Exhumation of Ultrahigh-pressure metamorphic rocks in the Western Gneiss Region, Norway

This activity is used in an undergraduate igneous/metamorphic petrology course. In our department, the course is not required for a BS or BA, but it remains a popular elective course. The course typically has enrollments of ~15-20 students each year. Each lab section has up to ~10 students.

• Students should have taken a mineralogy course and have familiarity with petrographic microscopes.

• Students should have already begun developing the following skills: identification of metamorphic minerals in thin section, interpretation of metamorphic textures (inclusion-host relationships, shape-preferred orientation of minerals).
• Students should have been exposed to the following concepts: Pressure-Temperature paths, prograde and retrograde metamorphic reactions, polymorphic phase transformations, continental subduction and ultrahigh-pressure metamorphism. Students should also have some knowledge of fabrics (foliation, lineation) in rocks and ideally some knowledge of mineral compositional zoning.

This activity is currently completed toward the second half of our metamorphic petrology section. It functions as a stand-alone exercise, but builds on the skills developed in the previous metamorphic labs (introduction to metamorphic rocks, metamorphic textures, metamorphic reactions).

Students who complete this lab will gain experience "thinking like a geologist": using a suite of samples collected from a single field area to make interpretations of the area's metamorphic and tectonic history, and communicating their observations to their classmates. In doing so, students will learn to recognize UHP minerals (including coesite), and gain confidence interpreting disequilibrium textures associated with polymorphic phase transformations (coesite to quartz), exsolution, and the formation of symplectites. Students will also be able to practice making interpretations of inclusion-host-matrix textures to determine if minerals grew along prograde, peak, or retrograde portions of the P-T path. These skills will all support generation and interpretation of a P-T path traveled by the terrane during exhumation.

1. Identification and description of common metamorphic minerals and textures found in UHP terranes: This is a fundamental skill for success in the lab activity. Students will practice mineral ID and extend their knowledge to UHP minerals, beyond what students will have seen in earlier petrology labs.
2. Interpretation of mineral assemblages and textures to identify protolith, estimate P-T conditions of metamorphism, and determine relative timing of fabric formation: Students will build on previous experience in the course to determine protoliths, determine the metamorphic facies of each sample, and determine whether mineral growth was pre-, syn-, or post-kinematic.
3. Reading and plotting on a P-T diagram: Students will be asked to use mineral stability fields to help plot their samples on a P-T diagram, and to connect their observations to generate a reasonable P-T path for terrane exhumation. The prograde portion of the path is provided as a rate of heating/depth and crustal density so that students can calculate a prograde path starting at mid-crustal conditions.
4. Recognition of the differential preservation of UHP minerals in UHP terranes: Students will be asked to consider why certain samples preserve evidence of UHP metamorphism while others do not.

1. Synthesis of ideas: A large component of the lab entails students synthesizing their observations for 3-5 samples into a coherent picture of UHP terrane exhumation.
2. Formulation of hypotheses: For certain questions, students will be asked to propose different mechanisms by which textures could be formed in a sample, and to argue for more reasonable formation process.
3. Analysis of data: Students will be asked to look at features of thin sections and supplied element maps to determine when in the terrane's subduction-exhumation history minerals grew

This lab involves communication of information on rock textures in both written and visual form, Additionally, the lab is designed in a "jigsaw" format so that students will have the chance to communicate their observations to other students who have looked at different rocks. In this way, students will teach each other and be more invested in their own work
A major goal of this lab is to have students effectively communicate their observations as figures. We ask that students either create annotated sketches using pencil, or use cameras in the petrographic microscopes to generate annotated photomicrographs. In either case, this lab serves as an opportunity for students to continue developing visual communication skills while generating figures and figure captions to support their written answers to lab questions.

The goal of this lab activity is to provide students with direct contact with a suite of related rocks that have undergone ultrahigh-pressure (UHP) metamorphism associated with continental subduction. The samples include two UHP eclogites, one retrogressed eclogite, and two metasedimentary rocks (one with kyanite, one with sillimanite), all collected from Norway's Western Gneiss Region, a large UHP terrane. The shorter version of the lab focuses only on the two eclogites and the retrogressed eclogite, consistent with metamorphic petrology courses that investigate rocks with one protolith at a time.

Two formats

Individual option: more time is focused on mineral identification and sample observations. This option is better if the goal is for students to log more time identifying metamorphic minerals and making observations. This is also a makeup option for students that miss the jigsaw lab. Students work through the whole handout primarily on their own.

Jigsaw option- more time is focused on constructing overall P-T path for the region. This option is better if the goal is for students to practice taking observations from one location and relating them to the overall geology of the region. With the jigsaw format, students work individually and as a small group to investigate one of the rock samples in detail.They will become the experts for that sample and be responsible for teaching students from the other groups about their sample (individual accountability). This cooperative learning strategy encourages a deeper understanding of their sample, allows students to practice communicating with their peers in a Teaching as Learning role, and allows more time for the culminating analysis of the overall P-T path for the region.

