ORIGIN OF DINANTIAN ZEBRA DOLOMITES SOUTH OF THE BRABANT-WALES MASSIF, BELGIUM

Zebra dolomites, characterized by a repetition of dark grey (a) and li ght (b) coloured dolomite sheets building up abbabba-sequences, occur in Dinantian strata from deep boreholes (> 2000 m) south of the Braban t-Wales Massif in Belgium. These zebra dolomite sequences are several tens of metres thick. The dark grey dolomite sheets (a) consist of non -planar crystals, 80-150 mu m in diameter. These crystals display a mo ttled red-orange luminescence and are interpreted to be replacive in o rigin. The white dolomite sheets (b) consist of coarse crystalline non planar b(1) dolomite, which evolves outwards into transparent saddle s haped b(2) dolomite. The b(1) dolomites possess a mottled red-orange l uminescence similar to the a dolomites, while the saddle shaped bz rim s display red to dark brown luminescent-zones, The b(1) dolomites are possibly partly replacive and partly cavity filling. Their b2 rims dis play criteria typical for a cement origin. Locally, cavities exist bet ween two succeeding white dolomite sheets. These cavities make up appr oximate to 5% of the zebra rocks and are locally filled by saddle shap ed ankerite and/or xenomorphic ferroan calcite. Geochemical and fluid inclusion data (T-h approximate to 120 degrees C) indicate a burial di agenetic origin for these zebra dolomites. The a and b(1) dolomites ar e characterized by similar geochemical compositions and fluid inclusio n data pointing toward a related origin. To explain the development of the zebra textures, suprahydrostatic pressures in conjunction with la te Variscan tectonic compression are invoked. A model involving dolomi tizing fluids expelled during the Variscan orogeny is proposed. An ove rpressured system is also invoked to explain the important porosity de velopment, the creation of centimetre-scale subvertical displacements of the zebra pattern and the microfractures affecting the b(1)-b(2) do lomite crystals.