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

Koenraad de Jong 1*, Seokyoung Han 1, Gilles Ruffet 2, Keewook Yi 3

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 3 Division of Earth and Environmental Sciences, Korea Basic Science Institute, 162 Yeongudanji-ro, Ochang, Cheongwon, Chungbuk 363-883, Republic of Korea

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. 593-609 (October 2014) DOI: http://dx.doi.org/10.14770/jgsk.2014.50.5.593

Submitted: 2014, September 30 Modified: 2014, October 2 Accepted: 2014, October 10

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

koendejongCross-Out

Abstract

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 (1) 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.

Keywords:

Multi-mineral Geochronology, Titanite SHRIMP, 40Ar/39Ar laser probe, Triassic, Korean

Peninsula

1. Introduction

Much of Korea consists of Precambrian continental crust that is subdivided into three terranes,

viz. the Nangrim, Gyeonggi and Yeongnam Massifs, from North to South (Fig. 1). These

terranes mainly comprise Palaeoproterozoic high-grade gneiss with minor Meso- and

Neoproterozoic additions and rare Paleozoic rocks (e.g., Chough, 2013; Lee and Cho, 2012;

Oh, 2012; Choi, 2014). The Precambrian terranes are separated by two belts of multiple-

deformed and metamorphosed sedimentary and volcanic rocks of late Neoproterozoic to

middle and late Palaeozoic age (e.g., Chough et al., 2000; Lim et al., 2005; Cho et al., 2007,

2013; Choi et al., 2012; Chough, 2013; Choi, 2014): the Imjingang Belt and the Ogcheon

Metamorphic Belt (Fig. 1). Additional multiple-deformed meta-sedimentary rocks, forming the

Taean Formation, crop out discontinuously along the western margin and structurally

uppermost part of the Gyeonggi Massif (Choi et al., 2008; Kee, 2011; Na et al., 2012; So et al.,

2013) in the Taean-Seosan-Dangjin, Anmyeondo-Boryeong areas, and the Yeongheung-

Seonjae-Daebu Islands (Fig. 1). In Anmyeondo at least four generations of more or less

undeformed and partly metamorphosed intrusive rocks occur in the Taean Formation. Sensitive

High-Resolution Ion Micro-Probe (SHRIMP) dating of zircon has shown that one magmatic

system is Jurassic and another Late Triassic (Han, 2014) in age. Until quite recently, the Taean

Formation has been regarded as Precambrian in age because of absence of fossils (Na, 1992),

but are now known to have been deposited after the late Silurian on the basis of the youngest

concordant UPb SHRIMP spot ages in (rims of) detrital zircons (Cho, 2007; Cho et al., 2010;

Kee, 2011; Na et al., 2012; So et al., 2013; Han, 2014). The zircon age distribution in the these

rocks is similar to that for sandstones of the Imjingang Belt (Cho et al., 2005; Han, 2014) and

some parts of the Ogcheon Metamorphic Belt (Cho et al., 2013). However, SHRIMP analysis

has not been able to provide precise estimates of the age of metamorphism of these rocks,

because rims of newly formed zircon around older cores are too thin to be analyzed with high

accuracy. This probably indicates that metamorphic recrystallization did not occur at

sufficiently high temperatures, viz. the upper amphibolite-facies or higher grade (Parrish and

Noble, 2003; Williams, 2001) needed to form significant overgrowths around older crystals.

The best estimates until now using zircon rims are ~229 Ma (Cho, 2007) and ~280 Ma (Kee,

2011). Therefore, in the current contribution, we used 40Ar/39Ar laser probe dating of

muscovite and SHRIMP analysis of metamorphic titanite to establish the timing of

metamorphism of the Taean Formation on Anmyeondo (Fig. 2). The use of rock forming

minerals, like mica, for isotopic dating has an advantage over the use of accessory minerals

because their growth can be more easily related to the metamorphic and tectonic evolution of

rocks. This makes it easier to interpret the meaning of isotopic dates obtained. The concordant

233 3 Ma (titanite, U-Pb SHRIMP) and 230.7 1.0 and 228.8 1.0 Ma (muscovite, 40Ar/39Ar) dates obtained by Han (2014) and that we report in this paper are the first high-

quality age estimates for metamorphism of the Taean Formation. In addition, we discuss the

geodynamic meaning of these isotopic dates.

