Origin of late Cenozoic basalts at the Cima volcanic field, Mojave Desert, California

Major element, trace element, and isotopic data from late Cenozoic alkali basalts comprising the Cima volcanic field, southeastern California, are used to characterize basalt sources beneath this portion of the Mojave Desert over the past 8 m.y. The basalts are dominantly trachybasalts with trace element compositions similar to modern ocean‐island basalts (OIB), regardless of the presence or absence of mantle‐derived xenoliths. In detail, the basalts can be divided into three groups based on their ages and on their trace element and isotopic characteristics. Those basalts <1 m.y. in age, and the majority of those 3–5 m.y. old, belong to Group 1 defined by high εNd values (7.6 to 9.3), low 87 Sr/ 86 Sr (0.7028 to 0.7040), low whole rock δ 18 O (5.8‰ to to 6.4‰), and a restricted range of Pb isotopic compositions that generally plot on the mid‐ocean ridge basalt (MORB) portion of the northern hemisphere reference line. The 3 to 5‐m.y.‐old basalts have rare earth element (REE) and other incompatible element abundances that increase regularly with decreasing %MgO and apparently have undergone more extensive differentiation than the younger, <1 m.y.‐old basalts. The Group 2 and 3 basalts are minor constituents of the preserved volcanic material, but are consistently older (5–7.6 m.y.) and have lower εNd (5.1 to 7.5) values than the Group 1 basalts. These basalts have distinctive trace element signatures, with the Group 2 basalts having higher Ni, lower Hf, and slightly lower middle REE abundances than the Group 1 basalts, while the Group 3 basalts are characterized by higher and more fractionated REE abundances, as well as higher Ca, P, Ti, Th, Ta, and Sc contents. The isotopic 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 Pacific MORB and for late Cenozoic basalts in the California Coast Ranges interpreted to have been derived from upwelling MORB asthenosphere. We suggest that the Group 1 basalts were all derived from light REE (LREE)‐enriched portions of the Pacific MORB source, which rose into the slab “gap” that developed beneath the southwestern United States during the late Cenozoic transition from a convergent to a transform plate margin. The Group 2 and 3 basalts either represent smaller degrees of melting of the MORB source, or melting of mafic portions of the subcontinental lithospheric mantle currently present beneath the region. Ancient, LREE‐enriched mantle lithosphere has not been a primary source of basaltic magmatism in this region at any time over the past 8 m.y.