Lower Crustal Xenoliths, Chinese Peak Lava Flow, Central Sierra Nevada

An assemblage of pyroxenite, peridotite, and mafic granulitexenoliths contained in the toe of a 10 Ma trachybasalt flowremnant overlying Late Cretaceous granitoids indicates the presenceof a mafic-ultramafic complex beneath the Sierra Nevada batholith.Olivine-free pyroxenites that include orthopyroxenites, websterites,and clinopyroxenites are dominant. Primary igneous texturesare displayed by some pyroxenites, but commonly are masked byrecrystallization. Fe-rich harzburgites and lherzolites arerare. A few of the ultramafic xenoliths contain ovoid opaquepatches that are apparently pseudomorphs after garnet and havepyralspite garnet compositions. A pressure corresponding toa lower crustal depth of approximately 40 km has been determinedfrom two of these xenoliths using a garnet-orthopyroxene geobarometer.Abundant mafic granulites can be subdivided into those containing12 per cent or less A₁₂O₃ and chemically gradational with pyroxenitesand others containing more than 15 per cent A₁₂O₃ and showingconsiderable scatter on oxide variation diagrams. The high-aluminagranulite xenoliths have relatively low ⁸⁷Rb/⁸⁶Sr but high ⁸⁷Sr/⁸⁶Sr,whereas low-alumina and ultramafic xenoliths have a wide rangeof ⁸⁷Rb/⁸⁶Sr, but lower ⁸⁷Sr/⁸⁶Sr; the isotopic data indicatean age for the complex roughly the same as that of overlyinggranitoid plutons. However, the granitoids have initial ⁸⁷Sr/⁸⁶Srratios intermediate between the high-alumina and ultramaficxenoliths, suggesting that they may have resulted from mixingof basaltic magma, represented by the ultramafics, and crustalmaterials, with subsequent crystal fractionation. The trachybasaltmay represent a partial melt of the ultramafic rocks.Rocks analogousto the Chinese Peak xenoliths are exposed in the Giles complexof central Australia, a series of several deformed layered maficand ultramafic intrusions, emplaced in a granulite facies terrain.Contemporaneous development of mafic-ultramafic complexes andthe Sierra Nevada batholith may explain the present day thick({small tilde} 50 km) crust in this region