Archean high-pressure metamorphism in the western Canadian Shield

The 'upper deck' of the 3200-2600 m.y. E Athabasca mylonite triangle in N Saskatchewan consists mainly of crustal rocks metamorphosed at >><$I> 15 kbar and 1000 degrees C. The T is constrained to > or = 1000 degrees C by homogenization thermometry of ternary feldspars in felsic gneisses and by garnet-clinopyroxene exchange thermometry in mafic rocks; the minimum P of 15 kbar is indicated by the presence of matrix kyanite in association with the high-T feldspars. Primary corundum in certain garnet-clinopyroxenites also indicates a P of > or = 15 kbar. Unusual mineral compositions such as Ti-rich muscovite, Cr-bearing kyanite and possible zircon exsolution from rutile are found in several locations; oriented rutile needles in garnet, interpreted as exsolution, are pervasive. The upper deck is a discrete metamorphic terrain, 400 km (super 2) in area and 10 km thick; elevation of this terrain into juxtaposition and fusion with granulite-facies lower crustal mylonites occurred within a 2600 m.y. intracontinental strike-slip shear zone. The upper deck is one of the very few high-P crustal metamorphic provinces known in the Americas and at 2600 m.y. is among the oldest such, worldwide.