Lapland Granulite Belt Research Articles

Geoheritage of the Kandalaksha region (Kola Peninsula, White Sea, Arctic Russia), Evaluation, and Geotourism Opportunities

The subarctic Kandalaksha region abounds in many tourist values and is notably characterized by a richness of geology, nature, history, and landscape. Its mountains are located directly on the coast of the White Sea. The skerry-fiord coast is the habitat of many seabirds that nest on its many islands. There is also large mammals such as seals and bears. The coastal area and islands are a reserve within the National Park that was established to protect the breeding areas of the seabirds. Despite its location near the Arctic Circle, the area has a relatively mild climate. Many of the villages and towns in the area have a centuries-old history inextricably linked to the indigenous peoples (Karelians, Sami) who inhabited the area, as well as to the “Pomors,” the first Russian colonizers of the area. On top of this, this is a location with an environment that contains numerous examples of flora and fauna, as well as a geologically and geomorphologically interesting landscape connected with glaciation, abrasion, and isostatic movements. Here, the geological substrate, made up of Paleoproterozoic collisional structures of the Lapland Granulite Belt, was repeatedly tectonically rejuvenated within the forming aulacogen of the White Sea and the Kola-Lapland Mobile Belt, along with numerous Paleozoic intrusions of kimberlite and alkaline rocks. The youngest processes are associated with Pleistocene glaciation and isostatic relaxation in the Holocene. All these features contribute to the high tourist potential of the area. At present, it is being developed in a more or less organized way with the participation of private capital. This article presents a proposal for the systematic development of the area in question and the formalization of tourist routes in the area. This article aims to meet this need by proposing a tourist trail in the Kandalaksha area so as to enable tourists to explore this remarkable area.

Metamorphism of the Korvatundra Structure of the Lapland–Kola Orogen (Arctic Zone of the Fennoscandian Shield)

AbstractWe study the P–T conditions and age of metamorphic evolution of the rocks that make up the Korvatundra structure in the northeast of the Fennoscandian Shield. The rocks underwent progressive metamorphism of the amphibolite facies at 625–660 ºC and 8.7–8.8 kbar 1945 ± 34 Ma (Sm–Nd data). The pegmatite cutting the metamorphic paragenesis that formed at this stage has an age of 1917 ± 6 Ma (zircon U–Pb data). Metamorphic transformations after 1917 Ma are manifested locally as discrete zones of blastomylonites in the rocks of the northern part and some inner sites of the Korvatundra structure. Both local increases and decreases in temperature and pressure are possible in these zones. The formation of light titanite with an age of 1863 ± 44 Ma marks the next stage of shear strain. Low-temperature alterations (chloritization and silicification) took place in the zones of final deformations 1722 ± 5 Ma (Rb–Sr data). Beginning from 1.94 Ga, the general deformational and metamorphic history of the Korvatundra structure, Lapland Granulite Belt, and Tana Belt confirms the assumption of the formation of a single inverted metamorphic zoning within the Korvatundra structure and the overlying Lapland–Kolvitsa Collision Belt in the Paleoproterozoic. The obtained data supplement the idea of the Paleoproterozoic geodynamic evolution of the Lapland–Kola orogen.

Geochemical systematics and U–Pb zircon age of the Vulvara anorthosite massif, Lapland granulite belt, Baltic shield: magmatic sources and metamorphic alteration of the rocks

ABSTRACT The Vulvara anorthosites occur among the rocks of intermediate composition, in contrast to most Lapland granulite belt (LGB) anorthosites, associated with mafic granulites. The U-Pb, Sm-Nd and REE geochemical data were used to compare Vulvara with other LGB anorthosites. The intrusion of the Vulvara massif occurred in the interval of 2100–1965 Ma. The first metamorphic processing took place 1947 ± 11 Ma ago under granulite facies. Postgranulite transformations occurred 1900–1850 Ma ago under amphibolite facies; earlier (1896 ± 15 Ma) and later (1852 ± 17 Ma, 1846 ± 16 Ma) thermal events were established. The Vulvara anorthosites, in comparison with other LGB anorthosites, are distinguished by a more acidic composition of plagioclase, a higher F number of rocks and mafic minerals, a higher content of SiO2, Na2O, K2O, Ba, Sr, Zr, Nb, Rb, and a lower content of CaO. The Sm-Nd and REE geochemistry suggest that there are three groups of LGB anorthosites, distinguished both by age and magma source.

