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

Koenraad de Jong 1*, Gilles Ruffet 2

1 School of Earth and Environmental Sciences, Seoul National University, 599 Gwnangno, Gwanak-gu, 151-747 Seoul, Republic of Korea E-mail: keuntie@yahoo.com 2 CNRS (CNRS/INSU) UMR 6118, Gosciences Rennes, 35042 Rennes Cedex, France and Universit de Rennes I, Gosciences Rennes, 35042 Rennes Cedex, France

This is the original English typescript with the original illustrations from which the published Korean paper was translated, which appeared in Journal of the

Geological Society of Korea (inserted at the back of this PDF):

Journal of the Geological Society of Korea, v. 50, no. 5, p. 611-626, (October 2014) DOI http://dx.doi.org/10.14770/jgsk.2014.50.5.611

Submitted: 2014, October 6 Modified: 2014, October 6 Accepted: 2014, October 10

ISSN 0435-4036 (Print) ISSN 2288-7377 (Online)

koendejongCross-Out

Abstract

We obtained identical 40Ar/39Ar (pseudo)plateau ages of 230.11.0 and 229.81.0 Ma (1) on

two hornblendes from garnet-bearing corona-textured amphibolites in the Hongseong area.

These ages are concordant with the 228.11.0 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-

150C/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.

Keywords:

Geochronology, 40Ar/39Ar laser probe, very fast cooling, Triassic, Korean Peninsula

1. Introduction

Many mountain belts formed by crustal thickening since the Paleoproterozoic were later in

their tectonic evolution affected by horizontal crustal extension and intruded by mantle-

sourced potassic Mg-rich magmatic rocks (e.g., Ligeois and Black, 1987; von Blanckenburg

and Davies, 1995; Brown and Dallmeyer, 1996; Platt et al., 1998; Grbacea and Frisch, 1998;

Turner et al., 1999; Ledru et al., 2001; Bodorkos et al., 2002; Schulmann et al., 2008; Dilek

et al., 2009; Molnar and Stock, 2009; von Raumer et al., 2014). Such magmatism typically

evolves over a short time, amongst others in post-collisional settings where an elevated

orogen may undergo reduction of lithospheric thickness due to convective thinning (Platt et

al., 1998; Molnar and Stock, 2009), thermo-mechanical removal of lower crust and

uppermost mantle (delamination) (Grbacea and Frisch, 1998; Turner et al., 1999), or break-

off of the dense oceanic part of subducted slabs and separation from stuck buoyant

continental portions (Ligeois and Black, 1987; von Blanckenburg and Davies, 1995; Brown

and Dallmeyer, 1996; Ledru et al., 2001; Schulmann et al., 2008; Dilek et al., 2009).

Although not limited to continental collision belts and the architecture of the Korean tectonic

system being yet far from clear, the Late Triassic magmatism in Korea is usually interpreted

as due to a change of tectonic regime subsequent to plate collision from compressional to

tensional (Williams et al., 2009; Kim et al., 2011a), often linked to asthenospheric upwelling

induced by lithospheric delamination (Choi et al., 2009), or oceanic slab break-off (Seo et al.,

2010; Oh, 2012; Choi, 2014). Crustal thinning in the Korean Peninsula is suggested by rare

ductile normal faulting in the top of the Gyeonggi Massif (Kim et al., 2000; Han et al., 2013)

and by metamorphic gaps in a pile with rocks with a downward increasing metamorphic

grade in the Imjingang Belt (Ree et al., 1996).

The consequence of replacement of lithosphere by hot asthenospheric mantle by any

of these processes is a steepening of the lithosphere geotherm, leading to mid-crustal felsic

and mafic plutonism (Bodorkos et al., 2002). The effect of such perturbed thermal regimes is

significant heating of the lower crust and creation of a thermal anomaly that propagates

upwards into the middle and upper crust (e.g. Bakker et al., 1989; Loosveld and Etheridge,

1990; van Wees et al., 1992; Bodorkos et al., 2002). Advection of voluminous magmas has

the potential to raise temperatures in the middle crust very quickly (Loosveld and Etheridge,

1990; Bodorkos et al., 2002). Such mechanisms could thus create temperatures in the range of

700900C at depths of only 20-30 km, typical for high-temperature/lowmedium-pressure

metamorphism.

The high temperatures attained during these processes in the lower and middle crust

limit the value of most low and medium temperature chronometers, like the 40Ar/39Ar system,

for the dating of peak and near-peak metamorphism and the following exhumation.

Exceptions are rapidly evolving high-grade metamorphic terranes and tectonic systems, for

which isotopic dating often yielded a small range of ages for mineral geochronometers that

have very different closure temperatures (e.g. Dallmeyer et al., 1986; Dokka et al., 1986;

Goodwin and Renne, 1991; Baldwin et al., 1993, 2004; Brown and Dallmeyer, 1996; Platt et

al., 1998; Charles et al., 2012; Cubley et al., 2013a, b; Daoudene et al., 2013). Numerical

modeling implies that very rapid cooling (>100 C/Ma) is linked to tectonic processes like

extension or gravity spreading (e.g. van Wees et al., 1992; Rey et al., 2009), instead of

exhumation principally by erosion.

