Light iron isotopes in high-silica granites record fluid evolution in magmatic-hydrothermal systems

Abstract

High-silica (SiO₂ > 70 wt%) granites in continental collisional zones are crucial for understanding formation and evolution of the upper continental crust. Fluids released from felsic magmas can help drive the transition from magmatic to magmatic-hydrothermal systems in shallow magma chambers. However, the influence of these fluids on compositional variations and Fe isotope fractionation during the later stages of evolution of felsic magmas are unclear. In this contribution, we report stable Fe–Mg isotope compositions for the Paleocene Zhengga leucogranite pluton, part of the Gangdese batholith in southern Tibet. The pluton consists of both biotite granite and garnet-bearing two-mica granite, the latter of which contains zoned plagioclase and patchy K-feldspar that show fluid-mediated partial replacement textures, recording complicated magmatic and hydrothermal processes. Compared to high-silica granites worldwide (δ⁵⁷Fe = +0.10 ‰ to +0.74 ‰), all rocks from the Zhengga leucogranite pluton have light and variable δ⁵⁷Fe values (+0.03 ‰ to +0.28 ‰ relative to IRMM-014), which display wave-shaped variations with progressive magmatic differentiation. However, their δ²⁶Mg values (relative to DSM-3) decrease from −0.12 ‰ to −0.72 ‰ with increasing SiO₂. The variable Fe and Mg isotope signatures of the Zhengga pluton can be best explained by a three-stage process, comprising initial fractional crystallization of biotite and magnetite, followed by deuteric fluid exsolution with decreasing temperatures and pressures, and final interaction between trapped fluids and residual melts in the highly crystalline magma mush. In combination with previously published Sr–Nd–Mo isotopes on the same samples, our new results suggest that fluid exsolution is required to elevate the δ⁵⁷Fe of the felsic melts by up to 0.15 permil, but subsequent fluid-melt reaction reduces the Fe isotopes and leads to similar light-Fe isotope compositions of final residual melts to their primary magma. Therefore, the high-silica granites can be enriched in light Fe isotopes due to the effects of magmatic fluids, which make a significant contribution to the formation and evolution of upper continental crust.