The Froderyd Group forms part of the Vetlanda-Oskarshamn belt (also known as the Oskarshamn-Jonkoping Belt), which is a piece of Palaeoproterozoic crust that is completely encapsulated by 1.81-1.77 Ga granitoids of the Transscandinavian Igneous Belt in the southern part of the Fennoscandian Shield. New U-Pb zircon data from a felsic metavolcanic rock in the Froderyd Group have been acquired using LA-ICP-MS single collector. The age is determined to 1853 +/- 11 Ma. The Froderyd Group is interpreted to represent a volcanic arc that was located southwest of the margin to the proto-continent Fennoscandia. Tonalitic magma, identified in the Eksjo-Backaby regions, formed the middle crust in this arc complex and intruded the volcanic arc rocks at ca. 1.83-1.82 Ga. When this arc complex gradually approached the margin to the proto-continent Fennoscandia, parts of it were uplifted above sea level and initiated lacustrine sedimentation in restricted basins, which now are found in the Vetlanda region. Parallel with the development of this arc complex, 1.86-1.85 Ga granitoids intruded the margin to the proto-continent Fennoscandia and 1.87-1.86 Ga elastic metasedimentary rocks in the Vastervik area in an Andean-type active continental margin. It can be concluded that the Vetlanda and Vastervik sedimentary basins formed in two completely different geological environments during two separate events. The Vastervik sediments formed along the margin to the proto-continent Fennoscandia before the Froderyd arc system had developed while the Vetlanda sediments formed in a post-arc environment outboard to the southwest of the margin to the proto-continent Fennoscandia. It is suggested that the mafic volcanic rocks close to the lake Nommen should be excluded from the Vetlanda supergroup and instead be related in time to the Froderyd Group. This paper presents an interpretation of the tectonic evolution including volcanic arc and rifted volcanic arc during the 1.87 to 1.77 Ga time span with relevance to the evolution of the active southwestern margin of the proto-continent Fennoscandia depicted as a sequence of schematic profiles.

ABSTRACTPolymetallic quartz veins, with up to 1500 ppm indium, have been discovered recently in the Sarvlaxviken area within the 1.64 Ga anorogenic multiphase Wiborg rapakivi batholith and adjacent 1.90 Ga Svecofennian crust in SE Finland. Evidence from primary fluid inclusions in the Sarvlaxviken area provides new information on the hydrothermal transport and depositional processes of metals in anorogenic granites. Fluid inclusions with variable aqueous liquid and vapour proportions (5–90 vol.% vapour) occur in quartz, cassiterite and fluorite belonging to three generations of polymetallic quartz veins. Microthermometry indicates that the veins were deposited at temperatures that range from ~500°C down to <100°C and salinities from 0 to 16 eq. mass% NaCl. Fluid inclusion data show that the depositional conditions were similar regardless of vein generation. The interpreted depositional processes involve phase separation with a combination of condensation, cooling and boiling of an initially low-salinity (<3 eq. mass% NaCl) aqueous magmatic vapour phase enriched in CO2-F-Cl-S and metals. Fluid inclusions with low salinities dominate, but higher salinities are recorded in metal-rich parts of the veins. The turbulent fluid flow, with complex geometry and temperature-salinity patterns, may explain why sulfide and/or oxide opaque minerals occur irregularly, and are locally the dominating vein minerals, but disappear completely into barren parts of the quartz veins. All fluids are considered to have been generated by the F-rich Marviken granite (and related granite dykes), which show all geochemical criteria for an ore-fertile granite. The quartz veins investigated in the adjacent Svecofennian country rocks are considered to represent the very last stage of a fluid with similar characteristics to the fluid responsible for the ore formation in the Sarvlaxviken area, but that had cooled to <100°C.

