Journal of
GEOsciences
  (Formerly Journal of the Czech Geological Society)

Original Paper

Accessory columbite to tantalite, tapiolite and zircon: products of extreme fractionation in highly peraluminous pegmatitic granite from the Považský Inovec Mountains, Western Carpathians, Slovakia

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Abstract:
Accessory Fe-rich columbite-group minerals, tapiolite and Hf-rich zircon occur in Hercynian pegmatitic leucogranite near Duchonka, Považský Inovec Mts., western Slovakia. The host rock represents highly peraluminous and fractionated S-type pegmatitic leucogranite with ASI = 1.27, Eu_N/Eu^*_N = 0.16, Rb/Sr = 7.2, Ta/Nb = 1.1 and Zr/Hf = 21, but not enriched in Li, B, Be or P. Columbite-tantalite, tapiolite and hafnian zircon form discrete crystals, 30 to 350 m in size, in association with quartz, plagioclase, K-feldspar, muscovite, sillimanite, almandine-spessartine and fluorapatite. Columbite-tantalite crystals show coarse oscillatory zoning, usually with border parts enriched in Ta. Locally, there is a reversal trend of zoning (decrease of Ta towards the border parts), or irregular convoluted zoning as a result of late-magmatic to subsolidus dissolution-reprecipitation. The composition of columbite-tantalite shows a relatively constant Mn/(Mn + Fe) ratio (0.20-0.27, locally 0.35-0.40), but extreme variations of the Ta/(Ta + Nb) ratio (0.18-0.72). Ratios higher than 0.63 plot inside the tantalite-tapiolite miscibility gap. Ferrotapiolite is mainly homogenous and shows relatively consistent compositions with Mn/(Mn + Fe) = 0.03-0.04 and Ta/(Ta + Nb) = 0.88-0.97. Metamict zircon (5 to 120 μm in size) exhibits tiny uraninite inclusions, high Hf contents (6 to 23 wt. % HfO₂, 0.06-0.23 Hf apfu), and locally elevated P, As and U contents, whereas Y and REE concentrations are low. Unusually wide and high Ta/Nb and Hf/Zr ratios in the accessory minerals are probably the product of extreme local Nb-Ta and Zr-Hf fractionation in highly peraluminous granite-pegmatite system