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The Knaften area is situated in the northern part of the Bothnian Basin in the central Svecofennian province of Sweden. Sedimentary rocks are here conformably intercalated with mafic to intermediat...

The Paleoproterozoic Central Lapland Granitoid Complex (CLGC) forms the northern core of the Fennoscandian Shield. We present here new U-Pb age data and review the existing ones from plutonic and metasedimentary rocks of the CLGC, and integrate the results with the structural field observations to decipher the tectonic history of this region. Granitoids vary in age from 2.43 Ga to 1.78 Ga, whereas metamorphic zircon overgrowths, both in granites and metasedimentary rocks, vary between 1.90 Ga and 1.76 Ga with several age peaks indicating prominent metamorphic pulses. The almost continuous growth of metamorphic zircon domains favors that the central part of the CLGC was near solidus for 100–120 Ma. A succession of five deformations and associated foliation-producing events, which were near-orthogonal to each other, have been identified. East-vergent thrusting and stacking (D1) commenced at ca. 1.92 Ga, and led to the onset of Barrovian-type metamorphism followed by a radical tectonic shift that yielded E-W trending D2 structures. Exhumation at 1.88–1.87 Ga caused decompression melting of thermally evolved thickened crust and led to the emplacement of large volumes of granites and triggering of Buchan-type metamorphic conditions. NE-SW collision (D3) at the southwestern margin of Fennoscandia heterogeneously overprinted the S3 fabric on D2 structures. Orogen parallel (NNW-SSE) buckling at 1.87–1.85 Ga formed oroclines in the hot linear arc segments. Renewed NNW-SSE shortening (D4) at 1.84–1.81 Ga produced long wavelength folds and caused the formation of domal structure in the CLGC. The final 1.78–1.76 Ga far-field collision (D5) in the west overprinted all the pre-existing structures. This complex multi- directional tectonic scenario provoked the intermittent exhumation phases between different contraction events of the CLGC.

The geodynamic system of Earth is controlled by the horizontal motion of fragmented tectonic plates due to complex interaction between the mantle and the crust. Land and marine paleomagnetic signatures, along with extensive petrochronological and structural data, suggest non-continuous, multi-directional convergence histories for modern orogenic belts, as seen in the Himalayas and Betic Cordillera, among others. The consequences of radical changes in the direction of plate motions are obscured by complex structural settings and repeated cycles of metamorphism in orogenic belts. Although, subduction was operational on Earth in the Paleoproterozic, one of the major questions is: how far back in geological time can we trace the scale of modern-style subduction, collisional tectonics and poly-metamorphism? The metasedimentary and volcanic rocks (2.5 to 2.05 Ga) of the Sodankyla belt of northern Finland were deformed and metamorphosed during a prolonged period, now represented by the Svecofennian orogen (ca. 1.92–1.77 Ga). The orogenic belt represents a window into the deep levels of a Paleoproterozoic crust, where the interplay between deformation and metamorphism can be compared with the modern orogenic settings. In this study, five discrete heterogeneously developed superimposed, near-orthogonal, deformation events (D1 to D5) have been identified and mapped based on the high-resolution aeromagnetic data and field studies. Regional correlation of tectonic fabrics, along with metamorphic textures across the Sodankyla belt and northern Finland, suggest that each deformation event was a regional-scale phenomenon rather than a local one. Previous geochronological data obtained from the Sodankyla belt and adjacent Central Lapland Granitoid Complex on different Paleoproterozoic deformed rocks allow us to propose a time-constrained tectono-metamorphic model for the Sodankyla belt. The sequence of near-orthogonal deformation history and the polymetamorphic evolution offer comparisons with modern-style orogenic settings. The most notable aspect is the occurrence of Mesoarchean and Paleoproterozoic eclogitic high-pressure granulites reported from the Belomorian belt of Fennoscandian Shield, which lies to the east of the Sodankyla belt. From a tectono-metallogenic viewpoint, the Sodankyla belt is located in the centre of Ni-Cu-PGE, orogenic gold-cobalt deposits of the northern part of Fennoscandian Shield, therefore, the tectonic processes and overprinting role of multiple deformation phases need to be well understood as they directly link mineral exploration to the timing and controls on mineralization.

The Western Finland (WF) supersuite, characterized by supracrustal rocks and intruding plutonic rocks, forms the core component of the coupled Bothnian oroclines in the central part of the Svecofennian orogen. We present here geochemical-isotope data, detrital zircon age data on sedimentary rocks and age data on granitoid plutons within the WF supersuite in the Pirkanmaa (PB) and Pohjanmaa (PoB) belt representing the southernmost limb of the southern and the hinge zone of the northern Bothnian orocline, respectively. The Tampere belt (TB) in the north and Hame belt (HB) in the south bound the PB. Basaltic to picritic volcanic rocks in the WF supersuite, with a proposed eruption age at ca. 1.91 Ga (1.92–1.90 Ga), show locally pillow-structures and are typically associated with black shales ± cherts ± carbonate-bearing rocks. Based on geochemistry and Sm-Nd data a variable MORB-WPB-type origin is evident and an evolution scenario from earlier WPB-type (rifting) to more MORB-affinity (ocean/sea opening) is proposed. Detrital zircon age distributions from sedimentary rocks with ≥1.91 Ga maximum depositional age in the TB, PB and HB have been compared and typically the age distributions of

Ore textures provide direct clues for tracking ore-forming processes. In this regard, most of our knowledge is generally based on two-dimensional (2-D) image analyses, leaving a considerable gap in comprehending three-dimensional (3-D) in-situ textural settings. Recent advances in lab-based and synchrotron radiation–based X-ray computed microtomography and nanotomography have made it possible to visualize and quantify rock volumes in a 3-D space. In this study, we first analyzed microscale textures in oriented drill cores from the world-class Suurikuusikko orogenic gold deposit of northern Finland using lab-based X-ray computed microtomography. The technique revealed a kinematic history and a number of in-situ 3-D quantitative aspects including size, shape, spatial distribution, and geometrical orientation of arsenopyrite and pyrite in a highly altered host-rock matrix. For 3-D nanotomography, the experimental procedure known as holotomography was adopted. Individual arsenopyrite crystals were separated and scanned with voxel sizes ranging from 50 nm to 150 nm using the X-ray nanoprobe beamline (ID16B) at the European Synchrotron Radiation Facility, France. This ultrahigh-resolution technique illustrated the 3-D distribution of micron- to nanoscale gold inclusions, mostly associated with primary rutile or along secondary microfractures inside arsenopyrite. The workflow, from micro- to nanotomography, outlined in this study offers an indispensable new technique in quantifying and characterizing 3-D textural settings of ores, which is otherwise impossible with conventional 2-D imaging devices. The method can also be highly useful in evaluating the amenability of ores to treatment with different processing options.

The Central Lapland belt (CLB), Perapohja belt (PB), and Kuusamo belt (KB) form an important part of northern Fennoscandia and record ca. 700 Ma (2.5–1.8 Ga) of Precambrian tectonic processes. The 2.5–2.1 Ga sedimentary-volcanic successions in these belts form a basin-wide surfaces with a wide lateral continuum. Tidal influenced shallow epeiric sea setting dominated at 2.4–2.1 Ga followed by a continental breakup and subsequent formation of a passive margin at 2.1 Ga. The collisional evolution (1.93–1.76 Ga) started with an E-W, locally NW-SE and NE-SW, compressional phase (D1; 1.93–1.90 Ga) that resulted in at least 600 km long and in part over 300 km wide E-vergent Lapland foreland fold and thrust belt (FTB). The KB is the best preserved example of this FTB and shows E-vergent imbricated structures in the east and E-vergent out of sequence thrusting in the west. The southern part of the KB has been affected by N-S shortening (D2) seen as upright to slightly inclined open to tight F2 folds and as oval-shaped dome and basin structures. The D3 overprinting causes folding with NW-SE trending spaced foliation in less competent rocks and parallel folds and faults in orthoquartzites. The D4 NE-SW trending overprinting deformation is characteristic for the high grade areas of the KB and is seen in outcrop to map scale tight folding. In general, the easternmost peripheral part of the E-vergent Lapland foreland FTB, formed upon non-stretched Archean basement, is only moderately affected by the later deformation events. In the stretched part of the Archean basement the earlier rift structures have facilitated the N-S shortening and the D2 structures dominate the map pattern. The D3, due to a SW-NE shortening, shows strong strain partitioning and localized deformation. After prolonged heating the Archean basement started to act ductile and long wavelength crustal-scale NE-SW trending D4 folds formed. The metamorphic zonation in the KB from greenschist facies in the south to medium pressure amphibolite facies in the north is explained by a major D4 anticlinorium (wavelength 70–100 km).

The Paleoproterozoic, 2.0–1.9 Ga Talvivaara formation of Finland was deposited during the Shunga Event, a worldwide episode of enhanced accumulation of organic-rich sediments in the aftermath of the Lomagundi–Jatuli carbon isotope excursion. Sulfidic carbonaceous mudstones in the Talvivaara formation contain one of the largest known shale-hosted nickel deposits. In order to gain new insight into this Shungian sedimentary environment, sedimentological, petrographical and in situ S and Fe isotopic microanalyses were carried out on samples representing depositional and early-diagenetic conditions. The event-bedded lithology with tidal signatures in the organic-rich mudstones strongly indicates deposition from predominantly river-delivered mud on a highly-productive coastal area, below storm-wave base. The riverine supply of phosphorus, sulfate and iron supported high primary productivity and resulted in strong lateral and vertical chemical gradients in the nearshore waters with a shallow oxic surface layer underlain by euxinic water. The stratigraphic upper part of the Talvivaara formation contains banded intervals of thin alternating pyrite beds and carbonaceous mudstone beds. The pyrite beds were deposited by seaward excursions of the concentrated, acidic Fe-rich river plume subsequent to droughts or dry seasons, which led to intense pyrite precipitation upon mixing with euxinic waters. δS34 and δFe56 values of the bedded pyrite (median δS34=−10.3‰ and δFe56=−0.79‰) are consistent with the reaction of dissolved Fe(II) with H2S from bacterial sulfate reduction. Organic-rich clayey Fe-monosulfide-bearing granules were transported from the muddy estuary, and enclosed in Fe (oxyhydr)oxide aggregates that were forming by wave and current reworking in nearshore accumulations of river-delivered iron. The isotopic composition of these presently pyrrhotitic inclusions (median δS34=−3.3‰ and δFe56=−1.6‰) indicates microbial iron reduction. The Fe (oxyhydr)oxide aggregates were transported in muddy debris flows to the distal euxinic seafloor. Their Fe (oxyhydr)oxide matrix was replaced by pyrite (median δS34=+5.8‰ and δFe56=+0.81‰) at shallow sediment depths with 34S and 56Fe-enriched porewater. Wavy-crinkly laminae of possible microbial origin developed on the euxinic seafloor during low sedimentation. These results indicate episodic deposition at seasonal to multiannual time scales. δS34 and δFe56 values in the studied Fe-sulfides provide evidence of microbial isotope fractionation processes and syndepositional and early-diagenetic origin, finding no support for the previously proposed local hydrothermal activity in the Talvivaara mudstones.

Structural mapping integrated with aeromagnetic and Anisotropy of Magnetic Susceptibility (AMS) data form complimentary and robust tools for analysis of regional structures and fabric overprinting relationships. This multi-scale approach is used to constrain the geometry and tectonic evolution of sparsely exposed and flat-lying outcrops of the Martimo belt (MB) of northern Fennoscandia. The MB records a history of near-orthogonal deformations upper greenschist to lower amphibolite facies metamorphism between ca 1.92–1.79 Ga. The earliest deformation (D1) involved east-directed thin-skin thrusting, and the development of recumbent F1 folds and a N-S trending S1 fabric. D2 deformation is prominent in the central part of the MB and is characterized by N-S shortening with the development of E-W trending F2 folds, an associated pervasive steeply-dipping S2 foliation and type-2 fold interference pattern. The MB and Mellajoki suite exhibit regional scale D2 synformal and antiformal refolded structures, respectively. D3 is more heterogeneously developed and manifests bimodal orientations with NNW-SSE and WNW-ESE structural trends. It is pronounced in the central and eastern parts of the MB but weakly developed in the western part. The steep dip of S3 in the region suggests that it formed in response to NE-SW bulk shortening. D4 structures have a conspicuous NNE-SSW trend that is visible as lineaments on the aeromagnetic image and locally in the field. The last deformation event (D5) affected almost the entire MB and was characterized by broadly WSW-ENE oriented crustal shortening. The N-S oval-shape geometry of the eastern part of the MB is attributed to the D5 event. Contrasting with well developed overprinting relationships between D1 and D3 structures, the relative timing of D3–D5 could not be established in the study area, but can be inferred from structures with similar trends in the Central Lapland Granitoid Complex, which is located north of the MB. The D1–D5 deformation scheme can be readily reconciled with the tectonic evolution of the Lapland-Savo, Fennian, Svecobaltic and Nordic orogens of the composite Svecofennian orogen. The age of D1 in the northern part of the Lapland-Savo orogen is ≤1.91 Ga based on crosscutting granitoids and maximum deposition ages of sedimentary rocks. D2 (1.90–1.89 Ga) has been linked to orogen parallel shortening during the late stage of southern part of the Lapland-Savo orogen. Several orthogonal tectonic events in northern Fennoscandia indicate amalgamation of crustal blocks around the core of the Archean Karelia continent at 1.92–1.77 Ga, and that Fennoscandia was a key component of the Columbia/Hudsonia/Nuna supercontinent.

Archean–Proterozoic (AP) collision boundary in central Fennoscandia is a complex collage of Archean basement rocks, its Paleoproterozoic supracrustal cover rocks, and Svecofennian (1.93–1.78 Ga) supracrustal and plutonic rocks. The apparent steep nature and complexity of the AP boundary has led to contradictory tectonic models varying from arc-continent collision via back-arc closure to accretion of arc terranes along a strike-slip shear zone. We present new geochronology data from detrital zircons combined with whole-rock geochemical and Sm–Nd isotopic data on Paleoproterozoic sedimentary and volcanic rocks across the boundary to revise its nature. The 2.06 Ga Siilinjarvi volcanic complex and associated turbidites, with an inferred deposition age at 2.1–2.06 Ga, are part of the cover sequence. The Siilinjarvi volcanic rocks derive from both plume (OIB) and sub-continental lithospheric mantle (SCLM) magma sources and are often contaminated by the Archean crust. The Sahara suite sedimentary cover rocks are derived from local 1.95–1.92 Ga bimodal volcano-plutonic source, which source also explains the common age population found in younger sedimentary rocks. The Nalantojarvi suite psammites, with maximum depositional ages of 1.97–1.93 Ga, are continental slope to deep water sediments deposited at the continental margin. Contemporaneous mafic magmatism, seen as lavas and dykes, show a change from SCLM dominated (present east) to asthenospheric depleted mantle (DM) dominated (present west) melts. The interpretation of the Nalantojarvi suite as cover rocks of the lower plate indicates that the Archean crust continues at least 50 km more to the west than thought earlier. Parautochthonous sedimentary rocks from Salahmi, Eastern Kansanneva and Loutemaki with maximum depositional ages at 1.92–1.91 Ga are inferred as collision-related foredeep deposits from local sources. Both the Lampaanjarvi suite and Pyhajarvi suite sedimentary rocks have deposited at 1.89–1.88 Ga or later, whereas the Lampaanjarvi suite rocks are dominated by a local source and the Pyhajarvi suite rocks derive from a mixture of older sediments and 1.89–1.88 Ga granitoids. At least two tectono-metamorphic stages separated with a major unconformity can be differentiated in the study area. The early deformation and metamorphic stage in the Nalantojarvi suite has occurred before the intrusion of the late- to post-collisional pyroxene granitoids at 1.88 Ga. The Pyhajarvi suite sedimentary rocks have been deposited after the intrusion of these granitoids between 1880 and 1875 Ma and then subsequently deformed and metamorphosed at ≤1875 Ma. We propose a tectonic model where the continental breakup occurred at 2.1–2.06 Ga followed by a new spreading within the pre-existing passive margin at 1.98–1.93 Ga. During initial continent-arc collision the subducted slab steepened strongly at high angle, subduction ceased rapidly, and finally detached and shallow slab breakoff occurred at 1.92–1.91 Ga followed subsequently by ophiolite obduction and a formation of a foreland fold and thrust belt. Mantle-crust decoupling of the Archean lithosphere occurred and a crocodile jaw formed, where the arc rocks are partly buried beneath the bounding Archean continent. Similar features have been recorded in other AP collision zones, which could indicate that the buoyant nature of the Archean lithosphere combined with denser Paleoproterozoic lithosphere may be a general cause for decoupling of Archean crust and mantle during arc-continent collision in Paleoproterozoic.

