Growth of metamorphic and peritectic garnets in

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    Growth of metamorphic and peritectic garnets in ultrahigh-pressure metagranite during continental subduction and exhumation in the Dabie orogen Qiong-Xia Xia, Haozhen Wang, Li-Gang Zhou, Xiao-Ying Gao, YongFei Zheng, James Ashton Van Orman, Haijun Xu, Zhaochu Hu PII: DOI: Reference:

S0024-4937(16)30324-3 doi: 10.1016/j.lithos.2016.08.043 LITHOS 4093

To appear in:

LITHOS

Received date: Accepted date:

7 April 2016 23 August 2016

Please cite this article as: Xia, Qiong-Xia, Wang, Haozhen, Zhou, Li-Gang, Gao, Xiao-Ying, Zheng, Yong-Fei, Van Orman, James Ashton, Xu, Haijun, Hu, Zhaochu, Growth of metamorphic and peritectic garnets in ultrahigh-pressure metagranite during continental subduction and exhumation in the Dabie orogen, LITHOS (2016), doi: 10.1016/j.lithos.2016.08.043

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Growth of metamorphic and peritectic garnets in

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ultrahigh-pressure metagranite during continental subduction

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and exhumation in the Dabie orogen Qiong-Xia Xia1, 2*, Haozhen Wang3, Li-Gang Zhou1, 3, Xiao-Ying Gao1, Yong-Fei Zheng1,

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James Ashton Van Orman4, Haijun Xu5, Zhaochu Hu5

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1. CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China

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2. State Key Laboratory of Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing, China 3. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics,

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Chinese Academy of Sciences, Beijing 100029, China 4. Department of Earth, Environmental and Planetary Sciences, Case Western Reserve University, 10900 Euclid Avenue, Cleveland OH 44106, USA 5. State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China

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Corresponding author. Email: [email protected] 1

ACCEPTED MANUSCRIPT Abstract Two generations of garnet are recognized in ultrahigh-pressure (UHP) metagranite from the Dabie orogen by a combined study of petrography, major and trace element profiles in garnet,

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and phase equilibrium modeling for metagranite. The results enable distinction between

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metamorphic and peritectic garnet on the basis of BSE images, and major and trace element compositions. Our research provides new insights into the growth of anatectic garnet due to

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dehydration melting of UHP metamorphic rocks during exhumation from mantle depths. The first generation of garnet (Grt-I) occurs as a broad domain in the center, which is related to metamorphic growth during prograde subduction. This garnet is dark in BSE images, rich in

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grossular and poor in almandine and pyrope. The chondrite-normalized rare earth element (REE) patterns show LREE depletion and flat MREE-HREE patterns. The second generation

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of garnet (Grt-II) occurs as a rim of euhedral garnet, or as patches in Grt-I domains, recrystallized after dissolution of preexisting metamorphic garnet in the presence of anatectic melts during exhumation. It is bright in BSE images, poor in grossular, and rich in almandine

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and pyrope contents. Trace element analyses on Grt-II domains yield high contents of Sc, Cr,

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Y and HREE and low contents of Ti and MREE. The chondrite-normalized REE patterns exhibit LREE depletion, and steep MREE-HREE patterns. Based on REE partitioning

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between garnet and zircon/titanite, the last growth times for metamorphic and anatectic garnets are constrained by zircon and titanite U-Pb ages to be ~240 Ma and ~220 Ma, respectively. Based on anatectic microstructures and a modeled P-T pseudosection, it is

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suggested that dehydration melting occurred at 2.0-2.5 GPa during exhumation. Melting occurred through the breakdown of phengite via the peritectic reaction: garnet (I) + phengite + plagioclase + quartz → garnet (II) + biotite + K-feldspar + melt.

Keywords: partial melting; peritectic garnet; metamorphic garnet; ultrahigh pressure; dissolution-reprecipitation

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ACCEPTED MANUSCRIPT 1. Introduction Dehydration melting of deeply subducted continental crust during exhumation from mantle depths has been demonstrated by several lines of evidence at different scales. These include

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the occurrence of multiphase solid (MS) inclusions in ultrahigh-pressure (UHP) metamorphic

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refractory minerals such as garnet, omphacite and kyanite (e.g., Ferrando et al., 2005; Frezzotti et al., 2007; Gao et al., 2012a; Chen et al., 2013a), felsic veinlets in UHP eclogites

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and gneisses (e.g., Zhao et al., 2007; Xia et al., 2008; Chen et al., 2012, 2013b; Zhang et al., 2015), and migmatitic leucosomes and granitic pegmatites in UHP metamorphic rocks (e.g., Wallis et al., 2005; Liu et al., 2010a; Zong et al., 2010; Zhao et al., 2012; Song et al., 2014;

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Wang et al., 2014). Dehydration melting of UHP metamorphic rocks commonly produces felsic melts with very small ratios of melt to residue (Li et al., 2013; Zheng and Hermann,

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2014). Such melts are referred to as anatectic melts and are different from differentiated magmatic melts, in that they did not completely separate from their parent metamorphic rocks and experienced little crystal fractionation (Li et al., 2013; Zheng and Hermann, 2014). In

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contrast, magmatic melts have completely separated from their parental rocks, represent

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mixing of different batches of anatectic melts, and have often experienced significant chemical differentiation during their transport upward. The composition of anatectic melts

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varies dramatically depending on source mineralogy and fluid regime (Clemens and Droop, 1998; Stevens et al., 1997, 2007; Zheng and Hermann, 2014), which exerts a strong control on the composition of peritectic and anatectic minerals. For instance, zircon grown from

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peritectic reaction during continental exhumation can be distinguished from that grown from magmatic melts and metamorphic fluids in terms of mineral inclusions, U-Pb ages and trace element compositions (e.g., Xia et al., 2010; Liu et al., 2010a; Chen et al., 2013b). Garnet is a very useful major phase in metamorphic rocks. Its stability over a wide range of temperatures and pressures permits its use in remarkably diverse tectonic settings and rock types (e.g., Baxter and Scherer, 2013). It can preserve strong zonation in major and trace elements as well as mineral inclusions, which bears information on re-equilibration with coexisting minerals during high-grade metamorphism (e.g., Skora et al., 2006; Konrad-Schmolke et al., 2008a, b; Dragovic et al., 2012; Baxter and Caddick, 2013). Thus garnet plays an important role in revealing the evolution of subduction-zone processes, including the pressure, temperature and duration of metasomatism, deformation and anatexis (e.g., Angiboust et al., 2014; Caddick et al., 2010; Caddick and Kohn, 2013; Dragovic et al., 2015; Habler et al., 2007; Martin et al., 2014; Pollington and Baxter, 2010; Storey and Prior,

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ACCEPTED MANUSCRIPT 2005). As a major phase in UHP metamorphic rocks, metamorphic garnet is unavoidably modified when anatectic melting has taken place (e.g., Perchuck et al., 2005, 2008; Liu et al., 2014). Some garnet may be of peritectic origin in the presence of anatectic melts. However,

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the characteristic features of peritectic garnets that allow them to be distinguished from

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metamorphic or magmatic garnets in UHP metamorphic rocks are still uncertain. Identifying such features that constrain the origin of garnets in UHP metamorphic rocks is critical to the

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P-T-t reconstruction of deeply subducted crustal rocks.

