the tectonic-setting of ophiolites in the western qinghai-tibet plateau, china

Journal of Asian Earth Sciences 29 (2007) 215–228

www.elsevier.com/locate/jaes

The tectonic-setting of ophiolites in the western Qinghai-Tibet Plateau, China

R.Z. Qiu a,b,¤, S. Zhou c, T.D. Li b, J.F. Deng c, Q.H. Xiao d, Z.X. Wu e, Z.Y. Cai f

a Development and Research Center, China Geological Survey, Beijing 100037, Chinab Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

c Key Laboratory of Lithospheric Tectonics, Deep-level Process and Exploration Ministry of Education, China University of Geosciences, Beijing 100083, China

d Information Center of Ministry of Land and Resources, Beijing 1000812, Chinae Institute of Geology, State Seismological Bureau, Beijing 100029, China

f Exibei Institute of Geology and Mineral Resources of Hubei Geoexploration Bureau, Xiangfan 441003, China

Received 4 January 2004; accepted 21 June 2006

Abstract

The study of geology, geochemistry, rare earth elements, trace elements, Pb and Sr isotopes of representative ophiolite bodies fromfour ophiolitic belts in the western Qinghai-Tibetan Plateau, shows that the mantle peridotites of these ophiolites are mainly harzburgitein composition, with minor dunite. They are characterized by high magnesium (MgO) and low aluminum, calcium and alkali oxide con-tents. Enrichment of light rare earth elements in mantle peridotites may be due to two geological processes: relatively strong partial melt-ing; and later metasomatism by the liquids released during the subduction of oceanic crust. Mantle peridotites are characterized by lowcontents of the trace elements Sr, Ti and Y and relatively high contents of Rb, Nb, Zr, Hf and Th, similar to metasomatic pyrolite. Theisotopic compositions of Sr and Pb show evidence of contamination by a crustal component. All the evidence indicates that the four ophi-olite belts in the western Qinghai-Tibetean Plateau have undergone metasomatism by liquids released during the subduction of oceaniccrust, suggesting that they were formed in a supra-subduction zone (SSZ) tectonic setting. The mantle peridotites in ophiolite belts locatedin eastern Qinghai-Tibetan Plateau, e.g. Sanjiang and West Kunlun, may be compared with the Troodos, which is regarded as a typicalSSZ complex, having the same geochemical characteristics, i.e. high MgO and LREE-rich. The geochemistry, combined with the occur-rence of boninite and adakite rocks, which are associated with subduction magmatism, suggest that ophiolites formed at diVerent times inQinghai-Tibetan Plateau, including Sanjiang and West Kunlun, are all SSZ-type ophiolites formed in a supra-subduction zone tectonicsetting.© 2006 Elsevier Ltd. All rights reserved.

Keywords: Ophiolite; Geology; Geochemistry; Tectonic setting; SSZ type ophiolite; Western Qinghai-Tibetan Plateau

1. Introduction

Ophiolites exposed widely in the Qinghai-Tibetan Pla-teau, provide the most important petrologic record of theevolution of Tethys during the Mesozoic. However, mostprevious research has been directed at the eastern part of

* Corresponding author. Tel.: +86 10 6230 2995; fax: +86 10 6230 3002.E-mail address: [email protected] (R.Z. Qiu).

1367-9120/$ - see front matter © 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.jseaes.2006.06.007

the area (Wang et al., 1987). The detailed geology of thewestern area is still poorly known compared with otherareas of China. The ophiolite complexes and their distribu-tion in the western part of the area have now been outlined(Guo et al., 1991; Bureau of Geology and MineralResources of Xizang Autonomous Region, 1993), but thegeochemistry of only a few of these complexes has beenstudied (Guo et al., 1991; Deng, 2000; Xia et al., 1998).

Based on detailed geological mapping and the study ofmajor, trace and rare earth elements and some Sr, Pb, Nd

mailto: [email protected]

mailto: [email protected]

216 R.Z. Qiu et al. / Journal of Asian Earth Sciences 29 (2007) 215–228

isotopes from typical ophiolites in the western Qinghai-Tibetan Plateau, a geotectonic setting is proposed for theirformation. Combining ophiolite data from the eastern partof the Qinghai-Tibetan Plateau with that from the Sanjiangand West Kunlun ophiolites, the ophiolite type, environ-ments of formation and ages are discussed brieXy in thispaper.

2. Geological characteristics and rock assemblages

The part of western Qinghai-Tibetan Plateau covered bythis paper is the area of Tibet Province between longitude88°E and the western boundary of China (Fig. 1). FourMesozoic ophiolite belts on diVerent scales are exposed inthis area. On a large-scale the Bangong-Nujiang and Yar-lung Zangpo ophiolites form the boundaries to the majortectonic units in Tibet. The Bangong-Nujiang ophioliteseparates the Qiangtang and Gangdese terranes and theYarlung Zangpo ophiolites mark the collision suturebetween the Indian Plate and Asian continent. Smaller scaleophiolite belts are exposed on the southern side of each ofthe major belts. The Shiquanhe Ophiolite Belt lies to thesouth of the Bangong-Nujiang Ophiolite. Some authorscombine the Bangong-Nujiang and the Shiquanhe Ophio-lite Belt as the North Tibet Ophiolite Belt (Deng and Wang,1987; Zhang and Zhou, 2001; Xiao and Li, 2000). ThePulan-Daba Belt lies to the south of the western segment ofthe Yarlung Zangpo Ophiolite Belt. For simplicity, we referto the complexes studied in this paper, from north to south,as the west segment of Bangonghu-Nujiang, the Shiquanhe,the west segment of Yarlung Zangpo and the Pulan-Dangqiong ophiolite belts of the western Qinghai-TibetanPlateau.

2.1. Bangonghu-Nujiang Ophiolite Belt

The Bangonghu-Nujiang Ophiolite Belt is 2400 km longand has an obvious geophysical expression (Xiao andLi, 2000). It consists of three segments from west to east,the Bangonghu-Gaize, Dongqiao-Anduo and Dingqing-Nujiang segments. The western segment of the Bang-onghu-Nujiang Ophiolite Belt includes about 35 exposedmaWc-ultramaWc bodies showing very strong serpentiniza-tion (Guo et al., 1991; Bureau of Geology and MineralResources of Xizang Autonomous Region, 1993), concen-trated mostly in the Bangonghu and Gaize areas. Someophiolite bodies are altered entirely to serpentinite or toserpentine schist, due to intense shearing, and areassociated with mélanges consisting of fragments ofmetamorphosed ophiolitic rocks, basic lava, gabbro,sandstone, limestone and siliceous rocks. Completeophiolite sequences are not found, but each member ofthe ophiolite suite can be found in diVerent segments of theophiolite belt. In a complete individual ophiolite complexthe sequence from base to the top includes metamorphicmantle peridotite, maWc-ultramaWc crystal-cumulates,sheeted diabase dykes, maWc lavas (pillow lavas occurlocally) and radiolarian cherts. The ophiolitic bodies in thewestern belt consist mainly of metamorphic peridotites,including harzburgite, dunite and lherzolite. Peridotitic andgabbroic cumulates are relatively widely distributed in theGaize area. Diabase sills or dyke swarms are of limitedoccurrence, and are found only at Dongjiri on the east bankof Bangong Lake and in the Jiegela and Qushenla areas.Pillow basalts are generally absent towards the western endof the belt, but occur at the eastern end at Shemalagou inthe Qushenla area. Gabbroic magmatism is generally

Fig. 1. Outline map to show the main crustal blocks, suture zones and faults in the area of the Qinghai-Tibet Plateau.

R.Z. Qiu et al. / Journal of Asian Earth Sciences 29 (2007) 215–228 217

regarded as a product of lithospheric extension (Qiu et al.,2002). A Sm–Nd isochron age of gabbro from Shemalagouophiolite gave 191§22 Ma (Qiu et al., 2002), suggestingthat the ocean basin in western segment of Bangong-Nuji-ang Ophiolite Belt opened in the Early Jurassic. Most of theophiolite bodies have tectonic contacts with the countryrocks on their southern sides. In some places the ophiolite iscovered unconformably by the Late Cretaceous (K2)Yuduo Formation, which Diplaraea sp. and Toucoria sp.also have been found, so that the oceanic basin must haveclosed by the end of the Cretaceous (K1) (Guo et al., 1991).

