Origin and Tectonic Implication of Ophiolite and Eclogite in the Song Ma Suture Zone Between the South China and Indochina Blocks

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, metasedimentary 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

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) showing 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, conglomerate, sandstone & shale; Lower–Middle Triassic, shale, siltstone, sandstone & limestone; 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-

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. The Tamky-Phuoc Son is a me´lange Gabbro (SM03B) and metamorphosed basalt (SM03A) zone consisting of serpentinite, mafic rocks and asso- Gabbro is closely associated with basalt and is com- ciated volcanic rocks that are mixed with continental posed of augite and calcic plagioclase (An59–62) with a material (Lepvrier et al., 2004). It has been considered fine-grained ophitic texture. Both augite and plagio- an oceanic domain (Tri, 1979; Sengo¨ r et al., 1988; Le & clase have prismatic shape (Cpx: 0.4–0.8 mm long and Ngo, 1995) or a paleorift (Gatinsky & Hutchison, 0.1–0.3 mm wide, some up to 1.5 · 0.5 mm). Augite is 1987). The Truong Son metamorphic belt and Kontum replaced by hornblende at the margins; minor plagio- Massif are composed of various amphibolite facies clase crystals are replaced by oligoclase, quartz and schists and gneisses, marble, granulite and ultrahigh-T zoisite. The associated basalts are extensively recrys- pelitic granulite (Osanai et al., 2008). High-T meta- tallized to greenish hornfelsic rock that is composed of morphism of the Truong Son belt and Kontum Massif fine-grained albite, chlorite & epidote (Fig. S1). took place at 250–240 Ma indicated by biotite and muscovite 40Ar-39Ar ages (Lepvrier et al., 1997, 2004; Maluski et al., 2005). SHRIMP dating yielded similar Foliated metabasite U–Pb ages (258–244 Ma) for gneiss, charnockite and Foliated metabasites of sub-ophiolite are deformed migmatite (Carter et al., 2001). Based on these U–Pb metamorphic mafic rocks that may represent oceanic data, together with published argon dating results, crust subducted to different depths. Metabasites in Carter et al. (2001) suggested that peak metamorphism Unit 1 consist of greenschist and plagioclase–amphib- of the Vietnamese basement and exhumation to upper olite schist. Greenschist comprises fine-grained (10– crustal levels occurred at the time interval between 15 lm across) albite, chlorite, epidote and minor tita- 258 ± 6 and 243 ± 5 Ma. Much older SHRIMP U– nite (11SM7). Greenschist shows folded metamorphic Pb ages of 452–407 Ma for gneisses from the Truong lamination fabric. Plagioclase–amphibolite schist Son belt and Kontum Massive have also been reported (SM05A, SM05B, SM06) consists of orientated horn- (Carter et al., 2001; Usuki et al., 2009); however, the blende (50–65 vol%), plagioclase (30–45 vol%, An35–43) tectonic significance of these ages is unclear. with minor quartz and titanite, and has schistose structure (Fig. S1). Plagioclase and quartz occur as interstitial phases between subhedral hornblende OPHIOLITE AND FOLIATED METABASITE grains. These rocks were metamorphosed at 4–6 kbar and 620–645 C estimated here using our mineral Ophiolite compositions and the amphibole–plagioclase ther- The ophiolite suite in the Song Ma suture zone consists mometer and amphibole barometers (Blundy & Hol- of serpentinized peridotite, gabbro, gabbroic diorite, land, 1990). diabase dyke, basalt, chert & banded quartzite. These ophiolitic rocks have been subjected to ocean-floor metamorphism under greenschist to lower amphibolite ECLOGITE AND ASSOCIATED ROCKS IN NAM CO facies conditions. Some serpentinite bodies contain ANTIFORM (UNIT 2) small lenses of rodingite and altered pyroxenite. The Outcrops of eclogite were found in the northwestern major members of the ophiolite are described below. Nam Co antiform, northeast of Dien Bien Phu (N21.476131, E103.30486). The eclogite block (>50 m in size) is enclosed in strongly altered garnet–mica– Peridotites quartz schist (but direct contact is unclear). Garnet– Peridotites are dominantly composed of dunite and hornblende fels occurs as layer or lens within metabasite harzburgite with minor lherzolite; they occur as elon- in the nearby area. Most eclogites show porphyrob- gated (Fig. 3), lens-like blocks (0.4–10 km long and lastic texture, that is, euhedral porphyroblastic garnet 0.2–0.8 km wide) along NW-trending faults or shear of 0.5–1.5 mm cross is set in relatively fine-grained zones (Fig. S1). The largest serpentinite body (2– matrix of Omp + Grt ± Ph + Qtz + Rt ± Brs 3 · 10 km) in the Thanh Hoa area contains chromite (mineral abbreviations after Whitney & Evans, 2010; pods or lenses (for location see TH in the inserted map Fig. 2a). The porphyroblastic garnet has a large

2012 Blackwell Publishing Ltd 52 R. Y. ZHANG ET AL. inclusion-rich core and a thin inclusion-free rim phacite is mostly fresh, but some shows partial (Fig. 2b). Based on mineral assemblage and retro- replacement of clinopyroxene and plagioclase. One gressive texture, these eclogites are divided into three sample (11SM5I) contains rare phengite. Fine-grained types: (i) phengite-bearing; (ii) phengite-free and (iii) zoisite (±amphibole ± quartz) grains form aggre- extensively retrograded eclogites. Phengite-bearing gates (Fig. 2b), which probably are pseudomorphs eclogite (11SM5H, 11SM5E & 11SM5C) is character- after lawsonite(?). Extensively retrograded eclogite ized by >5 vol% phengite and consists of (11SM5D) contains only relict garnet. All porphyrob- Grt + Omp + Ca–Na Amp + Qtz + Ph + Zo + lastic garnet is totally or partially replaced by amphi- Rt. Porphyroblastic garnet contains abundant inclu- bole, epidote, biotite, muscovite & K-feldspar sions of quartz, taramite (or barroisite), epidote and (Fig. S2). Omphacite is totally replaced by symplectites rutile with rare mica and chlorite. Omphacite occurs as of amphibole and plagioclase (Fig. 2c). Four occur- ﬁne-grained (<0.5 mm in size) crystals in the matrix. rences of amphibole in all types of eclogite were iden- Barroisitic amphibole has sharp boundaries with gar- tiﬁed: (i) inclusions in garnet; (ii) subhedral matrix net and omphacite showing textural equilibrium. phase; (iii) interstitial retrograde phase; and (iv) Phengite-free eclogite (11SM5F) consists of garnet, retrograde phase occurring in symplectite or as thin omphacite, barroisite and minor quartz, zoisite & ru- rim around garnet (Fig. 2b) or after omphacite. Some tile. Inclusions in coarse-grained garnet include rutile, subhedral barroisite in the matrix could be stable with quartz, epidote, K-feldspar, zircon & apatite. Om- other eclogitic phases.

