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Geology of the southern Sierra San Francisquito, Baja California, Mexico

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Geology of the southern Sierra San Francisquito, Baja California, Mexico

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Content INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UM I films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type o f computer printer. The quality o f this reproduction is dependent upon the quality o f the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely afreet reproduction. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Each original is also photographed in one exposure and is included in reduced form at the back o f the book. Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6” x 9” black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. UMI A Bell & Howell Information Company 300 North Zeeb Road, Ann Arbor MI 48106-1346 USA 313/761-4700 800/521-0600 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GEOLOGY OF THE SOUTHERN SIERRA SAN FRANCISQU1TO, BAJA CALIFORNIA MEXICO by Samantha Lee Goldfarb A Thesis Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment o f the Requirements for the Degree MASTER OF SCIENCE (Geology) December 1996 Copyright 1996 Samantha Lee Goldfarb Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 1383527 Copyright 1996 by Goldfarb, Samantha Lee All rights reserved. UMI Microform 1383527 Copyright 1997, by UMI Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. UMI 300 North Zeeb Road Ann Arbor, MI 48103 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UNIVERSITY O F SOUTHERN CALIFORNIA T H E GRADUATE SCHOOL. U N IV ERSITY PARK L O S A N G E L E S. C A LIFOR N IA 9 0 0 0 7 This thesis, written by Samantha Lee Goldfarb under the direction of h 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 D ate November 14, 1 9 96 THESIS COMMITTEE a Chairman 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS page LIST OF FIGURES..................................................................................................................iv LIST OF TABLES....................................................................................................................vi LIST OF PLATES....................................................................................................................vi ABSTRACT..............................................................................................................................vii CHAPTER I Introduction................................................................................................................................I Geologic History O f Peninsular California.................................................................. 7 Prebatholithic Rocks...................................................................................... 9 Metamorphism...............................................................................................11 Batholithic Rocks......................................................................................... 12 M ethodology.................................................................................................15 CHAPTER n Country Rock Descriptions...................................................................................................16 Introduction................................................................................................................16 Metasedimentary Rocks........................................................................................... 16 M etabasalt.................................................................................................................. 19 CHAPTER HI Plutonic Units..........................................................................................................................27 Punta Final Pluton.....................................................................................................27 La Guera Pluton........................................................................................................ 28 Contacts........................................................................................................ 31 Geochemistry............................................................................................... 32 Geochronology............................................................................................. 38 CHAPTER IV Structure..................................................................................................................................41 Introduction............................................................................................................... 41 D i................................................................................................................................ 41 D2................................................................................................................................ 42 D3................................................................................................................................ 45 D4.................................................................................................................................50 ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Dikes...........................................................................................................................54 Fractures.....................................................................................................................55 CHAPTER V Metamorphism........................................................................................................................ 57 Introduction............................................................................................................... 57 Mineral Assemblages................................................................................................ 57 Temperature-Pressure Conditions...........................................................................62 Timing Relationships................................................................................................ 62 CHAPTER VI Discussion...............................................................................................................................67 Stratigraphic Correlations........................................................................................ 67 Depositional Setting....................................................................................70 Structural Correlations.............................................................................................71 Causes O f Deformation............................................................................................75 Pluton Emplacement................................................................................................ 78 CHAPTER V n Conclusions.............................................................................................................................83 REFERENCES C ITED ................................................................................................. 85 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES 1. Map showing distribution o f Jurassic-Cretaceous arc rocks in northern Baja; 300-km Neogene translation restored (from Gastil, 1993).........................................4 2. Index map showing location of study area................................................................... 5 3. General geology map o f the study area........................................................................ 6 4. Distribution o f prebatholithic terranes in northern Baja (from Gastil, 1993)....... 10 5. Chart comparing the eastern and western phases o f the Peninsular Ranges Batholith (from Silver and others, 1979).................................................................. 14 6. Geochemical plots o f metabasalt data........................................................................ 24 7. Discrimination diagrams o f Pearce and Cann (1973) with triangles representing metabasalt samples in the study area and the closed circle representing a basaltic sill in Canal de Las Ballenas (Campbell and Crocker, 1993). In A, the Ti/100 - Zr - Y.3 graph, with-in plate basalts (continental or oceanic island) plot in field B, low potassium tholeiites in fields A and B, and caic-alkali basalts plot in fields C and B. In B, the Ti versus Zr graph, ocean-floor basalts plot in fields A and B, and calc-alkali basalts plot in fields C and B ......................26 8. Map illustrating location of samples collected for geochemical analyses...............33 9. Geochemical plots o f data from the La Guera and Punta Final Pluton.................. 35 10. Concordia plot o f four zircon fractions from the La G uera.................................... 37 11. Photographs o f country rock. In A, note the boudinage o f quartz-rich layers on outcrop dominated by schist. Also note the floating fold hinge in the left side o f photo. In B, just above the pen, are discontinuous D2 folds preserved in outcrop dominated by schist.................................................................................... 43 12. Well preserved D 2 similar folds in quartz-rich outcrop............................................44 13. Synoptic equal-area stereonet o f D2 structure. *Note - fold axes have been adjusted to compensate for internal inconsistencies..................................................46 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14. Northwest-southeast cross section through study area (see Plate 1 for exact location o f cross section). Horizontal scale is 1:10,000. Vertical scale is 1:4,000 ........................................................................................................................... 47 15. Figure A is an equal-area scatter plot o f poles to D2 foliations. B is a Kamb contour o f the same data............................................................................................... 48 16. Equal-area plot o f axial planes to D3 folds................................................................. 49 17. Photograph and sketch o f outcrop showing D2 folds being affected by D3 axial planar cleavage. D2 folds are best defined by darker, mica-rich layers. Pencil in photo is approximately 15 cm long..............................................................51 18. Equal-area scatterplot o f D3 hinge lines. Total o f 71 data points...........................52 19. Map illustrating trends o f regional cleavage surrounding the La Guera. Note deflection along western contact.................................................................................. 53 20. Equal-area stereonet o f two dominate fracture sets in the La Guera...................... 56 21. Map showing distribution of metamorphic minerals identified in the study area... 58 22. AFM diagram of pelitic schists in study area suggesting stable metamorphic mineral assemblages in the study area.......................................................................... 59 23. Sketch o f thin section showing deflection o f main D2 foliation around D4 andalusite due to reactivation of the foliation during emplacement. Also note the small deflection o f biotite porphyroblasts around the large andalusite in lower left o f figure. Stippled pattern in andalusite represents seriticization 64 24. Chart illustrating timing relationships between deformation events and porphyroblast growth in the metasedimentary units. No assumptions are to be made as to the length o f time of each deformation event......................................... 66 25. Map showing location o f studies performed in nearby areas that have been referred to in the text.....................................................................................................68 26. Diagram outlining hypothetical evolution o f the Jura-Cretaceous arc in Peninsular Baja.................................................................................................................................. 77 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES 1. List o f minerals present in metasedimentary units....................................................... 18 2. Major and trace element XRF data o f the pelitic schists in area............................... 20 3. Major and trace element XRF data for m etabasalt....................................................23 4. Modal mineralogy o f Punta Final and La Guera plutons...........................................30 5. Major oxide data for the Punta Final and La Guera plutons ....................................34 LIST OF PLATES 1. Geologic and foliation map of study area in back pocket 2. Geologic and lineation map o f study area................................................in back pocket vi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT The La Guera pluton has a discordant U/Pb age o f 81 Ma. The homogeneous granodiorite is undeformed, ilmenite-series, and zoned with a granitic core. Wall rocks consist o f Triassic - early Jurassic, intra-arc pelitic schists and quartzites with metabasalt sills, all deformed by at least three periods o f deformation. The D2 event caused transposition o f an earlier layering, pervasive continuous cleavage, and tight - isoclinal folds. A later folding event produced a map-scale overturned anticline and associated smaller scale folds. The youngest deformation event is related to emplacement o f the La Guera pluton, creating a thermal aureole o f sillimanite, andalusite, garnet, and mica as well as a structural aureole. Metamorphism reached upper greenschist to lower amphibolite grade. Evidence exists for both ductile flow and stoping as mechanisms for emplacing the La Guera pluton. Map relationships suggest ductile flow is responsible for ~ 24% o f the material transfer around the pluton. vii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I INTRODUCTION Geologists have recognized that arc settings are an area o f extensive continental growth. It is estimated that anywhere from 20% to 90% o f orogenic belts worldwide consist o f intrusive material added to the crust (Paterson and Fowler, 1993a). Understanding the kinematic and mechanical processes associated with magma emplacement is key to understanding the development of orogens. Pluton emplacement is also linked to ore mineralization and volcanic activity, making its study applicable to various disciplines in geology. The role o f magmas in the development o f orogenic belts is becoming increasingly important to geologists. Emplacement o f plutons has been one o f the greatest controversies o f geology for over two centuries. Understanding their behavior is of great importance. Plutons are a great source of crustal growth. The heat produced by them affects interpretations of regional metamorphism. Transferring material out o f the way of an ascending and emplacing pluton affects the structural fabric o f an area and the method by which a pluton is emplaced could affect the structural interpretation o f a region. Several approaches hold promise for unraveling emplacement: 1) examining the pluton- wall rock system in three dimensions, 2) looking for multiple emplacement mechanisms, 3) trying to quantify the space made by each mechanism during emplacement o f plutons, and 4) linking internal processes in the pluton and external processes related to material transfer. 