MAGMATIC AND GEOTECTONIC EVOLUTION OF A PROTEROZOIC OCEANIC BASIN SYSTEM - THE CAPE SMITH THRUST-FOLD BELT (NEW-QUEBEC)

The Cape Smith Thrust-Fold Belt (1999-1840 Ma, Parrish, 1989) is divided into two tectonostratigraphic domains separated by the major east-west Bergeron fault. In the southern domain, the Povungnituk Group lies on the Archean craton of the Superior Province. It contains a sedimentary sequence (Lamarche sub-group) grading to a continental LREE-enriched tholeiitic basaltic sequence (Beauparlant sub-group, MgO < 10%, TiO2 = 1.2-3.6%) that is locally overlain by phyllites and alkaline volcanic rocks. The Chukotat Group to the north includes several sequences evolving from olivine-phyric komatiitic basalts (MgO = 19-11 %, TiO < 0.9%) to pyroxene-phyric tholeiitic basalts (MgO = 12.5 - 7%, TiO2 = 0.8-1.1%) with moderately enriched to depleted LREE profiles. Finally, in its northernmost part, the Chukotat Group contains several sequences of LREE-depleted tholeiitic plagioclase-phyric basalts (MgO < 8%, TiO2 = 1.3-2.8%). The northern domain presents: (1) a vast dismembered ophiolitic complex (Watts Group) with plutonic peridotitic to gabbroic rocks, sheeted dykes and associated LREE-depleted tholeiitic basalts (MgO = 9.8-5.7%, TiO2 = 0.3-2.2%); and (2) a volcano-sedimentary sequence of shales, siltstones and greywackes intercalated by basaltic to rhyolitic calc-alkaline lavas and volcaniclastites (Parent Group). With respect to their structural, petrological and geochronological characteristics, the two tectono-stratigraphic domains are interpreted as having resulted from a rifting process with (1) the formation of an early oceanic crust (the Purtuniq ophiolite) in the northern domain, and (2) the subsequent opening of the more southern Povungnituk-Chukotat basin. A comparison with present-day rifting processes suggests that the younger Povungnituk-Chukotat basin evolved by an asymmetric rifting model implying: (1) the creation of a fault-bound basins into which shallow water sediments (Lamarche sub-group) accumulated; (2) the formation of an ensialic proto-rift into which LREE-enriched tholeiitic basalts of the Beauparlant sub-group were emplaced, with local emission of alkaline volcanic rocks on volcanic islands; (3) the progressive opening of an oceanic rift and subsequent eruption of weakly LREE-enriched then depleted olivine- and pyroxene-phyric basalts of the Chukotat Group; and (4) the formation of an oceanic crust characterized by the LREE-depleted plagioclase-phyric basalts of the Chukotat Group. Finally, in response to a north-south compression, the northern Chukotat basalts were probably subducted beneath the northern domain, thus initiating the development of a magmatic arc system (Parent Group). © 1990.