Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Ivo Macek, Milan Novák, Petr Gadas, Peter Bačík, Michaela Vašinová-Galiová

Late fibrous tourmaline from granitic pegmatites enclosed in compositionally contrasting Mg- and Ca,Mg-rich rocks; a monitor of late external fluids

Journal of Geosciences, volume 71 (2026), issue 1, 39 - 56

DOI: http://doi.org/10.3190/jgeosci.0062.26

Late fibrous tourmalines from three granitic pegmatites with similar size, internal structure and compositionally contrasting Mg- and Mg,Ca-rich host rocks (serpentinite - barren pegmatite Dolní Bory North, dolomite-calcite marble + calc-silicate rock - elbaite-subtype pegmatite Bližná I, both localities in Czech Republic, and spodumene-subtype pegmatite Tamponilapa Ambany, Sahatany Valley, Madagascar) were examined. We focused on their morphology, zonal patterns, chemical composition (EMPA, LA-ICP-MS), powder XRD and RAMAN spectroscopy as well as on the chemical composition of the associated early tourmalines (schorl - Dolní Bory, Mg-rich liddicoatite-elbaite - Bližná, dravite-schorl - Tamponilapa) to reveal textural and compositional relations. Fibrous tourmalines are Mg-rich and F- and Ca-poor: Dolní Bory - greyish Fe-rich dravite with Al_tot 6.07-6.33 apfu, Mg 1.45-1.62 apfu, Fe_tot 0.97-1.26 apfu; Bližná - blue to colorless dravite with Al_tot 6.15-6.36 apfu, Mg 1.97-2.22 apfu, Fe_tot 0.20-0.34 apfu; Tamponilapa - colorless to brownish dravite-oxy-dravite and rare magnesio-foitite with Al_tot 6.42-6.77 apfu, Mg 1.78-2.23 apfu, Fe_tot 0.16-0.24 apfu; low Ca ≥ 0.13 apfu, moderate X-site vacancy 0.31-0.51 pfu, very low F ≥ 0.01 apfu (Dolní Bory, Tamponilapa) are typical. The contents of major, minor and trace elements manifest that the following factors play a substantial role for composition of fibrous tourmalines: external fluids from host rocks with contrasting chemistry (e.g., Mg, Cr, Ni, V), internal residual fluids from pegmatite (H₂O, B, Li, Si, Al), and the chemical compositions of early tourmalines (B, Si, Al, Fe). Fibrous habitus of tourmalines was facilitated by crystallization from low-T aqueous fluids and controlled chiefly by crystal-structural constraints and rate of crystallization. The complex textural relations do not allow to reliably define their origin - direct replacement of early tourmaline, its overgrowing after its partial dissolution or combination of both processes.