FLUID INCLUSIONS IN GARNET, MIGMATITIC PARAGNEISS, ALGONQUIN DOMAIN, GRENVILLE PROVINCE, ONTARIO

Fluid inclusions in garnet have recently been discovered in migmatitic paragneisses in the polycyclic Algonquin domain (McClintock subdomain), Central Gneiss Belt, Grenville Province, Ontario. The region was affected by upper amphibolite to granulite facies metamorphism at ca. 1450 Ma and ca. 1080-1050 Ma. The host rocks are metapelitic gneisses, locally associated with pink felsic gneisses of probable volcanic origin, amphibolite sheets and deformed dykes, megacrystic orthogneisses, and rare calc-silicate boudins. The metapelites contain the assemblage garnet + biotite + K-feldspar + plagioclase + quartz + rutile + ilmenite ± sillimanite. Leucosomes are strongly deformed and both leucosomes and mesosomes contain garnet. Garnet in some samples appears to have grown from a melt, while in other samples post-garnet deformation has obscured original textural relationships. P-T estimates from these rocks range from 750-850°C and 8-10 kbar. Two sets of fluid inclusions are recognized: 1) negative-crystal, monophase inclusions; and 2) irregular-shaped L-V inclusions. Both sets occur along discrete planes in garnets. Type 1 single-phase inclusions consist of nearly pure CO₂, based on thermometric (T_mCO₂=-57°C) and laser Raman analysis, whereas Type 2 inclusions are aqueous. The density of Type 1 inclusions is 0.876-1.028 g/cm³; isochores indicate maximum P of ca. 6 kb for 700°C entrapment. Work is in progress to determine the composition and homogenization conditions of the fluid in the Type 2 inclusions, and the relative and absolute times of garnet growth and fluid entrapment. Textural relationships between the two sets of inclusions and mineral inclusions in the same garnets suggest that they may have formed at about the same time, at or near peak metamorphic conditions. Monazite inclusions in the same garnets may allow the absolute age of garnet growth, and thus of fluid entrapment, to be established. The source of the CO₂-rich fluid and its role in granulite and migmatite petrogenesis have yet to be established.