Baltic Shield Research Articles

Mesoarchaean banded iron formations of the Fennoscandian Shield: new zirconU-Pb (ID-TIMS and SHRIMP-II) isotope ages of noble metal mineralization and Nd-Sr data on whole rocks

ABSTRACT Banded iron formations (BIFs) are widespread over the world (3.81 Ga to 1.80 Ga) as the Algoma and Superior types. The paper provides new isotope (Nd-Sr, U-Pb) data on associated noble-metal mineralization in Algoma-type BIFs of the Fennoscandian Shield. U-Pb dating of accessory zircon applied to estimate the formation time of noble-metal-bearing skarn and related rocks in the Olenegorsk and Kirovogorsk BIFs, NW Fennoscandian Shield show that they occurred in the interval between the formation of basalt protolith (2.81 Ga amphibolites) and metamorphism in BIFs (2.75 Ga) to the formation of gold-bearing iron-rich skarn with native Au, Ag, Bi and tellurides (2.65 Ga) and low-temperature quartz-zeolite veins with redeposited Au (2.55 Ga). REE distribution in zircons and corresponding rocks indicated an important role of redox reactions in Fe and REE redistribution affected by reducing fluids on haematite-quartz protolith of BIFs.

Cold subduction recorded by the 1.9 Ga Salma eclogite in Belomorian Province (Russia)

When plate tectonics emerged on Earth is one of the most fundamental questions in geosciences. Nowadays, different arguments from diverse research fields (petrology, structural geology, geochemical and numerical modeling) postulate that the lithospheric subduction began sometime in the Archean-Paleoproterozoic with different styles. This study provides robust petrologic evidence for the modern-style plate tectonics onset in the Paleoproterozoic featured by low geothermal gradients. We investigated the Salma eclogite association from the Precambrian Belomorian Province (BP) on the Fennoscandian Shield (Russia). Based on the garnet chemical annuli (major and trace elements) and therein mineral inclusions, plus comprehensive zircon isotopic-chemical analyses, we establish the prograde metamorphic history of the eclogite, leading to peak conditions of 2.2-2.3 GPa, 650-670°C prior to 1.93 Ga. We thus, for the first time, conclude cold subduction (303 ± 16°C/GPa) proceeded in the BP, connected with the Lapland-Kola Orogen (LKO). Such conclusions are clearly against the previously argued Archean subduction scenario and “warm” tectonic setting in the BP, and are very important for understanding the geodynamics of the early Earth. Furthermore, in the light of other scarce coeval “cold” Paleoproterozoic high pressure (HP) metamorphic rocks elsewhere, we posit that modern-style plate tectonics already operated in the form of the Columbia Supercontinent.

The Depths to the Lithospheric Magnetic Sources Along the Kovdor–GSZ-76 Profile (Baltic Shield–Barents Sea)

The Depths to the Lithospheric Magnetic Sources Along the Kovdor–GSZ-76 Profile (Baltic Shield–Barents Sea)

Ordovician of the Eastern Baltic palaeobasin and the Tornquist Sea margin of Baltica

Abstract This paper summarizes recent knowledge on the palaeontology, biostratigraphy, correlation, sea-level and climate history and isotopic geochemistry of the Ordovician rocks in the western and central parts of the East European Craton, in the area extending from the southern margin of the Fennoscandian Shield to the western margin of the Ukrainian Shield. The regional chronostratigraphic standard is briefly summarized and its correlation to the global standard of the Ordovician is addressed. A two-part correlation chart of 10 areas with unique local Ordovician successions is aligned with the most recent international correlation standard of the Ordovician System and presented against the regular timescale. An updated summary of the evolution of the marine assemblages is provided, the principal gaps in the existing extremely rich palaeontological database are identified and the main bioevents are discussed.

The Great Dyke of the Kola Peninsula as a Marker of an Archean Cratonization in the Northern Fennoscandian Shield

The Great Dyke of the Kola Peninsula as a Marker of an Archean Cratonization in the Northern Fennoscandian Shield

Re-Os Isotope and HSE Abundance Systematics of the 2.9 Ga Komatiites and Basalts from the Sumozero-Kenozero Greenstone Belt, SE Fennoscandian Shield: Implications for the Mixing Rates of the Mantle

Re-Os Isotope and HSE Abundance Systematics of the 2.9 Ga Komatiites and Basalts from the Sumozero-Kenozero Greenstone Belt, SE Fennoscandian Shield: Implications for the Mixing Rates of the Mantle

Mineralogical, geochemical, and isotopic data of a new special agpaitic dyke, enriched in high field strength elements (Eastern Part of Baltic Shield, Russia)

