札幌市 - 2014年 07月 29日

40Ar/39Ar Single Grain Laser-Probe Mineral Ages: Slab Detachment-related Magmatism and Reheating

Followed by Very Fast Cooling in the Imjingang Belt and Uppermost Gyeonggi Massif (Korea)

Koen de Jong Seoul National University, Korea keuntie@yahoo.com

Seokyoung Han Seoul National University, Korea

Gilles Ruffet Ar/Ar - CNRS, University of Rennes, Rennes, France

Keewook Yi SHRIMP - Korea Basic Science Institute, Ochang, Korea

진리는 나의 빛

Asia Oceania Geoscience Society 2014 Sapporo, July 28 – August 1, 2014

Dépasser les frontières

진리는 나의 빛

Outline

Discuss their regional tectonic meaning in terms of:

•  Concordance of Triassic metamorphic and magmatic ages

•  Heating, very fast cooling and exhumation

•  Slab detachment as reason regional thermal pulse

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Present the 40Ar/39Ar laser probe single grain age spectra of white mica, biotite and amphibole from the northern and western margin of the Gyeonggi Massif

Dépasser les frontières

SHRIMP U-Pb ages metamorphic titanite, and magmatic zircon

札幌市 - 2014年 07月 29日

3 K. de Jong – Asia Oceania Geoscience Society 2014 Sapporo, July 28 – August 1, 2014

Ar/Ar single grain ages (1σ) •  northernmost Gyeonggi massif (strongly

retrogressed), and Imjingang belt, •  westernmost Gyeonggi massif in the

Hongseong belt (migmatites) and Anmyeondo (greenschist facies)

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Hongseong belt (Korea’s only eclogite relics):

§  Western Gyeonggi Massif: Neoproterozoic Deokjeongri Gneisses

•  Amphibolite layer in a gneiss •  Foliated garnet corona amphibolite

K. de Jong – Asia Oceania Geoscience Society 2014 Sapporo, July 28 – August 1, 2014

40Ar/ 39Ar laser step-heating dating of single grains (Hongseong)

K. de Jong – Asia Oceania Geoscience Society 2014 Sapporo, July 28 – August 1, 2014 5

Widespread anatexis (Hongseong area)

Garnet amphibolite

Amphibolite-bearing Gneiss

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•  Biotite in amphibolite JK02 gave a 228 Ma 1σ plateau age.

The age concordance points to fast cooling in the top of the Gyeonggi massif in Hongseong area.

•  The biotite plateau age is concordant to both 230 Ma hornblendes

K. de Jong – Asia Oceania Geoscience Society 2014 Sapporo, July 28 – August 1, 2014

•  230-235 Ma SHRIMP U-Pb ages in zircon rims in neighboring Neoproterozoic Deokjeongri gneisses.

40Ar/ 39Ar laser step-heating dating of single grains (Hongseong)

de Jong K, Ruffet G, (2014) Jou. Geol. Soc. Kor. 50(5)

Taean Formation - Middle Paleozoic Turbidite Sequence (Anmyeondo)

7 K. de Jong – Asia Oceania Geoscience Society 2014 Sapporo, July 28 – August 1, 2014

Anmyeon Island:

•  Taean Formation: thick rhythmically layered metasandstone - metapelite sequence

•  Along western boundary Gyeonggi Massif •  Likely equivalent of the Imjingang Belt

Taean Formation - Middle Paleozoic Turbidite Sequence (Anmyeondo)

8 K. de Jong – Asia Oceania Geoscience Society 2014 Sapporo, July 28 – August 1, 2014

•  Meta-sandstone and minor intercalated metapelites

•  Greenschist-facies metamorphism (400-450˚C)

9 K. de Jong – Asia Oceania Geoscience Society 2014 Sapporo, July 28 – August 1, 2014

40Ar/ 39Ar laser step-heating dating of single grains (Taean Formation)

•  Muscovite from a meta-pelite gave a 231 Ma 1σ plateau age.

•  Age is concordant to biotite and hornblende ages in the Hongseong area, pointing to a similar cause.

