Diverse subduction and exhumation of tectono-metamorphic slices in the Kalatashitage area, western Paleozoic Dunhuang Orogenic Belt, northwestern China

Journal Pre-proof Diverse subduction and exhumation of tectono-metamorphic slices in the Kalatashitage area, western Paleozoic Dunhuang Orogenic Belt, northwestern China

Qian W.L. Zhang, Hao Y.C. Wang, Jia-Hui Liu, Meng-Yan Shi, Yi-Chao Chen, Zhen M.G. Li, Chun-Ming Wu PII:

S0024-4937(20)30071-2

DOI:

https://doi.org/10.1016/j.lithos.2020.105434

Reference:

LITHOS 105434

To appear in:

LITHOS

Received date:

13 August 2019

Revised date:

11 February 2020

Accepted date:

16 February 2020

Please cite this article as: Q.W.L. Zhang, H.Y.C. Wang, J.-H. Liu, et al., Diverse subduction and exhumation of tectono-metamorphic slices in the Kalatashitage area, western Paleozoic Dunhuang Orogenic Belt, northwestern China, LITHOS(2020), https://doi.org/10.1016/j.lithos.2020.105434

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© 2020 Published by Elsevier.

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Diverse subduction and exhumation of tectonometamorphic slices in the Kalatashitage area, western Paleozoic Dunhuang Orogenic Belt,

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northwestern China

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Qian W.L. Zhang a, Hao Y.C. Wang a,b, Jia-Hui Liu a, Meng-Yan Shi a,c, Yi-Chao Chen a, Zhen

College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, P.O.

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a

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M.G. Li a, and Chun-Ming Wu a,*

Box 4588, Beijing 100049, China State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics,

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Institute of Resources and Environment, Henan Polytechnic University, Jiaozuo 454000,

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Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China

*

China

Corresponding author. E-mail address: [email protected] (C.-M. Wu)

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ABSTRACT High-pressure mafic granulite and garnet amphibolite are identified as small-scale tectonic slices within pelitic or semi-pelitic gneiss in the Kalatashitage area, which is located in the western Paleozoic Dunhuang Orogenic Belt, northwestern China. These rocks retain three generations of metamorphic mineral assemblages: prograde assemblage (M1) preserved as inclusions within garnet porphyroblasts, metamorphic peak assemblage (M2) consisting of

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matrix minerals and garnet porphyroblasts, and retrograde assemblage (M3) mainly

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represented by the symplectic minerals surrounding the embayed garnet and the retrograded

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hornblende rimming matrix-type clinopyroxene. Metamorphic pressure and temperature (PT) paths of high-pressure mafic granulite, amphibolite, and metapelite retrieved by

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thermobarometry are all clockwise, passing from 640–720 ºC/6.2–12.6 kbar (M1) through

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840–920 ºC/14.6–16.2 kbar (M2) to 750–815 ºC/5.5–7.9 kbar (M3) for high-pressure mafic granulite, from ~650 ºC/5.7 kbar (M1) through ~750 ºC/9.2 kbar (M2) to ~780 ºC/8.1 kbar

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(M3) for amphibolite, and from ~615 ºC/7.9 kbar (M1) through 730–820 ºC/8.6–11.7 kbar

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(M2) to 675–740 ºC/5.4–8.7 kbar (M3) for pelitic and semi-pelitic gneiss. Furthermore,

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pseudosection modeling of high-pressure mafic granulite indicates that the growth zonation of garnet porphyroblast exhibits prograde metamorphism in a P-T range of 510–800 ºC/8.5– 13 kbar and demonstrates peak metamorphic P-T conditions of ~850 ºC/16 kbar, which are consistent with the thermobarometric estimates. The significant pressure differences in peak metamorphism observed in different rocks indicate that the rocks initially subducted to remarkably different depths and were subsequently juxtaposed at shallower crustal levels during exhumation. Sensitive high-resolution ion microprobe (SHRIMP) analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating of metamorphic zircon indicates that the metamorphic events occurred at ca. 430–420 Ma (M2) and ca. 400–390 Ma (M3), respectively. Metamorphism was followed by the intrusion of

Journal Pre-proof granitic dykes at ca. 244 Ma. Moreover, the metamorphic evolution indicates that the Kalatashitage area was involved in the subduction, collision and subsequent tectonic exhumation in the Paleozoic. Combined with previous literature, it is inferred that the discrepant subduction and exhumation of high-grade metamorphic rocks is a universal phenomenon in the Paleozoic Dunhuang Orogenic Belt, supporting the ubiquitous existence of subduction-collision complexes in this orogenic belt.

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Keywords : Dunhuang Orogenic Belt; thermobarometry and pseudosection modeling;

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zircon; subduction; collision; exhumation

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1. Introduction Orogens are the most prominent tectonic bodies distributed in the Earth’s surface. Similar to encrypted hard disks, orogens record abundant information regarding the Earth’s evolutionary history and deep geodynamic mechanism (e.g., Brown, 2001; Xiao et al., 2010), including metamorphic, magmatic, and sedimentary material records of oceanic crustal

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subduction, continental collision, and intracontinental extension stages (e.g., Şengör et al.,

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1993; Brown, 2001; Xiao et al., 2010, 2015). Compared with magmatic and sedimentary

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rocks, which mainly provide geochemical information, geodynamic setting, tectonic regime of the Earth’s interior, and material transfer at the Earth’s surface, metamorphic rocks always

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preserve mineral records of successive metamorphic stages; therefore, they are suitable for

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depicting the evolutionary processes of an orogenic belt (e.g., Brown, 2001, 2007) and for evaluating the thermal state of the deep crust and the upper mantle. Furthermore,

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metamorphic rocks can record more than one episode of tectono- metamorphic events. The

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peak metamorphic pressure-temperature (P-T) conditions combined with the determined

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metamorphic ages can reflect the maximum pressure or temperature experienced by metamorphic rocks or terranes, which could possibly represent the depth of subduction or the degree of crust thickening during corresponding geological periods (e.g., O’Brien and Rötzier, 2003; Brown, 2007). Even in a limited area of an orogenic belt, the metamorphic P-T conditions and ages of peak metamorphism can be considerably diverse (e.g., Wang et al., 2017a). Such phenomena are not uncommon for orogens formed during different geological episodes (Caby et al., 2003; Liu et al., 2009; Hajná et al., 2010; Zhang et al., 2014; Klemd et al., 2015; Wan et al., 2015; Li et al., 2016a, 2016b; Wang et al., 2017a). The juxtapositions of metamorphic rocks with distinct metamorphic grades and/or ages mainly have two structural formats: (1)

Journal Pre-proof tectono- metamorphic mélange showing discontinuous “block-in- matrix” fabrics (Festa et al., 2012; Wakita, 2012, 2014; Klemd et al., 2015; Wakabayashi, 2015; Wan et al., 2015; Li et al., 2016a, 2016b; Wang et al., 2017a, 2018b) and (2) tectonic slices occurring as macroscopically continuous imbricated thrust nappes (Arne et al., 1997; Caby et al., 2003; Malusà et al., 2005; Liu et al., 2009; Hajná et al., 2010; Zhang et al., 2014). These occurrences of metamorphic rocks possibly indicate the mixing of rocks derived from significantly different depths during exhumation (Klemd et al., 2015; Li et al., 2016a, 2016b;

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Wang et al., 2017a). Metamorphic and geochronological studies provide important clues that can be used for identifying diverse metamorphosed slices and thoroughly understanding the

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orogenic process. The Dunhuang Orogenic Belt (DOB), which is located in the southernmost

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Central Asian orogenic belt (Fig. 1a), is bound by the Beishan orogen to the north, Qiemo-

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Xingxingxia Fault to the northwest, and Altyn Tagh Fault to the south (Fig. 1b; Zhao et al., 2016). In the past, the Dunhuang area was regarded as the Archean-Paleoproterozoic

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metamorphic basement of the Tarim Craton (Mei et al., 1997, 1998; Long et al., 2014) or

tonalite-trondhjemite-granodiorite

(TTG)

gneiss,

and

metamorphic

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Paleoproterozoic

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North China Craton (Zhang et al., 2013), from which a series of Mesoarchean-Neoarchean,

supracrustal rocks were identified (Mei et al., 1997, 1998; Zhang et al., 2012, 2013; He et al., 2013; Wang et al., 2013a, 2013b, 2014; Zhao et al., 2013, 2015a, 2015c, 2019a, 2019b; Zong et al., 2013; Yu et al., 2014). However, recent studies indicate that the “Dunhuang block” was actually a Paleozoic orogenic belt (Zhao et al., 2016; Wang et al., 2017a) that underwent extensive Paleozoic metamorphism (Fig. 2; Meng et al., 2011; Zong et al., 2012; He et al., 2014; Peng et al., 2014; Wang et al., 2016a, 2017a, 2017b, 2018a, 2018b; Pham et al., 2018) and magmatism (Fig. 2; Zhang et al., 2009; Zhu et al., 2014, 2019; Zhao et al., 2015b, 2017; Wang et al., 2016b, 2016c, 2017c; Bao et al., 2017; Shi et al., 2017, 2018, 2019; Feng et al., 2018). The Archean-Paleoproterozoic rocks that are sporadically exposed in the Dunhuang

Journal Pre-proof block may represent exotic tectonic slices derived from circumjacent ancient cratons (the Tarim Craton or North China Craton) during the Paleozoic orogeny. Previous studies have mainly focused on the central and eastern parts of the DOB (e.g., Sanweishan, Dongbatu-Mogutai, Hongliuxia, Qingshigou, and Duobagou areas; Fig. 2) and workers have identified some important metamorphic rocks such as eclogite, high-pressure mafic granulite, medium-pressure mafic granulite, amphibolite and metapelite (Meng et al., 2011; Zong et al., 2012; He et al., 2014; Peng et al., 2014; Wang et al., 2016a, 2017a, 2017b,

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2018a, 2018b; Pham et al., 2018). These rocks all exhibited clockwise P-T paths, with some characterized by isothermal decompression (ITD). Such phenomenon is typically indicative

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of subduction- or collision-related metamorphism formed during 445–365 Ma (Meng et al.,

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2011; Zong et al., 2012; He et al., 2014; Peng et al., 2014; Wang et al., 2016a, 2017a, 2017b,

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2018a, 2018b; Pham et al., 2018). However, metamorphic or geochronologic data for the western DOB are not available. This lack of data hampers our understanding of the tectono-

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metamorphic evolution of the entire belt. Recent studies revealed that the Aketashitage region

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(located at the southwestern margin of the DOB; Fig. 1b) is a Paleoproterozoic orogenic belt

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and reveals the presence of extensive Paleoproterozo ic metamorphic and magmatic events, which are possibly related to the assembly of the Columbia supercontinent during the Paleoproterozoic global orogeny (Wu et al., 2019; Zhang et al., 2019). Therefore, it is unclear whether the Kalatashitage area belongs to the Paleozoic DOB or to the Paleoproterozoic metamorphic basement. In this paper,

we present an integrated study of the field

investigation,

micropetrography, thermobarometry, pseudosection modeling, as well as sensitive highresolution ion microprobe (SHRIMP) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating of metamorphic zircon from the Kalatashitage area, to thoroughly understand the tectono- metamorphic evolution of the western DOB and to

Journal Pre-proof provide important data for understanding the subduction, collision, and exhumation of metamorphic rocks.

