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

摘要

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.