Journal of Asian Earth Sciences 19 (2001) 233±248 www.elsevier.nl/locate/jseaes

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

aISTEM-CNRS, Universite Montpellier 2, Place EugeÁne Bataillon, 34095, Montpellier, France bLaboratoire de Tectonique, Universite Pierre et Marie Curie, 4 Place Jussieu, case 129, 75252 Paris cedex 05, France cLondon Fission Track Research Group, Department of Earth Sciences, Birkbeck and University College, Gower Street, London, WC1E 6BT, United Kingdom dLaboratoire de GeÂologie, Ecole Normale SupeÂrieure, 24 rue Lhomond, 75231 Paris cedex 05, France eNational University of Vietnam, Hanoi, 334 Nguyen Trai Str., Thanh Xuan, Hanoi, Viet Nam fGeological 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 the 40Ar± 39Ar method using puri®ed mica separates. Across the dome ages range from 236 Ma at the southern edge to 160 Ma in the core, attesting to a strong imprint in the Early Triassic time. A clear difference is seen between these Mesozoic ages and the Eocene to Miocene ages (from 40 to 24 Ma) that obtained in the nearby Red River Shear Zone using the same method. These data show that the Song Chay Massif was already high in the crust when the high temperature deformation of the Red River Shear Zone took place. The ®nal exhumation of the Song Chay orthogneiss constrained by ®ssion-track analysis on samples along the same transect occurred during the Early Miocene and could be interpreted as the consequence of a ®rst normal sense of motion along the fault which bounds the massif to the south. Timing is similar to that of exhumation in the Red River Shear zone. q 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Ar±Ar method; Fission-track ages; Song Chay Massif; Vietnam

1. Introduction studies (Jolivet et al., 1999) south of the Red River Shear Zone have identi®ed a large metamorphic core complex (the The Indochina peninsula, particularly northern Vietnam, Bu Khang Dome) and also evidence for extension during the is in a key-position for understanding the geodynamic Early Miocene. A number of structures in Vietnam are evolution of South Eastern Asia. Crossed by the southern known to date back to the Early Triassic (240 Ma, Lepvrier termination of the Red River Shear Zone it has been et al., 1997). Other thermotectonic episodes which may strongly affected by the India-Asia collision and by South have affected the region (e.g. during the Cretaceous, Lep- China Sea rifting. The precise role and extent of in¯uence of vrier et al., 1997; Lacassin et al., 1998) are more obscure, the Red River Shear Zone is not yet fully known and is the but this may be due to the current paucity of geochronolo- subject of ongoing debate (Tapponnier et al., 1982; 1986; gical and ®eld data. Thus, to decipher the geodynamic Briais et al., 1993; Leloup and Kienast, 1993; Leloup et al., evolution of Indochina it is essential that we understand 1995; Harrison et al., 1996; Dewey et al., 1989; Molnar and the timing and interaction between the different phases of Gipson, 1996; England and Molnar, 1990; Murphy et al., deformation and structures. In this context we have studied 1997; Rangin et al., 1995; Chung et al., 1997). The penin- the deformation and exhumation history of a large meta- sula is classically considered as a rigid block but recent morphic massif, close to the Red River Fault (RRF). The Song Chay Massif is located about 10 km north-east * Corresponding author. Tel.: 133-0467545926; fax: 133-0467547362. of the Day Nui Con Voi, east of the town of Lao Cai (Fig. 1). E-mail address: [email protected] (H. Maluski). It is a large domal structure which on ®rst examination

1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(00)00038-9 234 H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

Fig. 1. Location map and topography of northern Vietnam. The Song Chay Massif is close to the Day Nui Con Voi and the Red River. appears similar to the Bu Khang dome, and therefore, may Kontum Block, in the South (Fromaget, 1941). These extend have had a similar history. To understand the temporal rela- into the metamorphic ranges of Burma, Thailand, eastern tionship between this structure, the Red River Shear Zone Laos and Vietnam, as well as the extreme south-western part and Miocene extension found in the Bu Khang Dome south of China. The northern region is occupied by a complex of the fault (Jolivet et al., 1999) we have used a combination realm (Figs. 1 and 2), in which the NW±SE RRF zone is of ®eld observation, 40Ar± 39Ar mica dating (Maluski et al., central. Parallel to the active RRF is the Cenozoic Red River 1999), and apatite ®ssion-track analysis. The results are Shear Zone. The elongate Day Nui Con Voi Dome is compared with those from the Red River Shear Zone in bounded by the RRF to the west and by the Song Chay the Dai Nui Con Voi. 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 2. Geology the eastern side of the Red River and extends into China. It has a dome-like shape, roughly trending in a NE±SW direc- The major structures within the Indochina peninsula are tion and is bounded on its western ¯ank by the Song Chay the Truong Song belt (CordillieÁre Anamitique of the early Fault and on its southern ¯ank by an E±W trending mylo- French authors), in North to Central Vietnam, and the nite zone, which on geological maps appears to be H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248 235

