The South Altun orogenic belt is an important subduction- collision complex belt in northwest China, located between the Tarim and the Qaidam basins. Its early Paleozoic tectonic evolution has been one of the research hotspots in recent years. However, the timing of oceanic crust subduction has been controversial. In this paper, we study the petrology, geochemistry, zircon U- Pb chronology and zircon Lu- Hf isotope geochemistry of the Manya quartz monzonite, and discuss its petrogenesis and the tectonic environment. The samples show high alkali, potassium- rich, low- titanium, iron- poor and Nd- Ta- Ti anomalies similar to shoshonitic rocks. The quartz monzonites were generated between 511 Ma to 495 Ma, and the values of εHf(t) mainly range from 3. 51 to 0. 08, with some positive values ranging from 0. 04 to 1. 69. Based on our research, we infer that the subducting oceanic crust released large amounts of water when arriving at amphibolite facies boundaries, and triggered the mantle wedge peridotite hornblende metasomatism. Due to the dragging of the subducting oceanic crust, the temperature of the mantle wedge rose, causing the melting of the hornblende metasomatized peridotite, which triggered the partial melting of upper crust materials. Finally, crustal melts mixed with smaller mantle melts to form quartz monzonite. The ~500 Ma granites are petrological response to transition from oceanic island arc environment to active continental margin. Thus, the southern Altun oceanic crust may have begun subducting at around 517 Ma.