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Part o f the chain o f Circum-Pacific batholiths, the Peninsular Ranges batholith extends for more than 1000 km from southern California into Baja California. The 140-80 m.y. batholith is part o f a Jura-Cretaceous arc that extends from westernmost Baja California into the western edge of mainland Mexico. This arc presents an excellent natural laboratory in which to study the effects of magma transport, the growth of arcs, and the effect on the resulting structural grain and metamorphic character o f the arc. The Baja arc can be divided into two northwest-trending belts defined by systematic lithologic, temporal, geochemical, and structural variations. The dividing line between the two belts coincides with the location o f a steep crustal discontinuity. Rocks in the western belt have a clear oceanic affinity and different geochemical signature than the eastern belt rocks which have continental affinities. Prebatholithic rocks in the western terrane consist o f volcanic and volcaniclastic rocks. Eastern zone prebatholithic rocks consist o f metasedimentary rocks o f miogeosynclinal origin. Larsen (1948) indicated that the contact between the arc terrane and the eastern terrane may be depositional. Gastil and others (1981) found no evidence for depositional contacts between the two terranes and suggested that the contact is tectonic. It was first suggested by Gastil and others (1978, 1981) that the boundary between the prebatholithic terranes represents a suture zone created when the Jurassic-Cretaceous arc terrane collided with western North America. This hypothesis of the prebatholithic boundary has been supported by Todd and others (1988), Griffith and Goetz (1987), Griffith (1987), and Goetz (1989). 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The distribution o f arc rocks (Figure 1) has led to various hypotheses on the tectonic evolution of the proposed suture zone. Further complicating the history o f the peninsula is emplacement o f the voluminous Peninsular Ranges batholith. Explaining the variations across the suture zone and the origin o f the suture itself has been the focus o f structural geologists working in the peninsula over the last 15 years. The present study examines the La Guera pluton and its surrounding country rock; neither have been previously mapped in detail. The study area is in the southern Sierra San Francisquito range in east-central Baja (Figure 2), approximately 15-20 km northeast o f the proposed suture zone. Originally, the area was chosen as a possible location to study pluton emplacement mechanisms. Preliminary study o f geologic maps (Figure 3), suggested that there was 3-D exposure of a pluton roof and walls, providing an ideal setting to study emplacement. However, after reconnaissance work, I have concluded that the proposed roof exposures were in fact a thin septum between two different plutonic bodies. The main focus o f this study was then changed to address several regional problems. The host rocks o f the pluton in question, herein referred to as the La Guera pluton, are multiply deformed and hold clues that help clarify regional deformational events and timing. The La Guera itself had never been studied previously and this project aims to characterize it petrographically and compare it to other eastern zone plutons. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. * « * • v v v v s*s* S;VS ^yy» ,s * s » s ^y y » i*yy» » % • s*vs»s ^•sVs-s ■ < ? < ? •%«s« • /y y ■s«s« ^y» <yj,vyy •s« •vs«\*vs*vs i w i w w i - i • vs»\(as« vvms« study area % • % • ft* ftft ? ? ••y* •vm S '* , s-v s«s •W • f t •ftftft* * v v s * A ’V N 'v v ' V?/?A VAW?/?/ s*s%s-%-s*s«s-s*v • / ; / • ^ ■ «*• /■ ^ / s • s • % • s • % • s ■ % • s • « « • • < • ✓ • * * • * » ■ • * ; * •s « s • s • > • > • s < ? < ? < ? < ? < ? < ? * : • s » s * s « s « s » s » * ■/*/*/*/*/* •S»S "S*> ■ '■ .'■ .'I W; T % (% a \ %*%*s* T /i Vvs(s * % « % « * n 'W : W V . * \S“ S "S * S « S « ^. *^yyy: W » i'iS ^ y y . * ^ 6sisi% iV W i ^ yy \ w v €s-s,s Figure 1. Map showing the distribution of Jurassic-Cretaceous arc rocks in northern Baja; 300-km Neogene translation restored. From Gastil (1993). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. lexicali / Tijuana San Felipe Puertodtos Catavina' kilometers J Purifa’ Final Bahia \ Calamajue Baja California Ija California Sur study area Figure 2. Index map showing location o f study area. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. miscellaneous granitic bodies N vssxva i 114*15’ Figure 3. General geology map o f the study area. 6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. G e o l o g ic H ist o r y O f P en in su la r C a l ifo r n ia During most of the Paleozoic, the continental margin along what is now eastern Baja California was the site o f passive margin miogeoclinal deposition o f quartz-rich carbonate rocks. Arc activity began in the Middle Jurassic with emplacement of subduction-related plutons along the eastern margin o f what is now the peninsula and in western mainland Mexico. These earliest plutons are -140 - 135 m.y. old (Gastil and others, 1981) and intrude mostly epiclastic rocks o f Paleozoic age (Gastil and others, 1975). By 130 Ma, the main phase o f arc activity was concentrated along the western edge o f the peninsula and plutons were being emplaced into coeval volcanogenic rocks and incorporating the geochemical signature o f the underlying oceanic crust. At the same time, flysch and intra-arc elastics were deposited in a basin formed by back - arc extension, between the western arc terrane and the older plutons in the east. At -105 Ma, contraction resumed and the intra-arc basin closed, juxtaposing the western volcanic terrane, underlain by oceanic crust, with basinal and miogeoclinal deposits in the east and their underlying Proterozoic continental crust. The contact between the these two terranes is most likely a steeply dipping suture (Gastil and others, 1981; Griffith, 1987; Goetz, 1989). Various hypotheses as to its origin have included the existence of two parallel and contemporaneous subduction zones; one o f which created an arc along the western mainland o f Mexico and one along the western peninsula and the continental borderland. Between the two arcs existed an intra-arc basin. Collision o f the two arcs closed the basin and produced the suture zone (Gastil and others, 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1981). A variation on the coilisional model was proposed by Griffith (1987). His model states that the Jura - Cretaceous arc was separated from the continent by a narrow, back- arc basin created by slow subduction. Increased plate velocity at circa 97 Ma caused collapse of the arc against the craton. Gastil (1993) proposes that the arc was initiated along the eastern margin o f the peninsula and that during the Jurassic, the axis of magmatism moved to the southwest, opening a back-arc basin behind it. Closure o f the basin would account for the formation o f the -105 Ma suture (Gastil, 1993). The suture is most often obscured by younger geologic events (Sedlock and others, 1993). Where it is thought to be exposed, it is characterized by steeply plunging Iineations, asymmetric kinematic indicators indicating reverse displacement, and upper greenschist-to lower amphibolite-facies metamorphism (Griffith, 1987; Goetz, 1989; Sedlock and others, 1993). Emplacement o f the Peninsular Ranges batholith transcends the hypothesized extensional and contractional phases of the arc, providing an important age constraint on this deformation. After suturing o f the western oceanic and eastern continental terrane, pluton emplacement continued in the eastern terrane until -8 0 m.y.. These post-tectonic plutons are typically undeformed and intrude Paleozoic and older continental crust as well as the clastic rocks deposited in the intra-arc basin. The youngest plutons recognized in the peninsula are 80 m.y. (Gastil, 1983). 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Prebatholithic Rocks The most recent description o f prebatholithic rocks in Baja divides the peninsula into four northwest-trending belts based upon depositional environment (Gastil, 1993). Figure 4 shows the modern distribution o f these units after restoration o f 300 km of right- lateral displacement associated with opening o f the Gulf o f California. The miogeoclinal rocks along the eastern peninsula are quartz-rich, carbonate- bearing metasedimentary units resting on Proterozoic cratonal crust (Todd and others, 1988; Gastil, 1993). Rock types include amphibolite, plagioclase-rich schist, quartzite, and various gneissic rocks. The miogeoclinal rocks range in age from latest Precambrian to Ordovician and are similar to rocks found in the Death valley region and in the San Bernardino Mountains o f southern California (Stewart and others, 1984). Ordovician to Permian slope-to-basin rocks consist o f mature quartzite, chert, and carbonate. Rock types indicate a shallow-to-deep-water paleoenvironment as well as submarine debris flows and flysch deposits (Gastil, 1993). Pillow basalt found in Devonian to Mississippian sequences suggest eruption close to the continent (Campbell and Crocker, 1993; Leier-Englehardt, 1993). Triassic to Cretaceous back- and intra-arc units include shallow marine and nonmarine strata ranging from schists, paragneiss, calc-silicate-bearing feldspathic quartzite, amphibolite, marble, and metaconglomerate (Todd and Shaw, 1979). Flysch- type units such as the Bedford Canyon Formation, Julian Schist, and Frenchman Valley Formation have been identified and named north of the international border. Equivalent 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LEGEND J-K arc volcanics ft-K back-arc elastics O-P slope-basin M exjcaJl U tePC-P miogeoclinal Figure 4. Distribution o f prebatholithic terranes in northern Baja. From Gastil (1993). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. units are present south o f the border, including an area 10 km from the present study area (Gastil and Miller, 1984; Griffith, 1987), although very few o f these units have been given formal names. The volcanic and volcanogenic rocks of the western terrane are divided inio two formations — the Santiago Peak Volcanics and the Alisitos Formation. The Santiago Peak Volcanics are a non-marine suite of calc-alkalic and tholeiitic, andesitic-to rhyolitic massive flows, tuff, agglomerate, and breccia with small amounts o f basalt and volcaniclastic rocks (Herzig and Kimbrough, 1991). Recent dating ( a concordant U-Pb date o f 115-130 Ma) has constrained the age of Santiago Peak rocks as Early Cretaceous, making it broadly coeval with earliest phases of the batholith (Herzig and Kimbrough, 1991; Kimbrough and others, 1990). The Alisitos rocks, first seen 11 km south of the international border, are thought to be the marine equivalent o f the Santiago Peak. This unit consists of > 6 km thick sequence of calc-alkalic rhyolitic to andesitic flows, pyroclastic rocks, and breccias with interbedded volcaniclastic sandstone, shale, and limestone (Gastil and others, 1981; Sedlock and others, 1993). Metamorphism The Baja Peninsula has undergone regional dynamothermal metamorphism synchronous with batholith emplacement (Gastil and others, 1975; Todd and others, 1988). Based on studies of the arc north o f the international border, metamorphic grade 11 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. increases in degree from west to east across the peninsula. Lower greenschist facies metamorphism is found in the western volcanic sequence, with assemblages o f chlorite - albite - epidote/ clinozoisite - serpentinite - calcite - stilpnomelane - ankerite ± micas (Todd and others, 1988). Basinal rocks, which include Ordovician to Permian quartzite, chert, and carbonate, are metamorphosed to low-to-intermediate pressure amphibolite facies, producing the assemblage o f quartz - biotite - muscovite - andalusite - sillimanite - graphite - plagioclase ± cordierite ± garnet (Todd and others, 1988). These assemblages indicate peak metamorphic conditions o f 2-4 kb and 600-700°C in the basinal terranes (Todd and others, 1988). The miogeoclinal rocks in the east are metamorphosed to low and intermediate pressure amphibolite facies producing the following assemblage: quartz - diopside - epidote / clinozoisite - plagioclase - tremolite / actinolite-calcite± alkali feldspar ± grossularite ± wollastonite ± dolomite ± hornblende ± biotite ± phlogopite ± scapolite ± sphene ± muscovite ± graphite (Todd and others, 1988). Peak conditions reached between 2 - 4.5 kb and 620-800°C in the eastern terrane (Todd and others, 1988). Batholithic Rocks The Peninsular Ranges batholith is widely recognized to be organized into two roughly parallel, northwest-trending belts. The western belt contains plutons that range in age form 130-105 Ma with no systematic space-time progression, implying a static 12 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. magmatic arc (Silver and others, 1979). Eastern belt plutons are less than 105 Ma and progressively decrease in age to the east due to a 10 km/m.y. eastward migration of the arc (Silver and others, 1979). Although there are a small number o f plutons that yield middle to Late Jurassic ages in the eastern terrane, the main phase of batholith plutonism ranges from 130-80 Ma. The division o f the batholith into two belts has also been based on the gravity, geochemical, and structural evidence. Some o f the factors used to divide the batholith are: crustal thickness (Oliver, 1980); 5 ^ 0 and initial ^ S r / ^ S r (Silver and others, 1979); REE variations (Gromet and Silver, 1979) I-type versus S-type granitoids (Todd and Shaw, 1979); and magnetite- versus ilmenite-series rocks (Gastil and others, 1989). Figure 5 summarizes differences across the batholith from west to east. The general location o f the boundary coincides with the proposed suture thought to represent the contact between the western volcanic arc and the eastern continental margin. Composition o f the batholith varies from west to east although low K^O granodiorites and tonalites make up more than 75% o f the batholith. Western plutons range from gabbro and quartz gabbro to tonalite, with moderate amounts o f leucogranodiorite (Silver and others, 1979). Eastern plutons show a much more regular composition o f sphene-homblende-biotite-bearing tonalites and granodiorites (Silver and others, 1979). Gabbro is rare in the eastern zone. Two-mica granitoids and garnet- bearing rocks are found only in the eastern zone (Gastil, 1990a). These eastern belt plutons have been termed La Posta type after a large tonalitic body just north o f the 13 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Feature Western plutons country rock enenl tiara cter ages rock type geochemistry source area Cretaceous volcanic arc rocks of the Santiago Peak and Alisitos formations metamorphosed to greenschist grade smaller in size; shallower level of emplacement; more deformed; zoned horn mafic cores to felsic rims range in age from -130 -105 Ma with no systematic space-time progression, implying a static arc range from abundant tonalite to gabbro; prominent hornblende moderate REE fractionation patterns; low Sr values; low 5180 (+6 to +8.5) I-type granitoids signifying a mantle or oceanic source; magnetite bearing Eastern plutons con tin en tally derived deep marine basinal, intra-arc ana miogeodinai rocks metamorphosed to amphibolite grade middle-crustal emplacement; little or no solid-state deformation; zoned from mafic rims to felsic cores range in age from -105 - 90 Ma* and young progressively to the east implying a migrating dominated by tonalite and low K20 nodiorite, some two-mica and garnet ring tonalites mostly heavy REE fractionation with a zone of light REE patterns to the east; high initial Sr;high 5180 (-t-9 to +12) S-type granitoids indicating either partial melting of continental crust or assimilation of metasedimentary country rod:; mostly flmenite series Figure 5. Chart comparing the eastern and western phases o f the Peninsular Ranges Batholith (modified from Silver and others, 1979). