Directory UMM :Data Elmu:jurnal:J-a:Journal of Asian Earth Science:Vol19.Issue1-2.Feb2001:
Journal of Asian Earth Sciences 19 (2001) 77±84
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First SHRIMP U±Pb zircon dating of granulites from the Kontum massif
(Vietnam) and tectonothermal implications
Tran Ngoc Nam a,*, Yuji Sano b, Kentaro Terada b, Mitsuhiro Toriumi c, Phan Van Quynh d,
Le Tien Dung e
b
a
Department of Geosciences, Hue University of Science, 27-Nguyen Hue, Hue City, Viet Nam
Department of Earth and Planetary Sciences, Hiroshima University, Kagamiyama 1-3, Higashi-Hiroshima 739-8526, Japan
c
Geological Institute, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan
d
Department of Cartography and Remote Sensing, Hanoi National University, 334-Nguyen Trai, Hanoi, Viet Nam
e
Department of Geology, University of Mining and Geology, Tu Liem, Hanoi, Viet Nam
Abstract
The Kontum massif in Central Vietnam represents the largest continuous exposure of crystalline basement of the Indochina craton. The
central Kontum massif is chie¯y made of orthopyroxene granulites (enderbite, charnockite) and associated rocks of the Kannack complex.
Mineral assemblages and geothermobarometric studies have shown that the Kannack complex has severely metamorphosed under granulite
facies corresponding to P±T conditions of 800±8508C and 8 ^ 1 kbars. Twenty-three SHRIMP II U±Pb analyses of eighteen zircon grains
separated from a granulite sample of the Kannack complex yield ca 254 Ma, and one analysis gives ca 1400 Ma concordant age for a zoned
zircon core. This result shows that granulites of the Kannack complex in the Kontum massif have formed from a high-grade granulite facies
tectonothermal event of Indosinian age (Triassic). The cooling history and subsequent exhumation of the Kannack complex during Indosinian times ranged from ,8508C at ca 254 Ma to ,3008C at 242 Ma, with an average cooling rate of ,458C/Ma. q 2001 Elsevier Science
Ltd. All rights reserved.
1. Introduction
Granulites are recognized as a potential source of important information toward understanding deep crustal compositions and processes, and early tectonothermal events. The
Kontum massif (Central Vietnam) is composed of such
granulites. Orthopyroxene granulites (charnockites, enderbite) and associated rocks, which are similar to those of
Eastern Ghats (India) and East Antarctica, form the central
part of the Kontum massif (Fig. 1). They are grouped into
the gneiss complexes under local names (Figs. 1 and 2;
Department of Geology and Minerals of Vietnam Ð
DGMVN, 1989, 1995). As it was once held that granulites
were only formed in Archean and Early Proterozoic times,
the granulites of the Kontum massif were thought to be of
Archean age by previous workers (e.g., Tran Quoc Hai,
1986; DGMVN, 1989, 1995; Hutchison, 1989). The
Archean ages of these granulites have been suggested
based on the common occurrence of charnockitic rocks
* Corresponding author. Current address: Geological Institute, the
University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan. Fax: 181-35841-4569.
E-mail address: [email protected] (T.N. Nam).
(Tran Quoc Hai, 1986; Hutchison, 1989; DGMVN, 1989,
1995). However, the Archean ages are hardly accepted
because no available geochronological data have yet been
documented for these granulites.
Recent advanced Ar±Ar and K±Ar geochronological
studies have revealed a wide occurrence of three tectonothermal episodes in Indochina. They are the Triassic
Indosinian, Late Jurassic±Early Cretaceous in the Truong
Son belt north of the Kontum massif (e.g., Maluski et al.,
1995, 1997; Rangin et al., 1995; Lepvrier et al., 1997; Nam,
1998), and Tertiary in the Bu Khang gneiss dome north of
the Truong Son belt (e.g., Jolivet et al., 1999) and in the Red
River shear zone (e.g., Nam, 1998; Wang et al., 1998). PreIndosinian tectonothermal events have been poorly documented by geochronological data in this region, and
geochronological data of the Kontum massif remain scarce.
Since granulites were formed at high temperature conditions
under deep crustal levels, U±Pb zircon dating of such granulites could provide valuable evidence for high-temperature
stages of the tectonothermal evolution of the Kontum massif
that may be dif®cult to obtain by Ar±Ar or K±Ar techniques. This present study was undertaken in an attempt to
determine the timing of tectonothermal events recorded in
zoned zircons from a main granulite complex in the central
1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S 1367-912 0(00)00015-8
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T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
Fig. 1. Simpli®ed map of the northern Kontum massif (after Nam, 1998). Box corresponds to Fig. 2.
Kontum massif. This is the ®rst SHRIMP U±Pb zircon data
documented for the complex, which was previously thought
to be the oldest unit (Archean in ages) in Indochina (e.g.,
Tran Quoc Hai, 1986; Hutchison, 1989; DGMVN, 1989,
1995).
2. Geological setting
The Kontum massif represents the largest continuous
exposure of crystalline basement of the Indochina craton,
which was once regarded as the stable continental core of
South-East Asia (Hutchison, 1989). The Kontum massif is
mainly composed of high-grade metamorphic rocks which
were thought to be Precambrian in ages by previous workers
(e.g., Tran Quoc Hai, 1986; Hutchison, 1989; DGMVN,
1989, 1995). Paleozoic sedimentary rocks are mostly absent
there. The massif is partly covered by Mesozoic volcanosedimentary formations and Neogene±Quaternary basalts,
and is intruded by Paleo-Mesozoic granodiorite bodies
(Figs. 1 and 2; DGMVN, 1989, 1995; Nam, 1998).
High-grade gneiss rocks of the Kontum massif (Vietnam)
are grouped into different units under local names, including
the Kannack complex and associated intrusive complexes,
the Ngoc Linh complex, the Kham Duc and Poko formations (DGMVN, 1989, 1995). The Kannack complex occupies the central and southern Kontum massif (Fig. 1). It is
subdivided into three parts: (1) lower, (2) middle and (3)
upper (Fig. 2). The lower part is composed of two-pyroxene
and hyperthene-garnet-bearing granulites in close connection with autochthonous plutons of orthopyroxene-bearing
granites (charnockites and enderbites), and cordierite±sillimanite-bearing gneisses (khondalites). The thickness of the
lower part is estimated to be 700±1000 m. The middle part
is mainly composed of plagioclase±biotite±hypersthene
gneiss, two-pyroxene plagioclase gneiss, biotite±sillimanite±cordierite gneiss and migmatite rocks, with an estimated thickness of 500±900 m. The upper part is chie¯y
made of garnet±cordierite±sillimanite±biotite gneiss,
cordierite±sillimanite schist, and quartzite. Several calciphyre and marble intercalations are found among the gneiss
and schist. The upper part is estimated to be 700 m thick.
Autochthonous plutons of gabbrogranulite, enderbites
and charnockites, and biotite±garnet±cordierite granite are
distinctly placed into the Konkbang, Songba and Pleimanko
complexes, respectively (Fig. 2). Plutonic bodies show
massive, granitic texture in the center and gradually change
to strongly foliated rocks along the peripheries. Foliation of
these rocks is commonly parallel to those of host gneisses.
