Evidence for a Carbonatite-Influenced Source Assemblage for Intraplate Basalts from the Buckland Volcanic Province, Queensland, Australia

Joshua J Shea,Stephen F Foley

doi:10.3390/min9090546

Abstract

Eastern Australia contains a widespread suite of primitive (MgO ≥ 7.5 wt.%) intraplate basaltic provinces, including those sited along the longest continental hotspot track on Earth (≈2000 km), the Cosgrove track. The Buckland volcanic province is the most southerly basaltic province on the Cosgrove track before a >1600 km stretch that contains only sparse leucitite volcanism. Buckland is also situated just northeast of the edge of thick cratonic lithosphere where it transitions to a thinner continental lithosphere (<110 km) to the east, which may influence the production of plume-derived melts. Here, analysis of minor and trace elements in olivines in alkali basalts and basanites from the Buckland Province are combined with whole-rock compositions to elucidate the mantle source assemblages, and to calibrate minor and trace element indicators in olivine for application to source mineralogy. Olivine xenocrysts show element concentration ranges typical for peridotites; Mn and Al concentrations indicate that the ambient mantle is spinel, rather than garnet, peridotite. High modal pyroxene content is indicated by high Ni, Zn/Fe, and Fe/Mn in olivines, while high Ti/Sc is consistent with amphibole in the source. Residual phlogopite in the source of the basanites is indicated by low K/Nb in whole rocks, while apatite contains high P2O5 and low Rb/Sr (≥0.015) and Sr/La (≥13). The basanite source assemblage probably contains apatite, phlogopite, olivine, clinopyroxene and orthopyroxene, whereas the alkali basalt source assemblage is probably amphibole, olivine, orthopyroxene and clinopyroxene ± phlogopite ± apatite. Both source assemblages correspond broadly to olivine websterite, with the basanite source lying deeper than that for alkali basalt, explaining the occurrence of phlogopite in the source. This mineralogy, along with whole-rock Ti/Eu, Zr/Hf and P2O5/TiO2 values approaching those of natural carbonatites, provide evidence showing that the Buckland source consists of a peridotite that has interacted with a carbonate-rich melt whose origin may be in the deep lithosphere or asthenosphere beneath the craton. Similar enrichment processes are probably common throughout eastern Australia, controlling trace element characteristics in basaltic provinces. The topography of the underside of the lithosphere may play a significant role in determining mantle source assemblages by diverting and concentrating melt flow, and thus influence the location of basaltic provinces.

Highlights

The identification of primitive basalt source assemblages provides a way to understand mantle processes, such as the recycling of crustal material and mantle metasomatism, that are not directly observable on the surface [1,2]
From the 1970s onwards, it was recognised from the trace element compositions of volcanic rock and peridotite xenoliths of southeastern Australia that the origin of the more alkaline members of these series could not be explained without enrichment of the source by mantle metasomatism [10,11]
We aim to identify the source assemblages for the Buckland alkali basalts and basanites, shed light on mantle processes and produce a holistic and robust method capable of detecting diverse mantle source assemblages beneath eastern Australia

Summary

Introduction

The identification of primitive basalt source assemblages provides a way to understand mantle processes, such as the recycling of crustal material and mantle metasomatism, that are not directly observable on the surface [1,2]. From the 1970s onwards, it was recognised from the trace element compositions of volcanic rock and peridotite xenoliths of southeastern Australia that the origin of the more alkaline members of these series could not be explained without enrichment of the source by mantle metasomatism [10,11]. Experiments showed that H2 O and CO2 in the source were required to explain melilitites [12]. These early models of melt production in a homogeneously metasomatised peridotite source have been replaced by the acceptance that hydrous (and possibly carbonate) minerals are sequestered in separate lithologies rich in amphibole, mica and clinopyroxenes [13], with minor apatite and oxide minerals. It is worthy of note here that eastern Australian volcanics contain xenoliths rich in amphibole and apatite [14,15]

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