2. Regional Geology

The Gyeonggi Massif (Fig. 1) is a poly-metamorphic terrane that 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). Some

serpentinites are associated with rare bodies of strongly retrogressed mafic granulite with

exceptional omphacite relics in some garnet porphyroblasts, recording pressures and

temperatures of 1.652.1 GPa and 775850C (Oh et al., 2005; S.W. Kim et al., 2006; Zhai et

al., 2007), acquired during the Triassic (Guo et al., 2005; S.W. Kim et al., 2006).

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

Massif, but as part of the so-called Hongseong suture. However, for other authors these high-

pressure metamorphic rocks are associated with Neoproterozoic (770742 Ma) intrusive rocks

(Deokjeongri gneisses; Oh et al., 2005; S.W. Kim et al., 2006, 2008) and part of the Gyeonggi

Massif. Work in progress by Park et al. (2013), in contrast, suggests that the precursor of one

of the mafic eclogites intruded Paleoproterozoic gneisses, which may have belonged to the

continental margin of the Gyeonggi Massif in the Neoproterozoic. The Gyeonggi Massif,

including the strongly retrogressed high-pressure granulites in the Hongseong area, seems to be

the only of the three Precambrian terranes significantly affected by Triassic metamorphism.

This is shown by isotopic ages of U-bearing accessory minerals in the 250-215 Ma range, but

most between 235 and 231 Ma (e.g. S.W. Kim et al., 2006, 2008; Oh et al., 2006b; J.M. Kim et

al., 2008; Kim et al., 2009; Suzuki, 2009; Yi and Cho, 2009; Kee, 2011; Cho et al., 2013b; Lee

et al., 2014; Yengkhom et al., 2014).

The Imjingang Belt (Fig. 1), recrystallized under syn-tectonic medium-pressure,

medium- to high-temperature Barrovian type conditions (T= 500800 C; P=

affected by several phases of superimposed deformation and metamorphism, but at lower

maximum temperature and pressure (T= 500-650C; P= 0.4-0.8 GPa: Cho and Kim, 2005).

The isotopic mineral ages vary from ~290 to ~160 Ma (errors of 10-15%), which is similar to

range of ages in the top of the southern Gyeonggi Massif below the belt (e.g. Cliff et al., 1985;

Cheong et al., 2003; Oh et al., 2004; Kim, 2005; Kim et al., 2007). The pressure and

temperature conditions indicate that metamorphism and deformation of the Taean Formation,

Imjingang Belt and Ogcheon Metamorphic Belt occurred at depths in the order of 20-35 km,

which implies a collisional setting. The protoliths of these rocks were probably deeply buried

sedimentary units along the margins of the Precambrian Gyeonggi Massif with Paleozoic

additions, or were exotic sedimentary terranes accreted to that margin. The Imjingang and

Ogcheon Belts have been regarded as suture zones (Chough et al., 2000), in a tectonically

complex system (Chough et al., 2013). The Imjingang Belts and the Gyeonggi Massif 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; Kim et al., 2006b, 2008, 2011a-c; Zhai et al.,

2007; Kwon et al., 2009; Oh, 2012; Chough et al., 2013; Choi, 2014; Lee et al., 2014;

Yengkhom et al., 2014).

All major Korean tectonic terranes have been intruded by Triassic and voluminous

Jurassic plutonic rocks (Fig. 1; Kee, 2011; Kim et al., 2011a; Park et al., 2010; Sagong et al.,

2005). 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; Choi et al., 2009; Williams et

al., 2009; Seo et al., 2010; Kee, 2011; Kim et al., 2011a). This type of Mg-rich potassic

magmatism has its source in the mantle and typically evolves over a short time in an

extensional tectonic setting during plate convergence, amongst others in a post-collisional

setting (Bonin, 1986; Ligeois and Black, 1987; Davies and von Blanckenburg, 1995;

Turner et al., 1996; Ligeois et al., 1998; Gill et al., 2004; Dilek and Altunkaynak, 2009;

von Raumer et al., 2014). Although not limited to continental collision belts and the

architecture of the Korean tectonic system being yet far from clear, the Late Triassic

magmatism 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).