The historic mining district of Pitkäranta in the Ladoga region, Fennoscandian Shield, was exploited for Fe, Cu, Zn, Pb, Sn and Ag in the nineteenth to twentieth centuries. The Pitkäranta region is dominated by Palaeoproterozoic supracrustal rocks, which, together with gneissic Archaean dome structures, constitute an allochthonous terrane complex that amalgamated to the Archaean continent during the Svecokarelian orogeny at 1.9–1.8 Ga. This crustal complex was intruded by 1.8 Ga Late orogenic granites, 1.54 Ga anorogenic rapakivi granites and 1.45 Ga dolerites. The polymetallic skarn ores of Pitkäranta extend over a 25-km-long zone in Palaeoproterozoic supracrustal rocks and formed from hydrothermal solutions, which emanated from the anorogenic rapakivi granites and reacted with marble layers. Four major ore types are recognised after the dominating metal: Fe, Cu, Sn and Zn, respectively. These types are not restricted to individual mines or mine fields but represent end members in zonation patterns within each ore body. Pitkäranta was the second discovery site in the world for indium but has been without modern documentation for more than 75 years. The indium contents in the ores are up to 600 ppm, in most cases sphalerite-hosted. The only roquesite-bearing sample in this study had an indium grade of 291 ppm and an In/Zn ratio of 51 (close to the criteria for the limiting conditions for creating an In-rich mineral). The Pitkäranta ores have a potential for future small-scale exploitation, but all such plans are hampered by high contents if Bi, Cd and As.

Several promising exploration targets in the western parts of the Wiborg batholith, southeastern Finland, have been studied with respect to Zn, In, Ag, As, Sn and Cu. Ores occur both as massive In-bearing magnetite–sphalerite (Getmossmalmen), as greisen-style veins (Jungfrubergen) and as Cu-dominant polymetallic quartz veins (Korsvik-1 and -2, Sarvlaxviken area). The Cu-dominant (chalcopyrite–bornite), Zn-poor quartz veins are characterised by high In/Zn ratios (>3000); roquesite (CuInS2) being a major indium-carrier, alongside sphalerite, chalcopyrite and arsenopyrite. In contrast, sphalerite is the dominant In-carrier in the greisen veins and massive ores characterised by lower In/Zn ratios (<100). In such cases, it appears that all indium partitioned into sphalerite at concentrations of 500–1500ppm, and no roquesite formed. In the veins of the Sarvlaxviken area, we observe the exsolution of a high-temperature precursor sulphide to form co-existing sphalerite and roquesite. These exsolution textures extend down to the sub-microscopic scale and there is no evidence of extensive solid solution along the ZnS–CuInS2 join beyond a couple of wt.% In (and Cu) in sphalerite. The co-existing roquesite is close to an ideal composition and contains μm-scale exsolutions of sphalerite, indicating negligible solid solution at the CuInS2 end of the join. The new data are compared with the published data on the mineralogical distribution of the indium in sulphide ores.

A wide application of modern precision research techniques to the studies of Pitkyaranta ores allowed us to find increased contents of indium (to 0.33%), silver (447 g/t), gold (0.2–0.4 g/t), and palladium (0.2 g/t). A series of rare minerals previously not found here was also discovered. Among ore minerals, these are roquesite, zavartskite, electrum, stutzite, altaite, bismite, glaucodot, cervelleite, hedleyite, pavonite, cannonite, plantnerite, lindkvistite, ashoverite, etc. The discovery of roquesite and electrum is the most important in terms of metallogeny. Roquesite (indium sulfide) is found in Karelia for the first time. The highest indium contents in direct correlation to those of zinc are characteristic for polymetallic ores of the Pitkyaranta ore fields with sphalerite as the concentrating mineral (to 0.5% of In). The predicted zinc resources are evaluated to ∼2.5 million t for the Pitkyaranta group of ore deposits, and to 400 000 t for the Hopunvaara region. Respectively, the resources of indium amount to ∼2400 t (total) and 600 t for the Hopunvaara region.

