Limited influence of subducted continental material on mineralogy and elemental geochemistry of primitive magmas from Indonesia-Australia collision zone

Melt inclusions within forsterite-rich olivine crystals in two mixed magmas from the Indonesia-Australia collision zone provide information on the primary melts formed within this tectonic environment. Although whole rock Sr, Nd and Pb isotope data show a strong influence of subducted continental material, the melt inclusion major and trace element compositions are only subtly different from typical subduction-related magmas. The presence of low-Ca olivine crystals with inclusions of aluminous spinel in one sample can be explained by localised reactions during influx of hot basaltic magma in a partially crystallised magma chamber. The mineralogy of the samples is similar to that of other mixed arc magmas. Major and trace element data for reheated melt inclusions show subtle distinctions compared to non-collisional magmas in terms of CaO/Al₂O₃, Th/Nb and Sr/Y ratios, reflecting partial melting processes during slab to wedge transfer of subducted continental material. Magmas influenced by the subduction of continental material are therefore very similar to normal subduction-related magmas, apart from their radiogenic isotope signature. Magmas with crustal isotope ratios and highly unusual (e.g. peraluminous) geochemical compositions must therefore be formed by other processes, such as upper crustal contamination.