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Origin of Late Cenozoic Basalts at the Cima Volcanic Field, Mojave Desert, California

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Origin of Late Cenozoic Basalts at the Cima Volcanic Field, Mojave Desert, California JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. B5, PAGES 8399-8415, MAY 10, 1995 Origin of late Cenozoic basalts at the Cima volcanic field, Mojave Desert, California 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 mantle-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 basalt (MORB) portionof the northernhemisphere 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 lithosphere 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 history of the been widely appliedin the westernUnited States[Perry et al., southwestern United States 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 erosion 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 subduction-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 felsic 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 Proterozoic gneissesand significant Nd, St, and Pb isotopic variations, but these granites [Wooden and Miller, 1990], Late Proterozoicand variations are locally the result of mafic crustalcontamination Paleozoicmiogeoclinal and cratonicmetasedimentary rocks and not of differences in the isotopic compositionsof their [Stewart, 1970; Stewart and Poole, 1975], the Mesozoic mantle sources [Glazner et al., 1991; Glazner and Farmer, Teutonia batholith [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 Cima volcanic field (Figure 1), which migrationin the magmatismhas beenidentified [Turrin et al., is not only the largest Cenozoic basalt center in the Mojave 1984]. The lava flows are primarily alkali basalts and Desert but also the longest lived, having been active since the hawaiites 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 xenoliths, a substantialproportion of which past 8 m.y. are compositexenoliths containingveins of pyroxenite and GeologicSetting/Previous Work gabbro [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 crust [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 pyroxene 35ø 116000' 15o45 ' 115030 ' I ..::i.:.:.:.:.:.::,: 30' I ß :.: :.: To Valley ................ Death Valley junction 00,0/,/.,,Well.,__ :................................ 1:1:1:1:7." • c•2ø o Halloran 25 Springs (314 •5.• 27> 16 35 ø 15' Cim; To ß :. 30 29,31 :i:i?:i:S:':'::::':5•!:.58:i?i}!:i}?i?i?i:i•. Lake • :•?'"''...'.'•...... ." ". ...... iiiii!iiiii::iiiiiii!ili::iiiiiiiiiiiiiii::iiiii!• "...... : ......... •i•i•iii::::::? To Kelso 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
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