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For permission to copy, contact [email protected] 2001 Geological Society of America1486

GSA Bulletin; November 2001; v. 113; no. 11; p. 1486–1502; 14 figures; 2 tables; Data Repository item 2001125.

Geology and geochemistry of mafic to felsic plutonic rocks in theCretaceous intrusive suite of Yosemite Valley, California

Kent Ratajeski*Allen F. GlaznerBrent V. MillerDepartment of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599-3315, USA

ABSTRACT

The intrusive suite of Yosemite Valleyprovides an excellent example of coeval maf-ic and felsic magmatism in a continental-margin arc. Within the suite, hornblendegabbros and diorites associated with theCretaceous El Capitan and Taft Granitesoccur as scattered mafic enclaves, enclaveswarms, small pods, synplutonic dikes, anda 2 km2 mafic complex known as the ‘‘di-orite of the Rockslides.’’ Field evidence sug-gests that most of the mafic rocks are tem-porally related to the El Capitan Graniteand that significantly less mafic magma ac-companied the slightly later intrusion of theTaft Granite. Concordant zircon fractionsfrom the diorite of the Rockslides yield anage of 103 6 0.15 Ma, which is the sameage as the El Capitan Granite. Initial iso-topic compositions of the mafic and felsicrocks are similar; the mafic rocks exhibitonly slightly higher 87Sr/86Sr, lower 143Nd/144Nd, and higher 206Pb/204Pb ratios than thegranites. Because the mafic magmas areonly slightly more isotopically evolved thanthe granites, geochemical variation withinthe granites is not easily explained in termsof contamination of a depleted-mantle com-ponent by partial melts of ancient, high-sil-ica continental crust. Rather, these data areconsistent with an interpretation that the ElCapitan Granite was derived by partialmelting of relatively young mafic sourcesbroadly similar to the mafic rocks of thesuite.

Keywords: granite, isotopes, mafic mag-

*Present address: Department of Geology, North-ern Arizona University, Box 4099, Flagstaff, Ari-zona 86011, USA; e-mail: [email protected].

mas, Sierra Nevada batholith, YosemiteNational Park.

INTRODUCTION

The importance of mafic magmatism for thegeneration of calc-alkalic plutons and batho-liths in convergent-margin settings is widelyacknowledged. Mafic magmas that are under-plated beneath or injected into the lower crusthave been called upon as thermal triggers forlower-crustal partial melting (e.g., Hildreth,1981; Bergantz, 1989), parental magmas forcrystallization differentiation (e.g., Bowen,1928, Bateman and Chappell, 1979), and end-member components of magma mixing (e.g.,Allegre and Othman, 1980; Castro et al.,1991).

Historically, large-scale mafic magmatismin ancient arc settings has been inferred large-ly from geochemical and isotopic studies ofthe granites (sensu lato) that dominate the up-per-crustal parts of arcs. Because of the rela-tive rarity of mafic plutons in granitic batho-liths, and perhaps also because of theentrenched notion that mafic plutons are ‘‘pre-cursors’’ to the granites, only a few studies ofthe Sierra Nevada, for example, have exam-ined the relationships between larger expo-sures of mafic rocks and the nearby granites(Frost and Mahood, 1987; Bradford, 1995;Coleman et al., 1995; Sisson et al., 1996).Much more common are studies of smaller-scale mafic enclaves—quenched volumes ofbasaltic to dioritic magma entrained within si-licic magma chambers (e.g., Furman andSpera, 1985; Barbarin, 1990; Dorais et al.,1990; Didier and Barbarin, 1991). Geochro-nologic studies of mafic plutons also lag be-hind those of the more voluminous granites.Relating map-scale exposures of mafic rockswithin batholiths to crustal-scale arc petrogen-esis remains an important goal in igneous pe-

trology. In this paper we present new field,geochronologic, geochemical, and isotopicdata bearing on the relationship of the graniticrocks in western Yosemite Valley to nearbymafic plutonic rocks.

GEOLOGIC SETTING

Yosemite Valley is located in the west-cen-tral part of the Sierra Nevada batholith, Cali-fornia. The geology of the western half of thevalley (Fig. 1) is dominated by middle Cre-taceous (ca. 100 Ma; Stern et al., 1981) gra-nitic rocks associated with the intrusive suiteof Yosemite Valley (Bateman, 1992). In thevicinity of Yosemite National Park, plutonicrocks of this suite intrude Paleozoic metase-dimentary rocks and granodioritic and tonali-tic plutons of the Fine Gold Intrusive Suite(ca. 115 Ma; Bateman, 1992). In the easternhalf of Yosemite Valley, granites of the intru-sive suite of Yosemite Valley are intruded byLate Cretaceous granodiorites and granites re-lated to the Tuolumne Intrusive Suite (Bate-man, 1992), which were emplaced from 95 to85 Ma (Fleck et al., 1996; Coleman and Glaz-ner, 1997). Intrusive relationships (Calkins etal., 1985) and geochronologic data (Stern etal., 1981) indicate an eastward shift in the lo-cus of magmatism through the Yosemite re-gion during the Cretaceous, a pattern that isreflected in the Cretaceous batholith as awhole (Bateman, 1992).

ROCK UNITS AND FIELDRELATIONSHIPS

Rock units that are currently assigned to theintrusive suite of Yosemite Valley are the ElCapitan and Taft Granites (Bateman, 1992).The El Capitan Granite is the older and morevoluminous unit of the two and tends to sur-round smaller masses of Taft Granite. Our

Geological Society of America Bulletin, November 2001 1487

CRETACEOUS INTRUSIVE SUITE OF YOSEMITE VALLEY, CALIFORNIA

Figure 1. Geology of the western part of Yosemite National Park, modified from Huber,Bateman, and Wahrhaftig (Huber et al., 1989). Diorite and gabbro are abundant in theregion and are colored black on the map, regardless of age.

field work suggests that mafic enclaves,schlieren, small pods of mafic rocks, and thediorite of the Rockslides mafic complex arespatially and temporally associated with the ElCapitan Granite, whereas the Taft Granite issimilarly associated with mafic enclaves,schlieren, and the large mafic dike complexthat is visible on the North America Wall ofEl Capitan (Reid et al., 1983). A steeply dip-ping, north-northwest–striking foliation is pre-sent in many of these rocks; we interpret allfoliation described herein to be magmatic.Mafic rocks continue to be spatially associated

with these granites for ;10 km to the south,but we have focused almost exclusively on theYosemite Valley, where a variety of mafic tofelsic rock units associated with the intrusivesuite of Yosemite Valley are exposed nearlycontinuously over a 1.2 km vertical section.

Granites

The El Capitan Granite, the dominant geo-logic unit within the western half of YosemiteValley (Fig. 2), ranges from coarse-grained bi-otite granodiorite to biotite granite and is equi-

granular to porphyritic with potassium feld-spar megacrysts (Bateman, 1992). Althoughthe granite shows some limited variability incomposition and texture, these features do notappear to describe a regular pattern as forzoned plutons. Concordant U-Pb zircon datasuggest an age of 102–103 Ma for the unit(Stern et al., 1981). We attempted to date theEl Capitan Granite for this study (see below),but our attempts resulted in discordant data,which plotted slightly below concordia in thevicinity of 102–105 Ma, possibly indicating acombination of Pb loss and inheritance ofslightly older zircons.