Fig. 2. Plane light photomicrograph (a, c) and backscattered electron images (BEI b, d) showing assemblage and texture of eclogite and garnet–hornblende fels. (a) Eclogite (11SM5E). (b) Porphyroblastic garnet rimmed by amphibole contains inclusions of amphibole, quartz and rutile and shows compositional zone (eclogite 11SM5I). Fine-grained zoisite aggregates (+Amp + Qz) are also shown. (c) Omphacite is totally replaced by symplectite of very ﬁne-grained amphibole and plagioclase (11SM5D). (d) Garnet– hornblende fels (11SM4D). Coarser garnet in the hornblende matrix contains inclusions of amphibole and quartz and is surrounded by secondary amphibole, plagioclase and chlorite.

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shows similar petrographic feature with sample 11SM4B and 4D, but contains more garnet (30 vol%) and quartz (‡10 vol%).

WHOLE ROCK AND MINERAL CHEMISTRY

Major and trace elements of whole rock Analytical methods that have been reported in Lin et al. (2012) are described briefly in Appendix S1. Oxide totals of serpentinized peridotites are <90 wt% due to extensive serpentinization characterized with high MgO ⁄ (MgO + FeOtot) ratios of 0.82–0.85 (Ta- ble S1). All metabasites from Unit 1 and Unit 2 including eclogite and garnet–hornblende fels have 97 wt% anhydrous totals because of the presence of hydrous phases, and have basaltic compositions with one exception (SM06). In contrast to relatively uni- form SiO2 contents (45.7–49.4 wt%), these rocks vary in other major elements (wt% values: TiO2 = 0.76– tot 2.6, Al2O3 = 12.2–15.1, FeO = 9.4–15.4 (converted from Fe2O3), MgO = 4.4–11.0, CaO = 8.5–11.9, Na2O = 0.83–2.99 and K2O = 0.15–1.12; see Table S2). Eclogites have little variation in bulk compositions and have higher Al2O3 (14.2–15.1 wt%), and lower FeOtot (9.44–10.19 wt%) relative to the metabasites. Two garnet–hornblende fels samples have higher MgO (11 wt%) than those of eclogites; one garnet–hornblende fels sample (11SM3D) is rich in FeOtot (15.39 wt%) and low in MgO (4.4 wt%). The exceptional plagioclase–amphibolite schist (SM06) has high SiO2 (54 wt%) and alkali contents (Na2O+ K2O = 2.2 wt%), corresponding to the composition of basaltic andesite. Song Ma serpentinites are slightly depleted in LREE Fig. 3. REE patterns of serpentinite (a), metabasalt (b), and compared with MREE and HREE (Fig. 3a). The eclogite and garnet–hornblende fels (c). All abundances are contents of MREE and HREE are 1–2 times higher normalized to the chondrite composition of Sun & McDonough (1989). (a) REE patterns of harzburgite (I4-04) and serpentinites than those of the chondrite, thus indicative of a mantle from the Thanh Hoa body (location, see Fig. 1), and of me- origin. Most greenschists and plagioclase–amphibole tabasites (b). Data of N-MORB and E-MORB are from Sun & schists show LREE-depleted REE patterns [(La ⁄ Lu)N McDonough (1989). ratios of 0.44–0.83], with flat MREE and HREE comparable to the normal MORB (N-MORB) REE Garnet–mica–quartz schist is composed of musco- pattern. One amphibolite sample (SME03) in the Nam vite, quartz & garnet with minor rutile, apatite & Co antiform is an exception, which has an LREE-en- monazite. Garnet (10 vol%) ranges from 0.3 to riched pattern [(La ⁄ Lu)N = 3.92] and a positive Eu 1.3 mm in size and is surrounded by muscovite and anomaly. In addition, the basaltic andesite (SM06) has quartz within pressure shadows. Garnet with many higher REEtot abundance than that of N-MORB cracks is partially overprinted by a white mica and (Fig. 3b). chlorite fabric. Muscovite is locally replaced by biotite REE patterns of the eclogites show two types. and chlorite. Rutile is rimmed by ilmenite. Phengite-free eclogite (11SM5F) has a flat pattern Garnet–hornblende fels (11SM4B & D) consists of [(La ⁄ Lu)N = 1.1]; phengite-bearing eclogites (11SM5C hornblende (70 vol%), garnet 12–20 vol%), minor and 11SM5H) show LREE-enriched patterns with quartz and rutile with massive granoblastic texture. (La ⁄ Lu)N ratios of 5.8–10.9. Garnet–hornblende fels Hornblende is prismatic in shape and ranges from 0.3 have flat REE patterns [(La ⁄ Lu)N = 1.9–2.1], coupled to 1.2 mm in size. Garnet of 0.3–1.5 mm across is with variable REE abundances. A Fe–Ti-rich garnet– extensively fractured. Coarse-grained garnet contains hornblende fels (11SM3D) has particularly higher total inclusions of Ep + Hbl + Ts + Qz, and some garnet REE than other garnet–hornblende fels. Except for is partially replaced by Hbl + Pl + Chl (Fig. 2d). sample 11SM3D, both garnet–hornblende fels and Rutile is locally replaced by ilmenite. Sample 11SM3D eclogite have negative Eu anomalies.

