Multiple‐step recrystallization within massive ancient dolomite units: an example from the Dinantian of Belgium

Abstract

ABSTRACT South of the Caledonian Brabant‐Wales Massif a more than 200 m thick Tournaisian to Lower Visean replacive dolomite unit can be followed for several hundred kilometres from the Boulonnais (France) to Aachen (Germany). Field observations, of features such as karst cavities occurring at the top of the Lower Visean dolomite which are filled by Lower Visean crinoidal limestone, indicate that dolomitization and karstification took place during the Early Visean. This early development of the dolomite is in agreement with the presence of stylolites cutting the dolomite fabric. The minor element composition of the majority of the 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 δ 13 C values (range‐0.72 to +5.31% o ) mainly reflect the carbon isotopic composition of the precursor limestones. The δ 18 O values, in contrast, are highly variable: ranging from‐19.15 to +0.85% o . This rather large range of δ 18 O values is explained by multiple‐step re‐equilibration/recrystallization during progressive burial and subsequent uplift of the dolomites. These processes are also responsible for the high 87 Sr/ 86 Sr values of the dolomites which range from about 0.7088 to 0.7098. They are distinctly more radiogenic than Lower Visean marine carbonates (0.7076–0.7078). Correlation, however, of δ 18 O values or 87 Sr/ 86 Sr ratios with dolomite and/or cathodoluminescenec (CL) textures has not been very successful. This suggests that recrystallization may remain unrecognized if only petrographic techniques are used. Nevertheless, certain CL textures can be related to specific interactions with the ambient recrystallizing fluids.