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Volume 30 Issue 6
Dec.  2019
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Petrology and Metamorphic P-T Paths of Metamorphic Zones in the Huangyuan Group, Central Qilian Block, NW China

  • The Central Qilian Block is a Precambrian block in the Qilian Orogen,which has long drawn international attention for the study of orogeny and continental dynamics. The Huangyuan Group in the Datong area is one of the Precambrian metamorphic basement units in the Central Qilian Block and reflects metamorphism in the Barrovian garnet zone and sillimanite zone from south to north. Based on detailed fieldwork,this study presents a systematic study of petrography,mineral chemistry and phase equilibria of schists and gneisses from the two metamorphic zones. The garnet metamorphic zone is composed of micaschist,garnet-bearing micaschist and felsic leptynite,with in-terlayered plagioclase amphibolite. The sillimanite metamorphic zone consists of garnet-bearing biotite micaschist,sillimanite-bearing biotite-plagioclase gneiss and felsic leptynite. Garnet from the garnet metamorphic zone shows growth zoning with increasing almandine and pyrope and decreasing spessartine from core to rim. Garnet from the sillimanite metamorphic zone is almost homogeneous. Towards the outer rim,the contents of almandine and pyrope slightly decrease and grossular slightly increase. Biotite in both metamorphic zones is ferro-biotite. Plagioclase is oligoclase in garnet metamorphic zone and andesine in sillimanite metamorphic zone. Phase equilibrium modeling of a sample from garnet metamorphic zone resulted in a clockwise P-T path with a prograde stage (4.5-5.0 kbar,520-530℃),a peak P stage (9.8-10.2 kbar,560-570℃),a stage of thermal relaxation (8.0-8.5 kbar,580-590℃) and finally a retrograde stage (6.8-7.0 kbar,560-580℃). Thermodynamic modeling of a sample from the sillimanite metamorphic zone indicates a prograde stage (5.5-6.0 kbar,540-550℃) and a peak stage (7.8-8.5 kbar,660-690℃). The results indicate that the Huangyuan Group experienced medium-pressure amphibolite-facies metamorphism,which resulted from continental-continental collision between the Qaidam Block and the Central Qilian Block.
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Petrology and Metamorphic P-T Paths of Metamorphic Zones in the Huangyuan Group, Central Qilian Block, NW China

    Corresponding author: Yilong Li, yilong.li@qq.com
  • 1. School of Earth Sciences, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China
  • 2. Geology & Geochemistry Group, Department of Earth Sciences, VU Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

Abstract: The Central Qilian Block is a Precambrian block in the Qilian Orogen,which has long drawn international attention for the study of orogeny and continental dynamics. The Huangyuan Group in the Datong area is one of the Precambrian metamorphic basement units in the Central Qilian Block and reflects metamorphism in the Barrovian garnet zone and sillimanite zone from south to north. Based on detailed fieldwork,this study presents a systematic study of petrography,mineral chemistry and phase equilibria of schists and gneisses from the two metamorphic zones. The garnet metamorphic zone is composed of micaschist,garnet-bearing micaschist and felsic leptynite,with in-terlayered plagioclase amphibolite. The sillimanite metamorphic zone consists of garnet-bearing biotite micaschist,sillimanite-bearing biotite-plagioclase gneiss and felsic leptynite. Garnet from the garnet metamorphic zone shows growth zoning with increasing almandine and pyrope and decreasing spessartine from core to rim. Garnet from the sillimanite metamorphic zone is almost homogeneous. Towards the outer rim,the contents of almandine and pyrope slightly decrease and grossular slightly increase. Biotite in both metamorphic zones is ferro-biotite. Plagioclase is oligoclase in garnet metamorphic zone and andesine in sillimanite metamorphic zone. Phase equilibrium modeling of a sample from garnet metamorphic zone resulted in a clockwise P-T path with a prograde stage (4.5-5.0 kbar,520-530℃),a peak P stage (9.8-10.2 kbar,560-570℃),a stage of thermal relaxation (8.0-8.5 kbar,580-590℃) and finally a retrograde stage (6.8-7.0 kbar,560-580℃). Thermodynamic modeling of a sample from the sillimanite metamorphic zone indicates a prograde stage (5.5-6.0 kbar,540-550℃) and a peak stage (7.8-8.5 kbar,660-690℃). The results indicate that the Huangyuan Group experienced medium-pressure amphibolite-facies metamorphism,which resulted from continental-continental collision between the Qaidam Block and the Central Qilian Block.

0.   INTRODUCTION
  • The Qilian orogenic belt, at the northern margin of the Greater Tibetan Plateau, is an important component of the Central China orogenic belt. It is located in the tectonic corridor between the North China, South China and Tarim cratons, and is thought to result from the convergence and collision of the Alxa, Central Qilian and Qaidam blocks (Xia et al., 2016; Xu et al., 2006; Yang et al., 2002) (Fig. 1a). From north to south, the Qilian orogenic belt can be tectonically divided into the North Qilian accretionary belt, the Central Qilian Block and the South Qilian accretionary belt (Fig. 1b).

