DSpace at VNU: U-Pb dating of high temperature metamorphic episodes in the Kon Tum Massif (Vietnam)

Journal of Asian Earth Sciences 30 (2007) 565–572 www.elsevier.com/locate/jaes U–Pb dating of high temperature metamorphic episodes in the Kon Tum Massif (Vietnam) Franc¸oise Roger a,*, Henri Maluski a, Andre´ Leyreloup a, Claude Lepvrier b, Phan Truong Thi c a Laboratoire Geosciences Montpellier, UMR 5243-CNRS – Universite´ Montpellier 2, place Euge`ne Bataillon, 34095 Montpellier cedex, France b Laboratoire de Tectonique, UMR 7072-CNRS-Universite´ P&M Curie, Place Jussieu, 75252 Paris cedex 05, France c National University of Vietnam, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam Received May 2006; received in revised form 24 January 2007; accepted 24 January 2007 Abstract In Vietnam, recent studies demonstrated that the Kon Tum Massif was part of the Indosinian mountain system, together with the Truong Son Belt One sector of the Kon Tum Massif (Song Ba valley) is a granulite facies terrain for which Ar–Ar and U–Pb ages cluster around 245–250 Ma In the area of Song Bien river, also metamorphosed under granulite facies conditions, Ar–Ar ages of 410 Ma are displayed by biotite In this study, U–Pb dating of monazite and zircon has been used to understand the reason for these two groups of ages, taking into account the metamorphic mineral assemblages related to granulite facies metamorphism in the two districts For granulites of Song Ba valley, ages of 245–250 Ma are confirmed from monazites In the Song Bien district, ages of 465–470 Ma displayed by monazites are interpreted to correspond to the metamorphic climax Zircons from one of these samples record the age of anatexis that has affected the granulitic rocks The younger age recorded in the Song Ba valley is considered to reflect high temperature related to charnockitic intrusions emplaced near the end of the Indosinian orogeny, overprinting the earlier formed granulites and resetting the U–Pb and Ar–Ar ages For the first time, two HT events are dated in the Kon Tum Massif, one occurring during the Ordovician and the other during the Permo-Triassic Ó 2007 Elsevier Ltd All rights reserved Keywords: Granulite; U–Pb dating; Indosinian; Ordovician; Vietnam Introduction The Truong Son range, formerly known as Cordillie`re Annamitique, occupies the eastern side of the Indochinese Peninsula From North to South, several metamorphic structure can be described (Fig 1) All of them formed during the Indosinian episode, which was first recognized and described by Fromaget (1941) as ante-Norian in age The metamorphism peak up to amphibolitic facies has been linked to this orogeny and dated at around 245–250 Ma * Corresponding author E-mail address: francoise.roger@gm.univ-montp2.fr (F Roger) 1367-9120/$ - see front matter Ó 2007 Elsevier Ltd All rights reserved doi:10.1016/j.jseaes.2007.01.005 with different radiometric methods (Maluski et al., 1995; Lepvrier et al., 1997; Nagy et al., 2001; Tran Ngoc Nam et al., 2001; Carter et al., 2001) In the northern to central part of Vietnam, these structures exhibit a NW–SE orientation marked in the field by a well-developed subvertical foliation and horizontal mineral lineation, in association with shear planes, generating mylonitic to ultra-mylonitic rocks In the Kon Tum Massif that forms the south-central part of the system, two complexes are classically distinguished: the Kan Nack complex and the Ngoc Linh complex (Fig 1b) Both have experienced granulitic metamorphic conditions As described formerly by Maluski et al (2005), the peak of granulite facies metamorphism is represented in the Kan Nack complex by garnet–cordie- 566 F Roger et al / Journal of Asian Earth Sciences 30 (2007) 565–572 Fig (a) Sketch map of Vietnam showing the localisation of the Kon Tum Massif (b) Simplified geological map of the Kon Tum