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 Pielinen domain (FIRE 3 CMP 3700–6300)

Geographic domain constraints

The FIRE 3 line transects the south-central Finnish part of the Archean Ilomantsi subterrane of the Archean Central Karelian subprovince in a roughly E-W direction. The Pielinen domain covers the upper crustal CMP range 3700–6300 of the FIRE 3 profile. It commences in the Archean sanukitoid suite Sysmäjärvi quartz diorites (CMP 6300) in the east and terminates in the west at the Archean–Proterozoic boundary (~CMP 3700) towards the Proterozoic Höytiäinen domain (CMP 3700–2500).

General geology

The upper crust of the Pielinen domain belongs to the Neoarchean Ilomantsi subterrane of the Archean Central Karelia subprovince (Sorjonen-Ward, 1993; 2006). Most of the surface rocks along the FIRE 3 profile belong to the Ilomantsi complex, which consists of several Neoarchean granitic intrusions (tonalites, granodiorites, and quartz diorites) with sanukitoid affinities (Lobach-Zhuchenko et al., 2005; Heilimo et al., 2011; 2012), migmatitic TTG (tonalite-trondhjemite-granodiorite) gneisses (Hölttä et al., 2012), and GGM (granodiorite-granite-monzogranite) series granites (Käpyaho et al., 2006; Hölttä et al., 2012) intercalated by greenstone belts (Sorjonen-Ward, 1993; 2006; Hölttä et al., 2016).

The granitic rocks of the domain include the Sysmäjärvi (Möhkö) quartz diorite (CMP 6300–6100) and Pogosta (Ilomantsinjärvi) granodiorite (CMP 5900–4700) that belong to the Kuittila sanukitoid suite, undiscriminated tonalitic TTG series migmatites (CMP 4900–3800), and the dominantly monzogranitic rocks of the Kutsu (CMP 4800–3700) and Naarva (CMP ~6100) granite suites that belong to the Archean GGM series (Käpyaho et al., 2006; Hölttä et al., 2012).

The Ilomantsi greenstone belt has two major branches. The Hattu schist belt (CMP 6100–5900) lies in the eastern part of the domain, between the Kuittila Suite sanukitoids; the Sysmäjärvi quartz diorite (Möhkö tonalite) in the east and the Pogosta (Ilomantsinjärvi) granodiorite in the west. The Kovero branch of the greenstone belt in the western part of the domain is separated into three discrete zones: Sonkaja (CMP ~4600), Otravaara/Kuusijärvi (CMP 4400–4200), and Ylinen/Keskijärvi (CMP ~3800) zones, which are intercalated within TTG series tonalitic migmatites and Kutsu suite granites.

The rocks of the domain are dominantly Archean but are intruded by minor Paleoproterozoic gabbro and diabase bodies and dikes designated to three distinct arrays that intersect the FIRE 3 profile (Sorjonen-Ward, 2006): the Marjovaara array (CMP 5100–4500), Havukkakallio array (CMP ~5500), and Kuuksenvaara array (CMP 6100–5600).

Lithology

Greenstone belts

The eastern branch of the Ilomantsi greenstone belt, the Hattu schist belt, consists principally of feldspathic volcaniclastic deposits with andesitic, basaltic, and locally also ultramafic volcanic interlayers (Sorjonen-Ward, 1993, 2006). In its southern part the belt is dominated by turbiditic and pelitic metasedimentary rocks, mainly greywackes. The Kovero greenstone belt is dominated by mafic and ultramafic volcanic and subvolcanic units with minor felsic and sedimentary interlayers (Sorjonen-Ward, 1993, 2006).

Kuittila suite sanukitoids

The sanukitoid rocks of the Kuittila suite (Lobach-Zhuchenko et al., 2005; Heilimo et al., 2011, 2012) make up the majority of the eastern part of the Pielinen domain. They belong to the older (2.75–2.73 Ga) Eastern Sanukitoid Zone (Heilimo et al., 2011, 2012) of the Central Karelian Province. Major Kuittila suite sanukitoids along the FIRE 3 profile include the Sysmäjärvi quartz diorite (Möhkö tonalite, Sorjonen-Ward, 2006; CMP 6300–6000), Kuittila tonalite (CMP ~5900), and the Pogosta granodiorite (Ilomantsinjärvi granodiorite, Heilimo et al., 2011; CMP 5800–4900).

TTG gneisses and GGM granites

Tonalite-trondhjemite-granodiorite (TTG) gneisses and migmatites make up most of the Archean Karelian Province and are prominent also in the western part of the Pielinen domain along the FIRE 3 profile. Granodiorite-granite-monzogranite (GGM) suite rocks (Käpyaho et al., 2006; Hölttä et al., 2012) are also more prominent in the western part of the domain towards the contact of the Proterozoic Höytiäinen domain (CMP 3800–3700). The largest GGM suite intrusion is the Kutsu granite that lies about 5 km south of the FIRE 3 profile at CMP ~4650. In the eastern part of the domain, the Sysmäjärvi quartz diorite is intruded by the Lukanvaara leucogranite (CMP ~6100) that belongs to the Naarva GGM granite suite.

