JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. B8, PAGES 17,425-17,445, AUGUST 10, 1996
Evolution of a mafic volcanic field in the central Great Basin, south central Nevada
G. M. Yogodzinski,T. R. Naumann,E. I. Smith,and T. K. Bradshaw3 Centerof Volcanicand Tectonic Studies, Department of Geosciences,University of Nevada,Las Vegas
J. D. Walker IsotopeGeochemistry Laboratory, Department of Geology,University of Kansas,Lawrence
ABSTRACT. Evolutionof a mafic volcanicfield is investigatedthrough a studyof Pliocene agerocks in the ReveilleRange in southcentral Nevada. Plioceneactivity began with the eruptionof relativelyabundant hawaiite (episode 1, 5-6 Ma), whichwas followed by trachytic volcanism(4.3 Ma) andby a secondepisode of lower-volumehawaiite and basanite (episode 2, 3.0-4.7 Ma). Incompatibleelements indicate an asthenosphericsource. Isotopically, episode 2 basaltscluster around 87Sr/86Sr=0.7035 and œNd=+4.2, but episode 1 samplesvary to high 87Sr/86Sr(up to 0.7060)over a narrowrange of end(+0.8 to +4.5). Trachyticrocks (MgO--0.5%) are isotopicallyakin to the episode1 basalts.Geochemical variation requires the additionof a crustalcomponent (high 87Sr/86Sr, Sr/Nd, Pb/La, low œNd) to theepisode 1 hawaiitesand trachyticsamples, probably by assimilationof carbonate-richsedimentary wall rock. The volcanic field developedin at leasttwo eruptivecycles of approximatelyequal duration. Basanites (deeper andlower percentage melts) appear only in theyounger episode. Eruptive episodes were apparently linkedto separatemelting events in the mantle. Throughtime, basaltswere produced in diminishingvolumes by lower percentagemelting, magma generation and storage was at greater depths,and magma ascent was at highervelocities. Spatially, the meltinganomalies were large in the Pliocenebut progressivelydiminished in sizeso that by Pleistocenetime, volcanismwas restricted to a small area near the northern end of the initial outbreak.
Introduction 1988; Farmer et al., 1989]. The assimilation of continental The studyof Miocene and Pliocene age mafic volcanicrocks crust is regardedby most workers to be of minor petrogenetic associated with Cenozoic crustal extension in the western importance [e.g., Leeman, 1982; Menzies et al., 1983; Fitton United States continuesto provide insight into a variety of et al., 1988], thoughthere are caseswhere crustalassimilation geologically important processesand systems. These include is thought to have been a primary control over Basin and lithospheric-scaletectonic features of rifting on continental Range basalt geochemistry[e.g., Glazner et al., 1991; Glazner crust, important aspects of the crust-mantle geochemical and Farmer, 1992]. system,and clues to the nature of magmaticdifferentiation and End-membersin the geochemical spectrum of Basin and the genesis of igneous rocks in the continental rift Rangebasalts are well representedin mafic volcanicrocks that environment [e.g., Leeman, 1982; Menzies et al., 1983; occur within the NNE trending zone of Pliocene and younger Fitton et al., 1988; Glazner et al., 1991; Glazner and Farmer, mafic volcanism that extends from Death Valley on the south, 1992; Bradshaw et al., 1993]. to the PancakeRange and Lunar Crater Volcanic Field on the Previous and ongoing studies have shown that there is a north (Figure 1). This is the Death Valley-PancakeRange twofold geochemical division among Pliocene and younger basalt zone of Vanimanet al. [1982] and Farmer etal. [ 1989]. mafic volcanic rocks in the Basin and Range region [Menzies Basalts from the central part of this zone in the area around et al., 1983; Fitton et al., 1988; Orrnerod et al., 1988; CraterFlat (southernNevada province of Menzieset al. [1983]) Rogers et al., 1995]. This twofold division is most often are among the most isotopically enriched in the region interpretedto reflect compositionallydistinct sourcesin the (87Sr/86Sr•- 0.707, œNd< -8.5 [see Farmeret al., 1989; asthenosphericand lithospheric mantle [see also Leeman, Livaccariand Perry, 1993]) and have all of the major and trace 1970; Hedgeand Noble, 1971; Leeman, 1982; Fitton et al., element features that characterize basaltic rocks derived from the lithospheric mantle (hypersthene-normative with low •Now at Departmentof Geology,Dickinson College, Carlisle, FeO*, TiO2, Rb/Ba, and Ti/Hf and high La/Ta and Ba/Nb [see Pennsylvania. Fitton et al., 1988]). In contrast, basaltic rocks from the 2Nowat Departmentof Geology,University of Idaho,Moscow. 3Now at House of Lords Committee Offices, London. northernend of the zone, in the Reveille and Pancakeranges (including the Lunar Crater Volcanic Field), are isotopically Copyright1996 by theAmerican Geophysical Union. depleted (87Sr/86Sr•- 0.7035, œNd> +3 [seeFarmer et al., 1989; Foland and Bergman,1992]) and have major and trace Papernumber 96JB00816. element featureslike averageocean island basalt (nepheline- 0148-0227/96/96JB-00816509.00 normative with high FeO*, TiO2, Ti/Hf, and Rb/Ba and low
17,425 17,426 YOGODZINSKIET AL.: ORIGIN OF GREAT BASIN BASALTS
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WESTERN & CENTRAL Cima GREAT BASIN BASALTS Includesbasalts less than approximately 8 m.y. old. Modifiedfrom Luedke & Smith(1981).
Figure1. Distributionof basaltic volcanic rocks less than approximately 8 m.y. old in thewestern and centralGreat Basin of centraland southwestern Nevada and southeastern California. Death Valley-Pancake Rangebasalt zone from Vaniman etal. [1982]and Farmer etal. [1989].Modified from Luedke and Smith [1981.
La/Taand Ba/Nb [see Fitton et al., 1988, 1991]). Basaltsfrom Folandand Bergman, 1992] and from the Crater Flat area to the the Reveille and Pancakerange area are generallyregarded as south[Vaniman et al., 1982;Farmer et al.,1989; Bradshaw the asthenosphericallyderived end-member in the region[e.g., andSmith, 1994]. Two broadly different themes are developed. Fitton et al., 1988; Farmer et al., 1989]. First,the data are interpreted within the context of a changing In thispaper we examinethe geologyand geochemistry of sourcechemistry for the Pliocene volcanic rocks, with Plioceneage mafic volcanic rocks in the Reveille Range (Figure1). We comparethe Pliocene age Reveille Range rocks emphasison theaddition of a crustalcomponent to theoldest to Pleistoceneage basaltic rocks from the Lunar Crater of the basaltsin the Reveille Range area. Second,the data are' Volcanicfield to the north[Bergman, 1982; Lumet al., 1989; interpretedin the contextof volcanicfield evolution,with YOCK)DZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS 17,427
I 116 ø 15'
38 ø 10' -
38 ø 00'-
GEOLOGIC MAP OF REVEILLE RANGE MAFIC VOLCANIC ROCKS
Episode#2 basalts(3.0-4.6 Ma) Trachyte- lava/pyroclastic(4.3 Ma) Episode#1 basalts(5.1-5.9 Ma) Basaltic andesites(Miocene?) 0 2 4 hawaiitevent trachytevent basanitevent 116o 15' I
Figure 2. Geologic map of Pliocenevolcanic rocks in the Reveille Rangeof southcentral Nevada (see also Figure 1). Mapping is modifiedslightly from that of Naumannet al. [ 1991]. Seealso Martin and Naumann [1995].
