Multiple episodes of anatexis in a collisional orogen: Zircon evidence from migmatite in the Dabie orogen Ren-Xu Chen, Binghua Ding, Yong-Fei Zheng, Zhaochu Hu PII: DOI: Reference:
Please cite this article as: Chen, Ren-Xu, Ding, Binghua, Zheng, Yong-Fei, Hu, Zhaochu, Multiple episodes of anatexis in a collisional orogen: Zircon evidence from migmatite in the Dabie orogen, LITHOS (2014), doi: 10.1016/j.lithos.2014.11.004
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Multiple episodes of anatexis in a collisional orogen: zircon
1. CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and
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Space Sciences, University of Science and Technology of China, Hefei 230026, China 2. State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth
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Sciences, China University of Geosciences, Wuhan 430074, China
ACCEPTED MANUSCRIPT Abstract A combined study of mineral inclusions, U-Pb ages and trace elements was carried out on zircon from migmatites in the Dabie orogen. The results provide insights into multistage
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anatexis of ultrahigh-pressure metamorphic rocks in the continental collision orogen. Zircon
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grains in thin sections and mounts record four episodes of magmatic, metamorphic and anatectic events: (1) middle Neoprotrozoic U-Pb ages for domains that contain Qz ± Ap ± Pl
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inclusions and exhibit high Th/U ratios (>0.1), steep HREE patterns with marked negative Eu anomalies, dating protolith emplacement; (2) late Triassic U-Pb ages of 212 ± 5 to 219 ± 4 Ma for domains that contain Cpx ± Grt inclusions and show low Th/U ratios (<0.1), flat HREE
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patterns without negative Eu anomalies, dating quartz eclogite-facies metamorphism during the early exhumation of deeply subducted continental crust; (3) U-Pb ages of 192 ± 4 to 200 ±
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4 Ma for domains that contain multiphase solid inclusions of Qz + Pl + Hem + Cal, Qz + Kfs + Ep, Qz + Bt + Ap and Qz + Pl + Bt, and exhibit low Th/U ratios (<0.1), steeper HREE patterns with negative Eu anomalies and high Nb and Ta contents, dating the first episode of
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anatexis in association with granulite-facies overprinting during the late exhumation; and (4)
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early Cretaceous U-Pb ages of 124 ± 1 to 140 ± 4 Ma for domains that contain Qz inclusion and show variable Th/U ratios, typical magmatic REE patterns, dating the second episode of
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anatexis which occurred during the Cretaceous in the postcollisional stage. The zircon domains in the two episodes of anatexis exhibit a large range of U-Pb ages, suggesting protracted durations of anatexis in the both exhumational and postcollisional stages. There are
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considerable differences in the compositions of zircons between the two episodes of anatexis, suggesting differential behaviors of their anatexis. The first episode of anatexis is caused by dehydration melting in association with decompressional exhumation, resulting in coprecipitation of zircon and REE-rich minerals such as allanite/epidote and monazite from the anatectic melt. The second episode of anatexis is caused by both dehydration and hydration melting due to breakdown of monazite and hydrous minerals such as allanite/epidote and amphibole with local focus of aqueous solutions. This leads to variable Th/U ratios but relatively consistent magmatic zircon-like trace element compositions, with the widespread occurrence of poikilitic amphibole in the leucosome. In either case, the anatectic zircons record the partial melting of ultrahigh-pressure metamorphic rocks at different times and P-T conditions.
ACCEPTED MANUSCRIPT 1. Introduction Partial melting has been increasingly recognized in ultrahigh-pressure (UHP) metamorphic rocks of continental collision orogens (e.g., Zheng et al., 2011 and references
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therein). In some high-T/UHP terranes, anatexis is likely to occur close to the peak UHP stage
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(e.g., Labrousse et al., 2011). In some low-T/UHP and mid-T/UHP terranes, on the other hand, anatexis may occur during exhumation of deeply subducted continental crust (e.g., Hermann
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and Green, 2001; Hermann, 2002a; Auzanneau, et al. 2006; Lang and Gilloti, 2007; Zhao et al., 2007; Xia et al., 2008; Ragozin et al., 2009; Chen et al., 2013a, 2013b; Li et al., 2014). Recognition and dating of crustal anatexis at the uppermost mantle and lower crustal depths
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are therefore crucial not only for deciphering the exhumational mechanism of UHP metamorphic rocks but also for understanding the relationships between partial melting,
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granitic magmatism and orogenic processes (e.g., Keay et al., 2001; Wallis et al., 2005; Whitney et al., 2009; Zheng et al., 2011). However, it is hardly straightforward to demonstrate explicitly that the incipient melting indeed took place in UHP metamorphic rocks because
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many UHP rocks suffered extensive retrograde reaction and reequilibration during
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exhumation. This is particularly so for those UHP rocks that experienced amphibolite-facies overprinting. Nevertheless, such refractory minerals as zircon may provide robust records of
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anatexis.
Zircon is a refractory mineral and has very low rates of Pb diffusion (Cherniak, 2010), thus its U-Pb age can readily reflect its growth from anatectic melts rather than simple cooling
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along a metamorphic P-T path (e.g., Baldwin et al., 2007; Wu et al., 2007; Liu et al., 2012). The extremely stable property of zircon and its high closure temperature of Pb diffusion leave its isotopic system completely or partly undisturbed by subsequent metamorphism and migmatization. In situ U-Pb dating of zircon domains can thus provide important age information on the complicated evolution history of its metamorphic/migmatic host (e.g., Keay et al., 2001; Wu et al., 2007; Rubatto et al., 2009; Liu et al., 2012; Li et al., 2014). Furthermore, based on mineral inclusions and trace elements in zircon, it is feasible to distinguish metamorphic zircon from magmatic and anatectic zircons, and further link the different origins of zircon domains to specific geological processes (e.g., Rubatto, 2002; Rubatto and Hermann, 2003; Zheng, 2009; Xia et al., 2009, 2010, 2013; Chen et al., 2010, 2011, 2013a; Liu and Liou, 2011; Li et al., 2013, 2014). Zircon studies have been extensively performed on UHP metamorphic rocks from the Dabie-Sulu orogenic belt, one of the largest UHP metamorphic terranes on Earth (e.g., Liou et
ACCEPTED MANUSCRIPT al, 2009; Zheng, 2012). In terms of the differences in metamorphic P-T conditions, the Dabie UHP slices are subdivided into low-T/UHP, mid-T/UHP and high-T/UHP zones, respectively (Zheng, 2008; Liu and Li, 2008). While amphibolite-facies retrogression is prominent in the
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mid-T/UHP zone, granulite-facies overprinting is remarkable in the high-T/UHP zone.
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According to the experimental data available for the stability of white micas such as muscovite and phengite (Hermann, 2002a; Auzanneau et al., 2006; Zheng et al., 2011) and the
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P-T paths of the three UHP zones (Zheng, 2008; Liu and Li, 2008), partial melting is expected to take place during their exhumation to lower crust levels. This is confirmed by microscale studies of petrography, mineralogy and geochemistry on UHP eclogite and granitic gneiss in
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the mid-T/UHP zone of Central Dabie (Gao et al., 2012a; Liu et al., 2013) and the low-T/UHP zone of South Dabie (Xia et al., 2008). However, it is still unclear whether the high-T/UHP
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zone of North Dabie also underwent partial melting during the exhumation. Partial melting is very prominent in the eastern part of the Sulu orogen, where UHP rocks underwent late Triassic migmatization (Liu et al., 2010a, 2012; Zong et al., 2010a; Zeng et al., 2011; Chen et
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al., 2013a, 2013b) and granitic magmatism (e.g., Yang et al., 2005; Zhao et al., 2012). In view
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of the middle Triassic U-Pb ages for UHP metamorphism in the coesite stability field (Zheng et al., 2009; Liu and Liou, 2011), it is evident that the partial melting of UHP rocks primarily
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took place during the exhumation of deeply subducted continental crust. Nevertheless, early Cretaceous migmatization and magmatism are remarkable in the North Dabie high-T/UHP zone (e.g., Zhang et al., 1996; Wu et al., 2007; Zhao and Zheng, 2009; Wang et al., 2013). Thus, it is critical to distinguish the postcollisional anatexis from the synexhumation anatexis
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in the UHP metamorphic rocks.
This paper presents an integrated study of zircon mineragraphy, U-Pb geochronology and trace element geochemistry for migmatites from the Dabie orogen. The results indicate that the migmatites have a Neoproterozoic protolith age and a late Triassic metamorphic age, implying their involvement in the Triassic continental collision. In addition, there are two groups of U-Pb ages in the late Triassic to early Jurassic and the early Cretaceous, respectively, for anatectic zircons from the migmatites. This indicates multistage anatexis of the UHP metamorphic rocks in the exhumational and postcollisional stages, respectively. Because of the significant difference in the time of crustal anatexis, the present study provides insights into the difference in anatectic P-T conditions between the exhumational and postcollisional stages and thus into the tectonic evolution of the continental collision orogen.
2. Geological setting
ACCEPTED MANUSCRIPT The Dabie-Sulu orogenic belt in east-central China was built by the Triassic continental collision between the South China Block and the North China Block (e.g., Li et al., 1993; Zheng et al., 2003; Xu et al., 2006). Identifications of coesite and microdiamond in eclogites
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(e.g., Wang et al., 1989; Xu et al., 1992) and granitic gneiss from this belt (e.g., Liu and Liou,
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2011) demonstrate that crustal materials were subducted to mantle depths of at least 120 km for UHP metamorphism. The Tanlu Fault separates the Dabie-Sulu orogenic belt into eastern
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and western segments, which are named the Sulu and Dabie orogens, respectively (Fig. 1). The Dabie orogen is composed of five fault-bounded metamorphic units that are named the main tectonic zones from north to south (Zheng et al., 2005): (1) the Beihuaiyang
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low-T/low-P greenschist-facies zone, (2) the North Dabie high-T/UHP granulite-facies zone, (3) the Central Dabie mid-T/UHP eclogite-facies zone, (4) the South Dabie low-T/UHP
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eclogite-facies zone, and (5) the Susong low-T/high-P blueschist-facies zone. The majority of these metamorphic rocks have igneous protoliths, which have been dated to have zircon U-Pb ages of 740 to 780 Ma (e.g., Zheng et al., 2009). Postcollisional magmatic rocks are
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dominated with felsic rocks with sporadic mafic rocks, which occur in all the five
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metamorphic units (e.g., Zhao and Zheng, 2009).
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The North Dabie high-T/UHP zone is primarily composed of granitic orthogneiss, with minor eclogite, peridotite, pyroxenite, marble and granulite. These metamorphic rocks were intruded by voluminous granitoids and minor mafic-ultramafic bodies of early Cretaceous age (e.g., Bryant et al., 2004; Zhao and Zheng, 2009). Geochronological studies indicate that
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protoliths of the eclogite, granulite and regional orthogneiss are mostly Neoproterozoic igneous rocks with minor Paleoproterozoic rocks (e.g., Hacker et al., 1998; Wu et al., 2008; Xie et al., 2010; Jian et al., 2012). The eclogite and granitic gneiss contain not only mineral exsolutions in metamorphic garnet and clinopyroxene but also diamond and coesite inclusions in metamorphic zircon (Tsai and Liou, 2000; Xu et al., 2003, 2005; Liu et al., 2007a, 2007b, 2011a, 2011b), indicating the UHP metamorphism of these rocks. Triassic zircon U-Pb and mineral Sm-Nd ages were obtained for the eclogite and granitic gneiss (e.g., Li et al., 1993; Xie et al., 2004, 2006, 2010; Liu et al., 2007a, 2011b; Zhao et al., 2008), indicating that the North Dabie zone was involved in the Triassic continental subduction (Zheng et al., 2005). Subsequent to the UHP eclogite-facies metamorphism, eclogite and gneiss in the North Dabie zone were first subject to granulite-facies overprinting, and then to amphibolite-facies retrogression (e.g., Tsai and Liou, 2000; Liu et al., 2007a, 2011a, 2011b), with local overprinting of migmatization (e.g., Zhang et al., 1996; Zhao et al., 2008). This differs from
ACCEPTED MANUSCRIPT metamorphic rocks in the Central and South Dabie UHP zones, where the granulite-facies overprinting has been not recognized so far. Accordingly, four metamorphic stages are recognized in the North Dabie zone (Liu et al., 2007a, 2011a, 2011b; Tong et al., 2011): (1)
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an UHP eclogite-facies stage, with P = 2.5-4.0 GPa and T = 800-980 °C, witnessed by the
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occurrence of diamond; (2) a retrograde HP eclogite-facies stage, with P = 2.0 GPa and T = 800-990 °C, (3) a retrograde granulite-facies stage, with P = 0.9-1.4 GPa and T = 690-960 °C;
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(4) a retrograde amphibolite-facies stage, with P = 0.5-0.7 GPa and T = 500-700 °C. On the other hand, early Cretaceous thermal overprinting and magmatism are widespread in the North Dabie zone (e.g., Hacker et al., 1998; Bryant et al., 2004; Zhao and Zheng, 2009). The
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postcollisional magmatic rocks intruded into the UHP rocks are composed of voluminous granitoids and minor mafic-ultramafic rocks, with intrusive ages of 117 to 143 Ma (e.g., Zhao
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and Zheng, 2009; Dai et al., 2011). The migmatization of early Cretaceous is prominent in the North Dabie zone, with significant occurrences on the outcrop scale (Wang et al., 2002; Wu et al., 2007).
