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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. B5, PAGES 8399-8415, MAY 10, 1995

Origin of late Cenozoic at the Cima , ,

G. L. Farmer, 1A. F. Glazner,2 H. G. Wilshire,3 J. L. Wooden,3W. J. Pickthom,3 and M. Katz4

Abstract. Major element,trace element, and isotopicdata from late Cenozoicalkali basalts comprisingthe Cima volcanicfield, southeasternCalifornia, are usedto characterizebasalt sourcesbeneath this portion of the Mojave Desertover the past8 m.y. The basaltsare dominantlytrachybasalts with traceelement compositions similar to modernocean-island basalts (OIB), regardlessof the presenceor absenceof -derivedxenoliths. In detail,the basalts can be dividedinto threegroups based on theirages and on theirtrace element and isotopic characteristics.Those basalts <1 m.y. in age, and the majorityof those3-5 m.y. old, belongto Group1 definedby high end values (7.6 to 9.3), low s7sr/SSSr (0.7028 to 0.7040), low whole rock•5•SO (5.89'oo to6.49'oo), and a restrictedrange of Pbisotopic compositions thatgenerally plot on the mid-oceanridge (MORB) portionof thenorthern hemisphere reference line. The 3 to 5-m.y.-oldbasalts have rare earthelement (REE) andother incompatible element abundances thatincrease regularly with decreasing%MgO andapparently have undergone more extensive differentiationthan the younger,<1 m.y.-oldbasalts. The Group2 and 3 basaltsare minor constituentsof thepreserved volcanic material, but are consistentlyolder (5-7.6 m.y.) andhave lower end (5.1 to 7.5) valuesthan the Group 1 basalts.These basalts have distinctive trace elementsignatures, with the Group2 basaltshaving higher Ni, lowerHf, and slightlylower middleREE abundancesthan the Group 1 basalts,while the Group3 basaltsare characterizedby higherand more fractionated REE abundances,as well ashigher Ca, P, Ti, Th, Ta, andSc contents.The isotopicand trace element characteristics of all thebasalts are interpretedto have beenlargely inherited from their mantle source regions. The isotopiccompositions of theGroup 1 basaltsoverlap the values for PacificMORB andfor lateCenozoic basalts in theCalifornia CoastRanges interpreted to havebeen derived from upwelling MORB asthenosphere.We suggestthat the Group 1 basaltswere all derivedfrom light REE (LREE)-enrichedportions of thePacific MORB source,which rose into the slab"gap" that developed beneath the southwesternUnited Statesduring the late Cenozoictransition from a convergentto a transform platemargin. The Group2 and3 basaltseither represent smaller degrees of meltingof the MORB source,or meltingof maficportions of thesubcontinental lithospheric mantle currently presentbeneath the region. Ancient, LREE-enriched mantle has not been a primary sourceof basalticmagmatism in thisregion at any time over the past8 m.y.

Introduction spatial isotopic and trace element variations of syntectonic and posttectonic,mantle-derived basalts. This approachhas An important question in the Cenozoic of the been widely appliedin the westernUnited States[Perry et al., southwestern concerns whether subcontinental 1987; Farmer et al., 1989; Wilshire, 1990; Livaccari and lithospheric mantle was at least partially eroded, either Perry, 1993] and the results of these studieshave cast some thermally or tectonically, during inferred early Tertiary low- doubt on the assertion that wholesale or delamination anglesubduction [Bird, 1988;Livaccari and Perry, 1993]. One of ancient mantle lithosphereoccurred during the Cenozoic indirect method of studying the past behavior of the deep [Livaccari and Perry, 1993]. A key area in further assessing continental lithosphere involves assessing temporal and the extent of Cenozoic lithosphere erosion in the western United States is the Mojave Desert of southern California. This region contains numerous Cenozoic basalt vents and is close to the Cenozoic continental margin, where the likelihood of -related thermal and tectonic ' CooperativeInstitute for Researchin Environmental Scienceand Department of GeologicalSciences, University modification of the lithospheric mantle was greatest. of Colorado,Boulder, Colorado. However, the isotopicand trace elementcompositions of the 2 Departmentof Geology,University of NorthCarolina, Cenozoic igneousrocks are not easily interpretedin terms of ChapelHill. • U.S. GeologicalSurvey, Menlo Park, California. variationsin their mantle sourceregions. For example, the 4 Camarillo, California. voluminousearly and middle Miocene volcanic rocks in this region generally contain large crustal componentsand Copyright1995 by theAmerican Geophysical Union. thus provide little unequivocal information regarding the Papernumber 955B00070. nature of the subcontinentalmantle [Glazner and O'Neil, 1989; 0148-0227/95/95J B-00070505.00 Glazner, 1990]. Younger, late Cenozoicbasalts in the Mojave

