Oxygen isotope trends of granitic magmatism in the Great ...Department of Geography-Geology,...

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GSA Bulletin; March/April 2004; v. 116; no. 3/4; p. 451–462; doi; 10.1130/B25324.1; 9 figures; 1 table; Data Repository item 2004046.

Oxygen isotope trends of granitic magmatism in the Great Basin:Location of the Precambrian craton boundary as reflected in zircons

Elizabeth M. King†

Department of Geography-Geology, Illinois State University, Campus Box 4400, Normal, Illinois 61790, USA

John W. Valley‡

Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA

Daniel F. Stockli§

Department of Geology, University of Kansas, 120 Lindley Hall, Lawrence, Kansas 66045, USA

James E. Wright#

Department of Geology, University of Georgia, Geography-Geology Building, Athens, Georgia 30602, USA

ABSTRACT

The d18O values of magmas in the GreatBasin help to decipher the tectonic assem-bly of North America and to determine themagmatic source and potentially the com-position of the crust at depth. Igneous zir-cons from Precambrian, Jurassic, Creta-ceous, and Tertiary intrusive bodies of thenorthern Great Basin best preserve the rec-ord of magmatic oxygen isotope ratios. Thevariation of d18O values in zircon with agereconciles previous differences in interpre-tation based on d18OWR and radiogenic iso-tope data. The new d18OZrc data support theresults of previous radiogenic isotope stud-ies documenting the increased availabilityof crustal and sedimentary components tomagmas in the Cretaceous during the Se-vier orogeny and a return to a larger pro-portion of mantle-derived components inthe Tertiary during Basin and Rangeextension.

In the Great Basin, d18OZrc values alsovary systematically with crustal structureas determined by radiogenic isotope sys-tematics. Plutons emplaced east of the 87Sr/86Sri 5 0.706 isopleth have higher d18OZrc

values than plutons intruded west of the0.706 line. However, analyses of d18O valuesin quartz do not show the bimodal distinc-tion across the 0.706 line owing to subsoli-dus alteration. On the basis of d18OZrc, plu-

†E-mail: [email protected].‡E-mail: [email protected].§E-mail: [email protected].#E-mail: [email protected].

tons intruding the Walker Lane belt areindistinguishable from other plutons em-placed west of the 0.706 line despite signif-icant tectonic displacement and rotation ofthe belt.

The difference in d18O values across the0.706 line reflects the involvement of high-d18O Precambrian and Paleozoic sedimen-tary rocks of the continental margin andPrecambrian craton in magmas intrudedeast of the 0.706 line, whereas magmas westof the 0.706 line are dominated by lower-d18O rocks derived from juvenile volcanicarcs. Crustal boundaries and discontinu-ities in the Great Basin determined by ra-diogenic isotope systems agree with discon-tinuities observed in d18OZrc values; thisagreement indicates that the 0.706 linemarks a geochronologic and compositionaldiscontinuity in the basement rocks.

Keywords: Great Basin, oxygen isotopes,zircon, terrane boundaries.

INTRODUCTION

The Great Basin of northern Nevada andUtah has undergone extensive tectonic activitysince the middle Paleozoic. Abundant mag-matism throughout the Great Basin’s historydocuments the variability in thickness and ageof the continental crust through which plutonshave ascended. The location of the westernedge of Precambrian crystalline basement hasbeen the topic of many isotopic studies, yetthere is still disagreement as to the placementof this crustal boundary (Kistler and Peter-

man, 1973, 1978; Zartman, 1974; Lee et al.,1981; Farmer and DePaolo, 1983; Solomonand Taylor, 1989; Barton, 1990; Elison et al.,1990; Wright and Wooden, 1991; Elison,1995). The 87Sr/86Sri 5 0.706 isopleth (where87Sr/86Sri is the initial Sr isotope ratio of theplutonic rocks) defined by Kistler and Peter-man (1973, 1978), commonly known as the0.706 line, has been interpreted to mark thetransition from continentally derived rocks onthe east to volcanic-arc–derived rocks on thewest (Kistler and Peterman, 1973, 1978;Farmer and DePaolo, 1983; Elison et al.,1990). Kistler and Peterman (1973) interpret-ed the 0.706 line as the western edge of thePrecambrian crystalline basement (Fig. 1A).Further work by Farmer and DePaolo (1983),however, has defined the edge of the crystal-line basement farther to the east at the «Nd 527 line, which coincides with the 87Sr/86Sri 50.708 (Fig. 1A). The zone between the west-ern edge of the crystalline basement (87Sr/86Sri

5 0.708) and the western edge of the conti-nentally derived sedimentary rocks (87Sr/86Sri

5 0.706) is a transition zone containing con-tinentally derived sedimentary rocks, thethickness of which generally increases west-ward to the 0.706 line; west of that line, thethickness of arc-derived rocks increases (Fig.2).

Solomon and Taylor (1989) interpretedwhole-rock (WR) d18O values of felsic igne-ous rocks to place the craton edge even farthereast, along their eastern zone/central zoneboundary in Utah (Fig. 1B). The d18OWR val-ues range from 7‰ to 9‰ in their easternzone and 9‰ to 13‰ in the central zone (Sol-

452 Geological Society of America Bulletin, March/April 2004

KING et al.

Figure 1. Simplified map of isotopic discontinuities in the western United States. (A) The87Sr/86Sri (ISr) 5 0.706 and 0.708 lines of Kistler and Peterman (1973) as labeled. The «Nd

5 27 of Farmer and DePaolo (1983) is coincident with the 0.708 line. The thick, graylines denote the boundaries of the Pb isotope zones Ia, Ib, II, and III of Zartman (1974).The area between the 0.706 and 0.708 lines is the transition zone between continentallyderived rocks and arc-derived rocks. (B) The boundaries of the oxygen isotope zonesdefined by Solomon and Taylor (1989) on the basis of whole-rock analyses. The line labeled1 is the «Nd 5 27 line from A. The d18OWR values of the western zone range from 6‰ to8.5‰, those of the central zone range from 9‰ to 13‰, and those of the eastern zonerange from 7‰ to 9‰. Modified from Farmer and DePaolo (1983) and Solomon (1989).

omon and Taylor, 1989). The presence of highd18OWR values in eastern Nevada and westernUtah is interpreted as requiring a primarilysedimentary magma source; therefore Solo-mon and Taylor (1989) suggested that the Pre-cambrian craton basement is located signifi-cantly farther east than radiogenic studiesindicate. The d18OWR data indicate unalteredaccreted volcanic-arc rocks at depth in thecrust in the western zone (d18OWR 5 6‰ to8.5‰) (Fig. 1B). Additional isotopic andstratigraphic studies (i.e., Barton, 1990; Elisonet al., 1990) have agreed more with the Farm-er and DePaolo (1983) and Kistler and Peter-man (1973) crustal boundaries than with thestable isotope boundaries of Solomon andTaylor (1989).

