Journal of Earth Science  2019, Vol. 30 Issue (3): 603-620   PDF    
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Multi-Stage Metamorphism of the UHP Pelitic Gneiss from the Southern Altyn Tagh HP/UHP Belt, Western China: Petrological and Geochronological Evidence
Yuting Cao 1,2,3, Liang Liu 2, Chao Wang 2, Cong Zhang 1,3, Lei Kang 2, Wenqiang Yang 2, Xiaohui Zhu 4     
1. Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals, College of Earth Science & Engineering, Shandong University of Science and Technology, Qingdao 266590, China;
2. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China;
3. Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
4. Xi'an Center of Geological Survey, China Geological Survey, Xi'an 710054, China
ABSTRACT: The kyanite-bearing garnet pelitic gneiss from the Jianggalesayi area in southern Altyn Tagh high pressure/ultra-high pressure belt was proved to have been experienced UHP metamorphism (>12 GPa) by the discovery of kyanite and spinel exsolution microstructure in quartz (precursor stishovite). In this study, three stages of retrograded metamorphism (M2-M4) after the UHP metamorphism (M1) were identified for the UHP pelitic gneiss. The HP granulite-facies stage (M2) was characterized by the mineral assemblage of garnet+kyanite+K-feldspar+rutile+quartz±ilmenite, recording the P-T condition of >1.12 GPa and~850-930℃. The granulite-facies stage (M3) was represented by the mineral assemblage of garnet rim+K-feldspar+sillimanite (Sill1)+biotite (Bt1)+plagioclase (Pl1)+ilmenite+quartz, and confined under P-T conditions of 0.5-0.8 GPa and~770-795℃. The late cooling stage M4 was accompanied by the appearance of fine-grained Pl2, Sill2 and Bt2 in the matrix, and the P-T conditions were 0.4-0.6 GPa and < 675℃. A clockwised P-T path was obtained for the pelitic gneiss in the P-T pseudosection, which showed a deep subduction/collision processes with subsequent exhumation and cooling. Combined with the corresponding multistage metamorphic assemblages, the age dating results implied that the zircons from the gneiss have integrated the recording peak metamorphic (M1, 484±3 Ma) and retrograded metamorphic ages (M2 to M3, 450±2 Ma). There was about 32 Ma interval during the first exhumation from the upper mantle depth (>350 km) to the lower crust depth (~40-20 km), resulting in an average exhumation rate of 9.11-9.70 mm/yr. In the southern Altyn Tagh region, the HP and UHP rocks from different areas had identical peak metamorphic ages. Therefore, contemporary UHP and HP rocks with different metamorphic evolutions were recognized coexisting in the same orogenic belt, which can be interpreted by the model of subduction channel. The continental crustal were subducted to different depths along the direction of the subduction channels at~500 Ma, suffered different grade metamorphism, and then returned to the surface along the subduction channel.
KEY WORDS: southern Altyn Tagh HP/UHP belt    kyanite-bearing garnet pelitic gneiss    P-T pseudosection    subduction channel    continental deep subduction and exhumation    
0 INTRODUCTION

In the past two decades, ultra-high pressure (UHP) metamorphic minerals, such as coesite and diamond, have been discovered in the worldwide metamorphic terranes (Meng et al., 2018a; Wang et al., 2018; Liou et al., 2009; Zheng, 2008; Chopin, 1984; Smith, 1984), indicating that the low density continental crust can be subducted to mantle depths of 80–200 km for large-scale UHP metamorphism and subsequent exhumation to the shallow crust. As a result, UHP and non-UHP metamorphic rocks (include ultra-high temperature (UHT) and high pressure (HP) rocks) are presently exposed together along the margin of convergent continents, forming the basic tectonic framework of continental collisional orogens (Lei and Xu, 2018; Meng et al., 2018a; Zheng, 2008), and always record multiple events including continental subduction, collision and exhumation. These results can supply geodynamical information, petrographical and geochronological evidences for the formation and evolution of the orogens (Meng et al., 2019; Chen et al., 2018; Sun et al., 2018; Wang et al., 2010; Zhao et al., 2010; Ernst, 2006, 2001; Song et al., 2006; Carswell and Zhang, 1999; Maruyama et al., 1996).

The South Altyn Tagh region in the Northwest China is a typical subduction/collision complex belt formed at Early Paleozoic. The various HP/UHP metamorphic rocks in this region are mainly located in the Jianggalesayi area, the Danshuiquan area, the Yinggelisayi area and the Munabulake area (Cao et al., 2019, 2013, 2009; Liu et al., 2018, 2012, 2007a, 2005, 2004, 2002; Gai et al., 2017; Wang et al., 2011; Zhang and Meng, 2005; Zhang et al., 2002, 2001), including the UHP eclogite, garnet peridotite, granitic gneisses and pelitic gneisses. Recent investigations of these HP/UHP rocks had made significant advancements on continental subduction depth, subduction/collision time, metamorphic evolution and properties of the subducted continental crust (Wang et al., 2014, 2013; Liu et al., 2013a). The majority of HP/UHP rocks have similar peak metamorphic ages of 480–504 Ma (Cao et al., 2019, 2015, 2013, 2009; Wang et al., 2014, 2011; Zhu et al., 2014; Liu et al., 2013a, 2012, 2010, 2009; Zhang and Meng, 2005; Zhang et al., 2004). In the Jianggalesayi area, the UHP eclogites showed an integrated clockwise P-T-t path with a protolith age of 752±7 Ma, peak eclogite-facies age of 500±7 Ma, and HP granulite-facies retrograded age of 455±2 Ma (Liu et al., 2012), which outlined an entire process from the continental deep subduction to the two exhumation stages of the southern Altyn Tagh HP/UHP belt (Liu et al., 2012). To date, besides the UHP eclogites, no retrograde metamorphic age was reported for the other HP/UHP rocks in the southern Altyn Tagh HP/UHP belt, which inhibited a proper understanding of the metamorphic evolution and exhumation rate of these HP/UHP rocks. In especial, although the UHP evidence had been documented (Liu et al., 2007a) in the Jianggalesayi area, the detailed metamorphic evolution of the UHP gneiss after the UHP metamorphism was indistinct. With this in view, we present a detailed investigation of the UHP pelitic gneiss (kyanite-bearing garnet pelitic gneiss) in the Jianggalesayi area, including petrography, mineralogy, and in situ U-Pb ages studies and aiming to constrain the multi-stage metamorphic ages, to evaluate the P-T-t evolution, and to document the metamorphic history related to exhumation.