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MINERAL CHEMISTRY OF ECLOGITE AND Amphibole and zoisite GARNET–HORNBLENDE FELS All Fe is expressed as Fe2+ in garnet and omphacite Amphibole and zoisite in eclogite formulae. For end-member calculation of clinopy- Most amphibole inclusions in garnet are taramite that 3+ roxene, Fe =Na) (Al + Cr) if Na > (Al + Cr), contains 3–4 wt% Na2O and are 0.56–0.63 in NaM4. and if Na £ Al + Cr, all Fe is expressed as Fe2+ Barroisite or pargasite ⁄ tschermakite inclusions in based on six oxygen ions. The amounts of ferric and garnet are rare (11SM5H & 11SM5I); they have sim- ferrous iron for amphibole formulae were calculated ilar Na2O (2.96–3.83 wt%) content with taramite, using the procedure described by Schumacher (1991). but lower in (Na + K)A (0.38) for barroisite and in 3+ Iron is expressed as Fe in the zoisite ⁄ epidote for- NaM4 (0.46–0.47) for pargasite ⁄ tschermakite. The mula. Mineral compositions of eclogites are listed in matrix amphibole contains 3 wt% Na2O and has a Tables 1 and S4, and those of garnet–mica–quartz composition intermediate between hornblende and schist and garnet–hornblende fels are shown in Ta- barroisite. Most subhedral amphibole grains are bar- ble S3. roisite. Retrograde amphibole shows significant variation in SiO2,Al2O3 &Na2O. Retrograde amphibole in the symplectite of garnet is pargasite or tscher- Garnet makite with high Al2O3 (17.69–21.18 wt%) and low Garnet from garnet–mica–quartz schist, garnet–horn- SiO2 (38.50–40.91 wt%). Amphibole in the symplec- blende fels and eclogite has distinct variations in tites after omphacite and thin rims after garnet is composition (Fig. 4). Garnet in garnet–mica–quartz pargasite or hornblende with low Na2O content (0.64– schist is almandine (Alm80–83Prp1–4Grs14–16, Table S3), 1.18 wt%). All zoisite has similar composition with whereas that from garnet–hornblende fels is almandine- 2.23–2.57 FeOtot wt%. Epidote contains 7.64–8.93 rich with variable spessartine component (Table S3). wt% FeOtot. Garnet from sample 11SM3D has increasing almandine with decreasing pyrope and spessartine from core Amphibole in garnet–hornblende fels (Alm58Prp12Sps5Grs25) to rim (Alm65Prp7Sps2Grs26). In contrast, garnet from sample 11SM4D has increasing Amphibole inclusions in garnet and matrix amphibole pyrope and almandine with decreasing spessartine and are hornblende with a few exceptions, and have low constant grossular from core (Alm55Prp13Sps10Grs22)to Na2O content (0.83–1.75 wt%). Amphibole after gar- rim (Alm58Prp19Sps2Grs21). net and retrograde amphibole rims of hornblende are Eclogitic garnet has low spessartine (Sps0–4) and tschermakite having very low SiO2 (most <43 wt%) displays significant variation in almandine (Alm40–54) and high Al2O3 content. and pyrope (Prp17–42) components with mild variation in grossular (see Tables 1 & S4). Most euhedral, porphyroblastic garnet exhibits pronounced composi- P–T ESTIMATE tional zoning with much higher almandine and lower grossular cores than the rims. X-ray mapping shows a Methods wide Fe-rich core with a narrow, relatively Mg-rich rim P–T conditions were estimated using both geother- (Fig. 5a). The compositional profile shows pyrope in- mobarometry and isochemical phase diagram (pseudo- creases with decreasing almandine, spessartine and section). For the Grt–Cpx thermometer (Krogh Ravna, 3+ 3+ grossular from core (Alm52–53Prp17–18Sps3–4Grs24–28) 2000), the Fe in Omp was calculated as Fe = Na– to rim (Alm39–42Prp36–42Sps1Grs18–24) line A–B Al–Cr. For the application of Grt–Omp–Ph–Coe ⁄ Qz (Fig. 5b). The fine-grained matrix garnet has compo- thermobarometry (Krogh Ravna & Terry, 2004), the sitions similar to rim compositions of garnet por- ferric iron of clinopyroxene was calculated assuming phyroblasts. The relict garnet rimmed by symplectite four cations and six oxygenP ions. The phengite formula (Fig. S2) corresponds to the core composition of pri- was normalized to Si–Al–Ti–Cr–Fe–Mn–MgP = mary garnet. 12.00. Garnet was normalized to Ca–Mn–Fetot–Mg– Al–Ti–Cr = 5.00, where Ca + Mn + Fe2+ +Mg=3 and Al + Ti + Cr + Fe3+ = 2.00. Here, Fe3+ = Omphacite and phengite 3.00 ) (Al + Ti + Cr) and Fe2+ =Fetot ) Fe3+. Omphacite and phengite only occur in eclogite and The P–T pseudosection of eclogite was calculated have relatively homogeneous compositions. The jadeite using the computer program Perple_X (Connolly, component of most omphacite ranges from 33 to 35 2005; update in April 2010) and the internally consis- mol% except for sample 11SM5I (Jd30). Si values of tent thermodynamic data set (Holland & Powell, 1998; phengite range from 3.35 to 3.40 pfu. Retrograde and their updates). The model system Na2O–K2O– clinopyroxene has low jadeite component (Jd23), and CaO–FeO–MgO–Al2O3–SiO2–TiO2–H2O (NKCFM- retrograde white mica in symplectite after garnet has ASTH) was assumed for eclogite. The following low Si pfu of 3.15. solution models are used: phengite, olivine, orthopy-

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Table 1. Mineral compositions of the Song Ma eclogite.

Sample 11SM5C 11SM5D 11SM5E 11SM5H 11SM5H 11SM-5I

Mineral Grta Ompa Pha Hbl Grt Chl Grta Ompa Pha Grta Ompa Pha Grta Ompa Pha Grta Ompa Pha Brs Trm Ilm Ep Grta Ompa Pha Grta Ompa Pha Ts Prg Brs

No. A-21 A-12 A1-2 B.53-4 B.93 B-77 B-62 B37-8 D.L24 D.177 D.192 A.35 A.60 A.48 B.21 B.38 B.23 105-6 107-8 B113 B1.25 B.20 A21 C.190 C.193 C.1-4 A.73-4 A.76 B.118

Note r mtx relc in ⁄ g r c big ir r m in ⁄ gin⁄ gin⁄ gin⁄ g r r mtx av4 in ⁄ gin⁄ gin⁄ g