An intrinsic limitation of studying microstructures in thin section is that their spatial (three-dimensional, 3-D) distribution, shape, and orientation have to be inferred by combining 2-D data from different sections. This procedure always involves some degree of interpretation that in some cases can be ambiguous. Recent advances in high-resolution X-ray computed microtomography have made possible the direct imaging in 3-D of volumes of rock to centimeter scale. This rapidly evolving technology is nondestructive and provides a holistic approach of microstructural analysis that eliminates interpretative procedures associated with 2-D methods. Spatial images can be generated through any part of the rock sample and used as virtual petrographic sections. Our application of this technique to an oriented drill core sample from the classic Orijarvi metamorphic region of southern Finland reveals a number of in situ 3-D aspects, including: (1) the spatial distribution and shape of andalusite porphyroblasts, (2) the geometry of a matrix foliation anastomosing around the porphyroblasts, (3) a millimeter-scale compositional layering that controlled the oscillation of porphyroblasts and sulfide mineralization, and (4) distinct inclusion trail patterns characterizing porphyroblast core versus rim zones. The combined data indicate that the steeply dipping bedding-subparallel foliation that characterizes the Orijarvi area formed by bulk north-south crustal shortening and associated vertical stretching.

Altogether 1130 U–Pb analyses on zircons from 25 samples across the Pajala shear zone in northern Fennoscandia were performed by LA-ICPMS methodology to study the origin of the zone. The samples were also analyzed for their Sm–Nd isotope systematics and geochemically. The new data is tested with existing geological information to elaborate the provenance and maximum depositional ages, the stratigraphic position of studied samples, and finally to assess the tectonic evolution in the Pajala shear zone area. Chromium-enriched metasedimentary rocks are found within and to the east and west of the N-S trending Pajala shear zone. No material originating from Proterozoic felsic igneous (zircons) or mafic (Sm–Nd) rocks was found in these rocks and thus, only Neoarchean or very early Paleoproterozoic ages of deposition are possible. Our new results indicate that the bimodal 1.99–1.97 Ga supracrustal rocks of the Rovaniemi Supersuite cover a large area east of the Pajala shear zone. Based on the absence of Archean zircons typical of the metasedimentary rocks covering the Archean basement, it seems that the basement and its sedimentary cover were not exposed at the time of deposition of the Rovaniemi Supersuite. The metasedimentary rocks of the Uusivirka Supersuite, located within the Pajala shear zone, vary from hornblende-bearing metasedimentary rocks, metapelite, and meta-arkose to orthoquartzite. Their characteristic features include a dominant Proterozoic zircon population (65–87%), with a high proportion of analyzed grains yielding ages between 1.96 Ga and 1.91 Ga, and maximum depositional ages of 1.92–1.91 Ga. We record at least three metamorphic events; at ca. 1.92–1.90 Ga, ca. 1.85 Ga and at 1.82–1.78 Ga, of which the youngest can possibly be separated into two events at 1.83–1.82 Ga and 1.79–1.77 Ga, respectively. We propose a tectonic model in which continental breakup occurred at 2.1–2.05 Ga, followed by the development of a magmatic arc (Kittila arc) at ca. 2.0 Ga above a subduction zone. Two Archean continental units (Norrbotten and Karelia) collided at ca. 1.92–1.91 Ga, the Kittila arc was obducted upon the Karelia continent as a foreland fold and thrust belt, and the metasedimentary rocks of the Uusivirka Supersuite were emplaced as foreland deposits. The proposed tectonic model is based on the absence of 2.44 Ga layered intrusions, and especially 2.06 Ga rocks and 1.99–1.97 Ga rocks of the Rovaniemi Supersuite that are found only east of the Pajala shear zone. In our model the Pajala shear zone originated as a divergent plate boundary, and was multiply reactivated after continental collision with both lateral and vertical movements.

The accretion of magmatic arcs gives rise to elongate, linear orogens and is a key process in forming new continental crust. Many Precambrian continents are, however, presently equidimensional or have large areas without any clear linearity, such as the central part of the Palaeoproterozoic Svecofennian Orogen (1.92–1.77 Ga). One way of forming an equidimensional continental domain is by buckling of a linear orogen about vertical axes of rotation into one or more coupled oroclines. Here, we reinterpret existing data and demonstrate the occurrence of coupled Bothnian oroclines in the Svecofennian Orogen. Palinspastic restoration of the southern and northern Bothnian oroclines brings a 1000-km-long segment of the Svecofennian Orogen into an originally linear, NW-striking geometry that restores the lithological belts, metamorphic zones and structural vergences to a common direction, and which indicates that the orogen consists of a SW-facing arc, which has been shortened along NE-verging folds and thrust faults.

The metasedimentary basin-fill successions of the Central Lapland belt, Perapohja belt, and Kuusamo belt form an important part of the northern Fennoscandia Shield. They each record ca. 600 Ma (2.5–1.9 Ga) of sedimentation history and tectonic evolution, providing important information about past depositional conditions and tectonic settings. Detrital zircon geochronology shows that Archean age populations between 3.00 and 2.70 Ga have provenances from the surrounding basement complexes of granitoid/TTG rocks, whereas the Paleoproterozoic age populations (≤2.25–1.86 Ga) can be linked to the surrounding granitoids in northern Finland and Sweden. Zircon morphology studies indicate that basin sedimentation and/or provenance area(s) were likely influenced by eolian processes ca. 2.3–2.1 (or possible earlier). Major and trace element geochemistry indicates minor to moderate weathering trends, closely resembling Archean-Proterozoic sandstones and shales. Provenance signatures of tectonic settings vary from rift to arc and collisional. We propose five basin evolution stages for the Central Lapland belt, Perapohja belt, and Kuusamo belt: a rifting stage from 2.5 to 2.1 Ga including (i) initial rifting/early syn-rift, (ii) syn-rift, and (iii) syn-rift to early post-rift; (iv) a passive margin (post-rift) stage from 2.1 to 1.94–1.92 Ga; and (v) a foreland basin system 1.94–1.92 to 1.88 Ga. The rifting stage includes bimodal subaerial to subaqueous volcanism with continental to marine sedimentation, and the initial accumulation of carbonate and evaporate successions, indicating later local tectonic stability and restricted hydrological circulation. The early development of an epeiric sea setting was followed by a passive margin stage and the development of aulacogens, controlled by cyclic sea-level fluctuations superimposed on long-term thermal subsidence. This stage was characterized by the accumulation of hemipelagic deposits together with gravity-driven sandstone deposits and associated subaqueous volcanism. The foreland basin system comprises an initial compressional stage with the development of forebulge and foredeep depozones and magmatic arc collision. This was followed by the main compressional stage, where basin inversion resulted in a foreland fold-thrust belt. The foreland basin system includes syn- and post-orogenic deposystems and a transition from underfilled marine sedimentation to overfilled non-marine deposits.

