Differences between magnetic and mineral fabrics in low-grade, cleaved siliciclastic pelites: A case study from the Anglo-Brabant Deformation Belt (Belgium)

Theoretically, within a given pelitic rock in which the main paramagnetic carriers are white mica and chlorite and in which, judging from the AMS parameters, AMS is controlled by the paramagnetic carriers, the results of X-ray pole figure goniometry should show a qualitative relationship with AMS. This idea is tested on single-phase deformed, low-grade pelites of the Lower Palaeozoic Brabant Massif (Belgium), representing the southeastern part of the Anglo-Brabant Deformation Belt. For the vast majority of the samples investigated, the different LF-AMS parameters suggest a paramagnetic control on LF-AMS. Different parameters of LF-AMS measured at 77 K (low-T LF-AMS) and a comparison of these with the LF-AMS parameters indicate that LF-AMS is controlled virtually entirely by paramagnetic carriers. High-field torque magnetometry (HF-AMS) indicates further that AMS is controlled entirely by paramagnetic carriers for the vast majority of the samples. Two apparent discrepancies are observed, however. These are 1) a frequent mismatch between the LF-AMS fabric on the one hand and the (paramagnetic) fabric indicated by low-T LF-AMS and HF-AMS on the other hand, and 2) a rather common mismatch between the preferred orientation of chlorite and white mica suggested by X-ray pole figure goniometry on the one hand and the LF-AMS fabric, low-T LF-AMS fabric and HF-AMS paramagnetic fabric on the other hand. These apparent discrepancies may partly be attributed to the composite nature of both the LF-AMS fabric and the paramagnetic susceptibility fabric. The presence of different populations of paramagnetic carriers, possibly in combination with minor traces of ferromagnetic (s.l.) carriers which are not picked up by the methods applied, may explain some of the mismatches observed and are compatible with the basin evolution history of the Brabant Massif. In addition, it is possible that the low temperatures (low-T LF-AMS) and high fields (HF-AMS) are capable of enhancing one of the paramagnetic phases, thus leading to discrepancies between methods that define the mineral and the magnetic fabrics. The results of this study show that, even when the LF-AMS data suggest that the LF-AMS of a given pelitic rock in which phyllosilicates form the main paramagnetic carriers, is controlled entirely by paramagnetic carriers, alternative methods, such as phyllosilicate X-ray pole figure goniometry, will not necessarily reflect the paramagnetic carrier orientation. Although a number of cases have successfully correlated LF-AMS with mineral fabric and occasionally even strain, this study serves as an example for how methods to define mineral and magnetic fabrics can be sensitive in detecting different paramagnetic phases. (C) 2008 Elsevier B.V. All rights reserved.