| Summary: | Recent advances in the understanding of petrogenetic processes at plate boundaries, coupled with improved trace-element and geochronological analytical techniques, permit a more sophisticated approach to paleogeographic reconstructions of ancient volcanic environments. This paper reviews published geochemical data on Cambro-Ordovician volcanic rocks of the Canadian Appalachians and uses these data to interpret the plate tectonic setting of the volcanic activity. Synthesis of the data suggests that some spatial relationships among the various volcanic suites may have been maintained with respect to the Laurentian and Avalonian continental margins despite considerable crustal shortening across, and transcurrent displacement parallel to, the Appalachian Orogen. Six tectonovolcanic environments are recognized, including passive-margin rift volcanic rocks, oceanic island volcanic rocks, sub-arc ophiolites, island-arc volcanic rocks, back-arc ophiolites, and continental-margin back-arc volcanic rocks. Passive-margin, rift-related volcanic rocks include Late Precambrian to Early Cambrian, within-plate, tholeiitic to transitional alkalic basalts and associated per alkaline rhyolites of the Sutton Mountains in Québec and the Cloud Mountain and Deer Lake are as of Newfoundland. These volcanic rocks were extruded onto Grenville crust along the northwestern margin of the Appalachian Orogen and, together with the Late Precambrian Long Range dyke swarm, indicate that a period of crustal extension, related to the formation of an ancient ocean basin, persisted for at least 60 million years. Sub-arc ophiolites include Early Ordovician ophiolite complexes from the Eastern Townships of Québec and Baie Verte Peninsula of northwestern Newfoundland. The presence of boninites and primitive-arc volcanic rocks suggests that these ophiolites represent the foundation to arc systems rather than ocean floor generated at a mid-oceanic ridge. Volcanic-arc suites in central Newfoundland range from Late Cambrian to Early Ordovician in age and ...
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