Arsenian monazite-(Ce) and xenotime-(Y), REE arsenates and carbonates from the Tisovec-Rejkovo rhyolite, Western Carpathians, Slovakia:: Composition and substitutions in the (REE,Y)XO4 system (X = P, As, Si, Nb, S)

A unique REE-Y-(Th)-P-As-(Si)-(Nb)-(S) accessory assemblage was identified in a small body of Lower Triassic A-type rhyolite of the Silicic Superunit near Tisovec-Rejkovo, Central Slovakia. Arsenian monazite-(Ce)-phosphatian gasparite-(Ce) and arsenian xenotime-(Y)-phosphatian chemovite-(Y) solid solutions in association with REE carbonates (bastnasite, parisite, rontgenite?, synchysite) and rare cerianite-(Ce) form anhedral to subhedral grains and aggregates (<= 0.3 mm), scattered in the groundass or as intergrowths with zircon and Fe-Ti oxides. Compositions show a wide AsP-1 substitution in monazite-gasparite and xenotime-chemovite solid solutions; atom. % As/(As+P)=0.00 to 0.73 and 0.10 to 0.94, respectively. The presence of S in monazite-gasparite s.s. (<= 0.14 apfu), together with Ca enrichment indicates clinoanhydrite substitution, CaS(REE,Y)(-1) (PAs)(-1), rather than the incorporation of brabantite, along the CaTh(REE,Y)(-2) exchange vector. Moreover, the Th- and Nb-rich members reveal thorite and fergusonite substitutions, ThSi(YREE)(-1) (PAs)(-1) and Nb(PAs)(-1), respectively. The As-rich REE phases originated probably during the post-magmatic alteration of primary monazite-(Ce) and xenotime-(Y) by As-rich, high fO(2) fluids, whereas cerianite-(Ce) and the REE carbonates are products of a later hydrothermal overprint of the rhyolite by CO2-rich solutions. (C) 2006 Elsevier B.V. All rights reserved.