A petrographic study of thermally metamorphosed pelitic rocks in the Burke area, northeastern Vermont

High-grade metamorphic rocks in the Burke area include staurolite-, andalusite-staurolite-, sillimanite-andalusite-staurolite-, sillimanite-garnet-staurolite-, sillimanite-garnet-potash feldspar (rare)-, kyanite-sillimanite-staurolite-, and kyanite-sillimanite-andalusite-staurolite-bearing assemblages. These rocks are interpreted as having been formed from low-grade schist and phyllite under load conditions higher than those characterizing normal hornfels aureoles. The various assemblages indicate the metamorphic trends produced as a result of increasing temperature. The one occurrence of a kyanite-sillimanite-andalusite-bearing assemblage suggests that these polymorphs were probably formed under closely similar conditions during thermal metamorphism but not necessarily simultaneously. Change of temperature is easier to envisage than change of pressure during the crystallization of this assemblage. Temperature and pressure conditions must have hovered near to the assumed triple point of the alumino-silicates.