Abstract Thirty years of research, and especially the refinements of many geological, geochemical and geophysical techniques, have uncovered many new facets of the geology of the Scandinavian Caledonides, also correcting some fundamental misconceptions. Our present understanding is that of a sequence of allochthons, some derived from Baltica, but others of probably exotic origin, in part from the Laurentian margin that collided with Baltica, but perhaps also from other parts of Rodinia. The present paper summarizes the main features of the Scandinavian Caledonides, proposing some rethinking of the traditional schemes, which were developed lacking a substantial amount of the information we have today, and discusses the main advances since the last major synthesis in 1985.

Abstract The Caledonides of Britain and Ireland are one of the most intensively studied orogenic belts in the world. This review considers all the tectonic events associated with the development and closure of the Iapetus Ocean. It first summarizes the tectonic evolution of each segment involved in the Scottish–Irish sector of the Caledonides and then reviews the temporal evolution of the Caledonian Orogeny. Three main tectonic phases are recognized in the Scottish–Irish Caledonides: an Early–Middle Ordovician (475–465 Ma) phase termed the Grampian Orogeny; a phase of Silurian (435–425 Ma) tectonism restricted to the Northern Highland Terrane of Scotland termed the Scandian Orogeny; and an Early Devonian (395 Ma) phase termed the Acadian Orogeny. The Grampian Orogeny was caused by the collision of the Laurentian continental margin with an oceanic arc terrane and associated suprasubduction zone ophiolites during the latest Cambrian–Early Ordovician. Following the Grampian arc–continent collision event, there was a subduction polarity reversal. This facilitated continued subduction of Iapetan oceanic lithosphere and an Andean-type continental margin developed on and adjacent to the Laurentian margin in the Middle Ordovician along with a substantial thickness of accretionary prism sediments (the Southern Uplands–Longford Down Terrane). The Iapetus Ocean is believed to have disappeared by the Late Silurian based on the faunal record and a continent–continent collision ensued. The absence of significant regional deformation and metamorphism associated with the Late Silurian collision between Avalonia and the Scottish–Irish margin of Laurentia suggests that the continental collision in this sector of the Caledonian–Appalachian orogen was ‘soft’ or highly oblique. The exception is the Northern Highlands Terrane of Scotland that was believed to have been situated 500–700 km to the north along orogenic strike. This terrane records evidence for significant Silurian regional deformation and metamorphism attributed to the collision of the Laurentian margin of East Greenland with Baltica (the Scandian Orogeny). Current controversies in the Laurentian Caledonides of Scotland and Ireland are discussed at the end of this review.

Abstract The Greenland and Svalbard Caledonides make up an important part of the Palaeozoic Caledonian orogen, and preserve a complex history of Palaeoproterozoic arc accretion, Proterozoic to Palaeozoic sedimentation within various basins and extensive magmatism, metamorphism and deformation during the Caledonian orogeny. In this summary, the current understanding of the structure and lithological content of the Greenland and Svalbard Caledonides is first reviewed, and open questions are highlighted. The Greenland Caledonides are divided into three different segments, and the term terrane is abandoned for the Svalbard Caledonides. Then, other Caledonian fragments in the Arctic region are discussed, including Bjørnøya, Pearya and Cordilleran terranes and parts of the Barents Shelf. Finally, a regional synthesis covering the geological evolution of the Greenland and Svalbard Caledonides from the Palaeoproterozoic to the end of the Caledonian orogeny is presented and controversial issues and open questions are discussed.

