Contributions to Mineralogy and Petrology

, Volume 57, Issue 3, pp 277–295 | Cite as

Isograd Patterns and Pressure-Temperature Distributions During Regional Metamorphism

  • Peter H. Thompson


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.


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The paper has benefited from critical reviews by H.J. Greenwood, M. Frey, E. Froese, B.W. Evans, V. Trommsdorff, E. Niggli, and E. Wenk. J. Hunsiker, E. Jaeger, and A. Streckeisen also contributed helpful criticism. I am grateful to E. Niggli for providing working space in the Mineralogisches-Petrographisches Institut, Universität Bern, and to Mr. and Mrs. Codoni (Casa Ohme, Cama) and their son Antonio for their hospitality during the summer of 1973. In addition, Antonio Codoni opened my eyes to the wonders of Al-silicate segregations. The financial support provided by an Overseas Postdoctorate Fellowhsip (1972–74) from the National Research Council of Canada and from J. M. Moore (N. R. C. Grant A 1506) (1975) is gratefully acknowledged. I wish to thank Robbie Lennox (Geological Survey of Canada) for her expert typing of the final draft. I would like to acknowledge with special emphasis Judy MacKay Thompson for her assistance in the field, for typing the paper numerous times, and for moral support throughout the project.


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Copyright information

© Springer-Verlag 1976

Authors and Affiliations

  • Peter H. Thompson
    • 1
  1. 1.Mineralogisches-Petrographisches InstitutUniversität BernBernSwitzerland

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