Litho-stratigraphic effect on Variscan fluid flow within the Prague synform, Barrandian: Evidence based on C, O, Sr isotopes and fluid inclusions

The Palaeozoic sedimentary sequence of the Prague synform (Ordovician–Devonian) in the centre of the Bohemian massif underwent Variscan deformation and thermal overprint events. Variscan veins widespread throughout the sedimentary strata have precipitated from syntectonic aqueous and hydrocarbon-rich fluids. Homogenization temperatures of aqueous inclusions increased from 70 up to 226 °C in the Cambrian rocks underlying the Prague synform. Seawater, modified due to intensive water–rock interaction, was the main fluid component. Fluid flow was limited and restricted to the lithostratigraphic compartments forming a rock-buffered system. Stable isotopic modelling (C, O) and final interpretations of the confined hydrostratigraphic fluid migration was supported by the 87Sr/86Sr ratio in veins and wall rocks. Siliciclastic Cambrian and Ordovician rocks and the associated intersecting veins yielded similar isotopic signatures, and consequently the fluid migration is restricted to layer- and formation-scales. Gradually downwards increasing fluid temperature and compositional changes reflect burial at oil-window conditions. An open fluid system could be expected in proximity of major faults of the Prague syncline and at the top of the sedimentary sequence.

► Variscan fluids determinated are the aqueous- and hydrocarbon-rich.
► Variscan vein minerals crystallized in a closed, rock-buffered fluid system.
► An open fluid-buffered system was defined for fault-related veins.
► Vertical fluid pathways in the Prague syncline were restricted or entirely missing.
► Fluid migration was confined to litho-stratigraphic units (layer-scale migration).