emphasison time-space-compositionaltrends as they relate to rocks in the central Great Basin of the western United States. magmaevolution in the mantle and crust. This area lies along the axis of geophysicalsymmetry outlined by Eaton et al. [1978] for the central Great Basin, and is Location, Volcanic Stratigraphy, isolatedfrom the well-developedvolcanic fields of the Sierran Province/Western Great Basin to the west, and the transition and Petrography zone/Colorado Plateau to the east [Leeman, 1970, 1982; Pliocene and Pleistoceneage basaltsof the Reveille Range Menzies et al., 1983; Fitton et al., 1988]. and PancakeRange (includingthe Lunar Crater Volcanic Field) Geologic mapping and K-At dating in the Reveille Range constitutethe largestvolume of Late Cenozoicmafic volcanic and adjacentareas [Naumann et al., 1991; Martin and Naumann, 17,428 YOGODZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS
Table 1. RockNames, Partial Norms, Phenocryst Assemblages, and Locations for ReveilleRange Samples Sample RockName neph* hyp* Phenocrysts? LatitudeNorth LongitudeWest Elevation,feet (m) Episode 1 Basalts R9-1-48 hawaiite 0.0 0.0 plag,olv, trace cpx 38ø 9.8' 116ø 6.1' 6200(1890) R9-3-60 hawaiite 0.5 0.0 plag,olv 37ø 54.4' 116ø 7.2' 6690(2039) R9-4-61 hawaiite 0.4 0.0 plag,olv 37ø 59.1' 116ø 6.9' 6200(1890) R8-I-4 hawaiite 3.4 0.0 plag,olv 38ø 4.0' 116ø 7.7' 6340(1932) R8-I-17 hawaiite 1.8 0.0 plag,olv 38ø 4.3' 116ø 7.5' 6100(1859) R9-I-56 hawaiite 0.0 5.4 plag,olv, trace bio, cpx 38ø 10.6' 116ø 5.6' 5280(1609) R0-I-73 hawaiite 0.0 4.1 plag,olv 38ø 4.7' 116ø 13.1' 6950(2118) R8-I-37 hawaiite 1.0 0.0 plag,olv, cpx 38ø 6.2' 116ø 8.2' 5920(1804) R8-1-7 hawaiite 3.5 0.0 plag,olv 38ø 3.7' 116ø 7.9' 5980(1823) R8-I-25 hawaiite 0.6 0.0 plag,olv, trace bio 38ø 3.1' 116ø 8.6' 5800(1768) R9-2-59 hawaiite 3.0 0.0 plag,olv 37ø 57.3' 116ø 6.0' 5800(1768) R8-1-14 hawaiite 0.0 4.2 plag,olv 38ø 4.0' 116ø 6.8' 5500(1676) R0-1-77 hawaiite 0.0 8.7 plag,olv 37ø 50.7' 116ø 12.9' 6000(1829) R8-1-29 hawaiite 0.1 0.0 plag,olv 38ø 5.6' 116ø 8.7' 6000(1829) R8-1-39 hawaiite 2.3 0.0 plag 38ø 6.9' 116ø 7.6' 5860(1786) R8-I-28 hawaiite 2.2 0.0 plag,olv, oxd 38ø 5.7' 116ø 8.1' 5650(1722) R9-I-66 hawaiite 0.3 0.0 plag,trace olv 38ø 9.6' 116ø 11.7' 6200(1890) Episode 2 Basalts R8-I-27 basanite 9.4 0.0 cpx,olv, oxd 38ø 3.2' 116ø 9.4' 6472(1973) R8-I-23 basanite 9.3 0.0 cpx,olv, oxd 38ø 3.4' 116ø 9.3' 6320(1926) R8-I-26 basanite 10.I 0.0 cpx,olv, oxd 38ø 3.2' 116ø 9.7' 6280(1914) R8-I-22 basanite 8.7 0.0 cpx,olv, oxd 38ø 3.5' 116ø 9.3' 6500(198 I) R8-1-11 basanite 10.6 0.0 cpx,olv, oxd 38ø 4.5' 116ø 8.8' 6320 (1926) R8-I-30 basanite 7.0 0.0 cpx,olv, plag,oxd 38ø 5.2' 116ø 8.4' 6030 (1838) R9-1-47 basanite 6.4 0.0 olv 38ø 10.4' 116ø 6.4' 5320(1622) R9-I-55 basanite 6.1 0.0 plag,olv, oxd 38ø 8.2' 116ø 7.7' 5540(1689) R8-1-13 hawaiite 0.0 0. I plag,olv, cpx,oxd 38ø 4.4' 116ø 7.3' 6120 (1865) R9-I-46 hawaiite 3.1 0.0 olv,plag 38ø I0.5' 116ø 6.6' 5455(1663) R8-1-32 hawaiite 0.0 0.4 plag,olv 38ø 2.2' 116ø 7.0' 5580 (1701) R9-144 hawaiite 2.1 0.0 plag,cpx, oxd, olv 38ø 7.I' 116ø 6.0' 5300 (1615) R8-I-12 hawaiite 0.4 0.0 olv, plag,cpx, oxd 38ø 4.25' 116ø 7.0' 5930 (1807) R8-1-19 hawaiite 2.4 0.0 plag,otv, el}X, oxd 38ø 2.4' 116ø 7.2' 5730 (1747) R8-I-6 hawaiite 3.7 0.0 plag,olv, oxd,cpx 38ø 4.7' 116ø 7.6' 6160 (1878) R8-I-18 hawaiite 3.0 0.0 plag,cpx, olv, oxd 38ø 4.4' 116ø 7.4' 6240 (1902) >,8-1-1 hawaiite 3.2 0.0 plag,olv, oxd,cpx 38ø 3.6' 116ø 8.0' 6000 (1829) TrachyticRocks R8-1-16 trachyandesite 2.6 0.0 plag,kspr, oxd, cpx, trace olv 38ø 3.5' 116ø 7.6' 5840 (1780) R8-1-40 trachyandesite 10.6 0.0 no phenocrysts 38ø 7.7' 116ø 6.1' 5841 (1780) R9-I-43 trachyte 8.0 0.0 traceplag, ksp, cpx 38ø 7.75' 116ø 5.3' 5240 (1597) R9-1-62 trachyte 4.4 0.0 traceplag, cpx, oxd 38ø 7.6' 116ø 6. I' 5700 (I 737) R8-I-41 trachyte 4.4 0.0 plag,kspr, cpx, oxd 38ø 7.9' 116ø 5.7' 5380 (1640) R8-1-42 trachyte 5.9 0.0 plag,kspr, cpx, oxd 38ø 7.6' 116ø 6. I' 5600 (1707) Abbreviations:nepheline (neph), hypersthene (hyp), plagioclase(plag.), olivine (olv.), clinopyroxene(cpx.), iron-ttitanium oxide (oxd.), biotite (bio.), potassiumfeldspar (kspr.) *Nonnativecompositions based on Fe 2+ / (Fe2+ + Fe3+) = 0.80. tPhenocrystslisted in orderof decreasingabundance.
1995] indicate that mafic volcanism began in middle to late Basalts of episode 1 are the most abundantof the Pliocene Miocene time with scatterederuptions of volumetricallyminor age volcanic rocks in the Reveille Range. They comprise a basaltic andesite. The early basaltic andesites occur in the minimumvolume of approximately8 km3 (estimateof outcrop northwesternmostReveille Range (Figure 2) and in scattered volume) and were erupted from --52 vents located throughout locationsto the south. Near the town of Rachel (Figure 1) this the range (Figure 2). Most episode 1 basalts contain unit has been dated at approximately 14 Ma [Naumann et al., phenocrystsof olivine and plagioclase only (25-35 modal 1991]. Based on petrographicand chemical similarities, we percent), but some also contain minor phenocrysts of anticipatea similar middle to late Miocene age for the early clinopyroxene, Fe-Ti oxide, and occasionally biotite. Large basaltic andesitesin the Reveille Range. Geochemicaldata on phenocrysts(>5 mm) and/or megacrysts (>1 cm) of calcic the Miocene rocks are not presentedhere, and these rocks will feldspar (labradorite) are also common, and these sometimes not be considered further in this work. occur in glomerocrystictrains that range up to 15 cm in long Plioceneactivity in the Reveille Rangecommenced with the dimension. The common presence of biotite in the eruptionof a relatively large volume of alkalic basalt(5.1-5.9 groundmass of episode 1 basalts is notable. Alteration Ma), which was followed by trachyticvolcanism (4.3 Ma) and minerals include iddingsite and less commonly serpentineor finally by a seconderuptive episodeof lower volume basalt bowlingite (both after olivine) and calcite. (3.0-4.7 Ma, see Naumann et al. [1991] for information on In the northeasternReveille Range, basaltsof episode 1 are dates). These map units are shownin Figure 2 and will be overlainby two trachyticdome-like lava flows (--0.1 km3)and referredto throughoutthis paper as (1) the episode1 basalts, associatedpyroclastic surge deposits [see Naumann et al., (2) the trachytic rocks, and (3) the episode 2 basalts. 1990]. The trachytic lavas are sparsely phyric (<5 modal Petrographic information on these units is summarizedbelow percent) with phenocrysts of sanidine, plagioclase, green- and in Table 1. colored clinopyroxene,Fe-Ti oxides, and occasionallyapatite. YOCR)DZINSKIET AL.: ORIGIN OF GREAT BASIN BASALTS 17,429
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Table 2c. Major andTrace Element Abundances in ReveilleRange Trachytic Rocks R8-1-16 R8-1-40 R9-1-43 R9-1-62 R8-1-41 R8-1-42 Trachy-andesite Trachy-andesite Trachyte Trachyte Trachyte Trachyte SiO2 55.77 55.71 58.92 59.74 59.83 60.14 TiO2 1.11 0.44 0.47 0.51 0.45 0.51 A1203 16.11 15.93 16.94 17.28 17.35 17.23 Fe203 9.82 6.86 6.74 7.11 6.85 7.20 MnO 0.20 0.17 0.17 0.18 0.16 0.18 MgO 1.20 0.47 0.52 0.47 0.28 0.38 CaO 3.80 4.60 3.62 2.26 1.97 2.04 Na20 5.53 5.98 6.25 6.50 6.22 6.58 K20 4.28 5.33 5.58 5.24 5.83 5.68 P205 0.30 0.17 0.11 0.24 0.16 0.18 LOI 0.01 2.57 1.64 0.57 0.27 1.12 Total 98.13 98.23 100.96 100.10 99.37 101.24
FeO* 8.83 6.17 6.06 6.39 6.16 6.47 Mg# 0.23 0.15 0.16 0.14 0.09 0.12
Rb 71.2 87.0 90.4 89.6 Th 8.46 9.77 10.3 10.3 Ba 849 353 364 264 $r 308 101 78 79 La 62.9 73.0 82.3 72.6 Ce 130 137 155 150 Nd 69.7 64.8 71.4 60.9 $m 14.7 13.4 13.7 13.6 Eu 3.95 2.30 2.64 2.46 Tb 2.37 2.03 1.85 1.89 Yb 5.33 5.11 4.70 4.32 Lu 0.75 0.68 0.68 0.60 Y 51.9 48.0 48.5 45.9 Ta 4.85 6.41 6.46 6.49 Nb 64 84 88 90 76 Hf 16.7 18.2 18.8 18.5 Zr 879 953 947 814 $c 12.4 5.9 6.5 5.8 Cr 19 11 3 16 Ni 1 1 1 1 Zn 129 124 108 102 Co 9 2 2 1 See Table 2a footnotes,
