Lithium Isotope Fractionation During Intensive Felsic Magmatic Differentiation

The Xihuashan and Yaogangxian granitic plutons in South China comprise highly evolved multiphase Li‐rich granites and host quartz‐vein‐type tungsten deposits. The δ7Li values of Phase A (early stage), B (middle stage), and C (late stage) from the Xihuashan pluton are 1.0–1.2‰, 1.1–3.0‰, and 2.4–2.8‰ respectively, increasing through chemical evolution. The granites from the Yaogangxian pluton also display gradually enriched in heavy Li isotopes in a later stage, although systematically lighter than those of the Xihuashan pluton. In both plutons, the δ7Li shows good correlations with SiO2 and Li concentrations as well as Rb/Sr, Nb/Ta, and Zr/Hf ratios, indicating Li isotopic fractionation most likely caused by magmatic differentiation. In situ analyses show that the minerals of Xihuashan pluton record a continuous elemental spectrum, reflecting the results of progressive magmatic differentiation. The δ7Li values of quartz, feldspar, mica, and zircon all correlate well with the chemical evolutions of granitic magma, systematically elevated in Phases B and C relative to Phase A. The Li isotope data of the mineral separates further document that the enrichment of 7Li in the residual melt was most likely due to the equilibrium fractionation between the mineral and melts. The data are interpreted to reflect that intense magmatic differentiation was responsible for Li isotopic variations coupled with the enrichment in the Li, F, P, and rare metals in the late‐phase granites of the Xihuashan pluton. The lithium isotope behavior documented in this study provides new insights into magmatic differentiation and associated rare‐metal mineralization. Plain Language Summary In the past, magmatic differentiation was thought not to produce resolvable Li isotopic fractionation based on studies of basaltic rocks. Overall, only sparse studies on Li isotopes of highly evolved granites have been reported, and thus possible isotope fractionation of Li isotopes during differentiation of felsic magma could have been neglected. In this study, we present evidence of Li isotope fractionation during the differentiation of granitic magma. The most evolved (late‐stage) rare‐metal‐rich granites are systematically enriched in heavier Li isotopes. The chemical and isotopic compositions of both bulk rocks and mineral separates (e.g., zircon and mica) indicate that equilibrium fractionation during fractional crystallization governs Li isotopic behavior and enriches heavier Li isotopes in the