Age of eclogites formed by the subduction of the Mesoarchaean oceanic crust (Salma, Belomorian Eclogite Province, eastern Fennoscandian Shield, Russia): A synthesis

Competing evolutionary models and age of eclogite facies metamorphism, Mesoarchaean, Neoarchaean or Palaeoproterozoic, of the subducted Mesoarchaean oceanic crust (Salma association, Belomorian Eclogite Province) are discussed on a basis of systematic analysis of previously known and newly obtained data. Four main types of zircons were distinguished in eclogites: porous crystals with numerous inclusions from eclogite-metagabbro; wide-rimmed zircons with relict porous cores similar to previous type separated from garnetites; round-oval zircons from eclogite-metagabbronorite that are characteristic for granulite facies rocks and zircons with euhedral oscillatory zoning cores and oval grains that are characteristic for the eclogite facies pillow basalts. Regular changes in REE patterns and in crystallization-recrystallization temperatures of certain domains of the porous zircons display sequence of magmatic and metamorphic events. The ∼ 2.9 Ga domains retain magmatic-type REE patterns. Low- and medium-temperature inclusions of prenite, pumpelliite, albite, actinolite, chlorite, diaspore and saponite in garnet and abundant microinclusions of the prenite-pumpelliite and greenschist facies in zircons with LREE-MREE enrichment indicate hydrothermal metamorphism in the spreading ridge and ocean floor at 2.9–2.82 Ga. Disappearance of Ce positive anomaly from REE pattern in zircon, change negative to positive Eu anomaly and LREE-MREE enrichment caused by plagioclase removal and replacement of rutile with sphene evidence eclogite facies metamorphism linked with subduction at 2.82–2.78 Ga. Temperatures in the 700–900 °C range of the round-oval zircons from eclogite-metagabbronorite records the Neoarchaean granulite facies overprint at 2.77–2.70 Ga. Series of the high temperature Palaeoprpoterozoic events was terminated by 2.1–1.7 Ga event marked by the rims with lowest REE that frame all types of zircons. Change from positive to negative Eu anomaly, retrieval of negative Ce anomaly indicate the presence of plagioclase, reduction type of fluids and low water activity characteristic of high-temperature metamorphism under stretching condition and mantle-plume activity. The deep reworking of the Sm-Nd isotope system in the Belomorian tectonic province at ∼ 1.9 Ga, including the Salma eclogite association, was caused by the enormous crustal heating that spread from the Lapland granulite belt southward. Radiogenic 176Hf enrichment of 1.9 Ga zircon indicates recrystallization of a long-existed garnet with release of significant amount of 176Hf.

Soil water drives distribution of northern boreal conifers Picea abies and Pinus sylvestris

Considering the climate change the role of edaphic drivers for the distribution of cold-hardy conifers, Norway spruce (Picea abies (L.) Karst.), and Scots pine (Pinus sylvestris L.) is crucial. Even though both species reach latitude 70oN, P. abies forest line is located at exceptionally lower latitudes in Fennoscandia compared to P. sylvestris tree line. Hence the divergence in mafic (basic) and felsic (acidic) lithologies and thereby physicochemical properties of parent glacial tills may play a critical role in tree species distributions. The objective of this study was to explore soil water content (SWC) and nutrient regimes in Finnish Lapland by transecting the main lithological units; mafic Central Lapland Greenstone Belt (CLGB) in the south and felsic Lapland Granulite Belt (LGB) in the north. Eight one ha in size plots were sampled at 10 m intervals for soil chemistry and texture and measured for SWC, gamma-ray, electrical conductivity, temperature and matric potential. In binary decision tree (DT) analysis, soil physics explained the growth sites of P. abies and P. sylvestris with 97% accuracy, the most important variable being SWC. The constraint for P. sylvestris was SWC > 0.24 m3m−3. Based to the soil chemistry DT, 95.4% of the P. abies and P. sylvestris growth sites were correctly classified. The most important variables were Cr, Ti, Mn and P. Distribution of conifers follows lithology, such that P. abies occupies mesic-wet tills derived from ultramafic and mafic rocks of the CLGB, whereas P. sylvestris thrives only on dry and coarse-textured, often granitoid tills. We argue that dry and nutrient-poor tills of felsic garnet gneisses of the LGB constitute a dispersal barrier for P. abies. The above tree species specific constraints provide guidance for future dispersal models.

