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Geological Society, London, Special Publications; 2004; v. 238; p. 61-68;
DOI: 10.1144/GSL.SP.2004.238.01.06
© 2004 Geological Society of London

Magnetic Fabric Characterization Methods and Mineral Sources

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

Norihiro Nakamura1 & Graham J. Borradaile2

1 Department of Geo-Environmental Science, Tohoku University, Sendai 980-8578, Japan n-naka{at}mail.tains.tohoku.ac.jp
2 Geology & Physics Dept., Lakehead University, Thunder Bay, P7B 5E1, Canada gjborrad{at}lakeheadu.ca

Magnetic fabric changes due to progressive metamorphism are poorly understood. Bulk magnetic susceptibility ({kappa}) 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: {kappa}, 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 ({kappa}) 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 {kappa}, Pj and Tj in the new projection.