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 parts: The Kianta (1), Kainuu (2), Iisalmi (3), Savo (4), and Keitele (5) domains.

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 References

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Gaál, G., 1986. 2200 million years of crustal evolution: the Baltic Shield. Bulletin of the Geological Society of Finland 58, 149–168.

Gaál, G., 1990. Tectonic styles of Early Proterozoic ore deposition in the Fennoscandian Shield. In: G. Gaál, D.I. Groves (eds.), Precambrian ore deposits related to tectonics. Special Issue. Precambrian Research 46, 83–114.

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