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The voluminous (~6,600 km3) Late Ordovician to Silurian Topsails igneous suite is a diverse group of bimodal igneous rocks, consisting mainly of peraluminous to peralkaline, subsolvus to hypersolvus granites. It intrudes and overlies Early to Middle Ordovician subduction-related calc-alkaline intrusive and volcanic rocks. Petrogenetic modeling suggests the peralkaline phases formed by fractional crystallization from an initially peralkaline magma, while other phases formed by mixing with basaltic magma, represented by dikes, and subsequent fractional crystallization. Sidewall crystallization and wall-rock assimilation may have contributed to the present configuration of the suite. The A-type geochemical signature, a source-related characteristic, probably derives...

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