8399 8400 FARMER ET AL.: ORIGIN OF MOJAVE BASALTS

Desert interacted little with silicic crustal material and do show The volcanic field is underlain by gneissesand significant Nd, St, and Pb isotopic variations, but these [Wooden and Miller, 1990], Late Proterozoicand variations are locally the result of crustalcontamination Paleozoicmiogeoclinal and cratonicmetasedimentary rocks and not of differences in the isotopic compositionsof their [Stewart, 1970; Stewart and Poole, 1975], the mantle sources [Glazner et al., 1991; Glazner and Farmer, Teutonia [Beckerman et al., 1982], and Miocene 1992]. Furthermore, the late Cenozoic centers were small- terrestrial sedimentary rocks and minor volcanic rocks volume and generally short-lived [Glazner et al., 1991] and so [Wilshire, 1992a]. Eruptive activity apparentlyoccurred in provide few insights into any temporal variations in the two distinctpulses from 7.6 to 3.0. Ma and from 1.0 Ma. to basalt sources. the present[Wilshire et al., 1991]. No systematicspatial One exceptionis the (Figure 1), which migrationin the magmatismhas beenidentified [Turrin et al., is not only the largest Cenozoic basalt center in the Mojave 1984]. The flows are primarily alkali basalts and Desert but also the longestlived, having been active since the that straddlethe boundarybetween hypersthene- and late Miocene [Turrin et al., 1985]. Althougha few isotopic nepheline-normativecompositions [Katz, 1981;Turrin et al., analysesfrom the younger basaltsat this center are available 1985; Wilshire et al., 1991]. In this respect, the basalts are [Peterman et al., 1970; Farmer et al., 1989], no systematic similar to other late Cenozoic basalts in the northern and chemical or isotopic studiesof the basaltsexist. We present centralBasin and Range [Best and Brimhall,1974; Vaniman et here the resultsof a comprehensivesurvey of the chemicaland al., 1982; Farmer et al., 1989] and to "straddle-A" type isotopicvariations of the Cima basaltswhich we have used to transitional alkali basalts [Miyashiro, 1978]. Two of the characterize their source regions and to determine if flows, and about 10% of the cones, contain abundant mantle- significant variations in the basalt sourcesoccurred over the derivedperidotire , a substantialproportion of which past 8 m.y. are compositexenoliths containingveins of pyroxenite and GeologicSetting/Previous Work [Wilshire et al., 1988, 1991]. Large pyroxeniteand gabbro xenoliths, compositionally similar to the vein The Cima volcanicfield (Figure 1) consistsof more than 70 material, are abundantand probably representportions of the cinder cones and roots of vents with associated basalt flows lower [Wilshire, 1990; Wilshire et al., 1991]. Many thatcover approximately 150km 2 of the!vanpah uplift in other flows and cones contain small proportions of southeasternCalifornia [Hewett, 1956; Wilshire et al., 1987]. xenocrysticmaterial as well as plagioclaseand

35ø 116000' 15o45 ' 115030 ' I ..::i.:.:.:.:.:.::,: 30' I ß :.: :.:

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iiiii!iiiii::iiiiiii!ili::iiiiiiiiiiiiiii::iiiii!• "...... : ...... •i•i•iii::::::?

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Figure 1. Schematicmap of the Cima volcanicfield compiledfrom Wilshire [1992c, 1992a, 1992b, 1992d]. Numberscorrespond to samplelocations (sample numbers given in Table 1). FARMER ET AL.' ORIGIN OF MOJAVE BASALTS 840•

megacrysts. No trace element data have previously been United States Geological Survey [Baedecker and McKown, publishedfor the basaltsof the Cima volcanicfield, but Sr and 1987]. Nd isotopic data indicate that they have amongstthe lowest The analytical proceduresfor the isotopic analyses are 87Sr/86Sr(<0.7030) and highest end (>+9) values yet given elsewhere[Farmer et al., 1991' Glazner et al., 1991] and determined for Cenozoic basalts in the western United States are only briefly reviewed here. The Sr and Nd chemical [Peterman et al., 1970; Farmer et al., 1989]. separations and isotopic analyses were performed at the University of Colorado, Boulder. Average total procedural Samples blanksfor Rb, St, Sm, and Nd were 2 ng, 700 pg, 200 pg, and 500 pg, respectively,and were all negligiblefor the analyzed Samples were obtained from lava flows, cinder cones, and samples. The Nd and Sr isotopicanalyses were conductedon a hypabyssalbasaltic intrusive rocks throughout the volcanic six-collector Finnigan-MAT solid sourcemass spectrometer. field to characterizethe basaltsover as wide a geographicand The Sr isotopic analyses were four-collector static-mode age span as possible (Figure 1). The samplesfor which data measurements.Replicate analysesof the SRM-987 standardin are presentedwere from datedunits, and a few (samplenumbers thismode yielded a mean 87Sr/86Sr of0.71025+._2(2•j) during prefixed by "MC") were previously studied by Turrin et al. the study period. The Nd isotopic compositionsare reported [1985] (Table 1). In general, the basalts are sparsely asend valuesusing a reference14JNd/144Nd ratio of porphyritic with and + clinopyroxene 0.512638. The Nd isotopic analyses were three-collector . The olivine, clinopyroxene, and some dynamic-modemeasurements. Repeated measurement of the La plagioclase phenocrysts show normal zoning. Other JollaNd standard inthis mode yielded amean 143Nd/144Nd of plagioclase phenocrysts show weak to strong oscillatory 0.511840+_3(20). zoning. In contrast to the phenocrysts, plagioclase, Pb and O isotopicdeterminations were conductedat the U.S. clinopyroxene, orthopyroxene, spinel, and less common Geological Survey, Menlo Park. The proceduresused for the olivine and apatite megacrystsare embayed and show strong Pb analysesare describedelsewhere [Wittke et al., 1989]. The reaction textures. Plagioclasemegacrysts typically have an U.S. GeologicalSurvey standard basalt BCR-1 gavean average intermediate spongy zone of partial melt and an outer, more 206pb/204pb=18.82, 207pb/204pb=15.63, and calcic, clear zone. Orthopyroxenemegacrysts have olivine + 208pb/204pb=38.7during the period of analysis.Powders glass coronas that most likely represent the products of and coarselycrushed and sievedportions of whole rock basalts reaction with the host basalt. The groundmassesof the basalts were used for the oxygen isotopeanalyses. Both powdersand

