Lapland-Kola Orogen Research Articles

PROVENANCE AND AGE CONSTRAINTS OF THE RIPHEAN SANDSTONES FROM THE IVANOVSKY GRABEN (THE KOLA PENINSULA) BASED ON THE U-Pb LA-ICP-MS DATING AND RAMAN SPECTROSCOPY OF DETRITAL ZIRCON

The results of U-Pb LA-ICP-MS dating of 150 detrital zircon grains from the Precambrian (Riphean?) sandstones of the Kildin group on the coast of the Ivanovka Bay (northeastern Kola Peninsula) are presented. A group of three youngest grains yielded constraint on the sandstone maximum depositional age of 1179±45 Ma. Detrital zircon age distribution is dominated by three maximums of 1.9, 2.4 and 2.7 Ga. The authors consider two complementary scenarios of interpretation of the data obtained. The first scenario involves the contribution of distant provenance out of granitoids in the Lapland-Kola orogen (~1.9 Ga) and adjacent areas of the Kola-Karelia region (~2.7 Ga), Imandra-Varzuga structure (~2.4 Ga), and Sveconorwegian belt (~1.2 Ga); the off-Baltic (Greenland?) sources of the detrital zircon also cannot be excluded. According to the second scenario, the Ivanovsky graben received zircons also from the proximal sources – sialic complexes formed upon melting of granite-gneiss substratum of the upper crust of the Murmansk craton during largescale episodic mafic magmatism 1.86, 2.50 and 2.68 Ga. The Raman spectroscopy showed that about 25 % of the analyzed detrital zircon grains experienced a 1.4-Ga-old thermal effect which did not lead to Pb losses but partially restored the crystalline structure of zircon. This result can be used as an additional constraint in the detailed provenance analysis of the northeastern East European Platform sedimentary basins and is indicative of 1.4-Ga-old large-scale magmatic events related to the breakup of the supercontinent Nuna.

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.

BELOMORIAN MOBILE BELT AS PART OF THE NUNA/COLUMBIA SUPERCONTINENT: NEW PALEOMAGNETIC AND GEOLOGICAL DATA

Analysis of the evolution of the Belomorian Mobile Belt (BMB) suggests that the metamorphism of gabbroic rocks was accompanied by their remagnetization and that lateral variations in magnetization constituents carry evidence for the time sequence of the extension of the western foreland of the Lapland-Kola orogen (LKO) to upper crustal levels. The latter formed while the Nuna/Columbia Supercontinent was being made up. The combined study of metamorphosed Paleoproterozoic igneous complexes has revealed a remagnetization trend in BMB rocks. The study has shown that the remagnetization front spread here from the north-west to the south-east and that it seems to be due to the collision pattern upon LKO formation. In addition, remagnetization in the southern BMB and in the southeastern Karelian Craton, dated at 1.65-1.60 Ga, was revealed. It seems to be due to the final episodes of formation of the Svecofennian orogen.

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.

A revised geodynamic model for the Lapland-Kola Orogen

The Paleoproterozoic Lapland-Kola Orogen in Fennoscandia has been studied for decades and several plate tectonic models have been proposed including one-sided subduction zone, either towards SW or NW, or two opposite-verging subduction zones before the collision. Based on new structural and isotope data from Finland and recently published data from Russia, we propose a revised tectonic model for the Paleoproterozoic Lapland-Kola Orogen. The main components are foreland in the NE followed by cryptic suture, Inari arc, retro-arc basin and retro-arc foreland in the SW. The latter three constitute the Inari Orocline.Subduction towards present SW and subsequent arc magmatism (Inari arc) started at ca. 1.98 Ga followed by voluminous sedimentation in the deepening retro-arc basin. Underplating of a mid-ocean ridge caused flat subduction and magmatic flare at 1.92 Ga over a broad distance in the retro-arc basin. Rapid heating led to melting of the retro-arc basin sediments and voluminous amounts of granulite-facies diatexites formed. During collision (D1) at 1915–1910 Ma, large thrust nappes formed on the foreland. Deformation in the retro-arc basin is seen as recumbent folding and shearing of diatexites in the lower parts of the basin and thrusting of metatextite-diatexite packages in the upper parts. A post-collisional stage is seen as 1904 Ma appinites and decompression derived granites at 1.90–1.89 Ga. Renewed shortening (D2), due to far-field effects in SW at 1.88–1.87 Ga, led to thick-skin shortening of the Archean middle crust, large-scale crustal duplexing of already cooled granulites towards the retro-arc foreland and inclined upright folding of granulites in the opposite direction towards the Inari arc. A switch in the stress field from NE-SW to NW-SE led to orogen-parallel contraction and buckling started along a dextral strike-slip fault zone to form the Inari Orocline. Buckling is seen in the bending of pre-orocline fabrics and formation of syn-orocline fabrics: radial conical folds (D3), radial fractures, a strike-slip fault zone and thrusting at the hinge zone. The end-result is a mega-scale parallel multi-layer fold composed of the Inari arc, retro-arc basin and possibly also the heated retro-arc foreland.

