Multistage metamorphic events in granulitized eclogites...

Lithos 122 (2011) 107–121

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Multistage metamorphic events in granulitized eclogites from the North Dabiecomplex zone, central China: Evidence from zircon U–Pb age, trace element andmineral inclusion

Yi-Can Liu a,b,⁎, Xiao-Feng Gu a, Shu-Guang Li a, Zhen-Hui Hou a, Biao Song b

a CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, Chinab Beijing SHRIMP Center, Chinese Academy of Geological Sciences, Beijing 100037, China

⁎ Corresponding author. CAS Key Laboratory ofEnvironments, School of Earth and Space SciencesTechnology of China, Hefei 230026, China. Tel./fax: +86

E-mail address: [email protected] (Y.-C. Liu).

0024-4937/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.lithos.2010.12.005

a b s t r a c t
a r t i c l e i n f o
Article history:Received 3 July 2010Accepted 9 December 2010Available online 16 December 2010

Keywords:Multistage metamorphic eventsU–Pb datingZirconEclogiteNorth Dabie complex zone

The studied ultrahigh-pressure (UHP) eclogites, located in the southwestern part of the North Dabie complexzone (NDZ) in central China, represent deeply subducted mafic lower continental crust of the South ChinaBlock and display a multiple metamorphic evolution. However, the exact timing of the UHPmetamorphism inthe NDZ is poorly constrained, and thus impedes our understanding of the tectonic evolution of this area. Inorder to constrain the ages of peak UHP metamorphism and subsequent retrogression during continentalsubduction and exhumation, zircon from the eclogites in the NDZ has been investigated by a combinedpetrological, trace element and U–Pb isotopic study. In combination with petrological data, the present zirconSHRIMP and LA-ICPMS U–Pb dating provides precise constraints on the timing of multistage metamorphicevents on the eclogites in the region. The U–Pb isotope in zircon from the eclogites records the times ofmultiple discrete events in the history of the rock such as Neoproterozoic, 238±2 Ma, 222±4–227±2 Ma,210±4–215±2 Ma, 199±2 Ma and 176±2–188±2 Ma. Neoproterozoic ages defined by relic igneous coresrepresent their protolith time and the other age-groups reflect their metamorphic records by CL images, lowTh/U ratios and mineral inclusions. By U–Pb age, trace element and mineral inclusion of zircon, at least twoepisodes of eclogite-facies metamorphism have been identified from the eclogites, best estimated at 226±3and 214±3 Ma, respectively. The younger ages of 199±2 and 176–188 Ma most likely record the granulite-facies overprinting and amphibolite-facies retrogression occurring during exhumation whereas the age of238±2 Ma probably reflects the prograde metamorphic timing prior to the UHP metamorphism. Thus,Neoproterozoic mafic lower continental crust was subducted to depths greater than 120 km (correspondingto the lowest pressure for diamond formation) and suffered from UHP metamorphism at 226±3 Ma. Thenthese UHP metamorphic rocks were exhumed to about 60 km depth at 214±3 Ma and experienced high-pressure quartz eclogite-facies retrograde metamorphism, and subsequently to lower- and upper-crustallevels and overprinted by granulite- and amphibolite-facies metamorphism, respectively.

Crust–Mantle Materials and, University of Science and551 3600367.

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1. Introduction

Eclogites and related high-grade rocks in high-pressure (HP) toultrahigh-pressure (UHP) metamorphic belts are the most importantwitnesses of continental subduction, collision and subsequentexhumation processes and thus provide important information onthe geodynamics of orogens. Reliable geochronological data arecrucial for an understanding of their multistage metamorphicevolution. However, accurate and precise dating of HP–UHP meta-morphism is known to be a difficult task bymeans of Rb–Sr, Ar–Ar andSm–Nd geochronology, due to retrograde metamorphism, multiple

growths and/or recrystallization and isotopic disequilibrium (e.g.,Jahn et al., 2005; Li et al., 2000; Liu et al., 2005; Rubatto and Hermann,2003; Thöni and Jagoutz, 1992). By comparison, in-situ U–Pb dating ofzircon is a powerful method to obtain credible ages for the multiplemetamorphic events of rocks involved in complex processes (e.g.,Ayers et al., 2002; Gebauer et al., 1997; Möller et al., 2002; Rubattoet al., 1999; Wu et al., 2006).

Zircon is a ubiquitous accessory mineral in crustal rocks. Due to itshighly refractory nature, high closure temperature and extremelyslow diffusion rate of Pb, zircon is able to preserve multiple stages ofmagmatic and metamorphic records in complex metamorphicterranes (e.g., Hermann and Rubatto, 2003; Katayama et al., 2001;Möller et al., 2002), thereby readily amenable to be used for obtainingU–Pb ages in poly-metamorphic rocks. However, zircon in high-grademetamorphic rocks displays a wide diversity and complexity oftextures that reflect variations in the physicochemical conditions and

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108 Y.-C. Liu et al. / Lithos 122 (2011) 107–121

the duration of each metamorphic event, and are caused bymodifications of pre-existing structures and/or by growth of newzircon (Chen et al., 2010; Corfu et al., 2003; Xia et al., 2009). It is thusnecessary to link zircon crystallization to particular metamorphicconditions to understand the timing and geodynamic implications ofdifferent geological events. But, inclusions of metamorphic mineralsin zircon can provide a direct link between dating andmetamorphism,and the complicated internal structures, including irregular bound-aries and various core, mantle and rim domains, of zoned zircon grainscan be revealed by cathodoluminescence (CL) images (e.g., Ayerset al., 2003; Gebauer et al., 1997; Hermann et al., 2001; Katayamaet al., 2001), recorded complicated magmatic and metamorphichistories of the rocks. The trace element composition of zircon isadditional information for establishing such links and has successfullybeen used to constrain the conditions of zircon growth or monitor thecoexisting paragenesis (e.g., Rubatto, 2002; Schaltegger et al., 1999;Sun et al., 2002; Whitehouse and Platt, 2003). Consequently, acombination of CL imaging, mineral inclusions and in-situ measure-ments of U–Pb ages and trace elements on different zircon domainscan differentiate different generations of zircon for which the ages ofdifferent metamorphic stages can be determined (e.g., Hermann et al.,2001; Liu et al., 2006a, 2006b; Sun et al., 2002) and, most importantly,helps to avoid analyzing more than one domain at a time andobtaining mixed ages. These techniques thus have the potential togreatly improve our understanding of subduction and exhumation ofUHP rocks in a collisional orogen.

Numerous geochronological studies on UHP metamorphic rocksfrom the Dabie orogen, central China indicate that the HP–UHPeclogite-facies metamorphism occurred between 210 and 245 Ma(e.g., Ayers et al., 2002; Chavagnac and Jahn, 1996; Hacker et al., 1998;Li et al., 1993, 2000; Liu et al., 2005, 2006a, 2006b, 2007b; Rowley et al.,1997; Wan et al., 2005; Wu et al., 2006; Zheng et al., 2005a, 2009).These data generally reflect Triassic metamorphism though with abroad age spectrum. Being aware of the large variation in apparentages and highMSWD (mean square of weighted deviates) values up to13, Hacker et al. (1998) found more than one single age populationin their Dabie samples. Accordingly, these authors argued for twoepisodes of metamorphic zircon growth occurred at ~240 Ma and~219 Ma, respectively. Caution should be exercised with any arbitraryweighted mean age calculation without consideration of possiblemultiple metamorphic age components in UHP rocks that would yieldquestionable or geologically meaningless mixed ages (Liu et al.,2006a). In fact, multiple zircon growth during Triassic metamorphismhas been observed in theUHP rocks from theDabie and Sulu belts (e.g.,Liu et al., 2006a, 2006b; Wan et al., 2005; Wu et al., 2006; Zheng et al.,2005a).

The Dabie UHP metamorphic belt in central China can be dividedinto three eclogite-bearing UHP crustal slices from north to south: theNorth Dabie high-T/UHP complex zone (NDZ), the Central Dabie mid-T/UHP metamorphic zone (CDZ), and the South Dabie low-T/UHPeclogite zone (SDZ) (Li et al., 2004; Liu and Li, 2008; Liu et al., 2005,2007a, 2007b; Xu et al., 2003, 2005; Zheng, 2008). The Pb isotopicmapping on the Dabie UHP belt has revealed the detachment betweendeeply subducted upper continental crust (the CDZ) and lower crust(the NDZ) (Li et al., 2003; Zhang et al., 2002). Thus the termination ageof peak UHP metamorphism is probably different in different slices ofdeep-subducted slab (Li et al., 2004; Liu and Li, 2008; Liu et al., 2007b).Similar conclusion also has been drawn from the Sulu terrane by Liuet al. (2009). Intensive Sm–Nd and U–Pb geochronological studiesconsistently indicated the reliable UHP metamorphic ages of 225–238 Ma for the coesite-bearing rocks from the CDZ (e.g., Ayers et al.,2002; Hacker et al., 1998; Li et al., 1993, 2000; Liu et al., 2006a, 2006b;Rowley et al., 1997). Whereas it is noteworthy that reportedmetamorphic ages of UHP rocks in the CDZ exhibit two distinctpeaks at ~233–240 and ~226–228 Ma (Liu et al., 2006a; Wu et al.,2006; Zheng et al., 2009). By contrast, the Sm–Nd garnet–omphacite

age of 212±4 Ma for eclogite and U–Pb zircon age of 218±3 Ma forgneiss from the NDZ (Liu et al., 2005, 2007b) is significantly youngerthan the Sm–Nd age of 226±3 Ma (Li et al., 2000), and SHRIMP U–Pbages of 227±1 to 234±3 Ma on coesite-bearing domains ofmetamorphic zircon (Liu et al., 2006a, 2006b) for UHP rocks fromthe CDZ, and the Sm–Nd age of 236±4 Ma andU–Pb age of 242±3 Ma(Li et al., 2004) for low-T eclogite from the SDZ, respectively. Asdescribed above, the UHP age of the CDZ was well-dated by a numberof independent methods whereas there is comparatively lack ofreliable high-precision geochronological information for the UHProcks on the NDZ and SDZ. Based on the available geochronologicaldata from the NDZ, however, it remains uncertain as to whether thepeak metamorphic ages for the UHP rocks from the NDZ are youngerthan those for the UHP rocks from the CDZ, as previously suggested byLiu et al. (2007b). It is needed to precisely date more UHP rocks fromthe NDZ and clarify them, thereby helping to better understand themechanism for exhumation of UHP rocks from different slices ofdeeply subducted slab in the Dabie orogen.

