DSpace at VNU: LA-ICPMS zircons U Pb dating of Permo-Triassic and Cretaceous magmatisms in Northern Vietnam - Geodynamical implications

Journal of Asian Earth Sciences 48 (2012) 72–82

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Journal of Asian Earth Sciences
journal homepage: www.elsevier.com/locate/jseaes

LA-ICPMS zircons U/Pb dating of Permo-Triassic and Cretaceous magmatisms
in Northern Vietnam – Geodynamical implications
Françoise Roger a,⇑, Henri Maluski a, Claude Lepvrier b, Tich Vu Van c, Jean-Louis Paquette d
a

Université Montpellier 2, CNRS UMR 5243, Géosciences Montpellier, 34095 Montpellier Cedex 5, France
ISTEP, UMR 7193-CNRS-Université P&M Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France
c
National University of Vietnam, 334 Nguyen Trai Thanh Xuan, Hanoi, Viet Nam
d
Laboratoire ‘‘Magmas et Volcan’’ (CNRS UMR 6524), Université B. Pascal, F-63 038 Clermont-Ferrand Cedex, France
b

a r t i c l e

i n f o

Article history:
Received 15 March 2011
Received in revised form 7 December 2011
Accepted 20 December 2011
Available online 8 January 2012
Keywords:
Nappes

Indosinian orogeny
Cretaceous magmatism
Paleo-Pacific subduction
Paleotethys
NE Vietnam

a b s t r a c t
In northeastern Vietnam, the major tectonic episode responsible for nappes emplacement is Triassic.
These allochtonous structures are intruded by granitic melts. Two post-tectonic massifs showing no sign
of deformation have been dated by the LA-ICPMS zircon U–Pb techniques. Dating reveals a multiphase
history with zircon cores showing evidence of Proterozoic magmatism. The emplacement of the Phia Bioc
granite intrusive in allochtonous units is 248–245 Ma, an age which assesses a younger limit for the
major nappes tectonic. This tectonic could be synchronous of the tectonometamorphic strike-slip faulting
events (250–245 Ma) defined in the Truong Son Belt as the Indosinian orogen. The Phia Bioc intrusion is
probably linked with the intra-plate magmatism of the Emeishan Large Igneous Province or with magmatism associated with the Paleotethys closure. The age of the Phia Oac granite intrusion in displaced units
is much younger, at 87.3 ± 1.2 Ma. This granite is probably linked to the magmatic activity produced during the Paleo-Pacific plate subduction under the SE Asia continental plate during the Mesozoic. Although
the Cenozoic Red River fault system is close to these two plutons, this last thermotectonic episode has not
been strong enough to disturb the U/Pb system. Zircons rims do not show any Tertiary magmatic or metamorphic overprint.
Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction
It is commonly accepted that the major geodynamic event
responsible for the build up of SE Asia took place in the Triassic
period during the Indosinian orogeny (Fromaget, 1941a,b), resulting from continental collisions between several Gondwana-derived
blocks subsequent to the closure of the Paleotethys ocean. One
branch of the Paleotethys is represented by the Permo-Triassic
Song Ma ophiolitic suture (Fig. 1A and B), which separates the
South China and Indochina blocks. In Vietnam, North and South
of the suture, a thermotectonic overprint has been demonstrated
through radiometric investigations (U–Pb and Ar–Ar methods)

applied to metamorphic and magmatic minerals. As a result, the
major periods of thermotectonic activities are ca. 420–470 Ma,
240–250 Ma and 20–35 Ma (Nagy et al., 2001; Vu Van Tich,
2004; Maluski et al., 2005, 1995, 2001; Roger et al., 2007; Maluski
and Lepvrier, 1998; Lepvrier et al., 1997; Carter et al., 2001; Leloup
et al., 1993, 1995, 2001; Searle, 2006, 2007; Searle et al., 2010).

⇑ Corresponding author.
E-mail address: (F. Roger).
1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jseaes.2011.12.012

Two distinct structural domains coexist on each side of this suture.
Along and south of the Song Ma suture, in the Truong Son range
(Fig. 1), the Triassic structures are characterized by high strain ductile deformation and metamorphism along partially mylonitic
NW–SE trending strike-slip faults (Lepvrier et al., 1997, 2008).
Ar–Ar and U–Pb dating have been conducted in this belt establishing that this major thermotectonic event (the Indosinian orogeny)
took place around 245–250 Ma (Maluski et al., 1995; Lepvrier et al.,
1997; Maluski and Lepvrier, 1998; Roger et al., 2007). The northeastern part is dominated by originally flat structures corresponding to large nappes, which have been later refolded. This geometry
was first described through classical field geology (Deprat, 1915)
and has been confirmed recently by Lepvrier et al. (2011). What
is the age of this nappe tectonics? Does it occur synchronously
with the thermotectonic event responsible of the development of
ductile strike-slip faults in the Truong Son belt? An answer to these
questions can be indirectly provided by LA-ICP-MS U–Pb zircon
dating of intrusive granitic bodies emplaced through the allochtonous metasedimentary terranes of NE Vietnam. Is there any influence of the Cenozoic tectonics as locally observed in Vietnam (Bu
Khang massif and Red River Shear Zone Fault) on these U/Pb data
(Maluski et al., 1997; Jolivet et al., 1999; Leloup et al., 1993,
1995, 2001)?

73

F. Roger et al. / Journal of Asian Earth Sciences 48 (2012) 72–82

2. Geological outline of the Northern Vietnam

2.2. Magmatic bodies

2.1. Tectonics

Several granitic and gabbro-syenitic to ultramafic bodies of various sizes intrude the Paleozoic metasedimentary rocks of the nappe and the Song Hien displaced unit (Fig. 2). The larger intrusion is
represented, West-Northwest of Bac Can, by the Phia Bioc granite
which forms a crescent-shaped body, convex to the East and
bounded by a thrust fault. This magmatic body intruded into the
Silurian series, consists in a biotite–muscovite bearing granite with
occurrences of amphiboles bearing melts. This granite is porphyroid, undeformed and locally contains enclaves of microdiorites.
It can be considered as a post-tectonic granite surrounded by a
thermal metamorphic zone including horns. Two samples of the
granite have been collected for dating: sample VT19
(N22°240 1800 ; E105°410 1500 ) and sample VT16 (N22°070 4700 ;
E105°460 4500 ) (Fig. 2). According to Tran Van Tri (1979), the Phia
Bioc granite is Pre-Ladinian (235–230 Ma) because it locally crosscuts Lower Triassic sediments but occurs as pebbles in the basal
conglomerates of the Ladinian sedimentary formation. The same
author obtained K–Ar ages scattered of 230–306 Ma for this granite. Moreover to the South, the Cho Chu granite, which belongs to

