Magma Mixing and Cretaceous Crustal Growth: Geology and Geochemistry of Granites in the Central Sierra Nevada Batholith, California

Abstract

Major- and trace-element data suggest that some granites exposed in the Sierra Nevada batholith represent appropriate end members in a mixing process that generated voluminous granodiorites. Geochemical trends in a range of rock types indicate that the highest-silica granites have been variably modified by partial melting and/or fractional crystallization. Trends in initial Sr, Nd, and common Pb isotope ratios reveal little variation within a wide range of rock types in the same suite, but significant spatial and isotopic variation among individual suites of rocks. These observations are consistent with the interpretation that granites in the Sierra are generated by partially melting juvenile, mantle-derived diorites and mixed with penecontemporaneous mantle melts. With the exception of one spatially limited suite in this study, there is little evidence that ancient continental crust was involved in the generation of the central Sierra Nevada batholith. Instead, isotopic trends from east to west in this study indicate that the majority of granites in the central Sierra Nevada are likely juvenile crust. These observations may dramatically increase estimates of crustal growth rates in the western United States during the Mesozoic.