3. Taean Formation

On Anmyeon Island, the Taean Formation (Fig. 2) comprises rhythmically layered series of

light-coloured sandstone and dark gray pelite with intercalations of calcareous psammite and

minor carbonate, and finally rare thin black very-fine-grained tuff horizons. These rocks were

probably originally deposited by deep-water turbidites in a submarine distal fan/lobe

environment (Lim et al., 1999; Choi et al., 2008; So et al., 2013). The youngest Paleozoic

peaks of concordant SHRIMP UPb spot ages are between 431 and 420 Ma (errors 1%) in

(rims of) detrital zircon (Cho, 2007; Cho et al., 2010; Kee, 2011; Na et al., 2012; So et al.,

2013; Han, 2014), which these authors take as at evidence that the Taean Formation has been

deposited after the late Silurian.

The main metamorphic minerals in meta-pelites are biotite and muscovite; garnet is

extremely rare, whereas aluminum-silicates (kyanite or andalusite) are absent. In the six-

component system K2O-FeO-MgO-Al2O3-SiO2-H2O, biotite appears in Al-poor metapelites

near 400C and garnet around 450C (Bucher and Grapes, 2011). But these authors indicate

that garnet can be formed at significantly lower temperatures by the preferential incorporation

of manganese and calcium. Calc-silicate rocks, like the one sampled for isotopic dating, are

made up of carbonate, light-green actinolitic amphibole, titanite and clinozoisite-epidote,

whereas (grossular-rich) garnet is absent. Taken together, this suggests that metamorphism of

the Taean Formation occurred below 450C under lower to middle greenschist facies

conditions.

Two phases of superimposed deformation are present in the Taean Formation. The

earliest cleavage S1 is only locally developed in meta-pelites as the axial plane cleavage of rare

recumbent isoclinal folds. But generally S1 is a well-developed quartz-mica differentiation

foliation that is often parallel to the bedding. In meta-sandstone the development of a quartz

shape preferred orientation is generally minor. Most calc-silicate rocks do not show a clear

tectono-metamorphic fabric, but locally a foliation is formed that contains an amphibole

lineation. F2 folds are megascopic open to tight disharmonic flexural slip structures, which in

meta-pelites may have a well-developed crenulation cleavage S2 as axial plane foliation that

makes an angle with layers of meta-sandstone.

The Taean Formation on Anmyeon Island contains abundant NE to ENE striking

massive up to several metres thick mafic dykes. Their texture varies from coarse-grained

doleritic with randomly oriented plagioclase crystals to fine-grained without visible plagioclase

crystals. The dykes have no amygdales, nor chilled margins, suggesting relatively deep

intrusion levels. These dykes truncate the locally well-developed S1 foliation and mesoscopic

F2 folds and their axial plane cleavage S2. All dykes have abundant biotite upto a few mm

wide, implying that they were probably metamorphosed.

At Mongsanpo (Fig. 2) an inequigranular pinkish-orange coloured syenite cuts the

discordant intrusion contact between a 7-metres-thick mafic dyke and the layering in the Taean

Formation. This shows that the dyke intruded the sediments before the syenite. Zircon from the

syenite gave a SHRIMP 206Pb/238U age of 229.6 3.5 Ma (Han, 2014), which is concordant to

the 229 Ma U-Pb age obtained on zircon by Cho (2007). The syenite contains abundant fine-

grained mafic rocks that form elongated dm-long xenoliths upto metre-scale bodies sometimes

with chilled margins. The mafic rocks are extensively veined by the syenite. Cuspate-concave

contacts between both magmatic rocks, with a flow banding developed in the mafic rocks

parallel to the contact suggests that both were present in liquid state, and did not mix due to the

viscosity contrast. Syenites contain a number of dm-wide zones of well-lineated and well-

foliated ultra-mylonites, pointing to solid-state tectonic deformation. This shows that even

though these magmatic bodies intruded after the folding of the metasediments they were not

completely post-tectonic.