Gently sloping shear zones in the Belomorian Mobile Belt: Geology, structure, and P–T parameters

The Belomorian Mobile Belt (BMB) in northern Karelia mostly consists of gently sloping shear zones, whose gneisses and migmatized amphibolites and blastomylonites are typically thinly banded, with their banding consistently dipping north- and northeastward. These gently sloping shear zones were not affected by folding after they were produced and are not cut by Paleoproterozoic metabasite dikes. Intrusive metabasites in the gently sloping shear zones make up relatively small (usually <5 m) equant or elongate bodies and occur as fragments of larger bodies. These fragments are often concentrated in stripes. Metabasites in the gently sloping shear zone are sometimes also found as lenses and tabular bodies of relatively small thickness, which are conformable with the foliation of the host rocks. The gently sloping shear zones cut across older domains of more complicated structure, which suggests that these zones are gently sloping ductile shear zones. Along these zones, the nappes were thrust south- and southwestward, and this process was the last in the origin of major structural features of BMB when the Paleoproterozoic Lapland–Kola orogen was formed. Practically identical age values were obtained for the gently sloping shear zone in the two widely separated Engonozero and Chupa segments of BMB: 1879 ± 21 Ma (40Ar/39Ar amphibole age of amphibolite whose protolith was mafic rock) and 1857 ± 13 Ma (Sm–Nd mineral isochron age of garnet amphibolites after gabbronorite). The P–T metamorphic parameters in these gently sloping shear zones are remarkably different from the metamorphic parameters outside these zones: the pressure is 3–4 kbar lower and the temperature is 60–100°C lower. Thrusting-related decompression triggered the transition from the older high-pressure episode of Paleoproterozoic metamorphism to a younger syn-thrusting higher temperature metamorphic episode. The peak metamorphic parameters corresponding to the boundary between the amphibolite and granulite facies were reached only in the central portions of the shear zones: T= 680–760°C, P = 8.0–11.9 kbar. In areas of the most intense migmatization, temperature estimates in the central portions of the shear are as high as 810–830°C. The marginal portions of the shear zones were formed at lower temperatures of 610–630°C. The temperature heterogeneous and rock heating in the gently sloping shear zones may have resulted from flows of high-temperature metamorphic fluid that were focused to the central portions of the zones.

Archaean and palaeoproterozoic migmatizations in the Belomorian Province, Fennoscandian Shield: Petrology, geochronology, and geodynamic settings

Multiple repetitions of migmatization processes are an important indication of the polychronous evolution of Precambrian Mobile Belts: this is certainly true for the Belomorian Belt. In the Belomorian Province of the Fennoscandian Shield, newly obtained data demonstrate the effect of two stages of melting of the Earth’s crust under conditions of higher pressure up to 8–14 kbar. The early stage of the migmatization and genetically related leucogranite formation took place in the Neoarchaean (2710 ± 15 and 2706 ± 14 Ma, U–Pb zircon ages), while the younger one happened in the Palaeoproterozoic (1944 ± 12 and 1882 ± 9 Ma, U–Pb zircon ages of leucosomes). The early stage of crust melting is related to collision in the Belomorian Neoarchaean orogen, while the later stage occurred during formation of the Lapland–Kola orogen.

