Gneisses from the granulite terrane of the central Boothia Uplift, Arctic Canada

Orthopyroxene-bearing granitic (s. l.) gneiss and metamafic rocks and garne t-sillimanite paragneiss, which underwent granulite-facies metamorphism and intrusion by syenitic magma 1.9 Ga ago, constitute the bulk of the highly deformed crystalline core of the Boothia Uplift. The sample suite investiga ted is representative of the central part of the uplift, exposed in norther n Boothia Peninsula and southern Somerset Island. Two types of fluid inclus ions predominate in quartz of the gneisses: CO2 with admired methane or nit rogen, and brine. Temperatures and pressures calculated from garnet - ortho pyroxene -plagioclase - quartz and garnet - sillimanite - plagioclase - qua rtz equilibria mostly fall in the ranges 740 degrees-850 degrees C and 6-8 kbar. One sample of garnet-orthopyroxene tonalite gneiss has preserved a re cord of what may be peak or near-peak conditions of metamorphism: 960 degre es C and 8.7 kbar. Cordierite-bearing paragneisses yield less than or equal to 650 degrees C and <5 kbar, reflecting the retrograde formation of cordi erite during decompression and cooling. The Boothia-Somerset granulite terr ane probably represents an extension of the 1.9-2.0 Ga Taltson-Thelon oroge n, noted far its high-temperature metamorphism and magmatism, but is marked by a distinctive syntectonic intrusive suite and the presence of gneissic protoliths of earliest Proterozoic age.