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Journal of Asian Earth Sciences 25 (2005) 653–677 www.elsevier.com/locate/jaes 40 Ar–39Ar geochronology of the charnockites and granulites of the Kan Nack complex, Kon Tum Massif, Vietnam Henri Maluskia,*, Claude Lepvrierb, Andre´ Leyreloupa, Vu Van Ticha,c, Phan Truong Thic a Laboratoire de Ge´ochronologie, UMR5567, CNRS-ISTEEM, Universite´ Montpellier 2, Place Euge`ne Bataillon 34095, Montpellier, France b Laboratoire de Tectonique, ESA 7072, Universite´ Pierre et Marie Curie, Place Jussieu, 75230 Paris, France c National University of Vietnam, Hanoi, 334 Nguyen Trai Thanh Xuan, Hanoi, Vietnam Received 24 March 2004; accepted 20 July 2004 Abstract The Truong Son Belt forms the eastern rim of the Indochina Block in Southeast Asia The age of the metamorphism, mainly along NW–SE mylonitic shear zones that affects this belt, has been formerly determined at about 240–250 Ma This age corresponds to the Indosinian tectonometamorphic episode The Kon Tum Massif, situated to the south of this belt, comprises high-temperature rocks, the Kan Nack Complex, including charnockites and granulites The main charnockitic outcrops, restricted to the Song Ba Valley, establish the intrusive nature of these magmatic rocks within granulite facies material Basic charnockitic rocks are mainly quartz enderbites to norites and hornblende–pyroxene granulite facies rocks The 40Ar–39Ar age of intrusion-cooling of charnockitic magmas is determined from primary magmatic biotites at about 245 Ma In the east of the Kan Nack Complex some granulite facies rocks exhibit relicts of primary granulite facies parageneses, whereas others show evidence of overprinting by a retrogressive low-grade metamorphism Ar–Ar dating confirm this evolution, giving ages of 400 Ma for primary relict granulite facies phases and 260–270 Ma from the most retrogressed samples establishing the youngest limit for the granulite facies metamorphism Granulites intruded by charnockites in the Song Ba Valley yield ages of about 250 Ma, equivalent to the ages of the charnockites, and have evidently been completely reset by these high temperature intrusions Therefore, the Kan Nack Complex of the Kon Tum Massif is not an independent unit with respect to the Indosinian orogen, but represents the deepcrustal part of this belt q 2004 Elsevier Ltd All rights reserved Keywords: Vietnam; Kon Tum; Granulite facies metamorphism; Charnockites; Indosinian; Ar–Ar geochronology Introduction The tectonic development of the Indochinese peninsula was the result of two main orogenic events As defined by Fromaget (1941), the Indosinian orogeny occurred during a major episode in Late Permian and Triassic times and was the expression of the collision of several Gondwana-derived continental terranes (Indosinia, Sibumasu and South China), after narrowing and suturing of different branches of Paleotethys (Metcalfe, 1996, 1999; Lepvrier et al., 1997) During the Tertiary period the collision of India with Eurasia induced the subsequent extrusion of Indochina, after resorption of the correlative part of Neotethys * Corresponding author Tel.: C33 467 14 45 68; fax: C33 467 14 36 46 E-mail address: maluski@dstu.univ-montp2.fr (H Maluski) 1367-9120/$ - see front matter q 2004 Elsevier Ltd All rights reserved doi:10.1016/j.jseaes.2004.07.004 The geology of the territory of Vietnam, which forms the eastern border of the peninsula, reflects this tectonic evolution (Fig 1) The Truong Son Belt (Annamitic Cordillera of the former French geologists—Fromaget, 1941) in the north-central part of the country, was built by the Indosinian movements The Red River Fault Zone in northern Vietnam was the site of important left-lateral shearing, due to the lateral extrusion of the Indochina Block during the Tertiary The Kon Tum Massif forms the south-central part of Vietnam and is commonly regarded as an old, stable Precambrian basement (Tien et al., 1989) Because of the occurrence of metasedimentary and meta-igneous granulites and charnockites, this block has classically been interpreted as a fragment of Gondwana, equivalent in age to similar facies rocks which are exposed in southern India and 654 H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 Fig Main geological units in Vietnam Major metamorphic complexes; suture zones; main mylonitic fault zones Antarctica (Katz, 1993) Previous ages determined by the U–Pb, Rb–Sr and K–Ar methods fall in the range 1650–1810 Ma (Thi, 1985; Hai, 1989) Unfortunately, these results were published without sufficient documentation and the data are imprecise or unreliable New results obtained on the high-grade rocks, using both Ar–Ar and U–Pb methods, are coincident with the ages of w250 Ma formerly obtained in the Truong Son Belt H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 to the north of the Kon Tum Massif (Lepvrier et al., 1997; Maluski and Lepvrier, 1998; Maluski et al., 1999, 2000, 2002; Carter et al., 2001; Nagy et al., 2001; Nam et al., 2001) The aim of this study is to constrain the age of the thermotectonic phase which has affected the Kon Tum Massif Up-to-now, radiometric results related to this massif have been obtained along classical sections and easy-toreach outcrops, as for example the Song Ba Valley, and without any reference to their petrology In this work, samples for dating were selected after careful petrological analysis, in order to distinguish clearly primary minerals which developed during high-grade conditions, from those which reflect an overprint under lower-grade conditions This preliminary investigation of parageneses is essential with regards to the interpretation of their ages We thus present an extensive geochronological study of the charnockitic and granulite facies material which forms the Kan Nack Complex in the Kon Tum Massif, using the 40Ar–39Ar radiometric method, with reference to results recently obtained by the U–Pb method on identical samples The studied area extends roughly between 148 and 158 N latitude (Fig 1) The surrounding low- to-high-grade metasedimentary series (Ngoc Linh Complex—Tien, 1989) is not included in this work These rocks extend to the northwards to the latitude of Tra Bong, and constitute a link with the Truong Son terranes The Paleozoic Dien Binh series, which lies to the W, represents an independent and older unit (Lepvrier et al., in press) Geology of Northern-Central Vietnam and Kon Tum Massif: an overview 2.1 Northern-Central Vietnam The Truong Son Belt (Fig 1) occupies North-central Vietnam and Eastern Laos to the north of the Kon Tum Massif Various types of rocks comprise this mountain range Amphibolite facies quartzites, micaschists, para- and ortho-gneisses, amphibolites and marbles are restricted to relatively narrow NW- to W-trending shear zones, marked by the development of mylonites and ultra-mylonites (Lepvrier et al., 1997) The mylonitic foliation is very steep in these zones, and bears a subhorizontal NW–SE stretching lineation Various, clear kinematic indicators reveal a constant dextral shear Following a similar trend, along Song Ma and between Tamky and Phuoc Son (Fig 1), ultrabasic and basic rocks have been interpreted as the remains of dismembered and metamorphosed ophiolites, defining Paleotethyan or older suture zones Whatever the age of the suturing, the same dextral ductile strike-slip movements affected the rocks in these suture zones (Fig 1) (Lepvrier et al., 1997) The metamorphic rocks are unconformably overlain locally, by undeformed Upper-Triassic detrital red beds 655 This unconformity provides an upper age limit to the thermotectonic event Many 40Ar–39Ar ages obtained from synkinematic metamorphic minerals in different parts of the belt cluster around 245–250 Ma (Maluski et al., 1995; Lepvrier et al., 1997) This indicates that the Truong Son Range was mainly developed during a Late Permian–Early Triassic thermotectonic event, which is also exhibited in the Song Chay Massif in northern Vietnam (Maluski et al., 2001) Such a tectonic and metamorphic climax is significantly older than the classical Late Triassic (Neotriassic) pre-Norian Indosinian phase (Fromaget, 1941) This has been confirmed by Late Permian U–Pb crystallization ages on zircons from granites in north-central Vietnam (Nagy and Schaărer, 1999) and on zircons from orthogneiss, on the western bank of the Red River (Carter et al., 2001) Taking into account the right-lateral shear movements marking the NW- to Wtrending zones, the Late Permian–Early Triassic event likely results from oblique collision of Indochina with the neighboring South China and Sibumasu blocks (Lepvrier et al., 1997; Carter et al., 2001) More recent strike-slip or normal brittle movements reactivated some of the Triassic shear zones, principally during Neogene and even Quaternary times In addition, Oligo-Miocene ductile extension has deeply affected the Bu Khang massif within the Truong Son Belt (Fig 1) (Maluski and Lepvrier, 1998; Jolivet et al., 1999) In the North of Vietnam, the Red River Fault Zone, which represents a major Cenozoic sinistral shear zone linked to the extrusion of the Indochina Block, has been extensively studied and