Magmatic and metamorphic evolution of tourmaline-rich rocks of the Sopron area, Eastern Alps (W-Hungary)

Abstract

Tourmaline-rich pegmatitic orthogneisses, tourmalinites, kyanite-chlorite-muscovite schists and quartzites crosscut by subordinate quartz-tourmaline veins and layers were newly described from the Sopron area, Western Hungary. The orthogneisses mainly consist of quartz, plagioclase, tourmaline, garnet and white mica. In smaller amounts K-feldspar, beryl, Mg-rich chlorite, kyanite, lazulite, florencite, monazite and apatite also are present. Magmatic cores and two generations of metamorphic tourmaline (Fe-rich and Mg-rich) were distinguished. Tourmaline in tourmalinites is generally large (several cm), deformed, contains chlorite inclusions and shows oscillatory zoning or polygonal fabric. Large tourmaline crystals often contain dark brown mica-shaped relic areas with higher amount of Ti and Fe than the adjacent parts, interpreted as relics of micas from the protolith. Besides tourmaline, quartz, white mica, plagioclase, apatite, garnet, rutile, ilmenite, scheelite, zircon and monazite are also present in the tourmalinites. Deformed tourmaline-quartz bands and veins occur in kyanite-chlorite-muscovite schists and quartzites. Euhedral, zoned and deformed schorl-dravite is accompanied with kyanite, Mg-chlorite (leuchtenbergite), rutile muscovite and sillimanite. Narrow colorless tourmaline rims enriched exclusively in Mg (FeO < 1 wt. %) can be identified. Coarse-grained orthogneisses with a significant amount of primary tourmaline-beryl assemblage indicates a fluid-rich, B-Be-bearing environment during the final crystallization of the Variscan peraluminous leucogranite. The formation of tourmalinites can be explained by the related boron metasomatism. Phengitic white mica rims and calcic garnet rims in orthogneisses and tourmalinites indicate high-pressure Alpine metamorphic overprint. The presence of REE-rich phosphate mineralisation and leuchtenbergite in the orthogneisses imply that high salinity fluids metasomatized the orthogneisses along the pre-existing shear zones after the Alpine metamorphic peak. Tourmaline grains in kyanite-bearing quartzites and schists may have originated from a micaschist that underwent a strong Mg-metasomatism during the formation of leucophyllites described from the area.