Metasomatism and partial melting in a subduction complex: Catalina schist, southern California.

Migmatitic blocks containing leucocratic zones, veins and pods with albitic plagioclase + quartz + muscovite are found in the amphibolite unit of the Catalina schist, a subduction-zone metamorphic terrain. Field relations, mineral and bulk chemistry, phase-equilibrium along with element partitioning and fluid inclusion data, suggest that the migmatites and dykes are partial melts of variably metasomatized amphibolites at P approx 8-11 kbar and T approx 640-750oC, in the presence of low-salinity aqueous fluid. The Catalina metamorphism occurred at much shallower levels than those inferred for subduction-related magmatism and is one of multi-stage, slab-derived metasomatic contamination of the mantle in the 'hanging wall' of a subduction zone. -L.C.H.Dept. of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington DC 20560, USA.