Sveconorwegian Orogen Research Articles

Molybdenite Re–Os dating constrains gravitational collapse of the Sveconorwegian orogen, SW Scandinavia

Re–Os dating of molybdenite from small deposits is used to define crustal domains exhibiting ductile versus brittle behaviour during gravitational collapse of the Sveconorwegian orogen in SW Scandinavia. A 1019 ± 3 Ma planar quartz vein defines a minimum age for brittle behaviour in central Telemark. In Rogaland–Vest Agder, molybdenite associated with deformed quartz and pegmatite veins formed between 982 ± 3 and 947 ± 3 Ma in the amphibolite-facies domain (three deposits) and between 953 ± 3 and 931 ± 3 Ma west of the clinopyroxene-in isograd (two deposits) in the vicinity of the 0.93–0.92 Ga Rogaland anorthosite complex. The data constrain the last increment of ductile deformation to be younger than 0.95 and 0.93 Ga in these two metamorphic zones, respectively. Molybdenite is the product of an equilibrium between biotite, oxide and sulfide minerals and a fluid or hydrated melt phase, after the peak of 1.03–0.97 Ga regional metamorphism. Molybdenite precipitation is locally episodic. A model for gravitational collapse of the Sveconorwegian orogen controlled by lithospheric extension after 0.97 Ga is proposed. In the west of the orogen, the Rogaland–Vest Agder sector is interpreted as a large shallow gneiss dome, formed slowly in two stages in a warm and structurally weak crust. The first stage at 0.96–0.93 Ga was associated with intrusion of the post-collisional hornblende–biotite granite suite. The second stage at 0.93–0.92 Ga, restricted to the southwesternmost area, was associated with intrusion of the anorthosite–mangerite–charnockite suite. Most of the central part of the orogen was already situated in the brittle upper crust well before 0.97 Ga, and did not undergo significant exhumation during collapse. In the east of the orogen, situated against the colder cratonic foreland, exhumation of high-grade rocks of the Eastern Segment occurred between 0.97 and 0.95 Ga, and included preservation of high-pressure rocks but no plutonism.

Timing of Late Neoproterozoic glaciation on Baltica constrained by detrital zircon geochronology in the Hedmark Group, south‐east Norway

AbstractThe Moelv Tillite is the Late Neoproterozoic Varanger glacial deposit recorded in the Hedmark Group, SE Norway. Paired U–Pb and Lu–Hf data collected on detrital zircons in the Rendalen Formation underlying the Moelv Tillite have identified an uncommon 677 ± 15 to 620 ± 14 Ma population, that constrain the deposition of the Moelv Tillite to be younger than 620 ± 14 Ma. The youngest detrital zircons may be derived from granite magmatism related to the 616 ± 3 Ma Egersund dolerite magmatism, situated in the western part of the Sveconorwegian orogen. The Moelv Tillite, which is not overlain by a cap carbonate, possibly correlates with the c. 580 Ma Squantum‐Gaskiers glacial deposits of Avalonia. Available palaeomagnetic data for the Late Neoproterozoic suggest that Baltica was located at intermediate to high latitude between 620 and 555 Ma.

Petrology and ion microprobe U‐Pb chronology applied to a metabasic intrusion in southern Sweden: A study on zircon formation during metamorphism and deformation

