Directory UMM :Data Elmu:jurnal:J-a:Journal of Asian Earth Science:Vol19.Issue1-2.Feb2001:

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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, 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 the40_Ar±39_{Ar 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 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). It is a large domal structure which on ®rst examination

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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

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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

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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

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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

40

Ar±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 160mm 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, (2282405200; 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

39

Ar/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 (228290 4100; 1048510 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 (2283202600; 10484902400). 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 (2284403900; 10483800200). 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 theY-axis de®ning an atmos-pheric 40Ar/36Ar ratio.

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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

%39Ar cumulative VN 322 MUSCOVITE

%39Ar cumulative VN329 BIOTITE

%39Ar cumulative < _{164±2 Ma} >

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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

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) Step age (Ma) Total age (Ma)

VN322 MUSCOVITE 234_^0.8 208_^2

204^1 60^5

VN324 MUSCOVITE 236^0.5 228^1 230^2

VN329 BIOTITE 201^2 200^2 200^2

VN333 BIOTITE 166^2 166^2 165^1.7

VN335 MUSCOVITE 164^2 160^3 167^2 163^1.7

VN335 BIOTITE 176^2 176^2 174^2

VN337 MUSCOVITE 198^2 195^2 194^2

0 20 40 60 80 100

AGE

(Ma)

%39Ar cumulative VN 107 BIOTITE < 40±1 Ma >

0 10 20 30 40 50

0 20 40 60 80 100

AGE

(Ma)

%39Ar cumulative VN 106 MUSCOVITE < 33.1±0.8 Ma >

0 10 20 30 40 50

0 50 100

AGE

(Ma)

%39Ar cumulative

VN 110 MUSCOVITE < 24.1±1 Ma >

a

b

c

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Table 2

Ar isotopic results for analysed minerals. Correction interference used for36_Ar/37_Ar

Cais 2:93£10 24

:Mass discrimination correction factor is calculated for a

40_Ar/36_{Ar ratio of 291}

Temperature (8C) 40Arp_/39_Ar 36_Ar/40_Ar 37_Ar/39_Ar _{% Atm.} _%39_Ar _Age

^1sd

VN322 MUSCOVITE (J0.018342)

500 1.931 1.18 0.015 34.8 0.6 62.79^20.36

550 2.336 0.295 0.019 8.7 1.2 75.71^19.90

600 3.228 0.195 0.008 5.7 2.4 103.77^14.30

650 4.057 0.084 0.006 2.4 4 129.49^8.17

700 4.84 0.096 0.006 2.8 7.2 153.45^18.32

750 5.586 0.101 0.005 3 12.8 175.99^2.63

800 6.739 0.092 0.002 2.7 26.8 210.28^.98

850 6.552 0.156 0.002 4.6 41.4 204.77^.98

900 6.32 0.074 0.004 2.2 50.2 197.88^1.51

950 6.722 0.072 0.004 2.1 57.7 209.78^1.97

1000 7.129 0.067 0.006 1.9 65.5 221.72^1.53

1050 7.386 0.074 0.004 2.1 83.5 229.23^.78

1100 7.551 0.064 0.004 1.9 98.5 234.02^.79

1150 7.276 0.316 0.037 9.3 99.9 226.01_^8.76

Total age208.6_^2.1

500 2.268 2.062 0.021 60.9 0.3 73.54 ^45.90

550 4.507 1.007 0.015 29.7 0.6 143.3^47.04

600 6.189 0.067 0.008 2 1.1 193.99^ 26.68

650 6.234 0.142 0.005 4.1 2.1 195.32^14.45

700 6.742 0.210 0.005 6.2 4.2 210.35^6.41

750 7.008 0.144 0.003 4.2 8.4 218.17^3.17

800 7.383 0.111 0.002 3.3 22.6 229.13^1.02

900 7.334 0.083 0.001 2.4 34.9 227.72^1.39

950 7.400 0.090 0.001 2.6 47.5 229.63^1.30

1000 7.505 0.070 0.001 2 59 232.68^1.40

1050 7.619 0.056 0.001 1.6 92.4 236.00^.54

1100 7.598 0.116 0.003 3.4 97.9 235.38^2.54

1150 7.581 0.160 0.012 4.7 99.9 234.90^8.28

Total age230.0^2.2

VN329 BIOTITE (J0.018342)

