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Development of magnetic fabrics during hydrothermal alteration in the Soultz-sous-Forêts granite from the EPS-1 borehole, Upper Rhine Graben

By
Jana Just
Jana Just
Geologisch-Paläontologisches Institut, Ruprecht-Karls Universität, INF 234, 69120 Heidelberg, Germany e-mail: jana.just@urz.uni-heidelberg.de
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Agnes Kontny
Agnes Kontny
Geologisch-Paläontologisches Institut, Ruprecht-Karls Universität, INF 234, 69120 Heidelberg, Germany e-mail: jana.just@urz.uni-heidelberg.de
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Helga De Wall
Helga De Wall
Institut für Geologie, University in Würzburg, Pleicherwall 1, 97070 Würzburg, Germany
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Ann M. Hirt
Ann M. Hirt
Institut für Geophysik, ETH Hönggerberg, HPP 01.2, 8093 Zürich, Switzerland
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Fatíma Martín-Hernández
Fatíma Martín-Hernández
Faculty of Geosciences, Utrecht University, Budapestlaan 17, 3584 CD Utrecht, The Netherlands
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Published:
January 01, 2004

Abstract

The Variscan, magnetite-bearing Soultz-sous-Forêts granite is found between 1420 and 2230 m of the EPS-1 borehole situated in the Upper Rhine Graben (France). Our study focuses on the changes of magnetic properties that occur during the progressive hydrothermal alteration and fracturing of the Soultz granite after emplacement. The magnetic susceptibility (κ) of the granite is between 10 and 80 × 10−3 SI, and suggests that ferrimagnetic minerals are the primary carrier. During cooling and later tectonic and hydrothermal overprints, including the formation of the Rhine Graben, the granite was deformed under brittle conditions and partially altered by hydrothermal fluids. Along with this fluid activity, oxidation of magnetite to hematite occurred and reduced κ (< 1 × 10−3 SI). AMS analysis on oriented samples documents the history of progressive transformation from primary magmatic fabric to tectonic fabric during hydrothermal alteration and faulting. The fresh granite with multidomain magnetite grains shows sub-horizontal magnetic foliations and randomly oriented magnetic lineations within the foliation plane. This fabric is similar to the magmatic fabric reflected by biotite. Transformation of the magnetic fabric started with localized magnetite oxidation along NW-SE oriented micro-cracks, which are probably associated with a late-magmatic alteration (stage I). Elongated and co-aligned magnetite relics within the newly formed hematite caused a well-defined NW-SE trending magnetic lineation and steeper magnetic foliation. Later alteration associated with intense brittle deformation (stage II) initially adopted this magnetic fabric, but intense cataclasis destroyed it. The geometry and orientation of magnetic fabric clearly indicate a hydrothermal alteration, which relates to the acting tectonic stresses in the post-emplacement history of the Soultz granite.

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Contents

Geological Society, London, Special Publications

Magnetic Fabric: Methods and Applications

F. Martín-Hernández
F. Martín-Hernández
Faculty of Geosciences, Utrecht University, The Netherlands
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C. M. Lüneburg
C. M. Lüneburg
Department of Geology and Geophysics, University of New Orleans, USA
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C. Aubourg
C. Aubourg
Laboratoire de Tectonique, Université de Cergy-Pontoise, France
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M. Jackson
M. Jackson
Institute for Rock Magnetism, University of Minnesota, USA
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Geological Society of London
Volume
238
ISBN electronic:
9781862394865
Publication date:
January 01, 2004

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