Balmoral Beach Aboriginal shell midden, Port Jackson, Australia: pumice petrology and sources
Abstract
Palaeopumice strand deposits (1800 to 4000 years B.P.) occur in the Balmoral Beach Aboriginal Shell Midden excavations. In composition, the pumice clasts are low-potassium dacites and contain up to 5% xenocrysts, phenocrysts and glomerophenocrysts of orthopyroxene, pyroxene, plagioclase and iron oxide minerals. The matrix includes smaller grains of these minerals in an abundant, highly vesicular rhyodacite volcanic glass. The 3300 year old and younger pumices typically contain rare xenocrysts and phenocrysts of magnesian olivine, forming an unusual mineral assemblage. One older pumice clast is a quartz-bearing rhyodacite, with distinctive trace element geochemistry. The typical dacites, resemble those erupted from the Tonga-Kermadec island arc in the Southwest Pacific, due both to the presence of olivine and the variably depleted trace and rare earth element contents. This implies a 4000 km drift caused by the East Australian oceanic current, before the pumice was stranded in Port Jackson.