1 GEOLOGICAL BACKGROUND AND SAMPLES

The Altyn Tagh Orogen marks the northern margin of the Qinghai-Tibet Plateau (Fig. 1a). It can be subdivided into four units from the north to the south (Fig. 1b): I. the northern Altyn Tagh terrain; II. the northern Altyn Tagh subduction-collision belt; III. the central Altyn (Milanhe-Jinyanshan) massif; IV. the southern Altyn Tagh subduction-collision belt (Liu et al., 2009). The southern Altyn Tagh subduction-collision belt is further subdivided into the southern Altyn Tagh HP/UHP metamorphic belt (Liu et al., 2018, 2013a, 2012, 2009; Wang et al., 2013, 2011), and the southern Altyn Tagh ophiolite tectonic melange belt (Liu et al., 2015, 1998; Kang et al., 2014; Yang et al., 2012; Ma et al., 2011, 2009; Li et al., 2009). The southern Altyn Tagh HP/UHP metamorphic belt is a typical subduction/collision complex belt formed at Early Paleozoic. The UHP metamorphic rocks include the UHP eclogite and the UHP pelitic gneiss from the Jianggalesayi area (Liu et al., 2018, 2007a; Gai et al., 2017), and the HP/UHP magnesite-bearing garnet peridotite, potassium feldspar-bearing garnet clinopyroxenite, and granitic gneiss from the Yinggelisayi area (Liu et al., 2005, 2004, 2002); the HP metamorphic rocks are composed of the pelitic granulite and the granitic granulite from the Danshuiquan area (Zhu et al., 2014; Cao et al., 2009), and the HP pelitic granulite from the Munabulake area (Cao et al., 2013). These HP/UHP rocks occur as lenses in the "Altyn Complex", and have the protolith ages of 750–1 000 Ma and peak metamorphic ages of 484–505 Ma (see the reviews by Liu et al., 2013a, 2010, 2009). The chronological data suggest an Early Paleozoic age for the subduction-collision complex in the southern Altyn Tagh region.

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Figure 1. Geological and tectonic map of the Altyn Tagh Orogen (a), (b) and profile map of the Jianggalesayi area (c) (modified after Liu et al., 2012, 2009). I. Northern Altyn Tagh terrain; II. northern Altyn Tagh subduction-collision belt; III. central Altyn (Milanhe-Jinyanshan) massif; IV. southern Altyn Tagh subduction-collision belt; IV1. southern Altyn Tagh HP/UHP metamorphic belt; IV2. southern Altyn Tagh ophiolite tectonic melange belt

The studied Jianggalesayi area located in the southern Altyn Tagh HP/UHP metamorphic belt, which is composed of a supracrustal metamorphic rock series, metamorphic intrusions, and a mafic volcanic series. The UHP metamorphic rocks of the eclogite and the pelitic gneiss occur as lenses parallel to the regional foliation. The country rocks mainly consist of granitic gneiss, pelitic gneiss and marble (Fig. 1c). Recently, Gai et al. (2017) found the coesite inclusions in omphacite from the UHP eclogite, which provided direct evidence for the UHP metamorphism; Liu et al. (2007a) identified oriented kyanite (Ky)+spinel (Spl) exsolutions in quartz in the UHP pelitic gneiss, which was inferred to be pre-existing Al-Fe-bearing stishovite (P > 12 GPa); Liu et al. (2018) recognized the paramorphs of the former stishovite in omphacite and garnet from the UHP eclogite with the P-T conditions of P > 8–9 GPa and T=800–1 000 C. These findings all suggested an ultra-deep subduction and exhumation of the southern Altyn continental rocks to and from the mantle depths in the stability field of the stishovite. Based on previously detailed petrological and geochronological data of the area, a clockwise P-T-t path was obtained for the UHP eclogite, which indicated that the Neoproterozoic protolith of the UHP eclogite (752±7 Ma) had been successively subjected to deep continental subduction and UHP metamorphism at ~500 Ma, underwent HP granulite-facies metamorphism at 455 Ma and later amphibolite-facies retrograde metamorphism. Furthermore, the exhumation rate for the eclogite was estimated to be ~1.2 mm/year (Liu et al., 2012).

The kyanite-bearing garnet pelitic gneiss in the present study occurs as residual inclusions in the granitic gneiss in the same profile of the UHP eclogite (Liu et al., 2012) (Figs. 1c and 2a). But, the gneiss is not directly in contact with the UHP eclogite. According to the detailed field mapping and litho-tectonic studies, the two rock types should belong to the same UHP rock suite.

2 PETROGRAPHY AND MINERALOGY

The kyanite-bearing garnet pelitic gneiss is grey-white in color and shows a porphyroblastic texture and gneissose structure (Fig. 2b). The coarse-grained prophyroblasts consist of garnet (Grt) (15%–25%), potassium feldspar (Kfs) (perthite and microcline, 10%–15%), banded quartz (Qz) (15%–25%) and kyanite (Ky) (10%–15%). The fine-grained matrix is mainly composed of quartz, plagioclase (Pl), microcline, sillimanite (Sill) and flaky biotite (Bt). The accessory minerals are zircon, apatite, ilmenite (Ilm) and rutile (Rt). Mineral abbreviations are after Whitney and Evans (2010).

