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

摘要

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 K_2O, 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 contents of high field strength elements (HFSE), such as Zr, Y and Nb. The fine-grained granitoidsare calc-alkaline characterized by high Sr, low Y and HREE, similar to adakitic rocks, and with Na_2O/K_2O<2 (1.76-1.91), high I_(Sr) (0.70689-0.70981), and negative e_(Nd)(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.