Gothian Research Articles

Precambrian fault reactivation revealed by structural and K-Ar geochronological data from the spent nuclear fuel repository in Olkiluoto, southwestern Finland

Integrated structural/geochronological studies help unraveling complex brittle deformation histories. We have analysed the structural geological database of brittle faults from the ONKALO™ underground facility for spent nuclear fuel in Olkiluoto in southwestern Finland. Based on the structural geological data from eleven representative fault zones, we classify the Olkiluoto brittle structural features into four fault systems, referred to as Fault system I to IV. The classification is based on their structural properties and tectonic history, crosscutting relationships, fault rock mineralogical characterization and 3D modelling. Some constraints on the timing of faulting are provided by K-Ar dates on synkinematic illite from fault gouge samples. Our results show that the bedrock in southwestern Finland experienced numerous brittle deformation phases between ca. 1.75 and 0.9 Ga. N-S strike-slip faults (Fault systems I and II) formed at mid-crustal levels ca. 1.79–1.75 Ga ago in response to NW-SE/NNW-SSE compression soon after the Svecofennian orogeny. Later E-W striking oblique dextral/normal faults (Fault system III) are tentatively associated with the Gothian orogeny 1.6 Ga ago. These three fault systems were reactivated during NE-SW compression ca. 1.3–1.2 Ga ago, coeval with intrusion of a regional swarm of olivine diabase sills. E-W compression at the onset of the Sveconorwegian orogeny ca. 1.1–1.0 Ga ago resulted in the formation of SE dipping low-angle thrust faults (Fault system IV) and the selective reactivation of fault system II and III. Overall E-W extension during the collapse of the Sveconorwegian orogen ca. 0.97–0.87 Ga ago caused the localised reactivation of fault systems III and IV. Our research approach, which is integral to the siting process of repositories for spent nuclear fuel, demonstrates that the basement in southwestern Finland experienced repeated reactivation since the Mesoproterozoic, suggesting that future deformation localization is likely to be also accommodated by reactivation of existing brittle structures rather than formation of new faults.

Age, geochemistry, and origin of the mid-Proterozoic Häme mafic dyke swarm, southern Finland

We have reappraised the age and composition of the mid-Proterozoic Häme dyke swarm in southern Finland. The dominant trend of the dykes of this swarm is NW to WNW. Petrographic observations and geochemical data indicate uniform, tholeiitic low- Mg parental magmas for all of the dykes. Nevertheless, the variability in incompatible trace element ratios, such as Zr/Y and La/Nb, provides evidence of changing mantle melting conditions and variable crustal contamination. Our ID-TIMS 207Pb/206Pb ages for four low-Zr/Y-type dykes indicate emplacement at 1639 ± 3 Ma, whereas the most reliable previously published ages suggest emplacement of the high-Zr/Y-type dykes at 1642 ± 2 Ma. We propose that the Häme dyke swarm, and possibly also the other mid- Proterozoic mafic dyke swarms in southern Finland, records a progressive decrease in Zr/Y values due to magma generation under developing areas of thinned lithosphere. We consider that the formation of mafic magmas was most probably associated with the upwelling of hot convective mantle in an extensional setting possibly related to the nearby Gothian orogeny. The generation of tholeiitic magmas below continental lithosphere was probably promoted by the elevated mantle temperature underneath the Nuna supercontinent. We speculate that the origin of most of the relatively small mid-Proterozoic mafic dyke swarms, anorthosites, rapakivi granites, and associated rocks found across Nuna was similarly triggered by extensional plate tectonics and the convection of anomalous hot upper mantle below the supercontinent.

Tracing the palaeoredox conditions at Forsmark, Sweden, using uranium mineral geochronology

