Two major clay-rich fault zones (Betic Cordillera, SE Spain) exhumed under arid conditions were studied to establish the origin and nature of phyllosilicates in deformed mica schists and the role of fault zones in the genesis of rocks rich in clay minerals. Both areas contain wide zones (≈200 m across) of alternating fault gouge and protolith creating a complex arrangement of sheared lenses along fault strand cores. Minerals in schists were characterized by X-ray diffractometry and scanning and transmission electron microscopy, and two samples were selected for 40Ar/39Ar dating of four clay grain-size fractions each.

Quartz, chlorite and K-micas are the principal phases in all samples (crystalline protolith and faulted schists). Paragonite and Fe and Ti-oxides are present in some samples and, additionally, most of the damaged samples have kaolinite, illite and gypsum. Backscattered electron images show irregularly oriented stacks of phyllosilicates with curved shapes in the fault rocks and quartz grains deformed by brittle fracturing. Clay fabrics in the damaged rocks are poorly developed. Lattice fringe images of faulted damaged samples revealed defect-free dioctahedral mica and Fe-rich chlorite packets which were generally with no more than several tens of nm thickness, although chlorite grains were usually thicker than micas. Chlorite showed more obvious deformation features. The calculated ages of the authigenic clay component in the two samples are 6.3 ± 0.8 Ma and 6.2 ± 0.4 Ma, while the ages calculated for the protolith clays are 22.8 ± 1.9 Ma and 11.7 ± 1.3 Ma. The age of the authigenic clay overlaps the range of ages from the nearby Cabo de Gata volcanic series (SE Spain). Textural and chemical results point toward that although the fault gouge is mostly a product of mechanical crushing of the protolith, the presence of quartz provided a strong and fragile behaviour to the faulted rocks increasing their permeability, and creating permeable paths through which low-temperature hydrothermal fluids circulated producing an intense leaching of the parent material and promoting crystallization of new authigenic minerals (illite, kaolinite, smectite and gypsum).

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