The coarse-grained garnet, kyanite and potassium feldspar prophyroblasts occur as relic grains surrounded or separated by retrograde minerals (Sill, Bt, Qz and Pl) (Figs. 2c–2e), indicating that they were once coexisted together. Locally, kyanite has partially transformed to sillimanite (Fig. 2f). The Bt, Sill and Pl all occur in two textural forms: as retrograded productions distributed around the garnet, kyanite and K-feldspar prophyroblasts (Bt1, Pl1, Sill1) (Figs. 2c–2f); or as fine-grained minerals coexisted together in the matrix (Bt2, Pl2, Sill2) (Figs. 2g and 2h). In the matrix, some biotite and sillimanite occur as intergrowth to form a symplectic texture (Fig. 2h). These features indicate the existence of the reactions of Grt+2Ky+Qz=3Pl and Grt+Kfs+H2O=Bt+Sill+Qz among the garnet, kyanite and K-feldspar prophyroblasts.

Like the documented by Liu et al. (2007a) (Figs. 3a and 3b), in the cores of the coarse-grained quartz, abundant oriented Ky+spinel (Spl) rods or needles are also identified (Figs. 3c and 3d). The boundaries of these quartz domains against other minerals are relatively free of inclusions (Fig. 3d). Representative pairs of quartz grains also show parallel oriented rods and needles crossing a high angle grain boundary, implying an inherited exsolution microstructure from a pre-existing phase. Energy-dispersive (EDS) X-ray analyses for the kyanite, spinel and rutile are shown in Fig. 3e (parts of O peaks and parts or all of Si peaks are from the enclosing quartz).

3 ANALYTICAL METHODS

Mineral compositions for the kyanite-bearing garnet pelitic gneiss in this study were analyzed with an JXA-8230 microprobe at the State Key Laboratory of Continental Dynamics in Northwest University, Xiʼan. The instrument was operated at an accelerating voltage of 15 kV, 10 nA probe current and a 1-μm diameter beam. The data were calibrated by natural and synthetic standards from SPI Company.

Whole rock major element analyses were carried out using X-ray fluorescence spectrometry at the State Key Laboratory of Continental Dynamics, Northwest University, Xiʼan. The standards used were BCR-2 and GBW07105.

Zircon grains were separated from samples by heavy liquid and magnetic techniques. The selected zircon grains were fixed in epoxy resin and polished down to expose internal structures. Cathodoluminescence (CL) images were taken using Quanta 400FEG environmental scanning electron microscope equipped with an Oxford energy dispersive spectroscopy system and a Gatan CL3+ detector at the State Key Laboratory of Continental Dynamics, Northwest University, Xiʼan.

Zircon trace element and U-Pb isotope analyses were performed using a Agilient 7500a ICPMS coupled with a ComPex102 Excimer 193 nm ArF laser and MicroLas GeoLas 200M optics. The synthetic NIST610 was used to optimize the machineto obtain a maximum signal intensity (238U signal intensity > 460 cps/ug·g-1) and low oxide production (ThO/Th < 1%). The samples were analyzed by the ablation of a single spot with 32 μm width and 20–40 μm depth, and the data were acquired in peak-jumping-pulse-counting mode with one-point measured per peak. Ratios of 206Pb/238U, 207Pb/206Pb, 207Pb/235U and 208Pb/232Th were calculated using the ICPMSdataCal program (Liu, 2011), and then corrected using the Harvard zircon 91500 as external standard with a recommended 206Pb/238U age of 1 065.4±0.6 Ma (Wiedenbeck et al., 2004). NIST610 was used as the external calibration standard and 29Si as the internal standard. The concordia diagrams and weighted mean calculations were made using ISOPLOT 3.0 (Ludwig, 2003). The detailed analytical procedures are described in Chang et al. (2006) and Meng et al. (2018b).

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Figure 2. Photographs (a)–(b) and microphotographs (c)–(h) showing the occurrence and the microstructures of the kyanite-bearing garnet pelitic gneiss from the Jianggalesayi area. Mineral abbreviations are after Whitney and Evans (2010)
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Figure 3. Microphotographs showing spinel, kyanite and rutile rods and needles in quartz. (a), (b) Revised from Liu et al. (2007a); (c) spinel, kyanite and rutile rods and needles in quartz; (d) the rim of the quartz with fewer and smaller inclusions; (e) energy dispersive (EDS) X-ray analyses (parts of O peaks and parts or all of Si peaks are from the enclosing quartz)
4 MINERAL CHEMISTRY 4.1 Garnet

The garnet prophyroblast of the examined samples was dominated by 66.25 mol%–71.60 mol% almadine (Alm), 19.32 mol%–25.44 mol% pyrope (Prp), 3.14 mol%–3.63 mol% spessartine (Sps) and 4.43 mol%–5.83 mol% grossular (Grs) (Tables 1 and S1). The garnet shows a weak compositional zoning profile, without a pronounced variation in the core, but with a slight rim-ward increase in XAlm (Fe2+/(Fe2++Mn+Mg+Ca)), and a decrease of XPrp (Mg/(Fe2++Mn+Mg+Ca)) from the core to the rim (Fig. 4, Tables 1 and S1). The flat compositional zoning in the core might probably be caused by the re-equilibration during the high temperature metamorphism. The increase in Fe2+ at the garnet rim is considered as a result of an ion exchange with surrounding ferromagnessium minerals (such as biotite) during the retrograded metamorphism, which indicates a retrograde growth with the decreased temperature and pressure (e.g., Xia and Zheng, 2011; Carswell et al., 2000; Enami, 1998; Spear et al., 1984).

Table 1 Respectively mineral composition (wt.%) of the Ky-bearing Grt pelitic gneiss from the Jianggalesayi area
4.2 Biotite

The biotite around the garnet prophyroblast (Bt1) has a TiO2 content of 2.86 wt.%–2.95 wt.%, a FeO content of 13.10 wt.%–13.17 wt.%, a MgO content of 14.89 wt.%–14.42 wt.% and an XMg [Mg/(Mg+Fe2+)] of 0.66–0.67; while, the biotite in the matrix (Bt2) has slightly higher TiO2 content (5.14 wt.%–5.26 wt.%) and MgO content (12.28 wt.%–12.50 wt.%) and higher FeO content (14.40 wt.%–13.72 wt.%) than the biotite around the garnet prophyroblast. The XMg ranges from 0.60 to 0.62 (Table 1). The characteristic of the low FeO and high MgO content of the biotite surrounding the garnet prophyroblasts suggests that intense Fe2+-Mg2+ exchange existed between the two minerals (Zhou et al., 2003; Spear, 1991).

