Petrology, Geochemistry, and Origin of Metamorphosed Mafic Rocks of the Trans Vietnam Orogenic Belt, Southeast Asia

Journal ofWhole Mineralogical rock chemistry and Petrological of metamorphosed Sciences, mafic Volume rocks 108, of the page TVOB 55─ 86, 2013 55

Petrology, geochemistry, and origin of metamorphosed mafic rocks of the Trans Vietnam Orogenic Belt, Southeast Asia

* ** ** *** # Kazuhiro Yonemura , Yasuhito Osanai , Nobuhiko Nakano , Masaaki Owada and Sotaro Baba

*Division of Polar Region Environment, Graduate school of Social and Cultural Studies, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan **Division of Earth Sciences, Faculty of Social and Cultural Studies, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan *** Department of Earth Sciences, Yamaguchi University, Yamaguchi 753-8512, Japan # Department of Natural Environment, University of the Ryukyus, Okinawa 903-0213, Japan

The Trans Vietnam Orogenic Belt (TVOB) of Southeast Asia is thought to have formed by a continent-continent collision between the South China and Indochina cratons in Permian-Triassic times. Here we focus on the nature and origin of metamorphosed mafic rocks that are widely distributed throughout the TVOB, including rocks from the Cangshan Mountains (northern TVOB), the Red River Shear Zone and Song Ma Suture Zone (central TVOB), and the Kontum Massif (southern TVOB). Amphibolite facies metamorphosed mafic rocks are widely distributed throughout all of these areas, while granulite facies and eclogite facies metamorphosed mafic rocks occur only in the Kontum Massif and Song Ma Suture Zone, respectively. Major and trace element compositions of these TVOB metamorphosed mafic rocks indicate an overall tholeiitic affinity, but suggest a wide array of tectonic settings for the precursor mafic magmas, including volcanic-arc basalt (VAB), mid-ocean ridge basalt (MORB), and within-plate basalt (WPB). In addition, Fe-rich gabbro-derived mafic metamorphic rocks occur in the southern part of the TVOB (in the Kontum Massif). On the basis of the distributions of each type of mafic metamorphic rocks and their metamorphic grades, there were arc and oceanic crust between the South China Craton and the Indochina Craton.

Keywords: Collision zone, Metamorphosed mafic rocks, Trans Vietnam Orogenic Belt, Indochina Craton, South China Craton

INTRODUCTION setting of the source magma and process of igneous rock formation (i.e., basaltic lava flow versus gabbroic intru- Continental-continent collision zones are characterized by sion) of these rocks is generally determined based on pe- a variety of crustal materials including metamorphic trography, geochemistry, and geological relationships rocks, granitoids, and accretionary prisms, in which deci- with other lithologies observed in the field (e.g., Pearce phering the metamorphic evolution and history of magma and Cann, 1973). generation are critical in unraveling the continental colli- Asian continent is considered to be a composite of a sion process. By evaluating the tectonic settings of the ig- number of smaller aggregated microcontinents (Metcalfe, neous protoliths of metamorphic rocks caught up between 1999), and in Southeast Asia alone, a total of five micro- two collided continents, or even metamorphic rocks of the cratons have been identified, including the South China collided continental fragments themselves, we can place craton, Indochina craton, Shan-Thai craton, Sibumasu important constraints on the processes involved in conti- craton, and West Burma craton. Metamorphic rocks relat- nental collision, and this technique works especially well ed to the continental collisions in Southeast Asia are ex- for mafic metamorphic rocks. Such rocks are typically posed along the boundaries or sutures between these mi- considered to be derived from igneous rock precursors crocontinents (Fig. 1), and undoubtedly preserve important (e.g., Bucher and Frey, 1994), and the original tectonic information pertaining to the continental collision pro- doi: 10.2465/jmps.120813 cesses that affected each of these microcontinents during K. Yonemura, [email protected] Corresponding author their amalgamation. Y. Osanai, [email protected] Recent studies of metamorphic rocks preserved 56 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba

each of these six terranes are different, both the P-T path West trajectories and the timing of metamorphism are similar Burma 1 Fig. 2 CHINA among all terranes (i.e., Permo-Triassic with a clockwise craton 2 South China P-T path; Osanai et al., 2001, 2004a, 2004b, 2008; Na- craton kano et al., 2004, 2007a, 2007b, 2008, 2009a, 2010). Osa- nai et al. (2008) suggested that all of these terranes were LAOS 3 MYANMAR formed by a single tectonic event, and accordingly they 4 proposed the term Trans Vietnam Orogenic Belt (TVOB) to collectively describe all of the metamorphic terranes THAILAND sandwiched between the Indochina and South China cratons. Metamorphic rocks of the Ailao Shan Zone and 5 Indochina Cangshan Mountains of China are recognized as the craton northernmost extension of this orogenic belt (Osanai et Sibumasu craton 6 al., 2008), although the metamorphic rocks encompassing Fig. 3 the whole TVOB are considered to have formed simulta- neously by continent-continent collision between the CAMBODIA South China and Indochina cratons. As stated above, recent geochronological and petro- VIETNAM logical investigations of the TVOB have given rise to our Shan-Thai craton understanding of the collisional processes involved in that orogeny, such as the regional timing and conditions of metamorphism and exhumation of the subducted craton. 250 km However, the tectonic settings of protolith formation for these various metamorphic terranes prior to collision in the TVOB orogeny remain poorly understood. With re- MALAYSIA 1 Cangshan Mountains gards to the ages of the protoliths of these metamorphic Ailao Shan Zone Trans Vietnam 2 Orogenic Belt rocks, some papers have obtained preliminary ages that 3 Red River Shear Zone Collision boundary are much older than the timing of the TVOB orogeny 4 Song Ma Suture Zone Collision zone (~ 430 Ma: e.g., Osanai et al., 2003; Roger et al., 2007; 5 Truong Song Belt metamorphic rocks 6 Kontum Massif Nakano et al., 2010; ~ 680 Ma: Nakano et al., 2003; Osa- nai et al., 2003), which were interpreted as being related Figure 1. Tectonic domain map showing the distribution of micro- - - continental blocks and metamorphic rocks in Southeast Asia, to the so called Pan African Orogeny and the formation highlighting sixFig.1_Yonemura field areas in the TVOB et that al. contain abundant of the Gondwana supercontinent (e.g., Lan et al., 2003; metamorphosed mafic rocks, which are the subject of this study. Osanai et al., 2004a, 2008; Nakano et al., 2007a, 2008, Regional geology after Osanai et al. (2010). 2010). A few recent studies have also employed Sr and Nd isotopes and trace element geochemistry of amphibo- along the boundary between the South China and Indo- lites to constrain the origin of their protoliths. For in- china cratons have revealed that these metamorphic rocks stance, Lan et al. (2003) regarded the protolith of the am- were formed during a Permo-Triassic continental colli- phibolites from the Kontum Massif as being represented sion event (Osanai et al., 2001, 2004a, 2008; Nakano et by mafic magmas underplated and emplaced into the low- al., 2007a, 2008, 2010; Lepvrier et al., 2008). Along this er crust of the Indochina craton in association with rift-re- cratonic join, metamorphic rocks are exposed within the lated magmatism. In contrast, Lin et al. (2007) and Miya- Cangshan Mountains and the Ailao Shan Zone in China, moto et al. (2009) reported that the amphibolites from the as well as along the Red River Shear Zone, the Song Ma Song Ma Suture Zone were derived from MORB type Suture Zone, the Truong Son belt, and the Kontum Massif magmatism. Collectively, these previous studies seem to in Vietnam (Fig. 1). The protoliths of these metamorphic suggest that the metamorphosed mafic rocks of the TVOB rocks are diverse, and include pelitic, felsic, mafic, and are composed of both lower crustal materials of the Indo- ultramafic rocks, as well as carbonate and calc-silicate china craton, and Paleo-Tethys oceanic crust. However, rocks, and they were metamorphosed in various crustal these results cover only a limited area of the entire TVOB. condition ranging from greenschist to granulite facies, as Consequently, in this study, using petrography and whole- well as eclogite facies. Although the peak pressure-tem- rock geochemistry, we investigate the nature of the proto- perature (P-T) conditions of metamorphism preserved in lith assemblages for metamorphosed mafic rocks sampled Whole rock chemistry of metamorphosed mafic rocks of the TVOB 57

105ºE Kunming N 100ºE 25ºN Yangtze river

32602D, 32602F 815T01A, 815T01B 914T01G Cangshan Dali 913T01E, 913T01J 32406A, 32406B, 81501A2 Mountains 324061, 32406D 816T01

1206T01V, 1206T01AN one Z 81801C 1207T01I, 1207T01L, 1207T01S, r Shea r Song Chay fault ive Hanoi 1207T01AD, 1207T01AH d R 32401A – Re Song Hong fault Ailao Shan Yen Bai 1207T04C, 1207T04H, 32803C, 328T01C, 1207T04I 901T01D 328T01D 20ºN Mojiang 830T03C, 830T01E Nujiang river SOUTH 830T01A 813T02B CHINA Song Da fault50401 SEA

Dien Bien Phu Song Ma Song Ma F. Simao Suture zone 226T02G Dien Bien Phu F. 22504A 822T03J 821T03E 831T01T 22601A 22508 Sedimentary rocks Mekong river Que Phong Mafic-ultramafic rocks 819T04C Granitoids Rivers Faults Gneisses and schists Song Ca fault Slates and phyllites Sampling points 100 km

Metamorphic rocks of Ailao ShanFigure - Red 2. Simplified geological map of the Ailao Shan-Red River Shear Zone (Cangshan Mountains, Ailao Shan Zone, Red River Shear Zone) River Shear Zone and their northern and Song Ma Suture Zone. Sample locations are also shown. Compiled after United Nations (1990a, 1990b), Leloup et al. (1995), Tran Ngoc extention metamorphic complexes Fig.2_Yonemura et al. Nam et al. (1998) and Lepvrier et al. (2008).

across a vast tract of the TVOB (i.e., in Vietnam, Laos, that the regional metamorphism in the Cangshan Moun- and China), including the Cangshan Mountains, the Red tains area was related to an earlier (Permo-Triassic) conti- River Shear Zone, the Song Ma Suture Zone, and the nental collision event involving the South China and In- Kontum Massif. Ultimately, we proposed new tectonic dochina cratons (e.g., Osanai et al., 2008; Searle et al., evolutional model through subduction to continental col- 2010). lision determined from the distribution of each protolith Metamorphic rocks of the Cangshan Mountains con- tectonic settings and metamorphic grades of metamor- tain mineral assemblages that indicate greenschist facies phosed mafic rocks throughout the TVOB. Mineral abbre- to amphibolite facies metamorphic conditions. Gneisses viations discussed in this paper are after Whitney and Ev- are exposed along the eastern side of the mountain range, ans (2010). while schists are occur predominantly on the western side (e.g., Yunnan Bureau of Geology and Mineral Resources, GEOLOGICAL BACKGROUND OF THE STUDY 1990). The main lithologies present along the eastern side AREAS are Bt-Ms-Chl schist, Grt-Bt-Ms schist, and Ep-Hbl schist, whereas those on the western side are Grt-Sil-Bt Cangshan Mountains gneiss, Grt-St-Bt-Ms gneiss, Grt-Cpx amphibolite, and amphibolite. The Cangshan Mountains (Diancangshan Mountains) are The estimated peak P-T conditions in metamorphic situated in southwestern China, where the metamorphic rocks from the Cangshan Mountains area are 550-570 °C rocks outcrop within a NW-SE trending corridor measur- and 5-7 kbar (Leloup et al., 1993) and 540-610 °C (Gillay ing 80 km long by 10-15 km wide (Fig. 2). This area is et al., 2003) from Grt-St-Bt-Ms series schist, and 637 °C regarded as a part of the northern extension of the Ailao and 6.5 kbar from amphibolite (Cao et al., 2007). Eocene Shan-Red River Shear Zone, which is considered to rep- to Early Miocene ages, which are related to the sinistral resent the boundary between the South China and Indo- shearing event, have been reported for some of these met- china cratons (e.g., Tapponnier et al., 1990). Mylonitized amorphic rocks using various radiometric dating methods metamorphic rocks found in this shear zone exhibit struc- including 40Ar/39Ar (e.g., Leloup et al., 1993; Cao et al., tures which indicate left-lateral (sinistral) shearing, prob- 2011a), Th-Pb Mnz (Gilley et al., 2003), and SHRIMP ably in relation to far-field collision of the Indian subcon- and LA-ICP-MS U-Pb Zrn (Cao et al., 2011b; Searle et tinent with the Eurasian continent during the Eocene (e.g., al., 2010) techniques. In addition, significantly older LA- Leloup et al., 1993). Furthermore, it has been suggested ICP-MS U-Pb Zrn ages of 233-243 Ma were determined 58 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba

40 39 by analyzing the cores of some igneous Zrn grains from previously reported Eocene-Early Miocene Ar/ Ar metamorphic rocks of the Cangshan Mountains (Li et al., ages, K-Ar age, and Th-Pb Mnz age determined on rocks 2008; Searle et al., 2010). from the Red River Shear Zone are regarded as being Metamorphosed mafic rocks analyzed in the present linked with the main episode of left-lateral shearing (e.g., study were collected from the southern part of the Cang- Tran Ngoc Nam et al., 1998; Leloup et al., 2001; Gilley et shan Mountains (Fig. 2). Ep-Hbl schist, as well as am- al., 2003). Some older ages, between 220 and 265 Ma, phibolite from this study area, commonly shows a well- have also been reported for rocks from this shear zone developed schistosity (Figs. 4a and 4b; Color version of (e.g., Carter et al., 2001; Gilley et al., 2003), possibly re- Figure 4 is available online from http://japanlinkcenter. lated to older continental collision event, as suggested by org/DN/JST.JSTAGE/jmps/120813.). In addition, meta- Osanai et al. (2008) and Searle et al. (2010). morphosed pelitic and felsic rocks are commonly ob- The metamorphosed mafic rocks of the Red River served in outcrop adjacent to the rock exposures of meta- Shear Zone analyzed in this study were collected from the morphosed mafic rocks. Although their direct stratigraphic Song Hong and Xuan Dai groups (Fig. 2). Several am- or structural relationships with one another are unclear, phibolites in the Song Hong Group exhibit a clearly dis- these mafic rock units might represent isolated blocks or cernible gneissosity. In some cases, contact relationships layers intercalated within the pelitic and felsic gneisses between the amphibolites with other lithologies were di- and schists. rectly observed, whereby several metamorphosed mafic rock units were found as layers or blocks intercalated Red River Shear Zone within pelitic or felsic gneisses (Figs. 4c and 4d).

