DSpace at VNU: Ar-Ar and fission-track ages in the Song Chay Massif: Early Triassic and Cenozoic tectonics in northern Vietnam

Ar±Ar and ®ssion-track ages in the Song Chay Massif: Early Triassic andCenozoic tectonics in northern Vietnam H.. Avigadd a ISTEM-CNRS, UniversiteÂ Montpellier 2, Place EugeÁne Bataillon

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Ar±Ar and ®ssion-track ages in the Song Chay Massif: Early Triassic and

Cenozoic tectonics in northern Vietnam

H Maluskia,*, C Lepvrierb, L Jolivetb, A Carterc, D Roquesc, O Beyssacd, Ta Trong Tange,

Nguyen Duc Thangf, D Avigadd

a ISTEM-CNRS, UniversiteÂ Montpellier 2, Place EugeÁne Bataillon, 34095, Montpellier, France

b Laboratoire de Tectonique, UniversiteÂ 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 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 the40Ar±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

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

penin-sula is classically considered as a rigid block but recent

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, Lep-vrier et al., 1997; Lacassin et al., 1998) are more obscure, but this may be due to the current paucity of geochronolo-gical 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 meta-morphic 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)

PII: S1367-9120(00)00038-9

www.elsevier.nl/locate/jseaes

* Corresponding author Tel.: 133-0467545926; fax: 133-0467547362.

E-mail address: maluski@dstu.univ-montp2.fr (H Maluski).

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appears similar to the Bu Khang dome, and therefore, may

have had a similar history To understand the temporal

rela-tionship 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 direc-tion and is bounded on its western ¯ank by the Song Chay Fault and on its southern ¯ank by an E±W trending mylo-nite zone, which on geological maps appears to be

H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 234

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.

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terminated by the Song Chay Fault The eastern and

south-eastern 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 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 40 Ar± 39 Ar ages and ®ssion track ages.

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trending NE-SW and is steeper in the southern rim

Hori-zontally 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

K-feldspars 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

mica-schists (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

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).

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fabrics where the foliation is least visible The most

common shear criteria are S±C relations, asymmetric

pres-sure 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

Viet-nam 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

evolu-tion 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

emplace-ment 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

40Ar±39Ar age spectrum also suggested a phase of rapid

cooling in the late Jurassic

4.1 40Ar±39Ar results

The radiometric40Ar±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

mica-grains In these conditions the grain-size effect, as

mentioned in McDougall and Harrison (1988), is

mini-mised, concerning dimension controlling gas loss in diffu-sive loss conditions

Sample VN 322 (Fig 4a) is located in a subvertical shear zone which bounds the dome to the south, (228 2405200; 1048

4205500) 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 tempera-tures 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

39Ar/40Ar ratios, is also re¯ected in the isochron diagram normalised to40Ar, 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 (228 290 4100; 1048 510 4300) 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 released39Ar, 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 3202600; 1048 4902400) 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

36Ar/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 4403900; 1048 3800200) 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 well-de®ned plateau age at 166 ^ 2 Ma for near 95% of the39Ar released The closure of the mineral vs Ar occurred at that time, without subsequent reopening of the system An iden-tical age of 166 ^ 2 Ma is obtained through the isochron diagram, with an intercept on the Y-axis de®ning an atmos-pheric 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 Musco-vites and biotites give, respectively, 164 ^ 2 Ma and 176 ^

2 Ma: For muscovite, the plateau age is calculated over 60%

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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 radiogenic40Ar 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

0

50

100

150

200

250

300

%39Ar cumulative

VN 322 MUSCOVITE

S SONG CHAY

234±0.8 Ma

204±1 Ma

60 Ma

0 50 100 150 200 250 300

%39Ar cumulative

SONG CHAY

236±0.5 Ma 228±1 Ma

0

50

100

150

200

250

300

%39Ar cumulative

VN329 BIOTITE

SONG CHAY

0 50 100 150 200 250 300

%39Ar cumulative

VN333 BIOTITE

SONG CHAY

0

50

100

150

200

250

300

%39Ar cumulative

VN335 BIOTITE

SONG CHAY

0 50 100 150 200

%39Ar cumulative

< 164±2 Ma >

VN 335 MUSCOVITE SONG CHAY

167±2Ma

0 50 100 150 200 250 300

%39Ar cumulative

SONG CHAY 198±2 Ma

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

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(values differ slightly according to different authors;

Harri-son et al., 1985; McDougall and HarriHarri-son, 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

well-de®ned isochron, especially for the Y intercept value,

connected with the40Ar/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 musco-vite layers, clearly visible under the microscope Musco-vites 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

40Ar/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

204 ^ 1

60 ^ 5

0 20 40 60 80 100

%39Ar cumulative

VN 107 BIOTITE

0 10 20 30 40 50

%39Ar cumulative

0 10 20 30 40 50

%39Ar cumulative

< 24.1±1 Ma >

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

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Table 2

Ar isotopic results for analysed minerals Correction interference used for 36 Ar/ 37 Ar Ca is 2:93 £ 10 24 : Mass discrimination correction factor is calculated for a

40 Ar/ 36 Ar ratio of 291

Temperature (8C) 40 Ar p / 39 Ar 36 Ar/ 40 Ar 37 Ar/ 39 Ar % Atm % 39 Ar Age ^ 1sd

VN322 MUSCOVITE (J 0.018342)

Total age 208.6 ^ 2.1 VN324 MUSCOVITE (J 0.018342)

Total age 230.0 ^ 2.2 VN329 BIOTITE (J 0.018342)

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Table 2 (continued)

Total age 174.7 ^ 1.80 VN335 MUSCOVITE(J 0.018342)

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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

seri-cites 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 loca-tions 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 dosi-meter Counting and track length measurements used a micro-scope total magni®cation of 1250 £ with a 100 £ dry objective Central ages were calculated using the IUGS-recommended zeta calibration approach (Hurford, 1990)

Table 2 (continued)

Total age 40.3 ^ 1 VN110 MUSCOVITE(J 0.012158)

Total age 23.9 ^ 0.9

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