The Belomorian eclogite province (eastern Fennoscandian Shield, Russia): Meso-Neoarchean or Late Paleoproterozoic?

A critical discussion of competing models of the geodynamic nature (oceanic or continental subduction) and age (Meso-Neoarchean or Late Paleoproterozoic) of the eclogite facies metamorphism in the Belomorian eclogite province (BEP) is based on the systematic analysis of the sum of previously known and newly obtained data characterizing the geological structure of the Salma eclogite association and features of zircons from eclogites, including the isotopegeochronological and geochemical characteristics, composition and distribution of mineral inclusions. Regular changes in the REE trends and crystallization-recrystallization temperature of porous zircons in eclogite-metagabbro illustrate the sequence of magmatic and metamorphic events in the Meso-Neoarchean and Paleoproterozoic. The susceptibility to recrystallization of zircons is due to partial metamictness and porous structure. The earliest (~2.9 Ga) zircon zones retain mag-matic-type REE trends. The microinclusions of the prenite-pumpelliite and greenschist facies minerals and the increase in the LREE and MREE concentrations indicate hydrothermal metamorphism in the spreading ridge and on the ocean floor at 2.9–2.82 Ga. Prenite, pumpelliite, albite, actinolite, chlorite, diaspore and saponite also form inclusions in the eclogitic garnet. An increase of LREE and MREE, the disappearance of the Ce positive anomaly, a change from negative to positive Eu anomaly at 2.82–2.78 Ga indicate that plagioclase was removed during the formation of the ‘garnet + omphacite’ eclogite association and the replacement of sphene with rutile. The eclogite facies metamorphism linked with subduction of the oceanic crust is also indicated by the microinclusions of garnet and rutile in zircon. The crystallization temperature in 700–900 °C range of the round-oval zircons from eclogites-metagabbronorites records the Neoarchean granulite facies metamorphism at 2.77–2.70 Ga, the negative Eu anomalies in the cores and rims of zircons indicate the participation of plagioclase in the metamorphic crystallization. Late (2.1–1.7 Ga) rims of porous zircons that occurred at 600–680 °C are distinguished by minimal REE concentrations, a change from a positive Eu anomaly to a negative one, and the appearance of a negative Ce anomaly, which indicates the presence of plagioclase, reducing type of fluids and, accordingly, low water activity that is characteristic of high-temperature metamorphism under stretching condition and mantle-plume activity. The deep Sm-Nd system reworking in the Belomorian tectonic province, including BEP, at ~1.9 Ga was caused by the crustal heating that spread from the Lapland granulite belt border in the west-south-westward direction. The Lu-Hf system in zircon reworking with a significant increase in radiogenic Hf indicates the recrystallization of a long-existing garnet, in which a significant amount of radiogenic 176Hf accumulated by 1.9 Ga as a result of the 176Lu decay. This contradicts the earlier suggestion of the eclogite garnet primary crystallization in the late Paleoproterozoic (1.94–1.89 Ga).

Metallogenic Setting and Evolution of the Pados-Tundra Cr-Bearing Ultramafic Complex, Kola Peninsula: Evidence from Sm–Nd and U–Pb Isotopes

