DECIPHERING TECTONOMAGMATIC EVENTS IN THE GANDERIAN ASPY TERRANE OF CAPE BRETON ISLAND, NOVA SCOTIA, CANADA: JUXTAPOSED EARLY EDIACARAN THROUGH DEVONIAN ARC AND WITHIN-PLATE PLUTONS

The Aspy terrane of north-central Cape Breton Island contains abundant plutons and, in some places, comagmatic volcanic rocks, with ages ranging from Early Ediacaran through Late Devonian. Aspy terrane plutons display wide range in petrological and isotopic characteristics, reflecting varying proportions of mantle melts and crustal inheritance in their petrogenesis, as well as differences in tectonic setting. The oldest dated units are ca. 625 Ma leucotonalite plutons with evolved epsilon Nd signatures and unknown host rocks. At ca. 570-560 Ma, I- and S-type granitoid plutons in the western part of the terrane record continental margin subduction, likely correlative with events of the same age recorded in the Bras d'Or terrane of Cape Breton Island and in the Exploits terrane of west-central Newfoundland. Early Ordovician (490-475 Ma) dioritic and tonalitic plutons and their host Late Cambrian low-grade metawacke and MORB-like metavolcanic rocks record igneous activity in a back-arc basin, likely representing the Penobscottian tectonomagmatic event recognized in the Exploits terrane of central Newfoundland and New Brunswick but not previously recognized in Cape Breton Island. Ordovician-Silurian plutons, now orthogneiss, and associated high-grade metamorphic rocks occur in the northwestern part of the terrane, in tectonic contact with Silurian low-grade volcanic rocks of the Sarach Brook and related formations, and widely distributed co-magmatic dioritic and granitic plutons; both of these contrasting associations may be manifestations of the Salinic orogeny but at different crustal levels. Widespread mid-to late Devonian plutons include S-type collisional plutons and associated pegmatite through the northern part of the terrane, coeval with but unrelated to bimodal volcanic rocks and comagmatic A-type granite/gabbro plutons in the southwestern part of the terrane. The youngest event is represented by the ca. 364 Ma Margaree pluton, largest in the Aspy terrane and formed in an extensional tectonic environment, following the orogenic activity and during regional uplift and cooling. Hence, plutonic rocks of the Aspy terrane apparently record both pre-Appalachian Ganderian orogenic events and the entire spectrum of Paleozoic Appalachian orogenic events.