Tectonic provenance of the Palaeoproterozoic Plum Tree Volcanics: implications for the initiation of the McArthur Basin

Abstract

The Palaeoproterozoic (1825 ±4Ma) Plum Tree Volcanics are a bimodal suite of basalt and rhyolite lavas forming part of the fluvial conglomerate-sandstone sequence of the upper Edith River Group. They are preserved in remnants unconformably overlying the Pine Creek Orogen north of Katherine in the Edith River, Mt Callanan and Birdie Creek Basins. These sequences directly post-date the convergent deformation of the Pine Creek Orogen and mark the beginning of the extensional regime that initiated the McArthur Basin. The tectonic setting of the Plum Tree volcanism, whether divergent intraplate rift or mantle hotspot, may suggest how the formation of the McArthur Basin began and provide insight into how the Pine Creek Orogen compression ceased.

In this paper, geochemical methods were used to determine the tectonic setting of the Plum Tree Volcanics. Whole rock geochemical data were collected via XRF, ICP-MS and ICP-OES. Nd-Sm and Sr isotopic data were collected via column chromatography and TIMS. Petrographic data were collected via optical petrography.

Radiogenic Sr (87Sr/86Sr= ~0.708) and non-radiogenic Nd (εNd(i)= -6 to -8) isotopes suggest a crustal component in melt evolution. Modelling of melt evolution by pure fractional crystallisation presents well-fitting liquid lines of descent, suggesting a fractional crystallisation driven melt evolution. Tholeiitic basalts and trace element geochemistry suggests a mantle derived melt driven by a mantle plume and intraplate continental rifting. Modelling of AFC processes suggest a lower crust sourced assimilant. Ambiguous basalt geochemistry supports a continental rift derived melt and an oxygen fugacity of FMQ -1 suggests a primitive, reduced melt reflecting a mantle parent. Optical petrography presents a plagioclase and clinopyroxene rich mineral assemblage reflecting a mantle parent.