Pre- and synorogenic burial metamorphism in the Ardenne and neighbouring areas (Rhenohercynian zone, central European Variscides)

• Published in: Tectonophysics 1999-08, Vol.309 (1), p.227-256
• Main Authors: Fielitz, Werner, Mansy, Jean-Louis
• Format: Article
• Language: English
• Subjects: Ardenne; Brabant Massif; diastathermal metamorphism; Eifel; palaeofluids; preorogenic metamorphism; synorogenic metamorphism
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/S0040-1951(99)00141-9

The Rhenohercynian zone of the central European Variscides contains in the central Ardenne/Eifel/Brabant/Campine region of Belgium, France, Germany, and Luxemburg an omnipresent thermal overprint, which reaches in places uppermost epizonal conditions. Because of its distribution and structural relationships this metamorphism cannot be explained by a singular tectonometamorphic event. Field and microstructural observations, new illite crystallinity and other data relating to this metamorphism are presented. Together with a review of existing petrological, illite crystallinity, vitrinite reflectance, conodont alteration index, and fluid inclusion data, and their relationship to small- and regional-scale geological structures three distinct geotectonic situations can be described, which overlap and alternate in space and time. (1) The oldest metamorphism, which reached epizonal conditions, can be localized in the pre-Devonian basement rocks of the Brabant Massif and, with minor evidence, of the Stavelot Massif further south. It is clearly related to a pre-Variscan (Caledonian) deformation event and could be of burial origin. (2) The main regional metamorphism affected all Devonian and Carboniferous rocks and the lower Palaeozoic basement highs of the Ardenne/Eifel area. It does not exceed diagenetic conditions in some areas, but reaches maximal values of 500°C and 400 MPa in the Serpont and Rocroi Massif areas ( Theye and Fransolet, 1993; Robion et al., 1995). It is pre- to synkinematic to the main Variscan contractional penetrative deformation and correlates well with the age and thickness of the sedimentary cover rocks. The metamorphism is associated with the pre-Carboniferous preorogenic rifting stage of the Rhenohercynian basin and documents the peak of subsidence and sediment accumulation. We interpret it as diastathermal metamorphism. It reaches its maximum in the low-grade metamorphic zones of the Ardenne area, which are inverted synsedimentary structures. (3) The diastathermal metamorphism is overprinted in the central part of the Ardenne/Eifel area (Middle Devonian to Carboniferous rocks of the Dinant Synclinorium) by an anchizonal/beginning epizonal metamorphism. In the foreland basins north of the Midi Fault Zone it reaches diagenetic to lowermost anchizonal conditions. It is the expression of the burial by thick synorogenic mid- to Upper Carboniferous cover rocks, which were deposited in piggyback and foreland basins. This clastic wedge relates to