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Volume 30 Issue 6
Dec.  2019
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Shaoting Ma, Xu-Ping Li, Hao Liu, Fanmei Kong, Han Wang. Ultrahigh Temperature Metamorphic Record of Pelitic Granulites in the Huangtuyao Area of the Huai'an Complex, North China Craton. Journal of Earth Science, 2019, 30(6): 1178-1196. doi: 10.1007/s12583-019-1245-6
Citation: Shaoting Ma, Xu-Ping Li, Hao Liu, Fanmei Kong, Han Wang. Ultrahigh Temperature Metamorphic Record of Pelitic Granulites in the Huangtuyao Area of the Huai'an Complex, North China Craton. Journal of Earth Science, 2019, 30(6): 1178-1196. doi: 10.1007/s12583-019-1245-6

Ultrahigh Temperature Metamorphic Record of Pelitic Granulites in the Huangtuyao Area of the Huai'an Complex, North China Craton

doi: 10.1007/s12583-019-1245-6
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  • Pelitic granulite from the Huangtuyao area, occurrs in the Huai'an Complex, is located in the Trans-North China Orogen of the North China Craton. On the basis of petrolography, mineral com-ponent, and phase equilibrium modeling studies, the P-T conditions and mineral assemblages of pelitic granulites can be divided into four metamorphic stages:the prograde metamorphic stage M1 defined by the stable mineral assemblage of Grt1 (garnet core)+Pl+Bt+Kfs+Qz+Rt, the peak pressure Pmax stage M2 indicated by Grt2 (garnet mantle)+Kfs±(Ky)+Rt+Qz+Liq (melt), peak temperature Tmax stage M3 characterized by Grt3 (garnet rim)+Sill+Pl+Kfs+Qz+Ilm+Liq, and retrograde stage M4 represented by Grt (in matrix)+Kfs+ Sill+Bt+Pl+Qz+Ilm. By using the THERMOCALC V340, the P-T conditions are estimated at ~13.8-14.1 kbar and ~840-850℃ at stage M2, and 7-7.2 kbar and 909-915℃ for the Tmax stage M3, indicating an ultra-high temperature (UHT) metamorphic overprinting during decompression and heating process after high pressure granulite facies metamorphism. The mineral assemblages and their P-T conditions presented a clockwise P-T trajectory for the Huangtuyao pelitic granulites. The major metamorphic events at ~1.95 and ~1.88 Ga obtained by the zircon U-Pb dating suggest that pelitic granulites from the Huangtuyao area has undergone HP granulite metamorphism which probably occurred in the prograde metamorphism and related to the collision between the Ordos and the Yinshan blocks, and afterwards UHT metamorphism is related to crustal extension after continental-continental collision.
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Ultrahigh Temperature Metamorphic Record of Pelitic Granulites in the Huangtuyao Area of the Huai'an Complex, North China Craton

doi: 10.1007/s12583-019-1245-6
    Corresponding author: Xu-Ping Li

Abstract: Pelitic granulite from the Huangtuyao area, occurrs in the Huai'an Complex, is located in the Trans-North China Orogen of the North China Craton. On the basis of petrolography, mineral com-ponent, and phase equilibrium modeling studies, the P-T conditions and mineral assemblages of pelitic granulites can be divided into four metamorphic stages:the prograde metamorphic stage M1 defined by the stable mineral assemblage of Grt1 (garnet core)+Pl+Bt+Kfs+Qz+Rt, the peak pressure Pmax stage M2 indicated by Grt2 (garnet mantle)+Kfs±(Ky)+Rt+Qz+Liq (melt), peak temperature Tmax stage M3 characterized by Grt3 (garnet rim)+Sill+Pl+Kfs+Qz+Ilm+Liq, and retrograde stage M4 represented by Grt (in matrix)+Kfs+ Sill+Bt+Pl+Qz+Ilm. By using the THERMOCALC V340, the P-T conditions are estimated at ~13.8-14.1 kbar and ~840-850℃ at stage M2, and 7-7.2 kbar and 909-915℃ for the Tmax stage M3, indicating an ultra-high temperature (UHT) metamorphic overprinting during decompression and heating process after high pressure granulite facies metamorphism. The mineral assemblages and their P-T conditions presented a clockwise P-T trajectory for the Huangtuyao pelitic granulites. The major metamorphic events at ~1.95 and ~1.88 Ga obtained by the zircon U-Pb dating suggest that pelitic granulites from the Huangtuyao area has undergone HP granulite metamorphism which probably occurred in the prograde metamorphism and related to the collision between the Ordos and the Yinshan blocks, and afterwards UHT metamorphism is related to crustal extension after continental-continental collision.

Shaoting Ma, Xu-Ping Li, Hao Liu, Fanmei Kong, Han Wang. Ultrahigh Temperature Metamorphic Record of Pelitic Granulites in the Huangtuyao Area of the Huai'an Complex, North China Craton. Journal of Earth Science, 2019, 30(6): 1178-1196. doi: 10.1007/s12583-019-1245-6
Citation: Shaoting Ma, Xu-Ping Li, Hao Liu, Fanmei Kong, Han Wang. Ultrahigh Temperature Metamorphic Record of Pelitic Granulites in the Huangtuyao Area of the Huai'an Complex, North China Craton. Journal of Earth Science, 2019, 30(6): 1178-1196. doi: 10.1007/s12583-019-1245-6
  • Granulite facies rocks undergoing extreme conditions, with the peak temperature reaching more than 900–1 100 ºC in the deep crust (20–40 km), are referred to as UHT rocks (e.g., Li X-P et al., 2019; Santosh et al., 2012; Harley, 2008, 2004, 1998). The UHT rocks usually contain indicative mineral assemblages, such as, sapphirine+quartz (Liu et al., 2010; Santosh et al., 2009a, b; Tsunogae et al., 2003; Dallwitz, 1968), spinel+quartz (e.g., Liu et al., 2012; Zhang et al., 2012; Hensen, 1987), hypersthene+sillimanite+quartz (White et al., 2008; Kelsey et al., 2004; Tateishi et al., 2004), osumilite-bearing granulite (Korhonen et al., 2013; Das et al., 2001; Ellis et al., 1980), and inverted pigeonite (Sandiford and Powell, 1986). The recent progressive studies reported that the pelitic granulites without the above diagnostic mineral assemblages may have also undergone UHT metamorphism, in particular, with ternary feldspar such as mesoperthite and antiperthite (e.g., Liu et al., 2019; Li and Wei, 2016; Jiao and Guo, 2011). For the UHT granulite with ternary feldspar, it must be formed in anatexis during high grade metamorphism, rather than the magmatic residues; therefore, residual mesoperthite and antiperthite from orthogneiss before metamorphism are not the indicative minerals of UHT metamorphism (e.g., Li X-P et al., 2019; Harley, 2008).

    The North China Craton (NCC) includes three Paleoproterozoic orogenic/mobile belts such as the Jiao-Liao-Ji belt, the khondalite belt and the Trans-North China Orogen (TNCO) (Fig. 1a; Zhao et al., 2005). The Huaiʼan Complex is located in the northwestern margin of the TNCO, and adjacent to the khondalite belt of the Western Block. It consists of 80% tonalite- trondhjemite-granodiorite (TTG) gneisses, and a few pelitic and mafic granulites (Figs. 1b, 2) (Zhao et al., 2005). The relationships of these three rock types in the Huaiʼan Complex are controversial. The main point of view is that pelitic granulites in the Huai'an Complex may have derived from the rocks in the western khondalite series during the collision of the Eastern and Western blocks at about 1.85 Ga (Liu et al., 2019; Zhao et al., 2010, 2005; Zhang et al., 1994). The previous petrologic studies indicate that both the mafic and pelitic granulites in the Huaiʼan Complex present a clockwise P-T path, and zircon U-Pb dating reveals the protolith age of these granulites at ~2.0 Ga, and metamorphic ages of ~1.95–1.85 Ga (Liu et al., 2019; Wang et al., 2016).

    Figure 1.  (a) Major tectonic units of the North China Craton (after Zhao et al., 2005); (b) schematic geological map of the Huaiʼan Complex in the northern segment of the TNCO of the NCC (modified after Guo et al., 2002). TNCO. Trans-North China Orogen; NCC. North China Craton.

    Figure 2.  Geological lithologic distribution of Huangtuyao area, Huaiʼan Complex (after Zhang et al., 2014).

    The pelitic granulites from Huangtuyao of the Huai'an Complex were selected for this study. We present results from petrological and mineralogical analyses, thermodynamic modeling results of P-T conditions and zircon U-Pb data. Despite lack of the diagnostic UHT mineral assemblages, the results of pelitic granulites show that they may have experienced early high-pressure (HP) granulite metamorphism, and subsequently overprinted by an UHT event.

