Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Fracture-related calcites as indicators of palaeofluid circulation in granites of the Krkonoše-Jizera Pluton (Czech Republic)
Published online: 31 October 2017
AbstractOur study aimed to assess the late tectonothermal history of the Krkonoše-Jizera Pluton by means of the palaeohydrogeological interpretation of geochemical data (fluid inclusions, stable isotopes, trace elements, radiocarbon dating) of fracture-related calcites from the Jizera Granite in the Bedřichov water-supply tunnels in the Jizerské hory Mts. Apatite fission track analysis (AFTA) was used to characterize a cooling history of this granite.
The hydrothermal fracture fillings represent vertical or subvertical quartz-carbonate and carbonate veins, often with zoned or brecciated structure. The major vein minerals are quartz and calcite; less abundant are chlorite, hematite, dolomite, siderite, rhodochrosite, ankerite, fluorite and clay minerals.
The hydrothermal veins form a subvertical orthogonal network of: a) NW-SE trending veins, 0.5 to 15 cm wide, filled with quartz and calcite; b) NE-SW trending thin veins filled with fine-grained carbonates only. The former veins are more frequent than the later ones.
Homogenization temperatures of fluid inclusions in calcites of the NW-SE veins are in the range between 107-147 °C, and in quartz up to 184 °C. The inclusions in both minerals have variable salinity, from 0.4 to 29 wt. % NaCl equiv. The eutectic temperatures of -50 to -56 °C show a significant proportion of CaCl2 in the solution. Homogenization temperatures of fluid inclusions in calcites of the NE-SW veins are in the range from 90 to 142 °C , and in quartz up to 184 °C. All inclusions in both minerals have low salinity from 0.2 to 5.4 wt. % NaCl equiv. Na-K-Mg-Fe chlorides are involved on the composition of the solution.
The calculated oxygen isotopic composition of parental paleofluids shows that a part of the calcites formed from fluids with δ18O from +6 to +10 ‰ (V-SMOW); fluids affected by an interaction with magmatic/metamorphic rocks were likely to be their source solution. The other calcites have the calculated fluid δ18O slightly positive, +0.6 to +3.5 ‰ (V-SMOW). Brines circulating in the Jizera Granite at lower temperatures were apparently their source solutions. Calcites mainly from NE-SW veins have negative calculated δ18O values of their mother solutions down to -5 ‰ (V-SMOW). It probably matches the late meteoric waters circulating in cooled granite.
A considerable variability of the carbon isotopic composition (δ13C values) of calcite from all veins proves their multiple carbon sources. Some of them are likely to have carbon derived from a deep resource; the others are probably influenced by organic carbon.
The Sr contents in calcite are mostly higher than 100 ppm (57 up to 574 ppm) and indicate the hydrothermal origin of calcites.
14C dating revealed that the hydrothermal calcites from all the fractures have no measurable activity of radiocarbon. Therefore they are older than the detection limit of the method, which is 47 to 51 ka BP.
According to AFTA analysis the Jizera Granite was uplifted from zone of total annealing (≤120 °C) at approximately 80 Ma, then its temperature slowly decreased to 80 °C, subsequently stagnated, and finally about 10 Ma ago quickly dropped to the recent temperature.
Published online: 31 October 2017