U–Pb and 40Ar/39Ar mineral ages from Cape North, northern Cape Breton Island: implications for accretion of the Avalon Composite Terrane

Abstract

Isotopic studies have been used to determine protolith and tectonothermal ages in rocks of the Cape North and Money Point groups and two plutons, that form part of the Avalon Composite Terrane. Colourless, euhedral zircon crystals from low-grade metarhyolite in the Money Point Group record a concordant U–Pb age of 427.5 ± 4 Ma, which is interpreted to date the time of extrusion. This provides a maximum age of middle Silurian for polyphase deformation and concomitant greenschist–amphibolite-facies metamorphism that affected the Cape North area in the eastern half of a major, sinistral, positive flower structure. Monazite from the Cape North granite, intruded during peak metamorphic conditions, yielded nearly concordant data with a 207Pb/235U age of 414 ± 3 Ma. This dates the time of crystallization of the Cape North granite. Metamorphic hornblende from the Cape North and Money Point groups records variably discordant 40Ar/39Ar age spectra that define plateau and (or) isotope correlation ages of ca. 380–390 Ma. Muscovite from these units displays generally concordant spectra that define plateau ages ranging between ca. 365 and 375 Ma. Muscovites from the syntectonic Cape North granite and the posttectonic Relay Station granite record plateau ages of ca. 380–382 Ma. The 40Ar/39Ar ages suggest that relatively rapid postmetamorphic cooling occurred during the Devonian. These isotopic results and petrological data indicate that burial of the Cape North units to ca. 18 km, metamorphism, uplift, and erosion occurred in ca. 60 Ma. Such rates are comparable with those recorded in modern transpressive tectonic regimes. The calc-alkaline nature of the Money Point volcanic rocks suggests derivation above a subduction zone. A correlative Silurian–Devonian tectonothermal event recorded in southern Newfoundland probably records the accretion of the Avalon Composite Terrane with the Dunnage Zone.