5 to 2.0 Ga and the following compressional orogenic phase that started at ca. 2.0–1.89 Ga and took place in two, at least partly overlapping, stages (Ward et al., 1989; Sorjonen-Ward et al., 1997; Patison et al., 2006; Hölttä et al., 2007).
The first orogenic stage was dominated by northeastward compression related to the Svecofennian orogeny that accreted the Paleoproterozoic supracrustal formations of the Central Lapland greenstone belt (CLGB) onto the Archean basement. The second stage is characterized by deformation related to the overthrusting of the Lapland granulite belt towards present day southwest (Hölttä et al., 2007).
The resulting structural framework recorded by the rocks in the Central Lapland region is rather complicated and makes interpretation of the major tectonic features challenging. Archean rocks are most likely underlying most of the FIRE 4A profile but are exposed only in the northeasternmost part of it (Patison et al., 2006). The juvenile Lapland granulite belt and the reworked Inari area have also been proposed to form a part of the orogenic core of the Paleoproterozoic Lapland–Kola orogen between the Archean Kola and Karelian cratons (Daly et al., 2006).
The deeper contacts between these orogenic blocks are somewhat debatable but it is evident that the CLGB is separated from the Lapland granulite belt along a large-scale listric overthrust (the Tanaelv zone) that dips to the northeast (Daly et al., 2006; Patison et al., 2006). The relationship between the Inari area and the Lapland granulite belt has been interpreted either as a northeastward subduction of the granulite belt under the Inari area (Daly et al., 2006 and references therein) or as a jagged margin preserving the suture zone between the Karelian and Kola cratons (Patison et al., 2006).
The FIRE 4A profile runs for 319 km in a southwest to northeast direction from Sirkka to Näätämö across the areas of central and northeastern Lapland (Patison et al., 2006). The profile commences in the Kittilä suite metavolcanic rocks and crosses over the older formations of the Central Lapland Greenstone Belt (Lehtonen et al., 1998) into the Lapland Granulite Belt (Meriläinen, 1976; Hörmann et al., 1980) and further into the diverse Archean and Proterozoic rocks of the Inari Area (Nironen et al., 2002).
The upper crust along the FIRE 4A profile has been divided into three domains:
The Kittilä domain (CMP 9920–13000) 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 southern part of the FIRE 4A profile commences in and crosses over the metavolcanic rocks of the Kittilä suite just north of the Sirkka (Kittilä) shear zone that is interpreted to be the thrust surface of the allochtonous metavolcanic units (Patison et al., 2006). Further in the northeast, the profile crosscuts some of the autochtonous/parautochtonous units of the CLGB grouped collectively as the Vuotso complex that comprises a heterogeneous amalgamation of mostly Paleoproterozoic metasedimentary rocks deformed and metamorphosed during the overthrusting of the Lapland granulite belt over the CLGB.
The Ivalo domain (CMP 13000–19000) encompasses the metasedimentary and enderbitic rocks of the Lapland granulite belt and the heterogeneous metamorphic rocks of the Tanaelv zone (Marker et al., 1990) that is inferred to represent the approximate fault surface along which the belt was thrusted onto the Central Lapland rocks of the Kittilä domain in the southwest (Meriläinen, 1976; Hörmann et al., 1980; Patison et al., 2006).
The granulite belt has been interpreted as the root of a juvenile arc system related to the Lapland-Kola orogeny (Daly et al., 2006). The metasedimentary granulitic rocks of the LGB represent an arc-related greywacke basin and the enderbitic rocks represent arc-magmas intruded into the metasedimentary rocks at around 1.92–1.90 Ga (Tuisku et al., 2006; Tuisku and Huhma, 2006).
The Inari domain (CMP 19000–22700) represents the margin of the Archean Kola craton that was reworked in the Lapland-Kola orogeny at ca. 1.93 Ga (Daly et al., 2006). The region is rather poorly studied but consists of an amalgamation of reworked Archean basement rocks and Paleoproterozoic, dominantly metasedimentary, units (e.g., Meriläinen, 1976).
Due to several differing tectonic and geological interpretations based on limited data, unified nomenclature and division of geological units has not been established. It seems, however, that the southwestern part of the domain consists dominantly of Paleoproterozoic rocks, whereas the northeastern/eastern parts are dominantly Archean.
Northeastern/eastern part of the domain comprises the Archean Inarijärvi or Inari complex (Korsman, 1997; Heilimo et al., 2009) also interpreted as a part of the Sørvaranger terrain and named as the Garsjøen gneiss complex (Sorjonen-Ward and Luukkonen, 2005). In the latter scheme, the southwestern part of the Inari domain is interpreted as the dominantly Archean Inari terrain (or terrane; Daly et al., 2006) and comprises the ca. 2.5 Ga Suorre–Tievja gneiss complex (Sorjonen-Ward and Luukkonen, 2005) or a tectonic package of mixed Archean and Proterozoic units (Daly et al., 2006). These two terrains are separated by the metavolcanic rocks of the Opukasjärvi group that belongs to the Polmak–Pasvik–Pechenga(–Imandra–Varzuga; Daly et al., 2006) belt (Sorjonen-Ward and Luukkonen, 2005).
The northeastern part of the FIRE 4A profile runs parallel and also briefly crosscuts the Vainospää granite that belongs to the ca. 1.8 Ga Nattanen granite suite (Heilimo et al., 2009).
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