COMPOSITIONAL EVOLUTION OF ZONED TOURMALINE CRYSTALS FROM POCKETS IN COMMON PEGMATITES OF THE MOLDANUBIAN ZONE, CZECH REPUBLIC
| Authors | |
|---|---|
| Year of publication | 2012 |
| Type | Article in Periodical |
| Magazine / Source | Canadian Mineralogist |
| MU Faculty or unit | |
| Citation | |
| Doi | http://dx.doi.org/10.3749/canmin.50.4.895 |
| Field | Geology and mineralogy |
| Keywords | foitite; schorl; dravite; magnesio-foitite; compositional evolution; zoning; pocket; common pegmatite; Moldanubian Zone; Czech Republic |
| Description | Common pegmatites commonly occur in migmatized biotite-sillimanite gneisses and felsic granulites of the Strazek Unit, Moldanubian Zone, in the Czech Republic. Mostly concordant dikes or irregular bodies 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 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 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 Fe-tot/(Fe-tot + Mg) and Al contents but sharp increase of Ti from the core to intermediate zone and outer rim, respectively, are typical. Nearly all Fe was determined as Fe2+ (3% of Fe3+ in both core and intermediate zones of the tourmaline) using Mossbauer spectroscopy. The mineral assemblages (andalusite + cordierite) of common pegmatites and their geological and petrographic features indicate P-T conditions of similar to 500-650 degrees C and less than similar to 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 tourmalinedo not fit the current classifications of granitic pegmatites. |
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