Mineral Corona Formation During High-P Retrogression of Granulitic Rocks, Ungava Orogen, Canada

Crystalline basement exposed in tectonic windows within theUngava Orogen records a polycyclic Archean granulite-faciesto Paleoproterozoic amphibolite-facies history. Amphibolite-faciesassemblages comprise garnet coronas around plagioclase, clinopyroxeneor cummingtonite coronas on orthopyroxene, hornblende coronason clinopyroxeneorthopyroxene, sodic rims on calcic plagioclase,and/or titanite coronas on ilmenite. Petrographic observationsand model reactions suggest that growth of coronitic garnetis closely associated with amphibolitization of twopyroxenegneisses. Calcic plagioclase constitutes a key reactant in allgarnet-producing reactions and possibly acted as a rate-controllingphase. Multi-equilibrium thermobarometric calculations showgood convergence of possible equilibria in the amphibolite-faciesrocks, indicating that coronitic textures need not imply completechemical disequilibrium. P—T determinations for the amphibolite-faciesgneisses beneath the thrust belt of Ungava Orogen are in therange 7.7–9.8 kbar and 585–723C. These values areconsistent with prograde determinations from pelitic schistswithin the thrust belt. Estimates of water activity clusterinto two populations. High a_H2O values are obtained for highlystrained basement rocks adjacent to the thrust belt whereaslow a_H2O values are derived for orthogneiss samples which showno thrust-related fabrics and are distal to the thrust belt. KEY WORDS: corona; high-P retrogression; multi-equilibrium thermo-barometry ^*Corresponding author. Telephone: (613) 995-4935. Fax: (613) 995-9273. Internet: mstonge{at}cc2smtp.emr.ca