Evolution of the Svecofennian Bedrock in Southern Finland - Spatial and temporal changes in the mantle-derived magmatism and mantle-crust interaction

The Svecofennian orogen in the central Fennoscandian Shield was formed between 1.96–1.77 Ga. This study focuses on characterizing the temporal and spatial changes of the mantle derived and related magmas in southern Finland during the early stages (1.92–1.86 Ga) of the orogeny. The findings are further used to create a magmatic evolution model for the early Svecofennian orogen in southern Finland. Based on the on their age, geochemical features and Sm-Nd and zircon Lu-Hf isotope signature this work provides a classification of the early orogenic intrusives into six categories: (i) 1.89 Ga rift-related rocks, 1.89–1.87 Ga arc-related (ii) mafic and (iii) felsic rocks, 1.86 Ga within-plate-type rock association including (iv) high-Nb gabbros (HNB), (v) high-Mg gabbros (HMG) and (vi) adakite-lite rocks. The 1.89 Ga rocks show E-MORB type geochemistry suggesting a rift-related setting in a forearc region. The 1.89–1.87 Ga arc-related rocks are the most abundant and carry distinctive subduction-related signatures. The rare association of the 1.86 Ga igneous rocks are characterized followingly: the HNB show OIB-type geochemical features, positive initial εNd value, and near-chondritic initial zircon εHf values; the HMG showhigh MgO, Cr and Ni contents, positive εNd, and positive zircon εHf values, and adakite-like rocks show slight enrichment in Sr and La, relative depletion in some HFSEs, positive εNd value, and chondritic to negative zircon εHf values. Lower crustal (rutile-bearing) garnet pyroxenites, i.e., arclogites, are suggested to be the source of 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. In this work, I suggest that arc magmatism prevailed during contractional stages of the orogeny, whereas the extensional stages were characterised by MORB/withinplate type magmatism. The timing, compositional and isotopic changes of earlyorogenic magmatism are broadly compatible with intervals of contraction and extension, i.e., tectonic switching model and may provide a perspective to rapid build-up of Paleoproterozoic crust. In addition, I suggest a model of formation, delamination and partial melting of arclogites to describe the evolution and shift from ~1.88 Ga arc magmatism to the 1.86 Ga within-plate type magmatism. The rutile-bearing arclogites were formed during 1.89–1.87 Ga arc magmatism followed by the arclogite delamination and partial melting during extension of the thickened Svecofennian crust at 1.86 Ga. This model explains the features of the rare 1.86 Ga magmatic association as well as the extremely thick crust and the high velocity lower crust encountered in the central Fennoscandian Shield and possibly in other Paleoproterozoic orogens.Fennoskandian kilven keskiosassa sijaitseva Etelä-Suomen kallioperä syntyi Svekofennisen vuorijonon poimutuksen eli orogenian aikana 1.96–1.77 miljardia vuotta sitten. Tämä tutkimus keskittyy orogenian alkuvaiheessa syntyneiden, 