range from glassy through cryptocrystallineto intergranular. coarsely-crushed, rock were leached in HC1 and HF prior to Rare sampleshave coarseintergranular textures. Identifiable analysis. The oxygen isotopedata are reportedin the standard componentsof the groundmassesare olivine, clinopyroxene, per mille notationrelative to the Vienna standardmean ocean plagioclase,and equant opaqueminerals, and, in some cases, water. All the oxygen isotopemeasurements were performed apatite. Some,but not all, eruptivesstudied contained crustal in duplicatewith a reproducibilityof approximately+_0.1%oo. and/or mantle xenoliths,as noted in Table 1. Sample Ci-56 is listed as nonxenolithic but representsa flow derived from a pluggedvent that does contain sparsemantle xenoliths. Results Resultsof the major and trace elementanalyses are plotted in Figures2-6 accordingto the groupingsdefined below. The Analytical Procedures sampled flows are predominantlytrachybasalts, ranging from Each samplewas 'disaggregatedand hand-pickedto avoid hawaiites to potassic trachybasalts(Figure 2 ) [Le Maitre, xenolithsand obviousxenocrysts. All of the sampleswere 1989], and includeboth nepheline-and hypersthene- analyzedfor major elementsand Sr and Nd isotopicratios normafive samples (Figure 3). Ne-normative compositions (Tables 1, 4). Lead and oxygenisotopic measurements (Table dominatethe <1 Ma basalts(Figure 3). All of the basaltsare 4) and trace element data (Tables 2, 3) were obtained for a moderatelyenriched in light rare earth elements(LREE; Figure subsetof the samples. 4, 5) and have trace element abundancessimilar to those of ocean-island basalts (OIB' Figure 6). The basalts show a Major element chemistry for all "CD" samples was restrictedrange of end values (5.1 to 9.3), 87Sr/86Sr (0.7028- obtainedby electronmicroprobe analysis of samplesfused on 0.7050),/5180 (5.8 to 7.8%0;Figure 7), andPb isotopic Pt wire loops in the hot spot of a Deltech 1-atm gas mixing compositionsthat cluster around the values defined by the furnace [Glazner and Ussler, 1989]. Oxygen fugacity was northernhemisphere reference line (NHRL;Figures 7, 8). maintainedat Ni-NiO duringfusion. Sampleswere fusedfor There are no consistentdifferences between the isotopic or 30-120 min at 1400-1450øC. After fusion,sample glass was elementalcompositions of -bearing and/xenolith-free analyzedon the UNC 3-spectrometer,fully automatedMAC-5 basaltsof a givenage (Figures 2, 3). microprobe,at 15 kV and a samplecurrent of 20 hA, using While all the basaltsare similarcompositiønally, basalts Bence-Albeematrix correctionand a defocussed(25 Ixm)beam. younger than 5 Ma have distinctly different isotopic and Other sampleswere analyzedfor major elementsat the U.S. chemicalcompositions than the older basalts(5-7 Ma), which GeologicalSurvey by high-precisionwavelength-dispersive X providesthe basis for separating the basalts into Groups 1-3. ray spectrometry[Taggart et al., 1987]. FeO,H2 O+, H20-, and CO2 were determinedby classicalwet-chemical analysis Group I [< 5 m.y.] [Jacksonet al., 1987]. FeO values are uncertain becausetotal dissolution of the samples was commonly not achieved. The Group 1 basalts(<5.0 Ma) constitutethe dominantrock Instrumentalneutron activation analysesof minor and trace type of the volcanic field and are characterizedby their dement compositions•(Tables 2, 3) were determinedat the uniformly high end values(7.7 to 9.3) and generallylow, but 8402 FARMER ET AL.' ORIGIN OF MOJAVE BASALTS

ß c:;Od c:; o oc:3dod c:;oo

o• I'• ee• o oa0o

ß 0

06•• oo•

z>-zzzzzz>.zz >,,z• zzz FARMER ET AL.: ORIGIN OF MOJAVE BASALTS 8403 variable,87Sr/86Sr ratios (0.7028 to 0.7040). The Sr and Nd isotopic compositionsof these basaltsdo not eovary (Figure 9). The high end value of the Group i basaltsrepresents the key distinctionfrom the older basalts,the latter (Groups2 and 3) havingend values<7.5 (Figure 7). The Group i basaltscan be further subdivided on the basis of their age and spatial distribution into Group l a (basalts <1 Ma in age from the southernhalf of the volcanicfield; Figure 1) and Group lb (3-5 Ma basalts from the northern half of the field). These two groupsare distinguishablechemically in the following ways. 1. Group lb, but not Group la, basalts have major and trace element variationsthat correlatewith %MgO (Figure 4a and 4b). 2. Group lb basalts trend towards lower percentagesof MgO, FeOt, CaO,and TiO 2 andhigher percentages of K20, Na2¸, A1203, andP205 than the Group la basalts(Figure 4a), as emphasizedby the fact that the Group lb samplesinclude the few basaltic traehyandesitesanalyzed from the volcanic field (Figure 2). 3. Group lb basalts trend towards lower Ni and So, but higherrare earth element(REE) and Hf (Zr) contents,than the Group la basalts(Figure 4b).

Groups 2 and 3 (>5 Ma) TheGroup 2 and 3 basaltsconstitute asmall fraction ofthe preservedvolcanic material. Group 2 consistsof 5.5 to 6.1 Ma basalts found east of the main volcanic field'and Ci-16, whichis a 6.1Ma olddike sample from the western portion of thefield (Figure 1). One younger,3.0 Ma basalt(Ci-12) is includedwith this groupon the basisof its isotopicand chemicalcharacteristics. Group3consists ofweathered basalt flowsfrom the southeast comer of the field and represents the oldestbasalts sampled (6.5 to 7.6 Ma). BothGroups 2 and 3 aredistinguishable from their Group 1 counterpartsin their lower end values(5 ß 1 to 7.5) that correlate with their Sr isotopic compositions (0.7030 to 0.7050;Figure 9). Chemically,theGroup 3 basalts range towardsrelatively high MgO (6.4% to 9.4%)and at theseMg contentsare distinguishable from the Group 1 and2 basaltsin their(1) generallylower percentages of FeO t andNa20, and higherpercentages of TiO2, P205, andCaO (Figure4a), (2) higherTh, Ta, Se, REE, and Hf (Zr) contents(Figure 4b), (3) morefractionated REE abundances(i.e., higherCeffbN; Figure5), and (4) higher $180 values (6.2 to 7.8%o) and high 20•/204pband 206pb/204pb ratios (Figures 7, 8). TheGroup 2basalts haveelemental compositions similar to the younger,Group l a, basaltsbut can be distinguished from the latter by their (1) lower percentagesof SiO2 and TiO2, andhigher percent FeO t (Figure 4a), and (2) higher Ni, lowerHf (Zr),and slightly lower middle REE (Sin, Eu, Gd, Tb) contents(Figures 4b, 5). Discussion We discussbelow the origin of the isotopicand chemical variationsin the Group 1 andGroup 2 and3 basaltsseparately, UU followedbya discussionofwhat implications theoverall data set has for deternfiningthe sourcesof the parentalmagmas and for the Cenozoictectonic evolution of the Mojave Desert. Origin of Group 1 Basalts The high•Nd values,low $180, and the common associationwith mantle-derivedxenoliths suggestthat the 8404 FARMER ET AL.: ORIGIN OF MOJAVE BASALTS

