AGE, METAMORPHISM AND TECTONIC SIGNIFICANCE OF SYN-OROGENIC MAFIC ROCKS FROM THE MANICOUAGAN SECTION (NE GRENVILLE PROVINCE)

New U-Pb and metamorphic data on grenvillian age mafic intrusions add constraints on the tectonothermal evolution across a crustal section through the orogen’s hinderland, exposed in the Manicouagan area. The lower levels of the section (Manicouagan Imbricate Zone, MIZ) tectonically overlie parautochthonous units to the north, and include tectonic slices of 1650 Ma and 1450 Ma old mafic rocks. MIZ was metamorphosed under high-T eclogite facies conditions at ~1050-1030 Ma, during the Ottawan event of the Grenvillian orogeny. The middle levels (Canyon terrane, CT) consist of migmatite and ca.1440 Ma diorite metamorphosed under mid-P granulite facies conditions at 1040 Ga (new ages), i.e. at the same time as MIZ. The structurally highest Hart Jaune terrane (HJT) crosscuts the MIZ-CT contact and consists of 1450 Ma granulites. Two types of broadly syn-orogenic (~1.2-1.0 Ga) mafic intrusions were identified. All levels are intruded by ~1250 to 1160 Ma Fe-Ti gabbro and anorthosite (new ages for CT and HJT). These rocks are eclogitized in MIZ, and transformed to mid-P granulite in CT, in accordance with the intensity of the Ottawan event in each of the two units. In contrast, they lack high-T overprint in HJT. Fe-Ti mafic dykes, dynamically recrystallized to mid-P granulite, occur in major shear zones in both MIZ and CT; they are interpreted to be of Ottawan age based on structural relations. In CT, in addition, a gabbro dyke with ophitic texture yielded a crystallization age of 1040 Ma, coeval with the Ottawan event in the area. Broadly syn-metamorphic mafic intrusions and high metamorphic temperatures achieved at the same time in lower and middle levels of thick crust, imply fast heat transfer from the mantle, aided by transfer of mafic magmas, before advanced post collisional thinning. Lack of high-T Grenvillian overprint in HJT and structural relations with the underlying units, suggest that HJT was juxtaposed with the rest late in the orogenic evolution, and was at high structural levels throughout the Grenvillian orogeny.