Geochronology of the southern Québec Dunnage zone, Canadian Appalachians – new U-Pb zircon data, synthesis and implications for the paleogeography and tectonic setting of the Taconian orogeny

Abstract

Ordovician oceanic crustal rocks of the peri-Laurentian Dunnage zone crop out for > 300 km in the southern Québec Appalachians and consist of ophiolites (Thetford-Mines, Asbestos, Lac-Brompton, and Mont-Orford), Ascot Complex volcanics, and sedimentary rocks of the Saint-Daniel Mélange and Magog Group. The Ascot Complex is the sole occurrence of an Ordovician volcanic arc in the Québec Appalachians and consists of volcanic rocks overlain by black phyllites of the Saint-Daniel mélange. Both the Ascot Complex and Saint-Daniel mélange are overlain by the Magog Group. Laurentian zircon inheritance is found in all these rocks but the nature of the basement over which the ophiolites were obducted, the Ascot complex was formed and accreted, and the overlying sediments have been emplaced has been recently questioned; some studies suggesting that it has a Gondwanan affinity. To confirm the affinity of these rocks, we conducted a new structural and geochronological study for the southern Québec Dunnage zone to propose an up-to-date vision of the tectonic evolution of the Québec Dunnage zone during the Taconian orogeny. We obtained a CA-ID-TIMS U-Pb zircon age of 505.99 ± 0.16 Ma for a syn-volcanic granitic intrusion of the Mont-Orford ophiolite, confirming its late Cambrian age. High-precision U-Pb zircon ages of 474–470 Ma from the Ascot Complex suggest that it belongs to a single volcanic edifice contaminated by Laurentian continental rocks. This explains better the age of the overlying Saint-Daniel Mélange and the detrital zircon ages found at the base of the Magog Group. Field relationships indicate that the ophiolites, the Ascot Complex and the Saint-Daniel Mélange were accreted to Laurentia from c. 475 to c. 462 Ma, before the deposition of the Magog Group. The Québec Dunnage zone units show a strong Laurentian inheritance and/or provenance, which indicate that the nature of their basement is Laurentian, and that the peri-Gondwana Moretown terrane outcropping further south in Vermont does not extend into the Quebec Appalachians.