Hot dolomites in a Variscan foreland belt: Hydrothermal flow in the Cantabrian Zone (NW Spain)

The Cantabrian Zone (CZ) in NW Spain represents the foreland belt of the Variscan Iberian Massif. It consists of a Precambrian basement covered by Palaeozoic sediments. These underwent intense thin-skinned tectonics, diagenetic to epizonal thermal events, and several episodes of fluid flow causing large-scale hydrothermal dolomitization. Aim of this research is to trace the carbonate diagenesis in the Carboniferous Barcaliente and Valdeteja Formations in the Bodón Unit, and to define type and origin of the dolomitizing fluids. Employed methods include petrography, cathodoluminescence (CL), XRD, stable isotopes and fluid inclusion (FI) microthermometry/Raman spectrometry. The dolomitizing fluid was possibly hot (100 to 150°C), saline, Mg-rich modified seawater, operating in a burial environment. It is assumed that the dolomitization occurred during late- to post-Variscan extensional phases. Main pathways for the fluids were the Variscan thrust and fault planes, as well as stratification/lamination joints of the host limestones. One of the main tectonic lineaments, the Leon Line, played an effective role for fluid circulation, as reflected by the highest temperatures and often almost complete dolomitization close to this fault. Extensional tectonics may have promoted a gravity driven flow of fluids, which circulated deeply down, underwent heating and depletion in 18O and dolomitized the primary carbonates.