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Geologic And Metamorphic Evolution Of The Basement Complexes In The Kontum Massif, Central Vietnam

Geologic and metamorphic evolution of the basement complexes in the Kontum Massif, central Vietnam

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  Geologic and metamorphic evolution of the basement complexes in theKontum Massif, central Vietnam  N. Nakano a, ⁎ , Y. Osanai a  , M. Owada  b , Tran Ngoc Nam c , T. Toyoshima d , P. Binh e ,T. Tsunogae f  , H. Kagami d a   Division of Evolution of Earth Environment, Faculty of Social and Cultural Studies, Kyushu University, 4-2-1 Ropponmatsu, Chuo-ku, Fukuoka 810-8560 Japan  b  Division of Earth Sciences, Graduate School of Science and Technology, Yamaguchi University, 1677-1 Yoshida, Yamaguchi 753-8511 Japan c  Department of Geosciences, Hue University, 77 Nguyen Hue Street, Hue, Vietnam d Graduate school of Science and Technology, Niigata University, 8050 Ikarashi-2-nocho, Niigata 950-2181 Japan e  Research Institute of Geology and Mineral Resources, Thanh Xuan, Hanoi, Vietnam f   Institute of Geoscience, University of Tsukuba, 1-1-1 Ten-nou-dai, Ibaraki 305-8571 Japan Received 29 June 2005; accepted 31 January 2007Available online 9 February 2007 Abstract This paper presents a regional scale observation of metamorphic geology and mineral assemblage variations of Kontum Massif, centralVietnam, supplemented by pressure – temperature estimates and reconnaissance geochronological results. The mineral assemblage variations andthermobarometric results classify the massif into a low- to medium-temperature and relatively high-pressure northern part characterised bykyanite-bearing rocks (570 – 700 °C at 0.79 – 0.86 GPa) and a more complex southern part. The southern part can be subdivided into western andeastern regions. The western region shows very high-temperature ( N 900 °C) and -pressure conditions characterised by the presence of garnet andorthopyroxene in both mafic and pelitic granulites (900 – 980 °C at 1.0 – 1.5 GPa). The eastern region contains widespread medium- to high-temperature and low-pressure rocks, with metamorphic grade increasing from north to south; epidote- or muscovite-bearing gneisses in the north( b 700 – 740 °C at   b 0.50 GPa) to garnet-free mafic and orthopyroxene-free pelitic granulites in the south (790 – 920 °C at 0.63 – 0.84 GPa). ThePermo-Triassic Sm –  Nd ages (247 – 240 Ma) from high-temperature and -pressure granulites and recent geochronological studies suggest that thesouth-eastern part of Kontum Massif is composed of a Siluro-Ordovician continental fragment probably showing a low-pressure/temperaturecontinental geothermal gradient derived from the Gondwana era with subsequent Permo-Triassic collision-related high-pressure reactivationzones.© 2007 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.  