SPHENE AND ZIRCON PHASE EQUILIBRIA IN SIERRAN GRANODIORITES: IMPLICATIONS FOR PLUTONIC-VOLCANIC SYSTEMS

The Mesozoic Sierra Nevada batholith preserves a long and widespread record of continental arc magmatism that can be used as a global model. This region is important in that it is possible to study early intrusive suites of the batholith and corresponding volcanic rocks, including ash-flow tuffs, that may have originated from the same magmatic system. The petrogenesis and phase equilibria of cogenetic zircon and sphene in granodiorites can provide limits on the role of sphene in generating eruptible silica-rich melts. Hornblende + plagioclase + quartz thermobarometry in sphene and zircon-bearing granodiorites indicate near-solidus crystallization of the volumetrically dominant silicates at ~680-730 °C and 3 to 5 kb. Titanium thermometry was used to calculate zircon crystallization temperatures using activities of a_SiO2,zircon=0.9 and a_TiO2,zircon=0.5, between zircon and quartz saturation. Further, the Zr-in-sphene thermometer provided sphene-crystalization temperatures at 3 kb and activities of a_SiO2,sphene=1 and a_TiO2,sphene=0.3. These calculations yielded crystallization temperatures of ~690-800 °C for zircon and ~630-710 °C for sphene, results that are consistent with Th/U abundance patterns indicating that the bulk of zircon crystallization preceded growth of sphene. These zircon and sphene equilibria data suggest a general crystallization model for hydrous and oxidized Sierran granodiorites that attributes the lack of sphene in related volcanic rocks to near-solidus sphene crystallization (<30% melt) after the granodiorite system “locked up” and was unable to erupt voluminous ash-flow tuffs.