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

Year, pages: 1995, 79 - 94

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 microprobe- based study of substitution relations showed that allanite compositions are dominated by the substitutions Ca + Fe³⁺ = ^Fe2+ REE³⁺ (Fe-epidote-allanite) and Ca + Al³⁺ = Fe²⁺ + REE³⁺ (Fe-epidote-ferriallanite). While bright domains are thought to be primary, the dark grey domains are interpreted as oxidated, the oxidation being compensated by partial REE escape. This interpretation is based on a new type of diagram (total REE+Th vs. Al) contoured with F_ox = Fe³⁺/(Fe³⁺ + Fe²⁺) isolines. The diagram enables a realistic estimation of the F_ox ratio of spot analyses of Fe-allanites as confirmed by Mössbauer-derived Fe³⁺/Fe²⁺ ratios of two allanites. While igneous allanites seem to have Fox ratios buffered at about 0.4, vein allanites are variably oxidated, and metamorphic allanites both oxidated and reduced. The distribution of LREE in allanites is characterized by Ce-dominance, the La/Nd ratio being rather low. The composition of the parental granitic liquid inferred from the compositions of the primary allanite domains and known LREE partitioning coefficients shows a strongly fractionated LREE distribution pattern.

Petrik, I., Broska, I., Lipka, J., Siman, P. 1995. GRANITOID ALLANITE-(Ce): SUBSTITUTION RELATIONS, REDOX CONDITIONS AND REE DISTRIBUTIONS (ON AN EXAMPLE OF I-TYPE GRANITOIDS, WESTERN CARPATHIANS, SLOVAKIA). In Geologica Carpathica, vol. 46, no.2, pp. 79-94. 1335-0552.

Petrik, I., Broska, I., Lipka, J., Siman, P. (1995). GRANITOID ALLANITE-(Ce): SUBSTITUTION RELATIONS, REDOX CONDITIONS AND REE DISTRIBUTIONS (ON AN EXAMPLE OF I-TYPE GRANITOIDS, WESTERN CARPATHIANS, SLOVAKIA). Geologica Carpathica, 46(2), 79-94. 1335-0552.