1.92–1.86 miljardia vuotta vanhojen, vaippaperäisten magmojen luonnehdintaan sekä vaipan ja kuoren vuorovaikutuksen tutkimiseen. Näiden perusteella luon magmatismia ja sen alueellista sekä ajallista muutosta kuvaavan mallin Etelä-Suomesta. Magmakivet luokiteltiin niiden (i) kiteytymisiän, (ii) kemiallisen koostumuksen ja (iii) Sm-Nd ja zirkonien Lu-Hf isotooppikoostumuksen perusteella seuraaviin luokkiin: 1.89 miljardia vuotta vanhat kuoren repeytymiseen liittyvät kivet, 1.89–1.87 miljardia vuotta vanhat vulkaanisen kaaren kivet, 1.86 miljardia vuotta vanhat kuoren sisäisen magmatismin kivet, sisältäen korkean Nd-pitoisuuden gabbroja (HNB), korkean Mg-pitoisuuden gabbroja (HMG) ja adakiittisia kiviä. 1.89 miljardia vuotta vanhat kuoren repeytymiseen liittyvät kivet sisältävät E-MORBtyyppisen geokemiallisen koostumuksen ja ehdotan niiden syntyneen kaaren edustaaltaan repeämisen seurauksena. 1.89–1.87 miljardia vuotta vanhat vulkaanisen kaaren kivet ovat selkeästi yleisimpiä ja ne sisältävät selkeän subduktiovyöhykkeen sormenjäljen. Harvinaista 1.86 miljardia vuotta vanhaa magmakiviseuruetta voidaan luonnehtia seuraavasti: HNB-kivet sisältävät OIB-tyyppisen geokemiallisen koostumuksen, positiivisen εNd-arvon ja kondriittisen zirkonien εHf-arvot; HMGkivet sisältävät korkean MgO-, Cr- ja Ni-pitoisuuden, korkean εNd-arvon ja positiiviset zirkonien εHf-arvot; adakiittiset kivet ovat rikastuneita Sr:in ja La:in ja köyhtyneet HFSE-alkuaineista, sisältävät positiivisen εNd-arvon ja hieman negatiiviset zirkonien εHf-arvot. HNB-kivet ovat syntyneet arclogiittien, eli alakuoren rutiilipitoisten granaatti-pyrokseniittikumulaattien osittaissulamisen seurauksena. HMG-kivet ovat syntyneet subduktion rikastaman peridotiitin (ylävaippa) osittaissulamisesta, kun taas adakiittisten kivien lähde on mafinen alakuori. Tulosten perusteella orogenian puristusvaiheessa (kuoren paksuuntuminen) kaarityyppinen magmatismi vallitsee, kun taas ekstension aikana E-MORB/kuoren sisäinen magmatismi on vallitseva tyyppi. Kivien kiteytymisiät sekä koostumuksellinen syklinen vaihtelu vastaa hyvin niin sanottua tectonic switching mallia, jossa kuoren puristus/lyhentyminen ja ekstensio vaihtelevat. Tämä voi tarjota uuden mekanismin nopealle kuoren kasvulle Paleoproterotsooisella ajalla. Lisäksi malli tiheän arclogiittikerroksen muodostumisesta, hajoamisesta ja osittaissulamisesta selittää muutoksen 1.88 miljardia vuotta vanhasta kaarityypin magmatismista 1.86 miljardin vuoden ikäiseen kuoren sisäiseen magmatismiin. Mallissa rutiilipitoinen arclogiittikerros muodostui 1.89–1.87 miljardia vuotta sitten yleisen kaarityyppisen magmatismin seurauksena. Tätä seurasi tiheän arclogiittikerroksen osittainen vajoaminen ylävaippaan paksuuntuneen kuoren ekstension aikana ja arclogiittien osittaissulamista. Tämä malli selittää harvinaisen 1.86 miljardia vuotta vanhan kivilajiassosiaation geokemialliset piirteet, kuoren paksuuntumisen mekanismin sekä suuren tiheyden omaavan alakuoren syntymisen Fennoskandian kilven alueella sekä mahdollisesti muissa Paleoproterotsooisissa orogenioissa

