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

作者：Lukas Krmcek,Milan Novak,Robert B.Trumbull,Jan Cempirek,Stanislav Houzar

摘要：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.TypeⅠcomprises individual,euhedral,prismatic grains and grain aggregates in a carbonate-dominant(calcite±dolomite)matrix poor in silicates.TypeⅡis characterized by euhedral to subhedral grains and coarse-to fine-grained aggregates in silicate-rich layers/nests within metacarbonate bodies whereas typeⅢoccurs as prismatic grains and aggregates at the contact zones between carbonate and associated silicate host rocks.TypeⅣis in veins crosscutting metacarbonate bodies,and typeⅣtourmaline occurs at the exocontacts of elbaite-subtype granitic pegmatite.Tourmaline from the different settings shows distinctive compositional features.Typical for typeⅠare Mg-rich compositions,with fluor-uvite>dravite>>magnesio-lucchesiite.Tourmalines from typeⅡsilicate-rich layers/nests are highly variable,corresponding to oxy-schorl,magnesio-foitite,Al-rich dravite and fluor-uvite.Typical for typeⅢtourmalines are Ca,Ti-bearing oxy-dravite compositions.The typeⅣveins feature dravite and fluor-uvite tourmaline compositions whereas typeⅤtourmaline is Li,F-rich dravite.Tourmaline is the only Bbearing phase in paragenetic typesⅠ-Ⅳ,where it is characterised by two principal ranges of B-isotope composition(δ^11B=-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(δ^11B=-18‰to-14‰),whereas metacarbonates in the Poli?ka unit(Teplá-Barrandian Zone)and Olesnice unit(Moravicum of the Moravo-Silesian Zone)has exclusively isotopically heavier tour

发文机构：Department of Geological Sciences Institute of Geology of the Czech Academy of Sciences Brno University of Technology GFZ German Research Centre for Geosciences Department of Mineralogy and Petrography

关键词：BoronisotopesTOURMALINEMetacarbonatesMoldanubicumVariscanorogeny

分类号： G63[文化科学—教育学]