LATE PROTEROZOIC TECTONIC MODEL FOR THE AVALON TERRANE IN MARITIME CANADA

The Avalon Terrane in Maritime Canada extends from Cape Breton Island through northern Nova Scotia (Antigonish and Cobequid highlands) to southern New Brunswick. The terrane contains abundant late Proterozoic volcanic rocks (approximately 570-680 m.y. old). The geochemistry of the volcanic rocks of southern Cape Breton Island and southern New Brunswick indicates that they were erupted in a volcanic arc environment. In both regions the mafic rocks display a remarkably similar compositional zonation which resembles the across-arc variations observed in modern volcanic arcs. The progressive compositional changes that include a transition from island arc tholeiites along the southeastern coasts to calc-alkaline rocks inland to the northwest are probably due to a northwest dipping subduction zone with a trench located to the southeast. In the Antigonish and Cobequid highlands, continental tholeiites and calc-alkaline volcanic rocks are interbedded and overlain by turbidites that have probably been deposited in a back arc or intra-arc environment. The superimposed positive gravity and magnetic anomalies that may be traced discontinuously from the Grand Banks through Nova Scotia (Collector Anomaly) and along the northern side of the Gulf of Maine to near Boston, probably represent late Proterozoic ophiolitic remnants preserved by incomplete suturing along a late Proterozoic plate boundary. Northeast trending segments are inferred to mark subduction zones, and the E-W portions could correspond to transform faults. Thus the present geographical distribution of the Late Proterozoic volcanic rocks is explained by a tectonic model involving a single subduction zone offset by two late Proterozoic transform faults located approximately along the Minas Fault and the Scatarie Ridge.