Source for suprachondritic Nb/Ta and Zr/Hf values in 1.86 Ga monzogabbros in south-central Fennoscandian Shield

The ratios of refractory element such as Nb/Ta and Zr/Hf (17,5-19.9 and 34.3-36, respectively [1,2]), in the unfractionated and in the silicate Earth are assumed to follow those of the chondrites. However, the Earth’s crust and other major silicate reservoirs show subchondritic Nb/Ta values (~12-15.5) suggesting a mass imbalance for Nb and Ta in Earth which have led to so-called “Nb-Ta paradox” and searching for a suprachondritic Nb/Ta reservoir and “missing Nb” [3]. During its formation Earth’s core might have fractionated Nb making it a potential reservoir for Nb [4]. The Nb/Ta fractionation, however, is an ongoing process during continent formation so other reservoirs such as refractory rutile-bearing eclogites [2], subcontinental lithospheric mantle [5] or rutile-bearing deep arc cumulates [6] could explain part of the HFSE mass inbalance. We have studied Nb, Ta, Zr and Hf concentrations of 1.86 Ga monzogabbros from southwestern Finland. The rocks show elevated Nb/Ta (~16-38) and Zr/Hf (~ 41-48) ratios and OIB-like enriched geochemical features with low initial zircon εHf values but high εNd values. This is in contrast to the older 1.90-1.88 Ga magmatism within the same area [7]. Our results suggest a residence of a high Nb/Ta and Zr/Hf reservoir under the central Fennoscandian shield during the Paleoproterozoic and a link between the voluminous 1.90-1.88 Ga synorogenic magmatism with low Nb/Ta ratios and elevated Nb/Ta and Zr/Hf ratios in the 1.86 Ga gabbroic magmatism.</p

Critical Metals in Strategic Energy Technologies - Assessing Rare Metals as Supply-Chain Bottlenecks in Low-Carbon Energy Technologies

Due to the rapid growth in demand for certain materials, compounded by political risks associated with the geographical concentration of the supply of them, a shortage of these materials could be a potential bottleneck to the deployment of low-carbon energy technologies. In order to assess whether such shortages could jeopardise the objectives of the EU’s Strategic Energy Technology Plan (SET-Plan), an improved understanding of these risks is vital. In particular, this report examines the use of metals in the six low-carbon energy technologies of SET-Plan, namely: nuclear, solar, wind, bioenergy, carbon capture and storage (CCS) and electricity grids. The study looks at the average annual demand for each metal for the deployment of the technologies in Europe between 2020 and 2030. The demand of each metal is compared to the respective global production volume in 2010. This ratio (expressed as a percentage) allows comparing the relative stress that the deployment of the six technologies in Europe is expected to create on the global supplies for these different metals. The study identifies 14 metals for which the deployment of the six technologies will require 1% or more (and in some cases, much more) of current world supply per annum between 2020 and 2030. These 14 significant metals, in order of decreasing demand, are tellurium, indium, tin, hafnium, silver, dysprosium, gallium, neodymium, cadmium, nickel, molybdenum, vanadium, niobium and selenium. The metals are examined further in terms of the risks of meeting the anticipated demand by analysing in detail the likelihood of rapid future global demand growth, limitations to expanding supply in the short to medium term, and the concentration of supply and political risks associated with key suppliers. The report pinpoints 5 of the 14 metals to be at high risk, namely: the rare earth metals neodymium and dysprosium, and the by-products (from base metals) indium, tellurium and gallium. The report explores a set of potential mitigation strategies, ranging from expanding European output, increasing recycling and reuse to reducing waste and finding substitutes for these metals in their main applications. A number of recommendations are provided which include: • ensuring that materials used in significant quantities are included in the Raw Materials Yearbook proposed by the Raw Materials Initiative ad hoc Working Group, • the publication of regular studies on supply and demand for critical metals, • efforts to ensure reliable supply of ore concentrates at competitive prices, • promoting R&D and demonstration projects on new lower cost separation processes, particularly those from by-product or tailings containing rare earths, • collaborating with other countries/regions with a shared agenda of risk reduction, • raising awareness and engaging in an active dialogue with zinc, copper and aluminium refiners over by-product recovery, • creating incentives to encourage by-product recovery in zinc, copper and aluminium refining in Europe, • promoting the further development of recycling technologies and increasing end-of-life collection, • measures for the implementation of the revised WEEE Directive, and • investing broadly in alternative technologies. It is also recommended that a similar study should be carried out to identify the metal requirements and associated bottlenecks in other green technologies, such as electric vehicles, low-carbon lighting, electricity storage and fuel cells and hydrogen.JRC.F.7-Energy systems evaluatio

