U-Pb zircon ages from metasedimentary and plutonic rocks in the Bras d'Or terrane of Cape Breton Island, Nova Scotia, Canada: insights into the Ediacaran–Cambrian tectonomagmatic evolution of Ganderia

Abstract

The Bras d’Or terrane of central Cape Breton Island, Nova Scotia, Canada, contains a well-preserved record of the Ediacaran to Early Cambrian evolution of Ganderia, a Gondwana-derived terrane in the northern Appalachian orogen. A complex assemblage of low- to high-grade metasedimentary rocks have varied detrital zircon signatures from LA-ICP-MS U-Pb zircon dating but combining 3 or more samples yielded representative age spectra which support correlation of the low- and high-grade metasedimentary rocks throughout the Bras d’Or terrane and the corresponding Ganderian Brookville terrane of southern New Brunswick. In quartzite samples from the McMillan Flowage Formation in the northwestern Bras d’Or terrane the youngest detrital zircons have ages >900 Ma, in contrast to previously studied psammitic and semi-pelitic samples from correlative units in the eastern Bras d’Or terrane in which youngest detrital ages are 620–600 Ma. Both quartzite and semi-pelitic samples from the McMillan Flowage Formation contain Neoproterozoic dates from zircon rims which reflect metamorphic overgrowths during peak metamorphism at ca. 550 Ma, providing a robust age for peak metamorphism in the Bras d’Or terrane that supports similar, albeit sparse, ages reported previously from monazite and titanite samples. This metamorphism is coeval with the emplacement of voluminous dioritic to granitic plutons that occur throughout the Bras d’Or terrane and formed in an Andean-type continental margin subduction zone. New U-Pb zircon ages presented here from plutons in the northern Bras d’Or terrane, combined with previously published ages, are consistent with subduction-related magmatism and associated metamorphism between ca. 575 Ma and 540 Ma.