Interpretation of zircon coronae textures from metapelitic granulites of the Ivrea–Verbano Zone, northern Italy: two-stage decomposition of Fe–Ti oxides
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...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-07-01
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| Series: | Solid Earth |
| Online Access: | https://www.solid-earth.net/8/789/2017/se-8-789-2017.pdf |
| Summary: | 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. |
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| ISSN: | 1869-9510 1869-9529 |