The groundmass in the trachytic rocks is dominated by standard analyses are in Table 3. Isotopic analyses for feldspar, pyroxene, oxides, and colorlessto pale green glass. ReveilleRange rocks are presentedin Table 4, alongwith new Except for the presenceof calcite in the groundmassof some isotopicdata on Pleistocene-agebasalts of southernNevada samples, the trachytic rocks are mostly free of alteration Crater Flat area (trace elementdata for the Crater Flat samples minerals. are presentedin Bradshawand Smith, 1994). Basaltsof episode2 are the youngestvolcanic rocks in the Major Elements Reveille Range. They comprise a minimum volume of approximately1 kin3 and were erupted from 14 ventslocated Pliocenebasalts of episode1 (Table 2a) are mildly alkaline with silica-saturated and undersaturatedvarieties (Table 1). only in the northeasternpart of the range (Figure 2). Most episode2 basaltscontain phenocrystsof plagioclase,olivine, Episode1 basaltshave 45.7-49.4% SiO2, 1.0-2.2%K20, and total alkalis of 4.3-6.5% (K20+Na20, Figure 3 and Table 2a). clinopyroxene,and Fe-Ti oxides,but some samples(which we classify below as basanites)lack phenocrystsof plagioclase. They are relativelyhigh in FeO* (9.6%-14.0%) andTiO2 (2.5- Episode2 basaltsare distinguishedby the commonpresence of 3.6%) and have low-to-moderatecontents of AI20 3 (15.1- 17.4%), MgO (2.6-6.1%), and CaO (6.6-9.4%). Normative large clinopyroxene crystals (phenocrysts or megacrysts) compositionsand total alkali contents indicate that the rock and/or cognate xenoliths of medium-grain plagioclase- name "hawaiite" is appropriatefor all episode1 basalts(Table clinopyroxene-oxidegabbro. At some locations, lavas of 1 and Figure 3; seealso MacDonaldand Katsura[ 1964] and Le episode 2 also contain phenocrystsand/or megacrystsof Maitre [ 1989]). amphibole and abundant ultramafic inclusions (dunite, harzburgite). Biotite doesnot occur as a phenocrystphase in Pliocene basalts of episode 2 (Table 2b) are nearly all the episode 2 basalts and is less common and less well nepheline-normative(Table 1). On average,episode 2 basalts developed in the groundmassthan in basalts of episode 1. are thereforeslightly more alkaline than episode1 (i.e., lower Alteration minerals are like those in episode 1 basalts,though SiO2, A1203, andCaO andhigher TiO 2, FeO*, Na20, andMgO), calciteappears to be lesscommon in the episode2 samples. but the major element similarities between the stratigraphic groups are generally more striking than are their differences. Geochemistry One exception is the subsetof episode 2 sampleswhich are Major and trace elementdata for Reveille Rangevolcanic stronglyundersaturated (>5% normafivenepheline) with <45% rocksare presentedin Tables2a, 2b, and 2c. Replicateand SiO2 and/orhigh total alkali contents(Table 2b and Figure3). 17,432 YOC•DZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS
Table 3. Data Precision and Accuracy INAA Replicate INAA USGS Standard XRF USGSStandard R9-1-47 (n--4) BHVO-1 (n=4) SCO-1 (n=6) Meanppm Precision* Meanppm Precision* Accepted Meanppm Precision* Accepted Th 6.38 4.1% 1.13 16.3% 1.08 na na na Ba 768 3.8% 140 11.5% 139 na na na Ta 5.27 4.8% 1.24 7.6% 1.23 na na na Hf 8.97 6.1% 4.68 1.7% 4.38 na na na La 62.6 6.1% 15.7 2.8% 15.8 na na na Ce 123 4.0% 39.4 11.4% 39.0 na na na Sm 11.0 5.8% 6.47 2.3% 6.20 na na na Eu 3.19 3.0% 1.97 5.9% 2.06 na na na Tb 1.21 2.3% 0.91 12.3% 0.96 na na na Yb 2.47 2.9% 1.81 3.8% 2.02 na na na Lu 0.36 3.4% 0.28 19.8% 0.40 na na na Sc 16.5 5.7% 30.9 1.2% 31.8 na na na Cr 37.8 5.1% 293. 2.5% 289 na na na Co 31.5 10.4% 43.2 2.1% 45.0 na na na Rb na na na na na 115 4.2% 112 Sr na na na na na 168 6.6% 174 Nb na na na na na 12.4 2.2% 11.0 Y na na na na na 34.8 2.2% 26.0 Zr na na na na na 157 10.2% 160 Ni na na na na na 38 17.8% 27 *Analyticalprecision is two standarddeviations expressed as a percentageof the meanfor repeatanalyses of samplesand standards. Othervalues are in partsper million. Acceptedvalues for the primaryINAA standard(NIST 1633fly-ash) are Th (24.7 ppm),Ba (1420), Ta (2.00), Hf (7.40), La (84.0), Ce (175), Sm (17.0), Eu (3.70), Tb (2.50), Yb (7.40), Lu (1.12), Sc (39.0), Cr (196), Co (43.0).
These lavas are petrographicallydistinct in that plagioclaseis An extendedplot of OIB-normalized concentrations(Figure generally not an abundantphenocryst phase (Table 1). These 7) shows again that the Reveille Range basanites have featuresjustify the rock name "basanite"to distinguishthem relatively high concentrations of incompatible elements from the less alkaline and more plagioclase-phyrichawaiite compared to the hawaiites. Episode 2 hawaiites have, on basalts. average, higher incompatible element concentrations than The Reveille Range trachytic rocks (55-62% SiO2) have episode 1 hawaiites, but these differencesare small compared total alkalis of 9.8-12.3% (Na20+K20) and contain 3-11% to the distinctivebasanites. Figure 7 also pointsout the broad normativenepheline (Figure 3 and Table l c). Comparedto the similarity in interelement ratios among all Reveille Range Reveille Range basalts,the trachyticrocks have K20 contents basalts. The only clear exception to this is the relative that are higherby a factorof 4 (K20--4.3-5.8%). The trachytic concentrationof Pb (i.e., Pb/La, Pb/Ce) which is substantially rocks are moderateto high in A1203 (15.9-17.4%) and FeO* higher in episode 1 hawaiites than in any of the episode 2 (6.7-9.8%), variably low in TiO 2 (0.44-1.11%) and CaO (2.0- samples(hawaiites or basanites). 4.6%), and very low in MgO (0.4-1.2%). The Reveille Range The rare earth element characteristicsof the Reveille Range trachyticrocks resembletristanites and other evolvedrocks of basalts are consistent with other incompatible element the alkaline ocean island basalt (OIB) igneous series [e.g., features. All of the Reveille Range basalts show an overall Wilkinson, 1974]. light REE-enriched pattern (Figure 8) with uniformly low abundancesof heavy REE (Yb, Lu) at approximately8 times Incompatible Elements chondritic. On average, the episode 2 basaniteshave the Incompatible element concentrations (Tables 2a-2c) and highest concentrations of light REE at 100-155 times interelement ratios are broadly similar for episode 1 and chondritic. The episode 1 hawaiites have light REE episode2 hawaiites,episode 2 basanites,average OIB, and for concentrationsof 56-130 times chondrites,and the episode 2 Pleistoceneage basaltsof the Lunar Crater Volcanicfield. This hawaiites fall within a relatively restricted compositional is clear from the average compositions plotted on extended range with light REE at 66-100 times chondrites(see also mid-ocean ridge basalt (MORB)-normalized incompatible La/Yb versusMgO in Figure 6). element diagrams (Figure 4), and from ratio-ratio plots of all In the Reveille Range trachyticrocks, concentrations of the the available samples (Figure 5). The similarity of the most incompatibleelements are significantlyhigher than in Reveille Range and Lunar Crater data to averageOIB contrasts the associatedbasalts (Figure 7). The highestconcentrations with that of Pleistoceneage basaltsfrom Crater Flat, which are in Zr (-926 ppm), Hf (-18 ppm), Rb (-83 ppm),and K20 have higher concentrationsof Ba, Th, and light rare earth (5.4 wt %) which range between 2.5 and 3.8 times the elements(LREE) but lower relative concentrationsof high field concentrationsin the basalts. Concentrationsof La (73 ppm), strengthelements (HFSE), especiallyTa and Ti (Figures4 and Nb (82 ppm), Y (49 ppm), and Yb (4.8 ppm) are 1.4-2.4times 5). higher than in the basalts. The trachyticrocks have negative Important differences among Reveille Range basalts are Eu anomalies(Eu/Eu* = 0.54 - 0.83), but their REE patternsare revealed by their incompatible and compatible element otherwise parallel to those of the basalts at higher concentrations. Nearly all the basanites have higher concentrations(Figure 9). In contrast,concentrations of Ba in concentrationsof Sr, La, Th, Zr, and Ta (relative to MgO) than the trachyticrocks (•-420 ppm) are similar to or slightlylower the hawaiites (Figure 6), and in this way the episode 2 than in the basalts, and concentrationsof Sr (-160), P205 basanites are similar to the youngest basalts of the Lunar (0.19 wt %), and TiO2 (0.56 wt %) are all substantiallylower Crater Volcanic Field. (Table 2 and Figure 7). YOCK)DZINSKI• AL.:ORIGIN OF GREAT BASIN BASALTS 17,433
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0 9.0•8.0 ['•-"'•-"-•"-'-'•[ • :'•*:•:'"':'"•:"'trachyticrocksN • 7.0 '- + 6.0 O•cq5.0 2; 4.0 - 3.0 ...... CraterFlat basalts _
1.0
0.0 ,' '•' ''• ' ' I ''' ','' '','' ' ,,,l,•,l• 40 42 44 46 48 50 52 54 56 58 6O 62 SiO2
Figure 3. Total alkalis versusSiO 2 for Plioceneage volcanicrocks of the Reveille Range comparedwith Pleistoceneage basaltsfrom CraterFlat and the Lunar CraterVolcanic Field. Reveille Rangerocks designated here as basanites(solid circles) are distinguishedby their relatively low SiO2 and high alkali contentsand by petrographicfeatures, especially the absenceof plagioclasephenocrysts (see text and Table 1). Reveille Range data are from Table 2 and unpublishedUniversity of Nevada,Las Vegas,data. Crater Flat samplesare Red Cone and Black Coneanalyses from Bradshawand Smith[ 1994]. LunarCrater Volcanic Field dataare from Bergman[1982] and Kargel [1987].
AVERAGE BASALT COMPOSITIONS Sr, Nd, and Pb Isotopes
Isotopic compositionsof episode 1 and episode2 basalts 100.0- ,• • episode#I,g2 hawaiites are somewhat variable considering the overall similarity in I e•g2 basanites their incompatibleelement features. Episode1 hawaiitesshow a widerange in 87Sr/86Sr(0.7043 - 0.7061), whereasthe range in episode 2 (basanites and hawaiites) is more restricted (0.7035 - 0.7037, Figure 10). Neodymium isotopesare also I0.0 - more variable in basalts of episode I than episode2, though the differencesare not as great as for Sr (Figure 10). A similar patterncan be seenin the Pb isotopedata; mostof the episode 2 basaltsform a linear trendimmediately above the mantle reference line at high 2o6pb/2OnPb(..-19.2), whereas the episode1 basaltsscatter from the mantle trend toward high 1.0i 2O7pb/2O4pband high A7/4Pb(Figure 11). The trachyticrocks in the Reveille Rangeare also variable(high 87Sr/86Srand Rb Th K Ce Sm Ti Y Lu 2O7pb/2O4pb)and in this regardare like the episode1 hawaiites
400.0 (Figures 10 and 11).
