The Composition, Development and Geological Setting of the Early Permian Boggabri Volcanics, Gunnedah Basin, NSW

Abstract

The Boggabri Volcanics comprise: basalts andesites, dacites, pyroxene rhyolites and biotite rhyolites. These outcrop mainly as coherent lava flows or domes, less commonly as ignimbrites and rarely as scoria or as small intrusives and are accompanied by minor occurrences of sedimentary rock and rare airfall tuff. At least 80 mappable rock units (mostly lavas) are recognisable at 1:85,000 scale and these together form eight discrete domains. These volcanic rocks probably developed in a terrestrial setting at or adjacent to numerous minor eruptive centres that were largely manifest as swarms of lava domes, and less commonly as calderas and minor intrusives. The Boggabri Volcanics are of subalkaline to mildly alkaline affinity but have unusual features: primitive MORB-like basalt petrology, plagioclase chemistry, high Zr/Nb and other ratios; arc-like high Al₂0₃, high MORB-normalised Ba, Pb and Sr and low MORB normalised Nb; and a Hawaiian Nepheline Melilitite-like Zr-Hf-Sm anomaly. Other unusual features include: depleted HREE; high Mg/Cr; high Ni/Cr; high Cs; low Hf; and intermediate MORB-normalised K, Rb and Pb. Their compositional affinity is equivocal because discriminant analysis, petrogenetically significant ratios, REE analysis, elemental abundance plots and spectra, and comparative bivariate plots yield contradictory results, variously indicating or negating all common melt environments depending on the parameters used.