Massif rite and cordierite–spinel Generally, granulite facies rocks are kondalitic and kinzigzitic banded gneisses and granulitic quartzites After this high-grade episode, granulites experienced a low to medium grade retrogression, more or less expressed according to their location On the center and western side of this complex, a large volume of charnockitic rocks intrude the granulite Recent studies have established (Maluski and Lepvrier, 1998; Vu Van Tich, 2004; Maluski et al., 2005) that the Kon Tum Massif was affected by the Indosinian orogeny (Fig 1a) In the Kon Tum Massif, radiometric data were obtained using the U–Pb method on zircons and Ar–Ar method on biotite Zircons from two gneisses of the Song Ba valley (Fig 1b), yielded concordant ages of 248 ± and 243 ± Ma (Shrimp method, Carter et al., 2001) From the same area, a granulitic gneiss analysed with Shrimp by Tran Ngoc Nam et al (2001) yielded 23 concordant ages at 253.7 ± 11.6 Ma, obtained on 18 zircons An age of 1404 ± 34 Ma was obtained on the core of an oscillatoryzoned zircon, surrounded by a younger rim at 250 ± Ma In the same outcrop, Ar–Ar ages of Ti–biotites are in the same range of around 245 Ma (Maluski et al., 2005) Up to now, all U–Pb ages obtained on granulite metamorphic rocks of the Kon Tum Massif were acquired from samples from the Song Ba valley, due to easy access of the area, the excellent quality of the outcrops and the variety of exposed lithologies But if one takes the trouble to examine other outcrops from more remote areas, the ages of biotites obtained there are much more scattered In the area of the Song Bien river, located on the eastern side of the Kan Nack complex (Fig 1b), two biotites from granulites display Ar–Ar plateau ages of 403 ± and 405 ± Ma (Maluski et al., 2005) As these two groups of granulites correspond to two main groups of ages, e.g., 245 Ma for the Song Ba group with Ar–Ar and U–Pb, and 405 Ma for the Song Bien group with Ar–Ar (Maluski et al., 2005), we try to understand the significance of these ages in conjunction with their respective metamorphic history by applying the U–Pb dating method In addition, a detailed mineralogical study was conducted in order to solve the problem Petrological setting The granulite facies rocks studied in this work occur within the Kan Nack Complex in the eastern sector along the Song Bien river (A, Fig 1b), and in the western sector along the Song Ba river (B, Fig 1b) F Roger et al / Journal of Asian Earth Sciences 30 (2007) 565–572 2.1 Granulites of the eastern sector: the Song Bien river group (VN 414 and VN 413) Along the NE–SW section of Song Bien (as exposed from 14° 14 1000 ; 108° 55 1000 to 14° 13 5900 ; 104° 54 5500 ), the granulitic rocks display a regular WNW to NNW-striking foliation with a steep dip toward the SW A mineral lineation can be observed plunging strongly to the NW with a normal sense of shear (Maluski et al., 2005) Rocks from the Song Bien group are composed only of fresh anatectically banded granulites, corresponding to pelitic or semi-pelitic protoliths All these rocks are strongly foliated parallel to their banding (VN 414) Sample VN 413 is representative of the leucosome of these anatexites The granulite facies assemblages are dominated by quartz, antiperthitic plagioclase, mesoperthitic K–feldspar, Ti–biotite, almandine–pyrope garnet, prismatic sillimanite, Fe–Zn spinels, Mg-rich cordierite, Fe–Ti oxides, graphite, rutile and zircon Some rocks also contain scarce monazites Primary muscovite has disappeared and is replaced with prismatic sillimanite, K–feldspar and anatectic melt by reaction between quartz and muscovite (Fig 2, No 1) The absence of a Sil/Opx/Bt assemblage shows that pressure never exceeds kb Garnet–cordierite–K–feldspar and cordierite–spinel in the presence of anatectic liquid are the most probable critical stable