The Age of the Sept Iles layered mafic intrusion, Canada: implications for the Late Neoproterozoic/Cambrian history of southeastern Canada

The large Sept Iles layered mafic intrusion of eastern Canada has been precisely dated by U‐Pb zircon methods at 565 ± 4 Ma. Consideration of other magmatic events at that time in southeastern Canada reveals three different phases of magmatism. The first phase was composed of two tholeiitic dike swarms, at 615 Ma and 590 Ma. The second, including the Sept Iles intrusion, comprised a widespread series of mostly alkaline intrusions, emplaced around 575–565 Ma, along the rift faults of the St Lawrence graben. The last phase was composed predominantly of alkali and transitional basalts, now exposed in the Appalachians, erupted during the latest Neoproterozoic and early Cambrian. The second and third phases are best accounted for by the arrival of a major mantle plume near the town of Sept Iles 565 million years ago. The widespread, but volumetrically minor, alkali magmatism that peaked 10 m.y. earlier is typical of other plumes elsewhere. The plume model is reinforced by sedimentological evidence for a crustal dome centered near the town of Sept Iles that persisted for 80 m.y. The positions of the earlier dike swarms are problematical. They may represent vertical and/or lateral transport of magmas into the crust from earlier independent plumes or precursors to the Sept Iles plume