Additionally, recent studies of UHP gneisses from the NDZcombing CL imaging with SHRIMP U–Pb dating (Liu et al., 2007b)have revealed a complex, multi-episodic nature of zircon in theserocks, resulting in a great challenge for precise age determinations ofvarious growth components on isolating discrete domains of singlezircon grains. Eclogites enclosed within gneisses, especially from thesouthwestern NDZ, however, have so far not received much attentionto their geochronology owing to the poor or bad outcrop exposure andpervasive overprinting of granulite- and amphibolite-facies. Up tonow, only two Sm–Nd ages (Liu et al., 2005; Xie et al., 2004b) and onezircon age (conventional isotope dilution multigrain or single zirconanalysis or ID-TIMS) of 230±6 Ma (Liu et al., 2000a) for eclogiteswere reported in the northern part of the NDZ. Published ID-TIMS dataof zircon from the granitic gneiss defined the Triassic metamorphicages of 226±6 Ma, 218±9 Ma and 213±4 Ma at Ta'erhe (Liu et al.,2000a), Manshuihe (Jiang et al., 2002) and Zhujiapu (Zheng et al.,2004). In view of the multistage evolution of the eclogites (Liu et al.,2005, 2007a) resulting in poly-phase overgrowth on zircon, the data,which were obtained with conventional zircon U–Pb dating method,probably dated amixed or ambiguous age because of a mixed signal ofdifferent age domains (Gebauer et al., 1997; Liu et al., 2002). Recently,Xie et al. (2004a) reported a less precise age of 212±21 Ma and Liuet al. (2007b) obtained a more precise data of 218±3 Ma by zirconSHRIMP U–Pb dating on the gneiss. As a result, the exact age of UHPmetamorphism on the eclogite is poorly constrained and it remains totest whether the 218±3 Ma represents the best estimation of peakmetamorphic time in the NDZ, as supposed by Liu et al. (2007b).Therefore, more reliable and precise age data are required in order to:(i) better constrain the peak UHP metamorphic age, and (ii) betterunderstand the complex histories of the UHP metamorphic rocks inthis area.

In this paper, we present a combined study of CL image, U–Pb age,trace element and mineral inclusion of zircon from the UHP eclogitesin the south-west of the NDZ. The results not only provide precisedeterminations of the ages of multiple episodes of metamorphicevents on the eclogites, but also shed new light on the subduction andexhumation processes of the continental crust in this area.

2. Geological background and sample description

The Dabie orogen is located in the intermediate segment of theQinling–Dabie–Sulu orogenic belt formed by the collision of the NorthChina Block and South China Block (SCB) during early Mesozoic. Itcomprises several fault-bounded rock units with varying metamor-phic grades. It is generally subdivided into five major lithotectonicunits from north to south (e.g., Li et al., 2004; Liu and Li, 2008; Liuet al., 2005, 2007a, 2007b;Malaspina et al., 2006; Okay, 1993; Xu et al.,2003, 2005; Zheng, 2008; Zheng et al., 2005b): (1) the Beihuaiyang

109Y.-C. Liu et al. / Lithos 122 (2011) 107–121

zone (BZ); (2) the North Dabie high-T/UHP complex zone (NDZ);(3) the Central Dabie mid-T/UHP metamorphic zone (CDZ); (4) theSouth Dabie low-T/UHP eclogite zone (SDZ); and (5) the Susongcomplex zone (SZ). These five zones are separated by the respectiveXiaotian-Mozitan fault, Wuhe-Shuihou fault, Hualiangting-Mituofault and Taihu-Shanlong fault (Fig. 1). Zone (1) is a low-gradecomposite unit comprising the meta-flysch of Foziling (or Xinyang)Group and the Luzhenguan (or Guishan) complex, whereas Zones (2),(3), (4) and (5) belong to the subducted SCB.

The UHP metamorphic rocks, including coesite-bearing eclogite,UHP gneiss, quartz jadeitite and marble with eclogite nodules, areobserved in the CDZ and SDZ (e.g., Li et al., 2004; Okay, 1993; Okayet al., 1993; Rolfo et al., 2004; Xu et al., 1992). The occurrence ofdiamond and coesite in the UHP rocks in the CDZ indicates the UHPmetamorphism occurred at 700–850 °C and N2.8 GPa (e.g., Okay,1993; Okay et al., 1989; Rolfo et al., 2004; Wang et al., 1989; Xu et al.,1992), whereas the peak P–T conditions on the eclogites in the SDZwere estimated at 670 °C and 3.3 GPa (Li et al., 2004). Both the CDZand SDZ units experienced UHP eclogite-facies, and subsequent HPeclogite- and amphibolite-facies retrograde metamorphism (e.g., Liet al., 2004; Rolfo et al., 2004; Xu et al., 1992).

Fig. 1. Schematic geological map of the Dabie orogen, with inset showing the location of thilocalities with sample numbers are described in detail in the text. BZ= Beihuaiyang zone, NDzone, SDZ = South Dabie low-T/UHP eclogite zone, SZ = Susong complex zone, HMZ = Hucomplex, XMF = Xiaotian-Mozitan fault, WSF = Wuhe-Shuihou fault, HMF = Hualiangting

The NDZ consists predominantly of banded tonalitic and granitoidgneiss and post-collisional magma intrusions with subordinate meta-peridotite (including dunite, harzburgite and lherzolite), garnetpyroxenite, garnet-bearing amphibolite, granulite and eclogite. Theoriented mineral exsolutions in garnet and clinopyroxene, and micro-diamond imply that the eclogites from the NDZ also underwent theUHPmetamorphism at a possible pressure of N3.5–4.0 GPa (Malaspinaet al., 2006; Xu et al., 2003, 2005). The Triassic zircon U–Pb (Liu et al.,2000a, 2007a) and Sm–Nd (Liu et al., 2005) ages of the eclogites fromthe NDZ suggest that the eclogites formed by the Triassic SCBsubduction, similar to those from the CDZ and SDZ. The Triassicmetamorphic ages (Bryant et al., 2004; Liu et al., 2000a, 2007b; Xieet al., 2004a) and occurrence of micro-diamond in zircon (Liu et al.,2007b) from the banded gneisses in the NDZ suggest that the gneissessurrounding the eclogiteswere also involved into the continental deepsubduction of the SCB. Following the UHP and HP eclogite-faciesmetamorphism, however, the eclogites from the NDZ were firstsubjected to granulite-facies overprinting, and later to amphibolite-facies retrogression (e.g., Liu et al., 2004, 2005; Xu et al., 2000). Thesecorroborate the case for a different metamorphic evolution in thedifferent slices (e.g., NDZandCDZ) of theDabieUHPbelt (see below for

s area within the Triassic Qinling–Dabie–Sulu collision orogen in central China. SampleZ=North Dabie HT/UHP complex zone, CDZ= Central Dabie mid-T/UHPmetamorphicwan mélange zone, HZ = Hong'an low-T eclogite zone, DC = amphibolite-facies Dabie-Mituo fault, TSF = Taihu-Shanlong fault, and TLF = Tan-Lu fault.


P–T–t paths in detail). Therefore, although both the CDZ andNDZ unitsexperienced UHP metamorphism, they had different exhumationhistories, suggesting that the CDZ and NDZ are two decoupled UHPslices (Li et al., 2003; Liu and Li, 2008; Liu et al., 2005, 2007a, 2007b; Xuet al., 2000, 2003, 2005). The Pb isotope investigations show that theUHP rocks from the CDZ are characterized by high radiogenic Pb butbanded gneiss from theNDZ are characterized by low radiogenic Pb (Liet al., 2003; Zhang et al., 2002). This suggests that the exhumed UHProcks in the CDZ were derived from subducted upper crust, while theUHP rocks from the NDZwere from subducted felsic lower continentalcrust with minor mafic boudins or lenses (Li et al., 2003; Liu et al.,2007a, 2007b).