The current paper contributes to that discussion by supplying the first high-quality 40Ar/39Ar single grain laser probe data on biotite (228 Ma) and hornblende (230 Ma) from

amphibolites in the Hongseong area along the western margin of the Gyeonggi Massif.

Instead of trying to date peak conditions using U-bearing accessory minerals, we focused on 40Ar/39Ar laser-probe step-heating dating of single grains of fabric-forming minerals formed

during retrograde recrystallization and exhumation. This approach helps to meet a major

geochronological challenge of obtaining age estimates for the duration and speed of tectonic

and metamorphic processes in the Korean orogenic system - information that is currently

essentially lacking. In a companion paper, de Jong et al. (2014) revealed concordant 233-230

Ma 40Ar/39Ar and U-Pb ages for metamorphic muscovite and titanite from greenschist facies

metapelites in Anmyeon Island (Fig. 1), located about 50 km to the west of the Hongseong

area, which are concordant to the age of a syenite intruding these rocks. This shows that the

late Triassic metamorphic and magmatic event has strongly affected rocks at structurally

different levels, underlining the speed of the processes involved.

2. Regional Geology

Much of Korea consists of Precambrian continental crust that was extracted from the mantle

around ~2.7 Ga, with major additions at ~2.5 Ga (Lee and Cho, 2012), which is subdivided

into three terranes, from North to South: the Nangrim, Gyeonggi and Yeongnam Massifs (Fig.

1). They mainly comprise Paleoproterozoic (2.3-1.8 Ga) high-grade gneiss and supracrustal

rocks, forming stable cratonal continental platforms after ~1.9-1.8 Ga (Lee and Cho, 2012)

until at least the early Paleozoic. The Gyeonggi Massif mainly comprises middle

Paleoproterozoic (1.931.83 Ga) in part high-grade gneiss and variably metamorphic metasediments (e.g., Lee and Cho, 2012; Lee et al., 2014) and minor Neoproterozoic (0.9-

0.75 Ga) magmatic and sedimentary rocks in its western and central parts (Lee et al., 2003;

Kim et al., 2008; Oh et al., 2009) and, at least partly Paleozoic orthogneiss, metasediments,

including marble, as well as metabasites, felsic rocks, lens-shaped bodies of highly

serpentinized ultramafic rocks (Weolhyeonri complex; Kim and Kee, 2010; Kim et al., 2011b,

c).

The three gneiss terranes are separated by two belts of multiple-deformed and

metamorphosed sedimentary and volcanic rocks of late Neoproterozoic to middle and late

Palaeozoic age: the Imjingang and Ogcheon Belts (Fig. 1; e.g., Kim, 2005; Lim et al., 2005;

Cho et al., 2007, 2013a; Kee, 2011; Choi et al., 2012; Lee and Cho, 2012; Chough, 2013;

Choi, 2014). Multiply deformed middle Paleozoic greenschist facies metamorphic turbidites,

which are comparable to similar series in the Imjingang Belt (Taean Formation; So et al.,

2013, en references therein), crop out discontinuously along the western margin, and

structurally uppermost part, of the Gyeonggi Massif (Fig. 1). All this material has been

variously reworked in the Late Triassic (Kee, 2011; Kim et al., 2011; Lee and Cho, 2012; Oh,

2012). Especially rocks of the Gyeonggi Massif, including the strongly retrogressed high-

pressure granulites in the Hongseong area, and the Imjingang Belt, recrystallized under syn-

tectonic medium-pressure, medium- to high-temperature Barrovian type conditions (T=

500800 C; P=

porphyroblasts, recording pressures and temperatures of 1.652.1 GPa and 775850C

(Bibong and Baekdong bodies (Fig. 2): Oh et al., 2005; S.W. Kim et al., 2006; Zhai et al.,

2007; Kwon et al., 2009), acquired during the Triassic (Guo et al., 2005; S.W. Kim et al.,

2006). For these reasons, Kim et al. (2011b, c) do no longer regard these rocks as part of the

Gyeonggi Massif, but defined this complex area as the Hongseong suture and argued that it

was the site of prolonged subduction followed by some kind of plate collision in terminal

Paleozoic to earliest Mesozoic time. However, for other authors these high-pressure

metamorphic rocks are associated with Neoproterozoic (770742 Ma) intrusive rocks (Oh et

al., 2005; S.W. Kim et al., 2006, 2008) and part of Gyeonggi, possibly as MORB-like basalt

or gabbro in a back-arc basin (Oh et al., 2009). Moreover, Park et al. (2013) suggested that

the precursor of the mafic Bibong eclogite intruded Paleoproterozoic gneisses, which, judging

from 803 24 Ma old ages in cores of zircon (S.W. Kim et al., 2006b) may have belonged to

the continental margin of the Gyeonggi Massif in the Neoproterozoic. The eastern contact of

the Weolhyeonri complex with this so-called Yugu orthogneiss complex is tectonic (Fig. 2;

