NB-TA-TI OXIDES IN GRANITIC PEGMATITES FROM THE TOPSHAM PEGMATITE DISTRICT, SOUTHERN MAINE

Numerous bodies of granitic pegmatite intrude gneissic rocks in the To psham pegmatite district, in southern Maine. Many of these pegmatites contain metamict, REE-bearing, complex Nb-Ta-Ti oxides. Initial report s identified this mineral as samarskite-(Y), an ABO, mineral with Nb: Ta and Ti at the B site and REE + Y dominant at the A site. However, e lectron-microprobe and X-ray-diffraction analyses of seven recently co llected samples from various locations in the district reveal that the oxide mineralogy is complex, consisting of several oxide phases. When heated, five of the specimens examined recrystallized to high-T samar skite plus other phases. However, these specimens have U + Th greater than REE + Y at the A site, and thus are ishikawaite rather than samar skite-(Y). Ishikawaite was found in all of the locations studied excep t the Consolidated Number 2 Quarry. The only metamict-appearing sample collected from the Consolidated Number 2 Quarry is ferrocolumbite. an AM(2)O(6) oxide. At the Stand Pipe Hill location, polycrase-(Y), an A B(2)O(6) oxide with Y dominant at the A site and Ti dominant at the B site, occurs in addition to ishikawaite. Samarskite-(Y) is notably abs ent from the pegmatites studied.