THERMOBAROMETRY OF CA-CLINOAMPHIBOLES IN THE BARCROFT GRANODIORITIC PLUTON, CENTRAL WHITE MOUNTAINS, EASTERN CALIFORNIA

The Ca-clinoamphibole + biotite ± augite-bearing Middle Jurassic Barcroft pluton consists of intergradational gabbro/diorite, metadiorite, voluminous mafic-to-felsic granodiorite, and rare alaskite-aplite. It was emplaced along the Barcroft structural break—a major ENE-striking suture. This fault juxtaposes mid-Mesozoic bimodal metavolcanics on the north against metamorphosed Lower Cambrian platform strata on the south. The Cretaceous McAfee Creek Granite and related bodies intruded the section on the east. Microprobe analyses of Ca-clinoamphiboles from 16 Barcroft rocks (ten granodiorites, four gabbro/diorites, and two metadiorites) allow the assignment of P-T conditions of crystallization and/or recrystallization based on theoretically calculated and experimentally determined Ti- and Al-clinoamphibole thermobarometers. Combined with earlier studies of country-rock metamorphism, and oxygen isotopic investigations of mineral assemblages in both plutons and wall rocks, four stages in the P-T evolution of the Barcroft pluton have been identified: Stage (1), partial melting of a mafic protolith (or at least equilibration of a rising mafic igneous column) at ~25 km depth, then onset of crystallization of refractory hornblendes and other ferromagnesian phases at ~915 ± 25°C attending magma ascent. Stages (2)—>(3), crystallization of progressively more actinolitic hornblende during decompression cooling, then stalling of the pluton at a subvolcanic depth of 10-12 km as temperature declined over the interval ~760-650°C. Stage (4), subsolidus recrystallization and exsolution of nearly stoichiometric actinolite attending deuteric alteration over the range ~545-395°C at shallower depths. Mineral assemblages of stages (1)—>(4) are spatially disposed progressively inward in the pluton, indicating that the Barcroft pluton lost heat along intrusive contacts with the wall rocks; it cooled more slowly inward, annealing and re-equilibrating in the process. The Cretaceous McAfee Creek Granite transected this ENE-trending thermal zonation, but exerted little effect on delta 18/O values and Ti + Al contents of analyzed Barcroft clinoamphiboles.