The Rajkonkoski ore occurrence is located within the region of the Karelian craton (AR2) and the Svecofennian folded belt (PR1) conjugation. It is presented by quartz-carbonate veins in metadoleriles and a zone of brecciation, crumple, and silification of carbonaceous shales within the volcanites of the Soanlakhtinsky suite (PR1). Ore mineralization in black shales and quartz veins has features of genetic similarity presenting different levels of the ore system controlled by different range strike-slip fault dislocations. At the Rajkonkoski ore occurrence, 41 ore minerals have been identified: 12 tellurides (native tellurium, hedleyite, pilsenite, tsumoite, tellurobismuthite, hessite, stuetzite, radclidzhite, joseite-B, altaite, volynskite, petzite); 4 bismuth-tellurides of the following compositions Bi3Te, Bi3Te2, BiTe4, PbBiTe; 3 selenides (clausthalite, tellurolaitakarite, native selenium); and 12 native metals (gold, silver, electrum, copper, iron, lead, tin, bismuth, osmiridium). The contents of the main ore minerals in places exceed 10%, and the concentrations of elements reach as follows: Cu and Pb, 5%; Zn, Bi, 1%; Se, 219 ppm; Te, 171 ppm; Sb, 3 ppm; As, 5 ppm; Ag, >0.1%; Au, 35.28 ppm. Ore mineralization is formed during the temperature interval from 550°C up to <170oC in the conditions of high activity of Se and Te, and beginning from medium temperatures (>300°C) complete miscibilities galenite-clausthalite and galenite-altaite are observed. In aggregate with a wide temperature interval (>400°C) of ore process evolution and mineral specia variety of telluride and native metal mineralizations, the original “torsion” of different temperature mineralizations makes it possible to determine the affiliation of the Rajkonkoski ore occurrence to the xenothermal type deposits or epithermal “alkaline,” gold-telluride A-type characterized by a close connection with magmatism of increased alkalinity and the original geochemical (Te-V-F) and mineral (tellurides of gold, silver and other metals, fluorite, roscoelite, vanadium-containing sulfides) associations. Taking into consideration that many of the xenothermal and epithermal A-type gold and silver deposits are large commercial objects, the prospects of the Rajkonkoski ore occurrence and the region of the Karelian craton and Svecofennian folded belt conjugation seem to be significant for noble metal mineralization.

Compounds containing essential Bi, Ph, Te and S are rare in nature. Aleksite, PbBi2Te2S2, is known from less than ten localities worldwide, and the single other recognized mineral, saddlebackite, Pb2Bi2Te2S3, is known only from the type locality, the Boddington Au deposit, Western Australia. Another phase, unnamed PbBi4Te4S3, had earlier been recognized as homogeneous grains and lamellar intergrowths within an assemblage consisting of aleksite, tellurobismuthite and tetradymite from St. David's mine Clogau, Wales. U.K. Re-investigation of this assemblage, including careful micro-analysis to avoid obvious intergrowths of phases. reveals an almost continuous range of compositions between tetradymite and aleksite. Investigations of complex sulfosalt-telluride assemblages from lilijarvi, a satellite deposit within the Orijarvi orefield, southwest Finland, have revealed compositions approximating to the range Pb5Bi4Te4S7 - Pb7Bi4Te4S9. These occur as fine intergrowths, rarely as larger single lamellae. also with aleksite, within a matrix of giessenite, galena and cosalite. The samples contain abundant gold, seen also as symplectite intergrowths with rutile. Investigation of the microparagenesis of precious-metal -bearing galena - chalcopyrite - pyrite mineralization in quartz veins at Fragant ("Langenleiten"), Carinthia Province of Austria, revealed the presence of several compositionally different Bi-Pb tellurosulfides. Aleksite is the most abundant, followed by unnamed phases with compositions close to Pb3Bi4TC4S5, Pb5Bi4Te4S7 and Pb6Bi4Te4S8. They occur as inclusions in galena and are variably associated with sulfosalts such as lillianite. cosalite, felbertalite and aikinite. The compositional dataset from the above occurrences is difficult to interpret without accompanying structural data. The data, however, Suggest the existence of an incremental chemical series with the general formula PbNBi4Te4SN+2. Alternatively, these are not discrete, essentially immiscible phases within a chemically defined modular series, but are simply compositions across a continuous compositional series. If the existence of a series can be proven, tetradymite, Bi4Te4S2, would correspond to N = 0, unnamed PbBi4Te4S3, to N = 1, aleksite, to N = 2, unnamed Pb3Bi4Te4S5, to N = 3, and saddlebackite, to N = 4, where the N values reflect chemical composition, rather than structurally defined homologous order. Indirect support for such a hypothesis comes from the recognition in the literature of four phases (N = 1, 2, 3 and 4) as synthetic products (phases D, E, F and J) obtained at 500 degrees C. The unnamed phases from Iilijarvi and Fragant may correspond to unspecified higher members of the same series. The lamellar banding with galena and tetradymite, and extended compositional fields observed in the Clogau and Iilijarvi specimens, are highly reminiscent of similar issues in Bi-sulfosalt series, allowing LIS to speculate that we may be looking at a typical accretional homologous series, with incremental growth in the thickness of layers.In such a scenario, random sequences of stacking of discrete memers of the series at the lattice scale are considered to apply, causing chemical variation.

