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 Maluski a,*, C Lepvrier b, L Jolivet b, A Carter c, D Roques c, O Beyssac d, Ta Trong Tang e, Nguyen Duc Thang f, D Avigad d a ISTEM-CNRS, Universite Montpellier 2, Place EugeÁne Bataillon, 34095, Montpellier, France Laboratoire de Tectonique, Universite Pierre et Marie Curie, Place Jussieu, case 129, 75252 Paris cedex 05, France c London Fission Track Research Group, Department of Earth Sciences, Birkbeck and University College, Gower Street, London, WC1E 6BT, United Kingdom d Laboratoire de GeÂologie, Ecole Normale SupeÂrieure, 24 rue Lhomond, 75231 Paris cedex 05, France e National University of Vietnam, Hanoi, 334 Nguyen Trai Str., Thanh Xuan, Hanoi, Viet Nam f Geological Survey, Hanoi, Viet Nam b Received 14 October 1999; revised May 2000; accepted 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 Introduction The Indochina peninsula, particularly northern Vietnam, is in a key-position for understanding the geodynamic evolution of South Eastern Asia Crossed by the southern termination of the Red River Shear Zone it has been strongly affected by the India-Asia collision and by South China Sea rifting The precise role and extent of in¯uence of the Red River Shear Zone is not yet fully known and is the subject of ongoing debate (Tapponnier et al., 1982; 1986; Briais et al., 1993; Leloup and Kienast, 1993; Leloup et al., 1995; Harrison et al., 1996; Dewey et al., 1989; Molnar and Gipson, 1996; England and Molnar, 1990; Murphy et al., 1997; Rangin et al., 1995; Chung et al., 1997) The peninsula is classically considered as a rigid block but recent * Corresponding author Tel.: 133-0467545926; fax: 133-0467547362 E-mail address: maluski@dstu.univ-montp2.fr (H Maluski) studies (Jolivet et al., 1999) south of the Red River Shear Zone have identi®ed a large metamorphic core complex (the Bu Khang Dome) and also evidence for extension during the Early Miocene A number of structures in Vietnam are known to date back to the Early Triassic (240 Ma, Lepvrier et al., 1997) Other thermotectonic episodes which may have affected the region (e.g during the Cretaceous, Lepvrier et al., 1997; Lacassin et al., 1998) are more obscure, but this may be due to the current paucity of geochronological and ®eld data Thus, to decipher the geodynamic evolution of Indochina it is essential that we understand the timing and interaction between the different phases of deformation and structures In this context we have studied the deformation and exhumation history of a large metamorphic massif, close to the Red River Fault (RRF) The Song Chay Massif is located about 10 km north-east of the Day Nui Con Voi, east of the town of Lao Cai (Fig 1) It is a large domal structure which on ®rst examination 1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd All rights reserved PII: S 1367-912 0(00)00038-9 234 H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 Fig Location map and topography of northern Vietnam The Song Chay Massif is close to the Day Nui Con Voi and the Red River appears similar to the Bu Khang dome, and therefore, may have had a similar history To understand the temporal relationship between this structure, the Red River Shear Zone and Miocene extension found in the Bu Khang Dome south of the fault (Jolivet et al., 1999) we have used a combination of ®eld observation, 40Ar± 39Ar mica dating (Maluski et al., 1999), and apatite ®ssion-track analysis The results are compared with those from the Red River Shear Zone in the Dai Nui Con Voi 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 and 2), in which the NW±SE RRF zone is central Parallel to the active RRF is the Cenozoic Red River Shear Zone The elongate Day Nui Con Voi Dome is bounded by the RRF to the west and by the Song Chay Fault to the east To the west of the RRF, alkaline granites intrude the gneissic Phang Si Pan Massif Our main study area, the Song Chay Massif, is located on the eastern side of the Red River and extends into China It has a dome-like shape, roughly trending in a NE±SW direction and is bounded on its western ¯ank by the Song Chay Fault and on its southern ¯ank by an E±W trending mylonite zone, which on geological maps appears to be H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 235 Fig Geological map (from Geological map Vietnam, 1/200,000) and cross-section of the studied area showing the major structures, small scale structures in the Song Chay Massif as well as the location of samples and the 40Ar± 39Ar ages and ®ssion track ages terminated by the Song Chay Fault The eastern and southeastern limits of the dome correspond to the Lo river valley which also occupies a major fault Sample collection and observations of the structural and deformational history were made along the single road that crosses the dome, from the city of Bac Quang to the villages of Hoang Su Phi and Xin Man Terranes surrounding the dome, to the south and east, consist of greywackes and micaschists to slaty schists overlain by a karstic formation of Cambrian limestones The Ordovician and Silurian are represented by limestones and quartzite, and are unconformably over- lain by Devonian conglomerates, slates and limestones The Permo-Carboniferous is represented by carbonates Deformation in the Song Chay dome We describe a cross-section of the dome from the SE to the NW (Fig 2) The southern limit of the dome is a narrow EW trending fault, which cuts strongly lineated quartzites, micaschists and marbles The foliation is folded into a broad antiform with an axis 