triassic eclogites from central qiangtang, northern tibet, china: petrology, geochronology and...

Lithos 125 (2011) 173–189

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Triassic eclogites from central Qiangtang, northern Tibet, China: Petrology,geochronology and metamorphic P–T path

Qing-Guo Zhai a,b,c,⁎, Ru-Yuan Zhang c,d, Bor-Ming Jahn c,d, Cai Li e, Shu-Guang Song f, Jun Wang b

a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, Chinab Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, Chinac Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwand Department of Geosciences, National Taiwan University, Taipei 106, Taiwane School of Earth Science, Jilin University, Changchun, Jilin, 130061, Chinaf School of Earth and Space Sciences, Peking University, Beijing, 100871, China

⁎ Corresponding author at: Institute of Geology, ChSciences, Beijing, 100037, China.

E-mail address: [email protected] (Q.-G. Zhai).

0024-4937/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.lithos.2011.02.004

a b s t r a c t
a r t i c l e i n f o
Article history:Received 19 July 2010Accepted 12 February 2011Available online 19 February 2011

Keywords:TibetQiangtangEclogiteZircon U–Pb and 40Ar/39Ar datingOceanic subduction

High-pressure (HP)/low-temperature (LT) metamorphic rocks, such as eclogite and blueschist, are generallyregarded as an indicator of subduction-zone metamorphism. Eclogites have recently been discovered in thecentral Qiangtang Block. Their occurrence is highly significant to the understanding of the closure of the Paleo-Tethys and tectonic evolution of northern Tibet. We report the results of petrological, mineralogical andgeochronological investigations of these rocks, and discuss their tectonic implications. The Qiangtang eclogiteoccurs as blocks and lenses in Grt–Phn schist and marble, and is composed of garnet, omphacite, phengiteand rutile. Eclogitic garnet contains numerous inclusions, such as glaucophane and phengite in the core, andomphacite in the mantle or inner rim. In strongly retrograded eclogite, the omphacite is replaced byglaucophane, barroisite and albite. Four stages of metamorphic evolution can be determined: (1) progradeblueschist facies; (2) peak eclogite facies; (3) decompression blueschist facies and (4) retrograde greenschistfacies. Using the Grt–Omp–Phn geothermobarometer, a peak eclogite facies metamorphic condition of 410–460 °C and 2.0–2.5 GPa was determined. Zircon U–Pb dating gave ages of 230±3 Ma and 237±4 Ma for twoeclogite samples. The ages are interpreted as the time of eclogite facies metamorphism. Moreover, 40Ar/39Ardating of phengite from the eclogite and Grt–Phn schist yielded ages about 220 Ma, which are probablyindicative of the time of exhumation to the middle crust. We conclude that the HP/LT metamorphic rockswere formed by northward subduction of the Paleo-Tethys Ocean and they marked a Triassic suture zonebetween the Gondwana-derived block and Laurasia.

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1. Introduction

Geological studies of the Qiangtang area in the north-centralTibetan plateau have been severely limited by the poor accessibilitydue to its high elevation (N5000 m) and rudimentary road network.A N500 km long east–west trending HP/LT Triassic metamorphic beltin central Qiangtang had been recently documented and studied(Kapp et al., 2000, 2003; Li et al., 1995; Zhang et al., 2006a,b), andrecently eclogite was discovered (Li et al., 2006; Zhang et al., 2006a).Based on preliminary petrological and geochronological studies,several tectonic models have been proposed to explain the

formation of the Qiangtang metamorphic belt. These models are:(1) the Qiangtang metamorphic belt marks an in situ Paleo-Tethyssuture, produced by the amalgamation of the south Qiangtang Blockand the north Qiangtang Block (Li et al., 1995, 2006; Zhang et al.,2006b). (2) The Qiangtang metamorphic belt represents theSongpan–Ganzi flysch deposit that was underthrust ~200 kmsouthward from the Jinsha suture and exhumed in the interior ofthe “Qiangtang terrane” (Kapp et al., 2000, 2003). (3) The Qiangtangmetamorphic belt was regarded as a westward extension of a MiddleTriassic continental collision zone between Laurasia (Kunlun) andGondwana (Qiangtang), which was then rifted from Kunlun to opena second oceanic flysch basin during the Late Triassic (Pullen et al.,2008).

Eclogite is generally considered to be produced in subduction-zonemetamorphism. Some works of age determination and tectonicimplications of the eclogite from the central Qiangtang Block hadbeen reported by several authors (Li et al., 2006; Pullen et al., 2008;

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174 Q.-G. Zhai et al. / Lithos 125 (2011) 173–189

Zhang et al., 2006a). Li et al. (2006) suggested that the peak stagemetamorphism took place in a condition of b500 °C and 1.56–2.35 GPa and considered the eclogite to be a retrograded low-temperature eclogite. On the contrary, Zhang et al. (2006a) obtaineda minimum pressure of 2.0–2.5 GPa at 482 to 625 °C, and regardedthe eclogite to have undergone unambiguous UHP metamorphism.Pullen et al. (2008) obtained two Lu–Hf isochron ages of 244±11 Maand 233±13 Ma for eclogite from the Gemu area and considered theeclogite as HP to UHP metamorphic rocks, but they did not estimatetheir P–T conditions.

The purposes of this paper are to give a more detailed account ofthe metamorphic petrology and to report new ages of the eclogitesand associated Grt–Phn schist, using U–Pb and 40Ar/39Ar techniques.The results will be used to constrain the P–T conditions andmetamorphic evolution of the HP rocks, and finally, to discuss someimplications for a Triassic oceanic closure of the Paleo-Tethys. The

Gemu

Rongma

Gangma Co

Guog

Kunlun Block

Longmu Co-Shuang

B

Jinsha Suture

Laurasiaaffinity rocks

Gondwanaaffinity rocks

Late Triassicvolcanic rocks

Permian Ophioliticmelange and OIB

High pressurerocks

Eclogite

Blueschist

Early PaleazoicOphiolitic melange

84

84

86

86

32

34

Pianshis

P2205

P2201

P22

P2214

Fig.1(c)Fig.1(b)

(a)N

NCaboniferouswhite marble

Eclogite, blackmarble &Grt-Phn schist

Greenschist &quartz schist

200 m

GMC0701

(c)

Fig. 1. (a) Simplified geological map of the Qiangtang area, northern Tibet, (b) a

mineral abbreviations used in this paper are after Kretz (1983), exceptPhn for phengite and Bar for barroisite.

2. Geological outline

From north to south, Tibet is composed of the Kunlun (includingthe Songpan–Ganzi flysch in its southern margin), Qiangtang andLhasa Blocks. The Qiangtang is located in the northern Tibetan plateau,bounded by the Jinsha suture to the north and Bangong–Nujiangsuture to the south (Fig. 1a) (Sengör and Nataľin, 1996; Tapponnieret al., 2001; Yin and Harrison, 2000). In recent years, geologicalinvestigations of 1:250,000 scale were carried out by Chinesegeologists in the northern Tibetan plateau (Pan et al., 2004). A suturezone was identified with the occurrence of eclogite, blueschist andophiolitic mélange (Li et al., 1995, 2006; Pan et al., 2004; Zhai et al.,2007, 2009, 2010; Zhang et al., 2006b). The suture zone is named as

Kunlun Block

Qaidam BlockTarim Block

N. Qiangtang Block

S. Qiangtang Block

Lhasa BlockIndia Block

Longmu Co

Gerze

Shuanghu

LhasaFig.1(a)

Altyn Tagh Fault

Yarlung Zangbu Suture

Kunlun Suture

Shuanghu

anjianian

South Qiangtang Block

North Qiangtang Block

hu suture

angong-Nujiang Suture

Lhasa Block50 100 km0

32

88

34

88

han

E0601E0622

E0665

E1011

Qtz-Phn schist& quartzite

E063904

Eclogite,blueschist,marble &Grt-Phn schist

E0636

0 1 2 km

E0635

(b)

nd (c) sampling locations. The legends in (b) and (c) are the same as in (a).

175Q.-G. Zhai et al. / Lithos 125 (2011) 173–189

the Longmu Co-Shuanghu suture zone, and it separates the Qiangtanginto the north and south Qiangtang Blocks (Li et al., 1995) (Fig. 1a).The north Qiangtang Block consists of Late Devonian to Triassicsedimentary sequences overlain by Jurassic to Cenozoic sedimentaryrocks (BGMR, 1993; Li and Zheng, 1993; Li et al., 1995). SomeLate Paleozoic sedimentary rocks contain fusulinid and/or coralfossils (BGMR, 1993; Li et al., 1995). These fossils show charac-teristics of warm-water biota of the Cathaysian affinity (BGMR,1993; Li and Zheng, 1993; Li et al., 1995; Zhang et al., 2009). The LateTriassic volcanic rocks from the Nadigangri Formation are exposedalong the south margin of the north Qiangtang Block (Zhai and Li,2007).

