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| Titel |
Zircon geochronology and Hf-O isotope geochemistry from granites in the Iapetus Suture Zone in Ireland and the Isle of Man |
| VerfasserIn |
Tobias Fritschle, J. Stephen Daly, Martin J. Whitehouse, Brian McConnell, Stephan Buhre |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250086864
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| Publikation (Nr.) |
 EGU/EGU2014-801.pdf |
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| Zusammenfassung |
| Late Caledonian syn- to post-orogenic granites located in the Iapetus Suture Zone (ISZ)
in Ireland and Britain have been related to A-type subduction and possible slab
breakoff [1] following the Laurentia-Avalonian collision. Lack of reliable age data
(especially in Ireland) has inhibited petrogenetic investigations of these rocks. Hence, ion
microprobe U-Pb and oxygen isotope analyses as well as LA-MC-ICPMS Lu-Hf
isotopic measurements on zircons from Irish and Isle of Man granites have been
undertaken to provide better constraints on this enigmatic episode of the Caledonian
Orogeny.
Four stages of Late Caledonian granitic magmatism (c. 435, 417, 410 and 394 Ma) are
indicated by U-Pb dating of oscillatory-zoned magmatic zircons. The Crossdoney,
Kentstown, Drogheda and Ballynamuddagh granites together with a rhyolite from
Glenamaddy have yielded U-Pb concordia ages, interpreted as intrusion-ages, between
419.9 ± 4.3 Ma (Glenamaddy) and 415.8 ± 2.0 Ma (Crossdoney) with a weighted average of
417.5 ± 0.9 Ma (MSWD = 1.3). The Glenamaddy Granite – which intruded the Glenamaddy
Rhyolite – yielded an age of 410 ± 2.1 Ma. In addition, the Rockabill Granite yielded a
younger age of 393.9 ± 1.9 Ma, whereas the Carnsore Granite yielded an older age of
434.6 ± 1.9 Ma.
Inherited zircons (487 to 453 Ma) occur in several of the granites, and are interpreted to
have been derived from Ordovician arc magmatic rocks accreted within the ISZ. A
younger group of c. 440 Ma inherited zircons occurs in the c. 417 Ma Crossdoney and
Ballynamuddagh granites. These grains could be related to continued or renewed Silurian arc
magmatism.
Hf-O isotopic measurements on the dated zircon grains range between -2 and +7 ÉHfi units
and 5.5 to 8.5 oδ18O. These are interpreted to indicate the contribution of juvenile mantle
melts – possibly derived from the Ordovician arc – to some of the granites. Significant
heterogeneities in zircon oxygen isotopes in at least four of the granites further suggest the
involvement of isotopically distinct protoliths.
The Dhoon and Foxdale granites in the Isle of Man were previously regarded to be of Late
Caledonian age. Surprisingly, zircons from these granites yielded concordant U-Pb ages of
455.6 ± 2.1 Ma (Dhoon) and 455.9 ± 2.1 Ma (Foxdale), respectively. Inherited cores are of
Meso- and Palaeoproterozoic age. Hafnium isotopic analyses of the dated magmatic zircon
rims from both intrusions are slightly more radiogenic than those from the Late Caledonian
granites. Their δ18O values range between 5.5 to 7.5 o. The Isle of Man granites are now
suggested to be the plutonic equivalents of a Late Ordovician (Caradocian) volcanic arc
generated in the Iapetus Ocean.
[1] Atherton & Ghani (2002), Lithos 62, 65-85. |
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