扬子板块北缘下志留统龙马溪组重晶石结核特征及其成因机制分析

An analysis of barite concretions from lower Silurian Longmaxi Formation on the northern margin of the Yangtze block and their genetic mechanism

在扬子板块北缘城口明中剖面及巫溪徐家坝剖面下志留统龙马溪组底部的硅质岩、泥岩中发现椭球状重晶石结核。其矿物组成主要为重晶石颗粒与作为"基质"的黄铁矿、粘土矿物和石英。通过重晶石岩石学、矿物学及锶同位素分析表明,重晶石结核形成于早期成岩阶段松软沉积物的孔隙水中。上升洋流带来丰富的营养及富钡物质,表层海水的高初始生产力促使生物繁盛,海水中的钡通过生物作用富集形成生物钡,生物钡(bio-barite)在埋藏过程中的硫酸盐耗竭区(sulfate depleted zones)通过硫酸盐细菌作用(BSR)溶解被激活提供了钡的来源。围岩岩性(黑色泥岩和硅质泥岩)表明重晶石结核形成于缺氧的环境中。上述研究对深入理解早志留世时期扬子板块北缘古海洋环境有一定的启示作用。

Barite concretions were found in siliceous rocks and mudstones in the lower Silurian Longmaxi Formation on the northern margin of the Yangtze block. Their mineral compositions are barite crystals, pyrite, clay or quartz.Based on an analysis of the macroscopic and microscopic morphology of barite and strontium isotopes, the authors hold that barite deposits precipitated from the enclosed pore water in sedimentary column. The high productivity and anoxic bottom water provided the environment for the formation of barite. Bio-barite could be formed by biological effect in the seawater. Afterwards, bio-barite was dissolved by sulfate bacteria（BSR） in the sulfate depleted zones.Sulfate in pore water was evolved from the penecontemporaneous seawater. The host mudstone indicates high preservation potential in an anoxic depositional environment. This study has provided some implications for the further understanding of the paleoenvironment of the northern margin of the Yangtze block in early Silurian.