The Longmu Co-Shuanghu suture zone is an elongate, WNW-trending belt of ~500 km long and ~100 km wide (Li et al., 1995;Zhang et al., 2006b). It is also named as the Qiangtang metamorphic

Fig. 2. Photographs showing field relations between the eclogite blocks and their country rocthe Gemu area; (b) eclogite block in faulted contact with white marble in the Gangma Co armarble (bedded) from the Gemu area; (e) a close-up view of a Grt–Phn schist in the Gemu

belt as defined earlier. The belt is composed of blueschist, eclogite,ophiolitic mélange, and metasedimentary rocks (including greens-chist, Gln-bearing marble and minor chert) (BGMR, 1993; Kapp et al.,2000, 2003; Li et al., 1995, 2006, 2008; Zhai et al., 2004, 2006, 2007,2010; Zhang et al., 2006b). Blueschist occurrences had beendocumented for decades in different areas of the Longmu Co-Shuanghu suture zone, such as Shuanghu, Rongma and Gangma Co(Fig. 1a) (Kapp et al., 2000, 2003; Li et al., 1995; Zhai et al., 2009).The available age data include 40Ar/39Ar ages of 223±4 Ma and227±4 Ma for Na-amphibole from blueschist (Li et al., 1995; Zhaiet al., 2009), 40Ar/39Ar ages of 203 to 222 Ma for white micafrom metapelite (Kapp et al., 2000, 2003; Zhai et al., 2009) andLu–Hf mineral isochron ages of 233±13 Ma and 244±11 Ma foreclogite in the Gemu area (Pullen et al., 2008). All these ages havebeen interpreted as the time of metamorphism for the Qiangtang

ks in the Qiangtangmetamorphic belt. (a) Eclogite block in marble and Grt–Phn schist inea; (c) an eclogite lens in Grt–Phn schist from the Gemu area; (d) and eclogite block inarea and (f) folded marble in the Gangma Co area.


metamorphic belt. The ophiolitic mélange comprises cumulategabbro, basalt and plagiogranite (Zhai et al., 2007, 2010). A zirconSHRIMP U–Pb dating on cumulate gabbros yielded a weighted meanage of 438±10 Ma for the Guoganjianian ophiolite (Li et al., 2008;Zhai et al., 2007) and 467±4 Ma for the Gangma Co ophiolite (Zhaiet al., 2010). In addition, Permian ocean island basalt (OIB) withinterbedded radiolarian chert and limestone of the Lugu Formationis widespread in the Qiangtang metamorphic belt (BGMR, 1993; Panet al., 2004).

The south Qiangtang Block consists of Middle Ordovician toPermianmetasedimentary rocks and Jurassic to Cenozoic sedimentarycover (BGMR, 1993; Li and Zheng, 1993; Li et al., 1995). The southQiangtang Block contained glaciomarine deposits and cold-waterbiota, which probably correspond to the Permo-Carboniferoussediments of southern Tibet (Jin, 2002; Li and Zheng, 1993; Li et al.,1995; Zhang et al., 2009). The ages of detrital zircon from theCarboniferous arkosic sandstone and quartzite in the south QiangtangBlock are similar to those from the Carboniferous strata in the Lhasaterrane (Leier et al., 2007; Pullen et al., 2008). The Lhasa terrane was apart of Gondwana (Yin and Harrison, 2000).

3. Sample description

3.1. Occurrence of eclogite

In this paper, the studied samples are not only from previouslyreported eclogite locality in the Gemu area, but also from a newlocality in the Gangma Co area that is ~150 km to the west of the lasteclogite locality (Fig. 1a).

In theGemuarea, eclogite is distributed in a belt of about 10 km-longand 3 km-wide. In this belt, eclogite occurs mainly as blocks or small

Fig. 3. Photomicrographs showingmineral assemblage (a and b) of eclogite in the Gemu area(b) eclogite sample (E0601) consists of Grt+Omp+Phn+Rt with retrograded phases of barcore, Qtz and Ep in the mantle of garnet (E0639) and (d) glaucophane, and barroisite inclu

lenses in marble and Grt–Phn schist (Fig. 1b). Eclogite blocks areisolated, and have sharp contact with the country marble and Grt–Phnschist (Fig. 2a, d). The size of eclogite block ranges from 1m to 100 m(Fig. 2a, d). Eclogite from most blocks are massive, and locally banded.The eclogite lenses of tens of centimeters long are enclosed in marbleand Grt–Phn schist (Fig. 2c–e). Eclogites in the northern marginof the belt are strongly retrograded, locally, to blueschist. Theoccurrence of blueschist is similar to that of eclogite, as large blocks orsmall lens. Most Grt–Phn schists are foliated and generally fine-grainedwith garnet grains less than 1 mm. However, coarse-grained schistwithlarge garnet grains ranging from 2 mm to 3 mm is occasionally found(Fig. 2c, e).

The newly discovered eclogite lies in the southeast of theGangma Co area (Fig. 1a). Blueschist and ophiolitic mélange hadbeen reported about 30 km west of the eclogite location (Kapp et al.,2000, 2003; Zhai et al., 2010). Eclogite in this belt is about 1 km longand 300 m wide and occurs as blocks within strongly deformedmarble and Grt–Phn schist (Fig. 2b, f). Eclogite blocks range from10 cm to 50 m in size. Most blocks exhibit a strong deformation andretrograded alteration in the margins. Marbles of the Gangma Co areashowwhite and black varieties. Strong deformation is observed in theimmediate contact with eclogite (Fig. 2f).

3.2. Petrography

Both eclogites in the Gemu and Gangma Co show porphyroblastictexture, and have similar petrographic feature. Eclogites have ex-perienced retrogradedmetamorphism, from the least altered, throughstrongly retrograded eclogite to blueschist. The pristine eclogite iscomposed of 30–40 vol.% garnet, 20–40 vol.% omphacite, 2–5 vol.%rutile and b5 vol.% phengite, with or without quartz. Zircon and

; back-scattered electron (BSE) images showing inclusions in garnet (c and d). In (a) androisite and titanite (plane polarized); (c) Grt inclusion-bearing phengite inclusion in thesions in garnet (P2205).

image of Fig.�3


apatite are accessory minerals. Secondary Amp, Ep/Czo, Ttn, Ab,Pg, and Chl are present in some eclogites due to retrogrademetamorphism.

Garnet occurs as euhedral or subhedral crystals of ~50–500 μmin eclogite from the Gemu area (Fig. 3a, b). Most fine-grainedgarnet in eclogite from the Gemu area is inclusion-free. In contrast,coarse-grained garnet contains inclusions of Gln, Phn and Bar(Fig. 3c, d). Although the size of most inclusion is b10 μm, largeinclusions may reach 20–100 μm in the core of porphyroblasticgarnet (Fig. 3c, d). Omphacite is generally subhedral with highlyvariable grain size and modal abundance in eclogite samples. Mostomphacite grains range from ~50 μm to ~1.5 mm in diameters(Fig. 3a, b and d), and are partially replaced by symplectites ofamphibole (barroisite and actinolite) and albite with or withoutquartz (Figs. 3a, b, d and 4a–c). Phengite is a minor phase in eclogite

Fig. 4. Photomicrographs (a and b) and BSE image (c) showing that the omphacite was replaeclogite in the Gemu area. (d) glaucophane, epidote, and omphacite occur in the garnet coreby the symplectitic intergrowths of actinolite and albite (GMC0701); (f) a close-up view of

from the Gemu area, and it generally occurs as flakes of 10–100 μmin length. In some samples phengite is rimmed by paragonite.Retrograded barroisite and actinolite are interstitial between garnetand omphacite or around omphacite (Fig. 3a, b). Some coarse-grained amphibole grains contain abundant inclusions of Grt, Ompand Rt, suggesting that the amphiboles were crystallized aftereclogitic garnet and omphacite.

In the strongly retrograded eclogite in the Gemu area, in additionto the common eclogitic minerals of garnet, phengite, rutile withminor or without omphacite, many secondary phases, such as,glaucophane, barroisite, actinolite, epidote, albite and chlorite havedeveloped. Omphacite in the strongly retrograded rocks is partiallyreplaced by Gln, Bar, Act, Ab and Qtz (Fig. 4a–c), and the secondaryamphibole was formed in the order of Gln, Bar, and Act (Fig. 4b, c).Blueschist in the Gemu area is volumetrically minor. It is similar to

ced by amphiboles, and in turn, by glaucophane, barroisite and actinolite (P2201) fromand rim (GMC0701) from eclogite in the Gangma Co area. (e) Omphacite was replacedthis area shown in (e).

image of Fig.�4

Table 1Chemical analyses of minerals in eclogites, blueschists and Grt–Phn schists in the Qiangtang metamorphic belt.

Rock E0601 E0622 E0639

Eclogite Eclogite Eclogite

Mineral Grt Omp Phn Bar Grt Omp Phn Grt Omp Phn Grt(C) Grt Omp Phn Phn(IG) Bar(IG)

SiO2 38.01 54.86 50.25 47.71 37.99 56.36 51.28 38.00 55.46 51.06 37.38 37.92 55.10 51.95 50.86 45.21TiO2 0.14 0.11 0.80 0.31 0.17 0.05 0.46 0.11 0.07 0.79 bdl 0.13 0.10 0.67 bdl bdlAl2O3 21.11 7.69 26.68 9.56 21.35 7.77 27.48 21.34 9.30 26.68 20.61 21.30 9.98 26.01 26.97 13.11Cr2O3 0.03 0.02 bdl 0.06 bdl 0.01 0.01 bdl bdl bdl bdl 0.01 0.02 bdl 0.01 bdlFeO 29.08 8.24 3.46 13.60 29.04 8.43 3.73 28.85 7.68 3.08 29.09 29.31 8.04 2.94 3.14 15.48MnO 0.30 0.03 bdl 0.05 0.46 0.01 0.01 0.30 0.02 0.01 0.07 0.44 0.01 bdl bdl 0.03MgO 2.14 8.44 3.08 11.80 2.19 8.38 3.27 1.89 7.07 3.14 1.32 2.10 6.47 3.37 3.16 9.58CaO 9.29 14.04 bdl 7.79 9.41 11.75 0.01 10.21 12.51 0.03 10.92 9.60 11.29 bdl 0.01 7.51Na2O bdl 6.06 0.65 3.81 bdl 6.39 0.60 bdl 7.52 0.63 bdl bdl 8.10 0.67 0.62 4.12K2O bdl bdl 10.04 0.55 bdl bdl 8.40 bdl bdl 9.97 bdl bdl bdl 10.10 10.50 0.53Total 100.09 99.50 94.96 95.24 100.61 99.16 95.25 100.69 99.64 95.40 99.38 100.81 99.10 95.72 95.26 95.56O 12 6 11 23 12 6 11 12 6 11 12 12 6 11 11 23Si 3.01 1.99 3.38 7.03 3.00 2.05 3.40 3.00 1.99 3.41 3.01 2.99 1.98 3.45 3.41 6.69AlIV 0.00 0.01 0.62 0.97 0.00 0.00 0.60 0.00 0.01 0.59 0.00 0.01 0.02 0.55 0.59 1.31AlVI 1.97 0.32 1.50 0.69 1.99 0.33 1.55 1.98 0.38 1.51 1.95 1.98 0.41 1.49 1.54 0.97Ti 0.01 0.00 0.04 0.03 0.01 0.00 0.02 0.01 0.00 0.04 – 0.01 0.00 0.03 – –