We present the petrography, bulk major and trace elements, and isotopic compositions for the recently identified Mohnatye Roga agpaitic syenite dyke, located 30 km from the Kovdor carbonatite complex, Kola Peninsula, Russia. The dyke was identified below Quaternary deposits as the result of a gravimetric survey and subsequent drilling. A K–Ar age on lamprophyllite of 368 ± 9 Ma, coincides with the age of the Devonian Kola Alkaline Carbonatite Province (KACP). In the total alkalis vs. silica diagram, the Mohnatye Roga dyke exhibits a compositional trend from lamprophyre to peralkaline phonolite as well as mineralogical and geochemical affinities with the nearby Niva intrusion. The dyke is composed of orthoclase, Ti-rich aegirine-augite, aenigmatite, lamprophyllite-group minerals, K-amphibole, and astrophyllite with accessory noonkanbahite, calcite, ilmenite, rutile, barite, apatite, and sulfides. Late stage minerals include natrolite and ferripyrophyllite. A notable feature of this dуke is its extremely high agpaicity and enrichment in HFSEs (high-field strength elements). The concentrations of a number of HFSEs are higher in this dyke than in the Lovozero rare metal deposit. Mohnatye Roga dyke and Niva intrusion are examples of a gradual transition from melt to hydrothermal fluid. Isotopic data indicate a mantle source for the dyke, and seemingly an absence of contamination by continental crust during its formation. The geochemical and isotopic data suggest that the source of the KACP rocks, including the Mohnatye Roga dyke, is FOZO mantle which is believed to be related to plume magmatism. • Origin and age of the recently identified lamprophyllite-bearing agpaitic syenite Mohnatye Roga dyke (Kola peninsula) • Great similarity with the Niva intrusion and the link to Devonian Kola Alkaline Carbonatite province • Agpaitic SiO 2 -undersatured melts preserving the depleted FOZO plume signature • Enrichment in HFSEs of Mohnatye Roga dyke and comparison with gigant Lovozero and Khibiny deposits.

Hydrothermal alteration, lithogeochemical marker units and vectors towards mineralisation at the Svärdsjö Zn-Pb-Cu deposit, Bergslagen, Sweden

ABSTRACT The Svärdsjö Zn-Pb-Cu deposit is situated in the heavily mineralised Bergslagen lithotectonic unit of the Fennoscandian shield, south-central Sweden. Intense hydrothermal alteration followed by a strong overprint by deformation and metamorphism during the Svecokarelian orogeny complicate interpretation of the local geology. Integration of whole-rock lithogeochemical and petrographic methods has allowed the mainly dacitic volcanic host succession and effects of ore-related hydrothermal alteration to be characterised. Mineralisation is hosted by 2–15 m thick, commonly skarn-altered dolomitic marble interbeds. Zones of strong–intense hydrothermal chlorite-sericite alteration envelop the marble units, recording mass gains of Fe and Mg, as well as Na depletion. Minerals such as cordierite, anthophyllite and sillimanite formed in these rocks during regional metamorphism. Mineralisation via sub-seafloor replacement is suggested for the deposit based on alteration zoning and the irregular, stratabound, marble-hosted style of sulphide lenses. It is inferred that mineralisation formed via neutralisation of hot, acidic metalliferous fluids. Geochemically and lithologically distinct units adjacent to the mineralised zones can serve as marker units to aid further exploration in the area. Mass change calculations reveal that Fe and Mg enrichment, as well as Na depletion exhibit detectable changes extending up to 100 m from the mineralised lenses, providing exploration vectors.

Paleoseismic Events Reflected in Late Pleistocene and Holocene Deposits of the Terraces of Imandra Lake (Kola Region, Fennoscandian Shield)

Paleoseismic Events Reflected in Late Pleistocene and Holocene Deposits of the Terraces of Imandra Lake (Kola Region, Fennoscandian Shield)

The Radial Anisotropy of the Continental Lithosphere From Analysis of Love and Rayleigh Wave Phase Velocities in Fennoscandia

