Zinc isotopic compositions of migmatites and granitoids from the Dabie Orogen, central China: Implications for zinc isotopic fractionation during differentiation of the continental crust

Abstract

Some high silica granitic rocks (SiO2 > 70 wt%) show larger Zn isotopic variations (δ66ZnJMC 3-0749L) than less silicic granitic rocks, for which the reasons are unclear. Here we report Zn isotopic data for leucosomes (L, melt) and melanosomes phases (M, residue) in migmatites as well as granitoids from the Dabie orogenic belt, central China. Leucosomes always have higher δ66Zn than their associated melanosomes, with Δ66ZnL–M (δ66ZnL – δ66ZnM) varying widely from 0.08‰ ± 0.07‰ to 0.76‰ ± 0.07‰. High (Th/U)L/M (Th/UleucosomeTh/Umelanosome) ratios of four migmatite samples indicate modification of the leucosomes by fluid exsolution. Rayleigh fractionation modeling suggests that fluid exsolution could be responsible for the elevated δ66Zn in the four leucosomes. Three samples with large Δ66ZnL–M values and positive Eu anomalie, also have high Δ66ZnL-M values, probably due to accumulation of isotopically heavy plagioclase. Equilibrium Zn isotopic fractionation caused by partial melting is modeled to be 0.14‰–0.18‰ (Δ66Znmelt–residual) at 704–820 °C, consistent with the measured diatexite data (Δ66ZnL-M = 0.14‰ ± 0.07‰). The remaining two samples that were not significantly affected by fluid exsolution or plagioclase accumulation yield Δ66ZnL-M values larger than those expected in equilibrium melting and may reflect disequilibrium fractionation between melt and residue during the rapid melt segregation from the source. Our study confirms that Zn isotopes could be significantly fractionated during crustal anatexis and the magnitude of fractionation can be further enhanced by late-stage magmatic differentiation. Therefore, zinc isotope may act as a potential indicator of crust differentiation.