U-Pb isotope systems have been used to constrain the timing of formation, alteration, and oxidation of U minerals from the meta-granitic bedrock at Forsmark, eastern Sweden. Secondary ion mass spectrometry (SIMS) has been used to collect U-Pb data from uraninite. Discordant data suggest a ~1.8 Ga emplacement of uraninite-bearing pegmatites and an event of uraninite alteration at ~1.6 Ga. The latter age is contemporaneous with the Gothian orogeny in Scandinavia, which was associated with hydrothermal fluid circulation in the Fennoscandian Shield. Ca-uranyl silicates haiweeite and uranophane predominately formed 1.3–1.2 Ga, contemporaneous with the emplacement of the Satakunta complex of the Central Scandinavian Dolerite Group. A Palaeozoic group of Ca-U(VI)-silicates is also present, which indicates that the geochemical composition of geologic fluids was heterogeneous throughout the fracture network during this time. Low Pb concentrations in the U(VI) silicates of several samples are compatible with a recent (<100 Ma) alteration or precipitation of these minerals in connection to reaction with carbonate-rich fluids. The results support a geologically early oxidation of U(IV) to U(VI) and provide insight into the palaeoredox conditions that may impart an on-going influence on the mobility of natural U in the Forsmark fracture network.

Multiple post-Svecofennian 1750–1560 Ma pegmatite dykes in Archaean-Palaeoproterozoic rocks of the West Troms Basement Complex, North Norway: Geological significance and regional implications

The West Troms Basement Complex, North Norway, consist of Meosarchaean, Neoarchaean and Palaeoproterozoic crust which likely represent the continuation of the Karelian craton of the Fennoscandian Shield beneath and across the Caledonian nappes. The complex is transected by several meta-sedimentary/volcanic belts with ages from ca. 2.7 to 1.9Ga, and linear ductile shear zones that record polyphase deformation after emplacement of several late-Svecofennian plutonic complexes (1.80–1.79Ga). The Astridal supracrustal belt of the Senja shear zone was intruded by various generations of granite pegmatite dykes, which yield zircon and titanite U–Pb ages ranging between 1758 and 1562Ma, with evidence of a weak Caledonian overprint at about 400Ma that formed or reset some, but not all, titanites. The age range indicates that formation of the pegmatites occurred after termination of the main Svecofennian orogeny (1.92–1.79Ga) of the Karelian craton. Comparison within the larger scale context of the Fennoscandian Shield suggests that emplacement of these pegmatites may have been caused by late-stage reactivation of intracratonic shear zones, possibly reflecting distal effects of deeper magmatic processes such as intrusion of coeval aged rapakivi granite suites (ca. 1.65–1.5Ga) in Finland, and/or the Mesoproterozoic Gothian orogeny and magmatism affecting the southwestern parts of Fennoscandia. The Mesoproterozoic ages, however, also correspond to late Laxfordian (ca. 1.7–1.6Ga) tectonic events in the Lewisian basement on the Laurentian margin of Scotland, suggesting a potential early link between these basement complexes.

Proterozoic basement and Palaeozoic sediments in the Ringkøbing–Fyn High characterized by zircon U–Pb ages and heavy minerals from Danish onshore wells

New data from the Proterozoic basement and scattered Palaeozoic sediments in the Ringkøbing–Fyn High including zircon U–Pb geochronometry, heavy mineral compositions and whole rock geochemistry is presented here to provide a frame of reference for detrital provenance studies. The Ringkøbing–Fyn High is a WNW–ESE trending structural high including subcropping basement rocks, and the results indicate that it is a southerly extension of the Fennoscandian Shield. The zircon data show matching age distribution patterns in crystalline basement rocks obtained from two drill sites, the Glamsbjerg-1 and Grindsted-1 wells. They both record a characteristic Telemarkian accretionary event at 1.51 and 1.48 Ga and a Sveconorwegian metamorphic overprinting at 1.08 Ga. Furthermore, the dominant age intervals in the Glamsbjerg High (1.55–1.48 Ga) and the Grindsted High (1.51–1.44 Ga) suggest that rocks of the Gothian orogeny (that ended at 1.52 Ga) are only present in the eastern part of the Ringkøbing–Fyn High. Thus, the buried basement in central Denmark may be youngest towards the west, which is consistent with the general westward age progression trend in the Sveconorwegian Orogen. The basement breccia in the Arnum-1 well on the southern flank of the Ringkøbing–Fyn High has zircon ages (c. 1.54–1.53 Ga) that resemble those of gneiss in the Glamsbjerg High. The conglomeratic sandstone in the Ringe-1 well on the Glamsbjerg High has a dual age distribution as the matrix has late Palaeoproterozoic to early Mesoproterozoic ages, whereas the granitic clasts have a distinct middle Neoproterozoic age (c. 0.76 Ga) that may indicate an Avalonian source. The quartzite in the Slagelse-1 well on the northern flank of the Ringkøbing–Fyn High has a broad age span with late Palaeoproterozoic to late Mesoproterozoic zircon ages. Supplementary material: Detailed documentation of U/Pb analytical procedures, results and analysed zircon spots are available below.