The Luotian dome in the southwestern segment of the NDZ is adeeply eroded area with abundant felsic and mafic granulites (Fig. 1).Unusual eclogites occur as lenses in garnet-bearing banded tonaliticgneisses from the Luotian dome (Liu et al., 2007a). These eclogitespreserve early granulite-facies mineral relics and have been over-printed by regionally extensive HP granulite-facies metamorphism,followed by penetrative amphibolite-facies metamorphism duringexhumation. Therefore, the eclogites in the region are often stronglyretrogressed to (garnet) amphibolite. The eclogite-facies mineralassemblage is garnet, relict omphacite and rare coesite or quartzpseudomorphs after coesite, with rutile, quartz, allanite and fluor-apatite as common constituents whereas omphacite, coesite/quartzpseudomorphs and fluor-apatite commonly occur as inclusion ingarnet or zircon, and omphacite was occasionally found in the matrix(Liu et al., 2007a, 2011). Three samples of granulized eclogite werecollected from Luotian (samples 031201-1A and 031201-1B) andJinjiapu (sample 06LT03-2), respectively (Fig. 1), for this study.Samples 031201-1A and 031201-1B were collected from the sameoutcrop and are strongly retrograded eclogites, which are mainlycomposed of garnet, rutile, diopside, hornblende and plagioclase withminor quartz, hypersthene and ilmenite. Omphacites were retro-graded to diopside with/without quartz needles in garnet and in thematrix or a symplectite of diopside and plagioclase; most orientedrods or needles in garnet are rutile, although those may beaccompanied by clinopyroxene+amphibole+apatite needles,which attest to the presence of a Si–Ti–Na–P-rich precursor garnetphase (majorite) stabilized at UHP conditions (N5–7 GPa) (see Liuet al., 2011 in detail). By comparison, sample 06LT03-2 is a lessretrogressed eclogite and is composed of garnet, omphacite, diopsideand rutile, with minor hypersthene, hornblende, plagioclase, quartzand ilmenite. Omphacite grains in the sample normally occur asinclusion in garnet and some of them are transformed to diopsidewithoriented needles of quartz or quartz+plagioclase+amphibole+hypersthene or quartz+hypersthene in the matrix, indicative of aformer UHP stage as suggested by Tsai and Liou (2000). Diopside withquartz needles sometimes contains retrograded hypersthene margin;rutile is partly transformed into ilmenite. In addition, a number ofmonocrystalline and polycrystalline quartz inclusions are enclosed ingarnet with well-developed radial fractures, which are interpreted asdecompression features and are commonly considered to be diagnos-tic for the identification of quartz pseudomorphs after coesite within arigid host (see Liu et al., 2011 for a review). This hypothesis is alsosupported by the presence of relic coesite in zircon from the sample06LT03-2 (see below). However, more details of the petrography andmineral chemistry of the granulized eclogites studied were given in twoseparate papers (Liu et al., 2007a, 2011) and are only summarized here.Despite the strongly pervasive granulite- and amphibolite-faciesoverprint, five metamorphic stages have been recognized for theeclogites in the area (Liu et al., 2007a, 2011; and Liu Y.-C. et al.,unpublished data): (1) a granulite-facies stage, with P=~0.8 GPa; (2) aUHP eclogite-facies stage, with P=4.0 GPa and T=910–980 °C, wit-nessed by the occurrence of diamond (Liu et al., 2007b; Xu et al., 2003,2005) and oriented mineral exsolutions in garnet and clinopyroxene(Tsai and Liou, 2000; Liu et al., 2007a, 2011; Malaspina et al., 2006; Xu

et al., 2003, 2005); (3) a HP eclogite-facies stage, with P=2.0 GPa andT=940–990 °C, characterized by the coexistence of garnet, sodicclinopyroxene or jadeite-poor omphacite and rutile with quartz; (4) aretrograde granulite-facies stage, with P=1.1–1.4 GPa and T=900–920 °C, indicated by the presence of hypersthene, plagioclase anddiopside symplectite, and oriented hypersthene+quartz needles inclinopyroxene; (5) a retrograde amphibolite-facies stage, withP=0.6–0.7 GPa and T=600–700 °C.

3. Analytical methods

Zircon was separated from approximately 2 to 5 kg of each sampleby crushing and sieving, followed by magnetic and heavy liquidseparation and hand-picking under a binocular microscope. Approx-imately 500 zircon grains for each sample, together with a referencezircon U–Pb standard TEMORA 1 (417 Ma: Black et al., 2003), weremounted using epoxy, which was then polished until all zircon grainswere approximately cut in half. The internal zoning patterns of thecrystals were observed by CL image, which was analyzed at theInstitute of Mineral Resources, Chinese Academy of GeologicalSciences (CAGS). The representative CL images for the studiedsamples are presented in Fig. 2. Mineral abbreviations in figures andtables are after Kretz (1983).

Mineral inclusions in zircon were identified using Ramanspectroscopy at the Continental Dynamics Laboratory, CAGS and/orsubstantiated using the electron microprobe analyzer (EMPA) at theInstitute of Mineral Resources, CAGS, the Analytical Centre, ChinaUniversity of Geosciences (Wuhan) and the State Key Laboratory forMineral Deposits Research, Nanjing University. The analytical condi-tions on the Raman and EMPA were reported by Liu et al. (2005) andXu et al. (2005). Representative Raman spectra of inclusion mineralsin zircon from three samples are reported in Fig. 3.

LA-ICPMS zirconU–Pb dating for sample 031201Awas carried out atthe StateKey Laboratory of ContinentalDynamics, NorthwestUniversityin Xi'an. The detailed analytical procedure was described by Yuan et al.(2004). The GeoLas 200M laser-ablation system equipped with a193 nm ArF-excimer laser was used in connection with ELAN6100 DRCICP-MS. The spot sizes were 32 μm in diameter. Heliumwas used as thecarrier gas to enhance the transport efficiencyof theablatedmaterial. Allmeasurements were performed using zircon 91500 as the externalstandard with a 206Pb/238U age of 1065.4±0.6 Ma (Wiedenbeck et al.,1995). Analyses of zircon 91500 during our measurements in thisstudy gave a weighted mean 206Pb/238U age of 1062.7±4.6 Ma(MSWD=0.014, n=14). The common Pb correction was appliedusing the method of Andersen (2002). Apparent and discordia U–Pbages were calculated by the ISOPLOT program (Ludwig, 2001). Theresults are listed in Table 1,with 2σ errors of themean for isotopic ratiosand calculated ages. The errors given in Table 1 and Fig. 6 for individualanalyses are quoted at the 1σ level, whereas the error for weightedmean ages given in Fig. 6, and in the text, are quoted at 2σ (95%confidence level).

SHRIMP zircon U–Pb dating for samples 031201-1B and 06LT03-2was performed on a SHRIMP II at the Beijing SHRIMP Center.Instrumental conditions and data acquisition were generally asdescribed by Compston et al. (1992) and Williams (1998). The U–Pbisotope datawere collected in sets of five scans throughout themassesand a reference zircon TEMORA 1 with an age of 417 Ma (Black et al.,2003) was analyzed every fourth analysis. Our measurement ofstandard zircon TEMORA 1 during this study yielded a weighted206Pb/238U age of 416.8±2.6 Ma (MSWD=1.6, n=14), which is ingood agreement with the recommended isotope dilution-thermalionization mass spectrometry (ID-TIMS) age of 416.75±0.24 Ma(Black et al., 2003). Common Pb was corrected using the measured208Pb. The U–Pb isotope data were treated following Compston et al.(1992) with the ISOPLOT program of Ludwig (2001). The results arelisted in Table 2. The errors given in Table 2 and Fig. 6 for individual

Fig. 2. Representative images of zircon in samples 031201-1A (a–d), 031201-1B (e–h) and 06LT03-2 (i–l): cathodoluminescene (CL) images (a, c, e, g and i–k) and plane-polarizedlight (PL) photos (b, d, f, h and l). Zircons (a) and (b), (c) and (d), (e) and (f), (g) and (h), and (k) and (l) are the same grain, respectively. The open circles are analysis spot withavailable 206Pb/238U ages.


analyses are quoted at the 1σ level, whereas the error for weightedmean ages given in Fig. 6, and in the text, are quoted at 2σ (95%confidence level).

Zircon trace element analyses for samples 031201-1B and 06LT03-2were conducted by the laser ablation ICP-MS at the CAS Key Laboratoryof Crust–Mantle Materials and Environments, University of Science andTechnology of China in Hefei. The Geolas Pro laser-ablation system wasused for the laser ablation experiments. The Laserwavelength is 193 nmand ablation spot size is 40 μm. The laser frequency and beam energyare 10 Hz and 140 mJ respectively. The ICP-MS used was an Elan DRCIIfrom PerkinElmer Sciex. Detailed analytical procedure was reported byYuan et al. (2004). Element concentrations of zircons were calculatedusing Pepita software with the zircon SiO2 contents as internal standardand the NIST610 as external standard. The simultaneous analysis dataon NIST612 show that the accuracy and precision of trace elementsare better than 10%. The limit of detection for the different REE variedfrom 0.02 to 0.09 ppm. The analytical data are listed in Table 3.

4. Results

4.1. CL imaging, mineral inclusion and trace element in zircon

The dated samples contain comparatively abundant, clear androunded zircon. By CL images, almost all zircon grains reveal ahomogeneous zircon population with no core characterized by nearlyspherical to multifaceted morphology and internal sector- to fir-tree

zoning (Fig. 2), which may be ascribed to the complete dissolution orlosing of inherited zircon and formation of newmetamorphic zircon asa result of temperatures high enough to cause partial melting (Chenet al., 2010; Rubatto andHermann, 2007; Xia et al., 2009), suggestive ofmetamorphic origin (e.g., Corfu et al., 2003). Rare grains from sample031201-1A show distinct cores that display weakly oscillatory zoningwith high Th/U ratio of 1.09 (Table 1), indicating they are igneouszircons that have been variably modified by metamorphic recrystal-lization (Bauer et al., 2007; Corfu et al., 2003). However, unlike zirconfrom some of the UHP rocks (e.g., quartz jadeitite or dolomitic marbleor paragneiss) from the CDZ (e.g., Liu et al., 2006a, 2006b) or thetonalitic gneisses from the NDZ (Liu et al., 2007b), it is generallydifficult to obviously distinguish from different core–mantle–rimtexture only by CL in a single zircon grain for the studied samples(Fig. 2). But, combined with geochronologial results and CL images,except for rare igneous core, two types of metamorphic mantledomains, named as inner- and outer-mantle domains (M1≥220 Maand M2b220 Ma), in zircon can clearly be recognized from each ofthree samples. They fall into two fields by age-cluster on the Concordiadiagrams and define two consistent discrete age-groups at 222±4–227±2 Ma and 210±3–215±2 Ma, respectively (see below). It isfurther supported by garnet compositions, mineral inclusion assem-blages and REE patterns of zircon. The former usually retain the UHPassemblage of garnet, rutile, aragonite and coesite, and the lattercontain garnet, rutile, quartz and clinopyroxene inclusions, suggestingquartz eclogite-facies metamorphism in origin (Figs. 2 and 3). Relict

image of Fig.�2

Fig. 3. Representative Raman spectra of mineral inclusions in zircon of eclogites from the Luotian dome. (a) Garnet; (b) rutile; (c) aragonite and rutile; (d) coesite (Cs) and quartz.These spectra also contain host zircon peaks at 355–357, 439–442, 975–976 and 1007–1010 cm−1.

