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

Liu Z, Shea J, Foley SF, Bussweiler Y, Rohrbach A, Klemme S, Berndt J

Abstract

Many Cenozoic basaltic rocks in Eastern Australia exhibit an age-progressive trendfrom north to south, leading to the suggestion that one or more mantle plumes passedbeneath the Australian plate. Trace element patterns indicate that the source regionshave been metasomatised by infiltrating melts, but the source rock assemblages havenever been closely identified. Here, trace element analyses of olivine and whole rockgeochemistry for several occurrences in New South Wales (Bingara-Inverell, Dubbo,Barrington and Ebor) are combined to characterize the mineralogy of the source andidentify the nature of the melts that caused the metasomatic enrichment. According toNi/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 inolivine 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 amixed pyroxenite+peridotite source based on their high FC3MS values. In contrast, thesource of basanite and picrobasalt from Barrington was peridotite with little pyroxenite.High Li and Zn in olivines, high P2O5/TiO2 and Zr/Hf at low Ti/Eu in whole rocksillustrate that the pyroxenite sources of basanites from Bingara/Inverell, Barrington andDubbo resulted from variable degrees of carbonatitic metasomatism. Partial melting ofperidotite metasomatised by carbonatite melts at around the spinel-garnet peridotitetransition depth produced basalts and basanites from Dubbo, Barrington, Ebor,Bingara/Inverell and Buckland (Queensland). Carbonatitic metasomatism iswidespread in the eastern Australian mantle lithosphere, occurring seaboard of a ledgebetween thick lithosphere beneath the Australian continent that stretches fromQueensland, through New South Wales to Victoria.