Conodont colour alteration indices (CAI) of Upper Ordovician limestones from the Iberian Peninsula

Conodont colour alteration index (CAT) data in Upper Ordovician rocks from several areas of the Variscan domain in the Iberian Peninsula indicate cond itions ranging from diagenesis to low-grade metamorphism. In most of the ar eas, where studies using other indicators, such as illite crystallinity (IC ) or vitrinite reflectance are lacking, the CAT method has permitted a prel iminary estimation of the metamorphic grade. In the Almaden syncline (Centr al-Iberian Zone), where IC studies are available, the thermal conditions in ferred from CAI data agree with those obtained by the IC method. In the Pue rtollano-Almuradiel syncline, the thermal interval obtained primarily from fluid inclusions (270-370 degrees C) overlaps considerably with that obtain ed from CAI data (180-340 degrees C). In general, cleavage in rocks is pres ent in anchizonal or epizonal conditions, whereas in diagenetic conditions with CAT greater than or equal to 2.5, cleavage is scarce. The conodont tex ture changes with increasing metamorphism, and apatite recrystallisation ap pears in general with CAI greater than or equal to 5. Variation of CAI valu es within a single sample and/or within short stratigraphic distances obser ved at several localities is due to hydrothermal activity.