In this paper, we combine petrology and geochemistry in garnet and related minerals from UHP metagranite (granitic orthogneiss) in the Dabie orogen to evaluate the origin of different

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types of garnet. The metagranite is a direct host of UHP eclogites and biotite paragneiss in this region. Partial melting has been identified in the eclogitic garnet based on MS inclusions

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with radial cracks (Gao et al., 2012a, 2013; Liu et al., 2013). It is possible that partial melting also occurred in the country rock metagranites. Garnet porphyroblasts from the UHP metagranite show evident core-rim zoning or patchy structures in back scattered electron

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(BSE) images, major and trace element profiles. These observations provide a sound basis for

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distinction between garnets of different origins in the UHP granitic gneiss, with insights into

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the P-T-t regime of deeply subducted continental crust in the continental collision orogen.

2. Geological setting

The Dabie-Sulu orogenic belt is located in east-central China (Fig. 1), and was built by the

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continental collision between the North China Block and the South China Block in the Triassic (e.g., Wang et al., 1995; Cong, 1996; Liou et al., 1996; Li et al., 1999; Zheng et al., 2003, 2009). The orogenic belt is separated into eastern and western segments by the Tanlu Fault, and are referred to as the Sulu and Dabie orogens, respectively (Fig. 1a). The Dabie orogen extends across the Anhui, Henan and Hubei provinces in the west. It consists of a series of fault-bounded metamorphic units that can be subdivided into five main tectonic zones from north to south (Zheng et al., 2005a): (1) the Beihuaiyang 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 eclogite-facies zone, and (5) the Susong low-T/high-P blueschist-facies zone.
The eclogite occurs not only as small lenses and enclaves in the granitic orthogneiss in the southern unit, but also as large-scale blocks interlayered with the granitic orthogneiss, biotite

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ACCEPTED MANUSCRIPT paragneiss or marble in the northern unit (Fig. 1b). Coesite and its pseudomorphs are observed as inclusions in garnet, jadeite and dolomite from the eclogites, marbles and surrounding gneisses (e.g. Gao et al., 2012a; Liou et al., 1997; Liu et al., 2001; Wang et al.,

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1989; Wang and Liou, 1991; Schertl and Okay, 1994). Petrological studies suggest that the

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peak metamorphic pressures for the UHP eclogites are greater than 3.0 GPa at temperatures of 700 to 800 °C, based on garnet-clinopyroxene Fe-Mg partitioning geothermometry (Okay,

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1993; Cong et al., 1995; Carswell et al., 1997). The granitic gneiss was re-equilibrated under P-T conditions of 400±50oC and 400 MPa (Cong et al., 1995; Liu et al., 2003), corresponding to retrograde metamorphism at low amphibolite-facies. However, several observations

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indicate that the granitic orthogneiss shares the same UHP metamorphic evolution as the eclogite-paragneiss units that are enclosed by it. Carswell et al. (2000) reported the occurrence

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of rutile inclusions in titanite, high-Ca garnet (XCa up to 0.49) and high-Si phengite (up to 3.49) in zoisite from the granitic orthogneiss at this locality. In addition, Liu et al. (2001) discovered coesite inclusions in zircon from all types of para- and orthogneisses in the central

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Dabie zone. Zircon U-Pb dating for the granitic orthogneiss also yields two groups of Triassic

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ages of ~240 Ma and ~220 Ma (e.g., Zheng et al., 2005b). A detailed description of the eclogite, gneiss and jadeite-quartzite at Shuanghe has been

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made by Okay (1993), Cong et al. (1995) and Liou et al. (1997). The UHP eclogite slice at Shuanghe is separated into two units by a dextral strike-slip fault (Fig. 1b). Both units, with outcrop area about 1 km2, are surrounded by the UHP granitic orthogneiss. The present study deals with garnet-bearing granitic orthogneiss at Shuanghe. Three samples (95M16, 12DB37

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and 12DB38) were collected from the granitic orthogneiss in the northern unit, whereas another two samples (95M18 and 95M19) were collected from the granitic orthogneiss in the southern unit (Fig. 1b). The five granitic gneisses have the similar mineral assemblages and modal abundances. They are mainly composed of plagioclase, K-feldspar and quartz with minor amounts of garnet, muscovite, biotite, titanite and epidote/allanite (Fig. 2). The detailed modal abundances (%) for minerals in granitic gneisses are listed in Table 1. However, detailed observations under the microscope show that the granitic gneiss in the northern unit contains fewer hydrous minerals such as epidote, muscovite and biotite than those in the southern unit (Table 1).
Mineral abbreviations used in this paper are after Whitney and Evans (2010). Abbreviations for trace elements follow the common conventions: light rare earth elements (LREE), middle 5

ACCEPTED MANUSCRIPT rare earth elements (MREE) and heavy rare earth elements (HREE).

3. Analytical methods

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3.1 Whole-rock major elements

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The whole-rock major elements were analyzed at ALS Mineral/ALS Chemex Co. Ltd. at Guangzhou, using XRF spectrometry. Fused glass disks with lithium borate were used and

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repeated analyses of the standards GSR-2 and GSR-3 indicate that the analytical precisions were better than ±0.01%.

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3.2 Major and trace elements in garnet and other minerals

Garnet and mineral inclusions therein from the target samples were carefully examined in

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thin sections under a microscope and by Raman spectroscopy before major and trace element analyses were obtained. The laser Raman analysis was made on a Renishaw RM 2000 Raman spectrometer at CAS Key Laboratory of Crust-Mantle Materials and Environments in

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University of Science and Technology of China, Hefei. The beam size for Raman

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spectroscopy was ~2 μm.

Major element mapping of garnet was undertaken using the Fei Quanta 450 FEG scanning

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electron microscope (SEM) at State Key Laboratory of Geological Processes and Mineral Resources in China University of Geosciences, Wuhan, based on energy dispersive spectrometry (EDS) using an Oxford Inca X-Max 50 silicon drift detector (SDD). The

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measurements were carried out with an accelerating voltage of 20 kV, a spot size of 6.0 μm and a working distance of 12 mm. Major element analyses were made for some garnet, titanite, allanite, epidote, micas and feldspar using a JEOL JXA-8100 electron microprobe (EMP) at State Key Laboratory of Mineral Deposits Research in Nanjing University, Nanjing. Major elements for other garnets were also analyzed on a JEOL JXA-8230 EMP at Hefei University of Technology, Hefei. The working conditions were set at 15 kV of accelerating voltage and a beam current of 2×10-8 A, with a beam size of 1-5 μm. Each element includes a background acquisition of ~ 10-20s (gas blank) followed by 20s data acquisition from the sample. The precision for most major elements (with atomic numbers greater than 10 and concentrations higher than 10%) are better than ±2% (1σ). Trace elements in garnet and titanite were analyzed by means of the laser ablation-inductively coupled plasma mass spectrometer (LA-ICPMS) at State Key Laboratory

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ACCEPTED MANUSCRIPT of Geological Processes and Mineral Resources in China University of Geosciences, Wuhan. These analyses were made on the same garnet grains that had been analyzed for major elements by EMP, and BSE images were used to efficiently avoid cracks and mineral

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inclusions. Detailed operating conditions for the laser ablation system and the ICP-MS

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instrument and data reduction are the same as described by Liu et al. (2008). Laser sampling was performed using a GeoLas 2005 with a diameter of ~ 24 or 44 μm depending on the grain

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size of the target mineral, to a depth of ~10-20 μm. The laser frequency was 8 Hz and the energy density was 14 J·cm-2. An Agilent 7500a ICP-MS instrument was used to acquire ion-signal intensities. Helium was applied as a carrier gas. Argon was used as the make-up

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gas and mixed with the carrier gas via a T-connector before entering the ICP. Nitrogen was added into the central gas flow (Ar+He) of the Ar plasma to decrease the detection limit and

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improve precision (Hu et al., 2008).

