Petrogenesis of granitoids from the Lachlan Fold Belt, southeastern Australia: the role of disequilibrium melting

Abstract S- and I-type granites from the Lachlan Fold Belt, southeastern Australia, have been investigated to assess the role of disequilibrium melting in their petrogenesis. Differences between the median initial eHf compositions of magmatic zircon populations and the host bulk-rock (ΔeHfblk-zrc) range from -0.6 to +2.5 e units, providing evidence for intra-sample (and hence inter-phase) Hf-isotopic heterogeneity. Linear variations on Harker diagrams and O and Hf isotope compositions of magmatic zircon preserved in many I- and S-type granites are inconsistent with assimilation or simple mixing hypotheses. In contrast, isotopic disequilibrium between the melt and a restite assemblage can explain the bulk-rock versus zircon differences observed in these samples. Assuming that magmatic zircon records the melt composition, differences between the bulk-rock eHf and eHf of magmatic zircon (ΔeHfblk-zrc values) measured for I-type granites (0.4-2.5) can largely be explained by disequilibrium amphibole dehydration melting of meta-igneous protoliths that were either isotopically heterogenous at the time they were formed, or perfectly homogeneous before being aged in the crust for 0.4-1.0 billion years prior to partial melting. The Currowong Suite exhibits petrographic features and preserves geochemical and isotopic compositions that do not lend themselves to simple restite model or magma mixing explanations; however, these observations could be explained by the restite unmixing of magma batches generated from a single source rock if, as modelling has suggested, separate batches contain different melt compositions. By investigating the application of disequilibrium melting to granite genesis, this study demonstrates that isotopic heterogeneity at various sampling scales should actually be expected for the production of granites from a single source, rather than necessitating the involvement of multiple sources and mixing processes. As a result great care should be taken in the interpretation of isotope data from granitic bulk-rocks or their zircons.