Kim et al., 2011b, c). Consequently, all these terranes are often regarded as possible eastward

extension of the Qinling-Dabie-Sulu ultrahigh-pressure metamorphic belt in China, though in

often sharply conflicting models (e.g. Ree et al., 1996; Lee and Cho, 2003; Oh et al., 2005,

2006a, 2009; Kim et al., 2006b, 2008, 2011b; Zhai et al., 2007; Kwon et al., 2009; Oh, 2012;

Chough et al., 2013; Choi, 2014; Lee et al., 2014; Yengkhom et al., 2014).

Late Triassic (Carnian to early Norian) magmatism is widespread and affects all major

tectonic terranes (Fig. 1). This gabbromonzonite and syenitegranite suite has yielded 237 to

219 Ma isotopic ages, with part of this medium- and high-K calc-alkaline magmatic suite

having shoshonitic affinity (Oh et al., 2006b; Jeong et al., 2008; Peng et al., 2008; Choi et al.,

2009; Williams et al., 2009; Seo et al., 2010; Kee, 2011; Kim et al., 2011a). The Daedong

Supergroup (Bansong, Nampo and Gimpo groups) is deposited in the late Early to earliest

Middle Jurassic (187172 Ma; Han et al., 2006; Jeon et al., 2007). The most prominent

peak in age probability diagrams for detrital zircons corresponds to the Paleoproterozoic age

range found in the Gyeonggi Massif, and prominent peaks in the Early Permian, Late Triassic

Early-Middle Jurassic, and very subordinate Archean, Neoproterozoic and middle Paleozoic

peaks (Jeon et al., 2007), suggesting that most rock type currently cropping put were already

at erosional level in the Jurassic.

3. Polyphase tectono-metamorphic evolution of the Gyeonggi Massif

The Gyeonggi Massif (Fig. 1) is a poly-metamorphic terrane. The Paleoproterozoic rocks

experienced two superimposed tectono-metamorphic cycles: M1 under lower granulite-facies

to high-grade conditions of Paleoproterozoic age (1.93-1.85 Ga) and a second and weaker one

(M2) probably of Permo-Triassic age that is characterized by reheating during

decompression (Cho et al., 1996; Cho et al., 2013b; Lee et al., 2000; Lee and Cho, 2003; Oh

et al., 2006a; Lee et al., 2014; Yengkhom et al., 2014). During the second tectono-

metamorphic cycle decompression occurred. This is shown by the formation of cordierite in

some gneisses in symplectitic coronas around older garnet, or in the garnet-embedding

matrix, which suggests metamorphic conditions of 0.50.35 GPa at 700750C (Cho et al.,

2013b; Lee et al., 2014). These conditions agree with the absence of significant or widespread

dehydration melting of biotite and partial melting during this event. Biotite dehydration

reactions occur over a temperature interval that spans over 100C reflecting differences in Ti,

Mg and fluorine content of the various solid solutions in the mineral (see reviews by Chen

and Grapes, 2007 and Sawyer, 2008), and generally takes place below 850C at moderate

pressure. Local far more extreme metamorphic conditions are, however, recorded by rare

spinel granulites (T= >900C; P = 0.75 GPa) in the eastern Gyeonggi Massif (Odesan area)

that occur

4. 40Ar/ 39Ar geochronology 4.1 Hongseong area

Two amphibolites from the Hongseong area along the Gyeonggi massifs western margin

(Fig. 2) were dated through biotite and hornblende analyses. A remarkable suite of rocks

crops out in this area: (1) strongly deformed and migmatised Neoproterozoic intrusive rocks

(Deokjeongri gneisses); Oh et al., 2005; S.W. Kim et al., 2006, 2008), and (2) at least partly

Paleozoic orthogneiss, meta-sediments, including marble, as well as metabasites, felsic rocks,

and highly serpentinized ultramafic rocks (Weolhyeonri complex; S.W. Kim et al., 2006,

2008, 2011b, c; Kim and Kee, 2010).

4.2 Sample descriptions

4.2.1 JK02

Foliated biotite-bearing amphibolite JK02 (Fig. 2; 3637'20.17"N; 12646'26.68"E)

forms a fragment wrapped by the well-developed main foliation of the surrounding tonalitic

gneiss (Deokjeongri Gneiss Formation; the SHRIMP U-Pb zircons intrusion age is 815-850

Ma, errors 1-1.5%, Kee, 2011). Sampled minerals are relatively large inclusion-free black

amphibole, and large biotite crystals present as aggregates of a few grains, which are

distinctly larger than the matrix. Local partial melting is manifest at the contact between

gneisses and amphibolite. The gneisses themselves show anatexis localized in shear zones

that deflect the main foliation, and also as irregular veins and patches along the main

foliation. Irregular veins of fine-grained essentially undeformed light pink granitic rocks cut

the shear zones, the main foliation, and earlier partial melt zones. Kee (2011) obtained ages of

234 2 and 235 8 Ma metamorphic rims of some zircons.

