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Dating polyphase deformation across low-grade metamorphic belts: An example based on ⁴⁰Ar/³⁹Ar muscovite age constraints from the southern Quebec Appalachians, Canada

Sébastien Castonguay^,1, Gilles Ruffet^,2 and Alain Tremblay^,3

¹ Geological Survey of Canada, GSC-Québec, 490 rue de la Couronne, Québec G1K 9A9, Canada
² Geosciences Rennes, CNRS-Université de Rennes 1, Campus de Beaulieu, Avenue Général Leclerc, Rennes Cedex 35042, France
³ Département des Sciences de la Terre et de l'Atmosphère, Université du Québec à Montréal, C.P. 8888, Succursale Centre-Ville, Montréal, Québec H3C 3P8, Canada

Geochronologic ⁴⁰Ar/³⁹Ar data of fabric-forming metamorphic minerals, in conjunction with structural and metamorphic studies, are being increasingly used to constrain the deformation and tectonometamorphic evolution of polyphase low-grade orogens. Careful data interpretation is needed to extract meaningful age constraints from neo- and recrystallized minerals affected by isotopic disturbances. In the southern Quebec Appalachians, the Sutton Mountains anticlinorium exposes the metamorphic core of the early Paleozoic continental margin of Laurentia. Orogenesis in this part of the Appalachians was the result of tectonic events that have been classically attributed to the combined effects of the Middle to Late Ordovician Taconian and the Middle Devonian Acadian orogenies; however, evidence of separate and distinct Silurian–Early Devonian tectonism has also been recently documented. Laser step-heating ⁴⁰Ar/³⁹Ar data on single-grains of muscovite from polydeformed greenschist facies samples of the Sutton Mountains anticlinorium indicate that these tectonometamorphic events are heterogeneously preserved as a prograde Taconian event at ca. 456 Ma and an Acadian overprint at ca. 390 Ma. The integration of ⁴⁰Ar/³⁹Ar age spectra analyses with structural relationships provides precise age constraints on the duration, propagation, and evolution of Silurian–Early Devonian hinterland-directed deformation as it migrated across the anticlinorium, including back thrusting from ca. 433 Ma to ca. 420 Ma and extensional faulting from ca. 417 Ma to ca. 405 Ma. Along the Laurentian margin of the northern Appalachians, such Silurian–Early Devonian tectonism is currently attributed either to the collapse of the Taconian orogen triggered by the delamination of the subducted slab or to the outboard accretion of peri-Gondwanan terranes during the Salinic orogeny.

Key Words: Quebec Appalachians • ⁴⁰Ar/³⁹Ar geochronology • polyphase deformation • deformation age • greenschist facies metamorphism • dynamic recrystallization