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Volume 30 Issue 6
Dec.  2019
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Metamorphism and Zircon Geochronological Studies of Metagabbro Vein in the Yushugou Granulite-Peridotite Complex from South Tianshan, China

  • Yushugou granulite-peridotite complex, located at the east part of the northern margin of South Tianshan, may represent an ophiolitic slice subducted to 40-50 km depth with high-pressure granulite facies metamorphism. Although a lot of studies have been conducted on rocks in this belt, the rock as-sociation and tectonic background of the ophiolitic slice are still in dispute. A detailed study on petrology, phase equilibrium modeling and U-Pb zircon ages have been performed on the metagabbro vein in peridotite unit to constrain the tectonic evolution of the Yushugou granulite-peridotite complex. Three stages of mineral assemblage in the metagabbro were defined as stage Ⅰ:CpxA+OpxA+PlA, which represents the original minerals of the metagabbro vein; stage Ⅱ:CpxB+OpxB+PlB+Spl, which represents the mineral assemblage of granulite facies metamorphism with peak P-T conditions of 4.2-6.9 kbar and 940-1 070℃; stage Ⅲ is characterized by the existence of prehnite, thomsonite and amphibole in the matrix, indicating that the metagabbro vein may be influenced by fluids during retrograde metamorphism. Combined with the crosscut relationship, it can be deduced that the metagabbro vein, together with the peridotite in Yushugou granulite-peridotite complex has experienced similar high-temperature granulite facies metamorphism. The zircon chronological data shows that the protolith age of the metagabbro vein is 400.5±6.2 Ma, reflecting Devonian magmatism event and the granulite facies metamorphism occurred at~270 Ma which may be related to the post-collisional magmatism.
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Metamorphism and Zircon Geochronological Studies of Metagabbro Vein in the Yushugou Granulite-Peridotite Complex from South Tianshan, China

    Corresponding author: Lu Zhang, lu.zhang@hpstar.ac.cn
  • 1. Center for High Pressure Science and Technology Advanced Research, Beijing 100094, China
  • 2. Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing 100871, China
  • 3. Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
  • 4. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
  • 5. College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China

Abstract: Yushugou granulite-peridotite complex, located at the east part of the northern margin of South Tianshan, may represent an ophiolitic slice subducted to 40-50 km depth with high-pressure granulite facies metamorphism. Although a lot of studies have been conducted on rocks in this belt, the rock as-sociation and tectonic background of the ophiolitic slice are still in dispute. A detailed study on petrology, phase equilibrium modeling and U-Pb zircon ages have been performed on the metagabbro vein in peridotite unit to constrain the tectonic evolution of the Yushugou granulite-peridotite complex. Three stages of mineral assemblage in the metagabbro were defined as stage Ⅰ:CpxA+OpxA+PlA, which represents the original minerals of the metagabbro vein; stage Ⅱ:CpxB+OpxB+PlB+Spl, which represents the mineral assemblage of granulite facies metamorphism with peak P-T conditions of 4.2-6.9 kbar and 940-1 070℃; stage Ⅲ is characterized by the existence of prehnite, thomsonite and amphibole in the matrix, indicating that the metagabbro vein may be influenced by fluids during retrograde metamorphism. Combined with the crosscut relationship, it can be deduced that the metagabbro vein, together with the peridotite in Yushugou granulite-peridotite complex has experienced similar high-temperature granulite facies metamorphism. The zircon chronological data shows that the protolith age of the metagabbro vein is 400.5±6.2 Ma, reflecting Devonian magmatism event and the granulite facies metamorphism occurred at~270 Ma which may be related to the post-collisional magmatism.

0.   INTRODUCTION
  • The Yushugou granulite-peridotite complex is located at the northern margin of South Tianshan, western China. Former petrographical and geochemical studies indicate that the complex represents an ophiolitic slice subducted to 40-50 km depth and underwent high-pressure granulite facies metamorphism (Wang J L et al., 1999; Wang R S et al., 1999a, b), or represents a segment of the crust-mantle transition zone and exhumed by diapirism processes (Ji et al., 2014; Jian et al., 2013). The coexistence of granulite and peridotite is rarely reported in the subduction zone, their forming relationships is significant to understand the processes at the mantle-crust interface and the tectonic evolution of the Paleozoic Chinese South Tianshan (Kusbach et al., 2015; Jahn et al., 1999; Brueckner, 1998).

    Previous geochronological data of the Yushugou granulite-peridotite complex by zircon U-Pb dating, Sm-Nd and Rb-Sr mineral isochron and amphibolite 40Ar/39Ar (Li T F et al., 2011; Zhou et al., 2004; Wang et al., 2003, 1999b, 1998) has yielded a widespread age range, from 401 to 310 Ma, demonstrating that the complex had experienced multiple geothermal events. Wang et al. (1999a) suggested that the granulite-peridotite complex, characterized by multiple deformation and metamorphism, is an integrated relic paleo-oceanic slice. Other researchers agreed that there exists an ophiolite belt, which extends from Yushugou to Tonghuashan and Liuhuangshan. However, they claimed that the granulite unit may represent a separated block with different metamorphic histories from the ophiolite (Zhang et al., 2018a, b; Xu et al., 2011; Yang et al., 2011; Shu et al., 1998, 1996; Wu et al., 1992). On the other hand, Jian et al. (2013) and Ji et al. (2014) interpreted the granulite-peridotite complex to be a part of the continental mantle and the lower crust adjacent to the Moho. Geochemical data indicated that the protoliths of the Yushugou ophiolitic units derived from MORB and SSZ tectonic settings, underwent subduction fluid alteration and may be related to the formation of island-arc volcanic rocks (Xu et al., 2011; Yang et al., 2011). From above, the definition and tectonic background of the suture and related ophiolites in the South Tianshan orogenic belt are still in dispute.

    The studies about the metamorphic evolution of the peridotite are poor due to the partly or completely serpentinization and lack of characteristic metamorphic minerals which can be used to decipher metamorphic conditions. A good way to reconstruct the P-T conditions of the serpentinite is to study its associated rock types. In this study, we found a lot of metagabbro veins intruding into the peridotite unit of the Yushugou granulite-peridotite complex, which can be used to constrain the metamorphic evolution of the peridotite unit. These metagabbro veins had been found for years (Dong et al., 2001; Xu et al., 2011; Yang et al., 2011; Wang Y et al., 1999), but the studies on them are still insufficient.

