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

Abstract

Tertiary volcanic rocks were erupted across the Mojave Desert andsoutheastern California in the early to middle Miocene after a longperiod of magmatic quiescence. Eruption of these rocks generallycoincided with regional extension. New Sr, Nd, and whole-rock Pbisotopic data for Miocene rocks, from a transect from the westernMojave Desert to the Colorado River trough, provide clues to crustaland mantle structure during this important time period. Volcanism wascalc-alkalic but transitional to alkalic in the eastern part of thearea. Trace element contents, trace element ratios, and radiogenicisotopic compositions vary across the study area, and isotopiccomposition is correlated with both bulk chemical composition andlongitude. Correlation of isotopic composition with bulk compositionresults from widespread contamination of mantle-derived basalts withcrust. Variation of isotopic composition versus longitude for rocks ofall bulk compositions reflects longitudinal variations in both mantleand crustal structure. Geochemical signatures of mafic rocks east ofabout long 116 degrees W indicate an ancient (Precambrian) enrichedlithosphere source. Some thinned and/or reworked Precambrian crustextends west of 116 degrees W. Mafic rocks west of 116 degrees W werederived from mantle with oceanic geochemical and isotopic signatures.There is no clear, coincident, major crustal structure or tectonicboundary in surface geology at 116 degrees W; however, modern-dayseismicity is restricted to areas west of this longitude, and there isa change in the dominant orientation of mountain ranges at thislongitude. The data here also indicate that the long-standing problemof drawing the Sr (sub i) = 0.706 line through the Mojave Desert canbe resolved if the line represents the west edge of the North Americanmantle rather than Precambrian crust. By this definition, the Sr (subi) = 0.706 line would be the isotopic boundary at 116 degrees W. Thecause of early Miocene volcanism in the Mojave Desert remainsenigmatic. An oceanic mantle source for early Miocene basalts in thewestern Mojave Desert suggests a possible connection to asthenosphericmantle volcanism in the California Coast Ranges. Simple models of decompression melting of enriched mantle following convective thinningand extension of the lithosphere cannot be applied across the MojaveDesert.

73 samples from this reference