The boundary between plutonic bodies and host gneisses or
migmatites is often unclear because of the gradual change.
Several small lenses of host gneisses (two-pyroxene and
hypersthene granulites) ranging from 1±2 cm to 2±3 m in
thickness are found within the enderbite±charnockite
bodies.
Mineral assemblages of the Kannack complex include: (i)
plagioclase (Pl) 1 clinopyroxene (Cpx) 1 orthopyroxene
(Opx) 1 quartz (Q), (ii) Pl 1 hypersthene (Hyp) 1 garnet
(Gt) 1 Q, and (iii) Pl 1 Hyp 1 K-feldspar (Kfs) 1 Q in
the lower part; (iv) Q 1 Pl 1 Kfs 1 sillimanite (Sil) 1
Gt 1 Bi ^ cordierite (Cord), (v) Q 1 Pl 1 Kfs 1 Cord 1
Hyp 1 Sil 1 Gt, and (vi) Q 1 Pl 1 Kfs 1 spinel 1 Hyp 1
Cord 1 Gt in the middle part; (vii) calcite 1 diopside,
(viii) calcite 1 oligoclase 1 garnet ^ dolomite, and (ix)
plagioclase 1 diopside 1 quartz in the upper part. These
assemblages are typical of granulite facies metamorphism.
Geothermobarometric studies of the complex have yielded
P±T conditions of 800±8508C (by garnet±cordierite and
T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
79
Fig. 2. Structural sketch map and cross-section of Kannack area, the central Kontum massif (see Fig. 1 for location). Kannack granulite complex: 1. Twopyroxene granulites, interbeds of biotite±sillimanite±cordierite±garnet gneiss, enderbite and charnockite; 2. Plagioclase±biotite±hypersthene gneiss, interbeds of two-pyroxene gneiss, quartz±biotite±sillimanite±garnet±cordierite schist and migmatites; 3. Garnet±cordierite±sillimanite±biotite gneiss, cordierite±sillimanite schist and quartzite; 4. Marble and calciphyre. Igneous rocks associated with the Kannack complex: 5. Gabbrogranulite (the Konkbang
complex); 6. Enderbite and charnockite (the Songba complex); 7. Biotite±garnet±cordierite granite (the Pleimanko complex); 8. Paleo-Mesozoic granite;
9. Middle Triassic rhyolite; 10. Neogene±Quaternary basalt; 11. Quaternary alluvia; 12. Fault; 13. Foliation; 14. Sampling locality.
two-pyroxene geothermometry), and 8 ^ 1 kbars (by
garnet±orthopyroxene±plagioclase±quartz geobarometry)
(DGMVN, 1989). Some additional calibrations using
garnet±biotite and garnet±hornblende geothermometry
give estimations of 700±7508C, regarded as indicating the
conditions of a later retrograde stage of metamorphism. The
above mineral assemblages and geothermobarometric
results therefore indicate peak metamorphism P±T conditions for the Kannack complex to be 800±8508C and 8 ^ 1
kbars. The Kannack complex and associated intrusive
complexes were previously proposed to be Archean in age
on the basis of petrological correlation with classic Archean
granulites in other parts of the world (Phan Truong Thi,
1985; Tran Quoc Hai, 1986; DGMVN, 1989, 1995).
However, the proposed Archean age is hardly accepted
without available geochronological data. Few geochronological data have been reported for the complex, including a
K±Ar age of 242 Ma obtained on a biotite separate (Nam,
1998), and a K±Ar age range of 1650±1810 Ma, which was
from `a personal communication' source but often cited by
previous workers (e.g., Phan Truong Thi, 1985; Tran Quoc
Hai, 1986; Hutchison, 1989).
Other metamorphic formations, which were thought to be
Precambrian (e.g., DGMVN, 1989, 1995), are widely
distributed in the northern Kontum massif (Fig. 1). They
are composed of biotite±sillimanite gneiss, amphibolite,
biotite schist, migmatite rocks and lenses of marble. This
rock assemblage suggests amphibolite facies metamorphism. Some Rb±Sr ages of 1400±1600 Ma have been
reported for these formations (Phan Truong Thi, 1985).
80
T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
The Poko formation south-east of Kontum town (see location in Fig. 1) contains some characteristic algal stromatolites, suggesting an Upper Proterozoic age (DGMVN,
1989). Precambrian rocks are strongly foliated (DGMVN,
1989). The foliation is folded to form gentle folds with
commonly North±South trending axes.
Mesozoic volcano-sedimentary formations are composed
of andesite, rhyolite and tuffs, and conglomerates, sandstone
and siliceous shale of Late Triassic±Early Jurassic and
Cretaceous ages. Neogene±Quaternary basalt is mainly of
tholeiitic and sub-alkaline olivine compositions, ranging
from 16 to 0.2 Ma in age (DGMVN, 1989, 1995; Hoang
and Flower, 1998; Lee et al., 1998).
3. Sample and mineral descriptions
Zircons analyzed in this study were separated from a
granulite sample (KT13108 in Fig. 2) collected from the
middle part of the Kannack complex in the central Kontum
massif, Vietnam (Fig. 2). The sample is coarse grained and
has a gneissic texture. Mineral constituents of the sample are
quartz (,25±30%), plagioclase (,25±35%), K-feldspar
(,10±15%), garnet (,10%), sillimanite (,7±10%), biotite
(,5±10%), and cordierite. Apatite and zircon are common
accessory minerals in the sample.
Zircons were concentrated by crushing, sieving, and
mineral separation with an isodynamic separator and
heavy-liquid, and hand-picking using a binocular microscope. Zircons occur as an individual crystal with an euhedral shape, and range from 0.2 to 0.5 mm in size (Fig. 3).
The perfectly euhedral shape suggests that zircons have
®nally grown in an environment that supposes presence of
liquid melt (i.e., likely magmatic zircon). Fig. 4 shows
representative EMPA backscatter images of zircon grains,
in which three types of zoning patterns can be seen: (a)
perfectly homogeneous from central to outer part, (b)
large homogeneous core and a narrow rim, and (c) oscillatory zoning. Among eighteen grains analyzed in this
study, twelve grains (67% of zircon population) have
type (a), ®ve grains (27% of zircon population) display
type (b) and one grain does display type (c). Types (b)
and (c) suggest likely overgrowths of zircon, whereas the
type (a) indicates one-stage growth. SHRIMP U±Pb
analyses have been performed on both cores and rims of
overgrowth zircons.
4. Analytical methods and results
Analyzed zircon grains were mounted in epoxy-resin disk
with several grains of the standard zircon SL13 and QGNG.
