Svecofennian Crust Research Articles

Tracing arclogites in the Paleoproterozoic Era – A shift from 1.88 Ga calc-alkaline to 1.86 Ga high-Nb and adakite-like magmatism in central Fennoscandian Shield

Arclogites, i.e., lower crustal garnet-pyroxenite cumulates, are suggested to play an important role in controlling magma differentiation in modern continental arcs. Until now, arclogite-related magmatism has only been described from the Phanerozoic Era. The Svecofennian orogen in the central Fennoscandian Shield hosts a rare association of 1.86 Ga igneous rocks geochemically distinct from the surrounding and much more abundant 1.90–1.87 Ga subduction-related calc-alkaline magmatism. The 1.86 Ga magmatic rocks are divided into three groups: 1) high-Nb gabbros (HNB) which are enriched in Fe2O3T, TiO2, P2O5, F, LILE, and HFSE (especially Nb; 18.9–44 ppm), show positive initial εNd value, and near-chondritic but variable initial zircon εHf values; 2) high-Mg gabbros (HMG) which are characterised by high MgO, CaO, Cr and Ni contents, slight enrichment in LILE, positive εNd, and positive but variable zircon εHf values; 3) adakite-like rocks showing high Al2O3 and Na2O contents, slight enrichment in LILE, relative depletion in some HFSE, positive εNd value, and chondritic to negative zircon εHf values. The three groups yield zircon U-Pb ages of ~1.86 Ga and exhibit undeformed textures in contrast to the surrounding supracrustal rocks metamorphosed at ~1.88 Ga. The ages and compositions are clearly different from the adjacent 1.90–1.87 Ga arc-related igneous rocks suggesting a distinct origin. Despite similar ages and close spatial relationship, separate sources are required for each of the different 1.86 Ga rock groups. Trace element modelling of partial melting suggests that arclogites, with compositions similar to pyroxenite xenoliths found in the kimberlite pipes of eastern Finland, are the source for the HNB rocks. In contrast, subduction-modified mantle peridotite is the source for the HMG rocks, and a mafic lower crustal source is suggested for the adakite-like rocks. The following geodynamic model is suggested: (rutile-bearing) arclogite formation at 1.90–1.87 Ga followed by arclogite delamination and partial melting during extension of the thickened Svecofennian crust at 1.86 Ga.

Deposition conditions for the indium-bearing polymetallic quartz veins at Sarvlaxviken, south-eastern Finland

ABSTRACTPolymetallic quartz veins, with up to 1500 ppm indium, have been discovered recently in the Sarvlaxviken area within the 1.64 Ga anorogenic multiphase Wiborg rapakivi batholith and adjacent 1.90 Ga Svecofennian crust in SE Finland. Evidence from primary fluid inclusions in the Sarvlaxviken area provides new information on the hydrothermal transport and depositional processes of metals in anorogenic granites. Fluid inclusions with variable aqueous liquid and vapour proportions (5–90 vol.% vapour) occur in quartz, cassiterite and fluorite belonging to three generations of polymetallic quartz veins. Microthermometry indicates that the veins were deposited at temperatures that range from ~500°C down to <100°C and salinities from 0 to 16 eq. mass% NaCl. Fluid inclusion data show that the depositional conditions were similar regardless of vein generation. The interpreted depositional processes involve phase separation with a combination of condensation, cooling and boiling of an initially low-salinity (<3 eq. mass% NaCl) aqueous magmatic vapour phase enriched in CO2-F-Cl-S and metals. Fluid inclusions with low salinities dominate, but higher salinities are recorded in metal-rich parts of the veins. The turbulent fluid flow, with complex geometry and temperature-salinity patterns, may explain why sulfide and/or oxide opaque minerals occur irregularly, and are locally the dominating vein minerals, but disappear completely into barren parts of the quartz veins. All fluids are considered to have been generated by the F-rich Marviken granite (and related granite dykes), which show all geochemical criteria for an ore-fertile granite. The quartz veins investigated in the adjacent Svecofennian country rocks are considered to represent the very last stage of a fluid with similar characteristics to the fluid responsible for the ore formation in the Sarvlaxviken area, but that had cooled to <100°C.

Magmatic age of rare-earth element and zirconium mineralisation at the Norra Kärr alkaline complex, southern Sweden, determined by U–Pb and Lu–Hf isotope analyses of metasomatic zircon and eudialyte