nB
ien
Phu

N

Ye
n

Fa
ult

tur

e

Son

a

SOUTH CHINA SEA

R

S

Hai Nan

Thanh Hoa

gC

T

O

n

t

Die

-

t

ul

A

ul

Su

Bu Khang
Massif

L

Ti
e

HANOI

Fa

Ma

Fa

er

Da

iv

20°N

ng

R

ng

So

ed

So

R

22°N

A

CHINA

ng
Ba

Fault

Cao

Song Chay Massif

Vinh

U
O
N
G

Mek

18°N

S

ong

O
N

Rive

Dailoc Massif

B
E

r

Da Nang

L

THAILAND

T

16°N

Tamky-Phuoc Son Suture
. Tra Bong

Dien Binh
Complex

KONTUM BLOCK
PO

14°N

K

o

CAMBODIA

g
an
W

Ch
ao

12°N

Fa
ult
Zo
ne

Fa
ult

Dalat
HO CHI MINH TOWN

10°N

Suture

Fault

102°E

104°E

106°E

QAIDAM

108°E

110°E

B

NORTH CHINA

1 BH

SG

QIANGTANG

Lon
gm
Sha en
n

9

2

LHASA

Yi
INDIA

6

Qinling

ckk
looc
bbl
ee
tz
g
n
Ya

Da10
bie

ck
lo

SOUTH CHINA
CHINA sia b
3 SOUTH

ay
tha
Ca

Si

4

5
7

A

INDOCHIN

SIBUMASU

West Burma

Three different main structures define a prominent NW–SE
structural orientation south of the Red River Fault: the Tule volcano-detrital basin, limited to the South by the Song Da continental
rift system which bounds the Song Ma shear zone (Figs. 1A and 2).
The latter joins to the NW with the Dien Bien Phu Fault. Volcanic
rocks in the Tule massif emplaced during two main periods, in
the middle-late Jurassic (176–145 Ma) and the late Cretaceous
(80–60 Ma) (Anh et al., 2003). The main tectono-metamorphic
activity of the Song Ma shear zone has been precisely dated using
Ar–Ar dating and relates to the Indosinian orogeny at ca. 250 Ma
(Lepvrier et al., 1997; Maluski and Lepvrier, 1998; Maluski et al.,
1999). Up to now, in NW Vietnam, no Paleozoic magmatism has

been found south of the Red River Fault.
When crossing the Cenozoic massif of the Day Nui Con Voi
bounded to the northeast by the Song Chay Fault (Fig. 2), the
NW–SE structural direction disappears. The most prominent structure is represented by the Dulong-Song Chay massif which extends
towards the North in South China (Roger et al., 2000; Maluski et al.,
2001; Carter et al., 2001; Yan et al., 2006): granitic and orthogneissic series form the crystalline core with a dome-like structure overlain by muscovite bearing marbles in its northern part and
cordierite-sillimanite bearing schists in its southernmost edge.
Orthogneissic rocks were dated by Roger et al. (2000) who obtained on zircons a U–Pb emplacement age of 428 ± 5 Ma for the
porphyritic monzogranitic protolith. An overprinted metamorphism was dated by Ar–Ar at 234–236 Ma, clearly related to the
Indosinian tectonometamorphic episode, as elsewhere displayed
in the Song Ma structure and southerly by the entire Truong Son
Belt (Maluski et al., 2001; Lepvrier et al., 2004, 2008). Finally, a
slow-doming episode at 180 Ma is also recorded by Ar–Ar analysis
on micas along a N–S cross section (Roger et al., 2000; Maluski
et al., 2001). Low temperature apatite fission-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 (Maluski et al., 2001). To the East of the Song Chay
Massif, the geological map displays an arch-shaped structure convex to the SE (Fig. 2). Cambrian formations surround the eastern
and southern limits of the crystalline core, with micaschists,
quartzites and limestones. To the East and South East, these lower
Paleozoic formations are overlain by lower to middle Devonian
limestones and quartzites. More to the East, a crescent-shaped
middle Ordovician shales and sandstones series is intruded by
granitoids. Finally, still to the East, lower and upper Paleozoic
schists are covered by a large Triassic detrital formation with clayish shales, sandstones and conglomerates. All these arched formations are bounded by mylonitic faults p.p. Following Deprat (1915),
this geometry corresponds to a major nappe structure. In spite of
errors pointed out by Bourret (1922), as for example for the Pia
Oac massif, this interpretation is still partially valid, except for
the sense of displacement of the nappes system which is N to
NE-directed (Lepvrier et al., 2011). To the East of the major basal

contact of the nappe, the Lower Triassic Song Hien Formation
develops and forms another displaced unit (Lepvrier et al., 2011)
(Fig. 2). Upper Triassic to Lower Jurassic formations, are largely exposed in the external zones and also form isolated outcrops resting
on the Paleozoic formations of the nappe. To the south of Lang SonThai Nguyen city these upper Triassic sedimentary rocks unconformably overlie the Lower–Middle Triassic series of the displaced
unit.
In the external part of the area, to the East of Cao Bang and to
the North of Yen Minh the Paleozoic formations represent a distinct autochtonous domain (Lepvrier et al., 2011).

Cretaceous
granitoids
Emeishan Large
Igneous Province

8

EAST MALAYA

Fig. 1. Simplified maps of Vietnam (A) and of SE Asia (B) showing major blocks and
structures discussed in the text. SG: Songpan-Garzê belt, BH: Bayan Har terrane, Yi:
Yidun (or Litang-Batang) block, Si: Simao terrane. Main sutures of Paleotethys: r:
Jinsha, s: Yushu-Batang, t: Ailaoshan, u: Song Ma, v: Nan-Uttaradit, w:
Changning-Menglian, x: Sra Kaeo, y: Bentong-Raub, z: Kunlun-Anyenaqen, {:
Qinling-Dabie. (After Roger et al., 2003, 2008, 2010 and Metcalfe, 2002.).


74

F. Roger et al. / Journal of Asian Earth Sciences 48 (2012) 72–82

103°

Dulong-Song Chay Massif

CHINA

23°

105°

104°

106°
Suoi Cun Complex

YEN MINH
MINH
YEN

Fan Si Pan Granite

Phia Oac

HA GIANG
GIANG
HA

Phia Oac
Oac Granite
Granite
Phia

v

v

v
v
v
Phu
Phu
Sa
Sa Phin
Phin
v v
Complex
Complex

v

O
NG

F

Permian-Triassic

Neogene

Paleozoic

Jurassic-Cretaceous

Proterozoic

J

THAI NGUYEN
J

PHU THO
J

VIET TRI

K

HANOI

∼

∼
∼

HAI PHONG

HOA BINH
BINH
HOA

∼

∼ ∼

Kim Boi Massif

∼
∼ ∼ ∼ ∼ ∼∼
∼

Magmatic Rocks

Quaternary

O

J

D

50 km

Sedimentary Rocks

LANG SON

Nui Chua Complex
CH
AY

K

0

O

LTT
UUL
FFAA

NGHIA
NGHIA LO
LO

v

RI
VE
R

LLTT
UU
AA

21°

∼

SON LA

v

SS

∼ ∼
∼ ∼

K

I

∼∼

RE
D

v
v

TUYEN QUANG

VO

DIEN
DIEN BIEN
BIEN PHU
PHU

∼

N

v
v

Cho Chu
Phu Ngu

O

IC

U

v

VT19 BAC CAN

T
UL
FA

N

D

O

TULE v

v

BAC CAN
D
O

LUC YEN

Phia Bioc
Bioc
Phia

*

CHO DON
DON
CHO

N
IEEN
HHI

AY

∼
∼∼ ∼
∼ ∼ ∼ RE
∼
∼U∼
∼ ∼ ∼∼ ∼∼A SU∼T
∼ M∼
LTT ∼

UUL
∼ SONG∼
FFAA ∼
A
DDA ∼
∼
G
NNG ∼
∼∼ ∼ ∼
SSOO
∼ ∼ ∼∼ ∼ ∼ ∼
∼ ∼ ∼
∼ ∼∼ ∼ ∼ ∼∼ ∼
∼ ∼∼ ∼

v

v

LAOS

CHO DON

v

v

Phia Bioc
Bioc Granite
Granite

Phia

*

D

G
AN
OB
CA

v

LAI CHAU

O

D

K

G
NG
ON
SSO

K

SAPA
SAPA

v

D

DIIE
D
EN
NB
BIIE
EN
NP
PH
HU
U FFA
AU
ULLTT

22°

BABE LAKE
K

CHO RA

VT16
VT16

CAO BANG

CHO RA

*

VT 235
BABE LAKE

Paleocene

Late Permian-Late Triassic

Cretaceous

Paleozoic

Jurassic-Early
Cretaceous

Proterozoic
Mafic rocks

∼ ∼

Thrusts
(nappe contact)

} Foliation

Strike-slip fault

Gneiss Song Chay
v v

Volcano-detritic

*

Sample

Fig. 2. Simplified geological map of NE Vietnam (adapted from Geology of Vietnam (North Part) Hanoi, General Department of Geology 1979 and Lepvrier et al. (2011).

the same magmatic complex is unconformably covered by Upper
Triassic sandstones and conglomerates (Fig. 2). More to the
North–Northeast, the Phia Oac granite, located South of Nguyen
Binh and Tinh Tuc villages is also an undeformed post-tectonic
granite (Bourret, 1922). It is a two-micas leucocratic granite, with
phenocrysts of feldspars (sample VT 235; N22°370 2900 ;
E105°520 42) which has been emplaced through the ductilely deformed allochthonous Devonian series. This formation is exposed
on the western flank of the massif and to the East through the Lower Triassic Song Hien Formation that also forms a displaced unit
(Lepvrier et al., 2011) (Fig. 2). The pluton is surrounded by a contact metamorphic aureole formed by cordierite–andalusite–biotite
schists and horns. Izokh et al. (1964) obtained K–Ar ages of 85–
95 Ma on biotite and muscovite.

