Exogenous and endogenous construction of the subaqueous Glenwood felsic lava flow complex; Abitibi greenstone belt, Québec, Canada

Abstract

Both intrusive and extrusive volcanic facies of a felsic lava flow complex are exposed as the outcrops of the “Glenwood Rhyolite” within the Blake River Group of the Abitibi greenstone belt, Québec, Canada. Sub-vertically-dipping volcanic facies allows for the identification of the internal architecture of an Archean subaqueous felsic flow complex. Detailed facies analyses show volcanic facies and volcanic textures and structures atypical of subaqueous pyroclastic deposits or deposits of dome complexes. Additionally, a complex network of mafic dykes cross-cut the felsic facies. When integrated with facies-specific geochemical analyses, a coherent model for all volcanic and intrusive facies and supplemental information regarding the host volcanic complex has been developed.Detailed mapping reveals extrusive or “exogenous” aphanitic felsic volcanic facies and intrusive or “endogenous” quartz- and feldspar-phyric felsic volcanic facies. Exogenous facies are comprised of massive lobate, in situ brecciated, flow breccia and flow-front breccia facies, whereas the only visible endogenous facies is a quartz- and feldspar-phyric unit that intrudes the brecciated facies on the eastern side of the complex. All felsic facies demonstrate a west to east flow direction. Both exogenous and endogenous facies have similar chemical compositions ranging from dacite to rhyodacite with a transitional affinity. Mafic to intermediate dykes predominantly strike north–south and east–west with occasional east–northeast-trending units. Wispy terminations at some dyke margins and dismembered dykes indicate they were intruded into a still cooling viscoelastic medium. Geochemical analyses further defines that most east–west-trending dykes have a tholeiitic affinity, whereas north–south-trending dykes are dominantly calc-alkaline and east–northeast-trending dykes are transitional.The subaqueous Glenwood felsic flow complex was emplaced in a fault-bounded depression and consists of stacked exogenous dacite/rhyodacite lobes and intruded by a late quartz- and feldspar-phyric endogenous lobe. The intrusion of mafic dykes occurred relatively soon after the cessation of felsic activity, as evidenced by primary textures consistent with intrusion into a viscoelastic medium. Combined, the extrusive and intrusive facies of the complex attests to the complexity of the host volcanic environment.