we refer to a full bibliography in Leloup et al (2001) 2.2 The Kon Tum Massif The Kon Tum Massif in south-central Vietnam represents an uplifted block of metamorphic rocks, covered partly by Mesozoic continental red beds or even directly capped by Neogene to Quaternary lava flows (Lee et al., 1998) To the West, in Cambodia, the Kon Tum Massif probably extends beneath the Khorat Basin A fission track analysis indicates that the present-day topography is linked to late Neogene crustal uplift associated with basaltic eruptions (Carter et al., 2000) In contrast to the Truong Son Belt where dominant NW–SE to E–W tectonic directions are conspicuous features, the Kon Tum Massif (Fig 2), as shown in the central and western portions, is controlled by N–S fractures and shear zones The central-eastern part of the Kon Tum Block is occupied by the Kan Nack Complex (Tien et al., 1989) formed by anatectic granulitic and charnockitic rocks These rocks are surrounded, sometimes in fault contact (Ba To fault), by amphibolite facies gneisses and schists belonging to the Ngoc Linh Formation (Tien et al., 1989) that (to the N and W of the massif) consists principally of biotite–hornblende gneisses, amphibolites, biotite–sillimanite–garnet–bearing schists, graphitic schists As shown 656 H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 Fig Geological sketch map of the Kon Tum Massif: Precise GPS locations are given in Tables and along the cross-section from Kon Tum city to Song Re´ River, through Konplong, these rocks form a wide antiform that exhibits normal shear movements on both flanks, while a large granitic Cretaceous undeformed body occupies the core of the structure (Fig 3.1) Similarly, the granulitic rocks of the Kan Nack Complex exposed along the upper course of the Song Ba River (Fig 2) are represented by Al-rich metapelites, quartzites and marbles, have a foliation with a low to moderate dip, locally mylonitic, and form gentle dome structures (Fig 3.2) Charnockitic bodies, represented by enderbites (quartz–plagioclase–pyroxene high-grade magmatic rocks) and locally norites (hypersthene clinopyroxene gabbros), occur in the core of these structures The structural relationships between the charnockites and granulites has been observed clearly at several sites and the intrusive nature of the contact is now well established Along the Song Ba River, near Kan Nack, inclusions of foliated granulites occur within the charnockites Intrusive charnockitic lenses, although moderately deformed, exhibit gneissic or banded structures in their outer parts and become more massive in their centres The western part of the Kon Tum Massif is bounded by a major N–S fault (Tri, 1986), which can be followed for at least 100 km, from Phuoc Son along the Po Ko River; this is named the “Po Ko Fault Zone” (Fig 1) At the latitude of Dak To, the Po Ko Fault swings eastwards and splays in different branches towards the Sathay Valley and towards the Kon Tum Basin and probably extends more to the south beneath the Pleiku plateau basalts reaching the lower course of the Song Ba To the east of the fault recently discovered charnockitic rocks are exposed, which constitute the westernmost occurrence of these rocks To the west, there are mainly non-granulite facies meta-igneous rocks that belong to an independent tectonic unit, known as the Dien Binh Complex, which has older Paleozoic ages (Maluski et al., 2002; Lepvrier et al, in press; Tich, 2004) The foliated and sometimes mylonitic rocks, cropping out on both sides of the structure, follow the strike of the fault and display a constant N-trending and W-dipping steep to moderate foliation, except in the Dak To segment, where the foliation bends to WNW–ESE When exposed, stretching and mineral lineations are systematically WNW- to NWoriented Late Indosinian movements along this fault are H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 657 Fig Two E–W cross-sections through the Kon Tum Massif (see location (1) and (2) on Fig 2.1 Kan Nack Complex (granulite facies gneisses, quartzites, metapelites, marbles intruded by charnockites (in black) and both invaded by migmatites (in grey); (2) Ngoc Linh Complex: orthogneisses, amphibolites, micaschists (locally with S/C normal shear bands) invaded by migmatitic granites; (3) ophiolites (peridotites and pyroxenites in a sheared serpentinite matrix); (4) Dien Binh Complex: Paleozoic ortho-granodiorites to diorites, surrounded by amphibolite facies metasedimentary foliated rocks; (5) non-foliated Mesozoic granite and felsic rocks; (6) Permo-Triassic lavas and Upper Triassic terrigenous sediments; (7) Neogene sediments of the Kon Tum basin; (8) Quaternary basalts A,B,C,D refer to the samples location (Fig 2) dated at 232G2 Ma by synkinematic biotites sealed by late undeformed granodioritic intrusions which were emplaced at 204G1 Ma (Lepvrier et al., in press) In the northern part of the Kon Tum Massif, which forms the transition with the Truong Son Belt (Fig 1), the E–W dextral shear zones of Quang Ngai-Trabong and TamkyPhuoc Son progressively bend north-westwards to connect with the Po Ko Fault (Fig 1.2) the north of Kan Nack town (Fig 2) This area lies on the western rim of the Kan Nack Complex where enderbites and two pyroxenes–plagioclase granulite facies rocks occur In addition, two further localities were visited (Fig 2C,D) that exhibit two pyroxene–hornblende and clinopyroxene–hornblende–plagioclase granulite facies rocks The different charnockitic rocks are described in Table according to their mineralogy A careful observation of thin sections has enabled separation of primary and late secondary parageneses: The Kan Nack Complex We present the main data related to parageneses found in the charnockites and in the granulitic facies metamorphic rocks Then ages of both rock types are discussed in conjunction with their metamorphic assemblages Granulitic rocks, and to a lesser extent the charnockites, contain complex assemblages developed during ambient P–T conditions after the peak of metamorphism and during overprinting in low-grade conditions Consequently, the interpretation of the isotopic ages depends of the attribution of the parageneses to particular metamorphic conditions Petrological data were obtained after an extensive study with Camebax 50 Microprobe (Service Commun Microsonde Sud, Montpellier, France) Ar–Ar methodology applied in this work is described in Appendix 3.1.1 Primary paragenesis Whatever their location, the charnockitic rocks contain the primary assemblage: plagioclase, clinopyroxene, Gorthopyroxene, Ggarnet, Gquartz, GTi-rich red biotite and Gcalcic amphibole In more basic samples, biotite and amphibole were formed later than and surround the pyroxenes 3.1.2 Late retrograde paragenesis Some retrograde phases occur close to late microcracks Orthopyroxene has been transformed to cummingtonite and magnesio-cummingtonite, brown hornblende to yellow pistacite, and feldspars to secondary white micas The growth of these secondary minerals is linked to a retrogressive event, likely due to magma cooling and decompression during exhumation of the Kon Tum Massif 40 Ar–39Ar geochronology of the charnockites 3.1 Mineralogy of charnockitic rocks 3.2 The occurrence of charnockitic rocks is restricted to the area along the upper course of the Song Ba River, to Sixteen samples of charnockites have been dated in this work; only two from the Song Ba Valley: VN522 from 658 H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 Table Mineralogy of main charnockitic rocks Rock type Mineralogy Quartz Enderbite Without garnet With garnet Quartz, antiperthitic sodic plagioclase, hypersthene brown hornblende, green biotite Quartz, antiperthitic sodic plagioclase (oligoclase/andesine), hypersthene, garnet (almandine-pyrope) Red Ti-biotite Minor: apatite, zircon, ilmenite, sphalerite Norite Without quartz With quartz Pyroxene-Amphibole Granulite Rock Amphibole-Plagioclase Granulite Rock Andesine/labradorite, orthopyroxene (bronzite–hypersthene), amphibole Quartz, andesine /labradorite, orthopyroxene, (bronzite/hypersthene) salitic clinopyroxene, red Ti-biotite,Gbrown pargasitic amphibole Minor: apatite, zircon, ilmenite, magnetite,Gsphene Labradorite, brown pargasitic hornblende, Ca-clinopyroxene, Gorthopyroxene, Gred Ti- biotite Labradorite, brown pargasitic hornblende Ca-clinopyroxene, orthopyroxene, Gred Ti-biotite Minor: apatite, zircon, Gsphene the Bu Nu Brook in the western part of the Kan Nack complex (Fig 2D), and VN389 from the Dak To area, outside the Kan Nack Complex to the west (Fig 2C) All other samples were collected along the upper course of the Song Ba River (B, in Fig 2) A summary of the 40Ar–39Ar ages of biotites from the charnockites and their respective GPS locations is given in Table 3.2.1 Song Ba River Samples VN 810, 294, 358, 796, 812, 814, 799, 357, 798, 800, 813, 808, 364 (Between 14816 55 00 N; 108829 43 00 E and 14808 18 00 N; 108835 15 00 E): Nearly all types of charnockite are represented in these outcrops: norites, quartz-two pyroxene–plagioclase granulite facies rocks and quartz–enderbites The biotite ages of these charnockitic rocks yield very comparable results (Fig 4a–m): each sample defines a flat plateau age which clusters between 238 and 245 Ma, except for sample VN364, which yields an older age of 251.5G2.7 Ma Only VN294, VN358, VN800 and VN364 (Fig 4b,c,j,m) present significantly younger ages in low to intermediate temperatures, due to argon loss from less retentive domains and consequently diffusion prior to the primary closure of the mineral VN 294, 358 and 800 commonly show a slight alteration of biotites and orthopyroxenes that are partly transformed to green chlorite, and very tiny secondary white micas Also, VN358 has secondary carbonates in the matrix This minor alteration may explain the trend of age spectra related to the low temperature steps (7–10% of 39Ar) rather than a late thermal episode at w100–150 Ma Nevertheless, this alteration was not strong enough to disturb the more retentive sites, as indicated by the plateau ages The biotite of sample VN357 (Fig h) was analysed as a population and gives a complex age spectrum for which a plateau age can be calculated only between 1050 8C and the fusion temperature, corresponding Table Location and summary of ages of dated charnockites Charnockites Song BA River VN810 VN294 VN358 VN796 VN812 VN814 VN799 VN357 VN798 VN800 VN813 VN808 VN364 SW SONG BA VN295 BU NU BROOK VN522 DAK TO VN389 GPS position (N latitude; E longitude) Rocks AGE (Ma) BiZBiotite 14813 08 00 ; 108831 12 00 14808 16 00 ; 108836 06 00 14820 07 00 ; 108844 16 00 14816 55 00 ;108829 43 00 14808 18 00 ;108835 16 00 14807 51 00 ; 108833 40 00 14808 18 00 ;1088 00 35 15 00 14815 45 00 ; 108830 17 00 14808 18 00 ; 108835 15 00 14808 18 00 ; 108835 15 00 14808 18 00 ; 108835 15 00 14808 18 00 ; 108835 15 00 14814 18 00 ; 108830 21 00 Norite Enderbite Proxene-pl-norite Quartz Enderbite Norite Pyroxene-pl-Norite Quartz Enderbite Quartz Enderbite Quartz Pyroxene-pl Quartz Enderbite Pyroxene-pl Pyroxene-pl Quartz Enderbite 237.9G2.3 Bi 238.5G3.2 Bi 240.2G4.7 Bi 240.7G2.3 Bi 242.0G2.8 Bi 242.2G2.8 Bi 243.0G2.5 Bi 243.3G2.4 Bi 243.1G2.4 Bi 245.2G2.7 Bi 245.4G2.4 Bi 245.6G2.4 Bi 251.5G2.7 Bi 13858 44 00 ; 108831 38 00 Quartz Pyroxene-pl 226.5G3.6 Bi 14816 05 00 ; 108851 01 00 Amphibole-pl 264.3G3.2 Bi 14843 03 00 ; 108849 12 00 Quartz Enderbite 259.5G4.8 Bi H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 to w50% of released 39Ar at 243.3G2.4 Ma A secondary amphibole has developed in this sample at the expense of the pyroxene VN364, which is a quartz enderbite, yields the oldest plateau age at 251.5G2.7 Ma Concerning its mineralogy, this sample is a unique charnockite containing garnet in equilibrium with orthopyroxene and Ti-rich biotite 659 Sample VN295 (13858 44 00 N; 108831 38 00 E): This basic charnockite, belongs to the Kan Nack Complex, but is located SW of the Song Ba River (Fig 2) The biotite yields a plateau age at 226.5G3.6 Ma (Fig 5) with a slight Ar depletion in low temperature steps, giving younger ages at w110–150 Ma, probably without any thermotectonic meaning The plateau age is younger than that of previous samples Fig Age spectra of charnockites from the Song Ba River 660 H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 Fig (continued) 3.2.2 NW Dak To Sample VN389 (14843 03 00 N; 108849 12 00 E): Orthopyroxene-free enderbites to clinopyroxene–amphibole–plagioclase granulite facies rocks occur within the metamorphic series of the Ngoc Linh Formation, to the North of Dak To town The dated sample is an enderbite with a brown pargasitic hornblende, salitic clinopyroxene and red Ti-rich biotite A secondary paragenesis in this rock is represented by chlorite, actinolite and sphene, developed at the expense of amphiboles and Ti-biotites Biotites, which constitute an equilibrium phase, give an age of 259.5G4.8 Ma (Fig 6b), obtained on a plateau fraction corresponding to 80% of released 39Ar, clearly older than the average age of charnockites from the Song Ba Valley Low temperature increments yield younger ages between 40 and 187 Ma, with a step at 164 Ma, related to 12% of 39Ar released This trend may be related to Fig Age spectrum of the charnockite VN295, Dak To, SW of the Kan Nack complex H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 661 Fig Age spectra of charnockites from Bu Nu Brook (VN522) and Dak To (VN389) the secondary paragenesis described above and in this case would not be related to a precise thermotectonic event 3.2.3 Bu Nu Brook Sample VN522 (14816 05 00 ; 108851 01 00 ): Along the Bu Nu Brook, these rocks are associated with amphibolites and quartzites They are diopside–augite–amphibole–plagioclase granulite facies rocks Much brown hornblende has developed after the pyroxene The biotite yields a plateau age of 264.3G3.2 Ma (Fig 6a), without indication of argon loss on low temperature steps, nor any evidence of an excess Argon component This result constitutes the oldest age we have found in the charnockitic suite 3.3 Mineral assemblages of the metasedimentary granulite facies gneisses 3.3.1 The primordial granulite facies evolution We have observed granulites along the Song Bien Brook, south of Hoai An town, in the eastern part of the Kan Nack Complex They are also very well exposed along the upper course of the Song Ba River where they occur in close relationship to the charnockitic rocks (Fig 2) and constitute the basement of the Kan Nack Complex To the south, garnet–cordierite and cordierite–spinel assemblages linked to the peak of the granulite facies metamorphism are well preserved, while to the north, granulites experienced polymetamorphic conditions and now exhibit only various relict granulite facies assemblages (Table 3) The protoliths of the granulites were sedimentary Al-rich rocks including pelites, semi-pelites, sandstones and Al-quartzites intercalated with calcareous and dolomitic sediments The more siliceous granulite facies rocks are khondalitic and kinzigitic banded gneisses, granulite facies metaquartzites, and scarce calcareousdolomitic rocks are represented by forsterite–humite– clinopyroxene calc-silicates The high-grade metapelitic rocks are foliated, strongly layered, and were deformed under ductile conditions This is demonstrated by a strong stretching lineation that contains elongated prismatic sillimanite, quartz ribbons and oval garnets in the main mylonitic shear zones, similar to those described by Ji and Martignole (1994) In metapelites muscovite is absent and has been replaced by prismatic sillimanite and K-feldspar K-feldspar is generally mesoperthitic and sodic plagioclase is locally Table Primary paragenesis in granulites Sample Location 00 00 Rock type Mineralogy qtz, pl.ap, Ksp.mp, Ti-bi, grt, sill.p, crd, zr, graph, rt, chl2, pn2, mus2 qtz, pl.ap, Ksp, bi, sill.p, grt, spl, zr, ap qtz, pl, Ksp.mp, bi, grt, sill.p, crd, spl, dsp2, pn2 qtz, Ksp.mp, Ti-bi, grt, crd, white mica, chl2 qtz, pl.ap, Ksp, crd, Ti-bi, grt, pn2, sc2, chl2 qtz, grt, spl, pl, zr, ap, mus2, chl2 qtz, chld, mus, grt, chl, sill.p, qtz, pl.ap, Ksp.mp, bi, sill.p, grt, spl, rt, Fe–Ti ox, chl, mus qtz, pl.ap, Ksp.mp, Ti-bi, grt, sill.p, crd, spl, chl2, mus2, pn2, dsp2 qtz, pl.ap, Ksp.mp, Ti-bi, grt, crd, spl, chl2, mus2, pn2, dsp2 qtz, pl.ap, Ti-bi, grt, zr, ap, chl2 VN413 14818 04 N, 108829 01 E Pelitic granulite VN414 VN415 VN514 VN515 VN505 VN512 VN362 VN363 VN805 VN811 14818 04 00 N, 108829 01 00 E 14818 04 00 N, 108829 01 00 E 14818 04 00 N, 108829 01 00 E 14818 04 00 N, 108829 01 00 E 14815 04 00 N, 108850 32 00 E 14815 04 00 N, 108850 32 00 E 14813 47 00 N, 108830 53 00 E 14813 47 00 N, 108830 53 00 E 14813 47 00 N, 108830 53 00 E 14812 07 00 N, 108832 04 00 E Granulite facies quartzite Pelitic granulite Anatectic granulite facies gneiss Granulite facies gneiss Quartz micaschist Quartz micaschist Quartzo-feldspathic granulite Metatectic granulite Semi-pelitic granulite Granulite facies quartzite List of abbreviations: Qtz, quartz; pl.ap, antiperthitic plagioclase; Ksp.mp, mesoperthitic K-feldspar; Ti-bi, Ti–Biotite; grt, garnet; sill.p, prismatic sillimanite; crd, cordierite; zr, zircon; graph, graphite; rt, rutile; chl, chlorite; pn, pinite; mus, muscovite; spl, spinel; ap, apatite; dsp, diaspore; sc, sericite; ox, Fe–Ti oxides; (2) refers to secondary paragenesis 662 H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 antiperthitic Frequently, garnet coexists in equilibrium with Mg-rich cordierite and cordierite with Fe-rich spinel All these observations indicate that granulite facies was reached The compositional layers in the outcrops contain different parageneses, for instance garnet–biotite–sillimanite–spinel, cordierite–biotite–sillimanite, and garnet, cordierite, biotite, sillimanite, cordierite, spinel in others These banded granulites suffered local anatexis, giving rise to quartz, sodic plagioclase, K-feldspar, biotite, and sometimes garnet in granulite facies leucosomes Anatectic conditions were reached within the stable biotite/quartz and garnet/ K-feldspar/sillimanite/Zn–Fe rich spinel stability fields Orthopyroxene is lacking in these leucosomes In some places, sillimanite and biotite, and garnet and sillimanite have reacted together, giving rise respectively to garnet– cordierite and cordierite–spinel coronas, which