陕南地区埃迪卡拉纪陡山沱组下部海绿石的成因机制:对古海水氧化还原条件的启示  

The genesis of glaucony in the lower Ediacaran Doushantuo Formation,southern Shaanxi:implication to seawater redox condition

作　　者：刘宇晨 徐东滔 王新强[2,3] 刘金高 LIU Yuchen;XU Dongtao;WANG Xinqiang;LIU Jingao(School of Science,China University of Geosciences(Beijing),Beijing 100083,China;School of Earth Sciences and Resources,China University of Geosciences(Beijing),Beijing 100083,China;State Key Laboratory of Biogeology and Environmental Geology,China University of Geosciences(Beijing),Beijing 100083,China;Institute of Earth Science,China University of Geosciences(Beijing),Beijing 100083,China;State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences(Beijing),Beijing 100083,China)

机构地区：[1]中国地质大学(北京)数理学院,北京100083 [2]中国地质大学(北京)地球科学与资源学院,北京100083 [3]中国地质大学(北京)生物地质与环境地质国家重点实验室,北京100083 [4]中国地质大学(北京)科学研究院,北京100083 [5]中国地质大学(北京)地质过程与矿产资源国家重点实验室,北京100083

出　　处：《地质通报》2023年第2期363-375,共13页Geological Bulletin of China

基　　金：国家自然科学基金项目《华南埃迪卡拉纪氮同位素的时空变化及环境背景》(批准号:41872032);《新元古代中期华南及其近缘板块古地理位置动态重建》(批准号:41830215);高等学校学科创新引智计划《深时生命与环境演化学科创新引智基地》(编号:B20011)。

摘　　要：海绿石是海洋环境中一类重要的自生矿物,在古环境研究方面具有广泛的应用。基于中国陕南地区埃迪卡拉纪陡山沱组下部海绿石的分布特征,对其开展了综合的岩相学、原位微区成分定量分析、X-射线衍射(XRD)分析等研究,旨在探讨海绿石的形成机制,剖析研究区埃迪卡拉纪早期的氧化还原环境。偏光显微镜和扫描电镜(SEM)观察结果表明,海绿石多以胶体沉淀物的形式充填于石英、长石等碎屑矿物颗粒之间的孔隙中,为早期成岩阶段自生沉淀成因。由于海绿石的形成需要Fe(Ⅱ)和Fe(Ⅲ)的同时存在,Fe氧化还原界面附近(次氧化)最有利于海绿石的发育,因此研究区陡山沱组下部海绿石的形成指示了次氧化的孔隙水条件。能谱(EDS)定量分析表明,研究区陡山沱组的海绿石具有高K 2 O和Al_(2)O_(3)、低Fe_(2)O_(3)含量,该化学组分是前寒武纪海绿石的典型特征。碎屑矿物溶解及海水与孔隙水之间的物质交换提供了海绿石演化过程所需的元素。与贵州瓮安地区同时期的含海绿石地层相比,研究区海绿石的分布层位相对局限,表明古海水氧化还原环境和古地理环境对于海绿石分布位置具有共同调控作用。Glaucony is an important authigenic mineral in marine environment and has been widely used in palaeoenvironmental research.In this study,we report the occurrence of the glaucony in the lower Ediacaran Doushantuo Formation in southern Shaanxi Province for the first time.Integrated studies including petrography,in-situ microanalysis and X-ray diffraction(XRD)are carried out to constrain the diagenesis of the glaucony and the oceanic redox condition in the early Ediacaran of this area.Microscopic observations using polarizing microscope and scanning electron microscope(SEM)show that the glaucony mainly fills the pores of quartz,feldspar and other clastic minerals in the form of colloidal precipitates,suggesting authigenic precipitation during the early diagenesis.Since both Fe(Ⅱ)and Fe(Ⅲ)are simultaneously required for glauconitization,glaucony prefers to precipitate around the Fe redoxcline(suboxic conditions).Accordingly,the occurrence of glaucony in the lower Doushantuo Formation of study section implies suboxic pore water environments.The results of energy dispersive spectroscopy(EDS)show that the glaucony is characterized by high K 2 O and Al_(2)O_(3)but low Fe_(2)O_(3)contents which are typical of Precambrian glaucony.The dissolution of detrital minerals along with ion exchange between the seawater and pore-water supplied the elements required for glaucony formation.Compared with contemporaneous glaucony-bearing strata in Weng'an area,Guizhou,the limited vertical distribution of glaucony in the study area indicates that the marine redox condition and paleogeography may have played a joint role in regulating the formation and spread of glaucony.

关 键 词：古海洋学 两河口 陡山沱组 粘土矿物 氧化还原环境 

分 类 号：P578.963[天文地球—矿物学]