The Åker metabasite occupies a key position in a major tectonic lineament in southernmost Sweden, the Protogine Zone, which coincides closely with the eastern boundary of the late Mesoproterozoic Sveconorwegian orogen of southwest Scandinavia. Metamorphic reactions, associated with the transformation from isotropic gabbro to foliated garnet amphibolite, were identified from disequilibrium textures of which some involved release of zirconium (Zr) and growth of metamorphic zircon. Ion microprobe dating of igneous zircon gave 1562 ± 6 Ma, whereas metamorphic zircons yielded ages of 1437 ± 21, 1217 ± 75, and 1006 ± 68 Ma. The presence of baddeleyite pseudomorphs made up of saccharoidal zircon and a higher abundance of older rather than younger metamorphic zircons suggest redistribution of Zr into new zircon, first by the breakdown of baddeleyite (ZrO2) and later by the consumption of igneous phases containing trace amounts of Zr. Several generations of metamorphic zircon and the presence of 1.56 and 1.22 Ga mafic intrusions along the Protogine Zone call for a complex tectonic history probably reaching back to at least ∼1.56 Ga. Growth of metamorphic zircon at ∼1.44 Ga may relate to a regional, compressional event. The WNW trending deformational structures on both sides of the Protogine Zone may possibly relate to that event. The ∼1.22 Ga metamorphic zircons are coeval with the emplacement of numerous granitic, syenitic, and mafic intrusions along and parallel to the Protogine Zone. The age around 1.0 Ga, finally, marks Sveconorwegian metamorphism for which thermobarometry of the Åker garnet‐amphibolite suggests 1000–1200 MPa at 600°C–630°C. Thereafter, significant relative uplift of the rocks to the west of the Protogine Zone occurred on nearly vertical, north‐south trending deformation zones.

Age and emplacement of late Sveconorwegian monzogabbroic dykes, SW Sweden

The monzogabbroic (jotunitic) Göteborg dykes in SW Sweden display igneous mineralogy and texture, with chilled margins that crosscut all ductile structures in the wall rock. Morphological features suggest emplacement into brittle crust along a set of pre-existing WNW- to W-trending fractures. U–Pb dating of baddeleyites from the largest of these dykes, the Tuve dyke, yields a late Sveconorwegian (Grenvillian) age of 935±3 Ma, which is considered as the crystallisation age of the dyke swarm. The Göteborg dykes and the Hakefjorden Norite–Anorthosite Complex on the Swedish west coast show many similarities with the jotunitic rocks in the Rogaland Anorthosite Complex of south-western Norway, in terms of age, major element composition and Fe–Ti oxide concentrations. The Blekinge–Dalarna dyke swarm east of and along the Protogine Zone in south-eastern Sweden, and the Hunnedalen dykes in south-western Norway, also record the characteristic high Ti–P–K, evolved “jotunitic” character. A distinct feature of all these late Sveconorwegian mafic rocks is the low content of normative diopside (low Ca), but in other aspects they resemble modern continental flood basalts. Ages around 930 Ma for many late Sveconorwegian intrusions correspond in time with the latest phase of extensional movements along crustal-scale shear zones, suggesting that this magmatism is related to extensional collapse and exhumation of the Sveconorwegian orogen.

Magnetic fabrics in the Holum granite (Vest-Agder, southernmost Norway): implications for the late evolution of the Sveconorwegian (Grenvillian) orogen of SW Scandinavia

The Holum granite (Vest-Agder, southernmost Norway) has been considered to be the oldest (980 Ma) undeformed (“post-tectonic”) pluton of a late-Sveconorwegian granitoid series that form two belts in southern Norway and SW Sweden. An integrated structural study of this granite pluton, based on a survey of its anisotropy of low-field magnetic susceptibility (AMS), complemented by microstructural observations, optical microscopy and rock magnetism investigations (including measurements of partial anhysteretic remanent magnetization and its anisotropy, pAARM), is presented here. The magnetic susceptibility and the AMS of the Holum granite are carried by large magnetite grains. Coaxiality between the magnetite-controlled AMS fabric and the rock fabric is verified, which allows the use of the former as a proxy for the latter. Microstructural evidence of a deformation continuum in the granite from magmatic to solid state, and homogeneity and continuity of the AMS fabrics across the pluton demonstrate that the Holum granite is not post-tectonic, but that it was emplaced in a tectonic strain field. A model of emplacement and deformation during the last Sveconorwegian regional folding phase evidenced in the country rocks of the pluton is proposed.

