Northern Ladoga Region Research Articles

T-t Evolution of the Early Proterozoic Rocks in the Northern Ladoga Region from the Data on U-Pb, Rb-Sr and Sm-Nd Systems in Minerals

This paper presents the results of a study of isotopic systems in minerals and rocks in southern margin of the epi-Archean Karelian craton in the zone of its junction with the Svecofennian mobile belt. U-Pb, Sm-Nd and Rb-Sr mineral ages of metamorphic rocks allowed reconstructing a T-t trend during ~1.88–1.61 Ga, which reflects a wide-ranging cooling history of metamorphic rocks from the peak values of about 650–700 °C at 1.88–1.79 Ga (U-Pb age of monazites and apparent oldest Sm-Nd age of amphiboles) to 300–400 °C at 1.61 Ga (model Rb-Sr age of biotites) in zones of low- and medium-temperature metamorphism. The specificity of removal of deep-seated rocks to the present-day erosion surface and the reconstructed T-t trend comply with the development of thrust-nappe structures during the exhumation of the Svecofennids. It is also assumed that differential vertical block movements played a significant role during the post-orogenic extensional collapse and neorifting.

Geochemical Aspects of the Frictional Melting of Metapsammites during Seismic Slips (with Reference to Pseudotachylytes of the Ladoga Region)

Based on analytical data on the geochemistry of tectonic pseudotachylytes and their host rocks, the specific trends of redistribution of major, trace, and rare earth elements in the course of seismogenic frictional melting of the arkosic metaterrigenous rocks from three zones of regional metamorphism with different temperatures (greenschist, amphibolite, and granulite) in the Northern Ladoga region are discussed. Oppositely directed trends of changes in the contents of the oxides of major elements in the protolith–blastocataclasite–pseudotachylyte triad were revealed, and a unidirectional increase in the basicity of the frictional melt as compared with the protolith was established at the same time. Geochemical evidence for partial selective melting of the protoliths is discussed. The specific features of the fractionation of trace and rare-earth elements during the transition of the protolith into the melt, as well as during its subsequent partial recrystallization, are shown. The emergence of a peak of elevated europium contents relative to the protolith in the melt matrix of all three sampling points is noted. Based on changes in the concentrations of these elements in the zones of pseudotachylyte substrate generation and in the areas of its moving and injection, the estimates of their differential mobility during frictional melting in a dynamic slip zone are given.

Almandine jewelry garnet from the Kitelya deposit (Karelia): composition and spectroscopic properties

Research subject. The chemical composition and spectroscopic properties of almandine jewelry garnets from the Kite-lya deposit in the Northern Ladoga region (Karelia). Materials and methods. The chemical composition, impurity elements, mineral inclusions and spectroscopic properties of almandine jewelry crystals were studied using IR and Mössbauer spectroscopy. Results. Garnet crystals were found to exhibit a weakly pronounced zonal composition, varying from Alm75Pir15Sps7Grs3 in the center to Alm80Pir14Sps4Grs2 at their edges. Therefore, the Ca and Mn contents decrease towards the grain edges. This zonality of garnets is characteristic of the processes of progressive metamorphism of their host rocks. The garnet crystals feature small inclusions of quartz, chlorite, mica FACI (biotite), ilmenite, rutile, monazite, zircon and pyrrhotite. The composition of chlorite, biotite and zircon was established. The parameter of the cube unit cell ao = 11.522 ± 0.003 Å was calculated. The IR absorption spectra of 995, 966, 901, 878, 638, 568, 528, 476 and 455 cm–1 are characteristic of the pyrope-almandine difference. Mössbauer spectroscopy revealed an insignificant admixture of trivalent iron (Fe3+) in the structure of Kitelya garnets (≈1 % of the amount of isomorphic iron). The obtained optical absorption spectra of garnet plates in the visible light spectrum indicate that Fe2+ ions in dodecahedral positions, to a lesser extent dodecahedral Mn2+ ions, as well as possibly octahedral Fe3+ ions are responsible for the bright red-crimson color of pyrop-almandine from the Kitelya deposit. Conclusions. A “portrait” of typomorphic features (composition and properties) of the pyrope-almandine jewelry garnet from the Kitelya deposit was obtained. This portrait can be used when analyzing the historical finds of faceted or cabochonized differences of almandine in jewelry, church utensils in both Russia and Europe (where this jewelry material was exported during the 17th century). The preservation of garnet jewelry differences in the host rock is due to the presence of thin amorphous kelefite shells or soft minerals (sericite, chlorite, kaolinite etc.).

