Magnetofabric and textural analysis from highly anisotropic magnetite bearing shear zones in metamorphic nappes of the Scandinavian Caledonides

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The first documentation of very high anisotropies (P´ > 4) of the magnetic susceptibility in metamorphic nappes from the Scandinavian Caledonides raises questions about the causative processes. Magnetite in a platy shape was identified as the main carrier of the magnetic susceptibility and the main reason for these high degrees of anisotropy. A major question is whether these extremely flattened magnetite grains are controlled by mimetic growth, ductile deformation, or both. Since ductile deformation can influence the grain shape of magnetite and thus the extrinsic susceptibility the AMS can be linked with ductile deformation processes. This is in marked contrast to paramagnetic minerals, which will attain fabric saturation at very high strain and are thus suitable for quantification of low to moderate strain. The exhumation of high-grade metamorphic rocks is commonly accompanied by ductile shearing localized in zones of various dimensions, geometries, lithologies, p-T-conditions and activity periods. Total strain is expected to be high in major shear zones within high-grade metamorphic nappes, but difficult to quantify. The study of shear zones with very high anisotropies of the magnetic susceptibility provides a chance to monitor systematic changes of rock magnetic parameters and petrofabrics – quantified by low- and high-field AMS, IRM, and AARM – and to link them with field observations, meso- to microscale structural data, textural data, magnetomineralogy (SEM, EMP), and image analysis. For the reintegration of the results into the geological realm a few 40Ar/39Ar-datings of recrystallized, synkinematic white micas are considered.