MELTING PROCESSES IN CONTINENTAL LITHOSPHERE: EFFECTS OF MANTLE METASOMATISM ON MELT COMPOSITION

Small volumes of high-potassium, silica-undersaturated lavas occur in a range of tectonic settings worldwide. We have conducted phase equilibrium experiments on a primitive olivine leucitite (OL) from the central Sierra Nevada, California and a primitive absarokite from the Mascota region in western Mexico (PA). Both compositions are in equilibrium with mantle olivine: Fo₉₁ (OL) and Fo₉₀ (PA). The near-liquidus phase relations were determined from 1 to 3.5 GPa and at temperatures from 1300 °C to 1460 °C in piston-cylinder experiments. The OL 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 OL magma was derived from a hydrous source at greater than 100-km depth. The PA composition is multiply saturated with olivine and orthopyroxene with ~2% H₂O at 1.6 GPa and 1400 °C. At an H₂O content of 5 wt. % the multiple saturation point with orthopyroxene and olivine occurs at 1.7 GPa and 1300 ^oC. For the OL magma we propose that subduction-derived fluids drive a reaction that consumes garnet + orthopyroxene to create clinopyroxene + phlogopite, and that these high-potassium Sierran magmas are created by melting of this phlogopite - clinopyroxene metasomatized peridotite. In contrast, the primitive Mexican absarokite was segregated from a depleted harzburgite residue at low extents of melting (< 5 %) and a depth of 50 km. Thus, mantle source mineralogy exerts important controls on the compositions of silica-undersaturated magmas. Highly silica-undersaturated melts (like OL) are apparently not in equilibrium with orthopyroxene-saturated source residues, and represent melts that consumed only the metasomatized part of the mantle source region. At lower amounts of silica undersaturation, melts like PA may be derived from orthopyroxene saturated residues.