4.2.2 JK04B

Foliated garnet-bearing corona-textured amphibolite JK 04B (Fig. 2; (3637'17.11"N;

12646'57.70"E) is associated with a several metres long garnetite fragment that occurs in

layered marbles that also contain boudinaged amphibolite bands. The rocks occur in the

Deogjeongri Gneiss Formation, but lithology-wise are more like the Weolhyeonri Complex.

We therefore assign JK 04B, tentatively, to the Weolhyeonri Complex. The amphibolite

contains relics of clinopyroxene and feldspar, with amphibole rimming 2-3 mm diameter

garnet. Different generations of decimetre-centimetre thick feldspar-rich veins cut the

structure without apparent deformation. Diffuse and discontinuous 1-2 cm thick feldspar-rich

veins that are parallel to the main foliation in layered amphibolites also reveal localized

anatexis.

4.3. Analytical Procedure

Following thorough ultrasonic rinsing in distilled water single mineral grains, obtained by

handpicking the 0.3-2.0 mm size fraction of crushed rock under a binocular zoom

microscope, were wrapped in Al foil envelopes (11 mm 11 mm 0.5 mm), which were

stacked in an irradiation can, with neutron flux monitors inserted after every 8 to 10 samples.

Samples and standards (Amphibole Hb3gr; age: 1081.0 0.11% Ma; Renne et al., 2010,

2011) were co-irradiated with Cd-shielding for 298 hours at the McMaster reactor (Hamilton,

Canada, location 8E) with a J/h of 5.86 x 10-6 h-1. The sample arrangement allowed

monitoring of the neutron flux gradient with a precision of 0.2%. Mineral grains were 40Ar/39Ar step-heated with a Synrad CO2 continuous laser at Geosciences Rennes, following

the procedure outlined by Ruffet et al. (1991, 1995). Blanks were performed routinely at the

start of an experiment and repeated typically after each third run, and subtracted from the

subsequent sample gas fractions. Isotopic analyses were performed on a MAP215 noble gas

mass spectrometer. The five argon isotopes and the background baselines were measured in

eleven cycles, in peak-jumping mode. All isotopic measurements are corrected for mass

discrimination and atmospheric argon contamination, following Lee et al. (2006) and Mark et

al. (2011), as well as K, Ca and Cl isotopic interferences. Decay constants used: Renne et al.

(2011). Apparent age errors are plotted at the 1 level and do not include the errors on the 40Ar*/39ArK ratio and age of the monitor and decay constant. Plateau ages were calculated if

70% or more of the 39ArK was released in at least three or more contiguous steps, the apparent

ages of which agreeing to within 1 of the integrated age of the plateau segment. The errors

on the 40Ar*/39ArK ratio and age of the monitor and decay constant are included in the final

calculation of the error margins on the pseudo-plateau age or on apparent ages individually

cited. The 40Ar/39Ar analytical data are listed in Table 1, and shown as age spectra in Fig. 3.

4.4 Results

4.4.1 JK02

40Ar/39Ar step-heating dating of JK 02 yielded a saddle-shaped age spectrum for

hornblende, the lower flat part of which defines a plateau age of 230.1 1.0 Ma (81.6% 39Ar

release), which is concordant to the 228.1 1.0 Ma plateau age (79.0% 39Ar release) obtained

on biotite (Fig. 3; Table 1).

4.4.3 JK04B

We separated a dark brownish green relatively inclusion-free hornblende crystal up to 5

mm long that is part of the linear fabric of the rock. Laser step-heating yielded a saddle-

shaped 40Ar/39Ar age spectrum, the lower flat part of which defines a pseudo-plateau age of

229.8 1.0 Ma (47.3% 39Ar release; Fig. 3; Table 1), which is fully concordant with the 228.1

1.0 Ma and 230.1 1.0 Ma plateau ages of biotite and hornblende from amphibolite JK02.

5. Interpretation

We obtained concordant 230.1, 229.8 (hornblende) and 228.1 Ma (biotite) 40Ar/39Ar

(pseudo)plateau ages. This shows that rocks in the Hongseong area have been cooled rapidly

during the Carnian (earliest Late Triassic) according to the most recent international

chronostratigraphic chart of the International Commission on Stratigraphy (Cohen et al.,

2013). But how fast did these rocks cool?

Values for the closure temperatures for the minerals we dated that are usually quoted in

the literature are: ~500C for amphibole (Harrison, 1981; Baldwin et al., 1990) and ~300C

for biotite (e.g. Purdy and Jger, 1976; Harrison et al., 1985) for moderate cooling rates.