GEOCHEMICAL ASPECTS OF METAPSAMMIT FRICTIONAL MELTING DURING SEISMIC MOVEMENTS (BY THE EXAMPLE OF PSEUDOTACHYLYTES OF THE LADOGA REGION)

Based on geochemical analyses of tectonic pseudotachylytes and their host rocks the features of redistribution of major, rare and rare-earth elements during seismogenic frictional melting of arkose-type metaterrigenous rocks from different-temperature zones of regional metamorphism) of Northern Ladoga region are considered. Multidirectional trends in variations of the major elements oxides contents in the triad protolith-blastocataclasite-pseudotachylyte were revealed, but a unidirectional increase of the frictional melt basicity in comparison with the protolith was established. Geochemical evidence of partial selective melting of source rocks is considered. Peculiarities of rare and rare-earth elements fractionation during transition to the melt of protolithic material, as well as during its subsequent partial crystallization are shown. The appearance of positive europium anomaly in the melt matrix of all three sampling points is noted. By changes of these elements’ concentrations in zones of pseudotachylyte substrate generation and in areas of its injection their differential mobility during frictional melting in a zone of dynamic movement is estimated.

GEOCHEMICAL MARKERS OF THE JOINT STRUCTURAL AND COMPOSITIONAL EVOLUTION OF THE BASEMENT AND COVER (NORTHERN LADOGA SWECOFENNIDES, RUSSIA)

Detailed studies of the deeply metamorphosed Early Precambrian rocks of the Northern Ladoga region allowed us to distinguish three deformation stages of the Svecofennian tectogenesis during which there occurred significant structural and compositional transformations of the "cover (Paleoproterozoic) – basement (Archean)" system. In addition to the structural-paragenetic analysis, which allowed to allocate transversal structural paragenesis in both floors, there are some other opportunities in the recognition of their-hosted granitoid veined bodies with a positive Eu anomaly. The rock varieties with this anomaly are always high in barium and do not show a direct correlation between the Eu anomaly and (La/Yb)n, Ca and Sr. This is contrary to the ideas about the occurrence of a positive Eu anomaly due to the substitution of divalent strontium by Eu++ and suggests that the formation of such rocks took place under the influence of deep reduced fluids. It was found that granitoids with a positive Eu anomaly were formed during the first and last stages of the structure evolution, with a predominance of brittle deformations and a deep-reduced fluid breakthrough. At the second stage, with the dominant manifestation of plastic deformations, when such fluids could be "blocked" within the system, there was a formation of granitoids with low barium concentrations and a negative Eu anomaly.

Evolution of the Meyeri Thrust Zone of the Northern Ladoga Region (Republic of Karelia, Northwest Russia): PT Conditions for the Formation of Mineral Parageneses and Geodynamic Reconstructions

Evolution of the Meyeri Thrust Zone of the Northern Ladoga Region (Republic of Karelia, Northwest Russia): PT Conditions for the Formation of Mineral Parageneses and Geodynamic Reconstructions

Two stage of gold-bearing sulfide ores of early Proterozoic gabbroids in the north Ladoga area

In a study based on Pb isotopes, it has been shown that the formation of gold-sulfide mineralization in the Early Proterozoic gabbros of the Northern Ladoga region is associated with different tectonic phases. This conclusion is based on a comparative study of isotopic data for feldspars and lead sulfides in two Early Proterozoic intrusions; Velimäki and Alattu-Päkylä. The investigated intrusions have similar geological settings at the junction of the Svecofennian accretionary complex and the Karelian craton. Both massifs are characterized by manifestations of noble metal sulfide mineralization associated with zones of local shear deformations. The formation of the studied ore massifs is attributed to the Svecofennian and Caledonian tectonic stages. Feldspars of Velimäki intrusion have parameters and model age of Pb significantly older than U-Pb age of zircon (1.9 Ga), although they correspond to the Svecofennian time of magmatic crystallization of gabbro and clinopyroxenites, while sulfide pyrite-pyrrhotite mineralization is associated with gold in these rocks with the Caledonian stage of fluid-thermal processing of the Early Proterozoic magmatic mineral paracenteses. Feldspars and sulfides of the Alattu-Päkylä intrusion have similar lead isotopic parameters and Pb model age (~ 2 Ga), also somewhat older than the U-Pb age of zircon, but at the same time indicating the formation of rock-forming and ore sulfide associations with gold during the Svecofennian (Early Proterozoic) plutonic and tectonic-thermal events. The source of lead sulfides of the Velimäki massif is characterized by the parameters of the upper crust with high µ2 = 238U / 204Pb, while the µ2 parameter of the minerals of the Alattu-Päkylä massif is slightly lower, which indicates the participation of the mantle-lower crustal reservoir material in the formation of magma.

