High-pressure, high-temperature rocks from the base of thick continental crust: Geology and age constraints from the Manicouagan Imbricate Zone, eastern Grenville Province

The Grenville Province of the Canadian Shield is composed of crustal material with Laurentian affinities, involved in continental collision during the Grenvillian Orogeny between 1.19 and 0.98 Ga. In the eastern Grenville Province,the Manicouagan Imbricate Zone (MIZ) is composed of Paleoproterozoic (Labradorian, similar to 1.6 Ga) and Mesoproterozoic (Pinwarian, similar to 1.4 Ga) crustal segments that were metamorphosed under high-pressure, high-temperature (high-P-T) conditions (P similar to 1800 MPa and 800 degrees<T<900 degrees C) and intruded by synmetamorphic gabbroic stocks and dykes and postmetamorphic granite, during a similar to 1.0 Ga crustal shortening event in the Grenvillian Orogeny. The gabbroic dykes display high-P mineral assemblages and a withinplate tholeiite signature attesting to the intrusion of asthenospheric melts during the high-P metamorphism. The structural configuration of MIZ is that of a thrust stack (Lelukuau terrane) overlain by an extensional assembly of slices (Tshenukutish terrane). To the north, Lelukuau terrane overlies Archean basement and Paleoproterozoic supracrustal rocks of the Gagnon terrane along a thrust contact. Tshenukutish terrane is tectonically overlain to the south by crustal slices composed of pre-Grenvillian and early Grenvillian lithologic units that are correlative with those in MIZ, but which were at midcrustal levels during the Grenvillian Orogeny (low-P segment of the eastern Grenville Province), and were intruded by anorthosite and granitoids farther south. A likely tectonothermal evolution of MIZ at similar to 1.0 Ga involves crustal shortening by imbrication in a postsubduction phase of continental collision, followed by (convective ?) removal of the underlying thickened lithospheric mantle and the rise of hot asthenospheric material close to the base of the crust. Exhumation of MIZ likely occurred from the basal levels of the crust and was achieved by (1) NW directed thrusting over a crustal-scale ramp (Archean basement of the Gagnon terrane) with coeval extension at the higher levels of the pile (stage 1 extension) and (2) a second extension event (stage 2 extension) coeval with emplacement of postmetamorphic granite at the boundary between MIZ and the low-P segment. Copyright 1998 by the American Geophysical Union.