The Taft Granite (Calkins, 1930) has a low-er color index than the El Capitan Granite anda finer-grained, equigranular texture. Most ofthe Taft Granite is medium grained, but someoutcrops are as coarse grained as the equi-granular part of the El Capitan Granite.Throughout the western half of Yosemite Val-ley, Taft Granite forms dikes and larger mas-ses that sharply cut the El Capitan Granite andassociated mafic rocks. The precise age of theTaft Granite is unknown. A discordant date of95 Ma was reported by Stern et al. (1981), andour attempts to date the Taft resulted in dis-cordant data similar to those obtained for theEl Capitan Granite.

Granite-Hosted Mafic Enclaves, Schlieren,and Pods

Mafic rocks are locally plentiful in the ElCapitan pluton. Enclaves are typically ellip-soidal, dioritic, medium grained, and equi-granular to porphyritic; they occur as small(;0.1 m), isolated bodies and in large swarms(Fig. 3A). Some have chilled margins. Thosein the larger swarms are usually surroundedby a foliated, granitic matrix, which is deplet-ed in potassium relative to normal El Capitangranite. Porphyritic enclaves contain plagio-clase and quartz xenocrysts from the El Cap-itan Granite (Barbarin, 1990). Plagioclase xen-ocrysts in porphyritic enclaves are essentiallyunzoned and have compositions (An35–40) sim-ilar to those of the unzoned plagioclase grainsin the surrounding granitic matrix and to cal-cic cores within the El Capitan Granite (TableDR6 in GSA Data Repository1). Mafic en-claves, which are less common within the TaftGranite than in the El Capitan Granite, arecommonly sheared and grade into schlieren.

A complete gradation in size exists from

1GSA Data Repository item 2001125 is avail-able on the Web at http://www.geosociety.org/pubs/ft2001.htm. Requests may also be sent [email protected].

1488 Geological Society of America Bulletin, November 2001

RATAJESKI et al.

Figure 2. Geologic map of the western part of Yosemite Valley.

decimeter-scale gabbroic to dioritic enclaveshosted by the El Capitan Granite to mappablepods of gabbro and diorite. Mafic pods areconcentrated in the El Capitan Granite west ofthe Rockslides cliffs and across the MercedRiver near the Pohono Trail (Fig. 2). Pods ofmafic rocks associated with the El CapitanGranite are typically composed of medium-grained, equigranular to porphyritic (probablyxenocrystic; see below) gabbros and dioritessimilar to those in mafic enclaves. Pods rangein size from small bodies a few meters across(Fig. 3B) to larger bodies hundreds of metersacross. Many of these pods are elongated par-allel to the regional magmatic foliation. Somediorite pods have relatively sharp contactsagainst El Capitan Granite; foliation withinthe diorite generally parallels these contacts.Fine-grained, quenched margins at these con-tacts are locally present.

Mafic pods are porphyritic only in smallerpods and in the marginal regions of larger

pods (Fig. 3C), although it is possible that lessnoticeable, smaller xenocrysts are presentwithin otherwise equigranular varieties. Ob-vious xenocrysts in these rocks include round-ed quartz with hornblende reaction rims, andplagioclase with core compositions that matchcalcic plagioclase cores within the El CapitanGranite (An;40) (Table DR6 in Data Reposi-tory; see text footnote 1). Cores of xenocrysticplagioclase are successively mantled by ahighly calcic zone (An;84), which probablyformed during magma mixing, and then by arim of An40 plagioclase. Groundmass plagio-clase in these rocks contains An;79 cores andAn;41 rims. The interiors of plagioclase xen-ocrysts contain tiny hornblende inclusions(Fig. 4), the origin of which is uncertain, ashornblende is exceedingly rare in the granite.One possibility is that hornblende was a li-quidus phase in the granite, but reacted toform biotite during cooling. However, the factthat the hornblende inclusions are identical in

composition to hornblende in the groundmassof the mafic rocks suggests that these inclu-sions may have formed in channel-like cavi-ties within partially resorbed plagioclase dur-ing the introduction of the xenocrysts into themafic magma (R. Wiebe, 1999, personalcommunication).

Also present within some diorite pods areirregular, foliated ‘‘stringers’’ of leucocraticbiotite tonalite or quartz diorite, which com-monly separate finger-like splays of horn-blende diorite (Fig. 3B). Some stringers canbe traced into marginal regions of foliated,leucocratic tonalite to quartz diorite that sur-round mafic pods within El Capitan Granite(Fig. 3B). These ‘‘marginal sheaths’’ are a fewmeters thick, and many have sharp contactsagainst the surrounding, coarser-grained gran-ite. The K-depleted stringers and sheaths prob-ably represent El Capitan Granite that inter-acted with the mafic magma in the liquid andsolid states.

Diorite of the Rockslides Mafic Complex

The diorite of the Rockslides mafic com-plex (Calkins, 1930; Calkins et al., 1985) oc-curs in cliff-forming outcrops on the northside of the valley west of El Capitan (Fig.5A). The extensive talus deposits after whichthe unit is named are almost entirely com-posed of boulders derived from this unit; theyoffer fresher samples and more clearly ex-posed contact relationships than do the weath-ered outcrops above. An examination of thetalus and outcrops indicates that the dioriteconstitutes a composite mafic intrusion com-posed of a variety of different rock types. Theterm ‘‘diorite of the Rockslides’’ does not re-fer to a restricted rock type, but to the maficcomplex as a whole (Calkins, 1930).

Much of the lithologic diversity within thediorite of the Rockslides mafic complex oc-cupies a broad spectrum between two end-member rock types: (1) a fine- to medium-grained, weakly porphyritic, biotitehornblende gabbro or diorite and (2) a coarser-grained, more leucocratic, equigranular, horn-blende biotite quartz diorite or tonalite. Bothtypes are largely free of pyroxene, as clino-pyroxene occurs only rarely as small relictcores within hornblende. The finer-grained,more melanocratic gabbro generally occurs asmafic enclaves of various sizes within the leu-cocratic rock. These mafic enclaves rangefrom randomly oriented, centimeter-scale bod-ies to meter-thick, pillowed layers similar tothose described in other layered mafic-felsiccomplexes in the Sierra (Bradford, 1995; Co-leman et al., 1995; Sisson et al., 1996) and



