Sr-sulphate and associated minerals found from kyanite-bearing eclogite in the Moldanubian Zone of the Bohemian Massif, Czech Republic

Abstract

Sr-sulphate, celestine, was newly found in a kyanite-bearing eclogite associated with the Nove Dvory peridotite mass in the Moldanubian Zone of the Bohemian Massif, Czech Republic. Celestine is closely associated with anhydrite, pyrite, pyrrhotite and chalcopyrite, and those minerals occur in the matrix, where fine-grained omphacite aggregate and kelyphites after garnet develop. A common Sr reservoir in eclogite is known to be epidote, but the maximum pressure-temperature (P-T) conditions of the studied eclogite was estimated as about 1050-1150 °C, 4.5-4.9 GPa. In such extremely high P-T conditions, epidote should be unstable. In fact, epidote is absent from most of eclogites in the Moldanubian Zone of Czech Republic. These facts suggest that a Sr-reservoir after the epidote-breakdown in subducting eclogite might be celestine, or its high-P polymorph, although the formation stage of celestine in the study sample and maximum stability limits of celestine are not known. Another idea is that celestine in the study sample was formed at relatively shallow levels after the ascent of the eclogite. In this case, omphacite and apatite would have contained significant amount of Sr under the eclogite-facies conditions. Otherwise, metasomatic infiltration of Sr-rich fluids into the eclogite is necessary for the formation of celestine in order to provide sufficient amount of Sr. A Ba-rich alumino-silicate, probably celsian, was also found as a retrograde product around pyrrhotite in kelyphite and in symplectites mainly composed of augite and plagioclase after omphacite. Several grains of biotite are also present along the margin of garnet and in the symplectites after omphacite. In addition, small amounts of muscovite and amphibole are present in the symplectites after omphacite. These findings suggest that metasomatism with Sr- and Ba-rich fluids may have occurred during decompression of the eclogite and may not be indicative of the UHP history of these rocks.