    Figure 1.  (a) Tectonic framework of China and location of the Qilian orogenic belt; (b) simplified geological map of the Qilian orogenic belt and adjacent areas (modified from Song et al., 2017; Xia et al., 2016; BGMR-QP, 1991). 1. Precambrian basement; 2. granite; 3. Mid–Late Neoproterozoic (848–604 Ma) rift-related volcanics; 4. obducted slice of Late Neoproterozoic–Cambrian (550–497 Ma) oceanic crust; 5. Middle Cambrian–Ordovician rift-related volcanics; 6. Middle Cambrian–Ordovician (514–446 Ma) arc volcanics; 7. Middle Cambrian–Ordovician (517–449 Ma) back-arc volcanics; 8. high pressure (HP)- ultrahigh pressure (UHP) metamorphic complex; 9. high pressure (HP) blueschist with eclogite; 10. Late Ordovician–Early Silurian (445–428 Ma) post-collisional rift volcanics; 11. Silurian incipient molasse formation; 12. Devonian molasse; 13. fault/inferred fault; 14. Carboniferous–Cenozoic cover.

    The Precambrian basement in the Qilian Orogen mainly consists of the Maxianshan Group, the Hualong Group, the Huangyuan Group, the Tuolai Group and the Beidahe Group. Previous work in the Central Qilian Block principally focused on the protolith formation age of basement rocks (Li et al., 2018a; Yan et al., 2015; He et al., 2010; Tung et al., 2007; Xu et al., 2007; Gehrels et al., 2003; Guo et al., 2000), the tectonic affinity of the block (Li et al., 2018a; Yan et al., 2015; Tung et al., 2013, 2012, 2007; Lu et al., 2009; Zhang et al., 2006; Wan et al., 2003, 2000) and Paleozoic magmatism (Wang et al., 2017; Huang et al., 2016, 2015; Song et al., 2013; Wu et al., 2011; Tseng et al., 2009).

    By geothermobarometer, Lin et al. (2009) derived a peak P-T condition of 6.5 kbar and 618 ℃ from an amphibolite, and a core temperature of 418 ℃ and a rim temperature of 545 ℃ from a biotite amphibolite in the Maxianshan Group. Qi et al. (2004) obtained peak P-T conditions of 6.2–7.7 kbar and 651–763 ℃ from a felsic mylonite in the Huangyuan Group. Bai et al. (1998) obtained an anticlockwise P-T path with peak P-T conditions of 3.8–4.8 kbar and 558–645 ℃ and suggested the supracrustal rocks in the Huangyuan Group were formed in a continental-rift environment. The previous studies regarding metamorphism of the Precambrian basement rocks in the Central Qilian Block were mostly based on traditional geothermobarometry. There are several uncertainties for defining mineral assemblages of different metamorphic stage and P-T conditions when using geothermobarometer, while phase equilibrium modeling can well avoid the influence of the disequilibrium between different mineral assemblages (Wei, 2011).

    To decipher the tectonic evolution of an orogen requires a precise knowledge of the P-T path of rocks that were metamorphosed and deformed during orogeny (Harris et al., 2004). The present tectonic scenarios about the Central Qilian Block were primarily postulated based on the Mid–Late Neoproterozoic to Early Paleozoic volcanism (Xia et al., 2016) or petrological constraints on the North Qilian accretionary belt and the North Qaidam HPM-UHPM belt (Song et al., 2014, 2013), lacking in petrological information from the basement rocks in the Central Qilian Block itself. Here we present a study by phase equilibrium modeling to decipher metamorphic P-T paths of rocks from different metamorphic zones in the Huangyuan Group, which is an important Precambrian basement unit in the Central Qilian Block. The results will provide constraints on the tectonic evolution of the Qilian Orogen.

1.   GEOLOGICAL SETTING
  • The Central Qilian Block, located in the center of Qinghai Province, is one of several Precambrian blocks in NW China. Precambrian basement rocks are the main component of the Central Qilian Block and are unconformably overlain by thin layers of Phanerozoic supracrustal strata (BGMR-QP, 1991; BGMR-GP, 1989). The Precambrian basement rocks in the Qilian Orogen can be divided into three sections as the Maxianshan Group in the east, the Hualong and Huangyuan groups in the middle, and the Tuolai and Beidahe groups in the west. All of these units consist of strongly deformed gneisses, migmatites, schists, marbles, quartzites, amphibolites and Proterozoic granitoids. The Phanerozoic cover strata include Early Paleozoic volcano-sedimentary associations, later Paleozoic to Triassic shallow marine to continental facies formations, and Jurassic to Quaternary swamp to continental facies deposits (BGMR-QP, 1991; BGMR-GP, 1989).

    The Huangyuan Group occurs dispersed throughout Huang- yuan County, Huangzhong County and Ledu County of Qinghai Province. From bottom to top, the Huangyuan Group can be subdivided into the Liujiatai Formation and the Dongchagou Formation, and both consist of schist, gneiss, amphibolite, granite gneiss, quartzite, marble and migmatite (BGMR-QP, 1991). By study of whole-rock geochemistry, mineral assemblage and geothermobarometry of garnet-bearing micaschists, the protolith of Huangyuan Group was suggested to be originated from intermediate-basic volcano-sedimentary rocks deposited in an epicontinental back-arc basin (Guo et al., 1999). Medium- pressure amphibolite facies metamorphism at 3.8–4.8 kbar and 558–645 ℃ affected the rocks (Bai et al., 1998).