Abstract In central parts of the Scandinavian Caledonides, detrital zircon signatures provide evidence of the change in character of the Baltoscandian crystalline basement, from the characteristic Late Palaeoproterozoic granites of the Transscandinavian Igneous Belt (TIB, c. 1650–1850 Ma) in the foreland Autochthon to the typical, mainly Mesoproterozoic-age profile ( c. 950–1700 Ma) of the Sveconorwegian Orogen of southwestern Scandinavia in the hinterland. Late Ediacaran to Early Cambrian shallow-marine Vemdal quartzites of the Jämtlandian Nappes (Lower Allochthon) provide strong bimodal signatures with TIB (1700–1800 Ma) and Sveconorwegian, sensu stricto (900–1150 Ma) ages dominant. Mid-Ordovician turbidites (Norråker Formation) of the Lower Allochthon in Sweden, sourced from the west, have unimodal signatures dominated by Sveconorwegian ages with peaks at 1000–1100 Ma, but with subordinate components of older Mesoproterozoic zircons (1200–1650 Ma). Latest Ordovician shallow-marine quartzites also yield bimodal signatures, but are more dispersed than in the Vemdal quartzites. In the greenschist facies lower parts of the Middle Allochthon, the Fuda (Offerdal Nappe) and Särv Nappe signatures are either unimodal or bimodal (950–1100 and/or 1700–1850 Ma), with variable dominance of the younger or older group, and subordinate other Mesoproterozoic components. In the overlying, amphibolite to eclogite facies lower part of the Seve Nappe Complex, where the metasediments are dominated by feldspathic quartzites, calcsilicate-rich psammites and marbles, most units have bimodal signatures similar to the Särv Nappes, but more dispersed; one has a unimodal signature very similar to the Ordovician turbidites of the Jämtlandian Nappes. In the overlying Upper Allochthon, Lower Köli (Baltica-proximal, Virisen Terrane), Late Ordovician quartzites provide unimodal signatures dominated by Sveconorwegian ages ( sensu stricto ). Further north in the Scandes, previously published zircon signatures in quartzites of the Lower Allochthon are similar to the Vemdal quartzites in Jämtland. Data from the Kalak Nappes at 70°N are in no way exotic to the Sveconorwegian Baltoscandian margin. They do show a Timanian influence (ages of c. 560–610 Ma), as would be expected from the palinspastic reconstructions of the nappes. Thus the detrital zircon signatures reported here and published elsewhere provide supporting evidence for a continuation northwards of the Sveconorwegian Orogen in the Neoproterozoic, from type areas in the south, along the Baltoscandian margin of Baltica into the high Arctic. Supplementary material: LA-ICP-MS U–Pb analyses are available at http://www.geolsoc.org.uk/SUP18699 .

Abstract LA-ICP-MS U–Pb and Hf-isotope data on detrital zircons from the Ediacaran and Cambrian Dividal Group demonstrate that the autochthonous cover sequence above the Fennoscandian Shield in northernmost Scandinavia is not derived from an easterly Archaean and Palaeoproterozoic source within Baltica as commonly thought. Detrital zircon age populations on four samples from the Dividal Group are dominated by Mesoproterozoic zircons with relatively few Palaeoproterozoic and Archaean zircons. Two samples, from Altevann and Reisadalen have, in addition, a significant population of Ediacaran zircons ( c. 570–560 Ma), indicating a Timanian source area for most of the lower Cambrian Dividal Group sediments. The 176 Hf/ 177 Hf isotope data show the Ediacaran zircons to be derived from two separate plutonic complexes within the Timanides. It is argued that the Dividal Group sediments were deposited in a foreland basin south and SW of the Timanide Orogen. Similarities with clastic zircon age populations from Cambrian deposits in Akkajaure, Ladoga/White Sea and St Petersburg areas indicate that this foreland basin possibly extended southward for at least 1000 km. A foreland basin setting for the Ediacaran and Cambrian deposits in Central and Southern Scandinavia can thus account for the enigmatic Neoproterozoic detrital zircons in these deposits. Supplementary material: Analytical LA-ICP-MS data on detrital zircons from Dividal Group samples AA12-1, AA12-2, AA11-29 and AA11-30 are available at http://www.geolsoc.org.uk/SUP18731 .

Abstract This article describes the occurrence, bulk geochemistry and phenocryst compositions of Caledonian volcanic ash beds (bentonites) in the sedimentary sections of the Palaeozoic Baltic sedimentary basin. Four periods of volcanism are recognized in the eastern Baltic region: (a) Late Sandbian with sources derived from the convergent margin between Avalonia and Baltica; (b) Late Katian with sources from the margin of the Iapetus Ocean (Norwegian Caledonides); (c) Aeronian (with extension into Telychian and Sheinwoodian) with sources in the Central European Caledonides; and (d) Telychian to Early Ludlow with sources derived from the convergent margin between Laurentia and Baltica (Norwegian Caledonides). Trace element compositions in bentonites indicate mostly evolved source magmas of rhyolitic and dacitic composition. The volcanism in the Aeronian is characterized by less evolved basaltic and trachyandesitic compositions. Sanidine compositions indicate the existence of potassium-dominated (over sodium) source magmas in Late Sandbian and from the late Homerian to Early Ludlow. During other periods both potassium- and sodium-dominated source magmas occur. The presence of sodium-rich sanidine in many bentonites combined with the scarcity of biotite suggests that the source magmas were water-undersaturated. Biotite phenocrysts are mostly Mg-rich, but Fe-rich varieties occur in the Late Sandbian and Early Telychian.

