西昌及邻区晚二叠世－中三叠世盆地沉积演化及其构造意义

During Late Permian period, there erupted large Emeishan basalt in upper Yangtze region affected by the mantle plume, and presercved a lot of volcanic materials and clastic sediments. Angular unconformity between Leikoupo Formation and Xujiahe Formation indicates tectonic regime transfer after Middle Triassic.In Late Triassic, terrestrial foreland basin formed. Xichang and adjacent areas are well-preserved and -exposed Late Permian to Middle Triassic sedimentary records, which completely reflected deep mantle plume and shallow geological processes. In view of the weak study on depositional and tectonic evolution, sections of Late Permian to Middle Triassic clastic rocks and unconformity were measured using the method of sedimentology. Various sedimentary systems, depositional environments and evolution processes were constructed which showed obvious time and spatial variations, based on regional sedimentary characteristics. Analysis of detrital composition trends, heavy minerals, geochemistry of sedimentary rocks, restored paleo-current direction and detrital zircon geochronology traced sedimentary provenance,confirmed rock type of source rocks, summarized tectonic settings, concluded tectonic events in different geologic time and revealed basin-range pattern. Research on structural event deposits and tectonic unconformity integrated with isotope geochronology and provenance analysis results implicates regional structural attributes and denudation processes in adjacent orogenic belt, establishing the corresponding relationship between tectonic framework and sedimentary fill. Further, we can discuss dynamic background of basin-range systems evolution and correlation between deep and shallow geological processes.

晚二叠世时期，上扬子地区喷发大面积的峨眉山玄武岩，受地幔柱影响，保存大量火山物质和碎屑沉积，而中三叠世之后，构造体制发生转变，形成陆相前陆盆地。西昌及邻区是晚二叠世－中三叠世地层保存和出露较为完整的地区，沉积记录很好的反映了深部地幔柱和浅部地质作用过程。针对研究区沉积－构造演化薄弱环节，本项目拟采用沉积学方法，通过对晚二叠世－中三叠世碎屑岩和不整合面的构造分析，并结合区域同时期沉积发育特征，（1）恢复西昌及邻区晚二叠世－中三叠世不同时空的沉积作用类型、沉积环境及演化过程；（2）通过碎屑成分统计、重矿物分析、碎屑岩地球化学、古水流恢复和碎屑锆石年代学，示踪沉积物源区和古盆山格局；（3）对构造－事件沉积层和构造不整合面分析，结合同位素年代学研究和物源恢复结果，揭示区域构造运动学属性和相邻构造带的剥露过程，建立构造格局与沉积充填的对应关系，探讨盆山系统演化的动力学背景和深、浅部地质作用的相关性。

西昌盆地及其周缘晚二叠世发育地幔柱影响的峨眉山玄武岩，而晚三叠世为前陆盆地。晚二叠世至中三叠世时期，深部地幔柱和浅部地质作用过程相互联系略显薄弱。本项目通过沉积学方法，建立晚二叠世－中三叠世地层沉积序列，确定其物源区以及剥蚀过程，揭示区域构造与沉积充填的相互关系，确定晚二叠世－中三叠世构造环境，探讨深、浅部地质作用的相关性。. 沉积序列表明，晚二叠世宣威组主要为冲积扇－河流－三角洲，且由北向南逐渐过渡。飞仙关组沉积环境为冲积扇－河流，嘉陵江组沉积环境为潮坪；雷口坡组由灰绿色砂岩以及白云岩组成，沉积环境为三角洲前缘－潮坪。沉积序列自下而上，显示伸展构造环境。. 交错层理恢复古水流方向指示，宣威组和嘉陵江组物源主要来自于西北，飞仙关组来自于西南或西北，雷口坡组来自于东南。. 重矿物组成、电子探针分析结果、地球化学以及碎屑锆石U-Pb测年结果指示，宣威组主要来自241-252Ma峨眉山玄武岩，飞仙关组主要来自715-997Ma OIB特征基性岩、248-272Ma峨眉山玄武岩以及次要峰值512-542Ma凝灰岩，少量来自1793-2431Ma经过搬运的变砂屑岩（源岩为岩浆岩）；嘉陵江组主要来自746-975Ma基性岩浆岩，雷口坡组来自236-268Ma峨眉山玄武岩、716-812Ma具有OIB特征的岩浆岩以及1666-2117Ma、2391-2483Ma经过搬运的变砂屑岩（源岩为岩浆岩）。综合分析表明，晚二叠世宣威组－早三叠世嘉陵江组构造环境为大陆裂谷，雷口坡组形成于被动大陆边缘。