Multistage anatexis during tectonic evolution from oceanic subduction to continental collision: A review of the North Qaidam UHP Belt, NW China

Earth-Science Reviews 191 (2019) 190–211

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Multistage anatexis during tectonic evolution from oceanic subduction to continental collision: A review of the North Qaidam UHP Belt, NW China

T

Shengyao Yua,b, , Sanzhong Lia,b, Jianxin Zhangc, Yinbiao Penga, Ian Somervilled, Yongjiang Liua,b, Zhengyi Wanga, Zhuofan Lia, Yong Yaoa, Yan Lia ⁎

a

Key Lab of Submarine Geosciences and Prospecting Techniques, MOE, Institute for Advanced Ocean Study, College of Marine Geosciences, Ocean University of China, Qingdao 266100, China b Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qngdao 266061, China c Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China d UCD School of Earth Sciences, University College Dublin, Belfield, Dublin 4, Ireland

ARTICLE INFO

ABSTRACT

Keywords: North Qaidam UHP belt Multiple anatexis Early Paleozoic Oceanic subduction Continental collision

The North Qaidam ultra-high pressure (UHP) metamorphic belt in the northern Tibetan Plateau is considered as a typical Alpine-type UHP metamorphic belt due to the Early Paleozoic subduction of the Qaidam Block beneath the Qilian Block. The well-preserved Paleozoic metatexite migmatite, diatexite migmatite, felsic sheet/dyke and anatectic granite in the North Qaidam UHP metamorphic belt provide us with an excellent opportunity to study the generation, transport and final fate of crustal melt and geodynamic processes associated with orogenesis. Based on structural relationships, petrology, geochronology, whole-rock geochemistry and SreNd isotope data, the Early Paleozoic anatexis during oceanic subduction to continental collision in the North Qaidam UHP metamorphic belt can be further divided into three stages of development ~470 Ma, 446–428 Ma and 432–420 Ma. The first-stage granitic leucosomes are rich in K, poor in Na with low Sr/Y and enrichment of large ion lithophile elements (LILEs), which were probably derived from partial melting of ancient felsic gneiss in continental arc circumstances during oceanic subduction. The second-stage Na-rich leucosomes and tonalite plutons are characterized by high Na, Sr, Sr/Y and La/Yb and low heavy rare earth elements (HREEs), with positive εNd(t) values of 0.1–4.3 and zircon εHf(t) values of 8.3–10.2, similar to typical tonalite-trondhjemite-granodiorites (TTGs). These TTG-like magmas were produced through partial melting of newly emplaced gabbroic rocks with arc affinity under high-pressure (HP) granulite-facies conditions in a thickened lower crust during continental collision. The volumetrically significant migrated plutons evolved from TTG-like melt will become segments of continental crust and contribute to crustal growth, with partial residual products of HP granulite and/or garnet pyroxenite to the mantle by delamination. The third stage of anatexis was preserved in both migmatitic UHP gneiss and eclogite in the Xitieshan and Luliangshan terranes. The Kfs-rich felsic leucosomes inside UHP gneisses in the third stage are characterized by high alkali contents and low mafic component with FeOT + MgO + TiO2 < 2%. In trace element distribution diagrams, these Kfs-rich leucosomes exhibit parallel patterns to their host gneisses but lower element contents and slightly positive Eu and Sr anomalies. The Pl-rich leucosomes within the retrograde eclogite have geochemical features as follows: (1) rich in CaO, Na2O and poor in K2O, with Na2O/K2O ratios > 2.0; (2) high La/Yb and Sr/Y, and low Y and HREEs; (3) high Al2O3 and low Mg# values; and (4) enriched in LILEs(e.g., Rb, Ba, K, Sr, Pb) and poor in high field strength elements (HFSEs). Partial melting of UHP eclogite and felsic gneiss during the initial retrogression stage with Pl-rich and Kfs-rich leucosome formation was triggered by dehydration melting involving predominant zoisite and muscovite. The anatectic melts from partial melting of deeply subducted UHP gneiss have accumulated and migrated outside the deeply subducted crustal slice in the form of syn-collisional plutons. The melt evolution from the leucosomes produced at the early exhumation stage to syn-collisional granitoids produced at the late exhumation, might contribute greatly to the exhumation of the North Qaidam UHP metamorphic belt from mantle depths to the lower crustal levels.

⁎ Corresponding author at: Key Lab of Submarine Geosciences and Prospecting Techniques, MOE, Institute for Advanced Ocean Study, College of Marine Geosciences, Ocean University of China, Qingdao 266100, China. E-mail address: [email protected] (S. Yu).

https://doi.org/10.1016/j.earscirev.2019.02.016 Received 24 April 2018; Received in revised form 4 December 2018; Accepted 14 February 2019 Available online 18 February 2019 0012-8252/ © 2019 Elsevier B.V. All rights reserved.

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1. Introduction

subduction/collision because most of UHP metamorphic rocks have experienced extensive retrograde reactions and re-equilibration during exhumation (Zheng et al., 2011). Partial melting of deep continental crust may occur during prograde heating, the peak stage or decompression (Whitney et al., 2004, and references therein). Thus, microstructural analysis and direct dating of felsic leucosomes formed by partial melting in an orogen are therefore crucial for understanding the relationships among partial melting, metamorphic evolution and orogenic processes. The Early Paleozoic North Qaidam UHP metamorphic belt (NQD) is widely considered to result from continental collision between the Qilian Block and the Qaidam Block at 460–420 Ma (Li et al., 2017). Abundant leucosomes or quartzo-feldspathic dykes have been recognized in UHP eclogite and gneiss, HP/MP granulite, and amphibolite-facies gneiss/metabasite, which are ideally suited to decipher multistage anatexis during orogenesis. Based on geochemical and geochronological data of leucosomes, one school of thought proposes that partial melting occurred during exhumation of subducted oceanic crust or continental crust (Chen et al., 2012a, b; Liu et al., 2014; Song et al., 2014a, b; Yu et al., 2015a; Yu et al., 2015b; Zhang et al., 2015; Cao et al., 2017). However, field relationships, microscopic petrology and geochemistry of leucosome and residuum in the Dulan terrane suggest that partial melting also occurred in the thickened lower crust during the UHP metamorphic stage (Yu et al., 2012, 2014). Although the timing and mechanism of partial melting is still controversial, the wellpreserved metatexite migmatite, diatexite migmatite, felsic sheet/dyke and anatectic granite provide us with an excellent opportunity to study the generation, transport and final fate of crustal melt and geodynamic processes associated with orogenesis. In this contribution, we present an overview of multistage anatexis together with some new data in the North Qaidam Belt in order to: (1) evaluate the formation mechanism of multistage partial melting, melt extraction and syn-collisional granite emplacement, and (2) understand the relationships among partial melting, metamorphic evolution and orogenic processes. Integrated studies on migmatites, HP-UHP metamorphic rocks and magmatic rocks also allow us to profoundly understand and reconstruct the tectonic history of the North Qaidam UHP belt during the Early Paleozoic.

Partial melting is a common geological phenomenon in high-grade metamorphic orogens that strongly influences the thermal and rheological behavior of orogenic crust (Andersson et al., 2002; Whitney et al., 2003) and may contribute to orogenic collapse and is responsible for the geochemical differentiation of continental crust (Rey et al., 2001; Vanderhaeghe and Teyssier, 2001; Sawyer et al., 2011). The generation, transport and final fate of crustal melts are controlled by tectonic forces (Brown, 2001; Vanderhaeghe and Teyssier, 2001) and thus may have different dynamic and temporal relationships with tectonic events during orogenesis (Keay et al., 2001; Whitney et al., 2003). Partial melting is closely linked to (1) high-temperature metamorphism at relatively deep crustal levels under thickened crustal conditions (England and Thompson, 1984; Patiño Douce et al., 1990; Thompson and Connolly, 1995; Aikman et al., 2008; Zeng et al., 2009), (2) decompressional heating and melting associated with rapid exhumation of high-grade metamorphic rocks (Harris and Massey, 1994; Patiño Douce and Harris, 1998; Ganguly et al., 2000), (3) heating of metasediments that are highly enriched in radiogenic elements (U, Th and K) (Chamberlain and Sonder, 1990; Searle and Szulc, 2005) or (4) fractional heating associated with rapid movement along major boundary faults (Harrison et al., 1997). In recent years, hydrous silicate melts by dehydration-driven in situ partial melting constrained from experiments and natural rocks have been increasingly recognized in many UHP metamorphic terranes, e.g., the Kokchetav UHP terrane of Kazakhstan, the Sulu-Dabie UHP terrane of China, the Western Gneiss Region of Norway and the North Qaidam Mountains (Hermann et al., 2001; Labrousse et al., 2011; Zheng et al., 2011; Chen et al., 2012a, 2012b; Yu et al., 2012, 2014, 2015a, 2015b; Song et al., 2014a, 2014b; Zhang et al., 2015). Partial melting of UHP rocks has great effects on the tectonothermal evolution and lithospheric rheology of continental collisional zones, which will lead to the generation of a two-phase medium allowing melt–solid segregation, and thus plays a crucial role in accelerating the exhumation of UHP slabs and triggering crust-mantle interaction or syn-exhumation magmatism (Labrousse et al., 2011; Zhao et al., 2012). However, it is difficult to determine explicitly when the incipient melting occurred during 40 N 90 E

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Fig. 1. Geological sketch map showing the geological setting of the North Qaidam Mountains (modified after Zhang et al., 2017). 191

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2. Regional geologic setting

isoclinal folds. A locally developed sub-horizontal to shallowly northwest-plunging lineation trends to the northwest (Mattinson et al., 2007, 2009). Song et al. (2003) subdivided the DLU into the South Dulan Belt (SDB) and the North Dulan Belt (NDB), based on the post-peak HP granulite-facies overprinting of the SDB eclogite. However, the new data suggest that the eclogites from the two sub-belts experienced a similar metamorphic history without HP granulite-facies overprinting (Zhang et al., 2009b, 2010). Moreover, a HP granulite unit, which predominantly consists of mafic HP granulite, paragneiss, orthogneiss and amphibolite, was recognized in the western margin of the SDB (Fig. 2a). The HP granulite unit is presumed to be in fault contact with the eclogite-bearing unit (Yu et al., 2009, 2011b, 2014). The existing mapping shows that the HP granulite only outcrops to the westnorth of the fault whereas the eclogite is restricted to the southeast. The peak metamorphic P-T conditions of the HP granulites are 1.4–1.8 GPa and 800–950 °C (Yu et al., 2011b), which is distinct from those of eclogite with peak metamorphic conditions of 2.6–3.5 Ga and 600–760 °C (Zhang et al., 2010). The Wulan Complex, located at the northern margin of the Dulan eclogite-gneiss terrane, consists of granitic gneiss, perlitic gneiss, quartzite and marble, with minor enclosed amphibolite and mafic granulite in the felsic gneiss. The Precambrian metamorphic basement of the Wulan Complex is composed of the Delingha Complex, the Dakendaban Group and the Wandonggou Group (Fig. 2b), which are in tectonic contact (Lu et al., 2002). The Delingha Complex mainly consists of Paleoproterozoic granitic gneiss and enclosed amphibolite and mafic granulite (Lu et al., 2002; Gong et al., 2012; Yu et al., 2017). Abundant zircon UePb age data suggest that these granitic gneisses formed between 2.47 and 2.35 Ga (Lu et al., 2002, 2008; Gong et al., 2012). The Dakendaban Group can be further divided into lower and upper sub-groups. The lower Dakendaban sub-group, which is composed of a set of high amphibolite-facies volcano-sedimentary rocks, is located in the northern part of Wulan and Delingha (Chen et al., 2012a, 2012b), whereas the upper sub-group is mainly exposed in the Delingha region and comprises supracrustal rocks similar to the khondalites (Chen et al., 2012a, 2012b). The Wandonggou Group is composed of greenschist-facies metasedimentary rocks and minor mafic metavolcanic rocks with metamorphic age of 1022 ± 64 Ma (Yu et al., 1994). The Wulan Complex also experienced granulite-facies metamorphism and arc magmatism in the Early Paleozoic. The peak metamorphic conditions of a sillimanite-garnet-bearing paragneiss were constrained at 677–696 °C and 3.5–4.2 kbar during 483–450 Ma (Li et al., 2015). The Early Paleozoic magmatic rocks predominantly include gabbro, gabbro-diorite and granite (Kang et al., 2015; Sun et al., 2015).

The northwest–southeast-trending North Qaidam UHP Belt, located along the northern margin of the Qinghai-Tibet Plateau (Fig. 1), is a typical Alpine-type UHP metamorphic zone due to the Early Paleozoic subduction of the Qaidam Block beneath the Central Qilian Block (Zhang et al., 2016, 2017). The North Qaidam UHP Belt mainly consists of paragneiss, orthogneiss, with minor intercalated, blocks or lenses of eclogite, mafic granulite and garnet peridotite, similar to most of the continental-type UHP terranes worldwide (e.g., the Dabie-Sulu, the Western Gneiss Region). They thrust over, or are overlain by, Lower Paleozoic volcanic and sedimentary rocks, and are intruded by granitic plutons. The eclogites exhume from > 550 km depth near the localities of Dulan, Xitieshan, Luliangshan and Yuka and record metamorphic ages between 423 and 460 Ma (Yang et al., 2002, 2006; Song et al., 2003, 2006, 2014b; Zhang et al., 2005, 2006; Zhang et al., 2008a, 2008b, 2009a, 2010, 2011, 2016, 2017; Mattinson et al., 2006, 2007; Yu et al., 2013). Garnet peridotite outcrops in the Luliangshan area and appears to have been exhumed from mantle depths > 200 km (Song et al., 2004, 2005; Xiong et al., 2011). Based on rock association, petrologic criteria, and field relationships, four UHP metamorphic subunits can be distinguished along the North Qaidam UHP Belt from east to west (Zhang et al., 2008a). (1) The Dulan eclogite-gneiss terrane (DLT) consists of granitic gneiss, paragneiss, eclogite and ultramafic lenses that are enclosed within gneiss. (2) The Xitieshan eclogite-gneiss terrane (XTT) is dominated by kyanite- and sillimanite-bearing paragneiss (schist) and orthogneiss with rare marble and amphibolite and is intruded by granite plutons that have been dated to 428 Ma (Meng et al., 2005). (3) The Luliangshan garnet peridotite-gneiss terrane (LLT) is defined by sillimanite-bearing paragneiss and orthogneiss, with ultramafic rocks (garnet peridotite and garnet pyroxenite) as lenses, and is intruded by Silurian granite plutons (Zhang et al., 2008a). (4) The Yuka eclogite-gneiss terrane (YKT) comprises eclogite, metapelite, orthogneiss and rare marble. The coesite inclusions and coesite pseudomorphism have been identified in zircon, omphacite and garnet from metapelites and eclogites in the Dulan, Xitieshan and Yuka terranes, indicating the subduction of continental crust to a depth > 80 km (Yang et al., 2002; Song et al., 2003; Zhang et al., 2009a, 2009b, 2010; Yu et al., 2013). The microdiamond inclusions in zircon and garnet exsolution structures have also been observed in garnet peridotite from the Luliangshan Terrane, suggesting that both supracrustal rocks and metabasite were subjected to UHP metamorphism (Song et al., 2004, 2005). The eclogites from the various UHP terranes record similar MT (670–800 °C) and UHP (27–33 kbar) peak metamorphic conditions (Song et al., 2003, 2006; Chen et al., 2009; Zhang et al., 2009a; Zhang et al., 2009b; Zhang et al., 2010; Zhang et al., 2011). However, the different UHP terranes exhibit two types of disparate retrograde P-T paths. The UHP rocks at the Yuka and Dulan terranes record near-adiabatic decompression to amphibolite-facies conditions (Zhang et al., 2005, 2009a, 2009b; Chen et al., 2009). In contrast, the eclogites at the Xitieshan and Luliangshan terranes experienced high-temperature (HT) granulite-facies overprint during decompression (Song et al., 2014b; Zhang et al., 2017).

2.2. Xitieshan eclogite-gneiss terrane (XTT) The Xitieshan eclogite-gneiss terrane is bounded by thrusts with the Tanjianshan Group, greenschist- to amphibolite-facies metamorphosed sequence of Early Paleozoic arc-related volcanic-sedimentary rocks (Fig. 2c). The Xitieshan Terrane mainly consists of garnet-kyanite ( ± sillimanite) - biotite paragneiss and granitic gneiss intercalated with lenses of eclogite and ultramafic rocks. The predominant deformation fabrics are SSE–NNW-trending foliations (S1) and subhorizontal stretching lineations that are defined by some oriented sillimanite and biotite in paragneisses, suggesting high-temperature deformation (Zhang et al., 2008a). Two types of eclogite including bimineralic and phengite eclogites have been recognized in the Xitieshan Terrane, which occurs as boudins/ lenses or interlayer within granitic gneiss and paragneiss. In contrast to other UHP terranes, fresh eclogites in the Xitieshan Terrane are rarely preserved in the center of large-scale blocks. From the center to rim of large-scale mafic boudins, it is easy to recognize the transition from fresh eclogite to (garnet) amphibolite via garnet granulite. Rare coesite inclusions have been found in zircons from eclogites (Liu et al., 2012a). The peak UHP conditions for eclogites

2.1. Dulan eclogite-gneiss terrane (DLT) The Dulan eclogite-gneiss terrane is located approximately 30 km northeast of Dulan Town, which contains ortho- and paragneiss (schist) enclosing eclogite lenses and minor ultramafic rocks, and was intruded by ca. 400 Ma granite plutons (Fig. 2a; Wu et al., 2004; Yu et al., 2011a). The coesite inclusions in zircon grains from metapelite and eclogite support that the North Qaidam is a typical continental subduction complex exhumed from > 80 to 100 km depths (Yang et al., 2001, 2002; Song et al., 2003; Zhang et al., 2009a, 2009b, 2010). An amphibolite-facies foliation in the gneiss strikes northwestward and dips steeply to the northeast, which exhibits a modification by tight to 192

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Fig. 2. Schematic geological map showing major litho-tectonic units in the Dulan Terrane (a), Wulan Complex (b), Xitieshan Terrane (c), Luliangshan Terrane (d) and Yuka Terrane (e).

were constrained at 2.7–3.2 GPa and 750–790 °C by the relict peak UHP mineral assemblage garnet + omphacite + phengite + rutile and rare coesite pseudomorphs in omphacites (Zhang et al., 2011; Liu et al., 2012a).