The article presents new Sm–Nd and U–Pb geochronological data on rocks of the poorly studied Pados-Tundra Cr-bearing complex. It is part of the Notozero mafic–ultramafic complex (western Kola Peninsula) and occurs at the border of the Paleoproterozoic Lapland Granulite Belt and the Archean Belomorian composite terrain. The Pados-Tundra complex hosts two major zones, the Dunite and Orthopyroxenite Blocks. Dunites are associated with four levels of chromite mineralization. Isotope Sm–Nd studies of dunites, harzburgites, and orthopyroxenites from the central part of the complex have been carried out. The isochron Sm–Nd age on 11 whole-rock samples from a rhythmically layered series of the complex is 2485 ± 38 Ma; the mineral Sm–Nd isochron for harzburgites shows the age of 2475 ± 38 Ma. It corresponds with the time of large-scale rifting that originated in the Fennoscandian Shield. When the rhythmically layered series of the intrusion and its chromite mineralization were formed, hornblendite dykes intruded. The U–Pb and Sm–Nd research has estimated their age at ca. 2080 Ma, which is likely to correspond with the occurrence of the Lapland–Kola Ocean. According to isotope Sm–Nd dating on metamorphic minerals (rutile, amphibole), the age of postmetamorphic cooling of rocks in the complex to 650–600 °C is 1872 ± 76 Ma. The U–Pb age on rutile from a hornblendite dyke (1804 ± 10 Ma) indicates further cooling to 450–400 °C. The conducted research has determined the early Proterozoic age of rocks in the rhythmically layered series in the Pados-Tundra complex. It is close to the age of the Paleoproterozoic ore magmatic system in the Fennoscandian Shield that developed 2.53–2.40 Ga ago. Later episodes of alterations in rocks are directly related to main metamorphic episodes in the region at the turn of 1.9 Ga. Results of the current study expand the geography of the vast Paleoproterozoic East Scandinavian Large Igneous Province and can be applied for further studies of similar mafic–ultramafic complexes.

Fluids in High-Pressure Granulites

Properties of fluids in high-pressure granulites were studied in HP granulites (~8.7–11 kbar, ~800–900°С) and syngranulite fluid infiltration-driven HP metasomatites (~11–9 kbar, ~920–850°С) from the Lapland granulite belt of the Fennoscandian Shield. The study involved large-scale mapping of the rocks, microthermometry of mineral-hosted fluid inclusions, multiequilibrium mineral thermobarometry, and calculations of H2O activity based on mineral equilibria. The mafic pyroxene granulites and syngranulite metasomatites (quartz blastomylonites with orthopyroxene, sillimanite, and garnet; veins and vein-like bodies of orthopyroxene–garnet and diopside–scapolite rocks) contain similar assemblages of syngenetic fluid inclusions (which are hosted mostly in quartz and also in garnet, orthopyroxene, and scapolite) of contrasting chemical composition: nearly pure СО2 (distinctly predominant), brines (the dominant salts are CaCl2 and NaCl), and N2 ± H2O. These three types of inclusions coexist in the same generations of early inclusions: rarer primary (p) and predominant primary–secondary (ps). The CO2 inclusions have either high or low densities, and the N2 inclusions are of low density. The brine inclusions show a wide range of total salt contents (up to 30–35 wt%) and variable concentration proportions of the dominant salts: p-inclusions with a salinity of 20 wt% CaCl2 + 10 wt% NaCl; ps-inclusions with a salinity of 5 wt% CaCl2 + 20 wt% NaCl; p- and ps-inclusions with a salinity of 5–23 wt% NaCl eq; and p-inclusions with halite (up to 35 wt% NaCl). In general, CaCl2 is the predominant salt component in the early p- and ps-inclusions of the rocks. Considered together, currently available data (including Sr, Nd, and O isotope systems) on these rocks indicate that the external fluid flow during the origin of granulites was evidently of mantle origin. At the peak P–T parameters, the inclusions were entrapped from a heterogeneous fluid in which immiscible water–salt and CO2-rich fluids, which initially contained N2, coexisted. Data on the chemical composition and salt concentrations of the fluids, $${{a}_{{{{{\text{H}}}_{2}}{\text{O}}}}}$$ = 0.40–0.51, are compared with the theoretically predicted phase state of the fluids and the properties of the coexisting immiscible fluid phases at the estimated P–T parameters of granulite petrogenesis on the basis of numerical models in the H2O–CO2–NaCl and H2O–CO2–CaCl2 ternary systems. The location of the tie-lines and solvus were calculated to subsequently use for the thermodynamic prediction. The paper discusses similarity and the reasons for the difference between the theoretical compositions of the generated fluid phases and the composition of fluid inclusions, geochemical consequences of the heterogenization of granulite fluids (the formation of concentrated alkaline brines and a potentially acidic CO2-rich fluid phase, the values of the mass and volume fractions of these phases depending on variations in the composition of the initial homogeneous fluid, etc.). It follows that an extensive region of the compositions of aqueous fluids with different concentrations of CO2 and Na and Ca chlorides exists at the P–T parameters of HP granulites in which originally homogeneous fluid splits into compositionally contrasting fluid phases with different properties. This region of coexisting immiscible fluids significantly expands with increasing CaCl2 concentration. Hence, the lower crust at the level of the HP granulite facies may be the region where high-temperature immiscible fluids are generated. One of these fluids is a denser phase of alkaline brines, and the other is a less dense potentially acidic phase of H2O–CO2 fluids rich in CO2. Ascending along regional permeable zones, these fluid phases of deep origin can play an important role in magmatic, metamorphic, metasomatic, and ore-forming processes in the middle and upper crust.

