Abstract: The Triassic granites in the Lancang-Menghai area of western Yunnan belong to the southern part of the Lincang granite belt, which are distributed in a nearly north-south direction. The main lithology is biotite monzogranite with weathered crusts, which has favorable metallogenic conditions for ionic rare earth deposits. In order to understand the internal relationship between the Triassic granite and REE mineralization, the petrological and geochemical characteristics of the Triassic biotite monzogranite and its weathered crust have been studied. The medium-grained (medium-coarse) or porphyritic biotite monzogranite in the inner facies zone of Triassic granite is more favorable for mineralization than the fine-grained biotite monzogranite in the outer facies zone. Under the effect of weathering and leaching, rare earth elements in biotite monzogranite are separated from rare earth independent minerals or rare earth-containing accessory minerals and then adsorbed into the whole weathered regolith clay minerals for mineralization. The biotite monzogranite is characterised by high silicon, rich potassium and rich aluminum. The completely weathered layer is more aluminum-rich, while the contents of MgO, TFeO and CaO decrease, and the content of trace element Ce decreases significantly. The characteristics of other major elements, trace elements and rare earth elements inherit the characteristics of parent rocks. A comprehensive study shows that the Triassic biotite monzogranite in the study area has the basic condition for the mineralization of adsorbed rare earth deposits, and the contents of dissociable rare earth independent minerals and rare earth-containing accessory minerals are important factors for the formation of ionic rare earth deposits.

Key words: Triassic, granite, geochemical, rare earth mineralization, Lancang-Menghai area