2.5–2.05 Ga) to convergent amalgamation and accretion of reworked Archean and newly generated Proterozoic crustal components (ca. 2.0–1.85 Ga; Patison et al., 2006).
Archean crust along the surface of the FIRE 4 profile is limited to the rocks of the Ranua complex (Hölttä et al., 2012) in the southern part and to the presumably Archean metasedimentary rocks of the Vuojärvi group (Lahtinen et al., 2015) along the northern flank of the Central Lapland granitoid complex (CLGC). Metasedimentary and -volcanic rocks deposited during the Paleoproterozoic extensional stage are represented by the rocks of the Peräpohja belt in the south and the Central Lapland greenstone belt (CLGB) in the north (Patison et al., 2006; Kyläkoski et al., 2012; Lahtinen et al., 2015). These crustal blocks were further deformed and metamorphosed during accretion onto the margins of the Archean craton and followed by the intrusion of the granitic rocks of the CLGC between ca. 2.13–1.76 Ga (Patison et al., 2006; Ahtonen et al., 2007; Lauri, 2013). The significance and detailed origin of the CLGC is still unclear but most likely it records a rather diverse magmatic and tectonic history that lasted over 300 m.y. (e.g., Lauri, 2013 and references therein).
From isotopic evidence (Huhma, 1986; Ahtonen et al., 2007; Lauri et al., 2012) Archean middle and lower crust of the Karelian craton is inferred to underlie the entire FIRE 4 profile and the Proterozoic rocks to occupy only the topmost 5–20 km of the section (Patison et al., 2006). The seismic fabric of the Archean middle crust underneath the Archean Ranua complex and the adjacent Peräpohja schist belt has been inferred to record the Paleoproterozoic extension in the form of partially melted domains and abundant mafic dykes (Patison et al., 2006). Otherwise the tectonic deep features of the entire profile have been interpreted to reflect the later compressional stages (Patison et al., 2006).
The FIRE 4 profile runs for 249 km from Ranua in the south to Sirkka in the north. The southern end of the FIRE 4 profile transects the northern part of the Archean Ranua complex (Hölttä et al., 2012) and runs towards north across the Paleoproterozoic Peräpohja belt and dissects the entire Proterozoic Central Lapland granitoid complex (CLGC). On the northern flank of the CLGC the profile first transects the presumably Archean metasedimentary rocks of the Vuojärvi group (Lahtinen et al., 2015) and then terminates in the supracrustal rocks of the Central Lapland greenstone belt (CLGB).
The upper crust along the FIRE 4 profile has been divided into five domains from south to north:
Geological subdivision of the dominantly Neoarchean (ca. 2.7 Ga) Pudasjärvi complex (e.g., Nironen et al., 2002) has been problematic as the region is poorly exposed and remains relatively unstudied (Sorjonen-Ward and Luukkonen, 2005; Hölttä et al., 2012).
The Ranua domain (CMP 0000–1700) coincides with the northern part of the Pudasjärvi complex, which was coined as the Ranua complex by Hölttä et al. (2012), and separated from the partly Paleoarchean (Mutanen and Huhma, 2003) Siurua complex further south. The Siurua complex also hosts the oldest known rock in the Fennoscandian Shield, the 3.5 Ga Siurua trondhjemite gneiss (Mutanen and Huhma, 2005). Majority of the Ranua complex consists of diverse migmatitic tonalite-trondhjemite-granodiorite (TTG) series gneisses with some minor amphibolites and greenstones (Hölttä et al., 2012)
Considerable part of the FIRE 4 profile within the Ranua complex transects the ca. 2.7 Ga Ranua diorite that is a massive medium-grained metaigneous rock (Mutanen and Huhma, 2003). The northern contact of the Ranua complex towards the Peräpohja schist belt is characterized by early Paleoproterozoic (ca. 2.44 Ga) intrusions of the Portimo layered igneous complex of the Tornio-Näränkävaara belt related to the rifting of the Archean Karelian craton (Iljina and Hanski, 2005).
The Peräpohja domain (CMP 1700–3100) coincides with the Paleoproterozoic (ca. 2.5–1.9 Ga) Peräpohja schist belt that comprises metasedimentary and -volcanic sequences related to the rifting of the Archean basement (Kyläkoski et al., 2012; Lahtinen et al., 2015, Ranta et al., 2015) and deposited on the Archean basement of the Ranua (Pudasjärvi) complex (Hölttä et al., 2012).
