In the Heathcote Greenstone Belt of central Victoria, a sequence of boninites and low-Ti andesites is overlain and intruded by tholeiitic basalts with affinities to backarc basin basalts. Two suites of boninites have been identified: one (Type A) with Ti/Zr ratios of 63±4, (La/Yb)N of 2–3 and HREE 5 times chondritic levels. The other suite (Type B) overlies Type A boninites and has Ti/Zr ratios of 23±3 and lower TiO2 and HREE contents (2–3 × chondrite), but shows significantly greater LREE enrichment, with (La/Yb)N greater than 5. Fractionation within both suites was largely controlled by the low-Ca pyroxenes protoenstatite and enstatite. Plagioclase-phyric low-Ti, high-Mg andesites occur in fault contact with the boninites, and have Ti/Zr and (La/Yb)N ratios very close to those of Type B boninites, but at higher absolute abundances of TiO2 and HREE. They are not related to either boninite suite by any realistic fractionation scheme, but originated from the same source as Type B boninites by approximately half the degree of partial melting that generated the boninites.
Type A boninites could have been generated when LILE-enriched hydrous fluids derived from a subducted slab invaded depleted, clinopyroxene-poor lherzolite at depths less than 30 km, and initiated H2O-undersaturated partial melting. In a later partial melting event at similar depths, continued influx of metasomatic fluids into by now highly-depleted peridotite could have generated Type B boninites and low-Ti andesites. The presence of boninites and low-Ti andesites in the Cambrian Heathcote and Mount Wellington Greenstone Belts in southeastern Australia suggests that the early history of the Lachlan Foldbelt took place in a subduction-related, intraoceanic setting.