A suit of volcanic rocks outcrop in Xi Ujimqin Qi and is considered to be the west extension of the well-known Dashizhai Formation whose tectonic setting remains controversial. To constrain the age of formation and tectonic nature of the volcanic rocks, petrology, geochronology and geochemistry researches were conducted. The results revealed that the volcanic rocks were mainly composed of andesites and rhyolites. LA-MC-ICP-MS zircon U-Pb dating yielded ages of 275‒311 Ma. The andesites are rich in alkali but poor in MgO, slightly enriched in LREE, K, Rb, Ba but depleted in Nb and Ta, with slight negative Eu anomalies. Geochemical characteristics indicated the andesites were generated by fractional crystallization of synchronous basic magma, but contaminated by continental crust. The rhyolites are characterized by low CaO and MgO contents but with high SiO2, K2O and Na2O contents. Chondrite- normalized REE patterns of the rhyolites exhibit slight LREE enrichment, with striking negative Eu anomalies. The rhyolites are also enriched in LILE but strongly depleted in Nb, Ta, Sr, P, Ti, which suggests the acid magma was originated from partial melting of the mid-lower continental crust and underwent subsequent fractional crystallization. Geochemical plots of the andesites exhibit characteristics of within-plate basalt, while the rhyolites show an affinity with A2-type granites. Geochemical characteristics of the volcanic rocks in Xi Ujimqin Qi suggest a post-collisional setting. Combined with previous published regional geological data, we concluded that the Dashizhai Formation in Xi Ujimqin Qi was formed in a post-collisional extensional setting and the Paleo-Asian Ocean was closed no later than the late Permian.%内蒙古西乌旗地区发育一套中酸性火山岩,空间展布特征显示其为大石寨组火山岩的西延部分。为查明该火山岩的形成时代及构造属性,对其进行了岩石学、年代学和岩石地球化学研究。研究结果表明该火山岩主要由安山岩及流纹岩组成。锆石LA-MC-ICP-MS U-Pb定年结果显示其喷发时代为275~311 Ma,属晚石炭世–早二叠世。岩石地球化学特征表明中性岩富钙贫镁,富集LREE及K、Rb、Ba,亏损Nb、Ta,具有弱Eu异常,安山质岩浆可能是由基性岩浆分离结晶形成的,但在形成过程中受到了陆壳物质混染。酸性岩贫钙镁,富硅碱,具有显著的Eu负异常,亏损Nb、Ta、Sr、P、Ti,反映岩石成因与中下地壳的熔融及其后期的分离作用有关。在构造判别图解中,中性岩具有板内玄武岩特征,而酸性岩具有A2型花岗岩特征。结合区域已发表资料推断,西乌旗大石寨组火山岩形成于造山后伸展环境,暗示古亚洲洋至少在早二叠世之前已经闭合。
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