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

An exposure of sillimanite-rich, strongly deformed, stromatic, K-feldspar-bearing migmatites in the Monashee Terrane west of Revelstoke, British Columbia, has been examined to determine the process of migmatization and to evaluate whether the system was open or closed during leucosome formation. An anatectic origin for the migmatites is supported by: (I) the minimum melt composition of the leucosomes; (2) textures suggesting a fluid behavior of the leucosomes and local pegmatitic textures; and (3) P-T estimates (720-820 degrees C; 7 . 5-9 kbar) above vapor-absent melting conditions of muscovite + puartz. To establish whether melt was extracted or added during migmatization, measured volume percents of leucosome were compared with estimates of melt production modeled by muscovite + quartz dehydration melting. Quantitative estimates of volume percent of leucosome at present in the outcrop are between 20 and 30%. The amount of melt produced from the model muscovite dehydration melting reaction is constrained by measured modal percent of sillimanite (15-25%) in the outcrop and is dependent on modal proportion of muscovite in the unmelted protolith and the melt water content. Using a muscovite-rich protolith and a melt water content of 4 wt %, complete 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 % melt and similar to 23 vol % sillimanite, indicating a melt loss of 16 vol %. Melt loss may have occurred by melt movement along foliation planes during flattening, during formation of shear bands or locally along subvertical fractures. Spatial proximity of the outcrop to the Monashee decollement suggests that thrusting was localized to Zones of high melt production, which in turn facilitated melt migration.