The agpaitic Norra Kärr alkaline complex in southern Sweden is rich in heavy rare-earth elements and zirconium. Despite generally containing high concentrations of Zr, agpaitic rocks sensu stricto are devoid of igneous zircon. During the late stages of magmatic activity at Norra Kärr, metasomatic Na- and F-rich fluids transporting Zr complexes caused fenitisation (syn-magmatic alkali metasomatism) of the granitic wall rocks, which formed new metasomatic zircon. Fenite zircon was dated by LA-MC-ICP-MS with the U–Pb method at 1.49±0.01Ga, while the unaltered country rock granite was dated at 1.79±0.01Ga. Zircon in the fenites exhibits εHf+6.58±0.36 at 1.49Ga; significantly above the expected value for Svecofennian crust at that time (ca. −5±2), but identical, within error, of that measured in eudialyte from Norra Kärr. This suggests a common source of Hf for fenite zircon and eudialyte from Norra Kärr, which isotopically strengthens the genetic link between fenite and nepheline syenite. The 1.49Ga age dates not only the fenitisation, but also the relatively small agpaitic intrusion, which caused the fenitisation. This also dates the origin of the rare-earth element and Zr ore within the agpaitic nepheline syenite, which probably formed from an evolved magma derived from the mantle. A few zircon xenocrysts, which survived in the agpaitic nepheline syenite magma, may indicate the presence of an Archaean component in the basement.

Fingerprinting Gondwana versus Baltica provenance: Nd and Sr isotopes in Lower Paleozoic clastic rocks of the Małopolska and Łysogóry terranes, southern Poland

Neodymium and strontium isotope compositions of clastic material in Lower Paleozoic rocks, in addition to detrital zircon and mica geochronology and faunal data are used to constrain the geotectonic provenance of the Małopolska and Łysogóry terranes in southern Poland. Being characterized by markedly different geological histories during Early Paleozoic times, Małopolska and Łysogóry were most often interpreted as exotic terranes, but their identity, provenance and accretionary history are still being discussed. This study presents the first systematic Nd and Sr isotope survey of the Lower Paleozoic siliciclastic rocks. Cambrian clastic rocks of Małopolska differ in their Nd and Sr isotope compositions from those of Łysogóry. The former are characterized by a wide range of the εNd(505Ma) values, from −11.7 to −5.7, Sr isotope ratios from 0.712 to 0.767, and model ages (T2DM) predominantly younger than 1.9Ga, whereas the εNd(t) values of the latter vary within a fairly narrow range of the εNd(505Ma), from −11.0 to −8.3, Sr isotope ratios from 0.722 to 0.754, and model ages are older than 1.9Ga. New Nd data combined with detrital zircon ages provide evidence that Małopolska must have been supplied with clastic material from Amazonian sources during Cambrian times, being also progressively sourced from the Baltic Svecofennian crust in the Middle Cambrian. Detritus recognized in Łysogóry has mostly been derived from the Svecofennian basement of Fennoscandia, but it has also a small addition from Cadomian sources. All provenance and biogeographic data suggest that Małopolska was situated close to the Amazonian Craton near the western end of the Cadomian Belt during the Early Cambrian, and it was not until the late Middle Cambrian that Małopolska collided with the Baltica margin. During the Late Cambrian Łysogóry occupied a position between Baltica and the Cadomian Belt but it was probably not yet accreted to the Baltic crust at that time.

The Borodinskii Massif of potassium porphyric granite: Results of U–Pb dating and its tectonic position (Fennoscandian Shield)

It has been demonstrated for the first time that the Svecofennian crustal porphyric granites in the southeastern part of the Fennoscandian Shield are clearly subdivided into two age groups of 1.87 and 1.80 Ga. The representative of the first group is the Kuznechenskii Massif of porphyric granites with a U–Pb age of 1874 ± 4 Ma belongs to the group of plutons formed during the orogenic stage of Svecofennide evolution. The Borodinskii Massif with an age of 1797 ± 2 Ma is a member of the second group: these plutons were formed under tectonically stable (epiplatform) conditions. Therefore, the formation of porphyric granites occurred as the result of repeated generation of crustal magmas during multistage accretional growth of the Svecofennian crust.

The mode of deformation in the orogenic mid‐crust revealed by seismic attribute analysis

AbstractThe processes and the structures within the middle and the lower crust are intimately related to the evolution of orogenies, but more detailed knowledge is needed of these deep crustal processes. Seismic reflection data, chiefly displayed in amplitude, are commonly used to interpret deep crustal structures. The strongest amplitude events are probably related to lithological changes and do not necessarily correspond to geological structures, tectonic boundaries, or deformation fabrics. Furthermore, the detailed structures and the strain distribution between the interpreted structural boundaries remain obscure. Here, we show an example of how seismic attributes, combined with the seismic facies interpretation technique, can be used to enhance 2D seismic reflection data from the Palaeoproterozoic Svecofennian crust of southern Finland, to reveal unprecedentedly detailed information about the deformation fabrics within the mid‐crust of a collisional orogen. The images are plausibly interpreted to show that the extension/lateral flow of the orogenic middle and lower crust was mainly accommodated by kilometer‐scale S‐C′ structures. The structures form penetrative deformation fabrics which are correlated with outcrop observations. The successful enhancement of the seismic data confirms the ductile extension affecting hot orogenic crust, and gives new information about the strain distribution of the regional, syn‐ to late‐orogenic deformation. The seismic attribute method and the seismic facies interpretational approach described in this paper should be applicable to other seismic datasets from the crystalline basement.