Cr 0.00 0.00 – 0.01 – 0.00 0.00 – – – – 0.00 0.00 – 0.00 –

Fe3+ 0.00 0.10 0.10 0.56 0.00 0.12 0.16 0.00 0.13 0.05 0.05 0.02 0.14 0.02 0.10 0.68Fe2+ 1.96 0.15 0.09 1.12 1.93 0.14 0.05 1.90 0.10 0.12 1.90 1.91 0.10 0.14 0.08 1.23Mn 0.02 0.00 – 0.01 0.03 0.00 0.00 0.02 0.00 0.00 0.00 0.03 0.00 – – 0.00Mg 0.25 0.46 0.31 2.59 0.26 0.45 0.32 0.22 0.38 0.31 0.16 0.25 0.35 0.33 0.32 2.11Ca 0.79 0.55 – 1.23 0.80 0.46 0.00 0.86 0.48 0.00 0.94 0.81 0.44 – 0.00 1.19Na – 0.43 0.09 1.09 – 0.45 0.08 – 0.52 0.08 – – 0.57 0.09 0.08 1.18K – – 0.86 0.10 – – 0.71 – – 0.85 – – – 0.86 0.90 0.10Cations 8.02 4.00 6.99 15.42 8.01 4.00 6.89 8.00 4.00 6.97 8.01 8.01 4.00 6.97 7.01 15.47P (GPa) 2.0 2.5 2.1 2.3T (°C) 453 462 412 441

Notes: C = core, R = rim, IG = inclusion in garnet, IZ = inclusion in zircon from eclogite. bdl = below detection level. P–T estimates (coexistent mineral pairs) were calculatedusing the Grt–Omp–Phn geobarometer (Ravna and Terry, 2004).


the strongly retrograded eclogite in mineral assemblage and texture,but omphacite is totally replaced by symplectite of Gln, Bar andAb±Chl.

Garnet occurs as idiomorphic porphyroblast in eclogite from theGangma Co area (Fig. 4d). The grain size of garnet grains varies from0.5 to 2 mm, and is generally larger than those from the Gemu area.Most garnet grains contain numerous inclusions: Gln and Bar are inthe core, Omp in the rim or inner rim, and Ep, Rt Qtz and Pg randomlydistributed (Fig. 4d). Omphacite occurs as subhedral or anhedralcrystals of ~100–1000 μm in diameters. Most omphacite grains ineclogite from the Gangma Co area are more strongly retrograded thanthose from the Gemu area. In the extensively retrograded eclogite(GMC0701) omphacite is rimmed by symplectites of fibrous actinoliteand albite (Fig. 4e, f). Furthermore, in some samples with strongretrograded texture, amphibole was recrystallized from symplectites.Consequently, the increase of amphibole minerals in the rock wasaffected by the continuous breakdown of omphacite. Garnet is lessalterated than omphacite. Phengite is also a minor phase in eclogitefrom the Gangma Co area.

The Grt–Phn schist consists of garnet (20–40 vol.%), phengite (30–40 vol.%) and quartz (5–20 vol.%) with minor rutile (~2 vol.%). Garnetoccurs as porphyroblast and contains inclusions of quartz andphengite. Quartz shows a wavy extinction.

4. Mineral chemistry and P–T estimates

Mineral chemical compositions were analyzed by electron probemicro-analyzer (EPMA) JEOL JXA-8800 at the Institute of Geologyand Mineral Resources, Chinese Academy of Geological Sciences(Beijing), and JEOL JXA-8900R at the Institute of Earth Sciences,Academia Sinica (Taipei). The operating conditions were 20 kV,20 nA beam current, and 5 μm probe diameter. Mineral inclusionsin garnet and zircon were analyzed using 2 μm beam. Elemental

mapping was performed using a focused beam with 15 kV and 20 nAprobe current. X-ray intensities were counted in each 1 μm spot for0.025 s. Ferric iron in garnet and clinopyroxene was determinedbased on the scheme of Droop (1987) and Fe3+ = Na–Al–Cr ofCawthorn and Collerso (1974), respectively. Normalized pyroxeneend-member components were calculated on the basis of jadeite(XJd) = AlVI/(Na+Ca), aegirine (XAe) = (Na–AlVI)/(Na+Ca) andaugite (XAu) = (Ca+Mg+Fe2+)/2 (Morimoto, 1988). Fe3+ inamphibole was estimated on the basis of structural formulae of 23oxygens following the charge balance method of Robinson et al.(1982). Representative mineral compositions of various rocks arelisted in Table 1.

4.1. Garnet

In order to examine its chemical variation, compositional mappingand profile measurements were performed on garnet (Figs. 5 and 6).The analytical data of garnet are listed in Table 1 and plotted in Fig. 7.Garnet in the eclogites from both the Gemu and Gangma Co localitiesis almandine- and grossular-rich (Alm48–66Prp2–11Grs25–32Sps0–21).The compositions of garnet from strong retrograded eclogite(Alm61–65Prp6Grs27–29Sps24) and blueschist (Alm63–66Prp7–8Grs26–30Sps0) are similar to those of garnets from eclogite. Some eclogiticgarnets exhibit a pronounced zoning in composition. In general,the almandine and pyrope components increase with decreasingspessartine component from core to rim (Figs. 5 and 6). In sampleGMC0701, Sps component decreases from~20 mol% in the core to Sps-free in the outer rim (Fig. 6a). The grossular component, how-ever, shows a small but irregular variation. Garnet from the Grt–Phn schist (Alm66–78Prp7–9Grs8–20Sps5–7) is relatively Alm-rich andGrs-poor. The compositions of garnet from eclogite, strong retro-graded eclogite, blueschist and Grt–Phn schist all plot in the C-typeeclogite field as defined by Coleman et al. (1965). However, garnet in


E1011 E0665 P2205 P2214

Eclogite Eclogite Eclogite Eclogite

Grt Omp Phn Omp(IZ) Omp(IZ) Phn(IZ) Phn(IZ) Bar Grt Omp Phn Grt Omp Phn Phn(IZ) Bar Act Grt Omp Phn

38.19 54.94 51.90 53.04 53.73 51.29 51.78 47.95 37.68 55.02 51.17 37.86 55.08 49.25 54.68 49.00 54.86 37.89 54.63 51.940.13 0.10 0.37 0.14 0.08 0.27 0.46 0.42 0.17 0.03 0.26 0.04 0.07 0.48 0.06 0.35 0.72 0.14 0.05 0.2620.70 9.43 26.16 10.47 7.46 26.64 27.70 10.44 21.21 8.62 25.52 21.39 8.49 26.34 25.62 11.16 2.35 20.91 8.72 25.42bdl 0.02 0.05 bdl bdl bdl 0.04 0.03 0.02 0.06 bdl bdl 0.01 0.03 0.04 bdl bdl bdl bdl 0.0329.14 7.52 3.06 10.81 10.86 3.08 3.20 13.87 28.82 6.28 2.97 29.22 7.55 3.56 2.41 15.79 11.99 27.47 7.27 40.42 0.01 0.02 0.08 0.13 0.02 0.03 0.07 0.20 bdl bdl 0.06 0.05 bdl 0.02 0.10 0.02 0.28 bdl 0.031.82 7.41 3.41 6.46 7.34 3.88 3.25 11.41 2.75 8.46 3.64 2.50 7.80 3.31 3.57 9.48 15.67 2.30 8.01 3.879.96 12.56 0.01 11.27 13.39 bdl bdl 8.35 9.11 14.14 0.19 8.87 13.19 bdl 0.01 6.13 9.29 10.69 13.60 0.03bdl 7.26 0.47 7.29 6.82 0.37 0.68 3.71 bdl 6.70 0.94 bdl 7.00 0.62 0.43 5.03 1.78 bdl 7.27 0.46bdl bdl 10.20 0.12 bdl 10.50 9.98 0.41 bdl bdl 10.61 bdl bdl 10.53 8.24 0.36 0.14 bdl bdl 10.69100.36 99.24 95.64 99.68 99.81 96.04 97.10 96.65 99.96 99.31 95.31 99.94 99.24 94.12 95.07 97.40 96.82 99.68 99.55 96.7312 6 11 6 6 11 11 23 12 6 11 12 6 11 11 23 23 12 6 113.03 1.98 3.45 1.92 1.95 3.41 3.39 6.98 2.99 1.98 3.44 3.00 1.99 3.36 3.58 7.09 7.79 3.01 1.96 3.450.00 0.02 0.55 0.08 0.05 0.59 0.61 1.02 0.01 0.02 0.56 0.00 0.01 0.64 0.42 0.91 0.21 0.00 0.04 0.551.94 0.38 1.51 0.37 0.27 1.50 1.53 0.78 1.97 0.34 1.46 2.00 0.35 1.48 1.56 0.99 0.18 1.96 0.33 1.440.01 0.00 0.02 0.00 0.00 0.01 0.02 0.05 0.01 0.00 0.01 0.00 0.00 0.02 0 0.04 0.08 0.01 0.00 0.01– 0.00 0.00 – – – 0.00 0.00 0.00 0.00 – – 0.00 0.00 0 – – – – 0.000.00 0.11 0.06 0.22 0.26 0.16 0.10 0.42 0.03 0.10 0.08 0.00 0.13 0.18 0 0.46 0.54 0.01 0.14 0.171.95 0.12 0.11 0.11 0.07 0.00 0.07 1.27 1.88 0.09 0.08 1.95 0.10 0.03 0.13 1.45 0.89 1.82 0.08 0.050.03 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 – – 0.00 0.00 – 0 0.01 0.00 0.02 – 0.000.21 0.40 0.34 0.35 0.40 0.38 0.32 2.48 0.33 0.45 0.36 0.30 0.42 0.34 0.35 2.05 3.32 0.27 0.43 0.380.85 0.48 0.00 0.44 0.52 – – 1.30 0.77 0.54 0.01 0.75 0.51 – 0 0.95 1.41 0.91 0.52 0.00– 0.51 0.06 0.51 0.48 0.05 0.09 1.05 – 0.47 0.12 – 0.49 0.08 0.05 1.41 0.49 – 0.51 0.06– – 0.87 0.01 – 0.89 0.83 0.08 – – 0.91 – – 0.92 0.69 0.07 0.03 – – 0.918.01 4.00 6.96 4.01 4.00 7.00 6.97 15.43 8.01 4.00 7.05 8.01 4.00 7.05 6.79 15.43 14.93 8.00 4.00 7.032.2 2.1 2.0 2.3432 411 411 419


