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Continental margin magmatism and migmatisation in the west-central Fennoscandian Shield

Högdahl, Karin LU ; Sjöstrom, Håkan; Andersson, Ulf B and Ahl, Martin (2008) In Lithos 102(3-4). p.435-459
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... (More)
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. (Less)
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Contribution to journal
publication status
published
subject
keywords
migmatite, granitoid, Sm-Nd-Sr isotopes, geochemistry, Svecokarelian orogeny, U-PbSIMS and TIMS zircon geochronology
in
Lithos
volume
102
issue
3-4
pages
435 - 459
publisher
Elsevier
external identifiers
  • wos:000256823100003
  • scopus:42949126717
ISSN
0024-4937
DOI
10.1016/j.lithos.2007.07.019
language
English
LU publication?
yes
id
e4017642-92a7-445f-ba78-f30f32681138 (old id 1190602)
date added to LUP
2008-09-08 10:33:10
date last changed
2017-09-10 03:43:30
@article{e4017642-92a7-445f-ba78-f30f32681138,
  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.},
  author       = {Högdahl, Karin and Sjöstrom, Håkan and Andersson, Ulf B and Ahl, Martin},
  issn         = {0024-4937},
  keyword      = {migmatite,granitoid,Sm-Nd-Sr isotopes,geochemistry,Svecokarelian orogeny,U-PbSIMS and TIMS zircon geochronology},
  language     = {eng},
  number       = {3-4},
  pages        = {435--459},
  publisher    = {Elsevier},
  series       = {Lithos},
  title        = {Continental margin magmatism and migmatisation in the west-central Fennoscandian Shield},
  url          = {http://dx.doi.org/10.1016/j.lithos.2007.07.019},
  volume       = {102},
  year         = {2008},
}