However, isotopic closure does not only depend on temperature but also on the chemistry of a

mineral (e.g., Fe/Mg ratio, halogen content), diffusion geometry, grain size, and finally the

cooling rate (e.g., Harrison et al., 1985; Baldwin et al., 1993; Lister and Baldwin, 1996; Villa,

1998). In rapidly cooled geological systems the above generally used values are likely to be

significantly higher (Harrison, 1981; Baldwin et al. 1993; Lister and Baldwin, 1996). Because

we dated biotite grains of 0.5-1 mm diameter, we use a value of 360C for an elevated

cooling rate calculated using the activation energy and Do/a2 parameters of Grove and

Harrison (1996). The closure temperature for amphibole is even more difficult to estimate.

Harrison (1981) predicted that the closure temperature for hornblende should be between

500C and 580C for cooling rates between 10 and 500C/Ma, using an effective diffusion

radius of 80 m, and diffusion parameters he obtained from isothermal-hydrothermal 40Ar*

loss experiments. In addition to the effect of cooling rate and the size of diffusion domains,

Dahl (1996) used the concept that volume diffusion in a mineral relates to the openness of its

lattice, indicated by the percentage of open space in a unit cell (ionic porosity). On the basis

of natural hornblende compositions he suggested a closure temperature range of 520600C,

calculated for an effective diffusion radius of 80 m and a cooling rate of 200C/Ma. This

temperature range has been revised upward to 550650 C by Villa (1998) using

experimental data in the literature. In the light of the 0.5-1 mm diameter of the hornblende

grains dated we adopt the higher value of 650C.

The concordant 230.1 1.0, 229.8 1.0 Ma (hornblende) and 228.1 1.0 Ma (biotite) 40Ar/39Ar (pseudo)plateau ages we obtained are almost identical within uncertainty with the

234-235 Ma UPb ages (errors 1-3 %,) of metamorphic rims on zircons in adjacent Deokjeongri gneisses (Kee, 2011). The latter are similar to the average 234.5 Ma U-Pb zircon

(rim)age (range: 243 6 and 229 10 Ma; Guo et al. 2005; S.W. Kim et al. 2006, 2008,

2011a, b; Kee, 2011) of various rocks in the Hongseong area, and to SHRIMP U-Pb zircon

rim ages of 237235 in the Gyeonggi Massif (Kee, 2011). Oh et al. (2006a) obtained 245 Ma

with 4 % error on zircon from the spinel granulite in the Odesan area. The temperature during M2 was probably around 700750C, but locally likely as high as 900950C for the

spinel granulite. Combined with the fact that the UThPb closure temperature of unaltered

zircon is very high (well over 900C; Ireland and Williams, 2003), this means that the zircon

rim dates reported in the literature should be regarded as formation ages. Thus, cooling from

the metamorphic conditions during the M2 overprint to the 40Ar/39Ar closure temperature for

amphibole was completed in about 5 Ma, taking an average age for M2 of 235 Ma and a

temperature of 750C. Using the 40Ar/39Ar hornblende age and a temperature estimate for M2

on the basis of the Odesan granulite (245 Ma at 950C), despite its large error, suggests that

cooling took 15 Ma. Taken together this shows cooling rates in the order of 20-60C/Ma,

applying the closure temperature of 650C for amphibole, as discussed previously. A

subsequent very rapid temperature decrease is suggested by the synchronous closure of the

KAr isotopic system of hornblende and biotite (228230 Ma). Using the 2 Ma difference between the average values of the hornblende and biotite single grain dates gives a cooling

rate in the order of 150C/Ma, using the closure temperatures estimates mentioned above.

Using the lowest estimate for hornblende closure in the K-Ar system (500C) would still

imply a cooling rate of about 100C/Ma.

Rates of post-metamorphic cooling are controlled primarily by exhumation

mechanisms. Terranes exhumed by erosion record slow cooling, whereas tectonic exhumation

leads to rapid cooling (e.g. Dallmeyer et al., 1986; Dokka et al., 1986; Baldwin et al., 1993,

2004; Brown and Dallmeyer, 1996; Platt et al., 1998; Charles et al., 2012; Cubley et al.,

2013a, b; Daoudene et al., 2013). Many of these examples of very rapidly cooled rocks are

from metamorphic core complexes. Metamorphic core complexes are crustal-scale features

that formed by exhumation of thick ductilely deformed medium- to high-grade metamorphic

rocks associated with partially molten middle to lower continental crust from beneath

overlying relatively thin, highly extended and significantly lower-grade or unmetamorphosed

upper crustal rocks along low-angle normal faults (detachments) (e.g. Vanderhaeghe et al.,

2003; Rey et al., 2009). Metamorphic core complexes have been documented in areas marked

by crustal extension interpreted as the result of gravitational collapse of a previously

thickened crust, like the North American Cordillera (e.g. Norlander et al., 2002;

Vanderhaeghe et al., 2003; Kruckenberg et al., 2008; Rey et al., 2009; Cubley et al., 2013a,

b), the eastern Mediterranean (Cyclades, western Turkey-Aegean) (Thomson et al., 2009;

Dilek et al., 2009), the Variscan Orogen of Europe (Brown and Dallmeyer, 1996; Ledru et al.,

2001), and the central part of eastern Asia (Charles et al., 2012; Daoudene et al., 2013).