The Red River Shear Zone is a prominent NW-SE trend- Song Ma Suture Zone ing shear zone in northern Vietnam that measures up to 10 km wide and around 250 km long (Fig. 2). Greenschist to The Song Ma Suture Zone is located in northwestern granulite facies metamorphic rocks are the dominant li- Vietnam and northeastern Laos, and is composed of main- thologies (Osanai et al., 2008). Nguyen Vinh et al. (2004) ly greenschist to amphibolite facies metamorphic rocks divided the rocks from this shear zone into two separate (Osanai et al., 2008), although some eclogites and granu- lithostratigraphic groups: the Song Hong Group in the lites have also been reported (Nakano et al., 2008, 2010). northern part of the shear zone and the Xuan Dai Group This suture zone is divided into two formations, based in the western part, separated by the Song Hong Fault largely on differences in metamorphic grade: the Nam Co (Fig. 2). In the Song Hong Group, various high-grade Formation and the Nam Su Lu Formation (Phan Son et metamorphic rocks are observed, such as Grt-Sil-Bt al., 2005). The Nam Co Formation consists mainly of Ms gneiss, Grt-Crd-Bt gneiss, Grt-Opx granulite, Grt-Opx- schist and Grt-Ph schist, with lesser amounts of mafic and Cpx granulite, and Opx-Cpx-Hbl granulite, as well as a calc-silicate rocks (Osanai et al., 2008). Nakano et al. wollastonite-bearing calc-silicate rock (Osanai et al., (2010) also found eclogites within this formation. In con- 2008). In contrast, the Xuan Dai Group is composed trast, the main lithologies of the Nam Su Lu Formation mainly of low-grade metamorphosed pelitic rocks with are amphibolite, Grt amphibolite, Grt-Opx amphibolite, subordinate intercalations of both Ep amphibolite and Grt-Bt gneiss, and Grt-Sil-Bt gneiss (Osanai et al., 2008), quartzite (Osanai et al., 2008). and some high-pressure granulites have been found with- Peak metamorphic conditions of the Red River Shear in the northwestern part of this formation (Nakano et al., Zone have been estimated at 690 ± 30 °C and 6.5 ± 1.5 2008). kbar (Tran Ngoc Nam et al., 1998), 735 ± 65 °C and 4.7 ± The metamorphic conditions during the formation of 1.7 kbar (Leloup et al., 2001), and 780-830 °C and 7.5- these rocks were estimated at 910-930 °C and 19-20 kbar 8.8 kbar (Gilley et al., 2003), as determined from the Grt- based on analyses of high-pressure granulite from the Sil-Bt gneisses present within this shear zone. Further- Nam Su Lu Formation (Nakano et al., 2008), 580-600 °C more, preliminary thermodynamic calculations for and 19-23 kbar based on Grt-Cld-Ph schist, and 600-620 metamorphosed bauxite from this shear zone yielded 18 °C and 21-22 kbar based on eclogite from the Nam Co kbar (at 790 °C) as the peak pressure and higher than Formation (Nakano et al., 2010). The age of metamor- 1000 °C (at 15 kbar) as the peak temperature (Nakano and phism of the Nam Su Ku Formation was determined at Osanai, 2011). Parts of the Red River Shear Zone experi- 233 ± 5 Ma based on a U-Th-Pb Mnz age (Nakano et al., enced ultrahigh-temperature (UHT) conditions, as in- 2008), while the age of metamorphism of the Nam Co ferred from the coexistence of Grt, Crn, and Qtz in Ky- Formation was determined at 243 ± 4 Ma, also based on a bearing Grt-Crn-Sil-Spl gneiss (Osanai et al., 2008). The U-Th-Pb Mnz age (Nakano et al., 2010). A few Mnz core Whole rock chemistry of metamorphosed mafic rocks of the TVOB 59 domains found within Mnz grains within the Grt-Ph bearing Grt-Sil gneiss (Osanai et al., 2008). Amphibolite schist record a much older thermal event at 424 ± 15 Ma facies metamorphic rocks predominate in the Ngoc Linh (Nakano et al., 2010). complex, including Grt-Sil-Bt gneiss, Grt-Bt gneiss, am- Metamorphosed mafic rocks such as amphibolite and phibolite, Grt amphibolite, and Ca Amp-Bt gneiss. Gran- Grt-bearing amphibolite are found as main lithologies in ulite facies metamorphic rocks, including UHT granulites, the Nam Su Lu formation (Nakano et al., 2008), and the have been reported from the Kontum Massif, but only amphibolites commonly exhibit a clearly defined foliation along the Duc To Kan Shear Zone (Nakano et al., 2004, (Figs. 4e and 4f). Eclogite and other metamorphosed maf- 2007a; Osanai et al., 2004a, 2004b). The Kham Duc com- ic rock (amphibolite, Grt-bearing amphibolite) from the plex is composed mainly of greenschist to amphibolite fa- Nam Co formation have been found as isolated blocks or cies metamorphic rocks that have been subdivided into a lenses within pelitic schists (Nakano et al., 2010). The low pressure unit in the southwestern part of the complex, metamorphosed mafic rocks of the Song Ma Suture Zone and a medium pressure unit in the north (Long et al., analyzed in this study were collected from both the Nam 1995). In the low pressure unit, the main lithologies are Co and Nam Su Lu formations. Grt-Bt-Ms schist and Grt-Sil-Bt-Ms gneiss, whereas the medium pressure unit is composed mainly of metamor- Kontum Massif phosed pelitic, felsic, and mafic schists and gneisses, and minor calc-silicate rocks (Nakano et al., 2009a). The Kontum Massif occurs mainly within central Viet- Peak metamorphic conditions in the Kannak com- nam, extending partly into eastern Laos (Fig. 3; Color plex have been estimated at 1000-1050 °C and 11-12 version of Figure 3 is available online from http://japan- kbar from pelitic granulites (Osanai et al., 2001, 2004b), linkcenter.org/DN/JST.JSTAGE/jmps/120813). On the and retrograde metamorphic stage with conditions of 800- basis of the metamorphic grade, it has been divided into 900 °C and 8-9 kbar also affected these rocks, as calculat- three separate metamorphic complexes: the Kannak com- ed by Osanai et al. (2001, 2004b). In the Ngoc Linh complex in the southeastern part of the massif, the Ngoc Linh plex, the initial ultrahigh-pressure (UHP) rock complex in the central part, and the Kham Duc complex assemblage, from eclogitic UHP mafic granulite, indicates in the northern and southwestern parts (e.g., United Na- conditions of ~ 900 °C and ~ 40 kbar. Furthermore, UHT tions, 1990a). The Kannak complex is composed mainly metamorphic conditions of 1000-1050 °C (at 13 kbar) of amphibolite and granulite facies metamorphosed pelitic were determined from rocks of the Ngoc Linh complex, rocks, including Grt-Opx-Sil-Crd granulite, Grt-Crd-Sil- as a later ‘peak temperature’ stage on the clockwise P-T Bt gneiss, Grt-Opx ± Bt charnockitic gneiss, and Gr- path associated with the continental collision event that caused this regional metamorphism (Nakano et al., 2004,

108ºE 109ºE 2007b). In contrast, the Kham Duc complex of the Kon- Sedimentary rocks tum Massif has yielded peak pressure conditions of 620- Granitoids Hue SOUTH N - Metamorphic rocks CHINA 650 °C at 11 12 kbar and peak temperature conditions of Kannak Complex SEA 730-750 °C at 7-8 kbar from the Grt-Ged-Ky gneisses Ngoc Linh Complex Danang-Dailoc Danang Kham Duc Complex shear zone 16ºN present within the complex (Nakano et al., 2009a). Unclassfied (in Laos) Numerous radiometric age determinations, have Faults Tam Ky-Phuoc Son shear zone Sampling points been made using multiple isotopic systems to estimate the 91001B, 91001A1, 90906A timing of metamorphism in the Kannak and Ngoc Linh 91001A2 Kham Duc Shear zone 910T02A Quang Ngai complexes. Obtained ages form an age cluster at ca. 240- - - 15ºN 260 Ma, and this includes U Th Pb Mnz dating (Osanai 902T02E, 902T02P 01090301H 82701B et al., 2001), SHRIMP U-Pb Zrn dating (e.g., Carter et al., 41602A 2001; Tran Ngoc Nam, 2001), IDTIMS U-Pb Zrn dating 319T01A, 319T01B, 41401BB 319T01C, 319T01D, - D (Nagy et al., 2001), the Sm Nd isochron method (Nakano 319T01E, 319T01F u c 40 39 Kontum To K et al., 2003, 2007a), Ar/ Ar dating (Nagy et al., 2001; an sh P ea - 904T02H, 904T02T o r z Maluski et al., 2005), and K Ar dating (Tran Ngoc Nam, K one o 14ºN su 42001H Qui Nhon 1998). In contrast, existing geochronological data on tu re zo ne rocks from the Kham Duc complex show two distinct age 100km clusters at 407-418 Ma (based on SHRIMP U-Pb Zrn dat- - Figure 3. Simplified geological map of the Kontum Massif with ing: Carter et al., 2001) and ca. 230 250 Ma (based on 40 39 sample locations in this study. Compiled after United Nations Ar/ Ar dating: Lepvrier et al., 1997; and U-Th-Pb Mnz (1990a, 1990b). Fig.3_Yonemura et al. dating: Nakano et al., 2009a). 60 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba

Figure 4. Field photographs highlighting the modes of occurrence of several of the metamorphic rocks from the TVOB analyzed in this study. (a) Ep-Hbl schist from the eastern side of the Cangshan Mountains. (b) Amphibolite from the western side of the Cangshan Mountains. (c) and (d) Amphibolite layer within pelitic gneiss (c) and felsic gneiss (d) from the Red River Shear Zone. (e) and (f) Amphibolite from the Nam Su Lu Formation in the Song Ma Su- ture Zone. (g) Grt amphibolite from the eastern Kannak complex. (h) Mafic granulite block within felsic gneiss from the Ngoc Linh complex. Color version of Figure 4 is available online from http://japan linkcenter.org/DN/JST.JSTAGE/ jmps/120813.

To provide a representative sample suite, the meta- plex, large blocks of Grt amphibolites (Fig. 4g) are sur- morphosed mafic rocks analyzed in this study from the rounded by pelitic gneisses (Osanai et al., 2008). In addi- Kontum Massif were collected from all three metamor- tion, the mafic granulites found within the Ngoc Linh phic complexes, including some samples that were col- complex generally occur as isolated blocks or lenses lected from Laos (Fig. 3). Contact relationships between within pelitic or felsic gneisses (Fig. 4h; Nakano et al., mafic rocks and other lithologies in the Kannak complex 2004; Osanai et al., 2004a). The metamorphosed mafic have been documented previously, whereby Cpx- and rocks within the Kham Duc complex have a prominent Opx-bearing mafic granulites were observed in outcrop as gneissosity and an overall layered structure consisting of thin intercalations within pelitic gneisses (Osanai et al., Hbl-, Pl-, and Ep-dominant layers, which, notably, are 2004a). Moreover, in the eastern part of the Kannak com- also commonly intercalated with felsic and pelitic schists Whole rock chemistry of metamorphosed mafic rocks of the TVOB 61 or gneisses (Nakano et al., 2009a). shown in Appendix 3 (Appendixes 1-3 are available online from http://japanlinkcenter.org/DN/JST.JSTAGE/ PETROGRAPHY AND MINERAL CHEMISTRY jmps/120813).