    In this article, we report a detailed work on petrology, phase equilibrium modeling and zircon U-Pb geochronology on the metagabbro veins in the peridotite unit from the Yushugou granulite-peridotite complex, the tectonic implications for the Yushugou granulite-peridotite complex are discussed. Mineral abbreviations are after Kretz (1983).

1.   GEOLOGICAL SETTING
  • In the central Asia, the Chinese Tianshan orogenic belt is more than 1 500 km long, extending from Fan-Karategin of Tajikistan in the west to Chinese western Tianshan HP/UHP belt in the east (Volkova and Budanov, 1999; Gao et al., 1995; Sobolev et al., 1986). It seperates the Tarim Craton to the south and Junggar Block to the north (Fig. 1). The Tianshan Orogen was formed by the collision of Tarim Craton and Junggar Block in Late Paleozoic (Gao et al., 1998; Coleman, 1989). Two Paleozoic suture zones were identified by Windley et al. (1990) and Allen et al. (1993) in the Chinese Tianshan: the Late Devonian-Early Carboniferous South Tianshan suture zone between the Tarim Craton and Yili-central Tianshan Block; the Late Carboniferous-Early Permian North Tianshan suture zone between the Yili-central Tianshan Block and the Junggar Block.

    Figure 1.  (a) Tectonic framework of Chinese South Tianshan (modified after Lü et al., 2012; XBGMR, 1960, 1959). (b) Schematic geological map of Yushugou granulite-peridotite complex (modified after XBGMR, 1960), the red star represents the sampling locality of the metagabbro.

    The South Tianshan Orogen was formed as a result of tectonic amalgamation between the Tarim Craton and the Yili-central Tianshan Block during Late Paleozoic (Han et al., 2011). The subduction of the South Tianshan paleo-ocean underneath the Yili-central Tianshan generated a paired metamorphic belt, which comprises a HP-UHP belt to the south and coeval low-P granulite-facies rocks to the north (Su et al., 2018; Xia et al., 2018, 2014a; Zhang et al., 2007; Li and Zhang, 2004). The Early Silurian-Early Carboniferous subduction of the South Tianshan paleo-ocean (Zhang et al., 2017; Xia et al., 2014a, b; Han et al., 2011; Gao et al., 2008) created a magmatic arc along the south margin of the Yili-central Tianshan (Ma et al., 2014; Xu et al., 2013, 2010; Long et al., 2011; Yang and Zhou, 2009; Zhu Y F et al., 2009, 2006; Yang et al., 2006; Zhu Z X et al., 2006).

    The Yushugou granulite-peridotite complex is located in the Yushugou-Tonghuashan-Liuhuangshan mélange belt at the northern margin of South Tianshan, western China. It shows a lensed tectonic slice in NW-SE direction, which is about 10 km long and 1-3 km wide (Fig. 1). Granulite and meta-peridotite predominate in this complex (Zhang et al., 2018a, b; Yang et al., 2011; Zhou et al., 2004; Wang et al., 1999a, b). The granulite unit mainly consists of mafic and felsic granulite (Fig. 2a), with minor interbedded layers of amphibolites and marble lenses in some places. The peridotite unit is in tectonic contact with the granulite unit. The Yushugou granulite-peridotite complex mainly consists of two rock units according to the field observations and petrological studies (Dong et al., 2001; Wang et al., 1999a, b): (ⅰ) metamorphosed peridotite unit with protolith of residual mantle peridotite, intruded by metagabbro vein; (ⅱ) granulite unit with mafic granulite, felsic granulite, amphibolite and marble. The detailed field relationships between the granulite unit and peridotite unit are shown in Fig. 2a. The peridotites were almost serpentinized. The peridotite unit was cut by lots of metagabbro veins (Fig. 2b). The metagabbro vein extends several meters, and with width ranging from a few to tens of centimeters. No metagabbro veins were found in the granulite unit.

    Figure 2.  Field photographs and photomicrographs of metagabbro veins in Yushugou. (a) Field photograph of the peridotite unit and granulite unit. (b) Field photograph of the metagabbro vein and peridotite unit. (c)-(f) Photomicrographs of the metagabbro vein samples, (c) porphyritic clinopyroxene (0.3-0.6 mm in diameter, denoted as CpxA), orthopyroxene (0.4-0.6 mm in diameter, denoted as OpxA) and plagioclase (0.6-0.8 mm in diameter, denoted as PlA); (d) matrix of fine-grained clinopyroxene (denoted as CpxB), orthopyroxene (denoted as OpxB), plagioclase (denoted as PlB) and spinel; (e) spinel intergrowth with CpxB; (f) amphibole, prehnite and thomsonite in the matrix.

2.   METHODS
  • Major element compositions of whole-rock samples were obtained using X-ray fluorescence (XRF) at Hebei Institute of Regional Geology and Mineral Resources Survey. Trace element compositions of whole-rock samples were determined as solute using Agilent 7500ce inductively coupled plasma mass spectrometry (ICP-MS) at Peking University. The major and trace elements data are presented in Table 1.

    Sample Y14-18 Y14-19 Y14-24
    Major oxides (wt.%)
    SiO2 45.58 40.76 43.8
    TiO2 0.12 0.11 0.25
    Al2O3 19 14 16.82
    Fe2O3 0.89 1.75 2.38
    FeO 3.09 2.13 4.35
    MnO 0.08 0.08 0.12
    MgO 12.25 22.72 13.9
    CaO 11.96 8.41 11.34
    Na2O 1.91 0.99 1.72
    K2O 0.24 0.14 0.13
    P2O5 0.01 0.01 0.01
    LOI 4.72 8.64 4.86
    Total 99.86 99.73 99.7
    Mg# 0.88 0.95 0.85
    Trace element (ppm)
    Rb 6.62 4.49 0.63
    Sr 588.5 612.25 1255.57
    Y 4.35 3.31 6.05
    Zr 2.43 4.36 8.92
    Cs 1.67 1.76 0.99
    Ba 71.75 27.58 158.56
    La 0.09 0.38 0.72
    Ce 0.25 0.85 2.02
    Pr 0.06 0.13 0.36
    Nd 0.43 0.73 1.95
    Sm 0.26 0.28 0.68
    Eu 0.24 0.2 0.46
    Ti 728.98 717.18 1789
    Gd 0.5 0.44 1.04
    Tb 0.11 0.09 0.21
    Dy 0.76 0.62 1.44
    Ho 0.17 0.14 0.32
    Er 0.5 0.38 0.92
    Tm 0.08 0.06 0.15
    Yb 0.49 0.38 0.91
    Lu 0.07 0.06 0.14
    Hf 0.13 0.18 0.4
    Ta 0.65 0.23 0.26
    Pb 2.32 0.86 2.37
    U 0.02 0.02 0.03
    δEu 2.01 1.72 1.68

