 SIO , (w l% ) O..70.39 •c •a g - c | 10J^Ma8!L u , 1 U Ce Nd Sm E u Tb Yb Lu Figure 42. Chondrite-normalized REE patterns for rocks of the La Mixtequita batholith. (a) Permian rocks, samples are as follows: ISTEH-41= homblende-biotite quartz monzodiorite and CHOX-75= homblende-biotite quartz diorite. (b) Early Jurassic rocks, samples are as follows: ISTEH-37= homblendc-biotite-hypersthene gabbro, CHOX-74= clinopyroxene-biotitc monzodiorite, ISTEH-38= homblende-pyroxene- biotite quartz diorite and ISTEH-39= biotite granite. 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The intermediate samples display basically similar REE pattern, that is, a moderate HREE depletion. The monzodiorite (54.31 wt% S i0 2) contains La= 26 x chondrite, Lu= 9 x chondrite and a (La/Lu)N= 2.8 and no Eu anomaly. Hence, for these three samples, REE abundances increase with silica and the Eu anomaly changes from positive to negative. The quartz diorite (56.45 wt% S i0 2) has La= 31 x chondrite, Lu= 11 x chondrite, (La/Lu)N= 2.8 and a weak negative Eu anomaly. The more evolved sample, a granite (70.39 wt% S i0 2), displays a strong fractionated REE pattern with La= 58 x chondrite, Lu= 3 x chondrite, (La/Lu)N= 21.0 and a discrete positive Eu anomaly. DISCUSSION OF THE GEOCHEMISTRY The available geochemical data of the La Mixtequita batholith allows discrimination of the two major magmatic suites of this plutonic complex. It is important to note that just one sample of the Early Jurassic suite falls into the compositional range of the Permian plutons (58.93 to 67.75 wt% S i0 2). Otherwise both suites seem to form a continuous compositional pattern (Figure 33). The geochemistry indicates that they correspond to two distinct magmatic suites. This is consistent with field relationships and geochronologic data. The geochemical differences between the two plutonic suites are given below. In terms of major oxide geochemistry, the Permian granitoids have higher contents o f K20 (up to 3.67 wt%) and Na20 (up to 6.08 wt%) than the Early Jurassic suite. Their main difference is their high P2Os content (0.15 to 0.32 wt%) relative that to that of the Early Jurassic suite (0.00 to 0.58 wt%) (Figure 33). In terms of the Peacock index, the Permian suite is calcic while the Early Jurassic suite is calc-alkalic. The Permian plutons also display a higher Sr content (352-878 ppm) than the Early Jurassic suite (183-680 ppm) (Figures 33 and 43). The Ba content of the Permian granitoids shows a steep increase with increasing S i0 2 and has the moderate Ba enrichment of the Early Jurassic suite. For intermediate 1 18 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1000 800 ■ ■ Permian 4? 0 | f 600 - ■ 1 U h 400-■ Early Jurassic 200-- 0 20 40 60 80 100 Rb (ppm) Figure 43. Rb versus Sr for the La Mixtequita batholith. Symbols as in Figure 33. 1 1 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. compositions, the Permian plutons exhibit a high Ba content (up to 1336 ppm, with one extraordinary sample having a Ba content of 4051 ppm). The REE abundances are similar relative to silica except for the presence of a positive Eu anomaly in the Permian rocks and the absence and presence of a discrete negative Eu anomaly in the Early Jurassic rocks. Although quantitative geochemical modeling for the La Mixtequita batholith was not tested, some insights about its petrogenesis may be outlined. The well-defined geochemical trends in both the Permian and the Early Jurassic suites suggest that crystal fractionation played an important role in their magmatic evolution. However, such well-defined lineages could also result by partial melting (Cox et al., 1979) or by a combination of both processes (Green, 1980). Moreover, assimilation of crustal rocks coupled with fractional crystallization (AFC) is a commonly considered process in the generation of felsic to mafic igneous suites above subduction zones (Brown et al., 1984). Xenoliths from the Middle Proterozoic Guichicovi complex occurring in both plutonic suites and the Grenville-age upper intercept U-Pb ages also in both plutonic suites imply that crustal contamination probably has changed the original composition of the magmas. Confirmation of crustal contamination necessitates isotopic analysis (e.g., Sr, Nd, Sm, O; Cox et al., 1979; W ilson, 1989). Fractional crystallization is considered an important process in the evolution of both plutonic suites of the La Mixtequita batholith. For both the Permian and Early Jurassic suites, the decrease of CaO and AI2O3 as Si0 2 increases indicates fractionation of plagioclase and clinopyroxene. The decrease in FeO* (all Fe) and M gO also suggests pyroxene fractionation. Decreasing of TiO: and MnO with increasing Si0 2 can be explained by separation of Fe-Ti oxides and/or mafic silicates, and the decrease of P2O5 can be attributed to fractionation of apatite. For the Permian granitoids, the decrease of Zr with increasing Si0 2 content suggests zircon fractionation. Other elemental trends are less well defined for the Permian phases of the batholith including variations in K2O, Na2 0 , 120 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. AI2O 3 , Sr, Ba and A/CNK ratio. This may be due to variable degrees of accumulation of specific minerals, such as plagioclase as indicated by REE abundance (Cox et al., 1979; Cullers and Graf, 1984). Depletion of Y when increasing differentiation could be accounted for fractionation of hornblende and apatite (Green, 1980). The behavior of the Early Jurassic trace elements patterns is somewhat inconsistent. Sr remains constant over a range o f SiC> 2 , as in the Permian suite, this could be related to competing effects of fractionation of plagioclase (Dsr > l) and mafic silicates such as pyroxene and hornblende (Dsr < l). D refers to the distribution coefficient of the trace element. Rb increases with silica which requires that biotite (DRb <1) was not an important fractionating phase. Depletion of Y, as also observed in the Permian plutons, suggests crystal fractionation of hornblende and apatite (Green, 1980). Increasing LREE with silica also reflects an increasing degree of fractionation of plagioclase and pyroxene. Depletion of the HREE in the more felsic