SW Grenville Province, Canada: the case against post–1.4 Ga accretionary tectonics

Abstract

Seven accretionary sutures, formed between 1.16 and 1.03 Ga, have been identified by different authors in the Ontario–Quebec–Adirondack (OQA) segment of the Mesoproterozoic Grenville orogen in Canada. With one exception, the inferred accretionary terrane boundaries lie within, or at the margins of the Central Metasedimentary Belt (CMB), located between the Central Gneiss Belt and the Adirondack Highlands (Central Granulite Terrane). However, geological, geochronological, and petrological data suggest that the Grenville orogen on both sides of the proposed terrane boundaries (sutures) preserves a common 1.4–1.03 Ga tectonomagmatic history, inconsistent with its origin as a post-1.4 Ga collage of exotic tectonic blocks. Features which straddle the proposed 1.16–1.03 Ga ‘sutures’, from the Central Gneiss Belt, via the Adirondack Highlands, to the Mauricie area, include: (1) Mesoproterozoic continental crust (1.5–1.4 Ga) forming the host and/or basement to younger magmatic and supracrustal suites. (2) A 1.35–1.3 Ga continental arc, remnants of which occur from the CMB boundary zone (CMBBZ) in Ontario to the Appalachians in the United States, built on the 1.5–1.4 Ga continental crust. (3) Intrusions of 1.17–1.13 Ga age in the Central Gneiss Belt (mafic suite), and the Adirondack Highlands and their Quebec extension (AMCG suite, i.e. anorthosite massifs and related granitoids). (4) Relics of 1.18–1.14 Ga sedimentary basins in the northwestern CMB and the Mauricie area. We propose that an alternative model can adequately account for the observed geology of this part of the Grenville orogen wherein, the rocks of the OQA segment were part of an Andean-type margin between 1.4 and 1.2 Ga. At 1.35–1.3 Ga, a continental magmatic arc was built upon the southeastern margin of Laurentia represented by the 1.5–1.4 Ga Mesoproterozoic continental crust. The arc split at 1.3 Ga forming an ensialic back arc basin, relics of which now occur in the northwestern part of the CMB, and the back arc basin was flanked to the southeast by an active 1.28–1.25 Ga arc. Collision between the Laurentian margin and another continent (Amazonia?) occurred at 1.2 Ga, resulting in closure of the back arc basin and initiation of thrusting along the CMBBZ. Post-collisional lithospheric shortening led to convective removal of thickened subcontinental lithosphere, upper mantle melting, and extension of the overlying crust, resulting in widespread magmatic activity at 1.17–1.13 Ga, including emplacement of the AMCG massifs. Crustal extension generated sedimentary basins now represented by the St Boniface sediments in the Mauricie area (1.18 to between 1.15 and 1.09 Ga), and the penecontemporaneous Flinton Group in the northwestern CMB. Renewed, post-collisional, granulite facies shortening commenced at 1.12 Ga, manifested as nappes in the Central Gneiss Belt, and thrusting in the Mauricie area. Continued post-collisional shortening at 1.08–1.05 Ga was more localised, resulting in reactivation of thrusting in the CMBBZ, and initiation of the kinematically compatible Tawachiche shear zone along the eastern border of the Quebec extension of the Adirondack Highlands. The characteristics of the OQA segment of the Grenville orogen can all be accounted for in the context of: (1) a 1.4–1.2 Ga, southeast facing Andean-type margin to a Laurentian upper plate, associated with northwest dipping subduction; (2) continental collision at 1.2 Ga; and (3) subsequent, continued, post-collisional shortening, without invoking accretion of exotic terranes between 1.4 and 1.0 Ga.