Latest Paleozoic (former Early Permian) volcanic rocks of Arbasay Formation are represented by typical intermediate-basic, acidic volcanic rocks and pyroclastic rocks, in south margin of Junggar Basin, northern to Bayingou ophiolite belt. By means of SHRIMP zircon U-Pb dating, the age of Arbasay volcanic rocks is (307.2±1.3) Ma (MSWD=1.2), which indicates that Arbasay volcanic rocks were erupted in Late Carboniferous, not Early Permian. With the similarity between Arbasay volcanic rocks, pyroclastic rocks and post-collisional granites in petrology and geochemistry, the proto-type of Late Paleozoic southern Juggar Basin should be extensional depression basin.