Abstract

Geochronological studies on the timing of deformation and metamorphism along the Laurentian margin have shown that the ages of metamorphism events change along the strike of the Newfoundland–New England segment of the Appalachian orogen. The Quebec Appalachians represent a critical transition zone, where new single-grain laser 40Ar/39Ar plateau ages show that both Middle Ordovician and Silurian deformation and metamorphism are recorded. Parts of the analyzed samples give 40Ar/39Ar muscovite ages ranging from 468 to 461 Ma. These samples were taken within structural windows of the eastern internal Humber zone that are overlain by ophiolitic and melange rocks of the Dunnage zone. Other analyzed samples yielded Silurian 40Ar/39Ar muscovite ages clustering from 425 to 421 Ma. These samples were taken across the Notre Dame Mountains anticlinorium, which represents a structural and metamorphic dome within the internal Humber zone. The Middle Ordovician ages of southern Quebec are coeval with data from western New England and are interpreted as representing cooling shortly after the peak Taconian regional metamorphism. The Silurian ages, which have never been previously documented in the Quebec Appalachians, are comparable to those described in Newfoundland; however, they are not attributed to tectonism that shows characteristics of an orogen-scale continent-continent collision (Salinian orogeny), but rather those of late Taconian backthrusting and normal faulting or a period of crustal extension that followed the main Taconian crustal thickening.

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