Abstract

Common pegmatites with pockets lined with crystals of smoky quartz, feldspars, muscovite, and black tourmaline commonly occur in migmatized biotite–sillimanite gneisses and felsic granulites of the Strážek Unit, Moldanubian Zone, in the Czech Republic. Mostly concordant dikes or irregular bodies (0.1–2 m thick) consisting of an outer granitic unit, a graphic unit and a pocket unit with large crystals (up to 1 m long) of minerals show transitional to locally sharp contacts to the host migmatized rocks. Thin muscovite-rich veins, 1–10 cm thick, with the assemblage muscovite ≥ albite, quartz and tourmaline are closely associated spatially. Variable prismatic to lens-shaped tourmaline crystals and their aggregates, up to 30 cm across, from pockets and muscovite-rich veins exhibit striking zoning in the optical microscope (distinct colors and pleochroism) and in BSE images: core (foitite – schorl – magnesio-foitite), intermediate zone (schorl – dravite) and narrow outer rim (schorl – dravite); tourmaline from muscovite veins is relatively homogeneous (dravite – magnesio-foitite). A slight decrease in X-site vacancy, in Fetot/(Fetot + Mg) and Al contents but sharp increase of Ti from the core to intermediate zone and outer rim, respectively, are typical. Also, the LA–ICP–MS study of a single crystal of tourmaline from a pocket demonstrates a very similar chemical composition of core and intermediate zone, but a distinct composition of outer rim; Mn, Ni, Zn, Sc and Ga decrease, and V, Cr, Co and Sr increase in the rim. Nearly all Fe was determined as Fe2+ (<3% of Fe3+ in both core and intermediate zones of the tourmaline) using Mössbauer spectroscopy. The mineral assemblages (andalusite + cordierite) of common pegmatites and their geological and petrographic features indicate P–T conditions of ∼500–650°C and less than ∼3–4 kbar for massive units, but lower for tourmalines and other minerals from pockets and in particular for associated muscovite-rich veins. The distribution of pegmatites in migmatitic rocks, their concordant and transitional (diffusive) contacts to the country rocks, simple assemblages of minerals, primitive chemical composition of minerals (all typical for abyssal pegmatites), and common large pockets lined with well-developed crystals of smoky quartz, feldspars, muscovite, and tourmaline (which is typical of miarolitic pegmatites), do not fit the current classifications of granitic pegmatites.

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