Baltica-Laurentia link during the Mesoproterozoic: 1.27 Ga development of continental basins in the Sveconorwegian Orogen, southern Norway

Recent models suggest that Laurentia and Baltica were contiguous during the Mesoproterozoic and shared a long-lived active continental margin, subsequently reworked during the Grenvillian orogeny. Around 1.25 Ga, the geological record is dominated by dyke-swarm intrusion, continental rift basin formation, A-type felsic magmatism, and arc – back-arc basin development. It points to a dominantly extensional tectonic regime over most of the craton and the Grenvillian margin, suggesting a retreating subduction boundary at that time. In the westernmost allochthonous domain of the Sveconorwegian Orogen, southern Norway, the Sæsvatn–Valldal supracrustal sequences are interpreted as rift or pull-apart basins. They formed at and after 1.27 Ga, in a continental setting, at the margin of Baltica. This interpretation is based on geological, geochemical, and new secondary ion mass spectrometry (SIMS) zircon U–Pb data. A subvolcanic quartz porphyry at the base of the Sæsvatn sequence yields a 1275 ± 8 Ma intrusion age. Metarhyolite samples in the lower part of the sequences yield equivalent extrusion ages of 1264 ± 4 Ma (Sæsvatn sequence) and 1260 ± 8 Ma (Valldal sequence). The metarhyolite units are overlain by sequences of metabasalt and metasandstone. An angular unconformity between the metarhyolites and overlying rocks is locally observed and possibly reflects rift tectonics during formation of the basin. A sample of arkosic metasandstone at the top of the exposed Sæsvatn sequence yields a few Archaean detrital zircon grains and a large spectrum of 2.2–1.2 Ga Proterozoic grains. These data point to a varied continental provenance and constrain sedimentation to later than 1211 ± 18 Ma.

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.

1.05–1.01 Ga Sveconorwegian metamorphism and deformation of the supracrustal sequence at Sæsvatn, South Norway: Re-Os dating of Cu-Mo mineral occurrences

Abstract Re-Os dating of a suite of nine molybdenite samples from two small Cu-Mo mineral occurrences in the epidote-amphibolite facies Sæsvatn supracrustal sequence provides a temporal record of Sveconorwegian metamorphism and deformation. The sequence is situated in the western allochthonous lithotectonic domain of the Sveconorwegian orogen, the Rogaland-Hardangervidda terrane of south Norway. Onset of metamorphism occurred at about 1047 ± 2 Ma, as recorded in small gash veinlets, followed by a deformational peak at about 1032 ± 2 Ma, as recorded in mineralized breccia and mineralized metagabbro comprising a ductile shear zone constituting the Langvatn deposit. Deformation waned significantly by about 1017 ± 2 Ma, based on mineralization hosted in a brittle fault zone within stratigraphically higher metabasalts exposed at the Kobbernuten deposit. The origin of the mineralization at both deposits is most probably metamorphic with ore constituents provided by metasomatism of hosting basalt and gabbro. A metamorphic origin is supported by an array of Re-Os ages that can be related to structural features and the stratigraphic sequence, the absence of plutons related in time and space, the confinement of ore occurrences to mafic sequences in a bimodal supracrustal package that includes rhyolites and clastic units, and clear evidence for Cu mobility in mafic units. The ‘main’ Sveconorwegian orogenic event, probably a continent-continent collision involving imbrication, stacking, and burial of terranes took place at about 1.05 Ga and thereafter. Peak deformation in the Sæsvatn supracrustal sequence in the western part of the Sveconorwegian orogen (South Norway) may correlate with thermal metamorphism in the Idefjorden terrane in the eastern part of the orogen (SW Sweden), with a timing of about 1.03 Ga for both regions. The results of this study indicate that comparatively low grade domains in the orogen (greenschist to epidote-amphibolite facies), corresponding to upper crust, were deformed in a ductile fashion at about 1.03 Ga and were affected by brittle deformation as early as 1.025–1.015 Ga. In the high-grade domains (amphibolite to granulite facies), corresponding to middle and lower crust, ductile deformation is younger, beginning at about 1.025 Ga and persisting until 0.97 Ga.

