Early Paleozoic magmatic rocks are sporadically distributed in the western section of Langshan Mountains of Inner Mongolia, while Late Silurian two- mica monzonitic granite pluton intruding the Baoyintu Group is mainly exposed in the Chaogewenduer region. LA- ICP- MS zircon U- Pb age shows that the 206Pb/238U weighted average age of two- mica monzonitic granite is 419. 6±4. 5~416. 0±2. 0 Ma. The monzonitic granite with dark biotite and visible muscovite shows massive structure and is characterized by high Si (SiO2=73. 14%~75. 76%), rich alkali (Na2O+K2O=5. 85%~8. 20%), strong peraluminous, and no diorite enclaves, indicating the features of S- type granite. The geochemical characteristics of the pluton shows that enrichment in Sr, K, Pb, depletion in Nb, Ta, P, Ti, slightly enrichment in LREE, without obvious Eu anomalies. Negative εHf(t) values (-8. 3~-4. 0) and old tDM2 values (1658~1930 Ma) of zircon imply characteristic of melting of Paleoproterozic- Mesoproterozic crust. R2- R1 diagram shows that the rocks are characterized by syn- collision granites, suggesting that the rocks likely formed in the tectonic setting of syn- collision amalgamation and are consistent with the structure nature of Late Silurian granite in the Shandaimiao area. Compared with the Late Silurian quartz diorite in the Bainaimiao area, both rocks have similar geochemical characteristics, reflecting the same tectonic setting.