2. Geological background The DOB is composed of several discrete northeast-southwest trending terranes (Fig. 2) formed by the Mesozoic-Cenozoic dislocation of circumambient strike-slip faults (i.e., the

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Qiemo-Xingxingxia Fault to the northwest, and Altyn Tagh Fault to the south; Fig. 1b). The

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DOB was formerly considered to be formed during the Precambrian, and the metamorphic

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rocks exposed in this region were labeled as Archean-Paleoproterozoic “Dunhuang Complex” or “Dunhuang Group,” consisting of a series of medium to high- grade metamorphic

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supracrustal rocks and subordinate gneissic granitoid (Mei et al., 1997; Lu et al., 2008; Zhao

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and Cawood, 2012; Zhang et al., 2013; Zong et al., 2013). Supracrustal rock series are mainly composed of pelitic schist and gneiss, felsic gneiss, mafic granulite, amphibolite, and marble

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(Lu et al., 2008). The granitoid gneiss in the DOB includes TTG gneiss, monzogranitic

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gneiss, and granitic gneiss (Lu et al., 2008; Zhao and Sun, 2018). A few younger

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undeformed, unmetamorphosed granitoid plutons intrude into the Dunhuang metamorphic complex (Zhao et al., 2016, 2017). Until recently, age dating has indicated that metamorphic and magmatic rocks of the Dunhuang region recorded orogenic and magmatic events of the Paleozoic (the ages are depicted in Fig. 2), which led to renaming this area as the Paleozoic DOB (Zhao et al., 2016; Wang et al., 2017a, 2017b, 2018a, 2018b; Pham et al., 2018). The Kalatashitage area is located in the western part of the DOB (Fig. 2). Exposed rocks in this area constitute a ~40 km long, ~5–10 km wide, east-west trending belt surrounded by the Cenozoic desert (Fig. 3). The Kalatashitage area is mainly composed of various types of medium to high- grade metamorphic rocks, including marble, felsic gneiss, semi-pelitic gneiss, pelitic gneiss, amphibolite, and mafic granulite. These rocks usually display a clear

Journal Pre-proof foliation, from which two stages of deformation are recognized (Figs. 4b and 4i). The first stage of deformation (D1 ) is represented by tight or rootless folds (S 1 ), along which a few tiny leucosomes are distributed (Figs. 4b). The pervasive NWW-SEE trending foliation (S2 ) constitutes the second stage of deformation (D2 ; Figs. 4b and 4i). The eastern part of the Kalatashitage area (Figs. 4a–4e) shows high-pressure granulite facies metamorphism evidenced by the occurrence of considerable quantities of clinopyro xene and the evident partial melting in metabasite and metapelite, whereas the western part mainly indicates

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amphibolite facies metamorphism (Figs. 4f–4i). The high-pressure mafic granulite and amphibolite in the Kalatashitage area mostly occur as interlayers or small-scale tectonic slices

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(Figs. 4a, 4f, and 4h) preserved within intensely deformed pelitic gneiss or felsic gneiss (Figs.

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4b and 4i). In some outcrops, parts of the high-pressure mafic granulite intensively

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retrograded to garnet amphibolite. These high- grade metamorphic rocks are subsequently intruded by granitic plutons (Fig. 3) or granitic dykes and sills (Figs. 4c and 4g).

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In this study, we focus on the garnetiferous high-pressure mafic granulite, amphibolite,

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metafelsic, and metapelitic rocks to recover their metamorphic P-T paths and metamorphic

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ages. The representative garnet-bearing samples are as follows: (1) high-pressure mafic granulite samples KL76 (92°30′43.6″E, 39°33′54.4″N), DHS46 (92°27′8.3″E, 39°31′37.2″N), DHS33 and DHS32 (92°30′43.6″E, 39°33′54.4″N); (2) garnet amphibolite sample DHS45 (92°27′8.3″E, 39°31′37.2″N) and DHS24 (92°24′24.2″E, 39°31′34.3″N); (3) garnet-biotitehornblende gneiss sample DHS43 (92°27′2.5″E, 39°32′5.1″N); (4) garnet- hornblende-biotite gneiss sample DHS17 (92°19′34.9″E, 39°31′19.2″N), DHS06 (92°18′41.5″E, 39°30′47.4″N). Furthermore, to determine the upper age limit of metamorphism, a granitic dyke sample DHS31 (92°30′43.6″E, 39°33′54.4"N) was also collected from the Kalatashitage area. The locations of these samples are depicted in Fig. 3.

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3. Petrography and metamorphic mineral assemblages A micropetrographic study reveals two or three generations of metamorphic mineral assemblages formed in the successive metamorphic stages, including the prograde mineral assemblage (M1) composed of the core and/or mantle of garnet porphyroblasts and inclusions preserved within them, peak mineral assemblage (M2) consisting of matrix minerals and the garnet, and retrograde mineral assemblage (M3) mainly consisting of symplectic minerals

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surrounding embayed garnet and retrograde hornblende rimming matrix-type clinopyroxene.

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The high-pressure mafic granulite and amphibolite samples contain all three mineral

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assemblages ‒ M1, M2, and M3 ‒ whereas some pelitic and semi-pelitic gneiss samples only preserve two stages of mineral assemblages ‒ M1/M2 or M2/M3. The primary mineral

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components and modal contents (vol. %) of the representative metamorphic samples studied

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are listed in Table 1. Detailed petrographic features, different meta morphic mineral assemblages, and metamorphic reactions are described in the following, with mineral

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abbreviations after Whitney and Evans (2010).

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3.1 High-pressure mafic granulite

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The prograde metamorphic assemblage (M1) mainly consists of hornblende (Hbl1 ) + plagioclase (Pl1 ) + quartz (Qz1 ) + epidote (Ep1 ) + ilmenite (Ilm1 ) + minor clinopyroxene (Cpx1 ) inclusions preserved within garnet and core/mantle of garnet porphyroblast, representing an amphibolite facies metamorphic precursor (Figs. 5a, 5f, and 5g). It should be noted that some coarse or irregular mineral grains that grew along thick cracks of garnet porphyroblast might not be authentic inclusions but were formed during later retrogression or fluid alternation. The peak metamorphic assemblage (M2) in most high-pressure mafic granulite samples is composed of coarse-grained garnet porphyroblast and coarse-grained matrix minerals (~15–20% garnet (Grt2 ) + ~25–30% clinopyroxene (Cpx2 ) + ~20–25% hornblende (Hbl2 ) +

Journal Pre-proof ~15–20% plagioclase (Pl2 ) + ~15–20% quartz (Qz2 )), as well as some accessory minerals such as rutile, zircon, titanite, and apatite (Fig. 5). Notice that some zircon crystals crystallized between matrix-type plagioclase and quartz displaying “triple-junction” equilibrium texture as shown in Fig. 5i. It is common that some clinopyroxene crystals in the matrix retrograded to hornblende (Figs. 5d, 5f, and 5h). Sample KL76 (Figs. 5a–5c) has a much higher content of clinopyroxene (~65%) than other samples. From M1 to M2, possible metamorphic reactions are deciphered as follows:

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Ep1 + Pl1 + Hbl1 + Ilm1 → Grt2 + Cpx2 + Pl2 + Rt2 + H2 O

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Hbl1 → Cpx2 + H2 O

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The retrograde metamorphic assemblage (M3) is mainly characterized by the fine-

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grained, worm- like coronitic symplectite rimming the garnet porphyroblast. Such coronitic symplectites around garnet porphyroblasts were named as the “white-eye socket” texture by

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Ma and Wang (1994) in Chinese literature. The tiny, intergrown “white-eye socket”

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symplectites mainly consist of hornblende (Hbl3 ) + plagioclase (Pl3 ) + quartz (Qz3 ) + minor

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biotite (Bt3 ) + magnetite (Mag3 ) ± clinopyroxene (Cpx3 ) (Fig. 5). Replacement of the edge of matrix- type clinopyroxene (Cpx2 ) by hornblende (Hbl3 ) is another retrogression phenomenon (Figs. 5d, 5f and 5h), which is common in most high-pressure mafic granulite samples except KL76. Some “matrix- type” coarse- grained hornblende (Hbl2 ) was possibly produced by the exhaustive retrogression of preexisting matrix-type clinopyroxene (Cpx2 ). The garnet amphibolite sample DHS45, closely neighboring the high-pressure mafic granulite sample DHS46, displays typical amphibolite facies metamorphic mineral assemblage without any clinopyroxene relicts and with some pseudomorphs of garnet porphyroblasts (Figs. 6a–6b). Such retrograde metamorphic reaction texture is common in high-pressure mafic granulite

Journal Pre-proof (e.g., Zhao et al., 2001; Guo et al., 2002). The following possible metamorphic reactions are responsible for the transformation from M2 to M3 assemblages: Grt2 (rim) + Cpx2 + Pl2 → Cpx3 + Pl3 Grt2 (rim) + Cpx2 + H2 O → Hbl3 + Pl3 ± Bt3 ± Mag3 Cpx2 + H2 O → Hbl3 (rimming matrix Cpx2 )

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3.2 Garnet amphibolite The M1 mineral assemblage consists of hornblende (Hbl1 ) + plagioclase (Pl1 ) + quartz

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(Qz1 ) + epidote (Ep1 ) as tiny inclusions preserved in garnet, possibly indicating an epidote-

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amphibolite facies metamorphic precursor. These inclusions are randomly distributed in the garnet porphyroblast and show no orientation.

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The peak metamorphic assemblage (M2) mainly comprises ~15% porphyroblastic

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garnet (Grt2 ) and medium- to coarse- grained matrix minerals, including ~35% hornblende

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(Hbl2 ) + ~25% plagioclase (Pl2 ) + ~15% quartz (Qz2 ) + ~5% biotite (Bt2 ), as well as accessory minerals such as zircon, titanite, and apatite (Figs. 6a–6d). The aligned matrix

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minerals, such as hornblende (Hbl2 ) and plagioclase (Pl2 ), define the main foliation. Furthermore, the ubiquitous nearly 120° triple junction between the matrix minerals indicates that the peak metamorphic assemblage has reached thermodynamic equilibrium. It is inferred that the peak assemblage (M2) is formed from the prograde assemblage (M1) via the following reaction: Ep1 + Hbl1 + Pl1 → Grt2 + Hbl2 + Pl2 The M3 mineral assemblage is represented by the coronitic symplectites surrounding the garnet porphyroblast, formed through retrograde metamorphic reactions between the garnet rim and adjacent matrix minerals. The fine-grained symplectic minerals commonly include

Journal Pre-proof hornblende (Hbl3 ), plagioclase (Pl3 ), and quartz (Qz3 ) (Figs. 6a–c). The symplectic assemblage (M3) is randomly distributed around the garnet porphyroblast. A possible reaction from M2 to M3 is listed as follows: Grt2 (rim) + Hbl2 → Hbl3 + Pl3 ± Bt3 3.3 Pelitic gneiss and semi-pelitic gneiss Various types of paragneiss account for a large proportion of rock located in the

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Kalatashitage area. They are mainly composed of pelitic and semi-pelitic gneiss, which have similar mineral associations (garnet + biotite + plagioclase + quartz ± hornblende) but

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different modal contents.

