geologic and metamorphic evolution of the basement complexes in the kontum massif, central vietnam

Available online at www.sciencedirect.com

(2007) 438–453www.elsevier.com/locate/gr

Gondwana Research 12

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 Japanb 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, Vietnamd 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, Vietnamf 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 (N900 °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(b700–740 °C at b0.50 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. Zhang et al., 2000; Dabie–Sulu belt, China)although much older extreme metamorphic rocks have been

⁎ 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 Rdoi:10.1016/j.gr.2007.01.003

found 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: Zhang et al., 2000, Sulu Complex: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–

esearch. Published by Elsevier B.V. All rights reserved.

mailto:[email protected]

http://dx.doi.org/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.

439N. Nakano et al. / Gondwana Research 12 (2007) 438–453

440 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 hasbeen subdivided into Kannak, Ngoc Linh and Kham Duccomplexes (Fig. 1b) 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

Table 1Representative rock assemblages from the Kannak, Ngoc Linh and Kham Duccomplexes in the Kontum Massif

Rock types

Complexes andmetamorphic facies

Pelitic/felsicrocks

Mafic/intermediate rocks

Calc–silicaterocks

Western Kannak Grt–Opxgranulite

Opx–Cpxgranulite

Wo–Cpxgranulite

Granulite to UHTgranulite-facies

Grt–Opx–Crd–Sil granulite

Grt–Opx–Cpxgranulite

Wo–Cpx–Splgranulite

[Granulite-facies] Grt–Opx–Btgranulite

Opx–Cpx–Hblgranulite

Ol–Spl–Phlmarble

Grt–Crd–Sil–Bt gneiss

Cpx–Scpgneiss

Eastern Kannak Grt–Sil–Btgneiss


Amphibolite- togranulite-facies

Sil–Bt gneiss Opx–Hbl granulite

[Granulite-facies] Grt–Crd–Sil–Bt gneiss

Amphibolite

Grt–Bt gneiss Grt amphiboliteSil–Crd–Btgneiss

Cpx amphibolite

Hbl–Bt gneiss

Western Ngoc Linh Grt–Opx–Btgneiss

Grt–Opx–Cpxgranulite

Wo–Cpxgranulite

Granulite to UHTgranulite-facies

Opx–Bt gneiss Grt–Opx–Cpx–Hbl granulite

Ol marble

[Amphibolite-facies] Grt–Crd–Sil–Bt gneiss


Grt–Cpx–Ttngneiss

Grt–Sil–Btgneiss

Amphibolite Cpx–Trgneiss

Eastern Ngoc Linh Grt–Bt gneiss Cpx amphibolite Cpx–Ttn gneissEp amphibolite- toamphibolite-facies

Bt–Sil gneiss Amphibolite Cpx–Epgneiss

[Amphibolite-facies] Bt gneiss Hbl–Bt gneiss Cpx–Tr gneisBt–Ms gneiss Hbl–Bt–Ep gneiss Ms–Gr

marble

Kham Duc Grt–Bt gneiss Amphibolite Hbl–Cpx–Epgneiss

Grt–Ky–Bt–Msgneiss

Grt amphibolite Grt–Hbl–Ttngneiss

Greenschist- toamphibolite-facies

Grt–St–Btgneiss

Grt–Epamphibolite

Cpx marble

[Greenschist- toblueschist-facies)

Grt–Bt–Msschist

Grt–Hbl–Bt gneiss Ol–Cpx marble

Grt–Ms schist Hbl–Ep–Bt gneiss

Metamorphic facies in brackets are after Department of Geology andMinerals ofVietnam (1998a,b,c,d,e,f).

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 extrememetamorphic conditions in this massif with evidences forUHT 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 andpetrography of the metamorphic rocks distributed in theKontum Massif. We also estimate the P–T conditions basedon reaction textures and mineral chemistry of garnet and otherphases 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 ofthe massif based on the present data and recently reportedpetrological and geochronological data in an attempt to trace thehistory of continental collision in Southeast Asia.

Mineral abbreviations used in figures and tables in this paperfollow those of Kretz (1983).

2. Geological outline

In this section, we describe the distribution and modes ofoccurrence of metamorphic rocks in the three differentcomplexes (Fig. 1b; 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 ofthe Kontum Massif (Fig. 1b). 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 KannakComplexes.

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. 3b). In theeastern 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).

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.


Hornblende-free orthopyroxene–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 (southeastof 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. 1b), 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 presentknowledge, 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


observed as blocks, lenses and disrupted layers in themylonitised felsic gneisses (Fig. 3e,f). In the eastern NgocLinh Complex (west of Ba To town and Quang Ngai city; 14°30′N–15°16′N; 108°15′E–108°32′E), metamorphic rocks are

Table 2Major mineral assemblages of analyzed samples

No. Fig. no. Region Rock type GPS position Foliation Qtz Pl Kf

Kannak Complex (mafic-intermediate)

