Tectonic versus mineralogical contribution to the magnetic fabrics of epimetamorphic slaty rocks: an example from the Ardennes Massif (France-Belgium)

The discrimination of the respective contribution of strain and mineralogy of the bulk Anisotropy of low field Magnetic Susceptibility (AMS) of rocks is addressed by a study of the Ardennes Variscan Thrust Belt. Unmetamorphosed to low-medium-grade rocks in the study area define a succession of magnetic parageneses: hematite + maghemite (?), hematite + maghemite (?) + magnetite, magnetite + pyrrhotite. A direct control of the magnetic mineralogy on the magnitude of AMS axes is shown. In addition, in all cases, the interpretation of the directions of AMS axes as kinematic indicators also applies to these rocks. We show a progressive change in shearing direction from the south of the Rocroi massif, where the nappe transport is N-directed; in the north, the vergence changes to northwestward.