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.
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