Early Mesoproterozoic magmatism in northwestern Lithuania: a new U–Pb zircon dating

We present new geochronological evidence of latest Palaeoproterozoic-earliest Mesoproterozoic magmatism in the Telsiai Deformation Zone, NW Lithuania. Employing the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the University of Tasmania, Australia, we demonstrate that a granodiorite, which had previously been considered to belong to the 1850-1820 Ma Kursiai charnockitic batholith, has a U-Pb zircon age of ca 1.62 Ga. The weighted mean Pb-207/Pb-206 age obtained from eight concordant zircon grains is 1625 +/- 6 Ma (MSWD = 0.6), while the upper-intercept concordia age is 1619 +/- 27 Ma (MSWD = 0.56). Like similar ages of granitoids from several small intrusions in Estonia, this once more indicates extensive melting of the crust at that time. We suggest that both the 1.62 Ga magmatism and the emplacement and crystallization of the large Riga anorthosite-mangerite-charnockite-rapakivi batholith in Latvia and western Estonia at 1580 Ma were far-field feedback effects of the formation of the new Palaeoproterozoic continental crust during the Gothian orogeny at the present SW margin of the East European Craton. (Less)

Late Paleoproterozoic terrane accretion in northwestern Canada and the case for circum-Columbian orogenesis

The reconstruction of the paleocontinental configuration involving ancestral North America (Laurentia) at the Paleoproterozoic–Mesoproterozoic boundary has been developed in the last 30 years with different scenarios being proposed and different combinations of landmasses assembled together. However, the lack of information for the northwestern side of the North American craton has so far been an obstacle for the complete paleocontinental reconstruction and its tectonic history. Here we provide new age determinations on rocks of the Wernecke Supergroup and of the Wernecke Breccia of the Wernecke Mountains in Yukon to provide a more complete picture of the entire North American craton and its possible conterminous at 1600Ma. The six youngest U–Pb ages of the detrital zircon from quartz sandstones of the Wernecke Supergroup suggest that the sedimentary succession is as old as 1640Ma. Lu–Hf garnet ages on garnet bearing schists of the Fairchild Lake Group (lower Wernecke Supergroup) give a bimodal population of ages of approximately 1600Ma and 1370Ma: the first age is related to the Racklan Orogeny, and the younger event is likely attributable to a reheating episode (Hart River Sills emplacement). The younger age of the Wernecke Supergroup puts into question the previous model concerning the emplacement of the Bonnet Plume River Intrusions, and requires the development of a new tectonic model for the northwestern margin of Laurentia. This new model involves obduction of an exotic terrane on top of the Wernecke Supergroup during the latest phases of the Racklan Orogeny (ca. 1600Ma). This exotic terrane, herein called Bonnetia, contains rocks of the Bonnet Plume River intrusions and of the Slab volcanics. During the hydrothermal event that led to the emplacement of the Wernecke Breccia, clasts and megaclasts of the overlying Bonnetia foundered down to the breccia pipes to the level of the Wernecke Supergroup, and this dynamic explains the existence of older rocks engulfed within a younger sedimentary succession. The Racklan Orogeny is now interpreted as a northwestern expression of the Mazatzal Orogeny of southwestern United States, and of the Labradorian Orogeny of eastern Canada which was in turn connected with the Gothian Orogeny of Scandinavia. The connection among these orogenic events makes plausible the hypothesis of a circum-Laurentian orogenic belt with possible extensions in other landmasses (Australia, Antarctica, Siberia, or China) where coeval deformation belts are present.

