Evidence for the Formation of Pliocene Sierran High-Potassium Magmas from Deep Melting of a Phlogopite-Clinopyroxene Metasomatized Peridotite

Phase equilibrium experiments have been conducted on a primitive olivine leucitite (WC-1) from the central Sierra Nevada, California. The near-liquidus phase relations were determined from 1.2 to 3.4 GPa and at temperatures from 1350°C to 1460°C in piston-cylinder experiments. The composition with about 2% water is multiply saturated with olivine and clinopyroxene at approximately 2.8 GPa and 1460°C, and with 6% water phlogopite is stable. Xenoliths carried by other young Pliocene lavas in the vicinity of WC-1 have yielded temperatures of equilibrium from 700 to 900°C, with one outlier at 1060°C. These xenoliths support the hypothesis that the lower lithosphere under the Sierra Nevada delaminated just prior to the Pliocene, and fluid metasomatized mantle melted to produce the high-potassium Pliocene lavas. We suggest that subduction-derived fluids drive a reaction that consumes garnet + orthopyroxene to create clinopyroxene + phlogopite, and that the high-potassium Sierran magmas are created by melting phlogopite - clinopyroxene metasomatized peridotite.