GRANULITE-FACIES METAMORPHISM IN THE QUETICO SUBPROVINCE, NORTH OF MANITOUWADGE, ONTARIO

A zone of granulites, defined by an orthopyroxene-in isograd and exten ding more than 100 km in length and about 10 km in width, occurs near the southern margin of the Quetico Subprovince, north of Manitouwadge, Ontario. Mineral assemblages in metasedimentary rocks and associated migmatites consist of quartz, plagioclase, garnet, orthopyroxene, biot ite, cordierite, sillimanite, K-feldspar, hercynite, magnetite, ilmeni te, and other accessory phases. Minor mafic gneisses and calc-silicate pods or lenses are also present. From equilibrium phase relations and thermobarometry, the granulites experienced a thermal-peak event (4-6 kbar (1 bar = 100 kPa), 680-770 degrees C, a(H2O) of 0.15-0.25 and fO (2) of 1-2 log units above the FMQ buffer) in association with Da defo rmation, followed by a retrogression (550-660 degrees C and 3-4 kbar) and a later hydrothermal alteration (1-2 kbar and 200-400 degrees C). The distribution and calculated peak metamorphic conditions of the gra nulite zone in the Quetico Subprovince are similar to those of granuli tes in the English River Subprovince and other proposed accretionary t erranes. The low-pressure, high-temperature metamorphism in the Quetic o Subprovince is interpreted to be related to both crustal thickening and addition of heat from subduction-related magmatism.