4. Geochronology

4.1 Sample descriptions

4.1.1 13JK09 (calc-silicate) 13JK09 is a pale-green coloured calc-silicate rock that is intercalated with biotite-rich red-

coloured meta-pelites that contain a layer-parallel cleavage S1. Titanite occurs as 100250 mm

long, subhedral to euhedral, rhomboid to elongate, somewhat rounded individual crystals that

contain few (?ilmenite) inclusions. Back-scattered electron images of subhedral titanite grains,

likely broken during processing, reveal a patchy planar banding or sector zoning, generally

with straight zone boundaries (Fig. 3), but sometimes with a more spotted aspect with rounded

boundaries and diffuse zoning. The patchy zoning is probably due to in-situ replacement or

recrystallization in reaction to changes physical conditions during metamorphism (Ueda et al.,

2012).

4.1.2 12JK20, 12JK70 (biotite-bearing metapelites)

Sample 12JK20 is an undeformed, 1-2 mm diameter grain of metamorphic muscovite from a

dark grey mica-rich layer that contains a cm-spaced crenulation cleavage S2, which makes an

angle with the layering in grey well-bedded sandstones that do not contain the cleavage.

Sample 12JK70 is one of the undeformed and randomly oriented 2.5-3 mm diameter muscovite

crystals that cross-cut the main quartz-biotite-muscovite fabric S1 in a decimeter-thick

carbonaceous metapelite. S1 is almost parallel to the layering in neighbouring meta-sandstone.

4.2 U-Pb geochronology

4.2.1 U-Pb dating of Titanite Titanite (sphene) is a calcium titanium silicate (CaTiSiO5) that can incorporate uranium in its

lattice and has similar properties to zircon and monazite. The mineral was first applied as a U

Pb dating tool by Tilton and Grunenfelder (1968) and has since been applied widely in tectono-

metamorphic studies (e.g. Corfu and Muir, 1989; Getty and Gromet, 1992; Cliff et al., 1993;

Scott and St. Onge, 1995; Resor et al., 1996; Frost et al., 2000; Tanner and Evans, 2003; Ueda

et al., 2012; Spencer et al., 2013; Yi et al., 2014). It can yield useful isotopic data because it

has a high closure temperature (c. 650-700C) and is of fairly widespread occurrence in

metamorphic rocks, (mafic and calc-silicate rocks, (impure) carbonates), in which it is stable to

the highest temperatures (Scott and St. Onge, 1995; Frost et al., 2000; Tanner and Evans,

2003). Because titanite is forming part of a metamorphic assemblage, other minerals may

exchange Ti and Ca, with it when it (re)crystallises (Scott and St. Onge, 1995). Therefore,

titanite newly formed under amphibolite facies or lower conditions is more likely to have

recorded the timing of crystallization instead of its closure to diffusion (Resor et al., 1996;

Frost et al., 2000; Tanner and Evans, 2003).

4.2.2 Analytical Procedure Titanite grains were collected using conventional mineral separation techniques. Rock samples

were crushed in a hydraulic crusher and pulverized using a ring mill. Rock powder was sieved

to obtain grains in the size fraction of 63 - 245 m. In order to remove clay minerals and

concentrate heavy minerals, panning technique was used. After hand-magnetic separation and

applying heavy liquid techniques, about 100-150 titanite grains were hand-picked under a

stereoscopic zoom microscope, and subsequently mounted on an epoxy disc with reference

titanite BLR-1 (Ontario metamorphic megacryst, 1047.1 0.4 Ma, 206Pb/238U = 0.1764, U =

261 ppm; Aleinikoff et al., 2007). The epoxy mount was polished to expose the approximate

center of grains and then coated with gold. In order to investigate the internal structures of

grains, Second Electron Images (SEI) and backscattered electron (BSE) images were obtained

using a JEOL JSM-6610LV scanning electron microscope (SEM) at Korea Basic Science

Institute (KBSI). A SHRIMP IIe installed at KBSI was used to obtain the in-situ U-Pb ages.

