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Prograde epizonal clay mineral assemblages and retrograde alteration in tectonic basins controlled by major strike-slip zones (W Iberian Variscan chain)

Published online by Cambridge University Press:  09 July 2018

M. Vázquez ,
I. Abad ,
J. Jiménez-Millán ,
F. T. Rocha ,
P. E. Fonseca  and
H. I. Chaminé
M. Vázquez
Affiliation:
Centro de Minerais Industriais e Argilas (MIA), Departamento de Geociências, Universidade de Aveiro, 3810-193 Aveiro, Portugal
I. Abad*
Affiliation:
Departamento de Geología, Universidad de Jaén, Campus Universitario, Edificio B-3, 23071 Jaén, Spain
J. Jiménez-Millán
Affiliation:
Departamento de Geología, Universidad de Jaén, Campus Universitario, Edificio B-3, 23071 Jaén, Spain
F. T. Rocha
Affiliation:
Centro de Minerais Industriais e Argilas (MIA), Departamento de Geociências, Universidade de Aveiro, 3810-193 Aveiro, Portugal
P. E. Fonseca
Affiliation:
Departamento de Geologia, Faculdade de Ciências da Universidade de Lisboa and LATTEX, Portugal
H. I. Chaminé
Affiliation:
Centro de Minerais Industriais e Argilas (MIA), Departamento de Geociências, Universidade de Aveiro, 3810-193 Aveiro, Portugal Departamento de Engenharia Geotécnica, Instituto Superior de Engenharia do Porto (ISEP), Portugal
*
*E-mail: miabad@ujaen.es
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Abstract

We have carried out optical microscopy, X-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM) studies of phyllosilicates from black slates of very low to low-grade metamorphism. Such slates belong to a Middle/Late Devonian basin and an Early Carboniferous basin associated with the Porto–Tomar–Ferreira do Alentejo strike-slip shear zone (Ossa-Morena Zone, Portuguese Iberian Variscan Massif). These black slates are imbricated in an Upper Proterozoic substratum of higher metamorphic grade. Kübler Index values of white micas and mineral assemblages deduced from the XRD, SEM and TEM data (muscovite, chlorite and pyrophyllite) indicate high anchizonal and epizonal metamorphic conditions for slates from these basins. The b parameter and the phengitic contents of mica suggest the occurrence of low pressures (1–2 kbar) related to an extensional geotectonic setting. The dense fracture network shown by SEM images and the high density of crystal defects revealed by the TEM study in the eastern basin, adjacent to faults produced by shearing, suggest that their epizonal phyllosilicates were more affected during deformation than those belonging to the western basin, favouring the development of a retrograde association (siderite, kaolin group minerals and Al-smectite) on the epizonal paragenesis. Microcavities formed along phyllosilicate cleavage acted as channels for fluid transport favouring alteration under low-temperature conditions.

Keywords

TEMSEMshear deformationIberian Massifmuscovitechloritedickitekaoliniteretrograde alterationlow-T metamorphism
Type
Research Article
Information
Clay Minerals , Volume 42 , Issue 1 , March 2007 , pp. 109 - 128
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2007

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