500 3.023 1.057 0.058 31.2 1 97.36_^21.27

550 5.826 0.304 0.007 9 3.7 183.17^8.28

600 6.248 0.114 0.004 3.3 11 195.76^3.42

650 6.451 0.065 0.001 1.9 31.1 201.76^1.16

700 6.479 0.052 0.002 1.5 48.7 202.6^1.29

750 6.445 0.069 0.005 2 56.7 201.58^2.69

800 6.395 0.055 0.013 1.6 60.2 200.11^.55

850 6.501 0.307 0.015 9 64.1 203.23^6.63

900 6.394 0.102 0.011 3 70.8 200.08^.90

995 6.421 0.090 0.004 2.6 84 200.88^1.70

1050 6.560 0.074 0.002 2.1 90.7 205^3.41

1100 6.570 0.086 0.004 2.5 98.3 205.29^3

1150 6.748 0.695 0.059 20.5 99.9 210.55^14.25

Total age200.3^2.0

550 4.466 0.564 0.025 16.6 2.1 142.04_^7.18

600 5.221 0.088 0.007 2.6 7.2 164.99_^2.78

650 5.24 0.128 0.002 3.8 19.5 165.57_^1.14

700 5.263 0.049 0.002 1.4 38.8 166.26^.81

750 5.261 0.099 0.003 2.9 52.2 166.22^1.15

800 5.237 0.104 0.014 3.0 56.6 165.48^3.18

850 5.242 0.164 0.039 4.8 60.8 165.64^3.26

900 5.288 0.130 0.018 3.8 68.1 167.03^1.92

950 5.256 0.090 0.011 2.6 78.7 166.06^1.39

995 5.288 0.044 0.01 1.3 86.7 167.03^1.70

1050 5.242 0.120 0.02 3.5 94.6 165.63^1.81

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Table 2 (continued)

^1sd

1150 4.792 1.436 0.031 42.4 99.9 152.01^20.29

Total age165.4^1.70

450 2.426 1.348 0.037 39.8 0.1 78.56_^92.24

500 0.824 1.146 0.000 33.8 0.3 27.06^139.99

550 3.352 0.666 0.031 19.6 0.5 107.67^76.70

600 4.336 0.569 0.022 16.8 0.9 138.06^43.06

650 5.121 0.040 0.014 1.2 1.7 161.98^19.55

700 5.225 0.061 0.006 1.8 3.1 165.12^11.87

750 5.320 0.228 0.007 6.7 6.5 167.99^5.44

800 5.361 0.142 0.006 4.2 11 169.22^3.80

850 5.496 0.177 0.003 5.2 22.2 173.29^1.64

950 5.565 0.071 0.002 2.1 38.6 175.36^1.14

995 5.594 0.067 0.002 1.9 49.7 176.23^1.41

1050 5.593 0.054 0.001 1.6 90.7 176.20^.42

1100 5.719 0.078 0.004 2.3 96.9 179.99^2.92

1150 5.886 0.192 0.023 5.6 100 184.98^5.33

Total age174.7^1.80

VN335 MUSCOVITE(J0.018342)

450 4.081 1.944 0.042 57.4 0.3 130.23_^40.60

500 2.981 1.076 0.020 31.8 1.2 96.06_^17

550 4.521 0.360 0.005 10.6 3.1 143.75_^8.36

600 5.076 0.148 0.001 4.3 7.9 160.6^2.83

650 5.162 0.072 0.001 2.1 19.9 163.22^1.36

700 5.211 0.048 0.000 1.4 41.5 164.68^.79

750 5.217 0.050 0.001 1.4 56.2 164.89^1.20

800 5.149 0.086 0.004 2.5 60.8 162.81^.85

850 5.171 0.056 0.007 1.6 64.4 163.5^5.20

900 5.205 0.079 0.004 2.3 73.8 164.5^2.01

950 5.137 0.069 0.004 2.0 75.0 162.45^3.30

1000 5.341 0.073 0.002 2.1 87.8 168.62^1.15

1050 5.281 0.040 0.011 1.1 93.2 166.81^3.25

1100 5.279 0.029 0.003 0.8 98.0 166.76^3.03

1150 5.083 0.081 0.009 2.4 99.0 160.82^3.89

1200 4.879 0.030 0.008 0.9 99.9 154.65^14.13

Total age163.6^1.7

500 0.957 2.652 2.188 78.3 0.2 31.41_^89.35

600 3.692 0.922 7.948 27.2 0.7 118.21_^42.14

700 5.551 0.109 1.845 3.2 4.1 174.94^5.53

750 5.662 0.091 0.011 2.7 9.6 178.26^3.88

800 5.988 0.064 0.008 1.8 16.1 188.01^2.80

850 6.284 0.057 0.005 1.6 25.4 196.81^2.00

900 6.395 0.082 0.007 2.4 31.7 200.11^3.02

950 6.316 0.079 0.007 2.3 39.8 197.77^2.45

995 6.264 0.062 0.003 1.8 51.8 196.22^1.62

1050 6.310 0.064 0.003 1.9 68.3 197.60^1.37

1100 6.308 0.072 0.002 2.1 92.7 197.54^1.00

1150 6.364 0.152 0.004 4.5 99.9 199.19^2.57

Total age194.3^2.0

450 0.711 3.100 0 91.6 0.3 15.54^40.08

500 1.311 1.962 0 58 0.5 28.53^49.21

550 2.800 1.765 0.007 52.1 0.7 60.41_^59.51

600 1.304 1.339 0 39.5 1.6 28.38_^13.52

650 1.544 1.295 0 38.2 2.4 33.56_^15.54

700 1.390 0.533 0 15.7 4.7 30.23_^4.69

750 1.528 0.063 0 1.8 9.6 33.22^2.45

800 1.492 0.159 0 4.7 15.2 32.45^1.86

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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 % of39Ar.

