抄録
REE-bearing accessory minerals of representative leucogranites of the ilmenite series were examined in the Naegi granite of the Sanyo Belt and Busetsu granite of the Ryoke Belt. Major REE-carriers appear to be rim of zircon, then monazite, xenotime and fluorite in the Naegi biotite granite, but monazite, zircon and xenotime in the Busetsu muscovite-biotite granite. Vein dikes cutting the Naegi granite (main phase) contain ferugsonite and zircon as major REE-carrier, then small amounts of thorite, rutile and fluorite. Thus, the dikes are enriched in HREE, which is due to LREE consumed to form monazite in the main phase. U and Th as well as HREE are enriched in rim of the zircon, which implies that these components were added to the host zircon crystals during the deuteric stage. Yamaguchilite and Naegite from pegmatite localities, which have been known traditionally as varieties of zircon, were also examined by EPMA. The Yamaguchilite contains up to 20.94 wt.% Σ REE2O3, 3.87 wt.% ThO2 and 2.46 wt.% UO2, while the Naegite reveals up to 3.31 wt.% Σ REE2O3, 12.63 wt.% ThO2 and 6.19 wt.% UO2. Formerly reported anomalous Nb and Ta contents in the Naegite fall below the detection limit in this study. It is thus proposed that these elements came from micro-inclusions of the Naegite in the previous analyses, which were done by wet-methods on the mineral separates.
