Y,REE,Nb,Ta,Ti-oxide (AB2O6) minerals from REL–REE euxenite-subtype pegmatites of the Třebíč Pluton, Czech Republic; substitutions and fractionation trends

Aeschynite-group minerals (AGM) and euxenite-group minerals (EGM) occur in REL–REE euxenite-subtype pegmatites from the Třebíč Pluton, Czech Republic. They form strongly metamictized, light brown to black, equigranular to needle-like, subhedral to anhedral grains enclosed in blocky K-feldspar and less commonly in albite, and blocky quartz, and in the graphic unit (quartz and K-feldspar). Both AGM and EGM are homogeneous to slightly heterogeneous in BSE images. They are not commonly associated with the other primary Y,REE,Ti,Nb-bearing minerals, i.e. allanite-(Ce), monazite-(Ce), titanite, and ilmenite, which occur within the same textural-paragenetic unit. Aeschynite-(Y), aeschynite-(Ce), aeschynite-(Nd), nioboaeschynite-(Ce), tantalaeschynite-(Ce), vigezzite and polycrase-(Y) were identified using EMP and canonical discrimination analysis [Ercit, T.S., 2005a. Identification and alteration trends of granitic–pegmatite-hosted (Y,REE,U,Th)–(Nb,Ta,Ti) oxide minerals: a statistical approach. Can. Mineral. 43, 4 1291–1303.]. The exchange vector ACa B(Nb,Ta) A(Y,REE)− 1 BTi− 1 or its combination with the exchange vector ACa2 B(Nb,Ta)3 A(U,Th)− 1 A(Y,REE)− 1 BTi− 3 have been elucidated for the AGM. The exchange vector ACa A(U,Th) A(Y,REE)− 2 is predominant in the EGM. The AGM are enriched in HREE, whereas LREE are concentrated in the EGM. Weak to none-existent geochemical fractionations, as expressed by the U/(U + Th), Y/(Y + REE), Ta/(Ta + Nb) and (Nb + Ta)/(Ti + Nb + Ta) ratios, were noted for single grains from both the AGM and EGM, as well as in grains of polycrase-(Y) from four different textural-paragenetic units located in the Vladislav pegmatite. Simultaneous increase of U/(U + Th) and Y/(Y + REE) in the AGM during fractionation is typical. The Ta/(Ta + Nb) fractionation is usually weak and contradicts the Y/(Y + REE) and U/(U + Th) fractionation trends. This unusual behavior of Nb and Ta may be controlled by associated Ti-rich minerals (titanite, ilmenite, rutile), the composition of parental melt and/or by elevated F activity. The AGM and EGM from pegmatites of the Třebíč Pluton are quite similar in composition to those from REL–REE euxenite-subtype pegmatites in the Trout Creek Pass, Chaffee County, Colorado, USA, which are generally Ca,U,Th-depleted, show lower Ta/(Ta+Nb), and lower variation in HREE/LREE.