Svecofennian Age Research Articles

The structural position and age of eclogite rocks in the area of Gridino village in the Belomorian mobile belt

Data on the localization of eclogitized mafic bodies in steeply and superposed gently dipping ductile shear zones in the area of Gridino village are considered. The process of eclogitization in steeply dipping ductile shear zones, which affect basites of rootless bodies (fragments of tectonic melange) and dikes, was determined by deformations. Such a relation between them is most evident in narrow ductile shear zones that cross Early Proterozoic (age of ca. 2.4 Ga) gabbronorite dikes. Gabbronorites involved in deformations of ductile shear zones experienced eclogitization, reflected in the total recrystallization of magmatic minerals with the formation of equilibrium high-pressure mineral parageneses, ordered metamorphic textures, and linear strain patterns. Synchronous metamorphic transformations in gabbronorites, which avoided plastic deformations and retained massive structures (in wall rocks of shear zones) resulted in the formation of reaction coronas at the boundaries of magmatic mineral grains. Younger Fe gabbro dikes, localized in steeply dipping ductile shear zones, which cross gabbronorite dikes, are totally eclogitized. This determines the temporal lag between the formation of gabbonorite dikes and eclogitization in steeply dipping ductile shear zones and indicates its Svecofennian age. As in steeply dipping ductile shear zones, eclogitized basites constituting rootless bodies in their gently dipping counterparts are characterized by ordered metamorphic textures indicating the deformation nature of eclogitization. Gently dipping ductile shear zones are never crossed by Early Proterozoic basite dikes, although they contain boudins of them, which implies the younger (Svecofennian) age of these zones. The absence of eclogitization features in flat ferruginous basite bodies, which take part in the structure of gently dipping ductile shear zones and demonstrate orientation of metamorphic structures similar to that of gneisses, indicates that bodies of eclogitized basites were involved into these zones after eclogitization. An Archean age of these seems unlikely, since rootless bodies of eclogitized basites are never crossed by Proterozoic basite dikes in the study area. The textural data imply the deformation nature and Svecofennian age of eclogitized basites of the area of Gridino village.

New U-Pb and Sm-Nd ages and P-T estimates for eclogitization in the Fe-rich gabbro dyke in Gridino area (Belomorian Mobile Belt)

New data from isotope geochronology (U-Pb, Sm-Nd) petrological study provide evidence of the Svecofennian age (∼1.9 Ga) for eclogitization in the Fe-gabbro dyke inductile shear zones of Gridinrea. P-T estimates of eclogitization were computed using the THERIAK/DOMINO software. A close timing relationship between dyke magmatism and eclogitization is inferred.

Paleoproterozoic eclogites in the salma area, Northwestern Belomorian Mobile Belt: Composition and isotopic geochronologic characteristics of minerals and metamorphic age

The paper represents results of a comprehensive geochemical and isotopic-geochemical (SIMS) study of eclogites from the northwestern part of the Belomorian Belt (Salma eclogites). A detailed fieldwork was carried out at the quarry of the Kuru-Vaara deposit of ceramic pegmatite in the northwestern part of the study area, in which tonalite-trondhjemite gneisses include bodies and blocks of eclogite and Grt-Aug eclogite-like clinopyroxenite and are cut across by numerous pegmatite veins. The least altered types of the Grt-Cpx rocks selected for our further research included: (1) widespread massive homogeneous fine-grained Grt-Omp eclogite that replaced gabbro and contained symplectites of Pl + low-Na-Cpx around omphacite and Pl-Hbl kelyphite rims around Grt; and (2) coarse-grained eclogite-like Grt-Hbl-Aug clinopyroxenite beds up to 20 cm thick in the central parts of high-Mg metaultrabasites, which are mostly tremolite-actinolite schists. The REE patterns of garnet, clinopyroxene, and amphibole from the eclogites confirm that they crystallized simultaneously, under a high pressure, and in the absence of plagioclase. Local U-Pb dates of the zircons and their geochemistry are at variance with the earlier hypothesis that the eclogite metamorphism occurred in the Archean. The eclogites and Grt-Hbl-Aug clinopyroxenite were determined to contain zircons of Svecofennian age (approximately 1900 Ma), which show all geochemical characteristics of classic eclogitic zircons and occur either as individual crystals or as rims around Archean magmatic zircons from the primary gabbroids.

The age of HP metasomatism in shear zones during collision-related metamorphism in the Lapland Granulite Belt: The U/Pb SHRIMP II dates on zircons from sillimanite-hypersthene rocks of the Porya Guba nappe

This study presents the first U/Pb SHRIMP II dates on zircons from metasomatic sillimanitehypersthene rocks within the Porya Guba nappe of the Lapland Granulite Belt. The results yielded a Paleoproterozoic Svecofennian age (1913–1914 Ma) of the HT/HP metasomatism in shear zones during collision-related metamorphic events in the Lapland Granulite Belt.