    The study area is located in Datong County, northwest of Xining City in Qinghai Province, and is dominated by the Dongchagou Fomation of the Huangyuan Group (Fig. 2). The Triassic Nanyinger Formation and several Neoproterozoic and Paleozoic magmatic plutons are present in outcrops. The Huang- yuan Group in the study area mainly consists of garnet-bearing micaschist, mica schist, feldspar-mica schist and sillimanite- bearing biotite-plagioclase gneiss, with localized occurrences of lenticular marble and garnet-bearing plagioclase amphibolite. Combined with previous work (BGMR-QP, 1991), the Dongchagou Formation along the Baoku River from south to north can be subdivided into garnet metamorphic zone and sillimanite metamorphic zone. The garnet metamorphic zone is dominated by garnet-bearing micaschist and felsic leptynite, with layers of garnet-bearing plagioclase amphibolite (Figs. 3a, 3b). The rocks were intruded by granitic veins (Fig. 3b). The sillimanite metamorphic zone is composed of garnet-bearing biotite schist, sillimanite-bearing biotite-plagioclase gneiss and felsic leptynite. Masses of felsic veins occur among biotite-plagioclase gneiss (Fig. 3c). Due to ductile shear, the rocks were commonly mylonitic and deformed with apparent migmatization (Fig. 3d). To constrain the metamorphic P-T conditions, a garnet-bearing micaschist (25015) from the garnet metamorphic zone and a sillimanite-bearing biotite-plagioclase gneiss (25029) from the sillimanite metamorphic zone were collected for further analysis.

    Figure 2.  Geological map of the Huangyuan Group in Datong area (modified from BGMR-QP, 1991). 1. Huangyuan Group; 2. Tuolai Group; 3. Mesozoic; 4. Cenozoic; 5. Neoproterozoic intrusive rocks; 6. Caledonian intrusive rocks; 7. Ordovician volcanic rocks; 8. fault; 9. isograd; 10. Baoku River; 11. study area; 12. sample location.

    Figure 3.  Photographs of field occurrences of representative rocks from the Huangyuan Group. (a) Interbedded garnet-bearing micaschist and felsic leptynite; (b) plagioclase amphibolite layers between micaschist with later granitic veins; (c) occurrence of felsic lenses in biotite plagioclase gneiss; (d) migmatization phenomena in sillimanite-bearing biotite plagioclase gneiss.

2.   ANALYTICAL METHODS
  • Whole-rock compositions of the studied rocks were determined at the National Taiwan University. Oxides of major elements were determined by X-ray fluorescence (XRF) (Rigaku RIX- 2000) with an analytical uncertainty of 5%. Mineral compositions were analyzed using a JEOL JXA 8900 electron probe microanalyzer at the Institute of Earth Sciences, Academia Sinica, with operating conditions of 15 kV accelerating voltage and 10 nA beam current. The probe diameter was set at 1 μm for garnet and 5–10 μm for other minerals with a count time of 10 s for peak and background.

3.   PETROGRAPHY
  • Garnet-bearing micaschist sample 25015 from the garnet zone shows a porphyroblastic texture and mainly consists of garnet porphyroblast (5%), quartz (55%), biotite (20%), muscovite (10%), plagioclase (5%) and chlorite (3%) with minor ilmenite and rutile (2%) (Fig. 4a). The equidimensional garnet porphyroblasts are 0.4–1.5 mm in size. The big garnet grain has an inclusion-rich core and a nearly clear rim, indicating that they were formed in different stages. The inclusions in garnet are identified as quartz, biotite, muscovite and ilmenite and are thought to represent the mineral assemblage during prograde metamorphism (Fig. 4b). The garnet porphyroblast is surrounded by a foliated matrix of muscovite+biotite+quartz+ plagioclase+ilmenite (Fig. 4a). Biotite occurs in the cracks of garnet (Fig. 4c). Some big biotite grains in the matrix were replaced by chlorite and ilmenite (Fig. 4d), suggesting that retrograde metamorphism affected the rock.

    Figure 4.  Photomicrographs of the schist and gneiss. (a) Garnet-bearing micaschist (sample 25015, garnet zone) with a porphyroblastic texture consists of quartz, biotite, muscovite, plagioclase, garnet, chlorite and minor ilmenite and rutile. (b) Porphyroblastic garnet in sample 25015 contains inclusions of quartz, biotite and ilmenite. (c) Biotite occurs in a crack of a garnet crystal in sample 25015. (d) Some big biotite grains in the matrix have retrogressed to chlorite and ilmenite with small rutile relics in sample 25015. (e) Irregularly shaped garnet is 0.3–0.4 mm in size with conclusions of biotite, quartz and ilmenite in sample 25029. (f) Tiny sillimanite is often associated with biotite. (g), (h) Some amphibole grains have retrogressed to biotite and exsolved ilmenite±titanite, and some biotite grains broke down to chlorite±ilmenite±titanite in sample 25029. The mineral abbreviations used in this paper are as follows: Grt. garnet; Pl. plagioclase; Qz. quartz; Amp. amphibole; Cpx. clinopyroxene; Opx. orthopyroxene; Bt. biotite; Chl. chlorite; Ilm. ilmenite; Kf. K-feldspar; Ky. kyanite; Ms. muscovite; Pg. paragonite; Rt. rutile; Sil. sillimanite; Ttn. titanite; Zo. zoisite; Ab. albite; An. andesite; Alm. almandine; Grs. grossular; And. andalusite; Prp. pyrope; Sps. spessartine (Whitney and Evans, 2010).

