GENESIS AND TRANSFORMATION OF DICKITE IN PERMO-TRIASSIC SEDIMENTS (BETIC CORDILLERAS, SPAIN)

In the Malaguide Complex (Betic Cordilleras, Spain), dickite is widely developed in elastic Permo-Triassic sequences. The lateral extent str etches at least 300 km along the Betic range, whilst vertical extent i s variable and appears limited to the lowest 20-150 m of these sequenc es. Textural, chemical and crystallochemical characteristics of the di ckite, illite/mica and chlorite in dickite-bearing rocks and in the ov erlying (Permo-Triassic) and underlying (Carboniferous) rocks have bee n investigated to determine the approximate conditions in which dickit e has developed. Using the chlorite geothermometer (based on Al-IV con tents), temperatures of 146+/-28 degrees C and 169+/-12 degrees C have been deduced for two Permo-Triassic members, and 305+/-12 degrees C f or Carboniferous. The Si content in illites has been used as a geobaro meter, and pressures of 4.8+/-2 kbar have been estimated in Carbonifer ous rocks and tentative pressures of 2.7+/-3 and 2.1+/-2 kbar in Permo -Triassic members. Chemical evolution of phyllosilicates is accompanie d by increasing illite crystallinity and % 2M(1) polytype.