Laurentian Provenance of Archean Mantle Fragments in the Proterozoic Baltic Crust of the Norwegian Caledonides

The Proterozoic gneisses of theWestern Gneiss Region (WGR) of western Norway experienced HP^UHP metamorphism during the 435^390Ma Caledonian (Scandian) orogeny, and locally enclose numerous large bodies of Archean peridotite. Competing models for the emplacement of these peridotites into the gneisses involve either (1) upthrusting of subcontinental mantle into overlying gneisses or (2) ‘sinking emplacement’ of peridotites from the Laurentian mantle wedge into the upper surface of the subducting Baltica plate. The first model implies the existence of Archean lower crust below the outcropping gneisses. To evaluate these models we have carried out a regional survey of the U^Pb age, Hf isotope composition and trace-element compositions of detrital zircon grains collected from drainages in the northern half of theWGR.The zircon data indicate that the gneisses that make up the crust of theWGR were originally generated during the 1·7^1·5 Ga Gothian orogeny. The Hf isotope signatures of these zircons indicate a juvenile (i.e. mantle) origin; there is no evidence, from either inherited zircons or the Hf isotope data, that Archean crustal materials were involved in the genesis of these gneisses. The Sveconorwegian orogeny (1·3 to51·0 Ga) that overprinted the Gothian gneisses involved both juvenile magmatic additions to the crust and remelting of the Gothian basement; these Sveconorwegian-age magmas also show no evidence of Archean contributions. A population of zircons collected from a drainage area containing large mantle-derived peridotite bodies includes anhedral to rounded grains with distinctive trace-element patterns consistent with derivation from depleted rocks; these are inferred to be derived from the peridotites and/or their enclosed eclogites and pyroxenites. These zircons give Archean Hf model ages, but Pb/Pb ages ranging from Archean to Caledonian, suggesting that the younger ages reflect resetting during later thermal events. The Archean zircon ages are consistent with Archean Re^Os model ages previously obtained on the peridotites. In the absence of any evidence for Archean crust (and hence Archean mantle) beneath southern Baltica, we infer that the peridotite massifs represent fragments of the subcontinental lithosphere beneath Laurentia, and were introduced tectonically into the gneisses during the Caledonian subduction of Baltica beneath Laurentia.

Correction to “Unraveling 1.5 Ga of brittle deformation history in the Laxemar‐Simpevarp area, southeast Sweden: A contribution to the Swedish site investigation study for the disposal of highly radioactive nuclear waste”

[1] The Swedish Nuclear Fuel and Waste Management Company (SKB) is undertaking site investigation at two locations in Sweden, Forsmark and Laxemar-Simpevarp, with the aim of identifying a suitable area for the construction of a deep repository for the disposal of highly radioactive nuclear waste. Fault slip data from outcrops and oriented drill cores were used to compute paleostress states and to unravel the sites' brittle deformation history. Results from the Laxemar-Simpevarp area show that its suggested brittle history results from multiple reactivation of fracture and fault sets caused by the many orogenic episodes that affected the region during at least 1.5 Ga of geological evolution in the brittle deformational regime. Two compressional, approximately NW/NNW-SE/SSE and NNE-SSW oriented shortening events generated sets of conjugate, steep strike-slip fractures. These sets formed during the late stages of the Svecokarelian and possibly also of the Gothian orogeny, soon after the region entered the brittle deformation domain. The Mesoproterozoic Sveconorwegian orogeny generated fractures and faults that are assigned to a third set of conjugate strike-slip faults, which constrain an approximately E-W σ1. The Caledonian shortening, oriented approximately NW-SE to E-W, reactivated the latter but also formed a new, similarly oriented set of subvertical strike-slip fractures. Permian transtension was oriented NW-SW and caused a prominent set of moderately dipping NW-SE trending normal faults in the Precambrian basement of the study area. Two other approximately NW-SW and NW-SE oriented shortening events are recorded in Ordovician limestones and can be tentatively linked to the far-field effects of the Laramide and Alpine orogenies.

Unraveling 1.5 Ga of brittle deformation history in the Laxemar‐Simpevarp area, southeast Sweden: A contribution to the Swedish site investigation study for the disposal of highly radioactive nuclear waste

