Acta Musei Moraviae

Variscan syntectonic types of fluids generated from Palaeozoic sequences of the Moravian Karst and Drahany Upland

Variscan hydrothermal calcite±quartz, quartz±chlorite±calcite veins are widespread in Palaeozoic sedimentary sequences of the Moravian Karst and the Drahany Highland. Veins developed in two main lithological types, in limestones and siliciclastic rocks (greywackes, conglomerates, shales), and their hydrothermal minerals have been investigated using microthermometric methods. The revealed variability in fluid types trapped in inclusions reflects the lithology of host rocks and/or structural aspects. The H₂O–NaCl fluid system is responsible for precipitation of veins in limestones and coarse-grained siliciclastics (greywackes). Inclusions in vein minerals from the sequences with shales within shear zones contain CH₄±CO₂ and H₂O–NaCl±CH₄ fluids. Organic matter in rocks has been transformed and aqueous and organic fluids have been derived by deformation processes at elevated temperatures. The coexistence of those fluids enabled calculation of trapping P-T conditions. Revealed temperatures (T_h = 85–275 °C) point to regional trends as documented by reflectivity of organic matter reported before. Some local conditions, however, differ from the trend. Within a shear zone, highly variable temperatures (up to 283 °C) and pressures (180–1200, max. up to 2040 bar) may result from friction overheating and pressure fluctuations along thrusts planes.