Conodont thermal alteration patterns in Palaeozoic rocks from Belgium, northern France and western Germany

Abstract

AbstractIn this paper thermal patterns based on conodont Colour Alteration Indices (CAI) have been studied in the Devonian and Carboniferous rocks from Belgium, northern France and western Germany. Four maps (with the exception of the southeastern Rheinisches Schiefergebirge) have been compiled for the investigated area: one for the Eifelian and Givetian, a second for the Frasnian, another for the Famennian and one for the Tournaisian and Viséan. Conodonts have not been found in Cambrian rocks from Belgium and northern France and as they are scarce in the Ordovician to Lower Devonian formations of this area, it was impossible to integrate these CAI data into an isograd map. In Germany (in the eastern Rheinisches Schiefergebirge) only a few CAI values for the Lower Devonian strata are available. In contrast to the Ardennes where in general conodonts show CAI values of 3.0–5.0, indices in areas adjacent to the Anglo-Brabant Massif, e.g. in the western Namur Synclinorium and in the Eifel Hills, range between 1.5 and 2.0. Slightly higher values occur in the Campine Basin north of the Anglo-Brabant Massif. By comparison with CAI data from Belgium, conodont colour alteration indices from the same stratigraphic horizons in some areas of the northern and eastern Rheinisches Schiefergebirge are up to one index higher. In most areas of the Rheinisches Schiefergebirge the level of metamorphism corresponds to the anchizone. Regional variations in CAI data appear to be related to buried intrusions and associated hydrothermal activity. In the southeastern Rheinisches Schiefergebirge CAI data are not completely consistent but it is possible to discern an increase in coalification from the north to the south. High index values in the southern Rheinisches Schiefergebirge are believed to result from regional metamorphism and heating related to numerous intrusive bodies.