SL13 is the well-known Sri Lanka 572 Ma megacryst extensively used by the Australian National University SHRIMP
group as a U/Pb and abundance calibration standard
(Roddick and van Breemen, 1994; ClaoueÂ-Long et al.,
1995; Williams, 1998), and QGNG is a new multicrystal
zircon standard from Quartz±Gabbro±Norite±Gneiss
(QGNG) from Cape Donnington, Eyre Peninsula, South
Australia whose TIMS U/Pb age is 1850 ^ 2 Ma (2s )
(C.M. Fanning, personal communication, 1997). Zircon
grains were polished to provide a ¯at surface for sputtering
of secondary ions until they were exposed through their
mid-sections. After polishing (by using 0.25 mm diamond
paste), they were coated by thin gold plate to prevent charging of the sample surface by the primary ion beam.
The samples were evacuated in the sample lock overnight
and introduced into the sample stage in the ion source chamber. A 3-nA mass-®ltered O22 primary beam was focused to
sputter a 30-mm-diameter area with positive ions extracted.
Before the actual analysis, the sample surface was rastered
for 3 min in order to clean up the surface of the grain and
eliminatory possible contaminants. The magnet was cyclically peak-stepped through a series of mass numbers
ranging from mass 196 for 90Zr216O 1 to mass 254 for
238 16 1
U O , including the background at mass number
204.1, and Pb isotopic mass numbers at 204, 206, 207,
and 208, and the atomic U peak at the number 238 and
232
Th 16O peak at number 248. The 206Pb/ 238U ratios in the
samples were calibrated using an empirical relationship
(ClaoueÂ-Long et al., 1995) as follows:
206
Pb=238 U A £ 206 Pb1 =238 U1 obs = 238 U16 O1 =238 U1 2obs
where A, ( 206Pb 1/ 238U 1)obs, and ( 238U 16O 1/ 238U 1)obs denote
a constant, observed secondary 206Pb 1/ 238U 1 and
238 16 1 238 1
U O / U ratios, respectively. The constant A was
obtained by repeated measurements of standard zircons.
Subtraction of common Pb from measured Pb is required
to estimate the accurate age. In this study, measured
208
Pb/ 206Pb ratio in each grain was used for the correction
of common Pb (Compston et al., 1984). This method is
effective for low Th/U samples such as zircon. Although
204
Pb/ 206Pb was measured to con®rm the presence of
common Pb, the correction of common Pb using
204
Pb/ 206Pb causes a large analytical uncertainty because
of minor 204Pb abundance. Use of measured 207Pb/ 206Pb
for the correction of common Pb is also possible, but radiogenic 207Pb/ 206Pb in sample must be assumed. An advantage
of the use of 208Pb/ 206Pb correction is that measured
207
Pb/ 206Pb can be used to calculate the radiogenic
207
Pb/ 206Pb. Experimental details are given elsewhere
(Sano et al., 1999a,b).
Table 1 lists zircon data of measured U concentration,
204
Pb/ 206Pb, 207Pb/ 206Pb, 208Pb/ 206Pb and 238U/ 206Pb ratios,
and radiogenic 238U/ 206Pb ages for the Kannack complex
of the Kontum massif, Vietnam. Fig. 5 shows a Tera±
Wasserburg U±Pb zircon concordia diagram for twentyfour analyses listing in Table 1. Most of the zircon grains
are concordant within an experimental error. Twenty-three
analyses yield an age of 253.7 ^ 11.6 Ma. There is only one
age of 1404 ^ 34 Ma, which is distinct from the others
(VTM08.1 in Table 1; Fig. 5).
T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
Fig. 3. Microphotographs of zircon under transmitted light, showing euhedral shape of individual zircon grain (three grains 15, 16 and 17 are not shown). Labeled-grain numbers are the same as those in Table 1.
Scale bar is 0.5 mm.
81
82
T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
Fig. 4. Representative EPMA backscatter images of zircons from the granulite complex of the Kontum massif. Three types of zoning pattern: (a) homogeneous, (b) large core and narrow rim, and (c) oscillatory zoning, corresponding to grain (3), (8) and (18) in Fig. 3, respectively. (d) Enlarge zoned core of (c).
Positions of spots of SHRIMP U±Pb analyzing are shown. Scale bars on (a, b and c) are 100 mm.
5. Discussion
Zircon is the most commonly used mineral for dating
both high-grade metamorphic events and magmatic solidi®cation. The closure temperature of zircon U±Pb system is
recently proposed to be greater than 8508C (e.g., ClaoueÂLong et al., 1995; Lee et al., 1997; Sano et al., 1999a),
though it has previously been commonly accepted to be
,7508C (e.g., Mattinson, 1978). Lee et al. (1997) have estimated the closure temperature for U±Th±Pb isotopic
system in natural zircon to be greater than 9008C. The
peak metamorphic conditions recorded in the studied granulite were estimated to be 800±8508C, and analyzed zircons
have perfectly euhedral shapes (Fig. 3), and mostly appear
to be homogeneous in zoning pattern (type (a)), suggesting
their growth in a melting environment, as mentioned in
previous sections. Assuming the closure temperature for
zircon U±Pb system is higher than 8508C, ages of
253.7 ^ 11.6 Ma obtained from twenty three analyses in
this study (Table 1; Fig. 5) are therefore interpreted to be
the age of a high-grade metamorphic event that formed
granulites of the Kontum massif.
An apparent concordant U±Pb age of ca. 1400 Ma was
obtained on the core of oscillatory zoned zircon (Fig. 4c).
This old core was surrounded by a younger (250 ^ 8 Ma)
rim, which is consistent with the ages of other analyzed
zircons. The oscillatory zoned zircon could recrystallize
during later high-grade metamorphism (Pidgeon, 1992).
Radiogenic Pb can be partially lost during recrystallization
and other processes, including radiation damage, selfannealing and chemical reaction (e.g., Pidgeon, 1992;
Sano et al., 1999a). Therefore, the inherited age of ca.
1400 Ma could be tentatively interpreted as a minimum
age for the protolith of the granulites. Furthermore, the
inherited age obtained here is generally consistent with
previous Rb±Sr ages of 1400±1600 Ma reported for Proterozoic formations of the Kontum massif (Phan Truong Thi,
1985; DGMVN, 1989). There is also a possibility that the
old inherited zoned core has formed from an earlier highgrade metamorphism occurring in Middle-Proterozoic times
(1400±1600 Ma).
