Alekssev and two co-authors reported in 2011 that the research history of allanite-(Y) has been considered. The composition and genesis of this mineral is discussed with allowance for new findings in Li–F granites of the Russian Far East. Compositional anomalies in allanite-(Y) reflect the metastable character of its structure. Types of zoning and trends in the chemical substitution of major components are described. Two crystal chemical modes of allanite-(Y) formation and the two-step isomorphism of its components are suggested.
The main tendencies in the chemical evolution of allanite-(Ce) and allanite-(Y) in granitic rocks of the Far East are pointed out. Allanite-(Y) is formed in the anomalous geochemical setting that characterizes crystallization of fluid-saturated subalkaline granitic melt and is regarded as an index mineral of rare-metal ongonite magmatism completing the Late Cretaceous Pacific Orogeny.
The authors concluded that:
1) The research history of allanite-(Y) shows that this mineral is not abundant because its formation requires special geochemical conditions. Allanite-(Y) is characterized by anomalous and steadily nonstoichiometric chemical composition. The enrichment in Y, HREE, and Fe2+ and depletion in Ca, Fe3+, Al, and Th are typical, as well as a yttrium-type chondrite- normalized REE pattern with Dy and Nd anomalies.
2) The structure of allanite-(Y) is metastable due to metamictization and the complex two-step isomorphism of HREE, Y, Ca, and Fe, which leads to formation of structural defects. Allanite-(Y) was formed in two ways: allanite-(Ce) → allanite-(Y) and anomalous allanite-(Y) → allanite-(Y).
3) The evolution of allanite composition reflects the geochemical evolution of the granitic series of the Far East, from high-Al allanite(Ce) in early biotite granite to ferroan allanite-(Y) in rare-metal zinnwaldite granite. Allanite-(Y) is formed in the anomalous geochemical setting inherent to crystallization of fluid-saturated subalkaline granitic melt and serves as a guide of rare-metal ongonitic magmatism completing the Late Cretaceous Pacific Orogeny.
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