Using Petrogeochemical Modeling to Understand the Relationship between Paleozoic Magmatism in the Kola Region and Its Precambrian History

The Kola region hosts numerous Paleozoic massifs of ultrabasic alkaline rocks and carbonatites with deposits of commercially valuable metals, such as iron, tantalum, niobium, and rare earth elements. These magmatic complexes are characterized by high contents of alkaline elements at generally low contents of SiO2 and/or Al2O3. In this study, we examined the precursors to the formation of the unique Paleozoic alkaline province through studying the early Precambrian stages in the evolution of the Kola collision area, from where these unique features probably originated. We mathematically modeled the changes in the chemical composition of these rocks. The obtained data can be used for metallogenic forecasting, which indicated a number of Precambrian objects in the region, namely, the Lapland Granulite Belt of the Kola region and granulite belts in Eurasia. The mathematical modeling performed during this research depicted a linear trend that defined the style of the changes in the chemical composition at the transition from the metaultrabasic-basic rocks of the Lapland granulite belt to the group of belts in Eurasia. These differences are statistically significant with respect to the obtained trend (chemical composition projected on the trend), mainly manifested as increased SiO2 and Al2O3 contents with a decreasing total alkalis content, which is opposite to the indicated trends of the changing chemical composition in the Paleozoic alkaline rock units of the Kola region. We concluded that one of the reasons for the unique composition of the Paleozoic magmatism products could be a specific feature of the earlier Neoarchean stages of the tectonic-magmatic activity in the northeastern Baltic Shield, which implies a close relationship between later geological events and the early Precambrian history, at least in the study area.

Postglacial tectonics and palaeoseisic dislocation in the “Kovda” area (Kandalaksha bay, eastern Fennoscandian shield)

«Вестник Санкт-Петербургского университета. Науки о Земле» — научно-теоретический рецензируемый журнал, обобщающий результаты исследований в области геологии, гидрогеологии и геохимии, петрологии и экологии, ландшафтного планирования и геоморфологии, экономической и социальной географии.

Thermobarometric and Petrological Study of Mafic Xenolithes in Plagiogranites of the River Lotta Area of the Central Zone of the Lapland Granulite Belt

Thermobarometric and Petrological Study of Mafic Xenolithes in Plagiogranites of the River Lotta Area of the Central Zone of the Lapland Granulite Belt

Geochronology of Metamorphic Events in the Lower Crust beneath NW Russia: a Xenolith Hf Isotope Study