SiO2 40.02 54.59 50.11 51.8 38.16 28.62 40.23 56.28 50.41 39.78 55.11 49.56 40.47 55.02 49.62 39.89 55.36 49.27 42.20 41.30 0.61 38.60 39.35 56.14 50.56 40.06 54.41 50.10 41.646 43.20 45.87 TiO2 0.15 0.12 0.46 0.07 0.18 0.03 0.00 0.17 0.35 0.04 0.12 0.49 0.06 0.07 0.47 0.00 0.07 0.60 1.04 1.02 52.99 0.03 0.15 0.02 0.38 0.09 0.10 0.58 0.44 0.59 0.64 Cr2O3 0.00 0.01 0.17 0.00 0.08 0.00 0.00 0.00 0.10 0.02 0.00 0.14 0.00 0.11 0.06 0.00 0.05 0.09 0.07 0.07 0.00 0.00 0.07 0.00 0.01 0.09 0.00 0.55 0.06 0.00 0.19 Al2O3 22.64 8.49 27.46 7.08 21.55 11.89 22.65 8.75 26.94 22.70 8.54 27.40 22.80 8.33 26.89 23.19 8.49 27.17 18.11 18.58 0.35 26.28 22.18 9.03 27.00 22.51 8.51 27.83 17.84 16.91 15.24 FeOtotal 18.97 3.91 1.42 6.22 23.79 12.70 20.68 3.76 1.70 21.41 4.47 1.90 19.24 4.15 1.80 20.10 3.63 2.15 13.49 14.71 39.80 8.93 22.65 3.59 1.57 22.38 3.91 1.38 16.22 13.10 11.38 MnO 0.15 0.00 0.02 0.00 1.60 0.10 0.38 0.02 0.00 0.43 0.07 0.01 0.26 0.13 0.09 0.28 0.03 0.08 0.05 0.04 5.98 0.05 0.89 0.00 0.00 0.26 0.04 0.00 0.07 0.00 0.03 MgO 10.39 10.40 3.54 17.52 4.44 3.39 9.00 10.95 3.69 8.31 10.34 3.63 11.19 10.96 3.43 9.13 10.83 3.57 9.76 9.00 0.06 0.11 6.59 10.61 3.81 7.37 10.95 3.66 8.88 10.69 12.66 CaO 7.88 16.83 0.07 10.15 9.96 28.41 8.52 16.28 0.02 8.88 16.86 0.01 7.10 16.74 0.01 8.77 16.69 0.02 8.64 8.80 0.34 23.35 9.68 16.51 0.00 9.10 16.98 0.04 9.53 9.71 9.27 Na2O 0.01 4.81 0.77 2.48 n.d 0.09 0.00 4.88 0.68 0.00 4.81 0.75 n.d 4.73 0.65 0.09 4.92 0.69 3.83 4.09 0.00 0.01 0.07 5.05 0.53 0.04 4.18 0.64 2.96 3.16 3.10 K2O 0.00 0.00 9.78 0.22 n.d 0.04 0.00 0.00 10.17 0.00 0.00 10.16 n.d 0.00 10.20 0.00 0.00 9.96 0.56 0.53 0.00 0.02 0.00 0.00 10.29 0 0.01 10.02 0.32 0.50 0.39 Total 100.21 99.15 93.80 95.55 99.76 85.27 101.45 101.08 94.07 101.56 100.33 94.06 101.1 100.23 93.24 101.44 100.12 93.59 97.76 98.14 100.14 97.38 101.63 100.94 94.14 101.9 99.09 94.81 97.952 97.86 98.764 RGNO H OGM PILT N ECLOGITE AND OPHIOLITE MA SONG THE OF ORIGIN Si 3.00 1.97 3.38 7.38 2.99 6.43 3.00 1.99 3.40 2.98 1.97 3.35 3.00 1.97 3.38 2.97 1.98 3.35 6.12 6.02 0.02 6.48 2.99 1.99 3.40 3.01 1.97 3.35 6.27 6.36 6.63 Ti 0.01 0.00 0.02 0.01 0.01 0.00 0.00 0.00 0.02 0.00 0.00 0.03 0.00 0.00 0.02 0.00 0.00 0.03 0.11 0.11 0.99 0.00 0.01 0.00 0.02 0.01 0.00 0.03 0.05 0.07 0.07 Cr 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.03 0.01 0.00 0.02 Al 2.00 0.36 2.18 1.19 1.99 3.15 1.99 0.36 2.14 2.01 0.36 2.18 1.99 0.35 2.16 2.04 0.36 2.18 3.10 3.19 0.01 5.20 1.98 0.38 2.14 1.99 0.36 2.19 3.16 2.94 2.60 Fe3+ 0.00 0.00 0.00 0.07 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.17 0.17 0.00 1.25 0.00 0.00 0.00 0.00 0.00 0.00 0.22 0.13 0.17 Fe 1.19 0.12 0.08 0.60 1.56 2.40 1.29 0.11 0.10 1.34 0.13 0.11 1.19 0.12 0.10 1.25 0.11 0.12 1.29 1.46 0.83 0.00 1.44 0.11 0.09 1.40 0.12 0.08 1.61 1.36 1.03 Mn 0.01 0.00 0.00 0.00 0.11 0.02 0.02 0.00 0.00 0.03 0.00 0.00 0.02 0.00 0.00 0.02 0.00 0.00 0.01 0.00 0.13 0.01 0.06 0.00 0.00 0.02 0.00 0.00 0.01 0.00 0.00 Mg 1.16 0.56 0.36 3.72 0.52 1.14 1.00 0.58 0.37 0.93 0.55 0.37 1.24 0.59 0.35 1.01 0.58 0.36 2.11 1.96 0.00 0.03 0.74 0.56 0.38 0.82 0.59 0.36 1.99 2.35 2.73 Ca 0.63 0.65 0.00 1.55 0.84 6.84 0.68 0.62 0.00 0.71 0.65 0.00 0.56 0.64 0.00 0.70 0.64 0.00 1.34 1.38 0.01 4.20 0.79 0.63 0.00 0.73 0.66 0.00 1.54 1.53 1.44 Na 0.00 0.34 0.10 0.68 0.00 0.04 0.00 0.33 0.09 0.00 0.33 0.10 0.00 0.33 0.09 0.01 0.34 0.09 1.08 1.16 0.00 0.00 0.01 0.35 0.07 0.01 0.29 0.08 0.86 0.90 0.87 K 0.00 0.00 0.84 0.04 0.00 0.01 0.00 0.00 0.87 0.00 0.00 0.88 0.00 0.00 0.89 0.00 0.00 0.86 0.10 0.10 0.00 0.01 0.00 0.00 0.88 0.00 0.00 0.85 0.06 0.09 0.07 Total 8.00 4.01 6.97 15.24 8.01 20.02 8.00 3.99 6.99 8.01 4.01 7.02 8.00 4.01 7.00 8.01 4.01 7.01 15.45 15.56 1.99 17.17 8.02 4.00 6.98 7.99 3.99 6.98 15.78 15.73 15.63

av, average; c, core; m, mantle; r, rim; ir, inner rim; af, after; por, porphyroblast; in ⁄ g, inclusion in garnet; mtx, matrix; ret, retrograde phase; cp-sym; g-sym, symplectite after omphacite and garnet, respectively; f.g, ﬁne-grained; relc, relict; n.d, not determined. aData were used for P–T estimate by using thermobarometer. 55 56 R. Y. ZHANG ET AL.