portion of the batholith indicates fractionation of zircon. The positive Eu anomaly in the most mafic rocks indicates accumulation of plagioclase. Finally, the transition from metaluminous to peraluminous character is consistent with crystal fractionation of hornblende and clinopyroxene ( Davis, 1989; Fox and Miller, 1990). The two major plutonic suites from the La Mixtequita batholith probably were derived from slightly different sources. The lack of strong HREE depletion and the lack of a negative Eu anomaly are attributes consistent with a source that lacked significant garnet and plagioclase at the stage of melt separation. Although many elemental abundances overlap, the Permian granitoids consistently have high K, Na, Rb, and Sr indicating either a more enriched source, a lower degree of melting, or a greater abundance o f residual phases that do not incorporate these elements, such as pyroxene. However, whether the source of these rocks was the mantle wedge overlying the subducting oceanic slab or the lower continental crust cannot be concluded due to the lack of appropriate isotopic data (e.g. Nd, Sr, O, etc.). 121 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Lithology, mineralogy and the available geochemistry data supports the conclusion that the La Mixtequita batholith represents a subduction-related magmatic complex. The Permian granitoids typically contain hornblende and biotite, a common feature of arc- related rocks (Wilson, 1989). The entire batholith is calc-alkaline, has low TiC>2 (Green, 1980) with trace element abundances typical of island arcs and Pacific batholiths associated with marginal continental subduction (Wilson, 1989). Finally, the presence o f post-Early Jurassic dikes with similar mineralogical and geochemical characteristics to those of the two plutonic suites indicates that the entire magmatic activity in the La Mixtequita batholith was a consequence of continental margin subduction. 1 2 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TECTONIC IMPLICATIONS OF THE GUICHICOVI COMPLEX AND THE LA MIXTEQUITA BATHOLITH Important tectonic implications result from the present study. Although previous work on the Guichicovi complex lacked petrographic detail (Murillo and Torres, 1990; M urillo and Navarrete, 1992; Murillo et al., 1992), the complex is now lithologically characterized as a granulite facies crystalline terrane composed of mafic and felsic granulite, paragneiss, amphibolite, a granulitic dioritic pluton (meta-jotunite) and sporadic intercalated calcareous metasediments. The U-Pb geochronology brackets the age o f this terrane to the Middle Proterozoic (Grenville-age). These rocks were previously considered a portion of the La Mixtequita batholith. Thermobarometry indicates pressure-temperature conditions of the granulite grade metamorphism at 7.4 ± 0.5 kbar and 837 ± 59 °C. Assuming average crustal densities of 2.67 g/cm3, these calculations suggest that at the time of metamorphism, the Guichicovi complex was -28 km deep in the continental crust. Lithology, metamorphic grade, and age of the Guichicovi complex suggest a reliable correlation with the granulites of the Grenville belt of eastern North America. Similar lithology and metamorphic facies occur in the Adirondacks where a segment of the Grenville belt is well exposed (Bohlen et al., 1985). Moreover, the Guichicovi complex correlates well with the previously known Grenville-age granulitic terranes of eastern and southern Mexico (Murillo-Munetdn and Anderson, 1994; Murillo-Munetdn et al., 1994) including: the Novillo gneiss (State of Tamaulipas), the Huiznopala gneiss (State of Hidalgo) and the Oaxacan complex (State of Oaxaca) (Fries et al., 1962 and 1966; Fries and Rinc6 n-Orta, 1965; Anderson and Silver, 1971; Ortega-Gutierrez, 1981a and 1981b; Patchett and Ruiz, 1987; Ruiz et al., 1988; Goel et al., 1991). In addition, similar metamorphic conditions to those of the Guichicovi complex were estimated for the Oaxacan complex (730 ± 50 °C and 7 ± 1 kbar, Mora and Valley, 1985; Mora et al., 1986). The present geographic distribution of the Guichicovi complex suggests that it now represents 1 2 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the southeastemmost known occurrence of Grenville-age granulitic rocks in continental North America. M exico has been divided into several tectono-stratigraphic terranes (Figure 1). The crystalline rocks of the La Mixtequita area have been included as part of the basement of the Maya terrane (Campa and Coney, 1983). When the Maya terrane was defined, its basement was considered to be a meta-plutonic complex of Permo-Triassic age (Campa and Coney (1983). In the tectono-stratigraphic configuration of Campa and Coney (1983) another terrane, the Juarez terrane, separates the Oaxaca and the M aya terranes (Figure 1). Recently, Ortega-Gutierrez et al. (1990) and Sedlock et al. (1993) reanalyzed the distribution of the tectono-stratigraphic terranes in southern Mexico, but both studies did not consider the existence of Precambrian rocks in the M aya terrane. The present study confirms the extensive distribution of Grenville-age granulites in the La Mixtequita area. In terms of regional geology, the La Mixtequita region displays some tectonic similarities to other terranes of southern Mexico. The Oaxacan complex is in tectonic contact with Lower Paleozoic polymetamorphic rocks (predominantly greenschist metamorphic grade) known as the Acatlan complex (Ortega-Gutierrez, 1981a and 1981b; Ruiz et al., 1988; Yaiiez et al, 1991). The Paleozoic Acatlan complex comprises the basement of the Mixteco terrane which lies to the west of the Oaxacan complex (Ortega- Gutierrez, 1981a ; Campa and Coney, 1983; Ruiz et al., 1988; Yaiiez et al, 1991). The western region of the La Mixtequita area also consists of greenschist grade rocks (mainly pelitic in character) grouped as the Mazatlan complex (Murillo and Navarrete, 1992; Murillo et al., 1992). Reported K-Ar ages for these rocks include Triassic (Ruiz, 1978), Permian (Murillo and Navarrete, 1992; M urillo et al., 1992) and Devonian (Vazquez et al., 1989). Thus, the greenschist facies pelites laying to the western side of the Guichicovi complex might be Lower