Abstract This paper summarizes the main geochronological features of the Caledonian nappes in southern Norway, discusses their similarities and differences, and reports new U–Pb isotope dilution thermal ionization mass spectrometry data. The latter include ages of 1670 Ma for orthogneiss from the Upper Finse Nappe, 1500, 1250 and 950 Ma for intrusive rocks from the Hallingskarvet Nappe, and 1037 and 997 Ma for volcanic units from the Suldal and Lower Finse nappes. The Caledonian nappes can be subdivided into units formed at 1700–1600 Ma, having an affinity with Gothian crust, and units formed at about 1500 Ma, correlating with Telemarkian crust. A Sveconorwegian tectonometamorphic overprint is ubiquitous, but with large differences in the intensity and relative timing of the overprint. Some complexes (such as Kvitenut–Dyrskard) were affected at around 1000 Ma and others (such as Jotun and Lindås) between 970 and 930 Ma. There is also considerable variety in the Caledonian effects. Ordovician events affected some nappes (Jæren, Revsegg), thought to be of exotic origin, while Silurian and Devonian events of variable intensity are observed in all nappes. The emerging patterns offer the basis for a qualitative discussion of the provenance of the nappes that can eventually be combined with quantitative structural criteria to reconstruct pre-Caledonian palaeogeographies.

Abstract The Espedalen Nappe is located in the east of the Jotun–Valdres Nappe Complex in southern Norway. Its main component, the Espedalen Complex, consists of an early suite of jotunite, charnockite and augengneiss, a main anorthosite–ultramafite–norite suite, and a late gabbronorite suite. Mafic dykes and lamprophyres cut the complex, some being feeders to the Stylskampen mafic to felsic suite at the western margin of the complex. The complex hosts Ni–Cu mineralizations which were mined in past centuries. Two crystalline outliers occur at Ormtjernkampen and Røssjøkollen further south. Caledonian convergence thrust the nappe on to Early Palaeozoic sedimentary rocks. Rocks of the Espedalen Complex are covered and/or tectonically interleaved with Late Precambrian conglomerates and arkoses of the Valdres Group. Dating of zircon and titanite by ID-TIMS U–Pb indicates formation of the complex at 1520–1510 Ma, followed by deformation and metamorphism between 1050 and 950 Ma during the Sveconorwegian orogeny, before the eventual translation during the Caledonian orogeny. These data indicate an affinity of the nappe with Telemarkian crust in the autochthonous Baltic basement, in contrast to the rocks of the overlying Jotun Complex which correlate instead to Gothian crust formed between 1600 and 1700 Ma.

