Evidence for deep melting of hydrous metasomatized mantle: Pliocene high-potassiuim magmas from the Sierra Nevadas

Phase equilibrium experiments have been conducted on a primitive Pliocene 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° to 1460°C in a piston-cylinder apparatus. The composition with ∼2% H₂O is multiply saturated with olivine and clinopyroxene at approximately 3.1 GPa and 1460°C and with 6% water in the coexisting melt phlogopite is stable. These results indicate that the magma was derived from a hydrous source at greater than 100 km depth. 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 are consistent with 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.