Zircon O-Hf isotope data from the Bega Batholith, southeastern Australia

Subduction-related magmatic arcs are considered principal sites of continental growth, but the origin of the voluminous felsic (granitic) igneous rocks emplaced in these settings is uncertain. Probing the magmatic history of these granites can provide insight into the magma plumbing systems responsible for differentiation and, potentially, generation of continental crust. Seminal studies suggest derivation of the Lachlan Fold Belt I-type granites in eastern Australia by partial melting of infracrustal sources, yet these same granites show field and chemical evidence for interaction between mantle-derived magmas and older source components, obfuscating the link between I-type granite petrogenesis and crustal growth. To evaluate these competing petrogenetic scenarios, we integrate U-Pb (zircon) geochronology, zircon εHf–δ18O, and bulk-rock Nd-Sr-O isotopic and geochemical data to explore the formation of the ‘Cordilleran-style’ Bega Batholith, the largest I-type batholith in the Lachlan Fold Belt. Secular compositional trends are identified where the granites transition from isotope signatures characteristic of melting old continental sources in the west, to more mantle-like isotope compositions in the east. These compositional shifts, and correlated zircon εHf–δ18O arrays, are consistent with open-system magma generation and mantle-derived magmatic input, where the composition of the mantle-derived component changed, and the proportion of supracrustal source material diminished, with ongoing arc extension and oceanward migration of the subduction zone. Isotopic mass balance, incorporating constraints from whole rock Nd data, suggests a cumulative mantle input of ~85–111 x 103 km3 representing up to 75% of the Bega Batholith by volume at a magma production rate of ~60 km3 km-1 Ma-1 and indicating considerable addition of juvenile crust along the eastern margin of Australia between ca. 420–385 Ma. Our results highlight the key role of crust-mantle interaction in the petrogenesis of I-type granites in extensional back-arcs, and that these environments represent important sites of Phanerozoic continental growth.