Nlatitude

Elongitude

[1] 4a–b West Grt–Opx–Cpx granulite

14° 17′35″

108° 29′05″

EW52°N

▵ ⌾ ▵

[2] – West Amphibolite 13° 51′25″

108° 28′45″

N28°W68° E

– ⌾ –

[3] 4c East Opx–Cpx–Hbl gneiss

14° 14′05″

108° 52′10″

N45°W52°W

▵ ○ –

[4] – East Cpxampibolite

14° 37′35″

108° 53′00″

E W20°N

– ○ –

Kannak Complex (pelitic–felsic)[5] 4d–e West Grt–Opx–

Crd–Sil–Btgneiss

14° 18′05″

108° 28′55″

N68°W30°N

⌾ ○ ⌾

[6] – West Grt–Crd–Sil–Btgneiss

14° 10′00″

108° 35′05″

N34°W64°E

▵ ▵ –

[7] 4f East Grt–Crd–Sil–Bt gneiss

14° 14′05″

108° 52′10″

N45°W52°W

▵ ⌾ –

[8] – East Grt–Crd–Si–Bt gneiss

14° 38′45″

108° 51′40″

E W20°N

⌾ ○ ▵

[9] – East Grt–Btgneiss

14° 15′50″

109° 01′00″

N 6 5 °W64°S

⌾ ○ ○

Ngoc Linh Complex (mafic-intermediate)[10] 4g,j West Grt–Opx–

Cpx granulite14° 47′20″

107° 51′50″

NS 74°E ⌾ ▵ –

[11] 4h,k West Grt–Opx–Cpx–Hblgranulite

14° 46′50″

107° 53′50″

N2°W70°E

▵ S –

[12] 4i West Opx–Cpx–Hbl granulite

14° 46′50″

107° 53′50″

N2°W70°E

▵ ⌾ –

[13] – East Cpx–Hbl–Btgneiss

14° 44′20″

108° 39′35″

N60°E60°S

○ ○ ▵

[14] – East Hbl–Bt–Epgneiss

15° 01′10″

108° 29′45″

N 4 6 °W36°N

○ ○ ▵

Ngoc Linh Complex (pelitic–felsic)[15] 4l–m West Grt–Opx–Bt

gneiss14° 46′50″

107° 53′50″

NS 67°E ⌾ ○ ○

[16] – East Grt–Btgneiss

14° 44′15″

108° 33′55″

N38°E34°E

⌾ ▵ ▵

[17] – East Grt–Bt–Msgneiss

15° 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

▵ ○ –

[19] – Grtamphibolite

15° 34′05″

107° 49′30″

N81°E69°S

▵ ○ –

[20] – Cpx–Hbl–Bt gneiss

15° 15′40″

108° 29′50″

N70°E55°S

○ ○ ▵

exposed 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).

s Grt Cpx Opx Hbl Bt Sil Crd Spl Ms St Ky Ep Remarks andreactiontextures

○ ○ S – ▵ – – – – – – – Opx+Pl; Opxrods in Cpx

– – – ⌾ ▵ – – – – – – – –

– ▵ S ⌾ ▵ – – – – – – – Opx+Cpx+Pl

– ▵ – ⌾ – – – – – – – –

○ – S – ▵ ○ ○ S – – – – Opx+Crd;Opx+Pl; Crd+Qtz; Spl+Crd;Spl+Crd+Pl

○ – – – ▵ ⌾ ○ ▵ – – – – Spl+Crd

⌾ – – – ▵ ○ ○ ▵ – – – – Spl+Crd

○ – – – ○ ○ ○ – – – – – –

○ – – – ○ – – – – – – –

⌾ ○ S I – – – – – – – – Opx+Pl; Opxrods in Cpx

⌾ ⌾ S ○ ▵ – – – – – – – Opx+Pl; Opx+Cpx+Pl;Opx rodsin Cpx

– ○ ⌾ ○ ▵ – – – – – – – Opx+Pl (Grtpseudomorph);Opx rodsin Cpx

– ▵ – ▵ ⌾ – – – – – – – –

– – – ○ ⌾ – – – – – – ▵ –

⌾ – ▵ – ⌾ S – S – – – – Elongate Grtassociatedwith Sil+Spl

▵ – – – ○ – – – – – – – –

▵ – – – ○ – – – ▵ – – – –

○ – – ⌾ – – – – – – – ▵ Hbl+Pl

○ – – ⌾ – – – – – – – – –

– ○ – ⌾ ○ – – – – – – – –

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 andreactiontextures

Kham Duc Complex (pelitic–felsic)[21] 4o Grt–Ky–Bt

gneiss15° 32′15″

108° 01′20″

N72°E18°S

⌾ ▵ – ○ – – – ⌾ ○ – – ▵ – ○ – Deformedfibrolite

[22] – Grt–St–Btgneiss

15° 32′15″

108° 01′20″

N72°E18°S

⌾ ○ ○ ○ – – – ⌾ – – – ▵ ○ – – –

[23] – Grt–Bt–Msschist

15° 15′10″

108° 07′15″

N72°E80°S

⌾ – – ○ – – – ▵ – – – ○ – – – –

⌾, abundant; ○, moderate; ▵, poor;–, absent; S, symplectite; I, inclusion.Localities of each sample are shown in Figs. 2 and 3. Brackets and parentheses indicate presence only as symplectites and as inclusion, respectively.Foliations of boulders, undeformed rocks and strongly migmatized rocks were measured from neighbor outcrops.

Table 2 (continued )


In general, mafic rocks from the western Ngoc LinhComplex are characterised by a high-pressure granulite-faciesmineral paragenesis (garnet–clinopyroxene–orthopyroxene–quartz–plagioclase± spinel±hornblende). Nakano et al. (2004)obtained UHT metamorphic conditions of c. 1050 °C and1.3 GPa from hornblende-free mafic granulites. Amphibolite-facies rocks of clinopyroxene–hornblende–biotite, pyroxene-free amphibolite and hornblende–biotite gneisses are predom-inantly distributed in the eastern Ngoc Linh Complex (west ofBa To town) and the epidote-bearing mafic rocks exposed in thenorth-eastern part (west of Quang Ngai city; Fig. 2).

High-temperature mineral parageneses such as garnet+orthopyroxene and garnet+cordierite+sillimanite are identifiedfrom unmylonitised pelitic and felsic gneisses. On the otherhand, many garnet-absent and muscovite-bearing gneisses occurin the north-eastern Ngoc Linh Complex (Fig. 2). Muscovite-free garnet–biotite and biotite–sillimanite gneisses are exposedin the south-eastern part of the complex (around Ba To town).

