Abstract: The magmatic-hydrothermal evolution during the differentiation of granitic magma is a key stage to the formation of magmatic pegmatites and related rare-metal mineralization. However, the controlling factors during this process are still unclear. Numerous Paleozoic felsic plutons and rare-metal-mineralized granitic pegmatites occur in the East Qinling tectonic belt, and their genetic relationship has attracted much attention. In order to examine the magmatic-hydrothermal evolution at the late stage of the felsic plutons and shed light on the formation of rare-metal mineralization in East Qinling, we carried out comprehensive analyses on in situ major and trace elements for tourmaline from the Piaochi pluton, the largest S-type granite in East Qinling. Tourmaline from the Piaochi pluton can be divided into four types, i.e., disseminated tourmaline (Tur-D), quartz-tourmaline nodules (Tur-N), tourmaline-quartz veins (Tur-V), and tourmaline along schistosity (Tur-d). The substitution of major elements of Tur-N and Tur-V tourmalines is mainly controlled by (Na, Mg) (X_vac, Al)_-1 and (Mg, OH) (Al,O)_-1, instead of Fe³⁺Al_–1 substitution. This indicates that the hydrothermal fluids of the Piaochi pluton are in a reducing environment with medium-high salinity content. Based on the occurrence and composition of Tur-D tourmaline, it can be further divided into Tur-D1 and Tur-D2. Tur-D1 is schorlite with a considerable amount of Al in the Y-site, indicating that they crystallize at an early stage of magma cooling. In contrast, Tur-D2 is dravite and replaces Tur-D1 in local places with Tur-D1 tourmaline inclusions, possibly due to the interaction with external fluids at a late stage. Tur-N tourmaline can be further divided into Tur-N1 and Tur-N2. Tur-N1 is schorlite with high Fe, Sn, Nb, Ta, and Zn and low Mg, V, and Sr, and crystallizes from immiscible aqueous boron-rich fluids at the late stage of magma differentiation. Tur-N2 and Tur-V tourmaline have similar compositions, varying from schorlite to dravite, with higher Mg, V, Sr, and Pd compared to Tur-N1. This suggests the influence of the external fluids from country rocks on the composition of magmatic-hydrothermal evolution of the Piaochi pluton. Tur-d tourmaline (dravite) is closely related to biotite, with magnetite occurring along its rims and fractures, indicating that the metasomatism of biotite by boron-rich fluids. Additionally, the high Nb and Ta concentrations of Tur-N1 tourmaline suggest a transient Nb-Ta enrichment on cooling of the Piaochi pluton. Nevertheless, the reduced condition and intensive interaction with country rocks may impede the Nb-Ta mineralization at a late stage of cooling. Therefore, rare metals would be enriched by the felsic melt differentiation and the exsolution of immiscible fluid during the magmatic-hydrothermal stage. However, the potential of rare-metal mineralization is mainly controlled by the redox condition of the fluids and the interaction with wall-rocks.