Homogenization of zircon Hf isotopes during late-stage granite crystallization

Abstract

Zircon Hf isotope compositions provide valuable constraints on granite petrogenesis and the evolution of continental crust. However, the causes of variations in zircon Hf isotope ratios during magma evolution are poorly understood. Here we compiled zircon U–Pb ages and Lu–Hf isotope compositions of the Permian-Triassic granites in South China to demonstrate that Hf isotope variability is common in the early stages of granitic magma evolution, but Hf isotope ratios are homogenized during the final stages of granite crystallization. Late-grown zircons have relatively high concentrations of U and have εHf(t) values approximating the median values of multiple analyses for the low-U zircon in the same samples. Hafnium isotope variations measured in low-U zircon could be related to incremental assembly of multiple isotopically distinct magma batches derived from partial melting of heterogeneous crustal rocks; these discrete magma batches may have remained isolated from each other during segregation and extraction in the source and during ascent through the crust. After emplacement, these magma batches may have experienced variable degrees of mixing resulting in homogenization of Hf isotopes in late crystallized zircon. Coupled with whole-rock geochemical data of the Permian-Triassic granites, we argue that the Hf isotope homogenization is related to crystallization of zircon from late-stage magmas or precipitation of hydrothermal zircon from exsolved hydrothermal fluids during the final stages of magma solidification. Hafnium isotope homogenization in late-stage melts/fluids may have been driven by an increased concentration of fluorine and the efficient diffusion of hafnium-fluoride complexes through silicic melt. These results provide a deeper understanding of the mechanisms that contribute to Hf isotope heterogeneity in zircon during granite crystallization.