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 from a crustal protolith from which I-type granitic melts had been previously extracted. Large, generally barren, A-type suites of Topsails type should be distinguished from highly fractionated granites with A-type chemical characteristics. Such fractionated granites, which are often mineralized, are volumetrically insignificant phases of large I- and S-type granite suites.

The Topsails suite formed during a complex collisional event that produced virtually continuous magmatism between Early Ordovician and Silurian time. The suite probably reflects partial melting of depleted continental crust induced by emplacement of mafic magmas under tensional conditions into a continental plate overriding a subduction zone. Thus this A-type suite indicates that classic theory of alkaline magmatism in rifted continental blocks can be applied to orogenic settings.