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Migmatites are common in the regional orthogneiss of the North Dabie high-T/UHP zone,
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mainly occurring in the Yuexi dome and the Luotian dome (Zhang et al., 1996; Wang et al., 1998, 2002; Faure et al., 2003; Wu et al., 2007). This indicates the presence of partial melting
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in the UHP metamorphic rocks, but the time with relationship to the P-T-t evolution and mechanism of their formation remains elusive. Zhang et al. (1996) suggested that migmatization occurred coeval or shortly after the UHP metamorphism on the basis of the observations that some metamafic rocks were overprinted by amphibolite- to granulite-facies
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metamorphism to yield Sm-Nd isochron ages of 224-244 Ma for retrograded eclogites (Li et al., 1993), and that migmatic gneisses were intruded by early Cretaceous granites. Based on deformation features, the age of migmatization was assumed to be late Triassic to early Jurassic (Faure et al., 2003). Wang et al. (2002) obtained ID-TIMS U-Pb dates for multigrain zircon fractions from migmatites that defined a weighted mean
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Pb/238U age of 137.1 ± 1.1
Ma, and regarded it as the age of migmatization. Wu et al. (2007) dated different domains of zircon from migmatites and defined the time of partial melting at 120-145 Ma with two different peaks at 123 ± 1 and 139 ± 1 Ma. It seems that there are two episodes of partial melting in the early Cretaceous. Migmatites in the North Dabie zone usually preserved mineral assemblages that are consistent with anatectic conditions at 700-800°C and 500-700 MPa (Zhang et al., 1996; Wang et al., 2002, 2013). However, the following questions are unresolved: (1) whether the North Dabie zone underwent partial melting during the exhumation in the late Triassic; (2)
ACCEPTED MANUSCRIPT how crustal anatexis is different between the exhumational and postcollisional stages; and (3) what is the relationship between the granite, migmatite and granulite of similar ages in the
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Dabie orogen?
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3. Sample description
Migmatite samples used in this study were collected from Manshuihe in the Yuexi dome
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(Fig. 2). In this outcrop, the migmatites are mainly stromatic metatexites according to the definition of Sawyer (2008). They are characterized by layered structures made up of several millimeters to tens of centimeters concordant or discordant leucosomes (Fig. 2a and 2b).
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Local places of the migmatite body exhibit larger extent of deformation and flow, with former leucosome being cut by later leucosome (Fig. 2b). Accumulation of amphiboles and biotite
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forms the concordant shearing layers or melanosomes interlayer with leucosomes (Fig. 2a and 2c). Locally, K-feldspar-rich or plagioclase-rich pegmatite occurs as boudins within the migmatite (Fig. 2c and 2d). We sampled two leucosomes (04NDB17 and 09DB84) and two
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migmatites composed of both melanosome and leucosome (04NDB19 and 09DB82). Mineral
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abbreviations throughout are after Whitney and Evans (2010).
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Sample 04NDB17 and 09DB84 are deformed leucosomes. Distribution of the minerals in the leucosome is heterogeneous not only between the different samples but also within single samples. Both samples generally contain a mineral assemblage of Qz + Kfs + Pl + Bt + Ttn +
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Ap + Zrn +Mgn. Nevertheless, sample 04NDB17 contains minor amphibole whereas sample 09DB84 contains minor monazite. In sample 04NDB17, equant subhedral to euhedral crystals of plagioclase are in contact along their faces rather than corners and have straight, rational faces against the interstitial quartz and biotite to form a framework structure (Fig. 3a). In sample 09DB84, feldspar occurs as grains variable in size and shape, or forms a complex intergrowth with quartz (granophyre) (Fig. 3b). All these suggest their growth from former melt (Holness et al., 2011; Chen et al., 2013a, 2013b). Biotite in the leucosome is generally fine-grained and forms as flake or cluster dispersed in the quartz-feldspathic matrix. Zircon mainly occurs as inclusion in or matrix with quartz and feldspar.
Samples 04NDB19 and 09DB82 are both composed of melanosome and leucosome. The melanosome in both samples contains large amount of amphibole and biotite with minor amount of Qz + Pl + Kfs + Cpx (Figs. 3c and 3e). In the melanosome of 04NDB19,
ACCEPTED MANUSCRIPT amphibole grains variable in size and shape form intergrowth or interlayer with biotite and contain clinopyroxene inclusions (Fig. 3c). The leucosome in 04NDB19 contains amphibole and clinopyroxene, which are millimeter and commonly poikiloblastic and anhedral (Fig. 3d).
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Amphibole contains inclusions of plagioclase, quartz, and/or biotite. In the melanosome of
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sample 09DB82, euhedral to anhedral amphibole grains variable in size and shape together with small biotite grains form foliation with a few quartz-feldspathic minerals filling between
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them (Fig. 3e). Some feldspar grains exhibit good crystal faces (Fig. 3e), suggesting their growth from melts (Holness et al., 2011). Small anhedral clinopyroxene and epidote generally occur among amphiboles. The leucosome in sample 09DB82 is predominantly composed of
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Qz + Pl + Kfs + Bt, with a minor amount of amphibole and clinopyroxene and a trace amount of epidote, titanite, apatite and zircon. Anhedral amphibole grains with variable size and
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shape dispersed in the quartz-feldspathic matrix. Clinopyroxene occurs as anhedral grains dispersed in the quartz-feldspathic matrix (Fig. 3f) and as small anhedral inclusion in amphibole. Some feldspar grains show well crystal faces (Fig. 3f), suggesting their
4. Analytical methods
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crystallization from melts (Holness et al., 2011).
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Cathodoluminescence (CL) imaging for zircon was performed using a FEI Sirion200 Scanning electron microscope at Central Laboratory of Physics and Chemistry in University of Science and Technology of China, Hefei. The working conditions during the CL imaging
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were maintained at 20 kV. Inclusions in zircon were observed under an optical microscope and identified by Laser Raman spectroscopy (ThermoFisher DXR) with the 532 nm line of the laser and TESCAN MIRA3 FEG-SEM Scanning electron microscope with EDAX GENESIS APEX Apollo System at CAS Key Laboratory of Crust-Mantle Materials and Environments, University of Science and Technology of China, Hefei. Back-scattered electron (BSE) images were acquired at accelerating voltage of 10 kV, while semi-quantitative energy dispersive X-ray spectroscopy chemical microanalyses (EDS) analyses were acquired at accelerating voltage of 12 kV. Zircon U-Pb dating and trace element analysis on mount and thin sections were made by the LA-ICPMS technique at State Key Laboratory of Geological Processes and Mineral Resources in China University of Geosciences, Wuhan. Laser ablation sampling was performed using a Geolas 2005 system equipped with a 193 nm ArF-excimer laser. An Agilent 7500a ICP-MS was used to acquire ion-signal intensities. Detailed instrumental
ACCEPTED MANUSCRIPT conditions and data acquisition were described by Liu et al. (2010b, 2010c) and Zong et al. (2010b). The laser analyses were conducted with a beam diameter of 24 μm and a 4 Hz repetition rate. The Agilent Chemstation was utilized for the acquisition of each individual
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analysis. The off-line selection and integration of background and analyte signals, and
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time-drift correction and quantitative calibration were conducted by ICPMSDataCal (Liu et al., 2010b). Trace element concentrations were calibrated by using
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Si as internal calibrant
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and NIST SRM610 as the reference material. Zircon 91500 was used as the external standard for U-Pb dating. Time-dependent drifts of U-Th-Pb isotopic ratios were corrected using a linear interpolation for every five analyses according to the variations in zircon 91500 (Liu et
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al., 2010b). Preferred U-Th-Pb isotope ratios used for zircon 91500 are from Wiedenbeck et al. (1995).Uncertainty of preferred values for the external standard 91500 was propagated to the ultimate results of samples. Zircon standard GJ-1 was analyzed as an unknown samples. The 206
Pb/238U ages for GJ-1 in this study are 601±2 Ma (MSWD=0.69, n=10) for
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obtained mean
thin section analyses and 601±2 Ma (MSWD=0.12, n=10) for mount analyses, consistent with
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the recommended value of 599.8±1.7 Ma (Jackson et al., 2004). Common Pb correction was
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carried out by using the EXCEL program of ComPbCorr#_151 (Andersen, 2002). Apparent and discordia U-Pb ages were calculated by the ISOPLOT program (Ludwig, 2003).
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Uncertainties of individual analyses are reported with 1σ errors; weighted-mean ages were calculated at 2σ confidence level. During the time-resolved analyses of minerals, caution was taken to constrain the signal to chemically homogeneous parts of the crystals and to avoid any
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inclusions and heterogeneities that could be potentially present in the analyzed minerals.
5. Results
Zircons from all the four samples were analyzed on thin sections, and zircons from the two samples 04NDB17 and 04NDB19 were also analyzed on mounts. Zircon U-Pb isotope and trace element data are listed in Tables A1 to A4 of Appendix I. The weighted mean U-Pb age plots of zircons on the thin sections and mounts for each of the samples are presented in Figures A1 and A2 of Appendix II, respectively. Table 1 presents a summary of zircon mineragraphy (external morphology and internal structure), mineral inclusions, U-Pb isotopes and trace elements for the migmatites.
The zircons were first analyzed on the thin section, and then analyzed on the grain mount to confirm. Weighted mean U-Pb ages were calculated for the same group of zircons. For a
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the relative lack of zircon growth in some samples. Thus, there are no enough zircon domains
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for the U-Pb dating. Nevertheless, if the U-Pb ages used in the weighted mean calculation are the same within analytical errors, the weighted mean age obtained by calculation from
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different numbers of analyses yields the same age result within the analytical errors though one obtained from more analyses is closer to the true weighted mean with a smaller error than that obtained from less analyses. Therefore, the weighted mean ages obtained by several spot
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analyses are generally the same within analytical errors though they are not the true weighted mean. In this regard, the weighted mean ages are still calculated for the groups of zircon U-Pb
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ages with only several analyses.
The Ti-in-zircon thermometry is applied to the samples following the calibration of Ferry and Watson (2007). It is known that the activity of SiO2 and TiO2 (aSiO2 and aTiO2, respectively),
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pressure, and solid-state volume diffusion have different effects on the Ti-in-zircon
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thermometry (Ferry and Watson, 2007; Ferriss et al., 2008; Tailby et al., 2011). Due to the low rate of Ti diffusion in zircon (Cherniak and Watson, 2007), the Ti-in-zircon thermometry
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can yield zircon growth temperature even during ultrahigh-temperature metamorphism (e.g., Baldwin et al., 2007). All the samples used in this study contain quartz, hence the activity of SiO2 is buffered at unity. If rutile is present, the activity of TiO2 is also unity. If titanite is present instead of rutile, the activity of TiO2 is estimated to be ~0.5, forcing an upward
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correction of 50°C on the Ti-in-zircon temperature. Titanite instead of rutile was present in the studied samples. Therefore, the Ti-in-zircon temperatures are calculated at the assumption of aTiO2 = 0.5.
5.1 Thin Section 5.1.1 Leucosome 04NDB17 Based on the CL images (Fig. 4a and b), three distinct domains of zircons were recognized: (1) cores, showing blurred oscillatory zoning and middle CL brightness; (2) cores or mantles, showing unzoning or faint growth zoning and dark CL luminescence; (3) rims, with unzoning or weakly zoning and relatively strong luminescence. One analysis on the middle luminescent core yields discordant U-Pb age of 234 ± 3 Ma. It shows a high Th/U ratio of 0.19, steep HREE pattern with (Lu/Ho)N ratio of 4.67, positive Ce and negative Eu anomalies with Eu/Eu* = 0.25 (Table 1 and Fig. 5b). The U-Pb isotope data for the CL-dark cores and
ACCEPTED MANUSCRIPT mantles show small scattering towards high
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Pb/235U ratios in the concordia diagram,
yielding an isochron age of 192 ± 2 Ma (Fig. 5a). Four of them yield concordant U-Pb ages of 191 to 196 Ma with a weighted mean of 192 ± 2 Ma (Fig. 5a). They are characterized by low
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Th/U ratios <0.1 (except one with a Th/U ratio of 0.14), steeper MREE-HREE patterns with
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(Lu/Ho)N ratios of 10.3 to 16.6, negative Eu anomalies with Eu/Eu* ratios of 0.15 to 0.62, and high Nb contents of 56.5 to 210 ppm and Ta contents of 21.4 to 202 ppm (Table 1 and Fig.
to 874°C (Table 1). The rims are too thin to analyze.
