Transition from diagenesis to metamorphism near the front of the Variscan regional metamorphism (Cantabrian Zone, northwestern Spain)

Distribution of paleotemperatures has been studied in Paleozoic rocks of th e Variscan fold and thrust belt of the NW Cantabrian Zone (Spain) by a comb ined application of illite crystallinity (IC) and conodont color alteration index (CAI). Results show that the diagenetic-metamorphic grade increases with the increase in rock age, as marked by decreasing IC values and increa sing CAI values. Moreover, the CAI isogrades roughly reflect the structural traits of the area, indicating that maximum temperatures were attained dur ing sedimentary burial previous to folding. The geothermal paleogradient ob tained suggests conditions near the boundary between low and middle P/T con ditions. There is evidence of at least two thermal events during the tecton othermal development of the belt. The main stage is interpreted as the resu lt of the burial of the rocks characterized by late diagenesis conditions t hat occurred under a geothermal gradient of similar to 35 degreesC km(-1). The narrow zone with low anchizonal conditions in the western part of the a rea could represent the front of the Variscan orogenic metamorphism. This s tage was followed by thrusting that juxtaposed higher-grade rocks of the in ternal zone (Westasturian-Leonese Zone) over lower-grade rocks of the exter nal one (Cantabrian Zone), giving rise to an inverted metamorphic pattern. Subsequently, rocks in the southern part of the area were affected by conta ct metamorphism associated with emplacement of granitoid bodies. This late- Variscan thermal event led to local enhancement of CAI values and to change s in the clay mineralogy by metasomatism.