Mineralogical and δ18O–δD isotopic study of kaolinized micaschists at Penestin, Armorican Massif, France: New constraint in the kaolinization process

Abstract

Many kaolinite occurrences developed from Paleozoic rocks forming the basement of the Armorican Massif in western France. Their origin, hydrothermal alteration and/or weathering, is still discussed. In order to provide new constraints to this debate, we studied the kaolinite occurrence at Penestin in the southern part of the Armorican Massif using several approaches, including field and petrological observations, mineralogical analyses (XRD) and stable oxygen–hydrogen isotope analyses. An alteration profile was developed from faulted micaschists showing, from the bottom to the top of the profile, a reddish saprolite zone with pure white kaolinite fissures cutting across the schistosity and a white kaolinite zone, 5–10m thick. The detailed study by XRD of the layer silicates enabled us to characterize a mineralogical sequence with increasing alteration intensity, which is coherent with a weathering evolution: mica, chlorite→illite/vermiculite (chlorite) mixed-layer mineral→vermiculite→vermiculite/kaolinite mixed-layer mineral→kaolinite. Moreover, an intense iron leaching process occurs within the kaolinite zone, which is characterized by a bleaching of this zone combined with iron oxyhydroxide precipitation along/within fractures and cracks. The isotopic analyses of kaolinite from the zone and fissures gave δ18O values ranging from 19.64 to 21.21‰, and δD values ranging from −69.4 to −65.8‰. This confirms that kaolinite was formed by low-temperature water–rock interactions in contact with meteoric fluids during weathering processes. Isotopic and stratigraphic data suggest that this weathering occurred before the Messinian (~7Ma) and probably dates back to the Eocene, during which the climate was sub-tropical at this location.