Abstract: The Jiujingzi ophiolite belongs to the eastern segment of the Xar Monron River ophiolite belt in southeastern Inner Mongolia. In this paper, the authors carried out zircon U-Pb dating of a gabbro dike of the Jiujingzi ophiolite and the surrounding siltstone that is in fault contact with the ophiolite. The results indicate that the gabbro dike was formed in late Early Permian (274.7± 1.7 Ma, MSWD=0.079). Considering previous geological data on strata, paleontology and magmatic rocks, the authors hold that there was still an ocean basin in late Early Permian in southeastern Inner Mongolia. The detrital zircons of the adjacent siltstone constitute four age peaks:2350-2700 Ma, 1700-2100 Ma, 370-470 Ma and 250-290 Ma. A comparison with regional tectonothermal events shows that the provenance of the siltstone was mainly from the north margin of the Sino-Korean paleoplate. The age of the youngest zircon is late Early Permian-early Early Triassic (249±4.7 Ma), which is similar to the ages of the disappearance of the marine strata in southeastern Inner Mongolia, the mixing between the Cathaysian and Angaran floras, the convergence of the palaeolatitude curves of the Siberian and Sino-Korean paleoplates and the formation of the collision-related magmatic rocks. It is thus suggested that the tectonic emplacement of the Jiujingzi ophiolite might have occurred in late Early Permian-early Early Triassic, which might represent the final closure time of the Paleo-Asian Ocean.