Petrogenesis of granitic pegmatite from the Lianyunshan complex, South China: Insights from apatite and muscovite chemistry

Abstract

The Jiangnan Orogenic Belt (JOB) in the Northeastern Hunan Province (NHP), the one of the most significant regions for rare metal mineral resources in South China Block (SCB). It is renowned for abundant Li-Be-Nb-Ta granitic pegmatite deposits, However, the magmatic process of the Lianyunshan pegmatites remain poorly understood. This study focuses on various lithologies of the Lianyunshan complex pluton and pegmatite through zircon LA-ICP-MS UPb dating, whole-rock major and trace element geochemistry, and in situ trace element analyses of muscovite and apatite. These experiments offer unique insights into the mechanisms driving lithium-enriched pegmatite formation in this region. We elucidated the genesis of pegmatite and the relationship between Li-rich pegmatite and granite in Lianyunshan region. Zircon UPb dating yielded the concordia ages of 147.4 ± 0.69 Ma for biotite monzonitic granite, 145.5 ± 0.73 Ma and 144.0 ± 1.3 Ma for two-mica monzonitic granite, respectively. The whole rock geochemical analyses reveal that granites and pegmatites exhibit obvious characteristic of fractional crystallization. In situ trace element analysis for muscovite reveal Rb, Cs, Ta concentrations increase progressively but the ration of Nb/Ta, Li/Rb, K/Cs, and K/Rb decrease in sequence of BMG → TMG → pegmatite→spodumene pegmatite in Lianyunshan region. These trends indicate varying degrees of fractional crystallization, with pegmatites and spodumene pegmatites representing more evolved magmatic differentiation products. Fractional crystallization simulation using muscovite compositions supports a rayleigh fractionation process, wherein the initial granitic melt progressively evolved to form TMG, pegmatites, and ultimately spodumene pegmatites. Apatite can be divided into magmatic and hydrothermal types, both identified as fluorapatite. The rare earth element (REE) distribution patterns of apatite are consistent with whole-rock trends. However, apatite from spodumene pegmatites and some other pegmatites exhibit anomalous REE patterns. From BMG → TMG → pegmatites to spodumene pegmatites, Sr content (23–617 ppm) in apatite decreases while REE content increases (1001–3159 ppm). Apatite in TMG, and pegmatites share similar trace element characteristics, consistent with the fractionation trends observed in muscovite modeling. Apatite from pegmatites with relative lower SiO₂ content (65–70 wt%) show reduced REE concentrations and significantly elevated Eu/Eu* (0.01–0.7) and La/Yb (0.1–26) ratios. This suggests that pegmatite formation involved plagioclase decomposition and separation crystallization of HREE-enriched minerals.