Progressive metamorphism of pelitic rocks: A lab assignment to facilitate translation from AFM space to P-T space

Jane Selverstone

University of New Mexico

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Initial Publication Date: June 11, 2008 | Reviewed: December 15, 2014 (see revision history: 2 events)

Summary

This is an example of a lab activity aimed at teaching students how to go from natural samples to AFM diagrams to P-T conditions of equilibration.

Metamorphic Processes, PTt Relationships, Metamorphic Rock Suites and ...(more) | College Lower (13-14), College Upper (15-16)

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Context

Audience

This exercise is designed for a sophomore or junior level petrology course that is required for majors.

Skills and concepts that students must have mastered

How the activity is situated in the course

The lab is designed to be completed in two 3-hour lab sessions, at the end of which the students will be able to:

Estimate the P-T conditions of equilibration of pelitic schists
Determine whether different samples are related to each other via changes in pressure and temperature or changes in bulk composition
Test assumptions about reaction histories.

The lab follows an in-class lecture in which we graphically derive the AFM projection and talk in general terms about pelitic schists.

Goals

Content/concepts goals for this activity

Students will see representative samples of pelitic rocks metamorphosed to greenschist, amphibolite, and low granulite (second-sillimanite) facies conditions. (My students look at high-pressure metapelites in another lab).
Students will understand that rocks of similar bulk composition develop dramatically different mineral assemblages in response to different sets of pressure-temperature conditions.
Students will understand that rocks with subtle differences in bulk composition will develop different mineral assemblages in response to the same pressure-temperature history.
Students will be able to translate between assemblages developed in pelitic rocks and those in mafic rocks metamorphosed under the same conditions. For example, staurolite-bearing assemblages in pelitic rocks develop in the amphibolite facies, and thus are consistent with hornblende-plagioclase assemblages but not actinolite-chlorite assemblages in interlayered mafic rocks. (We have already discussed the facies concept and done a lab on mafic rocks by the time we get to this assignment).
Students will be able to use their thin section and hand sample observations to determine AFM topologies. They will then use the AFM topologies to constrain the pressure-temperature conditions of final equilibration of each sample.
Students will develop and evaluate hypotheses to explain the coexistence of "too many minerals" on an AFM diagram. This goal is designed to blend understanding of thermodynamic concepts with interpretation of thin section textures.
Students will develop some appreciation for map and outcrop patterns, and for causes of sample heterogeneity in individual suites of rocks.
Students who choose to do the extra credit question will gain experience in constructing PTt paths from thin section observations, and in thinking about the tectonic implications of these paths.

Higher order thinking skills goals for this activity

This activity involves critical thinking and data analysis.

Other skills goals for this activity

Description of the activity/assignment

This lab assignment helps students to develop a detailed understanding of AFM diagrams and how they relate to pressure-temperature conditions of metamorphism of pelitic rocks. The lab is designed to be completed in two 3-hour lab sessions, at the end of which the students will be able to:

Estimate the P-T conditions of equilibration of pelitic schists
Determine whether different samples are related to each other via changes in pressure and temperature or changes in bulk composition
Test assumptions about reaction histories.

The lab follows an in-class lecture in which we graphically derive the AFM projection and talk in general terms about pelitic schists. The lab emphasizes going from real rocks to the phase diagrams to pressure-temperature space. I particularly emphasize extracting P-T information from pelitic schists using the AFM diagram (we revisit this qualitative approach after going through a quantitative thermobarometry exercise later in the course). I introduce the laboratory assignment via a PowerPoint presentation that is available from the Teaching Petrology website. My own version of this PowerPoint file includes several slides on the tectonic histories of the orogens from which my lab samples are derived (New England, Alps, New Mexico, etc.).
The lab as written is obviously not directly transferable to other users as it is tied to specific thin sections and hand samples. However, the section below on Adaptations (in the instructors' notes, see below) suggests ways for other instructors to achieve the same goals, using their own sample suites. The lab as included in this volume contains brief answers in order to show the nature of the samples that I use and the level of expertise that I expect of my students.
The AFM diagrams and P-T diagram are redrafted after Spear (1993). The AFM movie in the PowerPoint presentation is from Worley and Powell (1998), but similar movies can also be obtained from Spear (1999).

Determining whether students have met the goals

Students have successfully completed this exercise if they completely and accurately answer the questions and follow instructions given in the exercise handout.

More information about assessment tools and techniques.