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Volume 30 Issue 6
Dec.  2019
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Petrology, Metamorphic P-T Paths and Zircon U-Pb Ages for Paleoproterozoic Mafic Granulites from Xuanhua, North China Craton

  • The studied mafic granulites are located at Xiwangshan, Xuanhua region in the north of the Trans-North China Orogen (TNCO), occurring as lens within tonalite-trondhjemite-granodiorite (TTG) gneisses in the eastern part of the Xiwangshan area. The rocks contain the representative granulite-facies minerals such as garnet, clinopyroxene, orthopyroxene, plagioclase, amphibolite, rutile and quartz, and also well-developed melt pseudomorph and antiperthite. Although the prograde metamorphic stage (M1) cannot be retrieved due to rare preservation of pre-peak-stage mineral associations, three distinct mineral assemblages that formed in different metamorphic stages can be identified, based on petrography and mineralogy characteristics. The peak stage (M2) is characterized by Grt2+Cpx2+Amp2+Pl2+Rt+melt pseudomorphs, and a post-peak decompression stage (M3) contains a mineral assemblage of Grt3+Opx3+ Cpx3+Amp3+Pl3, while a later-retrogression stage (M4) is featured by coronas of Amp4+Pl4 surrounding garnet. By calculating metamorphic P-T conditions using THERMOCALC, stage M2 was constrained to be 13.2-14.8 kbar and 1 050-1 080℃, and M3 recorded P-T conditions of 5.7-7.3 kbar and 825-875℃, while M4 yielded P of~5 kbar and T of~660℃, consistent with amphibolite facies metamorphism. Taking into account of all these petrological data, we propose that the mafic granulite experienced a high-pressure (HP) and ultra-high temperature (UHT) granulite-facies metamorphism during the peak metamorphism, which was accompanied with a clockwise P-T path. U-Pb dating of metamorphic zircons in the granulites yields two groups of ages at 1 853±14 and 1 744±44 Ma, respectively. We suggest that the older age corresponded to the HP-UHT metamorphism, while the younger age represented an retrograde metamorphic event during cooling.
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Petrology, Metamorphic P-T Paths and Zircon U-Pb Ages for Paleoproterozoic Mafic Granulites from Xuanhua, North China Craton

    Corresponding author: Fanmei Kong, kongfanmei56@sdust.edu.cn
  • Shandong Key Laboratory of Depositional Mineralization and Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, China

Abstract: The studied mafic granulites are located at Xiwangshan, Xuanhua region in the north of the Trans-North China Orogen (TNCO), occurring as lens within tonalite-trondhjemite-granodiorite (TTG) gneisses in the eastern part of the Xiwangshan area. The rocks contain the representative granulite-facies minerals such as garnet, clinopyroxene, orthopyroxene, plagioclase, amphibolite, rutile and quartz, and also well-developed melt pseudomorph and antiperthite. Although the prograde metamorphic stage (M1) cannot be retrieved due to rare preservation of pre-peak-stage mineral associations, three distinct mineral assemblages that formed in different metamorphic stages can be identified, based on petrography and mineralogy characteristics. The peak stage (M2) is characterized by Grt2+Cpx2+Amp2+Pl2+Rt+melt pseudomorphs, and a post-peak decompression stage (M3) contains a mineral assemblage of Grt3+Opx3+ Cpx3+Amp3+Pl3, while a later-retrogression stage (M4) is featured by coronas of Amp4+Pl4 surrounding garnet. By calculating metamorphic P-T conditions using THERMOCALC, stage M2 was constrained to be 13.2-14.8 kbar and 1 050-1 080℃, and M3 recorded P-T conditions of 5.7-7.3 kbar and 825-875℃, while M4 yielded P of~5 kbar and T of~660℃, consistent with amphibolite facies metamorphism. Taking into account of all these petrological data, we propose that the mafic granulite experienced a high-pressure (HP) and ultra-high temperature (UHT) granulite-facies metamorphism during the peak metamorphism, which was accompanied with a clockwise P-T path. U-Pb dating of metamorphic zircons in the granulites yields two groups of ages at 1 853±14 and 1 744±44 Ma, respectively. We suggest that the older age corresponded to the HP-UHT metamorphism, while the younger age represented an retrograde metamorphic event during cooling.

0.   INTRODUCTION
  • Subduction and collision during continent-continent convergence is commonly associated with high-pressure metamorphism of the continental crust (Wang S J et al., 2017, 2016; Anderson et al., 2012; Chopin et al., 2012; Brown, 2007). The high-grade metamorphic rocks come from the deep continental crust, which are thus the "recorder" of thermal-chemical processes and tectonic events of the deep earth. Granulite facies metamorphic rocks have critical tectonic significance in the continental evolution history, especially in convergent settings from subduction to collision (Shen et al., 2018, 2014; Ernst and Liou, 2008). A thorough understanding of petrology, metamorphic P-T conditions and geochronology for a specific granulite is helpful to decipher crustal evolution and chemical dynamics in convergent plate boundaries (Brown, 2014; Tsunogae et al., 2014; Korhonen et al., 2013; Wei and Powell, 2004).

    Paleoproterozoic high pressure (HP) mafic granulites are widely distributed in the North China Craton (NCC), which are well preserved as the basement along the Trans-North China Orogen (TNCO) in the central part of the NCC (Guo et al., 2002; Zhai et al., 1992). Some scholars believed that these metamorphic rocks were formed during collisional events due to cratonization of the NCC in the Late Paleoproterozoic (Chen et al., 2018a; Guo et al., 2015; Zhao et al., 2012, 2000). However, until now, there are many controversies for the mafic granulites in the TNCO, including the metamorphic P-T conditions, metamorphic ages and the associated geodynamic setting that induced the 'unusal' high-T metamorphism (Guo et al., 2015; Zhai, 2009). Previous U-Pb dating on metamorphic zircons from various granulite-facies rocks mainly yielded two groups of metamorphic ages at 1.93–1.90 and 1.85–1.79 Ga, respectively (Liu et al., 2019; Tang et al., 2017; Qian and Wei, 2016; Zhang H F et al., 2016; Qian et al., 2015; Peng et al., 2014; Zhao et al., 2006; Guo et al., 2005). The older age might indicate the collisional assembly of major blocks in the NCC at ca. 1.93–1.90 Ga, while the younger age may represent a retrograde event (Tang et al., 2017; Zhang H F et al., 2016). Since the identification of HP granulite in the Huai'an-Xuanhua region by Zhai et al. (1992), many scholars have conducted integrated petrological, geochemical and geochronological studies on the mafic granulites. As a representative research site, the Huai'an Complex is located in the northwestern part of the TNCO (Tang et al., 2017; Zhao et al., 2005; Guo et al., 2002). The complex is dominantly composed of pelitic and mafic granulites that are enclosed in TTG gneisses (Zhao et al., 2005; Li et al., 1998). By studying petrology and chronology of mafic granulite from the Manjinggou and Xiwangshan areas within the Huai'an Complex, Guo et al.(2005, 2002) attained a clockwise P-T path with peakmetamorphic conditions of 11–14.5 kbar and 750–870 ℃ at 1.84–1.83 Ga. In addition, Huang et al. (2018) identified two types of mafic granulites in terms of petrological features from the Xiwangshan and Xuanhua regions, and obtained the highest pressure of 1.1–1.2 kbar for amphibole-rich mafic granulites.

    Since the report of ultra-high temperature (UHT) metamorphism in the pelitic granulite from the khondalite belt in the NCC (Liu et al., 2019; Li and Wei, 2018; Santosh et al., 2012, 2007b; Guo et al., 2006), the UHT metamorphism has attracted much attention. In some localities, the khondalite series is closely associated with mafic HP granulites (Wu et al., 2016), although some scholars suggested that the pelitic granulites associated with mafic granulites had an independent metamorphic history (Zhao et al., 2010, 2005; Zhao, 2009). However, Wu et al. (2016) considered that these pelitic and mafic granulites have experienced a similar metamorphic P-T evolution. In the past, there was no definite UHT rocks reported in the TNCO. Recently, Liu et al. (2019) proposed that the high pressure pelitic granulites from the Manjinggou area in the Huai'an Complex recorded evidence of UHT metamorphism. Besides, Liao and Wei (2019) showed the mafic granulites from the Huangtuyao area in the Huai'an Complex have also undergone UHT metamorphism, with achieved temperatures at 980–1 010 and 1 000–1 030 ℃.

