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

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Solid Earth, 8, 789-804, 2017
https://doi.org/10.5194/se-8-789-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
25 Jul 2017
Interpretation of zircon coronae textures from metapelitic granulites of the Ivrea–Verbano Zone, northern Italy: two-stage decomposition of Fe–Ti oxides
Elizaveta Kovaleva1,2, Håkon O. Austrheim3, and Urs S. 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, Althanstrasse 14, Vienna, 1090, 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 coronae 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 coronae and fine-grained chains, hundreds of micrometers long and ≤ 1 µm thick, spatially associated with the larger zircon grains. Formation of such textures is a result of zircon precipitation during cooling after peak metamorphic conditions, which involved: (1) decomposition of Zr-rich ilmenite to Zr-bearing rutile, and formation of the vermicular-shaped zircon during retrograde metamorphism and hydration; and (2) recrystallization of Zr-bearing rutile to Zr-depleted rutile intergrown with quartz, and precipitation of the submicron-thick zircon coronae during further exhumation and cooling. We also observed hat-shaped grains that are composed of preexisting zircon overgrown by zircon coronae during stage (2). 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 coronae (2) was coeval with, or immediately after, brittle deformation as coronae are found to fill fractures in the host rock. The latter is evidence of local, fluid-aided mobility of Zr. This study demonstrates that metamorphic zircon can nucleate and grow as a result of hydration reactions and mineral breakdown during cooling after granulite-facies metamorphism. Zircon coronae textures indicate metamorphic reactions in the host rock and establish the direction of the reaction front.

Citation: Kovaleva, E., Austrheim, H. O., and Klötzli, U. S.: Interpretation of zircon coronae textures from metapelitic granulites of the Ivrea–Verbano Zone, northern Italy: two-stage decomposition of Fe–Ti oxides, Solid Earth, 8, 789-804, https://doi.org/10.5194/se-8-789-2017, 2017.
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Short summary
This is a study of unusual coronae 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 coronae. Both are found in the same petrological context, so that the difference between two generations is very conspicuous. Formation of zircon coronae is attributed to the two-stage decomposition of Fe–Ti oxides, a rich source of Zr.
This is a study of unusual coronae textures formed by zircon in granulitic metapelites,...
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