DELINEATION AND CHARACTERIZATION OF TECTONIC DOMAINS IN THE WESTERN GRENVILLE PROVINCE - PRELIMINARY RESULTS OF GEOLOGICAL MAPPING ALONG HIGHWAY 117, QUÉBEC, CANADA

The Grenville Province is subdivided into the Allochthonous Belt (AB), composed of peri-cratonic to exotic rocks accreted to Laurentia that were deformed during the Ottawan (1090-1040 Ma) phase of the Grenvillian Orogeny, and the Parauthochtonous Belt (PB), composed of cratonic rocks reworked during the second, Rigolet (1000-960 Ma) phase. The AB is juxtaposed above the PB along the regionally continuous Allochthon Boundary Thrust (ABT).

Along HW 117 in western Quebec, which provides an orogen-perpendicular transect through the Grenville Province, the location of the ABT has not been determined and the extent of Grenvillian reworking within the PB is contentious. Fieldwork conducted in 2017 along the highway within undifferentiated PB and AB rocks has led us to distinguish various subdomains.

From north to south the first two subdomains encountered are characterized as: (1) Grt-bearing and (2) Opx-bearing (charnockitic) dioritic orthogneisses, in the NW and SE respectively. Both domains exhibit granulite grade assemblages and contain folded, stromatic migmatites and discrete shear zones. The presence of mafic dykes inferred to be pre-Grenvillian and a new U-Pb age from a late leucosome that yielded an Archean age, suggests that Grenvillian reworking in these domains was minimal. The third subdomain consists of regionally shallow dipping, amphibolite-grade orthogneisses and their migmatitic equivalents. Migmatites are abundant, displaying a range of textures from connected networks of patchy to stromatic, to nebulitic leucosomes, the latter of which, commonly contains large 10x meter-scale rotated mafic boudins. This migmatitic subdomain is overlain by HP-granulite (Grt-Cpx) mafic rocks of the Renzy Terrane, reportedly forming a tectonic klippe of the AB, which would imply it would be bounded by the ABT. The fifth subdomain is characterized by rocks with high magnetic susceptibility and the occurrence of variably deformed charnockites and amphibolite-grade gneisses with a subhorizontal, penetrative, and composite fabrics.

Modern geochronological techniques linking in-situ monazite and zircon dates with specific textures, structures and/or chemical composition associated with metamorphic reactions are underway to understand accommodation of deformation in this ancient orogen.