    Xiwangshan mafic granulites are located in the Xuanhua region at the northern part of the TNCO, to the east of Huai'an region (Fig. 1) (Huang et al., 2018; Guo et al., 2002). The metamorphic evolution of the Xiwangshan mafic granulites is crucial to understand the tectonic setting and thermal evolution of the TNCO. Huang et al. (2018) obtained the highest temperature of 940 ℃ for one amphibole-rich mafic granulite from the Xiwangshan area. However, they considered that there are significant uncertainties related to the results. In this case, whether the mafic granulites in Xiwangshan have suffered the UHT metamorphism is questionable. Have the HP mafic granulites in the TNCO widely experienced ultra-high temperature metamorphism? What was the timing for the HP/UHP metamorphism? Combined petrological, geochemical and geochronological studies using targeted samples are required to answer these questions.

    Figure 1.  (a) Sketch map showing the tectonic subdivision of the North China Craton (modified after Zhao et al., 2005); (b) geological sketch map of the Huai'an-Xuanhua terrain, showing Precambrian lithological units, the red box shows location of the study area (modified after Guo et al., 2002).

    The collected samples of mafic granulite in the Xiwangshan area contain many high Ti amphiboles and high Al clinopyroxenes, as well as some antiperthites. These features may indicate that their forming temperature was more than 900 ℃ (Prakash et al., 2007; Ernst and Liu, 1998; Nickel et al., 1985). Melt pseudomorphs surrounding garnet, orthopyroxene, clinopyroxene or amphibolite are also observed. In this paper, we present the results of petrography, mineral chemistry, P-T pseudosection modeling and zircon U-Pb geochronology of the collected mafic granulites. Our work contributes to construct its metamorphic evolution, and provides valuable information to understand the Precambrian collisional process of the TNCO.

1.   GEOLOGICAL SETTING
  • As one of the oldest cratons in the world, the NCC is mainly composed of Archean–Paleoproterozoic metamorphic basement, Mesoproterozoic–Cenozoic sedimentary cover and Mesozoic intrusive dykes (Wang et al., 2019a, b; Santosh et al., 2015; Zhao et al., 2012, 2005; Zhai and Santosh, 2011). Previous studies suggested that cratonization of the NCC basement was within 2.5–1.8 Ga by assembly of some Archean microblocks (e.g., Santosh et al., 2016). A popular early structural model of the NCC divided the Precambrian basement of the NCC into the Eastern and Western blocks separated by the intervening TNCO (Fig. 1a) (Zhao et al., 2005, 2000).

    The NCC is composed of four Archean micro-continental blocks that are separated by three Paleoproterozoic orogenic belts, namely the khondalite belt, the Jiao-Liao-Ji belt and the TNCO belt (Zhao et al., 2012, 2005). The three belts formed at ca. 1.95, ca. 1.90 and ca. 1.85 Ga, respectively (Zhao et al., 2012, 2005). However, the khondalite belt and the TNCO were considered to be distinct, since the former has peak metamorphic or collisional ages at ca. 1.95 Ga, while the latter has peak metamorphic or collisional ages at ca. 1.85 Ga (Wang H Z et al., 2016; Wang L J et al., 2011; Zhao, 2009; Zhao et al., 2006, 2005).

    The basement rocks in the TNCO include Neoarchean to Paleoproterozoic TTG gneiss, lenticular granulite, metamorphic surface crustal rocks and deformed granite and meta-mafic rocks (Wei et al., 2014; Wang et al., 1996), as well as ca. 1.85–1.76 Ga mafic dykes (Peng et al., 2007). The TNCO is composed of several low- to high-grade metamorphic terrains, which are, from the north to south, the Chengde, Huai'an-Xuanhua, Hengshan, Wutai, Fuping, Lüliang, Zanhuang, Zhongtiao, Dengfeng and Taihua terrains. The metamorphic complexes occurring in these terrains form a giant metamorphic belt (Zhao, 2009). Pelitic granulite and HP mafic granulite are widely exposed in the basement of these terrains (Zhao, 2009). Previous studies showed that the main metamorphic ages in the TNCO are in a wide range from 1.96 to 1.75 Ga (Liao and Wei, 2019; Liu H et al., 2019; Wei, 2018; Qian and Wei, 2016; Wang H Z et al., 2016; Wu et al., 2016; Liu Y C et al., 2009; Zhao et al., 2006; Liu S W et al., 2006; Guo et al., 2005).

    The studied samples are collected from Xiwangshan in the Xuanhua region, which is located in the northern part of the TNCO. Further to the east the Huai'an region is located, and further to the south, the Hongqiyingzi Group is located (Fig. 1b). The complex in the Xuanhua and Huai'an regions are known as Sanggan Group (Guo et al., 2002; Zhai et al., 1992), which is separated from the khondalite belt in the Western Block by the Gushan fault, and is separated from the Hongqi-yingzi Group by the Sangyi-Chicheng fault. The complex that occurs in the Xiwangshan area is dominated by ca. 2.5 Ga TTG and diorite gneisses (Liu et al., 2012). Mafic granulites are widely outcropped as lenses or dykes in the TTG gneisses (Huang et al., 2018; Guo et al., 2002), and these rocks have undergone Paleoproterozoic HP granulite metamorphism and recorded a typical clockwise P-T path including a nearly isothermal decompression (ITD) segment (Huang et al., 2018; Guo et al., 2002). Their metamorphic ages range from 1.87 to 1.80 Ga (Huang et al., 2018; Guo et al., 2005).

    Two mafic granulite samples (17XWS1-1, 17XWS1-2; 40°44′40″N, 115°3′43″E) are collected at the eastern part of the Xiwangshan area, (Fig. 2), where vimineous mafic granulites apparently occur as small dyke-like bodies or lenticular enclaves within the TTG gneisses (Figs. 3a, 3b). The mafic granulites are dark grey in color with massive and weakly gneissic structures (Figs. 3a, 3b). The edges of the lenticular enclaves show evidence of intense shearing deformation. The usually E-W trending foliation with a steep dip to the south is defined by elongate garnet, pyroxene and plagioclase. Mineral lineation is also well developed that usually plunges to the SSE or SSW, except for the fold in the southwest part. The elongate lenses of mafic granulites are parallel to each other and probably originated from mafic dykes (Guo et al., 2002; Li et al., 1998).

    Figure 2.  Geological map of the Xiwangshan area showing the major lithological units and the sample locations (modified from Guo et al., 2002; Li et al., 1998).

    Figure 3.  Field photographs (a), (b), (c) and microphotographs (d)–(l) of the mafic granulites from the eastern part of the Xiwangshan area. Mineral abbreviations after Whitney and Evans (2010). (a) The mafic granulites crop out as dyke-like bodies within the TTG gneisses; (b) dark grey mafic granulite lenticle with a granoblastic texture; (c) massive and epigranular structure of hand specimen; (d) garnet porphyroblast and internal inclusions, the dotted red line indicates the point of the garnet profile; (e) M3 stage fine-grained symplectite mineral assemblages clinopyroxene (Cpx3)+orthopyroxene (Opx3)+plagioclase (Pl3) around the garnet; (f) plagioclase as a moat around the garnet separate the large porphyrites of clinopyroxene and amphibolite; (g) orthopyroxene and plagioclase occur in the interior of large clinopyroxene in sample 17XWS1-2; (h) antiperthite structure, the K-feldspar as a small cluster needle-shaped exsolution texture occurring in large plagioclase phenocryst of sample 17XWS1-2; (i) the melt occurring at the edge of garnet; (j) corona assemblages consisting of amphibolite+ plagioclase surrounding the garnet phenocryst in sample 17XWS1-2; (k) corona of amphibolite (Amp4) around the fine-grained garnet in sample 17XWS1-1; (l) amphibolite developed from the interior or along the tiny cracks of clinopyroxene.