Distinguishing Precambrian Active Morphostructures in the Northern Ladoga Region from Comprehensive Geological–Geomorphological Data

On the basis of analyzed and generalized geological–geomorphological data, including new data obtained by the authors, a scheme of the recently active Precambrian morphostructures of the Northern Ladoga region has been compiled. The following factors were taken into account when compiling this scheme: topographic expression of ancient structures; the occurrence of earthquake epicenters and the location of paleoseismodisloсations and He and Rn anomalies within these structures; the results of GPS points movement analysis; and the results of computer and physical modeling on equivalent materials.

Two genetic types of pseudotachylytes

The study of two varieties of pseudotachylytes (PST) in granitoids of the Riphean complex on the Barents Sea coast of the Kola Peninsula (Rybachii and Srednii peninsulas) and in metapsammite of the Paleoproterozoic complex in the Northern Ladoga region by a few independent analytical methods has made it possible to establish that they belong to different genetic forms, such as mechanically crushed rocks and melting products, respectively. As for the melting differences, we have given a detailed description of the mineral and material transformations of the original rock into the PST glass matrix and obtained evidence for the initial melting out of the micaceous eutectics with its subsequent shift to the granite type. The conclusion has been made on the most likely formation of molten PST due to frictional rock melting under rapid rise of its blocks from a depth of 12–15 km to the crustal surface (less than 3 km) along the faults of presumably seismogenic nature. It is suggested that crushing and frictional melting can be complementary, rather than mutually exclusive processes, and the formation of molten PST is commonly preceded by the mechanical rock crushing stage.

Faults in the quarterly deposits in Northern Ladoga region as indicator of seismotectonic processes related to neotectonic activation of Precambrian structural plan

Faults in the quarterly deposits established during field investigations indicate that seismic events with magnitude of no less than 5 occurred in the Northern Ladoga region during neotectonic time. Computer modeling has shown that the modern seismicity of this territory is associated with the neotectonic activation of Precambrian disjunctive structures in conditions of NW compression. Relief analysis allows us to distinguish faults limiting the basin of the Ladoga Lake, which belongs to active neotectonic structures.

The Northern Ladoga region as a prospective tourist destination in the Russian-Finnish borderland: Historical, cultural, ecological and economic aspects

The work reported here has examined the transformation of the Northern Ladoga region (a natural and historical region in the Russian-Finnish borderland) from ‘closed’ border area into a prospective tourist destination in the face of changes taking place in the 1990s. Three periods to the development of tourism in the region are identified, while the article goes on to explore general trends and features characterising the development of a tourist destination, with the focus on tourist infrastructure, the developing types of tourism and tourismoriented projects. Measures to further stimulate tourism as an economic activity of the region are suggested.

Platinoids in the Kaalamo Differentiated Massif in the Northern Ladoga Region, Karelia, Russia

The Kaalamo massif is located in the Northern Ladoga region, Karelia, on the extension of the Kotalahti Belt of Ni-bearing ultramafic intrusions in Finland. The massif, 1.89 Ga in age, is differentiated from pyroxenite to diorite. Nickel–copper sulfide mineralization with platinoids is related to the pyroxenite phase. The ore consists of two mineral types: (i) pentlandite–chalcopyrite–pyrrhotite and (ii) chalcopyrite, both enriched in PGE. Pd and Pt bismuthotellurides, as well as Pd and Pt tellurobismuthides, are represented by the following mineral species: kotulskite, sobolevskite, merenskyite, michenerite, moncheite, keithconnite, telluropalladinite; Pt and Pd sulfides comprise vysotskite, cooperite, braggite, palladium pentlandite, and some other rare phases. High-palladium minerals are contained in pentlandite–chalcopyrite–pyrrhotite ore. Native gold intergrown with kotulskite commonly contains microinclusions (1–3 μm) of Pd stannides: paolovite and atokite. Ore with 20–60% copper sulfides (0.2–6.0% Cu) contains 5.1–6.6 gpt PGE and up to 0.13–2.3 gpt Au. Pd minerals, arsenides and sulfoarsenides of Pt, Rh, Ir, Os, and Ru are identified as well. These are sperrylite, ruthenium platarsite, hollingworthite, and irarsite; silvery gold and paolovite have also been noted. All these minerals have been revealed in the massif for the first time. The paper also presents data on the compositions of 25 PGE minerals (PGM) from Kaalamo ores.

