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| Titel |
Very high anisotropies of magnetic susceptibility in a shear zone from the Scandinavian Caledonides: A magnetofabric and textural analysis |
| VerfasserIn |
R. Engelmann, J. Grimmer, A. Kontny, A. M. Hirt, R. O. Greiling |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250025912
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| Zusammenfassung |
| Magnetic fabric investigations of mylonitic garnet mica schists of the Seve-slab N and W of
Lake Slipsiken (Swedish Caledonides) showed very high anisotropies of the magnetic
susceptibility (AMS) with P’-values up to 4.5. The high degree of anisotropy in this shear
zone raises questions about its origin and provides a chance to monitor systematic changes of
rock magnetic parameters and petrofabrics. Microscopic investigations revealed that
metamorphic foliation is defined by muscovite and biotite. Magnetite and ilmenohematite
(fine hematite exsolution lamellae in ilmenite hosts) comprise less than 3 per cent
by volume of the rock. They are aligned subparallel to the mica fabric, forming
flattened and elongated mineral grains of 20 to 250 μm size. Their long axis are
aligned in the plane of metamorphic foliation. Because bulk susceptibilities are very
high (max. value 65*10-3 SI) only magnetite with a TV of –152°C and TC of
580°C can be seen in temperature-dependence of magnetic susceptibility. However,
IRM acquisition with following thermal demagnetization of the orthogonal IRM,
revealed a significant contribution of ilmenohematite on remanence, although NRM
demagnetization is dominated by a soft magnetic behaviour (MDF-values < 15 mT). Low-
and high-field AMS and AARM axes are nearly coaxial, and kmax and kint align in the
plane of metamorphic foliation. This observation clearly confirms a syntectonic
formation of mica minerals and Fe-Ti oxides during the main stage of shear zone
deformation. The shape factor indicates consistent oblate shapes for the susceptibility
ellipsoid. We observed a field dependency (up to 10 per cent) of magnetic susceptibility
parallel to kmax, especially for samples with high P’. We suggest that this field
dependence and the high AMS are produced by preferentially oriented exsolution lamellae
in ilmenohematite in combination with flattened and elongated magnetite grains. |
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