Magnetic susceptibility fabrics in slates: Structural, mineralogical and lithological influences

Abstract

A single, penetrative tectonic deformation of Archean greywackes, accompanied by low grade metamorphism, generated low-field magnetic susceptibility fabrics that are coaxial with the rock's schistosity although there are multiple sources of magnetic susceptibility. Pre-metamorphic magnetite and the metamorphic minerals pyrrhotite, biotite and chlorite (variety thuringite: daphnite-aphrosiderite) are responsible for the anisotropy of susceptibility. These minerals grew with preferred crystallographic orientations influenced by the prevailing stress regime. On its own, rotation cannot be invoked to explain the alignment of the metamorphic minerals. There are varying proportions of at least two of these minerals in most specimens. For 153 specimens the degree of anisotropy increases as the mean susceptibility increases over the range from 100 to 700 (in units of 10 −6 S.I.)- Theory shows that this is to be expected either due to increasing the traces of strongly oriented ferrimagnetics or due to increasing the proportion of chlorite. In the latter case this simultaneously increased its preferred orientation by closer packing. Stress-influenced crystallization of chlorite rapidly achieves saturation alignment and a limiting magnetic anisotropy although strain may continue to increase. Samples taken through individual graded beds show that the susceptibility ellipsoid varies from oblate in slaty rocks to a neutral shape in sandier rocks. However, its orientation is always structurally controlled, with the minimum susceptibility axis perpendicular to schistosity.