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Oxygen isotopic evidence for fluid infiltration in the Mount Stuart batholith, Washington

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Oxygen isotopic evidence for fluid infiltration in the Mount Stuart batholith, Washington

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Content 1 i INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter free; 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 subm itted. 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 w ill indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand com er and continuing from left to right in equal sections with small overlaps. 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OXYGEN ISOTOPIC EVIDENCE FOR FLUID INFILTRATION IN THE M OUNT STUART BATHOLITH, W ASHINGTON ty Julie C hristine Francis A Thesis Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA ha P artial Fulfillm ent o f the R equirem ents for the D egree MASTER OF SCIENCE (Geological Sciences) A ugust 1997 C opyright 1997 Julie C hristine Francis R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. UMI Number: 1387818 UMI Microform 1387818 Copyright 1998, 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 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. U N IV E R SIT Y O P S O U T H E R N C A L IF O R N IA T H E GBA.OUA.TE S C H O O L u n i v e r s i t y p a r k LOS ANGELES. C A L IFO R N IA M O O T This thesis, written by under the direction of Hjsx. 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 o f Science Julie C. Francis Dm a D ate— J lz g s 5 l2 t R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Acknowledgements I th an k Law ford A nderson, Jean M orrison, an d Scott P aterson for guidance, collaboration, a n d helpful discussions o f this project, as w ell as critical review s o f this m anuscript; N am i K itchen fo r laboratory assistance; D ave M ayo, Eric H ovanitz, C hris H ill, N am i K itchen, and John M cRaney for advice and discussions; T racy A llen fo r field assistance. This research was m ad e possible b y generous contributions o f m aterials and equipm ent. Law ford A nderson m ad e sam ples and d ata from th e entire batholith available. Scott P aterson and Bob M iller provided sam ples from deform ed zones o f the b atholith. Jean M orrison perm itted u se o f th e stable ; isotope geochem istry laboratory at the U n iv ersity of Southern C alifornia. i I S u p p o rt from the G eological Society o f A m erica, the Foss Foundation, f and the U niversity of S outhern C alifornia D epartm ent of E arth Sciences L G raduate S tu d en t Research Fund is gratefu lly acknow ledged. ii R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Table of Contents Acknowledgements ii List of Figures v List of Tables v ii List of Appendicies v ii Abstract v iii In tro d u ctio n 1 R egional G eology 4 M ount S tu art B atholith 6 Rock Types 6 C hem ical T rends 8 C hem ical E volution 9 S tru ctu ral Features 10 ‘ C hiw aukum Schist 11 I Ingalls C om plex 16 | W indy Pass T hrust 17 i Stable Isotope System atics 18 Basic C oncepts of Stable Isotope G eochem istry 18 Isotopic fractionation 19 G eotherm om etry an d G eobarom etry 21 I . . E lem ental exchange therm obarom etry 21 G eneral form o f isotopic therm om eters 21 C alibration o f isotopic therm om eters 23 Em pirical C alibrations 23 - T heoretical C alibrations 23 Experim ental C alibrations 24 C om positional C onsiderations 25 M ethods 26 Sam ple C ollection 26 Sam ple P reparation 26 W hole Rock 27 ; M ineral Separates 27 Laser T echnique 27 Sam ple P reparation and L oading 29 The Extraction Line 30 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. C onventional Silicate E xtraction T echnique 31 L aboratory S tandards 31 XRF a n d M icroprobe D ata 33 A nalysis o f Isotopic a n d C hem ical D ata 35 M o u n t S tu art B ath olith 35 W hole R ock 35 E lem ental C h em istry 41 M ineral S eparates 42 M ajor M inerals 42 M inor M inerals 48 Secondary M inerals 49 R esults o f Stable Isotope T herm om etry 50 D eform ed Zones 55 C hiw aukum S chist 56 In terp retatio n s 57 C auses o f Isotopic Z onation 57 i C rustal M elting 57 \ M agm atic A ssim ilation 58 ; Subsolidus F lu id In filtratio n 59 | Low T em perature 59 H igh T em perature 61 Source of Fluid 63 M echanism of F lu id In filtratio n 64 R eturn Flow o f A ureole Fluid 66 C ontact M etam orphism 67 H igh P ressu re M etam orphism 68 E xtent o f A lteration 70 Tectonic Im plications 72 "Baja B ritish C olum bia" H ypothesis 72 D eterm in in g P aleo h o rizo n tal 72 B earing of Isotopic Study 76 O th er Plutons of th e N o rth C ascades 77 C onclusions 80 B ibliography 82 IV R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. List of Figures F igure 1-1 M ap o f W ashington, show ing th e locality o f the M ount S tu art b ath o lith (black) an d its c o u n try rocks (C hiw aukum S chist, dashed; Ingalls O p h io lite, shaded). 5 F igure 1-2 M ap show ing d istrib u tio n o f silica contents w ith in th e M ount S tu art b ath o lith . 7 F igure 1-3 Foliation p a tte rn w ith in th e M ount S tu a rt b ath o lith . 12 Figure 4-1 Schem atic d iag ram o f laser extraction line. 28 F igure 4-2 D aily averages o f UWG-2 stan d ard analyses. 34 Figure 5-1 H istogram s o f w hole rock an d m ineral values. 38 Figure 5-2 M ap of M ount S tu a rt b ath o lith show ing w h o le rock §18o values. 39 F igure 5-3 W hole rock 5 ^ 0 vs. silica co n ten t g rou p ed b y rock type. 40 F igure 5-4 Elem ental C h em istry of the M ount S tu art b ath o lith and C hiw aukum schist. 41 Figure 5-5 G eneral sequence o f average values w ith in m inerals an d rocks o f the M ount S tuart b a th o lith . 45 F igure 5-6 C om parison o f q u artz, plag io d ase, and w h o le rock S180 values w ith in the M ount S tuart b ath o lith . 46 F igure 5-7 Biotite, ho rn b len de, and pyroxene isotopic com positions com pared to w hole rock 8^80 for the M ount S tu art b ath o lith . 48 F igure 5-8 C om parison o f chlorite and b io tite isotopic com positions. 50 Figure 5-9 Stable isotope therm om etry o f the M ount S tu art b ath o lith . 52 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. t } t Figure 5-10 D istribution o f tem peratures w ith in th e M o u n t Stuart b a th o lith . 54 Figure 6-1 H ypothetical m odel o f m agm atic assim ilation of high m aterial. 60 Figure 6-2 O bserved p a tte rn o f m ineral 8 ^ 0 values w ith in the M ount S tu a rt b atho lith. 60 Figure 6-3 H ypothetical m odel o f low tem perature, subsolidus fluid in filtratio n . 62 Figure 6-4 H ypothetical m odel o f h ig h tem perature, subsolidus fluid in filtratio n . 62 Figure 6-5 P hotograph o f q u artz vein in tru d in g M ount Stuart batho lith from die C hiw aukum schist. 65 Figure 6-6 P hotograph o f a bleached zone surrounding a quartz v ein w ith in th e M ount S tu art batholith. 65 Figure 6-7 Schem atic representation o f retu rn flow o f originally m agm atic fluids into a b ath o lith . 66 Figure 6-8 Extent o f flu id infiltratio n in to the M ount S tu art b atholith from th e C hiw aukum schist. 71 Figure 7-1 D epth contours calculated u sin g m ineral therm obarom etry w ith in th e M ount S tu art batholith. 74 Figure 7-2 Location o f high phitons and gneisses in the Glacier P eak W ilderness area o f the N orth C ascades, W ash in g to n . 79 vi R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. List of Tables Table 4-A Com parison, o f w hole rock S ^ O values m easu red u sin g conventional and laser techniques. Table 4-B D aily UW G-2 Standard A nalyses. T able 5-A W hole rock S ^ O , silica content, lithology, an d m ineralogy o f th e M ount S tu art b ath olith an d C hiw aukum schist. 32 33 36 Table 5-B M ineral isotopic d ata for th e M ount S tu art batholith an d C hiw aukum Schist. 43 Table 5-C W hole Rock analyses of sam ples w ith du ctile d efo rm atio n . 55 L ist of A ppendides A ppendix A C orrelation coefficients and significance of correlation am ong d ata sets from th e M ount S tu art b atho lith 92 A ppendix B A ppendix C F ractionations betw een m inerals w ith in rocks o f the M ount S tuart b ath o lith and C hiw aukum schist. O xygen stable isotope therm om etry for the M ount S tu art batholith 94 96 vii R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. A bstract The late Cretaceous M ount S tuart batholith, located in the N o rth C ascades o f W ashington, is zo n ed w ith respect to oxygen isotopes. B atholith m argins p reserv e h ig h valu es (-10-12 % o), w hile th e in terio r has m ore n orm al isotopic com positions (-7-9 %o). A nalyses o f m in eral separate d a ta rev eal th a t elevated values resu lt from hig h tem p eratu re subsolidus in filtratio n o f flu id from the neighboring C hiw aukum schist. Fluid in filtratio n cou ld be associated w ith retu rn flow o f m agm atic aureole flu id s, contact m etam orphism , o r h igh-pressure regional m etam orphism trig g ered b y post-em placem ent loading. This process occurred betw een 350 and 625°C, resettin g isotopic com positions an d , to a lesser extent, elem ental chem istry. | Paleom agnetic studies focused on th e M ount S tu art bath olith su g g est th a t the p lu to n m ay have been transported 3000 km n o rth w ard to its p resen t ■ latitu d e. A ssociated barom etric studies yield conflicting solutions of r paleohorizontal. Isotopic resettin g du rin g fluid infiltration suggests th a t r ; therm obarom etry reflects subsolidus conditions, and req u ire cautious u se. i i L 9 I i I viii R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Introduction The M ount S tuart b ath o lith , a late C retaceous tonalitic to granodioritic p lu to n in central W ashington, h as been th e subject o f m uch in terest a n d research due to its location in th e tectonically com plex N o rth C ascades. A p relim inary oxygen isotope stu d y o f th e b ath o lith revealed a zo n atio n in w hole rock 8ls O values (Francis e t al., 1996; A nderson e t al., 1996). The so u th ern and w estern portio ns o f the b a th o lith show values th a t are norm al fo r granitoids ( ~ 7 - 9 % o ), b u t m u ch o f the m arg in o f th e b ath o lith has elevated values (~ 1 0 -1 2 % o ). The goal o f th is stu d y is to determ ine th e exten t o f oxygen isotopic zonation w ith in th e M o u n t S tu art b ath o lith an d to d eterm in e the o rig in of this zonation. ■ There are tw o hypotheses th a t m ay ex p lain th e isotopic v ariatio n w ith in the M ount Stuart. The country ro ck, dom in an tly C hiw aukum schist, h as m uch higher 8 l s O values ( 1 2 - 1 8 %o) th a n th e b ath o lith , in dicatin g a likely » i source for 180 enrichm ent. P erh ap s assim ilation o f w allrock d u rin g em placem ent o f th e M ount S tu a rt has led to elevated 8180 zones a ro u n d the rim o f the b atholith. E lem ental exchange betw een the C hiw aukum sch ist and th e m agm a w o u ld contam inate th e chem ical an d isotopic sig natu re o f the plu to n ic rocks. A lternatively, flu id from th e C hiw aukum schist, an d flow ing in to the batholith, could exchange oxygen w ith the M ount S tuart, causing zones o f 180 enrichm ent. I A careful stu d y of in d iv id u a l m ineral 8180 values for sam ples from \ * d ifferen t portions o f the b ath o lith is the k ey to d istin gu ishin g betw een these hypotheses. In addition, fractionations b etw een m inerals can be u sed to 1 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. determ ine isotopic exchange tem p eratu res, w hich sh o u ld further co n strain form ation m echanism s. By d istin gu ishin g the roles o f assim ilation an d flu id infiltration in the form ation o f this p lu to n , com m only accepted m odels fo r these processes m ay also be evaluated. If assim ilation is th e cause o f th e isotopic zonation, th a t allow s testin g of the v alid ity of AFC (assim ilative fractional crystallization) m odels. O n the o th er han d , a conclusion th at flu id infiltratio n is responsible raises the questions o f how fluid flow s into a b ath o lith . The M ount S tu art batholith is am ong the larg e st of N orth C ascade plutons an d has typical chem ical sig n atu re of a volcanic arc granitoid (Paterson e t al., 1994). U nderstanding th e processes involved in the form ation o f the M ount Stuart m ay y ield clues a b o u t geologic events related to the com plicated tectonics of the N o rth Cascades, a n d of arc-system form ation. In a bro ader sense, this stu d y w ill b ear u p o n u n derstan d in g o f the generation o f new cru stal m aterial, a critical E arth process. A m ong the tectonic com plications is the p ro blem of em placem ent pressure an d paleolatitude of the bath o lith . B arom etric studies have yielded conflicting results, suggesting th at th e p lu to n w as tilted o r dom ed, (A gue and B randon, 1992,1996; Paterson e t al., 1994; A nderson an d Sm ith, 1995). B arom etry has been com bined w ith paleom agnetic d a ta (Beck and N oson, 1972; Beck e t al., 1981; L und et al., 1994; Paterson e t al., 1994) to indicate th a t th e entire tectonic block containing th e M ount S tu a rt w as translated n o rthw ard over long distances to reach its current latitu d e (e.g., Beck e t al., 1981; A gue and B randon, 1992,1996). Problem s ex ist w ith these stu d ies d u e to tem p eratu re dependence and o th er uncertainties in the alum inum -in- hom blende calibration. Portions o f th e batholith record subsolidus 2 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. tem peratures th a t are in ap p ro p riate for u se w ith the barom eter (A nderson, 1997). Sim ilar investigations o f the C hiw aukum schist indicate th a t th e N ason terran e, w hich includes th e M ount S tu art batholith, w as overpressured b y a tectonic th ru st block, o r overlain b y ad d itio n al p lu to n s on th e northern en d (e.g., E vans an d Berti, 1986; Brown an d W alker, 1993; M iller e t al., 1993). W hole rock and m ineral S ^ O d ata w ill provide an in d ep en d en t d ata se t to evaluate th e reliability o f data used in barom etric an d paleom agnetic studies. This stable isotopic stu d y should also be useful in in teg ratin g the histo ry o f the M ount S tu art b atholith w ith larg er scale regional tectonic ev en ts. t f i l I R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. I R egional G eology The late C retaceous M ount S tu art b atholith is th e largest p lu to n in th e N orth C ascades o f W ashington, w ith -500 km ? o f a re al exposure. I t is located -200 km e ast of Seattle in central W ashington, n ear th e tow n of L eavenw orth. The b ath o lith extends from n o rth o f th e Steven's P ass area dow n to M ount S tu art and the E nchantm ent Peaks in th e south (Figure 1-1). M uch of th e p lu to n lies w ithin the A lpine Lakes W ilderness A rea an d Snoqualm ie N ational Forest. The bath o lith is com prised o f tw o nearly connected lobes. T he w estern lobe is elongate w ith a NW trend. T he eastern lobe is m ore com plex, w ith a ! large bulbous region in the southeast connected b y a n arro w stem to a hook- I [ shaped reg io n in the northw est. A n arro w band o f C hiw aukum schist, the country rock for n o rth ern portion o f th e batholith, sep arates these tw o lobes. To th e so u th , the p lu to n is surrounded b y the Ingalls ophiolite com plex, w hich w as stru ctu rally em placed onto the C hiw aukum along the W indy Pass thrust. T hrusting b eg an before m agm a em placem ent, as seen in th e cross cutting relationship o f the batholith across the fault; how ever, activ ity on the L th ru st continued d u rin g em placem ent (Taylor, 1994; Paterson e t al., 1994). The block com prised of the M ount S tuart b ath o lith and th e C hiw aukum schist is know n as the N ason terrane. The terrane is b o un d ed on the east b y the L eavenw orth fault an d o n the w est b y the S traight C reek | fault, b o th o f w hich are Tertiary. Rocks o f this block are the so u th ern t extension o f the C oast Plutonic C om plex in B ritish C olum bia an d A laska, left laterally offset by the Straight Creek-Fraser fault system (e.g., M isch, 1966). R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. BRITISH COLUMBIA 118' 117' 119' 120° 122° 121 123' 40 mi 50 km Mt. Bakei WASHINGTON Glacier Mt. Olympus enw orth /enatchee * J j r 1 Seattle \ ^ •/•T a c o m a Mt. Stuart Mt. Rainier River A Mt, Adams Mt. St. Helens Columbia OREGON Portland Figure 1-1. M ap of W ashington, show ing the locality of the M ount Stuart Batholith (black) and its country rocks (Chiwaukum Schist, dashed; Ingalls O phiolite, shaded). Cities and tow ns are m arked w ith dots. Major m ountain peaks are m arked w ith triangles. SCF = Straight Creek Fault, RLF = RossLake Fault, LF = Leavenworth Fault. T he Ingalls com plex, associated w ith other Ju rassic ophiolites o f w estern W ashington (Vance e t al., 1980), is p art of th e N orthw est C ascades th ru st sy stem (NWCS) th a t extends w e st to the San Ju an Islands (B randon an d C ow an, 1985; B randon et al., 1988). Two sm aller plutons th a t neighbor th e M ount S tuart m ay be related to it, b u t are n o t considered w ith th e m ain b ath o lith for the p u rpo ses of th is stu d y . The Big Jim com plex is a sm all ultram afic to felsic p lu to n just e a st o f th e M ount S tuart batholith, n e ar th e central sill-like region. It m ay be a n e arly phase of the M ount S tu art based on continuous chem ical trends across ro ck types (Pongsapich, 1974; Erikson, 1977; K elem an and G hiorso, 1986), b u t is independently zoned. The Beckler Peak sto ck lies w est o f th e M ount S tu art, I sep arated from it b y the E vergreen fault. T abor an d others (1993) argue th a t I j th e Beckler Peak m ay have b een connected w ith th e eastern lobe of the M o u n t Stuart, based on p lu to n geom etries a n d location o f th e Beckler Peak sto ck in a stru ctural graben. ■ M o u n t Stuart Batholith Rock types The M ount S tuart is a calc-alkaline in tru sio n com posed prim arily of to n alite. The rock types range in com position from tw o-pyroxene dio rite in th e so uthern p o rtio n, through q u artz diorite a n d tonalite, to granodiorite in th e northeast, representing a ran g e o f silica co n ten t from 51.8-70.8 w eight % ‘ (A nderson, 1992; Paterson e t al., 1994) (Figure 1-2). These are generally m etalum inous, b u t peralum inous lithologies (such as b io tite-, biotite-gam et, a n d garnet-tw o m ica granodiorites) occur a ro u n d the edges o f the batholith. E rikson (1977) recognized m afic m em bers in clu d in g gabbro an d tw o-pyroxene 6 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. SiO , (wt. % ) >66 laS il Granodiorite 59-66 Tonalite 5 1 - 5 9 H Diorite F igure 1-2. M ap show ing d istrib u tio n of silica contents