We followed previous workers analysis protocol (e.g., Williams, 1998; Ireland and Williams,

2003; Yi et al., 2014). The intensity and size of the primary ion beam (O2-) were 2.9 - 3.4 nA

and 25 m. Each analysis included five scans through the Zr, Pb, U, and some species. 207Pb/206Pb common lead ratios were calculated by indirect estimation from Y-intercept of the

regression line in the Terra-Wasserburg diagram. The data were reduced using the SQUID

Excel macro program (Ludwig, 2009) and plotted on the Concordia diagram using the

ISOPLOT/Ex program (ver. 3.75) of Ludwig (2012).

4.2.3 Results

14 spots from 12 titanite grains of calc-silicate 13JK09 sample were analyzed (Table 1).

In the Tera-Wasserburg Concordia diagram, ages are distributed along a well-defined

regression line with a lower intercept age of 232.5 3.0 Ma (n=14; MSWD = 1.2) (Fig. 4). The

Y-intercept of the regression line is 0.8400 0.013, which is the derived initial 207Pb/206Pb in

the analysis. The corresponding model Pb value of 0.851 is within uncertainty of the calculated

ratio (Stacey and Kramers, 1975).

4.3 40Ar/39Ar geochronology

4.3.1 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).

The 40Ar/39Ar analytical data are listed in Table 2, and shown as age spectra in Figs. 5a,

b. 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. We calculated pseudo-plateau

ages because less than 70% of the 39Ar was released in at least three or more contiguous steps,

the apparent ages of which agreed 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.

4.3.2 Results 40Ar/39Ar laser probe dating of both muscovite single grains gave saddle-shaped age spectra

(Figs. 5a,b). Sample 12JK70 has concordant apparent ages in the low and high temperature

steps (95% 39Ar released) with a pseudo-plateau age of 230.7 1.0 Ma (1) and a youngest

apparent age in the intermediate temperature range of 226.8 0.3 Ma (Fig. 5a; Table 2).

Sample 12JK20 has a more pronounced saddle-shaped age spectrum. The central part is flat

and has a pseudo-plateau age of 228.8 1.0 Ma (1, 25% 39Ar released), whereas the low and

high temperature steps have identical ages of 237.5 0.4 Ma and 237.1 0.4 Ma, respectively

(Fig. 5b; Table 2).

5. Interpretation

The 232.5 3.0 Ma 206Pb/238U lower intercept age of metamorphic titanite is concordant with

the 230.7 1.0 Ma and 228.8 1.0 Ma pseudo-plateau 40Ar/39Ar ages of muscovite grains that

cross-cut the main tectono-metamorphic fabric S1 (12JK70) or occur as undeformed

recrystallized grains in S2 (12JK20) in metapelites. Usually, isotopic dates in metamorphic

rocks are interpreted to record when the temperature in a geological system drops below a

critical threshold, permitting minerals to start accumulating isotopes formed by radioactive

decay in their crystalline lattices. These are referred to as the mathematically obtained closure

temperature (Dodson, 1973), or the empirically defined blocking temperature (e.g., Purdy and

Jger, 1976). Values for the closure temperatures that are usually quoted in the literature for

the minerals we used are: 660-700C for titanite (Scott and St. Onge, 1995; Frost et al., 2000)

and ~350C for muscovite (Robbins, 1972) for moderate cooling rates. However, isotopic

closure does not only depend on temperature but also on the chemistry of a mineral (Fe/Mg

ratio, halogen content), diffusion geometry, grain size, cooling rate and pressure (e.g., Lister

and Baldwin, 1996; Harrison et al., 2009; Villa et al., 2014). We used the most recent diffusion

coefficients for muscovite obtained by Harrison et al. (2009) to estimate the closure

temperature in our case (Fig. 6). The muscovite closure temperature for diffusion dimensions

of 0.5 to 1.0 mm (the size of the grains we analyzed) varies between 420 and 520C for

cooling rates between 1-100C/Ma (Fig. 6). In a companion paper de Jong and Ruffet (2014)

obtained identical 40Ar/39Ar (pseudo)plateau ages between 230 and 228 Ma (1 errors: 1 Ma)

on hornblende and biotite from amphibolites in the Hongseong area, pointing to very rapid

cooling (100-150C/Ma). The efficiency of cooling is also shown by the near-coincidence of

these ages with Late Triassic U-Pb zircon ages in the Gyeonggi Massif and the Hongseong

area, reported in the literature (e.g. Guo et al., 2005; S.W. Kim et al., 2006, 2008, 2011a, b;

