A PRELIMINARY REPORT ON THE METAMORPHIC GEOLOGY OF THE POMFRET DOME AREA, VERMONT

The Pomfret Dome, a 10 km long by 5 km wide, north-trending antiformal structure of Acadian age, lies within the Connecticut Valley–Gaspe Synclinorium (CVGS) in east-central Vermont. The CVGS in this area consists predominantly of the Waits River, Standing Pond, and Gile Mountain Formations. In the Pomfret Dome area, the Waits River Formation consists of interlayered calc-schists, graphitic metapelites, and marbles, in various proportions. The Standing Pond Formation, thought to be of volcanic origin, consists primarily of a well-foliated biotite, hornblende, plagioclase, +/- garnet gneisses with subordinate amounts of garnet-bearing garbenschiefer. Locally a metafelsite is associated with the more mafic metavolcanic rocks. The rocks mapped as Gile Mountain Formation consist of metapelites, metapsammites, and quartzites.

Mineral assemblages in the metapelitic rocks indicate a range in metamorphic grade from garnet zone to sillimanite zone. We mapped staurolite-in, kyanite-in, and sillimanite-in isograds that are dominantly north trending and indicate an increase in metamorphic grade towards the center of the dome. Metamorphism was accompanied by a period of deformation that resulted in the development of a pervasive crenulation cleavage. Inclusion textures in garnet are complex but suggest multiple phases of garnet growth, some of which accompanied deformation. Textural relationships, such as crenulated inclusion trails and broken or folded crystals, suggest syn- to post-deformational growth of staurolite, kyanite, and plagioclase porphyroblasts.