    Sillimanite-bearing amphibole-biotite-plagioclase gneiss sample 25029 from the sillimanite zone shows a granoblastic texture and consists of plagioclase (35%), quartz (40%), amphibole (3%), biotite (15%), garnet (5%) with minor chlorite, titanite, ilmenite, rutile and sillimanite (2%). Irregularly shaped garnet is 0.3–0.4 mm in size with inclusions of biotite, quartz and ilmenite (Fig. 4e). Tiny sillimanite is often associated with biotite (Fig. 4f). Some amphibole grains have retrogressed to biotite and separated out ilmenite±titanite, and some biotite grains have retrogressed to chlorite±ilmenite± titanite (Figs. 4g, 4h). These petrographic observations indicate that the rock has experienced cooling after crystallization of the main mineral assemblage.

4.   MINERAL CHEMISTRY
  • Garnet in both samples is mainly composed of almandine with subordinate amounts of pyrope, grossular and spessartine (Table 1).

    Sample 25015 25029
    Mineral Grt Grt Grt Grt Grt Grt
    Core Mid Rim Core Mid Rim
    n 3 3 2 3 8 6
    SiO2 37.10 37.50 37.31 37.85 37.72 37.77
    TiO2 0.10 0.08 0.03 0.04 0.06 0.02
    Al2O3 20.90 21.11 21.10 21.61 21.68 21.77
    Cr2O3 0.15 0.02 0.05 0.00 0.00 0.00
    TFeO 26.31 29.79 34.01 27.71 27.94 27.62
    MnO 10.88 6.96 2.39 1.91 1.86 1.94
    MgO 0.93 1.17 1.61 2.77 2.71 2.55
    CaO 3.09 3.27 3.12 7.62 7.61 8.11
    Na2O 0.02 0.02 0.02 0.02 0.01 0.02
    K2O 0.00 0.01 0.01 0.01 0.01 0.01
    Total 99.46 99.89 99.61 99.60 99.68 99.85
    Si 3.02 3.03 3.02 3.01 3.00 2.99
    Ti 0.01 0.00 0.00 0.00 0.00 0.00
    Al 2.01 2.01 2.01 2.02 2.03 2.03
    Cr 0.01 0.00 0.00 0.00 0.00 0.00
    Fe3+ 0.00 0.00 0.00 0.00 0.00 0.00
    Fe2+ 1.79 2.01 2.30 1.84 1.86 1.83
    Mn 0.75 0.48 0.16 0.13 0.12 0.13
    Mg 0.11 0.14 0.19 0.33 0.32 0.30
    Ca 0.27 0.28 0.27 0.65 0.65 0.69
    Na 0.00 0.00 0.00 0.00 0.00 0.00
    K 0.00 0.00 0.00 0.00 0.00 0.00
    Total 7.97 7.96 7.97 7.98 7.98 7.99
    XAlm 0.61 0.69 0.79 0.63 0.63 0.62
    XPry 0.04 0.05 0.07 0.11 0.11 0.10
    XSps 0.26 0.16 0.06 0.04 0.04 0.04
    XGrs 0.09 0.10 0.09 0.22 0.22 0.23
    Number of ions on basis of 12 oxygen atoms. XAlm=Fe2+/(Fe2++Mn+Mg+Ca), XSps=Mn/(Fe2++Mn+Mg+Ca), XPrp=Mg/(Fe2++Mn+Mg+Ca), XGrs=Ca/(Fe2++Mn+Mg+Ca).

    Table 1.  Chemical compositions (wt.%) of garnet from the studied schist and gneiss from Huangyuan Group

    Garnet in garnet zone sample 25015 has a composition of Alm61–81Grs9–10Prp4–7Sps3–26. From core to rim, there is no obvious variability in the contents of grossular, but the contents of almandine and pyrope increase and the contents of spessartine decrease (Fig. 5a), showing growth zoning characteristics (Spear, 1993; Deer et al., 1982) and indicating the garnet was formed during prograde metamorphism, likely with a temperature of < 650 ℃ (Spear, 1993; St-Onge, 1987; Tracy, 1982).

    Figure 5.  (a) Compositional profiles of a garnet in garnet zone sample 25015. (b) Compositional profiles of a garnet in sillimanite sample 25029. (c) Mica classification diagram after Foster (1960), showing the compositions of biotite in samples 25015 and 25029. (d) Feldspar classification diagram after Smith (1974), showing the compositions of plagioclase in samples 25015 and 25029. An=Ca/(Ca+Na+K), Ab=Na/(Ca+Na+K), Or=K/(Ca+Na+K); XAlm=Fe2+/(Fe2++Mn+Mg+Ca), XSps=Mn/ (Fe2++Mn+Mg+Ca), XPrp=Mg/(Fe2++Mn+Mg+Ca), XGrs=Ca/(Fe2++Mn+Mg+Ca).

