Metamorphic History of the Archean Pikwitonei Granulite Domain and the Cross Lake Subprovince, Superior Province, Manitoba, Canada

Abstract

The Pikwitonei granulite domain and parts of the Cross Lake subprovince, located along the northwestern margin of the Archean Superior Province, expose an oblique cross-section through ∼20 km of Archean continental crust. The area has been investigated using phase equilibrium and geochronological techniques to derive quantitative pressure-temperature-time paths as a function of depth in the crust. Ages from metamorphic minerals indicate that metamorphism lasted at least from ∼2744 Ma to ∼259O Ma, but growth of garnet and zircon occurred only during short intervals at 2744–2738, 2700–2687, 2660–2637, and 2629–2591 Ma. Constraints from experimentally calibrated geobarometers and geothermometers and phase petrology indicate that ‘peak’ conditions for the last metamorphism, at ∼2640 Ma, were 575°C/3kb at Utik Lake, 750°C/7kb at Cauchon Lake, 830°C/7·5–8 kb at Natawahunan Lake, and 9 kb close to the Thompson mobile belt. High-grade metamorphism was associated with intrusion and possibly underplating of magmas that had temperatures in excess of 1100°C and contributed significant amounts of heat that promoted high-grade metamorphism. Mineral textures indicate that during prograde metamorphism, the terrane passed from the andalusite into the sillimanite stability field. After ‘peak’ metamorphism at ∼2640 Ma the terrane cooled nearly isobarically at a rate of 1–2°C/Ma. The observed characteristics of the amphibolite to granulite terrane are consistent with a model where metamorphism o…