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

terminated by the Song Chay Fault. The eastern and south- lain by Devonian conglomerates, slates and limestones. The eastern limits of the dome correspond to the Lo river valley Permo-Carboniferous is represented by carbonates. 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, 3. Deformation in the Song Chay dome from the city of Bac Quang to the villages of Hoang Su Phi and Xin Man. Terranes surrounding the dome, to the We describe a cross-section of the dome from the SE to south and east, consist of greywackes and micaschists to the NW (Fig. 2). The southern limit of the dome is a narrow slaty schists overlain by a karstic formation of Cambrian EW trending fault, which cuts strongly lineated quartzites, limestones. The Ordovician and Silurian are represented micaschists and marbles. by limestones and quartzite, and are unconformably over- The foliation is folded into a broad antiform with an axis 236 H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

Fig. 3. Photographs of outcrops in the Song Chay Massif showing top-to-the-north kinematic indicators. All sections are parallel to the lineation and perpendicular to the foliation. (a) Orthogneiss near Huang Xu Phi in the northern part of the section. (b)±(d) Orthogneiss from the southern side of the dome. Photograph (c) shows a high strain zone slightly oblique on the foliation in the less strained gneiss (lower). The button with the star gives the scale (2 cm).

trending NE-SW and is steeper in the southern rim. Hori- stretching lineation and a weak planar anisotropy. These zontally foliated orthogneisses and migmatites are found orthogneisses are not ubiquitously deformed and locally near the core of the antiform, as shown on the cross section, occur in an unfoliated facies with large feldspars in an Fig. 2. To the North, upper levels of the core are made of undeformed groundmass. This rock has been considered biotite and muscovite-bearing orthogneisses containing K- to be an intrusive granite, but its occurrence suggests to feldspars several centimeters in length. Close to the village us that it is simply the undeformed equivalent of the of Xin Man, horizontally sheared micaschists are directly orthogneiss. Gradients of strain are seen at the scale of overlain by muscovite-bearing marbles that alternate with tens of meters and a general increase in deformation is pelitic schists, considered as Cambrian (Geological Survey observed from the undeformed granite toward the north of Vietnam, 1999 (Geological map 1/200,000); Tran Van and south. The most intense deformation is observed in Tri, 1977; Phan Cu Tien et al., 1989). the northern part of the section between Xin Man and A conspicuous NW- or N-trending stretching lineation is HuangSuPhi. recognised all along the section in orthogneisses and mica- Orthogneisses yield consistent kinematic indicators schists (Fig. 2, map). In the internal parts of the dome the showing a top-to-the-north or northeast sense of shear orthogneiss fabric is often constrictional with a strong (Fig. 3) even in regions characterised by constrictional H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248 237 fabrics where the foliation is least visible. The most mised, concerning dimension controlling gas loss in diffu- common shear criteria are S±C relations, asymmetric pres- sive loss conditions. sure shadows on alkali feldspar, sigmoidal foliation when Sample VN 322 (Fig. 4a) is located in a subvertical shear approaching zone of shear localisation. zone which bounds the dome to the south, (228 240 5200; 1048 This simple deformation pattern suggests that a nearly 420 5500). It is a sillimanite±cordierite bearing micaschists horizontal shear zone has been active between the basement with ¯exuose biotites and muscovites. Muscovite de®nes a and the cover, with a top-to-the-north shear sense, and has very irregular shaped degassing spectrum with increasing been lately folded into a broad antiform. Comparable ages since 60 Ma for low temperatures up to 234 Ma in ¯at-lying shear zones on this scale are not common in Viet- the last signi®cant step. Intermediate degassing tempera- nam and its age is unknown. tures display an age of 204 Ma. This spectrum relates to a East of Bac Quang, cordierite±sillimanite±muscovite closure of the system at an age of 234 Ma, which then micaschists and quartzites displaying a N808E-trending suffered a subsequent Ar loss. The strong scattering of the foliation and a gently west-dipping lineation occupy the 39Ar/40Ar ratios, is also re¯ected in the isochron diagram southern rim of the dome. normalised to 40Ar, in which no linear array can be de®ned. Sample VN 324 (Fig. 4b) is a typical orthogneiss from the southern rim of the dome (228 290 4100; 1048 510 4300). Its 4. Geochronological data mineralogical content is quartz, K-feldspar and biotite, with very few muscovites. Micas are oriented in the foliation and The Song Chay Massif and the surrounding area have present the shape of late to post deformational minerals. The been relatively unexplored by geochronology: gneisses, age spectrum of the muscovite does not de®ne a plateau age schists and migmatites were dated by the U±Pb method, but displays, for 90% of released 39Ar, increasing ages from at 2652 and 1000 Ma. (Tran Van Tri, 1977; Tran Ngoc 73 Ma to a ®rst integrated age of 228 ^ 1Ma; and a second Nam, 1997). These Archean U±Pb ages most probably at 236 ^ 0:5Ma: As for the previous sample, this mineral relate to inherited Pb. Tugarinov et al. (1979) further suffered inhomogeneous Argon loss, which affects mainly found a U±Pb zircon and apatite upper intercept age of low temperature degassing sites. For this sample, the 625 ^ 20 Ma; and a lower intercept at 30 Ma. Nguyen and isochron plot does not reveal a well-de®ned straight line. Dao (1995) published an age of 350 Ma on biotite without Sample VN 329 (Fig. 4c) is a ®ne-grained gneiss with information on the dating method. More recently, the evolu- quartz, plagioclase, K-feldspar, coarse biotites and few tion of this massif was investigated using the Ar±Ar method muscovites (228 320 2600;1048 490 2400). This facies is locally and the ®rst age data relating to Triassic metamorphism intercalated within the orthogneisses. The biotite displays a were presented by Maluski et al. (1999). The protolith age very regular age spectrum for which an age plateau can be of the Song Chay orthogneisses was measured by Leloup et de®ned at 201 ^ 2 Ma for near 80% of the 39Ar degassed. al. (1999) using the zircon U±Pb method. Dated at 428 ^ The ®rst degassing step gives an age around 100 Ma. This 5Ma; this age probably corresponds to the time of emplace- pattern attests to a closure of the mineral at 200 Ma, ment of the protolithic granite. The same study also followed by a very weak subsequent Ar loss. In a diagram measured a Rb±Sr age and 40Ar± 39Ar mica plateaux ages 36Ar/40Ar, 39Ar/40Ar, we can de®ne an isochron giving an on a single sample from the southern part of the dome. The age of 200 ^ 2Ma; identical to the one displayed by the results gave ages that span a period between 209 ^ 9 and integrated plateau age. 176 ^ 5 Ma and were interpreted as documenting a Late Sample VN 333 (Fig. 4d) is a migmatitic gneiss to the Triassic shearing event around 210 Ma. A K-feldspar west of Wang Xu Phy village (228 440 3900; 1048 380 0200). It 40Ar± 39Ar age spectrum also suggested a phase of rapid contains quartz, plagioclase, muscovite and biotite. Micas cooling in the late Jurassic. develop in the foliation and appear to have formed syn- to 4.1. 40Ar± 39Ar results post-deformation. The biotite of this sample yields a well- de®ned plateau age at 166 ^ 2 Ma for near 95% of the 39Ar The radiometric 40Ar± 39Ar stepwise heating method was released. The closure of the mineral vs. Ar occurred at that used on pure mineral aliquots. Results are presented from time, without subsequent reopening of the system. An iden- the southern cover to the northern one, crossing the whole tical age of 166 ^ 2 Ma is obtained through the isochron antiform (Figs. 4 and 5). Analytical conditions have been diagram, with an intercept on the Y-axis de®ning an atmos- formerly described in Maluski et al. (1995) and Lepvrier et pheric 40Ar/36Ar ratio. al. (1997). A summary of results is presented in Table 1. Sample VN 335 (Fig. 4e and f) was taken 5 km east of Argon isotopic results are given in Table 2. All the samples Xin Man village (Fig. 1). It is a ®ne-grained orthogneiss, of orthogneisses and migmatites described here and used for from the northernmost part of the dome. It is composed of radiometric dating are coarse grained. The granulometric quartz, plagioclase, biotite and muscovite. Mica¯akes are fraction used for dating was 160 mm in diameter for mica- developed in the foliation, with undeformed shapes. Musco- grains. In these conditions the grain-size effect, as vites and biotites give, respectively, 164 ^ 2 Ma and 176 ^ mentioned in McDougall and Harrison (1988), is mini- 2Ma: For muscovite, the plateau age is calculated over 60% 238 H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

