Hf isotopic fingerprinting of geodynamic settings: Integrating isotopes and numerical models

Abstract

Abstract Hf isotopes have proven invaluable in understanding the evolution of Earth s crust-mantle system, but their use in reconstructing tectonic environments, in many cases, remains equivocal. In this study, we introduce a new approach to predict the Hf isotopic evolutionary pattern for rifting and collision based on the integration of numerical models and 176Hf/177Hf isotopes. The geodynamic numerical models allow us to estimate the proportion of juvenile material added to the crust through time. On the basis of this proportion, we calculate changing 176Hf/177Hf ratios using mixing models. Predicted Hf isotopic patterns generated through this numerical approach imply that juvenile signals are observed during back-arc extension, whereas evolved signatures dominate collisional settings. We use this novel modeling approach in the case study region of the Halls Creek Orogen to elucidate its tectonic setting through time. In addition, the geochemical features of magmatic rocks in the case study region imply partial melting of a sub-arc mantle wedge with magma-crust interaction on ascent in a convergent margin setting. The links between predicted Hf isotopic evolution, geodynamic numerical models, whole rock geochemistry and measured zircon Hf isotopic evolution trend resolve three discrete stages in the tectonomagmatic development of the Halls Creek Orogen: (1) oceanic crust subduction; (2) back-arc formation with addition of juvenile mantle input; and (3) docking of the North Australian and Kimberley cratons resulting in the development of mixed-source magmatism formed in a collisional setting. We provide a new method to validate geodynamic models with isotopic datasets, which should lead to more rigorous understanding of crustal evolution.