FIRE 1 Tectonic overview

The FIRE 1 profile crosses three tectonic regions: the Archean nucleus in the east, the Archean–Proterozoic transition zone, and the Paleoproterozoic (Svecofennian) rocks of the Savo belt and Central Finland granitoid complex (Korja et al.

, 2006). The Archean nucleus comprises Archean tonalite-trondhjemite-granodiorite (TTG) and granite-granodiorite-monzogranite (GGM) series granitic rocks of the Eastern and Western Kianta blocks and the interlaying Kuhmo greenstone belt (Korja et al., 2006), which belong to the Archean Lentua complex (Hölttä et al., 2012). The general tectonic interpretation is that the Archean nucleus underwent early Proterozoic ~2.45 Ga extension and break-up reflected by mafic dykes and intrusions and related basin development (Kontinen and Paavola, 2006; Lahtinen et al., 2015).

The Archean-Proterozoic transition zone consists of a diverse amalgamation of Proterozoic (Kainuu Belt) and reworked Archean crustal blocks (Kajaani or Manamansalo, Iisalmi, and Rautavaara Complexes) deposited on, broken off, and accreted onto the margin of the Archean nucleus during the early Proterozoic. Three tectonic models have been proposed for the Archean–Proterozoic transition (cf., Lahtinen et al., 2015): 1) continent–continent collision (Gaál, 1990; Lahtinen, 1994; Lahtinen et al., 2005), 2) back-arc basin related to a subduction in the present-day southwest (Hietanen, 1975; Gaál, 1986), and 3) strike slip geometry (Park, 1985; Kontinen and Paavola, 2006).

On the Paleoproterozoic side of the transition zone, the supracustal rocks of the ca. 1.93–1.92 Ga Savo Belt represent a primitive island arc (Pyhäsalmi Island Arc; Kousa et al., 1994; Lahtinen, 1994) accreted onto and possibly partially thrusted under the margin of the Archean craton (Iisalmi Complex) during the Svecofennian collision (Kukkonen et al., 2006). The granitic rocks of the Central Finland granite complex (CFGC) include syn- (1.89–1.88 Ga) to postkinematic (1.88–1.87 Ga) granitic and gabbroic intrusions (Nironen et al., 2000) and related supracrustal rocks generated within the proposed Keitele microcontinent during the Svecofennian orogeny (Lahtinen et al., 2005).

    FIRE 1 Geographic constraints

    The eastern part of the 500 km long FIRE 1 line runs east to west and transects the Neoarchean Lentua complex (Hölttä et al., 2012) that includes the Eastern and Western Kianta blocks separated by the Kuhmo greenstone belt (Korja et al., 2006).

    Towards the west the profile crosses the Paleoproterozoic Kainuu belt and the intertwining slivers of the prominently Archean Kalpio (Kajaani), Manamansalo, and Rautavaara complexes (Hölttä et al., 2012). Further to the southwest, the profile crosses the partly Mesoarchean Iisalmi complex (Hölttä et al., 2012) and the Archean–Proterozoic boundary to the Paleoproterozoic Savo belt.

    The profile continues southwestward, towards the center of the Paleoproterozoic Central Finland Granitoid Complex and terminates north of the town of Keuruu in Central Finland.

      FIRE 1 Geological and tectonic division

      The upper crust along the FIRE 1 profile has been divided into five domains:

      • The Kianta (CMP 0000–3500) (1),
      • Kainuu (CMP 3500–6200) (2),
      • Iisalmi (CMP 6200–9500) (3),
      • Savo (CMP 9500–11000) (4), and
      • Keitele (CMP 11000–20000) (5) domains.
      Geological description of the Kianta domain (FIRE 1 CMP 0000–3500)

      The Kianta domain (CMP 0000–3500) corresponds to the Neoarchean Lentua complex (Hölttä et al., 2012) that is divided into the eastern Kianta (CMP 0000–2000; Vartius and Kuhmo blocks of Kontinen and Paavola, 2006) and western Kianta (2000–3500; Hyrynsalmi block of Kontinen and Paavola, 2006) blocks by the interlying Kuhmo greenstone belt (e.g., Korja et al., 2006; Lehtonen, 2016).

      A major west-dipping tectonic boundary between the crustal blocks has been interpreted to lie on the western margin of the Kuhmo greenstone belt (CMP ~2000; Käpyaho et al., 2006 and references therein, Kontinen and Paavola, 2006).

      The eastern Kianta block is dominated by the ca. 2.7–2.8 Ga Haasianvaara suite tonalite-trondhjemite-granodiorite (TTG) series tonalites and greenstone belt remnants, whereas the western Kianta block along the FIRE 1 profile comprises a larger proportion of somewhat younger (ca. 2.68 Ga) granite-granodiorite-monzogranite (GGM) series rocks of the Konivaara suite (Käpyaho et al., 2006; Hölttä et al., 2012).