Table3. InstrumentalNeutron Activation Analy?es ofRare Earth Elements From SAMPLE La Ce Nd Sm Eu Gd Tb Tm Yb Lu Group I a Ci-6-1 32.9 69.8 33.8 7.96 2.45 7.8 1.06 0.448 2.62 0.359 Ci-20 38.5 78.6 34.5 7.75 2.48 7.8 1.1 0.473 2.7 0.363 Ci- 15 21.9 48.5 24.2 5.73 1.93 6.2 0.88 0.38 2.27 0.313 Group lb Ci-12 30.3 60.5 24.7 5.24 1.75 5.3 0.78 0.387 2.39 0.329 MC-1 31.3 72.6 36.4 9.15 2.72 9.5 1.26 0.523 30.7 0.419 Ci-60 34 71.7 34.6 7.55 2.54 7.93 1.14 0.504 2.98 0.41 Ci-57 34 72.2 34.9 7.85 2.59 8.35 1.18 0.523 2.98 0.43 Ci-56 40 86.2 40.6 8.77 2.86 8.74 1.3 0.56 3.33 0.46 Ci-25 33.7 68.3 37.8 9.48 2.94 9.2 1.3 0.561 3.26 0.442 Ci-19 32.6 70.1 35 8.8 2.67 8.6 1.21 0.498 3.05 0.42 Group 2 Ci-9-102 32.7 65.9 28.6 5.97 1.85 5.7 0.82 0.405 2.4 0.352 Ci-10-101 31.5 66.3 27.2 5.58 1.77 5.7 0.82 0.4 2.31 0.331 Ci- 16 32.5 62.6 28 6.13 1.94 6 0.84 0.407 2.44 0.347 Ci 9-101 32.8 64.2 27.6 5.59 1.83 5.7 0.85 0.414 2.47 0.328 Group 3 Ci-53 50 94.3 39.6 7.87 2.53 7.33 1.02 0.433 2.55 0.35 Ci-49 51 102 44.8 8.86 2.69 8.16 1.13 0.462 2.67 0.37 Uncertainty% 1 I 2 1 1 3 2 10 2 1 Concentrationsin ppm

Group la and lb basaltsrepresent mantle-derived that mineral phases, an observation consistent with the low experienced little preeruptive crustal contamination. Our solubilityof Nd in groundwaters[Michard, 1989] and with the preferred interpretationfor the relatively wide range of Sr incompatibilityof REE in carbonates[DePaolo, 1988]. isotopic compositionsin these basalts is that they do not Based on these arguments,we concludethat the magmas representoriginal characteristicsof the parental magmas. arentalto the Group1 basaltshad •Nd valuesfrom 7.5 to 9.5, M,any of the analyzed flows containedsecondary carbonate 7Sr/86Sr from 0.7028 to 0.7030,and Pb isotopic precipitatedfrom local groundwaters,and while suchmaterial compositionsthat clusteredaround the northern hemisphere was avoided for the purposesof the isotopic analyses,the referenceline. The high •Nd values clearly preclude ancient, presenceof any Sr-bearingsecondary material in the analyzed LREE-enrichedlithospheric mantle as the primarysource of rock powers could have affected the measuredSr isotopic thesebasalts and point instead towards a mantlesource with a compositions[Schucker and Foland, 1989], given that only long-termLREE depletion,most likely either young(i.e., the rock powdersused for the oxygen isotopedeterminations Cenozoic) lithosphericmantle or asthenosphericmantle. underwentacid leachingprior to dissolution. Significantly, the Group 1 basalts have isotopic To test for the influence of secondarycarbonate, the Sr compositionssimilar to that inferred for asthenospheric isotopic compositionof an HC1 leach from sampleCD-5 mantleupwelling beneath the eastern Pacific Ocean. The •Nd (Table 4) was determined. Approximately10% of the total valuesof the Group1 basaltsoverlap the valuesobserved for whole rock Sr dissolved in the leach solution, and the leached PacificOcean mid-ocean ridge basalts (MORB) (Figure 9), the Sryielded an 87Sr/86Sr value (0.7083) that was considerably latter generally having lower •Nd valuesthan seafloor basalts higherthan the whole rock value (0.7033; Table 4). The HC1 from other ocean basins [Hart, 1984; White et al., 1987]. The and HF leached whole rock yielded a significantly lower basalt Sr and Pb isotopic compositionsalso overlap those 87Sr/86Srof 0.7029,which we consider to be representative found for Pacific MORB (Figures 8, 9). Therefore it seems of the actual magmatic value. Based on these results, it is likely that the magmasparental to the Group 1 basaltswere likely that much of the scatterobserved in the Sr isotopic derived exclusively from asthenosphericsources, without compositionsof the Group 1 basalts is the result of interaction with the continental lithosphere, and that they contamination by relatively radiogenic Sr present in were derivedfrom a sourceisotopically equivalent to the long- secondarycarbonate. The Sr containedin the carbonatecould term LREE-depletedmantle source from which PacificMORB have originally leached, by downward percolating were derived. The latter source correspondsto the depleted groundwaters,from airbornedust depositedon the flow tops, MORB mantle(DMM) definedby Zindlerand Hart [1986]. We given that soils developedon lava flows at Cima are largely recognize, however, that the OIB-like trace element eolian in origin [McFadden et al., 1984]. Eolian materialhas abundancesof the Group 1 basaltsseem inconsistent with this beensuggested as one sourceof pedogenicSr elsewherein the conclusion,given that the small degreesof partial meltingof Basin and Range [Marshall and Mahan, 1994]. Someof the a REE-depletedgarnet lherzolite source required to generatethe scatter in the basalt Pb isotopic compositions,particularly observedLREE-enrichment in the Group 1 basalts(0.1% to the occurrenceof very radiogenicPb in sampleMC-1 (Figures 1%) [Clague, 1987;Salters and Hart, 1989] wouldproduce 7, 8), could also be due to the contributionof Pb presentin more Si-undersaturatedmagmas than observed[Edgar, 1987]. secondarymineral phases. In contrast,the lack of any We can only resortto the usualargument [Frey et al., 1978] covariationbetween the Sr and Nd isotopiccompositions of that a crypticenrichment in incompatibleelements must have the Group ! basaltssuggests that little Nd was mobilized by occurredin the mantlesource just beforethe meltingevent that low-temperature fluids and incorporated into secondary spawnedthe Group 1 basalts. FARMER ET AL.: ORIGIN OF MOJAVE BASALTS 8405