Analytical methods for quantitative chemical analysis Results of petrographic studies and quantitative min- of minerals eral chemistry

The mineral assemblages present within the TVOB sam- Cangshan Mountains. There are three distinct types of ples collected in this study are shown in Table 1, where metamorphosed mafic rocks documented from the Cang- the rock types listed are defined as follows: shan Mountains in this study: Grt-bearing amphibolite, 1) Amphibolite, which is medium- to coarse- amphibolite, and schist. grained and does not contain Grt or Opx. The Grt-bearing amphibolites are mainly composed 2) Grt-bearing amphibolite, which is amphibolite of Grt, Hbl, and Pl, with minor Ttn, Ap, and Ilm; several containing Grt but not Opx. Grt-bearing amphibolite samples also contain Bt or Cpx. 3) Granulite, which is mafic granulite containing These Grt-bearing amphibolites generally exhibit a grano- Opx. blastic texture (Fig. 5a), and the Grt grains themselves are 4) Grt-bearing granulite, which is mafic granulite anhedral and medium- (0.5-1.0 mm) to coarse-grained containing Opx and Grt. (up to 4 mm). The rims of the medium-grained Grt

5) Schist, which is fine-grained and exhibits a (Alm54−55Prp13−18Grs27−29Sps1−3) crystals have higher Prp

prominent schistosity. contents than in their cores (Alm53−59Prp5−12Grs29−32Sps3−6), 6) Eclogite, which contains Grt and Omp. whereas the coarse-grained Grt grains have a relatively

7) Metagabbro, which is a metamorphosed gabbro homogeneous chemical composition (cores: Alm60−63

showing clear relict igneous gabbroic textures. Prp8−10Grs25−29Sps2−3; rims: Alm59−62Prp8−10Grs26−30Sps2−3). For in situ quantitative chemical analysis of minerals Most Grt grains contain inclusions of other minerals, such in polished petrographic thin sections of TVOB rock sam- as Ttn, Pl, and Qz. The Hbl crystals in these Grt-bearing ples, we used a scanning electron microscope equipped amphibolites are fine- to medium-grained, and are classi- with a wavelength dispersive spectrometry system (JEOL fied into Fprg to Fts {Si = 6.09-6.56 p.f.u.; (Na + K)A = 2+ JXA-8530F) at Kyushu University, Japan. These quanti- 0.30-0.65 p.f.u.; XMg [Mg/(Mg + Fe )] = 0.34-0.47; Ti = tative analyses were performed using an accelerating volt- 0.03-0.25 p.f.u.}. Pl crystals are anhedral and have inter- age of 15 kV, a 12 nA probe current, and a 2 µm beam di- mediate An contents (An41−50), and have no sign of any ameter, and all data were processed using a standard ZAF clear chemical zonation. Bt (XMg = 0.29-037; Ti = 0.26- correction program. Natural mineral samples (ASTI- 0.59 p.f.u.) is also observed within the matrix, and occurs MEX-MINM-53) and synthesized oxide samples (P&H as inclusions within larger Grt and Hbl crystals. Cpx was Block No. SP00076) were used as standards for the quan- observed in at least one sample of Grt-bearing amphibo- titative chemical analyses, and representative mineral lite, and this particular rock exhibits a nematoblastic tex- compositions of the analyzed rocks from the TVOB are ture (Fig. 5b). The Cpx grains in this sample are anhedral listed in Table 2. The ‘Amp’ nomenclature is after Leake and have relatively homogeneous chemical compositions et al. (1997), and for the abbreviation of ferrohornblende, (XMg = 0.63-0.68; Al2O3 = 0.5-1.0 wt%). Grt, which coex- which is unlisted in Whitney and Evans (2010), we used ists with Cpx in this rock, is medium-grained (0.2-0.5 ‘Fhb’. When denoting the colors of minerals throughout mm) and anhedral (Fig. 5b), and chemical analyses indi- this section of the paper, we are referring to the mineral cate that the rims of these Grt grains have higher Alm and color as it appears in polished thin section, as viewed un- Prp contents, but lower Sps contents (Alm48−51Prp7−10 der a petrographic microscope in plane polarized light. Grs30−34Sps5−15) than their cores (Alm43−45Prp4−5Grs31−34 3+ The Fe calculations for Amp in this study follow those Sps19−21). Hbl crystals in this Grt-amphibolite are classi- 3+ of Leake et al. (1997), whereas the Fe calculations for fied into Ts to Mhb [Si = 6.3-6.8 p.f.u.; (Na + K)A = 0.29- other minerals follow Droop (1987). Basis oxygen num- 0.69 p.f.u.; XMg = 0.52-0.65] and have high Ti contents ber of each minerals are shown in Table 2. The mineral- (0.06-0.23 p.f.u.); Act was observed on some of the Hbl ogical data for another three samples of the TVOB rims. The cores (An54−55) of Pl in this Grt-amphibolite

(93001H, 830T01C, and 830T01E) are also available for have higher An contents than the rims (An40−46). comparison in previously published datasets (Nakano et Amphibolites of the Cangshan Mountains contain al., 2004, 2010). The variety of calcic Amp in each rocks Hbl and Pl, with minor Qz, Ilm, and Ttn, and exhibit a shown in Appendix 1, Pl shown in Appendix 2, and Grt nematoblastic texture. These amphibolites are grouped 62 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba

Table 1. Mineral assemblages of the analyzed metamorphosed mafic rocks

Table 1. (Continued)

●, Abundant; ○, Rich; +, Moderate; ×, Poor; -, Absent; S, As symplectite; ( ), secondary mineral. A, amphibolite; GA, Grt-bearing amphibo- * † lite; G, granulite; GG, Grt-bearing granulite; S, schist; EC, eclogite; MG, metagabbro; n.a., not analyzed. in symplectite; Nakano et al. (2010); †† Nakano et al. (2004). into two distinct varieties: a pale brownish Hbl-dominant this study are mainly composed of Hbl, Pl, Qz, and minor amphibolite and a greenish Hbl-dominant amphibolite. Ilm. Several schists contain Ep. The Hbl in these schists, The brownish Hbl is medium-grained (0.3-0.8 mm) with which is greenish and fine-grained (up to 0.3 mm), de- compositions of Ts and Mhb [Si = 6.31-7.46 p.f.u.; (Na + fines the schistosity (Fig. 5c). These Hbl grains are classi-

K)A = 0.00-0.39 p.f.u.; XMg = 0.52-0.81], and has relative- fied into several compositional members such as Ts, Fts, ly high Ti contents (up to 0.39 p.f.u.). Greenish Hbl Mhb, and Fhb [Si = 6.33-7.20 p.f.u.; (Na + K)A = 0.02- grains, on the other hand, are fine- to medium-grained (up 0.52 p.f.u.; XMg = 0.47-0.87], and are relatively Ti-poor to 4 mm), anhedral, and pale greenish to greenish in color. (up to 0.08 p.f.u.). The Pl crystals in these schists are fine-

This greenish Hbl is classified as Ts and Mhb in composi- grained and have low An contents (An21−42). Fine-grained 3+ 3+ tion [Si = 6.20-7.34 p.f.u.; (Na + K)A = 0.02-0.49 p.f.u.; Ep [Fe /(Fe + Al) = 0.24-0.30] grains occur as aggre-

XMg = 0.50-0.87; Ti = 0.01-0.12 p.f.u.]. An compositions gates within several of these schists (Fig. 5c). of Pl grains in the pale brownish Hbl-dominant amphibo- Red River Shear Zone. The main metamorphosed lite are variable among grains (An43−94), whereas Pl in the mafic rocks in the Red River Shear Zone are Grt-bearing greenish Hbl-dominant amphibolite samples have shows amphibolite and amphibolite. The Grt-bearing amphibo- low to intermediate An content (An28−56). There are no sig- lite contains Grt, Cpx, Hbl, Pl, Qz, Ttn, and Ilm, and ex- nificant differences of P-T conditions and whole rock hibits a granoblastic texture; one Grt-bearing amphibolite compositions between An-rich Pl-bearing and -absent sample shows a layered structure composed of Grt- and samples. An content in An-rich Pl gradually decreases Cpx-dominant layers (Fig. 5d). The Grt grains are subhe- from core (An91−94) to mantle (An81−73) and dramatically dral to anhedral, medium- to coarse-grained (up to 1 decreases in the rim (An43−57). These An values are similar mm), and contain inclusions of Hbl and Qz. The Grt to those of Pl in the greenish Hbl-dominant amphibolite. chemical compositions vary widely among samples (Alm

This suggests that the An-rich Pl is igneous relict and the 44−62Prp11−20Grs20−37Sps2−8). Furthermore, the Grt grains in An-poor rim was formed during amphibolite-facies meta- several Grt-bearing amphibolite samples are composition- morphism. ally homogeneous, while Grt grains in other samples ex- The schists of the Cangshan Mountains sampled in hibit pronounced chemical zonation, whereby the Grs 64 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba - III. - III, symplectite - I; Symp. - I, Symplectite - ). CA, Cangshan Mountains; RR, Red River Shear Zone; SM, Song Ma Suture Zone; KM, Kontum Massif; A, amphibolite; GA, Grt ). CA, Cangshan Mountains; RR, Red River Shear Zone; SM, Song Ma Suture KM, Kontum Massif; 2+ bearing granulite; S, schist; Symp. = Mg/(Mg + Fe - morphosed mafic rocks from the TVOB 2. Representative electron microprobe results of minerals in meta morphosed mafic rocks from the Table Mg X . 3 O 2 total Fe as ** Total Fe as FeO; Total * bearing amphibolite; G, granulite; GG, Grt Whole rock chemistry of metamorphosed mafic rocks of the TVOB 65 - III. - 1) - III, symplectite - I; Symp. - Table 2. (Continued Table I, Symplectite - ). CA, Cangshan Mountains; RR, Red River Shear Zone; SM, Song Ma Suture Zone; KM, Kontum Massif; A, amphibolite; GA, Grt ). CA, Cangshan Mountains; RR, Red River Shear Zone; SM, Song Ma Suture KM, Kontum Massif; 2+ bearing granulite; S, schist; Symp. = Mg/(Mg + Fe - Mg X . 3 O 2 total Fe as ** Total Fe as FeO; Total bearing amphibolite; G, granulite; GG, Grt * 66 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba

Table 2. (Continued-2)

* ** 2+ Total Fe as FeO; total Fe as Fe2O3. XMg = Mg/(Mg + Fe ). CA, Cangshan Mountains; RR, Red River Shear Zone; SM, Song Ma Suture Zone; KM, Kontum Massif; A, amphibolite; GA, Grt-bearing amphibolite; G, granulite; GG, Grt-bearing granulite; S, schist; Symp.-I, Symplectite-I; Symp.-III, symplectite-III. content increases and the Alm content decreases from and are classified into several end-member compositions core to rim. The Cpx porphyroblasts present in these such as Prg, Fprg, Ts, Fts, Mhb, and Fhb[Si = 6.19-7.01 rocks are anhedral, contain inclusions of brownish Hbl p.f.u.; (Na + K)A = 0.26-0.65 p.f.u.; XMg = 0.42-0.69]. and Ttn, and are homogeneous in composition (XMg = Some of the rims of these Hbl grains are partially replaced

0.56-0.73; Al2O3 = 0.6-2.7 wt%). Cpx + Pl symplectites by Act. Some Pl grains in the Grt-bearing amphibolites are found around Grt (Fig. 5d). Several Grt rims, which contain inclusions of Grt, Cpx, and Hbl, and the composi- are in direct contact with the symplectite, show slightly tion of Pl grains is variable (An42−76) and heterogeneous, knurled shape. This observation seems to indicate that Grt with the rims of Pl grains characterized by higher An con- was consumed by the formation of the symplectite. Cpx tents than the cores. in this symplectite has lower-XMg content (XMg = 0.58- Amphibolites of the Red River Shear Zone mainly

0.59; Al2O3 = 0.5-1.9 wt%) than Cpx grains in matrix comprise a mineral assemblage that includes Hbl, Pl, and (outlined above) and Pl in the symplectite has high An Qz, with minor Ttn and Opq. Most of these amphibolites content (An80−91). The symplectite is observed between contain fine- to medium-grained (up to 0.7 mm) Hbl Grt and Hbl or Qz. These occurrences suggest the possi- crystals, although in several samples the Hbl grains are ble reaction of Grt + Hbl +Qtz = Cpx + Pl, which should 0.5-2.0 mm in size. The amphibolites containing fine- to occur during the decompression process. Medium-grained medium-grained Hbl exhibit nematoblastic textures (Fig. (up to 0.8 mm) brownish Hbl grains (Fig. 5d) present in 5e). These Hbl grains are greenish in color (Fig. 5e) and these rocks have high Ti contents (Ti = 0.11-0.32 p.f.u.) show wide chemical variation. The variation should de- Whole rock chemistry of metamorphosed mafic rocks of the TVOB 67 pend on their whole-rock chemical compositions rather contents [Si = 3.30-3.34 (O=11); Nakano et al., 2010]. than difference of their microstructural relationships. Amp The Grt-bearing amphibolites that occur within the in amphibolites containing high alkali elements (Na2O + Song Ma Suture Zone are mainly composed of Grt, pale

