EMPLACEMENT OF THE LAC-SAINT-JEAN ANORTHOSITIC SUITE, GRENVILLE PROVINCE, CANADA

The Lac-Saint-Jean Anorthositic Suite (AMCG suite) (1160-1140 ma) consists of multiple injections of magma which together form a huge mass covering 20 000 km2. Its volume is constrained by two major systems of shear zones. The NE-SW system comprises three major sub parallel shear zones each of which is 200 to 300 km long (St-Fulgence, Pipmuacan, Chute des Passes). The second system is composed of several NNE-SSW trending shear zones (Peribonka river, Begin and Chabot megadykes). The NE-SW shear zones split the anorthositic mass into two domains, each with several lobes. Field works show that each lobe has its own compositional, textural, structural and metamorphic characteristics. We propose that these major structures have played an important role during the emplacement of the Lac-St-Jean Anorthositic Suite. These major structures were later reactivated during the Grenvillian orogeny resulting in the thrusting of the different lobes towards NW, followed by sinistral strike-slip movements. New geochronological data show that the majority of the felsic and intermediate intrusive rocks within the Lac-Saint-Jean Anorthositic Suite are not part of this AMCG suite but are contemporaneous with the adjacent St-Urbain (1082-1050 Ma) or Labrieville (1020-1010 Ma) AMCG suites. Furthermore, these intrusive rocks are mainly associated with major regional linemaments.