constitute the stable critical parageneses during the granulite facies metamorphic peak 3.3.2 Late retrograde paragenesis: the low-to mediumgrade overprinting All the pelitic and semipelitic granulite facies rocks have experienced minor, heterogeneous, low- to mediumgrade retrogression according to their location in the Kan Nack Complex The less retrogressed samples are always located in the Song Bien Brook and along the Song Ba River (Fig 2) The most retrograded samples are located on the eastern rim of the block (Kim Son area, Fig 2) The low-grade alteration firstly begins along microfractures or by the way of shear zones and progressively invades the granulite facies coronas, and then the whole rocks The main retrogressive phases are listed in the Table This retrogressive process results in damouritization (a variety of muscovite) of plagioclase, and transformation of K-feldspar to white micas The garnet is only partially chloritized, whereas biotite has been completely altered The cordierite has been transformed to pinite, and then to white mica Locally, diaspore replaces spinel in the cordierite–spinel reaction coronas Late chloritoid, Table Mineralogy of retrogressive phases in the granulites Granulites Mineralogy of low-grade overprint VN413 VN514 VN515 VN505 VN512 VN362 VN363 VN805 VN811 mus2, pinite,chl white micas, chl pinite, sericite, chl mus replaces feldspar, chl in grt white micas, chl, ctd ctd, mus chl, mu, pinite, dsp mus, pinite, chl Chl ctd, Chloritoid; Other abbreviations same as for Table muscovite sensu stricto and chlorite may also nucleate from the older granulite facies minerals of which only prismatic sillimanite and garnet have survived This late nucleation strongly suggests a new low- to medium- grade metamorphic overprint that is especially well marked in the Kim Son region (Fig 2), where only granulite facies garnet and prismatic sillimanite have survived in the new metamorphic assemblage, generally as inclusions in Fe-chloritoid Granulite facies biotites have been totally transformed to chlorite and rutile needles Locally, the coronas appear more or less deformed by a younger tectono-metamorphic event First, forming a prominent ellipsoidal-shaped fabric Late chloritoid, muscovite, chlorite and exceptionally diaspore may also nucleate on the older granulite facies minerals such as biotite, garnet, cordierite, prismatic sillimanite, K-feldspar and spinels, and partially replace them along cleavages or microcracks; this suggests H20 circulation This process seems to have affected mostly the northeastern margin of the Kan Nack Complex We attribute this to polymetamorphism rather than to simple cooling linked to exhumation, as found in the Song Ba Valley and Song Bien Brook, taking into account the huge size of the neoblasts (e.g chloritoid cm long) that define the rough new foliation We present results of 40Ar–39Ar analysis from eleven samples of biotites and secondary muscovites from the granulite facies gneisses according to their geographic provenance Ages and locations of samples are given in Table The pressure conditions of this late retromorphic event cannot be precisely estimated: diaspore and quartz are not in textural equilibrium, which rules out high-pressure conditions (Theye et al., 1997) Garnet is no longer in equilibrium with plagioclase, thus no thermobarometric calculation is possible Because secondary white mica is muscovite sensu stricto without phengite substitution, the pressure regime is likely to have been low 3.4 40Ar–39Ar radiometric data of metasedimentary granulite facies gneisses 3.4.1 Song Bien and Nuoc Dang Brook Samples VN413, VN414, VN415, VN514, VN515 (14814 25 00 N; 108855 30 00 E): All these samples were collected along a cross-section, following the Song Bien Brook, over a distance of approximately km They are situated on the eastern rim of the Kan Nack Complex (A in Fig 2) No charnockitic intrusions have been found in this area VN414 and VN415 are metasedimentary granulites, like VN413 VN514, VN515 are ribonned granulite facies paragneisses; age spectra obtained from their biotites have a similar trend and present a flat plateau age, related to more than 50% of 39Ar released, and are between 304 and 405 Ma (Fig 7a–e) H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 663 Table Location and ages of biotites and muscovites from granulites Metapelitic granulite facies rocks GPS position 00 00 Rock type Ar–Ar age (Ma) (Mineral) VN413 VN414 VN415 VN514 VN515 14818 04 N, 108829 01 E 14818 04 00 N, 108829 01 00 E 14818 04 00 N, 108829 01 00 E 14818 04 00 N, 108829 01 00 E 14818 04 00 N, 108829 01 00 E pelitic granulite quartzitic granulite Pelitic granulite Granulite facies anatectic gneiss Granulite facies gneiss 325.6G3.1 Bi 405.7G3.8 Bi 403.4G3.8 Bi 304.2G3.6 Bi 343.3G4.2 Bi VN505 VN512 VN362 VN363 VN805 VN811 14815 04 00 N; 108850 32 00 E 14815 04 00 N; 108850 32 00 E 14813 47 00 N; 108830 53 00 E 14813 47 00 N; 108830 53 00 E 14813 47 00 N; 108830 53 00 E 14812 07 00 N, 108832 04 00 E quartz micaschist quartz micaschist quartzo-feldspathic granulite metatectic granulite semi-pelitic granulite quartzitic granulite 262.7G3.2 Mus 270.7G2.5 Mus 241.1G2.4 Bi 244.8G2.4 Bi 243.6G2.4 Bi 247.8G2.4 Bi Bi for Biotite; Mus for Muscovite Fig Age spectra of granulites from the Song Bien Brook VN414 and VN415 contain the most preserved granulite facies assemblages 664 H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 Samples VN505, VN512 (14815 04 00 ; 108850 32 00 ): VN505 and 512 were collected along the Nuoc Dang Brook (A, in Fig 2) Both are micaschists with relict prismatic sillimanite and secondary muscovite They yield plateau ages of 262.7G3.2 and 270.7G2.5 Ma, respectively, for 40% and 80% of released Ar (Fig 8a,b) Concerning muscovite from VN 505 muscovite, most of the radiogenic argon was released during the third step, due to the high transparency of this mineral, which influences dissipation of the laser beam energy The age related to this step is slightly older than the plateau age at 269G1 Ma The trend of these spectra does not present any Ar loss, and is interpreted as reflecting the age of the closure of the system 3.4.2 Granulitic gneisses from the Song Ba River Samples VN362, VN363, VN805, VN811 (14813 47 00 ; 108830 53 00 ): These samples were collected along the Song Ba River, to the north of Kan Nack town (B in Fig 2) in an area where charnockites intrude the granulitic gneisses Well-defined plateaux ages were obtained for these four biotites, for 50–90% of 39Ar released with ages clustered between 241.1G2.4 and 247.8G2.4 Ma (Fig 9a–d) VN363 and VN 811 (Fig 9b,d) present a peculiar trend with slight Ar diffusion affecting the low temperature steps, resulting in non-representative ages (Table 6) Interpretation and conclusions 4.1 Charnockites Plateaux ages obtained from the biotites in the charnockites are reported in Fig 11 Samples are plotted according to their geographical location 4.1.1 Song Ba River Group The magmatic rocks, which represent the charnockitic suite, are intrusive, as said above, into the granulite facies metamorphic series This observation is confirmed by radiometric data obtained on primary matrix Ti-rich biotites from the charnockites, the ages of which span 238 and 245 Ma (Fig 10) These ages, which are younger than those obtained for granulites from the Song Bien Group, probably mark the youngest age limit for the intrusions of charnockite, or at least, the time when different charnockitic intrusions formed as a closed system after cooling The retrogressive event responsible for the development of chlorite, actinolitic amphibole, pistacite and secondary carbonates and a more intense CO2-rich fluid circulation may be responsible for the slight argon loss, which affected the less retentive sites in some samples (e.g VN294, VN358, VN800, VN364 and VN389) The time interval, between 238 and 245 Ma determined in this work is confirmed by two independent U–Pb results (Nagy et al., 2001) previously obtained on zircons from the same sample, VN357, the biotite of which gives an Ar–Ar age of 243G Ma Five zircon fractions gave concordant dates, while two fractions were slightly discordant The average age obtained was 249G2 Ma With the SHRIMP technique, Carter et al (2001) obtained an age of 258G6 Ma, related to the rims and cores of zircons, although there was some evidence of late stage resorption The close agreement of these ages confirms the rapid cooling of these intrusions The local occurrence of foliated charnockites demonstrates that some magmas began to be emplaced during Indosinian tectonic activity Among samples dated in the Song Ba Valley, only VN364, which is the unique garnet-bearing enderbite, yields an older plateau age of 251.1G2.7 Ma for the primary biotites Despite the position of this sample, closely interbedded with other younger intrusions, this age could correspond to an earlier phase of magmatic activity, and thus reflects an intrusion/cooling Ar–Ar age There is no evidence for interpreting this age as the result of an excess argon component Special attention must be paid to the coronitic norite VN 295 that outcrops in the extreme SW of