K2O in whole-rock = 3.75-4.48 wt%) are classified into greenish to greenish Hbl, and minor Qz and Ttn, although mostly Prg, Fprg and Ed with minor Ts and Mhb [Si = several Grt-bearing amphibolites also contain Ep or Pl. In

6.21-6.65 p.f.u.; (Na + K)A = 0.43-0.65 p.f.u.; XMg = 0.44- the Grt-bearing amphibolite samples that contain Ep 3+ 3+ 0.62; Ti = 0.06-0.29 p.f.u.], and Amp in alkali elements grains, Pl is notably absent, and the Fe /(Fe + Al) ratios poor amphibolites (Na2O + K2O in whole-rock = 3.01- of the Ep grains are 0.15-0.16. The Grt porphyroblasts in 3.14 wt%) are classified into mostly Ts, FTs, Mhb and this rock type are subhedral and coarse-grained (1-2 mm), Fhb with minor Prg and Fprg [Si = 6.26-7.07 p.f.u.; (Na + and exhibit compositionally distinct core and rim do-

K)A = 0.23-0.53 p.f.u.; XMg = 0.44-0.77; Ti = 0.04-0.27 mains, whereby the Grt rims have a composition p.f.u.]. Coexisting Pl that occurs within this variety of (Alm51−52Prp5−8Grs39−43Sps1−2) that is lower in Fe and Mn, amphibolite (i.e., fine- to medium-grained Hbl) yields and higher in Mg than the Grt cores (Alm54−56Prp5−6Grs35−37 chemical compositions with low to intermediate An con- Sps3−5). The Hbl grains exhibit Fe-rich and Ti-poor chem- tents (An16−49). One of the amphibolite samples also con- ical compositions (Si = 6.49-7.08 p.f.u.; (Na + K)A = tains Cpx and Ep. Furthermore, Chl and Act are observed 0.24-0.47 p.f.u.; XMg = 0.41-0.50; Ti = 0.03-0.07 p.f.u.), as secondary minerals within this type of amphibolite. For and are classified as Fts and Fhb. However, the Ep-free the Cpx-bearing sample, Cpx crystals are medium- Grt-bearing amphibolites of the Song Ma Suture Zone grained (0.2-0.5 mm), and anhedral, and often contain in- contain two distinct varieties of Hbl, including medium- clusions of Ep. The chemical composition of the Cpx grained pale greenish Hbl and Pl-coexisting fine-grained grains is XMg = 0.80-0.85, Al2O3 = 0.6-1.7 wt%. In addi- greenish Hbl (Fig. 5g). The cores of the medium-grained 3+ 3+ tion, Ep [Fe /(Fe + Al) = 0.15-0.21] grains occur in the Hbl are classified as Mhb (Si = 6.71-7.02 p.f.u.; (Na + matrix of this sample, and have a fine-grained, and granu- K)A = 0.11-0.12 p.f.u.; XMg = 0.62-0.68; Ti = 0.05-0.07 lar appearance. In the other amphibolite (i.e., that contain- p.f.u.), while the rims show more compositional variation ing coarse-grained Hbl), Hbl [Si = 6.72-7.16 p.f.u.; (Na + and fall within the Ts to Mhb compositions (Si = 6.47-

K)A = 0.06-0.19 p.f.u.] shows a granoblastic texture and 7.24 p.f.u.; (Na + K)A = 0.04-0.26 p.f.u.; XMg = 0.59-0.71; has high XMg (0.81-0.89) and low Ti contents (Ti = 0.04- Ti = 0.03-0.08 p.f.u.). The fine-grained Hbl is mainly

0.07 p.f.u.), while co-existing Pl has a high An content classified as Mhb (Si = 6.15-7.20 p.f.u.; (Na + K)A =

(An86−88). 0.03-0.63 p.f.u.; XMg = 0.53-0.80) and has low Ti contents Song Ma Suture Zone. In the Song Ma Suture (Ti = 0.02-0.07 p.f.u.). The fine-grained Pl that coexists

Zone, the main varieties of metamorphosed mafic rocks with this fine-grained Hbl, has low An contents (An16−31). are eclogite, Grt-bearing amphibolite, and amphibolite. The Grt in this rock type is medium-grained, subhedral to The eclogites of the Song Ma Suture Zone contain anhedral, and compositionally homogeneous, and has

Grt, Omp, Amp, Ph, and fine-grained Rt (Fig. 5f), of Alm- and Prp-rich, and Sps-poor compositions (Alm51−57 which the Grt and Omp crystals exhibit a granoblastic Prp10−27Grs21−31Sps1−6). texture. In general, the minerals in these eclogites are sub- The amphibolites that crop out in the Song Ma Su- hedral, and commonly lack mineral inclusions. Nakano et ture Zone exhibit nematoblastic textures (Fig. 5h) and al. (2010) reported that the Grt grains in same rocks have contain greenish Hbl, Pl, and minor Ttn, Ilm, Po, and Qz. chemically homogeneous wide core and mantle zones (Al The Hbl in these amphibolites is classified mainly as m40−48Prp29−35Grs20−25Sps0−3), in addition to thin, surround- Mhb, and to a lesser extent as Ts and Fts [Si = 6.13-7.43 ing rim zones that are notably Mg-rich and Mn-poor (Al p.f.u.; (Na + K)A = 0.01-0.50 p.f.u.; XMg = 0.48-0.74]. m40−47Prp32−40Grs18−22Sps0−2). The Omp grains in these Most amphibolites contain Pl showing low- to medium- eclogites have a high Jd component {cores: An component (An16−67). However, some amphibolite

Jd27−32Ae3−7CaTs2−5; XNa [Na/(Na + Ca)] = 0.39-0.41; XMg samples contain high-An Pl (An84−91). Although there is

= 0.85-0.91; rims: Jd26−30Ae7−12CaTs3−6; XNa = 0.40-0.42; no critical evidence from the texture and chemical zoning,

XMg = 0.91-0.98; Nakano et al., 2010}, and Pl (An7−18) oc- the high-An Pl is possibly igneous relict as similar to am- curs with Na-poor Omp to form symplectites that sur- phibolites from the Cangshan Mountains. round the more Na-rich Omp grains (Nakano et al., Kontum Massif. The metamorphosed mafic rocks 2010). These Omp grains and surrounding symplectites within the Kontum Massif are Grt-bearing granulite, are together hosted by larger crystals of Na-Ca Amp (Na- granulite, Grt-bearing amphibolite, and amphibolite; in kano et al., 2010). The Ph grains present in these eclogites addition, metagabbros are exposed within the Kham Duc are chemically homogeneous and have slightly high Si complex. 68 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba Pl 1 mm Grt 1 mm Cpx 0.5 mm 0.5 mm Hbl Ttn Cpx Cpx+Pl Cpx Pl Hbl Fig. 5 Yonemura et al. Ttn Pl Ilm Pl Qz Ilm Cpx Hbl Prh+Spl Prh (l) (p) (h) (d) Qz 1 mm 1 mm 1 mm 0.2 mm Srp Qz Grt Ol Ep Hbl Cpx Ttn Ilm Opx Hbl Pl Hbl+Pl Bt Chl Ilm Pl Ilm (k) (c) (o) (g) Opx Qtz 0.5 mm 0.25 mm 1 mm 0.25 mm Cpx Omp Grt Grt Ol Opx+Spl+Hbl Ttn Grt Hbl Na–Ca amp Ol Qz Ph Opx+Spl+Pl+Mag Pl Cpx Hbl (j) (f) (n) (b) Ttn Cum Qz Ilm 0.5 mm 1 mm 1 mm 0.25 mm Pl Hbl Opx (rich) +Pl Ttn Hbl Opx+Pl (rich)+Mag Po Hbl Grt Hbl Pl (i) (a) (e) (m) Whole rock chemistry of metamorphosed mafic rocks of the TVOB 69

The Grt-bearing granulites contain Grt, Opx, Cpx, mm; Fig. 5l). Opx is anhedral and has homogeneous

Qz, Rt, and Opq, as well as Opx + Pl symplectites. Pl is chemical compositions (XMg = 0.50-0.53; Al2O3 = 0.8-1.8 only observed as a symplectite component. Several sam- wt%). Cpx is also chemically homogeneous, having ples contain Hbl in the matrix. There are Opx + Pl + Spl + slightly high XMg values (0.60-0.67) and low Al2O3 con- Mag symplectite-bearing, and Cpx- and Qz-abundant, tents (0.8-1.8 wt%), and several Cpx grains contain Opx Grt-bearing granulites. Detailed studies of the petrogra- lamellae. Only small amounts of Hbl are observed in the phy and metamorphic evolution of these Grt-bearing medium-grained granulites, whereas the coarse-grained granulites were performed by Nakano et al. (2004). The granulites contain abundant Hbl. The Hbl in these granu-

Grt in Opx + Pl + Spl + Mag symplectite-bearing Grt- lites is classified as Mhb [Si = 6.64-7.16 p.f.u.; (Na + K)A bearing granulite is coarse-grained (up to 3mm) and high- = 0.10-0.38 p.f.u.; XMg = 0.50-0.63; Ti = 0.11-0.21 p.f.u.]. er Prp content (core: Alm39−43Prp33−39Grs18−24Sps0−1; rim: The Pl grains present within the medium-grained granu-

Alm42−53Prp27−37Grs11−23Sps1−4) than the Grt in the other two lite have homogeneous An-rich compositions (An82−89), Grt-bearing granulite (Cpx-abundant Grt-bearing granu- whereas Pl within the coarse-grained granulites has large, lite: Alm58−63Prp14−19Grs20−22Sps1−2; Qtz-abundant Grt-bear- chemically homogeneous cores (An50−65), and thin, slight- ing granulite: Alm58−63Prp6−14Grs17−25 Sps1−4). The Cpx pres- ly An-rich rims (An60−71). ent within Grt-bearing granulites is quite coarse-grained The Grt-bearing amphibolites contain Grt, Hbl, and (up to 3 mm) and occurs mainly as aggregates. Some of Pl, with minor Qz, Ttn, and Ilm. The amphibolites show these Cpx crystals exhibit lamellae-like textures com- nematoblastic or granoblastic textures. The Grt grains in posed of Opx, Amp and Pl. The Al2O3 (up to 2.8 wt%) the amphibolite having a nematoblastic fabrics, are and Na2O (up to 6.1 wt%) contents of Cpx crystals vary coarse-grained (4-5 mm), anhedral, and contain inclu- significantly among grains in these rocks. Nakano et al. sions of Ttn in their rims. These Grt crystals are charac- (2004) subdivided the symplectites into three types: Opx- terized by high Alm and Grs contents, and show a slight rich Opx + Pl symplectites (symplectite I; Fig. 5i), Pl-rich decrease in Sps content from core (Al60Prp7−8Grs26−28

Opx + Pl symplectites (symplectite II; Fig. 5i), and Opx + Sps7−10) to rim (Al58Prp7−8 Grs27−28Sps5−6). Hbl within these Pl + Spl + Mag symplectites (symplectite III; Figs. 5i and nematoblastic Grt-bearing amphibolites is fine- to medi-

5j). The Opx in symplectite III has higher Al2O3 contents um-grained (0.1-0.4 mm), appears greenish in color, and (3.1-5.5 wt%) than in the other two symplectites varieties ranges in composition from Fts to Fhb [Si = 6.48-6.72

(Al2O3 = 0.5-2.7 wt%). Some medium-grained Opx crys- p.f.u.; (Na + K)A = 0.38-0.48 p.f.u.; XMg = 0.48-0.50; Ti = tals are observed to surround individual Qz grains, and 0.11-0.15 p.f.u.]. The Pl grains are anhedral and have in- have low Al2O3 contents (0.7-1.2 wt%). The XMg values termediate An contents (An39−48). The Grt grains present and ZnO contents of Spl in symplectite III range from within the granoblastic amphibolite contain inclusions of 0.37 to 0.48 and 0.4 to 1.3 wt%, respectively. The Pl in Hbl and Ilm, and the chemical composition of this Grt

Opx + Pl + Spl + Mag symplectite-bearing Grt-bearing equates to an Alm- and Grs-rich (Alm61−69Prp11−12 granulite present within symplectite III has higher An Grs19−27Sps1−3). Medium-grained (up to 2 mm) Hbl grains contents (An83−93) than the Pl of symplectites I and II are classified as - Ti rich Fts to Fhb compositions [Si =

(An42−53) (Nakano et al., 2004). The Pl in Cpx- and Qz 6.44-6.61 p.f.u.; (Na + K)A = 0.22-0.39 p.f.u.; XMg = 0.43- abundant Grt-bearing granulite is only observed within 0.49; Ti = 0.11-0.24 p.f.u.], and the Pl present in these the Opx + Pl symplectites, and has high An contents rocks has low An contents (An28−39).

