ND ISOTOPE MAPPING OF LARGE SCALE FOLDING AND THRUSTING IN THE LAC DUMOINE AREA OF THE GRENVILLE PROVINCE, WESTERN QUEBEC

The Allochthon Boundary Thrust (ABT) represents a major tectonic boundary within the Grenville province that separates crustal terranes that remained more-or-less stationary during the Grenville orogeny from those that underwent major lateral displacement. Despite its major significance as a tectonic boundary, the ABT is often hard to locate in the field, since it usually juxtaposes high grade grey gneisses on both sides. However, the ABT can be delineated by the use of Neodymium (Nd) model ages, since it usually juxtaposes terranes with distinct crustal formation ages. Defining the correct location of the ABT through Nd isotopic analysis will aid in the overall understanding of the orogenic history of the Grenville province. Thirty-four strategically chosen samples were used to map the ABT in the poorly understood Lac Dumoine region of southwestern Quebec, where prior investigation suggested the need for a revision in the regional mapping. Granitoid orthogneisses of the ‘stationary' Parautochthon yield Nd model ages >1.8 Ga, compared to granitoid orthogneisses of the laterally displaced Allocthonous Polycyclic Belt (APB) that yield Nd model ages < 1.8 Ga. Under these constraints, a newly identified salient of parautochthonous crust ~100 km long was found to cut into the centre of the allochthonous Lac Dumoine thrust sheet, separating it into two discrete lobes, which we refer to as the Lac Watson and Lac Bryson nappes. This evidence has important implications for the large-scale structure of the Grenville orogen, since it suggests that the main ramp for exhumation of deeply buried allochthonous crust during orogenic shortening was probably located about 100 km further to the southeast than previously proposed. The Lac Watson and Lac Bryson nappes are then relatively thin-skinned thrust sheets that were thrust sub-horizontally over the Parautochthon.