METAMORPHIC GEOLOGY OF THE POMFRET DOME AREA, EAST-CENTRAL VERMONT

The Pomfret Dome is an Acadian, north-trending, approximately 10 km long by 5 km wide antiformal structure within the Connecticut Valley-Gaspe Synclinorium in east-central Vermont. The dome is comprised of the Waits River (calcareous and metapelitic rocks), Standing Pond (mafic gneiss and amphibolite ± garnet), and Gile Mountain (metapelitic schist, quartzite) Formations.

Mineral assemblages in metapelitic rocks indicate a range in metamorphic grade from garnet zone around the periphery of the dome, through staurolite, staurolite + kyanite, to kyanite zone rocks in the core of the dome, with the local development of sillimanite zone assemblages. Geothermobarometric calculations for staurolite + kyanite and kyanite zone rocks suggest peak metamorphic conditions were approximately 600 °C and 7-8 kbar. Curved inclusion trails and discontinuities in inclusion trail patterns in garnet suggest syn-deformational garnet growth. Curved inclusion trails also occur in deformed staurolite and kyanite. This texture implies that deformation continued after growth of these porphyroblasts. Late stage chlorite and muscovite crosscut the crenulated foliation.

Garnets in the metapelitic rocks are almandine-rich and compositional profiles show decreasing spessartine contents and Fe/Mg ratios from cores to rims, features typically associated with growth zoning. X-ray maps reveal garnet compositions were modified along rims, mineral inclusion boundaries, and fractures. There is a marked decrease in grossular content of garnet with a concomitant increase in the pyrope and almandine components in these regions. However, there is little to no change in the ratio of the almandine/pyrope components. In some, but not all samples, the compositional discontinuity is accompanied by the presence of abundant fluid inclusions in the garnet, both along the garnet rim and in the garnet interior near mineral inclusion contacts. These observations suggest high-temperature interaction between garnet and fluid.