chlorite in garnet core indicative of amphibolite facies assemblage. (II) The eclogite stage is characterized by the presence of eclogite paragenesis of Grt + Omp + Ph + Rt + Qz + Brs. (III) The retrograde stage showing overprint of amphibolite to greenschist facies mineral assemblages. P–T conditions for stages I and III are approxi- mately constrained by their mineral assemblages and stability fields in the pseudosection (Fig. 6). For stage II, in addition to pseudosection calculation, the Grt– Cpx–Ph–Coe ⁄ Qz thermobarometer (Krogh Ravna & Terry, 2004) and Grt–Cpx thermometer (Krogh Rav- na, 2000) were also applied to estimate P–T conditions of phengite-bearing eclogites. The calculated pressure Fig. 4. Plot of garnet composition in Alm–Grs–Prp space. ranges from 26 to 28 kbar and temperature ranges 11SM5A, garnet–mica–quartz schist; 11SM3D, 11SM4B & 4D, garnet–hornblende fels; other samples, eclogite. from 650 to 710 C for T1 (Krogh Ravna & Terry, 2004) and from 660 to 795 C for T2 (Krogh Ravna, roxene and ilmenite after Holland & Powell (1998), 2000), respectively (Table 2). garnet after White et al. (2007), omphacite after Green The P–T pseudosection for eclogite sample 11SM5H et al. (2007), amphibole after Dale et al. (2005), is shown in Fig. 6; its topology is slightly different chlorite after Holland et al. (1998), biotite after Ta- from that calculated by Nakano et al. (2010). The jcmanovaét al. (2009), plagioclase after Newton et al. minimum pressure of eclogitic assemblages (1980). Talc is assumed to be ideal solution. Zoisite, Omp + Ph + Grt + Qz + Rt and Omp + titanite, rutile and lawsonite are assumed to be Ph + Grt + Amp + Qz + Rt rises from 17 kbar at end-member compositions. 760 C (Nakano et al., 2010) to 22 kbar at 790 C (Fig. 6). The main reason is that our eclogite has higher CaO and Al2O3 contents and 2 wt% lower RESULTS FeOtot than the Nakano et al. (2010) sample. In addi- According to petrographic observation, eclogite dis- tion, Nakano et al. assumed Fe3+ ⁄ (Fe3+ +Fe2+)= plays a three-stage metamorphic evolution. (I) The pre- 0.14; however, we assume total Fe as Fe2+, because eclogite stage is defined by the inclusions of taramite, garnet and omphacite have very little Fe3+– based on barroisite, quartz, epidote, rutile and rare mica and formula calculations (see Table 1).

Fig. 5. Zoned garnet: a (left side), X-ray element mapping of zoned garnet; and b (right side), compositional proﬁle line A–B of zoned garnet.

2012 Blackwell Publishing Ltd ORIGIN OF THE SONG MA OPHIOLITE AND ECLOGITE 57

Fig. 6. P–T pseudosection for eclogite (11SM5H) calculated in the NKCFMASHT system at aH2O = 1 using bulk composition of eclogite and a software Perple-X by Connolly (2005, update in April 2010). FeOtot is expressed as FeO. Assemblages: 1, Omp Ph Tlc Amp Grt Law Rt; 2, Omp2 Chl Ph Tlc Amp Law Rt; 3, Omp Chl Ph Tlc Amp Law Rt Stp; 4, Omp Chl Ph Tlc Amp Law Qz Rt; 5, Omp Chl Ph Amp Grt Zo Qz Rt; 6, Omp Chl Ph Amp Zo Qz Rt; 7, Omp2 Chl Ph Amp Zo Ttn Qz; 8, Bt Chl Ph Amp Zo Ttn Qz; 9, Omp Ph Amp Grt Zo Law Qz Rt; 10, Bt Chl Ph Amp Grt Zo Ttn Qz; 11, Bt Ph Amp Grt Zo Qz Rt; 12, Omp Chl Ph Zo Ttn Ab Qz; 13, Bt Omp Chl Zo Ttn Ab Qz; 14, Bt Chl Amp2 Zo Ttn Ab Qz; 15, Bt Chl Pl Amp2 Zo Ttn Qz; 16, Bt Omp Chl Pl2 Amp2 Ttn; 17, Bt Chl Pl2 Amp2 Ttn; 18, Bt Chl Pl2 Amp Ilm; 19, Bt Pl Amp Zo Ttn Qz. Amp2, Pl2 and Omp2: these minerals have two different compositions in the assemblage due to immiscibility of solid solution. H2Ois in all assemblages in the pseudosection. Open ellipses mark the P–T ﬁelds of pre- eclogite stage (I), peak eclogite stage (II) and retrograde stage of amphibole to greenschist facies (III), respectively. Mineral abbreviations are after Whitney & Evans (2010).

Table 2. P–T estimates for the Song Ma eclogite.

Sample T1 (C) T2(C) P (kbar)

11SM5C 700 795 26.7 11SM5E 725 725 28.2 11SM5E 695 770 26.5 11SM5H 660 795 26.1 11SM5H 635 725 26.8 11SM5I 655 655 27.1 11SM5I 665 665 26.1

P-T1 estimate after Krogh Ravna & Terry (2004). T2, Fe3+ = Na–Al–Cr (Krogh Ravna, 2000).

