167-8 Using geochronology and geochemistry to unravel the tectonomagmatic history of Cape Breton Island, Nova Scotia, Canada and its role in understanding the Appalachian-Caledonide orogen.

Session: Integrating Geochronology and Geochemistry to Decipher the Tectonic Evolution of Orogenic Belts

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Cape Breton Island (CBI) occupies a unique position in the Appalachian-Caledonide orogen, both geographically and geologically. Its highly compressed section through the orogen preserves four major components: the Laurentian Blair River Inlier, Ganderian Aspy and Bras d’Or terranes, and Avalonian Mira terrane. Its importance in Appalachian models, particularly related to the two Ganderian terranes, has not always been appreciated. Extensive work in the Cape Breton Highlands has revealed significant complexity in the Ganderian terranes that sheds light on processes and chronology of events in other areas. The Bras d’Or terrane consists of a complex assemblage of low- to high-grade metamorphic rocks with protoliths older than ca. 600 Ma. They were intruded and metamorphosed by voluminous arc-related, I-type dioritic to granite plutons formed in a subduction zone ca. 575-555 Ma. The Ediacaran to Early Cambrian evolution of Ganderia is well preserved in the Bras d’Or terrane and can be correlated with similar events in New Brunswick and Newfoundland. In contrast, the Aspy terrane contains abundant plutons and, locally, comagmatic volcanic rocks, with ages ranging from Early Ediacaran through Late Devonian that vary widely in petrology, geochemistry, and tectonic setting. The oldest units (ca. 625 Ma, host rocks unknown) are leucotonalite plutons with evolved epsilon Nd signatures. Continental arc-related I- and S-type granitoid plutons (ca. 570-560 Ma) can be correlated with parts of the Exploits terrane of west-central Newfoundland. Early Ordovician (490-475 Ma) dioritic and tonalitic plutons along with MORB-like metavolcanic rocks are interpreted to represent a back-arc basin setting, likely related to the Penobscottian event recognized in the Exploits terrane of central Newfoundland and New Brunswick.  Ordovician-early Devonian arc-related magmatic activity and metamorphism reflect the Salinian and Acadian orogenies, orogeny.  Voluminous collision-related S-type granitoid plutons (ca. 375 Ma) are present throughout the Aspy terrane and as stitching plutons in the Aspy-Bras d’Or boundary. The S-type plutons are coeval with but geochemically unrelated to bimodal volcanic rocks and comagmatic A-type granite/gabbro plutons in the southwestern part of the terrane. By ca. 365-360 Ma evolved I-type plutons were emplaced in an extensional tectonic environment. The increased understanding of the wide spectrum of tectonic and magmatic events in Cape Breton Island is in the process of being updated to better integrate into tectonic models of the whole orogen.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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