Tourmalinites in the metamorphic complex of the Svratka Unit (Bohemian Massif): a study of compositional growth of tourmaline and genetic relations

Tourmalinites (rocks with an association Tu + Qtz + Ms +/- Grt +/- Bt +/- Ky +/- Sil +/- Pl +/- Kfs) from the Svratka Unit form stratiform layers hosted in mica schists. The chemical composition of tourmaline from tourmalinites varies from Al-rich schorl to dravite. The tourmaline usually exhibits three compositional domains, which are, from centre to the rim: a chemically inhomogeneous brecciated core (zone I), a volumetrically minor internal rim zone II, and a dominant outermost zone III. The compositional variability of tourmaline in all the zones is controlled by the ((X)square Al-Y (OH)-O-W) (Na-X Mg-Y F-W)(-1) and (FeMg-1)-Fe-Y-Mg-Y substitutions. The tourmaline of the zone I corresponds to highly vacanced X-site, Al-rich schorl with lower F (up to 0.43 apfu), which is interpreted as an older, low-temperature hydrothermal tourmaline. Tourmaline of the zone II corresponds to dravite rich in F (reaching up to 0.66 apfu) crystallizing during the prograde metamorphism. Lastly, the Al-rich schorl-dravite of the zone III grew most likely during retrograde metamorphism. The tourmaline from the host mica schists has a similar chemical composition. The central dravite part corresponds to the zone II, and the predominant schorl-dravite rim to the zone III, in tourmalines from tourmalinites. Overall, the dravite exhibits compositional characteristics of the prograde, amphibolite-facies metamorphic event characterised by the mineral assemblage Qtz + Ms + Bt + Tu +/- Ky +/- St +/- Grt. The second generation of tourmaline (schorl-dravite), as well as the rim of garnets present in both the mica schists and tourmalinites, formed during exhumation of the Svratka Unit accompanied by decreasing pressure and temperature. In the mica schists, this event resulted in decompression breakdown of staurolite, according to the reaction St + Ms + Qtz = Grt + Sil + Bt + H2O. Breakdown of muscovite could have released B and F used for tourmaline formation. The P-T conditions of this retrograde metamorphism were calculated at 600-640 degrees C and 5-6 kbar. Tourmalinites are interpreted as a part of a metamorphosed volcano-sedimentary complex primarily rich in F and B, however, the derivation of all the F-and B-rich fluids from the neighbouring migmatites and metagranites is unlikely. Similarities in the chemical composition of the tourmalinites and the mica schists suggest a similar protolith to both rock types. The variation in most of the elements reflects the mineral composition associated with the transition from mica schist to tourmalinite.