I00.0 Figure 4. MORB-normalizedincompatible element diagram. Normalizing values and plotting order are from Sun and McDonough[ 1989]. (a) Comparisonof averagePliocene age Reveille Range hawaiites (episode 1 and episode 2) to the I0.0 averagePliocene age basanite,average Pleistocene age basalt from the LunarCrater Volcanic Field [Bergman, 1982; Kargel, 1987], and average ocean island basalt (OIB from Sun and McDonough[1989]). (b) Comparisonof averagePliocene age Reveille Range hawaiites (episode 1 and episode 2) to the .averageRed Cone and Black Cone analysesfrom Crater Flat [Bradshawand Smith, 1994]. Notice the similarity among OIB, Lunar Crater, and all the Reveille Rangedata (Figure4a), !,0 [ and contrast those relatively smooth patterns with the spiky Rb Th K Ce Sm Ti Y Lu patternproduced by the Crater Flat data (Figure 4b). YOC,ODZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS 17,435
al., 1988]. The presence of the OIB-type incompatible element signaturein Pleistoceneage basaltsof the Lunar Crater _[ (MORB& OIB) • _ Volcanic field [Bergman, 1982; Foland et al., 1987; Lum et al., 1989] indicates that the mantle source of mafic volcanism in this part of central Nevada has remained largely unchanged for the past 5-6 Myr. In general, the isotopic data for these rocks are consistent with the incompatible element data and the asthenospheric source interpretation. The episode 2 samples form a tight clusteraround 87Sr/86Sr-0.7035 and œNd---+4,and mostof the Pb isotopesfor the episode2 rocks fall on a trend immediately 11e_Rang_e__ [ above and parallel to the northern hemispherereference line (Figures 10 and 11). Isotopic characteristicsof Pleistocene episode•• ' age basaltsin the Lunar Crater Volcanic field [Bergman, 1982; 1 Foland et al., 1983; Foland and Bergman,1992] are like those o.ool O.Ol o. 1 of episode 2 and are thus consistent with the asthenospheric Rb / Ba source interpretation.
140 There are, however, isotopic features of the episode 1 hawaiites (5-6 Ma) that probably do not reflect geochemical CraterFlat B 120 - variation within the asthenosphericsource. Specifically, the wide variation in Sr isotopesrelative to end in the episode1 ReveilleRange - samplescontrasts with the tight cluster of the episode2 data. I[• episode#1 hawaiites The flat trend in the episode 1 data (shallow negativeslope) on 80- [C)episode#2hawaiites Lunar Crater the Nd-Sr isotopediagram (Figure 10) is toward a high Sr/Nd component and, in this regard, is unlike well-correlatedtrends L•episodeg2basanites basalts -_ that are generally seen in largely mantle-derived volcanic • OceanicBasalt systems(e.g., southernNevada area basaltsin Figure 10). The 40- , (MORB& OIB) _ absence of this radiogenic Sr signature from the younger basaltsin the area (episode2, Lunar Crater) arguesthat is not a 20- feature of the asthenosphericsource and must therefore have been acquired when episode 1 basalts moved through the o I ! I I 1 I lithosphere. 500 lOOO 15oo 2000 2500 3000 3500 4000 4500 The high87Sr/86Sr and high SffNd featuresof the radiogenic Ti / Hf component in the episode 1 hawaiites are relatively well Figure 5. Incompatibleelement ratio-ratio plots comparing constrainedby the data array and concave-downwardcurvature Pliocene Reveille Range basalts to Pleistocenebasalts from of the mixing line and are similar to Sr-Nd componentsin Crater Flat and the Lunar Crater Volcanic Field (locations in modemmarine sediment and/or average upper crest (Figure 12). Figure 1). (a) K/Ba versus Rb/Ba; note the similarity of the The high SffNd requirementis particularlyimportant because it Reveille Range the Lunar Crater basalts to oceanic basalt and disqualifiesenriched lithospheric mantle (or basaltsfrom such the distinctive character(high Ba relative to Rb and K) of the mantle) as likely sourcesof this component(Figure 12). We Crater Flat data. (b) La/Ta versus Ti/Hf; note again the believe therefore that the episode 1 basaltsacquired an upper similarity of the Reveille Range the Lunar Crater basalts to oceanic basalt and the distinctive character (low relative Ti and crustal componentthrough assimilation of wall rock in a Ta) of the Crater Flat data. Reveille Range data are from Table shallowmagma chamber. Foland et al. [1991] and Folandancl 1. Lunar Crater data are from Bergman [1982] and Kargel Bergman [1992] showedthat Sr and Nd isotopesin basaltsof [1987]. Crater Flat data are Red Cone and Black Cone analyses the Reveille Range and PancakeRange-Lunar Crater Volcanic from Bradshaw and Smith [ 1994]. Oceanic basalt data are from Fieldare stronglycorrelated with oxygenisotopes (•5180), and Hofmann and White [1983]. Data regardedas "suspect"by they too arguefor assimilationof crustby the older (Pliocene Hofmann and White (in parenthesesin their Table 1) are age) rocksin the area. excluded. Case for Carbonate Assimilation in Episode 1 Hawaiites
Petrogenesis The isotopic data presented here and from Foland and Bergman [1992] indicate that the crustal contaminant in the Asthenospheric Basalts With a Component From the Upper Crust episode1 basaltshad high 87Sr/86Sr,Sr/Nd, and •5180. It must additionally have had relatively low concentrationsof most Pliocene age basalts of episode 1 and episode 2 in the other incompatible elements, becausealthough the episode 1 Reveille Range have incompatible element concentrationsand lavas have relatively radiogenicSr, their overall incompatible interelementratios that are nearly identicalto thoseof average elementprofile is nearly the sameas that for the episode2 and OIB (Figures5 and 6). The incompatibleelement data therefore Lunar Crater samples(Figures 5 and 6). indicate that the predominantsource for Pliocenebasalts in the One possibility is that the episode 1 lavas were Reveille Range was asthenosphericmantle [see also Fitton et contaminated by limestone or some other carbonate-rich rock. 1200 7.00-•
400•...•:•^a•//• •,•,,• youngJtLunar b'asanites• 3.00• I La400t/I I 4.00• 30 Ta3.00 • 20 2.00•
400 La/Yb2ø 15
100• ',.... I .... ',.... I 2.00 4.00 6.00 8.00 10.00 12.00 2.00 4.00 6.00 8.00 10.00 12.00 MgO MgO Figure 6. Incompatibleelements (Sr, La, Zr, Th, Ta, La/Yb) versusMgO for alkalic basaltsof the Reveille Range and for the youngestLunar Crater Volcanic Field basalts(Qb-1 units from Bergman [1982]). Hawaiite samples from episode 1 (open squares)and episode 2 (open circles) generally have similar incompatible element concentrationsrelative to MgO. Episode 2 basanites(solid circles) have relatively high concentra- tions of incompatibleelements and high La/Yb comparedto MgO. Data from Table 2 and Bergman [ 1982].
AVERAGE REVEILLE RANGE 4.0
3.0
2.0
1.0
0.0 Rb Th Ta La • P Sm Hf Y Lu
Figure 7. Average incompatibleelement concentrationsfor Reveille Range basaltsnormalized to average oceanisland basaltof Sun and McDonough[ 1989]. Average episode2 basaniticsamples includes episode 2 basanitesand episode 2 hawaiites with relatively high incompatibleelement concentrations(see Figure 6). Notice the similarity for ratios amongmost elementsexcept Pb which is relatively high in only the episode1 average(e.g., high Pb/Ce in episode 1 hawaiites). YOC•DZINS• ET AL.: ORIGIN OF GREAT BASIN BASALTS 17,437
160• I I I I, ,I 160 EPISODE #1 HAWAIITES PISODE #2 BASANITES EPISODE #2 HAWAIITES 140 140
120 120
100 100• X [I-]R9-1-661 lSX• I 'R9-1-47I _[:•• [AR8-1-281 • •x,•X• I s R8-1-30I /o R8-1-1I { 80 [: •l!• I*RS-l-ll I . . 9-1-48] . .
40
20 20
I I [ I I I I I I I I I I •0 La Ce Nd Sm Eu Tb Yb Ce Nd Sm Eu Tb Yb Ce Nd Sm Eu Tb Yb Lu
Figure 8. Chondrite-normalizedrare earth element concentrationsfor Reveille Range basalts(Table 1). Samplesplotted here were selectedto show the full range of light rare earth elementconcentration within each stratigraphic-geochemicalgrouping (see also Figure 6). Normalizing values are the Leedy Chondrite: La (0.378), Ce (0.976), Nd (0.716), Sm (0.230), Eu (0.0866), Tb (0.0589), Yb (0.249), Lu (0.0387). See also Masuda et al. [1973].