2.   ANALYTICAL METHODS
  • The whole-rock chemical compositions were determined by X-ray fluorescence spectrometry (XRF, Axiosmax) analysis combined with standard wet chemical methods at the Hebei Institute of Regional Geological and Mineral Survey, China.

    Mineral compositions were analyzed at Tongji University, Shanghai, using a JXA-8230 electron microprobe with conditions of 15 kV accelerating voltage, 10 nA probe current, and a 1–3 μm diameter beam except for biotite (5 μm). The analytical spots for garnet profiles and representative mineral analyses are presented in Table 1.

    Mineral Grt1 Grt2 Grt3 Pl2 Pl3 Pl4 Amp2 Amp3 Amp4 Cpx2 Cpx3 Opx3
    Position Core Mantle Rim Core In symplectite Corona Core In symplectite Corona Inclusions In symplectite In symplectite
    (wt.%) Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max
    SiO2 39.16 38.90 39.28 38.84 38.96 38.73 52.13 51.73 50.99 50.22 50.71 47.62 40.72 40.47 41.23 42.05 41.42 41.13 47.13 44.55 51.64 48.20 51.14 51.17
    TiO2 0.05 0.09 0.02 0.06 0.04 0.06 0.03 0.00 0.00 0.00 0.04 0.00 1.76 1.97 1.46 1.56 1.34 1.11 1.29 2.32 0.32 0.86 0.01 0.07
    Al2O3 22.12 22.11 22.49 21.94 21.90 22.14 30.14 30.58 31.31 32.09 29.09 33.43 13.00 13.89 13.71 13.88 13.82 13.83 9.67 12.73 3.58 7.31 3.21 4.13
    Cr2O3 0.07 0.12 0.06 0.16 0.12 0.12 0.00 0.00 0.01 0.01 0.03 0.02 0.13 0.11 0.14 0.17 0.34 0.30 0.01 0.10 0.16 0.13 0.10 0.14
    FeO 18.46 18.03 19.48 18.50 20.72 21.18 0.16 0.12 0.21 0.00 1.25 0.00 12.59 12.00 11.98 10.76 11.77 11.17 7.30 7.23 6.49 8.07 20.31 17.19
    MnO 0.42 0.42 0.43 0.50 0.55 0.50 0.03 0.00 0.02 0.00 0.10 0.03 0.07 0.04 0.01 0.06 0.11 0.04 0.06 0.00 0.11 0.07 0.18 0.21
    MgO 9.02 8.80 9.88 8.70 10.26 9.51 0.01 0.00 0.01 0.00 0.01 0.00 12.74 12.94 13.12 13.95 13.19 13.70 11.13 9.88 14.08 12.41 23.24 24.9
    CaO 10.61 10.85 8.29 10.57 6.88 7.67 11.98 12.95 14.15 15.10 16.47 16.00 11.02 10.83 11.18 11.36 10.98 10.95 23.04 23.01 22.11 21.71 0.33 0.26
    Na2O 0.00 0.01 0.01 0.01 0.02 0.00 4.52 4.13 3.47 2.60 3.23 2.41 2.76 2.98 2.95 3.00 2.91 2.71 0.57 0.61 0.60 0.92 0.00 0.01
    K2O 0.00 0.01 0.00 0.00 0.00 0.02 0.00 0.00 0.02 0.00 0.00 0.00 0.01 0.00 0.01 0.01 0.19 0.19 0.00 0.00 0.00 0.00 0.00 0.00
    Total 99.91 99.34 99.94 99.28 99.45 99.93 99.00 99.51 100.19 100.02 100.93 99.51 94.80 95.23 95.47 96.53 96.07 95.13 100.20 100.43 99.09 99.68 98.52 98.08
    Oxygen 12 12 12 12 12 12 8 8 8 8 8 8 23 23 23 23 23 23 6 6 6 6 6 6
    Si 2.96 2.96 2.96 2.96 2.96 2.94 2.38 2.36 2.32 2.20 2.23 2.19 6.15 6.07 6.14 6.16 6.15 6.13 1.75 1.65 1.92 1.79 1.91 1.89
    Ti 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.20 0.22 0.16 0.17 0.15 0.12 0.04 0.07 0.01 0.02 0.00 0.00
    Al 1.97 1.98 2.00 1.97 1.96 1.98 1.62 1.64 1.68 1.80 1.64 1.81 2.32 2.46 2.41 2.40 2.42 2.43 0.42 0.56 0.16 0.32 0.14 0.18
    Cr 0.00 0.01 0.00 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.01 0.02 0.02 0.04 0.04 0.00 0.00 0.01 0.00 0.00 0.00
    Fe 3+ 0.10 0.09 0.07 0.09 0.11 0.12 0.01 0.00 0.01 0.00 0.05 0.00 0.39 0.39 0.34 0.31 0.37 0.46 0.05 0.05 0.02 0.11 0.03 0.03
    Fe 2+ 1.06 1.05 1.15 1.08 1.20 1.21 0.00 0.00 0.00 0.00 0.00 0.00 1.16 1.07 1.11 0.97 1.05 0.88 0.17 0.17 0.18 0.13 0.598 0.505
    Mn 0.03 0.03 0.03 0.03 0.04 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.01
    Mg 1.02 1.00 1.11 0.99 1.16 1.08 0.00 0.00 0.00 0.00 0.00 0.00 2.87 2.89 2.91 3.05 2.92 3.05 0.62 0.55 0.78 0.69 1.29 1.37
    Ca 0.86 0.88 0.67 0.86 0.56 0.62 0.59 0.63 0.69 0.76 0.79 0.79 1.78 1.74 1.79 1.78 1.75 1.75 0.92 0.91 0.88 0.87 0.01 0.01
    Na 0.00 0.00 0.00 0.00 0.00 0.00 0.40 0.37 0.31 0.24 0.28 0.22 0.81 0.87 0.85 0.85 0.84 0.78 0.04 0.04 0.04 0.07 0.00 0.00
    K 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 0.00 0.04 0.04 0.00 0.00 0.00 0.00 0.00 0.00
    XMg 0.49 0.49 0.50 0.48 0.49 0.47 0.71 0.73 0.72 0.76 0.74 0.78 0.78 0.76 0.82 0.84 0.68 0.73
    Grs 0.29 0.30 0.23 0.29 0.19 0.21 Wo 0.54 0.56 0.48 0.51 0.01 0.11
    Prp 0.34 0.34 0.37 0.33 0.39 0.37 En 0.36 0.34 0.42 0.41 0.68 0.73
    Alm 0.36 0.36 0.40 0.36 0.41 0.42 AlM1 0.17 0.21 0.08 0.11 0.05 0.07
    An 0.59 0.63 0.69 0.76 0.74 0.79
    Ab 0.41 0.37 0.31 0.24 0.26 0.21
    Grs=Ca/(Fe2++Mg+Ca+Mn); Prp=Mg/(Fe2++Mg+Ca+Mn); Alm=Fe2+/(Fe2++Mg+Ca+Mn); An=Ca/(Ca+Na+K); Ab=Na+/(Ca+Na+K); Wo=Ca/(Fe2++Mg+Ca); En=Mg/ Fe2++Mg+Ca); XMg=Mg/(Mg+Fe2+); AlM1=Al–(2–Si).