w ithin th e M ount S tu a rt B atholith. gabbro-norite. T he batholith also contains m afic dioritic enclaves, both in d iv id u al and in sm all sw arm s (A nderson, 1992). Xenoliths o f C hiw aukum sch ist are com m on on the edge of the b a th o lith an d occur locally throughout. The m ineralogy of the batholith is fairly uniform . T onalite sam ples are com posed m ainly of plagioclase, biotite, q u artz, and hornblende. Q uartz ten d s to be fine-grained and interstitial, w h ile larg er plagioclase grains are su b h ed ral to euhedral. M afic rocks contain orthopyroxene an d lesser am ounts of clinopyroxene th a t are fine-grained, euhedral, an d occasionally co n tain ilm enite inclusions. Plagioclase com positions are o lig o d ase to lab rad o rite (An21-53) throughout the p lu to n , an d average An40 in biotite- 7 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. hornblende tonalite, the d om in an t phase. M ost sam ples contain little to no potassium feld sp ar (generally < 2%). P eralum inous rocks have no hornblende, b u t contain b io tite along w ith g a rn e t a n d /o r m uscovite. The w idespread accessory phases are a p atite and zircon, w hile allan ite an d sphene occur occasionally an d tourm aline is rare. D m enite is the d o m in an t Fe-Ti oxide m ineral, w ith alm ost n o m agnetite p resen t th ro u g h o u t the b atholith. Pyrrohotite is a com m on su lfid e accessory phase. A lteration of plagioclase an d o rth o d ase to se ria te is typical, ranging from m inor occurrence in m ineral cores to significant am ounts along g rain boundaries. Sm all am ounts o f chlorite are fo u n d only w ith biotite crystals. Sphene is com m on as a secondary m ineral, a sso d ated w ith biotite and ! h o rn b len d e. f Chemical trends A nderson (1992), E rikson (1977), an d Pongsapich (1974) have analyzed the chem ical tren d s w ith in the M ount S tuart batholith, fin d in g it to be a fairly typical volcanic arc p luton. It is a m edium -K series w ith a caltic alkali-lim e index a t 62% S i0 2 (Paterson, e t al., 1994). E rikson noted th a t the batholith also has a h ig h ratio of N a /K , as seen in th e predom inance o f sodium p lag io d ase ov er potassium feldspar. The rocks are generally m etalum inous, b u t th e felsic sam ples w ith greater than 68% S i0 2 are slig h tly peralum inous. | The M ount S tu art has a n u n u sually high-M g signature, w hich is higher th an j j m ost C ordilleran batholiths. Trace d em en ts reveal a volcanic arc granite com position w ith in the dassificatio n of P earce an d others (1984) (A nderson, 1992; P aterson e t al., 1994). R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Chemical evolution The range of ro ck types w ithin th e batholith h as been explained largely b y m eans o f fractionation. Erikson (1977) suggested th a t th e p aren t m agm a w as eith er a quartz d io rite (a dom inant rock type) o r a tw o-pyroxene gabbro (the o ld est phase). H e in terp reted th e o ld er, m ore m afic rocks as a p a re n t m agm a, w ith a com position like th e high-alum ina b a sa lt described b y Irvine an d B arragar (1971). M odeling fractional cry stallizatio n w ith m ajor elem ent chem istry dem onstrated th a t the tw o-pyroxene gabbroic p aren t w o u ld yield th e p ro p o rtio n s and com positions o f rocks observed in th e M ount S tu art b ath o lith . U nfortunately, it w ould also produce v e ry large quantities o f resid u e an d cum ulate fo r w hich there is no evidence a t the cu rren t level of [ exposure. I { A nderson (1992) a n d Paterson a n d others (1994) also m odeled fractionation in the M o u n t Stuart, in clu d in g trace elem en t and rare e arth r elem ent (REE) com positions. C hrom ium and nickel tren d s ruled o u t a b asaltic p aren t, because th e positive relationship p ro h ib ited rem oval o f olivine th a t w ould be expected w ith a m afic p arent. Fractionation of orthopyroxene, plagioclase, and dinopyroxene o r h o rn b len de w as m ore com patible w ith the d a ta . REE trends w ith negative E u anom alies indicated plagioclase fractionation for m ost o f th e rocks, b u t p o sitiv e Eu anom alies in i | the m ore m afic diorites suggested th a t these u n d erw en t plagioclase [ accum ulation. The E u anom aly reversal w as used to bracket the p a re n t ! m agm a as a diorite w ith 54-56% S i02 (Paterson e t a l., 1994). • O th er processes h av e been fou nd to contribute locally to b ath o lith com position. Mafic enclaves that rep resen t a sep arate, elem entally different m agm a show lim ited, o u tcro p scale evidence of m agm a m ixing. Several 9 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. sam ples o f tw o-m ica granodiorite from the b ath o lith have abnorm al elem en tal values (high K, F e/M g) an d elevated oxygen isotopic com positions, in d icativ e o f local assim ilation o f th e C hiw aukum schist (P aterson e t al., 1994). T hese occurrences are lim ited to the b ath o lith m argin. Structural features For the m ost p art, the M ount S tu art b ath o lith preserves m agm atic tex tu res a n d stru ctu res. These features include in tern al com positional zo n in g , m ineralogic layering, m icrocrystalline enclaves, m agm atic foliations an d lineations, am ong others (e.g., M iller an d Paterson, 1992; P aterson et al., 1994). Internal contacts betw een pulses are m ostly sharp, b u t also m ay be ; g rad atio n al (Pongsapich, 1974). D eform ational m icrostructures, su ch as i q u artz u n d u lato ry extinction o r rare kinked b iotite, an d sub -so lid us alteration of m inerals (m ostly biotite to chlorite an d plagioclase to se ria te ) are present, b u t m in o r th ro u g h o u t m uch of th e pluton. In general the rocks have ig n eo u s, hypdiom orphic textures. B roadly, igneous foliations are roughly NW striking w ith steep dips on th e n o rth ern en d o f the batholith. The southern, bulbous p o rtio n show s m arg in p arallel foliations th a t becom e stronger tow ard the p lu to n m argin. O ne exception is an area of shallow dips th at occurs near a large inclusion of C hiw aukum schist in the Pioneer C reek area. M iller and P aterso n (1992) an d P aterso n an d others (1994) described the com plex foliation an d lineation | p a ttern s th a t occur throughout th e b atholith (Figure 1-3). F oliations w ithin th e p lu to n ic rocks are defined predom inantly b y th e m afic m inerals, especially biotite and hornblende. T abular plagioclase crystals also show alig nm en t in m any cases (e.g., Pongsapich, 1974; Erikson, 1977). M argin 10 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. parallel foliations m ay be th e result of orientation by m agm a flow (E rikson, 1977), bu t th e presence of overp rin tin g solid-state foliations, parallel to igneous features, indicates th a t foliations hav e resulted h o rn a com bination o f m agm atic processes an d regional deform ation (M iller an d Paterson, 1994). A lthough th e m ajority o f the b ath o lith preserves igneous features, th ere are fo u r regions of no tab le solid-state deform ation, including th e Rock Lake shear zone (near H ighw ay 2), the Icicle R idge shear zone, the T um w ater M ountain sh ear zone, and th e Pioneer C reek area near th e p lu to n 's intersection w ith the W indy Pass th ru st (M iller and Paterson, 1992,1994; Paterson e t al., 1994) (Figure 1-3). The Pioneer Creek dom ain records stro n g shallow -dipping foliations (both m agm atic an d subsolidus) in a dom e-like p attern , w hich roughly su rro u n d s a large inclusion of the C hiw aukum schist. The other th ree zones, lying o n the NE edge o f the pluton, preserve approxim ately NW striking steeply dippin g subsolidus foliations, lineations, an d SW verg en t asym m etrical folds (M iller an d Paterson, 1992,1994; P aterson e t al., 1994). Features in these deform ed m argins indicate th a t the b ath o lith w as em placed du rin g regional SW-NE contraction, paired w ith extension in th e NW-SE direction (M iller and Paterson, 1992). C hiwaukum Schist The co u n try rock su rro u n d in g the n o rth ern tw o-thirds of the b ath o lith is the C hiw aukum schist, a pelitic m etasedim entary unit. It consists dom inantly o f graphite-bearing biotite-gam et-quartz schist, w ith lesser am ounts of schistose and gneissic am phibolite as w ell as sm all pods of calc- silicate and m arble (Tabor e t al., 1987,1993). h i places, the schist also contains porphyroblasts of staurolite, andalusite, cordierite, kyanite a n d /o r sillim anite. 11 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Rock Lake Shear Zone W indy Pass T hrust D eform ational Zone D ip of Foliation: <30° _l_ 3 0 -6 0 ° --- >60° / Icicle Ridge Shear Zone T um w ater M ountain Shear Zone Pioneer C reek Area Figure 1-3. Foliation p attern w ithin the M ount Stuart batholith. Four regions o f solid-state deform ation are also show n: Ross Lake Shear Zone, Icicle C reek, Tum w ater M ountain Shear Zone, and Pioneer C reek Area. A fter M iller and Paterson (1994). 12 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The C hiw aukum sch ist is m u ltip ly deform ed, preserving o p e n to isoclinal folds alo n g w ith stro n g foliations, lineations an d S-C fabrics, b u t rarely original b ed d in g (Tabor e t al., 1987, Paterson e t al., 1994) M etam orphism o f the C hiw aukum schist occurred in sev eral stages. O verall, m etam orphic grade increases from so u th to north, ra n g in g from u p p er greenschist facies to am phibolite facies (e.g., Plum m er, 1980; Evans an d Berti, 1986; B endixen e t al., 1991). Plum m er (1980) has in terp reted th is tren d to result from an early perio d o f B arrovian-style regional m etam orphism , follow ed b y contact m etam orphism caused by em placem ent o f th e M ount S tuart b ath o lith . This has been show n to be false b y later stu d ies. Evans an d Berti (1986) p resen ted textural evidence that co n tact m etam orphism occurred | before h ig h p ressu re regional m etam orphism . | A lthough th ere is disagreem ent over ex ten t and co n ditio ns o f the I aureole, it seem s clear th at contact m etam orphism w as a dy n am ic event I ' involving deform ation. Low p ressu re porphyroblasts, m ost n o tab ly ( andalusite, grew n ear batholith m argins during th is stage. C o rd ierite along w ith m inor sillim anite an d fibrolite also form ed, su rro u n d in g o r replacing f andalusite in th e inn er aureole (Evans and Berti, 1986), su g g estin g th at these l i m inerals crystallized slightly later. Foliations a n d lineations w ere transposed parallel to b ath o lith contacts, w ith increasing in ten sity to w ard th e p lu to n (Plum m er, 1980; P aterson e t al., 1994). R egional m etam orphism closely follow ed, o v erp rin tin g earlier m etam orphism in th e NE p a rt o f th e C hiw aukum Schist. G ro w th o f new m inerals g arn et, stau ro lite, and kyanite represents a significant increase in pressure. T his has been attrib u ted to a post-em placem ent stru c tu ra l loading event (E vans and B erti, 1986) Em placem ent o f overlying p lu to n s (m agm atic 13 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. loading) (Evans an d Berti, 1986; Brown a n d W alker, 1993) o r stacking o f th ru st sheets (as in th e N orthw est C ascades th ru st system ) (e.g., B randon an d C ow an, 1985; B randon e t al., 1988) are speculated causes o f such high p ressures. S tructural analyses o f M iller an d P aterson (1994) su g g est th at th e C hiw aukum sc h ist has u n dergone a t lea st five stages o f d u ctile deform ation. The first tw o are th e resu lt o f regional NE-SW contraction th a t p rim arily p red ated em placem ent o f th e M ount S tu a rt batholith. T he th ird deform ation, w hich form ed sub-horizontal structures, is associated w ith m ovem ent o n th e W indy Pass th ru st. Subsequently deform ation cau sed by M ount S tuart em placem ent sh ifted stru ctu res into p arallelism w ith b ath o lith contacts. The fin al phase o f deform ation relates to reverse faulting o n th e NE m argins of th e b ath o lith , representative o f post-em placem ent NE-SW shortening (P aterson & M iller, 1994; P aterson e t al., 1994). Peak m etam orphic co ndition estim ates from therm ob arom etric calculations th ro u g h o u t the C hiw aukum schist include a w id e range o f pressures and tem peratures, from 3-10 k b ar and 500-700°C. M ost | tem peratures fall w ith in n arro w er lim its o f 550-650°C. T here is a p ressu re | increase aw ay from the b ath o lith tow ard th e NE, w ith 4.0 kbar pressures I w ith in 1 km o f th e batholith an d 7.5 kbar a t a distance (B endixen e t al., 1991). I This d ata is com patible w ith o th er p ressu re estim ates (e.g., Evans a n d B erti, t 1986; M agloughlin, 1986; B row n and W alker, 1993), su g gesting a general I tren d o f m oderate pressures (3-4 kbar) n e a r the pluton th a t grade to h ig h er pressures (6-8 kbar) farther aw ay, w hich Paterson and M iller (1994) lin k to the sillim anite an d kyanite-sillim anite stab ility fields respectively. Such observations in several stu d ies corroborate Evans and B erti's kyanite-form ing 14 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. I t event. C ontact m etam orphic conditions can be estim ated in areas w h ere no la te r m etam orphism is recorded, as in the outcrops o f C hiw aukum betw een th e tw o b ath o lith lobes. Several calculations in this v icin ity yield 3-4.5 kbar a n d 550-685°C fo r th e contact aureole in the schist (Evans an d Berti, 1986; M agloughlin, 1986; Brown a n d W alker, 1993; Paterson e t al., 1994). Tim ing o f th e m ajor p hases of m etam orphism in th e C hiw aukum sch ist has been fairly w ell bracketed. The age o f the b ath o lith at 96-93 M a clearly constrains th e tim ing o f contact m etam orphism a n d the g ro w th of associated an d alu site, cordierite, and sillim anite porphyroblasts (e.g., P aterson an d M iller, 1994). Post-em placem ent regional m etam orphism began after 93 M a. K-Ar b io tite ages in the schist indicate th a t m etam orphism en d ed and th e area cooled below the blocking tem perature of biotite (300-350°C) b y 80-85 M a (Evans a n d B erti, 1986; B row n and W alker, 1993). C renulated inclusion trails w ithin an d alu site h in t o f pre-em placem ent deform ation, b u t th e tim e at w hich m etam orphism first began is poorly know n (Evans and Berti, 1986; Paterson e t al., 1994). The p a re n t rocks w ere dom inantly sedim entary, probably shale an d greyw acke d eriv ed from a volcanic arc. This origin is su p p o rted by petrochem ical stu d ies yielding high A /C N K ratios indicative of w eathering (Paterson e t al., 1994). The depositional age o f the p ro to lith , how ever, is p o o rly constrained. M etam orphism predates the M ount S tuart b ath o lith a t 96 M a, thus the p ro to lith m ust have been deposited significantly earlier, possibly d u rin g the Jurassic. No fossils rem ain to bracket the p erio d of sedim entation, b u t a Rb-Sr "scatterchron" d ate by M agloughlin (1986) h in ts th at the depositional age could be 210 ± 22 Ma. Possible correlations w ith the o th er 15 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. u n its, su ch as the 193±11 M a Tonga form ation (e.g., D uggan an d Brown, 1994), fu rth er com plicate u n d erstan d in g of the p a re n tal sedim ent age. In g a lls C om plex The co u n try rock exposed a t the so u th ern end o f th e M ount Stuart b ath o lith is th e Ingalls com plex (Pratt, 1958; Southw ick, 1974; M iller, 1985), one o f m any Jurassic ophiolites th roughout W ashington a n d O regon (e.g., D avis e t al., 1978; Vance e t al., 1980). The Ingalls com plex w as em placed onto th e C hiw aukum Schist by n o rth w ard m ovem ent on th e W indy Pass th ru st (M iller, 1985), although field evidence locally suggests SSW -directed m otion (T aylor, 1994). [ The Ingalls com plex p reserves the com m on ophiolite sequence i in clu d in g ultram afic rocks (harzburgite, lherzolite, d u n ite, an d hornblende p eridotite) w ith gabbros, diabase dikes, basalts, an d pillow basalts. C hert and argillaceous sedim ents, along w ith basaltic breccia, are also present. ; Sedim ents are particu larly com m on to the e a st of the b ath o lith (Southwick, 1974; M iller, 1977; M iller, 1985; M iller et al., 1993b). Z ircons from gabbros in th e com plex have yielded U-Pb dates of 155-156 M a (M iller, 1985; M iller et al., 1993b). R adiolarians (152-163 Ma) in chert associated w ith the Ingalls confirm th is m id- to late-Jurassic form ation age (Tabor e t al., 1987a; M iller e t al., 1993b). The Ingalls com plex is considered to h ave originated in an ocean or m arg in al b asin (Southwick, 1974; M iller, 1977; M iller 1985), an d m odified in a fractu re zone (M iller, 1985, M iller and M ogk, 1987). R egional geology suggests a sou th erly source o f the Ingalls com plex, since th ere are several o th er ophiolites to the south, b u t no p o ten tial seafloor sources in the N ason 16 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. terran e an d C oast plutonic com plex to th e n o rth (M iller, 1985; Davis e t al., 1978; V ance e t al., 1980). W in d y P a ss T hrust The W indy Pass th ru st juxtaposes th e C hiw aukum schist and th e stru c tu ra lly overlying Ingalls ophiolite. T he E-W tren d in g fau lt is ex p o sed in a ro o f p e n d a n t o f the M o u n t Stuart b ath o lith , w hich cross-cuts (and in p a rt is sh eared by) the th ru st (P aterson e t al., 1994). The fau lt chiefly has a g en tle so u th d ip , w hich is shallow er than th at o f im bricate slices p resen t in th e u p p e r p late (M iller, 1980, 1985). N orthw ard m ovem ent on the W indy P ass th ru st (M iller, 1985) occurred betw een 96 M a, the age o f m etagabbros in the u p p e r p late (W. H oppe, 1981 in M iller, 1985), and 93 M a, the age of th e M ount S tu art (Engels and C row der, 1977 up dated b y M iller, 1985), b u t the fau lt rem ain ed active d u rin g b ath o lith em placem ent (P aterson e t al., 1994). f \ 17 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Stable Isotope Systematics Stable isotopic com positions can b e useful indicators o f geologic process, su ch as fluid-rock interaction, w h en o th er chem ical tracers m ay n o t be effective. O xygen is ab u n dan t in th e E arth, b o th in silicate an d carbonate rocks, as w ell as in w a ter an d the atm osphere. Laser m icroprobe extraction techniques (Sharp, 1990) allow analysis o f sm all in dividual m ineral grains. Basic C oncepts o f Stable Isotope G eochem istry T here are three stab le isotopes o f oxygen, 160 , 1 ^ 0 an d 18o. 16o is by far the m o st abundant (99.763%), b u t 1 ^ 0 an d 1 8 0 exist in m easurable am ounts (0.0375% an d 0.1995%, respectively) (G arlick, 1969). The relative am ounts o f each isotope present in a substance can be m easured using m ass spectrom etry, then expressed as a ratio . Since 1 ^ 0 occurs in n early negligible am ounts o n Earth, an d th ere is a g reater m ass difference betw een 1 ^ 0 and 180, isotopic ratios are given as 1 8 0 /1 ^ 0 . The 8 term inology expresses isotopic com positions relative to a sta n d a rd value, so th a t com parisons can be m ade betw een sam ples. For oxygen isotopes, th e universal sta n d a rd u sed in non-oceanographic system s is SMOW, "sta n d ard m ean o cean water"(81 8 O sM O W = 0%o). D ue to the m ass differences b etw een the oxygen isotopes, there are slig h t differences in chem ical behavior. Isotope exchange reactions (in w hich th ere are no chem ical changes other th a n red istrib u tio n o f isotopes) and kinetic processes, w hich involve d ifferen t reaction rates for each isotope, can 5 (1) O stan d ard 18 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. cause th e distribution o f oxygen isotopes to vary betw een tw o phases (Hoefs, 1987). This difference in com position is called isotopic fractionation, expressed as (2) Aa-b = § a -8 b w here a and b are tw o phases. Isotopic fractionation Isotopic fractionations betw een phases are large a t low tem peratures an d decrease a t increasing tem perature. This is du e to the tem perature dependence o f the equilibrium constant, K. For an exchange reaction (after Kieffer, 1982; Hoefs, 1987; U rey, 1947) (3) cC1 + d D 2 « c C 2 + d D l w here K = w 1 2 2 1 (Dz/Di^ an d C, D = tw o phases c, d = stoichiom etric coefficients the equilibrium constant can be expressed in term s o f th e partition function, Q, of a m olecule (from statistical m echanics) (4) k = 1 S q /5 £ i ) w here Q = y e-E ,/w (Q d,/Q d.) and Ei = energy level of ith state relative to rest state k = B oltzm ann constant T = T em perature (K). In isotopic system s, the fractionation factor is considered m ore often th an the equilibrium constant, alth o u g h they are essentially the sam e. The fractionation factor, a , is defined as ( 1 8 o/, 6 o ) (5) a C -D - C ( i8o / “ o ) d ' If oxygen isotopes are random ly d istrib u ted throughout C and D, th en (6) a - K1/n w here n = num ber of atom s exchanged. 