  • The evolution of Paleoproterozoic orogenic belts in the NCC is an issue worthy of attention (Meng et al., 2018). The previous studies mainly focus on the tectonic process and timing of the amalgamation of various blocks. There are generally three models that are proposed to understand the magmatic and metamorphic events for the evolution of the Paleoproterozoic orogenic belts in the NCC. Zhai and Santosh (2011) suggested that three mobile belts, including Jiao-Liao-Ji, Fengzhen and Jinyu were divided, and all of them were formed around ~1.95 Ga, mainly on the basis of the age dating from granulite-facies terrains. Zhao et al.(2005, 2001) also suggested that there were three collisional belts, the Jiao-Liao-Ji belt, the TNCO and the khondalite belt from east to west NCC (Fig. 1a). This model, however, considered that the khondalite belt of the Western Block was formed at ~1.95 Ga by collision of the Yinshan and Ordos blocks, and the TNCO was formed at ~1.85 Ga by the collision of the Eastern and Western blocks. The ages of ~1.95 and ~1.85 Ga were referred as the peak metamorphic stages of granulites for the khondalite belt and TNCO, respectively. In addition, there is another model, which proposes an Inner Mongolia-northern Hebei Orogen (IMNHO), extending from east to west. The IMNHO is developed by collision of an arc setting with the northern margin of the NCC at ~2.3 Ga ago; afterwards, the closure of the back arc basin leads to a convergent deformation at 1.92–1.85 Ga ago (e.g., Wei, 2018; Kusky, 2011; Kusky and Li, 2003).

    The khondalite belt, an EW-trending orogenic belt, extends from Jining in the east, to Qilianshan and Helanshan in the west. The dominant lithologies of the belt consist of more than 80% khondalite-series rocks, which are garnet-sillimanite gneiss, garnet quartzite, felsic paragneiss, calc-silicate rock and marble, and less amount of Late Archean to Paleoproterozoic TTG gneisses, mafic granulites, chainockites and S-type granites (Guo et al., 2012; Li et al., 2011; Zhao et al., 2010, 2005). The TNCO consists of ~80% TTG gneisses, and a small amount of greenschist facies mafic rocks, amphibolites, high-pressure granulites and retrograded eclogites (Zhao et al., 2010; Guo et al., 2002; Zhai et al., 1995, 1992).

    Huai'an Complex is located in the northern portion of the TNCO, at the junction of the khondalite belt and the Trans-North China Orogen (Fig. 1b). It thus becomes an important window to explore the Paleoproterozoic crust evolution of the NCC.

    The major lithologies in the Huai'an Complex are: (1) gray TTG gneisses; (2) meta-granites and mafic granulite; (3) some meta-sedimentary rocks. All of these lithological units experienced granulite facies metamorphism (Zhang et al., 2011; Zhao et al., 2010; Wu et al., 1998; Liu et al., 1996). The formation ages of the TTG gneisses are ~2.53–2.45 Ga (Li et al., 2018; Liu et al., 2012; Zhang et al., 2012; Zhao et al., 2008). Some garnets bearing potassic granite intrusions show the magmatic ages of ~2.5 Ga and the metamorphic age of ~1.80 Ga (Zhang et al., 2011). The mafic high-pressure (HP) granulites have formation ages of ~2.0 Ga and metamorphic ages of ~1.95–1.85 Ga (Wang et al., 2016). The pelitic granulites include garnet-sillimanite gneiss, garnet-feldspar gneiss, and garnet-bearing quartzitic gneiss with minor calc-silicate rocks (Zhang et al., 2012, 2011; Zhao et al., 2010; Wu et al., 1998). The magmatic zircon U-Pb ages of these meta-sedimentary rocks are ~2.15–2.0 Ga (Zhang et al., 2014; Li et al., 2011; Wan et al., 2006), and the metamorphic ages are ~1.95–1.82 Ga (Liu et al., 2019; Zhao et al., 2010). The mineral assemblage of kyanite-garnet-K-feldspar was reported from the pelitic granulite of the Huai'an Complex, suggesting that rocks experienced HP granulite facies metamorphism (Wu et al., 2016).

  • The chemical composition of minerals was analyzed by using a JEOL JXA-8230 electron microprobe at Tongji University, Shanghai, under the conditions of 10 nA probe current of a 1–3 μm diameter beam, 15 kV accelerating voltage. The analytical data for garnet profiles and represented mineral analyses are presented in Tables 12. The bulk-rock compositions of the samples was analyzed by X-ray fluorescence spectrometry (XRF) at the Hebei Institute of Regional Geological and Mineral Survey, China.

    Mineral Garnet profile Figs. 3a, 3b (corresponding to Table S1) Biotite Plagioclase K-feldspar
    Position Grt1 in core Grt2 in mantle Grt3 in rim Inclusion in Grt1 In matrix In garnet core In garnet rim In perthite Pl in matrix Kfs in perthite
    Stage M1 M2 M3 M1 M4 M1 M3 M3 M4 M3
    Min Max Min Max Min Max
    SiO2 38.68 38.81 39.20 38.57 39.44 38.42 38.52 35.69 36.06 59.20 59.39 60.41 60.80 61.04 59.77 62.27 62.58 65.04 62.98 65.04 62.98
    TiO2 0.04 0.00 0.06 0.00 0.06 0.00 5.17 6.78 7.08 0.00 0.01 0.02 0.07 0.00 0.01 0.00 0.02 0.06 0.03 0.06 0.03
    Al2O3 22.52 21.77 22.14 22.50 22.68 22.42 13.15 14.77 14.85 25.06 25.14 24.87 23.95 23.67 25.60 23.94 23.36 18.40 22.57 18.40 22.57
    Cr2O3 0.03 0.03 0.04 0.03 0.00 0.00 0.06 0.10 0.13 0.01 0.00 0.03 0.06 0.01 0.00 0.00 0.02 0.00 0.00 0.00 0.00
    FeO 27.04 27.23 26.75 26.54 27.10 27.15 9.11 13.28 13.56 0.00 0.00 0.00 0.00 0.00 0.16 0.00 0.03 0.02 0.00 0.02 0.00
    MnO 0.23 0.23 0.25 0.25 0.21 0.29 0.04 0.00 0.00 0.02 0.00 0.00 0.02 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    MgO 10.15 10.25 10.39 9.54 10.16 9.70 16.94 12.26 12.37 0.01 0.00 0.01 0.02 0.02 0.01 0.00 0.00 0.00 0.01 0.00 0.01
    CaO 1.16 1.25 1.38 1.56 0.83 1.11 0.04 0.00 0.00 6.94 7.12 6.89 5.78 5.85 7.62 5.25 5.21 0.08 4.21 0.08 4.21
    Na2O 0.01 0.02 0.01 0.03 0.02 0.02 0.29 0.19 0.19 7.09 7.33 6.70 8.27 8.18 6.53 8.57 8.41 1.25 7.51 1.25 7.51
    K2O 0.01 0.02 0.00 0.04 0.01 0.01 8.53 9.05 9.25 0.58 0.50 0.48 0.21 0.14 0.90 0.16 0.20 14.76 2.46 14.76 2.46
    Total 99.88 99.61 100.22 99.06 100.51 99.12 92.47 92.12 93.51 99.00 99.58 99.63 99.51 99.40 100.72 100.25 99.86 99.97 99.90 99.97 99.90
    Oxygen 12 11 8
    Si 2.96 2.98 2.99 2.98 3.00 2.97 2.89 2.74 2.74 2.67 2.66 2.70 2.72 2.73 9.95 2.75 2.78 2.72 2.72 3.00 3.01
    Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.29 0.39 0.40 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Al 2.03 1.97 1.99 2.05 2.03 2.04 1.16 1.34 1.33 1.33 1.33 1.31 1.26 1.25 5.03 1.25 1.22 1.28 1.28 1.00 0.99
    Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Fe3+ 0.04 0.07 0.03 0.00 0.00 0.02
    Fe2+ 1.69 1.68 1.68 1.72 1.72 1.73 0.57 0.85 0.86 0.00 0.00 0.01 0.01 0.02 0.03 0.00 0.00 0.00 0.00 0.00 0.00
    Mn 0.02 0.02 0.02 0.02 0.02 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Mg 1.16 1.17 1.18 1.10 1.16 1.12 1.89 1.40 1.40 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Ca 0.10 0.10 0.11 0.13 0.07 0.09 0.00 0.00 0.00 0.34 0.34 0.33 0.28 0.28 1.36 0.25 0.25 0.27 0.28 0.00 0.01
    Na 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.03 0.03 0.62 0.64 0.58 0.72 0.71 2.11 0.74 0.72 0.71 0.69 0.11 0.09
    K 0.00 0.00 0.00 0.00 0.00 0.00 0.82 0.89 0.90 0.03 0.03 0.03 0.01 0.01 0.19 0.01 0.01 0.02 0.02 0.87 0.88
    XCa 0.032 0.034 0.037 0.044 0.022 0.031
    XMg 0.77 0.62 0.62
    Ti 0.29 0.39 0.40
    An 0.34 0.34 0.35 0.28 0.28 0.37 0.25 0.25 0.27 0.29 0.00 0.01
    Or 0.03 0.03 0.03 0.01 0.01 0.05 0.01 0.01 0.02 0.02 0.88 0.90
    Ab 0.63 0.63 0.62 0.71 0.71 0.58 0.74 0.74 0.71 0.70 0.11 0.09
    XMg=Mg2+/(Fe2++Mg2++Mn2++Ca2+); XCa=Ca2+/(Fe2++Mg2++Mn2++Ca2+).