2.3. Kham Duc Complex

The Kham Duc Complex is located in the northern andsouth-western parts of the Kontum Massif (Fig. 1b). Only thenorthern Kham Duc Complex (hereafter, Kham Duc Complex)was investigated in this study. Metamorphic rocks in thesouthern part of the Kham Duc Complex were mylonitisedalong the Kham Duc shear zone (Fig. 3h). In general, themetamorphic rocks show an E–W trending foliation with ahigh-angle dip towards north or south. Felsic mylonite iscomposed mainly of plagioclase, K-feldspar porphyroclasts andstrongly elongated ribboned-quartz with or without hornblende.In the western part of this complex, pelitic and psammitic schistsare dominant. Various metamorphosed layers of mafic, ultra-mafic and felsic rocks are locally intercalated with pelitic andpsammitic rocks (Fig. 3i).

Through detailed field investigations, it was identified thatamphibolite- to epidote amphibolite-facies (partly greens-chist-facies) mafic rocks are widely exposed in this complex,which often contain garnet and/or clinopyroxene.

Garnet–muscovite assemblage is usually observed to bewidespread in the pelitic rocks. In addition, these pelitic rocksare characterised by the presence of kyanite and staurolite

(Fig. 2) indicating low- to medium-temperature with moder-ate- to high-pressure metamorphic conditions. Preliminary P–T estimates indicate a clockwise P–T path (Osanai et al., 2005;Nakano et al., in review).

3. Petrography and reaction textures

The major mineral assemblages and other information onanalysed samples are summarised in Table 2. Only character-istic microstructures are described in this section. Referencenumbers in brackets in the description given below correspondto those in Table 2.

3.1. Kannak Complex

3.1.1. Mafic rocksIn the western Kannak Complex, some mafic granulites

contain granoblastic garnet coexisting with clinopyroxene,plagioclase and minor quartz. Garnet–orthopyroxene–clinopyr-oxene granulite ([1]) consists of orthopyroxene+plagioclasesymplectite between garnet and clinopyroxene (Fig. 4a), whichindicates decompression process during granulite-facies meta-morphic condition. All clinopyroxenes in the granulite ([1])include orientated orthopyroxene rods (Fig. 4b).

In the eastern Kannak Complex, mafic rocks commonlyshow granoblastic texture; however, symplectitic intergrowth isnot observed. Only orthopyroxene–clinopyroxene–hornblendegneiss ([3]) has orthopyroxene+clinopyroxene+plagioclasesymplectite around hornblende (Fig. 4c). These symplectiticpyroxenes contain numerous fine-grained hornblende (Fig. 4c)inclusions, which indicate hornblende-dehydration.

3.1.2. Pelitic rocksGarnet–orthopyroxene–cordierite–sillimanite–biotite gneiss

([5]) from the western Kannak Complex is composed of garnet,cordierite, sillimanite, biotite, quartz, K-feldspar and minorplagioclase. Orthopyroxene and spinel are observed only assymplectites. Orthopyroxene+cordierite and spinel+cordieritesymplectite occur around garnet porphyroblasts (Fig. 4d).Intergrowths of orthopyroxene+plagioclase, cordierite+quartzand spinel+cordierite±plagioclase are also observed betweengarnet and K-feldspar (Fig. 4e). These cordierite or spinel

Fig. 4. Photomicrographs and backscattered images showing representative microstructures of metamorphic rocks from the Kannak (a–f), Ngoc Linh (g–m) and KhamDuc (n–o) complexes. (a) Opx+Pl symplectite between Grt and Cpx (sample [2] in Table 2; W. Kannak), (b) Opx rods in Cpx ([2]; W. Kannak), (c) Opx+Cpx+Plsymplectite around Hbl ([3]; E. Kannak), (d) Opx+Crd and Spl+Crd symplectite around Grt porphyroblast ([5]; W. Kannak), (e) Various symplectites (Opx+Crd,Opx+Pl, Crd+Qtz and Spl+Crd±Pl) between Grt and Kfs ([5]; W. Kannak), (f) Spl+Crd±Sil symplectite replacing Grt ([7]; E. Kannak), (g) Opx+Pl symplectiteamong Grt, Cpx and Qtz ([10]; W. Ngoc Linh), (h) Opx+Pl symplectite among Grt, Cpx, Hbl and Qtz ([11]; W. Ngoc Linh), (i) Grt pseudomorph replaced by Opx+Plin Grt-free granulite ([12]; W. Ngoc Linh), (j) Opx rods in Cpx ([10]; W. Ngoc Linh), (k) Opx+Cpx+Pl symplectite between Hbl and Qtz. Note that secondary Cpxalso contains Opx rods ([11]; W. Ngoc Linh), (l) Porphyroblastic Opx in pelitic gneiss ([15]; W. Ngoc Linh), (m) Strongly elongated Grt associated with Sil and Spl([15]; W. Ngoc Linh), (n) Pl-rich Hbl+Pl corona around Grt ([18]; Kham Duc), (o) Porphyroblastic Grt and Ky with deformed Sil ([21]; Kham Duc).


Table 3Representative microprobe analyses of garnet

Complexes Kannak Ngoc Linh Kham Duc

Referenceno.

