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Transmission Electron Microscopy Study of Very Low-Grade Metamorphic Rocks in Cambrian Sandstones and Shales, Ossa-Morena Zone, Southwest Spain

Published online by Cambridge University Press:  28 February 2024

A. López-Munguira  and
F. Nieto
A. López-Munguira
Affiliation:
Area de Cristalografía y Mineralogía, Facultad de Ciencias, Universidad de Extremadura, Avda. de Elvas s/n, 06071-Badajoz, Spain
F. Nieto
Affiliation:
Departamento de Mineralogía y Petrología and I.A.C.T., Universidad de Granada-CSIC, Avda. Fuentenueva s/n, 18002-Granada, Spain
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Abstract

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.

Keywords

AEMAnchizoneChloritic PhasesClastic RocksMicaPolytypesVariscan Belt
Type
Research Article
Information
Clays and Clay Minerals , Volume 48 , Issue 2 , April 2000 , pp. 213 - 223
Copyright
Copyright © 2000, The Clay Minerals Society

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References

Abad, M.M. and Nieto, F., 1995 Genetic and chemical relationships between berthierine, chlorite and cordierite in nodules associated to granitic pegmatites of Sierra Albarrana (Iberian Massif, Spain) Contributions to Mineralogy and Petrology 120 327–336 10.1007/BF00306511.Google Scholar
Ahn, J.H. and Peacor, D.R., 1986 Transmission and analytical electron microscopy of the smectite to illite transition Clays and Clay Minerals 34 165–179.Google Scholar
Ahn, J.H. Peacor, D.R. and Essene, E.J., 1985 Coexisting paragonite-phengite in blueschist eclogite: A TEM study American Mineralogist 70 1193–1204.Google Scholar
Baronnet, A., Buseck, P. and Ribbe, P.H., 1992 Polytypism and stacking disorder Minerals and Reactions at the Atomic Scale: Transmission Electron Microscopy, Reviews in Mineralogy, Volume 27 Washington, D.C. Mineralogical Society of America 231–282 10.1515/9781501509735-011.CrossRefGoogle Scholar
Baronnet, A. and Kang, Z.C., 1989 About the origin of mica polytypes Phase Transitions 16 447–493 10.1080/01411598908245724.CrossRefGoogle Scholar
Champness, P.E. Cliff, G. and Lorimer, G.W., 1981 Quantitative analytical electron microscopy Bulletin of Mineralogy 104 236–240.CrossRefGoogle Scholar
Cliff, G. and Lorimer, G.W., 1975 The quantitative analysis of thin specimens Journal of Microscopy 103 203–207 10.1111/j.1365-2818.1975.tb03895.x.CrossRefGoogle Scholar
Dalla-Torre, M. Livi, K. Veblen, D.R. and Frey, M., 1996 White K-mica evolution from phengite to muscovite in shales and shale matrix melange, Diablo Range, California Contributions to Mineralogy and Petrology 123 390–405 10.1007/s004100050164.CrossRefGoogle Scholar
Franceschelli, M. Mellini, M. Memmi, I. and Ricci, C.A., 1989 Sudoite, a rock-forming mineral in Verrucano of the Northern Apennines (Italy) and the sudoite-chloritoid-pyrophyllite assemblage in prograde metamorphism Contributions to Mineralogy and Petrology 101 274–279 10.1007/BF00375312.CrossRefGoogle Scholar
Fransolet, A.M. and Schreyer, W., 1884 Sudoite, di/trioctahedral chlorite: A stable low-temperature phase in the system MgO-Al2O3-SiO2-H2O Contributions to Mineralogy and Petrology 86 409–417 10.1007/BF01187144.CrossRefGoogle Scholar
Guidotti, C.V. Yates, M.G. Dyar, M.D. and Taylor, M.E., 1994 Petrogenetic implications of the Fe3+ contents of muscovite in pelitic schists American Mineralogist 79 793–795.Google Scholar
Iijima, S. and Buseck, P.R., 1978 Experimental study of mica structures by high-resolution electron microscopy Acta Crystallographica A34 709–719 10.1107/S0567739478001473.CrossRefGoogle Scholar
Inoue, A. and Utada, M., 1991 Smectite-to-chlorite transformation in thermally metamorphosed volcanoclastic rocks in the Kamikita area, Northern Honshu, Japan American Mineralogist 76 628–640.Google Scholar
Jiang, W.T. Nieto, F. and Peacor, D.R., 1992 Composition of diagenetic illite as defined by analytical electron microscope analyses: Implications for smectite-illite-muscovite transitions 29th International Geological Congress Abstract 1 100.Google Scholar