Zircon geochronology in polymetamorphic gneisses in the Sveconorwegian orogen, SW Sweden: ion microprobe evidence for 1.46–1.42 and 0.98–0.96 Ga reworking

Ion microprobe U–Th–Pb analyses of zircons in variably metamorphosed and veined orthogneisses in the southern part of the parautochthonous Eastern Segment of the Sveconorwegian (1.20–0.90 Ga) orogen, SW Sweden, broadly define two age groups, oscillatory and sector zoned magmatic zircon cores yield 1.70–1.68 Ga while overgrowths, homogeneous crystals, and recrystallized domains in primary zircon yield 1.46–1.42 Ga. In addition, a late-kinematic pegmatite was dated at 0.96 Ga, while a penetratively deformed granite dyke contains both 1444±8 Ma magmatic and 982±15 Ma metamorphic zircons. The 1.70–1.68 Ga ages date the orthogneiss protoliths and fall in the same age range as well-preserved rocks of the Transscandinavian Igneous Belt that forms a major part of the crust east of the Sveconorwegian orogen. Despite Sveconorwegian penetrative deformation under granulite to upper amphibolite conditions, secondary zircons yielding Sveconorwegian ages are virtually absent in the 1.70–1.68 Ga orthogneisses but are abundant in rocks younger than ca. 1.45 Ga. It is suggested that Zr hosted in magmatic phases was redistributed to form new zircon during the 1.46–1.42 Ga event, resulting in a mineralogy in which the main minerals were depleted in Zr. These data, therefore, imply that high-grade metamorphism may occur without associated growth of new zircon. Furthermore, the absence of secondary zircons with ages >ca. 1.46 Ga suggests a re-assessment of models calling for extensive Gothian deformation and metamorphism in the Eastern Segment.

Correlation of supracrustal sequences and origin of terranes in the Sveconorwegian orogen of SW Scandinavia: SIMS data on zircon in clastic metasediments

Ion probe U–Pb data on detrital zircons were acquired in five pre-Sveconorwegian clastic metasedimentary units in the Sveconorwegian orogen in S Norway. Turbidite-type sediments in the Veme complex (Begna sector, west of the Oslo rift) display zircons between 1.67 and 1.53 Ga (9 grains). This complex is interpreted as a westward, younger extension of the Stora Le–Marstrand metasedimentary belt (≤1.58 Ga) of the Idefjorden terrane (east of the Oslo rift). The belt formed in an outboard setting, and the detritus in the sediments was probably mainly derived from subduction-related complexes exposed in the Idefjorden terrane. To the west, a quartzite of the Hallingdal complex in the Telemark sector has zircon populations at 3.13–2.68 and 2.03–1.71 Ga (34 grains). It was deposited between 1.7 and 1.5 Ga and attests the presence of differentiated continental crust older than 1.5 Ga in S Norway. Two samples of the Hettefjorden and Festningsnutan Groups from the Hardangervidda plateau have zircon populations at 3.25–2.42 and 2.03–1.54 Ga (47 grains). The 1.90–1.85 Ga frequency maximum of the age distribution in these three epicontinental sediments corresponds with the peak of Svecofennian magmatic activity in central Sweden, and to the age of the main zircon populations in late-Svecofennian sediments. This is an indication of a Fennoscandian origin for the Telemark–Bamble and Rogaland–Hardangervidda terranes, at the margin of the Svecofennian domain; alternative exotic provenances are possible. A quartzite sample of the Modum complex in the Kongsberg sector displays 2.07–1.48 Ga Proterozoic zircon populations (28 grains) and minor Archaean populations. The Modum complex represents, probably together with the Kragerø complex in the Bamble sector and the Seljord Group in the Telemark sector, parts of a quartzite-rich epicontinental cover deposited after 1.48 Ga on the Telemark–Bamble terrane. The juxtaposition of the Telemark–Bamble and Rogaland–Hardangervidda terranes with the subduction-related Idefjorden terrane, can be explained by a transpressive translation of these terranes at the margin of the Fennoscandian shield during the Sveconorwegian orogeny. Sveconorwegian sinistral strike-slip shearing probably took place along an amphibolite-facies banded gneiss unit situated at the boundary.