Paleoproterotsooisten arclogiittien jäljillä - Muutos 1.88 Ga kalkkialkalisesta magmatismista 1.86 Ga korkean Nb:n ja adakiitti-tyyppiseen magmatismiin Fennoskandian kilven keskiosissa

Arclogites, i.e., lower crustal gamet-pyroxenite cumulates, are suggested to play an important role in controlling magma differentiation in modem 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 talc-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 epsilon(Nd) value, and near-chondritic but variable initial zircon epsilon(Hf) values; 2) high-Mg gabbros (HMG) which are characterised by high MgO, CaO, Cr and Ni contents, slight enrichment in LILE, positive epsilon(Hf), and positive but variable zircon epsilon(Hf) values; 3) adakite-like rocks showing high Al2O3 and Na2O contents, slight enrichment in LILE, relative depletion in some HFSE, positive CNd value, and chondritic to negative zircon epsilon(Nd) values. The three groups yield zircon U-Pb ages of similar to 1.86 Ga and exhibit undeformed textures in contrast to the surrounding supracrustal rocks metamorphosed at similar to 1.88 Ga. The ages and compositions are dearly 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. (C) 2020 Elsevier B.V. All rights reserved.Peer reviewe

Constraints over the age of magmatism and subsequent deformation for the Neoarchean Kukkola Gneiss Complex, northern Fennoscandia

The Archean crust in northern Fennoscandia preserves a fragmentary geological record, making direct correlation among Archean domains challenging. This study presents two new zircon U-Pb age determinations from the Archean Kukkola Gneiss Complex (KGC) that straddles the border between Finland and Sweden. The results indicate that crystallization of tonalites within the magmatic core of the complex occurred at 2711 +/- 8 Ma, somewhat earlier than previously considered. A new pulse of magmatism occurred at 2675 +/- 10 Ma as demonstrated by hornblende-tonalites cutting the 2.71 Ga rocks. The results further indicate that the first deformation event responsible for development of penetrative foliations occurred after the first magmatic event at 2.71 Ga and prior to the subsequent tectonothermal event at 2.68 Ga. These findings are in concert with the known major periods of magmatism (2.8-2.7 Ga) and deformation (2.7 Ga) within better-known Archean domains in northern Fennoscandia, and hence support their correlation with KGC. Three complementary age determinations on the Haparanda-suite granites and tonalites were conducted: the results indicate crystallization ages of 1.90-1.89 Ga, overlapping with the known age range of the suite and supporting its predominance over the 1.8 Ga Lina suite granites in the Tornio-Haparanda area.Peer reviewe

Lithosphere 2016: Ninth symposium on the Structure, Composition and Evolution of the Lithosphere in Fennoscandia

We describe here a newly discovered cross-cutting mafic dyke in the vicinity of Lohja, southern Finland. The dyke is E-W trending and about 6 m in width. Only a few zircons were recovered from the dyke despite the high Zr contents and they all were inherited. Therefore, the crystallisation age remains uncertain. The dyke is shoshonitic in composition and the closest analogues are the åva lamprophyre dykes in SW Finland.</p

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 gamet-pyroxenite cumulates, are suggested to play an important role in controlling magma differentiation in modem 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 talc-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 εHf, 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 CNd value, and chondritic to negative zircon ε​​​​​​​Nd values. The three groups yield zircon U-Pb ages of similar to 1.86 Ga and exhibit undeformed textures in contrast to the surrounding supracrustal rocks metamorphosed at similar to 1.88 Ga. The ages and compositions are dearly 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. </p

Constraints over the age of magmatism and subsequent deformation for the Neoarchean Kukkola Gneiss Complex, northern Fennoscandia

