【作者】 董进；

【导师】 张世红；

【作者基本信息】 中国地质大学（北京） ， 构造地质学， 2009， 博士

【摘要】 湖北宜昌震旦系陡山沱组四段黑色页岩中发育大量以白云石为主的碳酸盐结核,扫描电镜和岩石磁学方法发现结核中的磁性矿物以胶黄铁矿为主。结核由自生碳酸盐胶结原始沉积碎屑而成,而胶黄铁矿作为一种自生矿物,常形成于早期还原成岩环境,其形成伴随着原有含铁碎屑矿物的溶解。因此研究结核的生长演化及其磁学性质对揭示早期埋藏和成岩环境具有重要意义。碳酸盐结核的矿物成分和微观组构分析表明,结核与围岩中含有相似的碎屑矿物成分,结核通过碳酸盐胶结原始沉积碎屑生长而成。围页岩中的高有机质含量和草莓状黄铁矿揭示了缺氧沉积环境。结核中保存很好的纸房状构造(card-house)和白云石球状构造暗示结核形成于沉积早期沉积物-水界面下0-3米处。在硫酸盐还原带通过有机质分解和甲烷厌氧氧化可产生大量碳酸盐,其充填由有机质分解或发酵产生的气泡或孔洞可形成白云石球状构造。陡山沱组结核的块状构造以及其中透入性分布的球状构造揭示了结核以透入性模式生长,碳酸盐胶结在整个结核中同时成核结晶并生长。扫描电镜和岩石磁学方法发现结核与围岩中都含有胶黄铁矿,且结核中的磁性矿物以胶黄铁矿为主,另含少量磁铁矿。磁化率各向异性测量显示,结核与围岩具有完全不同的磁组构特征。围岩磁化率各向异性为扁球状且磁面理与层面平行的正常沉积磁组构,结核磁化率各向异性为长球状且磁线理与层面垂直的反转磁组构。为了解释结核中反转磁组构的成因机制,除磁化率各向异性外还测量了等温剩磁各向异性。测量结果显示,围岩与结核的等温剩磁各向异性具有与磁化率各向异性相似的磁组构特征。岩石磁学分析表明胶黄铁矿控制了结核的反转磁组构特征。与围岩相比,结核并未受到强烈的压实作用,而是通过结核生长保存了原始的沉积组构,表明结核的反转磁组构在早期沉积时就已存在。结核的反转磁组构由胶黄铁矿微晶晶轴定向排列而成,并推测这可能与生长环境有关。在早期还原成岩时,垂向上的化学环境变化以及流体流动使胶黄铁矿的[100]轴垂直层面生长。这种原始沉积磁组构通过结核生长被保存下来,而围岩却由于压实作用使[100]轴平行层面排列,从而形成正常沉积压实磁组构。更多还原

【Abstract】 Abundant carbonate concretions are distributed in the black shale of the upper Ediacaran Doushantuo Formation in South China. Scanning Electron Microscope (SEM) and rock magnetism studies reveal that the predominant magnetic minerals in the concretions are greigites. Concretion grows through authigenic carbonates cementing detrital materials, and greigite usually forms authigenically in anoxic sedimentary environments in association with dissolution of reactive detrital iron-bearing minerals. So the study of growth and magnetic property of concretions are significant in revealing the early burial and diagenetic environment.Mineralogical and textural characteristics reveal the detrital components in concretions are similar to that of the host rocks, which suggest concretions were formed by authigenic carbonate cementation of detrital materials. High organic carbon content and abundance of framboidal pyrites in the hosting shale suggest an anoxic depositional environment. Well-preserved cardhouse clay fabrics in the comcretions suggest their formation at 0-3 m burial depth, likely associated with microbial decomposition of organic matter and anaerobic oxidation of methane. Gases through decomposition of organic matter and/or from methanogenesis created bubbles and cavities, and anaerobic methane oxidation at the sulfate reduction zone resulted in carbonate precipitation, filling in bubbles and cavities to form spherical structures of the concretions. Massive textures and pervasive spherical structures of the Doushantuo concretions suggest the pervasive model of the concretionary growth. Isolated carbonate crystals in the concretions are nucleated and crystallized simultaneously.Observation of SEM and results of rock magnetic study all indicate greigite exists in the concretions and host rocks. Especially in the concretions, the magnetic minerals are dominated by greigite, with trace magnetite. The measurement of anisotropy of magnetic susceptibility (AMS) reveals concretions have different magnetic fabric from host rocks. The host rocks show oblate AMS shape with magnetic foliation parallel to the bedding plane, which is referred to as a normal sedimentary magnetic fabric. The concretions show prolate AMS shape with magnetic lineation perpendicular to the bedding plane, which is referred to as an inverse magnetic fabric. Besides AMS, anisotropy of isothermal magnetic remanence (AIRM) are also measured to the sample of concretions and host rocks. The orientation of the principal axes of the AIRM ellipsoid is very similar to those of the AMS ellipsoid. Rock magnetism studies show that greigite contribute to the inverse magnetic fabric of the concretions. As compared with host rocks, concretions had resisted compaction through concretionary growth and preserved primary sedimentary fabric. This evidences that the inverse magnetic fabric have occurred at early sediment. Preferred crystallographic orientation of the greigite produces inverse magnetic fabric of the concretions, which is possibly related with early sedimentary environment. During early reductive diagenesis, vertical change of geochemistry in the sediment column and fluid flow possibly cause [100] axis perpendicular to the bedding plane. This primary magnetic fabric was preserved in the concretions during concretionary growth and change into normal sedimentary-compaction magnetic fabric in the host rocks being greigite [100] axis parallel to the bedding plane by compaction.更多还原