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In garnet-biotite-hornblende gneiss sample DHS43, three generations of mineral assemblages (M1, M2 and M3) are observed. M1 consists of fine- grained inclusions (Bt1 +

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Pl1 + Qz1 ) in garnet. M2 consists of ~15% garnet porphyroblast (Grt2 ) and matrix minerals

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such as, ~10% biotite (Bt2 ) + ~15% hornblende (Hbl2 ) + ~10% plagioclase (Pl2 ) + ~40%

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quartz (Qz2 ) + minor accessory zircon and apatite (Fig. 6g). The metamorphic reaction from the M1 to M2 is anticipated to be:

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Bt1 + Hbl1 + Pl1 → Grt2 + Bt2 + Hbl2 + Pl2 M3 is characterized by fine-grained “white-eye socket” symplectites (Hbl3 + Bt3 + Pl3 + Qz3 ) rimming the garnet (Fig. 6g). The possible metamorphic reactions forming symplectite are inferred as follows: Grt2 (rim) + Hbl2 → Hbl3 + Pl3 Grt2 (rim) + Bt2 + Pl2 → Bt3 + Pl3 In garnet- hornblende-biotite gneiss samples DHS06 and DHS17, only the peak (M2) and retrograde (M3) assemblages were identified. The lack of prograde metamorphic assemblage

Journal Pre-proof (M1) might indicate a long duration of prograde or peak metamorphism, which leads to a complete consumption of preexisting prograde minerals (M1). The peak assemblage (M2) consists of ~15% garnet porphyroblast (Grt2 ) plus matrix minerals, including ~5% hornblende (Hbl2 ) + ~15–20% biotite (Bt2 ) + ~5–10% plagioclase (Pl2 ) + ~40–45% quartz (Qz2 ) + minor accessory minerals zircon and apatite (Figs. 6e–6f). Fine-grained symplectic intergrowth of biotite (Bt3 ) + plagioclase (Pl3 ) + quartz (Qz3 ) ± hornblende (Hbl3 ) on the rim of the embayed garnet porphyroblast, constitute the retrograde assemblage (M3) as shown in

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Figs. 6e–6f.

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4.1 Electron microprobe analysis

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4. Analytical techniques

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Compositional analysis and backscattered electron (BSE) imaging of representative minerals, and X-ray compositional mapping of representative garnet porphyroblasts were

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conducted on the JOEL JXA 8230 wavelength dispersive electron microprobe at the School

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of Resources and Environmental Engineering, Hefei University of Technology in China. The

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analytical conditions included an accelerating voltage of 15 kV, beam current of 20 nA, electron beam diameter of 1–5 μm, and counting time of 10–20 s. The standards utilized were natural and synthetic minerals. The program ZAF was used for matrix corrections. At least three grains were analyzed for each mineral, 3-10 spots on each grain and 40-80 spots on every compositional profile of garnet porphyroblast were analyzed, respectively. The representative mean chemical compositions of the minerals used for estimating P-T conditions are listed in Supplementary Table S1. 4.2 U-Pb dating of zircon Zircon crystals were separated from five representative samples through conventional density and magnetic techniques. Thereafter, zircon grains of the garnet amphibolite samples

Journal Pre-proof ‒ DHS45 and DHS24 ‒ and zircon standard TEM (Black et al., 2003) were mounted together in epoxy for the SHRIMP analysis. Zircon grains of high-pressure mafic granulite samples ‒ KL76, DHS46, and the granitic dyke sample DHS31 ‒ were mounted on other epoxy blocks for LA-ICP-MS analysis. Cathodoluminescence (CL) images and transmitted and reflected light micrographs of these zircon crystals were used to reveal internal structures and to facilitate the selection of appropriate analysis spots. 4.2.1 SHRIMP U-Pb dating of zircon

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The U-Pb dating of zircon crystals, separated from garnet amphibolite samples DHS45 and DHS24, was conducted on the SHRIMP II ion microprobe at the Beijing SHRIMP

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Center, Chinese Academy of Geological Sciences (CAGS). Analytical procedures were

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similar to those described by Wan et al. (2005). Mass resolution during the analytical sessions

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was ~5000 (1% peak height). The intensity of the primary O 2- ion beam was set to 5–6 nA and spot size was ~30 μm. Each site was rastered for 150 s prior to analysis to remove

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contamination from the gold coating. Five scans through the mass stations were made for

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each age determination. The calibration standard used was TEM with an age of 417 Ma 204

Pb abundances, assuming Broken Hill lead

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(Black et al., 2003). The measured

composition, were used to correct the content of common lead. Data processing was carried out using the Isoplot program v.4.15 (Ludwig, 2012). The uncertainties regarding the concordia diagrams for individual analyses are quoted at the 1 σ level, whereas the errors on weighted mean ages are given at the 95% confidence level. The SHRIMP dating results are listed in Supplementary Table S2. 4.2.2 LA-ICP-MS U-Pb dating of zircon LA-ICP-MS U-Pb dating and REE elemental analysis of zircon separated from highpressure mafic granulite sample DHS46 were conducted using an Agilent 7500a ICP-MS connected to a Geolas-193 UV laser system at the State Key Laboratory of Continental

Journal Pre-proof Dynamics, Northwest University, China. Liu et al. (2007) provide detailed descriptions of the instruments and analytical procedures. The spot diameter and laser repetition rate were set as 32 μm and 6 Hz, respectively. The zircon separated from another high-pressure mafic granulite sample KL76 was tested using a Thermo Fisher iCAP-RQ quadrupole ICP-MS connected to a resolution SE laser system at the Radiogenic Isotope Facility of University of Queensland, Australia. The spot diameter and laser repetition rate were set to 24 μm and 6 Hz, respectively. Geochronology data of granitic dyke sample DHS31 and zircon trace

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element data from samples KL76, DHS45, and DHS24 were obtained using a Thermo Element XR HR-ICP-MS and Agilent 7900 ICP-MS connected to a resolution LR laser

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system at the Tianjin Institute of Geology and Mineral Resources, Chinese Academy of

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Geological Sciences, China. The spot diameter and laser repetition rate were set as 29 μm and

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6 Hz, respectively. The Isoplot program v.4.15 (Ludwig, 2012) was used for data reduction.

5.1 Garnet

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5. Mineral chemistry

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S3 and Table S4, respectively.

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LA-ICP-MS U-Pb dating and zircon REE analysis results are listed in Supplementary Table

Four X-ray compositional mapping analyses (Fig. 7) and nine transverse compositional profile analyses (Fig. 8) were performed on representative garnet porphyroblasts. The chemical compositions of garnet from all samples are dominated by almandine [X Alm = Fe2+/(Fe2+ + Mg + Ca + Mn)], grossular [X Grs = Ca/(Fe2+ + Mg + Ca + Mn)], pyrope [XPy = Mg/(Fe2+ + Mg + Ca + Mn)] and minor spessartine [X Sps = Mn/(Fe2+ + Mg + Ca + Mn)] components. In some high-pressure mafic granulite samples (e.g., DHS32 and DHS33), the compositional profiles of garnet porphyroblasts display typical growth zoning (Figs. 7b, 8b and 8c), with X Sps and Fe# [=Fe/(Fe + Mg)] ratios gradually decreasing from the core (X Sps =

Journal Pre-proof 0.03, Fe# = 0.82) to the inner rim (X Sps = 0.01, Fe# = 0.69), which forms a typical “bellshaped” growth zonation in metapelites, amphibolites, and mafic granulites (e.g., Spear et al., 1990; Lu et al., 2017; Wang et al., 2017b; Liu et al., 2019). However, garnet from other highpressure mafic granulite samples has a relatively homogeneous chemical composition (Figs. 7a, 8a, and 8d). It is noticed that most garnet porphyroblasts also exhib it a weak trending increase of XAlm and X Sps at the outermost rim as a thin “kick-up,” indicating Fe-Mg reexchange (diffusion) between the garnet porphyroblast rim and adjacent matrix minerals

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(e.g., clinopyroxene and hornblende) during the retrograde stage (e.g., Spear and Florence, 1992; Kohn and Spear, 2000).

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In garnet amphibolite sample DHS24, both the X-ray mapping and compositional profile

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of a representative garnet porphyroblast show typical growth zoning (Figs. 7c and 8e) with

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XSps and Fe# ratios decreasing from the core (X Sps = 0.08, Fe# = 0.92) to the inner rim (X Sps = 0.03, Fe# = 0.81), while garnet in sample DHS45 does not display compositional zoning

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(Figs. 8f–8g). In the pelitic gneiss samples DHS06 and DHS17, the representative garnet

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porphyroblast exhibits negligible chemical zoning from core to rim (Fig. 8h). In semi-pelitic

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gneiss sample DHS43, the garnet porphyroblast displays weak growth zonation (Figs. 7d and 8i). The X Sps and Fe# ratios decrease slightly from the core (X Sps = 0.09, Fe# = 0.83) to the inner rim (XSps = 0.05, Fe# = 0.74) with a minor increase at the outermost rim (X Sps = 0.08, Fe# = 0.80), which can be interpreted as the result of retrograde rim re-equilibration (Spear and Florence, 1992; Kohn and Spear, 2000). 5.2 Plagioclase Plagioclase occurs as three types of minerals in high-pressure mafic granulite samples. The inclusion-type plagioclase (Pl1 ) is chemically heterogeneous between different samples, i.e., the andesine (XAn = 0.44) for sample KL76, the oligoclase (XAn = 0.26) for sample DHS46, and the andesine (XAn = 0.39) for sample DHS32. Although few matrix-type

Journal Pre-proof plagioclase (Pl2 ) grains have weak compositional zonation with XAn increasing from core to rim, the chemical composition of Pl2 is relatively homogeneous in different samples with XAn ranging from 0.42 to 0.44, excluding sample DHS32 (XAn = 0.52). The symplectite-type plagioclase (Pl3 ) has much higher anorthite content (XAn = 0.81–0.84) than that of Pl2 , and the rim of Pl3 usually has slightly higher XAn than the core. In the garnet amphibolite sample DHS24, the symplectite-type plagioclase (Pl3 ; XAn = 0.64) has an evidently higher anorthite content than that of the matrix-type plagioclase (Pl2 ;

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XAn = 0.42). In the pelitic gneiss sample DHS06, two types of plagioclase have been identified: matrix type (Pl2 ; XAn = 0.18) and symplectite type (Pl3 ; XAn = 0.19), both with

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similar anorthite content. Another pelitic gneiss sample DHS17, has three types of

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plagioclases, whose anorthite content increases from Pl1 (XAn = 0.31) and Pl2 (XAn = 0.31) to

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Pl3 (XAn = 0.36). The semi-pelitic gneiss sample DHS43 preserves three types of plagioclase. The anorthite content decreases from the inclusion-type (Pl1 , XAn = 0.42) to the matrix-type

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5.3 Clinopyroxene

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(Pl2 , XAn = 0.35); thereafter, it increases slightly to the symplectite-type (Pl3 , XAn = 0.42).

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Clinopyroxene mainly occurs as the matrix- type (Cpx2 ) in high-pressure mafic granulite, representing the product of peak metamorphism, but the inclusio n type (Cpx1 ) and symplectite type (Cpx3 ) are only identified in sample KL76. The Cpx1 has a higher Mg# [=Mg/ (Fe + Mg)] value of 0.72 than that of Cpx2 (Mg# = 0.62) and Cpx3 (Mg# = 0.60), while the Cpx3 has lower Al2 O3 contents (3.65 wt %) than Cpx2 (5.05 wt %). The matrix-type clinopyroxene (Cpx2 ) is a chemically homogeneous diopside according to the classification of Morimoto et al. (1988). 5.4 Hornblende Hornblende exists as a major rock- forming mineral in almost all rock types. Three types of hornblende have been identified in metamafic samples; the inclusion type (Hbl1 ), the

Journal Pre-proof matrix type (Hbl2 ), and the symplectite type (Hbl3 ). The FeO content has similar tendency that increases from Hbl1 to Hbl2 and Hbl3 (Table S1). In pelitic gneiss and semi-pelitic gneiss samples, hornblende is a secondary mineral, which only occupies a minor proportion of three generations of metamorphic mineral assemblage. Hornblende in these samples generally have a similar CaO content (~11–12 wt %) and is classified as tschermakite, magnesiohornblende, and pargasite following the nomenclature of Leake et al. (1997).

