Petrology and geochemistry of peridotites in the Zhongba ......Petrology and geochemistry of...

Chemical Geology 288 (2011) 133–148

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Chemical Geology

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Petrology and geochemistry of peridotites in the Zhongba ophiolite, Yarlung ZangboSuture Zone: Implications for the Early Cretaceous intra-oceanic subduction zonewithin the Neo-Tethys

Jin-Gen Dai a,⁎, Cheng-Shan Wang a,⁎, Réjean Hébert b, M. Santosh c, Ya-Lin Li a, Jun-Yu Xu d

a State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences and Resources, Research Center for Tibetan Plateau Geology,China University of Geosciences (Beijing), Beijing 100083, Chinab Département de géologie et de génie géologique, 1065 avenue de la Médecine, Université Laval, Québec, Canada G1V 0A6c Department of Natural Environmental Science, Faculty of Science, Kochi University, Akebono-cho, Kochi 780–8520, Japand National research center for geoanalysis, Beijing 100037, China

⁎ Corresponding author. Tel.: +86 10 82321612; fax:E-mail addresses: [email protected] (J.-G. Dai),

(C.-S. Wang), [email protected] (R. Hébert).

0009-2541/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.chemgeo.2011.07.011

a b s t r a c t
a r t i c l e i n f o
Article history:Received 13 September 2010Received in revised form 14 July 2011Accepted 17 July 2011Available online 23 July 2011

Editor: L. Reisberg

Keywords:TibetHarzburgitePlatinum group elementsYarlung Zangbo SutureOphiolite

The Zhongba ophiolite is located in the western part of the Yarlung Zangbo Suture Zone (YZSZ) separatingEurasia to the north from the Indian plate to the south. This ophiolite comprises a well-preserved mantlesequence dominated by harzburgites with minor dunites. Highly depleted modal, mineral and bulk rockcompositions of the harzburgites indicate that they are residues aftermoderate to high degrees of partialmelting(13–24%) mainly in the spinel-stability field. These rocks display typical U-shaped chondrite-normalized RareEarth Element (REE) patterns and fractionated chondrite-normalized Platinum Group Element (PGE) patterns.These characteristics, in combination with their hybrid mineral and whole-rock compositions intermediatebetween those of abyssal and forearc peridotites, indicate melt-rock interaction resulting in the selectiveenrichment of LREE and Pd. We propose a two-stage model to explain the generation of the Zhongbaharzburgites: 1) original generation from a MORB-source upper mantle, and 2) subsequent trapping as part of amantle wedge above a subduction zone. Comparable observations from the ophiolitic massifs along the wholeYZSZ allow us to propose that a ca. 2500-km long complex subduction systemwas active between India and theLhasa terrane, Burma, and the Karakoram microcontinent within the Neo-Tethys during the Early Cretaceous,similar to the modern active intra-oceanic subduction systems in the Western Pacific.

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1. Introduction

Peridotites from ophiolite suites provide important information onmelt extraction, melting, and melt-rock interaction in the uppermantle of paleo-oceanic lithosphere (e.g. Prinzhofer and Allègre,1985; Kelemen et al., 1992; Zhou et al., 2005; Arai et al., 2007; Dileket al., 2007; Ishiwaka et al., 2007; Eyuboglu et al., 2010), and thus offercritical clues to the tectonic setting of the ophiolite complexes (e.g.Melcher et al., 2002; Karipi et al., 2006; Pagé et al., 2009; Pearce andRobinson, 2010). Geochemical data from peridotites and theirconstituent minerals can be used to characterize the origin and thetectonic setting of ophiolitic rocks (e.g. Choi et al., 2008; Aldanmazet al., 2009; Caran et al., 2010; Ulrich et al., 2010).

Several studies have reported characteristic U-shaped REE patternsin peridotites associated with ophiolite complexes (e.g. Edwards andMalpas, 1995;Varfalvy et al., 1997;Monsef et al., 2010; Zhouet al., 2005),modern supra-subduction zones (e.g. Parkinson and Pearce, 1998;

Pearce et al., 2000), and ultramafic xenoliths in volcanic rocks (e.g. Abeet al., 2003; Downes et al., 2004). U-shaped REE patterns have beenreported from worldwide ophiolite suites, for instance, the NewCaledonia ophiolite in the southwest Pacific (Prinzhofer and Allègre,1985;Ulrichet al., 2010), the Speikophiolite in theeasternAlps (Melcheret al., 2002), the Trinity ophiolite in northern California (Gruau et al.,1998), the Antalya and the Ortaca ophiolite in southwestern Turkey(Uysal et al., 2007; Aldanmaz et al., 2009; Caran et al., 2010), theMauresmassif in southeastern France (Bellot et al., 2010), the Sapat peridotite inKohistan, northwestern Pakistan (Bouilhol et al., 2009), the Othris andthe Kallidromon ophiolite in Greece (Karipi et al., 2006; Barth et al.,2008), the Khoy ophiolite in northwestern Iran (Monsef et al., 2010),the Yushigou ophiolites in the North Qilian (Song et al., 2009), theWutaishan peridotites in North China (Polat et al., 2006), and theLuobusa and the Xigaze ophiolite in southern Tibet (Hébert et al. 2003;Xia et al., 2003; Dubois-Côté et al., 2005; Zhou et al., 2005; Bédard et al.,2009; Bezard et al., 2011). The high LREE/MREE ratios of theseperidotites are inconsistent with what is expected for residua of partialmelting of the primitive mantle (e.g. Prinzhofer and Allègre, 1985; Freyet al., 1991). The various models proposed to explain the U-shaped REEpatterns include: 1) sequential disequilibriummelting that began in the

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134 J.-G. Dai et al. / Chemical Geology 288 (2011) 133–148

garnet facies and ended in the plagioclase facies (Prinzhofer andAllègre,1985); 2) contamination through percolation of low-temperaturecontinental fluid (Sharma and Wasserburg, 1996; Gruau et al., 1998);and 3) mantle metasomatism (Navon and Stolper, 1987; Proenza et al.,1999; Zhou et al., 2005). Generally, the formation of the U-shaped REEpatterns in ophiolitic peridotites involves a two-stage process: 1) theperidotites were formed originally in the depleted MORB-sourcemantle after melt extraction; and 2) they were subsequently trappedas a forearc mantle wedge above a suprasubduction zone and weremetasomatized by LREE enriched melt from the subducted slab (e.g.Zhou et al., 2005; Uysal et al., 2007; Guilmette et al., 2009).

The ophiolites within the Yarlung Zangbo Suture Zone (YZSZ) insouthern Tibet have been the focus of several studies, as they provideimportant constraints for reconstruction of the geodynamic evolutionof the Neo-Tethyan Ocean, and also offer critical constraints on thetiming of the India-Asia collision. Previous studies of the YZSZophiolites have focusedmostly on the radiometric and paleontologicalages (e.g. Matsuoka et al., 2002; McDermid et al., 2002; Zhou et al.,2002; Malpas et al., 2003; Miller et al., 2003; Ziabrev et al., 2003;Wang et al., 2006; Wei et al., 2006b; Zhong et al., 2006; Xia et al.,2008b), the geochemistry of the volcanic and plutonic rocks (e.g.Girardeau et al., 1985; Chen et al., 2003; Malpas et al., 2003; Dubois-Côté et al., 2005; Dupuis et al., 2005; Niu et al., 2006; Xia et al., 2008a;Zhu et al., 2008), and the origin of metamorphic soles (e.g. Guilmetteet al., 2008, 2009). Although the geochemical features of theperidotites in the YZSZ have been documented in previous studies(Zhang et al., 2005a; Zhou et al., 2005; Liu et al., 2010), thepetrogenesis and tectonic setting of these rocks remain ambiguous.

In this study,we investigate the Zhongba peridotites in thewesternYZSZ through petrology, mineral chemistry, and whole-rock geo-chemistry (major, trace and platinum-group elements). The objectivesof this study are: (1) to elucidate the origin of these peridotites and thenature of the petrological processes; (2) to determine the geotectonicsetting of the Zhongba peridotites; and (3) to compare their tectonicsetting with those of other ophiolitic massifs along the whole YZSZ.

Lhasa

Qiangtang

Cretaceous-Eocene

Gangdese Arc Cretaceous-E

Paleozoic-Me

XFB

PzU

GdA

Unknown Un

Xigaze forea

AcPZhongba

Saga

Pulan

Zhada

Kailas

30°N

86°E84°E

X

AcPPzU

GCT

Fig.1CZGT

32°N

82°E

80°E

Dangqiong

Zhongba

Yungbwa

Saga

Xiugugabu

BNSZ

YZSZ

JSSZ

AKSZATF

MBF

A

Fig.1B

B

0 60

km

N

Asia

Tibet SouthChina

India

500 km200 km

Fig.1A

Himalaya

Fig. 1. A. Simplified tectonic map of the Tibetan Plateau showing major sutures (modified froKunlun suture zone; JSSZ, Jinshajiang suture zeone; BNSZ, Bangong-Nujiang suture zone; YZSFault. B. Schematic tectonic map of southern Tibet showing the ophiolitic massifs within thethrust; ZGT: Zhongba–Gyangze thrust; GT: Gangdese thrust. C. Simplified geological mapcentral location of the Zhongba ophiolite is 29°46.0′N, 83°46.9′E.

Our data reveal the presence of a large and complex intra-oceanicsubduction systemwithin theNeo-Tethys between India and the Lhasaterrane, Burma, and the Karakoram microcontinent, providing newinsights into the evolution of the Neo-Tethys.