Grt–Phn schist has lower grossular component than those in eclogite(Fig. 7).

4.2. Omphacite and phengite

The chemical compositions of omphacite and white mica fromeclogites in the Gemu and Gangma Co area are similar. Omphacitefrom both pristine and strongly retrograded eclogite (as relic) hasrather homogeneous compositions. The jadeite component ofomphacite ranges from 31 to 42 mol% (Table 1 and Fig. 8). Omphaciteinclusions (5–30 μm) in zircon and garnet show a similar range (27–39 mol%) of jadeite components (Fig. 8). The majority of white micafrom eclogite and Grt–Phn schist has Si values of 3.35 to 3.48 pfu.The inclusions of phengite in zircon and garnet have similar Si pfuwith an exception (3.58 pfu).

4.3. Amphibole

Three types of amphibole occurrence have been identified fromeclogite in the Gemu and Gangma Co area: (1) as inclusions ingarnet (Fig. 3d), (2) as an anhedral phase around omphacite andphengite in the matrix (Figs. 3 and 4a–c), and (3) as an anhedral orfibrous grained crystals in symplectites (Fig. 4e, f). The amphibolegrains of different stages show a large variation in composition(Fig. 9), but all analyzed amphibole grains have high Mg/(Mg+Fe)ratios ranging from 0.57 to 0.82. The amphibole inclusions ineclogitic garnet are glaucophane and barroisite, whereas the retro-graded amphiboles in the matrix of the rocks including blueschistare glaucophane, barroisite and actinolite (Fig. 9). The glauco-phane inclusions in eclogitic garnet have NaB values (1.51–1.72 pfu)lower than those in the matrix of retrograded eclogite (1.73–1.82 pfu) and blueschist (1.77–1.78 pfu). The barroisite is character-ized by Si ranging from 6.69 to 7.28 pfu and NaB from 0.70 to

1.08 pfu. The actinolite has high Si values of 7.62–7.79 and low NaBof 0.38–0.42.

4.4. P–T estimates

Petrographic observations indicate that the peak stage assemblagein the Qiangtang eclogite is Grt+Omp+Phn+Rt, whereas matrixbarroisite, glaucophane and epidote are obviously late-stage mineralsthat replaced omphacite in most studied samples. P–T estimatesare based on the combination of the garnet–clinopyroxene Fe2+–Mgexchange thermometer of Ravna (2000) and the internally consistentgeothermobarometric formulations derived from the reactions:

3 celadoniteþ 2 grossular þ 1 pyrope→6 diopsideþ 3 muscovite

(Ravna and Terry, 2004). Peak equilibrium temperatures wereestimated from the Fe2+–Mg partition (KD) between coexistinggarnet and clinopyroxene. The rim of the garnet and the adjacentomphacite and phengite are assumed to have been in equilibriumduring the peak metamorphism, hence they are used for P–Tcalculation. The Fe3+ of clinopyroxene was estimated to equal Na–(AlVI+Cr). The P–T conditions of the peak eclogite stage werecalculated using the Grt–Omp–Phn geobarometer (Ravna and Terry,2004) and Grt–Cpx geothermometer (Ravna, 2000). Temperatures of410–460 °C (average 430 °C) with ±60 °C uncertainty, and pressuresof 2.0–2.5 GPa (average 2.2±0.2 GPa) were obtained for eclogitesfrom the Gemu and Gangma Co areas (Table 1). The uncertainty of theGrt–Cpx Fe–Mg thermometry is related to the oxidation state of iron.Generally, the uncertainty of the temperature estimate for eclogite isabout ±60 °C using the calibration by Ravna and Paquin (2004). Theuncertainty of the Grt–Omp–Phn barometer couldn't be expressed asan absolute value (see Krogh Ravna and Terry, 2004, discussion).Ravna and Terry (2004) discussed the deference of pressure estimates

Table 1 (continued)

Rock P2214 GMC0701 P2204

Eclogite Eclogite Retrograded eclogite

Mineral Grt (C) Grt(R) Grt Omp Phn Grt (C) Omp(IG) Gln(IG) Gln(IG) Gln(IG) Act Ab Grt Omp Phn Grt Omp Phn Gln

SiO2 37.90 38.08 36.99 55.34 52.14 37.14 55.26 58.24 55.96 58.36 53.67 68.13 37.34 55.07 51.72 37.61 56.06 51.57 56.31TiO2 0.14 0.21 0.12 0.05 0.19 0.29 bdl 0.18 bdl 0.05 0.04 0.01 0.22 bdl 0.30 0.16 bdl 0.30 bdlAl2O3 21.03 20.99 20.45 9.83 25.49 18.62 8.96 10.95 10.70 10.30 3.78 19.41 20.56 9.16 26.18 21.46 8.97 27.02 10.94Cr2O3 0.02 0.07 bdl bdl bdl bdl 0.01 bdl bdl bdl bdl bdl 0.05 bdl 0.08 bdl bdl 0.40 bdlFeO 25.45 30.07 29.85 7.03 3.23 21.92 7.81 12.37 14.84 12.72 13.64 0.22 29.78 8.54 4.59 27.85 8.09 3.14 11.58MnO 4.00 0.69 0.10 bdl 0.01 9.30 0.01 bdl bdl bdl 0.02 0.01 0.73 bdl bdl 1.66 bdl 0.02 0.03MgO 0.76 1.26 1.73 6.81 3.43 0.45 7.43 9.24 7.46 9.48 14.75 bdl 1.41 7.08 3.35 1.60 8.11 3.30 9.68CaO 11.20 9.60 9.77 12.69 0.01 10.75 13.75 0.83 0.61 1.13 10.24 0.16 9.76 12.48 bdl 10.50 12.05 bdl 1.22Na2O bdl bdl bdl 7.32 0.28 bdl 6.50 5.83 5.60 5.70 1.38 11.31 bdl 7.15 0.36 bdl 7.05 0.43 6.39K2O bdl bdl bdl bdl 11.32 bdl bdl 0.03 bdl 0.03 0.13 0.04 bdl bdl 10.13 bdl bdl 9.60 0.02Total 100.50 100.97 99.00 99.08 96.08 98.46 99.73 97.65 95.17 97.76 97.64 99.29 99.85 99.47 96.71 100.84 100.34 95.78 96.17O 12 12 12 6 11 12 6 23 23 23 23 8 12 6 11 12 6 11 23Si 3.01 3.01 2.99 2.00 3.48 3.05 2.00 7.94 7.90 7.96 7.62 3.00 3.00 1.99 3.43 2.97 2.00 3.42 7.84AlIV 0.00 0.00 0.01 0.00 0.52 0.00 0.00 0.06 0.10 0.04 0.38 0.00 0.00 0.01 0.57 0.03 0.00 0.58 0.16AlVI 1.97 1.96 1.94 0.42 1.48 1.80 0.38 1.69 1.68 1.62 0.25 1.00 1.94 0.38 1.47 1.97 0.38 1.53 1.64Ti 0.01 0.01 0.01 0.00 0.01 0.02 – 0.02 0.00 0.01 0.00 0.00 0.01 – 0.01 0.01 – 0.01 –