Keywords:  Low- T   metamorphism; High- T   metamorphism; Siluro-Ordovician; Permo-Triassic; Kontum Massif  1. Introduction Permo-Triassic metamorphic events in East Asia are welldocumented in terranes such as China, the Korean Peninsula andthe south-western part of the Japanese Islands, which arecharacterised by ultrahigh-temperature (UHT) granulites (e.g.Osanai et al., 1998; Higo terrane, Japan) and ultrahigh-pressure(UHP)eclogites(e.g.Zhangetal.,2000;Dabie – Sulubelt,China)although much older extreme metamorphic rocks have beenfound in Asia recently (Santosh et al., 2006, in press). Meta-morphic pressure – temperature (  P  – T  ) evolution of these terranesshows a clockwise prograde path with a near-isothermal decom- pression(e.g.Dabie Complex:Zhangetal.,2000,SuluComplex:Yang and Jahn, 2000, North Dabie Complex: Xiao et al., 2001, Higo terrane: Osanai et al., 2006) after the peak metamorphism;the tectonics of these terranes has been correlated to continentalcollision between the North and South China cratons (e.g. Zhanget al., 1996; Faure et al., 1999; Kim et al., 2000; Oh, 2006; Ohet al., 2006; Osanai et al., 2006; Metcalfe, 2006).The geological framework of Vietnam is characterised byseveral units dissected by well-developed NW – SE to NNW –  Available online at www.sciencedirect.com Gondwana Research 12 (2007) 438 – 453www.elsevier.com/locate/gr  ⁎  Corresponding author. Tel.: +81 92 726 4818; fax: +81 92 726 4843.  E-mail address:  [email protected] (N. Nakano).1342-937X/$ - see front matter © 2007 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.doi:10.1016/j.gr.2007.01.003  Fig. 1. Distribution of NW – SE to NNW – SSE trending shear zones slightly modified after  Lepvrier et al. (2004) (a) and simplified geological map of the KontumMassif modified after  United Nations (1990) (b).Fig. 2. Distribution of orthopyroxene-, epidote-, muscovite- and kyanite-bearing rocks in the Kontum Massif. Localities of analysed samples are also shown.439  N. Nakano et al. / Gondwana Research 12 (2007) 438  –  453  SSE trending shear zones (Fig. 1a). The Kontum Massif incentral Vietnam, previously considered to be the Precambriancrystalline basement of the Indochina craton, is one of themajor geological units (e.g. Hutchison, 1989). The massif has been subdivided into Kannak, Ngoc Linh and Kham Duccomplexes (Fig. 1 b) based on metamorphic grades char-acterised by granulite-, amphibolite- and greenschist-faciesassemblages, respectively (e.g. United Nations, 1990).However, recent works have provided evidence for aPermo-Triassic tectono-metamorphic event in some parts of the massif (e.g. Maluski and Lepvrier, 1998; Tran Ngoc Nam,1998; Tran Ngoc Nam et al., 2001; Osanai et al., 2001; Nagyet al., 2001; Carter et al., 2001). Recently, Osanai et al.(2001, 2004) and Nakano et al. (2004) identified extreme metamorphic conditions in this massif with evidences for UHT metamorphic conditions ( T  =1050 °C at   P  =1.2 – 1.3 GPa). These recent breakthroughs suggest the need of aregional scale reassessment of the geological and tectono-metamorphic evolution of this massif.In this paper, we describe the regional scale occurrence and petrography of the metamorphic rocks distributed in theKontum Massif. We also estimate the  P  – T   conditions basedon reaction textures and mineral chemistry of garnet and other  phases in suitable samples. In addition, we perform reconnais-sance Sm –  Nd isotopic analyses for high-grade mafic and peliticgranulites. Finally, we re-evaluate the geological evolution of the massif based on the present data and recently reported petrological and geochronological data in an attempt to trace thehistory of continental collision in Southeast Asia.Mineral abbreviations used in figures and tables in this paper follow those of  Kretz (1983). 