Carbonates may have high concentrations of Sr but will (15.1-17.4%) contents in episode 1 basalts, suggestingthat generally have low concentrationsof other incompatible DSr = 1.0 +/- 0.15 is appropriate. elements. Carbonatesalso have high 5180 and Sr/Nd and Strontiumisotopes in the episode1 hawaiitesalso appearto thereby have all of the featuresof the putative contaminant[cf. be correlatedwith SiO2 content(Figure 14), and this may also Veizer, 1983; Shaw, 1985]. reflect an AFC control over 87Sr/86Sr. There are, however, no The carbonatecontaminant interpretation is supportedby clear correlations between 87Sr/86Sr and other general the observation that episode 1 samples with higher Sr indicatorsof crystal fractionation(e.g., decreasingMgO, CaO, concentrationsalso havehigher 87Sr/86Sr. Specifically, a plot CaO/A120 3, increasingBa, Th, K), but this may in part be of 87Sr/86Srversus Sr concentration(Figure 13) showsthat the becauseisotopic analysesare available for hawaiite samples episode 1 samplesare scatteredaround a mixing line between that are all similarly evolved (MgO 4.5-6.1%). an evolved basalt composition(Sr=550 ppm, 87Sr/86Sr= Mixing calculations indicate that the observed shift in 0.7035) and a carbonate assimilant (Sr=850 ppm,87Sr/ 87Sr/86Sr (from 0.7035 to 0.7060) requires 10-40% 86Sr=0.7085). The binary mixing line is equivalentto an assimilation of a carbonate that contains 850 ppm Sr with assimilation-fractional crystallization (AFC) model wherein 87Sr/86Sr=0.7085 and Sr/Nd = 85 (Figure 12). This is a the bulk distributioncoefficient for Sr is 1.0 (bulk Dsr=l.0 [see substantial amount of contamination, and it implies that DePaolo, 1981]). The data are somewhat scattered but are radiogenic Sr mobilized in the wall rock reactions was largely encompassedby AFC calculationsusing Dsr between efficiently incorporatedinto the basalticmelt. A large amount 0.85 and 1.15 (Figure 13). There is no systematicchange in of Sr may have been liberatedin wall rock reactionswherein Sr concentrationover the rangeof MgO (2.7-6.1%) and A1203 Ca-Mg carbonates(with variably high Sr) are replacedby Ca- Mg silicates(with relatively low St). Minerals formed in a melt-carbonate reaction zone (e.g., 1000 : I I I ,I I I wollastonite,garnet, Ti-Al-rich pyroxenes,nepheline) are not Reveille RangeTrachytes observedin the episode 1 basalts,nor do these basaltsshow • R9-1-62 shifts in major elementcomposition that might be anticipated O R8-1-41 as a consequenceof basalt-limestoneinteraction (e.g., Ca enrichment,Si-A1 depletion [see Wyllie, 1974]). Detailed • 100 studies of basalt-limestone interaction indicate, however, that thesepetrologic consequences of limestoneassimilation will be producedin only a very localizedpart of a magmaticsystem. Specifically,Baker and Black [1980] and Joesten[1977] found small veins and apophysesof strongly Ca-rich hybridized • ....%!i{iiii:%11iiiiiiiiii•i• ...... basalt in melt-limestone reaction zones, but they concluded • _Laverage Pliocene ...... that these melts were producedin only very small volumes becauseelements liberated by carbonatebreakdown (mostly 10evelll• R ...... '"•'"'"'"•'""::';:";'"3Ca) were readily accommodatedby crystallization within the La Ce Nd Sm Eu Tb Yb Lu reactionzone [see also Wyllie, 1974]. These studies of basalt-limestone interaction have Figure 9. Chondrite-normalizedrare earth element reemphasizedtwo importantpoints initially made by Bowen concentrationsfor Reveille Range trachyticrocks compared [1922, 1928] in his classic treatment of wall rock assimila- with basalts(data from Table 1). tion. These are, first, that the sluggishtransfer of heat from the 17,438 YOCK)DZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS
10- ::ii•11i?•ii:11•?•:i:i:!ii!!i:!!•i:11ô:!•-"•-• Oepisode #2hawaiites
5 ii•,:.:..-?•.:..i•;••. • [ IZI episode #1hawaiites zx '"
0 ':••'...•.•..:•[i!:...•.:•.••• ,A trachytic rocks end ...... 'iiiiY?•:.--.:•.•q-:%%,,!,:i :'5,::. -
-5 BasinBasalts::•i.-."i•:•iii•'"'""'"'•'?", t...... •- ""• "•:::.•g::i•::::.-.....•..:•!•.• :•:•::•??...:i::•,.\ \x,. modernsediment
-10 CraterFlat • BlackCone)
-
-15 I I I I I I I ! i [ I I J Ji ! J I iI I I I 0.703 0.705 0.707 0.709 0.711 0.713 87Sr/ 86Sr Figure 10. Nd -Sr isotopecorrelation diagram comparing Reveille Range data with basaltsfrom throughout the southernNevada area (< 7 m.y. old) and uppercrust (as sampledby modemmarine sediment). Notice the tightcluster of episode2 data(hawaiites and basanites) and the "fiat trend" of theepisode I basaltstoward high 87Sr/86Srrelative to œNa-Notice also that the Reveille Range trachytic rocks scatter to highrelative 87Sr/86Sr valuesand in this way are isotopicallyakin to the episode1 basalts.Reveille Range and CraterFlat data are from Table 2. Data for southernNevada area basalts (< 7 m.y. old) are from Farmeret al. [ 1989], Coleman [1990], Walker and Coleman [1991], Feuerbachet al., [1993], Hoffine [1993], Ormerod [1988], and unpublishedUniversity of Nevada,Las Vegas,University of Kansasdata. Modem marinesediment data are from Ben Othman et al. [ 1989].
15.80
modern manne 15.75 sediment
15.70
15.65 CraterF1;. . ' ' ' E.z33 15.60
low A7/4 samples 15.55 ß Southern Nevada ..... ß . area.bas.alg . . 15.50 ReveilleRange O episode#2 hawaiites 15.45 • episode#2 basanites /x, trachyticrocks 15.40 [3 episode#1 hawaiites
15.35 ,' ' ' ' I ' "' ' I ' ' 17.50 18.00 18.50 19.00 19.50 20.00 206pb/ 204pb
Figure11. Leadisotope correlation diagram (2O7pb/•O4pb versus 9-O6pb/o-O4Pb) comparing Reveille Range datato southernNevada area basalts and modern marine sediment. Notice that six of eightepisode 2 samples (includingboth hawaiites and basanites) fall alonga trendparallel to the mantlereference line at low A7/4Pb [seeHart, 1984], whereasthe episode1 samples,including trachytic rocks, scatter to high relative 2ø7pb/204pb(high A7/4Pb). References forbasalt data are the same as in Figure10. Leadisotope data for modernsediment from Ben Othman et al. [1989] andKay et al. [1978], andreferences therein]. YOCK)DZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS 17,439
10 I [ i , I i i i I I , [ i i I ] I [ I I Basalt End-member Sr=-550ppm Reveille Range Nd=26 ppm [• episode#1 (Sr/Nd=21) ,. Oepisode#2 hawaiite •) episode#2 basanite
_ Imixing with onate end
• Carbonate - lithosphericbasalt .•_ End-member .- -5I mixingwith "X Sr=850ppm . -10 • Nd=10ppm _ LithosphericEnd-member St=1400 ppm X•l•Nd=85) . Nd=97 ppm (Sr/Nd=14) I I I I I I I I I I I I I I I I I I -15 I I I 0.70200 0.70400 0.70600 0.70800 0.71000 87Sr/ 86Sr Figure 12. Neodymium-Srisotope correlation diagram for ReveilleRange basalts with binary mixing lines between low 87Sr/86Srepisode 2 basalts (hawaiites and basanites)and end-memberswith compositionsof lithosphericbasalt (high 87Sr/86Sr,low Sr/Nd) and carbonate(high 87Sr/86Sr,high Sr/Nd). Noticethat the downwardcurvature of the carbonatemixing line andthe trend toward high 87Sr/86Srrelative to œNdmatch the episode1 hawaiite trend well. Crosseson mixing lines are at incrementsof 10%. magma to the reaction zone means that energy for wall rock In the case of limestone assimilation, this means that Ca assimilation will generally come from crystallization within liberatedby carbonatebreakdown will generallybe crystallized the reaction zone itself and, second, that elements mobilized as clinopyroxene within or near to the basalt-limestone by melt-wall rock reactionswill generallybe accommodatedby reaction zone [Joesten, 1977; Baker and Black, 1980]. So solid solution in minerals crystallizing in the reaction zone. basaltic lavas erupted from a magma system that interacted
AFCmodel m Ma/Me=0.35 /l• AFCMa/Me odel:= 0 35 0.709f,,,I,,, ,1!!.,!.,),1,,,IDSr: 1.0 ! / \ ,,, I,i,,,I DSr=0.85'1,,, 0,708Ma/McAFCmodel: = 0.35 I[ (~binarym••_._•• / [-7-• DSr= 1.15 '",X•' CarbonateAssimilant
0.706 r•rn
0.705
_ I-! episode#1 hawaiites 0.704-- ¸ episode#2 hawaiites
-- o o 0.703•- -- Basalt Parent in AFC models
0702,, I '' '' ! '' '''' I '''' I ''' I '''' I ''' ''''I ''I ' ' 4OO 500 600 700 800 900 1000 1100 1200 Sr (ppm)
Figure 13. Sr isotopesversus Sr concentrationand assimilation-fractionalcrystallization modeling (AFC [DePaolo, 1981]) of episode1 basaltsfrom the Reveille Range. Parameter"r" in AFC calculationsis the mass assimilateddivided by the mass crystallized (Ma/Mc). Crosseson AFC curves are at incrementsof 10% crystallization. 17,440 YOCK)DZINSKIET AL.: ORIGIN OF GREATBASIN BASALTS
48.0E'"'' I, ,',, I .... I .... '1.... I,, •, I .... =] 0.709 .
