Li-Cs-Ta pegmatites form by fractional crystallization of granitic melts from thickened lower crust sources: Insights from the Triassic granite-pegmatite system in the Great Xing’an Range, NE China

Tracing Li-Cs-Ta (LCT) pegmatites to their ultimate sources is important for understanding their genesis and mechanisms of rare metal enrichment. This study presents a comprehensive dataset for the petrology, mineralogy, geochronology, and geochemistry of the newly discovered Daiqintala rare metal granitic pluton in the southern Great Xing’an Range of the eastern Central Asian Orogenic Belt in China, with the aim of tracing its ultimate sources and revealing the mechanisms of rare metal enrichment. This pluton is composed of biotite granites, leucogranites, ore-barren pegmatites, and rare metal−mineralized pegmatites. The biotite granites and leucogranites constitute the main body of the pluton, while the pegmatites typically occur as dikes within the pluton or intrude into the Permian metasedimentary rocks. Zircon-monazite-columbite U-Pb dating results show that the biotite granites, leucogranites, barren pegmatites, and mineralized pegmatites were emplaced simultaneously at 227−226 Ma, suggesting a close spatial-temporal relationship between them. These rocks have overlapping Sr-Nd-Hf isotopic compositions, indicating that they derived from a common magma source. Thermodynamic and Rayleigh fractionation modeling, coupled with geochemical and petrographic characteristics, demonstrate that the strongly peraluminous leucogranites were generated by continuous fractional crystallization of the weakly peraluminous granitic magma with a composition comparable to that of the biotite granites. Further differentiation of the strongly peraluminous granitic magma, driven by diking infiltration, spawned the barren pegmatites and mineralized pegmatites, with the latter being the most fractionated products of the Daiqintala granite-pegmatite system. The continuous increase in the degree of differentiation and fluxing components contributed significantly to the homogenization of zircon Hf isotopes and the mineralization of rare metals in the pegmatites. High Na2O/K2O and Sr/Y ratios and low Rb/Sr ratios and heavy rare earth element contents of the less evolved biotite granites, as well as the modeling results of batch melting and isotope mixing, suggest that primary magmas forming the Daiqintala granite-pegmatite system were derived by high-pressure partial melting of a predominantly juvenile basaltic lower crust, with a negligible contribution from the ancient metamorphic basement. Our findings reveal that not all LCT pegmatites are derived from metasedimentary sources; instead, weakly peraluminous granitic magma derived from rare metal−depleted, thickened basaltic lower crust can also generate rare metal pegmatites through multistage fractional crystallization. This mechanism may be applicable to many LCT pegmatites associated with high Sr/Y granitic complexes in postcollisional settings.