2. Geological setting

The Yarlung Zangbo Suture Zone (YZSZ), the southernmost offour sutures in the Tibetan Plateau, is a nearly E–W trendingsuture between India to the south, and the Lhasa terrane to the north(Fig. 1A, B). Traditionally, the YZSZ is believed to be composed ofremnants of theNeo-Tethys (e.g. Allègre et al., 1984; Bédard et al., 2009;McDermid et al., 2002; Miller et al., 2003). However, the newlydiscovered Upper Devonian gabbros in the western YZSZ reveal thepresence of Paleo-Tethys relicts within the YZSZ (Zhou, 2002; Dai et al.,2011). Thus, the YZSZ is a complex subduction system probablypreserving the imprints of both the Paleo-Tethys and the Neo-Tethys.The YZSZ is composed of three basic lithotectonic units. From north tosouth these are: 1) the Xigaze forearc basin; 2) the Yarlung Zangboophiolitic belt and associated mélange; and 3) and the accretionaryprism (Fig. 1B). The Gangdese arc to the north is related to thenorthward subduction of the Neo-Tethyan lithosphere beneath theLhasa terrane (e.g. Ji et al., 2009),whereas the Indianpassive continentalmargin to the south is inherited from fragmentation of supercontinent.The accretionary prism is dominantly composed of Triassic to Eocenesediments. However, in the eastern part of this suture, the Late TriassicLangjiexue Group is different from the strata of the accretionary prism,and may have been derived from the Lhasa terrane to the north ratherthan the Indian region to the south (Dai et al., 2008; Li et al., 2010).

The Yarlung Zangbo ophiolitic belt comprises well preserved todisrupted ophiolitic massifs, and most of these might represent theremnants of Neo-Tethys lithosphere. From east to west, theseophiolitic massifs include the Luobusa (LBS), the Zedang (ZD), theDazhuka (DZK), the Bailang (BL), the Jiding(JD), the Sangsang (SS),the Saga (SG), the Zhongba (ZB), the Dangqiong (DQ), the Xiugugabu

ocene Triassic-Eocene

sozoic Passive continental margin

AcP

IPCM Paleozoic-Eocene Indian Triassic Langjiexue

Accretionary prism

it

rc basin

Lhaze

XigazeRenbu

Lhasa

CuomeiGyantse

Qusong

88°E 90°E

GdA

IPCM

GT

GCT

92°E

ZedangLuobusa

DazhukaBailang

Jiding

Sangsang

FB

Peridotite Mélange

Quaternary

1kmN

Diabase dike

C

TerraneLT

OphiolitesOph

m Chung et al., 2005; Yin and Harrison, 2000). The major sutures are: AKSZ, Ayimaqin–Z, Yurlung Zangbo suture zone. Major faults: MBF, Main Boundary Fault; ATF, Altyn TaghYZSZ based on Pan et al. (2004) and Ding et al. (2005). Major faults: GCT, Great Countershowing the mantle peridotite body and the diabase dike discussed in this study. The

Table 1Summary of ages and tectonic settings from the YZSZ ophiolites.

Locality Rock type Method Age Tectonic setting Reference

Luobusa (LBS) Gabbro-diabase Bulk rock Sm-Nd 177±33 Ma originally Mid-Ocean Ridge,subsequently Supra-Subduction Zone

Zhong et al. 2006; Zhou et al. 2002, 2005Diabase Zircon U-Pb 162.9±2.8 Ma

Zedong (ZD) Andesite dykes Hornblende 40Ar–39Ar 156±0.3 Ma Intra-Oceanic Magmatic Arc McDermid et al. 2002; Wei et al. 2006aBasalt Bulk rock Sm–Nd 175±20 Ma

Dazhuka (DZK) Radiolarian fauna Radiolarian age ~130–112 Ma Supra-Subduction Zone Malpas et al., 2003; Ziabrev et al. 2003;Xia et al. 2003; Dubois-Côté et al. 2005Quartz diorite Zircon U–Pb 126±1.5 Ma

Bailang (BL) Amphiobolite Hornblende 40Ar–39Ar 123.6±2.9 Ma127.7±2.2 Ma127.4±2.3 Ma

Intra-Oceanic Supra-Subduction Zone Guilmette et al. 2009, 2008; Li et al. 2009

Gabbro Zircon U–Pb 125.6±0.8 MaJiding (JD) Gabbro Zircon U–Pb 128±2 Ma Intra-Oceanic Supra-Subduction Zone Wang et al. 2006; Hébert et al. 2003Sangsang (SS) Diabase Zircon U–Pb 125.2±3.4 Ma Intra-Oceanic Supra-Subduction Zone Xia et al. 2008b; Bédard et al., 2009Zhongba (ZB) Diabase Zircon U–Pb 125.7±0.9 Ma Intra-Oceanic Supra-Subduction Zone Dai et al. in revision; This studyDangqiong (DQ) Gabbro Zircon U–Pb 126.7±0.4

123.4±0.8Intra-Oceanic Supra-Subduction Zone Chan et al. 2007; Bezard et al., 2011

Diabase Zircon U–Pb 122.3±2.4 MaXiugugabu (XGGB) Tholeiitic basalt Bulk rock Sm–Nd 147±25 Ma Intra-oceanic supra-subduction zone Wei et al. 2006b; Bezard et al., 2011Yungbwa (YBW) Tholeiitic basaltic dike Hornblende 40Ar–39Ar 152±33 Ma originally Mid-Ocean Ridge,

subsequently Supra-Subduction ZoneMiller et al. 2003; Li et al. 2008;Liu et al. 2010Diabase Zircon U–Pb 120.2±2.3 Ma

Harzburgite

Dunite

Harzburgite

A

B

Fig. 2. Field photographs of the Zhongba peridotites in the western YZSZ. A. Alignmentof pyroxenes marks the foliation (dashed line) in the harzburgite; B. dunite dike inharzburgite. Tabular dunite is enclosed within the harzburgite.

135J.-G. Dai et al. / Chemical Geology 288 (2011) 133–148

(XGGB), and the Yungbwa (YBW). Recent geochronological studiesfrom these massifs indicate that their ages range from Mid-Jurassic toEarly Cretaceous, but most of them are clustered at 120–130 Ma(Table 1; Chan et al., 2007; Dai et al., in revision; Guilmette et al., 2009,2008; Li et al., 2008, 2009; Malpas et al., 2003; McDermid et al., 2002;Miller et al., 2003; Wang et al., 2006; Wei et al., 2006a, 2006b; Xiaet al., 2008b; Zhong et al., 2006; Zhou, 2002; Ziabrev et al., 2003).Petrological and geochemical studies reveal that most of these massifswere formed in an intra-oceanic supra-subduction zone (Table 1;Bédard et al., 2009; Bezard et al., 2011; Dubois-Côté et al. 2005;Guilmette et al., 2009, 2008; Hébert et al. 2003; McDermid et al. 2002;Liu et al. 2010; Xia et al. 2003; Zhou et al. 2005).

The Zhongba ophiolite is a newly discovered ophiolite suitelocated in the western segment of the YZSZ. It was mapped asCenozoic strata in previous geological maps. However, our detailedfieldwork has revealed that it is a dismembered ophiolite suite. Thisophiolite is characterized by the occurrence of a large peridotite bodywith associated crustal rocks, tectonically overlying a Lower Creta-ceous mélange that includes a wide variety of exotic sedimentary andvolcanic blocks. The mantle sequence is mostly composed ofharzburgites and minor dunites.

3. Field occurrence and petrography

The Zhongba ophiolitic massif occurs west of Zhongba county inthe YZSZ (Fig. 1B). It consists of fresh harzburgites withminor dunites,and associated mafic rocks. The harzburgite exhibits a strong foliation(Fig. 2A). Minor dunite is enclosed within the harzburgite and thecontact between the dunite and host harzburgite is mostly sharp(Fig. 2B). The peridotites are thrust over the diabase dikes and thepillow basalts. They are surrounded by a mélange zone containinglimestone, chert, and massive basalt (Fig. 1C).

The harzburgites are remarkably fresh with little serpentinisation.The modal content of olivine varies from 70% to 83%, orthopyroxenefrom 12% to 25%, clinopyroxene from 1% to 3%, and spinel from0.5% to 1%. The harzburgites show mainly porphyroclastic texture,characterized by millimeter-sized prophyroclasts of olivine (Ol)and orthopyroxene (Opx). Olivine also occurs as small neoblasts(Fig. 3B, C). Both the olivine and orthopyroxene grains show internaldeformation. Olivine displays undulose extinction or kink bands(Fig. 3A, B), whereas the orthopyroxene shows kinked or distortedlamellae and elongated shape (Fig. 3C). Orthopyroxene occurs as largeprophyroclasts or as smaller interstitial grains. They also containexsolutions of clinopyroxene and are typically in spatial associationwith spinel (Fig. 3F). Clinopyroxene occurs as prophyroclasts or as

irregular interstitial grains, or in some cases, as small blebs exsolvedfrom orthopyroxene. Some clinopyroxenes and orthopyroxenesare replaced by olivine (Fig. 3D, E). The spinel usually appears asbrown to dark red vermicular crystals associated with orthopyroxeneor occurs as euhedral crystals. The smaller spinels seem to be moreeuhedral than the larger ones (Fig. 3F). They also contain olivine asinclusions.

Ol

Ol

Ol Neoblasts

Ol

Kink-bandingKink-banding

Ol Neoblasts

Opx

Opx

Ol

OpxOl

OpxOl

Ol Ol

Ol

Opx

OpxOl

Opx

OpxOl

Ol

Ol

Opx

OpxOpx

Opx

Spl

Ol

Vermicular Spl

Spl

Spl

OpxOl

0.2 mm 0.5 mm

0.5 mm

0.5 mm0.2 mm

0.5 mm

A B

C D

E F

Fig. 3. Photomicrographs in cross-polarized light of the Zhongba harzburgite from the western YZSZ. A. Kink-banding in olivine (Ol); B. High-temperature recrystallization of olivine(Ol) neoblasts and kink-banding in olivine (Ol); C. Elongated orthopyroxene (Opx) with irregular and diffuse boundaries against the interstitial olivine (Ol) matrix; D. Olivine (Ol)embayments partly corroding the orthopyroxene (Opx) porphyroclast; E. Olivine (Ol) grains replacing orthopyroxene (Opx) porphyroclasts, showing extremely irregular outlines;F. Orthopyroxene (Opx) and spinel (Spl) symplectite, also showing euhedral to subhedral spinels.


4. Analytical techniques

Mineral analyses on the Zhongba harzburgites were performed ona Cameca SX-100 five-spectrometer electronmicroprobe at UniversitéLaval, Canada. Analytical conditions were 15 kV, 20 nA with a

Table 2Representative olivine compositions of the Zhongba harzburgites.