The Palaeoproterozoic crust and upper mantle in the region between the Ukrainian and Baltic shields of the East European Craton were built up finally during collision of the previously independent Fennoscandian and Sarmatian crustal segments at c. 1.8-1.7 Ga. EUROBRIDGE seismic profiling and geophysical modelling across the southwestern part of the Craton suggest that the Central Belarus Suture Zone is the junction between the two colliding segments. This junction is marked by strong deformation of the crust and the presence of a metamorphic core complex. At 1.80-1.74 Ga, major late to post-collisional extension and magmatism affected the part of Sarmatia adjoining the Central Belarus Zone and generated a high-velocity layer at the base of the crust. Other sutures separating terranes of different ages are found within Sarmatia and in the Polish-Lithuanian part of Fennoscandia. While Fennoscandia and Sarmatia were still a long distance apart, orogeny was dominantly accretionary. The accreted Palaeoproterozoic terranes in the Baltic-Belarus region of Fennoscandia are all younger than 2.0 Ga (2.0-1.9, 1.90-1.85 and 1.84-1.82 Ga), whereas those in Sarmatia have ages of c. 2.2-2.1 and 2.0-1.95 Ga. Lithospheric deformation and magmatism at c. 1.50-1.45 Ga, and Devonian rifting, are also defined by the EUROBRIDGE seismic and gravity models.

The Fennoscandian and Ukrainian shield areas contain some of the largest and richest gold deposits in Europe. The earliest records of gold mining in northern Europe go back more than 200 years, but it was not until the last two decades that gold has become the major target in the Fennoscandian shield for most exploration and mining companies. The extent of …

Gold-quartz veins occurring in the Mjosa-Vanern ore district, southeast Norway and southwest Sweden, represent early Neoproterozoic members of the orogenic gold type of deposit. The Harnas gold-quartz veins, in the central part of the ore district, are steeply dipping veins hosted in a local, west-northwest–east-southeast–trending brittle shear zone, which transects the north-south–trending deformational fabric in the surrounding greenschist grade orthogneisses. This deformation and subsequent vein formation occurred at around 1.0 Ga in a late phase of the Sveconorwegian (Grenvillian) orogeny. Fluid inclusions show that the ore-bearing vein system at Harnas developed essentially in three successive stages: a quartz stage at a depth of ≈ 4 km, a pyrite-gold stage at a shallower crustal level (≈1.5 km) after rapid exhumation of the area, and finally a galena stage. All stages involved fracturing subparallel to the strike of the host shear zone. During the first two stages, the ore fluid was an aqueous H 2 O-CO 2 fluid with a salinity of 4 to 10 wt percent NaCl equiv and a temperature of ≈200°C, whereas in the galena stage it was a purely aqueous fluid with a similar salinity and a temperature of ≈150°C. Oxygen and sulfur isotope results imply a predominantly metamorphic origin for the ore fluid and suggest that important ore constituents, such as lead and sulfur, were derived from the regional orthogneisses. Other gold-anomalous quartz veins in the Harnas area, as well as the Brustad gold-quartz vein in the northernmost part of the Mjosa-Vanern ore district, show some variation in fluid composition. However, aqueous fluid inclusions containing CO 2 and calcite were identified in all veins. This, and other similarities, strongly suggests that the veins throughout the district were formed contemporaneously and were controlled by deformation that, at least in part, affected the entire Mjosa-Vanern region. It is inferred from geologic evidence and pressure estimates that veins began to form during the final phase of Sveconorwegian continent-continent collision and were completed during incipient rapid exhumation of the thickened crust. A set of barren quartz-calcite veinlets, which crosscut the ore-bearing veins at Harnas, is unrelated to the ore formation. These veinlets were deposited from a surface-derived, low-temperature, saline aqueous fluid during some significantly later, but regionally extensive, hydrothermal event.

