STRUCTURAL AND MICROSTRUCTURAL ANALYSIS OF THE VALLANT ANORTHOSITIC SUITE, GRENVILLE PROVINCE, QUEBEC

The Vallant anorthositic Suite (U/Pb : 1148 Ma) is an anorthosite massif of the Grenville Province, located in the Baie-Comeau region of northern Québec. It is composed of anorthosite , leuconorite, leucotroctolite and troctolite facies and is genetically related to mangerite. The plagioclases range in composition from andesine to labradorite. The textures of the anorthosite facies are megacrystic and partially to strongly recrystallized. To the north, this suite is in tectonic contact with the Baie-Comeau Complex, a unit of migmatitic orthogneisses dated at ca. 1101 Ma (U/Pb). The contact between these two units dips southward and has been interpreted as a ductile thrust fault. To the south, the Vallant Suite is in contact, along a presumed normal fault, with the Varin plutonic Suite, a series of monzonites dated at 1059-1019 Ma (U/Pb zircon ages). To the west, the Vallant Suite is oriented east-west, parallel to the regional structural grain, but the orientation of the anorthosite massif is approximately north-south on the eastern side. In this sector, the Vallant Suite is surrounded by different lithological units, notably the Belinda plutonic Suite and Bignell Suite, which represents a monzodioritic to monzonitic sequences that appears as old suites.

A series of transects have been undertaken along the Vallant anorthositic Suite in order to define its internal geometry and determine the nature of its contact with the surrounding rocks. This work has confirmed the presence of a fault contact to the north, most likely in the form of a thrust fault. The southern contact shows important local shearing deformation but field observations suggest an intrusive contact between the Varin and the Vallant suites. Due to strong mineral recrystallization, macroscopic shear-sense indicators are difficult to observe. The microstructural analysis of oriented samples from different areas within the zone of contacts should aid with the characterization of the structures and determine the kinetics of deformation. In addition, it is likely that part of deformation observed in the Vallant anorthositic Suite is of syn-magmatic origin. The ongoing work also concerns the determination of the P-T conditions of regional metamorphism and the compositional variations of plagioclases within the anorthosite facies of this suite.