Lithofacies architecture and paleoenvironment of a Paleoproterozoic volcano-sedimentary sequence: Insight into rift-related volcanism during supercontinent assembly

Abstract

The greenschist to lower-amphibolite facies, Proterozoic (Orosirian: ca. 1883–1848 Ma) Aillik Group in Labrador, Canada, comprises a well-preserved example of subaqueous to intermittently subaerial volcanism and sedimentation. The group consists of interlayered sedimentary, volcaniclastic, pyroclastic and bimodal volcanic units that exhibit well-preserved depositional and structural relationships. Cumulatively, these lithofacies significantly demonstrate the evolution of Paleoproterozoic volcanism during the assembly of the supercontinent Nuna. A minimum stratigraphic thickness of 14.9 km of the Aillik Group is exposed along the coast of Labrador and is divided into two successions. The western stratigraphic succession consists of subaerial to subaqueous sandstone, siltstone, conglomerate, resedimented tuffaceous sandstones, and tuffaceous conglomerate with lesser rhyolite flows and felsic tuff. The eastern stratigraphic succession is dominated by felsic tuff and rhyolite, lesser pillowed to massive mafic lavas, and in situ and resedimented mafic tuff. Mafic rocks are basaltic, whereas felsic rocks comprise high-silica rhyolites and tuffs. Primary subaqueous pyroclastic deposits are indicative of explosive activity, an observation also supported by the presence of accretionary lapilli and an abundance of lithic fragments in epiclastic deposits. The volcano-sedimentary lithofacies of the Aillik Group are consistent with that observed in modern intra-arc or arc-rift settings, and the succession is interpreted to have formed in a localized back-arc rifting environment within an overall complex convergent margin.