Each analysis incorporated a background acquisition of approximately 20s (gas blank) followed by 50s data acquisition from the sample. The Agilent Chemstation was utilized for

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the acquisition of each individual analysis. Off-line selection and integration of background

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and analyzed signals, and time-drift correction and quantitative calibration were performed by ICPMSDataCal (Liu et al., 2008, 2010b). Trace element concentrations were calibrated by

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using 29Si as an internal calibrant and NIST SRM610 as the reference material. The precision and accuracy of the NIST-610 analyses are ±2-5% for most elements at the ppm concentration level. Element contents were calibrated against multiple reference materials (BCR-2G, BIR-1G and BHVO-2G) without applying internal standardization (Liu et al., 2008). The

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preferred values of element concentrations for the USGS reference glasses are from the GeoReM database (http://georem.mpch-mainz.gwdg.de/). Analyses of USGS rock standards (BCR-2G and BHVO-2G) indicate that the precision and accuracy (1σ) are better than ±10% for trace elements, and ±2% for major elements. Limits of detection (LOD) for these USGS standards were described in detail by Gao et al. (2002). LOD for each element and analysis were calculated individually as three times the standard deviation of the background signal (taken before ablation) divided by element sensitivity during the respective ablation. For non-significant analytical signals, LOD values are also reported (marked by < LOD). It has to be mentioned that the area and depth of laser ablation are much bigger than the volume sampled by the EMP analysis. Hence, the major element concentrations determined by EMP and LA-ICPMS are not exactly the same. Because the EMP analyses have better spatial resolution, and also may have higher precision than the LA-ICPMS analyses, the EMP results are used in this study for major element compositions. 7

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3.3 Zircon U-Pb ages and trace elements Zircon U-Pb dating and trace element analyses were simultaneously performed by the

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LA-ICPMS in-situ method at State Key Laboratory of Geological Processes and Mineral

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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

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Agilent 7500a ICP-MS was used to acquire ion-signal intensities. Detailed instrumental conditions and data acquisition were described by Liu et al. (2010b, 2010c) and Zong et al. (2010). For zircon trace element and U-Pb isotope analyses, the blank was very low because

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high purity argon and helium was used. The ICP-MS measurements were carried out by using time-resolved analysis and peak hopping at one point per mass and the dwell time for each isotope was set at 6 ms for Si, Ti, Nb, Ta, Zr and REE, 15 ms for 204Pb, 206Pb, 207Pb and 208Pb,

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and 10 ms for 232Th and 238U. Each spot analysis includes 20s of background acquisition and 40s sample data acquisition. Trace element concentrations were calibrated by using 29Si as an

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internal calibrant and NIST SRM610 as a reference material. The precision and accuracy of

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NIST-610 analyses are ±2-5% for most elements at the ppm concentration level. Isotope ratios, including 207Pb/206Pb, 206Pb/238U, 207Pb/235U and 208Pb/232Th, were calculated

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using Glitter 4.0 software (Macquarie University), which were then corrected using the zircon 91500 as an external calibrant with a

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Pb/238U age of 1065.4±0.6 Ma (Wiedenbeck et al.,

1995). All the measured isotope ratios of zircon 91500 were regressed over the course of the analytical session and used to calculate correction factors. These correction factors were then

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applied to each sample in order to correct both instrumental mass bias and depth-dependent elemental and isotopic fractionation. The common Pb correction was carried out by using the EXCEL program of ComPbCorr#_151 (Andersen, 2002), assuming that the observed 206

Pb/238U, 207Pb/235U and

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Pb/232Th ratios for a discordant zircon can be accounted for by a

lead loss at a defined time. Apparent and discordant U-Pb ages were calculated by the ISOPLOT program (Ludwig, 2003).

4. Results 4.1 Petrography The occurrence of quartz and feldspar in the target granitic gneiss shows petrographic evidence for partial melting. In most cases the two minerals are found in coarse-grained, xenomorphic and hypidiomorphic blocks. In some cases, plagioclase occurs as veinlets along

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ACCEPTED MANUSCRIPT the quartz-quartz and quartz-feldspar grain boundaries, showing low feldspar-quartz-quartz dihedral angles (Fig. 2a). Quartz is also fine-grained, or occurs as elongate polycrystalline aggregates filling in plagioclase-plagioclase grain boundaries (Fig. 2c). Some highly cuspate

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or vein-like quartz is also found to occur along plagioclase-biotite or plagioclase-plagioclase

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grain boundaries (Fig. 2b and c). EMP analyses show that plagioclase in the studied orthogneiss is generally Na-rich, with variable but generally low Ca contents (Table 2).

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Garnet grains in samples 12DB37, 12DB38 and 95M16 from the northern unit are idioblastic and euhedral with hexagonal prophyroblasts (Figs. 2d-e and 3a-c), whereas garnets in samples 95M18 and 95M19 from the southern unit occur as islands or peninsulas (Figs. 2f,

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2h and 3d-f). Quartz, K-feldspar and plagioclase are the most common mineral inclusions in the garnet porphyroblasts (Fig. 3a and f). Multiple phase inclusions of albite+K-feldspar are

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also occasionally found in the garnet (Fig. 3f). Fractures filled with later chlorite and carbonates are common in garnet (Fig. 4d). Most garnet grains are partially replaced by epidote, biotite and plagioclase (Fig. 2e and g).

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Epidote/zoisite also occurs as large prophyroblasts (>500 µm) or small inclusions (500 µm) (Fig. 2g). Polygenetic titanites have been identified in sample 95M19 by Gao et al. (2012b), in which magmatic and metamorphic origin were distinguished using BSE images, mineral inclusions, major and trace elements and U-Pb ages. The metamorphic

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titanite was explained as the product of retrograde metamorphism during exhumation.

4.2 Major and trace element zoning in garnet Major element X-ray mapping was made on two garnet grains in samples 12DB37 and 12DB38 (Fig. 4). Major element profiles were measured on six grains in five samples (Figs. 3 and 5; supplementary Table S1). BSE images show that these garnets have patchy microstructures (Figs. 3 and 4). Major element X-ray maps and profiles indicate that garnet crystals are strongly zoned in Ca and Fe, and weakly zoned or unzoned in Mn and Mg (Figs. 4 and 5). For instance, the contents of almandine vary from 20.3 to 39.3 mol.% and grossular from 20.7 to 48.0 mol.% for garnet grain 12DB38-2, but pyrope contents only vary from 0.20 to 1.29 mol.% (Table S1; Fig. 5c). Major element profiles on the six garnet grains show that the dark and bright BSE domains have distinct compositions, which are denoted as Grt-I and

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ACCEPTED MANUSCRIPT Grt-II, respectively (Figs. 3-6). The Grt-I domains occur as a broad domain in the center, with low almandine and pyrope concentrations, and high grossular concentrations. In contrast, the Grt-II domains occur as rims of euhedral garnet, or as patches within the Grt-I domains. They

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have relatively high almandine and pyrope concentrations and low grossular concentrations.

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Spessartine in the two garnet grains from samples 12DB37 and 12DB38 is weakly zoned (Fig. 4b and c). However, spessartine in the other garnets from samples 95M16, 95M18 and 95M19

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are strongly zoned, with high contents in the Grt-II domains (Figs. 5a, d, e and f).