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5b). They have Ti contents of 5.49 to 16.8, corresponding to Ti-in-zircon temperatures of 755
5.1.2 Leucosome 09DB84
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In the CL images (Fig. 4c-e), zircon grains can also be divided into three different domains similar to sample 04NDB17. The middle-luminescent cores show high Th/U ratios of 0.31 to 1.06 and apparent
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Pb/238U ages of 241 to 352 Ma (Table 1). They exhibit steep
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HREE patterns with (Lu/Ho)N ratios of 3.62 to 5.73, clearly positive Ce and negative Eu
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anomalies with Eu/Eu* = 0.23 to 0.54, suggesting their magmatic origin. One analysis yields a low content of Nb (2.17 ppm) and Ta (0.53 ppm), whereas the others give significantly high
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contents of Nb (135 to 214 ppm) and Ta (58.1 to 110 ppm) (Table 1). The CL-dark cores and mantles show concordant or nearly concordant U-Pb ages of 193 to 206 Ma with a weighted mean of 200 ± 4 Ma (Fig. 5c). They have low Th/U ratios of <0.1, steep HREE patterns with (Lu/Ho)N ratios of 10.5 to 18.4, marked negative Eu anomalies with Eu/Eu* ratios of 0.14 to
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0.42, and high contents of Nb (10.1 to 230 ppm) and Ta (6.47 to 154 ppm) (Table 1 and Fig. 5d). Two analyses of them show relative enrichment of LREE due to the presence of mineral inclusions. All of them have Ti-in-zircon temperatures of 697 to 807°C (Table 1). The CL-bright rims exhibit low Th contents of 49.8 to 59.7 ppm and intermediate U contents of 100 to 132 ppm with Th/U ratios of 0.38 to 0.60. They show HREE enrichment with (Lu/Ho)N of 5.88 to 8.25, low contents of Nb (1.28 to 6.69 ppm) and Ta (0.38 to 0.75 ppm) (Table 1). They have concordant U-Pb ages of 138 ± 3 to 139 ± 5 Ma with a weighted mean of 140 ± 4 Ma (Table 1 and Fig. 5c). They have Ti-in-zircon temperatures of 721-837°C (Table 1).
5.1.3 Migmatite 04NDB19 CL imaging reveals that most zircon grains show core-rim structures (Fig. 4f-h). The cores exhibit planar, oscillatory or blurred oscillatory zoning, whereas the rims exhibit weak zoning
ACCEPTED MANUSCRIPT or unzoning. A few grains exhibit weak zoning or unzoning. Two dated cores show concordant or nearly concordant U-Pb ages of 757 and 779 Ma (Table 1 and Fig. 5e). High Th/U ratios (0.20 and 1.06), steep MREE-HREE patterns, and marked positive Ce anomalies
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and negative Eu anomalies (Fig. 5f) reflect their magmatic origin (Hoskin and Schaltegger,
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2003; Chen et al., 2010). One zircon grain and two rims with unzoning or weak zoning have intermediate U contents and low Th contents with Th/U ratios of 0.009 to 0.09 (Table 1). They
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are poor in REE and have flat HREE patterns without negative Eu anomalies (Fig. 5f). Two analyses yield concordant U-Pb ages of 215 and 216 Ma with a weighted average of 216 ± 3 Ma, whereas the other one has younger age of 198 Ma (Fig. 5e). All of them have Ti-in-zircon
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temperatures of 630 to 709°C (Table 1).
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5.1.4 Migmatite 09DB82
Zircon grains show variable zoning patterns, from oscillatory zoning to unzoning and CL-bright. All of them yield concordant to nearly concordant but relatively variable U-Pb
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ages ranging from 123 ± 1 to 135 ± 2 Ma. They can be divided into two distinct populations,
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one at 124 ± 1 Ma and the other at 133 ± 2 Ma (Fig. 5g). However, they show similar trace element contents and patterns, with high Th/U ratios of 0.59 to 1.12, steep HREE patterns
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with (Lu/Ho)N ratios of 6.32 to 8.55, and marked negative Eu anomalies with Eu/Eu* = 0.53 to 0.71 (Table 1 and Fig. 5h). One analysis yields relative enrichment of LREE due to the presence of mineral inclusions. They have Ti contents of 9.98 to 21.9 ppm, corresponding to
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Ti-in-zircon temperatures of 660 to 794°C (Table 1).
5.2 Mount grain
5.2.1 Leucosome 04NDB17 Zircon grains are short-to-long prismatic and colorless (Fig. 6). CL imaging reveals that there are four types of zircon domains including: (1) relict core, showing planar zoning or blurred oscillatory zoning with middle luminescence; (2) grains, showing unzoning and relatively strong CL brightness; (3) cores or mantles, showing CL-dark with and/or without patched zoning; (4) rims, showing unzoning or weakly zoning with very strong CL brightness. The relict cores with planar zoning or blurred oscillatory zoning have high Th/U ratios of 0.20 to 1.06, steep MREE-HREE patterns with (Lu/Ho)N ratios of 3.96 to 5.30, marked positive Ce and negative Eu anomalies (Table 1 and Fig. 7b); and contain mineral inclusions of Qz ± Ap ± Pl. They have variable apparent 206Pb/238U ages ranging from 139 ± 6 to 656 ± 7 Ma, yielding a discordia intersecting the concordia curve at 721 ± 61 and 156 ± 16 Ma, respectively (Fig.
ACCEPTED MANUSCRIPT 7a). One CL-bright and unzoning zircon grain has a low Th/U ratio of 0.09 and a U-Pb age of 219 ± 4 Ma (Table 1 and Fig. 7a). It is characterized by a low REE content, a flat HREE pattern with a (Lu/Ho)N ratio of 1.48, and no negative Eu anomaly with an Eu/Eu* ratio of
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0.95 (Table 1 and Fig. 7b). The CL-dark cores and mantles exhibit steeper MREE-HREE
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patterns with (Lu/Ho)N ratios of 7.8 to 15.9, higher contents of U and lower Th/U ratios <0.1 (except one with a Th/U ratio of 0.14) and negative Eu anomalies (Table 1 and Fig. 7b).
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Except one analysis with a younger age of 133 Ma, all the other analyses yield concordant U-Pb ages scattering between 189 ± 2 and 207 ± 2 Ma with a weighted mean of 199 ± 4 Ma (Fig. 7a). They also show significantly high contents of Nb and Ta. They contain a lot of
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mineral inclusions of Qz ± Cal ± F-Ap ± Pl ± Kfs ± Ms and granitic multiphase solid (MS) inclusions of Qz + Pl + Hem + Cal, Qz + Kfs + Ep, Qz + Bt + Ap and Qz + Pl + Bt (Fig. 8).
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They have Ti-in-zircon temperatures of 807 to 875°C (Table 1). The CL-bright rims have low U contents of 70.6 to 175 ppm and Th/U ratios of 0.41 to 0.92 (Table 1). Compared to the relict cores, they have lower REE contents, but similar REE patterns (Fig. 7b). All of them
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have concordant U-Pb ages of 126 to 140 Ma with a weighted mean of 134 ± 4 Ma (Fig. 7a).
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They contain a few Qz inclusions and have Ti-in-zircon temperatures of 730 to 762°C (Table 1).
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5.2.2 Migmatite 04NDB19
Zircon grains are euhedral to subhedral (Fig. 6). Based on CL images, REE patterns, Pb/238U ages and Th/U ratios, four groups of zircon were recognized. Group-1 zircon
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206
domains consists of relict cores that exhibit oscillatory, blurred oscillatory or patched zoning. They exhibit variable apparent
206
Pb/238U ages ranging from 449 to 783 Ma, some of them
yield concordant U-Pb ages of 747 ± 17 to 783 ± 13 Ma (Table 1). They define a discordia chord with two intercept ages at 776 ± 71 and 130 ± 24 Ma, respectively (Fig. 7c). All these domains are characterized by enrichment of HREE (Fig. 7d) with (Lu/Ho)N ratios of 1.77 to 5.79, pronounced positive Ce anomalies with Ce/Ce* ratios of 7.37 to 152, clearly negative Eu anomalies with Eu/Eu* ratios of 0.08 to 0.62, and relatively high Th/U ratios of 0.16 to 1.22 (Table 1 and Fig. 7d). They contain a lot of mineral inclusions of Qz ± Ap ± Pl. These features are primarily consistent with those of magmatic zircon (Rubatto, 2002; Hoskin and Schaltegger, 2003). Group-2 zircon domains occurs as grains or mantles around the group-1 zircon domains with unzoned CL pattern (Fig. 6). Most of them have concordant U-Pb ages of 205 to 222 Ma with a weighted mean of 212 ± 5 Ma. Some of them show younger 206Pb/238U
ACCEPTED MANUSCRIPT ages of 159 to 199 Ma (Fig. 7c). All of these domains are characterized by low Th/U ratios of 0.01 to 0.07, relatively flat HREE patterns with (Lu/Ho)N ratios of 0.74 to 2.31, and relative lack of negative Eu anomalies with Eu/Eu* = 0.73 to 1.30 (Table 1 and Fig. 7d). They contain
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a few inclusions of Grt ± Cpx. Group-3 zircon domains occur as rims around the group-2
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zircon domains with bright and unzoned CL pattern (Fig. 6). They exhibit steeper REE patterns with (Lu/Ho)N ratios of 12.4 to 14.9, low Th/U ratios of 0.01 to 0.05, and marked
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negative Eu anomalies with Eu/Eu* ratios of 0.19 to 0.36 (Table 1 and Fig. 7d). Most of them have variable 206Pb/238U ages of 187 ± 17 to 204 ± 6 Ma with a weighted mean of 197 ± 7 Ma (Table 1 and Fig. 7c). One analysis exhibits a younger
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Pb/238U age of 173 ± 8 Ma, which
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may be due to the Pb loss. These zircon domains contain mineral inclusions of Qz ± Ap ± Kfs ± Pl ± Ms. Group-4 zircon domains occur as rims of the group-1 or group-2 zircon domains
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with bright unzoned CL pattern (Fig. 6). All of them show concordant U-Pb ages of 128 to 138 Ma with a weighted mean of 132 ± 3 Ma (Fig. 7c). They are characterized by high Th/U ratios of 0.04 to 0.85, steep REE patterns with (Lu/Ho)N ratios of 4.56 to 6.79 and negative Eu
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anomalies with Eu/Eu* ratios of 0.32 to 0.63 (Table 1 and Fig. 7d). They contain a few quartz
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inclusions.
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6. Discussion
Mineragraphy, mineral inclusions and microchemistry of metamorphic zircon can help to estimate its prevailing growth conditions. The analyses of zircons on thin section and grain
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mount give similar results; the four different groups of zircon domains are recognized. The middle-luminescent or CL-bright cores in leucosomes 04NDB17 and 09DB84 and migmatite 04NDB19 exhibit oscillatory, blurred oscillatory, planar or patched zonation (Figs. 4 and 6), indicating that they are the protolith zircon of magmatic origin that experienced different extents of metamorphic recrystallization (Hoskin and Schaltegger, 2003; Xia, et al., 2009; Chen et al., 2010). Since these zircon domains occur as cores and are relict protolith zircon of magmatic origin, this first group of zircon domains is denoted as relict cores or RC domains. They exhibit high Th/U ratios of >0.1, steep HREE patterns with marked positive Ce and negative Eu anomalies (Figs. 5b, 5d, 5f, 7b, 7d and 9) and contain mineral inclusions of Qz ± Ap ± Pl, consistent with their magmatic origin (e.g., Rubatto, 2002; Hoskin and Schaltegger, 2003; Chen et al., 2010). Their U-Pb dating yields upper intercept U-Pb ages of 721 ± 61 to 786 ± 140 Ma (Table 1 and Figs. 5 and 7). Some of them also have concordant U-Pb ages of 757 to 783 Ma (Table 1). The results are consistent not only with zircon U-Pb ages for
ACCEPTED MANUSCRIPT basement gneisses in the North Dabie zone (e.g., Hacker et al., 1998, 2000; Bryant et al., 2004; Liu et al., 2007a; Zhao et al., 2008; Xie et al., 2010), but also with zircon U-Pb ages for UHP metaigneous rocks in the Dabie-Sulu orogenic belt (e.g., Zheng et al., 2009; Liu and Liou.,
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2011). The large variation of U-Pb dates for these relict cores of magmatic origin suggests
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variable extents of reworking by later thermal events (Figs. 5a, 5c, 5e, 7a and 7c).
