Variscan veins: record of fluid circulation and Variscan tectonothermal events in Upper Palaeozoic limestones of the Moravian Karst, Czech Republic

Abstract

Numerous Variscan syntectonic calcite veins cross-cut Palaeozoic rocks in the Moravian Karst. A structural, petrographic and stable isotopic analysis of the calcite veins and a microthermometric study of fluid inclusions in these vein cements have been carried out to determine the origin of the Variscan fluids and their migration during burial and deformation. The isotopic parameters of white (older, more deformed) and rose (younger) calcites are: 87 Sr/ 86 Sr is between 0.7078 and 0.7082 (white) and 0.7086 (rose), δ 18 O is between +17.7 and +26.1 (white) and between +14.8 and +20.7 ‰ SMOW (rose), δ 13 C ranges from +0.1 to +2.5 (white) and from −0.3 to +1.6 ‰ V-PDB (rose). The isotopic signatures point to precipitation in an older fluid system buffered by the host rock (white calcites) and to an open, younger fluid-dominated system (rose calcites). Parent fluids (H 2 O-NaCl system) had salinities between 0.35 and 17.25 eq. wt % NaCl. The pressure-corrected and confined homogenization temperatures suggest formation of the calcite veins from a fluid with a temperature between 120 and 170 • C, a pressure of 300-880 bar at a depth between 2.1 and 3.2 km. The fluids were most likely confined to a particular sedimentary bed as a bed-scale fluid migration (white older calcite veins) or, later, to a pile of Palaeozoic sediments as a stratigraphically restricted fluid flow (rose younger calcite veins). The low temperatures and pressures during precipitation of calcites, which took place close to a peak of burial/deformation, confirm the distal position of the Moravian Karst region within the Variscan orogen.

Plumbotectonic aspects of polymetallic vein mineralization in Paleozoic sediments and Proterozoic basement of Moravia (Czech Republic)

“…This mechanism for Variscan veins has been recently proved as a smallscale (stratigraphicaly confined) fluid migration system active in the Moravian region (Slobodník et al 2006). On the other hand, coincidence of trends of the widespread post-Variscan veins and regional faults strongly suggest a large-scale fluid flow relating to a younger hydrothermal system.…”

Fault-controlled dolomitization in the Montagna dei Fiori Anticline (Central Apennines, Italy): record of a dominantly pre-orogenic fluid migration

“…Precipitation of D5 in microveins and cements in breccia zones, followed by late-stage calcite cementation in the Montagna dei Fiori Fault damage zone (C2, C3, and C4). migration, the excess of pore pressure at the base of the thrust ramp in the fold hinge and during fold tightening could promote the localization of the fractures (Smith and Wiltschko, 1996;Ghisetti and Vezzani, 2000), with fluid migration within this zone and eventually dolomitization. These fractures could have been corridors that later on formed the incipient NW-SE Montagna dei Fiori Fault.…”

Diagenesis to very low-grade metamorphism in lower Palaeozoic sediments: A case study from deep borehole Tobolka 1, the Barrandian Basin, Czech Republic

“…Comparable veins were also employed to constrain p-T conditions during Variscan orogenic deformations in other parts of the European Variscan orogen (e.g. Berwouts et al, 2008;Marsala et al, 2013;Slobodník et al, 2006).…”