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 Ounas domain (FIRE 4 CMP 3100–7700)

The Ounas domain (CMP 3100–7700) covers the FIRE 4 transect across the Paleoproterozoic Central Lapland granitoid complex (CLGC) that comprises a diverse suite of granitic, migmatitic, and supracrustal rocks generated and emplaced over roughly 350 million years from 2.13 to 1.76 Ga (Huhma, 1986; Ahtonen et al., 2007; Lauri, 2013).

The relatively un-radiogenic Sm–Nd and Lu–Hf isotope compositions of the granitic rocks (Huhma, 1986; Lauri et al., 2012) have been interpreted as evidence of Archean source and, by extension, of Archean middle and lower crust underlying the entire CLGC (Patison et al., 2006). Due to the lack of detailed mapping, distribution of different age groups and geochemical types within the CLGC are still eluding and require further research (Lauri, 2013). Nevertheless, it is apparent that the CLGC has a rather complicated crustal architecture, hosts also a substantial amount of non-granitic lithologies, and is a relatively shallow formation (Patison et al., 2006).

The northern edge of the CLGC is rimmed by the migmatitic supracrustal Sieppijärvi suite (Väänänen, 2004 and references therein) that was previously interpreted to be Archean in age (known as the Venejärvi complex). Supracrustal units similar to the Sieppijärvi suite (i.e., the Räväsjärvi suite) are also embedded as inclusions within the CLGC and the relationship of the suite to the Archean Vuojärvi group (Lahtinen et al., 2015) on its northern side is not entirely clear.

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.