[1] [5] [6] [7] [8] [9] [10] [11] [15] [16] [17] [18] a [19]a [21] [22] a [23] a

Rocktypes

M P P P P P M M P P P M M P P P

SiO2 38.93 40.52 39.72 38.79 38.33 38.15 38.08 38.54 38.88 38.04 37.55 37.78 38.65 38.25 38.04 37.90TiO2 0.00 0.00 0.00 0.13 0.00 0.00 0.00 0.42 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.10Al2O3 21.72 22.77 22.31 21.88 21.55 21.36 21.44 21.11 21.77 21.24 21.33 21.21 21.76 21.65 21.32 21.45Cr2O3 0.34 0.00 0.00 0.00 0.00 0.29 0.00 0.00 0.00 0.24 0.25 0.00 0.00 0.00 0.00 0.00FeO 26.20 21.92 24.47 29.84 30.36 31.20 26.21 27.98 30.21 31.90 28.25 24.88 29.09 28.40 31.01 26.99MnO 0.97 0.86 0.46 1.23 0.97 2.50 0.00 0.93 0.09 3.92 10.94 3.75 1.52 4.80 6.40 8.90MgO 6.40 13.37 11.50 7.61 6.94 5.39 3.27 4.42 8.41 4.55 1.82 1.75 4.38 4.79 3.04 2.66CaO 6.31 1.64 1.85 1.28 1.39 1.73 10.12 7.62 1.08 0.92 1.17 10.19 5.85 3.08 2.03 2.67Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.20K2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Total 100.87 101.08 100.31 100.76 99.54 100.62 99.12 101.02 100.44 100.81 101.31 99.56 101.25 100.97 101.84 100.87O 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12Si 3.000 3.001 3.000 3.000 3.010 3.002 3.012 3.004 3.005 3.009 3.005 3.011 3.008 2.999 3.006 3.012Ti 0.000 0.000 0.000 0.008 0.000 0.000 0.000 0.025 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.006Al 1.973 1.988 1.986 1.994 1.995 1.981 1.999 1.939 1.983 1.981 2.012 1.992 1.996 2.001 1.986 2.009Cr 0.021 0.000 0.000 0.000 0.000 0.018 0.000 0.000 0.000 0.015 0.016 0.000 0.000 0.000 0.000 0.000Fe 1.689 1.358 1.546 1.930 1.994 2.053 1.734 1.824 1.952 2.111 1.891 1.658 1.893 1.862 2.050 1.794Mn 0.063 0.054 0.029 0.081 0.065 0.167 0.000 0.061 0.006 0.263 0.742 0.253 0.100 0.319 0.428 0.599Mg 0.735 1.476 1.295 0.877 0.812 0.632 0.385 0.513 0.969 0.536 0.217 0.208 0.508 0.560 0.358 0.315Ca 0.521 0.130 0.150 0.106 0.117 0.146 0.858 0.636 0.089 0.078 0.100 0.870 0.488 0.259 0.172 0.227Na 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.031K 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Total

cation8.003 8.006 8.006 7.995 7.993 7.999 7.988 8.002 8.004 7.993 7.982 7.993 7.994 8.000 8.001 7.993

Fe3+ 0.008 0.017 0.019 0.000 0.000 0.000 0.000 0.007 0.012 0.000 0.000 0.000 0.000 0.000 0.002 0.000Fe2+ 1.681 1.341 1.527 1.930 1.994 2.053 1.734 1.817 1.940 2.111 1.891 1.658 1.893 1.862 2.048 1.794XMg 0.304 0.524 0.459 0.312 0.289 0.235 0.182 0.220 0.333 0.203 0.103 0.111 0.212 0.231 0.149 0.149Alm 0.560 0.447 0.509 0.645 0.667 0.685 0.582 0.600 0.646 0.706 0.641 0.555 0.633 0.621 0.681 0.611Prp 0.245 0.492 0.431 0.293 0.272 0.211 0.129 0.170 0.322 0.180 0.074 0.070 0.170 0.187 0.119 0.107Grs 0.174 0.043 0.050 0.035 0.039 0.049 0.288 0.210 0.030 0.026 0.034 0.291 0.163 0.086 0.057 0.077Sps 0.021 0.018 0.010 0.027 0.022 0.056 0.000 0.020 0.002 0.088 0.251 0.085 0.034 0.106 0.143 0.204

Reference numbers in brackets correspond with those in Table 2. Rock types of P and M indicate pelitic or felsic and mafic to intermediate rocks, respectively.a Rim composition.


forming reaction textures in the gneiss suggest decompression([5]).

In the eastern Kannak Complex, reaction textures aregenerally absent in most pelitic gneisses ([8],[9]), which aremainly composed of garnet, biotite, plagioclase and K-feldsparwith or without cordierite. However, in a few cases, garnetporphyroblasts are replaced by a decompression assemblage ofspinel+cordierite±sillimanite symplectite ([7]; Fig. 4f).

3.2. Ngoc Linh Complex

3.2.1. Mafic rocksMafic granulites from the western Ngoc Linh Complex

generally contain garnet+clinopyroxene+quartz±hornblendewith minor plagioclase. Decompression texture of orthopyrox-ene+plagioclase symplectite is commonly developed at thegrain boundaries of the primary minerals ([10]; Fig. 4g and [11];Fig. 4h). Orthopyroxene+plagioclase symplectite in garnet-freegranulite ([12]; Fig. 4i) is regarded as garnet pseudomorphstabilised at a high-pressure stage. Orthopyroxene is also

observed as thin rods within granoblastic clinopyroxene in suchgranulites ([10],[11],[12]; Fig. 4j). In addition, minor orthopyr-oxene+clinopyroxene+plagioclase symplectites are formedbetween hornblende and quartz ([11]; Fig. 4k). It is noted thatfine-grained orthopyroxene rods are also present withinsecondary clinopyroxene (Fig. 4k), which indicates that theorthopyroxene was formed during cooling after decompression.These decompression and cooling textures are similar to thosein mafic granulite observed from the western Kannak Complex.