300 300

250 234±0.8 Ma 250 228±1 Ma <236±0.5 Ma>

200 200

204±1 Ma 150 150 AGE (Ma) AGE (Ma) 100 S SONG CHAY 100 SONG CHAY 60 Ma VN 324 MUSCOVITE 50 VN 322 MUSCOVITE 50

0 0 0 50 100 0 50 100 39 % Ar cumulative %39 Ar cumulative a b

300 300

250 250 < 201±2 Ma > 200 200 < 166±2 Ma >

150 150 AGE (Ma) AGE (Ma) 100 SONG CHAY 100 SONG CHAY VN333 BIOTITE 50 VN329 BIOTITE 50

0 0 0 50 100 0 50 100 39 39 % Ar cumulative c % Ar cumulative d

300 200 167±2Ma 250 150 <> 200 < 176±2 Ma > 164±2 Ma

150 100 AGE (Ma) AGE (Ma) SONG CHAY 100 SONG CHAY 50 VN 335 MUSCOVITE 50 VN335 BIOTITE

0 0 0 50 100 0 50 100 39 39 % Ar cumulative e % Ar cumulative f

300

250 < 198±2 Ma > 200

150 AGE (Ma) 100 SONG CHAY 50 VN 337 MUSCOVITE 0 050100 % 39Ar cumulative g

Fig. 4. 40Ar± 39Ar age spectra from the Song Chay Massif. of 39Ar released. The last three signi®cant steps reveal an would re¯ect radiogenic 40Ar loss, less pronounced on the integrated age slightly older than the previous one at 167 ^ more retentive sites, resulting in the last old age of 167 Ma. 2Ma: The whole pattern of this age spectrum attests to an The result obtained on biotite is somewhat surprising Ar diffusion loss, resulting in younger ages in low extraction because the closure temperature of biotite is lower than temperatures (96, 143, 160 Ma). The plateau therefore, for muscovite. Even if this value is not precisely known H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248 239

