FIRE 3 Geographic constraints

The eastern part the FIRE 3 line transects the south-central Finnish part of the Archean Ilomantsi subterrane of the Archean Central Karelian subprovince in a roughly east-west direction. Towards the west the profile crosses the Archean-Proterozoic boundary into the North Karelian schist belt that consists of autochtonous and allochtonous Paleoproterozoic platform sedimentary sequences of the Karelian formations that are intercalated with Archean basement inliers and black schist interlayers associated with ophiolitic rocks. The profile terminates in the west-northwest in the Paleoproterozoic granitic rocks of the Heinävesi suite (Sorjonen-Ward, 2006).

FIRE 3 Geological and tectonic division

The upper crust along the FIRE 3 profile has been divided into three parts: the Outokumpu (1), Höytiäinen (2), and Pielinen (3) domains following the demarcation by Sorjonen-Ward (2006).

The middle crust has been divided into two parts: the Allochtonous Archean middle crust (4) and Autochtonous Archean middle crust (5) blocks.

The lower crust (6) is treated as a single block due to its lack of seismic fabric and observed structural features. Moho depth is approximated from Grad et al. (2009).

Geological description of the Outokumpu domain (FIRE 3 CMP 0000–2500)

Geographic domain constraints

The Outokumpu domain corresponds to the part of the FIRE 3 profile that transects the western part of the North Karelian schist belt from the eastern contact of the Archean Sotkuma basement inlier (CMP 2500) to the Svecofennian granodiorites of the Heinävesi intrusive suite in the WNW (CMP 0000). The profile is roughly E-W along CMP 2500–1200 and follows a NW-SE strike along CMP 1200–0000.

General geology

In the eastern end of the Outokumpu domain (CMP ~2500–1800) the FIRE 3 profile follows the southern contact of the Archean Sotkuma inlier with the metasedimentary rocks that have been interpreted to belong to the autochtonic lower Kaleva assemblages. Towards W the profile crosses over allochtnonous upper (western) Kaleva metasediments of the Savo Province (Viinijärvi suite) that are dominated by monotonous turbiditic biotite paraschists (CMP 1800–0450) intercalated by the serpentinites and black schists of the Outokumpu assemblage (CMP 0880–0840; Peltonen, 2005; Peltonen et al., 2008). In the WNW the upper Kaleva metasediments are intruded by the granites and granodiorites of the Paleoproterozoic (1870–1850 Ma) Heinävesi intrusive suite (CMP 0470–0000).

Lithology

Archean Sotkuma dome

The southern part of the Sotkuma inlier alongside the FIRE 3 transect is composed of migmatitic TTG gneiss but there are also some local occurences of (potentially basal) conglomerates in the southern contact towards the Kaleva metasediments (Huhma, 1975).

Kaleva sediments and Outokumpu assemblage

The upper (western) Kaleva allochtonous metasediments of the Outokumpu domain comprise a relatively homogeneous unit of metaturbiditic greywackes interbedded with local black schists (Laajoki, 2005; Peltonen et al., 2008; Lahtinen, 2010). The black schists are associated with a local sequence of highly strained ultramafic and mafic intrusions comprising the Outokumpu ophiolite complex that was over-thrusted alongside the allochtonous upper Kaleva metasediments onto the Archean basement and autochtonous lower Kaleva sequences (Peltonen, 2005; Peltonen et al., 2008).

Heinävesi suite granitic rocks

The Maarianvaara granites and granodiorites in the western end of the FIRE 3 transect belong to the Heinävesi intrusive suite (Huhma, 1986), which intruded into the upper Kaleva metasediments along the entire length of the North Karelian schist belt (Lahtinen, 2000).

Geochronology

The U-Pb zircon age of the Outokumpu ophiolite sequence is 1972 ± 18 Ma (Huhma, 1986). The maximum depositional age of the allochtonous upper Kaleva rocks within the Outokumpu domain is constrained by age of the ophiolite sequences in Outokumpu and Jormua and detrital zircon grains at around 1.95–1.90 Ga (Huhma, 1986; Kontinen, 1987; Claesson et al., 1993; Peltonen et al., 2008; Lahtinen et al., 2010). Their detrital zircon studies have recorded an Archean component (2.63–3.02 Ga) but are dominated by Paleoproterozoic ages (1.91–2.1 Ga; Lahtinen et al., 2010). Minimum age of deposition for the allochtonous western Kaleva sequence is defined by the plutonic rocks (Heinävesi suite) that intruded the formation at 1.87 Ga (Huhma, 1986; Lahtinen et al., 2010).

Structural and metamorphic geology

The suggested tectonic boundary between the Kalevian sequences coincides with the eastern contact of the Archean Sotkuma inlier and has been interpreted from the west-dipping reflectors that can be followed down to the depth of ~10 km (CMP 2500–1500). As a dominantly allochtnonous unit, tectonic stacking and multiple folding are common features of the upper Kaleva rocks (Lahtinen et al., 2010). The Outokumpu ophiolite assemblage with associated black schists is tectonically enclosed in the upper Kaleva metasediments but represents at least locally also their deposition surface (Lahtinen et al., 2010). The dominant structural feature in the upper crust west of the Sotkuma inlier is the Viinijärvi synform (hinge plane NE-NNE, moderate dip SW; surface expression at CMP ~1800–800; Sorjonen-Ward, 2006). The surface intersection of the interpreted western (east-dipping) limb of the synform coincides with the FIRE 3 intersection of the Outokumpu sequence (CMP ~800). Non-reflective zones underneath the Viinijärvi syncline have been correlated with the granitic rocks of the Heinävesi suite (Maarianvaara/Kermajärvi) in the west and south. Zones of high reflectivity between CMP 900–1200 have been correlated to Outokumpu-type sequences reached through a deep drill core (Kukkonen et al., 2006; Heinonen et al., 2011). The metamorphic grade increases from east to west across the domain and is seen in recrystallization, segregational banding, and migmatization, which become stronger in the western part (Lahtinen, 2000; Sorjonen-Ward, 2006).

Geochemistry and isotope geology

The allochthonous western Kaleva metasedimentary rocks are compositionally and isotopically very homogeneous but based on geochemical criteria they have been divided into two groups (Lahtinen, 2000; Lahtinen et al., 2010): 1) metapsammites and 2) SiO2-poor metapelitic rocks with higher levels of elements incorporated in clay minerals (Al2O3, MgO, FeO, K2O). Based on trace element evidence (high Sc/Th and Cr/Th ratios) the material of the western Kaleva rocks is interpreted to have a substantial mafic component (Lahtinen, 2000). The Sm-Nd model ages (TDMNd) of the Upper Kaleva metasediments ranges from 2.23 to 2.41 Ga and εNd values from −0.6 to −2.8 at 1.9 Ga (Lahtinen et al., 2010). A Heinävesi suite granodiorite from Viitalahti has a Sm-Nd mantle model age (TDMNd) of 2.20 Ga and εNd value of -2.1 at 1.87 Ga (Huhma, 1986). The geochemical and geochronological evidence suggests that the source of the western Kaleva sediments was dominantly juvenile but with some Neoarchean material (Lahtinen et al., 2010).

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