Paleozoic in age and could, therefore, represent an equivalent terrane to the Acatlan complex (L6 pez and Rodriguez, 1986; Torres, 1986; Murillo and Navarrete, 1992; 1 2 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Murillo et al., 1992). In other words, the present terrane configuration in the La Mixtequita area may be the same as that observed between the Oaxacan and Acatlan complexes. How the Guichicovi complex achieved its present position is enigmatic. The Acatlan complex represents an extension of the Appalachian system from North America (Ortega-Guti6 rrez, 1981a and 1981b; Campa and Coney, 1983; Yaiiez et al., 1991). The present configuration of the Grenvillian Oaxacan and the Appalachian Acatlan complexes is the opposite to that in the U.S. and Canada. The Appalachian belt lies along the eastern side of the Grenville belt in U.S. and Canada, but in eastern and southern Mexico, the Appalachian Acatlan complex lies along the western side of the Grenville metamorphic belt. Campa and Coney (1983) considered that about 80% of Mexico consists o f suspect terranes. In this perspective, the terranes comprising eastern and southern Mexico (including the Maya terrane) contain basement accreted during Late Paleozoic time. The terranes were later displaced by strike-slip translations during the opening of the Gulf of Mexico in Jurassic time (Campa and Coney, 1983; Silver and Anderson, 1983). Ruiz et al. (1988) assumed that the Grenville-age belt in Mexico is not suspect and proposed two models to explain the western position of the Appalachian belt. Model (1) suggests that the Appalachian belt is a suspect terrane like those occurring in the western North America Cordillera. Their model (2) states that the Acatlan complex is an extension of the Appalachian belt into Mexico and that the Grenville-age belt was either cut by the Proto- Atlantic ocean (origin of the Appalachian system) or was trapped during the Appalachian tectonic event. Yaiiez et al. (1991) suggested that the inverse configuration in the Grenville- Appalachian Mexican system is because this system was continuous with South America until the break-up of Pangea in Mesozoic time. This model implies that the Oaxacan complex and consequently the Guichicovi complex were derived from South America. Recently, Sedlock et al. (1993) proposed that the Grenville-age rocks in M exico are fault- bounded fragments of basement derived from the North American Grenville province. 1 2 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The present position of the Guichicovi complex may be associated with regional strike-slip translations occurring probably during Mesozoic time. Two speculative models are considered in which the origin of the Paleozoic greenschist facies rocks of the La Mixtequita area plays an important role. Model (1): the Guichicovi complex might represent a sliver (containing the Phanerozoic La Mixtequita batholith) derived from the Oaxacan complex. In this model, the Paleozoic greenschist rocks (Mazatlan complex) do not comprise a part of the Appalachian belt (Acatlan complex). The sliver would have traveled -300 km along the dextral strike-slip Valle Nacional fault during Cretaceous(?) time and this fault may represent a continental megashear. The translation along the boundary between the M aya terrane and Oaxacan-Juarez terranes seems to be possible (Figure 44). Model (2): the Guichicovi complex, the Paleozoic schists (Mazatlan complex), and the La Mixtequita Batholith comprise a composite sliver derived from the Acatlan and Oaxacan complexes. In this interpretation, the Paleozoic greenschist facies rocks are considered a part of the Appalachian belt (Acatlan complex). In this model, the sliver did not travel along the Valle Nacional Fault but along a different megashear that was not observed in the study area (Figure 44). An extensive mylonitic belt on the northeastern margin of the Oaxacan complex (Ortega-Gutierrez, 1981a) could represent an extension of the megashear (Figure 44). This mylonite belt that includes the Oaxacan fault, lies between the Oaxaca and Juarez terranes and contains protoliths of the Oaxacan complex (Ortega- Gutierrez, 1981a). Timing of this translation is uncertain but could be associated with the opening o f the Gulf of Mexico during late Middle Jurassic time. Displacement of the La Mixtequita batholith supports this interpretation. Lack of structural and geologic data from the western side of the La Mixtequita region and the Juarez terrane precludes the selection of any of these proposed models. A Permian granite pluton is emplaced into the northeastern margin of the Oaxacan complex (Rulz-Castellanos, 1979) and deformed Jurassic granitoids occur in the suture of the 126 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. MAYA Acatlan M IXTECO Guichicovi laxaca \ • x o l a p a : ■OAXACA \ JUAREZ EXPLANATION (0aXaCan m M y .o n i* b eUw1 , h r r a .m briM p,o,oU to p | | | p | Paleozoic metamorphic rocks (Acatlan and M azatlan complexes) ) ) ) \ \ \ | La M ixtequita batholith (Permian and Low er Jurassic) Xolapa metamorphic com plex (Mesozoic(?)J Permian pluton M esozoic low-grade metamorphic rocks (+++??«] Jurassic plutons Figure 44 (a). Tectonic Model 1. Translation of the Precambrian Guichicovi complex and the La Mixtequita batholith, as a single block, occurred along the boundary between the Maya and Juarez terranes. Circle indicates the possible original position of the block. The Guichicovi complex represents a sliver derived from the Oaxacan complex. See text for details. Terrane boundaries (— — ) from Campa and Coney (1983). Distribution of metamorphic belts modified from Ortega-Gutierrez (1984). 