3. U–Pb geochronology
3.1. LA-ICPMS: instrumentation and analytical method
Separated grains were mounted in epoxy resin disks, polished
to reveal equatorial cross sections. U–Th–Pb geochronology of zircon was conducted by laser ablation inductively coupled with plasma spectrometry (LA-ICPMS) at the Laboratoire Magmas et
Volcans, Clermont-Ferrand (France). The analyses involve the ablation of minerals with a Resonetics Resolution M-50 powered by an
ultra short pulse ATL Atlex Excimer laser system operating at a

wavelength of 193 nm (detailed description in Müller et al.,
2009). Spot diameters of 26 lm associated to repetition rates of
3 Hz with a laser energy of 4 mJ were used. The ablated material
is carried into helium, and then mixed with nitrogen and argon, before injection into a plasma source of an Agilent 7500 cs ICP-MS
equipped with a dual pumping system to enhance the sensitivity.
The analytical method is basically similar to that developed by
and reported in Tiepolo (2003) and Paquette and Tiepolo (2007).
The signals of 204(Pb + Hg), 206Pb, 207Pb, 208Pb, 232Th and 238U
masses are acquired. The occurrence of common Pb in the sample
can be monitored by the evolution of the 204(Pb + Hg) signal intensity, but no common Pb correction was applied owing to the large
isobaric interference from Hg. The 235U signal is calculated from
238
U on the basis of the 238U/235U = 137.88. Single analyses consisted of 30 s of background integration with laser off followed
by 1 min integration with the laser firing and a 30 s delay to wash
out the previous sample (approximately 10 s for six orders of magnitude) and to prepare the next analysis. Data are corrected for U–
Pb and Th–Pb fractionation occurring during laser sampling and for
instrumental mass discrimination (mass bias) by standard bracketing with repeated measurements of GJ-1 zircon standard (Jackson
et al., 2004). Data reduction was carried out with the software
Ò
package GLITTER (developed by the Macquarie Research Ltd.),
(Van Achterbergh et al., 2001; Jackson et al., 2004). For each analysis, the time-resolved signal of single isotopes and isotopic ratios
was monitored and carefully inspected to verify the presence of
perturbations related to inclusions, fractures, mixing of different


Table 1
Analytical results of LA-ICPMS U-Pb dating.
Spot no.

Zircon

Concentration (ppm)
Th

U

Th/U

Raw ratios
207

Pb/206Pb

Apparent ages (Ma)
±1r

206

Pb/238U

±1r

207

Pb/235U

±1r

208

Pb/232Th

±1r

207

Pb/206Pb

±1r

206

Pb/238U

±1r

207

Pb/235U

±1r

208

Pb/232Th

±1r

Z6-C
Z6-R

Z5-R
Z4-R
Z4-C
Z1-C
Z7-R1
Z7-C
Z7-R2
Z8-R
Z8-C
Z9-R
Z9-C
Z10-R
Z10-C
Z11-R1
Z11-C
Z11-R2
Z14-C
Z14-R
Z21-R
Z20-R1
Z20C
Z20-R2
Z18-R1
Z18-C
Z18-R2

136
271
482
229

237
579
198
177
224
224
168
606
529
222
213
116
300
176
237
313
163
180
156
203
200
54
301

284
1605
800
412
272
744

478
209
483
833
199
3944
481
389
288
332
882
396
519
393
245
564
135
376
611
128
1915

0.48
0.17
0.60
0.55
0.87
0.78
0.42
0.84

0.46
0.27
0.85
0.15
1.10
0.57
0.74
0.35
0.34
0.44
0.46
0.79
0.67
0.32
1.15
0.54
0.33
0.43
0.16

0.07224
0.05215
0.05191
0.05146
0.05152
0.05249
0.05169
0.05257
0.05294
0.05141

0.05427
0.05124
0.07077
0.05244
0.05397
0.05089
0.17409
0.05190
0.15295
0.05127
0.05049
0.05300
0.09554
0.05129
0.05405
0.07031
0.05113

0.00106
0.00072
0.00075
0.00087
0.00096
0.00076
0.00091
0.00115
0.00086
0.00076
0.00119
0.00066

0.00095
0.00094
0.00105
0.00093
0.00213
0.00090
0.00210
0.00095
0.00104
0.00089
0.00169
0.00095
0.00086
0.00111
0.00069

0.13885
0.03976
0.03797
0.03968
0.0392
0.03928
0.03955
0.03854
0.03909
0.03977
0.03906
0.03917
0.14509
0.03962

0.03889
0.03873
0.36343
0.03922
0.24238
0.03951
0.03919
0.03856
0.14095
0.03886
0.03822
0.14200
0.03899

0.00173
0.00049
0.00047
0.0005
0.00049
0.00049
0.0005
0.0005
0.00049
0.00049
0.0005
0.00048
0.0018
0.0005
0.0005
0.00049

0.00449
0.0005
0.00306
0.0005
0.0005
0.00049
0.00183
0.0005
0.00048
0.0018
0.00049

1.38336
0.28594
0.2718
0.28163
0.27847
0.28432
0.28189
0.27943
0.28537
0.28192
0.29229
0.27673
1.41596
0.28649
0.28948
0.27185
8.72533
0.28070

5.11234
0.27937
0.27287
0.28185
1.85705
0.27483
0.28484
1.37692
0.27492

0.02214
0.00436
0.00432
0.00505
0.00544
0.00453
0.00527
0.00630
0.00499
0.00456
0.00660
0.00400
0.02116
0.00542
0.00591
0.00526
0.12220
0.00519
0.07782
0.00547

0.00584
0.00505
0.03463
0.00537
0.00490
0.02350
0.00417

0.04399
0.01219
0.01183
0.01219
0.01200
0.01234
0.01283
0.01191
0.01214
0.01215
0.01214
0.01227
0.04271
0.01204
0.01263
0.01198
0.09439
0.01253
0.08243
0.01248
0.01249
0.01202

0.06440
0.01192
0.01377
0.04123
0.01300

0.00094
0.00027
0.00025
0.00027
0.00026
0.00026
0.00030
0.00028
0.00028
0.00029
0.00029
0.00028
0.00095
0.00029
0.00031
0.00031
0.00217
0.00031
0.00205
0.00031
0.00033
0.00032
0.00163
0.00032

0.00036
0.00111
0.00034

992.8
291.8
281.4
261.6
264
306.9
271.6
310.4
326.1
259
381.9
251.4
950.7
304.6
369.7
235.9
2597.4
281.1
2379.1
253
217.6
328.8
1538.7
253.8
372.9
937.5

246.8

29.6
31.2
32.9
38.2
42.1
32.8
40.0
48.9
36.6
33.7
48.3
29.2
27.1
40.1
43.5
41.8
20.3
39.3
23.2
42.2
46.9
37.4
32.9
42.0
35.5
32.0
30.8

