FIRE 2 Tectonic overview

The upper crust along the FIRE 2 profile consists of a diverse amalgamation of Proterozoic crustal blocks that have been accreted towards the present-day north-northeast during the Paleoproterozoic Svecofennian orogeny (Lahtinen et al., 2005). From north to south these blocks comprise the Central Finland granitoid complex (CFGC), the Arc complex of Western Finland (ACWF) that includes the Tampere and Pirkanmaa belts, and the Häme belt that makes up the northern part of the Arc complex of southern Finland (ACSF; Nironen et al., 2006). The Häme belt was intruded by the late to post-orogenic (1.85–1.82 Ga) alkali feldspar granites of the southern Finland granite suite (Huhma, 1986, Suominen, 1991, Kurhila et al., 2005; 2010; Kurhila, 2011).

The tectonic fabric of the upper crust along FIRE 2 has been interpreted to result from two-stage regional structural evolution: synorogenic (1.89–1.88 Ga) compression and late orogenic (1.87–1.85 Ga) extension (Nironen et al., 2006). The ACWF has been interpreted to represent a pop-up structure wedged between the ACSF and the CFGC during the main compressional stage of the orogeny (Nironen et al., 2006).

The FIRE 2 profile is 216 km long and it runs in a generally north to south direction across the southern part of the Central Finland granitoid complex (CFGC), the arc complex of western Finland, and the northern part of the arc complex of southern Finland. In its southernmost (CMP 7463–8999) part the profile turns into a northeast–southwest direction.

The upper crust along the FIRE 2 profile has been divided into four domains:

  • The Keitele (CMP 0000–3800) (1),
  • Tampere (CMP 3800–4200) (2),
  • Pirkanmaa (CMP 4200–6860) (3), and
  • Häme (CMP 6860–8999) (4) domains.

The Keitele domain (CMP 0000–3800) corresponds roughly to the area of the Central Finland granitoid complex (CFGC) that comprises a variety of orogenic, dominantly I-type granitic rocks and minor associated supracrustal rocks with a relatively evolved geochemical character (Nironen, 2005).

Majority of the CFGC consists of 1.89 to 1.87 Ga calc-alkaline granodiorites and granites synkinematic with the major orogenic stage corresponding to the collision of the Southern Finland arc complex (Bergslagen microcontinent and the Häme belt of the southern Svecofennia sub-province; Lahtinen et al., 2016) with the Western Finland arc complex (Keitele and parts of the presumed Bothnian microcontinents or the central Svecofennia sub-province; Lahtinen et al., 2016) at ca. 1.9 Ga.

The 1.88 to 1.86 Ga postkinematic granitic rocks with more alkaline geochemical character (Nironen, 2005) form smaller, often discordant, intrusions with transitional I- to A-type characteristics and can be divided into three geochemical types (types 1, 2, and 3; e.g., Elliott, 2003 and references therein).

The southern part of the CFGC crosscut by the FIRE 2 profile comprises a diverse lithology of dioritic, granitic, and supracrustal rocks (Nironen et al., 2006). The supracrustal Hirsilä belt in the southern part of the domain is lithologically similar to the Tampere belt further south (Kähkönen, 2005).

The Tampere domain (CMP 3800–4200) corresponds to the Tampere belt (Kähkönen, 2005), which is a narrow (10–20 km) E–W -oriented sliver comprised of ca. 1.90–1.89 Ga greywackes and medium-grade metavolcanic rocks with mid-ocean-ridge-basalt (MORB) and within-plate-lava (WPL) geochemical affinities adhered to the southern margin of the Central Finland granitoid complex (CFGC).

The eastern part of the greenstone belt is contiguous with a series of gabbroic intrusions related to the orogenic granitic rocks of the CFGC (Nironen, 2005). The Tampere belt is interpreted as a volcanic arc that was subducted towards the present day north under the CFGC. Structurally the belt is interpreted as a syncline or a synform with steep to nearly vertical foliation (Kähkönen, 2005).

The predominantly metasedimentary migmatitic rocks of the NE–SW -oriented Pirkanmaa belt (Kähkönen, 2005) make up the bulk of the Pirkanmaa domain (CMP 4200–6860).

