MULTIPLE-STEP RECRYSTALLIZATION WITHIN MASSIVE ANCIENT DOLOMITE UNITS- AN EXAMPLE FROM THE DINANTIAN OF BELGIUM

South of the Caledonian Brabant - Wales Massif a more than 200 m thick Tournaisian to Lower Visean replacive dolomite unit can be followed f or several hundred kilometres from the Boulonnais (France) to Aachen ( Germany). Field observations, of features such as karst cavities occur ring at the top of the Lower Visean dolomite which are filled by Lower Visean crinoidal limestone, indicate that dolomitization and karstifi cation took place during the Early Visean. This early development of t he dolomite is in agreement with the presence of stylolites cutting th e dolomite fabric. The minor element composition of the majority of th e dolomites remains almost uniform throughout the entire studied area. Values for Fe, Mn, Na and Sr are normally in the range 700-4700 ppm, 15-400 ppm, 80-300 ppm and 50-200 ppm, respectively. The deltaC-13 Val ues (range - 0.72 to + 5-3 parts per thousand) mainly reflect the carb on isotopic composition of the precursor limestones. The deltaO-18 val ues, in contrast, are highly variable: ranging from - 19.15 to + 0.85 parts per thousand. This rather large range of deltaO-18 values is exp lained by multiple-step re-equilibration/recrystallization during prog ressive burial and subsequent uplift of the dolomites. These processes are also responsible for the high Sr-87/Sr-86 values of the dolomites which range from about 0.7088 to 0.7098. They are distinctly more rad iogenic than Lower Visean marine carbonates (0.7076-0.7078). Correlati on, however, of deltaO-18 Values or Sr-87/Sr-86 ratios with dolomite a nd/or cathodoluminescence (CL) textures has not been very successful. This suggests that recrystallization may remain unrecognized if only p etrographic techniques are used. Nevertheless, certain CL textures can be related to specific interactions with the ambient recrystallizing fluids.