4.3 Plagioclase

Plagioclase in the kyanite-bearing garnet pelitic gneiss occurring as retrograded mineral surrounding the garnet (Fig. 2d) or coexisted with biotite and sillimanite in the matrix (Figs. 2f, 2g) have different compositions (Table 1). Plagioclase grains (Pl1) around garnet exhibit a composition of Ab38.97–40.22An59.23–60.64 Or0.40–0.55. Plagioclase grains (Pl2) in the matrix have higher CaO content (14.29 wt.%–14.54 wt.%) and lower Na2O content (2.64 wt.%–2.71 wt.%) than Pl1 with a composition of Ab24.63–25.44 An74.04–74.98Or0.39–0.51.

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Figure 4. Compositional profile of the garnet from the kyanite-bearing garnet pelitic gneiss
5 METAMORPHIC STAGES AND PSEUDOSECTION MODELING 5.1 Metamorphic Stages

Based on microstructures, reaction textures and mineral chemistry between mineral phases of the kyanite-bearing garnet pelitic gneiss, at least four metamorphic stages are recognized.

5.1.1 Peak UHP metamorphic stage (M1)

The peak UHP metamorphic stage was characterized by abundant oriented Ky+Spl rods or needles in the cores of the quartz grains. According to the microstructures, EBSD analyses and high temperature and high pressure experimental data (Liu et al., 2007a; Irifune et al., 1994; Irifune and Ringwood, 1993), the quartz grains were inferred to be the retrograded products from the pre-existing Al-Fe-bearing stishovite, and a minimum pressure was estimated to be of ∼12 GPa (Fig. 5a) (Liu et al., 2007a).

5.1.2 HP granulite-facies metamorphic stage (M2)

The mineral assemblage (M2) in this stage was represented by Grt (garnet core)+Ky+Kfs+Rt+Qz±Ilm. The garnet, kyanite and K-feldspar occurred as relic prophyroblasts (Fig. 2c), and always surrounded or separated by the retrograded minerals of Bt1, Pl1 and Sill1 (Figs. 2c–2e). The garnet cores were characterized by the high XPrp and low XAlm. In this stage, pre-existed (Al+Fe)- bearing stishovite had been transformed into quartz with abundant oriented Ky+Spl exsolutions in the cores. The mineral assemblage of Grt+Ky+Kfs+Rt+Qz±Ilm represented a typical HP granulite facies mineral assemblage (O'Brien and R tzler, 2003).

5.1.3 Granulite-facies stage (M3)

The mineral assemblage of Grt (garnet rim)+Kfs+Sill1+ Bt1+Pl1+Ilm+Qz was the representative features of stage M3. In this stage, the HP granulite facies minerals of Grt+Ky+Kfs became unstable, and consumed by the degenerative reactions of Ky=Sill1, Grt (garnet core)+2Ky+Qz=3Pl1 and Grt (garnet core)+ Kfs+H2O=Bt1+Sill1+Qz, which accounted for the formation of the Sill1, Bt1 and Pl1. These degenerated minerals occurred surrounding or separating the preexisted prophyroblasts (Figs. 2c–2f). The compositions of the garnet rims exhibited higher XAlm and lower XPrp than that of the garnet cores (Fig. 4).

5.1.4 Amphibolite-facies stage (M4)

The last stage was accompanied by the further regression of the minerals in M3 stage by the degenerative reactions of Grt (garnet rim)+2Sill1+Qz=3Pl2 and Grt (garnet rim)+Kfs+H2O= Bt2+Sill2+Qz. The garnet were totally consumed and disappeared in this stage. The newly formed Pl2, Bt2 and Sill2 occurred as fine-grained intergrowth in the matrix (Figs. 2g and 2h). Therefore, the resulting assemblage of Pl2+Bt2+Sill2+Ilm+Qz was the representative assemblage of this retrograded stage.

5.2 P-T Pseudosections

In this paper, the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2- H2O-TiO2-Fe2O3 (NCKFMASHTO) system was applied to the P-T pseudosection modeling in order to constrain the P-T conditions of the M2 to M4 stages of the studied gneiss. Calculations were performed using THERMOCALC 3.40 (Powell and Holland, 1988) with the internally consistent thermodynamic data set, ds62, of Holland and Powell (2011) and the re-parameterized a-x models for the NCKFMASHTO system (Powell et al., 2014; White et al., 2014). The contents, in wt.%, of the bulk-rock chemical compositions were Na2O=0.90, CaO=0.76, K2O=3.27, TFeO=7.79, MgO=2.45, Al2O3=17.41, SiO2=64.05, MnO=0.06, TiO2=0.90, which were then normalized into mol%, with H2O=3.47, Na2O=0.92, CaO=0.83, K2O=2.26, FeO=7.07, MgO=3.95, Al2O3=11.11, SiO2=69.45, TiO2=0.72, O=0.30. The content in mol% was used for the pseudosection modeling using the NCKFMASHTO system, where the O value for the bulk composition is equal to the moles of Fe2O3. The H2O content was adjusted using T-MH2O diagram to ensure that the final phase assemblage was stable just above the solidus (Korhonen et al., 2013, 2012). Assuming that ilmenite and quartz were in excess for subsolidus conditions, and H2O, ilmenite and quartz were in excess under the solidus. Minerals used in the equilibrium calculations were indicated with their abbreviations: garnet (g), K-feldspar (ksp), kyanite (ky), biotite (bi), plagioclase (pl), sillimanite (sill), corderite (cd), quartz (q), muscovite (mu), rutitle (ru), silicate melt (liq), ilmenite (ilm) and orthopyroxene (opx).