2.2. Shiquanhe Ophiolite Belt

The Shiquanhe Ophiolite Belt consists of nine bodies,Jiagang, Lameila, Shiquanhe, etc. The belt, about 470 kmlong, extends eastwards to Jiangma in Cuoqin County.Compared with the Bangonghu Ophiolite Belt, the Shiqu-anhe Ophiolite Belt is on a relatively small scale. It waspreviously regarded as a subbelt of Bangonghu-NujiangOphiolite Belt. The north and central igneous belts of LateYanshanian (Jurassic-Cretaceous) age in Gangdese arelocated on the southern sides of the Bangonghu andShiquanhe Ophiolite Belt, respectively. Although no otherophiolites have been found in the middle segment towardsthe east, middle Gangdese igneous arc rocks continue east-wards, so it is possible that the Shiquanhe Ophiolite Beltcould be connected to the Qielihu and Lanong bodies toform a large-scale ophiolite belt, more than one thousandkilometers long. Since the Shiquanhe has similar contactrelationships to the Bangonghu ophiolite (being coveredunconformably by the Cretaceous Yuduo Formation (K2)),and the middle Gangdese igneous belt has the same rangeof ages, 75–133.6 Ma, it is believed that the ocean basinrepresented by these ophiolites had closed by the end of theEarly Cretaceous (K1) (Qiu, 2002).

2.3. West segment of Yarlung Zangpo Ophiolite Belt

The Yarlung Zangpo Ophiolite Belt, composed of west-ern, central and eastern segments, is about 1500 km long.Small ophiolite bodies generally 10 km long and 1–2 kmwide are distributed sporadically in the western segment ofthe Yarlung Zangpo Belt. Most of them are strongly ser-pentinized. The ophiolites do not form complete sequences.The most complete sections consist of one to three units,with tectonic contacts between them. The main rock-typesare harzburgite, dunite, layered gabbro, pillow lava andchert.

Gabbros from key segments of the ophiolite suite havegiven Ar–Ar, Sm–Nd isotopic ages of 176–180 Ma (Zhou,2002), suggesting that the ocean basin opened in the EarlyJurassic. A relatively complete ophiolite sequence is foundin Rikangba, including metamorphic mantle peridotite, pil-low lava, radiolarian chert and Xysch.

The newest evidence is the 40Ar/39Ar age of 64.43 Maobtained from the lower volcanic rocks of Linzizong

Formation at the east segment of the Yarlung Zangpo Belt(Zhou et al., 2004), which is in accord with the regionalunconformity between the Linzizong Formation and theage of the Cretaceous Shexing Group, and is regarded asindicating the initiation of India-Eurasia collision.

Since the upper part of the ophiolite is covered uncon-formably by conglomerate, sandstone and limestone ofmiddle Eocene marine facies, it is suggested that the oceanbasin must have closed by the end of the Late Cretaceous.

2.4. Pulan-Dangqiong Ophiolite Belt

An ophiolite complex is located in the North Himalayatectonic belt from Zada to Pulan and Dangquezangbu,characterized by large exposed ophiolitic bodies, such asDaba, Dangqiong, Woerbacuo and Xiugugabu etc. (Fig. 2).They have structural contacts with Triassic or Jurassic-Cre-taceous rocks. The diVerent bodies have diVerent rocktypes: the Dangqiong Ophiolite is mainly pyroxenite-peri-dotite, with some peridotite and dunite, with gabbro anddiabase as dykes or dyke swarms on its southern side; thePulan Ophiolite consists of serpentinized harzburgite at thecenter and serpentinite at the margins. The lithologicalassemblage of this ophiolite belt is relatively unique beingcomposed mainly of mantle peridotite, comprising harz-burgite, gabbro and dunite. The age of formation of thisophiolite may be older than the ophiolites of the YarlungZangpo Belt (Guo et al., 1991). Based on the associatedrocks oceanic crust may have been formed in the Late Tri-assic Period, and the ocean closed in the Early CretaceousPeriod. The closure of the oceanic basin may have occurredat the same time as the closure of the Yarlung ZangpoSuture, i.e. by the end of the Late Cretaceous.

The above characteristics indicate that the ophiolitebelts exposed in the western part of the Qinghai-TibetanPlateau are fully comparable: (1) the ophiolites have beendismembered, with the separation of the various parts; (2)only the lower units can be clearly observed, and most ofthe upper units of an ophiolite suite are absent; (3) cumu-late crystalline rocks, gabbro and diabase dyke swarms areonly observed in a few places. The lowest unit of the ophio-lite suite, i.e. mantle peridotite, consists mainly of harzburg-ite, some of which shows a high degree of depletion. Theages of formation of the ophiolite belts are diVerent, theBangonghu and Shiquanhe ophiolite belts were formed bythe end of the Early Cretaceous, and the western segment ofthe Yarlung Zangpo and the Pulan-Dangqiong OphioliteBelt were formed in the Late Cretaceous.

3. Petrochemistry and geochemistry

Representative ophiolite bodies from each ophiolite beltwere chosen for study: Chalamula in the Bangonghu Belt;Siquanhe in the Shiquanhe Belt; Rikangba and Dajinsi inthe western segment of the Yarlung Zangpo Belt, Anren inmiddle; the Pulan and Dangqiong bodies in the Pulan-Dangqiong Belt; the Dongjiri gabbros on the northern side


of Bangonghu; and Dangqiong gabbro near the southernside of the Dangqiong body in the Pulan-Dangqiong Belt.Detailed sampling sites are shown in Fig. 2. Pb and Sr iso-topes have been determined for some of these samples.

3.1. Petrochemistry

Analytical results for fresh samples of mantle perido-tite and gabbro are listed in Table 1. Table 2 shows thestatistical results of petrochemical analysis from 112 sam-ples from the ophiolite belts in the western part of theQinghai-Tibetan Plateau. For convenience of comparison,because most of ophiolites are serpentinized to variousextents, the data listed in Tables 1 and 2 are the dry com-ponent data, from which the volatiles have been deducted.Compared with average values from ophiolite belts(Table2), most of the analytical results from the mantle perido-tite (Table 1) are consistent, indicating that the bodiesselected are representative of the ophiolite belts as awhole. Based on CIPW NORM calculations, clinopyrox-ene is 0–9.97% and orthopyroxene is 10.8–43.8%, and the

majority of the samples plot in the harzburgite Weld in theclassiWcation diagram (Fig. 3). These results are consistentwith the earlier research results from the Ngari area, fromlatitude 88 °E to the boundary of Tibet, adjacent to thepresent work (Guo et al., 1991). The mantle peridotiteis comprised mainly of harzburgite, with subordinatelherzolite (Table 2).

The major-element chemistry of the mantle peridotitesfrom the four ophiolite belts is similar (Table 1). Comparedwith the composition of pyrolite (Ringwood, 1975) andwith peridotite enclaves from Cenozoic alkali basalt in eastChina (E and Zhao, 1987), MgO is clearly on the high side,SiO2 is on the low side, and Al2O3, CaO, Na2O and K2O areclearly on the low side; i.e. these are mantle peridotitescharacterized by high MgO and low aluminum, calciumand alkalis. Based on the m/f values and areal compositionstatistics for the various rock belts (Table 2), except for afew samples from the Shiquanhe and Yarlung Zangpoophiolite belts, which are maWc–ultramaWc rocks with m/fvalues <6.5 [m/fD (Mg2+ + Ni2+)/(Fe2+ + Fe3+ + Mn2+)],most of the analyzed rocks, have an m/f ratio >6.5

Fig. 2. Sketch map showing the distribution of ophiolite belts and ophiolite bodies in the western Qinghai-Tibetan Plateau. Among them, (A) showing tec-tonic units; (B) showing the distribution of boninite and adakite have been found; (C) showing sampling spot and number for ophiolite bodies.

R.Z

. Qiu et al. / Journal of A

sian Earth S

ciences 29 (2007) 215–228219

Table 1Major and trace element composition of the mantle peridotite and gabbro rocks from the western Qinghai-Tibetan Plateau

ontents of Fe2O3 and FeO in rocks as pre-xa value; Fe2O3 and FeO contents do not

s way, [Fe2O3]D(1¡Oxr)(0.9 Fe2O3+FeO)/e way. Data analyzed by the chemistry lab-

11 12

-H2 DqTC9-H3 D12-2

rgite Dunite Gabbro

iong Pluton Dangqiong

angqiong Pulan-Dangqiong Pulan-Dangqiong

41.77 46.720.05 1.200.41 14.790.96 2.404.07 8.110.06 0.18

51.94 9.150.56 15.980.14 1.340.02 0.040.01 0.090.95 0.61b.d.l 70b.d.l 8.402.10 2500.98 410.69 2.40b.d.l <0.5

33 561.60 2.501.60 2.401.54 2.610.82 2.771.48 6.690.14 0.970.70 8.580.16 2.110.04 1.040.12 3.770.02 0.680.13 5.290.03 1.100.05 3.32

<0.01 0.450.03 2.68

<0.01 0.330.48 22.103.73 39.780.77 1.13

52.9 2.5

Note: Mg#DMg2+/[Mg2++total Fe2+]. Since many rocks were highly oxidized, before calculating the Mg#, we carry out an equilibrium calculation of the cscribed by Le Maitre (1976). Formula used to calculate the OxrDFeO/(FeO+Fe2O3) and OxaD0.88¡0.0016SiO2¡0.027(Na2O+K2O), compare Oxr and Oneed to be adjusted if Oxr>Oxa, and if Oxr<Oxa, it indicates that the Fe2O3 and FeO contents need to be adjusted. To get the Fe2O3 and FeO contents in thi(0.1Oxr+0.9), and [FeO]DFeO+0.9(Fe2O3¡ [Fe2O3]), then we can calculate total Fe2+ for Mg# calculation. Mg# in Tables 2 and 6 was calculated in the samoratory of Yichang Institute of Geology and Mineral Resources, China.Major oxides in wt%, trace elements in ppm, b.d.l.Dbelow detection limit, Eu/Eu*D2EuN/[SmN+GdN].