parageneses of the granulite metamorphic peak (Fig 2) 2.2 The granulites of the western sector: the Song Ba river group (VN 811 and VN 363) Along the Song Ba river section (14° 10 2500 /108° 34 4500 to 14° 18 2000 /108° 28 5000 ) the granulitic rocks generally Fig Simplified P–T grid calculated using Thermocalc 3.1 for the KFMASH (+Qz) model system The shaded area corresponds to the peak metamorphic (Tmax, Pmax) in the Kan Nack granulitic complex [ ] and d: KFMASH (+quartz) invariant points , reactions cited in the text 567 display a flat or slightly dipping foliation Like in the Song Bien group, the granulitic rocks are anatectic and banded Quartzofeldspathic leucosomes occur with or without foliation Sample VN 811 is undeformed and is rich in quartz, acidic plagioclase, garnet, biotite, Fe–Ti oxides, apatite, zircons and monazite As in Song Bien, some of these granulites (e.g., VN 363) show cordierite between sillimanite and biotite, and also cordierite–Fe spinel–plagioclase coronas around garnet or between garnet and sillimanite (Fig 3) 2.3 Metamorphism climax characterisation Samples VN 413 (Song Bien group), and VN 363 and 811 (Song Ba group) are Si-rich, Zn-poor (quartzites, semi-pelites and true metapelites as protolith) (Vu Van Tich, 2004) and can be studied in the KFMASH system with excess quartz (Powell et al., 1998) Three reactions Bt + Sil fi Grt + Crd + Kfs + liq (No 2), Grt + Sil + Water fi Crd + Spl (No 3) and Grt + Sill + Liq fi Crd + Spl + Kfs (No 4) have been successively crossed (Fig 2) Reactions 5, and (Fig 2) have never been crossed The granulite facies metamorphic peak took place in the garnet–cordierite–K feldspar or in the cordierite–spinel–K feldspar anatectic P–T stability field at approximately ± kbars and 750 ± 20 °C Sample VN 414 from the Song Bien group is Zn-rich (Vu Van Tich, 2004) and presents a different primary assemblage that cannot be represented in Fig Indeed, Zn contributes to an enlargement of spinel’s stability field towards higher pressures and lower temperatures (Nichols et al., 1992) Because the Song Bien and Song Ba groups contain the same metamorphic mineral assemblages, we can assume that these two regions have probably experienced about the same climax P–T conditions The question is whether there are one or two different HT metamorphic events Fig Cordierite (Crd) and hercynitic spinel (He) reaction corona between garnet (Grt) and prismatic sillimanite (Sil) in the VN 363 granulite of Song Ba river Such a texture indicates that the reaction No has been crossed (see text) 568 F Roger et al / Journal of Asian Earth Sciences 30 (2007) 565–572 The main difference between the Song Bien and Song Ba groups is the widespread occurrence of interlayered charnockitic intrusions in Song Ba granulites while the Song Bien group is free of these intrusions These charnockitic rocks represent igneous mineral assemblages (Opx-Grt-PlQz, Opx-Cpx-Pl-Qz) compatible in P–T space with those of the metamorphic granulites (Vu Van Tich, 2004) (Fig 4b) The same sample contains yellowish, transparent and rounded monazites, five of which were analysed The Pb content lies between 1800 and 8800 ppm and U content lies between 3200 and 18,000 ppm (Table 1) Representative points MZ1, 2, and define a discordia, with a lower intercept of 437 ± 10 Ma and an upper one at 1421 ± 120 Ma A single subconcordant monazite MZ4 gives an age of 465 ± Ma (Fig 4c) U–Pb Geochronology 3.1 U–Pb analytical procedure The zircon fractions and single monazites were dissolved and analysed by ID-TIMS at the University of ClermontFerrand (UMR 6524, Magmas et Volcans) following the technique described by Paquette and Pin (2001) Total blanks were 2–6 pg for Pb The U blanks were negligible The U and Pb isotopes were analysed on a Fisons VG Sector 54-30 Mass Spectrometer in multi-collector static mode Initial common Pb