Table 1Zircon U–Pb isotopic data obtained by LA-ICPMS for sample 031201-1A in the NDZ.

Spot no. Loc. Inclusion U (ppm) Th (ppm) Th/U Pb* (ppm) 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 206Pb/238U Age (Ma) Disc. (%)a

031201-1A1 M1 No 370 2.67 0.007 13.2 0.0498 0.0005 0.2448 0.0023 0.0357 0.0003 226±2 22 M1 No 549 13.1 0.024 20.2 0.0505 0.0006 0.2501 0.0023 0.0359 0.0003 227±2 03 M1 No 411 2.96 0.007 14.9 0.0486 0.0005 0.2333 0.0022 0.0348 0.0003 220±2 34 M1 Cs+Qtz 531 3.00 0.006 18.8 0.0504 0.0005 0.2410 0.0022 0.0347 0.0003 220±2 05 M2 No 433 3.96 0.009 15.0 0.0500 0.0005 0.2334 0.0022 0.0338 0.0003 215±2 16 M2 No 551 19.1 0.035 19.3 0.0519 0.0007 0.2382 0.0024 0.0333 0.0003 211±2 −37 M1 No 509 6.65 0.013 18.7 0.0489 0.0005 0.2421 0.0022 0.0359 0.0003 228±2 48 M1 No 461 3.45 0.007 16.6 0.0502 0.0005 0.2421 0.0022 0.0350 0.0003 221±2 09 M2 No 416 1.00 0.002 14.2 0.0501 0.0005 0.2307 0.0021 0.0334 0.0003 212±2 010 M2 No 762 11.7 0.015 27.0 0.0493 0.0005 0.2343 0.0021 0.0345 0.0003 218±2 211 R No 800 38.6 0.048 24.8 0.0494 0.0005 0.2011 0.0018 0.0296 0.0003 188±2 112 M1 Grt 483 2.56 0.005 18.5 0.0507 0.0005 0.2626 0.0024 0.0376 0.0003 238±2 013 M1 Grt 689 3.77 0.005 25.2 0.0506 0.0006 0.2498 0.0021 0.0358 0.0003 227±2 014 C Pl 958 1041 1.087 159 0.0628 0.0006 1.0386 0.0091 0.1200 0.0011 731±6 115 M1 Grt 331 1.87 0.006 12.5 0.0499 0.0005 0.2498 0.0023 0.0363 0.0003 230±2 216 M1 No 677 5.95 0.009 24.9 0.0491 0.0005 0.2444 0.0022 0.0361 0.0003 229±2 317 M1 Grt 546 5.95 0.011 19.9 0.0494 0.0005 0.2419 0.0022 0.0355 0.0003 225±2 218 R Grt 736 9.84 0.013 23.6 0.0499 0.0005 0.2163 0.0019 0.0314 0.0003 199±2 019 M2 No 891 4.60 0.005 31.1 0.0490 0.0005 0.2296 0.0020 0.0340 0.0003 215±2 220 M1 Grt 402 1.98 0.005 14.9 0.0507 0.0005 0.2537 0.0023 0.0363 0.0003 230±2 021 M2 No 472 5.56 0.012 16.0 0.0504 0.0006 0.2300 0.0021 0.0331 0.0003 210±2 022 M1 Grt 364 3.56 0.010 13.0 0.0501 0.0005 0.2425 0.0022 0.0351 0.0003 223±2 123 M2 No 588 4.11 0.007 20.5 0.0500 0.0006 0.2283 0.0021 0.0332 0.0003 210±2 024 M1 No 487 4.35 0.009 18.1 0.0506 0.0005 0.2537 0.0023 0.0364 0.0003 230±2 025 M1 No 786 14.1 0.018 28.6 0.0499 0.0005 0.2433 0.0022 0.0354 0.0003 224±2 126 M2 Cpx 783 12.8 0.016 26.6 0.0499 0.0005 0.2261 0.0021 0.0329 0.0003 208±2 027 M2 Grt 431 3.08 0.007 15.6 0.0498 0.0005 0.2372 0.0022 0.0345 0.0003 219±2 128 M2 No 282 2.26 0.008 9.5 0.0499 0.0005 0.2264 0.0021 0.0329 0.0003 209±2 1

M1, inner mantle; M2, outer mantle; C, core; R, rime. Grt, garnet; Cpx, clinopyroxene; Cs, coesite; Qtz, quartz; Pl, plagioclase; No, no inclusion.a Disc.(%) means discordant degree defined as [1−(207Pb/235U age)/(206Pb/238U age)]×100.


image of Fig.�3

Table 2Zircon U–Pb isotopic data obtained by SHRIMP for samples 031201-1B and 06LT03-2 in the NDZ.

Spot no. Loc. Inclusion 206Pbc(%)

U(ppm)

Th(ppm)

Th/U 206Pb*(ppm)

207Pb/206Pb ±(%) 207Pb/235U ±(%) 206Pb/238U ±(%) 206Pb/238UAge(Ma)

Disc. (%)a

031201-1B1.1 M1 No 2.63 216 1.11 0.005 6.79 0.0537 5.2 0.2640 5.5 0.0356 1.7 226±4 −52.1 M1 No 2.52 147 1.19 0.008 4.63 0.0472 6.4 0.2330 6.7 0.0358 1.9 226±4 63.1 M1 No 2.33 203 0.91 0.005 6.54 0.0489 7.5 0.2470 7.7 0.0366 1.8 231±4 34.1 M2 Grt 1.32 163 0.73 0.005 4.75 0.0515 5.2 0.2370 5.4 0.0334 1.5 212±3 −25.1 M2 Grt, Rt 2.12 311 8.04 0.026 9.21 0.0509 4.6 0.2360 4.8 0.0337 1.3 214±3 −16.1 M2 No 1.81 274 4.33 0.016 8.18 0.0480 10 0.2260 10 0.0341 1.3 216±3 47.1 M1 Grt 1.85 271 2.80 0.011 8.81 0.0503 4.2 0.2570 4.4 0.0371 1.3 235±3 18.1 M1 No 0.93 273 1.72 0.006 8.39 0.0505 4.9 0.2460 5.0 0.0354 1.3 224±3 09.1 M1 Grt, Ap 0.51 880 4.59 0.005 27.5 0.0518 2.1 0.2581 2.4 0.0361 1.1 229±3 −210.1 M2 No 1.00 578 6.35 0.011 17.1 0.0500 2.8 0.2351 3.0 0.0341 1.2 216±3 111.1 M2 Grt 1.04 621 8.64 0.014 18.2 0.0472 2.9 0.2190 3.1 0.0337 1.2 214±3 612.1 M1 No 1.43 348 5.11 0.015 10.8 0.0505 4.5 0.2470 4.6 0.0355 1.2 225±3 013.1 M1 No 2.95 208 0.88 0.004 6.52 0.0561 13 0.2730 13 0.0354 1.5 224±4 −1014.1 R Cpx 1.72 332 4.14 0.013 8.06 0.0479 6.1 0.1830 6.2 0.0277 1.3 176±2 315.1 M1 Grt, Cpx 1.57 207 1.63 0.008 6.47 0.0505 5.1 0.2480 5.3 0.0357 1.5 226±3 016.1 M1 No 1.36 401 4.79 0.012 12.9 0.0502 3.3 0.2559 3.5 0.0370 1.2 234±3 117.1 M2 No 3.83 158 0.88 0.006 4.82 0.0506 12 0.2390 12 0.0343 1.6 217±4 018.1 M2 Grt 1.18 494 4.93 0.010 14.7 0.0487 4.0 0.2300 4.2 0.0343 1.2 217±3 319.1 M1 No 1.13 432 2.58 0.006 13.6 0.0496 3.1 0.2482 3.4 0.0363 1.2 230±3 220.1 M1 Rt 1.83 364 4.98 0.014 11.3 0.0471 3.9 0.2306 4.1 0.0355 1.3 225±3 621.1 M2 Grt 1.48 429 1.86 0.004 12.6 0.0481 4.0 0.2231 4.2 0.0336 1.3 213±3 422.1 M1 Grt 1.40 275 2.41 0.009 8.65 0.0501 4.0 0.2500 4.2 0.0361 1.3 229±3 123.1 M2 No 2.35 345 4.24 0.013 10.2 0.0540 10 0.2510 10 0.0337 1.8 214±4 −624.1 M1 No 2.23 213 1.15 0.006 6.52 0.0501 6.9 0.2400 7.1 0.0348 1.4 220±3 125.1 M1 No 1.20 448 4.54 0.010 13.5 0.0506 3.1 0.2417 3.4 0.0346 1.2 220±3 026.1 M2 No 1.04 311 3.53 0.012 9.06 0.0528 3.7 0.2441 3.9 0.0336 1.3 213±3 −427.1 M2 Grt 11.8 57.2 0.84 0.015 1.82 0.0534 18 0.2410 18 0.0327 2.5 207±8 −65.2 M2 Grt, Rt 2.73 132 0.34 0.003 3.84 0.0502 7.3 0.2280 7.5 0.0330 1.8 209±4 014.2 M1 Grt 2.88 169 0.46 0.003 5.24 0.0571 11 0.2760 11 0.0351 1.7 222±4 −1127.2 M2 Grt 2.87 213 1.10 0.005 6.39 0.0569 4.9 0.2660 5.2 0.0340 1.6 215±4 −11

