The tectonic unit division of northern Lesser Khingan Mountains is located in the Songnen block, and Jiushushan-Xiaoxing an intrusive arc. The intrusive rocks in this area are of great significance for constraining the subduction process and mechanism of the Nanjing Ocean. This work conducted zircon U-Pb geochronology and lithogeochemical studies on alkali-feldspar granite and monzonite in northern Lesser Khingan Mountains to determine the formation age of this episode of intrusive rocks and discussed the petrogenesis and tectonic setting. The results show that the alkali-feldspar granites and monzonite in the study area both represent a Late Carboniferous magmatic evolution event from 309 to 298 Ma. Geochemical analysis results indicate that the Late Carboniferous monzonite and alkali-feldspar granite in the study area are both characterized by high differentiation. Comprehensive geochronology and geochemical results show that the alkali-feldspar granite in the study area (located on the southeast side of the Hegen Mountain-Heihe suture zone) is A-type granite, and the monzonite is highly differentiated I-type granite. These findings align with the known suite of I- and A-type granites emplaced during Late Carboniferous post-orogenic extension in the Longjiang, Nanjing, and Taxi areas. The Nanjing Ocean was completely closed and entered an extensional stage in the Late Carboniferous in the northern Lesser Khingan area. At the same time, the Nanjing Ocean also subducted toward the Songnen block in the southeast direction. This discovery provides strong evidence for the bidirectional subduction model of the Nanjing Ocean's closure.