There is still in great controversial about the formation process of the Xing' an-Mongolian Orogenic Belt, which involves where the suture position is and when the final closure of the Paleo-Asian Ocean happens. In order to reconstruct the sedimentary paleogeography in the northern margin of Xar Moron River, detailed sedimentary environment and sedimentary facies analyses together with zircon U-Pb geochronological studies on the Zhesi Formation in Hexigten Banner have been carried out. Sample 170917-07 and 170915-05 are collected from the tuffaceous sandstone and andesite of this formation, respectively, to study their zircon ages. Based on the zircon CL images and U-Pb concordia diagrams, the zircons in sample 170917-07 can be divided into four groups : 247 similar to 309Ma (N = 25) with a peak age of 263Ma, 401 similar to 475Ma (N =26) with a peak age of 440Ma, 581 similar to 1424Ma (N = 10) , and 1731 similar to 2446Ma (N = 7) ; while the 21 zircons in sample 170915-05 analyzed are concordant, and show four groups : group A (258 similar to 261Ma, N = 2) , group B (281 similar to 298Ma, N =5) with a concordant age of 285 +/- 2Ma, group C (315 similar to 485Ma, N = 8) , and group D (1033 similar to 2180Ma, N = 8). The youngest concordant age groups of the samples analyzed are 262 +/- 1 Ma and 258 +/- 17Ma, respectively, which constrains that the maximum depositional time of the Zhesi Formation is mid-late Permian. With regard to the Permian tectonic pattern in study area, some researchers believe that the lithology across the Xar Moron River near Hexigten is dominated by broadly northeast trending Permian volcanoclastic turbiditic successions. However, this study shows that the Zhesi Formation is characterized by near-source rapid accumulation and fast subsidence. Accompanied by active volcanic activities, the sedimentary environment of this formation belongs to multiple alternations of delta front and prodelta facies. Detailed observations of field sections suggest that the Zhesi Formation, alternatively developed with thick layered olistostrome, is not characterized by a continuous Bouma Sequence which consists of monotonous graded bedding, but rather than an irregular interfacial sequence from sandstone to siltstone or mudstone. As a result, the Permian paleogeographic frame in the study area doesn't have conditions for the development of deep-sea turbidite fans, which implied that there was no extensive continental margin to the south during the Middle Permian. On the other hand, according to paleocurrent recovery and provenance analysis, the main sources of the Zhesi Formation are composed of the volcanic activities in the same period, the North China Craton and the Early Paleozoic continental margin arc in the south of the study area, which suggests that the studied area has been connected with the North China plate in Middle Permian, and there is no such barrier effects as an oceanic subduction zone. To sum up, the paleogeographic frame in Middle Permian doesn't possess the developmental conditions of deep-sea turbidite fan, but the extensional environment of rapid accumulation in a near source environment. Furthermore, the broad southward continental margin and the subduction zone representing the plate boundary don't exist in Middle Permian.
