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 Höytiäinen domain (FIRE 3 CMP 2500–3700)

Geographic domain constraints

The Höytiäinen domain (Sorjonen-Ward, 2006) is constrained in the east by the surface expression of the boundary (CMP ~3700) between the Neoarchean Ilomantsi subterrane (Pielinen domain) and the Proterozoic autochtonous Höytiäinen province (Ward, 1987; 1988; Kohonen, 1995). In the west it is limited by the Suhmura thrust contact to the Savo province (Outokumpu domain; CMP 2500–0000) and the Archean Sotkuma basement inlier (Kohonen, 1995; CMP ~2500, coincident also with an acquisition break in the FIRE 3 transect between 2700–2500).

General geology

Höytiäinen domain belongs to the southern part of the Paleoproterozoic North Karelian Schist Belt and includes (par)autochtonous eastern (lower) Kaleva (in the east) with autochtonous upper Kaleva (in the west) and their underlying Jatuli platform rocks (easternmost part of the domain; Ward, 1987; Kohonen, 1995; Lahtinen, 2000; Lahtinen et al., 2010). The rocks have been interpreted as syn- and post-rift sequences deposited within a marginal basin of the Archean basement during early Proterozoic multistage extension and deformed during the convergent stage of a subsequent basin inversion (Kohonen, 1995; Lahtinen et al., 2010).

The sequence consists of autochtonous pelitic and psammitic eastern Kaleva (or Kalevian) metasediments (greywackes and mica schists; CMP 3660–2700) underlain by a thin occurrence of Jatuli platform quartzites (CMP 3680–3660) that delineate the eastern contact towards the Archean rocks of the Pielinen domain. The Jatuli quartzites and conglomerates do not necessarily represent a basal unit for the sequence but most likely the location of a W–SW dipping fault zone that was reactivated multiple times during both extensional and convergent stages (Kohonen, 1995; Laajoki, 2005).

Lithology

Lithologically the eastern Kaleva is heterogeneous in different scales with interbedded metagreywacke and metapelite dominated units and metaquartzitic intercalations (Lahtinen et al., 2010). The metasediments in the northern part of the Höytiäinen province have been separated in five distinct lithologic assemblages (LA 1–5; Kohonen, 1995). LA 1, 2, and 3 are dominated by laminated pelitic schists with quartzite interbeds, minor carbonate rocks (LA 2; Kohonen, 1995), and graphite-bearing interlayers (LA3; Kohonen, 1995). LA 4 and 5 include a larger amount of psammitic material but are interbedded with pelitic layers (Kohonen, 1995).

Geochronology

Based on the U-Pb geochronology of volcanic rocks underlying the lower Kaleva (Tohmajärvi volcanic complex, 2105 ± 15 Ma; Huhma, 1986), maximum deposition age of the autochtonous Kalevian sequences is traditionally assumed to be ~2.1 Ga (Pekkarinen, 1979; Huhma, 1986; Pekkarinen and Lukkarinen, 1991; Lahtinen et al., 2010). Detrital zircon studies have, however, only yielded Neoarchean U-Pb ages (Claesson et al., 1993; Lahtinen et al., 2010). The maximum depositional age of the autochtonous upper Kaleva rocks within the domain is constrained by ages of the ophiolite sequences and detrital zircon grains from the western Kaleva (Outokumpu domain) 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 record dominantly Paleoproterozoic ages (1.91–2.1 Ga; Lahtinen et al., 2010). Minimum age of deposition for the whole Kalevian (Kaleva) sequence is defined by intrusive plutonic rocks (Heinävesi suite) 1.87 Ga in age (Huhma, 1986; Lahtinen et al., 2010).

Structural and metamorphic geology

The general structural framework of the Kaleva sequences suggests extension and subsidence were followed by an eastward convergence with over-thrusting of the western allochtnonous Kaleva sequences and basement nappes on top of the (par)autochtonous eastern Kaleva  sequences. Two-stage tectonic evolution is evident in the structure of the rocks of the Höytiäinen domain: early extensional features during the rifting of the Archean basement (~2.1–2.05 Ga) and convergent structures related to the inversion of extension into a collisional tectonic setting (~1.95–1.9 Ga). The convergent stage reactivated many of the earlier extensional features as the originally west-dipping eastern contact developed into an eastward over-thrust and the western east-dipping marginal faults into rotational basement thrusts (Kohonen, 1995). Poorly-defined near-vertical reflectors in the Höytiäinen domain (CMP 3000–3100) are interpreted as correlatives of mafic volcanic sequences (Tohmajärvi) exposed on the surface southeast of the profile transect (Sorjonen-Ward, 2006).The Höytiäinen province is characterized by lower to medium amphibolite facies metamorphic conditions that increase towards the west (Kohonen, 1995; Lahtinen et al., 2010).

Geochemistry and isotope geology

The major element and Sm-Nd isotope geochemistry of the lower Kaleva rocks in the Höytiäinen domain suggests a source dominated by Neoarchean weathered paleosols and derived sedimentary rocks with local input from Neoarchean unweathered and Proterozoic volcanic and subvolcanic rocks (Huhma, 1987; Kohonen, 1995; Lahtinen, 2000; Lahtinen et al., 2010). The upper Kaleva rocks in the western part of the domain have somewhat lower TDMNd values and differ also geochemically from the lower Kaleva assemblages, which suggests a dominantly Paleoproterozoic source for them (Lahtinen et al., 2010).

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