The discovery of Middle-Late Ordovician syenogranite on the southern margin of Altun orogenic belt and its geological significance

The syenogranite is located in the Paxialayidang ditch of the Altun Mountains. The U-Pb dating of zircons from the syenogranite using LA-ICP-MS yielded a group age of 455.1±3.6Ma, indicating that the crystallization of the intrusion occurred in Middle-Late Ordovician period. The geochemical analysis shows that major elements are characterized by high SiO₂, Al₂O₃ and K₂O and low TiO₂, CaO and MgO, which suggests that syenogranite belongs to the typical high-K calc-alkaline series with deeply peraluminous feature. In addition, the rocks are enriched in total REE. The samples are enriched in LREE (light rare earth elements) and depleted in HREE (heavy rare earth elements) with Eu anomalies. The chondrite-normalized REE patterns show right-oblique type. The syenogranite is enriched in large ion lithophile elements of Rb, Th, K and depleted in high field strength elements of Ba, Sr, Ti, with the characteristics of S-type granite. In combination with the diagrams for discriminating compositions of original rocks, the authors hold that the rocks were formed by the partial melting of meta-pelitic sedimentary rocks from the lower crust. Combined with the data of regional geological characteristics, the authors consider that the syenogranite was formed in the transitional tectonic setting from the compressional to the extensional regime, thus belonging to the post-collisional granites. It is shown that Azhong Block and Qaidam Block entered into a transformation period from compression to extension during Middle-Late Ordovician period.