PARAGENESIS OF ALLANITE-(Y) AND ALLANITE-(Ce) IN TOURMALINITES OF THE SEVERNY MASSIF (CHUKOTKA) AND RELATIONSHIP BETWEEN YTTRIUM AND LANTHANIDES IN ALLANITE
Morphology, optical properties and composition of allanite-(Ce) and allanite-(Y) have been studied for these minerals from rare-metal tourmalinites of the Severniy Massifin Chukotka. Composition and structure of host metasomatites, associations of rock-forming and accessory minerals were investigated also. One of obtained result is the conclusion about hydrothermal origin of both allanite species. There have been revealed the partial replacement of allanite-(Y) by allanite-(Ce), and detected metasomatic rims of 2-10 μm width enriched in LREE around allanite-(Y). The degree of isomorphic substitution for Y+HREE and LREE in allanite-(Y) is estimated 16 %, in averages, whereas the maximum value of the ratio (Y+HREE)/LREE does not exceed 0.25. The assumption is made that transition of allanite-(Y) into allanite-(Ce) might be caused by an increase of acidity with decreasing water fluid temperature at the late stage of the hydrothermal process.