DSpace at VNU: Early Paleozoic medium-pressure metamorphism in central Vietnam: evidence from SHRIMP U-Pb zircon ages

Geosciences Journal
Vol. 13, No. 3, p. 245 − 256, September 2009
DOI 10.1007/s12303-009-0024-2
ⓒ The Association of Korean Geoscience Societies and Springer 2009

Early Paleozoic medium-pressure metamorphism in central Vietnam: evidence from SHRIMP U−Pb zircon ages
Tadashi Usuki*
Ching-Ying Lan
Tzen-Fu Yui
Yoshiyuki Iizuka
Van Tich Vu
Tuan Anh Tran
Kazuaki Okamoto†
Joseph L. Wooden
Juhn G. Liou

}

Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan 115, Republic of China
Faculty of Geology, Hanoi University of Science, Hanoi, Vietnam
Institute of Geological Sciences, National Center for Natural Sciences and Technology, Hanoi, Vietnam
Department of Earth, Planetary and Space Sciences, Faculty of Education, Saitama University,
Saitama 338-8570, Japan

} Department of Geological and Environmental Sciences, Stanford University, CA 94305-2115, USA

ABSTRACT: To constrain the timing of collisional event in the
Indochina block, SHRIMP U–Pb dating and REE analyses of zircon were carried out for two paragneiss samples of the Kham Duc
Complex, central Vietnam. Both samples contain kyanite, staurolite, and zoisite as relics from an early metamorphic stage (M1),
and biotite and sillimanite as major minerals constituting the foliation formed during the late metamorphic stage (M2). The change
in mineral assemblages indicates a clockwise P-T path composed

of a high- or medium-P + low-T stage (M1) and a subsequent lowP + high-T stage (M2). The U−Pb concordia ages of zircon rims are
447 ± 6 Ma and 452 ± 6 Ma for the two samples, respectively. These
results are distinctly different from the available Ar–Ar mineral
ages of 254–225 Ma. Following the clockwise P-T path and phase
equilibrium analyses of the Complex, we suggest that the zircon
rims were formed near peak temperatures during the decompression. The ~450 Ma zircon rim thus gives the minimum age constraint
for a possible crustal thickening event during Early Paleozoic,
whereas the reported Permo-Triassic Ar–Ar ages would result from
an Indosinian overprint. This Early Paleozoic event is most likely
related to a collisional orogeny between the Indochina and South
China blocks. Late Neoproterozoic to Neoarchean ages are recorded
from detrital zircon cores of the Kham Duc Complex, the Kontum
Massif and Truong Son Belt, suggesting that their protoliths might
have derived from sediments at the Gondwana margin.
Key words: zircon, SHRIMP analyses, Kham Duc Complex, Vietnam,
Indochina block

1. INTRODUCTION
Southeast Asia has been formed by amalgamation of several continental blocks, including the Indochina, South
China, and Sibumasu blocks (Fig. 1a). However, the timing
of amalgamations, in particular, that between the Indochina
*Corresponding author:
Also at Division of Natural Sciences, The United Graduate School
of Education, Tokyo Gakugei University, Tokyo 184-8501, Japan
†

and South China blocks, has been a subject of debate for
decades (e.g., Hutchison, 1989; Findlay and Phan Trong
Trinh, 1997; Metcalfe, 1999; Carter et al., 2001; Carter and
Clift, 2008; Lepvrier et al., 2008; Nakano et al., 2008;

Osanai et al., 2008), although recent geochronological studies revealed that two separate magmatic or tectonic events
occurred respectively during the Ordo-Silurian (Nagy et al.,
2001; Carter et al., 2001; Lan et al., 2003; Roger et al., 2007)
and the Permo-Triassic (Carter et al., 2001; Tran Ngoc Nam
et al., 2001; Lan et al., 2003; Lepvrier et al., 2004; Maluski
et al., 2005; Nakano et al., 2006, 2008) (Fig. 1b). On the
basis of recent discovery of high-P metamorphic rocks from
the Kontum massif and the Song Ma suture zone, a PermoTriassic collision between the Indochina and South China
was proposed (e.g., Nakano et al., 2008; Osanai et al., 2008).
On the other hand, based on regional seismic and thermochronological evidence combined with Early Mesozoic
paleogeographic constraint, Carter and Clift (2008) proposed a reactivation event in the Indochina block instead of
the Indochina-South China collision in Triassic. Findlay
and Phan Trong Trinh (1997), however, proposed a SiluroDevonian docking event of an island arc/forearc terrain to
the South China block, based on a review of geological and
chronological data of the Song Ma suture zone. Lepvrier et
al. (2008) further argued that two possible collisional events
occurred at Lower Triassic and Ordo-Silurian, respectively.
One reason for this apparent discrepancy comes from the
fact that the pre-Indosinian tectonics in Indochina is not at
all clear. For example, the Ordo-Silurian event was suggested
to be related with an arc magmatism (Nagy et al., 2001) or
an early extension prior to Gondwana breakup (Carter et al.,
2001). In previous studies, Ordo-Silurian ages were retrieved
only from magmatic or low-P metamorphic rocks. To address
whether high- or medium-P metamorphic rocks existed at


246

Tadashi Usuki et al.

