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Geochronology of high-pressure mafic granulite dykes in SW Sweden; tracking the P-T-t path of metamorphism using Hf isotopes in zircon and baddeleyite

Söderlund, Ulf LU ; Fredrik, Hellström and Sandra, Kamo (2008) In Journal of Metamorphic Geology 26(5). p.539-560
Abstract
Although the U-Pb zircon chronometer has been widely used for dating metamorphism in moderate to high-grade rocks, it is generally difficult to link the U-Pb age of zircon to specific metamorphic reactions. In this study, the initial Hf isotopic composition of secondary zircon is compared with the evolution of Hf isotopic composition of the bulk sample, back-projected from the measured value through time. This approach may enhance the interpretation of radiometric ages performed on metamorphic mineral assemblages. Here, U-Pb, Sm-Nd and Lu-Hf geochronology and thermobarometry have been integrated and applied to two metamorphosed diabase dykes in the Sveconorwegian orogen, SW Sweden. The dykes are located ~ 5 km east of the NNE-trending Göta... (More)
Although the U-Pb zircon chronometer has been widely used for dating metamorphism in moderate to high-grade rocks, it is generally difficult to link the U-Pb age of zircon to specific metamorphic reactions. In this study, the initial Hf isotopic composition of secondary zircon is compared with the evolution of Hf isotopic composition of the bulk sample, back-projected from the measured value through time. This approach may enhance the interpretation of radiometric ages performed on metamorphic mineral assemblages. Here, U-Pb, Sm-Nd and Lu-Hf geochronology and thermobarometry have been integrated and applied to two metamorphosed diabase dykes in the Sveconorwegian orogen, SW Sweden. The dykes are located ~ 5 km east of the NNE-trending Göta Älv deformation zone in the Idefjorden terrane, and trend parallel to this zone. The Lunden dyke is recrystallized into a coronitic granulite-facies assemblage. U-Pb isotopic analyses of baddeleyite in this dyke indicate an emplacement age of ~1300 Ma. Thermobarometric techniques applied to garnet and omphacitic clinopyroxene coronas indicate high-pressure metamorphism at c. 15 kbar and c. 740 °C. Growth of polycrystalline zircon at the expense of baddeleyite occurred at 1046 ± 6 Ma. Identical Hf isotopic composition of polycrystalline zircon and baddeleyite shows that the baddeleyite-to-zircon transition took place before Hf equilibration among the other metamorphic minerals and, hence the c. 1046 Ma age of polycrystalline zircon sets an upper age limit of metamorphism of this sample. The Haregården dyke is recrystallized into a granoblastic transitional upper amphibolite to granulite-facies assemblage. The estimated P-T conditions are c. 10 kbar and c. 700 °C. Analyses of small (~30 µm), clear and round zircon in this sample yield a Concordia U-Pb age of 1026 ± 4 Ma, which is indistinguishable from the Lu-Hf and Sm-Nd mineral isochron ages of 1027 ± 9 Ma and 1022 ± 34 Ma. This type of secondary zircon plots at the lower end of the Lu-Hf isochron and indicates simultaneous growth with garnet at c. 1026 Ma, a time when Hf isotopic equilibrium among minerals must have been reached. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
thermobarometry, high-pressure granulite, U-Pb, Lu-Hf, baddeleyite, Sm-Nd
in
Journal of Metamorphic Geology
volume
26
issue
5
pages
539 - 560
publisher
Wiley-Blackwell
external identifiers
  • wos:000256122200004
  • scopus:44349131560
ISSN
0263-4929
DOI
10.1111/j.1525-1314.2008.00776.x
language
English
LU publication?
yes
id
ce4e04df-bebe-4e61-9b52-ecc5de28a882 (old id 1151855)
date added to LUP
2008-09-15 13:46:13
date last changed
2017-07-02 04:03:52
@article{ce4e04df-bebe-4e61-9b52-ecc5de28a882,
  abstract     = {Although the U-Pb zircon chronometer has been widely used for dating metamorphism in moderate to high-grade rocks, it is generally difficult to link the U-Pb age of zircon to specific metamorphic reactions. In this study, the initial Hf isotopic composition of secondary zircon is compared with the evolution of Hf isotopic composition of the bulk sample, back-projected from the measured value through time. This approach may enhance the interpretation of radiometric ages performed on metamorphic mineral assemblages. Here, U-Pb, Sm-Nd and Lu-Hf geochronology and thermobarometry have been integrated and applied to two metamorphosed diabase dykes in the Sveconorwegian orogen, SW Sweden. The dykes are located ~ 5 km east of the NNE-trending Göta Älv deformation zone in the Idefjorden terrane, and trend parallel to this zone. The Lunden dyke is recrystallized into a coronitic granulite-facies assemblage. U-Pb isotopic analyses of baddeleyite in this dyke indicate an emplacement age of ~1300 Ma. Thermobarometric techniques applied to garnet and omphacitic clinopyroxene coronas indicate high-pressure metamorphism at c. 15 kbar and c. 740 °C. Growth of polycrystalline zircon at the expense of baddeleyite occurred at 1046 ± 6 Ma. Identical Hf isotopic composition of polycrystalline zircon and baddeleyite shows that the baddeleyite-to-zircon transition took place before Hf equilibration among the other metamorphic minerals and, hence the c. 1046 Ma age of polycrystalline zircon sets an upper age limit of metamorphism of this sample. The Haregården dyke is recrystallized into a granoblastic transitional upper amphibolite to granulite-facies assemblage. The estimated P-T conditions are c. 10 kbar and c. 700 °C. Analyses of small (~30 µm), clear and round zircon in this sample yield a Concordia U-Pb age of 1026 ± 4 Ma, which is indistinguishable from the Lu-Hf and Sm-Nd mineral isochron ages of 1027 ± 9 Ma and 1022 ± 34 Ma. This type of secondary zircon plots at the lower end of the Lu-Hf isochron and indicates simultaneous growth with garnet at c. 1026 Ma, a time when Hf isotopic equilibrium among minerals must have been reached.},
  author       = {Söderlund, Ulf and Fredrik, Hellström and Sandra, Kamo},
  issn         = {0263-4929},
  keyword      = {thermobarometry,high-pressure granulite,U-Pb,Lu-Hf,baddeleyite,Sm-Nd},
  language     = {eng},
  number       = {5},
  pages        = {539--560},
  publisher    = {Wiley-Blackwell},
  series       = {Journal of Metamorphic Geology},
  title        = {Geochronology of high-pressure mafic granulite dykes in SW Sweden; tracking the P-T-t path of metamorphism using Hf isotopes in zircon and baddeleyite},
  url          = {http://dx.doi.org/10.1111/j.1525-1314.2008.00776.x},
  volume       = {26},
  year         = {2008},
}