U–Pb ages of some crystalline rocks from the Burlington Peninsula, Newfoundland, and implications for the age of Fleur de Lys metamorphism

Abstract

U–Pb measurements on minerals from the Burlington Peninsula indicate that volcanic rocks of the Grand Cove Group and the probably cogenetic Cape Brule porphyry have primary crystallization ages of 475 ± 10 Ma (Early Ordovician). Later intrusive rocks, including the Dunamagon granite, the Burlington granodiorite and the Seal Island Bight syenite were intruded between 445 and 435 Ma ago. The Grand Cove Group and the Cape Brule porphyry completely predate deformation and metamorphism of the eastern division of the Fleur de Lys Supergroup; the Seal Island Bight syenite, Burlington granodiorite, and Dunamagon granite were evidently emplaced during the early stages of this orogeny. The new age results therefore suggest that the orogeny is Taconic in the classic sense. Some of the isotope systems show evidence of a strong thermal event about 340 Ma ago (Early Carboniferous, Acadian). This event may have caused resetting of some of the Rb–Sr isochron ages recently reported for this area.