Results of thermo-mechanical modelling of the evolution of some metamorphic core

complexes (e.g. Rey et al., 2009) show very rapid cooling (>100C/Ma) following near-

isothermal decompression under high-temperature conditions. As we will discuss below this

evolution is related to mantle dynamics.

6. Discussion

Oh et al. (2006a) indicated that the Late Triassic hypersthene-bearing monzonite pluton alone

may not have been a sufficient heat source to produce the temperature conditions for granulite

metamorphism (T= >900C; P = 0.75 GPa) in the Odesan area in the eastern Gyeonggi

Massif. They suggested that regional heating may result from the intrusion of large mafic-

ultramafic complexes at a deeper level. The Late Triassic regional metamorphism in Korea is

also too extensive to be only related to the granitoids intrusions. This suggests that the

fundamental thermal anomaly induced by the convective thinning of the lithosphere was

ultimately responsible for both the metamorphism and magmatism. Brown (2007) indicated

that some granulites in low-pressure high-temperature belts that show near-isobaric cooling

paths, could be the result of mid-crustal magmatic accretion in arc or rift settings. Coexistence

of hercynitic low-Zn spinel (ZnO=1.6-2.6 for the Odesan rocks (Oh et al. 2006a)) and quartz

is one of the criteria for ultrahigh temperature metamorphism (Brown, 2007; Harley, 2008;

Kelsey, 2008). These authors showed that one of the ways to create such extreme

metamorphic conditions is to add heat by advection in the form of basaltic melts or diapiric

emplacement of asthenosphere. This may be the result of delamination of the thickened

lithospheric root and/or slab break-off. The lithospheric thinning induced by such processes

provokes a massive increase in the geothermal gradient and high heat flow (e.g. Bodorkos et

al., 2002). The previously mentioned petrological data for M2 suggest a thermal gradient in

the order of 30-40C/km. Such values correspond to thermal conditions found in the

hyperthermal basin of Europe (Cermak, 1993; e.g., Tyrrhenian Sea, western Turkey-Aegean,

and the Pannonian Basin in the Alpine-Carpathian collision belt) that are formed in response

to roll back and detachment of slabs (Edwards and Grasemann, 2009). Much of the Neogene-

Quaternary potassic volcanism of the Mediterranean region (Keller 1983; Peccerillo, 2005;

Bianchini et al., 2008) occurs in areas underlain by detached slabs (Spakman, 1990; Wortel

and Spakman, 1992). The characteristics of this magmatism, namely the high-K calc-alkaline

signature with subordinate amount of shoshonitic components and its generation from

subduction-modified (metasomatised) crust or lithosphericasthenospheric mantle, are shared

with the Late Triassic plutons in Korea. Interestingly, the Late Triassic M2 metamorphic

event in the Gyeonggi Massif is also recorded as a metamorphic event in the tectonically

overlying middle Paleozoic greenschist facies Taean meta-sediments on Anmyeon Island.

Han (2014) and de Jong et al. (2014) reported a concordant titanite U-Pb age of 233 and a muscovite 40Ar/39Ar age of 230 Ma in greenschist facies metamorphic rocks (T< 450C), on this island located about 50 km to the west of the Hongseong area. Syenitic magmatism on

Anmyeondo is of the same age (Han, 2014). These authors concluded that ca. 233-230 Ma-

aged metamorphism and magmatism, including intrusion of mafic dykes, took place after the

area was folded and was due to the combined advective and conductive asthenospheric heat

transport associated with post-collisional delamination of the lower crust and uppermost

mantle, and/or oceanic slab break-off. In combination with the age data provided in the

present paper, this points to a relatively short lived, tectonically induced, magmatic and

metamorphic pulse after contractional deformation that had influence on deeper and more

superficial levels of the crust in Korea in the Late Triassic.

Two collision belts in Europe that show the short-lived pulse-like magmatic and

metamorphic evolution particularly well are the Betic-Rif orogen (westernmost

Mediterranean), and the Variscan orogen. Both belts show the development of granulites and

other high-temperature metamorphic rocks due to the advection of heat from the underlying

mantle, linked to slab detachment or delamination, which also produced K-Mg rich magmatic

rocks, following plate collision and associated high-pressure (eclogite) metamorphism. For

the Betic-Rif orogen Platt et al. (1998) established that the formation of granulites took less

than 2 Ma following an increase of temperatures in the deep crust by more than 100C, and

that the entire process including subsequent isothermal decompression took place in about 15

Ma. Decompression and cooling of the high-pressure granulitic rocks in the Bohemian Massif

(Variscan Belt) occurred within 10 Ma of the metamorphic peak (Schulmann et al., 2008).