    Table 1.  Major and trace element compositions for the metagabbro in Yushugou, Chinese South Tianshan

  • Major element compositions of minerals were obtained using a JEOL JXA-8100 electron microprobe at the Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, Peking University, Beijing. The operating conditions were 15 kV acceleration voltage, 10 nA beam current and 2 μm beam diameter. The counting time was 20-30 s. Natural jadeite (Si), forsterite (Mg), hematite (Fe), albite (Na, Al), rutile (Ti), rhodonite (Mn) and sanidine (K) served as standards. The representative mineral analyses data are presented in Table 2.

    Mineral Y14-18 Y14-19 Y14-24
    CpxA CpxB OpxA OpxB PlA PlB Spl Amph Amph Prh Tmp CpxA CpxB OpxA OpxB Spl Amph Tmp CpxA CpxB OpxA OpxB Spl Spl Tmp
    SiO2 51.17 52.58 53.34 53.39 49.92 53.71 0.03 46.11 48.01 42.45 37.86 51.57 51.92 54.47 54.86 0.09 56.09 38.17 51.33 50.36 53.19 53.37 0.07 0.10 38.67
    TiO2 0.40 0.19 0.03 0.08 0.00 0.00 0.09 0.82 0.06 0.01 0.00 0.35 0.36 0.00 0.00 0.00 0.18 0.03 0.39 0.61 0.05 0.05 0.02 0.06 0.04
    Al2O3 5.62 3.19 4.25 4.50 31.56 29.44 66.54 10.39 10.12 24.66 29.43 5.48 5.75 3.49 2.98 66.99 1.92 29.64 6.09 6.88 4.00 4.28 63.05 62.10 29.97
    Cr2O3 0.02 0.02 0.00 0.04 0.01 0.04 0.16 0.08 0.06 0.02 0.00 0.29 0.33 0.02 0.03 0.70 0.14 0.04 0.06 0.24 0.00 0.02 0.03 0.35 0.01
    Fe2O3 3.36 1.46 2.35 2.35 0.09 0.03 0.61 3.88 3.93 0.00 0.00 2.65 1.19 2.04 1.68 0.11 0.99 0.00 2.97 1.45 3.01 1.51 3.28 3.48 0.00
    FeO 0.45 2.00 8.47 8.47 0.00 0.00 14.01 3.12 2.17 0.00 0.05 0.52 1.81 7.33 7.82 12.16 1.46 0.04 1.83 4.03 10.99 12.63 17.28 18.34 0.00
    MnO 0.13 0.14 0.20 0.22 0.02 0.00 0.08 0.11 0.08 0.01 0.00 0.13 0.08 0.22 0.19 0.06 0.06 0.00 0.16 0.16 0.29 0.21 0.14 0.14 0.00
    MgO 14.58 16.62 30.30 30.74 0.00 0.00 18.71 17.86 18.64 0.03 0.07 15.17 15.07 32.23 32.07 19.91 22.48 0.06 14.61 13.47 29.17 28.39 15.94 15.11 0.02
    CaO 22.92 22.88 1.01 0.32 14.18 11.71 0.01 12.43 12.12 26.30 12.09 22.78 22.65 0.25 0.34 0.06 13.19 11.76 22.41 21.05 0.30 0.33 0.06 0.21 11.91
    Na2O 1.21 0.38 0.07 0.04 3.81 5.15 0.00 1.91 1.70 0.09 3.24 1.10 1.00 0.00 0.00 0.00 0.19 2.73 1.07 1.20 0.00 0.00 0.04 0.03 2.28
    K2O 0.00 0.06 0.00 0.00 0.03 0.00 0.01 0.23 0.10 0.00 0.02 0.00 0.00 0.00 0.00 0.01 0.05 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.04
    Total 99.52 99.39 100.03 100.16 99.62 100.09 100.19 96.56 96.59 93.58 82.76 99.78 100.04 100.04 99.80 100.08 96.65 82.50 100.63 99.32 100.69 100.64 99.58 99.57 82.94
    Oxygen 6.00 6.00 6.00 6.00 8.00 8.00 4.00 23.00 23.00 10.00 20.00 6.00 6.00 6.00 6.00 4.00 23.00 20.00 6.00 6.00 6.00 6.00 4.00 4.00 20.00
    Si 1.87 1.90 1.88 1.87 2.29 2.43 0.00 6.59 6.78 2.71 5.26 1.87 1.88 1.90 1.92 0.00 7.76 5.29 1.86 1.85 1.88 1.89 0.00 0.00 5.32
    Ti 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.09 0.01 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.02 0.00 0.01 0.02 0.00 0.00 0.00 0.00 0.00
    Al 0.24 0.19 0.18 0.19 1.70 1.57 1.98 1.75 1.69 1.85 4.82 0.23 0.25 0.14 0.12 1.98 0.31 4.85 0.26 0.30 0.17 0.18 1.93 1.92 4.86
    Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.01 0.01 0.00 0.00 0.01 0.02 0.00 0.00 0.01 0.00 0.00 0.00 0.01 0.00
    Fe3+ 0.09 0.07 0.06 0.06 0.00 0.00 0.01 0.42 0.42 0.00 0.00 0.07 0.03 0.05 0.04 0.00 0.10 0.00 0.08 0.04 0.08 0.04 0.06 0.07 0.00
    Fe2+ 0.01 0.04 0.25 0.25 0.00 0.00 0.30 0.37 0.26 0.00 0.01 0.02 0.06 0.21 0.23 0.26 0.17 0.00 0.06 0.12 0.32 0.37 0.38 0.40 0.00
    Mn 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.01 0.00 0.01 0.01 0.01 0.01 0.00 0.00 0.00
    Mg 0.79 0.82 1.59 1.61 0.00 0.00 0.70 3.80 3.92 0.00 0.01 0.82 0.81 1.68 1.67 0.74 4.64 0.01 0.79 0.74 1.53 1.50 0.62 0.59 0.00
    Ca 0.90 0.90 0.04 0.01 0.70 0.57 0.00 1.90 1.83 1.80 1.80 0.89 0.88 0.01 0.01 0.00 1.96 1.75 0.87 0.83 0.01 0.01 0.00 0.01 1.76
    Na 0.09 0.07 0.01 0.00 0.34 0.45 0.00 0.53 0.47 0.01 0.87 0.08 0.07 0.00 0.00 0.00 0.05 0.73 0.08 0.09 0.00 0.00 0.00 0.00 0.61
    K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01
    Sum 4.00 4.00 4.01 4.00 5.03 5.02 3.00 15.66 15.54 6.37 12.77 4.00 4.00 4.00 4.00 3.00 15.07 12.65 4.00 4.00 4.00 4.00 3.00 3.00 12.56
    Xan 0.67 0.56
    XMg 0.98 0.94 0.86 0.87 0.70 0.98 0.94 0.89 0.88 0.74 0.93 0.86 0.83 0.80 0.62 0.59
    Note: Xan=Ca/(Ca+Na), XMg=Mg/(Mg+Fe2+). Amph. amphibole; other mineral abbreviations are after Kretz (1983).