Sample no. ZB067 ZB068 ZB069 ZB070 ZEOS-01–02 Z

SiO2 40.54 40.80 40.33 40.45 41.28 4TiO2 0.00 0.00 0.00 0.00 0.00 0Al2O3 0.00 0.01 0.00 0.00 0.00 0Cr2O3 0.01 0.00 0.00 0.04 0.00 0MgO 50.62 48.92 50.30 50.65 49.71 4CaO 0.01 0.01 0.02 0.02 0.02 0MnO 0.05 0.13 0.06 0.10 0.13 0FeO 8.22 9.26 8.08 8.65 8.71 8CoO 0.01 0.01 0.00 0.02 0.00 0NiO 0.37 0.44 0.36 0.40 0.47 0ZnO 0.03 0.01 0.00 0.04 0.01 0Total 99.85 99.59 99.15 100.35 100.32 9Mg# 0.92 0.90 0.92 0.91 0.91 0

Mg#=Mg/(Mg+Fe2+).

counting time of 20s on the peaks and 10s on the background. Themain minerals obtained from the Zhongba harzburgites were olivine,spinel, orthpyroxene, and clinopyroxene. About 280 spots wereanalyzed. Only the representative data are presented in Tables 2–5.The complete results are given in online Supplementary material A.

EOS-01–05 ZEOS-01–06 ZEOS-02–01 ZEOS-02–03 ZEOS-02–04

1.09 40.84 40.68 40.50 40.91.00 0.00 0.00 0.00 0.00.00 0.00 0.00 0.00 0.00.00 0.01 0.00 0.03 0.009.62 49.55 48.77 49.38 49.80.01 0.02 0.01 0.00 0.02.09 0.20 0.13 0.18 0.21.53 8.94 8.92 9.37 9.29.00 0.02 0.03 0.02 0.00.45 0.43 0.38 0.40 0.41.03 0.00 0.02 0.00 0.009.81 100.01 98.94 99.88 100.64.91 0.91 0.91 0.90 0.91

Table 3Representative spinel compositions of the Zhongba harzburgites.

Sample no. ZB067 ZB068 ZB069 ZB070 ZEOS-01–02 ZEOS-01–05 ZEOS-01–06 ZEOS-02–01 ZEOS-02–03 ZEOS-02–04

SiO2 0.01 0.04 0.05 0.03 0.05 0.04 0.04 0.02 0.01 0.02TiO2 0.01 0.05 0.01 0.00 0.02 0.02 0.04 0.02 0.05 0.01Al2O3 40.19 23.67 37.84 33.67 25.92 37.73 35.49 29.87 30.64 33.94Cr2O3 26.93 44.56 30.58 34.41 42.94 30.32 32.16 38.63 38.08 34.22Fe2O3 2.40 2.03 1.07 2.77 1.85 1.12 1.56 1.82 1.95 1.59MgO 15.88 12.34 15.27 15.10 12.97 15.62 14.68 13.61 14.89 14.69CaO 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.01MnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00FeO 13.43 16.39 14.11 13.85 16.12 13.24 14.44 15.55 13.65 14.23CoO 0.00 0.07 0.06 0.21 0.00 0.08 0.00 0.10 0.00 0.09NiO 0.12 0.13 0.11 0.14 0.06 0.13 0.18 0.05 0.10 0.10ZnO 0.28 0.05 0.14 0.16 0.14 0.19 0.19 0.21 0.19 0.16Na2O 0.01 0.03 0.00 0.00 0.00 0.00 0.00 0.01 0.02 0.04Total 99.27 99.37 99.25 100.34 100.08 98.48 98.77 99.87 99.56 99.10Cr# 0.31 0.56 0.35 0.41 0.53 0.35 0.38 0.46 0.45 0.40Mg# 0.68 0.57 0.66 0.66 0.59 0.68 0.64 0.61 0.66 0.65

Cr#=Cr/(Cr+Al); Mg#=Mg/(Fe2++Mg).

Table 4Representative orthopyroxene compositions of the Zhongba harzburgites.

Sample no. ZB067 ZB068 ZB069 ZB070 ZEOS-01–02 ZEOS-01–05 ZEOS-01–06 ZEOS-02–01 ZEOS-02–03 ZEOS-02–04

SiO2 55.81 55.65 55.74 55.52 56.17 55.01 54.91 56.68 55.55 55.70TiO2 0.04 0.06 0.03 0.02 0.02 0.02 0.04 0.00 0.02 0.03Al2O3 2.81 3.46 2.67 2.92 2.88 2.87 2.91 2.33 2.49 2.22Cr2O3 0.51 0.87 0.71 0.73 0.68 0.64 0.69 0.43 1.18 0.31MgO 33.97 33.30 34.01 34.11 33.98 33.58 33.82 34.22 34.65 34.90CaO 0.74 2.63 1.68 1.59 0.62 1.71 1.15 0.69 0.95 0.57MnO 0.14 0.13 0.15 0.12 0.13 0.11 0.10 0.14 0.15 0.10FeO 5.99 5.47 5.56 5.67 5.65 5.51 6.24 6.03 5.97 6.23NiO 0.08 0.09 0.07 0.13 0.07 0.13 0.07 0.11 0.10 0.10Na2O 0.00 0.02 0.00 0.01 0.01 0.01 0.01 0.02 0.02 0.03K2O 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00Total 100.09 101.67 100.60 100.81 100.20 99.59 99.93 100.63 101.08 100.20Mg# 0.91 0.92 0.92 0.91 0.91 0.92 0.91 0.91 0.91 0.91

Mg#=Mg/(Mg+Fetotal); total iron as FeO.


The least-altered samples were powdered to less than 200 meshfor whole-rock analyses. Major element oxides and trace elementswere analyzed at the State Key Laboratory of Continental Dynamics,Northwest University in Xi'an, China. Whole rock major elementcompositions were determined by X-ray fluorescence (XRF) infused glass disks using a Rigaku RIX 2100 spectrometer. Analyses ofthe USGS reference material (BCR-2) indicate precision and accuracybetter than 2% for SiO2, TiO2, Al2O3, Fe2O3, and CaO, and better than 4%for MnO and MgO (Rudnick et al., 2004). This is supported by thereproducibility of sample ZB070 (Table 6). Whole rock trace elementcompositions were analyzed by Elan 6100-DRC inductively coupled

Table 5Representative clinopyroxene compositions of the Zhongba harzburgites.

Sample no. ZB067 ZB068 ZB069 ZB070 ZEOS-01–02 ZE

SiO2 51.65 52.31 51.57 51.99 51.44 53TiO2 0.08 0.06 0.05 0.03 0.06 0.Al2O3 2.70 3.66 3.49 3.51 3.05 2.Cr2O3 0.78 1.10 1.11 0.99 0.98 0.MgO 18.04 17.33 17.00 17.29 18.04 18CaO 23.36 23.58 24.36 24.14 22.81 23MnO 0.07 0.10 0.08 0.07 0.09 0.FeO 2.32 2.22 2.06 2.31 2.82 2.NiO 0.02 0.06 0.10 0.06 0.06 0.Na2O 0.07 0.10 0.08 0.03 0.04 0.K2O 0.00 0.00 0.00 0.00 0.00 0.Total 99.09 100.51 99.90 100.40 99.36 10Mg# 0.93 0.93 0.94 0.93 0.92 0.

Mg#=Mg/(Mg+Fetotal).

plasma mass spectrometry (ICP-MS). The samples were dissolved in amixture of HF and HNO3 in Teflon bombs at 190 °C for 48 h. Thisprocedure was repeated using smaller amounts of acids. Afterdigestion, the sample was evaporated to incipient dryness, refluxedwith 6 N HNO3, and heated again to incipient dryness. The USGSreferencematerials (BCR-2, BHVO-2 andAGV-2)were used tomonitorthe analytical accuracy. These results indicate that the accuracy isbetter than 10% for most elements, with many elements agreeing towithin 2% of the recommended values (Table 6; only the results ofBHVO-2 are presented). Analytical details and analysis of referencematerials are reported in (Rudnick et al., 2004; Liu et al., 2007).

OS-01–05 ZEOS-01–06 ZEOS-02–01 ZEOS-02–03 ZEOS-02–04

.23 52.15 53.04 53.34 52.8006 0.05 0.06 0.04 0.0600 2.79 2.41 1.93 2.1357 0.90 0.68 0.60 0.49.27 17.83 18.09 18.29 18.54.99 23.74 23.91 24.13 23.9709 0.06 0.07 0.07 0.0607 2.32 2.18 2.13 2.4302 0.05 0.01 0.09 0.0503 0.13 0.03 0.07 0.0500 0.00 0.00 0.00 0.000.33 100.02 100.49 100.69 100.5794 0.93 0.94 0.94 0.93

Table 6Whole-rock major, trace element compositions of the Zhongba harzburgites from western YZSZ.

Sample no. ZEOS-01–01 ZEOS-01–02 ZEOS-01–05 ZEOS-01–06 ZEOS-02–01 ZEOS-02–03 ZB067 ZB068 ZB069 ZB070 ZB070R

XRF-major element (wt.%)SiO2 43.85 43.31 43.71 44.33 43.91 41.76 42.51 42.53 43.55 42.91 42.84TiO2 0.01 0.01 b0.01 0.01 0.01 b0.01 b0.01 0.02 b0.01 0.01 0.01Al2O3 1.03 1.02 1.14 1.11 1.05 0.81 0.92 1.06 1.01 0.97 0.96Fe2O3

b 9.21 9.08 8.99 9.03 9.01 9.18 9.26 8.91 9.15 9.13 9.11MnO 0.12 0.11 0.12 0.11 0.11 0.11 0.12 0.11 0.11 0.11 0.12MgO 43.66 43.87 43.31 42.67 42.93 43.39 44.59 43.20 43.86 43.95 43.96CaO 1.32 1.16 1.21 1.34 1.33 1.00 0.93 1.22 1.09 1.13 1.13Na2O b0.01 b0.01 b0.01 0.01 0.03 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01K2O b0.01 b0.01 b0.01 b0.01 0.02 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01P2O5 0.01 0.01 0.01 0.01 0.01 0.01 b0.01 0.01 0.01 0.01 0.01LOI −0.20 0.48 0.61 0.56 0.63 2.91 0.68 2.19 0.43 0.92 0.93Total 99.01 99.05 99.10 99.18 99.04 99.17 99.01 99.25 99.21 99.14 99.07Mg# 90.38 90.54 90.52 90.35 90.42 90.35 90.51 90.57 90.47 90.51 90.53

ICP-MS trace element (ppm) BHVO-2Meas.

