Petrogeochemical characteristics, zircon SHRIMP U—Pb ages and Lu—Hf isotopic compositions of Late Carboniferous A‐type granitoids, Yili area, Inner Mongolia (China)

Abstract

A large Late Palaeozoic A‐type granite belt lies along the Hegenshan‐Nenjiang‐Heihe belt in the Central Asian Orogenic Belt (CAOB). The Kuleqi, Naimanhe, and Gejiadian plutons at Yili are developed within this A‐type granite belt. The genesis and formation environment of these A‐type granites are controversial. The results of zircon SHRIMP UPb dating analyses indicate that the A‐type granites at Yili formed during the Late Carboniferous (302–307 Ma), and their geochemical data indicate that they principally represent a weakly peraluminous (A/CNK 1.01–1.21), high‐K calc‐alkaline series. These granites are enriched in HFSEs, Th, Rb, and K and depleted in LILEs, Ba, Nb, Sr, P, and Ti, which suggests that they record signatures of evolution of continental magma arc. The Nb/Ta ratios of these granites range from 6.84 to17.51, with an average value of 11.65, indicating that they represent crustal‐sourced magma. Their Zr/Hf ratios range from 37.05 to 88.70, with an average value of 59.72 (all values >39), suggesting that these rocks are dominated by mantle‐sourced components. The zircon Hf isotopic compositions of these rocks show εHf(t) values ranging from 6.8 to 12.5 (all positive), with an average value of 9.9, and depleted mantle two‐stage model ages ranging from 518 to 887 Ma, implying that these zircons may contain Early Palaeozoic–Neoproterozoic depleted mantle material. Based on these data, combined with their trace element characteristics, we conclude that the A‐type granitoids at Yili were mainly produced by the mixing of crust and mantle materials; together with their regional tectonic characteristics, these data indicate that these granitoids are related to the collision of the Xing'an and Songliao blocks, which was triggered by the subduction of the Palaeo‐Asian Ocean.