Two Stage Felsic Volcanism in the Lower Proterozoic Upper Aillik Group, Labrador, Canada: Its Relationship to Syn- and Post-Kinematic Plutonism

Summary The Aillik Group, situated in the Lower Proterozoic Makkovik Province, Labrador, Canada, is a bimodal volcanic suite consisting of a lower mafic volcanicsedimentary sequence and an upper felsic volcanic-volcaniclastic association. These rocks have been intruded by syn-and postkinematic granitoid rocks. Previous mapping and stratigraphic interpretations suggest that the upper Aillik Group is itself divisible into ‘early’ and ‘late’ parts separated by a mafic volcanic/tuffaceous unit. Both involve dacitic to rhyolitic volcanism, but the earlier one has a much higher proportion of metasedimentary material and the later one is dominated by rhyolitic volcanic rocks. Major and trace element data suggest that these two sequences are compositionally distinct. The late sequence is compositionally similar to the postkinematic granites, but data are inadequate to establish a link between the early sequence and synkinematic granites, yet do not preclude it. Although geochronological data suggest c. 1800 Ma and 1660 Ma events in both the upper Aillik Group and granitoid plutons, a straightforward model linking early upper Aillik Group with synkinematic granites and late upper Aillik Group with postkinematic granites is complicated by c. 1800 Ma ages from some felsic volcanic rocks assigned as late upper Aillik Group. Inadequate stratigraphic control, insufficient petrochemical data and (or) inherited ages could account for the contradictions; or the model may be invalid. Detailed stratigraphic analysis and careful geochemical studies are required to resolve the conflicts.