J. metamorphic Geol., 2013, 31, 49–62 doi:10.1111/jmg.12012 Origin and Tectonic Implication of Ophiolite and Eclogite in the Song Ma Suture Zone between the South China and Indochina Blocks R. Y. ZHANG,1 C.-H. LO,1 S.-L. CHUNG,1 M. GROVE,2 S. OMORI,3 Y. IIZUKA,4 J. G. LIOU2 AND T. V. TRI5 1Department of Geosciences, National Taiwan University, Taipei 106, Taiwan ([email protected]) 2Department of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305, USA 3Department of Earth and Planetary Sciences, Faculty of Sciences, Institute of Technology, Meguro, Tokyo 152-8551, Japan 4Institute of Earth Sciences, Academia Sinica, Taipei, 11529, Taiwan 5Department of Geology and Minerals of Vietnam, No. 6, Pham Ngu Lao St., Hanoi, Vietnam ABSTRACT Southeast Asia consists of several microcontinents that detached from the northeastern margin of Gondwanaland. The Song Ma belt in northern Vietnam consists of ophiolite, metabasite, metasedi- mentary rocks and eclogite, and it is thought to be a suture zone between the Indochina and South China blocks. However, the nature and boundaries of the Song Ma belt and the collision age of the two blocks have long been debated. In this article, petrological and geochemical studies on the Song Ma ophiolite and eclogite and first sensitive high-resolution ion microprobe (SHRIMP) age dating of eclogite provide new light to resolve such debate. Eclogite consisting of garnet, omphacite, phengite, quartz, barroisite and rutile is closely associated with garnet–phengite–quartz schist in the ÔNam Co antiformÕ, a northern subunit of the Song Ma belt. The eclogite experienced a three-stage metamorphic evolution: (I) pre-eclogite stage (amphibolite facies) defined by inclusions of taramite, barroisite, quartz, zoisite ⁄ epidote, mica, rutile & rare chlorite in garnet, (II) eclogite stage and (III) retrograde stage of amphibolite to greenschist facies. The P–T conditions of the three stages are of 14–16 kbar and 520– 550 °C (I), 24–27 kbar and 650–750 °C (II), and 3–7 kbar and 430–510 °C (III), and show a clockwise P–T path based on their mineral assemblages and stability fields in the P–T pseudosection. Thermobarometric results yield similar peak pressure and temperature (26–28 kbar and 650–710 °C). These data suggest that the Song Ma eclogite underwent high-pressure metamorphism in subduction ) zone with a low thermal gradient 8 °Ckm 1. The Song Ma ophiolite is composed of serpentinized peridotite, gabbro, basalt, mafic dyke and chert, and experienced ocean-floor metamorphism. Metabasalt and gabbro of ophiolite suite and eclogite all have MORB-type geochemical affinities. Zircon separates from eclogite have very low Th ⁄ U ratios of 0.01–0.05, indicating a metamorphic origin. SHRIMP U–Pb isotopic analyses of this zircon yield a 206Pb ⁄ 238U weighted mean age of 230.5 ± 8.2 Ma. This age is interpreted as the closure age of the Paleotethys Ocean that separated the South China and Indochina blocks, and the subsequent collision of the two blocks that took place at the Middle Triassic corresponding to the major episode of the Indosinian Orogeny. Key words: eclogite; ophiolite; sensitive high-resolution ion microprobe dating; Song Ma suture zone; Vietnam. Phu (Fig. 1) was defined as a suture zone (classical) INTRODUCTION between the South China and Indochina blocks (e.g. Southeast Asia consists of several microcontinents that Hutchison, 1975; Tri, 1979; Lepvrier et al., 2004). detached from the northeastern margin of Gondw- Most previous studies have focused on the large- analand (Metcalfe, 1996). Ophiolite, generally found scale tectonic evolution of Southeast Asia (e.g. along suture zone, is widespread in Southeast Asia and Hutchison, 1975; Sengo¨ r, 1979; Sengo¨ r & Hsu, 1984; marks the amalgamation of various Gondwana- Tapponnier et al., 1990; Metcalfe, 1996, 1999; for a derived microcontinents (Hutchison, 1975). Because of summary see Metcalfe, 2011). More recently, the the identification of Song Ma ophiolite in northern metamorphic rocks have been used to further elucidate Vietnam, a narrow NW-trending belt from the west of timings and tectonic evolution of Vietnam and Thanh Hoa in northern Vietnam through a part of the Southeast Asia (Lepvrier et al., 1997, 2004; Osanai Laos territory and extending to the north of Dien Bien et al., 2008; Nakano et al., 2009, 2010). The nature and Ó 2012 Blackwell Publishing Ltd 49 50 R. Y. ZHANG ET AL. Fig. 1. Simplified geological map of the Song Ma suture zone and adjacent areas (modified after Lepvrier et al., 2004) show- ing the eclogite location. DQS, Dian-Qiong suture, RRF, Red River fault belt; SDF, Song Da fault, SMF, Song Ma fault. SMSZ, Song Ma suture zone. WCF, Wang Chao fault. TS, Truong Son belt; TPS, Tamky- Phuoc Son suture; KM, Kontum Massif. TH, the location of the biggest Thanh Hoa serpentinite body (2–3 km wide and 10 km long) in the Song Ma suture zone (see insert small map in the upper right). Upper Cre- taceous, red bed; Upper Triassic, conglom- erate, sandstone & shale; Lower–Middle Triassic, shale, siltstone, sandstone & lime- stone; Permian–Lower Triassic, shale, tuff, mafic volcanic rock & peridotite; Carbonif- erous–Permian, limestone, sandstone & shale; Middle Devonian–Lower Carbonifer- ous, shale & limestone; Silurian–Lower Devonian, shale, conglomerate, sandstone & limestone. The lithology of the classical Song Ma suture & Nam Co Formation see text. boundaries of the Song Ma belt and collision age of the eclogite originally formed and peak P–T conditions of Indochina and South China blocks have long been eclogite metamorphism; and (ii) which is the northern debated due to extremely poor exposures and the unit of the Song Ma suture zone and the closure time resulting lack of petrological, geochemical and geo- of the Lao-Vietnamese Paleotethys branch and subse- chronological data. The Song Ma belt is considered by quent collision of the South China and Indochina most scientists to be a suture zone created by the clo- blocks. sure of the Lao-Vietnamese Paleotethys branch that separated South China from Indochina (e.g. Tri, 1979; Lepvrier et al., 2004). The Song Ma suture and the GEOLOGICAL OUTLINE Dian-Qiong suture in South China (see Fig. 1) were In this study, the Song Ma suture zone includes two interpreted as an originally contiguous Triassic suture subunits. Unit 1, the classical fault-bounded Song Ma between the South China and Indochina blocks (Cai & suture zone (or belt), is a narrow NW–SE-trending belt Zhang, 2009), which was later disrupted by Cenozoic (Fig. 1). To the south, the Song Ma fault separates the strike-slip telescoping. In contrast, Findlay & Trinh suture zone from Upper Triassic intrusions of unde- (1997) proposed that the Song Ma Anticlinorium is a formed granite. Unit 2, the Nam Co antiform, lies to polymetamorphosed Lower Palaeozoic island the north of the Unit 1 and is bordered by the Song Da arc ⁄ forearc terrane accreted to the South China plate fault in north. This unit together with the classical in Siluro-Devonian times, and based on regional pal- Song Ma belt (Unit 1) was named as the Song Ma aeogeographical, palaeontological and thermochrono- Anticlinorium (Findlay & Trinh, 1997). Unit 1 is logical data, Carter & Clift (2008) concluded that there composed of an ophiolite suite consisting of serpenti- is no definitive evidence for Triassic collision between nized peridotite, pyroxenite, gabbro, diabase, basalt the South China and Indochina blocks; alternatively and plagiogranite (Hutchison, 1975; Lepvrier et al., the ÔIndosinian orogenyÕ represents a reactivation event 2004), metabasite and metasedimentary rocks includ- driven by accretion of the Sibumasu block to Indo- ing chert. Metamorphic rocks in Unit 1 consist mainly china rather than a major mountain-building event. of greenschist and plagioclase–amphibole schist with The discovery of eclogite in the Song Ma area (Nakano minor quartz–mica schist and quartzite. The Nam Co et al., 2010) provides an opportunity to study the ori- antiform (Unit 2) consists of greenschist, mica–quartz gin and metamorphic evolution of eclogite, collision schist, quartzite & phyllite (Nam Co Formation) in age of the South China and Indochina blocks, in turn addition to recently discovered various garnet-bearing to reevaluate the tectonic evolution of Southeast Asia. metapelites, garnet–hornblende fels and eclogites in the In this study, we determine petrological and geo- northwestern antiform (Nakano et al., 2010; this chemical characteristics of both ophiolite complex and study). The eclogite experienced high-pressure (HP) eclogite and provide first eclogite age using sensitive metamorphism at 21–22 kbar, 600–620 °C estimated high-resolution ion microprobe (SHRIMP). We then by P–T pseudosection with composition isopleth cal- discuss (i) in what tectonic setting the ophiolite and culation, and 26–28 kbar, 620–680 °C by thermoba- Ó 2012 Blackwell Publishing Ltd ORIGIN OF THE SONG MA OPHIOLITE AND ECLOGITE 51 rometer (Nakano et al., 2010). U–Th–Pb electron in the upper right of Fig. 1). All peridotites have been microprobe analyses of monazite from garnet–chlori- partially or totally serpentinized. Relicts of olivine, toid–phengite schist yielded a metamorphic age of enstatite, diopside and Cr-spinel are preserved in some 243 ± 4 Ma, and the cores and rims of monazite serpentinized peridotites. Lherzolite comprises olivine crystals from a garnet–phengite schist yielded (Fo92), diopside, enstatite (En92) & minor Cr-spinel 424 ± 15 and 238 ± 11 Ma, respectively (Nakano with trace Fe–Ni sulphides. Harzburgite (I4-04) con- et al., 2010). The eclogite has never been dated until sists of coarse-grained olivine (Fo90–91) & enstatite the present study described below. (En91), and contains abundant talc and lizardite of The Indochina block in Vietnam can be divided into secondary origin (Fig. S1). three units. From north to south, they are Truong Son metamorphic belt, Tamky-Phuoc Son suture zone and Kontum Massif.