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Acadian metamorphism in the Dunnage zone of southern Québec, northern Appalachians: ⁴⁰Ar/³⁹Ar evidence for collision diachronism

Alain Tremblay^*,1, Gilles Ruffet^,2 and Sébastien Castonguay^,1

¹ Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada
² UMR-Géosciences Azur, Centre National de Recherche Scientifique, Université de Nice-Sophia Antipolis, 06108 Nice Cedex 02, France

In New England, the Acadian orogen is divided into Western and Eastern metamorphic belts on the basis of contrasting pressure-temperature (P-T) paths, peak metamorphism conditions, and ages. Along-strike correlations of structures indicate that rock units affected by Devonian metamorphism and deformation in southern Québec belong to the Western Acadian belt, and share the same tectonic evolution as in New England. Muscovite ⁴⁰Ar/³⁹Ar ages obtained from greenschist-grade, Ordovician volcanic and plutonic rocks of the Ascot Complex in southern Québec indicate that Acadian metamorphism and deformation peaked ca. 380–375 Ma. Ordovician muscovite ages of ca. 462 Ma are locally well preserved in the granitic intrusion of the Ascot Complex, and provide constraints on the timing of island-arc plutonism in the Dunnage zone of southern Québec. Ordovician ⁴⁰Ar/³⁹Ar age spectra do not show important losses of radiogenic argon, indicating that Acadian metamorphism has been low grade, with temperatures below the Ar isotopic closure temperature proposed for muscovite. Acadian peak metamorphism is significantly younger, and P-T conditions of regional deformation have been lower in southern Québec than in amphibolite-grade rocks of the Western Acadian belt of New England. In the northern Appalachians, Acadian metamorphism and deformation are attributed to Devonian age crustal overthickening related to plate convergence. The comparison of Devonian tectonic fabrics in New England and southern Québec suggests that progression in the timing and P-T conditions of Acadian peak metamorphism in both areas are the result of the irregular geometry of the collision zone and of the northward migration of metamorphic and structural fronts along the orogen.

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