Serial No. 1 2 3 4 5 6 7 8 9 10

Sample No. DJTC-H3 D78-1 D80-1 D46-1 D20-2 D109-1 D24-1 D34-2 DqTC1-H1 DqTC4

Lithology Gabbro Harzburgite Harzburgite Harzburgite Harzburgite Harzburgite Harzburgite Harzburgite Dunite Harzbu

Pluton Dongjiri Chalamula Shiquanhe Rikangba East Dajinsi South Anren county Pulan pluton Pulan pluton Dangqiong pluton Dangq

Rock belt Bangonghu Bangonghu Shiquanhe Yarlung Zangpo Yarlung Zangpo Yarlung Zangpo Pulan-Dangqiong Pulan-Dangqiong Pulan-Dangqiong Pulan-D

SiO2 46.80 44.57 46.41 44.81 42.91 46.61 45.21 45.78 41.18 45.07TiO2 0.48 0.10 0.10 0.06 0.06 0.06 0.05 0.05 0.05 0.05Al2O3 14.88 1.65 1.13 0.97 1.18 2.30 1.89 1.67 0.88 0.99Fe2O3 1.82 1.57 1.41 1.52 1.65 1.43 1.58 1.73 2.00 1.63FeO 6.79 6.89 6.17 6.74 7.65 6.19 6.78 7.31 8.76 6.72MnO 0.15 0.12 0.07 0.12 0.17 0.12 0.13 0.14 0.14 0.12MgO 9.40 43.37 44.33 44.64 45.91 41.12 43.04 41.59 46.27 44.32CaO 18.88 1.65 0.24 1.08 0.34 1.95 1.17 1.56 0.48 0.76Na2O 0.66 0.02 0.09 0.02 0.04 0.14 0.05 0.07 0.04 0.16K2O 0.10 0.01 0.01 0.01 0.01 0.02 0.01 0.02 0.01 0.01P2O5 0.03 0.03 0.02 0.02 0.09 0.05 0.10 0.06 0.18 0.15Mg# 0.67 0.90 0.91 0.91 0.90 0.91 0.90 0.89 0.89 0.91Sr 35 B.d.l B.d.l B.d.l 3.20 4.70 b.d.l 1.10 b.d.l b.d.lBa 10 b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.lV 200 40 16 28 34 55 34 39 13 24Sc 41 11 5.40 9 9.20 11 9.50 12 4.80 7.10Nb 1.40 0.94 1.10 1.20 1.60 0.88 1.60 1.40 1.60 1.30Ta b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.lZr 60 41 32 28 33 29 33 30 27 35Hf 2.40 1.40 1.40 1.10 1.60 1.10 1.30 1.60 1.60 1.30Th 2.00 1.80 1.60 1.60 1.10 1.60 1.40 1.60 1.60 1.60Rb2O 3.56 1.78 1.78 2.37 0.71 2.08 1.31 2.38 2.26 1.54La 6.62 5.56 1.55 0.82 0.73 1.24 0.66 1.26 5.19 3.62Ce 8.10 5.54 0.99 1.46 1.46 0.82 1.20 1.95 5.26 3.60Pr 0.98 0.38 0.14 0.14 0.17 0.06 0.11 0.14 0.49 0.34Nd 4.54 1.51 0.57 0.71 0.96 0.30 0.50 0.57 1.44 1.36Sm 1.38 0.35 0.14 0.16 0.18 0.08 0.14 0.17 0.35 0.35Eu 0.67 0.07 0.04 0.04 0.05 0.07 0.05 0.06 0.06 0.07Gd 1.70 0.17 0.12 0.16 0.17 0.09 0.21 0.19 0.37 0.23Tb 0.31 0.03 0.02 0.03 0.03 0.02 0.03 0.04 0.06 0.04Dy 2.22 0.17 0.18 0.17 0.12 0.24 0.13 0.25 0.27 0.19Ho 0.47 0.04 0.06 0.04 0.03 0.04 0.03 0.05 0.06 0.07Er 1.16 0.11 0.10 0.10 0.08 0.13 0.08 0.15 0.13 0.15Tm 0.18 0.02 0.01 0.01 0.01 0.02 0.01 0.02 0.01 0.02Yb 1.03 0.11 0.03 0.06 0.07 0.17 0.09 0.10 0.07 0.08Lu 0.11 0.02 <0.01 0.01 0.01 0.03 0.01 0.01 0.02 0.02Y 9.55 0.87 0.64 0.79 0.80 1.41 0.99 0.84 0.89 0.73�REE 29.47 14.07 3.97 3.91 4.07 3.31 3.24 4.96 13.78 10.14Eu/Eu* (�Eu) 1.35 0.83 1.02 0.74 0.85 2.52 0.90 1.10 0.48 0.71(Ce/Yb)N 7.864 50.4 30 22.8 21.2 4.82 13.6 20.1 77.4 43.9


(Qiu, 1991), with an m/f ratio from 8.7 to 12.66. It is sug-gested that the ophiolites in the west Qinghai-Tibetan Pla-teau belong to the magnesian peridotite–serpentiniteassemblage of Alpine ophiolite type. Two of the gabbros,have Mg# of 63.3 and 68.9, indicating that they werederived from original magma sources.

The Mg# ratio and MgO content of mantle peridotites isgenerally regarded as indicating the degree of mantle deple-tion, or the degree of partial melting. The larger the Mg#

ratio, the higher the MgO content and the higher the pro-portion of low melting point components, the greater thedegree of partial melting and the more depleted the residualmantle (Coleman, 1977; Nicolas and Prinzhofer, 1983;Hartmann and Wedepohl, 1993). The signiWcance forpetrology is that components which are easily melted, suchas CaO, Al2O3, SiO2, are easily taken into the melt duringthe process of extraction of basaltic magma from the sub-continental lithosphere. The more basaltic magma isextracted, the richer the remnant mantle peridotite is inMgO, and the higher the degree of mantle depletion. Theaverage MgO contents (wt%) in harzburgites from theBangonghu, Shiquanhe and the western segment of Yar-lung Zangpo and Pulan-Dangqiong belt are 43.52%,44.07%, 43.72% and 42.97%, respectively, and the Mg# val-ues are 92.3, 95.5, 93.73 and 91.92. Although the diVerentdegree of partial melting in each ophiolite belt is shown bythe average MgO content (wt%) and the Mg# value, theyare higher than the MgOD37.67% and the Mg#D0.89 of

Fig. 3. Al2O3–CaO–MgO diagram of mantle peridotites (Based on lectureby J.A. Pearce in Beijing, October 2002). SSZ ophiolites mostly Harzburg-ite Ophiolite Type (HOT), MORB ophiolites mostly Lherzolite OphioliteType (LOT).

pyrolite (Ringwood, 1975). The four ophiolite belts exposedin the western Qinghai-Tibetan Plateau all show a relativelyhigh degree of mantle depletion or a high degree of partialmelting.

Chromian spinel is not sensitive to alteration and its rel-ative Cr2O3 and MgO contents are indicative of the extentof partial melting. Based on electron-microprobe results(Table 2) from spinels in harzburgite from the ophiolites,the average MgO content (wt%) of spinels from the Bang-onghu, the western segment of the Yarlung Zangpo andPulan-Dangqiong belts is 16.13%, 14.79%, 16.27%, respec-tively, and the Cr2O3 contents are 25.61, 36.49 and 34.51,respectively. The relationship of Cr2O3 and MgO in spinels(Dick and Bullen, 1984) also indicates that the ophiolitebelts exposed in the western Qinghai-Tibetan Plateau havea relatively high degree of mantle depletion, or a highdegree of partial melting.

According to the tectonic setting in which they wereformed, ophiolites can be classiWed into Mid-Ocean RidgeBasalts (MORB) or supra-subduction (SSZ) types (Pearceet al., 1984). The mantle peridotites of SSZ ophiolites arecomposed mainly of harzburgite (HOT) and the peridotitesof MORB ophiolites are composed mainly of lherzolite.Most of samples from the western Qinghai-Tibetan Plateauplot in the (HOT) Weld in the diagram of Al2O3–CaO–MgO(Fig. 3) implying the ophiolites were produced in supra-subduction zone setting.

