内蒙古狼山西北缘乌兰敖包地区“阿木山组”沉积时限、物源特征及其地质意义

The Age and Potential Provenance Information of Amushan Formation in Wulanaobao Area,Northwestern of Langshan,Inner Mongolia and Its Geological Significance

沉积盆地的研究对于理解造山带的构造演化具有重要意义.内蒙古狼山地区位于中亚造山带中段,构造演化历史复杂,晚古生代沉积盆地的大地构造背景存在争议.在对狼山西北缘乌兰敖包地区出露的"阿木山组"岩性特征、沉积构造及沉积环境的系统研究基础上,将该组分为3个岩性段.一段主要为粗碎屑岩,交错层理发育,沉积环境为辫状河三角洲;二段底部主要为粗碎屑岩,向上沉积物粒度变细,顶部出现生物碎屑灰岩,在岩屑石英砂岩中新发现芦木Calamites sp.,座延羊齿Alethopteris sp.,带科达Cordaites principalis Gein等植物化石,沉积环境由辫状河三角洲向滨海环境过渡;三段主要岩性为结晶灰岩,含生物碎屑,沉积环境为滨-浅海环境."阿木山组"一、二段砂岩碎屑锆石年龄可分为～2.5Ga、～1.8～1.2Ga、～826Ma、461～440Ma和313～273Ma五个时间段,其中最小年龄为273Ma,结合植物化石鉴定结果以及上覆火山岩地层年代,确定"阿木山组"沉积时代为265～273Ma.研究区内同时代的岩浆岩指示该区在早中二叠世处于古亚洲洋向华北板块俯冲的岩浆弧环境,结合"阿木山组"的沉积特征、沉积时代及所处构造位置,对其地层划分与对比方案提出建议.

The study of sedimentary basins is important for understanding the tectonic evolution of orogenic belts.The Langshan region of Inner Mongolia is located in the middle central Asian orogenic belt and has a complicated structural evolutionary history.The tectonic setting of Late Paleozoic sedimentary basin is still in controversy.Based on the study of the lithology,sedimentary structure and sedimentary environment of Amushan Formation in Wulanaobao area in the northwestern of Langshan,we divide the Formation into three members.The first member is mainly composed of coarse clastic rocks with cross-bedding developed,and the sedimentary environment is braided river delta.The bottom of the second member is mainly composed of coarse clastic rocks with increasingly finer grain-size in the upper sediments and the bioclastic limestone at the top of second member,and the depositional environment is from braided river delta to littoral circumstances.At the same time,we firstly discovered plant fossils such as Calamites sp.,Alethopteris sp.,Cordaites principalis Geinfrom the lithic quartz sandstone of the second member.The third member consists of crystalline limestone and bioclastic limestone,and the sedimentary facies type is littoral-neritic facies.The detrital zircon ages of sandstone from the first and second members of Amushan Formation are divided into five groups of about^2.5Ga,~1.8-1.2Ga,~826Ma,461-440 Ma and 313-273 Ma,respectively.The youngest age of Amushan Formation is 273 Ma.Combining the results of plant fossils and the age of overlying volcanicrocks,the age of'Amushan Formation'is limited between 265 Ma and 273 Ma.The contemporaneous magmatic rocks in the study area were formed in the continental marginal arc.With the research of sedimentary characteristics,sedimentary age and tectonic position of Amushan Formation,we propose the method of stratigraphical divisions and correlation.