Julivert, M. Fontbote, J.M. Ribeiro, A. and Conde, L., 1974 Mapa Tectónico de la Península Ibérica y Baleares Madrid Instituto Geológico y Minero de España.Google Scholar
Kisch, H. and Frey, M., 1987 Correlation between indicators of very lowgrade metamorphism Low Temperature Metamorphism Glasgow Blackie Press 227–300.Google Scholar
Lee, J.H. Ahn, J.H. and Peacor, D.R., 1985 Textures in layered silicates: Progressive changes through diagenesis and low-temperature metamorphism Journal of Sedimentology and Petrology 55 532–540.Google Scholar
Lindgreen, H. and Hansen, P.L., 1991 Ordering of illitesmectite in Upper Jurassic claystones from the North Sea Clay Minerals 26 105–125 10.1180/claymin.1991.026.1.10.CrossRefGoogle Scholar
López-Munguira, A. Nieto, F. and Sebastian-Pardo, E., 1993 Caracterizacion de las pizarras cámbricas de la Unidad Alconera (Zona de Ossa-Morena). Su utilidad como indicadores de las condiciones metamorficas Geogaceta 13 69–71.Google Scholar
López-Munguira, A. Morata, D. and Nieto, F., 1996 Geoquímica de los materiales pelíticos cámbricos al noroeste de Zafra (Badajoz) Geogaceta 22 149–152.Google Scholar
López-Munguira, A. Nieto, F. and Morata, D., 1998 Metamorphic evolution from diagenesis to epizone in Cambrian formations from NW Zafra (Ossa-Morena zone, SW Spain) Neues Jahrbuch für Mineralogie 174 131–157.CrossRefGoogle Scholar
Merriman, R.J. Roberts, B. and Peacor, D.R., 1990 A transmission electron microscope study of white mica crystallite size distribution in a mudstone to slate transitional sequence, North Wales, UK Contributions to Mineralogy and Petrology 106 27–40 10.1007/BF00306406.CrossRefGoogle Scholar
Nieto, F., 1997 Chemical composition of metapelitic chlorites: X-ray diffraction and optical property approach European Journal of Mineralogy 9 829–842 10.1127/ejm/9/4/0829.CrossRefGoogle Scholar
Nieto, F. Ortega-Huertas, M. Peacor, D.R. and Aróstegui, J., 1996 Evolution of illite/smectite from early diagenesis through incipient metamorphism in sediments of the Basque-Cantabrian Basin Clays and Clay Minerals 44 304–323 10.1346/CCMN.1996.0440302.CrossRefGoogle Scholar
Peacor, D., Buseck, P. and Ribbe, P.H., 1992 Diagenesis and low-grade metamorphism of shales and slates Minerals and Reactions at the Atomic Scale: Transmission Electron Microscopy, Reviews in Mineralogy, Volume 27 Washington, D.C. Mineralogical Society of America 335–376 10.1515/9781501509735-013.CrossRefGoogle Scholar
Ross, M. Takeda, H. and Wones, D.R., 1966 Mica polytypes: Systematic description and identification Science 151 191–193 10.1126/science.151.3707.191.CrossRefGoogle ScholarPubMed
Shau, Y.H. Peacor, D.R. and Essene, E.J., 1990 Corrensite and mixed-layer chlorite/corrensite in metabasalt from northern Taiwan: TEM/AEM, EMPA, XRD, and optical studies Contributions to Mineralogy and Petrology 105 123–142 10.1007/BF00678980.CrossRefGoogle Scholar
Van Der Pluijm, B.A. Lee, J.H. and Peacor, D.R., 1988 Analytical electron microscopy and the problem of potassium diffusion Clays and Clay Minerals 36 498–504 10.1346/CCMN.1988.0360603.CrossRefGoogle Scholar
Vidal, O. Goffé, B. and Theye, T., 1992 Experimental study of the stability of sudoite and magnesiocarpholite and calculation of a new petrogenetic grid for the system FeO-MgO-Al2O3-SiO2-H2O Journal of Metamorphic Geology 10 603–614 10.1111/j.1525-1314.1992.tb00109.x.CrossRefGoogle Scholar
Warr, L.N. and Nieto, F., 1998 Crystallite size distributions in very low grade metamorphic pelites: A HRTEM and XRD study of clay mineral crystallinity index standards Canadian Mineralogist 36 1453–1474.Google Scholar
Yau, Y.C. Peacor, D.R. and McDowell, S.D., 1987 Smectite-to-illite reactions in Salton Sea shales: A transmission and analytical electron microscopy study Journal of Sedimentology and Petrology 57 335–342.Google Scholar