the Song Ba Valley Fig Age spectra of granulites from the Song Bien brook overprinted by low-to medium grade metamorphism H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 665 Fig Age spectra of granulites from the Song Ba River Its biotite age of 226.5G3.6 Ma represents the youngest age obtained from the charnockites Coronas are formed of vermicular orthopyroxene–plagioclase symplectites, which could be interpreted in terms of a decompression reaction due to adiabatic uplift, or to a locally enhanced erosion rate Moreover, this sample shows a late overprint with development of tiny secondary white micas This late development under different and new P–T conditions may account for the younger age of the biotite In this case, we can assume the age is related to the final phase of Indosinian thermotectonism 4.1.2 Dak To and Bu Nu areas Sample VN389 from the Dak To area is situated in the westernmost outcrop of the studied charnockitic rocks in the Kon Tum Massif (C in Fig 2) It is separated from the Kan Nack Complex by the Ngoc Linh Metamorphic Series The age yielded by Ti-rich biotites is 259.5G4.8 Ma, and is therefore clearly older than the age of charnockites from the Song Ba River This result may correspond, as for the biotite of VN364, to the age of earlier intrusion-cooling of these charnockites The thermal conditions occurring during the development of a secondary paragenesis can be invoked to explain the trend of younger ages in low extraction temperatures The same situation concerns charnockitic rock VN522 (D in Fig 2) that displays the same petrological features as VN389 and is the only sample with a charnockitic affinity found in the Bu Nu area, in the easternmost part of the Kan Nack Complex It is slightly altered (chlorite after biotite) and does not contain orthopyroxene The age of 264.3G3.2 Ma obtained on biotite may indicate, as for VN 389, that the intrusions of charnockites were initiated some 20 Ma earlier than the main intrusions in Song Ba, or may represent a differential uplift between the rim and the core of the massif For these last two samples of biotite, as for biotite VN364, we have no evidence to attribute this older age to an excess Ar component 4.2 Metasedimentary granulites All ages obtained on granulites are reported in Fig 11 Results cover a large interval between 405.7G3.8 and 244.2G2.7 Ma Three age groups can be distinguished, corresponding to three distinct areas: (a) the Song Bien Brook (VN 414, VN415, VN413, VN514, VN515), related to the oldest ages, (b) to the west of this area along the Nuoc Dang Brook (VN505, VN512), and (c) along the Song Ba 666 H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 Table Isotopic data for Ar–Ar analyses 40Ar* is radiogenic Argon VN505 MUSCOVITE Laser VN512 MUSCOVITE Laser Sample VN362 BIOTITE Laser VN363 BIOTITE Laser 10 11 12 13 14 15 16 17 40Ar*/39Ar 36Ar/40Ar 39Ar/40Ar 37Ar/39Ar %39Ar 10 8.848 7.569 9.038 9.060 8.890 8.848 8.748 8.840 8.835 9.133 !1000 JZ.017734 2.505 1.942 0.196 0.025 0.045 0.040 0.070 0.044 0.134 0.012 0.029 0.056 0.104 0.109 0.110 0.111 0.111 0.111 0.108 0.109 0.222 0.103 0.000 0.001 0.000 0.000 0.009 0.000 0.003 0.000 0.0 74.0 1.0 57.3 6.5 5.8 59.5 0.7 63.4 1.3 80.8 1.1 84.0 2.0 93.1 1.3 94.1 3.9 100.0 0.3 Total ageZ266.4G3.3 Ma 262.9G49 227.2G5.5 268.2G9.5 268.8G1.2 264.1G2.0 262.9G1.1 260.2G4.0 262.7G1.5 262.6G6.3 270.8G1.3 10 11 12 13 14 15 16 17 18 8.243 9.942 9.273 9.549 9.403 9.128 9.113 8.979 9.062 9.336 9.186 9.329 9.467 9.200 9.106 9.436 9.204 9.087 JZ.017672 0.829 0.282 0.250 0.220 0.067 0.032 0.005 0.023 0.061 0.002 0.003 0.030 0.025 0.029 0.082 0.005 0.003 0.071 0.0915 0.0921 0.0998 0.0979 0.1042 0.1084 0.1095 0.1105 0.1083 0.1070 0.1087 0.1062 0.1048 0.1077 0.1071 0.1058 0.1085 0.1077 0.119 0.032 0.110 0.018 0.010 0.006 0.002 0.009 0.001 0.049 0.000 0.001 0.013 0.000 0.000 0.000 0.000 0.000 0.1 24.5 0.9 8.3 1.8 7.3 6.4 6.5 19.0 2.0 27.6 0.9 43.0 0.1 51.6 0.7 54.2 1.8 55.6 0.1 57.4 0.1 61.2 0.9 64.9 0.7 91.8 0.8 93.8 2.4 95.6 0.1 97.4 0.1 100.0 2.1 Total ageZ272.3G3.0 Ma 245.3G64 292.0G12 273.8G2.6 281.3G2.7 277.3G1.1 269.8G1.1 269.4G1.4 265.7G1.6 268.0G1.7 275.5G2.4 271.4G1.5 275.3G1.0 279.1G1.8 271.8G1.1 269.2G4.2 278.2G1.5 271.9G1.5 268.7G0.4 40Ar*/39Ar 36Ar/40Ar 39Ar/40Ar 37Ar/39Ar %39Ar % Atm Age (Ma) !1000 JZ.017791 4.449 7.217 8.412 7.893 7.869 7.985 7.991 8.011 8.006 8.034 8.222 7.967 7.975 8.113 7.955 8.101 7.959 !1000 1.181 0.651 0.446 0.147 0.166 0.082 0.096 0.065 0.075 0.052 0.030 0.061 0.104 0.069 0.092 0.091 0.071 0.146 0.111 0.103 0.121 0.120 0.121 0.121 0.122 0.122 0.122 0.120 0.123 0.121 0.120 0.122 0.120 0.123 – 0.053 0.054 0.036 0.047 0.052 0.064 0.140 0.039 0.022 0.016 0.010 0.014 0.008 0.005 0.000 0.006 0.2 0.1 2.5 8.2 11.0 19.1 25.0 30.3 39.7 47.3 60.0 74.4 77.4 81.9 84.0 87.0 100.0 34.9 19.2 13.1 4.3 34.9 2.4 2.9 1.9 2.2 1.6 0.9 1.8 3.1 2.1 2.7 2.7 2.1 137.4G15.5 217.9G7.6 251.6G4.8 237.0G1.8 236.4G1.4 239.6G1.7 239.8G2.6 240.3G2.2 240.2G2.8 241.0G1.7 246.3G3.4 239.1G1.2 239.3G2.7 243.2G2.9 238.8G2.6 242.9G2.6 238.9G0.9 1.609 0.948 1.129 0.121 0.088 0.137 0.233 0.040 0.170 0.2 0.7 1.3 47.5 28.0 33.3 133.44G47 243.22G14 149.45G16 (continued on next page) JZ.017791 4.315 8.112 4.854 % Atm AGE G1sd H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 667 Table (continued) Sample 10 11 12 13 14 15 16 17 VN805 BIOTITE Laser VN811 BIOTITE Laser VN413 BIOTITE Laser 10 11 12 13 14 15 16 10 11 12 13 14 15 40Ar*/39Ar 36Ar/40Ar 39Ar/40Ar 37Ar/39Ar %39Ar % Atm Age (Ma) 4.673 6.682 8.241 8.309 8.077 8.199 8.258 8.410 8.228 8.084 8.146 8.071 8.028 8.146 !1000 1.895 0.888 0.141 0.024 0.084 0.030 0.047 0.009 0.043 0.061 0.041 0.031 0.084 0.052 0.094 0.110 0.116 0.119 0.120 0.120 0.119 0.118 0.119 0.121 0.121 0.122 0.121 0.120 0.328 0.091 0.001 0.010 0.012 0.015 0.013 0.011 0.010 0.004 0.009 0.003 0.015 0.001 1.5 3.1 6.1 10.0 15.3 20.4 28.0 35.9 45.1 57.7 61.0 74.4 77.5 100.0 56.0 26.2 4.1 0.7 2.4 0.8 1.4 0.2 1.2 1.8 1.2 0.9 2.5 1.5 144.1G42 202.66G6.0 246.82G3.7 248.73G2.7 242.24G1.8 245.67G2.0 247.31G2.6 251.57G3.5 246.47G1.4 242.42G2.4 244.17G3.1 242.07G1.7 240.88G3.9 244.18G1.8 0.460 0.081 0.035 0.049 0.085 0.140 0.099 0.046 0.036 0.036 0.042 0.029 0.001 0.024 0.020 0.073 0.114 0.129 0.124 0.130 0.127 0.122 0.119 0.121 0.121 0.121 0.120 0.123 0.125 0.124 0.123 0.126 0.012 0.012 0.016 0.015 0.021 0.002 0.025 0.017 0.021 0.025 0.009 0.003 0.008 0.008 0.001 0.005 2.1 8.4 14.6 23.4 27.9 31.8 37.1 43.6 50.8 58.5 65.6 74.1 78.2 81.4 86.8 100.0 13.6 2.4 1.0 1.4 2.5 4.1 2.9 1.3 1.0 1.0 1.2 0.8 0.0 0.7 0.6 2.1 226.7G2.5 227.1G1.5 239.8G1.3 228.5G4.0 231.7G1.7 235.9G1.6 244.7G2.1 243.6G3.4 245.6G1.3 244.6G1.2 246.7G3.4 241.3G1.9 240.4G1.0 241G2.7 242.2G2.3 232.9G2.5 JZ.017791 2.136 2.497 6.199 8.119 8.235 8.247 8.259 8.355 8.346 8.183 8.112 8.216 8.031 8.381 8.049 2.408 1.899 0.29 0.045 0.045 0.048 0.042 0.048 0.024 0.012 0.108 0.013 0.035 0.003 0.052 0.135 0.176 0.147 0.121 0.120 0.120 0.120 0.118 0.119 0.122 0.119 0.121 0.123 0.119 0.122 0.000 0.022 0.180 0.011 0.047 0.043 0.070 0.091 0.116 0.044 0.101 0.082 0.006 0.319 0.430 0.5 1.2 6.3 25.6 37.0 46.7 58.2 67.8 76.3 79.2 80.6 86.5 89.7 90.4 99.9 71.1 56.1 8.6 1.3 1.3 1.4 1.2 1.4 0.7 0.3 3.2 0.3 1.0 0.1 1.5 67.2G4.0 78.4G17 188.7G1.7 243.4G1.2 243.4G1.4 243.4G1.3 243.4G1.2 243.4G1.6 243.4G0.9 243.4G2.2 243.5G6.9 243.5G1.5 243.5G2.4 243.5G2.3 243.5G2.2 40Ar*/39Ar 36Ar/40Ar 39Ar/40Ar 37Ar/39Ar !1000 JZ.017791 3.848 9.756 9.334 11.281 10.622 10.976 11.079 1.416 0.775 0.495 0.011 0.213 0.085 0.061 0.151 0.079 0.091 0.088 0.088 0.089 0.089 0.410 0.216 0.133 0.000 0.000 0.000 0.000 JZ.017791 7.528 7.542 7.993 7.591 7.703 7.852 8.167 8.128 8.197 8.162 8.238 8.044 8.013 8.034 8.077 7.746 !1000 % 39Ar 0.6 1.5 3.3 7.1 11.3 26.2 42.5 % Atm 41.8 22.9 14.6 0.3 6.3 2.5 1.8 AGEG1sd 119.5G22 288.8G15 277.2G32 330.0G3.3 312.3G2.8 321.8G1.7 324.6G1.4 (continued on next page) 668 H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 Table (continued) 10 11 12 13 14 15 40Ar*/39Ar 36Ar/40Ar 39Ar/40Ar 37Ar/39Ar % 39Ar % Atm AGEG1sd 11.197 11.225 11.067 11.057 11.027 10.951 11.096 11.679 0.074 0.088 0.057 0.055 0.066 0.131 0.306 0.065 0.087 0.087 0.089 0.089 0.089 0.088 0.082 0.084 0.000 0.000 0.000 0.029 0.012 0.028 0.168 0.058 60.7 2.1 67.5 2.6 74.8 1.7 79.6 1.6 89.4 1.9 95.8 3.8 96.8 9.0 100.0 1.9 Total ageZ325.6G3.1 Ma 327.7G1.3 328.5G2.4 324.3G2.6 324.0G2.6 323.2G1.8 321.2G3.8 325.0G13 340.6G3.8 1.273 0.780 0.243 0.128 0.040 0.064 0.043 0.045 0.051 0.112 0.056 0.034 0.169 0.167 0.012 0.077 0.064 0.039 0.070 0.071 0.069 0.070 0.070 0.070 0.068 0.068 0.069 0.073 0.073 0.072 0.001 0.040 0.011 0.011 0.011 0.001 0.085 0.000 0.000 0.000 0.007 0.002 0.031 0.000 0.028 0.3 37.6 1.0 23.0 4.8 7.2 8.6 3.8 17.4 1.1 42.6 1.9 51.4 1.2 60.0 1.3 68.8 1.5 72.4 3.3 78.3 1.6 94.3 1.0 95.4 5.0 97.5 4.9 99.9 0.3 Total ageZ411.9G3.8 Ma 243.8G18 351.4G11 634.8G3.5 397.1G3.3 397.6G6.2 404.3G2.4 405.7G2.9 401.6G2.0 404.8G1.7 408.0G5.7 410.6G4.3 408.2G2.9 375.9G6.0 377.4G3.4 395.8G2.3 2.902 2.833 0.114 0.044 0.106 0.059 0.094 0.049 0.106 0.068 0.048 0.136 0.008 0.048 0.138 0.186 0.100 0.074 0.071 0.073 0.068 0.069 0.069 0.070 0.069 0.070 0.070 0.069 0.069 0.071 0.069 0.067 0.455 0.886 0.002 0.001 0.000 0.010 0.125 0.000 0.000 