Cr 0.00 0.00 – – – – 0.00 – – – – – 0.00 – 0.00 – – 0.02 –

Fe3+ 0.00 0.00 0.08 0.09 0.04 0.09 0.07 0.54 0.71 0.57 0.60 0.00 0.05 0.11 0.18 0.05 0.11 0.10 0.42Fe2+ 1.71 2.02 1.93 0.12 0.14 1.42 0.16 0.87 1.05 0.88 1.02 0.01 1.95 0.15 0.07 1.79 0.13 0.07 0.93Mn 0.27 0.05 0.01 – 0.00 0.65 0.00 – – – 0.00 0.00 0.05 – – 0.11 – 0.00 0.00Mg 0.09 0.15 0.21 0.37 0.34 0.06 0.40 1.88 1.57 1.93 3.12 – 0.17 0.38 0.33 0.19 0.43 0.33 2.01Ca 0.95 0.81 0.85 0.49 0.00 0.95 0.53 0.12 0.09 0.17 1.56 0.01 0.84 0.48 – 0.89 0.46 – 0.18Na – – – 0.51 0.04 – 0.46 1.54 1.53 1.51 0.38 0.96 – 0.50 0.05 – 0.49 0.06 1.73K – – – – 0.96 – – 0.01 – 0.01 0.02 0.00 – – 0.86 – – 0.81 0.00Cations 8.01 8.02 8.02 4.00 7.01 8.02 4.00 14.67 14.63 14.68 14.96 4.98 8.01 4.00 6.98 8.01 4.00 6.93 14.91P (GPa) 2.3 2.1 2.2T (°C) 427 423 439


between their method and other methods. For example, the baro-meter gives higher pressures than the corresponding method ofWaters andMartin (1993), because they use a different garnet activitymodel. If the garnet activity model of either Berman (1990) orGanguly et al. (1996) is used with the method of Waters and Martin(1993), pressure becomes about 0.2 GPa higher (Cuthbert et al.,2000). In addition, temperatures of 472–511 °C (average 485±20 °C)was obtained by the geothermometer of Powell (1985) at a givingpressure of 2.2 GPa (detailed data were shown in Appendix A). Theseresults lie mainly in the lawsonite–eclogite field and represent thepeak stage of the Qiangtang eclogite.

5. U–Pb geochronology

5.1. Zircon and inclusions

Zircon grains were separated from eclogite according to thefollowing procedure. (1) Rock samples of 30–40 kg were crushed andsieved to a grain size of ~100 μm. (2) Iron fillings and metal oxideswere removed by hand magnet from the washed and dried sample.(3) Zircon and apatite were separated from other minerals with aFrantz magnetic separator. (4) Apatite was separated from zirconusing heave liquid. Finally zircons were purified by hand-pickingunder a binocular microscope.

Although zircon grains from the Qiangtang eclogite are generallyb10 μm in the matrix of eclogite, a few larger grains (~100 μm) wereobtained from two eclogite samples (E1011 and P2205) of the Gemuarea. Zircon grains from sample E1011 and P2205 have similarcharacteristics; they range in size from 50 to 150 μm, and have anelongated euhedral form and colorlessness (Fig. 10a, b). The CL imagesshow that they are mostly uniform without oscillatory-zoning butsome grains have high luminescent bright stripes (Fig. 10a, b). Zircongrains from the Qiangtang eclogite contain inclusions of phengite,

garnet and omphacite (Table 1). The sizes of mineral inclusions rangefrom 5 to 50 μm (Fig. 10c).

5.2. Analytical method

Zircon crystals were embedded in 25 mm epoxy disks andpolished down to approximately half the zircon thickness. The zirconCL images were obtained on a FEI PHILIPS XL30 SFEG SEM with 2 minscanning time at conditions of 15 kV and 12 nA at Peking University.The U–Pb isotope analyses of zircon for sample E1011were performedusing SHRIMP II at the Beijing SHRIMP Center of Institute of Geology,Chinese Academy of Geological Sciences. Instrumental conditions andmeasurement procedures were the same as described by Compstonet al. (1992). The diameter of the ion beam was 30 μm and the datawere collected in sets of five scans through the masses with 2 nAprimary O2 beams. The reference zircon was analyzed first and thenafter every three unknowns. Standard zircon sample SL13 (572 Ma)was measured to calibrate U, Th and Pb concentrations, and standardzircon TEM (417 Ma) was used for isotopic fractionation correction(Black et al., 2003). The data were processed with the SQUIDand ISOPLOT software of Ludwig (1999, 2001). The errors given inTable 2 for individual analyses are quoted at the 1σ level, whereas theerrors for the weighted mean ages are quoted at 2σ (95% confidencelevel).

The zircon from sample P2205 was analyzed using an Agilent7500a quadruple (Q)-ICP–MS and 193 nm laser ablation system at theInstitute of Geology and Geophysics (IGG), Chinese Academy ofSciences (CAS). The analytical method for U–Pb age is the same asthose described in Xie et al. (2008). The beam size was 40–60 μm. Thefractionation correction and results were calculated using GLITTER 4.0(Jackson et al., 2004) and then corrected using the Harvard zircon91500 as external standard. Common Pb was corrected according tothe method proposed by Andersen (2002). The weighted mean ages


P2204 P2201 E0636

Retrograded eclogite Blueschist Grt-Phn schist

Gln Gln(IG) Gln Bar(R) Act(R) Ab Grt Grt Phn Phn Gln Gln Bar Grt Grt Phn Phn

55.92 55.80 56.36 51.09 54.78 68.37 38.16 37.67 50.14 50.42 56.75 56.03 49.31 36.34 36.94 50.82 51.11bdl bdl bdl bdl 0.04 bdl bdl bdl 0.59 0.43 bdl bdl bdl 0.09 0.19 0.62 0.6010.70 9.55 10.78 9.76 2.67 19.42 21.44 21.18 25.47 28.57 11.21 11.03 12.74 20.11 20.02 28.22 28.100.04 0.06 bdl bdl 0.02 bdl bdl 0.01 0.02 0.05 bdl 0.01 0.04 0.01 bdl bdl 0.0411.84 13.36 11.46 13.63 10.60 0.13 30.09 28.60 4.65 2.83 11.34 12.14 15.68 35.09 29.86 2.39 2.310.02 0.02 bdl 0.03 0.03 bdl 0.05 0.03 0.01 0.04 0.01 bdl 0.01 2.25 3.02 bdl 0.0310.10 9.15 9.67 11.56 16.66 0.02 2.11 1.84 3.28 2.88 9.77 9.76 9.55 2.35 1.73 2.89 2.911.63 1.86 1.19 6.93 9.91 0.24 9.10 10.56 0.01 0.11 0.96 1.52 6.04 2.75 7.15 bdl bdl6.73 7.08 6.80 4.05 1.73 11.41 bdl bdl 0.34 0.87 6.61 6.76 4.86 bdl bdl 1.06 0.950.02 0.02 0.02 0.20 0.15 0.07 bdl bdl 9.64 8.82 0.03 0.02 0.30 bdl bdl 9.69 9.4296.99 96.90 96.29 97.23 96.60 99.66 100.95 99.89 94.15 95.02 96.68 97.27 98.52 98.98 98.91 95.69 95.4723 23 23 23 23 8 12 12 11 11 23 23 23 12 12 11 117.77 7.891 7.87 7.28 7.77 3.00 3.01 3.00 3.42 3.35 7.85 7.77 6.99 2.99 3.01 3.37 3.390.23 0.109 0.13 0.72 0.23 0.00 0.00 0.00 0.58 0.65 0.15 0.23 1.01 0.01 0.00 0.63 0.611.53 1.482 1.65 0.92 0.21 1.00 1.99 1.98 1.46 1.59 1.67 1.57 1.12 1.93 1.92 1.57 1.58– – – – 0.00 – – – 0.03 0.02 – – – 0.01 0.01 0.03 0.030.00 0.007 – – 0.00 – – 0.00 0.00 0.00 – 0.00 0.01 0.00 – – 0.000.40 0.112 0.28 0.53 0.50 0.00 0.00 0.03 0.19 0.10 0.42 0.38 0.67 0.11 0.06 0.03 0.020.98 1.468 1.06 1.09 0.76 0.01 1.98 1.88 0.07 0.06 0.89 1.03 1.19 2.29 1.97 0.10 0.110.00 0.003 – 0.00 0.00 – 0.00 0.00 0.00 0.00 0.00 – 0.00 0.16 0.21 – 0.002.09 1.929 2.02 2.46 3.52 0.00 0.25 0.22 0.33 0.29 2.02 2.02 2.02 0.29 0.21 0.29 0.290.24 0.282 0.18 1.06 1.51 0.01 0.77 0.90 0.00 0.01 0.14 0.23 0.92 0.24 0.62 – –

1.82 1.941 1.84 1.12 0.48 0.97 – – 0.04 0.11 1.77 1.82 1.33 – – 0.14 0.120.00 0.003 0.00 0.04 0.03 0.00 – – 0.84 0.75 0.01 0.00 0.05 – – 0.82 0.8015.06 15.227 15.03 15.21 15.01 4.99 8.00 8.01 6.97 6.93 14.92 15.05 15.31 8.02 8.01 6.98 6.95


and Concordia plots were processed using ISOPLOT 3.0 (Ludwig,2003). In this study, zircon 91500 as an unknown sample yielded aweighted 206Pb/238U age of 1063±6 Ma (n=8), which is similar tothe reference age of 1065 Ma (e.g., Nebel-Jacobsen et al., 2005;Wiedenbeck, et al., 1995).

5.3. Results

Eleven to fourteen zircon grains from two eclogite samples(E1011 and P2205) were analyzed, and the results are listed inTables 2 and 3. In most analyzed gains, U and Th contents of sampleP2205 (185–2111 ppm and 144–2903 ppm) are higher than those ofsample E1011 (84–569 ppm and 63–804 ppm). Ten analyses fromsample E1011 yielded 206Pb/238U ages ranging from 222 to 237 Mawith high Th/U values of 0.77–1.77 (Table 2). These analyses areconcordant (Fig. 11a), and define a weighted mean age of 230±4 Ma(MSWD=1.6). The meanings of four outliers (spot 1.1, 4.1, 12.1 and14.1) are not clear and are excluded from the calculation of theweighted mean age. Eleven analyses from sample P2205 show 206Pb/238U ages of 231–242 Ma and gave a weighted mean 206Pb/238U age of237±4 Ma (MSWD=0.64) (Table 3, Fig. 11b). These zircons are alsocharacterized by high Th/U ratios ranging from 0.78 to 2.04 (Table 3).In summary, the zircon U–Pb ages from eclogite in the Gemu area arebetween 230±4 Ma and 237±4 Ma.

