Origin of Dinantian zebra dolomites south of the Brabant‐Wales Massif, Belgium

Abstract

Zebra dolomites, characterized by a repetition of dark grey (a) and light (b) coloured dolomite sheets building up abbabba‐sequences, occur in Dinantian strata from deep boreholes (> 2000 m) south of the Brabant‐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 μm in diameter. These crystals display a mottled red–orange luminescence and are interpreted to be replacive in origin. The white dolomite sheets (b) consist of coarse crystalline nonplanar b1 dolomite, which evolves outwards into transparent saddle shaped b2 dolomite. The b1 dolomites possess a mottled red–orange luminescence similar to the a dolomites, while the saddle shaped b2 rims display red to dark brown luminescent‐zones. The b1 dolomites are possibly partly replacive and partly cavity filling. Their b2 rims display criteria typical for a cement origin. Locally, cavities exist between two succeeding white dolomite sheets. These cavities make up ≈5% of the zebra rocks and are locally filled by saddle shaped ankerite and/or xenomorphic ferroan calcite. Geochemical and fluid inclusion data (Th ≈ 120 °C) indicate a burial diagenetic origin for these zebra dolomites. The a and b1 dolomites are characterized by similar geochemical compositions and fluid inclusion data pointing toward a related origin. To explain the development of the zebra textures, suprahydrostatic pressures in conjunction with late Variscan tectonic compression are invoked. A model involving dolomitizing fluids expelled during the Variscan orogeny is proposed. An overpressured system is also invoked to explain the important porosity development, the creation of centimetre‐scale subvertical displacements of the zebra pattern and the microfractures affecting the b1–b2 dolomite crystals.

The Ranero Hydrothermal Dolomites (Albian, Karrantza Valley, Northwest Spain): Implications on Conceptual Dolomite Models

“…Besides, some zebra dolomite clasts look very similar to fragments of corals and may have been formed by mimic replacement of the latters (cf. Nielsen et al, 1998).…”

Zebra pattern in rocks as a function of grain growth affected by second-phase particles

“…The stable grain size achieved in the light and dark layers is in good agreement with the natural samples (Figure 9). The grain sizes are found to vary between 80 and 150 µm in the dark and 0.5-5 mm in the light layers [12]. The crystals in the simulations have final grain sizes ranging from ≈ 200 µm in the impurity rich to 2 mm in the impurity free layers.…”

“…It is widely accepted that the formation of the structure consists of several phases of which the dark fine grained dolomite is the first one. This first phase represents the replacement of the initial limestone by dolomite [5][6][7][8][9][10][11][12][13][14][15]. What exactly triggers the development of the subsequent phases, which build up the coarse grained regions and FIGURE 1 | Examples of zebra textures at different scales.…”

“…During the replacement of the limestone by dolomite the fluid flow was focused in these high permeability zones leading to higher dissolution and dolomitization rates and the formation of layers. Studies carried out on zebra dolomites in Belgium, Spain and Canada concluded that the pattern evolved during fracturing due to tectonic stress [8,9] or hydrofracturing during the pulsed influx of dolomitizing brines [12,13,15]. The light coarse grained dolomite bands are interpreted as being at least partly void filling cements which precipitated into extensional fractures [12].…”

“…Studies carried out on zebra dolomites in Belgium, Spain and Canada concluded that the pattern evolved during fracturing due to tectonic stress [8,9] or hydrofracturing during the pulsed influx of dolomitizing brines [12,13,15]. The light coarse grained dolomite bands are interpreted as being at least partly void filling cements which precipitated into extensional fractures [12]. Based on the high symmetry of the banding it has been stated that the process which leads to the formation of the zebra texture has to involve self-organization [7,10,11].…”