Caledonian formation of gold-bearing sulfide depositions in Early Proterozoic gabbroids in the northern Ladoga region

We have studied Pb isotopic systems of K-feldspar, pyrite, and pyrrhotine from gabbroids and ore of the Velimyaki Early Proterozoic massif in the northern Ladoga region in the southeastern part of the Fennoscandian Shield. The isochronous Pb–Pb age of sulfides has been determined as ∼450 Ma, which corresponds to intersection of the regression line with the lead accumulation curve with μ = 10.4–10.8; the model Pb age of sulfides is close to isochronous under the condition that the composition of lead evolved from a geochemical reservoir with an age of 1.9 Ga. The isotopic parameters of the lead in sulfides and K-feldspar indicate their formation in upper crust conditions (μ = 238U/204Pb > 10). From the obtained data, it follows that the isotopic composition of lead in K-feldspar corresponds to a Proterozoic age (1890 Ma) of magmatic crystallization of the rocks in the massif, and strongly radiogenic lead sulfides testify, with the greatest probability, to the later (Caledonian) formation of sulfide ores.

Zonal REE + Y profiles in garnet and their genetic implications: A case study of metapelites from the northern Ladoga region

The objective of this study is to provide insights into the REE and Y behavior during garnet porphyroblast formation in staurolite-bearing schists as a constituent of Late Paleoproterozoic metapelites of the Ladoga Complex. The MnNCKFMASH P–T pseudosection for a single sample and Grt–Bt thermometry indicate that the garnet core grew at 520°C and under 7.0–7.2 kbar in the Grt–Bt–Pl–Chl–Ms–Zo field, whereas the garnet rim was equilibrated at 590–600°C and under 3.5–4.0 kbar. The measured zoning profiles are strongly depleted in REE + Y in the garnet core containing high Mn and Ca concentrations. The intermediate zone of garnet is enriched in La, Ce, Pr, and Nd (inner LREE + Nd annulus), as well as in Dy, Er, Yb, Lu, and Y (outer HREE + Y + Dy annulus). According to pseudosection analysis, these peaks were probably produced owing to breakdown of epidote-group minerals (allanite, REE-rich epidote) at T 6.5 kbar. Towards the rim, the HREE + Y contents gradually decrease, whereas MREE (Sm, Eu, Gd) display an inverse trend. The rim also exhibits a negative Eu anomaly. The former tendency reflects an increase in temperature during garnet crystallization and partitioning of elements between garnet and monazite. It is thought that the latter is linked to oppositely directed change in garnet-monazite partition coefficients for HREE and MREE with increasing temperature.

From migmatites to crustal granites: Pb–Pb isotopic system of feldspars of partial melting products, Northern Ladoga region, Fennoscandinavian Shield

The isotopic–geochemical features of late and postorogenic granites of the S type and ambient migmatites are studied within the Russian part of the Svecofennian orogen of the Fennoscandinavian Shield. The spatial association of leucosomes of migmatites and granites of the S type and their similar petro- and geochemistry and distribution of Pb isotopes are evidence of the genetic similarity of their parental melts. The Borodinskoe pluton has a more primitive 206Pb/207Pb ratio, which indicates the presence of upper and U-poor lower crustal material in the source of granitic magmas. This conclusion is supported by the ɛNt(t) lower value of granites of this pluton relative to those of other plutons of the region.

THE REORGANIZATION OF TIMBER INDUSTY OF THE SOVIET KARELIA AFTER THE WINTER WAR

The paper focuses on the changes in timber industry management in the Soviet Karelia in 1940-1941. In the 1930’s the government of the republic lost control over logging. However, after the Winter War the former Finnish territories the Karelian Isthmus and the Northern Ladoga region with several large pulp and paper mills and woodworking enterprises were annexed to the newly formed Karelo-Finnish Union Republic. It resulted in the changes of the logging scheme and flows of raw wood. The governing body of the Karelo-Finnish SSR prevented the formation of a departmental “enclave” with union People’s Commissars for timber and paper industries entirely controlling logging and wood processing on the new territories. The leaders of the Karelo-Finnish SSR managed to persuade the central authorities in their ability to supply raw wood for the new factories and regained control over logging on the whole territory of the republic.