The western margin of the Karelia Province of the Fennoscandian Shield consists of Archean crust covered by remnants of Paleoproterozoic (c. 2.5–1.9 Ga) metasedimentary rocks. The study area in the eastern part of the margin shows 2.3–2.1 Ga cratonic sedimentation followed by 2.1–2.05 Ga rift sedimentation and c. 1.91–1.92 Ga foredeep to foreland sedimentation related to continent–arc/continent collision. Extreme LREE mobility (loss up > 95% of La) is recorded by many samples from these units. Monazite, bastnasite and, in some cases, allanite have been the prealteration LREE host phases in the LREE depleted samples. Monazite was altered to Th-silicates with variable loss of the LREE, the remaining part of which was incorporated in Th-poor monazite. The depositional age of the sedimentary units was studied by dating (LA-MC-ICPMS) detrital zircon grains, and the age of LREE mobility by dating monazite and xenotime and by whole-rock Sm–Nd modeling. The maximum deposition age of the foredeep to foreland basin is 1.91–1.92 Ga based on detrital zircon data and the minimum age of the main foliation in the study area is 1.87–1.86 Ga based on the younger age limit of late-to post-tectonic xenotime and the ages of few probable metamorphic zircons. Coeval bastnasite, with xenotime in a LREE enriched sample, suggests that this was also one important stage of LREE mobility. Monazite crystallizations have occurred 1.76–1.78 Ga ago while the Rb–Sr whole rock data suggest that the closure of this isotope system took place 1732 ± 21 Ma ago. The Sm–Nd modeling gives ages at 0.4–1.0 Ga for LREE depletion. The Sm–Nd age of 412 ± 27 Ma can be considered as the age of the latest major LREE depletion event. We propose that alkali-bearing oxidizing fluids started to form in the cratonic sequences due to diagenetic reactions maybe as early as 2.3–2.1 Ga. Alkalinity of fluids was later increased by interaction with alkaline 2.05 Ga source materials before and during the c. 1.91–1.92 Ga foredeep to foreland basin stage. Basin inversion and metamorphic crystallization occurred at 1.91–1.87 Ga. Episodes of fluid migration at 1.78–1.73 Ga and afterwards were focused along fracture zones and in microfracture networks. The 0.4 Ga stage is the most prominent of the

A Paleoproterozoic lateritic paleosol overlain by ultra-mature/mature quartzite is reported for the first time from three localities in the Svecofennian province in southern Finland. These are important indicators for warm paleoclimate and continental paleoenvironment. Ultra-mature quartzites above the lateritic paleosols are overlain by matrix-supported conglomerates followed by mature quartzites. These latter rocks are related to incipient rifting followed by main rift stage starting with increasingly more immature meta-arkose. We present a tectonic model in which exhumation, after 1.88–1.87 Ga contraction, unroofed plutonic rocks. Volcanism and exhumation-related erosion produced deposition of both volcanic and sedimentary material. Weathering of these rocks and deposition of ultra-mature/mature quartzites took place between 1.87 and 1.85/1.84 Ga followed by rifting at 1.84 Ga. Tectonic slicing during basin inversion thrust the pre-rift quartzites and rift-related volcanic and sedimentary rocks over older mid-crustal rocks and produced a low-angle stacking system. To accommodate voluminous magmatism at deeper crustal levels and widespread sedimentation at the surface, we propose that exhumation due to faulting followed by stabilization of the upper crust (0–15/20 km) occurred simultaneously as the detached hot middle and lower crust experienced vertical shortening via ductile flow and extrusion. The ca. 1.85 Ga paleosol-quartzite successions in southern Fennoscandia have many similarities with the widespread ca. 1.7 Ga paleosol-quartzite successions in the United States. Both are intra-orogenic, formed on hot orogens and have simultaneous magmatism at deeper crustal levels and sedimentation at the surface. Accretion-type models presented for southern Fennoscandia at 1.87–1.80 Ga are not compatible with the available data and either far-field setting during outboard subduction or a newly formed continental interior setting are proposed. Anyhow tectonic models proposed for southern Fennoscandia between 1.87 and 1.80 Ga should take into account the occurrence of lateritic paleosols and ultra-mature quartzites.

The Paleoproterozoic cover sequence at the 100–150 km wide western margin of the Archean Karelian Province is dominated by deep water Lower and Upper Kaleva metasediments. We present here an interpretation of Sm–Nd isotope and geochemical data on 36 samples, TIMS multi-grain U–Pb zircon analyses on nine samples, and ca. 100 SIMS analysis of detrital zircon grains from four Upper Kaleva and one Lower Kaleva samples. The Lower Kaleva is characterized by autochthonous–parautochthonous, lithologically heterogeneous metaturbidites showing common enrichment in quartz. All the analysed detrital zircons are of a local Neoarchean source but tDM variation up to 2.4 Ga combined with geochemical data indicate abundant mixing of Paleoproterozoic mafic material, presumably from 2.1 Ga plateau lavas and dykes, in most of the Lower Kaleva samples. The Upper Kaleva is dominantly allochthonous with tectonically enclosed fragments of ophiolite bodies, and it is characterized by lithological and geochemical-isotopic homogeneity. Geochemical, isotopic and detrital zircon data favour material derived from an orogenic domain, comprising both Archean and Proterozoic units, followed by effective mixing during the transport. The Archean zircon grains (25%) are mostly Neoarchean. The Paleoproterozoic grains lack zircons at 2.5–2.2 Ga and plot dominantly (92%) between 1.92 and 2.05 Ga. The indicated maximum deposition ages vary from 1.95–1.94 Ga to 1.92 Ga. The main source area proposed is the Himalaya-type Lapland-Kola orogen (now) in the northeast, which experienced mountain building and erosion at 1.95–1.91 Ga. The western margin of the Karelian Province shows evidence of rifting and lithosphere thinning from 2.1 to 1.95 Ga but it is still under debate whether the craton breakup occurred at 2.06 Ga in a volcanic or later at 1.95 Ga in a non-volcanic margin setting. One hypothesis is that the onset of collision in the northeast changed plate motion and lead to a new spreading within the pre-existing passive margin at 1.97–1.95 Ga. Thus, both a volcanic margin at 2.06 Ga and a non-volcanic margin at ca. 1.95 Ga could have been operated at the western margin of the Karelian Province.

