Archean ultrahigh temperature event in the northwestern Superior Province, Canada: Metamorphic evolution and tectonic implications of the Pikwitonei Granulite Domain

The Pikwitonei Granulite Domain (PGD), located in the northwestern Superior Province, is one of the largest Neoarchean high-grade metamorphic domains in the world, and is a key to understanding the Neoarchean crustal evolution of the Superior Province. Here we report results of a study on ultrahigh temperature (UHT) granulites with a sapphirine + quartz-bearing peak assemblage from the Sipiwesk Lake area in the PGD. Three stages of metamorphic assemblage development are recognized based on petrographic observations: Pre-peak stage is marked by garnet, sillimanite, K-feldspar, and biotite inclusions within sapphirine and orthopyroxene. Peak stage is charactered by the typical UHT associations of sapphirine + quartz and sapphirine + orthopyroxene. The retrograde stage is represented by the retrograde formation of cordierite, biotite and sillimanite in the matrix. Phase equilibrium modelling based on the bulk compositions of sapphirine-bearing granulites suggest that the rocks have experienced extensional UHT metamorphism in excess of 1035 degrees C at pressures of 7.9-9.0 kbar, followed by isobaric cooling process. SHRIMP U-Pb dating of metamorphic zircons in sapphirine-bearing granulite record the timing of the UHT event in the PGD with a weighted mean Pb-207/Pb-206 age of 2681 +/- 13 Ma. This study in combination with other metamorphic P-T paths and age information reveals that this UHT metamorphism at 2.68 Ga in PGD was generated by upwelling asthenosphere due to slab break-off in collision during the amalgamation of the Superior Province.