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. international border. The La Posta type plutons are similar in petrography, zonation, and age. In northern Baja, depth o f emplacement o f the batholith increases from west to east (Silver and others, 1979; Todd and others, 1994). Thermobarometry on plutons and country rock has yielded peak conditions of 3.0 ± 1.2 kb and 550°± 35°C for the western zone and pressures and temperatures of 4.5 ± 1.5 kb and 650°± 50°C, respectively, for the eastern zone (Grove, 1989; Grove, 1987) However, there is a breakdown in these data south of the Puertocitos volcanic field where metamorphic pressures are much shallower and temperatures are lower (Rothstein and Manning, 1995). M eth o d o lo g y Approximately seven weeks of field mapping on the scale of 1:10,000 was done for the study. More than seventy thin sections o f both the country rocks and the La Guera were examined for general petrography and detailed microstructural studies. X-ray fluorescence geochemistry was performed on 6 samples o f the country rock and 11 plutonic samples. 15 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER II COUNTRY ROCK DESCRIPTIONS In t r o d u c t io n The country rocks surrounding the La Guera pluton are mostly metasedimentary with local metabasalt sills. Pelitic schists and quartzite comprise almost all o f the metasedimentary units in the study area, with local limestone or calc-silicate layers interbedded with the metasedimentary units. During mapping, the metasedimentary units were mapped together as no mappable contacts between the lithologic units could be discerned and the metabasalt sills were delineated on the map where their dimension made it possible. Reconnaissance work on the eastern side o f the pluton revealed more carbonate-rich rocks that were not studied in this project. M e t a se d im e n t a r y R ock s Metasedimentary rocks in the study area range from pelitic schist/phyllite to quartzite and a few limestone layers. These units are interbedded on a variety o f scales (cm to m) and are generally laterally discontinuous, making the mapping o f layers for long distances difficult. The schists are silica-rich and the quartzites are often micaceous suggesting a range of protolith compositions between pure argillite and clay-rich silt, and possibly chert. Compositional layers range in thickness from mm-scale mica partings in quartzite to several meter-thick quartzite layers. No primary stratigraphic features were 16 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. discovered in the field or in thin section, supporting the observation that the compositional layering represents transposed bedding and metamorphic banding. The schist/phyllite units primarily contain quartz, biotite, muscovite, and opaques ± the following minerals: plagioclase, orthoclase, chlorite, andalusite, sillimanite, garnet, fibrolite, pyroxene (Table 1). There may or may not be more feldspars than reported in Table 1 due to the extremely fine-grained nature of the schists. Quartz grains are mostly polygonal in shape and show veiy little intracrystalline strain, but are commonly fractured. Feldspars, where recognized, are also polygonal in shape. Detrital biotite and muscovite usually occur as elongate grains and show a strong grain preferred orientation parallel to the main foliation in the rock. Biotites were commonly pleochroic. Most o f the coarser biotite grains have zircon inclusions with alteration halos. The opaques are assumed to be graphite based on the common occurrence of rectangular grains and the bulk composition o f the rock. Chlorite occurs locally in veins and along mica-rich layers. Grain size o f all minerals is < 0.1 mm in the majority of samples. The mica-rich layers are less resistant to weathering and often are eroded out to form small depressions or valleys. Quartzites grade from almost pure (98%) quartz to micaceous quartzites, with up to 30% biotite and muscovite. The quartz grains are generally polygonal and show very little intracrystalline strain. There was no observed flattening o f grains, no formation o f quartz trains or ribbons, and very minor undulatory extinction. Grain boundaries are typically irregular and show evidence o f grain boundary migration. The more resistant quartz-rich layers are often discontinuous due to boudinage or pinch-and-swell textures. 17 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE 1. MINERAL ASSEMBLAGES OF METASEDIMENTARY UNITS. |sanf“,''v bio muse _ 2 E _ chi and fibr -gag; BG 6 14 18 22 23 29 32 40 62 78 99 102 122 145 158 169 181 196 203 232 238 241 245 274 282 288 290 315 318 x t x = present in abundance o f > 2%; t = ess than 2% o f thin section area Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. This feature has been described in similar rocks near San Felipe (Anderson, 1993) and Canyon Calamajue (Griffith, 1987; Griffith and Hoobs, 1993). Gray-blue limestone beds were found at three locations interbedded with schist and quartzite. One locality contained a laterally discontinuous block o f the limestone that measured approximately 20 m by 10 m. The other two outcrops o f limestone were laterally continuous and parallel to the regional trends. Near the pluton contact, the rocks become gneissic, with distinct coarse-grained felsic and mafic banding. Composition remains generally the same but grain size increases by over an order o f magnitude. Grain sizes of ~ 5mm were measured in the gneissic rocks. Three samples o f phyllite were analyzed for major oxides and trace elements (Table 2). Discussion o f the data as it pertains to metamorphism will be discussed in a later chapter. M eta b a sa lt Local interbeds o f amphibolite-grade metabasalt occur on the western side o f the La Guera pluton. These rocks are black in hand sample and very fine-grained. They are concordant with the metasedimentary units and are also affected by a continuous cleavage generally parallel to that in the metasedimentary rocks, although less pervasive. Plagioclase constitutes 40-50% o f the rock and shows signs o f recrystallization. The remainder o f the rock is presently composed o f epidote and chlorite, although remnant hornblende is preserved in the samples. The hornblende is the result of 19 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE 2. WEIGHT % OXIDES AND TRACE ELEMENT ANALYSES (ppm) FOR PELITIC SCHISTS. i m i i i i i i i i i i i i i ! I B B I M S SiO , 65.35 65.98 7 3 . 0 2 TiO? 0.91 0.7 0.61 Ab.Os 18.02 15.72 12.51 FeO 4.2 3.86 4.36 MgO 1.57 1.88 2.36 MnO 0.049 0.043 0.1 CaO 0.53 1.16 0.45 N a70 0.9 1.92 0.71 k 2o 3.8 2.99 2.89 P 2 O 5 0.08 0.08 0.03 LO I 3.11 4.68 2.65 Total 98.52 ; 99.69 Fe* 0.728 .. 0,672 . 0,649 ' &7CNK ' v 2.76 1.86 ' 2.46" Rb 142 132 107 Sr 93.4 229 54.7 Ba 970 531 647 Ga 21.8 20.5 17.3 Y 35.3 32.2 24.8 La 37.2 31.2 25.5 Z r 277 153 138 Nb 12.4 9.8 15.4 Cu 10.8 31.2 109.2 Zn 95 113 164 C r 76 55.6 66.7 Ni 7.8 18.1 30.3 Th 11.4 13.7 8.4 U 4.6 5.7 2.4 Pb 12 23 14.1 S 1033 106 57.2 Cl 324 181 143 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. amphibolite-grade metamorphism and the epidote and chlorite are secondary. Opaques constitute less than 2% o f the rock. Plagioclase in the unit has an approximate An content o f 40, which is consistent for a basaltic composition. Three samples o f the metabasalt were analyzed by x-ray fluorescence to determine major and trace-element compositions (Table 3). Geochemical plots are presented as Figure 6. The metabasalts can be characterized as Iow-K, metaluminous, and calc-alkaline. On the Si02 vs. K^O + N a ^ plot, the samples plot as subalkaline. Using the classification o f Miyashiro (1974), the basalts plot as calc-alkaline, which is indicative o f intra-arc magmatism. This contrasts with the tectonic setting o f metabasalts in surrounding areas described by Campbell (1985) and Anderson (1993) which indicate a rifted margin setting. Looking at the Pearce-Cann discrimination diagrams (Pearce and Cann, 1973) in Figure 7 it is clear how different the metabasalt is from the basalt described by Campbell (1985). On the Ti, Zr, and Y diagram, the metabasalt falls into the within-plate field. On the Ti vs. Zr plot, the sample fits into the calc-alkali basalt. This is noticeably different from Campbell's sample which indicates ocean-floor affinity. There is a discrepancy in characterizing the tectonic setting o f the metabasalt between the Miyashiro and Pearce-Cann classifications. Resolving this problem may not be possible in the present study, however, it is clear that the metabasalts are related to arc activity as they are concordant with the intra-arc elastics. Also, the metabasalts are not 21 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. indicative o f a rifted margin setting and do not have oceanic affinities as do the metabasalts described by Campbell (1985) and Anderson (1993). 22 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE 3. WEIGHT % OXIDES AND TRACE ELEMENT ANALYSES (ppm) FOR METABASALT SAMPLES AND COMPARISON WITH METABASALTS IN SAN FELIPE AND SIERRA ASAMBLEA. ^ M ^ g j g ! 1® f i H B i SiO? 48.97 50.55 54.60 44.83 4 7 . 0 T iO i 0.72 0.68 0.64 3.58 2 . 0 AI2 O 3 12.97 17.43 12.69 14.91 15.54 FeO 9.49 8.52 8.33 14.51 10.9 M gO 12.08 7.54 9.54 10.54 7.0 M nO 0.178 0.162 0.171 0.19 0.21 CaO 10.5 11.63 10.57 4.71 8.15 Na^O 1.95 2.61 0.85 1.82 4.0 K ? 0 0.45 0.15 0.49 4.01 1.3 P2O5 0.07 0.06 0.12 1.16 0.69 L O I 1.44 1.47 1.69 Total 98.82 iloo&ol 99.69 100.24 96.79 Fefc v ' 0.440 0.531 0.466. 0.579 0.609 A/CNK 0.569 0.681 0.600 0.936 0.700 Rb 14.8 3.6 26.6 Sr 150.2 165.4 333.6 Ba 140.2 110.8 151.8 G a 13.3 15.1 13.2 Y 16.5 16.4 17 La 8.8 8.7 13.1 Z r 41.5 35 69.7 Nb 3.2 4.1 1.6 Cu 61.1 22 1.9 Zn 82.7 63.8 96.6 C r 639.9 211.9 158 Ni 365.8 107.1 13.3 Th 0.2 0.3 3.2 U 0.3 0.1 0.5 Pb 2.9 2.9 9 S 150 129 77 ci 153.3 134.8 110.9 * data from metabasalt sample in the San Felipe area (Anderson, 1993) ° data from pillow basalt in the Sierra Asamblea (Campbell, 1985) 23 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. • metabasalt medium-K low-K 55 60 S i0 2 (wt %) 70 I I F I V | 1—1 I I y | I "I 1 Alkaline • metabasalt ep 4 £ O S' z + o £ Subalkaline 45 50 55 60 Si02 (w t% ) 65 70 Figure 6. Geochemical plots o f metabasalt data. Field boundaries in the top diagram from Gill (1981). In the bottom diagram, the field boundary is taken from Irvine and Barager (1971). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70 • metabasalt 65 calc-alkaline 60 55 tholeiitic 50 45 0.2 0.4 0.6 0.8 FeO*/(FeO*+MgO) 1.0 * z u 3 peraluminous • metabsalt metaluminous 0.5 45 50 55 60 S iC > 2 (wt % ) 65 70 Figure 6 (continued). Calc-alkaline - tholeiitic boundary in top figure from Miyashiro (1974). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. T i/IO O A. Zr Y.3 15000 B. 10000 ~ 5000 H 50 100 150 200 Zrppm Figure 7. Discrimination diagrams of Pearce and Cann (1973) with triangles representing metabasalt samples in the study area and the closed circle representing a basaltic sill in Canal de Las Ballenas (Campbell and Crocker, 1993). In A, the Ti/100 - Zr - Y.3 graph, with-in plate basalts (continental or oceanic island) plot in field B, low potassium tholeiites in fields A and B, and calc-alkali basalts plot in fields C and B. In B, the Ti versus Zr graph, ocean-floor basalts plot in fields A and B, and calc-alkali basalts plot in fields C and B. 26 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER m PLUTONIC UNITS Pun ta F in a l P l u t o n Part o f the Punta Final pluton (herein named) falls within the study area (Figure 3) and will be described briefly. The portion o f the pluton studied is a homogeneous biotite granodiorite. One sample o f the pluton and one sample o f the transitional zone around the pluton were analyzed by XRF to determine major element concentrations. The results are plotted on the diagrams with the La Guera data presented in the next section. The fine grained, equigranular unit has a "salt and pepper" appearance with a color index o f approximately 40. Subhedral plagioclase grains, averaging 0.5 cm in size, comprise approximately 45% o f the unit and exhibit both normal and oscillatory zoning. Anhedral quartz makes up approximately 33% o f the unit. Many o f the grains have irregular, lobate grain margins as well as moderate amounts o f undulatory extinction. Average grain size for quartz is 0.5 cm. Oikocrystic potassium-feldspar comprises less than 5% o f the pluton and displays tartan twinning. Biotite is the single mafic phase, comprising approximately 20% o f the pluton. Many biotites show minor kink banding and almost all have abundant zircon inclusions. Opaques (mostly ilmenite) and myrmekite make up the remainder o f the unit. Mafic enclaves ranging in diameter from 1 cm to 6 m are present in the pluton. The enclaves are rounded to irregular in shape and show no preferred orientation. 27 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A weak foliation is present in the pluton, although, no consistent orientation was measured. This weak foliation, along with the deformation mentioned previously (kinked biotites, undulatory extinction and lobate margins in quartz) may be the result of regional deformation or the result of emplacement of the La Guera pluton. Further detailed mapping, geochemical, and geochronological studies o f the Punta Final would be required to understand the causes o f deformation. Surrounding the Punta Final pluton along its western edge is a 0.5 km wide mixed zone composed o f blocks o f gneissic wall rock and intrusive pods and dikes o f the pluton. The pluton in this zone has a stronger foliation, although no consistent orientation. There are distinct stoped blocks o f wall rock in the zone where the orientation o f the cleavage is at angles to regional trends. Other zones o f wall rock are not clearly separate blocks and still have a cleavage consistent with regional trends. L a G u er a Pl u t o n The La Guera pluton is an undeformed, ilmenite-series, granodiorite body that has a surface exposure o f approximately 36 km2. Access to the pluton is limited by the topographically steep contacts around the body. More than half of the contact is covered by recent alluvial deposits, obscuring much of the contact relationships. The pluton consists o f a main phase of biotite-bearing granodiorite with a small core of muscovite- biotite-bearing granite. Both facies are medium-to-coarse grained, holocrystalline, and equigranular. 