50 100

40 80

30 60 < 33.1±0.8 Ma > < 40±1 Ma > AGE (Ma) 20 AGE (Ma) 40

10 VN 106 MUSCOVITE 20 VN 107 BIOTITE

0 0 0 20406080100 0 20406080100 39 39 % Ar cumulative a % Ar cumulative b

50

40

< 24.1±1 Ma 30 >

AGE (Ma) 20

10 VN 110 MUSCOVITE

0 0 50 100 %39 Ar cumulative c

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

(values differ slightly according to different authors; Harri- marbles, close to Xin Man village. The foliation of the son et al., 1985; McDougall and Harrison, 1988; Hames and marble is very slight, being underlined by very thin musco- Bowring, 1995), we should expect a younger age for the vite layers, clearly visible under the microscope. Musco- biotite than for the muscovite. An excess Ar component vites give a well-de®ned plateau age at 198 ^ 2 Ma for may be suspected in this biotite, in reference with the age 80% of 39Ar released. A similar age is obtained with the of the muscovite. It means that if such a component occurs isochron diagram, but without any precision on the in the biotite, its distribution is nearly homogeneous on the 40Ar/36Ar ratio, due, as for the earlier sample, to the strong whole sites of the mineral, and results in an increase of age clustering of 40Ar/39Ar. The pattern of this age spectrum of 12 Ma, vs. the coexisting muscovite. For both samples, attests for an argon loss subsequent to the closure of the the extreme clustering of data prevents de®nition of a well- system, with regularly increasing ages from 31 Ma up to de®ned isochron, especially for the Y intercept value, the plateau age. We discuss the signi®cance of those ages connected with the 40Ar/36Ar ratio. in the last section of this paper. Sample VN 337 (Fig. 4g) is located in the northern cover In addition to the samples taken from the Song Chay of the crystalline core, represented by muscovite bearing 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 ^ 2 240 H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

Table 2 36 37 24 Ar isotopic results for analysed minerals. Correction interference used for Ar/ ArCa is 2:93 £ 10 : Mass discrimination correction factor is calculated for a 40Ar/36Ar ratio of 291

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

VN322 MUSCOVITE (J ˆ 0.018342) 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 age ˆ 208.6 ^ 2.1 VN324 MUSCOVITE (J ˆ 0.018342) 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 age ˆ 230.0 ^ 2.2 VN329 BIOTITE (J ˆ 0.018342) 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 age ˆ 200.3 ^ 2.0 VN333 BIOTITE (J ˆ 0.018342) 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 1100 5.27 0.114 0.034 3.3 99.2 166.49 ^ 3.70 H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248 241

Table 2 (continued)

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

1150 4.792 1.436 0.031 42.4 99.9 152.01 ^ 20.29 Total age ˆ 165.4 ^ 1.70 VN335 BIOTITE (J ˆ 0.018342) 450 2.426 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 age ˆ 174.7 ^ 1.80 VN335 MUSCOVITE(J ˆ 0.018342) 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 age ˆ 163.6 ^ 1.7

VN337 MUSCOVITE (J ˆ 0.018342) 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 age ˆ 194.3 ^ 2.0 VN106 MUSCOVITE (J ˆ 0.012158) 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 ^ 1.86 850 1.546 0.177 0 5.2 27.7 33.61 ^ .77 242 H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

Table 2 (continued)

Temperature (8C) 40Arp/39Ar 36Ar/40Ar 37Ar/39Ar % Atm. %39Ar Age ^ 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 age ˆ 32.9 ^ 0.8 VN107 BIOTITE (J ˆ 0.012158) 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 age ˆ 40.3 ^ 1 VN110 MUSCOVITE(J ˆ 0.012158) 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 Total age ˆ 23.9 ^ 0.9

