Objectives: Two- mica monzonite granite, which is the direct surrounding rock of the mineralized pegmatite veins, is widely distributed within the Bieyesamasi Li—Nb—Ta mining area, Xinjiang Altay orogenic belt, China. This rock is the only identified Ordovician granite so far in the North Altay. The aim of this paper is to investigate the petrogenesis and geodynamic significance.Methods:Based on the petrologic work in the field and under the microscope, the whole- rock major and trace elements (including the rare earth elements) and the Nd isotopes were carried out on the Bieyesamasi two- mica monzonite granite in this paper.Results: The Bieyesamasi two- mica monzonite granite is characterized by high SiO2 (72.93%~73.14%) and Al2O3(14.60%~14.82%), relatively high potassium (K2O/Na2O=1.17~1.27), moderate total alkali (Na2O+K2O=7.96%~8.30%) contents, and high A/CNK values (1.21~1.28), belonging to the high- K calc- alkaline strongly peraluminous series; it is rich in LREE (light rare earth elements), depleted in HFSE (high field strength elements) (Zr, Hf, especially Ti), has low La/Yb values, and exhibit obvious negative Eu anomalies (δEu=0.43~0.47); the samples yield negative εNd(t) values (-4.2~-2.9) and high Nd mode ages (1.417~1.523Ga).Conclusions: The above geochemical and isotopic compositions indicate that the Bieyesamasi two- mica monzonite granite belongs to a highly fractionated I- type granite. It was mainly derived from the partial melting of the ancient crust, and mixed with some newly accreted crustal sources. The Bieyesamasi two- mica monzonite granite was formed in an initial continental arc setting, which is consistent with the tectonic environment drew from the other Ordovician granites identified in the Central and South Altay. The Bieyesamasi two- mica monzonite granite Mineralization of the Bieyesamasi deposit did not occur in the Ordovician continental arc setting. Combined with the regional studies, the Ordovician continental arc is not favorable for the occurrence of rare metal mineralization in the Xinjiang Altay.