236 H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 Fig Photographs of outcrops in the Song Chay Massif showing top-to-the-north kinematic indicators All sections are parallel to the lineation and perpendicular to the foliation (a) Orthogneiss near Huang Xu Phi in the northern part of the section (b)±(d) Orthogneiss from the southern side of the dome Photograph (c) shows a high strain zone slightly oblique on the foliation in the less strained gneiss (lower) The button with the star gives the scale (2 cm) trending NE-SW and is steeper in the southern rim Horizontally foliated orthogneisses and migmatites are found near the core of the antiform, as shown on the cross section, Fig To the North, upper levels of the core are made of biotite and muscovite-bearing orthogneisses containing Kfeldspars several centimeters in length Close to the village of Xin Man, horizontally sheared micaschists are directly overlain by muscovite-bearing marbles that alternate with pelitic schists, considered as Cambrian (Geological Survey of Vietnam, 1999 (Geological map 1/200,000); Tran Van Tri, 1977; Phan Cu Tien et al., 1989) A conspicuous NW- or N-trending stretching lineation is recognised all along the section in orthogneisses and micaschists (Fig 2, map) In the internal parts of the dome the orthogneiss fabric is often constrictional with a strong stretching lineation and a weak planar anisotropy These orthogneisses are not ubiquitously deformed and locally occur in an unfoliated facies with large feldspars in an undeformed groundmass This rock has been considered to be an intrusive granite, but its occurrence suggests to us that it is simply the undeformed equivalent of the orthogneiss Gradients of strain are seen at the scale of tens of meters and a general increase in deformation is observed from the undeformed granite toward the north and south The most intense deformation is observed in the northern part of the section between Xin Man and Huang Su Phi Orthogneisses yield consistent kinematic indicators showing a top-to-the-north or northeast sense of shear (Fig 3) even in regions characterised by constrictional H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 fabrics where the foliation is least visible The most common shear criteria are S±C relations, asymmetric pressure shadows on alkali feldspar, sigmoidal foliation when approaching zone of shear localisation This simple deformation pattern suggests that a nearly horizontal shear zone has been active between the basement and the cover, with a top-to-the-north shear sense, and has been lately folded into a broad antiform Comparable ¯at-lying shear zones on this scale are not common in Vietnam and its age is unknown East of Bac Quang, cordierite±sillimanite±muscovite micaschists and quartzites displaying a N808E-trending foliation and a gently west-dipping lineation occupy the southern rim of the dome Geochronological data The Song Chay Massif and the surrounding area have been relatively unexplored by geochronology: gneisses, schists and migmatites were dated by the U±Pb method, at 2652 and 1000 Ma (Tran Van Tri, 1977; Tran Ngoc Nam, 1997) These Archean U±Pb ages most probably relate to inherited Pb Tugarinov et al (1979) further found a U±Pb zircon and apatite upper intercept age of 625 ^ 20 Ma; and a lower intercept at 30 Ma Nguyen and Dao (1995) published an age of 350 Ma on biotite without information on the dating method More recently, the evolution of this massif was investigated using the Ar±Ar method and the ®rst age data relating to Triassic metamorphism were presented by Maluski et al (1999) The protolith age of the Song Chay orthogneisses was measured by Leloup et al (1999) using the zircon U±Pb method Dated at 428 ^ Ma; this age probably corresponds to the time of emplacement of the protolithic granite The same study also measured a Rb±Sr age and 40Ar± 39Ar mica plateaux ages on a single sample from the southern part of the dome The results gave ages that span a period between 209 ^ and 176 ^ Ma and were interpreted as documenting a Late Triassic shearing event around 210 Ma A K-feldspar 40 Ar± 39Ar age spectrum also suggested a phase of rapid cooling in the late Jurassic 4.1 40Ar± 39Ar results The radiometric 40Ar± 39Ar stepwise heating method was used on pure mineral aliquots Results are presented from the southern cover to the northern one, crossing the whole antiform (Figs 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 Argon isotopic results are given in Table All the samples of orthogneisses and migmatites described here and used for radiometric dating are coarse grained The granulometric fraction used for dating was 160 mm in diameter for micagrains In these conditions the grain-size effect, as mentioned in McDougall and Harrison (1988), is mini- 237 mised, concerning dimension controlling gas loss in diffusive loss conditions Sample VN 322 (Fig 4a) is located in a subvertical shear zone which bounds the dome to the south, (228 24 H 52 HH ; 1048 42 H 55 HH ) It is a sillimanite±cordierite bearing micaschists with ¯exuose biotites and muscovites Muscovite de®nes a very irregular shaped degassing spectrum with increasing ages since 60 Ma for low temperatures up to 234 Ma in the last signi®cant step Intermediate degassing temperatures display an age of 204 Ma This spectrum relates to a closure of the system at an age of 234 Ma, which then suffered a subsequent Ar loss The strong scattering of the 39 Ar/ 40Ar ratios, is also re¯ected in the isochron diagram normalised to 40Ar, in which