BHVO-2Rec.

Li 1.27 1.35 1.55 1.32 1.44 1.22 1.35 1.57 1.35 1.40 4.77 4.80Be 0.0066 0.0024 0.0033 0.0033 0.0066 0.0028 0.0011 0.0044 0.0030 0.0043 1.15 1.10Sc 7.39 5.64 11.2 8.83 13.5 9.80 9.88 10.1 8.66 10.7 31.8 32.0V 47.2 47.7 49.4 52.0 53.3 39.9 39.9 43.5 45.9 44.9 316 317Cr 2425 2578 2754 2798 2580 2553 2120 2528 2494 2598 281 280Co 117 117 114 115 112 119 119 113 116 114 44.5 45.0Ni 2299 2331 2268 2244 2225 2356 2341 2289 2302 2280 118 119Cu 20.3 20.0 19.2 22.4 12.9 16.5 12.0 22.5 19.3 24.7 128 127Zn 47.6 48.7 48.4 49.8 49.4 51.5 48.5 47.5 48.4 49.7 100 103Ga 1.00 1.01 1.07 1.10 1.07 0.90 0.93 1.08 1.00 0.96 21.4 21.7Ge 0.94 0.97 0.97 0.99 0.95 0.91 0.92 0.93 0.96 0.93 1.61 1.60Rb 0.14 0.18 0.075 0.080 0.12 0.085 0.049 0.074 0.049 0.13 10.09 9.80Sr 0.57 0.60 0.35 0.31 1.20 0.29 0.29 0.49 0.66 1.02 394 389Y 0.31 0.28 0.25 0.28 0.33 0.20 0.24 0.35 0.25 0.23 26.1 26.0Zr 0.40 0.43 0.23 0.28 1.45 0.17 0.42 0.41 0.31 2.02 169 172Nb 0.075 0.068 0.041 0.045 0.19 0.029 0.028 0.087 0.041 0.039 18.7 18.1Cs 0.022 0.0036 0.0020 0.0036 0.0032 0.0056 0.0020 0.0019 0.0030 0.0040 0.14 0.10Ba 2.13 2.82 0.53 0.40 1.88 0.51 0.25 0.53 1.17 1.15 134 130La 0.092 0.047 0.025 0.040 0.15 0.028 0.014 0.054 0.028 0.044 15.5 15.0Ce 0.19 0.10 0.052 0.086 0.31 0.058 0.030 0.12 0.059 0.090 38.0 38.0Pr 0.022 0.013 0.0071 0.011 0.038 0.0069 0.0040 0.015 0.0076 0.012 5.54 5.29Nd 0.074 0.054 0.028 0.042 0.15 0.028 0.015 0.063 0.030 0.044 24.6 25.0Sm 0.017 0.015 0.0094 0.011 0.033 0.0088 0.0064 0.018 0.011 0.014 6.28 6.20Eu 0.0049 0.0044 0.0025 0.0032 0.011 0.0022 0.0022 0.0060 0.0027 0.0033 2.06 2.07Gd 0.018 0.019 0.010 0.012 0.036 0.0089 0.010 0.023 0.012 0.015 6.20 6.30Tb 0.0038 0.0040 0.0027 0.0029 0.0061 0.0026 0.0030 0.0047 0.0030 0.0027 0.94 0.90Dy 0.038 0.034 0.029 0.032 0.048 0.020 0.024 0.045 0.029 0.029 5.36 5.31Ho 0.010 0.010 0.0093 0.010 0.012 0.0070 0.0085 0.013 0.0094 0.0093 1.06 1.04Er 0.039 0.034 0.033 0.037 0.043 0.028 0.033 0.048 0.033 0.034 2.53 2.54Tm 0.0075 0.0066 0.0068 0.0077 0.0082 0.0056 0.0062 0.0086 0.0071 0.0068 0.36 0.33Yb 0.063 0.059 0.061 0.063 0.062 0.051 0.054 0.067 0.061 0.056 2.02 2.00Lu 0.012 0.011 0.011 0.012 0.012 0.0093 0.010 0.012 0.011 0.011 0.31 0.28Hf 0.010 0.011 0.0053 0.0087 0.024 0.0062 0.0038 0.013 0.0064 0.0078 4.30 4.10Ta 0.047 0.035 0.029 0.027 0.034 0.027 0.025 0.026 0.027 0.027 1.19 1.14Pb 1.38 0.37 3.46 0.67 0.54 0.96 0.060 0.92 0.098 0.14 1.84 2.60Th 0.016 0.0080 0.0044 0.0058 0.031 0.0079 0.0031 0.0085 0.0064 0.016 1.31 1.20U 0.0060 0.0031 0.0016 0.0021 0.0076 0.0022 0.0016 0.0025 0.0023 0.0034 0.46 0.42

Mg#=Mg/(Mg+Fe); total Fe as Fe2+; bbelow detection limit, LOI: loss-on-ignition; total Fe as Fe2O3; Meas.: measured value; Rec.: recommended value. The values of BHVO-2samples (USGS standard) are the average of two analyses. ZB070R is the reproducibility of sample ZB070.Recommended values are from http://minerals.cr.usgs.gov/geo_chem_stand/ and Govindaraju (1994).


Platinum group elements (PGE) were concentrated using thenickel sulfide fire assay and Te-coprecipitation method of Jacksonet al. 1990 and Sun et al. 1993 at the National Research Center forGeoanalysis, Chinese Academy of Geological Sciences, Beijing, China.Ten grams of each sample were well mixed with 2 g carbonyl nickelpowder, 20 g sodium borate, 12 g sodium carbonate, 1.2 g sulfur, 2 gglass powder (SiO2) and 1 g wheat flour (Flour is used to reduce themetal oxides to metal or alloy, and to reduce the high valence oxidesto low valence). Because the formation and loss of highly volatile OsO4

were unavoidable during the following chemical preparation, anappropriate amount of 190Os spike (from Oak Ridge NationalLaboratory) was added to allow Os concentration determination by

isotope dilution (e.g. Sun et al., 2001 and Sun and Sun, 2005). Themixture was transferred into a ceramic crucible, covered with a thinlayer of sodium borate and sodium carbonate and baked at 1100 °C for1–1.5 h in the muffle furnace. The NiS button formed during fusionwas collected, crushed with a RETSCHMM301 oscillator and placed ina 150 mL glass beaker. 60–80 mL HCl was added to dissolve thedisintegrated button at ~120 °C until the solution was clear andwithout bubbles. 0.5–1 mL Tellurium solution and 1–2 mL stannouschloride were added. Then the solution was filtered and theprecipitate was washed many times with water and HCl. Theprecipitate together with the membrane was transferred into a 7 mlTeflon beaker. 1 mL aqua regia was added to the Teflon beaker and

http://minerals.cr.usgs.gov/geo_chem_stand/


then the beaker was sealed and heated at ~120 °C for 2–3 h. Aftercooling, the beaker was opened and the solution was diluted to 10 mlwith H2O for measurement by inductively coupled plasma-massspectrometry (ICP-MS) on a TJA PQ-EXCELL instrument. An analysistypically consisted of 100 scans over the mass sequence 99 (Ru), 101(Ru), 103 (Rh), 105(Pd), 108(Pd), 190(Os), 192(Os), 191(Ir), 193(Ir),194(Pt), 195(Pt). During these analyses, 175Lu was used as internalstandard. 192Os/190Os ratios were used to calculate Os concentrationsby isotope dilution. Other PGE were determined by comparison ofsignal intensities with those of a standard solution. The detailedprocedure was described by Sun et al., 2008; Sun and Sun, 2005;Zhang et al., 2005b. The results of analyses of reference material Gpt-3are consistent with the recommended values (Table 7). To evaluateoverall reproducibility of the sample preparation process andinstrumental measurement, the sample ZEOS-02-01 was replicatedfor all PGE. The relative errors between the first analyses and thereplicates are: 4.5% for Os, 8% for Ir, 5% fro Ru, 1.5% for Pd, 2.2% for Rh,and 12.5% for Pt (Table 7). The analyses of reference materialsillustrate that this method yields precisions that range from 4.2% to10.3% (Please see Sun and Sun, 2005 for details). The PGEconcentrations are given in Table 7.

5. Results

5.1. Mineral chemistry

Representative compositions of olivine from the Zhongba harz-burgites are listed in Table 2. Their Mg# values [Mg/(Mg+Fe2+)atomic ratio] range from 0.90 to 0.92 with NiO contents from 0.36 to0.48 wt.% (Fig. 4A). Spinels have Cr# values [Cr/(Cr+Al) atomic ratio]ranging from 0.30 to 0.56 and Mg# values [Mg/(Mg+Fe2+) atomicratio] varying from 0.57 to 0.72 (Table 3; Fig. 4B). Using calculationssimilar to those given in Hirose and Kawamoto (1995), the Zhongbaspinel composition suggests moderate to high degrees of partialmelting (13–24%) (Fig. 4B). A similar conclusion is also provided bythe plots of spinel Cr# and olivine Mg# (Fig. 4C). Orthopyroxeneshave Mg# values [Mg/(Mg+Fetotal)] varying from 0.91 to 0.92, alongwith Al2O3, Cr2O3 contents ranging from 1.49 to 4.49 wt.%, 0.25 to1.18 wt.%, respectively (Table 4; Fig. 5A, B). Clinopyroxenes show highMg# values [Mg/(Mg+Fetotal)] (0.92–0.94), and Cr2O3 contents spanbetween 0.32 and 1.41 wt.% (Table 5; Fig. 5C).