(",1 (",1 (",1 (",,I (",,I (",,I (",,I •"• ('• ('• ('• (",,I 8406 FARMER ET AL.: ORIGIN OF MOJAVE BASALTS

12 Group Xenoliths stronglycompatible, arguing for the role of clinopyroxene+ A la N olivine fractionation,while the REE, and Hf, Zr, P, and K were ß la Y incompatible. The abundancesof some elements,such as Sr, O lb N basaltic Ta, Th, show no clear variationswith %MgO and may have ß lb Y trachy- trachy- andesite beenweakly compatibleduring diffentiation. Using calculated El2 N bulk partition coefficients [Nielsen, 1992], and with 12 Y +3 N clinopyroxene(titanaugite) compositionsfrom [Katz, 1981], we attempted to model the variation in the REE abundances observedbetween the least and most differentiatedGroup lb basalts (samples Ci-60 and Ci-54, respectively) through crystal fractionationof mineral assemblagescontaining, in + + varying proportions,the major phenocrystphases. Those solutions that best mimic the absolute REE abundances and the basaltic basalt andesite modestincrease in Ce/3rbNbetween these two samplesrequire 20% crystallization and the removal of an assemblage containing between 20% and 40% clinopyroxene,with the

o remainderconsisting of olivine + plagioclase. Plagioclase 37 41 45 49 53 57 61 cancompose up to 50% of the fractionatingassemblage, but at higherpercentages a markedEu anomalydevelops after small SiO2 wt% degreesof fractionationwhich is not observedin the analyzed samples. We note that the occurrenceof both pyroxenitesand (with isotopic compositionssimilar to those of the Figure 2. Total alkalies (Na20+K20 wt %) versus silica basalts[Mukasa and Wilshire, 1993]) as veins in peridotire (SiO 2 wt %) for basalts from the Cima volcanic field. xenoliths contained in the youngest basalts supports the Classification scheme from Le Maitre [1989]. Group assertion that mafic crystalline material accumulatedin the designations based on trace element and isotopic data, as uppermantle beneaththe volcanicfield during fractionationof describedin text. Basalts are also segregatedon the basis of the Cenozoic-agebasaltic magmas. whether or not they contain mantle-derivedxenoliths. The younger,Group la, basaltstend to have higher MgO, higher Ni contents, and lower incompatible element abundancesthan the Group lb basaltsand probablyrepresent less differentiated versions of the older Group 1 basalts. Origin of Chemical Distinctions Between However, the poor correlationbetween MgO and traceelement Groups la and lb abundances(Figure 4b) precludes any simple fractionation The chemical differencesbetween the Group l a and lb mechanismin explainingthe rangeof major and traceelement basalts can be simply explained as the result of more abundancesof these basalts. Instead, more complicated protracted crystal fractionation of the older (Group lb) scenariosin which the various flows represent differing basalts. No significant differences in the elemental degrees of and later fractionationmay be compositionsof the magmas parental to either group of required. It may also be necessaryto assessthe role of basaltsare required. Supportinga crystalfractionation origin interaction between <1 Ma. magmas and the mafic vein for the range of compositionsamong the Group lb basaltsis material that accumulatedin the uppermostmantle and crust the strongcovariation between %MgO and many other major duringthe earlier periodsof magmatism,a possibilitythat can and trace elements. The chemicalvariations clearly illustrate only be addressedwhen extensivechemical and isotopicdata that duringdifferentiation of the Group lb basalts,Sc, Ni were of compositemantle xenolithsfrom Cima are available.

Ne Di

GroupXeno!iths, _• N

o N m2 N i /• • n2 v + N

O! Hy

Figure 3. Ternary plots of CIPW normativecompositions of basaltsfrom the Cima volcanicfield. FARMER ET AL.: ORIGIN OF MOJAVE BASALTS 8407

54 0 I SiO2 K20 ß o o o o o

ß ß ß ß 5o

ß*l*:--A•z Crater E3 [] +

A1203 18 . Na20 ß 4- o o

ß A

ß -f-

I , I ! + , ! , I , I FeO t TiO 2 4- o o A A

ß ß A o o ß o ß [] ß [] o , I CaO

oo P205 O0 0 A o AA o•&o o o

[] 00% A

6 , O i , i i 2 4 6 8 10 2 4 6 8 10

MgO MgO

Figure 4a. Selectedmajor elementcompositions versus %MgO. Symbolsas in Figure 2.