Replacement of graphic pegmatite by graphic albite-actinoliteclinopyroxene intergrowths (Mjavatn, southern Norway)

After peak metamorphism, the Sveconorwegian orogen in the Bamble sector, southern Norway, underwent three separate events, viz., the intrusion of granitic pegmatites, regional but locally acting metasomatism, and, finally, intrusion of post-tectonic granites. Metasomatism resulted in localized but widespread formation of albite-actinolite/clinopyroxene-quartz assemblages, with associated monomineralic veins of actinolite, Ti-minerals, etc. At one locality, the Mjavatn pegmatite, graphic quartz-K-feldspar intergrowths were replaced by graphic albite-actinolite-clinopyroxene intergrowths. The preservation of the graphic texture, implying conservation of volume, enables quantification of mass transfer. Application of Gresens9 composition-volume relationships of metasomatism (isocon diagrams) demonstrates massive influxes of Al, Na, Ca, Mg and Fe, accompanied by removal of K and, to a lesser extent, Si. Fluid inclusion studies demonstrate that NaCl-rich brines are the most likely candidates to transport the observed mass. Metasomatism is estimated to have occurred at 350–450°C, 2-4 kbar.

Composition and U–Th–total Pb model ages of polygenetic zircons from the Vånga granite, south Sweden: An electron microprobe study

U–Th–total Pb dating of zircons from the Vånga granite, south Sweden, by electron microprobe (EMP) revealed three different zircon generations. U–Th–Pb analyses of finely zoned zircons, existing both as rounded cores and overgrowth rims, yielded a well defined U–Th–total Pb model age of 1448±25 Ma (1[sgrave]). This age is interpreted to date the intrusion of the Vånga granite, which is notably older than previously suggested. Hf- and F-rich overgrowths yielded a U–Th–total Pb model age of 1128±51 Ma (1[sgrave]). Based on the Hf- and F-enrichment and morphological features, it is suggested that the overgrowths formed by a dissolution–precipitation process in a hydrothermal environment. In few cases magmatic zircon overgrows clear, unzoned cores, which could hardly be dated by EMP due to low U, Th, and Pb contents. A single analysis of one of these cores, however, yielded a U–Th–total Pb model age of c. 1700 Ma, which fits with a previously determined age of refractory zircons in a c. 1445–1465 Ma old Karlshamn-type granite. This agreement supports the previous interpretation that the Vånga granite crystallized from a residual liquid separated from the Karlshamn granite magma. The new intrusion age of c. 1450 Ma indicates that the Vånga granite intruded contemporaneously with an extensive thermomagmatic activity further west in the Eastern Segment of the Sveconorwegian orogen. The c. 1130 Ma age of zircon overgrowths has tectonic implications on the N–S trending Protogine Zone; a major shear zone that marks the eastern boundary of Sveconorwegian reworking. Since equally old syenites and dolerites occur within the Protogine Zone, the age of these hydrothermal overgrowths indicates that the crust affected by Protogine Zone deformation is not exotic relative to the crust of Blekinge, nor is it conceivable to relate the metamorphic contrast across the Protogine Zone to deformation east thereof.

