Abstract:

Several rift basins developed in the western part of Xing’an-Mongol orogenic belt during late Paleozoic, in which volcanic-sedimentary series is important to understand the late Paleozoic tectonic evolution, however, there is not a clear space-time distribution of the series. Stratigraphy and geochronology of Xilimiao Formation have been researched to understand the basin filling process and tectonic setting. According to the field sections, Xilimiao Formation can be divided into three segments. Upper and lower segments are composed of crystal tuffs and rhyolitic lithic crystal tuffs, and middle segment are dark sedimentary rocks. The zircon ²⁰⁶Pb/²³⁸U age of 282±2 Ma achieved from the rhyolitic tuff of upper segment represents the forming age. The two peak ages of 304 Ma and 450 Ma represent the upper limit of the sedimentary event, the forming age of early Paleozoic orogenic belt, respectively, and the 870 Ma represents the age of Neoproterozoic basement. The data of middle detrital rocks indicate that the forming age of Xilimiao Formation was between 304?282 Ma, older than Zhesi Formation. Provenance of the detrital zircon from the middle Xilimiao Formation provides sedimentary evidence for transform process from compress of the middle Paleozoic orogenic belt to extension of the late Paleozoic rift basin.

Key words: Xing’an-Mongol orogenic, Arigin Sum Area, Xilimiao Formation, Permian, volcanic-sedimentary rock series, zircon geochronology

摘要：

为了查明兴蒙造山带西段二连?满都拉裂陷盆地的充填过程以及其中火山?沉积岩系的时空分布, 对西里庙组开展地层层序和同位素年代学研究, 探讨盆地形成的构造背景。实测剖面查明, 二叠纪西里庙组地层可分为3段, 上段和下段为晶屑凝灰岩及流纹质岩屑晶屑凝灰岩, 中段为暗色沉积岩, 构成独特的火山?沉积岩系。上段流纹质凝灰岩的锆石²⁰⁶Pb/²³⁸U年龄为282±2 Ma, 代表形成年龄; 中段碎屑岩的两组锆石年龄峰值分别为304 Ma和450 Ma, 另有1颗锆石的年龄为870 Ma, 分别代表最大沉积年龄、早古生代造山带的形成时代和晚元古代基底的年龄。中段碎屑岩的年代学测试结果表明, 西里庙组火山?沉积岩系的形成时代为304~282 Ma, 其层位应置于哲斯组之下。西里庙组中段沉积地层碎屑锆石的物源分析结果为从中古生代构造挤压过程向晚古生代构造伸展过程转化的沉积响应提供了证据。

关键词: 兴蒙造山带, 艾力格庙地区, 西里庙组, 二叠纪, 火山?沉积岩系, 锆石年代学