Palaeoweathering of Lower Palaeozoic rocks of the Brabant Massif, Belgium:a mineralogical and petrographical analysis

The Lower Palaeozoic low-grade metamorphic rocks of the Brabant Massif are largely buried below a thick cover of post-Palaeozoic strata. Along the top of the subcrop, they comprise remnants of Cretaceous to Tertiary weatherin g profiles that represent the lower part of thick saprolites. The alteratio n of the chlorite- and muscovite-dominated Palaeozoic rocks was characteriz ed by the destruction of chlorite, accompanied by the formation of kaolinit e and iron oxides and/or iron hydroxides. The first product of chlorite wea thering seems to have been regularly interstratified chlorite-vermiculite o r chlorite-smectite, which is now represented by interstratified chlorite-m uscovite with regular ordering. Outside the thin transitional zones in whic h this mineral occurs, the rubefied intervals show only little variation in composition, which is due to the replacement of chlorite by kaolinite over short vertical distances and the stability of muscovite throughout the pre served parts of the saprolite. The rubefied rocks do have a somewhat differ ent composition along the top of some profiles. which is related to an inte raction with groundwater after burial, resulting in smectite formation, fel dspar weathering and iron dissolution. Groundwater interaction is also resp onsible for the occurrence of weathering without rubefaction, outside the a reas with saprolite remnants, which resulted in vermiculite, smectite and k aolinite formation. Copyright (C) 1999 John Wiley & Sons, Ltd.