Biotitická plagioklasová rula z Koutů nad Desnou a Rejhotic (vrbenská skupina v zóně Červenohorského sedla): nové mineralogické údaje

The paper deals with metamorphic rocks of the Vrbno Group in the Červenohorské sedlo Zone near the Kouty nad Desnou village (north-eastern part of the Bohemian Massif). The studied rocks are named simply as biotite plagioclase gneiss in geological maps. Research has shown that most of the rocks in the area correspond to the graphite-bearing staurolite-biotite plagioclase gneiss. This gneiss merges into mica schist with the same mineral association but with a higher content of biotite. Plagioclase is a major and sometimes even dominant constituent of the rocks. Its composition varies from An39 to An84. Accessory minerals include ilmenite, apatite-(CaF), less often zircon, tourmaline (dravite), garnet (almandine), pyrite, and chalcopyrite. In the mentioned rocks, thin graphite-bearing chlorite plagioclase gneiss intercalations were found. Plagioclase has an extremely high basicity (An88-91). It is obvious from the chemical and mineral composition of the studied rocks that their protolith had to be rich in iron, aluminum and calcium. Clay rocks can be assumed as protolith. The origin of clay sediments or clay residue was possibly linked to the weathering of basic volcanism products