06LT3-21.1 R No 18.6 21.2 1.37 0.065 0.64 0.0570 34 0.2240 34 0.0284 3.5 180±12 −142.1 M2 Rt 4.48 108 2.18 0.020 3.23 0.0478 8.4 0.2190 8.6 0.0333 1.8 211±4 53.1 R No 13.5 37.5 5.56 0.148 1.12 0.0950 20 0.3940 20 0.0300 3.1 191±10 −774.1 M1 Cpx 9.33 43.3 34.0 0.785 1.47 0.0448 17 0.2220 18 0.0359 2.7 227±10 115.1 M1 Grt 2.91 140 8.69 0.062 4.35 0.0525 7.9 0.2540 8.1 0.0351 1.7 222±4 −36.1 M1 No 18.9 17.4 9.31 0.535 0.65 0.0420 47 0.2070 47 0.0355 3.8 225±14 157.1 M1 No 1.49 304 7.55 0.025 9.22 0.0515 3.9 0.2470 4.1 0.0348 1.3 220±3 −29.1 M2 No 3.69 112 17.9 0.160 3.3 0.0475 8.2 0.2150 8.4 0.0329 1.8 209±4 510.1 M2 No 15.9 27.1 3.58 0.132 0.93 0.0870 16 0.4030 17 0.0334 3.2 212±11 −6211.1 M2 Rt 5.77 81.8 6.18 0.075 2.43 0.0510 21 0.2290 21 0.0326 2.1 207±5 −114.1 M1 Rt 18.7 9.58 1.80 0.188 0.38 0.1120 23 0.5700 24 0.0371 4.9 235±17 −9615.1 M1 No 33.2 8.33 3.96 0.475 0.39 0.0660 79 0.3300 79 0.0364 5.1 230±25 −2616.1 M2 Rt, Cpx 13.7 33.6 10.7 0.318 1.08 0.0420 39 0.1850 39 0.0323 3.0 205±9 1617.1 M1 Grt 8.62 65.4 8.23 0.126 2.2 0.0539 12 0.2660 13 0.0357 2.5 226±7 −618.1 M2 No 29.3 13.9 3.86 0.278 0.58 0.0120 200 0.0600 200 0.0339 4.1 215±18 7519.1 M2 Ap 1.19 1596 1200 0.752 46.2 0.0567 1.8 0.2605 2.1 0.0333 1.0 211±3 −1120.1 M2 Rt 33.7 16.0 2.46 0.154 0.65 0.0360 87 0.1600 87 0.0315 3.8 200±18 2621.1 M2 Grt 3.41 129 4.85 0.038 3.76 0.0573 6.8 0.2580 7.1 0.0327 1.7 207±4 −12

M1, inner mantle; M2, outer mantle; R, rime. Pbc and Pb⁎ indicate the common and radiogenic portions, respectively. Common Pb corrected using measured 208Pb. Grt, garnet; Cpx,clinopyroxene; Rt, rutile; Ap, apatite; No, no inclusion.

a Disc.(%) means discordant degree defined as [1−(207Pb/235U age)/(206Pb/238U age)]×100.


coesite from the M1 domain could be distinguished from quartz by itshigher relief, lower birefringence and Raman spectrum. As shown inFig. 3d, the Raman spectrum of the coesite inclusion contains thetypical peak of 521 cm−1 associatedwith the quartz peak (466 cm−1),suggesting that coesite was evidently transformed to quartz accom-panying a fracture through it (Fig. 2d) during decompression. It shouldbe noted that the intensity of the diagnostic 521 cm−1 band of coesiteis not as strong as expected. This may be due to the very fine-grainednature of the relict coesite accompanying with almost completelytransformed quartz, which may affect the Raman diffraction, asobserved by Ghiribelli et al. (2002), Liu et al. (2002) and Zhang et al.(2005). In addition, the garnets within themetamorphic inner-mantledomains (M1) of zircon are rich in grossular and poor in spessartinewith their end members of 16.99–21.91 mol% and 0.93–2.28 mol%,respectively; while another are relatively rich in spessartine and poorin grossular with their end members of 1.76–5.67 mol% and 11.94–

16.26 mol%, respectively (Table 4 and Fig. 4). Carswell et al. (2000)have documented that more calcic Mn-poor garnet formed at UHPconditions in many other UHP rocks from the CDZ. Liu et al. (2007a)also determined that the garnets formed at UHP and HP conditionsshow higher and lower CaO and grossular components, respectively,e.g. 8.27 wt.% and 20.02 mol% for peak UHP conditions in the area,similar to those of garnets occurred in the inner mantle domains ofdated zircon (M1). Similar cases also have been observed in UHP rocksfrom other orogens (e.g., Hermann et al., 2001). It is thereforesuggested that both types of garnet here should be ascribed to beformed at different pressure conditions, i.e. UHP and HP eclogite-facies, respectively, because it has been experimentally demonstratedthat the Ca content of garnet increases with increasing pressure at afixed temperature (Hermann and Green, 2001).

Another direct link between metamorphic history and theformation of zircon is provided by its trace elements, because it has

Table 3LA-ICPMS trace element analysis for zircon in samples 031201-1B and 06LT03-2 (ppm).

Spot no. Loc. Y Nb Ta La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Th U

031201-1B5.2 M2 20.3 1.24 0.03 b.d.l. 0.84 b.d.l. b.d.l. 0.16 0.31 1.47 0.31 2.37 0.48 1.31 0.22 1.52 0.34 13,939 0.18 1296.1 M2 37.5 1.39 0.03 0.05 1.84 0.02 0.32 0.41 0.45 3.20 0.69 4.23 0.92 3.06 0.55 4.74 0.87 13,507 5.85 3279.1 M1 65.3 1.25 0.09 0.01 2.69 b.d.l. b.d.l. 0.99 1.07 7.51 1.52 9.00 1.91 6.08 1.02 6.74 1.25 11,313 2.75 52713.1 M1 65.7 1.49 b.d.l. 0.04 1.22 b.d.l. 0.11 0.77 1.08 7.14 1.52 9.50 2.07 5.69 0.94 8.17 1.65 14,140 1.10 22614.1 M2 30.0 1.49 0.09 0.03 1.75 0.01 0.23 0.51 0.48 2.74 0.64 4.10 0.88 2.71 0.44 3.26 0.65 12,458 3.81 27615.1 M1 62.3 1.21 0.03 0.01 0.90 b.d.l. 0.06 0.78 0.88 7.70 1.66 9.83 2.03 5.51 0.96 7.02 1.53 12,659 1.79 19517.1 M2 36.3 1.30 0.06 0.02 1.29 b.d.l. 0.05 0.34 0.50 3.89 0.83 5.20 1.06 3.49 0.63 5.09 0.99 12,517 2.38 21219.1 M1 55.4 1.73 0.10 0.14 2.46 0.04 b.d.l. 0.58 0.90 6.56 1.34 8.41 1.62 5.12 0.76 5.20 0.92 13,469 3.82 52720.1 M1 38.2 1.48 0.13 0.01 2.58 0.04 b.d.l. 0.55 0.52 3.14 0.66 4.81 1.02 3.26 0.55 3.71 0.72 12,316 5.60 40521.1 M2 40.9 1.33 0.04 0.08 1.39 0.01 0.12 0.32 0.61 4.13 0.89 5.28 1.11 3.15 0.54 4.31 0.84 12,914 1.64 26022.1 M1 21.6 0.91 0.08 0.04 1.87 b.d.l. 0.10 0.60 0.53 2.70 0.48 3.04 0.60 1.81 0.27 1.87 0.36 12,141 3.45 28823.1 M2 32.5 1.42 0.07 0.04 2.15 0.01 0.39 0.61 0.59 3.15 0.72 4.65 1.14 3.33 0.56 3.61 0.61 12,102 5.95 38024.1 M1 57.1 1.17 0.09 0.04 1.89 0.01 0.18 0.93 0.94 7.32 1.61 9.85 1.99 5.59 0.93 6.92 1.14 11,674 2.37 43125.1 M1 26.5 1.39 0.08 0.02 2.32 b.d.l. 0.19 0.32 0.46 3.25 0.63 4.13 0.81 2.30 0.37 2.81 0.51 12,624 3.69 42226.1 M2 32.6 1.17 0.11 b.d.l. 1.60 0.02 0.22 0.70 0.56 3.49 0.72 4.42 1.11 2.98 0.48 3.49 0.58 12,749 3.81 30827.1 M2 21.8 0.92 0.09 0.04 0.46 0.03 0.25 0.15 0.15 0.86 0.25 2.36 0.56 1.89 0.43 3.49 0.67 12,727 0.81 94.427.2 M2 60.3 1.10 0.03 0.02 1.11 0.02 0.33 0.73 0.78 6.93 1.53 9.30 1.92 5.88 0.98 7.46 1.45 11,265 1.38 22528.1 M2 22.8 1.08 0.06 0.05 0.43 0.02 0.23 0.20 0.27 1.56 0.33 2.79 0.67 1.85 0.28 2.24 0.33 12,708 0.83 57.9