    Table 1.  Representative EMP analyses data (wt.%) of major minerals for samples 17HTY3-1 and 17HTY3-2 from the pelitic granulite in Huangtuyao, Huai'an Complex

    Sample 17HTY3-1-1 17HTY3-1-2 17HTY3-1-3 17HTY3-2-1 17HTY3-2-2
    Texture Perthitic feldspar Perthitic feldspar Perthitic feldspar Perthitic feldspar Perthitic feldspar
    Kfs
    (host)
    Pl
    (lamellae)
    Kfs
    (host)
    Pl
    (lamellae)
    Kfs
    (host)
    Pl
    (lamellae)
    Kfs
    (host)
    Pl
    (lamellae)
    Kfs
    (host)
    Pl
    (lamellae)
    SiO2 65.04 62.58 65.25 62.44 64.53 61.99 65.97 62.76 65.62 62.89
    TiO2 0.06 0.02 0.00 0.01 0.01 0.00 0.02 0.03 0.03 0.00
    Al2O3 18.40 23.36 18.27 23.55 18.12 23.38 18.57 23.57 18.44 23.12
    Cr2O3 0.00 0.02 0.01 0.01 0.03 0.04 0.01 0.00 0.00 0.00
    FeO 0.02 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    MnO 0.00 0.00 0.00 0.00 0.04 0.01 0.00 0.01 0.00 0.00
    MgO 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.02 0.00
    CaO 0.08 5.21 0.12 5.12 0.19 5.30 0.09 5.25 0.13 4.96
    Na2O 1.25 8.41 1.01 8.64 0.78 8.71 1.04 8.40 1.24 8.71
    K2O 14.76 0.20 14.96 0.18 15.14 0.18 15.21 0.19 15.10 0.27
    Total 99.97 99.86 99.63 99.95 98.87 99.65 100.94 100.23 100.58 99.99
    Si 3.000 2.775 3.009 2.767 3.005 2.760 3.005 2.771 3.002 2.785
    Ti 0.002 0.001 0.000 0.000 0.000 0.000 0.001 0.001 0.001 0.000
    Al 1.001 1.221 0.993 1.230 0.995 1.227 0.997 1.227 0.994 1.207
    Cr 0.000 0.001 0.000 0.000 0.001 0.001 0.000 0.000 0.000 0.000
    Fe2+ 0.001 0.001 0.000 0.000 0.000 0.001 0.000 0.001 0.000 0.001
    Mn 0.000 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.000
    Mg 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.001 0.000
    Ca 0.004 0.247 0.006 0.243 0.009 0.253 0.005 0.248 0.006 0.236
    Na 0.111 0.723 0.091 0.742 0.070 0.752 0.092 0.719 0.110 0.748
    K 0.868 0.011 0.880 0.010 0.899 0.010 0.884 0.011 0.881 0.015
    An 0.004 0.252 0.006 0.244 0.009 0.249 0.005 0.254 0.006 0.236
    Ab 0.113 0.737 0.093 0.746 0.072 0.741 0.094 0.735 0.110 0.749
    Or 0.883 0.011 0.901 0.010 0.919 0.010 0.901 0.011 0.884 0.015

    Table 2.  Representative EMP analyses data (wt.%) for perthite from pelitic granulite in the Huangtuyao terrane (data for the mineral pair of plagioclase lamellae and host K-feldspar)

    Zircon separation and CL imaging were carried out at the Mineral Separation Laboratory of the Hebei Institute of Regional Geological and Mineral Survey, Langfang, Hebei. The U-Th-Pb analyses and trace element analyses were done using GeoLas Pro HD and Agilent 7900 housed at the Wuhan Sample Solution Analytical Technology Co. Ltd., Wuhan, China. The analytical procedures and quality control follow Wang S J et al.(2019, 2018), Wang X et al. (2018), Hu et al. (2011), and Liu S J et al. (2010), Liu Y S et al. (2008). Zircon 91500 was used as an external standard to correct instrumental mass discrimination and elemental fractionation during the ablation. Zircon GJ-1 was treated as a quality control for geochronology. The lead abundance of zircon was externally calibrated using NIST SRM 610 with Si as internal standard, while Zr as an internal standard for trace elements. The age calculations and the concordia diagrams were conducted by the ISOPLOT 4.15 (Ludwig, 2003). The REE concentrations of zircons were normalized using chondrite values after Sun and McDonough (1989).

    Mineral abbreviations used in this paper are taken from Whitney and Evans (2010).

  • The representative samples were taken from the same outcrop of pelitic granulites in the Huangtuyao area (Fig. 3a), showing medium-fine grained and gneissic structure (Figs. 3a–3b). The major minerals of the rocks are garnet (20%–30%), feldspar (20%–30%), quartz (15%–20%), sillimanite (10%–15%), minor component of biotite (< 5%), and accessory minerals consist of rutile, ilmenite, graphite, apatite and zircon. The mineral assemblages can be classified into four generations as follows, which represent four metamorphic stages from M1, M2, M3 to M4 correspondingly.

    Figure 3.  Sillimanite-garnet-K-feldspar gneisses from Huangtuyao area of the Huaiʼan Complex. Photographs showing (a) the outcrop of pelitic granulite in Huangtuyao, and (b) hand specimen of pelitic granulite; microphotographs showing (c) garnet porphyroblast with inclusions and minerals in the matrix, (d) Rt replaced by ilmenite, graphite in fracture; BSE images showing (e) the mantle and rim portion of a garnet porphyroblast and inclusions, and (f) inclusions in porphyroblastic garnet; and microphotographs showing (g) plagioclase, biotite and K-feldspar grains in matrix, (h), (i) melting textures at garnet boundary and in between quartz and feldspar. Hol for hollow, Per for perthite, other abbreviations after Whitney and Evans (2010).

    Most of the garnet porphyroblasts are 1–2 mm except for a few fine grains. The garnet porphyroblasts are commonly anhedral-subhedral in shape with melting corroded rims and generally have inclusions of plagioclase, sillimanite, K-feldspar, quartz, biotite, ilmenite and rutile (Figs. 3d–3f, 4). The mineral inclusions are distributed in different zones of garnet: (1) a swarm of minerals, involving quartz, biotite, plagioclase, K-feldspar, occur in the garnet core, representing the products of prograde metamorphism (M1, Figs. 3c, 4a–4b); (2) a few inclusions, such as quartz, K-feldspar, sillimanite, and rutile, which are occasionally found in the garnet mantle, are produced from peak pressure metamorphism (M2, Figs. 3c, 3e and 3f); (3) minor inclusions, such as sillimanite, plagioclase, ilmenite and quartz, are present in the rim of garnets, which were formed in the decompression process after peak pressure stage (M3, Figs. 3e–3f, 4). In addition, small grain peritectic garnets are of typical melting structure, also representing metamorphic stage M3 (Figs. 3h, 3i). Moreover, the assemblage of sillimanite, plagioclase, quartz, K-feldspar and ilmenite surrounding porphyroblastic garnet, represents the products of the retrograde cooling metamorphism (M4, Figs. 3c and 3d).

    Figure 4.  Representative compositional profiles of porphyroblastic garnets from samples 17HTY3-1 and 17HTY3-2.

    Sillimanite shows three occurrences: (1) as fine grained inclusions at the rim of the porphyroblastic garnet (M3, Fig. 4a); (2) subhedral crystals intergrowth with K-feldspar, biotite and quartz as retrogressive corona texture of garnet at stage M4 (Figs. 3c, 3d); (3) as euhedral crystals in matrix (M4, Fig. 3c).