•  As the metamorphic temperature 400-450˚C is < closure temperature: metamorphic age

de Jong K, Han S, Ruffet G, Yi K, (2014) Jou. Geol. Soc. Kor. 50(5)

10 K. de Jong – Asia Oceania Geoscience Society 2014 Sapporo, July 28 – August 1, 2014

•  Intercalation of biotite-rich layers (brown) and amphibole-carbonate-epidote-titanite rock (green).

•  Meta tuffaceous rock

•  Prismatic metamorphic titanite gave a lower intercept age of 232.5 Ma

•  Concordant to 231 Ma muscovite age

•  As the metamorphic temperature 400-450˚C is < closure temperature for titanite 650-700˚C: metamorphic age

de Jong K, Han S, Ruffet G, Yi K, (2014) Jou. Geol. Soc. Kor. 50(5)

KBSI - SHRIMP U-Pb dating titanite (Taean Formation)

11 K. de Jong – Asia Oceania Geoscience Society 2014 Sapporo, July 28 – August 1, 2014

Post-collisional syenite mingled with mafic intrusives, cut all earlier-formed folds and cleavages.

KBSI - SHRIMP U-Pb dating zircon (syenite in Taean Formation)

•  Concordia age 229.6 Ma

•  Intrusion age syenite

•  Concordant to the metamorphic 231 Ma muscovite and 233 Ma titanite ages

Data KBSI SHRIMP – Han S, MSc. Thesis (2014)

Regional Tectonic Meaning

Haemi pluton 226-233

K. de Jong – Asia Oceania Geoscience Society 2014 Sapporo, July 28 – August 1, 2014 12

50 km

Kim et al. Lith 2006 Kim et al. PR 2008

Kee et al. KIGAM 2011

Zhai & Guo 2006

•  Rims of zircons have ages between 231 and 237 Ma.

This age concordance points to heating, magmatism, metamorphism, followed by fast cooling in the top of the Gyeonggi massif and Anmyeondo.

•  Dates formation of rims around zircon, at anatectic conditions, maybe as high as 700-800ºC.

•  231 – 237 Ma metamorphism

•  226 – 233 Ma magmatism

Regional Tectonic Meaning

•  Regional 226-233 Ma magmatism in Korea

Haemi pluton 226-233

(Triassic intrusives and ages KIGAM; Kim et al. GR 2011)

K. de Jong – Asia Oceania Geoscience Society 2014 Sapporo, July 28 – August 1, 2014 13

50 km

•  Gabbro–diorite–monzonite–syenite–granite association

•  High-K calc-alkaline and shoshonitic compositions, evolved from enriched mantle in a post-collisional setting (slab detachment) (Williams et al., Lithos 2009;

Kim et al. GR 2011; Oh JPSK 2012).

•  Himalayas, Alps, Betics, Tianshan, Dabie-Sulu

Kim et al. Lith 2006 Kim et al. PR 2008

Kee et al. KIGAM 2011

Zhai & Guo 2006

Take-home Message

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Gravity-controlled extensional collapse triggered by slab detachment, delamination, or convective removal of lithosphere

Concordance of: §  ∼230 Ma Ar/Ar amphibole and biotite ages (Hongseong),

§  ∼233-231 Ma Ar/Ar muscovite and U-Pb metamorphic titanite (Anmyeondo)

§  ∼231 - 237 Ma zircon rims:

Fast cooling.

Post-collisional evolution in Korea:

Coeval with 226 - 233 Ma post-collisional magmatism, injected into a spreading orogen. Active until ∼220 Ma (youngest shear zone muscovite)

Oldest Ar/Ar ages ∼243 - 239 Ma: early exhumation phase.

Metamorphism and recrystallization: ∼231 - 237 Ma zircon ages (Hongseong).

