Indicadores de metamorfismo de bajo grado en el carbonífero de la Cuenca Central Asturiana

The Central Coal Basin (Cantabrian Zone) comprises mostly Westphalian sequences which developed in the external zone of the Variscan Orogen Belt. Sedimentation is thought to have occurred in a foreland basin in front of the developing mountain chain. Isovol maps show the highest volatile contents in the shouthern part o f the area. The isovols cut stratigraphic boundaries and structures, indicating that heating is a late event. Clay assemblages are dominated by illite with smaller amounts o f kaolinite, chlorite, pyrophyllite, paragonite and mixed-layers illite/smectite and paragonite/muscovite, across the entire coalfield. Illite crystallinity (1C) values in the Carboniferous sequence are in the range 0.635-0.257°20 for the air-dried mounts (77% of the values < 0.42°20), and within the range 0.539-0.251 °20 for the glycolated mounts (97% of the values < 0.42°2Q). Most of the values are therefore within the anchizone. These data are incompatible with previous correlations of coal rank/volatile matter and illite crystallinity. Discrepant 1C values are interpreted as a feature inherited from the source area.