was determined for each fraction using the Stacey and Kramers (1975) two-step model Individual fraction ellipse errors (±2r) and regression calculations were determined using PbDat 1.24 and Isoplot/Ex 2.49 programs, respectively (Ludwig, 1993, 2001) Uncertainties in ages are given at the 95% confidence level The decay constants used for the U–Pb system are those determined by Jaffrey et al (1971) and advised by IUGS (Steiger and Jaăger, 1977) Four samples were analysed, two in the eastern sector (Song Bien river group) and two in the western sector (Song Ba river group) (Fig 1b) 3.2 The Song Bien river granulitic group (VN 414 and VN 413) For sample VN 414, seven single monazites were analysed They are yellowish, transparent and rounded They have U and radiogenic Pb concentrations of 7022– 44,589 ppm and 6471–19,748 ppm, respectively, and probably exhibit inheritance (Table and Fig 4a) Five monazites are concordant or sub-concordant (Nos 1, 2, 3, and 6) with a mean concordant 206Pb/238U age of 468 ± Ma (MSWD = 7), and a pooled 207Pb/206Pb age of 468.6 ± 1.5 Ma (MSWD = 4) The small amounts of inherited Pb likely account for the discordance of points Nos and Two regression lines were fit through these points and the concordant points at 468.5 ± Ma Upper intercepts are, respectively, 635 ± 160 Ma and ± 0.3 Ga (Fig 4a) For sample VN413, four zircon fractions were analysed They are pink, transparent and facetted, typical of granulite facies rocks (Pupin, 1976) Pb concentration is bracketed between 156 and 372 ppm U concentrations are between 2300 and 5000 ppm (Table 1) In a concordia diagram, all points are discordant and probably indicate an inherited component Regression yields a lower intercept of 444 ± 25 Ma and an upper intercept of 1488 ± 150 Ma 3.3 The Song Ba river granulitic group (VN 811 and VN 363) Two selected single monazites from sample VN 811 are yellowish transparent and rounded The U content of the grains range between 500 and 800 ppm while the radiogenic Pb concentration lies around 1100–650 ppm (Table 1) Plotted on a concordia diagram, they give concordant points at 250.5 ± 0.8 Ma (MSDW = 0.35) (Fig 5a) Monazites from sample VN 363 are yellowish, transparent and generally euhedral Two analysed single monazites show U and radiogenic Pb concentrations of 21,000– 25,000 ppm and 12,500–5600 ppm, respectively (Table 1) They yield concordant points, with an age of 247 ± Ma (MSWD = 0.13) (Fig 5b) Interpretation and discussion 4.1 Song Bien group Concerning this group, the lower intercept age for the VN 414 monazite is 468 ± Ma, which is confirmed by the 207Pb/206Pb age at 468.6 ± 1.5 Ma for five concordant monazites and indicate a minimum age for granulitic metamorphism This age is similar within error bars to the concordant age of monazite MZ4 from sample VN 413 at 465 ± Ma.The mineralogy of the granulitic paleosome (VN 414) corresponds to the better preserved granulite facies peak assemblage The oldest ages for monazites are in agreement with Ti–biotite ages based on Ar–Ar dating The difference in ages between the U–Pb and Ar–Ar methods (Fig 6) reflects different closure temperatures for these two systems Unlike VN 414, the leucosome VN 413 displays discordant monazites and zircons, which give a lower intercept around 440 Ma This minimum age may be related to an anatectic episode that has affected some of these granulitic rocks VN 413, like VN 414, contains relic primary assemblages that correspond to an anatectic leucosome, implying a slightly more complex history than VN 414 This explains the younger U–Pb ages obtained from zircons and monazites These minerals yield an age probably close to the age of the anatectic episode, during which some biotites were transformed into chlorite with the growth of rutile needles, cordierite was altered to pinite and secondary muscovite developed This