06LT03-21.1 M2 7.1 1.20 0.01 0.03 0.07 b.d.l. 0.06 b.d.l. 0.06 0.47 0.11 0.71 0.15 0.42 0.07 0.41 0.07 15,312 1.07 37.12.1 M2 22.4 1.11 0.01 b.d.l. 0.60 0.01 0.23 0.41 0.49 2.97 0.58 3.63 0.69 2.09 0.33 2.24 0.39 16,208 3.23 1124.1 M1 6.6 1.21 b.d.l. 0.01 0.12 0.03 b.d.l. 0.07 0.09 0.49 0.08 0.60 0.13 0.29 0.04 0.27 0.05 16,552 2.53 14.25.1 M1 20.9 1.00 b.d.l. 0.01 0.84 b.d.l. 0.07 0.37 0.53 3.09 0.59 3.34 0.71 1.96 0.29 2.43 0.39 15,521 5.88 1328.1 M1 24.2 1.51 3.91 0.06 0.36 b.d.l. 0.13 0.40 0.39 3.14 0.74 3.90 0.77 2.39 0.30 2.17 0.36 16,198 8.73 97.79.1 M2 10.7 1.30 0.03 b.d.l. 0.48 0.01 0.08 0.09 0.30 1.67 0.30 1.52 0.35 0.96 0.16 1.26 0.26 16,484 5.67 93.110.1 M2 9.0 1.20 0.02 0.04 0.62 0.02 0.11 0.16 0.17 1.12 0.22 1.10 0.20 0.64 0.10 0.67 0.13 18,010 5.34 38.911.1 M2 23.6 1.66 0.04 0.05 0.68 b.d.l. 0.22 0.24 0.46 2.79 0.42 3.42 0.72 2.05 0.34 2.47 0.42 17,070 10.3 14116.1 M2 7.9 1.52 0.01 b.d.l. 0.45 b.d.l. 0.05 0.10 0.19 0.98 0.21 1.08 0.21 0.62 0.10 0.70 0.16 17,735 11.0 37.417.1 M1 16.2 1.27 0.04 0.02 0.34 b.d.l. b.d.l. 0.94 0.60 2.46 0.44 2.49 0.60 1.46 0.24 1.53 0.33 18,397 5.96 76.5

LT104.1 C 244 0.31 0.16 0.004 2.93 0.02 0.27 0.55 0.48 2.94 1.16 17.69 7.87 41.30 11.88 150.1 24.9 4566 81 1514.2 C 617 0.34 0.15 0.04 4.19 0.10 1.74 2.83 1.80 13.88 4.63 59.55 21.69 94.39 24.12 266.3 37.6 4397 243 1494.5 C 582 0.31 0.13 0.06 3.95 0.17 2.31 2.98 1.96 12.87 4.44 51.88 19.79 88.13 21.93 239.5 38.7 4739 141 101

M1, inner mantle; M2, outer mantle; C, core. b.d.l. = below detection limit.


been documented that the REE patterns of zircon reflect theconcurrent growth of minerals such as garnet (depletion in HREE)or feldspars (negative Eu anomaly) and their formation environment,and are not affected by the retrograde history or secondary alteration(Cherniak et al., 1997; Hermann and Rubatto, 2003; Liu et al., 2009;Rubatto, 2002; Schaltegger et al., 1999). It is therefore possible thatthe trace element composition and the REE patterns of zircon canmonitor the coexisting paragenesis and characterize the chemicalenvironment at the time of zircon formation. As described above,zircon grains from the present dated samples are metamorphic inorigin and only rare have igneous core from sample 031201-1A. Inorder to compare the trace element difference of igneous core withmetamorphic overgrowth domains, the rare earth elements of zirconfrom the previously dated eclogite (sample LT10) (Liu et al., 2007a)were performed (Fig. 5). The zircon cores from sample LT10 showhigh REE contents and typical enrichment in HREE of magmatic zircon(Rubatto, 2002). Trace element compositions of two kinds ofmetamorphosedmantle domains (M1 andM2) in zircon from samples031201-1B and 06LT03-2 exhibit obviously consistent REE patternswith clearly flat HREE and no significance negative Eu anomaly(Fig. 5), which is compatible with absence of feldspar and contem-poraneous growth of zircon and garnet, suggesting that all themetamorphic M1 and M2 domains formed under eclogite-faciesconditions (e.g., Hermann et al., 2001; Rubatto, 2002; Rubatto andHermann, 2003; Sun et al., 2002; Whitehouse and Platt, 2003).

Therefore, based on garnet compositions, inclusion assemblages,REE patterns and their discrete ages (as detailed below), there are twotypes of metamorphic M1 and M2 domains in zircon from threesamples. They formed in separate stages of the metamorphicevolution of the rock and recorded UHP and HP eclogite-faciessignatures, respectively. These are quite consistent with the abovepetrographic observations.

4.2. Zircon U–Pb ages

The zircon grains from sample 031201-1A were too small to allowmore than one analysis per grain by LA-ICPMS U–Pb dating; twenty-eight analyses were carried out on 28 zircon grains (Table 1). Spot 17from the core domain yielded a 206Pb/238U concordant age of 731±6 Ma, representing the protolith age supported by igneous oscillatoryzoning and high Th/U ratio of 1.087. The other 27 analyses on themetamorphic domains show lowTh contents (1–31 ppm)and lowTh/Uratios of 0.01–0.05, consistent with metamorphic origin (e.g., Rubatto,2002). All the 28 analyses define a discordiawith an upper intercept ageof 684±30Ma and a lower intercept age of 229±8 Ma (MSWD=2.9)(Fig. 6a). The 22 concordant ages from themetamorphic zircondomainscan be further subdivided into two groups with peaks in the probabilityplot at 226±3 Ma (MSWD=0.57, n=13) (M1) and 213±3 Ma(MSWD=0.88, n=9) (M2), respectively (Fig. 6b). In addition, spots15, 21 and 13 yield 206Pb/238U concordant ages of 238±2, 199±2 and188±2 Ma, respectively.

A total of 30 U–Pb spot analyses were made on 27 zircon grainsfrom sample 031201-1B (Table 2; Fig. 6c). Except spot 14.1, 29concordant age-spots from the metamorphic mantle domains can befurther grouped in 227±2 Ma (MSWD=1.6, n=15) and 215±2 Ma(MSWD=0.77, n=14), respectively (Fig. 6c). The spot 14.1 gives ayoung concordant age of 176±2 Ma.

Eighteen U–Pb spot analyses were obtained from 18 grains fromsample 06LT03-2 (Table 2). The concordant age-spots from themetamorphic mantle domains can be further grouped in 222±4 Ma(MSWD=0.33, n=4) and 210±4 Ma (MSWD=0.19, n=4)(Fig. 6d). The spot 1.1 gives a young concordant age of 180±12 Ma.

In summary, the zircon U–Pb age results indicate that the studiedsamples experienced the same metamorphic events and have theidentical multiple 206Pb/238U concordant age-groups within analytical

Table 4Electron microprobe analyses of representative garnets in zircon from samples 031201-1A, 031201-1B and 06LT03-2 in the NDZ.

