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

Seven accretionary sutures, formed between 1.16 and 1.03 Ga, have been iden tified 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 t he Central Metasedimentary Belt (CMB), located between the Central Gneiss B elt and the Adirondack Highlands (Central Granulite Terrane). However, geol ogical, 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 straddl e the proposed 1.16-1.03 Ga 'sutures', from the Central Gneiss Belt, via th e 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 are, remnan ts of which occur from the CMB boundary zone (CMBBZ) in Ontario to the Appa lachians 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) Relies of 1.18-1.14 Ga sed imentary basins in the northwestern CMB and the Mauricie area. We propose that an alternative model can adequately account for the observe d geology of this part of the Grenville orogen wherein, the rocks of the OQ A segment were part of an Andean-type margin between 1.4 and 1.2 Ga. At 1.3 5-1.3 Ga, a continental magmatic are was built upon the southeastern margin of Laurentia represented by the 1.5-1.4 Ga Mesoproterozoic continental cru st. The are split at 1.3 Ga forming an ensialic back are basin, relies of w hich now occur in the northwestern part of the CMB, and the back are basin was flanked to the southeast by an active 1.28-1.25 Ga are. Collision betwe en the Laurentian margin and another continent (Amazonia?) occurred at 1.2 Ga, resulting in closure of the back are 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 activit y at 1.17-1.13 Cia, including emplacement of the AMCG massifs. Crustal exte nsion generated sedimentary basins now represented by the St Boniface sedim ents in the Mauricie area (1.18 to between 1.15 and 1.09 Ga), and the penec ontemporaneous Flinton Group in the northwestern CMB. Renewed, post-collisi onal, granulite facies shortening commenced at 1.12 Ga, manifested as nappe s in the Central Gneiss Belt, and thrusting in the. Mauricie area. Continue d post-collisional shortening at. 1.08-1.05 Ga was more localised, resultin g in reactivation of thrusting in the CMBBZ, and initiation of the kinemati cally compatible Tawachiche shear zone along the eastern border of the Queb ec extension of the Adirondack Highlands. The characteristics of the OQA segment of the Grenville orogen can all be a ccounted for in the context of. (1) a 1.4-1.2 Ga, southeast facing Andean-t ype margin to a Laurentian upper plate, associated with northwest, dipping subduction; (2) continental collision at 1.2 Ga; and (3) subsequent, contin ued, post-collisional shortening, without invoking accretion of exotic terr anes between 1.4 and 1.0 Ga. (C) 2000 Elsevier Science B.V. All rights rese rved.