late evolution is reflected by a meaningless Ar–Ar age of 326 Ma, intermediate between the anatexis age of 440 Ma and Indosinian orogeny at 245–250 Ma Table U–Pb analytical results for zircons and single monazites from VN 414, VN 413, VN 811 and VN 363 samples Samplea Weight (mg) Concentrations U (ppm) 206Pb/204Pb (raw) Pbc (ppm) Atomic ratiosb (±2r) 207Pb/ 235U 206Pb/ 238U % err 207Pb/ 206Pb % err 0.7 1.1 0.0396 0.0395 0.3 0.6 0.05121 0.05150 0.6 0.9 Apparent ages (Ma) 207c/ 206c Err 251 250 250 263 14 22 207Pb/ 235U 206Pb/ 238U 0.57 0.56 251 251 VN 363: metapelitic granulite MZ 0.02 24461 MZ 0.01 21054 5585 12482 962 851 0.2771 0.2722 1.8 2.4 0.0392 0.03856 0.9 1.5 0.05128 0.05119 1.5 1.8 0.56 0.66 248 244 248 244 253 249 35 42 Song Bien river VN 414: metapelitic granulite MZ 0.01 7022 MZ 0.01 26922 MZ 0.01 44589 MZ 0.01 11441 MZ 0.02 33838 MZ 0.01 11645 MZ 0.01 17321 1538 4720 7238 2460 7404 1940 3886 6471 10517 19748 8036 9837 11532 7775 0.5884 0.5840 0.5875 0.6128 0.5873 0.5842 0.5991 0.26 0.21 0.32 0.12 0.46 0.24 0.14 0.07561 0.07513 0.07562 0.07754 0.07540 0.07516 0.07665 0.22 0.20 0.32 0.11 0.36 0.22 0.13 0.05643 0.05634 0.05641 0.05732 0.05642 0.05637 0.05669 0.13 0.08 0.06 0.05 0.03 0.08 0.04 0.87 0.98 0.99 0.92 0.99 0.94 0.96 470 467 469 485 469 468 477 470 467 470 482 469 467 476 469 466 468 504 469 467 479 1 leucosome in metapelitic granulite (in same outcrop as VN 414) 0.004 2740 205 1154 0.6948 0.005 2306 156 2861 0.5832 0.005 2508 251 5060 1.0862 0.002 5019 372 2068 0.6953 0.01 3199 1605 2458 0.6818 0.005 4595 1895 2782 0.7615 0.001 15458 7482 1505 0.6010 0.002 11994 3267 1491 0.5831 0.001 18065 8770 1246 0.5527 0.21 0.28 0.25 0.26 0.32 0.30 0.43 0.29 0.40 0.08119 0.07410 0.10784 0.08074 0.08012 0.08594 0.07450 0.07483 0.07132 0.19 0.26 0.24 0.23 0.20 0.26 0.24 0.17 0.22 0.06206 0.05708 0.0730 0.06246 0.06172 0.06426 0.05851 0.05651 0.05620 0.11 0.08 0.07 0.12 0.24 0.30 0.34 0.23 0.31 0.88 0.95 0.96 0.86 0.66 0.68 0.62 0.64 0.61 536 467 747 536 528 575 478 466 447 503 461 660 501 497 531 463 465 444 676 495 1015 690 664 750 549 473 460 1 7 VN 413: Z1 Z2 Z3 Z4 MZ MZ MZ MZ MZ a b c F Roger et al / Journal of Asian Earth Sciences 30 (2007) 565–572 Song Ba river VN 811: leucosome in metapelitic granulite (in same outcrop as VN 363) MZ 0.02 795 690 1052 0.2800 MZ 0.02 504 327 675 0.2807 Rho % err The individual analyses were performed on unbroken, crack-free, and best quality grains of the population Ratio corrected for mass discrimination, spike contribution, 2–6 pg of Pb blank and initial common Pb deduced from the model of Stacey and Kramers (1975) Radiogenic 569 570 F Roger et al / Journal of Asian Earth Sciences 30 (2007) 565–572 (Fig 4a–b) The 635 Ma intercept can be compared to the Sm–Nd age of 678 ± 23 Ma of an underformed garnet amphibolite located in the eastern side of the Kan Nack Complex along the coast (Nakano et al., 2002; Osanai et al., 2004) 4.2 Song Ba group From the Song Ba granulites, several implications arise – the concordance between both monazite U–Pb ages for the Song Ba river granulites suggests that a high temperature metamorphic event occurred at circa 250 Ma These ages, obtained by concordant points, reflect the best estimate of the closure of the U–Pb system for these monazites They are similar to the Shrimp age of 253.7 ± 11.6 Ma obtained for the rims of zircons extracted from a granulite from the same outcrop (Tran Ngoc Nam et al., 2001) (Figs and 6a) – Samples VN 363 and VN 811 from Song Ba yield similar ages of 247–250 Ma via the U–Pb method on monazites (this study) and 244 ± and 247 ± Ma, respectively, based on Ar–Ar dating of Ti–biotites (within the error bars) (Fig 6a) (Maluski et al., 2005) This slight difference in age is linked to different closure temperatures between the two radiometric systems These observations suggest rapid cooling of the granulites