Fig. 1. a: Simplified tectonic terrane
map of East and Southeast Asia showing major blocks discussed in this
study (after Metcalfe, 1999). Box
corresponds to Figure 1b. The boundary between the Yangtze craton and
the Cathaysia fold belt is after Xu et al.
(2007). RRSZ denotes Red River
shear zone. b: The Indochina block in
relation to the South China block
showing shear zones around Vietnam
(Lepvrier et al., 2004). Box
corresponds to Figure 2. Solid squares
denote the sample localities of published U−Pb zircon and monazite ages
obtained by the SHRIMP or TIMS
method in nearby areas. Data from
Roger et al. (2000; 2007), Carter et al.
(2001), Tran Ngoc Nam et al. (2001),
Nagy et al. (2001), Lan et al. (2003)
and Nakano et al. (2006). Star marks
the location of Archean crustal fragment of the Yangtze block (Lan et al.,
2001; Tran Ngoc Nam et al., 2003).

the Ordo-Silurian time is a key to understand the tectonics
in the Indochina block.
The Kham Duc Complex (Fig. 2) in central Vietnam consists primarily of medium-P metamorphic rocks (Phan Cu
Tien, 1991), which is regarded as the product of a collisional event between the Indochina and South China blocks
(e.g., Osanai et al., 2008). Despite recent petrologic studies
on the P-T evolution of Kham Duc Complex (e.g., Usuki et
al., 2004; Nakano et al., 2007; Osanai et al., 2008), the timing of metamorphism of this complex is poorly constrained.

Several Permo-Triassic Ar–Ar amphibole, biotite, and muscovite ages (254−225 Ma) were reported from the Kham
Duc Complex and were regarded as the timing of collision
between the Indochina and South China blocks (Fig. 2;
Lepvrier et al., 2004). However, Roger et al. (2007) and
Carter and Clift (2008) argued that these Ar–Ar ages could
be alternatively interpreted as a result of complete resetting
during a superimposed tectonic event. Obviously, more
robust age data are prerequisite to tackle the issue.
Having the above question in mind, in-situ zircon U−Pb


Early Paleozoic medium-pressure metamorphism in central Vietnam

247

Fig. 2. Simplified geological map around the Kham Duc Complex (modified after Lepvrier et al., 2004). Sample localities (VNTS178 and VNTS18-2) and their U−Pb zircon ages (this study) are indicated by bold characters. Also shown are Ar−Ar ages (Amp: amphibole, Mus: muscovite, Bt: biotite) of the Phuoc Son, Tien Phuoc and Tra Bong areas, reported by Lepvrier et al. (2004). TPSZ: TamkyPhuoc Son Shear Zone, TBSZ: Tra Bong Shear Zone.

chronology and REE analyses were carried out for two
gneiss samples from the Kham Duc Complex in order to
constrain the timing of the medium-P metamorphism. We
also discuss the possible source provenance for the protoliths of the Kham Duc Complex.
2. GEOLOGICAL SETTING
The Indochina block in Southeast Asia is bounded to the
north by the NW-trending Song Ma suture zone with the
South China block (Fig. 1). The Truong Son Belt and Kontum Massif in north-central Vietnam belong to the Indochina block and are affected by the collisional event between
the Indochina and South China blocks (e.g., Lepvrier et al.,
2004; 2008; Osanai et al., 2008). In this collision zone, a
series of NW to W-trending subparallel shear zones developed (Lepvrier et al., 2004). The Kham Duc Complex (Fig.
2) in the southern part of the collision zone is exposed ~70
km along the east-west direction between Truong Son Belt

and Kontum Massif. To the north, the Complex is unconformably covered by unmetamorphosed sediments, and to
the south it is bounded by the Tra Bong shear zone next to
the Kontum Massif. The lithology of the Complex is mainly
composed of metapelites, amphibolites, and granodioritic
orthogneisses. Along the Tamky-Phuoc Son and Tra Bong
shear zones (Fig. 2) within the Complex, minor amounts of
serpentinite and gabbro also occur.
Along the Ho Chi Minh road in the Phuoc Son area (Fig.
2), a typical cross-section of the Kham Duc Complex crops
out, where metapelites and amphibolites are intercalated on

a centimeter to meter scale in thickness. These metamorphic rocks have an EW-trending schistosity with a steep dip
to N or S. Usuki et al. (2004) studied metapelites in this
area and estimated a clockwise P-T path from the early
higher-P and lower-T metamorphism (M1) to the later lowerP and higher-T metamorphic stage (M2) (Fig. 3), based on
the occurrences of relict kyanite, staurolite and zoisite (M1)
and foliation-forming sillimanite and biotite (M2). Usuki et
al. (2004) estimated the P-T conditions of M1 and M2 as 11.8
kbar/464 oC and 6.8−7.0 kbar/711−722 oC, respectively, on
the basis of garnet-biotite (Bhattacharya et al., 1992) and
garnet-chlorite (Dickenson and Hewitt, 1986) geothermometers and garnet-plagioclase-aluminosilicate-quartz geobarometric calculation using a MS-Excel spread sheet by D.
Waters ( />From the same area and the Hiep Duc area (~10 km SW of
Tan An town, Fig. 2), similar clockwise P-T paths have
been also proposed (Fig. 3, Nakano et al., 2007; Osanai et
al., 2008). Ar–Ar ages of the late Permian to middle Triassic period (254−225 Ma) for amphibole, muscovite, and
biotite were reported from the mylonitized schist, kyanitebearing schist, sillimanite-bearing schist, mylonitic dioritic
gneiss, granodioritic orthogneiss, and post-tectonic granite
(Lepvrier et al., 2004) (Fig. 2).
3. SAMPLE DESCRIPTIONS
We chose two paragneiss samples from the Phuoc Son

area (Fig. 2 and Table 1) in the Kham Duc Complex for the
zircon SHRIMP analysis. Both gneisses contain kyanite and


248

Tadashi Usuki et al.

