Proterozoic subduction and temane amalgamation in the southwestern Grenville province, Canada: Evidence from ultrapotassic to shoshonitic plutonism Louise Corriveau* Department of Geological Sciences, McGill University, Montreal, Quebec H3B 2A7, Canada ABSTRACT  A late Grenvillian (1089-1076 Ma) subduction regime followed by terrane amalgamation  is postulated as the paleoenvironment of a 400-km-long belt of potassium-rich alkaline and  shoshonitic plutons in the Central metasedimentary belt of the southwestern Grenville prov- ince, Canada. Emplacement of the plutons postdates the regional metamorphism in the country  rock, but predates major shear zones that form a structural boundary for the plutonic belt. The  extent, timing, and magmatic affinities of the suite delineate the Gatineau domain within the  current Mont-Laurier terrane of Quebec and are compelling evidence for the allochthonous  nature of the Elzevir terrane and its extension eastward to Rideau Lake in Ontario and  northward into the Gatineau domain. The belt trends northeast, the emplacement ages are  younger to the southeast, and the magmatic affinities are those of island-arc ultrapotassic to  shoshonitic rocks. This is interpreted to reflect the existence of a southeast-dipping, northeast- trending subduction zone beneath the combined Elzevir-Gatineau terrane between 1089 and  1076 Ma. Subsequent to subduction, amalgamation of the Elzevir terrane to the other terranes  formed the Central metasedimentary belt, which then accreted and collided with the allochtho- nous polycyclic belt. These events provide evidence that the Ottawan orogeny commenced at  ca. 1090 Ma in the Central metasedimentary belt. A modern analogue of this plutonism and its  tectonic setting may have been the magmatism and arc-continent collision and subduction  setting of the Sunda arc, Indonesia.  INTRODUCTION  In the Central metasedimentary belt of the Grenville province of Canada, the identification of a 1089 to 1076 Ma, K-rich alkaline and sho- shonitic, intrusive suite provides a petrotectonic assemblage for further progress in our under- standing of the tectonic history of the Grenville province. The following questions arise concern- ing this magmatism: Are these plutons anoro- genic (Baragar, in Baer, 1974) or orogenic? Do they represent the waning of the Grenvillian orogenic cycle (Windley, 1986) or the initiation of a new orogeny? Does the Mont-Laurier ter- rane of Quebec (Rivers et al., 1989) exist? De- lineation of terranes, their allochthonous nature and paleotectonic environment, and the accre- tionary history of the Central metasedimentary belt are investigated herein; I also propose changes to current terrane boundaries in this belt and a revised tectonic model for the Ottawan orogeny. Moreover, the role of alkaline mag- matism as a tool in tectonic modeling is emphasized. CENTRAL METASEDIMENTARY  BELT  The Central metasedimentary belt (Wynne- Edwards, 1972; Fig. 1) is part of the allochtho- *Present address: Department of Geology, Univer- sity of Toronto, Toronto, Ontario M5S 1A1, Canada.  nous monocyclic belt and consists of supra- crustal rocks (ca. 1.3 to 1.2 Ga) and plutonic suites formed during the Grenvillian orogenic cycle (ca. 1.3-1.0 Ga; Moore and Thompson, 1980). The Central metasedimentary belt has been thrust onto the allochthonous polycyclic belt along the Central metasedimentary belt boundary zone; a stage of this thrusting has been dated at 1060 Ma (syntectonic pegmatites; van Breemen and Hanmer, 1986). The ages cited in this paper are U-Pb zircon or baddeleyite ages (with 2 a error ca. +2 m.y.). Currently the belt is divided from west to east into the Bancroft, Elzevir, and Frontenac terranes in Ontario, and the Mont-Laurier terrane in Quebec, and is separated from the Morin and Adirondack ter- ranes by the Labelle shear zone and the Carthage-Colton mylonite zone, respectively (Brock and Moore, 1983; Rivers et al., 1989). The Elzevir terrane currently extends from the Bancroft terrane to the Sharbot Lake mylo- nite zone (Davidson, 1986) and includes the Sharbot Lake block (Brock and Moore, 1983) between the Robertson Lake and the Sharbot Lake mylonite zones. The Elzevir terrane com- prises supracrustal sequences that contain tho- leiitic and calc-alkaline metavolcanic rocks (1287-1248 Ma; Davis and Bartlett, 1988), which were deformed, metamorphosed, and in- truded by calc-alkaline tonalitic plutons during the Elzevirian orogeny (Moore and Thompson, 1980). This orogeny has been interpreted as the collision of an island arc (the Central meta- sedimentary belt) with a continent to the east to form an Andean-type continental margin (Windley, 1986). Emplacement of 1240 Ma calc-alkaline and peralkaline granitic plutons (Heaman et al., 1986; van Breemen and Davidson, 1989; Lumbers et al, 1990) and continental sedimentation of limited extent (Flinton Group) took place after the Elzevirian orogeny. The regional metamorphism (Ottawan orogeny; Moore and Thompson, 1980) post- dates the 1240 Ma plutonism and deposition of the Flinton Group and ranges from greenschist to granulite facies. Postmetamorphic stocks (1-22; Fig. 1) and pegmatite and carbonatite dikes (Lumbers et al., 1990) appear to represent the last Proterozoic igneous activities in the ter- rane. Windley (1986) interpreted the post- Flinton regional metamorphism as reflecting terminal continental collision of the Central metasedimentary belt and the allochthonous polycyclic belt at ca. 1050-1000 Ma and attrib- uted the late intrusive suites to crustal thickening at terminal collision. This regional metamor- phism has recently been shown to be older than 1089 Ma (Corriveau et al., 1990) and distinct from the ca. 1070 to 1050 Ma (Davidson and van Breemen, 1988; McLelland et al., 1988) metamorphic episode in the Adirondack terrane and in the allochthonous polycyclic belt that represents the culmination of the Ottawan orog- eny in the southwestern Grenville province. The Frontenac terrane (currently east of the Sharbot Lake mylonite zone) is characterized by equilibration in the granulite facies and a lack of metavolcanic rocks and tonalite present in the Elzevir terrane. It contains abundant marble, quartzite, and metapelite (Davidson, 1986) and a granitic to gabbroic plutonic suite of 1177— 1165 Ma (van Breemen and Davidson, 1989; Marcantonio et al., 1988). The Mont-Laurier terrane (north of the Ottawa River; Fig. 1) has been previously correlated with the Frontenac terrane on the basis of the presence of amphibo- lite to granulite facies assemblages and the abundance of marble, quartzite, and metapelite (Davidson, 1986; Rivers et al., 1989). The Ban- croft terrane is distinguished by a belt of nepheline-bearing gneisses and a belt of carbon- atite dikes (1070-1040 Ma; Lumbers et al., 1990). 614 GEOLOGY, v. 15, p. 614-617, July 1990