1. Introduction Of the several parameters that may be used to establish thermal evolution in very low- to low-grade rocks, the more widely applied in sedimentary sequences is illite crystallinity (IC), used as a reference for delimiting the anchizonal boundaries (Kübler, 1967a). However, the application of IC is limited to pelitic rocks due to the influence of the bulk chemistry on its value (Kisch, 1980, 1983). This leaves a great proportion of sedimentary rocks which cannot be studied in this way. The conodont colour alteration index (CAI) (Epstein, Epstein & Harris, 1977; Rejebian, Harris & Huebner, 1987) enables the study of other lithologies, mainly carbonates, and thus the methods are complementary. From the microstructural point of view, an important characteristic of the regional metamorphism is the devel- opment of cleavage. Knowledge of its distribution and characteristics in low-grade areas provides a good com- plement to the above mentioned methods (Coombs, 1961; Kübler, 1967b; Aller et al. 1987; Kisch, 1989, 1991b; Durney & Kisch, 1994). Whilst studies of the separate parameters are common, data on their correlation based on simultaneous studies of several parameters are scarce (Buggisch, 1986; Kovacs & Arkai, 1987; Nöth, 1991; Keller, Lehner & Buggisch, 1993). For this reason we have undertaken such a study on low-grade Palaeozoic rocks of the Variscan Belt of northwest Spain. For an integrated study of this kind, the Cape Peñas–Cape Torres sector (Figs 1, 2), in the northwest of the Iberian Peninsula, has been selected as a suitable area because the transition from diagenesis to metamorphism has been documented (Brime & Pérez-Estaún, 1980). Further, it shows a nearly complete Palaeozoic sequence (Fig. 2), with alternating carbonate and siliciclastic for- mations, and a variable development of foliation. 2. Geological setting The Cape Peñas–Cape Torres section is located in the Cantabrian Zone, which constitutes the external part of the Variscan Belt in the northwest Iberian Peninsula (Fig. 1). The Cantabrian Zone is characterized by thrust nappes and associated folds typical of a thin-skinned tectonic style (Julivert, 1971; Pérez-Estaún et al. 1988 ). However, this section is bounded to the west by the Westasturian Leonese Zone that marks the beginning of the internal zone of the orogen, where foliation and regional meta- morphism are widespread (Fig. 1). Hence, the Cape Peñas–Cape Torres transect has stratigraphical and struc- tural characteristics that are transitional between the Westasturian–Leonese Zone and Cantabrian Zone, and shows the initial stages of development of a regional foli- ation and metamorphism in the orogen. The studied transect has a nearly complete succession from the Lower Ordovician (Arenig) to the Upper Carboniferous (Namurian) (Truyols & Julivert, 1976) (Fig. 2). Nevertheless, there are some important differ- ences between the northwestern and the southeastern parts of the section. In the northwest, close to Cape Peñas, the Middle and Upper Ordovician are represented by a Geol. Mag. 134 (3), 1997, pp. 323–334. Copyright © 1997 Cambridge University Press 323 Simultaneous use of thermal indicators to analyse the transition from diagenesis to metamorphism: an example from the Variscan Belt of northwest Spain SUSANA GARCIA-LOPEZ*, COVADONGA BRIME*, FERNANDO BASTIDA* & GRACIELA N. SARMIENTO† * Departamento de Geología, Universidad de Oviedo, 33005 Oviedo, Spain † Departamento de Paleontología, Universidad Complutense–Instituto de Geología Económica (CSIC-UCM), Facultad de CC Geológicas, 28040 Madrid, Spain (Received 6 June 1996; accepted 13 January 1997) Abstract – The transition from diagenesis to very low-grade metamorphism has been characterized by con- odont colour alteration index (CAI), illite crystallinity (IC), clay-mineral assemblages and rock fabric in a transect (Cape Peñas–Cape Torres) located in the Cantabrian Zone (northwest Spain). IC values show a northwest to southeast change from epizonal to diagenetic values. CAI values show a general congruence for low- and high-grade areas. This correlation indicates that CAI values for the epizone are > 5.5, the anchizone 5.5 ≥ CAI > 4, and the diagenetic zone ≤ 4. Cleavage develops in incompetent rocks with CAI values greater than 3, mainly by pressure solution. Temperature increases towards the inner parts of the oro- gen (towards the northwest) and is considered to be related to the tectonic superposition caused by thrusting and folding. A gradient of 35 °C/km is inferred for the transect. In the southeastern sector of the section there are several thermal anomalies that are attributed to the presence of epithermal fluids.