Tectonothermal analysis of a mainly carbonate unit located in the external part of the Variscan orogen in NW Spain is dealt with using the conodont colour alteration index (CAI) and the study of textures of these microfossils. The Kübler index of the illite (KI) is used as a complementary method. The area is characterized by a great uniformity in the CAI values, which in most cases are <2, indicating diagenetic conditions. In spite of the low CAI values, textures show great variety and were mainly originated by diagenetic processes of apatite dissolution and precipitation. The conodonts underwent a long period of heating (probably from the Pennsylvanian to the Cenozoic) to low temperatures (<100°C) to reach the low CAI values measured. Assuming a normal geothermal gradient, these temperatures required an overburden <3 km that in part was due to burial and in part to tectonic superimposition. Minor local anomalies in the CAI values and some textural alterations, related to dissolution and precipitation of authigenic minerals, could be due to epithermal activity that gave rise to various ore deposits in the studied area mainly during Permian times.