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Journal of Asian Earth Sciences 19 (2001) 233±248
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Ar±Ar and ®ssion-track ages in the Song Chay Massif: Early Triassic and
Cenozoic tectonics in northern Vietnam
H. Maluski a,*, C. Lepvrier b, L. Jolivet b, A. Carter c, D. Roques c, O. Beyssac d, Ta Trong Tang e,
Nguyen Duc Thang f, D. Avigad d
a
ISTEM-CNRS, Universite Montpellier 2, Place EugeÁne Bataillon, 34095, Montpellier, France
Laboratoire de Tectonique, Universite Pierre et Marie Curie, 4 Place Jussieu, case 129, 75252 Paris cedex 05, France
c
London Fission Track Research Group, Department of Earth Sciences, Birkbeck and University College, Gower Street, London, WC1E 6BT, United Kingdom
d
Laboratoire de GeÂologie, Ecole Normale SupeÂrieure, 24 rue Lhomond, 75231 Paris cedex 05, France
e
National University of Vietnam, Hanoi, 334 Nguyen Trai Str., Thanh Xuan, Hanoi, Viet Nam
f
Geological Survey, Hanoi, Viet Nam
b

Received 14 October 1999; revised 9 May 2000; accepted 7 July 2000

Abstract
The Song Chay Massif is the northeasternmost metamorphic complex in Vietnam, to the east of the Red River Shear Zone. It shows a large
antiformal structure involving orthogneisses and migmatites overlain, on its northern ¯ank, by muscovite bearing marbles. An E±W striking
fault bounds the dome to the South. Kinematic indicators along a S±N section reveal top-to-the-N shear sense along the interface between the
orthogneissic core and the overlying metasediments. Radiometric ages were obtained by the 40Ar± 39Ar method using puri®ed mica separates.
Across the dome ages range from 236 Ma at the southern edge to 160 Ma in the core, attesting to a strong imprint in the Early Triassic time. A
clear difference is seen between these Mesozoic ages and the Eocene to Miocene ages (from 40 to 24 Ma) that obtained in the nearby Red
River Shear Zone using the same method. These data show that the Song Chay Massif was already high in the crust when the high
temperature deformation of the Red River Shear Zone took place. The ®nal exhumation of the Song Chay orthogneiss constrained by
®ssion-track analysis on samples along the same transect occurred during the Early Miocene and could be interpreted as the consequence of a
®rst normal sense of motion along the fault which bounds the massif to the south. Timing is similar to that of exhumation in the Red River
Shear zone. q 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Ar±Ar method; Fission-track ages; Song Chay Massif; Vietnam

1. Introduction
The Indochina peninsula, particularly northern Vietnam,
is in a key-position for understanding the geodynamic
evolution of South Eastern Asia. Crossed by the southern
termination of the Red River Shear Zone it has been

strongly affected by the India-Asia collision and by South
China Sea rifting. The precise role and extent of in¯uence of
the Red River Shear Zone is not yet fully known and is the
subject of ongoing debate (Tapponnier et al., 1982; 1986;
Briais et al., 1993; Leloup and Kienast, 1993; Leloup et al.,
1995; Harrison et al., 1996; Dewey et al., 1989; Molnar and
Gipson, 1996; England and Molnar, 1990; Murphy et al.,
1997; Rangin et al., 1995; Chung et al., 1997). The peninsula is classically considered as a rigid block but recent
* Corresponding author. Tel.: 133-0467545926; fax: 133-0467547362.
E-mail address: maluski@dstu.univ-montp2.fr (H. Maluski).

studies (Jolivet et al., 1999) south of the Red River Shear
Zone have identi®ed a large metamorphic core complex (the
Bu Khang Dome) and also evidence for extension during the
Early Miocene. A number of structures in Vietnam are
known to date back to the Early Triassic (240 Ma, Lepvrier
et al., 1997). Other thermotectonic episodes which may
have affected the region (e.g. during the Cretaceous, Lepvrier et al., 1997; Lacassin et al., 1998) are more obscure,
but this may be due to the current paucity of geochronological and ®eld data. Thus, to decipher the geodynamic
evolution of Indochina it is essential that we understand

the timing and interaction between the different phases of
deformation and structures. In this context we have studied
the deformation and exhumation history of a large metamorphic massif, close to the Red River Fault (RRF).
The Song Chay Massif is located about 10 km north-east
of the Day Nui Con Voi, east of the town of Lao Cai (Fig. 1).
It is a large domal structure which on ®rst examination

1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S 1367-912 0(00)00038-9

234

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

Fig. 1. Location map and topography of northern Vietnam. The Song Chay Massif is close to the Day Nui Con Voi and the Red River.

appears similar to the Bu Khang dome, and therefore, may
have had a similar history. To understand the temporal relationship between this structure, the Red River Shear Zone
and Miocene extension found in the Bu Khang Dome south
of the fault (Jolivet et al., 1999) we have used a combination

of ®eld observation, 40Ar± 39Ar mica dating (Maluski et al.,
1999), and apatite ®ssion-track analysis. The results are
compared with those from the Red River Shear Zone in
the Dai Nui Con Voi.

2. Geology
The major structures within the Indochina peninsula are
the Truong Song belt (CordillieÁre Anamitique of the early
French authors), in North to Central Vietnam, and the

Kontum Block, in the South (Fromaget, 1941). These extend
into the metamorphic ranges of Burma, Thailand, eastern
Laos and Vietnam, as well as the extreme south-western part
of China. The northern region is occupied by a complex
realm (Figs. 1 and 2), in which the NW±SE RRF zone is
central. Parallel to the active RRF is the Cenozoic Red River
Shear Zone. The elongate Day Nui Con Voi Dome is
bounded by the RRF to the west and by the Song Chay
Fault to the east. To the west of the RRF, alkaline granites
intrude the gneissic Phang Si Pan Massif.

Our main study area, the Song Chay Massif, is located on
the eastern side of the Red River and extends into China. It
has a dome-like shape, roughly trending in a NE±SW direction and is bounded on its western ¯ank by the Song Chay
Fault and on its southern ¯ank by an E±W trending mylonite zone, which on geological maps appears to be

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

235

Fig. 2. Geological map (from Geological map Vietnam, 1/200,000) and cross-section of the studied area showing the major structures, small scale structures in
the Song Chay Massif as well as the location of samples and the 40Ar± 39Ar ages and ®ssion track ages.

terminated by the Song Chay Fault. The eastern and southeastern limits of the dome correspond to the Lo river valley
which also occupies a major fault. Sample collection and
observations of the structural and deformational history
were made along the single road that crosses the dome,
from the city of Bac Quang to the villages of Hoang Su
Phi and Xin Man. Terranes surrounding the dome, to the
south and east, consist of greywackes and micaschists to
slaty schists overlain by a karstic formation of Cambrian

limestones. The Ordovician and Silurian are represented
by limestones and quartzite, and are unconformably over-

lain by Devonian conglomerates, slates and limestones. The
Permo-Carboniferous is represented by carbonates.

