ACADIAN OROGENESIS AND HIGH GRADE METAMORPHISM IN THE CENTRAL MOBILE BELT OF CENTRAL NEWFOUNDLAND

Early Devonian orogenic activity (Acadian ss) in the Newfoundland Appalachians generally did not lead to any significant metamorphism and plutonic activity, except in the southern and eastern parts of the Central Mobile Belt (Gander Zone and parts of Exploits Subzone). Here our mapping shows that medium to high-grade rocks comprising various combinations of staurolite, kyanite, sillimanite and garnet schist, garnet amphibolite, migmatite and granite plutons are thrust towards the west over low greenschist or sub-greenschist facies rocks from Cape Ray to central Newfoundland (c. 200 km). U-Pb TIMS ages of monazite from metamorphic rocks and U-Pb zircon ages from synmetamorphic plutons consistently yielded Lower Devonian ages between 420 and 411 Ma, whereas post-kinematic granite plutons gave ages of ca. 404 Ma and younger. In general, Lower Devonian plutons are concentrated in the medium to high-grade rocks and are relatively rare or absent elsewhere. Based on our investigations, regional correlations and geochronology, we propose that a significant part of the infrastructure of the Acadian Orogen was thrust westwards to its hinterland in a large-scale hot nappe that now has a Z-shaped sinusoidal shape parallel to the Hermitage flexure. This hypothesis is consistent with seismic data, which indicates a truncation of the northwest-dipping reflectors in the eastern Central Mobile Belt by reflectors associated with the proposed nappe. The Acadian infrastructure is bounded to the east by the Hermitage-Dover fault system, which separates low grade Avalon Zone rocks from the high grade Gander Zone. Although the exact tectonic cause of the Acadian Orogeny is not well constrained at present in the Northern Appalachians, it is either due to one collision between Ganderia and Avalon or to two rapid successive collisions involving both Avalon and Meguma.