3.2. Rare earth elements (REE)

The total REE of the harzburgites show a large variationwithin each ophiolite belt, from less than chondrite in theShiquanhe body in the Shiquanhe Ophiolite Belt, and theRikangba and Dajinsi in the Yarlung Zangpo, to less, orthe same as chondrite, in the Pulan body in the Pulan-Dangqiong Ophiolite Belt and 3.5 times chrondrite in theChalamula body in the Bangonghu Ophiolite Belt(REED 3.95£ 10¡6 normalizing value of chondrite fromLeedey, quoted in Masuda et al., 1973) (Table 1). ThePulan-Dangqiong rock belt shows the greatest variation,from less than chondrite (REED 3.24–13.78£ 10¡6) to 2.5times chondrite (REED13.78£ 10¡6). There may be adiVerence in the total REE even in the same body, i.e. thePulan body, which has a total REE of 3.24£10¡6 in theeast and 4.96£ 10¡6 in the west. These small diVerences in

Table 2Average major element composition of the mantle peridotite rocks from the ophiolite belts in western Qinghai-Tibetan Plaeeau (major oxides in wt%)

Note: Number in brackets is the Statistic Samples; data source the 1M geological reports of Bureau of Geology and Mineral Resources of Xizang Auton-omous Region (1985, 1986, 1987a,b), Deng (2000), Guo et al. (1991) and this paper. The contents of MgO and Cr2O3 shown in black are the results fromspinels in harzburgites analyzed by the electronic microprobe, from the ophiolites. Analyses in the laboratory of China University of Geosciences, Wuhan.

Ophiolite belt SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 Mg# MgO Cr2O3

Bangonghu(23) 46.10 0.09 1.27 1.52 6.62 0.11 43.44 0.68 0.10 0.05 0.03 0.906 16.13(16.42–15.58)(2) 25.61(24.35–26.86)(2)Shiquanhe(19) 45.15 0.06 1.48 1.62 7.02 0.11 43.93 0.38 0.13 0.08 0.03 0.902Yarlung Zangpo(24) 45.44 0.08 1.45 1.56 7.19 0.12 42.92 1.15 0.05 0.02 0.03 0.900 14.79(12.61-17.59)(7) 36.49(24.35-43.51)(7)Pulan-Dangqiong(46) 44.58 0.04 1.10 1.22 6.82 0.12 45.00 0.98 0.08 0.02 0.02 0.909 16.27(14.51-17.70)(15) 34.51(23.53-47.25)(15)Ringwood, 1975 45.48 0.72 3.57 0.46 8.10 0.14 37.67 3.10 0.57 0.13 0.06 0.890


the same body may be caused by the mineral composition,or by diVerences in the intensity of serpentinization, buttwo dunite samples from Dangqiong body have total REEcontent of 3.73£ 10¡6 and 13.78£10¡6. This great rangeimplies that the body was aVected by major inhomogenei-ties in the degree of partial melting, or that the ophiolitebody is complex.

(Ce/Yb)N ratio values vary from 1.23 to 13.43. Most ofthe harzburgites and dunites show light REE-rich patternsin the chondrite-normalized diagram (Fig. 4), only thedegree of light REE enrichment is diVerent in each ophio-lite belt. For example, some harzburgite samples in theBangonghu Ophiolite Belt and the Pulan-Dangqiong rockbelt have relatively strong light REE enrichment, e.g. D78-1from Bangonghu, DqTC1-1 and DqTC4-H2 from theDangqiong bodies ((Ce/Yb)ND 11.16–19.66), whileothers from Shiquanhe and the western segment of theYarlung Zangpo belt have only weak light REE enrichment((Ce/Yb)ND1.23–7.62). Enrichment in light REE has a pos-itive correlation with the MgO content, and may be relatedto the degree of partial melting.

The total REE in gabbros associated with the Dongjiribody in the Bangonghu Ophiolite Belt and the Dangqiongbody in the Pulan-Dangqiong belt, are higher than in peri-dotite with 7–10 times as much REE as in chondrite, andhave a positive Eu anomaly (Eu > 1), caused by the crystal-lization and accumulation of plagioclase. The Dongjiribody has light REE enrichment, ((Ce/Yb)ND2), while theDangqiong body is weakly depleted ((Ce/Yb)ND 0.63)(D12-2).

The rock types, MgO contents and Mg# ratio valuessuggest that the four ophiolite belts exposed in the westQinghai-Tibetan Plateau belong to the Alpine type, inwhich harzburgite is the main component of the peridotiteassemblage, with a higher degree of depletion, but theREE compositions of the harzburgite and dunite in all theophiolite belts are diVerent from the equivalent rocks ofAlpine-type (Figs. 4 and 5). In Alpine ophiolites (Fig. 5)light REE depletion in peridotite, middle REE depletion

Fig. 4. Chrondrite normalized REE patterns of harzburgite and dunitefrom ophiolite belts in the western Qinghai-Tibetan Plateau. Normalizingvalue after Leedey quoted in Masuda et al. (1973).

and strong depletion patterns show a special ‘U’-shape indunite and harzburgite, indicating a higher degree of par-tial melting. In that situation, the main mineral species,such as garnet and clinopyroxene are incorporated intothe melt, resulting in depleted REE. While the mantle per-idotites of the ophiolites in the western Qinghai-TibetanPlateau consist mainly of harzburgite with a relativelyhigh MgO content and Mg# ratio compared with pyrolite(Ringwood, 1975) indicating that they are the relics fromthe melting of subcontinental lithosphere and have under-gone a high degree of partial melting. These bodies shouldshow the same extent of depletion of REEs and lightREEs as Alpine ophiolite, but now they all show enrich-ment in light REEs. A possible explanation is that theyhave undergone two processes. First the REEs weredepleted by partial melting, and then light REEs wereintroduced by later stage geological processes. These latestage geological processes may have included: (1) alter-ation; (2) recrystallization of magma melt; and (3) mantlemetasomatism, e.g. serpentinization.

(1) Most of the ophiolites in the western Qinghai-TibetanPlateau show a high degree of serpentinization. How-ever, after studying the peridotites, Frey (1984)reported that there is no relationship between thedegree of alteration and the total REE in these rocks,which have a diVerent amount of serpentinizationcompared to the Ronda ophiolite. Similarly, Loubetet al. (1975) have suggested that alteration during ser-pentinization might activate REE (mostly light REE)to a certain extent, but the essential characteristics ofthe REE, such as the total REE and their distributionpattern, cannot be aVected. Serpentinization is there-fore unlikely to change the REE signature of therocks to any great extent.

(2) The recrystallization of magma melts, which have notbeen extracted during partial melting of the mantle,but this is not supported by the rock assemblages andthe petrochemistry of the mantle peridotites men-tioned above.

Fig. 5. REE patterns of mantle peridotites in Alpine ophiolite, quotedfrom Li (1992).


(3) Mantle metasomatism. Compared with the aboveprocesses, it is most likely that mantle metasomatismhas introduced light REE. If a hydrous Xuid contain-ing light REE and which was LILE ion-rich wasintroduced into the mantle during the process of oce-anic crust subduction, the REE signature of rockscould be changed to a great extent. Since the REEabundance of pyrolite is very low, the addition of ahydrous Xuid containing a large amount of lightREEs and LILE ions would change the REE signa-ture of the rock, resulting in light REE enrichmentand an increase in total REE.

Therefore, the REE characteristics of harzburgite anddunite suggest that the ophiolites in western Qinghai-Tibetan Plateau Wrst underwent strong partial melting,resulting in light REE depletion, and later underwent Xuidmetasomatism during the process of oceanic crust subduc-tion, resulting in light REE enrichment and an increase intotal REE.

A mantle wedge located in a supra-subduction zone isthe most likely tectonic setting for this sequence of events.This interpretation is consistent with the results of thechemical analyses which indicate that the ophiolites wereformed in a SSZ setting, as discussed above, and that mostof the samples of mantle peridotite from the western Qing-hai-Tibet Plateau plotting in the HOT Weld of the Al2O3–CaO–MgO diagram (Fig. 3).

3.3. Trace elements

The abundance of the trace elements Sr, Ba, Ta in man-tle peridotite is very low (Table 1). Compared with N-MORB and the original mantle rocks (Condie, 1989),harzburgite and dunite in mantle peridotites have high Zr/Nb, Zr/YTh/Yb and Ti/Y ratios, and low Ti/V, Ti/Zr, Hf/T and K/Rb ratios (Table 3). The abundance of trace ele-ments in mantle peridotite with the characteristics of thelarge ion lithophile Rb, inactive elements Nb, Zr, Hf and

radioactive elements is relatively high. Strongly incompat-ible elements Sm, Ti, Y and Yb are depleted (Table 1,Fig. 6), similar to pyrolite (quoted after Li, 1992). Thissupports the explanation that metasomatism at a latestage inXuenced the REE abundance. Comparing traceelements in gabbros, harzburgites and dunites on spider-grams, their total characteristics are consistent (Fig. 7)

Fig. 6. Primitive mantle-normalized trace element spidergrams of harz-burgite and dunite. Normalized values after Sun and McDonough (1989).