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One CL-bright zircon grain in leucosome 04NDB17 and the CL-dark mantles and a few rims around the RC domains and a few CL-bright zircon grains in migmatite 04NDB19 exhibit no zoning or cloudy zoning (Figs. 4 and 6), low Th/U ratios (<0.1), flat HREE patterns
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with slightly or no negative Eu anomalies (Table 1 and Figs. 5f, 7b, 7d and 9), and contain mineral inclusions of Grt + Cpx. These features suggest their growth during eclogite-facies
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metamorphism. This second group of zircon domains is denoted as D1 domains. While four analyses yield younger U-Pb ages of 159 ± 4 to 199 ± 9 Ma, the others yield late Triassic U-Pb ages of 212 ± 5 to 219 ± 4 Ma (Figs. 5e, 7a and 7c). Previous geochronological studies
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of eclogite-facies metamorphism in the North Dabie zone also yield mineral Sm-Nd isochron
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ages of 219 ± 11 to 244 ± 11 Ma (Li et al., 1993; Xie et al., 2004) and concordant U-Pb ages of 212 ± 21 to 229 ± 18 Ma (Jiang et al., 2002; Zheng et al., 2004; Liu et al., 2007a). Liu et al.
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(2011b) presented a combined study of zircon U-Pb ages, trace elements and mineral inclusions for three eclogites, and argue for that the UHP metamorphic event at 226 ± 3 Ma based on the occurrence of coesite inclusion in metamorphic zircon and interpret the weighted mean age of 214 ± 3 Ma as recording a later HP eclogite-facies metamorphic event during
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exhumation. The eclogite-facies metamorphic age of 224 ± 2 Ma was also obtained on zircon domains that contain mineral inclusions of Cpx ± Grt ± Rt in the Qingtian granulite of North Dabie (Wang et al., 2012). In either case, the eclogite-facies metamorphism is evident during exhumation in the late Triassic. On the other hand, the metamorphic zircon of Jurassic U-Pb ages exhibits the same REE patterns as those of Triassic ones (Figs. 5f and 7d), suggesting the loss of radiogenic Pb due to later reworking during the early Cretaceous tectonothermal event. The decoupling between REE composition and U-Pb dates was also reported in metamorphic zircon (e.g., Chen et al., 2010; Flowers et al., 2010). The CL-dark cores, mantles and grains in leucosomes 04NDB17 and 09DB84 and a few CL-bright rims in migmatite 04NDB19 exhibit no zoning, faint growth zoning or patched zoning (Figs. 4 and 6), low Th/U ratios of <0.1, steeper HREE patterns with (Lu/Ho)N ratios of 7.81 to 18.4, negative Eu anomalies with Eu/Eu* ratios of 0.15 to 0.70, and high contents of Nb and Ta (Table 1 and Figs. 5b, 5d, 7b, 7d and 9). They contain not only inclusions of
ACCEPTED MANUSCRIPT quartz, plagioclase, K-feldspar, apatite and calcite, but also MS inclusions of Qz + Pl + Hem + Cal, Qz + Kfs + Ep, Qz + Bt + Ap and Qz + Pl + Bt (Fig. 8). Their trace element features and mineral inclusions are significantly different from the RC and D1 domains (Table 1 and
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Fig. 9). This third group of zircon domains is denoted as D2 domains. The MS inclusions in
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these domains occur as granophyric intergrowth of more than two phases (Fig. 8). As minerals in the MS inclusions are quartz, feldspar, apatite and biotite as identified by Laser Raman and
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EDS analyses, it is reasonable to infer that they have granitic composition. This feature is similar to the fully crystallized melt inclusions in migmatite and granulite (Cesare et al., 2009). Furthermore, their trace element characteristics (e.g., low Th/U ratios of <0.1, steeper HREE
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patterns, high contents of REE and marked negative Eu anomalies) (Table 1 and Figs. 5b, 5d, 7b, 7d and 9) are consistent with those for the zircon of anatectic origin (e.g., Rubatto et al.,
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2009; Liu et al., 2010a, 2012; Zeng et al., 2011; Chen et al., 2013a, 2013b; Li et al., 2013, 2014), but different from metamorphic zircon grown from aqueous solutions (e.g., Rubatto and Hermann, 2003; Wu et al., 2009; Chen et al., 2010, 2012). This group of zircon domains
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has Ti-in-zircon temperatures of 697 to 875°C (Fig. 10c), which are higher than the wet
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solidus of pelitic and granitic rocks (e.g., Hermann et al., 2006; Zheng et al., 2011) and thus consistent with their growth from hydrous melts (Li et al., 2013). Therefore, the majority of
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U-Pb ages for the D2 zircon domains are 192 ± 4 to 200 ± 4 Ma (Figs. 5a, 5c, 7a and 7c), which record zircon growth from anatectic melts. In contrast, two younger U-Pb ages of 133 and 173 Ma may be caused by later reworking in the early Cretaceous. The old group of zircon U-Pb ages indicates the existence of partial melting during the exhumation of deeply
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subducted continental crust, registering the first episode of anatexis in the North Dabie zone.
The D2 zircon domains exhibit significantly high contents of Nb and Ta (Fig. 9d). In terms of the solubility of HFSE in aqueous solutions and hydrous melts (Zheng et al. 2011), the high contents of Nb and Ta for these zircon domains are consistent with their growth from hydrous melts rather than aqueous solutions (Zheng and Hermann, 2014). On the other hand, the Nb and Ta contents of zircon would also depend on the presence or absence of rutile. Low contents of Nb and Ta in zircon imply the presence of rutile prior to or during zircon growth, whereas high contents of Nb and Ta may imply the decomposition of rutile. In this regard, the low contents of Nb and Ta for the D1 zircon domains may suggest the existence of rutile during their growth, whereas the high contents of Nb and Ta in the D2 zircon domains suggest the decomposition of rutile during their growth. The experimental studies have indicated that halogen can significantly enhance the solubility of rutile in
ACCEPTED MANUSCRIPT fluid/melt (e.g., Manning, 2004). The presence of voluminous F-Ap and Cal inclusions in this group of zircon domains implies the high contents of halogen in the hydrous melt from which the zircon domains grew with the high contents of Nb and Ta.
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The CL-bright rims in leucosome 09DB84, grains in migmatite 09DB82, CL-bright rims
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and grains in leucosome 04NDB17, and a few CL-bright rims in migmatite 04NDB19 exhibit no zoning, faint growth zoning or oscillatory zoning (Figs. 4 and 6). They have high Th/U
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ratios of 0.38 to 1.13, steep HREE patterns with (Lu/Ho)N ratios of 4.38 to 8.55, positive Ce anomalies, and negative Eu anomalies with Eu/Eu* ratios of 0.15 to 0.71 (Table 1 and Fig. 9h). These trace element characteristics are similar to the RC domains, but significantly
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different from the D1 and D2 domains (Fig. 10), and thus this fourth group of zircon domains is denoted as D3 domains. This group of zircon domains exhibit early Cretaceous U-Pb ages
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of 124 ± 1 to 140 ± 4 Ma (Figs. 5c, 5g, 7a and 7c). Those from the same sample 09DB82 can be further subdivided into two groups of 124 ± 1 to 133 ± 2 Ma (Fig. 5g). It appears that there are two distinct episodes of zircon growth during the early Cretaceous. Oscillatory zoning
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(Figs. 4 and 6) and magmatic trace element characteristics (high Th/U ratios>0.1, steep REE
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patterns, significant positive Ce anomalies and negative Eu anomalies) (Figs. 5d, 5h, 7b, 7d and 9) for these zircon domains suggest their growth from hydrous melts. They exhibit
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Ti-in-zircon temperatures of 660 to 837°C (Fig. 10d), consistent with the migmatization temperatures of 700-820°C obtained by petrologic thermometers (Zhang et al., 1996; Wang et al., 2013). The present age result is consistent with that of Wu et al. (2007), who dated eight migmatites to yield two age peaks at 123 ± 1 and 139 ± 1 Ma. The two groups of early
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Cretaceous ages were also obtained from overgrown zircon rim in orthogneiss, with one at 129-120 Ma and the other at 137-146 Ma; and magmatic zoning was well developed in some early Cretaceous overgrown rims, suggesting the presence of anatexis in the orthogneiss (Hacker et al., 1998; Bryant et al., 2004; Xie et al., 2006, 2010; Zhao et al., 2008). The younger group of ages is consistent with known dates for voluminous emplacement of mafic and felsic magmas in the North Dabie zone (e.g., Hacker et al., 1998; Bryant et al., 2004; Xie et al., 2006; Zhao and Zheng, 2009). The early Cretaceous ages were also obtained from granulite at Huilanshan (Hou et al., 2005). These early Cretaceous magmatic and metamorphic ages support the notion that the widespread magma emplacement and HT granulite-facies metamorphism in the North Dabie zone are associated with contemporaneous migmatization (Tong et al., 2011), suggesting extensive anatexis in the early Cretaceous. Despite the difference in zircon U-Pb ages, the D3 zircon domains of the two age peaks do not exhibit significant difference in Th/U ratios (Fig. 11) and trace element compositions
ACCEPTED MANUSCRIPT (Fig. 9). This suggests that this episode of migmatization is induced by the same mechanism with a protracted duration. Previous studies have demonstrated that the anatectic temperature of orogenic crust can remain above the solidus for time as long as 30 Myr (e.g., Slagstad et al.,
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2004; Sawyer et al., 2011). As a consequence, zircon U-Pb dating usually yields a range of
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ages for migmatization that encompass the ages of initial anatexis, extensive anatexis and subsequent crystallization (e.g., Baldwin et al., 2007). In this regard, these migmatic zircon
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domains have grown from the anatectic melts that were produced in a protracted duration.
In summary, the occurrences of Neoproterozoic protolith ages and late Triassic
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eclogite-facies metamorphic ages in the migmatite indicate that the migmatite in the North Dabie zone was involved into the Triassic deep subduction of the South China Block and
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would undergo similar P-T evolution to the UHP eclogite and gneiss in this zone. Considering the similarity in zircon U-Pb ages between the migmatite and UHP gneiss, it is reasonable to deduce that the migmatite was produced by partial melting of the precursor UHP gneiss.
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Anatexis of felsic gneiss in the North Dabie zone has also been described in some studies
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(Wang et al., 2002; Wu et al., 2007). Following the middle Triassic UHP eclogite-facies metamorphism, the migmatite underwent the first episode of anatectic events in the late
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Triassic to early Jurassic during exhumation and the second episode of anatexis in the early Cretaceous during the postcollisional tectonism.
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7. Constraints on the time of granulite-facies overprinting The construction of P-T-t path is critical for understanding the tectonic evolution of continental collision orogen. As illustrated in Fig. 12, the time of eclogite-facies and amphibolite-facies metamorphism for the North Dabie zone has been well constrained (Liu et al., 2011b). Abundant Triassic metamorphic ages and UHP metamorphic evidence have been reported to support that the North Dabie zone is a UHP slice that was involved in the Triassic deep subduction of the South China Block with peak eclogite-facies metamorphism at ~226 Ma (e.g., Xu et al., 2003, 2005; Liu et al., 2007a, 2007b, 2011b; Wang et al., 2012). A common consensus also has been reached that the North Dabie zone was exhumed to middle crustal level with amphibolite-facies retrogression at 180-200 Ma (Liu et al., 2011b; Jian et al., 2012; Wang et al., 2012). However, it is still uncertain with regard to the time of granulite-facies overprinting during the exhumation of deeply subducted continental crust in this high-T/UHP zone.
ACCEPTED MANUSCRIPT Felsic granulite at Huangtuling and mafic granulite at Huilanshan in the North Dabie zone were dated to yield granulite-facies metamorphic events in the Paleoproterozoic and
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Cretaceous, respectively (Chen et al., 1998; Hou et al., 2005; Wu et al., 2008). However,
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these two events are not related to the exhumation of UHP metamorphic rocks in the late Triassic (Fig. 12). Nevertheless, Wang et al (2002) reported a biotite Ar/Ar plateau age of 195
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± 2 Ma for felsic granulite at Huangtuling, and recommended it as the minimum age for granulite-facies overprinting. Liu et al. (2005) reported a mineral Sm-Nd isochron age of 212 ± 4 Ma (defined by garnet + omphacite from eclogite) and inferred it as a cooling age for
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granulite-facies metamorphism, but Liu et al. (2011b) reinterpreted it as HP eclogite-facies metamorphic age. Liu et al (2011b) also presented zircon U-Pb age, trace element and mineral
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inclusion analyses for three eclogites from Luotian in the North Dabie zone and speculated that the granulite-facies and amphibolite-facies metamorphic events took place at 199 ± 2 Ma and 176-188 Ma, respectively. In contrast, Wang et al. (2012) suggested that the
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granulite-facies and amphibolite-facies retrograde events occurred at ~213 Ma and ~200 Ma,
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respectively.
