Evolution of a submerged composite arc volcano: volcanology and geochemistry of the Normétal volcanic complex, Abitibi greenstone belt, Québec, Canada

Abstract

The 4 km-thick Archean Normétal volcanic complex (NVC), composed of basaltic andesite, dacite, and rhyolite, is represented by five distinct volcanic phases and one sedimentary phase. Initial volcanic construction features effusive mafic volcanism characterized by massive, pillowed and pillow breccia flows and local massive dacite (phases 1 and 2a). Prominent felsic volcanism of phase 2 commences locally with tuffs, lapilli tuffs and lapilli tuff breccias derived either from hydroclastic or autoclastic fragmentation processes (phase 2b). The principal constructive phase of the NVC (phase 2c) is composed of pillowed andesite, massive dacite, and dominant massive, flow banded and lobate rhyolite flows. Autoclastic or hydroclastic brecciation of the former have produced rhyolitic tuff, lapilli tuff and lapilli tuff breccia. Rhyolitic volcanism continued with eruption of lava flows (phase 3) and the intrusion of dykes and felsic endogenous domes (phases 3 and 4). A subsequent 20–70 m-thick sedimentary unit, composed of volcaniclastic turbidites and pelagic background sediments, constitutes a marker horizon indicating volcanic quiescence. Renewed volcanism of phase 5 is characterized by mafic to felsic turbiditic lapilli tuffs and tuffs, and mafic to felsic flows or intrusions. The felsic lapilli tuffs, tuffs and flows host the Normétal VMS deposit. The geometry and volcanic stratigraphy of the NVC suggests emission of viscous, phenocryst-rich felsic flows from three principal centers, including a parasitic western vent, the major central 6 km-wide cauldron structure and an eastern vent. Voluminous viscous felsic lava over a large area supports the inference of numerous vents whereby individual centers coalesced to produce a composite or complex stratovolcano. Proximal to distal facies changes, variable rhyolitic unit and lobe closures argue for multiple conduits. The VMS deposits are located at the western edge of the central cauldron. Geochemical analyses show two complete compositional spectrums (phases 1, 2, 4 and 5) from basaltic andesite to rhyolite. The Zr/Y and LaN/YbN ratios of phases 1, 2, 4 and 5 show a transitional affinity whereas phase 3 is tholeiitic to slightly transitional. Multi-element diagrams suggest that all phases are consistent with subduction-related processes. The mafic-felsic NVC, a composite volcano that formed upon a shield type volcano, displays subaqueous effusive dominant volcanic construction at depth below storm wave base, as indicated by pillowed flows, turbiditic and pelagic sedimentary rocks, and massive sulphide deposits. Geochemistry and physical volcanology of the NVC are consistent with construction of an immature arc volcano. The submerged Izu-Bonin arc volcanoes may be modern analogues.