Granulite occurrences have been known for a wide range
of ages, including ages as young as 700±500 Ma (e.g., PanAfrican, India and Sri Lanka) and 300 Ma (Hercynian
83
T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
Table 1
SHRIMP II U-Pb analyses of zircons from a granulite of the Kontum massif, Vietnam a
Sample
U (ppm)
204
Pb/ 206Pb
VTM01.1
VTM01.2
VTM02.1
VTM03.1
VTM04.1
VTM05.1
VTM06.1
VTM06.2
VTM07.1
VTM08.1
VTM08.2
VTM09.1
VTM10.1
VTM11.1
VTM11.2
VTM12.1
VTM13.1
VTM13.2
VTM14.1
VTM15.1
VTM16.1
VTM17.1
VTM18.1
VTM18.2
37
214
88
225
98
110
272
139
630
335
117
185
161
393
132
180
186
763
374
209
237
179
861
189
0.00293 ^ 0.00078
0.00031 ^ 0.00010
0.00091 ^ 0.00029
0.00032 ^ 0.00009
0.00093 ^ 0.00028
0.00071 ^ 0.00030
, 0.00006
0.00070 ^ 0.00025
0.00007 ^ 0.00002
0.00004 ^ 0.00002
0.00075 ^ 0.00033
, 0.00018
0.00053 ^ 0.00016
0.00013 ^ 0.00006
0.00050 ^ 0.00020
0.00047 ^ 0.00017
0.00032 ^ 0.00015
0.00003 ^ 0.00001
0.00016 ^ 0.00009
0.00040 ^ 0.00016
0.00027 ^ 0.00012
0.00052 ^ 0.00018
0.00008 ^ 0.00003
0.00031 ^ 0.00013
207
Pb/ 206Pb
208
0.0658 ^ 0.0045
0.0532 ^ 0.0011
0.0604 ^ 0.0015
0.0539 ^ 0.0010
0.0572 ^ 0.0019
0.0567 ^ 0.0017
0.0523 ^ 0.0009
0.0541 ^ 0.0011
0.0519 ^ 0.0005
0.0921 ^ 0.0011
0.0555 ^ 0.0018
0.0533 ^ 0.0011
0.0544 ^ 0.0012
0.0528 ^ 0.0009
0.0544 ^ 0.0015
0.0545 ^ 0.0012
0.0533 ^ 0.0012
0.0530 ^ 0.0005
0.0528 ^ 0.0007
0.0533 ^ 0.0010
0.0542 ^ 0.0014
0.0567 ^ 0.0012
0.0522 ^ 0.0007
0.0537 ^ 0.0011
Pb/ 206Pb
0.403 ^ 0.022
0.696 ^ 0.012
0.367 ^ 0.015
0.212 ^ 0.005
0.244 ^ 0.011
0.240 ^ 0.013
0.161 ^ 0.004
0.186 ^ 0.005
0.034 ^ 0.001
0.181 ^ 0.002
0.217 ^ 0.010
0.127 ^ 0.005
0.191 ^ 0.009
0.129 ^ 0.005
0.188 ^ 0.006
0.162 ^ 0.005
0.177 ^ 0.007
0.085 ^ 0.002
0.073 ^ 0.003
0.146 ^ 0.005
0.135 ^ 0.004
0.209 ^ 0.008
0.031 ^ 0.001
0.157 ^ 0.005
238
U/ 206Pb
24.04 ^ 0.77
25.95 ^ 0.90
25.17 ^ 0.93
24.62 ^ 0.36
25.68 ^ 0.90
25.02 ^ 0.55
25.44 ^ 0.57
25.47 ^ 0.67
24.41 ^ 0.53
4.10 ^ 0.11
25.13 ^ 0.79
25.28 ^ 0.72
24.50 ^ 0.70
24.31 ^ 0.79
25.04 ^ 0.52
24.91 ^ 0.69
24.29 ^ 0.54
24.86 ^ 0.89
24.22 ^ 0.43
24.70 ^ 0.50
24.33 ^ 0.62
23.67 ^ 0.48
24.32 ^ 0.89
25.54 ^ 0.91
238
U± 206Pb * age (Ma)
256.5 ^ 9.3
247.9 ^ 14.7
246.7 ^ 10.1
256.8 ^ 3.9
244.0 ^ 8.9
251.8 ^ 5.9
246.8 ^ 5.5
246.8 ^ 6.5
259.0 ^ 5.5
1404 ^ 34
249.5 ^ 8.1
248.8 ^ 7.1
256.6 ^ 7.4
259.6 ^ 8.4
252.1 ^ 5.3
253.2 ^ 7.0
258.5 ^ 5.8
254.2 ^ 9.0
260.5 ^ 4.5
255.8 ^ 5.1
259.9 ^ 6.6
265.7 ^ 5.5
259.4 ^ 9.3
245.7 ^ 8.7
a
Sample numbers show each grain of zircon analyzed. Sub-numbers such as VTM01.1 and VTM01.2 indicate a different pit position in a single grain;
generally X.1 shows core and X.2 is rim of the zircon. Note that there is a signi®cant change of U concentration even in the single grain. Errors assigned to the
isotopic, elemental ratios and the radiogenic ages are two-sigma level.
granulites of Europe), although Archean and Proterozoic
ages are most common for granulites (see Harley, 1989;
and references therein). This present study strongly indicates an Indosinian age for granulites of the Kontum massif
instead of Archean as proposed by previous workers. The
Indosinian orogeny (ca. 250±240 Ma) has affected the Indochina craton and is well documented by recent advanced
geochronological data for several metamorphic belts (e.g.,
Maluski et al., 1995, 1997; Rangin et al., 1995; Lepvrier et
al., 1997; Nam, 1998). The granulites in this study could
provide the ®rst example of an Indosinian age for granulites
worldwide.
From the above discussions, it is inferred that the
Kannack complex in the Kontum massif has suffered a
tectonothermal event, that occurred at granulite facies metamorphism conditions of 800±8508C during Indosinian times
(254±240 Ma). The cooling history and subsequent exhumation of the Kannack complex that occurred during Indosinian times could be calculated by using present U±Pb 254
Ma zircon ages and the K±Ar 242 Ma biotite age (Nam,
1998). Calculated result shows that the Kannack complex
started to cool from ,8508C (peak of metamorphic conditions) at 254 Ma to 3008C (the closure temperature of
biotite K±Ar system) at 242 Ma, with an average cooling
rate of ,458C/Ma.
Fig. 5. Tera±Wasserburg U±Pb zircon concordia diagram for the granulite
complex of the Kontum massif, Vietnam. Twenty-three data cluster at
253.7 ^ 11.6 Ma, and one other from a zircon core yields concordant age
of 1404 ^ 34 Ma. Errors are depicted at two-sigma level. The solid curve
indicates concordant age.
6. Conclusions
1. Granulites of the Kannack complex of the Kontum
massif (Vietnam) have been generated during Indosinian
84
T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
times as shown by the ca. 254 Ma age of most SHRIMP
U±Pb zircon analyses.
2. An inherited age of ca. 1400 Ma recorded in an oscillatory zoned zircon core indicates a minimum age for the
protolith of studied granulites as being Middle Proterozoic.
3. The cooling history and subsequent exhumation of the
Kannack complex (the Kontum massif, Vietnam)
occurred during Indosinian times with an average cooling
rate of ,458C/Ma between ,8508C at 254 Ma and 3008C
at 242 Ma.
Acknowledgements
Authors thank Dr. Trinh Van Long for helpful discussion
and Dr. H. Yoshida for EPMA assistance. Careful reviews
by Dr. Henri Maluski and an anonymous reviewer improved
the manuscript. A post-doctoral fellowship from the Japan
Society for the Promotion of Science to TNN is gratefully
acknowledged. This study is supported in part by a Grant-in
Aid (ID. No. P98376) from the Ministry of Education,
Science, Sport and Culture, Government of Japan
(Monbusho). This is contribution No. 8 of the SHRIMP
laboratory at Hiroshima University.