Seismic imaging in the frontal part of the Sveconorwegian orogen, south-western Sweden

Reflection seismic data have been acquired along a 17 km profile in the eastern frontal part of the Sveconorwegian orogen, south-western Sweden. Receiver spacing was 25 m and nominal shot spacing 100 m. Shots ranging in size from 0.5 to 1.0 kg were fired in 3 m deep shot holes. The data image the structure of the upper part of the crust from depths of a few 100 m down to depths of approximately 10 km. The profile crosses the central axis of an inferred fan-like structure, indicated from earlier, detailed surface mapping results. Deformational fabrics within this structure dip approximately 15–40°E in the western part of the profile and 60–80°W in its eastern part. The seismic data display a bivergent geometry with the central axis situated approximately 8 km east of a major fault (Protogine Zone in earlier studies). Surface structural data predict the hinge axis to be located about 3 km farther east. Despite this discrepancy, there is good general agreement with the structure inferred from the surface data. The image obtained from the reflection seismic data establishes that the fan-like structure is a major, upper crustal feature. The present data in combination with previous deep seismic data sets exclude a single-stage extensional model, with an east-dipping extensional deformation zone protracting eastwards from the western part of the profile. The combined seismic data provide support for a two-stage compressional model for the development of the fan-like structure, with crustal thickening and stacking prior to 955 Ma and later compression with a stronger horizontal component of displacement during 930–905 Ma. The latter gave rise to the development of the complex system of retrogressive deformation zones referred to as the Sveconorwegian Frontal Deformation Zone (SFDZ), a tectonic analogy to the Grenville Front in the eastern part of North America. The SFDZ dips westwards, may become listric at depth and possibly sole in a zone of lower crustal reflectivity west of the present profile. The kinematics and geometry of the SFDZ suggest that reverse displacements (thrusting) may be responsible, at least in part, for the exhumation of the medium- to high-grade rocks in the western part of the fan-like structure. Both the seismic data and the surface structural geology show many similarities to the Grenville Front.

Sveconorwegian (-Grenvillian) deformation, metamorphism and leucosome formation in SW Sweden, SW Baltic Shield: constraints from a Mesoproterozoic granite intrusion

The presence of partly well-preserved Mesoproterozoic (∼1.4 Ga) granites in the eastern part of the Sveconorwegian orogen in SW Scandinavia has previously been used as evidence for a Gothian age (1.70–1.55 Ga) for the regional ductile structures and leucosome formation in the region. Accordingly, deformation during the Sveconorwegian (Grenville-) orogeny (1.1–0.9 Ga) has been described as subordinate and mainly restricted to discrete ductile shear zones. Structural, metamorphic and U–Pb zircon data from the Mesoproterozoic Tjärnesjö (meta-) granite demonstrate that the granite intruded prior to a major gneiss-forming event. The U–Pb age of 1368±4 Ma for igneous zircon from the granite defines the upper limit of this deformation. The Tjärnesjö massif is composed of coarse-grained, K-feldspar megacrystic granite to quartz monzonite in various strain states. The heterogeneous deformation of the massif, with tectonic lenses of better-preserved but rarely completely undeformed granite, is triggered by the high-competence of the coarse-grained intrusion. Ductile deformation structures include penetrative or near-penetrative upper amphibolite and high-pressure granulite facies gneissic foliation, as well as discrete amphibolite facies shear zones. Leucosome formation is observed throughout the massif; continuous exposures show transformation of the granite by deformation and migmatization into a veined gneissic rock similar to the country rock gneisses. Zircon from strongly deformed veined Tjärnesjö granite, and from pegmatoid filling in boudinaged amphibolite within the granite, date the partial melting at 992±24 Ma and 954±21 Ma respectively. The regional foliation in the surrounding country rock gneisses is concordant with the gneissic foliation in the Tjärnesjö meta-granite and the regional foliation locally cross-cuts the lithological boundary between these units. The lower age limit for the deformation and metamorphism is set by a post-tectonic pegmatite dyke within the massif dated at 947±12 Ma. The data from the Tjärnesjö meta-granite show that the Sveconorwegian high-grade ductile deformation and metamorphism was pervasive, or near pervasive in competent lithologies, and commonly associated with partial melting. These data call for re-evaluation of previous tectonic models inferring that the main ductile deformation and veining of the crust in SW Sweden is Gothian in age (1.70–1.55 Ga).