It is known that granulite-facies metamorphism is capable of inducing the anatexis of
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crustal rocks. A number of studies also have documented that there is genetic relationship between migmatite, granite and granulite (e.g., Brown, 2010). In this regard, the time of anatexis/migmatization may date the granulite-facies metamorphism. The occurrences of Neoproterozoic protolith ages (757 to 783 Ma) and late Triassic eclogite-facies metamorphic
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ages (212 ± 5 to 219 ± 4 Ma) in the migmatite indicate that the migmatite in the North Dabie zone was also involved in the Triassic continental deep subduction and would undergo similar P-T evolution to the UHP eclogite and gneiss in this zone. The Ti-in-zircon temperatures of 697 to 875°C for the D2 zircon domains are also comparable with petrological temperatures of 690 to 920°C for granulite-facies metamorphism in the North Dabie zone (Chen et al., 2006; Liu et al., 2007b, 2011b). In addition, this group of zircon domains exhibit low Th/U ratios of <0.1, steep REE patterns with (Lu/Ho)N ratios of 7.8 to 18.4, and marked negative Eu anomalies (Table 1 and Figs. 5b, 5d, 7b and 7d). These trace element characteristics are similar to the anatectic zircons grown from partial melts produced by granulite-facies metamorphism during the late Triassic from the other areas in the Dabie-Sulu orogenic belt (e.g, Liu et al., 2010a, 2012; Chen et al., 2013a, 2013b). Therefore, this episode of anatexis recorded in the D2 zircon domains is associated with the granulite-facies metamorphism during the exhumation of deeply subducted continental crust.
ACCEPTED MANUSCRIPT The U-Pb dating yields concordant U-Pb ages of 192 ± 4 to 200 ± 4 Ma for the D2 zircon domains in the migmatite (Figs. 5a, 5c, 7a and 7c), representing the time of migmatization during the “hot” exhumation (Fig. 12). The few younger late Jurassic U-Pb ages may be
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caused by the loss of radiogenic Pb due to later reworking during the early Cretaceous
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tectonothermal event. On the other hand, it has been demonstrated that the U-Pb dating of anatectic zircon usually yields a range of ages for the anatectic event that encompass the ages
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of initial anatexis, extensive anatexis and subsequent crystallization (e.g., Baldwin et al., 2007). The U-Pb ages of 192 ± 4 to 200 ± 4 Ma are nearly synchronous with, but slightly earlier than, the amphibolite-facies metamorphism at 180 to 200 Ma, suggesting either that
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there is a wide transition between the granulite-facies metamorphism and amphibolite-facies metamorphism (Touret and Huizenga, 2012) or that the growth of anatectic zircon protracted
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with extension to the amphibolite-facies metamorphism. In this regard, the U-Pb dating of anatectic zircon in this study may only give the lower limit of anatectic event. In view of the petrogenetic relationship between granulite-facies metamorphism and migmatization and the
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protracted growth of anatectic zircons, it is inferred that the granulite-facies metamorphism
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during the exhumation in the North Dabie zone would also occur in the late Triassic.
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8. Implications for multistage anatexis of UHP metamorphic rocks Partial melting of UHP metamorphic rocks can dramatically affect the rheology of deeply subducted continental crust and thus play a crucial role in accelerating the exhumation of
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UHP slices (e.g., Hermann et al., 2001; Labrousse et al., 2002, 2011; Wallis, et al. 2005; Teyssier, 2012). Thermal-mechanical models and field-based studies support a link among continental subduction, partial melting, and crustal flow in subduction channels and their overlying lithosphere and suggest that partial melting may be a significant process in the exhumation of UHP metamorphic rocks and reworking of collisional orogens in general (Whitney et al., 2009; Zheng, 2012). In this regard, the partial melting of exhumed metamorphic rocks plays an important role in the tectonic evolution of collisional orogens (Brown, 2010). As such, the geochemical differentiation of continental crust is realized by the partial melting with the products of various migmatites, granulites and granites (Zheng et al., 2011). The collisional orogenesis leads to crustal thickening, which is generally followed by extension and thinning of orogenic lithosphere (Brown, 2010). Short-lived granulite-facies overprinting may take place in the exhumational stage of UHP metamorphic rocks. As a
ACCEPTED MANUSCRIPT consequence, the exhumed crustal rocks may be subject to low extent of anatexis. On the other hand, in the postcollisional stage, the change from compression to extension in orogenic lithosphere is commonly associated with an increase in heat flow and thus occurrence of
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extensive crustal anatexis (e.g., Keay et al., 2001; Wu et al., 2007). While the crustal anatexis
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in the postcollisional stage is common in many collisional orogens (e.g., Zhang et al., 1996; Wu et al., 2007; Zheng et al., 2013a), the anatexis of UHP metamorphic rocks during the
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exhumation of deeply subducted continental crust has been increasingly recognized in continental collision orogens (e.g., Labrousse et al., 2002, 2011; Lang et al., 2007; Zheng et al., 2011; Gao et al., 2012; Liu et al., 2012, 2013; Chen et al., 2013a, 2013b; Gordon et al.,
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2013; Li et al., 2014).
Multistage anatectic events have been identified in migmatites from collisional orogens
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such as the central Alps and far-eastern Nepal Himalaya (Rubatto et al., 2009; Imayama et al., 2012). Crustal anatexis in the both exhumational and postcollisional stages was reported in the Sulu UHP metamorphic zone (Liu et al., 2012), where zircon records two anatectic events
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in separated grains (overgrowths around cores exhibit uniform U-Pb ages within individual
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zircon grains). In the North Dabie high-T/UHP zone, on contrary, the D2 and D3 zircon domains in the migmatite exhibit the two distinct, successive overgrown rims around the relict
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cores (Figs. 4 and 6). More importantly, the two episodes of zircon growth occur in the same UHP metamorphic rocks. The MS inclusions, magmatic zoning and trace element composition of these zircon domains suggest their growth from anatectic melts and thus record two anatectic events. The zircon U-Pb dating yields the two groups of anatectic ages,
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respectively, in the early Jurassic to late Triassic and in the early Cretaceous (Figs. 5 and 7). The two separated anatectic events occur in the same samples, providing us an opportunity to figure out the differences in crustal anatexis between the exhumational and postcollisional stages. The D2 and D3 zircon domains in the migmatite show different compositions (Fig. 9), suggesting their growth from different origins of anatectic melts. The D2 zircon domains have low Th/U ratios (<0.1), steeper HREE patterns with marked negative Eu anomalies, consistent with those for the zircon of anatectic origin during the exhumation of deeply subducted continental crust in the Dabie-Sulu orogenic belt (Liu et al., 2012; Chen et al., 2013a, 2013b; Li et al., 2014). As phengite is usually the primary hydrous phase in UHP felsic rocks, its stability during continental collision is very important for the anatexis of such rocks (Auzanneau et al., 2006; Hermann et al., 2006; Zheng et al., 2011). This group of zircon domains exhibit high Ti-in-zircon temperatures of 697-875°C (Fig. 10c), concordant with
ACCEPTED MANUSCRIPT temperatures of phengite dehydration melting. It is likely that this episode of anatexis would be induced by phengite breakdown in the exhumational stage of late Triassic to early Jurassic. This interpretation is consistent with muscovite inclusion and MS inclusion of biotite
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associated with quartz and feldspar in the anatectic zircon domains. The late Triassic anatexis
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of UHP gneiss during the exhumation of deeply subducted continental crust has been widely reported from the Sulu orogen and also ascribed to the dehydration melting of phengite (Liu
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et al., 2010a, 2012, 2013; Zong et al., 2010a; Zeng et al., 2011; Gao et al., 2012; Chen et al, 2013a; Li et al., 2014). There are low MREE-LREE contents and steeper REE patterns for these zircon domains (Fig. 9), implying simultaneous growth of allanite/epidote and monazite
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with zircon from the first episode of anatectic melts.
The D3 zircon domains exhibit trace element compositions significant different from the
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D2 zircon domains but similar to those relict protolith zircon cores of magmatic origin (Fig. 9). They exhibit higher Th, LREE and MREE contents, and higher Th/U ratios (Fig. 9), suggesting that they might have crystallized from hydrous melts (e.g., Vavra et al., 1996), in
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which MREE- and LREE-rich minerals such as allanite/epidote, monazite and amphibole, and
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Th-rich minerals such as allanite/epidote and monazite from the host rocks were dissolved during partial melting at high temperatures. On the other hand, the anatexis of metamorphic
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rocks generates anatectic melts that have not separated from parental rocks and thus did not achieve the saturation of incompatible trace elements by fractional crystallization (Zheng and Hermann, 2014). With transport and accumulation of the hydrous melts, the anatectic melts would evolve to the magmatic melts to achieve the saturation of water and incompatible trace
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elements. Burda and Gaweda (2009) found two populations of zircons from stromatic migmatite based on Th/U chemistry and interpreted that zircons with low Th/U ratios record the incipient crystallization of melts, whereas those with high Th/U ratios crystallized during the later cooling accompanied by melt crystallization. Both high and low Th/U ratios have been observed in the anatectic zircons of early Cretaceous ages from the present study (Fig. 11). Similar two populations of zircons showing older U/Pb ages with low Th/U ratios but younger U-Pb ages with high Th/U ratios have also been observed in a single migmatite from the North Dabie zone (Wu et al., 2007; Wang et al., 2013). It appears that zircons grew from the later evolved melt would have elevated Th contents and Th/U ratios. It is expected that the other incompatible elements would also behave similarly. In this regard, the higher contents of incompatible element in these D3 domains suggest that the second episode of anatexis would be associated with elevated extent of partial melting, with significant evolution of partial melts for zircon growth. As such, these D3 zircon domains may be better termed as the
ACCEPTED MANUSCRIPT migmatic zircon than the anatectic zircon. The widespread occurrence of poikilitic and porphyritic amphibole in the leucosome (Fig. 3d) suggests that the amphibole is the peritectic phase of melting reaction. Experiment studies
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have demonstrated that hornblende is stable as a crystallizing phase concomitant with melt
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production only when external fluid was added (e.g., Gardien et al., 2000). The Ti-in-zircon temperatures for the D3 zircon domains are 660 to 837°C (Fig. 8d), which appear to across
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the wet solidus of granite and to reach the dehydration melting temperatures of biotite. In this regard, this episode of migmatization was induced not only by hydration melting but also by dehydration melting in the postcollisional stage of early Cretaceous. Combined with the
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inclusions of Bt, Pl and Qz in the amphibole (Fig. 3d), it is suggested that the melting reaction during the second episode of migmatization may be written as follows: Bt + Pl + Qz ± H2O =
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melt ± Hb ± Cpx ± Ttn. The high Th, LREE and MREE contents and high Th/U ratios for some of the D3 domains suggest that Th-rich minerals and LREE-and MREE-rich minerals were also involved in this episode of anatexis.
