Magmatism and metamorphism in a Grenvillian fragment: U-Pb and 40Ar/39Ar ages from the Blair River Complex, northern Cape Breton Island, Nova Scotia, Canada

Abstract

The Blair River Complex in northern Cape Breton Island, Nova Scotia, is interpreted to be an exposure of Grenvillian basement which formed the southeastern tip of a promontory on the proto-Atlantic continental margin of North America. U-Pb (zircon) data reported here confirm Grenvillian ages for the major components in the Blair River Complex. The locally granulite-facies Sailor Brook gneiss has a minimum protolith age of 1217 Ma and was metamorphosed at 1035 +12/−10 Ma. The Lowland Brook Syenite has an igneous crystallization age of 1080 +5/−3 Ma, and the Otter Brook granitic orthogneiss yielded an igneous crystallization age of 978 +6/−5 Ma. An igneous crystallization age could not be obtained for anorthositic rocks in the Blair River Complex, but the Red River Anorthosite Suite was metamorphosed at 996 +6/−5 Ma. Silurian thermal activity is recorded in the Blair River Complex by granitic magmatism dated at 435 +7/−3 Ma and a widespread amphibolite-facies metamorphic overprint of similar age. These events are corroborated by 206Pb/238U ages of ca. 423 Ma for titanite from seven widely distributed samples from both Silurian and Proterozoic units, interpreted to represent cooling from igneous or peak-metamorphic conditions through temperatures of ca. 550–600°C. Rutile in one sample gave a U-Pb age of 410 ± 2 Ma, which may represent postmetamorphic cooling through 405°C. The U-Pb cooling ages are supported by similar 40Ar/39Ar ages of ca. 420 Ma for phlogopite and muscovite from rare calc-silicate lenses in the Blair River Complex, and for hornblende from a Silurian diorite intrusion. The widespread Silurian metamorphism of this inlier is similar in age to that in western Newfoundland and indicates involvement of the Blair River Complex in Silurian orogenesis. Our data support the interpretation that the Blair River Complex represents Laurentian Grenvillian basement, analogous to basement exposures in the Humber Zone of western Newfoundland.