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| Titel |
Arc petrogenesis in southern Ireland and the Isle of Man: Implications for Ordovician accretionary history and constraints from Late Caledonian plutonism |
| VerfasserIn |
Tobias Fritschle, J. Stephen Daly, Martin J. Whitehouse, Brian McConnell, Stephan Buhre |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250136028
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| Publikation (Nr.) |
 EGU/EGU2016-16977.pdf |
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| Zusammenfassung |
| Peri-Laurentian and peri-Gondwanan magmatic arcs and microcontinents, and their attendant
sedimentary basins were assembled during the Caledonian Orogeny (c. 490 - 400 Ma) to
form the Irish and British lithosphere. Accretion of these terranes to Laurentia and
subsequent closure of the Iapetus Ocean initiated the generation of widespread Late
Caledonian plutons (c. 425 - 400 Ma). Petrogenetic investigation of Ordovician arc-related
rocks aims to test possible terrane affinities, using geochemical data from the arcs
and related rocks as well as isotopic signatures preserved within Late Caledonian
granites.
SIMS zircon U-Pb geochronology has provided middle to early Ordovician ages for
volcanic rocks with arc affinities from Avoca (Ireland, c. 463 Ma) and a newly
discovered volcanic sequence from Port-e-Vullen (Isle of Man, c. 473 Ma). Granitic
rocks from Leinster (Ireland), interpreted as arc plutons, yielded late to middle
Ordovician ages of c. 457 - 454 Ma (Croghan Kinshelagh) and c. 462 - 459 Ma
(Graiguenamanagh), similar to the c. 457 Ma age of the Dhoon Granite (Isle of
Man).
Oxygen isotopic compositions of zircons from the Ordovician volcanic and plutonic rocks
are close to or slightly heavier than mantle values (δ18O generally < 7 ‰). Lu-Hf zircon
compositions suggest different terrane affinities: relatively juvenile ɛHfT values (c. +8.5 -
+5.3) for the Avoca volcanics are similar to those of the older unit of the Croghan Kinshelagh
Granite, whereas the Port-e-Vullen volcanics and the Graiguenamanagh Granite have less
radiogenic ɛHfT values (c. +4.4 - +1.3). The present-day geographic distribution of these
rocks and petrogenetic inferences from their North American correlatives invite
comparison with the Avalonian and the Ganderian microcontinent, respectively[1].
These constraints are supported by inherited zircons and corresponding isotopic
analyses.
ɛHfT values (c. +11.5 - +1.5) from magmatic zircons of the Dhoon Granite and the
younger unit of the Croghan Kinshelagh Granite span the range defined by the two arc
terranes, suggesting that these younger rocks formed after terrane amalgamation.
This is in accordance with structural constraints: volcanic rocks from Avoca and
Port-e-Vullen, as well as those from the Graiguenamanagh Granite, are strongly
foliated, whereas the Dhoon and Croghan Kinshelagh granites show little or no
deformation. This suggests a major accretion event in the early part of the Upper
Ordovician.
Petrogenetic investigations of the Late Caledonian plutons reveal major differences in
their inferred source compositions. Granites in the centre and to the south of the Iapetus
Suture Zone (ISZ) faithfully preserve Ordovician arc isotopic signatures and are interpreted
to have formed almost exclusively by reprocessing peri-Gondwanan arc-related
rocks. In contrast, granites to the north of the ISZ suggest the amalgamation of
peri-Laurentian continental arc-rocks with relatively unradiogenic Palaeoproterozoic
Laurentian basement. Granites located in the Irish Sea are distinct due to the significant
involvement of sedimentary protoliths, consistent with a mixed signature from both
peri-Gondwanan arcs and Laurentian detritus. This threefold distinction is also
supported by age and isotopic constraints from inherited zircons in the Late Caledonian
plutons.
[1]van Staal et al. (2009, and references therein): J. Geol. Soc. London 327, 271-316. |
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