The inclusions in garnet and retrograde metamorphic assemblage in sample 11SM5H are coincident with the fields of Chl–Ph–Amp–Zo–Qz–Rt (14–16 kbar and 530–550 C), and Bt–Chl–Amp–Pl–Zo–Ttn–Qz (3– 7 kbar and 430–520 C), respectively (Fig. 5). The eclogite assemblage of Omp + Ph + Grt + Qz + Rt + Brs corresponds to the field of Omp–Ph–Amp–Grt–Qz– Rt that is within a P–T field of 22–28 kbar and 640– 790 C; in the field, the pressure will have a negative correction with temperature when >700 C (Fig. 6). Fig. 7. P–T path of the Song Ma eclogite showing a metamor- The results for stage II are consistent with the P–T phic history of subduction zone. Stages I and III are estimated estimates (26–28 kbar and 650–710 C) by the Grt– based on the P–T fields of inclusions in garnet and retrogressive assemblage shown in the pseudosection in Fig. 5, respectively. Cpx–Ph–Coe ⁄ Qz thermobarometer (Krogh Ravna & Peak P–T conditions of eclogite facies metamorphism are esti- Terry, 2004). By combining the pseudosection and mated by combining pseudosection and thermobarometric cal- thermobarometric calculations, the average peak P–T culations. P–T boundaries of various metamorphic facies are condition of eclogite facies metamorphism in the Song indicated: GR, granulite, AM, amphibolite, EA, epidote– amphibolite, BS, blueschist schist, GS, greenschist. The subdi- Ma area is 26 ± 2 kbar and 700 ± 50 C. The P–T visions of eclogite (EC) of amphibole eclogite (Amp-EC), epi- estimates of three stages define a clockwise P–T path dote eclogite (Ep-EC), lawsonite eclogite (Lws-EC), and dry with peak pressure of 26 ± 2 kbar (Fig. 7) suggesting eclogite are also indicated (after Maruyama et al., 1996).

2012 Blackwell Publishing Ltd 58 R. Y. ZHANG ET AL. that the Song Ma eclogite underwent HP metamor- geometry (SHRIMP-RG) in the Stanford-United phism in subduction zone and experienced retrograde States Geological Survey cooperative ion microprobe metamorphism during exhumation of subducted slab. facility. The speciﬁc analytical method is listed in Appendix S1. Zircon grains separated from eclogite (11SM5I) range from 30 to 100 lm in size and have GEOCHRONOLOGY rounded, elliptical or subhedral forms. The CL images U–Th–Pb isotopic analyses were performed with the of all zircon grains show stubby internal texture sensitive high-resolution ion microprobe-reverse (Fig. 8). Concentrations of U (0.62–2.40 ppm) and Th (0.00–0.52, most <0.07) in the 28 grains analysed are extremely low, and 232Th ⁄ 238U ratios range from 0.00 to 0.23 but most 0.01–0.04 (Table 3), and are dis- tinctly different from magmatic zircon. Three data with large errors have been excluded from the age calculation due to either insufﬁcient analysis time or unknown reason. Available data of 25 zircon grains yielded a 207Pb corrected 206Pb ⁄ 238U weighted mean age of 230.5 ± 8.2 Ma (Fig. 9).

DISCUSSION

Tectonic setting for the formation of ophiolite and metabasites Chemical characteristics of peridotites, a major member of the Song Ma ophiolite, described above suggest that they are residual mantle and represent remnants of former Paleotethys oceanic lithosphere prior to the Fig. 8. Cathodoluminescence images and U and Th concentra- collision of the South China and Indochina blocks. tions of zircon separated from eclogite (11SM5I) analysed by sensitive high-resolution ion microprobe-reverse geometry. All The REE patterns of metabasalt and gabbro of the indicate a metamorphic origin. ophiolite suite (Fig. 3b) are compatible with N-MORB

Table 3. U–Th–Pb microanalyses of zircon from the Song Ma eclogite (11SM5I).

207Pb corrected

206Pb ⁄ 238U Spot name %comm Pb U (ppm) Th (ppm) Th ⁄ U Age (Ma) 1r err 206Pb ⁄ 238U1r err Total 238U ⁄ 206Pb % err Total 207Pb ⁄ 206Pb % err

11SM5I-17a 16.91 1.63 0.01 0.01 163.0 20.3 .0256 0.0032 32.44 6.6 0.1841 38.4 11SM5I-6a 39.20 1.12 0.01 0.01 182.2 35.1 .0287 0.0056 21.21 7.4 0.3625 24.1 11SM5I-13 6.39 1.19 0.08 0.07 198.9 23.1 .0313 0.0037 29.88 7.7 0.1011 65.8 11SM5I-5 )2.37 1.88 0.02 0.01 200.3 15.4 .0316 0.0025 32.43 7.0 0.0312 86.8 11SM5I-2 7.03 1.73 0.02 0.01 204.8 20.1 .0323 0.00302 28.80 9.4 0.1064 21.3 11SM5I-21 2.57 1.38 0.01 0.01 209.5 21.5 .0330 0.0035 29.49 7.1 0.0709 83.1 11SM5I-4 8.45 2.40 0.01 0.00 215.6 16.3 .0340 0.0026 26.91 5.7 0.1180 32.0 11SM5I-11 21.62 0.62 0.01 0.01 219.0 37.9 .0345 0.0061 22.69 9.3 0.2234 41.5 11SM5I-15 )1.27 1.63 0.00 0.00 220.5 17.5 .0348 0.0028 29.10 6.4 0.0404 99.0 11SM5I-24 16.80 1.80 0.03 0.02 223.1 21.7 .0352 0.0035 23.63 5.5 0.1849 29.2 11SM5I-14 11.51 1.59 0.03 0.02 224.9 21.3 .0355 0.0034 24.92 5.9 0.1427 37.5 11SM5I-9 2.76 1.36 0.02 0.01 226.0 19.7 .0357 0.0032 27.25 7.4 0.0727 50.4 11SM5I-20 1.38 1.37 0.04 0.03 226.6 18.2 .0358 0.0029 27.56 6.7 0.0618 58.0 11S05I-27 5.18 1.37 0.02 0.02 226.5 21.3 .0358 0.0034 26.52 6.8 0.0921 54.4 11SM5I-3 7.91 2.10 0.02 0.01 230.3 18.9 .0364 0.0030 25.32 7.9 0.1141 17.5 11SM5I-22 4.44 1.10 0.00 0.00 230.9 21.3 .0365 0.0034 26.21 7.4 0.0863 50.4 11SM5I-30 4.06 1.89 0.01 0.00 234.3 21.7 .0370 0.0035 25.91 5.5 0.0833 70.1 11SM5I-8 )3.55 1.84 0.01 0.01 236.3 16.0 .0373 0.0026 27.73 6.6 0.0225 70.9 11SM5I-18 )0.95 0.79 0.01 0.02 237.2 25.6 .0375 0.0041 26.94 9.5 0.0433 99.0 11SM5I-10 6.54 1.20 0.02 0.02 240.9 23.7 .0381 0.0038 24.55 7.8 0.1034 45.2 11SM5I-12 2.62 1.68 0.07 0.04 241.7 18.1 .0382 0.0029 25.49 5.7 0.0720 53.7 11SM5I-19 7.19 1.08 0.01 0.01 246.9 26.9 .0390 0.0043 23.77 7.0 0.1087 58.0 11SM5I-7 )2.38 1.54 0.01 0.01 254.9 21.0 .0403 0.0034 25.39 7.4 0.0323 99.0 11SM5I-16 39.87 2.29 0.52 0.23 259.4 26.6 .0411 0.0043 14.64 4.3 0.3710 12.3 11SM5I-25 )0.54 1.70 0.05 0.03 263.7 20.7 .0418 0.0033 24.08 5.9 0.0472 91.3 11SM5I-23 3.58 2.39 0.16 0.07 263.6 17.2 .0417 0.0028 23.10 4.9 0.0802 42.2 11SM5I-29 5.47 0.67 0.01 0.02 278.6 30.7 .0442 0.0050 21.40 8.4 0.0958 58.1 11SM5I-1a 5.68 2.21 0.21 0.10 341.7 26.9 .0544 0.0044 17.33 7.7 0.0990 16.7 aData unused to determine age; total ratios are uncorrected for Pb, but normalized to standard zircon R33 (419 Ma).

