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Tectonometamorphic evolution of the southern Quebec Appalachians: ⁴⁰Ar/³⁹Ar evidence for Middle Ordovician crustal thickening and Silurian–Early Devonian exhumation of the internal Humber zone

¹ Geological Survey of Canada, Centre Géoscientifique de Québec, C.P. 7500, Sainte-Foy, Quebec G1V 4C7, Canada
² Unité Mixte de Recherche-Géosciences Azur, Centre National de la Recherche Scientifique–Université de Nice–Sophia Antipolis, 06108 Nice Cedex 02, France
³ Institut National de la Recherche Scientifique–Géoressources, Centre Géoscientifique de Québec, C.P. 7500, Sainte-Foy, Quebec G1V 4C7, Canada
⁴ UMR-Géosciences Azur, Centre National de la Recherche Scientifique–Université de Nice–Sophia Antipolis, 06108 Nice Cedex 02, France

Published age data from the Humber zone of the southern Quebec Appalachians indicate that deformation and metamorphism occurred during Middle to Late Ordovician time. This paper presents the results of a regional-scale geochronological study of 70 amphibole and mica single grains by the ⁴⁰Ar/³⁹Ar laser step-heating method, which brings forward a more complete Middle Ordovician–Early Devonian tectonometamorphic evolution of the Quebec Appalachian Humber zone. Biotites have plateau ages varying between 420 and 411 Ma. Amphiboles are often compositionally rimmed and/or contain biotite inclusions. Their age spectra are generally disturbed, but may suggest the occurrence of two distinct events: Silurian (429–424 Ma) and Middle Ordovician (462–460 Ma). Two main groups of muscovite ages have been obtained: Middle Ordovician (469–461 Ma) and Silurian–Early Devonian (431–411 Ma), but Middle to Late Devonian (387–376 Ma) ages are also locally present. Middle Ordovician ages are interpreted to record crustal thickening by nappe emplacement during regional Taconian metamorphism. Silurian–Early Devonian ages are attributed to consequences of backthrusting and normal faulting. A statistical analysis of Silurian–Early Devonian muscovite age spectra reveals an irregular decrease in weighted apparent age maxima across the internal Humber zone. This age-decreasing trend is interpreted to result from the temporal (ca. 431–411 Ma) and possibly spatial (northwest to southeast) progression of deformation, recrystallization, and fluid circulation during tectonic exhumation of the internal Humber zone in southern Quebec. Tectonic models for the Northern Appalachian internal Humber zone should take into account the strong body of evidence for Silurian–Early Devonian hinterland-directed and extensional deformation as documented in southern Quebec.

Key Words: Ar/Ar • extension tectonics • Northern Appalachians • Quebec • Silurian • Taconian orogeny

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