28 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Compositionally, the La Guera differs very little from the Punta Final. The main differences are the lack o f mafic enclaves in the La Guera, and its coarser grain size. The two plutons may be genetically related at depth, but may have been emplaced at different times under slightly different ambient conditions. The biotite facies makes up more than 80% o f the pluton and the mineralogy is consistent in all the samples analyzed for this project. This facies is characterized by 1-to 4 mm euhedral biotite. Plagioclase is the most common phase and usually displays oscillatory zoning in sub-to euhedral crystals. Quartz occurs as anhedral grains with minor undulatory extinction. Potassium-feldspar is a minor phase and mostly occurs as oikocrysts containing inclusions of plagioclase, quartz, and biotite. Many o f the potassium-feldspar crystals display well-developed "tartan" twinning. Myrmekite is present in all o f the samples. Modal mineralogy o f the samples examined are presented in Table 4. The central two-mica granite is recognized by the visibility o f muscovite in outcrop. In thin section, the subhedral muscovite crystals average 1 mm in length. The granite phase also has a noticeable increase in the amount of orthoclase (see Table 4). Orthoclase grains exhibit both tartan twinning and perthitic texture in some samples. Plagioclase lacks the oscillatory zoning that is characteristic in the biotite facies. Quartz is present as anhedral grains with minor undulatory extinction. The only occurrence o f hornblende in the pluton is in a series o f isolated hornblende diorite stoped blocks found dispersed throughout the pluton. Outcrops o f the 29 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE 4. MODAL ABUNDANCES FOR PLUTONIC UNITS. Punta Final Mixed zone La Guera biotite facies mus-bio hbld- diorite BG 146 BG276 BG 284 j BG 360 BG 361 BG 362 BG 363 BG 366 | BG 364 BG 365 qtz 30 40 37 41 35 40 37 40 35 t plag 48 41 45 47 50 40 44 43 27 50 orth 2 4 5 2 3 13 12 5 33 0 bio 20 15 13 12 12 7 7 12 3 25 muse 0 t 0 0 0 0 0 0 2 0 hbld 0 0 0 0 0 0 0 0 0 21 myrm t t t t t t t t t 0 zir t t t t t t t t t 0 aP........ 0 0 0 0 0 t t 0 0 2 0P.9_.. t t t t t t t t t 2 *values based on visual estimation; t = < 1% mus-bio = muscovite biotite facies o f La Guera hbld-diorite = hornblende diorite blocks in La Guera Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. diorite are discontinuous and disrupted. Plagioclase is the prominent phase in the diorite, occurring as anhedral-to subhedral, discontinuously zoned grains. Hornblende and biotite occur in almost equal amounts (Table 4). The hornblende have an inclusion trail o f dark material along the inside margin o f the grain. Some o f the biotites are kinked and many show signs o f chlorite alteration. Quartz occurs in trace amounts. Apatite is also present in these blocks, the only noted appearance o f apatite in the pluton. Kimzey (1982) noted similar blocks of hornblende diorite in the La Posta pluton and attributed them to the existence o f a zone o f hornblende accumulation that was later disturbed by inward crystallization from the walls o f the magma chamber and emplacement o f younger, more felsic phases. A similar explanation is plausible for the blocks found in the present study. The La Guera pluton is unfoliated and shows no solid-state deformation. There is also a lack o f magmatic structures with the exception o f layering in associated dikes that will be discussed in a later section. There is some late stage alteration o f biotite to chlorite and inter-crystalline fractures that are probably related to Cenozoic events. Contacts Where visible, the contacts o f La Guera are characterized by numerous dikes and a small gneissic zone. The contact dips steeply to the west on the western side and steeply to the south along the northern contact. The southern contact is eroded along a steep canyon and subsequently covered by sedimentation. On the east, the contact is almost 31 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. entirely covered by younger alluvium. Where it is exposed on the east, the contact dips steeply to the west. The northern contact is extremely discordant, cutting orthogonally across is generally concordant, however it varies from concordant to discordant due to irregularity o f the contact with respect to the orientation o f the wall rock structure. The concordance along the west and east sides is coincidental, as the margin o f the pluton is parallel to the regional foliation. Geochemistry Ten samples o f the pluton were analyzed by x-ray fluorescence to determine major element concentrations. The location of these sample sites can be found in figure 8. All of the plots that follow in this section contain data points from the Punta Final sample, and the mixed zone sample (Table 5). Harker diagrams o f various elements versus Si02 are presented in this section (Figure 9). All of the samples clearly plot as subalkaline on the Na2 0+K 2 0 vs. Si02 plot. On the Si02 versus Fe# (FeO*/(FeO + Mg)) plot most of the samples plot as calc-alkaline except for the hornblende diorite and the Guera samples which straddle the boundary between calc-alkaline and tholeiitic. Using the Peacock classification (1931), all o f the samples fall in the calcic field except for the hornblende block which plots as calc-alkalic. Negative slopes can be seen in both the FeO vs. S i02 and the MgO vs. Si02 diagrams. On the A/CNK versus Si02 graph, the hornblende diorite falls into the metaluminous field while the remainder o f the samples are peraluminous. This explains 32 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. •32 •53 •84 N \ \ V \ . - - _ / / ✓ / / , - XXXXXXXXXXXN „ / // / / / / / / ///] /\ \ N N \ N \ \ \ \ \ \ H If f f f f f f f f f f f ft •276 . XXXNNX f f f f f f f * ' k V V c - • ' •368' ffff/fff/ff f f S f f f f f / f f f f f f f f Evl Punta Final pluton ['Vj La Guera pluton I I undifferentiated — country rock and alluvium •32 sample number •18 •284. . ... f f f f f . * / / / / / ^ / / / / fffffffffffffffff. \ N \ \ \ S N \ V \ \ \ N \ \ \ \ t •362 f f f f f f * f f sz \ \ \ \ N S \ \ X j / / / / / / / / / •110 f f f f f f f f f f f f f f f s s * f f , . . . / /. /. f / / / / / / / / / / / / / / / / / / / f X v V V X V V \ N\ V\ \ \ N\ \ \ \\ \\ \\ \ \ \ I [/ ^./ / / / / / / // // // / / // / / // // // •361 \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ / / / / / / / / / / / / / / , __ N V \ S \ \ N \ \ \ N \ \ \ \ . . •363 \ V \ \ , f * f f f\ * t 4 * S f f S f * f f f ’f'f'f S f f *\ > V V V V > \ \ N,\ XXXXXXXXXXXXXXXXX / / / / / ✓ / / _ . S S \ \ S \ \ N X« 3a£< ’/ / / / / / / / Jw.................. N \ N \ N \ > V \ \ \ \ \ \ \ \ \ \ / / / / / / / / / ✓/ // / / // /, • XXXXXXXXXXXXXXX* «/•(• / / / / < / / / / / / / / / / / *365 j X X X X X V N \ .N \ \ N \ VN * ^ ' \ V \ \ N \«0/ “. / / / / / N A \ \ \ \ \ V \ \ \ \ V \ \ \ \ \ N N \ N V 'T . / / / / / / / / / / / / / / / / / / / / / / > 360' / ^ / / / / / / / / / / ^ - \ N\ \ S \ V \ \ \ N \ \ J # « r / / / / / / / ✓/ / / / / / 1 km . / / / / / / / / / / / / / / / / / / / / / / / / / . , - - - - - S f f f f f f f f f f S f f f f S f f f * S f f S f\ ,X XXXXXN \ \ V N \ \ S S V \ \ \ \ \ \ \ V V N \ N \ \ \ \ \ \ > . / ' / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /I * f * f f f f f f f f f f f f f f S * * * f S * f s s X- > X XXXX X \ \ \ N \ N N \ \ \ \ \ V \ S N \ S S X \ \ \ N S \ \ \ \ \ x , . / / / / / / s s f f * f f f f s f f S f f S S S S S * S S f s s s s s s s\ L XX XV XX x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x * * * * s f S f f f f f f f f f f f s S S S * * S S S f S s s + s x XX NX x x x x x x x x x x x x x x x x x x x x x x x x x x x x v x x x A / / / / ^ / * _vi / / / / / / / / / / / / / / / / / / / / ✓ / / / / / / NXXVXXX N# OCQv x x x x x x x x x x x x x x x x v x x x x x v x x x x 1 /y . ^ /. / ✓/ / / / / / / / / / / / / / / / / / / / / / / / / \X X X X X X X x x x x x x x x x x v x x x x x x x x x x x x x x x x k/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / ✓ / / / X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X XXX Figure 8. Map illustrating location o f samples collected for geochemical analyses. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE 5. VALUES FOR MAJOR ELEMENT OXIDES OF PLUTONIC UNITS. Punta Final Mixed zone La Guera biotite facies mus-bio hbld- diorite BG 146 BG 276 BG 284 BG 360 BG 361 I BG 362 ! BG 363 1 BG 366 BG 364 BG 365 SiO? 68.36 65.52 69.47 68.28 67.91 68.67 68.40 68.89 75.43 59.89 Ti0 9 0.47 0.61 0.28 0.26 0.34 0.27 0.25 0.22 0.03 0.95 AI9 O3 16.54 16.59 16.49 17.04 16.81 16.91 16.87 16.89 14.27 16.12 FeO 2.86 3.77 2.56 2.45 3.28 2.64 2.35 2.21 0.42 ~0.i0 5.6_ ...4 89 MgO 1.27 1.64 0.66 0.54 0.81 0.61 0.56 0.55 MnO 0.07 0.07 0.06 0.07 0.07 0.06 0.06 0.03 0.09 CaO 3.74 4.36 3.34 3.44 3.47 3.08 2.96 3.16 0.85 5.87 n S 2° K?0 3.72 2.84 3.15 4.09 4.34 4.01 4.04 4.17 4.21 4.28 3.33 ... i 5 -... 2.12 2.64 2.68 2.57 3.51 2.98 2.84 4.44 P2.9.3_ _ Total 0.12 99.99 0.15 0.11 i 0.10 1 0.13 i 0.10 i 0.11 ! _ 0.10 I 0.04 99.70 = §9.19 1 99.40 i 99.90 T 98.71 f 99.13 I 99.89" 0.21 9833 Fe# 0.692 0.697 0.795 I 0.819 I 0:802 ; 0.812 1' 0.808 i 0.801 I 0.808 0.534 A/CNK 103 1.08 1.05 I L05 1 105 | 1.09 ,1 1.07 1 1.07 0.902 musc-bio = muscovite biotite facies o f La Guera pluton hbld-diorite = hornblende diorite blocks in the La Guera pluton mixed zone = sample o f Punta Final pluton collected in transition zone Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. lOp- t - ---------1 - - - - - - - - - - 1 - - - - - - - - - - r O 1 2 + o 3 2 O « U .01 55 calc- alkalic caldc 60 65 70 Si02 (w t% ) 75 80 peraluminous 8 1 . 0 < V - metaluminous 55 60 65 70 S i0 2 (wt %) 75 80 ■ La Guera a Punta Final o 2-mica core ♦ mixed zone + hornblende diorite block Figure 9. Geochemical plots o f data from the La Guera and Punta Final plutons. 35 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ts 1 o C M + o CM « 2 Alkaline > S Subalkaline 80 65 70 75 60 55 S iO j (wt%) high-K o C M medium-K low-K S i 0 2 (w t % ) ■ La Guera a Punta Final o 2-mica core ♦ mixed zone + hornblende diorite block Figure 9 (continued). Field boundaries in top graph is from Irvine and Barager (1971). Field boundaries in bottom graph are from Gill (1981). 36 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70 - * S * * * * • | ' g 60 3 5 calc-alkaline tholeiitic 50F^ 7 / f / ° / 40 U.55 0 .6 5 0.75 0 .8 5 FeO/(FeO+MgO) o I5 55 60 65 70 75 80 Si02 (W t % ) ■ La Guera a Punta Final o 2-mica core • mixed zone + hornblende diorite block Figure 9 (continued). Calc-alkaline - tholeiitic boundary in top figure from Miyashiro (1974). 37 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the absence o f sphene and hornblende in the main phase of the pluton which is characteristic o f the La Posta series plutons. In general, the La Guera can be characterized as a medium - to - high potassium, peraluminous pluton. The pluton is subalkaline, however, it cannot be clearly classified as calc-alkaline or tholeiitic based upon the Miyashiro classification (1974). Geochronology Based on the fact that the La Guera is geographically located in the eastern belt o f the batholith and has similar petrographic and structural characteristics to the other plutons there, it would be expected that the La Guera would fall into the same age range as the eastern plutons (-120 - 80 Ma). U/ Pb dating o f the La Guera was done by G. Girty at San Diego State University. Four zircon fractions were extracted and the results plotted on a concordia curve. The fractions are discordant and fall along a chord whose lower intercept is 81 ± 121 Ma and upper intercept o f 519± 560 Ma (Figure 10). Reasons for the uncertainties on both intercepts are two fold: 1) there is an inheritance problem that needs to be isolated, and 2) the data are too clustered to derive a precise regression. The first problem o f inheritance is common in plutons o f the eastern zone as there is a fair amount o f interaction between the magma and the continental crust. The second problem o f clustering poses a problem in that to get an accurate age determination, there needs to be spread in the data. 38 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 108 .017 104 .016 100 .015 .014 .013 Intercepts at 519± 560 and 81 ± 121 Ma - 8 Q . .012 .08 .09 .10 .11 207Pb/235U Figure 10. Concordia plot o f four zircon fractions from the La Guera. 39 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Although the uncertainties are very high, the upper inheritance age and the lower crystallization age can be considered fairly good estimates (G. Girty, pers. comm.). At the very least, it may be assumed that the La Guera is a Cretaceous magma. If this age is accurate then the La Guera is one o f the youngest plutons recognized in the peninsula (Gastil, 1983). 40 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER IV STRUCTURE In t r o d u c t io n Wall rocks in the study area have been subjected to at least three periods of deformation producing S-tectonite fabrics. Deformation in the metabasalts is typically nonpenetrative, whereas the metasedimentary units are intensely foliated and folded. As mentioned earlier, the La Guera shows no signs of solid-state deformation. The deformation events are denoted Dj-D3 . The term "D^1 is used here to represent the earliest phase o f deformation discerned in the area, as there may be earlier phases o f deformation that have been obliterated by later events. Planar fabrics are denoted by "S" followed by a numerical subscript (i.e. S3 for the planar fabric associated with D,). The same system is used for linear fabrics. Plates 1 and 2 present structural data collected during this study. D, Description of the oldest recognized structures is only based on thin section evidence, where remnants o f a planar fabric are observed being transposed to D2 orientations. Evidence for the St fabric was observed in one sample where a crenulation cleavage associated with transposition o f S( into S2 is preserved. Aligned biotites and opaques define the S( fabric. I was not able to determine if S( is a tectonic fabric or an original sedimentary feature. 41 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. d2 The main deformation event to have affected this area produced tight to isoclinal folds and a related continuous cleavage (S2 ) that is the most pervasive structure in the field area. During this event, S, was fully transposed, creating the dominant banding and continuous cleavage now present in the rocks. The metamorphic banding created consists of alternating mica-rich phyllite and quartzite layers described earlier. The banding is parallel to the S2 continuous cleavage. In rocks rich in mica, S2 cleavage is pervasive and the parallel foliation is best expressed by the alignment of mica minerals. In quartzite rich units, the cleavage is much less pervasive. It is best defined by the existence o f mm-thick mica-rich partings. In thin section, the alignment of elongate mica grains best defines the S2 foliation in quartz-rich units. Units dominated by pelite commonly show boudinaged and discontinuous quartz- rich layers as well as floating fold hinges o f quartz-rich material (Figure 11). In quartz- rich outcrops, tight to isoclinal folds of the pelitic layers are preserved, illustrating the intensity o f this folding. D2 folds are best preserved in quartz-rich rocks (Figure 12). Using Ramsay's classification (1967) which distinguishes folds based on variations in layer thickness and the curvature o f folds in profile, the majority o f D2 folds are class 2 similar folds. There is extreme thickening o f the layers in hinge zones and dip isogons are nearly parallel. 42 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 11. Photographs o f country rock. In top photo, note the boudinage o f quartz-rich layers in outcrop dominated by schist. Also note floating fold hinge in the left side of photo. In bottom photo, just above the pen, are discontinuous D2 folds preserved in outcrop dominated by schist. 43 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 12. Well preserved D2 similar folds in quartz-rich outcrop. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. D 2 axial planes trend northwest and dip moderately to the northeast. The mean axial plane has an orientation o f 326°, 63° (Figure 13). Fold axes to the D 2 folds are varied in orientation and therefore the mean vector orientation o f 72°, 223° (Figure 13) is not an accurate representation of the orientations. D3 A major folding event produced much o f the large folds now dominant in the field. S2 cleavage is folded into a map-scale overturned antiform (Figure 14). The fold is overturned to the southeast; foliation can be observed dipping more steeply than the layers in the southeastern limb. Abundant second-order folds related to this event are present in the area many o f which show eastward vergence. In the hinge area, distinct "m" folds are found, although, on the limbs, the small scale folds show no consistent sense o f asymmetry. When the poles to S2 foliations are plotted on D-diagram (Figure 15), the poles generally fall along a great circle girdle indicating that the large- scale fold is cylindroidal. The tightness of the fold can be described as tight based on an interlimb angle o f 25°. The axial surface o f the fold drawn bisecting the interlimb angle and through the fold axis (the heavy line in Figure 15) has an orientation o f 334°, 80°NE. Axial surfaces to the D 3 folds measured in the field (S3) yield a mean orientation o f 002°, 72°SW (Figure 16). This discrepancy between the statistical axial surface and the mean axial surface o f small scale folds measured in the field is problematic. This may indicate that small scale features are 45 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. N mean axial plane = 32 6°, 6 3 ' + pole to axial plane • fold h in g e!* ■ mean fold hinge = 5 9 °, 077° Figure 13. Synoptic equal-area stereonet o f D 2 structure. *Note - fold axes have been adjusted to compensate for internal inconsistencies. 