| 1. |
- Högdahl, Karin, et al.
(author)
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Geological relations and U-Pb geochronology of Hyttsjo granites in the Langban-Nordmark area, western Bergslagen, Sweden
- 2007
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In: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 129:1, s. 43-54
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Journal article (peer-reviewed)abstract
- The Hyttsjo granites occur in the extensively mineralised Langban-Nordmark area in the westernmost part of the Bergslagen ore province. They have been classified as late Svecokarelian granites due to their homogeneous and generally isotropic appearance in addition to a WR Rb/Sr age. Moreover, they have been considered as possible candidates for supplying essential metals to epigenetic mineralisation in this classic ore district. Two Hyttsjo granites yield U-Pb zircon data ages of 1791 2 and 1793 3 Ma, respectively, which overlap with emplacement ages of the adjacent 1.80-1.78 Ga Filipstad suite belonging to the Transscandinavian Igneous Belt (TIB). Mafic rocks occur quite abundantly associated with the Filipstad-type granite (sensu lato) and various types of mafic enclaves as well as hybrid rocks are present, suggesting a co-magmatic origin. Such mafic intrusives are also exposed in the vicinity of most known Hyttsjo-type plutons. Not least our observations that the former exhibit back-veining by granitic melts suggest intimate causal and temporal relationships between granite formation and mafic TIB rocks. The Hyttsjo granites were probably produced through partial melting related to the intrusion of hot, mafic magmas in and along the border between the TIB and the Svecofennian supracrustal and subvolcanic rocks. Thus, all available observations and data suggest that the Hyttsjo granites are intimately related to and most probable a product of TIB magmatism. Also, they do not lend any support for the hypothesis that the formation of the Hyttsjo granites represent a temporally separate intrusive episode. The Hyttsjo granites are therefore unlikely to be discernibly responsible for specific mineralisation in this area.
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| 2. |
- Bergman, S., et al.
(author)
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Transpressive shear related to arc magmatism: The Paleoproterozoic Storsjön-Edsbyn Deformation Zone, central Sweden
- 2006
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In: Tectonics. - 0278-7407 .- 1944-9194. ; 25
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Journal article (peer-reviewed)abstract
- The polyphase, 15–30 km wide and >200 km long Storsjön-Edsbyn Deformation Zone (SEDZ) apparently separates two major Paleoproterozoic provinces in central Sweden. Four main phases of deformation have been recognized along the zone; associated high-strain rocks are characterized by their mineralogy, microstructures, kinematic patterns, magnetic signatures, and relative ages. The mineral and stretching lineations are consistently steeply to moderately plunging. Analysis of mesoscopic shear zone populations of different generations suggests that most displacements are compatible with north-south to northeast-southwest bulk regional shortening. The structural analysis suggests that the main ductile deformation along the SEDZ was due to dextral transpression with a steep coaxial component. Field evidence combined with published age data constrain the main deformation to the time interval circa 1.7–1.3 Ga. Synkinematic titanite from a ductile protomylonite in the SEDZ yields an U-Pb age of 1674 ± 6 Ma. The combined results imply transpressive deformation broadly synchronous with, and spatially related to, volcanism and plutonism in a continental margin magmatic arc setting.
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| 3. |
- Högdahl, Karin, et al.
(author)
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Continental margin magmatism and migmatisation in the west-central Fennoscandian Shield
- 2008
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In: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 102:3-4, s. 435-459
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Journal article (peer-reviewed)abstract
- The Ljusdal Batholith (LjB) is a major component of the central Svecofennian Domain in Sweden. It is separated from the Bothnian Basin to the north by the 1.82-1.80 Cia crustal-scale Hassela Shear Zone (HSZ). The LjB has emplacement ages of 1.86-1.84 Cia, is mainly alkali-calcic, metaluminous, has epsilon(Nd) values between -0.3 and + 1.2 and was formed in a magmatic arc setting. During the Svecokarelian orogeny the LjB was affected by at least three fold episodes. Large-scale folded screens of migmatised metasedimentary rocks occur in the eastern part of the batholith, and to the north of the HSZ, there is a 50 km wide diatexite belt. The Transition Belt (TrB), consisting of 1.88-1.85 Ga granitoids, is located at the northwestern extension of this belt. A calc-alkaline and peraluminous composition combined with negative epsilon(Nd) values (- 1.7 to -0.8) indicates a large proportion of metasediments in the source for these granitoids. U-Pb SIMS data on zircon rims from migmatites and leucogranites to the north and east of LjB yield ages of 1.87-1.86 Ga, i.e. coeval with the granitoids of the LjB and the TrB. There is thus a close relationship between the LjB, the TrB and the migmatites in both space and time. Syn-migmatitic shearing along the HSZ indicates that a proto-HSZ was initiated already at c. 1.86 Ga, and the location of the proto-HSZ is inferred to be controlled by two older nuclei present in the lower parts of the crust. As crustal-scale shear zone systems are known to act as ascent pathways for sheet-like flow in active orogenies the TrB may represents accumulations of melts that were attracted and extracted by the proto-HSZ and intruded in a block that was not pervasively affected by subsequent shear along the HSZ. An active continental margin setting for the LjB implies subduction at c. 1.86 Ga, and provides a heat source for both the migmatites and the TrB. A later migmatisation at 1.82 Ga has been recorded to the south of the HSZ. Within the LjB the 1.82 Ga stromatic migmatites are folded by F-2 folds, and the fabric is truncated by 1.80 Cia pegmatites.
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| 4. |
- Högdahl, Karin, et al.
(author)
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Ductile shear zones related to crustal shortening and domain boundary evolution in the central Fennoscandian Shield
- 2009
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In: Tectonics. - 0278-7407 .- 1944-9194. ; 28, s. TC1003-
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Journal article (peer-reviewed)abstract
- The Paleoproterozoic part of the Fennoscandian Shield is composed of crustal components formed in different tectonic settings and generally separated by well-defined shear zone systems. An anomalous transitional boundary has been investigated by integrating structural analysis and geochronology with published geophysical data. The nature of this boundary is interpreted to be a consequence of an apparent stacking in the lower and middle crust initiating 1.87–1.86 Ga dextral shear along the Gävle-Rättvik Zone (GRZ) and adjacent shear zones, resulting in an arcuate northern boundary of the Bergslagen province. This boundary coincides with geophysical anomalies and temporal and metamorphic breaks. Owing to continuous convergence the pure-shear overprint component increased on the GRZ and caused a shift of dextral shear to the Hagsta Gneiss Zone with recorded shear at 1809 ± 2 Ma. Most likely, both these structures are related to coeval shear zones farther to the east as a part of an ∼1500 km long crustal, or possibly terrane, boundary.
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