430 ± 8 Ma through early crust subduction and subsequent thermal relaxation and exhumation (Meng and Zhang, 2008). The ultramafic complex occurs as irregular lenses (700 m in length × 250 m in width) in the gneisses, being dominated by garnet lherzolite with minor interlayered dunite and crosscutting garnet pyroxenite dykes (Yang and Deng, 1994; Song et al., 2004, 2005; Zhang et al., 2008a). Mineral exsolution lamellae of rutile + two pyroxene + sodic amphibole in garnet and ilmenite + Al-chromite in olivine (Song et al., 2004, 2005), as well as the presence of diamond inclusions in zircon, suggest that this peridotite body experienced ultrahigh pressure metamorphism at depths in excess of 200 km. The peak metamorphic conditions of 45–65 kbar and 980–1130 °C have been calculated for the garnet peridotite (Song et al., 2004, 2005). The mafic granulite lenses, ranging in

2.3. Luliangshan peridotite-gneiss terrane (LLT) The Luliangshan garnet peridotite-gneiss terrane is distributed ~20 km south of Da Qaidam Town (Fig. 2d), and mainly consists of kyanite/sillimanite-bearing paragneisses, orthogneiss, mafic granulite lenses and an ultra-mafic complex. This terrane is intruded by granitic plutons on the western side, which were suggested to have formed by partial melting of the HP-UHP rocks from the middle and lower crust at 193

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length from < 1 m to tens of meters, are mainly composed of garnet, clinopyroxene, orthopyroxene, plagioclase, amphibole, and quartz, with minor rutile, ilmenite, and zircon. Petrographic data and P-T calculations suggest a HP granulite-facies stage (10–14 kbar and 720–830 °C), followed by a medium-pressure granulite-facies stage (6.2–8.5 kbar and 730–850 °C) (Zhang et al., 2008a). In addition, a possible earlier eclogite-facies stage (P > 15 kbar) is inferred, based on rare omphacite inclusions within garnets in mafic granulites enclosed in paragneisses (Zhang et al., 2007; Cao et al., 2017). Recently, fresh eclogite has also been recognized in the LLT (Cao et al., 2017; Chen et al., 2008). The eclogite experienced at least three metamorphic stages with peak metamorphic P-T conditions of P > 3.0GPa and T = 647–768 °C (Chen et al., 2018).

of garnet-cumulate and/or meta-ultramafic, which are considered to be the segregated neosomes consisting of leucosome and surrounding residue (or melanosome) and are interpreted as principal evidence for partial melting (Fig. 3g). Pre-partial-melting structures, such as foliation and bedding, occur only in the scattered schollen of paleosome or residue. The tonalite pluton is an elongated, irregular-shaded body that is 1–2 km wide and 7–8 km long and intrudes into amphibolite and felsic gneiss with clear, fine-grained chilled margins (Fig. 3h). Local mafic enclaves have been observed in the tonalite pluton, which are angular to oval in shape and range from sub-millimeter to tens of centimeters in size (Fig. 3i). The main minerals in the mafic enclaves are garnet, clinopyroxene, plagioclase, quartz, and rare zoisite and rutile, similar to the melanosome in the migmatite. Discordant dyke-like bodies or patches of the tonalite pluton that range from several centimeters to a few tens of meters wide crosscut the foliation and stromatic layering at variable angles. These discordant tonalite bodies exhibit petrographic continuity with millimeter- to centimeter-scale foliation-parallel leucosome, which suggests that the material in the leucosomes and the discordant tonalites crystallized at the same time and formed parts of the same melt extraction network (Yakymchuk et al., 2013). The paleosome shows a porphyroblastic texture and consists of plagioclase and clinopyroxene, amphibole, orthopyroxene, quartz, and rare garnet and rutile (Fig. 4a). The melanosome is mainly composed of garnet + clinopyroxene + plagioclase + quartz + rutile, with or without minor kyanite, zoisite, amphibole and accessories of zircon (Fig. 4c and d). Highly cuspate plagioclase, K-feldspar and quartz occur as isolated pockets between garnet-clinopyroxene boundaries, which could be interpreted as pseudomorphs of former melts (Fig. 4c and d). The peak P-T conditions of the melanosome have been constrained at 1.4–1.8 GPa and 800–950 °C (Yu et al., 2011b, 2014; Song et al., 2014a). Three types of felsic leucosome have been identified based on field relationships, mineral assemblage and geochemistry (Yu et al., 2011b, 2014). The garnet-bearing leucosome is composed of mediumto fine-grained plagioclase, K-feldspar, quartz, garnet and rutile, with or without kyanite, and minor zoisite/clinozoisite, amphibole, and scapolite (Fig. 4e). In the diatexite migmatite, elongated, irregular plagioclase and quartz grains occur in the form of veinlets along the garnetgarnet boundaries and exhibit low garnet-plagioclase-garnet dihedral angles (Fig. 4g). Polycrystalline inclusions of plagioclase and quartz were also recognized in the core of host garnet (Fig. 4h), which have been interpreted as the products of incongruent reactions that produced an anatectic melt that was trapped by growing peritectic minerals (Cesare et al., 2009). The dyke-like bodies or patches and massive tonalite plutons show similar microscopic texture and mineral assemblage, which are medium-grained and typically consist of plagioclase, K-feldspar, quartz, muscovite, biotite, amphibole and rare zircon and epidote (Fig. 4i)(Yu et al., 2012).

2.4. Yuka eclogite-gneiss terrane (YKT) The Yuka eclogite-gneiss terrane is situated at the northwestern end of the North Qaidam UHP Belt, which mainly consists of granitic gneisses and pelitic schists/gneisses that are in fault contact with the Early Paleozoic island arc volcanic rocks of the Tanjianshan Group, dunites and Cambrian gabbros in the east (Fig. 2e). Two types of eclogite have been recognized as layer- or lens-shaped blocks and boundinaged dykes within granitic and pelitic schists/gneisses (Chen et al., 2005; Menold et al., 2009; Ren et al., 2017). The coarse-grained phengite eclogite is mainly composed of garnet +ompacite + phengite + rutile + quartz, whereas the fine-grained massive eclogite mainly consists of garnet +ompacite + rutile +quartz. Lawsonite (pseudomorph)-bearing eclogite has been identified, which indicates that its protolilth is likely to be an oceanic crustal slice (Ren et al., 2017). Traditional thermobarometers and phase equilibrium modeling predict that the eclogite experienced a clockwise P-T evolution with a peak stage at 570–680 °C, 27–34 kbar, near or slightly higher than the quartz–coesite transition boundary. The coesite inclusions in garnet from the coarse-grained eclogite and the calculated P-T paths suggest that the Yuka UHP Terrane experienced deep subduction (~100 km) at a lower geothermal gradient (6–7 °C/km) (Zhang et al., 2009c). The garnet-kyanite bearing pelitic gneisses followed a metamorphic P-T path similar to that of the enclosed eclogite, with peak conditions of 615–700 °C and 2.3–3.1GPa (Zhang et al., 2004). 3. Field and microscopic petrology of anatexis 3.1. Dulan eclogite-gneiss terrane (DLT) 3.1.1. Migmatite in Yematan HP granulite unit In the DLT, the felsic leucosome generally appears as thin layers, veinlets or small leucocratic patches oriented sub-parallel to the main foliation within the meta-gabbro and HP mafic granulite (Fig. 3a and b). A few large outcrops show arrays of interconnected shear band structures that contain leucosome and scattered garnet (Fig. 3c). These arrays are interpreted to be remnants of the network of channels through which melts escaped (Sawyer, 2001). In this case, although the anatectic melt has migrated away from the position of generation and accumulated in larger scale, more stable low-pressure sites, but still within the confines of its source layers. In some outcrops, the segregated and unsegregated neosome were identified, where the garnetbearing leucosome did not migrate far from its source layer (Fig. 3d). On this occasion, in situ leucosome, residue and unsegregated neosomes could be distinguished clearly. The leucosome in the stromatic metatexite migmatite flows and becomes folded together with rootless isoclinal folds of residue (Fig. 3e), indicating that partial melting and deformation were contemporaneous, with the melt enhancing the deformation, and the deformation aiding the concentration of melt in lowstress region. Peritectic garnets up to 2–3 cm occur in both leucosome and melanosome components of the nebulitic diatexite migmatite (Fig. 3f). The felsic components locally contain lenses, blocks or layers

3.1.2. Migmatite in the Wulan arc complex In the Wulan arc complex, felsic leucosome generally appears as thin layers, veinlets or small leucocratic patches oriented subparallel to the main foliation of both biotite-amphibole orthogneiss and paragneiss (Fig. 5a). The veinlet-like leucosomes have migrated away from the position of generation and have accumulated in larger scale in the form of sheets or dykes (Fig. 5b), but still within the source. Highly cuspate, irregular elongated feldspar and quartz grains distribute along the boundaries between plagioclase and quartz (Fig. 5c), which are also interpreted as pseudomorphs of the former melt (Sawyer, 2010). The felsic leucosome is composed of medium- to coarse-grained plagioclase, K-feldspar and quartz, with minor amphibole, biotite and zircon (Fig. 5d). The melanosome is composed of garnet, sillimanite, biotite, plagioclase, K-feldspar, quartz, and rare rutile and zircon (Fig. 5e). The mafic granulite in studied area shows porphyroblastic texture, and consists of clinopyroxene, orthopyroxene, amphibole, plagioclase, quartz and rare rutile (Fig. 5f). 194

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Metagabbro

Leu

coso

me

Leucosome

Le uc os om e

Neosome

a Resid

Leucosome

uum

Neosome Le

Ne

os

uc

om

os

c

b

Leucosome Diatexite migmatite

om

e

e

d

e

f

Leucosome Tonalite pluton

Tonalite

Residuum

g

h

Mafic granulite

i

Fig. 3. Field relationships of metagabbro (Paleozoic), HP granulite (residue) and TTG-like leucosome and tonalite pluton in the Dulan Terrane. (a) Leucosome occurs as patch or vein in the meta-gabbro. (b) Thin bands of garnet-bearing leucosome within the stromatic metatexite migmatite is oriented parallel to the major plane of anisotropy in the melanosome. (c) Scattered coarse-grained garnet and leucosome forming arrays of interconnected shear band structures. (d) Textural relations of leucosome, residuum, paleosome and unsegregated neosome. (e) The leucosome in the stromatic metatexite migmatite flows and becomes folded together with rootless isoclinal folds of the residue. (f) Coarse-grained euhedral-subhedral garnet grains within the diatexite migmatite. (g) Blocks or lenses of HP granulite residue surrounded by quartz-feldspathic leucosome. (h) Discordant dyke-like bodies or patches of the tonalite pluton crosscut the amphibolite. (i) HP mafic granulitic enclaves in the tonalite pluton.

Felsic sheets or dykes that range from several decimeters to a few meters wide crosscut the foliation and stromatic layering at variable angles (Fig. 6f). Microscopic texture related to partial melting could be recognized in both migmatized metabasite and gneiss. In the metabasite, elongated quartz and/or feldspar grains occur like veinlets along the phengitephengite and phengite-clinopyroxene (garnet) grain boundaries, showing low dihedral angles (Fig. 6g). In the core of the garnet, polymineral inclusions of muscovite + feldspar + quartz could represent crystallization of a leucogranitic magma droplet trapped by garnet during growth (Yu et al., 2015a). Irregularly shaped feldspar and quartz has been recognized between garnet and clinopyroxene (Fig. 6h). These microstructures are interpreted as trapped melt pockets that crystallized within the residual melanosome, and they provide strong evidence for in situ partial melting (Sawyer, 2008; Johnson et al., 2013). In the migmatitic gneiss, phengite is rimmed by biotite, feldspar and quartz (Fig. 6i and j). Cuspate, wedge-shaped pockets consisting of K-feldspar + quartz + plagioclase occur between quartz and plagioclase boundaries and indicate that the microscale migration of the melt was perhaps limited to several millimeters (Fig. 6k) (Sawyer, 2008; Holness et al., 2011). Similarly wedge-shaped pockets diagnostic of anatexis were also reported in anatectic UHP gneiss from the Sulu orogen and anatectic quartzite from the Scottish Highlands (Holness and Clemens, 1999; Chen and Zheng, 2013). Felsic veinlets composed of quartz, feldspar and biotite were recognized across the boundaries of feldspar and quartz, which may represent melt channels (Fig. 6l).

3.2. Xitieshan eclogite-gneiss terrane (XTT) In the large mafic boudins, quartz-feldspathic leucosomes (Pl-rich leucosomes) generally occur as numerous thin layers, veinlets or patches in the metabasite layers (Fig. 6a).The proportion of thin Pl-rich leucosomes varies greatly from outcrop to outcrop, ranging in thickness from millimeters to decimeters. At the regions of contact between eclogite and leucocratic veins, the eclogite is generally retrograded into HP granulite or garnet amphibolite. The Pl-rich leucosomes are generally parallel to the foliation in the metabasite. Euhedral-subhedral garnet grains within the leucosomes are commonly larger than those in the melanosome (Fig. 6b). Locally coarse-grained patches consist of plagioclase + quartz ± K-feldspar commonly enclosing large crystals of pink garnet or occasionally clinopyroxene. These patches are interpreted to be in situ neosome consisting of leucosome surrounding peritectic phases formed by partial melting (Yu et al., 2015a). The felsic gneiss corresponds mainly to lit-par-lit metatexites, grading locally to diatexites. The leucosomes generally occur as numerous thin veinlets, layers, or patches in the metatexite migmatite, which are mostly aligned subparallel to the gneissosity or foliation (Fig. 6c). The leucosome in the stromatic metatexite migmatite flows and becomes folded and concentrates in the low-stress region (Fig. 6d). The diatexite migmatites have undergone a textural homogenization that has destroyed the primary centimeter-scale bedding typical of the felsic gneiss (paleosome) and metatexite. Pre-partial-melting structures, such as foliation and bedding, occur only in the scattered centimeterscale mafic schlieren that is dominated by biotite and garnet (Fig. 6e). 195

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0.3 mm

0.3 mm

0.3 mm

Pl Pl

Opx

Amp

Pl Amp Grt

Cpx

Pl

Cpx

a

b

c

0.6 mm

0.3 mm

0.3 mm

Fsp Cpx

Pl

Ky

Zo

Grt

Grt Pl

Grt

e

d 0.3 mm

f 0.3 mm

10µ m

Amp

Grt

Pl

Qtz Fsp

Grt Qtz Qtz Grt

Fsp

g

h

i

Fig. 4. Microphotographs showing the textures of in situ partial melting in the Dulan Terrane. (a) Meta-gabbro is composed of dominantly clinopyroxene, orthopyroxene and plagioclase, with rare amphibole and quartz. (b) Beaded droplets, pudding or tubes of leucosome (plagioclase + quartz) interspersed in the amphibole. (c) and (d) Highly cuspate, elongated plagioclase, K-feldspar and quartz grains between garnet-clinopyroxene boundaries represent pseudomorphs of former melts in mafic granulitic residue. (e) Felsic leucosome contains plagioclase, K-feldspar, quartz, and rare peritectic garnet and kyanite. (f) Relict of zoisite is surrounded by plagioclase and quartz. (g) Vein-like, elongated, irregular plagioclase and quartz grains occur along garnet-garnet boundaries and exhibit low garnet-plagioclasegarnet dihedral angles. (h) Vein-like, elongated, irregular plagioclase and quartz grains occur along garnet-garnet boundaries and exhibit low garnet-plagioclasegarnet dihedral angles in mafic granulite. (i) Tonalite pluton comprised primarily of plagioclase, quartz, K-feldspar, amphibole and rare biotite. The mineral Abbreviations is after Whitney and Evans, 2010: Amp = amphibole, Cpx = clinopyroxene, Fsp = feldspar, Grt = garnet, Ky = kyanite, Opx = orthopyroxene, Pl = plagioclase, Qtz = quartz, Zo = zoisite.