This study investigates oxygen isotope ra-tios of magmatic zircons across the northernGreat Basin to address the tectonic evolutionof the crust in the region as reflected in ig-neous rocks and the disagreement betweencrustal boundaries delineated by previous sta-ble and radiogenic isotope studies. Zircon ox-ygen isotope analyses have successfully beenused to determine crustal structure and mag-matic evolution in numerous studies of igne-ous and metamorphic rocks from a variety ofgeologic settings (King et al., 1998, 2002;Bindeman and Valley, 2000, 2003; Peck et al.,2000, 2001; King and Valley, 2001; Valley etal., 2002, Valley, 2003).

Zircon is a common trace mineral in gra-nitic magmas and can preserve oxygen isotoperatios representative of magmatic conditionseven after geologic events that alter oxygenisotope ratios of many other rock-formingminerals (see Valley, 2003). Such geologicevents include hydrothermal alteration (Valleyand Graham, 1996; King et al., 1997, 2000;Monani and Valley, 2001) and high-grade re-gional metamorphism (Valley et al., 1994;Peck et al., 2000, 2003). Even if rocks arerelatively unaltered and undeformed, ex-change of oxygen isotopes can still occur in arock by volume diffusion between mineralsduring cooling (Eiler et al., 1992). Slow dif-fusion rates of oxygen through the zircon lat-tice and high closure temperature relative tomost other minerals makes zircon an idealmineral to preserve magmatic d18O values(Dodson, 1973; Watson and Cherniak, 1997;Peck et al., 2003; Valley, 2003). Plutons inregional studies such as this, can be subject tovariable cooling rates, alteration, and tectonichistories. As a result, correlations based onquartz and whole-rock stable isotope analysesmay not accurately represent the magmaticvalues that are evident with analyses of d18Oin zircon.

Geological Society of America Bulletin, March/April 2004 453

OXYGEN ISOTOPE TRENDS IN THE GREAT BASIN AND LOCATION OF CRATON BOUNDARY

Figure 2. Simplified, interpretive cross section of the Great Basin, western United States,before the Jurassic. RMT—Roberts Mountains Thrust; GT—Golconda Thrust; CSFT—Calaveras-Shoofly Thrust; A—Antler flysch; M—mantle. The edge of the Precambriancrystalline basement is located at the «Nd 5 27 line of Farmer and DePaolo (1983). Thewestern edge of the transition zone is located approximately at the 0.706 line of Kistlerand Peterman (1978). Figure of modified from Farmer and DePaolo (1983).

REGIONAL GEOLOGY

The Great Basin preserves a full record ofpre-Cenozoic tectonic activity along the con-tinental margin of North America. Shelf sed-imentation was continuous from the Precam-brian through the Triassic; the thickness ofsedimentary rocks exceeds 15 km in easternNevada and thins eastward toward the craton(Gans and Miller, 1983; Miller et al., 1988).Passive-margin sedimentation along the west-ern edge of the North American continent wasdisrupted by two collisional events prior to theMesozoic. During the Late Devonian to EarlyMississippian, the Antler orogeny thrust lowerPaleozoic sedimentary rocks eastward alongthe Roberts Mountains Thrust (Figs. 2 and 3)(Speed et al., 1988; Dilek and Moores, 1999).The second collisional event was the Sonomaorogeny during the Permian–Triassic when theGolconda allochthon was thrust along the Gol-conda Thrust over the Roberts Mountainsallochthon.

During the Middle to Late Triassic, active-margin tectonic activity began again along thewestern edge of the North American conti-nent, as indicated by the igneous rocks in theGreat Basin, perhaps due to a subduction zonedipping eastward at the western margin of thecontinent (Saleeby et al., 1992). The westernGreat Basin was subjected to crustal shorten-ing during the Jurassic (between 200 and 150Ma) (Wyld and Wright [2001] and the refer-ences therein). A second middle Mesozoic

compressional event occurred primarily in theeastern and central Great Basin with the onsetof contractional deformation associated withthe Sevier orogeny. This orogeny is markedby ;50% crustal thickening and eastwardthrusting along the Sevier Thrust belt (Fig. 3)from the late Early Cretaceous to the earlyTertiary; the maximum intensity occurred atca. 100–90 Ma (Armstrong, 1968; Stewart,1980; Allmendinger and Jordan, 1981; All-mendinger, 1992; Wyld and Wright, 2001).After crustal thickening during the Sevierorogeny, extension began during the Oligo-cene and continued through the Miocene(Wernicke, 1992) along low-angle detachmentfaults throughout much of the Great Basin.Extension averaged 40% across the region butis highly variable, reaching ;250% in theeastern Great Basin (Gans and Miller, 1983;Coney and Harms, 1984; Miller et al., 1988).This extension was oriented east-west with lit-tle or no rotation.

The Walker Lane belt of the western GreatBasin (Fig. 3) has undergone more complextectonic activity than the simple east-westcontraction and extension that affected themajority of the Great Basin. The Walker Lanebelt is a northwest-trending zone ;700 kmlong and 100–300 km wide in western Nevadaand eastern California (Fig. 3). The belt con-tains complex dextral strike-slip faults alongwhich Mesozoic and Cenozoic offset has oc-curred (Stewart, 1988; Schweickert and Lah-

ren, 1990; Wyld and Wright, 2001).The Walk-er Lane belt currently accommodates 1.1 60.1 cm/yr of right-lateral slip between the sta-ble craton to the east and the Sierra Nevadato the west (Argus and Gordon, 1991).

