A Sr, Nd, and Pb isotopic study of mantle domains and crustal structure from Miocene volcanic rocks in the Mojave Desert, California

Tertiary volcanic rocks were erupted across the Mojave Desert and southeast ern California in the early to middle Miocene after a long period of magmat ic quiescence, Eruption of these rocks generally coincided with regional ex tension. New Sr, Nd, and whole-rock Pb isotopic data for Miocene rocks, fro m a transect from the western Mojave Desert to the Colorado River trough, p rovide clues to crustal and mantle structure during this important time per iod. Volcanism was calc-alkalic but transitional to alkalic in the eastern part of the area, Trace element contents, trace element ratios, and radioge nic isotopic compositions vary across the study area, and isotopic composit ion is correlated with both bulk chemical composition and longitude, Correl ation of isotopic composition with bulk composition results from widespread contamination of mantle-derived basalts with crust. Variation of isotopic composition versus longitude for rocks of all hulk compositions reflects lo ngitudinal variations in both mantle and crustal structure. Geochemical sig natures of mafic rocks east of about long 116 degrees W indicate an ancient (Precambrian) enriched lithosphere source. Some thinned and/or reworked Pr ecambrian crust extends west of 116 degrees W. Mafic rocks west of 116 degr ees W were derived from mantle with oceanic geochemical and isotopic signat ures. There is no clear, coincident, major crustal structure or tectonic bo undary in surface geology at 116 degrees W; however, modern-day seismicity is restricted to areas west of this longitude, and there is a change in the dominant orientation of mountain ranges at this longitude. The data here a lso indicate that the longstanding problem of drawing the Sr-i = 0.706 line through the Mojave Desert can be resolved if the line represents the west edge of the North American mantle rather than Precambrian crust. By this de finition, the Sr-i = 0.706 line would be the isotopic boundary at 116 degre es W. The cause of early Miocene volcanism in the Mojave Desert remains eni gmatic. An oceanic mantle source for early Miocene basalts in the western M ojave Desert suggests a possible connection to asthenospheric mantle volcan ism in the California Coast Ranges. Simple models of decompression melting of enriched mantle following convective thinning and extension of the litho sphere cannot be applied across the Mojave Desert.