The age of the inherited component at 1.4 Ga for the Song Bien (VN 413 and VN 414) area shown by the upper intercept of monazites and zircons, is similar to the single U–Pb age obtained by Tran Ngoc Nam et al (2001) in the Song Ba valley – The fact that the 1.4 Ga event has been recorded in Song Bien and Song Ba area, while no record of the 460 Ma episode was obtained in the Song Ba sector, has to be emphasized An unlocalized and unprecise 480–500 Ma monazite CHIME age, detected among some grains, has been recently reported (Osanai et al., 2004) Fig Concordia diagrams of granulites from Song Bien river (eastern sector) (a) Sample VN 414 on single monazites; (b) sample VN 413 on zircons and (c) sample VN 413 on single monazites (Maluski et al., 2005) This late event did not reach temperatures high enough to completely reset the Ar systematics of the biotites, and/or the U–Pb systematics of zircons and monazites The upper intercepts of 635 ± 160 Ma, ± 0.3 Ga and 1421 ± 120 Ma are interpreted as minimum ages of an inherited component related to the sedimentary protolith age or to the age of a previous metamorphic event Several hypotheses can be invoked to explain the absence of ages around 450 Ma in the Song Ba area The first explanation, which we not favor, is that the amount of data is insufficient Another explanation could be tectonic The two areas (Song Ba and Song Bien) might have been far apart during this period of time (Ordovician) and later brought to their present positions The displacement could have been taken up along major faults like the N–S Song Con Fault, which occurs between them (Fig 1b) Perhaps the most likely hypothesis is total overprinting of the high-temperature event in the Song Ba area, due to the widespread occurrence of intrusive charnockitic magmas in the granulites As shown in Maluski et al (2005), Ar– Ar ages of charnockites in Song Ba represent an average age of 242 ± 1.5 Ma (±1r) This age is approximately the same as U–Pb ages on zircons in the same area obtained by Nagy et al (2001) at 249 ± Ma and by Carter et al F Roger et al / Journal of Asian Earth Sciences 30 (2007) 565–572 571 Fig Concordia diagrams of granulites from Song Ba river (western sector) (a) sample VN 811 and (b) sample VN 363 ones, leading to complete resetting of the isotopic clock and giving clustered ages around 245–250 Ma However, we cannot discard the possibility that a new granulitic episode occurred around 245–250 Ma and was responsible for the U–Pb and Ar–Ar ages This was accompanied and closely followed by a charnockitic intrusive magmatic event Petrological data not provide any information, because granulites of the Song Ba and Song Bien sectors display the same mineral assemblage and imply similar P–T climax conditions (Fig 2) More probably, the 465 Ma granulitic episode dated using U–Pb in Song Bien was also responsible for the development of the granulitic assemblage in the Song Ba sector, with selective resetting at Song Ba by the charnockitic intrusions during the Indosinian episode at 245–250 Ma Conclusions Fig Synthesis of U–Pb and Ar–Ar ages for granulites from Song Ba (a) and Song Bien (b) river (2001) at 258 ± Ma with the Shrimp technique (Fig 6a) This implies that a charnockitic episode overprinted the older granulitic series, equivalent in age to the Song Bien The granulite metamorphic rocks of the Kan Nack Complex outcrop within two areas along the Song Bien and the Song Ba rivers (Fig 1b) The main geological difference between these groups is the widespread occurrence of interlayred charnokitic intrusions within granulites of Song Ba river These two regions experienced similar climax P–T conditions (750 ± 20 °C for ± kbars) (Fig 2) The different ages of the PT climax (460 vs 250 Ma) in Song Bien and Song Ba suggests either two granulite facies events or overprinting of the