6. 40Ar/39Ar dating

6.1. Analytical method

As described in the former section, phengite was replaced byparagonite in some samples. In order to obtain reliable 40Ar/39Ar age ofphengite, we selected a sample whose phengite grains were idiomor-phic and without experiencing retrograde metamorphism. Phengitic

micas from two eclogites (E0635 and GMC0701) and a country rock(Grt–Phnschist, E0636)were selected for 40Ar/39Ardating. Thephengiteflakes were purified using magnetic separation, and further cleanedin an ultrasonic bath with ethanol. The purity is better than 99%. Themica samplewaswrapped in aluminum foil and loaded into a tube of Alfoil, and then sealed into a quartz bottle (height: 40 mm; diameter:50 mm). The bottle was irradiated for 65 h in a nuclear reactor (theSwimming Pool Reactor, Chinese Institute of Atomic Energy, Beijing).The reactor delivered a neutron flux of ~6×1012 n/cm2/s and theintegrated neutron flux is about 1.16×1018 n/cm2. Ar isotope analysiswas carried out on a MM-1200B mass spectrometer at the Institute ofGeology, Chinese Academy of Geological Sciences, Beijing. The mea-sured isotopic ratios were corrected for mass discrimination, atmo-spheric Ar component, procedural blanks and mass interferenceinduced by irradiation. The blanks of m/e of 40Ar, 39Ar, 37Ar and 36Arare less than 6×10−15, 4×10−16, 8×10−17 and 2×10−17 mol, res-pectively. The correction factors of interfering isotopes produced duringthe irradiationwere determined by the analysis of irradiated K2SO4 andCaF4 pure salts. Their values are (36Ar/37Aro)Ca=0.0002389, (40Ar/39Ar)K=0.004782 and (39Ar/37Aro)Ca=0.000806. All 37Ar were cor-rected for radiogenic decay (half-life 35.1 days). The 40K decay constantused is 5.543×10−10 a−1 (Steiger and Jäger, 1977). The standardsample is ZBH-25 which has an age of 132.7±1.2 Ma and a potassiumcontent of 7.6% (Chen et al., 2002). The uncertainties on the apparentages on each step are quoted at the 1σ level, butweightedmeanplateauages and isochron ages are given at the 2σ level (Ludwig, 2001).

6.2. Results

The results of 40Ar/39Ar dating are shown in Table 4. The phengitesamples yielded well-defined plateau ages of 214.1±1.8 Ma and219.5±1.7 Ma for eclogite and 223.2±1.7 Ma for Grt–Phn schist. The40Ar/36Ar intercept values are comparable with the atmosphere value

Fig. 5. X-Ray element mappings showing the composition zoning of eclogitic garnet (E0639).


(295.5) within error for both samples (Fig. 12). These ages representthe time when the temperature of phengite was cooled to the closuretemperature of 350–400 °C during the retrograde metamorphism(Hames and Bowring, 1994; Harrison et al., 2009). The occurrence ofexcess argon has been reported for phengite from HP/UHP metamor-phic rocks (Jahn et al., 2001; Li et al., 1994; Ruffet et al., 1997),therefore, caution must be warranted when interpreting 40Ar/39Arstep-heating data from phengite in eclogite. However, the agreementbetween the plateau and isochron ages, the atmosphere compositionof trapped 40Ar/36Ar values argue against a significant excess argoncomponent in the phengite of this study. We conclude that the

0

0

0

0

0

0

0

1.5mm0

0.1

0.2

0.3

0.4

0.5

0.6GMC0701(Garnet)

Chl EpChl

Xalm

Xgrs

Xprp

Xsps

A B0.7

rim core rim

Fig. 6. Compositional profiles from core to rim for two eclogitic garnets. From core to rim the gFigs. 3d and 5).

obtained ages are significant and represent the time of cooling in theretrograded metamorphism.

7. Discussion and conclusion

7.1. Age of HP metamorphism

Mineral inclusions in zircons from eclogite are useful for under-standing the origin and P–T condition of zircon (re)crystallization (Liuet al., 2004, 2007). As described above, zircon crystals from theQiangtang eclogite contain numerous inclusions of garnet, omphacite

0.55mm0

.1

.2

.3

.4

.5

.6

.7

Xalm

Xgrs

Xprp

Xsps

E0639(Garnet)C D

rim core rim

rossular component decreases with increasing Alm component (for profile location, see

image of Fig.�6

Retrograded eclogite

Blue schist

Grt-Phn schist

EclogiteGrs

Prp

C-type

B-type

A-type

Alm Sps+

Fig. 7. Composition of garnets in eclogite, blueschist and Grt–Phn schist. The threegarnet groups are defined by Coleman et al. (1965).


and phengite (Fig. 10). EPMA analyses show that the compositionsof omphacite and phengite inclusions are similar to those of Ompand Phn in the matrix of eclogite. The presence of inclusions of theeclogite facies assemblage suggests that the zircons were formedduring eclogite facies metamorphism. However, these zircons haveunusually high Th/U ratios, with some even greater than unity(Tables 2 and 3). Note that high ratios are mostly found in zircons ofigneous rocks (e. g. gabbros) but are unusual in metamorphic zircons(Hoskin and Black, 2000). This study indicates that zircon with highTh/U ratio (N1) could also be formed in metamorphic rocks. It hasbeen reported that such kind of zircons were found in oceanic-typesubduction zones where fluid is enriched (Dubinska et al., 2004; Liatiand Gebauer, 1999; Rubatto and Hermann, 2003; Rubatto et al., 1999).As the fluid could enhance component transport, zircons could alsocrystallize at low temperatures (b550 °C) (Rubatto and Hermann,2003; Rubatto et al., 1999; Tsujimori et al., 2005). Consequently, theU–Pb zircon ages of 230±4 Ma and 237±4 Ma are interpreted as thetime of the peak eclogite stage.

Two eclogite samples from the Gemu area had been dated usingthe Lu–Hf isochron method. Pullen et al. (2008) obtained two mineral

WEF

Jd Ae

Quad

OmphaciteAegirine

Augite−

Jadeite Aegirine

Eclogite

Inclusionin zircon

Inclusionin garnet

Retrogradedeclogite

Fig. 8. Omphacite compositions plot on a WEF (Wo, En, and Fs)-Jd-Ae ternary diagramfor eclogite and blueschist. Clinopyroxene classification is after Morimoto (1988).

isochron ages of 233±13 and 244±11 Ma. These ages are similar tothe U–Pb zircon ages of this study. Although we did not obtain azircon age for eclogite from the Gangma Co area, the new 40Ar/39Arage is similar to those of eclogite from the Gemu area. The eclogitefrom the two localities, though separated by 150 km, have similarmineral assemblages and P–T conditions. Consequently, the twoeclogites most probably underwent the peak metamorphism at thesame time.

The 40Ar/39Ar ages of phengite (214–220 Ma) from the Qiangtangeclogite are about 10–15 Ma younger than the zircon ages. Theyrepresent the time of eclogite retrogression to greenschist. Similar40Ar/39Ar ages (203–222 Ma) of white mica have been reported formetapelite from the Qiangtang metamorphic belt (Kapp et al., 2000,2003; Zhai et al., 2009). Apparently, the Late Triassic age (203–222 Ma) can be interpreted as the exhumation time of the entireQiangtang metamorphic belt.

7.2. Metamorphic evolution of the Qiangtang eclogite

Petrographic observations, P–T estimate and ages of variousstages are critical for understanding the metamorphic evolutionand tectonic significance of any metamorphic rock or terrane. Basedon the textural relations and mineral compositions described above,the eclogites from the Gemu and Gangma Co areas seem to haveshared a common metamorphic evolution. Four stages of evolutioncan be recognized: (1) pre-eclogite (blueschist) facies progrademetamorphism; (2) peak eclogite facies metamorphism; (3) decom-pression blueschist facies metamorphism and (4) retrograde greens-chist facies metamorphism (Fig. 13). The four stages are describedbelow:

Stage I (prograde blueschist stage): The inclusions of glaucophane,barroisite and phengite in the Sps-rich cores of zoned garnet (Fig. 3c, dand Fig. 4d) suggest that the eclogites from the Gemu and Gangma Coareas underwent a blueschist facies metamorphism prior to theeclogite facies metamorphism (Fig. 13).

Stage II (peak eclogite stage): The peak eclogite facies metamor-phism is characterized by eclogitic equilibriummineral assemblage ofGrt+Omp+Phn+Rt. Omphacite crystals not only occur as inclusionin the mantle and inner rim of garnet, but also in the matrix. Themantle and inner rim of garnet contains much higher Alm and lowerSps components than the core. The peak P–T condition of all eclogitesamples mainly lies in the lawsonite–eclogite field (Fig. 13). U–Pbdating of zircons from the eclogite gave mean ages of 230±4 Ma and237±4 Ma, which are interpreted as the time of the eclogite faciesmetamorphism. Note that the metamorphism did not reach thecondition of ultrahigh-pressure (UHP) metamorphism, because thecritical index minerals of UHP metamorphism, such as coesite ordiamond (Liou et al., 2004, 2009), have not been identified.