New U-Pb titanite age data on the rocks from the Karelian craton and the Belomorian mobile belt, Fennoscandian Shield

We test the possibility of applying titanite as indicator of the boundaries between the Belomorian mobile belt and the Karelian craton of the Fennoscandian Shield. U-Pb isotope dating established wide variations of titanite ages in the Belomorian mobile belt and the Karelian craton. The titanites from the Karelian craton are mainly Archean in age (2.52–2.86 Ga), whereas the Belomorian mobile belt contains, with few exceptions, Paleoproterozoic titanites (1.74–1.95 Ga). In the Karelian craton, the age of titanite, in general, records the cratonization of the Earth’s crust (2.8 Ga and 2.6–2.7 Ga, respectively). In the Belomorian mobile belt, it presumably reflects the timing of the exhumation of tectonic nappes from the mid-crustal depths during the collisional stage of the evolution of the Lapland-Kola orogen.

Paleoproterozoic mafic dyke swarms of the Belomorian Province, eastern Fennoscandian Shield

Paleoproterozoic mafic igneous rocks (2450–1970 Ma) are exposed in the form of layered intrusions, dykes, and volcanic rocks in the Karelian, Kola and Murmansk provinces and in the form of dykes and small intrusions in the Belomorian Province, Eastern Fennoscandian Shield. The age and sequence of mafic dyke emplacement during the Paleoproterozoic are very similar in these regions. Further comparisons of geochemical characteristics of mafic dyke swarms in the Belomorian Province and neighboring cratons show considerable similarities. At least three distinct groups of Paleoproterozoic mafic dykes exist in the Belomorian Province of the Eastern Fennoscandian Shield. They are (i) early Mg-tholeiitic dykes, (ii) ca. 2.44–2.36 Ga high-Mg gabbronorites and (iii) ca. 2.12 Fe-tholeiitic dykes. Research into mafic dykes of the Belomorian Province and their comparison with Paleoproterozoic dyke swarms of the Karelian, Kola and Murmansk provinces has provided evidence for at least two extensive shared magmatic events dated to ca. 2.5–2.4 Ga and ca. 2.1 Ga in the Paleoproterozoic in the Eastern Fennoscandian Shield confirming that the Belomorian Province does not represent a Paleoproterozoic suture but rather a deep-eroded inner part of the Lapland-Kola orogen foreland.

New constraints for the source characteristics, deposition and age of the 2.1–1.9 Ga metasedimentary cover at the western margin of the Karelian Province

The Paleoproterozoic cover sequence at the 100–150 km wide western margin of the Archean Karelian Province is dominated by deep water Lower and Upper Kaleva metasediments. We present here an interpretation of Sm–Nd isotope and geochemical data on 36 samples, TIMS multi-grain U–Pb zircon analyses on nine samples, and ca. 100 SIMS analysis of detrital zircon grains from four Upper Kaleva and one Lower Kaleva samples. The Lower Kaleva is characterized by autochthonous–parautochthonous, lithologically heterogeneous metaturbidites showing common enrichment in quartz. All the analysed detrital zircons are of a local Neoarchean source but t DM variation up to 2.4 Ga combined with geochemical data indicate abundant mixing of Paleoproterozoic mafic material, presumably from 2.1 Ga plateau lavas and dykes, in most of the Lower Kaleva samples. The Upper Kaleva is dominantly allochthonous with tectonically enclosed fragments of ophiolite bodies, and it is characterized by lithological and geochemical-isotopic homogeneity. Geochemical, isotopic and detrital zircon data favour material derived from an orogenic domain, comprising both Archean and Proterozoic units, followed by effective mixing during the transport. The Archean zircon grains (25%) are mostly Neoarchean. The Paleoproterozoic grains lack zircons at 2.5–2.2 Ga and plot dominantly (92%) between 1.92 and 2.05 Ga. The indicated maximum deposition ages vary from 1.95–1.94 Ga to 1.92 Ga. The main source area proposed is the Himalaya-type Lapland-Kola orogen (now) in the northeast, which experienced mountain building and erosion at 1.95–1.91 Ga. The western margin of the Karelian Province shows evidence of rifting and lithosphere thinning from 2.1 to 1.95 Ga but it is still under debate whether the craton breakup occurred at 2.06 Ga in a volcanic or later at 1.95 Ga in a non-volcanic margin setting. One hypothesis is that the onset of collision in the northeast changed plate motion and lead to a new spreading within the pre-existing passive margin at 1.97–1.95 Ga. Thus, both a volcanic margin at 2.06 Ga and a non-volcanic margin at ca. 1.95 Ga could have been operated at the western margin of the Karelian Province.