47.0i46.5 ._• •.• • • •*' •_• 0.7070.708• hav•aiites x•/ / :: SiO246.0• io • • • • • • • • • • • ]episode•l] • I 1= I i 44.5• • •1 [Ohawaiites] 43.5 O.V03• ...... • •piso•e•2 :: 0.70300.7035o.70ao 0.7045 0.7050 0.7055 0.7060 0.7065 0.702 • hawaiires :: 87Sr/ 86Sr 12 14 16 18 20 22 24 26 28 30 Figure 14. Sr isotopesversus SiO2 content for Reveille A1203 Range basalts. Note the generalincrease in 87Sr/86Srwith Figure 15. Strontium isotopes versus A120 3 content for increasingSiO 2 among Reveille Range hawaiites. Data from Tables 2 and 4. Reveille Range basalts compared with hypothetical Ca- feldsparwith high 87Sr/86Sr.The trendof the mixingline fails to passthrough the episodeI data indicatingthat mechanical incorporation of Ca-feldspar (i.e., plagioclase megacrysts) extensivelywith limestoneare generallynot expectedto show with high 87Sr/86Sr cannot explain the Sr isotopic anomalousbehavior in CaO or othermajor elements. composition of the episode I samples. Strontium For our purposesit is perhapsmost interestingthat the concentrationin feldsparused in mixing calculationsis 1200 strongly hybridized basalts analyzed by Baker and Black ppm and is similar to the Sr concentrationin a labradorite [1980] were enrichedin CaO by approximatelya factorof 2.4 megacrystanalyzed by Bergman [ 1982]. Only unreasonably high Sr concentrationsfor plagioclase (>10,000 ppm) will (overthe unhybridizedbasalts), but wereenriched in Sr by up bendthe mixing line to passthrough the episode1 data. to a factorof 15 [seeBaker and Black, 1980, Table II]. This profound enrichment in Sr over CaO in the small-volume hybridizedmelts providesclear and tangibleevidence that Sr compared to other incompatibleelements (i.e., a carbonate- liberatedin carbonatebreakdown is preferentiallyexcluded rich wall rock). from the Ca-silicate minerals that crystallize in a basalt- limestone reaction zone. We conclude that in the case of the Other Geologic and Petrographic Support episode1 hawaiitesin the Reveille Rangesuch displaced Sr for the Carbonate Assimilation Model may have been efficientlyincorporated into the basalticmelts. Assumingthat significantSr may be liberatedby melt- The olivine + plagioclasephenocryst assemblage in the limestoneinteraction, we turn to the questionof how that Sr episode 1 basalts contrasts with the higher pressure wasincorporated into the episode1 hawaiitemagmas. Much of phenocryst-megacryst-xenolith assemblage present in the radiogenicSr diffusing away from the wall rock reaction younger basalts in the area (see petrographic descriptions zone may have been scavengedby feldsparscrystallizing in above). The phenocrystassemblage in episode 1 hawaiitesis the cumulate mush. Pieces of this mush zone in the form of therefore consistentwith evolution in a low-pressuremagma megacrysticand large phenocrysticplagioclase are commonin chamber in the upper crust. The explosive eruption of the the episode 1 basalts. Aluminum-87Sr/86Sr mixing highly evolved trachytic rocks isotopically akin to the relationshipsindicate, however, that mechanicalincorpora- episode1 hawaiites(87Sr/86Sr > 0.7040) probablymarked the tion of feldspar from the mush zone cannot account for the coolingand death of that high level magmasystem. shift in Sr isotopecompositions observed in the episode 1 We know therefore that among Pliocene-Pleistocenebasalts samples(Figure 15). in the area, the episodeI hawaiiteswere eruptedin the largest If radiogenicSr was not carriedinto the basalticmelt by volume (see geologic map in Figure 2), they resided in the plagioclasecrystals, then it must have been transferredfrom shallowest magma chamber (plagioclase-olivine-dominated the reaction zone to the melt largely by diffusion. Other phenocryst assemblage,see Table 1), and they eventually elementsthat may moveefficiently by diffusiondo not appear, cooledto producethe mostevolved melts (the trachyticrocks). however,to have been as stronglyaffected as does Sr. There All of these features are consistent with the model that the are no clear relationshipsbetween Ba or Ba/La and 87Sr/86Sr, episode I hawaiites appearto have experiencedcontamination and correlations between K or K/La and 87Sr/86Sr are weak in the upper crust whereasyounger basalts in the area (i.e., despitethe fact that the diffusivemobility of K is thoughtto be episode2 and Lunar Crater)do not. relatively high in systemswhere basalt is assimilatingcrustal Petrographic support of the assimilation model is also rocks [Watson, 1982; Watson and Jurewicz, 1983]. The Pb present. Small phenocrystsof biotite appearin some episode data do show a trend toward higher Pb/La with increasing I hawaiitesbut not in basaltsof episode2 (Table 1). Biotite is 87Sr/86Sr,but one samplenonetheless has very radiogenicSr also well-developedgroundmass phase in the episode1 basalts and also low relative Pb (Figure 16). We conclude therefore but is much lesscommon in basaltsof episode2. The presence that the episode I hawaiiteswere altered by assimilationof a of groundmasscalcite in the Reveille basalts may also be wall rock that had high concentrationsof Sr (and possiblyPb) significant. We have interpretedcalcite in episodeI basalts YOCd)DZINSKIETAL.: ORIGIN OF GREAT BASIN BASALTS 17,441
0.710
0.709 - La=10 ppm .._ ili:'""'":• •iii!iiii!iiii
0.708 -
0.707- Carbonate end-member La= 10 ppm -I - Pb=10 ppm
0.706-- _ - - 0705-- ReveilleR'-••-••ge ] 0.704 -- • episode#1hawaiites •_ 0.703 -- • O•-B-2•npd;member0episode #2hawaiites • I [ [ • [ I[I Pb=2.5I ppmI I • [ • [•[ I I I 0.702I I I I 0.0
Figure16. Strontiumisotopes versus Pb/La for Reveille Range rocks with mixing lines between upper crust compositionsandepisode 2 basalts. Notice the trend for most of the episode 1 data toward increasing Pb/La withincreasing 87Sr/86Sr. Reveille Range data are from Table 2 andBen Othrnan etel. [1989]. as largely alteration-related(see petrographicdescriptions experienced significant contamination in the uppercrust, and above),but in somecases, groundmass calcite is intergrown that youngerbasalts in the area(episode 2 andLunar Crater alongstraight crystal boundaries with biotiteand feldspar in basalts)generally did not [see also Folandand Bergman, areas where there is no hint of alteration (aside from the 1992]. presenceof calcite). The texturessuggest that some of the groundmasscarbonate could magmatic or deuteric,though on Discussion thispoint the texturalevidence is probablynot conclusive. Perhapsmost importantly,quartzo-feldspathic xenoliths Alternatives to the Carbonate Assimilation Model andxenocrysts are not seenin the ReveilleRange episode 1 hawaiites,even in the presenceof cleargeochemical evidence The geochemicalvariation in theepisode 1 lavasconstrains for crustalcontamination. In this regard,the Reveille Range fairlytightly the natureof the high87Sr/86Sr component in basalts are unlike the Pliocene Colville Mesa basalts of the theserocks and would appearto eliminatethe mantle as a LakeMead Area andcertain Miocene age hawaiites in theBasin potentialsource for thissignature. The EMII enrichedmantle andRange of Mexico[Feuerbach et el., 1993; Luhret el., component,which is widespreadin OIB of certainwestern 1995]. In these areas, geochemicalevidence for crustal PacificIslands [ Zind!erand Hart, 1986],produces a Nd-Sr array assimilationis supportedby abundantpetrographic evidence that falls directlyover the episode1 data. However,the in the form of gneissicxenoliths and quartz xenocrysts, absenceof this isotopicsignature from episode2 and Lunar presumablyfrom the deepcrust. Craterbasalts (which have incompatible element characteris- Finally,it is importantto recognizethat Paleozoic and late ticslike thoseof episode1) arguesthat the radiogenicSr did Precambriancarbonates constitute a great thicknessof the not come from the asthenosphericsource. Enriched uppermostcrust beneath central Nevada (6-12 km [Langenheim lithospheric mantle beneaththe westernUnited States and Larson, 1972]) and that they are therefore the most generallyhas low Sr/Nd and Sr-Nd isotope ratios that produce a probablesource of uppercrustal contamination in young steepnegative trend on theNd-Sr isotope correlation diagram basalts in the area. Most of the limestone section has (e.g.,Crater Flat data in Figure10) whichis unlikethe trend probablyundergone diegenesis and may not therefore have producedbythe episode 1 data. In addition,the high 878r/86Sr particularlyhigh Sr concentrations(aragonite is Sr-rich, basaltsof episode1 showno signsof havingacquired the calcite and dolomite are not), but the great thicknessof distinctive trace element characteristics of Great Basin miogeoclinalcarbonate in south-centralNevada at leastraises lithosphericmantle (see Figures 4 and5). the possibilityfor the preservationor formationof Sr-rich Assimilationof plagioclase-richrocks in thedeep crust may carbonatehorizons beneath the Reveille Range. be another alternative to the carbonate assimilation model We concludethat available geologicand petrographicdata outlinedabove. Geochemically,an anorthositewould have the providebroad support to the idea,developed on thebasis of characteristicsrequired of the assimilant(high 87Sr/86Sr, geochemistry,that the episode 1 basaltsin theReveille Range Sr/Nd, 8180, low concentrationsof other incompatible 17,442 YOC•DZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS elements), but with no a priori evidence for an anorthosite singleeruptive episode and that this episodewas 1.0-1.5 Myr body beneath central Nevada, this can only be an ad hoc in duration. interpretation. Glazner et al. [1991] and Glaznerand Farmer Becausethe episode2 basaltsimmediately overlie but are [1992] argue that extensiveassimilation of gabbroicrocks in isotopicallydistinct from the trachyticrocks and the basaltsof the deepcrust may accountfor diverseisotopic characteristics episode1, we believethat the episode2 basaltsrepresent the in basalts from the Mojave area of southeasternCalifornia. beginningof a new eruptiveepisode for the volcanicfield. It Indeed, some Mojave basaltsfollow a Sr-Nd isotopictrend is unlikely that episode2 basaltscould have reoccupiedthe similar to that seen in episode 1 hawaiitesfrom the Reveille shallow episode 1 storage system without also showing Range. There is, however,no clearrelationship between Sr-Nd isotopic evidence