3. Deformation in the Song Chay dome
We describe a cross-section of the dome from the SE to
the NW (Fig. 2). The southern limit of the dome is a narrow
EW trending fault, which cuts strongly lineated quartzites,
micaschists and marbles.
The foliation is folded into a broad antiform with an axis

236

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

Fig. 3. Photographs of outcrops in the Song Chay Massif showing top-to-the-north kinematic indicators. All sections are parallel to the lineation and
perpendicular to the foliation. (a) Orthogneiss near Huang Xu Phi in the northern part of the section. (b)±(d) Orthogneiss from the southern side of the
dome. Photograph (c) shows a high strain zone slightly oblique on the foliation in the less strained gneiss (lower). The button with the star gives the scale

(2 cm).

trending NE-SW and is steeper in the southern rim. Horizontally foliated orthogneisses and migmatites are found
near the core of the antiform, as shown on the cross section,
Fig. 2. To the North, upper levels of the core are made of
biotite and muscovite-bearing orthogneisses containing Kfeldspars several centimeters in length. Close to the village
of Xin Man, horizontally sheared micaschists are directly
overlain by muscovite-bearing marbles that alternate with
pelitic schists, considered as Cambrian (Geological Survey
of Vietnam, 1999 (Geological map 1/200,000); Tran Van
Tri, 1977; Phan Cu Tien et al., 1989).
A conspicuous NW- or N-trending stretching lineation is
recognised all along the section in orthogneisses and micaschists (Fig. 2, map). In the internal parts of the dome the
orthogneiss fabric is often constrictional with a strong

stretching lineation and a weak planar anisotropy. These
orthogneisses are not ubiquitously deformed and locally
occur in an unfoliated facies with large feldspars in an
undeformed groundmass. This rock has been considered
to be an intrusive granite, but its occurrence suggests to

us that it is simply the undeformed equivalent of the
orthogneiss. Gradients of strain are seen at the scale of
tens of meters and a general increase in deformation is
observed from the undeformed granite toward the north
and south. The most intense deformation is observed in
the northern part of the section between Xin Man and
Huang Su Phi.
Orthogneisses yield consistent kinematic indicators
showing a top-to-the-north or northeast sense of shear
(Fig. 3) even in regions characterised by constrictional

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

fabrics where the foliation is least visible. The most
common shear criteria are S±C relations, asymmetric pressure shadows on alkali feldspar, sigmoidal foliation when
approaching zone of shear localisation.
This simple deformation pattern suggests that a nearly
horizontal shear zone has been active between the basement
and the cover, with a top-to-the-north shear sense, and has
been lately folded into a broad antiform. Comparable

¯at-lying shear zones on this scale are not common in Vietnam and its age is unknown.
East of Bac Quang, cordierite±sillimanite±muscovite
micaschists and quartzites displaying a N808E-trending
foliation and a gently west-dipping lineation occupy the
southern rim of the dome.
4. Geochronological data
The Song Chay Massif and the surrounding area have
been relatively unexplored by geochronology: gneisses,
schists and migmatites were dated by the U±Pb method,
at 2652 and 1000 Ma. (Tran Van Tri, 1977; Tran Ngoc
Nam, 1997). These Archean U±Pb ages most probably
relate to inherited Pb. Tugarinov et al. (1979) further
found a U±Pb zircon and apatite upper intercept age of
625 ^ 20 Ma; and a lower intercept at 30 Ma. Nguyen and
Dao (1995) published an age of 350 Ma on biotite without
information on the dating method. More recently, the evolution of this massif was investigated using the Ar±Ar method
and the ®rst age data relating to Triassic metamorphism
were presented by Maluski et al. (1999). The protolith age
of the Song Chay orthogneisses was measured by Leloup et
al. (1999) using the zircon U±Pb method. Dated at 428 ^

5 Ma; this age probably corresponds to the time of emplacement of the protolithic granite. The same study also
measured a Rb±Sr age and 40Ar± 39Ar mica plateaux ages
on a single sample from the southern part of the dome. The
results gave ages that span a period between 209 ^ 9 and
176 ^ 5 Ma and were interpreted as documenting a Late
Triassic shearing event around 210 Ma. A K-feldspar
40
Ar± 39Ar age spectrum also suggested a phase of rapid
cooling in the late Jurassic.
4.1. 40Ar± 39Ar results
The radiometric 40Ar± 39Ar stepwise heating method was
used on pure mineral aliquots. Results are presented from
the southern cover to the northern one, crossing the whole
antiform (Figs. 4 and 5). Analytical conditions have been
formerly described in Maluski et al. (1995) and Lepvrier et
al. (1997). A summary of results is presented in Table 1.
Argon isotopic results are given in Table 2. All the samples
of orthogneisses and migmatites described here and used for
radiometric dating are coarse grained. The granulometric
fraction used for dating was 160 mm in diameter for micagrains. In these conditions the grain-size effect, as

mentioned in McDougall and Harrison (1988), is mini-

237

mised, concerning dimension controlling gas loss in diffusive loss conditions.
Sample VN 322 (Fig. 4a) is located in a subvertical shear
zone which bounds the dome to the south, (228 24 0 52 00 ; 1048
42 0 55 00 ). It is a sillimanite±cordierite bearing micaschists
with ¯exuose biotites and muscovites. Muscovite de®nes a
very irregular shaped degassing spectrum with increasing
ages since 60 Ma for low temperatures up to 234 Ma in
the last signi®cant step. Intermediate degassing temperatures display an age of 204 Ma. This spectrum relates to a
closure of the system at an age of 234 Ma, which then
suffered a subsequent Ar loss. The strong scattering of the
39
Ar/ 40Ar ratios, is also re¯ected in the isochron diagram
normalised to 40Ar, in which no linear array can be de®ned.
Sample VN 324 (Fig. 4b) is a typical orthogneiss from the
southern rim of the dome (228 29 0 41 00 ; 1048 51 0 43 00 ). Its
mineralogical content is quartz, K-feldspar and biotite, with
very few muscovites. Micas are oriented in the foliation and
present the shape of late to post deformational minerals. The
age spectrum of the muscovite does not de®ne a plateau age
but displays, for 90% of released 39Ar, increasing ages from
73 Ma to a ®rst integrated age of 228 ^ 1 Ma; and a second
at 236 ^ 0:5 Ma: As for the previous sample, this mineral
suffered inhomogeneous Argon loss, which affects mainly
low temperature degassing sites. For this sample, the
isochron plot does not reveal a well-de®ned straight line.
Sample VN 329 (Fig. 4c) is a ®ne-grained gneiss with
quartz, plagioclase, K-feldspar, coarse biotites and few
muscovites (228 32 0 26 00 ; 1048 49 0 24 00 ). This facies is locally
intercalated within the orthogneisses. The biotite displays a
very regular age spectrum for which an age plateau can be
de®ned at 201 ^ 2 Ma for near 80% of the 39Ar degassed.
The ®rst degassing step gives an age around 100 Ma. This
pattern attests to a closure of the mineral at 200 Ma,
followed by a very weak subsequent Ar loss. In a diagram
36
Ar/ 40Ar, 39Ar/ 40Ar, we can de®ne an isochron giving an
age of 200 ^ 2 Ma; identical to the one displayed by the
integrated plateau age.
Sample VN 333 (Fig. 4d) is a migmatitic gneiss to the
west of Wang Xu Phy village (228 44 0 39 00 ; 1048 38 0 02 00 ). It
contains quartz, plagioclase, muscovite and biotite. Micas
develop in the foliation and appear to have formed syn- to
post-deformation. The biotite of this sample yields a wellde®ned plateau age at 166 ^ 2 Ma for near 95% of the 39Ar
released. The closure of the mineral vs. Ar occurred at that
time, without subsequent reopening of the system. An identical age of 166 ^ 2 Ma is obtained through the isochron
diagram, with an intercept on the Y-axis de®ning an atmospheric 40Ar/ 36Ar ratio.
Sample VN 335 (Fig. 4e and f) was taken 5 km east of
Xin Man village (Fig. 1). It is a ®ne-grained orthogneiss,
from the northernmost part of the dome. It is composed of
quartz, plagioclase, biotite and muscovite. Mica¯akes are
developed in the foliation, with undeformed shapes. Muscovites and biotites give, respectively, 164 ^ 2 Ma and 176 ^
2 Ma: For muscovite, the plateau age is calculated over 60%