Ancient and juvenile components in the continental crust and mantle: Hf isotopes in zircon from Svecofennian magmatic rocks and rapakivi granites in Sweden

The sources of igneous rocks in the continental crust are elusive, but they may be traced by radiogenic isotopes, which convey a message about the age and composition of the concealed parts of the continent. We investigated the Hf-isotope composition of zircon in ten rocks from central and southern Sweden. Two felsic metavolcanic rocks and two metagabbros (ca. 1.89 Ga) from Bergslagen, southern Sweden, show epsilon(Hf)(t) ranges of -1.8 to +5.1 and +2.6 to +6.8, respectively, suggesting that juvenile sources have contributed to both. A 1.85 Ga granite from southern Bergslagen shows a epsilon(Hf)(t) range of -2.6 to +4.6 for magmatic zircons, but both highly negative and positive values for inherited grains, providing evidence for both Archean and juvenile crustal sources. These and previous data confirm the existence of juvenile proto-Svecofennian crust (<2.2-1.9 Ga) with a minor Archean component, from which later crustal magmas were generated. The Hf-isotope evolution curve for this crust can be approximated by epsilon(Hf)(1.90) = 3 +/- 3 and (176)Lu/(177)Hf = 0.018. Similarly, the present data, together with data for younger mafic intrusions, can be used to infer the presence of a mildly depleted sub-Svecofennian mantle evolution curve with epsilon(Hf)(1.90) = 4.5 +/- 2.5 and (176)Lu/(177)Hf = 0.0315. Zircons from four out of five rapakivi intrusions (1.53-1.50 Ga) in central Sweden yield negative epsilon(Hf)(t) in the range -9.8 to -4.6, suggesting mixed Archean and juvenile Svecofennian sources. One intrusion farther south ranges between epsilon(Hf)(t) of -4.1 and -1.6, and has a larger contribution from Svecofennian crust. The data suggest that the crust in Bergslagen, southern Sweden, is dominantly Paleoproterozoic, while higher proportions of Archean material are present below central Sweden.

1.8 Ga magmatism in southern Finland: strongly enriched mantle and juvenile crustal sources in a post-collisional setting

Whole-rock and isotope geochemistry of six ∼1.8 Ga post-kinematic intrusions, emplaced along the ∼1.9 Ga Southern Svecofennian Arc Complex (SSAC) and in the SW part of the Karelian Domain in Finland, was studied. The intrusive age [U–Pb secondary ion mass spectrometer (SIMS)] of one of these, the Petravaara Pluton, was determined as 1811 ± 6 Ma. Basic-intermediate rocks are alkali-rich (K2O + Na2O > 4 wt.%) and typically shoshonitic, strongly enriched in large ion lithophile elements and light rare earth elements, but relatively depleted in high field strength elements and heavy rare earth elements. The enrichment is much higher than can be accounted for by crustal contamination and requires previously melt-depleted mantle sources, subjected to variable metasomatism by carbonate-rich fluids and sediment-derived melts. These sources are inferred to consist of phlogopite ± amphibole-bearing peridotites from depths below the spinel–garnet transition, as shown by the high Ce/Yb ratios. 87Sr/86Sr(1.8 Ga) ratios in the range 0.7027–0.7031 and ‘mildly depleted’ ϵNd(1.8 Ga) values (+0.1 to +1.4), with T DM values <2.1 Ga, suggest that mantle enrichment was associated with the previous Svecofennian subduction–accretion process, when enriched sub-Svecofennian mantle sections developed, dominantly characterized by 147Sm/144Nd ratios of 0.14–0.17. The associated granitoids are diversified. One group is marginally peraluminous, transitional between I (volcanic-arc) and S (syn-collisional) types, and was derived from mixed igneous and sedimentary, but juvenile Svecofennian source rocks, as supported by near-chondritic ϵNd(1.8 Ga) and somewhat elevated 87Sr/86Sr(1.8 Ga). The other group is transitional between I and A (within-plate) types in character and had dominantly igneous protoliths. The whole-rock geochemistry and isotopes suggest that the compositional variation between ∼50 and 70 wt.% SiO2 may be explained by hybridization between strongly enriched mantle-derived magmas and anatectic granitic magmas from the juvenile Svecofennian crust. One intrusion in the east contains a significant portion of Archaean, mostly igneous protolithic material (ϵNd(1.8 Ga) = –2.8 and ϵHf(t) for zircons between +2.8 and −11.9, with an average of −4.9). The ∼1.8 Ga post-kinematic intrusions were emplaced within the SSAC subsequent to the continental collision with the Volgo-Sarmatia craton from the SE, during a shift from contraction to extension, that is, in a post-collisional setting.

