MIGMATITES FROM GRENVILLE, QUEBEC - METAMORPHIC P-T-X CONDITIONS IN TRANSITIONAL AMPHIBOLITE GRANULITE-FACIES ROCKS/

Migmatitic rocks near Grenville, Quebec, preserve features indicative of reactions at the onset of granulite facies metamorphism. In this ar ea, metapelites and metacarbonates of the classic Grenville Series are spatially associated with granitic gneiss and metabasite, and flank a Paleozoic, Fe-rich syenite stock. Near this intrusion, the metapelite is diatexitic and nearly devoid of biotite, indicating the involvemen t of biotite during melting in the contact aureole of this intrusion. Outside of the contact aureole, metapelites and associated rocks conta in biotite and are metatexitic. These features suggest two episodes of migmatization, the earlier predating the syenite, the later, synchron ous with this intrusion. Homblende-rich metabasites near the syenite c ontain a two-part neosome consisting of coarse-grained leucosome veins and patches that are enclosed by fine-grained, pyroxene-rich envelope s. Migmatization is attributed to dehydration melting in the presence of CO2-rich fluids possibly derived from nearby carbonate rocks prior to and/or during emplacement of the syenite. The occurrence of isolate d mafic clots in the mesosome and rarity of melanosome seams on leucos omes suggest that some melts were mobile on an outcrop scale. These ob servations suggest that the leucosomes formed by the segregation of me lts, which, coupled with CO2 flux, dehydrated the wallrock along narro w margins, forming the pyroxene-rich neosomes. Back-reaction with resi dual fluids led to the local scapolitization of plagioclase and the co ncomitant formation of coronal garnet on pyroxene in neosomes. Thermob arometry of corona structures within the contact aureole generates dif fusional Mg-Fe blocking temperatures (similar to 550 degrees C at 5.5 kbar). Extrapolated up-temperature, P-sensitive equilibria for the cor onas yield similar pressures (8-9 kbar) as texturally-equilibrated ass emblages for which high temperatures (similar to 750 +/- 50 degrees C; X-CO2 = 0.90-0.95) were determined for rocks sampled inside and outsi de of the contact aureole. This suggests that the Grenville migmatites had not been substantially decompressed by the time that the syenite was emplaced. (C) Elsevier Science B.V.