Extreme P-, Bi-, Nb-, Sc-, U- and F-rich zircon from fractionated perphosphorous granites: The peraluminous Podlesí granite system, Czech Republic

Abstract

The strongly peraluminous and P-rich, protolithionite and zinnwaldite leucogranites from Podlesí, western Krušné Hory Mts., Czech Republic, contain accessory zircon with extraordinary enrichment of several elements, which constitute trace elements in common zircon. Elements showing a not yet reported anomalous enrichment include P (up to 20.2 wt.% P 2O 5; equivalent to 0.60 apfu, formula calculated on the basis of 4 oxygen atoms), Bi (up to 9.0 wt.% Bi 2O 3; 0.086 apfu), Nb (up to 6.7 wt.% Nb 2O 5, 0.12 apfu), Sc (up to 3.45 wt.% Sc 2O 3; 0.10 apfu), U (up to 14.8 wt.% UO 2; 0.12 apfu) and F (up to 3.81 wt.% F; 0.42 apfu). Strong enrichment of P preferentially involved the berlinite-type substitution (2 Si 4+ ⇔ P 5+ + Al 3+) implying that significant Al may enter the Si position in zircon. Incorporation of other exotic elements is primarily governed by the xenotime (Si 4+ + Zr 4+ ⇔ P 5+ + Y 3+), pretulite (Sc 3+ + P 5+ ⇔ Zr 4+ + Si 4+), brabantite-type (Ca 2+ + (U, Th) 4+ + 2P 5+ ⇔ 2Zr 4+ + 2Si 4+), and ximengite-type (Bi 3+ + P 5+ ⇔ Zr 4+ + Si 4+) substitution reactions. One part of the anomalous zircons formed late-magmatically, from a strongly peraluminous, P–F–U-rich hydrous residual melt that gave rise to the zinnwaldite granite. Interaction with aggressive residual fluids and metamictization have further aided in element enrichment or depletion, particularly in altered parts of zircon contained in the protolithionite granite. Most of the zircon from F-rich greisens have a composition close to endmember ZrSiO 4 and are chemically distinct from zircon in its granite parent. This discrepancy implies that at Podlesí, granitic zircon became unstable and completely dissolved during greisenization. Part of the mobilized elements was reprecipitated in newly grown, hydrothermal zircon.