Sample VN 106 (Fig. 5a) is a quartzite occurring close to 4.2. Fission-track data the Pho Lu city, on the Red River. A very strong lineation occurs in these rocks, which exhibit an E±W foliation. It Apatite ®ssion-track analysis was undertaken on samples contains layers of ®ne grained muscovites and biotites from the Song Chay Massif and RRF zone, to complement the underlining the foliation. The muscovite displays a plateau argon data-set and constrain the low temperature cooling de®ned for near 90% of 39Ar released at 33:1 ^ 0:8Ma: history. The sensitivity of the system to closure at low Sample VN 107 (Fig. 5b) is a mylonitic orthogneiss with temperatures (,60±1108C) enables detection of weak (in a N130 vertical foliation from the road section between Lao magnitude) cooling events that may not be otherwise detected Cai and Sa Pa. Plagioclase is partly transformed with seri- by higher temperature methods. The results and sample loca- cites. Intersticial muscovites occur in the matrix. A biotite tions are given in Table 3. Sample preparation and analysis yields an age of 40 ^ 1 Ma for 90 % of 39Ar. followed procedures given in Storey et al. (1996) with samples Sample VN 110 (Fig. 5c) was taken near Bao Yen on the irradiated in the thermal facility of the Risù Reactor, National border of the Dai Nui Con Voi massif. This is a ®ne grained Research Centre, Rosklide, Denmark, (cadmium ratio for gneiss with a developed N15 trending lineation. Muscovites Au . 200±400†; using Corning glass CN-5 as a neutron dosi- are coarse grained, with ®sh-like shapes. Very ®ned grained meter. Counting and track length measurements used a micro- biotites and plagioclase occur, with garnets and tourmalines. scope total magni®cation of 1250 £ with a 100 £ dry An age of 24 ^ 1 Ma was obtained on a muscovite for near objective. Central ages were calculated using the IUGS- 60 % of released argon. recommended zeta calibration approach (Hurford, 1990). H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248 243 For the Song Chay Massif samples (Table 3), data quality which cooled at the fastest rate between 110 and 608C is mixed. Although adequate numbers of individual grain (sample VN 9807), comes from the maximum elevation ages have been measured for all samples, track length (at ,800 m). Regionally cooling for most samples measurement was affected by low spontaneous track densi- occurred at a similar rate (within experimental uncer- ties. Thus, only six samples (VN 9801, 9805, 9807, 9811, tainties), to between 3 and 58C/Myr. 9812 and 9814) yielded adequate numbers of horizontally Samples from the RRF were also analysed to complement con®ned tracks to suitably de®ne length distributions. the new argon data. Some of the apatite samples from this Nevertheless, given the similarity within the data-set, region were dif®cult to analyse because of lower than between central ages and mean track lengths, it is reason- normal uranium concentrations (often ,5 Uppm) and this able to infer that similar thermal histories were experienced affected the quality of some of the track length data. Never- by those samples which did not contain adequate numbers theless the resultant data-set is of suitable quality to provide of con®ned tracks. meaningful constraints on the regions cooling/exhumation Central (modal) ages range between 16 ^ 3 Ma and 24 ^ history. 2Ma; with mean track lengths (for samples with more than Samples VN 9818±9821 are from the road section 50 measurements), between 13:60 ^ 0:31 mmand14:12 ^ between Lao Cai and Sapa along which the argon sample 0:15 mm: Qualitatively, the relatively long mean track lengths VN 107 was also collected. The four samples range in suggest that cooling through the apatite partial annealing zone central age from 37 ^ 2Mato27^ 3Ma: Track lengths (,110±608C) was relatively rapid. The cooling paths may be for those samples with adequate numbers of measurements further constrained by modelling utilising the procedure of range from 13:72 ^ 0:27 mmto14:31 ^ 0:14 mm; and are Gallagher (1995). This is a probabilistic approach that predicts consistent with moderately rapid cooling Ð hence the ages thermal histories from within speci®ed time-temperature approximate to the time of cooling. Sample age and lengths bounds. Each thermal history is used to predict ®ssion-track show no correlation with elevation, and therefore, the age parameters which are quantitatively compared with observed distribution is unrelated to simple uniform erosional values and ranked according to goodness of ®t. Maximum denudation. likelihood is used in order to compare each individual The ®ssion-track data from the undeformed granites (VN observation. 9824±9827) adjacent to the main Phan Si Pang granite, west Only those samples with statistically well-de®ned of Sapa give central ages between 32 ^ 4 and 30 ^ 3Ma: length distributions were modelled i.e. VN 9801, Sample VN 9827 has suitable numbers of measured 9805, 9807, 9811, 9812 and 9814 and representative con®ned tracks that comprise a mean length of 14:18 ^ plots are shown in Fig. 6. The modelled results show 0:14 mm; consistent with rapid cooling. The similarity the portion of a samples thermal history (between ,60 among the four data suggest they experienced the same and 1108C) that is constrained by the ®ssion-track data. thermal history. Any variation in temperature below ,608C is unresol- Two samples (VN 9846 and VN 9848) were analysed vable (highlighted by the grey shading and dashed from locations near the town of Bao Yen close to the edge time±temperature path). The shaded areas surrounding of the Day Nui Con Voi. These gave similar central ages the constrained time±temperature paths (solid line) 22 ^ 2 Ma and 18 ^ 1 Ma and both have mean track represent the 95% con®dence regions. The oldest track lengths longer than 14 mm indicative of rapid cooling. 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 5. Interpretation and discussion through the 1108C isotherm. For samples VN 9811, 9812 and 9814 this took place between 20 and 21 The 40Ar± 39Ar and ®ssion-track data-sets from the Song Ma, and for samples VN 9807, 9805 and 9801, between Chay Massif are signi®cantly different in age, and therefore, 22 and 28 Ma. Table 4 summarises the main relate to different aspects of the regions geodynamic evolu- time±temperature information extracted from the modelled tion. Due to the different closure temperature for Ar and cooling data. FissionTtrack systems (350±4008C vs. 608C for exhuma- A plot of sample location against ®ssion-track central age tional FT cooling), it is possible to recognise both Mesozoic (Fig. 7) suggests a possible trend of increasing age to the and Cenozoic events in the Song Chay Massif. We now south-east. This is seen more clearly in the modelling which discuss the signi®cance of these ages. shows the older ages record an earlier cooling than The geographic distribution of the 40Ar± 39Ar results samples to the north-west. There is no evidence for a shows ages that are younger in the central part of the systematic correlation between age and elevation as dome. Muscovite and biotite from the southernmost would be expected from a terrain that experienced a samples, VN322 and VN324, record ages of 234 and slow to moderate uniform rate of denudation i.e. the 236 Ma, respectively, corresponding to the last increments cooling pattern is not caused by variable depths of of experimental degassing. A similar range of ages can be erosion. But, it is interesting to note that the sample found throughout Vietnam (Lepvrier et al., 1997); the Song 244 H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248 m) Tracks measured m m) S.d. ( m geometry correction factor; (iii) Ages p /2 p 0.14 1.31 86 0.31 0.99 11 0.31 1.70 31 0.18 1.63 81 0.16 1.43 83 0.39 1.66 19 0.15 1.42 90 0.15 1.36 79 0.25 1.35 30 0.19 1.42 59 0.27 2.38 80 0.27 1.98 54 0.18 1.48 69 0.14 0.98 53 0.45 1.49 12 0.21 0.83 17 0.14 1.38 103 0.16 1.14 51 0.14 0.92 45 ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ 2 13.93 1 13.51 2 No data 2 13.60 2 13.14 2 14.01 3 13.39 2 13.41 2 14.12 1 13.81 2 13.65 2 No data 3 13.72 2 13.75 4 13.77 3 14.31 5 No data 4 13.73 4 14.98 3 14.18 2 14.24 1 14.41 ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ I Ni % R.E. Central Age (Ma) Mean track length (