127 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. MAYA Acatlan MIXTECO Guichicovi •axaca % x o l a p a ; iOAXACA \ JUAREZ EXPLANATION Precambrian meiamorphic rocks (Oaxacan and Guchicovi complexes) Mylonitic belt with Precambrian protoliths ^ Paleozoic metamorphic rocks (Acatlan and MazatUn complexes) \ \ si ✓ / ✓ v v La Mixtcquita batholith (Permian and Lower Jurassic) ^ Xolapa metamorphic complex [Mesozok(?)) Permian pluton nnvrH Mesozoic low-grade metamorphic rocks Jurassic plutons Figure 44 (b). Tectonic Model 2. Translation of the Precambrian Guichicovi complex, the Paleozoic greenschist facies Mazatlan complex and the La Mixtequita batholith, as a single block, occurred along a right strike-slip megashear. The megashear corresponds to the boundary between the Oaxaca and Juarez terranes and probable extends to the southeast («■--). Circle indicates the possible original position of the block. The sliver was derived from the Oaxacan and Acatlan complexes. See text for details. Terrane boundaries (— — ) from Campa and Coney (1983). Distribution of metamorphic belts modified from Ortega-Gutierrez (1984). 1 2 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Oaxacan and Acatlan complexes (Torres and Murillo, 1986). This suture is located in the northern boundary of the Oaxacan complex (Figure 44). The distribution of the metamorphic belts and plutons is similar to that of the basement rocks of the La Mixtequita area. Permian and Early Jurassic plutons of the La Mixtequita batholith are emplaced into the Guichicovi complex. Permian and Triassic K-Ar ages on micas of the Paleozoic pelitic schists (Ruiz, 1978; Murillo and Navarrete, 1992; Murillo et al., 1992) suggest that the schists were also affected by the magmatic activity of the La Mixtequita batholith. These stratigraphic similarities between the La Mixtequita area and the region of the contact between the Oxacan and Acatlan support the second hypothesis. This indicates that the Grenville Guichicovi complex, the Paleozoic greenschist pelites (Mazatlan complex) and the La Mixtequita batholith might have constituted a single continental block derived from the Oaxacan and Acatlan metamorphic complexes and associated Permian and Jurassic plutons. This block was probably derived from the northern margin of the Grenville Oaxacan complex and the northeastern part of the Appalachian Acatlan complex (Figure 44). The Permian suite of the La Mixtequita batholith represents an Andean-type magmatic arc emplaced into the Proterozoic Guichicovi complex. The geochemistry and lithology of this suite are consistent with a continental arc tectonic setting. Permian plutonism occurs extensively in both outcrop and subsurface in northern, northeastern and southern Mexico (Damon et al., 1981; Murillo and Torres, 1987; Torres et al., in progress). This Permian arc affected the Proterozoic and Paleozoic basement, including the Grenvillian granulites and the Appalachian-affinity rocks (Ruiz-Castellanos; 1979; Murillo and Torres, 1987; Torres et al., in progress). The Permian granitoids of the La Mixtequita batholith could either be associated with Pacific subduction (Cordilleran affinity) or related to the final closure of the proto- Atlantic ocean (Appalachian affinity). Permian magmatic activity in California has been 1 2 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. documented by Cox and Morton (1980), Walker (1988) and Snow et al. (1991). Permian plutonism (Alleghanian Orogeny) also occurs in the Appalachian belt of U.S. related to final closure o f the proto-Atlantic ocean with continental collision of Africa and South America against North America (Sinha and Zietz, 1982; Hatcher et al., 1989). The Permian plutonism in Mexico was first correlated with the latter event (Damon et al., 1981; Murillo and Torres, 1987). Recent studies, however, based on paleogeographic reconstructions suggest that the Permian arc in Mexico is more consistent with a Pacific regime (Sedlock et al., 1993; Torres et al., in progress). The present study does not preclude either o f the two models. The Early Jurassic plutonic suite of the La Mixtequita batholith also represents a continental-margin magmatic arc. The geochemical and lithologic characteristics o f this suite clearly indicate a magmatic arc tectonic setting. Evidences of a Jurassic arc associated with the Pacific-margin tectonics are widespread in both outcrop and subsurface over a wide region of Mexico (Damon et al., 1981; Torres and Murillo, 1986; Murillo and Torres, 1987; Tosdal et al., 1989). The Early Jurassic plutonism of the La Mixtequita batholith is related to a Pacific subduction regime as interpreted by Damon et al. (1981). The onset of an Andean-type arc associated with eastward subduction beneath the western North America margin is well recognized (Burchfiel and Davis, 1972 and 1975; Davis et al., 1978). Coeval Jurassic calc-alkaline volcanism and plutonism in the southern U.S. Cordillera are also well documented. For instance, Jurassic plutonism occurs along the eastern side of the Sierra Nevada batholith (Inyo-While Mountains and Argus Range) in California (Chen and Moore, 1982; Anderson et al., 1992), Jurassic meta-volcanic rocks occur interstratified with quartz arenites in the Baboquivari in southern Arizona (Wright et al., 1981), and Jurassic volcanics and plutons are associated with sediments in southeastern Arizona (Tosdal et al., 1989; Asmeron et al., 1990). This Cordilleran Early Jurassic arc extends into northern Sonora (Tosdal et al., 1989). 1 3 0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CONCLUSIONS The most important conclusions of this study are: 1. The Guichicovi complex from the La Mixtequita area in the state of Oaxaca corresponds to a Grenville-age granulite terrane. This terrane was formerly considered as part of the La Mixtequita batholith. The complex consists of a varied lithology including two-pyroxene felsic and mafic granulites, paragneiss, amphibolite, a granulite dioritic pluton (meta-jotunite) and minor marble, calc-silicate gneiss and quartzite. Common metamorphic mineral phases include garnet, orthopyroxene, and clinopyroxene, graphite and perthite. 2. U-Pb and K-Ar geochronology constrains a Grenville-age for the Guichicovi complex. Zircon fractions from two gamet-clinopyroxene-homblende felsic gneisses indicate an U-Pb age of 980-990 M a for this complex. A K-Ar hornblende age o f 911 ± 46 M a from a mafic granulite is considered to indicate rapid cooling. Younger K-Ar dates including hornblende ages o f 309 ± 7 Ma from an amphibolite and 236 ± 6 Ma from a clinopyroxene-homblende quartzo-feldspathic gneiss and a potassium feldspar age of 146 ± 3 M a from a gamet-homblende felsic gneiss are interpreted to be thermal events associated with the emplacement of the La Mixtequita batholith. 