In the eastern Ngoc Linh Complex, mafic rocks showporphyroblastic texture and do not contain any symplectiticintergrowths ([13], [14]).

3.2.2. Pelitic rocksOrthopyroxene in garnet–orthopyroxene–biotite gneiss

([15]) from the western Ngoc Linh Complex occurs only asporphyroblast (Fig. 4l), and no reaction textures are preserved inpelitic gneisses from throughout the complex. The garnet–orthopyroxene–biotite gneiss ([15]) consists of two types ofgarnet porphyroblast: rounded-shape porphyroblast including

Fig. 5. Chemical compositions of Grt plotted on the almandine–pyrope–grossular (or spessartine) ternary diagrams. Reference numbers in brackets correspond withthose in Table 2.


quartz and biotite and elongated porphyroblast associated withspinel and sillimanite (Fig. 4m). The former is in equilibriumwith orthopyroxene but the latter might have formed during thedeformation stage at low-pressure condition.

3.3. Kham Duc Complex

3.3.1. Mafic rocksDecompression texture is recognized only in coarse-grained

garnet amphibolites ([18]) with plagioclase-rich hornblende+plagioclase coronas around garnet (Fig. 4n). This indicatesdecompression under amphibolite-facies condition.

3.3.2. Pelitic rocksKyanite-bearing rocks always contain sillimanite (Fig. 4o).

Garnet and kyanite porphyroblasts generally show subhedral

Table 4KD values and Ti contents in hornblende for each rock

KD (Grt–Hbl) Kannak Complex Ngoc Linh Complex

Reference no. [2] [3] [4] [10] [11]Range – – – – 2.49–3.14Ti in Hbl (pfu) 0.21–0.27 0.24–0.31 0.21–0.31 0.26–0.36 0.31–0.36

KD (Grt–Bt) Kannak ComplexReference no. [5] [6] [7] [8] [9]Range 1.53–2.05 1.55–2.14 1.51–1.72 2.15–2.76 2.12–2.45

KD (Grt–Hbl) and KD (Grt–Bt) were calculated by the formulas of KD=(Mg/Fe)Hbl /Reference numbers in brackets correspond with those in Table 2.

shape (Fig. 4o), whereas sillimanite (or fibrolite) is highlydeformed and forms gneissose fabric with fine-grained biotite.The sillimanite is considered to have formed through decom-pression from kyanite stability field during the deformationstage.

4. Thermobarometric constraints

4.1. Background

Fe–Mg partitioning between garnet and other ferromagnesianminerals is known as one of the powerful tools for estimatingmetamorphic temperatures (e.g. Ferry and Spear, 1978; Thomp-son, 1979;Graham andPowell, 1984).Mineral composition-basedthermobarometric indicators are also widely used in recent times(e.g. Harley and Motoyoshi, 2000; Sajeev and Osanai, 2004).

Kham Duc Complex

[12] [13] [14] [18] [19] [20]– – – 7.18–9.90 4.54–5.91 –0.25–0.35 0.10–0.14 0.08–0.12 0.14–0.18 0.05–0.10 0.13–0.19

Ngoc Linh Complex Kham Duc[15] [16] [17] [21] [22] [23]2.69–3.05 2.60–3.20 3.71–4.32 3.02–4.09 4.63–6.04 5.01–6.20

(Mg/Fe)Grt and (Mg/Fe)Bt / (Mg/Fe)Grt, respectively. pfu means per formula unit.

Fig. 6. Distribution of KD values between Grt and Bt or Hbl and Ti contents in Hbl. Reference numbers in brackets correspond with those in Table 2.


Recent experimental studies pointed out that the Ti content inhornblende increases with increasing temperature (e.g. Ernst andLiu, 1998; López and Castro, 2001). In this study, we performed aregional scale estimation of KD values using garnet–biotite orgarnet–hornblende pairs depending upon the rock type examinedand the equilibrium assemblages preserved. In addition, weexamined the Ti content in hornblende for all appropriate samplesto plot the variations in metamorphic temperature. The method ofKD estimation follows that of Sajeev and Osanai (2005) bycarefully selecting equilibrated Fe–Mg minerals surrounded byfelsic minerals in the matrix assuming that the pair preserves thepeak-metamorphic composition. Hence, symplectitic mineralswere not used in KD and P–T calculations. Ti contents ofhornblende in all mafic rocks are buffered byminor constituents ofilmenite or titanite, so that the Ti contents can be regarded as theindicator of the metamorphic temperature conditions.

4.2. Mineral chemistry

Electron microprobe analyses were performed using ascanning electron microscope with an energy dispersivespectrometry system (JEOL JED2140-JSM 5301S) at KyushuUniversity. The quantitative analyses of rock-forming mineralswere performed with an accelerating voltage of 15 kV, usingdata processing by the oxide-ZAF model correction program.

In general, distinct chemical zoning is absent in all ferromag-nesian minerals except garnet, however, a negligibly thin Mg-richrim is observed in some clinopyroxenes, biotites, hornblendes and

orthopyroxenes. Most plagioclases observed in this study haveAn-rich thin rims. Garnets in almost all samples have Mg-rich,Mn- and Fe-poor wide cores with thin Mg-poor, Mn- and Fe-richrims. Based on detailed petrographic observation and careful dataselection, we assume that the Mg-rich homogeneous cores ofgarnet are in equilibrium with the homogeneous core offerromagnesian minerals and preserve the peak-metamorphicconditions. Some garnets (samples [18], [19], [22] and [23] fromthe Kham Duc Complex) exhibit chemical zonation showing anincrease of Mg and a decrease of Mn from core to rim, which mayimply that chemical diffusion has not strongly affected because ofthe low-temperature conditions. In these cases, the cores are not inequilibrium with any matrix minerals, and generally rimcompositions are used to obtain peak-temperature conditions(e.g. Matsumoto et al., 2003; Liu et al., 2004).