Fennoscandian gold deposits have been successfully explored in a wide range of Precambrian geological environments in Sweden and Finland during the last two decades. Under-explored areas still exist at other sites in the Fennoscandian Shield, particularly in Russia and Norway, and there is a high potential for future new discoveries of economic gold in northern Europe. Careful geological documentation of a number of previously productive gold deposits, such as Haveri and Saattopora (Finland), Boliden, Adelfors, and Enasen (Sweden), and Eidsvoll and Bidjovagge (Norway), is now available in the literature. Kutemajarvi, Pantavaaraa, and Suurikuusikko (Finland), and Bjorkdal, Akerberg, Harnas, and Pahtohavare (Sweden) represent a new generation of gold mines, and recent research on these enables an improved base for formulating regional and local metallogenetic models. Younging trends from northeast to southwest characterize both the crust- and ore-forming regional patterns in the Fennoscandian Shield. Orogenic (or mesothermal) gold deposits are found in a wide range of host rocks and are closely linked to either the Paleoproterozoic Svecokarelian or the Neoproterozoic Sveconorwegian orogeny. Unlike these shear zone-related deposits, significant amounts of gold were concentrated in volcanogenic massive sulfide (VMS) deposits by magmatic-hydrothermal processes in conjunction with formation of Paleoproterozoic Svecofennian juvenile crust. Hundreds of gold deposits of a variety of ages and genetic styles are, therefore, now known in the Precambrian of northern Europe, of which some are economic, others subeconomic, and still others only of scientific interest. One hundred representative deposits in Sweden, Norway, Finland, and Russia are discussed in this review. Literature references are provided for each of them, and detailed maps show the location of each deposit or prospect. Listed data on tonnage and grades for the most recent exploration targets, as well as the economically and historically most significant deposits, show that they were formed during the entire Precambrian. Yield of gold in the Fermoscandian Shield is, so far, confined to Proterozoic deposits, but the potential to find economically viable gold deposits in the Russian parts of the Archean greenstone belts should not be underestimated.

The Dala granitoids and their associated volcanic products are part of a major Palaeoproterozoic igneous complex (the 1.85–1.67Ga Transscandinavian Igneous Belt; TIB) in the Fennoscandian Shield. TIB constitutes the south-western border of the Svecofennian Domain, which was formed by an orogenic episode at 1.93–1.83Ga. Earlier plate tectonic models proposed that TIB is a postorogenic batholith complex, which was formed in a compressional tectonic regime representing an Andino-type environment, whereas an extensional tectonic regime has been proposed for the anorogenic (1.65–1.51Ga) rapakivi granites in the Svecofennian Domain. In this paper, a key segment of TIB (the Dala granitoids) is discussed in order to focus attention on the post- to anorogenic magmatic evolution in the Svecofennian Domain. Three types of granitoids (Järna, Siljan and Garberg) can be distinguished within the Dala granitoid complex. The Järna granitoids are the most primitive and were emplaced at a significant depth in the crust at ca 1.79Ga. The 1.70–1.68Ga Siljan and Garberg granites are more evolved and were emplaced at a shallow level in the crust, closely associated in space, time and origin with the volcanic Dala porphyries. Field relationships, as well as geochemical and geochronological data, indicate magma genesis within a compressional tectonic regime for the 1.79Ga Järna granitoids. However, most of the Dala granitoids appear to have formed in an extensional tectonic regime, which was initiated during the last phase of TIB magmatism. This extensional igneous component is represented by the 1.70–1.68Ga high-level Siljan and Garberg granites and their closely associated volcanic products (the Dala porphyries). An improved model for the formation of the post- to anorogenic magmatism in the Svecofennian Domain suggests that the Dala granitoids represent a post- to anorogenic igneous key complex, which reflects the transition from a compressional to an extensional tectonic environment along a plate boundary.

