Mobilization and fractionation of HFSE and REE by high fluorine fluid of magmatic origin during the alteration of amphibolite

High field strength elements (HFSE) and rare earth elements (REE) are normally immobile in hydrothermal fluids whereas experimental studies indicate some anionic ligands can significantly enhance their solubility in hydrothermal fluids. However, the role of fluorine (F) in hydrothermal fluids revealed by experiments and natural samples is different or even contradictory, especially in the mechanism of elements mobilization and fractionation. Here, we report a field-based geochemical study of amphibolites with extremely high Nb/Ta ratios (up to 98.7) and enriched HREE concentrations. The Paleoproterozoic amphibolites are intruded by a granitic dike with zircon UPb age of 132 ± 3 Ma. Felsic veinlets are observed in thin-sections of the amphibolite. Biotites with high F contents occur as aggregates along the boundary between the felsic veinlets and amphiboles, which indicates they are formed by reaction between amphibole and the granitic melt. The amphiboles have core-mantle-rim structure with high F actinolitic mantle surrounding low F magnesio-hastingsitic core. Some actinolites intergrow with biotites, titanites and magnetites. These evidences indicate that the actinolites are produced by hydrothermal metasomatism or interface-coupled dissolution–precipitation. The metasomatism occurred at 0.3–0.9 kbar and 500–650 °C within the hornfels facies. Hydrothermal titanites from amphibolites yield a lower intercept age of 131 ± 4.3 Ma, which is consistent with the crystallization age of the granitic dike. These observations indicate that the fluids are derived from fractional crystallization of a felsic magma for diking, and the released fluids are channeled along the interface between dike and amphibolite. The metasomatic minerals show high F and no Cl contents, indicating the hydrothermal fluids are high in fluorine (0.04–0.12 wt%) and low in chlorine. The F contents of minerals negatively correlate with HFSE, REE contents and (La/Yb)N ratios, but positively with Nb/Ta ratios. These reveal the progressive leaching and fractionation of HFSE and REE by the hydrothermal fluids. Mass balance estimates that most trace elements lose 20–60% during the hydrothermal alternation of amphibole. The calculation results also reveal that more Ta and LREE are lost than Nb and HREE, indicating Ta and LREE are more soluble than Nb and HREE, respectively, in the hydrothermal fluids. All these observations suggest that HFSE and REE are not only mobilized but also fractionated because of different degrees of mobilization by fluoride-bearing fluids.