ELSEVIER Precambrian Research 76 (1996) 93-113 Pre(umbriun Reseurth 550-580 Ma magmatism in Cape Breton Island (Nova Scotia, Canada)" the product of NW-dipping subduction during the final stage of amalgamation of Gondwana J. Dostal a, J.D. Keppie b, B.L. Cousens c, J.B. Murphy b "DepartmentofGeology, Saint Mary's University, Halifax, N.S. B3H 3C3, Canada b Department of Geology, Saint Francis Xavier University, Antigonish, N.S. B2G 2W5, Canada c Department of Earth Sciences, Carleton University, Ottawa, Ont. KIS 5B6, Canada Received 27 September 1994; revised version accepted 18 July 1995 Abstract Cape Breton Island is host to widespread magmatic activity dated between ~ 550 and 580 Ma. In the south, its products include the Capelin Cove pluton and host Coastal Volcanic Belt; both are composed of bimodal, tholeiitic--calc-alkaline suites that were emplaced on a thin sialic crust in a volcanic arc system. Their trace element and Nd isotope variations are consistent with fractional crystallization accompanied by lower crustal contamination in a compositionally zoned magma chamber. In contrast, the Indian Brook and Kerrs Brook plutons and Price Point volcanic rocks in central Cape Breton Island have higher contents of strongly incompatible trace elements and resemble modem arc-related calc-alkaline rocks emplaced on a moderately thick continental crust. Comparison with available data for other 550-580 Ma igneous rocks across Cape Breton Island indicates that they were produced above a NW-dipping subduction zone. This convergence was synchronous with the final stage of global- scale orogenic activity that culminated with the final amalgamation of Gondwana. Available detrital zircon ages in late Neopro- terozoic ( ~ 650-545 Ma) sediments in the Antigonish Highlands (Avalon Composite Terrane) suggest derivation from the Amazon craton. It is proposed that the Avalon Composite Terrane formed off northwestern South America during the late Neoproterozoic, 1. Introduction Neoproterozoic calc-alkaline igneous rocks are an important component of Pan-African and Brasiliano orogens. Those that occur in the Avalon Composite Terrane of Atlantic Canada (Fig. 1) have generally been correlated with northwest Africa (e.g., O'Brien et al., 1983). However, igneous activity in the Trans- Sahara orogen occurred between 800 and 600 Ma (Caby and Andreopoulos-Renaud, 1987), and between 700 and 580 Ma in the Reguibat Shield (Ducrot and Lancelot, 1978). In contrast, igneous activity in the 0301-9268/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI0301-9268 (96)00040-2 Avalon Composite Terrane lasted from ~ 700 Ma until ~550 Ma (Keppie et al., 1989; Dunning et al., 1990; Barr et al., 1990; Bevier et al., 1993). Such a span of Neoproterozoic calc-alkaline magmatism is also to be found in the Amazon craton (Teixiera et al., 1989) and is attributed to the final amalgamation of Gondwana. In this context, the 550-580 Ma magmatic arc rocks in the Avalon Composite Terrane probably formed during the closing stages of amalgamation of Gondwana. In southern Cape Breton Island, Neoproterozoic rocks occur in several fault-bounded blocks termed (from southeast to northwest) the Coastal, Stirling,