显生宙碳酸盐岩中燧石结核的几种成因模式

显生宙以来,碳酸盐岩地层中有燧石结核产出的现象十分普遍。现有研究认为,硅质生物壳体是燧石结核最主要的硅质来源,随着地质历史上主要硅质生物类型的演变,燧石结核的产出环境逐渐从浅水变为深水。不同地区不同层位的燧石结核往往具有一些共同特征,包括呈孤立分散的结核状产出、硅质选择性交代方解石颗粒而保留晶形完好的白云石、硅质矿物具有隐晶硅质-微晶石英-粗晶石英的规律性变化等。基于上述主要特征及不同研究实例的特点,前人总结出了有机质氧化模式、半透膜模式、混合水硅化模式、重结晶应力控制交代模式等燧石结核成因模式,从不同角度对燧石结核的典型特征进行了解释。然而由于燧石结核成因的复杂性及其可形成于沉积-成岩的不同阶段,各个成因模式均存在一定的局限性,只可用于解释部分地质特征。鉴于燧石结核对研究区的沉积环境、成岩历史等具有很好的指示作用,对其成因的研究具有重要意义,尽管上述模式的提出时间较早,但针对特定问题的研究非常深入,在以后的研究中应加以借鉴。

Several genetic models of nodular chert hosted in Phanerozoic carbonate

Carbonate-hosted nodular chert is very common in Phanerozoic strata. Siliceous organisms are the main source of silica for the chert. The sedimentary environment of chert evolved from shallow water to deep water with the evolution of main siliceous organism spices during the geological history. Some typical characteristics exist among nodular cherts in different districts and different ages. The nodular chert usually exists in isolation and dispersion. The authors could observe from the thin section that the calcite grains are replaced selectively and the dolomite crystals are still euhedral. The siliceous minerals exhibit regular change from crypto-crystalline to microcrystalline and finally to macrocrystalline form. Based on the typical characteristics and specific phenomena of the research area, four main genetic models are proposed by different researchers, which are Organic Matter Oxidation Model, Semiper-meable Membrane Model, Mixing Zone Model and Force of Crystallization Controlled Replacement Model. However, due to the complexity of the genesis and various possibilities of formation stage during the sedimentary and diagenetic history, those models all have some limitations and can only explain the characteristics partially. Since the nodular chert can indicate the sedimentary environ-ment and diagenetic history well, the research on its origin is meaningful. Although these models have been proposed for decades, they still have reference value in the future because of the careful and profound thinking.