Ferocolumbit z frakcionovaneho P-bohateho turmalinickeho leukogranitu z vyderskeho plutonu na Sumave

This paper reports a unique occurrence of macroscopic ferrocolumbite in the fractionated phosphorus- and boron-rich leucogranite related to the Vydra Pluton (SW branch of Moldanubian Batholith, Klomínský et al. 2010, Žáček 2018). A small 1-2 m dyke of leucogranite cuts Weinsberg-type biotite granite of the Vydra Pluton (peraluminous S-type granite) at the GS117a site (49.01432° N, 13.41735°E), situated on the Czech-German border in the Šumava National Park (Fig. 1). The leucogranite consists of albite (˜35 %), K-feldspar (˜25 %), quartz (˜30 %), minor muscovite (5-10 %), tourmaline (2-3 %), apatite (1-2 %), accessory U-rich zircon (mostly enclosed in tourmaline) and the ferrocolumbite described below (Fig. 2a-d). The leucogranite is peraluminous, A/CNK = 1.20, La_N/Sm_N = 2.03, Eu/Eu* = 0.33, rich in SiO₂ (72.91 wt.%), Na₂O (4.55 wt.%), K₂O (4.18 wt.%), and boron (˜0.2-0.3 wt.% B₂O₃, corresponding to ˜2-3% of tourmaline in the rock), see Table 1. The high phosphorus content (0.645 wt.% P₂O₅) is combined with Ca and is partially bound in apatite, with the excess phosphorus subsequently being bound in K-feldspar and albite. The leucogranite has elevated concentrations of Rb (521 ppm), Sn (86 ppm), As (64 ppm), Nb (24 ppm) and U (16 ppm), and low TiO₂ (0.03 wt.%) and MgO (0.08 wt.%). It is extremely depleted in V, Cr, Ni, Sr, Ba, Y, Th (1.7 ppm) and total REE (24 ppm). The albite has elevated Rb₂O (˜0.1 wt.%) and P₂O₅ (up to 0.4 wt.%), and the K-feldspar contains ˜0.85 wt. % P₂O₅ and also increased Rb₂O (˜0.1 wt.%). The muscovite also contains elevated Rb₂O (˜0.2 wt. %), Cs₂O (˜0.1 wt. %), and FeO^tot(˜2.5 wt.%) (see Table 2). The tourmaline is zoned X-deficient schorl (centre) to Na-rich foitite (rim): Mg/(Mg+Fe) = 0.19-0.20, X_Na = 0.46-0.58 apfu, X_vacancy = 0.39-0.53 apfu (Table 3, Fig. 3). It frequently encloses radioactive zircon rimmed by conspicuous pleochroic circles (Fig. 2c). The ferrocolumbite appeared as a unique aggregate of prismatic crystals up to 1.5 mm long, grown in the K-feldspar and plagioclase (Fig. 2d). It consists of three generations, which are well distinguishable in a BSE and by chemical composition (Figs 2d, 4, Table 4). The oldest Ta-Mn-poor ferrocolumbite I (approximately 80 % of the sample, see Fig. 2d) is enriched in W (0.04-0.05 apfu) and Ti (0.12-0.15 apfu) and has the average empirical formula (2 points): (Fe_0.79Mn_0.17) (Nb_1.67Ta_0.17Ti_0.13W_0.04) O₆.\r\nTa-rich ferrocolumbite II forms part of one of the prismatic crystals and irregular patches on the edge of ferrocolumbite I and has the empirical formula: (Fe_0.77Mn_0.20) (Nb_1.31Ta_0.61Ti_0.07W_0.02) O₆. The third generation forms irregular patches on the edges of ferrocolumbite I and corresponds to Nb-rich ferrotantalite with the empirical formula: (Fe_0.77Mn_0.23) (Ta_1.07Nb_0.86Ti_0.06W_0.01) O₆. The compositional trend of the studied ferrocolumbite-ferrotantalite is a strong increase in Ta/(Ta + Nb), slight increase in Sn and Sc, and moderate to slight decrease in Ti, W, Zr, Y, Pb and U (Fig. 4, Table 4).

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