Anhydrite-bearing rocks from the Rožná district (Moldanubian zone, Czech Republic): high-grade metamorphosed exhalites?

Abstract

Several types of anhydrite-bearing rocks have been found in the amphibolite-facies metamorphosed rocks at the north-eastern margin of the Moldanubian Zone. Anhydrite either forms monomineralic bands up to 40 cm thick, or occurs in the form of disseminated grains in surrounding calc-silicate gneiss together with feldspar, scapolite, amphibole, pyroxene, epidote and pyrite. The isotopic composition of sulphur (δ34S=30.6 to 32.3‰) and strontium (87Sr/86Sr=0.70797 to 0.70781) in anhydrite may indicate a marine source of sulphate. The isotopic ratio of strontium is in the same range as that of metamorphosed strata-bound barite–sulphide ores, which have been previously described in the same area. The δ34S values of coexisting pyrite range from 21.4 to 22.5‰, the Δ34Sanhydrite–pyrite corresponding to the metamorphic temperature of 600 to 660 °C. In contrast to many submarine-exhalative deposits, the oxygen isotopic compositions of anhydrite (δ18O=9.3 to 10.2‰) are lighter than that of barite (δ18O=10.4 to 13.8‰). This indicates that the both minerals are not in isotopic equilibrium. Therefore, it is probable that anhydrite and barite from the Rožna district were deposited from fluids that contained different proportions of seawater and hydrothermal fluids or from hydrothermal fluids that underwent variable extent of oxygen isotope exchange with seafloor rocks. The δ13C values in calcite (δ13C=–17.2 to –18.7‰) from anhydrite-bearing rock are lower than those in distant marbles. As graphite is absent in anhydrite- and calcite-bearing rocks, impoverishment in the 13C isotope cannot be attributed to the graphite–carbonate isotopic exchange during metamorphism. It is proposed that low δ13C values in carbonates are caused by pre-metamorphic oxidation of organic matter in course of hydrothermal processes. Anhydrite and anhydrite-bearing calc-silicate gneiss from the north-eastern part of the Moldanubian Zone are interpreted to be the high-grade metamorphosed analogue of anhydrite-rich exhalites commonly found in submarine-exhalative hydrothermal deposits.