3. P-T metamorphic conditions estimated by Mg-Fe cation exchange and solvus pyroxene thermometry and mineral equilibria barometry have been conducted on the Guichicovi complex. The barometric calibration by Newton and Perkins (1982) for OPX- GAR-PLA-QTZ equilibria yielded 7.4 ± 0.3 kbar at 837 ± 59 °C based on pyroxene solvus thermometry (Lindsley, 1983). At 7 kbar, cation-exchange thermometers yielded 755 ± 25 °C (GAR-OPX; Lee and Ganguly, 1988), 717 ± 21 °C (GAR-CPX; Ellis and Green, 1979), 649 ± 11 °C (GAR-BIO; Hodges and Royden, 1984), and 631 ± 18 °C (GAR- HBL; Graham and Powell, 1984). Peak metamorphic conditions were 7.4 ±0.3 kbar at 837 ± 59 °C. 13 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4. The systematic thermometric trend T t w o -p x > T G a r -o p x > T q a r -c p x > T G a r -b i o > T G a r -h b l suggests either cation re-equilibration or incomplete equilibration during rapid cooling. All the thermometers utilized show chemical evidence of cation diffusion. Temperature estimated using core compositions are generally higher than temperatures estimated using rim compositions by 30-87 °C depending on the thermometer employed. 5. Lithology, metamorphic conditions and age of the Guichicovi complex suggest correlation with other Grenville-age granulite terranes in eastern and southern Mexico, including the Novillo gneiss (State of Tamaulipas), Huiznopala gneiss (State of Hidalgo) and Oaxacan complex (State of Oaxaca). 6 . The La Mixtequita batholith represents a post-granulite plutonic complex emplaced into the Guichicovi granulites. Field relationships, lithology, and geochronologic analysis suggest that the La Mixtequita batholith is composite and includes three magmatic suites: Permian granitoids (southern region), Early Jurassic plutons (central and northern areas), and minor post-Early Jurassic dikes. U-Pb ages of 254 ± 7 M a and 189 ± 3 Ma were obtained for a quartz monzonite and a quartz diorite respectively from the southern and northern margins of the La Mixtequita batholith. These ages constrain two major magmatic episodes in this batholith during Permian and Early Jurassic time. K-Ar ages show the same distribution as the U-Pb dates. 7. The Permian suite consists mostly of homblende-biotite quartz monzonite and quartz monzodiorite with minor quartz diorite and granite. This suite shows calc-alkaline and metaluminous geochemical characteristics and spans from middle- to high-K. The Early Jurassic suite shows an extensive lithologic range including gabbro, monzodiorite, quartz diorite and pink leucogranite. Typical accessory minerals in the Early Jurassic plutons include hornblende, biotite, clinopyroxene and hypersthene. These plutons also show calc-alkaline characteristics, range from metaluminous to peraluminous and spans from low- to high-K. Post-Early Jurassic dikes compose a minor part of the La Mixtequita 1 3 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. batholith consisting of diorite, monzodiorite, andesite and latite also showing calc-alkaline affinity. 8 . M ajor oxide, trace element and REE geochemistry indicates that the La Mixtequita batholith represents two Andean-type magmatic arcs active during Permian and Early Jurassic times. Permian plutonism may be either Cordilleran or Appalachian in affinity whereas the Early Jurassic plutonism is considered to be associated with a subduction zone along the Pacific margin (Cordilleran affinity). 133 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REFERENCES A nderson, J. L., Barth, A. P., Young, E. D., Bender, E. E., Davis, M. J., Farber, D. L., Hayes, E. M., and Johnson, K. A. 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Further reproduction prohibited without permission. © 1 a> S 2 C 2 * 3 i s 54 < v-j g £ * C sn & n ° £ 6 C 'C I I o Si s ■§ E <N •G r J * § m C to ^ & O u — rf I 2 ^ 2 I (9 S i jo — 2 3 S3 ■= S 3 •= 2 c S3 ■= S S £ =i g r? £ o n o ' < f> »n 5 C; r) o r ^ fO d 2 £ R g S ® c d 2 S ? s ! O 1 V© J N :ls s i l i l ° S « § : < = c d® s s s s " i d 2 S 8 ‘ O s C • fO I SSFiga! d d S => C ' § s £ o S o * ' ! ri o’ 2 3 g s S S ® K r i d s i s s § ; o M < 9 * 1 d 3 . 3 8: . * 0 si m Ig *§ . f*i a s = S B j ; o r i d “ s i S g = s g S d r t d ® $ s o a s g s r i O P r * " S 3 * a g 3 g: ] s ” a s i < = > 8 3 s g B 9 S =* s 3 a 3 3: o' c ’ £ ci S 5 a 2 | S s « ! g =' J S l a I s 5 : S = g 2 *o o' 5 Jj < = > 3 S S 3 * £ d ri o' 2 ia.L : o s i s s a s o ^ s ; ! = i c « * R S s S 5 a R f l S -• < g 3 a o n o “ ” s ^ s s i g S d ri d 2 " ci g 8 : ci „-. 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R S o ^ 2 £ 8 a | 5 d r i 6 5 « d ° c ( S S § | g g = « »orio2*oS d8 i-I d d ci ' ' ' a 8 8 S — o d d — d d d <N = K On tfv — cn r* o — V O o d o’ 5 ^ S C O 00 O o d d 00 N V £ \ ' $ 3 8 o d d — oo O ' vo ro oo — ooooooooo 8 8 8 S — odd § § 8 1 — o d d r- r» - *£ j N 7 1 O — v o O O O O O © I odd go m 2 ov r» o co oo © 1 o d d 8 3 8 2 S 2 2 8 2 3 2 00 © © — — o d d 3 2 o d d § S 8 «n 2 — : O N O i d d o' i $ S § 3 vo o co r— = 8 2 S o o o o o o o o o o 00 © O — — o d d § § § § ■ o r- o ' d d o ' i r* vo co co — « 09 O d d d < a t vf S 3 5 ■ d d d i O n CO CO l 3 3 © i d d d i r i r j o o x O O r v Q O ^ ^ O o p a ^ m H s UUi S a U * 5 Z < /> 15 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. a a . < i— o e o * a .5 c 3 I 1 ® § 'S cm a £ I c m a cm a $ I a 2 S 33 d 2 « ^ S sj ^ s ^ a s r- S 3 ^ H O O ' 2 3 s vo O O' ri d O ' 2 S 3 2 ? - o O' 2 s « = * ?i - a s ; a ; =* a ' a o | si cn 3 » n 3 si as 3 si I 3 £ 8 g § ri ci 2 C i 3 °° In ® § S $ S " H rid ° 2 s a g ; ® -> ci 00 2 2 8 * 3 - C i ci = ^ r a d ci ?3 c i Si — so ’* .] >r < = > a 3 § 2 ® a l 2 o ri a 8 3 s 2 s - a R ■ • — ci a o o o 2 2 S 3 £ g In ® 8 s * n ® g a 3 ® I 5? S O w a ? C i ? S P S g R - o- ci z 2 8 * S - ci ci ci a s g a - c> ci a s 8 R a « o “ O' 8 0 3. S ri d 2 w 3 8 ? S ri O 2 ® Tf O v, m ri o 2 » ^ d r i d S o d O ^ a 3 < a s d dl 31 8 3 a ? ® r» 8l „ m osi O' Ip 9| £ S S " d s do r 5 R §1 R cs a a § a S s a ? o a 3 s 2 a a §8 d 2 isi s a S ci ? o “ ? ri d 22 *> P 8 ° 2 « d £ oi 8 8 * S ri o ^ » cm © 2 00 ' O O ^ i n ri o 22 °o s a 3 2 ri d 22 °o O' 2 - s a ' 2 «? ; d £ ( = R 38 2 i i I a 2 1 3 a 1 a » O ' 2 8 o _ r< i f a *r* I s r< r ) S o i O O n s s i s S i ’i S S 8 S d d d o § 8 d o d i § 8 § o d d © © *n oBS o d d i d d d !s§i ; d d d o 3 o o d d : § 8 § 0 0 0 § — r» S 3 d d d 5 rn m , 8 8 8 o d d 8 8 8 8 — do'©' O' C O « S 8 8 3 — o d d S 8 § I — o' d o' 8 8 S d o' d — 0 0 0 — 0 0 0 S m — \ q o m 5 >5 n r O a r* bo ci o o' o d «n r * * 00 m 3 3 ?