Figure 5b shows the specific P-T pseudosection constructed based on chemical compositions of the kyanite-bearing garnet pelitic gneiss, with the pressure and temperature ranging from 2 to 14 kbar and 600 to 950 C, respectively. The peak UHP stage (M1) was represented by abundant oriented Ky+Spl exsolutions in the cores of the quartz grains after the Al-Fe-bearing stishovite (Figs. 3a–3d), and a minimum pressure was ~12 GPa (Fig. 5a) (Liu et al., 2007a). The HP granulite-facies (M2) stage was represented by the mineral assemblage of Grt (garnet core)+Ky+ Kfs+Rt+Qz±Ilm, with P-T conditions of > 1.12 GPa and ~850– 930 C constrained by the chemical compositions of garnet cores in the pseudosection field of "g-ky-ksp-ru-ilm-q-liq" (Fig. 5b, M2). The granulite-facies (M3) stage was featured by the mineral assemblage of Grt (garnet rim)+Kfs+Sill1+Bt1+Pl1+Ilm+Qz. It fits the pseudosection field of "g-ksp-sill-bi-pl-ilm-q-liq" and its P-T conditions were constrained by X(g) of garnet rims and Ti in Bt1 at ~5–8 kbar and ~770–795 C (Fig. 5b, M3). The amphibolite-facies (M4) stage was characterized by the mineral assemblage of Pl2+ Bt2+Sill2+Ilm+Qz, of which the P-T conditions were limited at 4–6 kbar and < 675 C in the P-T pseudosection (Fig. 5b, M4). By connecting the pseudosection fields that match the three retrograded metamorphic stages, a clockwise P-T path for the kyanite- bearing garnet pelitic gneiss from the Jianggalesayi area has been constructed, with cooling-decompression following the peak UHP metamorphism. This implies that the pelitic gneiss have undergone deep subduction/collision processes with subsequent exhumation and cooling.

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Figure 5. (a) P-T path for the UHP pelitic gneiss by Liu et al. (2007a); (b)P-T pseudosection using the NCKFMASHTO system to calculate the equilibrium among minerals from the kyanite-bearing garnet pelitic gneiss
6 ZIRCON U-PB DATING RESULTS 6.1 CL Images

Zircon grains from the kyanite-bearing garnet pelitic gneiss are rounded and 50–100 μm long. In CL images, they generally display a obviously core-rim structure, and are divided into three types. Type I is characterized by a dark-grey luminescent core surrounded by a grey-white luminescent rim (Fig. 6Ⅰ). All cores and rims with uniform internal structure without zoning, showing the typical metamorphic zircon features (Corfu et al., 2003). Type II grains have an analogous magmatic cores with broad oscillatory magmatic zoning, surrounded by medium-luminescent metamorphic rims (Fig. 6Ⅱ). The magmatic cores are interpreted to be inherited zircon from the protolith of the gneiss. Type III grains preserved three textural domains, showing a tiny inherited core surrounded by a dark-grey luminescent metamorphic mantle and a grey-white luminescent rim (Fig. 6Ⅲ). The dark-grey metamorphic mantle and grey-white rim are respectively correspond to the metamorphic core and metamorphic rim from type I zircon grains.

6.2 Trace Element Compisitions

Based on the CL images, thirty LA-ICP-MS trace-element analyses were done on zircons from the kyanite-bearing garnet pelitic gneiss. Six locations from the inherited cores of types II and III zircons have high REE (667 ppm–1 918 ppm) and HREE (645 ppm–1 845 ppm) concentrations, and low GdN/YbN ratios (0.01–0.05) (Table 2). Chondrite-normalized REE patterns showed a steep slope HREE pattern with pronounced positive Ce anomalies and moderate negative Eu anomalies (Eu/Eu*= 0.42–0.69) (Fig. 7a). The Th/U ratios are high and range from 0.14–0.73. Six analysis spots from the metamorphic cores of types I and III zircons show a flat HREE pattern and negative Eu anomalies (Fig. 7a). They have lower REE (84.1 ppm to 203 ppm) and HREE (77.1 ppm to 198 ppm) content levels than the zircon cores, with GdN/YbN between 0.27 and 1.41, and low Th/U ratios of 0.06–0.09, as detailed in Table 2 and Fig. 7b. Sixteen analysis spots from the metamorphic rims of all zircons also show flat HREE patterns and negative Eu anomalies in the chondrite- normalized REE pattern diagram (Fig. 7a), with GdN/YbN between 0.16 and 5.62 (Table 2), and low Th/U ratios of mostly less than 0.1 (0.04–0.11). The zircon trace element features imply that metamorphic cores and rims are typical metamorphic overgrowth, and have been formed together with the coexisting garnet (Whitehouse and Platt, 2003; Rubatto, 2002; Rubatto and Gebauer, 2000; Schaltegger et al., 1999).

Table 2 LA-ICP-MS trace element (×10-6 ug/g) compositions of the zircons from the Ky-bearing Grt pelitic gneiss from the Jianggalesayi area
6.3 Zircon U-Pb Ages

The results of the LA-ICP-MS zircon U-Pb dating of the kyanite-bearing garnet pelitic gneiss are presented in Table 3, and plotted on U-Pb concordia diagram (Fig. 7c). The dating results for the different zircon domains defined two age groups at the bottom of the concordia diagram, and some scattered ages plotted in grey (Fig. 7c). Six analyses of the inherited cores yielded 206Pb/238U ages ranging from 645 to 1 086 Ma (Table 3); six analyses of metamorphic cores yielded coherent ages of 480 to 487 Ma (Table 3), with a weighted mean of 484±3 Ma (Fig. 7c); sixteen analyses of metamorphic rims exhibited apparent 206Pb/238U ages from 444 to 458 Ma (Table 3), with a weighted mean of 450±2 Ma (Fig. 7c).