same 465 Ma metamorphic event in the Song Ba region due to charnockite intrusions at around 250 Ma The monazite U–Pb ages of 465–470 Ma obtained on the Song Bien river granulite are interpreted as the minimum age of HT metamorphism They are the first Ordovician ages obtained on HT metamorphism in the Kan Nack Complex They can also be compared to the 450 and 444 Ma (U– Pb, Nagy et al., 2001; Carter et al., 2001) and 425–340 Ma (Ar–Ar, Vu Van Tich, 2004) ages displayed by the Paleozoic series of the Dien Binh Complex, located in western 572 F Roger et al / Journal of Asian Earth Sciences 30 (2007) 565–572 Kontum, west of the Po Ko suture (Fig 1a) The Dien Binh rocks did not experience any granulitic conditions nor record any Indosinian metamorphic ages The granulitic rocks located east of the suture belong to the Ngoc Linh complex and comprise HP rocks, in particular an eclogitic mafic granulite, that displays Sm–Nd ages of 240–250 Ma (Nakano et al., 2003), similar to those found in the Song Ba group (Fig 1b) Such a contrast might be explained by the existence of a west-dipping subduction zone at the actual locaties of the Po Ko fault as part of the Permo-Triassic Indosinian event This explanation was already invoked on the basis of several lines of evidence, including structural arguments (Lepvrier et al., 2004) The Ordovician ages can be compared to U–Pb ages obtained throughout Vietnam for a magmatic event dated at 428 ± Ma in the Song Chay (Roger et al., 2000) and at 418–407 Ma in the Dailoc Massif of the Central Truong Son Belt (Carter et al., 2001) (Fig 1a) The geodynamic significance of these magmatic and metamorphic events is not presently clear and requires further investigation of Gondwana-derived blocks With regard to the Song Ba granulite, the younger U–Pb ages (250.5 ± 0.8 and 247 ± Ma) are likely related to intrusion of charnockitic magmas corresponding to the end of the Indosinian orogeny Acknowledgements This work was granted by the French–Vietnamese cooperative program CNRS/ASTV No 17617 Authors are indebted to J.P Paquette for using facilities of the laboratoire ‘‘Magmas et Volcans’’ of Clermont-Ferrand The authors thank D Pattison and M Jolivet for discussion and correcting the English Thanks to Prof W Siebel, A Carter and an anonymous reviewer for critical reading of this manuscript References Carter, A., Roques, D., Bristow, C., Kinny, P., 2001 Understanding Mesozoic accretion in Southeast Asia: significance of Triassic thermotectonism (Indosinian orogeny) in Vietnam Geology 29 (3), 211–214 Fromaget, J., 1941 L’Indochine franc¸aise, sa structure ge´ologique, ses roches, ses mines et leurs relations possibles avecla tectonique Bulletin du Service Ge´ologique de l’Indochine 26 (2), 140 p Jaffrey, A.H., Flynn, K.F., Glendenin, L.E., Bentley, W.C., Essling, A.M., 1971 Precision measurement of half-lives and specific activities of 235U and 238U Physical Review C4, 1889–1906 Lepvrier, C., Maluski, H., Nguyen Van Vuong, Roques, D., Axente, V., Rangin, C., 1997 Indosinian NW-trending shear zones within the Truong Son belt (Vietnam) Tectonophysics 283, 105–127 Lepvrier, C., Maluski, H., Vu Van Tich, Leyreloup, A., Phan Truong Thi, Nguyen Van Vuong, 2004 The early Triassic Indosinian orogeny in Vietnam (Truong Son Belt and Kon Tum Massif); implications for the geodynamic evolution of Indochina Tectonophysics 393, 87–118 Ludwig, K.R., 1993 Pbdat: a computer program for processing Pb–U–Th isotope data, version 1.24, United States Geological Survey open-file report 88–542, 32 pp Ludwig, K.R., 2001 User manual for Isoplot/Ex rev 2.49 A geochronological toolkit for Microsoft Excel., Bereley Geochronology Center Special Publication 1a, 1–56 Maluski, H., Lepvrier, C., 1998 Overprinting metamorphism in Vietnam, AGU Fall Meeting 1998, Eos 79, 45,795 Maluski, H., Lepvrier, C., Roques, D., Nguyen Van