Abstract The study of complex orogenic belts commonly begins in the frontal regions with well-defined tectonostratigraphy, and relatively simple structure and metamorphism, and proceeds into the progressively more complex hinterland, which nevertheless may contain the best geochronological record of the most intense orogenic events. The northern part of the Western Gneiss Region in the hinterland of the Scandian orogen contains a robust U–Pb zircon geochronological framework on rocks subjected to high-pressure (HP) and ultra-high-pressure (UHP) metamorphism that has implications for tectonic development both there and towards the foreland. HP and UHP eclogite crystallization occurred at 415–410 Ma (Early Devonian, Lochkovian to Pragian), followed by pegmatite crystallization at c. 395 Ma (Late Emsian) during exhumation and return to amphibolite-facies conditions, thus limiting the process to 15–20 myr. The nature and sequence of events are much more complex than in the foreland, causing difficulty in correlation, yet the combined tectonics in the two regions provides the necessary context to explain, for example, how rocks were subjected to deep-seated, high-temperature metamorphism and then exhumed to shallower levels. Here, we suggest how a recently recognized extensional detachment fault and a recently recognized out-of-sequence thrust might be linked to the timing of HP metamorphism and later exhumation. The postulated Agdenes extensional detachment in its footwall has basement gneisses containing Mesoproterozoic igneous titanite fully reset at 395 Ma, as well as Devonian pegmatites, and in the hanging wall Ordovician to Early Silurian granitoids of the Støren Nappe containing igneous titanite barely influenced by Devonian recrystallization and no evidence of post-Ordovician melts. This implies removal of a significant crustal section on a large-scale detachment. Rocks both above and below are overprinted by the same late, subhorizontal, sinistral ductile extensional fabric, obscuring any fabrics produced during development of the detachment itself. Eastern Trollheimen escaped the late, strong, subhorizontal overprint, and shows: (1) early emplacement of thrust nappes of Lower and Middle Allochthons over Baltican basement and its Late Neoproterozoic quartzite cover; (2) major, SE-directed, recumbent folding of the entire thrust-imbricated sequence; and (3) major, out-of-sequence, SE-directed thrusting (Storli Thrust), for an 80 km minimum transport across-strike, of the recumbent-folded sequence over deeper, less deformed, lower basement gneisses and unconformable Neoproterozoic quartzite cover. The upper basement contains boudins of eclogite and garnet-corona gabbro lacking in the lower basement. Similar basement imbrications occurred in the Tømmerås window, the Skarddøra Antiform, the Mullfjället Antiform and the Grong–Olden Culmination, up to 240 km NE of Trollheimen, as well as in the Reksdalshesten antiform 100 km west, all within the postulated minimum 400×180 km area of the Agdenes detachment.

Abstract Branch line restoration of the external Finnmark Caledonides, northern Norway, gives 232 km linear displacement of the Middle Allochthon, rising to 252 km when including thrusting-direction changes. A 108 km SSE/SE-directed deformation, mostly ductile, and 144 km ENE/ESE-directed deformation, mostly brittle, occurred. The North Varanger Region/Terrane restores to under Porsangerhalvøya, with the Trollfjorden–Komagelva Fault a possible precursor of the Kokelv Fault. Dextral offset on the Trollfjorden–Komagelva Fault was c. 207 km. Three major sediment depositional areas are recognized: Timan Basin (including Barents Sea–Løkvikfjellet, Einovskaya–Manjunnas–Digermulen and Laksefjord sub-basins); Gaissa Basin (including Finnmark Ridge and Baltica Coast); and Gaissa Promontory. The Barents Sea–Løkvikfjellet Sub-basin comprises 15.9 km of sediments of the North Varanger Terrane. The Einovskaya–Manjunnas–Digermulen Sub-basin comprises 4 km of Rybachi Terrane sediments overlain by the Manjunnas Group (3 km) and then a Gaissa Basin equivalent succession (4.2 km). The 7.7 km-thick Laksefjord Group (Laksefjord Sub-basin) lies directly WNW of the Finnmark Ridge. Maximum clastic sedimentation ( c. 3.7 km) in the Gaissa Basin was in the NE and maximum dolomite accumulation (235 m) in the SW, where the basin closed off. The Finnmark Ridge and Baltica Coast have condensed Gaissa Basin successions. The Gaissa Promontory lies north of the Gaissa Basin, probably with the same lithostratigraphy.

Abstract The scientific drilling project COSC (Collisional Orogeny in the Scandinavian Caledonides), designed to study key questions concerning orogenic processes, aims to drill two fully cored boreholes to depths of c. 2.5 km each at carefully selected locations in west-central Sweden. The first of these, COSC-1, is scheduled for start late spring 2014 and will target the Seve Nappe Complex, characterized by inverted metamorphism and with parts that have evidently been subjected to hot ductile extrusion. In this study available seismic sections have been combined with surface geology to produce a 3D interpretation of the tectonic structures in the vicinity of the COSC-1 borehole. Constrained 3D inverse gravity modelling over the same area supports the interpretation, and the high-density Seve Nappe Complex stands out clearly in the model. Interpretation and models show that the maximum depth extent of the Seve Nappe Complex is less than 2.5 km, consistent with reflection seismic data. The gravity modelling also requires underlying units to comprise low-density material, consistent with the Lower Allochthon, but the modelling is unable to discern the décollement separating the allochthons from the crystalline Precambrian basement.