no linear array can be de®ned Sample VN 324 (Fig 4b) is a typical orthogneiss from the southern rim of the dome (228 29 H 41 HH ; 1048 51 H 43 HH ) Its mineralogical content is quartz, K-feldspar and biotite, with very few muscovites Micas are oriented in the foliation and present the shape of late to post deformational minerals The age spectrum of the muscovite does not de®ne a plateau age but displays, for 90% of released 39Ar, increasing ages from 73 Ma to a ®rst integrated age of 228 ^ Ma; and a second at 236 ^ 0:5 Ma: As for the previous sample, this mineral suffered inhomogeneous Argon loss, which affects mainly low temperature degassing sites For this sample, the isochron plot does not reveal a well-de®ned straight line Sample VN 329 (Fig 4c) is a ®ne-grained gneiss with quartz, plagioclase, K-feldspar, coarse biotites and few muscovites (228 32 H 26 HH ; 1048 49 H 24 HH ) 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 ^ 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 ^ Ma; identical to the one displayed by the integrated plateau age Sample VN 333 (Fig 4d) is a migmatitic gneiss to the west of Wang Xu Phy village (228 44 H 39 HH ; 1048 38 H 02 HH ) It contains quartz, plagioclase, muscovite and biotite Micas develop in the foliation and appear to have formed syn- to post-deformation The biotite of this sample yields a wellde®ned plateau age at 166 ^ Ma for near 95% of the 39Ar released The closure of the mineral vs Ar occurred at that time, without subsequent reopening of the system An identical age of 166 ^ Ma is obtained through the isochron diagram, with an intercept on the Y-axis de®ning an atmospheric 40Ar/ 36Ar ratio Sample VN 335 (Fig 4e and f) was taken km east of Xin Man village (Fig 1) It is a ®ne-grained orthogneiss, from the northernmost part of the dome It is composed of quartz, plagioclase, biotite and muscovite Mica¯akes are developed in the foliation, with undeformed shapes Muscovites and biotites give, respectively, 164 ^ Ma and 176 ^ Ma: For muscovite, the plateau age is calculated over 60% 238 H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 300 300 234±0.8 Ma 200 204±1 Ma 150 100 S SONG CHAY 60 Ma 50 0 50 39 % Ar cumulative 150 SONG CHAY VN 324 MUSCOVITE 50 0 100 50 39 % Ar cumulative a 100 b 300 300 250 250 > 201±2 Ma AGE (Ma) < AGE (Ma) 200 100 VN 322 MUSCOVITE 228±1 Ma 250 AGE (Ma) AGE (Ma) 250 200 150 SONG CHAY 100 VN329 BIOTITE 50 0 50 39 % Ar cumulative 200 166±2 Ma < > 150 100 SONG CHAY 50 VN333 BIOTITE 0 100 50 39 % Ar cumulative c 100 d 200 300 167±2Ma 250 200 < 176±2 Ma AGE (Ma) AGE (Ma) 150 > 150 100 < 0 SONG CHAY SONG CHAY VN 335 MUSCOVITE VN335 BIOTITE 50 39 % Ar cumulative > 100 50 50 164±2 Ma 0 100 e 50 39 % Ar cumulative 100 f 300 250 198±2 Ma AGE (Ma) < > 200 150 100 SONG CHAY 50 VN 337 MUSCOVITE 0 100 50 % 39 Ar cumulative g Fig 40Ar± 39Ar age spectra from the Song Chay Massif of 39Ar released The last three signi®cant steps reveal an integrated age slightly older than the previous one at 167 ^ Ma: The whole pattern of this age spectrum attests to an Ar diffusion loss, resulting in younger ages in low extraction temperatures (96, 143, 160 Ma) The plateau therefore, would re¯ect radiogenic 40Ar loss, less pronounced on the more retentive sites, resulting in the last old age of 167 Ma The result obtained on biotite is somewhat surprising because the closure temperature of biotite is lower than for muscovite Even if this value is not precisely known 50 100 40 80 AGE (Ma) AGE (Ma) H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 30 33.1±0.8 Ma < > 20 10 0 20 40 60 < 40±1 Ma 60 80 % Ar cumulative > 40 20 VN 106 MUSCOVITE 239 0 100 VN 107 BIOTITE 20 40 39 a 60 80 39 % Ar cumulative 100 b 50 AGE (Ma) 40 30 < 24.1±1 Ma > 20 10 VN 110 MUSCOVITE 0 50 39 % Ar cumulative 100 c Fig 40Ar± 39Ar age spectra from the Red River Shear Zone (values differ slightly according to different authors; Harrison et al., 1985; McDougall and Harrison, 1988; Hames and Bowring, 1995), we should expect a younger age for the biotite than for the muscovite An excess Ar component may be suspected in this biotite, in reference with the age of the muscovite It means that if such a component occurs in the biotite, its distribution is nearly homogeneous on the whole sites of the mineral, and results in an increase of age of 12 Ma, vs the coexisting muscovite For both samples, the extreme clustering of data prevents de®nition of a wellde®ned isochron, especially for the Y intercept value, connected with the 40Ar/ 36Ar ratio Sample VN 337 (Fig 4g) is located in the northern cover of the crystalline core, represented by muscovite bearing marbles, close to Xin Man village The foliation of the marble is very slight, being underlined by very thin muscovite layers, clearly visible under the microscope Muscovites give a well-de®ned plateau age at 198 ^ 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 Summary of Ar±Ar ages of analysed minerals in the Song Chay Massif Sample no Plateau age (Ma) Isochron age (Ma) VN322 MUSCOVITE VN324 MUSCOVITE VN329 BIOTITE VN333 BIOTITE VN335 MUSCOVITE VN335 BIOTITE VN337 MUSCOVITE 236 ^ 0.5 201 ^ 166 ^ 164 ^ 176 ^ 198 ^ 200 ^ 166 ^ 160 ^ 176 ^ 195 ^ Step age (Ma) Total age (Ma) 234 ^ 0.8 204 ^ 60 ^ 228 ^ 208 ^ 167 ^ 230 ^ 200 ^ 165 ^ 1.7 163 ^ 1.7 174 ^ 194 ^ 240 H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 Table Ar isotopic results for analysed minerals Correction interference used for 36Ar/ 37ArCa is 2:93 £ 1024 : Mass discrimination correction factor is calculated for a 40 Ar/ 36Ar