3.3. Luliangshan peridotite-gneiss terrane (LLT)

may be continuous on an outcrop scale. They are commonly subparallel to the foliation and compositional layering (Fig. 7e), but may crosscut these early structures at moderate to steep angles. These felsic sheets or dykes exhibit petrographic continuity (similar mineral assemblage and microstructure) with millimeter- to centimeter-scale foliation-parallel leucosomes (Fig. 7e). The felsic sheets accumulated to form granitic plutons connecting and combining with each other (Fig. 7f). In some retrograde eclogite samples, felsic veinlets, composed of quartz and feldspar, were recognized across the boundaries of garnet, clinopyroxene, amphibole, and clinopyroxene + plagioclase symplectite, which may represent melt channels (Fig. 7g). Similar textures have been reported for mid-temperature, UHP retrograde eclogite from the Sulu Orogen, suggesting microscale transport of silicic melts relative to the rest of the rock and partial melts that did not escape from the rock (Zhao et al., 2007). In the migmatitic gneiss, the coarsegrained phengite is locally rimmed by biotite and quartz (Fig. 7h), and irregular K-feldspar and quartz generally occur on the rims of biotite and muscovite (Fig. 7i). Similar textures have also been recognized in the Dabie Orogen, and those textures were interpreted as the breakdown of phengite to biotite and melt (Xia et al., 2008; Zheng et al., 2011). Highly cuspate, arcuate K-feldspar grains were found wrapping the quartz grains (Fig. 7j). Abundant patches of highly cuspate plagioclase, quartz, and K-feldspar, with/without muscovite, occur as

In the Luliangshan area, felsic veins are observed within the retrograde eclogite (Fig. 7a). The veins, about 5–20 cm in width, are weakly deformed, and confined within the host retrograde eclogite without extending into the country gneiss. At the contact with the felsic vein, the retrograde eclogite is strongly amphibolized into garnet amphibolite (Cao et al., 2017). In the outcrop, the felsic gneiss is mainly composed of garnet, biotite, K-feldspar, plagioclase, kyanite/sillimanite, quartz, rare phengite and amphibole. Outcrops of felsic gneiss throughout this region preserve features diagnostic of anatexis and, in local areas, display strong migmatization. The felsic leucosomes are generally aligned subparallel to the gneissosity or foliation (Fig. 7b), but also locally crosscut these early structures at moderate to steep angles. Peritectic garnets up to 3 cm across occur in both leucosome and melanosome components of diatexite migmatite (Fig. 7c). K-feldsparrich pegmatite generally crosscut the early structures at variable angles, with thickness > 3 cm (Fig. 7d). The grain size of quartz and feldspar in the K-feldspar-rich pegmatite is proportionally coarser (2–3 mm) than that in thinner leucosomes (1–2 mm). The felsic leucosome probably represents earlier crystallization of quartz and plagioclase, whereas the K-feldspar-rich pegmatite represents percolating fractionated melt trapped during cooling. Large sheets are decimeters to meters thick and 196

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a

b

0.3 mm

0.3 mm

Qtz Pl Pl

Kfs Qtz

Fsp+Qtz

c

d

0.3 mm

0.3 mm

Cpx Grt

Bt Sil

Qtz

Opx Pl

e

f

Fig. 5. Field photographs and microtexture of migmatitic gneiss in the Wulan Complex. (a) and (b) Field pictures showing the migmatitic of felsic gneisses. (c) Irregular arcuate K-feldspar (Kfs) grains along boundaries of plagioclase and quartz. (d) Elongated veinlet of quartz + plagioclase between boundaries of plagioclaseplagioclase. (e) The residue is composed of garnet, sillimanite (Sil), biotite, feldspar and quartz. (f) Coeval mafic granulite consists of orthopyroxene, clinopyroxene, plagioclase, amphibole and quartz.

isolated pockets between plagioclase-plagioclase boundaries (Fig. 7k). Elongated felsic veinlets composed of K-feldspar + quartz ± plagioclase ± muscovite, which strongly argue for the existence of felsic melts (Vernon, 2010), further indicate the existence of partial melting (Fig. 7l).

age of 470 ± 3 Ma (MSWD = 0.46) and low Th/U ratios of 0.03–0.12 (Tables 1). Rare inherited zircon cores gave 207Pb/206Pb age ranging from 1836 to 2462 Ma. Sixteen anatectic zircon domains have 176Hf/177Hf ratios ranging from 0.282270 to 0.282334. Their age-corrected εHf(t) values are between −7.4 and − 5.2 at 470 Ma with TDM ranging from 1275 to 1367 Ma and TDMC ranging from 1581 to 1703 Ma (Fig. 8b) (Tables 2).

4. Zircon UePb geochronology and LueHf isotope of multistage anatexis in the NQD UHP metamorphic belt

The second-stage anatexis (446–428 Ma) was distributed in the DLT. The timing of anatexis in the DLT has been documented by SHRIMP and LA-ICP-MS UePb dating of zircon in the felsic leucosomes and HP mafic granulite residue, and a compilation of the reported geochronological data reveals that anatexis mainly occurred between 446 and 428 Ma with a peak of 434 Ma (Fig. 9B) (Yu et al., 2009, 2012, 2013, 2014; Song et al., 2014a), which overlaps the ages of UHP metamorphism ranging from 445 to 430 Ma (Zhang et al., 2010, 2016). The age of ~470 Ma has also been observed in inherited zircon cores of felsic leucosome and HP mafic granulite residue, which records high U and Th concentrations and high Th/U ratios, conforming that they are of magmatic origin. The concordant UePb age of ~470 Ma agrees well with the magmatic crystallization age of gabbros with arc affinity in the Dulan area (Zhu et al., 2010). The LueHf isotopes were analyzed on

Based on field relationships, petrology, geochronology, whole-rock geochemistry and SreNd isotope, the Early Paleozoic anatexis in the NQD can be further divided into three development stages of ~470 Ma, 446–428 Ma and 435–420 Ma (Figs. 8 and 9). The third-stage anatexis at 435–420 Ma is stronger than those at ~470 Ma and 446-428 Ma. 1. The first-stage anatexis (~470 Ma) mainly occurred in the Wulan Complex of the DLT. Zircon grains in the leucosome sample DL15–21-6.1 are subeuhedral to euhedral prismatic crystals. CL images reveal that most zircon grains contain bright, irregular zoning with or without dark, unzoned cores (Fig. 8a). A total of 13 laser ablation spots on zoned rims yielded 206Pb/238U ages ranging from 462 ± 5 to 475 ± 5 Ma, with a weighted mean 206Pb/238U 197

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Eclogite Leucosome Leucosome

Leucosome

Eclogite

b

a

Leucosome

d 200µ m

c

Felsic sheet

e

f

200µ m

200µ m

Bt

Cpx

Qtz

Ms

Ms

Grt Fsp

g

h 100µ m

200µ m

i 200µ m

Bt

Qtz Ms

Fsp+Qtz Kfs

j

k

l

Fig. 6. Field pictures and photomicrographs showing anatexis of the UHP eclogite and felsic gneiss in the Xitieshan Terrane. (a) Field occurrences of felsic leucosomes are parallel to the foliation of retrograde eclogite. (b) Peritectic garnets within the leucosomes are commonly larger than those within the melanosome. (c) Thin layers of leucosomes are aligned subparallel to the gneissosity or foliation. (d) Folding deformation leads to the migration and aggregation of felsic leucosome in migmatite. (e) Mafic schlieren within diatexite migmatite. (f) Array of interconnected leucosomes coalesce in felsic sheet. (g) Elongated veinlets of feldspar (Fsp) grains along boundaries of phengite-phengite and phengite-clinopyroxene. (h) Felsic veinlet composed of feldspar and quartz across the boundary of garnet, amphibole, and clinopyroxene + plagioclase symplectite, representing melt channel. (i) and (j) Phengite is rimmed by biotite, Kfeldspar and quartz. (k) Patch of plagioclase, K-feldspar, quartz occurs as pocket between boundaries of plagioclase-quartz. (l) Melt pseudomorph, now veinlet of feldspar + quartz + biotite, interpreted based on its location between quartz grains with rounded edges.

both magmatic core and anatectic rim of zircons in leucosomes. Primary magmatic zircon cores from the leucosomes have 176Hf/177Hf ratios from 0.282714 to 0.282768, with εHf(t) values between 8.3 and 10.2 at 470 Ma and TDM ranging from 669 to 743 Ma (Fig. 8b). The anatectic rims have 176Hf/177Hf ratios (0.282734 to 0.282877) similar with or slightly higher than those in the magmatic cores, suggesting that the melt might have been sourced from the metagabbros in a closed system without the addition of Hf through external fluids. The εHf(t) values for anatectic rims range from 8.1 to 13.3, and the TDM values range from 518 to 724 Ma. The third-stage anatexis was preserved in both migmatic UHP gneiss and eclogite in XTT and LLT, which exhibit a peak of 426 Ma. Zircons from granitic leucosomes in the UHP gneisses recorded three age clusters with distinct internal textures, mineral inclusions, Th/U ratios and trace-element system (Fig. 9C). The cathodoluminescence-bright inherited cores exhibit clear oscillatory or patchy zoning, high Th/U ratios of > 0.1, steep HREE-MREE patterns and negative Eu anomalies

(Yu et al., 2015b; Zhang et al., 2015). These features indicate that they represent relict magmatic zircon domains. Their UePb analyses yielded protolith ages of 900–950 Ma, which is consistent with a previous protolith age of granitic gneiss during Grenvillian Orogeny when the supercontinent Rodinia assembled (Song et al., 2012; Yu et al., 2013). This is also similar in age to the Jinning Orogeny commonly recognized in the South China Block. In contrast, the dark mantle exhibits no zoning or weak zoning, with low Th/U ratios, flat HREE patterns without Eu anomalies, suggesting that zircon crystallization in equilibrium with garnet and plagioclase was not stable under eclogite facies conditions (Rubatto, 2002). These metamorphic zircon domains yielded eclogite-facies metamorphic ages of 444–449 Ma, which are consistent with the well-documented ages (440–460 Ma) of HP/UHP metamorphism of eclogite previously reported in the NQD UHP terrane (Zhang et al., 2011). The anatectic zircon rims display pronounced oscillatory zoning and mineral inclusions of K-feldspar + plagioclase + quartz and low Th/U ratios, which recorded subsequent anatectic ages 198

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Migmatitic gneiss Eclogite Leucosome

Leucosome

a

b

c

Leucosome

Kfs-rich pegmatite

Granitic pluton

Felsic sheet

d

e

200µ m

f

200µ m

200µ m

Ms

Qtz

vei

n

Bt Kfs

Fel

sic

Bt Ms Pl

g

i

h

200µ m

200µ m

200µ m

Kfs+Pl+Qtz+Ms

Kfs Qtz Phn

Grt

Ms

Kfs

Kfs+Pl+Qtz

Pl

j

k

l

Fig. 7. Field photographs and photomicrographs documenting microstructures, mineral assemblages and former melt textures in the Luliangshan Terrane. (a) Field occurrences of felsic leucosomes in the retrograde, euhedral-subhedral garnet grains within the leucosomes are larger than those in the melanosome. (b) Felsic leucosomes generally occur as thin layers in the stromatic metatexite migmatite. (c) Euhedral-subhedral garnet in the nebulous diatexite migmatite. (d) Kfs-rich pegmatite generally crosscut foliation without obvious direction. (d) The granitic leucosome merges into the felsic sheet with petrographic continuity. (f) Array of interconnected leucosomes in the felsic gneiss coalesce in small pluton. (g) Irregularly shaped films of K-feldspar between garnet and clinopyroxene. (h) and (i) Phengite is rimmed by K-feldspar and biotite, which indicate that the partial melting is related to breakdown of phengite. (j) The phengite (Phn) is rimmed by feldspar and/or quartz in the core of garnet, demonstrating breakdown of phengite. (k) Patches of highly cuspate plagioclase, quartz, and K-feldspar, with/without muscovite, occur as isolated pockets between plagioclase-plagioclase boundaries. (l) Elongated, veinlet-like comprise of K-feldspar + plagioclase + quartz between plagioclase boundaries.

of 421–435 Ma, (Yu et al., 2015b; Zhang et al., 2015). The magmatic zircon cores in leucosome show 176Hf/177Hf ratios of 0.282138–0.282243, with εHf(t) values between −2.5 and 1.2 at 940 Ma and TDM ranging from 1.58 to 1.78Ga. In contrast, the metamorphic mantle has higher 176Hf/177Hf ratios (0.282341–0.282443) and εHf(t) values of −5.5 to −1.2, demonstrating that the metamorphic zircon mainly newly grew from decomposition of other minerals or most of the magmatic zircon was preserved during UHP metamorphism and only a portion of it was dissolved (Yu et al., 2015b). The anatectic rims have LueHf isotopic features indistinguishable from the metamorphic domains with 176Hf/177Hf values of 0.282359–0.282401. These features demonstrate a closed system, in which no Hf was added via external fluids, dissolution or breakdown of the pre-metamorphic cores or hafnium-rich minerals such as garnet and titanite (Flowerdew

et al., 2006; Zheng et al., 2006), which are still the main Hf carrier in the felsic gneiss but were not dissolved during partial melting. Similar to the migmatic gneiss, zircon cores of granitic leucosomes in the retrograde UHP eclogite also recorded eclogite-facies metamorphic ages of 444–452 Ma, and internal textures and mineral inclusions and trace-element system of zircon rims yielded anatectic ages of 422–433 Ma (Chen et al., 2012a; Yu et al., 2015a; Zhang et al., 2015; Cao et al., 2017). The anatectic zircons in felsic leucosome have Hf isotope composition (176Hf/177Hf = 0.282375 ± 0.000021) and εHf(t) values of −5.37 ± 0.75, which is similar to those of the metamorphic domains from the host retrograde eclogite (Cao et al., 2017). The similarity of Hf isotope composition suggests that the melt might be sourced from the retrograde eclogite in a closed system without external Hf adding, and Hf-isotopic homogenization occurred within the 199

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characterized by high alkali contents and low mafic component with FeOT + MgO + TiO2 < 2%, which are mostly plotted in the granite regions on the An-Ab-Or diagram (Fig. 11). In trace element distribution diagrams, these leucosomes exhibit parallel patterns to their host gneisses but lower element contents and slightly positive Eu and Sr anomalies. Geochemical studies demonstrate that the Pl-rich leucosomes within the retrograde eclogite in the XTT and LLT have geochemical features as follows: (1) rich in CaO, Na2O and poor in K2O, with Na2O/K2O ratios > 2.0 (Fig. 10a); (2) high La/Yb and Sr/Y, and low Y and HREEs (Fig. 12); (3) high Al2O3 and low Mg# values; and (4) enriched in LILEs (e.g., Rb, Ba, K, Sr, Pb) and poor in HFSEs. The Pl-rich leucosomes may be divided into two subgroups according to their distinct REE patterns: (a) higher total REE content with conspicuous positive Eu anomalies, and (b) lower total REE content with conspicuous positive Eu anomalies. The Eu-rich group also shows higher Sr content than those in the Eu-poor group (Yu et al., 2015a).

490

0.079 480

206

238

Pb/ U

0.077 470

0.075 460

0.073 450

0.071 a 0.55

Mean=470±3Ma, n=13 MSWD=0.46

0.57

0.59 207

0.61

0.63

2 35

Pb/ U

6. Discussion

20 15 D e p le te

10 1.8

6.1. Geochronology of HP-UHP metamorphism in the NQD UHP metamorphic belt

e

Ga

HP-UHP metamorphic rocks within orogenic belts record geodynamic processes of subduction and exhumation of both oceanic and continental lithospheric materials. A large number of zircon UePb ages have been reported for UHP eclogites and country felsic gneisses of the NQD (Fig. 14). In the NQD, except for rare zircon TIMS UePb and AreAr data giving older metamorphic ages of 486–495 Ma and 466–477 Ma for eclogites in the YKT, which was probably caused by mixing ages of metamorphic domains and residual cores, and excess Ar (Zhang et al., 2005), most of the zircon SHRIMP and LA-ICP MS UePb data yield similar 420–460 Ma eclogite-facies metamorphic ages for eclogites, felsic gneisses and garnet peridotites in different terranes in the NQD. A large metamorphic age span of 30–40 Myr from 420 to 460 Ma leads to different interpretations on the evolutionary process. Some researchers explained that the NOD UHP metamorphism may be diachronous or protracted during continental subduction and collision (Mattinson et al., 2006, 2007; Zhang et al., 2008a, 2010, 2017). Based on two episodes of metamorphic ages recorded in metamorphic zircons with core-rim structures from the DLT eclogite, one school of thought linked two episodes of eclogite-facies metamorphism to an earlier oceanic subduction (440–460 Ma) following continental subduction (440–420 Ma), respectively (Song et al., 2006; Song et al., 2014b). However, Li et al. (2018) reported only one Early Paleozoic tectonic event along the orogen based on tectonic relationships and structural analysis. There is no evidence for two cycles of crustal subduction in the NQD, such as those in the Alps (Rubatto et al., 2011). Most of the published data show that continent-type felsic gneiss have a similar metamorphic age of 420–460 Ma with the enclosing eclogite, suggesting that the continental subduction occurred during this period (Mattinson et al., 2006; Zhang et al., 2010; Yu et al., 2013; Ren et al., 2016). This leads to another interpretation that the prolonged eclogitefacies metamorphism during continental collision lasted from 460 to 420 Ma (Mattinson et al., 2006; Zhang et al., 2010; Yu et al., 2013; Ren et al., 2016). Based on mineral index and REE patterns, Zhang et al. (2016) suggested that the oceanic-type eclogites share a similar metamorphic evolution with the continental-type eclogites with peak metamorphic ages of 430–445 Ma in the DLT, which indicates that the exhumed oceanic-type eclogite was detached from the subducted oceanic crust and then entrained by the exhuming continental crust.