    Table 1.  Representative EMP analyses data for sample 17XWS1-1 from the mafic granulite in Xiwangshan area

    Zircon separation and cathodoluminescence (CL) imaging were carried out at the Mineral Separation Laboratory of the Hebei Institute of Regional Geological and Mineral Survey, China. The zircon grains were separated from 2–3 kg of each sample using standard heavy-liquid and magnetic techniques, with subsequent handpicking under a binocular microscope. All grains were examined using transmitted and reflected light photomicrographs under microscope to ensure that the included mineral inclusions could be avoided. CL images were carefully collected to identify internal structures prior to U-Pb dating. The zircon U-Pb dating and in situ trace element analyses were performed by LA-ICP-MS at the Nanjing FocuMS Technology Co. Ltd. Australian Scientific Instruments Resolution LR laser-ablation system (Canberra, Australian) and Agilent Technologies 7700x quadrupole ICP-MS (Hachioji, Tokyo, Japan) were combined for the experiments. The 193 nm ArF excimer laser, homogenized by a set of beam delivery systems, was focused on zircon surface with fluence of 3.5 J/cm2. Ablation protocol employed a spot diameter of 33 μm at 6 Hz repetition rate for 40 s (equating to 240 pulses). Helium was applied as carrier gas to efficiently transport aerosol to ICP-MS. Zircon 91500 was used as external standard to correct instrumental mass discrimination and elemental fractionation during the ablation. Zircon GJ-1 was treated as quality control for geochronology. Lead abundance of zircon was external calibrated against NIST SRM 610 with Si as internal standard, while Zr as internal standard for other trace elements (Hu et al., 2011; Liu et al., 2010). Raw data reduction was performed off-line by ICPMSDataCal software (Liu et al., 2010). The calculation of ages, weighted mean and concordia diagrams were conducted by the ISOPLOT 3.0 (Ludwig, 2003).

3.   PETROGRAPHY
  • The mafic granulite samples contain garnet, pyroxene and plagioclase, and show nearly uniform petrological and mineralogical characteristics (Fig. 3c). The major rock-forming minerals include garnet (~20%), clinopyroxene (~25%), orthopyroxene (~5%), amphibolite (~25%), plagioclase (~20%); quartz, rutile, magnetite, ilmenite, titanite and zircon are present as accessory minerals (~5%). The studied samples are medium grained and generally show granoblastic-porphyroblastic texture. 'White eye socket' texture can be observed in these mafic granulites. Most grains are full and round observed in thin sections (Figs. 3b, 3c).

    The porphyroblastic garnet grains have grain sizes of 1–4 mm and contain inclusions of rutile, quartz and clinopyroxene. However, rutile and quartz are absent in the matrix, indicating that these minerals were products of prograde or peak metamorphic stages. Furthermore, most garnet porphyroblasts are surrounded by irregular, anhedral or worm-like amphibolite, clinopyroxene and plagioclase that show intergrowth features; these minerals are the so-called seriate poikilitic or symplectic texture, separating the larger clinopyroxene and amphibolite with garnet (Figs. 3e3f). The proportion of amphibolite is relatively high; the medium-grained amphibolite occurs in the matrix and is associated with larger clinopyroxene and garnet, a part of the amphibolite also intergrew with plagioclase as a corona around garnet (Figs. 3j, 3k). Most of the clinopyroxene grains are medium to coarse grained as a matrix phase, and a few fine grains occur as inclusions in the mantle of garnet porphyroblast (Fig. 3d). Whereas there are a few fine-grained amphibole developed at the edge of large clinopyroxene or along fractures (Fig. 3l). Plagioclase with rounded edge usually occurs as fine-medium grain in the matrix. By observation under the microscope, we recognized the presence of melt pseudomorphs developing between plagioclase and garnet (Fig. 3i). In sample 17XWS1-2, some rounded inclusions of orthopyroxene and plagioclase occur in the interior of large clinopyroxene grains (Fig. 3g). And in other locations, small rod-shaped or cluster needle-shaped potassium feldspar dissolves in plagioclase porphyroblast, which was identified as antiperthite forming antiperthitic structure (Fig. 3h).

    Three metamorphic stages are recognized according to the petrographic and composition characteristics of the rock-forming minerals. Since the composition of most garnet prophyroblasts is very homogeneous with only a few garnets that contain inclusions, we are not able to determine any prograde metamorphic mineral assemblage. The M2 mineral assemblage includes the mantle of coarse-grained prophyroblasts garnet (Grt2), clinopyroxene (Cpx2), plagioclase (Pl2) and amphibolite (Amp2) and rutile as minor inclusions in the garnet porphyroblasts, the Cpx2, Pl2 and Amp2 are all the core of their prophyroblasts, respectively. A fine-grained symplectite mineral assemblage, such as clinopyroxene (Cpx3)+orthopyroxene (Opx3)+ plagioclase (Pl3)+amphibolite (Amp3)+rim of the porphyroblast garnet (Grt3) (Fig. 3e) are considered to be formed during the post-peak decompression (M3) stage. The final retrograde (M4) stage is marked by the symbiotic corona, the mineral assemblage of which comprises amphibolite (Amp4)+plagioclase (Pl4) surrounding garnet porphyroblasts (Figs. 3j, 3k).

4.   MAJOR MINERAL CHEMISTRY
  • Two samples 17XWS1-1 and 17XWS1-2 were selected for detailed mineral composition analyses, and the representative mineral compositions are given in Table 1.

  • The garnet occurs mainly as medium-coarse grained prophyroblast (~3 mm), and some garnets exhibit obvious variational composition zoning. The dominated compositions of garnet are Alm (0.36–0.47), Prp (0.33–0.40), Grs (0.16–0.30), Sps (0.008–0.013), and XMg (0.43–0.51). The porphyroblastic garnet profile from sample 17XWS1-1 was analyzed to evaluate the variation of compositional zoning corresponding to the each metamorphic stage (Fig. 4, the full profile composition data are shown in Table S1). The chemical analyses show that Grs gradually decreases from the core to the mantle and then to the rim, whereas, Alm and Pyr show opposite trends (Fig. 4a). The variation of Sps is not obvious due to low Mn content in the garnet. The analyzed garnet is unevenly, with mantle and a wide rim developing on the left side, whereas the mantle and a very narrow rim growing on the right side (Fig. 3d); for the rim on the left side, XMg reaches a peak of 0.51 at the center of rim, then decreases towards the outer rim and the crack in the internal garnet (Fig. 4a). However, the garnet right side with a wide rim is associated with Cpx2 and Pl2, and the left side coexists with Pl3 (Fig. 3d).

    Figure 4.  Garnet porphyroblast composition diagrams for mafic granulite sample 17XWS1-1. (a) Garnet chemical zoning profiles of XMg, Alm, Prp, Grs, and Sps; (b) compositional variation of garnet porphyroblast from core to rim showing variations of the four end members.

  • Clinopyroxene has Wo of 0.45–0.57, En of 0.34–0.49 and Fs of 0.06–0.12, belonging to diopside (Fig. 5). Two types of clinopyroxene are distinguished in the samples based on the petrography and compositional features. Firstly, clinopyroxene as inclusions in the mantle of the garnet and the core of the coarse-grained clinopyroxene porphyroblasts have higher Al2O3 contents (9.67 wt.%–12.86 wt.%), which may have formed during the peak metamorphic stage (Cpx2). Secondly, the rim of the coarse-grained clinopyroxene porphyroblasts and the poikiloblastic fine-grained clinopyroxene that show intergrowth with orthopyroxene and plagioclase contain Al2O3 component of 3.58 wt.%–7.31 wt.% with CaM2 (0.87–0.89). This type of clinopyroxene should have crystallized during a post-peak metamorphic stage (Cpx3). In addtion, the clinopyroxene porphyroblasts show a slight increase in XMg (0.76–0.89) from the core to the rim.

    Figure 5.  Mineral compositional diagram of clinopyroxene; Wo=Ca/(Fe2++ Mg+Ca); En=Mg/(Fe2++Mg+Ca); Fs=Fe2+/(Fe2++Mg+Ca). Classification is after Morimoto (1988).

  • Two occurrences of orthopyroxene have been recognized. Fine-grained (< 1 mm) and poikiloblastic orthopyroxene is mainly intergrowth with clinopyroxene and plagioclase, all of them coexist with the garnet. The other occurrence of orthopyroxene occurs as inclusions along the cracks in clinopyroxene in sample 17XWS1-2 (Fig. 3g), and their chemical compositions are similar to the former occurrence (Fig. 3g). Orthopyroxenes contain Al2O3 component of 2.72 wt.%–4.13 wt.% with XMg from 0.68 to 0.73, and En is 0.52–0.73, belonging to hypersthene. The textural relationship indicates both the two orthopyroxenes form in the post-peak metamorphic stage (M3).