Fig. 7. Primitive mantle-normalized trace element spidergrams of gabbros.Normalizing values after Sun and McDonough (1989).

Table 3Ratios of trace elements from mantle peridotite and gabbros

Note: Serial numbers in Table 3 are the same as in Table 1; the ratio of the original mantle and N-MORB (Serial numbers 13–14) after Condie (1989).

No. Sample Zr/Nb Zr/Y Th/Yb Ti/Y Ti/V Ti/Zr Hf/Th K/Rb

1 DJTC-H3 42.86 6.28 1.94 288.76 13.79 45.96 1.20 255.012 D78-1 43.62 47.13 16.36 620.17 13.49 13.16 0.78 51.003 D80-1 29.09 50.00 48.48 843.05 33.72 16.86 0.88 51.004 D46-1 23.33 35.44 25.00 379.43 10.71 10.71 0.69 38.315 D20-2 20.63 41.25 15.94 374.69 8.82 9.08 1.45 127.876 D109-1 32.95 20.57 9.41 212.59 5.45 10.34 0.69 87.297 D24-1 20.63 33.33 15.91 302.78 8.82 9.08 0.93 69.308 D34-2 21.43 35.71 16.49 356.84 7.69 9.99 1.00 76.299 DqTC1-H1 16.88 30.34 23.53 336.80 23.06 11.10 1.00 40.17

10 DqTC4-H2 26.92 47.95 19.51 410.62 12.49 8.56 0.81 58.9511 DqTC9-H3 47.83 68.75 57.14 624.48 142.74 9.08 1.00 117.9012 D12-2 23.33 2.53 0.90 314.67 27.82 124.18 1.04 139.1313 Original mantle 18 2.3 0.25 313 17 139 3.8 29314 N-MORB 26 8 0.06 300 39 100 12 625


with an abundance of trace elements, with the characteris-tics of slightly higher large ion lithophile Rb, inactive ele-ments Nb, Zr, Hf and radioactive elements, and arerelatively high especially in the strongly incompatible ele-ments Sm, Ti, Y, Yb. These characteristics indicate thatthe gabbros, harzburgites and dunites are all consanguine-ous and had a common origin.

4. Geochemical characteristics of Pb and Sr isotopes

4.1. Pb isotopic geochemistry

The Pb isotopic compositions of harzburgite, duniteand gabbro from the western Qinghai-Tibet Plateau areshown in Table 4. Most samples plot between the lowercrust and orogenic lines on the plumbotectonic model(after Zartman and Doe, 1981) (Fig. 8). The content ofhighly radiogenic Pb isotopes is high, and can be com-pared with the average composition of modern crustal Pb:(206Pb/204PbD 18.75, 207Pb/204PbD 15.64, 208Pb/204PbD 38.7) (Stancey and Kramers, 1975) and the aver-age value of the sea-Xoor sediments from Java (206Pb/204PbD 18.82, 207Pb/204PbD 15.68, 208Pb/204PbD 39.03)(Ben et al., 1989), while the crust-derived source sedimenteven corresponds with the EMII component, deWned byHart (1984). So, it is obvious that the Pb isotopic compo-sition of harzburgite, dunite and gabbro has been aVectedby a crust-derived component, probably crustal materialor seaXoor sediment, which was incorporated in the peri-dotite during the process of oceanic crust subduction in aSSZ setting.

4.2. Sr isotopic geochemistry

Rb, Sr content and 87Sr/86Sr ratios of harzburgite anddunite in mantle peridotite from the western Qinghai-Tibet Plateau are extremely low, and discrepancies in theinitial ratio of (87Sr/86Sr)i are very large (Table 5), the min-imal value is 0.70811 from harzburgite in Chalamula, andthe maximum value is 0.90483 of dunite from Dangqiong.Sometimes The Sr isotopic composition of ophioliticrocks has a great range. Some authors (Liu et al., 1995;Xing et al., 1997) attribute this variation to the formationof these rocks on the seaXoor in contact with sea water,increasing the 87Sr/86Sr ratio. But such a great variation inthe isotopic composition of Sr in the research area is diY-

cult to explain by sea-water alteration. It is more probablethat the ophiolite has been metasomatized by Xuids dur-ing the process of formation. Since the Rb and Sr contentsare extremely low, it is only possible for the Sr composi-tion to be greatly changed if a REE and LILE ion-richXuid has been added. In other words, the variation of theinitial (87Sr/86Sr)i ratios of harzburgite and dunite indi-cates that the ophiolites in western Qinghai-Tibetan Pla-teau were formed in a SSZ-setting, with the addition ofXuids during subduction.

The abundance of Rb and Sr, especially of Sr, in gab-bros from Dongjiri and Shemalagou in the west Bang-onghu-Nujiang Belt is low as a whole (Table 5), buthigher than mantle peridotite; while the 87Sr/86Sr and Rb/Sr ratio is less than that of mantle peridotite. The data of87Sr/86Sr and Rb/Sr did not show a clear linear relationship(Qiu et al., 2004a). This may indicate that their genesis wasdue to the crystallization-diVerentiation of maWc-ultra-maWc magma. Strontium initial ratios (87Sr/86Sr)i of fourwhole rock samples, including the Shemalagou ophiolite,are 0.7059, 0.7061, 0.70459, 0.70549, with an average of0.7055 (Table 5). These are very close to the strontium ini-tial ratios of Jurassic volcanics (0.70372–0.707988)obtained from Dingqing in the east segment of the Bang-onghu-Nujiang Belt and gabbro from the Troodos Ophio-lite (0.7048–0.7053); the latter are both typical SSZophiolites (Zhang and Zhou, 2001; Pearce et al., 1984),associated with boninite.

Fig. 8. 207Pb/204Pb versus 206Pb/204Pb for harzburgite, dunite and gabbro(after Zartman and Doe, 1981).

Table 4Pb isotope analytical results from harzburgite, dunite and gabbro from the ophiolite belts in the western Qinghai-Tibetan Plateau

Note: Analyzed by isotopic laboratory of Yichang Institute of Geology and Mineral Resources. Pb isotope ratios are reported as present-day values with2 errors for each analysis. Pb isotope ratios are corrected for fractionation, using the values of Todt et al. (1996) for the NBS981 standard.

Ophiolite belt Pluton Sample No. Lithology 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

Bangonghu Dongjiri DJTc-H3 Gabbro 18.147 § 0.01 15.537 § 0.01 38.210§ 0.01Chalamula D78-1 Harzburgite 17.765 § 0.01 15.525 § 0.01 37.918§ 0.01

West segment of Yarlung Zangpo Rikanba D46-1 Harzburgite 17.398 § 0.02 15.343 § 0.04 37.361§ 0.09Pulan D24-1 Harzburgite 17.727 § 0.18 15.453 § 0.06 37.791§ 0.38

Pulan-Dangqiong Dangqiong DqTc1-H1 Dunite 17.82 § 0.01 15.554 § 0.01 38.027§ 0.04


5. Tectonic setting of the formation of the ophiolites in Qinghai-Tibetan Plateau

5.1. Ophiolite setting in western Qinghai-Tibetan Plateau

Ophiolites in the western Qinghai-Tibetan Plateau,including the western segment of the Bangonghu,Shiquanhe, the western segment of the Yarlung Zangpoand Pulan-Dangqiong ophiolite belts, are Alpine-typeophiolites, comprising mainly Harzburgite (HOT), with thepetrochemical characteristics of high magnesium oxide(MgO) enrichment and low contents of aluminum, calciumand alkali oxides. The light REE enrichment diVers fromthat of Alpine ophiolites, indicating that the mantle perido-tite underwent two processes: Wrst partial melting, and laterXuid metasomatism during the process of subduction ofoceanic crust. Trace elements in mantle peridotite with thecharacteristics of large ion lithophile Rb, inactive elementsNb, Zr, Hf and radioactive elements are relatively enriched,while strongly incompatible elements, Sm, Ti, Y and Yb aredepleted (Table 1 and Fig. 6). These characteristics are sim-ilar to those of metasomatic pyrolite, and indicate the peri-dotites have undergone metasomatism, and may be due tothe contamination of the magmatic source by a crust-derived component, such as crustal material or seaXoorsediment. This is consistent with a high radiogenic Pb isoto-

pic composition, and is also consistent with Sr initial (87Sr/86Sr)i values with a large range (0.70459–0.90483). Thismight be due to Xuid added in a subduction setting, result-ing in a variation of the Sr isotopic composition. In addi-tion, the Sr initial (87Sr/86Sr)i values of gabbros fromShemalagou and Dongjiri in the western segment of Bang-onghu-Nujiang Ophiolite Belt are close to those from thevolcanic rocks in Dingqing (0.70372–0.707988) and thegabbro in Troodos (0.7048–0.7053), which are regarded ashaving been formed in a typical SSZ setting. All of theabove features suggest that the ophiolites in the westernQinghai-Tibetan Plateau, including the Bangonghu, theShiquanhe, the western segment of Yarlung Zangpo andPulan-Dangqiong ophiolites, were formed in a supra-subduction zone (SSZ) setting and are therefore SSZophiolites.

