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Contact metamorphic rocks of Orijärvi

Finland

Photo from 1908 taken by Pentti Eskola himself during his field work in the Orijärvi area. As Eskola himself stated in his 1914 publication, the image shows: "Folded layers of limestone intercalated with leptite (now fine-grained paragneiss) and amphibolite. Hepolahdensaari, Orijarvi. The photograph was taken from the West." Note the hammer for scale in the photo (GTK, Vanhatkuvat / GTK, old photographs).

Photo from 1908 taken by Pentti Eskola himself during his field work in the Orijärvi area. As Eskola himself stated in his 1914 publication, the image shows: “Folded layers of limestone intercalated with leptite (now fine-grained paragneiss) and amphibolite. Hepolahdensaari, Orijarvi. The photograph was taken from the West.” Note the hammer for scale in the photo (GTK, Vanhatkuvat / GTK, old photographs).

Geological Period

Paleoproterozoic (Orosirian)

Main geological interest

History of geosciences
Igneous and Metamorphic petrology

Location

Orijärvi, Finland
60°13’00”N, 023°32’00”E

Photo from 1908 taken by Pentti Eskola himself during his field work in the Orijärvi area. As Eskola himself stated in his 1914 publication, the image shows: “Folded layers of limestone intercalated with leptite (now fine-grained paragneiss) and amphibolite. Hepolahdensaari, Orijarvi. The photograph was taken from the West.” Note the hammer for scale in the photo (GTK, Vanhatkuvat / GTK, old photographs).

Study of these rocks led to the foundation of the metamorphic facies concept, used in nearly all metamorphic studies.

Eskola (1920) introduced the concept of metamorphic facies to the petrological community as a product of his work on the metamorphic rocks of the Orijärvi region (Eskola, 1915). This work took inspiration from similar studies in Norway by Goldschmidt (1911), and represented a conceptual breakthrough to treat rocks as chemical systems that obeyed the phase rule and possessed a mineral association as an indication of metamorphic grade (Evans, 2007). The metamorphic facies concept is now one of the central concepts in metamorphic petrology, and the classification provides a common language to discuss metamorphic rocks by specialists and non-specialists alike.

Cordierite gneiss xenolith (20 cm across) in the c. 1.89 Ga Orijärvi granodiorite.

Cordierite gneiss xenolith (20 cm across) in the c. 1.89 Ga Orijärvi granodiorite.

  • Geological description

The Orijärvi area forms part of the Southern Svecofennia terrane in southern Finland. This terrane is one of several terranes that were collectively accreted to the Fennoscandian (aka Baltic) shield during the long-lived Svecofennian orogen at c. 1.96-1.77 Ga, during formation of the Nuna (aka Columbia) supercontinent (Kara et al., 2018). Rocks in the Orijärvi area mainly comprise volcanic rocks with sedimentary intercalations and are thought to have formed in an arc rift setting. These rocks were folded and regionally metamorphosed to amphibolite-facies conditions of 3 kbar and 650 °C at c. 1.89-1.88 Ga as part of the Svecofennian orogen (Schneiderman and Tracy, 1991). Synorogenic intrusion of granodiorite caused contact metamorphism at the same or lower pressure-temperature conditions (Eskola, 1915). Metasedimentary rocks are composed dominantly of quartz, feldspar, muscovite, biotite, cordierite, hornblende and garnet. Metavolcanic rocks mainly consist of hornblende, cummingtonite and plagioclase and subordinate biotite and ilmenite. In skarn zones associated with the contact aureole of the granodiorites, cordierite-bearing orthoamphibolite gneisses also occur. These chemically-unusual rock types are attributed to both alteration of the protolith prior to regional metamorphism and metasomatism during contact metamorphism (Schneiderman and Tracy, 1991).

  • Scientific research and tradition

The site has continued to be studied as the associated mineral assemblages are relatively rare (e.g. Schneiderman and Tracy, 1991). However, the main legacy from analysis of metamorphic rocks in Orijärvi is the development of the metamorphic facies concept, which is taught on every metamorphic petrology course in the world.

Example modern-day metamorphic facies diagram. P-P = prehnite-pumpellyite.

Example modern-day metamorphic facies diagram.
P-P = prehnite-pumpellyite.

  • Reference

Eskola, P. (1914) ‘On the petrology of the Orijärvi region in southwestern Finland’, Bulletin de la Commission geologique de Finlande, 40, p. 277.

Eskola, P. (1915) ‘On the relations between the chemical and mineralogical composition in the metamorphic rocks of the Orijarvi region’, Bulletin de la Commission geologique de Finlande, 44, pp. 109–145.

Eskola, P. (1920) ‘The mineral facies of rocks’, Norsk Geologisk Tidsskrift, 6, pp. 143–194.

Evans, B.W. (ed.) (2007) Landmark Papers 3: Metamorphic Petrology. Twickenham, UK: Mineralogical Society of Great Britain and Ireland.

Goldschmidt, V.M. (1911) Die kontaktmetamorphose im Kristianiagebiet. Kristiania: In kommission bei J. Dybwad (Videnskapsselskapets skrifter. I. Mat.-naturv. klasse 1911, no. 1).

Kara, J. et al. (2018) ‘1.90-1.88Ga arc magmatism of central Fennoscandia: geochemistry, U-Pb geochronology, Sm-Nd and Lu-Hf isotope systematics of plutonic-volcanic rocks from southern Finland’, Geological Acta, 16(1), pp. 1–XIV.

Schneiderman, J.S. and Tracy, R.J. (1991) ‘Petrology of orthoamphibole-cordierite gneisses from the Orijärvi area, southwest Finland’, American Mineralogist, 76(5–6), pp. 942–955.

  • Author(s)

Owen Weller.
University of Cambridge. United Kingdom.

Richard White.
University of St Andrews. United Kingdom.

Richard Palin.
University of Oxford. United Kingdom.

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