安徽巢湖地区中二叠统栖霞组灰岩中燧石成因

Origin of Nodular Cherts in Limestones in Middle Permian Qixia Formation,Chaohu,Anhui Province

安徽省巢湖地区中二叠统栖霞组以发育一套滨海沼泽-浅海碳酸盐台地环境为主的细碎屑岩至碳酸盐岩沉积建造为特征,灰岩中常见结核状及条带状燧石。燧石多为椭球状和串珠状,部分燧石与灰岩间发育宽约0.5 cm的过渡带。镜下观察灰岩为微晶生物碎屑灰岩;过渡带也多由微晶方解石组成,多数钙质生物壳体被石英充填或半充填,扫描电镜下可见方解石微溶、石英充填溶孔的现象;燧石主要为隐晶及微晶石英,生物碎屑类型与灰岩中基本一致,且多被石英交代。岩石学特征表明死亡的生物在腐烂降解过程中形成的有机酸抑制了碳酸钙的沉淀,并使部分生物碎屑及灰泥发生溶蚀,胶质二氧化硅沉淀,形成燧石结核。燧石中w(Al)/w(Al+Fe+Mn)平均值为0.63,远大于热水沉积硅质岩的最大值0.35;Fe/Ti平均值为9.5,小于热水成因的最小值20;Al-Fe-Mn三角图投点位于非热液成因区域;燧石中Al2O3含量平均值为0.20%,远高于MgO、Na2O、K2O的含量。分析认为,该区燧石结核的二氧化硅可能来源于陆源物质。

The Middle Permian Qixia Formation in Chaohu area, Anhui Province, was mainly composed of fine-grained clastic rocks and limestones which were deposited in the coastal marsh and carbonate platform. Limestones were characterized by nodular and banded cherts. Most of cherts were axiolitic and moniliform in outcrops. Among some cherts and limestones developed transitional belts with 0.5 cm width around cherts. The limestones were composed of packstone and wackestone which consisted mainly of fragments of fossils and microcrystalline calcite through polarizing microscope observation. The transitional belts were mainly composed of micrites and the aragonite-shell fragments which were replaced or half replaced by quartz prior to the micrites. Meanwhile the amorphous silica was found to be precipitated into the pores due to the micrites’dissolution under SEM observation in the transition belts. The chert consists of microcrystalline or cryptocrystalline quartz together with the fossil fragments. The categories of the organic matter and fossil fragments in the cherts were similar to those found in limestones, however, most of them were replaced by quartz completely. The petrology analysis of the cherts and limstones indicates that the organic acid which was produced by degradation of organic matters, would greatly increase the saturation of CaCO3, promote the dissolution of aragonitic fossils and marls in the sediment. As a result, the calcium precipitation was inhibited yet gelatinous silica precipitated as well. The geochemistry of nodular cherts revealed that the average ratio of Al/（Al＋Fe＋Mn） was 0.63, which was higher than the maximum value （0.35） of hydrothermal deposition. The average ratio of Fe/Ti was 9.5 also lower than the minimum value （20） of hydrothermal deposition. The Al-Fe-Mn data of the cherts fell into in the non-hydrothermal field in the Al-Fe-Mn ternary diagram. The Al2O3 value （average 0.20%） was far beyond the other contents such as MgO,Na2O,K2O in the cherts and similar to the terrigenous silicon. Such geochemistry features indicated that the silica of the cherts in limestones was probably sourced from the continental materials in research area.