Transformation of Fe-Ti gabbro to coronite, eclogite and amphibolite in the Baie du Nord segment, Manicouagan Imbricate Zone, eastern Grenville Province

Fe-Ti gabbros from the Bale du Nord Segment of the Manicouagan Imbricate Zo ne, metamorphosed under high P-T conditions during the Grenvillian orogeny, have been the focus of a detailed micropetrological study. Textures and mi neral chemistry suggest that the mineral assemblages represent progressive stages of metamorphic transformation resulting in the formation of coronas, pseudomorphs after igneous phases (transitional) and true, granoblastic ec logites. The transitional and eclogitic samples also have coronas which are developed locally around igneous xenocrysts of plagioclase and olivine. Th e deformed margins of coronitic Fe-Ti gabbros are transformed to amphibolit e and contain clinopyroxene-bearing leucosomes with garnet poikiloblasts th at are indicative of high-P-T dehydration melting. Interpretation of garnet zoning and thermobarometry suggest that the highest P-T conditions are rec orded by coronas around xenocrysts (c. 720-800 degrees C at 14-17 kbar) and garnet-clinopyroxene cores in granoblastic assemblages (c. 740-820 degrees C at 13-17 kbar) in the eclogitic samples. Re-equilibration during the ear ly stages of exhumation at high-T conditions (>700 degrees C) affected all samples, and is evidenced by the widespread development of pargasite-bearin g plagioclase collars in the coronitic and transitional metagabbros and by widespread re-equilibration of the eclogites giving lower P-T estimates at grain boundaries. However, the difference in calculated pressure conditions between coronite and eclogite samples is consistent with increasing pressu re (depth) from the coronites (11-13 kbar) to the eclogites (13-17 kbar). T he P-T conditions recorded by these rocks define a metamorphic field gradie nt which suggests high heat flow through the lower crust during the Grenvil lian orogeny.