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)

^1sd

900 1.469 0.272 0 8 41.9 31.95^.57

950 1.520 0.149 0 4.4 57.1 33.03^.77

1000 1.522 0.157 0 4.6 68.8 33.10_^.90

1100 1.546 0.056 0 1.6 86 33.61_^.72

1400 1.543 1.287 0.001 38 100 33.53_^1.18

Total age32.9_^0.8

450 3.174 3.324 0.038 98.2 0.2 68.31^82.93

500 0.768 3.341 0.036 98.7 0.5 16.78^51.80

550 1.548 3.258 0.034 96.2 1.9 33.66^7.83

600 1.823 2.996 0.007 88.5 7.9 39.56^1.81

650 1.851 1.808 0.003 53.4 24.4 40.15^.59

700 1.839 0.483 0.002 14.2 54.3 39.90^.39

750 1.839 0.171 0.002 5.0 79.8 39.91^.43

800 1.848 0.219 0.003 6.4 85.3 40.10^1.90

850 1.831 0.409 0.040 12.1 88.2 39.72^3.79

900 1.936 0.508 0.296 15.0 90.9 41.97^4.88

950 1.754 0.557 0.090 16.4 94.6 38.07^3.63

1000 1.708 0.344 0.043 10.1 96.4 37.08^5.41

1100 2.260 0.557 0.093 16.4 99.3 48.91_^4.07

1400 3.748 2.532 20.203 74.8 100 80.39_^16.67

Total age40.3_^1

VN110 MUSCOVITE(J0.012158)

450 0.374 3.023 0.014 89.3 1.3 8.20^11.89

500 0.115 3.541 0.014 100 2.5 2.52^13.77

550 1.014 1.769 0.015 52.3 4.2 22.12^11.35

600 1.437 0.307 0.007 9.0 6.4 31.26^8.43

650 1.235 0.418 0.007 12.3 10.3 26.90^4.96

700 1.143 0.327 0.008 9.3 16.4 24.90^3.07

750 1.077 0.435 0.006 12.8 24.4 23.48^2.80

800 1.117 0.328 0.006 9.7 34.5 24.34^1.67

850 1.104 0.317 0.005 9.3 46.7 24.05^1.49

900 1.121 0.323 0.004 9.5 56.9 24.44^1.74

950 1.102 0.335 0.002 9.8 63.9 24.03^2.50

1000 1.008 0.636 0.001 18.8 70.1 21.97^2.63

1100 1.291 1.329 0.002 39.2 79.7 28.10^2.28

1400 1.061 2.404 0.008 71.0 100 23.13_^1.18

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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 densi-ties. 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 reason-able 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 13:60^0:31mm and 14:12^

0:15mm: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

unresol-vable (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

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 uncer-tainties), 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. Never-theless 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 13:72^0:27mm to 14:31^0:14mm;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^

0:14mm; 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 14mm 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 evolu-tion. Due to the different closure temperature for Ar and FissionTtrack systems (350±4008C vs. 608C for exhuma-tional 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

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H.

Maluski

et

al.

/

Journal

of

Asian

Earth

Sciences

19

(2001)

233

±

248

Table 3

Fission track apatite analytical data for the Song Chay Massif

Notes: (i) Track densities are (£106tr cm22) numbers of tracks counted (N) shown in brackets; (ii) Analyses by external detector method using 0.5 for the 4p/2pgeometry correction factor; (iii) Ages calculated using dosimeter glass CN-5; analyst CarterzCN5339^5;(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) Tracks measured

Song Chay Massif

VN9801 22.29.74 104.51.74 115 20 1.481 4159 0.135 218 1.496 2418 15.4 23^2 13.93^0.14 1.31 86

VN9802 22.29.77 104.51.49 150 20 1.482 4159 0.365 664 4.106 7473 13.3 22^1 13.51^0.31 0.99 11

VN9803 22.31.68 104.50.32 410 20 1.484 4159 0.053 94 0.585 1039 0.04 23^2 No data