238

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248
300

300

234±0.8 Ma

200
204±1 Ma

150
100

S SONG CHAY
60 Ma

50
0
0

50
39
% Ar cumulative

150

SONG CHAY
VN 324 MUSCOVITE

50
0
0

100

50
39
% Ar cumulative

a

100

b

300

300

250

250

>

201±2 Ma

AGE (Ma)

<
AGE (Ma)

200

100

VN 322 MUSCOVITE



228±1 Ma

250
AGE (Ma)

AGE (Ma)

250

200
150

SONG CHAY

100

VN329 BIOTITE

50
0
0

50
39
% Ar cumulative

200

166±2 Ma

<

>

150
100

SONG CHAY

50

VN333 BIOTITE

0
0

100

50
39
% Ar cumulative

c

100

d

200

300

167±2Ma

250
200

<

176±2 Ma

AGE (Ma)

AGE (Ma)

150

>

150
100

<

0
0

SONG CHAY

SONG CHAY

VN 335 MUSCOVITE

VN335 BIOTITE
50
39
% Ar cumulative

>

100

50
50

164±2 Ma

0
0

100

e

50
39
% Ar cumulative

100

f

300
250

198±2 Ma

AGE (Ma)

<

>

200
150
100

SONG CHAY

50

VN 337 MUSCOVITE

0
0

100

50
%

39

Ar cumulative

g

Fig. 4. 40Ar± 39Ar age spectra from the Song Chay Massif.

of 39Ar released. The last three signi®cant steps reveal an
integrated age slightly older than the previous one at 167 ^
2 Ma: The whole pattern of this age spectrum attests to an
Ar diffusion loss, resulting in younger ages in low extraction
temperatures (96, 143, 160 Ma). The plateau therefore,

would re¯ect radiogenic 40Ar loss, less pronounced on the
more retentive sites, resulting in the last old age of 167 Ma.
The result obtained on biotite is somewhat surprising
because the closure temperature of biotite is lower than
for muscovite. Even if this value is not precisely known

239

50

100

40

80
AGE (Ma)

AGE (Ma)

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

30

33.1±0.8 Ma

<

>

20
10
0
0

40

40±1 Ma

<

60
80
% Ar cumulative

>

40
20

VN 106 MUSCOVITE
20

60

0
0

100

VN 107 BIOTITE
20

40

39

a

60
80
39
% Ar cumulative

100

b

50

AGE (Ma)

40
30

<

24.1±1 Ma

>

20
10

VN 110 MUSCOVITE

0
0

50
39
% Ar cumulative

100

c

Fig. 5. 40Ar± 39Ar age spectra from the Red River Shear Zone.

(values differ slightly according to different authors; Harrison et al., 1985; McDougall and Harrison, 1988; Hames and
Bowring, 1995), we should expect a younger age for the
biotite than for the muscovite. An excess Ar component
may be suspected in this biotite, in reference with the age
of the muscovite. It means that if such a component occurs
in the biotite, its distribution is nearly homogeneous on the
whole sites of the mineral, and results in an increase of age
of 12 Ma, vs. the coexisting muscovite. For both samples,
the extreme clustering of data prevents de®nition of a wellde®ned isochron, especially for the Y intercept value,
connected with the 40Ar/ 36Ar ratio.
Sample VN 337 (Fig. 4g) is located in the northern cover
of the crystalline core, represented by muscovite bearing

marbles, close to Xin Man village. The foliation of the
marble is very slight, being underlined by very thin muscovite layers, clearly visible under the microscope. Muscovites give a well-de®ned plateau age at 198 ^ 2 Ma for
80% of 39Ar released. A similar age is obtained with the
isochron diagram, but without any precision on the
40
Ar/ 36Ar ratio, due, as for the earlier sample, to the strong
clustering of 40Ar/ 39Ar. The pattern of this age spectrum
attests for an argon loss subsequent to the closure of the
system, with regularly increasing ages from 31 Ma up to
the plateau age. We discuss the signi®cance of those ages
in the last section of this paper.
In addition to the samples taken from the Song Chay
Massif we also report data from the Day Nui Con Voi.

Table 1
Summary of Ar±Ar ages of analysed minerals in the Song Chay Massif
Sample no.

Plateau age (Ma)

Isochron age (Ma)

VN322 MUSCOVITE

VN324 MUSCOVITE
VN329 BIOTITE
VN333 BIOTITE
VN335 MUSCOVITE
VN335 BIOTITE
VN337 MUSCOVITE

236 ^ 0.5
201 ^ 2
166 ^ 2
164 ^ 2
176 ^ 2
198 ^ 2

200 ^ 2
166 ^ 2
160 ^ 3
176 ^ 2
195 ^ 2

Step age (Ma)

Total age (Ma)

234 ^ 0.8
204 ^ 1
60 ^ 5
228 ^ 1

208 ^ 2

167 ^ 2

230 ^ 2
200 ^ 2
165 ^ 1.7
163 ^ 1.7
174 ^ 2
194 ^ 2

240

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

Table 2
Ar isotopic results for analysed minerals. Correction interference used for 36Ar/ 37ArCa is 2:93 £ 1024 : Mass discrimination correction factor is calculated for a
40
Ar/ 36Ar ratio of 291
Temperature (8C)

40

Ar p/ 39Ar

36

Ar/ 40Ar

37

Ar/ 39Ar

% Atm.