Trace elements analyses show that Grt-I and Grt-II domains also have much different

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compositions, except in grain 12DB38-2 (Figs. 7 and 8). The Grt-I domains have high Ti (342-939 ppm) and MREE (119-296 ppm) concentrations, and are low in Sc (4.94-32.8 ppm),

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Cr (0-2.72 ppm), Y (948-3901 ppm) and HREE (335-718 ppm) (Table 3; Fig. 7). The chondrite-normalized REE patterns show increasingly enriched LREE to MREE, negative Eu anomalies with Eu/Eu* = 0.22-0.52, and flat MREE-HREE distributions with (Yb/Dy)N =

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1.68-2.86 (Fig. 8). In contrast, the Grt-II domains have low Ti (132-515 ppm) and MREE

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(78-415 ppm) concentrations, and high Sc (12.4-141 ppm), Cr (0-10.6 ppm), Y (1079-5004 ppm) and HREE (763-3044 ppm) (Table 3; Fig. 7). The chondrite-normalized REE patterns

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show depletion in LREE, negative Eu anomalies with Eu/Eu* = 0.09-0.60, and steep middle to heavy REE patterns with (Yb/Dy)N = 2.40 to 12.5 (Fig. 8). Both domains exhibit obviously negative Eu anomalies with Eu/Eu* = 0.09-0.60. For garnet grain 12DB38-2, Grt-I and Grt-II domains have comparable trace element compositions. There is no obvious distinction

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between the two domains in trace elements such as Sc, Ti, Cr, Y and REE (Table 3; Fig. 7).

4.3 Zircon U-Pb ages and trace elements Detailed U-Pb, Sm-Nd and Rb-Sr geochronological studies have been published for the UHP granitic gneiss at Shuanghe (e.g., Li et al., 2000; Chavagnac et al., 2001; Ayers et al., 2002; Zheng et al., 2005b). In this study, we have only analyzed some zircon grains from three samples: 12DB37, 12DB38 and 95M19. These zircon grains are short to long prismatic, light yellow and translucent, with lengths of 100 to 200 μm. They are euhedral, and display obvious core-rim structure (Fig. 8a, d and g). CL images show that most grains have large cores with obscured oscillatory zoning and very thin rims without zonation. Raman analysis on these zircon grains indicates the presence of many separate mineral inclusions (e.g., apatite, quartz, feldspar and muscovite) and MS inclusions (e.g., Qtz+Kfs, Qtz+Pl, Mus+Kfs) in the 10

ACCEPTED MANUSCRIPT core, but only a few grains of quartz and apatite in the rim. Forty-eight LA-ICPMS analyses were made on zircon core and/or rim from the three metagranites for U-Pb ages and trace elements (Tables 4 and 5). The U-Pb dating yields

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similar intercept ages, with upper intercept ages of 771±84 to 809±72 Ma and lower intercept

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ages of 215±29 Ma to 255±140 Ma (Fig. 9b, e and h). However, there are different age distributions for the three samples. Most zircon from the northern unit orthogneiss (12DB37

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and 12DB38) exhibits discordant U-Pb ages of 316 to 809 Ma. Concordant Triassic ages of 234 to 257 Ma are only obtained from one spot (#20) among eighteen analyses from sample 12DB37 and two spots (#7 and #14) among sixteen analyses from sample 12DB38. On the

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other hand, seven spots among fourteen analyses from the southern unit orthogneiss (95M19) yield concordant Triassic ages of 224 to 254 Ma (Fig. 9).

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The analyses on the zircon cores from three orthogneisses give discordant U-Pb ages of 270 to 809 Ma, and relatively high Th and U contents with variably high Th/U ratios of 0.14 to

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1.05 (Table 4). Their chondrite-normalized REE patterns show features characteristic of

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magmatic zircon, with positive Ce anomalies (Ce/Ce*=1.1-328), negative Eu anomalies (Eu/Eu*=0.05-0.40), and steep MREE-HREE patterns (Fig. 9c and f). Therefore, these zircon

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cores are interpreted as protolith domains of magmatic origin, but likely suffered varying degrees of modification by metamorphic recrystallization depending on the accessibility of metamorphic fluids.

The Triassic U-Pb ages of 257 to 222 Ma can be further categorized into two subgroups of

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243-257 Ma and 227-234 Ma, respectively, which were denoted as Zir-I and Zir-II. Their weighted mean ages for these two subgroups are 248 ± 6 (Zir-I) and 227 ± 10 Ma (Zir-II), respectively. These two subgroups of Triassic zircons show similarly low Th/U ratios of 0.01 to 0.21, variable Th contents of 3.26 to 327 ppm and high U contents of 327 to 2688 ppm. In the chondrite-normalized REE distribution diagram, both of them exhibit pronounced positive Ce anomalies (Ce/Ce* = 33.3 to 142), negative Eu anomalies (Eu/Eu* = 0.15 to 0.71), and steep MREE-HREE patterns with lower MREE and HREE contents relative to the magmatic cores (Fig. 9c, f and i). Ti-in-zircon thermometry for the Zir-I and Zir-II yields similar temperatures of 588-788oC (Table 5).

5. Phase equilibrium modeling and P-T evolution Numerous studies show that the mineralogical evidence for UHP metamorphism in the

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ACCEPTED MANUSCRIPT continental subduction zone is mainly recorded in eclogite and garnet peridotite, but their country rocks, often ordinary gneiss, show only very rare hints of the UHP metamorphism in common rock-forming minerals (Massonne, 2009). This has often resulted in an

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underestimate of peak P-T conditions for the country rocks (e.g., Cong et al., 1995; Liu et al.,

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2003). However, many studies have demonstrated that such gneisses do share the same P-T paths during continental subduction and exhumation. For example, based on the coesite

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inclusions in zircon from all types of para- and orthogneisses in the Dabie-Sulu orogenic belt, the country rock orthogneiss would also have experienced the same UHP metamorphic evolution as the eclogite-paragneiss units that are enclosed by it (Ye et al., 2000; Liu et al.,

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2001; Liu and Liou, 2011).

The construction of P-T pseudosections has turned out to be a valuable tool in

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understanding the phase relations, changes in mineral compositions, and processes of partial melting, metamorphic dehydration and so on under high-grade metamorphic conditions (e.g., Tinkham and Ghent, 2005; Massonne, 2009; Wei et al., 2010; Zhou et al., 2014). In order to

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calculate the phase relations of metagranites for a P-T range of 1.0-4.0 GPa and 500-900 °C,

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the PERPLE_X computer software package (see Connolly, 1990, 2005; version downloaded from http://www.perplex.ethz.ch, August 17, 2006) was used in this study. The model system

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NCKMnFMASHT is chosen to calculate the P-T pseudosection for the UHP granitic gneiss at Shuanghe in the Dabie orogen. The data set of Holland and Powell (1998; updated in 2002) for many mineral endmembers (file hp02ver.dat) is applied. The following solid-solution series, which are compatible with the above data set, were selected from this file: amphibole

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(White et al., 2003), biotite (Powell and Holland, 1999), clinopyroxene (Green et al., 2007), garnet (White et al., 2005), ilmenite (White et al., 2000), phengite (Coggon and Holland, 2002) and feldspar (Holland and Powell, 2003). For the fluid phase, H2O (Holland and Powell, 1998) and the silicate melt model (White et al., 2001) were considered. These equilibrium thermodynamic calculations can be used to model the mineral assemblages on some time and length scales (Guiraud et al., 2001). The NCKMnFMASHT P-T pseudosection calculated for the Shuanghe UHP metagranite (12DB38) is presented in Fig. 10. Also shown are the change of mineral assemblages, the isopleths of Ca and Fe in garnet and associated modal contents (vol.%) for melt and some minerals (garnet, phengite and biotite) in the P-T plane. The effective bulk compositions used in the P-T construction are obtained from the whole-rock compositions analyzed by XRF in sample 12DB38 (Table 2). Based on the low grossular and high almandine components (gr: 20, alm: 40) in the Grt-II domains, retrograde conditions are plotted to 1.2 GPa / 700 oC (point 12