Seven 187Re-187Os ages were determined for molybdenite and pyrite samples from two well-dated Precambrian intrusions in Fennoscandia to examine the sustainability of the Re-Os chronometer in a metamorphic and metasomatic setting. Using a new 187Re decay constant (1.666 × 10−11y−1) with a much improved uncertainty (±0.31%), we determined replicate Re-Os ages for molybdenite and pyrite from the Kuittila and Kivisuo prospects in easternmost Finland and for molybdenite from the Kabeliai prospect in southernmost Lithuania. These two localities contain some of the oldest and youngest plutonic activity in Fennoscandia and are associated with newly discovered economic Au mineralization (Ilomantsi, Finland) and a Cu-Mo prospect (Kabeliai, Lithuania). Two Re-Os ages for vein-hosted Kabeliai molybdenite average 1486 ± 5 Ma, in excellent agreement with a 1505 ± 11 Ma U-Pb zircon age for the hosting Kabeliai granite pluton. The slightly younger age suggests the introduction of Cu-Mo mineralization by a later phase of the Kabeliai magmatic system. Mean Re-Os ages of 2778 ± 8 Ma and 2781 ± 8 Ma for Kuittila and Kivisuo molybdenites, respectively, are in reasonable agreement with a 2753 ± 5 Ma weighted mean U-Pb zircon age for hosting Kuittila tonalite. These Re-Os ages agree well with less precise ages of 2789 ± 290 Ma for a Rb-Sr whole-rock isochron and 2771 ± 75 Ma for the average of six Sm-Nd TDM model ages for Kuittila tonalite. Three Re-Os analyses of a single pyrite mineral separate, from the same sample of Kuittila pluton that yielded a molybdenite separate, provide individual model ages of 2710 ± 27, 2777 ± 28, and 2830 ± 28 Ma (Re = 17.4, 12.1, and 8.4 ppb, respectively), with a mean value of 2770 ± 120 Ma in agreement with the Kuittila molybdenite age. The Re and 187Os abundances in these three pyrite splits are highly correlated (r = 0.9994), and provide a 187Re-187Os isochron age of 2607 ± 47 Ma with an intercept of 21 ppt 187Os (MSWD = 1.1). It appears that the Re-Os isotopic system in pyrite has been reset on the millimeter scale and that the 21 ppt 187Os intercept reflects the in situ decay of 187Re during the ∼160 to 170 m.y. interval from ∼2778 Ma (time of molybdenite ± pyrite deposition) to ∼2607 Ma (time of pyrite resetting). When the Re-Os data for molybdenites from the nearby Kivisuo prospect are plotted together with the Kuittila molybdenite and pyrite data, a well-constrained five-point isochron with an age of 2780 ± 8 Ma and a 187Os intercept (−2.4 ± 3.8 ppt) of essentially zero results (MSWD = 1.5). We suggest that the pyrite isochron age records a regional metamorphic and/or hydrothermal event, possibly the time of Au mineralization. A proposed Re-Os age of ∼2607 Ma for Au mineralization is in good agreement with radiometric ages by other methods that address the timing of Archean Au mineralization in deposits worldwide (so-called “late Au model”). Molybdenite, in contrast, provides a robust Re-Os chronometer, retaining its original formation age of ∼2780 Ma, despite subsequent metamorphic disturbances in Archean and Proterozoic time.

Molybdenite is a common accessory mineral in shear-hosted Au deposits. To test the Re-Os molybdenite chronometer in structurally and thermally complex localities, we have selected three important Au-Mo deposits in large scale shear structures. These are the Kasperske Hory deposit in the Bohemian Massif (Czech Republic), the Ridgeway and Haile deposits in the Carolina Slate Belt (southeastern USA), and the Antoinette deposit in the southwestern part of the Fennoscandian Shield (southeast Norway). The Re-Os chronometer in molybdenite, unlike most other chronometers, does not appear to be disturbed by subsequent regional metamorphic events. In addition to directly dating the time of Au-Mo deposition, the Re-Os method should prove useful in untangling structurally complex geologic terranes.

A mineralized Palaeoproterozoic volcanic sequence has been identified southwest of Vetlanda in southern Sweden. The sequence, which is defined as the Fröderyd Group, is surrounded by 1.78 Ga granitoids and may be considered as the southernmost segment of Svecofennian crust in Sweden. The Fröderyd Group has a bimodal geochemical character and is dominated by pillow basalts and associated gabbros but rhyolites, thin intercalations of marble and stratiform sulphide ores do also occur. The carbonate intercalations and the stratiform sulphide ores are interpreted as submarine exhalites formed syngenetically with the volcanic activity. The mafic rocks have a typical mid-ocean ridge basalt geochemistry with flat rare earth elements patterns. The lead isotopic compositions of the mafic rocks and the associated stratiform sulphide ores support such a close affinity to juvenile sources with mantle character. In contrast, the rhyolites display more evolved geochemical signatures and isotopic patterns and are interpreted to represent remelted products of slightly older sialic crust. A model for the formation of the Fröderyd Group is presented, suggesting rifting along a continental margin which may have occurred in the final stage of the Svecofennian crust-forming period.