西秦岭中志留统含铀岩系中硅质岩的岩石学、地球化学及其成因

PETROLOGY, GEOCHEMISTRY AND ORIGIN OF CHERTS IN THE URANIFEROUS FORMATIONS, MIDDLE SILURIAN WEST QINLING RANGE

根据产状和成因,本文将西秦岭中志留统含铀岩系中的硅质岩分为,(1) 成岩过程中硅化交代碳酸盐岩所形成的硅质岩,产于碳酸盐岩和泥质岩类所组成的透镜层的顶部或碳酸盐岩层内;(2) 早期成岩阶段形成的纹层状硅质岩,主要产于碳酸盐岩和泥质岩透镜层底部或呈薄层夹于粉砂岩、泥质岩层中。根据沉积环境分析及硅质岩的化学成分、REE分布模式和氧同位素组成,提出本区硅质岩形成于受大气淡水影响的局限浅海环境。

According to the occurrences and origin, two types of cherts have been recognized in the Uraniferous Formations of Middle Silurian from West Qinling range. (1) The cherts originated from secondary silicified carbonates, which occur chiefly on the top of lenticular strata of chents, carbonate rocks and mudrocks, or within the carbonate rocks, and they are characterized by light color under microscope, coarse-grained crystalline with relic textures of limestone such as pelletoidal textures, oolotic textures, biologic textures and so on. (2)The platy or laminated cherts are characterized by dark color fine-grained crystal, high content of organic matter and laminated structures. The analyses of scanning electron microscope, infrared adsorption spectra and X-ray powder diffraction for cherts indicate that both kinds of cherts are composed of low temperature quartz and its crystallinity is high. It is suggested that the cherts in this area were formed in the restricted shallow marine environment which was possibly influenced by meteoric water based on the chemical composition, REE patterns and oxygen isotope compositions of cherts as well as its sedimentary environment. In addition, a model for the origin of cherts is also proposed here. The genetic model can explain successively the origin of both kinds of cherts in this area. The platy or laminated cherts were formed under reducing conditions of intermediate environment of terrigenous clastic environment changing towards clear water environment for carbonate sediments, in which silica partially precipitated and further enriched during the early diagenetic stage. The cherts originated from secondary silicified carbonates were formed in the mixing zones (meteoric water and saline water)of diagenetic environment within the isolated carbonate platform in the restricted shallow marine environment, as a result of secondary silicification in large scale during the stage of early diagenisis. The silica sources appear to have come mainly from the continent by means of dissolved silica in slightly alkaline meteoric water(surface runoff and ground water).