Middle to Upper Ordovician volcanic rocks in the Arisaig area of Nova Scotia, Canada, constitute the only known record of volcanism in West Avalonia during that interval. Hence, they have been extensively studied to test paleocontinental reconstructions that consistently show Avalonia as a drifting microcontinent during that period. Identification of volcanic rocks with an intermediate composition (the new Seaspray Cove Formation) between upper Darriwilian bimodal volcanic rocks of the Dunn Point Formation and Sandbian felsic pyroclastic rocks of the McGillivray Brook Formation has led to a reevaluation of magmatic relationships in the Ordovician volcanic suite at Arisaig. Although part of the same volcanic construction, the three formations are separated by significant time-gaps and are shown to belong to three distinct magmatic subsystems. The tectonostratigraphic context and trace element contents of the Dunn Point Formation basalts suggest that they were produced by the high-degree partial melting of an E-MORB type source in a back-arc extensional setting, whereas trace element contents in intermediate rocks of the Seaspray Cove Formation suggest that they were produced by the low-degree partial melting of a subduction-enriched source in an arc setting. The two formations are separated by a long interval of volcanic quiescence and deep weathering, during which time the back-arc region evolved from extension to shortening and was eventually onlapped by arc volcanic rocks. Based on limited field constraints, paleomagnetic and paleontological data, this progradation of arc onto back-arc volcanic rocks occurred from the north, where an increasingly young Iapetan oceanic plate was being subducted at an increasingly shallow angle. Partial subduction of the Iapetan oceanic ridge is thought to have subsequently generated slab window magmatism, thus marking the last pulse of subduction-related volcanism in both East and West Avalonia.