Partial melting accompanied high-pressure amphibolitization of eclogitic blocks and metasomatism of blocks and their peridotite matrix in a unit of the Catalina Schist, a subduction zone metamorphic terrane. Migmatitic blocks contain leucocratic zones, veins, and pods with albitic plagioclase ? quartz ? muscovite. Mineralogically similar dikes and veins cut the ultra-mafic matrix; a few veins can be traced back into migmatitic blocks.

Uniform phase proportions and igneous textures indicate that the dikes and veins crystallized from silicate melts. Field relations, mineral and bulk compositions, and comparisons with experimental data suggest that the migmatites and dikes are partial melts of variably metasomatized amphibolites. Element-partitioning, phase-equilibrium, and fluid-inclusion data are compatible with partial melting of the amphibolites at P = ∼8–11 kbar and T = ∼640–750 °C, in the presence of a low-salinity aqueous fluid.

Although the Catalina metamorphism occurred at much shallower levels than those inferred for subduction-related magmatism, the process envisioned is one of multistage, slab-derived metasomatic contamination of the mantle in the “hanging wall” of a subduction zone.

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