838.2
251.4
240.2
250.9
247.8
248.3
250
243.8
247.2
251.4
247
247.7
873.4
250.5
246
245
1998.4
248
1399.1
249.8
247.8
243.9
850
245.7
241.8
856
246.6

9.8
3.0

2.9
3.1
3.1
3.0
3.1
3.1
3.0
3.1
3.1
3.0
10.1
3.1
3.1
3.1
21.3
3.1
15.9
3.1
3.1
3.0
10.4
3.1
3.0
10.2
3.0

881.9
255.4
244.1
252

249.4
254.1
252.2
250.2
254.9
252.2
260.4
248.1
895.7
255.8
258.2
244.2
2309.7
251.2
1838.2
250.2
245
252.1
1065.9
246.5
254.5
879.1
246.6

9.4
3.4
3.5
4.0
4.3
3.6

4.2
5.0
4.0
3.6
5.2
3.2
8.9
4.3
4.7
4.2
12.8
4.1
12.9
4.4
4.7
4.0
12.3
4.3
3.9
10.0
3.3

870.1
244.9
237.8
244.9
241.2
247.9
257.7
239.2

244
244.1
243.8
246.4
845.4
241.8
253.7
240.7
1823.1
251.7
1601
250.7
250.9
241.6
1261.4
239.5
276.5
816.7
261.1

18.3
5.5
4.9
5.4
5.3
5.2
6.0
5.6
5.5
5.7

5.9
5.6
18.5
5.8
6.2
6.2
40.2
6.3
38.4
6.3
6.5
6.4
31.0
6.3
7.2
21.5
6.8

VT 19
16
17
18
19
20
21
22
23
24
27
28

29
30
31
32
33
34
35
38
39
40
41
42
43
44

Z9
Z10-R1
Z10-C
Z10-R2
Z11-R1
Z11-R2
Z12-C
Z1-R
Z1-C
Z3-R1
Z3-C
Z3-R2
Z4-R1
Z4-R2
Z7-C

Z8-R1
Z8-C
Z8-R2
Z14-R1
Z14-R2
Z15-R
Z15-C
Z6-R
Z6-C
Z1-R

600
397
799
638
250
773
995
967
457
1127
829
795
349
182
495
1435
1458
1257
736

837
631
510
1109
979
844

1031
621
888
811
1435
1155
1547
961
775
1645
1408
1379
1360
232
628
1632
1933
1364
1729
1563
728
716
1707

1488
1090

0.58
0.64
0.90
0.79
0.17
0.67
0.64
1.01
0.59
0.68
0.59
0.58
0.26
0.79
0.79
0.88
0.76
0.92
0.43
0.54
0.87
0.71
0.65
0.66
0.77

0.05664

0.05111
0.05169
0.05272
0.05241
0.05609
0.05199
0.05219
0.05139
0.05217
0.05286
0.05333
0.05472
0.05506
0.05280
0.05193
0.05256
0.05110
0.05179
0.05346
0.05269
0.05250
0.05180
0.05362
0.05149

0.00077
0.00078
0.00070
0.00071
0.00068

0.00077
0.00066
0.00068
0.00070
0.00069
0.00069
0.00070
0.00091
0.00199
0.00084
0.00067
0.00068
0.00065
0.00079
0.00142
0.00074
0.00076
0.00072
0.00091
0.00071

0.0366
0.03932
0.03848
0.03814
0.03797
0.03828
0.03841
0.03839
0.03851

0.03812
0.03718
0.03734
0.03784
0.03633
0.03859
0.03884
0.03783
0.03835
0.03768
0.03340
0.03797
0.04056
0.03890
0.03739
0.03945

0.00045
0.00048
0.00047
0.00046
0.00046
0.00047
0.00047
0.00047
0.00047
0.00046
0.00045
0.00046
0.00047

0.00052
0.00048
0.00047
0.00046
0.00047
0.00047
0.00045
0.00047
0.00050
0.00048
0.00047
0.00049

0.28586
0.27715
0.27433
0.2773
0.27444
0.29608
0.27538
0.27632
0.27296
0.27424
0.27104
0.27459
0.28552
0.27584
0.28100
0.27816
0.27421

0.27027
0.26912
0.24625
0.27585
0.29361
0.27787
0.27645
0.28017

0.00428
0.00452
0.00408
0.00412
0.00394
0.00445
0.00392
0.00400
0.00409
0.00401
0.00394
0.00398
0.00506
0.00991
0.00478
0.00398
0.00394
0.00387
0.00445
0.00659
0.00427

0.00466
0.00424
0.00496
0.00424

0.01261
0.01218
0.01176
0.01196
0.01415
0.01330
0.01182
0.01166
0.01184
0.01185
0.01205
0.01224
0.01321
0.01171
0.01222
0.01190
0.01194
0.01194
0.01237
0.01258
0.01152
0.01227
0.01220
0.01175
0.01213

0.00022
0.00021
0.00020
0.00020
0.00026
0.00023
0.00020
0.00020
0.00021
0.00021
0.00021
0.00022
0.00029
0.00032
0.00023
0.00021
0.00022
0.00022
0.00025
0.00031
0.00022
0.00024
0.00024
0.00024
0.00024

476.7
245.9
271.8

316.8
303.2
455.6
284.9
293.8
258.4
293
322.9
342.7
400.1
414.6
320.4
282.1
309.8
245.4
276.2
348.2
315.2
307.1
276.8
355
263

30.2
34.6
30.7
30.4
29.2
29.8
28.9

29.5
31.0
29.8
29.4
29.1
37.3
78.3
35.7
29.0
29.0
29.2
34.7
58.8
31.7
32.8
31.4
37.8
31.2

231.7
248.6
243.4
241.3
240.3
242.1
243
242.8
243.6
241.1
235.3

236.3
239.4
230
244.1
245.7
239.4
242.6
238.4
211.8
240.2
256.3
246
236.6
249.4

2.8
3.0
2.9
2.9
2.9
2.9
2.9
2.9
2.9
2.9
2.8
2.8
2.9
3.3
3.0

2.9
2.9
2.9
2.9
2.8
2.9
3.1
3.0
2.9
3.0

255.3
248.4
246.1
248.5
246.2
263.3
247
247.7
245.1
246.1
243.5
246.4
255
247.3
251.5
249.2
246.1
242.9
242

223.5
247.4
261.4
249
247.8
250.8

3.4
3.6
3.3
3.3
3.1
3.5
3.1
3.2
3.3
3.2
3.1
3.2
4.0
7.9
3.8
3.2
3.1
3.1
3.6
5.4
3.4
3.7
3.4

4.0
3.4

253.4
244.6
236.3
240.2
284
267
237.5
234.4
238
238.1
242.1
245.8
265.2
235.3
245.5
239.1
239.8
239.9
248.5
252.7
231.6
246.6
245.1
236.1
243.8

4.3

4.3
3.9
4.0
5.2
4.6
4.0
4.0
4.2
4.2
4.3
4.3
5.7
6.5
4.6
4.3
4.3
4.3
5.0
6.2
4.4
4.8
4.7
4.9
4.7

75

(continued on next page)

F. Roger et al. / Journal of Asian Earth Sciences 48 (2012) 72–82

VT 16
49
50
51
52
53
54
55
56
57
60
61
62
63
64
65
66
67
68
71
72
73
74
75
76
77
78
79

76

Table 1 (continued)
Spot no.