VN9804 22.32.04 104.50.10 580 20 1.485 4159 0.098 206 1.139 2399 3.8 22^2 13.60^0.31 1.70 31

VN9805 22.32.50 104.49.34 750 20 1.487 4159 0.121 163 1.246 1682 0.33 24^2 13.14^0.18 1.63 81

VN9807 22.32.94 104.48.77 800 20 1.490 4159 0.515 116 0.671 1511 19.7 20^2 14.01^0.16 1.43 83

VN9810 22.41.80 104.29.38 391 16 1.490 4159 0.034 44 0.543 709 20.0 16^3 13.39^0.39 1.66 19

VN9811 22.43.05 104.32.07 365 18 1.492 4159 0.435 329 5.704 4309 22.4 19_^2 13.41_^0.15 1.42 90

VN9812 22.44.75 104.35.68 395 20 1.493 4159 0.094 111 1.262 1496 0.75 19_^2 14.12_^0.15 1.36 79

VN9813 22.44.89 104.37.29 470 20 1.495 4159 0.221 442 3.045 6086 0.8 18_^1 13.81_^0.25 1.35 30

VN9814 22.43.97 104.42.20 480 17 1.497 4159 0.119 191 1.620 2585 17.9 19_^2 13.65_^0.19 1.42 59

VN9815 22.35.01 104.46.89 720 20 1.498 4159 0.057 121 0.896 1913 22.9 17^2 No data

Lao Cai to Sapa

VN9818 22.26.29 103.55.50 595 20 1.586 8792 0.052 89 0.517 897 10.1 27^3 13.72^0.27 2.38 80

VN9819 22.25.64 103.55.01 730 20 1.263 7004 0.086 131 0.492 749 29.1 37^2 13.75^0.27 1.98 54

VN9820 22.24.40 103.54.06 900 20 1.586 8792 0.082 110 0.781 1045 21.7 30^4 13.77^0.18 1.48 69

VN9821 22.22.21 103.52.12 1285 20 1.586 8792 0.131 127 1.275 1241 24.9 29^3 14.31^0.14 0.98 53

West of Sapa

VN9824 22.21.24 103.45.92 2200 11 1.263 7004 0.175 70 1.245 497 25.6 32^5 No data

VN9825 22.21.30 103.46.47 2000 21 1.263 7004 0.098 80 0.691 561 0 31^4 13.73^0.45 1.49 12

VN9826 22.21.68 103.45.86 1905 16 1.263 7004 0.015 94 1.013 639 11.3 32^4 14.98^0.21 0.83 17

VN9827 22.21.59 103.45.26 1670 20 1.586 8792 0.125 192 1.150 1772 28.2 30^3 14.18^0.14 1.38 103

Bao Yen

VN9846 22.11.94 104.23.52 320 14 1.263 7004 0.335 176 3.259 1712 24.5 22_^2 14.24_^0.16 1.14 51

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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 there-fore, we consider 234±236 Ma to record Triassic tectono-metamorphism.

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

28 Ma 107 ¡C Obs. Mean length : 13.92 Pred. Mean length: 13.83 Obs. S.D. : 1.31 Pred. S.D. : 1.33 Oldest track (Ma) : 25

VN 9801 Obs. Mean length : 13.56 Pred. Mean length: 13.57 Obs. S.D. : 1.49 Pred. S.D. : 1.33 Oldest track (Ma) : 21

VN 9811 Obs. Mean length : 13.64 Pred. Mean length: 13.54 Obs. S.D. : 1.42 Pred. S.D. : 1.43 Oldest track (Ma) : 21

VN 9814

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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

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 Indosi-nian 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 young-est 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 diffu-sion 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 evolu-tion of the southern part of the Song Chay Massif. The cordierite±sillimanite bearing schist constitutes the south-ernmost 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 308to N 240. The trend of the age spec-trum 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 deformation-metamorphism 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

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 9801 23 25 10 3.3

VN 9805 24 28 17 4.5

VN 9807 20 22 17 10

VN 9811 19 21 10 4.5

VN 9812 19 20 11 5

VN 9814 19 21 11 5

0 200 400 600 800 1000

Elevation

(m)

22.25 22.3 22.35 22.4 22.45 22.5 22.55

Central Age (Ma)

10 15 20 25 30

104.2 104.3 104.4 104.5 104.6 104.7

East

West North

South

Latitude

Longitude

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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 exhuma-tion 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 13:72^0:27mm and

14:31^0:14mm 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 tempera-tures .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 ages30_^ 4 and 32^5 Mameasured on undeformed granites adja-cent 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

with exhumation from signi®cant crustal depths. In contrast the data from the Song Chay Massif show Cenozoic exhuma-tion was limited, preserving an earlier Indosinian thermotec-tonic 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 loca-lised 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 meta-morphism and shearing, observed along a NW±SE cross-section. 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 Univer-sity of Vietnam, Hanoi. Funding for DR and ®ssion-track analysis was provided by the University of London South-east Asia Research Group.

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