    Feldspars are plagioclase and K-feldspar. Plagioclase shows three occurrences, which are in the garnet core (M1, Fig. 3c), garnet rim (M3, Fig. 3e) and in the matrix of the pelitic granulite (M4, Fig. 3g). In addition, plagioclase also occurs as lamellae of solid solutions within perthite, which are formed at M3 stage (Figs. 3c–3d). K-feldspars can occur at any metamorphic stages, showing as inclusions in all zones of the garnet porphyroblasts from core to rim (Figs. 3c–3d, 4a) and in the matrix of pelitic granulite (Figs. 3c and 3g).

    Biotite occurs as small inclusion in the core of the garnet porphyroblast (M1, Fig. 3f), and as flake in the porphyroblastic garnet rim (M4, Fig. 3e), or intergrowth with plagioclase, K-feldspar and quartz in the matrix (Fig. 3g).

    Rutile is observed in the garnet mantle, clearly representing the pressure peak stage M2 (Fig. 3f); but it is also found at the rim of garnet grain (Figs. 3c–3d). Most rutile grains are replaced by ilmenite during decompression process from M2 to M3 stages or at M3 stage (Figs. 3c, 3d, 3f). Other accessory minerals like graphite, apatite and zircon are observed as inclusions in garnet, feldspar and quartz, or in the matrix.

    Melting evidence is widely observed in samples, including (1) the melt film at the garnet boundary or in grain boundaries of quartz and feldspar, formed at M3 stage (Figs. 3h and 3i, Liu et al., 2019; Sawyer, 1999); (2) the presence of rounded quartz in garnet mantle suggesting that the melt involves in the growth of garnet porphyroblast during the M2 and M3 stages (Fig. 3e; Liu et al., 2019).

    Above all, petrographic studies provide information of four metamorphic stages. The mineral assemblages are: ① Grt1+Pl+ Bt±Kfs+Qz+Rt: present in the core of the porphyroblastic garnet, representing the prograde metamorphic stage M1; ② Grt2+Kfs± (Ky)+Rt+Qz+Liq (melt): included in the mantle of porphyroblastic garnet, showing the characteristic assemblage of the peak pressure metamorphic stage (M2); ③ Grt3+Sill+Pl+Kfs+Qz+ Ilm+Lid: occurring as inclusions in garnet thin rim, representing the third metamorphic stage M3; ④ Grt+Kfs+Sill+Bt+Pl+ Qz+Ilm, recrystallized as small grains in the matrix and in the melt coronas of the porphyroblastic garnet rim, representing the fourth metamorphic stage M4.

  • Samples 17HTY3-1 and 17HTY3-2 were selected for detailed EMP analysis of mineral compositions. The compositions of major minerals are given in Tables S1 and 1.

    Garnet is dominated by almandine (XFe) ~56 mol.%–58 mol.%, pyrope (XMg) ~37 mol.%–40 mol.%, a small amount of grossular (XCa) ~2.2 mol.%–4.4 mol.% and inappreciable spessartine (XMn) ~0.3 mol.%–1.4 mol.% (Table S1). In order to find out the compositional variations corresponding to the texture of garnet zoning, two compositional profiles of porphyroblastic garnet were analyzed and plotted in Fig. 4. The results show that the XMg, XFe and XMn contents are basically invariable from core to rim. The XCa contents show distinguishable variations from the core to rim. In the garnet profile of sample 17HTY3-1 (Fig. 4a), the XCa content is ~3.2 mol.%–3.4 mol.% in the garnet core (M1), while a higher value of ~3.7 mol.%–4.4 mol.% in the garnet mantle (M2) with a few transitional compositions from M2 to M3 (~3.2 mol.%–3.3 mol.%, Table S1) and ~2.2 mol.%–3.1 mol.% in the garnet rim (M3). In the second garnet profile of sample 17HTY3-2 (Fig. 4b), the XCa content from the garnet core, mantle to rim are 4.2 mol.%–4.4 mol.%, 4.2 mol.%–4.4 mol.% and 2.6 mol.%–3.1 mol.%, respectively, except for the transitional compositions from M2 to M3 (3.4 mol.%–4.0 mol.%). In the garnet profile 17HTY3-2, the composition from the core to mantle of garnet profile seems to preserve better information for M2 stage metamorphism (Fig. 4b). The low content of XCa in the garnet mantle of the profile 17HTY3-1 may be caused by the development of fractures across through rim to mantle (at A end of the profile 17HTY3-1, Fig. 4a).

    Biotite presents XMg value of 0.77 and TiO2 content of 5.17 wt.% as inclusion in the core of the garnet, representing the prograde metamorphism (M1); it also occurs as small patch in the matrix, showing XMg ~0.62 and TiO2 contents of 6.78 wt.%–7.08 wt.%.

    Plagioclase presents as inclusions with XAn content ~0.34– 0.35 in the garnet core, ~0.27–0.37 in the garnet rim, and XAn content is ~0.25 as solid solution lamella within perthite crystal in the matrix. The plagioclase occurs as subhedral crystals in the matrix with XAn of 0.27–0.28.

  • Phase equilibria are modeled in the system NCKFMATSHO (Na2O-CaO-K2O-FeO-MgO-Al2O3-TiO2-SiO2-H2O-Fe2O3) by considering the mineral assemblages and compositions for the representative of sillimanite-garnet-K-feldspar gneiss sample 17HTY3-1. Thermodynamic calculations of pseudosections were performed using THERMOCALC V340 (Powell and Holland, 1988), and the internally consistent thermodynamic database used is the ds62 of Holland and Powell (2011) with the re-parameterized a-x models for calculated system (White et al., 2014).

    The bulk composition of sample 17HTY3-1 presents SiO2 69.18 wt.%, Al2O3 15.01 wt.%, TiO2 0.66 wt.%, MgO 1.86 wt.%, CaO 0.84 wt.%, Fe2O3 1.51 wt.%, FeO 4.40 wt.%, K2O 3.65 wt.%, Na2O 1.93 wt.%, MnO 0.03 wt.%, P2O5 0.03 wt.% and LOI 0.69 wt.%. The MnO and P2O5 contents are not taken into account for the construction of pseudosections, and other compositions are normalized to 100% in order to establish phase equilibrium in the NCKFMATSHO system.

  • A T-XH2O diagram was computed at 7.5 kbar from a near- anhydrous composition to excess H2O (XH2O=0–1) to evaluate the involvement of melt in the NCKFMATSHO system (Fig. 5). At this pressure, the final metamorphic stage M4 (Grt+Kfs+ Sill+Bt+Pl+Qz+Ilm) is closest to the solidus of pressure range (Figs. 5 and 6). Melt loss has occurred before granulite reaches its peak temperature metamorphic conditions (i.e., Liu et al., 2019; White and Powell, 2007). There is no distinct change of the mineral compositions and assemblages above solidus fields in comparison of the measured bulk composition and melt re-integrated bulk composition controlled the P-T pseudosection (e.g., Wu et al., 2017, 2016; Indares et al., 2008).

    Figure 5.  T-XH2O diagram at 7.5 kbar for sample 17HTY3-1 of the garnet-sillimanite-K-feldspar gneisses.

    Figure 6.  (a) P-T pseudosections calculated from the measured whole-rock composition of sample 17HTY3-1. The isopleth of Xca in garnet, XMg in biotite and XAn in plagioclase for relevant assemblages are plotted in the P-T pseudosection(Tables 1 and 2), orange oval taken from Wang et al. (2016); (b)the further constraint of Tmax (M3)defined by two-feldspar geothermometer.

    The rock was formed at a low oxygen fugacity condition due to the existence of graphite in the mineral assemblage. The fO2=10-16 was chosen for further calculation of the P-T pseudosection for pelitic granulite of this study. This value is based on the diagrams of fO2-XH2O and fO2-T, which are calculated for pelitic graphite-bearing gneisses by Pattison (2006) and Glassley (1982), respectively. The stable field of graphite is at a range fO2 ~10-12–10-25, while fO2=10-16 is corresponding to the conditions of T < 900 ºC and XH2O < 0.4, which is consistent with the pelitic granulite of this study. Rutile is absent as predicted from the investigated assemblages, and seems to be affected more by pressure rather than by temperature or H2O content (Fig. 5). The muscovite exists when XH2O > 0.5, but no muscovite is found in this study, the definition of XH2O=0.25, therefore is chosen as a median value from 0 to 0.5 for further constraint of the P-T pseudosection.