Zircon

Concentration (ppm)
Th

U

Th/U

Raw ratios
207

Pb/206Pb

Apparent ages (Ma)
±1r

206

Pb/238U

±1r

207

Pb/235U

±1r

208

Pb/232Th

±1r

207

Pb/206Pb

±1r

206

Pb/238U

±1r

207

Pb/235U

±1r

208

Pb/232Th

±1r

Z1-C
Z2

438
1225

629
1471

0.69
0.83

0.05152
0.05111

0.00097
0.00070

0.03921
0.03878

0.00050
0.00048

0.27856

0.27334

0.00548
0.00412

0.01226
0.01208

0.00026
0.00024

264.1
245.6

42.6
31.0

247.9
245.3

3.1
3.0

249.5
245.4

4.4
3.3

246.3

242.7

5.3
4.8

VT 235
5
6
10
11
12
13
42
43
44
46
48
49
50
53
54
57
58
59
60
61

Z15-R1
Z15-C
Z14-R

Z7-C
Z7-R
Z6-R
Z1-R1
Z1-C
Z1-R2
Z2-C
Z3-R1
Z3-C
Z3-R2
Z9-R1
Z9-C
Z19-C
Z19-R
Z12-R1
Z12-C
Z12-R2

178
201
854
208
318
86
304
355
1181
263
1160
483

339
234
1021
136
274
431
3200
247

584
982
414
841
1893
436
5914
1334
2718
2058
9121
1952
8243
4604
827
1847
2386
3081
1644
1736

0.305
0.205
2.062
0.247
0.168
0.198
0.051
0.266
0.435
0.128
0.127
0.248
0.041
0.051
1.235
0.074
0.115
0.140
1.946
0.142

0.05021
0.04835
0.04975
0.06707
0.04834
0.04979
0.0478
0.04707
0.04883

0.04899
0.04862
0.04758
0.04909
0.05305
0.05834
0.04766
0.04791
0.05063
0.05019
0.04793

0.00111
0.00084
0.0012
0.00079
0.00108
0.00126
0.00057
0.00079
0.00067
0.0007
0.00057
0.00081
0.0006
0.00072
0.00119
0.00074
0.0008
0.00076

0.00087
0.00083

0.01334
0.01327
0.01331
0.12819
0.01404
0.01339
0.01357
0.01372
0.01342
0.0137
0.01367
0.01337
0.01347
0.01387
0.01711
0.0136
0.01313
0.01303
0.01454
0.01326

0.00017
0.00016
0.00017
0.00154
0.00018
0.00017

0.00016
0.00017
0.00016
0.00016
0.00016
0.00016
0.00016
0.00017
0.00021
0.00016
0.00016
0.00016
0.00018
0.00016

0.09239
0.08847
0.09128
1.18561
0.09357
0.09191
0.08945
0.08904
0.09035
0.09256
0.09169
0.08774
0.0912
0.1015
0.13762

0.08941
0.08678
0.09097
0.10062
0.08766

0.0021
0.00162
0.00224
0.0158
0.00213
0.00236
0.00118
0.00157
0.00134
0.00141
0.00119
0.00156
0.00123
0.00149
0.00287
0.00147
0.00152
0.00146
0.00183
0.00159

0.0042
0.00428
0.00407

0.0376
0.00451
0.00474
0.00448
0.0045
0.00422
0.00509
0.00479
0.00456
0.00569
0.01307
0.00565
0.00507
0.00412
0.00461
0.00389
0.00447

0.0001
0.0001
0.00007
0.00063
0.00013
0.00016
0.00009
0.00009
0.00007
0.0001
0.00008
0.0001

0.00012
0.00028
0.00012
0.00014
0.00012
0.00012
0.00009
0.00013

204.5
116.5
183.2
839.8
116
185.1
88.5
52.5
139.7
147.6
129.5
77.6
152.3
330.9
541.9
81.4
93.7
224
203.8
94.8

50.7
40.6
55.2
24.3
51.7
57.7
29.0
39.0
32.0
33.0
27.1
40.7
28.2
30.4
44.6
37.2
40.1
34.3
39.7
41.5

85.5
85
85.2
777.5
89.9
85.7
86.9
87.8
85.9

87.7
87.6
85.6
86.3
88.8
109.3
87.1
84.1
83.4
93
84.9

1.1
1.0
1.1
8.8
1.1
1.1
1.0
1.1
1.0
1.0
1.0
1.0
1.0
1.1
1.4
1.1
1.0
1.0

1.1
1.0

89.7
86.1
88.7
793.9
90.8
89.3
87
86.6
87.8
89.9
89.1
85.4
88.6
98.2
130.9
87
84.5
88.4
97.3
85.3

2.0
1.5
2.1
7.3
2.0
2.2

1.1
1.5
1.3
1.3
1.1
1.5
1.1
1.4
2.6
1.4
1.4
1.4
1.7
1.5

84.8
86.3
82.1
746
90.9
95.5
90.3
90.8
85.1
102.5
96.6
91.9
114.7
262.5
113.8

102.2
83.1
93
78.4
90.1

2.1
2.0
1.4
12.3
2.7
3.1
1.7
1.8
1.4
2.1
1.7
2.0
2.4
5.6
2.4
2.9
2.4
2.4
1.8
2.6

R = rim and C = center.

75

78

Zr1

A

Zr20

74

61
49

Zr8

100 μm 55

47

Zr9

Zr2

B

46

C 60

79

Zr18

45

G
54

50

Zr6

100 μm

D

57

Zr7

50

49

56

Zr3

48

100 μm

age domains or common Pb. Calculated ratios were exported and
Concordia ages and diagrams were generated using the Isoplot/
Ex v. 2.49 software package by Ludwig (2001). The analytical data
are provided in Table 1 where errors are given at ±1r. In the text
and figures, all uncertainties in ages are given at the 95% confidence level (±2r). The discordant data were considered only if they
allowed possible discordia lines to be defined on the Concordia diagrams; otherwise they were not taken into account because of
doubtful interpretation. In laser-ablation ICPMS analyses several
factors that cannot be easily detected from the inspection of the
time-resolved signals might contribute to discordance (e.g. common Pb, mixing of different age domains, small cracks or
inclusions).

3.2. Results

44

43

Zr15

6

53

3.2.1. Description of zircons
The zircons crystals from the three samples (VT 19, VT16 and VT
235) dated by LA-ICPMS were mostly euhedral, transparent, colorless, stubby to elongate in shape, ranging in size between 100 and
300 lm. Before isotopic analyses, backscatter electron (BSE) and

cathodoluminescence (CL) images were acquired for all grains
using a scanning electron microscope (SEM) in order to check spot
positions with respect to the internal microstructures. BSE and CL
images show that most zircons had complex internal structures
(sector zoning and inherited core for VT 235) (Fig. 3). Well
preserved euhedral growth zones, with unperturbed oscillatory
zoning typical of magmatic growth (Hanchar and Miller, 1993)
were also present. No non-magmatic mechanism has been

76

100 μm

77

100 μm

E

42

100 μm

F
5

100 μm

Fig. 3. Cathodoluminescence images of dated zircons crystals from: (A–D) the Phia
Bioc granite (VT16) and (E–G) the Phia Oac granite (VT 235). Circles indicate the

analytical spots with a diameter of about 26 lm. The numbers in italics refer to
analytical data in Table 1.

F. Roger et al. / Journal of Asian Earth Sciences 48 (2012) 72–82

45
46


77

F. Roger et al. / Journal of Asian Earth Sciences 48 (2012) 72–82

demonstrated to produce oscillatory zoning in zircon (Hoskin,
2000). Non-magmatic zircon (i.e. metamorphic, recrystallized, or
hydrothermal) tends to have poorly-defined internal zoning with
sometimes a non-geometric patchy zoning (Pidgeon, 1992; Hanchar and Miller, 1993; Hoskin and Black, 2000). Igneous zircon
has typically higher Th/U ratios (>0.1) that usually do not overlap
with Th/U ratios of non-igneous zircon (Williams and Claesson,
1987; Vavra et al., 1996; Hoskin and Black, 2000; Hartmann and
Santos, 2004).

events (Neo-Proterozoic) in the region. 21 analyses (rims and
cores) (Table 1) form a concordant to sub-concordant cluster yielding an age of 248.5 ± 2 Ma (MSWD = 0.91) and a weighted average
207
Pb/206Pb age of 247 ± 1.5 Ma (MSWD = 1.03) (Fig. 4A). The Th/U
ratio of the concordant Permo-Triassic core (Th/U = 0.741–0.872) is
indistinguishable from the range observed in concordant PermoTriassic rims analyses (Th/U = 0.154–0.872). These values are typical of igneous zircons and the U–Th–Pb age of 248.5 ± 2 Ma is
interpreted as the emplacement age of the magmatic protolith.