The Swedish Nuclear Fuel and Waste Management Company (SKB) is undertaking site investigation at two locations in Sweden, Forsmark and Laxemar‐Simpevarp, with the aim of identifying a suitable area for the construction of a deep repository for the disposal of highly radioactive nuclear waste. Fault slip data from outcrops and oriented drill cores were used to compute paleostress states and to unravel the sites' brittle deformation history. Results from the Laxemar‐Simpevarp area show that its suggested brittle history results from multiple reactivation of fracture and fault sets caused by the many orogenic episodes that affected the region during at least 1.5 Ga of geological evolution in the brittle deformational regime. Two compressional, approximately NW/NNW‐SE/SSE and NNE‐SSW oriented shortening events generated sets of conjugate, steep strike‐slip fractures. These sets formed during the late stages of the Svecokarelian and possibly also of the Gothian orogeny, soon after the region entered the brittle deformation domain. The Mesoproterozoic Sveconorwegian orogeny generated fractures and faults that are assigned to a third set of conjugate strike‐slip faults, which constrain an approximately E‐W σ1. The Caledonian shortening, oriented approximately NW‐SE to E‐W, reactivated the latter but also formed a new, similarly oriented set of subvertical strike‐slip fractures. Permian transtension was oriented NW‐SW and caused a prominent set of moderately dipping NW‐SE trending normal faults in the Precambrian basement of the study area. Two other approximately NW‐SW and NW‐SE oriented shortening events are recorded in Ordovician limestones and can be tentatively linked to the far‐field effects of the Laramide and Alpine orogenies.

A palaeomagnetic and geochemical study of basic intrusions in northern Sweden

A palaeomagnetic and geochemical study has been performed on basic dykes in northern Sweden. The dykes and a gabbro formation were sampled in 28 sites and characteristic magnetizations could be defined in 23 of them. The dykes form a part of a swarm that trends in NE–SW to E–W. From differences in palaeomagnetic signatures and composition it is concluded that this swarm is composed of two generations of dykes, group A and B, trending in similar directions. The dykes of group A have compositions that are similar to rapakivi related dykes, while those of group B are different from most rapakivi dykes in Fennoscandia. The calculated pole positions may suggest that the group B dykes are older than those of group A and both groups intruded within the time span 1.77 Ga to 1.50 Ga. The trend of the dykes is more or less parallell to a palaeo-compressional stress field that may be expected from the collisional tectonics related to the Gothian orogeny. The intrusion of the rapakivi formations in Fennoscandia has been suggested to be related with the Gothian orogeny and the intrusion of the dykes may thus be guided by the stress field generated by the collisional tectonics.

Seismic evidence for Late Proterozoic orogenic structures below the Phanerozoic sedimentary cover in the Kattegat area, SW Scandinavia

Commercial seismic reflection profiles from SW Scandinavia image Sveconorwegian orogenic structures in the late Precambrian basement beneath the thin Phanerozoic cover. We interpret (1) pronounced west to WNW dipping reflections as Sveconorwegian localized ductile shear which overprint; (2) weaker east dipping reflections, interpreted as Gothian or early Sveconorwegian compressional structures. Folded and laminar seismic fabrics may represent F and C type crystalline thrust sheets emplaced during the Gothian and Sveconorwegian events. Some of the dipping reflections are observed to sole into (3) an interval of subhorizontal reflections at 2.5–4.5 s two‐way travel time, which we interpret as a “fossil” ductile to brittle transition that was unroofed during the Gothian Orogeny. (4) NW‐SE striking thrusts cutting the Sveconorwegian structures may indicate a shift in the late Sveconorwegian stress field. They may represent an early stage in the subsequent Phanerozoic basin evolution in the Danish area.

Relations between granitoid magmatism and migmatization: U–Pb geochronological evidence from the Western Gneiss Complex, Norway

Gneiss and migmatite samples from the southern part of the Western Gneiss Complex of Norway define a single U–Pb zircon discordia line with lower and upper intercepts at 1633 ± 8 Ma and 969 ± 8 Ma. The upper intercept is interpreted as the magmatic age of the protoliths, showing that the basement rocks were generated during the Gothian orogeny (1750–1500 Ma). The migmatites yield titanite ages that are similar to the lower intercept zircon age, and we interpret this to be the age of migmatization during the Sveconorwegian orogeny. U-rich titanite cores yield an age of 973 ± 3 Ma whereas less U-rich rims gave a slightly younger age of 961 ± 4 Ma. A granitoid batholith in the area gave a U–Pb zircon age that is coeval with the migmatite age (966 ± 2 Ma). The metaluminous granitic magma of the pluton formed by melting at relatively high temperatures in the lower crust. In contrast, the migmatites formed during upper amphibolite-facies conditions at relative high crustal levels. The close temporal and spatial relations between the migmatites and the granite batholith suggest that migmatite formation in the lower parts of the upper crust was related to the heat and mass flux linked to the rise of granitic magma from the lower crust.