morphism. Based on the modal compositions (Table 1),
these two samples were named as biotite gneiss (VNTS17-8)
and garnet-biotite gneiss (VNTS18-2), respectively.
3.1. Sample VNTS17-8 (Biotite Gneiss)
This sample was collected from a meter-scale biotite
gneiss layer hosted within a gneiss-amphibolite outcrop
along Ho Chi Minh Road, 6.5 km northeast of Kham Duc
town (Fig. 2). The gneiss is mainly composed of biotite (31
vol%), quartz, plagioclase, muscovite, sillimanite, with minor
or trace amounts of garnet, kyanite, staurolite, zoisite, K-feldspar, allanite, zircon, apatite, monazite, xenotime, rutile and
ilmenite (Table 1). Garnet occurs as subhedral fine grains
(~0.5−1 mm in diameter) and shows high XFe [Fe/(Fe + Mg)
= 0.82] (Table 1, Usuki et al., 2004). Biotite and sillimanite
define the major foliation (Fig. 4a). In addition, kyanite
(Fig. 4b), staurolite (Fig. 4c), zoisite and rutile were found
as inclusions in plagioclase (XAn = 0.33). These occurrences
demonstrate that staurolite, kyanite and zoisite are relict
minerals from an early higher-P condition (M1), in contrast
to sillimanite and biotite formed subsequently at a lower-P
and higher-T condition (M2). Zircon, allanite, apatite, monazite, rutile and ilmenite occur in the matrix. Some grains of
zircon and apatite in the matrix are surrounded by monazite
or xenotime.

Fig. 3. Clockwise P-T paths proposed for medium-pressure metamorphic rocks of the Kham Duc Complex. Thick solid arrow is the
P-T path of metapelites, and M1 and M2 indicate early higher-P/
lower-T and later lower-P/higher-T metamorphic conditions,
respectively (Usuki et al., 2004). Dot-dashed and dashed lines indicate P-T paths reported by Nakano et al. (2007) and Osanai et al.
(2008), respectively. The breakdown reactions of staurolite, zoisite
and margarite are adapted from Spear (1993). Light gray area indicates a melt-present field in association with garnet, biotite, sillimanite, albite, K-feldspar and quartz in the NaKFMASH model
system. Thin solid lines in light gray area indicate isopleths of Fe/
(Fe + Mg) in garnet (adapted from Spear et al., 1999). In this
divariant field, garnet would grow along the P-T path that crosses
the isopleths during decompressional heating and be consumed along
the path during decompressional cooling. Formation of metamorphic
zircon (~450 Ma) during garnet breakdown and in the presence of
a melt phase probably occurred during the decompression near the
peak metamorphism (M2). Abbreviations are the same as those in
Table 1, except for Ab = albite, An = anorthite, And = andalusite,
As = aluminosilicate, Mrg = margarite, and L = liquid.

3.2. Sample VNTS18-2 (Garnet-Biotite Gneiss)

staurolite as relics, and abundant zircon crystals showing
wide metamorphic rims as described later. These samples
are suitable for studying the timing of the medium-P meta-

This sample was collected along Ho Chi Minh Road
about 0.5 km east of sample VNTS17-8 (Fig. 2). The gneiss
outcrop consists of alternating lenses of 5−30 cm thick
biotite-rich and felsic layers. The sample was collected
from a 15 cm-thick biotite-rich layer, and is composed of
biotite (51 vol%), plagioclase, garnet, quartz and sillimanite

with minor or trace amounts of kyanite, staurolite, chlorite,
muscovite, K-feldspar, zircon, apatite, monazite, rutile and
ilmenite. Garnet grains, a few mm to 20 mm in size, are
partially replaced by biotite, sillimanite and plagioclase
(XAn = 0.41) (Fig. 4d). Large garnet porphyroblasts are homogeneous (XFe = ~0.78) apart from a strongly retrograded rim
(XFe = 0.82−0.83) (Table 1). Similar to sample VNTS17-8,

Table 1. GPS positions, mode of minerals, XFe (= Fe/(Fe + Mg)) of garnet and XAn (= Ca/(Ca + Na + K)) of plagioclase for the analyzed samples
Sample

Lat.

Long.

mode of minerals*
Grt St Zo Bt Chl Mus Ky Sil Qtz Pl Kfs Aln Zrn Ap Mnz Xno Rt Ilm

Biotite gneiss
15°29'24"N 107°49'33"E + +
(VNTS17-8)
Garnet-biotite
gneiss
15°29'21"N 107°49'49"E 16 +
(VNTS18-2)

+ 31 +

1

+

4 29 24 +

+ 51 +

+

+

1

9 22 +

+

+

+

+

+

+

+

+

+

+

+

+

XFe of
Grt

XAn
of Pl

0.82

0.33

0.78 at core,
0.41
0.82−0.83 at rim

*numbers indicate %. + means less than 1%. Abbreviations are Grt = garnet, St = staurolite, Zo = zoisite, Bt = biotite, Chl = chlorite, Mus = muscovite, Ky = kyanite, Sil = sillimanite, Qtz = quartz, Pl = plagioclase, Kfs = K-feldspar, Aln = allanite, Zrn = zircon, Ap = apatite, Mnz =
monazite, Xno = xenotime, Rt = rutile, and Ilm = ilmenite.


