The ‘Port aux Basques Gneiss’ of southwest Newfoundland comprises three paragneiss and schist assemblages: the Grand Bay and Port aux Basques complexes, and the Harbour le Cou Group. They are intruded by large volumes of basic sheets and coeval Middle Ordovician granitoids: the Margaree and Kelby Cove orthogneiss, which also cut a tectonic slice of earlier ultrabasic to basic rocks within the Grand Bay Complex: the Big Barachois assemblage. The Grand Bay and Port aux Basques complexes are juxtaposed by the Late Silurian Isle aux Morts Fault Zone against the Harbour le Cou Group to the east, which dominantly comprises sillimanite and garnet-bearing, pyritiferous metasediments with amphibolitic intrusions and meta-pillow basalts in its lower stratigraphic units.
Geochemical studies have shown that metabasites of the Grand Bay Complex, Port aux Basques Complex, Margaree and Kelby Cove orthogneiss comprise fractionated and crustally contaminated MORB-like and within plate tholeiites, suggestive of an ensialic Okinawa type back-arc basin. These have comparable characteristics to metabasites in the Harbour le Cou Group and suggest that these units comprise remnants of an extensive Mid-Ordovician marginal basin developed within a promontory on the peri-Gondwanan continental margin. In contrast, the Big Barachois assemblage in part comprises a distinct suite of pre-Mid-Ordovician Island Arc Tholeiites