| 秭归地区震旦系陡山沱组碳酸盐岩结核成因新认识及其地质意义 |
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| 引用本文： | 张明正,彭松柏,张利,方家松,张先进,韩庆森.秭归地区震旦系陡山沱组碳酸盐岩结核成因新认识及其地质意义[J].地球科学,2016,41(12):1977-1994. |
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| 作者姓名： | 张明正 彭松柏 张利 方家松 张先进 韩庆森 |
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| 作者单位： | 1.中国地质大学地球科学学院，湖北武汉 430074 |
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| 基金项目： | 中国地质大学(武汉)教学科研项目2014A08 |
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| 摘 要： | 扬子克拉通秭归地区震旦系陡山沱组第四段黑色泥页岩中广泛发育具明显δ13C负异常的碳酸盐岩结核，其是否与古甲烷天然气水合物渗漏有关值得深入研究.对该碳酸盐岩结核开展的沉积结构构造、岩相学和地球化学研究表明，碳酸盐岩结核具典型韵律环带结构，普遍发育有亮晶球体结构、草莓状黄铁矿，以及与渗漏系统有关的凝块组构，δ13C具明显负异常(-5.65‰~-6.76‰)，U、Mo元素强烈富集(UEF=8~26，MoEF=99~320)，Y/Ho比值为31.05~37.31，稀土配分型式为平缓左倾，主微量元素K、Sc、V、Cr、Co、Ni、Rb、Sr、Ba、Th、U和Mo等总体显示为缺氧-硫化环境，与冷泉碳酸盐岩的形成环境和特征一致.碳酸盐岩结核环带SiO2、MgO、CaO、CO2等地球化学元素含量呈阶段性连续增减变化，显示碳酸盐岩结核形成经历了初始形成、成岩-交代、成岩后改造3个连续演化阶段.据此，提出碳酸盐岩结核是新元古代末噶斯奇厄斯冰期(582~551 Ma)结束温度回暖，黑色泥页岩中低温封存固态天然气水合物发生分解释放和成岩-交代形成的冷泉碳酸盐岩结核，也是古天然气水合物存在的重要地质记录和标志，这一新认识为华南扬子克拉通在震旦系和下古生界沉积盖层中寻找页岩气(甲烷天然气)储集层位提供了重要地质依据.
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| 关 键 词： | 扬子克拉通 秭归地区 陡山沱组第四段 古天然气水合物 冷泉碳酸盐岩结核 地球化学 |
| 收稿时间： | 2016-05-24 |
New Recognition of Carbonate Nodules Genesis in Sinian Doushantuo Formation in Zigui Area and Its Geological Implication |
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| Abstract: | The carbonate nodules with strongly negative δ13C anomalies are widely developed in the black muddy shale in the forth member of the Doushantuo Formation from the Sinian System in the Zigui area, Yangtze Craton. Whether they were related to leakage of gas hydrate deserves further studies. Studies of sedimentary structure, petrography and geochemistry of these carbonate nodules show that these nodules have typical oscillatory zoning, sparry sphere structure, framboidal pyrite and agglomerate fabrics, which are related to venting system. The carbonate nodules display obvious negative δ13C anomalies (-5.65‰ to -6.76‰), strong enrichment of U and Mo elements (UEF=8-26, MoEF=99-320) and Y/Ho ratios between 31.05 and 37.31. Their REE distributions display gentle left-sloping patterns and their major and trace elements such as K, Sc, V, Cr, Co, Ni, Rb, Sr, Ba, Th, U and Mo indicate a hypoxia and sulfidic setting, which is consistent with characteristics and formation environment of cold seep carbonates. SiO2, MgO, CaO and CO2 contents of oscillatory zoning within the nodules show continuously increase or decrease changes, respectively. They reflect three stages of continuous evolution processes including primary formation, subsequent diagenesis-metasomatism and final diagenetic reworking. Accordingly, it can be proposed that these carbonate nodules were ultimately developed as decomposition, diagenesis and metasomatism of the low temperature stored solid gas hydrate sediment in the black muddy shale when the temperature became higher after the Neoproterozoic Gaskiers glaciation (582-551 Ma). This study provides an important record that confirms existence of the ancient natural gas hydrate and offers an important geological evidence for exploring shale gas (methane gas) reservoirs in the Sinian System and Lower Paleozoic sedimentary covers in the Yangtze Craton of South China. |
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