The Ganfang granite batholith formed in the Yanshan period is the most important ore- hosting rock in the Yifeng Li- Nb- Ta rare metals ore field. The aplite, being product of the late stage of magmatic evolution in the Ganfang granite, is closely related to the mineralization of aplite- vein type rare metals in the Yifeng area. However, little research has been done on these rocks. In this paper, petrographic, cassiterite U- Pb chronological and whole- rock geochemical analysis have been performed on the Tongan granitic aplite in the Tongan mining area, south of the Ganfang granite. Results of cassiterite LA- ICP- MS U- Pb dating show that the aplite formed at about 138.3 Ma. According to the results of geochemical analysis, aplite samples are characterized by high Al2O3, P2O5, CaO, and alkali (Na2O+K2O) contents and low Mg, Fe, and Ti contents. They have high A/CNK ratios (varying from 1.71 to 1.88) and low σ values (between 1.40 and 1.72), belonging to high potassium calc- alkaline rocks that are also highly peraluminous. In terms of trace and rare earth elements, the Tongan granitic aplite is enriched in Rb, U, Ta, Pb, P, and Hf elements and depleted in Ba, Th, Sr, and Ti elements, as well as highly depleted in rare earth elements. These characteristics indicate that the Tongan granitic aplite belongs to highly fractionated S- type granites. It was probably derived from the partial melting of metasedimentary rocks in the shallow crust, and experienced high degree of fractional crystallization in early stages, and magmatic- hydrothermal interaction in late stage. In addition, we believe that the early highly- fractional crystallization played an initial enrichment role in lithium mineralization, while the magmatic- hydrothermal interaction in the late evolution stage was the key to high enrichment of lithium and the eventual mineralization.