Magmatism in the Great Basin has occurredprimarily during three intervals, not includingthe subduction-related magmatism in the Tri-assic on the far western edge of the Great Ba-sin. The first pulse of igneous activity wasduring the Middle Jurassic–Early Cretaceous(ca. 170–100 Ma) preceding the Sevier orog-eny (Wright and Wooden, 1991). Late Creta-ceous (ca. 83–75 Ma) magmatism occurredduring the Sevier orogeny. This period ofmagmatism abruptly shifted to the east withchanges in the angle of subduction at the onsetof the Laramide orogeny (Barton, 1990). An-other magmatic pulse during the Cenozoic (ca.39–20 Ma) was concurrent with the onset ofBasin and Range extension.

PREVIOUS ISOTOPE STUDIES

Previous regional isotope studies focusedon correlations between geographic locationand isotopic composition. Zartman (1974) an-alyzed Pb isotopes in Mesozoic and Cenozoicigneous rocks in the Great Basin and definedthree isotope provinces from east to west: (Ia)contains Pb derived from Archean basementrocks; (Ib) contains Pb isotope ratios indica-tive of Proterozoic basement rocks; (II) con-tains Pb isotope ratios indicative of sedimen-tary rocks derived from the Precambriancraton; (III) contains Pb derived from youngerarc-derived igneous and sedimentary rocks(Fig. 1A). Kistler and Peterman (1973, 1978)identified a boundary between different crustaldomains based on the 87Sr/86Sri (i.e., the 0.706line) (Fig. 1A) in central Nevada, which theyinterpreted as the western edge of the Precam-brian continental crust.

Farmer and DePaolo (1983) coupled Ndand Sr isotopes to map the crustal structure ofthe Great Basin. A Nd isotope discontinuity(«Nd 5 27) (Fig. 1A), interpreted as the edgeof the Precambrian crystalline basement, liesalong the Sr isotope discontinuity shown bythe initial 87Sr/86Sr 5 0.708 isopleth, slightlyeast of the originally suggested boundary de-lineated by initial 87Sr/86Sr 5 0.706 (Kistlerand Peterman, 1973, 1978). The Nd isotopediscontinuity coincides roughly with the Rob-erts Mountains Thrust (Fig. 3). The edge ofthe Precambrian crystalline basement occurs;200 km west of a Sr isotope discontinuitythat is interpreted as the boundary betweenRb-depleted lower crust to the east and un-depleted lower crust to the west (Farmer and


KING et al.

Figure 3. Simplified map of the northern Great Basin, western United States, showing theoutlines of mountain ranges sampled in this study. Sample locations and map numberscorrespond to sample descriptions in Table DR1 (see footnote 1 in text). The 0.706 line isfrom Kistler and Peterman (1978); the Walker Lane belt is from Stewart (1988). RMT—Roberts Mountains thrust; STB—Sevier Thrust belt. Solid symbols for ages of igneousrocks sampled: triangles—Tertiary; circles—Cretaceous; squares—Jurassic; stars—Precambrian. Open squares—city and town locations.

DePaolo, 1983). The zone between the 0.708and 0.706 lines was interpreted by Farmer andDePaolo (1983) as a transition zone acrosswhich the thickness of continentally derivedsediment increases westward. The westernedge of the transition zone, which correlateswith the 0.706 line, was interpreted by Farmerand DePaolo (1983) to indicate the increase ofvolcanic-arc–related rocks in the crust. Strati-graphic and structural analyses of sedimentaryrocks of the Great Basin also place the tran-sition from continentally derived sedimentaryrocks to volcanic-arc–derived sedimentaryrocks at the 0.706 line (Elison et al., 1990).

Solomon and Taylor (1989) measured thed18OWR values of plutons in the Great Basin,and from these data they defined three crustalzones that reflect magma sources (Fig. 1B).They proposed on the basis of the whole-rockd18O values that the western zone (d18OWR 56‰ to 8.5‰) had an upper-mantle magmasource and the eastern zone (d18OWR 5 7‰ to9‰) had a magma source from Precambriancontinental crust. The central zone is transi-tional and divided into two subzones: thewestern part had a magma source influencedby volcanogenic accreted terranes (V-type),and the eastern part had d18O values indicativeof a Precambrian metasedimentary magmasource (i.e., S-type) (Solomon and Taylor,

1989). Both the V-type and S-type centralzones have d18OWR values from 9‰ to 13‰,and the division between the subzones isbased on the location of the 0.706 line. Thereis very little variation in the d18OWR values ofplutons on either side of the 0.706 line withinthe central zone.

Crustal zones defined previously on the ba-sis of oxygen isotope ratios do not correspondwith crustal structure interpreted from radio-genic isotope data. The central zone of Solo-mon and Taylor (1989)—the transition zonebetween continentally derived magmas andarc-derived magmas—is significantly widerthan the radiogenic transition zone betweenthe 0.708 and 0.706 lines (Farmer and De-Paolo, 1983). The division of subzones in thecentral zone is based on Sr isotope data mod-ified by the interpretation that the d18O com-ponent was derived from volcanic-arc–relatedrocks at depth for the V-type central zone andfrom continentally derived sedimentary rocksat depth for the S-type central zone. On thebasis of high-d18OWR plutons in eastern Ne-vada and western Utah, Solomon and Taylor(1989) also placed the western edge of the cra-ton farther east than the «Nd 5 27 disconti-nuity and closer to the Sr discontinuity indi-cating the western edge of the Rb-depletedlower crust.

Additional isotopic studies have focusedmore closely on crustal evolution in the GreatBasin with respect to age rather than geo-graphic distribution. Barton (1990) document-ed an increase of crustal input into graniticmagmas during the Cretaceous as representedby increased 87Sr/86Sri ratios, increased d18Ovalues, a transition from metaluminous to per-aluminous magmatism, and decreased «Nd.Wright and Wooden (1991) further document-ed radiogenic isotope trends with age by an-alyzing Jurassic, Cretaceous, and Cenozoicplutons. Jurassic plutons are dominated by asubcontinental lithospheric mantle componentwith the addition of a minor crustal compo-nent that has primitive Nd and Sr isotope com-positions but a high d18OWR value (Wright andWooden, 1991). Cretaceous magmatism in theGreat Basin is the result of crustal thickening,and plutons of this age have evolved Sr andNd isotope ratios that reflect a significantcrustal component. Cenozoic magmatism is amixture of mantle and crustal reservoirs witha significantly increased mantle componentcompared to Cretaceous plutonism.

