Abstract: This study carried out analyses of petrology, zircon U−Pb isotopic geochronology and rock geochemistry of the Late Carboniferous monzogranite in Xiaolulangou of Xilinhot, in order to make the discussion of tectonic environment and provide a new basis for studying the closure time of the Paleo−Asian Ocean. Based on the LA−ICP−MS zircon U-Pb dating results, determines that the age of emplacement of the granite is (319.5 ±1.2) Ma, and equivalent to Late Carboniferous. Petrological and geochemical data reveal that the rock is characterized by high silicon, high potassium, poor magnesium, low calcium, low phosphorus, and titanium, It belongs to the high−K calc−alkaline and peraluminous series of rocks; The rocks are rich in large Ionic lithophilic elements Rb and high field strength elements Th, Ta, Hf, while depleting Sr, Ba, Nb; Rare earth elements are characterized by slightly enriched light rare earth, Eu losses. The geochemical characteristics reflect that it is partial melting from the low crust. The tectonic environment discriminant diagrams show that this rock is I-type granite representing active continental margin, indicating that the subduction of the oceanic crust during the Late Carboniferous period still existed, which implying that the Paleo−Asian Ocean didn’t close in Late Carboniferous.