The southern margin of the belt is characterized by intrusions of the early Paleoproterozoic (ca. 2.44 Ga) Portimo layered igneous complex of the Tornio-Näränkävaara belt, which are related to the rifting of the Archean Karelian craton (Iljina and Hanski, 2005). Majority of the bedrock along the FIRE 4 transect consists of Kivalo group Palokivalo quartzites and Jouttiaapa metavolcanic rocks interpreted as Jatulian formations (Ranta et al., 2015). In the northern part of the domain the profile also transects the Pöyliövaara mica schists of the Paakkola group, which is considered to be a Kalevian metasedimentary formation (Ranta et al., 2015) belonging to the Rovaniemi supersuite (Lahtinen et al., 2015).
Metamorphic grade increases towards the north across the belt from greenschist to amphibolite facies (Ranta et al., 2015) towards the migmatitic contact of the Central Lapland granitoid complex (CLGC).
The Ounas domain (CMP 3100–7700) covers the FIRE 4 transect across the Paleoproterozoic Central Lapland granitoid complex (CLGC) that comprises a diverse suite of granitic, migmatitic, and supracrustal rocks generated and emplaced over roughly 350 million years from 2.13 to 1.76 Ga (Huhma, 1986; Ahtonen et al., 2007; Lauri, 2013).
The relatively un-radiogenic Sm–Nd and Lu–Hf isotope compositions of the granitic rocks (Huhma, 1986; Lauri et al., 2012) have been interpreted as evidence of Archean source and, by extension, of Archean middle and lower crust underlying the entire CLGC (Patison et al., 2006). Due to the lack of detailed mapping, distribution of different age groups and geochemical types within the CLGC are still eluding and require further research (Lauri, 2013). Nevertheless, it is apparent that the CLGC has a rather complicated crustal architecture, hosts also a substantial amount of non-granitic lithologies, and is a relatively shallow formation (Patison et al., 2006).
The northern edge of the CLGC is rimmed by the migmatitic supracrustal Sieppijärvi suite (Väänänen, 2004 and references therein) that was previously interpreted to be Archean in age (known as the Venejärvi complex). Supracrustal units similar to the Sieppijärvi suite (i.e., the Räväsjärvi suite) are also embedded as inclusions within the CLGC and the relationship of the suite to the Archean Vuojärvi group (Lahtinen et al., 2015) on its northern side is not entirely clear.
The Vuojärvi domain (CMP 7700–9100) corresponds to the metasedimentary Vuojärvi supersuite the age of which is debatable but in the most recent studies (Lahtinen et al., 2015) suggested to be Neoarchean or very early Paleoproterozoic. This interpretation is based on the lack of Proterozoic detrital zircon grains in all of the studied samples and relatively high (ca. 2.8 Ga) Sm–Nd model ages (Räsänen and Huhma, 2001; Lahtinen et al., 2015).
The deep seismic fabric underneath the northern CLGC in the FIRE 4 profile has been interpreted as a Paleoproterozoic reworked domain but it also permits interpretation of the Vuojärvi area as an extension of the Neoarchean middle crust (Patison et al., 2006). In earlier studies the Vuojärvi rocks have been included in the Paleoproterozoic Sodankylä group (e.g. Hanski and Huhma, 2005).
The Kittilä domain (CMP 9100–9920) corresponds to the Paleoproterozoic metasedimentary and -volcanic sequences of the Central Lapland greenstone belt (CLGB), which are at least partly correlated with the rocks of the Peräpohja schist belt (Lehtonen et al., 1998; Lahtinen et al., 2015 and references therein) and share a similar geological history.
The rocks of the ca. 2.0 Ga Kittilä suite (a.k.a. the Kittilä allochton) form a group of dominantly metavolcanic rocks with diverse geochemical signatures that are inferred to originate in a plume environment and, in contrast to the generally extensional tectonic regime, to have been emplaced in a convergent tectonic event on top of the rift-related supracrustal sequences (Lehtonen et al., 1998; Hanski and Huhma, 2005; Patison et al., 2006; Lahtinen et al., 2015).
The northern part of the FIRE 4 profile crosscuts the southern part of the CLGB, which comprises metasedimentary rocks of the Savukoski and Kumpu groups (Lehtonen et al., 1998; Hanski and Huhma, 2005; Patison et al., 2006).
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