AbstractRadial anisotropy (RA) in the upper mantle of the Fennoscandian Shield is analyzed by joint inversion of Love and Rayleigh wave phase velocities measured from recordings of teleseismic events at the ScanArray network. The phase velocities are measured by beamforming using three geographical subsets of the network as well as the full network. We analyze how different procedures for determining the phase velocities influence the final result and uncertainty. Joint inversion of the phase velocities in the period range 22–100 s reveals the presence of similar RA in the three subregions, with an average ξ value of about 1.05 in the subcrustal lithosphere down to at least 200 km depth. This corresponds to SH waves faster than SV by 2%–3%, a value very similar to those found in other continental regions. Considering this anisotropy together with other observables pertaining to seismic anisotropy in the area, we cannot propose a unique model satisfying all data. We can show, however, in which conditions different types of olivine crystallographic preferred orientations (CPOs) commonly observed in natural samples are compatible with the observations. CPO types associated with the preferred orientation of the a‐axis, in particular the common A‐type CPO, require a‐axes dipping not more than 25° from the horizontal plane to explain our observations. AG‐type CPO, characterized by preferred orientation of the b‐axis and occurring in particular in compressional settings, can be considered as an interesting alternative interpretation of continental lithospheric anisotropy, provided the olivine b‐axis is dipping by at least 60°.

Neoarchean high-pressure granulite-facies anatexis of continental rocks in the Belomorian Eclogite Province, Russia

Eclogites exposed along the northeastern boundary of the Belomorian orogen in the eastern Fennoscandian Shield formed as a result of Mesoarchean–Neoarchean subduction. Highly-deformed banded TTG gneisses with eclogitized mafic pods, lenses and dykes in the Gridino association of metamorphic rocks have been modified locally to have typical migmatite structures that formed as HP rocks decompressed through HP granulite-facies metamorphic conditions (16–12 kbar, 800–850 °C) to amphibolite-facies conditions (10–9 kbar, 600–700 °C). The migmatites are located with the boundaries between felsic and mafic lithologies, which are the most suitable place for partial melting, fluid migration, and component diffusion. Symplectic intergrowths of hydrous minerals (mica, epidote) together with quartz in the leucosome are important markers of arrested reaction textures that formed by reversed hydration crystallization of residual melts or by fluids where their infiltrated through migmatite areas. There are two potassic granitic leucosomes with contrasting geochemical signatures. Garnet and phengite-bearing leucosome replaces the host TTG gneiss and percolates mafic rocks. This leucosome is distinguished by a high Ba content, striking positive Eu and Sr anomalies and corresponding low concentrations of all other trace elements. The compositions of the leucosome record initial segregation and migration of melt away from the residual source and subsequent crystal fractionation, dominated by feldspars, of the escaped melt. Migration of anatectic melts led to the formation of small bodies of leucogranite that are characterized by high contents of trace elements and negative Eu and Sr anomalies. Leucogranites formed from portions of fractionated melt that percolated the migmatites and solidified. Anatexis occurred during the Neoarchean time (∼2.7 Ga). U-rich zircon domains were partly or completely affected by radiation damage that yield discordant scattered dates between 2.7 and 1.9 Ga, which are interpreted as reflecting a thermal and fluid overprint during evolution of Belomorian province that produced recrystallization and Pb loss in Neoarchean zircons.

PALEOPROTEROZOIC STRUCTURE OF THE WINDY BELT - UNRESOLVED ISSUES

The Windy Belt rift was formed in the Baltic Shield in the Early Proterozoic as a result of the destruction of the continental crust. Investigations of its sedimentary-volcanogenic complex have shown that the belt was formed on the continental crust within the Karelian craton. Sediments and volcanics were formed in an intraplate coastal-marine environment. Reconstruction of the sedimentation sources for clastic formation indicates continental conditions all along the formation of the structure. However, comparison of the section of the Windward Belt with analogous depressions of the Baltic Shield still allows us to question the accepted stratigraphic sequence of its associations.

ARCHAEAN AND PALEOPROTEROZOIC OROGENS OF THE FENNOSCANDIAN SHIELD: COMPARATIVE ANALYSIS OF ARCHITECTURE AND CRUSTAL EVOLUTION

Comparative analysis of the structure and mineralogical composition of major structural-mineralogical complexes and the evolution of the Meso-Neoarchean Karelian-Belomorian-Kola and the Paleoproterozoic Lapland-Kola and Svecofennian orogen of the Fennoscandian Shield was performed. It is noted that the lithosphere of the former (Archean) is 200-250 km thick, while the latter is 150- 200 km thick. Overlapping-overthrust structures contribute mostly to the earth’s crust architecture of both orogens. 3.0-2.6 and 2.0-1.65 Ga ago, each of them displayed spreading, subduction, collision and mantle plumes, and the general sequence of these events is comparable to Phanerozoic. The formation of both the Archean and Paleoproterozoic orogens took about 350 Ma. This shows that the Archean and Paleoproterozoic orogens do not differ in geodynamic nature from each other and from modern structures of this type.

