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Metamorphic control of magnetic susceptibility and magnetic fabrics: a 3-D projection

By
Norihiro Nakamura
Norihiro Nakamura
Department of Geo-Environmental Science, Tohoku University, Sendai 980-8578, Japan e-mail: n-naka@mail.tains.tohoku.ac.jp
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Graham J. Borradaile
Graham J. Borradaile
Geology & Physics Dept., Lakehead University, Thunder Bay, P7B 5E1, Canada e-mail: gjborrad@lakeheadu.ca
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Published:
January 01, 2004

Abstract

Magnetic fabric changes due to progressive metamorphism are poorly understood. Bulk magnetic susceptibility (κ) is known to increase with metamorphic grade but anisotropy changes have been neglected. To combine information on anisotropy with bulk susceptibility, we introduce a projection with three axes: κ, ellipsoidal eccentricity (Pj, the so-called ‘anisotropy degree’, despite the fact that this is quantified) and ellipsoid symmetry (Tj) as independent variables. The projection reveals that metamorphic facies can be discriminated successfully in the 3-D projection, with distinct, significant regression surfaces for crustal metamorphic rocks metamorphosed successively in greenschist, amphibolite, and granulite facies. This emphasizes that bulk magnetic susceptibility (κ) and its anisotropies (‘magnetic fabric’) evolve in response to metamorphic process, not just strain. Moreover, post-tectonic granitic plutons, upper mantle harzburgites and serpentinized mantle rocks also have characteristic regression surfaces relating κ, Pj and Tj in the new projection.

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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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