Sample no. 031201-1A 031201-1B 06LT03-2

Spot no. 1 2 3 4 5 1 2 3 4 5 6 7 8 1 2 3 4 5 6

Locality M1 M2 M1 M2 M2 M2 M2 M1 M2 M2 M2 M1 M2 M1 M2 M1 M1 M2 M1

SiO2 38.14 38.90 38.37 37.53 37.94 38.13 37.84 37.90 38.44 38.49 38.28 38.22 38.99 39.70 39.14 39.11 38.52 39.14 39.03TiO2 0.04 b.d.l. b.d.l. 0.02 b.d.l. 0.03 b.d.l. 0.01 0.01 0.03 b.d.l. 0.02 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 0.01 b.d.l.Al2O3 21.30 21.35 20.83 21.24 21.74 22.01 21.86 21.63 21.78 21.22 21.99 21.84 20.91 22.26 21.64 22.94 22.04 21.92 22.03FeO 26.24 26.48 26.83 27.97 27.55 27.20 27.17 27.18 26.38 26.66 25.98 25.75 26.75 22.82 25.12 22.47 24.53 24.31 23.25Cr2O3 0.14 b.d.l. 0.02 b.d.l. 0.02 0.05 b.d.l. b.d.l. 0.06 0.06 b.d.l. 0.01 b.d.l. b.d.l. b.d.l. 0.05 0.02 b.d.l. b.d.l.MnO 0.85 1.96 1.02 1.36 1.10 1.44 1.33 0.57 1.28 1.41 2.48 0.48 1.40 0.49 1.05 0.59 0.66 0.79 0.42MgO 4.39 4.82 4.50 4.64 5.39 4.46 5.17 4.74 5.09 4.81 4.83 4.56 4.78 7.61 6.96 7.72 6.55 7.80 7.57CaO 8.85 6.92 8.41 7.18 6.21 6.89 5.89 7.95 6.90 6.79 6.65 9.06 7.16 7.80 6.07 7.20 7.64 5.99 7.62Na2O 0.03 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.l. b.d.l. b.d.l. b.d.l. b.d.l. 0.02 b.d.l. 0.01 b.d.l.Total 99.84 100.43 99.98 99.94 99.95 100.21 99.26 99.98 99.94 99.47 100.21 99.94 99.99 100.68 99.98 100.10 99.94 99.97 99.92O 12.000 12.000 12.000 12.000 12.000 12.000 12.000 12.000 12.000 12.000 12.000 12.000 12.000 12.000 12.000 12.000 12.000 12.000 12.000Si 2.988 3.038 3.011 2.952 2.969 2.987 2.984 2.967 3.007 3.034 2.993 2.986 3.059 3.021 3.028 2.988 2.977 3.010 2.996AlIV 0.012 0.000 0.000 0.048 0.031 0.013 0.016 0.033 0.000 0.000 0.007 0.014 0.000 0.000 0.000 0.012 0.023 0.000 0.004AlVI 1.953 1.963 1.925 1.919 1.973 2.018 2.014 1.962 2.006 1.970 2.017 1.995 1.932 1.995 1.972 2.052 1.983 1.985 1.987Fe3+ 0.086 0.086 0.088 0.092 0.090 0.089 0.089 0.089 0.086 0.088 0.085 0.084 0.088 0.073 0.081 0.072 0.079 0.078 0.075Ti 0.002 0.002 0.000 0.001 0.000 0.002 0.000 0.001 0.001 0.002 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.001 0.000Fe2+ 1.633 1.643 1.673 1.748 1.713 1.693 1.702 1.691 1.639 1.670 1.614 1.598 1.667 1.380 1.544 1.364 1.506 1.485 1.418Cr 0.009 0.000 0.001 0.000 0.001 0.003 0.000 0.000 0.004 0.004 0.000 0.001 0.000 0.000 0.000 0.003 0.001 0.000 0.000Mg 0.513 0.561 0.526 0.544 0.629 0.521 0.608 0.553 0.594 0.565 0.563 0.531 0.559 0.863 0.803 0.879 0.755 0.894 0.866Mn 0.056 0.130 0.068 0.091 0.073 0.096 0.089 0.038 0.085 0.094 0.164 0.032 0.093 0.032 0.069 0.038 0.043 0.051 0.027Ca 0.743 0.579 0.707 0.605 0.521 0.578 0.498 0.667 0.578 0.573 0.557 0.758 0.602 0.636 0.503 0.589 0.633 0.494 0.627Na 0.005 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.003 0.000 0.001 0.000Alm 55.37 56.40 56.24 58.50 58.36 58.63 58.77 57.34 56.61 57.52 55.68 54.75 57.08 47.40 52.90 47.47 51.29 50.76 48.26Adr 4.19 4.21 4.37 4.57 4.36 4.21 4.25 4.33 4.11 4.25 4.04 4.04 4.34 3.51 3.95 3.37 3.84 3.78 3.62Grs 20.57 15.67 19.35 15.69 13.32 15.67 12.93 18.39 15.68 15.32 15.19 21.91 16.26 18.34 13.28 16.99 17.65 13.09 17.72Prp 17.38 19.27 17.70 18.21 21.42 18.04 20.98 18.76 20.50 19.47 19.43 18.19 19.14 29.66 27.50 30.58 25.70 30.56 29.48Sps 1.91 4.45 2.28 3.03 2.48 3.31 3.07 1.28 2.93 3.24 5.67 1.09 3.19 1.08 2.36 1.34 1.47 1.76 0.93

M1 and M2 represent inner- and outer-mantle of zircon, respectively. b.d.l. = below detection limit.

115Y.-C.Liu

etal./

Lithos122

(2011)107

–121

Fig. 4. End members of grossular (XGrs) and spessartine (XSps) of garnet occurred inmetamorphic inner-(M1) and outer-(M2) domains of zircon from eclogites in theLuotian dome.


uncertainty, suggesting six discrete age records, i.e. 731±6 Ma, 238±2 Ma, 222±4–227±2 Ma with a weighted mean age of 226±3 Ma(MSWD=0.63, n=3), 210±3–215±2 Ma with a weighted meanage of 214±3 Ma (MSWD=0.67, n=3), 199±2 Ma and 176±2–188±2 Ma. However, the age-groups of 238±2 Ma, 199±2 Ma and

Fig. 5. Chondrite-normalized REE patterns of zircons from samples 031201B (a) and06LT03-2 (b). Solid and open circles denote metamorphic inner-(M1) and outer-(M2)domains of zircon from samples 031201B and 06LT03-2; and open squares representigneous core domains of zircon from sample LT10. Chondrite-normalization values arefrom Sun and McDonough (1989).

176±2–188±2 Ma are needed to be further constrained by moredata.

5. Discussion

5.1. Peak UHP metamorphic age in the NDZ

As described above, in spite of the relatively well-constrainedmetamorphic and magmatic evolution in the NDZ, geochronologicaldata on the precise timing of multistage metamorphic events areeither not available or inconclusive. An important, and still verycontroversial question, concerns the actual time of UHP metamorphicclimax in the area.

Owing to the relatively high closure temperature (Tc) for Pbdiffusion in it (N800 °C, even N900 °C) and its sluggish kineticproperties for U, Th and Pb (Burton et al., 1995; Lee et al., 1997),zircon can retain high-fidelity records of its crystallization andmetamorphic history and cannot be systematically reset under mostgeological conditions, even during granulite-facies metamorphismand anatexis (e.g., Burton et al., 1995; Mezger and Krogstad, 1997).This implies that zircon that formed at any time during a high-grademetamorphic event generally records the time of zircon growth and/or overgrowth rather than a cooling age and its geochronologicalinformation related to earlier events is unaffected or not completelyerased by the retrograde evolution or secondary processes such asthermal overprint (Ayers et al., 2002; Burton et al., 1995; Hermannand Rubatto, 2003; Möller et al., 2002), and thus has the potential toreveal the original crystallization age as well as the ages of subsequentmetamorphic events within a single rock by in-situ U–Pb dating (e.g.,Rudnick and Williams, 1987). In contrast, Sm–Nd isochrones of HProcks are unlikely to relate to crystallization and often reflect onlycooling ages because of their lower closure temperatures (e.g., Burtonet al., 1995; Klemd and Bröcker, 1999; Mezger et al., 1992). Thus, thetiming of peak UHP metamorphism is most reliably dated by zirconin-situ U–Pb analysis (Ayers et al., 2002, 2003; Klemd and Bröcker,1999). In this context, published Sm–Nd data of the UHP rocks in theDabie orogen, especially in the NDZ, most likely represent coolingages or reworked ages.

In addition, the complexity of Sm–Nd isotopic systematics ineclogite and related high-grade rocks has been discussed by Li et al.(2000) and Thöni and Jagoutz (1992) in details. It was mainlyinvolved in the current debate on Tc of the Sm–Nd isotopic system ingarnet. Whereas some earlier studies indicate relatively hightemperatures in excess of 850 °C, more recent studies favor lowertemperatures (700–800 °C) (cf. Jung and Mezger, 2001 for review).On the other hand, Burton et al. (1995) and Mezger et al. (1992)argued for an even lower temperature of 600–650 °C for Nd closure.Actually, the closure temperature for any given element in garnet isdependent on a number of factors, including grain size, availability ofpore fluid, major element composition, the nature of coexistingphases, initial temperature, and the cooling rate experienced by eachindividual sample (Burton et al., 1995; Dodson, 1973). Therefore,garnet is unlikely to possess a unique closure temperature for Nd, butmay have a range from 500 to 850 °C, depending on variableconditions (for a review, see Li et al., 2000).

Furthermore, most of published Sm–Nd ages for UHP rocks in theDabie orogen are essentially isochrones given by two UHP minerals(i.e. one garnet+one omphacite) (e.g., Chavagnac and Jahn, 1996; Liet al., 1993). Because the possibility of Nd isotopic disequilibriumbetween UHP minerals and garnet+whole rock containing retro-grademetamorphic minerals often give older or unreasonable ages (Liet al., 2000; Liu et al., 2005), only the Sm–Nd isochron ages defined bythree or more UHPmineral phases for eclogite and related rocks couldbe more effective and reliable, and thereby chosen for the discussionin the text.

image of Fig.�4

image of Fig.�5

Fig. 6. Concordia diagrams of zircon U–Pb dating by means of the LA-ICPMS (a and b) and SHRIMP (c and d) technique for eclogites from the Luotian dome. Data-point error ellipsesare 1σ.


If the Tc of 700–800 °C for the Sm–Nd system in garnet, suggestedby Henson and Zhou (1995) and Jung and Mezger (2001), isreasonable and applicable also to the UHP rocks in the Dabie orogen,it may be close to the peakmetamorphic temperature of the UHP rocksin the CDZ but obviously lower than the one in the NDZ. Therefore, theSm–Nd isochrones of garnet+omphacite+rutile for coesite-bearingeclogite from the CDZ yielded an age of 226±3 Ma (Li et al., 2000),probably representing the UHP metamorphic time at or near peakmetamorphic conditions; whereas a reliable high-precision four-pointSm–Nd isochron age of 212±4 Ma with a relatively low MSWD valueof 1.01 (Liu et al., 2005), defined by two garnet+two omphacite fromthe eclogite at Huangweihe in the northern part of the NDZ, points to acooling time orHP eclogite-facies timebut did not record the peakUHPmetamorphic one in the area, as indicated by a striking compositionaldifference in jadeite end member of matrix- and inclusion-omphacitegrains (20–30 and40–50 mol%) for eclogite in theNDZ (Liu et al., 2004,2005; Xu et al., 2000). Such an interpretation is completely consistentwith the present results of 214±3 Ma recorded on the M2 domains ofzircon, formed at quartz eclogite-facies conditions. Moreover, the M1

domains of zircon formed the peakUHPage of 226±3 Ma, as evidentlysupported by coesite-bearing inclusion assemblage and trace ele-ments. In this case, our reported zircon U–Pb age of 218±3 Ma for theUHP gneiss in the NDZ should not be the best estimation for the peakmetamorphic time of the NDZ as supposed by Liu et al. (2007b) and itsgeochronological significance must be reevaluated. The main reasons

probably arise from an inappropriate weighted mean age calculationwithout consideration of possible poly-metamorphic age componentsin zircon from the sample. Although all the analyzed data wereobtained from the metamorphic mantle domains of zircon in sampleLT8-3 and are 206Pb/238U concordant ages, they span a larger age rangefrom 213 to 230 Ma with a relatively high MSWD value (2.7), whichmay be composed of more than one population with peaks at 224±3 Ma (MSWD=0.67, n=7) and 214±3 Ma (MSWD=1.02, n=8),respectively. The recalculated age results on the gneiss are in excellentagreement with the above data on the eclogites within error.Therefore, the 226±3 and 214±3 Ma can be considered to be thebest estimation of the UHP and HP eclogite-facies metamorphic time,respectively in the NDZ, as strongly supported by the strikingdifference in end members of garnet and mineral assemblage withinboth domains of metamorphic zircon described above. In this regard,the eclogites and surrounding gneisses in the NDZ should share acommon metamorphic evolutional process and hence have similargeochronological records, demonstrating that at least in this regionall the metamorphic rocks record an in-situ development of UHPmetamorphism.