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These different behaviors of anatexis between the two episodes of migmatization were
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caused by different tectonothermal regimes. The granulite-facies metamorphism is evident during the exhumation of UHP metamorphic rocks in the Dabie-Sulu orogenic belt (e.g., Yao
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et al., 2000; Zheng et al., 2011; Chen et al., 2013a, 2013b). The local anatexis of UHP rocks is petrogenetically associated with this “hot” exhumation, resulting in the first episode of migmatization. Because even small melt fractions could strongly affect the rheological behavior of deeply subducted crustal rocks (e.g., Rosenberg and Handy, 2005), this anatexis
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event of UHP rocks can significantly reduce the rock strength and thus facilitate the buoyant rise of the subducted rocks and further the exhumation of deeply subducted continental crust in the continental subduction channel (Zheng et al., 2013b). In contrast, the extensive anatexis took place in the postcollisional stage, recording the thermal pulse in the early Cretaceous. This tectonism may be caused by the regional extension of orogenic lithosphere and the tectonic collapse of thickened orogenic root (Xie et al., 2006; Wu et al., 2007; Zhao et al., 2013), leading to the second episode of migmatization. The postcollisional migmatization in the Dabie orogen occurs in the early Cretaceous. There are also extensive occurrences of contemporaneous magmatism and granulite-facies metamorphism in the North Dabie zone (e.g., Hou et al., 2005; Zhao and Zheng, 2009). On the other hand, the granulite of late Triassic age, contemporaneous with the first episode of migmatization, has also been found in the North Dabie zone (Wang et al., 2012). Although the granites of late Triassic age have not been reported in the Dabie orogen, they have been
ACCEPTED MANUSCRIPT documented to occur in the Sulu orogen (e.g., Yang et al., 2005; Zhao et al., 2012). Nevertheless, these magmatic and metamorphic rocks share a common feature that contain the relict magmatic zircon cores of middle Neoproterozoic U-Pb ages in these different rocks of
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either early Cretaceous or late Triassic, including migmatites (Wu et al., 2007; Liu et al., 2012;
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Wang et al., 2013 and this study), granites (e.g., Yang et al., 2005; Zhao and Zheng, 2009; Zhao et al., 2012) and granulites (Hou et al., 2005; Wang et al., 2012). In addition, the
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postcollisional and synexhumational magmatic rocks show similar Sr-Nd isotope compositions to those UHP metamorphic rocks, suggesting their origination from the UHP metamorphic rocks (Zhao and Zheng, 2009; Zhao et al., 2012). In this regard, the migmatite
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in this study has a genetic relationship to the both granite and granulite. The anatexis of UHP metamorphic rocks would produce anatectic melts and result in the formation of migmatites;
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if the anatectic melts segregates from restites and further transports upwards as with mineral crystallization, this yields the magmas of granitic compositions and leaves granulite as the restite. Therefore, the tectonic extension and collapse of collisional orogenic lithosphere may
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be responsible for the contemporaneous metamorphism and magmatism in the postcollisional
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9. Conclusions
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stage.
Polyphase zircon domains from migmatites in the Dabie orogen record not only eclogite-facies metamorphism but also two episodes of anatexis. The first episode of anatexis
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occurred in the late Triassic to early Jurassic, and the second episode of anatexis occurred in the early Cretaceous. The two episodes of anatexis are evident in the Dabie UHP metamorphic rocks, with a ca. 50 Myr interval in anatectic time. The first episode of anatexis is caused by dehydration melting due to the breakdown of hydrous UHP minerals during the “hot” exhumation, corresponding to the granulite-facies overprinting. The second episode of anatexis is induced by both hydration and dehydration melting due to the breakdown of hydrous minerals with local focus of aqueous solutions in the postcollisional stage. The two episodes of crustal anatexis record the particularity of tectonothermal regime for the continental collisional orogen in the exhumational and postcollisional stages, respectively. The first episode of anatexis may have facilitated the exhumation of deeply subducted continental crust from the mantle depth, and the second episode of anatexis may witness the thermal pulse of orogenic root in the postcollisional stage. There are also the contemporaneous occurrences of early Cretaceous migmatite, granulite and granite in the
ACCEPTED MANUSCRIPT Dabie orogen, and their precursors share the same origin of the deeply subducted continental crust. This suggests a genetic relationship between migmatite, granulite and granite.
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Acknowledgments
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This study has been supported by funds from the Chinese Ministry of Science and Technology (2015CB856106), the Natural Science Foundation of China (41221062,
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41173013 and 41422301), the Fundamental Research Programs for the Central Universities (WK2080000018 and WK2080000032), A Foundation for the Author of National Excellent Doctoral Dissertation of PR China (201229) and Program for New Century Excellent Talents
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in University (NCET-11-0875). Thanks are due to Yongsheng Liu for his assistance with the LA-ICPMS analyses and to Fukun Chen and Wan-Cai Li for their assistances with the laser
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Raman analyses. Comments by two anonymous reviewers greatly helped the improvement of
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the presentation. We are grateful to Marco Scambelluri for his editorial handling.
ACCEPTED MANUSCRIPT References Andersen, T., 2002. Correction of common lead in U-Pb analyses that do not report
204
Pb.
Chemical Geology, 192, 59-79.
T
Auzanneau, E., Vielzeuf, D., Schmidt, M.W., 2006. Experimental evidence of decompression
IP
melting during exhumation of subducted continental crust. Contributions to Mineralogy and Petrology, 152, 125-148.
SC R
Baldwin, J.A., Brown, M., Schmitz, M.D., 2007. First application of titanium-in-zircon thermometry to ultrahigh-temperature metamorphism. Geology, 35, 295-298. Bea, F., 1996. Residence of REE, Y, Th and U in granites and crustal protoliths; implications
NU
for the chemistry of crustal melts. Journal of Petrology, 37, 521- 552. Brown, M., 2010. Melting of the continental crust during orogenesis: the thermal, rheological,
MA
and compositional consequences of melt transport from lower to upper continental crust. Canadian Journal of Earth Science, 47, 655-694. Burda, J., Gaweda, A., 2009. Shear-influenced partial melting in the Western Tatra
D
metamorphic complex: geochemistry and geochronology. Lithos, 110, 373–385.
TE
Cesare, B., Ferrero, S., Salviolo-Mariani, E., Pedron, D., Cavallo, A., 2009. "Nanogranite" and glassy inclusions: The anatectic melt in migmatites and granulites. Geology, 37,
CE P
627-630.
Chen, N.S., Sun, M, You, Z.D., Malpas, J., 1998. Well-preserved garnet growth zoning in granulite from the Dabie Mountains, central China. Journal of Metamorphic Geology, 16,
AC
213-222.
Chen, Y., Ye, K., Liu, J.B., Sun, M., 2006. Multistage metamorphism of the Huangtuling granulite,
Northern
Dabie
Orogen,
eastern
China:
implications
for
the
tectonometamorphic evolution of subducted lower continental crust. Journal of Metamorphic Geology, 24, 633-654. Chen, R.-X., Zheng, Y.-F., Xie, L.W., 2010. Metamorphic growth and recrystallization of zircon: distinction by simultaneous in-situ analyses of trace elements, U-Th-Pb and Lu-Hf isotopes in zircon from eclogite-facies rocks in the Sulu orogen. Lithos, 114, 132-154. Chen, Y.-X., Zheng, Y.-F., Chen, R.-X., Zhang, S.-B., Li, Q., Dai, M., Chen, L., 2011. Metamorphic growth and recrystallization of zircons in extremely
18
O-depleted rocks
during eclogite-facies metamorphism: Evidence from U-Pb ages, trace elements, and O-Hf isotopes. Geochimica et Cosmochimica Acta 75, 4877-4898. Chen, R.-X., Zheng, Y.-F., Hu, Z., 2012. Episodic fluid action during exhumation of deeply
ACCEPTED MANUSCRIPT subducted continental crust: Geochemical constraints from zoisite–quartz vein and host metabasite in the Dabie orogen. Lithos, 155, 146-166. Chen, Y.-X., Zheng, Y.-F., Hu, Z., 2013a. Synexhumation anatexis of ultrahigh-pressure
T
metamorphic rocks: petrologic evidence from granitic gneiss from the Sulu orogen. Lithos,
IP
156-159, 69-96.
Chen, Y.-X., Zheng, Y.-F., Hu, Z., 2013b. Petrological and zircon evidence for anatexis of
SC R
UHP quartzite during continental collision in the Sulu orogen. Journal of Metamorphic Geology, 31, 389-413.
Cherniak, D.J., Watson, E.B., 2007. Ti diffusion in zircon. Chemical Geology, 242, 470-483.
NU
Cherniak, D.J., 2010. Diffusion in accessory minerals: zircon, titanite, apatite, monazite andxenotime. Reviews in Mineralogy and Geochemistry, 72, 827-869.
MA
Dai, L.-Q., Zhao, Z.-F., Zheng, Y.-F., Li, Q., Yang, Y., Dai, M., 2011. Zircon Hf-O isotope evidence for crust–mantle interaction during continental deep subduction. Earth and Planetary Science Letters, 308, 224-244.
D
Faure, M., Lin, W., Schärer, U., Shu, L.S., Sun, Y., Arnaud, N., 2003. Continental subduction
TE
and exhumation of UHP rocks. Structural and geochronological insights from the Dabieshan (East China). Lithos, 70, 213-241.
CE P
Ferriss, E.D.A., Essene, E.J., Becker, U., 2008. Computational study of the effect of pressure on the Ti-in-zircon geothermometer. European Journal of Mineralogist, 20, 745-755. Ferry, J.M., Watson, E.B., 2007. New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers. Contributions to Mineralogy and Petrology,
AC
154, 429-437.
Flowers, R.M., Schmitt, A.K., Grove, M., 2010. Decoupling of U-Pb dates from chemical and crystallographic domains in granulite facies zircon. Chemical Geology, 270, 20-30. Gao, X.-Y., Zheng, Y.-F., Chen, Y.-X., 2012a. Dehydration melting of ultrahigh-pressure eclogite in the Dabie orogen: evidence from multiphase solid inclusions in garnet. Journal of Metamorphic Geology, 30, 193-212. Gao, X.-Y., Zheng, Y.-F., Chen, Y.-X., Guo, J., 2012b. Geochemical and U–Pb age constraints on the occurrence of polygenetic titanites in UHP metagranite in the Dabie orogen. Lithos, 136-139, 93-108. Gardien, V., Thompson, A.B., Ulmer, P., 2000. Melting of biotite + plagioclase + quartz gneisses: the role of H2O in the stability of amphibole. Journal of Petrology, 41, 651-666. Gordon, S.M., Whitney, D.L., Teyssier, C., Fossen, H., 2013. U-Pb dates and trace-element geochemistry of zircon from migmatite, Western Gneiss Region, Norway: significance for
ACCEPTED MANUSCRIPT history of partial melting in continental subduction. Lithos, 170-171, 35-53. Hacker, B.R., Ratschbacher, L., Webb, L., Ireland, T., Walker, D., Dong, S.W., 1998. U/Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling-Dabie orogen,
T
China. Earth and Planetary Science Letters, 161, 215-230.
IP
Hacker, B.R., Ratschbacher, L., Webb, L., McWilliams, M.O., Ireland, T., Calvert, A., Dong, S., Wenk, H.R., Chateigner, D., 2000. Exhumation of ultrahigh-pressure continental crust
Geophysics Research, B105, 13339-13364.
SC R
in east central China: Late Triassic–Early Jurassic tectonic unroofing. Journal of
Hermann, J., Green, D.H., 2001. Experimental constraints on high pressure melting in
NU
subducted crust. Earth and Planetary Science Letters, 188, 149-168. Hermann, J., Rubatto, D., Korsakov, A., Shatsky, V., 2001. Multiple zircon growth during
MA
fast exhumation of diamondiferous, deeply subducted continental crust (Kokchetav Massif, Kazakhstan). Contributions to Mineralogy and Petrology, 141, 66-82. Hermann, J., 2002a. Experimental constraints on phase relations in subducted continental
D
crust. Contributions to Mineralogy and Petrology, 143, 219-235.
TE
Hermann, J., 2002b. Allanite: thorium and light rare earth element carrier in subducted crust. Chemical Geology, 192, 289-306.
CE P
Hermann, J., Spandler, C., Hack, A., Korsakov, A.V., 2006. Aqueous fluids and hydrous melts in high-pressure and ultra-high pressure rocks: implications for element transfer in subduction zones. Lithos, 92, 399-417. Holness, M.B., Cesare, B., Sawyer, E.W., 2011. Melted rocks under the microscope:
AC
microstructures and their interpretation. Elements, 7, 247-252. Hoskin, P.W.O., Schaltegger, U., 2003. The composition of zircon and igneous and metamorphic petrogenesis. Reviews in Mineralogy and Geochemistry, 53, 27-62. Hou, Z.-H., Li, S.-G., Chen, N.-S., Li, Q.L., Liu, X.M., 2005. Sm-Nd and Zircon SHRIMP U–Pb dating of Huilanshan mafic granulite in the Dabie Mountains and its zircon trace element geochemistry. Science China (D), 48, 2081-2091. Imayama, T., Takeshita, T., Yi, K., Cho, D.-L., Kitajima, K., Tsutsumi, Y., Kayama, M., Nishido, H., Okumura, T., Yagi, K., Itaya, T., Sano, Y., 2012. Two-stage partial melting and contrasting cooling history within the Higher Himalaya Crystalline Sequence in the far-eastern Nepal Himalaya. Lithos, 134-135, 1-22. Jackson, S.E., Pearson, N.J., Griffin, W.L., Belousova, E.A., 2004. The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U-Pb zircon geochronology. Chemical Geology, 211, 47-69.
ACCEPTED MANUSCRIPT Jian, P., Kröner, A., Zhou, G.Z., 2012. SHRIMP zircon U-Pb ages and REE partition for high-grade metamorphic rocks in the North Dabie complex: Insight into crustal evolution with respect to Triassic UHP metamorphism in east-central China. Chemical Geology, 328,
T
49-69.