% 39Ar

Age ^ 1sd

VN322 MUSCOVITE (J ˆ 0.018342)
500
1.931
550
2.336
600
3.228
650
4.057
700
4.84
750
5.586
800
6.739
850
6.552
900
6.32
950
6.722
1000
7.129
1050
7.386
1100
7.551
1150
7.276

1.18
0.295
0.195
0.084
0.096
0.101
0.092
0.156
0.074
0.072
0.067
0.074
0.064
0.316

0.015
0.019
0.008
0.006
0.006
0.005
0.002
0.002
0.004
0.004
0.006
0.004
0.004
0.037

34.8
8.7
5.7
2.4
2.8
3
2.7
4.6
2.2
2.1
1.9
2.1
1.9
9.3

0.6
1.2
2.4
4
7.2
12.8
26.8
41.4
50.2
57.7
65.5
83.5
98.5
99.9

62.79 ^ 20.36
75.71 ^ 19.90
103.77 ^ 14.30
129.49 ^ 8.17
153.45 ^ 18.32
175.99 ^ 2.63
210.28 ^ .98
204.77 ^ .98
197.88 ^ 1.51
209.78 ^ 1.97
221.72 ^ 1.53
229.23 ^ .78
234.02 ^ .79
226.01 ^ 8.76
Total age ˆ 208.6 ^ 2.1

VN324 MUSCOVITE (J ˆ 0.018342)
500
2.268
550
4.507
600
6.189
650
6.234
700
6.742
750
7.008
800
7.383
900
7.334
950
7.400
1000
7.505
1050
7.619
1100
7.598
1150
7.581

2.062
1.007
0.067
0.142
0.210
0.144
0.111
0.083
0.090
0.070
0.056
0.116
0.160

0.021
0.015
0.008
0.005
0.005
0.003
0.002
0.001
0.001
0.001
0.001
0.003
0.012

60.9
29.7
2
4.1
6.2
4.2
3.3
2.4
2.6
2
1.6
3.4
4.7

0.3
0.6
1.1
2.1
4.2
8.4
22.6
34.9
47.5
59
92.4
97.9
99.9

73.54 ^ 45.90
143.3 ^ 47.04
193.99 ^ 26.68
195.32 ^ 14.45
210.35 ^ 6.41
218.17 ^ 3.17
229.13 ^ 1.02
227.72 ^ 1.39
229.63 ^ 1.30
232.68 ^ 1.40
236.00 ^ .54
235.38 ^ 2.54
234.90 ^ 8.28
Total age ˆ 230.0 ^ 2.2

VN329 BIOTITE (J ˆ 0.018342)
500
3.023
550
5.826
600
6.248
650
6.451
700
6.479
750
6.445
800
6.395
850
6.501
900
6.394
995
6.421
1050
6.560
1100
6.570
1150
6.748

1.057
0.304
0.114
0.065
0.052
0.069
0.055
0.307
0.102
0.090
0.074
0.086
0.695

0.058
0.007
0.004
0.001
0.002
0.005
0.013
0.015
0.011
0.004
0.002
0.004
0.059

31.2
9
3.3
1.9
1.5
2
1.6
9
3
2.6
2.1
2.5
20.5

1
3.7
11
31.1
48.7
56.7
60.2
64.1
70.8
84
90.7
98.3
99.9

97.36 ^ 21.27
183.17 ^ 8.28
195.76 ^ 3.42
201.76 ^ 1.16
202.6 ^ 1.29
201.58 ^ 2.69
200.11 ^ .55
203.23 ^ 6.63
200.08 ^ .90
200.88 ^ 1.70
205 ^ 3.41
205.29 ^ 3
210.55 ^ 14.25
Total age ˆ 200.3 ^ 2.0

VN333 BIOTITE (J ˆ 0.018342)
550
4.466
600
5.221
650
5.24
700
5.263
750
5.261
800
5.237
850
5.242
900
5.288
950
5.256
995
5.288
1050
5.242
1100
5.27

0.564
0.088
0.128
0.049
0.099
0.104
0.164
0.130
0.090
0.044
0.120
0.114

0.025
0.007
0.002
0.002
0.003
0.014
0.039
0.018
0.011
0.01
0.02
0.034

16.6
2.6
3.8
1.4
2.9
3.0
4.8
3.8
2.6
1.3
3.5
3.3

2.1
7.2
19.5
38.8
52.2
56.6
60.8
68.1
78.7
86.7
94.6
99.2

142.04 ^ 7.18
164.99 ^ 2.78
165.57 ^ 1.14
166.26 ^ .81
166.22 ^ 1.15
165.48 ^ 3.18
165.64 ^ 3.26
167.03 ^ 1.92
166.06 ^ 1.39
167.03 ^ 1.70
165.63 ^ 1.81
166.49 ^ 3.70

241

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248
Table 2 (continued)
Temperature (8C)

40

Ar p/ 39Ar

1150

4.792

36

Ar/ 40Ar

37

Ar/ 39Ar

% Atm.

% 39Ar

Age ^ 1sd

1.436

0.031

42.4

99.9

152.01 ^ 20.29
Total age ˆ 165.4 ^ 1.70

VN335 BIOTITE (J ˆ 0.018342)
450
2.426
500
0.824
550
3.352
600
4.336
650
5.121
700
5.225
750
5.320
800
5.361
850
5.496
950
5.565
995
5.594
1050
5.593
1100
5.719
1150
5.886

1.348
1.146
0.666
0.569
0.040
0.061
0.228
0.142
0.177
0.071
0.067
0.054
0.078
0.192

0.037
0.000
0.031
0.022
0.014
0.006
0.007
0.006
0.003
0.002
0.002
0.001
0.004
0.023

39.8
33.8
19.6
16.8
1.2
1.8
6.7
4.2
5.2
2.1
1.9
1.6
2.3
5.6

0.1
0.3
0.5
0.9
1.7
3.1
6.5
11
22.2
38.6
49.7
90.7
96.9
100

78.56 ^ 92.24
27.06 ^ 139.99
107.67 ^ 76.70
138.06 ^ 43.06
161.98 ^ 19.55
165.12 ^ 11.87
167.99 ^ 5.44
169.22 ^ 3.80
173.29 ^ 1.64
175.36 ^ 1.14
176.23 ^ 1.41
176.20 ^ .42
179.99 ^ 2.92
184.98 ^ 5.33
Total age ˆ 174.7 ^ 1.80

VN335 MUSCOVITE(J ˆ 0.018342)
450
4.081
500
2.981
550
4.521
600
5.076
650
5.162
700
5.211
750
5.217
800
5.149
850
5.171
900
5.205
950
5.137
1000
5.341
1050
5.281
1100
5.279
1150
5.083
1200
4.879

1.944
1.076
0.360
0.148
0.072
0.048
0.050
0.086
0.056
0.079
0.069
0.073
0.040
0.029
0.081
0.030

0.042
0.020
0.005
0.001
0.001
0.000
0.001
0.004
0.007
0.004
0.004
0.002
0.011
0.003
0.009
0.008

57.4
31.8
10.6
4.3
2.1
1.4
1.4
2.5
1.6
2.3
2.0
2.1
1.1
0.8
2.4
0.9

0.3
1.2
3.1
7.9
19.9
41.5
56.2
60.8
64.4
73.8
75.0
87.8
93.2
98.0
99.0
99.9

130.23 ^ 40.60
96.06 ^ 17
143.75 ^ 8.36
160.6 ^ 2.83
163.22 ^ 1.36
164.68 ^ .79
164.89 ^ 1.20
162.81 ^ .85
163.5 ^ 5.20
164.5 ^ 2.01
162.45 ^ 3.30
168.62 ^ 1.15
166.81 ^ 3.25
166.76 ^ 3.03
160.82 ^ 3.89
154.65 ^ 14.13
Total age ˆ 163.6 ^ 1.7

VN337 MUSCOVITE (J ˆ 0.018342)
500
0.957
600
3.692
700
5.551
750
5.662
800
5.988
850
6.284
900
6.395
950
6.316
995
6.264
1050
6.310
1100
6.308
1150
6.364