0.000 0.005 0.014 0.008 0.005 0.030 0.040 0.3 85.7 0.4 83.7 11.0 3.3 31.2 1.3 35.2 3.1 42.8 1.7 49.2 2.8 58.0 1.4 63.4 3.1 75.1 2.0 78.7 1.4 81.5 4.0 85.9 2.3 95.3 1.4 98.1 4.0 99.9 5.5 Total ageZ397.6G3.8 Ma 44.9G25 69.2G58 390.0G2.6 389.0G5.1 405.1G6.0 405.6G2.4 404.3G3.8 405.0G3.2 404.5G3.9 403.0G1.9 402.0G4.9 399.1G4.9 406.6G5.9 399.9G3.0 398.4G5.8 404.7G11 0.199 0.045 0.027 0.035 0.021 0.023 0.033 0.033 0.022 0.017 0.007 0.002 0.111 0.091 0.095 0.095 0.094 0.093 0.094 0.097 0.096 0.097 0.096 0.095 0.094 0.090 0.004 0.011 0.002 0.000 0.025 0.017 0.007 0.000 0.000 0.014 0.000 2.090 0.020 13.5 30.2 38.5 44.4 54.3 63.8 71.4 79.4 89.5 93.4 96.5 99.3 100.0 !1000 VN414 BIOTITE Laser 10 11 12 13 14 15 JZ.017791 8.133 12.087 23.707 13.840 13.860 14.120 14.174 14.014 14.141 14.266 14.366 14.272 13.022 13.079 13.791 !1000 VN415 BIOTITE Laser 10 11 12 13 14 15 16 JZ.017791 1.417 2.200 13.565 13.527 14.151 14.170 14.122 14.148 14.128 14.071 14.032 13.918 14.212 13.949 13.889 14.137 !1000 VN514 BIOTITE Laser 10 11 12 13 JZ.017734 10.274 10.31 10.428 10.474 10.61 10.486 10.138 10.222 10.221 10.356 10.485 10.616 10.715 5.9 1.3 0.8 1.0 0.6 0.7 0.9 0.9 0.6 0.5 0.2 0.2 3.3 302.0G1.5 303.0G1.4 306.1G1.3 307.4G1.1 311.0G2.0 307.7G1.6 298.3G2.2 300.6G1.9 300.5G1.7 304.2G1.9 307.7G2.7 311.2G2.5 313.9G6.1 (continued on next page) H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 669 Table (continued) 40Ar*/39Ar 36Ar/40Ar 39Ar/40Ar 37Ar/39Ar % 39Ar % Atm AGEG1sd Total ageZ304.3 G3.6 Ma !1000 VN515 BIOTITE Laser JZ.017734 11.860 11.999 11.541 11.740 12.174 11.533 11.005 0.161 0.012 0.032 0.077 0.039 0.202 0.127 40Ar*/39Ar 36Ar/40Ar 39Ar/40Ar 37Ar/39Ar %39Ar %Atm Ar Age(Ma)G2 s 10 11 12 13 14 15 !1000 JZ.022995 4.086 3.504 3.208 5.235 5.676 5.820 5.823 5.810 5.731 5.650 5.682 5.323 5.858 5.801 6.032 0.582 0.182 0.420 0.262 0.110 0.073 0.040 0.036 0.034 0.073 0.090 0.067 0.036 0.052 0.065 0.202 0.270 0.272 0.176 0.170 0.168 0.169 0.170 0.172 0.173 0.171 0.184 0.168 0.169 0.162 0.410 0.356 0.382 0.129 0.051 0.028 0.017 0.024 0.033 0.127 0.150 0.130 0.101 0.159 0.196 0.2 17.2 0.7 5.3 2.9 12.4 5.7 7.7 9.9 3.2 19.6 2.1 33.8 1.1 41.9 1.0 47.7 1.0 52.1 2.1 57.3 2.6 64.4 2.0 68.3 1.0 89.1 1.5 99.9 1.9 Total ageZ221.8G3.5 Ma 162.0G52.0 139.8G28 128.4G6.8 205.0G5.4 221.3G3.3 226.6G1.6 226.7G1.1 226.2G1.7 223.3G3.0 220.3G3.7 221.5G2.7 208.3G2.1 228.0G3.8 225.9G0.9 234.3G1.5 10 11 12 13 JZ.017791 4.780 7.945 7.956 7.800 8.064 7.932 7.932 7.895 7.665 7.668 7.849 7.962 7.972 1.474 0.164 0.143 0.18 0.045 0.111 0.111 0.065 0.195 0.174 0.117 0.004 0.063 0.118 0.119 0.120 0.121 0.122 0.121 0.121 0.124 0.122 0.123 0.122 0.125 0.120 0.160 0.000 0.005 0.004 0.014 0.026 0.026 0.041 0.091 0.000 0.000 0.000 0.000 0.8 43.5 8.2 4.8 17.5 4.2 30.2 5.3 41.2 1.3 48.2 3.2 55.2 3.2 59.2 1.9 61.2 5.7 65.3 5.1 68.8 3.4 71.9 0.1 100.0 1.8 Total ageZ237.2G2.3 Ma 147.2G23.1 238.5G2.7 238.8G2.1 234.4G1.9 241.8G2.0 238.1G2.6 238.1G2.6 237.1G1.4 230.6G4.1 230.7G8.3 235.8G2.7 239.1G2.8 239.3G1.2 10 JZ.0228 4.157 3.457 4.681 6.198 6.236 6.314 6.225 6.026 5.953 6.063 1.456 0.836 0.507 0.141 0.047 0.059 0.045 0.170 0.128 0.113 0.137 0.217 0.181 0.154 0.158 0.155 0.158 0.157 0.161 0.159 0.092 0.125 0.049 0.008 0.002 0.007 0.023 0.032 0.055 0.063 Sample VN295 BIOTITE Population 0.080 0.082 0.085 0.083 0.081 0.081 0.087 0.000 0.000 0.002 0.046 0.014 0.260 0.013 37.5 4.7 66.0 0.3 88.6 0.9 92.4 2.2 97.3 1.1 98.3 5.9 100.0 3.7 Total ageZ343.3G4.1 Ma 344.4G1.7 348.0G1.6 335.9G2.3 341.2G5.1 352.6G2.5 352.6G2.6 352.6G2.7 !1000 VN810 BIOTITE Laser !1000 VN294 BIOTITE Population 0.9 2.2 6.9 22.4 54.6 68.8 74.1 78.3 82.0 87.3 43.0 24.7 15.0 4.1 1.4 1.7 1.3 5.0 3.7 3.3 163.3G18.4 136.8G13.3 182.9G3.4 238.4G1.6 239.8G0.9 242.6G1.1 239.4G2.9 232.2G3.8 229.6G4.4 233.6G3.0 (continued on next page) 670 H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 Table (continued) Sample 40Ar*/39Ar 11 12 13 14 6.129 6.186 6.209 4.794 36Ar/40Ar 39Ar/40Ar 37Ar/39Ar %39Ar %Atm Ar Age(Ma)G2 s 0.108 0.084 0.135 0.745 0.158 0.157 0.154 0.162 0.060 0.076 0.193 0.754 92.5 3.1 96.7 2.4 99.3 4.0 100 22.0 Total ageZ233.6G12 Ma 236.0G3.2 238.0G4.4 238.8G6.1 187.1G24.7 1.274 0.416 0.088 0.026 0.010 0.009 0.003 0.001 0.002 0.006 0.211 0.171 0.148 0.149 0.149 0.148 0.144 0.129 0.141 0.146 0.335 0.130 0.035 0.029 0.000 0.036 0.133 0.115 0.079 0.184 2.4 37.6 9.1 12.3 16.9 2.6 35.7 0.7 63.7 0.3 73.3 0.2 76.8 0.1 80.7 0.0 95.8 0.1 99.9 0.1 Total ageZ233.6G12 Ma 110.3G22.7 187.6G9.4 237.4G6.7 239.0G3.9 240.8G1.9 242.7G1.2 248.0G.9 276.1G2.1 254.1G3.5 244.7G11.9 2.35 0.994 0.288 0.073 0.000 0.053 0.063 0.061 0.139 0.043 0.051 0.065 0.001 0.011 0.003 0.050 0.093 0.119 0.122 0.121 0.119 0.121 0.122 0.120 0.122 0.123 0.121 0.127 0.116 0.121 0.011 0.052 0.027 0.028 0.025 0.033 0.035 0.032 0.018 0.007 0.011 0.014 0.058 0.000 0.052 0.1 69.4 0.3 29.3 2.5 8.5 10.3 2.1 21.3 0.0 29.9 1.5 37.0 1.8 43.4 1.8 48.4 4.1 60.2 1.2 72.7 1.5 83.4 1.9 86.0 0.0 87.0 0.3 99.9 0.10 Total ageZ242.0G2.3 Ma 184.3G200 227.5G40 229.8G3.6 239.1G1.2 245.7G1.6 245.7G2.1 241.6G1.1 240.2G1.5 238.7G1.6 241.0G1.3 239.6G1.3 242.5G1.0 236.3G1.7 255.2G1.9 246.0G1.1 2.532 1.033 1.844 1.019 0.941 0.678 0.511 0.075 0.114 0.034 0.094 0.148 0.469 0.746 0.133 0.102 0.066 0.091 0.088 0.098 0.104 0.120 0.119 0.121 0.119 0.116 0.112 0.100 0.246 0.032 0.023 0.012 0.000 0.006 0.011 0.014 0.021 0.019 0.002 0.010 0.097 0.037 1.1 74.8 3.3 30.5 8.2 54.5 16.7 30.1 24.4 27.8 32.2 20.0 39.8 15.1 53.3 2.2 62.6 3.3 75.1 1.0 84.7 2.7 89.3 4.4 91.7 13.8 100.0 22.0 Total age Z238.1G2.6 Ma 59.9G14 207.2G7.6 209.9G4.6 230.7G2.7 245.1G2.9 245.0G2.8 245.8G2.3 244.7G1.0 244.3G1.1 245.6G1.3 244.9G1.8 247.5G2.5 231.7G5.6 234.0G3.5 0.194 0.049 0.103 0.132 0.079 0.056 0.118 0.118 0.116 0.119 0.121 0.122 0.000 0.012 0.000 0.000 0.000 0.001 !1000 VN358 BIOTITE Population 10 JZ.021358 2.954 5.130 6.584 6.634 6.687 6.743 6.901 7.745 7.081 6.802 !1000 VN796 BIOTITE Laser 10 11 12 13 14 15.00 JZ.017791 6.048 7.557 7.636 7.966 8.203 8.202 8.056 8.006 7.954 8.035 7.985 8.089 7.869 8.544 8.214 !1000 VN800 BIOTITE Laser 10 11 12 13 14 JZ.017832 1.894 6.827 6.921 7.650 8.162 8.158 8.188 8.146 8.135 8.181 8.153 8.247 7.688 7.768 !1000 VN812 BIOTITE Laser JZ.017791 7.981 8.341 8.354 8.045 8.062 8.071 5.4 17.9 26.5 34.9 42.1 49.8 5.7 1.4 3.0 3.9 2.3 1.6 239.5G3.7 249.6G1.9 250.0G1.5 241.3G3.5 241.8G2.5 242.0G3.0 (continued on next page) H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 671 Table (continued) Sample 40Ar*/39Ar 10 11 8.048 8.188 7.689 8.116 8.209 36Ar/40Ar 39Ar/40Ar 37Ar/39Ar %39Ar %Atm Ar Age(Ma)G2 s 0.060 0.031 0.062 0.041 0.022 0.122 0.121 0.128 0.122 0.121 0.020 0.032 0.044 0.000 0.000 54.1 2.0 56.8 0.9 68.2 1.8 73.1 1.2 100.0 0.6 Total ageZ243.4G2.6 Ma 241.4G4.4 245.3G5.9 231.3G8.5 243.3G2.8 245.9G2.5 0.044 0.063 0.007 0.367 0.043 0.000 0.113 0.125 0.085 0.015 0.005 0.050 0.451 0.063 0.113 0.117 0.117 0.121 0.125 0.119 0.119 0.120 0.123 0.123 0.147 0.115 1.608 0.063 0.072 0.068 0.040 0.050 0.044 0.000 0.036 0.000 0.000 0.010 0.084 0.5 1.3 4.6 1.8 10.3 0.2 17.6 10.8 33.3 1.2 50.2 0.1 58.3 3.3 70.2 3.7 83.4 2.5 87.6 0.4 96.9 0.1 98.6 1.5 100.0 13.3 Total ageZ242.6G2.7 Ma 438.5G25 257.6G7.4 254.2G1.9 229.0G5.7 244.1G4.2 239.5G4.5 242.4G5.3 242.0G3.6 242.31G2.4 241.5G2.8 242.5G4.0 202.3G5.8 225.5G33 2.462 0.294 0.256 0.082 0.114 0.122 0.155 0.167 0.366 0.112 0.176 0.071 0.116 0.120 0.123 0.118 0.119 0.118 0.116 0.110 0.117 0.117 0.200 0.036 0.021 0.020 0.004 0.018 0.000 0.047 0.038 0.084 0.014 2.3 72.7 9.6 8.7 20 7.5 30.9 2.4 44.4 3.3 55.2 3.6 63.2 4.5 69.2 4.9 74.5 10.8 79.1 3.3 100.0 5.2 Total ageZ237.7G2.5 Ma 117.2G10 236.8G3.2 230.3G7.1 237.8G3.8 244.1G3.0 242.5G2.0 241.2G2.9 244.9G3.5 241.3G3.8 246.2G9.9 242.2G1.4 1.99 1.241 0.279 0.108 0.058 0.091 0.007 0.149 0.021 0.097 0.057 0.152 0.06 0.366 0.117 0.125 0.126 0.125 0.122 0.110 0.115 0.122 0.126 0.122 0.328 0.25 0.022 0.008 0.008 0.016 0.034 0.129 0.071 0.074 0.02 0.033 0.1 58.8 2.7 36.6 8.2 8.2 19.2 3.1 31.4 1.7 36.3 2.6 39.3 0.2 43.3 4.4 46.7 0.6 61.5 2.8 93 1.7 99.9 4.5 Total age Z238.7G2.4 Ma 213.2G150 56.2G9.7 242.2G4.0 237.9G2.1 241.1G2.0 239.4G5.5 250.7G7.1 264.7G5.6 264.2G6.2 245.4G1.6 240.8G.8.0 240.3G3.2 3.192 1.165 0.061 0.048 0.130 0.098 0.124 0.113 0.097 0.120 0.121 0.122 0.121 0.120 0.004 0.000 0.000 0.000 0.010 0.055 0.049 !1000 VN814 BIOTITE Laser 10 11 12 13 JZ.017791 15.468 8.627 8.507 7.608 8.144 7.982 8.084 8.070 8.080 8.053 8.088 6.672 7.485 10 11 JZ.017791 3.775 7.884 7.655 7.919 8.145 8.086 8.041 8.172 8.047 8.220 8.078 !1000 VN799 BIOTITE Laser !1000 VN357 BIOTITE Population 10 11 12 JZ.018342 6.841 1.726 7.834 7.687 7.795 7.739 8.128 8.616 8.599 7.944 7.785 7.770 !1000 VN798 BIOTITE Laser JZ.017791 0.500 6.758 8.126 8.142 7.856 8.014 7.909 0.1 0.7 18.1 25.8 31.5 36.0 41.5 94.3 34.4 1.8 1.4 3.8 2.8 3.6 15.9G72 204.8G19 243.6G1.6 244.0G1.9 236.0G2.8 240.4G3.8 237.5G3.9 (continued on next page) 672 H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 Table (continued) Sample 40Ar*/39Ar 10 11 12 13 14 15 16 8.293 8.168 8.168 8.226 7.908 7.960 8.220 8.171 7.959 36Ar/40Ar 39Ar/40Ar 37Ar/39Ar %39Ar %Atm Ar Age(Ma)G2 s 0.001 0.024 0.005 0.045 0.135 0.098 0.070 0.051 0.036 0.120 0.121 0.122 0.119 0.121 0.121 0.119 0.120 0.124 0.039 0.011 