Early Paleozoic medium-pressure metamorphism in central Vietnam

249

Fig. 4. Photomicrographs of samples VNTS17-8 and VNTS18-2. a: Sillimanite (Sil) and biotite (Bt) constituting the major foliation in

VNTS17-8. b: Kyanite (Ky) relic surrounded by plagioclase (Pl) in VNTS17-8. c: Staurolite (St) relic enclosed in plagioclase (Pl) in
VNTS17-8. d: Garnet (Grt) partially decomposed to biotite (Bt), sillimanite (Sil) and plagioclase (Pl) in VNTS18-2.

kyanite, staurolite and rutile are included in plagioclase.
These observations clearly indicate that sample VNTS18-2
experienced a decompressional heating from the early higherP condition (M1). Coarse-grained quartz-plagioclase melt
patches, which are strongly deformed, occur in the biotiterich matrix. Zircon, allanite, apatite, monazite and ilmenite
occur in the matrix. Trace amounts of apatite and allanite
are included in garnet.
4. ANALYTICAL METHODS
Zircon U−Pb dating and REE analyses were conducted
on a SHRIMP-RG (reverse geometry) ion microprobe
housed at Stanford-U.S. Geological Survey Mass Analysis
Center (SUMAC). Zircon crystals from two gneiss samples
were separated by the conventional heavy liquid technique,
and were mounted in an epoxy disc with a 25-mm diameter
and a 4-mm thickness. All the grains were imaged, using a
petrographic microscope under the transmitted and reflected
lights. Cathodoluminescence (CL) and back-scattered electron images were taken using a JEOL 5600 scanning electron

microscope (SEM) to identify internal structures, inclusions
and physical defects. The mounted zircon crystals were
washed with a saturated EDTA solution, thoroughly rinsed
in distilled water, dried in a vacuum oven, and coated with
Au. Mounts were then placed in a loading chamber at high
vacuum pressure (10-7 torr) for several hours to remove possible hydride effect.
Secondary ions were generated from the target spot with
an O2- primary ion beam of 4−6 nA. The primary ion beam
typically produced a spot with a diameter of ~25 mm and
a depth of 1−2 mm for 9−12 minutes. The basic acquisition

routine was described in Mattinson et al. (2006). Selected
sets of REEs were also measured immediately after the age
determination. In general, the number of scans through the
mass sequence and the counting time on each peak varied
on the basis of the sample ages and U−Th concentrations to
improve counting statistics and age precision.
Concentration data for U, Th and REEs were calibrated
against the well-characterized homogeneous zircon standard
MAD-green (4196 ppm U) (Mazdab and Wooden, personal
communication). Age data are standardized against R33


250

Tadashi Usuki et al.

(419 Ma; Black et al., 2004), which is analyzed repeatedly
throughout the analytical session. Data reduction for geochronology followed the methods described by Williams
(1998) and Ireland and Williams (2003), and used the Squid
and Isoplot programs of Ludwig (2001, 2003). Common Pb
corrections were made using the 207Pb method for ages
younger than 1000 Ma and 204Pb method for the older ages
(Williams, 1998). Concordia ages were calculated using the
Isoplot (Ludwig, 2003). Data reduction for trace elements
was done with a MS Excel spreadsheet provided by SUMAC.
Averaged counting rate of each element of interest was
ratioed to the appropriate high mass normalizing species to
account for any primary current drift. The derived ratios for
the unknowns were compared to the averages of those for
the standards to determine concentrations. Spot to spot precisions (as measured on the standards) vary owing to elemental ionization efficiency and concentration. For the MADgreen zircon, precisions are generally in the range of ±5%

for HREE, ±10−15% for MREE, and up to ±40% for La
(all values at 2σ).

5. RESULTS
The U−Pb isotopic compositions and REE concentrations
of zircons are listed in Tables 2 and 3, respectively. Figure
5 shows the CL images of analyzed zircons. Figures 6 and
7 are the Tera-Wasserburg plots of metamorphic zircon
domains and the chondrite-normalized (McDonough and
Sun, 1995) REE patterns of zircons, respectively.
Zircon grains from two samples are ovoid in shape and
70−250 μm in long dimension. Each grain can be clearly
divided into a detrital core and a metamorphic rim in the
CL image. However, metamorphic rims of two samples
show different internal structures; those of VNTS17-8 are
dark (5−30 μm in thickness) and almost structureless (Fig.
5a), whereas those of VNTS18-2 occasionally show weak
oscillatory zoning (Fig. 5b). Some zircons from both samples have a very thin, bright outermost rim (1−5 μm in thickness) (Fig. 5). The U−Pb isotopes and REE contents of zircon
were measured from metamorphic rims and detrital cores
only, because the bright outermost rim is too thin for SHRIMP

Table 2. SHRIMP U−Pb−Th analytical data of zircon
U
Th
(ppm) (ppm)
Bt gneiss (VNTS17-8)
1
199
74
2

510
15
3
181
351
5
552
4
6
189
65
7
496
4
8
373
7
10
487
6
11
459
4
12
235
321
Grt-Bt gneiss (VNTS18-2)
2
253
22

3
286
158
7
73
51
8
196
30
9
300
40
21
659
355
23
183
53
24
440
129
25
195
51
26
248
141
30
244
48

31
250
67
32
152
21
33
136
53
34
244
43
Spot

a

206

b

238

206

207

206

238

207

206

Pb*
(ppm)

0.37
0.03
1.94
0.01
0.35
0.01
0.02
0.01
0.01
1.36

68
31
14
34
14
31
23
30
28
19

1.47

0.27
<0.01
0.00
<0.01
0.03
0.01
0.13
0.07
<0.01

2.52
14.17
10.92
13.85
11.28
13.63
13.91
14.15
14.16
10.61

0.03
0.19
0.15
0.18
0.16
0.18
0.18
0.19
0.19

0.14

0.1450
0.0579
0.0588
0.0559
0.0577
0.0564
0.0560
0.0567
0.0562
0.0591

0.0008
0.0007
0.0010
0.0006
0.0010
0.0007
0.0008
0.0007
0.0007
0.0009

2124
438
565
449
548
456

447
440
440
581

29
6
8
6
7
6
6
6
6
8

2287
488
542
436
458
446
424
397
434
599

9
33
38

26
47
29
40
40
31
34

detrital core
rim
detrital core
rim
detrital core
rim
rim
rim
rim
detrital core

0.09
0.55
0.69
0.15
0.13
0.54
0.29
0.29
0.26
0.57
0.20

0.27
0.14
0.39
0.18

16
42
29
12
19
135
61
28
12
59
15
16
10
20
15

<0.01
0.48
4.07
0.16
<0.01
0.38
4.59
0.18
0.00

<0.01
0.06
0.08
<0.01
0.35
0.27

13.49
5.91
2.15
13.81
13.64
4.18
2.57
13.74
14.16
3.61
14.03
13.44
13.71
5.87
14.10