19 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. R eactions are generally w ritten w ith on ly one atom exchanged, so th at a = K. Finally, th e fractionation factor is related to the fractionation, o r difference in isotopic com position, b y th e approxim ation (7) §q — — A c-d = 10001norc .D • This approxim ation is excellent for A < 10 %o, th at is, the difference betw een A and lOOOlna is m uch less th an analytical uncertainty. The tem p eratu re dependence o f the equilibrium constant, K (and in tu rn o f the fractionation factor, a , an d m ineral fractionations, A) is the basis o f stable isotope g eotherm om etry. Stable isotopes are useful for geotherm om etry, b u t n o t for geobarom etry, because isotopic exchange reactions are p ressure independent. E xperim ents by C layton an d others (1975) conducted up to pressures o f 20 kbar have sh o w n th at any p ressu re dependence is less th an the lim it o f detection. i i I 20 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Geothermometry and Geobarometry E lem en ta l exchange therm obarom etry Several therm obarom etric calibrations are applicable to sam ples w ith in th e M o u nt S tuart b ath o lith (A nderson, 1992; Paterson e t al, 1994; A n d erso n an d Sm ith, 1995). T hese include tw o-pyroxene (e.g., K retz, 1982; L indsley, 1983), ilm enite-pyroxene (e.g., Bishop, 1980), dinopyroxene-hom blende (K retz an d Jen, 1978), plagiodase-hom blende (B lundy and H olland, 1990; H o llan d an d B lundy, 1994), a n d gam et-biotite (e.g., Ferry and Spear, 1978; G anguly and Saxena, 1984) therm om eters. The only barom eter available for this p lu to n ic assem blage is alum inum -in-hom b lende (e.g., Schm idt, 1992; H olland a n d B lundy, 1994; A nderson an d Sm ith, 1995). The details o f these m ajor elem ent m ineral chem istry geotherm om eters an d geobarom eters can be fo u n d in A nderson (1996) an d n eed n o t be discussed here. D iscussion w ill focus, in stead, o n the system atics an d theoretical basis of stable isotope th erm o m e try . G eneral fo rm o f iso to p ic therm om eters O xygen isotope system s result in fairly sim ple relationships b etw een K (or a ) an d T. Early theoretical studies b y B igeleisen and M ayer (1947) found th at, a t h ig h tem peratures, a p lo t of In a vs. 1 /T ^ should b e a line th a t passes th ro u g h th e origin. L ater calculations o f K ieffer (1982) clarified the lim its of th is relationship, show ing th a t below ~700°C it departs slightly from linearity an d is less th an the K~ l/T ^ trend. B ottinga an d Javoy (1973) expressed the relatio n sh ip w ith an eq u atio n o f the g en eral form , 21 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. A vin6 (8) Ax.y slO O O ln ^y = ^ - + B w here A = fractionation coefficient for n on -h y d ro u s m inerals B = correction factor fo r hydrous p h ases T = tem perature (K) x, y = m ineral phases fri th e case o f h y drous m inerals, a p lo t of lOOOlna versus 1 /T 2 has an in tercep t g iv en b y the correction factor B. T hus, in o rd er to u n d erstan d isotope fractio n atio n s, eith er to determ ine tem peratures based o n m easu red fractionations o r to p redict fractionations a t a given tem p eratu re, the fractio n atio n coefficients, A and B, m ust be know n for a w id e variety of m inerals. D eterm ination of these coefficients h as been th e g o al o f m any ex p erim en tal, theoretical, and em pirical stud ies. U se o f m ineral pairs w ith larg e fractionation factors en su res th at an aly tical erro r incorporated in th e A-value w ill have little effect. By d ifferen tiatin g equation (8) w ith respect to tem perature, 3A -2 A 31 T3 31 1 31 T3 S ° ' 3A -2A *' 3 A X A ' W hen A is sm all, th ere is a large change in tem perature w ith fractionation; how ever, if A is large, tem perature changes are slight for changes (or errors) in fractionation. For th is reason, tem peratures calculated from m ineral p a irs w ith large A values are the m ost reliable (B ottinga and Javoy, 1975). Isotopic equilibrium is req u ired in o rd er to apply isotopic therm om eters. In o rd er to evaluate the degree o f eq u ilibriu m in a n atu ral system o f m inerals, C layton and E pstein (1967) and D eines (1977) have su g g ested th e use o f m ineral trip lets, rather th an pairs, to calculate three tem p eratu res. If the calculated tem peratures are in reasonable agreem ent, the m in erals can be considered an isotopic equilibrium assem blage. 22 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. C alibration o f Iso to p ic Therm om eters Empirical Calibrations The m o st com m only u sed em pirical calibration is th a t o f B ottinga an d Javoy (1975). T hey d eriv ed fractionation coefficients based o n the n atu rally occurring fractionations betw een m inerals from 73 previously analyzed sam ples th a t w ere evaluated to represent isotopic equilibrium , based o n th e agreem ent o f tem peratu res calculated from m ultiple m ineral pairs. The calibration provides fractionation coefficients for both h yd ro u s and non - h y d ro u s phases (quartz, plagioclase, pyroxene, olivine, g arn et, am phibole, b io tite, m uscovite, ilm enite, an d m agnetite), w hich are g iv en relative to plagioclase w ith An60 (Table 3-A). This approach was taken because th e early experim ents only d ealt w ith single or a few m inerals, and m any w ere in con sisten t w h en com bined. In general, the m odel gives tem peratures low er th an anticipated for m o st rocks, indicating th a t exchange m ay continue at sub-solidus tem peratures an d below p eak m etam orphic conditions (B ottinga and Javoy, 1975). Theoretical Calibrations Kieffer (1982) developed a theoretical m odel including fractionation relationships for thirteen m inerals, calculated using concepts of statistical m echanics. The sequence o f m ineral fractionations agrees w ith that given b y T aylor and E pstein (1962), an d is also com patible w ith experim entally determ ined coefficients. Z heng (1993a, b) has used the m odified increm ent m ethod to calculate fractionation coefficients for a w ide ran ge of hyd rou s and anh y d ro us silicates. 23 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Experimental Calibrations Isotopic exchange reactions betw een silicate phases proceed too slow ly to reproduce in a laboratory, so experim ents h av e been conducted u sin g w ater (M atsuhisa e t al., 1979; M atthew s e t al., 1983) o r cald te (C layton e t al., 1989; Chiba e t a l., 1989) as m edia fo r reaction, often a t high p ressures (e.g.. C layton e t al., 1975; M atthew s e t al., 1983; M atsuhisa e t al., 1978). Two phases o f know n isotopic com position exchange oxygen u n til equilibrium is reached, th en eq u ilib riu m fractionations can be m easured over a range of tem peratures to determ ine th e fractionation coefficients. Sim ple ad d itio n or subtraction o f m ineral-m edium fractionation coefficients y ield s those fo r silicate m in eral p airs. The tw o series of experim ents produced nearly eq ual results, b u t calcite-based experim entally calibrated therm om eters ten d to give m ore geologically reasonable tem peratures, indicating a problem w ith the q u artz- w ater fractionation. W ater-based experim ents suffer from b o th the in stab ility of m any m inerals w ith w ater an d the com plex fractionation m echanism s of aqueous system s (C layton e t al., 1989). U sing albite-m ineral fractionations rather th an quartz-m ineral, those phases only stud ied by w ater-based reactions w ere incorporated in to the calcite-m ineral fractionation schem e (M atthew s, 1994). E xperim entally determ ined fractionation coefficients are only k n o w n for tem peratures from 600-1300°C, so extrapolation of the calcite-based calibration m ay introduce e rro r (Clayton e t al., 1989). The close agreem ent w ith low er tem perature w ater-based experim ents indicates th at, although 24 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. in ten d ed fo r tem peratures > 500°C, extrapolation d o w n to 300°C sh o u ld not create significant erro r (M atthew s, 1994). C om positional C onsiderations F ractionations betw een a silicate m ineral an d an o th er p h ase d o not d ep en d o n com positional variations o f th e silicate, u n less th e tetrah ed ral cations are involved. Plagioclase fractionations d ep en d on %An, b u t those of alkali feldspars are independent of N a-K content (O 'N eil an d T aylor, 1967). The difference betw een fractionation coefficients o f albite and an o rth ite is sizeable, ~l%o (Kieffer, 1982; M atthew s e t al., 1983; C hiba e t al., 1989). O ne can solve for interm ediate com positions, u sin g the "R eciprocal S u bstitution M odel", theoretically derived b y G anguly (1982). B ased o n the [ coupled su b stitu tio n (NaSi<=>CaAl) involved in plagioclase solid-solution, i 1 G anguly show ed th a t in feldspars, | (10) Apl-min = A-Ab-min ~ ^AnC^Ab— An) I o r, | (11) Apl— min — ■ ^•A n-m in ~ 0-~^AnX^Ab— An) r w here p i = plagioclase w ith com position XAn i m in - an o th er m ineral p h ase. * This relationship is critical to the Bottinga & Javoy (1975) calibration, because fractionation coefficients are given in term s o f feldspar w ith A n = 60%. For th e m inerals p resen t in rocks o f th e M ount S tu art b ath o lith , the em pirical calibration of B ottinga & Javoy (1975) is m ost w idely applicable, 1 accounting for b o th hydrous an d non-hydrous phases. The experim ental ; calibrations (C hiba e t al., 1989; M atthew s, 1994) are u seful for therm om etry involving q u artz, plagioclase, pyroxene, a n d occasionally garnet. 25 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Methods Sam ple C ollection S am ple locations w ere chosen to p ro v id e a w ide d istrib utio n across the batholith. A ll rocks collected w ere as fresh as possible; strongly w eathered sam ples w ere avoided. E xposure in the M o u n t Stuart area is fairly good, especially in th e so u th -eastern lobe and a t th e higher elevations. Sam pling took place over four field seasons. Rocks from b o th lobes of the b ath o lith , as w ell as th e nearby Beckler Peak stock, C hiw aukum schist, and Ingalls ophiolite com plex w ere collected by L aw ford A nderson d u rin g the sum m ers o f 1990,1991, a n d 1993. Subsequent sam ples w ere gathered p red o m in an tly from th e eastern bath o lith lobe and th e C hiw aukum schist in 1995 b y th e author. In ad d itio n , Bob M iller and Scott P aterson contributed nine sam ples to the stu d y . Previously p a rt o f structural stu d ies, these w ere included to investigate th e relationship betw een isotopic com position and deform ation w ith in th e M ount S tu a rt b atholith. | Sam ple P reparation | A representative p o rtio n of each sam p le was coarsely crushed u sing a | large ja w crusher. The fine fraction of cru sh ed rock w as th en sieved off and | d iscarded because th e steel plates of the jaw crusher m ay contam inate the sam ple. T he rem aining cru sh ed rock w as p u t through a sm aller jaw crusher to attain a finely crushed sam ple. The sm aller crusher is equipped w ith tu n g sten carbide plates, so there is no risk o f contam ination from the 26 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. m achinery. Each jaw crusher w as carefully cleaned before an d after each sam ple to avoid m ixing o f rock fragm ents. Whole Rock Sam ples for w hole rock isotopic analysis an d X-Ray Fluorescence (XRF) m ust b e pow dered. A representative am ount o f th e finely cru sh ed sam ple w as reduced to p o w d er using a tu n g sten carbide ball m ill. Mineral Separates h i m ost cases, th e fine cru sh w as n o t sufficiently sm all fo r separating m inerals. Sam ples w ere fu rth er cru sh ed to m onom ineralic size (or slightly sm aller) w ith a ceram ic m ortar a n d pestle, w hich w as carefully cleaned before and a fter each sam ple. In d iv id u al m inerals w ere hand-picked w ith tw eezers u n d er a m icroscope. This w as preferable to m ineral separation b y heavy liquids, because only a few m g o f m aterial are needed in laser analysis, w hile heavy liq u id sep aratio n requires larg e quantities o f sam ple. T hus, hand- picking m inerals w as determ ined to be the cleanest an d m ost efficient m eth o d . Laser Technique O xygen isotopic com positions of b o th w hole rock and m ineral separates w ere analyzed using a laser m icroprobe, m odeled after Sharp (1990), at the U niversity of Southern C alifornia. This system uses a CO 2 laser along w ith brom ine pentafluo rid e (BrFs) reagent to b reak the bonds w ith in m inerals and release oxygen m olecules in a vacuum line (Figure 4-1). The 27 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. METAL GLASS Cold Waste finger trap Pressure Diffusion gauge pump Mercury pump Torougjh pump Pressure gauge To mass spectrometer To rough pump BrFs Reagent Sample Single Double Double Single Carbon canister Kel-Fs chamber U-trap U-trap | U-trap U-trap converter Cold finger Figure 4-1. Schematic diagram of laser extraction line. Metal and glass tubing connects all parts, so the entire line is kept at high vacuum w hen collecting sam ples. ^ indicates location of valves. D otted rectangles represent dew ars of liquid nitrogen. The laser (not shown) aligns above sam ple cham ber to heat sam ples in the presence of BrFs. Reagent is rem oved in m etal end of line, before transfering sam ple gas to the glass end, w here oxygen is isolated and converted to CO2 . Sample size is m easured in the cold finger, imm ediately before sending sam ple gas into the m ass spectrom eter. oxygen is then converted to carbon dioxide, w hich can be analyzed b y a VG P rism m ass spectrom eter th a t is connected directly to th e laser extraction line. Sample Preparation and Loading Sam ples (~ 0.8 - 2.2 mg) w ere loaded in a nickel sam ple h o ld er, w hich can hold u p to 30 stan d ard s or sam ples in in d iv id u al divots. The sam ple h o ld er w as placed in th e sam ple cham ber of the laser extraction line, w hich w as th en evacuated. O pening o r v en tin g the sam ple cham ber allow s atm ospheric w ater in to th e vented segm ent of th e line. Several b rief p retreatm ents of BrFs are required to rem ove th is w ater, h i ad d itio n , sam ples m u st be p retreated o v ern ig h t w ith ~ 2 p si o f BrFs to elim inate any b ackground reaction th a t m ight in terfere w ith an alysis. Because m ultip le sam ples are loaded in a single sam ple cham ber, th ere is p o ten tial for p artial reactio n o f som e m inerals. M ost m inerals are n o t affected by overnight p retreatm en t; how ever, feldspars are very sensitive to BrFs, an d tend to react read ily d u rin g stan d ard p retreatm en t techniques. Partially reacted feldspars y ield erratic, irrep ro d u d b le resu lts, indicating th a t pretreatm ent causes the oxygen to be fractionated. To lim it reaction d u rin g p retreatm ent, sam ple h o ld ers loaded w ith feldspars w ere given a very lig h t overnight p retreatm en t of only 0.5 psi BrFs. In addition, placin g feldspar sam ples in an o v en a t 200-300°C for several h o u rs before lo ad in g them into the sam ple cham ber im proved resu lts. Plagioclase responded favorably to these m odifications, y e t o rth o d ase d ata d id not. For this reason only plagioclase feldspars w ere analyzed. 29 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The Extraction Line Oxygen w as released b y h eatin g w ith a CO2 laser in the presence of BrFs. Since sam ples w ere lo ad ed in a com m on cham ber, several blanks w ere ru n before begin n in g laser reactio n o f any sam ples. A blank w as m easured by releasin g an aliq u o t of reag en t in to the sam ple cham ber w ith ou t laser heatin g, and th e resu ltan t g as w as collected as a sam ple. This assures th at background reactio n of th e m inerals is sufficiently low as not to contribute to th e oxygen yield . Some m in erals m ay react w ith BrFs a t room tem perature, contributing u n w an ted oxygen to a sam ple. P retreatm en t generally elim inates this problem , b u t a t least one blank is ru n a t the sta rt o f each analysis day for verification. W hen b lan k yields w ere acceptably low (generally less th a n 10 m torr, slig h tly higher fo r feldspars), sam ples w ere analyzed. Reagent w as adm itted to th e sam ple cham ber an d non-condensable gasses w ere pum ped aw ay from each aliquot o f reag en t before in troducing it to th e sam ple. The sam ple w as th e n heated w ith th e laser u n til en tirely reacted, a 1 to 3 m inute process. M ost m inerals excep t q uartz left som e residue in the sam ple h o ld er. The oxygen w as cryogenically iso lated b y Breezing the resu ltan t gas w ith liquid n itro g e n . O xygen a n d other non-condensable gases w ere transferred to the carbon converter, w here a h o t carbon ro d changes oxygen to CO2 . C arbon dioxide freezes a t liquid nitrogen tem p eratu res (-96°C), sep aratin g it from rem aining non-condensable gasses. F ollow ing approxim ation CO2 yield w ith a th erm isto r gauge, th e gas w e n t directly to the m ass spectrom eter, w hich m easured the isotopic com position of carbon an d oxygen. 