2.652
0.922
0.109
0.091
0.064
0.057
0.082
0.079
0.062
0.064
0.072
0.152

2.188
7.948
1.845
0.011
0.008
0.005
0.007
0.007
0.003
0.003
0.002
0.004

78.3
27.2
3.2
2.7
1.8
1.6
2.4
2.3
1.8
1.9
2.1
4.5

0.2
0.7
4.1
9.6
16.1
25.4
31.7
39.8
51.8
68.3
92.7
99.9

31.41 ^ 89.35
118.21 ^ 42.14
174.94 ^ 5.53
178.26 ^ 3.88
188.01 ^ 2.80
196.81 ^ 2.00
200.11 ^ 3.02
197.77 ^ 2.45
196.22 ^ 1.62
197.60 ^ 1.37
197.54 ^ 1.00
199.19 ^ 2.57
Total age ˆ 194.3 ^ 2.0

VN106 MUSCOVITE (J ˆ 0.012158)
450
0.711
500
1.311
550
2.800
600
1.304
650
1.544
700
1.390
750
1.528
800
1.492
850
1.546

3.100
1.962
1.765
1.339
1.295
0.533
0.063
0.159
0.177

0
0
0.007
0
0
0
0
0
0

91.6
58
52.1
39.5
38.2
15.7
1.8
4.7
5.2

0.3
0.5
0.7
1.6
2.4
4.7
9.6
15.2
27.7

15.54 ^ 40.08
28.53 ^ 49.21
60.41 ^ 59.51
28.38 ^ 13.52
33.56 ^ 15.54
30.23 ^ 4.69
33.22 ^ 2.45
32.45 ^ 1.86
33.61 ^ .77

242

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

Table 2 (continued)
Temperature (8C)

40

Ar p/ 39Ar

900
950
1000
1100
1400

1.469
1.520
1.522
1.546
1.543

36

Ar/ 40Ar

37

Ar/ 39Ar

% Atm.

% 39Ar

Age ^ 1sd

0.272
0.149
0.157
0.056
1.287

0
0
0
0
0.001

8
4.4
4.6
1.6
38

41.9
57.1
68.8
86
100

31.95 ^ .57
33.03 ^ .77
33.10 ^ .90
33.61 ^ .72
33.53 ^ 1.18
Total age ˆ 32.9 ^ 0.8

VN107 BIOTITE (J ˆ 0.012158)
450
3.174
500
0.768
550
1.548
600
1.823
650
1.851
700
1.839
750
1.839
800
1.848
850
1.831
900
1.936
950
1.754
1000
1.708
1100
2.260
1400
3.748

3.324
3.341
3.258
2.996
1.808
0.483
0.171
0.219
0.409
0.508
0.557
0.344
0.557
2.532

0.038
0.036
0.034
0.007
0.003
0.002
0.002
0.003
0.040
0.296
0.090
0.043
0.093
20.203

98.2
98.7
96.2
88.5
53.4
14.2
5.0
6.4
12.1
15.0
16.4
10.1
16.4
74.8

0.2
0.5
1.9
7.9
24.4
54.3
79.8
85.3
88.2
90.9
94.6
96.4
99.3
100

68.31 ^ 82.93
16.78 ^ 51.80
33.66 ^ 7.83
39.56 ^ 1.81
40.15 ^ .59
39.90 ^ .39
39.91 ^ .43
40.10 ^ 1.90
39.72 ^ 3.79
41.97 ^ 4.88
38.07 ^ 3.63
37.08 ^ 5.41
48.91 ^ 4.07
80.39 ^ 16.67
Total age ˆ 40.3 ^ 1

VN110 MUSCOVITE(J ˆ 0.012158)
450
0.374
500
0.115
550
1.014
600
1.437
650
1.235
700
1.143
750
1.077
800
1.117
850
1.104
900
1.121
950
1.102
1000
1.008
1100
1.291
1400
1.061

3.023
3.541
1.769
0.307
0.418
0.327
0.435
0.328
0.317
0.323
0.335
0.636
1.329
2.404

0.014
0.014
0.015
0.007
0.007
0.008
0.006
0.006
0.005
0.004
0.002
0.001
0.002
0.008

89.3
100
52.3
9.0
12.3
9.3
12.8
9.7
9.3
9.5
9.8
18.8
39.2
71.0

1.3
2.5
4.2
6.4
10.3
16.4
24.4
34.5
46.7
56.9
63.9
70.1
79.7
100

8.20 ^ 11.89
2.52 ^ 13.77
22.12 ^ 11.35
31.26 ^ 8.43
26.90 ^ 4.96
24.90 ^ 3.07
23.48 ^ 2.80
24.34 ^ 1.67
24.05 ^ 1.49
24.44 ^ 1.74
24.03 ^ 2.50
21.97 ^ 2.63
28.10 ^ 2.28
23.13 ^ 1.18
Total age ˆ 23.9 ^ 0.9

Sample VN 106 (Fig. 5a) is a quartzite occurring close to
the Pho Lu city, on the Red River. A very strong lineation
occurs in these rocks, which exhibit an E±W foliation. It
contains layers of ®ne grained muscovites and biotites
underlining the foliation. The muscovite displays a plateau
de®ned for near 90% of 39Ar released at 33:1 ^ 0:8 Ma:
Sample VN 107 (Fig. 5b) is a mylonitic orthogneiss with
a N130 vertical foliation from the road section between Lao
Cai and Sa Pa. Plagioclase is partly transformed with sericites. Intersticial muscovites occur in the matrix. A biotite
yields an age of 40 ^ 1 Ma for 90 % of 39Ar.
Sample VN 110 (Fig. 5c) was taken near Bao Yen on the
border of the Dai Nui Con Voi massif. This is a ®ne grained
gneiss with a developed N15 trending lineation. Muscovites
are coarse grained, with ®sh-like shapes. Very ®ned grained
biotites and plagioclase occur, with garnets and tourmalines.
An age of 24 ^ 1 Ma was obtained on a muscovite for near
60 % of released argon.

4.2. Fission-track data
Apatite ®ssion-track analysis was undertaken on samples
from the Song Chay Massif and RRF zone, to complement the
argon data-set and constrain the low temperature cooling
history. The sensitivity of the system to closure at low
temperatures (,60±1108C) enables detection of weak (in
magnitude) cooling events that may not be otherwise detected
by higher temperature methods. The results and sample locations are given in Table 3. Sample preparation and analysis
followed procedures given in Storey et al. (1996) with samples
irradiated in the thermal facility of the Risù Reactor, National
Research Centre, Rosklide, Denmark, (cadmium ratio for
Au . 200±400†; using Corning glass CN-5 as a neutron dosimeter. Counting and track length measurements used a microscope total magni®cation of 1250 £ with a 100 £ dry
objective. Central ages were calculated using the IUGSrecommended zeta calibration approach (Hurford, 1990).

