Dickite related to fluid-sediment interaction and deformation in Pyrenean thrust-fault zones
Buatier, Martine; Travé, Anna; Buatier, Martine; Potdevin, Jean Luc
European Journal of Mineralogy Volume 9 Number 4 (1997), p. 875 - 888
published: Jul 23, 1997
manuscript accepted: Jan 27, 1997
manuscript received: Apr 4, 1996
ArtNo. ESP147050904003, Price: 29.00 €
Abstract The syn-sedimentary Lower Eocene deformation of the Ainsa basin (Central Pyrenees, Spain) resulted in the emplacement of the Cotiella nappe, with thrust faults affecting Cuisian turbidites. Three thrustfault zones have been studied in order to investigate the role of deformation on mineral transformations. Two of the thrust faults contain a thin shear band comprised of intensely cleaved black marls with calcite veins which are distinct from the surrounding grey marls. The mineral assemblages in the grey marls are quartz, calcite, illite, chlorite with minor dolomite and feldspars, the clay fraction is composed of illite, chlorite and minor I/S mixed layers. No major mineralogical changes related to deformation are observed except in the most strained sediments in which kaolin has been detected. Kaolin is the major component of the black marls, with a minor amount of detrital quartz, illite and chlorite. The kaolin poly type is dickite according to powder XRD and IR spectroscopy. Texture analysis shows that the black marls are composed of millimetric lenses of undeformed silt embedded in strongly deformed illite-dickite-rich sediment. The low-magnification transmission electron microscope images reveal that clay minerals are oriented with their (001) plane parallel to the foliation and that dickite occurs as large (3 μm) deformed crystals, or as small defect-free crystals generally embedded in illite. The 2M poly type nature of dickite has been characterised by HRTEM and selected-area electron diffraction. Longsequence poly types and complex disordered structures are also locally observed in large crystals. Microtextural relationships suggest that dickite is a syn-kinematic mineral which precipitated after partial illite dissolution. The potassium released during the reaction may have left the system through fluid circulation in the shear zone. The occurrence of syn-kinematic dickite in highly strained shear zones of thrust faults suggests that deformation is an important factor promoting fluid-sediment interactions and clay mineral reactions.