MELT EXTRACTION DURING FORMATION OF K-FELDSPAR PLUS SILLIMANITE MIGMATITES, WEST OF REVELSTOKE, BRITISH-COLUMBIA

An exposure of sillimanite-rich, strongly deformed, stromatic, K-felds par-bearing migmatites in the Monashee Terrane west of Revelstoke, Bri tish Columbia, has been examined to determine the process of migmatiza tion and to evaluate whether the system was open or closed during leuc osome formation. An anatectic origin for the migmatites is supported b y: (I) the minimum melt composition of the leucosomes; (2) textures su ggesting a fluid behavior of the leucosomes and local pegmatitic textu res; and (3) P-T estimates (720-820 degrees C; 7 . 5-9 kbar) above vap or-absent melting conditions of muscovite + puartz. To establish wheth er melt was extracted or added during migmatization, measured volume p ercents of leucosome were compared with estimates of melt production m odeled by muscovite + quartz dehydration melting. Quantitative estimat es of volume percent of leucosome at present in the outcrop are betwee n 20 and 30%. The amount of melt produced from the model muscovite deh ydration melting reaction is constrained by measured modal percent of sillimanite (15-25%) in the outcrop and is dependent on modal proporti on of muscovite in the unmelted protolith and the melt water content. Using a muscovite-rich protolith and a melt water content of 4 wt %, c omplete dehydration melting of muscovite results in a production of 54 vol % melt and 25 vol % sillmanite, indicating a melt loss of 29 vol %. A melt water content of 6 wt % results in production of 41 vol % me lt and similar to 23 vol % sillimanite, indicating a melt loss of 16 v ol %. Melt loss may have occurred by melt movement along foliation pla nes during flattening, during formation of shear bands or locally alon g subvertical fractures. Spatial proximity of the outcrop to the Monas hee decollement suggests that thrusting was localized to Zones of high melt production, which in turn facilitated melt migration.