Secondary magnetizations in shear and fault zones in southern Finland

Studies on remanent magnetization of shear and fault zones of crystalline rocks in southern Finland have been carried out to test juvenile fluid activity that can be dated paleomagnetically. Locating in the central part of Baltica, the area has been tectonically relatively quiet since the Precambrian. However, tectonism at the margins of the craton during the Caledonian, Hercynian and Uralian orogenies probably reactivated faults and resulted in low-temperature hydrothermal activity also within the craton. We identified two consistent remanent magnetization components in four (out of five) studied localities. One remanence component has an intermediate to high coercivity NNW directed magnetization ( D = 334.7°; I = 35.2°; k = 92.1; α 95 = 9.6°) and unblocking temperatures suggest that it resides in (titano)magnetite. The direction is similar to the remanence that is commonly observed in Svecofennian age (ca. ~ 1.9 Ga) rocks all over the Precambrian Fennoscandian Shield, and probably represents the primary Svecofennian magnetization preserved in the host rocks. Another common component is directed NE ( I = 30.8°; D = 35.3°; k = 264.5; α 95 = 5.7°) and is carried by hematite and maghemite. Its resulting paleomagnetic pole compares well with Phanerozoic paleopoles of Baltica, and is suggested to represent a Permian remagnetization. We interpret that this remanence reflects the reactivation of the Svecofennian crust in the late Paleozoic due to tectonic events related to the formation of the supercontinent Pangea. In addition, remagnetizations due to Mesoproterozoic rapakivi magmatism and younger (Mesozoic to present) events were identified.

187Re-187Os evidence for crustal lenses of Svecofennian age within the Mylonite Zone, SW Sweden

Re and Os concentrations and isotopic composition were analysed in four ultramafic cumulates from the Kedum tectonic lens within the Mylonite Zone, SW Sweden. An 1887±36 Ma isochron is obtained for six samples and interpreted to date the Kedum body. This is significantly older than the ∼1600–1700 Ma surrounding crust and the first rocks of Svecofennian age west of the Trans Scandinavian Igneous Belt. The initial gOs of 15.7±4.0 indicates contribution from a radiogenic source such as old crust.

Detrital Zircon Geochronology: New Evidence for an Old Model for Accretion of the Southwest Baltic Shield.

Disruption of the original crustal architecture as a result of tectonic events often complicates studies of the crustal evolution of Precambrian cratons. Ion-probe U-Pb dating of detrital zircons is one way to circumvent this problem. It yields a "fingerprint" of the source of the metasedimentary host that can be used to tie the host to a counterpart elsewhere. In this study, ion-probe U-Pb dating of detrital zircons from segments of the late Mesoproterozoic Southwest Scandinavian Domain (SSD) of the Baltic Shield provides new evidence for an old model of crustal evolution and tectonic development. Zircon ages for both the pebble and matrix fraction of the Seljord conglomerate, Telemark sector, Norway, show a 1.9-1.7-Ga frequency maximum, as was previously recognized for metasediments from the adjacent Bamble sector. However, the Telemark metasediments lack evidence for significant input of 1.4-1.6-Ga-old crustal material, characteristic of some of the Bamble metasediments. A maximum depositional age of ca. 1450 Ma is inferred for the Seljord conglomerate and a quartzite from the Bamble sector. Definite evidence for significant amounts of pre-1.6-Ga crust in the SSD is absent. Instead, provenance from the early Mesoproterozoic Svecofennian Domain for the 1.9-1.7-Ga crust is proposed, supported by the omnipresence of small amounts of Archean zircons and the distribution of Nd model ages of these and other SSD metasediments. It is proposed that the SSD originally formed a single, continuous belt, fringing the Baltic Shield, with the northernmost segments transported to their current position west of the Oslo rift along north-south-trending shear zones, i.e., at odds with the commonly held view of progressive westward growth of the Baltic Shield. The detrital zircon ages from the Faurefjell quartzite, Rogaland-Vest Agder sector, cluster around 1.65 Ga and reveal a maximum depositional age of 1244+/-40 Ma. Its deviating depositional setting indicates that the Rogaland sector evolved in a different setting, shielded from the crust of Svecofennian age. Instead, its ages allow for a source terrain within the Mesoproterozoic crust of South Norway.

