Alekseev, et al. report that the morphology, optical properties, and composition of allanite-(Ce) and allanite-(Y) from raremetal tourmalinite of the Severnyi granitic pluton in the Chukchi Peninsula have been studied, as well as the composition and structure of host metasomatic rocks and the assemblage of rock-forming and accessory minerals. The hydrothermal origin of both allanite species and their stable combination have been established. Allanite-(Y) is partly replaced with allanite-(Ce), and metasomatic rims 2–10 μm wide enriched in LREE around allanite-(Y) have been identified. The degree of isomorphic Y+HREE substitution for LREE is estimated at 16%, on average; the maximum (Y+HREE)/LREE ratio does not exceed 0.25. It is assumed that the transition of allanite-(Y) into allanite-(Ce) might be caused by an increase in acidity and decrease in aqueous fluid temperature at the late stage of the hydrothermal process.
They concluded that:
1) The hydrothermal origin of allanite-(Y) has been established.
2) The allanite-(Y) and allanite-(Ce) paragenesis has been established in tourmalinite from the Severnyi granitic pluton in the Chaun District of the Chukchi Peninsula.
3) A restricted (Y + HREE)–LREE isomorphism in allanite has been recorded.
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