Paleoproterozoic sediment recycling during multiphase orogenic evolution in Fennoscandia, the Tampere and Pirkanmaa belts, Finland

Geochronology of detrital zircons and their overgrowths combined with whole-rock geochemical and Sm–Nd isotopic data can be used to distinguish different stages of sediment recycling and metamorphism during multiphase orogenic evolution. This approach is applied to the Paleoproterozoic sedimentary rocks of the Tampere and Pirkanmaa belts (southern Finland) in the center of the composite Svecofennian orogen. The lower part of the Tampere belt succession and bulk of the Pirkanmaa belt are characterized by turbidites whereas the upper part of the Tampere belt succession is dominated by 1.90–1.89 Ga mature arc-type volcanic rocks. Detrital zircon geochronology indicates that the Tampere and Pirkanmaa belts have a coeval 1.92–1.89 Ga depositional and tectonic history. Ages of pre-depositional zircon overgrowths vary from 1.91 Ga to 2.0 Ga with clusters at 1.92 Ga and 1.98 Ga. Within the Pirkanmaa belt, post-depositional zircon overgrowths indicate metamorphic culmination at c. 1885 Ma in the Vammala Ni-zone and at c. 1875 Ma in the northern part. The lower conglomerates and graywackes in the Tampere belt and their equivalents in the Pirkanmaa belt are rich both in Neoarchean and Paleoproterozoic grains, the latter ranging in age from 1.9 to 2.1 Ga. Compared to these, a sample from the Vammala Ni-zone has an exotic provenance with at least c. 1.90 Ga, 2.04–2.15 Ga, 2.38–2.48 Ga and 2.57–2.63 Ga components. A sedimentary recycling and tectonic model for the central Fennoscandia is proposed, in which the Paleoproterozoic Keitele + Bergslagen continent was formed during an unnamed orogeny at 1.98–1.97 Ga. The Archean Norrbotten microcontinent was attached to the continent at 1.97–1.93 Ga. Upper Kaleva turbidites, derived from the Lapland-Kola orogen in the north, were deposited before 1.92 Ga on a passive margin of the Archean Karelia craton. The Karelia craton collided with the Keitele + Bergslagen + Norrbotten continent at c. 1.92 Ga forming the Lapland-Savo orogen. Subsequent evolution led to rifting and break-up of the latter continent into two microcontinents in the hinterland. At 1.92–1.91 Ga the rift was developed into a subsiding passive margin of the Keitele microcontinent with voluminous turbidite deposition, now seen as graywackes in the Tampere, Pirkanmaa and Pohjanmaa (western Finland) belts. The turbidite material was derived from the rising Lapland-Savo orogen and included recycled Upper Kaleva, recycled (sandstones) and first-cycle 2.03–1.97 Ga detritus from Keitele, 1.93–1.92 Ga Savo arc material, as well as detritus from the Archean craton and its cover deposits. The collision between Karelia and Keitele caused a subduction reversal and the onset of Tampere arc volcanism at 1.90 Ga. Arc-derived materials started to deposit and were mixed with older sedimentary rocks, and trench-parallel distal turbidites from exotic source were being deposited in the accretionary wedge.

Apparent partial loss 40Ar/39Ar age spectra of hornblende from the Palaeoproterozoic Lapland-Kola orogen (arctic European Russia): insights from numerical modelling and multi-method in-situ micro-sampling geochronology