SAMPLE LOCATIONS

Granitic rocks throughout northern Utahand Nevada were sampled for this study (seeFig. 3 and Table DR1).1 In addition, one sam-ple was collected from the Albion Mountainsin southern Idaho. The goals of sampling weremaximum geographic distribution and varietyof age. The general sample locations withinmountain ranges with respect to the 0.706 lineare shown in Figure 3, and latitude/longitude,rock name, age, isotope data, and geochem-istry for the intrusions are given in Table DR1(see footnote 1). References for the radiogenicisotope data and geochemistry of intrusivesamples in this study are shown in Table DR2(see footnote 1). We analyzed quartz from 124samples from 47 intrusive bodies and zirconfrom 139 samples from 42 intrusive bodies.We analyzed quartz and zircon from the sameintrusion in a few cases (n 5 11) but were notable to analyze any pairs from the same sam-ple because zircon separates came from earlierstudies.

Samples from localities 1 through 31 alongwith 37a, 37b, and 37c (Fig. 3) are locatedeast of the 0.706 line as defined by Kistler andPeterman (1973). Sample localities 32–36 and

1GSA Data Repository item 2004046, Data tableswith sample location, isotope and geochemical data,age, and references for previously published dataincluded in this study, is available on the Web athttp://www.geosociety.org/pubs/ft2004.htm. Re-quests may also be sent to [email protected].



TABLE 1. SUMMARY OF OXYGEN ISOTOPE RATIOS FOR PLUTONS OF THE GREAT BASIN

East of the 0.706 line West of the 0.706 line Walker Lane belt

QuartzPrecambrian gneisses 8.55‰ 6 0.41‰ (n 5 7)Jurassic 10.80‰ 6 0.87‰ (n 5 29) 10.22‰ 6 0.76‰ (n 5 6) 9.77‰ 6 0.69‰ (n 5 4)Cretaceous 11.54‰ 6 1.19‰ (n 5 13) 10.31‰ 6 0.98‰ (n 5 17) 9.53‰ 6 0.59‰ (n 5 10)Tertiary 8.76‰ 6 3.19‰ (n 5 35) 5.68‰ (n 5 1)ZirconPrecambrian gneisses 6.22‰ 6 0.75‰ (n 5 12)Precambrian pegmatites 7.25‰ 6 1.38‰ (n 5 7)Jurassic 7.71‰ 6 0.71‰ (n 5 24) 5.76‰ 6 0.34‰ (n 5 7) 5.48‰ 6 0.27‰ (n 5 4)Cretaceous 8.24‰ 6 0.85‰ (n 5 24) 6.22‰ 6 0.78‰ (n 5 12) 5.99‰ 6 0.30‰ (n 5 11)Tertiary 6.41‰ 6 0.90‰ (n 5 32) 5.68‰ 6 0.46‰ (n 5 9)

Note: See Table DR1 (see footnote 1 in text) for data.

38, 39, 44, 47, 48, and 49 (Fig. 3) are locatedwest of the 0.706 line. Sample localities 40–43, 45, and 46 are located within the WalkerLane belt as delineated by Stewart (1988). Thedata for samples HIL-1, HIL-10, LC-2, LC-5,LC-9, LC-15, LC-33, LC-34, and NP-1 arefrom King et al. (2001). Precambrian gneissesare from two different basement provinces.All samples of Precambrian quartz (sample lo-calities 7 and 8) are from the northern GreatBasin, and the Precambrian zircon samples(sample localities 13 and 19) are from base-ment rocks exposed in the southern Great Ba-sin belonging to the Yavapai province. Thesouthern Great Basin zircon samples are theonly samples of this study belonging to thatsouthern province.

ANALYTICAL METHODS

Mineral separates were prepared at the Uni-versity of Wisconsin (by King), Stanford Uni-versity (by Stockli), and Rice University andStanford University (by Wright and J.H. Dil-les). Zircons were separated from 15 to 20 kgsamples by using standard crushing, hydrau-lic, magnetic, and heavy-liquid techniques.The least magnetic fraction of zircon availablewas analyzed to limit complications related toalteration that are typically associated withmagnetic zircons (Krogh, 1982; Valley et al.,1994).

Mineral separates (quartz, titanite, garnet,and biotite) were obtained from ;0.5 kg sam-ples that were crushed and sieved. All min-erals were handpicked under a binocular mi-croscope. Quartz was cleaned in 29Mhydrofluoric, 12M hydrochloric, or 8M fluo-roboric acid at room temperature to confirmpurity prior to final selection.

Oxygen isotope ratios of 1–2 mg mineralseparates were analyzed at the University ofWisconsin by laser fluorination (BrF5) andgas-source mass spectrometry (Valley et al.,1995). The oxygen isotope data are reportedin the standard d notation in per mil (‰) rel-ative to Vienna standard mean ocean water (V-SMOW). Zircon grains were analyzed afterpowdering in a boron carbide mortar and pes-tle to avoid any possible grain-size effects dur-ing analyses and to maximize the efficiency offluorination. Quartz grains were analyzed byusing rapid heating and a defocused laserbeam (Spicuzza et al., 1998).

The UWG-2 garnet standard analyzed on 27days concurrently with the mineral separatesyielded an analytical precision of 0.06‰ (1standard deviation, n 5 91). On several days,daily averages of UWG-2 differed (60.02‰to 0.26‰) from the accepted value of 5.80‰

(relative to 9.59‰ for NBS-28). Analyses ofunknown samples from these days have beencorrected by this small amount as recom-mended by Valley et al. (1995). The averagecorrection was 0.08‰. The average reproduc-ibility of duplicated zircon and quartz analysesis 60.05‰ (n 5 111) and 60.16‰ (n 5112), respectively. NBS-28 quartz, analyzedconcurrently with quartz samples, yielded ad18O value of 9.55‰ 6 0.04‰ (n 5 4).

RESULTS

Oxygen isotope ratios of analyzed zircon,quartz, titanite, garnet, and biotite are reportedin Table DR1 (see footnote 1). The averaged18O values for quartz and zircon by age andlocation are summarized in Table 1. Histo-grams of quartz analyses are plotted in Figure4. Histograms of zircon analyses are plottedin Figure 5. The only samples not plotted inFigures 4 and 5 are 99GB–100, 99GB–113,99GB–114, and 99GB–115 because of poorconstraint on the age of the intrusions.

