Abstract
Magnetite-bearing mylonitic garnet–micaschists close to the major suture between the Baltica and Iapetus terranes (Seve Nappe Complex, Scandinavian Caledonides) show very high anisotropy of magnetic susceptibility (AMS) with corrected degree of anisotropy (P′) up to 4.8. Three different magnetic fabric types can be distinguished. They correspond to protomylonite (type I, P′ < 2), mylonite (type II, 2 < P′ < 3), and ultramylonite (type III, P′ > 3), respectively. The orientation of the ellipsoid axes from all applied magnetic fabric methods in this study is similar with shallow dips of the metamorphic foliation toward WSW and subhorizontal, mostly NW–SE trending mineral lineation. Differences between subfabrics were minimized under high shear strain as all markers tend to align parallel with the shear plane. The very high anisotropies and mostly oblate ellipsoid shapes of type III correlate with high magnetic susceptibility (k mean up to 55 × 10−3 SI units) and are related to the concentration of magnetite aggregates with shape-preferred orientation. They show a distinct field dependence of magnetic susceptibility of up to 10% in the k max-direction. We attribute this field dependence to a “memory” of high strains in the domain walls of the crystals acquired during synkinematic magnetite growth during shear zone fabric development at temperatures of 550–570°C.
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Acknowledgments
We thank Frantisek Hrouda, Agico Ltd., Brno, and Ben van der Plujim, University of Michigan, for helpful discussions and use of their respective laboratories. Florian Heidelbach from BGI in Bayreuth is thanked for his support on the EBSD analysis. Thanks to Philipp Lied for help with drawing part of the figures. We are grateful to the DFG for funding this project (KO1514/5). Jiri Zák and an anonymous reviewer are thanked for their constructive comments. This paper is dedicated to Frantisek Hrouda.
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Kontny, A., Engelmann, R., Grimmer, J.C. et al. Magnetic fabric development in a highly anisotropic magnetite-bearing ductile shear zone (Seve Nappe Complex, Scandinavian Caledonides). Int J Earth Sci (Geol Rundsch) 101, 671–692 (2012). https://doi.org/10.1007/s00531-011-0713-8
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DOI: https://doi.org/10.1007/s00531-011-0713-8