火山灰蚀变对细粒混积岩元素异常的影响及对古环境恢复的作用

Impacts of volcanic ash alteration on element anomalies in fine-grained mixed deposits and paleoenvironmental reconstruction： A case study of the 2nd member of the Permian Lucaogou Formation， Tiaohu-Malang Sag， Santanghu Basin

潘永帅 柳波 杨易卓 屈童 黄志龙 徐雄飞

Yongshuai PAN Bo LIU Yizhuo YANG Tong QU Zhilong HUANG Xiongfei XU

火山灰输入对咸化湖盆沉积环境重建的影响较大，其丰富的微量元素随着沉降入湖并与沉积物共同保存，导致湖相地层的元素含量无法代表原始沉积特征。以三塘湖盆地条湖-马朗凹陷二叠系芦草沟组二段（芦二段）为例，开展主/微量元素、稀土元素、饱和烃气相色谱-质谱、全岩X射线衍射和有机碳含量测试分析以及薄片鉴定，研究火山灰蚀变对细粒混积岩元素异常的影响机制，恢复火山灰影响下的咸化湖相沉积环境。研究结果表明：芦二段由细粒凝灰质和碳酸盐两种组分混积形成，长英质和碳酸盐矿物总含量平均可达95 %，属于广义上的相缘渐变混合沉积类型。大量热力学性质不稳定的盆缘火山灰物质在早期入水溶解和中、晚期脱玻化及有机酸溶蚀作用的影响下，向孔隙流体中释放出了高丰度的包括金属离子在内的多种营养性离子，对芦二段细粒混积岩中Ni， Co， S， P和Ga等元素产生了明显干扰，导致这些元素无法反映原始沉积环境信息。结合生物标志化合物和筛选校正后的主/微量元素分析发现，芦二段沉积期气候炎热干旱，湖盆水体供给有限，湖水盐度极高且含氧量较低。火山活动对芦二段沉积环境起到了明显的调控作用，当火山由强烈喷发期向间歇期过渡时，气候炎热干旱程度增强，水体蒸发量增大，水体盐度增高，含氧量增加，沉积形成了不同的岩石组合类型。

Volcanic ash input holds significant impacts on the reconstruction of sedimentary environments in saline lacustrine basins. Specifically， when abundant trace elements from volcanic ash enter lakes and are preserved in tandem with sediment， the element contents in lacustrine strata cannot accurately reflect the original sedimentary setting. Focusing on the 2nd member of the Permian Lucaogou Formation （P） in the Tiaohu-Malang Sag within the Santanghu Basin， we investigate the mechanisms by which volcanic ash alteration affects element anomalies in fine-grained mixed deposits using major， trace， and rare earth element analyses， gas chromatography-mass spectrometry （GC-MS） of saturated hydrocarbons， whole-rock X-ray diffraction （XRD）， total organic carbon （） content determination， and thin section observation. Accordingly， the saline lacustrine sedimentary environment under the action of volcanic ash is reconstructed. The results indicate that the P consists of fine-grained tuffaceous materials and carbonates， with the total content of felsic and carbonate minerals reaching up to 95 % on average， suggesting gradational mixed sedimentation at facies margins in a broad sense. Under the influence of alteration such as early dissolution in water and mid-late devitrification and organic acid corrosion， substantial volcanic ash thermodynamically unstable released various high-abundance nutrient ions （including metal ions） into pore fluids， which significantly interfered with the elements including Ni， Co， S， P， and Ga in the fine-grained mixed rocks of the P. As a result， these elements cannot accurately reflect the original sedimentary environment. Biomarker compounds， together with the analysis of major and trace element data after screening and correction， reveal a hot and arid climate during the deposition of the P， featuring limited water supply to the lacustrine basin and extremely high salinity and low oxygen concentration of the lake. Volcanic activity played a significant role in regulating the sedimentary environment of the P. The transition from intense to intermittent volcanic eruptions corresponded to an increasingly hot and arid climate， along with elevated water evaporation， water salinity， and oxygen concentration， leading to the deposition of various types of rock assemblages.

10.11743/ogg20250213