The Archean crust in northern Fennoscandia preserves a fragmentary geological record, making direct correlation among Archean domains challenging. This study presents two new zircon U-Pb age determinations from the Archean Kukkola Gneiss Complex (KGC) that straddles the border between Finland and Sweden. The results indicate that crystallization of tonalites within the magmatic core of the complex occurred at 2711 +/- 8 Ma, somewhat earlier than previously considered. A new pulse of magmatism occurred at 2675 +/- 10 Ma as demonstrated by hornblende-tonalites cutting the 2.71 Ga rocks. The results further indicate that the first deformation event responsible for development of penetrative foliations occurred after the first magmatic event at 2.71 Ga and prior to the subsequent tectonothermal event at 2.68 Ga. These findings are in concert with the known major periods of magmatism (2.8-2.7 Ga) and deformation (2.7 Ga) within better-known Archean domains in northern Fennoscandia, and hence support their correlation with KGC. Three complementary age determinations on the Haparanda-suite granites and tonalites were conducted: the results indicate crystallization ages of 1.90-1.89 Ga, overlapping with the known age range of the suite and supporting its predominance over the 1.8 Ga Lina suite granites in the Tornio-Haparanda area

Early Svecofennian rift-related magmatism: Geochemistry, U-Pb-Hf zircon isotope data and tectonic setting of the Au-hosting Uunimäki gabbro, SW Finland

We characterise the geochemistry, zircon Lu-Hf composition, age and the structure of the Uunimaki gabbro (UGB) in south-western Finland to improve the understanding of i) the early Svecofennian (1.92-1.89 Ga) crustal evolution of the central Fennoscandian Shield, ii) the potential role of rift-related magmatism for the build-up of the Paleoproterozoic accretionary orogens and iii) evaluate, which geological features provide the primary control over the localization of an orogenic gold mineralisation. The zircon U-Pb geochronology defines an age of 1891 +/- 5 Ma for the UGB, which is slightly older than most mafic intrusions in south-western Finland. The obtained chondritic initial zircon eHf values with E-MORB type geochemical affinity suggest a sub continental lithospheric mantle source for the UGB. The overall geochemistry indicates that the UGB magma as well as other E-MORB type rocks in the Pirkanmaa and Hame belts were formed in a rift-related environment in a fore-arc region at 1.89 Ga, predated by arc-type magmatism at similar to ~1.90 Ga and back-arc magmatism at similar to ~1.92 Ga in the Tampere belt. Slab retreat due to roll-back is suggested to cause the extension and related magmatism in the forearc region. Moreover, the timing and compositional and isotopic changes of early-orogenic magmatism are broadly compatible with intervals of extension and contraction, i.e., a tectonic switching model, and may provide a perspective to rapid build-up of Paleoproterozoic crust. Structural characterisation provides a framework where gold mineralisations are preferentially located within the high-strain north-eastern domain of the UGB, within fracture networks adjoining the high-strain zones. Our results indicate that neither the geochemical composition nor age of the intermediate-mafic intrusive host rocks play a major role in controlling the formation of gold mineralisation. By contrast, the localization of orogenic gold is controlled by localised structures (shear zones, fractures), and the variation in lithological composition of the intrusive host may contribute to the style of the mineralisation.</p

LITHOSPHERE 2018: TENTH SYMPOSIUM ON STRUCTURE, COMPOSITION AND EVOLUTION OF THE LITHOSPHERE: PROGRAMME AND EXTENDED ABSTRACTS

The Uunimäki gabbro was studied by zircon U-Pb geochronology which yielded an age of ~1.89 Ga, making it one of the oldest plutonic rocks in the Häme Belt. Geochemical analysis of the gabbro reveals that it lacks several characteristics for typical subduction zone rocks: (i) it does not have a negative Ta-Nb anomaly compared to average NMORB-composition, (ii) it shows a rather unfractionated REE pattern, (iii) it lacks clear enrichment of fluid-mobile elements (e.g. Ba, Rb, Th, Pb). Structurally, the Uunimäki gabbro is located at the intersection of several regional features: (i) steep NE-plunging folds, (ii) a ENE-WSW-trending deformation zone immediately to the north and (iii) a large N-S-trending deformation zone to the west. The gabbro itself has been deformed under both brittle and ductile conditions by primarily NW-SE-trending faults and shears.</p