(An77−91). The amphibolite lithology is composed mainly of The granulites of the Kontum Massif contain Opx, Hbl, Pl, and Opq, although several rock samples also con- Cpx, Pl, Hbl, and Qtz, along with minor Ttn, Ilm, and Po. tain Ep and Cpx. One amphibolite sample in particular These granulites show granoblastic textures and are medi- has a number of Cum grains that are present within the um- (0.2-0.5 mm; Fig. 5k) to coarse-grained (0.5-2.0 matrix. This Cum-bearing amphibolite shows nemato-

Figure 5. Photomicrographs (plane polarized light for all but (o), which was taken under cross-polarized light) of the metamorphosed mafic rocks and metagabbro from the TVOB analyzed in this study. (a) Grt-bearing amphibolite with Bt from the Cangshan Mountains. (b) Grt- bearing amphibolite including Cpx from the Cangshan Mountains. (c) Schist including Ep from the Cangshan Mountains. (d) Grt-bearing amphibolite with Cpx + Pl symplectite from the Red River Shear Zone. (e) Amphibolite from the Red River Shear Zone. (f) Eclogite from the Song Ma Suture Zone. (g) Grt-bearing amphibolite from the Song Ma Suture Zone. (h) Amphibolite from the Song Ma Suture Zone. (i) Grt- bearing granulite with Opx + Pl symplectite from the Kontum Massif. (j) Grt-bearing granulite with Opx + Pl + Spl + Mag symplectite from the Kontum Massif. (k) Medium-grained granulite from the Kontum Massif. (l) Coarse-grained granulite from the Kontum Massif. (m) Cum-bearing amphibolite from the Kontum Massif. (n), (o) Ol-bearing metagabbro from the Kontum Massif. (p) Ol-free metagabbro from the Kontum Massif. Color version of Figure 5 is available online from http://japanlinkcenter.org/DN/JST.JSTAGE/jmps/120813. 70 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba blastic textures (Fig. 5m), and the Cum grains themselves al. (2012). During each run, He gas was used as a carrier are characterized by homogeneous, Mg-rich compositions for the LA-ICP-MS analyses. Major and trace element

(Si = 7.66-7.85 p.f.u.; XMg = 0.75-0.77). The Hbl grains data of samples 902T02E, 902T02P, 90301H, 904T02H, occurring in these amphibolites are chemically homoge- 830T03C, and 830T03E are referred from Nakano et al. neous, and are classified as Ti-poor Mhb [Si = 6.70-7.08 (2004, 2010). The whole-rock chemical compositions de- p.f.u.; (Na + K)A = 0.01-0.05 p.f.u.; XMg = 0.85-0.88]. The termined for all metamorphic rocks analyzed in the pres- An contents of the Pl grains show a narrow range of val- ent study on the TVOB are listed in Table 3. ues (An43−49). The Ol-bearing metagabbros of the Kontum Massif Results of whole-rock geochemistry contain Ol, Cpx, Spl, and minor Opx, Hbl, and Pl. The Ol in these rocks is fine- to medium-grained (up to 0.4 mm), Evaluating the effects of chemical alteration. In order subhedral to anhedral, and some grains are surrounded by to accurately estimate the geochemical and petrological Hbl and Opx crystals. Opx + Spl + Hbl symplectites exist nature of the protoliths of metamorphosed mafic rocks, it between the Ol and Pl grains (Fig. 5n), and in several in- is helpful if the rocks retained their original chemical stances Chl and Srp surround the Ol grains (Fig. 5o). In composition throughout the course of metamorphism. In contrast, an Ol-free variety of metagabbro also occurs in many natural systems, however, such rocks are suscepti- the Kontum Massif, containing Cpx, Opx, Spl, and Pl. In ble to the effects of element mobility and transfer during these metagabbros, Prh + Spl symplectite occurs between subsequent alteration events, or even during metamor- large crystals of Cpx and Pl (Fig. 5p). phism. Consequently, in order to estimate the degree to WHOLE-ROCK GEOCHEMISTRY which major element transfer may have taken place in the metamorphic rocks sampled from the TVOB, we have Analytical methods of whole-rock geochemistry used various alteration index values such as the Chemical

Index of Alteration [CIA: 100 × Al2O3/(Al2O3 + CaO +

For analysis of whole-rock chemical compositions, we Na2O + K2O); Nesbitt and Young, 1982] and the Chemi- removed any rock portions containing visible evidence of cal Index of Weathering [CIW: 100 × Al2O3/(Al2O3 + CaO veining or alteration in an effort to analyze only the fresh- + Na2O); Harnois, 1988] to gauge the magnitude of any est and most representative whole-rock samples of meta- major element geochemical open system behavior affect- morphic rocks. The measurement of major oxide and ing these rocks. In using these two indexes, the estimated trace element (V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, alteration thresholds of these rocks, above which the ma- Nb, and Ba) concentrations of whole-rock samples was jor element geochemical integrity of the protolith is in determined by X-ray fluorescence (XRF) spectrometry on question, corresponds to values higher than 50. The CIA a fused glass disk (2:1 dilution of sample) using a Philips and CIW values determined for the metamorphosed mafic PW2400 XRF spectrometer and a Rigaku Primus II XRF rocks and metagabbro in this study range up to 47.9. spectrometer at Kyushu University. The analytical condi- Therefore, we can say that the element transfer effects tions (instrumental set-up) and procedures followed those (i.e., open system behavior) caused by alteration on the outlined in Nakano et al. (2009b) for the Philips PW2400, major element compositions of these metamorphosed and Nakano et al. (2012) for the Rigaku Primus II. For mafic rocks and metagabbro are actually quite small. Ac- analysis of rare earth element (REE) concentrations (i.e., cordingly, this enables us make robust estimates of the La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and major element geochemical and petrological nature of the Lu), along with the concentrations of the trace elements protoliths of these metamorphic rocks, and ultimately ob- Hf, Ta, Pb, and Th, 15 samples were dissolved and ana- tain insights into the likely tectonic settings in which they lyzed by alkali fusion. Obtained solutions were analyzed formed, as outlined below. by ICP-MS (Agilent 7500a) at Niigata University. The It is also important to consider the possible effects detailed procedures of alkali fusion and ICP-MS analysis that alteration and metamorphism might have on the mo- employed in this study, followed closely those reported in bility and transfer (open system geochemical behavior) of Neo (2009). All other samples were analyzed using fused trace elements in these metamorphic rocks. For instance, glass discs (i.e., the same ones used for XRF analysis) by large ion lithophile (LIL) elements are generally consid- LA-ICP-MS (Laser: New wave co. UP-213, ICP-MS: ered to be mobile during alteration and metamorphism, Agilent 7500cx) at Kyushu University. The detailed ana- whereas incompatible high field strength (HFS) elements lytical procedures and accuracy of LA-ICP-MS analyses are generally considered as immobile (e.g., Winchester using this glass bead method are reported by Nakano et and Floyd, 1976). However, Hill et al. (2000) indicated Whole rock chemistry of metamorphosed mafic rocks of the TVOB 71

Table 3. Whole-rock chemical compositions of metamorphosed mafic rocks from the TVOB

A, amphibolite; GA, Grt-bearing amphibolite; G, granulite; GG, Grt-bearing granulite; S, schist; EC, eclogite; MG, metagabbro; RR, Red * ** River Shear Zone. Total Fe as Fe2O3; total Fe as FeO. n.a., not analyzed; n.d., not detected. Mg# = (MgO/40.32)/[(MgO/40.32) +

(FeO**/71.84)]. S/SAS = SiO2/(SiO2 + Al2O3 + Fe2O3). CIA = 100 × Al2O3/(Al2O3 + CaO + Na2O + K2O). CIW = 100 × Al2O3/(Al2O3 + CaO † †† + Na2O). Major element and trace element data are after Nakano et al. (2010). Major element and trace element data are after Nakano et al. (2004). 72 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba

Table 3. (Continued-1)

(FeO**/71.84)]. S/SAS = SiO2/(SiO2 + Al2O3 + Fe2O3). CIA = 100 × Al2O3/(Al2O3 + CaO + Na2O + K2O). CIW = 100 × Al2O3/(Al2O3 + CaO † †† + Na2O). Major element and trace element data are after Nakano et al. (2010). Major element and trace element data are after Nakano et al. (2004). Whole rock chemistry of metamorphosed mafic rocks of the TVOB 73

Table 3. (Continued-2)

(FeO**/71.84)]. S/SAS = SiO2/(SiO2 + Al2O3 + Fe2O3). CIA = 100 × Al2O3/(Al2O3 + CaO + Na2O + K2O). CIW = 100 × Al2O3/(Al2O3 + CaO † †† + Na2O). Major element and trace element data are after Nakano et al. (2010). Major element and trace element data are after Nakano et al. (2004). 74 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba

Table 3. (Continued-3)

(FeO**/71.84)]. S/SAS = SiO2/(SiO2 + Al2O3 + Fe2O3). CIA = 100 × Al2O3/(Al2O3 + CaO + Na2O + K2O). CIW = 100 × Al2O3/(Al2O3 + CaO † †† + Na2O). Major element and trace element data are after Nakano et al. (2010). Major element and trace element data are after Nakano et al. (2004). Whole rock chemistry of metamorphosed mafic rocks of the TVOB 75

Table 3. (Continued-4) that some HFS elements, for instance Y, Ti, and Zr, are actually mobile during alteration. Hill et al. (2000) proposed an additional index of alteration defined by the S/

SAF ratio [i.e., SiO2/(SiO2 + Al2O3 + Fe2O3)]. For example, altered basalts, whose S/SAF ratios are less than 0.5, are recognized as having had a significant amount of their HFS elements removed during alteration (Hill et al., 2000). The S/SAF ratios of metamorphosed mafic rocks from the TVOB in this study are all higher than 0.55. Therefore, based on the empirically determined S/SAF ratios, the metamorphosed mafic rocks in this study are also considered to have maintained their original HFS element concentrations, as were once present in their former protolith whole-rock geochemical compositions. We observed no clear correlations between Ba and

TiO2 contents in these metamorphic rocks (Fig. 6a). In comparison with HFS element contents, Rb and Sr concentrations also appear to be quite scattered (i.e., variable). Hence, the LIL elements (e.g., Ba, Rb, Sr) do not display any trends with HFS elements. In contrast, when

comparing HFS elements (e.g., TiO2 versus Zr) on the variation diagram (Fig. 6b), they display a distinct correlation that does not appear to be influenced by metamorphic grade. Other HFS elements also show positive correlations with one another. Consequently, it seems clear that some of the LIL element concentrations in several of these metamorphic rock samples might have been affected by element transfer related to alteration or metamorphism. On the other hand, the original concentrations of HFS elements in the protoliths of these metamorphic rocks were probably preserved throughout metamorphism and alteration. Major and trace element contents. The metamorphosed mafic rocks of the Cangshan Mountains have

whole-rock SiO2 contents of 46-52 wt%, and are relative-

ly low in alkalis (Na2O + K2O = 1-3 wt%). Consequently, on the total alkalis versus silica (TAS) diagram, these metamorphosed mafic rocks plot predominantly in the basalt field (Fig. 7a), and they also show characteristics of

tholeiitic basalts (Fig. 7b). The range of XMg values for schists and amphibolites is quite wide (0.34-0.67), while samples of Grt-bearing amphibolites yield slightly lower

values (0.26-0.53). The Al2O3 contents of metamorphosed mafic rocks from the Cangshan Mountains are 12-19 wt% (Table 3). Some of the trace element concentrations determined for these rocks (specifically Cr, Sr, Zr, and Nb) yield a wide range of values (Cr = 25-784 ppm; Sr = 105- 534 ppm; Zr = 6-395 ppm; Nb = 1.5-40 ppm). However,

(FeO**/71.84)]. S/SAS = SiO2/(SiO2 + Al2O3 + Fe2O3). CIA = 100 × Al2O3/(Al2O3 + CaO + Na2O + K2O). CIW = 100 × Al2O3/(Al2O3 + CaO † †† + Na2O). Major element and trace element data are after Nakano et al. (2010). Major element and trace element data are after Nakano et al. (2004). 76 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba

500 500 (a) (b) 400 400

300 300

200 200 Zr (ppm) Ba (ppm)

100 100 Figure 6. TiO2 versus Ba and Zr variation diagrams for the metamor- 0 0 phosed mafic rocks from the TVOB. 0123456 0123456 (a) TiO2 versus Ba variation dia- TiO2 (wt%) TiO2 (wt%) gram. There is no correlation be- Cangshan Red River Song Ma Kontum tween LIL elements and HFS ele- Schist Amphibolite Amphibolite Amphibolite Amphibolite Grt-bg. amphibolite Grt-bg. amphibolite Grt-bg. amphibolite ments. (b) TiO2 versus Zr variation Grt-bg. amphibolite Eclogite Granulite - Grt-bg. granulite diagram. The data exhibit well de- Metagabbro fined positive linear correlations. Fig.6_Yonemura et al. Figure 7. Major element classification Fe +Ti (b) total (a) diagrams for the metamorphosed 15 Phonolite mafic rocks from the TVOB. (a) TAS diagram for the metamor-

Foidite Tephriphonolite phosed mafic rocks from the TVOB Alkaline Trachyandesite Trachyte series (after Le Maitre et al., 1989). The 10 Phonotephrite High-Fe bold solid line, which subdivides tholeiite Basaltic Subalikaline, O (wt%) -

