Enriched mantle components in Proterozoic continental-flood basalts of the Cape Smith foldbelt, northern Québec

Magmatism associated with ca. 2.04 Ga rifting of the Superior Province resulted in the formation of a succession of tholeiitic and alkaline basalts, as well as minor rhyolites, collectively known as the Povungnituk Group (Cape Smith foldbelt). The tholeiites are not typical continental-flood basalts (CFB) as they lack significant negative Nb and Ta anomalies, indicating that crustal contamination has not played a major role in their evolution. Although crystal fractionation along a gabbroic cotectic can account for much of the compositional variation in the Povungnituk basalts, it cannot explain the range of observed trace-element ratios, which is similar to that from tholeiitic to highly alkaline lavas observed in modern ocean–island–basalt suites. Variations of La/Sm and Nb/Zr indicates the presence of a highly enriched component and that mixing between this enriched component and a non-enriched end-member can account for the range of trace-element ratios in the Povungnituk lavas. Although the most enriched tholeiites are spatially associated with an alkaline volcanic center, enriched tholeiites are widely interspersed in the Povungnituk Group and are also found in other parts of the eastern Trans-Hudson orogen. The Paleoproterozoic mantle source appears to have contained a dispersed but variable enriched component that was responsible for the variation of degree of enrichment of the Povungnituk tholeiitic basalts.