扬子地台北缘震旦系灯影组地球化学特征及其对热水沉积暗示  

Geochemical Characteristics of the Sinian Dengying Formation in Northern Margin of the Yangtze Platform and Their Implications to Hydrothermal Sedimentation

作　　者：陈宝赟[1] 李荣西[2] 余金杰[1] 刘帅杰[1] CHEN Bao-yun1, LI Rong-xi2 , YU Jin-jie 1, LIU Shuai-jie1(1.MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037,China; 2.School of Earth Science ＆ Resources, Chang＇ an University, Xi＇ an 710064, China)

机构地区：[1]中国地质科学院矿产资源研究所国土资源部成矿作用与资源评价重点实验室,北京100037 [2]长安大学地球科学与资源学院,西安710064

出　　处：《矿物岩石地球化学通报》2018年第4期770-778,共9页Bulletin of Mineralogy, Petrology and Geochemistry

基　　金：国家自然科学基金项目(41173055)

摘　　要：震旦系灯影组在扬子地台北缘广泛分布,其岩性主要为白云岩、硅质白云岩和少量硅质岩。为探讨灯影组沉积时期的氧化还原状态并对热水沉积作用的暗示,分析了白云岩、硅质白云岩和硅质岩的微量稀土元素特征。结果表明,这3类岩石均呈现负的Ce异常,其V/Cr〉2、Ni/Co〉5、V/（V＋Ni）为0.6~0.84,表明灯影组沉积于弱氧化的浅水碳酸盐岩台地环境。另外,白云岩、硅质白云岩具有明显的正Eu异常、U/Th〉1,与海水稀土元素分布特征相比,其Ce负异常和轻稀土亏损程度明显减弱。上述特征及前人对灯影组硅质岩的Si-O同位素温度的计算结果表明,灯影组沉积时期有海底热水的混合。晚元古代晚期震旦纪扬子北缘处于裂谷构造环境,在由裂谷活跃期向平静期演化的过程中,其表现形式也由活跃的火山活动转化为热水喷流事件。灯影组即是在扬子北缘裂谷演化相对平静时期沉积的一套浅水碳酸盐岩,混合了海底喷流热液。The Sinian Dengying Formation is distributed extensively in northern margin of the Yangtze Platform. It is mainly composed of dolomite and siliceous dolomite with minor chert. In order to discuss the redox conditions of the paleo-sea water during its sedimentation period and their implications for hydrothermal sedimentation trace elements and rare earth elements（ REE） of dolomite,siliceous dolomite,and chert of the Dengying Formation have been analyzed in this study.Results show that V/Cr and Ni/Co ratios of the dolomite,siliceous dolomite,and chert of the Dengying Formation are higher than 2 and 5,respectively,with V/（ V＋Ni） ratios varying from 0. 6 to 0. 84 and negative Ce anomalies,indicating that these rocks were deposited in a weakly oxidized shallow paleo-sea water carbonate platform environment at the time. In addition,dolomite and siliceous dolomite have obvious positive Eu anomalies and U/Th ratios of over 1. They have similar REE patterns to that of the sea water though their negative Ce anomalies and the depletion of LREE are relatively weak.The above characteristics together with the calculated temperatures of sea-water based on Si-O isotopic compositions of chert in the Dengying Formation by previous researchers imply that seafloor hydrothermal fluid was mixed in sea water in the sedimentation process of various rocks of the Dengying Formation. It is suggested that the northern margin of the Yangtze Platform was in a rift environment in the Sinian period of late Proterozoic era. The evolution from active stage to inactive stage of rifting was represented by the transformation from active volcanism to submarine hydrothermal sedimentation.The Dengying Formation,mainly composed of a series of shallow sea-water carbonates,was deposited in a mixture of seawater and submarine exhalative hydrothermal fluid in the inactive stage of rifting.

关 键 词：扬子地台北缘 灯影组 白云岩 微量元素 氧化还原条件 

分 类 号：P59[天文地球—地球化学] P612[天文地球—地质学]