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Metamorphic titanite-zircon pseudomorphs after igneous zirconolite

Urueña, Cindy L. LU orcid ; Möller, Charlotte LU and Plan, Anders LU orcid (2023) In European Journal of Mineralogy 35(5). p.773-788
Abstract

The formation of metamorphic zircon after baddeleyite is a well-known reaction that can be used to date the metamorphism of igneous silica-undersaturated rocks. By contrast, metamorphic minerals formed after igneous zirconolite have rarely been reported. In this paper, we document metamorphic titanite + zircon pseudomorphs formed from the metamorphic breakdown of igneous zirconolite in syenodiorite and syenite, in the southeastern Sveconorwegian Province, Sweden. Water-rich fluid influx during tectonometamorphism in epidote-amphibolite-facies metamorphic conditions caused the release of silica during a metamorphic reaction involving igneous feldspar and pyroxene and the simultaneous breakdown of igneous Zr-bearing phases. Typical... (More)

The formation of metamorphic zircon after baddeleyite is a well-known reaction that can be used to date the metamorphism of igneous silica-undersaturated rocks. By contrast, metamorphic minerals formed after igneous zirconolite have rarely been reported. In this paper, we document metamorphic titanite + zircon pseudomorphs formed from the metamorphic breakdown of igneous zirconolite in syenodiorite and syenite, in the southeastern Sveconorwegian Province, Sweden. Water-rich fluid influx during tectonometamorphism in epidote-amphibolite-facies metamorphic conditions caused the release of silica during a metamorphic reaction involving igneous feldspar and pyroxene and the simultaneous breakdown of igneous Zr-bearing phases. Typical titanite + zircon intergrowths are elongated or platy titanite crystals speckled with tiny inclusions of zircon. Most intergrowths are smaller than 15 μm; some are subrounded in shape. Locally, bead-like grains of titanite and zircon are intergrown with silicate minerals. The precursor igneous zirconolite was found preserved only in a sample of near-pristine igneous syenodiorite, as remnant grains of mainly < 2 μm in size. Two somewhat larger crystals, 8 and 12 μm, allowed semiquantitative confirmation using microprobe analysis. Analogous with zircon pseudomorphs after baddeleyite, titanite + zircon pseudomorphs after zirconolite potentially offer dating of the metamorphic reaction, although the small size of the crystals makes dating with today's techniques challenging. The scarcity of reports of zirconolite and pseudomorphs reflects that they are either rare or possibly overlooked.

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Contribution to journal
publication status
published
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in
European Journal of Mineralogy
volume
35
issue
5
pages
16 pages
publisher
Copernicus GmbH
external identifiers
  • scopus:85173247195
ISSN
0935-1221
DOI
10.5194/ejm-35-773-2023
language
English
LU publication?
yes
id
33361a84-beb8-4ec0-8eff-163f44aae496
date added to LUP
2023-12-18 09:40:54
date last changed
2023-12-18 09:42:57
@article{33361a84-beb8-4ec0-8eff-163f44aae496,
  abstract     = {{<p>The formation of metamorphic zircon after baddeleyite is a well-known reaction that can be used to date the metamorphism of igneous silica-undersaturated rocks. By contrast, metamorphic minerals formed after igneous zirconolite have rarely been reported. In this paper, we document metamorphic titanite + zircon pseudomorphs formed from the metamorphic breakdown of igneous zirconolite in syenodiorite and syenite, in the southeastern Sveconorwegian Province, Sweden. Water-rich fluid influx during tectonometamorphism in epidote-amphibolite-facies metamorphic conditions caused the release of silica during a metamorphic reaction involving igneous feldspar and pyroxene and the simultaneous breakdown of igneous Zr-bearing phases. Typical titanite + zircon intergrowths are elongated or platy titanite crystals speckled with tiny inclusions of zircon. Most intergrowths are smaller than 15 μm; some are subrounded in shape. Locally, bead-like grains of titanite and zircon are intergrown with silicate minerals. The precursor igneous zirconolite was found preserved only in a sample of near-pristine igneous syenodiorite, as remnant grains of mainly &lt; 2 μm in size. Two somewhat larger crystals, 8 and 12 μm, allowed semiquantitative confirmation using microprobe analysis. Analogous with zircon pseudomorphs after baddeleyite, titanite + zircon pseudomorphs after zirconolite potentially offer dating of the metamorphic reaction, although the small size of the crystals makes dating with today's techniques challenging. The scarcity of reports of zirconolite and pseudomorphs reflects that they are either rare or possibly overlooked.</p>}},
  author       = {{Urueña, Cindy L. and Möller, Charlotte and Plan, Anders}},
  issn         = {{0935-1221}},
  language     = {{eng}},
  month        = {{09}},
  number       = {{5}},
  pages        = {{773--788}},
  publisher    = {{Copernicus GmbH}},
  series       = {{European Journal of Mineralogy}},
  title        = {{Metamorphic titanite-zircon pseudomorphs after igneous zirconolite}},
  url          = {{http://dx.doi.org/10.5194/ejm-35-773-2023}},
  doi          = {{10.5194/ejm-35-773-2023}},
  volume       = {{35}},
  year         = {{2023}},
}