$ P So o d d d d o m 00 m o S 8 a IS S d o' o' d 0 C M an * 0 «n C M 8 8 a P S 8 d d d d d vo vp — m »n S 3 3 £ S8 d d d d d O' n O r t to SSIQff E o d d d d d — in <n *n «n 8 8 S P $ d d d d d m o o\ m — © 8 5 £ 8 d d d d d § « c ft o o © 3 8 S o © o’ o d d o o' I 8 8 § I § 0 0 0 0 0 0 >6 00 N to m — t ? ? io 22 S cn d 0 0 0 0 0 — 00 m 00 tn o 8 cn vo Eo o d 0 d 0 © d cn «o 00 0 0 0 0 0 H i O' C M cm co vn 00 0 0 0 0 0 § — C M — vp d d d o’ o' ~ § § § £ > " * * ^ ^ o 3 § — o' d o' o' d d d o o’ 1 8 8 8 o d d d o d a fO re to c- ^ 1 ^ O v 00 m Q 1 O © * T T f 00 O d d d d d d < O 4 f t N M S3 to d f t 00 n o 1 © o * < r 00 © d d d d d d • § 00 rO 00 f t to r i <o © « < * «n 00 o d o d d S § § s i s § l § d d d — 000 o d d o d d 0 0 0 0 0 3 t !5 ^ ! Q O * 00 n o — 00 d d o’ d d § ot n g m o < ft 9 » d d d d o' o 5 < M ^ J S ^ 2 W 5 152 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. I i i Continuation of Appendix II (d) SAM PLE M X62 c l rim MX62 c2 rim M X62 c3 core M X62 d l rim M X62 M X62 d 2 co re d 3 co re MX62 e l rim MX62 e2 rim M X62 e4 co re MX62 fl rim MX62 12 rim M X62 f3core M X62 M X62 MX62 f4core h i core h 2 co re MX62 il rim MX62 i2 rim MX62 i3 core MX62 i4 core M X63 al rim MX63 a2core MX63 a3 rim M X63 b lrim S i0 2 52.35 51.58 51.67 51.87 51.66 50.99 51.26 51.20 50.89 51.73 5 2 2 6 51.32 51.46 51.63 52.03 51.70 52.08 51.60 51.78 51.94 51.06 51.67 5 1 2 2 T i0 2 0.25 0.25 0.33 0.30 0.37 0.49 0.42 0.47 0.55 0.29 0.35 0.48 0.38 0.31 0.32 0.37 0.28 0.39 0.31 0.28 0.33 0.28 0.18 A1203 1.82 2.07 2 41 2 4 4 2 8 7 3.18 2 4 8 2 6 9 2 8 8 2 3 9 2 0 2 2 8 5 2 6 3 2 2 7 2 1 9 2 4 5 2 3 6 2.81 2.48 2.05 2.40 2.05 1.73 Cr203 0.01 0.02 0.03 0.00 0.00 0.00 0.01 0.00 0.06 0.00 0.01 0.00 0.06 0.03 0.00 0.03 0.00 0.02 0 0 2 0.04 0.01 0.02 0.00 FeO* 9.16 9.75 9.69 9.77 10.12 10.32 10.11 10.51 10.14 9.94 9.61 10.26 9.83 9.78 9.60 9.95 9.48 9.93 9.65 10.53 10.79 10.55 9.45 M gO 14.23 13.54 13.44 13.46 13.25 13.10 1298 1291 12.55 13.60 13.81 13.02 13.32 13.65 13.73 13.40 13.91 13.31 13.53 13.73 12.51 13.09 13.55 MnO 0.13 0.09 0.11 0.10 0.15 0.06 0.10 0.09 0.08 0.12 0.11 0.10 0.09 0.14 0.12 0.07 0.04 0.12 0.05 0.20 0.24 0.21 0.17 CaO 22.50 22.34 22.40 221 5 222 3 21.70 22 2 8 221 5 2211 21.98 2 2 3 9 21.59 21.84 21.89 22.28 2 2 2 0 21.76 22.05 22.13 20.79 21.38 21.56 2 1 4 8 Na20 0.26 0.29 0.34 0.33 0.35 0.39 0.30 0.37 0.39 0.32 0.30 0.37 0.32 0.32 0.30 0.35 0.30 0.36 0.34 0.29 0.41 0.35 0.35 SUM 100.71 99.93 Formula based oo 4 cations 100.42 100.42 101.00 100.23 99.94 100.39 99.65 100.37 100.86 99.99 99.93 100.02 100.57 100.52 100.21 100.59 100.29 99.85 99.13 99.78 100.13 Si 1.934 1.926 1.920 1.927 1.911 1.901 1.919 1.909 1.912 1.923 1.932 1.919 1.923 1.925 1.929 1.920 1.935 1.915 1.925 1.944 1.932 1.939 1.945 Ti 0.007 0.007 0.009 0.008 0.010 0.014 0.012 0.013 0.016 0.008 0.010 0.013 0.011 0.009 0.009 0.010 0.008 0.011 0.009 0.008 0.009 0.008 0.005 Cr 0.000 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.002 0.001 0.000 0.001 0.000 0.001 0.001 0.001 0.000 0.001 0.000 Aliv 0.066 0.074 0.080 0.073 0.069 0.099 0.081 0.091 0.088 0.077 0.068 0.081 0.077 0.075 0.071 0.080 0.065 0.085 0.075 0.056 0.068 0.061 0.055 Alvi 0.013 0.017 0.025 0.034 0.036 0.040 0.029 0.027 0.040 0.027 0.020 0.045 0.038 0.025 0.025 0.027 0.038 0.038 0.034 0.034 0.039 0.030 0.021 Fe3+ 0.057 0.064 0.060 0.046 O.Q58 0.060 0.050 0.064 0.043 0.057 0.050 0.036 0.039 0.055 0.050 0.056 0.033 0.051 0.048 0.026 0.010 0.041 0.050 Fe2+ 0.225 0241 0.241 0.258 0.255 0.262 0.266 0.264 0.275 0.252 0.247 0.285 0.268 0.250 0.247 0.253 0.262 0.257 0.252 0.304 0.301 0.290 0.245 M g 0.783 0.753 0.744 0.745 0.730 0.728 0.724 0.717 0.703 0.753 0.761 0.726 0.742 0.758 0.759 0.742 0.770 0.736 0.750 0.766 0.705 0.732 0.752 Ca 0.891 0.894 0.892 0.882 0.881 0.867 0.894 0.885 0.890 0.875 0.887 0.865 0.874 0.875 0.885 0.883 0.866 0.877 0.881 0.834 0.857 0.867 0.897 M n 0.004 0.003 0.003 0.003 0.005 0.002 0.003 0.003 0.003 0.004 0.003 0.003 0.003 0.004 0.004 0.002 0.001 0.004 0.002 0.006 0.008 0.007 0.005 Na 0.019 0.021 0.024 0.024 0.028 SLQ22 0.027 0.028 0.023 0.022 0.027 0.023 0.023 0.022 0.025 0.022 0.026 0.025 0.021 0.030 0.025 0.025 Sum 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 Ui u> 3 c s e S I 3 c S I 3 3 § U ■ 3 3 | •a 3 l< 3 s .§ i a 21; A < ! O I b : B < 3 o t o o 8 g 8 5 n o - sj 2 S = > s 2 £: S 9 : ed p; < 3. s , _, O' »n 1 3 8 « *0 m o 2 ^ rJ O 2 - 2 - ! o‘ ^ 3 < vO 3 1 M 2 ;li f j Ov 3 8! 3 0 0 1 O ' 2 ^ 3 $ 8 s « S ^ « o 2 2 dS . m -. a N 00 2 8 « ? « * » • o c 4 £ < = * 2 i= ' d n d 2 « o « 2 a 1 d ? ; a s 2 « n d : ^ F 8 S § r id 2 d 3 8 S 2 r l d 22 « 00)2 “ 1 | vO i | is ’ ~ - i s o ?; dls vn 2 3 < = s _, m 3 3 1 ? 2 R S S ^ 2 ® 00 o -< o o\ 2 S 8 S = « o 2 2 z o - o 2 2 : g g « S _j ed 2 2 £ =i■ s a s R 8 2 « S o' - d 2 2 a R s a s C > ci d ® — 2 ^ ^ o f j a g 2 3 SI P 3 s a § 2 S d S S 3 8: 2 S pi 3 = > ’ S « 8! rt *0 rs i *0 5 n Q S 'n 6 r3 ® S ' 8 O ' $ }< * < = > ■ fj a $ o ts 3 V O x 7 1 ! »o S: — o O' 2 ; 9 5 5 8 R - o d 2 2 8 3 ” 1 d o < 2 1 9 ! 8 S 3 n H o o ! 2 s s s a ri d 2 ~ 1 a 8 s. 5 3 ri d 2 « ISSEsq rl d 2 2 a s a s = > S n S r t S 3 d a s 2 ^ 8 ='«a§ = • a 3 s: 2 K pi S =i a 3§ S 3 = > s ■ 2 8 Q fl! d rl « 4 * 0 3 V O 2 8 i O ' f3 V I - «o O 8 8 3 i d d d § 0' fO s s o d d : 8 § § d o o § r * " » O' 8 3 o d d i 8 § 3 o d d § tO fO 8 3 d o d § 3 8 § S o d o d 3 : S — 'O'© 8 S S o d d § fO O' 3 5 o'dd s 00 -* 8 8 3 d o d § r - »o 8 3 d o d § r4 ^ 5 5 d o d — r - oo 8 8 3 d o* d V© O ' V© 0 0 5 < S oo O *n Q I n [> » O i 0 0 0 0 0 ' v© O t o O ' * f < 8 S f : S 8 ! 0 0 0 0 0 ' v© «© — r a m < I a s s 8 : d d d d d i § — t o 0 0 V © < N ? S 5 I 0 0 0 0 0 ' r - f o o \ ^ s i d d o d o' < § S I 5 § : 0 0 0 0 0 ' © v -5 to C < to • S o o v « o d ' to v © o o o i d d o d o ' t o 0 0 t o VO t o I 8 S ri 5 8 ! 