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www.elsevier.nl/locate/jseaes
First SHRIMP U±Pb zircon dating of granulites from the Kontum massif
(Vietnam) and tectonothermal implications
Tran Ngoc Nam a,*, Yuji Sano b, Kentaro Terada b, Mitsuhiro Toriumi c, Phan Van Quynh d,
Le Tien Dung e
b
a
Department of Geosciences, Hue University of Science, 27-Nguyen Hue, Hue City, Viet Nam
Department of Earth and Planetary Sciences, Hiroshima University, Kagamiyama 1-3, Higashi-Hiroshima 739-8526, Japan
c
Geological Institute, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan
d
Department of Cartography and Remote Sensing, Hanoi National University, 334-Nguyen Trai, Hanoi, Viet Nam
e
Department of Geology, University of Mining and Geology, Tu Liem, Hanoi, Viet Nam
Abstract
The Kontum massif in Central Vietnam represents the largest continuous exposure of crystalline basement of the Indochina craton. The
central Kontum massif is chie¯y made of orthopyroxene granulites (enderbite, charnockite) and associated rocks of the Kannack complex.
Mineral assemblages and geothermobarometric studies have shown that the Kannack complex has severely metamorphosed under granulite
facies corresponding to P±T conditions of 800±8508C and 8 ^ 1 kbars. Twenty-three SHRIMP II U±Pb analyses of eighteen zircon grains
separated from a granulite sample of the Kannack complex yield ca 254 Ma, and one analysis gives ca 1400 Ma concordant age for a zoned
zircon core. This result shows that granulites of the Kannack complex in the Kontum massif have formed from a high-grade granulite facies
tectonothermal event of Indosinian age (Triassic). The cooling history and subsequent exhumation of the Kannack complex during Indosinian times ranged from ,8508C at ca 254 Ma to ,3008C at 242 Ma, with an average cooling rate of ,458C/Ma. q 2001 Elsevier Science
Ltd. All rights reserved.
1. Introduction
Granulites are recognized as a potential source of important information toward understanding deep crustal compositions and processes, and early tectonothermal events. The
Kontum massif (Central Vietnam) is composed of such
granulites. Orthopyroxene granulites (charnockites, enderbite) and associated rocks, which are similar to those of
Eastern Ghats (India) and East Antarctica, form the central
part of the Kontum massif (Fig. 1). They are grouped into
the gneiss complexes under local names (Figs. 1 and 2;
Department of Geology and Minerals of Vietnam Ð
DGMVN, 1989, 1995). As it was once held that granulites
were only formed in Archean and Early Proterozoic times,
the granulites of the Kontum massif were thought to be of
Archean age by previous workers (e.g., Tran Quoc Hai,
1986; DGMVN, 1989, 1995; Hutchison, 1989). The
Archean ages of these granulites have been suggested
based on the common occurrence of charnockitic rocks
* Corresponding author. Current address: Geological Institute, the
University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan. Fax: 181-35841-4569.
E-mail address: [email protected] (T.N. Nam).
(Tran Quoc Hai, 1986; Hutchison, 1989; DGMVN, 1989,
1995). However, the Archean ages are hardly accepted
because no available geochronological data have yet been
documented for these granulites.
Recent advanced Ar±Ar and K±Ar geochronological
studies have revealed a wide occurrence of three tectonothermal episodes in Indochina. They are the Triassic
Indosinian, Late Jurassic±Early Cretaceous in the Truong
Son belt north of the Kontum massif (e.g., Maluski et al.,
1995, 1997; Rangin et al., 1995; Lepvrier et al., 1997; Nam,
1998), and Tertiary in the Bu Khang gneiss dome north of
the Truong Son belt (e.g., Jolivet et al., 1999) and in the Red
River shear zone (e.g., Nam, 1998; Wang et al., 1998). PreIndosinian tectonothermal events have been poorly documented by geochronological data in this region, and
geochronological data of the Kontum massif remain scarce.
Since granulites were formed at high temperature conditions
under deep crustal levels, U±Pb zircon dating of such granulites could provide valuable evidence for high-temperature
stages of the tectonothermal evolution of the Kontum massif
that may be dif®cult to obtain by Ar±Ar or K±Ar techniques. This present study was undertaken in an attempt to
determine the timing of tectonothermal events recorded in
zoned zircons from a main granulite complex in the central
1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S 1367-912 0(00)00015-8
78
T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
Fig. 1. Simpli®ed map of the northern Kontum massif (after Nam, 1998). Box corresponds to Fig. 2.
Kontum massif. This is the ®rst SHRIMP U±Pb zircon data
documented for the complex, which was previously thought
to be the oldest unit (Archean in ages) in Indochina (e.g.,
Tran Quoc Hai, 1986; Hutchison, 1989; DGMVN, 1989,
1995).
2. Geological setting
The Kontum massif represents the largest continuous
exposure of crystalline basement of the Indochina craton,
which was once regarded as the stable continental core of
South-East Asia (Hutchison, 1989). The Kontum massif is
mainly composed of high-grade metamorphic rocks which
were thought to be Precambrian in ages by previous workers
(e.g., Tran Quoc Hai, 1986; Hutchison, 1989; DGMVN,
1989, 1995). Paleozoic sedimentary rocks are mostly absent
there. The massif is partly covered by Mesozoic volcanosedimentary formations and Neogene±Quaternary basalts,
and is intruded by Paleo-Mesozoic granodiorite bodies
(Figs. 1 and 2; DGMVN, 1989, 1995; Nam, 1998).
High-grade gneiss rocks of the Kontum massif (Vietnam)
are grouped into different units under local names, including
the Kannack complex and associated intrusive complexes,
the Ngoc Linh complex, the Kham Duc and Poko formations (DGMVN, 1989, 1995). The Kannack complex occupies the central and southern Kontum massif (Fig. 1). It is
subdivided into three parts: (1) lower, (2) middle and (3)
upper (Fig. 2). The lower part is composed of two-pyroxene
and hyperthene-garnet-bearing granulites in close connection with autochthonous plutons of orthopyroxene-bearing
granites (charnockites and enderbites), and cordierite±sillimanite-bearing gneisses (khondalites). The thickness of the
lower part is estimated to be 700±1000 m. The middle part
is mainly composed of plagioclase±biotite±hypersthene
gneiss, two-pyroxene plagioclase gneiss, biotite±sillimanite±cordierite gneiss and migmatite rocks, with an estimated thickness of 500±900 m. The upper part is chie¯y
made of garnet±cordierite±sillimanite±biotite gneiss,
cordierite±sillimanite schist, and quartzite. Several calciphyre and marble intercalations are found among the gneiss
and schist. The upper part is estimated to be 700 m thick.
Autochthonous plutons of gabbrogranulite, enderbites
and charnockites, and biotite±garnet±cordierite granite are
distinctly placed into the Konkbang, Songba and Pleimanko
complexes, respectively (Fig. 2). Plutonic bodies show
massive, granitic texture in the center and gradually change
to strongly foliated rocks along the peripheries. Foliation of
these rocks is commonly parallel to those of host gneisses.