Figure 3. Smaller-scale occurrences of mafic rocks associated with granites of the intrusivesuite of Yosemite Valley. (A) Large mafic enclave swarm in El Capitan Granite besideCalifornia State Highway 140 near Elephant Rock. (B) Wedge-shaped pod of diorite withinEl Capitan Granite at the base of the North America Wall of El Capitan. Note the ‘‘string-ers’’ of foliated leucocratic rock, which trend from upper right to lower left within theleft part of the pod. These stringers can be traced into a 0.5–1.5-m-thick ‘‘sheath’’ offoliated leucocratic rock, which surrounds the pod. (C) Porphyritic diorite associated withthe El Capitan Granite. Feldspar xenocrysts within the diorite were probably derived fromthe adjacent, partially molten granite. (D) Leucocratic quartz diorite ‘‘stringer’’ withinpod of diorite near Artist Creek.

elsewhere (e.g., Wiebe, 1993, 1996). In looseboulders and in outcrop (Fig. 5, B and C),meter-thick mafic layers are commonlystacked on top of each other, separated by in-tervening, 10–25-cm-thick septa of leucocrat-ic, medium-grained tonalite. Outcrops and pa-leohorizontal indicators within loose blocks oftalus (e.g., density-driven structures such astonalite protrusions into the bottoms of maficlayers; Fig. 5D) indicate an initially subhori-zontal orientation of the gabbroic pillows andsheets within the complex. Fresh surfaces ontalus reveal quenched, fine-grained margins ofthe gabbroic layers against the leucocratic ton-alite septa. In addition to the rock types al-ready described, rarer orbicular, rhythmicallylayered, and pegmatitic varieties of maficrocks also occur within the diorite of theRockslides mafic complex.

Contacts between that diorite and the ElCapitan Granite are exposed in cliffs 0.75 kmwest of Ribbon Falls and outcrops 0.5 kmnortheast of Fireplace Bluffs. Near RibbonFalls, gabbro has chilled against foliated, leu-cocratic quartz diorite, which in turn gradesinto weakly foliated, coarse-grained El Capi-tan Granite. In the exposures near FireplaceBluffs, diorite is complexly intermingled withEl Capitan Granite within a diffuse zone about0.5 m wide. Porphyritic textures are present inthe diorite within this zone but not outside it,suggesting that the feldspar ‘‘phenocrysts’’originated as xenocrysts from the adjacentgranite. Field relationships at these two loca-tions, as well as the lack of cutting of the ElCapitan Granite by dikes related to the dioriteof the Rockslides mafic complex, are consis-tent with the hypothesis that the diorite wasemplaced during the crystallization interval ofthe El Capitan Granite.

Mafic Dikes Exposed on El Capitan

A sizable mafic dike complex is exposed onthe 900-m-high eastern wall (the ‘‘NorthAmerica Wall’’) and summit dome of El Cap-itan (Fig. 6A). Reid et al. (1983) groupeddikes within this large complex into two sets.The older set, which includes large xenolithicblocks of partially digested El Capitan Gran-ite, is generally moderately dipping, lightercolored, biotite bearing and hornblende poor,and dioritic to granodioritic in composition.The younger set is steeply dipping, generallymore mafic and hornblende rich but compo-sitionally diverse, and rich in enclaves. It iscomposed of two large dikes: a western oneshaped like the outline of North America (Fig.6A) and an eastern one surrounding a largecircular area of white El Capitan Granite,


RATAJESKI et al.

Figure 4. Photomicrograph of a porphyritic (xenocrystic) diorite (sample YOS-77) from asmall mafic pod (crossed polarizers). Groundmass consists mostly of plagioclase, horn-blende, biotite, and minor quartz. Plagioclase xenocrysts contain partially resorbed cores,calcic zones, and inclusions of hornblende (hb; biotite is absent).

known to rock climbers as the ‘‘Great Circle,’’which is separated from ‘‘North America’’ byan intervening ‘‘Atlantic Ocean’’ of El Capi-tan Granite. Following Reid et al. (1983), werefer to both of these large, steeply dippingdikes as North America dikes, and we denotedikes of the older, moderately dipping set aspre–North America dikes.

Our observations at the base of the NorthAmerica Wall largely confirm the interpreta-tions of Reid et al. (1983) with regard to thefield relationships of the mafic dikes and theEl Capitan Granite. Pre–North America dikesexposed at the base of this wall have clearlyintruded totally solidified El Capitan Granite,locally breaking off and partially assimilatingblocks of granite in the process (Fig. 6B).Some of these dikes have sharp contactsagainst the El Capitan Granite and containpartially mingled mafic and felsic magmas(Fig. 6C).

The younger, North America Wall maficdikes extend across the summit dome of ElCapitan as a series of enclave-choked dikesthat strike N608W (Fig. 7). Contacts with theEl Capitan Granite are locally sharp (Fig. 6D),but elsewhere are more gradational, indicatingthat some interaction—perhaps partial meltingand assimilation—with El Capitan Granite hasoccurred here as well as on the North AmericaWall. Contacts of the mafic dikes with the TaftGranite are diffuse and characterized by broadzones in which dikes grade into highly elon-gate schlieren within the Taft Granite (Fig.

6E). Mafic schlieren are also present in narrowdikes of the Taft Granite that cut the El Cap-itan Granite along the El Capitan Trail 0.5 kmnorth of the summit. In some places on thesummit dome, the ‘‘dikes’’ are actually planarswarms of mafic enclaves (Tobisch et al.,1997) suspended in a matrix of Taft Graniteor remobilized El Capitan Granite. Enclavesin these outcrops are compositionally and tex-turally diverse and record a complex historyof interaction between mafic and felsic mag-mas (Fig. 6F). The synplutonic character ofthe North America dikes can be reasonablyinferred from these field relationships.

Calkins et al. (1985) mapped a North Amer-ica mafic dike along the contact of the El Cap-itan and Taft Granites approximately midwaybetween El Capitan summit and the edge ofthe North America Wall. Our mapping indi-cates that a foliated, leucocratic biotite tonalitecrops out at this location. This unit displayssharp, interdigitated contacts against the ElCapitan Granite, but grades into the adjacentTaft Granite. This tonalite is compositionallyvery similar to the biotite-rich, hornblende-poor pre–North America mafic dikes that havepartially digested quantities of adjacent ElCapitan Granite, and it may have formed in asimilar manner. Because this unit grades intothe Taft Granite and is lithologically similar toit, however, we interpret this unit as repre-senting coeval mafic material (similar to thatwithin the pre–North America dikes) that was

mixed within the marginal part of the TaftGranite magma chamber at this location.

These field relationships suggest that theNorth America mafic dike complex is coevalwith the Taft Granite and does not postdate itas previous workers have suggested (Calkins,1930; Calkins et al., 1985; Bateman, 1992).This interpretation best explains the strikingspatial association of the two units on thenorth side of the valley, where North Americamafic dikes are commonly in contact with TaftGranite and generally follow its arcuate mappattern.

GEOCHRONOLOGY

Rationale

As a test of the field interpretations, fivesamples were selected for conventional U-Pbzircon dating: El Capitan Granite (YOS-180);coarse-grained Taft Granite (YOS-1); a melan-ocratic, fine- to medium-grained sample ofgabbro from the diorite of the Rockslides maf-ic complex (YOS-23c); a leucocratic, medi-um- to coarse-grained diorite collected fromtalus below the cliffs bearing the diorite of theRockslides mafic complex (YOS-206); and afine-grained gabbro from the younger set ofNorth America mafic dikes on El Capitan(YOS-104). The methods employed in ouranalyses are summarized in Appendix 1, andsample locations are tabulated in Table DR1in Data Repository (see text footnote 1).