Geochronology

The bulk of the Ilomantsi greenstone belt including the Hattu schist belt was formed at about 2750 Ma and most of the granitic rocks in the Kuittila suite intruded either contemporaneously or shortly after (Vaasjoki et al., 1993; Heilimo et al., 2011; Huhma et al., 2012).  The Sysmäjärvi quartz diorite and Kuittila tonalite in the eastern part of the domain have coinciding U-Pb TIMS and SIMS ages at around 2745 Ma [Sysmäjärvi quartz diorite TIMS age (Vaasjoki et al., 1993): 2744 ± 3 Ma and a SIMS age (Heilimo et al., 2011): 2744 ± 5 Ma, Kuittila tonalite TIMS age (Vaasjoki et al., 1993): 2745 ± 10 Ma and SIMS age (Heilimo et al., 2011): 2741 ± 6 Ma] and the Pogosta granodiorite seems to be slightly younger at about 2730 Ma [TIMS ages (Vaasjoki et al., 1993): 2733 ± 6 and 2724 ± 5 Ma and a SIMS age (Heilimo et al., 2011): 2728 ± 7 Ma]. Youngest Ilomantsi area GGM age for the Lukanvaara suite based on unpublished data quoted by Hölttä et al. (2016) is 2.63 Ga. The Kovero greenstone belt has two distinct age groups. In addition to felsic dykes and gabbroic rocks contemporaneous with the main part of the Ilomantsi greenstone belt at ~2.75–2.76 Ga, felsic volcanic rocks in the Kovero belt have been dated at 2878 ± 2 Ma (Huhma et al., 2012).

Structural and metamorphic geology

Due to the lack of major Paleoproterozoic deformation, the seismic fabric of the Archean rocks in the eastern part of FIRE 3 profile has been interpreted to principally record Neoarchaean orogenic processes (Sorjonen-Ward, 2006). The relatively featureless upper crust in the Archean Pielinen domain has been interpreted to reflect its granitoid-dominated geology. The greenstone belts along the profile do not correlate with any apparent reflectivities. Contacts between the granitoids and the greenstone belts in the Pielinen domain are not observed to be depositional but are either intrusional or tectonic (Hölttä et al., 2012). Greenstone belts have generally steeply dipping structures with N–NNE strike trends and their deformation follows the intrusional features of the granitic rocks (Sorjonen-Ward, 2006). The Archean rocks of the Pielinen domain experienced two metamorphic stages: a Neoarchean event at 2.66–2.62 Ga coeval with the late stage GGM magmatism and a Paleoproterozoic thermal event at ~1.84 Ga evidenced by resetting of K-Ar isotopic systems and monazite U-Pb ages (Hölttä et al., 2016). The Proterozoic overprinting is, however, less pronounced in the Ilomantsi subterrane than in the Western Karelia Subprovince (Hölttä et al., 2012). Deformation related to the Proterozoic metamorphosis in the Pielinen domain was in general weak and restricted mostly to fracture zones (Sorjonen-Ward, 2006). The southern part of the Ilomantsi subterrane lacks systematic P/T determinations but the central parts were metamorphosed in lower to mid-amphibolite facies conditions (<5.5 kbar/<600°C; Hölttä et al., 2012). Metamorphic grade is higher in the northern parts of the subterrane varying from upper amphibolite to granulite facies (>6 kbar/>600°C; Hölttä et al., 2012). In general, greenstone belts display consistently lower metamorphic grade (especially in the inner parts) than the granitic rocks surrounding them (Tuisku, 1988; Hölttä et al., 2012; 2016).

Geochemistry and isotope geology

Most of the Archean bedrock in Finland belongs to the Western Archean subprovince but the Ilomantsi subterrane belongs to the Central Karelian subprovince. This is also reflected in the geochemistry of the Ilomantsi complex rocks that differs somewhat from the corresponding rock types elsewhere in the Finnish Archean.

Greenstone belts

The volcanic rocks of the Ilomantsi greenstone belt have arc-type (BADR, basalt–andesite–dacite–rhyolite) geochemistry (Hölttä et al., 2012; 2016). The Ilomantsi komatiites have high LREE/HREE ratios and negative Nb, Ta, and Ti-anomalies. Compositional similarity between the komatiites and associated felsic volcanic rocks suggests a petrogenetic connection (Hölttä et al., 2012). The Ilomantsi komatiitic basalts, however, have less fractionated incompatible trace element patterns than the komatiites, which suggests simple crystal fractionation may not be adequate to explain their geochemical relationship (Hölttä et al., 2012). Kovero komatiites and basaltic komatiites are geochemically distinct from the main Ilomantsi belt komatiites as they have less fractionated REE patterns and higher Al2O3/TiO2 and Cr/MgO ratios.

Granitic rocks

Archean TTG:s that make up most of the western part of the Pielinen domain have dominantly adakitic geochemistry (Richards and Kerrich, 2007; Hölttä et al., 2012). In contrast to most Finnish TTG series rocks the Ilomantsi TTG:s also display the diagnostic higher Cr and Ni values (Ruotoistenmäki, 2012). Archean granitic rocks of the Ilomantsi subterrane are on average relatively high in SiO2 and alkalis. The adakitic rocks of the Ilomantsi area are defined more clearly by post-collisional–late-orogenic geochemical signatures in contrast to broader pre- to post-orogenic ranges observed elsewhere in the Finnish Archean (Ruotoistenmäki, 2012). The granitic rocks of the eastern part of the Ilomantsi complex are dominantly sanukitoids, which have a distinct geochemical signature in elevated contents of both compatible (e.g., Mg, Ni, Cr) and incompatible (e.g., K, Ba, Sr, LREE) elements and Mg# at a given SiO2 when compared to the TTG series rocks (Heilimo et al., 2010; 2012).

Isotope geology

Sm-Nd isotope systematics of the Ilomantsi complex rocks are strongly affected by later metamorphic remobilization but meticulous examination of a large sample volume (O’Brien et al., 1993; Huhma et al., 2012) has produced a consistent petrogenetic picture. Neodymium isotope mantle model ages (TDMNd ~2.75–2.9 Ga) of the Ilomantsi complex rocks suggest a predominantly juvenile origin for most of the granitic and volcanic rocks. Last stage GGM rocks have not been directly studied in the Ilomantsi area but are elsewhere suggested to be generated by remelting of recently generated Neoarchean crust.

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