Hf isotope data for zircons and whole-rocks from lower crustal mafic granulite and pyroxenite xenoliths from NW Russia are presented together with the results of U–Pb zircon dating, Sm–Nd and Rb–Sr isotopic compositions of bulk-rocks and minerals, and trace element compositions of minerals. Most zircons preserve a record of only the youngest metamorphic events, but a few Grt-granulite xenoliths retain Archean magmatic zircons from their protolith. Metamorphic zircons have highly variable ɛHf(t) values from –25 to –4. The least radiogenic zircons were formed by recrystallization of primary magmatic Archean zircons. Zircons with the most radiogenic ɛHf grew before garnet or were contemporaneous with its formation. Zircons with ɛHf(t) from –15 to –9 formed by various mechanisms, including recrystallization of pre-existing metamorphic zircons, subsolidus growth in the presence of garnet and exsolution from rutile. They inherited their Hf isotopic composition from clinopyroxene, pargasite, rutile and earlier-formed zircon that had equilibrated with garnet. Subsolidus zircons were formed in response to a major change in mineral association (i.e. garnet- and zircon-producing reactions including partial melting). Recrystallized zircons date the onset of high-temperature conditions without a major change in mineral association. Age data for metamorphic zircons fall into five groups: >1·91 Ga, 1·81–1·86 Ga, 1·74–1·77 Ga, 1·64–1·67 Ga and <1·6 Ga. Most ages correlate with metamorphic events in the regional upper crust superimposed onto rocks of the Belomorian belt during formation of the Lapland Granulite Belt. Zircon formation and resetting at 1·64–1·67 Ga significantly postdates Lapland–Kola orogenic events and may relate to the onset of Mesoproterozoic rifting. The youngest ages (1·6–1·3 Ga) correspond to an event that affected only a few grains in some samples and can be explained by interaction with a localized fluid. The observed garnet-granulite associations were formed at 1·83 Ga in Arkhangelsk xenoliths and 1·74–1·76 Ga in most Kola xenoliths. By the end of the Lapland–Kola orogeny, the rocks were already assembled in the lower crust. However, no addition of juvenile material has been detected and preservation of pre-Lapland–Kola metamorphic zircon indicates that some xenoliths represent an older lower crust. Granulites, pyroxenites and Phl-rich rocks have a common metamorphic history since at least c. 1·75 Ga. At about 1·64 Ga metasomatic introduction of phlogopite took place; however, this was only one of several phlogopite-forming events in the lower crust.

EARLY PRECAMBRIAN CRUSTAL EVOLUTION OF THE BELOMORIAN AND TRANS-NORTH CHINA OROGENS AND SUPERCONTINENTS RECONSTRUCTION

Comparative analysis of the crustal evolution of the Early Precambrian Belomorian and Trans-North China orogens (Fig. 1) has shown [Slabunov et al., 2015] that: Both belts were formed by the superposition of two Precambrian orogenies. The earth crust of the Belomorian belt was produced during the Mesoarchaean to Neoarchaean Belomorian collisional orogeny [Slabunov, 2008; Slabunov et al., 2006] and then was reworked during the Palaeoproterozoic Lapland-Kola collisional orogeny [Daly at al., 2006; Balagansky et al., 2014]. The earth crust of the Trans-North China orogen was formed during a Neoarchean accretionary orogeny and then was reworked during a Paleoproterozoic collisional orogeny [Zhao et al., 2012; Guo et al., 2012, 2005]. The Lapland granulite belt is the core of the Lapland-Kola Palaeoproterozoic collisional orogen in the Fennoscandian shield and the Khondolite belt occupies the same tectonic position in a Palaeoproterozoic collisional orogen in the North China craton.

Olivine-bearing rocks of the Lapland granulite belt (Baltic Shield)

Olivine-bearing varieties of garnet–clinopyroxene crystalline schists of the Lapland granulite belt have been studied in detail for the first time. Two types of olivine (iron mole fractions of 27 and 38%) are distinguished. Olivine with lower Fe content occurs as inclusions in clino- and orthopyroxene and in terms of СаО and Cr contents is close to magmatic minerals. Olivine with high Fe content presumably suffered highand moderate-temperature metamorphism. The olivine-bearing rocks contain several grains of omphacite with 30–37 mol % jadeite and garnet with 44–50 mol % pyrope, which can be regarded as relict assemblages of the early stage of eclogitization of a magmatic protolith. The presence of symplectites indicates their retrograde transformation during decompression. The protoliths of the studied rocks could be olivine gabbronorites and pyroxenites. It was found that the rocks contain high-alumina minerals: corundum, spinel, and sapphirine. In addition, Al2O3 content in some amphibole grains is as high as 19 wt %. This indicates that the ascent of the deep-seated rocks was accompanied by interaction with Al-rich fluid. The positive Eu anomaly in the olivine-bearing rocks and some of their minerals is indicative of the reducing character of fluid. Activation of fluid reworking leading to the formation and transformation of the olivine-bearing rocks, transfer of alumina and its precipitation at different depths are related to the processes at the base of the Paleoproterozoic rift system of Karelides.

