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Possible Ophiolite Slivers Embedded In The Southwestern Sierra Nevada Batholith, Kern And Bakersfield Counties, California


Whole-rock geochemical analyses of gabbros and peridotites collected near the western edge of the Kern Plateau (Sequoia National Forest, Sierra Nevada Mountains, California) suggest that rocks exposed on Blackrock Mountain are not the product of Mesozoic arc magmatism. Instead, elevated chromium and nickel contents (698-1999 ppm Cr; 153-346 ppm Ni; n=5), coupled with high values of magnesium number (70-77) and olivine-pyroxene-rich mineralogy, appear more consistent with an ophiolite origin. For comparison, hornblende-rich Mesozoic arc gabbros of the Kern Plateau have lower Cr (rarely approaching 130 ppm) and Mg-numbers (∼55-65). The nearest recognized outcrops of ophiolite occur ∼50 km to the northwest at the southern tip of the Kaweah ophiolite melange of the Foothills ophiolite belt. Additional Cr-rich (∼300 ppm) peridotites outcrop near Bodfish, CA, ∼50 km to the southwest of Blackrock Mountain. We hypothesize that the Bodfish and Blackrock peridotites are ophiolite slivers that decorate a major sinistral transform fault, possibly formed during Permo-Triassic truncation of the southwestern North American continental margin. If correct, these ophiolite slivers would provide constraints on the location of the California-Coahuila transform fault along which the Caborca Block was translated ∼950 km to the southeast into Sonora, Mexico. Future work aimed at separating zircons from Blackrock plagiogranite may provide a test of this model: if zircon is present, U-Pb zircon dates might discriminate between arc-related (∼250-80 Ma) and ophiolite-related Ordovician magmatism.