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

Abstract

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