Zircon geochronology of migmatite gneisses along the Mylonite Zone (S Sweden): a major Sveconorwegian terrane boundary in the Baltic Shield

The southern section of the Mylonite Zone (MZ) is a major lithotectonic terrane boundary in the SW Baltic Shield. It separates the parautochthonous Eastern Segment of the Sveconorwegian orogen from the allochthonous Western Segment. Complex zircons in migmatised and banded orthogneisses along the southern MZ were investigated by ion microprobe analyses guided by backscattered electron imaging to directly date the partial melting and associated penetrative ductile deformation in this zone. In the eastern part of the MZ (Eastern Segment), resorbed igneous zircon cores in stromatic orthogneiss are 1686±11 Ma old, whereas extensive overgrowths and abundant newly formed simple zircons are 969±13 Ma old. Migmatised K-feldspar megacrystic granite has 1359±26 Ma igneous protolith zircon cores and abundant 968±13 Ma overgrowths and simple grains. Both rock units contain amphibolitised mafic pods with remnants of garnet-clinopyroxene-bearing, high-pressure granulite facies parageneses. In the western part of the MZ (Western Segment), igneous protolith zircon cores in stromatic orthogneiss are dated at 1585±11 Ma and abundant new growth of zircon at 980±13 Ma. Enclosed mafic rocks have middle amphibolite facies parageneses. Secondary zircon in all three samples occurs as distinct, sub-idiomorphic overgrowths or as simple crystals with low Th/U ratios (<0.07). The morphology and high modal abundance of secondary zircon (25–50% of the total volume of zircon), the absence of early- or pre-Sveconorwegian secondary zircon, and field relations provide evidence for that anatexis and associated penetrative ductile deformation in the southern MZ took place between 980±13 and 968±13 Ma. These late-Sveconorwegian ages contradict previous interpretations of a pre-1.55 Ga age for the principal penetrative ductile deformation and stromatic layering of orthogneisses along the southern MZ. As a consequence, the role of the MZ as an important amalgamation zone for crustal growth in the Baltic Shield during the 1.70–1.55 Ga Gothian orogeny is questioned. Instead, the new data suggest that the MZ is a major Sveconorwegian lithologic and metamorphic terrane boundary along which middle-amphibolite facies supracrustals and orthogneisses of the Western Segment (protolith ages: 1.64–1.58 Ga) were juxtaposed against high-pressure granulite to upper-amphibolite facies orthogneisses of the Eastern Segment (protolith ages: 1.70–1.66 Ga) in late Sveconorwegian time. By implication, the pre-Sveconorwegian relation between crustal units west of the MZ and the margin of continent Baltica remains to be established. Understanding of the late Sveconorwegian tectonic evolution, particularly the amount of displacement along the MZ, is therefore, a pre-requisite for modelling the early and pre-Sveconorwegian tectonic evolution and crustal block configurations in the SW Baltic Shield.

Magnetic fabrics and paleomagnetism of the Storsjön–Edsbyn deformation zone, central Sweden

A regional deformation zone, separating tectonomagmatic blocks in the central part of the Fennoscandian Shield, has been studied using anisotropy of magnetic susceptibility and paleomagnetism in order to define the tectonic character and age of the zone and to put the deformation into a plate tectonic context. The deformation zone (the Storsjön–Edsbyn Deformation Zone; SEDZ) separates the Rätan granite of the Trans Scandinavian Igneous Belt (TIB) and the southern Svecofennian subprovince in the southwest from older Svecofennian granitoids of the central Svecofennian subprovince in the northeast. The SEDZ is the most prominent deformation zone in a regional pattern of NW–SE trending shear zones. On basis of AMS data the width of the zone, which cuts through 1.7–1.9 Ga old rocks, is estimated at 10–12 km in its central–southern part and 20–25 km in its northern part. The orientations of the susceptibility axes vary systematically across the zone, in a way that suggests that the SEDZ is a pure shear dominated transpression zone caused by a compressive stress from the present SSW. Paleomagnetic data from a dolerite sill (Plat=10°, Plon=166°) and from a dyke (Plat=25°, Plon=190°) that cut the SEDZ indicate that no plastic deformation has occurred along the zone during the past 1.25 Ga, and probably not for the past 1.5–1.6 Ga. AMS data do not point to any deformation of the 1.70 Ga old Rätan granite, which further limits the time span of deformation. The main phase of deformation most likely took place between ∼1.80 and 1.70 Ga ago. The geological evolution of this part of the Fennoscandian Shield at the time between 1.80 and 1.70 Ga ago is very complex and in part still enigmatic. This time period involves magmatism of the TIB, which is related to eastward subduction. It also comprises the beginning of the Gothian orogeny and the development of a regional system of transpressive deformation zones. The compressive stress field from the present SSW indicated in this study is interpreted to be related to an initial stage of plate collision between the southwestern Scandinavian subprovince and older parts of the Fennoscandian Shield. This collision resulted in a system of regional NW–SE trending intra-plate deformation zones, one of which is the SEDZ