    Garnet in sample 25029 from the sillimanite zone has a composition of Alm59–65Grs20–28Prp9–12Sps4–5 and is almost homogeneous (Fig. 5b). The contents of almandine and pyrope slightly decrease and the contents of grossularite slightly increase in the garnet rim, indicating that the garnet rim encountered late reformation of Mg and Fe exchanging with adjacent mafic minerals.

  • Biotite in both samples is classified as ferro-biotite (Fig. 5c). Biotite in sample 25015 has Mg# (Mg/(Mg+Fe2+)) of 0.38–0.41 and Ti of 0.05–0.10 (p.f.u.). Biotite in sillimanite zone sample 25029 has Mg# of 0.38–0.42 and higher Ti of 0.13–0.27 (p.f.u.) than those in sample 25015 from the garnet zone (Table 2).

    Sample 25015 25029 25015
    Mineral Bt1 Bt2 Bt1 Bt2 Ms Ms
    Matrix Matrix Matrix Matrix Inclusion Matrix
    SiO2 35.92 34.19 35.05 33.76 46.18 46.29
    TiO2 1.67 1.05 1.36 2.68 0.30 0.27
    Al2O3 19.77 19.57 19.67 16.74 35.21 34.29
    Cr2O3 0.00 0.02 0.01 0.04 0.08 0.03
    TFeO 22.25 24.44 23.35 26.17 1.64 1.96
    MnO 0.05 0.09 0.07 0.22 0.00 0.01
    MgO 7.58 7.74 7.66 7.53 0.66 0.77
    CaO 0.00 0.03 0.01 0.12 0.00 0.04
    Na2O 0.21 0.00 0.10 0.06 1.08 1.00
    K2O 8.71 7.90 8.31 7.64 9.73 9.56
    Total 96.13 95.16 95.65 94.95 94.89 94.23
    Si 2.73 2.63 2.68 2.64 3.08 3.11
    Ti 0.10 0.06 0.08 0.16 0.02 0.01
    Al 1.77 1.77 1.77 1.54 2.77 2.72
    Cr 0.00 0.00 0.00 0.00 0.00 0.00
    Fe3+ 0.00 0.22 0.11 0.21 0.00 0.00
    Fe2+ 1.41 1.35 1.38 1.50 0.09 0.11
    Mn 0.00 0.01 0.00 0.01 0.00 0.00
    Mg 0.86 0.89 0.87 0.88 0.07 0.08
    Ca 0.00 0.00 0.00 0.01 0.00 0.00
    Na 0.03 0.00 0.02 0.01 0.14 0.13
    K 0.84 0.77 0.81 0.76 0.83 0.82
    Total 7.73 7.70 7.72 7.72 7.00 6.99
    Mg# 0.38 0.40 0.39 0.37 0.42 0.41
    Number of ions for on basis of 11 oxygen atoms. Mg#=Mg/(Fe2++Mg).

    Table 2.  Chemical composition (wt.%) of biotite and muscovite from the studied schist and gneiss from Huangyuan Group

    Muscovite only occurs in garnet zone sample 25015. Tiny muscovite inclusions within a big garnet porphyroblast have an average Si content of 3.08 (p.f.u.), while flaky muscovite in the matrix has a higher average Si content of 3.11 (p.f.u.) (Table 2).

    Compositions of measured plagioclase crystals are shown in Fig. 5d and Table 3. Plagioclase in sample 25015 from the garnet zone is oligoclase. Plagioclase inclusions within the garnet porphyroblast have higher anorthite contents (An17–18) than those in the matrix (An13–16). Plagioclase in the matrix shows compositional zoning with anorthite contents increasing from core (An13) to rim (An16). In sillimanite zone sample 25029, plagioclase has higher An contents and is classified as andesine. Coarse-grained plagioclase in the matrix shows homogeneous composition (An45–46).

    Sample 25015 25029
    Mineral Pl Pl Pl Pl Pl Pl
    Inclusion Inclusion Matrix Matrix Core Rim
    SiO2 63.22 63.48 65.09 63.95 55.76 54.98
    TiO2 0.00 0.02 0.00 0.00 0.00 0.00
    Al2O3 22.09 21.91 21.45 22.20 27.32 27.65
    Cr2O3 0.00 0.00 0.01 0.00 0.04 0.10
    TFeO 0.22 0.07 0.09 0.10 0.00 0.00
    MnO 0.04 0.04 0.00 0.00 0.02 0.06
    MgO 0.00 0.00 0.00 0.00 0.02 0.02
    CaO 3.66 3.47 2.88 3.47 9.26 9.69
    Na2O 9.40 9.55 10.27 9.83 6.20 6.13
    K2O 0.10 0.09 0.02 0.05 0.06 0.07
    Total 98.72 98.65 99.81 99.61 98.68 98.70
    Si 2.83 2.84 2.87 2.83 2.54 2.51
    Ti 0.00 0.00 0.00 0.00 0.00 0.00
    Al 1.16 1.16 1.12 1.16 1.46 1.48
    Cr 0.00 0.00 0.00 0.00 0.00 0.00
    Fe3+ 0.01 0.00 0.00 0.00 0.00 0.00
    Fe2+ 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
    Mg 0.00 0.00 0.00 0.00 0.00 0.00
    Ca 0.18 0.17 0.14 0.17 0.45 0.47
    Na 0.82 0.83 0.88 0.84 0.55 0.54
    K 0.01 0.01 0.00 0.00 0.00 0.00
    Total 5.00 5.00 5.01 5.01 5.01 5.02
    An 0.18 0.17 0.13 0.16 0.45 0.46
    Ab 0.82 0.83 0.87 0.83 0.55 0.53
    Or 0.01 0.01 0.00 0.00 0.00 0.01
    Number of ions for on basis of 8 oxygen atoms. Inc means inclusions in garnet. An=Ca/(Ca+Na+K), Ab=Na/(Ca+Na+K), Or=K/(Ca+Na+K).

