Evolution of tourmaline from Li-poor, F-rich pegmatites at the SE border of the Moldanubian Zone, Bohemian Massif
|Fakulta / Pracoviště MU
|Eastern margin of the Moldanubian Zone is characterized by common occurrences of rare-element pegmatites, loosely grouped in two large pegmatite fields (Strážek and Vratěnín-Radkovice). The area is characterized by presence of S-type granites and leucogranites in the thermal aureole of the Třebíč durbachite pluton, and various metamorphosed, strongly migmatized paragneiss to migmatites of the Gföhl Unit of the Moldanubian Zone. We studied mineralogy and tourmaline evolution in three highly to moderately fractionated elbaite-subtype pegmatites – Ctidružice, Hrotovice and Křižínkov. The three localities show several common features such as F-rich mineral assemblages (topaz, F-rich tourmaline), metasomatic micas with locally very high fractionation of Cs and Mn (sokolovaite, masutomilite), and high contents of W present in Nb,Ta-oxides (W-rich columbite, ixiolite, qitianlingite). The observed evolution of tourmaline with high initial contents of Fe and high Na,F-contents corresponds to relatively high initial fractionation of the melt. The tourmaline composition and presence of HFSE such as Nb, Ta and W in combination with high F-contents supports their origin as fractionation products of the associated S-type granites and leucogranites present in the thermal aureole of the Třebíč melasyenite (durbachite) pluton.