Adakitic high-Al trondhjemites in the Proterozoic Østfold-Marstrand Belt, W Sweden

This paper investigates the first identified intrusives in SE Norway–W Sweden with the specific signature of adakitic arc magmas, which in recent settings are preferably explained as partial melts extracted from subducted oceanic crust. The studied adakitic high–Al trondhjemites occur as sheets in the Koster archipelago, W Sweden, where they form the oldest recognized granitoids in the metasupracrustals of the Stora Le–Marstrand formation. The trondhjemites were intruded during a short ca. 1.59–1.58 Ga interlude between the early and the main orogenic events of the Gothian orogeny (1.6–1.56 Ga, Åhäll et al. 1998). This interlude is otherwise characterized by ‘ordinary’ calcalkaline magmatism which on Koster is predated by the trondhjemites. The typical adakitic signature suggests that the trondhjemitic magma was extracted from a MORB (Mid Ocean Ridge Basalt) like source, and that a hornblende eclogite restite was left in the region of melting. The restite composition indicates melt extraction at PT conditions in the range of 18–25 kb/800°C to 13-15 kb/950–1050°C. These requirement can only be met by subduction of warm (young or shear heated) oceanic crust beneath a crust including early Gothian metamorphosed and deformed Stora Le–Marstrand formation or by melting of metabasaltic material at a deep crustal level. The latter is a less likely possibility and demands that the Stora Le–Marstrand formation at the time of melt extraction was part of a &gt; 45 km thick crust.

Timing of Gothian structural evolution in SE Norway: A Rb-Sr whole-rock age study

The polydeformed basement area in SE Norway is situated within the southwest border of the Baltic Shield. The investigated area covers the northern part of the Ostfold segment (the Ostfold Complex) and adjoining parts of the Median segment (the Romerike Complex). New Rb-Sr whole-rock age determinations are integrated with the structural evolution. The oldest rock unit is the Ostfold-St. Le-Marstrand formation. During successive stages the supracrustal unit was intruded by major granitic bodies now forming the biotite gneiss (oldest), the augen gneiss and metatonalite/granite complexes (youngest). The emplacement of the granitic units was separated by orogenic deformation events, F0-F3, recognized by the development of large-scale regional folding accompanied by amphibolite facies metamorphism. Rb-Sr whole-rock age determinations indicate a maximum age for the biotite gneiss (pre-F0) at 1.71 Ga; this age also marks the maximum age of the Gothian orogeny. Intrusion of the Nordstrand-Sormarka metatonalite/gr...

The Mesoproterozoic cratonization of Baltica — new age constraints from SW Sweden

Abstract Comparable early Mesoproterozoic tectonic histories resulted in the formation of 1.71–1.62 Ga Labradorian and 1.75–1.55 Ga Gothian crust in Laurentia and Baltica respectively, along a postulated contiguous margin. New U-Pb geochronology in southwest Sweden has constrained the timing of three consecutive stages of the Gothian Orogeny. Calc-alkaline magmatism, presumably related to subduction, occurred as late as 1587 ± 3 Ma, indicating convergence and consumption of oceanic crust until this time. These magmas intruded terranes of previously accreted arcs, thus providing a minimum age for their juxtaposition and accretion. The age of the latest stage of Gothian migmatization is constrained between 1555 ± 2 and 1553 ± 2 Ma, synchronous with the transition from ductile to brittle conditions at the presently exposed erosional level. This transition permitted the emplacement of a suite of mafic-felsic 1555–53 Ma intrusions during an extensional regime that may have coincided with the post-deformational collapse of the Gothian Orogen. These age refinements in southwest Sweden indicate that the final stage of the Gothian Orogeny in Baltica occurred considerably later than that of the Labradorian Orogeny in Laurentia. The preservation of Gothian-aged titanite from southwest Sweden indicates that post-Gothian metamorphism, including that of Sveconorwegian-Grenvillian age, was not penetrative in this region.

