Unusual baryte-bearing hybrid basalt, Bourke-Byrock area, northern New South Wales

Abstract

Drilling near Mount Oxley and Mullagalah in the Bourke-Byrock area, NSW, intersected basaltic breccia pipes. 'The Mount Oxley basalt is an unusual hybrid rock involving intimate veining and intermingling between a slightly evolved basanite and a strongly evolved, late-stage, baryte-bearing trachyte. 'The basanite consists of abundant phenocrysts of altered olivine and diopside-augite, and rarer phenocrysts of nepheline, anorthoclase and Ti-rich magnetite, in a groundmass of plagioclase laths and 'Ti-rich magnetite grains. 'The trachytic component is dominated by alkali feldspar, largely sanidine, with calcic amphibole (Ti-Mg-rich hastingsite), baryte (with up to 2% Sr in coarser crystals) and secondary carbonates. Olivine-microgabbro and microsyenite xenoliths in the basalt suggest that the cumulates were formed from both the basanitic and trachytic magmas prior to emplacement. Xenoliths of, and xenocrysts from, high pressure ultramafic metamorphic assemblages (spinel harzburgite and spinel websterite) indicate a mantle source for the basanitic magma. 'I'wopyroxene temperatures based on Wells thermometry suggest these ultramafic assemblages were re-equilibrated under an ambient paleogeotherm between 990-1035°C. Similar basalt appears in the Mullagalah breccia pipe, but lacks the phenocrystic nepheline and the hybrid baryte-bearing trachyitic component found in the Mount Oxley basalt. Xenoliths in the Mullagalah breccia include a cumulate-like olivine-bearing diopside-amphibole (K-'Ti-rich ferroan pargasite) assemblage. The Mount Oxley and Mullagalah intrusions are not well dated, but were probably formed during Late Mesozoic-Late Cenozoic intraplate basaltic activity that occurred in eastern Australia, from magmas generated at mantle depths exceeding 38 km.