  • The compositions of plagioclase in different morphological structure vary obviously. The fine-medium grained plagioclases in the matrix and in the corona have the highest CaO with An in a range of 0.74–0.79 (Pl4). The medium-grained poikiloblastic plagioclase which intergrows with clinopyroxene and orthopyroxene, or the plagioclase between garnet and clinopyroxene have medium CaO with An in a range of 0.69–0.74 (Pl3). It is interesting that the An of plagioclase on the garnet side is higher than that of plagioclase on the clinopyroxene side. The core of coarse-grained plagioclase have the lowest CaO with An in a range of 0.59–0.63; for the antiperthite, the host plagioclase also have low CaO and An is in the range of 0.45–0.51, the Or of K-feldspar varies from 0.92 to 0.99. Both kinds of plagioclase with low An are interpreted as the mineral of peak metamorphism stage (M2).

  • Medium-coarse grained amphiboles (2–4 mm) are ubiquitous in the studied samples, and the compositions reveal that they are pargasites (Fig. 6) with Si of 6.0–6.24 (p.f.u., O=23), XMg of 0.71–0.78 and Ti of 0.12–0.22. The core of the amphibolite prophyroblast has higher Ti than that of the rim. We consider that the core is related to the peak metamorphism (Amp2). Besides, the amphiboles in the core of large clinopyroxene prophyroblast also have high Ti, similar to the core of the amphibolite prophyroblast. It could also be formed during the peak metamorphism (Amp2). The rim of amphibolite prophyroblast with lower Ti crystallized in the post-peak metamorphism stage (Amp3). The amphibolite in symplectitic corona around garnet has the lowest Ti and represents the retrograde stage (Amp4).

    Figure 6.  Mineral compositional diagrams of amphibolite. Classification in (a) after Leake et al. (1997); (b) diagram distinguishing high/low-Ti types from all plotted amphibolite compositions.

5.   P-T EVOLUTION OF THE HP MAFIC GRANULITES
  • At present, phase equilibrium modeling is widely used in the study of metamorphic petrology (Li Y et al., 2019; Zhang et al., 2019; Chen et al., 2018b; Sun et al., 2018; Wei, 2018; Wei and Powell, 2004). In this study, the phase equilibrium modeling was performed in the NCKFMASHTO (Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2) system using THERMOCALC version 3.45 (Powell and Holland, 1988), and the internally consistent thermodynamic data set ds62 of Holland and Powell (2011). The a-x relations for the main phases are: metabasite melt, clinopyroxene and amphibolite (Green et al., 2016); garnet, orthopyroxene (White et al., 2014); feldspars (Holland and Powell, 2003); ilmenite (White et al., 2000); quartz and rutile are considered as pure phases.

    P-T pseudosection was calculated using bulk-rock compositions for sample 17XWS1-1 by integrating mineral compositions and modal abundance data of the phases, normalized on the basis of mole percent as: SiO2=46.47, Al2O3=9.36, CaO=15.01, MgO=15.21, FeO=9.46, K2O=0.02, Na2O=1.51, TiO2=0.46, O2=0.80, H2O=1.70. The minor MnO and P2O5 contents were ignored in constructing the pseudosection, and other compositions were normalized to 100%, to construct the phase equilibrium within the NCKFMASHTO system. the mineral assemblage in the sample involves garnet, clinopyroxene, orthopyroxene, amphibolite, plagioclase, quartz, rutile, ilmenite and melt in the model system. The O2 (Fe2O3) contents were determined by titration, appropriate water contents in mole were adjusted using T-M(H2O) pseudosections calculated at 10 kbar, the estimated H2O content ranges from a near-anhydrous composition (H2O=0) to excess (H2O=3) (Fig. 7). According to previous estimate of peak metamorphism for mafic granulites (Huang et al., 2018; Guo et al., 2002), pressure was set as 10 kbar. Due to the facts that (1) rutile and quartz are absent in the matrix; (2) rutile as inclusions in garnet may have formed in the peak stage; and (3) melt pseudomorphs indicate the involvement of hydrous melt, the suitable water contents should range from 1.2 mol% to 2.4 mol%. Finally, we chose the appropriate M(H2O) content at 1.7 mol% referring to a previous study by Huang et al. (2018).

    Figure 7.  T-M(H2O) diagram at 10 kbar for sample 17XWS1-1. The blue dotted line represents the selected water content.

    The P-T pseudosection for sample 17XWS1-1 is presented in Fig. 8, it was constructed in the P-T range of 5–15 kbar and 800–1 100 ℃. Amphibolite, plagioclase and clinopyroxene are stable in the whole P-T region, quartz is stable above 9 kbar under 920 ℃; orthopyroxene gradually disappear above 9 kbar; rutile is stable above 8 kbar, a narrow transition zone from rutile to ilmenite occur at about 8 kbar in the diagram; and the melt appear about at 870 ℃ in this pseudosection. The isopleths of cg=Ca/(Fe2++Mg+Ca) in garnet, ca=CaM2 in clinopyroxene, An of plagioclase, and Ti of amphibolite for relevant mineral assemblages. cg contents mainly increase as pressure rises, An increases from high pressure to low pressure, and the Ti of amphibolite is related to temperature, which increases with temperature.

    Figure 8.  P-T pseudosection calculated from measured bulk composition of sample 17XWS1-1. The isopleths of Grs in garnet, Ca in clinopyroxene, An in plagioclase and Ti in amphibolite. The yellow field for the restrictive peak M2 and post-peak M3 conditions, black line represents the metamorphic evolution P-T path of the studied HP mafic granulites. Mineral abbreviations after Whitney and Evans (2010).

    In the peak metamorphic stage (M2), the assemblage contains the following phases, (1) rutile, plagioclase and clinopyroxene are developed in the garnet mantle and in the core of the coarse-grained clinopyroxene porphyroblasts; (2) the core of amphibolite prophyroblast and the amphiboles in the core of large clinopyroxene prophyroblast; and (3) the melt pseudomorphs surrounding garnet and between/corase-grained amphibole and plagioclase that are in contact with garnet porphyroblast. Therefore, the peak stage mineral assemblage consists of Grt2+ Pl2+Cpx2+Amp2+Rt+Melt pseudomorph. In order to precisely restrict the temperature and pressure conditions for the peak metamorphism, here we applied several related isopleths such as cg in garnet, An in plagioclase and Ti in amphibolite. The cg content in garnet mantle (Grt2) is 0.29–0.23, and the core of plagioclase (Pl2) show An content of 0.59–0.63. However, the cg and An isopleths are positive sloped in the field, while the Ti (0.20–0.22) of the peak amphibole (Amp2) is in a nearly vertical line. Based on intersections of these isopleths, we constrain the narrow peak P-T conditions of approximately 13.2–14.8 kbar and 1 050–1 080 ℃.

    As for the post-peak decompression stage (M3) during cooling, the assemblage is composed of Grt3+Pl3+Cpx3+Opx3+ Amp3, plagioclase+clinopyroxene+orthopyroxene+amphibolite, these minerals make up poikilitic texture and are in contact with the garnet porphyroblast (Fig. 3c). The temperature and pressure of stage M3 are constrained by isopleths of ca (0.87–0.89) in clinopyroxene (Cpx3), An (0.70–0.76) in poikilitic plagioclases (Pl3), and the stability of the M3 stage mineral assemblage. We define a P-T range of 5.7–7.3 kbar and 825–875 ℃ for this stage. The final retrogression stage (M4), with an assemblage without garnet is characterized by symplectitic coronas of plagioclase+ amphibole surrounding the garnet, consistent with amphibolite facies metamorphic conditions. We estimate the P-T range using conventional thermobarometers instead of in P-T pseudosection.

  • In addition to the P-T pseudosection modeling, we further estimate the metamorphic conditions of each stage using conventional thermobarometers for sample 17XWS1-1, including the garnet-clinopyroxene-plagioclase-quartz (Grt-Cpx-Pl-Qz) barometer (Newton and Perkins, 1982), the garnet-clinopyroxene (Grt-Cpx) thermometer (Krogh, 1988), the garnet-orthopyroxene (Grt-Opx) thermometer (Harley, 1984), Al-in-hornblende geobarometer (Anderson and Smith, 1995) and the hornblende geothermobarometer (Gerya et al., 1997). The results are shown in Table 2.