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

For the Song Chay Massif samples (Table 3), data quality
is mixed. Although adequate numbers of individual grain
ages have been measured for all samples, track length
measurement was affected by low spontaneous track densities. Thus, only six samples (VN 9801, 9805, 9807, 9811,
9812 and 9814) yielded adequate numbers of horizontally
con®ned tracks to suitably de®ne length distributions.
Nevertheless, given the similarity within the data-set,
between central ages and mean track lengths, it is reasonable to infer that similar thermal histories were experienced
by those samples which did not contain adequate numbers
of con®ned tracks.
Central (modal) ages range between 16 ^ 3 Ma and 24 ^
2 Ma; with mean track lengths (for samples with more than
50 measurements), between 13:60 ^ 0:31 mm and 14:12 ^
0:15 mm: Qualitatively, the relatively long mean track lengths
suggest that cooling through the apatite partial annealing zone
(,110±608C) was relatively rapid. The cooling paths may be
further constrained by modelling utilising the procedure of
Gallagher (1995). This is a probabilistic approach that predicts
thermal histories from within speci®ed time-temperature
bounds. Each thermal history is used to predict ®ssion-track
parameters which are quantitatively compared with observed
values and ranked according to goodness of ®t. Maximum
likelihood is used in order to compare each individual
observation.
Only those samples with statistically well-de®ned
length distributions were modelled i.e. VN 9801,
9805, 9807, 9811, 9812 and 9814 and representative
plots are shown in Fig. 6. The modelled results show
the portion of a samples thermal history (between ,60
and 1108C) that is constrained by the ®ssion-track data.
Any variation in temperature below ,608C is unresolvable (highlighted by the grey shading and dashed
time±temperature path). The shaded areas surrounding
the constrained time±temperature paths (solid line)
represent the 95% con®dence regions. The oldest track
recorded in each sample correlates approximately with
the time at which tracks ®rst began to be retained
within an apatite crystal lattice as the sample cooled
through the 1108C isotherm. For samples VN 9811,
9812 and 9814 this took place between 20 and 21
Ma, and for samples VN 9807, 9805 and 9801, between
22 and 28 Ma. Table 4 summarises the main
time±temperature information extracted from the modelled
cooling data.
A plot of sample location against ®ssion-track central age
(Fig. 7) suggests a possible trend of increasing age to the
south-east. This is seen more clearly in the modelling which
shows the older ages record an earlier cooling than
samples to the north-west. There is no evidence for a
systematic correlation between age and elevation as
would be expected from a terrain that experienced a
slow to moderate uniform rate of denudation i.e. the
cooling pattern is not caused by variable depths of
erosion. But, it is interesting to note that the sample

243

which cooled at the fastest rate between 110 and 608C
(sample VN 9807), comes from the maximum elevation
(at ,800 m). Regionally cooling for most samples
occurred at a similar rate (within experimental uncertainties), to between 3 and 58C/Myr.
Samples from the RRF were also analysed to complement
the new argon data. Some of the apatite samples from this
region were dif®cult to analyse because of lower than
normal uranium concentrations (often ,5 Uppm) and this
affected the quality of some of the track length data. Nevertheless the resultant data-set is of suitable quality to provide
meaningful constraints on the regions cooling/exhumation
history.
Samples VN 9818±9821 are from the road section
between Lao Cai and Sapa along which the argon sample
VN 107 was also collected. The four samples range in
central age from 37 ^ 2 Ma to 27 ^ 3 Ma: Track lengths
for those samples with adequate numbers of measurements
range from 13:72 ^ 0:27 mm to 14:31 ^ 0:14 mm; and are
consistent with moderately rapid cooling Ð hence the ages
approximate to the time of cooling. Sample age and lengths
show no correlation with elevation, and therefore, the age
distribution is unrelated to simple uniform erosional
denudation.
The ®ssion-track data from the undeformed granites (VN
9824±9827) adjacent to the main Phan Si Pang granite, west
of Sapa give central ages between 32 ^ 4 and 30 ^ 3 Ma:
Sample VN 9827 has suitable numbers of measured
con®ned tracks that comprise a mean length of 14:18 ^
0:14 mm; consistent with rapid cooling. The similarity
among the four data suggest they experienced the same
thermal history.
Two samples (VN 9846 and VN 9848) were analysed
from locations near the town of Bao Yen close to the edge
of the Day Nui Con Voi. These gave similar central ages
22 ^ 2 Ma and 18 ^ 1 Ma and both have mean track
lengths longer than 14 mm indicative of rapid cooling.

5. Interpretation and discussion
The 40Ar± 39Ar and ®ssion-track data-sets from the Song
Chay Massif are signi®cantly different in age, and therefore,
relate to different aspects of the regions geodynamic evolution. Due to the different closure temperature for Ar and
FissionTtrack systems (350±4008C vs. 608C for exhumational FT cooling), it is possible to recognise both Mesozoic
and Cenozoic events in the Song Chay Massif. We now
discuss the signi®cance of these ages.
The geographic distribution of the 40Ar± 39Ar results
shows ages that are younger in the central part of the
dome. Muscovite and biotite from the southernmost
samples, VN322 and VN324, record ages of 234 and
236 Ma, respectively, corresponding to the last increments
of experimental degassing. A similar range of ages can be
found throughout Vietnam (Lepvrier et al., 1997); the Song

244
Table 3
Fission track apatite analytical data for the Song Chay Massif
Notes: (i) Track densities are ( £ 10 6 tr cm 22) numbers of tracks counted (N) shown in brackets; (ii) Analyses by external detector method using 0.5 for the 4p/2p geometry correction factor; (iii) Ages
calculated using dosimeter glass CN-5; analyst Carter zCN5 ˆ 339 ^ 5; (iv) Central age is a modal age, weighted for different precisions of individual crystals
Sample

Long

Latt

Elevation (m)

No. grains

rd

Nd

rs

Ns

rI

Ni

% R.E.

Central Age (Ma)

Mean track length (mm)

S.d. (mm)

1.31
0.99

86
11

1.70
1.63
1.43
1.66
1.42
1.36
1.35
1.42

31
81
83
19
90
79
30
59

Tracks measured

104.51.74
104.51.49
104.50.32
104.50.10
104.49.34
104.48.77
104.29.38
104.32.07
104.35.68
104.37.29
104.42.20
104.46.89

115
150
410
580
750
800
391
365
395
470
480
720

20
20
20
20
20
20
16
18
20
20
17
20

1.481
1.482
1.484
1.485
1.487
1.490
1.490
1.492
1.493
1.495
1.497
1.498

4159
4159
4159
4159
4159
4159
4159
4159
4159
4159
4159
4159

0.135
0.365
0.053
0.098
0.121
0.515
0.034
0.435
0.094
0.221
0.119
0.057

218
664
94
206
163
116
44
329
111
442
191
121

1.496
4.106
0.585
1.139
1.246
0.671
0.543
5.704
1.262
3.045
1.620
0.896

2418
7473
1039
2399
1682
1511
709
4309
1496
6086
2585
1913

15.4
13.3
0.04
3.8
0.33
19.7
20.0
22.4
0.75
0.8
17.9
22.9

23 ^ 2
22 ^ 1
23 ^ 2
22 ^ 2
24 ^ 2
20 ^ 2
16 ^ 3
19 ^ 2
19 ^ 2
18 ^ 1
19 ^ 2
17 ^ 2

