SIMULTANEOUS USE OF THERMAL INDICATORS TO ANALYZE THE TRANSITION FROMDIAGENESIS TO METAMORPHISM - AN EXAMPLE FROM THE VARISCAN BELT OF NORTHWEST SPAIN

The transition from diagenesis to very low-grade metamorphism has been characterized by conodont colour alteration index (CAI), illite cryst allinity (IC), clay-mineral assemblages and rock fabric in a transect (Cape Penas-Cape Torres) located in the Cantabrian Zone (northwest Spa in). IC values show a northwest to southeast change from epizonal to d iagenetic values. CAI values show a general congruence for low- and hi gh-grade areas. This correlation indicates that CAI values for the epi zone are > 5.5, the anchizone 5.5 greater than or equal to CAI > 4, an d the diagenetic zone less than or equal to 4. Cleavage develops in in competent rocks with CAI values greater than 3, mainly by pressure sol ution. Temperature increases towards the inner parts of the orogen (to wards the northwest) and is considered to be related to the tectonic s uperposition caused by thrusting and folding. A gradient of 35 degrees 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.