Abstract New evidence is presented for ultra-high-pressure metamorphism of kyanite–garnet pelitic gneiss in the Åreskutan Nappe of the Seve Nappe Complex, in the central part of the Scandinavian Caledonides. Modelled phase equilibria for a peak pressure assemblage garnet+phengite+kyanite+quartz (coesite) in the NCKFMMnASH system record pressure and temperature conditions of c. 26–32 kbar at 700–720 °C, possibly up to ultra-high-pressure conditions. Subsequent decompression, simultaneous with an increase of temperature to c. 800–820 °C, led to partial melting largely owing to the dehydration and breakdown of phengite. Based on existing isotope age data, we conclude that the Middle Seve Nappe in central Jämtland experienced deep subduction in the late(st) Ordovician, prior to decompression and partial melting of the pelitic protoliths during Early Silurian extrusion, giving way in the Mid to Late Silurian to thrusting on to the Baltoscandian platform. Nappe emplacement probably continued into and through the Early Devonian.

Abstract Secondary ionization mass spectrometry (SIMS) U–Pb dating of zircons from the Åreskutan Nappe in the central part of the Seve Nappe Complex of western central Jämtland provides new constraints on the timing of granulite–amphibolite-facies metamorphism and tectonic stacking of the nappe during the Caledonian orogeny. Peak-temperature metamorphism in garnet migmatites is constrained to c. 442±4 Ma, very similar to the ages of leucogranites at 442±3 and 441±4 Ma. Within a migmatitic amphibolite, felsic segregations crystallized at 436±2 Ma. Pegmatites, cross-cutting the dominant Caledonian foliation in the Nappe, yield 428±4 and 430±3 Ma ages. The detrital zircon cores in the migmatites and leucogranites provide evidence of Late Palaeoproterozoic, Mesoproterozoic to Early Neoproterozoic source terranes for the metasedimentary rocks. The formation of the ductile and hot Seve migmatites, with their inverted metamorphism and thinning towards the hinterland, can be explained by an extrusion model in which the allochthon stayed ductile for a period of at least 10 million years during cooling from peak-temperature metamorphism early in the Silurian. In our model, Baltica–Laurentia collision occurred in the Late Ordovician–earliest Silurian, with emplacement of the nappes far on to the Baltoscandian platform during the Silurian and early Devonian, Scandian Orogeny lasting until c. 390 Ma.

Abstract The metamorphic evolution of the Tjeliken eclogite, occurring within the Seve Nappe Complex of northern Jämtland (Swedish Caledonides), is presented here. The prograde part of the pressure and temperature ( P–T ) path is inferred from the mineral inclusions (pargasitic amphibole) in garnet and intracrystalline garnet exsolutions in omphacite. Peak metamorphic conditions of 25–26 kbar at 650–700 °C are constrained from geothermobarometry for the peak-pressure assemblage garnet+omphacite+phengite+quartz+rutile, using the garnet–clinopyroxene Fe–Mg exchange thermometer in combination with the net-transfer reaction (6 diopside+3 muscovite=3 celadonite +2 grossular+pyrope) geobarometer, the average P–T method of Thermocalc and pseudosection modelling. Quartz inclusions with well-developed radial cracks were identified within omphacite, which suggest that the studied rock could have been buried down to the coesite stability field. Post-peak P–T evolution is inferred from diopside–plagioclase symplectites and amphibole coronas around garnet. Previous studies in northern Jämtland suggest a substantial gap between the P–T conditions of the Lower and Middle Seve nappes: 14–16 kbar and 550–680 °C and 20–30 kbar and 700–800 °C, respectively. The Tjeliken eclogite has been considered previously to be a part of Lower Seve by most authors, but the new P–T data suggest that it may be an isolated klippe of Middle Seve.

