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

Abstract

Conodont colour alteration index (CAI) data in Upper Ordovician rocks from several areas of the Variscan domain in the Iberian Peninsula indicate conditions ranging from diagenesis to low-grade metamorphism. In most of the areas, where studies using other indicators, such as illite crystallinity (IC) or vitrinite reflectance are lacking, the CAI method has permitted a preliminary estimation of the metamorphic grade. In the Almadén syncline (Central-Iberian Zone), where IC studies are available, the thermal conditions inferred from CAI data agree with those obtained by the IC method. In the Puertollano-Almuradiel syncline, the thermal interval obtained primarily from fluid inclusions (270-370°C) overlaps considerably with that obtained from CAI data (180-340°C). In general, cleavage in rocks is present in anchizonal or epizonal conditions, whereas in diagenetic conditions with CAI ≥ 2.5, cleavage is scarce. The conodont texture changes with increasing metamorphism, and apatite recrystallization appears in general with CAI ≥ 5. Variation of CAI values within a single sample and/or within short stratigraphic distances observed at several localities is due to hydrothermal activity.