5.2. Whole-rock geochemistry

The Zhongba harzburgites have very low loss-on ignition (LOI)values (eight of them b1 wt.%, the other two b3 wt.%,), consistentwith their fresh nature in hand specimen (Table 6). They have

Table 7Whole-rock Platinum Group elements concentrates of the Zhongba harzburgites from west

Sample Os Ir Ru Pd Rh Pt

ZEOS-01–01 5.57 3.74 7.61 7.33 1.35 7.62ZEOS-01–02 6.26 3.63 8.22 11.4 1.48 8.47ZEOS-01–05 5.18 3.78 7.72 6.67 1.34 7.43ZEOS-01–06 5.4 3.48 6.93 9.35 1.32 7.86ZEOS-02–01 5.32 3.63 7.93 7.45 1.39 9.07ZEOS-02–01R 5.56 3.92 7.53 7.56 1.36 10.2ZEOS-02–03 5.3 3.74 7.55 6.08 1.25 7.48ZB067 5.59 2.85 5.66 6.04 1.03 5.81ZB068 6.24 3.54 7.01 6.4 1.24 6.75ZB069 5.03 3.64 7.15 6.72 1.26 8.36ZB070 5.84 3.3 6.9 7.39 1.21 6.08Gpt-3 Meas. 7.94 4.6 10.5 4.58 1.28 6.25Gpt-3 Rec. 9.6±2.0 4.3±0.5 14.8±2.7 4.6±0.6 1.3±0.3 6.4±0.9

All PGE abundances are reported in ppb. Subscripts N indicate ratios normalized to CI chondrZEOS-02-01. Gpt-3 is the reference material. Meas.: measured value; Rec.: recommended v

relatively homogeneous whole-rock compositions (Table 6; Fig. 6)and are depleted in magmaphile elements as shown by the very lowAl2O3 (0.81–1.14 wt.%, with an average of 1.01 wt.%), and CaO (0.93–1.34 wt.%, with an average of 1.17 wt.%) contents. The Mg# values[Mg/(Mg+Fetotal)] of the samples vary from 90.35 to 90.54.

All the samples show very low REE concentrations with chondrite-normalized values from 0.17 to 0.48 for HREE, from 0.04 to 0.24 forMREE, and from 0.03 to 0.62 for LREE (Table 6). They exhibit typicallyU-shaped REE patterns, characterized by a highly fractionated HREEto MREE segment (LuN/GdN=2.74–8.65) (N stands for chondrite-normalized, normalizing values of Sun and McDonough, 1989) andsignificant enrichment of LREE relative to MREE (LaN/SmN=1.38–3.52) (Fig. 7). The LREE and MREE contents are rather variablewhereas the HREE abundances are in general more homogeneous.All of these characteristics are similar to those of modern SouthSandwich forearc peridotites (Pearce et al., 2000) and the peridotitesfrom Dazhuka and Bailang, central YZSZ (Dubois-Côté et al., 2005;Fig. 7). The patterns are also similar to those of boninites (Hickey andFrey, 1982), although they have much lower total concentrations(Fig. 7).

The Zhongba harzburgites are also highly depleted in terms ofmost other moderately incompatible trace elements since theirconcentrations normalized to primitive mantle values are mostlybelow 0.1. However, in the primitive mantle-normalized spidergram,they display variable relative enrichment in the most incompatibletrace elements (Cs, Rb, Ba, Th, U, and Pb), with strong spikes of Cs,U and Pb (Fig. 8). All of these characteristics resemble the moderndepleted forearc peridotites from Izu–Bonin–Mariana arc (Parkinsonand Pearce, 1998) and the fossil forearc peridotites from Yushigou,north Qilian (Song et al., 2009; Fig. 8). We should point out that theextremely large Pb anomalies may reflect late stage contamination inthe crust rather than mantle processes. Measurement of Pb and Ndisotopic compositions of these samples may be a way to investigatethis possibility, but this is beyond the scope of this study.

The total PGE contents for the Zhongba harzburgites range from26.98 to 39.46 ppb with an average of 32.64 ppb, which are muchhigher than those of the primitive mantle (23.5 ppb, McDonough andSun, 1995; Table 7). The Os contents vary from 5.03 to 6.26 ppb, Irfrom 2.85 to 3.78 ppb, Ru from 5.66 to 8.22 ppb, Pd from 6.04 to11.4 ppb, Rh from 1.03 to 1.48 ppb, and Pt from 5.81 to 9.07 ppb(Table 7). Individual PGE concentrations range from 0.005 to 0.02times those of CI chondrite. In the CI chondrite-normalized PGE plot,the rocks display strong Pd and Rh enrichment relative to Pt, and theirPGE patterns are similar to those from Northwest Anatolianperidotites (Aldanmaz and Koprubasi, 2006) and Luqu peridotitesfrom central YZSZ (Chen and Xia, 2008), both of which representforearc peridotites (Fig. 9A). The rocks under study are also

ern YZSZ.

(Os/Ir)N (Ru/Ir)N (Pt/Ir)N (Rh/Ir)N (Pd/Ir)N (Rh/Pt)N (Pd/Pt)N

1.38 1.30 0.92 1.26 1.62 1.38 1.771.60 1.45 1.05 1.43 2.60 1.36 2.471.27 1.31 0.89 1.24 1.46 1.40 1.651.44 1.28 1.02 1.33 2.22 1.30 2.181.36 1.40 1.13 1.34 1.70 1.19 1.51

1.32 1.29 0.90 1.17 1.34 1.30 1.491.82 1.27 0.92 1.26 1.75 1.38 1.911.64 1.27 0.86 1.23 1.50 1.43 1.741.28 1.26 1.03 1.21 1.53 1.17 1.481.64 1.34 0.83 1.28 1.85 1.55 2.23

ite values of McDonough and Sun (1995). ZEOS-02-01R is the reproducibility of samplealue.

0.00.20.40.60.81.00.0

0.2

0.4

0.6

0.8

1.0

Abyssal peridotites

Back arc basin basalts

Forearc peridotites

10%

15%

25%

35%

Mg# in spinel

Cr#

in s

pine

l

0.850 0.875 0.900 0.925 0.9500

0.2

0.4

0.6

Mg# in Olivine

NiO

(w

t.%)

Forearc peridotites

0.00.96 0.94 0.92 0.90 0.88 0.86 0.84

0.2

0.4

0.6

0.8

1.0

Abyssal peridotites

Oceanic Supra-Subduction Zone peridotites

Partial melting

FMM

OSMA

10%

20%

30%

40%

Mg# in olivine

Cr#

in s

pine

lA

B

C

Sangsang

Fig. 4. A. Variations of NiO vs. Mg# in olivine from the Zhongba harzburgites. The fieldoutlining olivine compositions in forearc peridotites is from Ishii et al. (1992). B. Spinelcomposition in the Zhongba harzburgites. The fields for spinel in abyssal peridotites aretaken from Dick and Bullen (1984) and Juteau et al. (1990). The fields for forearcperidotites are from Ishii et al. (1992) and Ohara and Ishii (1998). The fields forSangsang are from Bédard, et al., 2009. The curve with ticks represents the percentageof partial melting of the parental peridotites as estimated from experimental studies(Hirose and Kawamoto, 1995). C. Compositional relationship between Cr# of spinel andMg# of olivine of the Zhongba harzburgites, compared with abyssal peridotites (Dickand Bullen, 1984), oceanic supra-subduction-zone peridotites (Pearce et al., 2000),olivine-spinel mantle array (OSMA) and themelting trend of Arai (1994). FMM is fertileMORB mantle. The plot shows that the Zhongba harzburgites plot both in the abyssalperidotite field and the oceanic suprasubduction peridotite field.

Abyssal peridotites

Forearc peridotites

0.00.80 0.85 0.90 0.95 1.00

2.0

4.0

6.0

Mg# in orthopyroxene

Sangsang

0.00.0 1.0 2.0 3.0 4.0 5.0 6.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Forearc peridotites

Abyssal peridotites

Forearc peridotites

Abyssal peridotites

Mg# in clinopyroxene

0.00.80 0.85 0.90 0.95 1.00

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

A

B

C

Al 2

O3

wt%

in o

rtho

pyro

xene

Cr 2

O3

wt%

in c

linop

yrox

ene

Cr 2

O3

in o

rtho

pyro

xene

Al2O3 in orthopyroxene

Fig. 5. A and B. Al2O3 (wt.%) vs. Mg# and Cr2O3 (wt.%) of orthopyroxenes from theZhongba harzburgites. The abyssal peridotite field is from Hamlyn and Bonatti (1980);Johnson et al. (1990); and Juteau et al. (1990); the forearc peridotite field is from Ishiiet al. (1992); the fields for Sangsang are from Bédard, et al., 2009. C. Cr2O3 (wt.%) vs.Mg# in clinopyroxene from the Zhongba harzburgites showing the abyssal peridotitefield from Hamlyn and Bonatti (1980); Johnson et al. (1990); and Juteau et al. (1990);and the foreare peridotite field from Ishii et al. (1992).


characterized by supra-chondritic Ru/Ir, Os/Ir, Pd/Ir and Rh/Pt, Pd/Pt,and near-chondritic Pt/Ir ratios. Rh/Ir ratios of our samples are higherthan those of carbonaceous condrites, but similar to those of ordinaryand enstatite chondrites (Fischer-Gödde et al., 2010). Pd/Ir values are

significantly high with an average of 1.76 times CI chondritic values(Fig. 9B; Fig. 10; Table 7).

