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Prograde metamorphic zircon formation in gabbroic rocks : The tale of microtextures

Beckman, Victoria LU and Möller, Charlotte LU (2018) In Journal of Metamorphic Geology 36(9). p.1221-1236
Abstract

U–Pb zircon dates of metagabbroic rocks, such as eclogite, mafic granulite, and garnet amphibolite, are used to constrain the timing of tectonometamorphic evolution in orogens worldwide. For such interpretation, however, it is imperative to define at which stage of the P–T evolution that zircon crystallization took place: the prograde, peak, or retrograde stage. In order to accurately interpret metamorphic zircon ages, it is necessary to assess how the zircon crystallized or recrystallized, as zircon can dissolve or grow under different metamorphic conditions. Zircon is robust to retrograde isotopic resetting under most crustal conditions, but equilibrium Zr mass balance models have suggested that zircon is largely produced during... (More)

U–Pb zircon dates of metagabbroic rocks, such as eclogite, mafic granulite, and garnet amphibolite, are used to constrain the timing of tectonometamorphic evolution in orogens worldwide. For such interpretation, however, it is imperative to define at which stage of the P–T evolution that zircon crystallization took place: the prograde, peak, or retrograde stage. In order to accurately interpret metamorphic zircon ages, it is necessary to assess how the zircon crystallized or recrystallized, as zircon can dissolve or grow under different metamorphic conditions. Zircon is robust to retrograde isotopic resetting under most crustal conditions, but equilibrium Zr mass balance models have suggested that zircon is largely produced during retrograde metamorphism. This study takes a textural approach and identifies and reviews zircon-forming textures and reactions in gabbro and metagabbro at different metamorphic grades, ranging from subgreenschist to upper amphibolite- and eclogite-facies, and at different stages of metamorphic recrystallization. The textural relationships demonstrate that, in metagabbro, metamorphic zircon grows during the early stage of metamorphic recrystallization, independent of pressure and temperature. The mode of zircon formation is remarkably similar throughout different stages of metamorphic recrystallization, and the most significant source of Zr is igneous baddeleyite. Hence, in contrast to the equilibrium mass balance model, most zircon in metagabbro forms by prograde metamorphic reactions that consume igneous phases, and not by late retrograde reactions, and the onset of zircon forming reactions is governed primarily by the introduction of a hydrous fluid, commonly accompanied by ductile deformation.

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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
baddeleyite, gabbro, metamorphism, prograde, zircon
in
Journal of Metamorphic Geology
volume
36
issue
9
pages
1221 - 1236
publisher
Wiley-Blackwell
external identifiers
  • scopus:85052476755
ISSN
0263-4929
DOI
10.1111/jmg.12443
language
English
LU publication?
yes
id
91d15d0a-6bc6-4d2d-82a9-3e5b4434136d
date added to LUP
2018-09-28 07:24:32
date last changed
2020-02-19 05:06:36
@article{91d15d0a-6bc6-4d2d-82a9-3e5b4434136d,
  abstract     = {<p>U–Pb zircon dates of metagabbroic rocks, such as eclogite, mafic granulite, and garnet amphibolite, are used to constrain the timing of tectonometamorphic evolution in orogens worldwide. For such interpretation, however, it is imperative to define at which stage of the P–T evolution that zircon crystallization took place: the prograde, peak, or retrograde stage. In order to accurately interpret metamorphic zircon ages, it is necessary to assess how the zircon crystallized or recrystallized, as zircon can dissolve or grow under different metamorphic conditions. Zircon is robust to retrograde isotopic resetting under most crustal conditions, but equilibrium Zr mass balance models have suggested that zircon is largely produced during retrograde metamorphism. This study takes a textural approach and identifies and reviews zircon-forming textures and reactions in gabbro and metagabbro at different metamorphic grades, ranging from subgreenschist to upper amphibolite- and eclogite-facies, and at different stages of metamorphic recrystallization. The textural relationships demonstrate that, in metagabbro, metamorphic zircon grows during the early stage of metamorphic recrystallization, independent of pressure and temperature. The mode of zircon formation is remarkably similar throughout different stages of metamorphic recrystallization, and the most significant source of Zr is igneous baddeleyite. Hence, in contrast to the equilibrium mass balance model, most zircon in metagabbro forms by prograde metamorphic reactions that consume igneous phases, and not by late retrograde reactions, and the onset of zircon forming reactions is governed primarily by the introduction of a hydrous fluid, commonly accompanied by ductile deformation.</p>},
  author       = {Beckman, Victoria and Möller, Charlotte},
  issn         = {0263-4929},
  language     = {eng},
  number       = {9},
  pages        = {1221--1236},
  publisher    = {Wiley-Blackwell},
  series       = {Journal of Metamorphic Geology},
  title        = {Prograde metamorphic zircon formation in gabbroic rocks : The tale of microtextures},
  url          = {http://dx.doi.org/10.1111/jmg.12443},
  doi          = {10.1111/jmg.12443},
  volume       = {36},
  year         = {2018},
}