Zircon U-Pb Dating and Petrogenesis of Alkali-feldspar Granite in Mangnai Area,South Altun,NW China

The Chaishuigou alkali-feldspar granite is occurred in Southern Altun subduction-collision complex, which belongs to high-K calc-alkaline and metaluminous series. These rocks have high SiO₂(75.52%~77.58%) and K₂O/Na₂O(1.32~1.56) ratios, but they have low TiO₂(0.16%~0.20%), Al₂O₃(11.14%~11.86%), MgO(0.15%~0.23%) and CaO(0.66%~1.45%) contents. These rocks are characterized by LREE enrichment and depleted HREE, with obvious Eu anomaly and "seagull" REE distribution. They are enriched in large ion lithophile elements (Rb,Th,U,K), while depleted in high field strength elements(Nb,Ti,P). Therefore, the alkali-feldspar granite in Mangnai area is the typical A-type granite. The LA-ICP-MS zircon U-Pb dating results reveal that the alkali-feldspar granite in Mangnai area was formed at ca. (408.7±1.7)Ma. Based on the testing data and some previous research results, it's suggested that alkali-feldspar granite in Mongnai area was formed by partial melting of low crust during the stage of mantle material underplating in extensional tectonic setting. A-type granites (424~385Ma) were formed in the same tectonic-magmatic events in south Altun, indicating that a continuous extensional tectonic setting in the region.