6. Discussion

6.1. Partial melting

In general, modal mineralogy, whole rock chemistry, and mineralcompositions of peridotites are useful indicators for tracing the partialmelting history (e.g. Dick and Fisher, 1984; Uysal et al., 2007). Theclinopyroxene in these rocks is considered as the most rapidlyconsumedmineral during anhydrous partial melting, at least in spinelfacies lherzolitic peridotites (Jaques and Green, 1980). However,

40

41

42

43

44

45

46

47

MgO

SiO2 Al O2 3

MgO

0

0.3

0.6

0.9

1.2

MgO

0.0

0.4

0.8

1.2

1.6CaO

MgO

0

42 43 44 45 46 47 42 43 44 45 46 47 42 43 44 45 46 47

42 43 44 45 46 47 42 43 44 45 46 47 42 43 44 45 46 47

10

20

30

40

50

60V

MgO

100

105

110

115

120Co

MgO

2200

2300

2400

2500

2600Ni

Forearc harzburgites

Abyssal peridotites

Fig. 6. Whole rock MgO vs. Al2O3, CaO, SiO2, V, Co, and Ni diagrams for the Zhongba harzburgites. Major elements are on an anhydrous basis in wt.% and trace elements are in ppm.Field of abyssal peridotites is from Niu (2004); field of forearc peridotites is from Parkinson and Pearce (1998).


minor amounts of clinopyroxene may persist in the residue duringhydrous melting of spinel peridotite (Gaetani and Grove, 1998).Notably, peridotites can also be refertilized (Bezard et al., 2011). Thelow modal amount of clinopyroxene in the Zhongba harzburgitessuggests that these rocks have undergone high degrees of partialmelting.

The compositions of spinels are regarded as a useful tool forrevealing the partial melting process inmantle peridotites (e.g Bédardet al., 2009). The plot of Cr# and Mg# of spinel in these rocks suggestsa moderate to high degree of partial melting (13–24%) (Fig. 4B).Moreover, the Mg# of the olivine is also an indicator of the degree ofpartial melting, since olivine-melt equilibrium is not changedsubstantially by H2O (Gaetani and Grove, 1998). In the plot of spinelCr# vs. coexisting olivine Mg#, all the samples fall within the olivine-

10.0

0.01La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

0.1

1.0

Roc

k/C

hond

rite

South Sandwich Forearc Peridotite

Peridotite from Dazhuku

Boninites

and Bailang

Fig. 7. REE contents of the Zhongba peridotites normalized to chondritic values (Sunand McDonough, 1989). Compositional field for the south Sandwich forearc peridotitesis from Pearce et al. (2000); field for Dazhuka and Bainang peridotites is from Dubois-Côté et al. (2005). Boninite field is drawn from Hickey and Frey (1982).

spinel mantle array (OSMA) of Arai (1994), which is regarded asevidence for their residual origin after moderate to high degrees ofpartial melting (13–21%) (Fig. 4C). Moderate to high degrees of partialmelting are also consistent with their high Mg# and low Al2O3

contents in orthopyroxenes and clinopyroxenes (Tables 4 and 5).The whole-rock compositions of these rocks are consistent with

their highly refractory nature. The low Al2O3 and CaO abundances,combined with the moderately incompatible trace element concen-trations below primitive mantle values suggest that the Zhongbaharzburgites aremantle residues (Figs. 6–8; Table 6). The variations ofMgO vs. Al2O3, CaO, V, Co and Ni concentrations indicate that partialmelting exerted a significant control on the compositions of these

0.001

0.01

0.1

1.0

10.0

100.0

Izu-Bonin-Mariana forearc peridotites

Yushigou forearc peridotites

Roc

k/P

rimiti

ve M

antle

Cs Rb Ba Th U Nb La Ce Pr Pb Sr Nd Zr Eu Gd Tb Dy Ho Er Yb

Fig. 8. Primitive mantle-normalized trace element patterns of the Zhongba harzbur-gites. The normalization values are from Sun and McDonough (1989). Data of Izu–Bonin–Mariana forearc peridotites are from Parkinson and Pearce (1998); data of theYushigou forearc peridotites are from Song et al. (2009).

0.001Os Ir Ru Rh Pt Pd

Os Ir Ru Rh Pt Pd

0.01

0.1

0.0

1.0

2.0

3.0

(X/Ir

)NR

ock/

CI C

hond

rite

Luqu peridotites

Northwest Anatolian peridotites

Sumail peridotites

A

B

Luobusa peridotites

Average CI chondrite normalized value

Fig. 9. A. CI chondrite-normalized PGE abundances of Zhongba harzburgites, Thenormalization values are after McDonough and Sun (1995). The fields of NorthwestAnatolian peridotites are from Aldanmaz and Koprubasi (2006); the fields of Luquperidotites, Xigaze ophiolites from Chen and Xia (2008); both of them representsubduction zone peridotites; the fields of Sumail peridotites, Oman ophiolite fromLorand et al., 2009 and the lines of Luobosa from Becker et al., 2006. B. Average PGEconcentrations normalized to Ir and chondrite.


rocks (Fig. 6). The degree of partial melting can be better inferred fromthe HREE composition because these elements are considered to beonly slightly affected by metasomatic processes (Hellebrand et al.,2001). Fig. 11 shows the chondrite-normalized REE patterns of theZhongba harzburgites and the curves calculated for variable degreesof spinel facies fractional melting of N-MORB mantle (Piccardo et al.,2007). The modeling results suggest that ca.15–20% partial melting ofan N-MORB-like source produces a residue with HREE content similarto those of the Zhongba harzburgites (Fig. 11).

6.2. Behavior of PGE

The platinum group elements (PGE) are classified into two groupsaccording to their melting temperatures: the Iridium group (IPGE, Os,Ir, and Ru, melting temperature N2000 °C) and the Palladium group(PPGE, Rh, Pt and Pd, melting temperature b2000 °C) (Woodlandet al., 2002). In peridotites that have experienced high degrees ofpartial melting that removed most of the original base metalsulphides, the two groups occur within different mantle phases: theIPGE are thought to occur as discrete minerals or sulphides, alloys oroxides, often hosted within silicate grains, whereas the PPGE are mostlikely to occur as sulphides and alloys, often formed as interstitial

grains (Alard et al., 2000). The IPGE are significantly more compatibleand thus their abundances should increase in the residual mantle withhigh degrees of partial melting (e.g. Rehkämper et al., 1999a, 1999b;Lorand et al., 2000), while the PPGE are less compatible and thereforethe residue after melt extraction should have subchondritic Rh/Ir andPd/Ir ratios (e.g. Crocket, 1979; Barnes et al., 1985; Shirey andWalker,1998). For the PPGE, Pt should be more compatible than Pd (Pearsonet al., 2004), while the compatibility sequence of the IPGE remainsunclear (Becker et al., 2006).

The PGE patterns of the Zhongba harzburgites are characterized byOs, Ru and Rh, Pd enrichment relative to Ir and Pt (Fig. 9). The relativehigh Os and Ru concentrations may be explained by the high degree ofmelting, consistent with the major element and mineral composi-tions. These samples also have high Os/Ir ratios, similar to those ofsome depleted harzburgites reported by Becker et al. (2006). Apossible reason for this feature is the change in the partitioningbehavior of these elements at high degrees of partial melting (Beckeret al., 2006). An alternate explanation is analytical underestimation ofIr abundances due to a failure to dissolve insoluble Ir-group PGEalloys. However, the Ir abundance of reference material Gpt-3analyzed with our samples is consistent with the recommendedvalue (Table 7), arguing against an analytical artifact in the presentcase.

Partial melting can cause correlations between CaO and Pd/Ir (andRh/Ir), but in this case the Pd/Ir (and Rh/Ir) ratios would always bebelow, rather than above, the chondritic value (e.g. Lorand et al.,2003). Except for sample ZEOS-01-02, all of the other samples displaypositive correlations between Rh, Pd, Rh/Ir and Pd/Ir and CaO. The thePd/Ir (and Rh/Ir) ratios are above the chondritic value (Fig. 10). Thesecorrelations displayed by incompatible PGE with CaO and Al2O3 (notshown) indicate that the Zhongba harzburgites may have beenaffected by melt-rock interaction (e.g. Pearson et al., 2004; Fischer-Gödde et al., 2011). This interpretation is supported by the PGEpatterns of our samples, resembling those of Sumail harzburgites(Oman ophiolite) which are believed to be rejuvenated by melt-rockinteraction (Fig. 9A; Lorand et al., 2009). Although Rh abundances ofLuobusa peridotites were not reported by Becker et al., 2006, the restof the PGEs analyzed in that study also show patterns similar to thoseof our samples (Fig. 9A). As pointed out in the previous study by Zhouet al. (2005), the Luobusaperidotites probably underwent strongmelt-rock interaction. Therefore, the good correlations between incompat-ible PGE and lithophile elements and the high Rh/Ir and Pd/Ir ratioscannot be simply explainedby a partialmeltingprocess alone. They areconsidered to have resulted from secondary processes, the mostplausible being melt-rock interaction in the mantle (e.g. Luguet et al.,2001, 2003; Aldanmaz and Koprubasi, 2006; Chen and Xia, 2008).Melt-rock interaction could modify the depleted residues either bypercolation of sulfide melt or by refertilization by silicate melts(Luguet et al., 2003; Becker et al., 2006; Fischer-Gödde et al., 2011).

6.3. Melt-rock interaction

Traditionally, harzburgites have been regarded as the depleted,refractory residues produced by partial melting in the mantle (e.g.Himmelberg and Loney, 1973). However, various observationsconcerning the field, textural and whole rock geochemical featuresof the Zhongba harzburgites indicate that they have also undergonemelt-rock interactions. The occurrence of minor dunites has beenidentified within the harzburgites (Fig. 2B). The formation of dunitesin the mantle sections of many ophiolites is explained by theinteraction between infiltrating melts and upper mantle peridotites(e.g. Quick, 1982; Edwards and Malpas, 1995; Kelemen and Dick,1995), and this process results in the selective dissolution of pyroxeneand precipitation of olivine. Furthermore, numerous mineralogicaland textural characteristics indicative of melt-rock interactions havebeen identified within our samples, such as: 1) high-temperature

1.0

1.5

2.0

2.5

3.0

3.5

0.280.5 0.7 0.9 1.1 1.3 1.5

0.5 0.7 0.9 1.1 1.3 1.5 0.5 0.7 0.9 1.1 1.3 1.5

0.5 0.7 0.9 1.1 1.3 1.5

0.30

0.32

0.34

0.36

0.38

0.40

0.42

6

7

8

9

10

11

12

1.0

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1.5

1.6

CaO (wt.%) CaO (wt.%)

Rh

(ppb

)

Pd

(ppb

)

CaO (wt.%) CaO (wt.%)

ZEOS-01-02ZEOS-01-02

ZEOS-01-02

ZEOS-01-02

CI chrondritic ratio CI chrondritic ratio

Pd/

Ir

Rh/

Ir

Fig. 10. Correlation of CaO versus Rh, Pd, Rh/Ir, and Pd/Ir ratios for the Zhongba harzburgites. The incompatible PGEs show good correlation with lithophile elements (e.g. CaO andAl2O3), indicating melt-rock interaction processes. Rh/Ir and Pd/Ir of CI chondrite are from McDonough and Sun (1995).


recrystallization of olivine neoblasts (Fig. 3B); 2) olivine embaymentspartly corroding the orthopyroxene porphyroclast (Fig. 3D); 3)orthoproxene and spinel symplectite (Fig. 3F); and 4) olivine asinclusions in the spinels (e.g.Edwards and Malpas, 1995; Hellebrandet al., 2002; Zhou et al., 2005; Bédard et al., 2009; Dilek and Morishita,2009).