Table 3 LA-ICP MS U-Pb data of the zircons from the Ky-bearing Grt pelitic gneiss from the Jianggalesayi area
7 DISCUSSION 7.1 Age Interpretations

In this study, three discrete age groups were obtained for the different zircon domains from the kyanite-bearing garnet pelitic gneiss using zircon U-Pb analyses: 645–1 086, 484±3 and 450±2 Ma, respectively (Fig. 7c). The inherited zircon cores exhibited the characteristics of typical detrital zircons with steep HREE patterns and high Th/U ratios (Figs. 7a and 7b). These zircons yielded a 206Pb/238U age range from 645 to 1 086 Ma. However, these ages were all distributed along the discordia (Fig. 7c), which might be the mixing age of the different zircon zones and have not any geological meanings.

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Figure 6. Cathodoluminescence images of zircon crystals from the kyanite-bearing garnet pelitic gneiss
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Figure 7. (a) Chondrite-normalized REE pattern of zircons; (b) Th-U diagram for the different zircon zones; (c) zircon U-Pb concordia diagram for the kyanite- bearing garnet pelitic gneiss. Normalized after Sun and McDonough (1989)

The first age group of 484±3 Ma plotted as blue circles in Fig. 7c, which was obtained from the metamorphic cores. These domains exhibit planar structure on the CL images (Fig. 6), low Th/U ratios (< 0.1) (Fig. 7b) and flat HREE patterns (Fig. 7a), which are typical of the metamorphic zircons co-existed with the Grt-bearing assemblage (Xiang et al., 2018; Rubatto, 2002). This age is consistent with the documented peak metamorphic ages (484–505 Ma) of the HP/UHP rocks reported previously in the southern Altyn Tagh HP/UHP belt (Table 4) (Cao et al., 2019, 2013, 2009; Liu et al., 2013a, 2012, 2010, 2009; Wang et al., 2013, 2011; Zhang and Meng, 2005; Zhang et al., 2004). Therefore, although no UHP mineral inclusions were found in the zircons, according to the characteristics of the metamorphic zircon cores, the age of 484±3 Ma should be represented or close to the peak metamorphic age (M1) of the gneiss.

Table 4 Chronological data of the HP/UHP rocks from the southern Altyn Tagh HP/UHP belt

The second age group at 450±2 Ma was found in metamorphic rims of the zircons, which also exhibited the features of metamorphic zircons with low Th/U ratios (mainly less than 0.1) and flat HREE patterns (Figs. 6, 7a, 7b). The characteristics of the zircon rims also indicated the equilibrium symbiosis with the garnet. This age was identical to the granulite-facies metamorphic age (455±2 Ma) of the UHP eclogite from the Jianggalesayi area (Table 4) (Liu et al., 2012). Due to the lack of the mineral inclusions in zircons, the 450 Ma was supposed to be the metamorphic age of M2 to M3 stages based on the Grt-bearing mineral assemblage (Xiang et al., 2018; Rubatto, 2002).

The aforementioned geological results suggested that the gneiss had experienced similar thermal event as the peak UHP, HP granulite-facies and two stages of retrograded metamorphism during the exhumations. Combined with the corresponding multistage metamorphic assemblages identified in the kyanite-bearing garnet pelitic gneiss, the age dating results implied that the zircons from the gneiss have integrated recording peak metamorphic (M1, 484±3 Ma) and retrograded metamorphic ages (M2 or M3, 450±2 Ma) (Fig. 8a). These findings suggested that the rock masses had successively experienced from the UHP to HP metamorphisms during the continental subduction/exhumation processes together with the UHP eclogite (Liu et al., 2012) outcropped in the same section.

7.2 Metamorphic Evolution and Exhumation Rate

The mineral assemblage of the four metamorphic stages and their metamorphic P-T conditions, combined with zircon U-Pb dating data can be used to reconstruct the P-T-t path of the kyanite-bearing garnet pelitic gneiss (Fig. 5). The peak metamorphic stage M1, represented by the oriented Ky+Spl exsolutions in quartz grains after pre-existing Al-Fe-bearing stishovite, and a minimum pressure was estimated to be ~12 GPa (Liu et al., 2007a). The metamorphic cores recorded the peak metamorphic age of 484 Ma. The HP granulite-facies stage M2, which was characterized by the mineral assemblage Grt+Ky+Kfs+Rt+ Qz±Ilm, recorded the P-T condition of > 1.12 GPa and ~850–930 C. The granulite-facies stage M3, was represented by the mineral assemblage of Grt (garnet rim)+ Kfs+Sill1+Bt1+Pl1+Ilm+Qz, and confined under P-T conditions of 0.5–0.8 GPa and ~770–795 C. The metamorphic zircon rims may had formed during the M2 or M3 stages, and records ages of ~450 Ma the retrograde metamorphism. The late cooling stage M4 was accompanied by the appearance of fine- grained Pl2, Sill2 and Bt2 in the matrix and occurred under P-T conditions of 0.4–0.6 GPa and < 675 C.

The above results suggested that the gneiss was subducted to mantle depth (> 350 km) and subjected to UHP metamorphism at ~484 Ma; then, successively exhumed to ~40 or to ~20 km depth to subjected to high-pressure to medium-pressure granulite-facies metamorphism at ~450 Ma; finally, continuely exhumed to < 20 km and subjected to amphibolite-facies metamorphism at some time later than 450 Ma. Therefore, the studied UHP gneiss exhumed about a minimum of 310–330 km from the deep mantle depth (> 350 km) to lower crust depths (~40–20 km) within ~34 Ma. The exhumation rate of 9.11–9.70 mm/yr was calculated for the UHP pelitic gneiss.

