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- Andersson, Jenny, et al.
(författare)
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Zircon geochronology of migmatite gneisses along the Mylonite Zone (S Sweden): a major Sveconorwegian terrane boundary in the Baltic Shield
- 2002
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Ingår i: Precambrian Research. - 0301-9268. ; 114:1-2, s. 121-147
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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- Bergelin, Ingemar, et al.
(författare)
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Mesozoic rift magmatism in the North Sea region: Ar-40/Ar-39 geochronology of Scanian basalts and geochemical constraints
- 2011
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Ingår i: International Journal of Earth Sciences. - : Springer Science and Business Media LLC. - 1437-3254 .- 1437-3262. ; 100:4, s. 787-804
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Tidskriftsartikel (refereegranskat)abstract
- More than 100 volcanic necks composed of basanites and melanephelinites occur in Scania, southern Sweden, at the junction of two major tectonic lineaments, the Phanerozoic Sorgenfrei-Tornquist Zone (STZ) and the Proterozoic Protogine Zone. New Ar-40/Ar-39 isotope analyses of whole rock fragments of nine selected basalt necks suggest that the Mesozoic alkaline volcanism in the Scanian province commenced earlier than previously reported and comprised three separate volcanic episodes that span a total period of ca. 80 Myr: a first Jurassic (191-178 Ma), a second at the Jurassic/Cretaceous boundary (ca. 145 Ma), and a final middle Cretaceous episode (ca. 110 Ma). The new results allow for precise time correlations between eruption events in the Scanian and those in the North Sea volcanic provinces. The older, early Jurassic event in Scania is largely synchronous with that in the Egersund Basin and the Forties field whereas the event at ca. 145 Ma is correlated with activity in the Central Graben. These volcanic episodes also correlate in age with Kimmerian tectonic activity. Volcanic activity in the middle Cretaceous period has also been dated in the triple junction in the North Sea and offshore in the Netherland Sector. The correlation of basalt volcanism in Scania with the Egersund nephelinites strongly suggest that volcanism was triggered by repeated tectonic activity along the STZ. Geochemical data of alkaline mafic rocks in the Scanian and the North Sea volcanic provinces imply that different provinces have largely unique geochemical signatures in favour of a heterogeneous mantle in the North Sea volcanic region. However, basalts of different generations in one and the same province cannot be readily separated on the basis of geochemistry, suggesting that the same lithospheric mantle was the source of repeated volcanism over time in each province. The data suggest a low degree of melting of a volatile-bearing mantle lherzolite enriched in incompatible elements with the exception of the Forties basalts in the rift centre, produced by larger degree of melting and evolved by fractional crystallization.
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- Eneroth, Erik, et al.
(författare)
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Determination of magnetic properties of rocks by analysis of demagnetization curves: Hematite-ilmenite bearing rocks from SW Sweden
- 2008
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Ingår i: Journal of Geophysical Research. - 2156-2202. ; 113:B2, s. 24-02101
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Tidskriftsartikel (refereegranskat)abstract
- Granulites from the high-grade Sveconorwegian (corresponding to Grenvillian) gneiss terrane of SW Sweden were investigated in order to relate magnetic properties to the granulite facies mineralogy and the amphibolite facies retrograde mineralogy. We used standard paleomagnetic techniques, susceptibility measurements, electron and optical microscopy, and a new way to analyze demagnetization curves based on vector difference sums. The new technique allowed quantitation of the relative contributions of partial natural remanent magnetization (NRM) components as well as numerical extraction of thermal and coercive force (alternating field, AF) unblocking data. Two major components were identified, one stable which is carried by hematite-ilmenite, and one less stable carried by multidomain (MD) magnetite. The age of the hematite-ilmenite component is circa 930 Ma, and it has steep negative inclination, coercivities in excess of 150 mT, and unblocking temperatures between 540 degrees C and 640 degrees C. Two corresponding poles are reported (1) paleolatitude/paleolongitude (plat/plong) (218 degrees/- 24.3) with Dp/Dm (9.4 degrees/9.9 degrees) (paleopole) and (2) plat/plong (203 degrees/-51 degrees) with D-p/D-m (4.4 degrees/5.6 degrees) (VGP). The MD magnetite component is a recent or Cretaceous-Paleogene, steeply positive inclined NRM component associated with strong induced magnetization (plat/plong 2 degrees/-74 degrees and D-p/D-m 11 degrees/14 degrees (VGP). The MD magnetite was unblocked at conditions of 6-40 mT and 60-400 degrees C. Our new analysis method was successful in showing that retrograde amphibole formation strongly decreases the importance of the hematite-ilmenite component on behalf of the MD magnetite. It has a diminishing effect on a negative aeromagnetic anomaly in the area. The 930 Ma components constitute 80-0% of the vector difference sums depending on rock composition. However, as this fraction goes down to 60%, the induced magnetization seems to outweigh the hematite-ilmenite influence on aeromagnetic anomalies due to its opposite direction. These rocks still have a strong and stable hematite-ilmenite dominated NRM.
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- Estifanos, Benyam, et al.
(författare)
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A PIXE study of clouded plagioclase from southern Sweden
- 1997
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Ingår i: Nuclear Instruments & Methods in Physics Research. Section B: Beam Interactions with Materials and Atoms. - 0168-583X. ; 129, s. 83-91
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Tidskriftsartikel (refereegranskat)
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