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REFERENCES FOR DATA AND FURTHER READING de Jong K, Ruffet G, 2014. 243-220 Ma 40Ar/39Ar laser probe muscovite single grain

ages in the northernmost Gyeonggi Massif (Juksung area) and their tectonic implications. Journal of the Geological Society of Korea 50 (6), 771-782, DOI 10.14770/jgsk.2014.50.6.771 <download here>

de Jong K, Ruffet G, 2014. Tectonic implications of the very fast cooling shown by concordant 230-228 Ma 40Ar/39Ar laser probe hornblende and biotite single grain ages in the Hongseong area. Journal of the Geological Society of Korea 50 (5), 611-626, DOI 10.14770/jgsk.2014.50.5.611 <download here>

de Jong K, Han S, Ruffet G, Yi K, 2014. First age constraints on the timing of metamorphism of the Taean Formation, Anmyeondo: concordant 233 Ma U-Pb titanite and 231-229 Ma 40Ar/39Ar muscovite ages. Journal of the Geological Society of Korea 50 (5), 593-609, DOI 10.14770/jgsk.2014.50.5.593 <download here>

de Jong K, Han S, Ruffet G, 2015. Fast cooling following a Late Triassic metamorphic and magmatic pulse and their bearing on the tectonic evolution of the Korean Collision Belt. Invited paper in: Kusky, T., W.J. Xiao, Eds.; Comparative tectonic and dynamic analysis of cratons, orogens, basins, and metallogeny: Tectonophysics – a special volume to honor the career of Brian F. Windley. Accepted pending minor revisions.

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de Jong K, Ruffet G, 2014. 243-220 Ma 40Ar/39Ar laser probe muscovite single grain ages in the northernmost Gyeonggi Massif (Juksung area) and their tectonic implications. Journal of the Geological Society of Korea 50 (6), 771-782, DOI 10.14770/jgsk.2014.50.6.771

Laser step-heating of muscovites from strongly retrogressed and ductilely deformed rocks in the top of the northern Gyeonggi massif (Juksung area) yielded 1σ 40Ar/39Ar (pseudo)plateau ages of: 242.8 ± 1.0 Ma and 240.3 ± 1.0 Ma (mica schists) and 219.7 ± 0.9 Ma (mylonitic quartzite). A biotite single grain yielded a hump-shaped age spectrum with ∼245-250 Ma step ages pointing to 39Ar recoil, an irradiation artifact. It is possible that the ∼243-240 Ma muscovite ages record an early phase of exhumation following or during collision, and that the much younger muscovite age from the mylonitic quartzite implies extended or renewed recrystallization. These ages are ∼10 million years older and younger, respectively, than 40Ar/39Ar ages from amphibolites in anatectic gneisses (Hongseong area) and low-grade metasediments on Anmyeondo, linked to post-collisional tectonic and magmatic processes. Our study thus shows that cooling, exhumation and recrystallization in the Triassic occurred in distinct phases that were not coeval in all areas. This underscores that the younger age sets in metamorphic terranes cannot always be simply interpreted as due to passive post-tectonic cooling, but rather reflects distinct tectonic phases.

<download here>

ABSTRACTS

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de Jong K, Ruffet G, 2014. Tectonic implications of the very fast cooling shown by concordant 230-228 Ma 40Ar/39Ar laser probe hornblende and biotite single grain ages in the Hongseong area. Journal of the Geological Society of Korea 50 (5), 611-626, DOI 10.14770/jgsk.2014.50.5.611

We obtained identical 40Ar/39Ar (pseudo)plateau ages of 230.1 ± 1.0 and 229.8 ± 1.0 Ma (1σ) on two hornblendes from garnet-bearing corona-textured amphibolites in the Hongseong area. These ages are concordant with the 228.1 ± 1.0 Ma plateau age of biotite in the slightly older amphibolite. The concordant ages of hornblende and biotite, minerals with very different closure temperatures, show that the samples cooled very rapidly, probably in the order of 100-150°C/Ma. The efficiency of cooling is further underlined by the near-coincidence of these 40Ar/39Ar ages with 243-229 Ma (error 2-4%, average: 234.5 Ma) zircon U-Pb ages in the Gyeonggi Massif and the Hongseong belt, reported in the literature.

Very fast cooling rates require a fundamental tectonic control. Consequently, we discuss our data in the context of a relatively short-lived, tectonically induced, magmatic and metamorphic pulse that affected the crust in Korea in the Late Triassic. This could have been post-collisional delamination of the lower crust and uppermost mantle, and/or oceanic slab break-off to which the 237-219 Ma mantle-sourced potassic Mg-rich magmatic rocks that are widespread in Korea, also points.