ratio of 291 Temperature (8C) 40 Ar p/ 39Ar 36 Ar/ 40Ar 37 Ar/ 39Ar % Atm % 39Ar Age ^ 1sd VN322 MUSCOVITE (J ˆ 0.018342) 500 1.931 550 2.336 600 3.228 650 4.057 700 4.84 750 5.586 800 6.739 850 6.552 900 6.32 950 6.722 1000 7.129 1050 7.386 1100 7.551 1150 7.276 1.18 0.295 0.195 0.084 0.096 0.101 0.092 0.156 0.074 0.072 0.067 0.074 0.064 0.316 0.015 0.019 0.008 0.006 0.006 0.005 0.002 0.002 0.004 0.004 0.006 0.004 0.004 0.037 34.8 8.7 5.7 2.4 2.8 2.7 4.6 2.2 2.1 1.9 2.1 1.9 9.3 0.6 1.2 2.4 7.2 12.8 26.8 41.4 50.2 57.7 65.5 83.5 98.5 99.9 62.79 ^ 20.36 75.71 ^ 19.90 103.77 ^ 14.30 129.49 ^ 8.17 153.45 ^ 18.32 175.99 ^ 2.63 210.28 ^ 98 204.77 ^ 98 197.88 ^ 1.51 209.78 ^ 1.97 221.72 ^ 1.53 229.23 ^ 78 234.02 ^ 79 226.01 ^ 8.76 Total age ˆ 208.6 ^ 2.1 VN324 MUSCOVITE (J ˆ 0.018342) 500 2.268 550 4.507 600 6.189 650 6.234 700 6.742 750 7.008 800 7.383 900 7.334 950 7.400 1000 7.505 1050 7.619 1100 7.598 1150 7.581 2.062 1.007 0.067 0.142 0.210 0.144 0.111 0.083 0.090 0.070 0.056 0.116 0.160 0.021 0.015 0.008 0.005 0.005 0.003 0.002 0.001 0.001 0.001 0.001 0.003 0.012 60.9 29.7 4.1 6.2 4.2 3.3 2.4 2.6 1.6 3.4 4.7 0.3 0.6 1.1 2.1 4.2 8.4 22.6 34.9 47.5 59 92.4 97.9 99.9 73.54 ^ 45.90 143.3 ^ 47.04 193.99 ^ 26.68 195.32 ^ 14.45 210.35 ^ 6.41 218.17 ^ 3.17 229.13 ^ 1.02 227.72 ^ 1.39 229.63 ^ 1.30 232.68 ^ 1.40 236.00 ^ 54 235.38 ^ 2.54 234.90 ^ 8.28 Total age ˆ 230.0 ^ 2.2 VN329 BIOTITE (J ˆ 0.018342) 500 3.023 550 5.826 600 6.248 650 6.451 700 6.479 750 6.445 800 6.395 850 6.501 900 6.394 995 6.421 1050 6.560 1100 6.570 1150 6.748 1.057 0.304 0.114 0.065 0.052 0.069 0.055 0.307 0.102 0.090 0.074 0.086 0.695 0.058 0.007 0.004 0.001 0.002 0.005 0.013 0.015 0.011 0.004 0.002 0.004 0.059 31.2 3.3 1.9 1.5 1.6 2.6 2.1 2.5 20.5 3.7 11 31.1 48.7 56.7 60.2 64.1 70.8 84 90.7 98.3 99.9 97.36 ^ 21.27 183.17 ^ 8.28 195.76 ^ 3.42 201.76 ^ 1.16 202.6 ^ 1.29 201.58 ^ 2.69 200.11 ^ 55 203.23 ^ 6.63 200.08 ^ 90 200.88 ^ 1.70 205 ^ 3.41 205.29 ^ 210.55 ^ 14.25 Total age ˆ 200.3 ^ 2.0 VN333 BIOTITE (J ˆ 0.018342) 550 4.466 600 5.221 650 5.24 700 5.263 750 5.261 800 5.237 850 5.242 900 5.288 950 5.256 995 5.288 1050 5.242 1100 5.27 0.564 0.088 0.128 0.049 0.099 0.104 0.164 0.130 0.090 0.044 0.120 0.114 0.025 0.007 0.002 0.002 0.003 0.014 0.039 0.018 0.011 0.01 0.02 0.034 16.6 2.6 3.8 1.4 2.9 3.0 4.8 3.8 2.6 1.3 3.5 3.3 2.1 7.2 19.5 38.8 52.2 56.6 60.8 68.1 78.7 86.7 94.6 99.2 142.04 ^ 7.18 164.99 ^ 2.78 165.57 ^ 1.14 166.26 ^ 81 166.22 ^ 1.15 165.48 ^ 3.18 165.64 ^ 3.26 167.03 ^ 1.92 166.06 ^ 1.39 167.03 ^ 1.70 165.63 ^ 1.81 166.49 ^ 3.70 H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 241 Table (continued) Temperature (8C) 40 Ar p/ 39Ar 1150 4.792 36 Ar/ 40Ar 37 Ar/ 39Ar % Atm % 39Ar Age ^ 1sd 1.436 0.031 42.4 99.9 152.01 ^ 20.29 Total age ˆ 165.4 ^ 1.70 VN335 BIOTITE (J ˆ 0.018342) 450 2.426 500 0.824 550 3.352 600 4.336 650 5.121 700 5.225 750 5.320 800 5.361 850 5.496 950 5.565 995 5.594 1050 5.593 1100 5.719 1150 5.886 1.348 1.146 0.666 0.569 0.040 0.061 0.228 0.142 0.177 0.071 0.067 0.054 0.078 0.192 0.037 0.000 0.031 0.022 0.014 0.006 0.007 0.006 0.003 0.002 0.002 0.001 0.004 0.023 39.8 33.8 19.6 16.8 1.2 1.8 6.7 4.2 5.2 2.1 1.9 1.6 2.3 5.6 0.1 0.3 0.5 0.9 1.7 3.1 6.5 11 22.2 38.6 49.7 90.7 96.9 100 78.56 ^ 92.24 27.06 ^ 139.99 107.67 ^ 76.70 138.06 ^ 43.06 161.98 ^ 19.55 165.12 ^ 11.87 167.99 ^ 5.44 169.22 ^ 3.80 173.29 ^ 1.64 175.36 ^ 1.14 176.23 ^ 1.41 176.20 ^ 42 179.99 ^ 2.92 184.98 ^ 5.33 Total age ˆ 174.7 ^ 1.80 VN335 MUSCOVITE(J ˆ 0.018342) 450 4.081 500 2.981 550 4.521 600 5.076 650 5.162 700 5.211 750 5.217 800 5.149 850 5.171 900 5.205 950 5.137 1000 5.341 1050 5.281 1100 5.279 1150 5.083 1200 4.879 1.944 1.076 0.360 0.148 0.072 0.048 0.050 0.086 0.056 0.079 0.069 0.073 0.040 0.029 0.081 0.030 0.042 0.020 0.005 0.001 0.001 0.000 0.001 0.004 0.007 0.004 0.004 0.002 0.011 0.003 0.009 0.008 57.4 31.8 10.6 4.3 2.1 1.4 1.4 2.5 1.6 2.3 2.0 2.1 1.1 0.8 2.4 0.9 0.3 1.2 3.1 7.9 19.9 41.5 56.2 60.8 64.4 73.8 75.0 87.8 93.2 98.0 99.0 99.9 130.23 ^ 40.60 96.06 ^ 17 143.75 ^ 8.36 160.6 ^ 2.83 163.22 ^ 1.36 164.68 ^ 79 164.89 ^ 1.20 162.81 ^ 85 163.5 ^ 5.20 164.5 ^ 2.01 162.45 ^ 3.30 168.62 ^ 1.15 166.81 ^ 3.25 166.76 ^ 3.03 160.82 ^ 3.89 154.65 ^ 14.13 Total age ˆ 163.6 ^ 1.7 VN337 MUSCOVITE (J ˆ 0.018342) 500 0.957 600 3.692 700 5.551 750 5.662 800 5.988 850 6.284 900 6.395 950 6.316 995 6.264 1050 6.310 1100 6.308 1150 6.364 2.652 0.922 0.109 0.091 0.064 0.057 0.082 0.079 0.062 0.064 0.072 0.152 2.188 7.948 1.845 0.011 0.008 0.005 0.007 0.007 0.003 0.003 0.002 0.004 78.3 27.2 3.2 2.7 1.8 1.6 2.4 2.3 1.8 1.9 2.1 4.5 0.2 0.7 4.1 9.6 16.1 25.4 31.7 39.8 51.8 68.3 92.7 99.9 31.41 ^ 89.35 118.21 ^ 42.14 174.94 ^ 5.53 178.26 ^ 3.88 188.01 ^ 2.80 196.81 ^ 2.00 200.11 ^ 3.02 197.77 ^ 2.45 196.22 ^ 1.62 197.60 ^ 1.37 197.54 ^ 1.00 199.19 ^ 2.57 Total age ˆ 194.3 ^ 2.0 VN106 MUSCOVITE (J ˆ 0.012158) 450 0.711 500 1.311 550 2.800 600 1.304 650 1.544 700 1.390 750 1.528 800 1.492 850 1.546 3.100 1.962 1.765 1.339 1.295 0.533 0.063 0.159 0.177 0 0.007 0 0 0 91.6 58 52.1 39.5 38.2 15.7 1.8 4.7 5.2 0.3 0.5 0.7 1.6 2.4 4.7 9.6 15.2 27.7 15.54 ^ 40.08 28.53 ^ 49.21 60.41 ^ 59.51 28.38 ^ 13.52 33.56 ^ 15.54 30.23 ^ 4.69 33.22 ^ 2.45 32.45 ^ 1.86 33.61 ^ 77 242 H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 Table (continued) Temperature (8C) 40 Ar p/ 39Ar 900 950 1000 1100 1400 1.469 1.520 1.522 1.546 1.543 36 Ar/ 40Ar 37 Ar/ 39Ar % Atm % 39Ar Age ^ 1sd 0.272 0.149 0.157 0.056 1.287 0 0 0.001 4.4 4.6 1.6 38 41.9 57.1 68.8 86 100 31.95 ^ 57 33.03 ^ 77 33.10 ^ 90 33.61 ^ 72 33.53 ^ 1.18 