Evolution of chemical composition of tourmaline from the peraluminous orthogneisses from Olešenka, NE Moldanubian unitArticle PDF
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 (An7–9) 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 (Alm75–80 Sps12–19 Prp5–6 Adr1–3), 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).
David Buriánek, Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic, e-mail: firstname.lastname@example.org
Stanislav Houzar, Department of Mineralogy and Petrography, Moravian Museum, Zelný trh 6, 674 01 Brno, Czech Republic
Lenka Buřivalová, Department of Geological Sciences, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic