Magnetite grain shape fabric and distribution anisotropy vs rock magnetic fabric: a three-dimensional case study

Shape fabric and distribution anisotropy of magnetite grains in a highly ferromagnetic (s.l.) syenite are determined for three mutually perpendicular planes on about 600 magnetite grains. The grains are lenticular with an average aspect ratio of about 2.0. Their preferred orientation, 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 of the shape fabric ellipses (e.g. 1.41 and 1.39 in the section plane perpendicular to the magnetic foliation and parallel to the magnetic lineation). The anisotropic wavelet transform can detect and quantify the anisotropic distribution of the magnetite grains which are located interstitially along the feldspar grain boundaries. This anisotropic distribution has no noticeable effect on the rock’s anisotropy of magnetic susceptibility (AMS), since only a few percent of grains are close enough to interact magnetically. Therefore, it is realistic to consider that the AMS of ferromagnetic granitic rocks originates mainly from the shape fabric of anisotropic magnetite grains and a close correlation between the magnetic fabric and the magnetite grain shape fabric is expected.

[1]  Philippe Gaillot,et al.  The normalised optimised anisotropic wavelet coefficient (NOAWC) Method: An Image processing tool for multi‐scale analysis of rock fabric , 1997 .

[2]  G. Borradaile.  Magnetic susceptibility, petrofabrics and strain , 1988 .

[3]  Yves Meyer,et al.  Progress in wavelet analysis and applications , 1993 .

[4]  Alain Villemeur,et al.  Methods and techniques , 1992 .

[5]  J. Darrozes,et al.  Software for multi-scale image analysis: the normalized optimized anisotropic wavelet coefficient method , 1997 .

[6]  Bernd Jähne,et al.  Digital Image Processing: Concepts, Algorithms, and Scientific Applications , 1991 .

[7]  B. Henry,et al.  Tectonic applications of magnetic susceptibility and its anisotropy , 1997 .

[8]  Christina Y. Chan-Park,et al.  Distribution anisotropy: The cause of AMS in igneous rocks? , 1991 .

[9]  R. Ketcham,et al.  Three‐dimensional quantitative textural analysis of metamorphic rocks using high‐resolution computed X‐ray tomography: Part I. Methods and techniques , 1997 .

[10]  A. Nédélec,et al.  Shape anisotropy versus magnetic interactions of magnetite grains: Experiments and application to AMS in granitic rocks , 1995 .

[11]  B. Ellwood,et al.  Magnetic fabric of the Elberton Granite, northeast Georgia , 1980 .

[12]  A. Stephenson.  Distribution anisotropy: two simple models for magnetic lineation and foliation , 1994 .

[13]  F. Hrouda.  Magnetic anisotropy of rocks and its application in geology and geophysics , 1982 .

[14]  M. Fuller,et al.  Anisotropy of magnetic susceptibility of rocks and minerals , 1963 .

[15]  E. Cañón‐Tapia.  Single-grain versus distribution anisotropy: a simple three-dimensional model , 1996 .

[16]  P. Launeau,et al.  Magnetic fabric vs. magnetite and biotite shape fabrics of the magnetite-bearing granite pluton of Gameleiras (Northeast Brazil) , 1995 .

[17]  B. Pluijm,et al.  Plastic behavior of magnetite and high strains obtained from magnetic fabrics in the Parry Sound shear zone, Ontario Grenville Province , 1995 .

[18]  J. Bouchez.  Granite is Never Isotropic: An Introduction to AMS Studies of Granitic Rocks , 1997 .

[19]  A. Fallick,et al.  The Panafrican Stratoid Granites of Madagascar: Alkaline Magmatism in a Post-Collisional Extensional Setting , 1995 .