Boron isotopic variations in tourmaline from metacarbonates and associated calc-silicate rocks from the Bohemian Massif: Constraints on boron recycling in the Variscan orogen

Various metacarbonate and associated calc-silicate rocks form minor but genetically significant components of the lithological units in Bohemian Massif Variscan orogen Central Europe. These vary terms their lithostratigraphy, chemical composition mineral assemblage (dolomite/calcite ratio, silicate abundance). Tourmaline is present five paragenetic settings within units. Type I comprises individual, euhedral, prismatic grains grain aggregates a carbonate-dominant (calcite ​± ​dolomite) matrix poor silicates. II characterized by euhedral to subhedral coarse- fine-grained silicate-rich layers/nests bodies whereas type III occurs as at contact zones between carbonate host rocks. IV veins crosscutting bodies, V tourmaline exocontacts elbaite-subtype granitic pegmatite. from different shows distinctive compositional features. Typical for are Mg-rich compositions, with fluor-uvite ​> ​dravite>> ​magnesio-lucchesiite. Tourmalines highly variable, corresponding oxy-schorl, magnesio-foitite, Al-rich dravite fluor-uvite. tourmalines Ca,Ti-bearing oxy-dravite compositions. The feature compositions Li,F-rich dravite. only B-bearing phase types I–IV, where it characterised two principal ranges B-isotope (δ11B ​= ​−13‰ −9‰ −18‰ −14‰). correspond regionally Moldanubian Zone. Thus, Svratka Unit (Moldanubian Zone s.l.) contains isotopically lighter ​−18‰ −14‰), metacarbonates Polička unit (Teplá–Barrandian Zone) Olešnice (Moravicum Moravo-Silesian has exclusively heavier ​−9‰ −13‰). Variegated cover both isotope composition. light end B range may indicate presence continental evaporites individual investigated areas. On other hand, variations ~8 δ-units consistent reported shift isotopic metasedimentary due prograde metamorphism very-low grade eclogite facies. In contrast metacarbonate-hosted settings, exocontact pegmatites displays significantly δ11B (as low –7.7‰ –0.6‰), which attributed partitioning 10B cogenetic axinite and/or B-signature source pegmatite containing heavy signature.