Also other parts of the southern Variscan Orogen show fast cooling, in part in metamorphic

core complexes (Brown and Dallmeyer, 1996; Ledru et al., 2001).

Ductile normal faulting in rocks of the Korean Peninsula is rare, and has been

suggested in the top of the northern (Kim et al., 2000) and southern (Han et al., 2013)

boundaries of the Gyeonggi Massif. The Gyeonggi Shear Zone in the north (Juksung area)

has been dated at 226 1.0 Ma on the basis of a Rb-Sr muscovite whole-rock isochron

(Kim et al., 2000), whereas a mylonite zone in the south has been dated as 187.8 5.6 Ma by

the 40Ar/39Ar method on syn-tectonic muscovite (Han et al., 2013). The latter authors

conclude that the extensional exhumation of the Gyeonggi Massif took place in the 226-188

Ma period. This would imply a much longer period that suggested by our 40Ar/39Ar ages, and

point rather to a slow process. Kim et al. (2000) suggested that non-metamorphic sedimentary

rocks of Daedong Supergroup present in the Juksung area (Gimpo Group), not well dated at

the time as Late Triassic to Early Jurassic on the basis of plant fossils, could have been

deposited in a basin formed at the same time as ductile extensional deformation in the deeper

level of the crust, instead of during the early stages of the Daebo compressional regime in an

intra-arc setting, the current interpretation (Han et al., 2006; Jeon et al., 2007). During

sampling in the Juksung area for 40Ar/39Ar dating, we have found very-low grade

metamorphic sedimentary rocks, including conglomerate levels with large K-Fsp pebbles,

similar to the K-Fsp crystals present in the underlying Gyeonggi Massif, directly on top of

and in contact with the mylonites of the Gyeonggi Shear zone. These metasediments contain

brittle-ductile deformation structures that show the same sense of shear as in the mylonites of

the ductile shear zone immediately below. This may suggest that these sediments were

formed in extensional basins, as suggested by Kim et al., (2000) for the Gimpo Group, but

that due to the large extension values and rapid exhumation of the gneisses in the footwall

these sediments were affected by syn-tectonic metamorphism, as is described in many core

complexes. Assuming that the 226 Ma age obtained by these authors is correct, which is

currently being tested applying 40Ar/39Ar to the mylonites and low-grade sediments, these

observations in the Juksung area agree with the fast exhumation of the rocks along the margin

of the Gyeonggi Massif in the Hongseong area. Moreover, a series of slighltly

metamorphosed conglomerate on Deokjeok Island in the West Sea, which is cut by a 225 3

Ma granitic dyke, have been speculated to have been deposited as post-orogenic sediments

deposited in an extensional basin (Kim et al., 2014). Such rocks too may have evolved in a

rapidly cooling core-comple-like extensional regime. This also implies that the 188 Ma 40Ar/39Ar age for the mylonites in the southern margin of the Gyeonggi Massif obtained by

Han et al., (2013) does not refer to the same deformation phase. Isotopic ages in that part of

the Gyeonggi Massif and the Ogcheon Metamorphic Belt on top show a large spread

indicating a long and complicated tectonic and metamorphic history, which probably took

place in distinct phases. As a reminder of this, Han (2014) obtained single grain laser probe 40Ar/39Ar plateau ages of 231 1.0 Ma and 180 1.0 Ma (1) on two generations of distinct

muscovite in the same outcrop of Taean meta-sediments on Anmyeon Island.

This overview showed that, although the best indicators for continental collision, high-

pressure metamorphic rocks, are rare in Korea, and their age controversial, the thermal

evolution reconstructed in a number of small target areas, in combination with the magmatic

evolution clearly show that post-collisional processes, which characterize collisional belts

world-wide, also occurred in the Peninsula in the Late Triassic.

7. Conclusions

We obtained identical 230.1 1.0, 229.8 1.0 Ma (hornblende) and 228.1 1.0 Ma (biotite)

(pseudo)plateau 40Ar/39Ar ages from amphibolites in the Hongseong area, which are almost

concordant with UPb ages of metamorphic rims of zircons in the Gyeonggi Massif reported

in the literature. Taken together, these data indicate cooling from Late Triassic metamorphic

peak conditions of 750C, but locally maybe as high as 950C at 235 Ma (average zircon age) to 650C (hornblende closure temperature for KAr) at 230 Ma. This indicates a cooling rate of 20-60C/Ma. A subsequent very rapid temperature decrease (100-150C/Ma)

is shown by the synchronous closure of the KAr isotopic system of hornblende and biotite

(228230 Ma). Very rapid cooling points to tectonic exhumation mechanisms, with high-temperature, amphibolite facies decompression typically occurring in metamorphic core

complexes, which may be formed in previously tectonically thickened crust in collision

zones. The Late Triassic metamorphism and subsequent fast cooling are part of a relatively

short lived, tectonically induced, magmatic and metamorphic pulse that had influence on

deeper and more superficial levels of the crust in Korea after contractional deformation.