Geochronology of detrital zircons and their overgrowths combined with whole-rock geochemical and Sm–Nd isotopic data can be used to distinguish different stages of sediment recycling and metamorphism during multiphase orogenic evolution. This approach is applied to the Paleoproterozoic sedimentary rocks of the Tampere and Pirkanmaa belts (southern Finland) in the center of the composite Svecofennian orogen. The lower part of the Tampere belt succession and bulk of the Pirkanmaa belt are characterized by turbidites whereas the upper part of the Tampere belt succession is dominated by 1.90–1.89 Ga mature arc-type volcanic rocks. Detrital zircon geochronology indicates that the Tampere and Pirkanmaa belts have a coeval 1.92–1.89 Ga depositional and tectonic history. Ages of pre-depositional zircon overgrowths vary from 1.91 Ga to 2.0 Ga with clusters at 1.92 Ga and 1.98 Ga. Within the Pirkanmaa belt, post-depositional zircon overgrowths indicate metamorphic culmination at c. 1885 Ma in the Vammala Ni-zone and at c. 1875 Ma in the northern part. The lower conglomerates and graywackes in the Tampere belt and their equivalents in the Pirkanmaa belt are rich both in Neoarchean and Paleoproterozoic grains, the latter ranging in age from 1.9 to 2.1 Ga. Compared to these, a sample from the Vammala Ni-zone has an exotic provenance with at least c. 1.90 Ga, 2.04–2.15 Ga, 2.38–2.48 Ga and 2.57–2.63 Ga components. A sedimentary recycling and tectonic model for the central Fennoscandia is proposed, in which the Paleoproterozoic Keitele + Bergslagen continent was formed during an unnamed orogeny at 1.98–1.97 Ga. The Archean Norrbotten microcontinent was attached to the continent at 1.97–1.93 Ga. Upper Kaleva turbidites, derived from the Lapland-Kola orogen in the north, were deposited before 1.92 Ga on a passive margin of the Archean Karelia craton. The Karelia craton collided with the Keitele + Bergslagen + Norrbotten continent at c. 1.92 Ga forming the Lapland-Savo orogen. Subsequent evolution led to rifting and break-up of the latter continent into two microcontinents in the hinterland. At 1.92–1.91 Ga the rift was developed into a subsiding passive margin of the Keitele microcontinent with voluminous turbidite deposition, now seen as graywackes in the Tampere, Pirkanmaa and Pohjanmaa (western Finland) belts. The turbidite material was derived from the rising Lapland-Savo orogen and included recycled Upper Kaleva, recycled (sandstones) and first-cycle 2.03–1.97 Ga detritus from Keitele, 1.93–1.92 Ga Savo arc material, as well as detritus from the Archean craton and its cover deposits. The collision between Karelia and Keitele caused a subduction reversal and the onset of Tampere arc volcanism at 1.90 Ga. Arc-derived materials started to deposit and were mixed with older sedimentary rocks, and trench-parallel distal turbidites from exotic source were being deposited in the accretionary wedge.

Accretionary processes contributed to major continental growth in Fennoscandia during the Palaeoproterozoic, mainly from 2.1 to 1.8 Ga. The composite Svecofennian orogen covers c . 1×10 6 km 2 and comprises the Lapland–Savo, Fennia, Svecobaltic and Nordic orogens. It is a collage of 2.1–2.0 Ga microcontinents and 2.02–1.82 Ga island arcs attached to the Archaean Karelian craton between 1.92 and 1.79 Ga. Andean-type vertical magmatic additions, especially at c . 1.89 and c . 1.8 Ga, were also important in the continental growth. The Palaeoproterozoic crust is the end product of accretionary growth, continental collision and orogenic collapse. Preserved accretional sections are found in areas where docking of rigid blocks has prevented further shortening. The Pirkanmaa belt represents a composite accretionary prism, and other preserved palaeosubduction zones are identified in the Gulf of Bothnia and the Baltic Sea areas. In the southern segment of the Lapland–Savo orogen collision between the Archaean continent (lower plate) and the Palaeoproterozoic arc–microcontinent assembly (upper plate) produced a special type of lateral crustal growth: the Archaean continental edge decoupled from its mantle during initial collision and overrode the arc and its mantle during continued collision.

Detrital zircon U–Pb SIMS data on quartz-rich metasandstone units presumed to belong to the upper part of the Svecofennian stratigraphy in southeastern Finland and east-central Sweden suggest the existence of clastic sedimentary basins between the two main orogenic phases at 1.89–1.86 Ga and 1.83–1.79 Ga, during a period referred to as the intra-orogenic phase (1.86–1.83 Ga). Stratigraphically below the metasandstone at Hamrange, east-central Sweden, is a metadacite with an U–Pb zircon TIMS age of 1888 ± 6 Ma, which indicates the maximum age of sedimentation. It also indicates that an earlier proposed correlation of Hamrange metavolcanic rocks and 1.86 Ga equivalents at Los to the northwest must be rejected. Instead, there is a temporal affinity to the metavolcanic rocks in the Bergslagen Province to the south or Southern Finland to the east. Quartz-rich metasandstone samples from four localities, Luukkola, Pyhantaka and Tiirismaa in Finland and Hamrange in Sweden, yield multimodal detrital zircon age distributions with main populations at 2.95–2.60 Ga, 2.10–1.95 Ga and 1.92–1.85 Ga. The groups are similar in all four samples, and they are comparable to previously reported detrital ages in this part of the Fennoscandian Shield. The oldest zircon analysed gave an age of 3.32 Ga (Tiirismaa). The maximum ages of sedimentation (and of subsequent deformation and metamorphism), indicated by the youngest detrital zircon, from the four localities are 1842 ± 10 Ma (Luukkola), 1865 ± 11 Ma (Pyhantaka), 1848 ± 13 Ma (Tiirismaa), and 1855 ± 10 Ma (Hamrange), respectively. Possible source rocks for these zircon grains are found within and around the vast Ljusdal Batholith in Sweden, and in the Arc Complexes of Western and Southern Finland. It is concluded that several intra-orogenic sedimentary basins existed during the time interval 1.86–1.83 Ga ago, between two major orogenic events in the Fennoscandian Shield.

The Paleoproterozoic evolution of Fennoscandia and Greenland can be divided into major rifting and orogenic stages. The Paleoproterozoic rifting of Fennoscandia started with 2.505‐2.1 Ga, multiphase, southwest-prograding, intraplate rifting. Both Fennoscandia and Greenland experienced 2.1‐ 2.04 Ga drifting and separation of their Archean cratons by newly-formed oceans. The main Paleoproterozoic orogenic evolution of Fennoscandia resulted in the Lapland-Kola orogen (1.94‐1.86 Ga) and the composite Svecofennian orogen (1.92‐1.79 Ga). The Paleoproterozoic orogens in Greenland, from north to south, are the Inglefield mobile belt (1.95‐1.92 Ga), the Rinkian fold belt/Nagssugtoqidian orogen (1.88‐1.83 Ga) and the Ketilidian orogen (c. 1.8 Ga). The Lapland-Kola orogen, Inglefield mobile belt and the Rinkian fold belt/Nagssugtoqidian orogen are continent-continent collision zones with limited formation of new Paleoproterozoic crust, whereas the Ketilidian orogen displays a convergent plate-tectonic system, without subsequent collision. The composite Svecofennian orogen is responsible for the main Paleoproterozoic crustal growth of Fennoscandia.

Geology has been of profound importance for the Nordic countries since the Middle Ages. Strong economies were built on an understanding of the occurrence in bedrock of minerals containing metals, e.g., silver, copper, zinc and iron, and eventually led to the establishment of the first Geological Surveys in Norway and Sweden in the middle of the nineteenth century. The geology of Norden ranges from the oldest to youngest rocks on the planet. Based on the papers in this special issue, this introduction provides a brief summary of the geological evolution of Norden, from the Archean of Greenland and northern Fennoscandia to the on-going volcanicity in Iceland on the Mid-Atlantic Ridge. It also refers to aspects of Geoscience that are particularly important for society in Norden, including geo-resources (petroleum, geothermal energy, nuclear energy, metals, industrial minerals and groundwater) and environmental geology (including natural and anthropogenic processes, medical geology, geo-hazards and climate). Information on the early history of geology in Norden and the geological surveys is also included and, finally, an outline of the 33rd International Geological Congress with its main theme “Earth System Science: Foundation for Sustainable Development”. Validerad; 2008; 20080821 (pawe)

Based on an integrated study of geological and geophysical data, a tectonic model for the Palaeoproterozoic evolution of the Svecofennian orogen within the Fennoscandian Shield at the northwestern corner of the East European Craton is proposed. The Svecofennian orogen is suggested to have formed during five, partly overlapping, orogenies: Lapland-Savo, Lapland-Kola, Fennian, Nordic and Svecobaltic. The Svecofennian orogen evolved in four major stages, involving microcontinent accretion (1.92-1.88 Ga), large-scale extension of the accreted crust (1.87-1.84 Ga), continent-continent collision (1.87-1.79 Ga) and finally gravitational collapse (1.79 and 1.77 Ga). The stages partly overlapped in time and space, as different processes operated simultaneously in different parts of the plates. In the Lapland-Savo and Fennian orogenies, microcontinents (suspect terranes) and island arcs were accreted to the Karelian microcontinent, which itself was accreting to Laurentia in the Lapland-Kola orogeny. The formation of the Svecofennian orogen was finalized in two continental collisions producing the Nordic orogen in the west (Fennoscandia-Amazonia) and Svecobaltic orogen in the SSW (Fennoscandia- Sarmatia). The collisions were immediately followed by gravitational collapse.

