Pressure-temperature estimates of the blueschists from the Kopina Mt., northern Bohemian Massif, Poland - constraints on subduction of the Saxothuringian continental margin

The blueschist occurrence at the Kopina Mt. is situated at the eastern margin of the Karkonosze Izera Massif in the West Sudetes of the northern Bohemian Massif. Metabasic rocks with continental crust affinity occur as lenses within the prevailing metasediments. The metabasites consist mainly of garnet, blue amphibole, epidote, chlorite-I, titanite, hematite and quartz forming the high-pressure assemblage. Synkinematic garnet exhibits prograde zoning, a feature allowing for assessment of the prograde course of the pressure temperature (P-T) path. Phase equilibrium modelling has been used to estimate prograde and peak metamorphic conditions. Based on the P-T pseudosections, calculated in the system Na2O-CaO-K2O-FeO-MgO-MnO- Al2O3-SiO2-H2O-TiO2-O-2 (NCKFMMnASHTO), the garnet + glaucophane + epidote + chlorite + white mica(?) + Fe oxide + quartz titanite assemblage was formed between 12-15 kbar and 480-520 degrees C. These results are based on garnet and Na-amphibole compositional isopleths. Garnet shows a prograde zoning expressed by the rapid rim-ward decrease of spessartine, moderate increase of almandine and as light increase of other components. It is inferred here that white mica must have decomposed to secondary albite chlorite-II K-feldspar. The obtained results point to the formation of the Kopina blueschists along a low-temperature P-T gradient of 8-10 degrees C/km, typical of rocks from a subduction exhumation channel. We postulate that the Kopina blueschists were formed in a subduction system developed along the eastern termination of the Saxothuringian Ocean and operating beneath the Tepla Barrandian upper plate. When combined with previous data on continental crust affinity for the protolith, our results demonstrate a derivation of the Kopina blueschists from subducted part of the Saxothuringian margin.