2 trachyandesite basalt Tephrite Tholeiite the alkaline group from the sub al- (Ol<10%) series

O+K kaline, tholeiite group is after Irvine

2 Basanite (Ol>10%) Andesite Trachy- Rhyolite Na basalt and Baragar (1971). (b) Ternary dia- 5 Basaltic Dacite Komatiitic andesite basalt gram for classification of the meta- Dacite Tholeiite morphosed mafic rocks in this study Andesite Rhyolite Picro- Calc-alkaline High-Mg according to their cation percentag- basalt Andesite tholeiite Komatiite Dacite Basalt basalt es of Al, (Fetotal + Ti) and Mg (Jen- 0 Rhyolite 35 40 45 50 55 60 65 70 75 Al Mg sen, 1976). Symbols are the same as SiO (wt%) 2 in Figure 6. Fig.7_Yonemura et al. these trace element contents show no apparent differences these rocks are 11-16 wt%, their Mg# ranges from 0.38 to (patterns) with regards to what rock type was analyzed. 0.69, and there are no clear geochemical variations related This is also evident when examining the vertical spread of to rock type. In addition, each rock type exhibits a wide REE values on the C1 chondrite-normalized REE dia- range of Cr, Sr, and Zr concentrations (Cr = 74-1144 gram (Fig. 8a), especially for the amphibolites (squares) ppm; Sr = 9-691 ppm; Zr = 37-194 ppm). Samples of the and Grt-bearing amphibolites (circles). These REE data amphibolite rock type have a wide range of Nb concentra- show an overall flat to light - REE enriched pattern, and tions (3-41 ppm), whereas the Grt-bearing amphibolite the data for low-REE mafic rocks show significant posi- type has overall slightly lower and less varied Nb contents tive Eu anomalies (Fig. 8a). These low-REE content (4-13 ppm). The REE patterns of Grt-bearing amphibolite metamorphosed mafic rocks also have low contents of Zr and amphibolite from the Red River Shear Zone are flat and other HFS elements. to LREE-enriched (Fig. 8b). Samples of metamorphosed mafic rocks collected Metamorphosed mafic rocks of the Song Ma Suture from the Red River Shear Zone have SiO2 contents of 44- Zone have SiO2 contents and Mg# of 44-53 wt% and 34- 53 wt%, as well as relatively low total-alkali element 67, respectively, and their total-alkali element contents contents (Na2O + K2O = 1-4 wt%). Accordingly, when are relatively low (Na2O + K2O = 1-4 wt%). Most of shown on the TAS diagram, these whole-rock chemical these rocks plot within the basalt field on the TAS dia- compositions plot for the most part within the basalt field, gram (Fig. 7a), although three samples are basaltic andes- although one sample is a basaltic andesite (Fig. 7a). All of ites and one plots as a picrobasalt. Nevertheless, all of these rocks share geochemical characteristics with tholei- these Song Ma mafic samples show geochemical charac- itic basalts as plotted on the ternary classification diagram teristics that correspond to tholeiitic basalts (Fig. 7b). The in Figure 7b, although one sample did show an affinity to Al2O3 contents of the Grt-bearing amphibolite (~ 13.5- komatiitic basalts instead (Fig. 7b). The Al2O3 contents of 15.5 wt%) and eclogite (~ 13-14 wt%) rock types are Whole rock chemistry of metamorphosed mafic rocks of the TVOB 77

1,000 1,000 - (a) Cangshan Schist (c) Song Ma Amphibolite Figure 8. C1 chondrite normalized Amphibolite Grt bg. amphibolite - Grt bg. amphibolite Eclogite REE diagram (a) (d) for the meta-

100 100 morphosed mafic rocks and metagabbro from the TVOB. (a) The Cangshan Mountains. (b) The 10 10 Red River Shear Zone. (c) The

Rocks / C1 Chondrite N-MORB Rocks / C1 Chondrite E-MORB Oceanic arc ave. Song Ma Suture Zone. (d) The OIB Continental arc ave. Kontum Massif. Normalizing val- 1 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu ues of C1 chondrite are after Mc- 1,000 1,000 (b) Red River Amphibolite (d) Kontum Amphibolite Donough and Sun (1995). Values of Grt-bg. amphibolite Grt-bg. amphibolite - - Granulite N MORB, E MORB, and oceanic Grt-bg. granulite island basalt (OIB) are after Sun 100 100 Metagabbro and McDonough (1989). Values of continental arc average and oceanic 10 10 arc average are after Kelemen et al.

Rocks / C1 Chondrite Rocks / C1 Chondrite (2003). Symbols are the same as in Figure 6, but only in terms of their 1 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu shape, not their fill color. Fig.8_Yonemura et al. generally lower than that of the amphibolite (~ 15-16 including all of the amphibolite rocks and the remaining wt%). The eclogite samples and most of the amphibolites Fe-poor granulite and Grt-bearing amphibolite samples are low-Zr and low-Nb rocks (Zr = 56-88 ppm; Nb = 2-5 (i.e., those not referred to in the preceding sentence), the ppm), whereas the Grt-bearing amphibolite rocks and Fe2O3 contents range from 10 to 16 wt%. Cr contents are some of the amphibolite samples have significantly higher low for all rocks (1-239 ppm), with the exception of one Zr and Nb contents (Zr = 106-272 ppm; Nb = 11-28 amphibolite sample which yielded an exceptionally high ppm). The Cr and Sr concentrations in metamorphosed Cr concentration of 569.5 ppm. A number of other trace mafic rocks of the Song Ma Suture Zone exhibit wide elements also show a wide range of concentrations (Sr = variation (Cr = 80-720 ppm; Sr = 54-397 ppm), and show 18-302 ppm; Zr = 13-277 ppm; Nb = 1-18 ppm). Two of no clear geochemical particularities with respect to differ- the granulite samples show N-MORB-like, LREE-de- ent rock types. The REE patterns of eclogites are almost pleted REE patterns, and one Grt-bearing amphibolite perfectly flat (Fig. 8c), while most of the Grt-bearing am- shows OIB-like, LREE-enriched, and HREE-depleted phibolite and amphibolite samples show flat to LREE-en- REE patterns (Fig. 8d). All other metamorphosed mafic riched patterns (Fig. 8c). However, some of the amphibo- rocks show flat to slightly LREE-enriched REE patterns, lites show LREE-depleted REE patterns, which are and collectively exhibit a distinct ‘gap’ that indicates a bi- similar to the patterns exhibited by N-MORB (Fig. 8c). modal REE abundance pattern (Fig. 8d). Many of these

The whole-rock SiO2 contents of metamorphosed metamorphosed mafic rock samples from the Kontum mafic rocks of the Kontum Massif, with the exception of Massif, especially the high-Fe content Grt-bearing granu- the metagabbros, are in the range 44-54 wt%. These rocks lite, granulite, and Grt-bearing granulite, exhibit promi- also have low total-alkali element contents (up to 3 wt%). nent positive and negative Eu anomalies, whereby the As with all other field areas, the metamorphosed mafic positive Eu anomalies are displayed by the REE-enriched rocks from the Kontum Massif plot predominantly within rocks and the negative Eu anomalies are displayed by the the basalt field on the TAS diagram (Fig. 7a), with at least REE-depleted rocks (Fig. 8d). two samples plotting clearly in the basaltic andesite field The three metagabbro samples collected from the and two samples plotting as picrobasalts. Most metamor- Kontum Massif all plot within the Komatiite field on the phosed mafic rocks from the Kontum Massif exhibit geo- ternary classification diagram shown in Figure 7b, and chemical characteristics indicative of tholeiitic basalt af- have low Al2O3 contents (5-7 wt%). In addition, these finity (Fig. 7b) and Al2O3 contents are 11-16 wt%. Based metagabbro samples are high-Mg#, high-Cr, and high-Ni only on the high Fe contents (Fe2O3 = 18-24 wt%) ob- rocks (Mg# = 0.82-0.83; Cr = 956-1440 ppm; Ni = 487- served for the Grt-bearing granulites, some of the granu- 774 ppm). Furthermore, some of the other trace elements lite samples, and some of the Grt-bearing amphibolites, present in these rocks, such as Sr, Zr, and Nb, are notably most rocks from this field area (i.e., the Kontum Massif) quite low (Sr = 27-117 ppm; Zr = 11-24; Nb = 1-2 ppm). can be readily distinguished from the rocks present in all Collectively, these geochemical characteristics are taken other field areas. For the rest of the Kontum Massif rocks to indicate that these metagabbros were originally derived analyzed in this study (excluding metagabbro samples), from relatively primitive magmas. They also show flat 78 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba

REE patterns and contain low concentrations of REEs the protolith rocks for these metamorphosed mafic rocks (Fig. 8d). of the TVOB really were. However, if any of these metamorphosed mafic rocks preserve any ‘ghost’ igneous tex- DISCUSSION AND CONCLUSIONS tural features, or alternatively, any relict igneous minerals that still preserve their original chemical composition, Protoliths of metamorphosed mafic rocks of the TVOB then it is possible to evaluate whether the protoliths originated as basalt or gabbro. With regards to these two pos- In order to decipher the tectonic setting of the protoliths sibilities: i) It is clear that the metagabbros from the Kon- of metamorphosed mafic rocks, it is important to first try tum Massif analyzed in this study have actually retained and establish whether the rocks were derived from gabbro medium-grained relict Cpx, Ol, and Spl in igneous origin; or basalt. The main reason for this is because the tectonic ii) Several amphibolites analyzed in this study have An- settings of pristine basalts can be subdivided into a num- rich Pl, which also hints at the possibility that some of ber of categories based on whole-rock chemical composi- these Pl crystals are relict igneous grains that preserve the tions (e.g., Pearce and Cann, 1971, 1973). In this study, original geochemistry of Pl that was once growing within several of the amphibolite units were found to occur as a mafic magma. layers or lenses within surrounding pelitic gneisses (Figs. Even so, most of the textural and chemical features 4c and 4d). This mode of occurrence may indicate that of the minerals in these metamorphosed mafic rocks of precursor mafic magmas for these amphibolites were ei- the TVOB were formed during metamorphism. Hence, ther erupted on or intruded into pre-existing sediments. the bulk chemical compositions of these metamorphosed Therefore, we propose that the protoliths for these am- mafic rocks, with the exception of metagabbros, canbe phibolites can be regarded as having originally been vol- still be evaluated in terms of the compositions of mineral canic rocks (if not fine-grained shallow sills) to begin phases on an ACF diagram (e.g., Eskola, 1915), along with. In fact, metamorphosed pelitic rocks are generally with other whole-rock geochemical classification schemes distributed around metamorphosed mafic rocks in every for metamorphic rocks. Therefore, because it is difficult field area of the TVOB investigated in this study. This to classify every metamorphosed mafic rock from the lithological relationship could mean that most of these TVOB into volcanic rock-derived or igneous rock-de- metamorphosed mafic rocks, by inference, also started out rived subgroups by mode of occurrence or petrographical as basaltic lava flows (or shallow basaltic sills). Further- features alone (as such features may be scarce or some- more, several Grt-bearing granulite and granulite rocks what vague), we are usually left to classify these meta- are found as blocks or lenses within felsic gneiss (Fig. morphic rocks based only on their whole-rock geochemi- 4h), which might represent mafic magmas intruded into cal compositions, which may still be quite useful for crustal materials to begin with. deciphering the nature of their protoliths. Nevertheless, based only upon these modes of occur- In the case of similar chemical compositions be- rence in the field, it still remains unclear as to what all of tween basalt and gabbro protoliths, it is difficult to sepa-

25 35 50 (a) (b) (c) Fe-rich type 30 40 20

25 30 * (wt%) 3 O

2 15 Zr/Sm Fe 20 20 *+MnO+MgO (wt%) 3 O

10 2 15 10 Zr-poor type Fe

5 10 0 0123456 5 10 15 20 0 5 10 15 20 TiO (wt%) Al O (wt%) 2 2 3 Al2O3 (wt%)