When classified by age, analyses of d18OQtz

show significant variation from the Precam-brian through the Tertiary. Precambriand18OQtz values are .2‰ lower than Jurassicd18OQtz values (Figs. 4C, 4D). Cretaceousd18OQtz values have an average near the Juras-sic average with no clear distinction betweenplutons emplaced east or west of the 0.706line. Tertiary d18OQtz values are significantlylower and more variable than those of Creta-ceous samples (Figs. 4A, 4B). The averaged18O values of quartz from Tertiary plutonsreported in Table 1 includes the anomalousWarm Springs granite of the Egan Range (mapno. 24, see Fig. 3) with d18OQtz 5 27.63‰ 60.16‰. The d18OQtz of this sample is not rep-resentative of magmatic values. Zircons fromthis mountain range, however, have d18O val-ues typical for Tertiary intrusions in the region(6.78‰), and previous studies have suggestedthe modification of d18O values during Tertia-ry deformation (Lee et al., 1986). Excluding

the anomalous Warm Springs granite, d18OQtz

of Tertiary plutons east of the 0.706 line havean average value of 9.24‰ 6 1.45‰ (n 534).

The d18OZrc values for Precambrian gneissesin the Great Basin are the lowest of all suitesanalyzed, but almost all are higher than thosed18OZrc values from rocks in equilibrium withprimitive-mantle–derived magmas (d18O 55.3‰ 6 0.3‰) (Valley et al., 1998) (Fig. 5D).Jurassic d18OZrc values are nearly 1.5‰ higherthan Precambrian gneisses. Jurassic plutonsemplaced in the Walker Lane belt and west ofthe 0.706 line have distinctly lower d18O val-ues than plutons intruded east of the 0.706 line(Fig. 5C). The d18OZrc values of Cretaceousplutons are similar to Jurassic d18OZrc values,but the highest measured d18OZrc values in theGreat Basin are from Cretaceous rocks. Thed18OZrc values for Cretaceous plutons are bi-modal; plutons emplaced east of the 0.706 linehave distinctly higher d18OZrc than those em-placed west of the 0.706 line or in the WalkerLane belt. Tertiary plutons have a significantlylower average d18OZrc value than Cretaceousplutons (Figs. 5A, 5B). The Tertiary samplewest of the 0.706 line with the highest d18OZrc

value is from the Job Canyon Pass Tuff of theStillwater Range (d18O 5 6.55, map loc. 38,see Fig. 3).

Trends observed with d18OZrc roughly cor-relate with trends in molar Al2O3/(CaO 1Na2O 1 K2O) (A/CNK; Fig. 6). Overall, in-creasing d18OZrc coincides with increasing mo-lar A/CNK, indicating a larger proportion ofhigh-d18O peraluminous crustal rocks incor-porated into these magmas. Cretaceous plu-tons, intruded into the thickened crust of theSevier orogen, are typically peraluminouswith high d18OZrc values. Tertiary plutons gen-erated during Basin and Range extension tendto be metaluminous, although some samplesare peraluminous, and again are notable in thetypically low d18OZrc values compared to plu-tons of other ages.

Four plutons were analyzed for variations


KING et al.

Figure 4. Histogram of average d18O values for quartz from granitic rocks of the GreatBasin. Note there is no significant difference in d18O values between Cretaceous quartzfrom plutons intruding east or west of the 0.706 line. Cretaceous and Tertiary values arenot markedly different. Alteration of quartz after crystallization may have changed mag-matic d18O values and obscured information about the composition of the magmatic sourceor crust at depth.

in magmatic d18O values vs. elevation (LittleCottonwood stock, map loc. 1, 1801 m relief;Austin pluton, map loc. 37a, 400 m relief; IXLCanyon pluton, map loc. 38, 644 m relief;Bald Mountain, map loc. 40a, 1435 m relief).There is no consistent increase or decrease ofd18OZrc with increased elevation. The respec-tive magma chambers were homogeneous inmagmatic d18O values on the basis of the zir-con analyses of this study.

DISCUSSION

Alteration of Quartz and Whole-rockOxygen Isotope Ratios

The refractory nature, high closure temper-ature, and slow rates of oxygen diffusionthrough the crystal lattice allow zircon to behighly retentive of primary magmatic oxygenisotope ratios (see Valley, 2003). Hydrother-mal alteration and recrystallization during tec-tonic activity are two potential processes toalter d18O values in less refractory mineralssuch as quartz and feldspar, which is subse-quently reflected in whole-rock values (Valleyand Graham, 1996; King et al., 1997, 2000;Monani and Valley, 2001). The quartz andwhole-rock d18O values of some plutons inthis study may have been altered by hydro-thermal and tectonic activity (i.e., rocks in theEgan Range and the Stillwater Range). A thirdprocess that can change the d18OQtz relative tod18OZrc is assimilation during crystallization.The assimilation of country rock has beendocumented to change the d18Omagma value thatthen changes the oxygen isotope fractionationbetween early- and late-crystallizing minerals(King and Valley, 2001; Lackey et al., 2002).Zircon, a mineral that crystallizes relativelyearly in the crystallization sequence of a gra-nitic magma, can more accurately representthe original magmatic oxygen isotope ratiosthan minerals that crystallize later from amagma with a slightly altered d18O.

A fourth process that can elevate the quartzand whole-rock d18O values relative to refrac-tory minerals, such as zircon, is diffusionalexchange of oxygen during cooling. If plutonsare intruded at depth in the crust and allowedto cool slowly, minerals that crystallize earlyand have high closure temperatures to diffu-sion (i.e., zircon, garnet, titanite) (Dodson,1973; Coghlan, 1990; Valley et al., 1994;Morishita et al., 1996; Watson and Cherniak,1997; King et al., 2001; Peck et al., 2003) willpreserve d18O values more representative ofmagmas compared to minerals that crystallizelater and have low closure temperatures. Thefast grain boundary model predicts the ex-