ACCEPTED MANUSCRIPT B) in the field of K-feldspars + biotite + quartz + garnet + orthopyroxene + ilmenite + melt (Fig. 10a). This is consistent with the observation for mineral assemblages in sample 12DB38, except orthopyroxene which may have broken down to epidote + plagioclase during later

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decompression (Fig. 2e). Petrographic observations show that the mineral assemblage is

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garnet, K-feldspar, plagioclase, quartz, titanite, epidote and ilmenite. However, the composition of Grt-I domains with the highest grossular and lowest almandine (gr: 48, alm:

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20) is difficult to plot. The isopleths of almandine (20-50%) in the calculated garnet for sample 12DB38 can only be plotted to ~2.7 GPa at most in the coesite field (Fig. 10a). On the other hand, the highest grossular component (gr: 48) in the Grt-I domains can be plotted as

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high as 3.5 GPa in the UHP field of coesite stability (Fig. 10a). The inconsistency in the isopleths of grossluar and almandine for the Grt-I domains is probably related to the faster

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diffusion rate of Fe than Ca at the high temperature of 700oC during the initial exhumation, which will be further discussed later. The highest grossular content in garnet from the granitic gneiss commonly corresponds to the peak pressure during continental subduction (discussed

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in subsection 6.2.1). In addition, the peak P-T conditions for eclogite hosted by the granitic

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gneiss in this area has been estimated to be >3.3 GPa at temperatures of 750-800oC by means of the garnet-omphacite Fe-Mg exchange thermometer (Cong et al., 1995; Carswell et al.,

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1997). Thus the peak conditions can be plotted to 3.3 GPa/750oC (Point A) in the field of K-feldspars + phengite + garnet + omphacite + titanite + coesite (Fig. 10a), based on the highest grossular component (gr: 50) and the peak pressure of 3.3 GPa (Cong et al., 1995; Carswell et al., 1997). The mineral assemblages at peak conditions are consistent with the

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observation that many coesite and omphacite inclusions occur in zircon from the gneisses in this area (Liu et al., 2001), though no coesite or omphacite were found in this study.


6. Discussion 6.1 Partial melting of UHP metamorphic rocks Experimental studies of felsic rocks at 800-900oC and 0.5-5 GPa demonstrate that melting would happen at the eclogite-amphibolite facies transition during exhumation of the deeply subducted continental crust (Auzanneau et al., 2006). In fact, partial melting of UHP metamorphic rocks during continental collision is increasingly recognized in the field and under the microscope (Zheng et al., 2011 and references therein; Gao et al., 2012a, 2014; Chen et al., 2013a, b; Li et al., 2014; Liu et al., 2014; Wang et al., 2014). For example, felsic veinlets (Zhao et al., 2007; Xia et al., 2008; Chen et al., 2013b; Liu et al., 2014), migmatitic 13

ACCEPTED MANUSCRIPT leucosomes and granitic pegmatities (Wallis et al., 2005; Liu et al., 2010a; Zong et al., 2010; Xu et al., 2013; Song et al., 2014; Wang et al., 2014) are found in many UHP metamorphic rocks in the Dabie-Sulu orogenic belt. The time of partial melting in these UHP metamorphic

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rocks has been constrained by a combined study of zircon U-Pb dating and trace element

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analysis on peritectic zircon from UHP gneisses, migmatitic leucosomes and pegmatite in the Sulu orogeny. This study provides late Triassic ages of 217-228 (Liu et al., 2010a; Zong et al.,

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2010; Chen et al., 2013b; Li et al., 2013; Wang et al., 2014), slightly younger than the known ages of 225-240 Ma for major UHP metamorphism in the Dabie-Sulu orogenic belt (Zheng et al., 2009; Liu and Liou, 2011). Therefore, extensive partial melting of UHP metamorphic

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rocks did occur in the early stage of exhumation from the UHP to HP eclogite-facies regime in this orogenic belt.

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In the case of eclogite and its associated paragneiss and marble at Shuanghe in the Dabie orogen, coesite inclusions and its pseudomorphs have been observed in both garnet and dolomite in previous studies (Wang et al., 1989; Wang and Liou, 1991; Schertl and Okay,

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1994). Many types of MS inclusions, mainly composed of K-feldspar, quartz, plagioclase,

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epidote and barite, were also found in garnets from the UHP eclogites and paragneisses at Shuanghe (Gao et al., 2012a, 2013, 2014; Liu et al., 2013), demonstrating the onset of

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dehydration melting in the UHP metamorphic slice during the continental collision. However, direct evidence for partial melting of the granitic gneiss at Shuanghe was still lacking. In this study, we have found petrographic evidence for partial melting, in the form of anatectic microstructures in thin sections under the microscope (Figs. 2 and 3). For instance, feldspar

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and/or quartz occur as thin films or wide bands along the grain boundaries of quartz, feldspar and biotite (Fig. 2a and b). The highly cuspate or veinlike quartz and plagioclase are filled in the triple junctions and the grain fractures, forming blebs, branches and veins (Fig. 2c). MS inclusions of albite + K-feldspar also occur in some garnet grains (Fig. 3f). These petrographic features indicate that the feldspar and quartz would have grown from anatectic melts during the partial melting of granitic gneiss. In addition, muscovite is often partly replaced by biotite-feldspar aggregates (Fig. 2b), or with relicts in coexistence with cuspate feldspars (Fig. 2c), suggesting that the anatectic melts may be derived from dehydration melting due to the breakdown of phengitic muscovite. The calculated P-T pseudosection for granitic gneiss 12DB38 also indicates that partial melting occurred during exhumation of this continental crust (Fig. 10). Along the P-T path from A to B, the modeled melt abundance increases from 0 to ~2.0 vol. % (Fig. 10b), indicating dehydration melting at 2.0-2.5 GPa during exhumation. The modeled phengite 14

ACCEPTED MANUSCRIPT content decreases sharply from >8.5 vol. % to 3.0 GPa at temperatures >700 oC. This is consistent with the previous estimation for the peak P-T conditions of 3.0 GPa at temperature of 700 to 800 °C based on garnet-clinopyroxene Fe-Mg partitioning geothermometry in the hosted UHP eclogite (Okay, 1993; Cong et al., 1995; Carswell et al., 1997). Thus the Grt-I domains grew in the mineral

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assemblage Kfs + Phg + Grt + Omp + Ttn + Coe at peak conditions of >3.0 GPa and 700-800oC (Fig. 10a).

Flat major element patterns are evident in most Grt-I domains, especially those from the northern unit; all Grt-I domains in the granitic orthogneiss are nearly identical in composition (Fig. 6). This is very common in high-grade metamorphic garnet (e.g., Spear, 1993; Nyström and Kriegsman, 2003), indicating that the compositions of Grt-I domains became homogenized at a given stage during continental subduction-zone metamorphism. It has been suggested that rapid diffusion at temperatures higher than 650oC would partially or completely flatten the chemical zoning that grew during prograde metamorphism (Spear, 1991; Carlson and Schwarze, 1997; Carlson, 2002; Caddick et al., 2010). The temperature at peak pressure for the granitic orthogneiss at Shuanghe in the P-T pseudosection was estimated to be 700-800 oC (Fig. 10a). This high temperature would lead to rapid diffusion in garnet, producing nearly flat chemical zoning in Grt-I domains (Fig. 5), especially for Mn (Fig. 5a 16

ACCEPTED MANUSCRIPT and c). In this case, the Grt-I domains would have started to grow during prograde subduction and have been partially homogenized at the last stage of subduction.

