Ordovician and Silurian metamorphic cooling ages along the Laurentian margin of the Quebec Appalachians: Bridging the gap between New England and Newfoundland Available to Purchase

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.