MAGMATISM AND METAMORPHISM IN A GRENVILLIAN FRAGMENT - U-PB AND AR-40AR-39 AGES FROM THE BLAIR-RIVER COMPLEX, NORTHERN CAPE-BRETON ISLAND,NOVA-SCOTIA, CANADA/

The Blair River Complex in northern Cape Breton Island, Nova Scotia, i s interpreted to be an exposure of Grenvillian basement which formed t he southeastern tip of a promontory on the proto-Atlantic continental margin of North America, U-Pb (zircon) data reported here confirm Gren villian ages for the major components in the Blair River Complex. The locally granulite facies Sailor Brook gneiss has a minimum protolith a ge of 1217 Ma and was metamorphosed at 1035 +12/-10 Ma. The Lowland Br ook Syenite has an igneous crystallization age of 1080 +5/-3 Ma, and t he Otter Brook granitic orthogneiss yielded an igneous crystallization age of 978 +6/-5 Ma, An igneous crystallization age could not be obta ined for anorthositic rocks in the Blair River Complex, but the Red Ri ver Anorthosite Suite was metamorphosed at 996 +6/-5 Ma. Silurian ther mal activity is recorded in the Blair River Complex by granitic magmat ism dated at 435 +7/-3 Ma and a widespread amphibolite-facies metamorp hic overprint of similar age, These events are corroborated by (206)pb /U-238 ages of ca, 423 Ma for titanite from seven widely distributed s amples from both Silurian and Proterozoic units, interpreted to repres ent cooling from igneous or peak-metamorphic conditions through temper atures of ca, 550-600 degrees C, Rutile in one sample gave a U-Pb age of 410 +/- 2 Ma, which may represent postmetamorphic cooling through 4 05 degrees C. The U-Pb cooling ages are supported by similar Ar-40/Ar- 39 ages of ca, 420 Ma for phlogopite and muscovite from rare calc-sili cate lenses in the Blair River Complex, and for hornblende from a Silu rian diorite intrusion, The widespread Silurian metamorphism of this i nlier is similar in age to that in western Newfoundland and indicates involvement of the Blair River Complex in Silurian orogenesis. Our dat a support the interpretation that the Blair River Complex represents L aurentian Grenvillian basement, analogous to basement exposures in the Humber Zone of western Newfoundland.