46 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. c o in / V V V V V V V V / V 'V / x V \ v \ \ \ n \ \ v V . V / / / \ / y y / - y y y y / v W v W < n co in Figure 14. Northwest-southeast cross section through study area (see Plate 1 for exact location o f cross section). Horizontal scale is 1:10,000. Vertical scale is 1:4000. 47 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 15. Figure A is an equal-area scatter plot o f poles to D 2 foliations. B is a Kamb contour o f the same data. 48 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. N mean D 3 axial plane = j 002°, 72° / N = 1 3 • pole to D3 axial plane Figure 16. Equal-area plot of axial planes to D 3 folds. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. not necessarily representative o f larger features in this area or that some D2 folds were mistakenly measured as D3 folds. This S3 axial planar cleavage is characterized by the alignment o f mica grains, mostly in the hinge zone of folds. The S3 cleavage can be seen cutting across S2 foliation in thin section and across S2 layers in outcrop (Figure 17). The fold axis ( 71-point) chosen by the computer as the pole to the great circle girdle is 60°, 354°. This is very close to the mean o f the hinge lines measured in the field o f 64°, 359°. A scatter plot o f D3 hinge lines measured in the field is presented as Figure 18. D4 Deformation related to the intrusion o f the La Guera pluton is the last deformation event to affect the area. Intrusion caused amphibolite grade metamorphism in the aureole as well as minor deflection o f country rock structure. This deformation is limited to a thermal and structural aureole that extends approximately 0.5 km from the pluton margin. Detailed metamorphic descriptions associated with this event will be discussed in a later chapter. Deflection o f the main S2 foliation around the pluton is most evident along the southwestern edge o f the La Guera. It is difficult to trace any marker beds to determine the exact amount o f deflection, however, a general interpretation can be made by looking at the trend line map (Figure 19). 50 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. D3 folds 02 folds Figure 17. Photograph and sketch o f outcrop showing D2 folds being affected by D3 axial planar cleavage. D2 folds are best defined by darker, mica-rich layers. Pencil in photo is approximately 15 cm long. 51 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. N N = 76 □ mean fold hinge = 64°, 359° Figure 18. Equal-area scatterplot o f D3 hinge lines. Total o f 71 data points. 52 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Punta Final vM Figure 19. Map illustrating trends of regional cleavage surrounding the La Guera. Note deflection along western contact. 53 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rocks closest to the contact (within 5 m) are gneissic, bordering on migmatitic in some cases. Grain size has increased, as has the intensity o f metamorphic differentiation. In one thin section, the country rocks at the contact show near-magmatic textures, suggesting that the thermal pulse associated with the pluton was locally very strong. Dik e s Throughout the field area are numerous felsic dikes associated with the abundant plutonic bodies in the region. Surrounding the La Guera is an area o f abundant dikes with compositions ranging from aplite to granodiorite (plagioclase, quartz, orthoclase, and biotite bearing) to garnet-bearing pegmatite (quartz, plagioclase, biotite, muscovite, garnet). Sizes o f the dikes range from < 1 cm to several meters thick. The aplite and pegmatite are late in the geologic history o f this area as they cut across La Guera as well as country rock structures. Dikes with similar granodioritic compositions to La Guera can be found extending from the pluton and cutting discordantly across wall rock structures. The majority o f dikes in the area are unfoliated, however, some o f the intermediate composition dikes have a weak magmatic foliation defined by aligned mafics that, in one outcrop, is parallel to the main country rock foliation. At this outcrop, a late stage, undeformed pegmatite cuts across both the foliated dike and the country rock. Several o f the dikes exhibit schlieren layering. Some show a textural layering alternating between fine-and coarse-grained layers. Orientation o f the layering is unrelated to regional cleavage trends. 54 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A pocket o f euhedral, clear quartz crystals with inclusions o f needle-like tourmaline was found approximately 0.25 km from the pluton contact along the western margin. Some o f the pegmatites also contain similar needle-like tourmaline crystals. Fr a c t u r e s The La Guera pluton contains a predominant set of conjugate fractures. Both sets are obvious on air photos and in the field. The more dominant fracture orientation o f the pair has a mean orientation o f 95°, 83°. The conjugate set o f fractures has a mean orientation of 340°, 84° (Figure 20). Fractures like these have been described in other plutons in the northern part o f the peninsula (K. Schmidt, pers. comm.; J. Fletcher, pers. comm.). 55 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. N ««• m ean set 2 fracture = 84°, 340° m ean set 1 fracture = 83°, 095° N = 48 Figure 20. Equal-area stereonet o f two dominant fracture sets in the La Guera. 56 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER V METAMORPHISM I n t r o d u c t io n The metasedimentary rocks in the study area are mostly pelitic schists and quartzites. Two general phases o f metamorphism have affected the rocks o f the southern Sierra San Francisquito; an upper-greenschist to amphibolite grade regional dynamothermal event followed by contact metamorphism. Figure 21 illustrates the distribution o f metamorphic minerals in the study area. M in e r a l A ssem bla g es As described in an earlier chapter, many o f the quartzite layers have thin mica partings. Garnet preferentially grows in these mica seams. The euhedral garnets are small (<0.5 mm) and either lack internal inclusion trails or have minor, inconclusive inclusion trails. Biotite, muscovite, quartz, plagioclase, and andalusite are the common metamorphic minerals o f the pelitic units. Major oxide concentrations were analyzed for three of the pelitic samples. The results have been plotted on an AFM diagram which indicates minerals expected to be present in equilibrium based on bulk composition (Figure 22). The minerals predicted by the AFM plot are indeed present in the pelitic schists. Andalusite is found throughout the study area but higher concentrations o f crystals are found in thermal aureoles surrounding plutons. Euhedral, pink crystals range up to 57 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Punta Final pluton bm c b m ca m g m b ^ V o i n e mb 1 km bm a bmac m sc bm bma bma bma bma bm ac b mi •L a Guera pluton' bm ac. sillimanite in bma* Figure 21. Map showing distribution of metamorphic minerals identified in the study area (b = biotite, m = muscovite, a = andalusite, c = cordierite, g = garnet, s = sillimanite, f = fibrolite). 58 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Andalusite Kyanite A \ Sillimanite ■Cordierite Biotite Figure 22. AFM diagram of pelitic schists in study area suggesting stable metamorphic mineral assemblages in the study area. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10 cm in length. There is no correlation between distance to the pluton and size o f the crystals suggesting that bulk composition is responsible for the abundance and size o f andalusite. Most show good chiastolite forms in cross section. They grow in random orientations, sometimes in a radial pattern, mostly within the foliation plane, although some cut across the foliation plane. The crystals show no signs of deformation. Some o f the crystals have minor alteration to quartz and muscovite. Mica porphyroblasts include both muscovite and biotite with minor chlorite. Many samples contain three populations o f biotite and/or muscovite which are distinguishable by grain size, shape, and orientation. The earliest generation are metamorphosed original sedimentary grains that are an order o f magnitude smaller than the later generations, rectangular in shape, and are aligned parallel to the main S2 foliation. Many o f these first generation micas are kinked. Crystals o f the two younger generations o f metamorphic mica are anhedral, rounded grains that show no preferred alignment. Deformation o f grains is used to distinguish between the second and third generation. The second generation grains are stretched and are observed being deflected around other porphyroblasts. The youngest generation o f mica porphyroblasts show no signs o f deformation and completely overgrow earlier textures. Highly altered cordierite grains are found in two thin sections in the area. These are now mostly altered to graphite and muscovite and are stretched out parallel to the main S2 cleavage. The fact that the grains are deformed during the D2 event, suggests that 60 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. either they grew early in the D 2 event or during an earlier, low pressure-high temperature event. Sillimanite and fibrolite are found in a narrow aureole surrounding the La Guera pluton. Many o f the sillimanite grains have diamond shaped gray inclusions o f andalusite. Both the sillimanite and fibrolite overgrow any existing foliation. Crystals are observed growing subparallel to parallel to the main S 2 foliation, however the grains show no signs o f deformation. The presence o f sillimanite in the aureole indicates that condition reached upper amphibolite grade with temperatures of600-650°C (Ehlers and Blatt, 1982). Hornblende, chlorite and epidote are the dominant metamorphic minerals in the metabasalt unit. This assemblage indicates that the regional dynamothermal event reached at least lower amphibolite facies metamorphism (Ehlers and Blatt, 1982). Because o f overlapping thermal aureoles in the study area, it is difficult to draw Barrovian-type isograds. In the area surrounding the La Guera, the area of country rock between adjacent plutons is less than one half the radius o f many of the plutons. A sillimanite-in line can be easily drawn around the La Guera pluton (see Figure 21). A sample o f calcareous country rock preserved as a small piece o f roof in the Punta Final pluton contains metamorphic pyroxene, also indicative o f amphibolite grade metamorphism (Ehlers and Blatt, 1982). 61 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. T e m pe r a t u r e-P r e ssu r e C o n d it io n s Thermobarometry was done on a gamet-andalusite pair from a schist in the area by David Rothstein. The sample that Rothstein used for this analysis is located approximately 1 km from the Punta Final pluton and 1 .5 km from the La Guera pluton. Results yielded a pressure o f 2.5 kb and temperatures o f 455°C (Rothstein, personal comm.). Pressure is consistent with regional approximations o f 2-4 kb (Todd and others, 1988). However, temperatures are a bit lower than the regional approximations o f 600-700°C for peak conditions given by Todd and others (1988) for the arc north o f the international border. T im in g R ela tio n sh ips As described earlier, there have been at least three periods o f deformation in rocks o f the southern Sierra San Francisquito. It is clear that D 2 features are older than D 3 features. D 3 folds are folding the S2 foliation and the axial plane foliation related to D 3 deformation, S3 , clearly cuts across the S2 foliation. D 4 deformation related to emplacement of La Guera is clearly the latest event, as the pluton itself is undeformed and porphyroblasts associated with emplacement clearly overgrow earlier structures. The deflection around the pluton is not large enough to be responsible for much o f the deformation in the area. Locally, the foliation nearest the pluton was reactivated and a thin zone o f gneissic textures formed in the aureole. Cordierite porphyroblasts most likely grew before or during the D 2 event. They are strung out and stretched parallel to the orientation o f Sr This suggests that there 62 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. might have been a metamorphic/deformation event before the D 2 event. However, no other evidence o f this event was found so no conclusions may be drawn as to it's nature. Biotite and muscovite porphyroblasts show evidence o f growing during D 2 as well as during the D4 event. An older generation o f mica is observed strung out parallel to the S2 foliation. The youngest population of mica is observed overgrowing the S 3 axial planar foliation associated with the D3 folding event, suggesting that they grew during emplacement o f the La Guera pluton (D4 ). Timing o f andalusite growth is less clear and is a key piece o f evidence in determining the deformation history of these rocks. In outcrop, they appear to be post D 2 and D 3 deformation, growing in random orientations mostly within the foliation plane, however, some crystals do grow across the foliation plane. They show no signs of deformation (fracturing, ductile deformation, or preferred alignment). Alternatively, in thin section, andalusite in samples along the western side o f the pluton appear to be pre- to-syn-the main S2 foliation. The main S2 foliation is clearly deflected around andalusite (Figure 23) which at first suggests that andalusite are pre - D 2 deformation. However, a sample from the northern end of the pluton has similar looking andalusite that clearly overgrow the main S2 foliation with no deflection or wrapping of foliation around the crystals. This suggests that the andalusite is related to pluton emplacement (D4) and that the deflection of foliation around some of the andalusite is the result of the reactivation of S 2 foliation during D 3 deformation. 63 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 23. Sketch o f thin section showing deflection o f main D 2 foliation around D 4 andalusite due to reactivation o f the foliation during emplacement. Also note the small deflection o f biotite porphyroblasts around the large andalusite in lower left o f figure. Stippled pattern in andalusite represents seriticization. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Assuming this conclusion is correct and the andalusite is related to the D 4 emplacement, it would have had to grow early in the emplacement history before some o f the deformation associated with the pluton. Abundant aluminum in the system would allow for rapid growth of the andalusite during this early thermal pulse. The reactivated S2 foliation would have then been deflected around the andalusite along with the older generation o f mica. Sillimanite and fibrolite are clearly related to emplacement o f La Guera pluton. They are confined to a narrow thermal aureole around the pluton and overgrow all previous structures. The preferred alignment of the sillimanite and fibrolite could be explained by the fact that they are strain sensitive and were affected by late mechanical activity associated with emplacement. Euhedral garnet overgrow both the S2 foliation and the S 3 axial plane foliation indicating that they are also associated with pluton emplacement. If they were related to an earlier phase o f deformation, one would expect to see some deflection o f foliation around the grains. Figure 24 summarizes the timing relationships between structure and metamorphism in the study area. 65 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 0 &- T J 00 to Figure 24. Chart illustrating timing relationships between deformation events and porphyroblast growth in the metasedimentary units. No assumptions are to be made as to the length of time o f each deformation event. 