60t Thz Canadian M ineralo gist vol. 36, pp. 601-608(1998) Nb-Ta-Ti OXIDES lN GRANITIC PEGMATITES FROM THETOPSHAM PEGMATITE DISTRICT, SOUTHERN MAINE SARAHL. HANSON] Department of Geology and Geophysics, University of Utah 717 WBB, Salt lake Ciry, Unh 84112-1183, U.S.A. WILLIAM B. SIMMONS JR. ANDALEXANDER U. FALSTER Departrnent of Geology anl Geophysics, Universiry of New Orleans, New Orleans, ktuisiann 70148, U.S.A. AssrRAcr Numerous bodies of granitic pegmatite intrude gneissic rocks in the Topsham pegmatite district, in southern Maine. Many of these pegmatites contain metamict, REE-bearing, complex Nb-Ta-Ti oxides. Initial repons identified this mineral as samarskite-(Y), an ABO' mineral with M, Ta and Ti at the B site and REE + Y dominant at the A site. However, electron-microprobe and X-ray-diffraction analysesof seven recently collected samples from various locations in the district reveal that the oxide mineralogy is complex, consisting of several oxide phases. When heated, five of the specimensexamined recrystallized to high-T samarskiteplus other phases. However, these specimens have U + Th greater than REE + Y at the A site, and thus are ishikawaite rather than samarskite-(Y). Ishikawaite was found in all of the locations studied except the Consolidated Number 2 Quarry. The only metamict-appearing sample collected from the Consolidated Number 2 Quarry is ferrocolumbite, an AMrOu oxide. At the Stand Pipe Hill location, polycrase-(Y), an ABrOuoxide with Y dominant at the A site and Ti dominant at the B site, occurs in addition to ishikawaite. Samarskite-(Y) is notably absent from the pegmatites studied. Keywords: samarskite-(Y), ishikawaite, polycrase-(Y), Topsham pegmatite district, Maine. SOMMAIRE De nombreux massifs de pegmatite granitique ont 6t6 mis en place dans un socle gneissique dans le district de Topsham, dans le secteur sud de l'6tat du Maine. Dans plusieurs cas, ces pegmatites contiennent un oxyde complexe de M-Ta-Ti m6tamicte et porteur de terres rares. I-es rapports pr6liminaires font 6tat de samarskite-f), i stoechiom6trie ABO', un min6ral ayant Nb, Ta et Ti I la position B et les terres rares + Y i la position A. Toutefois, nos r6sultats d'analyses d la microsonde 6lectronique et par diffraction X de sept 6chantillons pr6lev6s r6cemment dans le district r6vBlent un assemblage d'oxydes complexe. Suite au chauffage, cinq des dchantillons ont recristallis6 d la samarskite et d'autres phases.Toutefois, ces dchantillons contiennent davantagede U + Th que de tenes rares + Y tr la position A, ce qui en font des exemples d'ishikawaite plut6t que de samarskite-(9. r--ishikawarle est presente a chaque endroit sauf la carridre Consolidated Number 2. l-e seul oxyde que parait m6tamicte h cet endroit est la ferroeolumbite, de stoechiom6trie AMrOu. Au site Stand Pipe Hill, nous avons trouvd le polycrase-(Y), oxyde d stoechiom6trie AB2O', avec Y dominant d la position A et Ti dominant a la position B, avec I'ishikawaite. La samarskite-(Y) serait absente des pegmatites 6tudi6es. (Traduit par la R6daction) Mots-cl6s: samarskite-(Y), ishikawarle, polycrase-(Y), district de pegmatites de Topsham, Maine. IxrRolucrroN Numerous bodies of granitic pegmatite intrude gneissic rocks in the Topsham pegmatite district, lo- cated in southern Maine, near the town of Topsham (Fig. 1). These pegmatites are generally tabular and commonly zoned. They range in thickness from 2 cm to at least 2 m (Shanin 1948). Wise & Frances (1992) have reported on the characteristics of granite pegmatites from the Brunswick pegmatite field in which the Topsham pegmatitesare located. Pegmatites in this district contain metamict, rare-earth-element- bearing complex Nb-Ta-Ti oxides. A single sample from Topsham, Maine has been analyzed, and identi- fied as samarskite-(Y) (Lane 1937); subsequently it has been generally assumed that all samples of a black, glassy, metamict mineral from this district consist of samarskite-(Y) (King & Foord 1994). Our purpose in I E -mai I add re ss: slhanson@binsham.mines.utah.edu