Total age ˆ 32.9 ^ 0.8 VN107 BIOTITE (J ˆ 0.012158) 450 3.174 500 0.768 550 1.548 600 1.823 650 1.851 700 1.839 750 1.839 800 1.848 850 1.831 900 1.936 950 1.754 1000 1.708 1100 2.260 1400 3.748 3.324 3.341 3.258 2.996 1.808 0.483 0.171 0.219 0.409 0.508 0.557 0.344 0.557 2.532 0.038 0.036 0.034 0.007 0.003 0.002 0.002 0.003 0.040 0.296 0.090 0.043 0.093 20.203 98.2 98.7 96.2 88.5 53.4 14.2 5.0 6.4 12.1 15.0 16.4 10.1 16.4 74.8 0.2 0.5 1.9 7.9 24.4 54.3 79.8 85.3 88.2 90.9 94.6 96.4 99.3 100 68.31 ^ 82.93 16.78 ^ 51.80 33.66 ^ 7.83 39.56 ^ 1.81 40.15 ^ 59 39.90 ^ 39 39.91 ^ 43 40.10 ^ 1.90 39.72 ^ 3.79 41.97 ^ 4.88 38.07 ^ 3.63 37.08 ^ 5.41 48.91 ^ 4.07 80.39 ^ 16.67 Total age ˆ 40.3 ^ VN110 MUSCOVITE(J ˆ 0.012158) 450 0.374 500 0.115 550 1.014 600 1.437 650 1.235 700 1.143 750 1.077 800 1.117 850 1.104 900 1.121 950 1.102 1000 1.008 1100 1.291 1400 1.061 3.023 3.541 1.769 0.307 0.418 0.327 0.435 0.328 0.317 0.323 0.335 0.636 1.329 2.404 0.014 0.014 0.015 0.007 0.007 0.008 0.006 0.006 0.005 0.004 0.002 0.001 0.002 0.008 89.3 100 52.3 9.0 12.3 9.3 12.8 9.7 9.3 9.5 9.8 18.8 39.2 71.0 1.3 2.5 4.2 6.4 10.3 16.4 24.4 34.5 46.7 56.9 63.9 70.1 79.7 100 8.20 ^ 11.89 2.52 ^ 13.77 22.12 ^ 11.35 31.26 ^ 8.43 26.90 ^ 4.96 24.90 ^ 3.07 23.48 ^ 2.80 24.34 ^ 1.67 24.05 ^ 1.49 24.44 ^ 1.74 24.03 ^ 2.50 21.97 ^ 2.63 28.10 ^ 2.28 23.13 ^ 1.18 Total age ˆ 23.9 ^ 0.9 Sample VN 106 (Fig 5a) is a quartzite occurring close to the Pho Lu city, on the Red River A very strong lineation occurs in these rocks, which exhibit an E±W foliation It contains layers of ®ne grained muscovites and biotites underlining the foliation The muscovite displays a plateau de®ned for near 90% of 39Ar released at 33:1 ^ 0:8 Ma: Sample VN 107 (Fig 5b) is a mylonitic orthogneiss with a N130 vertical foliation from the road section between Lao Cai and Sa Pa Plagioclase is partly transformed with sericites Intersticial muscovites occur in the matrix A biotite yields an age of 40 ^ Ma for 90 % of 39Ar Sample VN 110 (Fig 5c) was taken near Bao Yen on the border of the Dai Nui Con Voi massif This is a ®ne grained gneiss with a developed N15 trending lineation Muscovites are coarse grained, with ®sh-like shapes Very ®ned grained biotites and plagioclase occur, with garnets and tourmalines An age of 24 ^ Ma was obtained on a muscovite for near 60 % of released argon 4.2 Fission-track data Apatite ®ssion-track analysis was undertaken on samples from the Song Chay Massif and RRF zone, to complement the argon data-set and constrain the low temperature cooling history The sensitivity of the system to closure at low temperatures (,60±1108C) enables detection of weak (in magnitude) cooling events that may not be otherwise detected by higher temperature methods The results and sample locations are given in Table Sample preparation and analysis followed procedures given in Storey et al (1996) with samples irradiated in the thermal facility of the Risù Reactor, National Research Centre, Rosklide, Denmark, (cadmium ratio for Au 200±400†; using Corning glass CN-5 as a neutron dosimeter Counting and track length measurements used a microscope total magni®cation of 1250 £ with a 100 £ dry objective Central ages were calculated using the IUGSrecommended zeta calibration approach (Hurford, 1990) H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 For the Song Chay Massif samples (Table 3), data quality is mixed Although adequate numbers of individual grain ages have been measured for all samples, track length measurement was affected by low spontaneous track densities Thus, only six samples (VN 9801, 9805, 9807, 9811, 9812 and 9814) yielded adequate numbers of horizontally con®ned tracks to suitably de®ne length distributions Nevertheless, given the similarity within the data-set, between central ages and mean track lengths, it is reasonable to infer that similar thermal histories were experienced by those samples which did not contain adequate numbers of con®ned tracks Central (modal) ages range between 16 ^ Ma and 24 ^ Ma; with mean track lengths (for samples with more than 50 measurements), between 13:60 ^ 0:31 mm and 14:12 ^ 0:15 mm: Qualitatively, the relatively long mean track lengths suggest that cooling through the apatite partial annealing zone (,110±608C) was relatively rapid The cooling paths may be further constrained by modelling utilising the procedure of Gallagher (1995) This is a probabilistic approach that predicts thermal histories from within speci®ed time-temperature bounds Each thermal history is used to predict ®ssion-track parameters which are quantitatively compared with observed values and ranked according to goodness of ®t Maximum likelihood is used in order to compare each individual observation Only those samples with statistically well-de®ned length distributions were modelled i.e VN 9801, 9805, 9807, 9811, 9812 and 9814 and representative plots are shown in Fig The modelled results show the portion of a samples thermal history (between ,60 and 1108C) that is constrained by the ®ssion-track data Any variation in temperature below ,608C is unresolvable (highlighted by the grey shading and dashed time±temperature path) The shaded areas surrounding the constrained time±temperature paths (solid line) represent the 95% con®dence regions The oldest track recorded in each sample correlates approximately with the time at which tracks ®rst began to be retained within an apatite crystal lattice as the sample cooled through the 1108C isotherm For samples VN 9811, 9812 and 9814 this took place between 20 