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5.5 Biotite

Biotite occurs as a primary mineral in pelitic and semi-pelitic gneiss. Three types of

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biotite have been recognized, i.e., the inclusion type (Bt1 ), the matrix type (Bt2 ), and the

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symplectite type (Bt3 ). Bt1 contains a higher MgO content (14.46 wt %) than Bt2 (11.46 wt

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%) in semi-pelitic gneiss sample DHS43. However, the matrix-type biotite (Bt2 ) has similar chemical composition with the symplectite-type biotite (Bt3 ) in pelitic gneiss samples DHS06

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and DHS17.

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6. Metamorphic P–T paths

Two types of methods were employed for retrieving metamorphic P-T paths of various metamorphic rocks in the Kalatashitage area: (i) conventional thermobarometry, and (ii) pseudosection modeling. 6.1 Conventional thermobarometry Appropriate geothermometers and geobarometers were used to determine the metamorphic pressure-temperature (P-T) conditions for every metamorphic mineral assemblage described above, and then metamorp hic P-T paths were constructed for representative samples. The garnet-clinopyroxene geothermometer (Ravna, 2000) coupled with the garnet-clinopyroxene-plagioclase-quartz (GCPQ) geobarometer (Eckert et al., 1991)

Journal Pre-proof were both applied to the P-T computation of the prograde (M1) and peak (M2) assemblages in the high-pressure mafic granulite, which have random errors of ± 70 ºC (Ravna, 2000) and ± 1.5 kbar (Eckert et al., 1991), respectively. The hornblende-plagioclase (PH) geothermometer (Holland and Blundy, 1994) in concert with the garnet- hornblendeplagioclase-quartz (GHPQ) geobarometer (Dale et al., 2000) were employed for P-T calculation of the peak metamorphic assemblage (M2) in the garnet amphibolite, with random errors of ± 40 ºC (Holland and Blundy, 1994) and ± 1.1 kbar (Dale et al., 2000),

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respectively. The garnet-biotite geothermometer (Holdaway, 2000) combined with the garnet-biotite-plagioclase-quartz (GBPQ) geobarometer (Wu et al., 2004) were applied to the

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prograde (M1) and peak (M2) assemblages of pelitic gneiss or semi-pelitic gneiss with

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random errors of ± 25 ºC (Holdaway, 2000) and ± 1.2 kbar (Wu et al., 2004), respectively. In

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order to reduce the impacts of retrograde metamorphism, the inner rim composition of garnet porphyroblast (Grt2 ) with the lowest X Sps and Fe# values, and the core composition of matrix

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minerals, were employed to calculate the peak metamorphic (M2) P-T conditions.

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The retrograde assemblage (M3), such as the hornblende + plagioclase + quartz

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symplectite in metabasite and the biotite + plagioclase + quartz symplectite in metapelite, can hardly be in equilibrium with the garnet porphyroblast (Wu et al., 2014). Therefore, garnetabsent geothermobarometers should be applied to estimate the P-T conditions of the M3 assemblage. For example, the PH geothermometer (Holland and Blundy, 1994) combined with the PH geobarometer (Molina et al., 2015) with random errors of ± 40 ºC and ±1.5 kbar, respectively, were used to calculate the P-T conditions of the M1 and M3 assemblages. The Ti- in-biotite geothermometer proposed by Wu and Chen (2015) was adopted for quantifying the retrograde temperature of TiO 2 -saturated (ilmenite- or rutile-bearing) metapelite; otherwise, the monomineralic hornblende geothermobarometer (Hbl-PT; Gerya et al., 1997) was adopted for the M1 assemblage.

Journal Pre-proof In the computation, ferric iron content of garnet and clinopyroxene was determined by the stoichiometric and charge-balance method recommended by Droop (1987). The methods of Holland and Blundy (1994) and Dale et al. (2000) were adopted for hornblende. The metamorphic P-T conditions computed for different metamorphic assemblages are listed in Table 2 and shown in Fig. 9. 6.1.1 High-pressure mafic granulite For the high-pressure mafic granulite sample KL76 without evident retrogression

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overprint, the M1 P-T conditions were determined to be 638 ºC/11.9 kbar (HPQ) and 718 ºC/12.6 kbar (GCPQ), respectively. The M2 and M3 P-T conditions were estimated to be 919

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ºC/15.9 kbar (GCPQ) and 749 ºC/7.9 kbar (HPQ), respectively. For the other three high-

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pressure mafic granulite samples, i.e., DHS46, DHS33, and DHS32, the M1 P-T conditions

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were determined to be 682 ºC/7.6 kbar, 673 ºC/6.2 kbar, and 689 ºC/7.4 kbar, respectively (HPQ, Hbl-PT). The M2 P-T conditions were restricted to 843 ºC/14.6 kbar, 898 ºC/16.2

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kbar, and 818 ºC/13.4 kbar, respectively (GCPQ). Furthermore, application of HPQ

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geothermobarometry indicates that the M3 assemblage of these samples were formed at 814

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ºC/7.6 kbar, 791 ºC/5.5 kbar, and 786 ºC/6.8 kbar, respectively. The computed peak metamorphic P-T conditions of these representative high-pressure mafic granulite samples belong to the high-pressure granulite facies realm (O'Brien and Rötzler, 2003), reaching the lower limit of the high P/T facies series (Figs. 9a–9d; Miyashiro, 1961) according to the facies series scheme recommended by Spear (1993). The metamorphic P-T paths established for these samples successively underwent (high) amphibolite facies prograde metamorphism (640–710 ºC/6–12 kbar), then reached high-pressure granulite facies peak metamorphism (820–920 ºC/14–16 kbar), and finally retrograded at low-pressure amphibolite- granulite facies (750–810 ºC/6–8 kbar). These P-T paths were clockwise, and the retrograde stage reflects a nearly ITD process (Figs. 9a–9d).

Journal Pre-proof 6.1.2 Garnet amphibolite The P-T conditions of prograde (M1), peak (M2), and retrograde (M3) assemblage in garnet amphibolite sample DHS24 were determined to be 650 ºC/5.7 kbar, 752 ºC/9.2 kbar, and 781 ºC/8.1 kbar, using Hbl-PT, GHPQ and HPQ geothermobarometry, respectively. The M2 P-T conditions belong to typical amphibolite facies and intermediate P/T facies series (Spear, 1993; O'Brien and Rötzler, 2003). The retrieved P-T path is clockwise-type (Fig. 9e). 6.1.3 Pelitic gneiss and semi-pelitic gneiss

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For the pelitic gneiss samples DHS06 and DHS17, applications of the GBPQ, HPQ geothermobarometry, and Ti- in-biotite geothermometer yielded P-T conditions of 731–738

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ºC/8.6–10.1 kbar (M2) and 675–733 ºC/5.4 kbar (M3), respectively, indicating an ITD

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segment. However, the lack of appropriate inclusion mineral assemblage in the garnet

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porphyroblast hampered the estimation of M1 P-T conditions (dotted line in Figs. 9f and 9g). The M1, M2, and M3 P-T conditions of the semi-pelitic gneiss sample DHS43 were restricted

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to be 616 ºC/7.9 kbar, 823 ºC/11.7 kbar, and 738 ºC/8.7 kbar, respectively.

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The M2 P-T conditions of pelitic gneiss and semi-pelitic gneiss samples DHS06,

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DHS17, and DHS43 lie in the transition zone between the amphibolite facies and granulite facies, which belong to the intermediate P/T facies series (Figs. 9f–9h). The metamorphic P-T paths established from these samples display similar clockwise loops similar to that of the high-pressure mafic granulite (Fig. 9). 6.2 Pseudosection modeling To verify the results of the thermobarometry and decipher the metamorphic P-T paths of high-pressure mafic granulite in the Kalatashitage area in greater detail, sample DHS33, which preserved typical growth zonation of the garnet porphyroblast, for pseudosection modeling (Fig. 10), was chosen. The bulk rock compositions (in wt %) analyzed via X-ray fluorescence spectrometry (XRF) at the Institute of Geology and Geophysics, Chinese

Journal Pre-proof Academy of Sciences, Beijing, were SiO2 = 51.64, TiO 2 = 0.58, Al2 O3 = 12.75, TFe2 O3 = 13.94, MnO = 0.23, MgO = 7.40, CaO = 10.67, Na2 O = 1.30, and K2 O = 0.21. K2 O and MnO were neglected in the modeling owing to their low contents. According to the contents of Fe3+ cations in the main metamorphic minerals, 82% of the total Fe2 O3 was assumed to be FeO, which indicates a relatively high Fe2 O 3 content in the rock. The H2 O content was selected as a value that just stabilizes the peak mineral assemblage Grt + Cpx + Pl + Qz + Rt above the solidus, which assumes that the peak mineral assemblage has reached equilibrium with the

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remaining melt (e.g., White et al., 2004; Diener et al., 2008; Korhonen et al., 2013). Based on these reasonable assumptions, the final bulk rock co mpositions of sample DHS33 (in mol

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Na2 O = 1.31, O 2 = 0.50, and H2 O = 1.98.

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%) are SiO 2 = 53.76, TiO 2 = 0.46, Al2 O3 = 7.82, FeO = 10.93, MgO = 11.49, CaO = 11.76,

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Thermodynamic pseudosection modeling was calculated using the Na 2 O-CaO-FeOMgO-Al2 O3 -SiO2-H2 O-TiO2 -Fe2O 3 (NCFMASHTO) model system over a P-T range of 8–20

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kbar and 500–1000 ºC (Fig. 10), by employing the Perple_X program (Connolly, 2005;

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version 6.8.6) based on the internally consistent thermodynamic data set of Holland and

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Powell (2011; ds62, updated February 6, 2012). The minerals included in pseudosection modeling are garnet (Grt), clinopyroxene (Cpx), orthopyroxene (Opx), amphibole (Amp), plagioclase (Pl), epidote (Ep), rutile (Rt), ilmenite (Ilm), quartz (Qz), melt (Liq), and water (H2 O). The following activity-composition models were selected for each mineral: garnet (White et al., 2014), clinopyroxene (Green et al., 2016), orthopyroxene (White et al., 2014), amphibole (Green et al., 2016), plagioclase (Holland and Powell, 2003), epidote (Holland and Powell, 2011), ilmenite (White et al., 2014), and melt (Green et al., 2016). Rutile, quartz, and water are pure end-member phases. The calculated P-T pseudosection shows that the P-T space is dominated by four to six variance fields (Fig. 10). The prograde (M1) metamorphic P-T processes were quantitatively

Journal Pre-proof determined to be 8.5–13 kbar and 510–800 ºC, using the intersections of the isopleths of XPy [= Mg/(Fe2+ + Mg + Ca)] varying from 0.10 to 0.21 and X Grs [= Ca/(Fe2+ + Mg + Ca)] ranging between 0.27–0.34, from garnet core to mantle. There are some discrepancies between the prograde P-T paths retrieved from thermobarometry and pseudosection modeling, although together they record a heating and compression stage, which is possibly a result of the re-equilibration of Hbl inclusions at a higher temperature used for Hbl-PT thermobarometry estimation. The peak metamorphic mineral assemblage is Grt + Cpx + Amp

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+Pl +Ru + Liq, which is stable in the P-T space of 11.5–17 kbar/800–925 ºC. To quantitatively determine the peak metamorphic P-T conditions, the isopleths of XPy = 0.19,

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XGrs = 0.33 of the rim of garnet and Fe# [= Fe/(Fe + Mg)] = 0.34 of the matrix-type

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clinopyroxene, were used to yield an intersection field at the P-T condition of ~850 ºC/16

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kbar, which is consistent with the results of the thermobarometry computation. However, possibly because the effective bulk composition was intensively altered during the retrograde

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stage, this pseudosection could not provide a precise limit on the retrograde P-T path, which

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dotted line in Fig. 10.

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was simply speculated according to the thermobarometry computation and depicted by the

7. U-Pb dating results of zircon 7.1 High-pressure mafic granulite Zircon crystals separated from high-pressure mafic granulite sample KL76 are mostly elliptical in shape, with lengths ranging from 100–200 μm. In the CL images (Fig. 11a), most zircon grains exhibit relatively homogeneous internal structure, indicating metamorphic growth or recrystallization origin, and a few zircon grains exhibit relatively clear magmatic zonation, which suggests magmatic origin or incomplete metamorphic recrystallization (Vavra et al., 1999; Schaltegger et al., 1999; Hoskin and Black, 2000). Forty-two spot

Journal Pre-proof analyses were performed on 42 zircon grains. A majority of the analyzed spots indicate low Th and moderate U contents with low Th/U ratios (< 0.1), which also indicates a metamorphic origin (Williams, 2001; Rubatto, 2002). The analyzed results are mainly distributed on or close to the Concordia (Fig. 12a). It is noted that there are three obviously older age spots (No. 11, 25, and 32). After excluding significantly older ages with possibly inherited origin and discordant spots with large errors, the remaining

206

Pb/238 U ages of

thirty-two metamorphic growth or metamorphic recrystallization zircon grains yield a

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weighted mean age of 418.6 ± 1.8 Ma (MSWD = 1.02, Fig. 12b). Most zircon grains exhibit consistent REE characteristics, with low ΣREE (11–176 ppm) contents and flat HREE

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patterns, whereas a few zircon grains with typical magmatic zonation and significantly older

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ages have higher ΣREE contents and relatively steep HREE patterns (Table S4; Fig. 13a).

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Zircon crystals in another high-pressure mafic granulite sample DHS46 are ovoid or elliptical in shape, with lengths ranging between 80–150 μm. In the CL images (Fig. 11b),

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most zircon crystals exhibit bright-gray luminescence and homogeneous internal structure or

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display sector zoning, patchy zoning, and fir-tree zoning, which indicate solid-state

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recrystallization or newly growth/overgrowth origin during metamorphism (Vavra et al., 1999; Schaltegger et al., 1999; Hoskin and Black, 2000). A few crystals show typical corerim texture, which contains CL-bright or dark cores with weak oscillatory zonation, indicating an inherited origin, and CL-gray rim with homogeneous internal texture, suggesting metamorphic overgrowth origin. Forty- four spot analyses were conducted on 41 zircon grains. Most of the analyzed zircon grains have low Th contents (0.33–7.13 ppm) and moderate U contents (5.90–84.33 ppm) with low Th/U ratios (< 0.1), indicating metamorphic origin (Williams, 2001; Rubatto, 2002). The analyzed results are mostly distributed on or close to the Concordia (Fig. 12c). There are two spots on inherited zircon cores that yielded concordant ages of ~470 Ma, which represent the a ges inherited from the protolith of the

Journal Pre-proof high-pressure mafic granulite. After excluding significantly older and discordant spots, the remaining

206

Pb/238 U age of metamorphic origin zircon grains can be classified into two

major categories, i.e., ~430 Ma and ~385–400 Ma. Among them, analyses of 30 spots yielded a lower- intercept age of 429.4 ± 1.4 Ma (mean square of weighted deviates [MSWD] = 1.14) in the Tera-Wasserburg diagram (Fig. 12d), which is consistent with the weighted mean age of 429.6 ± 1.4 Ma (MSWD = 1.10) of 24 concordant spots. An additional seven analyses yielded a lower- intercept age of 390.9 ± 3.2 Ma (MSWD = 2.2), which agrees with the

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weighted mean age of 389.0 ± 3.2 Ma (MSWD = 0.29) of three concordant spots (Fig. 12e). The zircon grains with ages of ~430 Ma have similar REE characteristics with low ΣREE (8–

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102 ppm) contents (Table S4) and flat to moderate steep HREE patterns (Fig. 13b). However,

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those zircon grains with younger ages (~385–400 Ma) have diversiform REE contents (Table

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S4) and patterns (Fig. 13b). 7.2 Garnet amphibolite

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Zircon crystals separated from the garnet amphibolite sample DHS45 are elliptical or

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prismatic in shape and their lengths are 100–200 μm. In the CL images (Fig. 11c), zircon

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grains can be broadly divided into three types according to structural morphology: (1) internally homogeneous with CL-gray zircon grains, indicating metamorphic recrystallization or metamorphic growth origin; (2) CL-dark zircon grains with oscillatory zoning suggesting magmatic origin; and (3) zircon grains with CL-dark cores displaying oscillatory zonation and reflecting magmatic origin, and CL- gray rims with homogeneous internal texture indicating metamorphic overgrowth origin. Twenty-eight analyses were performed on 26 zircon grains. The Th and U contents of these analyzed spots are 3–36 ppm and 34–520 ppm, respectively, with low Th/U ratios ranging between 0.01–0.43. The analyses on metamorphic zircon grains or rims yielded a weighted mean of 206 Pb/238 U age of 430.3 ± 3.8 Ma (N = 19, MSWD = 0.78) after excluding extremely discordant spots with large errors. A few zircon

Journal Pre-proof grains displaying oscillatory zonation gave slightly older ages ranging from 440–479 Ma (Table S2; Fig. 12f). Zircon grains with typical metamorphic origin characteristics show consistent (left- inclined) REE characteristics and flat HREE patterns (Fig. 13c) with low ΣREE (4–28 ppm) contents (Table S4). Those zircon grains displaying oscillatory zonation also display higher ΣREE (8–277 ppm) contents (Table S4) with relatively steep HREE patterns (Fig. 13c). In another garnet amphibolite sample DHS24, zircon crystals were stubby or elliptical in

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shape with lengths ranging from 80–150 μm. Most crystals have similar CL images (Fig. 11d), exhibiting gray to dark luminescence and internally homogeneous or weak oscillatory

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zoning. Twenty- five analytical spots were conducted on 25 zircon grains, which have low Th

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(1–6 ppm) and moderate U (9–258 ppm), with low Th/U values (mostly < 0.1). Most of these

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results are distributed on or near Concordia (Fig. 12g). Similar to sample DHS46, these age results of metamorphic origin zircon grains in sample DHS24 can also be divided into two

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major categories, i.e., ~430 Ma and ~390–400 Ma (206 Pb/238 U ages). As shown in Fig. 12g, 206

Pb/238 U age of 431.3 ± 5.1 Ma (MSWD =

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fifteen analyses yielded a weighted mean of

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0.32), whereas six other spots provided a weighted mean of 206 Pb/238 U age of 398.2 ± 8.9 Ma (MSWD = 0.41) after filtering out age with large error and low concordance. All these zircon grains show similar REE characteristics with flat HREE patterns (Fig. 13d) and low ΣREE (3–49 ppm) contents (Table S4). 7.3 Granitic Dyke Zircon grains display a typically euhedral prismatic habit. They are 100–150 μm long, with an elongation ratio ranging from 2:1 to 3:1. In the CL image (Fig. 11e), all grains exhibit oscillatory zoning from core to rim. The Th and U contents of the analyzed spots are mainly 521–1836 ppm and 665–1858 ppm, respectively, with high Th/U ratios ranging between 0.42 and 1.28, which indicate magmatic origin. Twenty- four spots were analyzed on 24 zircon

Journal Pre-proof grains and yielded a weighted mean of 206 Pb/238 U age of 244.1 ± 1.6 Ma (MSWD = 2.3; Figs. 12h–12i).

8. Discussion 8.1 Tectonic significance of the metamorphic rocks and P-T paths The typical diagnostic metamorphic rocks formed in subduction zones are blueschist and

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eclogite. If the studied granulite and amphibolite of the Kalatashitage area actually derived

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from an eclogite protolith, they do record subduction. However, there are no evidences to

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prove that the granulite or amphibolite of the Kalatashitage area were transformed from eclogite, although eclogite has been found in the Hongliuxia area (Wang et al., 2017a), ~220

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km east of the study area. It is also known that blueschist and eclogite can be formed in

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cooler subduction zones; however, they are not necessarily formed in warmer subduction zones. Furthermore, it is known that in the same subduction zone, except for blueschist and

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eclogite, other types of metamorphic rocks can also be formed.

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The ubiquitous schistosity and gneissosity of the metamorphic rocks in the Kalatashitage

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area suggest the existence of deviatoric stress that prevailed coeval with metamorphic events, which were most likely formed in a subduction-collision zone environment. Furthermore, in the Kalatashitage area the high-pressure mafic granulite and garnet amphibolite usually occur as small- scale tectonic slices or lenses within the dominant paragneiss (e.g., pelitic gneiss and semi-pelitic gneiss). These lithological associations are common in orogens worldwide formed during different geological episodes, such as the Paleoproterozoic Trans-North China Orogen (Zhao et al., 2001; Wilde et al., 2002), Paleoproterozoic Trans-Amazonian Orogen in South America (Swapp and Onstott, 1989), Archean Limpopo Belt in Southern Africa (Kröner et al., 1999), and southeast Paleozoic DOB (Wang et al., 2017a), most of which were formed during the process of subduction or collision. The protolith of metamorphic rocks

Journal Pre-proof from the Kalatashitage area may be derived from either Paleozoic oceanic crust, oceanic sediment, or continental margin material which are difficult to identify, owing to intense overprint due to high-grade metamorphism and/or partial melting. From a metamorphic perspective, some metamorphic P-T paths can be diagnostic in the rebuilding process of geologic events (Harley, 1989; Brown, 1993). For example, clockwise P-T paths are usually characterized by convergent tectonic settings, e.g., subduction zone, collision zone, some island or continental arcs, and other analogous places in orogenic belts

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(England and Thompson, 1984; Thompson and England, 1984; Harley, 1989; Brown, 1993). In contrast, the anticlockwise P-T paths generally reflect various extensional tectonic settings

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with the addition of deep heat such as back-arc regions, intraplate rifts, and hotspots (Wells,

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1980; Sandiford and Powell, 1986; Hill et al., 1992). Different retrograde trajectories also

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possibly correspond to different tectonic explanations. For example, the retrograde P-T path for blueschist from the western Alps is characterized by near ITD, which most likely resulted

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from relatively rapid exhumation (Ernst, 1988); on the other hand, the hair-pin Franciscan-

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type retrograde P-T path is explained to represent relatively slow exhumation or refrigeration

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by underthrust sediments as the high P/T rocks were being exhumed (Ernst, 1988). Here, the high-pressure mafic granulite, garnet amphibolite, pelitic gneiss, and semipelitic gneiss all record similar clockwise P-T paths (Fig. 9), which indicate that they were possibly formed in the same convergent tectonic setting. Pseudosection modeling indicates that the high-pressure mafic granulite at the least records a pressure increasing process from 8.5 kbar to 16 kbar (Fig. 10). Furthermore, considering the sedimentary origin of their protolith, the dominant high- grade pelitic gneiss and semi-pelitic gneiss were significantly more likely to be formed during subduction or collision. These P-T paths possibly record the sequential subduction, collision, and post-collisional exhumation.

Journal Pre-proof Therefore, we can infer that in the Kalatashitage area, the amphibolite and metapelite might have been formed during subduction, and the granulite might have been formed during collision. However, we should admit it is currently difficult to differentiate between the rocks formed during subduction and those formed during collision, at least in this region. The exposed rocks in the western part of the Kalatashitage area mainly underwent amphibolite facies metamorphism and lack granulite facies rock association, whereas the eastern part of this area was mainly composed of upper amphibolite facies to high-pressure

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granulite facies metamorphic rocks. It is evident that prominent peak metamorphic pressure differences exist among high-pressure mafic granulite, amphibolite, and dominant pelitic and

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semi-pelitic gneiss, although the metamorphic rocks of this area record similar clockwise P-T

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paths. These calculated pressures indicate diverse subduction, collision, and exhumation

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processes affecting different tectonic slices within the Kalatashitage area, resulting in different tectonic slices reaching their different maximum depths and recording their distinct

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peak metamorphic P-T conditions. Afterwards, they were tectonically juxtaposed at relatively

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shallow levels during the exhumation stage. Similar phenomena were also observed in other

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orogenic belts. For example, Li et al. (2016b and references therein) used “subduction channel” models to explain the exhumation of high-pressure to ultrahigh-pressure metamorphic rocks with different peak metamorphic P-T conditions, southwest Tianshan orogenic belt. In the North Qaidam orogenic belt, different metamorphic slices underwent diverse metamorphism, including the Lüliangshan high temperature to high pressureultrahigh pressure (HP-UHP) metamorphic terrane and the adjacent Yuka-Luofengpo lowmiddle-temperature (LT-MT), UHP metamorphic terrane (Zhou et al., 2019a,b). These phenomena are not uncommon in global orogenic belts, with processes of juxtaposition also supported by numerical modeling (e.g., Gerya et al., 2002; Stöckhert and Gerya, 2005; Li et al., 2010).

Journal Pre-proof 8.2 Metamorphic ages High-grade metamorphism can create metamorphic zircon through two distinct ways: solid-state recrystallization of inherited zircon and new zircon growth, or overgrowth on older zircon (e.g., Vavra et al., 1999; Schaltegger et al., 1999; Wu et al., 2006). There are significant differences between zircon with a metamorphic origin and inherited magmatic zircon in several aspects such as their morphology, CL image features, Th-U chemistry, and REE patterns (e.g., Hoskin and Black, 2000; Hoskin and Schaltegger, 2003; Harley et al.,

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2007). This study chose four metamafic samples and one granitic dyke sample for zircon UPb dating to constrain metamorphic ages.

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In high-pressure mafic granulite samples KL76 and DHS46, most zircon grains display

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typical signs of metamorphic origin, such as ovoid or elliptical morphology, homogeneous

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internal structure, and typical core-rim textures in CL images (Figs. 11a–11b). A few zircon crystals exhibit weak oscillatory zonation, suggesting magmatic origin or incomplete

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metamorphic recrystallization (Vavra et al., 1999; Schaltegger et al., 1999; Hoskin and Black,

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2000). Petrographic observation reveals that zircon grains usually crystallize between matrix-

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type plagioclase and quartz, showing “triple-junction” equilibrium texture (Fig. 5i). Such phenomenon possibly suggests that most of the zircon crystals formed during peak metamorphism (e.g., Zhao et al., 1999; Wang et al., 2014). Moreover, the smaller zircon grains that closely coexist with retrograde metamorphic minerals probably suggest retrograde metamorphic origins (e.g., Wu, 2018). However, U-Pb dating of most zircon grains with typical metamorphic characteristics or a weak oscillatory zonation in sample KL76 yielded consistent ages within the error range. There are three possible reasons for this phenomenon: (1) the protolith ages recorded by zircon grains with an oscillatory zonation were close to the metamorphic ages yielded by metamorphic zircon grains; (2) the ages of zircon grains with oscillatory zonation were reset during metamorphism; (3) the zircon grains with oscillatory

Journal Pre-proof zonation were derived from the melt of high-pressure mafic granulite. Most of the analyzed zircon grains have low Th and moderate U contents with low Th/U ratios (< 0.1), also suggesting metamorphic origin (Williams, 2001; Rubatto, 2002). Furthermore, zircon grains showing metamorphic origins have similar REE characteristics and flat HREE patterns with low ΣREE contents (11–176 ppm), which suggest that they may have crystallized concurrently with garnet (Rubatto, 2002; Whitehouse and Platt, 2003). In contrast, the misty oscillatory zonation (Fig. 11a) and discrepant REE characteristics, including flat to steep

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HREE patterns (Fig. 13a), together suggest that those “magmatic” zircon grains are grea tly influenced by high-pressure granulite facies metamorphism, which may explain the

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consistency between the recorded ages and those of metamorphic zircon. After eliminating

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significantly older and discordant ages with large errors, the remaining consistent ages within

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the error range together yielded a weighted mean 206 Pb/238 U age of 418.6 ± 1.8 Ma (Figs. 12a–12b), representing the age of peak metamorphism. For sample DHS46, which is another

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high-pressure mafic granulite sample, U-Pb dating results of metamorphic origin zircon were

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classified into two major categories, i.e., ~430 Ma and ~385–400 Ma (Figs. 14c–14e). A

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majority of the analytical spots with ages of ~430 Ma exhibit similar REE characteristics with low ΣREE (8–102 ppm) contents (Table S4) and flat to moderately steep HREE patterns (Fig. 13b), indicating metamorphic origins under the presence of garnet (Rubatto, 2002; Whitehouse and Platt, 2003). As a contrast, zircon grains with younger ages (~385–400 Ma) have diversiform ΣREE contents (Table S4) and patterns (Fig. 13b), possibly suggesting the effects of a later fluid metasomatism during retrograde metamorphism (Rayner et al., 2005). Therefore, these two groups of U-Pb dating results, 429.6 ± 1.4 Ma and 389.0 ± 3.2 Ma, correspond to the ages of peak (M2) and retrograde (M3) metamorphic ages, respectively. In the garnet amphibolite sample DHS45, zircon crystals with typical metamorphic features yielded a weighted mean age of 430.3 ± 3.8 Ma (Fig. 12f), which is consistent with

Journal Pre-proof the peak metamorphic age of sample DHS46. A majority of zircon grains show consistent (left-inclined) REE characteristics and typical HREE depleted patterns (Fig. 13c) with low ΣREE contents (4–28 ppm; Table S4), implying that these zircon grains are metamorphic origins under the presence of garnet. Therefore, the age of 430.3 ± 3.8 Ma is interpreted as the peak metamorphic age. The metamorphic ages yielded by metamorphic origin zircon grains in another garnet amphibolite sample DHS24 can also be divided into two groups of 431.3 ± 5.1 Ma and 398.2 ± 8.9 Ma (Fig. 12g). However, the trace elements of all zircon

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grains consistently exhibit similar flat HREE patterns (Fig. 13d) and low ΣREE contents (4– 28 ppm; Table S4). These ages together with the REE patterns possibly suggest a long period

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of garnet growth (Rubatto, 2002; Whitehouse and Platt, 2003) or later fluid metasomatism

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during the retrograde metamorphism (Rayner et al., 2005). Regarding the granitic dyke

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sample DHS31 intruding into the high- grade metamorphic rocks in the Kalatashitage area, the typical magmatic zircon grains yielded a crystallization age of 244.1 ± 1.6 Ma (Figs. 12h–

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12i), postdating the metamorphic event. In almost all samples, there were few but non-

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negligible older ages (~440–480 Ma) of zircon grains with oscillatory zonation, whose CL

protolith.

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images, Th/U ratios, and REE patterns together indicate that they were inherited from

The above data indicate that the high-pressure granulite facies and amphibolite facies metamorphism occurred during 430–390 Ma, suggesting that the Kalatashitage area records Paleozoic subduction, collision, and exhumation. The ages (~440–480 Ma) of zircon grains with inherited origins constrain a lower limit of the protolith of metabasic samples. These geochronological results are similar with those obtained from the middle and southern parts of the DOB (Meng et al., 2011; Zong et al., 2012; He et al., 2014; Wang et al., 2016a, 2017a, 2017b, 2018a, 2018b; Zhao et al., 2016; Pham et al., 2018). 8.3 Tectonic implications

Journal Pre-proof Recent high-precision geochronological studies indicate that Paleozoic metamorphism prevailed in Early Silurian to Late Devonian in this ~400 km long, northeast-southwest trending belt (Meng et al., 2011; Zong et al., 2012; He et al., 2014; Wang et al., 2016a, 2017a, 2017b, 2018a, 2018b; Zhao et al., 2016; Pham et al., 2018). Studies also indicate that Paleozoic magmatism lasted from early Paleozoic to late Paleozoic (Zhang et al., 2009; Zhu et al., 2014, 2019; Zhao et al., 2015b, 2017; Wang et al., 2016b, 2016c, 2017c; Bao et al., 2017; Feng et al., 2018; Shi et al., 2018, 2019). Clockwise P-T paths of various metamorphic

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rocks of the DOB, indicative of an orogenic event, have been found recently in discrete tectonic blocks, such as Mogutai- Dongbatu (Zong et al., 2012; He et al., 2014; Wang et.,

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2018b; Pham et al., 2018), Hongliuxia (Wang et al., 2017a, 2017b), Shuixiakou (Wang et al.,

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2018a), and Qingshigou (Wang et al., 2016a). Combined with geochronologic data, we can

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conclude that the Dunhuang area was in fact a Paleozoic orogenic belt (Zhao et al., 2016;

constrained at ca. 445–365 Ma.

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Wang et al., 2017b, 2018a, 2018b; Pham et al., 2018), with the age of metamorphism

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However, diverse tectono- metamorphic features have been found among different

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tectonic blocks of the DOB. First, in some tectonic blocks (e.g., Hongliuxia, DongbatuMogutai, Qingshigou), mafic granulite, amphibolite, and eclogite (Wang et al., 2017a) occur as small- scale tectonic lenses intercalated within the matrix of metapelitic schist or gneiss, indicating typical block- in- matrix features of tectono- metamorphic mélange. However, in the Kalatashitage area, tectonic mélange was not observed, and this area is composed of larger tectonic slices. Second, clockwise P-T paths have been identified in all discrete tectonic blocks of the DOB, among which the Franciscan-type P-T paths were found in Qingshigou, and western Alpine- like P-T paths with ITD segments were found in Hongliuxia, DongbatuMogutai, and Kalatashitage. These findings suggest that the different tectonic blocks experienced different rates of uplift during exhumation. Third, in every tectonic block, such

Journal Pre-proof as Hongliuxia or Kalatashitage studied in this work, different metamorphic facies series have been found. These findings in turn suggest that P/T ratios may vary to some extent in the same subduction-collision zone. The diverse subduction, collision, and exhumation of metamorphic rocks, which appear as a universal phenomenon in many other orogens (e.g., Li et al., 2016b; Zhou et al., 2019a, 2019b) coupled with geochronological studies suggest that the Kalatashitage area belongs to the western part of the Paleozoic DOB. Until now, there was a lack of conse nsus whether the

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DOB was formed from the Paleozoic metamorphic and magmatic overprint of a preexisting Precambrian “Dunhuang Block” or entirely formed during the Paleozoic subduction-collision

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processes, as suggested above. Early studies mostly believed that the ages of protolith rocks

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in this area were early Precambrian, from which some Mesoarchean-Neoarchean and

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Paleoproterozoic gneissic granitoid and supracrustal rocks were identified (Mei et al., 1997, 1998; Zhang et al., 2012, 2013; He et al., 2013; Wang et al., 2013a, 2013b, 2014; Yu et al.,

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2014; Zhao et al., 2013, 2015a, 2015c, 2019a, 2019b; Zong et al., 2013). However, those

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early Precambrian intensively deformed granitoids are unrepresentative due to their

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occurrence and scale of formation, which are difficult to determine and cannot be used to date the protolith age of the entire Dunhuang Complex. Recent studies on detrital zircon from metasedimentary matrix in the DOB have identified Paleozoic detrital zircon in different areas (e.g., Hongliuxia and Duobagou), indicating that protolith ages are possibly not as old as people previously assumed (Zhu et al., 2018; Shi et al., 2019). In this study, it is noted that some inherited zircon grains yield slightly older but concordant ages (~440–480 Ma), which provide a lower age limit of the protolith of metabasic samples. Considering that metabasic rocks mainly occur as small-scale tectonic slices or interlays within metasedimentary matrix, it is believed that the entire structural format of the Kalatashitage area was formed during the Paleozoic

subduction-collision

processes.

The

heterogeneity

in

field

occurrence,

Journal Pre-proof metamorphic P-T paths, and peak metamorphic P-T conditions between different blocks in the DOB all support the view that this orogen was formed during Paleozoic subductioncollision processes, albeit high-pressure blueschist and eclogite facies rocks are missing in the majority of the discrete tectonic blocks. The north neighbored Beishan Orogenic Belt (Fig. 1b) was regarded as the southernmost margin of the Central Asia Orogenic Belt (Xiao et al., 2010), which experienced Paleozoic metamorphism and magmatism similar to the DOB. For example, all of these orogens exhibit

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Paleozoic high-pressure eclogite, such as ~465 Ma eclogite in the western area of the Liuyuan region in Beishan orogen (e.g., Liu et al., 2011; Qu et al., 2011; Saktura et al., 2017),

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and ~410 Ma eclogite, and ~445–430 Ma high-pressure granulite in the DOB (Zong et al.,

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2012; He et al., 2014; Wang et al., 2017a, 2017b, 2018b; this study). The high-pressure

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metamorphism recorded in the DOB is slightly younger than that in the Beishan orogen, possibly indicating the southward migration of the subduction zone. The DOB and Beishan

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orogen also have similar magmatic history during the Paleozoic, e.g., ca. 436–397 Ma

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granitoids in the Beishan orogen (e.g., Zhao et al., 2007) and ca. 460–410 Ma, 380–330 Ma

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granitoids in the DOB (Zhang et al., 2009; Zhu et al., 2014, 2019; Zhao et al., 2015b, 2017; Wang et al., 2016b, 2016c, 2017c; Bao et al., 2017). However, it is uncertain whether the DOB and Beishan orogen are a unified orogen or if they represent two independent orogenic belts formed during the Paleozoic, in which case additional data and regional correlation would be required.

9. Conclusions (1) Three generations of metamorphic mineral assemblages were identified in the newly found high-pressure mafic granulite, garnet amphibolite, pelitic gneiss, and semi-pelitic gneiss from the Kalatashitage area in the western Paleozoic Dunhuang Orogenic Belt. These

Journal Pre-proof assemblages were possibly formed during the subduction, collision, and exhumation processes. (2) The peak metamorphic P-T conditions of high-pressure mafic granulite can be attributed to the high P/T facies series, whereas the peak metamorphic P-T conditions of amphibolite and metapelite belong to the medium P/T facies series. Through pseudosection modeling and geothermobarometric computation, the retrieved metamorphic P-T paths of all the metamorphic rocks have a clockwise rotation, passing from 640–720 ºC/6.2–12.6 kbar

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(M1) through 840–920 ºC/14.6–16.2 kbar (M2) to 750–810 ºC/5.5–7.9 kbar (M3) for highpressure mafic granulite, from ~650 ºC/5.7 kbar (M1) through ~750 ºC/9.2 kbar (M2) to ~780

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ºC/8.1 kbar (M3) for garnet amphibolite, as well as from ~615 ºC/7.9 kbar (M1) through 730–

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820 ºC/8.6–11.7 kbar (M2) to 675–740 ºC/5.4–8.7 kbar (M3) for pelitic gneiss and semi-

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pelitic gneiss. Significant pressure differences of peak metamorphism suggest that the rocks subducted to remarkably different depths and were subsequently juxtaposed to identical

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shallower crustal levels during exhumation.

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(3) SHRIMP and LA-ICP-MS U-Pb dating of zircon reveal that the high-pressure mafic

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granulite and garnet amphibolite record metamorphic ages of ca. 430–390 Ma, and subsequent granitic dyke intruded the high-grade metamorphic rocks at ca. 244 Ma. (4) Thus, we conclude that metamorphic rocks of the Kalatashitage area were formed during the subduction, collision, and exhumation during the Paleozoic.

Declaration of interest: None Acknowledgments We sincerely thank Professor Yong-Hong Shi and Dr. Juan Wang, Hefei University of Technology, for their assistance in electronic microprobe analyses. We thank P rofessor YuSheng Wan and Dr. Shou-Jie Liu for their guidance in SHRIMP U-Pb dating of zircon at Beijing SHRIMP Center, Chinese Academy of Geological Sciences. We also thank Professor

Journal Pre-proof Xiao-Ming Liu and Drs. Hua-Dong Gong and Hong Zhang for their guidance in LA-ICP-MS U-Pb dating of zircon at State Key Laboratory of Continental Dynamics, Northwest University, China. We thank Professors Hong-Ying Zhou and Zhi- Bin Xiao for their help in LA-ICP-MS U-Pb dating of zircon at the Tianjin Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences, China. The authors also benefited from discussions with Professors Quan-Lin Hou and Quan-Ren Yan. Both the science and the English of this paper have been greatly improved through reviews by Professors Aley K. El-

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Shazly and Qiuli Li as well as the editorial review by Professor Marco Scambelluri. This work was financially supported by the National Natural Science Foundation of China

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(41730215) and the Key Research Program of Frontier Sciences from Chinese Academy of

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Sciences (QYZDJ-SSW-DQC036).

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Figure captions

Figure 1. (a) General location map of the Tarim Craton, south of the Central Asian

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Orogenic Belt; (b) Geological map of the Tarim Basin and adjacent orogenic belts. Note the

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location of the Dunhuang Orogenic Belt and its surrounding tectonic units (modified after Zhang et al., 2013).

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Figure 2. Geological map of the Dunhuang Orogenic Belt (modified after Zhao et al.,

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2016). Geochronologic data reported in literature are indicated. Ages in blue are magmatic

red rectangle.

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ages, whereas those in red are metamorphic ages. Area of this study (Fig. 3) is indicated by a

Figure 3. Simplified geological map of the Kalatashitage area, western Dunhuang Orogenic Belt (modified after Liu et al., 2010). Sample locations as well as magmatic and metamorphic age data are indicated. Figure 4. Field images of the Kalatashitage area. (a) High-pressure mafic granulite occurs as small-scale tectonic slices preserved within metapelitic gneiss. Sample DHS46 was collected on the back of this hill. (b) Metapelitic gneiss exhibits intense ductile deformation, tight folds, and partial melting. The first stage of deformation is represented by tight or rootless folds (S1 ), along which some tiny partially melted veins are distributed. The

Journal Pre-proof pervasive NWW-SEE trending schistosity or gneissosity (S 2 ) constitutes the second stage of deformation (D2 ). (c) High-grade metamorphic rock is intruded by later granitic dyke (sample DHS31). (d) Representative high-pressure mafic granulite (DHS33) indicating “white-eye socket” symplectite surrounding the garnet porphyroblast and partial melting. (e) The representative semi-pelitic gneiss (DHS43) shows gneissic and porphyroblastic structures. The round white pill (~0.6 cm) is a scale bar. (f) Felsic gneiss and hornblende-biotite gneiss show concurrent gneissic structures. (g) The hornblende-biotite gneiss is intruded by later

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granitic sill. (h) The garnet amphibolite is intercalated within the metapelite. (i) The metapelitic gneiss displays ductile deformation and tight folds, without obvious partial

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melting. The first stage of deformation (D1 ) is represented by a tight or rootless fold (S 1 ), and

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the pervasive NWW-SEE trending foliation (S2 ) constitutes the second stage of deformation

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(D2 ).

Figure 5. Photomicrographs and backscattered electron (BSE) images of the high-

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pressure mafic granulite from eastern Kalatashitage area. The yellow dotted line with arrow

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represents the analytical compositional profile of garnet porphyroblast, as depicted in Fig. 8.

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Sample number and rock type are labeled. Subscripts of the minerals represent formation of different metamorphic stages. (a–c) Sample KL76 preserves three generations of metamorphic mineral assemblages. The prograde metamorphic assemblage (M1) consists of the core and mantle of garnet and Hbl1 + Pl1 + Qz1 + Ep1 ± Cpx1 inclusions preserved within garnet. The peak metamorphic mineral assemblage (M2) mainly consists of Grt2 + Cpx2 +Pl2 + Qz2 . The retrograde assemblage (M3) is “white-eye socket” coronitic symplectite (Cpx3 + Hbl3 + Pl3 + Qz3 ) rimming garnet and Hbl3 riming Cpx2 . Similar reaction structures and mineral associations are also present in the samples below. (d) The rim of matrix-type clinopyroxene has been retrograded to hornblende in sample DHS32. (e) The rutile occurs as inclusions within matrix-type clinopyroxene in sample DHS32. (f) The rim of garnet

Journal Pre-proof porphyroblast has been broken down into “white-eye socket” coronitic symplectite and the rim of matrix-type clinopyroxene has been retrograded to hornblende in sample DHS33. (g) The BSE image of coronitic symplectite forming a rim around garnet in sample DHS33. (h) The rim of garnet porphyroblast was broken down into “white-eye socket” coronitic symplectite and the rim of matrix-type clinopyroxene has been retrograded to hornblende in sample DHS46. (i) Zircon crystal usually crystallized between matrix- type plagioclase and quartz with “triple-junction” equilibrium texture.

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Figure 6. Photomicrographs and backscattered electron (BSE) images of the garnet amphibolite, pelitic, semi-pelitic gneiss, and granitic dyke of the Kalatashitage area. The

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yellow dotted line with an arrow represents the analytical compositional profile of garnet

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porphyroblast, as depicted in Fig. 8. (a–b) Garnet amphibolite sample DHS45 shows “white-

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eye socket” coronitic symplectite (Hbl3 + Pl3 + Qz3 ± Bt3 ± Mag3 ) rimming the garnet. (c) Garnet amphibolite sample DHS24 shows prograde mineral assemblage (M1) of Hbl1 + Ep1 +

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Pl1 + Qz1 inclusions in garnet and peak mineral assemblage (M2) of Grt2 + Hbl2 + Pl2 + Qz2 .

