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Dumortierite associated with andalusite, dravite and chlorite were found in quartz veins at Holý vrch Hill near Psáry (central Bohemia, Czech Republic), in rocks belonging to Jílové metavolcanic belt. Two macroscopically distinct varieties of dumortierite were found: dark-violet aggregates of long prismatic crystals in dark tuffitic schists and pale violet to pink aggregates of fine-grained and fibrous dumortierite in rhyolitic metatuffs. Both dumortierite varieties are chemically and structurally very similar, with high contents of Mg (0.15 - 0.2 apfu) along with low amounts of Ti (0.04 - 0.06 apfu) and Fe (0.02 - 0.03 apfu). Rare inclusions of Mg,Ti-rich dumortierite (0.17 - 0.18 apfu Mg, 0.14 apfu Ti) were observed in thick dumortierite prisms. The veins originated by reaction of B-rich fluids derived from granites of the Central Bohemian Plutonic Complex with Al-rich metavolcanic rocks. The Psáry veins are unusual in the Bohemian Massif where dumortierite typically occurs in granitic pegmatites and related rocks. [ABSTRACT FROM AUTHOR]

In the Orlica-Śnieżnik Dome (NE Bohemian massif), alternating belts of orthogneiss with high-pressure rocks and belts of mid-crustal metasedimentary-metavolcanic rocks commonly display a dominant subvertical fabric deformed into a subhorizontal foliation. The first macroscopic foliation is subvertical, strikes NE-SW and is heterogeneously folded by open to isoclinal folds with subhorizontal axial planes parallel to the heterogeneously developed flat-lying foliation. The metamorphic evolution of the mid-crustal metasedimentary rocks involved successive crystallization of chlorite-muscovite-ilmenite-plagioclase-garnet, followed by staurolite-bearing and then kyanite-bearing assemblages in the subvertical fabric. This was followed by garnet retrogression, with syntectonic crystallization of sillimanite and andalusite parallel to the shallow-dipping foliation. Elsewhere, andalusite and cordierite statically overgrew the flat-lying fabric. With reference to a P-T pseudosection for a representative sample, the prograde succession of mineral assemblages and the garnet zoning pattern with decreasing grossular, spessartine and X are compatible with a P- T path from 3.5-5 kbar/490-520 °C to peak conditions of 6-7 kbar/∼630 °C suggesting burial from 12 to 25 km with increasing temperature. Using the same pseudosection, the retrograde succession of minerals shows decompression to sillimanite stability at ∼4 kbar/∼630 °C and to andalusite-cordierite stability at 2-3 kbar indicating exhumation from 25 km to around 9-12 km. Subsequent exhumation to ∼6 km occurred without apparent formation of a deformation fabric. The structure and petrology together with the spatial distribution of the metasedimentary-metavolcanic rocks, and gneissic and high-pressure belts are compatible with a model of burial of limited parts of the upper and middle crust in narrow cusp-like synclines, synchronous with the exhumation of orogenic lower crust represented by the gneissic and high-pressure rocks in lobe-shaped and volumetrically more important anticlines. Converging P- T- D paths for the metasedimentary rocks and the adjacent high-pressure rocks are due to vertical exchanges between cold and hot vertically moving masses. Finally, the retrograde shallow-dipping fabric affects both the metasedimentary-metavolcanic rocks and the gneissic and high-pressure rocks, and indicates that the ∼15-km exhumation was mostly accommodated by heterogeneous ductile thinning associated with unroofing of a buoyant crustal root. [ABSTRACT FROM AUTHOR]