U-Pb ages for two fractions of El CapitanGranite zircons have been reported by Sternet al. (1981): one is concordant at 103 Ma,and the other is slightly discordant at ca. 97Ma. Only one markedly discordant fraction ofthe Taft Granite (ca. 96 Ma) was analyzed byStern et al. Since this early work, improve-ments in the conventional U-Pb zircon tech-nique allow more precise ages to be obtainedfrom smaller, handpicked fractions. No pre-vious geochronologic analyses have been car-ried out on the mafic rocks.

Results

Isotopic ratios and calculated ages are tabu-lated in Table 1 and are plotted on a conven-tional concordia diagram (Fig. 8). Most of the206Pb/238U and 207Pb/235U ages fall between 101and 105 Ma. The 207Pb/206Pb ages are consider-ably older, ranging from 110 to 140 Ma, butthere are no linear trends within individual sam-ples for constructing meaningful discordia.Three fractions from the diorite of the Rock-slides mafic complex sample YOS-206 are con-cordant within error at 103.5 6 0.15 Ma



Figure 5. The diorite of the Rockslides mafic complex. (A) Outcrops and talus slopes in the Rockslides area. The El Capitan Graniteforms the steep cliffs adjacent to Ribbon Falls in the upper right, and the Taft Granite forms the gently sloping, mostly tree-coveredterrain immediately above the Rockslides cliffs (center). (B) Talus boulder in the Rockslides, displaying gabbroic pillows within leuco-cratic tonalite to quartz diorite host. (C) Subhorizontal gabbroic layers in outcrop, separated by thin, intervening leucocratic septa. Thebottommost layer contains dispersed mafic enclaves. Dark, vertical streaks are water. (D) Regularly spaced protrusions of leucocratictonalite to quartz diorite into mafic pillow within a boulder of talus. Paleo-up is down and to the left.

(MSWD 5 0.45, probability of concordance 50.83; method of Ludwig, 1998). Four additionalfractions from this rock form a sublinear arrayto the right of concordia that yields no mean-ingful regression. Two of these discordant frac-tions were analyzed at another geochronologylaboratory in order to ensure that discordance isnot an analytical artifact (see footnote to Table1). We, therefore, interpret the concordant frac-tions to date the crystallization of the diorite ofthe Rockslides mafic complex and the four otherfractions to show variable effects of Pb loss andzircon inheritance from a slightly older source.Our date for the diorite of the Rockslides maficcomplex is identical to the 103 Ma date obtainedby Stern et al. (1981). Our analyses from anothersample of the diorite of the Rockslides maficcomplex (YOS-23c), the North America diorite,the El Capitan Granite, and the Taft Granite plotjust to the right of concordia at 102–105 Ma(Fig. 8). With larger errors, it would be impos-sible to tell the difference between this complexdiscordance and concordant analyses. The pat-

tern of discordance from all five samples sug-gests that inheritance of ancient (Paleozoic orolder) zircons is not a major influence on the U-Pb systematics, but instead that Pb loss and in-heritance of only slightly older (Cretaceous orJurassic?) zircons is a common feature of SierraNevadan plutonic rocks. Although the four sam-ples with only discordant analyses provide noprecise age information, their true ages are likelyto be in the range of 102–105 Ma, which isroughly the range for rocks related to the intru-sive suite of Yosemite Valley (Stern et al., 1981).Careful grain selection and high-precision sin-gle-grain analyses will be required to better de-fine the ages of the other plutonic units.

GEOCHEMISTRY

Rationale

In order to document the whole-rock com-positions of the major rock types that com-prise the intrusive suite of Yosemite Valley

and to develop petrogenetic hypotheses, 39fresh samples were analyzed for major ele-ment, trace element, and isotopic composi-tions. Methods employed for the analyses aredescribed in Appendix 2, and the completedata set is available (Tables DR2, DR3, andDR4; see text footnote 1).

Major and Trace Elements

Sampled plutonic rocks associated with theintrusive suite of Yosemite Valley range from49 to 76 wt% SiO2 (representative data in Ta-ble 2). On SiO2 variation diagrams, trends inmost major elements across the suite are clear-ly nonlinear (Fig. 9). For Al, Na, and K, inparticular, breaks in slope occur at ;65–70wt% SiO2, the approximate lower limit of theEl Capitan and Taft Granites. Linear trendswithin the data are limited to the granites andto the mafic dikes exposed on El Capitan.Considerable scatter characterizes mafic sam-ples collected from the diorite of the Rock-


RATAJESKI et al.

Figure 6. Mafic dikes exposed on El Capitan. (A) Mafic dikes exposed on the North Amer-ica Wall. Older (pre–North America) dikes within the El Capitan Granite, which trendfrom lower left to upper right, are cut by the larger, darker-colored dike complex shapedlike North America (Reid et al., 1983). The vertical distance in this view is ;450 m. (B)Fairly homogeneous pre–North America mafic dike, frozen in the process of assimilatingwall rocks of El Capitan Granite. Unlike other mafic dikes on El Capitan, this dike con-tains biotite but lacks hornblende. (C) Mingled mafic and felsic magmas in pre–NorthAmerica mafic dike sharply cutting the El Capitan Granite at the base of the NorthAmerica Wall. (D) Enclave-choked North America mafic dike in El Capitan Granite onthe summit dome of El Capitan. A camera lens cap (;5 cm wide) is placed for scale atthe center right. (E) Sheared enclaves and schlieren in diffuse contact zone between TaftGranite and mafic dike photographed in D. (F) Diverse group of mafic enclaves withingranitic matrix in synplutonic dike on El Capitan.

slides mafic complex and smaller pods, a fea-ture observed in other mafic complexes in theSierra Nevada (Sisson et al., 1996).

Not surprisingly, Cr and Ni contents of themafic rocks (Fig. 10; representative data byDCP-AES [direct current plasma–atomicemission spectrometry] in Table 2) are lowcompared to values appropriate for primitivearc basalts. Sr contents for noncumulate maficrocks are in the 400–600 ppm range and do

not reach the high values (;650–800 ppm)attained by hornblende gabbros in the GoodaleCanyon and Onion Valley mafic complexes inthe eastern Sierra Nevada (Bradford, 1995;Sisson et al., 1996), suggesting that the Yo-semite mafic rocks are more geochemically re-moved from their mantle source(s). Elevatedcontents of Ba, Sr, and LREEs (light rare earthelements) characterize the interlayer septa inthe diorite of the Rockslides mafic complex

and the marginal sheaths associated with thecontacts of mafic pods against granite. Amongthe granites, Taft Granite sample YOS-210 isnotable for its low concentrations of Ba, Sr,and the REEs compared to the other graniticsamples.