ORE MINERALIZATION IN PALEOPROTEROZOIC GRANULITE COMPLEXES OF THE NORTH OF THE ALDAN-STANOVOY SHIELD

The problems of metallogenic specialization of the Early Precambrian continental crust in metamorphic complexes of various composition and conditions of formation are of urgent importance for regional metallogeny development. The paper considers the geology, features of the mineral and material composition of deposits and points of mineralization in the Paleoproterozoic granulite complexes of the Aldan-Stanovoy shield. A comparison of the features of composition with those of the ore deposits in the Svecofennides of the Baltic Shield as an well studied folded region has been performed. The conclusion was made about the similarities of metallogenic specialization of the metamorphic complexes.

Mapping of the loose sediments of glacial and periglacial formations in areas with boreal vegetation using remote sensing

The southeastern Fennoscandian Shield is a specific region dominated by glacial and periglacial originated Quaternary deposits. In dense boreal vegetation zones, direct remote decoding of Quaternary sediments is impossible. Our aim is to assess the efficiency of machine learning algorithms for identification of types of Quaternary deposits based on spectral data of vegetation cover and digital elevation model data. A comparative analysis of two classification methods, discriminant analysis (DA) and classification and regression trees (CARTs), was performed. Two models, DA and CART, were constructed based on a set of spectral variables. They included principal components and spectral indices, such as the normalized difference vegetation index, the normalized difference moisture index, and the clay minerals ratio. When the absolute height and the topographic position index (DA+ and CART+ models) are added to a set of independent variables, the classification of Quaternary sediments becomes more accurate. The nonparametric CART method was shown to be more accurate in differentiating loose deposits. The correctness of the CART+ model for the reference sample of observation points was as high as 90.5%. According to the trained data, mapping of Quaternary deposits was carried out. Kappa analysis showed that the agreement with the map of Quaternary deposits for the CART+ and DA+ models was 0.5 and 0.44, respectively. For the DA and CART models, the agreement was much lower. It is noted that the sets of spectral data for the vegetation cover more clearly show the grain-size composition of loose sediments. To make the classification of Quaternary deposits more correct, morphometric indicators were added. Furthermore, the CART+ method was used to estimate the maximum height of glaciolacustrine sediments (117.5 m), which corresponds to the maximum absolute mark of the periglacial reservoir.

The Lyavaraka Ultrabasic Complex, Serpentinite Belt, Kola Peninsula, Russia

The Paleoproterozoic Lyavaraka ultrabasic complex is one of several dunite–harzburgite–orthopyroxenite bodies exposed as shallow plutonic complexes in the Serpentinite Belt, Kola Peninsula, Russia. Lyavaraka and the other complexes are anorogenic, formed in a stable within-plate environment in the interval 2.5–2.4 Ga as members of a large igneous province formed in the Sumian cycle of igneous activity. This geotectonic setting accounts for the shallow emplacement of the strongly magnesian komatiitic magma in the Fennoscandian Shield. We recognize three stages of crystallization of the Al-undepleted magma, present as dislocated blocks. Zone I is the ultrabasic core-like zone in which olivine predominates. Orthopyroxene is the major mineral in Zone II, and Zone III contains the most evolved ultrabasic rocks in which recurrent olivine coexists with Cpx + Pl. Primocrysts of hypermagnesian Opx (Mg# 91–93) nucleated in central areas of Zone II as olivine (Mg# 89.1–90.3) was forming in Zone I. In Zone III, olivine grains of a second generation (Mg# 74.5–75.8) formed after the primocrystic Cpx (Mg# up to 88.0) appeared. The recurrence of olivine is attributed to the progressive buildup in fO2 as a result of degassing and conversion of Fe2+ to Fe3+, well documented in our earlier studies of oxide parageneses.

Architecture of the subcontinental lithospheric mantle of the Archean segment of the Fennoscandian Shield: Analysis of seismic data

Here, Karelian seismological network data were studied to reveal the architecture of the subcontinental lithospheric mantle (SCLM). The receiver functions method was used, which is based on the identification of converted waves from teleseismic earthquakes and their joint inversion into a velocity model. Original one-dimensional (1D) velocity models for the continental lithosphere of the central Karelian Craton (KC) and the western Belomorian Mobile Belt (BMB; both part of the Archean segment of the Fennoscandian Shield) were presented, analysed and interpreted in geological and petrological terms.The obtained results reveal that the SCLM is >200 km thick both in the KC and the BMB. The mantle in both structures was shown to contain a well-defined, contrasting boundary with the crust. However, the boundary between the SCLM and the asthenosphere was not identified in this study. The SCLM was found to be stratified and divided into upper, middle and lower layers in both structures. The boundary between the upper and middle layers was identified at a depth near the upper line of the garnet stability limit in mantle peridotites; the boundary between the middle and lower layers corresponds to the graphite/diamond equilibrium line.The SCLM of the BMB was shown to exhibit a distinct lowered velocity zone in the middle layer. The 1D velocity model of the central KC was found to be in good agreement with the evidence for the SCLM stratigraphy, based on the study of mantle xenoliths and xenocrysts from kimberlites and lamproites from the area.

