Abstract：Plentiful Caledonian and Indosinian intrusive rocks outcrop in the Longshan complex in the eastern Qilian Mountains, but accurate chronological data are lacking. LA-ICP-MS zircon U-Pb dating of the Yanjiadian diorite in the Zhangjiachuan-Qingshui area of the eastern Qilian Mountains indicates that the isotopic age of the diorite is 440.2±0.92 Ma（MSWD=1.13），which suggests that the island-arc complex represented by the Yanjiadian diorite formed in the Late Ordovician. Studies of regional geology and tectonic evolution indicate that in this area there occurred northward subduction of oceanic crust represented by the Hongtubao ophiolite during the Middle-Late Ordovician and island-arc magmatic complex zones represented by island-arc type volcanic-sedimentary series of the Middle-Late Chenjiahe Group and the Late Ordovician Yanjiadian diorite body.