The boundary between plutonic bodies and host gneisses or
migmatites is often unclear because of the gradual change.
Several small lenses of host gneisses (two-pyroxene and
hypersthene granulites) ranging from 1±2 cm to 2±3 m in
thickness are found within the enderbite±charnockite
bodies.
Mineral assemblages of the Kannack complex include: (i)
plagioclase (Pl) 1 clinopyroxene (Cpx) 1 orthopyroxene
(Opx) 1 quartz (Q), (ii) Pl 1 hypersthene (Hyp) 1 garnet
(Gt) 1 Q, and (iii) Pl 1 Hyp 1 K-feldspar (Kfs) 1 Q in
the lower part; (iv) Q 1 Pl 1 Kfs 1 sillimanite (Sil) 1
Gt 1 Bi ^ cordierite (Cord), (v) Q 1 Pl 1 Kfs 1 Cord 1
Hyp 1 Sil 1 Gt, and (vi) Q 1 Pl 1 Kfs 1 spinel 1 Hyp 1
Cord 1 Gt in the middle part; (vii) calcite 1 diopside,
(viii) calcite 1 oligoclase 1 garnet ^ dolomite, and (ix)
plagioclase 1 diopside 1 quartz in the upper part. These
assemblages are typical of granulite facies metamorphism.
Geothermobarometric studies of the complex have yielded
P±T conditions of 800±8508C (by garnet±cordierite and
T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
79
Fig. 2. Structural sketch map and cross-section of Kannack area, the central Kontum massif (see Fig. 1 for location). Kannack granulite complex: 1. Twopyroxene granulites, interbeds of biotite±sillimanite±cordierite±garnet gneiss, enderbite and charnockite; 2. Plagioclase±biotite±hypersthene gneiss, interbeds of two-pyroxene gneiss, quartz±biotite±sillimanite±garnet±cordierite schist and migmatites; 3. Garnet±cordierite±sillimanite±biotite gneiss, cordierite±sillimanite schist and quartzite; 4. Marble and calciphyre. Igneous rocks associated with the Kannack complex: 5. Gabbrogranulite (the Konkbang
complex); 6. Enderbite and charnockite (the Songba complex); 7. Biotite±garnet±cordierite granite (the Pleimanko complex); 8. Paleo-Mesozoic granite;
9. Middle Triassic rhyolite; 10. Neogene±Quaternary basalt; 11. Quaternary alluvia; 12. Fault; 13. Foliation; 14. Sampling locality.
two-pyroxene geothermometry), and 8 ^ 1 kbars (by
garnet±orthopyroxene±plagioclase±quartz geobarometry)
(DGMVN, 1989). Some additional calibrations using
garnet±biotite and garnet±hornblende geothermometry
give estimations of 700±7508C, regarded as indicating the
conditions of a later retrograde stage of metamorphism. The
above mineral assemblages and geothermobarometric
results therefore indicate peak metamorphism P±T conditions for the Kannack complex to be 800±8508C and 8 ^ 1
kbars. The Kannack complex and associated intrusive
complexes were previously proposed to be Archean in age
on the basis of petrological correlation with classic Archean
granulites in other parts of the world (Phan Truong Thi,
1985; Tran Quoc Hai, 1986; DGMVN, 1989, 1995).
However, the proposed Archean age is hardly accepted
without available geochronological data. Few geochronological data have been reported for the complex, including a
K±Ar age of 242 Ma obtained on a biotite separate (Nam,
1998), and a K±Ar age range of 1650±1810 Ma, which was
from `a personal communication' source but often cited by
previous workers (e.g., Phan Truong Thi, 1985; Tran Quoc
Hai, 1986; Hutchison, 1989).
Other metamorphic formations, which were thought to be
Precambrian (e.g., DGMVN, 1989, 1995), are widely
distributed in the northern Kontum massif (Fig. 1). They
are composed of biotite±sillimanite gneiss, amphibolite,
biotite schist, migmatite rocks and lenses of marble. This
rock assemblage suggests amphibolite facies metamorphism. Some Rb±Sr ages of 1400±1600 Ma have been
reported for these formations (Phan Truong Thi, 1985).
80
T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
The Poko formation south-east of Kontum town (see location in Fig. 1) contains some characteristic algal stromatolites, suggesting an Upper Proterozoic age (DGMVN,
1989). Precambrian rocks are strongly foliated (DGMVN,
1989). The foliation is folded to form gentle folds with
commonly North±South trending axes.
Mesozoic volcano-sedimentary formations are composed
of andesite, rhyolite and tuffs, and conglomerates, sandstone
and siliceous shale of Late Triassic±Early Jurassic and
Cretaceous ages. Neogene±Quaternary basalt is mainly of
tholeiitic and sub-alkaline olivine compositions, ranging
from 16 to 0.2 Ma in age (DGMVN, 1989, 1995; Hoang
and Flower, 1998; Lee et al., 1998).
3. Sample and mineral descriptions
Zircons analyzed in this study were separated from a
granulite sample (KT13108 in Fig. 2) collected from the
middle part of the Kannack complex in the central Kontum
massif, Vietnam (Fig. 2). The sample is coarse grained and
has a gneissic texture. Mineral constituents of the sample are
quartz (,25±30%), plagioclase (,25±35%), K-feldspar
(,10±15%), garnet (,10%), sillimanite (,7±10%), biotite
(,5±10%), and cordierite. Apatite and zircon are common
accessory minerals in the sample.
Zircons were concentrated by crushing, sieving, and
mineral separation with an isodynamic separator and
heavy-liquid, and hand-picking using a binocular microscope. Zircons occur as an individual crystal with an euhedral shape, and range from 0.2 to 0.5 mm in size (Fig. 3).
The perfectly euhedral shape suggests that zircons have
®nally grown in an environment that supposes presence of
liquid melt (i.e., likely magmatic zircon). Fig. 4 shows
representative EMPA backscatter images of zircon grains,
in which three types of zoning patterns can be seen: (a)
perfectly homogeneous from central to outer part, (b)
large homogeneous core and a narrow rim, and (c) oscillatory zoning. Among eighteen grains analyzed in this
study, twelve grains (67% of zircon population) have
type (a), ®ve grains (27% of zircon population) display
type (b) and one grain does display type (c). Types (b)
and (c) suggest likely overgrowths of zircon, whereas the
type (a) indicates one-stage growth. SHRIMP U±Pb
analyses have been performed on both cores and rims of
overgrowth zircons.
4. Analytical methods and results
Analyzed zircon grains were mounted in epoxy-resin disk
with several grains of the standard zircon SL13 and QGNG.
SL13 is the well-known Sri Lanka 572 Ma megacryst extensively used by the Australian National University SHRIMP
group as a U/Pb and abundance calibration standard
(Roddick and van Breemen, 1994; ClaoueÂ-Long et al.,
1995; Williams, 1998), and QGNG is a new multicrystal
zircon standard from Quartz±Gabbro±Norite±Gneiss
(QGNG) from Cape Donnington, Eyre Peninsula, South
Australia whose TIMS U/Pb age is 1850 ^ 2 Ma (2s )
(C.M. Fanning, personal communication, 1997). Zircon
grains were polished to provide a ¯at surface for sputtering
of secondary ions until they were exposed through their
mid-sections. After polishing (by using 0.25 mm diamond
paste), they were coated by thin gold plate to prevent charging of the sample surface by the primary ion beam.
