The Kontum Massif is considered as the most massive continuous exposure of Precambrian basement within Indochina (Tien et al., 1989) and has been inferred to be the stable continental core of Southeast Asia (Hutchinson, 1989). The Kontum Massif is mainly composed of high-grade metamorphic rocks which are partly covered by Mesozoic volcano-sedimentary formations and Neogene-Quaternary basalts. The metamorphic rocks were later intruded by Paleo-Mesozoic granodiorite bodies such as Ben Giang-Que Son, Van Canh and Hai Van Complexes (Fig. 2. 2, DGMVN, 1989, 1995, Nam, 1998).
Figure 2. 1. Simplified map of the Kontum Massif and location of study area (after Owada et al., 2019).
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The Kontum Massif has been subdivided into three different tectonostratigraphic terranes of the Kannak, Ngoc Linh, and Kham Duc complexes (Fig. 2. 1).
2.1 Kannak complex
The Kannak complex occupies the southeastern part of the Kontum Massif (Fig.
2. 1; Fig. 2. 2) and consists mainly of granulite facies metamorphic rocks. The principal lithologies for the Kannak complex are garnet ± cordierite ± biotite gneiss, garnet ± cordierite ± sillimanite ± biotite gneiss, garnet ± orthopyroxene ± cordierite gneiss, orthopyroxene ± clinopyroxene gneiss, and wollastonite-bearing calc-silicate gneiss.
These rocks have been subdivided into five subgroups with following sequence of Kon Cot mafic granulite, Xa Lam Co leptynite, Kim Son khondalite, Dak Lo calciphyr granulite and Song Ba orthopyroxene granulite (Figure 2. 2; DGMVN, 1989; Long, 2011).
The formation ages of these subgroups were poorly documented. The oldest subgroup (Kon Cot) has been considered to be formed during Precambrian, which evidenced by 1.4Ma detrital zircon ages for granulite (Nam et al., 2001) and 1.4-1.6Ga Rb-Sr ages (Thi, 1985; DGMN, 1989). This formation age of Kon Cot subgroup made up the upper intercept for all the samples which should be younger than 1.4-1.6Ga.
The Kannak Complex was considered as an Archean complex based on the similarities in metamorphic grade with other Archean granulite compositions in the world (Bao and Thang, 1979; Luong and Bao, 1982; Trang, 1986; Tien, 1991; Bao et al., 1994).
Recent studies documented Neo-Proterozoic to Lower Devonian and Permian-Triassic ages. The Neo-Proterozoic to Lower Devonian age is represented by 401-418Ma SHRIMP U-Pb age of orthogneiss (Carter et al., 2001). Different methods in recent studies also gave similar results. They are 678Ma Sm-Nd age for remaining amphibolite (Nakano et al., 2003), 479Ma U-Th-Pb CHIME age for monazite in UHT granulite (Osanai et al., 2001), 405-403Ma 40Ar-39Ar age for granulite (Maluski et al., 2005). The
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Permian-Triassic age was confirmed by 235-246Ma 40Ar-39Ar age for metapelitic granulites (Maluski et al., 2005), 238-250Ma U-Th- Pb CHIME age for UHT granulite (Osanai et al., 2001), 251Ma zircon age (Nagy et al., 2001), 243-258Ma SHRIMP U-Pb age (Carter et al., 2001) and 249-253Ma 40Ar-39Ar biotite age from charnockite (Nagy et al., 2001). The most recent results of Vuong et al. (2020) also confirmed the similar U-Pb zircon ages of 247-250Ma for granulite in this complex.
The P-T conditions for the Kannak complex have been estimated to be 800-850ºC and 8±1kbar, according to garnet-cordierite and two-pyroxene geothermometry (DGMVN, 1989; Roger et al., 2007; Nakano et al., 2007, 2013; Long, 2011).
2.2 Ngoc Linh complex
This subterrane occurred in the west of the Kannak subterrane and formed the western part of the Kontum Massif (Fig. 2. 1). The oldest rocks in the area comprise parts of the metamorphosed Ngoc Linh Complex, which exposes mainly along the upper course of the Đắk Mi and Re rivers in the north and northeast of Kontum Province and A Yun Pa area. Petrographically, this complex includes following rock groups: Đắk Mi crystalline schists, Song Ba amphibole gneiss, Ba Điền biotite gneiss, Ia Ban amphibolite, and Đèo Măng Rơi granulite (Long, 2011).
