MAGNETITE GRAIN SHAPE FABRIC AND DISTRIBUTION ANISOTROPY VS ROCK MAGNETIC FABRIC - A 3-DIMENSIONAL CASE-STUDY

Shape fabric and distribution anisotropy of magnetite grains in a high ly ferromagnetic (s.l.) syenite ale determined for three mutually perp endicular planes an about 600 magnetite grains. The grains are lenticu lar with an average aspect ratio of about 2.0. Their preferred orienta tion, determined by the inertia tensor method. is consistent with the principal directions of the magnetic susceptibility ellipsoid and the axial ratios of the magnetic fabric ellipses are very close to those o f the shape fabric ellipses (e.g. 1.41 and 1.39 in the section plane p erpendicular to the magnetic foliation and parallel to the magnetic li neation). The anisotropic wavelet transform can detect and quantify th e anisotropic distribution of the magnetite grains which are located i nterstitially along the foldspar grain boundaries. This anisotropic di stribution has no noticeable effect on the rock's anisotropy Of magnet ic susceptibility (AIC ISI, since only a few percent of grains are clo se enough to interact magnetically. Therefore. it is realistic to cons ider that the AMS of ferromagnetic granitic rocks originates mainly fr om the shape fabric of anisotropic magnetite grains and a close correl ation between the magnetic fabric and the magnetite grain share fabric is expected. (C) 1998 Elsevier Science Ltd. All rights reserved.