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

The Rhenohercynian zone of the central European Variscides contains in the central Ardenne/Eifel/Brabant/Campine region of Belgium, France, Germany, a nd Luxemburg an omnipresent thermal overprint, which reaches in places uppe rmost epizonal conditions. Because of its distribution and structural relat ionships this metamorphism cannot be explained by a singular tectonometamor phic event. Field and microstructural observations, new illite crystallinit y and other data relating to this metamorphism are presented. Together with a review of existing petrological, illite crystallinity, vitrinite reflect ance, conodont alteration index, and fluid inclusion data, and their relati onship to small- and regional-scale geological structures three distinct ge otectonic 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 a nd, with minor evidence, of the Stavelot Massif further south. It is clearl y 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 re aches maximal values of 500 degrees C and 400 MPa in the Serpent 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 roc ks. The metamorphism is associated with the pre-Carboniferous preorogenic r ifting stage of the Rhenohercynian basin and documents the peak of subsiden ce 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 m etamorphism is overprinted in the central part of the Ardenne/Eifel area (M iddle Devonian to Carboniferous rocks of the Dinant Synclinorium) by an anc hizonal/beginning epizonal metamorphism. In the foreland basins north of th e Midi Fault Zone it reaches diagenetic to lowermost anchizonal conditions. It is the expression of the burial by thick synorogenic mid-to Upper Carbo niferous cover rocks, which were deposited in piggyback and foreland basins . This elastic wedge relates to the Carboniferous contraction of the Rhenoh ercynian basin, whose metamorphosed rocks were themselves affected by the d eformation of the advancing orogenic front. (C) 1999 Elsevier Science B.V. All rights reserved.