FIRE 2A Tectonic overview

The upper crust along the FIRE 2A profile corresponds to two discrete allochtonous Proterozoic belts that make up the Arc complex of Southern Finland (ACSF) that has been accreted to the Keitele microcontinent block, towards the present day north during the Paleoproterozoic Svecofennian orogeny (Lahtinen et al., 2005), the Uusimaa belt in the southern part of the section and the Häme belt in the northern part (Kähkönen, 2005). Along the FIRE 2A profile the belts are separated by the Loppi shear zone (Nironen et al., 2006) running in an east–northeast to west–southwest direction. The Uusimaa belt and the southern part of the Häme belt have been 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; Kurhila, 2011). Several ~1.87 Ga mafic layered intrusions in the northern part of the Uusimaa belt form a roughly east–west -trending zone called the Hyvinkää–Mäntsälä Gabbroic Belt (Eerola, 2002).

The tectonic fabric of the upper crust in southern Finland 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 northwestern end of the FIRE 2A profile transects the Koijärvi and Kokkojärvi faults, which are interpreted to represent the southern edge of the Hämeenlinna shear zone, which marks the originally compressional border between the ACSF and the rocks of the Pirkanmaa belt that belongs to the Arc complex of Western Finland (ACWF) in the north (Nironen et al., 2006).

The FIRE 2A profile is 137.5 km long and it runs across two Paleoproterozoic belts in a northwest to southeast direction, the Häme belt in the north and the Uusimaa belt in the south (Nironen et al., 2006). In the northern part of the Uusimaa belt, the profile makes a kink into northeast–southwest direction.

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

  • The Häme (CMP 0000–2700) (1) and
  • Uusimaa (CMP 2700–5500) (2) domains.

The Häme domain (CMP 0000–2700) 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 (Lahtinen et al., 2005).

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 (ACSF; 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; Kurhila, 2011). 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).

The Uusimaa domain (CMP 2700–5500) corresponds to the metamorphic rocks of the Uusimaa belt (Kähkönen, 2005), which were intruded by the late orogenic (ca. 1.85–1.82 Ga) S-type granites of the Southern Finland granite suite (Kurhila et al., 2005; 2010; Kurhila, 2011), the ca. 1.87 Ga gabbroic rocks of the Hyvinkää-Mäntsälä belt (HMGB; Eerola, 2002), and the granites of the ca. 1.65–1.55 Ga southern Finland rapakivi suite (Rämö and Haapala, 2005).

Surface lithology of the Uusimaa domain along the FIRE 2A profile is dominantly (meta)igneous. Some metasedimentary and mafic volcanic rocks are exposed in the northern part of the profile section. The northern part (CMP 2900–3400) of the profile also crosses the Suivala layered intrusion of the HMGB intruding mafic metavolcanic rocks (Eerola, 2002; Nironen et al., 2006). The southernmost part (CMP 5300–5500) of the profile crosses the ca. 1.65 Ga Bodom rapakivi pluton that belongs to the southern Finland rapakivi suite (Vaasjoki, 1977; Kosunen, 2004; Rämö and Haapala, 2005).

Eerola, T., 2002. Mafic-silicic magma interaction in the layered 1.87 Ga Soukkio Complex in Mäntsälä, southern Finland. Bulletin of the Geological Society of Finland 74, 159–183.

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.

Kosunen, P., 2004. Petrogenesis of mid-Proterozoic A-type granites: Case studies from Fennoscandia (Finland) and Laurentia (New Mexico). PhD thesis, Department of Geology, University of Helsinki, Yliopistopaino, 129 p.

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., 2011. Late Svecofennian leucogranites of southern Finland: Chronicles of and orogenic collapse. PhD thesis, Department of Geosciences and Geography A8, 105 p.

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., 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.

Rämö, O.T. and Haapala, I., 2005. Rapakivi granites. 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, 533–562.

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.

Vaasjoki, M., 1977. Rapakivi granites and other post-orogenic rocks in Finland. Their age and the lead isotopic composition of certain associated galena mineralizations. Geological Survey of Finland, Bulletin 294, 64 p.