Abstract Phengite-bearing eclogites occur in the Richarddalen Complex of NW Spitsbergen, Arctic Caledonides. Phase equilibrium modelling and conventional geothermobarometry have been used to constrain the metamorphic evolution of these eclogites. Pseudosections are calculated for the peak-pressure assemblage garnet+omphacite+phengite+amphibole+dolomite quartz+rutile. Compositional isopleths for garnet and phengite constrain the pressure–temperature ( P–T ) conditions to 1.9–2.0 GPa and 720–730 °C, in good agreement with the results obtained from conventional thermobarometry (720–740 °C and 2.4–2.5 GPa). Further P–T pseudosection modelling of clinopyroxene+plagioclase±amphibole±clinozoisite symplectites after omphacite suggests that decompression to c. 1.2 GPa occurred along a steep exhumation path. The eclogite-bearing Richarddalen Complex constitutes the uppermost unit of a simple stack of thrust sheets where the metamorphic grade is increasing structurally upwards in the pile. Thrusting is the favoured uplift mechanism for the initial syn-orogenic exhumation to lower crustal levels. Constrictional north–south stretching in a transpressional regime is interpreted to be responsible for the final exhumation of the assembled stack of thrust sheets. Late Silurian–Early Devonian conglomerates were deposited directly on the eclogite-bearing gneisses of the Richarddalen Complex, and mark the end of exhumation of the nappe stack.

Abstract A transition from gabbro to eclogite has been investigated at Vinddøldalen in south-central Norway, with the aim to link reaction textures to metamorphic zircon growth and to obtain a direct U–Pb zircon age of the metamorphic process. In the different rocks of the transition zone zircon occurs as (I) igneous prismatic grains, (II) metamorphic polycrystalline rims and pseudomorphs after baddeleyite, and (III) as tiny (<10 µm) bead-like zircon grains. Textural relations suggest that type II zircon formed by breakdown of baddeleyite in the presence of silica, whereas Fe–Ti oxides were the main Zr source for the type III zircon. Subsolidus liberation of Zr and formation of bead zircon took place by oxyexsolution of titanomagnetite during fluid-assisted metamorphism, and by resorption of Fe–Ti oxide in rock domains that were completely recrystallized to eclogite. SIMS (secondary ion mass spectrometry) and TIMS (thermal ionization mass spectrometry) dating provides comparable U–Pb ages of magmatic zircon and baddeleyite. Baddeleyite (TIMS) yielded an age of 1457±11 Ma for the gabbro emplacement. Bead-type metamorphic zircon from eclogite gave 425±10 Ma (TIMS) dating the metamorphic transition from gabbro to eclogite in the upper basement of the Lower Allochthon in the south-central Scandinavian Caledonides.

Abstract The Flatraket Complex, in the ultra-high-pressure (UHP) domains of the Western Gneiss Region (WGR) of Norway, preserves granulite facies assemblages, which were locally overprinted by eclogite and amphibolite facies metamorphism. Zircon and monazite indicate magmatic crystallization of the rocks at 1680–1640 Ma and constrain the timing of the granulite facies overprint at 1100 Ma. This age is older than previously reported ages of 1000–950 Ma for regional metamorphism reaching anatexis and locally granulite facies in the WGR. The granulites at Flatraket may have developed as a consequence of local metasomatism, perhaps linked to metasomatism occurring at the same time in the nearby Sandvik peridotite. Granitic rocks from neighbouring Kråkeneset indicate magmatic emplacement at ≥1650 Ma, during the event that formed the Flatraket Complex and the bulk of the WGR. A gabbro body at Kråkeneset is dated at 1255±8 Ma by baddeleyite, which was not affected by the granulite event, implying that the rock remained impermeable to fluids, reacting instead to some degree during the Caledonian UHP event.

Abstract Norwegian eclogite-facies shear zones in the Lofoten islands served as major pathways for short-lived pulses of both 40 Ar-rich and 40 Ar-poor hot fluids during eclogitization and retrogression events related to the Caledonian orogeny. The open system for Ar affected most of the minerals leading to old 40 Ar/ 39 Ar ages, particularly in trioctahedral micas. 40 Ar concentration in the fluids appears to decrease during the amphibolite-facies retrogression and Ca-rich amphiboles yield 40 Ar/ 39 Ar ages of c. 415 Ma (Scandian event). Late- and post-Caledonian 40 Ar/ 39 Ar muscovite and K-feldspar ages and Rb/Sr biotite ages coincide with multiple extensional events, fluid infiltration and thermal activity during the final exhumation of the crustal rocks, potentially reflecting major tectonic episodes such as rifting of Pangaea and seafloor spreading between Europe and North America. Supplementary material: Sample coordinates from all the rocks, analytical methods, complete 40 Ar/ 39 Ar step-heating descriptions and electron microprobe results are available at http://www.geolsoc.org.uk/SUP18687 .