  • The P-T pseudosection calculated for sample 17HTY3-1 is constructed with the P-T range of 5–15 kbar and 700–1 000 ºC (Fig. 6). K-feldspar and quartz are present in all P-T range, and the fluid-absent solidus occurs at temperatures of ~750–810 ºC. Rutile occurs at high pressure above 8 kbar and is replaced by ilmenite as pressure decreases. Biotite is stable below ~780 ºC and 11 kbar. Plagioclase is absent above ~850–950 ºC in the medium pressure region. Melt appears above 770 ºC in this study.

    The isopleths of XCa [=Ca2+/(Ca2++Mg2++Fe2+)] in garnet are nearly parallel or medium slope to temperature coordinate in the most relevant regions, indicating that it is mainly affected by the variations of pressure. The XCa here is slightly different from that in the garnet described above petrography section because Mn is omitted from the system NCKFMATSHO. Plagioclase exists under medium to low pressure, and can be stable even under ultra-high temperature condition. The isopleths of XAn in plagioclase show medium slope in the plagioclase- bearing assemblages, and thus are used to constrain P-T condition together with XCa of garnet. The biotite composition is easily modified by high grade metamorphism (e.g., Liu et al., 2019), the isopleths of XMg in biotite can only be used as indicator of an evolutionary trend of P-T conditions.

    At the pre-peak M1 stage, the mineral assemblage (Grt1+ Pl+Kfs+Bt±Mc+Qz+Rt) together with XCa=0.032–0.034 obtained from the core of Grt1 in the first compositional profile (17HTY3-1) (Table S1). The XCa from the core of Grt1 in the second compositional profile (17HTY3-2) actually represents arising trend from M1 to M2 stages during prograde metamorphism. Since aluminosilicate minerals are not found in the Grt core, and the whole mineral assemblage Grt1+Pl+Kfs+Bt±Mc+ Qz+Rt, which is supposed for M1, is also not completely found out. In addition, the composition of plagioclase inclusion in garnet core (Table 1) has been revised by late high-grade HP-UHT metamorphism and is not suitable using for the constraint of M1 stage P-T conditions.

    The peak pressure metamorphic stage M2 consists of the mineral assemblage of Grt+Kfs+Qz±Rt±Ky+Liq. Although kyanite is not found in this study, it is possible to have been replaced by sillimanite during decompression process and late metamorphic stages since the fracture from rim to mantle of garnet could facilitate this transformation (Fig. 3e). The isopleths of XCa obtained from Grt2, the mantle of garnet profile 17HTY3-1 (0.037–0.043) and the core to mantle of garnet profile 17HTY3-2 (0.042–0.044) are plotted in the field of this mineral assemblage of peak pressure stage M2 (Table 1). In addition, Wang et al. (2016) reported that the mafic granulite of Huangtuyao area, which comes from the same metamorphic terrane with pelitic granulite in this study, contained kynite and experienced HP metamorphism (orange oval in Fig. 6a). We therefore take the intersecting point of pressure peak plot of the Huangtuyao mafic granulite (orange oval in Fig. 6a) and maximum isopleth XCa (=0.44) of garnet mantle that is in the stable domain of mineral assemblage in this stage. The peak pressure metamorphic conditions of M2 stage, therefore, are suggested at ~13.8–14.1 kbar. It is not possible to constrain the precise temperature in this study, so that we use dashed line to connect M2 with M3 stages. The of temperature stage M2, therefore, is estimated as ~840–850 ºC (Fig. 6).

    The metamorphic stage of Tmax (M3) has mineral assemblage of Grt+Kfs+Pl+Sill+Qz+Ilm+Liq. The XCa value (0.022–0.031) of Grt3 from garnet rim of the garnet profiles 17HTY3-1 and 17HTY3-2 and the XAn (0.28–0.37) of plagioclase inclusion define the maximum P-T conditions of the metamorphic stage M3 are ~7 kbar and ~910 ºC (Fig. 6a). Thus, it is suggested that the pelitic granulite of this study has experienced UHT metamorphism, which possibly overprints HP metamorphic stage M2.

    Perthite is widespread in the matrix of the Huangtuyao pelitic granulite. Four groups of perthite/mesoperthite compositions (including Pl lamellae and Kfs host) are analyzed, and the data are listed in Table 2. Microscope and BSE images show that plagioclase lamellae take ~11.2% to 20.5% in perthite of these samples (Fig. 7). Recent studies indicated that dissolution of apatite in the anatectic melt in pelitic granulites will affect the P-T estimate of metamorphic stage by using XCa of garnet in NCKFMASHTO system (e.g., Indares and Kendrick, 2018). Although apatite is not found as inclusion within garnet rim here, we still choose two-feldspar geothermometer to further constrain the metamorphic temperature condition of M3 stage (Fig. 6b). We choose the thermodynamic model of Fuhrman and Lindsley (1988) and Benisek et al. (2004) for further calculation, which are the robust calibrations for temperature estimation of granulite facies rocks. The reintegrated compositions of feldspars in pelitic granulite yield maximum temperature of 909–912 ºC/7 kbar and 911–915 ºC/8 kbar (Table 3, Fig. 8). On the basis of above results, the P-T conditions of M3 stage are estimated at ~7–7.2 kbar and 909–915 ºC.

    Figure 7.  BSE images (a)–(d) and microphotographs (e)–(f) showing perthite texture in the pelitic granulite from Huangtuyao of the Huaiʼan Complex.

    Sample Areal (%) EPMA data (mol.%) Re-integrated composition (mol.%) T (℃)/7 kbar T (℃)/8 kbar
    Pl
    (lamellae)
    Kfs
    (Host)
    Pl domin Kfs domin T
    (FL)
    T
    (BN)
    T
    (FL)
    T
    (BN)
    Ab Or An Ab Or An Ab Or An
    17HTY3-1-1 20.5 79.4 0.737 0.011 0.251 0.112 0.883 0.004 0.24 0.704 0.054 909 910 911 913
    17HTY3-1-2 17.7 82.2 0.745 0.01 0.244 0.093 0.9 0.006 0.209 0.742 0.048 895 897 897 900
    17HTY3-1-3 11.2 88.7 0.74 0.009 0.249 0.071 0.919 0.009 0.146 0.816 0.036 849 855 851 857
    17HTY3-2-2 20.4 79.5 0.748 0.015 0.236 0.11 0.015 0.236 0.241 0.705 0.053 911 912 913 915

    Table 3.  Re-integrated compositions of feldspar with areal proportion and chemical compositions of lamellae and host domains

    Figure 8.  Ternary plots of re-integrated feldspar compositions for all analyzed samples with the solvus calculated at 0.7 and 0.8 Ga using the model of Fuhrman and Lindsey (1988).

    At retrograde metamorphic stage M4 (Grt+Sil+Kfs+Bt+ Pl+Qz+Ilm), the P-T conditions can be constrained by the mineral assemblage in the matrix, combined with XMg (=0.62) of biotite of the matrix and XAn (0.27–0.29) of plagioclase in the matrix. Although TiO2 content in biotite is quite high (6.78 wt.%–7.08 wt.%), the isopleth of XMg in biotite from M4 satge doesn't pass over mineral assemblage stable area of Grt+Sill+ Kfs+Bt+Pl+Qz+Ilm+Liq. The P-T conditions of M4 stage are therefore constrained at < 780 ºC/5 kbar (Fig. 6a).

  • Zircon in the pelitic granulites of this study is present as inclusions in garnet, quartz, and feldspar, within the matrix between quartz and feldspar, or adjacent to garnet rim. Zircon grains in the sample are transparent or translucent, subhedral- anhedral in shape with the grain size of 80–120 μm. The CL patterns of zircon indicate patchy, structureless, irregular blurred zonation with the presence of some small dark cores. Rare grains record zoned cores, but some with thin and anhedral rims (Fig. 9a). The feature of zircon grains indicate that they have experienced the high-grade metamorphism (e.g., Harley et al., 2007; Hermann and Rubatto, 2003).

    Figure 9.  (a) Cathodoluminescence images of zircon grains of the pelitic granulite from Huangtuyao; (b) concordia diagram together with diagram of relative probability showing LA-ICP-MS U-Pb data of pelitic granulite; (c) zircon REE patterns of pelitic granulite from Huangtuyao, Chondrite values take from Sun and McDonough (1989).