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6.2.2 Peritectic growth during exhumation (Grt-II)

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The major element profiles across garnet show an abrupt decrease in grossular and a sudden increase in almandine from Grt-I to Grt-II (Fig. 5), indicating decrease of pressure and

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temperature during exhumation (Hermann, 2002; Kohn, 2003; Xia et al., 2012). The isopleths of grossular and almandine for Grt-II plot in the field of Kfs + Qtz + Bt + Grt + Opx + Ilm + melt (Fig. 9a). Partial melting of the Shuanghe UHP metagranites is indicated by the anatectic

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microstructures (Fig. 2a, b and c) and modeled P-T pseudosection (Fig. 9). In this regard, the Grt-II domains develop after metamorphic garnet (Grt-I) in the presence of anatectic melt

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during exhumation. Based on melting experiments at 4.0 GPa and 800-1100oC (Perchuk et al., 2005, 2008), the characteristic patchy microstructure of Grt-II (Fig. 3) is considered as diagnostic of peritectic garnet growth in the presence of hydrous melts. Similar patchy

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microstructures are also observed in peritectic garnets from the partial melted calc-gneisses in

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the Dabie orogen (Liu et al., 2014). This garnet also displays new domains with higher contents of almandine, spessartine, MREE, HREE and Y but lower contents of grossular,

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pyrope, P, Sc, Ti, V and Zr.

Some Grt-II domains show an increase in spessartine component (Fig. 5a, d, e and f). In general, Mn contents are typically low in the whole-rock and Mn is preferentially incorporated into garnet with respect to other major phases, resulting in progressive

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fractionation from the reactive matrix during garnet growth (e.g., Hollister, 1966; Caddick and Kohn, 2013). Therefore, elevated spessartine in some Grt-II domains may be related to the resorption of preexisting Grt-I components and/or back-diffusion of Mn into the residual grain (Kohn and Spear, 2000). Trace element zoning in garnet has been used to reconstruct the history of metamorphic rocks, and corresponds to changes in fluid action (e.g., Hervig and Peacock, 1989; Hickmott et al., 1992), trace element partition coefficients between garnet and matrix (e.g., Schwandt et al., 1996), and changes in mineral assemblage (e.g., Hickmott et al., 1987; Hickmott and Spear, 1992; Konrad-Schmolke et al., 2007, 2008a, 2008b; Zhou et al., 2011). Especially, trace element zoning in garnet can provide a sensitive monitor of dissolution and reprecipitation during mineral-fluid reaction (Kohn, 2003). During garnet resorption, release of garnet-compatible elements such as Mn, Y and HREE would elevate the activity of these elements in the matrix (Pyle and Spear, 1999). When garnet begins to grow again, the 17

ACCEPTED MANUSCRIPT garnet-compatible elements would again become strongly fractionated into garnet, resulting in an abrupt increase in these compatible elements. On the other hand, garnet-incompatible elements such as Ti and MREE would enter matrix phases and decline abruptly at the

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resorbed rim (Yang and Rivers, 2002). As illustrated in Figs. 7 and 8, the Grt-II domains show

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decreased Ti and MREE but increased Sc, Cr, Y and HREE compared to the Grt-I domains. This type of trace element zoning can be well explained by the processes of dissolution and

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reprecipitation. Trace elements such as Sc, Cr, Y, and HREE are compatible in garnet, so that resorption of the preexisting Grt-I domains increases their concentrations in the Grt-II domains (e.g., Kohn et al., 1997; Yang and Rivers, 2002). In contrast, Ti and MREE are

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incompatible in garnet, so that the resorption causes the concentrations of Ti and MREE to decrease in the Grt-II domains. Based on the variations of modal mineral abundances in

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pseudosection calculations, the peritectic reaction (1) might have occurred during dissolution of Grt-I domains and growth of Grt-II domains.

The coupled dissolution and reprecipitation are essentially the recrystallization in the

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presence of a fluid/melt phase (Putnis, 2002; Geisler et al., 2007; Xia et al., 2010), where the

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fluid/melt plays an important role in dissolving the preexisting minerals. It can serve as a metasomatic agent to transport components from reactant to product when the preexisting

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minerals are far from chemical equilibrium with the infiltrated fluid/melt system (Putnis, 2002). New garnet can overgrow a garnet in the presence of fluid/melt, which involves the dissolution of metamorphic minerals by water liberated from hydrous minerals. This mineral-water interface process can proceed through three mechanisms: (1) diffusion, (2)

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dissolution-reprecipitation, and (3) chemical reaction involving the formation of garnet (Putnis, 2002; Geisler et al., 2007; Martin et al., 2011; Chen et al., 2015). Diffusion is a thermally activated process and can flatten the growth zoning to different degrees depending on temperature and duration (Kohn, 2003), but it cannot modify the original morphology of garnet. On the other side, dissolution-reprecipitation and chemical reaction can modify the garnet crystal to an anhedral or skeletal morphology (Tomaschek et al., 2003; Geisler et al., 2007). During the reaction, grain boundaries and fractures within the garnet interiors provide pathways for chemical exchange, especially if they are hydrated, discordant, or filled with hydrates or fluid (Geisler et al., 2007; Xia et al., 2013). These are consistent with the skeletal or peninsula appearance for the studied garnets (Fig. 3), where the anatectic melt produced during exhumation would have played an important role in the resorption of Grt-I domains and reprecipitation of Grt-II domains. The spatial distribution of patchy microstructures in the Grt-II domains witnesses the possible infiltration of anatectic melts, which is indicated by the 18

ACCEPTED MANUSCRIPT occurrence of Grt-II domains in the garnet rims, fractures or around melt inclusions, such as quartz or MS inclusions of K-feldspar + albite (Fig. 3). The anatectic melt provides a medium for exchange of compatible and incompatible elements between the preexisting Grt-I domains

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and the overgrown Grt-II domains, resulting in increased or decreased concentrations of

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different elements in the Grt-II domains (Figs. 7 and 8).

In summary, the zoned garnets in the UHP granitic orthogneiss from Shuanghe in the Dabie

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orogen are inferred to have grown in two stages. The first generation of garnet (Grt-I) was produced by metamorphic growth during subduction. Some of the Grt-I cores suffered subsequent diffusional relaxation at the peak conditions to result in nearly flat chemical

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zoning. The second generation of garnet (Grt-II) was generated by peritectic reaction during exhumation, involving the dissolution-reprecipitation of Grt-I domains in the presence of

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anatectic melts.

