LAUSR

Abstract The southern Central Asian Orogenic Belt (CAOB) in South Mongolia consists of continental fragments sutured together by Paleozoic arc magmatism generated by the closing of the Paleo-Asian Ocean (PAO). Controversy persists regarding the timing and position of the final amalgamation of the Altai and Tianshan orogenic belts. This paper presents zircon U-Pb data of the Gurvan Sayhan accretionary complex, as well as the overlying sedimentary sequence, and a granite pluton intruding the complex to provide new insights into the Late Paleozoic evolution of South Mongolia. The detrital zircon grains from the Gurvan Sayhan accretionary complex yielded age spectra with predominant peaks at ca. 331 Ma, subdominant peaks at ca. 381, and 500 Ma, indistinct Precambrian peaks at ca. 848 Ma and the youngest group of 318 Ma. The Paleozoic zircon grains are interpreted to be derived from adjacent arc systems formed by the subduction of the PAO, whereas those with the Precambrian ages were possibly from the nearby Hangay and South Gobi microcontinent blocks. The proportion of Precambrian detrital zircon grains in the complex decreases with time, likely suggesting a change in the paleogeography during sedimentation of the complex. Detrital zircon grains from the overlying stratum produced an age spectrum characterized by containing the youngest peak at ca. 293 Ma, although older peaks at ca. 340 Ma and 466 Ma are also present. The granite pluton intruding the accretionary complex has an emplacement age of 292.9 ± 1.3 Ma, similar with the youngest detrital zircon age peak from the overlying stratum, which is interpreted from the post-collisional magmatic rocks. Therefore, our results demonstrate that the final closure of the PAO in southern Mongolia was Late Carboniferous–Early Permian (318–293 Ma), and the Gurvan Sayhan accretionary complex represents the suture zone of the Altai and Tianshan orogenic belts.