DICKITE RELATED TO FLUID-SEDIMENT INTERACTION AND DEFORMATION IN PYRENEAN THRUST-FAULT ZONES

The syn-sedimentary Lower Eocene deformation of the Ainsa basin (Centr al Pyrenees, Spain) resulted in the emplacement of the Cotiella nappe, with thrust faults affecting Cuisian turbidites. Three thrust-fault z ones have been studied in order to investigate the role of deformation on mineral transformations. Two of the thrust faults contain a thin s hear band comprised of intensely cleaved black marls with calcite vein s which are distinct from the surrounding grey marls. The mineral asse mblages in the grey marls are quartz, calcite, illite, chlorite with m inor dolomite and feldspars, the clay fraction is composed of illite, chlorite and minor I/S mixed layers. No major mineralogical changes re lated to deformation are observed except in the most strained sediment s in which kaolin has been detected. Kaolin is the major component of the black marls, with a minor amount of detrital quartz, illite and ch lorite. The kaolin polytype 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 deforme d illite-dickite-rich sediment. The low-magnification transmission ele ctron microscope images reveal that clay minerals are oriented with th eir (001) plane parallel to the foliation and that dickite occurs as l arge (3 mu m) deformed crystals, or as small defect-free crystals gene rally embedded in illite. The 2M polytype nature of dickite has been c haracterised by HRTEM and selected-area electron diffraction. Long seq uence polytypes and complex disordered structures are also locally obs erved in large crystals. Microtextural relationships suggest that dick ite 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 occurrenc e of syn-kinematic dickite in highly strained shear zones of thrust fa ults suggests that deformation is an important factor promoting fluid- sediment interactions and clay mineral reactions.