    Table 2.  Selected microprobe analyses data of rock-formation minerals from metagabbro in Yushugou, Chinese South Tianshan

  • Zircon grains for dating were handpicked under a binocular microscope from the selected individual zircon grains obtained using conventional heavy liquid and magnetic separation techniques from the crushed rock samples. These zircons were subsequently mounted in an epoxy resin, then polished to expose the inner surface of the zircon and prepared for dating. Cracks and inclusions of the zircons were carefully examined under microscope. Before U-Pb isotopic analysis, cathodoluminescence (CL) images were performed at Peking University to obtain their internal structures information.

    SIMS U-Pb zircon dating was performed with the Chinese Academy of Sciences Cameca IMS-1280 ion microprobe (CASIMS) at the Institute of Geology and Geophysics in Beijing, following the analytical procedures described by Li et al. (2009). The primary O2-beam was accelerated at 13 kV, with an intensity of ~10 nA. The elliptical spot is 20 µm×30 µm in size. Relative probability density and concordia diagrams were obtained using ISOPOLT 4.15 (Ludwig, 2003). The analytical data are shown in Table 3.

    Spot Position U (ppm) Th (ppm) Th/U f206% 207Pb/235U ±1σ (%) 206Pb/238U ±1σ (%) ρ 207Pb/206Pb ±1σ 207Pb/235U ±1σ 206Pb/238U ±1σ
    01 Core 48 42 0.88 0.18 11.684 90 1.61 0.487 2 1.51 0.934 08 2 596.0 9.6 2 579.5 15.2 2 558.5 31.9
    02 Rim 154 254 1.65 0.40 0.303 13 2.13 0.042 4 1.52 0.712 80 279.5 33.9 268.8 5.0 267.6 4.0
    03 Core 368 91 0.25 0.27 4.945 23 1.56 0.309 1 1.50 0.960 38 1 896.1 7.8 1 810.0 13.3 1 736.2 22.9
    04 Core 185 99 0.54 0.15 0.502 94 1.84 0.066 3 1.52 0.822 80 412.7 23.3 413.7 6.3 413.9 6.1
    05 Core 232 88 0.38 0.30 0.491 12 1.83 0.064 2 1.50 0.819 46 432.4 23.2 405.7 6.1 401.0 5.8
    06 Core 1 688 423 0.25 3.17 1.289 43 1.78 0.132 9 1.51 0.850 37 938.7 19.1 841.0 10.2 804.5 11.4
    07 Core 102 157 1.54 0.19 10.448 50 1.55 0.466 0 1.50 0.968 62 2 483.1 6.5 2 475.4 14.5 2 465.9 30.8
    08 Core 1 147 158 0.14 0.08 1.431 47 1.65 0.150 5 1.59 0.962 28 898.0 9.2 902.2 9.9 903.8 13.4
    09 Core 258 192 0.74 0.08 5.669 23 1.56 0.345 0 1.50 0.962 40 1 943.8 7.6 1 926.7 13.5 1 910.8 24.9
    10 Core 329 158 0.48 0.06 1.494 71 1.58 0.153 1 1.50 0.950 82 951.8 10.0 928.2 9.7 918.3 12.9
    11 Rim 1 396 717 0.51 3.44 0.310 88 2.58 0.043 6 1.50 0.581 65 270.7 47.5 274.9 6.2 275.3 4.1
    12 Rim 71 81 1.14 1.19 0.288 82 5.32 0.044 7 1.53 0.287 99 44.2 117.5 257.6 12.2 281.7 4.2
    13 Core 331 107 0.32 0.28 5.418 29 1.58 0.301 0 1.52 0.963 66 2 105.5 7.4 1 887.8 13.6 1 696.2 22.7
    14 Core 359 238 0.66 0.36 0.478 37 1.71 0.063 2 1.51 0.883 52 408.6 17.8 397.0 5.6 394.9 5.8
    15 Core 96 60 0.62 0.57 0.496 96 2.95 0.065 0 1.51 0.511 67 431.0 55.6 409.6 10.0 405.9 5.9
    16 Rim 1 104 438 0.40 4.75 0.315 96 3.56 0.043 1 1.52 0.426 57 334.3 71.4 278.8 8.7 272.2 4.1
    17 Rim 395 315 0.80 0.38 0.288 16 1.83 0.041 2 1.50 0.818 84 229.8 24.1 257.1 4.2 260.1 3.8
    18 Core 676 387 0.57 0.08 0.488 72 1.67 0.064 9 1.52 0.909 26 397.1 15.5 404.0 5.6 405.2 6.0
    19 Rim 145 113 0.77 1.65 0.297 09 4.68 0.044 2 1.50 0.320 54 134.7 101.1 264.1 11.0 278.9 4.1
    20 Core 557 66 0.12 0.59 0.467 96 1.90 0.062 6 1.50 0.790 10 379.4 26.0 389.8 6.2 391.5 5.7
    21 Core 275 168 0.61 0.28 0.485 64 1.83 0.064 1 1.50 0.819 34 411.4 23.3 401.9 6.1 400.3 5.8
    22 Core 1 305 338 0.26 0.07 1.158 18 1.60 0.126 9 1.51 0.943 26 812.7 11.1 781.1 8.8 770.1 11.0
    23 Core 1 963 476 0.24 0.71 1.208 21 1.74 0.129 6 1.53 0.880 64 856.8 17.0 804.4 9.7 785.6 11.3
    24 Rim 305 148 0.49 0.43 0.296 52 2.60 0.042 3 1.53 0.589 34 232.7 47.8 263.7 6.1 267.2 4.0
    25 Core 109 79 0.72 0.47 0.479 98 2.32 0.062 9 1.51 0.648 51 425.4 39.0 398.1 7.7 393.4 5.8
    26 Core 497 105 0.21 0.04 1.499 68 2.15 0.155 6 1.77 0.821 80 925.2 25.0 930.3 13.2 932.4 15.4
    27 Core 55 46 0.84 0.25 10.447 57 1.67 0.461 9 1.50 0.898 49 2 497.7 12.3 2 475.3 15.6 2 448.0 30.6
    28 Core 53 62 1.17 0.16 10.463 11 1.62 0.465 6 1.52 0.937 44 2 486.7 9.5 2 476.7 15.1 2 464.4 31.1
    29 Core 180 122 0.68 0.07 5.109 33 1.56 0.329 8 1.51 0.964 97 1 837.9 7.4 1 837.7 13.4 1 837.4 24.2
    30 Core 1 023 213 0.21 0.58 1.390 65 1.66 0.145 7 1.50 0.907 70 904.9 14.3 885.0 9.8 877.0 12.3
    31 Core 1 239 293 0.24 0.03 1.488 40 1.55 0.154 0 1.50 0.965 06 930.8 8.3 925.7 9.5 923.5 12.9
    32 Core 793 334 0.42 0.10 1.480 75 1.54 0.151 9 1.50 0.971 77 949.0 7.4 922.5 9.4 911.5 12.8