40Ar/39Ar age spectra with progressively increasing step ages are well known for metamorphic hornblende and have been classically interpreted by partial loss of radiogenic argon by diffusion processes during younger thermo-tectonic reworking. Application of a number of numerical modelling tools based on diffusion theory and that assume thermally activated loss of radiogenic 40Ar by solid-state volume diffusion suggests that staircase-shaped age spectra of Neoarchaean tschermakitic hornblende from the Lapland-Kola Orogen are due to argon losses of 40–50% during reheating to 450 ± 25 °C in Palaeoproterozoic time. However, in hornblende samples that yielded staircase-type age spectra, biotite occurs in the matrix, as well as intimately and abundantly intergrown with the amphibole along grain boundaries, cleavages, fractures and other defects. Drilling of 1.5 mm diameter discs from carefully selected hornblende grains in petrographic thin sections permitted to minimise the effects of contaminant biotite inclusions and/or compositional zoning of the amphibole. 40Ar/39Ar laser probe step-heating of drilled biotite-free hornblende discs yielded flat age spectra, suggesting absence of thermally activated radiogenic 40Ar loss. This would imply unrealistically contrasting temperature histories for neighbouring grains. Apparent-loss age spectra, thus, result from differential gas release of hornblende and an included, earlier degassing minor contamination of much younger biotite, which had apparently not been completely eliminated from the amphibole separate, despite careful handpicking. This is confirmed by the Ca/K ratio spectra — a proxy for 37ArCa/39ArK — of hornblende that are flat for drilled biotite-free hornblende grains, but initially low for hornblende separates. A drilled disc and a separate of hornblende from a biotite-free amphibolite did not yield apparent loss spectra, but flat age and Ca/K ratio spectra, confirming the interpretation of the role of biotite.

Paleoproterozoic evolution of Fennoscandia and Greenland

The Paleoproterozoic evolution of Fennoscandia and Greenland can be divided into major rifting and orogenic stages. The Paleoproterozoic rifting of Fennoscandia started with 2.505‐2.1 Ga, multiphase, southwest-prograding, intraplate rifting. Both Fennoscandia and Greenland experienced 2.1‐ 2.04 Ga drifting and separation of their Archean cratons by newly-formed oceans. The main Paleoproterozoic orogenic evolution of Fennoscandia resulted in the Lapland-Kola orogen (1.94‐1.86 Ga) and the composite Svecofennian orogen (1.92‐1.79 Ga). The Paleoproterozoic orogens in Greenland, from north to south, are the Inglefield mobile belt (1.95‐1.92 Ga), the Rinkian fold belt/Nagssugtoqidian orogen (1.88‐1.83 Ga) and the Ketilidian orogen (c. 1.8 Ga). The Lapland-Kola orogen, Inglefield mobile belt and the Rinkian fold belt/Nagssugtoqidian orogen are continent-continent collision zones with limited formation of new Paleoproterozoic crust, whereas the Ketilidian orogen displays a convergent plate-tectonic system, without subsequent collision. The composite Svecofennian orogen is responsible for the main Paleoproterozoic crustal growth of Fennoscandia.

Age of the Moncha Tundra fault, Kola Peninsula: Evidence from the Sm-Nd and Rb-Sr isotopic systematics of metamorphic assemblages

The first Sm-Nd and Rb-Sr dates were obtained for the dynamometamorphic processes associated with the origin and evolution of the Moncha Tundra fault, Kola Peninsula, which separates two large Early Paleoproterozoic layered intrusions: the Monchegorsk Ni-bearing mafic-ultramafic intrusion and the Main Range massif of predominantly mafic composition. The fault belongs to the regional Central Kola fault system, whose age was unknown. The material for the dating included metamorphic minerals from blastomylonitic rocks recovered by structural borehole M-1. Mineralogical thermobarometry suggests that the metamorphism occurred at 6.9–7.6 kbar and 620–640°C, which correspond to the amphibolite facies. The Sr and Nd isotopic systems were re-equilibrated, and their study allowed us to date the dynamometamorphic processes using mineral isochrons. It was established that the Moncha Tundra fault, and, respectively, the whole Central Kola fault system appeared in the middle of Paleoproterozoic ∼2.0–1.9 Ga, simultaneously with the Svecofennian orogen in the central part of the region and the Lapland-Kola orogen in its northeastern part. Another episode of dynamometamorphism that occurred at 1.60–1.65 Ga is envisaged.

