Geothermobarometry of the Northwestern Margin of the Superior Province: Implications for Its Tectonic Evolution

Geothermometry and geobarometry of orthogneisses and metasediments across the Pikwitonei granulite facies domain of central Manitoba shows an increase of paleopressure from about 6 kbar near the orthopyroxene isograd on the east to 12 kbar near the western boundary against the Thompson Nickel Belt. The most consistent geobarometers are based on the assemblage garnet-orthopyroxene-plagioclase-quartz. Associated temperatures determined from various geothermometers are mainly in the range of 700-800°C, indicating, with the paleopressures, a steep dP/dT gradient during late Archean metamorphism. Metasedimentary assemblages within the Thompson Belt indicate amphibolite facies metamorphic conditions near 650°C and 4.5 kbar, which probably resulted from a mid-Proterozoic (Hudsonian) closure of a continental rift. A few outliers of granulite facies within the Thompson Belt record metamorphic conditions similar to those of the Pikwitonei domain and are probably relics which survived the later event. It is postulated that Archean crust of the Superior Province, bearing the effects of a profound granulite facies metamorphism, was thrusted westward in mid-Proterozoic time onto the Churchill craton. Subsequent isostatic uplift of the thickened crust has exposed a coherent crustal block with a considerable paleopressure variation, and smaller isolated uplifted blocks (the Thompson Belt relics). Water for hydrous retrogression of these blocks probably came from underthrusted Thompson Belt sediments.