The samples were evacuated in the sample lock overnight
and introduced into the sample stage in the ion source chamber. A 3-nA mass-®ltered O22 primary beam was focused to
sputter a 30-mm-diameter area with positive ions extracted.
Before the actual analysis, the sample surface was rastered
for 3 min in order to clean up the surface of the grain and
eliminatory possible contaminants. The magnet was cyclically peak-stepped through a series of mass numbers
ranging from mass 196 for 90Zr216O 1 to mass 254 for
238 16 1
U O , including the background at mass number
204.1, and Pb isotopic mass numbers at 204, 206, 207,
and 208, and the atomic U peak at the number 238 and
232
Th 16O peak at number 248. The 206Pb/ 238U ratios in the
samples were calibrated using an empirical relationship
(ClaoueÂ-Long et al., 1995) as follows:
206
Pb=238 U A £ 206 Pb1 =238 U1 obs = 238 U16 O1 =238 U1 2obs
where A, ( 206Pb 1/ 238U 1)obs, and ( 238U 16O 1/ 238U 1)obs denote
a constant, observed secondary 206Pb 1/ 238U 1 and
238 16 1 238 1
U O / U ratios, respectively. The constant A was
obtained by repeated measurements of standard zircons.
Subtraction of common Pb from measured Pb is required
to estimate the accurate age. In this study, measured
208
Pb/ 206Pb ratio in each grain was used for the correction
of common Pb (Compston et al., 1984). This method is
effective for low Th/U samples such as zircon. Although
204
Pb/ 206Pb was measured to con®rm the presence of
common Pb, the correction of common Pb using
204
Pb/ 206Pb causes a large analytical uncertainty because
of minor 204Pb abundance. Use of measured 207Pb/ 206Pb
for the correction of common Pb is also possible, but radiogenic 207Pb/ 206Pb in sample must be assumed. An advantage
of the use of 208Pb/ 206Pb correction is that measured
207
Pb/ 206Pb can be used to calculate the radiogenic
207
Pb/ 206Pb. Experimental details are given elsewhere
(Sano et al., 1999a,b).
Table 1 lists zircon data of measured U concentration,
204
Pb/ 206Pb, 207Pb/ 206Pb, 208Pb/ 206Pb and 238U/ 206Pb ratios,
and radiogenic 238U/ 206Pb ages for the Kannack complex
of the Kontum massif, Vietnam. Fig. 5 shows a Tera±
Wasserburg U±Pb zircon concordia diagram for twentyfour analyses listing in Table 1. Most of the zircon grains
are concordant within an experimental error. Twenty-three
analyses yield an age of 253.7 ^ 11.6 Ma. There is only one
age of 1404 ^ 34 Ma, which is distinct from the others
(VTM08.1 in Table 1; Fig. 5).
T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
Fig. 3. Microphotographs of zircon under transmitted light, showing euhedral shape of individual zircon grain (three grains 15, 16 and 17 are not shown). Labeled-grain numbers are the same as those in Table 1.
Scale bar is 0.5 mm.
81
82
T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
Fig. 4. Representative EPMA backscatter images of zircons from the granulite complex of the Kontum massif. Three types of zoning pattern: (a) homogeneous, (b) large core and narrow rim, and (c) oscillatory zoning, corresponding to grain (3), (8) and (18) in Fig. 3, respectively. (d) Enlarge zoned core of (c).
Positions of spots of SHRIMP U±Pb analyzing are shown. Scale bars on (a, b and c) are 100 mm.
5. Discussion
Zircon is the most commonly used mineral for dating
both high-grade metamorphic events and magmatic solidi®cation. The closure temperature of zircon U±Pb system is
recently proposed to be greater than 8508C (e.g., ClaoueÂLong et al., 1995; Lee et al., 1997; Sano et al., 1999a),
though it has previously been commonly accepted to be
,7508C (e.g., Mattinson, 1978). Lee et al. (1997) have estimated the closure temperature for U±Th±Pb isotopic
system in natural zircon to be greater than 9008C. The
peak metamorphic conditions recorded in the studied granulite were estimated to be 800±8508C, and analyzed zircons
have perfectly euhedral shapes (Fig. 3), and mostly appear
to be homogeneous in zoning pattern (type (a)), suggesting
their growth in a melting environment, as mentioned in
previous sections. Assuming the closure temperature for
zircon U±Pb system is higher than 8508C, ages of
253.7 ^ 11.6 Ma obtained from twenty three analyses in
this study (Table 1; Fig. 5) are therefore interpreted to be
the age of a high-grade metamorphic event that formed
granulites of the Kontum massif.
An apparent concordant U±Pb age of ca. 1400 Ma was
obtained on the core of oscillatory zoned zircon (Fig. 4c).
This old core was surrounded by a younger (250 ^ 8 Ma)
rim, which is consistent with the ages of other analyzed
zircons. The oscillatory zoned zircon could recrystallize
during later high-grade metamorphism (Pidgeon, 1992).
Radiogenic Pb can be partially lost during recrystallization
and other processes, including radiation damage, selfannealing and chemical reaction (e.g., Pidgeon, 1992;
Sano et al., 1999a). Therefore, the inherited age of ca.
1400 Ma could be tentatively interpreted as a minimum
age for the protolith of the granulites. Furthermore, the
inherited age obtained here is generally consistent with
previous Rb±Sr ages of 1400±1600 Ma reported for Proterozoic formations of the Kontum massif (Phan Truong Thi,
1985; DGMVN, 1989). There is also a possibility that the
old inherited zoned core has formed from an earlier highgrade metamorphism occurring in Middle-Proterozoic times
(1400±1600 Ma).
