The source of A-type magmas in two contrasting settings: U–Pb, Lu–Hf and Re–Os isotopic constraints

Abstract

The sources of post-orogenic A-type magmas from two distinct geodynamic settings are compared. The end of the ca. 514–480Ma Delamerian Orogeny, southeastern South Australia, was marked by ~10Myr of bimodal A-type magmatism, driven by convective removal of thickened lithosphere. Initial Os and Hf isotope ratios record a heterogeneous lithospheric mantle source, with some input from aesthenospheric mantle. Mafic parental melts fractionated to produce the granites. In contrast, initial Os isotope ratios of the A-type magmas that comprise the ca. 1598–1583Ma Mesoproterozoic Gawler Felsic Large Igneous Province, central South Australia, record a dominant evolved lower crust component. However, initial Hf isotope ratios from these samples are depleted, indicating a mantle source for lithophile elements. This voluminous, bimodal magmatism lasted for ~15Myr, and ended the Wartakan Orogeny. In both cases the homogenisation of chemical (rheological) heterogeneities, inherited from terrain amalgamation and orogenic thickening, strengthened the lithosphere. The contemporaneous fusion of heterogeneous mantle±crust may represent a common, stabilising influence on the lithospheric column regardless of tectono-magmatic setting.