Finding of corundum-bearing rocks in the Lapland granulite belt

Corundum-bearing rocks are described for the first time in the Kandalaksha structure of the Lapland granulite belt. Corundum is confined to rocks of two types: metagabbro‒anorthosites constituting lenses among metaanarthosites of the Kandalaksha massif and basic granulites. Corundum crystals (up to 200 μm long) occur in plagioclase and garnet and differ from each other depending on the host mineral, which serves as evidence against their xenogenic nature. Some corundum crystals exhibit an axial zone, which may indicate their crystallization from the gaseous phase. Corundum-bearing rocks are accompanied by piclogites (pyroxene‒garnet varieties with olivine). Piclogites and their minerals (clinopyroxene, garnet) are characterized by a positive Eu anomaly, which implies rock reworking by fluids during corundum formation, when deep-seated complexes were subjected to exhumation.

Stress, rheological structure and earthquakes in the POLAR profile in the northern Fennoscandian Shield

We model stress fields and potential areas of deformation, use differences to analyse the possible variations in the modelling results and investigate the relations between the earthquakes and our modelling results in the POLAR profile in the northern Fennoscandian Shield. Calculated stress fields show rather uniform distribution, with significant deviations concentrated in rheologically weaker regions, mainly in the middle crust. Significant areas of deformation are also connected to these weaker regions, in the crust and also in the mantle. These regions control whether decoupled or coupled mechanical conditions exist. Further analysis shows that major differences between the stress fields calculated with dry and wet rheologies are found in the lower part of the middle crust extending from the Lapland Granulite Belt area towards the northeast. Similarly, differences between the models due to the varying heat production and the depth of the thermal lithosphere–asthenosphere boundary concentrate in rheologically weak crustal regions and in the mantle. These differences display importance of the rheological composition and thermal structure in rheological modelling. Our rheological model for reverse stress regime with typically used values of 0.35 for pore fluid factor and 0.75 for friction coefficient seems to result in too strong crust in order to explain satisfactorily the earthquakes focal depths in the POLAR profile. Most of the earthquakes can be explained by reverse stress regime if high pore pressure conditions exist. However, for the deepest events, it seems that also changes both in stress regime and in friction coefficients are needed. Rheologically, weak middle crust in the central and north-eastern parts of the POLAR could explain the absence of deep earthquakes in these parts of the profile.

Sequence of geologic events in the central and northeastern parts of the Lapland Granulite Belt: Isotopic geochemical zircon data and results of geological-petrological studies

Geological and petrological studies of rocks were conducted to reproduce the succession of endogenic processes in the central and northeastern parts of the Lapland Granulite Belt along the course of the Lotta River. Mineralogical-geochemical data on zircon from these rocks (the anatomy of zircon crystals seen in BSE and CL imagery; U, Th, Hf, and Y concentrations; and REE patterns) and their crystallization temperatures (calculated using the Ti concentration) make it possible to correlate the newly obtained geochronologic data with the sequence of metamorphic events inferred from petrological data. Three major episodes of enderbite-forming processes are distinguished: (i) early episode at 1.99 Ga, (ii) main episode at 1.940–1.925 Ga; and (iii) late episode at 1.89 Ga. The early medium-pressure metamorphic episode is dated at 1.97–1.96 Ga. The diatexis of the quartz-feldspar granulites with the derivation of garnet plagioclase leucogranites took place at 1.917–1.909 Ga.