2012 Blackwell Publishing Ltd ORIGIN OF THE SONG MA OPHIOLITE AND ECLOGITE 59

metamorphic complex (Fig. 10). The metabasalt, gabbro, plagioclase–amphibole schist and eclogite exhibit MORB or N-MORB chemical features, in spite of very limited displacement from the MORB-OIB array in the Th ⁄ Yb–Nb ⁄ Yb diagram (Fig. 10c), but garnet–hornblende fels plots in the E-MORB ﬁeld in both Th ⁄ Yb– Nb ⁄ Yb and TiO2 ⁄ Yb–Nb ⁄ Yb diagrams. Overall, the Song Ma ophiolite and eclogite are interpreted to have formed in the N-MORB-type geochemical afﬁnities, and the protoliths of garnet–hornblende fels could be interpreted to have an E-MORB origin.

Subduction zone metamorphism The Song Ma ophiolite comprises a thrust slice of upper mantle peridotite and oceanic crustal rocks. Peridotite was partially to totally serpentinized, and rodingite formed during serpentinization. Gabbroic augite was replaced by hornblende at its margins, and plagioclase (An59–62) was locally recrystallized to oligoclase + Qz + Zo. Metabasalt contains greenschist facies assemblage of Ab + Chl + Ep ± Act + Ttn. These petrological characteristics and related metamorphic reactions indicate that the Song Ma ophiolite Fig. 9. (a), Tera-Wasserburg concordia diagram for the SHRIMP analyses (uncorrected 207Pb ⁄ 206Pb v. uncorrected has been subjected to ocean-floor metamorphism 238U ⁄ 206Pb) of the zircon from eclogite. Data-point error ellipses under greenschist- to low amphibolite facies conditions. are 2r. Circular: data used for age determination; triangle: data On the other hand, combining pseudosection and 207 are not used for age determination. (b) Pb corrected thermobarometric calculations, the peak P–T condi- 206Pb ⁄ 238U weighted mean ages of eclogite (11SM5I). Twenty- five analysed data are used to determine the mean ages. tions of eclogite are 26 ± 2 kbar at 700 ± 50 C. These P–T estimates imply that the oceanic plate was subducted to mantle depth of 85 km for eclogitiza- that is characterized by slight depletion in LREE rel- tion. Previous P–T calculations for garnet–chloritoid– ative to MREE and HREE. phengite schist are 1.9–2.3 kbar and 580–600 C Plagioclase–amphibole schist, eclogites and garnet– (Nakano et al., 2010). Moreover, these rocks exhibit a hornblende fels have similar basaltic compositions clockwise P–T path similar to some UHP phengite- with one exception that has basaltic andesite com- bearing eclogites from the Dabieshan in central China position (SM06). REE patterns of plagioclase– (Carswell & Zhang, 1999). The protoliths of the eclogite amphibolite schist are similar to that of weakly with MORB affinity were subducted and recrystallized metamorphosed basalt. The REE patterns of pheng- through amphibolite prograde and peak eclogite facies ite-free eclogite (11SM5F) and garnet–hornblende fels recrystallization. During early stage decompression, (11SM4D) are similar to E-MORB pattern, but oth- these rocks were retrograded, while temperature in- ers show an oceanic island basalt (OIB)-like chemical creased. All these data suggest that the Song Ma feature (Fig. 3c) that is probably caused by differen- eclogite and associated garnet-bearing metapelite were tiation of MORB with minor crustal materials input subjected to HP metamorphism at low thermal gradient ) in the basalt. of 8 Ckm 1, which is coincident with the P–T Previous studies on high-P mafic rocks from New environment of subduction zones. Caledonia and other HP ⁄ UHP terranes (such as Western Alps and Sulu terranes) also indicated that there are no significant changes in HFSE and REE Age of collision and tectonic implication abundances during subduction zone metamorphism Our new data indicate that the Song Ma ophiolite has (Becker et al., 2000; Chalot-Prat et al., 2003; Spandler N-MORB-type geochemical affinities, and the eclogites et al., 2004; Liu et al., 2008). Furthermore, REE pat- represent subducted, slightly differentiated MORB terns of peridotite also suggest that there is no distinct incorporated within a low-grade metamorphosed sed- change during serpentinization. In this scenario, four imentary (+volcanic rocks) wedge during exhumation. discrimination diagrams of TiO2–FeO ⁄ MgO (Glassily, Thus, the classical Song Ma belt and ÔNam Co anti- 1974), 2Nb-Zr ⁄ 4-Y (Meschede, 1986), Th ⁄ Yb–Nb ⁄ Yb formÕ can be defined as a suture zone linked to closure and TiO2 ⁄ Yb–Nb ⁄ Yb (Pearce, 2008) are used to of the Paleotethys Ocean between South China and decipher the initial tectonic setting of ophiolite and Indochina. A part of the Nam Co Formation can be