0 0 0 0 0 ’ s s a s s : O n 00 O o' o' d d d ' O ' t N N 2 ' 8 a S 8 8 i 0 0 0 0 0 ' ^ v-> — r< < 8SPS8: d d o d o* < m oo r- — • 3T i 6 3 P §8 8 : 0 0 0 0 0 ' § 0 0 to © * Q ' r - v o t o Q n r- oo o 1 d d d c i d ' - - f n t o i n O « N « o 8 8 S « ^ S 8 ' © d o d o — o o o o s § § § § — d o d o £ 8 1 ] 8 3a — O (O 8 5 3 o d d — *o r- 8 8 3 d o d cf o o o o — d o d d § § § ! § — o o' d o' » t O N ^ 8 8 8 3 5 — d o d o 8 8 § d o d § © v ri 8 3 o d d — ro o* 8 8 3 o d d 8 8 5 g 8 o'o'dd d § — o o\ » © P 3 P 5 8 0 0 0 0 0 BOONS' 8 SQ £ § 8 d d d d d N n 5 Q 8 ^ # 8 8 : o c» d o o ^ © Q r* t o o n r> 3 S d d d d d § © ov cv » P ! ? 2 8 d d d o’ o’ n — to O V ) 8 S 3 3 8 ; d d d d d l ^ s § i 0 O ' O ' o' o' 0 0 t o Ov ^ »© ® to «0 v © S o r 4 r * 0 0 o o' d 0 o 0 m ~ * 3 N n 8 3 t O O n S S S a, . ^ + + 22g1!9 5,5 ^ « 3 l _ - - 3 ' 5 ' 3 tu (/ll->^Uu!S<U2i/l X , y iH U < ‘ 5u.U» sc3l 154 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. i i Lh U\ Continuation of Appendix n (f) SAMPLE MX64 f3 core MX64 e l rim MX64 e2 rim MX64 e3 core MX64 p.4 rim MX64 hi rim MX64 h2 rim MX64 h3 core Si02 50.39 51.32 51.64 51.05 51.22 51.74 51.58 51.56 T i02 0.54 0.37 0.37 0.51 0.45 0.34 0.29 0.37 A1203 3.36 2.48 2.35 3.16 2.79 2.20 2.30 2.51 Cr203 0.05 0.03 0.01 0.00 0.01 0.00 0.01 0.00 FeO* 11.26 10.87 10.30 11.03 10.74 10.30 10.14 10.42 MgO 12.50 13.03 13.15 12.57 12.82 13.22 13.18 13.09 MnO 0.13 0.11 0.14 0.19 0.16 0.08 0.15 0.12 CaO 21.33 21.85 22.11 21.34 21.60 22.18 22.05 22.04 Na20 £LM 0.41 0.41 0.42 0.39 0.39 0.37 0.41 SUM 100.00 100.47 100.48 100.27 100.17 100.45 100.06 100.53 Formula based on 4 cations Si 1.890 1.912 1.921 1.909 1.915 1.925 1.926 1.918 Ti 0.015 0.010 0.010 0.014 0.013 0.010 0.008 0.010 Cr 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.000 Aliv 0.110 0.088 0.079 0.091 0.085 0.075 0.074 0.082 Alvi 0.038 0.021 0.024 0.049 0.038 0.022 0.027 0.028 Fe3+ 0.072 0.075 0.063 0.044 0.050 0.062 0.056 0.064 Fe2+ 0.281 0.264 0251 0301 0.286 0.258 0.260 0.261 Mg 0.699 0.724 0.729 0.701 0.714 0.733 0.733 0.726 Ca 0.857 0.872 0.881 0.855 0.865 0.884 0.882 0.878 Mn 0.004 0.003 0.004 0.006 0.005 0.002 0.005 0.004 Na 0.032 0.030 0.030 0.031 0.028 0.028 0.027 0.030 Sum 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 a B n s ri . a ■ a c a> a . a < s I £2 £ i ! £2 £2 - " .o s S 5 2 * 3 « < 3 g s ! g a - X * I - > » m u §■*<3 f « • > i 22,8 o u i J j o Q £ ^ _ g t w s s 2 3 § ■ 2 a • = rpw -j — m S S 00 60 ^ O f f > o r r a « o o o d - d S £ : d d ^ S s S ^ S S s d - d S g o d V) o «o o — S O O n ° < Q S S SISSK . £ 0 - 0 8 - i S g 3 5 Q\ —: —• o o ^ ^ ■ o o o r ^ i N ' O “ 8 S 8 ^ J m o ' — ^ J < - “ 1 - o g 2 ° o S 2 S 3 S S S ^ o - o ^ ° o Si o < :2 K S ^ ' d - d - © - d ^ g o o P i A r i f ^ Q ' O ' O cx? 0 ' f 0 c?! “ ' o ' 0 C d o d 2 Sc > 0 s s 5 ; o o o 8 « " 2 s SS = o 5i S m q u -i ' ? g ? ^ i s s 0 0 = ^ 0 0 SBS8KP3!5i? £ d d d S 3 d d ^ — m o ^ ^ ' O v n S d d o S S o o <o 8 0 » n CM O o © ■ * CM CM o o o o NO o o M i s s u s » _ oo — ° d ^ d ? ; t - d i ^ r- d I 8 I o n T 8 I * 5 18 18 1 8 I rn 18 «n © cm p- r - vo — o o o o o w"j O n r*i c n O © O o o O n O O 0 0 0 0 0 0 0 cm cn M- O m 3 S S 8 S © O — O © r - no o ^ co V| On On N m O 0 0 O n O O o o o o d C T N C M oo C O N O r-* C O C M C O N o P * — o © 0 0 — 0 0 CM P - On o o V") f-» m CO CM C s O P- — O O 0 0 — 0 0 s o o o CM w o — $ 2 » ■ N N © p- — © © 0 0 — 00 o o o o 8 — — © oo r> o m = s s — o d C M P- C M a s s N t n m 2 S S — o o C M O S8S 3* -U * C M SS8 C M C M C M $SS — o o’ m o — Z , co > -r- + + .2 > C O C M 00 - Po^gdKdKSd s rt D 2 C O 156 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. I i : i Continuation of Appendix III (b) SAMPLE 14X15 M X I5 MX15 MX15 MX15 14X15 MX15 MX26 14X26 MX26 14X26 MX26 MX26 MX26 MX26 14X62 MX62 MX62 MX62 MX62 ___________ b2 rim b3 core b4 core cl rim c2 rim c3 core c4 core al rim a2 core bl rim b2 core b3 core dl core d2 core < 1 3 rim al core a2 core bl rim b2 rim b3 core Si02 50.30 50.29 50.25 49.60 50.63 5033 50.38 49.73 46.64 50.39 49.87 50.24 49.72 50.15 50.20 51.80 51.61 51.28 51.56 51.58 T i02 0.10 0.08 0.11 0.11 0.08 0.05 0.11 0.12 0.12 0.09 0.11 0.12 0.14 0.14 0.18 0.13 0.09 0.14 0.13 0.11 A1203 1.72 1.82 1.95 1.87 1.50 1.61 1.69 0.79 1.04 0.88 1.06 0.90 0.96 0.91 0.95 1.86 1.86 1.92 1.58 1.52 Cr203 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.01 0.03 0.02 0.04 0.04 0.08 0.06 0.01 0.00 0.02 0.02 0.03 0.01 FeO* 30.04 29.45 29.24 99.87 29.50 30.19 29.56 31.33 31.33 31.04 3131 31.40 31.68 31.57 31.44 25.71 25.88 25.94 26.43 26.64 MgO 17.59 17.60 17.60 17.02 17.64 1731 17.68 14.89 14.51 1633 16.25 16.00 16.09 15.99 15.96 20.38 20.49 19.94 19.83 20.08 MnO 0.97 0.98 1.02 0.98 1.05 1.03 0.99 0.75 0.71 0.73 0.74 0.80 0.76 0.76 0.68 0.18 0.21 0.19 0.25 0.16 CaO 0.64 0.74 0.75 0.70 0.67 0.72 0.70 2.18 1.00 0.82 1.06 0.87 0.86 0.90 0.90 0.80 0.93 0.99 0.72 0.81 N a20 0.04 0.03 0.02 0.06 0.01 0.02 0.05 0.03 0.04 0.03 0.01 0.04 0.05 0.03 0.02 0.05 0.06 0.03 0.04 0.03 SUM 101.40 Formula based on 4 100.99 cations 100.94 170.21 101.08 101.18 101.16 99.83 95.42 100.33 100.35 100.41 100.34 100.51 100.34 100.91 101.15 100.45 100.57 100.94 Si 1.910 1.915 1.914 1.221 1.928 1.920 1.915 1.947 1.912 1.950 1.930 1.947 1.928 1.942 1.947 1.935 1.923 1.929 1.941 1.933 Tt 0.003 0.002 0.003 0.002 0.002 0.001 0.003 0.004 0.004 0.003 0.003 0.003 0.004 0.004 0.005 0.004 0.003 0.004 0.004 0.003 Cr 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.001 0.001 0.001 0.001 0.002 0.002 0.000 0.000 0.001 0.001 0.001 0.000 Aliv 0.077 0.082 0.086 0.054 0.067 0.072 0.076 0.036 0.050 0.040 0.048 0.041 0.044 0.042 0.043 0.065 0.077 0.071 0.059 0.067 Alvi 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.017 0.005 0.014 0.011 0.000 Fe3+ 0.074 0.079 0.080 0.053 0.063 0.070 0.073 0.031 0.045 0.037 0.041 0.036 0.037 0.034 0.034 0.044 0.071 0.051 0.043 0.062 Fe2+ 0.880 0.859 0.852 2.003 0.876 0.893 0.867 0.994 1.029 0.968 0.969 0.982 0.990 0.989 0.986 0.759 0.735 0.765 0.789 0.773 Mg 0.996 0.999 0.999 0.624 1.001 0.978 1.002 0.869 0.887 0.942 0.937 0.924 0.930 0.923 0.923 1.135 1.138 1.118 1.112 1.122 Ca 0.026 0.030 0.031 0.018 0.027 0.029 0.029 0.091 0.044 0.034 0.044 0.036 0.036 0.037 0.037 0.032 0.037 0.040 0.029 0.033 Mn 0.031 0.032 0.033 0.020 0.034 0.033 0.032 0.025 0.025 0.024 0.024 0.026 0.025 