66 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER VI DISCUSSION St r a t ig r a p h ic C o r r e l a t io n s Very few formal formation names have been assigned to units in Baja California. However, an attempt will be made here to correlate units in the study area to rocks that have been named in adjacent areas. Figure 25 shows the location o f several studies close to the present area done by Campbell (1985), Crocker (1987), Griffith (1987), and Hoobs (1985). Campbell (1985) distinguished four mappable units in his study area, located less than 5 km east o f the present area, that along with the six units mapped by Crocker (1987), 4 km to the northeast, have been designated the Canal de las Ballenas Group. The rock types in their study areas are carbonates and metavolcanics which are essentially absent in the present study area. However, their unit E is predominantly phyllite with some bedded chert, limestone, quartz siltstone, and quartz arenite (Campbell and Crocker, 1993). It is likely that the units mapped by Crocker and Campbell are part o f the slope to basin sequence described by Gastil (1993). The slope to basin rocks are typically mature quartzite, chert, and carbonate. Conodonts collected in their study areas indicate a Middle to Late Devonian age for the metasedimentary units. Griffith (1987), who mapped less than 10 km to the south o f the present study area describes the Mision and Frailes Formations in the Sierra Calamajue. These formations are dominated by interbedded quartz-rich and mica-rich metasedimentary rocks with some 67 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. y Bahia Gonzaga Punta SFinal Bahia Calamjue Crocker k(1987) Griffith (1987) and V Hoobs (1985) Rancho’ Seco Chapa 1 1 a Figure 25. Map showing location of studies performed in nearby areas that have been referred to in the text. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. carbonates. These units lie along strike o f the present study area. He also describes the Canon Calamajue unit as interbedded chert and phyllite that contains boxcar size blocks o f limestone. This description of the limestone block is similar to a block found in the present study. Hoobs (1985) describes a 6 km thick section which includes six informal units. The only unit which resembles the present study area in any way is his Unit A which consists of interbedded phyllite and metachert that is weakly foliated. The other units are metavolcanics and more calcareous units. The units that sound most similar to those in the present study are those rocks in the San Felipe area described by Anderson (1993). Anderson named the metasedimentary sequence the Playa San Felipe Group which consists o f eight informal map units (A-H). Almost all o f these units have interbedded micaceous quartzite and quartz-biotite-graphite schist containing seriticized andalusite ciystals. North o f the international border, descriptions o f the Julian Schist are very similar to rocks in the present study. Germinario (1993) describes the schist as being 75% metapelite, ranging from phyllite to paragneiss. He also describes a coarsening o f schist to gneiss adjacent to plutonic bodies similar to that found in the present area. Porphyroblasts o f synkinematic andalusite, fibrolite, and sillimanite in pluton aureoles are described for the Julian schist, similar to descriptions made for the present study area. Bedding has been transposed and isoclinally folded in the Julian schist. Germinario (1982) identified preserved turbidite sequences indicating the unit originated as thin-bedded flysch deposits. 69 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The rock units described by Crocker and Campbell contain more carbonates than the present study and are part o f the Paleozoic miogeoclinal sequence based on the fossil correlations. Griffith and Hoobs describe some units that are part o f the Paleozoic section as well as some of the Triassic clastic sequences and a direct correlation cannot be made. Some o f the units described by Anderson are similar to those in the present area, however, the bulk o f the section in the San Felipe area described by Anderson is correlative with the Paleozoic miogeoclinal section. Based on rock type, the present study area can best be correlated with the Julian schist. More detailed stratigraphic studies are needed to definitively support this correlation. Depositional Setting The nature o f the metasedimentary rocks in the current study suggests a shallow- marine environment, sourced by continental rocks (Dickinson and Suczek, 1979), varying from sand-to mud-dominated. Based on rock types, the area falls into Gastil's (1993) late Triassic to Cretaceous intra and back arc clastic sequence (see figure 4). Quartz-bearing sandstone, argillite, and minor carbonate rocks characterize Gastil's sequence. Other quartzite-phyllite sequences, similar to those in the present study, have been describe nearby in the Sierra la Asamblea and Laguna Chapala. The age determination of the intra and back arc sequence is based on dating o f an ammonoid found in the Julian schist (Gastil and others, 1975). Other supporting evidence for a Triassic age is Rb-Sr and radiolarian dating (Gastil and others, 1981). Thomson and 70 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Girty (1994) also suggest a Triassic to Early Jurassic age for the elastics based upon U/Pb dating o f the Harper Creek Gneiss which engulfs the elastics north o f the international border. Unfortunately, no fossils were found in the present area to definitively correlate the study area with the intra and back arc sequence. In fact, fossils are extremely rare in any of the areas thought to fall into this sequence (Gastil and others, 1975). Gastil and others (1975) state "Without fossils, it will never be easy to establish the stratigraphic identity of the shale-sandstone (intra and back arc elastics) belt." However, the rock types in the present study are similar to other areas in the back and intra arc sequence and are distinctive from any other sequences. Therefore, the present area is located just west o f the contact between the clastic sequence and the slope to basin sequence. This conclusion can be based on the fact that reconnaissance mapping along the east side o f the La Guera revealed a higher concentration o f carbonate and volcanic units, along with the fact that the study areas o f Crocker and Campbell are located just to the east. St r u c t u r a l C o r r e l a t io n s Much o f the regional deformation o f the Baja Peninsula has been summarized in Gastil (1993). The following, is an attempt to correlate deformation in the study area with regional events in the peninsula. There are very few references that discuss regional deformation in Baja and all correlations made herein are preliminary. 71 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Griffith (1987) worked in the Sierra Calamajue, located less than 8 km from the present area. He describes three deformation events that affected the units in his study area. His D ( produced northwest-trending isoclinal folds with steep fold axes and an associated continuous axial planar cleavage. St has a mean orientation o f 315°, 71°. The linear fabric associated with this event is dominated by stretched Iithic fragments and mineral alignments with a mean orientation o f 74°, 088°. Most of this deformation is concentrated along fault zones in the area. Griffith can constrain timing o f D ( deformation to 107-92 Ma, based on Rb-Sr dating o f pre- and post-kinematic plutonic and metasedimentary units. He concludes that this deformation is related to collapse of the arc against North America. Griffith describes D2 as expressed by conjugate kink bands and faults and small scale folds with a spaced axial plane cleavage. Hinge lines to the folds plunge steeply north-northeast. His D3 event is a sinistral-verging map-scale flexure, most likely associated with faulting. Folds associated with this event were not recognized. Hoobs (1985) gives a brief structural description which includes a northwest- striking, northeast-dipping foliation and small isoclinal folds. He gave no measurements of data thus preventing any detailed comparisons. Anderson (1993) also gives a brief structural description o f the rocks in the San Felipe area which includes northwest trending isoclinal folds. Goetz (1989) describes four periods o f deformation in rocks of the Rancho El Rosarito area located approximately 70 km to the northwest, along the proposed suture 72 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. zone. His Dt event produced isoclinal folds with northwest striking axial planar foliation and northeast plunging fold axes. The S( axial planar foliation is parallel to S0 which Goetz interpreted as transposition of Su to S . His D 2 event produced open to tight, parallel to similar style minor folds with a northwest trending axial plane (149°, 80°E) and northwest, steeply plunging fold axes. Goetz states that D ( and D 2 may be part o f one continuous event, but that both indicate southwest-northeast contraction. Goetz's D3 event produced minor, open-to tight parallel folds and a crenulation cleavage. Axial surfaces are generally northeast trending and dip to the west. Fold axes plunge to the northeast. D4 consists o f a map scale flexure due to pluton emplacement. Crocker and Campbell (1993) worked in the Calamajue area, just on the eastern side o f the La Guera pluton. Their main focus was stratigraphic descriptions and correlations, however, some basic structural features are described. They describe three periods of deformation in the area. The first produced the regional penetrative cleavage found in the area and associated small scale asymmetric folds. The only statistical description of the folds is that they are northeast verging with a northwest trending axial plane. D2 is expressed by a plunging synform, oriented 40°NE, N63°E and an associated crenulation cleavage. The last deformation event produced east-west faults and related gentle anticlines with subhorizontal hinges. North o f the international border, (Thomson and Girty, 1994) similarly oriented deformational fabrics in the Cuyamaca-Laguna Mountain shear zone o f the Peninsular Ranges have been described. Structures that formed during their D t event include a 73 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. northwest striking, northeast dipping S( and a steeply plunging northeast trending L . Timing o f this event is restricted to between -118—115 Ma. D2 structures which represent northeast-southwest extension are concentrated along a 12-km long shear zone. Timing o f this event is placed between 105 and -94 Ma. Based on these descriptions, it is reasonable to assume that the D2 event described in the present study is related to other regional D, events. Common structural features include a northwest-striking pervasive cleavage, transposition o f previous layering, and v tight-to isoclinal folding with northwest striking axial planes. Almost all o f the structural studies described here call for this main deformation event to be related to collapse o f the arc against the craton between -110 and -95 Ma. The event described by Thomson and Girty (1993) north o f the international border is older (118-115 Ma) suggesting that collision may have progressed from north to south. Correlation o f the D3 event in the present study can best be correlated to the D3 event described by Goetz (1989). Both events have northeast-trending tight folds and fold axes that plunge steeply to the northeast. There are no time constraints on this D3 event, other than it occurs post the main phase of deformation (Goetz's D p present D2 ) and before pluton emplacement in the present area. The D4 event in the current study can also be best correlated to the D4 event of Goetz. This correlation is based more on. the cause o f this deformation (pluton emplacement) rather than the results. There are no detailed metamorphic or thin section descriptions by Goetz (1989) to confidently tie his D4 event to the current D4 event. 74 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. C auses O f De fo r m a t io n As noted earlier, the main phase o f deformation to affect the peninsula (D2) is thought to be caused by the suturing event at ca 105 m.y. There have been other hypotheses as to the cause o f this arc-wide deformation. One model is the internally imposed crustal scale ductile deformation o f country rocks by pluton rise and emplacement (Gastil, 1979). In this model, it is suggested that lateral contraction o f host rocks during emplacement allows for the addition of large volumes o f plutonic material. According to Gastil's model, within the zone o f pluton emplacement, host rocks are trapped between ascending and ballooning diapirs. This results in a steep foliation and steeply plunging folds. Goetz (1989) evaluated this model as a possible cause for the major deformational fabric found in the peninsula. He found no specific evidence to validate this hypothesis. However, this proposal stresses the importance of understanding pluton emplacement systems in the batholith. Other hypotheses as to causes for deformation have included a model by Gastil and others (1978, 1981) which proposed that two parallel, contemporaneous arcs existed, one oceanic and one continental, with contemporaneous subduction occurring beneath both arcs. Between the arcs existed a basin into which flysch units were deposited and then deformed when the two arcs collided. Gravity tectonics has also been suggested as a cause for major deformation o f the peninsula (Gastil and others, 1975; Goetz, 1989). According to the model, the clastic wedge along the margin of North America was the host rock for low density granitoids. 75 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. At approximately 100 Ma, temperatures within the zone o f pluton emplacement reached a critical level which allowed the hot and less dense plutons to rise buoyantly and "mushroom" into the overlying units creating the dominate structural orientation in the peninsula. This hypothesis lacks any concrete evidence related to the actual Baja peninsula and does not explain the numerous variations across the batholith. Goetz (1989) evaluated all o f the models presented here for major deformation of the peninsula. His basic conclusions were that data collected in his study are compatible with, but do not necessarily prove the collisional models o f Gastil and others (1978, 1981) and Griffith (1987) and the gravity tectonic models. Both Griffith (1987) and Goetz (1989) concluded that forcible pluton emplacement was not plausible for the regional deformation in the peninsula. In the present study area, pluton emplacement can also be ruled out as the cause of the regional deformation for the following reasons: (1) there is a strong discordance between wall rock trends and pluton margins in several areas, (2) porphyroblasts associated with emplacement overgrow the main foliation, and (3) the pluton itself shows no sign o f internal deformation. Therefore, it is necessary to have an external source for the regional deformation in the region. The most reasonable cause o f Mesozoic deformation is a variation on the back-arc collapse models of Gastil (1978, 1981) and Griffith (1987) which fail to take into account the presence o f Jurassic plutons emplaced into continental crust. The model proposed here (Figure 26) suggests that the arc began to be created along the margin o f North 76 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A. ■ € - P passive m argin sedimentation along a rifted m argin r-IT T l-X _I.T T 7X .ir x i i J - i i - L - ^ t r North America B. Late T p - J subduction related plutonism begins over North American plate Farallon North America C . 