r sNs

r ) shown in brackets; (ii) Analyses by external detector method using 0.5 for the 4 N (iv) Central age is a modal age, weighted for different precisions of individual crystals ; 5 dNd

^ r 339 ˆ CN5

z ) numbers of tracks counted ( 2 2 tr cm 6 10 £ Notes: (i) Track densities are ( Song Chay Massif VN9801 22.29.74 104.51.74 115 20 1.481 4159 0.135 218 1.496 2418 15.4 23 VN9802 22.29.77 104.51.49 150 20 1.482 4159 0.365 664 4.106 7473 13.3 22 VN9803 22.31.68 104.50.32 410 20 1.484 4159 0.053 94 0.585 1039 0.04 23 VN9804 22.32.04 104.50.10 580 20 1.485 4159 0.098 206 1.139 2399 3.8 22 Sample Long Latt Elevation (m) No. grains VN9805 22.32.50 104.49.34 750 20 1.487 4159 0.121 163 1.246 1682 0.33 24 VN9807 22.32.94 104.48.77 800 20 1.490 4159 0.515 116 0.671 1511 19.7 20 calculated using dosimeter glass CN-5; analyst Carter VN9810 22.41.80 104.29.38 391 16 1.490 4159 0.034 44 0.543 709 20.0 16 VN9811 22.43.05 104.32.07 365 18 1.492 4159 0.435 329 5.704 4309 22.4 19 Table 3 Fission track apatite analytical data for the Song Chay Massif VN9812 22.44.75 104.35.68 395 20 1.493 4159 0.094 111 1.262 1496 0.75 19 VN9813 22.44.89 104.37.29 470 20 1.495 4159 0.221 442 3.045 6086 0.8 18 VN9814 22.43.97 104.42.20 480 17 1.497 4159 0.119 191 1.620 2585 17.9 19 VN9815 22.35.01 104.46.89 720 20 1.498 4159 0.057 121 0.896 1913 22.9 17 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 VN9819 22.25.64 103.55.01 730 20 1.263 7004 0.086 131 0.492 749 29.1 37 VN9820 22.24.40 103.54.06 900 20 1.586 8792 0.082 110 0.781 1045 21.7 30 VN9821 22.22.21 103.52.12 1285 20 1.586 8792 0.131 127 1.275 1241 24.9 29 West of Sapa VN9824 22.21.24 103.45.92 2200 11 1.263 7004 0.175 70 1.245 497 25.6 32 VN9825 22.21.30 103.46.47 2000 21 1.263 7004 0.098 80 0.691 561 0 31 VN9826 22.21.68 103.45.86 1905 16 1.263 7004 0.015 94 1.013 639 11.3 32 VN9827 22.21.59 103.45.26 1670 20 1.586 8792 0.125 192 1.150 1772 28.2 30 Bao Yen VN9846 22.11.94 104.23.52 320 14 1.263 7004 0.335 176 3.259 1712 24.5 22 VN9848 22.13.65 104.26.88 260 20 1.263 7004 0.282 287 3.282 3343 0.0 18 H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248 245

0 0 VN 9801 VN 9811 20 0Ma 8¡C 20 0Ma 15 ¡C 2Ma 28¡C 1Ma 45¡C 40 4Ma 41¡C ? 40 20 Ma 84 ¡C ? 25 Ma 94 ¡C 21 Ma 122 ¡C 60 60 T(¡C) T(¡C) 80 80 PAZ PAZ 100 100

120 120

35 28 21 14 7 0 35 28 21 14 7 0 Time (Ma) Time (Ma)

30 20 Obs. Age : 22.5 Ma P(K-S) = 0.551 Obs. Age : 19.1 Ma P(K-S) = 0.970 Pred. Age : 22.5 Ma P(Chi) = 0.997 Pred. Age : 19.2 Ma P(Chi) = 0.994 Obs. Mean length : 13.92 Obs. Mean length : 13.56 20 Pred. Mean length: 13.83 N Pred. Mean length: 13.57 Obs. S.D. : 1.31 Obs. S.D. : 1.49 N 10 Pred. S.D. : 1.33 Pred. S.D. : 1.33 Oldest track (Ma) : 25 10 Oldest track (Ma) : 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Track Length (microns) Track Length (microns) 0 0 VN 9805 VN 9814 20 0Ma 12¡C 20 4Ma 60¡C ? 0Ma 10¡C ? 40 8Ma 34¡C 40 13 Ma 69 ¡C 28 Ma 107 ¡C 21 Ma 109 ¡C 60 60 T(¡C) 80 T(¡C) 80 PAZ PAZ 100 100