    Stage Mineral assemblage P (GPa) T (℃) Method References
    M2 Cpx2-Grt2-Pl2 1.4±0.2 1 021±50 Grt-Cpx thermometer Krogh (1988)
    Grt-Cpx-Pl-Qz barometer Newton and Perkins (1982)
    M3 Opx3-Grt3-Pl3-Amp3 0.74±0.2 865±50 Grt-Opx thermometer Harley (1984)
    Al-in-hornblende barometer Anderson and Smith (1995)
    M4 Amp4 0.5±0.2 660±50 Hbl geothermobarometer Gerya et al. (1997)

    Table 2.  Estimates of P-T conditions of different metamorphic stages for the mafic granulites sample 17XWS1-1

    As for the peak stage M2, the Grt-Cpx thermometer and Grt-Cpx-Pl-Qz barometer were applied using garnet compositions with the maximum Prp, clinopyroxene with the minimum XMg, and plagioclase with the minimum An. The results yield the maximum P-T estimate of 1.4±0.2 GPa/1 021±50 ℃. For the post-peak decompression stage (M3), the Grt-Opx thermometer shows temperature conditions in a range of 865±50 ℃ by calculating poikilitic orthopyroxene (Opx3) and garnet rim (Grt3). In addition, by calculating poikilitic amphibolite (Amp3) using Al-in-hornblende barometer (Anderson and Smith, 1995), the pressure was estimated to be about 0.74±0.2 GPa. For the last metamorphic stage (M4), the P-T condition is approximately 0.51 GPa/660 ℃ by using hornblende geothermobarometer (Gerya et al., 1997). Besides, the ternary feldspar thermometer (Fuhrman and Lindsley, 1988) was applied to the antiperthite (Table 3), the integrated compositions of ternary feldspar yield a temperature range from 1 020 to 1 060 ℃ at an expected pressure of 1.0 GPa (Fig. 9).

    Mineral Pl Pl Pl Pl Pl Kfs Kfs Kfs Kfs Kfs
    SiO2 56.63 55.31 56.46 56.93 56.40 64.79 65.08 65.65 64.28 64.45
    TiO2 0.00 0.00 0.00 0.00 0.06 0.04 0.07 0.15 0.06 0.00
    Al2O3 27.03 27.70 26.74 26.98 27.13 18.29 18.30 18.15 18.44 18.16
    Cr2O3 0.05 0.04 0.00 0.00 0.04 0.00 0.02 0.02 0.01 0.00
    FeO 0.18 0.15 0.11 0.13 0.16 0.04 0.02 0.34 0.11 0.59
    MnO 0.03 0.00 0.04 0.00 0.01 0.00 0.00 0.00 0.01 0.01
    MgO 0.01 0.00 0.00 0.02 0.03 0.02 0.00 0.01 0.00 0.00
    CaO 9.52 10.34 9.54 9.20 9.52 0.09 0.04 0.04 0.23 0.05
    Na2O 5.96 5.43 5.73 6.14 6.00 0.23 0.34 0.24 0.27 0.12
    K2O 0.00 0.00 0.00 0.00 0.00 16.14 16.64 15.78 15.89 16.59
    Total 99.41 98.97 98.62 99.40 99.35 99.64 100.51 100.38 99.30 99.97
    Oxygen 8 8 8 8 8 8 8 8 8 8
    Si 2.56 2.51 2.56 2.57 2.55 2.99 3.00 2.96 2.98 2.99
    Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00
    Al 1.44 1.48 1.43 1.43 1.44 1.01 0.99 1.03 1.02 0.99
    Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Fe 3+ 0.01 0.01 0.00 0.01 0.01 0.00 0.00 0.01 0.00 0.02
    Fe 2+ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Mg 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Ca 0.46 0.50 0.46 0.44 0.46 0.00 0.00 0.00 0.01 0.00
    Na 0.52 0.48 0.50 0.54 0.53 0.02 0.03 0.02 0.03 0.01
    K 0.00 0.00 0.00 0.00 0.00 0.97 0.98 0.97 0.95 0.98
    Sum 4.98 4.98 4.97 4.99 4.99 5.00 5.01 5.01 5.00 5.00
    An 0.47 0.51 0.48 0.45 0.47 0.00 0.00 0.00 0.01 0.00
    Ab 0.53 0.49 0.52 0.55 0.53 0.02 0.03 0.02 0.03 0.01
    Or 0.00 0.00 0.00 0.00 0.00 0.97 0.97 0.97 0.96 0.99

    Table 3.  Representative EMP analyses data of antiperthite for sample 17XWS1-2 from the mafic granulite in Xiwangshan area

    Figure 9.  Compositional plots of the integrated ternary feldspar from sample 17XWS1-2 (Table 3) with the solvus calculated at 1.0 GPa using the model of Fuhrman and Lindsley (1988).

6.   ZIRCON U-PB AGE AND REE CONTENTS
  • Zircon grains separated from the studied samples have no overgrowth structures. The zircons generally vary from 100 to 200 μm and show equiaxed oval or irregular crystal. The CL images are light gray or white because of the medium-strong luminescence. The grains exhibit planar zone structure without typically oscillatory zoning (Fig. 10a), indicating that they are metamorphic rather than igneous zircons. The age calculations and Concordia plots were carried out using Isoplot 3.0 (Ludwig, 2003).

    Figure 10.  Cathodoluminescence images of representative zircon grains, concordia diagrams of U-Pb zircon ages and zircon REE patterns for the Xiwangshan mafic granulite zircons. Chondrite values are from Sun and McDonough (1989).

    Fourteen analyses were performed on the metamorphic zircon grains, yielding 207Pb/206Pb ages between 1 881–1 722 Ma. According to the concordia diagrams (Figs. 10b, 10c), the metamorphic zircons can be divided into two groups. One group age is 1 853±14 Ma (MSWD=0.14) and the other group age is 1 744±44 Ma (MSWD=0.15) (Figs. 10b, 10c). Thus, the zircons record a two cycle of metamorphism, with the older age of 1 853±14 Ma representing a paleo-mesoproterozoic metamorphic event in the North China Craton (Li et al., 2018; Zhao et al., 2010; Guo et al., 2005).

    The zircon REE data are shown in Table S2. In chondrite-normalized REE diagrams, the data show the gradually ascending HREE patterns with weakly positive Ce and mostly moderate Eu negative anomalies for the two group zircons (Figs. 10d, 10e). These characteristics are similar to those observed in metamorphic zircon that crystallized contemporaneously with garnet (Liu et al., 2013).

7.   DISCUSSION
  • The presence of antiperthite is correlated with UHT determination in the mafic graulites (Prakash et al., 2007), and the ternary feldspar thermometer derivation from integrating antiperthite yields temperatures in a restricted range of 1 020–1 060 ℃, corresponding to an expected pressure of 1.0 GPa (Fuhrman and Lindsley, 1988) (Fig. 8). It has been proved that the content of Al2O3 in clinopyroxene has a positive relation with crystallization temperature and pressure as suggested by Nickel et al. (1985) and Yoshino et al. (1998). It is notable that the clinopyroxene (Cpx2) in the studied mafic granulites has high Al2O3 contents at the peak metamorphism. In this study, our samples show higher Al2O3 contents in the clinopyroxene than reported examples (or cases), such as the HP mafic granulites from Manjinggou (Guo et al., 2002), UHT mafic granulites from Qian'an gneiss dome (Liu and Wei, 2018) and Huangtuyao (Liao and Wei, 2019; Wang H Z et al., 2016), and even the mafic granulites from the same sampling location (Huang et al., 2018; Guo et al., 2002). The content of Al2O3 in the Cpx2 resembles that of clinopyroxene from UHT mafic granulite from the South Altyn Orogen (Dong et al., 2019). Based on A1M1 in the Cpx2 (0–0.15), Al-isopleth is above 1 000 ℃ (Nickel et al., 1985), the Cpx2 in the peak stage crystallized at above 1 000 ℃ (A1M1, 0.17–0.21, Table 1). Moreover, the content of TiO2 in the amphibole is also related to its formation temperature (Liao and Wei, 2019; Ernst and Liu, 1998), and the peak-stage amphibole (Amp2) also has high TiO2 content. In summary, the compositions of the peculiar minerals in the peak metamorphic stage indicate that they crystallized at ultrahigh temperature.