0.053 0.056 0.080 0.032 0.044 0.000 0.014 46.8 0.1 60.8 0.7 66.9 0.1 70.6 1.3 72.4 3.9 77.1 2.9 81.7 2.0 84.4 1.5 99.9 1.0 Total ageZ241.8G2.3 Ma 248.2G4.2 244.7G1.5 244.7G2.7 246.4G4.8 237.4G4.8 238.9G3.02 246.2G2.8 244.8G3.7 238.9G1.16 1.302 0.051 0.288 0.070 0.047 0.043 0.088 0.057 0.007 0.059 0.039 0.075 0.058 0.093 0.113 0.156 0.104 0.103 0.111 0.120 0.121 0.123 0.124 0.121 0.120 0.123 0.119 0.119 0.120 0.120 0.121 0.119 0.208 0.002 0.047 0.030 0.018 0.016 0.016 0.014 0.018 0.012 0.003 0.003 0.011 0.000 0.010 0.030 0.4 38.4 0.6 1.5 6.0 8.5 12.8 2.0 22.0 1.3 32.6 1.2 46.1 2.6 54.9 1.6 61.0 0.2 64.3 1.7 79.3 1.1 85.2 2.2 88.5 1.7 94.8 2.7 97.8 3.3 99.9 4.6 Total ageZ243.0G2.4 Ma 180.2G25 282.6G11 247.0G2.5 244.0G1.7 243.8G1.8 240.1G2.3 236.7G3.2 243.0G2.2 247.0G2.6 239.2G3.6 248.0G1.5 247.0G2.1 246.1G3.8 242.4G1.6 238.9G3.3 240.8G3.9 2.164 0.404 0.153 0.074 0.104 0.051 0.133 0.125 0.150 0.378 0.386 0.524 0.229 0.071 0.094 0.111 0.118 0.119 0.119 0.118 0.117 0.116 0.118 0.114 0.117 0.112 0.120 0.120 0.078 0.033 0.022 0.037 0.007 0.037 0.004 0.019 0.059 0.050 0.127 0.148 0.051 0.051 0.7 63.9 2.5 11.9 10.0 4.5 20.5 2.1 30.8 3.0 37.2 1.5 45.8 3.9 55.1 3.6 62.6 4.4 68.4 11.1 70.9 11.4 73.2 15.5 79.6 6.7 99.9 2.1 Total age Z242.2G2.3 Ma 118.7G8.4 237.7G4.7 243.5G1.7 245.3G1.9 244.8G1.2 249.0G2.3 246.3G2.5 247.7G2.0 242.8G2.4 234.9G3.0 227.1G5.2 227.7G4.7 234.0G2.5 245.0G1.3 0.342 1.745 0.886 0.147 0.131 0.112 0.165 0.197 0.216 0.147 0.128 0.148 0.123 0.052 0.151 0.113 0.122 0.119 0.118 0.116 0.112 0.109 0.115 0.115 0.114 0.115 0.002 0.122 0.016 0.006 0.006 0.003 0.016 0.038 0.053 0.044 0.053 0.100 0.193 !1000 VN813 BIOTITE Laser 10 11 12 13 14 15 16 JZ.017791 5.907 9.531 8.248 8.143 8.135 8.003 7.883 8.105 8.248 7.971 8.283 8.25 8.217 8.084 7.961 8.028 !1000 VN808 BIOTITE Laser 10 11 12 13 14 JZ.017791 3.823 7.916 8.123 8.189 8.171 8.320 8.224 8.273 8.100 7.818 7.540 7.564 7.785 8.178 !1000 VN364 BIOTITE Laser 10 11 12 13 JZ.017791 17.369 3.199 6.545 7.830 8.081 8.199 8.231 8.406 8.548 8.328 8.361 8.389 8.402 0.2 2.0 7.1 17.0 28.2 36.8 47.2 53.3 61.8 72.7 79.4 84.1 86.3 10.1 51.5 26.1 4.3 3.8 3.3 4.9 5.8 6.4 4.3 3.8 4.3 3.6 485.8G115 99.86G15 198.7G5.4 235.3G3.6 242.3G2.3 245.6G2.3 246.5G2.1 251.4G3.1 255.4G2.7 249.2G1.7 250.1G2.3 250.9G4.0 251.3G1 (continued on next page) H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 673 Table (continued) Sample 40Ar*/39Ar 14 15 16 17 8.428 8.416 8.422 8.176 36Ar/40Ar 39Ar/40Ar 37Ar/39Ar %39Ar %Atm Ar Age(Ma)G2 s 0.202 0.291 0.283 0.232 0.112 0.109 0.109 0.114 0.076 0.013 0.154 0.040 89.7 5.9 92.2 8.6 94.7 8.3 100.0 6.8 Total age Z242.8G2.4 Ma 252.0G6.1 251.7G10 251.9G7.1 245.0G3.8 2.176 0.503 0.892 0.918 0.000 0.001 0.015 0.000 0.016 0.004 0.025 0.000 0.021 0.342 0.181 0.143 0.162 0.14 0.138 0.14 0.137 0.133 0.137 0.137 0.137 0.134 0.699 0.202 0.385 0.394 0.001 0.011 0.040 0.006 0.025 0.007 0.047 0.046 0.142 0.8 64.3 1.8 14.8 3.6 26.3 12.1 27.1 27.5 0.0 38.1 0.0 42.0 0.4 44.7 0.0 48.8 0.4 56.9 0.1 78.3 0.7 95.3 0.0 99.9 0.6 Total age Z247.8 G4.7 Ma 39.7G53 172.5G42 187.4G23 164.8G5.7 255.9G2.7 258.0G0.6 254.8G1.4 259.8G3.1 265.9G0.6 261.1G5.4 259.6G2.0 260.9G0.7 264.7G0.9 3.130 1.761 0.983 0.658 0.466 0.447 0.452 0.477 0.376 0.320 1.122 0.008 0.054 0.074 0.088 0.095 0.094 0.097 0.098 0.100 0.088 0.078 0.085 0.074 0.057 0.076 0.052 0.083 0.164 0.289 0.096 0.366 0.108 9.7 92.5 22.5 52.0 33.7 29.0 48.7 19.4 62.0 13.7 69.5 13.2 82.5 13.3 90.5 14.1 96.1 11.1 99.3 9.4 100.0 33.1 Total ageZ267.7G3.3 Ma 276.9G42.3 261.7G3.3 278.2G3.9 268.1G1.6 266.3G1.9 269.6G3.4 261.9G2 256.4G2.8 259.4G3.7 298.3G3.6 251.4G21 !1000 VN389 BIOTITE Laser 10 11 12 13 JZ.021358 1.041 4.698 5.125 4.480 7.136 7.199 7.104 7.253 7.437 7.291 7.246 7.287 7.400 !1000 VN522 BIOTITE Laser 10 11 JZ.017482 9.490 8.928 9.536 9.165 9.098 9.220 8.938 8.734 8.845 10.284 8.551 Ar is radiogenic argon River (VN362, VN363, VN805 and VN 811) that yield the youngest ages 4.2.1 Song Bien Group Biotites in VN 414 and VN 415, have the oldest ages, of 405.7G3.8 and 403.4G3.8 Ma, respectively VN 414 is a slightly anatectic mesosome of a granulite facies quartzite with quartz, mesoperthitic microcline, garnet, biotite, prismatic sillimanite, cordierite, rutile and spinels (pure Fe and Fe–Zn), cordierite (only in some compositional layers), rutile, ilmenite and zircon VN 415 is a metasedimentary granulite, which looks like a boudinaged restite in which cordierite–hercynite coronas are well developed The mineralogy of these two samples corresponds to the better-preserved granulite facies primary assemblages in the granulites, which have been dated in this study Late secondary minerals in these two samples are only tiny grains of diaspore, muscovite and chlorite along microcracks No new foliation is observed They not show evidence for any later polymetamorphic reworking and their primary biotites yield the oldest ages found for these rocks Consequently, the plateau ages of these primary biotites record the youngest possible age limit for the granulite facies metamorphism As shown in Fig 11, biotites of VN515, VN413 and VN514 yield intermediate ages of 343.3G4.2, 325.6G3.1 and 304.2G3.6 Ma respectively Their host rocks are granulite facies paragneisses: VN514 and VN515 are slightly anatectic mesosomes and contain tiny muscovites and chlorites; VN413 is an anatectic leucosome In comparison with the two former samples, some biotites have been transformed to chlorites, or have suffered loss of Ti, giving rise to exsolution of rutile needles The cordierite is much altered, with formation of pinite The first explanation for these younger biotite ages is the influence of a late retrogressive event, as described above, which results in argon loss This conclusion is expressed in the Fig 11 by the symbol of increasing intensity of reworking However, if these younger ages are consistent with the petrology and mineralogy, the fact that plateau ages were 674 H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 relicts of Zn-rich spinel, and Ti-biotite is completely transformed to oxychlorite and chlorite Prismatic sillimanite is still present and occurs frequently as inclusions within chloritoid Plagioclase and K-feldspar are completely transformed to white micas and a secondary muscovite has developed as phenoblasts The new foliation is prominent and pervasive Samples VN505 and VN512 represent the most intense, low-to-medium grade overprint and are typical polymetamorphic granulites where secondary muscovites have nucleated These muscovites yield plateau ages at 262.7G3.2 and 270.7G 2.5 Ma As the muscovites represent a secondary phase, these ages demonstrate that the granulite facies metamorphism cannot be younger than 262–270 Ma These secondary muscovite ages are related to the total overprint by a late, low-grade facies, as indicated by metamorphic assemblages in those rocks Fig 10 Synthesis of plateaux ages of charnockites, according to their geographic position (1) refers to samples used for calculated average age (2) Younger age (VN295, W Song Ba), and older ages (VN800, 813, 808, 364, Song Ba); (VN389, Dak To); (VN522, Bu Nu) obtained for each sample has to be explained and several interpretations can be proposed: the plateau ages could represent the overprint of a subsequent thermotectonic, late, low-to medium-grade phase However, age spectra (Fig 7c– e) not provide evidence for a partial re-equilibration, especially for the less retentive sites, but yield a plateau related to 80–90% of the 39Ar released As the plateau ages are significantly different and span between 304.2G3.6 and 343.3G4.2 Ma, this result contradicts such a hypothesis which would imply a similar plateau age for all three samples; the plateau ages represent the closure of the systems during cooling But, as these samples are located in the same unit and very close to each other, such differences in age are surprising, unless they have been moved tectonically from their original position More probably, the trend of their flat age spectra, which does not indicate a significant argon loss related to the less retentive domains, would imply the closure of the Ar systems at 343, 325 and 304 Ma respectively, representing the decompressive-retrograde evolution of the granulite facies metamorphism Up-to-now, the conclusion is not obvious, due to the scarcity of continuous outcrops in this region that makes a clear knowledge of the structural imbrications of the series difficult 4.2.2 Nuoc Dang Group (Kimson formation) Samples VN505 and VN512 are located in the north west of the Song Bien area (Fig 2), along the Nuoc Dang Brook The samples are quartz-chloritoid micaschists and contain only relict assemblages of granulite facies metamorphism: the strongly broken garnets contain 4.2.3 Granulites of the Song Ba Group VN362, VN363, VN805 and VN811 (Fig 11) are granulite facies metapelites to quartzitic metatectic granulites They all contain granulite facies parageneses with Ti-rich biotite, garnet, and prismatic sillimanite Matrix biotites yield ages between 241.1G2.4 and 247.8G2.4 Ma that contrast markedly with those of other granulites from the Song Bien Group where they cluster around 244 Ma Because these rocks