IP
Jiang, L., Liu, Y.-C., Wu, W., Li, H., Fang, Z., 2002. Zircon U-Pb age and its geological implications of the gray gneiss to the northern Manshuihe in the North Dabie Mountains.
SC R
Geochimica, 31, 66-70 (in Chinese with English abstract).
Keay, S., Lister, G., Buick, I., 2001. The timing of partial melting, Barrovian metamorphism and granite intrusion in the Naxos metamorphic core complex, Cyclades, Aegean Sea,
partial melting at ultrahigh pressure. Geology, 39, 1171-1174.
TE
Lang, H.M., Gilotti, J.A., 2007. Partial melting of metapelites at ultrahigh-pressure conditions, Greenland Caledonides. Journal of Metamorphic Geology, 25, 129-147.
CE P
Li, S.G., Xiao, Y.L., Liu, D.L., Chen, Y.Z., Ge, N.J., Zhang, Z.Q., Sun, S.S., Cong, B.L., Zhang, R.Y., Hart, S.R., 1993. Collision of the North China and Yangtse Blocks and formation of coesite-bearing eclogites: timing and processes. Chemical Geology, 109, 89-111.
AC
Li, W.-C., Chen, R.-X., Zheng, Y.-F., Li, Q., Hu, Z., 2013. Zirconological tracing of transition between aqueous fluid and hydrous melt in the crust: constraints from pegmatite vein and host gneiss in the Sulu orogen. Lithos, 162-163, 157-174. Li, W.-C., Chen, R.-X., Zheng, Y.-F., Hu, Z., 2014. Dehydration and anatexis of UHP metagranite during continental collision in the Sulu orogen. Journal of Metamorphic Geology, 32, 915-936. Liou, J.G., Ernst, W.G., Zhang, R.Y., Tsujimori, T., Jahn, J.G., 2009. Ultrahigh-pressure minerals and metamorphic terranes – The view from China. Journal of Asian Earth Sciences, 35, 199-231. Liu, Y.-C., Li, S.-G., Xu, S.-T., 2005. Geochemistry and geochronology of eclogites from the north Dabie Mountains, central China. Journal of Asian Earth Science, 25, 431-443. Liu, Y.-C., Li, S.-G., Xu, S.-T., 2007a. Zircon SHRIMP U–Pb dating for gneisses in northern Dabie high T/P metamorphic zone, central China: implications for decoupling within
ACCEPTED MANUSCRIPT subducted continental crust. Lithos, 96, 170-185. Liu, Y.-C., Li, S., Gu, X., Xu, S., Chen, G., 2007b. Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central China. Journal of Metamorphic
T
Geology, 25, 975-989.
IP
Liu, Y.-C., Li, S.G., 2008. Detachment within subducted continental crust and multi-slice successive exhumation of ultrahigh-pressure metamorphic rocks: evidence from the
SC R
Dabie–Sulu orogenic belt. Chinese Science Bulletin, 53, 3105-3119. Liu, F.L., Robinson, P.T., Gerdes, A., Xue, H.M., Liu, P.H., Liou, J.G., 2010a. Zircon U-Pb ages, REE concentrations and Hf isotope compositions of granitic leucosome and
NU
pegmatite from the north Sulu UHP terrane in China: Constraints on the timing and nature of partial melting. Lithos, 117, 247-268.
MA
Liu, Y.S., Gao, S., Hu, Z.C., Gao, C.G., Zong, K.Q., Wang, D., 2010b. Continental and oceanic crust recycling-induced melt-peridotite interactions in the Trans-North China Orogen: U-Pb dating, Hf isotopes and trace elements in zircons from mantle xenoliths.
D
Journal of Petrology, 51, 537-571.
TE
Liu, Y.S., Hu, Z.C., Zong, K.Q., Gao, C.G., Gao, S., Xu, J., Chen, H.H., 2010c. Reappraisement and refinement of zircon U-Pb isotope and trace element analyses by
CE P
LA-ICP-MS. Chinese Science Bulletin, 55, 1535-1546. Liu, F.L., Liou, J.G., 2011. Zircon as the best mineral for P-T-time history of UHP metamorphism: A review on mineral inclusions and U-Pb SHRIMP ages of zircons from the Dabie-Sulu UHP rocks. Journal of Asian Earth Sciences, 40, 1-39.
AC
Liu, Y.-C., Gu, X.-F., Rolfo, F., Chen, Z.-Y., 2011a. Ultrahigh-pressure metamorphism and multistage exhumation of eclogite of the Luotian dome, North Dabie Complex Zone (central China): Evidence from mineral inclusions and decompression textures. Journal of Asian Earth Sciences, 42, 607-617. Liu, Y.-C., Gu, X.-F., Li, S.-G., Hou, Z.-H., Song, B., 2011b. Multistage metamorphic events in granulitized eclogites from the North Dabie complex zone, central China: evidence from zircon U-Pb age, trace element and mineral inclusion. Lithos, 122, 107-121. Liu, F.L., Robinson, P.L., Liu, P.H., 2012. Multistage partial melting events in the Sulu UHP terrane: zircon U–Pb dating of granitic leucosomes within amphibolite and gneiss. Journal of Metamorphic Geology, 30, 887-906. Liu, Q., Hermann, J., Zhang, J.F., 2013. Polyphase inclusions in the Shuanghe UHP eclogites formed by subsolidus transformation and incipient melting during exhumation of deeply subducted crust. Lithos, 177, 91–1093.
ACCEPTED MANUSCRIPT Ludwig, K.R., 2003. User's Manual for Isoplot 3.00: A Geochronological Toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication, Berkeley, No. 4, 70 pp.
T
Manning, C.E., 2004. The chemistry of subduction-zone fluids. Earth and Planetary Science
IP
Letters, 223, 1-16.
Ragozin, A.L., Liou, J.G., Shatsky, V.S., Sobolev, N.V., 2009. The timing of the retrograde
SC R
partial melting in the Kumdy-Kol region (Kokchetav Massif, Northern Kazakhstan). Lithos, 109, 274-284.
revisited: implications for the continental crust. Journal of Metamorphic Geology, 23, 19-28.
MA
Rubatto, D., 2002. Zircon trace element geochemistry: partitioning with garnet and the link between U-Pb ages and metamorphism. Chemical Geology, 184, 123-138. Rubatto, D., Hermann, J., 2003. Zircon formation during fluid circulation in eclogites
D
(Monviso, Western Alps): Implications for Zr and Hf budget in subduction zones.
TE
Geochimica et Cosmochimica Acta, 67, 2173-2187. Rubatto, D., Hermann, J., Berger, A., Engi, M., 2009. Protracted fluid-induced melting during
158, 703-722.
CE P
Barrovian metamorphism in the Central Alps. Contributions to Mineralogy and Petrology,
Rubatto, D., Chakraborty, S., Dasgupta, S., 2013. Timescales of crustal melting in the Higher Himalayan
Crystallines
(Sikkim,
Eastern
Himalaya)
inferred
from
trace
AC
element-constrained monazite and zircon chronology. Contributions to Mineralogy and Petrology, 165, 349-372. Sawyer, E.W., 2008. Working with migmatites: nomenclature for the constituent parts. Mineralogical Association of Canada Short Course Series, 38, 1-28. Sawyer, E.W., Cesare, B., Brown, M., 2011. When the continental crust melts. Elements, 7, 229-234. Slagstad, T., Jamieson, R., Culshaw, N., 2005. Formation, crystallization, and migration of melt in the mid-orogenic crust: Muskoka domain migmatites, Grenville Province, Ontario. Journal of Petrology, 46, 893-919. Sun, S.-s., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalt: implications for mantle composition and processes. Geological Society Special Publication, 42, 313-345. Tailby, N.D., Walker, A.M., Berry, A.J., Hermann, J., Evans, K.A., Mavrogenes, J.A., O'Neill,
ACCEPTED MANUSCRIPT H.St.C., Rodina, I.S., Soldatov, A.V., Rubatto, D., Sutton, S.R., 2011. Ti site occupancy in zircon. Geochimica et Cosmochimica Acta, 75, 905-921. Teyssier, C., 2012. Exhumation of deep orogenic crust. Lithosphere, 3, 439-443.
T
Tong, L.X., Jahn, B.M., Zheng, Y.-F., 2011. Diverse P-T paths of the northern Dabie complex
IP
in central China and its reworking in the early Cretaceous. Journal of Asian Earth Sciences, 42, 633-640.
journey. Gondwana Research, 21, 224-235.
SC R
Touret, Q.L.R., Huizenga, J.M., 2012. Fluid-assisted granulite metamorphism: A continental
Tsai, C.H., Liou, J.G., 2000. Eclogite-facies relics and inferred ultrahigh-pressure
NU
metamorphism in the North Dabie Complex, central-eastern China. American Mineralogist, 85, 1-8.
MA
Vavra, G., Gebauer, D., Schmid, R. & Compston, W., 1996. Multiple zircon growth and recrystallization during polyphase Late Carboniferous to Triassic metamorphism in granulites of the Ivrea Zone (Southern Alps): an ion microprobe (SHRIMP) study.
D
Contributions to Mineralogy and Petrology, 122, 337-358.
TE
Wallis, S., Tsuboi, M., Suzuki, K., Fanning, M., Jiang, L., Tanaka, T., 2005. Role of partial melting in the evolution of Sulu (eastern China) ultrahigh-pressure terrane. Geology, 33,
CE P
129-133.
Wang, X.-M., Liou, J.G., Mao, H.-K., 1989. Coesite-bearing from the Dabie Mountains in central China. Geology, 17, 1085-1088. Wang, J.H., Sun, M., Deng, S.X., 2002. Geochronological constraints on the timing of
AC
migmatization in the Dabie Shan, East-central China. European Journal of Mineralogist, 14, 513-524.
Wang, S.-J., Li, S.-G., An, S.-C., Hou, Z.-H., 2012. A granulite record of multistage metamorphism and REE behavior in the Dabie orogen: Constraints from zircon and rock-forming minerals. Lithos, 136-139, 109-125. Wang, S.-J., Li, S.-G., Chen, L.-J., He, Y.-S., An, S.-C., Shen, J., 2013. Geochronology and geochemistry of leucosomes in the North Dabie Terrane, East China: implication for post-UHPM crustal melting during exhumation. Contributions to Mineralogy and Petrology, 165, 1009-1029. Whitney, D.L., Teyssier, C., Rey, R.F., 2009a The consequences of crustal melting in continental subduction. Lithoshere, 1, 1-5. Whitney, D.L., Evans, B.W., 2010. Abbreviations for names of rock-forming minerals. American Mineralogist, 95, 185-187.
ACCEPTED MANUSCRIPT Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., von Quadt, A., Roddick, J.C., Spiegel, W., 1995. Three natural zircon standards for U-Th-Pb, Lu-Hf, trace element and REE analyses. Geostandards Newsletter, 19, 1-23.
ages and Hf isotope compositions of migmatite from the North Dabie terrane in China: constraints on partial melting. Journal of Metamorphic Geology, 25, 991-1009.
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Wu, Y.-B., Zheng, Y.-F., Gao, S., Jiao, W.-F., Liu, Y.-S., 2008. Zircon U-Pb age and trace element evidence for Paleoproterozoic granulite-facies metamorphism and Archean crustal rocks in the Dabie Orogen. Lithos, 101, 308-322.
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Wu, Y.-B., Gao, S., Zhang, H.-F. et al., 2009. U-Pb age, trace-element, and Hf-isotope compositions of zircon in a quartz vein from eclogite in the western Dabie Mountains:
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constraints on fluid flow during early exhumation of ultra-high-pressure rocks. American Mineralogist, 94, 303–312.
Xia, Q.-X., Zheng, Y.-F., Zhou, L.-G., 2008. Dehydration and melting during continental
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collision: constraints from element and isotope geochemistry of low-T/UHP granitic
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gneiss in the Dabie orogen. Chemical Geology, 247, 36-65. Xia, Q-X, Zheng, Y.-F., Yuan, H.L., Wu, F.Y., 2009. Contrasting Lu-Hf and U-Th-Pb isotope
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systematics between metamorphic growth and recrystallization of zircon from eclogite-facies metagranites in the Dabie orogen, China. Lithos, 112, 477-496. Xia, Q.-X., Zheng, Y.-F., Hu, Z.-C., 2010. Trace elements in zircon and coexisting minerals from low-T/UHP metagranite in the Dabie orogen: implications for action of
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supercritical fluid during continental subduction-zone metamorphism. Lithos, 114, 385-413.