Figure 5. Histogram of average d18O valuesfor zircon from granitic rocks of the GreatBasin, western United States. Precambrianzircons from basement gneisses show a sig-nificant spread in values, typical of Prote-rozoic crust rather than Archean crust(Peck et al., 2000). In contrast to quartz,Jurassic and Cretaceous zircons show dis-tinct populations east and west of the 0.706line. Plutons intruding the Walker Lanebelt are similar to those emplaced west ofthe 0.706 line. The bimodal d18O values ofJurassic and Cretaceous zircon are not ap-parent in quartz d18O analyses. There is asignificant drop of average d18O values inTertiary plutons east of the 0.706 line rel-ative to older plutons in the same area, in-dicating a relative increase of mantle com-ponents in the magma source to the eastduring the Tertiary. Figure 5D shows therange of d18OZrc values from rocks in equi-librium with primitive-mantle–derived mag-mas (Valley et al., 1998).N

change of oxygen isotopes between mineralsin a rock during the cooling history (Eiler etal., 1992, 1994). During cooling, minerals thathave not reached their blocking temperaturewill continue to exchange oxygen. Quartz and

feldspar are major constituents of graniticrocks and also possess low closure tempera-tures. During the cooling history of a graniticmagma, quartz and feldspar will exchange ox-ygen isotopes with other minerals in the rock

and preferentially take up 18O, resulting in ahigher d18O.

The majority of plutons in the Great Basinare likely affected by variable cooling histo-ries, particularly as tectonic activity haschanged crustal thickness significantly, thatcan lead to larger Qtz-Zrc fractionations thanthose representative of high-temperature mag-matic values. This study shows that broadtrends of d18O values and age are apparentwith quartz analyses, but not for the finer-scalestudy of crustal structure. Quartz and feldsparmay be modified by only a small amount fromprimary magmatic values, but the differenceis enough that subtle distinctions are obscuredby this diffusional resetting (King et al.,1998).

If cooling is closed system, the whole rockwill still preserve the magmatic d18O value,but open-system cooling with even minor ex-change with country rock or fluids will causemodification of the whole-rock d18O value. Ifthe country rock is sedimentary with a typi-cally high d18O value, the whole-rock willlikely rise because of exchange. Evidence forthe minor modification of whole-rock d18Ovalues in the Great Basin is apparent whenanalyzed d18OWR values are compared withcalculated d18OWR values based on d18OZrc andSiO2 content (Valley et al., 1994) (Table DR1[see footnote 1]). For the six plutons withd18OZrc, d18OWR, and SiO2 data available, thecalculated d18OWR values range from 1.5‰lower than the analyzed d18OWR values to


KING et al.

Figure 6. Plot of d18O values for zircon vs.molar Al/CNK [5 Al2O3/(CaO 1 Na2O 1K2O)] for plutons of various ages within theGreat Basin. Filled symbols are samples lo-cated west of the ISr 5 0.706 line. TablesDR1 and DR2 (see footnote 1 in text) pre-sent data and references. Plutons locatedwest of the 0.706 line typically have lowermolar Al/CNK ratios and lower d18OZrc val-ues. Cretaceous plutons have the clearesttrend of d18OZrc values that increase withmolar Al/CNK ratios, whereas the d18OZrc

values of Tertiary plutons appear relativelyinsensitive to molar Al/CNK ratios.

0.5‰ higher. The majority of the sampleshave lower calculated values than actual val-ues, typically ;1.0‰ lower. Such modifica-tions of actual whole-rock d18O values aresubstantial enough to obscure the small dif-ferences between magmatic sources.

Isotopic Trends of d18OZrc with Longitude

The zircon oxygen isotope data of thisstudy identify a major crustal boundary that isbroadly coincident with the 0.706 line andmarks the boundary between lower-d18O plu-tons on the west and high-d18O plutons on theeast. Farmer and DePaolo (1983) interpretedthe 0.706 line as the transition from primarilycontinental sedimentary rocks to volcanic-arc–dominated rocks. The location of thezircon-based oxygen isotope discontinuity dif-fers from that identified by using whole-rockdata (Solomon and Taylor, 1989). Whole-rockdata indicate high-d18O plutons farther westthan the 0.706 line. Previous models of crustalstructure incorporating oxygen and radiogenicisotope data proposed a zone of high-d18O arc-derived rocks west of the 0.708 line extendingnearly to the western border of Nevada. Thedata of this study are in better accord withradiogenic isotopes, do not require such abroad region of high-d18O rocks from volcanic

arcs, and indicate that the only major sourceof high-d18O sedimentary rocks assimilatedinto magmas of the Great Basin is continentalrocks east of the 0.706 line.

Approaching the 0.706 line from the east,the inferred thickness of Precambrian base-ment decreases while the thickness of thecraton-derived sedimentary rock section in-creases (Fig. 2). Elison et al. (1990) identifiedPaleozoic sedimentary rocks of the continentalshelf to the east of the 0.706 line and on thisbasis suggested the crystalline basement liesfarther east than the 0.706 line.

The increased thickness of craton-derivedsedimentary rocks is evident in the increasedd18OZrc as the 0.706 line is approached fromthe east (Fig. 7). In the Jurassic, Cretaceous,and Tertiary plutonic suites, the highest d18OZrc

values are nearest the 0.706 line and on theeastern side of the boundary. Jurassic plutonicrocks indicate a steady increase in d18OZrc val-ues as the 0.706 line is approached from theeast. The Cretaceous suite is unique becauseit displays high average d18OZrc values east ofthe 0.706 line (the highest in the region), andthe range of d18OZrc values increases as the0.706 line is approached from the east. Thisincreased presence of slightly lower d18OZrc

values near the 0.706 line suggests the influ-ence of low-d18O volcanic-arc rocks duringthis time period as the proportion of arc-derived rocks began to increase from east towest (Fig. 2).

Igneous rocks intruding the Great Basinwest of the 0.706 line all have significantlylower d18OZrc values than those east of the0.706 line, indicative of either less evolvedcrustal material involved in magma genesis ora lower proportion of evolved crustal materialin magmas. The Jurassic and Cretaceoussuites show bimodal d18OZrc distributions eastand west of the 0.706 line (Fig. 5). This bi-modality is not apparent in the d18OQtz distri-butions (Fig. 4). The difference between thequartz and zircon distributions could be theresult of subsolidus alteration of quartz d18Ovalues due to, for example, the diffusional ex-change of oxygen during cooling. Despite themore complex tectonic history of the WalkerLane belt, these plutons appear to have a mag-ma source similar to that of other plutons westof the 0.706 line. Jurassic and Cretaceous plu-tons emplaced in the Walker Lane belt all havelow d18OZrc values and are indistinguishablefrom Great Basin plutons located west of the0.706 line. The lower d18OZrc values of graniticrocks west of the 0.706 line suggest a lessevolved, juvenile arc magma source.