0.025 0.022 0.006 0.007 0.006 0.008 0.005 Na 0.003 0.002 0.001 0.003 0.001 0.001 o j m 0.002 0.003 0.002 0.001 0.003 0.004 0.002 0.002 0.004 0.004 0.002 0.003 0.002 SUM 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 L/l -J Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without perm ission. t i i Continuation of Appendix in (c) SAMPLE MX62 MX62 b4 core el core MX62 e2 rim MX62 n rim MX62 f2 rim MX62 MX62 il core i2 core MX64 c l rim MX64 MX64 c2 core fl core MX64 e l rim MX64 MX64 MX64 e2 core e3 core hi rim MX64 MX64 MX64 h2 rim h3 core h4 core Si02 51.94 50.43 51.09 52.00 51.96 52.66 51.47 51.49 51.13 50.99 51.37 51.57 51.43 51.47 51.62 51.35 51.01 Ti02 0.14 0.12 0.09 0.12 0.10 0.14 0.11 0.13 0.14 0.16 0.14 0.13 0.15 0.11 0.11 0.13 0.17 A1203 1.61 2.05 1.84 1.51 1.50 1.28 1.60 1.53 1.77 1.72 1.31 1.27 1.47 1.37 1.31 1.71 2.01 Cr203 0.00 0.02 0.04 0.01 0.00 0.00 0.04 0.00 0.00 0.00 0.00 0.04 0.01 0.05 0.00 0.01 0.05 FeO* 26.33 27.79 27.37 25.14 25.34 25.60 25.38 26.73 27.51 27.39 27.33 28.11 27.87 26.85 25.75 26.86 2653 MgO 20.13 18.68 19.20 21.13 20.85 20.95 20.66 20.01 19.52 19.20 19.46 19.65 19.47 20.01 20.84 19.71 19.41 MnO 0.18 0.19 0.22 0.16 0.21 0 2 0 0.23 0.22 0.22 0 2 3 0 2 7 0 2 6 0.27 0.26 0.20 0 2 2 0.25 CaO 0.77 1.10 0.76 0.70 0.67 0.69 0.83 0.73 0.75 0.70 0.61 0.64 0.73 0.86 0.72 0.80 1.55 N a20 0.01 0.01 0.02 0.01 0.02 0.04 0.02 0.01 0.03 0.00 0.01 0.03 0.00 0.02 0.03 0.02 0.02 SUM 101.11 100.39 100.63 100.78 100.65 101.56 100.34 100.85 101.07 100.39 100.50 101.70 101.40 101.00 100.56 100.82 101.00 Foimula based on 4 cations Si 1.942 1.914 1.930 1.938 1.942 1.952 1.931 1.933 1.921 1.931 1.943 1.929 1.930 1.930 1.933 1.930 1.915 Ti 0.004 0.003 0.003 0.003 0.003 0.004 0.003 0.004 0.004 0.005 0.004 0.004 0.004 0.003 0.003 0.004 0.005 Cr 0.000 0.001 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.001 0.000 0.000 0.001 Aliv 0.058 0.086 0.070 0.062 0.058 0.048 0.069 0.067 0.078 0.069 0.057 0.056 0.065 0.061 0.058 0.070 0.085 Alvi 0.013 0.006 0.012 0.005 0.008 0.008 0.002 0.001 0.000 0.008 0.001 0.000 0.000 0.000 0.000 0.006 0.004 Fe3+ 0.037 0.074 0.054 0.050 0.045 0.034 0.061 0.060 0.073 0.051 0.049 0.050 0.056 0.054 0.054 0.057 0.072 Fe2+ 0.786 0.809 0.811 0.733 0.747 0.759 0.735 0.779 0.792 0.816 0.815 0.830 0.818 0.788 0.752 0.788 0.760 Mg 1.122 1.057 1.081 1.174 1.162 1.158 1.155 1.119 1.093 1.084 1.097 1.095 1.089 1.118 1.163 1.104 1.086 Ca 0.031 0.045 0.031 0.028 0.027 0.027 0.033 0.029 0.030 0.028 0.025 0.026 0.029 0.035 0.029 0.032 0.062 Mn 0.006 0.006 0.007 0.005 0.007 0.006 0.007 0.007 0.007 0.007 0.009 0.008 0.009 0.008 0.006 0.007 0.008 Na 0.001 0.001 0.001 0.001 0.001 0.003 0.001 0.001 0.002 0.000 0.001 0.002 0.000 0.002 0.002 0.002 0.002 SUM 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 Ol 00 o a a Q .2 S 2 u .a T 3 S 3 a i a a < n — s -s r i _ a s •S .§ a .s § ^ s.§ a s OJ SO iJ ' f r i ift M v C- On O V)8 <N T f H « 0 N ° M N N V ) ^ 2 *■ ^ Q vO n J 8 S p o 2 n S — o f " * . © m oo 8 3ri © mi oo 8 m r - vO ~ o o d 8 *r O n t - i n o o i n no ri — © © © o M VO n « N t m p - © © no © r- m © © n r * in « o d d d £ 8 8 8 $ 8 r - in © © in r - c i - o o o o s 3 2 in r* Ov m m m in r - © o ON « N O n « ^ r* « . a r t ««f m in r* © d o d r i •- d r - On IN «r r - 8 r i -* d «T r i m m f*; r i 8 d r i - • R 8 mi O « 3 o o\ m m © © m m in o O H r* - i d o o o « n p « i n O i r o o ^ n n o o n r - d o d o s ? ^ § i S p l l S « - d d d d d l 5 S 3 8 $ S 3 *“ r* n o o n r a i r i - o o o o c l S ? 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S ? g « > i to < tu U S O Z dc co u . w < u , S U 2 ^ w 160 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. vo ro 45 00 V O r-t O' C M p S O ' q r f r* CO *r ro t-; '©I W > j o a C M 00 C M co *T r i - o co d 12 — • r i o l 8 d - r - c* * O ' cm r - n C M r*^ .89 vn O a C M ro q 3 q o *0 r- CO 8 ri 'T ri ri 12 r< o ri ri M i d l 8 d co o> oo co o o Q o o n i O ' O o o r o o « ri o - o s C M C O o o r- ro C M o o y r > O ' c o C M O ) ri O - ri ri ri ri o O P - O 0 0 1s S ' 0 0 ' 0 ' t * * p ' i n f n o » - H 2 } c o « — ' O ' © ' o q c © r - » r - o d r 2 N h i n N O ^ ' O o o o f f l ^ ^ , ^ - d d d « r j d « o d l |2 S O' $ f ° ! ^ ^ o ° ) ^ ' n v N f t i H ipss 1 olg ro cm \o r- r*- _-. cm _ ■ t s °° ■ * 2 o - - cMoorMO'cocMco^r- - 000 ' ' 0 l0 0 v 0 0 -0 0 r o d r O C M —> c M o o © o o >c-d6ci-Nd« I 12 — o S NO ■ n _ cm O' oo ............................... 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I I § s o S 3 • 9 O O if 5 o ( 9 o a - 1 3 i o _ > ’ > + C O + < N o o e 3 « * 3 . 3 f fi V } I I < u. 2 2 u z X u. U v) U . C O< < £ S 2 O u. 162 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. o 3 o a a Q a> A o u Q . O c o u S 3 > .a ■ a B a > a a < \ £ > # g a r* 1 S a 2 oo m oo ° ® 8 8 3 n S 2 !£ o o d 2 m ■ S s S S S g g o ^ : S ' i 2 2 2 o d o 2 - ! S » S S 2 - a « 2 « » ^ ^ 2 ;g o o d ® " ^N£?)P2 N « n 0 oo O N r w O O — (Mr, nrN'0 U K«j'n ' t S “ « ' o 8 8 S piif ^ S = 8 S S 8 g § 2 ? g S S « ' 2 2 2 o d d 2 Ndg; s s p s s g g g ^ a ^ oo .• r M N _ • _ • V Q . ! _ T g\ 5i S S s P 8 8 o 5 S g ' - r ^ d ^ o d © ® ^ vo _ ^ r~* n N » o l ^ w 8 8 3 o d d n 2 » n O o o « n 5 ^ 2 ° ® N ® d o >38 n Is ^ $: 3 <N . § 8 $ S = 8 S = S P M 3 2 m §2 |S Is o) S “ ^ o! S 5 8 2 S ' f j i C i i d d d S ilsag “ S S S n g s g J g a ^ = ^ dddSridlg; cm C M r*» o r*» m C M w > C M O v Ov m rJ — S £J o O £ c/3 P < O O “ N Q o § O s ! i3 z 2 b. 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Creator Murillo-Muñetón, Gustavo (author)

Core Title Petrologic and geochronologic study of Grenville-Age granulites and post-Granulite plutons from the La Mixtequita area, state of Oaxaca in southern Mexico, and their tectonic significance

School Graduate School

Degree Master of Fine Arts

Degree Program Geological Sciences

Degree Conferral Date 1994-12

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag geochemistry,mineralogy,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-1178

Unique identifier UC11340984

Identifier 1376489.pdf (filename),usctheses-c16-1178 (legacy record id)

Legacy Identifier 1376489.pdf

Dmrecord 1178

Document Type Thesis

Rights Murillo-Muneton, Gustavo

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA