Metamorphism and geochronology of garnet amphibolite from the Beishan Orogen, southern Central Asian Orogenic Belt: Constraints from P-T path and zircon U-Pb dating

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Metamorphism and geochronology of garnet amphibolite from the Beishan Orogen, southern Central Asian Orogenic Belt: Constraints from P-T path and zircon U-Pb dating Wenbin Kang a, Wei Li a, *, Lei Kang a, b, Yunpeng Dong a, Dazhi Jiang a, c, Jiwei Liang d, Haoqiang Dong b a

State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an, 710069, China MLR Key Laboratory of Genesis and Exploration of Magmatic Ore Deposits, Xi’an Center of Geological Survey, China Geological Survey, Xi’an, China c Department of Earth Sciences, Western University, London, Ontario, N6A 5B7, Canada d School of Earth Science & Resources, Chang’an University, Xi’an, 710054, China b

A R T I C L E I N F O

A B S T R A C T

Handling Editor: Richard M. Palin

Numerous lenses of garnet amphibolite occur in the garnet-bearing biotite-plagioclase gneiss belt in the Baishan area of the Beishan Orogen, which connects the Tianshan Orogen to the west and the Mongolia-Xing’anling Orogen to the east. The study of metamorphism in Beishan area is of great significance to explain the tectonic evolution of Beishan orogen. According to the microstructures, mineral relationships, and geothermobarometry, we identified four stages of mineral assemblages from the garnet amphibolite sample: (1) a pre-peak stage, which is recorded by the cores of garnet together with core-inclusions of plagioclase (Pl1); (2) a peak stage, which is recorded by the mantles of garnet together with mantle-inclusions of plagioclase (Pl2) þ amphibole (Amp1) þ Ilmenite (Ilm1) þ biotite (Bt1), developed at temperature-pressure (P-T) conditions of 818.9–836.5  C and 7.3–9.2 kbar; (3) a retrograde stage, which is recorded by garnet rims þ plagioclase (Pl3) þ amphibole (Amp2) þ orthopyroxene (Opx1) þ biotite (Bt2) þ Ilmenite (Ilm2), developed at P-T conditions of 796.1–836.9  C and 5.6–7.5 kbar; (4) a symplectitic stage, which is recorded by plagioclase (Pl4) þ orthopyroxene (Opx2) þ amphibole (Amp3) þ biotite (Bt3) symplectites, developed at P-T conditions of 732  59.6  C and 6.1  0.6 kbar. Moreover, the U-Pb dating of the Beishan garnet amphibolite indicates an age of 301.9  4.7 Ma for the protolith and 281.4  8.5 Ma for the peak metamorphic age. Therefore, the mineral assemblage, P-T conditions, and zircon U-Pb ages of the Beishan garnet amphibolite define a near-isothermal decompression of a clockwise P-T-t (Pressure-Temperature-time) path, indicating the presence of over thickened continental crust in the Huaniushan arc until the Early Permian, then the southern Beishan area underwent a process of thinning of the continental crust.

Keywords: Beishan orogen Garnet amphibolite P-T-t path Zircon U-Pb dating

1. Introduction The Central Asian Orogenic Belt (CAOB) extends from the Urals in the west to the Circum-Pacific Orogenic Belt in the east and from the Siberia Craton in the north to the Tarim-North China craton in the south. The CAOB is one of the largest and most complex accretionary orogenic belts in the world (Fig. 1a) (S¸eng€ or et al., 1993; Seng€ or and Natal’In, 1996; Jahn, 2000; Jahn et al., 2004; Windley et al., 2007; Xiao et al., 2009a, 2009b; Mao et al., 2012). The orogenic belt was formed via multiple subduction-accretion and collision processes from the early

Neoproterozoic period (ca. 1000 Ma) to the end of the Permian period (ca. 250 Ma) (Coleman, 1989; S¸eng€ or et al., 1993; Shu et al., 2003; Windley et al., 2007; Xiao et al., 2008; Glorie et al., 2011). The Beishan Orogen lies in the south of the CAOB, in Northwest China (Zuo et al., 1990; Xiao et al., 2009b, 2010) and connects the Tianshan suture to the west and the Solonker suture to the east (Fig. 1b). Understanding the Beishan Orogen is of great significance for elucidating the tectonic evolution of the CAOB and the final closure of the Paleo-Asian Ocean. However, despite its importance, there is little consensus about the tectonic history of the Beishan Orogenic Belt and a long-standing

* Corresponding author. E-mail address: [email protected] (W. Li). Peer-review under responsibility of China University of Geosciences (Beijing). https://doi.org/10.1016/j.gsf.2019.10.007 Received 31 July 2018; Received in revised form 4 September 2019; Accepted 30 October 2019 Available online xxxx 1674-9871/© 2019 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: Kang, W. et al., Metamorphism and geochronology of garnet amphibolite from the Beishan Orogen, southern Central Asian Orogenic Belt: Constraints from P-T path and zircon U-Pb dating, Geoscience Frontiers, https://doi.org/10.1016/j.gsf.2019.10.007

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Fig. 1. (a) Simplified tectonic map of the Central Asian Orogenic Belt (CAOB) and adjacent areas. (b) Geological map of the Beishan Orogen Belt (modified after Yuan et al., 2015; Zong et al., 2017). Zircon U-Pb age distribution of the Beishan Complex in the southern Beishan Orogen (Jiang et al., 2013; He et al., 2015; Liu et al., 2015; Yuan et al., 2015).

et al., 2014; Cleven et al., 2015). The Mazongshan block, the Hanshan arc, and the Queershan arc are located in the northern Beishan Orogen and formed during the collision and accretion of the Late Paleozoic island arc (Xiao et al., 2010; Tian et al., 2014; Cleven et al., 2015). While the Shibanshan arc and the Huaniushan arc are located in the southern Beishan and formed by the subduction and collision of microcontinent plates in the extension zone of the East Tianshan (Fig. 1b) (Liu et al., 2011; Qu et al., 2011). The investigated area is located in southwestern Beishan and belongs to the Huaniushan arc (Fig. 1b). To the north of the Baishan fault, the Late Carboniferous–Early Permian intermediate–basic igneous distributed and a large number of diabase dikes and syengranite dikes develope in the intermediate–basic igneous (Fig. 2a). The Beishan Complex is widely distributed in the southern Baishan area (Fig. 2a) and mainly contains high-grade metamorphic rocks (Zheng et al., 2018), which forms a complex northeast-southwest fold (Fig. 2a). The garnet-amphibolite, which is the subject of this study, is located in the belt of the garnet-bearing biotite-plagioclase gneiss as interlayer with folds locally developed (Figs. 2b and 3a). Two representative samples of garnet-amphibolite (PM5-51a and PM5-51b) were selected from one outcrop for zircon U-Pb isotope and electron microprobe analysis (Fig. 2b).

controversy focuses on whether the subduction-related orogeny of the Beishan Orogenic Belt terminated during the Devonian (Zuo, 1990; Zuo et al., 1990, 2003; Xu et al., 2001; Kheraskova et al., 2003) or the Carboniferous–Permian (Windley et al., 2007; Xiao et al., 2008; Ao et al., 2010; Mao et al., 2012). High-grade metamorphic rocks are widely distributed in the southern region of the Beishan Orogen (Zuo, 1990; Wei et al., 2000). However, these high-grade metamorphic rocks have been largely neglected in previous research, and their presence is poorly understood (Yuan et al., 2015). According to previous studies, Proterozoic and Neoproterozoic medium-grade metamorphic basement complexes are collectively referred to as “Beishan Complex” (Liu et al., 2011; Jiang et al., 2013; He et al., 2015; Yuan et al., 2015) (Fig. 1b). Liu et al. (2011) reported that eclogitic-facies metamorphism occurred in the subducted oceanic crust at ~465 Ma and retrograde metamorphism occurred at ~430 Ma in the Liuyuan area (Liu et al., 2011; Qu et al., 2011). Jiang et al. (2013) reported a metamorphic age of ~295 Ma for the amphibolite and gneiss in the Shibanshan arc. These works have provided important evidence for the tectonic evolution of the Beishan orogenic belt. However, detailed information on the metamorphic P-T conditions, ages, and geochemical signature of the high-grade metamorphic rocks are still missing in many areas in the Beishan Orogen, which is the key to constrain the probable nature of deep crust and provides insight into the important processes of tectonic evolution. This paper studies the metamorphism and geochronology of the garnet amphibolite distributed in the Baishan area. The new zircon age and P-T-t path of the garnet amphibolite provide critical geological constraints on the metamorphism and tectonic evolution of the Beishan Orogen (Fig. 1b).

3. Analytical techniques 3.1. Mineral chemistry The mineral chemical compositions of garnet-amphibolite were analyzed using a JEOL JXA-8100 wavelength-dispersive electron microprobe at the MLR Key Laboratory of Genesis and Exploration of Magmatic Ore Deposits at Xi’an Center of Geological Survey of China Geological Survey. The operating conditions were 15 kV accelerating voltage, 10 nA beam current and counting time of 10 s for each peak. The beam diameter was 1 μm for all minerals.

2. Geological background The Beishan Orogen is composed of Neoproterozoic–Phanerozoic accretionary orogenic collages (Xiao et al., 2003) and it can be divided into five tectonic units or terranes from south to north: the Shibanshan arc, the Huaniushan arc, the Mazongshan block, the Hanshan arc and the Queershan arc (Xiao et al., 2010; Liu et al., 2011; Qu et al., 2011; Tian

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Fig. 2. (a) Simplified geological map of the study area in the southern Beishan. (b) A simplified cross-section across the garnet amphibolites lenses of the study area.

3.2. Zircon U-Pb dating and analysis of trace elements

4. Petrology and sample description

Zircon grains were separated by heavy-liquid and magnetic techniques followed by hand picking under a binocular microscope. Zircon grains were mounted in epoxy and then polished. Before U-Pb dating, cathodoluminescence (CL) images of the zircons were taken at the MLR Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits of Xi’an Institute of Geology and Mineral Resources, using an Agilent 7500a ICP-MS. The ICP-MS was equipped with a unique shield torch, which provides higher sensitivity. The GeoLas 200 M laser ablation system was used for ICP-MS analysis, with a spot diameter of 24 μm. ICPMS operating conditions were optimized using continuous ablation of reference glass NIST SRM 610, to provide maximum sensitivity for high masses while maintaining low oxide formation and low background. U, Th, and Pb concentrations were calibrated by using 29Si as internal standard and NIST SRM 610 as an external standard. 207Pb/206Pb and 206 Pb/238U ratios were calculated using the GLITTER 4.0 program and then corrected using Harvard zircon 91500 as an external standard. U-Pb ages were calculated using the ISOPLOT program (Ludwig, 2003).

Garnet amphibolites outcrop as meter-sized elongated lenses. The long axis of the lenses are oblique to the gneissosity of the garnet-bearing biotite-plagioclase gneiss, and the lenses display a sharp contact with the gneisses, suggesting that the protolith of garnet amphibolites could represent a dike or a sill. The garnet amphibolite is melanocratic, fine-to medium-grained, schistose structure with strips locally developed, and inequigranular blastic texture (Fig. 3b). The rocks are mainly composed of plagioclase (Pl), amphibole (Amp), garnet (Grt), orthopyroxene (Opx), biotite (Bt), and ilmenite (Ilm). The mineral abbreviations used in this paper follow those of Whitney and Evans (2010). Plagioclase occurs in the following different crystal habits: (1) finegrained (0.1–0.2 mm) euhedral to subhedral inclusions in the core (Pl1) and mantle (Pl2) of garnet (Fig. 3c, d); (2) fine-grained (0.1–0.3 mm) euhedral to subhedral crystals, where it shows textural equilibrium with primary minerals (Fig. 3b) (Pl3), however, in most cases primary plagioclase is separated from garnets by Opx2-Pl4-Amp3 symplectites; (3) in Opx2-Pl4-Amp3 symplectites around garnet (Fig. 3c–f). Amphibole is classified into the following three types: (1) fine-

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Fig. 3. Representative rock outcrop, photomicrographs and back-scatter electron images of the garnet amphibolite. G-garnet; Amp-amphibole; Opx-orthopyroxene; Plplagioclase; Bt-biotite; Ilm-Ilmenite.

(Fig. 3d). A small amount of ilmenite (<5%) occurs in the matrix or as inclusions (Fig. 3d). According to detailed petrographic observations of mineral assemblages and reaction textures, we identified four main stages of metamorphism: prograde growth, peak metamorphism, retrogression, and symplectite development. (1) The prograde stage is represented by plagioclase, which is preserved as mineral inclusions within the garnet core. (2) The peak stage is represented by amphibole þ plagioclase þ biotite þ ilmenite preserved as mineral inclusions within the garnet mantle. The enclosed minerals within garnet show evidence for the adjustment of their boundary towards low-energy configuration (Fig. 3c–e), suggesting that these minerals, as well as the garnet mantle, are part of an equilibrium assemblage (Zuo et al., 2017). Coarse or irregular mineral grains randomly growing along cracks in garnet were excluded since these are likely later minerals that formed by retrogression or fluid alteration (Wang et al., 2016). (3) The retrograde stage is marked by the matrix mineral of garnet þ orthopyroxene þ amphibole þ plagioclase þ biotite þ ilmenite. These minerals show equilibrium textures (Fig. 3b) but garnets are more often separated by symplectites (Fig. 3c–f). (4) The symplektitic stage is

grained (0.1–0.2 mm) euhedral to subhedral inclusions in the mantle of garnet (Amp1) (Fig. 3c, d); (2) fine- or medium-grained (0.3–1 mm) euhedral or subhedral rhombic crystals (Amp2). It is in textural equilibrium with primary plagioclase, biotite, orthopyroxenes and ilmenite and may show sharp contacts with some garnets (Fig. 3b), however, in most cases primary amphibole is separated from garnets by Opx2-Pl4-Amp3 symplectites (Fig. 3b–f); (3) in symplectites with orthopyroxene and secondary plagioclase (Fig. 3c–f). Garnets occur as porphyroblasts and are medium-grained (0.5–1.5 mm), cracked euhedral hexagonal (Fig. 3b). However, in most case, garnets are corroded and rimmed by a symplectitic Opx2-Pl4-Amp3 (Fig. 3b–f). It is sometimes poikilitic, containing mineral inclusions of amphibole, plagioclase, biotite, and ilmenite (Fig. 3c, d). A small number of orthopyroxenes occur in the matrix (Fig. 3e, f). Others are mainly worm-like crystals in symplectites with secondary plagioclases and amphiboles between garnet and primary minerals (Fig. 3d–f). Biotite occurs as fine-grains (0.1–0.2 mm) in textural equilibrium with both primary amphibole and plagioclase (Fig. 3c, d). It can also form symplectitic rods with Opx2-Pl4-Amp3 intergrowths or as inclusions 4

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Table 1 Representative compositions of amphibole from garnet amphibolite samples. Sample

PM5-51b

Min.

Amp-edge

Amp-edge

Amp-edge

Amp-edge

Amp-in

Amp-in

Amp-in

Amp

Amp

Amp

Amp

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Totals Oxygens Si Ti Al Cr Fe3þ Fe2þ Mn Mg Ca Na K Sum AlⅣ AlⅥ XMg

49.65 0.24 4.47 0.00 29.28 0.97 10.13 1.58 0.52 0.03 96.88 23.00 7.59 0.03 0.81 0.00 0.06 3.68 0.13 2.31 0.26 0.16 0.01 15.05 0.40 0.41 0.38

52.30 0.19 3.24 0.00 25.64 0.45 13.79 1.17 0.38 0.04 97.20 23.00 7.78 0.02 0.57 0.00 0.00 3.19 0.06 3.06 0.19 0.11 0.01 14.97 0.22 0.35 0.49

52.66 0.00 2.02 0.00 25.77 0.64 14.62 0.74 0.12 0.04 96.60 23.00 7.88 0.00 0.36 0.00 0.00 3.23 0.08 3.26 0.12 0.03 0.01 14.96 0.12 0.24 0.50

54.14 0.05 0.81 0.00 25.08 0.34 15.32 0.77 0.09 0.01 96.62 23.00 8.04 0.01 0.14 0.00 0.00 3.12 0.04 3.39 0.12 0.03 0.00 14.89 0.00 0.19 0.52

42.10 0.83 13.83 0.02 18.30 0.08 8.68 10.81 1.56 0.56 96.76 23.00 6.33 0.09 2.45 0.00 0.43 1.87 0.01 1.94 1.74 0.45 0.11 15.57 1.61 0.86 0.46

42.58 0.79 13.70 0.04 17.71 0.16 9.13 10.81 1.50 0.56 96.98 23.00 6.36 0.09 2.41 0.01 0.44 1.77 0.02 2.03 1.73 0.43 0.11 15.55 1.58 0.86 0.48

43.16 0.83 13.08 0.01 16.57 0.14 10.16 10.43 1.19 0.63 96.21 23.00 6.43 0.09 2.30 0.00 0.53 1.54 0.02 2.26 1.67 0.34 0.12 15.47 1.49 0.83 0.52

43.18 0.63 13.05 0.05 17.86 0.15 9.43 9.79 1.50 0.64 96.30 23.00 6.46 0.07 2.30 0.01 0.57 1.66 0.02 2.10 1.57 0.44 0.12 15.51 1.46 0.87 0.48

44.50 1.02 10.94 0.10 17.83 0.25 9.86 10.03 1.06 0.60 96.20 23.00 6.68 0.12 1.94 0.01 0.40 1.84 0.03 2.21 1.61 0.31 0.12 15.38 1.26 0.69 0.50

45.99 0.00 10.93 0.00 18.51 0.02 10.22 9.11 1.33 0.35 96.47 23.00 6.83 0.00 1.92 0.00 0.44 1.86 0.00 2.26 1.45 0.38 0.07 15.36 1.10 0.83 0.50

43.88 0.98 12.23 0.00 17.54 0.24 9.80 10.14 1.73 0.37 96.91 23.00 6.52 0.11 2.14 0.00 0.51 1.67 0.03 2.17 1.61 0.50 0.07 15.50 1.41 0.76 0.50

Note: Letters used in texture are: in–inclusion of garnet, edge–near by the edge of garnet as symplektite. The mineral molecular formula and ion number are all calculated by AX program (Holland; http://www.esc.cam.ac.uk/astaff/holland/ax.html). Total Fe as FeO.

Fig. 4. (a) Classification of amphiboles in C(Al þ Fe3þ þ 2Ti) apfu vs A(Na þ K þ 2Ca) apfu diagram of Calcium amphiboles and Mg–Fe2þ-Mn end-member compositions diagram of monoclinic magnesium-iron-manganese amphiboles of Hawthorne et al. (2012); (b) chemical compositions of Pl plotted on albite–anorthite–orthose ternary diagrams; (c) Grt chemical compositions plotted on the grossular–pyrope–(almandine þ spessartine) ternary diagram of Coleman et al. (1965); (d) chemical compositions of Opx plotted on enstatite–ferrosilite–wollastonite ternary diagram of Morimoto (1988). Abbreviations: Amp, amphibole; Or, orthoclase; Ab, albite; An, anorthite; Alm, almandine; Sps, spessartine; Grs, grossular; Prp, pyrope; Wo, wollastonite; En, enstatite; Fs, ferrosilite. 5

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Table 2 Representative compositions of plagioclase from garnet amphibolite samples. Sample

PM5-51b

Min

pl-in-c

pl-in-m

pl-in-m

pl-in-m

pl

pl

pl

pl-edge

pl-edge

pl-edge

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Totals Oxygens Si Ti Al Cr Fe3þ Fe2þ Mn Mg Ca Na K Sum An Ab Or

55.19 0.00 29.06 0.04 0.00 0.00 0.00 10.42 5.51 0.03 100.25 8.00 2.48 0.00 1.54 0.00 0.00 0.00 0.00 0.00 0.50 0.48 0.00 5.00 51.01 48.81 0.18

49.72 0.00 32.17 0.01 0.32 0.05 0.00 15.02 2.94 0.02 100.30 8.00 2.26 0.00 1.73 0.00 0.01 0.00 0.00 0.00 0.73 0.26 0.00 5.00 73.75 26.11 0.13

47.44 0.00 33.77 0.00 0.52 0.08 0.01 16.37 1.93 0.04 100.22 8.00 2.17 0.00 1.82 0.00 0.02 0.00 0.00 0.00 0.80 0.17 0.00 4.99 82.24 17.54 0.23

48.10 0.00 32.96 0.01 0.24 0.00 0.00 15.75 2.40 0.01 99.50 8.00 2.21 0.00 1.79 0.00 0.01 0.00 0.00 0.00 0.78 0.21 0.00 5.00 78.38 21.58 0.04

46.05 0.00 33.36 0.02 0.26 0.00 0.02 16.67 1.84 0.02 98.27 8.00 2.15 0.00 1.84 0.00 0.01 0.00 0.00 0.00 0.84 0.17 0.00 5.01 83.24 16.66 0.10

46.98 0.00 33.07 0.01 0.10 0.01 0.01 16.09 1.97 0.03 98.28 8.00 2.19 0.00 1.82 0.00 0.00 0.00 0.00 0.00 0.80 0.18 0.00 4.99 81.68 18.12 0.21

46.92 0.09 33.12 0.00 0.29 0.00 0.01 16.25 2.04 0.04 98.78 8.00 2.18 0.00 1.81 0.00 0.01 0.00 0.00 0.00 0.81 0.18 0.00 5.00 81.32 18.47 0.21

44.92 0.14 35.35 0.00 0.35 0.04 0.02 18.73 0.71 0.00 100.32 8.00 2.07 0.01 1.92 0.00 0.01 0.00 0.00 0.00 0.92 0.06 0.00 4.99 93.60 6.38 0.01

44.75 0.00 35.89 0.00 0.46 0.01 0.01 18.64 0.61 0.00 100.41 8.00 2.06 0.00 1.94 0.00 0.02 0.00 0.00 0.00 0.92 0.05 0.00 4.99 94.4 5.60 0.00

44.59 0.00 35.72 0.00 0.57 0.00 0.00 18.94 0.59 0.00 100.48 8.00 2.05 0.00 1.94 0.00 0.02 0.00 0.00 0.00 0.93 0.05 0.00 5.00 94.65 5.35 0.00

Note: Letters used in texture are: in-c: inclusion at core of garnet, in-m: inclusion at mantle of garnet, edge: near by the edge of garnet as symplektite. The mineral molecular formula and ion number are all calculated by AX program (Holland; http://www.esc.cam.ac.uk/astaff/holland/ax.html). Total Fe as FeO.

represented by the formation of secondary minerals as a result of primary mineral breakdown. Garnet grains break down at the contact of quartz, which results in Opx2-Pl4 symplectites around garnet as in the following reaction:

Thost et al. (1991), Derridj et al. (2003) and Ayad et al. (2016) described similar reactions. Biotite may participate in reactions as follows (Ayad et al., 2016): Grt þ Amp2 þ Pl3 þ Qz þ Bt2 → Opx2 þ Pl4 þ Amp3 þ Bt3

Grt þ Qz → Opx2 þ Pl4 The reaction was reported to be an important reaction that defines the transition from high to lower pressures in mafic granulite rocks (O’brien and R€ otzler, 2003; Bendaoud et al., 2004; Koizumi et al., 2014; Ayad et al., 2016). Although quartz is absent in the garnet amphibolite, the texture suggests that the quartz present at the pre-decompression stage was completely consumed by the reaction above (Koizumi et al., 2014). When primary amphibole is involved, the garnet is separated from amphibole and plagioclase by Opx2-Pl4-Amp3 symplectites resulting from the reaction:

5. Results 5.1. Mineral compositions The chemical analysis results of inclusion-type, matrix-type, and symplectite-type amphibole are listed in Table 1. According to the amphibole nomenclature presented by Hawthorne et al. (2012), the inclusion-type amphibole is pargasite and tschermakite (XMg ¼ 0.46–0.54, Si ¼ 6.30–6.50, (Na þ K)A ¼ 0.46–0.57), the primary

Grt þ Amp2 þ Qz  Pl3 → Opx2 þ Pl4 þ Amp3

Fig. 5. Representative compositional zoning profiles of almandine (Alm), grossular (Grs), pyrope (Prp) and spessartine (Sps) across the three types of garnet. Type (1) is core-mantle-rim profiles across a non-symplektitic garnet; type (2) is rim-mantle-core-mantle-rim profiles across symplektitic garnet. 6

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Table 3 Representative compositions of orthopyroxene, biotite, ilmenite from garnet amphibolite samples. Sample

PM5-51b

Min

opx-edge

opx-edge

opx-edge

opx

bi

bi

bi

bi

bi

ilm

ilm

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Totals Oxygens Si Ti Al Cr Fe3þ Fe2þ Mn Mg Ca Na K Sum XMg En Fs Wo AlⅣ AlⅥ

50.70 0.03 1.30 0.06 31.98 0.58 14.61 0.58 0.11 0.04 100.00 6.00 1.98 0.00 0.06 0.00 0.00 1.05 0.02 0.85 0.02 0.01 0.00 3.99 0.45 43.65 54.70 1.24

50.75 0.07 1.17 0.03 31.98 0.48 14.71 0.52 0.04 0.01 99.75 6.00 1.99 0.00 0.05 0.00 0.00 1.05 0.02 0.86 0.02 0.00 0.00 3.99 0.450 44.02 54.70 1.13

50.84 0.03 1.41 0.00 30.99 0.52 15.50 0.47 0.02 0.00 99.77 6.00 1.98 0.00 0.07 0.00 0.00 1.01 0.02 0.90 0.02 0.00 0.00 3.99 0.47 46.15 52.79 1.00

51.55 0.10 1.03 0.03 31.14 0.34 15.02 0.54 0.07 0.01 99.83 6.00 2.00 0.00 0.05 0.00 0.00 1.01 0.01 0.87 0.02 0.01 0.00 3.97 0.46 45.14 53.43 1.16

36.52 1.58 16.09 0.01 20.53 0.04 10.87 0.00 0.31 8.58 94.53 11.00 2.80 0.09 1.45 0.00 0.09 1.23 0.00 1.24 0.00 0.05 0.84 7.79 0.49

36.17 2.36 15.75 0.18 19.22 0.11 10.99 0.00 0.19 9.21 94.18 11.00 2.79 0.14 1.43 0.01 0.00 1.24 0.01 1.26 0.00 0.03 0.91 7.82 0.50

35.45 2.51 15.70 0.17 19.56 0.09 10.72 0.05 0.22 8.58 93.08 11.00 2.77 0.15 1.44 0.01 0.02 1.26 0.01 1.25 0.00 0.03 0.86 7.79 0.49

35.54 2.70 15.79 0.12 19.18 0.06 10.92 0.01 0.22 9.06 93.60 11.00 2.76 0.16 1.45 0.01 0.00 1.25 0.00 1.27 0.00 0.03 0.90 7.82 0.50

36.15 2.77 15.79 0.16 19.11 0.08 11.23 0.00 0.24 9.47 95.01 11.00 2.77 0.16 1.43 0.01 0.00 1.22 0.01 1.28 0.00 0.04 0.93 7.84 0.51

0.03 50.04 0.05 0.09 47.35 1.25 0.10 0.12 0.08 0.02 99.11 3.00 0.00 0.95 0.00 0.00 0.09 0.91 0.03 0.00 0.00 0.00 0.00 2.00

0.00 51.90 0.00 0.00 47.29 1.34 0.17 0.00 0.00 0.00 100.69 3.00 0.00 0.98 0.00 0.00 0.05 0.94 0.03 0.01 0.00 0.00 0.00 2.00

1.19 0.27

1.20 0.23

1.23 0.22

1.24 0.21

1.23 0.20

Note: Letters used in texture are: in–inclusion, edge–nearby the edge of garnet as symplektite. The mineral molecular formula and ion number are all calculated by AX program (Holland; http://www.esc.cam.ac.uk/astaff/holland/ax.html). Total Fe as FeO.

amphibole is magnesio-hornblende, pargasite and tschermakite (XMg ¼ 0.48–0.50, Si ¼ 6.40–6.80, (Na þ K)A ¼ 0.43–0.57), and the amphibole in symplectites with plagioclase is either cummingtonite or grunerite (XMg ¼ 0.38–0.53, Si ¼ 7.50–8.00, (Na þ K)A ¼ 0.02–0.12) (Fig. 4a). Plagioclase show four different generations (Table 2). The plagioclase in the form of inclusion in the garnet core has labradorite compositions (An ¼ 51.01), the plagioclase in the form of inclusion in the garnet mantle has bytownite compositions (An ¼ 73.75–83.27), the plagioclases in the matrix with bytownite compositions (An ¼ 81.32–83.24), and the plagioclases preserved as symplectite has anorthite compositions (An ¼ 89.24–92.88) (Fig. 4b). There are two types of garnet based on their microstructures and components (Fig. 3b–f, 4c, 5; Supplementary Table S1): (1) garnet surrounded by matrix-types amphibole and plagioclase (Figs. 3b and 5a) with a core-mantle-rim zonal structure; (2) garnet surrounded by orthopyroxene þ plagioclase þ amphibole symplectites (Fig. 3c, d, 5b) with a core-mantle-rim zonal structure. The chemical compositions of garnet vary within a specific range, and the garnet shows the distinct compositional differences between core, mantle, and rim. The cores of garnet are predominantly almandine (55.33–63.19 mol. %), with grossular (19.04–24.57 mol. %), pyrope (7.08–14.09 mol. %) and minor spessartine (2.84–11.36 mol. %) components. The mantles of garnet are predominantly almandine (59.18–67.25 mol. %), with grossular (12.21–18.46 mol. %), pyrope (13.94–18.40 mol. %) and minor spessartine (1.62–5.11 mol. %) components. The rims of garnet are predominantly almandine (62.77–65.02 mol. %), with grossular (10.55–14.96 mol. %), pyrope (16.89–20.36 mol. %) and minor spessartine (1.69–3.42 mol. %) components (Fig. 4c). Chemical zoning profiles of garnet show a noticeable decrease in grossular and spessartine, and an increase in pyrope and almandine from the core to the rim (Figs. 4c and 5). The two types of garnet in the same rock show similar compositional characteristics in the core, mantle and rim, respectively. However, these components of type (2) vary at the contact of Opx2-Pl4

Fig. 6. CL images of typical zircon grains of the garnet amphibolite. The circles denote analytical spots, and the numbers denote analytical spot number.

symplectites in relative enrichment in almandine and spessartine, and depletion in grossular and pyrope (Fig. 5b). The compositional variations resulted from prograde metamorphism, retrograde metamorphism, and partial resorption of garnet. Orthopyroxenes of the matrix and those in the symplectic intergrowths with Pl4 þ Amp3 have similar chemical compositions (Fig. 4d; Table 3). Orthopyroxene has a ferrosilite composition (En ¼ 43.65–48.22) with an XMg of 0.448–0.493. Ferrosilite belongs to the secondary paragenesis resulting from the breakdown of garnetplagioclase-amphibole-quartz and allowing the development of Opx2Pl4-Amp3 symplectites.

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Fig. 7. U-Pb Concordia diagrams and Chondrite-normalized REE patterns of zircons from garnet amphibolite. Chondrite-normalization values are reproduced from McDonough and Sun (1995).

metamorphic zircons. Several individual grains are prismatic in shape with lengths of 70–170 μm. In CL images, the zircon grains are darkly luminescent or darkly luminescent rims with relict cores as a result of solid-state recrystallization (Hoskin and Black, 2000). The cores are weakly zoned, which indicates that the zircons are inherited magmatic zircons (Fig. 6b). Dark luminescence of rims or individual grains has U and Th concentrations of 270–2158 ppm and 36–254 ppm, respectively, and Th/U ranging from 0.04 to 0.32. Fifteen data points form a population with a weighted mean 206Pb/238U age of 284.4  2.2 Ma (MSWD ¼ 0.76) (Fig. 7b) and five other data points show older ages of 302  4 Ma, 302  4 Ma, 311  4 Ma, 384  7 Ma and 441  6 Ma with slightly higher Th/U (0.1–0.32), reflecting the effect of partial recrystallization which leaves a “memory” of the previous isotopic chemistry (Hoskin and Black, 2000). The Y abundances of the zircons range from 245 to 1440 ppm and the

5.2. Metamorphic geochronology The zircon grains from the garnet amphibolite are translucent, colorless, or light yellow. This study obtained 81 analytical spots on the 66 zircon grains (Supplementary Table S2). The CL images show that a few zircons with lengths of 70–100 μm and ellipsoidal display bright luminescence and no internal structures or weak sector zonation (Fig. 6a). The zircons have abundances of U ¼ 80–154 ppm and Th ¼ 4–33 ppm, and Th/U ratios ranging from 0.05 to 0.23. The data points yield a weighted mean 206Pb/238U age of 281.4  8.5 Ma (MSWD ¼ 0.03) (Fig. 7b). Moreover, trace element characteristics of the zircons show low Y abundances of 30–115 ppm. The chondritenormalized REE patterns show relatively flat HREE patterns (TbN/YbN ¼ 0.12–0.31) and have a negative Eu anomaly (δEu ¼ 0.36–0.69) (Fig. 7c; Supplementary Table S3). Therefore these grains are typical

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Fig. 8. P-T pseudosection for garnet amphibolite in the system NCKFMASHTO based on effective bulk compositions. Mineral abbreviations are as follow. L-melt, augaugite, hb-hornblende, g-garnet, opx-orthopyroxene, bi-biotite, ilm-ilmenite, pl-plagioclase, ksp-K-feldspar, mu-muscovite, q-quartz, ru-rutile, and H2O-aqueous fluid.

zircons display enriched HREE patterns (TbN/YbN ¼ 0.03–0.20) with a positive Ce anomaly and a negative Eu anomaly (δEu ¼ 0.02–0.72) (Fig. 7d; Supplementary Table S3). The remaining zircons are short prismatic in shape with 50–150 μm. They exhibit oscillatory zoning, which is consistent with the characteristic of magmatic zircons (Hoskin and Black, 2000; Brown, 2007; Wu et al., 2007; Diwu et al., 2014) (Fig. 6c). Oscillatory-zoned grains and relict cores have variable contents of U (56–1651 ppm) and Th (23–620 ppm), with Th/U ratios ranging from 0.23 to 1.45. Fifteen data points yield a weighted mean 206Pb/238U age of 301.9  4.7 Ma (MSWD ¼ 1.50) (Fig. 7b). Two additional populations show younger ages of 276.6  4.8 Ma and 211–240 Ma, due to partial Pb loss during late metamorphism and hydrothermalism. Moreover, several oscillatory-zoned zircons yield relatively older Concordia U–Pb ages with a large 206 Pb/238U age ranging from 340  6 Ma to 1877  26 Ma. Trace element characteristics of the zircons show Y abundances of zircons of 396–3832 ppm. They display enriched HREE patterns (TbN/YbN ¼ 0.04–0.22) and have a positive Ce anomaly and a negative Eu anomaly (δEu ¼ 0.02–0.73) (Fig. 7e; Supplementary Table S3). Furthermore, few zircons display flat LREE patterns, indicating the influence of late hydrothermalism (Hoskin, 2005).

6. Discussion 6.1. Metamorphic P-T path Petrographic and paragenetic analyses highlight four main metamorphic evolution stages. We could not obtain the P-T condition of the prograde stage due to lack of a suitable method for calculation. Thermobarometric calculations were conducted for the other three stages, using conventional mineral-based thermobarometric calculations in particular amphibole-plagioclase (AP) (Holland and Blundy, 1994) and garnet-orthopyroxene (GO) (Sen and Bhattacharya, 1984) thermometers, and garnet-amphibole-plagioclase-quartz (GAPQ) (Dale et al., 2000), and garnet-orthopyroxene-plagioclase-quartz (GOPQ) (Perkins and Chipera, 1985) barometers. Also, the Al-content of the Orthopyroxene for thermobarometers (RCLC) (Pattison et al., 2003) have been used to check the results. There are two calibrations for AP thermometers (Holland and Blundy, 1994) (A edenite þ 4 quartz¼tremolite þ albite; B edenite þ albite¼richterite þ anorthite). Reaction B can be applied to silica-undersaturated rocks. The quartz present in the garnet amphibolite, therefore we take the average value of two results of the two calibrations as the metamorphism temperature. The data of P-T conditions were calculated from random analyses of equilibrium assemblage. The

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Table 4 Result of the geothermobarometric calculations for garnet amphibolite samples. Metamorphism

Methods

Peak stage

AP: A edenite þ 4 quartz ¼ tremolite þ albite; B edenite þ albite ¼ richterite þ anorthite GAPQ: A 3 tschermakite þ 4 grossular þ 2 pyrope þ 12 quartz ¼ 4 tremolite þ 12 anorthite; B 3 pargasite þ 2 grossular þ pyrope þ 18 quartz ¼ 3 tremolite þ6 anorthite þ 3 albite; C 3 glaucophane þ 4 grossularþ 2 pyrope þ 12 quartz ¼ 3 tremolite þ 6 anorthite þ 6 albite phase equilibria modelling AP: A edenite þ 4 quartz ¼ tremolite þ albite; B edenite þ albite ¼ richterite þ anorthite GAPQ: A 3 tschermakite þ 4 grossular þ 2 pyrope þ 12 quartz ¼ 4 tremolite þ 12 anorthite; B 3 pargasite þ 2 grossular þ pyrope þ 18 quartz ¼ 3 tremolite þ6 anorthite þ 3 albite; C 3 glaucophane þ 4 grossularþ 2 pyrope þ 12 quartz ¼ 3 tremolite þ 6 anorthite þ 6 albite phase equilibria modelling GO: 3 enstatite þ 2 almandite ¼ 3 ferrosilite þ 2 pyrope

Retrograde stage

Fig. 9. P-T path of garnet amphibolite. The different metamorphic facies fields are reproduced from (Winter 2001). GAP denotes amphibole-plagioclase thermometers and garnet-amphibole-plagioclase-quartz barometers; GOP denotes garnet-orthopyroxene thermometers and garnet-orthopyroxene-plagioclase-quartz barometers; RCLC, which is short for ‘recalculation’, is a program that calculates P-T conditions based on Al-solubility in Opx in equilibrium with Grt.

Symplektitic stage

GOPQ: Agrossular þ 2 pyrope þ3 quartz ¼ 3 enstatite þ 3 anorthite; B grossular þ 2 almandite þ3 quartz ¼ 3 ferrosiliteþ 3 anorthite RCLC (Pattison et al., 2003)

single analysis of each mineral is used to calculate, and the results are computed as mean plus or minus two standard deviations. In addition to the conventional geothermobarametry, phase equilibria modelling were conducted using new a-x relations for hightemperature metabasites (Green et al., 2016; Palin et al., 2016). Effective bulk compositions for garnet amphibolite were determined by combining phase proportions with their representative compositions (Carson et al., 1999; Forshaw et al., 2019). The compositions, in mol.% oxide, are H2O ¼ 4.72, SiO2 ¼ 56.14, Al2O3 ¼ 9.47, FeO ¼ 10.57, MgO ¼ 8.43, CaO ¼ 7.23, Na2O ¼ 0.94, K2O ¼ 1.33, TiO2 ¼ 0.65 and O ¼ 0.51. The calculations were completed in the 10-component NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2) system using THERMOCALC version 3.45i (Powell and Holand, 1988) and the internally consistent data set of Holland and Powell (2011) (update ds62, February 6, 2012). The following a-x relations were used: melt (L), augite (aug), and hornblende (hb) (Green et al., 2016); garnet (g), orthopyroxene (opx) and biotite (bi) (White et al., 2014a); ilmenite (ilm) (White et al., 2000); plagioclase (pl) and K-feldspar (ksp) (Holland and powell, 2011), muscovite (mu) (White et al., 2014b). Pure phases included quartz (q), rutile (ru), and aqueous fluid (H2O). The temperature and pressure range from 700 to 900  C and 2–12 kbar, respectively. Phase diagrams are shaded according to the variance of each field, with darker shades indicating higher variance. The peak stage, revealed from inclusion-type minerals in the garnet mantle, lie well within the field of L-g-hb-bi-pl-ilm-q with P-T conditions of 7.3–9.2 kbar and 818.9–836.5  C. The retrograde stage is characterized by the assemblage of matrix minerals which matches the field of L-g-opx-hb-bipl-ilm-q. The P-T conditions for the retrograde stage can be defined at 5.6–7.5 kbar and 796.1–836.9  C (Fig. 8). While using GAPQ thermobarometers, the P-T conditions for peak stage can be defined at 789  37.5  C/7.8  0.8 kbar and the P-T conditions for the retrograde stage can be defined at 765  23.8  C/7.0  1.1 kbar. The symplektitic stage is represented by the secondary minerals around garnet. We used the GOPQ thermobarometers and RCLC thermobarometers to obtain the P-T

Pressure (kbar)

Temperature ( C) 789  37.5

7.8  0.8

7.3–9.2

818.9–836.5 765  23.8

7.0  1.1

5.6–7.5

796.1–836.9 730  59.9

5.4  0.5

6.1  0.6

732  59.6

conditions of symplektitic stage, and the results are 730  59.9  C/5.4  0.5 kbar and 732  59.6  C/6.1  0.6 kbar, respectively (Fig. 9; Table 4). The most calculated P-T conditions obtained from different thermobarometers are compatible. The temperatures obtained from phase diagrams are higher than the conventional geothermobarometry. The phase equilibria modelling define equilibrium relations between mineral and fluid/melt species at different P-T conditions for a particular bulk-rock composition. The phase diagrams are more reliable as they are simpler and easier to interpret than conventional geothermobarometry, which show many reactions. The pressure of symplektitic stage obtained from GO-GOPQ (5.4  0.5 kbar) is lower than RCLC (6.1  0.6 kbar), which is thought to be due to the Fe-Mg exchange of minerals after the metamorphism. The RCLC is a geothermobarometry scheme based on Alsolubility in Opx in equilibrium with garnet, corrected for later Fe-Mg exchange. Therefore, the result of RCLC is more reliable. In summary, the P-T path provides an isothermal decompression of a clockwise P-T-t path from high (7.3–9.2 kbar) to low pressures (5.6–7.5 kbar) at relatively constant high temperatures (790–840  C) (Fig. 8). The P-T path indicates that the garnet amphibolite in the Beishan area experienced an isothermal decompression, which commenced with peak granulite-facies metamorphism.

6.2. Interpretation of zircon U-Pb ages The inherited zircon cores in some zircon domains and some zircons with oscillatory-zoned display relatively high Th/U and HREE-enriched patterns with pronounced negative Eu anomalies of magmatic origin. These zircons yielded an age of 301.9  4.7 Ma (Fig. 7b). We interpret the data to be the protolith age for garnet amphibolite for it is the youngest 10

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Fig. 10. Schematic cartoons demonstrating the tectonic setting of the southern Beishan Orogenic Belt (modified after Van der Pluijm and Marshak, 2004).

Carboniferous (301.9  4.7 Ma) protolith of the garnet amphibolite emplaced in the Beishan Complex and went through granulite facies metamorphism (281.4  8.5 Ma) under over thickened continental crust, with the subsequent exhumation and cooling processes along ITD path. The first discovered Early Permian basic granulite in the Beishan orogen provide a key clue to infer the regional tectonic settings. The subduction and closure of the ocean between the Dunhuang block and the Huaniushan arc have started at early Paleozoic, which is mainly revealed by the presence of early Paleozoic eclogites (ca. 465 Ma) and HP granulites (440–430 Ma) in the southern Beishan orogen and the Dunhuang block respectively (He et al., 2018). The basic granulite suggests the presence of over thickened continental crust in the Huaniushan arc until the Early Permian; then the southern Beishan area underwent a process of thinning of the continental crust. The tectonic thinning processes are consistent with 295–270 Ma partial anatexis and low-pressure/high-temperature metamorphism (ca. 760  C and 5 kbar) of the Neoproterozoic granitic orthogneisses (Jiang et al., 2013; He et al., 2018). Moreover the Permian intermediate–basic complexes, bimodal volcanic rocks, and post-collision granites distributed in Beishan (Jiang et al., 2006; Ao et al., 2010; Zhang et al., 2010, 2011; Feng et al., 2012) also indicate the tectonic thinning process (Fig. 10).

and largest population of magmatic zircon age before metamorphism. The metamorphic zircons yielded ages of 281.4  8.5 Ma. The trace element composition is characterized by relatively flat HREE patterns with negative Eu anomalies (Fig. 7c), indicating that they grew together with garnet and plagioclase (Schaltegger et al., 1999; Rubatto, 2002; Rubatto and Hermann, 2003; Whitehouse and Platt, 2003; Liu et al., 2011). These zircons texture and trace element characteristics are consistent with the occurrence of garnet in peak stage and retrograde stage, so the age of the zircon of 281.4  8.5 Ma may record time of peak metamorphic stage or retrograde stage. Also, the zircon formed by partial solid-state recrystallization yielded ages of 284.4  2.2 Ma. The zircon age is a maximum estimate of the age of peak P-T conditions (Hoskin and Black, 2000), which agree with the age of 281.4  8.5 Ma in measuring error ranges. Therefore, we interpret the 281.4  8.5 Ma is approximately the age of peak granulite facies metamorphism.