30 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. C onventional Silicate E xtraction Technique A ll m ineral separates w ere analyzed u sin g th e laser, as w ere m any w hole ro ck specim ens. Forty w hole rock sam ples from the M o u n t Stuart b atholith, a n d 12 sam ples from th e C hiw aukum S chist w ere analyzed w ith a conventional silicate extraction line a t th e U n iv ersity o f S o uthern C alifornia, using externally heated N i reaction cham bers (e.g., C layton an d M ayeda, 1963). The sam ples w ere analyzed by Y eugin C hang, in a p ilo t stu d y for D rs. A nderson a n d M orrison. M ost values m easured o n the conventional line w ere tak e n to be accurate, unless discrepancies w ith o th er isotopic d a ta gave cause for doubt. S tandards for conventionally determ ined d ata a re acceptable; how ever, the n atu re o f th e individual cham bers in w hich each sam ple is reacted m akes stan d ard izatio n difficult (e.g., Valley e t al., 1995). Five of these sam ples w ere d uplicated o n the laser lin e w ith sufficient p recisio n to consider th e conventional d ata acceptable. Four w hole rock sam ples could n o t be reasonably duplicated w ith the laser technique a n d h a d values th a t w ere unlikely o r im possible based on m ineral separate d ata. In these cases, the conventionally determ ined 8lsO values w ere d iscard ed in favor o f laser analyses (Table 4-A). Laboratory Standards E ach day of laser analysis included the m easurem ent o f several U niversity o f W isconsin Gore M ountain G arnet #2 (UWG-2) stan d ard s w ith an accepted isotopic com position of 5 . 7 4 ± 0 .1 5 % o (V alley et al., 1 9 9 5 ) . UW G-2 stan d ard s w ere analyzed over 26 days, 20 of w hich included sam ple analysis (Table 4-B). In all, 136 stan d ard m easurem ents w ere m ade, av eraging 5.48 ± 3 1 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 0 .1 0 % o , w hich is 0 . 2 6 %o low er than expected for U W G - 2 (Figure 4 - 2 ) . The sam ples are precise a t the 0 .1 % > level, slightly b etter th an predicted, b u t are o n ly accurate to 0 .2 % o . It w ould be possible to im prove the accuracy by correctin g values, b ased on the difference betw een th e daily average and calib rated 81 8 0 com positions of U W G - 2 . Such a correction causes problem s w ith m any reversals o f m ineral 8180 values. D uplicates of stan d ard s ru n o n d ifferen t days u su ally produces resu lts v ery sim ilar to each o ther, b u t in m an y cases these diverge w hen a correction is applied. F or these reasons, no correction of values has been p u rsued . P t t i V T able 4-A. C om parison of w hole rock 5ls O values m easured using conventional and laser techniques. V alues th at have been stru ck are incom patible w ith m ineral separate d ata. Sample Conventional Line Laser Line Average 8"0 427 10.113 7.832 8.274 8.675 8.260 93SE-06 g ^ 10.607 10.607 93SW -04 8.553 8.526 8.540 93SW -06 fi.013 9.068 9.068 93SW -14 43*425 8.637 8.637 IC -10 8.666 7.100 6.060 7.275 KSE91-03 11.616 11.818 11.304 11.579 K SE91-09 10.124 9.399 9.626 9.716 K SW 91-06a 9.172 8.268 8.726 8.846 8.753 32 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. X R F and M icroprobe D ata W hole rock elem ental abundances w ere m easured in the X -ray fluorescence laboratory a t th e U niversity o f Southern C alifornia b y Law ford A nderson. M ineral com positions w ere also m easured b y A nderson, using an electron m icroprobe a t Texas A&M U niversity. T able 4-B. Daily UW G-2 Standard A nalyses. A verage values (% o) are given for each analysis d ay . SD = one stan d ard d eviation from th e m ean; n = num ber of sta n d a rd analyses. Italics indicate Date Analysis Day Average U S D n 4/21/95 1 5.25 0.20 3 217196 2 5.20 0.16 5 1/9/96 3 5.06 0.21 4 2/13/96 4 5.59 0.25 5 2/13/96 4 5.02 0.08 4 2/15/96 5 5.55 0.11 5 2/16/96 6 5.31 0.23 7 2/19/96 7 5.36 0.21 8 2/21/96 8 5.25 0.07 4 2/23/96 9 5.34 0.07 4 5/10/96 10 5.88 0.17 4 7/12/96 11 5.87 0.36 7 7/17/96 12 5.62 0.22 7 7/20/96 13 5.90 0.15 6 7/29/96 14 6.00 0.28 7 7/31/96 15 5.94 0.21 5 8/8/96 16 5.69 0.18 4 8/12/96 17 4.82 0.21 6 8/15/96 18 5.68 0.36 6 8/22/96 19 5.38 0.27 5 8/29/96 20 5.70 0.13 4 9/3/96 21 5.50 0.17 4 9/9/96 22 5.67 0.13 4 9/23/96 23 4.83 0.56 4 9/26/96 24 5.58 0.17 5 10/1/96 25 5.58 0.18 5 10/3/% 26 5.51 0.24 4 Average = 5.48 ± 0.10 Total number of days = 26 Total number of analyses = 136 33 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6 .4 0 6.20 £ 6 0 0 0 5 .8 0 2 ~ 5 .6 0 1 5 .4 0 § 5 .2 0 C O 5 .0 0 4 .8 0 4 .6 0 0 5 1 0 1 5 2 0 2 5 3 0 analysis day Figure 4-2. Daily averages of U W G - 2 standard analyses. Error bars show one standard deviation from the m ean. The expected range of values (solid and dashed lines) is 0.25% o higher than the m easured value range (heavy solid and dashed lines). £ i expected 5.74 ±0.15%, measured 5.48 ±0.10% . Analysis of Isotopic and Chemical Data M o u n t S tu a rt B atholith Whole Rock The batholith ran ges in w hole rock oxygen isotopic com position from 7.16 to 1 5 .5 2 % o (Table 5 - A ). O verall, g ranitic rocks, in clu d in g to n alite, q u artz diorite, a n d granodiorite, separate into several groups b ased on isotopic com position, b u t m ost h av e high 8 l ® 0 values from 7 . 8 to 1 0 .2 % o . T hose w ith the h ig h est 8 ^ 0 values (> 1 0 .2 % > ) are considered to req u ire som e u n u su a l process to elevate values (Taylor, 1 9 6 8 ) . O n the basis o f T aylor's classification, the rocks o f the M ount S tu art batholith fall in the interm ediate (8 sam ples), high (33 sam ples), and h ig h est (22 sam ples) g ro up s. A m ajority o f these (5 2 % ) are w ith in the ran g e determ ined to be norm al fo r plutonic com plexes. The d istrib u tio n of w hole rock 8 * 8 0 values is bim odal, w ith a p eak a t 8 . 5 %o and a second peak a t 1 1 .0 % o (Figure 5 -la). The highest 8 ^ 0 values (>10.2%o) are located n e ar the m argins of the plu ton , an d interm ediate values lie to w ard the core (Figure 5-2). H ig h est values are particularly d o m inant on the NE side o f th e batholith, w h ile the largely un zo n ed w estern lobe only includes interm ediate an d high com p ositio n s. S patially, the p a tte rn o f w hole rock isotopic v alu es som ew hat corresponds to the d istrib u tio n of lithologies and S i0 2 zonation. L ow SiC)2 m afic p h ases occur in th e core, associated w ith interm ediate v alu es, w hile h ig h SiC>2 felsic u n its are closer to p lu to n edges w here the h ig h est values are located. Since S i0 2 content correlates w ith igneous lithology, a 35 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. T able 5-A. W hole rock 8l® 0, silica content, lithology, an d m ineralogy o f the M ount S tu art batholith an d C hiw aukum schist. A nalysis technique is n o ted as laser, conventional (C onv), o r an average o f values determ ined by b o th m ethods (Avg). M inerals include all = allanite, b i = bio tite, ch = chlorite, cm = cum m ingtonite, cpx = dinopyroxene, ep = epidote, fb = fibrolite, gnt = g arnet, opx = orthopyroxene, q t = q u artz, sp = spinel?, srp = serpentine, ti = titan ite/sp h en e, zl = zeolite, * = secondary m ineral, ° = xenocrystic m ineral. M ount S tu art B atholith S am ple T ech n iq u e W R 5 " 0 W t % SiOx Rock T ype M ineralogy 427 L aser 8.3 64.2 tonalite qt, pi, hb. bi. ch* 329b Conv 10.6 68.6 eranodiorite 93SE-02 Laser 11.0 57.0 q uartz diorite bi, hb 93SE-04a Laser 11.0 67.7 tonalite bi, hb, ti* 93SE-04b Laser 10.6 67.2 tonalite bi, hb, ti* 93SE-06 Laser 10.6 68.1 tonalite qt, pi, ch*, bi, hb, zl* 93SE-07 Conv 11.2 69.0 tonalite bi, hb, all, ti* 93SE-09 Laser 7.2 62.0 au artz diorite bi, hb, ch*. ti*. ep*, sp*, qt, pi 93SE-10 Laser 7.9 61.8 au artz diorite bi, hb, opx, ti, am*, ti*, ep* 93SE-12a Laser 9.6 64.1 tonalite bi, hb, ch*, ti*, ep*, qt, pi 93SE-13 Laser 10.0 64.9 tonalite bi, hb, ch*, ti*, ep* 93SE-14 Conv 10.6 67.4 tonalite, altered b i,ch * .ti* ,ep * 93SE-15 Avr 11.1 68.6 tonalite ent,bi,ch*,ep* 93SE-17 Laser 10.9 71.0 biotite-tonalite bi. all 93SW-01 Laser 8.9 64.6 tonalite bi,hb 93SW-02 Avr 7.8 64.7 tonalite bi, hb. ti* 93SW-04 Avr 8.5 64.3 tonalite-altered bi, hb, ep*. ti*. ch*. pi, qt 93SW-06 Laser 9.1 63.0 tonalite bi, hb, ti*. ch*. pi, q t 93SW-08 Conv 9.2 70.2 tonalite bi, hb, ti*xh*, p i 93SW-10 Laser 8.7 65.5 tonalite bi, hb, ti*, ch*, pi. q t 93SW-12 Laser 9.7 67.3 tonalite bi, hb, ti*. ch*, pi 93SW-13 Laser 8.3 66.8 quartz diorite bi, hb, pi, ti*, ch* 93SW-14 Laser 8.4 65.2 tonalite bi, hb, pi, ti*, ch*, q t CR304-1 Conv 8.1 64.9 tonalite qt, pi, hb, bi CSP-01 Conv 11.3 66.9 tonalite qt, pi, hb, bi, ti* CSP-02 Conv 12.5 73.2 tonalite qt,pl,bi,m u,ep* CSP-06b Conv 12.7 65.4 2-mica tonalite ent, bi, mu, qt, pi, ch*, fb° CSP-16-1 Conv 16.8 72.1 peematite entm u,bi,ep* EL31-1 Laser 8.4 65.0 eranodiorite EL40-1 Laser 8.6 65.0 eranodiorite IC-01 Laser 7.7 59.6 quartz diorite hb, opx, bi, sp IC-03 Laser 8.6 54.3 diorite opx>cpx>hb IC-04 Laser 7.2 63.4 tonalite hb, bi, ti IC-10 Avr 7.3 53.7 diorite opx>hb>cpx, qt, pi KSE91-01 Conv 9.8 65.3 tonalite hb,bi KSE91-02a Conv 10.5 69.9 2-mica tonalite bi, ent, mu, qt, pi, ch* KSE91-02b Laser 8.3 61.1 mafic tonalite bi, hb, qt, pi, mu? KSE91-03 Avr 11.6 72.4 eranodiorite bi.qt, pl,ch* KSE91-04 Laser 10.6 70.5 eranodiorite bi, hbl, qt, pi, ch* KSE91-09 Avr 9.716 6 5 3 tonalite bi, hb, ti, (ep clot), qt, pi, ch* KSE91-10 Laser 8.045 52.4 diorite opx,cpx,hb,bi 36 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Table 5-A. (continued) M ount S tu art B atholith S a m p le T echnique W R d " 0 W t% SiOz Rock T ype M in era lo z v KSE91-11 Conv 9.8 70.7 tonalite bi, hb, ep , sp*, a t, pi, ch* KSE91-13 Conv 10.8 68.4 tonalite bi, hb, a t pi, ch* KSE91-15 Conv 12.1 63.3 tonalite hb, bi, opx. qt, pi, ch* KSE91-19 Laser 7.6 57.8 quartz diorite opx>cpx, hb, bi, qt, pi, ep* KSE91-22a Conv 12.6 6 7 5 ent-hbl tonalite Rnt, hb, bi, qt, pi K SE91-27a Conv 8.3 56.6 diorite opjocpx, hb, bi, at, pi KSE91-27b Conv 8.8 58.3 diorite a ltp x ,h b ,b i,q t,p l KSE91-27c Conv 10.4 58.9 bleached diorite hb,bi, cm*, srp*, ep* KSE91-30a Conv 8.5 56.1 diorite hb, bi, cm , qt, pL ch* KSE91-30c Conv 8.5 53.4 diorite hb,cm ,bi KSE91-33a Laser 7.1 51.8 diorite hb, cm , bi, qt, pi KSE91-42 Laser 8.3 57.9 quartz diorite hbl, opx, bi, qt, pi, ch* KSE91-43 Laser 8.2 65.7 tonalite hbl, bi, cpx KSE91-44 Conv 8.5 64.2 tonalite h b .b i KSW 91-02 Laser 7.7 66.7 tonalite bi. h b .sp , opx KSW 91-03 Conv 8.5 68.4 tonalite bi, hb, ti*, qt, pi KSW 91-05a Conv 9.3 52.8 diorite bi, hb, qt, pi KSW 91-06a Avr 8.8 63.3 tonalite bi, hb, qt, pi KSW 91-08 Conv 9.2 60.8 mafic tonalite bi, hb, qt, pi M S-181 Conv 9.8 64.8 tonalite hb, bi, ti. ep* # o f samples 60.0 average 9 J > 63.9 std dev 1.7 5.4 median 9.0 64.9 maximum 16.8 73.2 minimum 7.1 51.8 C hiw aukum Schist CH 91-01 Conv 18.0 65.7 pelitic schist CH 91-02 Conv 14.2 71.6 pelitic schist CH 91-05 Conv 13.1 59.5 pelitic schist KSE91-22C Conv 15.5 67.3 pelitic schist KSE91-40a Conv 13.7 68.3 pelitic schist LW-28 Conv 15.0 64.5 pelitic schist LW-53 Conv 18.8 63.6 pelitic schist M S-30A Conv 11.5 46.4 pelitic schist SP-10 Conv 12.3 54.5 pelitic schist SP-18 Conv 14.4 66.0 pelitic schist SPC H 91-02a Conv 12.2 63.0 pelitic schist SPCH 91-03 Conv 11.7 69.2 pelitic schist SPC H 91-04 Conv 12.4 68.7 pelitic schist SPC H 91-05b Conv 13.9 65.0 pelitic schist # o f samples 14.0 average 14.0 63.8 std dev 2.2 6.6 median 13.8 65.4 maximum 18.8 71.6 minimum 11.5 46.4 37 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. (a) Whole Rock 5ls O «io- • diai 14' (d) Hornblende 5^®0 * 1 l l I I l l ■ 1 - l l 2 - 7 8 9 10 11 12 13 14 15 16 Whole Rock Sls O (b) Quartz 8l®0 4 5 6 7 8 9 10 11 12 13 14 15 16 M ineral 5^®0 (e) Biotite 5180 14- 14- 12- 1 2- JS10' J10- "a. a. e 8 i s - A £ 6- o ■ ■ ■ J & o 1 • 4- 1 III 1 ♦ 4 - llll 2- o- ., , , J in llU , I , i 2 - 0 l* |l|l, ll. 4 5 6 7 8 9 10 11 12 13 14 15 16 M ineral 8^®0 (c) Plagioclase 8^®0 4 5 6 7 8 9 10 11 12 13 14 15 16 M ineral 5^®0 (f) Chlorite 6180 a a l O * 4 5 6 7 8 9 10 11 12 13 14 15 16 M ineral 8**0 4 5 6 7 8 9 10 11 12 13 14 15 16 M ineral 5*®0 Figure 5-1- H istogram s o f w hole rock and m in eral values. (a) w hole rock (b) q u artz (c) plagioclase (d) hornblende (e) biotite (f) chlorite. The distrib u tio n of whole rock valu es is bim odal. Biotite an d chlorite have distributions th at are very sim ilar to each other. 38 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Whole Rock 8 18 O < % o ) |H ig hest818O(>10.2 % o) |H igh 8 l80 (9 .0 -1 0 .2 % > ) pNorm al 8ls O (< 9.0 % o) 25 km u > v O Figure 5-2. M ap of M ount Stuart batholith show ing whole rock 8*80 values. Isotopic com positions have been contoured to reveal zonation w ith the highest 8 ^ 0 values around the m argins, and norm al values in the core. Plutonic sam ples are noted by bold text, w hile those from the Chiwaukum schist are italicized. relationship betw een rock type and w hole rock isotopic com position is also observed. A cross th e en tire sam ple suite, w hole rock 8 l80 values increase a t higher silica contents, b u t this correlation is only m oderately significant (r^ = 0.306, see A ppendix A for detailed correlation param eters) (Figure 5-3). C onsidering each lithology individually, only granodiorite records a relationship betw een w hole rock and SiC>2, w ith a stro n g positive correlation (r^ = 0.947). Generally, changes in isotopic com position for a given rock ty p e are n o t associated w ith silica content. V ariations in isotopic com position are p artially th e resu lt of changes in rock type an d the corresponding m ineralogical shifts, rath er th an silica content alone. Slightly elevated 8^®0 values could be generated u n d er 18.0 16.0 14.0 I 12.0 9 1 ■ 3 £ £ 1 0 . 0 8.0 6.0 50.0 55.0 60.0 65.0 70.0 75.0 Wt % SiOz Figure 5-3. W hole rock S^SO vs. silica content grouped b y rock type. The overall suite has a positive trend (dashed oval) b u t granodiorite (shaded) is the only individual rock type w ith a significant correlation. 40 W hole Rock 8l s O vs. S i0 2 O diorite □ tonalite A pegm atite in schist O quartz diorite + granodiorite I m afic tonalite , 2-mica o r gnt tonalite [ altered R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. ! equilibrium conditions by concentrating h ig h m inerals, p articu larly q u artz and p lag io d ase. Thus, m ore felsic rock types are o n average slig h tly elevated relativ e to m afic phases. The coefficient of determ ination fo r w hole rock 5180 v ersu s S i0 2 (r^ = 0.306) indicates th a t 30% o f th e isotopic v ariatio n can b e accounted in this w ay, b u t the full ran g e of isotopic com positions observed in th e M ount S tuart batholith can n o t be explained by lithologic v a ria tio n . Elemental Chemistry M ajor elem en t chem ical analysis o f th e batholith show s som e relatio n to isotopic com position (Figure 5-4). M ost elem ents, notably Fe, M g, an d M n, 0 0 0 % 1 0 1 " 3 Elemental Chemistry 20 . Mctaluminous 15 Chiwaukum Margin Eastern-high 8 ^ 0 ■Western 10 Penlum inous Eastern-normal 5 1 5 2.0 2 5 1.0 A/CNK Figure 5-4. Elem ental chem istry of the M ount Stuart batholith and C hiw aukum schist. W hole rock an d A /C N K values are elevated in the schist and batholith m argins relative to the eastern an d w estern lobes o f the pluton. 41 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. have no relationship to w hole rock 5 l® 0 . M olecular A l/(C a+ N a+ K ), o r A /C N K , ratio s are greater a t high w hole rock isotopic com positions (especially n e a r m argins), sh iftin g from m etalum inous to peralu m in o u s in som e sam ples. This indicates th at th e process th at cau sed elevated w hole rock 5 I 8O valu es around th e edges o f th e batholith also som ew hat altered elem ental abundances, o r th a t th e effected rocks preferentially h ad high A /C N K ratio s. Because resettin g involved only selected elem ents, chem ical an d evolutionary m odels fo r th e b ath o lith (e.g., Pongsapich, 1974; E rikson, 1977; A nderson 1992; P aterson e t al., 1994) should still h o ld , particu larly those based o n ferrom agnesian, ra re earth o r trace elem ents. Mineral Separates i Major Minerals h i o rd e r to u n d erstan d th e m echanism th at has created u n u sually h ig h oxygen iso to p ic com positions in the M ount S tuart b ath o lith , ind iv id u al m inerals (Table 5-B) w ere analyzed. Isotopic com positions o f the m inerals form a n orm al equilibrium fractionations sequence (e.g., T aylor, 1967; K ieffer, 1982) (Figure 5-5). Q uartz h as the highest Sl^O values, follow ed in descending o rd er by p lag io d ase, hornblende, and b iotite. W hole rock com positions are betw een th ose of q u artz an d p lagiodase, u sually quite d o se to p lag io d ase values. In sev eral instances, values d eviate from the expected o rd er, rev ersin g m ineral p ositions w ith in the sequence. T here are three quartz-w hole rock reversals, tw o hom blende-biotite reversals, and one qu artz-p lag io d ase reversal in the sam ples studied. 