    Table 3.  SIMS zircon U-Pb data for the metagabbro in Yushugou, Chinese South Tianshan

3.   PETROGRAPHY
  • Three samples, Y14-18, Y14-19 and Y14-24, obtained from the metagabbro vein were selected for petrological studies.

    The metagabbro sample Y14-18 displays a blastoporphyritic texture (Figs. 2c-2f). Minerals in the metagabbro are clinopyroxene (~24%), orthopyroxene (~19%), plagioclase (~45%), thomsonite (~10%), and minor spinel, amphibole, prehnite and pyrite (Figs. 2c-2f). Porphyritic clinopyroxene (0.3-0.6 mm, marked as CpxA, Fig. 2c), orthopyroxene (0.4-0.6 mm, marked as OpxA, Fig. 2c) and plagioclase (0.6-0.8 mm, marked as PlA, Fig. 2c) are distributed in the matrix of fine-grained clinopyroxene (marked as CpxB, Fig. 2d), orthopyroxene (marked as OpxB, Fig. 2d), plagioclase (marked as PlB, Fig. 2d) and spinel. We interpret the large particle size minerals (CpxA, OpxA and PlA) to represent the original minerals of the metagabbro vein and the smaller particle size minerals (CpxB, OpxB, PlB and spinel) to represent metamorphic minerals which were formed during granulite facies metamorphism. Spinel usually occurs as tiny grains in the matrix or intergrows with CpxB (Fig. 2e). Amphibole, prehnite and thomsonite distributed in the matrix (Fig. 2f), are interpreted to be formed due to retrogression. Both samples Y14-19 and Y14-24 have microstructure similar to that of the sample Y14-18, except for the plagioclases, which have transformed to thomsonites. All rocks show structural deformation of the porphyritic minerals and the directional arrangement of matrix minerals (Fig. 2c).

4.   RESULTS
  • In all the three samples, CpxA and CpxB are diopside with XMg (=Mg/(Mg+Fe2+)) of CpxA (0.89-0.99) slightly higher than that of CpxB (0.84-0.96) (Fig. 3a). OpxA and OpxB have similar XMg of 0.78-0.87, they are bronzite. PlA and PlB are labradorite, with higher An (=Ca/(Ca+Na)) of 0.67-0.69 for PlA than 0.51-0.58 for PlB (Fig. 3c). Spinel is pleonaste according to Gargiulo et al. (2013) (Fig. 3b). Amphibole has (Ca)M4=1.83-1.98 p.f.u., (Na+K)A=0.04-0.52 p.f.u. (O=23), Si=6.59-7.17 p.f.u. (O=22), XMg=0.91-0.96 and classified as tremolite, edenite or magnesiohornblende by Leake et al. (1997) (Fig. 3d).

    Figure 3.  Mineral chemistry diagrams showing variations of clinopyroxene, plagioclase, spinel and amphibole in the metagabbro vein from Yushugou. (a) Mg#-Ca/(Ca+Mg) diagrams showing variations of CpxA and CpxB in Y14-18, Y14-19 and Y14-24, respectively. (b) Fe2+/(Fe2++Mg2+) vs Fe3+/(Fe3++Al3+) diagram (Gargiulo et al., 2013) showing representative spinels from samples Y14-18, Y14-19 and Y14-24. (c) An-SiO2 diagram showing variations of PlA and PlB in Y14-18. (d) Si vs Mg/(Mg+Fe2+) diagram (Leake et al., 1997) showing representative amphiboles from the studied samples Y14-18 and Y14-19.

    To sum up, according to the petrographic characteristics and mineral composition, minerals in the metagabbro vein can be subdivided into three generations: Stage Ⅰ, CpxA+OpxA+PlA, which may represent the original minerals of the metagabbro vein; Stage Ⅱ, CpxB+OpxB+PlB+Spl, may be formed at granulite facies metamorphism; Stage Ⅲ, characterized by prehnite, thomsonite and amphibole in the matrix, may represent the retrograde mineral assemblage.

