Structures within large volume rhyolite lava flows of the Devonian Comerong Volcanics, southeastern Australia, and the Pleistocene Ngongotaha lava dome, New Zealand

Abstract

Abstract Many rhyolitic units within the Late Devonian Comerong Volcanics erupted as lava flows and domes, some up to 18 km long and 350 m thick. The textural and structural characteristics that distinguish the flows and domes as lava rather than rheomorphic ignimbrite include unbroken phenocrysts, zones of autobrecciation, and finely developed flow layering with individual layers continuous for several metres. The flow layering is typically contorted into isoclinal folds with forms suggesting fluidal deformation and is interlayered with and gradational into restricted zones of pumice-rich lapilli tuff and zones of lenticulite breccia. The lenticulite breccia comprises discontinuous, lenticular rhyolite fragments, the long axes of which define a foliation parallel to flow layering. Lenticles in the breccia vary from elongate layers up to 1 m long and several millimetres thick to short fragments less than 10 cm long and several centimetres wide. Similar zones of lenticulite breccia consisting of glassy lenticular clasts in a devitrified, spherulitic “matrix” of cristobalite and albite, exist within the Late Pleistocene Ngongotaha dome near Rotorua, New Zealand. The lenticulite breccia is considered to form by aqueous diffusion and selective devitrification of the rhyolite along anastomosing fluid paths and to be modified by mechanical fracturing of the lava in a zone of high shear stress. Geochemically the rhyolites are high-Si and A-type, with high Zr and Y contents indicating that they formed from high-temperature, relatively anhydrous, F-rich melts. A-type granitoids crop out intermittently along the length of, and adjacent to the volcanic complex and are comagmatic with the rhyolite. The high temperature, low bubble and phenocryst content, and a high eruptive rate of the rhyolite, likely resulted in a low effective viscosity and extensive flow units.