Zirconium isotope evidence for crystal-melt segregation during high-silica granitic magma differentiation

Abstract

Magma differentiation plays a crucial role in the chemical evolution of Earth's continental crust, with high-silica granites representing the product of extensive felsic magma differentiation. However, diagnostic evidence for crystal-melt segregation in felsic systems remains limited. This study investigates the potential of stable Zr isotopic composition as a novel tracer for high-silica magma differentiation processes, focusing on the Qitianling batholith and Yaogangxian pluton in the Nanling Range, South China, an area renowned for its large-scale distribution of high-silica granites (>70 wt.% SiO₂). We present comprehensive analyses of both bulk-rock and in-situ zircon Zr isotopic compositions, integrated with zircon U-Pb-Hf isotopic and trace element data. Zircon Zr isotopic compositions show systematic correlations with zircon Hf contents and Zr/Hf ratios, demonstrating that Zr isotopic fractionation during zircon crystallization is driven by the preferential incorporation of light Zr isotopes in zircon. This relationship is further supported by correlations between bulk-rock δ^94/90Zr_IPGP values and Zr contents and Zr/Hf ratios, which indicate progressive zircon separation from residual melts during magmatic differentiation. As a result, highly fractionated granites display significant heavy Zr isotope enrichments, characterized by elevated δ^94/90Zr_IPGP values in both bulk rocks (0.48 ‰ to 1.05 ‰) and zircons (up to 2.39 ‰). In contrast, common granites with insignificant zircon-melt segregation display primitive bulk-rock Zr isotopic compositions (0.04 ‰ to 0.27 ‰) similar to the upper continental crust. These results indicate that effective physical crystal-melt segregation leads to a remarkable elevation in the bulk-rock and zircon δ^94/90Zr_IPGP values of highly fractionated granites compared to those of common granites. Geochemical modeling suggests that >60 % segregation of zircon is required to account for the heavy Zr isotopic compositions in highly evolved granites. Furthermore, the high Zr isotopic values in zircon correlate with enriched U, W, Sn, and other incompatible elements, implying that Zr isotopic composition could serve as an indicator for mineralization potential associated with felsic magma differentiation.