and 21 Ma, and for samples VN 9807, 9805 and 9801, between 22 and 28 Ma Table summarises the main time±temperature information extracted from the modelled cooling data A plot of sample location against ®ssion-track central age (Fig 7) suggests a possible trend of increasing age to the south-east This is seen more clearly in the modelling which shows the older ages record an earlier cooling than samples to the north-west There is no evidence for a systematic correlation between age and elevation as would be expected from a terrain that experienced a slow to moderate uniform rate of denudation i.e the cooling pattern is not caused by variable depths of erosion But, it is interesting to note that the sample 243 which cooled at the fastest rate between 110 and 608C (sample VN 9807), comes from the maximum elevation (at ,800 m) Regionally cooling for most samples occurred at a similar rate (within experimental uncertainties), to between and 58C/Myr Samples from the RRF were also analysed to complement the new argon data Some of the apatite samples from this region were dif®cult to analyse because of lower than normal uranium concentrations (often ,5 Uppm) and this affected the quality of some of the track length data Nevertheless the resultant data-set is of suitable quality to provide meaningful constraints on the regions cooling/exhumation history Samples VN 9818±9821 are from the road section between Lao Cai and Sapa along which the argon sample VN 107 was also collected The four samples range in central age from 37 ^ Ma to 27 ^ Ma: Track lengths for those samples with adequate numbers of measurements range from 13:72 ^ 0:27 mm to 14:31 ^ 0:14 mm; and are consistent with moderately rapid cooling Ð hence the ages approximate to the time of cooling Sample age and lengths show no correlation with elevation, and therefore, the age distribution is unrelated to simple uniform erosional denudation The ®ssion-track data from the undeformed granites (VN 9824±9827) adjacent to the main Phan Si Pang granite, west of Sapa give central ages between 32 ^ and 30 ^ Ma: Sample VN 9827 has suitable numbers of measured con®ned tracks that comprise a mean length of 14:18 ^ 0:14 mm; consistent with rapid cooling The similarity among the four data suggest they experienced the same thermal history Two samples (VN 9846 and VN 9848) were analysed from locations near the town of Bao Yen close to the edge of the Day Nui Con Voi These gave similar central ages 22 ^ Ma and 18 ^ Ma and both have mean track lengths longer than 14 mm indicative of rapid cooling Interpretation and discussion The 40Ar± 39Ar and ®ssion-track data-sets from the Song Chay Massif are signi®cantly different in age, and therefore, relate to different aspects of the regions geodynamic evolution Due to the different closure temperature for Ar and FissionTtrack systems (350±4008C vs 608C for exhumational FT cooling), it is possible to recognise both Mesozoic and Cenozoic events in the Song Chay Massif We now discuss the signi®cance of these ages The geographic distribution of the 40Ar± 39Ar results shows ages that are younger in the central part of the dome Muscovite and biotite from the southernmost samples, VN322 and VN324, record ages of 234 and 236 Ma, respectively, corresponding to the last increments of experimental degassing A similar range of ages can be found throughout Vietnam (Lepvrier et al., 1997); the Song 244 Table Fission track apatite analytical data for the Song Chay Massif Notes: (i) Track densities are ( £ 10 tr cm 22) numbers of tracks counted (N) shown in brackets; (ii) Analyses by external detector method using 0.5 for the 4p/2p geometry correction factor; (iii) Ages calculated using dosimeter glass CN-5; analyst Carter zCN5 ˆ 339 ^ 5; (iv) Central age is a modal age, weighted for different precisions of individual crystals Sample Long Latt Elevation (m) No grains rd Nd rs Ns rI Ni % R.E Central Age (Ma) Mean track length (mm) S.d (mm) 1.31 0.99 86 11 1.70 1.63 1.43 1.66 1.42 1.36 1.35 1.42 31 81 83 19 90 79 30 59 Tracks measured 104.51.74 104.51.49 104.50.32 104.50.10 104.49.34 104.48.77 104.29.38 104.32.07 104.35.68 104.37.29 104.42.20 104.46.89 115 150 410 580 750 800 391 365 395 470 480 720 20 20 20 20 20 20 16 18 20 20 17 20 1.481 1.482 1.484 1.485 1.487 1.490 1.490 1.492 1.493 1.495 1.497 1.498 4159 4159 4159 4159 4159 4159 4159 4159 4159 4159 4159 4159 0.135 0.365 0.053 0.098 0.121 0.515 0.034 0.435 0.094 0.221 0.119 0.057 218 664 94 206 163 116 44 329 111 442 191 121 1.496 4.106 0.585 1.139 1.246 0.671 0.543 5.704 1.262 3.045 1.620 0.896 2418 7473 1039 2399 1682 1511 709 4309 1496 6086 2585 1913 15.4 13.3 0.04 3.8 0.33 19.7 20.0 22.4 0.75 0.8 17.9 22.9 23 ^ 22 ^ 23 ^ 22 ^ 24 ^ 20 ^ 16 ^ 19 ^ 19 ^ 18 ^ 19 ^ 17 ^ 13.93 ^ 0.14 13.51 ^ 0.31 No data 13.60 ^ 0.31 13.14 ^ 0.18 14.01 ^ 0.16 13.39 ^ 0.39 13.41 ^ 0.15 14.12 ^ 0.15 13.81 ^ 0.25 13.65 ^ 0.19 No data Lao Cai to Sapa VN9818 22.26.29 VN9819 22.25.64 VN9820 22.24.40 VN9821 22.22.21 103.55.50 103.55.01 103.54.06 103.52.12 595 730 900 1285 20 20 20 20 1.586 1.263 1.586 1.586 8792 7004 8792 8792 0.052 0.086 0.082 0.131 89 131 110 127 0.517 0.492 0.781 1.275 897 749 1045 1241 10.1 29.1 21.7 24.9 27 ^ 37 ^ 30 ^ 29 ^ 13.72 ^ 0.27 13.75 ^ 0.27 13.77 ^ 0.18 14.31 ^ 0.14 2.38 1.98 1.48 0.98 80 54 69 53 West of Sapa VN9824 22.21.24 VN9825 22.21.30 VN9826 22.21.68 VN9827 22.21.59 103.45.92 103.46.47 103.45.86 103.45.26 2200 2000 1905 1670 11 21 16 20 1.263 1.263 1.263 1.586 7004 7004 7004 8792 0.175 0.098 0.015 0.125 70 80 94 192 1.245 0.691 1.013 1.150 497 561 639 1772 25.6 11.3 28.2 32 ^ 31 ^ 32 ^ 30 ^ No data 13.73 ^ 0.45 14.98 ^ 0.21 14.18 ^ 0.14 1.49 0.83 1.38 12 17 103 Bao Yen VN9846 VN9848 104.23.52 104.26.88 320 260 14 20 1.263 1.263 7004 7004 0.335 0.282 176 287 3.259 3.282 1712 3343 24.5 0.0 22 ^ 18 ^ 14.24 ^ 0.16 14.41 ^ 0.14 1.14 0.92 51 45 22.11.94 22.13.65 H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 Song Chay Massif VN9801 22.29.74 VN9802 22.29.77 VN9803 22.31.68 VN9804 22.32.04 VN9805 22.32.50 VN9807 22.32.94 VN9810 22.41.80 VN9811 22.43.05 VN9812 22.44.75 VN9813 22.44.89 VN9814 22.43.97 VN9815 22.35.01 H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 0 Ma Ma Ma 25 Ma 40 60 VN 9811 20 ¡C 28 ¡C 41 ¡C 94 ¡C ? Ma Ma 20 Ma 21 Ma 40 T(¡C) 20 T(¡C) VN 9801 60 15 ¡C 45 ¡C 84 ¡C 122 ¡C ? 