    The ultrahigh temperature metamorphism in pelitic granulites occurs widely in the NCC (e.g., Li X P et al., 2019; Liu et al., 2019; Guo et al., 2012; Santosh and Kusky, 2010; Santosh et al., 2007a). However, whether the mafic rocks have undergone the UHT metamorphism is still a controversial issue (Li and Wei, 2018), due to the absence of spinel+quartz, sapphirine+quartz and other typical ultra-high temperature minerals in the mafic rocks. Whereas the mafic granulites in the South Altyn Orogen underwent the UHT metamorphism, the peak temperature was constrained at 1 000–1 070 ℃ (Dong et al., 2019). The mafic granulites in the Zunhua and Taipingzhai areas are well constrained at 1 000–1 100 ℃ using REE-in-clinopyroxene-orthopyroxene and REE-in-plagioclase-clinopyroxene thermometers (Liu and Wei, 2018; Yang and Wei, 2017). Recently, the discovery of ultrahigh-temperature mafic granulite in the Huai'an Complex (Liao and Wei, 2019) probably suggested that the mafic granulites all experienced ultrahigh-temperature metamorphism in the NCC.

    In the Xiwangshan and its surrounding areas, Guo et al. (2002) gave a constraint on the P-T conditions of the mafic granulites, and considered that the peak temperature reached ~750–870 ℃ and pressure at ~11–14.5 kbar. In contract, the peak temperature of one sample from the Xiwangshan area could have reached 900 ℃ (Guo et al., 2002). The peak P-T condition is about 16 kbar and 790 ℃ for the HP mafic granulites from the Womakeng area by using P-T pseudosection (Zhang D D et al., 2016). The Tmax stage in the condition of 980–1 010 ℃ at 8.5–10.5 kbar, and 1 000–1 030 ℃ at 7.5–9.5 kbar were constrained for the mafic granulites in the Huangtuyao area (Liao and Wei, 2019). The mafic granulites in the Xiwangshan area are estimated at a maximum temperature of 930 ℃ at 9.5 kbar; it reached ultra-high temperature (UHT), although these results of the mafic granulites are with uncertainties (Huang et al., 2018).

    In addition, a large amount of melt pseudomorphs and some antiperthite grains were observed in this study (Figs. 3h3i). Based on a combination of ternary feldspar thermometer and conventional geothermobarometers, we estimate the P-T conditions of the peak stage at 1.4 kbar/1 021±50 ℃ using the Grt-Cpx thermometer (Krogh, 1988). In addition, the post-peak stage was evaluated at a maximum P-T of 0.7 kbar/865±50 ℃ by using the Grt-Opx thermometer (Perkins and Chipera, 1985). Applying phase equilibria modeling (THERMOCALC version 3.45) and mineral isopleth methods (e.g., garnet (cg), plagioclase (An), clinopyroxene (ca) and amphibolite (Ti)), we determine the peak stage (M2) metamorphism conditions at P-T of 13.2–14.8 kbar and 1 050–1 080 ℃. To all appearances, the peak temperature has reached ultrahigh-temperature (≥900 ℃), and the pressure also have achieved high pressure. Obviously, the studied mafic granulite has experienced HP-UHT metamorphism in the peak stage.

    Although the prograde metamorphic stage (M1) cannot be determined due to the absence of typical mineral assemblage, the phase equilibrium modeling (THERMOCALC version 3.45), and mineral isopleth methods, as well as a comprehensive study were carried out on petrographic and mineralogical features. The phase equilibrium modeling and conventional geothermobarometer suggest three stages of metamorphic evolution for the studied mafic granulites (Fig. 11), and we obtain the peak metamorphic (M2) maximum T and maximum P of 1 050–1 080 ℃ and 13.2–14.8 kbar. Following the isothermal decompression (ITD) stage, the post-peak (M3) P-T is 825–827 ℃ and 5.7–7.3 kbar, and the last stage (M4) P-T condition is 0.51 kbar/660 ℃, revealing a clockwise P-T path. In summary, for the studied samples, HP-UHT granulite stage (M2), LP-HT granulite stage (M3), and a subsequent amphibolite facies stage (M4) were determined using the mineral assemblages and the phase equilibrium modeling as well as conventional geothermobarometers.

    Figure 11.  Diagram showing the metamorphic P-T paths for the mafic granulites in this study (the blue curve), as well as those from other studies (1–8) from the TNCO. 1. Hengshan Complex (Zhao et al., 2001); 2. Hengshan Complex (Zhang et al., 2013); 3. Huai'an Complex (Zhai et al., 1992); 4. Huai'an and Xuanhua complexes (Guo et al., 2002); 5. Hengshan Complex (O'Brien et al., 2005); 6. Chicheng Complex (Zhang D D et al., 2016); 7. Xuanhua Complex (Huang et al., 2018); 8. Huai'an Complex (Liao and Wei, 2019). P-T grid and facies boundaries are from Brown (2014), and the thermal gradients are from Brown and Johnson (2018).

  • Two groups of ages on metamorphic zircons were obtained in this study, which are 1 853±14 Ma (MSWD=0.14) and 1 744± 44 Ma (MSWD=0.15) (Fig. 10), respectively. For the older age, it is within the metamorphic age in a range of 1 882–1 847 Ma, which was interpreted as the age related to the collision between the Eastern and Western blocks in the NCC (Liu et al., 2006); and it is also similar to the age of Sil-Grt-K-Fsp mylonite at 1 866± 22 Ma (Li et al., 2011). In addition, the REE patterns of three zircons with obvious positive anomaly of Ce are similar to those from the HP mafic granulites in the Huai'an Complex (Zhang H F et al., 2016; Wang et al., 2010). Also, the REE patterns of the other zircons are similar to those from the UHT mafic granulites from the Huangtuyao and Huai'an Complex (Liao and Wei, 2019). In conclusion, these characteristics demonstrate that they could form at the peak stage of HP-UHT.

    Large amounts of granulites with ages of ca. 1.85 Ga widely occur in the NCC, especially in the TNCO. The age of ca. 1.85 Ga might be related to the amalgamation of the Western and Eastern blocks of the NCC (Zhao and Zhai, 2013; Zhao et al., 2012, 2005). Other researchers argued that this age represents the timing of the formation of the TNCO (Tang and Santosh, 2018; Zhao et al., 2010, 2006, 2005). In addition, Peng et al. (2005) reported a metamorphic age of 1 834±5 Ma in the mafic granulite from the Xiwangshan area, which related to the age of granulite-facies metamorphism, and they provided conclusive evidence to the age range of 1.85 to 1.77 Ga for mafic in the NCC. Thus, the ca. 1.85 Ga metamorphic age for HP granulite could correlate with the timing of mafic dyke emplacement also at 1.85 Ga. However, some scholars proposed the northern margin of the NCC was exposed by ca. 1.85 Ga exhumation after the collision (Santosh et al., 2007b). Additionally, Santosh et al. (2006) obtained one age peak from the UHT granulites at 1 819±11 Ma, and interpreted the age as the timing of exhumation under UHT conditions. As described above, 1.85 Ga metamorphic age may also response to the early-stage continental extension. Subsequently, lithospheric thinning and asthenosphere upwelling did have occurred following the collision event (Yang et al., 2008).

    As discussed above, we consider that the age of 1 853±14 Ma records the age of peak stage metamorphism (some zircons with high Ti), or represents the transition stage from peak to post-peak stage. This age is also consistent with the metamorphic event caused by emplacement of basic dyke swarms (Yang et al., 2008; Peng et al., 2005). These dykes represent early-stage continental extension, which is almost simultaneous or coeval compared to the collision between the Eastern and the Western blocks. At the bottom of the lower continental crust, the rocks were heated by upwelling of the mantle-derived magmas related to extension and subsequent collision. This tectonic-magmatism-metamorphism event shows that the tectonic setting changed from a collision background to an extensional post-collision background during the Paleoproterozoic.