120 120

35 28 21 14 7 0 35 28 21 14 7 0 Time (Ma) Time (Ma)

30 20 Obs. Age : 24.3 Ma P(K-S) = 0.863 Obs. Age : 18.7 Ma P(K-S) = 0.901 Pred. Age : 24.3 Ma P(Chi) = 0.999 Pred. Age : 18.7 Ma P(Chi) = 1.000 Obs. Mean length : 13.14 Obs. Mean length : 13.64 20 Pred. Mean length: 13.14 N Obs. S.D. : 1.62 Pred. Mean length: 13.54 N 10 Pred. S.D. : 1.48 Obs. S.D. : 1.42 10 Oldest track (Ma) : 28 Pred. S.D. : 1.43 Oldest track (Ma) : 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Track Length (microns) Track Length (microns)

Fig. 6. Modelling of ®ssion track data.

Ma complex to the west, the central and southern Truong Son The ages of 234±236 Ma obtained on two muscovites from Belt, and the Kontum massif, all reveal metamorphic ages ca. the outer rim of the dome are slightly younger than the average 240±245 Ma. These ages are found on syn- to late-kinematic age obtained from the Indosinian massifs located within the minerals and testify to the widespread in¯uence of Triassic north-south Truong Son Belt in Vietnam, but they are similar metamorphism in this southeastern part of Asia. Evidence for to the age range found in south-west China (Faure et al., 1996) this orogen can also be found in southern China and Thailand and Thailand (Dunning et al., 1995; Mickein, 1997), and there- (Mitchell, 1986; Hutchison, 1989; Arhendt et al., 1993; fore, we consider 234±236 Ma to record Triassic tectono- Dunning et al., 1995; Faure et al., 1996; Mickein, 1997). metamorphism. 246 H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

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

Sample Central age (Ma) Oldest track (time Approx time crossed Cooling rate for temperature crossed 1108C isotherm) (608C isotherm) 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

Although ages decrease through 200 Ma, to165 Ma in the very homogeneous all along the pro®le and we recognise a northern region, we infer that the whole granitic protolith, single ductile tectonic event. The evolution seen in micas which intruded at 428 ^ 5 Ma (Leloup et al., 1999), was ages along the pro®le is most probably the result of a slow sheared during the Indosinian. The deformation history is 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- 1000 nian orogenic episode, and have remained above this isotherm since that time. Intermediate zones crossed these 800 isotherms later, at 200 Ma, as inferred by sample VN 329, consistent with a moderately slow exhumation. The young- 600

Elevation (m) est sampled level, crossed the same isotherms much later, at ca. 165 Ma. Since biotite VN 333 and muscovite VN335 400 record the same age (166 and 164 Ma, respectively), it is probable that the cooling rate, at this late stage, increased. 200 The pattern of ages could be explained by a thermal diffu- sion effect, perhaps associated with a magmatic body 0 emplaced deeply under the core structure of the dome. 22.55 This would imply that 165 Ma of VN 333 and VN 335 is North a maximum age (oldest), related to partial loss of radiogenic 22.5 40Ar from the samples exhumed from the deeper crustal 22.45 levels. Latitude Sample VN 322 is important, in relation to the late evolu- 22.4 tion of the southern part of the Song Chay Massif. The cordierite±sillimanite bearing schist constitutes the south- 22.35 ernmost limit of the dome complex. It corresponds to a vertical E±W oriented mylonitic band. Muscovites occur 22.3 in the foliation plane and constitute a strongly developed South 22.25 lineation dipping 308 to N 240. The trend of the age spec- 104.7 trum attests to argon loss by diffusion processes, the ®rst age East obtained, 60 Ma being younger than the one obtained at the 104.6 end of the degassing procedure (234 Ma). The age of 60 Ma can be considered as an older limit for the late deformation- 104.5 metamorphism responsible for the development of this