Compared with present-day global plate tectonics, Archaean and Palaeoproterozoic plate tectonics may have involved faster moving, hotter plates that accumulated less sediment and contained a thinner section of lithospheric mantle. This scenario also fits with the complex geodynamic evolution of the Fennoscandian Shield from 2.06 to 1.78 Ga when rapid accretion of island arcs and several microcontinent–continent collisions in a complex array of orogens was manifested in short-lived but intense orogenies involving voluminous magmatism. With a few exceptions, all major ore deposits formed in specific tectonic settings between 2.06 and 1.78 Ga and thus a strong geodynamic control on ore deposit formation is suggested. All orogenic gold deposits formed syn- to post-peak metamorphism and their timing reflects the orogenic younging of the shield towards the SW and west. Most orogenic gold deposits formed during periods of crustal shortening with peaks at 2.72 to 2.67, 1.90 to 1.86 and 1.85 to 1.79 Ga. The ca. 2.5 to 2.4 Ga Ni–Cu ± PGE deposits formed both as part of layered igneous complexes and associated with mafic volcanism, in basins formed during rifting of the Archaean craton at ca. 2.5 to 2.4 Ga. Svecokarelian ca. 1.89 to 1.88 Ga Ni–Cu deposits are related to mafic–ultramafic rocks intruded along linear belts at the accretionary margins of microcratons. All major VMS deposits in the Fennoscandian Shield formed between 1.97 and 1.88 Ga, in extensional settings, prior to basin inversion and accretion. The oldest “Cyprus-type” deposits were obducted onto the Archaean continent during the onset of convergence. The Pyhasalmi VMS deposits formed at 1.93 to 1.91 Ga in primitive, bimodal arc complexes during extension of the arc. In contrast, the Skellefte VMS deposits are 20 to 30 million years younger and formed in a strongly extensional intra-arc region that developed on continental or mature arc crust. Deposits in the Bergslagen–Uusimaa belt are similar in age to the Skellefte deposits and formed in a microcraton that collided with the Karelian craton at ca. 1.88 to 1.87 Ga. The Bergslagen–Uusimaa belt is interpreted as an intra-continental, or continental margin back-arc, extensional region developed on older continental crust. Iron oxide–copper–gold (IOCG) deposits are diverse in style. At least the oldest mineralizing stages, at ca. 1.88 Ga, are coeval with calc-alkaline to monzonitic magmatism and coeval and possibly cogenetic subaerial volcanism more akin to continental arcs or to magmatic arcs inboard of the active subduction zone. Younger mineralization of similar style took place when S-type magmatism occurred at ca. 1.80 to 1.77 Ga during cratonization distal to the active N–S-trending subduction zone in the west. Possibly, interaction of magmatic fluids with evaporitic sequences in older rift sequences was important for ore formation. Finally, the large volumes of anorthositic magmas that characterize the Sveconorwegian Orogeny formed a major concentration of Ti in the SW part of the Sveconorwegian orogenic belt under granulite facies conditions, about 40 million years after the last regional deformation of the Sveconorwegian Orogeny, between ca. 930 and 920 Ma.

The Kangasjarvi Zn-Cu deposit is a highly deformed and metamorphosed Paleoproterozoic volcanogenic massive sulphide (VMS) deposit located in the Vihanti-Pyhasalmi base metal mining district of central Finland. The host sequence to the deposit, referred to as the Inner Volcanic Sequence (IVS), is comprised of a bimodal suite of metavolcanic rocks and a regionally extensive tonalite-trondhjemite gneiss (sub-volcanic intrusions?). A separate and perhaps younger sequence of mafi c volcanic rocks, with irregular intervals of undifferentiated intermediate to felsic schists and metalimestones, referred to as the Outer Volcanic Sequence (OVS), are separated from the IVS sequence by intervals of metagreywacke and U-P-bearing graphitic schists. A stratigraphic scheme for rocks within the IVS is proposed based on outcrop observations, locally preserved volcanic textures, aspects of seafl oor-related hydrothermal alteration and lithogeochemistry. In this scheme, rare andesites form the lowermost volcanic stratigraphy and are overlain by typical island-arc basalts that were erupted in a subaqueous setting. Tonalite-trondhjemite subvolcanic intrusions were locally emplaced within andesites and coeval rhyolites were extruded on the basaltic substrate. The extrusion of rhyolites, including high-silica rhyolites, was coeval with regional-scale, pre-metamorphic seafloor hydrothermal alteration and local sulphide mineralization. Extensively altered rhyolites envelope massive sulphides and are underlain by altered basalts. The latter rocks are now characterized by a variety of low-variance metamorphic mineral assemblages (e.g. orthoamphibole-cordierite rocks) and define a domain of intense pre-metamorphic chlorite ± sericite alteration in the stratigraphic footwall of the deposit. The altered nature of these rocks is attributed to reaction with seawater-related hydrothermal fluids within a zone of upflow at or near the seafloor. The fundamental controls on convective hydrothermal circulation and subsequent alteration and massive sulphide mineralization at Kangasjarvi, and possibly elsewhere in the district, share many characteristics with other well-described, ancient VMS deposits (e.g. massive sulphide deposits in the Flin Flon Belt, Manitoba, Canada). These characteristics include: 1) an association with bimodal volcanism developed in extensional settings; 2) a close spatial association with regionally extensive felsic subvolcanic intrusions; and 3) petrogenesis of ore-associated volcanic rocks (e.g. high-silica rhyolites, felsic subvolcanic intrusions) indicative of substantial heat transfer from the mantle to the upper crust and the development of anomalous thermal corridors. These features translate into geochemically distinctive rock types that, when combined with aspects of stratigraphy and pre-metamorphic alteration, may be used to develop regional exploration strategies in the Vihanti-Pyhasalmi district.