Granulite occurrences have been known for a wide range
of ages, including ages as young as 700±500 Ma (e.g., PanAfrican, India and Sri Lanka) and 300 Ma (Hercynian
83
T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
Table 1
SHRIMP II U-Pb analyses of zircons from a granulite of the Kontum massif, Vietnam a
Sample
U (ppm)
204
Pb/ 206Pb
VTM01.1
VTM01.2
VTM02.1
VTM03.1
VTM04.1
VTM05.1
VTM06.1
VTM06.2
VTM07.1
VTM08.1
VTM08.2
VTM09.1
VTM10.1
VTM11.1
VTM11.2
VTM12.1
VTM13.1
VTM13.2
VTM14.1
VTM15.1
VTM16.1
VTM17.1
VTM18.1
VTM18.2
37
214
88
225
98
110
272
139
630
335
117
185
161
393
132
180
186
763
374
209
237
179
861
189
0.00293 ^ 0.00078
0.00031 ^ 0.00010
0.00091 ^ 0.00029
0.00032 ^ 0.00009
0.00093 ^ 0.00028
0.00071 ^ 0.00030
, 0.00006
0.00070 ^ 0.00025
0.00007 ^ 0.00002
0.00004 ^ 0.00002
0.00075 ^ 0.00033
, 0.00018
0.00053 ^ 0.00016
0.00013 ^ 0.00006
0.00050 ^ 0.00020
0.00047 ^ 0.00017
0.00032 ^ 0.00015
0.00003 ^ 0.00001
0.00016 ^ 0.00009
0.00040 ^ 0.00016
0.00027 ^ 0.00012
0.00052 ^ 0.00018
0.00008 ^ 0.00003
0.00031 ^ 0.00013
207
Pb/ 206Pb
208
0.0658 ^ 0.0045
0.0532 ^ 0.0011
0.0604 ^ 0.0015
0.0539 ^ 0.0010
0.0572 ^ 0.0019
0.0567 ^ 0.0017
0.0523 ^ 0.0009
0.0541 ^ 0.0011
0.0519 ^ 0.0005
0.0921 ^ 0.0011
0.0555 ^ 0.0018
0.0533 ^ 0.0011
0.0544 ^ 0.0012
0.0528 ^ 0.0009
0.0544 ^ 0.0015
0.0545 ^ 0.0012
0.0533 ^ 0.0012
0.0530 ^ 0.0005
0.0528 ^ 0.0007
0.0533 ^ 0.0010
0.0542 ^ 0.0014
0.0567 ^ 0.0012
0.0522 ^ 0.0007
0.0537 ^ 0.0011
Pb/ 206Pb
0.403 ^ 0.022
0.696 ^ 0.012
0.367 ^ 0.015
0.212 ^ 0.005
0.244 ^ 0.011
0.240 ^ 0.013
0.161 ^ 0.004
0.186 ^ 0.005
0.034 ^ 0.001
0.181 ^ 0.002
0.217 ^ 0.010
0.127 ^ 0.005
0.191 ^ 0.009
0.129 ^ 0.005
0.188 ^ 0.006
0.162 ^ 0.005
0.177 ^ 0.007
0.085 ^ 0.002
0.073 ^ 0.003
0.146 ^ 0.005
0.135 ^ 0.004
0.209 ^ 0.008
0.031 ^ 0.001
0.157 ^ 0.005
238
U/ 206Pb
24.04 ^ 0.77
25.95 ^ 0.90
25.17 ^ 0.93
24.62 ^ 0.36
25.68 ^ 0.90
25.02 ^ 0.55
25.44 ^ 0.57
25.47 ^ 0.67
24.41 ^ 0.53
4.10 ^ 0.11
25.13 ^ 0.79
25.28 ^ 0.72
24.50 ^ 0.70
24.31 ^ 0.79
25.04 ^ 0.52
24.91 ^ 0.69
24.29 ^ 0.54
24.86 ^ 0.89
24.22 ^ 0.43
24.70 ^ 0.50
24.33 ^ 0.62
23.67 ^ 0.48
24.32 ^ 0.89
25.54 ^ 0.91
238
U± 206Pb * age (Ma)
256.5 ^ 9.3
247.9 ^ 14.7
246.7 ^ 10.1
256.8 ^ 3.9
244.0 ^ 8.9
251.8 ^ 5.9
246.8 ^ 5.5
246.8 ^ 6.5
259.0 ^ 5.5
1404 ^ 34
249.5 ^ 8.1
248.8 ^ 7.1
256.6 ^ 7.4
259.6 ^ 8.4
252.1 ^ 5.3
253.2 ^ 7.0
258.5 ^ 5.8
254.2 ^ 9.0
260.5 ^ 4.5
255.8 ^ 5.1
259.9 ^ 6.6
265.7 ^ 5.5
259.4 ^ 9.3
245.7 ^ 8.7
a
Sample numbers show each grain of zircon analyzed. Sub-numbers such as VTM01.1 and VTM01.2 indicate a different pit position in a single grain;
generally X.1 shows core and X.2 is rim of the zircon. Note that there is a signi®cant change of U concentration even in the single grain. Errors assigned to the
isotopic, elemental ratios and the radiogenic ages are two-sigma level.
granulites of Europe), although Archean and Proterozoic
ages are most common for granulites (see Harley, 1989;
and references therein). This present study strongly indicates an Indosinian age for granulites of the Kontum massif
instead of Archean as proposed by previous workers. The
Indosinian orogeny (ca. 250±240 Ma) has affected the Indochina craton and is well documented by recent advanced
geochronological data for several metamorphic belts (e.g.,
Maluski et al., 1995, 1997; Rangin et al., 1995; Lepvrier et
al., 1997; Nam, 1998). The granulites in this study could
provide the ®rst example of an Indosinian age for granulites
worldwide.
From the above discussions, it is inferred that the
Kannack complex in the Kontum massif has suffered a
tectonothermal event, that occurred at granulite facies metamorphism conditions of 800±8508C during Indosinian times
(254±240 Ma). The cooling history and subsequent exhumation of the Kannack complex that occurred during Indosinian times could be calculated by using present U±Pb 254
Ma zircon ages and the K±Ar 242 Ma biotite age (Nam,
1998). Calculated result shows that the Kannack complex
started to cool from ,8508C (peak of metamorphic conditions) at 254 Ma to 3008C (the closure temperature of
biotite K±Ar system) at 242 Ma, with an average cooling
rate of ,458C/Ma.
Fig. 5. Tera±Wasserburg U±Pb zircon concordia diagram for the granulite
complex of the Kontum massif, Vietnam. Twenty-three data cluster at
253.7 ^ 11.6 Ma, and one other from a zircon core yields concordant age
of 1404 ^ 34 Ma. Errors are depicted at two-sigma level. The solid curve
indicates concordant age.
6. Conclusions
1. Granulites of the Kannack complex of the Kontum
massif (Vietnam) have been generated during Indosinian
84
T.N. Nam et al. / Journal of Asian Earth Sciences 19 (2001) 77±84
times as shown by the ca. 254 Ma age of most SHRIMP
U±Pb zircon analyses.
2. An inherited age of ca. 1400 Ma recorded in an oscillatory zoned zircon core indicates a minimum age for the
protolith of studied granulites as being Middle Proterozoic.
3. The cooling history and subsequent exhumation of the
Kannack complex (the Kontum massif, Vietnam)
occurred during Indosinian times with an average cooling
rate of ,458C/Ma between ,8508C at 254 Ma and 3008C
at 242 Ma.
Acknowledgements
Authors thank Dr. Trinh Van Long for helpful discussion
and Dr. H. Yoshida for EPMA assistance. Careful reviews
by Dr. Henri Maluski and an anonymous reviewer improved
the manuscript. A post-doctoral fellowship from the Japan
Society for the Promotion of Science to TNN is gratefully
acknowledged. This study is supported in part by a Grant-in
Aid (ID. No. P98376) from the Ministry of Education,
Science, Sport and Culture, Government of Japan
(Monbusho). This is contribution No. 8 of the SHRIMP
laboratory at Hiroshima University.
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