    Table 3.  Chemical composition (wt.%) of plagioclase from the studied schist and gneiss from Huangyuan Group

5.   P-T PSEUDOSECTION CALCULATIONS
  • Metamorphic P-T conditions of the studied schists were calculated by phase equilibrium modeling with the measured bulk rock compositions (Table 4) using Perple_X thermodynamic modelling software (Connolly, 2005; version from March 2018) and the internally consistent thermodynamic data set of Holland and Powell (1998, updated 2011) in the model system MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 that closely matches the major element compositions of the two samples. For modeling the mineral activity-composition relationships selected are those defined for garnet, biotite and muscovite (White et al., 2014), feldspar (Holland and Powell, 2003), amphibole (Dale et al., 2000), pyroxene (Holland and Powell, 1996), chlorite and ilmenite (Holland and Powell, 1998). A pure H2O fluid and quartz are considered to be available in excess. Because P2O5 mainly exists in apatite and little apatite can be found in the samples, it was not considered an important system component, whilst Fe2O3 was neglected because the minerals present are thought to be low Fe3+.

    Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total
    25015 65.49 0.76 16.84 6.63 0.16 1.45 0.89 1.96 3.34 0.11 2.18 99.80
    25029 65.53 0.65 15.90 4.81 0.06 1.47 5.15 2.91 1.14 0.08 1.37 99.06

    Table 4.  Bulk compositions (wt.%) of schists from the Huangyuan Group

  • The P-T pseudosection of sample 25015 was calculated in a P-T range of 3.0–12 kbar and 450–700 ℃ (Fig. 6a). It is dominated by tri- and quadri-variant fields with a few uni- and bi- fields. Rutile is stable in the stability fields of > 7.0 kbar. Rutile relics are present in the observed mineral assemblage, suggesting > 7.0 kbar peak metamorphic conditions. Isopleths of XMg (=Mg/(Fe2++Mn+Mg+Ca)), XFe (=Fe2+/(Fe2++Mn+Mg+Ca)) and XMn (=Mn/(Fe2++Mn+Mg+Ca)) in garnet, and Si (p.f.u.) in white mica were contoured in order to further constrain the metamorphic P-T conditions (Fig. 6b). The measured XMg (0.04) and XMn (0.26) compositions in garnet cores constrain P-T conditions of 4.5–5.0 kbar and 520–530 ℃ in the stability field of garnet+ biotite+plagioclase+muscovite+albite+ilmenite+quartz, reflecting the prograde metamorphic conditions that affected the rock. Based on growth zoning of the garnet, peak P-T conditions were constrained by the measured XMg (0.07) in garnet and Si content (3.11) in muscovite, isopleths for which are located in the stability field of garnet+biotite+muscovite+paragonite+albite+rutile+ quartz, corresponding to P-T conditions of 9.8–10.2 kbar and 560–570 ℃. Therefore, the compositions of the garnet core recorded a prograde P-T path. According to the pseudosection, the present mineral assemblage of garnet+plagioclase+biotite+ muscovite+chlorite+ilmenite+quartz corresponds to P-T conditions of 6.5–7.2 kbar and 540–590 ℃. Compositions of XMg, XCa and XMn in rim of the garnet further constrain a P-T range of 6.8–7.0 kbar and 560–580 ℃ in this stability field, representing retrograde metamorphic conditions.

    Figure 6.  (a) P-T pseudosection for sample 25015 (garnet metamorphic zone) showing assemblage stability fields for the bulk rock composition reported in Table 4. (b) Contour intersections for compositions of garnet (XMg, XFe and XMn) and white mica (Si content) for sample 25015. (c) P-T pseudosection for sample 25029 (sillimanite metamorphic zone) showing assemblage stability fields for the bulk rock composition reported in Table 4. (d) Contour intersections for compositions of garnet (XMg, XFe and XCa) and plagioclase (XCa) for sample 25029. (e) Isopleths of H2O content (MH2O) and garnet content (MGrt) for sample 25015 showing MH2O decrease with T and MGrt decrease during decompression. (f) Isopleths of H2O content (MH2O) for sample 25029 showing MH2O decrease with T. XMg=Mg/(Fe2++Mn+Mg+Ca), XFe=Fe2+/(Fe2++Mn+Mg+Ca), XMn=Mn/(Fe2++Mn+Mg+Ca), XCa=Ca/(Fe2++Mn+Mg+Ca) for garnet, XCa=Ca/(Ca+Na+K) for plagioclase.

