Acta Musei Moraviae

Granitic NYF pegmatites of the Třebíč Pluton (Czech Republic)

Homogeneous segregations to complexly zoned dikes of granitic pegmatites, up to 2 m thick, are enclosed in Hercynian (ca. 340 Ma), I-type, ultrapotassic, porphyric, amphibol-biotite melasyenite to melagranite (durbachite) of the Třebíč Pluton (TP), Moldanubicum, western Moravia, Czech Republic. They typically show transitional contacts with the host syenites to granites. Mineral composition the pegmatites is characterized by dominance of K-feldspar (locally amazonite) over albite, common Mg-rich biotite (phlogopite) but absence of primary muscovite, common Y, REE-bearing minerals (allanite-(Ce), aeschynite and euxenite group minerals) and Ti-bearing minerals (titanite I, ilmenite). Further common accessory minerals include actinolite, zircon, monazite-(Ce), beryl, phenacite and niobian rutile. Tourmaline (abundant Ca, Ti, Fe³⁺-rich dravite to schorl, very rare Al-rich schorl) is a typical minor to accessory mineral in some euxenite pegmatites but rather rare in allanite pegmatites. Abundant alteration products include chlorite, pyrochlore group minerals, bastnäsite-(Ce), pseudorutile and titanite II. Specific zinnwaldite–masutomilite–elbaite pegmatite from Kracovice cuts gneiss nearby the western border of the TP. It exhibits similar mineralogical features – common Y, REE-bearing minerals (samarskite group minerals, fergusonite, monazite-(Ce), xenotime-(Y), yttropyrochlore, Y-rich spessartine), presence of amazonite but it also contains primary muscovite and abundant Li, B, Be, Mn, F-rich minerals such as zinnwaldite, masutomilite, Mn-rich lepidolite, elbaite, topaz, spessartine, beryl and hambergite. The examined pegmatites are classified as typical metaluminous, NYF pegmatites of the allanite and euxenite subtype; however, peraluminous, Li, Mn, F, B-rich pegmatite from Kracovice may also belong to the „Mixed“-family. Geochemical evolution from Mg-rich, metaluminous allanite subtype pegmatites, through less evolved euxenite subtype pegmatites and more evolved Mn, F, B-enriched albitized euxenite pegmatite from Klučov to highly evolved peraluminous Li, Mn, F, B-rich pegmatite from Kracovice is characterized by significant increase in activity of Li, F, Mn, Sn, Y, HREE and Nb, but depletion in Ca, Mg, LREE and Ti. Enrichment in B and Be during fractionation is less apparent. Granitic NYF pegmatites related to other durbachite bodies of the Bohemian Massif (Čertovo břemeno, Drahonín) show similar geochemical and mineralogical features, but they are generally slightly more evolved and they were classified as gadolinite subtype absent within the TP. Euxenite subtype pegmatites from Trout Creek Pass, Colorado, USA are similar to those from TP.