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Titel |
Peak metamorphic temperatures from Raman Spectroscopy on Carbonaceous Matter (RSCM) and δ18O and δ13C (carb) isotope composition of a major mélange zone in the South Norwegian Caledonides |
VerfasserIn |
Johannes Jakob, Olivier Beyssac, Philippe Boulvais, Torgeir B. Andersen |
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 |
250126974
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Publikation (Nr.) |
EGU/EGU2016-6774.pdf |
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Zusammenfassung |
A mélange in southern Norway comprises a matrix of garnet, mica- and black carbonaceous
schists and phyllites of abyssal origin, interlayered with originally coarser grained siliciclastic
metasediments, serpentinite conglomerates and sandstones, solitary metaperidotites and thin
slivers of gneisses. Several models for the formation of the mélange have been suggested,
including formation as a) an ophiolitic mélange formed during ophiolite obduction, b)
an unconformable post-obduction transgressive sequence or c) a mélange formed
during hyperextension along the pre-Caledonian margin of Baltica. In the past, the
mélange has therefore not been treated as one single tectonic unit, but has been
assigned to various tectonic positions with both outboard Iapetus and inboard Baltican
origins.
In this study we argue that the mélange occupies a tectonostratigraphic position below
major Baltican basement nappe-complexes previously assigned to the Middle Allochthon.
Furthermore, we present new consistent results on the peak metamorphic temperatures
(T ∼ 500∘ C), based on RSCM, and a characteristic δ18Ocarb isotope composition
(11-15.5 ‰ SMOW), both consistent for more than 250 km along strike of the mélange.
δ13Ccarb values fall within three clusters around 1, − 2 , and − 7 ‰ (PDB),
respectively.
The stable isotope investigation presented here was carried out in order to explore if
pre-Caledonian isotope signatures in various generations of carbonate veins and the early
Ordovician fossils at Otta, could have been preserved through a later Caledonian
metamorphic overprint. The results presented here however, suggest that re-equilibration of
the carbonates took place in the Silurian, most likely coeval with peak metamorphism of
∼ 500∘ C at ∼ 420 Ma, and the main fabric development close to the base of the
nappe-stack. Re-equilibration of the carbonates was assisted by the presence a pervasive
static fluid, allowing for oxygen isotope exchange with the surrounding schists and resulting
in an mélange-wide uniform δ18Ocarb signature. The carbon isotope composition was
re-equilibrated only within each lithological body and notably not beyond lithological
boundaries. |
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