Correlation of magnetic fabric and crystallographic preferred orientations of naturally deformed carbonate — mica rocks from the Alpi Apuane in Italy and the Damara Orogen in Namibia

Abstract

The anisotropy of magnetic susceptibility (AMS) is a time-efficient method to describe crystallographic preferred orientations of rocks and has been applied in a wide field of sedimentary, metamorphic and magmatic geology. The method, however, suffers from limitations which mainly result from the interference of diamagnetic, paramagnetic and ferromagnetic fabrics (de Wall 2005) — the term ferromagnetism is used in a wider sense here, including e.g. ferrimagnetism. The AMS is an integral parameter which describes a crystallographic preferred orientation as an ellipsoid. The quantitative correlation of the AMS with the crystallographic preferred orientations should help to allow a closer view at the applicability and the limitations of the AMS analysis (see also Schmidt et al. 2006 a, b)... The results of this study are based on a large variety of fabric types of carbonate-mica marbles and mylonites, i.e. varying mica content, grain sizes, grain shapes, types and intensities of the crystallographic preferred orientation. The presented first correlations of the AMS and CPO for the single mineral phases in general demonstrate a good matching. Regarding the comparison of texture types and the AMS, limitations are possible. While single c-axis maxima and girdle-like c-axis distributions can be also distinguished by the AMS, it is obvious that distinguishing between these types and the double c-axis type is not possible at the present stage.