Rare-element mineralogy of the Uzumine granitic pegmatite, Abukuma Mountains, northeastern Japan

The internal structure, textures, and rare-element mineralogy of the Uzumine granitic pegmatite from the Abukuma Mountains of northeastern Japan were examined to elucidate chemical compositions, processes of formation, and conditions of emplacement of a rare-element pegmatite associated with low-pressure and low-temperature metamorphism. The pegmatite dike is characterized by abundance of tourmaline, monazite, and euxenite, with small amounts of andalusite and corundum, and an absence of Li- and F-rich minerals. The dike is strongly zoned, comprising a wall zone, an intermediate zone, and a quartz core without miarolitic cavities. A border zone is not observed because of alteration and faults. The pegmatite probably was emplaced syntectonically around 96 million years ago in the Abukuma metamorphic rocks under shallow conditions in the crust (1-1.5 kbar and 300 degrees C). The mineral textures and compositions indicate formation by sequential crystallization from the wall zone to the quartz core with falling temperature. Compositional zones of tourmaline and garnet suggest crystallization during chemical differentiation consistent with decreasing temperature. The degree of Fe-Mn fractionation in oxide minerals is moderate, but the Nb-Ta fractionation is not very advanced.