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(d) Zircon crystal usually crystallized between matrix-type plagioclase and quartz with

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“triple- junction” equilibrium texture. (e) Grt-Hbl-Bt gneiss sample DHS06 shows diablastic garnet and peak mineral assemblage (M2) of Grt2 + Hbl2 + Bt2 + Pl2 + Qz2 . (f) Grt-Bt gneiss sample DHS17 has a garnet porphyroblast without inclusion minerals and peak mineral assemblage (M2) of Grt2 + Bt2 + Pl2 + Qz2 . (g) Grt-Bt-Hbl gneiss sample DHS43 shows peak mineral assemblage (M2) of Grt2 + Bt2 + Hbl2 + Pl2 + Qz2 . (h–i) Plane- and cross-polarized light photomicrographs of granitic dyke sample DHS31. Figure 7. X-ray compositional maps of the Fe, Mg, Ca, and Mn components of representative garnet porphyroblasts. Colors in the columns on the right side of each map indicate relative element concentrations (wt %).

Journal Pre-proof Figure 8. Chemical compositional profiles of representative garnet porphyroblasts along profiles indicated in Figs. 5–6. Figure 9. Metamorphic P-T paths retrieved from samples studied in the Kalatashitage area. Metamorphic facies and metamorphic facies series are obtained from O'Brien and Rötzler (2003) and Spear (1993), respectively. The Al2 SiO 5 phase transition lines are based on Holdaway and Mukhopadhyay (1993). The symbols for geothermobarometers are listed in Table 2.

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Figure 10. P-T pseudosection modeling for mafic granulite sample DHS33 calculated in the NCFMASHTO (+Qz) system using Perple_X program (Connolly, 2005; version 6.8.6)

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with an effective bulk composition generated according to XRF analysis. The intersections of

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isopleths of XPy [= Mg/(Fe2+ + Mg + Ca)] (0.10–0.21) and X Grs [= Ca/(Fe2+ + Mg + Ca)]

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(0.27–0.34) from the core to the mantle of garnet quantitatively yield prograde metamorphic P-T process in the P-T ranges of 8.5–13 kbar and 510–800 ºC. The red and yellow stars

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conditions, respectively.

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represent the intersections of isopleths for prograde metamorphic and peak metamorphic P-T

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Figure 11. Cathodoluminescence (CL) images of analyzed zircon crystals and corresponding ages (Ma) in yellow (concordant) and black (discordant). The red circles and numbers are analytical locations and corresponding spot numbers, respectively. (a) Highpressure mafic granulite sample KL76. (b) High-pressure mafic granulite sample DHS46. (c) Garnet amphibolite sample DHS45. (d) Garnet amphibolite sample DHS24. (e) Granitic dyke sample DHS31. Figure 12. Zircon U-Pb concordia diagrams for analyzed samples. (a–b) High-pressure mafic granulite sample KL76. (c–e) High-pressure mafic granulite sample DHS46. (f) Garnet amphibolite sample DHS45. (g) Garnet amphibolite sample DHS24. (h–i) Granitic dyke sample DHS31. Note: The red circles (ca. 430–420 Ma) and blue circles (ca. 400–390 Ma)

Journal Pre-proof represent two groups of metamorphic ages, respectively. The black circles are older ages of inherited zircon crystals. The black dotted circles are discordant ages that were not involved in the calculations. Corresponding data are listed in Tables S2–S3. Figure 13. Chondrite- normalized (Sun and McDonough, 1989) REE patterns of analytical zircon. (a) High-pressure mafic granulite sample KL76. (b) High-pressure mafic granulite sample DHS46. (c) Garnet amphibolite sample DHS45. (d) Garnet amphibolite sample DHS24. Note: The red lines (ca. 430–420 Ma) and blue lines (ca. 400–390 Ma) are

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the REE patterns of two metamorphic age groups, respectively. The black lines are the REE

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patterns of inherited zircon with older ages. Corresponding data are listed in Table S4.

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Table captions

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Table 1. Main mineral components and modal contents (vol. %) of representative rock samples.

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Table 2 Metamorphic P-T conditions retrieved for different metamorphic stages of

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Orogenic Belt.

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representative metamorphic rocks collected from the Kalatashitage area, western Dunhuang

Electronic appendices

Table S1. Chemical compositional data of representative minerals of high-pressure mafic granulite, garnet amphibolite, and pelitic and semi-pelitic gneiss from the Kalatashitage area in the western Dunhuang Orogenic Belt. Table S2. SHRIMP U-Pb geochronological data of zircon separated from garnet amphibolite samples DHS45 and DHS24 from the Kalatashitage area, western Dunhuang Orogenic Belt.

Journal Pre-proof Table S3. LA-ICP-MS U-Pb geochronological data of zircon separated from highpressure mafic granulite samples KL76 and DHS46 and granitic dyke sample DHS31 from the Kalatashitage area, western Dunhuang Orogenic Belt. Table S4. Trace elements of zircon U-Pb dating spots in high-pressure mafic granulite samples KL76 and DHS46 and garnet amphibolite samples DHS24 and DHS45 from the

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Kalatashitage terrane, western Dunhuang Orogenic Belt.

Journal Pre-proof Table 1. Main mineral components and modal contents (vol.%) of representative rock samples

Sample

Gr t

Cp x

Hb

Bt

l

Pl

Q

Others

z

HP mafic granulite

DHS46

(Ⅰ, Ⅱ)

20

DHS33

(Ⅰ, Ⅱ)

25

(Ⅰ, Ⅱ)

(Ⅰ, Ⅱ, Ⅲ)

35

rn

–

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DHS45

–

(Ⅰ, Ⅱ)

Ⅲ)

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amphibolite

(Ⅰ, Ⅱ)

2(

(Ⅰ, Ⅱ, Ⅲ)

5( Ⅱ)

25 (Ⅱ, Ⅲ)

Rt, Ilm, Zrn, Ap, Ttn

5( Ⅰ, Ⅱ, Ⅲ)

17 (Ⅰ, Ⅱ, Ⅲ)

5(

Ⅰ, Ⅱ, Ⅲ)

10

(Ⅱ, Ⅲ)

Zrn, Ap

Ⅰ, Ⅱ, Ⅲ)

15

(Ⅰ, Ⅱ, Ⅲ)

–

30

Grt

DHS24

Ⅲ)

20

25 (Ⅱ)

15

1(

(Ⅰ, Ⅱ, Ⅲ)

(Ⅱ) 16

DHS32

(Ⅰ, Ⅱ, Ⅲ)

(Ⅱ) 35

32

(Ⅰ, Ⅱ, Ⅲ)

5(

f

18

–

, Ⅲ)

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(Ⅰ, Ⅱ, Ⅲ)

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(Ⅰ, Ⅱ)

11

Pr

KL76

5 (Ⅰ

60

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15

Rt, Ilm, Zrn, Ap, Ttn

7( Ⅰ, Ⅱ, Ⅲ)

Rt, Ilm, Zrn, Ap, Ttn

15 Zrn, Ap, Ttn,

(Ⅰ, Ⅱ, Ⅲ Ilm )

45 (Ⅰ, Ⅱ, Ⅲ)

3( Ⅱ, Ⅲ)

25 (Ⅰ, Ⅱ, Ⅲ)

10 Zrn, Ap, Ttn,

(Ⅰ, Ⅱ, Ⅲ Ilm )

Grt-BtHbl gneiss 15 DHS43

15 –

(Ⅰ, Ⅱ)

(Ⅱ, Ⅲ)

10 (Ⅰ, Ⅱ, Ⅲ

10 (Ⅰ, Ⅱ, Ⅲ)

)

40 (Ⅰ, Ⅱ, Ⅲ

Mag, Chl, Ep, Zrn, Ap

)

Grt-HblBt gneiss DHS06

12

–

8(

23

10

40

Zrn, Ap

Journal Pre-proof (Ⅰ, Ⅱ)

Ⅱ, Ⅲ)

(Ⅱ, Ⅲ)

(Ⅱ, Ⅲ)

(Ⅰ, Ⅱ, Ⅲ )

10 DHS17

5( –

(Ⅰ, Ⅱ)

Ⅱ)

20 (Ⅱ, Ⅲ)

15 (Ⅰ, Ⅱ, Ⅲ)

45 (Ⅰ, Ⅱ, Ⅲ

Ilm, Zrn, Ap

) Note: Ⅰ, Ⅱ , Ⅲrepresent minerals formed at prograde (M1), peak (M2), and retrograde(M3)

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metamorphic stages, respectively.

Journal Pre-proof Table 2. Metamorphic P-T conditions retrieved for different metamorphic stages of metabasite and paragneiss samples from the Kalatashitage terrane, western Dunhuang Orogenic Belt

Prograde assemblage (M1)

7 18 D 82

16

HS24 D HS17

50

H PQ

7. 9

6

43

bl-PT

4

7

.2

CPQ

8

23

.7

52

9. 2

7 31

8. 6

7 38

10 .1

PQ

7

H PQ

8. 1

H PQ

6

Ti

75

G BPQ

H

8.

7

G

PQ

8

81

H

6.

7

G

BPQ

5.

7

38

H PQ

5

86

H PQ

6

91

ethod

7.

7

G

HPQ

9

14

M

7.

8

G

BPQ

kbar)

49

G

CPQ 11

7

D

13

.4

8

H

HS06

16

P(

7

G

CPQ

98

G

bl-PT

14

8

(℃)

G CPQ

.6

18

BPQ

5.

8

H

7.

6

D

PQ

2

89

H

6.

6

D HS43

6

73

.9

CPQ

ethod

15

G

7.

6

D HS32

.6 6

D HS33

12

9 19

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HS46

PQ

kbar)

f

.9

H

T

M

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38

11

(℃)

P(

pr

6

ethod

assemblage(M3)

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K

kbar)

T

M

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(℃)

P(

Pr

T

mple

Retrograde

(M2)

rn

Sa

L76

Peak assemblage

-in-Bt 7

33

5. 4

H PQ

Geothermobarometry symbols: GCPQ, the garnet-clinopyroxene (GC) geothermometer (Ravna, 2000) coupled with the garnet-clinopyroxene-plagioclase-quartz (GCPQ) geobarometer (Eckert et al., 1991); GBPQ, the garnet-biotite (GB) geothermometer (Holdaway, 2000) in concert with the garnet-biotite-plagioclase-quartz geobarometer (Wu et al., 2004); GHPQ, the hornblende-plagioclase geothermometer (Holland and Blundy, 1994) combined with the garnet-hornblende-plagioclase-quartz geobarometer (Dale et al., 2000).

Journal Pre-proof HPQ, hornblende-plagioclase geothermometer (Holland and Blundy, 1994) paired with hornblende-plagioclase geobarometer (Molina et al.,2015); Hbl-PT, the hornblende monomineralic geothermobarometers (Gerya et al., 1997);

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Ti-in-Bt, Ti-in-biotite geothermometer (Wu and Chen, 2015).

Journal Pre-proof

Highlights 

Three stages of metamorphic mineral assemblages have been found in the Kalatashitage area



These rocks record clock-wise P-T paths



The peak and retrograde events occurred at ca. 430–420 Ma and ca. 400–390 Ma, respectively Diverse subduction and exhumation were confirmed in NW Dunhuang Orogenic

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Belt, NW China

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

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13