The Zhongba harzburgites have well-developed, U-shaped REEpatterns indicating significant LREE enrichment following partial

10.0

0.001La Ce Pr Nd Sm EuGd Tb Dy Ho Er Tm Yb Lu

0.1

1.0

Roc

k/C

hond

rite

0.01

1%3% 7%

10%

15%

17%

20%

23%

25%

N-MORB Mantle

REE patterns of Zhongba Harburgites

Fig. 11. Partial melting modeling based on whole rock REE abundances of the Zhongbaharzburgites. The REE abundance curves calculated for variable degrees of spinel faciesfractional melting of N-MORB mantle are after Piccardo et al. (2007). HREE abundancesof the Zhongba harzburgites suggest ~15–20% partial melting.

melting. Their origin solely as a residuum from partial melting ofprimitive mantle is not consistent with the observed high LREE/MREEratios of these peridotites (e.g. Prinzhofer and Allègre, 1985; Freyet al., 1991). Prinzhofer and Allègre (1985) proposed the sequentialintegrated disequilibrium melting model beginning in the garnetlherzolite facies and ending in the plagioclase facies to explain the U-shaped patterns of the New Caledonian peridotites. However, ourmodeling result shows that the HREE concentrations in our samplescan be reproduced by mantle melting in the spinel-stability fieldwithout requiring residual garnet in the mantle (Fig. 11; Piccardoet al., 2007). Another explanation for the LREE enrichments is that themantle peridotites were contaminated through percolation of a low-temperature continental fluid (Sharma and Wasserburg, 1996; Gruauet al., 1998). However, the HFSEs (Th, Nb, Zr, and U), which arecommonly considered immobile during low-temperature alteration,also show relative enrichment in the primitive mantle-normalizedspidergram (Fig. 8). In general, the U-shaped REE patterns are nowbelieved to reflect partial melting coupled with melt-rock interactionas observed in other ophiolitic mantle peridotites worldwide (e.g. Xiaet al., 2003; Dubois-Côté et al., 2005; Zhou et al., 2005; Polat et al.,2006; Song et al., 2009; Caran et al., 2010; Ulrich et al., 2010). Thecharacteristics of melt-rock interaction in these samples are alsoconsistent with their PGE patterns displaying strong Pd enrichments(Fig. 9A).

The melt that reacted with the Zhongba harzburgites to generatethe U-shaped REE patterns and the enrichment of Rh and Pd isunlikely to be MORB magma, because such magmas have LREEdepleted chondrite-normalized REE patterns. However, boninitescommonly possess U-shaped chondrite-normalized REE patterns(e.g. Crawford, 1989; Hickey and Frey, 1982; Varfalvy et al., 1997;


Fig. 7), resembling those of the Zhongba harzburgites. This suggeststhat a boninitic melt could be the potential candidate which reactedwith the harzburgite and produced the U-shaped REE patternsobserved in this study. Moreover, boninitic lavas were generallyfound to have high Pd and low Ir contents (Hamlyn et al., 1985),which is consistent with the high Pd contents and the high Pd/Ir ratiosin our samples (Fig. 9). Boninitic lavas have been reported from theXigaze ophiolite in the central part of the YZSZ (Chen et al., 2003).

6.4. Tectonic setting

The compositions of mineral phases in host peridotites areconsidered as powerful petrogenetic indicators and thus can be usedto reveal their tectonic settings (e.g.Hébert et al., 2003; Bédard et al.,2009). In the Mg# vs. NiO of olivine plot, most of the Zhongbaharzburgites fall within the field of modern forearc peridotites(Fig. 4A; Ishii et al., 1992). The Mg# and Cr# of spinel fall near theintersection of the fields of modern abyssal peridotites (Dick andBullen, 1984; Juteau et al., 1990) and forearc peridotites (Fig. 4B; Ishiiet al., 1992; Ohara and Ishii, 1998). Moreover, in a plot of the Cr# ofspinel against the Mg# of olivine, the compositions of our samples areconsistent with those of both abyssal peridotites (Dick and Bullen,1984) and intra-oceanic subduction zone peridotites (Fig. 4C; Pearceet al., 2000). Similarly, the orthopyroxene and clinopyroxene com-positions of our samples are comparable to those of both abyssalperidotites and foreare peridotites (Fig. 5; Hamlyn and Bonatti, 1980;Johnson et al., 1990; Juteau et al., 1990; Ishii et al., 1992). Therefore, itseems that the Zhongba harzburgites have hybrid mineral composi-tions intermediate between those of forearc and abyssal peridotites.

The Zhongba harzburgites also have hybrid whole-rock composi-tions. In the MgO vs. Al2O3, CaO, SiO2, V, Co, and Ni diagrams (Fig. 6),most of these samples plot within both the abyssal peridotite field(Niu, 2004) and the forearc peridotite field (Parkinson and Pearce,1998). In the chondrite-normalized REE diagram, the REE patterns ofthe Zhongba harzburgites are similar to those of boninites (Fig. 7;Hickey and Frey, 1982). Thus, infiltration of LREE-enriched boniniticmelt is inferred to have produced the U-shaped REE patterns of theZhongba harzburgites. Generally, the ideal tectonic setting forgeneration of a boninitic melt is the mantle wedge beneath a forearcregion in a subduction zone during the initiation of subduction (e.g.Crawford et al., 1981; Hawkins et al., 1984; Crawford, 1989;Dilek et al.,2007; Dilek et al., 2008; Dilek and Furnes, 2009; Dilek and Thy, 2009;Pearce and Robinson, 2010; Wong et al., 2010). In addition, their REEpatterns also resemble those of modern South Sandwich forearcperidotites (Pearce et al., 2000) and the fossil forearc peridotites fromDazhuka and Bailang, central YZSZ (Fig. 7; Dubois-Côté et al., 2005). Inthe primitive mantle-normalized spidergram, the studied rocks arevery similar to the modern depleted forearc peridotites from Izu–Bonin–Mariana arc (Parkinson and Pearce, 1998) and the fossil forearcperidotites from Yushigou, north Qilian (Fig. 8; Song et al., 2009).Furthermore, in the CI chondrite-normalized PGE plot, their PGEpatterns are similar to those of Northwest Anatolian peridotites(Aldanmaz and Koprubasi, 2006) and Luqu peridotites from centralYZSZ (Chen and Xia, 2008), both of which represent fossil forearcperidotites (Fig. 9A). These characteristics indicate that the Zhongbaharzburgites were formed in the forearc mantle wedge above a supra-subduction zone.

The hybrid character of the mineral chemistry and whole-rockcomposition of the Zhongba harzburgites indicate that they wereprobably formed by a two-stage process: 1) derivation from aresidual MORB mantle that experienced 13–24% partial melting; and2) subsequent modification by interaction with boninitic melts withinthe mantle wedge. In the first stage, the Zhongba harzburgites weredepleted (e.g. low Al2O3 and CaO) by melt extraction, probably inthe MORB mantle. The high degree of melting also contributed tothe relatively high Os and Ru concentrations. In the second stage,

the abyssal peridotites were trapped in the mantle wedge abovean intra-oceanic subduction zone. Because Rh, Pt and Pd aremobilizedin the mantle wedge above a subduction zone (e.g. McInnes et al.,1999; Aldanmaz and Koprubasi, 2006), there they were modified byboninitic melts, resulting in the enrichment of LREE and incompatiblePGE (especially Pd). Furthermore, the dunites were formed bydissolution of pyroxene from the harzburgites (e.g.Bédard and Hébert,1998). Therefore, the most favorable tectonic setting for the Zhongbaharzburgites is a forearc above an intra-oceanic subduction zone, verysimilar to the modern active intra-oceanic subduction systems in theWestern Pacific and in the South Atlantic (Parkinson and Pearce,1998; Pearce et al., 2000; Reagan et al., 2010).

This tectonomagmatic evolution of the Zhongba harzburgitesbasically resembles that of the western Neo-Tethyan realm (theMirdita and the Kizildag; Dilek et al., 2007, 2008; Dilek and Furnes,2009; Dilek and Thy, 2009). Based on structural, petrological, andgeochemical studies, it has been proposed that most of the westernNeo-Tethyan ophiolites were generated in the infant arc-forearcregion above an intra-oceanic subduction zone. According to thismodel, in the initial stages of subduction, MORB-like magmas wereproduced from partial melting of relatively hot, depleted peridotitesin the upper plate. Subsequent retreat and sinking of the subductingslab resulted in asthenospheric upwelling and corner flow beneaththe forearc mantle, and finally facilitated shallow partial melting ofthe previously formed highly depleted harzburgites, generating theboninitic magmas (Dilek et al., 2007, 2008; Dilek and Furnes, 2009;Dilek and Thy, 2009; Dilek and Furnes, 2011). These boninitic meltspercolated and interacted with depleted harzburgites, which led tothe addition of LREE and incompatible PGE.