    Thirty-two analyses of thirty-two zircon grains were performed for sample 17HTY3-1. The zircon grains present low to moderate Th contents (11 ppm–158 ppm) and moderate U contents (158 ppm–573 ppm), giving rise to low Th/U ratios of 0.05–0.28 (Table 4). Although some Th/U ratios are somewhat higher than those typically originated metamorphic zircon (e.g., Chen et al., 2018; Wu and Zheng, 2004; Rubatto, 2002), they may reflect the melt involving reaction (Li and Wei, 2016; Wan et al., 2006). The zircon U-Pb dating of pelitic granulite indicates continuous and long age spectra from 1 976±28 to 1 815± 28 Ma, but exhibits 207Pb/206Pb age populations of 1 951 and 1 876 Ma according to the age distribution of the probability density histogram (Fig. 9b). The old group age of 1 951 Ma with seven zircons shows Th/U ~0.05–0.23; while young group age of 1 851 Ma with twenty-five zircons shows Th/U ~0.05–0.28 (Table 4). The old age group of 1 951 Ma shows weak in the diagram of relative probability (Fig. 9b), but seven zircons in the concordia diagram clearly indicate its existence as is also presented in CL images and Table 4.

    Spots Th (ppm) U (ppm) Th/U 207Pb/206Pb 1σ 207Pb / 235U 1σ 206Pb/238U 1σ 207Pb/206Pb (Ma) 1σ 207Pb/235U (Ma) 1σ 206Pb/238U (Ma) 1σ
    18HTY01-01 16 303 0.05 0.110 903 0.001 7 5.470 174 0.087 7 0.356 927 0.002 5 1 815 28 1 896 14 1 968 12
    18HTY01-02 24 191 0.13 0.116 329 0.001 9 5.507 144 0.084 3 0.342 773 0.002 2 1 902 29 1 902 13 1 900 11
    18HTY01-03 42 247 0.17 0.111 742 0.001 9 5.183 344 0.090 6 0.335 311 0.002 4 1 828 30 1 850 15 1 864 12
    18HTY01-04 24 170 0.14 0.115 536 0.002 2 5.560 939 0.107 2 0.347 972 0.003 1 1 888 34 1 910 17 1 925 15
    18HTY01-05 29 177 0.16 0.117 527 0.002 2 5.473 744 0.102 1 0.336 877 0.002 6 1 920 35 1 896 16 1 872 13
    18HTY01-06 44 154 0.28 0.112 375 0.001 9 5.243 177 0.096 1 0.337 091 0.003 3 1 839 31 1 860 16 1 873 16
    18HTY01-07 11 218 0.05 0.118 498 0.001 8 6.068 386 0.109 4 0.369 250 0.003 8 1 944 27 1 986 16 2 026 18
    18HTY01-08 25 261 0.10 0.114 410 0.001 7 5.431 316 0.081 6 0.342 977 0.002 9 1 872 27 1 890 13 1 901 14
    18HTY01-09 14 240 0.06 0.120 882 0.002 0 6.365 055 0.111 0 0.380 963 0.004 2 1 969 30 2 027 15 2 081 20
    18HTY01-10 39 239 0.16 0.117 806 0.002 1 5.801 464 0.102 9 0.355 422 0.003 0 1 924 32 1 947 15 1 960 14
    18HTY01-11 28 172 0.16 0.115 003 0.002 2 5.213 526 0.096 0 0.327 283 0.002 5 1 880 29 1 855 16 1 825 12
    18HTY01-12 24 225 0.11 0.112 294 0.003 1 5.320 907 0.159 2 0.340 074 0.002 7 1 837 50 1 872 26 1 887 13
    18HTY01-13 28 178 0.16 0.113 466 0.001 9 5.006 439 0.085 4 0.318 629 0.002 4 1 857 31 1 820 14 1 783 12
    18HTY01-14 39 246 0.16 0.112 088 0.001 7 5.395 204 0.086 6 0.347 614 0.002 7 1 835 28 1 884 14 1 923 13
    18HTY01-15 158 573 0.28 0.115 384 0.002 0 5.214 837 0.106 3 0.325 735 0.003 5 1 887 31 1 855 17 1 818 17
    18HTY01-16 21 171 0.12 0.115 756 0.001 9 5.565 723 0.099 4 0.347 714 0.003 3 1 892 30 1 911 15 1 924 16
    18HTY01-17 29 173 0.17 0.113 791 0.002 1 5.486 432 0.100 8 0.349 320 0.003 3 1 861 33 1 898 16 1 931 16
    18HTY01-18 54 235 0.23 0.119 143 0.002 1 5.853 520 0.113 6 0.355 263 0.003 6 1 944 27 1 954 17 1 960 17
    18HTY01-19 22 178 0.13 0.113 839 0.001 9 5.337 516 0.092 8 0.339 068 0.002 8 1 861 30 1 875 15 1 882 14
    18HTY01-20 15 308 0.05 0.120 514 0.001 8 6.093 891 0.099 7 0.365 416 0.003 2 1 965 26 1 989 14 2 008 15
    18HTY01-21 23 250 0.09 0.113 540 0.002 0 5.517 665 0.099 3 0.351 456 0.003 0 1 857 38 1 903 16 1 942 15
    18HTY01-22 15 194 0.08 0.121 276 0.001 9 6.217 868 0.100 5 0.370 547 0.002 9 1 976 28 2 007 14 2 032 13
    18HTY01-23 24 163 0.15 0.115 333 0.001 9 5.426 318 0.091 9 0.340 026 0.002 6 1 885 31 1 889 15 1 887 13
    18HTY01-24 27 153 0.17 0.116 756 0.002 3 5.296 049 0.101 2 0.327 768 0.002 5 1 907 35 1 868 16 1 828 12
    18HTY01-25 28 183 0.15 0.119 860 0.002 2 5.593 021 0.105 2 0.337 120 0.002 7 1 954 33 1 915 16 1 873 13
    18HTY01-26 26 164 0.16 0.112 366 0.002 1 5.283 961 0.102 9 0.339 823 0.002 5 1 839 34 1 866 17 1 886 12
    18HTY01-27 26 162 0.16 0.112 527 0.002 3 5.007 654 0.106 9 0.321 646 0.002 3 1 840 37 1 821 18 1 798 11
    18HTY01-28 21 158 0.13 0.118 309 0.002 8 5.661 907 0.134 8 0.346 935 0.002 7 1 931 42 1 926 21 1 920 13
    18HTY01-29 23 166 0.14 0.113 266 0.003 3 5.287 995 0.167 6 0.337 793 0.002 3 1 854 54 1 867 27 1 876 11
    18HTY01-30 25 203 0.12 0.113 193 0.004 3 4.810 277 0.194 1 0.308 280 0.002 5 1 852 67 1 787 34 1 732 12
    18HTY01-31 26 336 0.08 0.114 967 0.004 3 5.319 635 0.215 6 0.334 779 0.002 8 1 879 68 1 872 35 1 862 14
    18HTY01-32 21 198 0.11 0.116 939 0.003 9 5.310 402 0.188 9 0.329 203 0.002 5 1 910 59 1 871 30 1 835 12

    Table 4.  U-Pb isotopic data of zircons of the pelitic granulite from sample 17HTY3-1 in Huangtuyao, Huai'an Complex (dark rows for old group age, white rows for young group age)

    The chondrite-normalized REE diagrams of zircons present nearly flat HREE patterns (YbN/GdN=0.68–1.67 for old age group, 0.53–1.98 for young age group), and show positive Ce anomalies and moderate negative Eu anomalies (Table 5). These REE patterns are typical origin of high grade metamorphism, and imply that zircons formed simultaneously with the formation of garnet and feldspar (Wu et al., 2008; Hermann and Rubatto, 2003; Rubatto, 2002).

