TECTONIC SETTING AND REGIONAL SIGNIFICANCE OF THE PORT AUX BASQUES GNEISS, SW NEWFOUNDLAND

The 'Port aux Basques Gneiss' of southwest Newfoundland comprises thre e paragneiss and schist assemblages: the Grand Bay and Port aux Basque s complexes, and the Harbour le Cou Group. They are intruded by large volumes of basic sheets and coeval Middle Ordovician granitoids: the M argaree and Kelby Cove orthogneiss, which also cut a tectonic slice of earlier ultrabasic to basic rocks within the Grand Bay Complex: the B ig 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 silli manite and garnet-bearing, pyritiferous metasediments with amphiboliti c intrusions and meta-pillow basalts in its lower stratigraphic units. Geochemical studies have shown that metabasites of the Grand Bay Comp lex, Port aux Basques Complex, Margaree and Kelby Cove orthogneiss com prise Fractionated and crustally contaminated MORE-like and within pla te tholeiites, suggestive of an ensialic Okinawa type back-are basin. These have comparable characteristics to metabasites in the Harbour ie Con Group and suggest that these units comprise remnants of an extens ive Mid-Ordovician marginal basin developed within a promontory on the peri-Gondwanan continental margin. In contrast, the Big Barachois ass emblage in part comprises a distinct suite of pre-Mid-Ordovician Islan d Are Tholeiites.