Massive sulfide deposits in the Vihanti-Pyhasalmi district, central Finland, are hosted by a highly deformed and metamorphosed bimodal volcanic sequence. Alteration zones that envelop these deposits are composed of cordierite + orthoamphibole ± garnet-bearing rocks (referred to as COR rocks) belonging to upper amphibolite to granulite facies. Outcrops of mafic and felsic COR rocks (MCOR and FCOR) in the vicinity of the Ruostesuo Zn-Cu deposit define mappable layers with distinctive metamorphic mineral assemblages: orthoamphibole + cordierite ± plagioclase ± quartz (M1COR); garnet + orthoamphibole + cordierite ± plagioclase ± quartz (M2COR); quartz + cordierite + biotite ± orthoamphibole ± cummingtonite ± garnet ± plagioclase (F1COR); and quartz + cordierite + garnet + sillimanite (F2COR). These rocks are interlayered with, or grade into, typical amphibolites (i.e., hornblende + plagioclase) and quartz + plagioclase-bearing rocks that are interpreted as least altered mafic and felsic metavolcanic rocks, respectively. Whole-rock geochemical data for least altered metavolcanic rocks and COR rocks at Ruostesuo define three mafic suites (primitive basalts, basaltic andesites, and ferrobasalts) and two felsic suites (R1 and R2 rhyolites). Using a combination of high field strength element ratios and rare earth element profiles, COR rocks can be assigned to all five volcanic suites. Calculated mass changes for the majority of exposed COR rocks at Ruostesuo are similar to those reported for chlorite + quartz assemblages found within the inner core of alteration pipes located below some sea-floor massive sulfide deposits. Relative to least altered precursors, mafic COR rocks have gained Fe, Mg ± S and lost variable amounts of Si, Ca, Na, K, Rb, Ba, Sr, and Eu, which resulted in an average net mass loss of about 10 percent. The bulk composition of most altered mafic COR rocks (near total loss of Ca, Na, K) reflects the proportion of chlorite to quartz within the premetamorphic alteration assemblages and the composition of the chlorite. Mass-change results for altered mafic rocks indicate that basalts at Ruostesuo were altered by an evolved, Fe-rich hydrothermal fluid at high water/rock ratios. Mass changes for COR rocks of the R2 felsic volcanic suite are similar to those of the mafic rocks (enriched in Fe ± Mg and depleted in Ca, Na, K, Rb, Ba, Sr, and Eu) except for significant additions of Si that resulted in net mass gains of 20 to 30 percent. Felsic COR rocks of the R1 volcanic suite show gains in K, Rb, and Ba that are attributed to the formation of premetamorphic sericite during an alteration stage dominated by hydrothermal seawater. In comparison to least altered metavolcanic rocks, mafic and felsic COR rocks are depleted in 18O, with {Delta}18O values 2 to 5 per mil lower than precursor rocks. Estimated temperatures of alteration were 270° to 350°C. These data indicate that the signature of a high-temperature (~300°C) sea-floor hydrothermal upflow zone is preserved within upper amphibolite to granulite facies rocks at Ruostesuo. The stratigraphic boundary between strongly chloritized mafic volcanic rocks and silicified felsic volcanic rocks marks a transition from net mass losses associated with hydrothermal fluid-dominated alteration to net mass gains associated with seawater-dominated alteration. Silicification at or near the boundary between mafic and felsic volcanic rocks may have been the result of conductive cooling or mixing with seawater from above. In the absence of textural or crosscutting relationships indicative of a younging direction, a stratigraphic model is proposed on the basis of the above geochemical constraints. Altered basalts at Ruostesuo represent the exposed base of a chloritic stockwork zone that passes upsection into altered felsic volcanic rocks that host massive Zn-Cu sulfides. This stratigraphic model has significance for the interpretation of other massive sulfide deposits in the Vihanti-Pyhasalmi district and exploration strategies that couple chemostratigraphy with mass-change results. More significantly, the preservation of premetamorphic oxygen isotope values for strongly metamorphosed and locally migmatitic COR rocks indicate that oxygen isotope mapping is a viable exploration tool in the district and in other mineralized upper amphibolite to granulite facies terranes worldwide.

The presence of 1.91–1.93 Ga old granitoids at the Archean–Proterozoic boundary along the Raahe–Ladoga zone in Finland has been demonstrated on various occasions. These rocks have been considered to represent juvenile crustal material, as their e Nd values are markedly positive. However, as Svecofennian metasediments contain detrital zircons derived from a ca. 2 Ga old source, the possibility has existed that the 1.92 Ga age may have been a mixture between 2 and 1.89 Ga old zircon populations, as such mixing would not markedly affect their neodymium isotopic properties. Also, some syntectonic 1.89 Ga old Svecofennian granitoids contain heterogeneous zircon populations, but it has been impossible to determine the age and origin of the older zircons by conventional methods. NORDSIM ion probe results on three samples from the 1.92 Ga age group confirm the earlier conclusions. Especially important is that no zircons older than 1.95 Ga were detected in the 1.92 Ga group samples. Thus, the 1.92 Ga event was the beginning of the formation of new continental crust in the primitive Svecofennian island arc and these granitoids formed by partial melting of basaltic magmas derived from a depleted mantle source. One sample also contains a younger zircon population formed during the orogenic culmination at 1.89 Ga. In contrast, one grain from a sample representing the 1.89 Ga age group contains an Archean core, which is considered to represent sedimentary detritus assimilated during either magma formation or intrusion. While the results prove the true igneous nature of the 1.92 Ga event, they also rule out these rocks as a possible provenance for the ca. 2 Ga old zircons encountered in the Svecofennian metaturbidites. Thus, there is still no direct evidence from granitoid rocks for an extensive Svecofennian protocrust, the existence of which has been postulated on the basis of geochemical and SmNd isotopic data.

The Paleoproterozoic Svecofennian Orogen in the Fennoscandian shield can be regarded as one entity or as a collage of several accretionary units. To test for the possible occurrence of a suture zone dividing the Svecofennian Orogen into Central and Southern parts, we report new ion microprobe data on clastic zircons of nine samples of metasediments. Whole-rock Sm–Nd and geochemical data were acquired to characterize the source components of the metasediments. The Central Svecofennian metasediments in Finland can be divided to lower and upper types with inferred deposition ages at 1.92–1.91 and 1.90–1.88 Ga, respectively. Both types are immature and show a derivation from an unweathered orogenic source. The lower Central Svecofennian metasediments show a dominant Proterozoic age population at 1.93–2.02 Ga and a variation in eNd (1.9) values from about −3 to −0.5, which is mainly explained by variable amounts of Archean detritus (30–45%). The upper Central Svecofennian metasediments have eNd (1.9) at about 0, similar to values obtained from adjacent volcanic and plutonic arc rocks. One meta-arkose is tentatively classified as a molasse-type sediment with a source dominated by earlier Svecofennian sediments. Southern Svecofennian metasediments can be divided into mature and less mature types with dominant deposition ages at 1.90–1.88 Ga and eNd (1.9) values from −3 to +1. Mature quartzites seem to occur at least in two stratigraphic levels where the upper quartzites have maximum deposition ages at 1.87–1.86 Ga. A characteristic feature of some Southern Svecofennian metasediments is a dominant Proterozoic age population at 2.0–2.1 Ga. Coincident deposition of mature and immature sediments and volcaniclastic rocks indicate that different sources, weathered and non-weathered ‘basement’, and syn-depositional volcanic centers, produced simultaneously detritus to a large subsiding basin. Mature pelites and quartzites suggest a long residence time and stable passive margin or cratonic environment. The tectonic setting and possible provenance areas for the Central and Southern Svecofennian metasediments are also discussed. As a conclusion, the metasediments from the Central and Southern parts of the Svecofennian Orogen show different lithological characteristics and have contrasting source components. The presented isotope data indicate a different evolution of these parts prior to 1.89 Ga and the existence of a suture zone between them as proposed in earlier studies.