Chemical compositions of garnet used in KD calculation areshown in Table 3 and Fig. 5. Garnet in mafic rocks showsalmandine-rich composition (Alm53–65) with moderate to largeamounts of grossular molecules (up to Grs30; Fig. 5).Spessartine components in garnet from the Kham Duc complexare slightly higher (Sps3–10) than those from the Kannak andNgoc Linh complexes (Sps0–2). Garnets in pelitic gneisses havea wide range of Fe–Mg ratio (XMg 0.09–0.54) and spessartinemolecules (Sps0–30). Garnets in pelitic rocks from the westernKannak Complex have the highest-pyrope component in theanalysed samples (Fig. 5). Garnets from the western Ngoc Linh,eastern Kannak, eastern Ngoc Linh and Kham Duc complexesvary from pyrope-rich to spessartine-rich in that order (Fig. 5).

Table 5Thermobarometric results of suitable samples

Complexes Kannak Ngoc Linh Kham Duc

Region West East West East

Reference no. [1] [5] [6] [7] [8] [9] [10] [11] [15] [16] [18] [19] [21]

Rock types M P P P P P M M P P M M P

Thermometry (°C)Grt–Bt P&L 882 907 915 783 816 758 736 694

B92 841 869 915 759 797 728 708 652Grt–Opx H85 816

S&B 940Grt–Crd H&L 983 968 918 822

B88 904 914 897 830Grt–Cpx E&G 930 989 932

R 900 940 910Grt–Hbl G&P 774 600 573Pref. (GPa) 1.10 1.10 1.10 0.80 0.60 0.80 1.20 1.20 0.80 6.00 1.00 1.00 0.80

Barometry (GPa)Grt–Opx–Pl–Qtz P&C 0.79Grt–Als–Pl–Qtz H&C 1.03 1.12 0.84 0.63 0.86Grt–Bt–Pl–Qtz H90 0.82 0.50Grt–Hbl–Pl–Qtz K&S89 0.79Grt–Hbl–Pl–Qtz K&S90 0.86Grt–Cpx–Pl–Qtz N&P 1.04 1.50 1.15Tref. (°C) 1000 1000 900 900 800 800 1000 900 800 700 600 600 700

P&L, Perchuk and Lavrent'eva, 1983; B92, Bhattacharya et al. (1992); H85, Harley (1985); S&B, Sen and Bhattacharya (1984); H&L, Holdaway and Lee (1977);B88, Bhattacharya et al. (1988); E&G, Ellis and Green (1977); R, Ravna (2000); G&P, Graham and Powell (1984); P&C, Perkins and Chipera (1985); H&C, Hodgesand Crowley (1985); H90, Hoisch (1990); K&S89, Kohn and Spear (1989); K&S90, Kohn and Spear (1990); N&P, Newton and Perkins (1982).Rock types of P and M indicate pelitic or felsic and mafic to intermediate rocks, respectively.


These compositional variations of garnet in pelitic rocks form atrend from pyrope-rich to spessartine-rich via almandine-richcompositions (Fig. 5), which might reflect the change inmetamorphic grade from high to low, provided that there are nosignificant variations in bulk chemistry of the rock. Grossularcomponents of all garnets are rather low (less than Grs11);among these, samples from the Kham Duc Complex show aslightly higher grossular component (Grs6–11) as compared withother garnets (Grs2–6).

4.3. Results

The calculated KD values between garnet–biotite and garnet–hornblende pairs as well as the Ti content in hornblende arepresented in Table 4. The KD values of all analysed samples rangefrom 1.5 to 6.2 for garnet+biotite pairs and 2.3 to 9.9 for garnet+hornblende pairs (Table 4). Ti contents in hornblende vary from

Table 6Sm–Nd isotopic data for granulite and gneiss from the western Ngoc Linh Complex

Grt–Opx–Cpx–Hbl granulite [10]

W.R. Grt Hbl

Nd (ppm) 7.982 1.125 22.447Sm (ppm) 2.290 1.258 5.180147Sm/144 Nd 0.1735 0.6762 0.1395143Nd/143Nd 0.512477 0.513268 0.5124232σ error 0.000022 0.000067 0.000017

W.R. and F.F. indicate whole rock and felsic fraction, respectively.

0.05 to 0.36 throughout the massif (Table 4). The regionalvariations are illustrated in Fig. 6. The P–T conditions of thesegarnet-bearing samples, except the spessartine-rich garnet-bearing samples [17], [22] and [23], are shown in Table 5.Throughout the massif, the calculated temperatures range from570 to 990 °C and pressures vary from 0.5 to 1.5 GPa.

The western Kannak and Ngoc Linh complexes are char-acterised by the highest-grade metamorphism in the KontumMassif based on their low-KD values and high-Ti content inhornblende (Fig. 6). Thermobarometric estimates indicate thepossibility of high-pressure and high-temperature (c. 900–990 °Cat 1.03–1.50 GPa; Table 5) conditions.

The chemical features among garnet, biotite and hornblendefrom the eastern Kannak and Ngoc Linh complexes show low- tohigh-KD (Fig. 6), which correspond with granulite- to amphib-olite-facies P–T conditions (c. 700–900 °C) with medium- tolow-pressures (0.50–0.84 GPa; Table 5). In the Kham Duc

Grt–Opx–Bt gneiss [13]

Cpx W.R Grt F.F.

