Two-stage formation of kaolinite in shear-zone slates, southern Iberian Massif, SE Spain

Abstract

AbstractKaolinite aggregates are the major alteration product in the strongly deformed areas of a shear zone developed in contact-metamorphosed slates from the Southern Iberian Massif near Jaén (SE Spain). Muscovite and Fe-rich chlorite in strain shadows of broken andalusite crystals were observed in slates from less deformed areas. The presence of phyllosilicate packets nearly perpendicular to each other and veinlets parallel to {110} of andalusite reveal the importance of cleavage-controlled fracturing of andalusite to the mineral reactions. The compositional similarity between kaolinite and andalusite suggests a direct transformation of andalusite to kaolinite in the strongly altered area. The absence of clear preferred orientation of phyllosilicates relative to andalusite, and the compositional differences of andalusite with muscovite and chlorite from the less altered samples, indicate andalusite dissolution and chlorite-muscovite precipitation during a first mineral-growth stage. Muscovite-kaolinite and chlorite-kaolinite intergrowths indicate a later retrograde replacement stage.