Previous oxygen isotope studies suggesteda crustal structure for the western United

States that differed from structure delineatedby the radiogenic isotope data. On the basisof whole-rock powder analyses, there is nocorrespondence between the 0.706 line andany discontinuity in the oxygen isotope data(Solomon and Taylor, 1989). The whole-rockdata, if magmatic, would suggest that thehigh-d18O zone west of the 0.706 line is dueto magmas generated from an altered volcaniccomponent to explain the high d18O values butlow 87Sr/86Sr ratios (Solomon and Taylor,1989). The western edge of the Precambriancrystalline basement also was interpreted asbeing farther east on the basis of stable isotopeanalyses compared to radiogenic isotopes. Theresults of this study, however, indicate that thestructure of the crust inferred from radiogenicisotopes is compatible with stable isotopeanalyses of d18OZrc.

The d18OZrc data also indicate a d18O dis-continuity approximately coincident with the0.706 line in central Nevada that is not ap-parent with d18OQtz. This d18OZrc discontinuitylies considerably east of Solomon and Taylor’sboundary between the central and westernzone. Approaching the 0.706 line from theeast, d18OZrc values for Jurassic and Tertiaryplutons increase, which suggests a coincidentand gradual increase of high-d18O sedimentaryrock in the crust as the crystalline basementterminates or thins. West of the 0.706 line, thed18OZrc values dramatically decrease regardlessof the age of granitic rocks (Fig. 7). In contrastto zircon, the d18OQtz values of plutons west ofthe 0.706 line are difficult to distinguish fromthose east of the 0.706 line. The western edgeof the central high-d18O zone of Solomon andTaylor (1989) is at long. ;1198W, west of thedecrease in d18OZrc values. The eastern part ofthe central high-d18O zone of Solomon andTaylor (1989) includes a part of western Utah.The d18OWR values of plutons decrease dra-matically east of the eastern zone boundary.Zircons from Tertiary and Jurassic plutonsmaintain a fairly constant range of d18O values(5‰–7‰ and 6‰–8‰, respectively) acrossthe eastern to central zone boundary, which isapproximately equivalent to long. 1148W. Thed18OZrc data of this study narrow the high-d18Ozone of the northern Great Basin to lie justbetween the 0.706 and 0.708 lines, and thed18O values increase gradually as the 0.706line is approached from the east.

Isotopic Trends with Age

Oxygen isotope results from this study doc-ument the changing sources of magmas andthe availability of sedimentary rocks in thecrust in the Great Basin from the Precambrian



Figure 7. Plot of d18O values for zircon vs. longitude for plutons of various ages withinthe Great Basin, western United States. Jurassic plutons show an increase in d18O valuesas the 0.706 line is approached from the east. Cretaceous plutons become more variableas the 0.706 line is approached from the east. Jurassic, Cretaceous, and Tertiary plutonshave a significant drop in d18O values west of the 0.706 line.

to the Tertiary (Fig. 8). The thickness of thecrust in the Great Basin has changed as a re-sult of tectonic activity. After the Antler andSonoma orogenies, crustal thickening oc-curred during the Sevier orogeny; then thin-ning due to Basin and Range extension oc-

curred since ca. 40 Ma. The variable crustalthickness not only affects the geotherm butalso the amount of crust available for inter-action with melts and the amount of sedimen-tary rocks available for contamination of andassimilation into magmas. These variations in

crustal thickness are reflected in the stable iso-tope geochemistry of plutons intruding theregion.

Zircons from Precambrian rocks within thesouthern Great Basin have d18O values that aresimilar to those of other Precambrian terranesin North America (King et al., 1998; Peck etal., 2000). The d18OZrc values of Proterozoicrocks in the Great Basin are similar to calc-alkaline orthogneisses of early Grenville crust(6.9‰ 6 1.5‰) (Peck et al., 2000).

Wright and Wooden (1991) proposed thatthe source for Jurassic magmatism in theGreat Basin was primarily subcontinental lith-ospheric mantle. These plutons have a limitedrange of Sr and Nd isotope ratios that clusternear bulk Earth values. The radiogenic isotopedata, however, do not exclude a crustal com-ponent in the Jurassic magmas that is apparentin the peraluminous nature of some of the Ju-rassic granites and the presence of inheritedzircons. The Pb isotope data indicate a crustalcomponent in the Jurassic plutons of the east-ern Great Basin (Wright and Wooden, 1991).A crustal component in Jurassic magmas eastof the 0.706 line is evident in d18OZrc valuesbut is not evident in samples west of the 0.706line or in the Walker Lane. The average d18OZrc

value of Jurassic igneous rocks analyzed eastof the 0.706 line is 7.71‰ 6 0.71‰, over 2‰higher than the mantle (Fig. 5) (Mattey et al.,1994; Valley et al., 1998), and d18OZrc is highregardless of whether the pluton is metalu-minous or peraluminous (Fig. 6). Pb and Oisotope ratios of Jurassic plutons in the centraland western Great Basin do not indicate aslarge an old, high-d18O crustal component asin the eastern plutons. In the Walker Lane andother areas west of the 0.706 line, zircon d18Ovalues are similar to what would be expectedfrom primitive-mantle–derived melts. Thed18OZrc values indicate the necessity of a high-d18O crustal component in the magma, buthigh «Nd values also require that this crustalcomponent be relatively juvenile (Fig. 9).Young ocean crust altered at low temperatureor young sedimentary rocks are possible in-puts to magma that would result in high d18Ovalues and low 87Sr/86Sri ratios in the plutons.

Cretaceous plutons in the Great Basin aresimilar to Jurassic plutons in terms of theird18OZrc distributions (Fig. 8). Cretaceous plu-tons east of the 0.706 line have highly elevat-ed d18OZrc values, whereas plutons locatedwest of the 0.706 line, including those in theWalker Lane, are only slightly elevated rela-tive to mantle values (Table 1). Wright andWooden (1991) determined that Late Creta-ceous plutons are compositionally and isoto-pically (Sr, Nd, Pb) distinct from Jurassic plu-


KING et al.