Radiogenic Isotope Ratios

Initial Sr and Nd isotope ratios (correctedfor radiogenic growth since 103 Ma) for allrock types are similar (Fig. 11; representativedata in Table 2). Most of the data cluster tight-ly; (87Sr/86Sr)i ratios range from 0.7065 to0.7072, and eNd(t) values range from24 to26.Compared to the other Sierran intrusive suitesplotted in Figure 11, only the Mount Whitneyand Tuolumne intrusive suites have lowereNd(t) values, and only the Mount Whitney in-trusive suite reaches the high values of (87Sr/86Sr)i displayed by the intrusive suite of Yo-semite Valley. A notable feature of the dataset is that the mafic rocks from both the dioriteof the Rockslides mafic complex (except leu-cocratic septa) and the mafic dikes on El Cap-itan have slightly higher (87Sr/86Sr)i ratios(0.7071–0.7072) and lower eNd(t) values (–4.6to27.0) than the granites (0.7065–0.7066;25.5 to25.9). This trend is opposite to thatobserved in the nearby Tuolumne IntrusiveSuite, for which the most silicic compositionshave the most isotopically evolved composi-tions (Kistler et al., 1986; Coleman and Glaz-ner, 1997). However, the range in isotopic var-iability across the compositional range of theintrusive suite of Yosemite Valley is still quitesmall (Fig. 11). Similar flat trends in initialisotopic compositions versus SiO2 have beenreported for plutonic rocks associated with theLamarck Granodiorite (Coleman et al., 1992),the Goodale Canyon complex (Bradford,1995), and the mafic complex at Onion Valley(Sisson et al., 1996). These trends contrastwith the significant isotopic variation exhib-ited by the Fine Gold Intrusive Suite, whoseeNd(t) values span six units (Truschel, 1996),and by the Tuolumne Intrusive Suite, whoseeNd(t) values span seven units (Kistler et al.,1986; Coleman and Glazner, 1997).

In terms of Pb isotopes, plutonic rocks ofthe intrusive suite of Yosemite Valley plot atthe radiogenic, upper end (high 206Pb/204Pb) ofthe field for Sierra Nevada granitoids (Chenand Tilton, 1991). Like the Sr and Nd data,the Pb isotope data are fairly homogeneous(Fig. 12; representative data in Table 2) andare consistent with the Sr and Nd data in thatthe granitic rocks are slightly less radiogenicin composition (have lower 206Pb/204Pb ratios)



Figure 7. Geologic map of El Capitan summit.

than many of the mafic rocks associated withthe intrusive suite of Yosemite Valley.

DISCUSSION

Emplacement of the Intrusive Suite ofYosemite Valley

Mafic-felsic interaction in the intrusivesuite can be divided into two stages, early andlate, on the basis of the crystallinity of the ElCapitan Granite with which the mafic magmascame into contact. In the earlier stage, a par-tially molten El Capitan Granite carried and/or was invaded by abundant mafic magmas,which now form scattered enclaves, enclaveswarms, small mafic pods, and the mafic com-plex represented by the diorite of the Rock-slides mafic complex. Evidence of mafic-felsicmingling during this stage is provided by theleucocratic rocks (septa, stringers, and margin-al sheaths) associated with the mafic rocks.Other workers have interpreted leucocraticsepta as volumes of granitic mush that lostliquid by filter pressing after being trapped be-tween mafic layers (e.g., Wiebe, 1993, 1996).Field and geochemical evidence (presentedsubsequently) support this conclusion for theorigin of both the leucocratic septa within thediorite of the Rockslides mafic complex andthe stringers within the mafic pods.

The later stage of mafic-felsic magmatismoccurred after solidification of the El CapitanGranite and accompanied intrusion of the TaftGranite. The mafic magmas included in thisstage are represented by the mafic dikes andplanar enclave swarms exposed on El Capitan.Field and geochemical evidence (Reid et al.,1983; this work) suggests that these dikes par-tially melted solidified El Capitan Granite,producing hybrid rocks. In addition, field ob-servations on the summit dome of El Capitansuggest that other hybrid rocks produced dur-ing this stage were probably a result of magmamingling and mixing between coeval maficmagma and Taft Granite.

Suite-Wide Trends

The nonlinear variations among the rocksof the intrusive suite of Yosemite Valley ruleout large-scale magma mixing as an explana-tion for the chemical variation within thegranites (sensu stricto), because the granitesdo not project toward likely coeval mafic liq-uids, particularly those represented by thechilled, fine-grained, mafic layers in the dio-rite of the Rockslides mafic complex (Fig. 9).Similarly, the highly scattered compositions ofthe mafic rocks defy a simple two-component

mixing process involving a coeval granitemagma such as the El Capitan or Taft Granite.It is likely therefore that some of the scatteramong the mafic rocks results from crystal-liquid fractionation and/or more complex mix-ing behavior.

Granites

The mineralogic and isotopic similarity ofthe El Capitan and Taft Granites strongly sug-gests that these units are genetically related.The possibility that the variation within thegranites represents varying degrees of partialmelting from one homogeneous source is

ruled out by the fact that the Taft Granite isthe younger and more evolved of the two. TheTaft may represent a smaller degree of partialmelting of a similar source, partial melting ofa more silicic source, or perhaps derivation bythe separation of melt from early-formed solidphases within a crystalline mush of El CapitanGranite. A process of crystal-liquid fraction-ation may also be responsible for the chemicalvariation within the El Capitan Granite, whichtrends toward the Taft Granite on major ele-ment variation diagrams (Fig. 9) as well as ona modal plagioclase 1 biotite 1 (quartz 1 K-feldspar) diagram (Fig. 13).

Our attempts to model fractionation of low-


RATAJESKI et al.

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silica El Capitan Granite to produce TaftGranite liquid by simple models of fractional,equilibrium, and in situ crystallization werelargely unsuccessful (Ratajeski, 1999). For aparental liquid, we used typical low-silica ElCapitan Granite, and for a daughter liquid, weused typical Taft Granite. Because of the pos-sibility that the magmas may have lost liquidby filter pressing during solidification, how-ever, the use of common whole-rock compo-sitions in the modeling is not without prob-lems and may explain some of the negativeresults. Unambiguous samples of the originalgranitic liquids, such as might be found atquenched margins of dikes or plutons, werenot identified in the intrusive suite of Yosem-ite Valley, and until they are, our hypothesisthat the Taft and El Capitan Granites are ge-netically related remains untested.

Mafic and Associated Minor LeucocraticRocks

Some of the scatter among the mafic rockson the major element diagrams may be attri-buted to the fact that some of these samplesare cumulates, but distinguishing liquid com-positions from cumulates is difficult except forthe most obvious cases. For example, dioriteof the Rockslides mafic complex sampleYOS-95b, interlayer leucocratic septum YOS-55b, and marginal sheath YOS-103b all haveelevated modal plagioclase and high Al2O3,suggesting that they are partly composed ofcumulate plagioclase. Obvious hornblende cu-mulates are generally lacking from the dataset, as no samples greatly depart from thechilled mafic layers (representing the originalliquids) toward the hornblende apex on modalplagioclase 1 hornblende 1 biotite diagrams(Fig. 13). Except for the plagioclase cumulatesin rocks at mafic-felsic contacts, obvious cu-mulates are few, and we conclude that extremecrystal fractionation is not a common featurewithin the mafic rocks associated with the in-trusive suite of Yosemite Valley.