6.5. Implications for the evolution of the Neo-Tethys in southern Tibet

Several previous studies have also recognized two-stage evolutionof the ophiolitic peridotites along the Yarlung Zangbo Suture Zone,such as: 1) the Luobusa peridotites, eastern YZSZ (Zhou et al., 2005);2) the Xigaze peridotites, central YZSZ (Xia et al., 2003; Dubois-Côtéet al., 2005); and 3) the Yungbwa peridotites, western YZSZ (Liu et al.,2010). Although the two-stage evolution of the YZSZ mantleperidotites might record both opening and closing of the Neo-Tethys(e.g. Liu et al., 2010), only the second stage representing the initiationof intra-oceanic subduction in the Neo-Tethys could be well preserved.In fact, intra-oceanic subduction was first proposed by Allègre et al.(1984), and themodelwas further developed by Aitchison et al. (2000),Wang et al. (2000) and Hébert et al. (2003). It has also been proposedthat two or more intra-oceanic subduction zones existed within theNeo-Tethys (e.g.Hébert et al., 2003; Dubois-Côté et al., 2005; Bédardet al., 2009; Guilmette et al., 2009). However, the dynamics anddimension of this subduction system have not been considered so far.

Based on an integration of the geological and geochemical data aswell as paleontological and radiometric ages from ophiolitic massifsalong the whole YZSZ, we propose a geodynamic model for the originof these ophiolites involving the following stages: 1) during ~150–160 Ma, the LBS (Zhou et al., 2002, 2005; Zhong et al., 2006) and theZD (McDermid et al., 2002), in the eastern part of the YZSZ, began todevelop over an intra-ocean subduction zone in the eastern Neo-Tethys; 2) During ~120–130 Ma, the DZK (Malpas et al., 2003; Xiaet al., 2003; Ziabrev et al., 2003), the BL (Guilmette et al., 2008;Guilmette et al., 2009; Li et al., 2009), the SS and the SG (Xia et al.,2008b; Bédard et al., 2009), comprising the central part of the YZSZ,and the ZB, the DQ and the XGGB (Wei et al., 2006b; Bezard et al.,2011), and the YBW (Miller et al., 2003; Li et al., 2008; Liu et al., 2010),forming the western parts of the YZSZ, developed after the initiationof the intra-oceanic subduction (Fig. 12). All of these observationssuggest that a wide and continuous intra-oceanic subduction zonedeveloped along the whole YZSZ during the Late Jurassic and EarlyCretaceous, consistent with previous conclusions that an intra-

94°E 96°E 98°E92°E90°E88°E86°E84°E82°E80°E

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India

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Andaman

177±31 Ma (Sm-Nd, Zhou et al., 2002)162.9±2.8 Ma (U-Pb, Zhong et al., 2006)Peridotites formed in two stages: Originally MORB source;SSZ, mantle wedge (Zhou et al., 2005)

0 200

Between 152 and 156 Ma

(McDermid et al., 2002)

Intra-oceanic magmatic arc(Ar-Ar and U-Pb)

LBSZDDZKBL

126 ±1.5 Ma (U-Pb, Malpas et al., 2003)

From late Barremian to late Aptian

(Radiolarian,Ziabrev et al., 2003)SSZ (Xia et al., 2003)

125.6±0.8 Ma (U-Pb, Li et al., 2009)

Intraoceanic subduction zone

Guilmette et al. (2008, 2009)

SSSG

125.2±3.4 Ma (U-Pb, Xia et al., 2008b)

Intraoceanic suprasubduction zone

(Bédard et al., 2009)

125.7 ±0.9 Ma (U-Pb, Dai et al., in revision)

Intraoceanic subduction zone (This study)

122.3±2.4 Ma (U-Pb, Wei et al., 2006b)

DQXGGB

SSZ (Bezard et al., 2011)

147±25 Ma (Sm-Nd);152±33 Ma (Ar-Ar)(Miller et al., 2003)

120.2±2.3 Ma (U-Pb, Li et al., 2008)

Peridotites formed in two stages:

YBW

At MOR; and then at SSZ (Liu et al., 2010)

32°N

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92°E90°E88°E 94°E

Myanmar jadeitite:163-122 Ma (U-Pb)Mesozoic intra-subduction zone (Shi et al., 2009)

Sapat peridotite in Kohistan U-Shaped REE, SSZ(Bouilhol et al., 2009)

Cretaceous Intra-oceanicIsland arc(Khan et al., 1997)

Ophiolites

Kohistan

Karakoram

km

ZB

Fig. 12. A summary of the recent geochronologic and geotectonic data on the YZSZ region, showing the ages and the tectonic settings of the various ophiolite bodies (see text fordetails and references). The ophiolite outlines are modified from Pan et al. (2004), Khan et al. (1997) and Shi et al. (2009).


oceanic subduction systemwas active within the Neo-Tethys betweenthe Indian plate and Lhasa block (e.g. Aitchison et al., 2000; Allègre etal., 1984; Bédard et al., 2009; Dubois-Côté et al., 2005; Guilmette et al.,2009; Hébert et al., 2003; Fig. 12). However, the possibility of two ormore distinct intra-oceanic subduction zones cannot be precluded.

Furthermore, intra-oceanic subduction zones have also beenreported from the eastern and western continuation of the YZSZ.Based on the U–Pb ages and the Lu–Hf isotope signatures of zircons injadeitite from North Myanmar, Shi et al. (2009) proposed that thepresence of a Mesozoic (163–122 Ma) intra-oceanic subduction zonewithin the Indo-Burman Range was part of the same Neo-Tethystectonic regime. In the western part of the YZSZ, the Kohistan island

Australia

Great India

Eurasia

Lhasa

Burma

Zhedang arc

30°

Paleo-Equator

30°30°S

Early Cretaceous

(125-130 Ma)

Kohistan arc

Neo-Tethys

Karakorum

Continent

Subduction zone

Dazhuka

A

Fig. 13. A. Generalized plate tectonic reconstruction of the Early Cretaceous (modified from Ssuprasubduction system within the Neo-Tethys. The location of the Kohistan arc is after KDazhuka ophiolite (Abrajevitch et al., 2005). B. Geographic map of the western Pacific and1992). Our reconstructed tectonic setting is similar to the modern active intra-oceanic supr

arc is considered to be a Cretaceous intra-oceanic arc between Indiaand Karakoram (e.g. Khan et al., 1997; Bignold et al., 2006; Bouilholet al., 2009). Therefore, it is inferred that the intra-oceanic subductionzone extended eastward between India and Burma, and westwardbetween India and Karakoram at least during the Early Cretaceous.In addition, paleomagnetic data from Early Cretaceous rocks indicatethat the Dazhuka ophiolite was formed close to the equator(Abrajevitch et al., 2005), while the paleomagnetic and isotopic ageindicate that the position of the Kohistan island arc was south of theequator during the Cretaceous (Khan et al., 1997). These observationsallow us to reconstruct the paleogeography of the Neo-Tethys duringthe Early Cretaceous, which reveals a complex intra-oceanic

N

S

West Philippine Basin

Taiw

anLu

zon

Min

dana

o

crA nino

B-uzI Bon

in T

renc

hB

onin

Isla

nd

Mar

iana

Tr

ench

Arc

Mar

iana

Wes

tM

aria

naR

idge

Pal

auK

yush

uR

idge

Parece

BasinVela

Ryuky

u Arc

25°N

35°N

15°N

25°N

35°N

15°N

05°N 05°N

120°E 130°E 140°E

120°E 130°E 140°E 150°E

200 km

B

chettino and Scotese, 2001) of the eastern Neo-Tethys, showing the wide intra-oceanichan et al. (1997); the location of the Zhedang arc is inferred from the location of theSoutheastern Asia showing the subduction zones (modified from Stern and Bloomer,asubduction systems in the Western Pacific.


subduction system (Fig. 13A), resembling modern active intra-oceanic subduction systems in the Western Pacific (Fig. 13B). Thissubduction zone plunged northward, consuming most of the Neo-Tethyan lithosphere. This tectonic reconstruction is confirmed bytomographic images which reveal the presence of ancient subductedslabs beneath India (Van der Voo et al., 1999), and is also consistentwith evidence from radiolarian data (Baxter et al., 2010).

7. Conclusion

The Zhongba ophiolite mantle is dominated by harzburgites withminor dunites. The harzburgites are depleted in magmaphile majorelements, as shown by their low contents of Al2O3 and CaO. Themineral and whole-rock geochemistry indicates that these rocks areresidues left by ~13–24% of partial melting of a MORB-type mantlesource. The harzburgites are characterized by U-shaped, chondrite-normalized REE patterns and fractionated chondrite-normalized PGEpatterns with supra-chondritic ratios of PdN/IrN. These characteristics,as well as their hybrid mineral and whole-rock compositions, do nottally with simple in situ melt extraction, but probably reflect melt-rock interaction resulting in the selective enrichment of LREE and Pd.A two-stage evolution is proposed to explain the generation of theZhongba harzburgites: (1) initial formation by melting of MORB-source upper mantle; and (2) entrapment within the mantle above asubduction zone. New zircon U-Pb ages (125.7±0.92 Ma; Dai et al., inrevision) from the diabase dike occurring close to the harzburgitessuggest that the subduction zone should have been active during theEarly Cretaceous.

Comparable observations from the other ophiolitic massifs alongthe whole Yarlung Zangbo Suture and the similar ages of initiation ofsubduction allow us to propose that an extensive and complexsubduction system operated between India and the Lhasa terrane,Burma, and the Karakoram microcontinent within the Neo-Tethysduring the Early Cretaceous, similar to the modern active intra-oceanic subduction systems in the Western Pacific.

Acknowledgments

We appreciate Dr. YildirimDilek, Dr. Harry Becker, one anonymousreviewer and Editor Laurie Reisberg for their thorough and construc-tive comments. We thank Hanting Zhong, Yushuan Wei for theirassistance with the fieldwork and Raoult Guillaume, Rachel Bezard fortheir assistance with the mineral analyses andWenjun Qu for his helpwith PGE analyses. This study was supported by the FundamentalResearch Funds for the Central Universities (Project 2009PY03), 111Project of China Grant (Project B07011), and China Geological Survey(Project 12112011086037). Réjean Hébert benefited from NationalScience and Engineering Research Council of Canada (Grant No. 1253).

Appendix A. Supplementary data

Supplementary data to this article can be found online at doi:10.1016/j.chemgeo.2011.07.011.

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Petrology and geochemistry of peridotites in the Zhongba ......Petrology and geochemistry of...

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