Kee, 2011). The metamorphism of the Taean Formation on Anmyeondo, thus, seem to have

been acquired in a rapidly evolving Late Triassic tectonic system in Korea. Consequently, we

prefer the higher estimates of the closure temperature for muscovite in the dated rocks, which

are even for slowly cooled systems (>10C/Ma) above 450C (Fig. 6). These values are

calculated for pressures of 0.5 GPa; values of twice that amount increase the closure

temperature by 20C (Harrison et al. 2009). Values as high as 500C to 550C for muscovite

are also supported Villa et al., (2014) for muscovite that is not deformed or recrystallized after

cooling, which is the case for the crystals we dated. Because metamorphism of the sediments

of the Taean Formation occurred probably between 400 and 450C, thus well below the

closure temperature of titanite and more modern and realistic estimates for the closure

temperature of muscovite, we regard our isotopic mineral dates as metamorphic

recrystallization ages instead of cooling ages. Therefore, our isotopic dates show that the Taean

Formation has been metamorphosed during the Carnian (earliest Late Triassic) according to

the most recent international chronostratigraphic chart of the International Commission on

Stratigraphy (Cohen et al., 2013).

The 40Ar/39Ar age spectra of muscovite grains are slightly (12JK70) to moderately

(12JK20) saddle-shaped, the latter with concordant low and high temperature steps of 237.5

and 237.1 Ma, respectively (Fig. 5b; Table 2). Some other complex muscovite 40Ar/39Ar age

spectra obtained from the Taean Formation also show clear evidence of older age components

of 240 and 243 Ma for low and high temperature steps, compared to the main central parts

(Han, 2014), as will be discussed in detail elsewhere. Saddle-shaped 40Ar/39Ar age spectra are

well-known for muscovite from metamorphic and magmatic rocks (e.g. Cheilletz et al., 1999;

Alexandrov et al., 2002; Castonguay et al., 2007; de Jong et al., 2009; Tartse et al., 2011).

Cheilletz et al. (1999) and Alexandrov et al. (2002) explained such age spectra by the presence

of different reservoirs in a partially recrystallized mica grain. These reservoirs have distinct

argon compositions that degas over a different energy interval: a primary, unrecrystallized or

inherited domain (low and high temperature steps) and a newly formed or recrystallized one

(saddle minimum in the intermediate temperature steps). The younger subdomains formed by

growth or recrystallisation could characterise the last isotopic record during a lengthy

(re)crystallisation history. Ductile deformation and/or fluid circulation play an important role

during such processes (Tartse et al., 2011). In the case of the saddle-shaped age spectra

obtained for the Taean Formation, this could mean that the age spectrum of muscovite 12JK20

(Fig. 5b) shows a moderate recrystallization event around ~229 Ma that did not completely

overprint evidence for an earlier event of about 237 Ma old. In contrast, stronger crystallization

of muscovite 12JK70 (Fig. 5a) around ~231 Ma would have completely wiped out relics of an

older event. The age minimum of ~227 Ma in the spectrum of muscovite 12JK70 could

represent the last isotopic record of a lengthy recrystallization history. Field and thin section

evidence support this interpretation. The dated muscovites occur as undeformed grains that

cross-cut the main tectono-metamorphic fabric in greenschist facies metapelites (12JK70), or is

present as undeformed grains in samples with a well-developed S2 (12JK20). This suggests that

the two phases of ductile deformation that affected these Paleozoic sediments occurred earlier.

As we will discuss elsewhere in detail these age spectra imply that the Taean Formation has

been metamorphosed already during an earlier tectono-metamorphic phase around 240 3 Ma

and that muscovite recrystallized around 228-231 Ma.

