Abstract: The Carboniferous volcanic rocks of Benbatu Formation at Chagannuoer in Sonid Youqi, Inner Mongolia, consist of basalts with pillow and massive structure and dacites. The SiO₂ content of the volcanic rocks mainly fallen in two ranges 48. 20% -51. 11% and 64. 66% -65. 42% , showing a bimodal distribution composition. The mafic volcanic rocks are mainly alkaline basalt series, with Na-rich and potassium-deficient. The felsic volcanic rocks belong to calc-alkaline series with high alkali content and about 3% K₂O content. The pillow basalts are characterized by slightly loss LREE, enriched HREE, slightly positive Eu anomalies and high abundances in Hf, Zr and Th (HFSE). The massive basalts are characterized by enriched Rb, Th, Zr, Hf and loss Ta, Nb. The dacites have obvious severity of REE fractionation with Eu negative anomalies significantly, enriched Rb, Th, Pb, Hf, Zr and loss Eu, Sr, Nb, Ta. According to Rb-Sr isochron age, SHRIMP U-Pb zircon dating data and paleontological data, Chagannuoer bimodal volcanic rocks formed in Late Carboniferous, about 313-308 Ma. The characteristics of geochemistry and regional geology suggest that Chagannuoer bimodal volcanic rocks formed in post-collisional extensional setting, so it can be inferred that the Solonker suture zone had ever been closed before the Late Carboniferous.

Key words: Solonker suture zone, bimodal volcanic rocks, pillow basalt, Late Carboniferous, Sonid Youqi

摘要： 内蒙古查干诺尔地区石炭系本巴图组地层的火山岩, SiO₂含量集中分布在48. 20% ～51. 11% 和 64. 66% ～ 65. 42% 两个区间, 分别属于玄武岩和英安岩, 构成一套双峰式火 山岩组合。其中基性火山岩具有枕状和块状构造, 属于碱性玄武岩系列; 英安岩的碱含量较高, 属于钙碱性系列。基性火 山岩中, 枕状玄武岩轻稀土略亏损而重稀土略富集, 具有轻微 Eu 的正异常, 富集Hf,Zr, Th 等高场强元素, 亏损 Ba,Nb 等元素; 块状玄武岩富集 Rb, Th, Zr, Hf 等大离子亲石元素和高场强元素, Ta, Nb 亏损。酸性火山岩的轻重稀土分馏较 明显, Eu 负异常明显, 富集Rb, Th, Pb, Hf, Zr 等大离子亲石元素和高场强元素, 亏损Eu, Sr, Nb, Ta 等元素。根据 Rb-Sr 同位素等时线年龄和 SHRIMP 锆石 U-Pb 同位素定年数据, 结合古生物资料分析, 查干诺尔双峰式火山岩形成于晚石炭世, 介于 313 ～ 308 Ma 之间。岩石地球化学及其区域地质综合特征表明, 查干诺尔双峰式火 山岩可能形成于碰撞后的伸展环境, 由此推断索伦缝合带在晚石炭纪之前就曾经闭合过。

关键词: 索伦缝合带, 双峰式火山岩, 枕状玄武岩, 晚石炭世, 苏尼特右旗