内蒙林西地区中二叠世-早三叠世砂岩物源区示踪

Timing, position and process of final suturing of the eastern segment of the Central Asian Orogenic Belt (CAOB) is a remnant debated issue in recent years. Numbers of studies on petrology, paleontology, lithofacies and paleogeography, paleomagnetism and geophysics, which are related to subduction and collision between the North China and Siberian cratons, had been done. However, the study on clastic sedimentary, that response to the surrounding orogenic process, is still poor. The Linxi area in eastern Inner Mongolia, tectonically belongs to the eastern segment of the CAOB, and lies between the recent controversial suture zones of the Ondor Sum-Xar Moron suture and the Solonker-Sonidzuoqi-Linxi suture. The sediments developed in the area should record and preserve the information of closure of the Paleo-Asian Ocean. Therefore, the application will be mainly focusing on the provenance study for the sandstones derived from the Middle Permian-Lower Triassic Zhesi, Linxi and Laolongtou formations. The systemically field investigation, mapping, paleocurrent statistics and sampling will be done. Further, sandstone detrital clasts component analysis, geochemistry, LA-ICP-MS U-Pb dating and Hf isotope for detrital zircons will be carried out on the collected sandstones, respectively. The results expectably obtained will reveal not only the characteristics of source rocks, but also the sedimentary paleoenvironment and paleoclimate, and constrain their possible provenances and tectonic settings, as well as timings of the tectono-thermal events from the source areas. Combining with the previous studies, our results also contribute toward the consideration of the Late-Paleozoic tectonics of the study area, and provide a significant evidence on revealing the timing, position and process of final suture between the North China craton and the amalgamated Inner Mongolian microcontinents (Erguna-Xing'an-Songliao blocks) to its north.

中亚造山带东段的闭合时间、位置和过程一直是地质界争论的焦点问题。前人基于岩浆活动、古生物分布、岩相古地理、古地磁和地球物理等研究取得了一些进展，但从造山过程的碎屑沉积响应着手研究尚且不足。内蒙古林西地区位于中亚造山带东段，介于备受争议的索伦山—苏尼特左旗—林西缝合线与温都尔庙—西拉木伦河缝合带之间，保存了古亚洲洋构造域消亡的沉积记录。本申请选择区域内中二叠统—下三叠统哲斯组、林西组与老龙头组砂岩为研究对象，通过碎屑组分统计分析、地球化学特征、碎屑锆石U-Pb年龄和Hf同位素等物源示踪的研究手段，来探讨物源区母岩特征，了解碎屑物可能的来源及其大地构造背景和沉积时的古气候、古环境特点。结合区域已有的资料，理清内蒙微陆块群与华北板块碰撞拼贴过程相关的构造—热事件的重要时间节点，建立研究区晚古生代的构造演化模式，为探讨内蒙微陆块群与华北板块缝合带最终的闭合时间、位置和过程提供线索。

内蒙东部的克旗―林西―阿鲁科尔沁―扎鲁特―索伦―蘑菇气地区位于华北板块与西伯利亚板块所夹持的中亚造山带东段，西拉木伦缝合带与贺根山―黑河缝合带之间，是古亚洲洋晚古生代残余海盆最后消失的地区，保存了上述两缝合带构造演化过程的相关信息。本项目针对中、晚二叠―早三叠世砂岩开展了岩相学、地球化学、碎屑锆石LA-ICP-MS U-Pb年代学等方面研究。岩相学结果表明二叠系哲斯组和林西组与下三叠统陶海营子组、老龙头组砂岩成分均具有成熟度较低，近源快速沉积的特点，碎屑骨架成分显示多为火山岩区物源供给。地化判别图解揭示了物源区主要具活动大陆边缘和大陆岛弧型并含有少量被动大陆边缘物源区特点，且总体呈现由活动大陆边缘向大陆岛弧型物源区过渡的趋势。锆石CL图和Th/U值也指示了绝大多数锆石为岩浆成因锆石。结合前人研究成果，区域内已获得的2533粒碎屑锆石U-Pb年龄主要分为3组：~230-570 Ma （峰值年龄为~270-315 Ma、430 Ma和500 Ma）、~700-1100 Ma （峰值970 Ma）及 >1500 Ma （峰值~1800 Ma和~2500 Ma）。较古老的峰值年龄~1800Ma和~2500Ma，暗示了华北克拉通基底的物源信息。其余两组年龄则与西拉木伦缝合带北侧的额尔古纳―兴安―锡林浩特地块内部岩浆事件的时间相吻合。对比分析内蒙东部不同区域（克旗―林西地区、阿鲁科尔沁―扎鲁特地区和索伦―蘑菇气地区）、不同时代（中、晚二叠世―早三叠世）沉积物碎屑组分变化趋势，推测古亚洲洋最终沿西拉木伦缝合带自西向东呈 “剪刀”式闭合，时间持续至早三叠世。此外，索伦―蘑菇气地区晚石炭世―早三叠砂岩的岩相学和碎屑锆石U-Pb年代学结果，证实了额尔古纳―兴安地块与松辽―锡林浩特地块至少在中二叠世沉积以前就已经沿贺根山―黑河缝合带完成拼贴，应为前人提出的早石炭世末。上述成果为探讨中亚造山带东段的构造演化提供了重要的线索。