Archean subqueous high-silica rhyolite coulées: Examples from the Kidd-Munro Assemblage in the Abitibi Subprovince

Abstract

Subaqueous high-silica rhyolite in Prosser Township, southern Abitibi Subprovince, Ontario, consists of intercalated, thin (<15 m; low aspect ratio < 0.3) aphyric lavas with brown felsic tuffs, overlain by a pyroclastic pumice- and spatter-rich tuff breccia and are capped by quartz–porphyritic rhyolite. The quartz–porphyritic flows contain ∼10% phenocrysts whereas ‘aphyric’ flows have only sparse phenocrysts, typically 1%. The aphyric flows are characterized by features that are typical of subaqueous rhyolite flows, including well developed flow-top breccia, basal peperite, flow-banding, and hyaloclastite dominated breccia. Mass balance calculations and textural analysis demonstrate that the high-silica aphyric rhyolites were originally glassy having been recrystallized to a pseudo-spherulitic quartz–albite mixture which was subsequently sericitized and chloritized. The REE were moderately mobile (10–40%) whereas the high field strength elements were less mobile (<10%). The major elements within the flow interior are shown to have been relatively immobile and represent a reasonable estimate of the protolith composition. The viscosity of the aphyric lava protolith was estimated to be 7.8 × 10 6 Pa s based on an eruption temperature of ∼850 °C calculated from measurements using the Ti-in-zircon saturation thermometer. This viscosity estimate is similar to the calculated viscosity (∼4 × 10 7 Pa s) of the subaerial Námshraun rhyolite flow, Iceland. The Prosser and Námshraun rhyolites have comparable dimensions and eruption temperature. It is concluded that the low aspect ratio of the Prosser Township flows may have been controlled by a number of environmental factors including paleo-slope, magma rheology, eruption rate and the insulative effects of the flow breccia.