The south Altun Orogenic Belt, which is an important continental subduction- collision orogen belt, lies between the Qaidam basin and Qilian- Kunlun Orogenic Belt, and a number of early Paleozoic granites containing key information about the tectonic evolution of the orogenic belt have been discovered in this belt. The Mangya alkali- feldspar granites are ideal for research on the timing of the post- orogenic extensional environment and the crust mantle interaction; its genetic type, material source and tectonic environment need detailed study. In this paper, we present results of geochemical, LA- ICP- MS U- Pb and Lu- Hf isotopic analyses on the Mangya alkali- feldspar granites and discuss their tectonic implications. The samples are high in SiO2, iron, alkali, poor in calcium and magnesium, and depleted in Ba, Sr, P, Eu and Ti, belonging to A2- type granites; the granites were generated 403~424 Ma, and were product of partial melting of the Meso- Neoproterozoic juvenile crust materials (juvenile quartz- feldspathic materials or calc- alkaline granitoids). The magma source might have contained a small amount of Ca- rich plagioclase residuals. After 424 Ma, the South Altun was in post- orogenic extensional environment, and upwelling of the mantle materials was triggered by the isostatic adjustment between blocks, causing the partial melting of the crust materials which formed the A- type granites.