    La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
    18HTY01-01 6.86 45.30 10.27 67.61 26.70 5.23 34.39 7.25 52.28 12.31 38.64 6.85 52.93 8.72
    18HTY01-02 1.63 13.14 2.11 13.31 4.72 0.48 15.45 3.80 24.48 5.23 14.25 2.29 17.90 3.68
    18HTY01-03 7.44 46.55 13.72 115.42 50.11 5.26 36.68 4.98 29.38 6.26 17.59 2.67 17.46 2.59
    18HTY01-04 0.05 6.81 0.09 1.09 2.39 0.30 10.22 2.05 12.27 2.24 5.63 0.79 5.36 0.98
    18HTY01-05 0.00 5.97 0.02 0.49 2.14 0.19 11.82 2.36 14.49 2.91 8.10 1.14 8.26 1.40
    18HTY01-06 2.05 18.88 4.15 39.34 21.20 2.79 23.02 4.10 29.52 7.62 25.44 4.54 36.87 7.09
    18HTY01-07 0.39 6.31 0.77 5.79 3.30 0.52 14.25 4.19 32.44 7.05 18.45 2.81 22.33 5.29
    18HTY01-08 0.17 4.92 0.14 1.38 3.01 0.20 14.18 3.71 25.67 5.59 15.19 2.34 16.84 3.30
    18HTY01-09 3.15 22.47 4.20 25.58 8.18 1.25 17.75 4.33 30.99 6.21 15.99 2.41 21.69 5.46
    18HTY01-10 0.28 8.21 0.58 5.20 6.57 0.69 23.87 5.28 34.29 7.11 19.60 3.02 21.70 3.75
    18HTY01-11 0.06 6.51 0.06 0.95 2.76 0.23 12.33 2.83 17.24 3.52 9.84 1.59 11.65 2.06
    18HTY01-12 4.14 16.87 2.10 15.96 12.04 3.17 24.64 5.17 31.36 6.05 14.19 2.00 14.22 2.62
    18HTY01-13 0.00 5.00 0.02 0.68 1.72 0.16 11.11 2.28 14.63 2.82 7.80 1.11 8.24 1.45
    18HTY01-14 2.76 19.33 3.82 31.81 19.31 2.02 30.62 5.69 32.90 5.90 15.09 2.22 15.94 2.63
    18HTY01-15 49.20 291.30 51.57 335.33 122.86 18.25 86.10 11.44 77.26 19.53 67.63 12.88 114.83 22.33
    18HTY01-16 0.83 10.75 1.71 15.42 8.60 1.01 14.73 2.92 18.71 3.64 9.56 1.29 9.60 1.70
    18HTY01-17 4.36 21.15 3.36 19.20 6.77 1.24 14.57 2.84 16.05 3.19 8.94 1.33 10.04 1.74
    18HTY01-18 1.48 18.91 3.26 30.57 23.93 2.85 38.83 6.91 39.12 7.14 18.74 2.75 19.28 3.37
    18HTY01-19 0.01 5.59 0.03 0.66 2.22 0.34 15.60 3.79 25.62 5.16 12.94 2.04 16.88 3.38
    18HTY01-20 0.03 3.72 0.08 0.99 1.99 0.36 13.89 3.91 22.17 3.69 7.58 1.01 7.08 1.58
    18HTY01-21 0.04 7.14 0.20 2.21 3.30 0.45 11.03 2.43 13.59 2.62 6.57 1.00 7.17 1.25
    18HTY01-22 0.71 9.19 1.16 10.81 9.14 2.37 24.89 5.45 34.68 6.27 15.15 1.94 14.04 2.26
    18HTY01-23 1.37 15.17 3.01 24.67 13.26 1.56 17.79 3.18 17.84 3.10 7.39 1.20 7.75 1.24
    18HTY01-24 1.52 16.54 3.22 29.38 15.08 3.12 22.10 3.54 19.23 3.67 9.33 1.35 9.25 1.56
    18HTY01-25 0.00 6.60 0.04 0.74 3.12 0.29 12.82 2.75 17.46 3.51 9.36 1.36 10.35 1.84
    18HTY01-26 0.11 6.06 0.21 1.83 3.20 0.29 13.63 2.82 18.30 3.61 10.38 1.55 11.58 2.03
    18HTY01-27 0.70 8.33 0.51 2.44 2.55 0.26 10.35 2.15 12.06 2.19 5.78 0.85 6.50 0.93
    18HTY01-28 0.00 6.35 0.05 0.86 2.63 0.25 13.16 2.95 18.24 3.55 9.48 1.36 10.49 1.94
    18HTY01-29 1.14 15.58 2.21 19.86 13.60 2.07 20.37 3.54 20.92 3.89 9.61 1.50 10.87 1.80
    18HTY01-30 0.23 8.16 0.45 4.18 6.26 0.84 19.20 3.79 21.94 4.00 10.05 1.53 11.36 1.72
    18HTY01-31 9.09 87.00 22.45 199.11 126.41 28.41 123.44 19.13 117.74 23.80 63.19 9.22 59.09 8.89
    18HTY01-32 11.64 71.47 14.66 80.84 17.80 2.41 24.70 4.97 34.49 7.76 24.21 4.35 38.79 7.66
    18HTY01-33 0.00 5.42 0.02 0.64 2.34 0.27 14.07 3.82 26.46 5.67 14.29 2.13 16.81 3.06

    Table 5.  Trace-element compositions (ppm) of zircons of the pelitic granulite from sample 17HTY3-1 in Huangtuyao, Huaiʼan Complex (dark rows for old group age, white rows for young group age)

    The REE patterns as well as Th/U rotios of theses zircons are not able to distinguish two different groups of metamorphic ages because of their similar characteristics (Fig. 9c).

  • Previous studies indicate that the granulite facies rocks in the Huangtuyao area experienced medium-high pressure (MP-HP) and high temperature (HT) metamorphism at the peak metamorphic conditions of at ~860–890 ºC/10–13 kbar estimated by using conventional geothermobarometry of minerals (Wang et al., 2016; Liu, 1996). The difference in the pelitic granulite between the HP and MP metamorphism depends on whether kyanite presents or not. When investigated the pelitic granulite from khondalite series rocks of the Hongxibao- Huantuyao terrane, Liu (1996) reported the existence of kyanite. In this study, the metamorphic stages and the estimates of the P-T conditions for pelitic granulite of Huangtuyao are based on the thermodynamic pseudosection, mineral assemblage and mineral isopleth approaches as well as petrological observations, and the results upgrade the metapelite metamorphic grade to HP/ultra-high temperature (UHT) metamorphism.

    According to the petrological characteristics and phase equilibrium modeling of the samples from the Huangtuyao area, the P-T evolution can be divided into four metamorphic stages: the pre-peak stage M1, Pmax stage M2, Tmax stage M3 and the retrograde cooling stage M4. Mg2+ and Fe2+ in garnet have fast diffusion and are easily homogenized during HT-UHT stages (e.g., Li and Wei, 2018; Florence and Spear, 1991; Spear et al., 1990). It is accepted that Ca2+ in garnet and in plagioclase have slow diffusion rate and could preserve the early phases and have been used to constrain P-T conditions of granulite in HP-HT terranes (e.g., Liao and Wei, 2019; Liu et al., 2019; Zhang Y C et al., 2019; Li and Wei, 2018, 2016; Zhang Z M et al., 2018, 2017; Wu et al., 2017, 2016).

    The P-T conditions of pre-peak M1 stage could be determined by stable mineral assemblage in the garnet core, and mineral compositions at this stage. Biotite and plagioclase are present in the core of the garnet (Fig. 3f), but their compositions are all revised by late high-grade metamorphism and cannot be plotted on the supposed stable area of mineral assemblage (Table 1, Fig. 6a). In addition, no entire mineral assemblage Grt1+Ky+Pl+Kfs+Bt±Mc+Qz+Rt has been found in garnet core. We, therefore, could not define the P-T conditions of stage M1. Before reaching the Pmax stage M2, the pelitic granulite has experienced a progressive metamorphic process, which is indicated by increase of XCa from core to mantle in the garnet porphyroblast (Fig. 6a).

    At the P-T condition of the Pmax stage M2, kyanite was not found in this study, but was found by Liu (1996) in Hongxibao- Huangtuyao khjondalite belt. On the other hand, kyanite is possibly not preserved during retrograde metamorphism of late stages, and replaced by sillimanite during decompression and temperature increase process (Liu et al., 2019). According to the isopleth values of XCa from the garnet mantle (XCa=0.044–0.041) to garnet rim (XCa=0.022–0.031) of two compositional profiles (Table 1 and Figs. 4a–4b), The P-T path went along XCa decreasing trend during the decompression process (Fig. 6a). Combined XCa from the garnet mantle and Pmax conditions of mafic granulite from Huangtuyao (Wang et al., 2016), the Pmax stage of M2 for the pelitic granulite from Huangtuyao area is constrained at ~13.8–14.1 kbar and ~840–850 ºC (Fig. 6b).

    A temperature increasing decompression process is present to close to Tmax stage M3. The P-T conditions of the Tmax were determined by its stable mineral assemblage, isopleths of XCa in garnet rims (0.022–0.037), the XAn isopleths of plagioclase inclusions (0.28–0.37) and two-feldspar geothermometer (Fig. 5, Table 4). A P-T range of 909–915 ºC/7–7.2 kbar, therefore, was defined, and was upgraded to UHT metamorphic conditions in comparison with previous studies of metapelite from the Huangtuyao area (Wang et al., 2016; Liu, 1996).

    A retrograde cooling stage (M4) after Tmax M3 is figured out on the basis of the stable mineral assemblage in the matrix, XAn (0.27–0.29), and XMg (0.62) in biotite, which shows a trend to lower temperature outside of plotted P-T range of stable mineral assemblage Grt (in matrix)+Kfs+Sill+Bt+Pl+Qz+Ilm (Fig. 6). The P-T range of this stage is thus constrained as < 5 kbar and < 780 ºC.

