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 chernovite-(Y) solid solutions in association with REE carbonates (bastnäsite, parisite, röntgenite?, 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–chernovite 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 (P,As)− 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(Y,REE)− 1 (P,As)− 1 and Nb(P,As)− 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 fO2 fluids, whereas cerianite-(Ce) and the REE carbonates are products of a later hydrothermal overprint of the rhyolite by CO2-rich solutions.