      Geological description of the Kainuu domain (FIRE 1 CMP 3500–6200)

      The Kainuu domain (CMP 3500–6200) contains the Paleoproterozoic Karelian metasedimentary units of the Kainuu belt (Laajoki, 2005; Korja et al, 2006) and the ca. 2.05 Ga Otanmäki and ca. 1.8 Ga Kajaani granites (Korja et al., 2006; Kontinen and Paavola, 2006; Kontinen et al., 2013), which are interpreted to have tectonic and intrusive contacts, respectively, with the Archean Mananmansalo and Kalpio (Kajaani) complexes on the southwestern flank of the Kainuu belt. The Kainuu belt consists dominantly of Paleoproterozoic continental, epicontinental, and deep marine metasedimentary rocks of the Karelian formations deposited during and after the break-up of the Archean craton (Laajoki, 2005). Structurally the belt is located between the Archean Kuhmo block in the east and the Kalpio (Kajaani), Manamansalo, Rautavaara, and Iisalmi complexes in the west (Hölttä et al., 2012) and as such the bulk of it would represent a tectonically emplaced sliver of Proterozoic rocks overthrusted from the west and squeezed on top and between the Archean crustal blocks. In addition to the rift basin sediments the belt also includes the ca. 1.95 Ga Jormua ophiolite (Peltonen and Kontinen, 2004; Laajoki, 2005; Peltonen, 2005), which is suggested to represent a tectonically exposed sliver of Archean sub-continental lithosphere mantle (Peltonen and Kontinen, 2004). Both the Kainuu belt and the adjacent 2.05 to 1.8 Ga granitic rocks within the Kalpio (Kajaani) and Manamansalo complexes (Kontinen et al., 2013) have been interpreted to represent relatively thin (2–4 km) tectonic sheets in the upper levels of the crust underlain by rocks of the Archean Iisalmi and Rautavaara complexes exposed on the surface further in the west-southwest (Korja et al., 2006; Kontinen and Paavola, 2006).

      Geological description of the Iisalmi domain (FIRE 1 CMP 6200–9500)

      The Iisalmi domain (CMP 6200–9500) is here defined as containing the Archean rocks of the Rautavaara and Iisalmi complexes (Hölttä et al., 2012), their Paleoproterozoic Karelian metasedimentary cover rocks (Laajoki, 2005; Lahtinen et al., 2015), and the Proterozoic granitic rocks intruding them (Paavola, 2003) exposed along the FIRE 1 profile. The narrow sliver of the northern part of the Rautavaara complex transected by the FIRE 1 profile has also been called the Lahnajärvi block (Kontinen and Paavola, 2006).

      In contrast to the dominantly Neoarchean age of the Archean crust in Finland, the Iisalmi complex contains also Mesoarchean (ca. 3.2 Ga) material, which is absent for example from the adjacent Rautavaara complex (Hölttä et al., 2012; Lahtinen et al., 2015). The boundary between the Karelian formations and Svecofennian metasediments of the Savo belt in the west has been debated. Here we follow the demarcation of Lahtinen et al. (2015) who, alongside the Salahmi suite, consider also the psammitic eastern parts of the Näläntöjärvi suite to belong to the Kalevian cover and thus being a part of the Iisalmi domain and defining its western border at ca. CMP 9500.

      The Iisalmi complex has also been intruded by a N-S trending belt of Paleoproterozoic (ca. 1.88–1.86 Ga) granitic rocks of the Kaarakkala suite (Paavola, 2003). The FIRE 1 profile crosscuts one of these intrusions called the Kauppilanmäki granite at ca. CMP 8150–8600.

      Geological description of the Savo domain (FIRE 1 CMP 9500–11000)

      The Savo domain (CMP 9500–11000) corresponds to the part of the Paleoproterozoic (ca. 1.93–1.92 Ga) Savo belt (Kähkönen, 2005) that was redefined as the older Svecofennian supracrustal group (ca. 1.92 Ga) by Lahtinen et al. (2015).

      Along FIRE 1 profile the rocks of the Savo belt consist dominantly of the island-arc -type volcanic rocks of the Pyhäsalmi and Vihanti suites squeezed between the Archean Iisalmi complex (Hölttä et al., 2012; Lahtinen et al., 2015) and its Karelian metasedimentary cover (eastern Näläntöjärvi and Salahmi suites; Lahtinen et al., 2015) in the east and the Proterozoic Central Finland granitoid complex (CFGC) in the west (Kähkönen, 2005; Nironen, 2005). The Savo belt also coincides with the lithosphere-scale Raahe-Ladoga shear zone (RLSZ), which has been interpreted to represent the deep suture between the Archean Karelian province and the Proterozoic Svecofennian domain, or the A–P boundary (Lahtinen et al., 2015).

      Kontinen and Paavola (2006) interpret a steep lithospheric-scale Archean-Proterozoic boundary to lie between the eastern contact of the Savo belt towards the Kalevian metasedimentary sequences, concurrent with the strike-slip model for the cratonic boundary. In contrast, Korja et al. (2006) and Lahtinen et al. (2015) suggest that Archean middle and lower crust underlie the Savo belt as well, and that the deep Archean-Proterozoic border would be located further west, closer to the surface boundary of the Central Finland granitoid complex (CFGC) and Savo belt. This interpretation suggests that the original, collisional crustal architecture of the A–P boundary zone would have been preserved (Lahtinen et al., 2015).

      Geological description of the Keitele domain (FIRE 1 CMP 11000–20000)

      The Keitele domain (CMP 11000–20000) 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 reference therein).

      FIRE 1 References

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