Magmatic evolution and late hydrothermal activity in the Yashan rare-metal granites, South China: Insights from apatite geochronology and geochemistry

• Three genetically distinct types of apatite occur in the Yashan rare-metal granites. • Ap1 records magmatic crystallization at ∼152 Ma. • Ap2 formed via magmatic hydrothermal overprinting. • Ap3 records a discrete hydrothermal event at ∼87 Ma. The Yashan granitic complex, a multiphase, highly evolved granitic system, hosts one of the largest Ta-Nb-Li deposits in South China. Spatially, it comprises a composite pluton intruded by multiple albitite dikes (AD). The pluton exhibits a vertically zoned sequence of granitic rocks, progressing from two-mica granite (TMG) at the base through Li-mica granite (LG) and topaz lepidolite granite (TLG) to lepidolite quartz rock (LQR) at the top. Apatite is widespread in the Yashan complex and displays distinct morphologic and chemical variations. Three genetically distinct types have been identified: magmatic (Ap1), magmatic-hydrothermal overprinted (Ap2), and later hydrothermal (Ap3) apatite. Ap1 yields concordant U-Pb ages of 152.5 ± 0.6 Ma and exhibits geochemical characteristics similar to those of typical S-type granites. Fractional crystallization of plagioclase, monazite, and garnet controls the distribution of Sr, REE, and trace-elements. Ap2 displays porous textures, elevated Sr contents (up to 15,705 ppm), and high 87 Sr/ 86 Sr isotopic ratios, along with scattered U-Pb ages. The spatial association of Ap2 with hydrothermal Li-rich mica, as well as its enrichment in Nb and Ta, suggests fluid-mediated remobilization of rare metals. These features reflect metasomatic overprinting by hydrothermal fluids exsolved from the evolved magma. Ap3 records a discrete hydrothermal event with a late Cretaceous age of 87.2 ± 2.9 Ma. Collectively, these findings indicate that although hydrothermal alteration significantly modified mineral chemistry, magmatic differentiation remained the dominant control on rare-metal enrichment. Furthermore, the study highlights that Sr-rich apatite with low Th/U ratios (<1) in highly evolved granites can serve as a valuable indicator for constraining the timing of hydrothermal alteration.