Abstract: In order to understand the Late Paleozoic tectonic setting of eastern part of Central Asian Orogenic Belt more deeply, the Permian volcanic rocks from Xi Ujimqin Banner, Inner Mongolia were selected to study their geochronology and geochemistry. The contents of major elements, the age, the contents of trace elements and REEs and the Sr-Nd isotopes of the volcanic rocks were measured by X-ray fluorescence spectrometry (XRF), SHRIMP zircon U-Pb, ICP-MS and ISOPROBE-T, respectively, and the results are as follows. The volcanic rocks consist of subordinate basaltic andesites and dominant rhyolites, with the SiO₂ content ranges of 53.54%?54.71% and 72.61%?80.81%, respectively. The SHRIMP zircon U-Pb age of one rhyolite specimen is about 279 ± 4.3 Ma, indicating that the rhyolites erupted in Early Permian. The basaltic andesites are characterized by slightly enriched LREE, depleted HREE, slightly negative Eu, Nb, Ta and Sr anomalies, and high abundances of LILE and HFSE, with the (La/Yb)_N ratios of 2.06 to 2.34. The mafic rocks may be inferred to generate from deep non-primitive mantle largely mixed by crustal source according to these features above. The rhyolites are characterized by slightly enriched LREE, depleted HREE, intensively negative Eu, Sr, P and Ti anomalies, slightly depleted Nb and Ta, and high abundances in K, Zr and Hf, with the (La/Yb)_N ratios ranging from 1.70 to 3.81, relatively high positive ε_Nd(t) values (+3.98?+5.62) and relatively young T_DM (490?690 Ma). The felsic rocks show an A-type affinity and may be inferred to come from a juvenile crustal source. According to their geochemistry, regional geology and previous studies, these bimodal volcanic rocks were generated by intra-plate volcanism and formed in a post-collisional extensional setting.

Key words: Early Permian, bimodal volcanic rocks, extensional tectonic setting, Solonker suture zone

摘要： 为了加深对中亚造山带东段晚古生代构造背景的认识, 选取内蒙古西乌旗地区二叠纪火山岩进行年代学和地球化学研究。分别利用X光荧光光谱(XRF)、SHRIMP锆石U-Pb测年、ICP-MS和ISOPROBE-T方法测定研究区火山岩的主量元素含量、年龄、微量元素和稀土元素含量及Sr-Nd同位素。结果表明, 这套火山岩的SiO₂含量集中分布在53.54%~54.71%和72.61%~80.81%两个区间, 分别属于玄武安山岩和流纹岩, 构成双峰式火山岩组合。流纹岩SHRIMP锆石U-Pb年龄为279 ± 4.3 Ma, 说明它喷发于早二叠世。玄武安山岩轻稀土略富集而重稀土略亏损, (La/Yb)_N为2.06~2.34, 有轻微的Eu负异常, 富集大离子亲石元素和高场强元素, Nb, Ta和Sr有明显的负异常, 推断其来源于深部非原始地幔, 很大程度上受地壳成分的混染。流纹岩亦略富集轻稀土, 略亏损重稀土, (La/Yb)_N为1.70~3.81, 有很强的Eu负异常, Sr, P和Ti相对强烈亏损, Nb和Ta轻度亏损, K, Zr和Hf相对富集, 具有较高的正ε_Nd(t)值(+3.98~+5.62)和年轻的模式年龄T_DM(490~690 Ma), 具有A型花岗岩的特征, 推断其为新生陆壳来源。综合地球化学特征、区域地质特征和前人研究资料, 这套双峰式火山岩具有板内成因特征, 形成于碰撞后的伸展环境。

关键词: 早二叠世, 双峰式火山岩, 伸展构造环境, 索伦缝合带