A curved belt consisting of terranes or blocks and associated Late Paleozoic-Early Mesozoic sutures features the southeastern part of the Tibetan Plateau. It is the Sanjiang (Three-Rivers) multiple orogenic and ore-forming belt. In spite of numerous studies, its Paleotethyan (Late Paleozoic to Early Mesozoic) tectonic evolution remains a topic of much controversy. Recently published geochronological and geochemical data derived from modern techniques have revealed several magmatic zones with variable tectonic affinity. However, the temporal and spatial distribution pattern of the magmatic rocks didn't appear, mainly due to the absence of high quality data from some regions. As the Lanping Basin is located in the middle portion of the Sanjiang orogenic belt, the time and tectonic affinity of the volcanic rocks in this basin play a significant role in revealing the connection between the magmatic zones in the southern and northern segments of the Sanjiang belt. This study focused on the volcanic rocks exposed on the western margin of the Lanping Basin where high-quality geochronological and geochemical data are rare. The volcanic rocks in western Lanping Basin consist of andesitic tuff in the lower segment, andesite in the middle segment, and basaltic lava at the top. This volcanic sequence is developed upon the Carboniferous terrigenous clastics. The basaltic lavas are interlayered with reddish mudstone. The content of mudstone increases upwards. Geochemical data demonstrate that these volcanic rocks are of calc-alkaline or high-K calc-alkaline series, characterized by high Al₂O₃ and very low TiO₂, MnO₂, and P₂O₅ content. The andesitic samples exhibit arc-like chondrite- or primitive-mantle-normalized trace element pattern, whereas the basaltic lava displays enriched-middle-oceanic-ridge-basalt pattern. These features, combined with the field relationship, suggest that these volcanic rocks were formed in a continental arc setting. New LA-ICP-MS zircon U/Pb dating results indicate that the bottom crystal tuff was formed at 238 Ma, the andesite of the middle segment at 233 Ma, and the basaltic lava at the top of the volcanic sequence erupted at 229 Ma. These new data suggest that, at the end of the subduction, the breakup of the subducted slab might have taken place which led to the uprising of asthenosphere and induced partial melting of the enriched mantle wedge. The new data obtained by the authors have filled the spatial gap of geochronological data in the Sanjiang orogenic belt. As a result, a large scale (with a length >2 000 km) successive arc-like volcanic belt along the entire Sanjiang belt has become clear. Such a large arc-volcanic belt should be affiliated to a large-scale subduction zone (suture). Synthesizing available geological, geochemical, and geochronological data, the authors hold that the eastward or northeastward subduction of the Paleotethys along the Longmu Co-Shuanghu(-Lancangjiang)-Changning-Menglian suture probably resulted in the formation of the large-scale volcanic belt.