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

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 modern 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 globalscale orogenic activity that culminated with the final amalgamation of Gondwana. Available detrital zircon ages in late Neoproterozoic (∼ 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.