Ultimately, this metamorphism and magmatism maybe related to the combined advective and

conductive asthenospheric heat transport associated with post-collisional delamination of the

lower crust and uppermost mantle, and/or oceanic slab break-off.

Acknowledgements This research was supported by Basic Science Research Program through the National

Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2011-

0012900) to KdJ. KdJ thanks prof. OH Chang Whan for discussion during a post-sampling

excursion in the Hongseong area (Spring 2014). HAN Seokyoung and KIM Ju Hwan

contributed by translating the text into Korean.

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Table 1 40Ar/39Ar analytical data of laser step heating of hornblende and biotite single grains from amphibolites, Hongseong area.

JK02 Biotite 13 steps

Laser power Atm. Cont. 37ArCa/39ArK 37ArCa/39ArK %39ArK 40Ar*/39ArK 40Ar*/39ArK Apparent age Error (mW) %

Error

Error (Ma) (Ma)

300 95.67 0 0 0.11 5.76 3.60 179.4 106.5

350 94.05 0. 0 0.71 2.45 0.51 78.5 15.9

390 61.08 0.02 0.01 2.05 6.02 0.23 186.8 6.7

430 39.63 0 0 2.52 6.76 0.11 208.8 3.3

470 15.00 0 0 12.65 7.30 0.03 224.4 0.9

500 6.33 0.02 0.03 2.99 7.22 0.07 222.0 1.9

550 3.98 0.01 0.01 11.73 7.44 0.03 228.5 0.8

600 2.32 0.01 0.01 11.26 7.44 0.03 228.4 0. 9

650 2.81 0.01 0.01 9.58 7.44 0.03 228.3 0.9

700 2.25 0.01 0.01 13.93 7.44 0.02 228.3 0.6

760 2.19 0 0 4.71 7.38 0.07 226.5 2.2

850 2.33 0 0 4.9 7.33 0.08 225. 2 2.4

1111 1.24 0 0 22.87 7.41 0.02 227.5 0.6

JK02 Hornblende 10 steps

Laser power Atm. Cont. 37ArCa/39ArK 37ArCa/39ArK %39ArK 40Ar*/39ArK 40Ar*/39ArK Apparent age Error

(mW) %

Error

Error (Ma) (Ma)

500 36.74 2.05 0.46 0.15 65.01 1.93 1406.0 29.0

700 37.35 1.09 0.10 0.59 7.44 0.30 228.3 8.7

850 13.88 2.74 0.07 0.76 6.29 0.23 194.9 6.8

950 5.37 4.30 0.07 2.37 7.87 0.14 240.6 3.9

1000 1.05 4.13 0.03 8.13 7.67 0.04 235.0 1.2

1040 0.77 4.04 0.01 52.50 7.51 0.02 230.4 0.5

1070 0.55 4.01 0.01 19.29 7.48 0.02 229.4 0.6

1130 0.97 4.14 0.01 9.80 7.51 0.03 230.2 0.9

1250 8.05 4.43 0.10 1.31 7.53 0.18 231.0 5.1

2222 5.72 4.86 0.03 5.10 8.06 0.04 246.2 1.2

JK04B Hornblende 13 steps

Laser power Atm. Cont. 37ArCa/39ArK 37ArCa/39ArK %39ArK 40Ar*/39ArK 40Ar*/39ArK Apparent age Error

(mW) %

Error

Error (Ma) (Ma)

500 35.56 12.85 0.25 0.12 167.91 1.94 2525.3 15.8

650 45.83 19.36 0.18 0.20 51.80 0.88 1196.7 15.0

750 36.95 70.73 0.44 0.24 14.34 0.51 417.6 13.3

830 18.70 11.35 0.15 0.40 8.88 0.25 269.8 7.0

900 10.07 5.94 0.06 0.85 9.10 0.11 275.7 3.0

1000 2.02 4.07 0.01 5.47 7.90 0.02 241.8 0.5

1100 0.93 4.27 0.01 28.35 7.78 0.01 238.3 0. 4

1150 0.37 4.26 0.01 21.14 7.50 0.02 230.5 0.5

1200 0.46 4.29 0.01 14.74 7.50 0.02 230.3 0.5

1250 0.51 4.64 0.02 7.89 7.45 0.02 229.0 0.5

1320 2.43 5.70 0.02 3.56 7.47 0.03 229.1 0.9

1400 3.02 6.83 0.03 2.87 7.71 0.05 236.3 1.4

2222 9.92 6.95 0.01 14.18 7.74 0.03 237.2 0.7

J parameter error J Age monitor (Ma) Age monitor (Ma) Error

Mass Discrimination (1+e) Date Irradiation

1.81E-02 6.44E-05 1081 1.1891 1.007015 4/16/12

Figure 1. Simplified tectonic map of Korea.

Figure 2. Geologic sketch map of the Hongseong area with sample locations.

Modified after Kee (2011).

Figure 3. Laser step-heating age spectra of hornblende and biotite single grains JK02

and JK04B.