Stage III (decompression blueschist stage): This stage is character-ized by the presence of retrograded glaucophane, barroisite andepidote in strongly retrograded eclogite and blueschist in theGemu area. At this stage, omphacite was replaced or rimmed byglaucophane, barroisite and albite (Fig. 4a–c). The retrogradedassemblage suggests that eclogites in the Gemu areawere overprintedby the blueschist facies metamorphism. Meanwhile, the presence ofbarroisite suggests that the decompression P–T path may enter theepidote amphibolite facies (Matsumoto et al., 2003) (Fig. 13). The40Ar/39Ar ages of 223±4 Ma and 227±4 Ma of sodic amphibolesfrom the Qiangtang blueschists (Li et al., 1995; Zhai et al., 2009)may mark this decompression stage. The record of this stage is notclear for eclogites from the Gangma Co area, because they hadexperienced a stronger retrograded metamorphism in which ompha-cite was replaced by the intergrowths of fibrous actinolite and albite(Fig. 4e, f).

Stage IV (retrograde greenschist stage): The greenschist faciesretrogression is ubiquitous in all eclogite samples. Omphacite is

image of Fig.�7

0.0

0.5

1.0

1.5

2.0

6.57.07.58.0

Na B

Si

rim

Glaucophane

Winchite

Actinolite

Barroisite

Magnesio-hornblende

Matrix in eclogite

Inclusion in garnet from eclogite Matrix in blue schist

Inclusion in garnetfrom retrograde eclogiteMatrix in retrograde eclogite

Fig. 9. Plot of Si vs. NaB for amphiboles in the eclogite and blueschist. Classification of amphibole is after Leake et al. (1997).


replaced by symplectite of actinolite and albite (Fig. 4e, f) inretrograded eclogite from the Gangma Co area, and by intergrowthsof barroisite and actinolite (Fig. 4c) in eclogite from the Gemu area.The 40Ar/39Ar ages of phengite range from 214 to 220 Ma. This periodof time probably recorded the time of the last exhumation of theQiangtang eclogite.

7.3. An oceanic subduction zone

The origin of the blueschist and eclogite from the Qiangtangmetamorphic belt had been variably interpreted as a result of anoceanic (Li et al., 1995, 2006; Zhang et al., 2006b) or a continentalsubduction (Pullen et al., 2008; Zhang et al., 2006a). The peak P–Tcondition (2.0–2.5 GPa and 410–460 °C) of the Qiangtang eclogitemainly lies in the lawsonite–eclogite field (Fig. 13), and indicates thatthe eclogite was formed at an environment with a low geothermalgradient of ~6 °C km−1. This gradient is comparable with that of the

Table 2U, Th and Pb isotopic data of zircons from eclogite sample E1011 (data obtained at the Beij

Spot Common206Pb (%)

U(ppm)

Th(ppm)

232Th/238U

206Pb*(ppm)

207Pb⁎/206Pb⁎

±% 207Pb⁎/235U

±% 2

2

1.1 0.91 243 211 0.90 8.80 0.0559 3.8 0.321 4.8 02.1 0.99 470 569 1.25 14.3 0.0493 3.4 0.2385 3.8 03.1 1.55 457 475 1.07 14.7 0.0456 5.0 0.232 5.3 04.1 3.06 107 83 0.81 3.18 0.0506 13 0.234 13 05.1 2.20 233 195 0.87 7.35 0.0491 7.7 0.243 7.9 06.1 2.61 182 152 0.86 5.94 0.0508 8.1 0.259 8.3 07.1 4.72 136 125 0.95 4.35 0.0424 17 0.207 17 08.1 1.75 185 174 0.97 5.93 0.0573 6.4 0.289 6.6 09.1 1.59 204 165 0.84 6.61 0.0550 5.9 0.282 6.2 010.1 2.43 273 263 1.00 8.98 0.0454 9.4 0.234 9.6 011.1 1.39 395 678 1.77 13.9 0.0519 5.7 0.289 6.0 012.1 4.87 84 63 0.77 2.69 0.0492 16 0.240 16 013.1 0.60 569 804 1.46 18.0 0.0541 2.1 0.2728 2.7 014.1 1.09 414 378 0.94 14.2 0.0513 4.4 0.280 4.8 0

Notes: The radiogenic lead Pb* corrected for common Pb using 204Pb. All errors are 1σ.

Cenozoic to present-day oceanic subduction zones (Maruyama andLiou, 1998).

The Qiangtang metamorphic belt is a tectonic complex composedof metabasaltic rocks, ophiolitic mélange, minor chert and marble/limestone associated with blueschist and eclogite, metasedimentaryrocks and ultramafic rocks (BGMR, 1993; Kapp et al., 2000, 2003; Liet al., 1995, 2006, 2008; Zhai et al., 2004, 2007, 2010; Zhang et al.,2006b). Various lithologic units in the Qiangtang metamorphic beltcomplex exhibit different histories of deformation and metamor-phism. In fact, the scenario is similar to the Franciscan complex, whichis characterized by the occurrence of accretionary mélange formed byoceanic subduction at convergent plate margins (Cloos, 1982; Kappet al., 2003). Meanwhile, the geochemical features of eclogite andblueschist from the Qiangtang metamorphic belt suggest that theirprotoliths have an OIB and enrichedmid-ocean ridge basalt (E-MORB)affinity (Zhai et al., 2011). The lithological diversity of the Qiangtangmetamorphic belt, the petrological and geochemical characteristicsof these rocks, and the presence of low-temperature eclogite

ing SHRIMP Center).

06Pb⁎/38U

±% 206Pb/238U ±1σAge (Ma)

207Pb/206Pb ±1σAge (Ma)

208Pb/232Th ±1σAge (Ma)

Discordant(%)

.0417 2.9 263.2±7.4 448.0±84 287.0±10 41

.03510 1.6 222.4±3.6 162.0±79 217.7±4.8 −37

.03685 1.7 233.3±3.8 −22.0±120 225.5±6.1 1177

.03362 2.0 213.1±4.1 221.0±290 200.0±15 4

.03592 1.8 227.5±3.9 155.0±180 221.2±9.5 −47

.03699 1.8 234.2±4.2 234.0±190 240.0±11 0

.03538 2.0 224.1±4.4 −203.0±430 210.0±16 210

.03665 1.8 232.0±4.1 502.0±140 236.3±8.7 54

.03715 1.8 235.2±4.2 411.0±130 237.9±9.5 43

.03741 1.8 236.8±4.1 −36.0±230 231.0±10 757

.04034 1.7 254.9±4.3 283.0±130 259.2±6.0 10

.03545 2.1 224.6±4.7 157.0±380 219.0±22 −43

.03659 1.6 231.6±3.7 374.0±47 237.4±5.4 38

.03959 1.7 250.3±4.1 252.0±100 254.3±6.6 1

Fig. 10. (a) and (b) CL images showing the internal structure of the analyzed zircon grains from eclogites E1011 and P2205; (c) Photomicrographs showing mineral inclusions ofphengite (I, E1011), phengite and omphacite (II, E1011), and phengite (III, P2205) in zircon crystals from eclogites.


all point that the protoliths of the rock units were fragments of thePaleo-Tethys oceanic lithosphere. Moreover, a Late Triassic magmaticarc (i.e., the Nadigangri Formation) was present along the south

Table 3U, Th and Pb isotopic data of zircons from eclogite sample P2205 (data obtained using a LA

Contents 232Th/238U

Common-Pb corrected isotopic ratios (

Spot Th (ppm) U (ppm) Pb* (ppm) 207Pb*/206Pb* 1σ 207Pb*/235U

1 1364 799 46 1.71 0.05180 0.00101 0.271492 2373 1416 78 1.68 0.05152 0.00083 0.263453 3744 1916 108 1.95 0.05117 0.00086 0.255984 2170 1432 83 1.52 0.05235 0.00095 0.272445 794 530 29 1.50 0.05170 0.00110 0.262586 144 185 10 0.78 0.05267 0.00185 0.282917 2296 1615 87 1.42 0.05115 0.00120 0.263138 1784 1028 60 1.73 0.05109 0.00119 0.256729 3917 1921 117 2.04 0.05102 0.00116 0.2568410 1390 1097 61 1.27 0.05173 0.00130 0.2665811 2903 2111 115 1.37 0.05178 0.00135 0.27265

Notes: The radiogenic lead Pb* corrected for common Pb using 204Pb.

margin of the north Qiangtang Block (Wang et al., 2008; Zhai and Li,2007). The formation of magmatic arc is usually related to oceanicsubduction rather than continental subduction. Consequently, we

–ICP–MS at IGG, CAS, Beijing).

±1σ) Common-Pb corrected isotopic ages (Ma)

1σ 206Pb*/238U 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ

0.00486 0.03814 0.00095 277 28 244 4 241 60.00391 0.03723 0.00091 264 30 237 3 236 60.00395 0.03644 0.00089 248 29 231 3 231 60.00447 0.03792 0.00094 301 29 245 4 240 60.00505 0.03701 0.00094 272 27 237 4 234 60.00881 0.03912 0.00113 315 32 253 7 247 70.00546 0.03745 0.00097 248 27 237 4 237 60.00530 0.03655 0.00095 245 27 232 4 231 60.00515 0.03659 0.00094 242 27 232 4 232 60.00587 0.03743 0.00099 273 27 240 5 237 60.00622 0.03822 0.00102 276 27 245 5 242 6

0.150.05 0.25 0.350.028

0.032

0.036

0.040

0.044

0.048

206 P

b/23

8 U

206 P

b/23

8 U

207Pb/235U207Pb/235U

E1011

Mean = 230 4 Man=10, MSWD=1.6(Excluding 1.1, 4.1,11.1, and 14.1)

190

210

230

250

270

290

4.1

1.111.1

14.1

208

216

224

232

240

248

0.260.18 0.22 0.30 0.34 0.380.028

0.032

0.036

0.040

0.044

0.048

0.052

180

200

220

240

260

280

300

320

Mean = 237 4 Man=11, MSWD=0.64

P2205

210

220

230

240

250

260

270

Fig. 11. U–Pb concordia diagrams of zircons for samples E1011 and P2205.


conclude that eclogite from the central Qiangtang area was derivedfrom the subducted oceanic crust.