42 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. T able 5-B. M ineral isotopic data for the M ount Stuart batholith and Chiw aukum Schist. W hole rock values are included for com parison. All data are given in term s of 8 ^ 0 (%>). Q tz = quartz, Plag = plagiodase, H bl = hornblende, Biot = biotite, Px = pyroxene, Muse = m uscovite, G nt = garnet, C hit = chlorite, Tour = tourm aline, Ep = epidote, Fibro = fibrolite (xenocrystic). Sample Whole Major Minerals Minor Minerals Secondary Minerals Rock Qtz Plag Hbl Biot Px Muse Gnt Chit Tour Ep Fibro MOUNT STUART BATHOLIT1 i t o f samples 61 29 33 26 32 4 3 4 15 1 1 1 average 9.501 10.829 8.828 6.679 6.173 6.887 9.126 8.986 6.206 6.673 11.616 std dev 1.735 1.482 1.008 0.626 1.227 0.311 3.494 1.916 1.563 median 9.068 10.866 8.493 6.565 5.968 6.860 10.685 9.669 5.944 6.673 11.616 427 8.260 9.684 8.192 6.864 5.851 5.583 93SE-06 10.607 10.866 9.580 7.253 7.076 6.263 93SE-09 7.157 9.221 7.212 6.323 5.255 93SE-12a 9.648 11.921 8.668 7.833 5.629 5.476 93SW-04 8.540 10.197 7.938 6.550 4.029 4.278 93SW-06 9.068 11.076 9.205 6.611 5.968 5.245 93SW-10 8.682 10.246 8.308 6.048 4.663 4.171 93SW-14 8.365 10.414 8.465 5.916 5.214 CR304-1 8.051 10.268 8.578 6.355 5.188 CSP-01 11.311 11.097 8.244 5.977 CSP-02 12.491 13.708 10.776 8.901 11.569 CSP-06b 12.730 13.514 10.951 9.271 10.685 10.467 10.896 11.616 IC-10 7.275 8.146 7.578 6.507 6.538 KSE91-02a 10.459 11.171 8.395 5.968 6.823 5.526 KSE91~02b 8.270 10.941 9.311 6.528 5.237 5.124 with permission of the copyright owner. Further reproduction prohibited without permission. ■ o - 5 o Q . C o CD Q . T able 5-B. (Continued) Sample Whole Major Minerals Minor Minerals Secondary Minerals Rock Qtz Pla* Hbl Biot Px Muse Gnt Chit Tour Ep Fibro KSE91-03 11.579 10.488 9.347 5.112 5.919 KSE91-04 10.622 11.330 10.308 8.358 6.788 7.247 KSE91-09 9.716 12.214 8.902 7.582 6.788 6.720 KSE91-11 9.800 12.207 8.180 7.418 6.589 6.778 KSE91-13 10.755 12.356 9.995 7.020 6.650 KSE91-15 12.092 10.817 10.564 6.182 5.414 KSE91-19 7.563 8.202 6.307 7.480 7.288 6.673 KSE91-22a 12.600 13.892 10.925 5.893 9.017 9.669 KSE91-27a 8.336 8.220 6.225 6.169 6.916 KSE91-27b 8.849 9.191 8.021 6.939 5.451 6.804 KSE91-30a 8.497 11.054 8.520 6.685 6.072 6.391 KSE91-33a 7.080 8.131 8.668 7.168 6.712 KSE91-42 8.293 10.781 7.899 6.869 5.999 5.944 KSW91-03 8.500 9.204 8.197 5.985 5.070 KSW91-05a 9.297 9.102 6.751 6.456 KSW91-06a 8.753 8.924 7.908 5.922 5.300 KSW91-08 9.189 10.972 8.142 6.579 5.701 CHIWAUKUM SCHIST MS-30A | 11.486 | 13.354 10.821 12.204 14.403 O rder of 1 ^ 0 E nrichm ent in the M ount S tuart B ath o lith I I I £ 9 eo 0 1 S 8 a t i 7 - - 10.8 Quartz 9.7 Whole Rock □ 85 Plagiodase 8 6.6 Hornblende 63 Biotite M ajor M inerals 9.8 Gamet 9.1 Muscovite 6.9 Pyroxene 63 Chlorite M inor M inerals F igure 5-5. G eneral sequence o f average 8 1 ^ 0 values w ith in m inerals and rocks o f th e M ount S tuart b ath o lith . A lthough in d ividu al sam ples d em onstrate som e v ariation in th e sequence, o n average m in erals form a n orm al sequence of enrichm ent. O f th e m ajor m inerals, q u artz and plag io dase are volum etrically dom inant. T heir com positions, along w ith biotite an d hornblende, tend to contribute m ost strongly to w hole rock values. H istogram s rev eal th a t the bim odal w hole rock d istrib u tio n coinddes w ith peaks in the histogram s o f q u artz (11 %o) and plagiodase (8.5%o and ll% o) (Figure 5-1). O ther m ineral g l8o values differ from w hole rock distributions, dem onstrating the volum etric im portance o f quartz an d plagiodase to w hole rock co m p o sitio n s. Q u artz and plagiodase S^^O values show strong correlations w ith w hole rock isotopic com positions ( r^ = 0.612 an d 0.571, respectively) (Figure 5-6a), b u t the quartz trend (m = 0.6972) has a greater slope th an plagiodase 45 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. (a) Quartz and Plagiodase vs. Whole Rock 8 ^ 0 14 T □ Quartz ♦ Plagiodase 8 9 10 11 Whole Rock8l®0 12 y = 0.6972 x + 4.1638 R = 0.782 y - 03817 x + 3.4316 R = 0.755 13 (b) Plagiodase S ^O vs. Quartz 8 ^ 0 io ■ . 13 10 12 14 8 9 11 Quartz 81*0 Figure 5-6. C om parison of q u artz, plagiodase, and w hole rock S^^Ovalues w ithin the M ount S tuart b ath olith . (a) Q uartz an d p lag io d ase 8*80 com pared to w hole rock isotopic com position, (b) Plagiodase 8l® 0 com pared to q u artz S l^ o . Lines of constant fractionation (A qt-pl) are show n h av e a slope o f one. 46 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. (m = 0.5817). P erh ap s the m ost notable feature o f m ineral d a ta sets is th a t q u artz 5 ^ 0 values increase w ith w hole rock m ore rapidly th a n do p lag io d ase values. A p lo t o f q u a rtz v ersu s plagiodase 5 1 ^ 0 (Figure 5-6b) show s the tren d o f fractionation betw een q u artz and p lag io d ase (A qtz-plag)/ sh iftin g horn betw een 0 a n d l%o up to >4%o. In general, the v ariatio n results from increasing quartz S ^ O and to a lesser degree increasing p lag io d ase S ^ O ; therefore, those sam p les w ith elev ated w hole ro ck valu es also h av e high A qtz-plag v alu es. This indicates th at the process responsible for elevatin g w hole rock 8 l ® 0 values w as a relatively low tem perature process. T hus, larg er fractionations, reflecting low er tem peratures, form ed as w hole rock, q u artz, and p lag io d ase values increased. B iotite is m oderately correlated (r^ = 0.335) w ith w hole rock i com positions, d esp ite considerable variation w ith in the d a ta (Figure 5-7). Three sam ples h av e biotite S ^ O values th at are a t least 1.5 % o greater th a n any o th ers. All h av e very sim ilar isotopic com positions for b o th w hole rock and b io tite, plus th e y occur in zones o f subsolidus deform ation. If these three sam ples are d isreg arded , the b io tite trend is roughly flat w ith a g rea t deal 1 of scatter, quite lik e th a t of hornblende, w hich is n o t correlated w ith 8 ^ 0 (r^ I = 0.027). Biotite a n d hornblende (plus pyroxene, a m inor m ineral) have j sim ilar, o v erlap p in g 8l® 0 values in the 4 - 8%o range. i Because b io tite an d hornblende do not change as w hole rock com position does, th e y can n o t b e directly responsible for elevated w hole rock values. Instead, th e stro n g correlations and steep slopes o f q u artz and p lag io d ase d em onstrate th at th o se are the tw o critical m inerals to u n d erstan d in g th e cau se of isotopic zonation w ith in the M ount Stuart. 47 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. B iotile, H ornblende, an d Pyroxene vs. W hole Rock 5 ^ 0 10.0 o Hbl □ Biot • Px 9.0 ao □ □ 5.0 4.0 7.0 10.0 11.0 12.0 13.0 9.0 8.0 j W hole R ock 5 * 8 0 t F igure 5-7. Biotite, hornblende, and pyroxene com positions com pared to w hole rock for the M ount S tu art batholith. Minor minerals The isotopic com positions and trends o f m in o r m inerals are difficult to in terp ret because these m inerals do n o t occur in a ll sam ples. Pyroxene only occurs in th e m ore m afic lithologies, w hile g arn et a n d m uscovite are present in peralum inous rocks. Pyroxene 81^0 values are qu ite consistent, w ith <l%o variation = \ 6.54 - 7.29 %o). Because o f the mafic association, th e y also occur w ith in a * restricted range of w hole rock isotopic com positions, betw een 7 an d 9%o. The result is th a t pyroxene, like hornblende and b io tite, is n o t correlated w ith w hole rock (Figure 5-7). 48 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. { G arn et and m uscovite have v ery sim ilar d ata pattern s. Sam ples a t th e h ig h est w hole rock 8 ^ 0 values (> 1 2 % o ) have h ig h garnet a n d m uscovite isotopic com positions ( 8 ^ 0 = 9 .6 -1 1 .6 %o). O ne sam ple (KSE 91-2a) has a n elev ated w hole rock signature (10.46%>) b u t the g arnet an d m uscovite p resen t b o th have low values o f 6.83 a n d 5 .1 2 % o , respectively. A lthough o n ly th ree sam ples o f each m ineral w ere analyzed, b o th have series have stro n g positiv e correlations w ith steep slopes. Secondary minerals C hlorite occurs in m any sam ples as an alteration o f biotite. The fairly large cry stal size and distinctive g reen color m ake it an easy m ineral to visually separate, despite its low m o d al abundance. U nlike o th er secondary m inerals, sufficient chlorite sam ples have been analyzed to recognize som e d ata tren d s. B iotite and chlorite oxygen isotopic data are have a v ery strong p o sitiv e correlation (r^ = 0.861) w ith a slope close to 1 (m = 1.196) (Figure 5-8). This one to o ne relationship is n o t su rp risin g , because m ost o f th e chlorite in th ese sam ples is an alteration p ro d u ct o f biotite. The near equivalence of values, th o u gh , is im portant because it dem onstrates th a t the chlorite form ing alteratio n episode did n o t involve a n externally derived flu id such as m eteoric flu id, because th at w o uld h ave significantly low ered chlorite S ^ O values relative to biotite. C hlorite is also strongly correlated w ith w hole ro ck (r = 0.720), quartz (r = 0.801), an d plagiodase (r = 0.744) isotopic com position. E pidote also occurs as a secondary m ineral in som e sam ples, b u t its trace abundance, as w ell as sm all size, m ake it difficult to separate enough 49 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. C h lo rite vs. B iotite 5 ^ 0 y = 1 .1 9 6 3 x -1.1971 r = 0.928 5 7 4 5 6 7 9 8 10 Biotite 5ls O Figure 5-8. C om parison o f chlorite a n d biotite isotopic com positions. C hlorite is an alteratio n product o f biotite, and th u s 8l® 0 values have a one-to-one relationship. m aterial for analysis. In the one sam ple th at has b een exam ined, the low 818(3 v alu e of epidote corresponds w ith a low w h ole rock value. ' j Results of Stable Isotope Thermometry | O xygen isotope equilibration tem peratures have been calculated for all i m ajor m ineral fractionations (A ppendix B) in m an y sam ples from the M ount S tu art b atholith (A ppendix C). The calibration o f B ottinga and Javoy (1975) is favored, because it allow s incorporation of h y d ro u s phases (hornblende an d biotite) an d therefore is consistent fo r the m ajor m inerals of these rocks. O nly 50 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. quartz-plagioclase tem peratures can be calculated w ith the M atthew s (1994) o r C hiba a n d others (1989) calibration, as it is the o ne fractionation coefficient th at su its the m ineralogy of th e b atholith. The resu lts o f the calibrations are v irtu ally th e sam e, o n average ju st 12°C different, w ith B ottinga a n d Javoy (1975) consistently higher. T em peratures involving p lag io d ase could only be calculated for those sam ples w ith electron m icroprobe d a ta regarding anorthite co n ten t (An%). V ariations in plagiodase com position can significantly affect therm om etry, because fractionation coeffidents o f albite and an o rth ite differ b y as m uch as l%o (K ieffer, 1982; M atthew s e t al., 1983; Chiba e t al., 1989). For the isotopic com positions w ithin th e M ount S tu a rt b atholith, a 10% change in A n co n ten t could resu lt in 20-40°C difference in tem perature (based on A qtz-plag)- Tw o quartz-plagiodase p airs (IC-10 and KSE91-15) have im possibly high tem peratures resu ltin g from reversals. These tw o tem peratures have b een largely ignored for the pu rp o se of analysis. Stable isotopic tem p eratu res, calculated from 6 m ineral p airs dem onstrate w ide variability th a t generally has few patterns. T em peratures range from 165°C to 1079°C, d u ste rin g betw een 450 and 650°C w ith an average of 595 ± 218°C (A ppendix C). The rd atio n sh ip betw een isotopic tem perature and w hole rock isotopic com position is im portant to u n d erstan d the conditions at w hich isotopic zonation form ed. A verage tem p eratu res calculated from all m ineral pairs show n o co rrd atio n w ith w hole rock valu es (Figure 5-9a). Focus on tem peratures d eriv ed from quartz-plagiodase p airin g s (Tqt- pl) gives b etter insight, as these tw o m inerals h ave been show n to be m ost strongly reset. In fact, these tem peratures (365-900°C) tend to decrease 51 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. (a) Stable Isotope Thermometry 1400 WR 5**0 vs. T qj.pi ° hb-bl y = -34.709x - + - 872.61 x AvS r = 0.397 ■ qt-pl 1200 1000 a 800 ( J o H 600 X X 400 200 12.0 13.0 9.0 10.0 11. 0 7.0 8.0 W hole ro ck Sl s O (b ) Stable Isotope Thennometry Q u a rtz -m in e ra l p a in (c) Stable Isotope Thennometry P la g io c Ia M -m in e ra l p a in 1400 1200 1000 y 8 o o 600 400 200 7 .0 8 .0 9 j0 10.0 11-0 12 0 13.0 1400 1200 1000 o 600 400 200 7.0 8 .0 9 .0 10.0 11.0 1 2 0 13.0 Whole rock S“ 0 Whole rock S18© Figure 5-9. Stable isotope therm om etry o f the M ount Stuart batholih. (a) A verage tem peratures, plus those from quartz-plagiodase and hom blende- biotite p airs, (b) All quartz-m ineral tem peratures (c) A ll plagiodase-m ineral tem peratures. T em peratures are generally lower at high whole rock S^^O. 52 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. t som ew hat w ith w h ole rock 5 ^ 0 , having a w eak correlation betw een d a ta sets (r^ = 0.158). This tem perature change is slight, representing o n ly a 35°C drop p er l%o increase in isotopic com position, w hich translates to ro u g h ly a 100°C difference betw een norm al an d elevated w hole rock com positions. This g rad u al decrease in tem perature a t high w hole rock S ^ O can b e seen in all quartz-m ineral a n d plagioclase-m ineral pairs (Figure 5-9b,c), b u t it is im p o rtan t to n o te th a t this change is sm aller th an the range o f tem peratures calculated for a n y m ineral-pair. In ad d itio n to a decreasing tem perature tren d , hom blende-biotite tem peratures are consistently low er th an those o f other m in eral p airs (Figure 5-9a). O nly tw o sam ples record m agm atic T hb-bi; m ost sam ples range from 1 165-574°C. This indicates that although hornblende and b io tite seem to record | m agm atic 8 ^ 0 v alu es, they have undergone isotopic exchange after crystallization. C onsideration of sam ple locality reveals th a t low tem p eratu re sam ples occur in high 5 ^ 0 portions of the batholith (Figure 5-10). L arge quartz- plagioclase fractionations (reflective of low tem peratures) occur aro u n d the m argins of the b ath o lith , particularly to the NE, w hile the co re records sm aller A qt-pi a n d higher tem peratures. H om b lende-p lagioclase ■ | therm om etry, b ased o n elem ental abundances (B lundy and H olland, 1990), is I sim ilarly zoned, b u t records subsolidus tem peratures only in th e NE tip o f the p lu to n . C om parison of isotopic a n d elem ental therm om etry suggests th at t elem ental chem istry w as reset after crystallization to a lim ited extent, w hile isotopic equilibration generally occurred a t near- an d subso lid u s tem peratures dow n to about 450°C, especially in high w hole rock S ^ o regions. 53 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. mn*'i (b) Homblende-PIagioclase Thennometry (° C) Quartz-Plagioclase « 6 2 9 < 6 30; 25 km Figure 5-10. Distribution of tem peratures w ithin the M ount Stuart batholith. (a) H om blende-plagioclase therm om etry w as calculated w ith the calibration of Blundy and H olland (1990). (b) Q uartz-plagiodase fractionations. Large A values represent low tem peratures, w hile sm all A indicates high tem peratures. Deformed Zones Four deform ed areas o f th e M ount S tu a rt batholith (Figure 1-4) have b een noted, in clu d in g three sh ear zones o n th e NW edge o f th e p lu to n (Rock Lake shear zone, Icicle C reek sh ear zone, an d T um w ater M ountain shear zone) and one reg io n of deform ation associated w ith the W indy Pass T h ru st (Pioneer C reek area) (M iller a n d Paterson, 1992,1994). A ll o f th e shear zones lie w ith in regions o f elevated w hole rock com positions, b u t sam ples from the T um w ater M ountain (K SE 91-15, -22) and R ock Lake (CSP-6b, -2) shear zones are th e highest v alu es in th e bath o lith (12.1 - 12.7%o). N o elevated w hole rock 5 l8 o values exist in the Pioneer C reek area. in order to discern the relationship b etw een deform ation and high 5 I 8 0 values, n in e additional sam ples from th e Pioneer C reek, Icicle Ridge an d T um w ater M ountain zones w ere analyzed. These rocks w ere selected because they p reserv e evidence o f ductile deform ation b u t n o t of the loading ev en t th at o v erp rin ted the NE p a rt of the b ath o lith and C hiw aukum schist (Table 5-C). A ll o f these sam ples have norm al w hole rock S ^ O com positions (7.9 - 9.4%o). T able 5-C. W hole rock analyses of sam ples w ith ductile deform ation. S am p le D eform ational Z one W hole R ock 8lsO BJ 45-1 Icicle Creek 9.15 BJ 47-1 Icicle Creek 9.58 C A 6-1 Pioneer C reek 9.38 C A 16-1 Pioneer C reek 9.10 CA42-1 Pioneer C reek 7.94 CA45-1 Pioneer C reek 8.66 PC-1 Pioneer C reek 8.08 W I24-1 T u m w ater M o untain 8.11 55 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. C hiwaukum Schist The C hiw aukum Schist exhibits w hole rock values of 11.5 - 18.8 %o, w hich are ty p ical for pelitic lithologies (Table 5-A ). There is no ap p aren t geographic p a tte rn form ed by these isotopic com positions, based on the d ata available (14 sam ples). O ne sam ple (MS-30A) w ith m ineral 5 ^ 0 values (Table 5-B) show s th at biotite and plagiodase h av e significantly higher isotopic com positions th an their p lu to nic counterparts. The anorthite content o f p lag io d ase is unknow n, b u t isotopic tem peratures have been calculated for the w hole spectrum o f com positions. P lagiodase-biotite fractionation yields tem peratures from 460°C (albite) to 660°C (anorthite). 