Earliest Silurian supra-subduction magmatism in central Cape Breton Island

The Skye Mountain pluton intruded the Late Proterozoic Bras d'Or Gneiss in the Creignish Hills of central Cape Breton Island. It is composed of amphib ole-bearing mafic rocks (gabbro and diorite) with quartz diorite occurring in the northern portion. U-Pb isotopic analyses of zircon give an age of 43 8 +/- 2 Ma, i.e., earliest Silurian. The rocks are calc-alkaline with mantl e-normalized patterns characterized by high LILE/HFSE (large-ion-lithophile elements/high-field-strength elements) ratio and pronounced negative Nb an omalies. The pluton is inferred to have intruded a supra-subduction zone se tting. As such, it represents a southern extension of an Early Silurian are that extends along the axis of the Cape Breton Highlands. These calc-alkal ine rocks contrast with contemporaneous, tholeiitic rift-related volcanism that occurs in the Antigonish Highlands. A possible tectonic setting that e xplains both these suites suggests that oblique, sinistral south-dipping su bduction began north of Cape Breton Island and changed westwards into a sin istral transform boundary, which induced rifting in mainland Nova Scotia.