Metastability and Metamorphism
(reading a metamorphic petrology paper from the literature)
I use a problem set as a way to introduce students (mostly sophomores) to reading a technical article
from the literature. The paper is Austrheim, H, 1987, Eclogitization of lower crustal granulites by
fluid migration through shear zones, Earth and Planetary Science Letters 81:221-232. The paper describes
Grenville-age mafic and anorthositic granulites in Norway re-metamorphosed under eclogite facies conditions
during the Caledonian. The granulite-facies assemblages persist metastably, and only become eclogite
assemblages in or near shear zones where fluids allow the granulite 
eclogite reactions to occur.
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Granulite-facies mafic gneiss (lighter) cross-cut by an eclogite-facies shear zone
(darker) on Hølsnoy, Bergen Arcs, Norway. Note that the eclogite-facies mineralogy
(e.g. omphasite +garnet +kyanite +clinozoicite) extends several cm from the shear zone,
illustrating the role of the shear zone as a fluid conduit. Metastable granulite-facies
assemblages (e.g. plagioclase +diopside +garnet +orthopyroxene) persist through Caledonian
eclogite-facies metamorphism. A surprising insight for students is that during the
Caledonian the granulite was metastable through eclogite-facies conditions, but today at
the outcrop both the granulite and the eclogite are metastable.
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I try to have this problem set serve a number of purposes. First, I ask the students to answer a number
of guiding questions so that they work through the paper and look up terms as needed. Some of the questions
are directly from the text, and others more are open-ended. I wrap-up the problem set with a 'virtual field
trip' of the area discussed in the paper. This Powerpoint works out much better than similar ones I do in this
class, because the students have all read about the area and have tried to visualize the field relationships
already. The virtual field trip typically leads into interesting discussions about tectonics and metamorphism,
polymetamorphism, the meaning of facies, how petrology is done in the field, and how metastability is necessary
in order to interpret metamorphic history.
Article Describing this exercise:
Peck, WH, 2004, Teaching metastability in Petrology using a guided reading from
the primary literature: Journal of Geoscience Education, v. 52, p. 284-288.
click here for PDF version of this article.
Virtual Field Trip Powerpoint
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