The Lapland-Kola orogen: Palaeoproterozoic collision and accretion of the northern Fennoscandian lithosphere

Abstract A tectonic model is proposed for the Palaeoproterozoic Lapland-Kola orogen (LKO) in the northern Fennoscandian Shield. Although long regarded as an Archaean craton, integrated geological, geochemical and geophysical observations show that the Lapland-Kola orogen is a Palaeoproterozoic collisional belt containing both Archaean terranes and an important component of juvenile Palaeoproterozoic crust. Rifting, from 2.5 to 2.1 Ga, began under the influence of a mantle plume (&gt; 1000 km diameter), related to the break-up of the Kenorland supercontinent. Two linear suture zones within the orogeriic core mark the sites of continental separation, ocean formation and closure. One of these is identified as a belt of 1.98-1.91 Ga juvenile crust of both arc magmatic and sedimentary origin, marked by the Lapland Granulite, Umba and Tersk terranes. Palaeomagnetic data and ancient sedimentary detritus within these terranes suggest limited oceanic separation. Collision of juvenile terranes with the surrounding Archaean took place mainly between 1.93 and 1.91 Ga, resulting in a Himalayan-scale mountain belt, manifest by a thick-skinned region of high- P granulite-facies metamorphism, including the classical Lapland Granulite Belt and a broad zone of compressional deformation extending southwards into the Belomorian Mobile Belt. Protracted cooling and exhumation, possibly related to the buttressing effect of surrounding lithosphere, culminated in the intrusion of 1.80-1.77 Ga post-tectonic granites.

Crustal structure and tectonic model of northeastern Baltica, based on deep seismic and potential field data

Abstract The Early-Mid-Palaeozoic successions of the Pechora Basin along continent Baltica's northeastern margin unconformably overlie Neoproterozoic complexes of the Timanide orogen; further west, Palaeozoic platform strata cover the East European Craton basement. The latter is exposed in the northern parts of the Fennoscandian (Baltic) Shield and is also present beneath the Neoproterozoic (perhaps also Mesoproterozoic) and younger successions of the Mezen Basin. Geophysical data help define the crustal and upper mantle structures of this part of northwestern Russia, from the Kola Peninsula to the Uralide orogen. Within the Shield, seismic refraction and near-vertical reflection profiling allow the terranes of Archaean and Palaeoproterozoic age, defined at the surface (the Murmansk, Kola-Liinakhamary, Central Kola, Keivy and Belomorian terranes, and the Pechenga-Imandra-Varzuga and Lapland-Kolvitsa Granulite Belts constituting the Lapland-Kola orogen, and the West Karelian, Central Karelian and Vodlozer terranes forming the Karelian province), to be traced into the deeper crust, promoting interpretation of various early Precambrian compressional and extensional regimes. Thrusting, transcurrent and extensional faulting have been identified in a crust that has been partly thickened during late Archaean and Palaeoproterozoic collision and locally extended both in back-arc basin settings and during syn-post-collisional collapse. The main Fennoscandian terranes can be traced southeastwards beneath the Mezen Basin based on gravity and magnetic data. Seismic profiling shows that the Phanerozoic successions of this basin overlie intracratonic rifts of Neoproterozoic and possibly Mesoproterozoic age. The rifts are composed of a number of half-graben, the details of which are well defined by vibroseis reflection profiling. During the development of Mezen Basin rifts, a Neoproterozoic passive margin was established in the vicinity of what is now the Timan Range. Late Neoproterozoic (Vendian) orogeny emplaced slope-rise turbidite-dominated successions southwestwards onto the platform. Towards the hinterland of the Timanides (also referred to as Baikalides), beneath the Phanerozoic Pechora Basin, a combination of potential field and seismic surveys, both shallow reflection and deep refraction profiling, have allowed analysis of the crust and uppermost mantle. Neoproterozoic magmatic arcs, Grenville-age(?) microcontinents and possible oceanic domains are inferred to exist beneath the early Palaeozoic unconformity. A vast region of Neoproterozoic accretion reaches from the western Urals, northwards beneath the Pechora Basin and the eastern Barents Shelf.

