Geochemistry and zircon U-Pb age of the Permian radial diabase dyke from southern Xinyuan area, Western Tianshan Mountains, Xinjiang, and their geological significance

The accretionary processes of the Western Tianshan Mountains is a hot topic but there is no consensus on the final closure of the Paleo-South Tianshan Ocean basin and subsequent post-collision process. The Permian radial diabase dyke exposed near the North Nalati Fault provides an opportunity for understanding these problems. The dyke shows a typical ophitic texture. The phenocryst is composed of plagioclase and pyroxene, and the substrate is mainly composed of plagioclase, pyroxene, chlorite and some other accessory minerals. The diabase dyke hassodium-rich, potassium-poor and peraluminous characteristics, and hence belongs to the calc-alkaline series. The dyke is characterized by depletion of Nb and Ta and enrichment of Rb, Th and La, and has mantle source material with positive ε_Nd(t) values (+ 3.3~+ 3.8) and low initial Sr((⁸⁷⁹Sr/⁸⁶Sr)_i=0.70479~0.70548)). These geochemical characteristics are similar to those of the Carboniferous basalt of the study area, suggesting that the parental magma was formed by the interaction of asthenosphere and lithosphere. Zircon U-Pb dating of the dyke yielded an age of 280.4 ±3.6Ma. Combined with regional tectonic background, the authors hold that these Permian diabaseswere formed fromintraplate extension after the collision stage which was closely related to the oceanic lithosphere break-off.