Parallel Plumbing Systems Feeding a Pair of Coeval Volcanoes in Eastern Australia

Abstract

Eastern Australia hosts a long track of Cenozoic age-progressive volcanoes, mostly alkaline in composition. Of these, Warrumbungle and Comboyne are coeval and occur at the same latitude (31 degrees S), but they are similar to 300 km apart, on either side of the Great Dividing Range. The lavas from both volcanoes often contain complex crystal assemblages, including plagioclase, olivine and clinopyroxene, which permit a comparative study of pre-eruptive magma histories in a large, complex, continental setting. Here we combine mineral and whole-rock geochemistry with Ar-40/Ar-39 geochronology to temporally constrain the processes operating in the magma plumbing systems. Ar-40/Ar-39 geochronology indicates that volcanic activity took place for similar to 3 Myr, in two separate stages. The first stage (18-17.5 Ma) is evident only at the larger Warrumbungle volcano. In Stage 2 (similar to 17-15.5 Ma) the two volcanoes were active contemporaneously. The dominantly porphyritic and relatively evolved (MgO from 7.25 to 0.39 wt %) nature of the lavas suggests that the magmas stalled and differentiated in the crust prior to eruption. At the Warrumbungle volcano, Stage 1 magmas fractionated olivine and minor clinopyroxene and subsequently differentiated during ascent. The crystal cargo in Stage 2 magmas at the Warrumbungle volcano became increasingly more complex with time and the samples have been divided into two subgroups, according to age and petrological variation. Stage 2.1 magmas sampled olivine, clinopyroxene and plagioclase mushes at Moho depths of similar to 41 km. Disequilibrium textures in plagioclase and clinopyroxene macrocrysts indicate differences in composition between the mush and the ascending magmas. Stage 2.2 magmas, by contrast, carried a combination of antecrysts and phenocrysts. Clinopyroxene antecrysts show strong disequilibrium textures and are reversely zoned. In plagioclase, anorthite contents increase close to the rim of the crystals, to levels (An(60-55)) similar to those found at the core of primitive, normally zoned, euhedral antecrysts (An(53-50)). At the Comboyne volcano mineral phases have a similar complexity to those of Stage 2.2 at the Warrumbungle volcano, with disequilibrium textures and reversely zoned antecrysts providing evidence of magma mixing, only lacking the primitive, normally zoned, euhedral plagioclase crystals. The complex crystal assemblage evident in Stage 2.2 lavas at the Warrumbungle volcano and throughout Stage 2 at the Comboyne volcano indicates a coeval rejuvenation of evolving crystal-melt mushes with the intrusion of more primitive, hotter, and crystal rich or -poor magmas shortly before eruption. Forward modelling using Rhyolite-MELTS replicates the composition of melts and fractionated minerals along a polybaric fractional crystallization path at depths from 24 to 7 km at the Warrumbungle volcano and from 15 to 7 km at Comboyne, supported by barometry estimates on clinopyroxene crystals. This study has identified that the two temporally associated, but spatially discrete, continental alkaline volcanoes were fed by parallel plumbing systems, which become more complex throughout the life of the volcanoes. Multiple mush zones, in which magmas stagnated and fractionated, were periodically replenished with more primitive magmas, triggering eruptions intermittently over a protracted period of similar to 3 Myr.