A geochemical study of metabasalts from a subduction complex in eastern Australia

East of the Peel Fault System, in the New England Fold Belt of eastern Australia, a base faulted sequence of mafic igneous rocks conformably overlain by chert, siltstone and minor sandstone, occurs in repeated slices forming a stacked sequence, tens of kilometres thick. The mafic rocks show extensive mineralogical alteration, but apart from the highly mobile alkali elements, bulk whole-rock compositions retain a strong igneous imprint. Two metabasalt groups are recognised. Group A shows a distinct tholeiitic fractionation trend with increasing FeO{black star}, TiO₂, P₂O₅, FeO{black star} MgO, Zr and Y relative to only a small increase in SiO₂. The mean concentration of incompatible and immobile elements, and the composition of relict endiopsidic and augitic pyroxenes is similar to mid-ocean ridge tholeiites, particularly the more evolved ferrobasalts. Group-B metabasalts are alkalic in composition. They tend to have higher Al₂O₃, MgO, P₂O₅ and Zr, and lower FeO{black star} and Sr than Group-A basalts. Relict pyroxenes have high Ti and Al concentrations and are salites. These rocks were probably the product of oceanic within-plate volcanism. Both groups represent fragments of oceanic crust incorporated into a subduction complex along the eastern margin of the Australian continent.