CHUR

0

ε

Hf

(t)

5

d m a n tl

-5 First-stage leucosome in felsic gneiss

-10

Second-stage leucosome in metagabbro Third-stage leucosome in eclogite

-15

Third-stage leucosome in UHP gneiss

b

-20 0

500

1000

1500

2000

2500

t (Ma) Fig. 8. Cathodoluminescence images and zircon LA-ICP-MS results for leucosome of first stage in the Wulan Complex (a), and diagram of LueHf isotope for leucosomes in the NQD (b). Error ellipses are 1σ.

melt during the processes of leucosome formation and HP granulite metamorphism. 5. Whole-rock geochemistry e for migmatite in the NQD UHP metamorphic belt The granitic leucosomes in the first stage is high in SiO2 (68.08–82.04%) and K2O (4.35–6.41%), but poor in Na2O (0.85–1.85%), MgO (0.23–1.43%) and TiO2 (0.16–0.52%), which are all plotted in the granite regions on the An-Ab-Or diagram (Figs. 10a and 11). The leucosomes are weakly peraluminous with A/CNK ranging from 1.04 to 1.12 (Fig. 10b). In the chondrite-normalized REE diagram, the granitic leucosomes show large LREE-HREE fractionation with medium negative Eu anomalies. In the primitive-mantle normalized spider diagram, the leucosomes shows moderate LILE enriched pattern with negative anomalies in Nb, Ta and Ti. The felsic leucosomes in the second stage are rich in SiO2, Na2O and MgO, which are low - to medium-K and calc-alkaline in composition, with N2O/K2O ratios markedly > 1.0 (Fig. 10a). The leucosomes are metaluminous to weakly peraluminous with A/CNK mostly < 1.1 (Fig. 10b). When plotted on a normative feldspar classification diagram, the leucosomes are generally poor in Or, with most of them plotting within the compositional field of tonalite and trondhjemite and rare samples in the field of granodiorite (Fig. 11). The felsic leucosomes are also characterized by low HREEs, high La/Yb, Sr, and Sr/Y, and lack of negative Eu anomalies (Fig. 12). These geochemical features overlap with those of modern adakites and TTGs. The felsic leucosomes inside UHP gneisses in the third stage are

6.2. Possible partial melting mechanism for multi-stage anatexis 6.2.1. Partial melting during oceanic subduction As discussed above, the first-stage anatexis with high-K melt generation occurred at ~470 Ma during oceanic subduction. However, field 200

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Fig. 9. Diagram of zircon UePb ages for leucosomes during three stages of anatexis. Table 1 U-Th-Pb LA-ICP-MS data of zircons from first-stage leucosome in the North Qaidam UHP terrane. Sample

Content(ppm)

Th/U

DL15–21-6.1

Pb

Th

U

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

172 72 55 54 71 99 69 60 78 72 48 60 82 131 191 178

184 101 42 84 27 76 44 38 80 64 37 24 84 277 200 374

2295 914 697 697 902 1261 871 767 998 908 623 785 1048 429 370 1003

Measured ratios 206

Pb/

0.08 0.11 0.06 0.12 0.03 0.06 0.05 0.05 0.08 0.07 0.06 0.03 0.08 0.65 0.54 0.37

238

0.0743 0.0754 0.0752 0.0755 0.0765 0.0758 0.0757 0.0761 0.0754 0.0753 0.0763 0.0756 0.0759 0.2563 0.4564 0.1588

U

Ages(Ma)

1σ

207

235

0.0007 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0027 0.0047 0.0017

0.5864 0.5909 0.5844 0.5857 0.5968 0.5956 0.5860 0.6014 0.5888 0.5924 0.5918 0.5885 0.6009 3.9658 10.1035 2.6585

Pb/

U

1σ

207

0.0082 0.0086 0.0088 0.0089 0.0087 0.0082 0.0083 0.0086 0.0086 0.0085 0.0096 0.0087 0.0086 0.0629 0.1597 0.0426

0.0572 0.0568 0.0564 0.0563 0.0566 0.0570 0.0562 0.0573 0.0567 0.0571 0.0563 0.0565 0.0574 0.1122 0.1606 0.1214

relationships, whole-rock SreNd and zircon LueHf isotopic characteristics preclude the subducted oceanic slab as the dominant source for the granitic leucosomes. In contrast, field and microscopic observations support a common anatexis and migmatization for felsic gneisses. In the field, granitic leucosomes occur as thin layers in the felsic gneiss, and

Pb/

206

Pb

1σ

206

Pb/238U

0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0009 0.0008 0.0008 0.0014 0.0021 0.0016

462 469 467 469 475 471 470 473 468 468 474 470 472 1471 2423 950

1σ

207

Pb/235U

5 5 5 5 5 5 5 5 5 5 5 5 5 15 25 10

469 471 467 468 475 474 468 478 470 472 472 470 478 1627 2444 1317

1σ

207

Pb/206Pb

1σ

7 7 7 7 7 7 7 7 7 7 8 7 7 26 39 21

500 485 467 463 475 490 459 502 479 495 463 471 507 1836 2462 1977

29 30 32 31 31 29 30 31 30 30 34 31 30 23 22 23

the proportion of thin leucosomes range in thickness from millimeters to decimeters (Fig. 5a and b). The leucosomes are generally aligned subparallel to the foliation defined by oriented sillimanite and biotite. In thin section, highly cuspate, elongated K-feldspar grains or plagioclase + quartz vein along quartz-plagioclase boundaries probably 201

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Table 2 LA-MC-ICP-MS LueHf isotope data of zircons from first-stage leucosome in the North Qaidam UHP terrane. Sample

176

Yb/177Hf

DL15–21–6.1-1 DL15–21–6.1–2 DL15–21–6.1-3 DL15–21–6.1–4 DL15–21–6.1-5 DL15–21–6.1-6 DL15–21–6.1-7 DL15–21–6.1-8 DL15–21–6.1-9 DL15–21–6.1-10 DL15–21–6.1-11 DL15–21–6.1-12 DL15–21–6.1-13 DL15–21–6.1–14 DL15–21–6.1-15 DL15–21–6.1-16

0.011997 0.017861 0.021025 0.023275 0.025713 0.022222 0.013446 0.025962 0.018413 0.014661 0.019367 0.019652 0.023074 0.016213 0.034279 0.020260

2σ

176

Lu/177Hf

2σ

176

Hf/177Hf

0.000140 0.000092 0.000267 0.000088 0.000252 0.000061 0.000119 0.000299 0.000108 0.000109 0.000114 0.000047 0.000113 0.000052 0.000215 0.000124

0.000438 0.000534 0.000686 0.000727 0.000776 0.000674 0.000433 0.000820 0.000654 0.000525 0.000687 0.000618 0.000689 0.000572 0.001142 0.000609

0.000007 0.000001 0.000011 0.000003 0.000006 0.000001 0.000003 0.000005 0.000006 0.000001 0.000004 0.000001 0.000001 0.000003 0.000005 0.000002

0.282307 0.282306 0.282299 0.282276 0.282318 0.282321 0.282322 0.282278 0.282338 0.282327 0.282315 0.282340 0.282293 0.282319 0.282326 0.282293

2σ

176

Hf/177Hf(t)

0.000018 0.000017 0.000016 0.000014 0.000018 0.000015 0.000016 0.000015 0.000021 0.000016 0.000015 0.000016 0.000018 0.000017 0.000016 0.000017

0.282303 0.282301 0.282293 0.282270 0.282311 0.282315 0.282318 0.282271 0.282332 0.282322 0.282309 0.282334 0.282287 0.282314 0.282316 0.282288

The calculation of the “crust” (two-stage) Hf model age (TDMC) is based on the assumption of a mean (Wedepohl, 1995).

represent pseudomorphs after melt films, veinlets or pools (Fig. 5c and d). Anatexis can occur through the breakdown of hydrous minerals, such as amphibole, muscovite, biotite or zoisite, or through the influx of an externally derived fluid (Thompson, 1982; Weinberg and Hasalova, 2015). In this study, anhydrous peritectic minerals, such as garnet, have been recognized in the migmatitic gneiss in the DLT, inferred to be derived from fluid-absent hydrate-breakdown melting. The P-T path of the migmatitic gneiss crosses dehydration melting stability curves of muscovite constrained by experimental data, further demonstrating that dehydration melting is more likely to account for the first stage of anatexis. Microscopic observations show that the first stage of anatexis could be related to breakdown of muscovite, e.g. irregular K-feldspar, quartz on the rims of muscovite. The granitic leucosomes are rich in K, poor in Na with low Sr/Y, which are identical to the chemical characteristic of the melts produced by muscovite breakdown (Schmidt et al., 2004; Hermann et al., 2006). The dominated residual garnet + biotite + sillimanite + K-feldspar in the migmatized gneiss is also consistent with dehydration melting involving muscovite via peritectic reaction of muscovite + plagioclase + quartz → biotite + sillimanite 8

2σ

TDM (Ma)

TDMC (Ma)

Age(Ma)

−6.3 −6.3 −6.6 −7.4 −6.0 −5.8 −5.7 −7.4 −5.2 −5.6 −6.0 −5.2 −6.8 −5.9 −5.8 −6.8

0.6 0.6 0.6 0.5 0.6 0.5 0.6 0.5 0.7 0.6 0.5 0.6 0.6 0.6 0.6 0.6

1315 1319 1334 1367 1311 1303 1293 1367 1279 1290 1311 1275 1343 1302 1311 1339

1641 1644 1659 1703 1625 1617 1611 1701 1584 1604 1628 1581 1672 1619 1615 1669

470 470 470 470 470 470 470 470 470 470 470 470 470 470 470 470

176

Lu/177Hf value of 0.011 for the average continental crust

+ K-feldspar ± garnet +melts (Fig. 5e) (Vielzeuf and Holloway, 1988). The S-type granites intruded in Wulan arc complex show high A/CNK (1.10–1.12) and K2O/Na2O ratios (> 2), but low Sr/Y ratios. Zircon UePb dating yielded crystallization age of 460–480 Ma, with low εHf(t) values of −9.3~ −5.3, consistent with those of the first-stage granitic leucosomes. Further studies are required to link the first-stage anatexis to the coeval S-type magmatism. 6.2.2. Coveal partial melting and UHP metamorphism The timing for the second-stage anatexis with generation of TTGlike magmas in the DLT overlaps with the age of UHP metamorphism ranging from 446 Ma to 428 Ma. Some authors (e.g. Song et al., 2014a; Zhang et al., 2015) have proposed that the TTG-like melts in this region were derived from the partial melting of subducted oceanic crust, whereas others invoke remelting of metabasite in the thickened lower crust during continental collision (Yu et al., 2012, 2014). In this study, field relationships and microscopic petrological observations suggest that the TTG-like magmas in the DLT were produced through partial melting of the host metagabbros, rather than the subducted oceanic crust. The pronounced field evidence for in situ partial melting of 3.5

a

7

ƐHf(t)

b

I-S line

3.0

Al/(Na+K)(molar)

K 2 O(wt%)

6 5 4 3

2.5

Metaluminous

Peraluminous

2.0

1.5

2 1.0

1 0 0

1

2

3

4

5

6

7

0.5 0.5

8

Peralkaline 0.7

0.9

Na 2 O(wt%)

1.1.

1.3

1.5

1.7

Al/(Na+K+Ca)(molar)

First-stage leucosome in felsic gneiss

First-stage granitic pluton

Second-stage leucosome in metagabbro

Third-stage leucosome in UHP gneiss

Third-stage leucosome in eclogite

Third-stage felsic vein between amphibolite and gneiss

Second-stage tonalite pluton Syn-collisional granitoids

Fig. 10. Plots of K2O vs. Na2O and A/NK vs. A/CNK for for multiple leucosomes, felsic sheets and plutons in the NQD. Some data are from Chen et al., 2012, Yu et al., 2012, 2015a, Yu et al., 2015b, Song et al., 2014, Zhang et al., 2015, Cao et al., 2017. 202

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clinopyroxene or garnet-garnet boundaries, which are interpreted as the crystallized melt that remained to be trapped within the residue following migration of most of the melts away from their sources (Fig. 4c and d). (c) Thin irregular shaped films of quartz, plagioclase within garnetite showing a connected crystallization with the small dihedral angles between garnet-garnet (Fig. 4g). (d) Polymineral inclusions of quartz + feldspar in garnet cores could also represent crystallized leucogranitic magma trapped by garnet crystals during their growth (Fig. 4h). (e) The clear spatial association of peritectic garnet and clinopyroxene within the leucosome is critical to an interpretation of in situ melting, as opposed to an origin via injection of melt from a more distal source (Yu et al., 2014). In summary, field and microstructural observations suggest that the metagabbro and HP mafic granulite (or garnet pyroxenite) represent the potential starting material (protolith) and the residues of partial melting, respectively. This hypothesis is supported by the geochronological data, as zircons obtained from the leucosomes and HP mafic granulites contain inherited magmatic cores with an age of 470 Ma, which is similar to the protolith age of the metagabbro in the Dulan area. The presence of similar zircon populations in the metagabbros, HP mafic granulites, and leucosomes further confirms their common link. The affinity between metagabbro, HP mafic granulite and TTG-like leucosome is also supported by similar Nd and Hf isotopic values (Fig. 13; Yu et al., 2012). Moreover, comparison with dehydration melting experiments confirms that leucocratic melts in the DLT might have formed at temperatures higher than 900 °C and pressures > 1.5 GPa (Sen and Dunn, 1994), which are consistent with the peak conditions of the HP mafic granulite (Yu et al., 2011b, 2012, 2014; Song et al., 2014a). Thus, our field observations and analytical data demonstrate that leucosomes in this region were probably derived from the partial melting of metagabbro, with HP mafic granulite and garnet pyroxenite representing the residue. Experiments and field studies have suggested that the breakdown of hydrous minerals (e.g. mica, amphibole and zoisite) plays a principal role in triggering the partial melting of HP/UHP metamorphic rocks (Lang and Gilotti, 2007; Zheng et al., 2011). Although the P-T conditions cross the experimentally constrained boundaries of the stability curves of micas, partial melting of the metagabbro in the DLT cannot be dominated by the breakdown of mica, because the leucosomes are rich in Na and poor in K, quite distinct from those generated by mica breakdown. Instead, the anatexis of the metagabbro in this study could have been dominated by the breakdown of amphibole, as evidenced by the following observations. (a) Plagioclase and quartz occur locally as irregular grains around the rims of amphibole or as pudding, waterdrop shape interspersed in amphibole crystals, suggesting breakdown of amphibole with the melt generation (Fig. 4b). (b) High Sr tonalitic and trondhjemitic melts coexisted with residues that were dominated by clinopyroxene and garnet in this study, which is consistent with dehydration melting experiments of amphibole according to: amphibole + plagioclase → garnet + clinopyroxene ± epidote + melt (Lopez and Castro, 2001; Patiño Douce, 2005). (c) The P-T conditions of metagabbro crossed the dehydration melting stability curves of amphibole as constrained by experimental data (Fig. 15) (Lopez and Castro, 2001; Yu et al., 2014). The experimental study of Skjerlie and Patiño Douce (2002) on the zoisite-bearing eclogite indicates that zoisite-driven dehydration can take place to form LREE-Sr-rich tonalitic and trondhjemitic melt via the mineralogical reaction of zoisite + clinopyroxene + quartz →melt + kyanite + diopside ± garnet. This has been observed in our magmatic metagabbro, in which irregular cuspate zoisite is rimmed by irregular plagioclase and quartz, suggesting its breakdown in association with the dehydration melting during the exhumation (Fig. 4f). The presence of rare kyanite within the leucosome and restite further indicates that zoisite was another reactant during anatexis. In principle, whether the local anatexis occurs inside crustal slices primarily depends on the bulk composition and P-T path of metamorphic slices. The P–T path of the Dulan metagabbro crossed the amphibole and zoisite stability curves as constrained by experimental data

An

an

od

To n

ior

ali

ite

te

Winther 1996

Gr

Granite

Petford and Atherton, 1996

Trondhjemite

Or

Ab

Fig. 11. Plots of An-Or-Ab for multistage leucosomes, felsic sheets and plutons in the NQD. Symbols are as in Fig. 11. Some data are from Chen et al., 2012, Yu et al., 2012, 2015a, Yu et al., 2015b, Song et al., 2014, Zhang et al., 2015, Cao et al., 2017. 500

A=MORB partial melting curve with eclogite as a restite B=Crustal fractionation trend Ol+Pl+Cpx+Opx

To Sr/Y =1100

400

A Sr/Y

300

200

Adakite Andesite-dacite-rhyolite (ADR) field

100

B 0 0

5

10

15

20

25

30

35

40

45

50

Y(ppm) Fig. 12. Plots of Sr/Y vs. Y for multistage leucosomes, felsic sheets and plutons in the NQD. Symbols are as in Fig. 11. Some data are from Chen et al., 2012, Yu et al., 2012, 2015a, Yu et al., 2015b, Song et al., 2014, Zhang et al., 2015, Cao et al., 2017.

metagabbros is provided by innumerable thin layers, patches and lenses of leucosomes in the metagabbro similar to migmatization (Fig. 3a and b). The patches consisting of quartz + plagioclase generally enclose large garnet and/or local clinopyroxene crystals, which represent the in situ neosomes comprising peritectic phases and leucosomes that were generated during anatexis. The occurrences of HP mafic granulites and garnet pyroxenites as blocks or lenses within the quartz-feldspathic leucosomes further argues that they represent residues formed during partial melting (Fig. 3g). The field evidence for in situ partial melting of metagabbros in the DLT is incontrovertible. However, microstructural evidence is less clear, owing to slow cooling rates and reworking during retrograde evolution. The microscopic petrological observation for in situ partial melting of the meta-gabbros is evidenced by (a) pudding or water-drop shape of feldspar and quartz recognized interspersed amphibole grains (Fig. 4b). (b) Isolated patches or felsic veinlets that consist of highly cuspate feldspar and quartz between garnet203

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Fig. 13. SreNd isotope systematics of multistage leucosomes, felsic sheets and plutons in the NQD. Symbols are as in Fig. 11. Some data are from Yu et al., 2012, 2015a, Yu et al., 2015b, Zhang et al., 2015.