80 80 PAZ 100 PAZ 100 120 120 35 28 21 14 35 28 21 Time (Ma) N 10 P(K-S) = 0.551 P(Chi) = 0.997 N 10 10 11 12 13 14 15 16 17 18 19 20 Track Length (microns) 10 11 12 13 14 15 16 17 18 19 20 Track Length (microns) VN 9805 20 Ma Ma Ma 28 Ma 40 60 12 ¡C 60 ¡C 34 ¡C 107 ¡C 20 ? 40 T(¡C) T(¡C) P(K-S) = 0.970 P(Chi) = 0.994 Obs Age : 19.1 Ma Pred Age : 19.2 Ma Obs Mean length : 13.56 Pred Mean length: 13.57 Obs S.D : 1.49 Pred S.D : 1.33 Oldest track (Ma) : 21 80 PAZ 100 60 VN 9814 ? Ma 10 ¡C 13 Ma 69 ¡C 21 Ma 109 ¡C 80 PAZ 100 120 120 35 28 21 14 Time (Ma) 35 28 21 Time (Ma) 14 20 30 10 20 Obs Age : 22.5 Ma Pred Age : 22.5 Ma Obs Mean length : 13.92 Pred Mean length: 13.83 Obs S.D : 1.31 Pred S.D : 1.33 Oldest track (Ma) : 25 20 N 14 Time (Ma) 30 20 245 Obs Age : 24.3 Ma Pred Age : 24.3 Ma Obs Mean length : 13.14 Pred Mean length: 13.14 Obs S.D : 1.62 Pred S.D : 1.48 Oldest track (Ma) : 28 P(K-S) = 0.863 P(Chi) = 0.999 N 10 10 11 12 13 14 15 16 17 18 19 20 Track Length (microns) P(K-S) = 0.901 P(Chi) = 1.000 Obs Age : 18.7 Ma Pred Age : 18.7 Ma Obs Mean length : 13.64 Pred Mean length: 13.54 Obs S.D : 1.42 Pred S.D : 1.43 Oldest track (Ma) : 21 10 11 12 13 14 15 16 17 18 19 20 Track Length (microns) Fig Modelling of ®ssion track data Ma complex to the west, the central and southern Truong Son Belt, and the Kontum massif, all reveal metamorphic ages ca 240±245 Ma These ages are found on syn- to late-kinematic minerals and testify to the widespread in¯uence of Triassic metamorphism in this southeastern part of Asia Evidence for this orogen can also be found in southern China and Thailand (Mitchell, 1986; Hutchison, 1989; Arhendt et al., 1993; Dunning et al., 1995; Faure et al., 1996; Mickein, 1997) The ages of 234±236 Ma obtained on two muscovites from the outer rim of the dome are slightly younger than the average age obtained from the Indosinian massifs located within the north-south Truong Son Belt in Vietnam, but they are similar to the age range found in south-west China (Faure et al., 1996) and Thailand (Dunning et al., 1995; Mickein, 1997), and therefore, we consider 234±236 Ma to record Triassic tectonometamorphism 246 H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 Table Summary of time±temperature constraints and sample cooling rates derived from the modelled ®ssion track data Sample Central age (Ma) Oldest track (time crossed 1108C isotherm) Approx time crossed (608C isotherm) Cooling rate for temperature interval 110±608C (8C/Myr) VN VN VN VN VN VN 23 24 20 19 19 19 25 28 22 21 20 21 10 17 17 10 11 11 3.3 4.5 10 4.5 5 9801 9805 9807 9811 9812 9814 Although ages decrease through 200 Ma, to165 Ma in the northern region, we infer that the whole granitic protolith, which intruded at 428 ^ Ma (Leloup et al., 1999), was sheared during the Indosinian The deformation history is Elevation (m) 1000 800 600 400 200 22.55 Latitude North 22.5 22.45 22.4 22.35 22.3 South 22.25 104.7 Longitude East 104.6 104.5 104.4 104.3 West 104.2 10 15 20 25 30 Central Age (Ma) Fig Plots of ®ssion-track data against sample location and elevation Error bars are ^1sigma very homogeneous all along the pro®le and we recognise a single ductile tectonic event The evolution seen in micas ages along the pro®le is most probably the result of a slow doming after development of the Indosinian foliation rather than a succession of events The rims of the massif crossed the 450±3008C isotherms during or after the end of Indosinian orogenic episode, and have remained above this isotherm since that time Intermediate zones crossed these isotherms later, at 200 Ma, as inferred by sample VN 329, consistent with a moderately slow exhumation The youngest sampled level, crossed the same isotherms much later, at ca 165 Ma Since biotite VN 333 and muscovite VN335 record the same age (166 and 164 Ma, respectively), it is probable that the cooling rate, at this late stage, increased The pattern of ages could be explained by a thermal diffusion effect, perhaps associated with a magmatic body emplaced deeply under the core structure of the dome This would imply that 165 Ma of VN 333 and VN 335 is a maximum age (oldest), related to partial loss of radiogenic 40 Ar from the samples exhumed from the deeper crustal levels Sample VN 322 is important, in relation to the late evolution of the southern part of the Song Chay Massif The cordierite±sillimanite bearing schist constitutes the southernmost limit of the dome complex It corresponds to a vertical E±W oriented mylonitic band Muscovites occur in the foliation plane and constitute a strongly developed lineation dipping 308 to N 240 The trend of the age spectrum attests to argon loss by diffusion processes, the ®rst age obtained, 60 Ma being younger than the one obtained at the end of the degassing