  • The major and trace elements of the metagabbro vein are shown in Table 1. The metagabbro vein has SiO2 ranging from 40.76 wt.% to 45.58 wt.% and MgO of 12.25 wt.%-22.72 wt.%. Their K2O contents range from 0.13 wt.% to 0.24 wt.% and total alkalis (Na2O+K2O) from 1.13 wt.% to 2.15 wt.% with very low K2O/Na2O ratios (0.07-0.14) (Table 1). The TiO2 contents range from 0.11 wt.% to 0.25 wt.% and Al2O3 range from 14 wt.% to 19 wt.%. In TAS diagram (Fig. 4a), all the metagabbros are plotted in the gabbro area.

    Figure 4.  Geochemical discrimination diagrams of the metagabbro vein in Yushugou. (a) TAS plot; (b) FeOT-MgO-Al2O3 plot (Pearce et al., 1977). Some metagabbro data from Dong et al. (2001) and Xu et al. (2011) are also shown for comparison.

    The chondrite-normalized REE patterns and mantle-normalized trace element patterns of the metagabbros are shown in Fig. 5. They have relatively low total REE abundance and show flat chondrite-normalized REE patterns. Their chondrite-normalized rare earth element pattern displays a positive Eu anomaly (δEu ranges of 1.68-2.01) (Fig. 5a), indicating plagioclase accumulation (Dong et al., 2001; Zhang et al., 1992). In the primitive mantle normalized trace element patterns, they show positive Sr and Ba anomalies and negative U and Zr anomalies (Fig. 5b).

    Figure 5.  (a) Chondrite-normalized REE patterns and (b) primitive mantle (PM)-normalized trace element spider diagram of the metagabbro vein in Yushugou. Normalized values are from Sun and McDonough (1989). OIB, N-MORB and E-MORB are also shown for comparison.

  • Phase diagram represents the phase relations predicted for a given bulk-rock composition or composition range, which can be used in modeling how mineral proportions and mineral compositions change when a rock undergoes metamorphism (Powell and Holland, 2008). A P-T pseudosection for the chosen metagabbro Y14-18 was calculated using the software Perplex (Connolly, 2005) in the model system Na2O-CaO-FeO-MgO-Al2O3-SiO2-Fe2O3-H2O (NCFMASHO) (Fig. 6). Activity-composition relationships are those presented for garnet (Holland and Powell, 1998), clinopyroxene (Holland and Powell, 1996), orthopyroxene (Powell and Holland, 1999), amphibole (Dale et al., 2000), plagioclase (Newton et al., 1981), spinel (Holland and Powell, 1998), and melt (Holland and Powell, 2001). Other minerals in this P-T pseudosection are treated as pure end-member phases. The bulk composition used to calculate the pseudosection is presented in Table 1.

    Figure 6.  P-T pseudosection calculated for metagabbro vein sample Y14-18 in the NCFMASHO system, the used bulk-rock composition is shown in Table 1, normalized on the basis of weight percent as SiO2=45.58, Al2O3=19, CaO=11.96, MgO=12.25, FeO=3.09, Na2O=1.91, O=0.89. All fields are shaded by different colors, which represent different variants. Darker shaded field means higher variant, while lighter shaded field means lower variant. Red star shows the area of observed peak mineral assemblage in sample Y14-18. Some mineral assemblages with limited area are not shown. Mineral abbreviations: Hcrd. hydrouscordierite; Amph. amphibole; Mfr. magnesioferrite; Hcrd. hydrouscordierite; other mineral abbreviations follow Kretz (1983).

    This P-T pseudosection was calculated in the P-T range of 3-10 kbar and 700-1 200 ℃ (Fig. 6). It shows that the spinelbearing assemblages are stable at relatively high temperature and low pressure conditions. The observed mineral assemblage in sample Y14-18 involves orthopyroxene, clinopyroxene, plagioclase and spinel, corresponding to the modeled field with phase assemblage of Opx+Pl+Spl+Cpx+melt with P-T condition of 4.2-6.9 kbar and 940-1 070 ℃ in the model system (Fig. 6).

  • The obtained zircons from the metagabbro vein Y14-18 are transparent, colorless, about 50-100 μm long, and show subhedral or oval shapes. On CL images, most of the grains contain a core with oscillatory zoning interpreted to be of magmatic origin and a rim interpreted to be of metamorphic origin (Fig. 7). The unzoned or weakly zoned metamorphic zircons in Y14-18 should be formed in granulite facies according to Wu and Zheng (2004) and Vavra et al. (1999). The zircons have Th/U values of 0.12-1.65 (Table 3). Thirty-two analyzed spots are obtained on 25 grains. Among the analyses on zircon cores, 8 spots yield a weighted mean 206Pb/238U age of 400.5±6.2 Ma (MSWD=1.6; Fig. 7b), which may represent the time of the crystallization. The remaining 17 spots obtained from zircon cores yield ages of 2 596±9.6 to 812.7±11.1 Ma (Fig. 7a), indicating the inherited origin of these zircons. On the probability density diagram, the age spectrum can be subdivided into three major groups at 2 483.1-2 596, 1 837.9-2 105.5 and 812.7-925.2 Ma (Fig. 7a). For zircon rim of metamorphic origin, 7 analyses spots yield a weighted mean 206Pb/238U age of 271.5±7.0 Ma (MSWD=3.5; Fig. 7b), which is interpreted to represent the time of the granulite facies metamorphism.

    Figure 7.  (a) Relative probability density diagram and (b) concordia diagram for zircons from sample Y14-18. Representative CL images of the investigated zircons are shown in this figure. Analytical spots and measured ages are marked. The elliptical spot is 20 µm×30 µm in size.

