SHRIMP zircon U-Pb dating of the Gangou granitoids,Central Tianshan Mountains,Northwest China and tectonic significances

Two types of granitoids,highly deformed augen granitoids and undeformed fine-grained granitoids,are widely distributed in Gangou area,Central Tianshan Mountains,Northwest China. The augen granitoids are high-K calc-alkaline characterized by high K2O,Rb,Y and Th,and low Sr and Sr/Y. They also have high contents of large ion lithophile elements (LILE),such as Ba,Rb,K and Th,and relatively low con-tents of high field strength elements (HFSE),such as Zr,Y and Nb. The fine-grained granitoids are calc-alkaline characterized by high Sr,low Y and HREE,similar to adakitic rocks,and with Na2O/K2O<2 (1.76―1.91),high ISr (0.70689―0.70981),and negative εNd(t ) (-2.4―-5.3). High-precision SHRIMP U-Pb zircon dating results indicate that the Gangou augen granitiods were formed at 428 ± 10 Ma,and the fine-grained granitoids were emplaced at 361―368 Ma. These geochemical and U-Pb zircon data have significant implications for the timing of closure of the Mishigou-Gangou Ocean and the evolution of the Central Tianshan orogenic belt.