Contrib Mineral Petrol (1990) 105 :473~485 Contributions to Mineralogy and Petrology (C) Springer-Verlag1990 1.1 Ga K-rich alkaline plutonism in the SW Grenville Province U-Pb constraints for the timing of subduction-related magmatism Louise Corriveau 1., Larry M. Heaman 2, Franco Marcantouio 3"~*, and Otto van Breemen 4 1 Department of Geological Sciences, McGill University, Montr6al, Quebec H3B 2A7, Canada 2 Department of Geology, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada 3 Department of Geology, McMaster University, Hamilton, Ontario LSS 4MI, Canada 4 Geochronology Laboratory, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Received August 30, 1989 / Accepted February 5, 1990 Abstract. U-Pb zircon and baddeleyite dating of six syen- itic stocks establishes that the ultrapotassic, potassic al- kaline and shoshonitic magmatism with island-arc affini- ties in the Central Metasedimentary Belt (CMB) of the southwestern Grenville Province, Canada took place be- tween 1089 and 1076 Ma, along a 400-km-long, north- east-trending plutonic belt. These ages indicate that ul- trapotassic rocks with arc affinities are not unique to the Phanerozoic. West to east emplacement ages along a northern and southern cross-section of this belt range from 1083__2 Ma (Kensington), through 1081_+2 Ma (Lac Rouge) to 1076_+~ Ma (Loranger) in the north, and from 1089_+~ Ma (Loon Lake) and 1088_+2 Ma (Calabogie), to 1076_+ 2 Ma (Westport) in the south. Al- though closely spaced in time, in detail these ages suggest a slight younging of this magmatic activity to the south- east. Integration of the geochronological data with the spatial extent and potassic character of the plutons shows that the K-rich alkaline suite is distinct from the nepheline-syenite belt of the Bancroft terrane and from the syenite-monzonite suite of the Frontenac terrane of the CMB, and it is considered to be a magmatic episode unique to the Elzevir terrane and its Gatineau segment. The timing and the postmetamorphic emplacement of these plutons indicate that the regional greenschist to granulite-facies metamorphism of the country rock (pre- cise age unknown) is older than 1089 Ma throughout the entire Elzevir terrane. The potassic magmatism is interpreted as the initiation of the 1090 1050 Ma Ottaw- an Orogeny in the Elzevir terrane; thus, the regional metamorphism in this terrane, previously assigned to the Ottawan Orogeny, is an earlier event. The contem- * Current address : Centre G6oscientifique de Qu6bec, Commission G6ologique du Canada, 2700 rue Einstein, C.P. 7500, Ste-Foy, Qu6bec, 6IV 4C7, Canada ** Current address: Lamont-Doherty Geological Laboratory, Pal- issades, NY ] 0964, USA Offprint requests to: L. Corriveau poraneous emplacememt of this postmetamorphic plu- tonic belt with Keweenawan volcanism is at variance with current tectonic models which consider the Kew- eenawan rift to be formed at the same time as regional metamorphism in the CMB. Introduction Proterozoic ultrapotassic, potassium-rich alkaline and shoshonitic plutonism with subduction-related affinities has been recognized among equant (ca. 40 km 2) zoned syenitic stocks in the Central Metasedimentary Belt (CMB) of the southwestern Grenville Province, Canada (Corriveau 1989). The interpretation of the magmatic affinity and tectonic setting of these plutons has been approached from two different perspectives. The alka- line character, the presence of nepheline syenite, the sub- circular shape of the plutons, their postmetamorphic em- placement and their occurrence in Quebec and in Ontar- io have led to the currently held views that the plutons are part of two typical alkaline suites, spatially distinct: the nepheline-syenite belt of southwestern Quebec (Cur- rie 1976) and the monzonite-diorite suite of Ontario (Lumbers et al. 1990). These stocks were regarded as late- to post-tectonic with reference to the culmination of the Grenville Orogeny (Wynne-Edwards et al. 1966; Windley 1986) or as anorogenic and formed in an intra- plate extensional environment (Baragar, in Baer 1974; Anderson 1983; Anderson and Burke 1983; Lumbers et al. 1990). Three critical aspects arise from the integrat- ed geological-geochemical-geochronological study of these plutons: 1) the plutons have felsic to ultramafic magma-types with primary ultrapotassic, potassic alka- line and shoshonitic affinities, 2) they have extreme en- richment in large-ion-lithophile elements and depletion in Nb, Ta and Ti typical of island-arc magmatism, and 3) they appear to be emplaced along a 400-km-long,