Simultaneous use of thermal indicators to analyse the transition from diagenesis to metamorphism: an example from the Variscan Belt of northwest Spain

Abstract

The transition from diagenesis to very low-grade metamorphism has been characterized by conodont 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 orogen (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.