5.2. Comparison with other ophiolites

Ophiolites of diVerent ages are exposed in the Qinghai-Tibetan Plateau. In order to discuss more fully the tectonicsetting of these ophiolites, petrochemical data from mantleperidotite and REE data from the eastern Qinghai-TibetanPlateau, Sanjiang, western Kunlun and the Troodos,regarded as a typical ophiolite formed in a SSZ setting(Pearce et al., 1984), have been collected for comparison.

Table 5Rb–Sr isotope analysis results from harzburgite, dunite and gabbro from the ophiolite belts in the western Qinghai-Tibetan Plateau, Rb and Sr in ppm

Analyzed by isotopic laboratory of Yichang Institute of Geology and Mineral Resources, China.

Pluton Sample Rock Rb Sr 87Rb/86Sr (87Sr/86Sr)i (�) Data

Chalamula D78-1 Harzburgite 0.2738 1.711 0.4637 0.75526 4(1�) This paperkc-125 Harzburgite 0.1492 3.04056 0.1368 0.70811 8(1�) After Guo et al. (1991)

Rikangba D46-1 Harzburgite 0.1479 0.9053 0.4710 0.70976 6(1�) This paperDangqiong DqTc1-H1 Dunite 0.2592 0.4964 1.535 0.90483 4(1�) This paperDongjiri DJTc-H3 Gabbro 1.003 30.32 0.09534 0.70549 3(1�) This paper

04-1 Gabbro 6.006 286 0.0608 0.7059 6(2 �) After Qiu et al. (2002)Shemalagou 04-2a Gabbro 7.315 140.1 0.1511 0.7061 23(2 �) After Qiu et al. (2002)

04-2b Gabbro 2.533 172 0.0426 0.7045 13(2 �) After Qiu et al. (2002)

Table 6Characteristic components of Mantle peridotites from diVerent areas in the Qinghai-Tibetan Plateau, MgO and FeO* in wt%

Note: 1. Coleman (1977); 2. Bureau of Geology and Mineral Resources of Xizang Autonomous Region (1993); 3. Dong et al. (1995); 4. This paper.

Area Epoch Lithology Statistic number MgO FeO* Mg# Data

Dongqiao Mesozoic Harzburgite 1 44.48 6.99 0.919Dunite 1 47.19 6.82 0.926

Dingqing Mesozoic Harzburgite 2 47.13 7.21 0.921Dunite 1 50.21 6.93 0.928

Renbu Mesozoic Harzburgite 1 42.64 7.39 0.911 2Dunite 1 49.34 6.26 0.934

Luobusha Mesozoic Harzburgite 3 41.95 8.42 0.899Dunite 1 47.95 7.48 0.919

Zedang Mesozoic Harzburgite 1 42.72 7.33 0.912Dunite 1 41.74 6.95 0.915

Western Qinghai-Tibetan Plaeeau Mesozoic Harzburgite 11 43.51 6.38 0.924 4Dunite 2 49.09 7.73 0.919

Sanjiang Late Paleozoic–early Mesozoic UltramaWc rock 188 44.093 9.57 0.891 3Kudi in west Kunlun Paleozoic Dunite 1 46.23 7.733 0.914 4Troodos Harzburgite 8 45.7 8.06 0.910 1

Dunite 10 49.1 8.76 0.909


Data in Table 6 show that the mantle peridotites ineastern Qinghai-Tibetan Plateau have relatively high MgOcontents. The MgO content of the mantle peridotites is44.48–50.21% in Dongqiao and Dingqing in the eastern seg-ment of the Bangonghu-Nujiang Belt, Renbu in the middlesegment, and Luobusha and Zedan in the eastern segmentof Yarlung Zangpo Belt have an MgO contents of 41.95–49.34%, SanjiangD44.09% (Dong et al., 1995), TroodosD45.7–49.1% and the four ophiolite belts in the westernareaD43.51–49.09%. They all have relative high MgO con-tents, indicating they are remanent material, remainingafter a relatively high degree of partial melting, and areAlpine-type ophiolites.

From the review of the REE data shown in Table 7,although the data has diVerent precision, except for theharzburgite in Dingqing which did not show light REEenrichment ((La/Yb)N D 0.3), all the mantle peridotitesshow light REE enrichment (Fig. 9b), such as the easternsegment of Dongqiao in the Bangonghu-Nujiang Belt,Renbu in middle segment, Biannalie, Lieding and Song-duo ((La/Sm)N D 2.1–9.86; La/LuD 1.31–6.52)N, andLuobusha and Zedang in the eastern segment of the Yar-lung Zangpo Belt, Late Paleozoic-Early Mesozoic Sanji-ang ((La/Yb)N D 2.37), Paleozoic West Kunlun ((La/Yb)N D 18.67) and Troodos ((La/Yb)N D 2.79; (La/Yb)N D 1.77–4.59). Based on the above patterns of enrich-ment in light REE in mantle peridotites from westernQinghai-Tibetan Plateau, the genesis of mantle perido-tites in eastern Qinghai-Tibetan Plateau (Dongqiao inBangonghu-Nujiang, Renbu, Biannalie, Lieding andSongduo in the middle segment, and Luobusha and Zed-ang in eastern Qinghai-Tibetan Plateau, Late Paleozoic-Early Mesozoic Sanjiang ((La/Yb)N D 2.37)) and thePaleozoic West Kunlun are all the same as those in thewestern Qinghai-Tibet Plateau. These mantle peridotiteshave Wrst undergone relatively strong partial melting, andhave then undergone metasomatism by Xuids later duringthe process of oceanic crust subduction, resulting in lightREE depletion and then later enrichment. Therefore, itcan be concluded that ophiolites in Qinghai-TibetanPlateau, including Sanjiang and the West Kunlun, are allSSZ-type ophiolites formed in a supra-subduction zonesetting.

5.3. Evidence of magmatism associated with subduction in Qinghai-Tibetan Plateau

Boninite (high magnesium andesite) and adakite arecommonly regarded as the products of magmatism associ-ated with subduction (Pearce et al., 1984; Pushchin andKonovalov, 1994; Drummond and Defant, 1990; Kim andJacobi, 2002; Defant and Drummond, 1990; Sajona et al.,2000; Defant et al., 2001; Crawford et al., 1989; Dong andTian, 2004). Nearly all boninite or boninite-series rocks

Fig. 9. Chondrite-normalized REE patterns of mantle peridotites fromdiVerent areas as given in Table 7; apart from the harzburgites the datahas diVerent degrees of precision.

Table 7REE characteristics of mantle peridotites from diVerent areas in the Qinghai-Tibetan Plateau, rare earth elements in ppm

Note: 1. Areal geology of Xizang(Tibet) (1993); 2. This paper; 3. Dong et al. (1995); 4. Coleman (1977).

Statistic Area Lithology La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu � REE �L/�H �Eu (La/Yb)N Data

3 Rikaze Dunite 0.22 0.73 0.47 0.02 0.06 0.19 0.10 0.03 0.07 1.89 3.85 2.3 2.061 Luobusha Orthopyroxenite 1.83 3.62 1.76 0.54 0.42 0.18 0.58 0.13 0.68 0.41 0.05 0.32 0.05 10.57 3.76 1.13 3.751 Lherzolite 0.77 1.02 0.53 0.16 0.01 0.04 0.25 0.19 0.27 0.18 0.03 0.11 0.03 3.59 2.39 1.1 4.59 11 Dongqiao Dunite 0.15 0.04 0.03 0.06 0.06 0.01 0.28 2.99 1.7 1.771 Dingqing Harzburgite 0.04 0.04 0.10 0.01 0.19 0.79 1.05 0.302 West Tibet Dunite 3.01 3.37 0.32 1.07 0.26 0.05 0.25 0.04 0.20 0.04 0.09 0.01 0.05 0.02 8.75 11.66 0.6 41.07 28 Harzburgite 1.93 2.13 0.18 0.81 0.20 0.06 0.17 0.03 0.18 0.04 0.11 0.02 0.09 0.01 5.96 8.14 0.95 14.21 21 Sanjiang Harzburgite 0.43 0.88 0.55 0.12 0.05 0.03 0.12 0.02 2.20 12.18 1.12 2.37 31 West Kunlun Dunite 1.48 2.93 0.28 0.96 0.28 0.1 0.39 0.058 0.2 0.035 0.076 0.014 0.052 0.03 6.89 7.05 0.93 18.67 25 Troodos Metamorphic

peridotite0.07 0.72 0.01 0.03 0.01 0.00 0.01 0.00 0.02 0.00 0.02 0.00 0.02 0.00 0.91 10.92 1.32 2.79 4


have been found in fore-arc spreading regions (Hickey andFrey, 1982; Kerrich et al., 1998), and appear at an earlystage of the development of a fore-arc, rather than in aback-arc spreading environment (Hickey and Frey, 1982;Kerrich et al., 1998). Boninite commonly occurs in intra-oceanic subduction systems and indicates an immatureisland arc. Some researchers think that if boninite is foundin an ophiolite, the ophiolite is conWrmed as having formedin a fore-arc environment and to have been inXuenced bysubduction (Zhang and Zhou, 2001). Adakite is anotherimportant rock associated with subduction magmatism,which has recently aroused wide interest in China andabroad (Dong and Tian, 2004). Adakites are exposed involcanic arcs and are usually associated with an early stagein the subduction of oceanic crust.