Since the HP rocks were documented in the Qiangtang metamor-phic belt, several tectonic models had been suggested on the origin ofthe belt (Kapp et al., 2000, 2003; Li et al., 1995; Pullen et al., 2008;Zhang et al., 2006b). In recent years, typical, but not retrograded,eclogite was discovered in the Qiangtang metamorphic belt (Li et al.,2006; Zhang et al., 2006a), and the opinion of “blueschist-bearingmetamorphic core complex” suggested by Kapp et al. (2000, 2003) hasbeen superseded by a “subduction belt” (Li et al., 2006; Pullen et al.,

Table 4Results of 40Ar/39Ar stepwise heating dating for phengite from eclogite (E0635 and GMC07

T (°C) 40Ar/39Ar 36Ar/39Ar 37Ar/39Ar 38Ar/39Ar F

E0635 (eclogite, sample mass=45.11 mg, J value=0.009031±0.0000452)400 17.5037 0.0158 0.0744 0.0372 12.8245500 14.9267 0.0034 0.0609 0.0348 13.9336600 14.7416 0.0027 0.0340 0.0225 13.9309700 14.5932 0.0018 0.0159 0.0175 14.0467800 14.4459 0.0014 0.0060 0.0144 14.0376900 14.3274 0.0010 0.0066 0.0141 14.0150950 14.1979 0.0012 0.0130 0.0150 13.83141000 17.7676 0.0133 0.3884 0.1316 13.85801100 17.2070 0.0115 0.3490 0.1047 13.84381400 13.2253 0.0177 0.6170 0.1228 8.0429

E0636 (Grt–Phn schist, sample mass=45.94 mg, J value=0.009338±0.0000467)300 24.3329 0.0492 0.0863 0.0495 9.8087400 16.8218 0.0179 0.1192 0.0557 11.5413500 16.5157 0.0106 0.0463 0.0289 13.3910600 15.9295 0.0061 0.0194 0.0186 14.1272700 14.6372 0.0017 0.0077 0.0149 14.1274800 14.2850 0.0004 0.0025 0.0134 14.1636850 14.1527 0.0007 0.0121 0.0160 13.9335950 14.3963 0.0009 0.0073 0.0147 14.12741050 14.2717 0.0005 0.0030 0.0136 14.12851150 14.5027 0.0015 0.0271 0.0230 14.05051250 14.5043 0.0010 0.0688 0.0358 14.21941400 16.1417 0.0069 0.1298 0.0636 14.1061

GMC0701 (eclogite, sample mass=46.89 mg, J value=0.013117±0.0000656)800 28.9068 0.0725 1.4397 0.0369 7.6035900 16.1280 0.0222 0.1527 0.0163 9.56061000 19.0829 0.0312 0.0080 0.0170 9.85891050 12.3875 0.0085 0.0092 0.0116 9.86791100 10.6968 0.0031 0.0147 0.0131 9.76401150 10.4210 0.0020 0.0000 0.0125 9.81411200 10.1838 0.0010 0.0098 0.0122 9.88641300 11.2706 0.0038 0.4849 0.0161 10.1804

Notes: errors are 1σ; 40Ar* represents radiogenic 40Ar; F = 40Ar*/39Ar.

2008; Zhang et al., 2006b). However, it is still questionable for thetectonic setting of subduction, a continental (Pullen et al., 2008; Zhanget al., 2006b) or an oceanic subduction (Li et al., 2006). If it was acontinental subduction, the protoliths of the HP rocks must containcontinental granitic basement rocks and their overlying passive-margin sediments. Besides, island arc volcanic rocks are rarelyassociated with HP metamorphism (Ernst, 2001; Maruyama et al.,1996). In Qiangtang, the protoliths for HP metamorphic rocks com-prise bedded cherts, greenstones ofMORBorigin, seamount fragments,and trench turbidites. The rock assemblage belongs to an oceanic

01) and Grt–Phn schist (E0636).

39Ar (×10−14 mol) 39Ar (cumulative) (%) Age (Ma) ±1σ (Ma)

109.13 1.21 197.7 4.1118.29 2.52 213.8 2.9273.07 5.55 213.8 2.5660.61 12.88 215.5 2.4

2020.38 35.29 215.3 2.22969.98 68.23 215.0 2.32743.55 98.66 212.3 2.3

56.29 99.29 212.7 4.352.06 99.86 212.5 3.512.30 100.00 126.5 7.3

44.14 0.50 158.1 4.327.90 0.82 184.6 7.289.84 1.84 212.6 3.2

223.75 4.38 223.5 2.4678.17 12.07 223.5 2.1

2717.77 42.91 224.1 2.21015.88 54.44 220.7 2.3891.11 64.55 223.5 2.2

2570.98 93.72 223.6 2.2384.26 98.08 222.4 2.7116.97 99.41 224.9 3.652.36 100.00 223.2 5.1

1.26 0.30 172.0 17.05.01 1.47 213.0 12.0

31.34 8.83 219.4 3.369.97 25.24 219.6 3.5

135.48 57.03 217.4 2.3137.86 89.38 218.5 3.039.90 98.75 220.0 3.05.34 100.00 226.1 3.7

image of Fig.�11

Age (Ma)

Plateau age=223.2 1.7 Ma (2(Including 98.2% of Ar)39

0 20 40 60 80 100150

160

170

180

190

200

210

220

240

230

Age=223.3 3.2 MaAr/ Ar) =299 66MSWD=0.14

(2 )(40 36

o

0 1000 2000 30000

10000

20000

30000

40000

50000

Plateau age=214.1 1.8 Ma (2 )(Including 98.7% of Ar)39

Age (Ma)

Cumulative 39Ar (%)

180

190

200

210

220

230

240

0 20 40 60 80 100170

250

Age=214.9Ar/ Ar =282 53MSWD=0.0

4.6 Ma (2 )( )40 36

o

65

39Ar/36Ar

Cumulative 39Ar (%) 39Ar/36Ar

40Ar/36Ar

40Ar/36Ar

40Ar/36Ar

Cumulative 39Ar (%) 39Ar/36Ar

0 200 400 600 800 1000 12000

4000

8000

12000

16000

0 20 40 60 80 100

Plateau age=219.5 1.7Ma (2 )(Including 99.7% of Ar)39

140

160

180

200

220

240Age (Ma)

0

2000

4000

6000

8000

10000

12000

14000

0 400 800 1200

Age=218.2Ar/ Ar) =297 11MSWD=0.43

3.7 Ma (2 )(40 36

o

E0636 (Grt-Phn schist)

E0635 (Eclogite)

GMC0701 (Eclogite)

E0636 (Grt-Phn schist)

E0635 (Eclogite)

GMC0701 (Eclogite)

Fig. 12. 40Ar/39Ar age plateau and isochrons for the phengite samples from eclogite (E0635 and GMC0701) and Grt–Phn schist (E0636).


lithosphere. However, a Late Triassicmagmatic arcwas also developedalong the south margin of the north Qiangtang Block (Wang et al.,2008; Zhai and Li, 2007). Therefore, we suggest that an oceanicsubduction took place during the Triassic in central Qiangtang, but wecannot exclude the possibility of a continental collision after theoceanic subduction. This tectonic evolution is similar to that of theNorth Qilian andNorth QaidamHP/UHPmetamorphic belt (Song et al.,2006; Song et al., 2009).

Finally, we like to underline that the strata and fossil assemblagesof the south Qiangtang Block are entirely different from those of

the north Qiangtang Block (BGMR, 1993; Jin, 2002; Li and Zheng,1993; Li et al., 1995; Zhang et al., 2009). The occurrence of theeclogite, blueschist, ophiolitic mélange and OIB-derived rocks incentral Qiangtang (BGMR, 1993; Li et al., 1995, 2006, 2008; Zhai et al.,2006, 2007, 2010) suggest that a Paleo-Tethys Ocean shouldbe present between the north and south Qiangtang Block beforethe Triassic. The Qiangtang metamorphic belt was formed bynorthward subduction of the Paleo-Tethys oceanic crust and markeda Triassic suture zone between the Gondwana-derived block andLaurasia.

Fig. 13. P–T path for the eclogite in the Gemu (gray arrow) and Gangma Co area (blackarrow). The black diamond are the P–T data estimated based on the geothomobaro-metry of Ravna and Terry (2004); the gray circles are the P–T data estimated based onthe geothermometer of Powell (1985) at a giving pressure of 2.2 GPa. The geotherms,metamorphic facies fields and their abbreviations are after Liou et al. (2004) and Songet al. (2007).


Acknowledgements

We thank Z. Y. Chen, Y. Lizuka, Z. Q. Yang and W. Chen for theirassistance in the laboratory operation of EPMA, SHRIMP and 40Ar/39Ardating. We appreciate the reviews by two anonymous reviewers andeditorial comments of I. Buick, which have substantially improved themanuscript. This study was supported by the National ScienceFoundation of China (Grant no. 41072167, 40672147 and 40825007),Institute of Geology, Chinese Academy of Geological Sciences(Grant no. J0910) and Geological Survey Project of Chinese (Grantno. 1212010561605). Bor-ming Jahn acknowledges the financialsupport of the National Science Council (Taiwan) through grants NSC97-2116-M-001-011 and NSC 98-2116-M-001-009.

Appendix A. Supplementary data

Supplementary data to this article can be found online atdoi:10.1016/j.lithos.2011.02.004.

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