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Lotta_2018-10

Charlotte Möller

Professor

Lotta_2018-10

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

Författare

  • Victoria Beckman
  • Charlotte Möller
  • Ulf Söderlund
  • Jenny Andersson

Summary, in English

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.

Avdelning/ar

  • Berggrundsgeologi

Publiceringsår

2017-01-01

Språk

Engelska

Sidor

167-188

Publikation/Tidskrift/Serie

Journal of Petrology

Volym

58

Issue

1

Dokumenttyp

Artikel i tidskrift

Förlag

Oxford University Press

Ämne

  • Geology

Nyckelord

  • Baddeleyite
  • Gabbro
  • Metagabbro
  • Metamorphism
  • Sveconorwegian orogen
  • Zircon
  • Zircon formation

Status

Published

ISBN/ISSN/Övrigt

  • ISSN: 0022-3530