5.2. Geochronological implications

The above results suggest that zircon from the eclogites in the NDZdisplays complex internal texture, including rare igneous cores and

image of Fig.�6

Fig. 8. Sketched tectonic model for the detachment within deeply subductedcontinental crust and multi-slice successive exhumation in the Dabie orogen (modifiedafter Liu et al., 2007b). (a), (b) and (c) show successive subduction of underlying sliceswith nearly concomitant exhumation of overlying slices during the Triassic (242–199 Ma) collision between the NCB and the SCB. (d) Extrusion and thrusting ofsubducted slices onto the SCB during the late Triassic to middle Jurassic (199–170 Ma)convergence. (e) Doming and partial melting of eclogitic lower crust and enrichedupper lithospheric mantle, in response to a hot asthenosphere upwelling possiblyresulting from delamination of lithospheric mantle as well as the lowermost part oflower crust during 140–110 Ma. SCB = South China Block, NCB = North China Block,UC = upper continental crust, and LC = lower continental crust. The other abbreviatedsymbols are the same as in Fig. 1.

Fig. 7. Schematic P–T–t paths for three eclogite-bearing UHP slices in the Dabie orogen.The equilibrium lines for diamond = graphite (Kennedy and Kennedy, 1976) andcoesite = quartz (Bohlen and Boettcher, 1982) are shown. The P–T estimates and agesfor different metamorphic stages of the CDZ and SDZ were summarized from previousstudies (Ayers et al., 2002; Cong et al., 1995; Li et al., 2000, 2004; Liu et al., 2006a,2006b; Okay, 1993; Qiu et al., 2010; Rolfo et al., 2004; Wu et al., 2006). Themetamorphic P–T estimates and ages for different metamorphic stage of the NDZ wereestablished from published data (Xu et al., 2005; Liu et al., 2007a, 2011) and our presentstudy and unpublished data.


metamorphically grown mantle–rim domains, and recorded multi-stage age-clusters. The igneous core yields a Neoproterozoic age of731±6 Ma, suggestive of the formation time of their protoliths. Themeasured concordant ages from the metamorphic domains of zirconcan represent the timing of new growth or recrystallization of zirconduring subduction and exhumation as documented by Ayers et al.(2002), defined five discrete groups at 238±2, 226±3, 214±3 Ma,199±2 and 176±2–188±2 Ma. The second and third groupmetamorphic ages of these provide strongly chronological constraintson UHP and HP eclogite-facies metamorphism, characterized bymineral inclusion assemblages, lower Th/U ratios and typical REEpatterns with clearly depleted HREE and no significance negative Euanomaly. Minerals formed during granulite- and amphibolite-faciesretrogression occur commonly as symplectite assemblages for theUHP rocks in the NDZ (Liu et al., 2007a, 2011), indicating that eitherthe granulite- and amphibolite-facies metamorphism only lasted ashort period of time, or there were no or absence of fluids involved. Inthis regard, mineral recrystallization and zircon overgrowth weresignificantly limited at both stages, because the dissolution andovergrowth of zircon depend on the availability of fluids duringmetamorphism (Ayers et al., 2002; Liermann et al., 2002; Rubattoet al., 1999; Wu et al., 2006; Zheng et al., 2004). It is also documentedthat, for a given bulk composition, Zr solubility increases withincreasing temperature and reported that small overgrowths onzircon grains are present at granulite-facies grade (Rubatto et al.,2001). Hence, the ages of 199±2 and 176–188 Ma defined by a fewdata from the relevant domains of zircon, most of which are too thinto be analyzed by the SHRIMP II, may represent the retrograde time ofgranulite- and amphibolite-facies, respectively, as evidenced by the

granulite-facies mineral (garnet+diopside+whole rock) Sm–Ndisochron age of 199±2 Ma (Liu Y.-C. et al., unpublished data) andamphibole+whole rock Rb–Sr age of 172±3 Ma (Liu et al., 2000b)for the retrograded eclogites from the NDZ. Owing to thin and absenceof mineral and trace element evidence on zircon overgrowth rim,

image of Fig.�8

image of Fig.�7


more data are required in order to: (i) better constrain the ages of twoyounger events with additional methods such as Sm–Nd and Rb–Srdating, and (ii) better understand the metamorphic evolution of thestudied samples. Whereas a middle Triassic age of 238±2 Ma withlow Th/U ratio of 0.01 probably points to the prograde metamorphictiming prior to the UHP metamorphism. Due to the only one data andabsence of any supporting petrological observations or data, however,more data are needed to better constrain this age and its significance.

As described above and proposed by Liu et al. (2007b), in order tocompare the peak metamorphic ages with three different UHP slicesin the Dabie orogen, we chose the previous published data of zirconSHRIMP U–Pb dating and the Sm–Nd isochron ages defined by threeor more mineral phases for UHP rocks for the discussion below.Apparently, the present peak UHP age of 226±3 Ma in the NDZ isyounger than the zircon U–Pb ages of 234±3 and 242±3 Ma fromthe UHP rocks in the CDZ and SDZ, respectively (Li et al., 2004; Liuet al., 2006b). Accordingly, combined with previously published data(cf. Liu et al., 2007b in detail), there is a decrease trend of the peakmetamorphic and exhumation ages for the three UHP slices fromsouth to north, strongly supporting our previously proposed multi-slice exhumation model for UHP rocks from different slices of deeplysubducted continental crust in the Dabie orogen (Liu et al., 2007b). Itis further documented that there are several different tectonic slicesthat were at different crustal levels during the Triassic subduction andsubsequent exhumation and thus recorded UHP conditions atdifferent depths and times (Li et al., 2004; Liu and Li, 2008; Liuet al., 2007b; Okay et al., 1993; Xu et al., 2005) (Fig. 7). Similar caseshave also been observed in the Sulu terrane (Liu et al., 2009) and otherorogens (e.g., Bauer et al., 2007).

Therefore, the present and published data mentioned abovesupport a scenario in which the deeply subducted continental crustof the SCB did not remain a single coherent unit, but formed severalslices by multiple decoupling within it during subduction to mantledepths and subsequent exhumation, which may mainly be owing tothe difference of mechanic strength of rocks in various levels of thecontinental crust (Liu and Li, 2008; Liu et al., 2007b). Thisinterpretation is in agreement with experimental results showingthe existence of at least two low-viscosity zones within continentalcrust at different depths (Meissner and Mooney, 1998). In thiscontext, the detachment between the SDZ and CDZ, and the CDZ andNDZ occurred during the period of 242–226 Ma and 226–214 Ma,respectively, showing successive subduction of underlying slices withnearly concomitant exhumation of overlying slices in the Dabieorogen during the Triassic (242–199 Ma) collision between the NCBand the SCB (Fig. 8a, b, c). During the late Triassic to Jurassic (199–170 Ma) convergence, the three deep-subducted slices were extrudedand thrusted onto the SCB (Fig. 8d).

During the Cretaceous extension (140–110 Ma), doming andextensive plutonism resulted in the present structure and distributionof tectonic units (Fig. 8e), in response to a hot asthenosphereupwelling possibly resulting from delamination of lithosphericmantleas well as the lowermost part of lower crust in this period (Liu et al.,2007a; Wang et al., 2007; Zhao et al., 2008).

6. Concluding remarks

The present study for the first time constraints the precise timing ofUHP and HP eclogite-facies metamorphism for the NDZ in the Dabieorogenwith the ages of 222±4 Ma–227±2 Ma and 210±4 Ma–215±2 Ma, respectively. Accordingly, although three eclogite-bearing UHPslices in the Dabie orogen have similar peak UHP metamorphic ageswithin error, they have different P–T–t paths and exhumationalhistories resulting in various geochronological records on them afterthe peak stage. It is further documented that there are several differentslices formed by multiple decoupling within the subducted continentalcrust of the SCB during subduction to mantle depths and subsequent

exhumation, mainly resulted from the difference of mechanic strengthof rocks in various levels of the continental crust. Such an interpretationwill advance our general understanding of the tectonic evolution ofvarious parts of a continent–continent collision orogen, particularly inHP–UHP terranes.

Acknowledgements

This study was financially supported by the National BasicResearch Program of China (2009CB825002), the Chinese Academyof Sciences (kzcx2-yw-131), the PhD Foundation of the Ministry ofEducation of China (200803580001) and the National Natural ScienceFoundation of China (40572035, 40921002 and 40973043). Specialthanks are due to Dunyi Liu and Xiaoming Liu for their help in SHRIMPand LA-ICPMS U–Pb dating on zircon, to Zhenyu Chen for the CLimaging, to Ling Yan for the Raman analysis, and to Huifang Liu,Wenlan Zhang and Zhenyu Chen for electron microprobe analysis.Laura E. Webb and Christopher G. Mattinson are thanked for theirsuggestions and discussion on earlier version of this manuscript,which helped clarify some ambiguities as well as the Englishpresentation. Comments by Ian Buick, Armin Zeh and an anonymousreviewer have greatly improved the paper.

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