(Fig. 15). Thus, the partial melting of metagabbro in the Dulan unit with tonalitic and trondhjemitic melts and residues of garnet + clinopyroxene + plagioclase was triggered by the dehydration melting of predominant amphibole and rare zoisite.

simultaneously deformed with the host retrograde eclogite (Fig. 6a), and internally disappeared without extending to the country gneiss, locally displaying features diagnostic of migmatization. Remarkably, locally coarse-grained patches, consisting of quartz + plagioclase ± Kfeldspar and commonly enclose garnet (2–3 cm in diameter) (Figs. 6b and 7a), are interpreted as in situ neosomes consisting of felsic leucosome and surrounding peritectic phases. Microstructural evidence for in situ partial melting is demonstrated by the following evidences. (1) Irregularly elongated microclines have been recognized between clinopyroxene and/or garnet, or along garnet and garnetilmenite grain boundaries, suggesting crystallization from former melt (Fig. 6h) (Cao et al., 2017). (2) Polymineral inclusions or nanogranite of muscovite + quartz + feldspar in the core of the garnet could also be interpreted as crystallization of a leucogranitic magma droplet trapped by garnet during growth (Yu et al., 2015a). (3) Irregular elongated feldspar and quartz grains present along the muscovite-garnet (or clinopyroxene) boundaries (Fig. 6g), which are interpreted as pseudomorphs of the former melts. (4) Felsic veinlets, composed of quartz and feldspar, cross the boundaries of clinopyroxene, garnet, amphibole and clinopyroxene + plagioclase symplectite (Fig. 7g).

6.2.3. Partial melting during exhumation of deeply subducted continental crust 6.2.3.1. Possible partial melting mechanism for UHP eclogite. Experiments and natural investigations suggest that both metabasite and metapelite begin to melt to form felsic melts at temperatures of 650–700 °C in the presence of aqueous fluids (Sawyer, 2010; Zheng et al., 2011; Brown, 2013). However, in the absence of any influx of fluids, partial melting of metabasite rocks via reactions that consume hydrous minerals requires higher temperature than that of the metapelites and felsic rocks (Clemens, 2006). Macroscopic and microscopic observations provide strong evidence the Pl-rich felsic leucosomes were derived from in situ partial melting of the host retrograde eclogite in the XTT and LLT, rather than injection of melt from a more distal source. The pronounced field evidence for partial melting of metabasite in the XTT and LLT is provided by substantial numbers of thin veins

LLT

XTT

DLT

Eclogite

Paragneiss

Eclogite

Paragneiss

Paragneiss

Eclogite

Eclogite

Paragneiss

480

Migmatized gneiss

YKT 500

Migmatitic HP granulite

520

Subduction of oceanic crust

460

HP-UHP eclogite-facies metamorphism

440

Anatexis 420 400 Prograde metamorphism

Eclogite-facies metamorphism

Amphibolite-facies metamorphism

Anatexis

Fig. 14. Diagram of age statistics showing the relationship between multistage anatexis and HP-UHP metamorphism. 204

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+P he

n+ Gr

M el t

t+ Qt z/C

p+C o e (a H2 O lt + G <1) rt

(2 )C px

ut lt in

no Ph px+

Phe

M el

Me

(5)

) O =1

n+Q

t+ B

t+ P

(1 ) Z

tz

l+ G

rt

o ou

t

60

(7 )T on al

ut no

(6

)S

ol

id

o

bo

lit

e

Cp x+

B t+ Pl m p+

GR

f us

m ra

i ph

10

M el t+

(3 )A

O= 1)

600

A

515-460Ma

700

800

90 (Km)

Crust

30

K fs

+Q tz

Ph (4) r(a H 2

M el t

Ab +O

3

(Kbar)

L it ho sp he

Ex re m a ntle

hu

m

at

io

n

Granulite-facies metamorphism and partial melting

60

20

0

500

Slab-derived fluid

TTG-like tonalite

HGR

(8 )Q tz+

AM

GS

400

ic

Asthenosphere

900

UHP eclogite 90

Asthenosphere

30

300

an

2

BS

EA

30

G rt+

s( aH

2

lt

li du

(2 )C

Me

it e so

20 Q tz t+ Gr Ky o+ x+ )Z Cp (3 t+ el M

M el t

15

5

ce

Neoproterozoic mafic intrusions

t

Ep-EC

6 Amp-EC

10

Crust

us

4

(3)Am p+Qtz

Pressure(k bar)

10

Melt+ Cpx+G rt

20

(Kbar)

Dry-EC

cr

5 Co es ite Qu ar tz

Me

hen

1

30

(7 )P

Lw-EC

+Om

35

25

oe (a H

2

O< 1)

Volcanic rock (Km) S-type granite I-type granite Partial melting 30 Gabbro o

B

460-430Ma

1000

120

Temperature( ) Fig. 15. Schematic P-T paths for metamorphic processed illustrate multistage anatexis of UHP eclogite and gneiss, and HP granulite in the NQD. ① DLT eclogite, ② DLT HP granulite, ③ Wulan migmatized gneiss, ④ XTS eclogite, ⑤ LLT eclogite, ⑥ YKT eclogite. (1) Skjerlie and Patiño Douce, 2002 eclogite; (2) Auzanneau et al., 2006; metagreywacke; (3) Patiño Douce, 2005; tonalite; (4) Vielzeuf and Holloway, 1988; metapelite; (5) Hermann, 2002; metagreywacke; (6) Lopez and Castro, 2001; amphibolite; (7) Schmidt, 1993; tonalite; (8) Hermann and Green, 2001; metagreywacke. GR: granulite-facies, HGR: highpressure granulite-facies, GS: greenschist-facies, AM: amphibolite-facies, EA: epidote-amphibolite-facies, BS: blueschist-facies, Amp-EC: amphibole eclogitefacies, Ep-EC: epidote eclogite-facies, Lw-EC: lawsonite eclogite-facies. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(Km) (Kbar)

Crust Syn-collisional granitoids

30

10 60

Partial melting of subducted slab

20

90 30

Rising asthenosphere

430-415Ma

C

120

Continental-type eclogite

(Km)

These felsic veinlets may represent melt channels, indicating that the microscale migration of the melt was perhaps limited to several millimeters (Sawyer, 2008; Holness et al., 2011). The hypothesis that Pl-rich leucosome was derived from the host retrograde eclogite in the LLT and XTT is further supported by the similar Sr and Nd isotope composition for the Pl-rich leucosome and the retrograde eclogite (Fig. 13). In addition, the plagioclase-rich leucosome in the XTT and LLT shows low Rb/Sr rations compared to metasediment-derived melts, which commonly have elevated Rb/Sr (Harris and Inger, 1992; Zeng et al., 2005). Based on the fact that SreNd isotope of some felsic veins ranging between UHP gneiss and eclogite, some studies suggested that metabasite would be partially melted due to the fluid influx from the gneisses (Zhang et al., 2015). However, hydration melting could not be the dominated anatectic mechanism of the UHP eclogite for the following considerations. (1) Fluid-present melting and fluid-absent hydrate-breakdown melting may be distinguished based on the nature of the ferromagnesian minerals associated with leucosomes (Brown, 2013). Peritectic garnets have been recognized in Pl-rich leucosomes, demonstrating that they are more likely to be a product of fluid-absent hydrate-breakdown melting (Fig. 6b). (2) The P-T path of the Luliangshan and Xitieshan eclogites crosses the dry solidus of metabasalts and the dehydration melting stability curves of zoisite, phengite and amphibole constrained by experimental data (Fig. 15). (3) The same leucosome could record more than one zircon overgrowth during waterfluxed melting if melting conditions persist and fluids are available (Rubatto et al., 2009). This texture has not been observed in the zircons from leucosome within the metabasite in this study. (4) Most Pl-rich leucosomes show Sr and Nd isotopic values similar to those of the eclogite. Thus, the felsic veins with SreNd isotope between the felsic

(Kbar)

Crust 30

Oceanic-type eclogite

10 60 20 Rising asthenosphere

30

400-360Ma

90

D

Fig. 16. Schematic tectonic diagram showing multistage anatexis during tectonic evolution from oceanic subduction to continental collision in the NQD.

gneiss and eclogite, could arise from mixing of two melts from both the eclogite and gneiss during melt migration. Phengite and zoisite are the most important hydrous minerals in UHP eclogites, and the breakdown of these hydrous minerals play a principal role in triggering the partial melting of UHP rocks during continental collision (Lang and Gilotti, 2007; Zheng et al., 2011; Chen and Zheng, 2013). Microstructural observations generally demonstrate that partial melting of the eclogite could probably be related to the breakdown of phengite in the XTT and LLT, e.g., irregular elongated feldspar or quartz grains occur like veinlets along the phengite-phengite boundaries (Fig. 6g). Dehydration melting of phengite in metabasite generates melt rich in K, and is a key mechanism for hydrous potassic, K-rich or ultrapotassic magmatism (Hermann et al., 2006; Schmidt et al., 2004). However, geochemical characteristic shows that the Plrich leucosome contains little K-feldspar-bearing minerals and has low K2O content, which exclude the possibility that anatexis of the eclogite in the North Qaidam UHP Terrane was dominated by breakdown of phengite. In addition, most of the XTT and LLT eclogite is bimineralic (omphacite and garnet) with very low K2O contents (Meng et al., 2003; 205

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Yang et al., 2003), and correspondingly the phengite is almost absent. In this regard, rare phengite is insufficient for generating so many melts. The Pl-rich leucosomes in the XTT and LLT contain high contents of Sr with high Sr/Y and enrichment of LREEs, suggesting tonalitic geochemical features in the An-Ab-Or triangle diagram, which are identical to the geochemical characteristics of experimental melts produced by zoisite breakdown (Skjerlie and Patiño Douce, 2002). Experimental result indicates that zoisite and phengite could break down more or less simultaneously at similar conditions if sufficiently high temperatures are attained at P ≤ 2.5 GPa. The beginning temperature of melting when the two hydrous minerals coexist is ~100–200 °C lower than that in the case of only one hydrous phase (Vielzeuf and Schmidt, 2001; Zheng et al., 2011). The P-T paths of Xitieshan and Luliangshan eclogite intersect with the relevant dry solidus of the natural eclogite phengite and zoisite-dominated partial melting curves constrained by these experimental data. Thus, partial melting of eclogite in the XTT and LLT during exhumation was probably dominated by the breakdown of most zoisite and rare phengite through dehydration-melting reaction such as: zoisite + muscovite + quartz → garnet + kyanite +melt (Skjerlie and Patiño Douce, 2002). On the basis of higher Ba/Rb and Na/K, and lower Rb/Sr signatures for the leucosomes, sodic amphibole dehydration melting could also account for the formation of felsic leucosome in the XTT (Cao et al., 2017) (Tables 1 and 2).

anatexis (Zhang et al., 2015). In addition, coarse-grained phengite is locally rimmed by biotite, quartz and irregular K-feldspar, which also demonstrates breakdown of the muscovite (Figs. 6i, j, 7h and i). Experimental studies have shown that the decompressional partial melting of the granitic gneisses is related to the decomposition of phengite and the primary melt is granitic with a high K content, which is consistent with composition of the leucosomes in the study area (e.g., Huang and Wyllie, 1981, 1986; Stern and Wyllie, 1981; Auzanneau et al., 2006). Moreover, the P-T conditions of UHP felsic gneiss crossed the dehydration melting stability curves of muscovite as constrained by experimental data (Fig. 15). Thus, partial melting of UHP felsic gneiss in the XTT and LLT was triggered by the dehydration melting of predominant muscovite via by peritectic reaction of muscovite + plagioclase + quartz → biotite + kyanite/sillimanite + K-feldspar + garnet + melt (Vielzeuf and Holloway, 1988). 6.3. A link between anatexis and granitic magmatism 6.3.1. Implication for TTGs genesis and continental growth Most leucosomes and tonalite plutons in the DLT are characterized by high Na, Sr, Sr/Y and La/Yb and low HREEs and lack of negative Eu anomalies. These geochemical features overlap those of the TTGs. In field, arrays of the interconnected TTG-like leucosomes in the metagabbro coalesce in small tonalite pluton, which lead us to consider that the TTG-like tonalite pluton likely represents segregated, migrated melt derived from the molten metabasites. This hypothesis is supported by the following observations: (1) The tonalite bodies exhibit petrographic continuity with foliation-parallel leucosomes, suggesting that the discordant tonalites and leucosomes crystallized at the same time within different regions of the same melt extraction network (Yakymchuk et al., 2013); (2) the mafic enclave within the tonalite pluton has a mineral assemblage similar to the melanosome in the adjacent molten metagabbros; (3) the emplaced age of tonalite pluton is slightly younger or overlaps with the anatectic zircon age from the TTG-like leucosome; (4) the tonalite pluton and TTG-like leucosomes show similar wholerock major and trace element characteristics; (5) the affinity between TTG-like leucosome and tonalite pluton is also supported by similar Sr and Nd isotopic values with 87Sr/86Sr(t) and εNd(t) of 0.7033–0.7052 and 0.1–4.3. REE modeling results demonstrate that the TTG-like leucosome and tonalite pluton were derived from 20% partial melting of meta-gabbro with mafic granulite residue (Yu et al., unpublished data). It is widely accepted that much of the Earth's earliest-formed continental crust is dominated by magmatic intrusions of tonalitictrondhjemitic-granodioritic (TTGs) composition, inferred to have been derived from partial melting of hydrated meta-basalt (Jahn et al., 1981; Wolf and Wyllie, 1994; Rapp and Watson, 1995; Qian and Hermann, 2010). The Archean and younger TTG rocks can provide significant information on the differentiation of crust from mantle and continental growth regimes of the Earth (Defant and Drummond, 1990; Condie, 2005; Martin et al., 2005). For the genesis of TTGs, it is generally recognized that they were produced by partial melting of mafic rocks at relatively high pressure (Defant and Drummond, 1990). However, various geodynamic settings and P-T conditions have been proposed for the generation of TTG magmas and continental growth. Based on (1) experimental studies (Moyen and Stevens, 2006) and (2) the requirement for a hydrated source rock, some workers suggested TTGs formed in a subduction-related environment including melting of subducted oceanic crust or oceanic/continental arc crust. Thermodynamic calculations and the modeling of trace element contents also suggested that TTGs is best explained by partial melting of tectonically thickened mafic island-arc crust with pressure higher than 12 kbar (Xiong, 2006; Nagel et al., 2012). However, some workers suggested that the “island arc” model is not suitable for continental crust growth because of the following reasons. (1) The primary arc magmas or the bulk arc crust is basaltic whereas the bulk continental crust is andesitic; (2) the arc crust production is mass balanced by subduction erosion and sediment