3.2.2. Phia Bioc granite-Ba Be lake sample (VT 16)
A total of 27 analyses, carried out on 13 crystals are listed in Table 1 and plotted in Fig. 3. CL images show that most zircons have
complex internal structures (Fig. 3A–C). Six zircons show inherited
cores with U contents from 128 to 882 ppm and Th/U ratios from
0.340 to 1.155. Among these cores, three spots (no. 67, 71, 75) indicating inherited 207Pb/206Pb age components from 2.4 to 2.6 Ga and
1.6 Ga, were not retained in the discussion and the interpretation
(cf. III-1) (Figs. 3A and 4A). They probably correspond to a mixture
between an old core and the younger rim around 245–250 Ma
(spot no. 66, 68, 72; 74, 76) (Table 1). Rims and cores of three other
zircons (Zr: 6, 9 and 18) have been investigated and cores have a
sub-concordant position and yield 206Pb/238U age of 855 ± 44 Ma
(MSWD = 3.1), 207Pb/235U age of 886 ± 11 Ma (MSWD = 0.92) and
207
Pb/206Pb age 961 ± 33 Ma (MSWD = 0.92). Rims (spots no. 50,
62, 77, 79) have a concordant to sub-concordant position around
245–250 Ma and are characterized by higher U concentrations
ranging from 1605 to 3944 ppm, with lower Th/U from 0.154 to
0.169. All these features indicate the occurrence of old magmatic

3.2.3. Phia Bioc granite-Bac Can sample (VT 19)
A total of 33 spots were analyzed on 13 zircons from this sample. The U and Th contents of rims and cores are very similar, ranging from 621 to 1707 ppm and from 250 to 1458 ppm, respectively.
Rims and cores of zircons have Th/U ratios bracketed between
0.174 and 1 (Table 1), typical values of igneous zircon. All ellipses
form a concordant to sub-concordant cluster yielding an age of
245 ± 2 Ma (MSWD = 1.7) and the weighted average of 206Pb/238U
ages is 242 ± 2 Ma (MSWD = 1.7) (Table 1 and Fig. 4B), which is
interpreted to represent the emplacement age of the magmatic
protolith.

A

VT 19 : Phia Bioc granite - Bac Can

260

206

0.039

23 Spots

T = 245 ± 2 Ma
(MSWD = 1.7)

240

220

0.033
200

0.031
0.21

0.23

0.25

254
250

246
242
238
234
230
226

T = 242 ± 2 Ma

23 spots

MSWD = 1.7

207

0.27

Pb/235U

0.29

0.31

VT 235 : Phia Oac granite

VT 16 : Phia Bioc granite - BaBe

B

2200

206

0.14
71

78,49,63

0.039

75

220

4

6

0.23

0.25

8

256
252
248
244
240
236

232

0.016

0.27

100

300
90

0.014

0.04

80
0.012
(MSWD = 1.03)

100

Pb/235U

10

Fig. 4. Zircon U–Pb concordia diagrams from Phia Bioc granite. Both undeformed
granites are analyzed: VT 19 near Bac Can village (A) and VT 16 near the Ba Be lake
(B) (Fig. 2). Error ellipses and uncertainties in ages are ±2r.

54

12

12
43
46
48
57
4250
13
49
5
10
6 44
61
58

16 spots

T = 247.1 ± 1.3 Ma

207

120

T = 87.3 ± 1.2 Ma
110

(MSWD = 1.5)

206

200

130

0.020
0.018

21 spots
Pb/238U Age (Ma)

0.035

2

0.08

240

0.031
0.21

0

500

(MSWD = 0.91)

0.033

0.0

0.022
/235

T = 248.5 ± 1.8 Ma 260

0.037

0.1 600

11

700

78,49,63

207

Pb/238U

0.041

900

20 Spots

840

Pb U
0.13
1.15 1.25 1.35 1.45 1.55

206

0.2

0.12

920
T = 961±33 Ma
880

0.15

1800
1400

1000

960

67

206

0.3

27 spots

Pb/238U

Pb/238U

0.4

0.16

206

0.035

(206Pb/238U) Age (Ma)

0.037

0.010
0.07

0.09

54
60

53

(206Pb/238U) Age (Ma)

Pb/238U

0.041

Pb/238U

0.043

3.2.4. Phia Oac granite (VT 235)
CL images show growth zoning typically observed in magmatic
zircons (Fig. 3E–G). We performed a total of 20 spot analyses on
both cores and rims of 10 grains. Among these, 16 analyses
(Table 1) form a concordant to sub-concordant cluster yielding a
mean age of 87.3 ± 1.2 Ma (MSWD = 1.5) with a mean 206Pb/238U
age of 85.9 ± 0.7 Ma (MSWD = 1.7). The rims show highly variable
U and Th concentrations (U = 414–9121 ppm, Th = 86–3200 ppm),
whereas the core areas have more uniform U (982–2058 ppm)
and Th (201–483 ppm) concentrations. The Th/U ratios is ranging
from 0.127 to 2.062 except for three rims (spot 42, 50, 53) which
have lower Th/U ratios (0.041–0.051) as well as more homogeneous Th and U concentrations (Th = 234–339 ppm and
U = 4504–8243 ppm).
In zircon 7, the core (spot no. 11) has a sub-concordant position
and yields a 206Pb/238U age of 777.5 ± 18 Ma (±2r), and a 207Pb/235U
age of 794 ± 15 Ma. Rims are much younger at around 90 Ma
(Table 1, Fig. 5). The three analyses (spot 53, 54, 60) produced discordant ages, which probably correspond to a mixture between an
old core and a younger rim around 87 Ma (Fig. 3G). We interpret
the 87.3 ± 1.2 Ma age as reflecting the magma emplacement age
of the Phia Oac granite.

94
92

90
88
86
84
82
80

(12)

T = 86 ± 1 Ma

0.11

0.13

206

0.00
0.0

0.2

0.4

0.6

0.8

1.0

1.2

Pb/238U
1.4

Fig. 5. Zircon U–Pb concordia diagrams from Phia Oac granite (VT 235). Error
ellipses and uncertainties in ages are ±2r.