The composition of metamorphic rocks consists mainly of garnet ± sillimanite ± biotite gneiss with foliated quartz-feldspar layers corresponding to leucosome (Faure et al., 2018), garnet ± biotite gneiss, amphibolite, garnet amphibolite and hornblende ± biotite gneiss (Nakano et al., 2007; Osanai et al., 2008). These rocks were considered to have metamorphosed under amphibolite facies (Hutchinson, 1989). Some other rock types belong to granulite facies metamorphic rocks are also observed in this complex.
They are garnet ± sillimanite ± cordierite ± biotite gneiss, garnet ± orthopyroxene ± biotite
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gneiss, garnet ± orthopyroxene ± clinopyroxene ± hornblende granulite and garnet ± orthopyroxene ± clinopyroxene granulite (Nakano et al., 2004, 2007).
The age of the metamorphic rocks of the Ngoc Linh Complex is problematic.
Reported isotopic ages obtained by different methods range from Ordovician to Early Triassic (Nam, 1998, 2001; Nakano et al., 2003, 2007). U-Pb age of relict core of zircon grains in some units yielded 1455 and 2541 Ma. Furthermore, according to Lan et al.
(2003), the Nd-Sm TDM age of these metamorphic rocks lies in the interval from the Paleoproterozoic to Neoproterozoic (2.4-1.2Ga), and these certainly represent the earliest model age of the protoliths. Two metamorphic events were also recorded by Vuong et al.
(2020) by using U-Pb Zrn ages and U-Th-Pb Mnz ages for one mafic granulite and two pelitic gneisses. As such, the interpretation of the age for the Ngoc Linh Complex remains uncertain.
2.3 Kham Duc complex
The Kham Duc complex is distributed in the northern and western parts of the Kontum Massif (Fig. 2. 1). It is bounded by a granitic intrusion (Ben Giang-Que Son plutonic suit) in the south and Tra Bong strike-slip fault to the south (Lepvrier et al., 2004).
This complex consists mainly of hornblende ± biotite gneiss, epidote amphibolite, biotite
± muscovite schist, and biotite ± sillimanite schist. Garnet ± kyanite ± biotite schist, kyanite ± staurolite ± muscovite ± chlorite schist and garnet ± staurolite ± kyanite ± biotite schist are also presented as low- temperature and medium- pressure counterparts within this complex. Rocks in the Kham Duc complex is composed of greenschist to blueschist facies and low amphibolite facies metamorphic rocks with moderate pressure conditions (Long, 1995; Nakano et al., 2009; Usuki et al., 2009).
The age for the protoliths in the Kham Duc complex is minimal, such as the detrital zircon age for metapelite yields 558±7 Ma (Usuki et al., 2009). This age suggested
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that a part of the Kham Duc complex is younger than Late Neoproterozoic. The younger 240Ma Mnz age of Vuong et al. (2020) was interpreted as the timing of the high temperature/ medium pressure metamorphism of Grt ± Ky ± Bt gneiss which was considered to be occurred at 650-700°C and 8-8.6kbar by Nakano et al. (2007). This result is consistent with previous studies of Nakano et al. (2013) and Faure et al. (2018) with the peak metamorphism at 245-250Ma.
2.4 Hai Van complex
The Permian-Triassic Hai Van high-Al granite is widely distributed in geological structures that extending along the Truong Son Fold Belt. It located in the northern margin of the Kontum Massif (Fig. 2. 2). In the past, it was considered to belong to many different complexes, such as Late Triassic Phia Bioc complex (Dovjikov, 1965) or Carboniferous Truong Son complex (Tri and Bao, 1977). The most recent study of Faure et al. (2018) considered the Hai Van Complex to be the post- Ngoc Linh plutons. Rocks of Hai Van complex are mainly intrusive phase and exposed with large areas, composed of medium- to coarse-grained biotite granite, fine-grained two-mica granite, and biotite granodiorite.
The formation age of the Hai Van complex seems to fall within a short limited time span. The Early Triassic age was recorded based on the Rb-Sr isotopic age of granite in this complex with 236 ± 4.6 Ma (Anh et al., 1995) and 250 Ma (Hurley and Fairbairn, 1972). The similar ages have been acquired from monazite by U-Th-Pb geochronology at 239-219Ma and 237-222Ma (Nakano et al., 2013). Hieu et al. (2015) also reported zircon U-Pb ages within the range of 242-224Ma by using LA-ICP-MS. This formation ages of Hai Van Complex made up a lower intercept for all samples in this study and suggested that they can not be younger than Early Triassic.
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Figure 2. 2. Simplified geological map of the Kontum Massif (after D.G.M.V, 1989).
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