5.   DISCUSSION
  • Although a lot of studies have been performed on rocks in the Yushugou granulite-peridotite complex, the rock association and tectonic background of the ophiolitic slice are still in dispute. The main debate is whether these different rock units have experienced the same metamorphic evolution process or not. Former studies mainly focused on the metamorphism of the granulite unit in the complex, but it is key to know the metamorphic evolution of peridotite units. However, due to the partly or completely serpentinization of the peridotites and lacking of characteristic metamorphic minerals, it is hard to acquire the metamorphic conditions. A good way to reconstruct the P-T conditions of the serpentinite is to study its associated rock types (intruded metagabbro veins). Some researchers have calculated the P-T conditions of the metagabbro vein by conventional thermobarometry, their calculated results are 1 060 ℃, 17.6 kbar (Cpx-Opx and Ol-Cpx geothermobarometry) and 878 ℃, 12.6 kbar (Grt-Cpx and Grt-Cpx-Pl-Qtz geothermobarometry) (Wang Y et al., 1999). However, because the temperature obtained by using a geothermometer often marks a point on the cooling curve of a granulite body (Pattison, 2003; Frost and Chacko, 1989) and there are multi-stages mineral assemblages existing in the granulite (Zhao et al., 2001), it may cause much uncertainty to estimate the peak pressure and temperature conditions by conventional geothermobarometry (Powell and Holland, 2008). In contrast, pseudosection is a forward mineral equilibria calculation method for a given rock composition, which can obtain the certain P-T conditions of each specific mineral assemblage for a rock with complex metamorphic history (Powell and Holland, 2008).

    According to our petrological study, the observed peak metamorphic mineral assemblages in the metagabbros are orthopyroxene+clinopyroxene+plagioclase+spinel, which falls into the Opx+Pl+Spl+Cpx+melt region with P-T conditions of 4.2-6.9 kbar and 940-1 070 ℃ in the calculated pseudosection diagram (Fig. 6), and the pressure is much lower than the previous results calculated by conventional thermobarometry (Wang Y et al., 1999). Combined with petrographic observations, we identified three stages of mineral assemblage in the metagabbro. Stage Ⅰ: CpxA+OpxA+PlA, which represents the original minerals of the metagabbro vein; Stage Ⅱ: CpxB+OpxB+PlB+Spl, which represents the mineral assemblage of granulite facies metamorphism with the P-T conditions of 4.2-6.9 kbar and 940-1 070 ℃; Stage Ⅲ is characterized by the existence of prehnite, thomsonite and amphibole in the matrix, indicating that the metagabbro vein may be influenced by fluid during retrograde metamorphism.

    Phase equilibrium modeling indicates that the metagabbro vein in the Yushugou granulite-peridotite complex has experienced high-temperature granulite facies metamorphism. There are lots of studies on the metamorphic P-T conditions of the granulite unit in the Yushugou granulite-peridotite complex. The calculated results are 800-870 ℃ at 8.8-11.3 kbar using Grt-Cpx and Grt-Cpx-Pl-Qtz geothermobarometry (Shu et al., 2004), 795-964 ℃ at 9.7-14.2 kbar using Grt-Cpx and Grt-Cpx-Pl-Qtz geothermobarometry (Wang et al., 1999b), 724-826 ℃ at 6.4-8.8 kbar using Grt-Opx-Pl-Qtz geothermobarometry (Li T F et al., 2011), > 930 ℃ at 10.5-14.5 kbar (Zhang et al., 2016) and 860-920 ℃ at 9.8-10.6 kbar (Zhang et al., 2018a, b) by phase equilibrium modeling. Compared with the peak P-T conditions of the granulite unit, the metagabbro vein has higher peak temperature but lower peak pressure.

  • In the chondrite-normalized REE pattern, the metagabbros have relatively low total REE abundance and show flat chondrite-normalized REE patterns, and display a positive Eu anomaly (δEu ranges from 1.68-2.01) (Fig. 5a), indicating plagioclase accumulation. In the primitive mantle-normalized trace element pattern, the metagabbros display positive Sr and Ba anomalies, and have negative U and Zr anomalies (Fig. 5b). These geochemical characteristics are in accordance with the previous results of metagabbro from Yushugou (Xu et al., 2011; Dong et al., 2001; Wang et al., 1999a), which all suggest these geochemical characteristics are caused by plagioclase accumulation. In the FeOT-MgO-Al2O3 diagram (Fig. 4b), the metagabbros are all plotted in the ocean ridge and floor environment as well as the gabbro data after Dong et al. (2001) and Xu et al. (2011). In previous studies, Xu et al. (2011) suggested that the Yushugou-Tonghuashan ophiolite derived from mid-ocean ridge environment, underwent subduction fluid alteration and may be related to the formation of island-arc volcanic rocks. Yang et al. (2011) proposed that the ophiolitic units derived from MOR and SSZ tectonic settings. Wang et al. (1999a) suggested that the mafic rock is of oceanic ridge type. However, Dong et al. (2001) suggested that the meta-peridotites from Yushugou region represent the leftover of the original mantle from which basalts were extracted and the magma source of meta-mafic rocks may be related to the source of the oceanic island basalts. In this study, the metagabbros are plotted in the ocean ridge and floor environment in the FeOT-MgO-Al2O3 diagram. Further works are needed to be done to decipher the original environment of the metagabbro and its country rocks.

    Zircon is very stable and has excellent U-Th-Pb characteristics, so researchers commonly use zircon U-Pb dating for geochronology (Wu and Zheng, 2004). Especially for the magmatic rocks and high temperature metamorphic rocks, the U-Pb radiometric system is widely used because zircon has high closure temperature of U-Pb diffusion (Cherniak and Watson, 2001; Lee et al., 1997).

    The dating of zircons from Y14-18 indicates that they can be divided into three groups according to their ages: Precambrian (> 800 Ma), Early Devonian (~400 Ma), Middle Permian (~270 Ma). Eight zircon cores with oscillatory zoning from Y14-18 are interpreted to be of magmatic origin and yield a weighted mean 206Pb/238U age of 400.5±6.2 Ma, which may represent the time of the crystallization. This result is in accordance with the previous studies of metagabbro from Yushugou and dacite from Tonghuashan (Yang et al., 2011). There are also some older cores, which may be inherited.

    Other 7 spots of zircon rim give a weighted mean 206Pb/238U age of 271.5±7.0 Ma, which may represent the time of the granulite facies metamorphism. This result is later than the previous studies of quartz orthophyre from Tonghuashan (~290 Ma, representing post-collisional stages) (Yang et al., 2011) and may be related to post-collisional magmatism.