78

F. Roger et al. / Journal of Asian Earth Sciences 48 (2012) 72–82

4. Discussion
Among the zircons analyzed from Phia Bioc granite (VT 16) and
Phia Oac granite (VT 235), two inherited magmatic cores are in
concordant to sub-concordant position around 800–900 Ma. This
highlights the occurrence of Neo-proterozoic magmatic episodes
in NE Vietnam. A major Neo-proterozoic (1–0.75 Ga) event has already been recognized within the South China Block, correlated to
the amalgamation and then break-up of the Rodinia Continent (Li,
1999; Li et al., 2002, 2006; Zhou et al., 2002, 2006a,b, 2007; Roger
et al., 2010).
An outstanding fact is the lack of Paleozoic ages (around 460–
420 Ma) within the cores of the zircons, although a magmatic
intrusive event is known in northeastern Vietnam in the Song Chay
massif (428 ± 5 Ma) (Roger et al., 2000; Carter et al., 2001), at close
distance from these granites (Figs. 1 and 2). A similar range of
metamorphic and magmatic ages, related to the pre-Devonian
Early Paleozoic event are known in South China (Wang et al.,
2007b) as for example in the Xuefeng Shan belt where 420–

450 Ma ages are locally recorded. In the Yunkai massif a tectonothermal event occurred also during the Silurian: magmatic zircons
yield U–Pb ages ranging between 440 and 410 Ma and detrital zircons from paragneisses have been dated at 423 ± 7 Ma (Wang et al.,
2007c; Lin et al., 2008). This event is expressed in the South China
Block and in the autochtonous domain of North Vietnam (Dong
Van and Cao Bang areas) by the Devonian unconformity. In Vietnam (in the Indochina block), the Ordovician–Silurian major metamorphic and magmatic event has also been found in the Kontum
Block, where Ordovician ages (465–470 Ma) occur in granulites
and could represent a minimum age for a HT metamorphic episode
(Roger et al., 2007) (Fig. 1A). Silurian ages (420–440 Ma) have also
been found in the Dien Binh series which outcrop east of the Po Ko
fault zone, on the western edge of the Kontum massif (Nagy et al.,
2001; Vu Van Tich, 2004; Maluski et al., 2005; Roger et al., 2007)
(Fig. 1A). Silurian ages are also found in the Dailoc orthogneissic
massif in the central part of the Truong Song range (Carter et al.,
2001) (Fig. 1A).
Triassic ages are obtained on the Phia Bioc massif, where the
two samples VT19 and VT16, record an emplacement age of
245 ± 2 Ma, and 248.5 ± 2 Ma respectively for this granite intrusion
(Fig. 4A and B). In the studied area of NE Vietnam, the Ordovician
and Devonian shales and sandstones, which are part of the nappe
system, are intruded by the Phia Bioc granite. This granite is clearly
undeformed. These observations imply that the major tectonometamorphic event in the northeastern area of Vietnam, represented by nappes thrusting, cannot be younger than
248.5 ± 2 Ma. This result is in close agreement with the regional
unconformity of the Upper Triassic sediments on the Lower Paleozoic deformed metasedimentary rocks (Deprat, 1914; Fromaget,
1941b; Zhang, 1999; Liang and Li, 2005; Zhang et al., 2011). Furthermore, the Upper Triassic sandstones and conglomerates with
coal seams rest upon the undeformed Phia Bioc granite and the
Cho Chu granite (Fig. 2). To the SE of the South China block (Cathaysia block) (Fig. 2), equivalent Triassic ages have been found in
Early Mesozoic belts, formed in various geodynamic settings prior
the regional Upper Triassic unconformity but involving Lower Triassic strata (e.g., HNGBMR, 1988; Deng et al., 2004; Qiu et al., 2004;
Wang et al., 2005, 2007b; Li et al., 2007).
Several hypotheses can be invoked to explain this magmatism:

1. Several authors propose a common origin with the Emeishan
Large Igneous Province (ELIP) in the South China Block (Wang
et al., 2007a; Hoa et al., 2004, 2008; Polyakov et al., 2009; Shelepaev et al., 2010). The Phia Bioc massif intrusion is contemporaneous with the Nui Chua gabbronorite (U–Pb: 251 ± 3.4 Ma),
with the rhyolites exposed to the East of Cao Bang in the Suoi

Cun Massif (U–Pb: 248 ± 4.5 Ma) and Cho Don granite (Ar–Ar
(Bt): 250 ± 1 Ma) (Hoa et al., 2008) (Fig. 2). In the Luc Yen area,
the Tan Linh gabbrosyenite also yields similar ages of 247 and
243 Ma, according to Rb/Sr and Ar/Ar datings respectively
(Hoa et al., 2004) (Fig. 2). To the south of the Red River Fault,
the Kim Boi cordierite granite has been dated by U–Pb on zircon
at 242.4 ± 2.2 Ma (Hoa et al., 2008). For these authors (Hoa
et al., 2004, 2008), all these granitoids belong to the same magmatic complex namely the Phia Bioc complex. From these data,
the duration of this magmatism in NE Vietnam should be bracketed between around 255 and 240 Ma. This is a bimodal volcano-plutonic association with mafic–ultramafic rocks (Nui
Chua complex) and high alumina granites (Phia Bioc complex)
which intrude the nappe structure (Fig. 2). This bimodal magmatism is associated with the ELIP in the South China Block
and off which the Tule volcanic province is shifted by the left
lateral Red River Fault. It can be considered as a product of
the Permo-Triassic intra-plate magmatism of North Vietnam
(Wang et al., 2007a; Hoa et al., 2008; Polyakov et al., 2009;
Shelepaev et al., 2010). Nevertheless, the duration of the ELIP
in the South China is strongly debated: some authors (Ali
et al., 2002; Zheng et al., 2010; Shellnutt et al., in press) consider
that this magmatic event is extremely short in time between
260 and 257 Ma while other authors (Fan et al., 2008; Zhong
et al., 2007, 2009; Shellnutt and Zhou, 2008) propose a longer
duration around 10 Ma. Either this magmatism is continuous
up to around 250 Ma or the magmatism post-257 Ma would
be related to the collision between the Sibumasu and Indochina
Blocks (Shellnutt et al., 2011) during the closure of the Paleotethys ocean (Late Permian/Early Triassic) (Lepvrier et al., 2004).

2. The closure of the Paleotethys could be also the process responsible for the emplacement of Phia Bioc granite intrusive in the
allochtonous series. This ductile nappe tectonics is synchronous
with the tectonometamorphic and magmatic events already
described and dated along the Paleotethys sutures in western
Yunnan (Ailao Shan, and Lancangjiang) and to the Northern
Vietnam including the South of the Red River fault zone, in
the Song Ma shear zone and in the Truong Son Range, up to
the south in the Kontum Massif (Vu Van Tich, 2004; Maluski
et al., 2005; Lepvrier et al., 2008; Roger et al., 2007; Peng
et al., 2008; Fan et al., 2010; Wang et al., 2010) (Fig. 1A). More
recently Searle et al. (2010) obtained U/Pb ages between 243
and 239 Ma from metamorphic and magmatic rocks along the
Red River Fault zone, in Ailao Shan, Diancang Shan and Day
Nui Con Voi) (Figs. 1 and 2). In the Dulong – Song Chay Massif,
the Ar/Ar metamorphism ages have been found around 234–
237 Ma (Maluski et al., 2001; Yan et al., 2006). These younger
ages have to be understood as a cooling age linked to the
emplacement of the nappe.
3. Carter and Clift (2008) consider that in Vietnam the Indosinian
tectonics is linked to a continental accretion and tectonic reactivation event along an ancient suture (Song Ma Suture),
whereas in South China, Triassic thermotectonic events are
linked to the development of an active plate margin through
North to North-West directed subduction of the Paleo-Pacific
oceanic plate (Li et al., 2006, 2007). This interpretation is still
largely debated. The reactivation of the Song Ma suture would
be driven by the closure of the Paleotethys and the accretion
of the Sibumasu block to Indochina (Fig. 1) (Carter and Clift,
2008; Lepvrier et al., 2008). The recent discovery of eclogite
and high-pressure granulite facies metamorphism (dated at
243 ± 4 Ma) along the Song Ma suture zone in northern Vietnam

(Nakano et al., 2008, 2010) suggests that this structure corresponds to the suture zone between Indochina and South China
and that the collision occurred during the Early Triassic. Furthermore the peak P conditions estimated for the eclogite


