The Eastern Australian Volcanic Province, its primitive melts, constraints on melt sources and the influence of mantle metasomatism

Abstract

The Eastern Australian Volcanic Province (EAVP) is a ∼ 3800 km discontinuous belt of Cenozoic mafic intraplate volcanoes along the eastern seaboard of Australia. EAVP volcanic centres contain mantle-derived melts that show minor fractionation but have large ranges in major element concentrations. Current lava field and central volcano classifications of basaltic centres do not capture geochemical trends, and primitive melt compositions throughout the EAVP cannot be explained by partial melting of mantle peridotites alone. Based on a collated dataset of 3971 samples with chemical and spatial data, we classify its volcanism and identify mantle source assemblage compositions that give rise to its melts.We identify all late Mesozoic (∼110 Ma) and all Cenozoic mafic volcanism in eastern Australia as the EAVP. Three volcanic regions are recognised within the EAVP: (1) The Northern Region in northern Queensland is comprised predominately of basalts. (2) The Central Region, which includes volcanism lying between the Northern and Southern Regions, is a combination of dominantly basaltic rocks with some felsic plugs and intrusions. Three age progressive tracks are recognised in this region with intermittent non-age-progressive volcanism. (3) The Southern Region comprises all Cenozoic volcanism from Tasmania and Bass Strait. Here, lavas outcrop on lithosphere <60 km thick and are similar in composition to the Northern Region, but with a large temporal range. Collectively these three regions are made up of 67 separate volcanic centres, with two dominant volcanic centre compositions, namely basalt and leucitite.Primitive melt compositions in the EAVP can be produced by melting mantle source assemblages consisting of various proportions of three ‘end-members’: (1) a mixed anhydrous peridotite-pyroxenite, (2) a hydrous (phlogopite- and/or amphibole-bearing) pyroxenite, and (3) a hydrous pyroxenite + accessory Ti-oxides ± apatite. Primitive melts were identified with a filter on whole rock oxides Mg# > 58, FeOt > 6 wt%, CaO > 5.5 wt%, and MnO > 0.07 wt%, which is based on experimental melt compositions from realistic source compositions and P-T conditions. We suggest that the majority, if not all, of mantle source assemblages that gave rise to the EAVP underwent some degree of mantle metasomatism; this produced the three source end members introduced above. The metasomatic agents were probably silicate and carbonatite melts related to past subduction events during the accretion of eastern Australia while attached to eastern Gondwana between the mid-Cambrian to the late Triassic. Basalt and leucitite centres can be separated by TiO2 and K2O concentrations. Basaltic centres represent melts from the least metasomatised mantle sources on younger and thinner lithosphere to the east, whereas leucititic centres erupted on older and thicker lithosphere to the west, are sourced from the most pervasively metasomatised mantle assemblages at greater depths (120-160 km).Melts within the EAVP were generated at temperatures below those required to melt peridotite, as the solidi for metasomatised mantle assemblages are depressed by ∼300 °C compared to anhydrous peridotites. At these cooler temperatures, the melting process could be initiated by a slight increase above the ambient upper mantle temperature (∼1350 °C); this is compatible with shear-driven upwelling and edge-driven convection rather than mantle plume activity as the melting mechanism for intraplate volcanism in eastern Australia. Lower solidus temperatures increase the probability for future eruptions occurring at volcanic centres considered dormant.