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Zircon growth during progressive recrystallization of Gabbro to Garnet Amphibolite, Eastern Segment, Sveconorwegian orogen

Beckman, Victoria LU ; Möller, Charlotte LU ; Söderlund, Ulf LU and Andersson, Jenny (2017) In Journal of Petrology 58(1). p.167-188
Abstract

The interpretation of whether a dated metamorphic zircon generation grew during the prograde, peak or retrograde stage of a metamorphic cycle is critical to geological interpretation. This study documents a case at Herrestad, in the eastern part of the 1±0 Ga Sveconorwegian Province, involving progressive metamorphic recrystallization of gabbro to garnet amphibolite and associated behaviour of Zr-bearing minerals. In this case, textures show that baddeleyite is by far the main source of Zr for metamorphic zircon growth. The amount of metamorphic zircon formed was primarily controlled by the degree of metamorphic recrystallization, which in turn was controlled by deformation and the presence of a fluid as a transport medium. Zircon in... (More)

The interpretation of whether a dated metamorphic zircon generation grew during the prograde, peak or retrograde stage of a metamorphic cycle is critical to geological interpretation. This study documents a case at Herrestad, in the eastern part of the 1±0 Ga Sveconorwegian Province, involving progressive metamorphic recrystallization of gabbro to garnet amphibolite and associated behaviour of Zr-bearing minerals. In this case, textures show that baddeleyite is by far the main source of Zr for metamorphic zircon growth. The amount of metamorphic zircon formed was primarily controlled by the degree of metamorphic recrystallization, which in turn was controlled by deformation and the presence of a fluid as a transport medium. Zircon in the Herrestad rocks shows a range of morphologies and internal textures at different degrees of metamorphic recrystallization. Igneous zircon occurs together with baddeleyite in coarse-grained olivine-free facies of the gabbro. Metamorphic polycrystalline zircon rims on baddeleyite and minute (<5 μm) bead-like zircon grains at Fe-Ti oxide boundaries characterize the transition to coronitic metagabbro. With increasing metamorphic recrystallization, polycrystalline zircon rims grow at the expense of baddeleyite and the amount of minute bead-like zircon increases, forming strings of zircon beads with increasing distance from Fe-Ti oxide grains. The progressive breakdown of baddeleyite results in polycrystalline zircon aggregates that become denser and finally form single grains in completely recrystallized garnet amphibolite. Late magmatic zircon crystallized at 156765 Ma, whereas metamorphic zircon dates amphibolite-facies metamorphic recrystallization at 97067 Ma. The Herrestad case illustrates a general rule that the bulk Zr budget in originally baddeleyite-bearing rocks will rapidly become locked into metamorphic zircon during the first event of metamorphic recrystallization, when silica and Zr are released from the igneous minerals. Incomplete metamorphic recrystallization and partial preservation of baddeleyite, however, also allows later stages of zircon formation. Thus, in incompletely reacted rocks the final result may be highly complex with microscale zircon of several age generations.

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organization
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Contribution to journal
publication status
published
subject
keywords
Baddeleyite, Gabbro, Metagabbro, Metamorphism, Sveconorwegian orogen, Zircon, Zircon formation
in
Journal of Petrology
volume
58
issue
1
pages
22 pages
publisher
Oxford University Press
external identifiers
  • scopus:85019139840
  • wos:000398117400008
ISSN
0022-3530
DOI
10.1093/petrology/egx009
language
English
LU publication?
yes
id
6563a6b3-ca3c-4184-85ab-11be64e09b47
date added to LUP
2017-06-08 08:31:50
date last changed
2017-09-18 11:39:16
@article{6563a6b3-ca3c-4184-85ab-11be64e09b47,
  abstract     = {<p>The interpretation of whether a dated metamorphic zircon generation grew during the prograde, peak or retrograde stage of a metamorphic cycle is critical to geological interpretation. This study documents a case at Herrestad, in the eastern part of the 1±0 Ga Sveconorwegian Province, involving progressive metamorphic recrystallization of gabbro to garnet amphibolite and associated behaviour of Zr-bearing minerals. In this case, textures show that baddeleyite is by far the main source of Zr for metamorphic zircon growth. The amount of metamorphic zircon formed was primarily controlled by the degree of metamorphic recrystallization, which in turn was controlled by deformation and the presence of a fluid as a transport medium. Zircon in the Herrestad rocks shows a range of morphologies and internal textures at different degrees of metamorphic recrystallization. Igneous zircon occurs together with baddeleyite in coarse-grained olivine-free facies of the gabbro. Metamorphic polycrystalline zircon rims on baddeleyite and minute (&lt;5 μm) bead-like zircon grains at Fe-Ti oxide boundaries characterize the transition to coronitic metagabbro. With increasing metamorphic recrystallization, polycrystalline zircon rims grow at the expense of baddeleyite and the amount of minute bead-like zircon increases, forming strings of zircon beads with increasing distance from Fe-Ti oxide grains. The progressive breakdown of baddeleyite results in polycrystalline zircon aggregates that become denser and finally form single grains in completely recrystallized garnet amphibolite. Late magmatic zircon crystallized at 156765 Ma, whereas metamorphic zircon dates amphibolite-facies metamorphic recrystallization at 97067 Ma. The Herrestad case illustrates a general rule that the bulk Zr budget in originally baddeleyite-bearing rocks will rapidly become locked into metamorphic zircon during the first event of metamorphic recrystallization, when silica and Zr are released from the igneous minerals. Incomplete metamorphic recrystallization and partial preservation of baddeleyite, however, also allows later stages of zircon formation. Thus, in incompletely reacted rocks the final result may be highly complex with microscale zircon of several age generations.</p>},
  articleno    = {egx009},
  author       = {Beckman, Victoria and Möller, Charlotte and Söderlund, Ulf and Andersson, Jenny},
  issn         = {0022-3530},
  keyword      = {Baddeleyite,Gabbro,Metagabbro,Metamorphism,Sveconorwegian orogen,Zircon,Zircon formation},
  language     = {eng},
  month        = {01},
  number       = {1},
  pages        = {167--188},
  publisher    = {Oxford University Press},
  series       = {Journal of Petrology},
  title        = {Zircon growth during progressive recrystallization of Gabbro to Garnet Amphibolite, Eastern Segment, Sveconorwegian orogen},
  url          = {http://dx.doi.org/10.1093/petrology/egx009},
  volume       = {58},
  year         = {2017},
}