6.2.3.2. Possible partial melting mechanism for UHP felsic gneiss. For the UHP felsic gneiss, coarse-grained discordant K-feldspar-rich leucosomes provide macroscopic evidence for the former presence of melts, which locally show features of migmatization. Local irregular patches consist of plagioclase + quartz ± K-feldspar leucosomes that commonly surround garnet (Fig. 7c), which are interpreted to be in situ neosomes consisting of leucosomes surrounding peritectic phases. At the thin-section scale, the evidences for in situ partial melting of UHP felsic gneisses are as follow: (1) wedge-shaped patches or pockets consisting of K-feldspar + plagioclase + quartz ± muscovite between quartz or plagioclase boundaries represent pseudomorphs of the former melts and indicate that the microscale migration of the melts was perhaps limited to several millimeters (Figs. 6k and 7k) (Sawyer, 2008; Holness et al., 2011). (2) Irregular, elongated K-feldspar grains occur around the muscovite, or between the quartz-feldspar and quartzquartz boundaries, and cuspate interstitial films of feldspar grains occur at the quartz-quartz-feldspar triple junctions, which are also interpreted as pseudomorphs of the former melts (Figs. 6i, j, 7h and i). (3) Felsic veinlets composed of K-feldspar + quartz ± plagioclase ± muscovite occur along boundaries of plagioclase and quartz, which also represents channels for melt migration (Figs. 6l and 7l). In this regard, the UHP felsic gneiss host could be the source for the K-feldspar-rich leucosome in the XTT and LLT. This assumption is further proved by three discrete UePb ages recorded in zoned zircons from the K-feldspar-rich leucosome and melanosome: e.g. inherited magmatic cores of ~950 Ma, eclogite-facies metamorphic mantle of ~445 Ma and anatectic rims of ~430 Ma. The affinity between the K-feldspar-rich leucosome and UHP felsic gneiss is also supported by similar Nd and Hf isotope composition (Fig. 13) (Yu et al., 2015b; Zhang et al., 2015). Experiments and microscopic studies have suggested that muscovite is one of the most common hydrous minerals in UHP metapelite and metagranite, and partial melting of these rocks due to phengite breakdown is a common process under subduction-zone conditions (Hermann, 2002; Skjerlie and Patiño Douce, 2002; Auzanneau et al., 2006). In the XTT and LLT, relicts of muscovite occur between the boundaries of quartz and plagioclase in the magmatic gneiss, and they occasionally coexist with cuspate feldspar and quartz grains that are interpreted to have formed from anatectic melts (Yu et al., 2015b). Rounded muscovite inclusions and corroded matrix muscovite laths were observed, further suggesting that they may be residual reactants of 206

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recycling, thus contributing no net mass to continental crust growth; and (3) felsic igneous rocks in oceanic arcs are depleted in incompatible elements compared to average continental crust (Condie and Kroner, 2013; Niu et al., 2013). Alternative non-uniformitarian models for TTGs petrogenesis have been proposed, in which vertical forces were dominant, including anatexis of the lower levels of tectonically thickened oceanic crust or basaltic plateau (Smithies, 2000; Zegers and van Keken, 2001). Based on phase equilibria modeling of the Coucal basalts, Johnson et al. (2017) proposed that TTGs were produced by 20–30% melting of Councal basalts along high geothermal gradients near the base of thick, plateau-like basaltic crust, and subduction was not required to produced TTGs. The major petrological shortcoming for the non-uniformitarian model is the difficulty to supply enough water to deep crustal zones in the absence of subduction (Mayen et al., 2017). Furthermore, continental collision zones are argued to be primary sites for preserving juvenile crust and maintaining net continental crustal growth in the Phanerozoic (Niu et al., 2013; Song et al., 2014a). In this model, the juvenile crust is dominated by syn-collisional granitoids, such as the voluminous batholiths along many orogenic belts on the greater Tibetan Plateau (Mo et al., 2008; Niu et al., 2013; Song et al., 2014a), and continental collision with juvenile crust formation/preservation and super-continent amalgamation explains the episodic growth of the continental crust (Spencer et al., 2015). In the DLT, some researchers consider the mafic HP granulite residue to represent overprinted eclogite, and thus the TTG-like leucosome and tonalite were inferred to be derived from melting of a subducted oceanic crust (Song et al., 2014a). However, structural relationships, petrology, whole- rock major and trace element geochemistry indicated that the protoliths of the metagabbros (paleosomes) from the Dulan Unit formed in a continental arc environment at 470 Ma and the residual mafic HP granulite did not originate from oceanic-type eclogite (Yu et al., 2011b, 2014), ruling out the possibility that the TTG-like leucosome and tonalite were generated by subducted oceanic slab melting. In addition, the latest petrologic research also suggests that the Dulan oceanic- and continental-type eclogite directly retrogressed to amphibolite-facies during decompression without HP granulite-facies overprinting (Zhang et al., 2009a, 2010). The SreNd isotope of the TTG-like leucosome and tonalite in the DLT also contrast with melts derived from typical oceanic crust (Fig. 13). In addition, the TTG-like magmas in the DLT have Mg# ranging from 33 to 57, most of which are lower than 50. These low Mg#, Cr and Ni values further demonstrate that the magmas have not interacted with the mantle wedge, excluding the possibility that the Dulan TTG-like magmas were generated by the melting of oceanic slab or delaminated continental crust. Zircon UePb data in previous researches and this study suggest that anatexis with the formation of TTGs and associated HP mafic granulitic residue occurred during 446–428 Ma (Yu et al., 2012, 2014), which overlaps with the ages of the UHP metamorphism for the adjacent eclogite (445–430 Ma) in the Dulan unit during continental collision (Song et al., 2003; Zhang et al., 2010; Yu et al., 2013). The Dulan TTG-like leucosomes and tonalite plutons have positive wholerock εNd(t) and positive zircon Hf isotopes, which also preclude the possibility that the reworking of the ancient subducted continental crust is responsible for the origin of certain TTG rocks. Thus, a model of partial melting of tectonically thickened lower crust would adequately account for the generation of the TTG-like rocks in the DLT during continental collision. Distinct from the continental collision model proposed by Niu et al. (2013), in which the syn-collisional granitoid rocks are derived from partial melting of the upper ocean crust (i.e., the last fragments of underthrusting ocean crust upon collision) under amphibolite facies conditions, adding mantle-derived materials to form the juvenile continental crust mass. Our study in the DLT represents a case that TTG magmas could be conclusively derived from the partial melting of newly emplaced meta-gabbro with arc affinity under HP granulite-facies conditions in thickened lower crust that migrated upward during continental collision, evidenced by field, microstructural

and geochemical observation. Based on the fact that the sources of these TTG magmas are juvenile materials from mantle with depleted mantle isotopic signatures, the volumetrically significant migrated plutons evolved from TTG-like melt will become a segment of continental crust and contribute to net continental crustal growth, with partial residual products of HP granulite and/or garnet pyroxenite to the mantle by delamination. The model suggested in this paper explains the origin of TTG-like rocks and related continental growth genetically associated with continental collision zones. 6.3.2. Linking partial melting of UHP gneisses and syn-collisional granitoids during continental collision Crustal anatexis can give rise to granite bodies that range in scale from centimeter-wide leucosomes in migmatites, to intrusive plutons. However, the relationship between the syn-collisional granitic magmatism and partial melting of deeply subducted slab is not clear because of: (1) the common association of postorogenic extensional overprints (e.g., Costa and Rey, 1995; Foster and Fanning, 1997; Vanderhaeghe and Teyssier, 1997); (2) difficulties in constraining the exact timing of partial melting, metamorphism, and pluton emplacement; and (3) multiple episodes of metamorphism, magmatism and partial melting. The affinity between syn-collisional granitic magmatism and partial melting of deeply subducted slab can be understood through a combination of field, petrology, geochemistry and geochronology studies. In the NQD UHP metamorphic belt, syn-collisional granitoids occupy an area of ~60 km2 in the XTT and ~15 km2 in the LLT. Field observation shows an intrusive contact relation with the surrounding felsic gneiss. Geochemical data suggests that the granites exhibit characteristics of calc-alkaline and peraluminous, with high 87Sr/86Sr rations and low εNd(t) values, and was inferred to be derived from a deeper partially molten source with a composition similar to country felsic gneiss of the XTS and LLT (Meng et al., 2005; Meng and Zhang, 2008; Zhang et al., 2017). Geochronological studies revealed the syncollisonal granitoids intruded in the XTT and LLT during 430–415 Ma, which overlap with the timing of partial melting for deeply subducted slab. It provides us with a considerable opportunity to link the partial melting of UHP rocks to the syn-collisional magmatism. One school of thought linked these syn-collisional granitoids with partial melting of UHP eclogite or unsubducted felsic gneiss (Meng and Zhang, 2008; Chen et al., 2012a). However, zircons from the syn-collisional granitoids also record inherited age of 450–470 Ma, which is consistent with the well-documented ages of UHP metamorphism of eclogite and felsic gneisses previously reported in the XTT and LLT. This agreement indicates that the syn-collisional granites were derived from partial melting of deeply subducted continental slab including UHP eclogite and felsic gneiss, rather than those unsubducted felsic gneiss. Geochemical data demonstrate that the K-rich, peraluminous syn-collisional granite has major and trace element compositions similar to the K-feldspar-rich leucosome within the UHP gneiss, but distinct from the Pl-rich leucosome in the retrograde eclogite (Figs. 10, 11 and 12). A well-preserved progressive sequence documented the evolution process from partial melting of UHP felsic gneiss to emplacement of significant plutons on both outcrop and microstructural scales is also systematically documented, starting from metatexite migmatite, and continuing through diatexite migmatite, and then feeding felsic sheet and finally combining to form granitic pluton (Fig. 7d, e and f). The affinity between migmatized UHP gneisses and syn-collisional granitoids is also supported by similar Sr and Nd isotopic values with 87Sr/86Sr(t) and εNd(t) of 0.71–0.73 and −10~−5, respectively (Fig. 13). Thus, the anatectic melts from partial melting of deeply subducted UHP gneiss have accumulated and migrated outside the deeply subducted crustal slice in the form of syn-collisional pluton.

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6.4. Partial melting-induced exhumation of deeply subducted continental slab

Song et al. (2006) proposed that the two parallel belts represent a single north-dipping evolutionary sequence from oceanic subduction to continental collision. Yin et al. (2007) suggested a “diapiric flow” model, in which the North Qaidam UHP metamorphic eclogites originated from south-dipping subducted paleo-Qilian oceanic crust via diapiric flow across the mantle wedge. Although the detailed evolution of the NQD UHP Belt has not been unequivocally settled, published data was summarized to explore tectonic evolution of multi-stage anatexis and magmatism from oceanic subduction to continental collision in the NOD UHP Belt (Fig. 16).

Migmatized gneisses and retrograde eclogites from the XTT and LLT contain well-preserved macroscale and microscale structures that indicate partial melting of deeply subducted metabasite, metagranite and metapelite (Chen et al., 2012a, 2012b; Yu et al., 2015a; Yu et al., 2015b; Zhang et al., 2015; Cao et al., 2017). Although some researchers suggested that partial melting of deeply subducted slab could have occurred during the UHP metamorphic stage, e.g. coesite inclusions documented in the anatectic zircon in the Dabie-Sulu UHP Terrane (Chen and Zheng, 2013). Mounting evidence shows that partial melting was expected during the hot exhumation stage or late amphibolite-facies stage (Zong et al., 2010; Liu et al., 2012b). Numerous studies suggested that metamorphic zircon of felsic leucosomes within retrograde eclogite and gneiss in the NQD recorded the UHP metamorphism at 445–450 Ma, and some zircon grains or zircon rims recorded their growth from anatectic melts at 433–422 Ma, which was slightly younger than the known UHP metamorphic age and older than the age of amphibolite-facies retrogression. Thus, the NOD UHP metamorphic rocks underwent dehydration-driven incipient anatexis during phengite + zoisite or amphibole breakdown in the early stage of exhumation during continental collision. Experimental results have suggested that a small amount of melts dramatically affect the rheology of deeply subducted crust and thus can play a crucial role in accelerating the exhumation of ultrahigh-pressure slabs (Hermann et al., 2001; Labrousse et al., 2002; Chopin, 2003; Zheng et al., 2011). Numerical model results demonstrate that the average exhumation rate to crustal depths for the melt-bearing ultrahigh-pressure materials could reach 2–4.5 cm/yr (Ellis et al., 2011; Sizova et al., 2012). This rate is similar to the fastest exhumation rate determined for the initial exhumation of ultrahighpressure rocks to crustal depths in many ultrahigh-pressure terranes (Sizova et al., 2012). The average exhumation rate to crustal depths for the melt-bearing ultrahigh-pressure materials is 1.5–2 cm/yr in Papua New Guinea (Monteleone et al., 2007; Gordon et al., 2012). In the NQD, this exhumation rate has been constrained to be 1–2 cm/yr from mantle to crustal depths (Yu et al., 2014), which can be compared with exhumation rates of melt-bearing rocks in other ultrahigh-pressure terranes and data predicted by numerical models. In this regard, the melt evolution from the leucosomes produced at the early exhumation stage to syn-collisional granitoids produced at the late exhumation stage might contribute greatly to the exhumation of the NQD from mantle depths to the lower crustal levels. It is noted that the Pl-rich and Kfeldspar -rich leucosomes in UHP eclogite and felsic gneiss mainly composed of feldspar + quartz exhibit high LREE and LILE contents (Yu et al., 2015a, 2015b; Zhang et al., 2015). Field, petrological, geochronological and geochemical observations suggest that the anatectic melts have accumulated and migrated outside the deeply subducted crustal slice in the form of felsic sheet and granitic pluton. The migration of these melts will cause enrichment of LILEs, LREEs and radiogenic isotopes in the overlying mantle wedge. Such melt metasomatism has important implication on the crust-mantle interaction in continental subduction channel (Zheng et al., 2011).

(a). During 520–460 Ma, the area northward of the ocean beneath the southern margin of the Central Qilian Block produced arc volcanic rocks and plutons (Yuan et al., 2002; Shi et al., 2006; Zhu et al., 2010). The island-arc volcanic rocks in Jilusu and Shuangkoushan indicate initial oceanic subduction prior to 515 Ma. Some Precambrian basements, including orthogneiss and paragneiss/schist, might be partially molten due to thermo-disturbing related to intrusion of arc magmatism in the Wulan Complex. (b). Following the oceanic subduction, subduction of the continental crust was dragged by the oceanic slab with eclogite-facies metamorphism during 460–430 Ma. Neoproterozoic mafic rocks emplaced in the continental material were also subducted to mantle depths. Parts of the already exhumed Paleozoic oceanic material might have been entrained into the subducting continental crust (Zhang et al., 2016). Thickening of the lower crust leads to partial melting of newly emplaced arc-related metagabbro and its products, including TTG-like melt, volumetrically significant plutons evolved from melt and the associated mafic granulitic residues. The TTG-like pluton will become a segment of continental crust and contribute to crustal growth. (c). At~430 Ma, the deeply subducted continental crust was detached from the dense oceanic crust and started exhumation. The diachronous or protracted UHP metamorphism is attributed to a different exhumation path and mechanism in four different UHP terranes of the NQD (Zhang et al., 2017). The UHP rocks in the YKT and DLT record near-adiabatic decompression to amphibolite-facies conditions (Zhang et al., 2005; Chen et al., 2009; Ren et al., 2017). In contrast, The UHP rocks at the LLT and XTT followed more open clockwise P-T loops and experienced granulite-facies overprinting during exhumation. The subducted continental slab was subjected to partial melting during the “hot” exhumation in the XTT and LLT. The K-feldspar-rich leucosomes derived from partial melting of the UHP gneisses caused by muscovite breakdown will segregate, migrate and combine to form syn-collisional pluton. In contrast, partial melting of the UHP eclogite was likely attributed to breakdown of zoisite and rare phengite, with products of high Sr/Y, plagioclase-rich leucosomes and felsic sheet. The whole rock SreNd and zircon O isotopic features of some felsic veins are between the felsic gneiss and eclogite, compatible with their emanating from melts of both the eclogite and gneiss during the exhumation stage of the UHP rocks (Liu et al., 2014; Zhang et al., 2015). The melt evolution from the leucosomes produced at the early exhumation stage to the syn-collisional plutons produced at the late exhumation stage might contribute greatly to the exhumation of the NOD UHP Belt form mantle depths to the lower crustal levels. (d). During 400–360 Ma, an extensional collapse and delaminating of the orogen induce intrusion of a series of post-collisional plutonic magmas including granite, tonalite, granodiorite and enclosing contemporaneous mafic enclaves (Wang et al., 2014).