of crustal contamination, so we conclude isotopesand •5180in the Mojavebasalts [ Glazier et al., 1991, that episode2 eruptionswere fed from a separatestorage Figure 10] and in this way they contrastthe Reveille Range- location. The widespread occurrence of clinopyroxene LunarCrater rocks [ Foland et al., 1991; Folandand Bergman, phenocrystsand megacrystsin episode2 basaltsindicates that 1992]. Furthermore,the unusualelement-isotope correlations this storagelocation was deeperthan during episode1 time. seenin the Mojave basalts[Glazier et al., 1991, Figure 12] are This is confirmed,at least in part, by the presenceof mantle- absentfrom the episode 1 hawaiites. We concludetherefore derived xenoliths (dunites, harzburgites) and amphibole that the assimilationprocess that has operatedin the genesis megacrysts in some of the episode 2 basanites. These of the Mojave basalts is unlike that which has effected the xenoliths provide good evidence that their host basaltswere episode1 hawaiitesin the Reveille Range. storednear the crust-mantleboundary and wereerupted rapidly Postmagmaticalteration might also explain radiogenic Sr without a significant period of storage within the shallow and high •5180 in the episode1 basalts. One possibilityis crust. that meteoric waters have carried dissolved carbonate dust into The presenceof basanitesamong only the episode2 rocks cracks and other openingsin the episode 1 basalts(samples may provide further insight into the development of the were not leachedin acid prior to isotopicanalysis). Episode1 volcanic field. Specifically,the episode2 basanitesare more basaltsare slightly older, so pedogenicprocesses would have alkaline (>5% normative nepheline) than the hawaiites, had moretime to operateon them thanon the youngerepisode indicatinga greaterdepth of melting(higher pressure melts 2 rocks. Recall, however, that all of the Reveille Range [O'Hara, 1968; see also Takahashi and Kushiro, 1983; Klein basaltsare Pliocenein age, so if pedogenicprocesses have and Langmuir, 1987]). The basanites also have higher affected basalt compositions, it is surprising that the incompatible element concentrations than the hawaiites alterationaffects are so clearly presentin the episode1 rocks (relative to MgO, Figure 6), and assumingthat the mantle (4.5-6 m.y. old) but are completelyabsent from the episode2 source of Reveille Range hawaiites and basanites was rocks(3-4.5 m.y. old). A secondpossibility is that alteration compositionally similar (an assumption that is strongly in the episode 1 samples was producedby a hydrothermal supportedby the isotopic data presentedhere), these higher system establishedfollowing the eruption of the episode 1 incompatibleelement concentrations imply a lower percentage basaltsand trachyticrocks. If a hydrothermalsystem were not melting for the basanites. Other compositionalfeatures of the established following the eruption of the relatively small basanites, including steeper REE patterns (higher La/Yb, volume episode 2 basalts, then those basalts would have Figure 6) and higher NAB.0 (Table 2), also point to a low escaped alteration. percentagemelting origin for the basanitescompared to the The problem though, with any hydrosphericinterpretation hawaiites. The overall low volume of basanite in the Reveille for the origin of the crustalcomponent in episode1 basalts,is Range argues further that these basalts were produced by that it requiresthat all of the contaminantSr and Nd be carried smaller percentage melting than the associated hawaiites in the relatively small amount of (apparently alteration- which were produced in relatively large volumes, especially related) calcite that is presentin the rocks. The large amount during episode1 time. of contaminantSr and Nd required by massbalance (10-40%, The seculartrend toward deeper and lower volume melting see Figure 12), and the small amountof calcite present(less can be extended into the Pleistocene with the formation of the than -3%) therefore argue strongly against a hydrospheric youngestbasalts in the Lunar Crater Volcanic field (Qb-3 units origin for the observedisotopic shifts. In general, we believe from the Lunar Crater Volcanic Field [see Scott and Trask, that a magmaticorigin is far more likely to mobilize the large 1971]). These basalts occur in very small volumes, they are amountof Sr and Nd requiredto explain the observedisotopic more stronglyalkaline (9-14% normatire nepheline)than the variation. Reveille Range basanites, they have high MgO and high incompatible element contents (Figure 6), and they have Implications for the Evolution of the Volcanic relatively steep REE patterns (high La/Yb, Figure 6). The Field youngest Lunar Crater basalts also contain an abundantand diversesuite of both type I and type II megacrystsand nodules Geologic, petrographic,and geochemicalevidence outlined [Bergman, 1982]. These data clearly require that the youngest above indicates that between 5 and 6 m.y. ago, episode 1 Lunar Crater basaltswere formed by low percentagemelting of hawaiites in the Reveille Range were contaminatedby wall a relatively deep mantle sourceand that they traveledfrom the rock assimilation in an upper crustal magmatic plumbing mantle to the surface at relatively high velocities. system. This high level magma system is interpreted to have Comparisonof the Reveille Range data with the mappingof cooled and died 4.5 m.y. ago with the explosive eruption of Scott and Trask [1971] in the Pancake Range suggeststhat trachytic lavas and pyroclastic surges in the northeastern through time, there have been systematic shifts in the Reveille Range. The evidence therefore indicates that the geographicalshift distributionof eruptionsacross the region. episode 1 hawaiites and the trachytic rocks were part of a In its early history (3-6 Myr ago), the volcanic field covereda -YOGODZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS 17,443
eruptiveepisodes (first the hawaiites,then the basanites).The Plioceneand YoungerVolcanic Rocks: duration of the episode 2 episode is, however, poorly Reveille & Pancakerange area constrainedbecause there are relativelyfew radiometricages [• 5-6Ma (episode #1) and becausemany of the 3-4.5 Myr old basaltsare locatedin the PancakeRange where the Pliocenestratigraphy is lesswell ::":"•3-4.5 Ma (episode #2) ...... :.:.:.:.::....: established.It may be thatthe lengthsof the eruptiveepisodes 1 <3Ma (Lunar Crater field)..,. have changedwith time, such that as the volcanic field aged, eruptive episodes that produced the younger and smaller volume basalts were shorter than those that produced older, .%:' largervolume episodes. Only a moredetailed knowledge of the Pliocene stratigraphyin the PancakeRange will allow us to addressthis aspectof the volcanicfield history. / •.•:'•...•'•'•'•:'•%.A Overall, the volcanic field appearsto have developedin / :-:?:•: .'-'...... ½. 5>:: ..... responseto spatially and temporally discretemelting events in the mantle. The possible role of lithospheric extension or delamination in triggering these melting events cannot, however,be evaluatedwithout a substantialknowledge of local and regional structural/tectonic events at a resolution of approximately 1-2 Myr. The idea that the formation of small volume mafic volcanic fields may be coupled to specific . ?. -.... tectonic events therefore appears to be beyond our current understandingof tectonic events for most parts of the Basin and Range.
Acknowledgments. Helpful reviewsby Todd Housh,Britt Hill, and AssociateEditor William Melson are gratefully acknowledged. This •. • }T. ':'"•::•:• . t . .N.. manuscriptalso benefited from discussionswith Richard Carlson and Erik Christianson. A helpful review of an early version of this v t' J- manuscriptwas provided by Terry Plank. Thanks also to Shirley •e e]He•ange •.._. . Morikawa and Alex Sanchez for their assistancein the laboratory aspectsof this study. Pat Braught provided valuable assistancein preparing camera-readycopy. This research was supportedby the Nevada Nuclear Waste ProjectsOffice. We thank Carl Johnsonof that .....:::: office for his support. :•-•...... • ...... •• .....ß . . ..::•:. ,:.-"A•.; ...... :•:•:.•: References ...... •:? Baker,C.K., andP.M. Black,Assimilation and metamorphism at a basalt- limestonecontact, Tokatoka, New Zealand, Mineral. Mag., 43, 797- 807, 1980. BenOthman, D., W.M. White,and J. Patchett,The geochemistryof marine sediments,island arc magma genesis,and crust-mantle Figure 17. Time-space patterns for volcanism in the recycling,Earth Planet. Sci. Lett., 94, 1-21, 1989. Reveille and Pancake ranges from approximately6 Ma to Bergman,.S.C., Petrogeneticaspects of the alkali basalticlavas and present. Basedon mappingpresented here (Figure 2) and from included megacrysts and nodules from the Lunar Crater Scott and Trask [ 1971]. VolcanicField, Nevada,U.S.A., Ph.D. thesis,431 pp., Princeton Univ., Princeton,N.J., 1982. Bowen,N.L., Thebehaviour of inclusionsin igneousmagmas, J. Geol., broad area that encompassedwhat are today the Reveille and 30, 513-570, 1922. Pancakeranges. By the end of the Plioceneand in Pleistocene Bowen,N.L., TheEvolution of IgneousRocks, 332 pp.,Princeton U n iv. Press,Princeton, N.J., 1928. time, the areaover whichbasalts were eruptinghad retreatedto Bradshaw,T.K., andE.I. Smith,Polygenetic Quaternary volcanism at a small area in the north which today is marked by the CraterFlat, Nevada,J. Volcanol.Geotherm. Res., 63, 165-182, 1994. distributionof the youthfulcones of the Lunar Crater Volcanic Bradshaw,T.K., C.J. Hawkesworth,and K. Gallagher,Basaltic field (Figure 17). It appearsthat the initial melting anomaly volcanismin the southernBasin and Range:no role for a mantle plume,Earth Planet. Sci. Lett., 116, 45-62, 1993. that producedthe volcanicfield was large and that subsequent Coleman,D.S., and J.D. Walker, Geochemistryof Mio-Pliocene melting episodeswere smaller and centeredat the northernend volcanicrocks from around Panamint Valley, Death Valley a re a, of the initially large outbreak. These generaltime-space trends California,in Basin and Range Extensional TectonicsNear the were also noted by Naumann et al. [ 1991] and Foland and Latitudeof Las Vegas,Nevada, edited by B. Wemicke,Mern. Geol. Soc. Am., 176, 391-411, 1990. Bergman [1992]. DePaolo, D.J., Trace element and isotopic effects of combined Available age information indicates that in the Reveille wallrockassimilation and fractionalcrystallization, Earth Plant. Sci. Range the volcanic field developed in at least two eruptive Lett., 53, 189-202, 1981. episodeswhich were apparentlyboth 1.0-1.5 Myr long. The Eaton, G.P., R.R. Wahl, H.J. Prostka, and M.D. Kleinkopf, presenceof both hawaiite and basanite within the episode 2 Regional gravity and tectonic patterns: their relation to late Cenozoic epeirogeny and lateral spreading in the western sequence,and the relatively young age for the only basanite Cordillera,in CenozoicTectonics and RegionalGeophysics of the sample that has been dated (3.0 Ma [see Naumann et al., WesternCordillera, edited by R. B. Smith and G. P. Eaton, Mem. 1991]), suggeststhat episode 2 may itself be two separate Geol.Soc. Am., 152, pp. 51-92, 1978. 17,444 YOC•DZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS
Farmer,G.L., F.V. Perry, S. Semken, B. Crowe, D. Curtis,and D . J. Livaccari,R.F., andF.V. Perry,Isotopic evidence for preservationof DePaolo, Isotopic evidence on the structureand origin of Cordilleran lithospheric mantle during the Sevier-Laramide subcontinental lithospheric mantle in southern Nevada, J. orogeny,western United States, Geology, 21,719-722, 1993. Geophys.Res., 94, 7885-7898, 1989. Luedke,R.G., and R.L. Smith,Map Showing Distribution, Composition, Feuerbach,D.L., E.I. Smith, J.D. Walker,and J.A. Tangeman,The role and Age of Late Cenozoic Volcanic Centers in California and of the mantle during crustal extension: Constraints from Nevada,scale 1:1,000,000, U.S. Geol. Surv. Misc. Invest. Map, 1- geochemistryof volcanic rocks in the Lake Mead area, Nevada and 1091C, 1981. Arizona, Geol. Soc.Am. Bull., 105, 1561-1575, 1993. Lugmalt,G.W., andR.W. Carlson,The Sm-Ndhistory of KREEP,P roc. Fitton, J.G., D. James, P.D. Kempton, D.S. Ormerod,and W.P. LunarPlanet. Sci. Conf,, 9th, 689-704,1978. Leeman,The role of lithosphericmantle in the generationof late Luhr, J.F., J.G. Pier, J.J. Aranda-Gomez,and F.A. Podoseck,Crustal Cenozoic basic magmas in the western United States, J. contaminationin early Basin-and-Rangehawaiites of the Los Encinos PetroL,Special Lithosphere Issue, 331-349, 1988. VolcanicField, centralMexico, Contrib.Mineral. Petrol., 118,321- Fitton, J.G., D. James,and W.P. Leeman,Basic magmatism 339, 1995. associatedwith Late Cenozoicextension in the westernUnited States: Lum,C.L., W.P. Leeman,K.A. Foland, J.A. Kargel,and J.G. Fitton, Compositionalvariations in spaceand time, J. Geophys.Res., 96, Isotopicvariations in continentalbasaltic lavas as indicatorsof mantle 13,693-13,711, 1991. heterogeneity:Examples from the westernU.S. Cordillera,J. Foland,K.A., andS.C. Bergman, Temporal and spatial distribution of Geophys.Res., 94, 7871-7884, 1989. basalticvolcanism in the Pancakeand Reveilleranges north of MacDonald, G.A., and T. Katsura, Chemical compositionof Yucca Mountain, paper presented at Third International Hawaiian lavas,J. Petrol., 5, 82-133, 1964. Conferenceon High Level RadioactiveWaste Management, Martin, M.W., and T.R. Naumann,Tertiary Geology of the Reveille AmericanNuclear Society, LaGrange Park, Ill., 2366-2371,1992. Range Quadrangle,Northern Reveille Range, Nye County, Foland,K.A., S.C.Bergman, A.W. Hofmann,and I. Raczek,Nd andSr Nevada,Map 104, scale 1:24,000,Nev. Bur. of Mines and Geol., isotopicvariations in alkalibasalts and megacrysts from the Lunar Reno, 1995. CraterVolcanic Field, Nevada, EOS Trans. AGU, 64, 338, 1983. Masuda, A., N. Nakamura, and T. Tanaka, Fine structures of Foland,K.A., J.S. Kargel, C.L. Lum, and S.C. Bergman,Time- mutually normalizedrare-earth patterns of chondrites,Geochim. spatial-compositionrelationships among alkali basaltsin the Cosmochim.Acta, 37, 239-248, 1973. vicinity of the LunarCrater, south-central Nevada, GeoL Soc Am. Menzies,M.A., W.P. Leeman,and C.J. Hawkesworth,Isotope Abstr.Programs, 19, 666, 1987. geochemistry of Cenozoic volcanic rocks reveals mantle Foland, K.A., D.E. Schucker, B.M. Smith, W. Todt, and S.C. heterogeneitybelow western USA, Nature,303, 205-209, 1983. Bergman,Isotope geochemistry of Cenozoicalkali basaltsin the Naumann, T.R., E.I. Smith, and M. Shafiqullah, Post-6 Ma vicinity of the Lunar Crater volcanic field, south-central intermediate(trachytic) volcanism in the ReveilleRange, central Nevada:O andPb evidence for crustalcomponents, Geol. Soc. A m. GreatBasin, Nevada, GeoL Soc. Am. Abstr. Programs, 22, 72, 1990. Abstr.Programs, 23, 45, 1991. Naumann,T.R., E.I. Smith,M. Shafiqullah,and P.E. Damon, New K-Ar Glazner,A.F., andG.L. Farmer,Production of isotopicvariability in agesfor Pliocenemafic to intermediatevolcanic rocks in the Reveille continentalbasalts by crypticcrustal contamination, Science, 255, 72- Range, Nevada, Isochron West,57, 12-16, 1991. 74, 1992. O'Hara,M.J., The beatingof phas•equilibria studies in syntheticand Glazner,A.F., G.L. Farmer, W.T. Hughes, J.L. Wooden,and W. natural systems on the origin and evolution of basic and Pickthom,Contamination of basaltic magmaby mafic crust at ultrabasicrocks, Earth Sci..Rev.,4, 69-133, 1968. Amboy and Pisgah craters, Mojave Desert, California, J. Ormerod,D.S., Late-to post-subductionmagmatic transitions in the Geophys.Res., 96, 13,673-13,691,1991. westernGreat Basin, USA, Ph.D.thesis, Open Univ. 313 pp.,Milton Hart,S.R., A large-scaleisotope anomaly in thesouthern hemisphere Keynes,England, 1988. mantle,Nature, 309, 753-757, 1984. Ormerod,D.S., C.J. Hawkesworth,N.W. Rogers, W.P. Leeman,and Hedge,C.E., and D.C. Noble,Upper Cenozoic basalts with high M. Menzies,Tectonic and magmatictransitions in the WesternGreat 87Sr/86Srand Sr/Rb ratios, southern Great Basin, western United Basin,USA, Nature, 333, 349-353, 1988. States,GeoL Soc. Am. Bull., 82, 3503-3510, 1971. Rogers, N.W., C.J. Hawkesworth, and D.S. Ormerod, Late Hoffine,S.R., Geochemistry of the volcanic rocks of theSaline Range, Cenozoic basaltic magmatism in the western Great Basin, California:Implications for mantlecomposition and involvement in Californiaand Nevada, J. Geophys.Res., 100, 10,287-10,301,1995. extensionbeneath the Basin and Range, M.S. thesis, 98 pp., Univ.of Scott,D.H., andN.J. Trask,Geology of theLunar Crater Volcanic Fie ld, Kansas, Lawrence, 1993. Nye County,Nevada, U.S. Geol.Surv. Prof. Pap.,599-1, 122 pp., Hofmann, A.W., and W.M. White, Ba, Rb and Cs in the Earth's 1971. mantle,Z. Naturforsch.,38, 256-266, 1983. Shaw,H.F., Sm-Ndin marinecarbonates and phosphates: Implications Joesten,R., Mineralogicaland chemicalevolution of contaminated for Nd isotopesin seawaterand crustal ages, Geochim. Cosmochim. igneous rocks at a gabbro-limestonecontact, Christmas Acta, 49, 503-518, 1985. Mountains,Big Bendregion, Texas, GeoL Soc. Am. BulL, 88, 15 15- Sun,S.S., and W.F. McDonough,Chemical and isotopic systematics of 1528, 1977. oceanic basalts: implications for mantle compositionand Kargel,J.S., The geochemistryof basaltsand mantle inclusions from the processes,in Magmatismin the Ocean Basins,edited by A.D. Lunar Crater Volcanic Field, Nevada: Petrogeneticand Saundersand M. J. Norry,Geol. Soc. Spec. Publ., 42, 313-346,1989. geedynamicimplications, M.S. thesis,393 pp., Ohio StateUniv., Takahashi,E., and I. Kushiro,Melting of a dry peridotiteat high Columbus, 1987. pressuresand basaltmagma genesis,Am. Mineral., 68, 859-879, Kay, R.W., S.S. Sun,and C.N. Lee-Hu,Pb and Sr isotopesin 1983. volcanicrocks from the Aleutian Islandsand Pribilof Islands, Vaniman,D.T., B.M. Crowe, and E.S. Gladney,Petrology and Alaska,Geochim. Cosmochim. Acta, 42, 263-273, 1978. geochemistry of Hawaiite lavas from Crater Flat, Nevada, Klein,E.M., andC.H. Langmuir,Global correlations of oceanridge Contrib.Mineral Petrol., 80, 341-357, 1982. basalt chemistry with axial depth and crustal thickness,J. Veizer, J., Chemical diagenesisof carbonates:theory and Geophys.Res., 92, 8089-8115, 1987. applicationof traceelement technique, in Stable Isotopes in Langenheim, R.L., and E.R. Larson, Correlation of Great Basin SedimentaryGeology, edited by M. A. Arthur SEPM Short stratigraphicunits, Nev. Bur. Mines Geol. Bull., 36, 351pp., 1972. Course, 10, 1983. Leeman, W.P., The isotopic compositionof strontiumin late- Walker, J.D., and D.S. Coleman, Geochemical constraintson mode of Cenozoicbasalts from the Basin-Rangeprovince, western United extensionin the DeathValley region, Geology, 19, 971-974,1991. States,Geochim. Cosmochim. Acta, 34, 857-872, 1970. Wasserburg,G.J., S.B. Jacobsen,D.J. DePaolo, M.T. McCulloch,a n d Leeman, W.P., Tectonic and magmaticsignificance of strontium T. Wen, Precise determination of Sm/Nd ratios, Sm and Nd isotopic variations in Cenozoic volcanic rocks from the western isotopic abundancesin standardsolutions, Geochim. Cosmochim. UnitedStates, Geol. Soc.Am. Bull., 93, 487-503, 1982. Acta, 45, 2311-2323, 1981. Le Maitre,R.W., A Classificationof IgneousRocks and Glossaryof Watson, E.B., Basalt contamination by continental crust: Some Terms,193 pp., Blackwell Sci., Cambridge, Mass., 1989. experimentsand models,Contrib. Mineral. Petrol., 80, 73-87, 1982. YOC•DZINSKI ET AL.:ORIGIN OF GREAT BASIN BASALTS 17,445
Watson, E.B., and S.R. Jurewicz, Behavior of alkalies during T. R. Naumann,Department of Geology, University of Idaho,P.O. diffusiveinteraction of graniticxenoliths with basalticmagma, J. Box 3952,Moscow, ID 83843-1920. Geol., 92, 121-131, 1983. E. I. Smith,Center of Volcanicand Tectonic Studies, Department of Wilkinson, J.F.G., The mineralogy and petrographyof alkali Geosciences,University of Nevada,Las Vegas, NV 89154-4010. basalticrocks, in TheAlkaline Rocks, edited by H. Sorensen,pp. 67- J. D. Walker,Isotope Geochemistry Laboratory, Department of 95, JohnWiley, New York, 1974. Geology,University of Kansas,120 LindIcy Hall, Lawrence,KS Wyllie,P.J., Limestone assimilation, in The Alkaline Rocks, edited by H. 66045-2124. Sorensen,pp. 459-474, John Wiley, New York, 1974. G. M. Yogodzinski,Department of Geology,Dickinson College, Zindler, A., and S. Hart, ChemicalGeodynamics, Annu. Rev. Earth Planet. Sci., 14, 493-570, 1986. Carlisle,PA 17013-2896.(e-mail: yogodzin•dickinson.edu)
T. K. Bradshaw,House of LordsCommittee Offices, London SW1A (ReceivedJuly 17, 1995; revised February 15, 1996; OPW,England. acceptedMarch 12, 1996.)