  • Based on the occurrence of a HP-UHP belt in the south and a low-P, high-T metamorphic belt in the north, researchers proposed that there exists a paired metamorphic belt in the southwestern Tianshan (Xia et al., 2014a; Zhang et al., 2007; Li and Zhang, 2004). A magmatic arc lying at the south end of the Yili-central Tianshan was generated due to the Early Silurian-Early Carboniferous subduction of the South Tianshan paleo-ocean underneath Yili-central Tianshan (Xia et al., 2014a, b; Han et al., 2011; Gao et al., 2008). Lots of publications have reported magmatic plutons and metamorphic rocks related to this continental arc with age ranging from Early Silurian to Late Carboniferous (Ma et al., 2014; Xia et al., 2014a, b; Xu et al., 2013; Gou et al., 2012; Yang and Zhou, 2009; Zhu Y F et al., 2009; Gao et al., 2008; Yang et al., 2006; Zhu Z X et al., 2006). Combined with zircon age data in this study, we conclude that the formation of the metagabbro vein may be related to the emplacement of voluminous mafic magmas due to the continental arc magmation. The closure of South Tianshan paleo-ocean happened in the Late Carboniferous (Yang et al., 2013; Zhang et al., 2012; Han et al., 2011; Li Q L et al., 2011; Su et al., 2010), and the southwestern Tianshan orogenic belt may not reach stabilization until Mesozoic times (Xia et al., 2014a; Zhang et al., 2012). At the period of post-collisional, extension instead of compression was the main tectonic strength in Chinese southwestern Tianshan. There are a lot of plutons and metamorphic rocks related to post-collisional magmatism in the South Tianshan, such as ~290 Ma high-K calc-alkaline granitoids in Muzhaerte valley (Gou et al., 2012; Yang et al., 2012), 295-280 Ma A-type rapakivi granite and leucogranite in Kyrgyz South Tianshan (Konopelko et al., 2007), ~290 Ma quartz syenite porphyry in Tonghuashan (Yang et al., 2011), ~290 Ma granulite facies metamorphism (Xia et al., 2014a; Li and Zhang, 2004), and ~270 Ma syenite in Kekesu valley and S-type granite in Muzhaerte valley (Gou et al., 2012). The invasion of abundant granite to syenite magmas at 270-290 Ma could make this region heated, which may lead to high geothermal gradient and high temperature metamorphism at ~270 Ma. So, we conclude that the granulite facies metamorphism of the metagabbro veins happened at ~270 Ma related to the post-collisional magmatism. For the peridotite unit, most of the peridotites are partly or completely serpentinized, these rocks lack mineral assemblages characteristic of high temperature or high/ ultrahigh-pressure conditions. A good way to reconstruct the P-T conditions of the serpentinite is to study its associated rock types. In this paper, we present a detailed study of petrology, metamorphic evolution of the metagabbro vein which intruded into the peridotite unit of the Yushugou granulite-peridotite complex. The petrological characteristics indicate that the peridotite unit was cut by the metagabbro vein, which suggests that the peridotite should have experienced the same metamorphism with the metagabbro vein after its emplacement. The peak P-T conditions of the metagabbro indicate a high geothermal gradient of ~54 ℃/km, which suggests that the metagabbro was heated by the intrusion of a pluton related to arc in the subduction zone.

    In previous studies, researchers suggested that the Yushugou granulite-peridotite complex, which includes granulite unit and peridotite unit, is an integrated paleo-oceanic relic, experienced high-pressure granulite facies metamorphism (Wang R S et al., 2003, 1999a; Wang et al., 1999a). However, others proposed that the granulite unit represent a separated block from the ophiolite, which experienced different metamorphic history from the ophiolite (Zhang et al., 2018a, b; Xu et al., 2011; Yang et al., 2011; Wu et al., 1992). On the basis of our petrological and geochronological data, it indicates that the metagabbro vein, together with the peridotite in Yushugou granulite-peridotite complex has experienced similar hightemperature granulite metamorphism (4.2-6.9 kbar and 940-1 070 ℃, details see Section 5.1). The peak P-T conditions of the granulite unit are 6.4-14.5 kbar and 724-964 ℃ calculated by previous studies (Zhang et al., 2018a, b, 2016; Li T F et al., 2011; Shu et al., 2004; Wang et al., 1999b) and the metamorphic age of the granulite unit has a relatively large range of 310-390 Ma (Li T F et al., 2011; Zhou et al., 2004; Wang et al., 2003, 1999b). It can be concluded that, although the metagabbro vein, peridotite unit and granulite unit have experienced similar granulite metamorphism, the metamorphic P-T conditions and metamorphic age of the metagabbro vein and peridotite are different from those of the granulite unit. This indicates that the granulite unit and peridotite unit are not an integrated ophiolitic slice, but represent different rock units integrated by tectonic activity during the collision between the Tarim Craton and Yili-central Tianshan block (Zhang et al., 2018a, b; Xu et al., 2011; Yang et al., 2011).

6.   CONCLUSIONS
  • (1) Three stages of mineral assemblages were identified in the metagabbro vein intruding the peridotite unit. They are: Stage Ⅰ, CpxA+OpxA+PlA, which represents the original minerals of the metagabbro vein; Stage Ⅱ, CpxB+OpxB+PlB+Spl, may be formed at granulite facies metamorphism; Stage Ⅲ, characterized by prehnite, thomsonite and amphibole in the matrix, may represent the retrograde mineral assemblage.

    (2) The metagabbro vein, together with the peridotite in Yushugou granulite-peridotite complex may have undergone the same high-temperature granulite facies metamorphism with peak P-T conditions of 4.2-6.9 kbar and 940-1 070 ℃. The protolith age of the metagabbro vein is 400.5±6.2 Ma, reflecting Devonian magmatism; the granulite facies metamorphism of the metagabbro vein happened at ~270 Ma that may be related to the post-collisional magmatism.

    (3) The age and peak P-T conditions for the metagabbro vein are different from those of the neighboring granulite unit to the north in the Yushugou granulite-peridotite complex.

ACKNOWLEDGMENTS
  • We thank Profs. Chunjing Wei and Shuguang Song for their valuable discussion. We also thank Xiaoxiao Ling and Jiao Li for U-Pb zircon dating. This study was financially supported by the National Natural Science Foundation of China (Nos. 41802070, 41572051), the China Postdoctoral Science Foundation (No. 2018M631319) and the Fund from the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1254-5.

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