7. Multistage anatexis during tectonic evolution from oceanic subduction to continental collision Previous petrological, geochronological and geochemical data demonstrate that the NQD records two epochs of orogenies, i.e. the Neoproterozoic Grenville Orogeny related to amalgamation and breakup of the supercontinent Rodinia (Yu et al., 2013), and the Early Paleozoic Caledonian Orogeny (Song et al., 2014b; Zhang et al., 2017). Several models have been proposed to explain the Early Paleozoic tectonic evolution of the NQD. Yang et al. (2002) implied that the early Paleozoic North Qilian accretionary orogen and the NQD collisional orogen are two parallel independent subduction systems. In contrast,

Acknowledgments This study was financially supported by the National Key R&D Plan of China (Grant no. 2017YFC0601401), National Science Foundation of 208

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China (Grant no. 41572053, 41872050，41630207 and 91755212) and the Scientific and Technological Innovation Project Financially Supported by Qingdao National Laboratory for Marine Science and Technology (No. 2016ASKJ13).

timescales of melt crystallization in the D'Entrecasteaux Islands, southeastern Papua New Guinea. Earth Planet. Sci. Lett. 351, 237–246. Harris, N.B.W., Inger, S., 1992. Trace-element modeling of pelite-derived granites. Contrib. Mineral. Petrol. 110 (1), 46–56. Harris, N., Massey, J., 1994. Decompression and anatexis of Himalayan metapelites. Tectonics 13 (6), 1537–1546. Harrison, T.M., Lovera, O.M., Grove, M., 1997. New insights into the origin of two contrasting Himalayan granite belts. Geology 25 (10), 899–902. Hermann, J., 2002. Allanite: thorium and light rare earth element carrier in subducted crust. Chem. Geol. 192 (3–4), 289–306. Hermann, J., Green, D.H., 2001. Experimental constraints on high pressure melting in subducted crust. Earth Planet. Sci. Lett. 188 (1–2), 149–168. Hermann, J., Rubatto, D., Korsakov, A., Shatsky, V.S., 2001. Multiple zircon growth during fast exhumation of diamondiferous, deeply subducted continental crust (Kokchetav Massif, Kazakhstan). Contrib. Mineral. Petrol. 141 (1), 66–82. Hermann, J., Spandler, C., Hack, A., Korsakov, A.V., 2006. Aqueous fluids and hydrous melts in high-pressure and ultra-high pressure rocks: implications for element transfer in subduction zones. Lithos 92, 399–417. Holness, M.B., Clemens, J.D., 1999. Partial melting of the Appin Quartzite driven by fracture-controlled H2O infiltration in the aureole of the Ballachulish Igneous complex, Scottish Highlands. Contrib. Mineral. Petrol. 136 (1–2), 154–168. Holness, M.B., Cesare, B., Sawyer, E.W., 2011. Melted rocks under the microscope: microstructures and their interpretation. Elements 7 (4), 247–252. Huang, W.L., Wyllie, P.J., 1981. Phase relationships of S-type granite with H2O to 35 kbar: muscovite granite from Harney Peak, South Dakota. J. Geophys. Res. Solid Earth 86 (861), 10515–10529. Huang, W.L., Wyllie, P.J., 1986. Phase Relationships of Gabbro-Tonalite-Granite-Water at 15 Kbar with Applications to Differentiation and Anatexis. Springer, US, pp. 1217–1220. Jahn, B.M., Glikson, A.Y., Peucat, J.J., Hickman, A.H., 1981. REE geochemistry and isotopic data of Archean silicic volcanics and granitoids from the Pilbara Block, Western Australia: implications for the early crustal evolution. Geochim. Cosmochim. Acta 45 (9), 1633–1652. Johnson, T.E., Fischer, S., White, R.W., 2013. Field and petrographic evidence for partial melting of TTG gneisses from the central region of the mainland Lewisian complex, NW Scotland. J. Geol. Soc. 170 (2), 319–326. Johnson, T.E., Brown, M., Gardiner, N.J., Kirkland, C.L., Smithies, R.H., 2017. Earth's first stable continents did not form by subduction. Nature 543, 239–242. Kang, Z., Jiang, C., Ling, J., Zhao, Y., Song, Y., Zhou, W., 2015. Petrogenesis and ore genesis of the ilmenite-rich Kendelong mafic-ultramafic intrusion in Wulan, Qinghai. Acta Petrol. Sin. 31 (8), 2193–2210 (in Chinese with English abstract). Keay, S., Lister, G., Buick, I., 2001. The timing of partial melting, Barrovian metamorphism and granite intrusion in the Naxos metamorphic core complex, Cyclades, Aegean Sea, Greece. Tectonophysics 342 (3–4), 275–312. Labrousse, L., Jolivet, L., Agard, P., Hebert, R., Andersen, T.B., 2002. Crustal-scale boudinage and migmatization of gneiss during their exhumation in the UHP Province of Western Norway. Terra Nova 14 (4), 263–270. Labrousse, L., Prouteau, G., Ganzhorn, A.C., 2011. Continental exhumation triggered by partial melting at ultrahigh pressure. Geology 39 (12), 1171–1174. Lang, H.M., Gilotti, J.A., 2007. Partial melting of metapelites at ultrahigh-pressure conditions, Greenland Caledonides. J. Metamorph. Geol. 25 (2), 129–147. Li, X.C., Niu, M.L., Yan, Z., Da, L.C., Han, Y., Wang, Y.S., 2015. LP/HT metamorphic rocks in Wulan County,Qinghai Province: an early Paleozoic paired metamorphic belt on the northern Qaidam Basin? Chin. Sci. Bull. 60 (35), 3501–3513 (in Chinese with English abstract). Li, S.Z., Zhao, S.J., Liu, X., Cao, H., Yu, S., Li, X.Y., Somerville, I., Yu, S.Y., Suo, Y.H., 2017. Closure of the Proto-Tethys Ocean and early Paleozoic amalgamation of the microcontinental blocks in East Asia. Earth Sci. Rev. https://doi.org/10.1016/j. earscirev.2017.01.011. Li, S.Z., Zhao, S.J., Liu, X., Cao, H.H., Yu, S., Li, X.Y., Someville, I., Yu, S.Y., Suo, Y.H., 2018. Closure of the Proto-Tethys Ocean and Early Paleozoic amalgamation of microcontinental blocks in East Asia. Earth Sci. Rev. 186, 37–75. Liu, F.L., Robinson, P.T., Liu, P.H., 2012a. Multiple partial melting events in the Sulu UHP terrane: zircon U-Pb dating of granitic leucosomes within amphibolite and gneiss. J. Metamorph. Geol. 30 (8), 887–906. Liu, X.C., Wu, Y.B., Gao, S., Liu, Q., Wang, H., Qin, Z.W., Li, Q.L., Li, X.H., Gong, H.J., 2012b. First record and timing of UHP metamorphism from zircon in the Xitieshan terrane: implications for the evolution of the entire North Qaidam metamorphic belt. Am. Mineral. 97 (7), 1083–1093. Liu, X.C., Wu, Y.B., Gao, S., Wang, H., Zheng, J.P., Hu, Z.C., Zhou, L., Yang, S.H., 2014. Record of multiple stage channelized fluid and melt activities in deeply subducted slab from zircon U-Pb age and Hf-O isotope compositions. Geochim. Cosmochim. Acta 144, 1–24. Lopez, S., Castro, A., 2001. Determination of the fluid-absent solidus and supersolidus phase relationships of MORB-derived amphibolites in the range 4-14 kbar. Am. Mineral. 86 (11−12), 1396–1403. Lu, S.N., Yu, H.F., Zhao, Q.F., Jin, W., Li, H.K., Li, Q., Yang, C.L., Li, H.M., Zheng, J.K., Zhang, M.S., Jiang, M.M., Ge, X.H., Xiu, Q.Y., Zhang, W.Z., Guo, J.J., Liu, Y.J., 2002. Preliminary Study of Precambrian Geology in the North Tibet-Qinghai Plateau, 1st ed. Geological Publishing House, Beijing. Lu, S.N., Zhao, G.C., Wang, H.C., Hao, G.J., 2008. Precambrian metamorphic basement and sedimentary cover of the North China Craton: a review. Precambrian Res. 160 (1–2), 77–93. Martin, H., Smithies, R.H., Rapp, R., Moyen, J.F., Champion, D., 2005. An overview of adakite, tonalite-trondhjemite-granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution. Lithos 79 (1–2), 1–24.

References Aikman, A.B., Harrison, T.M., Lin, D., 2008. Evidence for early (> 44 Ma) Himalayan Crustal Thickening, Tethyan Himalaya, southeastern Tibet. Earth Planet. Sci. Lett. 274 (1–2), 14–23. Andersson, J., Moller, C., Johansson, L., 2002. Zircon geochronology of migmatite gneisses along the Mylonite Zone (S Sweden): a major Sveconorwegian terrane boundary in the Baltic Shield. Precambrian Res. 114 (1–2), 121–147. Auzanneau, E., Vielzeuf, D., Schmidt, M.W., 2006. Experimental evidence of decompression melting during exhumation of subducted continental crust. Contrib. Mineral. Petrol. 152 (2), 125–148. Brown, M., 2001. Crustal melting and granite magmatism: key issues. Phys. Chem. Earth Solid Earth Geod. 26 (4–5), 201–212. Brown, M., 2013. Granite: from genesis to emplacement. Geol. Soc. Am. Bull. 125 (7–8), 1079–1113. Cao, Y.T., Liu, L., Chen, D.L., Wang, C., Yang, W.Q., Kang, L., Zhu, X.H., 2017. Partial melting during exhumation of Paleozoic retrograde eclogite in North Qaidam, western China. J. Asian Earth Sci. 148, 223–240. Cesare, B., Ferrero, S., Salviolo-Mariani, E., Pedron, D., Cavallo, A., 2009. "Nanogranite" and glassy inclusions: the anatectic melt in migmatites and granulites. Geology 37 (7), 627–630. Chamberlain, C.P., Sonder, L.J., 1990. Heat-Producing elements and the thermal and Baric patterns of Metamorphic Belts. Science 250 (4982), 763–769. Chen, Y.X., Zheng, Y.F., 2013. Petrological evidence for crustal anatexis during continental collision in the Sulu orogen. Chin. Sci. Bull. 58 (22), 2198–2202. Chen, D.L., Sun, Y., Liu, L., Zhang, A.D., Luo, J.H., Wang, Y., 2005. Metamorphic evolution of the Yuka eclogite in the North Qaidam, NW China: evidences from the compositional zonation of garnet and reaction texture in the rock. Acta Petrol. Sin. 21 (4), 1039–1048 (in Chinese with English abstract). Chen, X., Xu, R.K., Zheng, Y.Y., Cai, P.J., 2008. Petrology and geochemistry of high niobium eclogite in the North Qaidam orogen, Western China: Implications for an eclogite facies metamorphosed island arc slice. J. Asian Earth Sci. 164, 380–397. Chen, D.L., Liu, L., Sun, Y., Liou, J.G., 2009. Geochemistry and zircon U-Pb dating and its implications of the Yukahe HP/UHP terrane, the North Qaidam, NW China. J. Asian Earth Sci. 35 (3), 259–272. Chen, D.L., Liu, L., Sun, Y., Sun, W.D., Zhu, X.H., Liu, X.M., Guo, C.L., 2012a. Felsic veins within UHP eclogite at Xitieshan in North Qaidam, NW China: partial melting during exhumation. Lithos 136, 187–200. Chen, N.S., Zhang, L., Sun, M., Wang, Q.Y., Kusky, T.M., 2012b. U-Pb and Hf isotopic compositions of detrital zircons from the paragneisses of the Quanji Massif, NW China: implications for its early tectonic evolutionary history. J. Asian Earth Sci. 54–55, 110–130. Chopin, C., 2003. Ultrahigh-pressure metamorphism: tracing continental crust into the mantle. Earth Planet. Sci. Lett. 212 (1), 1–14. Clemens, J.D., 2006. Melting of the continental crust: fluid regimes, melting reactions, and source-rock fertility. In: Brown, M., Rushmer, T. (Eds.), Evolution and Differentiation of the Continental Crust. Cambridge University Press, London, pp. 297–331. Condie, K.C., 2005. TTGs and adakites: are they both slab melts? Lithos 80 (1), 33–44. Condie, K.C., Kroner, A., 2013. The building blocks of continental crust: evidence for a major change in the tectonic setting of continental growth at the end of the Archean. Gondwana Res. 23 (2), 394–402. Costa, S., Rey, P., 1995. Lower crustal rejuvenation and growth during post-thickening collapse – insights from a crustal cross-section through a Variscan Metamorphic core complex. Geology 23 (10), 905–908. Defant, M.J., Drummond, M.S., 1990. Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347 (6294), 662–665. Ellis, S.M., Little, T.A., Wallace, L.M., Hacker, B.R., Buiter, S.J.H., 2011. Feedback between rifting and diapirism can exhume ultrahigh-pressure rocks. Earth Planet. Sci. Lett. 311 (3–4), 427–438. England, P.C., Thompson, A.B., 1984. Pressure-temperature-time paths of regional metamorphism I. Heat transfer during the evolution of regions of thickened continental crust. J. Petrol. 25 (4), 766–803. Flowerdew, M.J., Millar, I.L., Vaughan, A.P.M., Horstwood, M.S.A., Fanning, C.M., 2006. The source of granitic gneisses and migmatites in the Antarctic Peninsula: a combined U–Pb SHRIMP and laser ablation Hf isotope study of complex zircons. Contrib. Mineral. Petrol. 151 (6), 751–768. Foster, D.A., Fanning, C.M., 1997. Geochronology of the northern Idaho batholith and the Bitterroot metamorphic core complex: magmatism preceding and contemporaneous with extension. Geol. Soc. Am. Bull. 109 (4), 379–394. Ganguly, J., Dasgupta, S., Cheng, W.J., Neogi, S., 2000. Exhumation history of a section of the Sikkim Himalayas, India: records in the metamorphic mineral equilibria and compositional zoning of garnet. Earth Planet. Sci. Lett. 183 (3–4), 471–486. Gong, S.L., Chen, N.S., Wang, Q.Y., Kusky, T.M., Wang, L., Zhang, L., Ba, J., Liao, F.X., 2012. Early Paleoproterozoic magmatism in the Quanji Massif, northeastern margin of the Qinghai-Tibet Plateau and its tectonic significance: LA-ICPMS U-Pb zircon geochronology and geochemistry. Gondwana Res. 21 (1), 152–166. Gordon, S.M., Little, T.A., Hacker, B.R., Bowring, S.A., Korchinski, M., Baldwin, S.L., Kylander-Clark, A.R.C., 2012. Multi-stage exhumation of young HP-UHP rocks:

209

Earth-Science Reviews 191 (2019) 190–211

S. Yu, et al. Mattinson, C.G., Wooden, J.L., Liou, J.G., Bird, D.K., Wu, C.L., 2006. Age and duration of eclogite-facies metamorphism, North Qaidam HP/UHP terrane, western China. Am. J. Sci. 306 (9), 683–711. Mattinson, C.G., Menold, C.A., Zang, J.X., Bird, D.K., 2007. High- and ultrahigh-pressure metamorphism in the North Qaidam and South Altyn Terranes, western China. Int. Geol. Rev. 49 (11), 969–995. Mattinson, C.G., Wooden, J.L., Zhang, J.X., Bird, D.K., 2009. Paragneiss zircon geochronology and trace element geochemistry, North Qaidam HP/UHP terrane, western China. J. Asian Earth Sci. 35 (3–4), 298–309. Mayen, J.F., Laurent, O., Chelle-Michou, C., Couzinie, S., Vanderhaeghe, O., Zeh, A., Villaros, A., Gardien, V., 2017. Collision vs. subduction-related magmatism: two contrasting ways of granite formation and implications for crustal growth. Lithos 277, 154–177. Meng, F.C., Zhang, J.X., 2008. Contemporaneous of early Palaeozoic granite and high temperature metamorphism, North Qaidam Mountains, western China. Acta Petrol. Sin. 24 (7), 1585–1594 (in Chinese with English abstract). Meng, F.C., Zhang, J.X., Yang, J.S., Xu, Z.Q., 2003. Geochemical characteristics of cclogites in Xitieshan crea, north Qaidam of northwest China. Acta Petrol. Sin. 19, 435–452 (In Chinese with English abstract). Meng, F.C., Zhang, J.X., Yang, J.S., 2005. Tectono-thermal event of post-HP/UHP metamorphism in the Xitieshan area of the North Qaidam Mountains, western China: isotopic and geochemical evidence of granite and gneiss. Acta Petrol. Sin. 21 (1), 45–56 (in Chinese with English abstract). Menold, C.A., Manning, C.E., Yin, A., Tropper, P., Chen, X.H., Wang, X.F., 2009. Metamorphic evolution, mineral chemistry and thermobarometry of orthogneiss hosting ultrahigh-pressure eclogites in the North Qaidam metamorphic belt, Western China. J. Asian Earth Sci. 35 (3–4), 273–284. Mo, X.X., Niu, Y.L., Dong, G.C., Zhao, Z.D., Hou, Z.Q., Zhou, S., Ke, S., 2008. Contribution of syncollisional felsic magmatism to continental crust growth: a case study of the Paleogene Linzizong volcanic Succession in southern Tibet. Chem. Geol. 250 (1–4), 49–67. Monteleone, B.D., Baldwin, S.L., Webb, L.E., Fitzgerald, P.G., Grove, M., Schmitt, A.K., 2007. Late Miocene-Pliocene eclogite facies metamorphism, D'Entrecasteaux Islands, SE Papua New Guinea. J. Metamorph. Geol. 25 (2), 245–265. Moyen, J.F., Stevens, G., 2006. Experimental constraints on TTG petrogenesis: implications for Archean geodynamics. Archean Geody. Environ. 164, 149–178. Nagel, T., Hoffmann, J.E., Munker, C., 2012. Generation of Eoarchean tonalite-trondhjemite- granodiorite series from thickened mafic arc crust. Geology 40, 375–378. Niu, Y.L., Zhao, Z.D., Zhu, D.C., Mo, X.X., 2013. Continental collision zones are primary sites for net continental crust growth – a testable hypothesis. Earth Sci. Rev. 127, 96–110. Patiño Douce, A.E., 2005. Vapor-absent melting of tonalite at 15-32 kbar. J. Petrol. 46 (2), 275–290. Patiño Douce, A.E., Harris, N., 1998. Experimental constraints on Himalayan anatexis. J. Petrol. 39 (4), 689–710. Patiño Douce, A.E., Humphreys, E.D., Johnston, A.D., 1990. Anatexis and metamorphism in tectonically thickened continental-crust exemplified by the Sevier hinterland, western North-America. Earth Planet. Sci. Lett. 97 (3–4), 290–315. Qian, Q., Hermann, J., 2010. Formation of high-Mg diorites through assimilation of Peridotite by mozodiorite magma at crustal depth. J. Petrol. 51, 1381–1416. Rapp, R.P., Watson, E.B., 1995. Dehydration melting of metabasalt at 8-32-Kbar – implications for continental growth and crust-mantle recycling. J. Petrol. 36 (4), 891–931. Ren, Y.F., Chen, D.L., Hauzenberger, C., Liu, L., Liu, X.M., Zhu, X.H., 2016. Petrology and geochronology of ultrahigh-pressure granitic gneiss from South Dulan, North Qaidam belt, NW China. Int. Geol. Rev. 58, 171–195. Ren, Y.F., Chen, D.L., Kelsey, D.E., Gong, X.K., Liu, L., 2017. Petrology and geochemistry of the lawsonite (pseudomorph)-bearing eclogite in Yuka terrane, North Qaidam UHPM belt: an eclogite facies metamorphosed oceanic slice. Gondwana Res. 42, 220–242. Rey, P., Vanderhaeghe, O., Teyssier, C., 2001. Gravitational collapse of the continental crust: definition, regimes and modes. Tectonophysics 342 (3–4), 435–449. Rubatto, D., 2002. Zircon trace element geochemistry: partitioning with garnet and the link between U-Pb ages and metamorphism. Chem. Geol. 184 (1–2), 123–138. Rubatto, D., Hermann, J., Berger, A., Engi, M., 2009. Protracted fluid-induced melting during Barrovian metamorphism in the Central Alps. Contrib. Mineral. Petrol. 158 (6), 703–722. Rubatto, D.M., Regis, D., Hermann, J., Boston, K., Engi, M., Beltrando, M., McAlpine, S.R.B., 2011. Yo-yo subduction recorded by accessory minerals in the Italian Western Alps. Nat. Geosci. 4, 338–342. Sawyer, E.W., 2001. Melt segregation in the continental crust: distribution and movement of melt in anatectic rocks. J. Metamorph. Geol. 19 (3), 291–309. Sawyer, E.W., 2008. Atlas of Migmatites, the Canadian Mineralogist special Publication 9. In: Mineralogical Association of Canada, Quebec. NRC Research Press, Ottawa, pp. 371. Sawyer, E.W., 2010. Migmatites formed by water-fluxed partial melting of a leucogranodiorite protolith: microstructures in the residual rocks and source of the fluid. Lithos 116 (3–4), 273–286. Sawyer, E.W., Cesare, B., Brown, M., 2011. When the continental crust melts. Elements 7 (4), 229–233. Schmidt, M., 1993. Phase relations and compositions in tonalite as a function of pressure: an experimental study at 650 C. Am. J. Sci. 293, 1011. Schmidt, M.W., Vielzeuf, D., Auzanneau, E., 2004. Melting and dissolution of subducting crust at high pressures: the key role of white mica. Earth Planet. Sci. Lett. 228, 65–84. Searle, M.P., Szulc, A.G., 2005. Channel flow and ductile extrusion of the high Himalayan slab-the Kangchenjunga – Darjeeling profile, Sikkim Himalaya. J. Asian Earth Sci. 25