Lab materials (Instructor-only files) -- educator-only file

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Fjortoft digital materials (Zip Archive 80.7MB Mar31 20)

Saltaneset digital materials (Zip Archive 71.9MB Mar31 20)

Svartberget digital materials (Zip Archive 86.5MB Mar31 20)

Svartberget #2 digital materials (Zip Archive 87.3MB Mar31 20)

Verpeneset digital materials (Zip Archive 83.3MB Mar31 20)

Instructors who have taught this lab near the beginning of a petrology course have provided feedback on how this was difficult and recommend it later in the course, or in a metamorphic petrology course. The nature of the UHP samples requires that students already have an understanding of subduction and fabrics, so that they can focus their energy on understanding the P-T paths and the unique setting these rocks must have formed in, given the minerals and fabrics present.

This would be a difficult lab for a mineralogy course, as the students need to be familiar with petrogenetic grids (or extra time would be needed to practice).

The assessment materials are embedded throughout the lab (see the teaching materials download above).

• Braathen, A., Nordgulen,Ø., Osmundsen, PT., Andersen, T.B., Solli, A., and Roberts, D., 2000, Devonian, orogen-parallel, opposed extension in the Central Norwegian Caledonides: Geology, 28:615–618
• Chopin, C., 1984, Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some consequences: Contributions to Mineralogy and Petrology, v. 86, p. 107–118.
• Cruciani, G., Franceschelli, M., Groppo, C., and Spano, M.E., 2012, Metamorphic evolution of non-equilibrated granulitized eclogite from Punta de li Tulchi (Variscan Sardinia) determined through texturally controlled thermodynamic modelling: Granulitized eclogite from Variscan Sardinia: Journal of Metamorphic Geology, v. 30, p. 667–685, doi:10.1111/j.1525-1314.2012.00993.x.
• Fossen, H., 2010, Extensional tectonics in the North Atlantic Caledonides: A regional view: Geological Society of London special Publications, 355:767–793
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• Hacker, B.R., Andersen, T.B., Johnston, S., Kylander-Clark, A.R.C., Peterman, E.M., Walsh, E.O., and Young, D., 2010, High-temperature deformation during continental-margin subduction and exhumation: The ultrahigh-pressure Western Gneiss Region of Norway: Tectonophysics, 480:149–171
• Hermann, J., Spandler, C., Hack, A., and Korsakov, A.V., 2006, Aqueous fluids and hydrous melts in high-pressure and ultra-high pressure rocks: implications for element transfer in subduction zones: Lithos, v. 92, p. 399–417.
• Holtz, F., Becker, A., Freise, M., and Johannes, W., 2001, The water-undersaturated and dry Qz–Ab–Or system revisited. Experimental results at very low water activities and geological implications: Contributions to Mineralogy and Petrology, v. 141, p. 347–357.
• Huang, W.L. and Wyllie, P.J., 1981, Phase relationships of S-type granite with H₂O to 35 kbar: muscovite granite from Harney Peak, South Dakota: Journal of Geophysical Research, v. 86, p. 10515–10529.
• Liu, Q., Jin, Z., and Zhang, J., 2009, An experimental study of dehydration melting of phengite-bearing eclogite at 1.5–3.0 GPa: Science Bulletin, v. 54, p. 2090–2100, doi:10.1007/s11434-009-0140-4.
• Mibe, K., Kanzaki, M., Kawamoto, T., Matsukage, K.N., Fei, Y.W., Ono, S., 2005. Second critical endpoint in the basalt–H2O system. AGU Fall Meeting (abstract), V33C-03.
• Smith, D.C., 1984, Coesite in clinopyroxene in the Caledonides and its implications for geodynamics: Nature, v. 310, p. 641–644.
• Tucker, R.D., Robinson, P., Solli, A., Gee, D.G., Thorsnes, T., Krogh, T.E., Nordgulen, Ø., and Bickford, M.E., 2004, Thrusting and extension in the Scandian hinterland, Norway: New U-Pb ages and tectonostratigraphic evidence: Am. J. Sci., 304:477–532.
• Wang, Y., Zhang, L., Zhang, J., and Wei, C., 2017, The youngest eclogite in central Himalaya: P–T path, U–Pb zircon age and its tectonic implication: Gondwana Research, v. 41, p. 188–206, doi:10.1016/j.gr.2015.10.013.
• Zhou, G., Zhang, J., Li, Y., Lu, Z., Mao, X., and Teng, X., 2019, Metamorphic Evolution and Tectonic Implications of the Granulitized Eclogites from the Luliangshan Terrane in the North Qaidam Ultrahigh Pressure Metamorphic Belt, NW China: New Constraints from Phase Equilibrium Modeling: Journal of Earth Science, v. 30, p. 585–602, doi:10.1007/s12583-019-0897-6.