Pulsating Evolution of Metamorphism in Granulite Terrains: Kolvitsa Meta-anorthosite Massif, Kolvitsa Belt, Northeast Baltic Shield

From detailed petrographic and microprobe study and application of an internally consistent system of mineralogical thermometers and barometers (the TPF program), the pressure-temperature-time path of metamorphism of the Kolvitsa meta-anorthosites (metagabbro-anorthosites) (Kolvitsa belt of the Lapland-Kola orogen, Baltic shield) was estimated. The massif formed at a depth of more than 45 km. Crystallization of anorthosites (gabbro-anorthosites) occurred in the Paleoproterozoic (2450-2462 Ma) at a temperature of 1200°C. sif preserve evidence of several metamorphic events as relics of different (mostly kinetic, decomposition-crystallization) mineral reactions. The subsequent metamorphic history of the massif, spanning more than 600 Ma, evolved in two main stages, early Paleoproterozoic rifting (extensional conditions) and Svecofennian collision (compressional conditions), comprising five dated metamorphic (tectonothermal) events. Temperatures obtained are: for M1, 990°C-primary metamorphism soon after 2450-2462 Ma (time of magmatic crystallization of the massif); for M2, 870-915°C at 2423-2433 Ma; for M3, 750-830°C at 2374-2394 Ma; for M4, 670-710°C a; for M5, 570-600°C at 1827-1833 Ma. The rocks were affected by sub-isothermal decompression from a depth of ∼45 km to 40 km (P from 12.37 down to 11.2 kbar) during M2, from 40 km to 28.5-31 km during M3 (P from 11.2 down to 8-8.6 kbar), from 28.5 km to ∼23 km during M4 (P from 8-8.6 down to 6.5 kbar). Early Paleoproterozoic tectonothermal activation of the region (lasting from 55 to 75 Ma) was characterized by high rates of exhumation, 0.2-0.3 mm per year, and cooling, 3.3-4.5°C/m.y. During the Svecofennian stage, these rates were considerably lower, 0.045 mm per year and ca. 1°C/m.y., respectively. Both periods of regional development, despite differences in geodynamic conditions and exhumation mechanisms, are characterized by similar P-T paths with alternating periods of subisothermal decompression (activation) and subisobaric cooling (relaxation). Each decompression period was accompanied by intense tectonic deformation, with increase in rock permeability and acceleration of thermal, fluid, and mineral-forming processes.

Pre-1.91 Ga deformation and metamorphism in the Palaeoproterozoic Vammala Migmatite Belt, southern Finland, and implications for Svecofennian tectonics

A metamorphic event in the Vammala Migmatite Belt (VMB) at ~1.92 Ga, revealed by SHRIMP U-Pb analyses of both zircon overgrowths and monazite ,i s interpreted as post-depositional and is correlated with the development of the early high-grade schistosity .N either this Early Svecofennian deformation and metamorphism, nor the associated complex folding, is present in the overlying Ta mpere Schist Belt (TSB) sequence, consistent with the VMB being part of a pre-1.91 Ga basement complex. The ~1.92 Ga event provides a maximum deposition age for the TSB ,c onfirm ing earlier age estimates. Earlier stratigraphic correlations between parts of the VMB and TSB ,a nd associated tectonic interpretations, can no longer be sustained.The crustal thickening seen in the VMB ,a nd previously attributed to arc accretion at ~1.89 Ga, is now attributed to accretion of a large Svionian marginal basin during the ‘Early Svecofennian’ orogenic phase at ~1.92 Ga.This is of similar age to the deformation and metamorphism associated with collision in the Lapland-Kola Orogen to the north of the Karelian Province. The well-known post-TSB orogenic phase was also identified in the VMB by a monazite age of 1881±6 Ma. A granitoid intrusion gave an emplacement age of 1888±5 Ma, comparable to the age of granitoid clasts in the upper part of the TSB succession. The detrital zircon data are interpreted to suggest that deposition of the precursor VMB sediments probably took place soon after an earlier pre-depositional metamorphism at ~1.98 Ga, which affected igneous source complexes dated at ~1.99 Ga and ~2.01 Ga. Mafic rocks in the southern part of the VMB ,a nd probably also the Haveri basalts, represent a renewed episode of extensional magmatism, which might correlate with the 1.96–1.95 Ga Jormua and Outokumpu ophiolites. A pre-1.96 Ga older stage basin has an expression in Sweden and complexes of similar age occur in the concealed Palaeoproterozoic basement south of the Gulf of Finland. Similar rocks, deformed and metamorphosed before ~1.96 Ga, might be present beneath the Central Finland granitoid complex and the late Svecofennian granite-migmatite zone ,a nd were possibly more local sources for both the younger stage Svionian basin sediments and the post-1.91 Ga Bothnian Basin sediments. The TSB and other post-accretionary volcanic sequences, and the associated plutonism, are interpreted to reflect a ~40 m.y .e xtensional period, inboard of the contemporaneous active margin, between orogenic phases at ~1.92 Ga and ~1.88 Ga.This interpretation provides a more satisfactory explanation of the major heat input to the crust over a very wide area than does the arc accretion hypothesis.The tectonic evolution of the Svecofennian Province has strong similarities to that of the Palaeozoic Lachlan Fold Belt in eastern Australia.