Westward accretion of the Baltic Shield: implications from the 1.6 GaÅmål-Horred Belt, SW Sweden

Three new U-Pb zircon age determinations are reported from the Horred region, south-southeast of Göteborg, SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major N S trending shear zone and tectonic boundary, the Mylonite Zone, juxtaposes comparatively weakly migmatised lithologies in the west against more intensely migmatised gneisses in the east. West of the Mylonite Zone, a metavolcanic rock (the Mjösjödacite) yields an age of 1643 ± 29 Ma, whereas a cross-cutting plutonic rock (the Idala tonalite) has an age of 1584 ± 15 Ma. Together with a recent age for a volcanic rock from theÅmål region farther north (1.61 Ga, Lundqvist and Skiöld, 1992), these ages help to establish the existence of a coherent calc-alkaline igneous belt of 1.6 Ga age for which the nameÅmål-Horred Belt is proposed. East of the Mylonite Zone, a presumably metavolcanic rock (the Grimmared gneiss) yields an age of ∼ 1.61 Ga. The obtained age and the compositional similarity of rocks on each side of the Mylonite Zone indicate that more deformed and more strongly metamorphosed equivalents of the rocks in theÅmål-Horred Belt may occur also to the east of the Mylonite Zone in what is termed the Eastern Segment of the Southwest Scandinavian Domain. The new results establish theÅmål-Horred Belt as a major geological unit younger than most other crustal components in southern Sweden such as theÖstfold-Marstrand Belt (∼ 1.76 Ga), the Eastern Segment gneisses (> 1.66 Ga) and the three age groups of the Transscandinavian Igneous Belt (∼ 1.81 – 1.65 Ga; Larson and Berglund, 1992). The configuration of the crustal units in SW Sweden appears to necessitate more complex Proterozoic models than those with a persistent younging from the present east to the west. The present concept of the “Gothian orogeny” must be revised since at least two different orogenic episodes at ∼ 1.7 and 1.6 Ga can now be distinguished.

Isotopic constraints on the timing of crustal accretion of the Bamble Sector, Norway, as evidenced by coronitic gabbros

This paper presents the first SmNd data for coronitic gabbros (“hyperites”) from the high-grade Proterozoic Bamble Sector, Norway. SmNd whole-rock isochron ages of the Vestre Dale Gabbro, 1.11±0.14 Ga, the Jomåsknutene Gabbro, 1.77±0.19 Ga, and the Flosta Gabbro, 1.64±0.23 Ga indicate that, contrary to earlier ideas, intrusion of these gabbros is not exclusively bound to the Sveconorwegian orogeny (1.2-0.9 Ga). Some of the gabbros were already emplaced at the onset of or during the Gothian Orogeny (1.75-1.5 Ga). Initial ϵ Nd values of about +5.5 of Jomåsknutene and Flosta Gabbros provide clear evidence of a depleted mantle reservoir beneath this part of the Southwest Scandinavian Domain. This highly positive value differs from the Nd-isotope evolution curve for the Southwest Scandinavian mantle as postulated by Mearns et al. (1986), which assumes that depletion of the mantle beneath this part of Scandinavia started at c. 1.74 Ga. However, the initial ϵ Nd value of the Vestre Dale Gabbro plots close to this curve, whereas 143Nd 144Nd ratios of the mantle sources of the Tromöy and Arendal Gabbros evolved with approximately chondritic 144Sm 144Nd ratios; their ϵ Nd (1.1 Ga) values range from +5.1±0.3 to +6.0±0.3. Gothian metasomatism of a depleted mantle source by LILE and LREE-bearing fluids provides a mechanism to obtain low Nd-isotope ratios in magmas represented by the Vestre Dale Gabbro. Examples of low Nd-isotope magmas have also been found in the Telemark Sector, and in southern Sweden. The marked depletion in Nb indicates that the metasomatic fluids were possibly related to Gothian subduction.