Both boninite and adakite have been found on theQinghai-Tibetan Plateau (showing in Fig. 2B). Boninitehas been found in Lameila, in the volcanic belt between thewestern Bangonghu-Nujiang Ophiolite Belt and the Shiqu-anhe Ophiolite (Qiu, 2002; Qiu et al., 2004a,b), Dingqing ofthe eastern segment of Bangonghu-Nujiang Ophiolite Belt(Zhang and Yang, 1985), Rikaze of the middle segment ofthe Yarlung Zangpo Belt (Zhang and Zhou, 2001) andXiangcheng in the middle segment of the Sanjiang Belt(Mo, 1993). Boninite-like and adakite-like rocks have beenfound in the middle south Dangdese Belt (Chen et al.,1999), the boninite series has been found in Kudi of theWest Kunlun Ophiolite Belt (Yuan et al., 2002). Since bon-inite magmas are formed from a depleted mantle source(such as harzburgite), the Xuids must have been derivedfrom the partial melting of oceanic lithosphere in the‘mantle wedge’ (Crawford et al., 1989; Hickey and Frey,1982). Boninite or boninite-series rocks have been found inthe areas mentioned above, indicating that they had arefractory mantle source, such as harzburgite. This is con-sistent with the high MgO content of the mantle perido-tites (Table 6) and principal role of harzburgite in the rockassemblages of ophiolites of diVerent ages. In the igneouspetrotectonic assemblages of Late Yanshanian age whichare associated with the subduction of oceanic crust distrib-uted on the northern and southern sides of Gangdese(North belt and South belt of Gangdese), O-type adakiterocks have been recognized, with the isotopic characteris-tics of low initial 87Sr/86Sr values (<0.706), positive Nd(t)values (+2.5 to +5.7) and young TDM ages(312–562 Ma)(Qiu et al., 2003). Boninite, the boninite series andadakites can be regarded as recording the process of for-mation from an immature intra-arc to a mature volcanicarc, with the extension of continental growth on the north-ern and southern sides of the Gangdese Block during thelate Yanshanian. The universality of these two kinds ofrocks indicate that subduction was the main mechanism ofophiolite formation in Qinghai-Tibetan Plateau, mean-while, it is also evident that in these diVerent areas ophio-lites, including Sanjiang, West Kunlun, were formed in aSSZ setting associated with the subduction of oceaniccrust.

6. Conclusions

Ophiolites, exposed in the western Bangonghu, the Shi-quanhe, the western segment of the Yarlung Zangpo, andthe Pulan-Dangqiong ophiolite belts, have similar geologi-cal and geochemical characteristics, with harzburgite anddunite as the principal facies of the mantle peridotite rock-assemblages, petrochemically rich in magnesium oxide(MgO), and poor in aluminum, calcium and alkali oxides.This indicates they are Alpine-type ophiolites whichhave undergone a relatively high degree of mantle partialmelting.

The REE characteristics of the mantle peridotites indi-cate that they have undergone at least two processes, Wrststrong partial melting and later metasomatism by liquidsduring the process of oceanic crust subduction. From trace-element geochemistry most of the peridotites are characterizedby low Sr, Ti and Y and Rb, Nb, Zr, Hf and Th enrichment,implying they have been metasomatised. Relatively highradiogenic isotopes in the harzburgites, dunites and gab-bros indicate that the magmatic source was contaminatedby crustal material or by seaXoor sediments. The great vari-ation in the range of initial (87Sr/86Sr)i ratios of harzburgiteand dunite is probably due to an inXux of Xuids containinghigh REE and LILE ion contents, which were released dur-ing the process of oceanic subduction. However, the gab-bros have values very close to the strontium initial ratios ofJurassic volcanics from Dingqing and also the gabbros inthe Troodos Ophiolite, which is regarded as a typical SSZophiolite. If it is suggested that the ophiolites in the westernQinghai-Tibetan Plateau were formed by these two pro-cesses, and most suitable tectonic environment would be amantle wedge in a SSZ setting.

Based on the comparison of the petrochemistry of man-tle peridotites and the REE from the eastern and westernQinghai-Tibetan Plateau, Sanjiang, West Kunlun with theTroodos Ophiolite which is regarded as a typical ophioliteformed in a SSZ setting. The studied ophiolites have essen-tially the same geochemical characteristics with high MgOcontents or Mg# ratios and high total light REE. Moreover,boninite and adakite rocks, which are regarded as associ-ated with subduction magmatism, are distributed widely inthe Qinghai-Tibetan Plateau. Therefore, we conclude thatthe ophiolites in the Qinghai-Tibetan Plateau, includingSanjiang and West Kunlun, are all SSZ-type ophiolites,formed above a subduction zone.

Acknowledgments

The authors thank Prof. Zhang, Q. for helpful discus-sion. We thank Prof. Elsheikh M. Abdelrahman (Geologi-cal Research Authority of Sudan (GRAS)) for his kindlydiscussion and help on English. Our research is supportedby the National Fund of Nature Science of China (No.40572063), the Projects of China Geological Survey (Nos.200110200064, 1212010561502), the special program of theMinistry of Land and Resources of China (No. 200010103)


and the Key Laboratory of Lithospheric Tectonics andExploration, China University of Geosciences, Ministry ofEducation, China (Nos. 2003009; 2003010).

References

Ben, O.D., White, W.M., Patchert, J., 1989. The geochemistry of marinesediments, island arc magma genesis and crust-mantle recycling. Earthand Planetary Science Letters 94, 1–21.

Bureau of Geology and Mineral Resources of Xizang AutonomousRegion, 1985. The report of Regional Geology Survey of Rikaze Sheet1:1Million scale (unpublished, in Chinese).

Bureau of Geology and Mineral Resources of Xizang AutonomousRegion, 1986. The report of Regional Geology Survey of Gaize Sheet1:1 Million scale (unpublished, in Chinese).

Bureau of Geology and Mineral Resources of Xizang AutonomousRegion, 1987a. The report of Regional Geology Survey of GadakeSheet 1:1Million scale (unpublished, in Chinese).

Bureau of Geology and Mineral Resources of Xizang AutonomousRegion, 1987b. The report of Regional Geology Survey of Ritu Sheet1:1Million scale (unpublished, in Chinese).

Bureau of Geology and Mineral Resources of Xizang AutonomousRegion, 1993. Regional Geology of Xizang (Tibet) AutonomousRegion. Geological Publishing House, Beijing (in Chinese).

Chen Chang Hwa, Hsu, K.J., Shen Jason Jiun San, et al. 1999. The adakite-like and high-Ca boninite-like volcanic rocks in central and southernTibet; implication for Tibetan tectonic reconstruction[A]. IUGG 99(International Union of Geodesy and Geophysics General Assembly)abstracts[C] 74, Birmingham, United Kingdom.

Coleman, R.G., 1977. Ophiolites. Springer-Verlag, Berlin, Heidelberg, NewYork. 229p.

Condie, K.C., 1989. Geochemical changes in basalts and andesites acrossthe Archaean-Proterozoic boundary: identiWcation and signiWcance.Lithos 23, 1–18.

Crawford, A.J., Fallon, T.J., Green, D.H., 1989. ClassiWcation, petrogenesisand tectonic setting of boninites. In: Crawford, A.J. (Ed.), Boninites.Academic Division of Unwin Hyman Ltd., London, pp. 1–49.

Defant, M.J., Drummond, M.S., 1990. Derivation of some modern arcmagmas by melting of young subducted lithosphere. Nature 347, 662–665.

Defant, M.J., Sherman, S., Maury, R.C., 2001. The geology, petrology, andpetrogenesis of Saba Island, Lesser Antilles. Journal of Volcanologyand Geothermal Research 107 (1-3), 87–111.

Deng, W.M., 2000. Basite-superbasite. In: The Integrated ScientiWc Expe-dition to the Qinghai-Xizang Plateau, Chinese Academy of SciencesGeological evolution in Karakorum and Kunlun mountains. SciencePress, Beijing (in Chinese).

Deng, W.M., Wang, F.G., 1987. Bangonghu-Nujiang ophiolite in NorthTibet. In: Xibing, Wang (Ed.), Xizang (Tibet) Ophiolite. GeologicalPublishing House, Beijing (in Chinese).

