The Alxa occupies a critical position between the Tianshan-Beishan to the west and the Solonker suture to the east, and is significant for understanding the accretionary orogenesis of the southern Altaids. To unravel the tectonic history of the Alxa, we undertook an integrated study of field geology, geochemistry and geochronology of magmatic rocks and an accretionary complex. Six granites and one rhyolite from the Zhusileng-Hangwula Arc (ZHA) show peraluminous, (high-K) calc-alkaline composition, and one gabbro is tholeiitic. They show incompatible element-rich patterns and negative Nb-Ta anomalies on the primitive mantle-normalized trace element spider diagrams. LA-ICP-MS zircon U-Pb dating reveals 348-329 and 267-250 Ma magmatic events. These data indicate that the ZHA was a Japan-type island arc from late Paleozoic to Early Triassic. The Engger Us mélange comprises N-MORB-type pillow basalt, tuff, chert, siliceous mudstone, limestone and turbidite, which are tectonically juxtaposed by block-in-matrix structure. This mélange is interpreted as a Carboniferous–Permian accretionary complex. These new data, combined with previous studies, confirm that the Paleo-Asian Ocean subducted bidirectionally in the Alxa in Carboniferous–Early Triassic. A consistent divergent subduction system existed from Alxa to Solonker before the terminal closure of the Paleo-Asian Ocean in the Mid-Late Triassic.