Longitude mylonitic band and for the rejuvenation of the age of these older muscovites. This age of 60 Ma, is distinctly 104.4 older than Cenozoic ages between 35 and 25 Ma displayed by mylonitic gneisses along the RRF Zone, which cross cuts 104.3 this E±W structure. West In contrast to the argon data the ®ssion-track results 104.2 clearly show the region was affected by Cenozoic tectonism. 10 15 20 25 30 The time interval over which the Song Chay experienced Central Age (Ma) rapid cooling (28±20 Ma) coincides with the main phase of Fig. 7. Plots of ®ssion-track data against sample location and elevation. shear heating and sinistral movement along the Red River Error bars are ^1sigma. Shear Zone, and therefore, it is probable these two events H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248 247 are related. As stated in Section 4 the ®ssion-track data also with exhumation from signi®cant crustal depths. In contrast show a well developed geographical trend but there is no the data from the Song Chay Massif show Cenozoic exhuma- correlation between age and altitude which suggests that tion was limited, preserving an earlier Indosinian thermotec- simple erosion is not responsible for the distribution of ages. tonic signature. The two pulses of cooling correspond to an The gradient of ages from north to south (Fig. 7) suggests a increase in slip rate along the main fault which Harrison et al. northward tilt of the Song Chay block with an older exhuma- (1996) noted also coincides with the transtensional phase. It is tion in the south. This asymmetry would be consistent with probable that this transtensional environment has caused loca- block tilting perhaps caused by reactivation of bounding lised extensional unroo®ng of the Song Chay Massif as well as faults, a process that occurs isostatically after normal faulting the more pronounced extension recently identi®ed in the Bhu (e.g. Jackson and McKenzie, 1989). Khang Massif, southwest of the shear zone (Jolivet et al., The temporal relationship between the Red River Shear 1999). Zone and late stage exhumation of the Song Chay Massif is now explored further through new argon and ®ssion-track results. 6. Conclusion Sample VN 110 was collected from the Day Nui Con Voi The Song Chay Massif area has been affected by the and records a muscovite Ar±Ar age of 24 ^ 2Ma: Two Triassic orogeny, which is responsible for high-grade meta- ®ssion-track samples from the same area record central ages morphism and shearing, observed along a NW±SE cross- of 18 ^ 1and22^ 2 Ma showing that exhumation of the section. A shear zone formed during this orogeny at the main shear zone to shallow crustal levels was rapid. Published interface between metasediments and a granite intruded mica ages for the Day Nui Con Voi range from 24:9 ^ 0:2Ma ,430 Ma ago. The shear zone has a shallow dip and to 21:2 ^ 0:2 Ma (Harrison et al., 1996; Leloup et al., 1997; shows a consistent top-to-the-North sense of shear. Tran Ngoc Nam, 1988; Tran Ngoc Nam et al., 1998; Wang et 40Ar± 39Ar ages of micas from orthogneiss within the shear al., 1998); however, beyond the main shear zone there is little zone record a Triassic age on the southern area, but also published age data. Sample VN 107 from a road section show evidence for a younger cooling most probably related between Lao Ca and Sapa records mica age at 40 ^ 1Ma; to a slow doming in the Jurassic. Low temperature apatite whilst ®ssion-track data from the same area (Table 3) record ®ssion-track data from along the same transect record a later an age range between 27 ^ 3Maand37^ 2Ma: Track Cenozoic exhumation that involved some reactivation of lengths for these samples are between 13:72 ^ 0:27 mmand bounding faults, with a normal sense of movement. Timing 14:31 ^ 0:14 mm consistent with moderate to rapid cooling, is similar to the exhumation events in the RRF Zone and con®rmed by modelling the better quality data. The Oligocene implies a causal relationship. This study also reinforces the cooling recorded by both the argon and ®ssion-track data in importance of combining both low and high temperature this area thus relates to a cooling event associated with early dating methods in a single study. 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 Acknowledgements wide and up to 140 km long). This has previously been dated using K±Ar methods to between 41 and 58 Ma This study was supported by cooperative programs: 40 39 (Phan Cu Tien, 1977) however recent Ar/ Ar dating of Programme International de CoopeÂration Scienti®que phlogopite and biotite from the granite and fault bounded between CNRS (INSU) and CNST (Vietnam); Paris 6 metamorphic rocks show that rapid cooling from tempera- University, Montpellier 2 University and National Univer- tures .3008C occurred at ,34 Ma, an age that indicates sity of Vietnam, Hanoi. Funding for DR and ®ssion-track much younger emplacement (Leloup et al., 1997). This analysis was provided by the University of London South- age is identical (within error) to the ®ssion-track ages 30 ^ east Asia Research Group. 4 and 32 ^ 5Ma† measured on undeformed granites adja- cent to the main granite body. Such concordancy records geologically instantaneous cooling and is consistent with References the ®ssion-track length data (Table 3). This evidence, suggests early fault movement coincided with emplacement Arhendt, H., Chonglakmani, C., Hansen, B.T., Helmckke, D., 1993. of the Phan Si Pang granite, consistent with the observations Geochronological cross section through northern Thailand. Journal of Southeast Asian Earth Sciences 8, 207±217. of Leloup et al. (1997). Briais, A., Patriat, P., Tapponnier, P., 1993. Updated interpretation of The data from the RRF area show evidence for two phases magmatic anomalies and sea¯or spreading stages in the South China of cooling during the Cenozoic. The early phase occurred Sea: implications for the Tertiary Tectonics of SE Asia. Journal of during the Oligocene and is associated with emplacement of Geophysical Research 98, 6299±6328. the Phan Si Pang granite. The later phase is restricted to the Chung, S.L., Lee, T.Y., Lo, C.H., Wang, P.L., Chen, C.Y., Nguyen, Trong Y., Tran, Trong H., Wu, G.Y., 1997. Intraplate extention prior to conti- main shear zone along the Day Nui Con Voi and occurred nental extrusion along the Ailao Shan-Red River shear zone. Geology between ,25±21 Ma. In both cases cooling was associated 25 (4), 311±314. 248 H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248

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