2012 Blackwell Publishing Ltd 60 R. Y. ZHANG ET AL.

Fig. 10. Discrimination diagrams for the Song Ma metabasalt, eclogite and garnet–hornblende fels: (a) FeO ⁄ MgO v. TiO2 (Glassily, 1974), (b) 2Nb-Zr ⁄ 4-Y (Meschede, 1986), (c) Th ⁄ Yb v.Nb⁄ Yb and (d) TiO2 ⁄ Yb v.Nb⁄ Yb MORB (Pearce, 2008); N-MORB, normal MORB; OIB, oceanic island basalt; VAB, volcanic arc basalt; WPAB, within plate alkaline basalt; WPT, within plate tholeiite. interpreted as an accretionary complex, which contains North China and Yangtze cratons and between the both oceanic and South China continental elements. North and South Qiangtang blocks also took place at The previously reported collision time of the South Middle Triassic time (e.g. Li et al., 1993; Liu et al., China and Indochina blocks is controversial. The 2004, 2006; Zhang et al., 2005; Zhai et al., 2011). The reported ages range from the Devonian (Janvier Triassic orogeny resulted from closure of the Paleot- et al., 1996; Thanh et al., 1996), Carboniferous (Tri, ethys and subsequent amalgamation of continents in 1979; Metcalfe, 1999), Late Permian–Early Triassic Asia, not only in Southeast Asia. (Lepvrier et al., 1997; Chung et al., 1998) to Early Triassic (Carter et al., 2001; Lepvrier et al., 2004, Lepvrier et al., 2008). In contrast, Sengo¨ r & Hsu CONCLUSIONS (1984) suggested that the major closure of Paleotethys 1 Peridotites of the Song Ma ophiolite suite are of Ocean took place along Song Da rather than along residual mantle based on high Mg number (‡91) of Song Ma belt in the Late Triassic. Eclogite elsewhere olivine, enstatite and diopside, and flat MREE and has commonly been considered an index rock for HREE pattern with slightly depletion in LREE. The HP ⁄ UHP subduction zone metamorphism occurring sequence represents remnants of former Paleotethys in collision zones, and metamorphic age of eclogite is oceanic lithosphere prior to the collision of the significant to constrain the timing for collision of two South China and Indochina blocks. The other plates. The peak time of genesis and emplacement of members including basalt and gabbro of Song Ma most ophiolites in Earth history coincided with col- ophiolite and eclogite show N-MORB-type geo- lisional events (Dilek & Furnes, 2011). Our new data chemical affinities. imply that the formation of eclogite was related to 2 The P–T estimates of three-stage (pre-eclogite, subduction of sub-ophiolite oceanic basaltic rocks. eclogite and retrograde) metamorphism define a The U–Pb age of 230.5 ± 8.5 Ma may be used to clockwise P–T path with peak P–T conditions of constrain the closure time of the Lao-Vietnamese 26 ± 2 kbar and 650 ± 50 C, suggesting that the branch of the Paleotethys that separated the South Song Ma eclogite underwent high-P metamorphism China from Indochina. Thus, the collision time of the in subduction zone with a low thermal gradient ) two blocks is interpreted to 230 Ma, which is al- 8 Ckm 1. most coeval to Triassic collision of the Sibumasu to 3 The Song Ma suture zone consists of classical Song Indochina suggested by Sone & Metcalfe (2008). Ma suture and Nam Co antiform. A part of the Moreover, the continental collisions between the Nam Co Formation can be interpreted as an accre-

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tionary complex built on the margin of the South petrological calculations. Journal of Metamorphic Geology, 23, China block. 771–791. Dilek, Y. & Furnes, H., 2011. Ophiolite genesis and global tec- 4 New age dating (230.5 ± 8.2 Ma) for eclogite is tonics: geochemical and tectonic fingerpriting of ancient oce- highly significant. It indicates that the closure of the anic lithosphere. Geological Society of America Bulletin, 123, Paleotethys Ocean that separated Indochina from 387–411. South China and subsequent collision of the two Findlay, R.H. & Trinh, P.T., 1997. The structural setting of the blocks took place at Middle Triassic; at the same Song Ma region, Vietnam and the Indochina plate boundary problem. Gondwana Research, 1, 11–33. time, the ophiolite experienced ocean-floor meta- Gatinsky, Y.G. & Hutchison, C.S., 1987. Cathaysia, Gondw- morphism was emplaced onto the South China analand and Paleotethys in the evolution of continental block, corresponding to the major episode of the Southeast Asia. Bulletin of Geological Society, Malaysia, 20, Indosinian orogeny. 179–199. Glassily, W., 1974. Geochemistry and tectonics of the Grescent volcanic rocks, Olympic Peninsula, Washington. Geological Society of American Bulletin, 85, 785. ACKNOWLEDGEMENTS Green, E., Holland, T. & Powell, R., 2007. An order-disorder model for omphacitic pyroxenes in the system jadeite-diop- This research was supported by the National Science side-hedenbergite-acmite, with applications to eclogitic rocks. Council of Taiwan (NSC). We thank N. C. Pham and American Mineralogist, 92, 1181–1189. V. A. Tran for their kind help with fieldwork in the Holland, T.J.B. & Powell, R., 1998. An internally consistent Song Ma area, northern Vietnam. We sincerely thermodynamic data set for phases of petrological interest. Journal of Metamorphic Geology, 16, 309–343. appreciate B-M. Jahn and J. C. Wei for helpful dis- Holland, T., Baker, J. & Powell, R., 1998. Mixing properties and cussions in geochemistry and isochemical phase dia- activity-composition relationships of chlorites in the system gram, and J. Wood for SHRIMP data calibration. MgO-FeO-Al2O3-SiO2-H2O. European Journal of Mineralogy, Finally, we appreciate M. Sone and an anonymous 10, 395–406. Hutchison, C.S., 1975. Ophiolites in South-East Asia. Geological reviewer for their critical review and thank D. Rob- Society of American Bulletin, 86, 797–906. inson and G. Hoinkes for their editorial corrections Janvier, P., Pham, K. & Phuong, T.H., 1996. Decouverte dÕune and suggestions for revision. faune de Vertebres de type ‘‘sub-chinois’’ dans le Devonien inferieur de la basse River Noire (Song Da), Vietnam. Comptes Rendu Academy Science Paris, 323, 539–546. 2+ REFERENCES Krogh Ravna, E.J., 2000. 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