| 重庆秀山南沱冰期后的海陆环境变化 |
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| 引用本文： | 冯帆,关平,刘文汇,刘沛显.重庆秀山南沱冰期后的海陆环境变化[J].沉积学报,2018,36(3):531-541. |
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| 作者姓名： | 冯帆 关平 刘文汇 刘沛显 |
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| 作者单位： | 1.造山带与地壳演化教育部重点实验室 北京大学地球与空间科学学院, 北京 100871; |
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| 基金项目： | 国家重点基础研究发展计划("973"计划)项目(2012CB214801)National Key Basic Research Program of China(973 Pro-gram) |
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| 摘 要： | 扬子地区发育地层所记录的南沱冰期在时间上与Marinoan冰期相当，被认为是"雪球事件"的产物，受到广泛关注。借助于重庆秀山长河桥剖面的南沱组冰碛岩上覆陡山沱组盖帽白云岩和页岩样品的精细采集，选取冰碛岩之上2.5 m内的地层进行同位素比值和元素含量测试，并尝试性的使用酸不溶物的元素地球化学数据，对冰期后可能出现的环境变化进行了综合分析，结果表明:盖帽白云岩C同位素数据基本符合海水分层混合模式特征；U/Th值也反映出该地区在雪球后经历了由缺氧-贫氧环境向贫氧-氧化环境的迅速转变，可能反映了雪球后缺氧富有机质的深部大洋水随上升洋流上涌并被迅速氧化的过程；强烈的Eu正异常的出现，可能与埃迪卡拉纪海底火山、热液活动频繁出现或近源热液活动相关；盖帽碳酸盐岩样品中出现Ce轻微负异常，表明其形成于弱氧化环境；87Sr/86Sr值和Y/Ho值均呈现伴有大量陆源碎屑物输入的特点；盖帽碳酸盐岩的酸不溶物可以反映当时的大陆风化背景，其化学蚀变指数（CIA）稳定在72左右，说明当时的大陆环境具有温暖湿润的特征，化学风化作用强度中等。综上所述，在南陀冰期结束后，秀山长河桥剖面的沉积环境经历了缺氧-贫氧环境向贫氧-氧化环境的迅速转变，同时，随着上升洋流的出现原有的冰期海水分层被迅速破坏，并可能伴随着地表径流的不断增强，同时热液活动在这一时期也频繁发生。在这一时期，剖面附近的大陆环境也迅速由冰期过渡为温暖湿润的环境。
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| 关 键 词： | 雪球事件 南沱冰期 酸不溶物 环境变化 地球化学 |
| 收稿时间： | 2016-12-20 |
The Marine and Continental Environmental Changes after Nantuo Glaci-ation in Xiushan,Chongqing |
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| FENG Fan,GUAN Ping,LIU WenHui,LIU PeiXian.The Marine and Continental Environmental Changes after Nantuo Glaci-ation in Xiushan,Chongqing[J].Acta Sedimentologica Sinica,2018,36(3):531-541. |
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| Authors: | FENG Fan GUAN Ping LIU WenHui LIU PeiXian |
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| Institution: | 1.Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, China;2.SINOPEC Petroleum Exploration and Development Research Institute, Beijing 100083, China |
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| Abstract: | It is believed that the Nantuo glaciation represented by sediments in the Yangzte Platform could be as a Snowball Earth event and restricted to the Marinoan glaciation in age. The carbon, oxygen and strontium isotopes, and elements of whole-rock and acid-insoluble residua of the ～2.5 m-thick Doushantuo cap dolostone and shale from the Changheqiao section in Xiushan, Chongqing were measured and their geochemical features indicated some specific en-vironmental co-changes of ocean and continent during the post-Nantuo Glaciation warming. The δ13 CPDB varied from-3.4‰to-5.2‰, which could be explained by the model of stratification and mixing of ocean. The ratios of U/Th sug-gested that the sedimentary environments were rapidly changed from anoxic to oxic during the post-glacial warming. These changes might be related to the upwelling and oxidizing of the anoxic deep seawater. The lower 0.3 m samples have strong positive Eu anomaly, that may be related to the frequent hydrothermal activities in the Ediacaran ocean or proximal hydrothermal activity. The negative Ce anomaly in the cap dolostone might be due to oxic conditions. The val-ues of Y/Ho and 87 Sr/ 86 Sr probably displayed that a large number of terrigenous residuas were carried into ocean by the increasing surface runoff, during the post-glaciation. The acid-insoluble residuas of cap dolostone can be used to study the background of the average degree of continental chemical weathering. The CIA of acid-insoluble residuas ranges from 71.2 to 72.9, which indicated a warm and wet environment and medium chemical weathering. |
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