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* Trond.Slagstad@ngu.no. Current address: Geological Survey of Norway (NGU), Leiv Erikssons vei 39, 7491 Trondheim, Norway

The southwestern Grenville Province in Ontario consists of Paleoproterozoic and Mesoproterozoic rocks that formed along the southeastern margin of Laurentia during a significant period of growth of the North American continent. The area investigated consists of high-grade ortho- and paragneisses whose protoliths formed between 1500 and 1350 Ma. Geochemical and geochronological data from the rocks investigated point to a period of arc and back-arc magmatism along the Laurentian margin between ca. 1500 and 1450 Ma. A-type charnockites and granites are temporally and spatially associated with the arc rocks, and have compositions interpreted to indicate derivation from a tholeiitic basaltic underplate and from crustal melting, respectively. This interpretation implies that the arc underwent extension during part of its history. Between ca. 1450 and 1430 Ma, magmatism apparently ceased in the study area and was temporally associated with high-grade metamorphism in the back-arc region, possibly suggesting a change from an extensional to a compressional tectonic regime, although other interpretations are equally likely. Few data exist for the period after ca. 1430 Ma; however, a number of geochronological studies from nearby parts of the Grenville Province suggest that arc magmatism may have moved to a more outboard position on the continental margin. At ca. 1360 Ma, bimodal rhyolitic and basaltic magmatism suggests a return to an extensional tectonic regime. The geochemical data generally support, but suggest refinements to, previously proposed models for the evolution of the southeastern Laurentian margin in the Grenville Province that may be applicable to other parts of the margin.

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