6. Discussion

The 231 and 229 Ma old muscovites and the 233 Ma old metamorphic titanite are concordant

to the SHRIMP 206Pb/238U age of 229.6 3.5 Ma that Han (2014) obtained on zircon from the

syenite that intruded the Taean Formation at Mongsanpo (Fig. 2) after the second folding

phase. Consequently, metamorphism of the Taean Formation and syenitic magmatism in and

around Anmyeon Island are coeval. Mica and titanite are from rocks located at 6.5 to 20 km

from the dated syenite pluton, which does not show contact metamorphism. The mineral ages

are therefore probably not due to the heating by the relative small intrusion itself. Thus,

metamorphism and magmatism probably have a common tectonic cause. The Late Triassic

(237-219 Ma) gabbromonzonite and syenitegranite suite in Korea forms relatively small,

compositionally zoned, isolated plutons (Fig. 1). This spatial pattern and short time span points

to a focused heat source, limited in space and timing. SHRIMP dating of zircons in

(migmatitic) gneisses along the western Gyeonggi Massif has revealed that a number of rims

have 237228 Ma ages (errors: 3-5 Ma), pointing to a regional metamorphic overprint (Kim et

al., 2006, 2008; Kee, 2011). Metamorphic conditions during this event are not well known but

in order to enable formation of such rims rocks must have been at least in the upper

amphibolite facies (Williams, 2001; Parrish and Noble, 2003), in agreement with the moderate

but widespread anatexis observed in them. Rare spinel granulites occur in the eastern

Gyeonggi Massif (Odesan area) within 12 km of a hypersthene-bearing monzonite intrusion,

dated at 228.7 0.9 Ma (UPb on zircon; Jeong et al., 2008), record even higher temperatures

(T= >900C; P = 0.75 GPa, Oh et al., 2006a). This underscores that regional metamorphism

and magmatism in the Gyeonggi Massif, the Deokjeongri gneisses and Weolhyeonri complex,

as well as the Taean Formation at higher crustal level essentially took place during the same,

well-defined, short period in the early Late Triassic, and thus by the same tectonic process.

Field relations show that the syenite and the older mafic dyke swarm intruded the Taean

Formation after the two main ductile deformation phases that affected the meta-sediments. The

mafic dyke swarm and the mafic enclaves in the syenites formed in an extensional tectonic

regime after contraction-related structures were formed. Thus, magmatism and metamorphism

in the Late Triassic may reflect the rapid transfer of heat promoted by extension and magmatic

underplating. In an extensional tectonic regime granitoids and shoshonitic rocks may form

from potassic calc-alkaline magmas, which in turn were produced by partial melting of

previously subduction-metasomatised sub-continental mantle lithosphere by mafic magmas

and underplates created during adiabatic decompressional melting of upwelling hot

asthenosphere (e.g., von Blanckenburg and Davies, 1995; Turner et al., 1996; Gill et al., 2004;

Dilek and Altunkaynak, 2009; von Raumer et al., 2014). Ultimately, magmas may have been

derived from an asthenospheric window formed following delamination of thickened

lithosphere/slab detachment, which would expose the cooler lithosphere interior to

asthenospheric temperatures sufficient to initiate lithospheric melting. Replacement of

lithosphere by hot asthenospheric mantle increases the heat flux into the crust both by

conductive heat transport (hot asthenosphere) and by advective heat transport (magma)

(Bodorkos et al., 2002). This steepens the lithosphere geotherm, which leads to mid-crustal

felsic and mafic plutonism. The effect of such a perturbed thermal regime 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). This process could have been the trigger for the observed regional

metamorphism in the Late Triassic in the Gyeonggi Massif, and at higher crustal levels in the

Taean Formation.

There are a number of isotopic dates on U-bearing accessory minerals in the 245-260 Ma

(errors: 3-16 Ma) age range in the Gyeonggi Massif (Suzuki, 2009; Lee et al., 2014;

Yengkhom et al., 2014) and the Imjingang Belt and correlatives (Cho et al., 2005; Kim et al.,

2014). Although there is no indication that these dates refer to the high-pressure

metamorphism, many authors regard the c. 250 Ma isotopic ages as dating the collision (e.g.

Ree et al., 1996; Kwon et al., 2009; Chough et al., 2013; Choi, 2014; Yengkhom et al., 2014).

