POLYPHASED METAMORPHISM ON THE SOUTH-EASTERN BORDER OF THE WAKEHAM GROUP, EASTERN GRENVILLE PROVINCE, QUEBEC

Recent mapping centred on La Romaine and Musquaro lake in the Eeastern Grenville Province reveals a significant extension of the Wakeham Group south-eastward from its main corpus. Both area share a lithological assemblage of intermediate to felsic volcanic rocks (lapillistone, lapilli tuf), amphibolite, metapelite, marble and exhalite-likes units (garnetite, aluminous gneiss, hornblendite, albitite) locally mineralised. These rocks are intruded by granitic gneiss, probably affiliated to the 1.5 Ga granitoids surrounding the Wakeham Group. At La Romaine, they host a regional train of recristallized metagabbro and anorthosite enclaves which abuts at the eastern and northern ends of the transect into pre-metamorphic but little deformed mafic layered intrusions with Ni-Cu and PGE potential. Metamorphic grades of Musquaro and La Romaine supracrustals increase southeastward as recorded by the metapelites AFM assemblages. Musquaro metapelites to the north host medium amphibolite-facies assemblage (biotite, muscovite), whereas southern assemblages are characterized by upper-amphibolite facies (biotite, muscovite, sillimanite, garnet). The breakdown of muscovite and the garnet-cordierite leucosomes in La Romaine metapelites, associated with amphibolite hosting in-situ orthopyroxene leucosomes record granulite-facies metamorphism. This finding suggests that supracrustal units were metamorphosed at different crustal levels from North to South. Moreover, the granulitic supracrustal units and the recristallized anorthosite enclaves are surrounded by granitic gneiss, presenting medium amphibolite facies assemblages. This observation suggest that metamorphism was polyphased with an early event recorded by the Wakeham Group prior to granitic intrusions. The metamorphism recorded by the granitic gneisses is associated with regional 1.0 Ga Grenvillian event. 1469 and 1495 Ma high-grade metamorphism have been recognised respectively in the Hard-Jaune terrane and in the "basement to the Wakeham Group" in the eastern Grenville Province. Consequently, a first metamorphic event, not clearly recognised in the main corpus of the Wakeham Group, has been observed in the La Romaine area and could be affiliated with Pinwarian high-grade metamorphism.