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 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 domains:

  • Kittilä (CMP 9920–13000) (1),
  • Ivalo (CMP 13000–19000) (2), and
  • Inari (CMP 19000–22700) (3) domains.
Geological description of the Kittilä domain (FIRE 4A CMP 9920–13000)

The Kittilä domain (CMP 9920–13000) corresponds to the Paleoproterozoic metasedimentary and -volcanic sequences of the Central Lapland greenstone belt (CLGB), which are at least partly correlated with the rocks of the Peräpohja schist belt (Lehtonen et al., 1998; Lahtinen et al., 2015 and references therein) and share a similar geological history.

The rocks of the ca. 2.0 Ga Kittilä suite (a.k.a. the Kittilä allochton) form a group of dominantly metavolcanic rocks with diverse geochemical signatures that are inferred to originate in a plume environment and, in contrast to the generally extensional tectonic regime, to have been emplaced in a convergent tectonic event on top of the rift-related supracrustal sequences (Lehtonen et al., 1998; Hanski and Huhma, 2005; Patison et al., 2006; Lahtinen et al., 2015).

The southern part of the FIRE 4A profile commences in and crosses over the metavolcanic rocks of the Kittilä suite just north of the Sirkka (Kittilä) shear zone that is interpreted to be the thrust surface of the allochtonous metavolcanic units (Patison et al., 2006). Further in the northeast, the profile crosscuts some of the autochtonous/parautochtonous units of the CLGB grouped collectively as the Vuotso complex that comprises a heterogeneous amalgamation of mostly Paleoproterozoic metasedimentary rocks deformed and metamorphosed during the overthrusting of the Lapland granulite belt over the CLGB.

Geological description of the Ivalo domain (FIRE 4A CMP 13000–19000)

The Ivalo domain (CMP 13000–19000) encompasses the metasedimentary and enderbitic rocks of the Lapland granulite belt and the heterogeneous metamorphic rocks of the Tanaelv zone (Marker et al., 1990) that is inferred to represent the approximate fault surface along which the belt was thrusted onto the Central Lapland rocks of the Kittilä domain in the southwest (Meriläinen, 1976; Hörmann et al., 1980; Patison et al., 2006).

The granulite belt has been interpreted as the root of a juvenile arc system related to the Lapland-Kola orogeny (Daly et al., 2006). The metasedimentary granulitic rocks of the LGB represent an arc-related greywacke basin and the enderbitic rocks represent arc-magmas intruded into the metasedimentary rocks at around 1.92–1.90 Ga (Tuisku et al., 2006; Tuisku and Huhma, 2006).

Geological description of the Inari domain (FIRE 4A CMP 19000–22700)

The Inari domain (CMP 19000–22700) represents the margin of the Archean Kola craton that was reworked in the Lapland-Kola orogeny at ca. 1.93 Ga (Daly et al., 2006). The region is rather poorly studied but consists of an amalgamation of reworked Archean basement rocks and Paleoproterozoic, dominantly metasedimentary, units (e.g., Meriläinen, 1976).

Due to several differing tectonic and geological interpretations based on limited data, unified nomenclature and division of geological units has not been established. It seems, however, that the southwestern part of the domain consists dominantly of Paleoproterozoic rocks, whereas the northeastern/eastern parts are dominantly Archean.

Northeastern/eastern part of the domain comprises the Archean Inarijärvi or Inari complex (Korsman, 1997; Heilimo et al., 2009) also interpreted as a part of the Sørvaranger terrain and named as the Garsjøen gneiss complex (Sorjonen-Ward and Luukkonen, 2005). In the latter scheme, the southwestern part of the Inari domain is interpreted as the dominantly Archean Inari terrain (or terrane; Daly et al., 2006) and comprises the ca. 2.5 Ga Suorre–Tievja gneiss complex (Sorjonen-Ward and Luukkonen, 2005) or a tectonic package of mixed Archean and Proterozoic units (Daly et al., 2006). These two terrains are separated by the metavolcanic rocks of the Opukasjärvi group that belongs to the Polmak–Pasvik–Pechenga(–Imandra–Varzuga; Daly et al., 2006) belt (Sorjonen-Ward and Luukkonen, 2005).

The northeastern part of the FIRE 4A profile runs parallel and also briefly crosscuts the Vainospää granite that belongs to the ca. 1.8 Ga Nattanen granite suite (Heilimo et al., 2009).

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.