Xie, Z., Zheng, Y.-F., Jahn, B.-m., Ballevre, M., Chen, J.-F., Gautier, P., 2004. Sm–Nd and Rb–Sr dating for pyroxene–garnetite from North Dabie in east-central China: problem of isotope disequilibrium due to retrograde metamorphism. Chemical Geology, 206, 137-158. Xie, Z., Zheng, Y.-F., Zhao, Z.-F., Wu, Y.-B., Wang, Z.-R., Chen, J.-F., Liu, X.-F., Wu, F.-Y., 2006. Mineral isotope evidence for the contemporaneous process of Mesozoic granite emplacement and gneiss metamorphism in the Dabie orogen. Chemical Geology, 231, 214-235. Xie, Z., Chen, J.F., Cui, Y.R., 2010. Episodic growth of zircon in UHP orthogneisses from the North Dabie Terrane of east-central China: implications for crustal architecture of a collisional orogen. Journal of Metamorphic Geology, 28, 979-995.
ACCEPTED MANUSCRIPT Xu, S.T., Okay, A.I., Ji, S.Y., Sengor, A.M.C., Wen, S., Liu, Y.C., Jiang, L.L., 1992. Diamond from the Dabie-Shan metamorphic rocks and its implication for tectonic setting. Science, 256, 80-82.
New finding of micro-diamonds in eclogites from Dabie-Sulu region in central-eastern China. Chinese Science Bulletin, 48, 988-994.
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Xu, S.-T., Liu, Y.-C., Chen, G.-B., Ji, S.-Y., Ni, P., Xiao, W.-S., 2005. Microdiamonds, their classification and tectonic implications for the host eclogites from the Dabie and Su–Lu regions in central eastern China. Mineralogical Magazine, 69, 509-520.
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Xu, Z.-Q., Zeng, L.-S., Liu, F.-L., Yang, J.-S., Zhang, Z.-M., McWilliams, M., Liou, J.G., 2006. Polyphase subduction and exhumation of the Sulu high-pressure-ultrahigh-pressure
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metamorphic terrane. Geological Society of America Special Papers, 403, 93-113. Yang, J.H., Chung, S.L., Wilde, S.A., Wu, F.Y., Chu, M.F., Lo, C.H., Fan, H.R., 2005. Petrogenesis of post-orogenic syenites in the Sulu Orogenic Belt, East China:
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geochronological, geochemical and Nd-Sr isotopic evidence. Chemical Geology, 214,
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99-125.
Yao, Y.P., Ye, K., Liu, J.B., Cong, B.L., Wang, Q.C., 2000. A transitional eclogite- to high
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pressure granulite-facies overprint on coesite–eclogite at Taohang in the Sulu ultrahigh-pressure terrane, Eastern China. Lithos, 52, 109-120. Zeng, L.S., Gao, L.E., Yu, J.J., Hu, G.Y., 2011. SHRIMP zircon U/Pb dating on ultrahigh pressure rocks from Yangkou: implications for the timing of partial melting in the Sulu
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UHP metamorphic belt. Acta Petrologica Sinica (in Chinese with English abstract), 27, 1085-1094.
Zhang, R.Y., Liou, J.G., Tsai, C.H., 1996. Petrogenesis of a high temperature metamorphic terrane: a new tectonic interpretation for the north Dabieshan, Central China. Journal of Metamorphic Geology, 14, 319-333. Zhao, Z.-F., Zheng, Y.-F., Chen, R.-X., Xia, Q.-X., Wu, Y.-B., 2007. Element mobility in mafic and felsic ultrahigh-pressure metamorphic rocks during continental collision. Geochimica et Cosmochimica Acta, 71, 5244-5266. Zhao, Z.-F., Zheng, Y.-F., Wei, C.S., Chen, F.K., Liu, X.M., Wu, F.-Y., 2008. Zircon U–Pb ages, Hf and O isotopes constrain the crustal architecture of the ultrahigh-pressure Dabie orogen in China. Chemical Geology, 253, 222-242. Zhao, Z.-F., Zheng, Y.-F., 2009. Remelting of subducted continental lithosphere: Petrogenesis of Mesozoic magmatic rocks in the Dabie-Sulu orogenic belt. Science China (D), 52,
ACCEPTED MANUSCRIPT 1295-1318. Zhao, Z.-F., Zheng, Y.-F., Zhang, J., Dai, L.-Q., Li, Q., Liu, X.M., 2012. Syn-exhumation magmatism during continental collision: Evidence from alkaline intrusives of Triassic age
T
in the Sulu orogen. Chemical Geology, 328, 70-88.
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Zhao, Z.-F., Dai, L.-Q., Zheng, Y.-F., 2013. Postcollisional mafic igneous rocks record crust-mantle interaction during continental deep subduction. Scientific Reports, 3, 3413;
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doi:10.1038/srep03413.
Zheng, Y.-F., Fu, B., Gong, B., Li L., 2003. Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie-Sulu orogen in China: Implications for geodynamics
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and fluid regime. Earth-Science Reviews 62, 105-161.
oxygen isotope evidence for a large-scale 18O depletion event in igneous rocks during the Neoproterozoic. Geochimica et Cosmochimica Acta, 68, 4145-4165. Zheng, Y.-F., Zhou, J.-B., Wu, Y.-B., Xie, Z., 2005. Low grade metamorphic rocks in the
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Dabie–Sulu orogenic belt: a passive margin accretionary wedge deformed during
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continent subduction. International Geology Review, 47, 851-871. Zheng, Y.-F., 2008. A perspective view on ultrahigh-pressure metamorphism and continental
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collision in the Dabie-Sulu orogenic belt. Chinese Science Bulletin, 53, 3081-3104. Zheng, Y.-F., 2009. Fluid regime in continental subduction zones: petrological insights from ultrahigh-pressure metamorphic rocks. Journal of the Geological Society London, 166, 763-782.
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Zheng, Y.-F., Chen, R.-X., Zhao, Z.-F., 2009. Chemical geodynamics of continental subduction-zone metamorphism: insights from studies of the Chinese Continental Scientific Drilling (CCSD) core samples. Tectonophysics, 475, 327-358. Zheng, Y.-F., Xia, Q.-X., Chen, R.-X., Gao, X.-Y., 2011. Partial melting, fluid supercriticality and element mobility in ultrahigh-pressure metamorphic rocks during continental collision. Earth-Science Reviews, 107, 342-374. Zheng, Y.-F., 2012. Metamorphic chemical geodynamics in continental subduction zones. Chemical Geology, 328, 5-48. Zheng, Y.-F., Xiao, W.-J., Zhao, G.-C., 2013a. Introduction to tectonics of China. Gondwana Research, 23, 1189-1206 Zheng, Y.F., Zhao, Z.F., Chen, Y.X., 2013b. Continental subduction channel processes: Plate interface interaction during continental collision. Chinese Science Bulletin 58, 4371-4377.
2010a. Melting-induced fluid flow during exhumation of gneisses of the Sulu
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ultrahigh-pressure terrane. Lithos, 120, 490-510.
Zong, K.Q., Liu, Y.S., Gao, C.G., Hu, Z.C., Gao, S., Gong, H.J., 2010b. In situ U-Pb dating
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and trace element analysis of zircons in thin sections of eclogite: Refining constraints on the ultra high-pressure metamorphism of the Sulu terrane, China. Chemical Geology, 269,
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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. Sketch geological map of the Dabie orogen, showing sample location (after Wu et al.
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2007). NCB = North China Craton; SCB = South China Block; DB = Dabie orogen; YXD =
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Yuexi dome; LTD = Luotian dome.
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Fig. 2. Field photographs of migmatite at Manshuihe in the North Dabie zone. (a) Stromatic metatexites with concordant and discordant leucosomes. Samples 04NDB17 and 04NDB19 were taken nearby. (b) Local place of the migmatite body exhibits larger extent of
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deformation and flow with former leucosome was cut by later leucosome. (c) Accumulation of amphibole grains form concordant melanosome interlayer or boudin with leucosome in
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stromatic metatexites. Locally, K-feldspar-rich pegmatite occurs as boudin. Sample 09DB84 and 09DB82 were collected in this area. 09DB84 is leucosome, whereas 09DB82 is composed of both melanosome and leucosome. (d) Feldspar pegmatite occurs as boudin in amphibolite.
Fig. 3. Microphotographs of the migmatites at Manshuihe in the North Dabie zone. (a) Equant
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subhedral to euhedral feldspar crystals with fine-grained biotite in the grain boundary form the structure in leucosome 04NDB17. (b) Worm-like microstructures between feldspar and quartz, cuspate feldspar and quartz, and eroded monazite with quartz inclusion in leucosome
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09DB84. (c) Anhedral amphibole grains variable in size and shape with biotite + quartz + feldspar, and they contain clinopyroxene inclusions in the melanosome of sample 04NDB19. (d) Large and anhedral amphibole and clinopyroxene grains dispersed in the quartz-feldspathic matrix in the leucosome of sample04NDB19. Feldspar grains exhibit good crystal faces. (e) Mineral assemblage of the melanosome in sample 09DB82. (f) Anhedral clinopyroxene and crystal face feldspar in the leucosome of sample 09DB82.
Fig. 4. Cathodoluminescence images with
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Pb/238U ages for dated zircon crystals in thin
section. The white scale bar in each image represents 40 µm. Four distinct metamorphic domains (RC, D1, D2 and D3) are observed. RC denotes relict core. Fig. 5. U-Pb concordia diagrams and Chondrite-normalized REE patterns for zircon in thin sections of migmatite at Manshuihe in the Dabie orogen. Chondrite REE values are from Sun and McDonough (1989). Red lines denote the relict cores in leucosomes 04NDB17 and
ACCEPTED MANUSCRIPT 09DB84 and migmatite 04NDB19, olivine lines denote D1 domains with ages of ~215 Ma in migmatite 04NDB19, blue lines denote D2 domains of with ages of ~200 Ma in leucosomes 04NDB17 and 09DB84, and magenta lines denote D3 domain with Cretaceous ages in
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leucosome 09DB84 and migmatite 09DB82. Fig. 6. Representative Cathodoluminescence (CL) images and206Pb/238U ages for separated
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zircons mounted in epoxy. The white scale bar in each image represents 50 µm.
Fig. 7. U-Pb concordia diagrams and Chondrite-normalized REE patterns for zircon in grain
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mount. Chondrite REE values are from Sun and McDonough (1989). The symbols are defined
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as same as those in Fig. 5.
Fig. 8. Photomicrograph, BSE images and Raman Spectra of multiphase solid inclusions in D2 domains from migmatite at Manshuihe in the Dabie orogen. (a) Transmitted light image a
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D2 zircon grain, (b) and (c) microphotograph and Raman spectra of the multiphase solid
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inclusion in (a), (d) and (e) BSE images of multiphase solid inclusions included in D2
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domains.
Fig. 9 Plots of trace element compositions for the four groups of zircon domains. (a) Th/U vs. REE contents, (b) Th vs. U contents, (c) (Dy/Sm)N vs. (Lu/Ho)N, (d) Eu/Eu* vs. Nb+Ta contents. The four different groups of zircon domains can be distinguished by these
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representative trace element contents and ratios. The legend is valid for all plots.
Fig. 10. Frequency histogram of Ti-in-zircon temperatures for migmatite from Manshuihe in the North Dabie zone.
Fig. 11. Plots of U-Th-Pb systematics for the zircon of early Cretaceous U-Pb ages in migmatite. (a) The relationship between Th/U ratios and
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Pb/238U age; (b) the relationship
between Th and U contents.
Fig. 12. Summary of zircon U-Pb ages for UHP metamorphic rocks from the North Dabie zone. Grey bands indicate the age ranges associated with the labeled events. Reference: 1 = This study; 2 = Wu et al. (2007); 3 = Liu et al. (2011); 4 = Xie et al. (2010); 5 = Zhao et al. (2008); 6= Hou et al. (2005); 7 = Wu et al. (2008); 8 = Wang et al. (2012).
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ACCEPTED MANUSCRIPT Table 1 Summary of mineral assemblages, zircon U-Pb ages, representative trace element compositions and Ti-in-zircon temperatures for migmatites from Manshuihe in North Dabie Thin section Rock type
Notes: 1. Ages t1 and t2 denote the upper- and lower-intercept ages of the discordia dating, respectively. 2. Listed values are either variation ranges from multi-domains of the same origin or measured values from single domains. 3. The unit for the trace elements is ppm. 4. TTi was calculated by the Ti-thermometer in zircon of Ferry and Watson (2007).
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Table 1 (continued) Mount grain Leucosome
Sample
04NDB17
Assembl age
Qz, Pl, Kfs, Zrn, Ttn, Ap, Amp, Mag, Bt, Chl
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Th/U 206
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Pb/ U age (Ma) Discordi a age t1 (Ma) Discordi a age t2 (Ma) Sm Dy Lu Hf Y Nb Ta Ti P LREE MREE