Secondary magnetizations in shear and fault zones in southern Finland

Studies on remanent magnetization of shear and fault zones of crystalline rocks in southern Finland have been carried out to test juvenile fluid activity that can be dated paleomagnetically. Locating in the central part of Baltica, the area has been tectonically relatively quiet since the Precambrian. However, tectonism at the margins of the craton during the Caledonian, Hercynian and Uralian orogenies probably reactivated faults and resulted in low-temperature hydrothermal activity also within the craton. We identified two consistent remanent magnetization components in four (out of five) studied localities. One remanence component has an intermediate to high coercivity NNW directed magnetization ( D = 334.7°; I = 35.2°; k = 92.1; α 95 = 9.6°) and unblocking temperatures suggest that it resides in (titano)magnetite. The direction is similar to the remanence that is commonly observed in Svecofennian age (ca. ~ 1.9 Ga) rocks all over the Precambrian Fennoscandian Shield, and probably represents the primary Svecofennian magnetization preserved in the host rocks. Another common component is directed NE ( I = 30.8°; D = 35.3°; k = 264.5; α 95 = 5.7°) and is carried by hematite and maghemite. Its resulting paleomagnetic pole compares well with Phanerozoic paleopoles of Baltica, and is suggested to represent a Permian remagnetization. We interpret that this remanence reflects the reactivation of the Svecofennian crust in the late Paleozoic due to tectonic events related to the formation of the supercontinent Pangea. In addition, remagnetizations due to Mesoproterozoic rapakivi magmatism and younger (Mesozoic to present) events were identified.

Delamination of eclogitized lower crust: Control on the crust–mantle boundary in the central Fennoscandian shield

The crustal structure of the central part of the Fennoscandian shield is dominated by a high velocity lower crust (HVLC) ( V P 7.3–7.5 km/s, at the depth of 40–63 km in an area of ca. 400 km × 600 km) formed in the Palaeoproterozoic Svecofennian orogen at ∼ 1.91–1.87 Ga. In the area of the HVLC the Moho is deeper than 50 km and attains depths of up to 63 km in eastern Finland at the Archaean–Proterozoic boundary zone. We suggest that the present Moho dates back to late Svecofennian time and was determined by delamination of high-density eclogitized lower crust after tectonic collisional thickening of crust. Densities deduced from seismic and gravity data suggest a very small or completely lacking density contrast between lower crust and uppermost mantle. New geological evidence from kimberlite-hosted garnet xenocrysts supports the presence of a garnet population with compositions in agreement with crustal eclogitic garnets. In addition, small clinopyroxene–garnet mineral aggregates (microxenoliths) show omphacitic clinopyroxene compositions. Therefore, the presence of eclogites in the high velocity lower crust is implied, although the eclogite proportion as estimated from seismic velocity data is only a few percent in the present HVLC. With results on near-incidence and wide-angle seismic studies and geological and geochemical data on heat producing elements we modeled the Svecofennian collision and the thermal evolution of the crust to provide temperature constraints for the delamination model. The constructed thermal model simulates the genesis of Svecofennian synkinematic (at 1.89–1.88 Ga) and postkinematic (1.88–1.87 Ga) granitoids, as well as the general metamorphic grade of the surface rocks and lower crustal xenoliths. Moreover, the narrow distribution of the ages of the Svecofennian mafic–ultramafic magmatism (ca. 1.89–1.87 Ga) dispersed around the HVLC area is explained by the homogeneous warming of the lithosphere. The conditions of lower crustal delamination were estimated with a Rayleigh–Taylor convective instability model constrained with lower crustal and upper mantle rheological data. Delamination of a lower crustal eclogite layer 5–30 km thick takes place in about 7–50 Ma if the rock dynamic viscosity is ≤ 1·10 21 Pa s. It requires temperatures of ≥ 1000 °C in the lower crust and upper mantle. In the Svecofennian crust, such temperatures were attained at about 1860–1850 Ma, i.e. after the orogenic peak. We conclude that the Moho of the HVLC area was controlled by delamination of a ca. 5–10 km thick layer of high-density lowermost crust into the mantle. Such a density-controlled process may have controlled the Moho depth also in other Proterozoic shield areas.

