Fractionation mechanism of iron isotopes in highly fractionated granites from the Xinxian Pluton, Western Dabie Orogen, Central China

Iron isotopes are important for tracing the magmatic process. The fractionation of iron isotopes in granite is up to 0.55 ‰. In this study, Wangjiagou (XWJ) granite and Tayueping (XTY) granite in the Xinxian pluton of the Western Dabie orogen were evaluated. Both the XTY and XWJ granite belong to monzogranites, with high SiO 2 (74.42–76.82 wt.%) contents. The granites are depleted of Nb and Ti but enriched with Pb and K, and they display negative Eu anomalies (Eu/Eu* = 0.40–0.52) on REE plots that are normalized by chondrite. The δ 56 Fe values of the XTY granites vary from 0.19 ± 0.03 ‰ to 0.27 ± 0.04‰, and the δ 56 Fe values of the XWJ granites are 0.34 ± 0.02 ‰ and 0.36 ± 0.01 ‰, respectively. Both the XTY and the XWJ granites belong to highly fractionated granites due to their SI (solidification index), DI (differentiation index), and content of CaO. Evidence from the iron isotopes shows that neither fluid exsolution, alteration, weathering, nor partial melting can explain the enrichment of the heavy iron isotopes. The results modeled using the Rayleigh equation showed that fractional crystallization can produce Δ 56 Fe melt-crystal with the value of 0.08–0.15 ‰. In conclusion, fractional crystallization was the main factor controlling the fractionation of iron isotopes, and the change of melt composition may also lead to the enrichment of heavy iron isotopes in the residual melt.