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Figure 8. P-T-t paths of (a) the kyanite-bearing garnet pelitic gneiss and (b) the documentated HP/UHP rocks from the southern Altyn Tagh HP/UHP belt

In the same section, Liu et al. (2012) documented a clockwise P-T-t path of the UHP eclogite according to the multistage U-Pb ages and the estimated P-T conditions related to the subduction and exhumation. The Neoproterozoic protolith of the eclogite was hypothesized to have been subducted to at least ~100 km depth where subjected to UHP metamorphism at ~500 Ma. Subsequently, the eclogite was exhumed to ~45 km depth and experienced HP granulite-facies metamorphism at ~455 Ma. Finally (at < 455 Ma), the eclogite was continuely exhumed to mid-crustal levels (~25 km) and subjected to amphibolite-facies metamorphism. Therefore, the eclogite exhumed about a minimum of 55 km from upper mantle depth of ~100 km to lower crust depths of ~45 km within ~45 Ma, which corresponded to a exhumation rate of 1.2 mm/yr. Recently, however, Liu et al. (2018) identified quartz paramorphs after stishovite in the omphacites and garnets from the UHP eclogite, and estimated the minimum peak pressure of the eclogite to be > 8–9 GPa at 800 to 1 000 C based on the stability field of stishovite, rather than ≥2.8 GPa reported in previous studies (Liu et al., 2012). This new evidence suggested an ultra-deep subduction and exhumation of the eclogite to/from mantle depths (~300 km) in the stability field of stishovite (Liu et al., 2018). Assuming the peak metamorphic age was invariable, the eclogite should be exhumed from ~300 to ~45 km depth within ~45 Ma (Fig. 8b blue line). The exhumation rate was correspondingly changed to be ~5.67 mm/yr, which was appreciably slower than that of the studied UHP gneiss in this paper.

The exhumation rates of the UHP pelitic gneiss and UHP eclogite from the Jianggalesayi area were identical to those in the larger UHP terranes, such as 5.0–11.3 mm/yr for the Dabie-Sulu UHP terrane (Liu and Liou, 2010) and ~7 mm/yr for the Western Gneiss Region of Norway eclogites (Kylander-Clark et al., 2008), but slower than those in other smaller UHP terranes (see the summary by Liou et al., 2009). This suggests that the large and small UHP terranes probably process a fundamental different subduction and exhumation mechanisms (Liou et al., 2009).

7.3 Implications of the Coexisted Contemporary UHP and HP Rocks

Previous researches had reported that the UHP rocks the southern Altyn Tagh HP/UHP belt were exposed in the Jianggalesayi area (Liu et al., 2018, 2012, 2007a; Gai et al., 2017) and the Yinggelisayi area (Dong et al., 2018; Wang et al., 2011; Liu et al., 2005, 2004, 2002; Zhang and Meng, 2005), whereas, the HP rocks (various granulite types) was discovered in the Danshuiquan area (Zhu et al., 2014; Cao et al., 2009) and the Munabulake area (Cao et al., 2013). LA-ICP-MS and SHRIMP zircon U-Pb dating yielded the peak metamorphic ages of 484–500 Ma for the UHP rocks (this study; Wang et al., 2013, 2011; Liu et al., 2012, 2010, 2009; Zhang and Meng, 2005; Zhang et al., 2004), and of 486–505 Ma for the HP rocks (Cao et al., 2019, 2013, 2009; Zhu et al., 2014) as listed in Table 4. Figure 8 listed the P-T(-t) paths of all the documented HP/UHP rocks from the southern Altyn Tagh HP/UHP belt. Although all the HP/UHP rocks showed similar clockwise P-T paths, the occurrence and preservation, the P-T conditions of the peak and retrograded metamorphism were respectively different. For example, the UHP pelitic gneiss and the UHP eclogite in the Jianggalesayi area were subducted to the mantle depth (stishovite field) and subjected to UHP metamorphism (> 8–12 GPa) at ~484 Ma (Liu et al., 2018, 2007a), and subsequently subjected granulite- facies metamorphism at ~450 Ma and amphibolite-facies metamorphism at < 450 Ma (Liu et al., 2012); felsic granulites in the Yinggelisayi (Bashenwake) area was interpreted to record an early prograde evolution to the peak UHP eclogite facies stage (Liu et al., 2004), with P-T conditions of 3–7 GPa and 700–1 100 ℃, and later reflect decompression in HP-UHT granulite facies conditions (2.5–1.4 GPa and 1 000–1 090 C) (Dong et al., 2018); HP pelitic and granitic granulites from the Danshuiquan area and HP pelitic granulite from the Munabulake area were respectively experienced HP granulite-facies metamorphism (T > 850 C, P > 1.1 GPa) at ~500 Ma, and later retrograded metamorphism with rapid exhumation and fast cooling (Zhu et al., 2014; Cao et al., 2013, 2009). Therefore, contemporary UHP and HP rocks with different metamorphic evolutions were recognized coexisting in the same orogenic belt, which were also identified in some typical collision orogens, such as the North Qinling belt (Gong et al., 2016; Liao et al., 2016; Liu et al., 2016, 2013b, 2003, 1996, 1995; Chen et al., 2015, 2004; Chen and Liu, 2011; Zhang J X et al., 2011, 2009a; Yang et al., 2005, 2003; Hu et al., 1995, 1994), the North Qaidam terrane (Cao et al., 2017; Yu et al., 2014, 2013, 2012, 2011a, b, 2009; Chen et al., 2012, 2009; Zhang C et al., 2011; Zhang G B et al., 2009a, b, 2008; Zhang J X et al., 2009a, b; Song et al., 2009, 2006, 2005, 2004, 2003a, b), and the Songduo area of the Gangdese belt (Li Y et al., 2019; Meng et al., 2018c; Li P et al., 2017).

Zhang J X et al. (2009a) summarized the field relationships, metamorphic conditions and chronological data from the South Altyn, the North Qaidam and the North Qinling terranes, and established a tectonic model for the formation of high-pressure granulite and associated eclogite in continental collision orogens. In combination with similar situations in other collision orogens in the world, Zhang J X et al. (2009a) proposed that the HP granulite and the eclogite were formed in different thermal environments at the same time, i.e., eclogites in continental subduction slabs, whereas HP granulite at the base of the overriding continental crust thickened as a result of continental subduction. However, chronological data showed the protoliths of all the documented HP rocks from the southern Altyn Tagh region were formed at the Neoproterozoic Period (Table 4), which suggested that these HP rocks should belong to the continental subduction slabs rather than the thickened continental crust of the upper plate.