13.93 ^ 0.14
13.51 ^ 0.31
No data
13.60 ^ 0.31
13.14 ^ 0.18
14.01 ^ 0.16
13.39 ^ 0.39
13.41 ^ 0.15
14.12 ^ 0.15
13.81 ^ 0.25
13.65 ^ 0.19
No data

Lao Cai to Sapa
VN9818 22.26.29
VN9819 22.25.64
VN9820 22.24.40
VN9821 22.22.21

103.55.50
103.55.01
103.54.06
103.52.12

595
730
900
1285

20
20
20
20

1.586
1.263
1.586
1.586

8792
7004
8792
8792

0.052
0.086
0.082
0.131

89
131
110
127

0.517
0.492
0.781
1.275

897
749
1045
1241

10.1
29.1
21.7
24.9

27 ^ 3
37 ^ 2
30 ^ 4
29 ^ 3

13.72 ^ 0.27
13.75 ^ 0.27
13.77 ^ 0.18
14.31 ^ 0.14

2.38
1.98
1.48
0.98

80
54
69
53

West of Sapa
VN9824 22.21.24
VN9825 22.21.30
VN9826 22.21.68
VN9827 22.21.59

103.45.92
103.46.47
103.45.86
103.45.26

2200
2000
1905
1670

11
21
16
20

1.263
1.263
1.263
1.586

7004
7004
7004
8792

0.175
0.098
0.015
0.125

70
80
94
192

1.245
0.691
1.013
1.150

497
561
639
1772

25.6
0
11.3
28.2

32 ^ 5
31 ^ 4
32 ^ 4
30 ^ 3

No data
13.73 ^ 0.45
14.98 ^ 0.21
14.18 ^ 0.14

1.49
0.83
1.38

12
17
103

Bao Yen
VN9846
VN9848

104.23.52
104.26.88

320
260

14
20

1.263
1.263

7004
7004

0.335
0.282

176
287

3.259
3.282

1712
3343

24.5
0.0

22 ^ 2
18 ^ 1

14.24 ^ 0.16
14.41 ^ 0.14

1.14
0.92

51
45

22.11.94
22.13.65

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

Song Chay Massif
VN9801 22.29.74
VN9802 22.29.77
VN9803 22.31.68
VN9804 22.32.04
VN9805 22.32.50
VN9807 22.32.94
VN9810 22.41.80
VN9811 22.43.05
VN9812 22.44.75
VN9813 22.44.89
VN9814 22.43.97
VN9815 22.35.01

245

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248
0

0 Ma
2 Ma
4 Ma
25 Ma

40
60

VN 9811

20

8 ¡C
28 ¡C
41 ¡C
94 ¡C

?

0 Ma
1 Ma
20 Ma
21 Ma

40
T(¡C)

20

T(¡C)

0

VN 9801

60

15 ¡C
45 ¡C
84 ¡C
122 ¡C

?

80

80

PAZ

100

PAZ

100
120

120
35

28

21

14

7

0

35

28

21

Time (Ma)
30

20

Obs. Age : 22.5 Ma
Pred. Age : 22.5 Ma
Obs. Mean length : 13.92
Pred. Mean length: 13.83
Obs. S.D.
: 1.31
Pred. S.D.
: 1.33
Oldest track (Ma) : 25

20

N
10

1

2

3

4

5

6

7

8

P(K-S) = 0.551
P(Chi) = 0.997

N
10

9

10 11 12 13 14 15 16 17 18 19 20

2

3

4

5

6

Track Length (microns)

0 Ma
4 Ma
8 Ma
28 Ma

60

12 ¡C
60 ¡C
34 ¡C
107 ¡C

20

?

40
T(¡C)

T(¡C)

7

8

9

10 11 12 13 14 15 16 17 18 19 20

Track Length (microns)

VN 9805

40

80

PAZ

100

60

VN 9814

?

0 Ma 10 ¡C
13 Ma 69 ¡C
21 Ma 109 ¡C

80

PAZ

100

120

120
35

28

21

14
Time (Ma)

7

35

0

28

21
Time (Ma)

20

30

10

0

0

20

N

7

P(K-S) = 0.970
P(Chi) = 0.994

Obs. Age : 19.1 Ma
Pred. Age : 19.2 Ma
Obs. Mean length : 13.56
Pred. Mean length: 13.57
Obs. S.D.
: 1.49
Pred. S.D.
: 1.33
Oldest track (Ma) : 21

1

0

20

14
Time (Ma)

Obs. Age : 24.3 Ma
Pred. Age : 24.3 Ma
Obs. Mean length : 13.14
Pred. Mean length: 13.14
Obs. S.D.
: 1.62
Pred. S.D.
: 1.48
Oldest track (Ma) : 28

1

2

3

4

5

6

7

8

P(K-S) = 0.863
P(Chi) = 0.999

N
10

9

10 11 12 13 14 15 16 17 18 19 20

Track Length (microns)

2

3

4

5

6

7

8

7

0

P(K-S) = 0.901
P(Chi) = 1.000

Obs. Age : 18.7 Ma
Pred. Age : 18.7 Ma
Obs. Mean length : 13.64
Pred. Mean length: 13.54
Obs. S.D.
: 1.42
Pred. S.D.
: 1.43
Oldest track (Ma) : 21

1

14

9

10 11 12 13 14 15 16 17 18 19 20

Track Length (microns)

Fig. 6. Modelling of ®ssion track data.

Ma complex to the west, the central and southern Truong Son
Belt, and the Kontum massif, all reveal metamorphic ages ca.
240±245 Ma. These ages are found on syn- to late-kinematic
minerals and testify to the widespread in¯uence of Triassic
metamorphism in this southeastern part of Asia. Evidence for
this orogen can also be found in southern China and Thailand
(Mitchell, 1986; Hutchison, 1989; Arhendt et al., 1993;
Dunning et al., 1995; Faure et al., 1996; Mickein, 1997).

The ages of 234±236 Ma obtained on two muscovites from
the outer rim of the dome are slightly younger than the average
age obtained from the Indosinian massifs located within the
north-south Truong Son Belt in Vietnam, but they are similar
to the age range found in south-west China (Faure et al., 1996)
and Thailand (Dunning et al., 1995; Mickein, 1997), and therefore, we consider 234±236 Ma to record Triassic tectonometamorphism.

