The northern Appalachian orogen preserves evidence of a complex history of amalgamation of terranes with both Laurentian and Gondwanan affinities. The Ganderian Bras d’Or terrane of central Cape Breton Island is not well represented elsewhere in the orogen and its relationship to other Ganderian terranes is enigmatic, particularly with respect to its pre-Neoproterozoic history. The Boisdale Hills and Kellys Mountain areas contain the oldest metamorphic rocks in the Bras d’Or terrane. Quartzite units in the Boisdale Hills have detrital zircon populations with ages ranging from 3.2 to ca. 1 Ga. Paragneiss units from the Kellys Mountain area contain Meso- to Neoproterozoic detrital zircons, in which the youngest grains indicate that the maximum depositional age is <600 Ma. The detrital zircon populations of rocks from both areas are consistent with Gondwanan provenance for the protoliths, most likely the Amazonian craton. New U–Pb dates for subduction-related dioritic to granodioritic plutons in the Boisdale Hills yielded ages of 560 to ca. 540 Ma. Sedimentary, bimodal volcanic and plutonic rocks from the Bourinot belt in the Boisdale Hills and related plutons in the Kellys Mountain area have ages of ca. 510–490 Ma and are interpreted to have formed during extension related to separation of Ganderia from Gondwana. The southeastern Bras d’Or terrane preserves rocks formed in Pan-African subduction zones on a former passive margin of Gondwana as well as rocks formed during the initial stages of rifting of Ganderia from Gondwana and the subsequent opening of the Rheic Ocean.

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