  • For sample 25029, P-T pseudosections was calculated in a P-T range of 3.0–12.0 kbar and 400–800 ℃ (Fig. 6c). Isopleths of XFe (=Fe2+/(Fe2++Mn+Mg+Ca)), XMg (=Mg/(Fe2++Mn+Mg+Ca)) and XCa (=Ca/(Fe2++Mn+Mg+Ca)) in garnet and XCa (=Ca/ (Ca+Na+K)) in plagioclase are shown in Fig. 6d. Isopleths of XMg in garnet have steep slopes and vary mainly with temperature. Isopleths of XCa in plagioclase have moderately positive slopes increasing as pressure rising, and therefore mostly reflect changes in metamorphic pressure. Based on the compositional zoning of garnet, peak P-T conditions were constrained by the measured XMg (0.12) in garnet core and XCa in plagioclase (0.45–0.46), located in the stability field of garnet+biotite+plagioclase+ clinopyroxene+rutile+quartz with a P-T range of 3.1–3.3 kbar and 660–690 ℃. Compositions of XFe (0.59–0.61) and XCa (0.24–0.28) in garnet rims constrain a P-T range of 5.5–6.0 kbar and 540–550 ℃ in the stability field of garnet+biotite+plagioclase+ chlorite+amphibole+ilmenite+titanite+quartz, which represents the retrograde mineral assemblage and matches well with the present mineral assemblage in the matrix. Therefore, garnet in sample 25029 recorded a cooling during decompression.

6.   DISCUSSION
  • Based on petrography, mineral chemistry and phase equilibrium modeling, the metamorphic evolution of sample 25015 can be divided into prograde, peak and retrograde stages. The P-T conditions of the prograde metamorphic stage are constrained by garnet core compositions in a P-T range of 4.5–5.0 kbar and 520–530 ℃. The prograde P-T path constantly cuts the isopleths of H2O content reflecting progressive dehydration (Fig 6e), which is beneficial for the evolution of mineral assemblage. Chlorite broke down by dehydration reaction, and plagioclase broke down owing to the increasing pressure while garnet, muscovite and paragonite grew, and garnet formed growth compositional zoning. Peak metamorphic conditions of 9.8–10.2 kbar and 560–570 ℃ are constrained by garnet rim compositions and the maximum Si contents in white mica in the stability field of garnet+biotite+muscovite+paragonite+albite+rutile+quartz, whi- ch matches well with the presence of rutile relics in the rock (Fig. 4d). Garnet rim compositions constrain retrograde P-T conditions of 6.5–7.2 kbar and 540–590 ℃.

    The above inferences, combined with isopleths of H2O content, leave two possible P-T paths for the rock. After peak P-T conditions, if the rock evolved in P-T path A, it experienced isothermal decompression. In that case, the P-T path was subparallel to the isopleths of H2O content (Fig. 6e), suggesting that dehydration stopped and allowing preservation of the peak assemblage of garnet+biotite+muscovite+paragonite+ albite+rutile+quartz. This, however, is contrary to petrological observation of the presence of a retrograde mineral assemblage of garnet+biotite+muscovite+plagioclase+ilmenite+quartz in the matrix. If the rock evolved according to P-T path B after the peak stage, the P-T path continues to cross isopleths of H2O contents and evolved in the direction of reducing H2O contents (Fig. 6e), indicating that dehydration reactions continued, allowing the further evolution of mineral assemblages. The presence of post-peak P minerals in the rock matrix, in combination with the P-T pseudosection suggests that path B is more likely for the sample from the garnet zone.

    After the peak stage, thermal relaxation resulted in the breakdown of paragonite. At the same time, grossularite together with albite and quartz reacted to plagioclase and the rock entered the plagioclase stability field. With further heating during initial decompression rutile broke down to ilmenite. During cooling decompressive process, the rock was rehydrated by a small amount of intergranular fluid and chlorite appeared, probably by the reaction of Bt+fluid→Chl±Ilm. According to P-T path B, peak P and peak T conditions of garnet zone sample 25015 were not reached at the same time. Therefore, what is referred to as the peak P-T conditions above in fact are actually peak P conditions. Chemical diffusion in the garnet rim, or a lack of garnet growth during decompression (Fig. 6e) may be responsible for the fact that the peak T conditions were not recorded in the rock.

  • Phase equilibrium modeling indicates that peak P-T conditions constrained by compositions of garnet and plagioclase are corresponding to a P-T range of 3.1–3.3 kbar and 660–690 ℃ that located inside pyroxene-bearing stability field. After peak stage, decreasing temperature and pressure resulted in rutile breakdown to ilmenite. The P-T path cuts isopleths of H2O content in the direction of increasing H2O content (Fig. 6f), which means that retrograde metamorphic transformations could only occur if enough extraneous fluid affiliating was added to the rock (Thompson, 1983; Fyfe et al., 1978). The appearance of amphibole suggests extraneous fluid involving during cooling decompressive stage. With temperature and pressure further declining, amphibole regressed to chlorite or biotite, and biotite transformed to chlorite+ilmenite±titanite. Finally, the rock equilibrated in the stability field of garnet+biotite+plagioclase+ chlorite+amphibole+ilmenite+titanite+quartz, which is consistent with the present mineral assemblage in thin section.