135-110 Ma subduction shifts west; extension of Jurassic arc and creation of intra-arc basin Cretaceous a rc ^ ^ Jurassic arc ocean continental crust I cruji Farallon North America D. 110-95 Ma shallowing of subduting slab collapses basin and causes collision of Jurassic and Cretaceous arcs suture Farallon A North America Figure 26. Diagram outlining hypothetical evolution o f the Jura-Cretaceous arc Peninsular Baja Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. America, during emplacement o f the earliest Jurassic plutons into continental crust along mainland Mexico and eastern Baja. At ~ 135-130 Ma, extension was activated extending the arc and creating an intra-arc basin. The metasedimentary rocks in this study were deposited in this basin. An increase in plate velocity by 97 Ma between the Farallon and North American (Engebretson and others, 1985) caused a shallowing o f the subducting plate which then interacted with the overriding North American plate and caused collapse o f the arc and closure o f the intra-arc basin. The suture, which marks the closure of the basin, coincides with the older plate margin, separating oceanic and continental crust. This model explains the distribution o f rocks and the chemical and structural variations across the peninsula. Pl u t o n E m p l a c e m e n t The most common models o f magma emplacement in the literature today are ballooning, extension-facilitated emplacement, and stoping. Paterson and Fowler (1993a) give a critical review o f the criterion used to argue for each of these different mechanisms. Ballooning is a popular model that has been used to explain high strain aureoles and the shape and zonation of the magma chamber. Other characteristics used to argue for ballooning diapirs are concentric foliations in the pluton and the wall rock and the deflection o f country rock markers around the pluton. The La Guera system exhibits almost none of these features. There is a weak to moderate deflection o f country rocks around the southwestern margin o f the pluton, although, most of the pluton-wall rock 78 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. contact is discordant. There is no foliation or other strain indicators (flattened enclaves) in the La Guera which one would expect if ballooning was a major emplacement mechanism. Other features that are expected around ballooning diapirs are high temperature mylonites and melts around the pluton, both of which are not present around the La Guera. Recent studies by Paterson and Fowler (1993a) have shown that for many plutons this mechanism can only account for 10-40% of the space needed, thus requiring the addition o f another mechanism. Extension-facilitated emplacement has become a popular model although it fails to consider the shapes o f most plutons and the rates o f extension versus rates o f magma emplacement. If this mechanism is active during emplacement, one would expect to see extended or detached roof rocks (Paterson and Fowler, 1993b). Extensional models also favor the construction o f sheeted-dike complexes or layered plutons (Paterson and Fowler, 1993b). These features are a necessary result o f the interaction between fault rates and magma emplacement rates. Fault slip rates range from millimeters to centimeters per year, limiting the amount of magma that can be emplaced. Magma will be emplaced as a series of small, thin, sheet-like pulses. Extension may be eliminated right away as a mechanism for emplacement of La Guera, as there were no faults detected around the pluton. Also, the shape o f the body is not indicative o f extension related plutons which are typically sheeted and elongate in shape. Another mechanism, stoping, is characterized by the following features: discordant margins, thermally thinned aureoles, irregular roof contacts, migmatitic pluton 79 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. floors, and stoped blocks and zenoliths. If stoping is happening on a large scale, it is expected that chemical contamination o f the magma chamber would occur and evidence of movement o f the stoped blocks through the chamber would be present. However, Marsh (1982) states that if stoped blocks have radii greater than a few tens o f meters, they will settle quickly enough in a granitic magma to avoid thermal and chemical contamination. The velocity which blocks will sink is dependent upon the density contrast between the magma and the country rock. Many o f the features indicative of stoping are present in the La Guera system; discordant margins, stoped contacts, and thin aureoles are noted in the country rock surrounding the pluton. However, there was a noticeable lack o f stoped blocks found in the pluton. This is a common phenomena in plutons thought to have been largely emplaced by stoping. Daly (1933) answered this concern by suggesting that the stoped material is assimilated by the magma. To determine the extent, if any, o f contamination of the melt by the sinking blocks will require more detailed chemistry and trace element studies. Other less commonly called upon methods o f emplacement are cauldron subsidence, batch melting/zone melting, doming of roof, and depression o f floor. Cauldron subsidence is the process whereby a large central block o f the roof breaks off and sinks into the magma chamber. However, this method requires a system o f ring faults, absent from the present study area. Zone or batch melting is common at depth with thick mafic bodies (Marsh, 1982). The magma should take up the characteristics o f the rocks 80 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. that are being melted and migmatites should be common in the roof rocks. This process is unlikely to have been active during emplacement o f La Guera as it’ s shallow emplacement level and rock type preclude sufficiently high enough temperatures. Depression o f the floor o f a magma chamber is caused by intrusion of lopoliths. This mechanism is also an unlikely emplacement mechanism for the La Guera. Lopoliths are typically mafic to ultramafic magmas and are internally layered, neither of which is applicable to the La Guera. Laccoliths are emplaced during doming of the roof. This mechanism is usually limited to plutons emplaced at depths less than 3-4 km, as greater depths require unusually high pressures to uplift the roof. Commonly, the aureoles surrounding laccoliths are very narrow. Unfortunately, to really confirm the plausibility of this process, the roof o f a pluton needs to be preserved. In the Sierra San Francisquito, the lack o f roof rocks prevents the full evaluation of this mechanism. An important point that Paterson and Fowler make is that in almost every pluton system, one single mechanism is not adequate to explain emplacement, but rather multiple mechanisms are responsible for emplacement o f a single body. Based on the limited evidence available and the lack o f roof exposure, it is somewhat difficult to evaluate all of the possible emplacement mechanisms that could have been active during emplacement of the La Guera pluton. It is obvious that some ductile deformation associated with ballooning took place, as evidenced by the deflection o f country rock around the pluton. Using basic surface area calculations based on map relationships and the deflection o f the main foliation around the 81 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. pluton (see Figure 19), the amount of deflection around the west side o f the La Guera can 2 2 account for approximately 4 km o f the La Guera. This 4 km area represents only 12% 2 o f the total 33 km area o f the La Guera. Even if one assumes that an equal amount of deflection took place along the now covered east side o f the pluton, only 24% o f the area of the La Guera can be accounted for by the process of ductile flow. This requires the majority o f material transfer to have taken place by some other mechanism(s). Features discovered around the La Guera require that whatever processes were active during emplacement, transfer of material was probably in the vertical, rather than horizontal, direction. Discordance o f most o f the margin strongly suggests stoping was an active process, however, the lack o f stoped blocks makes it difficult to fully document this conclusion. One could argue that the stoped blocks sunk into the magma chamber and/or were assimilated, however, the lack of evidence o f these processes makes it difficult to estimate how much material transfer was accomplished by these mechanisms. The abundance o f granodioritic dikes extending from the pluton into the country rock suggests that the magma exploited fractures in the area which is conducive to the process of stoping. Stoped blocks are evident along the margin o f the Punta Final indicating that the process did occur in the area. 82 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER V n CONCLUSIONS Geology o f the southern Sierra San Francisquito is dominated by the La Guera pluton and it's country rock. The pluton is a homogeneous granodiorite typical o f other eastern Baja plutons. The ~ 81 Ma pluton is ilmenite-series, undeformed, and zoned with a more felsic core. All o f these features are similar to other plutons in the eastern belt of the peninsula. The metasedimentary country rocks in the area are dominated by pelitic schists and phyllites and quartzite o f probable Triassic to early Jurassic age. Minor interbeds o f limestone are present. Metabasalt sills are also present in the area, concordant to the metabasalt layers. The metasedimentary rocks in the study area are similar in rock type to units that are part o f the intra-arc clastic sequence described by Gastil (1975, 1993). Due to a lack o f fossils in the study area and in other units thought to be part o f this sequence, exact correlation is not possible. The metasedimentary units have been pervasively deformed by at least three periods o f deformation. The most pervasive tectonic event (D2) appear related to the closure o f an intra-arc basin and collapse o f the arc against North America. This event can be correlated with similar events in other areas o f northern Baja. Rocks deposited in the intra-arc basin, such as those in the study area, were subjected to intense, penetrative deformation and upper greenschist to lower amphibolite facies metamorphism. 83 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sometime after D2 but before emplacement o f the La Guera, another phase o f deformation (D3) created the map scale anticline and numerous small scale folds in the area and can be correlated to similar events in the Rancho El Rosario area o f western Baja (Goetz, 1989). The youngest deformation event in the area (D4) is related to emplacement of the pluton, which created a narrow thermal aureole containing sillimanite, andalusite, mica and garnet. A narrow structural aureole includes reactivation o f the dominant foliation and the development o f gneissic textures closest to the pluton. Locally, the structural aureole is absent. Constraining the emplacement mechanisms o f the La Guera is difficult, as there is no roof exposure, thereby limiting evaluation of many material transfer processes. There is evidence o f ductile flow due to ballooning, possibly enough to account for 24% of the pluton. Stoping was most likely an active process, however, with no visible stoped blocks or evidence o f their movement through the pluton, it is difficult to estimate how active stoping was. Assuming the La Guera was emplaced by a combination o f ductile flow (24%) and stoping, the latter could have accounted for 76% of the material transfer for the La Guera intrusion. 84 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REFERENCES CITED Anderson, P.V., 1993, Prebatholithic stratigraphy o f the San Felipe area, Baja California Norte, Mexico: Geological Society of America Special Paper 279, p. 1-10 Campbell, M„ 1985, Prebatholithic stratigraphy of the northeastern Sierra La Asamblea, Baja California, Mexico [M. Sc. Thesis]: San Diego State University, 112 p. Campbell, M. and Crocker, J., 1993, Geology west of the Canal de Las Ballenas, Baja California, Mexico: Geological Society o f America Special Paper 279, p. 61-76 Crocker, J.R., 1987, Prebatholithic geology o f the Bahia Calamajue area, Baja California, Mexico [M. Sc. Thesis]: San Diego State University, 129 p. Daly, R. A., 1933, Igneous rocks and the depths o f the earth: McGraw-Hill Book Company, Inc., New York, London, 598 p. Dickinson, W. R. and Suczek, C. A., 1979, Plate tectonics and sandstone compositions: The American Association o f Petroleum Geologists Bulletin, v. 63, no. 12, p. 2164-2182 Ehlers, E. G. and Blatt, H„ 1982, Petrology: Igneous, Sedimentary, and Metamorphic: USA, W. H. Freeman and Company, 732 p. Engebretson, D.C., Cox, A. and Gordon, R.G., 1985, Relative motions between oceanic and continental plates in the Pacific basin: 206, Geological Society o f America Special Paper 206, 59 p. Gastil, R.G., 1979, A conceptual hypothesis for the relationship o f differing tectonic terranes to plutonic emplacement: Geology, v. 7, p. 542-544. Gastil, R. G., 1983, Mesozoic and Cenozoic granitic rocks of southern California and western Mexico: Geological Society o f America Memoir 159, p. 265-275. Gastil, R.G., 1990, Zoned plutons o f the Peninsular Ranges in southern and Baja California: University Museum, University Tokyo, Nature and Culture, no. 2, p. 77-90. Gastil, R.G., 1993, Prebatholithic history of peninsular California: Geological Society of America Special Paper, v. 279, p. 145-156. 85 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Gastil, R. G., Phillips, R. F. and Allison, E. C., 1975, Reconnaissance geology o f the State o f Baja California: 140, Geological Society o f America Special Paper 140, 170 p. Gastil, R. G., Morgan, G. J., and Krummenacher, D., 1978, Mesozoic history o f peninsular California and related areas east of the Gulf o f California, in Paleogeography o f the western United States, Pacific Coast Paleogeography Symposium 2, Society o f Economic Paleontologists and Mineralogists, Pacific Section, p. 107-116. Gastil, R.G., Morgan, G. and Krummenacher, D., 1981, The tectonic history o f Peninsular California and adjacent Mexico, in Ernst, W. G., ed.. The Geotectonic Development o f California: Englewood Cliffs, New Jersey, Prentice-Hall, p. 284- 306. Gastil, R.G. and Miller, R„ 1984, Prebatholithic paleogeography o f peninsular California and adjacent Mexico, in Frizzell, V. A., ed.. Geology o f the Baja California peninsula: Society o f Economic Paleontologists and Mineralogists, Pacific Section, v. 39, p. 9-16. Gastil, G., Diamond, J., Knaack, D„ Walawender, M„ Marshall, M., Boyles, C., Chadwick, B., Erksine, B., Davis, T. and Smith, B., 1990, The problem o f the magnetite-ilmenite boundary in southern and Baja California, in Anderson, J. L., ed.. The Nature and Origin o f Cordilleran Magmatism. Geological Society o f America Memoir 174, p. 19-32. Germinario, M., 1982, The depositional and tectonic environments o f the Julian Schist, Julian California [M. Sc. Thesis]: San Diego State University, 95 p. Germinario, M„ 1993, The early Mesozoic Julian Schist, Julian, California: Geological Society o f America Special Paper, v. 279, p. 107-118. Gilbert, G. K., 1877, Geology o f the Henry Mountains, Utah: U. S. Geographical and Geological survey o f the Rocky Mountain Region, p. 170. Gill, 1981, Orogenic Andesite and Plate Tectonics: Springer-Verlag, New York. Goetz, C.W., 1989, Geology o f the Rancho El Rosarito Area: Evidence for latest Albian, east over west, ductile thrusting in the Peninsular Ranges [M.Sc. Thesis]: San Diego State University, 134 p. Griffith, R.C., 1987, Geology of the southern Sierra Calamajue area: Structural and stratigraphic evidence for latest Albian compression along a terrane boundary, Baja California, Mexico [M. Sc. Thesis]: San Diego State University, 119 p. 86 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Griffith, R.C. and Goetz, C.W., 1987, Structural and geochronologic evidence for mid- Cretaceous compressional tectonics along a terrane boundary in the Peninsular Ranges: Geological Society o f America, Abstracts with Programs, v. 19, no. 6, p. 384 Griffith, R. and Hoobs, J., 1993, Geology o f the southern Sierra Calamajue, Baja California Norte, Mexico: v, p. 43-60 Gromet, L.P. and Silver, L.T., 1979, Profile of rare earth element characteristics