Transmission Electron Microscopy Study of Very Low-Grade Metamorphic Rocks in Cambrian Sandstones and Shales, Ossa-Morena Zone, Southwest Spain
Transmission Electron Microscopy Study of Very Low-Grade Metamorphic Rocks in Cambrian Sandstones and Shales, Ossa-Morena Zone, Southwest Spain
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西班牙西南部奥萨-莫雷纳地区寒武系砂岩和页岩中极低品位变质岩的透射电子显微镜研究
DOI：
10.1346/ccmn.2000.0480207
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发表时间：
2000
影响因子：
2.2
通讯作者：
Francisco R. Nieto
中科院分区：
文献类型：
--
作者：
A. López;Francisco R. Nieto
本研究探讨了来自奥萨-莫雷纳带（西班牙Hesperian地块）的碎屑物质的岩石的纹理，结构和化学成分的演变。先前对这些岩石中层状硅酸盐的研究（通过X射线衍射、扫描电子显微镜和能量色散X射线分析以及电子探针）表明，岩石剖面的温度从底部（浅成条件）到顶部（成岩条件）呈下降趋势。在纳米尺度上，层状硅酸盐包在颗粒交叉处形成大角度，没有择优取向。与较低等级的包相比，变质等级的包宽且无缺陷。这些包在成岩条件下为15层至表带中>80层。二八面体富钾云母（白云母、多硅白云母和伊利石）具有共存的1 Md、1 M和2 M多型体。首次报道了富钾双八面体云母中的4、5和6层长周期多型体。云母的化学成分在锚定带和成岩带中几乎相同，包括伊利石（每个分子式单元0.8个原子，a.f.u.，K）和多硅白云母组分（0.15 a. f. u，Mg和0.13 a. f. u，Fe）。Fe可对应于铁白云母取代。表带样品具有高的白云母含量（Mg = 0.24 a.f.u.）几乎没有伊利石成分。其中一个成岩样品中同时存在菱钾长石、三八面体菱钾长石、苏多石和柯绿石。Berthieside和Escherichia在成分上是相同的。由于该系统的碎屑性质，corrensite的组成是不典型的其他corrensite，具有较高的Al含量，Fe/Mg比为0.01，和K作为可交换的阳离子。结构之间的差异成岩带和锚带是逐渐增加的双八面体富钾云母颗粒的大小，这涉及增加伊利石结晶度的基础上的Kübler指数。这些云母的化学成分为伊利石（成岩作用和锚定带）和浅成带中的多硅白云母。没有中间相，表明伊利石和多硅白云母之间的组成差距。寒武纪泥质岩中不同多型富钾云母的共存和化学均一性的缺乏表明了其不平衡性。
This study examines the evolution of the texture, structure, and chemical composition of rocks derived from clastic materials of the Ossa-Morena Zone (Hesperian Massif, Spain). Previous studies of phyllosilicates in these rocks (by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray analysis, and electron microprobe) indicated a temperature decrease from bottom (epizone conditions) to top (diagenetic conditions) of the rock section.At the nanometer scale, phyllosilicate packets form large angles where grains intersect with no preferred orientation. With metamorphic grade, packets are wide and defect free, compared to packets at lower grade. These packets are ∼ 15 layers under diagenetic conditions to >80 layers in the epizone. Dioctahedral K-rich micas (muscovite, phengite, and illite) have coexisting 1Md, 1M, and 2M polytypes. Long-period polytypes of 4, 5, and 6 layers are reported for the first time in dioctahedral K-rich micas. The chemical compositions of the micas are nearly identical in the anchizone and the diagenetic zone, comprising an illitic (0.8 atoms per formula unit, a.f.u., of K) and a phengitic component (0.15 a.f.u, of Mg and 0.13 a.f.u, of Fe). Fe may correspond to a ferrimuscovitic substitution. Epizone samples have a high phengitic content (Mg = 0.24 a.f.u.) and almost no illite component. One diagenetic sample has coexisting berthierine, trioctahedral chlorite, sudoite, and corrensite. Berthierine and chlorite are identical in composition. Because of the clastic nature of the system, the composition of corrensite is not typical of other corrensites, with higher Al content, Fe/Mg ratio at ∼1, and K as the exchangeable cation.Textural differences between the diagenetic zone and the anchizone are the progressive increase in the size of dioctahedral K-rich mica grains, which involves an increasing illite crystallinity based on the Kübler index. The chemical compositions of these micas are illite (diagenesis and anchizone) and phengite in the epizone. There are no intermediate phases, suggesting a compositional gap between illite and phengite. The coexistence of different polytypes of dioctahedral K-rich micas and the absence of chemical homogeneity indicate disequilibrium in the Cambrian pelitic rocks studied.