6.3. Tectonic significance Diagnostic mineral assemblages and metamorphic P-T-t paths may be used to infer the regional tectonic settings (e.g., O’Brien and R€ otzler, 2003; Ye et al., 2009; Ao et al., 2011; Wang et al., 2017). The pressure-temperature results of different stages show a near-isothermal decompression process of a clockwise P-T-t path for the Beishan garnet amphibolite (Fig. 9), and the P-T-t path suggests that the garnet amphibolite underwent pre-peak metamorphism, peak granulite-facies metamorphism, and two later stages of retrograde metamorphism. The isothermal decompression (ITD) paths are conventionally ascribed to the later stages of the thermal evolution of the over thickened continental crust, either with or without the additional effects of mantlederived magmas (Harley, 1989). Also, decompression paths may be generated in extensional settings within the footwall to a low-angle extensional detachment zone, if magmas accrete simultaneously onto the base of the extending crust (Wernicke, 1985; Harley, 1989). However, the latter setting would result in quite hot but shallow level (5–2 kbar) ITD paths (Harley, 1989), which are inconsistent with the pressure condition of the peak stage in this research. Therefore, the Late

7. Conclusion (1) Four periods of metamorphism and apparent mineral assemblages can be recognized in the garnet amphibolite. The pre-peak assemblage consists of garnet cores together with Pl1 in the cores. The peak assemblage consists of garnet mantles together with mineral inclusions in the mantles of Amp1 þ Pl2 þ Bt1 þ Ilm1. The retrograde stage assemblage consists of garnet rims together with minerals in the matrix of Amp2 þ Pl3 þ Opx1 þ Bt2 þ Ilm2. The symplektitic stage is represented by the growth of Pl4 þ Opx2 þ Amp3 þ Bt3 and the garnet rims where contact with symplektites. (2) Except for the pre-peak stage, the P-T conditions have been constrained by conventional geothermobarometer and phase

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equilibria modeling. The calculation results show a near isothermal decompression process of a clockwise P-T path. The PT path went from 818.9 to 836.5  C/7.3–9.2 kbar through 796.1–836.9  C/5.6–7.5 kbar to 732  59.6  C/6.1  0.6 kbar. (3) The U-Pb dating of the Beishan garnet amphibolite indicates an age of 301.9  4.7 Ma for oscillatory-zoned grains and inherited zircon cores provide protolith age of garnet amphibolite. The age of the peak stage is 281.4  8.5 Ma. (4) Mineral assemblages, P-T conditions, and zircon U-Pb data combined define a near-isothermal decompression process of a clockwise P-T-t path for the garnet amphibolite of Beishan, indicating that the presence of over thickened continental crust in the Huaniushan arc until the Early Permian, then the southern Beishan area underwent a process of thinning of the continental crust.

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Declaration of competing interest None. Acknowledgments We gratefully acknowledge the anonymous journal reviewers and would like to thank them for their constructive reviews and comments, which substantially improved this work. Financial support for this study was jointly provided by the Geological and Mineral Survey in NalatiYingmaotuo area of Tianshan-Beishan metallogenic belt (DD20160009) and the National Natural Science Foundation of China (Grant Nos.: 41572179, 41872218, 41421002 and 41372204). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.gsf.2019.10.007. References Ao, S.J., Xiao, W.J., Han, C.M., Li, X.H., Qu, J.F., Zhang, J.E., Guo, Q.Q., Tian, Z.H., 2011. Cambrian to early Silurian ophiolite and accretionary processes in the Beishan collage, NW China: implications for the architecture of the Southern Altaids. Geol. Mag. 149, 606–625. Ao, S.J., Xiao, W.J., Han, C.M., Mao, Q.G., Zhang, J.E., 2010. Geochronology and geochemistry of Early Permian mafic-ultramafic complexes in the Beishan area, Xinjiang, NW China: implications for late Paleozoic tectonic evolution of the southern Altaids. Gondwana Res. 18, 466–478. Ayad, B., Fettous, E.-H., Ouabadi, A., 2016. Isothermal decompression of garnet metabasites from Laouni terrane in the LATEA, Central Hoggar, Algeria. Arabian J. Geosci. 9, 238. Bendaoud, A., Derridj, A., Ouzegane, K., Kienast, J.R., 2004. Granulitic metamorphism in the Laouni terrane (central Hoggar, Tuareg shield, Algeria). J. Afr. Earth Sci. 39, 187–192. Brown, M., 2007. Crustal melting and melt extraction, ascent and emplacement in orogens: mechanisms and consequences. J. Geol. Soc. 164, 709–730. Carson, C.J., Powell, R., Clarke, G., 1999. Calculated mineral equilibria for eclogites in CaO–Na2O–FeO–MgO–Al2O3–SiO2–H2O: application to the Pou’ebo Terrane, Pam Peninsula, New Caledonia. J. Metamorph. Geol. 17, 9–24. Cleven, N.R., Lin, S., Xiao, W., 2015. The Hongliuhe fold-and-thrust belt: evidence of terminal collision and suture-reactivation after the Early Permian in the Beishan orogenic collage, Northwest China. Gondwana Res. 27, 796–810. Coleman, R.G., 1989. Continental growth of northwest China. Tectonics 8, 621–635. Coleman, R.G., Lee, D.E., Beatty, L.B., Brannock, W.W., 1965. Eclogites and eclogites: their differences and similarities. Geol. Soc. Am. Bull. 76, 483–508. Dale, J., Holland, T., Powell, R., 2000. Hornblende-garnet-plagioclase thermobarometry: a natural assemblage calibration of the thermodynamics of hornblende. Contrib. Mineral. Petrol. 140, 353–362. Derridj, A., Ouzegane, K., Kienast, J.-R., Belhaı€, D., 2003. P–T–X evolution in garnet pyroxenites from Tin Begane (central Hoggar, Algeria). J. Afr. Earth Sci. 37, 257–268. Diwu, C.R., Sun, Y., Zhao, Y., Liu, B.X., Lai, S.C., 2014. Geochronological, geochemical, and Nd-Hf isotopic studies of the Qinling complex, central China: implications for the evolutionary history of the north Qinling orogenic belt. Geosci. Front. 5, 499–513. Feng, J.C., Zhang, W., Wu, T.R., Zheng, R.G., Luo, H.L., He, Y.K., 2012. Geochronology and geochemistry of granite pluton in the north of Qiaowan, Beishan Mountain, Gansu province, China, and its geological significance. Acta Sci. Nauralium Univ. Pekin. 48, 61–70 (in Chinese with English abstract). 12

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