Heterogeneous Variscan deformation in late Silurian–early Devonian diabase and microgranite dikes of the Caledonian Rocroi Inlier (Ardenne Allochthon, France) quantified using anisotropy of magnetic susceptibility (AMS)

A swarm of m-scale dikes, made of diabase and porphyritic microgranite, related to a ca. 420 Ma bimodal tholeiitic magmatism crops out in the Rocroi Inlier, a Lower Paleozoic inlier exposed in the Ardenne Allochton (western Rhenohercynian Zone, European Variscan Belt). These dikes are affected by a more or less penetrative cleavage and their magmatic mineralogy has been replaced to various degrees by secondary minerals such as albite, chlorite, sericite, epidote and calcite, as a consequence of Pennsylvanian Variscan deformation and associated low-grade metamorphism. The thickest dikes (thickness from 3-4 to 10-15 m) display a deformation gradient from the border zones which has a penetrative cleavage to the core which is little or even apparently not deformed.

We have conducted measurements of the anisotropy of magnetic susceptibility on samples collected along transversal sections across five thick dikes from the Rocroi Inlier, two made of diabase and three of microgranite. The bulk magnetic susceptibility (K_m) has typical paramagnetic values, 5.8-14.0 x 10^-4 SI in the diabase and 0.4-6.4 x 10^-4 SI in the microgranite, except in the core of one diabase dike where magnetite occurs, ca. 580-890 x 10^-4 SI. The corrected anisotropy degree (P’) tends to have high values, up to ca. 1.8, in the deformed border zones and decreases towards the core. This parameter is therefore a proxy of the petrofabric strength. The shape parameter (T) reveals a predominantly oblate magnetic fabric, which suggests a prevailing coaxial deformation, in agreement with a previous finite strain study.

Magnetic foliation in the deformed border zones is parallel to dike margin, as well as to cleavage both in diabase or microgranite and in metapelites at the contact with the intrusion. It rotates a few degrees when moving away from the dike walls. Magnetic lineation is orientated down-dip on the foliation plane and is therefore also slightly deflected across the dikes. The mean magnetic foliation dip, hence the mean lineation plunge is ca. 30-60° to the S-SE. This orientation is roughly similar to that of the main (Variscan) cleavage in the metapelitic host rocks which bears a down-dip stretching lineation. However, magnetic fabric in the dikes and petrofabric in the country rocks outside the contact zone with the intrusions are slightly oblique (up to ca. 30°). Such a discrepancy possibly results from fabric refraction due to competence contrast with the host rocks. Incomplete transposition of the magmatic fabric by the Variscan deformation, as observed in particular in some microgranite samples, could also play a role here, by influencing the orientation of the magnetic fabric.