MINERAL CORONA FORMATION DURING HIGH-P RETROGRESSION OF GRANULITIC ROCKS, UNGAVA OROGEN, CANADA

Crystalline basement exposed in tectonic windows within the Ungava Oro gen records a polycyclic Archean granulite-facies to Paleoproterozoic amphibolite-facies history. Amphibolite-facies assemblages comprise ga rnet coronas around plagioclase, clinopyroxene or cummingtonite corona s on orthopyroxene, hornblende coronas on clinopyroxene +/- orthopyrox ene, sodic rims an calcic plagioclase, and/or titanite coronas on ilme nite. Petrographic observations and model reactions suggest that growt h of coronitic garnet is closely associated with amphibolization of tw o-pyroxene gneisses. Calcic plagioclase constitutes a key reactant in all garnet-producing reactions and possibly acted as a rate-controllin g phase. Multi-equilibrium thermobarometric calculations show good con vergence of possible equilibria in the amphibolite-facies rocks, indic ating that coronitic textures need not imply complete chemical disequi librium. P-T determinations for the amphibolite;facies gneisses beneat h the thrust Belt of Ungava Orogen are in the range 7.7-9.8 kbar and 5 85-723 degrees C. These values are consistent with prograde determinat ions from pelitic schists within the thrust belt. Estimates of water a ctivity cluster into two populations. High a(H2O) values are obtained for highly strained basement rocks adjacent to the thrust belt whereas low a(H2O) values are derived for orthogneiss samples which show no t hrust-related fabrics and are distal to the thrust belt.