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| Titel |
The origin and fate of eclogite-facies rocks in the SW Scandinavian Caledonides: a U-Pb and Rb-Sr study |
| VerfasserIn |
Matthijs Smit, Michael Bröcker, Ellen Kooijman, Erik Scherer |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040280
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| Zusammenfassung |
| Most large-scale deformational and metamorphic features in the Scandinavian Caledonides
occurred during Scandian times (430-395 Ma), which involved the collision between the
proto-continents Laurentia and Baltica and the deep (ca. 200 km) subduction of the
Baltoscandian margin. In spite of this pervasive regional metamorphism, some crustal
fragments in the orogen retain an intriguing geochronological record of short-lived
subduction-exhumation cycles from earlier stages of the Caledonian Wilson Cycle. One such
terrane fragment is the Jæren nappe, SW Norway, which contains ca. 470-Ma eclogites.
Because terrane correlations are ambiguous and not supported by geochronological evidence,
the geological significance of these eclogites remains unclear. To further unravel the history
of these rocks, U-Pb zircon and Rb-Sr mica geochronology were applied to their paragneiss
host rocks.
In each of the studied samples, some zircon analyses provided concordant or slightly
discordant Caledonian ages, providing a weighted mean 206Pb/238U age of 469±6 Ma
(2Ï). This age group, representing newly-formed rims and recrystallized patches
of older zircon, is identical to the HP age of the eclogites as recorded by Lu-Hf
geochronology (471±1 Ma, [1]). This observation is interpreted to document that the
eclogite protoliths and their host rocks underwent HP together as a coherent unit. The
majority of U–Pb zircon analyses provided detrital age populations around 0.61, 0.92,
1.0-1.4 and 1.6-1.9 Ga. In addition, some zircon grains yielded Archaean ages (0.3
mm) favor such a nearby source over the distal Archaean terranes of Baltica. This
illustrates the exotic nature of the Jæren nappe in relation to its current geological
setting.
Geothermobarometry [3] showed that the eclogites and paragneisses underwent
near-adiabatic exhumation from ca. 90 to 25 km depth at ca. 700 Ë C, followed by
supra-Barrovian overprinting, and steady cooling along a ‘hot’ geotherm (ca. 30 Ë
C/km). Our Rb-Sr dating of phengite, phlogopite, and biotite revealed mostly early
Scandian apparent ages (ca. 420 Ma). The results of this study suggest that the Jæren
nappe represents a crustal fragment that was subducted in a region that was distal
to Baltica. The eclogite-paragneiss association was rapidly exhumed through the
subduction channel before stalling at the base of the crust where it was thermally
overprinted. Subsequent tectonic transport over great distance and obduction onto the
Baltic hinterland was completed at the onset of Scandian continental collision and
subduction.
References: [1] Smit et al. (2008) J Metam Geol 26: 959-979; [2] Pedersen et al.
(1992) Terra Nova 4: 217-222; [3] Smit et al. (2010) Contrib Mineral Petrol: DOI:
10.1007/s00410-009-0440-3. |
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