Stefan Claesson

Isotopic evidence for the Precambrian provenance and Caledonian metamorphism of high grade paragneisses from the Seve Nappes, Scandinavian Caledonides

The Seve Nappes can be followed for more than 700 km along the Scandinavian Caledonides. Seve paragneisses are in part migmatitic, metamorphosed to granulite facies and eclogite-bearing. This probably reflects an ancient continent-continent collision. Zircon data presented here from a migmatitic paragneiss leucosome give almost concordant Caledonian U-Pb ages, and define together with strongly discordant zircons from the paragneiss a discordia line with the intercept ages 423±26 Ma and 1512±36 Ma. Strongly discordant zircon from another paragneiss, composed of cores overgrown with thick transparent mantles, also fits this chord. A stepwise dissolution experiment on these zircons demonstrates the existence of a component with Caledonian age in the crystals, and 3 of 5 dissolution steps plot along the discordia line defined by the conventional analyses of the same sample. The zircon data reflect high grade Caledonian metamorphism of sediments with Proterozoic protoliths. Sm-Nd model ages (TDM and TCHUR) for whole rock samples cluster around 1.9 and 1.55 Ga, respectively, demonstrating a Proterozoic average crustal provenance age for the detritus in these sediments. The zircon data support a correlation between the Seve Nappes and the Western Gneiss Region in Norway.

Svecofennian detrital zircon ages: implications for the Precambrian evolution of the Baltic Shield

Metasediments intruded by 1.90-1.87 Ga old plutonic rocks form the oldest major Proterozoic crustal component in the Svecofennian Domain of the Baltic (Fennoscandian) Shield. Their NdTDM model ages and conventional multigrain zircon UPb ages between 2.4 and 2.1 Ga have previously been interpreted either as mixing ages between ∼ 1.9 Ga old juvenile materials and a minor Archaean component, or as actual rock and protolith ages. To resolve the ensuing controversy, 120 individual detrital zircons from Svecofennian metasediments in Sweden and Finland were analysed using the SHRIMP ion microprobe. The oldest materials in this array are a 3.44 Ga old zircon from the Tampere Schist Belt in Finland and a 3.32 Ga old crystal from southeastern Sweden. About 30% of the analysed crystals are 2.97-2.60 Ga old, while ∼ 65% have ages between 2.12 and 1.88 Ga. Thus there is no evidence of 2.6-2.1 Ga old protoliths, but the age range of the Proterozoic zircons indicates that a major area of 2.1-1.9 Ga old crust was in erosional position 1.9 Ga ago. This implies that the formation of Palaeoproterozoic crust in the Baltic Shield or its one-time close neighbourhood must have commenced 100–200 Ma earlier than hitherto assumed. In conjunction with previously obtained isotopic data, the youngest detritus ages of the present study constrain the age of Svecofennian sedimentation. It can also be concluded that the Archaean zircons found in quartzites from southern Sweden may have been derived from source areas to the southwest of the central-Svecofennian marine depositional basin, the so-called Bothnian Basin, separating southern Sweden from the Archaean craton in the northeastern part of the Shield.

Delineation and character of the Archaean-Proterozoic boundary in northern Sweden