0.19
0.08
0.03
0.20
0.19
0.05
0.03

0.18
0.20
0.05
0.20
0.18
0.21
0.08
0.20

0.0557
0.0764
0.1894
0.0573
0.0546
0.0907
0.1657
0.0575
0.0557
0.0971
0.0563
0.0569
0.0547
0.0747
0.0579

0.0009
0.0007
0.0014
0.0011
0.0009

0.0005
0.0014
0.0007
0.0011
0.0006
0.0010
0.0009
0.0013
0.0011
0.0010

461
1003
2376
450
457
1379
2036
452
440
1578
444
462
455
1012
441

6
13
38

6
6
17
28
6
6
20
6
6
7
14
6

439
1106
2737
403
348
1440
2515
444
372
1569
482
488
386
1060
510

37

17
12
60
43
11
14
38
74
12
40
36
57
30
39

rim
detrital core
detrital core
rim
rim
detrital core
detrital core
rim
rim
detrital core
rim
rim
rim
detrital core
rim

Pbc
(%)

U/ Pb
(±1σ)

Pb/ Pb
(±1σ)

a

Radiogenic Pb
Common Pb component (%) of total

b

206

Th/U

206

Pb, determined using

207

Pb method

Pb/ U age

(Ma, ±1σ)

Pb/ Pb age
(Ma, ±1σ)

CL domain


Early Paleozoic medium-pressure metamorphism in central Vietnam

251

Table 3. REE data (concentrations in ppm) of zircon
Spot
La
Ce
Bt gneiss (VNTS17-8)
1
0.02
6.2
2
0.04
2.7
3
0.10
36.7
5
0.01
0.4
6

0.02
2.9
7
0.01
0.3
8
0.03
0.4
10
0.03
1.0
11
0.01
0.4
12
0.12
43.3
Grt-Bt gneiss (VNTS18-2)
2
0.01
2.2
3
0.09
13.6
7
0.04
15.2
8
0.01
2.8

9
0.01
2.5
21
0.10
39.3
23
0.05
5.8
24
0.10
7.9
25
0.01
4.8
26
0.03
12.6
30
0.02
4.2
31
0.01
5.7
33
0.01
8.8
34
0.01
3.3

1/2

Nd

Sm

Eu

Gd

Dy

Er

Yb

0.6
0.1
2.5
0.1
0.8
0.0
0.1
0.2
0.0
3.2

1.9
0.3
3.4

0.3
2.1
0.3
0.4
0.5
0.3
4.4

0.14
0.21
2.57
0.10
0.23
0.09
0.15
0.26
0.09
2.97

18
5
21
7
21
6
6
8
5
23

73
52
58
72
85
67
72
87
58
53

119
144
94
169
176
177
195
261
150
79

169
422
179
391
321
415
471
741

352
136

11
99
10
72
19
79
89
113
84
7

44
21
61
14
21
8
6
8
11
59

0.07
0.48
0.92
0.21
0.11

0.20
0.29
0.40
0.25
0.91

0.1
0.7
2.7
0.1
0.1
3.8
0.2
0.3
0.2
2.7
0.2
0.2
0.5
0.1

0.5
1.4
5.1
0.3
0.5
9.8
0.3
1.5
0.9

6.1
0.8
0.9
1.2
0.4

0.32
0.12
1.08
0.20
0.25
0.44
0.10
0.82
0.50
0.70
0.46
0.55
0.48
0.24

9
13
43
5
8
91
3
23
12

53
12
13
11
7

87
61
147
40
55
374
15
128
64
184
75
77
49
47

207
120
245
94
135
588
38
244
129

301
160
158
112
110

426
225
384
203
285
784
96
438
239
455
308
296
261
234

54
21
11
45
45
10
35
23
24

10
32
27
28
43

58
36
48
103
52
55
37
25
81
48
42
93
105
109

0.43
0.08
0.22
0.47
0.38
0.04
0.30
0.42
0.46

0.12
0.46
0.48
0.39
0.43

1/2

1/3

(Yb/Gd)N Ce/Ce*

Eu/Eu*

2/3

Ce/Ce* = CeN/(LaN × PrN) , Eu/Eu* = EuN/(SmN × GdN) , PrN = LaN × NdN

analyses.
The U−Pb isotopic compositions of metamorphic zircon
rims of both samples are plotted in a Tera-Wasserburg concordia diagram (Fig. 6), and are scattered in the range of
456−438 Ma and 462−440 Ma, respectively. The concordia
ages of zircon rims of both samples are consistent within
error; 447 ± 6 Ma (95% confidence, MSWD = 5.3, n = 6)
and 452 ± 6 Ma (95% confidence, MSWD = 1.7, n = 9),
respectively. The zircon rims of VNTS17-8 have the moderate U (373−552 ppm), low Th contents (4−15 ppm), and
low Th/U ratio (0.01−0.03), while those of VNTS18-2 have
lower U (152−440 ppm), higher Th (22−129 ppm) and
higher Th/U ratio (0.09−0.29). Dark-CL rims of VNTS17-8
with low Th/U may have formed through solid-state recrystallization (Hoskin and Black, 2000). On the other hand,