7. Constraints on the time of garnet growth

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Many mineral Sm-Nd and Lu-Hf isochron studies have been made to date garnet growth

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during subduction-zone metamorphism (e.g., Duchêne et al., 1997; Li et al., 2000; Blichert-Toft and Frei, 2001; Thöni, 2002; Vry et al., 2004; Zhao et al., 2006; Kylander-Clark

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et al., 2007; Cheng et al., 2008, 2011, 2015). However, many factors may violate the validity of isochron ages, yielding uncertainties on the Lu-Hf and Sm-Nd isochron ages (e.g., Thöni, 2002; Baxter and Scherer, 2013; Zheng et al., 2002, 2009; Wang et al., 2010). These include:

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(1) presence in garnet of mineral inclusions with high parent/daughter ratios; (2) presence of different generations of garnets in the same rock (or the same crystal); and (3) differential resetting of mineral isotope systems during retrograde metamorphism. Alternatively, some researchers have tried to date the multistage growth of garnet by dating mineral inclusions like zircon, monazite and titanite in different garnet domains (e.g., Cutts et al., 2010; Mahan et al., 2006; Zhou et al., 2011). It is possible to relate the ages of mineral inclusions to distinct stages of garnet growth if a genetic relationship between garnet and dated minerals is established (Hermann and Rubatto, 2003). As a major phase in eclogite, granulite or sometimes gneiss, garnet is an important sink of trace elements such as Y and HREE (e.g., Hickmott et al., 1987; Bea et al., 1994; Hermann, 2002; Otamendi et al., 2002; Rubatto and Hermann, 2003), though zircon can also be a sink of MREE and HREE as a minor phase. The relative depletion of HREE in zircon is commonly taken as evidence for the presence of garnet during metamorphic dehydration and partial melting (Otamendi and Patiño

19

ACCEPTED MANUSCRIPT Douce, 2001; Rubatto, 2002; Whitehouse and Platt, 2003). Therefore, the growth of either metamorphic or peritectic minerals in the presence of garnet during eclogite- and granulite-facies metamorphism would typically result in flat or depleted HREE patterns for

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the coexisting phases. This is referred to as the garnet effect. For example, HREE depletion is

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prominent for metamorphic zircon in eclogites (Rubatto, 2002) and peritectic zircon in mafic granulites (Whitehouse and Platt, 2003). Zircon and titanite are two common accessory

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minerals in granitic gneisses and are enriched in HREE (Rubatto, 2002; Gao et al., 2012b). In addition, they are good tools for U-Pb geochronology in high-grade metamorphic rocks. Therefore, a combined application of zircon and titanite U-Pb dating on thin sections can be

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used to constrain the time of garnet growth in UHP metamorphic rocks (e.g., Zhou et al., 2011; Chen et al., 2015).

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The zircon U-Pb dating and trace element analysis performed here for three Shuanghe orthogneiss samples (12DB37, 12DB38 and 95M19) yield two subgroups with Triassic ages of 248 ± 6 (Zir-I) and 227 ± 10 Ma (Zir-II), respectively, but with consistent steep

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MREE-HREE distribution patterns (Fig. 9c, f and i). The two subgroup ages are consistent

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with zircon U-Pb ages of 245 ± 3 to 240 ± 2 Ma and 226 ± 4 to 223 ± 2 Ma, respectively, for UHP eclogites from the Dabie orogen (Wu et al., 2006). They correspond to the final

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subduction prior to the onset of eclogite-facies UHP metamorphism and the transition from UHP to HP eclogite-facies during the initial exhumation, respectively. However, the two subgroups of newly grown zircons from our granitic orthogneisses do not show flat HREE patterns due to the garnet effect (Fig. 11a). This may be caused by much smaller amounts of

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garnet in felsic gneisses than in mafic eclogites and granulites. Microscopic observations show that the modal abundances of garnet and zircon in the granitic gneiss are ~ 1% and 700oC for about 20 Myr. In this regard, the UHP slice was exhumed from ~100 km to a 30 km crustal level with a timescale of about ~20 Myr. This timescale is consistent with previous estimates from mineral oxygen isotope exchange (Zheng

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et al., 1998, 2003) and zircon U-Pb dating (Wu et al., 2006) for the UHP metamorphic rocks

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in the Dabie orogen. On the other hand, previous studies have shown that the compositional zoning produced by garnet growth would be partially or completely homogenized at

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temperatures of >680oC via the intracrystalline diffusion of divalent cations (Carlson, 2002; Carlson and Schwarze, 1997). Thus it would be a big challenge for the preservation of major element zonings at 700oC for ~20 Myr during exhumation. The diffusion equation for a

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sphere, which describes the fraction of equilibration f with its surroundings, is (Crank, 1975): 6

2 2 2 2 𝑓 = 1 − (𝜋2 ) ∑∞ 𝑛=1(1/𝑛 )𝑒𝑥𝑝(−𝑛 𝜋 𝐷𝑡/𝑎 )

(2)

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where a is the radius of the sphere, D is the diffusion coefficient and t is the time. Only the first term in the series is significant for large degrees of equilibration: 6

𝑓 ≅ 1 − (𝜋2 ) 𝑒𝑥𝑝(−𝜋 2 𝐷𝑡/𝑎2 )

0.85 ≤ 𝑓 ≤ 1.

(3)

𝑡≅−

𝑎2 𝜋2 𝐷

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equilibration refers to f = 0.95): (1−𝑓)𝜋2

𝑙𝑛 [

6

D

Rearranging to solve for the time required for a given degree of equilibration (e.g. 95%

]

0.85 ≤ 𝑓 ≤ 1.

(4)

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The diffusion coefficient at various P-T conditions can be expressed in the form of an Arrhenian relation as follows,

D = D0 exp {-[Q (1bar) + P×ΔV+]/RT}.

(5)

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As presented by Table 6, at 700oC, the cation Mn has the highest diffusion rate, whereas Ca has the lowest diffusion rate. This is the reason why spessartine components for almost all garnet grains are weakly zoned, whereas the grossular components are obviously zoned (Fig. 5). The diffusion rate of Fe is a little lower than that of Mn, but evidently higher than that of Ca. It will take about 5 to 19 Myr for the almandine zoning to achieve 99% equilibration with grain radii of 200-400 μm at the temperature of 700oC (Table 6). But for grossular zoning, it will take about 34 to 135 Myr to achieve 99% equilibration at 700oC. In this regard, the observed almandine components in the studied garnets were almost at diffusion equilibrium during exhumation, whereas the grossular zoning was diffused to about 67-95% equilibration during ~20 Ma exhumation. This suggests that the peak pressure of 3.3 GPa estimated by the highest grossular content in Grt-I domains are underestimated. It also provides the best explanation for the inconsistency between almandine and grossular components plotted on the P-T psedosection (Fig. 10a). 22

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8. Conclusions (1) Metamorphic and peritectic garnets are recognized from the UHP metagranites in the

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Dabie orogen.

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(2) Metamorphic garnet (Grt-I) grew during prograde subduction and suffered subsequent diffusional relaxation at peak metamorphic conditions. Peritectic garnet (Grt-II)

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was recrystallized from the dissolution of metamorphic Grt-I domains in the presence of anatectic melts during early exhumation.

(3) In situ U-Pb dating of metamorphic zircon and titanite coexisting with garnet,

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together with the MREE and HREE partitioning between garnet and zircon/titanite, provides temporal constraints on the last growth of metamorphic and peritectic garnet at ~240 Ma and

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~220 Ma, respectively.

(4) Partial melting during the exhumation of UHP metagranites is responsible for the anatectic microstructures and modeled P-T pseudosection. The peritectic reaction is inferred

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to proceed in the following way: Garnet (I) + phengite + plagioclase + quartz → garnet (II) +

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Acknowledgments

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biotite + K-feldspar + melt.

This study was supported by funds from the Chinese Ministry of Science and Technology (2015CB856100) and the Natural Science Foundation of China (41272082 and 41590620).

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Thanks are due to Wenlan Zhang for her assistance with EMP analyses, to Wenlong Liu for his assistance with the SEM imaging, and to Wancai Li for his assistance with the LA-ICPMS. We thank editor M. Scambelluri and two anonymous reviewers for critical comments and suggestions, which greatly improved the presentation.

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extent of ultrahigh-pressure metamorphism in the Sulu ultrahigh-pressure terrane of East China: new implications from coesite and omphacite inclusions in zircon of granitic

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gneiss. Lithos 52, 157-164.

