华北古元古代末鲕铁岩:Columbia超大陆 裂解初期的沉积响应

铁建造和鲕铁岩是地史上两类主要的富铁沉积,不仅记录了地球大气与海洋氧化还原状态和化学条件演变,而且也反 应了构造运动、岩浆活动和生物的相互作用过程.过去对铁建造已有深入研究,而有关前寒武纪铁岩成因与古海洋和构造背景 研究甚少.运用扫描电镜(scanningelectronicmicroscopy,简称SEM)、X射线衍射(X-raydiffraction,简称XRD)、能谱(energy dispersive spectroscopy,简称EDS)技术分析铁鲕的微组构、矿物成分和化学组成,讨论华北串岭沟组(1.65~1.64Ga)鲕铁岩 的成因环境及其与Columbia超大陆裂解的关系.研究表明,铁鲕主要由赤铁矿和少量高岭石组成,贫陆源碎屑和Al2O3;鲕包 壳由微片状赤铁矿构成的致密和疏松纹层交互组成;Fe-Al呈明显的负相关性,表明铁主要源于缺氧富铁深海水体而非陆源 风化.鲕铁岩集中在快速海进和低陆源输入引起的沉积饥饿期,发育于氧化还原界面附近的潮下贫氧环境.与超大陆裂解伴生 的岩浆活动、基底沉降和快速海侵是促进鲕铁岩形成的重要因素.串岭沟组底部铁岩是华北地台响应Columbia超大陆裂解而 发生构造与环境转化的重要沉积记录. 

Terminal Paleoproterozoic Ooidal Ironstone from North China: A Sedimentary Response to the Initial Breakup of Columbia Supercontinent

As archives of atmosphere oxygenation and ocean chemistry, ironstones and iron formations have been used for trac- ing the processes of tectonics, magmatism, and microbe interactions.Precambrian iron formations have been extensively studied inpast decades, whereas non-cherty ironstones of the period have been rarely exp1ored in terms of their origin, paleo- environment and tectonic background.In this paper, micro-fabrics, minera1 and chemica1 composition of the ironstone from the Chuanlinggou Formation(1.65-1.64Ga) are studied using scanning electronic microscopy(SEM), X-ray diffraction(XRD) and energy dispersive spectroscopy(EDS) techniques, with emphases on their depositiona1 environment and potentia1 links to the breakup of Columbia supercontinent. The study shows that ooids in the ironstone consist mainly of hematite and minor amount of kaolinite, with rare terrestria1 detritus and very1ow A1203 content.0oida1 cortices are characterized by alternating dense and porouslaminae formed of hematite micro-flakes that arranged in different patterns.A marked negative correlation between Fe and A1 contents in ooida1 cortices suggests that ooid ironstones are of a Fe-source from anoxic and Fe-rich deep seawa- ters, rather than terrestria1 weathering as suggested previously. The ironstones concentrated in the intervals of sediment starva- tion were caused by rapid transgression and1ow detrita1 input, and formed in subtida1 dysoxic environment near to the chemo- cline.Magmatism, rapid basement subsidence and swift marine transgression accompanied with the initia1 breakup of the Co- 1umbia supercontinent at the time werelikely the major geo1ogic factors facilitating deposition of the ironstone. Thus the Chuan- 1inggou ironstone may have signi「ied major environment and tectonic changes o「 the North China Plat「orm responding to the breakup of the Columbia supercontine