oscillatory zonings and higher Th/U ratios for the zircon
rims of VNTS18-2 suggest that these rims have formed in
the presence of a melt. Chondrite-normalized REE patterns
of the metamorphic rims for VNTS17-8 (Fig. 7a) and
VNTS18-2 (Fig. 7b) are similar and characterized by steep
slope on the HREE [(Yb/Gd)N = 72−113 and 23−54, respec-

tively], negative Eu (Eu/Eu* = 0.20−0.48 and 0.38−0.48,
respectively) and positive Ce anormalies.
Zonations in detrital cores of zircon vary from oscillatory
(Fig. 5a, No. 6 and 12 and 5b, No. 3, 7, 21, 23, 26 and 33)
to concentric patterns (Fig. 5a, No. 1 and 3). The U−Pb isotopic compositions of four detrital cores for VNTS17-8 yielded
nearly concordant, Late Neoproterozoic 206Pb/238U ages of
581 ± 8, 565 ± 8, and 548 ± 7 Ma (1σ error) and the Paleoproterozoic 207Pb/206Pb age of 2287 ± 9 Ma (1σ error) in
comparison with six detrital cores of VNTS18-2 yielding
Mesoproterozoic and Neoarchean 207Pb/206Pb ages of 1060 ±
30, 1106 ± 17, 1440 ± 11, 1569 ± 12, 2515 ± 14, and 2737 ± 12
Ma (1σ error). The REE patterns of detrital cores show
larger variation compared to those of the metamorphic rims
(Fig. 7). Detrital cores of VNTS17-8 show a relatively gentle HREE slopes [(Yb/Gd)N = 7−19] than the rims (Fig. 7a).
Negative Eu anomalies are less conspicuous (Eu/Eu* =
0.91−0.92) in two spot analyses [No. 3 (565 ± 8 Ma) and
No. 12 (581 ± 8 Ma)] than in other two (Eu/Eu* = 0.07−0.11)
[No. 1 (2287 ± 9 Ma) and No. 6 (548 ± 7 Ma)]. In VNTS182, three cores [No.7 (2737 ± 12 Ma), 21 (1440 ± 11 Ma), and


252

Tadashi Usuki et al.

Fig. 5. Cathodoluminescence images of zircons. Circles indicate
the SHRIMP analytical spots with a diameter about 25 μm. a:
Sample VNTS17-8. b: Sample VNTS18-2.

26 (1569 ± 12 Ma)] show enrichments of REEs with strong
negative Eu anomalies (Eu/Eu* = 0.04−0.22) and other three
cores [No.3 (1106 ± 17 Ma), 23 (2515 ± 14 Ma), and 33 (1060
± 30 Ma)] show similar REE patterns as the metamorphic
rims, but with more negative and variable Eu anomalies
(Eu/Eu* = 0.08−0.39) (Fig. 7b).
6. DISCUSSION
6.1. Zircon Rim Formation and Garnet Decomposition
The metamorphic rims of two gneiss samples yielded a
consistent concordia age of ~450 Ma. The Kham Duc Complex may have experienced a clockwise P-T path composed
of a high- or medium-P and lower-T stage and a subsequent
low-P and high-T stage (Fig. 3; Usuki et al., 2004; Nakano
et al., 2007; Osanai et al., 2008). To link the dating results
with the petrologic information, it is important to ascertain
the position/time of zircon growth/recrystallization along
the estimated P-T path.
The metamorphic rims of zircon from two gneiss samples
have similar REE patterns characterized by enriched-HREEs
and negative Eu anomalies. The latter suggest that these
metamorphic rims of zircon were formed under a feldsparstable low-P condition (Eu/Eu* < 0.75, Rubatto and Hermann, 2007). The HREE-enriched steep pattern is typical for
zircon formed under a garnet-free or low-P metamorphic
environment (Rubatto and Hermann, 2007). Both samples

Fig. 6. Terra-Wasserburg plots of the SHRIMP data. Error ellipses
indicate 2σ. a: Sample VNTS17-8. b: Sample VNTS18-2. Shaded
ellipses indicate the averaged concordia age and error (95% confidence).

contain biotite, sillimanite and plagioclase resulting from the
garnet breakdown during decompression (Fig. 4d). The
characteristics of the REE patterns of these zircons could therefore be interpreted as a result of garnet decomposition,
although further studies including REE analyses of garnet
and other accessory minerals are necessary to test this postulation. In addition, the zircon overgrowth rims in sample
VNTS18-2 show oscillatory zonings and have high Th/U
ratios (>0.1). These features are comparable to those
described for magmatic zircons (Williams et al., 1996;
Rubatto and Gebauer, 2000). These characteristics therefore
suggest that the metamorphic rims of zircon in sample
VNTS18-2 may have formed in the presence of melt.
Light gray area in Figure 3 indicates a melt-present P-T
field with a mineral assemblage of garnet, biotite, silliman-


Early Paleozoic medium-pressure metamorphism in central Vietnam

253

to support this postulation.
From the above consideration, we interpret that metamorphic zircon rims dated at ~450 Ma from two gneisses may
have formed during the low-P and high-T stage (M2) along
the clockwise P-T path of the Kham Duc Complex, and
give the minimum age constraint for the medium-P (M1)
event. Strictly speaking, it is difficult to ascertain whether
the medium-P and low-T metamorphism (M1) and the lowP and high-T metamorphism (M2) took place during a single event or two separate events. However, we infer that a
crustal thickening (M1) event occurred during Early Paleozoic, because the youngest inherited detrital zircon age is
548 ± 7 Ma which gives the maximum age constraint.