The consistently greater degree of differentation of the contaminated,xenolith-poor lavas that stalled in the crust to Group lb basalts,and their relative scarcityof mantle-derived younger xenolith-bearing lavas that ascendedrapidly with xenoliths(Table 1), may indicate that they were derived from little crustal interaction or crystal fractionation [Glazner and magmasthat ponded and differentiatedat shallow levels in the Ussler, 1989]. mantle or in the lower crust, whereasthe younger basaltswere derived from magmasthat traversedthe upper mantle and crust Origin of Group 2 and 3 Basalts more quickly. In a sense, the Group 1 basalts mimic on a The fact that the Group 2 and 3 basalts have distinctly small scale the overall temporal evolution of basaltic different Nd isotopic compositionsfrom the Group 1 basalts volcanism in the Basin and Range, from older crustally suggeststhat differences exist between the sources of the 8408 FARMER ET AL.: ORIGIN OF MOJAVE BASALTS

11oo ! ß

Th + + lOOO Sr

900

8OO A + ß 700

6OO 00C•O0 ß 5OO ßmE3 m•A•• + PisgahCrater I ! I 4OO

Ta 5O + + A

o oco • oo & + ß ß 3O ß ßEl••isgah Crater []

10

I ! I i I i

o o 150 3.0

A ß A

lOO 2.5

o o o

f'• . 5o ß 2.0 I

Sc 25

ß ß A +A 2o [] []

& []

lO I • 3 , I I 8 10 4 6 MgO MgO

Figure 4b. Selectedtrace element abundancesversus %MgO. younger and older basalts at Cima. Such a conclusion is •withincreasing magmatic SiO2 (or decreasingMgO), while complicated,however, by the issueof whethertheir chemical closed system fractionation should produce no variation in and isotopiccharacteristics represent those of their parental P205/K20 becauseboth P and K are incompatiblewith respect magmas or were imposed during crustal contamination,or to near-liquidusmineral phases. The Group2 basaltsdo have evenduring posteruption weathering of the sampledmaterial. slightly lower P205/K2 ¸ than the Group 1 basalts(--0.3 Felsic crustalcontamination is unlikely to be responsible versus-0.4), but there is no obviouscovariation between this for the unique characteristicsof the Group 2 and 3 basalts. ratio andMgO (Figure 10). Instead,it appearsthat the Group2 Such contaminationshould produce decreasingmagmatic basalts were simply derived from parental magmas with a P205/K2 ¸ [Carlsonand Hart, 1987;Glazner and O'Neil, 1989] lower P205/K20 than the Group 1 basalts. The two Group3 FARMER ET AL.: ORIGIN OF MOJAVE BASALTS 8409

lOOO I I I I I I I ! I I I I I phases.Even the high õ180 of theGroup 3 basalts could be Groupla the result of the presenceof some secondarycarbonate in the • ci-6-1 analyzed material, or possibly of extensive low-temperature re-equilibration with meteoric fluids [Taylor and Sheppard, 1986]. However, becausethe trace element compositionsfor Group 2 and 3 basaltsdiffer from thoseof the Group 1 basalts, even for elements relatively immobile in groundwaters (Th, A1; Figure 4), some of the unique geochemicalcharacteristics of these basalts must have been inherited from the magmas from which the basaltscrystallized. We conclude, then, that the Group 2 and 3 basalts were derived from mantle source(s) with isotopic and element - -ffi- - ci-19 - -0- - ci-60 characteristics distinct from the source(s) of the Group 1 - -'•'- - Ci-25 - -0- - Ci-57 basalts. The small volume of eruptive material, the fact that the Group 2 and 3 basaltserupted, in many cases,from vents --•-- MC-1 peripheralto the main locus of magmatism(Figure 1), and the 1 • I I I I I I I I I I fact that they representthe oldest preservedvolcanic units La Ce Nd Sm Eu Gd Tb TmYb Lu (Figure 7), further suggest that these basalts represent the productsof incipient melting of their mantle source. Possible choices for this source include the same asthenopherethat later generated the Group 1 basalts, if that source were lOOO I I I I I I I I I I I ! I isotopically and chemically heterogeneous,or portions of the Group 2 mantle lithosphere which melted in response to the initial • 10-101 • 9-101 upwelling of asthenospherebeneath this region [Leemah and

• ci-12 • 9-102 Harry, 1993]. In the lattercase, the extremelyhigh end values of Cr-diopsidebearing spinel peridotirexenoliths found in the lOO Mojave Desert region (+16 to +18 [Menzies and Murthy, 1980; Nielson et al., 1993]) preclude lithosphericperidotite as a basaltsource, although the high end valuesare significant in that they could suggestthat the spinel peridotiresrepresent remnantsof ancient, LREE depleted,mantle still adheringto the base of the Mojave crust. An alternative lithospheric mantle source is mafic vein material [Wilshire et al., 1988; Group3 1991]. Much of this material, particularlyin the "green - -•- - ci-53 pyroxene" and Al-augite groups,has isotopic characteristics

