The Kutná Hora Complex (Moldanubian zone, Bohemian Massif): A composite of crustal and mantle rocks subducted to HP/UHP conditions

Abstract

Two tectonic lenses of garnet peridotites and one of eclogite, all occurring in high-pressure granulites from the Kutná Hora Complex, were investigated. These rocks are thought to be correlative with the high-grade metamorphic Gföhl unit of the Moldanubian zone. The garnet peridotites contain small bodies of eclogites and thin layers of garnet clinopyroxenite. Also present are two varieties of eclogites (kyanite-bearing and kyanite-rfree) and one variety of garnetite, which form blocks and boudins in the garnet peridotites. The eclogite occurring in granulite without garnet peridotite belongs to the kyanite-bearing variety. Textural relations and mineral compositional zoning of garnet in peridotites and in eclogites were used to constrain PT evolution of these rocks and to determine whether they passed a PT path similar to that in the host peridotite or represent fragments with different metamorphic histories. The results of thermobarometric calculations show that the peridotites were cooled during an increase of pressure at 4.5/950 °C, and the pressure and temperature obtained for garnet clinopyroxenite are also close to these PT values. The eclogites seem to be derived from different rock compositions and to have passed different thermal evolutions prior to their emplacement into garnet peridotites. Temperatures calculated for the eclogites hosted in peridotites range between 850 and 1000 °C, but pressure obtained for kyanite-bearing variety is about 3.4 GPa at 960 °C. The eclogite present in granulite contains prograde zoned garnet and gives temperature of 900 °C at 3.4 GPa. All these rocks seem to represent tectonic fragments that were subducted to different depths and tectonically assembled during their exhumation.