    Above all, the pelitic granulite of the Huangtuyao area records a clockwise P-T path. The rock experienced a decompression and temperature increasing process from peak pressure (Pmax) to peak temperature (Tmax) stages, and subsequent cooling path from the Tmax to retrograde stages.

    The granulite facies rocks in a collisional metamorphic zone is usually characterized by clockwise metamorphic P-T path (e.g., Zhao and Zhai, 2013; Zhao et al., 2012; Harley, 1989). The majority of UHT granulites in the khondalite belt of the North China Craton present clockwise P-T paths (Li and Wei, 2018, 2016; Wu et al., 2017; Jiao et al., 2015, 2013; Yang et al., 2014; Guo et al., 2012). While a few of the UHT outcrops show the records of the anti-clockwise P-T trajectories, including in the Tianpishan and Tuguiwula Park (Santosh et al., 2012), Xumayao in the Tuguiwula area (Zhang et al., 2012). Shimizu et al. (2013) believed that the realization of anti-clockwise P-T evolution requires a large amount of heat input to the lower crust at the subduction/collision stage, then extrusion and cooling, which may be related to magma underplating or the upwelling of high-temperature external fluid from asthenosphere. It is still unclear whether these two P-T trajectories represent specific tectonic background and geological significance, and what are their dynamic mechanisms. These two kinds of P-T evolution processes also exist in other UHT metamorphic zones in the other parts of the world (Li X-P et al., 2019).

  • Based on the metamorphic evolution of pelitic granulite in this study, the metamorphic stages of Pmax M2 and Tmax M3 occurred above the solidus (Fig. 6), indicating a melt-involved processes. The textural features from petrographic studies also support these melting intervenes during M2 and M3 stages (Figs. 4h, 4i). The CL images of zircons from two group ages show rounded, and luminescence of darkness, structureless and irregular blurred zonation, indicating that it is affected by high-grade metamorphism and melting (Fig. 9a, Cao et al., 2019; Li Y et al., 2019; Liu et al., 2019; Jiao et al., 2017, 2013; Li and Wei, 2016; Hermann and Rubatto, 2003). On the other hand, both age groups of zircon grains present positive slopes of LREE patterns from La to Sm, flat and even slight negative slopes HREE patterns from Gd to Lu (Fig. 9c). This HREE characteristics are consistent with increased uptake of HREE by simultaneously formed garnet between ca. 1.95 and 1.88 Ga, and are typical origin of HT-UHT granulite facies metamorphic zircons (Liu et al., 2019; Li and Wei, 2018, 2016; Jiao et al., 2017, 2013; Hermann and Rubatto, 2003). Pelitic granulites of this study, therefore, suggest that the collision event between the Yinshan and Ordos blocks occurred at ~1 951 Ma as represented by the post Pmax decompression P-T conditions, and the younger age of ~1 876 Ma is supposed to represent an exhumation and slow cooling process corresponding to the post-Tmax cooling P-T paths recorded by the pelitic granulites. The UHT metamorphism reaches in the process of decompression and increasing temperature after Pmax metamorphism. It reflects a heating and cooling process of thickened crustal rocks in the uplift process (Liu et al., 2019; Li and Wei, 2016).

    Both metamorphic P-T evolution and geochronological characteristics of pelitic and mafic granulites from Huanʼan Complex are comparable to those granulites from the Jining Complex of the western khondalite belt (Liao and Wei, 2019; Liu et al., 2019; Li et al., 2011; Wang et al., 2011; Zhang et al., 1994). Previous tectonic model studies show that there is an obvious cover and basement relationship between the granulites-bearing gray gneisses and the western khondalite belt with a decollement in between them (Liu et al., 2019; Zhang et al., 1994). It is possible that westward subduction of the Eastern Block underneath West Block of the NCC along the TNCO followed by exhumation/ denudation leaving the relicts of the gray gneisses of Huangtuyao, Manjinggou and Sifangdun in direct contact with the Huaiʼan gneissic basement (Liu et al., 2019; Zhang et al., 1994). Previous studies show that the pelitic and mafic granulites from Jining Complex records both normal HT and UHT metamorphic conditions. The examples of normal HT pelitic ganulites in Jining Complex are reported from Liangcheng area (Li et al., 2011; Wang et al., 2011), Datong (Wu et al., 2017) and from Xiaoshizi area (Jiao et al., 2013). The examples of the UHT metamorphic rocks, as we know, have been reported continuously in recent years in the Jining Complex. These UHT granulites could be up to very high temperature condition from 950 ºC to > 1 000 ºC (Li X-P et al., 2019; Li and Wei, 2018; Yang et al., 2014; Santosh et al., 2012, 2009b; Zhang et al., 2012; Jiao and Guo, 2011). The Huaiʼan and Jining Complexes share the same tectonic evolution, generally involved in ~1.95 Ga crustal thickening, ~1.92–1.91 Ga extension, which present a regional HT metamorphism with local UHT metamorphism, and subsequent cooling and uplifting (Liao and Wei, 2019; Liu et al., 2019; Zhang et al., 1994). The Huangtuyao pelitic granulite in this study may have been affected by local intrusive magma and present metamorphic conditions (909–915 ºC/7–7.2 kbar) in not very high local UHT metamorphism as some UHT granulites recorded.

    There are different views on the tectonic background of UHT metamorphism in the khondalite belt in the west of the North China Craton. Zhao et al. (2005) believed that the khondalite belt was formed by ~1.95 Ga collision between the Yinshan and the Ordos continental blocks, and the metamorphic evolution is characterized by clockwise P-T trajectories. The extension after the collision and the mantle upwelling caused by stretching may lead to the occurrence of UHT metamorphism at ~1.92 Ga (Zhai and Santosh, 2011). Several case studies provide evidences of ~1.92 Ga UHT metamorphic event, such as, the metamorphic age of the UHT sapphirine granulite at ~1.93–1.92 Ga from Dongpo of the Daqingshan area (Guo et al., 2012), UHT pelitic granulite formed ~1.92 Ga from Helingeer of Daqingshan area (Liu et al., 2012), sapphirine- bearing granulites formed ~1.92 Ga from the Jining Complex (Li and Wei, 2018; Jiao et al., 2013, 2011; Liu et al., 2010; Santosh et al., 2007). In the two-subduction model proposed by Santosh and Kusky (2010), it is believed that the metamorphic terrane experienced the early Pacific subduction-accretion and then final Himalayan collision-assemblage processes. Peng et al. (2012) and Guo et al. (2012) suggested that the metamorphic terrane of the Daqingshan area is a tectonic setting of the mid- ocean ridge subduction, which is accompanied by asthenosphere upwelling and leads to UHT metamorphism.

    However, recently discovered the sapphirine-bearing meta- pelites from Shaerqin of the Daqingshan terrane was reported to have experienced a ~1.85–1.86 Ga HT/UHT metamorphism in the central part of the khondalite belt, and is believed to have occurred in the geological background of crustal extension after continental-continental collision (Jiao et al., 2017, 2015). The identified pelitic granulite occurred in Hongsigou Village, south of Tuguiwula, present the typical UHT mineral assemblage of Spl+Qz with Tmax metamorphic temperature of 930–1 050 ºC and metamorphic age of ~1.88 Ga (Yang et al., 2014). Another example is at the boundary of Jining and Huaiʼan complexes, where the typical UHT mineral assemblage of Zhaojiayao recorded a metamorphic temperature of 950–1 000 ºC and a metamorphic age of 1.84 Ga (Li and Wei, 2016). Actually, the newly discovered structure and mineral assemblage of monazite of Dongpo UHT granulite and the corresponding SHRIMP U-Pb dating have confirmed the existence of two UHT metamorphic events which are in the ages of ~1.92 and 1.85 Ga, respectively (Jiao et al., 2018). It can be seen that the UHT metamorphic event in the khondalite belt of the Western Block in the NCC has become an important tectonic window of the crustal evolution in the Paleoproterozoic (Li X-P et al., 2019).

  • (1) The pelitic granulites in the Huangtuyao area of the Huai'an Complex present P-T conditions of Tmax metamorphism at ~7–7.2 kbar and ~909–915 ºC, which show UHT metamorphism after HP metamorphic conditions (Pmax) during decompression and temperature increase process, and record a clockwise P-T trajectory.

    (2) The zircon U-Pb dating of pelitic granulite indicates continuous age spectra from 1 976±28 to 1 815±28 Ma, but on the basis of probability density histogram, a major period of ~1.95 Ga is figured out, and followed by ~1.88 Ga granulite metamorphism.

    (3) The results of two major metamorphic events at ~1.95 and ~1.88 Ga may correspond to the collision and formation of the khondalite belt between the Ordos and the Yinshan blocks, and subsequent crustal extension after continental-continental collision.

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