FIRE 4A Geographic constraints

The FIRE 4A profile runs for 319 km in a southwest to northeast direction from Sirkka to Näätämö across the areas of central and northeastern Lapland (Patison et al., 2006). The profile commences in the Kittilä suite metavolcanic rocks and crosses over the older formations of the Central Lapland Greenstone Belt (Lehtonen et al., 1998) into the Lapland Granulite Belt (Meriläinen, 1976; Hörmann et al., 1980) and further into the diverse Archean and Proterozoic rocks of the Inari Area (Nironen et al., 2002).

FIRE 4A Geological and tectonic division

The upper crust along the FIRE 4A profile has been divided into three parts: Kittilä (1), Ivalo (2), and Inari (3) domains.

FIRE 4A Tectonic overview

The tectonic history of the upper crust along the FIRE 4A profile has been interpreted in the context of extension and prolonged Paleoproterozoic rifting of the Archean basement from 2.5 to 2.0 Ga and the following compressional orogenic phase that started at ca. 2.0–1.89 Ga and took place in two, at least partly overlapping, stages (Ward et al., 1989; Sorjonen-Ward et al., 1997; Patison et al., 2006; Hölttä et al., 2007).

The first orogenic stage was dominated by northeastward compression related to the Svecofennian orogeny that accreted the Paleoproterozoic supracrustal formations of the Central Lapland greenstone belt (CLGB) onto the Archean basement. The second stage is characterized by deformation related to the overthrusting of the Lapland granulite belt towards present day southwest (Hölttä et al., 2007).

The resulting structural framework recorded by the rocks in the Central Lapland region is rather complicated and makes interpretation of the major tectonic features challenging. Archean rocks are most likely underlying most of the FIRE 4A profile but are exposed only in the northeasternmost part of it (Patison et al., 2006). The juvenile Lapland granulite belt and the reworked Inari area have also been proposed to form a part of the orogenic core of the Paleoproterozoic Lapland–Kola orogen between the Archean Kola and Karelian cratons (Daly et al., 2006).

The deeper contacts between these orogenic blocks are somewhat debatable but it is evident that the CLGB is separated from the Lapland granulite belt along a large-scale listric overthrust (the Tanaelv zone) that dips to the northeast (Daly et al., 2006; Patison et al., 2006). The relationship between the Inari area and the Lapland granulite belt has been interpreted either as a northeastward subduction of the granulite belt under the Inari area (Daly et al., 2006 and references therein) or as a jagged margin preserving the suture zone between the Karelian and Kola cratons (Patison et al., 2006).

FIRE 4A References

Daly, S.J., Balagansky, V.V., Timmerman, M.J., and Whitehouse, M.J., 2006. The Lapland-Kola orogeny: Palaeoproterozoic collision and accretion of the northern Fennoscandian lithosphere. Geological Society of London, Memoirs 32, 579–598.

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.

Heilimo, E., Halla, J., Lauri, L.S., Rämö, O.T., Huhma, H., Kurhila, M., and Front, K., 2009. The Paleoproterozoic Nattanen-type granites in norhtern Finland and vicinity – a postcollisional oxidized A-type suite. Bulletin of the Geological Society of Finland 81, 7–38.

Hölttä, P., Väisänen, M., Väänänen, J., and Manninen, T., 2007. Paleoproterozoic metamorphism and deformation in Central Lapland, Finland. Geological Survey of Finland, Special Paper 44, 7–56.

Hörmann, P.K., Raith, M., Raase, P-. Ackermand, D., and Seifert, F., 1980. The granulite complex of Finnish Lapland: petrology and metamorphie conditions in the Ivalojoki - Inarijärvi area. Bulletin of the Geological Survey of Finland 308, 95 p.

Korsman, K., Koistinen, T., Kohonen, J., Wennerström, M., Ekdahl, E., Honkamo, M., Idman, H., Pekkala, Y., (comp.) 1997. Bedrock Map of Finland, 1: 1 000 000. Geological Survey of Finland.

Marker, M., Henkel, H., and Lee, M.K., 1990. Combined gravity and magmentic modelling of the Tanaelv and Lapland Granulite Belts, POLAR profile, northern Baltic. In: Freeman, R., Giese, P., Mueller, St. (eds.) The European Geotraverse: Integrative studies: results from the Fifth Earth Science Study Centre, Rauischholzhausen, Germany, 26 March – 7 April, 1990. Strasbourg: European Science Foundation, 67–76.

Merilänen, K., 1976. The granulite complex and adjacent rocks in Lapland, northern Finland. Bulletin of the Geological Survey of Finland 281, 129 p.

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.

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.

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.

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.

Sorjonen-Ward, P., Nironen, M., and Luukkonen, E. 1997. Greenstone Associations in Finland. In: Maarten de Wit & Lewis Ashwal (eds.) Greenstone Belts. Clarendon Press, London, 677–698.

Tuisku, P. and Huhma, H., 2006. Evolution of migmatitic granulite complexes: Implications from Lapland Granulite Belt, part II: isotopic dating. Bulletin of the Geological Society of Finland 78, 143–175.

Tuisku, P., Mikkola, P., and Huhma, H., 2006. Evolution of migmatitic granulite complexes: implications from Lapland Granulite belt, part I: metamorphic geology. Bulletin of the Geological Society of Finland 78, 71–105.

Ward, P., Härkönen, I., Nurmi, P., and Pankka, H. S. 1989. Structural studies in the Lapland greenstone belt, northern Finland and their applications to gold mineralizations. Geological Survey of Finland, Special Paper 10, 71–78.