crop out in a domain which is largely occupied by intrusive charnockites and the biotites yield a similar age, we interpret this age as the result of intrusion of the charnockitic material linked with a local increase of temperature, leading to a complete resetting of biotites vs radiogenic argon, and Fig 11 Synthesis of plateaux ages of granulites, according to their degree of reworking and their geographic position H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 not as the age of the peak granulite facies metamorphism This interpretation is also consistent with the age of the most recycled granulites from Nuoc Dang (260– 270 Ma), and are younger than the age of the less retrogressed granulites, the biotites of which have ages around 400 Ma Eighteen zircons from a granulite sampled in the same locality as VN811 has given 23 U–Pb SHRIMP ages around 253.7G11.6 Ma and one core age of 1404G 34 Ma (Nam et al., 2001) Despite the difference between the Ar–Ar average data at 244 Ma and U–Pb data at 253 Ma, which is related to the higher closure temperature of the zircon vs U–Pb system, the concordance of these results demonstrates that granulites of the Song Ba Group suffered strong reheating, followed by very rapid cooling Nevertheless, there is still doubt concerning the age of the granulite facies metamorphic peak: the oldest U–Pb core age of one zircon at 1404 Ma was interpreted by Nam et al (2001), either as a minimum age for the protolith of the granulites, or reflecting an earlier high-grade metamorphism in Mid-Proterozoic times (1400–1600 Ma) Lan et al (2003) presented new Sm/Nd data from a sample of granulite, close to our sample VN805, in the Song Ba Valley, they determined TDM which was 2.7 Ga Two significantly different TDM ages were obtained by the same authors on two charnockites, at 1.9 Ga and 1.5 Ga, both from the Song Ba Valley According to Lan et al (2003), the crust in central Vietnam is not rich in recycled Archean rocks, but mainly in Proterozoic rocks, whose ages are between and Ga But nevertheless, concerning the granulites, the TDM age of 2.7 Ga of Lan et al (2003), and the 2.7 Ga of Nagy et al (2001) still cast doubt on the protolith age of these early rocks Whatever the interpretation, the oldest Ar–Ar age of 405 Ma from the Song Bien Group probably reflects the youngest limit of the granulite facies metamorphic peak The Kan Nack Complex is formed by high-grade anatectic granulite facies for which the protolith age is, perhaps, as old as 1400 Ma This complex is intruded by charnockitic magmas The age of the granulite facies peak metamorphism appears to be debatable, considering results obtained by both radiometric methods U–Pb SHRIMP data on zircons from one granulite sample lead to an age of 254 Ma, but Ar–Ar data provide the youngest limit at 400 Ma, as indicated by samples containing the best preserved primary granulite facies assemblages This large discrepancy can be explained, as the U–Pb age determinations were not carried out on the best preserved granulite facies gneisses, which were used for the argon analysis U–Pb ages of 254 Ma relate only to the overprint of high-temperature charnockitic intrusions in the Song Ba Valley We thus infer that the Ar– Ar age of 400 Ma obtained on the best preserved granulite facies gneisses represents a minimum age (youngest) for the granulite facies metamorphic peak, 675 and may be intermediate between the true peak metamorphism age and the Permo-Triassic tectonometamorphic event Until now, we have not succeeded in separating zircons from this best preserved facies, which will probably yield an age older than 400 Ma Subsequently, this complex suffered a decompression P–T–t path that resulted in younger Ar–Ar ages: a low-tomedium grade metamorphic event overprinted these highgrade rocks, at w265–270 Ma This was followed by the widespread intrusion of charnockitic magmas at 240– 245 Ma in the core of the complex The emplacement of charnockites was contemporaneous with the thermotectonic episode which affected the entire Truong Son Belt around 245 Ma Except for one specific unit (Dien Binh Unit, to the east of the Po Ko Fault) the major part of the Kon Tum Massif, including the Kan Nack Complex was subjected to a Permo-Triassic tectono-metamorphic episode and therefore should not be viewed as an independent unit with respect to the Truong Son Belt The Kon Tum Massif clearly plays an integral part in the construction of the Permo-Triassic structures of the Indochina Block, although the paucity of continuous outcrops and the lack of exposed tectonic contacts makes precise reconstruction difficult According to the structural and kinematic data collected along the western border of the Kon Tum Massif, the N–S Po Ko Suture might have been the site of the westwards subduction of eastern Indochina (Kon Tum Block) beneath an intermediate block (western Indochina or the Khorat Block), located to the east of Sibumasu Left-lateral to top-to-the East shearing along Po Ko Suture, prior to extensional movements, are consistent with right-lateral shearing in the Truong Son Belt and accounts for the oblique collision of Indochina with both South China and Sibumasu-West Indochina, in Permo-Triassic times (Lepvrier et al., 2004 in press) Acknowledgements We are grateful for a fellowship grant awarded to Mr Vu Van Tich by the French Embassy in Vietnam We have benefited from the financial contribution of the Institut National des Sciences de l’Univers (Centre National de la Recherche Scientifique) through the Programme International de Coope´ration Scientifique “Vietnam” The cooperative programs between University Paris 6, University Montpellier and the National University of Vietnam, Hanoi, also greatly contributed to the field work We are deeply indebted to Mr Nguyen Xuan Bao and Mr Trinh Van Long for their help and knowledge in the field, and for logistical support Prof B Windley and Dr A Carter are warmly thanked for their valuable comments during the reviewing All the Vietnamese people we met in the field are thanked for their kindness 676 H Maluski et al / Journal of Asian Earth Sciences 25 (2005) 653–677 Appendix Experimental conditions Ar–Ar radiometric method was applied, using both population dating (with a classical HF furnace) and single grain dating (using a continuous laser) For population analyses 80–100 mg of pure separated mineral were encapsulated in evacuated quartz vials which were loaded as two or three superposed crowns, each of them containing one monitor In order to reduce the vertical irradiation gradient effect, the 40Ar/39Ar ratio measured on each monitor was used for age calculation for each relatedcrown samples The duration of irradiation under fast neutrons was 60 h at the Osiris reactor of Centre d’E´tudes Nucle´aires in Saclay (France) For laser analyses, single grain samples were wrapped in pure Al foil packets, loaded in the can as superposed layers, each containing a monitor Irradiation was undertaken in the Mac Master Reactor in Ontario (Canada) for 70 h under fast neutrons For both types of irradiation, monitors used were the 520.4G1.7 Ma MMHb-hornblende (Alexander et al., 1978) and the Caplongue Hornblende 344.5G3 Ma Laser single grain analyses were carried out using a LEXEL 3500 continuous W argon-ion laser for stepwise heating procedure and a MAP 215-50 noble gas mass spectrometer equipped with a Nier source and a JOHNSTON MM1 electron multiplier for the mass analysis We corrected measured isotopes from the blanks, atmospheric contamination, mass discrimination, and irradiationinduced mass interferences Radioactive decay of 37Cl and 39Ar was taken into account Age calculation was made using constants recommended by Steiger and Jaăger (1977) and McDougall and Harrison (1988) Reported errors are 1sigma for plateau and total ages, which include uncertainties of the monitors and their 40Ar/39Ar ratios References Alexander, E.C., Mickelson, G.M., Lanphere, M.A (1978) MM Hb-1: a new 40Ar/39Ar dating 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1986 The main tectonic features of Vietnam First Conference on Geology of Indochina, I pp 363–375 Tung-Yi Lee, Ching-Hua Lo, Sun-Lin Chung, Chin-Yu Chen, PeiLing Wang, Wen-Pin Lin, Nguyen Hoang, Cung Thuong Chi, Nguyen Trong Yem, 1998 40Ar/39Ar dating results of neogene basalts in Vietnam and its tectonic implication, in: Flower, M., SunLin Chung, Ching-Hua Lo, Tung-Yi Lee (Eds.), Mantle Dynamics and Plate Interactions in East Asia A.G.U Geodynamics Series 27, pp 317–330 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 202 ... towards the Sathay Valley and towards the Kon Tum Basin and probably extends more to the south beneath the Pleiku plateau basalts reaching the lower course of the Song Ba To the east of the fault... Geology of Northern-Central Vietnam and Kon Tum Massif: an overview 2.1 Northern-Central Vietnam The Truong Son Belt (Fig 1) occupies North-central Vietnam and Eastern Laos to the north of the Kon Tum. .. carbonates in the matrix This minor alteration may explain the trend of age spectra related to the low temperature steps (7–10% of 39Ar) rather than a late thermal episode at w100–150 Ma Nevertheless,