F. Roger et al. / Journal of Asian Earth Sciences 48 (2012) 72–82

(>2.1–2.2 GPa) indicate that continental subduction occurred,
attesting of a strong continental collision event incompatible
with the reactivation model of Carter and Clift (2008). In the
same way, Zhou and Li (2000) and Zhang et al. (2011) suggest
that westward subduction of the Paleo-Pacific plate was probably initiated around mid-Jurassic times. According to He et al.
(2010), in the coastal Southeast China the Triassic tectonics corresponds to N–S compression resulting from the northward collision of the Indochina Block and South China Block. The
tectonic transition from the Tethys to the Panthalassa (PaleoPacific) orogenic regimes was carried out in the Early Jurassic.
A detailed geochemical study of the Phia Bioc massif, will lead
us to choose between an origin linked to Emeishan plume activity or the magmatism products during the closure of the Paleotethys and continental collision between Indochina and South
China Blocks.
The Cretaceous U–Pb age of 87.3 ± 1.2 Ma obtained on the Phia
Oac granite confirms and precises the earlier K–Ar ages of 85–
95 Ma obtained on biotite and muscovite (Izokh et al., 1964). Those
ages are coherent with field relations between the granite and the
lower Triassic sedimentary rocks of the Song Hien displaced unit.
Up to now Cretaceous ages (79–105 Ma) on magmatic material
in Northern Vietnam were known through old K–Ar on biotite
and hornblende measurements in the Phu Sa Phin granite (Phan
et al., 1991; Dovjikov, 1965) (Fig. 2). In the Tule Basin, Anh et al.,
2003 obtained Ar–Ar ages of 80–60 Ma on magmatic rocks. Cretaceous granites are not restricted to NE Vietnam but are already
known in the Yidun block (eastern Tibet), in the Sibumasu block
(Shan Thai Block) and South China Block (Fig 1): Within the Yidun
block, the emplacement of the Chola Shan (105 ± 2 Ma) and Haizi
(94 ± 2 Ma) granitoids suggests that a magmatic episode occurred

during the middle Cretaceous (Reid et al., 2005) (Fig. 1). Both intrusions are possibly related to extension associated with the northward subduction of the Tethys underneath Asia (Reid et al.,
2007). However, the plutons are close to major faults (the Ganzi
and Litang faults) and it is possible that the Cretaceous magmatic
activity was related to the fault activity (Roger et al., 2010,
2011). The origin of those two Cretaceous granites remains very
uncertain however it is generally accepted that Jurassic – Cretaceous tectonics did not modify the general Triassic architecture
of eastern Tibet (e.g. Burchfield et al., 1995; Roger et al., 2004,
2010, 2011; Harrowfield and Wilson, 2005; Reid et al., 2005; Wilson et al., 2006).
In Thailand, an orogenic episode occurred during the Cretaceous, between 90–70 Ma. This tectonic has been dated in southern Thailand (Watkinson et al., 2008) from granitic intrusions. To
the North, the core complexes of Doi Ithanon and the Chiang
Mai-Licang Belt record ages of 85–70 Ma for the metamorphism
(Dunning et al., 1995; Macdonald et al., 2010) (Fig. 1). Up to
now, no clear origin for this orogeny has been expressed. For Dunning et al. (1995), the collision between Western Burma and the
Shan Thai Block could be one of the driving mechanisms (Fig. 1).
Many more Cretaceous ages have been found on granitic intrusions in the western end of the South China Block (South-Eastern
Yunnan and West Guangxi provinces) (Fig. 1A). Cheng and Mao
(2010) recently presented an exhaustive description of these granites. This magmatism occurred between 98 and 77 Ma, with a peak
between 80 and 95 Ma. For Cheng and Mao (2010), the magmas
possibly derived from partial melting of Meso-Proterozoic continental crust driven by lithospheric extension and asthenospheric
upwelling of the western Cathaysia block in Late Cretaceous. On
the same way, Cretaceous granites (around 100 Ma) occur along
the S-E coast of the Cathaysia Block (Jiang et al., 2011; Wong
et al., 2009; He et al., 2010). These granites are probably linked
to the Paleo-Pacific subduction. They were intruded during the Late

79

‘‘Yanshanian’’ episode, and most of them correspond to A-type
granites (Zhou et al., 2006a,b). Moreover, high K-calc alkaline
granitoids have been observed in Southern Vietnam, in the Dalat

area and along the coast (Fig. 1). Their U–Pb ages are bracketed between 112 and 88 Ma (Tuy Thi Bich Nguyen et al., 2004). This Andean-type magmatism is interpreted as a result of the NW-directed
subduction of the western Paleo-Pacific plate under the SE Asian
Continental margin (Taylor and Hayes, 1983). This offset of at least
500 km induced by the left-lateral Red River Fault of the Andeantype magmatism occurred along the S-E coast of the South China
block and the Dalat area (S Vietnam). Such an offset confirms the
model of Tapponnier et al. (1990) and Leloup et al. (1993, 1995,
2001, 2007). The value of this offset is debated. Recent studies
based on tectonic reconstructions also reported limited displacement along the Red River Fault (Wang et al., 1998; Searle, 2006,
2007; Searle et al., 2010). Whereas the granite of the Phia Oac represents the unique Cretaceous granite in NE Vietnam, it should be
logical to link this magmatic body with the magmatism associated
with the Paleo-Pacific subduction. A detailed geochemical study
would be necessary to confirm this hypothesis.
No Tertiary age has been found in the zircons of the Phia Bioc
complex and Phia Oac massif in spite of their vicinity from the
Red River Fault Zone. Metamorphic rocks have been exhumed during the Cenozoic along the Red River Fault but are not restricted to
the shear zone. In fact, high-grade marble around Sapa to the SW of
the fault zone and around Luc Yen to the NE of the fault zone (Garnier et al., 2002), the Fan Si Pan granitic massif (Searle, 2006, 2007)
and the Song Chay dome (Roger et al., 2000; Carter et al., 2001) also
show Tertiary regional metamorphism outside of the Red River
Shear zone (Figs. 1 and 2). However, the Tertiary metamorphic
and tectonic overprint was not strong enough to cause the (re)crystallization of zircon in these complexes. Once more these results
confirm that in Vietnam, the Cenozoic thermotectonic event is
essentially localized along the Red River Fault and the Bu Khang
metamorphic core complex and has no or only a limited influence
on the isotopic resetting in the high and middle temperature systems (U/Pb and Ar/Ar) outside these two areas (Figs. 1 and 2).
The building of Indochina is consequently mainly related to the
Triassic Indosinian orogeny.
5. Conclusion
Our new U/Pb ages demonstrate that in NE Vietnam, as in the
entire Truong Son Belt, an overprinting of several tectonometamorphic events is responsible for the current structure.

1. Ages of inherited cores in zircons attest for the involvement of
an old Neo-Proterozoic lithospheric crust related to the basement of the South China Block. Although a magmatic event is
known between 460 and 420 Ma, no such ages have been
obtained in zircons from these granites.
2. U–Pb dating of the undeformed, post nappes Phia Bioc granite
at 248–245 Ma implies that the major nappes tectonics in NE
Vietnam is older than 248.5 ± 2 Ma. This nappe tectonics is
older or synchronous of the Indosinian strike-slip tectonics as
observed in all the Truong Son range. This magmatism could
be linked to intra-plate activity of the Emeishan LIP or to the
closure of the Paleotethys during the Indosinian orogeny.
3. The Cretaceous age (87.3 ± 1.2 Ma) of the Phia Oac granite could
likely be related to the Cretaceous magmatism occurring at the
same time in South China, probably driven by the lithospheric
extension affecting the Cathaysia block as a consequence of
the late episode of the Paleo-Pacific plate subduction under
the continental margin of the SE Asia.
4. Zircons rims do not show any Cenozoic magmatic or metamorphic overprint. Although the Cenozoic Red River Fault system is


80

F. Roger et al. / Journal of Asian Earth Sciences 48 (2012) 72–82

close to these two plutons the Cenozoic tectonics has not been
strong enough to disturb the U/Pb system.
5. These data confirm that the structure of Indochina is mainly
related to the Indosinian tectonics.

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