procedure (234 Ma) The age of 60 Ma can be considered as an older limit for the late deformationmetamorphism responsible for the development of this mylonitic band and for the rejuvenation of the age of these older muscovites This age of 60 Ma, is distinctly older than Cenozoic ages between 35 and 25 Ma displayed by mylonitic gneisses along the RRF Zone, which cross cuts this E±W structure In contrast to the argon data the ®ssion-track results clearly show the region was affected by Cenozoic tectonism The time interval over which the Song Chay experienced rapid cooling (28±20 Ma) coincides with the main phase of shear heating and sinistral movement along the Red River Shear Zone, and therefore, it is probable these two events H Maluski et al / Journal of Asian Earth Sciences 19 (2001) 233±248 are related As stated in Section the ®ssion-track data also show a well developed geographical trend but there is no correlation between age and altitude which suggests that simple erosion is not responsible for the distribution of ages The gradient of ages from north to south (Fig 7) suggests a northward tilt of the Song Chay block with an older exhumation in the south This asymmetry would be consistent with block tilting perhaps caused by reactivation of bounding faults, a process that occurs isostatically after normal faulting (e.g Jackson and McKenzie, 1989) The temporal relationship between the Red River Shear Zone and late stage exhumation of the Song Chay Massif is now explored further through new argon and ®ssion-track results Sample VN 110 was collected from the Day Nui Con Voi and records a muscovite Ar±Ar age of 24 ^ Ma: Two ®ssion-track samples from the same area record central ages of 18 ^ and 22 ^ 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 ^ Ma; whilst ®ssion-track data from the same area (Table 3) record an age range between 27 ^ Ma and 37 ^ Ma: Track lengths for these samples are between 13:72 ^ 0:27 mm and 14:31 ^ 0:14 mm consistent with moderate to rapid cooling, con®rmed by modelling the better quality data The Oligocene cooling recorded by both the argon and ®ssion-track data in this area thus relates to a cooling event associated with early development of the RRF system a period that so far, is poorly constrained by high temperature geochronology West of Sapa is the Phan Si Pang granite body (10±15 km wide and up to 140 km long) This has previously been dated using K±Ar methods to between 41 and 58 Ma (Phan Cu Tien, 1977) however recent 40Ar/ 39Ar dating of phlogopite and biotite from the granite and fault bounded metamorphic rocks show that rapid cooling from temperatures 3008C occurred at ,34 Ma, an age that indicates much younger emplacement (Leloup et al., 1997) This age is identical (within error) to the ®ssion-track ages …30 ^ and 32 ^ Ma† measured on undeformed granites adjacent to the main granite body Such concordancy records geologically instantaneous cooling and is consistent with the ®ssion-track length data (Table 3) This evidence, suggests early fault movement coincided with emplacement of the Phan Si Pang granite, consistent with the observations of Leloup et al (1997) The data from the RRF area show evidence for two phases of cooling during the Cenozoic The early phase occurred during the Oligocene and is associated with emplacement of the Phan Si Pang granite The later phase is restricted to the main shear zone along the Day Nui Con Voi and occurred between ,25±21 Ma In both cases cooling was associated 247 with exhumation from signi®cant crustal depths In contrast the data from the Song Chay Massif show Cenozoic exhumation was limited, preserving an earlier Indosinian thermotectonic signature The two pulses of cooling correspond to an increase in slip rate along the main fault which Harrison et al (1996) noted also coincides with the transtensional phase It is probable that this transtensional environment has caused localised extensional unroo®ng of the Song Chay Massif as well as the more pronounced extension recently identi®ed in the Bhu Khang Massif, southwest of the shear zone (Jolivet et al., 1999) Conclusion The Song Chay Massif area has been affected by the Triassic orogeny, which is responsible for high-grade metamorphism and shearing, observed along a NW±SE crosssection A shear zone formed during this orogeny at the interface between metasediments and a granite intruded ,430 Ma ago The shear zone has a shallow dip and shows a consistent top-to-the-North sense of shear 40 Ar± 39Ar ages of micas from orthogneiss within the shear zone record a Triassic age on the southern area, but also show evidence for a younger cooling most probably related to a slow doming in the Jurassic Low temperature apatite ®ssion-track data from along the same transect record a later Cenozoic exhumation that involved some reactivation of bounding faults, with a normal sense of movement Timing is similar to the exhumation events in the RRF Zone and implies a causal relationship This study also reinforces the importance of combining both low and high temperature dating methods in a single study Acknowledgements This study was supported by cooperative programs: Programme International de CoopeÂration Scienti®que between CNRS (INSU) and 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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. .. N808E-trending foliation and a gently west-dipping lineation occupy the southern rim of the dome Geochronological data The Song Chay Massif and the surrounding area have been relatively unexplored