Earliest Silurian supra-subduction magmatism in central Cape Breton Island

Abstract

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 amphibole-bearing mafic rocks (gabbro and diorite) with quartz diorite occurring in the northern portion. U-Pb isotopic analyses of zircon give an age of 438 ± 2 Ma, i.e., earliest Silurian. The rocks are calc-alkaline with mantle-normalized patterns characterized by high LILE/HFSE (large-ion-lithophile elements/high-field-strength elements) ratio and pronounced negative Nb anomalies. The pluton is inferred to have intruded a supra-subduction zone setting. As such, it represents a southern extension of an Early Silurian arc that extends along the axis of the Cape Breton Highlands. These calc-alkaline rocks contrast with contemporaneous, tholeiitic rift-related volcanism that occurs in the Antigonish Highlands. A possible tectonic setting that explains both these suites suggests that oblique, sinistral south-dipping subduction began north of Cape Breton Island and changed westwards into a sinistral transform boundary, which induced rifting in mainland Nova Scotia.