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

The 4 km-thick Archean Normetal volcanic complex (NVC), composed of basalti c andesite, dacite, and rhyolite, is represented by five distinct volcanic phases and one sedimentary phase. Initial volcanic construction features ef fusive mafic volcanism characterized by massive, pillowed and pillow brecci a flows and local massive dacite (phases 1 and 2a). Prominent felsic volcan ism of phase 2 commences locally with tuffs, lapilli tuffs and lapilli tuff breccias derived either from hydroclastic or autoclastic fragmentation pro cesses (phase 2b). The principal constructive phase of the NVC (phase 2c) i s composed of pillowed andesite, massive dacire, 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 b reccia. 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 turb idites and pelagic background sediments, constitutes a marker horizon indic ating volcanic quiescence. Renewed volcanism of phase 5 is characterized by mafic to felsic turbiditic lapilli tuffs and tuffs, and mafic to felsic fl ows or intrusions. The felsic lapilli tuffs, tuffs and flows host the Norme tal VMS deposit. The geometry and volcanic stratigraphy of the NVC suggests emission of viscous, phenocryst-rich felsic flows from three principal cen ters, including a parasitic western vent, the major central 6 km-wide cauld ron structure and an eastern vent. Voluminous viscous felsic lava over a la rge area supports the inference of numerous vents whereby individual center s coalesced to produce a composite or complex stratovolcano. Proximal to di stal facies changes, variable rhyolitic unit and lobe closures argue for mu ltiple conduits. The VMS deposits are located at the western edge of the ce ntral cauldron. Geochemical analyses show two complete compositional spectr ums (phases 1, 2, 4 and 5) from basaltic andesite to rhyolite. The Zr/Y and La-N/Yb-N ratios of phases 1, 2, 4 and 5 show a transitional affinity wher eas phase 3 is tholeiitic to slightly transitional. Multi-element diagrams suggest that all phases are consistent with subduction-related processes. T he mafic-felsic NVC, a composite volcano that formed upon a shield type vol cano, displays subaqueous effusive dominant volcanic construction at depth below storm wave base, as indicated by pillowed flows, turbiditic and pelag ic sedimentary rocks, and massive sulphide deposits. Geochemistry and physi cal volcanology of the NVC are consistent with construction of an immature are volcano. The submerged Izu-Bonin are volcanoes may be modern analogues. (C) 2000 Elsevier Science B.V. All rights reserved.