56 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Interpretations Causes o f Iso topic Zonation T here are several processes th a t could be envisioned to create the elev ated w hole rock S ^ O values aro u n d the m argins of th e M o u n t Stuart b ath o lith . T he m ajor hypotheses th at w ill be considered are c ru stal m elting d u rin g m agm a generation, m agm atic assim ilation, an d flu id in filtration, w ith th e latter occurring a t either h ig h o r low tem peratures. Crustal Melting M elting o f high S ^ O crustal source m aterial in form ing th e original m agm a is a p o ten tial m eans to generate elevated oxygen isotopic com positions in th e plu ton ; how ever th is process is unlikely to yield the isotopic zo nation observed in the M ount Stuart batholith. In th is case the isotopic com position o f the crystallized rocks is a n inherent featu re of the m agm a. In d iv id u al batches of m agm a w ith different v alu es w ould be necessary to produce the isotopic zonation. Indeed the M ount S tu art b ath o lith is com prised of several sep arate m agm a pulses w ith lithologic zones th a t ro u g hly m irro r S ^ O zonation; how ever, rock type does n o t strictly correspond to isotopic com position, as w ould be predicted. Isotopic contours cross in tern al contacts an d gradational boundaries betw een p lu to n ic units (e.g., T abor e t al., 1993). As discussed previously, sim ple differences in rock type can only account for a sm all am ount (30%, r^ = 0.306) of th e isotopic v ariatio n w ithin th e b ath o lith (Figure 5-4). Each successive m agm a pulse needs to have su b stan tially h ig h er S ^ O values, created by additional crustal m elting, in this 57 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. m odel. Such an id ea is n o t com patible w ith elem ental chem istry and th e resu ltin g evolutionary m odels p u t forth b y P ongsapich (1974), Erikson (1977), A n d erso n (1992), an d Paterson an d others (1994). These w orkers have all concluded th a t fractional crystallization is responsible for chem ical tren d s w ith in the bath o lith , n o t continuous m elting o f a crustal source. This hy pothesis can be elim inated o n th e basis o f its incom patibility w ith accepted ev olutionary m odels, as w ell as lack of stro n g sp atial and statistical association w ith lithology a n d isotopic com position. Magmatic Assimilation M agm atic assim ilation o f country rock a t the level o f in trusion seem s a - likely explanation for the isotopic zonation o f th e batholith, because th ere is I field evidence of C hiw aukum schist inclusions w ith in g ran itic rocks. ! Incorporation of th e high pelitic schists w o u ld result in a n interm ediate isotopic com position recorded by the pluton. This could raise w hole rock § 1 8 o values aro u n d the m argins o f the b ath o lith , w here assim ilation w o u ld p o ten tially be greater. Elem ental com positions m ay also be sh ifted by r assim ilation, thus th is m odel w ould account fo r elevated A /C N K ratios in h ig h 8180 M ount S tu art sam ples. ! In order to evaluate th is hypothesis, th e isotopic tren d s of in d iv id u al i m inerals m ust be considered. As country rock is incorporated into the m agm a, the overall §180 w ould b e raised, increasing the isotopic com position t of a ll o f the m inerals. A ssim ilation of co u n try rock requires m agm atic 1 tem p eratu res and w ould, therefore, occur a t h ig h tem peratures before m o st m agm atic m inerals h ad com pletely crystallized. For this reason m ineral 58 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. fractionations (A) ought to b e constant a n d sm all, reco rding m agm atic cry stallizatio n tem peratures (Figure 6-1). A lthough portions o f th e p lu to n reco rd m agm atic A values, m u ch of the b ath o lith , particularly those areas w ith h ig h 5 ^ 0 h av e large fractionations consistent w ith tem peratures below th e so lid u s. The tre n d of fractionations betw een q u artz an d p lag io d ase, the m o st influential m inerals for w hole rock is n o t constant w ith in th e M ount S tu art b ath o lith (Figure 6-2). N early all q u artz-plagiodase p airs w ith d e v a te d w hole rock com positions yield subsolidus isotopic equilibration tem peratures (365 - 450°C), a t w hich m elting could n o t occur. The oxygen isotopic trends o f the b ath o lith do n o t fit the m odel predicted b y m agm atic assim ilation; therefore, d esp ite field evidence th at it w as locally active, assim ilation w as n o t th e d o m in an t process responsible for isotopic zonation o f th e batholith. Subsolidus Fluid Infiltration Low Temperature If h igh fluids en tered the b ath o lith after it h a d crystallized, exchange betw een fluid and rock w ould d e v a te oxygen isotopic com positions, affecting the m ore easily p en etrated m argins o f the p lu to n the m ost, fit this scenario, only th e readily altered m inerals w ill be affected, because resistan t m inerals are less prone to exchange a t low tem peratures (<300°C). P lag io d ase and th e h y d ro us m inerals (hornblende a n d biotite) sh o u ld have h ig h er valu es as the fluids increase w hole rock values, b u t th e m ore resistan t phases, p articu larly q u artz and pyroxene, m ay n o t b e changed. T his w ould re su lt in disequilibrium betw een the tw o critical m inerals, q u artz a n d p lag io d ase, w ith very large fractionations (Aqtz-plag) 59 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Magmatic Assimilation Quartz Plagiodase A is constant .o 2 1 2 Hornblende Biotite ASSIMILATION Figure 6-1. H ypothetical m odel o f m agm atic assim ilation o f hig h m aterial. As assim lation raises w hole rock isotopic com positions, all m ineral §18o values increase, as well. Sm all fractionations rem ain d u rin g assim ilation d u e to m agm atic tem peratures. M ineral Separates O i o. o 1 6 □ Qtz o Plag ♦ Hbl A Biot • Px \vvv viGt \v \\v v v \v v \\\\v v v \v \ \ \ \ Quartz Plagiodase bmblende Biotite Pyroxene Whole Rock 8*0 Figure 6-2. O bserved pattern of m ineral S^-^O values w ithin th e M ount S tuart batholith. M inerals h ave been gro u p ed to show general trend. Q uartz and plag io d ase increase w ith whole rock com position, w hile m afic m inerals are no t correlated w ith w hole rock 60 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. becom ing sm aller (possibly even negative) as flu id infiltration progresses (Figure 6-3). M ineral S ^ O values from the M ount S tu art batholith p reclu d e this hypothesis. Q uartz values are n o t constant, in fact, they increase m ore rapidly th a n does p lag io d ase w ith w hole rock com positions. Pyroxene, another alteratio n -resistan t m ineral, does have co n stan t 5 ^ 0 v alu es, b u t it occurs in v ery few sam ples. Fractionations (Aqt-pl an d Aqt-hb) also belie the m odel, becom ing larger rath er than sm aller as w hole rock com positions increase (Figure 5-6). Finally, isotopic therm om etry indicates tem peratures greater th a n 350°C. It is d e a r horn the data th at low tem perature subsolidus fluid in filtratio n is n o t an adequate m o d d to account for m ineralogical 5 ^ 0 variation in the M ount S tu art batholith. High Temperature A lthough low tem perature fluids are generally n o t capable of resetting quartz com positions, a t high er tem peratures q u artz exchanges m ore easily. H igh tem p eratu re fluid flow into the subsolidus b ath o lith w o u ld change the 5180 of a ll m inerals, because at tem peratures of 400-500°C, none are resistant to isotopic exchange. F luid-rock interaction w ould be m ore pervasive along th e m argin of the b ath o lith , leading to higher w hole rock 8 ^ 0 values and larg e m ineral fractionations consistent w ith low er tem peratures (Figure 6-4). The core, w hich is som ew hat shielded by the re st of the p lu to n from in filtratio n , should reco rd m agm atic A values. This m odel p red icts th at for in d iv id u al m inerals o u g h t to increase as fluid in filtratio n progresses. Cooling o f the initially h o t fluids w ould lead to low er tem peratures and 61 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Low Temperature, Subsolidus Fluid Infiltration Q uartz O 0 0 ’ c Q s 4 1 e t D isequilibrium Plagiodase H ornblende Biotite Whole Rock 61S0 FLUID IN F ILTR A TIO N Figure 6-3. H ypothetical m odel of low tem perature, subsolidus fluid infiltration. As infiltration of flu id raises w hole rock isotopic com positions, m ineral increases, except in resistan t m inerals, such as quartz. High Temperature, Subsolidus Fluid Infiltration Q uartz A increases Plagiodase 0 0 0 ’ S o s 4) e H ornblende Biotite Whole Rock 51S0 FLUID IN F ILTR A TIO N Figure 6-4. H ypothetical m odel of h ig h tem perature, subsolidus fluid infiltration. As this process raises w hole rock 5l& 0 values, all m ineral isotopic com positions are reset to higher values. Fractionations betw een m inerals w ill also increase. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. larg er fractionations w here interaction is greatest, th a t is, aro u n d the edges of th e b a th o lith . T he h ig h tem p eratu re flu id infiltration m o d el best fits the tren d s w ith in th e m in eral d a ta from th e M ount S tuart b atholith. T his hypothesis p red icts increases in b o th m in eral an d fractionations betw een m inerals a t h ig h er w hole rock com positions, w hich is observed in th e m easured data, h i ad d itio n , th is m odel resu lts in steep er trends fo r high S ^ O m inerals th an those w ith low er values. In this w ay, it satisfies th e data requirem ent th at q u artz hav e a steeper slope th an plagiodase, an d th at hornblende and b io tite values change v ery little w ith w hole rock com positions. Isotopic therm om etry indicates th a t q u artz an d plag io dase underw ent subsolidus exchange d o w n to 365°C, also com patible w ith a h ig h tem perature m odel. I This correspondence betw een m odel an d data indicates th a t the m ost f im p o rtan t process in form ing the elevated isotopic com positions aro u nd the | m argins o f the M ount S tu art b ath o lith w ere caused b y high tem perature, [ su b so lid u s flu id in filtratio n. I Source o f flu id | The source of the flu id is confined by the direction o f isotopic shift of I batholithic rocks. This flu id in filtratio n is not a case of alteration b y m eteoric I fluids. W ater th a t has gone th ro u g h the su rfid al circulation cy d e is observed [ to have negativ e S ^ O values. T hus, exchange betw een norm al granitoid rocks and m eteoric w ater a t h ig h tem peratures w o u ld low er oxygen isotopic com position o f th e rocks, rath er th an elevate it. Sim ilarly, fluids derived from th e Ingalls C om plex o u g h t to low er the b ath o lith com position as w ell, since o phiolites ten d to have low S ^ O values o f -5 -6 % o . Localized argillite 63 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. u n its in the Ingalls m ay have h ig h er S ^ O values than the b ath o lith , b u t q u an tities of th ese sedim ents are lim ited, h i contrast, fluids th a t equilibrated w ith th e C hiw aukum schist sh o u ld have values o f -1 2 - 18 % o an d exchange w ith these fluids can b e expected to increase isotopic com positions w ith in the p lu to n . The C hiw aukum schist represents th e o n ly local source o f h ig h S ^ O flu id s th a t could h av e created th e zonation w ith in the M ount S tu art b a th o lith . There is field evidence to su p p o rt this flu id infiltration source, h i places w ith in isotopically elevated contact regions, sm all v eins o f quartz p ro tru d e into th e p lu to n from th e sch ist (Figure 6-5). h i ad d itio n , a bleached zone su rro u n d in g a quartz vein n e a r Icicle C reek (sam ples KSE91-27a, b, c) show s alteration o f pyroxene to actinolite and b io tite to chlorite. M ore im portantly, it reveals higher values in bleached rocks (10.42% o) relative to th e un altered rock nearby (8 .3 4 -8 .8 5 %o) (Figure 6-6). The in terp retatio n of flu id s enterin g th e batholith from th e neighboring C hiw aukum schist is com patible w ith b o th isotopic d a ta an d field observations. . M echanism o f Fluid Infiltration | The conclusion th at fluids hav e come from the C hiw aukum schist r I lead s to the next logical question: W hat w as th e m echanism o r ev en t th at [ cau sed fluid m ig ratio n into the batholith? Several explanations seem feasible. 64 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. F ig u re 6-5. P ho to grap h o f q u artz v ein in tru d in g M ount S tu art b ath o lith from th e C hiw aukum Schist. Sledge ham m er show n for scale. Mount Stuart batholith F ig u re 6-6. P hotograph o f a bleached zone surrounding a q u artz vein w ith in th e M ount S tuart b ath o lith . M ineral alteration (pyroxene to actinolite; b io tite to chlorite) and h ig h 8 1 * 0 values (10.42) occur w ithin the zone, w hile ad jacent rocks h av e norm al (8.34-8.85) isotopic com positions. 65 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Return Flow of Aureole Fluid First, th is problem m ay suggest a local circulation system initiated by em placem ent o f th e bath o lith . M eteoric flu id s w ere n o t involved, d u e to th eir expected negative isotopic com position, b u t it is p o ssib le th at th e flu id s w ere originally given off b y th e batholith. T hus, aureole flu id s could h av e equilibrated w ith th e C hiw aukum schist w h ile th e m agm a crystallized, developing a h ig h S ^ O sig n atu re. A fter solidification th e bath o lith w o u ld be perm eable, allow ing fluids to re tu rn and create elevated isotopic com positions (Figure 6-7). The tw o-sided flu id exchange process should Return Flow of Aureole Fluid enriched 5 ^ 0 zone fluid flow p ath aureole Figure 6-7. Schem atic rep resen tatio n o f re tu rn flow o f originally m agm atic flu id s into a b atholith. Fluids released d u rin g em placem ent equilibrate w ith high S ^ O country rocks o f die aureole, retu rn in g to the p lu to n after crystallization, d u rin g cooling. This circu latio n should elevate S ^ O around th e b ath o lith m argin, w hile creatin g a depleted zone in the aureole. (Based o n H anson, 1995). 66 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. have produced an au reo le zone of d ep leted isotopic v alu es in the schist, m irro rin g the elevated regions in the b ath o lith . A d e ta ile d isotopic stu d y of th e C hiw aukum schist, to investigate th e presence o r lack o f such a depletion, w o u ld provide th e a d d itio n al data need ed to test th is flu id flow m odel. Contact Metamorphism C ontact m etam o rph ism o f the C hiw aukum sc h ist resu lted in th e g ro w th o f andalusite, follow ed by co rd ierite and m in o r sillim anite (Plum m er, 1980; Evans an d Berti, 1986). C orresponding deform ation in portions o f the b ath o lith also co ntinued h o rn m agm atic to subsolidus conditions (M iller and P aterson, 1992,1994). P rograde contact m etam orphic reactions w o u ld cause d eh y dratio n of I , the schist. Several w ater-producing reactions generally describe the * form ation of contact porphyroblasts, andalusite (12) m uscovite + q u artz = an d alu site + K -feldspar + H 2O (13) pyrophyllite = andalusite + q u artz + H 2O (14) paragonite + q u artz = and alu site + albite + H 2O an d cordierite (15) phlogopite + m uscovite = M g-cordierite + K -feldspar + H 2O f I (16) chlorite + m uscovite = cord ierite + b io tite + A l2 SiC>5 + H 2 O | (Spear, 1993). T extural evidence suggests th a t early an d alu site converted to | cordierite, possibly b y reactio n w ith b io tite o r chlorite, j (17) M g-chlorite + andalusite = M g-cordierite + K -feldspar + H 2 O (18) biotite + an d alu site = garnet + cordierite + H 2O . These exact reactions, ty p ical of low p ressu re pelite m etam orphism , m ay not have occurred th e C hiw aukum schist. If flu id -p ro d u cin g reactions continued 67 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. after solidification o f th e m agm a, o r if syn-em placem ent w aters rem ained in th e schist, fluids could b e released in to th e subsolidus batholith. High Pressure Metamorphism A th ird m echanism is suggested b y th e dom inance o f elevated values in th e n o rth eastern portion o f th e batholith. This area experienced a post-em placem ent reg io n al gam et-staurolite-kyanite-form ing m etam orphic event, d u e to tectonic lo ad in g (Evans a n d Berti, 1986; Brow n an d W alker, 1993). The staurolite a n d kyanite isograds, m apped by Plum m er (1980) and Evans an d B erti (1986), spatially coincide w ith high areas o f the batholith. Rocks o f th e C hiw aukum sc h ist outside o f the aureole, having n o t u n dergone prev io u s h ig h pressure m etam orphism , w ould experience prograde d eh y d ratio n reactions, h i th is scenario, fluids released by regional m etam orphism w ould b e draw n into th e batholith, leading to the observed isotopic zonation. Several p ro g rad e reactions ty p ical in B arrovian m etam orphism of pelites (Spear, 1993) th a t seem ap p ro p riate to the C hiw aukum schist assem blage m ay be capable of producing fluids. The grow th of staurolite from ch lo rito id (19 chloritoid = g arn et + chlorite + staurolite + H 2 O m ay not have been active, as virtually no chloritoid is preserved (S. Paterson, personal com m unication, 1997). S taurolite form ation from g arn et an d m ica (20) g arnet + chlorite = staurolite + biotite + H 2 O (21) g arn et + chlorite + m uscovite = staurolite + b io tite + plagiodase + quartz + H 2O 68 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. is m o re consistent w ith the regional m etam orphic assem blage. D avidson and E vans (1995) have proposed the breakdow n o f cordierite, consum ing w ater, (22) Fe-cordierite + sillim anite + HfcO = Fe-staurolite + q u artz as th e source of stau ro lite in the C hiw aukum schist. K yanite is th e other characteristic p orp h y ro b last of the lo ad ed assem blage. The inversion of an d alu site to k y an ite involves n o fluids; h o w ev er, the breakdow n of staurolite to kyanite does yield w ater. (23) stau rolite + chlorite = biotite + kyanite + H 2O (24) stau rolite = garnet + biotite + kyanite + H 2O. In ad ditio n to these general w ater-producing m etam orphic reactions, th e b reakdow n o f cordierite m ay release fluid. The channel-like atom ic stru c tu re o f cordierite traps H 2O , CC>2/ and large cations such as N a an d K (D eer, H ow ie, an d Z ussm an, 1966). Fluids o f th is com position w o u ld be capable n o t only o f oxygen isotope exchange, b u t also of resetting A /C N K ratio s as observed n ear batholith m argins. D avidson a n d Evans (1995) have found evidence linking flu id m ig ratio n w ith th e h ig h pressure m etam orphic event. Incipient staurolite an d sillim anite occur in quartz m icroveins o f th e M ount S tu art an d C h iw au ku m schist. This indicates th a t regional m etam orphism m ay have b een (1) controlled by fluid m igration and (2) m ore extensive th a n initially recognized by Evans and Berti (1986). R egions of post-em placem ent high-pressures coincide w ith the b a th o lith shear zones. Several deform ed sam ples o f the p lu to n have elevated w hole ro ck S ^ O values. Sam ples preserving evidence of ductile defo rm atio n , b u t n o t of the loading event record norm al isotopic 69 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. com positions. This indicates th a t w hile flu id infiltratio n w as n o t associated w ith ductile deform ation, it cou ld be linked w ith loading. Evidence from the C hiw aukum schist, in clu d in g the p o ten tial o f high p ressu re prograde m etam orphism to yield w a ter, an d m icroscopic evidence of flu id m igration, su p p o rts post-em placem ent lo ad in g as a m echanism for fluid in filtratio n o f th e M ount S tuart batholith. E xtent o f Alteration G eotherm om etry m ay be u sefu l for assessm ent of th e ex ten t to w hich flu id s have altered isotopic com positions w ith in th e b atholith. F ractionations indicate w hich m in eral phases w ere reset. A s o th er tren d s h av e also dem onstrated, A qtz-plag u nderw ent th e greatest deg ree o f exchange, p ro d u cin g subsolidus tem peratures. In ad d itio n , therm om etry reveals th at h o rnblende and b io tite 5 ^ 0 values equilibrated after crystallization, yielding th e low est isotopic tem peratures in the b ath o lith . C om parison o f isotopic an d elem ental exchange therm om eters reveals w hich sam ples record tru e m agm atic conditions. A qtz-plag exhibits geographic zonation w ith large fractionations (low tem peratures) aro u n d the edges o f the p lu to n (Figure 5-10a). The in d ep en d en t elem ental hom blende- p lag io d ase therm om eter, based o n Al-Si exchange (Blundy a n d H olland, 1990), likew ise show s high tem perature zones > 690°C tow ard th e core, th o u g h th e p attern is m ore p atchy (Figure 5-10b). W ith this calibration, tem p eratu res ranged from 613 to 780°C (A nderson, 1997), h ig h er th an those isotopic tem peratures, since coupled elem ental exchange m ay be m ore resistiv e to change th an isotopic exchange. A t 3-4 kb, how ever, tem peratures less th an 670°C are subsolidus. From this o bservation it follow s th a t areas 70 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. d o cum en tin g h ig h tem p eratu res w ith b o th m ethods are tru ly m agm atic, w hile low tem p eratu res in e ith e r case su g g est resettin g o f elem ental or isotopic com position. A reas o f non-alteration, estim ated in th is w ay, are restricted to several sm all areas preserving m agm atic conditions, b u t are n o t tig h tly con strain ed d u e to lim ited data (Figure 6-8). It is ap p aren t, based on these tw o ty p es o f therm om eters and th e o v erall w hole rock S ^ O zonation, th a t m uch o f th e M ount S tu a rt batholith, p articu larly the NE p o rtio n has b een altered b y h ig h tem p eratu re fluid in filtratio n . Extent o f Fluid Infiltration no fluid infiltration moderate fluid infiltration maximum fluid infiltration F ig u re 6-8. Extent o f flu id infiltration into the M ount S tu art b ath o lith from the C hiw aukum schist. E stim ated by com bining A q uartz-plagiodase a n d hom blende-plagioclase therm om etry. U naffected areas have b e en draw n as bro ad ly as possible. 