Evidence for the trapping of volumes of co-eval granitic material within the diorite of theRockslides mafic complex is provided by theleucocratic interlayer septa. On a modal pla-gioclase 1 biotite 1 (quartz 1 K-feldspar)ternary diagram, these rocks, along with otherleucocratic samples from stringers and mar-ginal sheaths around mafic pods, define a lin-ear array that projects away from the quartz1 K-feldspar apex, almost reaching the maficend of the range displayed by the El CapitanGranite (Fig. 13). These rocks probably rep-resent partially crystalline El Capitan Granitemagma that was trapped during injection of



Figure 8. U-Pb concordia diagram. Concordia is graduated in million years, and errorellipses are two standard deviations. None of the discordant data are sufficiently alignedto construct discordia.

mafic magmas at the base of a magma cham-ber (Wiebe, 1993, 1996). Bulk mixing be-tween mafic liquids (modeled by the fine-grained mafic layers) and felsic liquids(modeled by samples of El Capitan Granite)does not produce the compositions of the leu-cocratic septa or the marginal sheaths. Rather,the linear trend on the modal diagram proba-bly reflects some combination of (1) loss ofsilicic melt by filter pressing prior to or duringinjection and compaction of the mafic layers,(2) selective exchange of mobile alkalis be-tween the felsic and mafic magmas (Wiebe,1996), and (3) varying degrees of mafic com-ponent acquired during mingling. The cumu-late nature of these rocks is suggested by theirelevated Sr contents (which is compatible inplagioclase), yet these rocks also preserve anenriched LREE signature, possibly indicatingthat filter pressing took place in the presenceof LREE-rich trace phases, which remainedbehind in the solid residue. This idea is sup-ported by the identification of abundant apa-tite, sphene, and allanite within interlayer sep-ta samples YOS-105b and YOS-109b.

The origin of the mafic dikes on El Capitancan be addressed with the major and trace el-ement data. On the basis of field observationsand major element geochemistry, Reid et al.(1983) hypothesized that mixing of maficmagmas similar to those of the North Americadikes with alaskitic partial melts of El CapitanGranite host rock could explain the origin ofthe biotite-rich, ‘‘pre–North America Wall’’dikes of the first injection phase. Our majorelement data are consistent with this interpre-

tation, but our limited REE analyses indicatethat if this mixing process occurred, it did notresult in rocks with REE patterns intermediatebetween the mafic and felsic end members ofthe suite: among the two pre–North Americadikes sampled, one has an elevated LREE sig-nature similar to the granites, and one has aless enriched signature similar to the diorite ofthe Rockslides mafic complex and NorthAmerica dikes (Fig. 10). The REE data sug-gest that while El Capitan Granite was inter-acting with the margins of the dike complex,it retained most of its REEs (probably in un-melted trace phases) and therefore did notcontribute a large amount of REEs to the par-tial melts, which consequently mixed with themafic magmas. Similarly, interaction of NorthAmerica mafic dikes with Taft Granite on thesummit dome of El Capitan also did not resultin rocks with intermediate REE patterns: oursingle analysis of biotite tonalite YOS-67 re-sults in LREE values as enriched as most ElCapitan and Taft Granite samples. As for otherpetrologic processes, evidence for minor crys-tal fractionation within the North Americadike complex is not very great, as only oneanomalously hornblende-rich sample (YOS-214) is present in the data set.

Petrogenesis

Perhaps the most striking aspect of the geo-chemical data is the rather small amount ofisotopic variability that characterizes the intru-sive suite of Yosemite Valley across its entirepetrologic range. Coleman et al. (1992) and

Coleman and Glazner (1997) have interpretedisotopic variations within intrusive suites ofthe Sierra Nevada as reflecting different rolesof ancient continental crust. For suites thatshow significant isotopic contrast as well asinheritance in zircon systematics (e.g., Tuo-lumne), the involvement of significantamounts of preexisting, ancient crust is impli-cated. In contrast, a relatively small role forpreexisting crust is implicated for isotopicallyhomogeneous suites with few inherited zir-cons. Coleman et al. (1992) raised the possi-bility that most of the Lamarck Granodioritecould have been extracted relatively rapidlyfrom an enriched mantle source with (87Sr/86Sr)i ratios of ;0.7065 and eNd(t) of ;–4.5.Granitic rocks of the intrusive suite of Yosem-ite Valley have essentially this composition,which permits a similar source. However, asnoted by other workers (Ben Othman et al.,1989; Beard and Johnson, 1997), Sr and Ndisotopes are often not sufficient to distinguishamong the various processes that can result inenriched isotopic compositions, e.g., crustalcontamination of mantle magmas, mantle en-richment by sediment subduction, and foun-dering of crustal rocks into the mantle (e.g.,Beard and Glazner, 1995).

In Figure 12, we plot several compositionalfields representing Pb isotope reservoirs thatmay have been relevant to the genesis of theintrusive suite of Yosemite Valley, includingsubducted marine sediments, ancient metase-dimentary rocks, and enriched lithosphericmantle. Pb isotope compositions of intrusivesuite rocks are far removed from those of ma-rine sediments, suggesting that subducted sed-iments are not a likely source for Pb in theintrusive suite. In contrast, ancient metasedi-mentary Pb is a more likely component in theintrusive suite samples, as most of the samplesfall within or close to the Type II Pb provinceof Zartman (1974), a region that spans muchof north-central Nevada and contains greatthicknesses of miogeoclinal sedimentary rocksderived from Precambrian sources.

Whether this metasedimentary componentwas added by contamination of mantle-de-rived magmas (perhaps within the lower crust)or by direct contamination of the upper mantleitself (by foundering of crustal rocks into themantle at some time in the remote past) is un-known, but the fact that the Sr, Nd, and Pbisotope compositions become less crust-likewith increasing SiO2 argues against contami-nation of primitive mafic magmas by a high-silica, ancient crustal component. The westernYosemite Valley plutonic rocks are just abovethe upper 206Pb/204Pb limit of the field for theWestern Great Basin basalts (Beard and John-


RATAJESKI et al.

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Figure 9. Major element variation diagrams. Lines through the granites were fit by eye. Fe2O3* 5 total iron as Fe31.


RATAJESKI et al.

Figure 10. REE concentrations normalized to chondritic values (Anders and Grevesse, 1989). Note that this figure plots both INAA andICP-MS data; the same REEs were not analyzed by both methods.