2. Geological outline In this section, we describe the distribution and modes of occurrence of metamorphic rocks in the three different complexes (Fig. 1 b; Kannak, Ngoc Linh and Kham Duccomplexes) of the Kontum Massif, based on microscopicobservation of 1448 samples. The peak/near-peak metamorphicmineral assemblages are summarized in Fig. 2 and therepresentative assemblages of each complex are shown inTable 1. 2.1. Kannak Complex The Kannak Complex is situated in the south-eastern part of the Kontum Massif (Fig. 1 b). The present study identified bothgranulite- and amphibolite-facies mineral parageneses from thiscomplex, which contradicts the traditional concept of anArchean granulite-facies terrain. The mineral assemblagesshow slight variation between western and eastern Kannak Complexes.In the western Kannak Complex (northwest of Kannak andwest of An Khe towns; 13°50 ′  N – 14°19 ′  N; 108°27 ′ E – 108°36 ′ E), NW – SE to E – W trending tonalitic mylonite arefound (Fig. 3a), which shows characteristic right-lateralstrike – slip movement identified from asymmetrical deforma-tion textures. These mylonite outcrops rarely include calc – silicate blocks (Fig. 3a). The pelitic granulites in the westernKannak Complex are usually associated with garnet-bearingS-type tonalite (e.g. Owada et al., in press) (Fig. 3 b). In the eastern Kannak Complex (around the Phu My and Bong Sontowns; 14°5 ′  N – 14°39 ′  N; 108°45 ′– 109°6 ′ E), metamorphicrocks show NW – SE to E – W trending foliation in most cases.Some amphibolites show compositional layering composedof hornblende- and plagioclase-rich layers (Fig. 3c) with rareoccurrences of migmatitic structures (Fig. 3d). Table 1Representative rock assemblages from the Kannak, Ngoc Linh and Kham Duccomplexes in the Kontum Massif Rock typesComplexes andmetamorphic faciesPelitic/felsicrocksMafic/ intermediate rocksCalc – silicaterocksWestern Kannak Grt  – OpxgranuliteOpx – CpxgranuliteWo – CpxgranuliteGranulite to UHTgranulite-faciesGrt  – Opx – Crd – Sil granuliteGrt  – Opx – CpxgranuliteWo – Cpx – Splgranulite[Granulite-facies] Grt  – Opx – Bt granuliteOpx – Cpx – HblgranuliteOl – Spl – PhlmarbleGrt  – Crd – Sil – Bt gneissCpx – ScpgneissEastern Kannak Grt  – Sil – Bt gneissOpx – Cpx – HblgranuliteAmphibolite- togranulite-faciesSil – Bt gneiss Opx – Hbl granulite[Granulite-facies] Grt  – Crd – Sil – Bt gneissAmphiboliteGrt  – Bt gneiss Grt amphiboliteSil – Crd – Bt gneissCpx amphiboliteHbl – Bt gneissWestern Ngoc Linh Grt  – Opx – Bt gneissGrt  – Opx – CpxgranuliteWo – CpxgranuliteGranulite to UHTgranulite-faciesOpx – Bt gneiss Grt  – Opx – Cpx – Hbl granuliteOl marble[Amphibolite-facies] Grt  – Crd – Sil – Bt gneissOpx – Cpx – HblgranuliteGrt  – Cpx – TtngneissGrt  – Sil – Bt gneissAmphibolite Cpx – Tr gneissEastern Ngoc Linh Grt  – Bt gneiss Cpx amphibolite Cpx – Ttn gneissEp amphibolite- toamphibolite-faciesBt  – Sil gneiss Amphibolite Cpx – Epgneiss[Amphibolite-facies] Bt gneiss Hbl – Bt gneiss Cpx – Tr gneisBt  – Ms gneiss Hbl – Bt  – Ep gneiss Ms – Gr marbleKham Duc Grt  – Bt gneiss Amphibolite Hbl – Cpx – EpgneissGrt  – Ky – Bt  – MsgneissGrt amphibolite