Geologie en Mijnbouw 77: 77–91, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 77 Conodont colour alteration indices (CAI) of Upper Ordovician limestones from the Iberian Peninsula Graciela N. Sarmiento 1 , Susana Garc´ ıa-L´ opez 2 & Fernando Bastida 2 1 Museo Geominero and Departamento de Paleontolog´ ıa, Facultad de Ciencias Geol´ ogicas, Universidad Com- plutense, 28040-Madrid, Spain; 2 Departamento de Geolog´ ıa, Universidad de Oviedo, Arias de Velasco s/n, 33005-Oviedo, Spain Received 21 November 1997; accepted in revised form 13 November 1998 Key words: diagenesis, metamorphism, Ashgill biostratigraphy, Variscan belt 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 recrystallisation 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. Introduction The organic matter present in conodont elements shows colour changes with increasing temperature. These changes were used by Epstein et al. (1977) to establish a conodont colour alteration index (CAI), providing a geothermometer for temperatures ranging between 50 and 600 ◦ C. Hence, conodonts are use- ful for the study of the transition from diagenesis to metamorphism and for determining low- to medium- grade metamorphism. Several authors have described the origin, calibration and geological application of the CAI (e.g. Epstein et al. 1977, Rejebian et al. 1987, Burnett 1988, Nöth 1991, Königshof 1992). They have shown that the CAI is time- and temperature- dependent and results chiefly from burial, or regional or contact metamorphism. This simple and inexpen- sive method provides a determination of the degree of heating experienced by sedimentary and low- to medium-grade metamorphic rocks. Two physical characteristics accompanying colour changes in conodonts are correlated with increasing metamorphism: 1) deformation and fracturing, espe- cially common in delicate forms, 2) surface texture changes from smooth to sugary and granular (Burnett 1988, Königshof 1992, García-López et al. 1997). At present, the application of the CAI method and its calibration, with results obtained by other ther- mal indicators, are of great value in checking the suitability of the method for investigating the tectono- thermal history of rocks, especially in the transition from diagenesis to metamorphism. The purpose of this paper is to present CAI values of Asghill con- odonts from different tectonic domains in the Iberian Peninsula, and to analyse the geological significance of these values. Ordovician sequences in the Iberian Peninsula are dominated by clastic deposits. Locally, generally thin limestone levels occur in the Upper Or- dovician. Ashgill carbonate units have been studied from several localities in the Iberian Variscan belt, the