中文摘要：

内蒙古中部苏尼特左旗地区大石寨组酸性火山岩SHRIMP锆石U-Pb测年结果显示,2件流纹岩样品的206Pb/238U年龄加权平均值分别为272.1±3.1Ma和271.4±1.7Ma,说明大石寨组流纹岩喷发于早二叠世。锆石Hf同位素分析结果表明,εHf（t）值为＋10.8～＋13.1;Hf-Nd同位素耦合关系计算显示,其与同期形成的大石寨组玄武岩Nd同位素组成特点非常相似。全岩地球化学分析表明,该流纹岩高Si O_2,贫Ca O、Fe_2O_3＊和Mg O,大部分样品属于过铝质、钙碱性流纹岩;富集轻稀土元素,轻、重稀土元素分馏中等,具明显的负Eu异常,富集Cs、Rb、Th、U和Zr,亏损Ba、Sr、Nb、Ta、P和Ti,指示其是富水的钙碱性基性地幔岩浆分离结晶作用的产物。结合区域地质资料,认为苏尼特左旗流纹岩形成于大陆岛弧背景。

英文摘要：

SHRIMP zircon U-Pb ages of 272.1±3.1Ma and 271.4±1.7Ma from two rhyolite samples, which come from Dashizai Formation rhyolites from Sunid Left Banner in the middle part of Inner Mongolia, respectively uniformly indicate that these rhyolites of the Dashizhai Formation erupted in the early Permian. Their initial Hf isotopic ratios range from ＋10.8 to ＋13.1. According to the HfNd coupling relationship of terrestrial array, the authors obtained their corresponding conversion Nd values, which are very similar to Nd isotopic initial values of the coeval basalts exposed in the Dashizhai Formation and some other places, implying their common derivation from the depleted mantle. The rhyolites are characterized by high content of Si O_2, but low content of Ca O, Fe_2O_3＊, and Mg O.Most samples exhibit sub-alkaline and peraluminous characteristics, and show moderate fractionation REE patterns with pronounced negative Eu anomalies. In the spider diagram, they show enrichment in Cs, Rb, Th, U and Zr, and depletion in Ba, Sr, Nb, Ta, P andTi. All these features reveal that these rhyolites evolved from calc-alkaline mafic magmas with enriched water by fractional crystallization of mineral phases, and were probably generated in the continental arc system.