A different duration of the formation of leucosome of migmatites in the K- and Na-zones of the northern Ladoga region of the southeastern Svecofennian belt, Fennoscandian shield

The duration of migmatite formation in the K and Na geochemical zones was first estimated for the Svecofennides of the northern Ladoga region. The U-Pb (TIMS) age of monazite was determined for the most complete sequence of leucosomes with geological control of their relations and relative age observed in the outcrops. The duration of formation of the leucosomes in two zones is different and is not overlapped by the error of the age analysis: about 5 and no less than 10–15 Ma in the K and Na zones, respectively. The different duration of migmatite formation may reflect different temperature evolution of the rocks of the two zones and also the influence of the composition and rheological properties of migmatizing sequences, which govern the conditions of liquation, transportation, and crystallization of the anatectic melt.

Compositional zoning in garnet and kinetics of metamorphic crystallization

The kinetics of zoned garnet porphyroblast growth is exemplified in a sample of garnet-staurolite-biotite schist from the northern Ladoga region. The diffusion-controlled porphyroblast growth was accompanied by a decrease in the kinetic coefficient during phase reactions. Even at insignificant (1–2°C) thermal overstepping, the leading role of diffusion as a factor that controls kinetics of porphyroblast growth in medium-grade metapelites is consistent with the parameters of metamorphic crystallization: T = 500–650°C, t = 1 Ma; DA1app = 10−14 cm2/s, L = 0.2–0.6 cm, r = 1–3 mm, ΔCAl = 1.5 × 10−4–1.5 × 10−3 mol/cm3.

Morphological evidence for diffusion-controlled growth of garnet from metapelites

The surface microtopography of garnet from metamorphic schists of the northern Ladoga region has been investigated. The morphology of garnet is distinguished by a rough stepped surface on dodecahedral faces {110} and by the absence of sharp crystal edges. As follows from experimental results on crystal growth under hydrothermal conditions, the microtopography pattern gives evidence for a highly oversaturated inter-granular medium and the important role of diffusion as a factor controlling garnet growth.

Catagenesis of terrigenous rocks in the neoproterozoic intracratonic sedimentary basins and its effect on the formation of unconformity-type uranium mineralization

Mineral transformation of host rocks and localization of orebodies at the unconformity-type uranium deposits are considered for the Karku deposit in the northern Ladoga region. It is shown that the great depth of uranium mineral formation and the peculiar composition of host rocks, along with temperature and chemistry of fluids, played a critical role in variation of lithostatic and fluid pressure, porosity, and permeability. The compaction of quartz sandstone and gravelstone, which are typical host rocks at unconformity-type deposits, the development of microstylolithic sutures, conformal structures, pressure solution and deposition of quartz in free pores gave rise to the closure or constraint of pore space and to increase in pore pressure of fluids in the deep part of the Riphean troughs with approaching lithostatic loading. A transitional zone between hydrostatic and lithostatic pressure controlled localization of orebodies and was decisive for uranium mineral formation. This zone coincided with the Riphean-Paleoproterozoic unconformity and sank somewhat into the crystalline basement. Below this transitional zone, the intergranular fluid was under a pressure that was close to the pressure on solid phases, i.e., Ptot ≈ Pfl. The reliability of this phenomenon is confirmed by cessation of pressure solution-redeposition of quartz and distinct deceleration of dehydration of hydrous minerals. As is shown for the Karku deposit, the highly hydrated clay minerals of the illite-smectite series are widespread in its subore portion and lacking at the supraore levels along with termination of quartz regeneration. It is suggested that a zone of superhigh fluid pressure in deep parts of sedimentary basins constrains localization of uranium orebodies by structural and stratigraphic unconformity between Riphean and Paleoproterozoic rocks. It is stated that altered wall rocks at the unconformity-type uranium deposits cannot be identified with products of hydrothermal phyllic and argillic alteration of host rocks at the medium- and low-temperature endogenic deposits. The main distinctions consist in lack of wall-rock metasomatic reaction zoning and acid-alkaline evolution of solutions. All transformations of host rocks should be classified as diverse manifestations of deep catagenesis of sedimentary sequences and buried regolith. The carbon and oxygen isotopic compositions of calcite from host rocks at the Karku deposit are far from those of magmatic and hydrothermal carbonates. They are characterized by a high δ18O = +17 to +25‰ and a high dispersion of δ13C = −1.5 to almost −15‰. No granitoid magmatism is known in the regions, where the unconformity-type uranium deposits occur. Therefore, the rocks of the crystalline basement are the most probable source of uranium, which precipitated on the reductive barrier as a product of interaction with bitumen contained in the Riphean basal beds.