Earliest Silurian supra-subduction magmatism in central Cape Breton Island
DOI:
https://doi.org/10.4138/2042Abstract
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. RÉSUMÉ Le pluton du mont Skye fait intrusion dans le gneiss du protérozoique tardif de Bras D'Or des collines Creignish, dans le centre de l'ile du Cap-Breton. Il est constitue de roches mafiques amphibolifères (gabbro et diorite) accompagnées de diorite quartzique dans la partie septentrionale. Des analyses isotopiques au U-Pb du zircon le situent è 438 ± 2 Ma, c.- à-d. à la fin du Silurien précoce. Les roches sont des roches calco-alcalines presentant des structures normalises mantéliques caractérisées par un rapport élevé d'ELGI/EICE (éléments lithophiles à gros ions/éléments à intensite de champ éleveé) et par des anomalies négatives de Nb prononcées. On suppose que le pluton fait intrusion dans le cadre d'une zone de supra-subduction. Il représente comme tel un prolongement méridional d'un arc du Silurien précoce qui s'étend le long de l'axe des hautes-terres du Cap-Breton. Ces roches calco-alcalines font contraste avec le volcanisme tholéiitique contemporain lié à une fracture qui est survenu dans les hautes-terres d'Antigonish. Une compaction tectonique possible expliquant ces deux corteges laisse supposer que la subduction oblique sénestre inclinee vers le sud a commence au nord de Pile du Cap-Breton et s'est transformee vers l'ouest en une marge de cisaillement senestre qui a produit une distension continentale dans l'intérieur de la Nouvelle-Écosse. [Traduit par la rédaction]Downloads
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