ORIGIN OF LATE CENOZOIC BASALTS AT THE CIMA VOLCANIC FIELD, MOJAVE DESERT, CALIFORNIA

Major element, trace element, and isotopic data from late Cenozoic alk ali basalts comprising the Cima volcanic field, southeastern Californi a, are used to characterize basalt sources beneath this portion of the Mojave Desert over the past 8 m.y. The basalts are dominantly trachyb asalts with trace element compositions similar to modern ocean-island basalts (GIB), regardless of the presence or absence of mantle-derived xenoliths. In detail, the basalts can be divided into three groups ba sed on their ages and on their trace element and isotopic characterist ics. Those basalts <1 m.y. in age, and the majority of those 3-5 m.y. old, belong to Group 1 defined by high epsilon Nd values (7.6 to 9.3), low Sr-87/Sr-86 (0.7028 to 0.7040), low whole rock delta(18)O (5.8 pa rts per thousand to 6.4 parts per thousand), and a restricted range of Pb isotopic compositions that generally plot on the mid-ocean ridge b asalt (MORE) portion of the northern hemisphere reference line. The 3 to 5-m.y.-old basalts have rare earth element (REE) and other incompat ible element abundances that increase regularly with decreasing %MgO a nd apparently have undergone more extensive differentiation than the y ounger, <1 m.y.-old basalts. The Group 2 and 3 basalts are minor const ituents of the preserved volcanic material, but are consistently older (5-7.6 m.y.) and have lower epsilon Nd (5.1 to 7.5) values than the G roup 1 basalts. These basalts have distinctive trace element signature s, with the Group 2 basalts having higher Ni, lower Hf, and slightly l ower middle REE abundances than the Group 1 basalts, while the Group 3 basalts are characterized by higher and more fractionated REE abundan ces, as well as higher Ca, P, Ti, Th, Ta, and Sc contents. The isotopi c and trace element characteristics of all the basalts are interpreted to have been largely inherited from their mantle source regions. The isotopic compositions of the Group 1 basalts overlap the values for Pa cific MORE and for late Cenozoic basalts in the California Coast Range s interpreted to have been derived from upwelling MORE asthenosphere. We suggest that the Group 1 basalts were all derived from light REE (L REE)-enriched portions of the Pacific MORE source, which rose into the slab ''gap'' that developed beneath the southwestern United States du ring the late Cenozoic transition from a convergent to a transform pla te margin. The Group 2 and 3 basalts either represent smaller degrees of melting of the MORE source, or melting of mafic portions of the sub continental lithospheric mantle currently present beneath the region. Ancient, LREE-enriched mantle lithosphere has not been a primary sourc e of basaltic magmatism in this region at any time over the past 8 m.y .