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Solid Earth An interactive open-access journal of the European Geosciences Union

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© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
19 Dec 2016
Review status
This discussion paper has been under review for the journal Solid Earth (SE). The revised manuscript was not accepted.
Interpretation of zircon corona textures from metapelitic granulites of Ivrea-Verbano Zone, Northern Italy: Two-stage decomposition of Fe-Ti oxides
Elizaveta Kovaleva1,2, Håkon Austrheim3, and Urs Klötzli2 1Department of Geology, University of the Free State, Bloemfontein, 9300, 205 Nelson Mandela Drive, Free State, South Africa
2Department of Lithospheric Research, Faculty of Earth Sciences, Geography and Astronomy, University of Vienna, Vienna, 1090, Althanstrasse 14, Austria
3Section of Physics of Geological processes, Department of Geoscience, University of Oslo, Oslo, 0316, Norway
Abstract. In this study we report the occurrence of zircon corona textures in metapelitic granulites of the Ivrea-Verbano Zone. Unusual zircon textures are spatially associated with Fe-Ti oxides and occur as (1) vermicular-shaped aggregates 50–200 µm long and 5–20 µm thick, and as (2) zircon coronas and fine-grain chains, hundreds of µm long and ≤ 1 µm thick, spatially associated with (1). Formation of such textures is a result of mineral-fluid reactions, which occurred in two stages and involved: (1) decomposition of ilmenite to Zr-rich rutile and vermicular-shaped zircon during peak metamorphism and initial cooling stage, and (2) recrystallization of Zr-rich rutile to Zr-depleted rutile and submicron-thick zircon coronas during further exhumation and cooling. We also observed hat-shaped grains that are composed of preexisting zircon overgrown by zircon coronas during stage (2). Hat-shaped grains have a flat surface towards the oxide phase, which indicates partial dissolution of preexisting zircon grain. Formation of vermicular zircon (1) preceded ductile and brittle deformation of the host rock, as vermicular zircon is found both plastically- and cataclastically-deformed. Formation of thin zircon coronas (2) was coeval with or soon after the brittle deformation, as coronas occasionally fill fractures in the host rock.

Occurrences of zircon coronas has important implications in fundamental studies regarding metamorphism, metasomatism and element transport in the Earth’s crust. We demonstrate that metamorphic zircon can nucleate and grow as a result of hydration reactions at the cooling stage after granulite-facies metamorphism, and reflects stages of rock evolution. Zircon corona textures are the tool for indicating metamorphic and metasomatic reactions in the host rock, and establish the directions of the reaction front.

Citation: Kovaleva, E., Austrheim, H., and Klötzli, U.: Interpretation of zircon corona textures from metapelitic granulites of Ivrea-Verbano Zone, Northern Italy: Two-stage decomposition of Fe-Ti oxides, Solid Earth Discuss., doi:10.5194/se-2016-164, 2016.
Elizaveta Kovaleva et al.
Elizaveta Kovaleva et al.


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Short summary
It is a study of unusual coronary textures formed by zircon in granulitic metapelites, Ivrea-Verbano Zone (Northern Italy). Zircon coronas occur in two generations: (1) thick (5–20 µm) crescent-shaped aggregates and (2) thin (≤ 1 µm) thread-shaped and tangled coronas. Both are found in the same petrological context, so that the difference between two generations is very conspicuous. Formation of zircon coronas is attributed to the two-stage decomposition of Fe-Ti oxides, a rich source of Zr.
It is a study of unusual coronary textures formed by zircon in granulitic metapelites,...