Gadas, Petr

Novák, Milan

Zahradníček, Lukas

English

Zahradníček Lukas

Novák Milan

Gadas Petr

University of Szeged

Acta Mineralogica-Petrographica, Abstract Series, Szeged, Vol. 7, 2012 157 Joint 5th Mineral Sciences in the Carpathians Conference and 3rd Central-European Mineralogical Conference 20–21 April, 2012, University of Miskolc, Miskolc, Hungary PRIMARY VERSUS METASOMATIC TOURMALINE FROM ELBAITE-SUBTYPE PEGMATITE IN PIKÁREC NEAR KŘIŽANOV, CZECH REPUBLIC  ZAHRADNÍČEK, L.1 Department of Mineralogy and Petrology, National Museum, Cirkusová 1740, 193 00 Prague, Czech Republic 1*, NOVÁK, M.2 & GADAS, P.2 2 Department of Geological Sciences, Masaryk University, Kotlářská 2, 611 37, Brno, Czech Republic * E-mail: lukas_zahradnicek@nm.cz  High compositional variability of tourmaline in the individual textural-paragenetic units of granitic pegma-tites makes tourmaline excellent and widely used indi-cator of geochemical evolution from magmatic to hydrothermal stage and of external contamination of pegmatite melt as well. However, discerning of primary tourmalines and metasomatic ones are ignored in most cases. The elbaite-subtype pegmatite Pikárec is located in the easternmost part of the Moldanubian region, western Moravia, Czech Republic. Symmetrically zoned dike, ~ 3 m thick, is enclosed in amphibolite and con-sists of thin (i) coarse-grained outer granitic unit (Plg + Kfs + Qtz + Bt), volumetrically dominant (ii) graphic unit (Kfs + Qtz > Ab + Qtz) evolving to (iii) albite unit with blocky K-feldspar (Ab + Kfs + Qtz) and locally with (iv) small pockets. Tourmaline is a typical minor mineral evolving from the assemblage annite + schorl in outer parts of graphic unit through black schorl-foitite to Mn-rich foitite rimmed by pink elbaite in the inner part of graphic unit (Fig. 1a,b). Crystals of red, pink and green Mn-rich elbaite-rossmanite occur in pockets. Rare to common accessory minerals include fluorapatite, löllingite, spessartine, beryl, cassiterite, columbite, microlite, muscovite, polylithionite and pollucite. Al-teration of early tourmaline (+ feldspars) by me-tasomatic tourmaline (Fig. 1a,b) is a typical feature of most tourmaline grains and crystals except those from pockets. The reaction rims are always developed on the con-tact with plagioclase, albite or K-feldspar; no changes were found on the contact with quartz (Fig. 1a; left upper part). They originated in part instead of early tourmaline and in part instead of surrounding feldspars. These replacement processes may be expressed by the simplified reactions: Fig. 1a – Mg-schorl → schorl – (1) Mg → Fe 2+; anorthite component in plagioclase → liddicoatite – (2) 7An + 2Li2O + 3B2O3 + H2O → 2 liddicoatite + 5CaO + 2SiO2; Fig. 1b – Na-rich Mn-foitite → X-site vacant, F-enriched elbaite – (3) foitite + Na2O + Li2O + Al2O3 + F → elbaite + H 2O + 2FeO; elbaite → elbaite (slightly Ca,F -enriched). Stoichiome-try of the reactions is quite complicated; hence, some reactions are only approximate. Nevertheless, the reac-tions evidently require fluids rich in Li2O, B2O3, F, and H2O, what we can expect in early subsolidus stage at such complex (Li) pegmatite. The plot on Fig. 1c shows enrichment of metasomatic tourmaline by F relative to its primary precursors. In addition, slight to strong en-richment in Ca of metasomatic tourmaline is typical. Consequently, slight to moderate Ca,F-enrichment in late tourmaline may be an indication of  its metasomatic origin in complex granitic pegmatites. Acknowledgements. This work was supported by internal grant of National museum, Prague 2011/05/IG-PM to LZ and research project GAČR P210/10/0743 to MN and PG.       Fig. 1. Textural relations (a, b) and F/Mn plot (c) of primary and metasomatic tourmalines. Replacement of early  tourmaline + feldspars and surrounding feldspars by metasomatic tourmaline: a) outer part of graphic unit with  plagioclase An15; b) inner part of graphic unit with albite An01–00.    brought to you by COREView metadata, citation and similar papers at core.ac.ukprovided by University of Szeged

Primary versusmetasomatic tourmaline from elbaite-subtype pegmatite in Pikárec near Křižanov, Czech republic

http://acta.bibl.u-szeged.hu/33082/1/mineralogica_as_007_157.pdf

Primary versusmetasomatic tourmaline from elbaite-subtype pegmatite in Pikárec near Křižanov, Czech republic

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