In contrast to the prevailing arc-accretion hypothesis, it has recently been argued that the most extensive deformation and metamorphism (D
1) in the Skellefte region of north-central Sweden took ...place in older metasedimentary sequences before the eruption of the ∼1.90–1.87
Ga volcanic sequences. The new data presented here provide a basis for a refined hypothesis involving accretion of a pre-1.92
Ga marginal basin during an Early Svecofennian orogenic episode between ∼1.92–1.91
Ga.
To the south of the Skellefte district, in a part of the metasedimentary Robertsfors Group dominated by NE (D
1) structural trends, 40 SIMS analyses of zircon from a metasediment (Robertsfors-1) have been made. Fourteen analyses of overgrowths and structureless zircon grains provide strong evidence for a post-depositional metamorphic event dated to 1916
±
5
Ma
, which we correlate with the observed D
1 metamorphism and migmatisation. This age is indistinguishable from that of a corresponding event in the Vammala Migmatite Belt (VMB) in Finland. This is the most important crustal thickening episode in the evolution of the Svecofennian province, here distinguished as the early Svecofennian orogenic episode. Nine Archaean and 15 older Palaeoproterozoic (OP) detrital zircon grains also yielded data comparable to that from the VMB. The age of 1980
±
6
Ma obtained from the total OP population possibly indicates derivation from a single magmatic source area, with similarly aged metamorphic overgrowths on some of the grains.
Two sampled localities to the NE show D
1 schistosity overprinted by D
2 shears and granitic veining. Sample Rönnskär-3, from a relatively homogeneous granitoid, and sample Rönnskär-2, from a garnetiferous felsic layer in well-banded layered gneisses, both contain igneous zircon populations. These have been affected by D
1 and D
2 events but the SIMS data allow the age of magmatism in both rocks to be placed between 1940 and 1950
Ma, thus reinforcing the links to the Knaften/Storuman areas to the WNW. In the granitoid, six analyses from highly luminescent rims, yielding an imprecise age of 1856
±
25
Ma, are consistent with the earlier more precise
207Pb/
206Pb dating of the D
2 event at 1858
±
2 Ma. The D
2 event is also recorded by ID-TIMS analyses of monazite from five widely spaced localities in the Skellefte structural zone with ages ranging from 1849
±
3 to 1866
±
2
Ma. The D
2 deformation has similar characteristics in the Skellefte/Robertsfors and Tampere/Vammala areas. We suggest that it can be allocated to a second and Middle Svecofennian orogenic episode between 1.88 and 1.85
Ga, affecting a wide area.
It has been demonstrated that the protoliths of the Robertsfors Group were deposited before ∼1.95
Ga; the group was intruded by granitoids between ∼1.94 and 1.95
Ga; and it was strongly deformed and metamorphosed between ∼1.92 and 1.91
Ga. Proximity to the post-1.90
Ga Skellefte Group during the deposition of the latter is indicated for both Robertsfors and Knaften/Storuman areas by the presence of ∼1.90
Ga granitoids and ∼1.88
Ga conglomerates like those in the Skellefte district. The regional evidence for the continuity of metamorphic complexes over wide areas, and the similarity in pre- and post-depositional histories of the Robertsfors Group and the Vammala Migmatite Belt 300
km apart, support the hypothesis that accretion of a large (Svionian) marginal basin at ∼1.92–1.91
Ga provided the basement for the subsequent volcanism.
New single-grain and within-grain U-Pb zircon ages from the central Tauern Window help sorting out the time dimension among the various Variscan and pre-Variscan basement components that were ...strongly overprinted by Alpine orogeny. Single-grain isotope dilution (ID-TIMS) U-Pb zircon geochronology of three Basisamphibolit samples yield protolith formation ages of 351±2, 349±1 and 343±1 Ma. Laser ablation ICP-MS and ID-TIMS U-Pb detrital zircon dating of the Biotitporphyroblastenschiefer constrained the maximum time of sedimentation to between 362±6 Ma and 368±17 Ma. Paragneisses from the Zwölferzug yield maximum sedimentation ages from 345±5 Ma (ion microprobe data) to 358±10 Ma. Zircons from gabbroic clasts and detrital zircons from a meta-agglomerate from the Habach Phyllite give an upper intercept age of 536±8 Ma and a near-concordant age of 506±9 Ma, respectively. Hence, apart from the Habach Phyllite, the maximum sedimentation ages of the metasediments investigated range from Upper Devonian to Lower Carboniferous. Consequently, the Basisamphibolit, the Biotitporphyroblastenschiefer, and the paragneisses of the Zwölferzug form parts of the Variscan basement series. The Basisamphibolit (351-343 Ma) is distinct both in space and time of formation from the Zwölferzug garnet amphibolite (c. 486 Ma), which forms part of the pre-Variscan basement.PUBLICATION ABSTRACT
Extensive Sm–Nd isotope work has been carried out in northern Sweden with the intention of studying crustal reactivation and assimilation processes relative to juvenile arc accretion at an Archaean ...cratonic margin. Previous regional work has shown a gradual change from negative
ϵ
Nd(
t)
values in northernmost Sweden to positive values towards the Skellefte sulphide ore district farther south. Isotopic variation and geophysical data show a break in the character of the crust along a WNW-trending zone. It is suggested that this zone, which passes close to the towns of Luleå and Jokkmokk, represents the transition from the Archaean craton in the northeast to Proterozoic juvenile crust in the southwest. A profile across the eastern part of this transition zone has been studied in detail in the Luleå area, where Archaean rocks recently have been discovered. The distribution and character of the Archaean rocks have been studied. These occur as fragments, ranging in size from several km
2 to dm in scale, and consist of ca 2.7
Ga orthogneisses of granodioritic to dioritic composition and porphyritic granitoids of quartz monzodioritic to granodioritic composition. Plutonic rocks of ca 1.9
Ga age and Archaean rocks were analysed for major and trace elements, and for Sm–Nd isotopes. The calc-alkaline ca 1.9
Ga granitoids are dominated by granodiorites, tonalites, quartz diorites and quartz monzodiorites, and there is a distinct boundary rather than a transition zone between intrusions with postive
ϵ
Nd(
t)
values in the southwest and intrusions with negative
ϵ
Nd(
t)
values in the northeast. The locations of the outcropping Archaean fragments coincides with this distinct boundary, which is interpreted as a terrane boundary separating a ca 1.9
Ga juvenile arc terrane in the southwest from the Archaean continent in the northeast. A possible explanation for this is that the Proterozoic juvenile terrane was obducted onto the Archaean craton during the final stages of the Svecofennian orogeny.
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.
This paper presents U-Pb ages for an extended period of geological evolution between about 1860 and 1740 Ma in the northern part of the Fennoscandian Shield. It involved magmatism, ductile ...deformation and metamorphism along the border between Sweden and Finland. The study area separates into a western and an eastern domain roughly along a north-south trending structural boundary defined by the western margin of the Pajala shear zone. Approximately 1860-1850 Ma zircon ages from two granitoid rocks record contrasting post-emplacement deformation histories on either side of this boundary. While metamorphic monazite records c. 1850 Ma events in rocks of the western domain, metamorphic monazite and titanite as well as zircon overgrowths in the eastern domain verify deformation and high-grade metamorphism in the 1820-1780 Ma interval. This post-1820 Ma phase is most probably related to shear zones and to nearby intrusions. A titanite age at c. 1740 Ma in the eastern domain is suggested to exemplify a tectonic phase, which elsewhere in northern Fennoscandia is marked by fracturing, generation of pegmatites, and hydrothermal activity.
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