Whole-rock and zircon geochemistry of the Xiaoliugou granites, North Qilian Orogen (NW China): Implications for tectonic setting, magma evolution and W-Mo mineralization

摘要

Ordovician W-Mo polymetallic mineralization-related granites are widely developed in the western section of the North Qilian Orogen, NW China. In order to shed light on the tectonic setting, magma evolution and genesis of mineralization in the orogeny, highly evolved granitic rocks (monzogranites) from the Qibao and Qiqing deposits in the Xiaoliugou ore field were studied. Typical of subduction-related A-type granites, the Xiaoliugou granitic rocks are enriched in SiO2, Rb, Th, U, Ta, Nd, Zr and Hf but relatively depleted in Ba, Sr, P and Eu. The variable initial whole rock Sr-87/Sr-86 ratios (0.62937-0.70543) and wide range of zircon epsilon Hf(t) values (-18 to 6) suggesting mantle-crust interaction and mixing of depleted mantle and deep crust in the source area. Whole-rock elemental and Rb-Sr isotopic geochemistry suggest slow cooling, significant fractional crystallization, weak wall-rock assimilation/contamination and intense fluid differentiation/metasomatism during the genesis of the Xiaoliugou granitic rocks. Zircon U-Pb dating yielded three age clusters: Precambrian (> 540 Ma) (n = 32), Ordovician (similar to 450 Ma) (n = 40), and Cretaceous (similar to 135 Ma) (n = 18). Zircon internal textures indicate a range of sources including residual/detrital zircons from the magma source region and local sediments, magmatic and fluid-altered zircons, and late stage hydrothermal zircons. Zircon trace element and Lu-Hf isotopic compositions suggest fluid metasomatism during granite emplacement in the Ordovician and overprinting by a more evolved, oxidized fluid in the Cretaceous. Involvement of a late, high-temperature, highly fractionated, F-rich fluid could have enhanced W-Mo mineralization. Consequently, a two-stage genetic model is proposed for the Xiaoliugou granitic-minerogenic system involving primary mineralization in the Ordovician (similar to 450 Ma), followed by a Cretaceous magmatic hydrothermal overprint (similar to 135 Ma).