Dong, S.B., Tian, W., 2004. The nomenclature, characteristics and petro-genesis of adakite. Earth Science Frontiers 11 (4), 585–594 (in Chinesewith English abstract).

Dong, X.Y., Li, X., Ye, L.H., 1995. China UltramaWc Rocks. GeologicalPublishing House, Beijing (in Chinese).

Drummond, M.S., Defant, M.J., 1990. A model for trondhjemite-tona-lite-dacite genesis and crustal growth via slab melting: Archean tomodern comparisons. Journal of Geophysical Research 95, 21503–21521.

E, M.L., Zhao, D.S., 1987. Cenozoic Basalt and Deep-seated Enclaves inEastern China. Seismological Press, Beijing. pp.133–267 (in Chinese).

Frey, F.A., 1984. Rare earth element abundances in upper mantle rocks.In: Henderson, P. (Ed.), Rare Element Geochemistry, vol. 2. ElsevierScientiWc Publishing Co, Amsterdam, Netherlands, pp. 53–203.

Guo, T.Y., Liang, D.Y., Zhang, Y.Z., 1991. Geology of Ngari Tibet(Xizang). China University of Geosciences Press, Wuhan. 1991 (in Chi-nese with English abstract).

Hartmann, G., Wedepohl, K.H., 1993. The composition of peridotite tec-tonics from the Ivrea complex, northern Italy, residues from meltextraction. Geochemica et Cosmochemica Acta 57, 1761–1782.

Hart, S.R., 1984. Large-scale isotope anomaly in the southern Hemisphere.Nature 309, 753–757.

Hickey, R.L., Frey, F.A., 1982. Geochemical characteristics of boniniteseries volcanics: implication for their source. Geochemica et Cosm-ochemica Acta 46, 2099–2115.

Kerrich, R., Wyman, D., Fan, J., 1998. Boninite series: low Ti tholeiiteassociations from the 2.7 Ga Abitibi greenstone belt. Earth and Plane-tary Science Letters 164, 303–316.

Kim, J., Jacobi, R.D., 2002. Boninites; characteristics and tectonic con-straints, northeastern Appalachians. Physics and Chemistry of theEarth 27 (1–3), 109–147.

Le Maitre, R.W., 1976. M and Mg value in igneous rocks: Proposed usageand a comment on currently employed Fe2O3 corrections. Geochemicaet Cosmochemica Acta 40, 485–486.

Li, C.N., 1992. Petrology of Igneous Trace Elements. China University ofGeosciences Press, Wuhan (in Chinese).

Liu, C.Q., Jie, G.H., Zengtian, Z., 1995. Chemistry of Cenozoic basalt ineast China (II). Composition of Sr, Nd, Ce isotope. Geochemistry 24(3), 204–213 (in Chinese with English abstract).

Loubet, M., Shimizu, N., Allegre, C.J., 1975. Rare earth elements in Alpineperidotites. Contributions to Mineralogy and Petrology 53, 1–12.

Masuda, A., Nakamura, N., Tanaka, T., 1973. Fine structures of mutuallynormalised rare earth patterns of chondrites. Geochemica et Cosm-ochemica Acta 37, 239–248.

Mo, X.X., 1993. Tethyan volcanism and mineralization in the Sanjiangarea. Geological Publishing House, Beijing (in Chinese).

Nicolas, A., Prinzhofer, A., 1983. Cumulative or residual origin for thetransition zone in ophiolites, structural evidence. Journal of Petrology24 (2), 188–206.

Pearce, J.A., Lippard, S.T., Robert, S., 1984. Characteristics and tectonicsigniWcance of supra-subduction zone ophiolites. In: Kokelaar, B.P.,Howells, M.F. (Eds.), Marginal Basin Geology. Geological Society,London, Special Publication 16, 77–94.

Pushchin, I.K., Konovalov, Y.I., 1994. Magmatic rocks of the northinsular slope of the Tonga Trench. SOPAC Technical Bulletin 8,201–217.

Qiu, J.X., 1991. Applying magmatic petrology. University of GeosciencesPress, Wuhan, China. 387–389 (in Chinese).

Qiu, R.Z., Zhou, S., Xiao, R., 2002. Geochronology and Nd, Sr isotopecharacteristics of gabbro dikes from Shemalagou area: in the middle ofBangong-Nujiang ophiolite belt, north Tibet. Geological Review 48(Supplement), 64–68 (in Chinese with English abstract).

Qiu, R.Z., 2002. Igneous Rocks and Tectonic Evolution of the Neo-Teth-yan in the Western Tibetan Plateau. Unpublished Ph.D. thesis submit-ted to the China University of Geosciences, Beijing (in Chinese withEnglish abstract).

Qiu, R.Z., Deng, J.F., Zhou, S., 2003. Study on Sr-Nd isotopes of Meso-zoic-Cenozoic granites in Qinghai-Tibetan Plateau. Acta GeoscienticaSinica 24 (6), 611–617 (in Chinese with English abstract).

Qiu, R.Z., Zhou, S., Deng, J.F., 2004a. Dating of gabbro in the Shemalagouophiolite in the western segment of the Bangong Co-Nujiang ophiolitebelt, Tibet—with a discussion of the age of the the Bangong Co-Nuji-ang ophiolite belt. Geology in China 31 (3), 262–268 (in Chinese withEnglish abstract).

Qiu, R.Z., Cai, Z.Y., Li, J.F., 2004b. Boninite of ophiolite belts in WesternQinghai-Tibet Plateau and its geological implications. Geoscience 18(3), 305–308 (in Chinese with English abstract).

Ringwood, A.E., 1975. Composition and Petrology of the Earth’s Mantle.McGraw-Hill, New York.

Sajona, F.G., Maury, R.C., Pubellier, M., 2000. Magmatic source enrich-ment by slab-derived melts in a young post-collision setting, centralMindanao (Philippines). Lithos 54 (3–4), 173–206.

Stancey, J.S., Kramers, J.D., 1975. Approximation of terrestrial leadisotope evolution by a two-stage model. Earth and Planetary ScienceLetters 26, 207–221.


Sun, S.S., McDonough, W.F. 1989. Chemical and isotopic systematics ofoceanic basalt: implications for mantle composition and processes. In:Saunders, A.D., Norry, M.J. (Eds.). Magmatism in the Ocean Basins,Geological Society, London, Special Publication 42, 528 –548.

Todt, W., CliV, R.A., Hanser, A. et al., 1996. Evaluation of a 202Pb/205Pbdouble spike for high-precision lead isotopic analyses. In: Basu, A.,Hart, S. (Eds.). American Geophysical Union Monograph 95, p. 429.

Wang, X.B., Bao, P.S., Deng, W.M., 1987. Xizang (Tibet) ophiolite. Geo-logical Publishing House, Beijing (in Chinese).

Xia, B., Guo, L.Z., Shi, Y.S., 1998. Ophiolites and their terrane tectonics in south-west Tibet. Zhongshan University Press, Guangzhou. 1998 (in Chinese).

Xiao, X.C., Li, T.D., 2000. Tectonic evolution and uplift mechanism ofQinghai-Tibet Plateau. Science and Technology Press of Guangdong,Guangzhou (in Chinese).

Xing, G.F., Shen, W.Z., Wang, D.Z., 1997. Sr-Nd-Pb isotopic compositionand their source in characteristics of Mesozoic-Cenozoic magma inSouth Pole. Acta Petrologica Sinica 13 (4), 473–487 (in Chinese withEnglish abstract).

Yuan, C., Sun, M., Li, J.L., 2002. Tectonic background of the Kudiophiolite, western Kunlun: new constraints from boninite series rocks.Geochimica 31 (1), 43–48 (in Chinese with English abstract).

Zartman, R.E., Doe, B.R., 1981. Plumbotectonics—the model. Tectono-physics 75, 135–162.

Zhang, Q., Zhou, G.Q., 2001. Ophiolites of China. Science Press, Beijing (inChinese).

Zhang, Q., Yang, R.Y., 1985. The plutonite of High-Mg-andesite types inDingqing ophiolite and its geological signiWcance. Chinese ScienceBulletin 30 (16), 1243–1245 (in Chinese).

Zhou, S., 2002. Study on the Geochronology of several key regions ofGangdese magmatic and Yarlung Zangpo ophiolite belts, Tibet.Unpublished Ph.D. thesis submitted to the China University of Geo-sciences, Beijing (in Chinese with English abstract).

Zhou, S., Mo, X.X., Dong, G.C., Zhao, Z.D., Qiu, R.Z., Guo, T.Y., 2004.40Ar–39Ar geochronology of Cenozoic Linzizong volcanic rocks fromLinzhou Basin, Tibet, China, and their geological implications. ChineseScience Bulletin 49 (18), 1970–1979.

the tectonic-setting of ophiolites in the western qinghai-tibet plateau, china

Documents