Strong zinc isotope fractionation during felsic crust melting: Implications for tracing the origins of high-silica granites

Abstract High-silica granites, defined by SiO2 contents exceeding 70 wt%, are widely regarded as the end products of intracrustal differentiation. Despite their significance, the genetic mechanisms of these granites—whether they form from crustal melting or extensive fractional crystallization—remain a subject of ongoing debate. Here we show that these mechanisms can be distinguished by zinc isotopes, via investigating high-silica granites (leucogranites; SiO2 >70 wt%) from the Bengbu Uplift region in the North China Craton and granitoids with lower silica contents (SiO2 <70 wt%) from the adjacent Dabie-Sulu orogenic belt. The Dabie granitoids have an average δ66ZnJMC-Lyon value of 0.33‰ ± 0.01‰ (n = 19; two standard errors [2SE]), which is only marginally higher than that of their potential protoliths (mafic lower continental crust: 0.29‰ ± 0.02‰; 2SE). Batch melting simulations suggest that Zn isotope fractionation during crustal melting of a thickened mafic lower crust at ~1100 °C is nearly negligible. By contrast, the Bengbu leucogranites display δ66Zn values ranging from 0.28‰ to 0.49‰ (n = 20), which are up to ~0.20‰ higher than those of their potential protoliths (felsic gneisses; 0.30‰ ± 0.02‰; 2SE). The δ66Zn values are not correlated with indicators of fluid exsolution (Ba/Th and Ba/Nb) or fractional crystallization [MgO, TiO2, Fe2O3t, and (Dy/Yb)N], but instead reflect melting-induced fractionation, with a Δ66Znmelt–residue value of 0.20‰ at temperatures of ~700–710 °C. Zn isotope fractionation during crustal melting is likely driven by two mechanisms: (1) the preferential melting of biotite, which is isotopically heavier than other silicate phases, and (2) low melting temperatures, which can amplify isotope fractionation factors. Collectively, Zn isotopes provide new evidence indicating that the Bengbu leucogranites were formed via low-temperature crustal melting. Our observations also demonstrate that lower melting temperatures tend to induce a larger magnitude of Zn isotope fractionation during partial melting of felsic protoliths. The distinct Zn isotope fractionation during felsic crust melting has implications for tracing the origins of high-silica granites.