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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: [email protected] 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
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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
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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,
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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;
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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=
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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.
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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
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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
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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
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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
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Figure 1. Simplified tectonic map of Korea.
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Figure 2. Geologic sketch map of the Hongseong area with sample
locations.
Modified after Kee (2011).
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Figure 3. Laser step-heating age spectra of hornblende and
biotite single grains JK02
and JK04B.