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| Titel |
At what conditions does zircon grow/dissolve during high-T metamorphism? Relating zircon textures to PT-conditions |
| VerfasserIn |
Barbara E. Kunz, Daniele Regis, Paola Manzotti, Martin Engi |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250108501
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| Publikation (Nr.) |
 EGU/EGU2015-11154.pdf |
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| Zusammenfassung |
| A key question in ziconology is when and how zircon grows during metamorphism. To shed
light on zircon forming processes and the corresponding PT-conditions during high-T
metamorphism a case study was undertaken. The Ivrea Zone (N-Italy) exposes a lower
continental crustal section in which a continuous metamorphic field gradient from
amphibolite to granulite facies is documented. This field gradient is thought to reflect
protracted heating during late Paleozoic times, with a probable high-T peak in the
Permian.
We present first results from a primarily textural study supported by U-Pb ages, Th/U ratios
and Ti-in-Zrn thermometry. Four types of zircon were identified based on their overgrowth
proportions and the preservation of detrital cores. Zircon grains were thus classified
as
Type1 – detrital grains with no overgrowth or very narrow rims (300 Ma) and appears to reflect an early dehydration phase. Rim2b has
Permian ages (median 275 Ma), is by far the most common overgrowth type, found in a wide
PT-range. Its development appears related to biotite breakdown. Rim3 is texturally indicative
of magmatic zircon, occurs only in diatexites. Rim4 is the latest overgrowth and is locally
found at all metamorphic grades. Textural features suggest late fluid-related recrystallization
of existing zircon.
At lowest grade (675±35°C, 6±2 kbar) zircons show type1 only, overgrowths are too thin to
clearly identify the rim type. Further upgrade (~700°C, 7 kbar) type1 and type2
dominate. Type2 zircons show rim1, rim2a and occasionally rim4. At the Mu-out
isograd (750±50°C, 8.2±1.4 kbar) most zircons are of type2, now with rim2b instead
of 2a, in addition to rim1 and rim4. Near and in granulite facies (to 800°C, 8±2
kbar) mostly zircon type2 and type4 are present. While rim1 gets more narrow with
increasing metamorphic grade, rim2b grows significantly thicker. Occasionally
rim2a and rim4 occur. Close to the Bt-out isograd (~860°C, 9.2±1.7 kbar), mostly
type3 and type4 are found. Rim1 is absent, and rim2b is commonly overgrown
by rim3. Above the Bt-out isograd (>870°C, 9.5 kbar) zircon types3 and type4
dominate. Rim1 is missing, while Rim2b is the most common type, often with a thin
rim4.
Detailed textural features alone often are not sufficient to relate zircon growth zones to
PT-conditions. In combination with U-Pb ages, Th/U ratios and Ti-in-Zrn thermometry,
however it is possible to link spot ages to PT-conditions. |
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