Petrogenesis and geological significance of the Late Carboniferous granites from Gangou, Central Tianshan Mountains, Xinjiang

Granitoid plutons are widespread in Gangou of Central Tianshan orogen. Previous studies of the augen granite and the fine-grained granite from north Gangou revealed that their SHRIMP U-Pb dating ages were 428 Ma and 361-368 Ma, respectively. Comparatively, zircon LA-ICP-MS U-Pb dating of the south granitoid pluton in this study yielded the ²⁰⁶Pb/²³⁸U age of 301±1.0 Ma. Major and trace elements show that the granites belong to high-K calc-alkaline series, and their σ=2.67-3.22. In addition, they are enriched in LREE, Rb, K, Th, La, Zr and Hf, and depleted in Ba, Sr, Nb, Ta, P and Ti. They show a transitional features from I-type to A-type. Compared with the Silurian-Devonian granites, late-Carboniferous granits are characterized by lower I_Sr values (0.704771-0.705451), higher ε_Nd(t) (2.14-2.51) and younger T_DM2 (861-891 Ma), suggesting that more new depleted mantel-derived materials contributed to the generation of the granites during post orogen. It is therefore held that the petrogenesis of the late-Carboniferous granites was related to the underplating of the mantle-derived magmas. This study also provides new evidence for Phanerozoic vertical continental crustal growth in the Central Asian orogenic belt.