Clarifying source assemblages and metasomatic agents for basaltic rocks in eastern Australia using olivine phenocryst compositions

Abstract

Many Cenozoic basaltic rocks in Eastern Australia exhibit an age-progressive trend from north to south, leading to the suggestion that one or more mantle plumes passed beneath the Australian plate. Trace element patterns indicate that the source regions have been metasomatised by infiltrating melts, but the source rock assemblages have never been closely identified. Here, trace element analyses of olivine and whole rock geochemistry for several occurrences in New South Wales (Dubbo, Ebor, Bingara/Inverell, and Barrington) are combined with samples from Buckland (Queensland) to characterize the mineralogy of the source and identify the nature of the melts that caused the metasomatic enrichment. According to Ni/Mg against Mn/Fe and Zn/Fe ratios in olivines, Zn/Fe and FC3MS (FeOT/CaO-3*MgO/SiO2) parameters in whole rocks, tholeiite, alkali basalt, and basanite rich in olivine xenocrysts from Dubbo were derived from pyroxenite-dominated mixed source, mixed pyroxenite + peridotite source, and peridotite-dominated source, respectively. Similarly, basalts from Ebor and Bingara/Inverell are suggested to originate from a mixed pyroxenite + peridotite source based on their high FC3MS values. In contrast, the source of basanite and picrobasalt from Barrington was peridotite-dominated with less pyroxenite. High Li and Zn in olivines, high P2O5/TiO2 and Zr/Hf at low Ti/Eu in whole rocks illustrate that the pyroxenite sources of basanites from Bingara/Inverell, Barrington, Dubbo and Buckland resulted from variable degrees of carbonatitic metasomatism. Partial melting of peridotite metasomatised by carbonatite melts at around the spinel-garnet peridotite transition depth produced basalts and basanites from Dubbo, Ebor, Bingara/Inverell, Barrington and Buckland. Carbonatitic metasomatism is widespread in the eastern Australian mantle lithosphere, concentrated on the ledge of the thick lithosphere beneath the Australian continent that stretches from Queensland, through New South Wales to Victoria. Show more