However, the currently available U-Pb ages of zircon in the relict eclogitic and high-pressure

granulite metamorphic rocks in the Hongseong area are much younger: 231 3 Ma (Guo et al.,

2005; Kim et al., 2006). This age is the same as that of the magmatic pulse and associated

regional metamorphism related to lithospheric delamination (Choi et al., 2009), or oceanic slab

break-off (Seo et al., 2010; Oh, 2012). Numerical modelling by Gerya (2010) showed that it

takes over 15 Ma between the establishment of the peak pressure conditions in a subducted

crustal unit and initiation of the break-off process. This time span is similar to the time

required for destabilization and convective removal of the dense lithospheric layer beneath the

rigid lithosphere in the delamination model (England and Houseman, 1989; Bodorkos et al.,

2002). This strongly suggests that the ca. 231 Ma zircon ages in the pervasively retrogressed

eclogite and garnet granulite in the Hongseong area also reflect the metamorphism related to

the Late Triassic magmatic and thermal pulse. Gneisses in the area around the partially

retrogressed high-pressure rocks are particularly affected by magmatism and regional

metamorphism in the 237231 Ma range (Kim et al., 2006, 2008; Kee, 2011). This event

apparently has reset the U-Pb isotope system in zircon in the eclogite-relics. De Jong et al.

(2009) discussed a comparable case of fluid-mediated resetting of zircon in recrystallized high-

pressure metamorphic rocks in the Tianshan (China) also in association with widespread post-

collisional magmatism.

7. Conclusions

We have shown that the middle Paleozoic Taean Formation on Anmyeondo is affected by

regional greenschist facies metamorphism shown by concordant isotopic ages for metamorphic

titanite (232.5 3.0 Ma, SHRIMP U-Pb lower intercept age), muscovite (230.7 1.0 Ma and

228.8 1.0 Ma (1), 40Ar/39Ar pseudo-plateau ages). The metamorphism is coeval with the

intrusion of a syenite and mafic dykes after two folding phases in the meta-sediments. These

dates agree with isotopic ages for metamorphism (228-237 Ma), and gabbromonzonite and

syenitegranite magmatism (226-233 Ma) in the Gyeonggi Massif. This Late Triassic

metamorphic and magmatic event may reflect the rapid transfer of heat promoted by extension

and magmatic underplating, following slab detachment or lithospheric delamination. The effect

of such a perturbed thermal regime is significant heating of the lower crust and creation of a

thermal anomaly that propagates upwards into the middle and upper crust. This could have

triggered the observed regional metamorphism in the Gyeonggi Massif, and at higher crustal

levels in the Taean Formation. There are indications in the muscovite age spectra that the ca.

233-229 Ma event is superimposed on an earlier isotopic system of about 237 Ma, which could

relate to the observed pre-magmatic folding of the Taean Formation.

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). The paper benefitted from constructive comments by KIM Sung Won and an

anonymous reviewer.

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Table 1 U-Th-Pb isotopic results for metamorphic titanite (uncorrected) from calc-silicate 13JK09 at Kkotji Beach

Table 2 40Ar/39Ar analytical data of laser step heating of a muscovite single grains from a biotite-bearing metapelites Taean Formation (Sinon Unit) at Gonseom. 12JK20 < 3635'2.11"N; 12617'32.01"E>; 12JK70

Figure 1. Simplified tectonic map of Korea.

Figure 2. Geologic sketch map of Anmyeon Island and adjacent areas with sample

locations. Modified from So et al. (2013).

Figure 3. Cathodoluminescence and backscattered electron images of representative

zircon and titanite grains from calc-silicate rock 13JK09 at Kkotji Beach. Scale Bar

100 micron.

Figure 4. Concordia diagrams showing the SHRIMP spot analyses of titanite from

calc-silicate rock 13JK09 from the Taean Formation at Kkotji Beach.

Figure 5. Step-heating age spectra of an non-deformed muscovite grain in (a) 12JK70 cross-cutting the penetrative S1, and (b) 12JK20 with a well-developed crenulation cleavage S2, both from biotite-bearing meta-pelites in the Taean Formation at Gomseom.

Figure 6. Variation of closure temperature with cooling rate and diffusion dimensions

between 0.1 and 1 mm appropriate to 0.5 GPa pressure for Arrhenius parameters

(activation energy, E and diffusion coefficient, D0) adopted from Harrison et al. (2009)

and Pitra et al. (2010).