- -V- - Ci-49 similar to the Group 1 basalts and may be productsof this magmaticevent, but the older Cr-diopside •[roxenites trend • • ] [ I I I I I I I towardssignificantly lower end and higher ø•Sr/86Sr values La Ce Nd $m Eu Gd Tb Tm Yb Lu than observedfor the Group 1 basalts[Mukasa and Wilshire, 1993]. Consequently,melting of this dike material [Leemah Figure 5. (a) Chondrite normalized REE concentrationsfor and Harry, 1993], if presentin the shallowmantle during the Group 1 basalts.Normalizing values from [Wakita et al., early upwelling of asthenosphericmantle beneath the Cima 1972].(b) NormalizedREE concentrationsfor Group2 and3 area, couldhave been involvedin generatingthe Group 2 and 3 basalts. basalts. We are reluctant to further speculateon the origin of the distinctivecompositional characteristics of either the Group 2 basaltsfor which P abundancesare availablehave P205/K20 and 3 basalts,given the uncertaintiesin the sourcemineralogy ratios that range to much higher values(0.7) than observedfor and trace element abundances.For example, we note that the the Group i basalts. relatively high Ce/YbN of the Group 3 basaltscould either Contaminationby mafic crustalmaterial is also unlikely to reflect smaller degrees of partial melting of the LREE- be responsible for the unique characteristics of the older enriched, garnet peridotite from which the Group 1 basalts basalts. At Pisgah and Amboy craters, to the south of the were derived, or the presenceof apatite in either an ultramafic Cima volcanicfield, mixing betweenmafic crust, or anatectic or mafic mantle source,as also suggestedby the higher Ca and melts of such material, and mantle-derived magmas does P contentsin these basalts. Additional constraintsregarding account well for the basalt major and trace element the nature of the sources of these basalts await detailed characteristics [Glazner et al., 1991]. But unlike the Cima geochemical studies of the Cr-diopside xenolith suites from basalts, the inco_mpatibleelement abundances(i.e., K, REE; the Cima area. Figure4) and87Sr/86Sr ratios increase, and tNd values decrease,regularly with decreasing%MgO in the Pisgahand basalts. Implications for the Origin of Basaltic As suggestedfor the Group 1 basalts,the variability in the Magmatism in the Mojave Desert Sr and Pb isotopiccompositions of the Group 2 and 3 basalts Comparedto other asthenosphere-derivedbasalts in the could be due to the presenceof secondarymineral mineral Basin and Range,the Group 1 basaltshave consistently 8410 FARMER ET AL.: ORIGIN OF MOJAVE BASALTS

lOOO Sample Group --•'• Ci-15 la OIB ' Ci-19 lb ' Ci-25 lb : Ci-10-101 2 • Ci-49 3 • 100

,.• 10

1 Ba Yb Rb K Ta Ce Nd Sm Hf Tb Tm

Figure 6. Normalized trace element compositionsof selectedbasalts from the Cima volcanic field. The normalizationfactors given at bottomof diagramare chondritevalues, except for Rb, K, and P [Thompson et al., 1984]. Values for Nb and Y are extrapolated.Field for ocean-islandbasalts (OIB) and the patternfor the island-arcbasalt (IAB) from Thompsonet al. [1984]. Normal and enrichedmid-ocean ridge basalt (NMORB, EMORB) patternsfrom [Sunand McDonough,1989].

0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 10•Group" ' Group1,•.• • 39.11Group....la Gro:p1••-'• 'F•l1• ..... Gr••

•••. i . i . •i . i •. i . Group3• . ••.G •' ' ' ' ' Oo•' ' ' ' ' ' ' •up2' ' ' 0.705o.7o6•• . , . , . , . , ...... ts.7l...... 0.704[• • 15.6

0.7020.703•' ' ' , ..... • 15.5 O '

19.2•a'•'"-,• g• 19.0• •. 18.8•

• 18.4lS.6 t- ' ' ' ' ' • ' ' ' ' ' ' ' 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8

Age (Ma) Age (Ma)

Figure7a. The end, 87Sr/86Sr,and •5180 versus age for Figure 7b. Pb isotopic compositionsversus age for Cima Cima basalts. Symbolsas in Figure 2. basalts.Symbols as in Figure 2. FARMER ET AL.' ORIGIN OF MOJAVE BASALTS 8411

15.80

15.50

15.45.-

15.40

15.3517.0/ 17.5 18.0 18.5 '19.0 19.5 •' 20.0 206 204 Pb/ Pb

39.50

39.25

39.00 Amboy/PisgahCraters+ x+ 38.75 S.Nevad•• ake

38.50

38.25

38.00

37.75

37.50 17.0 17.5 18.0 18.5 19.0 19.5 20.0

206 204 Pb/ Pb

Figure8. (a)207pb/204pb versus 206pb/204pb and (b) 208pb/204pb versus 206pb/204pb for Cima basalts,other Cenozoic basaltsin the Mojave Desert (Deadman Lake, shown as crosses,Amboy crater, and Pisgahcrater), and Cenozoicbasalts in southernNevada [Farmer et al., 1989; Glazner et al., 1991]. Data from Pacific MORB from White et al. [1987]. NHRL is the northernhemisphere reference line [Hart, 1984]. Symbolsas in Figure 2.

higher œNd(>+8) values, higher even than values determined 9 [ Cole and Basu, 1992]). The Coast Ranges basalts have for basalts from the southern and the central Great long been attributed to melting of MORB-asthenosphere Basin (Menzies et al., [1991]; "Basin and Range" basalts upwelling into a slab "window" or "gap" that formed during shownon Figure 9). Therefore,while all of thesebasalts may the initial development of the San Andreas ultimately be related to partial melting within LREE-enriched system [Dickinson and Snyder, 1979; Pilger and Henyey, portionsof the mantle sourceparental to Pacific MORB, only 1979; Johnson and O'Neil, 1984; Severinghaus and Atwater, the isotopic data from the Group 1 basalts lead strongly to 1990].Young, low 87Sr/86Sr alkali basalts with OIB-type such a conclusion. On the other hand, late Cenozoic basalts trace elementpatterns have also been observedabove a slab in the CaliforniaCoast Ranges do have end values>+8 (Figure gap in Baja California [Rogerset al., 1985]. The similarity 8412 FARMER ET AL.' ORIGIN OF MOJAVE BASALTS

