The South Altun is a significant structural unit within the Altun Orogenic Belt in northwestern China. In this paper, we combine petrological and geochemical analyses with zircon LA–ICP–MS U–Pb and Lu–Hf data from the Mangya plutons, to understand the Paleozoic tectonic history of the South Altun Orogenic Belt. The Paleozoic magmatic events that formed the Mangya granitic rocks are divided into three episodes: 494–458 Ma; 450–436 Ma; and 416–404 Ma. We propose that the 484–458 Ma plutons record large-scale magmatic events associated with the northward subduction of the oceanic crust of the South Altun Ocean. Subduction-related magmatism produced voluminous high–K calc–alkaline series, high Sr/Y granitic rocks. After closure of the South Altun Ocean, collision of the Qaidam Block with the South Altun Continental Block from 450–436 Ma was driven by slab drag, and high–K calc–alkaline series, I-type granites were produced. Eventually, the subducted oceanic crust delaminated, and the 416–404 Ma A-type granites formed in a post–orogenic extensional environment.