Timing of deformation phases within the South Finland shear zone, SW Finland

The Palaeoproterozoic Svecofennian crust in southern and central Fennoscandia was established about 1.8 Ga ago after a prolonged history of accretion and intrusion. During late stages of the Svecofennian orogeny, deformation was partitioned into several crustal-scale shear zones in present-day Finland, Sweden and Estonia. One such major ductile deformation zone, ‘the South Finland shear zone’ (SFSZ) extends for almost 200 km through the Åland archipelago in southwestern Finland, and further along the southern and southwestern coast of Finland. This more than a kilometer wide transpressional zone appears to have been repeatedly reactivated. The deformation started with a period of regional, ductile dextral shearing of igneous rocks, producing striped granodioritic and tonalitic gneisses. The ductile phases are locally overprinted and followed by ductile to semi-ductile deformation evidenced by mylonite zones of variable width. The last stage of tectonic activity along the shear zone is recorded by pseudotachylites. Within this study, we dated zircons (SIMS U–Pb) and titanites (ID-TIMS U–Pb) from eight rock samples, and two pseudotachylite whole-rock samples ( 40Ar/ 39Ar) in order to reconstruct the deformation and (re)activation history of the shear zone. The results suggest that the medium-grained gneisses underwent three distinct deformation phases separated by time intervals without regional deformation. The ductile deformation within the study area initiated at ∼1.85 Ga. A second, more intensive deformation phase existed around 1.83 Ga, by which the shear zone was already well developed. Finally, the last ductile event is recorded by ∼1.79 Ga metamorphic titanites in relatively granoblastic granitoid gneisses that nevertheless already display protomylonitic textures, suggesting the initiation of large-scale mylonitisation around or soon after this time. The age of a pseudotachylite sample and, hence, the brittle deformation is bracketed between 1.78 and 1.58 Ga based on the age of pegmatites cut by pseudotachylites as well as 40Ar/ 39Ar minimum ages for the pseudotachylite, respectively. The data imply that the rocks within the study area entered the ductile–brittle transition zone due to rapid cooling and exhumation of the crust after ∼1.79 Ga.

A tomographic crustal velocity model of the central Fennoscandian Shield

SUMMARY In this paper, local seismic tomographic method is used to find the terrane distribution within the central parts of the accretionary Svecofennian Orogen. This study presents a crustal Pwave velocity model, and for the first time the S-wave velocity and Vp/Vs ratio model of SVEKALAPKO area that is 700 × 800 km 2 in southern and central Finland. The 3-D model is presented as P- and S-wave velocity as well as Vp/Vs ratio depth slices and vertical sections. The data set comprised of several subsets of crustal Pg- and Sg-wave traveltime data: from local events recorded by the SVEKALAPKO seismic tomography array in 1998‐1999, from older controlled source shots recorded at portable stations as well as at permanent stations, and from non-controlled chemical explosions recorded at permanent seismic stations. From 300 local explosions a total of 10 404 Pg-wave and 9361 Sg-wave arrival times were inverted to create independent 3-D Vp and Vs tomographic models, from which the Vp/Vs ratio was calculated. According to sensitivity tests, the optimal horizontal resolution of recognizable velocity structures in the central study area is at least 60 km down to depths of 40 km. In the final model, the seismic velocities vary smoothly. The lateral variations are larger for Vp (5.9‐6.6 km s −1 ) than for Vs (3.5‐3.8 km s −1 ) in the upper 20 km of the crust. At depths of 20‐40 km, Vp varies from 6.5 to 7.2 km s −1 and Vs from 3.7 to 4.1 km s −1 . The Vp/Vs ratio varies spatially more distinct than P- and S-wave velocities, usually from 1.70 to 1.74 in the upper crust and from 1.72 to 1.78 in the lower crust. Schist belts and their continuations at depth are associated with lower velocities and lower Vp/Vs ratios (Vp < 6.2‐6.8 km s −1 ; Vs < 3.6‐3.9 km s −1 ; Vp/Vs = 1.68‐1.73) than the granitoid areas (Vp = 6.3‐7.4 km s −1 ; Vs = 3.6‐4.2 km s −1 ; Vp/Vs = 1.72‐1.78). The Svecofennian Orogen was accreted from crustal blocks ranging in size from 100 × 100 km 2 to 200 × 200 km 2 in cross-sectional area. The intervening sedimentary belts have ca. 0.2 km s −1 lower P- and S-wave velocities and ca. 0.04 lower Vp/Vs ratios. Tomographic model supports the idea that the thick Svecofennian crust was accreted from several crustal terranes, some hidden, and that the crust was later modified by intra and underplating.

1.83—1.82 Ga formation of a juvenile volcanic arc—implications from U—Pb and Sm—Nd analyses of the Oskarshamn-Jönköping Belt, southeastern Sweden

The Oskarshamn-Jönköping Belt in southeastern Sweden is a geographically well-defined area comprising calc-alkaline intrusions and volcanic rocks together with units of coarse-grained clastic metasedimentary rocks. Deviating from the general composition is the Fröderyd Group with basalts of MORB character. The belt is surrounded by the 1.81-1.77 Ga Transscandinavian Igneous Belt. A conglomerate clast from the central part of the Oskarshamn-Jönköping Belt yielded a U—Pb zircon age of 1829±8. This age confirms the c. 1.83-1.82 Ga formation age of the belt. Sm—Nd whole rock analyses of various rock types throughout the Oskarshamn-Jönköping Belt show that no substantially older (>100 m.y.) continental material contributed to these rocks. Furthermore, the high positive εNd values of many of the analysed rocks points to the depleted mantle as the main component of their source. The Sm—Nd analyses also suggest that while some of the felsic units were formed by a high degree of magmatic fractionation from a mantle derived melt others were formed as the result of remobilisation of older Svecofennian crust. The new U—Pb data together with previously published ages imply that the formation of the Oskarshamn-Jönköping Belt was a rather quick process, possibly completed in c. 10 m.y. The most likely model of genesis for the area, based on the new U—Pb and Sm—Nd data, is formation at a continental margin subduction zone to the present south-west of a slightly older Svecofennian continent. In such a model the Fröderyd Group represents either a fore-arc setting or a back-arc rift.

Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland

In an effort to quantify the time parameter in the tectonomagmatic evolution of what has been called the Southern Svecofennian Arc Complex (SSAC) of SW Finland, advanced radiometric dating techniques have here been applied to rock groups of key importance in that area. In this paper we report the results of 131 high-resolution ion microprobe spot analyses (SIMS) of zircons, and 33 measurements using isotope dilution mass spectrometry (ID-TIMS) on zircon, monazite and titanite ,a nd employing both large-sample multi-grain as well as single-grain techniques. The To rsholma area of the Aland archipelago ,s ituated between southern Finland and central Sweden, is a key structural area significant to resolve the time dimension in Svecofennian tectonics.There a collage of imbricated rock slabs was formed by tectonic shortening representing the culmination of large-scale penetrative Svecofennian deformation. Another structurally significant feature investigated is the South Finland Shear Zone (SFSZ) that transects the southwest-Finnish archipelago and further east follows the southern coast of Finland.This shear zone forms the southern limit of the c. 1830 Ma Late Svecofennian Granite and Migmatite Zone (LSGM) and also features deformations of a later stage when the considered region of Svecofennian crust was consolidated. The obtained age results and thei rt ectonic analysis can be summarized as follows. The Enklinge volcanic sequence (1885±6 Ma) is within error limits coeval with the intrusion of abundant early-kinematic gneissose granodiorites whose average age of 1884±5 Ma marks the formation of new crust in this region .S ome of these geisses contain a significant amount of 2000–2080 Ma zircon. Although many Svecofennian granitoids are known to contain heterogeneous zircon populations, mainly formed c. 1890 Ma ago but also containing an inherited component, the Kokar gneiss is, to the best of our knowledge ,t he firs t case where inheritance from c. 2030 Ma sources has been unequivocally demonstrated in a syntectonic Svecofennian intrusive rock. At To rsholma ,g ranodiorites (1879±6 Ma) have similarly intruded the supracrustal series, but there they were later metamorphosed into granulite orthogneisses.The mesoscopic recumbent folds and subhorizonta ls chistosit yo ft hes eg neisses were transected by a set of steep amphibolitic dykes indicating an episode of extension. During a preceding stage ,t hese gneisses had been sandwiched like an allochthonous slab unit between the supracrustal rocks. Yo unger sheet-like granodioritic intrusions (1861±19 Ma) with associated dykes (1865±7 Ma) reflect an even later stage of collisional thrusting. Altogether this tectonic evolution lasted for approximately 15 Ma (c. 1875–1860 Ma) and records a period of significant deformations during the Svecofennian orogeny. Monazites and zircon rims yield concordant U-Pb ages of c.1830 Ma.This is in agreement with previously obtained ages of late Svecofennian granites and migmatites in the LSGM zone in S Finland. Weakl yd eformed pegmatites and even-grained granit ed ykes were emplaced 1790±6 Ma ago and intersect the other rocks of the area.They characterize the vaning stages of shearing along the northwestern part of the SFSZ, and the establishment of a consolidated crust where deformation was about to cease by this time.

Prolonged postcollisional shoshonitic magmatism in the southern Svecofennian domain – a case study of the Åva granite–lamprophyre ring complex

The Åva ring complex is one of four Paleoproterozoic postcollisional shoshonitic ring complexes in southwestern Finland. It is composed of ring dykes of K-feldspar megacryst-bearing granite, mingled in places with a shoshonitic monzonite, and lamprophyre dykes crosscutting all the rocks in a radial pattern. A survey was undertaken to trace the magma chamber beneath the ring complex to date it and measure some intensive parameters to clarify the crystallisation conditions at depth before the granite was emplaced in the upper crust. Mineral separates were extracted from the core zones of K-feldspar megacrysts in the granite, heavy mineral fractions (including zircons) from these separates were used for P- T assessment and age determinations, and the results were compared to data obtained from bulk rock samples. It appears that magma differentiation took place in a midcrustal magma chamber (at 4 to 7 kbar) possibly ∼30 Ma before the emplacement of the ring complex in the upper crust (deep assemblage ∼1790 Ma, shallow assemblage ∼1760 Ma). Relatively high activity of the alkalies and a low oxygen fugacity characterised the midcrustal chamber. The juvenile Svecofennian crust was invaded by shoshonitic magmas from an enriched lithospheric mantle over a long period of time. Some of these magmas were stored and differentiated in the middle crust before transportation to the upper crust. The results also show that coarse-grained granites may provide evidence for several magmatic evolutionary episodes, e.g., differentiation and crystallisation in different environments prior to final emplacement.

