FIRE 4 Geographic constraints

The FIRE 4 profile runs for 249 km from Ranua in the south to Sirkka in the north. The southern end of the FIRE 4 profile transects the northern part of the Archean Ranua complex (Hölttä et al., 2012) and runs towards north across the Paleoproterozoic Peräpohja belt and dissects the entire Proterozoic Central Lapland granitoid complex (CLGC). On the northern flank of the CLGC the profile first transects the presumably Archean metasedimentary rocks of the Vuojärvi group (Lahtinen et al., 2015) and then terminates in the supracrustal rocks of the Central Lapland greenstone belt (CLGB).

FIRE 4 Geological and tectonic division

The upper crust along the FIRE 4 profile has been divided into five parts from south to north: the Ranua (1), Peräpohja (2), Ounas (3), Vuojärvi (4), and Kittilä (5) domains.

Geological description of the Ranua domain (FIRE 4 CMP 0000–1700)

Geological subdivision of the dominantly Neoarchean (ca. 2.7 Ga) Pudasjärvi complex (e.g., Nironen et al., 2002) has been problematic as the region is poorly exposed and remains relatively unstudied (Sorjonen-Ward and Luukkonen, 2005; Hölttä et al., 2012).

The Ranua domain (CMP 0000–1700) coincides with the northern part of the Pudasjärvi complex, which was coined as the Ranua complex by Hölttä et al. (2012), and separated from the partly Paleoarchean (Mutanen and Huhma, 2003) Siurua complex further south. The Siurua complex also hosts the oldest known rock in the Fennoscandian Shield, the 3.5 Ga Siurua trondhjemite gneiss (Mutanen and Huhma, 2005). Majority of the Ranua complex consists of diverse migmatitic tonalite-trondhjemite-granodiorite (TTG) series gneisses with some minor amphibolites and greenstones (Hölttä et al., 2012)

Considerable part of the FIRE 4 profile within the Ranua complex transects the ca. 2.7 Ga Ranua diorite that is a massive medium-grained metaigneous rock (Mutanen and Huhma, 2003). The northern contact of the Ranua complex towards the Peräpohja schist belt is characterized by early Paleoproterozoic (ca. 2.44 Ga) intrusions of the Portimo layered igneous complex of the Tornio-Näränkävaara belt related to the rifting of the Archean Karelian craton (Iljina and Hanski, 2005).

FIRE 4 References

Ahtonen, N., Hölttä, P., and Huhma, H., 2007. Intracratonic Palaeoproterozoic granitoids in northern Finland: prolonged and episodic crustal melting events revealed by Nd isotopes and U-Pb ages on zircon. Bulletin of the Geological Society of Finland 79, 143–174.

Hanski, E. and Huhma, H., 2005. Central Lapland Greenstone belt. In: Lehtinen, M., Nurmi, P.A. & Rämö, O.T. (eds.) The Precambrian Geology of Finland − Key to the Evolution of the Fennoscandian Shield. Developments in Precambrian Geology 14, Amsterdam: Elsevier, 139–194.

Hölttä, P., Heilimo, E., Huhma, H., Juopperi, H., Kontinen, A., Konnunaho, J., Lauri, L., Mikkola, P., Paavola, J., and Sorjonen-Ward, P., 2012. Archean complexes of the Karelia Province in Finland. Geological Survey of Finland, Special Paper 54, 9−20.

Huhma, H., 1986. Sm-Nd, U-Pb, and Pb-Pb isotopic evidence for the origin of the Early Proterozoic Svecokarelian crust in Finland. Bulletin of the Geological Survey of Finland 337, 48 p.

Iljina, M. and Hanski, E., 2005. Layered mafic intrusions of the Tornio-Näränkävaara belt. In: Lehtinen, M., Nurmi, P.A. & Rämö, O.T. (eds.) The Precambrian Geology of Finland − Key to the Evolution of the Fennoscandian Shield. Developments in Precambrian Geology 14, Amsterdam: Elsevier, 101– 137.

Lahtinen, R., Huhma, H., Lahaye, Y., Jonsson, E., Manninen, T., Lauri, L.S., Bergma, S. , Hellström, F., Niiranen, T., and Nironen, M., 2015. New geochronological and Sm–Nd constraints across the Pajala shearzone of northern Fennoscandia: Reactivation of a Paleoproterozoic suture. Precambrian Research 256, 102–119.

Lauri, L.S., 2013. The myth of “the 1.8 Ga Central Lapland Granitoid Complex” unravels into discrete magmatic events during a 350 Ma period from 2.13 Ga to 1.76 Ga. Geological Survey of Finland, Report of Investigation 198, 105–107.

Lauri, L.S., Andersen, T., Räsänen, J., and Juopperi, H., 2012. Temporal and Hf isotope geochemical evolution of southern Finnish Lapland from 2.77 Ga to 1.76 Ga. Bulletin of the Geological Society of Finland 84, 121–140.

Lehtonen, M., Airo, M-L., Eilu, P., Hanski, E., Kortelainen, V., Lanne, E., Manninen, T., Rastas, P., Räsänen, J., and Virransalo, P., 1998. Kittilän vihreäkivialueen geologia, Lapin vulkaniittiprojektin loppuraportti. Summary: The stratigraphy, petrology and geochemistry of the Kittilä greenstone area, northern Finland. A report of the Lapland Volcanite Project. Geological Survey of Finland, Report of Investigation 140, 144 p.

Mutanen, T. and Huhma, H., 2003. The 3.5 Ga Siurua trondhjemite gneiss in the Archaean Pudasjärvi Granulite Belt, northern Finland. Bulletin of the Geological Society of Finland 75, 51–68.

Nironen, M., Lahtinen, R., and Koistinen, T., 2002. Suomen geologiset aluenimet – yhtenäisempään nimikäytäntöön, Summary in english: Subdivision of Finnish bedrock – an attempt to harmonize terminology. Geologi 54, 8–14.

Patison, N. L., Korja, A., Lahtinen, R., Ojala, V. J., and the FIRE Working Group 2006. FIRE seismic reflection profiles 4, 4A and 4B: Insights into the Crustal Structure of Northern Finland from Ranua to Näätämö. Geological Survey of Finland, Special Paper 43, 161–222.

Ranta, J-P., Lauri, L.S., Hanski, E., Huhma, H., Lahaye, Y., and Vanhanen, E., 2015. U-Pb and Sm-Nd isotopic constraint on the evolution of the Paleoproterozoic Peräpohja Belt, northern Finland. Precambrian Research 266, 246–259.

Räsänen, J., Huhma, H., 2001. U–Pb datings in the Sodankylä schist area, centralFinnish Lapland. Geological Survey of Finland, Special Paper 33, 153–188.

Sorjonen-Ward, P. and Luukkonen, E.J., 2005. Archean rocks. In: Lehtinen, M., Nurmi, P.A. & Rämö, O.T. (eds.) The Precambrian Geology of Finland − Key to the Evolution of the Fennoscandian Shield. Developments in Precambrian Geology 14, Amsterdam: Elsevier, 19−99.

Väänänen, J., 2004. Sieppijärven ja Pasmajärven kartta-alueiden kallioperä. Summary: Pre-Quaternary rocks of the Sieppijärvi and Pasmajärvi map-sheet areas. Suomen geologinen kartta 1:100 000 : kallioperäkarttojen selitykset lehdet 2624 ja 2642. Espoo: Geological Survey of Finland. 55 p.