Ion microprobe UPb zircon geochronology and isotopic evidence for a trans-crustal suture in the Lapland–Kola Orogen, northern Fennoscandian Shield

The Lapland–Kola Orogen (LKO; former Kola craton) in the northern Fennoscandian Shield comprises a collage of partially reworked late Archaean terranes with intervening belts of Palaeoproterozoic juvenile crust including the classic Lapland Granulite Terrane. Rifting of Archaean crust began at c 2.5–2.4 Ga as attested by layered mafic and anorthositic intrusions developed throughout the northernmost Fennoscandian Shield at this time. Oceanic separation was centred on the Lapland Granulite, Umba Granulite (UGT) and Tersk terranes within the core zone of the orogen. Importantly, SmNd data show that Palaeoproterozoic metasedimentary and metaigneous rocks within these terranes contain an important, generally dominant, juvenile component over a strike length of at least 600 km. Evidently, adjacent Archaean terranes, with negative ε Nd signatures, contributed relatively little detritus, suggesting a basin of considerable extent. Subduction of the resulting Lapland–Kola ocean led to arc magmatism dated by the NORDSIM ion probe at c 1.96 Ga in the Tersk Terrane in the southern Kola Peninsula. Accretion of the Tersk arc took place before c 1.91 Ga as shown by ion probe UPb zircon dating of post-D1, pre-D2 pegmatites cutting the Tersk arc rocks, juvenile metasediments as well as Archaean gneisses in the footwall of the orogen. Deep burial during collision under high-pressure granulite-facies conditions was followed by exhumation and cooling between 1.90 and 1.87 Ga based on SmNd, UPb and ArAr data. Lateral variations in deep crustal velocity and Vp/Vs ratio, together with reflections traversing the entire crust observed in reprocessed seismic data from the Polar Profile, may be interpreted to image a trans-crustal structure — possibly a fossilised subduction zone — supporting an arc origin for the protoliths of the Lapland Granulite, UGT and Tersk terranes and the location of a major lithospheric suture — the Lapland–Kola suture.

Titanite-rutile thermochronometry across the boundary between the Archaean Craton in Karelia and the Belomorian Mobile Belt, eastern Baltic Shield

U–Pb isotopic dating has been carried out on titanites and rutiles from the Karelian Protocraton, the Belomorian Mobile Belt and the intervening junction zone. These are some of the principal Archaean crustal units in the Baltic Shield which have undergone regeneration to various degrees during the Palaeoproterozoic. Palaeoproterozoic resetting of U–Pb titanite ages was complete in the Belomorian Belt and almost complete in the junction zone, while it hardly affected the Karelian Protocraton. In the latter, major crustal cooling occurred at 2.71–2.69 Ga after a major igneous event at 2.74–2.72 Ga, but a tectonothermal event at 2.65–2.64 Ga was less comprehensive. In the Belomorian Belt, a northeastern marginal zone immediately underlying the collisional-thrusting suture of the Lapland-Kola orogen has somewhat higher titanite ages of ca. 1.94–1.87 Ga than the central zone where these ages range between 1.87 and 1.82 Ga. Comparison between the titanite and rutile U–Pb ages suggests a postorogenic cooling rate between 2 and 4°/Ma in these parts of the Belt. The Neoarchaean junction zone between the Karelian and Belomorian provinces was a zone of particularly intense tectonic, magmatic and hydrothermal activity during or after the Palaeoproterozoic Lapland-Kola orogeny. Dominant, newly grown titanites in that zone have ages as young as 1.78–1.75 Ga, and the age differences between the titanite and rutile U–Pb ages are substantially smaller than elsewhere.