Transition from diagenesis to metamorphism in a calcareous tectonic unit of the Iberian Variscan belt (central massif of the Picos de Europa, NW Spain)

Abstract

Transition from diagenesis to metamorphism has been characterized in the central part of the Picos de Europa unit (Cantabrian Zone) by conodont colour alteration index (CAI), complemented with Kübler index (KI) data. This unit essentially comprises Carboniferous limestone. The study leads us to deduce two successive tectonothermal events. The first event generated a pattern with palaeotemperatures increasing towards the Pisuerga–Carrión unit, located south of the Picos de Europa unit. The present pattern is the result of an original dip to the north of the isothermal surfaces and the subsequent rising of the Picos de Europa unit along the frontal ramp of a deep Alpine thrust. This episode is interpreted as related to an extensional tectonic regime that occurred close to the Carboniferous–Permian boundary. The second event, which gave rise to thermal anomalies in the pattern of the first episode, was the result of hydrothermal processes in which fluid movement was facilitated by a dense network of fractures in the area close to the eastern section of the studied region. These processes gave rise to numerous mineralizations and have been related to a Permian extensional tectonic regime.