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.