High-pressure low-temperature metamorphism recorded in mica schists from the central part of the Fore-Sudetic Block (NE margin of Bohemian Massif, SW Poland)

The Lower Silesian Block outcrops in the NE part of the Bohemian Massif with its northeasternmost fragment, the Fore-Sudetic Block, buried under the Cenozoic sediments. The AMINV K-1 borehole drilled in 2013 and located in this area provides a unique insight into geology of the scarcely exposed part of Variscan crystalline basement. The borehole profile exhibits the metamorphosed volcano-sedimentary sequence composed mainly of quartz-rich schists, chlorite-schists and mica schists with garnet-rich layers covered with 100 m of Paleogene sediments. 

In this study, we have focused on the metamorphic record of garnet-bearing mica schists. Petrological investigation conducted with use of electron microscopy (SEM, EMPA) reveals the following, interesting features recurring in many studied samples: 1) presence of chloritoid inclusions in garnet; 2) progressive zoning of garnet; 3) diverse composition of white mica ranging from phengite to muscovite. Thermodynamic modeling shows that mineral parageneses including i.e. chloritoid, garnet and phengite crystallized in the conditions corresponding to high-pressure low-temperature (HP-LT) metamorphism, followed by the stage of regional metamorphism, marked by the growth of i.e. muscovite, feldspar and biotite. P-T conditions of HP-LT stage may have reached up to 17 kbar and 550^oC, while subsequent regional metamorphism most probably haven’t exceeded  10 kbar and 650^oC. QuiG Raman elastobarometry and Zr-in-rutile thermometers has been used to evaluate the modeled P-T conditions. EMPA U-Pb monazite dating determined the average age of crystallization at 339±12 Ma based on 45 point analyses of 18 monazite grains.