Figure 9. Variation diagrams for the metamorphosed mafic rocks and metagabbro from the TVOB. (a) TiO2 versus total Fe2O3 variation dia- Fig.9_Yonemura et al. gram. (b) Al2O3 versus total Fe2O3 + MgO + MnO variation diagram. (c) Al2O3 versus Zr/Sm variation diagram. Background gray areas represent the compositions for typical tholeiite and andesitic basalt data from the GEOROC database (http://georoc.mpch-mainz.gwdg.de) pro-

vided by the Max-Planck-Institute, when restricting the SiO2 range to 43-55 wt%. Symbols are the same as in Figure 6. On some of these diagrams, the Fe-rich rock type in (a) and Zr-poor rock type in (c) plot outside the domain of typical basalt, as do the metagabbros in two out of three plots in (b) and (c). Whole rock chemistry of metamorphosed mafic rocks of the TVOB 79

rate them without preservation of the original igneous from typical basalts on the Zr/Sm versus Al2O3 diagram rock texture during metamorphism. However, typical ba- (Fig. 9c). Several Grt-bearing amphibolite, amphibolite, salts may exhibit their own characteristic whole-rock and schist samples from the Cangshan Mountains have geochemical particularities and trends as depicted in Fig- significantly lower Zr/Sm ratios- (4 14) than typical ba- ure 9. Therefore, we may treat metamorphosed mafic salts (see dashed field in Fig. 9c). In addition, this Zr-de- rocks (whose chemical compositions fall inside the range pleted type grouping of Cangshan rocks trends towards of typical basalts) to originally have been basalt flows, Al2O3-rich, and other HFS element-poor (e.g., Ti) whole- which in turn allows us to estimate the tectonic setting of rock geochemical compositions (Figs. 9a and 9c). Fur- the protolith magma. On the other hand, metamorphosed thermore, this geochemical grouping also exhibits posi- mafic rocks whose geochemical compositions fall outside tive Eu anomalies (which is likely related to Pl accumu- the range of typical basalts, can probably be treated as lation/concentration in these rocks: Rollinson, 1993), but originally having been gabbro intrusions. Accordingly, for contains low overall abundances of all the REE (Fig. 8a). example, the metagabbro samples from the Kontum Mas- Based on low Zr/Sm ratios, high concentrations of Al2O3, sif of the TVOB all plot outside of (Figs. 9b and 9c) or at and the REE characteristics, the protolith of this Zr-de- the edge of (Fig. 9a) the geochemical fields for typical ba- pleted rock type is considered to have originally been a salts (Fig. 9). Pl-abundant gabbro. Furthermore, this Zr-depleted geo- As displayed in Figure 9a and b, typical basalts ex- chemical variety also has low Nb/La ratios (0.43-0.63). hibit distinct geochemical trends, whereby increasing Generally, Nb concentrations in arc magmas are depleted

Fe2O3 contents correlate positively with increasing TiO2 relative to the REE, such that Nb/La ratios tend to be low- concentrations, and Fe2O3 + MnO + MgO (wt%) contents er than in MORB (cf., N-MORB: Nb/La = 0.93, calculat- decrease with increasing Al2O3. The positive correlation ed from Sun and McDonough, 1989) (Kelemen et al., between Ti and Fe is considered to represent variations in 2003). Therefore, these lines of geochemical evidence magma geochemistry throughout crystallization. The in- suggest that the protoliths of these Zr-depleted metamor- verse correlation between Al2O3 and Fe2O3 + MgO + phosed mafic rocks found within the TVOB, were likely MnO might be related to increasing modal abundance of Pl-abundant gabbros derived from arc-related magma- Pl and decreasing modal abundances of mafic minerals in tism. the evolving basalts. On Figure 9a, several Grt-bearing The Mg# and Cr contents of metagabbros from the granulites, granulites, and Grt-bearing amphibolites from Kontum Massif indicate that the original magmas that the Kontum Massif plot within an Fe-rich compositional formed the protolith gabbros were quite primitive. Possi- field that is distinct from typical basalts, and therefore the ble tectonic settings for the protoliths of these metagab- protoliths of these rocks are considered to have been gabbros might include MORB or arc basalts, because these bros. These Fe-rich rock types also have total mafic rocks have MORB- or arc basalt-like flat REE patterns phase, such as Fe, Mg and Mn, rich trends (Fig. 9b). (Fig. 8d). Furthermore, these metagabbros also have Nb/ These major element compositions are considered to have La ratios that are slightly high (Nb/La = 0.71-1.08), as do been derived from the enrichment of these mafic rocks in MORB (Nb/La = 0.83-1.46). Consequently, based on minerals such as Ol, Opx, and Cpx, which have high-Fe, their REE patterns and HFS element compositions, the -Mn, -Mg compositions, perhaps as cumulate magmas. metagabbros of the Kontum Massif were probably geneti- Such Fe-rich compositions of mafic rocks from the Kon- cally related to a MORB-like tectonic setting. tum Massif have also been reported by Lan et al. (2003) We have also evaluated the possible tectonic settings for amphibolite rocks, which were interpreted to have of the protoliths of all metamorphosed mafic rocks from been derived from underplated magmas accumulated dur- the TVOB that plot within the geochemical field for typi- ing the break-up of the Gondwana supercontinent. cal basalts in Figure 9. To this end, on a C1 chondrite-

A Zr/Sm versus Al2O3 variation diagram with all of normalized Gdn-Ybn variation diagram (Fig. 10a), this the whole-rock geochemical data for metamorphosed ‘basalt origin’ group is divided into a high-(Gd/Yb)n mafic rocks from this study plotted along with the refer- group and a low-(Gd/Yb)n group. In this equation (as well ence field for typical basalts, is displayed in Figure 9c. Zr as throughout the main text, and also in Fig. 11), C1 displays a definite correlation with other HFS elements chondrite-normalized (McDonough and Sun, 1995) val-

(e.g., TiO2 in Fig. 6b), so that Zr/Sm means depletion of ues and ratios of normalized values are denoted with a HFS elements from middle REE. The Zr/Sm ratios of typ- subscript n. Incidentally, these same two groups of meta- ical basalts are 15-45 with a corresponding wide range of morphosed mafic rock samples are also recognized inde-

Al2O3 contents (10-18 wt%). The aforementioned Fe-rich pendently and plot as two separate fields on a n La -Ybn rock types display no apparent geochemical deviations variation diagram (Fig. 10b). In other words, the high- 80 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba

90 350 (a) (b) Figure 10. Trace element variation di- 80 300 agrams for use in the further classi- group group 70 n n fication of ‘basalt origin’ group 250 metamorphic rocks from the 60 TVOB. (a) Yb versus Gd variation 200 n n n n High-(Gd/Yb) 50 High-(Gd/Yb) diagram. (b) Ybn versus Lan varia- La Gd group tion diagram. The ‘basalt origin’ 40 group 150 n n Low-(Gd/Yb) group can be subdivided into a 30 100 high-(Gd/Yb)n group and a low-

20 Low-(Gd/Yb) (Gd/Yb)n group. (c) Ybn versus (Nb/ 50 10 La)n variation diagram, limited to only the low-(Gd/Yb)n group data 0 0 0 10 20 30 40 50 60 0 10 20 30 40 50 60 points. This low-(Gd/Yb)n group Yb can be further subdivided into a Ybn n 2.0 high-(Nb/La) subgroup and low- (c) n (Nb/La)n subgroup. The dashed line

High-(Gd/Yb) group field Cangshan Red River in (c) marks the intermediate (aver- n Schist Amphibolite age) value of (Nb/La)n between the 1.5 Amphibolite Grt-bg. amphibolite Grt-bg. amphibolite N-MORB and the ‘Oceanic arc ba-

n High-(Nb/La)n subgroup Song Ma Kontum salt average’ values. Subscript ʻnʼ 1.0 Amphibolite Amphibolite indicates mean C1 chondrite- (Mc- Grt-bg. amphibolite Grt amphibolite Eclogite Granulite Donough and Sun, 1995) normal- (Nb/La) ized values. The data plotted for E- Low-(Nb/La)n N-MORB - 0.5 subgroup E-MORB MORB and N MORB are from Sun Oceanic arc basalt average and McDonough (1989) and the Continental arc basalt average data plotted for ‘Oceanic arc basalt 0.0 average’ and ‘Continental arc basalt 0 10 20 30 40 50 60 average’ are from Kelemen et al.

Ybn (2003). Fig.10_Yonemura et al.

Ti/100 (a) 2Nb (b)

High (Gd/Yb)n group High (Gd/Yb)n group Low-(Gd/Yb)n group [High-(Nb/La) subgroup] Figure 11. Discrimination diagrams Whithin-plate n basalts for the ‘basalt origin’ group of meta- Low-(Gd/Yb)n group [low--(Nb/La) subgroup] IAT morphosed mafic rocks from the n E-type MORB Low-(Gd/Yb) group WPT n TVOB. Discrimination fields: (a) af- Low-(Gd/Yb)n group [low--(Nb/La)n subgroup]

[High-(Nb/La)n subgroup] ter Meschede (1986); (b) after MORB Within-plate basalts Pearce and Cann (1973); (c) after Calc-Alkaline &Volcanic-arc basalts N-type MORB basalts Wood (1980); (d) after Cabanis and & Volcanic-arc basalts Zr/4 Y Zr 3Y Lecolle (1989). Abbreviations: WPT, within-plate tholeiite; IAT, is- Hf/3 Y/15 (d) Back arc basin (c) N-MORB land arc tholeiite; VAB, volcanic arc Low-(Gd/Yb) group basalt; OL, overlap area between n Vorcanic arc [High-(Nb/La) subgroup] n tholeiites E-MORB volcanic arc and calc-alkaline ba- (weakly enriched) N-MORB salt. Symbols are the same as in Fig- OL E-MORB ure 10. The high-(Gd/Yb)n group Low-(Gd/Yb)n group VAB (enriched) [low--(Nb/La)n subgroup] - E-MORB plots in the within plate basalt field Low-(Gd/Yb)n group and [low--(Nb/La) subgroup] - n Low-(Gd/Yb) group in (a) (c) as well as the continental WPT n [High-(Nb/La) subgroup] n - High (Gd/Yb)n basalt field in (d). The high (Nb/La)n group subgroup exhibits N-MORB to E- Alkaline Calc-Alkaline Alkali basalts WPT basalts (intercontinental rift) MORB geochemical characteristics, Continental High (Gd/Yb)n - group basalts whereas the low (Nb/La)n subgroup Th Ta La/10 Nb/8 exhibits VAB affinity.

Fig.11_Yonemura et al. (Gd/Yb)n group also has high-Lan values, and vice versa. hibits an OIB-like LREE- and MREE-enriched pattern.

When examining this sample suite on the C1 chondrite- In contrast, the low-(Gd/Yb)n group exhibits LREE-poor normalized REE diagram, the high-(Gd/Yb)n group ex- or slightly LREE-rich patterns, which are similar to the Whole rock chemistry of metamorphosed mafic rocks of the TVOB 81

REE patterns exhibited by N-MORB, E-MORB, and av- these high-(Gd/Yb)n rocks plot within the continental ba- erage values of oceanic arc and continental arc rocks (Fig. salt field on that diagram (Fig. 11d). In contrast, the meta-

8). In general, the Nb and Ta concentrations in arc mag- morphosed mafic rocks of the low-(Gd/Yb)n variety, plot mas are depleted relative to REE contents, such that Nb/ within the MORB and arc-related fields (Figs. 11a-11d), La ratios tend to be lower than in MORB (e.g., Kelemen which is a trend that also conforms petrologically to the et al., 2003). On the (Nb/La)n-Ybn variation diagram, the flat to slightly LREE-rich REE pattern (Fig. 8). The high- low-(Gd/Yb)n group can be subdivided along the central (Nb/La)n subgroup plots mainly in the MORB field (Figs.

(average) dividing line between the values of (Nb/La)n for 11a, 11b and 11d), and notably, the geochemical field for

N-MORB and ‘average oceanic arc basalt’, into a high- the low-(Nb/La)n subgroup (which has more of an affinity

(Nb/La)n subgroup and low-(Nb/La)n subgroup (Fig. 10c). to island arc basalts) overlaps with the high-(Nb/La)n sub-

Based on this ratio (i.e., (Gd/Yb)n) and the observed vari- group in three of these discrimination diagrams (Figs. ations in REE patterns (Fig. 8), we propose that the high- 11a-11c). Furthermore, on the discrimination diagram of

(Nb/La)n subgroup of metamorphosed mafic rocks from Cabanis and Lecolle (1989), the low-(Nb/La)n subgroup the TVOB were probably derived from a MORB-like plots in the back-arc to arc basalt fields (Fig. 11d). Conse- protolith, and that the low-(Nb/La)n subgroup of rocks quently, we propose that the protoliths of the metamor- was probably derived from arc-related basalt lavas. phosed mafic rocks of the TVOB, which are thought to On three different discrimination diagrams, the meta- have originally been basalts, were actually formed in a morphosed mafic rocks of the high-(Gd/Yb)n group plot combination of three different types of tectonic settings of in the within-plate basalt (WPB) fields (Figs. 11a-11c). protolith magmatism, including WPB type [high-(Gd/Yb)n

This trend conforms to the petrological explanation for subgroup], MORB type [high-(Nb/La)n subgroup], and the observed REE patterns, which also involves LREE- arc type [low-(Nb/La)n subgroup] tectonic settings. enrichment. Moreover, another type of tectonic discrimi- The protolith rock types of metamorphosed mafic nation diagram (i.e., La-Y-Nb of Cabanis and Lecolle, rocks and metagabbros of the TVOB are broadly classi- 1989; Fig. 11d) adds support for a WPB origin for the fied into basalt and gabbro. The ‘basalt origin’ metamor- high-(Gd/Yb)n group of rocks from the TVOB, because phosed mafic rocks appear to have been derived from

South Cangshan Red River Song Ma Kontum Indochina SE China Kham Duc Ngoc Linh and Kannak Craton Craton Truong Song Laos

Protolith of metmorphosed mafic rocks WPB VAB (includig Sedimentary rock and Low-grade metamorphic rock Eclogite Zr-poor gabbro) Schist Granulite MORB Amphibolite Grt-bearing granulite Fe-rich gabbro Grt-bearing amphibolite Metagabbro