across the Peninsular Ranges batholith near the international border, southern California, U. S. A., and Baja California, Mexico, in Abbott, P. L., and Todd, V. R., eds., Mesozoic Crystalline Rocks:, Guidebook for Geological Society o f America Annual Meeting: San Diego, California, p. 83-110. Grove, M., 1987, Metamorphism and deformation o f prebatholithic rocks in the Box Canyon area, eastern Peninsular Ranges, San Diego County, California [M.Sc. Thesis]: University o f California Los Angeles, 174 p. Grove, M., 1989, Nature o f the metamorphic discontinuity across the Chariot Canyon fault, east-central Peninsular Ranges batholith, California: Geological Society of America Abstracts with Programs, v. 21, no. 5, p. 87. Herzig, C. T. and Kimbrough, D. L„ 1991, Early Cretaceous zircon ages prove a non- accretionary origin for the Santiago Peak volcanics, northern Santa Ana Mountains, California: Geological Society o f America Abstracts with Programs, v. 23, no. 2, p. 35. Hoobs, J. H., 1985, Carboniferous island arc and associated rocks from the Mission Calamajue area, Baja California, Mexico: [M.Sc. Thesis]: San Diego State University, 120 p. Irvine, T. N. , and Barager, W. R. A., 1971, A guide to the chemical classification o f the common volcanic rocks: Canadian Journal o f Earth Science, v. 8, p. 523 - 548. Kimbrough, D.L. and others, 1990, Early Cretaceous zircon U-Pb ages from the Santiago Peak volcanics, western Peninsular Ranges batholith, San Diego County, California: Geological Society of America Abstracts with Programs, v. 22, p. 35. Kimzey, J. A., 1982 Petrology and geochemistry of the La Posta granodiorite [M. Sc. Thesis]: San Diego State University, 81 p. 87 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Larsen, E. S., Jr., 1948, Batholith and associated rocks o f Corona, Elsinore and San Luis Rey quadrangles, southern California: Geological Society o f America Memoir 29, 182 p. Leier-Englehardt, P., 1993, Middle Paleozoic strata o f the Sierra Las Pintas, northeastern Baja California Norte, Mexico: Geological Society of America Special Paper, v. 279, p. 23-41. Marsh, B.D., 1982, On the mechanics o f igneous diapirism, stoping, and zone melting: American Journal o f Science, v. 282, p. 808-855. Miyashiro A., 1974, Volcanic rock series in island arcs and active continental margins: American Journal o f Science, v. 274, p. 321-355. Oliver, H.W., 1980, Peninsular Ranges, in Oliver, H.W., ed.. Interpretation o f the gravity map o f California and its continental margin: California Division o f Mines and Geology Bulletin 205, p. 17-20. Paterson, S.R. and Fowler, T.K., 1993a, Re-examining pluton emplacement processes: Journal of Structural Geology, v. 15, no. 2, p. 191-206. Paterson, S.R. and Fowler, T.K., 1993b, Extensional pluton emplacement models: do they work for large batholiths?: Geology, v. 21, p. 781-784. Peacock, M. A., 1931, Classification o f igneous rock series: Journal o f Geology, v. 39, p. 54-67. Pearce, J.A. and Cann, J.R., 1973, Tectonic setting o f basic volcanic rocks determined using trace element analyses: Earth and Planetary Science Letters, v. 19, p. 290- 300. Ramsay, J. G ., 1967, Folding and fracturing of rocks: New York, McGraw Hill, 568 p. Rothstein, D.A. and Manning, C.E., 1995, Compositional variations in metamorphic rocks from different crustal levels in the eastern Peninsular Ranges batholith, Baja California, Mexico: Third International Meeting on Geology o f Baja California Peninsula Abstracts, p. 178. Sedlock, R.L., Ortega-Gutierrez, F. and Speed, R.C., 1993, Tectonostratigraphic terranes and tectonic evolution of Mexico: Geological Society of America Special Paper 278, p. 153. 88 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Silver, L.T., Taylor, H.P. and Chappell, B„ 1979, Some petrological, geochemical and geochronological observations of the Peninsular Ranges batholith near the international border of the U.S.A. and Mexico, in Abbott, P. L., and Todd, V. R., eds., Mesozoic Crystalline Rocks: Guidebook for Geological Society o f America Annual Meeting: San Diego, California, p. 83-110. Stewart, J. H., McMenamin, M. A. S. and Morales-Ramirez, J. M., 1984, Upper Proterozoic and Cambrian rocks in the Caborca region, Sonora, Mexico-Physical Stratigraphy, biostratigraphy, paleocurrent studies, and regional relations: U.S. Geological Society o f America Special Paper 109, p. 36. Thomson, C.N. and Girty, G.H., 1994, Early Cretaceous intra-arc ductile strain in Triassic-Jurassic and Cretaceous continental margin arc rocks. Peninsular Ranges, California: Tectonics, v. 13, no. 5, p. 1108-1119. Todd, V.R. and Shaw, S.E., 1979, Structural, metamorphic and intrusive framework o f the Peninsular Ranges batholith in southern San Diego County, California, in Abbott, P. L„ and Todd, V. R., eds., Mesozoic Crystalline Rocks:, Guidebook for Geological Society of America Annual Meeting: San Diego, California, p. 177- 231. Todd, V.R., Erksine, B.G., and Morton, D. M„ 1988, Metamorphic and tectonic evolution o f the northern Peninsular Ranges batholith, in Ernst, W.G., ed.. Metamorphism and Crustal Evolution o f the Western United States: Englewood Cliffs, New Jersey, Prentice-Hall, Inc., p. 894-937. Todd, V.R., Kimbrough, D.L. and Herzig, C.T., 1994, The Peninsular Ranges batholith from western volcanic arc to eastern mid-crustal intrusive and metamorphic rocks, San Diego County, California: Geological Society o f America Field Trip Guidebook, Cordilleran Section, San Bernardino California, p. 227-235. 89 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PLEASE NOTE: Oversize maps and charts are filmed in sections in the following manne r LEFT TO RIGHT, TOP TO BOTTOM, WITH SM A L L OVERLAPS The follow ing map or chart has been refilmed in its entirety at the end o f this dissertation (not available on m icrofiche). A xerographic reproduction has been provided for paper copies and is inserted into the inside of the back cover. 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V * '•■ ‘V //C • /:■ / / ' ( / Y r " / ' -V ';" ; / / . , - • ■ ■ ' / • / > - . .7 / I / * • ' H K r ^ L A ' / ■ I ; ;• V \ / • ! • • ■ / / • , . . . \ \ - / •> ■ . - s '. « .• _X \ s " ' ' 0 # % .. ? if 1 - S I )• t '■ : f f - 1 1 V s > 3 ^ - Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PLATE 1 * f i V v \' . v ^ f e r f \ \ s ^~-v / V t Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. x ° h 1 f N f\ Qa \ y » r \ \ \ / H H V4 \ \ J r / \ X \ r ' x° t x s \ Y n o Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. > s ' Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I r * v . ■ M > T 'r ; r V v . \ / | . " / / • V f / C £ c s ; U - I . ^ — \ I S S r o i \ . . r x iG m b - j X ■\ \ . \ \ ~ / / \ \ s -) / ! v ' / * • c Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. itt.1 29° 35’ Reproduced with permission of the copyright owner. Further reproduction prohibited without permission KLGb v p q Reproduced with permission of the copyright owner. Further reproduction prohibited without permission I' \ GEOLOGY OF 1 1 km MAP SYMBC ^ 8 8 inclined continuous cleavage in interbedded pelitic • schists and quartzites coi vertii vertical continuous cleavage in interbedded pelitic schists and quartzites 7 8 ^ foliation in plutonic unit / • * contact; dashed where over approximate; dotted where arrow concealed *s. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3E SOUTHERN SIERRA SAN FRANCISQUITO FOLIATION MAP SAMANTHA GOLDFARB December 1996 \ LEGEND 73 inuous cleavage in metabasalt il continuous cleavage in metabasalt rtical foliation in plutonic unit imed anticline with ndicicating direction of plunge j*' Q a and KLGmb KLGb m z PF gr m b pq ROCK UNITS Quaternary alluvium andesite muscovite-biotite phase of La Guera biotite phase of La Guera mixed zone of Punta Final and country rock Punta Final undifferentiated granitic rocks metabasalt interbedded pelitic schist and quartzite A A W \ r v r r ■ r A , : '• V / / / ■ / ( i f = A -' ~ u A s . V ' ' ' ^ < 5 ® s - s * A a _ V f. J / J i - Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. V \ . s \ v \'\ ^ i V,:\ Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. a\ m V x ' : KLGb {■ v - V : 114° 15’ ( UjN \ I ) ' \ /V 80 v ( \ / I \ f a n^80 *7 M h * * > Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. lkm MAP SYMBOLS ^ 8 8 inclined continuous cleavage in interbedded pelitic • schists and quartzites vertical continuous cleavage in interbedded pelitic schists and quartzites 7^ foliation in plutonic unit continu ir vertical cc in vertic p it contact; dashed where overturn* approximate; dotted where arrow indi concealed 0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission SAMANTHA GOLDFARB December 1996 NX \ ) }- LEGEND S 73 Qa nuous cleavage in metabasalt and I continuous cleavage in metabasalt KLGmb KLGb tical foliation in plutonic unit mz PF med anticline with idicicating direction of plunge Sr mb pq J 3 * * 7, s / ' V \ Sl f o^ ■ J / ' ] j Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. V a « ? V ROCK UNITS Quaternary alluvium andesite muscovite-biotite phase of La Guera biotite phase of La Guera mixed zone of Punta Final and country rock Punta Final undifferentiated granitic rocks metabasalt interbedded pelitic schist and quartzite S i PLEASE NOTE: Oversize maps and charts are filmed in sections in the following manner: LEFT TO RIGHT, TOP TO BOTTOM, W ITH SMALL OVERLAPS The following map or chan has been refilmed in its entirety at the end o f this dissenation (not available on microfiche). A xerographic reproduction has been provided for paper copies and is insened into the inside of the back cover. Black and white photographic prints (17” x 23") are available for an additional charge. UMI Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. U t PF and QaVj it- mb mb Reproduced with permission of the copyright owner. Further reproduction prohibited without permission PF i m z Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ' \ r — ^ , i ’ > . i i •1 i ' < • f ' i s Y ' / V - \ / \ y • \ "s. V \ \ : . \ J 1 V 't , V _w >.. ^ \» u v n a \ * \ 0 , ' > \ l / l r,! /A O j W i w „ V ^ y ^ w / A . \ v / ? S t r ; U J k ^ Y "7 ; 1 '. I , V / 1 (! i ! r x J \ \ f r ^ \ \ \ ■ j ! / \ v / ; « u . - v \ ^ / f \ - / ^ » • • r . . . . f r'y / / i i i I y ; .-■ ;r/y -/? I - ’ \ - / ; ‘ - i ■ ■ ■ - • ’ / ■ * / 5 ' ^-' : ■ — -' ; * . ■ j. i ff '•<- • ■ • " \ v ' I 1 • » " \ . / r \ - / - ■ ■ / : • r a v i b , / / •■ * :— i \ : ; / y -. .. .. - -> / T —s - r »* • " ' . • / • / * ' - r , x / > • • - * .... ; • ;n\ - t - : —- ~r | i / :;y 7 + J -sjj / / \ / / V .- ■ /' y Y f y ..-— ' . /s, • ' * ' I / "A— *'' ; ’ 'Y! / 1 / ^ \ V \ \ \ % frVVT y , • -. \ \ V- 1 ! 1 > y v ; ' } \ \ v . I - • ■ -. i * vT's JAY'- ' v \ \ v r A \ \ v- j / > < / . s \ j / y y ^ ^ v ii- , . /■ * . ' v ' V r ^ ^ 0 {. < '\ v . ': ’C / f ' / . > . I * S A c < ■ ' t ; V ’ - < - * " • V* • * \ v -s \ / : - - \ ; / r:r:' •,/' « ; ' < • ■ ■ .- ^ s - 5 / r ' • ' 1 . • • v \ v / • • ‘ V : j /, / • i ! ; J ) J .'s~ - ■ j " /> / .1 N / v. p q f l - N > - . \ Nv > Y y V : : ^ : w . / : / X ' y ' -■: ' ■ / > / i ' '/ ■ ■ y ; . / I r A { i u / / ■ ' .':i , ' '-: u :•■ ■ _ ; / n ») v \ \ ' . Y j y ' i O - Y y - Y - >j-'~ • ■•/ s 'a j \ - / A i h /■ ■-:/■/y y \ / i //(// ; : / T ' ' y •.. . ' c \ \ ,J A i I I I j / f K ' .-'< ■ ; / j / 1 % ' ' S - r \ '. . \ • » » I j ? f ( ,y y / * . ’ / / V . ' ■ ; , • • \\v^. r / / / i ; ;</ /O .< / \ •*. (ysy> ))k l “v y V *— ^ / /V ; j jry b / / / / \ ^ tY 7 *^TJ*7'y rf-\~l^T/h — W '-PJ- 'A<j:y ' Y - ' - Y f i \ { t f ( < ‘ \ \ ( Y i ^ T ~ ~ s \ - < S l < s ' S J S S - t y / y . .; . ,v u C & S / • : • £ • > ; ; ^ A y c ; - y V r ~ » — y j . - • - .? ] \l\ \ \ r - v / ✓ f .- .■ ^ ! \i \ in; ^ —' \ \ , " Y . v t'w'V , S " S v \ (r-~ m rr^. y - • ____ y ; . - * • ■ j i \ \ . • ! “ v'-. % ^ ■ - ■ ■ ■ : ‘ . ■ / i l . / S _ • y § y s m y a y i L \ r • -^ y / / J ) w r Y s ^ ' j / s * j .. f ♦ \ t " S ’ • » - ■ * ' i > V 'i v ? Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A L i n I L L Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. / / KLGb p q / / i Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. * ^ 4 s y + f V y f ' O S ' : ' - ' ~ ^ J J / / ( J 1 7 ^ — ^ /V /-v v -^ 'v 55 \.-v - \ : • ; c - ^ s } i , - " X L s - ^ l / I f f * ' . - X X X «— ->\ iM N V' ' : s . ; ' ■ ' ■ ' ' > r-, v \ } r,A„' ^ ' ! / / .' — —• * , , • - , i k - - • - / f j y j / ^ v A ] I >y"~\ i V - . / / i .?• . * - . - < \ - k : - a \ x f l ^ i r \ \ l j c ^ r ^ ' : 1 1 ' r W - M ^ / A . A y f m ^ : : < y ) ; a . . ..r v , - v n - o ' / / - o T - ' * • ' * - 'is— •V - . .. v ■ - . - I : • ! c \ < \ s ) * C < • v > ' V . > * ' \ : ’ i ; - / L, ■ ■ - J V M k M » # - — v - fi . - / < - * * : i - * v / ' V r \ X * M M . / A lb - * 'N o . ■ ' / / N V J n X '^ n c ^ x \ "'N j/^/KLGm b • • - - * - ■ v L d L x : - X \ ■ \ X n 1 % \ J V -■'“ N -1 S - « T . \ - • > r j < ' , x x k I r j . .. .. Xj .— xx X f r l n O T n n V M T 7 rT 'T TT~ Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Qa 29° 35’ /• \ S t. i — * » •. * *-■. K J y ; * Reproduced with permission of the copyright owner. Further reproduction prohibited without permission \ 7A ✓T KLGb \ \ 114° 15' Reproduced with permission of the copyright owner. Further reproduction prohibited without permission GEOLOGY OF TB N 1 km MAP SYMBOI 86 \ fold hinge min 40 deavage-cleavage intersection cleava contact; dashed where over approximate; dotted where arrow concealed Reproduced with permission of the copyright owner. Further reproduction prohibited without permission [E SOUTHERN SIERRA SAN FRANCISQUITO LINEATION MAP SAMANTHA GOLDFARB December 1996 LEGEND 38 t Qa sral alignment 76, and ;e-lithologic layering intersection KLGmb KLGb mz urned anticline with indicicating direction of plunge PF gr ROCK UNITS Quaternary alluvium andesite muscovite-biotite phase of La Guera biotite phase of La Guera mixed zone of Punta Final and country rock Punta Final undifferentiated granitic rocks % f r r m r ^ v ; j \ ( i - V, , V . % » « » J \ \ j j r A \ / iS/J-J f / / / / £ £ £ ■ "v.. t O ' V ! v S * 'i ' - " ' I ! k : * • \ 1 \ metabasalt interbedded pelitic schist and quartzite . y ' r , \ 'o S \ - r / - s j ...... j * . — ■ — ’ ft V .. ,<V\* ^ '* v 5 r r - v ° v»— \ 1 ■ S J S 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Q a 29° 35’ Reproduced with permission of the copyright owner. Further reproduction prohibited without permission KLGb V - A p q Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. MAP SYMBOL N 0 0.5 1 km 86 \ fold hinge m int 40 deavage-deavage intersection cleavag contact; dashed where approximate; dotted where concealed overti arrow i '- s x * \ : A'" * \r Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SAMANTHA GOLDFARB December 1996 LEGEND 38 / al alignment Qa and -lithologic layering ntersection KLGmb KLGb mz :ned anticline with idicicating direction of plunge PF gr mb pq ROCK UNITS Quaternary alluvium andesite muscovite-biotite phase of La Guera biotite phase of La Guera mixed zone of Punta Final and country rock Punta Final undifferentiated granitic rocks metabasalt interbedded pelitic schist and quartzite ~— .-i x • : ! 3 " \ f - > t y % v . V / J ) \ Jr“ ~£2 t L ' % ^ \ I 1 tL \ v I i Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

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Creator Goldfarb, Samantha Lee (author)

Core Title Geology of the southern Sierra San Francisquito, Baja California, Mexico

School Graduate School

Degree Master of Science

Degree Program Geology

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

Tag Geology,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Paterson, Scott (committee chair), [illegible] (committee member), Davis, Gregory (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-8167

Unique identifier UC11337043

Identifier 1383527.pdf (filename),usctheses-c16-8167 (legacy record id)

Legacy Identifier 1383527.pdf

Dmrecord 8167

Document Type Thesis

Rights Goldfarb, Samantha Lee

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