Fig. 7. Chondrite (McDonough and Sun, 1995) normalized rare
earth element patterns of zircons. a: Sample VNTS17-8. b: Sample
VNTS18-2. Solid and dashed lines indicate metamorphic rims and
detrital cores, respectively. Pr was not analyzed and PrN was calculated by PrN = LaN × NdN .
1/3

2/3

ite, albite, K-feldspar and quartz, which is the same as that
of the M2 stage of VNTS18-2, in the NaKFMASH (Na2OK2O-FeO-MgO-Al2O3-SiO2-H2O) model system after Spear
et al. (1999). This NaKFMASH model system can adequately
describe partial melting reactions for the studied samples
(e.g., Indares and Dunning, 2001). The proposed P-T path
by Usuki et al. (2004; Fig. 3) passes through the melt-present
field. In this field, garnet would grow along the P-T path that
crosses the isopleths during the decompressional heating
and might be consumed along the path during the decompressional cooling (Spear et al., 1999). If zircon formed
during the garnet breakdown and in the presence of a melt
phase for VNTS18-2, the most probable P-T condition is
shown by an ellipse in Figure 3.
As mentioned above, the REE pattern may indicate that
the metamorphic zircons in VNTS17-8 also have formed
during garnet decomposition. However, in contrast to VNTS182, the dark rims of zircon in VNTS17-8 are structureless
and have low Th/U ratios (<0.1). They may therefore have
formed through metamorphic recrystallization (Hoskin and
Black, 2000) rather than the growth in the presence of melt.
Since the two gneisses must have experienced the same
metamorphic P-T path, one way to explain this apparent contrast
in zircon rims is the limited extent of partial melting in
VNTS17-8, although there is no any conclusive observation

6.2. Early Paleozoic Collisional Event in the Indochina
Block
This study revealed a possible collisional event in Early
Paleozoic in the Indochina block. As mentioned earlier, different interpretations for the Ordo-Silurian event have been
proposed, such as arc magmatism (Nagy et al., 2001), early
extension prior to Gondwana breakup (Carter et al., 2001)
or Caledonian collision (Lepvrier et al., 2008). These different interpretations may partly result not only from a complicated tectonic environment of the area in that period of
time, but also from the lack of adequate geologic information. For example, chemical characteristics of pre-Indosinian plutonic rocks need to be determined in order to verify
if they are related with subduction, collision, or rifting.
Those pre-Indosinian amphibolite- to granulite-facies gneisses
may not necessarily be related to rifting (Carter et al., 2001),
but in the light of the present study, may be a product of
decompressional heating following a presumed collisional
event. Although Triassic high-P rocks are reported from the
Song Ma suture zone (Nakano et al., 2008), these Triassic
ages are possibly the product of reactivation, as pointed out
by Carter and Clift (2008). Further petrologic studies with
robust age data, especially identification of mineral inclusions for the construction of early metamorphic stage and
P-T path, are necessary to give proper geodynamic interpretation on metamorphic rocks. By doing so, our study
firstly confirms a 548−450 Ma medium-P metamorphic
event in the collision zone between the Indochina and South
China blocks, suggesting a possible Early Paleozoic collisional event. This study therefore provides a piece of crucial
evidence to reconcile the existing discrepancy about the
timing of collisional event between the Indochina and
South China blocks.
Abundant 450−400 Ma zircons have been recently reported
from the South China block (e.g., Xu et al., 2007; Wang et
al., 2007) and a “Caledonian” intracontinental collisional
event between the Yangtze craton and the Cathaysia fold

belt was proposed (Wang et al., 2007). It should be noted
that the Indochina block has been offset by ~600 km southeastwardly relative to the South China block along the Red


254

Tadashi Usuki et al.

River shear zone since Tertiary (Chung et al., 1997; Wang
et al., 1998) and that an Archean crustal fragment of the
Yangtze craton has been reported between the Red River
shear zone and the Song Ma suture (Fig. 1b; Lan et al.,
2001; Tran Ngoc Nam et al., 2003). It is quite probable that
the Indochina block has been in contact with the Yangtze
craton long before Tertiary. Therefore, the collisional zone
in north-central Vietnam in this study could be a western
extension of the collisional belt between the Yangtze craton
and the Cathaysia fold belt of the South China block. Such
a possibility should be tested in the future with detailed and
systematic geological, petrological, and geochronological
studies.
6.3. Permo-Triassic Event in the Indochina Block
In previous studies, the timing of the medium-P metamorphism in the Kham Duc Complex was considered to be
Permo-Triassic on the basis of available Ar–Ar mineral
ages (Lepvrier et al., 2004; Nakano et al., 2007; Osanai et
al., 2008). The medium-P metamorphic rocks in the Kham
Duc Complex, as well as other high- and ultrahigh-P metamorphic rocks occurring in the Kontum Massif and Song
Ma suture zone, were regarded to have formed during the
Indosinian collisional event (Nakano et al., 2007, 2008;
Osanai et al., 2008). However, our result suggests that the

medium-P metamorphism in the Kham Duc Complex took
place in the Early Paleozoic. It is worthwhile to point out
that the thin, bright-CL, outermost rim of some zircons
(Fig. 5), which is undatable under the current spatial resolution of a SHRIMP machine, may have formed during the
Permo-Triassic event. All the above observations could be
reasonably interpreted as a result of the Permo-Triassic
overprinting on Early Paleozoic records. The overprinting
temperature condition was high enough to totally reset
Ar–Ar system (525−450 oC, 375−325 oC and 350−260 oC
for amphibole, muscovite and biotite, respectively; Spear,
1993), but not for zircon (re)crystallization. Rubatto et al.
(2001) reported no neocrystallization of zircon below ~700 oC
in regionally-metamorphosed metapelites, although in hydrothermal condition, new zircon could grow below 300 oC
(Dubi n´ ska et al., 2004; Tsujimori et al., 2005). It is likely
that the temperature condition for the Permo-Triassic
(Indosinian) overprint event never reached 700 oC in the
study area (Fig. 8), while the temperature conditions for the
Permo-Triassic event was high enough for zircon
(re)crystallization in some areas of the Kontum Massif
(Carter et al., 2001; Tran Ngoc Nam et al., 2001; Lan et al.,
2003; Nakano et al., 2006).
If the collision between the Indochina and South China
blocks indeed took place during Early Paleozoic as postulated
in this study, the geodynamic nature of the Permo-Triassic
event would be an interesting and vital issue to pursue. It is
possible that the Permo-Triassic overprint was also caused