The migmatites have been intruded by associated 1.89–1.87 Ga granitic rocks mostly of granodioritic composition (Nironen, 2005). The Pirkanmaa belt is interpreted as a subduction zone complex for the Tampere belt consisting of fore-arc sediments with material mostly derived from the Tampere arc and metamorphosed at medium- to high-grade during subduction (Kähkönen, 2005).

The Häme domain (CMP 6860–8999) corresponds to the Häme belt, which represents relatively juvenile island arc -type supracrustal material that was squeezed between two 2.1–2.0 Ga microcontinental blocks (Bergslagen and Keitele) around 1.84 Ga.

The Häme belt (Kähkönen, 2005) belongs to the southern Svecofennian sub-province (e.g., Lahtinen et al., 2016) or the Arc complex of Southern Finland (Korsman, 1997). Bulk of the Häme belt comprises metasedimentary and metavolcanic units with a relatively juvenile geochemical affinity (Kähkönen, 2005; Lahtinen et al., 2005).

Towards the south, the metasediments have been strongly migmatized by the 1.84–1.81 Ga, peraluminous, S-type lateorogenic granites (cf., Nironen, 2005; Kukkonen and Lauri, 2009; Kurhila et al., 2010). The granites were generated by partial melting and migmatization of the same metasedimentary rocks they are intruding, which possibly happened in response to extensional collapse during the final stages of the Svecofennian orogenic evolution (Kurhila, 2011).

Elliott, B.A. 2003. Petrogenesis of the Post-kinematic Magmatism of the Central Finland Granitoid Complex II; Sources and Magmatic Evolution. Journal of Petrology 44, 1681–1701.

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.

Kähkönen, Y., 2005. Svecofennian supracrustal rocks. In: Lehtinen, M., Nurmi, P.A., and Rämö, O.T. eds. Precambrian Geology of Finland – Key to the Evolution of the Fennoscandian Shield. Developments in Precambrian Geology 14, 343–406

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.

Kukkonen, I.T. & Lauri, L.S., 2009. Modelling the thermal evolution of a collisional Precambrian orogeny: high heat production migmatitic granites of southern Finland. Precambrian Research 168, 233-246.

Kurhila, M., Vaasjoki, M., Mänttäri, I., Rämö, T., & Nironen, M. 2005. U-Pb ages and Nd isotope characteristics of the Lateorogenic, migmatizing microcline granites in southwestern Finland. Bulletin of the Geological Society of Finland 77, 105–128.

Kurhila, M., Andersen, T., Rämö, O.T., 2010. Diverse sources of crustal granitic magma: Lu-Hf data on zircon in three Paleoproterozoic leucogranites of southern Finland. Lithos, 263-271.

Lahtinen, R., Korja, A. & Nironen, M., 2005. Paleoproterozoic tectonic evolution. In: Lehtinen, M., Nurmi, P.A., Rämö, O.T. (eds). Precambrian Geology of Finland – Key to the Evolution of the Fennoscandian Shield. Developments in Precambrian Geology, Volume 14. Elsevier, Amsterdam, 481–531.

Lahtinen, R., Huhma, H., Lahaye, Y., Lode, S., Heinonen, S., Sayab, M., and Whitehouse, M.J., 2016. Paleoproterozoic magmatism across the Archean-Proterozoic boundary in central Fennoscandia: Geochronology, geochemistry and isotopic data (Sm-Nd, Lu-Hf, O). Lithos 262, 507–525.

Nironen, M., 2005. Proterozoic orogenic granitic rocks. In: Lehtinen, M., Nurmi, P.A., and Rämö, O.T. eds. Precambrian Geology of Finland – Key to the Evolution of the Fennoscandian Shield. Developments in Precambrian Geology 14, 443–479.

Nironen, M., Korja A., Heikkinen P. and the FIRE Working Group, 2006. A geological interpretation of the upper crust along FIRE 2 and FIRE 2A. Geological Survey of Finland, Special Paper 43, 77–103.

Suominen, V., 1991. The chronostratigraphy of southwestern Finland, with special reference to Postjotnian and Subjotnian diabases. Bulletin of the Geological Survey of Finland 356, 100 p.