246

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

Table 4
Summary of time±temperature constraints and sample cooling rates derived from the modelled ®ssion track data
Sample

Central age (Ma)

Oldest track (time
crossed 1108C isotherm)

Approx time crossed
(608C isotherm)

Cooling rate for temperature
interval 110±608C (8C/Myr)

VN
VN
VN
VN
VN
VN

23
24
20
19
19
19

25
28
22
21
20
21

10
17
17
10
11
11

3.3
4.5
10
4.5
5
5

9801
9805
9807
9811
9812
9814

Although ages decrease through 200 Ma, to165 Ma in the
northern region, we infer that the whole granitic protolith,
which intruded at 428 ^ 5 Ma (Leloup et al., 1999), was
sheared during the Indosinian. The deformation history is

Elevation (m)

1000
800
600
400
200
0
22.55

Latitude

North
22.5
22.45
22.4
22.35
22.3

South
22.25
104.7

Longitude

East
104.6
104.5
104.4
104.3

West
104.2
10

15

20

25

30

Central Age (Ma)
Fig. 7. Plots of ®ssion-track data against sample location and elevation.
Error bars are ^1sigma.

very homogeneous all along the pro®le and we recognise a
single ductile tectonic event. The evolution seen in micas
ages along the pro®le is most probably the result of a slow
doming after development of the Indosinian foliation rather
than a succession of events. The rims of the massif crossed
the 450±3008C isotherms during or after the end of Indosinian orogenic episode, and have remained above this
isotherm since that time. Intermediate zones crossed these
isotherms later, at 200 Ma, as inferred by sample VN 329,
consistent with a moderately slow exhumation. The youngest sampled level, crossed the same isotherms much later, at
ca. 165 Ma. Since biotite VN 333 and muscovite VN335
record the same age (166 and 164 Ma, respectively), it is
probable that the cooling rate, at this late stage, increased.
The pattern of ages could be explained by a thermal diffusion effect, perhaps associated with a magmatic body
emplaced deeply under the core structure of the dome.
This would imply that 165 Ma of VN 333 and VN 335 is
a maximum age (oldest), related to partial loss of radiogenic
40
Ar from the samples exhumed from the deeper crustal
levels.
Sample VN 322 is important, in relation to the late evolution of the southern part of the Song Chay Massif. The
cordierite±sillimanite bearing schist constitutes the southernmost limit of the dome complex. It corresponds to a
vertical E±W oriented mylonitic band. Muscovites occur
in the foliation plane and constitute a strongly developed
lineation dipping 308 to N 240. The trend of the age spectrum attests to argon loss by diffusion processes, the ®rst age
obtained, 60 Ma being younger than the one obtained at the
end of the degassing procedure (234 Ma). The age of 60 Ma
can be considered as an older limit for the late deformationmetamorphism responsible for the development of this
mylonitic band and for the rejuvenation of the age of
these older muscovites. This age of 60 Ma, is distinctly
older than Cenozoic ages between 35 and 25 Ma displayed
by mylonitic gneisses along the RRF Zone, which cross cuts
this E±W structure.
In contrast to the argon data the ®ssion-track results
clearly show the region was affected by Cenozoic tectonism.
The time interval over which the Song Chay experienced
rapid cooling (28±20 Ma) coincides with the main phase of
shear heating and sinistral movement along the Red River
Shear Zone, and therefore, it is probable these two events

H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

are related. As stated in Section 4 the ®ssion-track data also
show a well developed geographical trend but there is no
correlation between age and altitude which suggests that
simple erosion is not responsible for the distribution of ages.
The gradient of ages from north to south (Fig. 7) suggests a
northward tilt of the Song Chay block with an older exhumation in the south. This asymmetry would be consistent with
block tilting perhaps caused by reactivation of bounding
faults, a process that occurs isostatically after normal faulting
(e.g. Jackson and McKenzie, 1989).
The temporal relationship between the Red River Shear
Zone and late stage exhumation of the Song Chay Massif is
now explored further through new argon and ®ssion-track
results.
Sample VN 110 was collected from the Day Nui Con Voi
and records a muscovite Ar±Ar age of 24 ^ 2 Ma: Two
®ssion-track samples from the same area record central ages
of 18 ^ 1 and 22 ^ 2 Ma showing that exhumation of the
main shear zone to shallow crustal levels was rapid. Published
mica ages for the Day Nui Con Voi range from 24:9 ^ 0:2 Ma
to 21:2 ^ 0:2 Ma (Harrison et al., 1996; Leloup et al., 1997;
Tran Ngoc Nam, 1988; Tran Ngoc Nam et al., 1998; Wang et
al., 1998); however, beyond the main shear zone there is little
published age data. Sample VN 107 from a road section
between Lao Ca and Sapa records mica age at 40 ^ 1 Ma;
whilst ®ssion-track data from the same area (Table 3) record
an age range between 27 ^ 3 Ma and 37 ^ 2 Ma: Track
lengths for these samples are between 13:72 ^ 0:27 mm and
14:31 ^ 0:14 mm consistent with moderate to rapid cooling,
con®rmed by modelling the better quality data. The Oligocene
cooling recorded by both the argon and ®ssion-track data in
this area thus relates to a cooling event associated with early
development of the RRF system a period that so far, is poorly
constrained by high temperature geochronology.
West of Sapa is the Phan Si Pang granite body (10±15 km
wide and up to 140 km long). This has previously been
dated using K±Ar methods to between 41 and 58 Ma
(Phan Cu Tien, 1977) however recent 40Ar/ 39Ar dating of
phlogopite and biotite from the granite and fault bounded
metamorphic rocks show that rapid cooling from temperatures .3008C occurred at ,34 Ma, an age that indicates
much younger emplacement (Leloup et al., 1997). This
age is identical (within error) to the ®ssion-track ages …30 ^
4 and 32 ^ 5 Ma† measured on undeformed granites adjacent to the main granite body. Such concordancy records
geologically instantaneous cooling and is consistent with
the ®ssion-track length data (Table 3). This evidence,
suggests early fault movement coincided with emplacement
of the Phan Si Pang granite, consistent with the observations
of Leloup et al. (1997).
The data from the RRF area show evidence for two phases
of cooling during the Cenozoic. The early phase occurred
during the Oligocene and is associated with emplacement of
the Phan Si Pang granite. The later phase is restricted to the
main shear zone along the Day Nui Con Voi and occurred
between ,25±21 Ma. In both cases cooling was associated

247

with exhumation from signi®cant crustal depths. In contrast
the data from the Song Chay Massif show Cenozoic exhumation was limited, preserving an earlier Indosinian thermotectonic signature. The two pulses of cooling correspond to an
increase in slip rate along the main fault which Harrison et al.
(1996) noted also coincides with the transtensional phase. It is
probable that this transtensional environment has caused localised extensional unroo®ng of the Song Chay Massif as well as
the more pronounced extension recently identi®ed in the Bhu
Khang Massif, southwest of the shear zone (Jolivet et al.,
1999).
6. Conclusion
The Song Chay Massif area has been affected by the
Triassic orogeny, which is responsible for high-grade metamorphism and shearing, observed along a NW±SE crosssection. A shear zone formed during this orogeny at the
interface between metasediments and a granite intruded
,430 Ma ago. The shear zone has a shallow dip and
shows a consistent top-to-the-North sense of shear.
40
Ar± 39Ar ages of micas from orthogneiss within the shear
zone record a Triassic age on the southern area, but also
show evidence for a younger cooling most probably related
to a slow doming in the Jurassic. Low temperature apatite
®ssion-track data from along the same transect record a later
Cenozoic exhumation that involved some reactivation of
bounding faults, with a normal sense of movement. Timing
is similar to the exhumation events in the RRF Zone and
implies a causal relationship. This study also reinforces the
importance of combining both low and high temperature
dating methods in a single study.
Acknowledgements
This study was supported by cooperative programs:
Programme International de CoopeÂration Scienti®que
between CNRS (INSU) and CNST (Vietnam); Paris 6
University, Montpellier 2 University and National University of Vietnam, Hanoi. Funding for DR and ®ssion-track
analysis was provided by the University of London Southeast Asia Research Group.
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