Abstract

Reverse-oblique overthrusting of high-grade Port aux Basques Gneiss over the lower-grade, supracrustal Windsor Point Group along the Cape Ray fault zone occurred during late Silurian to early Devonian time and is attributed to major oblique collision between Gondwanan and Laurentian continental blocks. New U-Pb zircon, 40Ar/39Ar, and K/Ar data limit the timing of this orogenic event. The megacrystic granitic phase of the Cape Ray Igneous Complex and the Cape Ray tonalite have been dated at 488 ± 3 Ma and 469 ± 2 Ma, respectively. A black rhyolite from the Windsor Point Group yielded an Ordovician age (453 +5/−4 Ma). A pretectonic gabbro sill within the Windsor Point Group, dated at 424 +4/−3 Ma, is cut by the Windowglass Hill granite dated at 424 ± 2 Ma. Synkinematic hornblende and biotite from the Grand Bay Complex mylonites of the Port aux Basques Gneiss have 40Ar/39Ar ages of 407 ± 4 Ma and 403 ± 4 Ma, respectively, which is indicative of a late Silurian–early Devonian age for the amphibolite grade deformation. These ages are compatible with a previously determined 415 Ma monazite age, interpreted as the time of peak metamorphism in the Port aux Basques Gneiss, and suggests rapid exhumation. The east-west flexure of the Cape Ray fault zone represents a tear fault accommodating differential shortening between the Cape Ray fault zone and the correlative fault system in central Newfoundland. Mid-Devonian granites (386 ± 3, 384 ± 2 Ma) intervene between the thrusting events and a younger sinistral transcurrent movement. This deformation and magmatic history can be correlated throughout the northern Appalachians along other major fault boundaries.

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