6.568 76.256 10.360 7.8091.645 13.700 8.397 1.2840.1514 0.1086 0.4901 0.09940.512446 0.511752 0.512370 0.5117380.000015 0.000014 0.000025 0.000019

Fig. 7. Isochron diagrams for Grt–Opx–Cpx–Hbl granulite [11] (a) and Grt–Opx–Bt gneiss [15] (b). Individual analyses are also shown. W.R. and F.F. indicate wholerock and felsic fraction (Pl+Kfs+Qtz), respectively.


Complex, estimated metamorphic conditions suggest the lowesttemperature in the massif (c. 570–700 °C) and relatively high-pressure (up to 0.86 GPa).

5. Sm–Nd geochronology

The lithologic characteristics, reaction textures and thermo-barometric results suggest that the metamorphic signatures ofthe western Kannak and Ngoc Linh complexes are quite similar.The timing of the high-grade metamorphism in the westernKannak Complex has been recognized as Permo-Triassic (e.g.Maluski and Lepvrier, 1998; Tran Ngoc Nam, 1998; Tran NgocNam et al., 2001; Osanai et al., 2001). Hence, metamorphic age

Fig. 8. Compiled age data from the Kontum Massif modified after Osan

from the western Ngoc Linh Complex is one of the key issues inunderstanding the tectono-metamorphic evolution of the massif.

Garnet–orthopyroxene–clinopyroxene–hornblende granu-lite block ([11]) and surrounding host garnet–orthopyroxene–biotite gneiss ([13]) from the western Ngoc Linh Complex wereanalysed using Sm–Nd internal isochron method. Garnet,clinopyroxene, hornblende and whole rock as well as garnet,felsic fraction (plagioclase+K-feldspar+quartz) and wholerocks were measured for samples [11] and [13], respectively.The isotopic analyses were carried out using thermal ionizationmass spectrometer (TIMS) equipped with five faraday cups(MAT-261) and nine faraday cups (MAT-262) at NiigataUniversity, Japan. 143Nd/144Nd ratio was corrected with

ai et al. (2004). The age data are represented by million years (Ma).


Geological Society of Japan standard JNdi-1 of 0.512116. Theresults are presented in Table 6. The isochron ages werecalculated using the Isoplot/Ex v.3 program (Ludwing, 2003).Individual 2σ errors for each mineral are used in thesecalculations.

The ages obtained, within error, are identical with 240±16Ma in the mafic block (Fig. 7a) and 247±11Ma in the peliticgneiss (Fig. 7b), which prove that the mafic block wasmetamorphosed together with pelitic gneiss during the samethermal event. In addition, it is noted that the Permo-Triassicages are also consistent with those of high-grade metamorphicrocks in the western Kannak Complex (Fig. 8).

6. Discussion

In our study, decompression textures were observed in allrocks within the massif (e.g. orthopyroxene+cordierite, ortho-pyroxene+plagioclase, spinel+cordierite and hornblende+pla-gioclase symplectites replacing garnet), whereas the KD valuesand estimated peak or near-peak conditions differed. On aregional scale, the highest-grade UHT and high-pressure (c.900–990 °C at 1.03–1.50 GPa) metamorphic rocks occurred inthe western Kannak and Ngoc Linh complexes (Fig. 6, Table 5).The UHT metamorphism in Kannak and the Ngoc Linhcomplexes were considered to be a simultaneous event basedon the results of their similar P–T paths (Fig. 9; Osanai et al.,2004; Nakano et al., 2004). The Sm–Nd ages (Fig. 7) also provethat the Permo-Triassic high-temperature and -pressure meta-morphism affected not only Kannak Complex but also the NgocLinh Complex (Fig. 8).

On the eastern Kannak Complex, amphibolite- to granulite-facies rocks are dominant (Fig. 4). In this region, temperature

Fig. 9. Presumed P–T paths for the highest-grade metamorphic rocks from theKannak, Ngoc Linh and Kham Duc complexes in the Kontum Massif afterOsanai et al. (2004, 2005) and Nakano et al. (2004, in review). P–T conditionsfrom eastern Kannak and Ngoc Linh complexes and their assumed P–T gradientare also shown. The reference numbers in brackets correspond with those inTable 5.

condition decreases from south (c. 900 °C) to north (c. 800 °C).In general, these rocks show low-pressure conditions of 0.63–0.84 GPa (Table 5). The eastern Ngoc Linh Complex, wheregarnet–biotite gneiss is the common rock type, showsconsistent P–T conditions of c. 700 °C at 0.50 GPa. In thenorth-eastern Ngoc Linh Complex, epidote amphibolite-faciesrocks are present. Although we were unable to obtain anyquantitative P–T data from these rocks, their mineral assem-blages and chemistries (garnet-free and epidote-bearing maficand muscovite-bearing and Mn-rich garnet-bearing peliticrocks) suggest much lower temperature and pressure conditionsthan the south-eastern Ngoc Linh Complex.

Garnet–cordierite–sillimanite–biotite gneiss from the east-ern Kannak Complex (14°14′25″ N; 108°55′30″ E) dated byMaluski et al. (2002, 2005) yielded Ar–Ar ages of 405–403 Ma(Fig. 8). This gneiss probably corresponds to sample [7](c. 900 °C at 0.84 GPa) based on the mineral assemblages(garnet+cordierite+sillimanite+biotite) and locality (14°14′05″N; 108°52′10″ E). A similar U–Pb SHRIMP zircon age wasalso reported by Tran Ngoc Nam et al. (2004) from hornblende–biotite gneiss in the eastern Ngoc Linh Complex (14°46′05″ N;108°31′07″ E), situated close to the locality of sample [16](c. 700 °C at 0.50 GPa; 14°44′15″ N; 108°33′55″ E). Thecombination of the results from this study and recentlydetermined ages indicates that the metamorphic rocks in theeastern Kannak and Ngoc Linh complexes are formed by thesame metamorphic event (Siluro-Ordovician). Moreover, thethermobarometric data, KD values and Ti content in hornblendeindicate that the metamorphic grade gradually decreases fromsouth to north (Fig. 6, Tables 4 and 5), i.e. from 900 °C at0.84 GPa in the south to 700 °C at 0.50 GPa or even lowertemperature and pressure in the north, which forms a low-P /Tmetamorphic gradient (c. 35–40 °C/km; Fig. 9).

