Fabric development and metamorphic evolution of lower Palaeozoic slaty rocks from the Rocroi massif (French-Belgian Ardennes): new constraints from magnetic fabrics, phyllosilicate preferred orientation and illite crystallinity data

This paper presents new results of a petrofabric study from Cambrian slates cropping out within the anchiepimetamorphic core of the Rocroi massif (Ard ennes Variscan fold-thrust belt of northern France and southern Belgium). T his study includes measurements of the Anisotropy of the Isothermal Remanen t Magnetisation (AIRM) and the phyllosilicate preferred orientation. It com pletes the previous results of Anisotropy of Magnetic Susceptibility (AMS) and of magnetic mineralogy which were obtained in 47 sites situated in and around this massif. These new data allow the characterisation of the differ ent subfabrics carried by the ferromagnetic minerals (metamorphic pyrrhotit e and magnetite) and the paramagnetic matrix (Fe-rich phyllosilicates). Com bined with some illite crystallinity data measured throughout the massif, t he petrofabric measurements evidence a polyphase tectonometamorphic evoluti on. A major Variscan compressional event is evidenced inducing the growth o f highly anisotropic pyrrhotite, coarse-grained hematite and phyllosilicate s. By contrast, the magnetite grains display a less well-organised fabric d ue to the existence of an inherited orientation pattern. In agreement with recent metamorphic studies, it argues for an early diastathermal metamorphi c event developed during a Devonian crustal-scale extension. (C) 1999 Elsev ier Science B.V. All rights reserved.