<download here>

ABSTRACTS

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de Jong K, Han S, Ruffet G, Yi K, 2014. First age constraints on the timing of metamorphism of the Taean Formation, Anmyeondo: concordant 233 Ma U-Pb titanite and 231-229 Ma 40Ar/39Ar muscovite ages. Journal of the Geological Society of Korea 50 (5), 593-609, DOI 10.14770/jgsk.2014.50.5.593

ABSTRACTS

Isotopic dating has established that the middle Paleozoic turbidites of the Taean Formation on Anmyeondo in the West Sea were affected by metamorphism the Late Triassic. We obtained a 206Pb/238U lower intercept age of 232.5 ± 3.0 Ma (95% confidence, MSWD = 1.2) of metamorphic titanite from a calc-silicate rock by Multi Collector Sensitive High-Resolution Secondary Ion Mass Spectrometry and 40Ar/39Ar laser probe pseudo-plateau ages of 230.7 ± 1.0 Ma and 228.8 ± 1.0 Ma (1σ) for two single grains of metamorphic muscovite. We estimate that the metamorphic temperature was below the closure temperatures of titanite and muscovite. Consequently, the concordant U-Pb and 40Ar/39Ar mineral ages are not cooling ages but demonstrate that the metamorphism of the Taean Formation on the island occurred in the earliest Late Triassic (Carnian), using the most recent international chronostratigraphic chart. The dated muscovite occurs as undeformed grains that cross-cut the main tectono-metamorphic fabric in greenschist facies metapelites, or from undeformed grains in rocks with a well-developed secondary crenulation cleavage. This suggests that the two phases of ductile deformation that affected these Paleozoic sediments occurred earlier. The muscovite age spectra show evidence of an earlier isotopic system of about 237 Ma, which could relate to the observed pre-magmatic folding of the Taean Formation.

<download here>

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Our 40Ar/39Ar study of the Gyeonggi Massif, the only Korean Paleoproterozoic high-grade gneiss terrane significantly affected by Permo-Triassic metamorphism, indicates 25 million years of post-collisional evolution, and distinct phases of cooling and exhumation. We obtained single grain ages for muscovite, biotite and hornblende in the range of ∼245-220 Ma, which are almost all plateaux (1σ errors <0.5%; ∼50-80% 39Ar release). ∼245 Ma age components in one hornblende grain suggest that retrograde hydration of a corona-textured garnet–clinopyroxene-rich mafic granulite in the lowermost Imjingang belt (northern margin of the massif) and its exhumation occurred in the earliest Triassic. Our age data further imply that at least part of the ductile deformation fabrics in the Gyeonggi Massif’s top formed around 240-245 Ma too. But the ∼220 Ma-old muscovite in a mylonitic quartzite implies extended or renewed recrystallisation in low-angle normal shear zones in the massif's uppermost part.

Hornblende and biotite in the Hongseong belt (uppermost western Gyeonggi Massif) yielded concordant plateau ages of ∼230 Ma, which are only marginally younger or concordant to 231-237 Ma SHRIMP U-Pb literature data of zircon in the area. This points to an important thermal event, followed by rapid cooling and exhumation, ∼10 million years later than that of the Imjingang Belt's high-pressure rocks. The late Triassic metamorphic mineral ages overlap with post-collisional 233-226 Ma-old syenite–monzonite–granite–gabbro magmatism with high-K calc-alkaline and shoshonitic affinity. Thus suggesting that tectono-metamorphic events may have occurred due to late Triassic slab detachment, following plate collision in Paleozoic-Mesozoic boundary times, suggested by ∼250 Ma U-Pb ages in zircon rims reported in the literature. Many of the late Triassic intrusives are ductilely deformed, so these magmatic rocks intruded into an orogen that was actively deforming by gravity spreading associated with upper crustal normal faulting around 220 Ma, following/accompanying slab-detachment.

ABSTRACTS