Fig. 8. The possible Early Paleozoic and Indosinian thermal histories of the Kham Duc Complex. The temperature range and age
of zircon (Zrn) refer to the overgrowth/recrystallization stage,
estimated from this study. The closure temperatures of Ar−Ar dating for amphibole (Amp), muscovite (Mus) and biotite (Bt) are

525−450 C, 375−325 C, and 350−260 C, respectively (Spear,
1993). Ar−Ar age data are after Lepvrier et al. (2004). O = Ordovician,
S = Silurian, D = Devonian, C = Carboniferous, P = Permian, and
T = Triassic.
o

o

o

by continental collision between the Indochina and South
China blocks (Nakano et al., 2007, 2008; Osanai et al., 2008).
The presence of high- and ultrahigh-P rocks from several
areas in the Kontum Massif in association with some PermoTriassic metamorphic rocks (Nakano et al., 2007, 2008;
Osanai et al., 2008) is in accord with this possibility. This
scenario requires opening and closure of an oceanic basin
along this suture/collision zone after Early Paleozoic and
during Indosinian time, respectively (Lepvrier et al., 2008).
Alternatively, the records of Permo-Triassic age may only
signify an overprinting effect on the pre-existing landmass
of Indochina and South China (Carter et al., 2001; Carter
and Clift, 2008). The overprinting effect might be caused by
a remote tectonic event, such as the collision between the
Indochina and Sibumasu blocks (Carter et al., 2001; Carter
and Clift, 2008). In either case, the Paleozoic geotectonic
history of the Indochina block is obviously more complicated
than previously thought. Further robust age data linked with
detailed petrologic studies are necessary to clarify the details.
6.4. Source Provenance of the Indochina Block
The ages of detrital zircon cores from the two gneiss

samples in the Kham Duc Complex cluster around 2.7, 2.5−
2.3, 1.6−1.4, 1.1 and 0.58−0.55 Ga. To the south, in the Kontum Massif (i.e., the Ngoc Linh and Kannack Complexes),
detrital zircon and monazite ages of 2.7, 2.5, 1.8, 1.5−1.4,
1.1, 0.87 and 0.64 Ga were reported (Nagy et al., 2001;
Tran Ngoc Nam et al., 2001; Lan et al., 2003; Roger et al.,
2007). To the north, in the Truong Son Belt, inherited core
ages of 2.6, 2.5, 1.8, 1.6, 1.2, 0.8 and 0.6 Ga were documented


Early Paleozoic medium-pressure metamorphism in central Vietnam

(Carter et al., 2001). The wide range of detrital zircon ages
observed in northern to central Vietnam is indicative of sedimentary protoliths formed at continental margin. Furthermore, in northwestern Australia, bedrock and detrital zircon
ages of 2.8−2.6, 1.8−1.5, 1.3−1.0 and 0.6−0.5 Ga are found
(Veevers, 2004). This age distribution is quite similar to that
of detrital zircon cores from the Kham Duc Complex, the
Kontum Massif and the Truong Son Belt. This similarity is
consistent with the postulation of Metcalfe (1999) that
Southeast Asian blocks were separated from Indian-Australian margin of Gondwana. In addition, the protolith of the
Kham Duc Complex could have been derived from sediments at the continental margin of Gondwana.
7. CONCLUSIONS
1. The SHRIMP U−Pb dating of zircon rims from two
paragneisses of the Kham Duc Complex consistently yielded
a concordia age of ~450 Ma.
2. The REE pattern, Th/U ratio and internal structure of
zircons, corroborated by the petrologic observation, suggest
that the zircon rims have (re-)crystallized during the decompression near the peak metamorphic condition.
3. This study gives the first unequivocal evidence for
Early Paleozoic crustal thickening event in the Indochina
block, which suggests a possible pre-Indosinian collisional

event between the Indochina and South China blocks. The
available Permo-Triassic Ar–Ar dates in the Kham Duc
Complex may result from the complete resetting caused by
the Indosinian tectonic event, whose geodynamic nature is
yet to be resolved.
4. The Permo-Triassic age is not detected from the zircons of the Kham Duc Complex. Thus the Indosinian overprint was not strong enough to cause the (re)crystallization
of zircon in this complex, although some Indosinian zircons
have been reported from the Kontum Massif.
5. The presence of Late Neoproterozoic to Neoarchean
zircon cores suggests that the protoliths of the Kham Duc
Complex could be derived from sediments at the continental margin of Gondwana.
ACKNOWLEDGMENTS: Thanks are due to F. Mazdab, A. Strickland and K. Maki for assisting the SHRIMP analysis at Stanford University. M.W. Yeh helped our field work in Vietnam. We are grateful
to A. Carter, Y. Kim and co-guest editor M. Cho for their thorough,
critical and constructive reviews which improved the manuscript significantly. This research was supported by the National Science Council
of the Republic of China and Academia Sinica. This paper is Contribution IESAS1387 of the Institute of Earth Sciences, Academia Sinica.

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Manuscript received November 13, 2008
Manuscript accepted August 9, 2009