Figure 11. Initial Sr and Nd isotope compositions of plutonic rocks from the western halfof Yosemite Valley compared with those of other intrusive suites from the central SierraNevada batholith. All data are corrected for 103 m.y. of radiogenic growth (based on theage determined by Stern et al., 1981). Symbols for the data from the intrusive suite ofYosemite Valley are the same as those in Figure 9. Data sources for the other intrusivesuites are as follows: Fine Gold Intrusive Suite (Truschel, 1996), Lamarck Granodiorite(Coleman et al., 1992), Tuolumne Intrusive Suite (Kistler et al., 1986; Coleman and Glaz-ner, 1997), and Mount Whitney Intrusive Suite (Hirt and Glazner, 1995).

son, 1997), which, along with ultrapotassicbasaltic lavas in the Sierran region (Van Koo-ten, 1981), are believed to have originatedfrom an enriched, lithospheric mantle (Ormer-od et al., 1988; Menzies, 1989). The isotopicproximity of the western Yosemite Valley plu-tonic rocks and the Western Great Basin ba-salts may hint at an enriched-mantle compo-nent in the granites and gabbros of theintrusive suite of Yosemite Valley, and in-volvement of metasedimentary Pb in this pro-cess would seem to be implicated by the Pbisotope data.

The broad isotopic similarity between theYosemite Valley granites and mafic rocks sug-gests that these rock types are related petro-genetically. One possibility that is consistentwith the geochemical data is that a parental,granodioritic melt (e.g., a melt similar in com-position to YOS-61) was produced by partialmelting of relatively young, LILE-enrichedmafic rocks in the lower crust (Fig. 14). Be-cause the El Capitan Granite is associated withcoeval gabbroic rocks (mafic enclaves, maficpods, and the diorite of the Rockslides), it isconceivable that the intrusion of the maficmagma caused the partial melting event that



Figure 12. Pb isotope data for feldspars from plutonic rocks associated with the intrusive suite of Yosemite Valley. Symbols are thesame as those in Figure 9; the additional open triangle represents a foliated, leucocratic quartz diorite collected from the contact betweenthe El Capitan Granite and the diorite of the Rockslides mafic complex. Reference lines shown on the diagrams are the present-daygeochron (Tatsumoto et al., 1973) and the Northern Hemisphere reference line (NHRL) of Hart (1984), which is regressed through mid-ocean ridge basalt (MORB) and oceanic-island basalt (OIB) data. The field for granitoids of the central Sierra Nevada batholith (Chenand Tilton, 1991) includes data only from feldspar separates, which are uncorrected for minor U decay to correspond with our data.Also shown for reference are the fields for modern marine sediments in the Pacific and Atlantic Oceans (Ben Othman et al., 1989), theType II Pb isotope province of Zartman (1974), which is thought to reflect sources associated with Precambrian metasedimentary rocks,and the fields for ultrapotassic basalts of the central Sierra Nevada (Van Kooten, 1981) and basalts from the western Great Basin (Beardand Johnson, 1997), both of which are thought to originate from the lithospheric mantle. Two-standard-error uncertainties are fromthe analysis of standard NBS 981.

Figure 13. Modal variations (inwt%). Modal data plotted in thisfigure were estimated by least-squares mass balance of bulkcompositions against phase com-positions determined by micro-probe for representative samples(Table DR6 in GSA Data Repos-itory; see text footnote 1). Thequartz-poor mafic rocks are plot-ted on the hornblende-plagio-clase-biotite ternary diagram(projected from quartz), and thehornblende-free leucocratic rocks(including granites) are plotted onthe plagioclase–biotite–(quartz 1K-feldspar) ternary diagram.Compositions of mafic enclaveswithin the El Capitan Granite arenot plotted because they have sig-nificant amounts of both quartzand hornblende.


RATAJESKI et al.

Figure 14. Petrogenetic model for the intrusive suite of Yosemite Valley.

in turn produced the granite. Experimentalpartial melting of a hornblende gabbro fromthe diorite of the Rockslides mafic complex at800–825 8C and 8 kbar (with no added fluid)produces partial melts similar in compositionto low-silica El Capitan Granite (Ratajeski andSisson, 1999), a preliminary finding that lendssupport to the model just outlined.

As shown by Coleman et al. (1992), the La-marck Granodiorite provides a similar exam-ple of isotopic near-homogeneity among coe-val mafic and felsic plutonic rocks. Theisotopic composition of the Lamarck is verysimilar to that of the intrusive suite of Yosem-ite Valley (Fig. 11), which raises the possibil-ity of a similar source for both intrusive suites.However, there is an important difference be-tween the two suites: the Lamarck containslarge amounts of intermediate magma (grano-diorite), which appears to have been producedby mixing of mafic and felsic magmas at afairly early or deep stage (Frost and Mahood,1987; G. Mahood, 2000, personal communi-cation). A later stage of mafic-felsic magma-tism produced several small mafic plutons,which are locally mingled with the LamarckGranodiorite, but on the whole are still largelyunhybridized with it. This later stage of mafic-felsic magmatism is very similar to that in-ferred for the intrusive suite of Yosemite Val-ley, during which coeval mafic and felsic

components remained largely unhybridized.The record of mafic-felsic magmatism in theintrusive suite of Yosemite Valley and othercomplexes in the Sierra Nevada underscoresthe considerable variability in the spatial andtemporal scales of mafic-felsic interactionwithin arcs.

CONCLUSIONS

1. The intrusion of the Yosemite Valleysuite involved two probably closely timedpulses of mafic-felsic magmatism. The firststage yielded the El Capitan Granite and maficmagmas that comprise scattered enclaves, en-clave swarms, small pods, and the diorite ofthe Rockslides mafic complex. The secondstage yielded the Taft Granite and the maficdike swarm exposed on El Capitan.

2. Mingling of mafic and felsic magmas hasproduced biotite-rich, leucocratic quartz dio-rites to tonalites, which form thin septa be-tween mafic layers in the diorite of the Rock-slides mafic complex, irregular masses withinmappable pods of mafic rocks, and contactzones between mafic rocks and the El CapitanGranite. Modal and geochemical data suggestthat some of these leucocratic rocks are pla-gioclase-rich cumulates. On the North Amer-ica Wall of El Capitan, some areas of the ElCapitan Granite interacted with a large mafic

dike complex and partially melted but at low-enough temperatures to prevent REE mobili-zation into adjacent mafic magmas.

3. Initial isotopic compositions of the maficand felsic rocks are very similar; the maficrocks exhibit only slightly more enrichedcompositions than the granites. This observa-tion argues against contamination of unevolvedmafic magmas by ancient continental crust,but could be reconciled with a two-stage pro-cess whereby melting of an enriched mantle(perhaps enriched by foundering of ancientcontinental crust) produced mafic rocks,which were remelted a short time later duringa subsequent pulse of mafic magmatism.

