Transition from Diagenesis to Metamorphism Near the Front of the Variscan Regional Metamorphism (Cantabrian Zone, Northwestern Spain)

Abstract

Distribution of paleotemperatures has been studied in Paleozoic rocks of the Variscan fold and thrust belt of the NW Cantabrian Zone (Spain) by a combined 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 increasing CAI values. Moreover, the CAI isogrades roughly reflect the structural traits of the area, indicating that maximum temperatures were attained during sedimentary burial previous to folding. The geothermal paleogradient obtained suggests conditions near the boundary between low and middle P/T conditions. There is evidence of at least two thermal events during the tectonothermal development of the belt. The main stage is interpreted as the result of the burial of the rocks characterized by late diagenesis conditions that occurred under a geothermal gradient of ∼35°C km−1. The narrow zone with low anchizonal conditions in the western part of the area could represent the front of the Variscan orogenic metamorphism. This stage was followed by thrusting that juxtaposed higher‐grade rocks of the internal zone (Westasturian‐Leonese Zone) over lower‐grade rocks of the external one (Cantabrian Zone), giving rise to an inverted metamorphic pattern. Subsequently, rocks in the southern part of the area were affected by contact metamorphism associated with emplacement of granitoid bodies. This late‐Variscan thermal event led to local enhancement of CAI values and to changes in the clay mineralogy by metasomatism.