A model for the granulite-migmatite association in the Archean basement of southwestern Montana

Abstract

The Archean basement of the northern Gallatin and Madison Ranges is dominantly composed of migmatitic, tonalitic gneisses, with interlayered pelitic schists, quartzites, and metabasites. Field, petrographic, mineral chemistry, and structural studies indicate that this system evolved along a clockwise P-T-t path. Peak metamorphic temperatures of 680-750°C and pressures of 6-8 kbar do not permit large-scale melting by means of vapor-absent reactions. The contemporaneous formation of anatectic migmatites and granulites occurs through complementary petrogenetic processes. Local vapor-present melting in the gneisses provides a local reservoir for water, which facilitates continuous mineral reactions in the amphibolite-granulite transition. Infiltration of water-rich solutions is required to produce the extensive migmatization. These melts, in turn, facilitate melt- enhanced deformation along pervasive ductile shear zones. -from Author