Song Chey Song Hong Song Da Song Ma Song Ca Tam Ky- Kham Duc Duc To Kan Po Ko fault fault fault fault fault Phuoc Son shear zone shear zone suture zone shear zone

South China Sea

N South China Craton

Basalt Gabbro Trans Vietnam Orogenic Belt Permo-Triassic MORB Zr poor type (VAB related) metamorphosed zone Indochina VAB Fe-rich gabbro Collision boundary Craton WPB Metagabbro Fault and shear zone

Figure 12. Schematic cross-section of the TVOB with rock types and protolith types and distribution map of various protolith types derived from metamorphosed mafic rocks and metagabbro from the TVOB. The cross-section is modified after Osanai et al. (2008). The base of distribution map is after Osanai et al. (2010). Abbreviations: MORB, mid-ocean ridge basalt; VAB, volcanic arc basalt; Fig.12_YonemuraWPB, within-plate et al.basalt. The main protolith tectonic settings are of VAB and WPB schist in the Cangshan Mountains from schist and amphibolite. VAB, WPB and MORB are main protolith tectonic settings in the Red River Shear Zone and in the Song Ma Suture Zone from schist, amphibolite, Grt- bearing amphibolite and eclogite. The Kontum Massif is composed by WPB and VAB from amphibolite, Grt-bearing amphibolite and MORB and Fe-rich gabbro from Grt-bearing amphibolite, granulite and Grt-bearing granulite. 82 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba

MORB, WPB, and arc basalts, whereas the ‘gabbro ori- MORB: middle crust to lower crust and Fe-rich gabbro: gin’ metamorphosed mafic rocks are divided into an Fe- lower crust and bottom. This large-scale vertical distribu- rich type, a Zr-poor type, and MORB-related metagab- tion of different crustal materials in the collision zone bros. The areal (map) distributions of each type of suggests subduction of oceanic crust prior to continental protolith tectonic setting and rock type are plotted on the collision (Figs. 13a and 13b; Color version of Figure 13 is map shown in Figure 12, in the context of all of the sur- available online from http://japanlinkcenter.org/DN/JST. rounding cratonic blocks and oceanic bodies. The assem- JSTAGE/jmps/120813). blage of protolith rock types in the Cangshan Mountains In the Cangshan Mountains, VAB related mafic rocks divided into WPB type and volcanic arc basalt (VAB) are collected from every sampling site (Fig. 12). On the type originated from basalt and third Zr-poor rock type other hand, WPB are collected in limited sampling sites possibly originated from a VAB gabbro protolith. Accord- (Fig. 12). The difference of distribution between VAB and ingly, the Cangshan Mountains are now characterized by WPB may indicate that the amount of WPB is less vol- an absence of any oceanic crustal material. However, in ume than VAB. In several sampling sites, both VAB and the Red River Shear Zone and the Song Ma Suture Zone, WPB are of similar rock types (Fig. 12), which suggest the distribution of tectonic setting (all of which are ‘basalt that WPB have been associated with VAB. These two evi- origin’ metamorphosed mafic rocks) are WPB, MORB, dences indicate a possibility that WPB intruded into VAB and arc basalt. Gabbro origin metamorphosed mafic rocks (Fig. 13a). Also in the other area, there are several assem- are absent in these area. Therefore, unlike the Cangshan blages of protolith in similar rock types; VAB, WPB and Mountains, the Red River Shear Zone and the Song Ma MORB derived Grt-bearing amphibolites from the Red Suture Zone contain a significant amount of oceanic River Shear Zone, VAB and MORB derived eclogites crustal materials. from Song Ma Suture Zone and MORB and Fe-rich gab- The variety of protolith tectonic settings for the bro derived granulites from the Kontum Massif. These metamorphosed mafic rocks of the Kontum Massif also complicated combinations of protolith in a single rock entails WPB, MORB, and VAB types (all of which per- type could be formed by mixing with subducted arc, oce- tain to the ‘basalt origin’ group), whereas an Fe-rich type anic crust and continental crust and coinstantaneous fold- and a MORB-related type are the proposed tectonic set- ing during continental collision as suggested by Osanai et tings of the protolith magmas of the ‘gabbro origin’ group al. (2008) (Fig. 13b). Although folding make apparent re- (i.e., the metagabbros). verse sequences as showing in Figure 13b, relationship between protolith and rock types are still confirmed in Tectonic implication large scale. This large-scale vertical distribution reflects structure of each protolith during the Permian-Triassic Here, we discuss about regional relationships between collision. The structure indicates that MORB subducted rock types and the tectonic settings of protolith, which under VAB and WPB during subduction stage, and Fe- may reveal crustal level of the analyzed rocks in the colli- rich gabbro subducted under them during collision stage. sion zone. The rock types of mafic rocks derived from After collision, several crustal materials were exhumed by WPB are schist, amphibolite and Grt-bearing amphibolite thrust faults (Fig. 13c). (Fig. 10a). The rock types of mafic rocks derived from The exhumed crustal materials in the Cangshan VAB, including VAB-related Zr-poor gabbro, are schist, Mountains are mainly upper to middle crustal materials amphibolite, Grt-bearing amphibolite and eclogite (Figs. with VAB, WPB derived metamorphosed mafic rocks. On 9c and 10c). The rock types of MORB origin mafic rocks the other hand, the exhumed crustal materials in the Red are amphibolite, Grt-bearing amphibolite, eclogite and River Shear Zone, the Song Ma Suture Zone and the Kon- granulite (Fig. 10c). Comparing with such ‘basalt origin’ tum Massif are mostly middle to lower crustal materials group and Zr-poor gabbro, the Fe-rich gabbro is higher- with VAB, WPB and MORB. Furthermore, Grt-bearing grade; Grt-bearing amphibolite, granulite and Grt-bearing granulite in the Kontum Massif are derived from Fe-rich granulite. In normal crustal geothermal gradient, schist, gabbro, which compose the Indochina Craton (Lan et al., amphibolite, Grt-bearing amphibolite are pissibly formed 2003). A number of metamorphosed sedimentary rocks by the subduction of upper to middle crustal materials. are found with metamorphosed mafic rocks. The meta- The different rock types should be controlled by the depth morphosed pelitic rocks have the Indochina Craton de- of the subduction of each rock type. Based on this con- rived age (~ 430 Ma) with the Permian-Triassic metamor- cept, the inferred crustal levels of each protolith in this phic age as Th-Pb Mnz age from the Red River Shear study are as follows; WPB: upper crust to middle crust, Zone (Gilley et al., 2003), U-Th-Pb Mnz age from the VAB and Zr-poor gabbro: upper crust to middle crust, Song Ma Suture Zone (Nakano et al., 2010) and Mnz Whole rock chemistry of metamorphosed mafic rocks of the TVOB 83

(a) Oceanic Crust subduction

Oceanic island

South China Craton Arc Oceanic Crust Indochina Craton

Upper Mantle Upper Mantle

Permo-Triassic metamorphism (b) Continental collision (ca. 250 Ma) Cangshan Red RIver Song Ma Kontum Permo-Triassic re-active zone* Truong Song Siluro-OrdovisianSiluro-Oldovisian continental fragment*

South China Craton Indochina Craton

Upper Mantle

South China Craton Indochina Craton

Figure 13. Schematic tectonic evolu- Upper Mantle tion of the TVOB. (a) Oceanic crust subduction stage. (b) Continental collision stage with folding crustal materials. (c) Thrust up stage after continental collision. Arc, oceanic Cangshan Red River Song Ma Truong Song Kontum crust and the subducted Indochina Kham Duc and SE Laos Ngoc Linh and Kannak Craton caught continental collision metamorphism during Permian- Triassic. The metamorphic rocks, which form the TVOB, are exhumed by thrusting. Color version Protolith of Permo-Triassic metamorphosed rocks Siluro-Oldovisian continental fragment Oceanic crust and Arc materials Indochina Craton related materials Indochina Craton related materials of Figure 13 is available online MORB Indochina Craton (metamorphic rock) Indochina Craton (metamorphic rock) VAB (including Zr-poor gabbro) Fe-rich gabbro Fe-rich gabbro from http://japanlinkcenter.org/DN/ WPB Sedimentary rock Fault Indochina*: Nakano et al.Craton (2007) JST.JSTAGE/jmps/120813.

Fig.13_Yonemura et al. CHIME age from the Kontum Massif (Osanai et al., tervened crustal materials, such as WPB, VAB and 2001). Hence, the Indochina Craton is the origin of ex- MORB, also subducted under the South China Craton. humed lower crustal metamorphosed mafic rocks and Collided crustal material were folding at that time (Fig. middle to lower crustal metamorphosed pelitic rocks 13b) (Figs. 13b and 13c). According to the vertical distribution 3) The Permian-Triassic collided zone were exhumed by of metamorphosed mafic rocks and the distribution of the thrust fault and formed the TVOB (Fig. 13c). Indochina Craton derived materials, the Indochina Craton We conclude that the TVOB consists of protoliths was considered to subduct under the South China Craton. originating from oceanic, island-arc, and within-plate tec- The tectonic evolution of the TVOB is divided at least tonic settings (in addition to an Fe-rich protolith rock three stages as follows: type). To a large degree we have deduced all of these vari- 1) Oceanic crust subducted beneath arc. The arc would ous protolith rock types and related tectonic settings, by distribute in the South China side (Fig. 13a). analogy with the various geodynamic settings and geo- 2) The Indochina craton collided and subducted under the chemical characteristics of modern plate tectonic environ- South China Craton during Permian-Triassic. The in- ments and their associated magmas. The one common de- 84 K. Yonemura, Y. Osanai, N. Nakano, M. Owada and S. Baba nominator bridging all three of these newly designated zircon U/Pb geochronology of magmatic rocks in the Dian- areas is that metamorphosed mafic rocks with a strong af- cang Shan massif, SE Tibet, China. Gondwana Research, 19, 975-993. finity to WPB are distributed throughout the entire region, Carter, A., Roques, D., Bristow, C. and Kinny, P. (2001) Under- - along with those that have a strong affinity to arc related standing Mesozoic accretion in Southeast Asia: significance basalts. Based on the relationship between areal distribu- of Triassic thermotectonism (Indosinian orogeny) in Vietnam. tion and metamorphic grade, arc was distributed in the Geology, 29, 211-214. 3+ South China Craton side and oceanic crust, mainly Droop, G.T.R. (1987) A general equation for estimating Fe con- MORB, subducted under the arc. The crustal components centrations in ferromagnesian silicates and oxides from microprobe analyses, using stoichiometric criteria. Mineralogi- - of the Permian Triassic South China/Indochina collided cal Magazine, 51, 431-435. zone folded during continental collision and were ex- Eskola, P. (1915) On the relations between the chemical and min- humed by thrusting to forming the TVOB. eralogical composition in the metamorphic rocks of the Ori- jarvi region. Bulletin de la Commission géologique de Fin- ACKNOWLEDGMENTS lande, 44, 109-145. Gilley, L.D., Harrison, T.M., Leloup, P.H., Ryerson, F.J., Lovera, O. and Wang, J.H. (2003) Direct dating of left-lateral defor- We would like to thank T. Toyoshima, T. Miyamoto, Nam mation along the Red River zone, China and Vietnam. Jour- Ngoc Tran, P. Binh, and Z. Li for assistance during field nal of Geophysical Research, 108, 14-1-14-21. surveys in the study areas; S. Kagashima, R. Imanaka, Y. Harnois, L. (1988). The CIW index: a new Chemical Index of Adachi, N. Neo, K. Shuto, and E. Tazawa for assistance Weathering. Sedimentary Geology, 55, 319-322. Hill, I.G., Worden, R.H. and Meighan, I.G. (2000) Yttrium : The with ICP-MS analysis; and T. Adachi for helpful com- immobility-mobility transition during basaltic weathering. ments. We would also like to thank two anonymous re- Geology, 28, 923-926. viewers for their constructive reviews and Prof. T. Tsuno- Irvine, T.N. and Baragar, W.R.A. (1971) A guide to the chemical gae for the editorial efforts. This work was partly classification of the common volcanic rocks. Canadian Jour- nal of Earth Sciences, 8, 523-548. supported by a Grant-in-Aid for Scientific Research (No. Jensen, L. (1976) A new cation plot for classifying sub-alkaline 21253008, 17253005 and 14340150 to Y. Osanai) from volcanic rocks. Ontario Division Mines Miscellaneous Paper, the Ministry of Education, Culture, Sports, Science, and No. 66. Technology, Japan. Kelemen, P.B., Hanghøj, K. and Greene, A.R. (2003) One view of the geochemistry of subduction-related magmatic arcs with DEPOSITORY MATERIALS an emphasis on primitive andesite and lower crust. In The Crust (Rudnick, R.L. Ed.). Vol. 3, Treatise on Geochemistry (Holland, H.D. and Turekian, K.K. Eds.). Elsevier-Pergam- Color version of Figures (4, 5, and 13) and Appendixes on, Oxford, 593-659. 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