GRANITOID ALLANITE-(CE) - SUBSTITUTION RELATIONS, REDOX CONDITIONS AND REE DISTRIBUTIONS (ON AN EXAMPLE OF I-TYPE GRANITOIDS, WESTERN CARPATHIANS, SLOVAKIA)

Seven allanites-(Ce) from the Sihla type s.l., a characteristic I-type granitoid of the Western Carpathians, were studied in detail. The BSE images have revealed their internal zoning characterized by repeated interactions of primary grains with late- to postmagmatic fluids which produced a patchwork of domains of variable brightness. The microprob e-based study of substitution relations showed that allanite compositi ons are dominated by the substitutions Ca + Fe3+ = Fe2+ + REE(3+) (Fe- epidote-allanite) and Ca + Al3+ = Fe2+ + REE(3+) (Fe-epidote-ferrialla nite). While bright domains are thought to be primary, the dark grey d omains are interpreted as oxidated, the oxidation being compensated by partial REE escape. This interpretation is based on a new type of dia gram (total REE + Th vs. Al) contoured with F-OX = Fe3+/(Fe3+ + Fe2+ i solines. The diagram enables a realistic estimation of the F-OX ratio of spot analyses of Fe-allanites as confirmed by Mossbauer-derived Fe3 +/Fe2+ ratios of two allanites. While igneous allanites seem to have F -OX ratios buffered at about 0.4, vein allanites are variably oxidated , and metamorphic allanites both oxidated and reduced. The distributio n of LREE in allanites is characterized by Ce-dominance, the La/Nd rat io being rather low. The composition of the parental granitic liquid i nferred from the compositions of the primary allanite domains and know n LREE partitioning coefficients shows a strongly fractionated LREE di stribution pattern.