Acta Musei Moraviae

Evolution of chemical composition of tourmaline from the peraluminous orthogneisses from Olešenka, NE Moldanubian unit

Medium-grained porphyroclastic, garnet-biotite to biotite-muscovite orthogneiss with tourmaline from Olešenka consist of quartz, K-feldspar (P up to 0.02 apfu and Na up to 0.14 apfu), plagioclase (An_7–9) and small amount mafic minerals. Biotite (Al 3.52–3.76 apfu, Fe/(Fe+Mg) 0.72–0.74) predominates over muscovite. Porphyroblasts of garnet (Alm_75–80 Sps_12–19 Prp_5–6 Adr_1–3), subhedral tourmaline and aggregates of fibrous sillimanite are locally present. Formation of central part of tourmaline grains type I; ((Fe/(Fe+Mg) = 0.55–0.64; Na = 0.68–0.85 apfu) can be explained as the product of pre-Variscan crystallization of granitic melt with low degree of geochemical fractionation. Tourmaline type II ((Fe/(Fe+Mg) = 0.43–0.45; Na = 0.68–0.82 apfu) and type III ((Fe/(Fe+Mg) = 0.52–0.55; Na = 0.61–0.65 apfu) are associated with formation of sillimanite, Mn- and Ca-rich garnet rims in orthogneiss indicate Variscan metamorphic overprint (747 ± 45 °C and 0.69 ± 0.18 GPa).