71 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Tectonic Implications "Baja British Columbia" H ypothesis The place o f origin o f th e C oast Plutonic Com plex a n d N orth C ascades is th e subject o f considerable controversy. Interpretations o f paleom agnetic d a ta from C retaceous plutons in the region have lead to d ie "Baja B ritish C olum bia" concept. This hypothesis holds th a t m any o f th e terranes in th e W ashington an d C anadian C ordillera w ere originally form ed a t the p resen t latitu d e o f Baja C alifornia w here they achieved their m agnetic sig n atu re, then traveled ab o u t 2000 - 3000 km n orth w ard to their cu rren t location (e.g. Beck et al., 1972; Irv in g e t al., 1985; U m hoefer, 1987). < Paleom agnetic studies o f the M ount S tuart b atho lith (Beck e t al., 1972; I i Beck e t al., 1981; L und et al., 1994) and a num ber of o ther m id to late | C retaceous plu to n s, such as th e Spuzzum an d Porteau p lu to n s (Irving e t al., t 1985) yield discordant paleodirection indicators. These m agnetizations differ significantly from th a t expected d u rin g the Cretaceous n o rm al su p erch ro n for N o rth A m erica. These paleom agnetic signatures can be explained e ith er by n o rth w ard translation over long distances (Baja B.C. theory) o r by tiltin g of the b ath o lith . A lthough m any in terp retatio ns favor tran slatio n over tilt (e.g. Beck e t al., 1972; Beck e t al., 1981; Irving e t al., 1985; U m hoefer, 1987), som e have fou n d evidence for about 30° o f southw est-dow n tilt around a ho rizo n tal axis striking 330° (B utler e t al., 1989). D eterm ining Paleohorizontal C alculations of paleom agnetic direction depend g reatly on know ing the o rien tatio n of the batholith relative to horizontal at the tim e of 72 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. i em placem ent. M ost p lu to n s hav e little internal stratig rap h y to give d u e s a b o u t paleohorizontal p lan es. Instead, country rocks o r barom etric variations w ith in the b ath o lith m u st p ro v id e inform ation regarding form er orientations (an d tilt su b seq u en t to em placem ent). h i the case of th e M o u n t S tu art batholith, country rocks p ro v id e little h elp , because b o th th e C hiw aukum schist an d th e Ingalls ophiolite are significantly deform ed. T he T ertiary Sw auk form ation adjacent to th e w est does indicate paleohorizontal, b u t presents a problem since it is folded (Tabor e t al., 1993) an d m ost likely in fau lt contact w ith the b ath o lith (M cD ougall, 1990; Tabor e t al, 1982,1993; M iller e t al., 1990). The tim e lapse betw een em placem ent an d erosion o f th e p lu to n could have involved tiltin g , so the paleohorizontal plane o f th e Sw auk form ation m ay n o t correspond w ith em placem ent orientation s. Increasing m etam orphic grade of C hiw aukum sch ist to the n o rth east m ig h t indicate southw est-dow n tilt, b u t Evans an d B erti (1986) show ed th at th is variation m ay be the result of a later m etam orphic even t. Ague an d B randon (1992,1996) used alum inum -in-hom blende barom etry (Schm idt, 1992; H olland and Blundy, 1994) to calculate d ep th s w ith in the M ount S tuart b ath o lith . Provided th a t the calculated p ressu res rep resen t crystallization conditions, they can b e used to determ ine a paleohorizontal surface to b e ap p lied in correcting paleom agnetic d a ta for post-em placem ent tilt. R esults o f this study indicated a range o f p ressures (ab o u t 0.1 - 0.5 GPa) w ith h ig h er pressures in the northw estern p a rt o f the b atholith. A gue and B randon concluded th a t the M ount S tu art b ath o lith has b een tilted gently (dip = 7.0°) to the southeast (strike = 43.2°) (Figure 7-1 a). R estoring the paleom agnetic d ata for the determ ined plane d id n o t elim inate 73 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. (b) Homblende-Plagioclase Thermometry ( * C) Quartz-Plagioclase 25 km Figure 5-10. D istribution of tem peratures w ithin the M ount Stuart batholith. (a) Hom blende-plagioclase therm om etry was calculated w ith the calibration of Blundy and H olland (1990). (b) Q uartz-plagioclase fractionations. Large A values represent low tem peratures, while sm all A indicates high tem peratures. the discordance w ith C retaceous norm al orientations. In stead , the conclusion called fo r 3100 km o f n o rth w ard translation, com bined w ith 11° o f clockw ise ro tatio n p lu s sh allo w southeast tilt to account fo r th e discordance. A lth o u g h th e concept o f u sin g barom etry to determ ine d ep th s, an d in tu rn p aleo h o rizo n tal surfaces, is a good approach, several problem s exist w ith the m ethods u se d b y A gue an d B randon. A nderson (1997) h as cited flaw s in the w ay in w h ich th e A l-in-hom blende calibration w as ap p lied , notably a lack of in co rp oratio n o f tem perature. A s in m any o th e r exchange system s, the su b stitu tio n o f alu m in um in th e hornblende stru c tu re is b o th p ressu re a n d tem p eratu re d e p en d e n t (A nderson an d Sm ith, 1995). A gue a n d B randon (1996) d isc o u n t tem p eratu re as a n im portant facto r and show th a t tem peratures fo r 10 sam ples do n o t v ary w ith p ressu re, leaving 3 /4 of | f sam ples un assessed . C alculations b y A nderson (1997) for these 10 rocks, p lu s | I m any o th ers, sh o w th a t m uch o f th e batholith records subsolidus | tem peratu res. A ll su ch subsolidus sam ples are unsuitable fo r u se w ith A l-in- | hornblende b aro m etry because th ey fall beyond th e lim its o f calibration. I A long w ith tem p eratu re considerations, stru ctu ral an d paleom agnetic I questions ex ist reg ard in g A gue a n d B randon's calculations. It seem s d o u b tfu l * | th at the re su lta n t paleohorizontal surface should b e a plane (Paterson an d | M iller, 1997), b ased o n pervasive folding and deform ation d u rin g and ju st | after the la st ph ases o f crystallization (M iller a n d Paterson, 1992,1994; I Paterson e t al., 1994). D ata also su g g est th at p ressu re increases d u e to load in g £ i ; of the b a th o lith b eg an d u ring cooling (Paterson e t al., 1994; D avidson an d Evans, 1995). h i ad d itio n P aterson an d M iller (1997) p u t fo rth m ultiple reasons for cau tio us use of paleom agnetic data (Beck e t al., 1981; Lund e t al., 1993,1994; P aterso n e t al., 1994). The batholith preserves on ly w eak m agnetic 75 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. rem anence w ith uncertainties regarding th e m agnetic m inerals preserving th e signature. M oreover, it records both n o rm al and rev ersed polarity, w hich occur in the SE a n d NW p o rtio n s o f the b ath o lith , respectively. Such paleom agnetic com plications, com bined w ith stru c tu ra l inform ation and th e un certain ties in v o lv ed in the A l-in-hom blende barom eter, cast d ou b t on p aleo h orizo n tal stu d ies. h i trying to u n derstan d th e pressure variations o f th e batholith, involved in paleohorizontal stu d ies, A nderson and colleagues (Paterson e t al., 1994; A nderson and Sm ith, 1995) com bined their d a ta w ith th e initial d ata o f A gue and B randon (1992). U sing the S chm idt (1992) A l-in-hom blende calibration to calculate pressures, they found a dom al p a tte rn w ith shallow p o rtio n s of the b ath o lith aro u n d the m argins an d deep zo n e in the core I (Figure 7-lb). T his pattern su p p o rts the assertio n th at th e paleohorizontal j surface m ay n o t b e planar, b u t m ore im portantly hints th a t another variable (possibly tem perature) m ay be involved in th e developm ent o f pressure k variations (Paterson e t al., 1994). Bearing o f Isotopic Study The conclusions of pervasive fluid flow into th e M ount S tuart I b ath o lith tfrom th e C hiw aukum schist, resu ltin g from th e isotopic data i | described herein, also raises concerns for b o th paleom agnetic and | paleohorizontal stu d ies. W hile isotopic analysis does n o t negate the : d isco rd an t directions found in th e M ount S tu a rt and o th e r pluto n s, the effects o f flu id infiltration o f the b ath o lith add an additional lay er o f com plexity to th e issue. 76 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. * Subsolidus fluid infiltration at tem peratures of 360 - 625° has changed n o t o n ly th e isotopic com position of m inerals b u t th e d istrib u tio n o f elem ents as w ell. The high tem peratures m ay have b een able to d estro y the m agnetic sig n atu re o f crystallization, resettin g paleom agnetic indicators to orien tatio n s a t th e tim e of flu id infiltration. It is clear th a t isotopic com positions o f m ost rocks w ithin th e b ath o lith do n o t rep resen t m agm atic conditions. Since elem ental chem istry w as also affected, therm obarom etric calculations reflect flu id in filtratio n rath e r th an m agm a crystallization. This can be filtered o u t by com parison of therm obarom etry w ith isotopic tem perature calculations— if both types o f calibration yield m agm atic tem peratures th e n the rock w as not altered by fluid flow . C aution m ust, therefore, be ap p lied in calculations o f ■ paleo h orizo n tal to choose sam ples recording m agm atic pressures. | In ad d itio n , use of alum inum -in-hom blende barom etry, in p articu lar, ? is ill-ad v ised for m uch of th e M ount S tu art batholith. H om blende-biotite yields th e low est isotopic tem peratures in th e batholith, recording subsolidus exchange. For this reason, it is a bad choice o f m ineralogy for m aking p ressu re calculations. U nfortunately, as y e t there are no other available calibrations th a t su it the m ineralogy of th e batholith. U n til such geobarom eters are developed o r corrections can be m ade for hornblende d iseq u ilib riu m , barom etric determ inations o f paleohorizontal w ill be u n reliab le. » ' f \ t Other P lutons o f the North Cascades W hole rock oxygen isotopic studies have been conducted b y W hite and others (1988) for m ost granitoid plutons a n d gneisses o f the G lacier Peak 77 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. W ilderness A rea, located to the n o rth of the M ount S tuart b ath o lith . Eight m ajor T ertiary o r o ld er plutons p lu s th e Sw akane biotite gneiss, exhibit h ig h 5 I 8 0 values (> 10%o). M ost are calc-alkaline, w ith higher SiC>2/ alkali content, and A /C N K th a n o th er plutons in th e vicinity. O n the w e st sid e o f the E n tiat fault, these h ig h 8 ^ 0 p lu to n s form a 15 km w id e N W -trending b elt (Figure 7-2) th at seem s to m atch th e location o f the 87/86gr = 0.704 line described b y A rm strong an d others (1977). The Skagit gneiss, w ith som e low to norm al as w ell as h ig h values, form s a separate, parallel b e lt about 15 km east of the first (W hite e t al., 1988). U nits like th e Skagit gneiss, w ith a range o f isotopic com positions, do n o t dem onstrate system atic geographic variation o f values. There is no association o f 5l® 0 values w ith p lu to n m argins, cores, n o r co un try rocks ; (W hite e t al., 1988; D aw es, 1993), as is observed in the M ount S tuart batholith. | These h ig h plutons in h erited the isotopic sig n atu re from m elting | of supercrustal rocks, based on low er country rocks an d the spatial i coincidence w ith th e 87/86gr = 0.704 line (D aw es, 1993). Studies by O 'N eil an d I others (1977) show ing th at S-type plu to n s have hig her 8l® 0 values( >10 %o) I than I-type (7.7 - 9.9 %o). | The M ount S tu art batholith, isolated from the belt o f h ig h 8 1 ^0 N o rth | C ascade plu to n s, differs due to its isotopic zonation and su rro u n d in g 1&0 t enriched co untry rocks. These contrasts m ean th a t the M ount S tu art isotopic I sig n atu re is n o t restricted to an in h erited origin. Thus, the flu id infiltration ' m odel proposed for th e batholith is perm issible w ith in the context of regional isotopic trends. 78 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 10 miles 10 kilom eters F iguie 7-2. Location, of h ig h plu to n s and gneisses in the G lad er Peak W ilderness area of th e N orth C ascades, W ashington. The d istrib u tio n of these bodies form s tw o N W trending belts thro u g h the region. H igh units, show n w ith lined p a ttern and *, include the Jordan Lakes (JO ), Cyclone Lakes (CY), Bench Lake (BE), Dow ney C reek (DO), H id d en Lakes (H I), an d Sulphur M ountain (SU) plu to n s plus Swakane gneiss(SW ). From W hite an d others (1988). 79 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Conclusions The M ount S tu art batholith w as em placed, in th e m id-C retaceous, across th e W indy Pass th ru st into C hiw aukum Schist an d Ingalls ophiolite com plex in the N orth C ascades of W ashington. The p lu to n is zoned w ith respect to 1 ® 0 /1^0, h av in g high w hole rock values n ear the m argins and m ore norm al values tow ard the core. M ineral separate d a ta provide th e key to u nd erstan d in g th e causes of th is zonation. Q uartz and plagioclase are the critical m inerals th a t have undergone exchange to p roduce elevated w hole rock 5 ^ 0 values. Fractionations betw een q u artz and plagioclase becom e larg er as w hole rock, quartz, a n d plagioclase 81^0 increase, recording a drop in tem perature. T hus, these tw o m inerals do n o t preserve m agm atic 8 l8 o values. | O xygen isotopic d a ta from the M o u n t S tu art batholith indicate th at the | zonation resu lted from flu id infiltration, w hich occurred a t fairly high | tem peratures (400-625°C). These fluids m u st have equilibrated w ith or been } derived from the C hiw aukum schist, as it is the only source capable of | elevating S ^ O values, b u t th e m echanism for d eriv in g the flu id is uncertain. \ A ureole flu id s liberated from the M ount S tu art m agm a m ay have i [ equilibrated w ith the schist) then retu rn ed to th e p lu to n after crystallization. £ \ A lternatively, high 5 ^ 0 fluid m ay h av e been generated by p ro grad e reactions I in the C hiw aukum sch ist d u rin g contact m etam orphism . Post-em placem ent ’ ■ regional m etam orphism d u e to loading m ay be th e m ost likely catalyst of fluid infiltration. Form ation o f high-pressure porphyroblasts su ch as staurolite a n d kyanite, p lu s the breakdow n of contact cordierite are d eh y d ratin g reactions, fri addition, h ig h p ressu re m inerals g rew in veinlets 80 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. w ith in the b ath o lith an d schist (D avidson, 1995). The geographic d istrib u tio n o f h ig h 5 I 8 0 w hole rock values coincides w ith the loaded region and deform ation zones, y et isotopic com positions are norm al in ductilely deform ed sam ples th a t w ere n o t loaded. W hatever the m echanism , fluid in filtratio n w as a pervasive ev ent th at affected nearly all o f th e bath o lith , except for lim ited areas in th e core, w here b o th elem ental an d isotopic geotherm om eters record m agm atic tem peratures. The M ount S tu art b ath o lith has b een th e subject of paleom agnetic controversy, because it yields a paleom agnetic direction quite different th an th a t expected for C retaceous rocks of N o rth Am erica. This has led som e to prop o se th at th e p lu to n form ed a t the p resen t latitude of Baja C alifornia • d u rin g the C retaceous, then w as translated n orthw ard to its site in central i I W ashington. A lternatively, post-em placem ent tilt could cause the I discordance. Paleohorizontal studies using geobarom etry have indicated th at r tilt w as m inor, calling u p o n the translation explanation for m agnetic I sig n atures. Fluid in filtration o f the M ount S tu art batholith has reset the | elem ental an d isotopic com positions w ith in m inerals of the p lu to n , lim iting [ th e areas useful for su ch studies and m aking application o f alum inum -in- f c u . j j h o rnblende barom etry questionable. Subsolidus infiltration a t high j tem peratures of flu ids equilibrated w ith th e C hiw aukum schist has obscured I som e im p ortant clues abo u t the form ation o f the M ount S tu art b atholith. f | 81 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Bibliography A gue, J .J., and B randon, M.T., 1992, T ilt and n o rth w ard offset o f C ordilleran batholiths resolved using igneous barom etry: N ature, v. 360, p . 146- 149. A gue, J.J., and B randon, M.T., 1996, Regional tilt o f th e M ount S tu art b ath o lith , W ashington, d eterm in ed u sin g alum in um -in -h om b lende barom etry: Im plications fo r n o rth w ard tran slatio n o f Baja B ritish Colum bia: G eological Society o f Am erica B ulletin, v. 108, p . 471-488. A nderson, J.L., 1992, C om positional variation w ith in th e high-M g, tonalitic M ount S tu art batholith, N o rth C ascades, W ashington: G eological Society of A m erica A bstracts w ith Program s, v. 24, p. 3. A nderson, J.L., 1996, Status of therm obarom etry in granitic batholiths: Transactions o f die Royal Society o f E dinburgh: E arth Sciences, v . 87, p. 125-138. < \ A nderson, J.L., 1997, Regional tilt o f the M ount S tu art batholith, W ashington, f determ ined u sin g alum inum -in-hom blende barom etry: Im plications i for n o rth w ard translation o f Baja B ritish C olum bia: D iscussion: Geological Society of A m erica B ulletin (in press). A nderson, J.L., an d Sm ith, D.R., 1995, The effects o f tem perature a n d /C>2 on the A l-in hornblende barom eter: A m erican M ineralogist, v . 