The age of the protolith of metamorphic rocks in the southeastern part of the Lapland granulite belt, southern Kola Peninsula: Correlation with the Belomorian mobile belt in the context of the problem of Archean eclogites

U-Pb zircon isotopic data on rocks from the Kandalaksha-Umba zone of the Lapland granulite belt in the Por’ya Bay area constrain the age of the protolith of the apodacite (apotonalite) Opx-Bt granulite gneisses at 2799 ± 4 Ma, and the age of the apogabbronorite Grt-Opx-Cpx-Hbl crystalline schists at 2315 ± 23 Ma. The U-Pb sphene age of the magmatic crystallization of the postmetamorphic granodiorites is 1901 ± 5 Ma. The zircon yields the U-Pb age of the contamination of xenogenic zircons, which were captured during the dissolution of xenoliths of the host Grt-Opx-Cpx-Hbl crystalline schists in granodiorite melt. The comparison of the most important attributes of the endogenic histories of the adjacent Lapland Granulite and Belomorian Mobile belts testifies to their similar evolutionary histories: (1) the protolith age of the acid Opx-Bt granulites of the Lapland Belt (2799 ± 4 Ma) coincides with the protolith age of acid gneisses in the Belomorian Belt (2890-2690 Ma); (2) the ages of the gabbronorite protolith of Grt-Opx-Cpx-Hbl granulites in the Lapland Belt (2315 ± 23 Ma) and gabbro-anorthosite in the Kolvitsa Massif (2462-2423 Ma) are close to the protolith age of eclogitized gabbronorites in the Belomorian coronite suite (2.46–2.36 Ga); (3) the age of granulite metamorphism of acid and mafic rocks in the Lapland Belt is 1912–1925 Ma, and the age of eclogite metamorphism of gneisses and metabasites in the Belomorian Belt is approximately 1.9 Ga, i.e., their metamorphism took place in Svecofennian time; (4) the peak pressure of granulite metamorphism in the Lapland Belt was 9–11 kbar at a temperature of 800–850°C, whereas the peak metamorphic parameters of eclogite metamorphism in the Belomorian Belt were 10–12 kbar and 640–700°C. This means that the metamorphic complexes of the Lapland and Belomorian belts had the same Mezo- and Neoarchean protoliths hosting bodies of Paleoproterozoic gabbroids and were completely formed largely by a single cycle of Svecofennian high-pressure zonal metamorphism within a temperature range from the lowest grade of the eclogite to the granulite facies.

Rare earth element and gold exploration in glaciated terrain: example from the Mäkärä area, northern Finland

The Mäkärä Au-rare earth element (REE) prospect area is located in the Tana Belt, south of the 1.9 Ga Lapland Granulite Belt, in northern Finland. The Belt has prominent lanthanum (La) and yttrium (Y) anomalies in regional till and bedrock geochemical data. High Y indicates enrichment of heavy REE in the bedrock. At Mäkärä, promising narrow Au-hematite-quartz veins occur in connection with tensional fractures in the great shear zone. Under the latest ice divide of the last glaciation, subglacial erosion was weak and glacial transport distance short. High Au contents in saprolite and till together with deep weathering have a strong positive correlation with the positive electromagnetic anomalies caused by the sulphidic gneisses. The highest La and Y contents in till correlate well with the Th maxima of airborne-radiometric datasets. Locally, the &lt;0.06 mm till size fraction contains up to 0.4% REE. A recent exploration project in Mäkärä revealed a 13 m wide Au-hematite-quartz vein with a mean of 3 ppm Au and 0.04–0.1% REE in kaolinitic saprolite derived from arkosic gneiss. The elevated REE content resembles that of ionic adsorption clays in China. Typical REE-rich minerals are monazite, rhabdophane, xenotime and kaolinite. Till geochemistry proved useful in REE exploration.

Noise directivity and group velocity tomography in a region with small velocity contrasts: the northern Baltic shield

Ambient noise tomography (ANT) is widely used to image strong velocity variations within the upper crust. Using careful processing, we obtained a 3-D model of shear velocities in the upper crust beneath northern Finland, where the lateral velocity variations are less than 3 per cent. As part of the tomography, the noise field is analysed. It is strongly heterogeneous but the signal-to-noise ratio is sufficient to obtain stable dispersion curves for all profile azimuths. Our results show that the directions of dominant noise sources of Rayleigh and Love waves are the same, but the amplitude distribution with azimuth is different for the two types of waves. More intriguingly, the high frequency Love waves are dominated by a mixture of higher modes rather than the fundamental mode. The reconstructed 3-D model shows the Lapland Granulite Belt as a high velocity body with a limit at surface in excellent agreement with geological observations at surface. Following this interface at depth, our results are compatible with previous studies suggesting an Archean north oriented subduction.