Abstract Before the deposition of a Proterozoic cover and the repeated Proterozoic reworking of the older rocks, the presently exposed Archaean areas in northern Sweden formed part of a coherent craton. In the present study, we have used SmNd isotopic analyses of Proterozoic granitoids and metavolcanics to delineate the Archaean palaeoboundary. In a regional context, the transition from strongly negative ϵ Nd( t ) values in the northeast to positive values in the southwest is distinct, and approximately defines the border of the old craton. The Archaean palaeoboundary extends in a WNW direction, and is subparallel to the longitudinal axis of the Skellefte sulphide ore district but it is situated ∼ 100 km farther to the north. The ∼ 1.9 Ga old granitoids on the two sides of the palaeoboundary were all formed in compressional environments, but those situated to the north have higher contents of LILE and LREE at similar contents of Si. This indicates that they were generated in an area with thicker crust and supports the location of the Archaean-Proterozoic palaeoboundary. There is no simple correlation between the Archaean palaeoboundary, as defined by the isotopic results, and any of the major fracture systems as interpreted from regional geophysical measurements. Reflection seismic work indicates that juvenile volcanic-arc terrains to the south have been thrust onto the Archaean craton. Possible thrust faults have been identified from aeromagnetic measurements. Rifting of the Archaean craton created a passive margin ∼ 2.0 Ga ago. Spreading shifted to convergence with subduction beneath the Archaean continent ∼ 1.9 Ga ago. Subsequently, the resulting juvenile volcanic arc collided with the old continent, and the Archaean palaeoboundary as existing today was formed by a collision characterized by overthrusting. The boundary then was disturbed by later deformation predominantly along NNE-trending fracture systems.

Nd isotope data on 1.9-1.2 ga old basic rocks and metasediments from the bothnian basin, central sweden

Abstract SmNd data are presented for basic rocks and metagreywackes from the early Proterozoic Bothnian basin in central Sweden. The basic rocks include synsedimentary metabasalts, early orogenic intrusions, and anorogenic dolerites, covering the age range 1.9-1.2 Ga. The samples thus span the entire period from the early part of the Svecokarelian thermo-tectonic episode to the time when this region of the Baltic Shield had been a stable craton for about 0.4 Ga. All these samples have slightly positive ϵNd-values, up to +2 for synorogenic rocks and up to +3.5 for anorogenic dolerites. If these values are representative for the contemporaneous mantle, they suggest a less depleted character for this mantle than that shown for several other Proterozoic shield areas. Three metagreywacke samples have ϵNd-values of −2.4 to −3.1 indicating a significant influx of detrital older crustal material into the Bothnian basin.

A Rb-Sr isotope study of granitoids and related mylonites in the Tännäs Augen Gneiss Nappe, southern Swedish Caledonides

Abstract The Tannas Augen Gneiss Nappe and parts of the related, subjacent Veman Nappe are composed of K-feldspar megacryst bearing granodiorites, which have been subject to low-grade metamorphism and varying degrees of deformation. They form parts of the Middle Allochthon — a long-transported nappe unit in the central Scandinavian Caledonides. The least-deformed granodiorite has a Rb-Sr age of 1610 ± 85 Ma and Sr1 = 0.7029, while zircons from the same rock give a discordia line with an upper intercept at 1685 ± 20 Ma. These dates reflect the age of intrusion of the rock. The granodiorite is comparable in composition and age with post-Svecokarelian granitoids and related volcanites east of the mountain belt. The more typically deformed augen gneiss does not yield isochrons, but shows disturbed Rb-Sr patterns both for specimens sampled regionally and on smaller scales. At the thrust contacts towards over- and underlying units, where the deformation was extreme, the gneisses have been transformed to dense m...

The age of the Ottfjället dolerites of Särv Nappe, Swedish Caledonides

Abstract Preliminary results of K-Ar and Rb-Sr age determinations of the Ottfjallet dolerites give K-Ar ages ranging from 650 to 2600 Ma and a Rb-Sr whole-rock isochron age of 735±260 Ma. The latter age is interpreted as the age of the dolerite intrusion. It is also suggested that the high K-Ar ages are due to excess Ar.

Caledonian metamorphism of Proterozoic Seve rocks on Mt. Åreskutan, southern Swedish Caledonides

Abstract Areskutan is a type locality for the complex of Seve nappes, which covers vast areas in the Swedish Caledonides. Here, an upper tectonic unit, the Areskutan Nappe, composed of migmatitic gneisses and metabasites metamorphosed in granulite facies, is separated by a thrust from a lower Seve unit. Various rocks in the Areskutan Nappe have been investigated using Rb-Sr and U-Pb techniques. Conventional Rb-Sr whole-rock analyses fail to yield isochrons but show that the rocks are clearly Precambrian. Small-scale whole-rock samples from the neosome part of a 2 kg migmatized metabasite specimen define an isochron with the age 414 ± 27 Ma, while paleosome samples from the same specimen scatter about this isochron. The age is considered to date the migmatization. A late, cross-cutting pegmatite gives an age of 395 ± 40 Ma, and minerals separated from a gneissic granite give 390 ± 45 Ma. Zircon from gneissic granite and from metabasite give a discordia line with lower and upper intercept ages of 386 ± 12 a...