Figure 8. Plot of d18O values for zircon vs. age of plutons (Tables DR1 and DR2 [seefootnote 1 in text] present geochronology data and references). The shaded area indicatesplutons located west of the 0.706 line. d18O values increase through the Jurassic andCretaceous until a break in plutonism at ca. 70 Ma. Tertiary magmatism returns to lowerd18O values.

Figure 9. Plot of d18O values for zircon vs. «Nd values for plutons of various ages withinthe Great Basin. Filled symbols are samples located west of the 0.706 line. Tables DR1and DR2 (see footnote 1 in text) present data and references. Cretaceous plutons show aclear trend of increasing d18OZrc with decreasing «Nd. The d18OZrc values of Tertiary plutonsappear insensitive to variation in «Nd; however, plutons located west of the 0.706 line havesignificantly lower «Nd values with little variation in d18OZrc compared to plutons east ofthe 0.706 line.

tons. The Late Cretaceous plutonic suite eastof the 0.706 line tends to be strongly peralu-minous, and isotope ratios indicate significantcrustal components in the magmas («Nd from213 to 223, initial 87Sr/86Sr ratios from

0.7109 to 0.7261 (Wright and Wooden,1991)). Some Cretaceous plutons are likelythe product of crustal melting resulting fromcrustal thickening and an elevated geothermduring the Sevier orogeny (Barton, 1990).

Barton (1990) documented a progressionduring the Cretaceous of an increasing sedi-mentary component in granitic magmas, par-ticularly those located east of the 0.706 line.Late Cretaceous plutons have higher initial87Sr/86Sr ratios, higher whole-rock d18O values,and lower «Nd values and are more peralumi-nous than Early Cretaceous plutons (Barton,1990). The highest d18OZrc values of plutonseast of the 0.706 line increase from ;8.5‰ toas high as 10‰ through the Cretaceous (Fig.8) and agree with the proposed trend of in-creasing sedimentary influence on magmasthrough the Cretaceous. The model proposedfor Cretaceous magmatism is a progressive in-crease of conductive heat in the region duringthe time period, with renewed magmatic ac-tivity leading to increased crustal componentsin magmas (Barton, 1990). The increase of acrustal signature in granitic magmas is not justdue to an elevated geotherm associated withcrustal thickening, but also an increased mag-matic travel path through the crust as crustalthickening occurred, resulting in longer crustalresidence times. Crustal thicknesses in theLate Cretaceous increased to as much as 50–60 km (Barton, 1990).

Magmatism in the Great Basin resumed inthe middle Tertiary after an abrupt break at theend of the Cretaceous and a .20 m.y. hiatus(Fig. 8). Tertiary plutonic d18OZrc values aresignificantly lower than the values of previousplutons (6.25‰ 6 0.87‰ compared to 7.20‰6 1.30‰). Plutons east of the 0.706 line arestill slightly elevated relative to mantle values,but nearly 2‰ lower than Cretaceous plutonseast of the 0.706 line. Nd and Sr isotope ratiosof Tertiary plutons indicate a mixture of man-tle and crustal reservoirs in the magma source,and these plutons plot between mantle-like Ju-rassic plutons and crustal Cretaceous plutonsin terms of initial 87Sr/86Sr and «Nd (Wright andWooden, 1991). This Tertiary suite of plutonswas generated and emplaced in the crust dur-ing Basin and Range extension. Althoughsome Tertiary plutons in the Great Basin arenonetheless peraluminous, a decreased crustalcomponent in the granitic magmas is in agree-ment with the model of mantle upwelling anddecreasing crustal thickness during extension.The significant decrease of d18OZrc between theCretaceous plutons and Tertiary plutons indi-cates a decreased crustal component in thesemagmas.

CONCLUSIONS

This study demonstrates that zircons fromigneous bodies in the Great Basin preservemagmatic d18O values and reveal temporal as



well as spatial trends of magmatic evolutionthat have not been seen by other oxygen iso-tope studies emphasizing analysis of whole-rock samples. Zircon d18O values are capableof seeing through subsolidus alteration thatfrequently affects quartz and whole-rock d18Ovalues and can indicate subtle source varia-tions. The 87Sr/86Sri 5 0.706 line has previ-ously been identified as the boundary betweenthe North American craton and accretedvolcanic-arc terranes. This crustal suture zoneis reflected by an oxygen isotope discontinuitywith higher d18OZrc values east of the 0.706line and lower d18OZrc values west of the 0.706line. The 0.708 line, or «Nd 5 27, has beeninterpreted to represent the westernmost extentof crystalline basement. Lying between the0.706 and 0.708 line is the thickest sequenceof continentally derived Precambrian and Pa-leozoic sedimentary rocks. As the 0.706 lineis approached from the east, the increasedthickness of high-d18O sedimentary rocks de-rived from the craton is also apparent in theGreat Basin plutons’ d18OZrc values that in-crease from east to west.

Oxygen isotope ratios of zircon also recordchanges in magma source through the pro-tracted geologic history of the Great Basin.For Jurassic plutons east of the 0.706 line, thecombination of radiogenic and stable isotoperatios suggests the mixture of a juvenile mag-ma source and high-d18O supracrustal rocks(Wright and Wooden, 1991). Cretaceousd18OZrc values indicate a greater involvementof crustal rocks in magma genesis as the crustwas thickened during the Sevier orogeny. Ter-tiary plutonic d18OZrc values decreased and be-came more mantle-like as the crust wasthinned during Basin and Range extension.Radiogenic isotope studies have identifiedcrustal discontinuities controlled by the dif-ference in the composition and age of thecrust. The stable isotope discontinuity in cen-tral Nevada identified by d18OZrc values furthersuggests a significant change in the lithologyof the crust.

ACKNOWLEDGMENTS

We thank M.J. Spicuzza for assistance in the sta-ble isotope laboratory, B.J. Kowallis for providingtitanite-bearing samples, and B. Hess for makingthin sections. We thank B. Tikoff and C. Johnsonfor thoughtful discussion and comments on thismanuscript. Reviews by J. Dilles, L. Farmer, and C.Miller and helpful editorial suggestions by L. Farm-er and Y. Dilek are gratefully acknowledged. Thisresearch was supported by National Science Foun-dation grants EAR-96–28260 and EAR-99–02973,Department of Energy FG02–93ER14389 (to Val-ley) and a Dean Morgridge Wisconsin Distin-guished Graduate Fellowship (to King).

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