80, p . 549- 559. : A nderson, J.L., Francis, J., and M orrison, J., 1996, B atholith m argin infiltration o f contact aureo le fluids: G eological Society o f A m erica A bstracts w ith [ Program s, v . 28, p. A-420. Beck, M.E., and N oson, L., 1972, A nom alous p aleo latitu d e in C retaceous granitic rocks: N ature, v. 235, p . 11-13. Beck, M .E.,Jr., B urm ester, R.F., an d Schoonover, R., 1981, Paleolm agnetism an d tectonics of the C retaceous M t. S tuart B atholith of W ashington: i Translation o r tilt: E arth an d Planetary Science L etters, v. 56, p . 336-342. B endixen, J.E., M orrison, J., and P aterson, S.R., 1991, T herm obarom etry of the pelitic C hiw aukum Schist, N o rth C ascades, W ashington: G eological Society of A m erica A bstracts w ith Program s, v. 23, p. A-445. 82 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Bigeleisen, J., and M ayer, M.G., 1947, C alculation o f eq u ilib riu m constants fo r isotopic exchange reactions: Journal o f C hem istry a n d Physics, v. 15, p . 261-267. B ishop, F.C., 1980, T he d istrib u tio n o f Fe^+ an d M g b etw een coexisting ilm enite and p yroxene w ith applications to geotherm om etry: A m erican Jo u rn al o f Science, v . 280, p . 46-77. B lundy, J.D ., an d H o llan d, T.J.B., 1990, Calcic am phibole e q u ilib ria and a new am phibole-plagiodase geotherm om eter: C o n trib u tio n s to M ineralogy an d Petrology, v . 104, p. 208-224. B ottinga, Y. and Javoy, M ., 1975, O xygen isotope p artitio n in g am ong the m inerals in ign eo u s and m etam orphic rocks: R eview s o f G eophysics an d Space Physics, v. 13, p . 401-418. Bottinga, Y., and Javoy, M ., 1973, C om m ents o n oxygen iso to p e geotherm om etry: E arth an d P lanetary Science L etters, v. 20, p. 250-265. B randon, M.T., and C ow an, D.S., 1985, The L ate C retaceous San Ju an Islands— N orthw estern C ascades T h ru st System : G eological Society of A m erica A bstracts w ith Program s, v. 17, p. 343. B randon, M .T., C ow an, D.S., and V ance J.A ., 1988, The la te C retaceous San Ju an T hrust System , San Ju an Islands, W ashington: A case history o f terran e in the w estern cordillera: G eological Society o f A m erica Special P aper 221,81p. B row n, E.H ., and W alker, N.W ., 1993, A m agm a loading m o d el for B arrovian m etam orphism in the SE C oast Plutonic C om plex, B ritish C olum bia an d W ashington: G eological Society o f A m erica B ulletin, v. 105, p . 479- 500. B utler, R.F., D ickinson, W.R., G ehrels, G.E., M cC lelland, W .C ., M ay, R.C., an d K lepacki, D., 1989, D iscordant paleom agnetic poles fro m th e C anadian C oast Plutonic Com plex: R egional tilt rath e r th an large-scale displacem ent?: Reply: G eology, v. 18, p. 1165-1166. C hiba, H ., Chacko, T., C layton, R.N ., G oldsm ith, J.R., 1989, O xygen isotope fractionations involving diopside, fo rsterite, m ag n etite, a n d caldte: A pplication to geotherm om etry: G eochem ica e t C osm ochem ica A cta, v. 53, p. 2985-2995. 83 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. C layton, R.N., a n d Epstein, S., 1961, The u se o f oxygen iso to p es in high tem perature geological therm om etry: Journal of G eology, v. 69, p. 447. 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M cDougall, J.W ., 1980, G eology an d stru ctu ral evolution o f th e Foss River- D eception C reek area, C ascade M ountains, W ashington: [M.S. thesis] C orvallis, O regon S tate U niversity, 86p. M iller, R.B., 1977, S tructure a n d petrology o f th e Ingalls m afic-ultram afic com plex an d associated pre-T ertiary rocks, C entral W ashington Cascades: G eological Society o f A m erica A bstracts w ith Program s, v. 9, I p. 468. t M iller, R.B., 1980, The W indy P ass th ru st an d th e em placem ent o f the Ingalls ophiolite com plex, cen tral W ashington C ascades: G eological Society o f A m erica A bstracts w ith Program s, v. 12, p . 141-142. i M iller, R.B., 1985, The o ph io litic Ingalls com plex, north-central C ascade M ountains, W ashington: G eological Society o f A m erica B ulletin, v. 96, p. 27-42. M iller, R.B., an d M ogk, D.W ., 1987, U ltram afic rocks of a fracture-zone ophiolite, N o rth C ascades, W ashington: Tectonophysics, v. 142, p. 261- [ 289. 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I V alley, J.W ., K itchen, N ., Kohn, M.J., N iendorf, C.R., and Spicuzza, M.J., 1995, 1 UW G-2, a garnet standard for oxygen isotope ratios: Strategies for high I precision and accuracy w ith laser heating: G eochim ica e t ; Cosm ochim ica A cta, v. 59, p . 5223-5231. » 1 V ance, J.A ., D ungan, M .A., Blanchard, D.P., and R hodes, JM ., 1980, Tectonic settin g and trace elem ent geochem istry of M esozoic ophiolite rocks in w estern W ashington: A m erican Journal o f Science, v. 280-A, p. 359- 388. 90 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. W hite, L.D., M aley, C.A., B arnes, I., an d F ord A.B., 1988, O xygen isotopic d ata for p lu to n ic rocks a n d gneisses o f th e G lacier Peak W ilderness an d vicinity, n o rth ern C ascades, W ashington: USGS O pen-file R eport 86- 76. Z heng, Y.F., 1993a, C alculation o f oxygen isotope fractionation in an hydrous silicate m inerals: G eochim ica e t C osm ochim ica A cta, v. 57, p . 1079- 1091. Z heng, Y.F., 1993b, C alculation o f oxygen isotope fractionation in hydroxyl- bearing silicates: E arth an d P lanetary Science L etters, v. 120, p . 247-263. 91. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A ppendix A. C orrelation coefficients and significance of correlation am ong data sets from the M ount Stuart batholith. = coefficient of determ ination; r = correlation coefficient; n = num ber of sam ples; d.f. = degrees of freedom ; r (0.5) and r (0.1) = m inim um value of r for correlation at the 5% and 1% confidence levels, respectively. v £ > N > Variables Calculated correlation Critical values of r S ig n M leant Correlation? Equation X y r * r n d.f. r (0,5) r (0.1) at 5 % level at 1 % level high/low W R 0.158 0.397 26 24 0.388 0.496 yes no moderate W R T (p l-h b r im s ) 0.102 0.320 20 18 0.444 0.561 no no none y=-8,2474xt747,33 W R T (p l-h b c o r e s ) 0.193 0.439 14 1 2 0.532 0.661 no no none y=-13,151x+843,92 W R qtz 0.612 0.782 29 27 0.367 0.470 yes yes high y=0.6972xt4.1638 W R P * 0.001 0.024 4 2 0.950 0.990 no no none ys0.011xt6.7983 W R plag 0.571 0.755 33 3 1 0.349 0.449 yes yes high y=0,5817x+3,4316 W R P (p l-h b r im s ) 0.000 0.004 16 14 0.497 0.623 no no none W R mu 0.970 0.985 3 1 0.997 1.000 no no none ysl.3715xt6.1849 W R hbl 0.027 0.165 25 23 0.396 0.505 no no none ys0.0764xt5.9939 W R gnt 0.274 0.524 4 2 0.950 0.990 no no none y=l.l039x-3.2554 W R Aqtz-plag 0.039 0.197 28 26 0.374 0.478 no no none ys0.1128xt0.9687 W R Aqtz-biot 0.068 0.260 28 26 0.374 0.478 no no none ys0.1558xt3.3054 W R Abiot-chlt 0.281 0.530 15 13 0.514 0.641 yes no moderate ys-0.2543xt2.4784 W R chit 0.518 0.720 14 12 0.532 0.661 yes yes high ys0.8566x-2.2035 W R biot 0.335 0.578 31 29 0.355 0.456 yes yes moderate ys0.5211xtl.3251 W R ___AvgT 0.034 0.183 32 32 0.349 0.449 no no none SiO, WR^h 0.280 0.529 31 29 0.355 0.456 yes yes moderate y=0,2787x-8.9076 SiO, V V R q m r t i d t o i d 0.129 0.359 7 5 0.754 0.874 no no none y=-0.1326xtl6.267 SiQ, W I^ a n lM i, 0.947 0.973 5 3 0.878 0.959 yes yes high ys0.4127x-18.226 SiO, W R **,. 0.175 0.418 9 7 0.666 0.798 no no none ys0.1368xt0.8289 SiO, W R 0.306 0.553 6 1 59 0.250 0.325 yes yes moderate ys0.1791x-l.9513 SiO, T (p l-h b r im s ) 0.490 0.700 18 16 0.468 0.590 yes yes high y=-7.1302xtll24.7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A ppendix A. (continued) vo C O Variables Calculated correlation Critical values of r SignilScant Correlation? Equation X y i * r n d.f. r (0.5) r ( 0 . 1 ) at 5 % level at 1 % level high/low SiO, T (p l-h b cores) 0.430 0.656 14 12 0.532 0.661 yes no m oderate y=-6.241x+1123.4 SiO, qtz 0.329 0.574 26 24 0.388 0.496 yes yes m oderate SiO, P X 0.494 0.703 4 2 0.950 0.990 no no none SiO, plag 0.172 0.415 32 30 0.349 0.449 yes no m oderate SiO, P (p l-h b r im s ) 0.012 0.107 14 12 0.532 0.661 no no none SiO, m u 0.716 0.846 3 1 0.997 1.000 no no none SiO, hbl 0.198 0.445 24 22 0.404 0.515 yes no m oderate SiO, Aqt-pl 0.044 0.209 28 26 0.374 0.478 no no none SiO, chit 0.012 0.110 15 13 0.514 0.641 no no none SiO, biot 0.132 0.363 28 26 0.374 0.478 no no none qtz P . , a g 0.518 0.719 29 27 0.367 0.470 yes yes high y=0,5076x+3.3643 qtz qtz hbl 0.059 0.243 22 20 0.423 0.537 no no none chit 0.614 0.784 14 12 0.532 0.661 yes yes high y=1,2153x+7,4949 qtz biot 0.546 0.739 26 24 0.388 0.4% yes yes high y=0,6613x-1.1077 plag < 1 * * 0.518 0.719 29 27 0.367 0.470 yes yes high y=l,01%x+1.7935 P . , a S PX 0.698 0.836 4 2 0.950 0.990 no no none plag hbl 0.030 0.174 17 15 0.482 0.606 no no none y=0.1215x+5,6285 plag gnt 0.965 0.982 4 2 0.950 0.990 yes no high y=1.088x-2,7098 P ! a g chit 0.554 0.744 14 12 0.532 0.661 yes yes high ysl.2653x-5.1302 plag biot 0.622 0.789 30 28 0.361 0.463 yes yes high y=0.9127x-1.9043 biot chit 0.861 0.928 14 12 0.532 0.661 yes yes high y=l,1963x-1.1971 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A ppendix B. Fractionations betw een m inerals w ithin rocks of the M ount Stuart batholith and C hiw aukum Schist. Fractionations are not show n for m inerals occurring in only a few sam ples, qtz = quartz, plag = plagioclase, hbl = hornblende, biot = biotite, px = pyroxene, chit = chlorite. Sample A (quaitz-miner*!) A (pUgiodaM-mincnl) A (homblcnde-mincral) A (biotite-mineral) qtz-plag qtz-hbl qtz-biot qtz-chlt qtz-px plag-hbl plag-biol plag-chlt plag-px hbl-biot hbl-chlt hbl-px biobchlt biot-px • oftampltt 27 22 27 15 2 25 31 14 4 24 11 4 15 3 average 2.057 3.797 4.808 5,068 1.998 1.967 2.690 2.753 1,119 0.721 1.270 -0.392 ■0.018 ■0,636 std dev 0.926 1.363 0.978 0.995 0.551 1.001 0.904 1.072 0.175 1.069 0.656 0.531 0,630 0.778 median 1.979 3.913 5.094 5.429 1.998 1.923 2.519 3.061 1.129 0.829 1.111 -0.361 0,068 -0.747 M OUNT STUART BATHOUT H 427 1.492 2.820 3.833 4.101 1.328 2.341 2.609 1.013 1.281 0.268 93SE-06 1.286 3.613 3.790 4.603 2.327 2.504 3.317 0.177 0.990 0.813 93SE-09 2.009 2.898 3.966 0.889 1.957 1.068 93SE-12a 3.253 4.088 6.292 6.445 0.835 3.039 3.192 2.204 2.357 0.153 93SW-04 2.259 3.647 6.168 5.919 1.388 3.909 3.660 2.521 2.272 -0.249 93SW-06 1.871 4.465 5.108 5.831 2.594 3.237 3.960 0.643 1.366 0.723 93SW-10 1.938 4.198 5.583 6.075 2.260 3.645 4.137 1.385 1.877 0.492 93SW-14 1.949 4.498 5.200 2.549 3.251 0.702 CR304-1 1.690 3.913 5.080 2.223 3.390 1.167 CSP-01 2.853 5.120 2.267 CSP-02 2.932 4.807 1.875 CSP-06b 2.563 4.243 2.618 1.680 0.055 -1.625 IC-10 0.568 1.639 1.608 1.071 1.040 -0.031 KSE91-02a 2.776 5.203 5.645 2.427 2.869 0.442 VO 4 * with permission of the copyright owner. Further reproduction prohibited without permission. CD ■ o - 5 O Q . Q . A ppendix B. (continued) Sam ple A (quaitz-minenl) A (plagioclue-mineral) A (hombtc nde-mincnri) |; A (biotitc-mincnO 1 qtz-plag qtz-hbl qtz-biot qtz-chlt qtz-px plag-px hbl-biot hbl-chlt hbl-px biot-chlt biot-px KSE91-02b 1.630 4.413 5.704 2.783 4.074 — ■ ........ 1.291 KSE91-03 1.141 5,376 4.569 4.235 3.428 -0.807 KSE91-04 1.022 2.972 4.542 4.083 1.950 3.520 3.061 1.570 1.111 i -0.459 KSE91-09 3.312 4.632 5.426 5.494 1.320 2.114 2.182 0.794 0.862 0.068 KSE91-11 4.027 4.789 5.618 5.429 0.762 1.591 1.402 0.829 0.640 -0.189 KSE91-13 2.361 5.336 5.706 2.975 3.345 0.370 KSE91-15 0.253 4.635 5.403 4.382 5.150 0.768 1 i KSE91-19 1 1.895 0.722 0.914 -1.173 -0.981 0.192 KSE91-22a 2.967 7.999 4.875 5.032 1.908 -3.124 1 KSE91-27a 1.995 2.051 1.304 0.056 -0.691 -0.747 KSE91-27b 1.170 2.252 3.740 2.387 1.082 2.570 1.217 1.488 0.135 -1.353 KSE91-30a 2.534 4.369 4.982 4.663 1.835 2.448 2.129 0.613 0.294 -0.319 KSE91-33a 0.963 1.419 1.500 1.956 0.456 .............. i KSE91-42 2.882 3.912 4.782 4.837 1.030 1.900 1.955 0.870 0.925 0.055 KSW91-03 1.007 3.219 4.134 2.212 3.127 0.915 KSW91-05a 2.351 2.646 .... , 0.295 ----- 1 KSW91-06a 1.016 3.002 3.624 1.986 2.608 0.622 KSW91-08 2.830 4.393 5.271 1.563 2.441 0.878 CHIWAUK1JM SC H I iT MS-30A 2.533 VO U l with permission of the copyright owner. Further reproduction prohibited without permission. CD ■ o - i O Q . Q . A ppendix C. Oxygen stable isotope therm om etry for the M ount Stuart batholith. Tem peratures are derived using the calibrations of Bottinga and Javoy (1975) and Chiba and others (1989). WR5"a 8.3 10.6 7.2 9.6 8.5 9,1 8,7 8.4 8.1 11.3 1Z5 1Z7 Sample#: 427 93SE-06 93SE-09 93SE-12a 93SW-04 93SW-06 93SW-10 93SW-14 C R304-1 CSP-01 CSF-02 CSF-06b Mineral-pair 4 values Aqt-pl 1.492 1.286 2.009 3.253 Z259 1.871 1.938 1.949 1.690 Z853 Z563 Z563 4qt-amp 2.820 3.613 Z898 4.088 3.647 4.465 4.198 4.498 3.913 13.514 ---- Aqt-blol dpl-amp 3.833 3.790 3.966 6.292 6.168 5.108 5.583 5.200 5.080 5.120 4.243 4.243 1.328 2.327 0.889 0.835 1.388 2.594 2.260 2.549 2.223 10.951 dpl-biot | Z341 2.504 1.957 3.039 3.909 3.237 3.645 3.251 3.390 Z267 1.680 1.680 damp-biot 1.013 0.177 1.068 2.204 Z521 0.643 1.385 0.702 1.167 •9,271 Plggioclase— Avg X * Fractionation coefficient* 0.306 0.304 0311 0.366 0.348 0.339 0.307 0.292 0.438 0.384 Chiba and others (1989) 1 Aqt-pl 1.2 61 1.259 1.267 1.324 1.305 1.296 1.262 1.247 1.400 1.343 Bottinga St Javoy (1975) B 0.00 Aqt-pl 1.288 1.286 1.293 1.351 1.332 1,323 1.289 1.274 1.426 1.369 B -030 Aqt-amp 3.148 3.148 3.148 3.148 3.148 3.148 3.148 3.148 3.148 3.148 3.148 3.148 B -0.60 Aqt-biot 3.690 3.690 3.690 3.690 3.690 3.690 3.690 3.690 3.690 3.690 3.690 3.690 B •030 Apl-amp 1.860 1.862 1.855 1.797 1.816 1.825 1.859 1.874 1.722 1,779 B •0.60 Apl-biot 2.402 2.404 2.397 Z339 2.358 Z367 Z401 2.416 Z264 Z321 B Temp -030 erature< Aamp-biot i.’ n a f ififlJ w u H t ’ C) 0.542 0.542 iM 'S S W ttW IW 0.542 W M tnsw w -ij 0.542 rKm zaW KK 0.542 t 0.542 t-visw ri,*. 0.542 ■l6t&5t29tMy 0.542 0.542 B W R -W M B iJ Y 0.542 ruauaawm 0.542 0.542 mrNejsrum Chiba and others (1989) quartz-plag 646 716 52 1 365 487 559 532 586 427 451 Bottinga & Javoy (1975) quartz-plag 656 727 529 371 495 568 540 595 434 458 quartz-amphibole 7 31 624 719 574 620 540 563 537 5 91 quartz-biotite 639 644 626 459 465 531 499 524 533 530 600 600 plag-amphibole 796 569 976 985 764 521 535 589 yii. plag-biotite 631 607 695 529 450 512 516 505 616 736 amphibole-biotite or all mineral pairs 369 W ttW U /M Y 637 793 660 356 r.,t fflS t.’ S R U fi 650 192 'waasr-u 518 165 493 485 526 294 .T .iM w atarii: 492 462 ii* * ? 519 335 525 526 th ao iM i& ao A 597 A tfttK U K ft; ’* • 598 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A ppendix C. (continued) v O WRS'th 7.3 10.5 8.3 11.6 10.6 9.7 9.8 10.8 12.1 12.6 8.3 8.8 Sample#; 1C-10 KSE91-02a KSE91-02b K SE91-03 K SE91-04 KSE91-09 KSE9M1 KSE91-13 KSE91-15 KSE91-22a KSE91-27a KSE91-27b Mineral-pair A values Aqt-pl 0.568 2.776 1.630 1.1 41 1.022 3.312 4.027 2.361 0.253 2.967 1.170 Aqt-amp 1.639 4.413 10.488 2.972 4.632 4.789 12.356 4.635 7,999 2.252 Aqt-biot 5.203 5.704 5.376 4.542 5.426 5.618 5.336 5.403 4.875 3.740 Apl-amp 1.0 71 2.783 9.347 1.950 1.320 0.762 9.995 4.382 5.032 1.995 1.082 Apl-biot 2.427 4.074 4.235 3.520 2.114 1.5 91 2.975 5.150 1.908 2.051 2.570 Aamp-biot 1.2 91 -5.112 1.570 0.794 0.829 •7.020 0.768 -3.124 0.056 1.488 Plagioclase— Avg X n. 0.512 0.248 0.378 0.333 0.388 fractionation coefficient* Chiba and others (1989) ]Aqt-pl 1.478 1.200 1,337 1.290 1.347 Bottinga & Javoy (1975) B 0.00 Aqt-pl 1.502 1.228 1.363 1.316 1,374 B -030 Aqt-amp 3.148 3.148 3.148 3.148 3.148 3.148 3.148 3.148 3.148 3.148 3.148 3.148 B -0.60 Aqt-biot 3.690 3.690 3.690 3.690 3.690 3.690 3.690 3.690 3.690 3.690 3.690 3.690 B -030 Apl-amp 1.646 1.920 1.785 1.832 1.774 B -0.60 Apl-biot 2.188 2.462 2.327 2,374 2.316 B Temp -030 erature( Aamp-biot : *.i *C ) 0.542 0.542 t'M S K S W W i 0.542 iw imm'#* 0.542 ■ ■ ■ 0.542 't.j.f.'rss’ B 'J i 0.542 WMWaftt 0.542 v r . ' - ’ S i ' S d . J T . v 0.542 0.542 W & t t ' W W K f t l 0.542 W fM U W f c tt tt 0.542 0.542 Chiba and others (1989) quartz-plag 1340 384 632 1985 401 Bottinga St Javoy (1975) quartz-plag 1353 392 641 2008 407 quartz-amphibole 1 0 0 1 544 708 526 513 526 343 837 quartz-biotUe 524 492 513 574 509 497 515 511 548 649 plag-amphibole 822 488 352 304 plag-biotite 629 432 369 688 At-j/ amphibole-biotite wall mineral pairs 1059 515 311 1 !i V ’ "Cj'f 485 5 88 265 ; A r t 5 ? . . C . f . l - n 587 431 546 420 ■ I ' S t S M H S V i i f c ; 589 O W i S i K J ' t - 1 - # 510 439 U i - W B l K f t W t 7 0 1 -273 458 % 1 729 277 667 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A ppendix C. (continued) vO 00 m s x b , 8.5 7.1 8.3 8.5 9.3 8.8 9.2 All Sample* Normal 6uO Elevated 8“ 0 Sample < h KSE91-30J KSE91-33a KSE91-42 KSW91-03 KSW 91-05a KSW91-06a KSW91-08 Average ±S,D, Average ±S.D. Average ±S.D. Mineral-pair A values Aqt-pl 2.534 -0337 2882 1.007 -9.102 1.016 2.830 Aqt-amp 4.369 0.963 3.912 3.219 -6.751 3.002 4.393 Aqt-biot 4.982 1.419 4.782 4.134 -6.456 3.624 5.271 Apl-amp 1.835 1.500 1.030 2.212 2351 1.986 1.563 Apl-biot | 2.448 1.956 1.900 3.127 2646 2608 2441 Aamp-biot 0.613 0.456 0.870 0.915 0.295 0.622 0.878 i l loelase— Avg X «, ■ H M M M t t t i S 0.466 N B M B H M w im m m n i a n m m i i m m u 0.420 0.322 ionation coefficients Chiba and others (1989) Aqt-pl 1.429 1.3 81 1.278 Bottinga & Javoy (1975) B 0.00 Aqt-pl 1.455 1.407 1.305 B •030 Aqt-amp 3.148 3.148 3.148 3.148 3.148 3.148 3.148 B -0.60 Aqt-biot 3.690 3.690 3.690 3.690 3.690 3.690 3.690 B -030 Apl-amp 1.693 1.741 1.843 B •0.60 Apl-biot 2.235 2283 2385 B sssa# Temp -0.30 jtw sia v eratmel Aamp-biot M W flttSM tfp T) 0.542 iv t!» n '.s c < 0.542 •jC W ttfS O fS W 0.542 iM W S K iW S U i 0.542 AlWrnSVKH 0.542 it.sb M W P in ;-* 0.542 ■.r.i.e.KKKftM 0.542 l.tMHKSHteJ .M A N U IN U AtMMta msamum H tnm M M M Chiba and others (1989) quartz-plag 893 399 666 415 659 272 647 551 Bottinga & Javoy (1975) quartz-plag 904 406 675 419 669 274 657 557 quartz-amphibole 548 1306 5 91 673 703 546 646 194 691 208 555 136 quartz-biotite 540 1079 555 610 662 520 570 115 595 147 551 52 plag-amphibole 697 600 722 648 201 683 1 5 1 553 311 plag-biotite 662 570 612 574 101 559 89 614 121 IM tiE A vgf amphibole-biotite SittK'CW ifmHt'i'M i'i or alt mineral pairs 497 545 574 863 407 586 395 609 681 t.'-r'cii 586 494 655 405 iH v M ito S W W tt 535 406 M/Sitivai 599 230 tttiU taiT 120 439 W 'fSO R i 620 181 146 283 K M tG U M I 571 388 /sH m m 69

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Creator Francis, Julie Christine (author)

Core Title Oxygen isotopic evidence for fluid infiltration in the Mount Stuart batholith, Washington

Degree Master of Science

Degree Program Geological Sciences

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

Tag geochemistry,Geology,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-15144

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