The small volumes of late Cenozoic basaltic magmatism Basalts elsewhere in the southern Basin and Range have also been ß Padtic MORB attributedto a sourceof the basaltsin DMM mantle passively [] Basinand Range z CoastRange upwelling beneath the region [Bradshaw et al., 1993], a conclusionsupported by Hf isotopic data for asthenosphere- derived basalts in the southern [Johnson and Beard, 1993]. We agreewith Bradshawet al. [1993] that there

[] is no need to invoke a "plume"source for any of the basaltic + magmatismin southernBasin and Range,not only becauseof the relativelysmall volume of magmagenerated during the late Cenozoic throughoutthe Mojave, but also becausesuch a model could not account for the fact the volcanism at Cima has remained in a more or less fixed position since -8 Ma. If produced by a hot spot in a fixed position relative to the 0.702 0.703 0.704 ' ;.705 0.706 Yellowstonehot spot, then a volcanic trail extendingfrom the Cima area to at least 200 km to the southwest should have beenproduced [Iyer, 1979]. 87Sr/86Sr While minor in volume, the Group 2 and 3 basalts are significant in that they representthe only basaltsat Cima that Figure9. InitialœNd versus 87Sr/86Sr for Cenozoiccould have been derived from lithosphericsources. However, basalts of the Cima volcanic field (symbols as in Figure 2), thesebasalts have œNd values significantlyhigher than those the California Coast Ranges [Cole and Basu, 1992], and the of Cenozoic volcanic rocks interpretedto have been derived Basin and Range [Menzies et al., 1985; Lure et al., 1989; from ancient (Precambrian)lithospheric mantle in the western Kemptonet al., 1991; Menzies et al., 1991]. United States (ENd -+2 to -10 [Alibert et al., 1986; Perry et al., 1987; Farmer et al., 1989; Menzies, 1989]). In addition, lithosphere-derivedbasalts elsewherein the western United States have trace element patterns akin to island arc basalts between the isotopic compositionsof these plate margin and not to OIB [Ormerod et al., 1988]. Therefore,if the Group basalts and the Group 1 basalts further supports a Pacific 2 and 3 basalts were derived solely from a LREE-enriched MORB mantle source for the latter. lithosphericmantle source,then the enrichment(1) must have Worldwide, magmatism above slab windows occurredduring the Phanerozoicand, (2) did not producelow has been recognized as the common product of passive high field strengthelement/large ion lithophile elementratios upwelling of asthenosphericmantle [Hole et al., 1991]. Such in the mantle source, as expectedfor enrichmentsassociated magmatism is characteristicallysmall in volume, and, as in with the subductionof oceanic lithosphere [McCulloch and the Antarctic Peninsula,often long-lived, and not necessarily Gamble, 1991]. Our data indicate that Precambrian accompaniedby extensional tectonism in the crust traversed lithosphericmantle, similar to that proposedelsewhere in the by the basaltic magmas [Hole, 1988; Hole et al., 1991]. A western United States, was not involved in basaltic similar origin for the basalts of the Cima volcanic field genesisin the Mojave Desert region at any time since 8 Ma. through melting of asthenosphereupwelling into a slab "gap" Such mantle lithosphere either never existed beneath this would accountfor the similarity in the isotopic compositions region or it was not preservedat depthswhere it could have of the Cima basalts to those of Pacific MORB, the relatively become involved in the genesis of late Cenozoic basaltic small volume of the magmatism, and for the occurrence of magmas,presumably as a resultof Phanerozoicthermal and/or basaltic magmatism at Cima (and in the Mojave Desert region tectonic events. in general) despite evidence that the Mojave block has been 0.80 under a transpressionalstress regime during the late Cenozoic [Bartley et al., 1990]. The issue is not totally resolved, Group3 --• however, as this model would predict that the slab-window basaltswould follow the trailing edge of the slab window as it 0.60 moved northward through California [Dickinson and Snyder, 1979], but no apparent age progressionsexist in the basaltic 0.40 eruptions of Mojave Desert region (A.F. Glazner et al., P manuscriptin preparation, 1995). Grouplb--•• We also note here that there is no evidence for the 0.20 0.1 involvement of slab-derived melts, as observed in other near- trenchareas [Defant and Drummond, 1990], or for the presence 0.7 0.3 '•'•2-com of metasomatized mantle sources from which island-arc basalts 0.5 mixing 0.00 are postulatedto have been derived [McCulloch and Gamble, 0 2 4 6 8 10 1991]. If such sourceswere present beneaththe Mojave just prior to the development of a slab gap in this region, then %MgO they must have been physically removed, perhapsby sinking Figure 10. P205/K20 versus %MgO for basalts from the or convecting back to greater depths in the mantle, by the Cima volcanicfield. Starredcurve is a two-componentmixing time basaltic magmatism at the Cima volcanic field was curve between Group 1 basalts and Precambrianfelsic crust initiated. [data from Glazner and O'Neil, 1989]. FARMER ET AL.: ORIGIN OF MOJAVE BASALTS 8413

Conclusions datingof the Cima volcanicfield, eastemMojave Desert,Califomia: late Cenozoicvolcanic history and landscapeevolution, Geology, 12, Our data confn'm that low end, LREE-enriched lithospheric 163-167, 1984. mantle has not been an important sourceof basalticmagmas at Edgar, A.D., The genesisof alkalinemagmas with emphasison their the Cima volcanic field since at least 8 Ma. Instead, most of sourceregions: Inferences from experimentalstudies, in Alkaline the basalts were derived from asthenosphereisotopically IgneousRocks, edited by J. G. Fitton and B. G. J. Upton, Geol. Soc. identical to the source of Pacific MORB, although the OIB Spec.Publ. London,30, 29-52, 1987. trace element patterns of the basalts may require that this Farmer, G. L., F. V. Perry, S. Semken,B. Crowe, D. Curtis, and D. J. DePaolo, Isotopic evidence on the structure and origin of sourcehave undergoneLREE-enrichment just before melting. subcontinentallithospheric mantle in southernNevada, J. Geophys. 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