Isograd Patterns and Pressure-Temperature Distributions During Regional Metamorphism

In rocks undergoing regional metamorphism, pressure and temperature gradients are not everywhere parallel, that is, isotherms are not everywhere parallel to isobars. Temperature gradients vary across the metamorphic terrain and decrease with increasing temperature. The angle between the temperature and the pressure gradients and the magnitude of the temperature gradient must be considered when isograd patterns are used to determine the distribution of pressure and temperature in the rocks when the isograds formed.In Val Mesolcina, southeastern Switzerland, post-tectonic isograds corresponding to mineral dehydration reactions dip steeply northward intersecting the more gently dipping isograd corresponding to the kyanite-andalusite transition. Distributions of pressure and temperature compatible with the isograd pattern have values of α (the angle between the pressure and the temperature gradients) of 50–70° and temperature gradients of 12–15 °C/km. These results indicate that at or near the peak of the Tertiary (Lepontine) phase of Alpine metamorphism isothermal surfaces in these rocks dipped steeply northward and isobaric surfaces were horizontal.The westward divergence of Lepontine mineral zone boundaries can be explained by decreasing values of α; that is, the flattening toward the west of a steep-sided thermal dome present in the eastern part of the Lepontine terrain at the time of metamorphism.