The 963 Ma Vinga intrusion and post-compressional deformation in the Sveconorwegian orogen, SW Sweden

In Scandinavia, the 1.15-0.90 Ga Sveconorwegian orogen is transected by major N-S trending shear zones separating lithotectonic segments. East of the Oslo Rift, the Idefjorden terrane hosts lithologically distinct, but spatially related post-compressional intrusions emplaced along similar WNW trends. The Vinga intrusion yields a 963±17 Ma age, which combined with other U-Pb results from this terrane, permits recognition of discrete post-compressional igneous episodes at 1041-1030, 984, 963, and 920 Ma. After final Sveconorwegian convergence and accompanying metamorphism by 969 Ma, gravitational collapse and related extension allowed ascent of the Vinga magma into a WNW fracture. We note that extensional movements with a sinistral strike-slip component confined to the N-S trending shear zones that bound the Idefjorden terrane, provide an emplacement mechanism for post-compressional magmas into the resulting WNW-trending fracture system.

Structural and geochronological evolution of the northeastern part of the Sveconorwegian orogen, south-central Sweden

An important tectonic model for the Sveconorwegian orogen north and northeast of Lake Vänern, south-central Sweden, was presented by Berthelsen (1980). Both the Mylonite Zone and the frontal area of the Sveconorwegian orogen were interpreted to be related to large-scale compressional tectonics. Earlier thrusting to the west was inferred to be followed by later thrust movement to the east.

Protolith ages and timing of deformation in the eastern, marginal part of the Sveconorwegian orogen, southwestern Sweden

U–Pb zircon and titanite ages from variably metamorphosed, igneous rocks in the eastern, marginal part of the Sveconorwegian orogen (Eastern Segment), north of lake Vänern, Sweden, suggest that these rocks belong to the younger (c. 1.71–1.65 Ga) igneous suite of the c. 1.85–1.65 Ga Transscandinavian Igneous Belt (TIB). Two porphyritic meta-granites and one isotropic monzonite yield U–Pb zircon ages of 1661±27, 1674±7 and 1674±7 Ma, respectively. These ages support earlier interpretations that the Eastern Segment north of lake Vänern contains tectonically reworked TIB rocks and that there is no major lithological discontinuity either across the Sveconorwegian Frontal Deformation Zone or within the Eastern Segment. Igneous, brown-coloured titanites from five samples yield similar U–Pb ages as the zircons. In contrast, colourless and oblate-shaped titanites from the western, penetratively deformed parts of the Eastern Segment give concordant ages of 976±4 and 956±6 Ma. Further to the east, similar titanite from both a penetratively deformed meta-granite and a ductile shear zone within a porphyritic quartz monzonite are strongly discordant close to late Sveconorwegian lower intercept ages. This type of titanite is interpreted to be syntectonic and formed during the upper greenschist (east)- to amphibolite (west)-facies, fabric-forming event in the Eastern Segment. Since the brown, igneous titanites and the zircons share a common igneous origin, and the single type of metamorphic titanites are either concordant or strongly discordant towards late Sveconorwegian intercept ages, it is inferred that no major tectonothermal event occurred between the time of TIB emplacement and the late Sveconorwegian tectonothermal reworking. Pre-Sveconorwegian, Palaeo- to Mesoproterozoic deformational and metamorphic events (Gothian), which have been inferred west of and further south within the Eastern Segment, appear to be absent in the northern part of this crustal unit.

