TSK 11 Göttingen 2006 Grimmer Very high anisotropies of the magnetic susceptibility in ductile shear zones: first quantitative results from a metamorphic nappe in the Central Scandinavian Cale- donides, Sweden Poster J.C. Grimmer1 Shear zones in the Seve crystalline base- ment nappes of the central Scandi- navian Caledonides contributed to ex- humation and translation of these high- grade metamorphic rocks. The Seve unit is considered to represent the for- mer distal passive margin of the conti- nent Baltica, which was subducted be- neath an island arc during Ordovician times and subsequently collided with the continent Laurentia during Silurian and early Devonian times. Strongly tex- tured mylonitic garnet mica schists with well developed mica fish and S–C-fabrics from a shear zone within the Seve unit show unusual high anisotropies of the magnetic susceptibility (AMS). The cor- rected degrees of anisotropy of the mag- netic susceptibility (P ′) range from 1.78 to 4.24. Bulk susceptibilities range from 2.8 × 10−3 to 96.9 × 10−3. The shape factors range from 0.32 to 0.62, docu- menting an oblate shape. Magnetic fo- liation is subparallel with metamorphic foliation. Magnetic lineation scatters due to permutations of the maximum and intermediate principal susceptibil- ity axes. Temperature-dependent sus- ceptibility measurements identify mag- netite as the carrier of the bulk sus- ceptibility. The temperature dependent susceptibility curves indicate a minor 1 Geologisch-Paläontologisches Institut, Ruprecht-Karls Universität Heidelberg, Im Neuenheimer Feld 234, 69120 Heidelberg contribution of iron carbonates. Such high anisotropies from natural samples have not yet been documented else- where. These high anisotropies result from apparently flattened magnetite. The relatively incompetent mylonite is ‘sandwiched’ between competent mafic- ultramafic rocks and localizes defor- mation. A multidisciplinary quantita- tive approach involving rock magnetic studies, geochemical data, digital image analysis, and X-ray texture goniometry (XTG) is carried out. A section across this ductile shear zone was sampled systematically from top to bottom over a vertical distance of ca. 0.8m (six samples). The bulk suscep- tibility and the degree of AMS increase from top to bottom. With the excep- tion of one sample this coincides with increasing total Fe-content. Other sam- ples from exposures of similar lithologies also exhibited high anisotropies (2.59 < P ′ > 4.15). Magnetite grains occur as inclusions in mica and garnet as well as in the fine-grained quartz-mica matrix. Grain sizes range from a few µm to ca. 0.5mm. First digital image analysis of SEM-pictures (x–z-sections) of a high- AMS sample shows ca. 2% modal mag- netite with a mean ellipticity of 3.3 and with ellipticities of up to 16.7. Statis- tically, the long axis of the magnetite grains is oriented subparallel with the S-planes. The magnetite grains can be thus used as kinematic indicators. The bulk susceptibility k and the anisotropy P ′ show a clear positive log-rhythmic correlation. The log-rhythmic relation- ship can be expressed by the general equation P ′ = a · log k + b. The geolog- ical meaning of the parameters a and b is not yet clear. These parameters could be a quantitative expression for: 1) the modal magnetite content, 2) the 1 Grimmer TSK 11 Göttingen 2006 distribution density of magnetite, and 3) the degree of deformation of magnetite grains and the associated shape. 2