242-4 Magma Genesis and Evolution of the Granite Mountains, Mojave National Preserve

Abstract:

Mesozoic magmatism in the Mojave Desert represents a southern extension of the Sierra Nevada Magmatic arc. The Granite Mountains in the eastern Mojave Desert are composed of Jurassic to Cretaceous intermediate to felsic plutonic rocks. We undertook a geochemical and geochronological study to better understand the petrogenesis of the Granite Mountains plutonic rocks and the source of Mesozoic Mojave Desert magmatism. In this study, fifteen samples from across the Granite Mountains were analyzed for zircon U-Pb, trace-element, and Hf isotopic compositions and whole-rock major- and trace-element concentrations. Zircon U-Pb analyses yielded both Jurassic (162–149 Ma; n = 87) and Cretaceous (80–75 Ma; n = 315) magmatic ages. Rare Proterozoic inherited grains ranged in age from 1780–1550 Ma (n = 4) with ε_Hf values of -28.4 to -35.0, corresponding to exposed Proterozoic wall rock surrounding the Granite Mountains. Hf analyses of magmatic zircon yielded a restricted range of ε_Hf values from -8.3 to -13.4 across all ages and compositions, consistent with derivation from an enriched lithospheric mantle source, with little impact from crustal assimilation. Whole-rock major- and trace-element data for Cretaceous and Jurassic samples each define single liquid lines of decent reflecting progressive evolution of the magmatic system through partial melting, magma mixing, and/or fractional crystallization. The Hf isotopic composition of the Granite Mountains plutonism is consistent with the upper limit of initial ε_Nd and ε_Hf values of Mesozoic plutonic rocks across the eastern Mojave and ε_Nd of nearby mantle xenoliths, potentially reflecting a wide-spread lithospheric mantle source. Such an enriched source composition would permit a more limited role of crustal recycling in isotopically enriched Mesozoic plutonic rocks and indicate that a larger volume of juvenile crust was generated by Sierran magmatism in this area.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-21681

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