  • Phase equilibrium modeling indicates that both samples from the Huangyuan Group experienced clockwise metamorphic P-T paths into amphibolite facies (Fig. 7), probably related to crustal thickening (Brown, 1993; Condie et al., 1992; England and Thompson, 1984; Thompson and England, 1984). Peak P-T conditions of garnet-bearing micaschist (sample 25015) from the garnet metamorphic zone and sillimanite-bearing biotite- plagioclase gneiss (sample 25029) from the sillimanite metamorphic zone are 8.0–8.5 kbar, 580–590 ℃, and 7.8–8.5 kbar, 660–690 ℃, respectively. In combination with a muscovite 40Ar/39Ar age of 422.3±1.3 Ma from the paragneisses in the Huangyuan Group (unpublished), the Huangyuan Group should record an Early Paleozoic metamorphic event.

    Figure 7.  P-T paths of the garnet and sillimanite metamorphic zones from the Huangyuan Group.

    Several hypotheses have been proposed for the Paleozoic tectonic evolution of the Qilian Orogen. Most researchers suggested that the North Qilian accretionary belt between the Alxa and Central Qilian blocks and the North Qaidam HPM-UHPM belt between the Central Qilian and Qaidam blocks are two independent subduction zones (Li et al., 2018a; Xia et al., 2016; Xu et al., 2006; Yang et al., 2002). Xia et al. (2016) concluded Mid–Late Neoproterozoic to Early Paleozoic volcanism in the Qilian Orogen and considered that the South Qilian Ocean was subducted northward beneath the Qilian Block at 540–446 Ma and the following continental-continental subduction and collision between the Qaidam and Central Qilian blocks occurred at 445–424 Ma. The muscovite 40Ar/39Ar age (422.3±1.3 Ma, unpublished data) indicates the metamorphic rocks in the Huang- yuan Group are products of the Early Palaeozoic collision between the Central Qilian Block and the Qaidam Block. By phase equilibrium modeling, Li et al. (2018b) reported garnet amphibolites in the Hualong Group experienced a clockwise P-T path with peak P-T conditions of ~4.9–6.3 kbar and ~755–820 ℃ in the upper amphibolite facies to low-temperature granulite facies, and retrograde cooling and decompression conditions of ~2.5–3.1 kbar and ~525–545 ℃, resulted from northward subduction of the South Qilian oceanic crust beneath the Central Qilian Block and the following continental collision between the Central Qilian and the Qaidam blocks.

    The continental subduction and collision between the Qaidam Block and the Central Qilian Block gave rise to melting of deep crust and formation of a series of 435–450 Ma syn- collisional granitoids in the middle Central Qilian Block (Huang et al., 2016; Shi et al., 2015; Yong et al., 2008). Crustal thickening and different intensity of granitic magmatism resulted in amphibolite-facies metamorphic zones of the Huangyuan Group. The metamorphic pressure of the rocks reached maximum first, and then the temperature reached maximum during relaxation stage, recorded by sample 25015 from garnet metamorphic zone. Peak P conditions of garnet-bearing micaschist (sample 25015) from the garnet metamorphic zone and sillimanite-bearing biotite-plagioclase gneiss (sample 25029) from the sillimanite metamorphic zone are 10.0 kbar, 560 ℃, and 8.0 kbar, 680 ℃, respectively, suggesting that they were buried to 30 and 24 km depth by crustal thickening, respectively. After the peak metamorphism, cooling decompressive process affected the rocks in the Huangyuan Group.

7.   CONCLUSION
  • (1) The metamorphic rocks of the Huangyuan Group exposed in the Datong area are divided into garnet metamorphic zone and sillimanite metamorphic zone from south to north. The garnet metamorphic zone is dominated by garnet-bearing mica quartz schist, garnet-bearing micaschist and felsic leptynite, with interlayers of plagioclase amphibolite. The rock assemblage of sillimanite metamorphic zone is garnet-bearing biotite-quartz schist, sillimanite-bearing biotite-plagioclase gneiss and felsic leptynite.

    (2) Garnet from the garnet metamorphic zone shows growth zoning with increasing almandine and pyrope and decreasing spessartite from core to rim. Garnet from the sillimanite metamorphic zone is almost homogeneous. Towards the rim, the contents of almandine and pyrope slightly decrease and grossularite slightly increase. Biotite in both metamorphic zones belongs to ferro-biotite. Plagioclase is oligoclase in the garnet zone and andesine in the sillimanite zone.

    (3) Phase equilibrium modeling of sample from the garnet metamorphic zone recorded a clockwise P-T path with a prograde stage (4.5–5.0 kbar, 520–530 ℃), a peak P stage (9.8–10.2 kbar, 560–570 ℃), a thermal relaxation stage (8.0–8.5 kbar, 580–590 ℃) and a retrograde stage (6.8–7.0 kbar, 560–580 ℃). A sample from the sillimanite metamorphic zone recorded a peak stage (7.8–8.5 kbar, 660–690 ℃) and a retrograde stage (5.5–6.0 kbar, 540–550 ℃). It is inferred that the Huangyuan Group experienced medium-pressure amphibolite-facies metamorphism, resulted from continental collision between the Qaidam Block and the Central Qilian Block.

ACKNOWLEDGMENTS
  • The authors are indebted to Dr. Chi-Yu Lee of Department of Geosciences, National Taiwan University for his help with whole- rock major element analysis. This study was funded by the National Natural Science Foundation of China (No. 41520104003), the National Key R & D Program of China (No. 2016YFC0600403), the China Geological Survey (No. DD20160201), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (Nos. CUGL170404, CUG160232). The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0879-0.

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