SEARCHING FOR MANTLE-DERIVED MAGMAS IN A GRANODIORITIC BATHOLITH: THE SUMMIT GABBRO AND ASSOCIATED PERIDOTITE OF THE KERN PLATEAU, SOUTHEASTERN SIERRA NEVADA BATHOLITH, CALIFORNIA

Gabbro and peridotite rocks outcrop as small pods (typically <1 km²) on the Kern Plateau, and may provide constraints on the character of the Mesozoic mantle source region and lower crust of the southeastern Sierra Nevada arc. Most outcrops of the Summit hornblende gabbro are texturally variable; rare magmatic layering is diffuse. Zircon U-Pb LA-ICP-MS dates define two Jurassic age populations, ~146 Ma and ~180 Ma, each with associated coeval granitoid rocks. Whole-rock (WR) magnesium numbers (Mg#) of 63-47 are too low to have formed from mantle-equilibrated magmas, opening the possibility of deep crustal differentiation.

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.