Grt  – Hbl – TtngneissGreenschist- toamphibolite-faciesGrt  – St  – Bt gneissGrt  – EpamphiboliteCpx marble[Greenschist- to blueschist-facies)Grt  – Bt  – Msschist Grt  – Hbl – Bt gneiss Ol – Cpx marbleGrt  – Ms schist Hbl – Ep – Bt gneissMetamorphic facies inbracketsareafter  Department of GeologyandMineralsof Vietnam (1998a,b,c,d,e,f).440  N. Nakano et al. / Gondwana Research 12 (2007) 438  –  453  Hornblende-freeorthopyroxene – clinopyroxene±garnetmaficgranulites are exposed only in the western Kannak Complex. Inthe eastern Kannak Complex, all mafic granulites containhornblende and either lack or have minor quartz. Intercalationsof amphibolite and hornblende –  biotite gneiss are frequentlyobserved mainly in the north-eastern Kannak complex (southeast of Ba To town).Garnet+orthopyroxene and garnet+cordierite bearing peli-tic rocks are observed mainly in the western Kannak Complex(Fig. 2). The highest-grade pelitic granulite (garnet  – orthopyr-oxene – sillimanite granulite; 1050 °C at 1.2 GPa) from theKontum Massif was found in the western Kannak area (Osanaiet al., 2004). In our field survey, sapphirine-bearing Mg – Al richgranulites were also identified in this area. On the other hand,orthopyroxene was not present in the pelitic and felsic gneissesof the eastern Kannak Complex (Fig. 2). Garnet  – cordierite – sillimanite –  biotite, garnet  – sillimanite±biotite and garnet  –  bio-tite gneisses are the main rock types exposed in this area. 2.2. Ngoc Linh Complex The Ngoc Linh Complex occurs in the western to central-eastern part of the Kontum Massif (Fig. 1 b), which has beenregarded as a Proterozoic amphibolite-facies metamorphiccomplex in previous studies (e.g. Hutchison, 1989). However,our investigations show that mostly high-grade metamorphicrocks are present in the western Ngoc Linh Complex (Fig. 2; 14°24 ′  N – 15°16 ′  N; 107°40 ′ E – 108°9 ′ E). Many rocks in thisregion are mylonitised, especially the exposures around theDac Glei and Dac Rve towns. Based on the present knowledge, it can be concluded that the distribution of thehigh-grade rocks is limited to the area north of Dac To town(Fig. 2).Pelitic or felsic mylonite is well exposed in the western NgocLinh Complex (Fig. 3e) characterised by N – S to NW – SEtrending right-lateral shear in most cases (Fig. 3e). Maficgranulites in the western Ngoc Linh Complex are usually 2 Fig. 3. Modes of occurrence of metamorphic rocks from the Kannak (a – d), Ngoc Linh (e – g) and Kham Duc (h – i) complexes. (a) Felsic mylonite including calc – silicate granulite blocks, (b) Grt-bearing tonalite (white) associated with pelitic granulite (grey), (c) Layered gneiss composed of Amp- and Pl-rich layers, (d)Migmatite composed of amphibolite (black) and granite (white), (e) Mafic granulite blocks within mylonitic felsic gneiss; the mylonite shows a sense of right-lateralstrike – slip movement, (f) Disrupted mafic layers intercalated with felsic gneisses, (g) Migmatite composed of leucosome (white) and melanosome (grey), (h) Maficmylonite along the Kham Duc shear zone, (i) Thin layer of felsic schist intercalated with pelitic schist.441  N. Nakano et al. / Gondwana Research 12 (2007) 438  –  453  observed as blocks, lenses and disrupted layers in themylonitised felsic gneisses (Fig. 3e,f). In the eastern NgocLinhComplex(westofBaTotownandQuangNgaicity;14°30 ′  N – 15°16 ′  N; 108°15 ′ E – 108°32 ′ E), metamorphic rocks areexposed as layered gneiss with distinct thin mafic and felsicgneisses. In addition, migmatitic structures comprising biotitegranitic leucosome and gneissose biotite or biotite – muscovitemelanosome are observed (Fig. 3g). Table 2Major mineral assemblages of analyzed samples No. Fig. no. Region Rock type GPS position Foliation Qtz Pl Kfs Grt Cpx Opx Hbl Bt Sil Crd Spl Ms St Ky Ep Remarks andreactiontexturesKannak Complex (mafic-intermediate) NlatitudeElongitude[1] 4a –  b West Grt  – Opx – Cpx granulite14° 17 ′ 35 ″ 108° 29 ′ 05 ″ EW52°N ▵  ⌾  ▵ ○ ○  S  –  ▵  – – – – – – –  Opx+Pl; Opxrods in Cpx[2]  –  West Amphibolite 13° 51 ′ 25 ″ 108° 28 ′ 45 ″  N28°W68° E –  ⌾  – – – –  ⌾  ▵  – – – – – – – – [3] 4c East Opx – Cpx – Hbl gneiss14° 14 ′ 05 ″ 108° 52 ′ 10 ″  N45°W52°W ▵ ○  – –  ▵  S  ⌾  ▵  – – – – – – –  Opx+Cpx+Pl[4]  –  East Cpxampibolite14° 37 ′ 35 ″ 108° 53 ′ 00 ″ E W20°N –  ○  – –  ▵  –  ⌾  – – – – – – – – Kannak Complex (pelitic – felsic)[5] 4d – e West Grt  – Opx – Crd – Sil – Bt gneiss14° 18 ′ 05 ″ 108° 28 ′ 55 ″  N68°W30°N ⌾  ○  ⌾  ○  –  S  –  ▵ ○ ○  S  – – – –  Opx+Crd;Opx+Pl; Crd+Qtz; Spl+Crd;Spl+Crd+Pl[6]  –  West Grt  – Crd – Sil – Bt gneiss14° 10 ′ 00 ″ 108° 35 ′ 05 ″  N34°W64°E ▵ ▵  –  ○  – – –  ▵  ⌾  ○ ▵  – – – –  Spl+Crd[7] 4f East Grt  – Crd – Sil – Bt gneiss14° 14 ′ 05 ″ 108° 52 ′ 10 ″  N45°W52°W ▵  ⌾  –  ⌾  – – –  ▵ ○ ○ ▵  – – – –  Spl+Crd[8]  –  East Grt  – Crd – Si – Bt gneiss14° 38 ′ 45 ″ 108° 51 ′ 40 ″ E W20°N ⌾  ○ ▵ ○  – – –  ○ ○ ○  – – – – – – [9]  –  East Grt  – Bt gneiss14° 15 ′ 50 ″ 109° 01 ′ 00 ″  N65°W64°S ⌾  ○ ○ ○  – – –  ○  – – – – – – –  Ngoc Linh Complex (mafic-intermediate)[10] 4g,j West Grt  – Opx – Cpx granulite14° 47 ′ 20 ″ 107° 51 ′ 50 ″  NS 74°E  ⌾  ▵  –  ⌾  ○  S I  – – – – – – – –  Opx+Pl; Opxrods in Cpx[11] 4h,k West Grt  – Opx – Cpx – Hblgranulite14° 46 ′ 50 ″ 107° 53 ′ 50 ″  N2°W70°E ▵  S  –  ⌾ ⌾  S  ○ ▵  – – – – – – –  Opx+Pl; Opx+Cpx+Pl;Opx rodsin Cpx[12] 4i West Opx – Cpx – Hbl granulite14° 46 ′ 50 ″ 107° 53 ′ 50 ″  N2°W70°E ▵  ⌾  – –  ○  ⌾  ○ ▵  – – – – – – –  Opx+Pl (Grt  pseudomorph);Opx rodsin Cpx[13]  –  East Cpx – Hbl – Bt gneiss14° 44 ′ 20 ″ 108° 39 ′ 35 ″  N60°E60°S ○ ○ ▵  –  ▵  –  ▵  ⌾  – – – – – – – – [14]  –  East Hbl – Bt  – Epgneiss15° 01 ′ 10 ″ 108° 29 ′ 45 ″  N46°W36°N ○ ○ ▵  – – –  ○  ⌾  – – – – – –  ▵  –  Ngoc Linh Complex (pelitic – felsic)[15] 4l – m West Grt  – Opx – Bt gneiss14° 46 ′ 50 ″ 107° 53 ′ 50 ″  NS 67°E  ⌾  ○ ○  ⌾  –  ▵  –  ⌾  S  –  S  – – – –  Elongate Grt associatedwith Sil+Spl[16]  –  East Grt  – Bt gneiss14° 44 ′ 15 ″ 108° 33 ′ 55 ″  N38°E34°E ⌾  ▵ ▵ ▵  – – –  ○  – – – – – – – – [17]  –  East Grt  – Bt  – Msgneiss15° 07 ′ 30 ″ 108° 35 ′ 05 ″  N56°E38°N ⌾  ○ ▵ ▵  – – –  ○  – – –  ▵  – – – – Kham Duc Complex (mafic-intermediate)[18] 4n Grt ampibolite15° 15 ′ 10 ″ 108° 07 ′ 15 ″  N84°E62°S ▵ ○  –  ○  – –  ⌾  – – – – – – –  ▵  Hbl+Pl[19]  –  Grt amphibolite15° 34 ′ 05 ″ 107° 49 ′ 30 ″  N81°E69°S ▵ ○  –  ○  – –  ⌾  – – – – – – – – – [20]  –  Cpx – Hbl – Bt gneiss15° 15 ′ 40 ″ 108° 29 ′ 50 ″  N70°E55°S ○ ○ ▵  –  ○  –  ⌾  ○  – – – – – – – – 442  N. Nakano et al. / Gondwana Research 12 (2007) 438  –  453