Vuong, Phan Van Quynh, Rangin, C., 1995 Ages 40Ar–39Ar du complexe plutonome´tamorphique de DaNang – Dai Loc (vietnam central) Processus de superposition des e´pisodes thermotectoniques cenozoăques et indosinien [Abstract] Cenozoic evolution of the Indochina Peninsula, Workshop Hanoi – Do Son p 65 Maluski, H., Lepvrier, C., Leyreloup, A., Vu Van Tich, Phan Truong Thi, 2005 40Ar–39Ar geochronology of the charnockites and granulites of the Kan Nack complex, Kon Tum Massif, Vietnam Journal of Asian Earth Sciences 25, 653677 Nagy, E.A., Maluski, H., Lepvrier, C., Schaărer, U., Phan Truong Thi, Leyreloup, A., Vu Van Tich, 2001 Geodynamic significance of the Kon Tum massif in central Vietnam: composite 40Ar/39Ar and U–Pb ages from Paleozoic to Triassic Journal of Geology 109, 755–770 Nakano, N., Osanai, Y., Owada, M., Tran Ngoc Nam, Pham Binh, Kagami, H., Suzuki, S., 2002 Two different metamorphic ages in the Kontum massif, central Vietnam, Abstract The 109th annual Meeting of the Geological Society of Japan, 309 Nakano, N., Osanai, Y., Owada, M., Tran Ngoc Nam, Suzuki, S., Kagami, H., 2003 Metamorphic evolution of high-pressure and ultrahigh-temperature mafic granulite from the Kon Tum Massif, Central Vietnam Earth Monthly 25, 236–243 Tran Ngoc Nam, Yuji, S., Kentaro, T., Mitsuhiro, T., Phan Van Quynh, Le Tien Dung, 2001 First SHRIMP U–Pb zircon dating of granulites from the Kon Tum massif (Vietnam) and tectonothermal implications Journal of Asian Earth Sciences 19 (1–2), 77–84 Nichols, G.T., Berry, R.F., Green, D.H., 1992 Internally consistent gahnitic spinel–cordierite–garnet equilibria in the FMASHZn system: geothermobarometry and applications Contribution of Mineralogy and Petrology 111, 362–377 Osanai, Y., Nakano, N., Tran Ngoc Nam, Toyoshima, T., Tsunogae, T., Pham Binh, 2004 Permo-Triassic ultrahigh-temperature metamorphism in the Kontum massif, central Vietnam, Journal of Mineralogical and Petrological Sciences, V 99, 225–241, Special issue Paquette, J.L., Pin, C., 2001 A new miniaturized extraction chromatography method for precise U–Pb zircon geochronology Chemical Geology 176 (1–4), 311–319 Powell, R., Holland, T.J.B., Worley, B., 1998 Calculating phase diagrams involving solid solutions via non-linear equations, with examples using Thermocalc Journal of Metamorphic Geology 16, 577–588 Pupin, J.P., 1976 Signification des caracte`res morphologiques du zircon commun des roches en pe´trologie Bases de la me´thode typologique Applications The`se d’Etat, Univ.de Nice, pp 1–393 Roger, F., Leloup, Ph., Jolivet, M., Lacassin, R., Phan Trong Trinh, Brunel, M., Seward, D., 2000 Long and complex thermal history of the Song Chay metamorphic dome (northern Vietnam) by multisystem geochronology Tectonophysics 321, 449–466 Stacey, J.S., Kramers, J.D., 1975 Approximation of terrestrial lead isotope evolution by a two stage model Earth Planetary Science Letters 26, 207221 Steiger, R.H., Jaăger, E., 1977 Subcommission on geochronology convention on the use of decay constants in geo- and cosmochronology Earth Planetary Science Letters 36, 359–362 Vu Van Tich, 2004 La Chaıˆne Indosinienne au Vieˆt Nam: Pe´trologie et ge´ochronologie du bloc me´tamorphique de Kon Tum The`se de Doctorat, Universite´ de Montpellier 2, pp.1–202  ... Tich, 2004; Maluski et al., 2005) that the Kon Tum Massif was affected by the Indosinian orogeny (Fig 1a) In the Kon Tum Massif, radiometric data were obtained using the U–Pb method on zircons and... between them (Fig 1b) Perhaps the most likely hypothesis is total overprinting of the high- temperature event in the Song Ba area, due to the widespread occurrence of intrusive charnockitic magmas in. .. et al / Journal of Asian Earth Sciences 30 (2007) 565–572 Fig (a) Sketch map of Vietnam showing the localisation of the Kon Tum Massif (b) Simplified geological map of the Kon Tum Massif rite and