Origin of chemically distinct granites in a composite intrusion in east-central Sweden: geochemical and geothermal constraints

A few tens of millions of years after the intrusion of the Early Svecofennian (∼1.87–1.85 Ma) granitoids in central Sweden, a renewed magmatic activity resulted in the emplacement of the Late Svecofennian granites, the tectonic setting of which remains obscure. S-type granites dominate this group, but both I-type and transitional granites are common. This study deals with one of these intrusions in east-central Sweden; a composite pluton that is insignificantly deformed and hosts both I- and S-type granites. One of the I-type granites shows a compositional trend from granodiorite to granite, which is uncommon among the Late Svecofennian granites. Major element and incompatible trace element compositions and ɛNd data show that two different sources, one igneous and one sedimentary, were involved. An important conclusion is that nearly coeval granites derived from different sources are found in close connection. The granites are suggested to have formed by partial melting in a thickened continental crust that was formed in an early stage of the Svecofennian event. Thermal models suggest that the slightly older, high-temperature I-type granite (granodiorite) was formed deeper in the crust than the S-type granite. The coexistence of essentially pure I- and S-type granites, rather than transitional mixtures, reflects the relative depths of the proposed sources and the varying thermal parameters of the lithologic units in the Svecofennian crust.

Metallogeny of Gold in the Precambrian of Northern Europe

Fennoscandian gold deposits have been successfully explored in a wide range of Precambrian geological environments in Sweden and Finland during the last two decades. Under-explored areas still exist at other sites in the Fennoscandian Shield, particularly in Russia and Norway, and there is a high potential for future new discoveries of economic gold in northern Europe. Careful geological documentation of a number of previously productive gold deposits, such as Haveri and Saattopora (Finland), Boliden, Adelfors, and Enasen (Sweden), and Eidsvoll and Bidjovagge (Norway), is now available in the literature. Kutemajarvi, Pahtavaaraa, and Suurikuusikko (Finland), and Bjorkdal, Akerberg, Harnas, and Pahtohavare (Sweden) represent a new generation of gold mines, and recent research on these enables an improved base for formulating regional and local metallogenetic models. Younging trends from northeast to southwest characterize both the crust- and ore-forming regional patterns in the Fennoscandian Shield. Orogenic (or mesothermal) gold deposits are found in a wide range of host rocks and are closely linked to either the Paleoproterozoic Svecokarelian or the Neoproterozoic Sveconorwegian orogeny. Unlike these shear zone-related deposits, significant amounts of gold were concentrated in volcanogenic massive sulfide (VMS) deposits by magmatic-hydrothermal processes in conjunction with formation of Paleoproterozoic Svecofennian juvenile crust. Hundreds of gold deposits of a variety of ages and genetic styles are, therefore, now known in the Precambrian of northern Europe, of which some are economic, others subeconomic, and still others only of scientific interest. One hundred representative deposits in Sweden, Norway, Finland, and Russia are discussed in this review. Literature references are provided for each of them, and detailed maps show the location of each deposit or prospect. Listed data on tonnage and grades for the most recent exploration targets, as well as the economically and historically most significant deposits, show that they were formed during the entire Precambrian. Yield of gold in the Fennoscandian Shield is, so far, confined to Proterozoic deposits, but the potential to find economically viable gold deposits in the Russian parts of the Archean greenstone belts should not be underestimated.

Age and ϵNd constraints on the Palaeoproterozoic tectonic evolution in the Baltic-Sea region

The metamorphic Precambrian basement in the western part of the East European Craton (EEC) has previously been considered to be Archaean in age. Recent U–Pb and Sm–Nd studies in Estonia, Poland and Belarus have, however, revealed a vast area of Palaeoproterozoic crust. In order to survey the crustal ages in Latvia and Lithuania, and to establish the relationships of these territories with adjacent regions of the EEC, particularly those exposed in the Baltic and Ukrainian Shields, three new U–Pb zircon age determinations and 21 ϵNd whole-rock analyses were performed.In Latvia, a granodiorite yielded an intrusion age of 1816±5Ma and a felsic metavolcanic rock an age of metamorphism at 1810±2Ma, with indications of a minimum protolith age of ca. 1840Ma. In Lithuania, a metasediment yielded a multigrain U–Pb zircon age of 2302+200−100Ma, very similar to multigrain zircon ages of Svecofennian metasediments in Scandinavia.The ϵNd survey of various rock types demonstrated relatively little variation, with depleted-mantle model ages between 2.4 and 1.9Ga for the igneous rocks and between 2.5 and 2.0Ga for the metasedimentary ones. This indicates only minor influence of Archaean crust but close isotopic similarity to rocks of the same lithologies in the Svecofennian Domain of Sweden and Finland.Together with recent U–Pb and Sm–Nd results from other parts of the EEC, the present study suggests a close relationship between the Palaeoproterozoic crust in Estonia, western Latvia and western Lithuania, and the southern part of the Svecofennian Domain within the Baltic Shield. All these regions have U–Pb zircon ages of rock formation between 1.90 and 1.82Ga, and were subjected to a major metamorphic event between 1.83 and 1.81Ga. These ages contrast with ages between 2.0 and 1.9Ga for similar rocks farther east and southeast, i.e. closer to the Ukrainian Shield. Altogether, the current results reaffirm an accretionary character of the Palaeoproterozoic crust in the western part of the EEC, as previously concluded by other investigators. In addition, there is basis to suggest that the crust in western Latvia and Lithuania was formed during the late stages of the Svecofennian orogeny and thus can be correlated with juvenile late Svecofennian crust in southeastern Sweden.

