PETROGENESIS OF THE BARCROFT PLUTON, NORTHERN WHITE-INYO MOUNTAINS, EAST-CENTRAL CALIFORNIA

The White-Inyo Range lies within the regional transition from Paleozoic and older North American continental basement to outboard Mesozoic and younger accreted terranes and a superimposed Andean-type arc. In the White Mountains, the metaluminous Barcroft granodiorite invaded a major NE-striking, SE-dipping high-angle reverse fault, the Barcroft break. The pluton was emplaced as a compositionally heterogeneous series of areally scattered melt pulses episodically injected over the approximate interval 167-161 Ma. The oldest dated rocks are relatively felsic, whereas the youngest is more mafic, suggesting progressive partial fusion of a relatively mafic protolith. Heavy rare earth-enriched zircons indicate that Barcroft melts were derived at mid-crustal depths from a mafic igneous source rock containing plagioclase and lacking garnet. Granodioritic magma genesis involved the possible mixing of mafic and felsic melts, as well as very minor assimilation of country rocks, but mainly by fractional fusion and crystallization. Bulk chemical, rare earth, and isotopic data suggest that analyzed Barcroft rocks are members of a single suite. The granodioritic rocks are slightly more magnetite-rich at higher elevations on the NE, approaching the roof of the pluton. Earlier thermobarometry (Ernst, 21002; Ernst and Rumble, 2003) chronicled the cooling and re-equilibration of the Barcroft pluton from its margins inward as well as from mid-crustal generation depths of ~25 km through ascent and stalling at ~10-12 km. Refractory phase assemblages consolidated along the pluton margins, whereas subsolidus minerals in the interior of the of body continued to re-equilibrate with upper crustal deuteric and surficial aqueous fluids during exhumation and cooling.