The Paxialayidang plutons are located from the Paxialayidang ditch on South Altyn Tagh. The granitic rocks are mainly composed of monzonitic and porphyritic monzonitic.The UPb dating of zircons from the Paxialayidang plutons using LAICPMS yields a group age of 460.1±3.9Ma , indicating the crystallizedage of the plutons in Middle—Late Ordovician epoch. The geochemical analysis shows that major element characterized by high SiO2(72.08%～72.27%), Al2O3(14.10%～14.79%), A/CNK=1.06～1.18 and K2O(5.55%～6.33%)contents; low TiO2(0.19%～0.23%),MgO(0.22%～0.28%)and TFeO(1.48%～1.79%）contents, which suggest that syenogranite belong to the typical highK calcalkaline series with weakly peraluminous. In addition，they are enriched in total REE contents (165.58×10-6～229.86×10-6).The samples are enriched in LREE (light rare earth elements) and depleted in HREE (heavyrare earth elements) with Eu anomalies (δEu=0.11～0.17). The chondrite normalized REE pattern curve shows rightsloping type. The syenogranitees are riched in large ion lithophile elements of Rb,Th,K,U and depleted in high field strength elements of Ba,Sr,Ti and Rb/Sr value(9.05～12.57), with the characteristics of Stype granite, combining with the diagrams for discriminating compositions of original rocks, indicate the rock is forned by the partial melting of meta pelitic sedimentary rock from lower crust. Combined with the dataes of regional geological characteristics, the Paxialayidang plutons were formed in the transitional tectonic setting from the compressional to extensional segimes, belonging to the postcollisional granites. It shows that Altyn Tagh orogenic belt between blocks on both sides of Azhong Block with Qaidam Block suggests that it entered into transformation period from compression to extension during Middle—Late Ordovician epoch.