Subduction channel, a dynamic model primarily proposed to explain the tectonic processes of oceanic subduction zones (Shreve and Cloos, 1986; England and Holland, 1979; Hsü, 1971). Materials in the subduction channel underwent deformation, metamorphism and even partial melting during the transporting crustal materials down to mantle depths and back to the surface. Similarly, the continental lithosphere also can be subducted to depths of 100–200 km (even > 350 km) and returned to crustal levels (Carswell and Compagnoni, 2003; Chopin, 2003; Ernst and Liou, 1999; Coleman and Wang, 1995). In this regard, Zheng (2012) proposed that the subduction channel model can be extended to continental subduction zones. In continental subduction channels, the subducting continental crust was detached at different depths in continental collision zones, and suffered different grade metamorphism, and then returned to the surface along the subduction channel (Zheng, 2012). As a consequence, different grades of metamorphic rocks occurred in the same collisional orogens.

Therefore, the contemporary UHP and HP rocks coexisted in the southern Altyn Tagh HP/UHP belt can be interpreted by the crustal detachment of subducting continental lithosphere at different depths in subduction channels. During the process of the continental subduction at ~500 Ma, the low-grade metamorphic rocks at greenschist to lower amphibolite facies were detached at shallow depths (Fig. 9a), the HP metamorphic rocks at granulite to eclogite facies were subducted to lower crustal depths, and the UHP eclogite-facies rocks were detached at the mantle depths of > 80–120 km, even of ~300 km (Fig. 9a). The direction of increasing metamorphic grade was also the direction of continental subduction (Fig. 9a). Subsequently, the continental subduction slab began to exhume accompanied due to some kind of exhumation mechanism (Ernst et al., 2007, 1997; Davies and von Blanckernburg, 1995). The UHP rocks (e.g., the UHP pelitic gneiss and the UHP eclogite from the Jianggalesayi area) retreated to the lower crust depth (~45 km) along the subduction channels (Fig. 9b), where overprinted granulite-facies metamorphism during exhumation out of the UHP regime at ~450 Ma. The petrological features and phase equilibria calculation recorded that the HP rocks have been exhumed to the upper crust where suffered the amphibolite-facies metamorphism after HP granulite-facies metamorphism (at 450–484 Ma) (Figs. 2 and 5). However, most HP rocks (HP granulite) from the Danshuiquan or Munabulake areas didn't record the retrograded metamorphic ages, which might be due to the direct exhumation to shallow depths, where the zircons from the HP rocks were not suffered recrystallization because of the low temperature. Finally, at < 450 Ma, all the HP and UHP rocks returned to the earth surface along the subduction channel, and coexisted together in the South Altyn orogeny (Fig. 9c).

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Figure 9. Schematic model for the continental subduction channel of the southern Altyn Tagh region. LPM. Low pressure metamorphism; HPM. high pressure metamorphism; UHPM. ultra-high pressure metamorphism

Summarily, the protoliths of the UHP rocks and HP rocks had been together subducted to different depths along the direction of the subduction channels at 484–505 Ma, and then returned to the surface with different exhumation P-T paths, which accounted for the coexisted contemporary UHP and HP rocks in the southern Altyn Tagh HP/UHP belt.

8 CONCLUSIONS

(1) Three retrograded metamorphism after UHP metamorphism were identified for the kyanite-bearing garnet pelitic gneiss. The HP granulite-facies stage M2 was characterized by the mineral assemblage Grt+Ky+Kfs+Rt+Qz±Ilm, recording the P-T condition of > 1.12 GPa and ~850–930 C. The granulite- facies stage M3 was represented by the mineral assemblage of Grt (garnet rim)+Kfs+Sill1+Bt1+Pl1+Ilm+Qz, and confined under P-T conditions of 0.5–0.8 GPa and ~770–795 C. The late cooling stage M4 was accompanied by the appearance of fine-grained Pl2, Sill2 and Bt2 in the matrix, and the P-T conditions was 0.4–0.6 GPa and < 675 ℃.

(2) Combined with the corresponding multistage metamorphic assemblages identified in the kyanite-bearing garnet pelitic gneiss, the age dating results implied that the zircons from the gneiss have integrated the recording peak metamorphic (M1, 484±3 Ma) and retrograded metamorphic ages (M2 or M3, 450±2 Ma). There was about 32 Ma interval during the first exhumation from the upper mantle depth (> 350 km) to the lower crust depth (~40–20 km), resulting in an average exhumation rate of 9.11–9.70 mm/yr.

(3) In the southern Altyn Tagh region, contemporary UHP and HP rocks with different metamorphic evolutions were recognized coexisting in the same orogenic belt, which can be interpreted by the model of subduction channel. During the process of the continental subduction at ~500 Ma, the low-grade metamorphic rocks at greenschist to lower amphibolite facies were detached at shallow depths, the HP metamorphic rocks at granulite to eclogite facies were subducted to the lower crustal depths, and the UHP eclogite-facies rocks were detached at mantle depths of > 80–120 km, even of ~300 km, along the direction of the subduction channels, and then returned to the surface.

ACKNOWLEDGMENTS

This study was supported by the National Natural Science Foundation of China (No. 41872053), the NSF of Shandong Province (No. ZR2019BD046), the Chinese Ministry of Science and Technology (No. 2015CB856103), the Opening Foundation of the State Key Laboratory of Continental Dynamics, Northwest University (No. 17LCD07), and SDUST Research Fund (No. 2015TDJH101). We are grateful to Drs. Xiaoming Liu, Jianqi Wang, Huadong Gong, and Chunrong Diwu for their help with chemical and isotopic analyses at Northwest University, China. Thanks are due to the editors and the two anonymous reviewers for their constructive comments that greatly helped to improve the manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-0896-7.

Electronic Supplementary Material: Supplementary material (Table S1) is available in the online version of this article at https://doi.org/10.1007/s12583-019-0896-7.


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