Mineral corona formation during high-P retrogression of granulitic rocks, Ungava Orogen, Canada

Abstract

Crystalline basement exposed in tectonic windows within the Ungava Orogen records a polycyclic Archean granulite-facies to Paleoproterozoic amphibolite-facies history. Amphibolite-facies assemblages comprise garnet coronas around plagioclase, clinopyroxene or cummingtonite coronas on orthopyroxene, hornblende coronas on clinopyroxene ±orthopyroxene, sodic rims on calcic plagioclase, and/or titanite coronas on ilmenite. Petrographic observations and model reactions suggest that growth coronitic garnet is closely associated with amphibolitization of twopyroxene gneisses. Calcic plagioclase constitutes a key reactant in all garnet-producing reactions and possibly acted as a rate-controlling phase. Multi-equilibrium thermobarometric calculations show good convergence of possible equilibria in the amphibolite-facies rocks, indicating that coronitic textures need not imply complete chemical disequilibrium. P-T determinations for the amphibolite-facies gneisses beneath the thrust belt of Ungava Orogen are in the range 7-7-9-8 kbar and 585-723°C. These values are consistent with prograde determinations from pelitic schists within the thrust belt. Estimates of water activity cluster into two populations. High aH2O values are obtained for highly strained basement rocks adjacent to the thrust belt whereas low aH2O values are derived for orthogneiss samples which show no thrust-related fabrics and are distal to the thrust belt.