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Titel |
New Ar/Ar single grain mineral ages from Korean orogenic belts with implications for the Triassic cooling and exhumation history |
VerfasserIn |
Koenraad de Jong, Gilles Ruffet, Seokyoung Han |
Konferenz |
EGU General Assembly 2013
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 15 (2013) |
Datensatznummer |
250080624
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Zusammenfassung |
The Korean peninsula is located in the eastern margin of the Eurasian continent where
major late Palaeozoic to early Mesozoic continental collision zones, like the Central
Asian Orogenic Belt and the Qinling-Dabie-Sulu Belt, merge with circum-Pacific
subduction-accretion systems. Deciphering the tectonic evolution of Korea is thus crucial for
the understanding of the amalgamation of East Asia. Classically, research in Korea has
focused on the search for (ultra)high-pressure metamorphic rocks and their isotopic
dating, most recently applying SHRIMP on Th- and U-bearing accessory minerals,
in order to substantiate links with the Qinling-Dabie-Sulu Belt across the Yellow
Sea in China. Instead of trying to date peak pressure conditions we focused on
40Ar/39Ar laser-probe step-heating dating of single grains of the fabric-forming
minerals muscovite, biotite and amphibole, formed during retrograde recrystallisation
and exhumation. This is a big advantage as their growth can be straightforwardly
correlated to major phases of the tectono-metamorphic evolution of rocks. This
approach helps to meet the major geochronological challenge of obtaining age
estimates for the timing of specific tectono-metamorphic events in the Korean orogenic
belts.
The Korean peninsula comprises a number of Palaeoproterozoic high-grade gneiss
terranes; only one of which has been affected by Permo-Triassic metamorphism: the
Gyeonggi Massif. We concentrated on the uppermost Gyeonggi Massif and the
overlying Imjingang Belt, to the North, and the ill-defined Hongseong zone to the West,
both constituted by younger metamorphic rocks. Both belts contain rare lenses
of mafic rocks with relics of high-pressure metamorphism. Hornblende from a
corona-textured amphibolite from the lowermost part of the Imjingang Belt yielded a
U-shaped age spectrum, the base of which is formed by four concordant steps with a
weighted mean age of 242.8 ± 2.4 Ma (15% 39Ar release). Muscovites from strongly
retrogressed and ductily deformed rocks in the mylonitised top of the Gyeonggi Massif
yielded different 1Ïă plateau ages: 242.8 ± 1.0 Ma and 240.3 ± 1.0 Ma for two
chlorite-mica schists, and 219.7 ± 0.9 Ma for a garnet-bearing micaceous quartzite.
Two amphibolites from Neoproterozoic orthogneiss in the Hongseong area yielded
concordant 1Ïă plateau ages of 228.1 ± 1.0 (biotite), 230.1 ± 1.0 (hornblende),
and 229.8 ± 1.0 Ma (hornblende from a foliated garnet-bearing corona-textured
amphibolite).
40Ar/39Ar laser-probe dating produced robust evidence that cooling and exhumation of
once deeply buried rocks in different parts of Korea essentially occurred in middle to late
Triassic time. The concordance of hornblende and mica ages in each of the target areas
implies a rapid cooling, during at least part of the history, which seems not to have been
coeval. This corroborates the observation that our Ar/Ar mineral ages are only a couple of
million years younger than CHIME and SHRIMP U-Pb ages in accessory minerals, which are
in the 230-255 Ma range in the uppermost Gyeonggi Massif and Imjingang Belt, and between
225-235 Ma in the Hongseong area. However, the much younger muscovite age
from the mylonitic quartzite implies a prolonged recrystallization in the ductile
shear zone in the uppermost Gyeonggi Massif. This is subject of ongoing research. |
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