PETROGENESIS OF POST-KINEMATIC PROTEROZOIC (1.69 TO 1.63 GA) GRANITOID PLUTONS FROM THE SOUTHEASTERN MOJAVE DESERT OF CALIFORNIA

Post-kinematic granitoid rocks found throughout the southeastern Mojave Desert comprise a variably metamorphosed and deformed sub-alkaline batholith, which have U/Pb crystallization (zircon) ages between 1.69 to 1.63 Ga, and which are cut by abundant mafic dikes of similar ages. They are highly distinctive rocks, with the common presence of 2 to 5 cm phenocrysts of K-feldspar. Compositionally, the suite is highly variable, ranging from a rather mafic diorite (50 wt.% SiO₂, 2.5 wt.% K₂O, 11% wt.% FeO*) to granite (70 wt. % SiO₂, 2 wt. % K₂O, 2.5 wt.% FeO*). All rocks, even the most mafic, are highly enriched in incompatible elements. REE are unusually high in the more mafic rocks, (La = 113, Lu = 0.78 ppm) but drop off rapidly in felsic rocks (to La = 40, Lu = 0.42 ppm). The primary mineral assemblage in addition to quartz and feldspars includes biotite +/- hornblende and abundant accessories. Course relicts and pseudomorphs of garnet occur locally, possibly as xenocrysts. The batholith intruded older metamorphic rocks (high-Al metasedimentary rock, 1694 and 1720 Ma) that were already at high grade. Subsequently it was metamorphosed during the Proterozoic and locally again during the Mesozoic. Elemental compositions require an incompatible element-rich but mafic (or ultramafic) source. Trace element models (AFC) suggest that the granitoids could have been largely derived by a fairly mafic melt with up to approximately 18% assimilation of the surrounding country rock. Assimilation rates this high seem not to be unreasonable in many circumstances. Based on measured Pb/Pb isotopes, it would seem that the assimilant would have to have elevated Pb ²⁰⁶/₂₀₄, ²⁰⁷/₂₀₄ and ²⁰⁸/₂₀₄ isotopes, which is consistent with it being older crustal material (and also based on available Nd isotope data).