Proterozoic sutures and terranes in the southeastern Baltic Shield interpreted from BABEL deep seismic data

A hitherto unknown terrane and its bounding sutures have been revealed by a combined study of normal-incidence and wide-angle seismic data along the BABEL profile in the Baltic Sea. This Intermediate Terrane is situated between a Northern Terrane of Svecofennian age and a Southwestern Terrane of Gothian age. It is delimited upwards by two low-angle and oppositely dipping sutures and occupies mainly middle and lower crustal levels with a width of ∼ 300 km at Moho level. The ∼ 1.86 Ga suture against the Northern Terrane is imaged by a prominent almost continuous NE-dipping crustal reflection from 3.5 to 14 s twt over 175 km. Where it downlaps on the Moho, sub-Moho velocities change from 8.2 to 7.8 km/s (±0.2) over less than 25 km. A relatively strong, NE-dipping normal-incidence and wide-angle reflection at 19–23 s twt indicates that the suture extends into the upper mantle. The pervasive NE-dipping reflection fabric of the Intermediate Terrane is interpreted as shear zones that developed during collision and possibly were reactivated by later events. High Poisson's ratios suggest a mafic composition or high fluid content. The ∼ 1.86 Ga collision was probably succeeded by continental break-up and removal of an unknown continent, except for the Intermediate Terrane. Subsequent formation of an east-dipping subduction zone further to the west led to the emplacement of 1.81-1.77-Ga-old granitoids in the southern part of the Transscandinavian Igneous Belt. The ∼ 1.65-1.60 Ga suture against the Southwestern Terrane is defined by a semi-continuous band of strong SW-dipping reflections between 3 and 8 s twt over 65 km, which are interpreted as a low-angle thrust zone along which Gothian crust overrode the Intermediate Terrane. The identification of three individual seismic terranes in the southeastern part of the Baltic Shield provides new evidence for Palaeoproterozoic plate tectonic processes.

Palaeomagnetism of Precambrian rocks in northern Sweden and its correlation to radiometric data

Abstract A palaeomagnetic study has been performed on Palaeo- to Mesoproterozoic basic intrusions and volcanic rocks from the Fennoscandian shield in northern Sweden. Three, possibly four, different generations of magnetizations were identified, the oldest assigned to a Svecofennian age (1.86-1.89 Ga). A second generation is related to the intrusion of granitoids of 1.80-1.76 Ga. In this geological event probably also the third group of directions has its origin. These different magnetizations may indicate that there are at least two different generations of basic intrusions in northern Sweden. A fourth group of directions is isolated as overprints. This magnetization is interpreted to be a Subjotnian magnetization, reflecting a previously unrecognized regional Subjotnian metamorphic event in the northwestern part of the Fennoscandian shield. The drift history for the Fennoscandian shield during the period 1.88-1.50 Ga has been defined based on these new palaeomagnetic data.

Gas seepages, gas eruptions and degassing structures in the seafloor along the Strömma tectonic lineament in the crystalline Stockholm Archipelago, east Sweden

The crystalline Stockholm Archipelago contains several deep seated tectonic lineaments of mainly Svecofennian age. Along parts of these lineaments, gas with a thermogenic signature rises to the surface creating pockmarks in the seabed. Ground water is known also to leak from intersecting submarine eskers in the area, but this phenomenon has so far not been proved to develop distinct pockmarks. The east-west direction is regarded to be the youngest out of four main lineament directions in the Archipelago. Investigations have been carried out along the eastwest Strömma lineament at Stavsnäs with respect to pockmark structures, location of main fractures within the lineament, and the gas content in clay samples along a line perpendicular to the lineament. Chemical analyses show that anomalous amounts of gas with thermogenic signature are contained in samples collected from within the lineament as compared to samples collected in the adjacent areas to the north and south. Gas eruptions are reported to occur frequently nearshore along the islands within the lineament at Stavsnäs.

Synpunkter på urbergsstratigrafin inom gränsområdena mellan Västerbottens och Norrbottens län

Abstract The divergencies between the stratigraphy as given in the description of the Precambrian areas of the Vasterbotten County (S. Gavelin 1955) and the stratigraphical succession of parts of the same area as presented by G. Kautsky (1957) are discussed. The most important problem concerns the age relations between the Revsund granite and the Vargfors sediments — the former of Svecofennian age, the latter of Karelian age according to Alvar Hogbom (1931, 1937) and Gavelin (1955). Kautsky has interpreted the Vargfors sediments as being formed prior to the Revsund granite, and in the present article the possibility of the existence of two separate divisions of the former Vargfors series is considered. Further, the consequences of a more regional application of Kautsky's table of formation to North-Swedish Archean areas are discussed.