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Geological Society, London, Special Publications; 2004; v. 238; p. 361-380;
DOI: 10.1144/GSL.SP.2004.238.01.19
© 2004 Geological Society of London

Tectonic Fabrics

Phyllosilicate preferred orientation as a control of magnetic fabric: evidence from neutron texture goniometry and low and high-field magnetic anisotropy (SE Rhenohercynian Zone of Bohemian Massif)

Martin Chadima1,2, Anke Hansen3, Ann M. Hirt4, Frantisek Hrouda5,6 & Heinrich Siemes7

1 Palaeomagnetic Laboratory, Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 135, CZ-16502 Praha 6, Czech Republic chadima{at}gli.cas.cz
2 Institute of Geological Sciences, Masarykova Univerzita, Kotlárská 2, CZ-61137 Brno, Czech Republic
3 GKSS Forschungszentrum Geesthacht GmbH, Max-Planck-Straße, Geb. 03, D-21502 Geesthacht, Germany
4 Institut für Geophysik, ETH Zürich, CH-8093 Zürich, Switzerland
5 AGICO Inc., Jecná 29a, CZ-62100 Brno, Czech Republic
6 Institute of Petrology and Structural Geology, Charles University, Albertov 6, Prague, Czech Republic
7 Institut für Mineralogie und Lagerstättenlehre, RWTH Aachen, Wüllnerstr. 2, D-52056 Aachen, Germany

The low- and high-field magnetic anisotropy (AMS, HFA) of the Rhenohercynian mudstones and greywackes is compared to the theoretical anisotropy calculated from neutron texture goniometry measurements. The magnetic anisotropy is predominantly carried by the paramagnetic phyllosilicates in the form of chlorite/mica stacks and the ferromagnetic contribution is insignificant. The respective principal directions of the theoretical anisotropy and the AMS and HFA are sub-parallel; magnetic foliation reflects the orientation of the maximal concentration of phyllosilicate basal planes, magnetic lineation is subparallel to the intersection axis of those planes. For the purpose of quantitative comparison, the infrequently used standard deviatoric susceptibility as a measure of the HFA degree is employed. A very good linear correlation of the degree of theoretical anisotropy and the measured AMS and HFA is found. The prolate and oblate shapes of the respective fabric ellipsoids are reasonably well correlated. Neutron texture goniometry justifies the use of the conventional magnetic anisotropy technique for the assessment of the mineral fabric of studied rocks. When compared with other works relating the magnetic anisotropy to the mineral preferred orientation (examined by e.g. U-stage or X-ray texture goniometry) neutron texture goniometry seems to be a preferable and very precise method fabric analysis.