Decompressed eclogites in the Sveconorwegian (–Grenvillian) orogen of SW Sweden: petrology and tectonic implications

Relict eclogites and associated high‐pressure rocks are present in the Eastern Segment of the SW Swedish gneiss region (the tectonic counterpart of the Parautochthonous Belt of the Canadian Grenville). These rocks give evidence of Sveconorwegian eclogite facies metamorphism and subsequent pervasive reworking and deformation at granulite and amphibolite facies conditions. The best‐preserved eclogite relics suggest a clockwise P–T –t history, beginning in the amphibolite facies, progressing through the eclogite facies, decompressing and partially reequilibrating through the high‐ and medium‐pressure granulite facies, before cooling through the amphibolite facies. Textures demonstrate the former coexistence of the plagioclase‐free assemblages garnet+clinopyroxene+quartz+rutile+ilmenite, garnet+clinopyroxene+ kyanite+rutile, and garnet+kyanite+quartz+rutile. The former existence of omphacite is evidenced by up to 45 vol.% plagioclase expelled as small grains within large clinopyroxene. Matrix plagioclase is secondary and occurs expelled from clinopyroxene or in fine‐grained, granulite facies reaction domains formed during resorption of garnet and kyanite. Garnet shows preserved prograde growth zoning with rimward increasing pyrope content, decreasing spessartine content and decreasing Fe/(Fe+Mg) ratio, but is partly resorbed and reequilibrated at the rims. P–T estimates from microdomains with clinopyroxene+plagioclase+quartz+garnet indicate pressures of 9.5–12 kbar and temperatures of 705–795 °C for a stage of the granulite facies decompression. The preservation of the prograde zoning suggests that the rocks did not reside at these high temperatures for more than a few million years, and chemical disequilibrium and ‘frozen’ reaction textures indicate heterogeneous reaction progress and overstepping of reactions during the decompression through the granulite facies. Together these features suggest a rapid tectonic exhumation. The eclogite relics occur within a high‐grade deformation zone with WNW–ESE stretching and associated oblique normal‐sense, top‐to‐the‐east (sensu lato) displacement, suggesting that extension was a main cause for the decompression and exhumation. Probable tectonic scenarios for this deformation are Sveconorwegian late‐orogenic gravitational collapse or overall WNW–ESE extension.

U-Pb zircon geochronological evidence for Neoproterozoic events in the Glenfinnan Group (Moine Supergroup): the formation of the Ardgour granite gneiss, north-west Scotland

The age and Precambrian history of the Moine Supergroup within the Caledonide belt of north-west Scotland have long been contentious issues. The Ardgour granite gneiss is essentially an in situ anatectic granite formed during deformation and regional high-grade metamorphism from Moine metasediments. High-precision TIMS and SHRIMP U-Pb zircon dating shows that the age of the anatectic Ardgour granite gneiss and its enclosed segregation pegmatites is 873 ± 7 Ma. This demonstrates the reality of a Neoproterozoic episode of high-grade metamorphism in the Glenfinnan Group Moine and, contrary to previous evidence, the absence of Grenvillian-aged metamorphism. This conclusion places constraints on Neoproterozoic palaeogeographic reconstructions of the North Atlantic region, indicating that the Moine rocks cannot be used as a link between the Grenvillian belt of North America and the Sveconorwegian orogen in Scandinavia. SHRIMP ages of between c. 1100 and 1900 Ma were obtained from detrital, inherited zircons and reflect the provenance of the Glenfinnan Group Moine sediments which must, therefore, have been deposited between c. 1100 and 870 Ma. Potential sources are found as relatively minor, tectonically bounded basement inliers within the British Caledonides, although more widespread source areas occur outside Britain in both Laurentia and Baltica. The most important feature of the provenance is the absence of detrital Archaean grains. This suggests that the Archaean Lewisian gneiss complex, which forms the basement component of the western foreland to the Caledonides in Britain, was not a major contributor to the Glenfinnan Group basin.

Age of the Vinga porphyry and indications of 963 Ma block movements in the Swedish part of the Sveconorwegian orogen

Age of the Vinga porphyry and indications of 963 Ma block movements in the Swedish part of the Sveconorwegian orogen