摘要:
上扬子地区晚震旦世灯影组及其相应地层白云岩中广泛发育一种外形奇特的具有圈层结构的肾球状体、椭球体、葡萄体及不规则状的球体(以下统称为葡萄体),宏观上呈钙华状成片产出,沿层面或裂隙分布,大量学者认为其为受大气淡水影响形成的。在详细野外观察描述的基础上,应用偏光显微镜、阴极发光和扫描电镜等分析测试手段,对四川南江杨坝地区灯影组一段白云岩中的葡萄体进行了研究,结果表明:葡萄体为具核心和明暗相间皮壳层的半球体,矿物组分以白云石为主,局部含少量自生石英,为围岩固结成岩后的产物;核心及暗色皮壳层中发育藻黏结集合体,并见大量不规则的微生物丝状体,且矿物组分中含有较高的碳元素,发较亮的红色阴极光,结合海底硬底的存在,说明其为海水环境中的生物-化学沉积物;葡萄体的外层主要为亮色皮壳层,呈等厚亮晶纤状胶结物,多发暗红色阴极光,并造成葡萄体间呈多面体交汇,说明后期葡萄体的形成以胶结作用为主,为海水潜流带的产物;葡萄体的大小和形态与基底上不规则凸起的形态及生长空间特征密切相关,固定的生长质点和规则的结构指示葡萄体形成于稳定的水体中;葡萄状白云岩的原始组构应为文石或高镁方解石,扫描电镜下呈晶粒结构,为白云石化作用的产物;葡萄集合体的组分、阴极发光性与其围岩相似,按照其产状和形态特征,把这种具有固定生长质点且具等厚皮壳层的半球体称作葡萄状构造,应为生物-化学成因的沉积-成岩构造,形成于稳定的海水中。
关键词:
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葡萄体 /
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白云岩 /
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成因 /
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灯影组 /
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震旦系
Abstract:
There is a special and extensively developed type of dolomite in Dengying Formation and its corresponding formations of Upper Sinian in Yangtze region, which is with an odd-looking like renal globule, ellipsoid, grape-like body and sphere in irregular shape(hereinafter referred to as the "grapes"). Many scholars in China believed that this kind of dolomite was affected by atmospheric fresh water during formation. "Grapes" are like pieces of tufa on the macro, distributed along interfaces of rock strata and fractures. On the basis of the detailed observation and description of outcrops, and using polarized light microscope, cathodoluminescence and scanning electron microscopy(SEM), the characteristics and genesis of grape-like dolomite in the First section of Sinian Dengying Formation in Yangba Town of Nanjiang County, were studied. The results show that the grapes were as the fillings in surrounding rocks through diagenetic ways into rocks. The grapes are usually like hemispheroids with cores and alternated dark and bright shells, and composed mostly of dolomite and a little of authigenic quartz. The cores and dark shells were formed from biological and chemical processes in sea water, according to the algal binding aggregates with irregular microbial filaments, high carbon elements in the mineral components, and in combination with the existence of the seabed hard ground. The bright shells were composed by fiber cements, which in the same thickness and formed the outer layers of grapes. Grapes are with polyhedral intersection and filled with fiber cements in the residual space, so the outer of grapes are forming in marine phreatic water and mainly due to cementation. Cores were the irregular bumps on the wall of dissolution holes and seams, the characteristics of grapes were closely related to the size and shape of bumps and the features of growth space. The fixed point and regular structure indicate that grapes were formed in stable water. Aragonite or high-magnesian calcite was the original component, because of the dolomitization, the grape-like dolomite was formed and with crystalline-granular texture. The characteristics cathode luminescence of grapes are similar to surrounding rocks, and according to the occurrence and the morphological characteristics of the grapes, the hemispheroids with cores and alternated dark and bright shells are identified as an sedimentary structure and named grape-like structure. The sedimentary structure is a deposition-diagenetic structure and due to biological and chemical effect, and formed in the stable sea environment.
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