Proterozoic and Archaean ages of detrital zircon from the Palaeoproterozoic Västervik Basin, SE Sweden: Implications for provenance and timing of deposition

Abstract Ages of detrital zircons, derived from Palaeoproterozoic metasedimentary rocks from Finland and Sweden are poorly represented in the presently exposed crust in the Baltic Shield. This study reports U-Pb ages of detrital zircons from the Svecofennian Västervik Basin. 41 spots from 31 zircon crystals were dated using U-Pb geochronology at the NORDSIM ion microprobe in Stockholm. Most analyses are concordant and the zircon grains commonly display well-developed magmatic oscillatory zoning. The ages documented are: ∼3.64 Ga, 3.03-2.95 Ga, 2.72-2.69 Ga, 2.12-1.87 Ga and 1.84 Ga. 75% of the grains are Palaeoproterozoic and 25% are Archaean. Ages gained from Proterozoic metasediments in Sweden, Finland, Svalbard, Greenland and Great Britain also report a large proportion of ∼2.1-1.9 Ga and a smaller proportion of Archaean zircons with ages around 2.7 and 3 Ga. These age groups probably represent major crust forming events. The here presented results provide an estimate of the time of deposition in the Västervik Basin for the time interval of 1882–1850 Ma, constrained by two concordant zircon analyses of 1872±24 and 1870±12 Ma, and the newly presented 1859±9 Ma age for the Loftahammar granite that intrudes the metasedimentary succession in the north. The young detrital zircon age of 1837±22 Ma might suggest that parts of the basin may be younger. As input of detrital grains may occur from several sources simultaneously (e.g. by rivers and by tidal currents from a marine source), the detrital grains were sampled from different depositional environments. Main fluvial sediment transport in the Västervik Basin was from present north whereas the tidal sediment transport was from the present south. The age groups documented in the Västervik Basin are poorly represented in the presently exposed crust in the Baltic Shield, but are represented in Sarmatia.

The oldest crust in the Ukrainian Shield – Eoarchaean U–Pb ages and Hf–Nd constraints from enderbites and metasediments

Abstract The oldest crust in the Ukrainian Shield occurs in the Podolian and Azov domains, which both include Eoarchaean components. U–Pb age data for Dniestr–Bug enderbites, Podolian Domain, indicate that these are c. 3.75 Ga old, and Lu–Hf isotope data indicate extraction from chondritic to mildly isotopically depleted sources with ɛHf up to c. +2. Nd model ages support their Eoarchaean age, while model ages for Dniestr–Bug metasedimentary gneisses indicate that these also include younger crustal material. Most of the Hf-age data for metasedimentary zircon from the Soroki greenstone belt, Azov Domain, reflects Eoarchaean primary crustal sources with chondritic to mildly depleted Hf isotope signatures at 3.75 Ga. A minor portion is derived from Mesoarchaean crust with a depleted ɛHf signature of c. +4 at 3.1 Ga. U–Pb zircon ages from Fedorivka greenstone belt metasediments are consistent with the Soroki age data, but also include a 2.7–2.9 Ga component. Nd whole rock model ages provide support for a younger crustal component in the latter. Both domains have been subject to Neoarchaean, c. 2.8 Ga, and Palaeoproterozoic, c. 2.0 Ga, metamorphism. The spatial distribution indicates that the Podolian and Azov domains evolved independently of each other before the amalgamation of the Ukrainian Shield.

Tectonometamorphic evolution of the Åreskutan Nappe–Caledonian history revealed by SIMS U–Pb zircon geochronology

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.