SEARCHING FOR MANTLE-DERIVED MAGMAS IN A GRANODIORITIC BATHOLITH: THE SUMMIT GABBRO AND ASSOCIATED PERIDOTITE OF THE KERN PLATEAU, SOUTHEASTERN SIERRA NEVADA BATHOLITH, CALIFORNIA
Rare two-pyroxene-hornblende gabbros (Mg#74; Cr < 130 ppm) have less evolved compositions. One outcrop contains olivine-rich zones surrounded by corona structures hypothesized to be mantle xenoliths. In another outcrop, poikiolitic hornblende anorthosite is cross-cut by a fine-grained olivine-porphyritic dike with a whole-rock Mg# of 69. Olivine phenocrysts display a bimodal distribution of Mg# (5 grains ~69; 3 grains ~96.5), the higher of which may represent mantle-derived xenocrysts.
Peridotites from nearby Blackrock Mountain have elevated whole-rock chromium and nickel contents (698-1999 ppm Cr; 153-346 ppm Ni; n=5), coupled with high Mg# (70-77), and may represent pyroxene-olivine cumulates of arc magmas. Alternatively, these rocks may have originated as part of an ophiolite complex; 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) gabbros 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. Future work focused on separating zircons from Blackrock quartz diorite and Bodfish two-pyroxene gabbro may provide a test of this model: Should zircon be present, U-Pb zircon dates might discriminate between Mesozoic arc magmatism and ophiolite-related Ordovician magmatism.