Growth-related 1.85–1.55 Ga magmatism in the Baltic Shield; a review addressing the tectonic characteristics of Svecofennian, TIB 1-related, and Gothian events

During the late Palaeoproterozoic, western Baltica was characterised by convergent-margin tectonics that resulted in the formation of N–S trending crustal units: the Transscandinavian Igneous Belt (TIB) and the westward younging Gothian growth zones. Together, these occupy a 200–300 km wide belt between Svecofennian (c. 1.9 Ga) crust in the east and the Skagerrak Sea/Oslo Rift in the west. Spatial and temporal constraints for 83 U–Pb dated rocks previously included in the TIB allow recognition of age groups at 1850, 1813–1766, and 1723–1657 Ma. However, the c. 1.85 Ga rocks should preferably not be included in the TIB because they form a tectonically distinct crustal unit (the Askersund suite). The 1.81–1.77 Ga age group is the major contributor to the TIB and corresponds to TIB 1. Westward younging of the TIB magmatism is demonstrated by the 1.72–1.66 Ga age group. Integration of the petrogenetic and spatial aspects for these rocks suggests an older magmatic phase at c. 1.72–1.69 Ga (TIB 2) and a final phase at c. 1.68–1.66 Ga (TIB 3). Recent structural, geochemical and isotopic data provide spatial and temporal constraints for the Svecofennian and Gothian developments, and for a bridging, c. 1.81–1.75 Ga tectonic stage we term the Smålandian. This tectonic stage includes the N–S trending, 1.81–1.77 Ga TIB 1 magmatism and manifests a marked shift by 1.81 Ga from previous north-directed convergence during the Svecofennian arc-accretionary event to east-directed convergence that also characterised the following Gothian evolution (c. 1.75–1.55 Ga). The early, c. 1.75–1.70 Ga Gothian evolution is still enigmatic but the subsequent development was due to westward stepping orogenesis, resulting in distinct c. 1.69–1.65, 1.62–1.58, and 1.56–1.55 Ga igneous belts between the TIB rocks and the Oslo Rift. The prolonged period of semi-continuous crustal growth during the Svecofennian-Smålandian-Gothian course appears to have ceased after the postulated docking of pre-1.60 Ga crust (“proto-SW Norway”) with Gothian crustal units of Baltica at c. 1.55 Ga. At the very least, the subsequent 1.50–1.20 Ga evolution lacks evidence for continental-margin processes east of the Oslo Rift.

Rapakivi-granite–anorthosite magmatism — a way of thinning and stabilisation of the Svecofennian crust, Baltic Sea Basin

In the Palaeoproterozoic, a 55–80 km thick layer of crust was formed in the Baltic Sea region during the Svecofennian orogeny at 1.9–1.8 Ga. Today, the remaining crustal thickness is 50–65 km. In the marginal parts of the 1,000,000 km 2 Svecofennian Domain, the Moho depth reached 40–75 km, which exceeds the 40 km crustal thickness of the southwestern edge of the Karelian Archaean Domain to which Svecofennia was accreted. The 1.65–1.50 Ga rapakivi magmatism of the Fennoscandian Province was limited to the Svecofennian Domain. The rapakivi igneous structures are confined to areally isolated subprovinces, which each have distinct 20–60 Ma long age spans of formation. The petrologic sequences of the subprovinces are alike, although similar petrological events occur at different times in the various subprovinces. The internal structure of a subprovince generally consists of a main igneous polyphase unit in a central position, with smaller felsic intrusions, as well as mafic dike swarms, spread over the peripheries of the subprovince. The rapakivi magmatism started in juvenile crust which was in a late stage of erosional thinning, 150–300 Ma after the period of maximal thickening. The most extensive rapakivi igneous activities were associated with crustal thinning down to 45–50 km. As a result, the thinner marginal parts of Svecofennia and the large interior rapakivi subprovinces were of similar thicknesses as the crust. The primary thickness of the original crust was maintained only in areas void of rapakivi magmatism. No major events destroyed the Svecofennian and rapakivi-related crustal structures subsequent to emplacement. Thus, it can be concluded that the extensive rapakivi igneous activity substantially thinned and stabilised the overthickened portions of the Svecofennian crust.