Garnet-bearing rocks are well distributed in the Kham DucComplex. In addition, the complex is characterised by thepresence of muscovite, kyanite and staurolite, which indicatesthat relatively low-temperature and high-pressure metamor-phism occurred in this complex. Thermobarometric results alsosupport this inference (c. 570–700 °C at 0.79–0.86 GPa;Table 5). The preliminary P–T path suggested by Osanai et al.(2005) and Nakano et al. (in review) shows similar shape to thatfrom the Kannak and Ngoc Linh complexes (Fig. 9). Althoughthe detailed metamorphic evolution and age of the Kham DucComplex are not well established at present, P–T conditionsestimated in this study and data from Ar–Ar chronology(Lepvrier et al., 1997, 2004) suggest that a Permo-Triassic low-to medium-temperature and relatively high-pressure metamor-phism affected the complex.

Based on the above results, the possible geological setting ofthe massif is proposed in Fig. 10. The Siluro-Ordovician low-pressure metamorphic rocks are observed only in eastern Kannakand Ngoc Linh complexes at present (Fig. 10). However, similarages were also obtained from U–Pb dating of the orthogneiss inthe Kham Duc Complex (Fig. 8; Carter et al., 2001; Nagy et al.,2001) and inherited U–Th–Pb ages were obtained from the coreof monazite in UHT granulite from the western Kannak Complex(Osanai et al., 2001). These ages suggest that the Siluro-

Fig. 10. Assumed new geological division of the KontumMassif. The line (A–B) on the map is equivalent to the cross section illustrated the lower side. P–T paths andgradient correspond with those in Fig. 9. Dotted lines indicate assumed isograd.


Ordovician thermal event affected the entire massif; however, theevidence for the Permo-Triassic metamorphic event is localisedand observed only in relatively high-pressure rocks (Fig. 10). Thisphenomenon probably indicates a reactivation/reworking of theexisting old continental crust. Although detailed structural studyis one of the keys to answer this question, it can be speculated thatthe high-temperature and high-pressure metamorphic rocks in theKannak and Ngoc Linh complexes are exposed only in narrowzones along the Dac To Kan shear zone (Fig. 10). The widespreadshear zones of the massif probably played a role to uplift thereactivated crust.

We conclude that the Kontum Massif as a whole composes acrustal section of Siluro-Ordovician, in which only granulite- toepidote amphibolite-facies (lower- to middle-crustal levels) areexposed at present (Fig. 10) based on the low-P /T metamorphicgradient (c. 35–40 °C/km) and the ages (450–400 Ma) observedin the eastern Kannak and Ngoc Linh complexes. The lower andupper crustal domains in this region were reactivated by laterhigh-grade and low-grade metamorphism, respectively during thePermo-Triassic (Fig. 10). Similar conclusions were alreadyderived based on detailed structural and geochronological studies

from the Kannak Complex (Maluski et al., 2002; Lepvrier et al.,2004; Maluski et al., 2005), where the c. 450–400 Ma wasinterpreted as the youngest limit of the Precambrian granulite-facies metamorphism and c. 250Mawas interpreted as reworkingof the crust caused by charnockite magma intrusion. However,most Permo-Triassic high-grade metamorphism in Asia isconsidered to be related to collision tectonics, and is representedby high-pressure granulite from the Imjingang belt (e.g. Ree et al.,1996), eclogite from the Gyeonggi massif in South Korea (Ohet al., 2005), UHT granulite from the Higo terrane in Japan (e.g.Osanai et al., 1998) andUHP eclogite from theDabie–Sulu belt inChina (e.g. Ernst and Liou, 1995). The Permo-Triassic high-grademetamorphism of the Kannak Complex is also considered to besimilar, occurring within continental collision setting (e.g. Osanaiet al., 2004). P–T conditions of the Kham Duc Complex(Barrovian-type) and of western Kannak and Ngoc Linhcomplexes (high-pressure granulite-facies) shown in the presentstudy strongly suggest a continental collision setting and crustalthickening. It is difficult to explain such high-pressure conditionsby charnockite intrusion alone. We therefore conclude that theKontum Massif was probably formed from Gondwana-derived


continental fragments that collided in the Permo-Triassic.Subsequently, part of the thickened crust was uplifted fromdeeper levels along the major shear zones.

Acknowledgements

We would like to thank T. Miyamoto, D. D. Kiem, N. V.Canh, Le Van De, Trinh Van Long and D. H. Hanh fordiscussion and help during field trip and M. Arima for financialsupport of the field trip. We are also grateful to S. Suzuki, Y.Hirahara, J. Ishioka and N. Nishi for helpful support on isotopicanalyses. Thanks are also due to B. Fitches, T. Kawakami, W. G.Ernst and C. Lepvrier for critical review of the manuscript aswell as K. Sajeev and M. Santosh to editorial assistance. Thiswork was partly supported by Grant-in-Aid for ScientificResearch (No. 14340150 and No. 17253005: to Y. Osanai) andJSPS Fellowship for Young Scientists (No. 05973: to N.Nakano) from the Ministry of Education, Culture, Sports,Science and Technology, Japan and by a Research Promotionfor Students (to N. Nakano) from Kyushu UniversityFoundation.

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