APPENDIX 1. ANALYTIC METHODS: U-PbZIRCON GEOCHRONOLOGY

Zircons were separated, picked, abraded, and dis-solved by standard techniques, a mixed 205Pb-233U-236U spike was added, and then Pb and U were col-lected by standard HBr column chemistry. Totalprocedural blanks were 2–18 pg Pb and ,1 pg U.Pb and U isotope ratios for all but two of the pickedfractions were determined with the VG Sector 54mass spectrometer at the University of North Car-olina (UNC) at Chapel Hill. Both U and Pb analyseswere conducted in static collection mode, using allFaraday detectors for U and combined Faraday(205Pb, 206Pb, 207Pb, and 208Pb) and Daly (204Pb) de-tectors for Pb. Long-term repeat measurements ofNBS 981 (n 5 32; errors are 1 S.D.) yield 206Pb/204Pb 5 16.937 6 0.16%, 207Pb/204Pb 5 15.48 60.16%, 208Pb/204Pb 5 36.69 6 0.17%, 207Pb/206Pb 50.9147 6 0.044, and 208Pb/206Pb 5 2.167 6 0.064.Data obtained at UNC were corrected for fraction-ation of 0.12% 6 0.08%/amu (atomic mass unit;Faraday) and 0.20% 6 0.07%/amu (Daly). As anindependent check on the accuracy of the UNCanalyses, two fractions (YOS-206–13 and YOS-206–14) were prepared at UNC but analyzed withthe VG Sector 54 mass spectrometer at SyracuseUniversity and corrected for fractionation of 0.18%6 0.07%. Raw data were reduced with the PbMacDat–2 program (D. Coleman, 1999, personalcommun.) using data-reduction and error-propaga-tion algorithms of Ludwig (1989, 1990). Concordiaplots, age estimates, and age errors were generatedwith a recently updated version of the Isoplot pro-gram (Ludwig, 1990).

APPENDIX 2. ANALYTIC METHODS:WHOLE-ROCK GEOCHEMISTRY

Only fresh samples were chosen for whole-rockgeochemical analysis. Major elements (Si, Ti, Al,Fe, Mn, Mg, Ca, Na, K, and P) and trace elements(Ba, Cr, Ni, Sc, Sr, V, Y, and Zr) were measured bydirect current plasma–atomic emission spectrometry(DCP-AES) for crushed powders of representativesamples of the main rock types described herein,using the method outlined by Beard and Glazner(1995). Rb analyses were attempted along with theother trace elements by DCP-AES, but flat calibra-tion curves (small signal to concentration ratio) didnot permit accurate analyses. Mafic samples weregenerally run separately from felsic samples. Cali-bration curves were calculated for each run by an-alyzing several standards spanning the expected



concentration range of the unknowns. For runs con-sisting of mafic samples, standards included five ofthe following: AGV-1, BIR-1, DNC-1, MAG-1,SDC-1, and STM-1. For runs analyzing intermedi-ate to silicic rocks, standards included granite stan-dard G-2 and/or rhyolite standard RGM-1, plus fourstandards from the preceding list. Along with oneof the standards, each sample was analyzed severaltimes (three to four times for major elements andfour times for trace elements), accounting for in-strumental drift, and means and standard deviationswere computed after anomalous data points werediscarded. The whole-rock major element data wereused in conjunction with microprobe analyses ofconstituent phases to determine precise modal abun-dances by mass balance.

Trace element concentrations were also obtainedfor several samples by instrumental neutron acti-vation analysis (INAA) at Oregon State Universityand by inductively coupled plasma–mass spectrom-etry (ICP-MS) at Duke University. Multiple ele-ments (including REEs) were analyzed by thesemethods. The reproducibility of the ICP-MS anal-yses, as judged by replicate dissolutions, is 61%–3% for REE and 61%–6% for other elements(Klein and Karsten, 1995). ICP-MS analyses of Zrand Hf resulted in extremely depleted concentra-tions in standard G-2 relative to their accepted con-centrations, an effect that probably resulted from in-complete dissolution of minor, refractory, traceelement–rich phases like zircon. As a result, Zr andHf data by ICP-MS are not included in this study.One-standard-deviation uncertainties for the INAAanalyses are generally less than 10%.

Rb, Sr, Sm, and Nd isotope compositions wereobtained for selected whole-rock samples, and Pbisotopes were obtained from feldspar separates, byusing the techniques described by Miller et al.(1995) and Fullagar et al. (1997). All isotopic anal-yses were performed with the eight-collector VGSector 54 mass spectrometer at the University ofNorth Carolina. Sr isotope compositions were nor-malized to 86Sr/87Sr 5 0.1194 by using an exponen-tial fractionation law and referenced to NBS 987 forwhich 87Sr/86Sr 5 0.710 250. Nd isotope composi-tions are normalized to 146Nd/144Nd 5 0.7219 byusing an exponential fractionation law and refer-enced to UNC Ames Nd metal with 143Nd/144Nd 50.512 14. Replicate analyses of NBS 987 run duringthe sample analyses gave 87Sr/86Sr 5 0.710 340 (60.000 022; two standard errors; n 5 4). Replicateanalyses of UNC Ames gave 143Nd/144Nd 5 0.512143 (6 0.000 0078; two standard errors; n 5 2).Long-term, internal run precision for Sr and Ndanalyses at UNC is better than 0.0010% (6 0.000007 for Sr and 6 0.000 005 for Nd). Measured 87Sr/86Sr ratios were corrected for 103 m.y. of radiogenicgrowth, based on the age of the El Capitan Graniteas determined by Stern et al. (1981). If the discor-dance of our preliminary zircon results is due to Pbloss (see earlier discussion), rocks of the intrusivesuite of Yosemite Valley may be slightly older, i.e.,their ages may be as great as 110–115 Ma. Thischange is not very significant for the initial 87Sr/86Srcalculation, however, and results in an increase ofonly 0.0001. For the Pb isotope analyses, total pro-cedural blank was 96 pg Pb, and analyses were ref-erenced to NBS 981 with 207Pb/206Pb 5 0.914 64and corrected 0.086 6 0.009%/amu for fraction-ation. Average uncertainties (two standard errors; n5 44) for the analyzed samples due to measurementuncertainty and propagation through the fraction-

ation correction are 0.0012 for 206Pb/204Pb, 0.012 for207Pb/204Pb, and 0.018 for 208Pb/204Pb.

ACKNOWLEDGMENTS

This work was partly supported by National Sci-ence Foundation grants 9219521 and 9526803 (toGlazner) and additional funding from Sigma Xi, theGeological Society of America, and the Universityof North Carolina Department of Geological Sci-ences. Field assistance by Jennifer Wenner and Wil-liam Hoyt is much appreciated. Discussions in thefield with John Bartley, Mark Brandon, David Haw-kins, Tom Sisson, and Robert Wiebe are much ap-preciated. Paul Fullagar, Kevin Stewart, JamesBeard, and Peter Malin are thanked for early re-views of this paper; reviews by Gail Mahood, Bren-dan McNulty, and Robert Wiebe greatly improvedthe revised version. We thank Scott Samson andGreg Wortman at the Syracuse University geochro-nology laboratory for analyses of two zircon frac-tions. Whole-rock INAA data were provided by theU.S. Department of Energy’s Reactor Sharing Grantto Oregon State University. Finally, Ratajeski wassupported by a National Science Foundation Grad-uate Research Fellowship.

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