Maturity Matters in Provenance Analysis: Mineralogical Differences Explained by Sediment Transport from Fennoscandian and Variscan Sources

The significance of mineralogical maturity as a provenance indicator has long been debated and we use this study to demonstrate that it can indeed be a powerful tool to track the distribution of sandstone reservoirs. We investigate the cause of the pronounced geographic and stratigraphic differences in mineralogical composition that are found in the Upper Triassic–Lower Jurassic Gassum Formation across the Norwegian–Danish Basin and surrounding areas. Zircon U-Pb dating of 46 sandstone samples including analysis of 4816 detrital grains are combined with quantifications of the detrital mineralogical composition and placed in a sequence stratigraphic framework. The results show that the Gassum Formation can be divided into a southeastern region with high mineralogical maturity and a less mature region to the northwest with more feldspars, rock fragments, micas, and heavy minerals. Both the mineralogical assemblage and the provenance signature have been thoroughly homogenized in the SE region where sediment supplies from the Fennoscandian Shield and the Variscan Orogen are evident. In the NW region, sediment was initially supplied from Fennoscandia only, but the provenance abruptly changed from the Telemarkia Terrane to comprising also the more distant Caledonian Orogen resulting in a different mineralogical assemblage. The change occurred during a basinwide regression and may be caused by tectonic movements in the hinterland that permanently changed the composition of the sediment supplied to the basin.

Terrigenous dissolved organic matter persists in the energy-limited deep groundwaters of the Fennoscandian Shield

The deep terrestrial biosphere encompasses the life below the photosynthesis-fueled surface that perseveres in typically nutrient and energy depleted anoxic groundwaters. The composition and cycling of this vast dissolved organic matter (DOM) reservoir relevant to the global carbon cycle remains to be deciphered. Here we show that recent Baltic Sea-influenced to ancient pre-Holocene saline Fennoscandian Shield deep bedrock fracture waters carried DOM with a strong terrigenous signature and varying contributions from abiotic and biotic processes. Removal of easily degraded carbon at the surface-to-groundwater transition and corresponding microbial community assembly processes likely resulted in the highly similar DOM signatures across the notably different water types that selected for a core microbiome. In combination with the aliphatic character, depleted δ13C signatures in DOM indicated recent microbial production in the oldest, saline groundwater. Our study revealed the persistence of terrestrially-sourced carbon in severely energy limited deep continental groundwaters supporting deep microbial life.

Evolution of the Gállojávri ultramafic intrusion from U-Pb zircon ages and Rb-Sr, Sm-Nd and Lu-Hf isotope systematics

The Karasjok–Central Lapland Greenstone Belt is one of the largest Palaeoproterozoic greenstone belts in the Fennoscandian Shield and includes multiple (ultra)mafic intrusions, some with notable ore reserves, formed during three episodes at 2.44, 2.22 and 2.05 Ga. This study presents new mineralogical, geochronological and isotopic data for the Gállojávri ultramafic intrusion, in the Karasjok Greenstone Belt, northern Norway. Previous petrogenetic modelling suggests that the intrusion was emplaced as a conduit system open for influx of melt with signs of polybaric fractionation and assimilation. Zircon U-Pb geochronology yields an age of 2051 ± 8 Ma, interpreted to reflect magmatic crystallisation. Large variations in isotopic signature over decimetres to metres indicate incomplete magma mixing. In bulk samples, εNd(t) ranges from −15 to 4. Zircon εHf(t) ranges from −14 to −1. Bulk 87Sr/86Sr(t) shows an apparent range from 0.5041 to 0.7072: the anomalously low values and general alteration indicates that 87Sr/86Sr is non-primary, whereas the less mobile Sm-Nd/ Lu-Hf systems are interpreted to represent primary magmatic signatures. We ascribe the large variations in the Nd and Hf isotopic signatures to local melting or dissolution of xenoliths and influx of variably contaminated melt into the semi-consolidated Gállojávri magma chamber, consistent with a conduit model involving variable replenishment and crustal interaction. The most evolved isotopic signatures cannot be accounted for by interaction with the local Archaean basement, indicating the presence of unidentified crustal components at depth. The Gállojávri intrusion shows many petrogenetic similarities to other c. 2.05 Ga (ultra)mafic intrusions in the Central Lapland Greenstone Belt.