Zhang, L., Chen, R.-X., Zheng, Y.-F., Hu, Z.C., 2015. Partial melting of deeply subducted continental crust during exhumation: insights from felsic veins and host UHP metamorphic rocks in North Qaidam, northern Tibet. Journal of Metamorphic Geology 33,

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Zhao, Z.-F., Zheng, Y.-F., Gao, T.-S., Wu, Y.-B., Chen, B., Chen, F.-K., Wu, F.-Y., 2006. Isotopic constraints on age and duration of fluid-assisted high-pressure eclogite-facies recrystallization during exhumation of deeply subducted continental crust in the Sulu orogen. Journal of Metamorphic Geology 24, 687-702. 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., Zhang, J., Dai, L.-Q., Li, Q., Liu, X., 2012. Syn-exhumation magmatism during continental collision: Evidence from alkaline intrusives of Triassic age in the Sulu orogen. Chemical Geology 328, 70-88. Zheng, Y.-F., Fu, B., Li, Y.-L., Xiao, Y.-L. and Li, S.-G., 1998. Oxygen and hydrogen isotope geochemistry of ultrahigh pressure eclogites from the Dabie Mountains and the Sulu 35

ACCEPTED MANUSCRIPT terrane. Earth and Planetary Science Letters 155, 113-129. Zheng, Y.-F., Wang, Z.R., Li, S.G., Zhao, Z.-F., 2002. Oxygen isotope equilibrium between eclogite minerals and its constraint on mineral Sm-Nd chronometer. Geochimica et

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Cosmochimica Acta 66, 625-634.

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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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Zheng, Y.-F., Zhou, J.-B., Wu, Y.-B., Xie, Z., 2005a. Low-grade metamorphic rocks in the 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., Wu, Y.-B., Zhao, Z.-F., Zhang, S.-B., Xu, P., Wu, F.-Y., 2005b. Metamorphic

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effect on zircon Lu-Hf and U-Pb isotope systems in ultrahigh-pressure metagranite and metabasite. Earth and Planetary Science Letters 240, 378-400. Zheng, Y.-F., Chen, R.-X., Zhao, Z.-F., 2009. Chemical geodynamics of continental

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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

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and element mobility in ultrahigh-pressure metamorphic rocks during continental collision. Earth-Science Reviews 107, 342-374. Zheng, Y.-F., Hermann, J. 2014. Geochemistry of continental subduction-zone fluids. Earth, Planets and Space 66, 93; doi:10.1186/1880-5981-66-93.

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Zhou, L.-G., Xia, Q.-X., Zheng, Y.-F., Chen, R.-X., 2011. Multistage growth of garnet in ultrahigh-pressure eclogite during continental collision in the Dabie orogen: constrained by trace elements and U-Pb ages. Lithos 127, 101-127. Zhou, L.-G., Xia, Q.-X., Zheng, Y.-F., Hu, Z.C., 2014. Polyphase growth of garnet in eclogite from the Hong'an orogen: Constraints from garnet zoning and phase equilibrium. Lithos 206-207, 79-99. Zong, K., Liu, Y., Hu, Z., Kusky, T., Wang, D., Gao, C., Gao, S., Wang, J., 2010. Melting-induced fluid flow during exhumation of gneisses of the Sulu ultrahigh-pressure terrane. Lithos120, 490-510.

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Supplementary Table

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Table S1. Major elements of garnets in granitic orthogneiss from Shuanghe in the Dabie

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orogen analyzed by electron microprobe.

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Dabie orogen (modified after Carswell et al., 1997), with details for sampling location.

Fig. 2. Petrographic observations for granitic orthogneiss from Shuanghe in the Dabie orogen.

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(a) Highly cuspate and veinlike plagioclase filling in quartz-quartz boundaries. (b) Thin films of quartz filling in the plagioclase-biotite boundaries. (c) quartz occurring as films and wide bands around the plagioclase. (d) Coarse garnet porphyroblast (grain G2 in

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sample 12DB38) in plane-polarized light photomicrograph, with K-feldspar and quartz included in the garnet, cross-polarized light photomicrograph. (e) Skeletal garnet

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intergrowth with biotite, plagioclase and epidote in sample 95M18, plane-polarized light photomicrograph. (f) Biotite occurs in the center of skeletal garnet. (g) Large grains of epidote/zoisite and titanite with patchy microstructure. (h) Multiphase inclusions of

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Whitney and Evans (2010).

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albite and k-feldspar in garnet from sample 95M19. Mineral abbreviations are from

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Fig. 3. BSE images for garnets from granitic orthogneiss in the Dabie orogen. The white spots denote the EMP analyses, and the red circles denote the domains for LA-ICPMS analyses.

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Fig. 4. EDS maps for Ca, Fe and Mn for two garnet grains from metagranites 12DB37 and 12DB38.

Fig. 5. Major element profiles for six garnet grains from granitic orthogneiss at Shuanghe in the Dabie orogen. (a)- (c) are for samples from the northern unit, and (d)-(f) are for samples from the southern unit.

Fig. 6. Ternary diagrams showing the molecular compositions for six garnet grains from granitic orthogneiss at Shuanghe in the Dabie orogen. Filled symbols denote the Grt-I domains, and open symbols denote the Grt-II domains.

Fig. 7. Trace element contents for the Grt-I and Grt-II domains in garnet from granitic orthogneiss at Shuanghe in the Dabie orogen. Filled symbols denote the Grt-I domains, 38

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Fig. 8. Chondrite-normalized REE distributions for garnet from granitic orthogneiss at

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Shuanghe in the Dabie orogen. The chondrite values for rare earth element normalization

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are from Sun and McDonough (1989). Filled symbols denote the Grt-I domains, and

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open symbols denote the Grt-II domains.

Fig. 9. Zircon CL-images, U-Pb Concordia diagrams and chondrite-normalized REE patterns for granitic orthogneiss at Shuanghe in the Dabie orogen. (a) to (c) Sample 12DB37, (d)

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to (f) sample 12DB38, (g) to (i) sample 95M19. The numbers in white circles in the CL images denote the analyzed spot number and the apparent 206Pb/238U ages. The chondrite

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values for rare earth element normalization are from Sun and McDonough (1989).

Fig. 10. The NCKMnFMASH P-T pseudosection calculated for metagranite 12DB38. Also

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shown are the change of mineral assemblages (a), the isopleths of grossular and

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almandine in garnet (a) and accompanying modal contents (vol.%) for melt (b), garnet (c), phengite (d) and biotite (e) in the P-T plane. The gray dotted lines with gray script

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are for almandine, and the blue dotted lines with blue script are for grossular.

Fig. 11. (a) The summary of REE distributions for two generations of zircon and garnet in the granitic orthogneiss from Shuanghe in the Dabie orogen. (b) Zircon/garnet partition

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coefficients (Dzir/grt) calculated from three generations of zircon and garnet. The experimental data at 850oC, 900oC, 950oC and 1000oC are from Rubatto and Hermann (2007). (c) Chondrite-normalized REE distributions for titanite in the granitic gneiss at Shuanghe in the Dabie orogen. Red lines are data in this study, and blue lines are data from Gao et al. (2012b). (d) The weighted mean ages for metamorphic titanite from sample 95M19, data from Gao et al. (2012b).

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ACCEPTED MANUSCRIPT Table 1 Modal abundances (%) of minerals in granitic gneisses from Shuanghe in the Dabie orogen. Upper unit

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95M16

95M18

95M19

Qtz Pl Kfs mus

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