华南西南地区晚二叠世源-汇沉积体系分析对峨眉山大火成岩省大规模剥蚀和晚期酸性火山活动的启示

There developed large-area terrigenous clastic sedimentary sequences during the Late Permian, overlying the ~260 Ma Emeishan basalts or Middle Permian carbonate-dominated strata, in the SW South China. These clastic sedimentary rocks are formed in a unique source-to-sink sedimentary system with detritus primarily derived from the Emeishan large igneous province, represent the geological records of the late Permian massive erosion of the Emeishan large igneous province and provide a chance to understand the late-stage silicic volcanism in the Emeishan large igneous province. This study will focus on the southeastward terrestrial-littoral-deep water basinal terrigenous clastic sedimentary system in the NW Guizhou-SW Guizhou-NW Guangxi region. Based on detailed sedimentology analysis and using sandstone modal composition, bulk rock and volcanic lithic fragment geochemistry, detrital apatite Sr isotopic and chemical composition, and detrital zircon U-Pb geochronology and trace element geochemistry, in combination with bulk rock and single mineral analysis from Emeishan volcanic tuffs, this study tends to document the configuration of facies and depositional evolution of this Late Permian terrigenous clastic sedimentary system, reveal the temporal and spatial trends of detrital sediment composition and the potential controls related to sedimentary processes within this sedimentary system, document the composition and nature of volcanic rocks eroded from the Emeishan large igneous province during the Late Permian, and reveal the potential volume and related magmatic evolution of the late-stage silicic volcanism in the Emeishan large igneous province.

华南西南地区分布有大面积的晚二叠世陆源碎屑沉积序列，覆盖在峨眉山玄武岩或中二叠世碳酸盐岩地层之上，代表了以峨眉山火山岩为源区的源-汇沉积体系，是峨眉山大火成岩省在晚二叠世大规模剥蚀的地质记录，为理解峨眉山大火成岩省晚期火山活动提供了宝贵研究素材。本项目拟以黔西北-黔西南-桂西北地区北东向的晚二叠世陆相-滨海相-深水海盆相陆源碎屑沉积体系为研究对象，通过详细的沉积学分析，运用砂岩碎屑组分、全岩和火山岩岩屑地球化学、磷灰石Sr同位素及化学组成、碎屑锆石U-Pb年代学及微量元素组成分析，配合峨眉山凝灰质火山岩的全岩和单矿物组成分析，揭示该碎屑沉积体系的环境构成和沉积演化，理解沉积体系内碎屑沉积物组成的时空变化趋势及可能的沉积控制机制，重建峨眉山大火成岩省晚二叠世剥蚀的火山岩岩石组成及属性，探讨峨眉山大火成岩省晚期酸性火山活动的规模和岩浆的形成演化过程。

峨眉山大火成岩省（LIP）在中-晚二叠世之交喷发后遭受了强烈的风化剥蚀，致使其火山喷发序列保存不完整，妨碍了对峨眉山LIP资源环境效应的全面认识。在黔西-桂北地区广泛分布有晚二叠世-早三叠世的陆源碎屑沉积岩，是峨眉山LIP强烈风化剥蚀的物质记录。为重建峨眉山LIP的晚期酸性火山活动和风化剥蚀过程，本项目对黔西-桂北地区晚二叠世陆源碎屑沉积岩和凝灰质岩石的沉积相、沉积时代、沉积物源和沉积体系等开展了较为系统地研究。完成了6条野外剖面的详细实测和10口钻孔岩心的精细编录，结合已有研究重新厘定了研究区晚二叠世从陆相-滨海相-深水盆地相的沉积古地理；获得了砂岩岩相组成、全岩地球化学组成、碎屑锆石U-Pb年龄和微量元素、Hf同位素等物源数据，明确了峨眉山LIP为主要物源区，确定了以高Ti玄武岩为主、酸性火山岩为辅的平均源岩组成；重新约束了峨眉山LIP的火山喷发时代，并基于碎屑锆石的物源数据随地层序列的变化，依次区分出晚二叠世早期、晚二叠世晚期和早三叠世早期三个剥蚀时段，揭示了峨眉山LIP晚期的火山岩浆作用经历了从地壳物质混染-分异结晶作用到高Ti玄武质岩浆分异结晶作用为主的岩浆转换；利用泥岩的全岩矿物、粘土矿物和地球化学组成随地层序列的变化，约束了峨眉山LIP剥蚀区的化学风化强度随时间的变化，揭示了晚二叠世早期和末期及早三叠世初期的三次化学风化趋势显著降低事件；基于现代玄武质基岩区化学风化速率与地表温度的相关关系，提出了定量约束玄武质基岩区物理侵蚀速率的方法，估算出了峨眉山LIP玄武岩剥蚀区在晚二叠世早期从200 t/km2/yr增大到~300 t/km2/yr，在晚二叠世末期从~300 t/km2/yr增大到~600 t/km2/yr并在早三叠世初期快速增大到>1500 t/km2/yr，揭示了区域强降雨或降雨模式的变化对热带玄武岩地貌区风化剥蚀和地貌演化的控制作用。