    Circa 1.78–1.76 Ga mafic dyke swarms and coeval volcanic rocks constituted a large igneous province in the central NCC (Peng et al., 2007). The igneous rocks were dated at 1 800–1 765 Ma that was in close proximity to the metamorphic age (1 795–1 755 Ma) (Liu Y C et al., 2009; Liu S W et al., 2006). Coincidently, this metamorphic age was in agreement with the flare-up timing of mafic swarms. The 1 800–1 765 Ma was interpreted as the timing of post-orogenic extension in the TNCO (Liu Y C et al., 2009; Liu S W et al., 2006). The age of 1 762 Ma from the HP granulite facies rocks most probably recorded late amphibolite facies retrogression occurring during the exhumation process in the southeastern margin of the NCC, and it corresponded with mafic dyke emplacement events (Liu et al., 2009). Thus, the younger age of 1 744±44 Ma (MSWD= 0.15) could represent the timing of amphibolite facies retrogression stage, and could be a response to the rapid exhumation of the granulite from the deep crust.

    The studied granulite rocks with Tmax and Pmax conditions of 1 050–1 080 ℃ and 13.2–14.8 kbar experienced HP-UHT peak stage (M2) and post-peak LP-HT stage (T-P conditions of 825–827 ℃, 5.7–7.3 kbar, stage M3), as well as the following amphibolite facies stage M4. The similar mafic granulites remained HP-UHT conditions and LP-UHT granulite stage during the slab uplifting to shallow crustal levels (Dong et al., 2019). This kind of HP-UHT metamorphic event may be interpreted to crustal thickening and large-scale heating related to mantle-derived magma upwelling at the base of the low crust, as evidenced by the widespread extension-related mafic magma emplacement in the NCC (Liu et al., 2009).

    The pressure yielded by the Xiwangshan UHT rocks is higher than that from the Huangtuyao (Liao and Wei, 2019), which indicates that the rocks were derived from a deeper level of the ultra-hot orogen. It is likely that the meta-mafic rocks occur at the base of the deep crust at the termination of the TNCO amalgamation. The deep crust was heated by mantle-derived magma which immediately followed the collision of the TNCO. The emplacing mafic dykes brought enormous heat to the granulite in the Xiwangshan and resulted in the UHT metamorphism. Subsequently, near-isothermal decompression occurred in a post-orogenic extensional setting (Liu et al., 2009), and the metamorphism converted into post peak-stage (M3). During this evolution process, the pressure fell quickly, whereas the temperature decreased very slowly. The feature indicates another thermal event that the granulite experienced during exhumation from the deep to the shallow crust.

    Similar to abundant clockwise P-T paths involving a near-isothermal decompression for the mafic granulites in the TNCO obtained from previous studies (Fig. 11) (Liao and Wei, 2019; Huang et al., 2018; Zhang D D et al., 2016; Guo et al., 2015; Zhang Y H et al., 2013; Guo et al., 2002; Zhao et al., 2001; Zhai et al., 1992), the clockwise P-T path modeled in this paper also records a near-isothermal decompression. However, the decompression occurred at ultra-high-temperature (1 050– 1 080 ℃) conditions, and post-peak metamorphism took place between 14.8 and 7.3 kbar within the temperature range of over 820 ℃. Consequently, the P-T path in this study is distinctly different and isolated from the others (Fig. 11).

    The Xiwangshan mafic granulite, located in the northern part of the TNCO (Fig. 1), is termed as the Xuanhua Complex by Huang et al. (2018). However, other researchers considered the mafic granulite (Liao and Wei, 2019; Liu et al., 2019; Wang H Z et al., 2016) as a part of the Huai'an Complex, which is separated from the easternmost part of the khondalite belt by the Gushan fault (Liao and Wei, 2019). Previously, the khondalite belt and the TNCO were considered to be two contrasting orogenic belts, because the former has collisional or peak metamorphic ages at ca. 1.95 Ga, while the latter indicates that the collision or peak metamorphism at ca. 1.85 Ga (Liao and Wei, 2019). However, recent studies on amphibolite-facies rocks from the Wutai-Hengshan region showed that the main collisional event within the TNCO belt also occurred at ca. 1.95 Ga (Wei, 2018; Qian et al., 2017; Qian and Wei, 2016). Some mafic granulites were considered to be intrusive dykes or sheets with a metamorphic age of 1.95–1.80 Ga, similar to those of meta-sedimentary rocks from the khondalite belt in the west of the Huai'an terrain (Wang H Z et al., 2016; Wang L J et al., 2015, 2011; Wang J et al., 2010; Zhao et al., 2010; Santosh et al., 2006). Based on the studies on ca. 1.91 Ga UHT mafic granulites, Liao and Wei (2019) proposed that the Huai'an Complex in the TNCO and Jining Complex in the khondalite belt shared a similar tectonic setting during ca. 1.95–1.85 Ga, including ca. 1.95 Ga crustal thickening related to HP granulite metamorphism and ca. 1.92–1.91 Ga extension related to regional or local UHT metamorphism.

    Multiple clues suggest that the Huai'an Complex in the TNCO and the Jining Complex in the khondalite belt have a similar tectonic evolution history. However, the age of ca. 1.85 Ga for UHT metamorphism in this study is much younger than those of UHT metamorphism in the khondalite belt, such as 1.92–1.91 and 1.87 Ga (Jiao et al., 2017; Li and Wei, 2016), or in the Huai'an Complex, such as ca. 1.91 and 1.87 Ga (Liao and Wei, 2019; Liu et al., 2019). Compared to the ages of upper-amphibolite facies in southern Hengshan Complex and lower-amphibolite facies in the Wutai Complex (Qian et al., 2017), the age of amphibolite facies metamorphism (ca. 1.74 Ga) is younger. Zhai and Santosh (2011) considered that the HP and HT-UHT granulites in the NCC demonstrated two main stages of metamorphism at ca. 1.95–1.89 and ca. 1.85–1.82 Ga, respectively. Therefore, it is probably a protracted orogenic process associated with the assembly of the NCC (Zhai and Santosh, 2011). In order to obtain new sights into this issue, more detailed studies are necessary in the future.

8.   CONCLUSIONS
  • Based on a comprehensive study on mineral assemblages, phase equilibrium modeling, geochronology, ternary feldspar thermometry and other conventional thermobarometry, three metamorphic stages are confirmed for the mafic granulites from the Xiwangshan area, Xuanhua, in the north of the TNCO belt. The main findings are concluded as follows.

    (1) The mafic granulites experienced HP-UHT metamorphism in the peak stage (M2) with P-T conditions of 13.2–14.8 kbar and 1 050–1 080 ℃, subsequently, a post-peak stage of decompression (M3) at 5.7–7.3 kbar and 825–875 ℃, and the final recrystallization at amphibolite facies (M4) P-T conditions about 5 kbar and 660 ℃. Based on these data, a clockwise P-T path is obtained.

    (2) The zircon U-Pb ages of 1 853±14 and 1 744±44 Ma from the mafic granulites records the age of UHT-metamorphic stage and a cooling process at amphibolite facies, respectively.

ACKNOWLEDGMENTS
  • We thank Dr. Jie Dong for help with ternary feldspar thermometer and phase equilibrium modeling. We thank Dr. Songjie Wang for help with paper polishing. We thank Dr. Lingmin Zhang for help during the operation of electron microprobe analysis and processing the analytical results. We also appreciate construtive reviews from two anonymous reviewers and the editors. This research was supported by the National Natural Science Foundation of China (No. 41761144061), the Shandong Provincial Natural Science Foundation (No. ZR2016DM04), and the University Students Innovation Program of SDUST (No. 2015TDJH101). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1251-8.

    Electronic Supplementary Materials: Supplementary materials (Tables S1–S2) are available in the online version of this article at https://doi.org/10.1007/s12583-019-1251-8.

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