We present results of a geochemical survey based on 1150 outcrop samples for heat production studies in a E–W oriented band ( 120 km ×500 km) located approximately between 62 and 63°N in Finland, the central Fennoscandian shield. The results provide representative averages of heat production rate in the studied tectonic units, but also demonstrate the heterogeneity and spatial variation in radiogenic heat production rate of the Precambrian lithosphere. The study area covers formations from the Archaean granite-greenstone terrain in the east via a Palaeoproterozoic mobile belt to a major granitoid complex surrounded by schist belts in central and western Finland. In this study we review the heat production rates as calculated from the U, Th and K total analyses and densities of the samples, and the relations of heat production rate values with major tectonic setting, lithological types, major composition and petrophysical properties of rocks. The data is further correlated with heat flow values measured in boreholes and applied in a lithospheric heat production model in the central part of the shield. Generally, heat production rate increases from Archaean to Proterozoic rocks in E–W direction but this trend is relatively weak and often overrun by lithological variations. The arithmetic means are 1.1±1.3 μW m−3 in the Archaean domain, 1.3±0.6 μW m−3 in the Hoytiainen autochthonous Proterozoic domain, 1.2±0.5 μW m−3 in the Suvasvesi domain characterized by allochthonous Proterozoic cover, 1.0±0.6 μW m−3 in the Proterozoic Raahe–Ladoga Mobile belt, 1.3±1.1 μW m−3 in the Proterozoic Rantasalmi–Haukivuori domain, 1.6±0.8 μW m−3 in the Central Finland Granitoid Complex and 1.4±0.8 μW m−3 in the Bothnian Schist Belt. The standard deviations of the mean values are considerable and reflect the heterogeneity of heat production. All distributions overlap. Heat production rate shows spatial variations which range in scale from kilometres to tens of kilometres. Well-defined systematic variations of heat production rate values with either SiO2 content, density or P-wave velocity could not be found, although the correlations are statistically significant at 1% risk level. The correlations are weak and scattered, and sometimes opposite in sign in the major rock groups (plutonic, metavolcanic and metasedimentary). Heat flow density measured in boreholes increases with increasing heat production rate, but the present data do not support a strict linear dependence between the variables, and the regression line parameters vary depending on the applied heat production values (drill core analyses versus hole site averages from the present study). This indicates that the heat production rate is spatially heterogeneously distributed in the scale of the boreholes and their formations. A lithospheric heat production model was constructed using data from the present study for the upper crust, literature data for lower crustal xenoliths in the study area and a xenolith-derived heat flow density value for the mantle. The total crustal contribution to the surface heat flow density (37 mW m−2) from radiogenic heat sources is calculated as 26 mW m−2, and the contributions from the lower, middle and upper crust are 4, 11 and 11 mW m−2, respectively.

The central part of the Fennoscandian Shield in Finland is composed of the Palaeoproterozoic Svecofennian domain and the Archaean Karelian craton with a Palaeoproterozoic allochthonous and autochthonous cover. A cryptic suture separating these areas and another tentative suture dividing the Svecofennian into central and southern parts have been proposed. The chemical composition of sedimentary rocks (N=300) within the study area, including the effects of palaeoweathering, hydraulic sorting, depositional environment and post-depositional processes, have been studied in order to delineate sediment source components. The main proposed source components for the Archaean sedimentary rocks are weathered 3.0–3.2 Ga greenstone+granite±TTG and local 2.7 Ga sources. Autochthonous 2.2–1.9 Ga cover rocks were mainly derived from a mixture of chemically weathered palaeosol (2.2–2.35 Ga), sedimentary rocks derived from the palaeosol, and mafic dykes and plateau volcanics (mainly 2.2–2.1 Ga) although in places locally derived non-weathered Archaean sources dominated. Archaean crust and 2.0–1.92 low-K bimodal rocks from a primitive island arc are the proposed source for the allochthonous Western Kaleva cover rocks. These formed in a subsiding foredeep during initial collision from orogenic detritus in the same oblique collision zone. The central Svecofennian sedimentary rocks can be divided into local arc-derived rocks (≤1.89 Ga) and older (≥1.91 Ga) rocks from a mixture of Western Kaleva sources and a 1.91–2.0 Ga mature island arc/active continental margin source. Rifting followed by increased subsidence during initial collision in the NE and subsequent arc reversal caused rapid erosion from the mountain belt, exposing diverse source compositions as seen in the large variation of Th/Sc (2–0.5), and deposition into an oblique hinterland basin further developing into a subduction related foredeep. Mature greywackes from the southern Svecofennian in the study area resemble passive margin sediments with a source dominated by inferred alkaline-affinity complexes and Archaean rocks. Less mature rocks also occur and had sources dominated either by island arc/active continental margin rocks or local picritic rocks. In the sedimentary record the Archaean–Proterozoic transition up to 2.1 Ga was dominated by input of mainly mafic plateau-type volcanic contribution to the Archaean detritus. Palaeoproterozoic sediments having a crustal component (≤2.1 Ga) show higher Th/Sc, Th/Cr, and lower Sm/Nd and Eu/Eu* relative to the Archaean rocks but locally low Th/Cr ratios complicate the situation. Ba depletion relative to K, Rb and Th is a characteristic feature of the sedimentary rocks of the central Fennoscandian Shield indicating high amounts of Ba lost from the clastic record during 2.3–1.9 Ga and further recycled back to the mantle forming a subduction component and an enriched mantle component. Ba depletion seems to have been especially characteristic of chemical weathering during 2.35–2.2 Ga under CO2-rich and low-O2 atmosphere. Whether this strong Ba depletion is characteristic of the Archaean–Proterozoic transition and quiet supercontinent stages in general remains to be determined.

The central part of the Svecofennian domain in Finland is characterized by thick lithosphere with crustal thickness up to 60–65 km. Major collisional events at 1.91-1.90 Ga and 1.89 Ga and a thrusting event at 1.86-1.84 Ga, the latter due either to continuation of shortening or to a separate intracrustal collision, are considered the major causes of crustal thickening. The origin of the Svecofennian domain has been formerly attributed to mixing of depleted mantle melts with variable amounts of Archaean crustal component via subduction, but the new data and reinterpretation of older data imply that, besides juvenile crust, the Svecofennian domain also contains older Palaeoproterozoic crustal components. No Archaean component is found in the 1.93-1.91 Ga gneissic tonalites and related felsic volcanics (eNd (t) + 1 – +4) which occur in the Savo Belt (SB), adjacent to the Archaean craton. The depleted mantle model ages vary from 1.94 Ga to 2.33 Ga with most values ≤2.0 Ga. The analysed syntectonic (post-thickening) granitoids (≤1.89 Ga) in the SB exhibit positive eNd (t) values ranging from +2.8 to +3.0 pointing to a juvenile source. The hypersthene granitoids are similar in age but show epsilon values around −1, which are only slightly lower than found in EM-derived mafic rocks. The isotopic and geochemical data for the Central Finland Granitoid Complex and Tampere Schist Belt indicate the occurrence of evolved thick crust (≥2.0 Ga) and associated lithospheric mantle already at 1.91 Ga. The southernmost part of the Southern Svecofennian in Finland also shows evolved-type crust (≥2.0 Ga). A tentative model for the development of crust and mantle is presented. It includes the occurrence of continental nucleus surrounded by more juvenile island arc accretions. Two successive collisional events thickened both the crust and the subcontinental lithospheric mantle, with the greatest relative amount of thickening occurring in the mobile zones. Convective removal of the lowermost parts of thickened lithosphere caused an upwelling of hot asthenosphere, increase in the geotherm, and melting of both the enriched subcontinental lithospheric mantle and the convective depleted asthenosphere. Mafic magmatism during the post-thickening stage (≤1.89 Ga) thus varied from dominantly enriched- to dominantly depleted-mantle melts and was associated with variable crustal contamination. The thickening of the lithosphere in the central part of the Svecofennian domain was not followed by large extension, suggesting that the thickness was already great 1.88-1.80 Ga ago. This, together with the thick high-velocity lower crust, could have provided the isostatic balance needed to stabilize the thick crust.

The Precambrian bedrock of Finland forms the core of the Fennoscandian Shield. The Archean area includes exposed Archean rocks (23%) and Archean crust overlain by Paleoproterozoic supracrustal and plutonic rocks (30%). The Proterozoic area is composed of the Paleoproterozoic Svecofennian Province (41%) intruded by rapakivi granites (4%).