(1), 173–185. Sen, C., Dunn, T., 1994. Dehydration melting of a basaltic composition amphibolite at 1.5 and 2.0 Gpa – Implications for the origin of Adakites. Contrib. Mineral. Petrol. 117 (4), 394–409. Shi, R.D., Yang, J.S., Wu, C.L., Iizuka, T., Hirata, T., 2006. Island arc volcanic rocks in the north Qaidam UHP belt, northern Tibet plateau: evidence for ocean-continent subduction preceding continent-continent subduction. J. Asian Earth Sci. 28 (2–3), 151–159. Sizova, E., Gerya, T., Brown, M., 2012. Exhumation mechanisms of melt-bearing ultrahigh pressure crustal rocks during collision of spontaneously moving plates. J. Metamorph. Geol. 30 (9), 927–955. Skjerlie, K.P., Patiño Douce, A.E., 2002. The fluid-absent partial melting of a zoisitebearing quartz eclogite from 1·0 to 3·2 GPa; implications for melting in thickened continental crust and for subduction-zone processes. J. Petrol. 43 (2), 291–314. Smithies, R.H., 2000. The Archaean tonalite-trondhjemite-granodiorite (TTG) series is not an analogue of Cenozoic adakite. Earth Planet. Sci. Lett. 182 (1), 115–125. Song, S.G., Yang, J.S., Liou, J.G., Wu, C.L., Shi, R.D., Xu, Z.Q., 2003. Petrology, geochemistry and isotopic ages of eclogites from the Dulan UHPM Terrane, the North Qaidam, NW China. Lithos 70 (3–4), 195–211. Song, S.G., Zhang, L.F., Niu, Y.L., 2004. Ultra-deep origin of garnet peridotite from the North Qaidam ultrahigh-pressure belt, Northern Tibetan Plateau, NW China. Am. Mineral. 89 (8–9), 1330–1336. Song, S.G., Zhang, L.F., Niu, Y.L., Su, L., Jian, P., Liu, D.Y., 2005. Geochronology of diamond-bearing zircons from garnet peridotite in the North Qaidam UHPM belt, Northern Tibetan Plateau: a record of complex histories from oceanic lithosphere subduction to continental collision. Earth Planet. Sci. Lett. 234 (1–2), 99–118. Song, S.G., Zhang, L.F., Niu, Y.L., Su, L., Song, B., Liu, D.Y., 2006. Evolution from oceanic subduction to continental collision: a case study from the Northern Tibetan Plateau based on geochemical and geochronological data. J. Petrol. 47 (3), 435–455. Song, S.G., Su, L., Li, X.H., Niu, Y.L., Zhang, L.F., 2012. Grenville-age orogenesis in the Qaidam-Qilian block: the link between South China and Tarim. Precambrian Res. 220, 9–22. Song, S.G., Niu, Y.L., Su, L., Wei, C.J., Zhang, L.F., 2014. Adakitic (tonalitic-trondhjemitic) magmas resulting from eclogite decompression and dehydration melting during exhumation in response to continental collision. Geochimica et Cosmochimica Acta 130, 42–62. Song, S.G., Niu, Y.L., Su, L., Wei, C.J., Zhang, L.F., 2014a. Adakitic (tonalitic-trondhjemitic) magmas resulting from eclogite decompression and dehydration melting during exhumation in response to continental collision. Geochim. Cosmochim. Acta 130, 42–62. Song, S.G., Niu, Y.L., Su, L., Zhang, C., Zhang, L.F., 2014b. Continental orogenesis from ocean subduction, continent collision/subduction, to orogen collapse, and orogen recycling: the example of the North Qaidam UHPM belt, NW China. Earth Sci. Rev. 129 (1), 59–84. Spencer, C.J., Cawood, P.A., Hawkesworth, C.J., Prave, A.R., Roberts, N.M.W., Horstwood, M.S.A., Whitehouse, M.J., 2015. Generation and preservation of continental crust in the Grenville Orogeny. Geosci. Front. 6 (3), 357–372. Stern, C.R., Wyllie, P.J., 1981. Phase relationships of I-type granite with H2O to 35 kilobars: the Dinkey Lakes biotite-granite from the Sierra Nevada Batholith. J. Geophys. Res. Solid Earth 86 (B11), 10412–10422. Sun, J.P., Chen, S.Y., Peng, Y.B., Shao, P.C., Ma, S., Liu, J., 2015. Determination of early Cambrian Zircon SHRIMP U-Pb Datings in Zongwulong Tectonic Belt, Northern margin of Qaidam Basin,and its Geological significance. Geol. Rev. 743–751 (in Chinese with English abstract). Thompson, A.B., 1982. Dehydration melting of pelitic rocks and the generation of H2Oundersaturated granitic liquids. Am. J. Sci. 282 (10), 1567–1595. Thompson, A.B., Connolly, J.A.D., 1995. Melting of the continental-crust – some thermal and petrological constraints on anatexis in continental collision zones and other tectonic settings. J. Geophys. Res. Solid Earth 100 (B8), 15565–15579. Vanderhaeghe, O., Teyssier, C., 1997. Formation of the Shuswap metamorphic core complex during late-orogenic collapse of the Canadian Cordillera: role of ductile thinning and partial melting of the mid- to lower crust. Geodin. Acta 10 (2), 41–58. Vanderhaeghe, O., Teyssier, C., 2001. Partial melting and flow of orogens. Tectonophysics 342 (3–4), 451–472. Vernon, R.H., 2010. Granites really are magmatic: using microstructural evidence to refute some obstinate hypotheses. J. Virtual Explor. 35https://doi.org/10.3809/jvirtex. 2011.00264. paper 1. Vielzeuf, D., Holloway, J.R., 1988. Experimental determination of the fluid-absent melting relations in the pelitic system. Contrib. Mineral. Petrol. 98 (3), 257–276. Vielzeuf, D., Schmidt, M.W., 2001. Melting relations in hydrous systems revisited: application to metapelites, metagreywackes and metabasalts. Contrib. Mineral. Petrol. 141 (3), 251–267. Wang, M.J., Song, S.G., Niu, Y.L., Su, L., 2014. Post-collisional magmatism: consequences of UHPM terrane exhumation and orogen collapse, N. Qaidam UHPM belt, NW China. Lithos 210, 181–198. Wedepohl, K.H., 1995. The compositions of the continental crust. Geochim.Cosmochim. Acta 59, 1217–1232. Weinberg, R.F., Hasalova, P., 2015. Water-fluxed melting of the continental crust: a review. Lithos 212, 158–188. Whitney, D.L., Evans, B.W., 2010. Abbreviations for names of rock-forming minerals. Am. Mineral. 95, 185–187. Whitney, D.L., Teyssier, C., Fayon, A.K., Hamilton, M.A., Heizler, M., 2003. Tectonic controls on metamorphism, partial melting, and intrusion: timing and duration of regional metamorphism and magmatism in the Nigde Massif, Turkey. Tectonophysics 376 (1–2), 37–60. Whitney, D.L., Teyssier, C., Fayon, A.K., 2004. Isothermal decompression, partial melting

210

Earth-Science Reviews 191 (2019) 190–211

S. Yu, et al.

supercontinent. Precambrian Res. 291, 42–62. Yuan, G.B., Wang, H.C., Li, H.M., Hao, G.J., Xin, H.T., Zhang, B.H., Wang, Q.H., Tian, Q., 2002. Zircon U-Pb age of the Gabbros in Luliangshan Area on the Northern margin of Qaidam Basin and its Geological Implication. Prog. Precambrian Res. 25 (1), 37–40 (in Chinese with English abstract). Zegers, T.E., van Keken, P.E., 2001. Middle Archean continent formation by crustal delamination. Geology 29 (12), 1083–1086. Zeng, L.S., Saleeby, J.B., Asimow, P., 2005. Nd isotope disequilibrium during crustal anatexis: a record from the Goat Ranch migmatite complex, southern Sierra Nevada batholith, California. Geology 33 (1), 53–56. Zeng, L.S., Liu, J., Gao, L., Xie, K.J., Wen, L., 2009. Early Oligocene anatexis in the Yardoi gneiss dome, southern Tibet and geological implications. Chin. Sci. Bull. 54 (1), 104–112. Zhang, J.X., Meng, F.C., Yang, J.S., 2004. Eclogitic metapelites in the western segment of the north Qaidam Mountains:evidence on "in situ" relationship between eclogite and its country rock. Sci. China 47 (12), 1102–1112. Zhang, J.X., Yang, J.S., Mattinson, C.G., Xu, Z.Q., Meng, F.C., Shi, R.D., 2005. Two contrasting eclogite cooling histories, North Qaidam HP/UHP terrane, western China: petrological and isotopic constraints. Lithos 84 (1–2), 51–76. Zhang, J.X., Yang, J.S., Meng, F.C., Wan, Y.S., Li, H.M., Wu, C.L., 2006. U-Pb isotopic studies of eclogites and their host gneisses in the Xitieshan area of the North Qaidam mountains, western China: new evidence for an early Paleozoic HP-UHP metamorphic belt. J. Asian Earth Sci. 28 (2–3), 143–150. Zhang, J.X., Meng, F.C., Yu, S.Y., Qi, X.X., 2007. Metamorphic history recorded in high pressure mafic granulites in the Luliangshan Mountains to the north of Qaidam Basin, Northwest China: evidence from petrology and zircon SHRIMP geochronology. Earth Sci. Front. 14 (1), 85–97 (in Chinese with English abstract). Zhang, J.X., Mattinson, C.G., Meng, F.C., Wan, Y.S., Tung, K., 2008a. Polyphase tectonothermal history recorded in granulitized gneisses from the north Qaidam HP/UHP metamorphic terrane, western China: evidence from zircon U-Pb geochronology. Geol. Soc. Am. Bull. 120 (5–6), 732–749. Zhang, G.B., Song, S.G., Zhang, L.F., Niu, Y.L., 2008b. The subducted oceanic crust within continental-type UHP metamorphic belt in the North Qaidam, NW China: evidence from petrology, geochemistry and geochronology. Lithos 104 (1–4), 99–118. Zhang, G.B., Ellis, D.J., Christy, A.G., Zhang, L.F., Niu, Y.L., Song, S.G., 2009a. UHP metamorphic evolution of coesite-bearing eclogite from the Yuka terrane, North Qaidam UHPM belt, NW China. Eur. J. Mineral. 21 (6), 1287–1300. Zhang, J.X., Meng, F.C., Li, J.P., Mattinson, C.G., 2009b. Coesite in eclogite from the North Qaidam Mountains and its implications. Chin. Sci. Bull. 54 (6), 1105–1110. Zhang, G.B., Zhang, L.F., Song, S.G., Niu, Y.L., 2009c. UHP metamorphic evolution and SHRIMP geochronology of a coesite-bearing meta-ophiolitic gabbro in the North Qaidam, NW China. J. Asian Earth Sci. 35 (3–4), 310–322. Zhang, J.X., Mattinson, C.G., Yu, S.Y., Li, J.P., Meng, F.C., 2010. U-Pb zircon geochronology of coesite-bearing eclogites from the southern Dulan area of the North Qaidam UHP terrane, northwestern China: spatially and temporally extensive UHP metamorphism during continental subduction. J. Metamorph. Geol. 28 (9), 955–978. Zhang, C., Zhang, L.F., Van Roermund, H., Song, S.G., Zhang, G.B., 2011. Petrology and SHRIMP U-Pb dating of Xitieshan eclogite, North Qaidam UHP metamorphic belt, NW China. J. Asian Earth Sci. 42 (4), 752–767. Zhang, L., Chen, R.X., Zheng, Y.F., Hu, Z., 2015. Partial melting of deeply subducted continental crust during exhumation: insights from felsic veins and host UHP metamorphic rocks in North Qaidam, northern Tibet. J. Metamorph. Geol. 33 (7), 671–694. Zhang, L., Chen, R.X., Zheng, Y.F., Li, W.C., Hu, Z., Yang, Y.H., Tang, H.L., 2016. The tectonic transition from oceanic subduction to continental subduction: Zirconological constraints from two types of eclogites in the North Qaidam orogen, northern Tibet. Lithos 244, 122–139. Zhang, J.X., Yu, S.Y., Mattinson, C.G., 2017. Early Paleozoic polyphase metamorphism in northern Tibet, China. Gondwana Res. 41, 267–289. Zhao, Z.F., Zheng, Y.F., Chen, R.X., Xia, Q.X., Wu, Y.B., 2007. Element mobility in mafic and felsic ultrahigh-pressure metamorphic rocks during continental collision. Geochim. Cosmochim. Acta 71 (21), 5244–5266. Zhao, Z.F., Zheng, Y.F., Zhang, J., Dai, L.Q., Li, Q.L., Liu, X.M., 2012. Syn-exhumation magmatism during continental collision: evidence from alkaline intrusives of Triassic age in the Sulu orogen. Chem. Geol. 328, 70–88. Zheng, J.P., Griffin, W.L., O'Reilly, S.Y., Yang, J.S., Zhang, R.Y., 2006. A refractory mantle protolith in younger continental crust, east-Central China: age and composition of zircon in the Sulu ultrahigh-pressure peridotite. Geology 34 (9), 705–708. Zheng, Y.F., Xia, Q.X., Chen, R.X., Gao, X.Y., 2011. Partial melting, fluid supercriticality and element mobility in ultrahigh-pressure metamorphic rocks during continental collision. Earth Sci. Rev. 107 (3–4), 342–374. Zhu, X.H., Chen, D.L., Liu, L., Li, D., 2010. Zicon LA-ICP-MS U-Pb dating of the Wanggaxiu gabbro complex in the Dulan area, northern margin of Qaidam Basin, China and its geological significance. Geolo. Bull. China 29 (2), 227–236 (in Chinese with English abstract). Zong, K.Q., Liu, Y.S., Hu, Z.C., Kusky, T., Wang, D.B., Gao, C.G., Gao, S., Wang, J.Q., 2010. Melting-induced fluid flow during exhumation of gneisses of the Sulu ultrahigh-pressure terrane. Lithos 120 (3–4), 490–510.

and exhumation of deep continental crust. In: Grocott, J., McCaffrey, K.J.W., Taylor, G.K., Tikoff, B. (Eds.), Vertical Coupling and Decoupling in the Lithosphere. vol. 227. Geological Society of London Special Publication, pp. 313–326. Wolf, M.B., Wyllie, P.J., 1994. Dehydration-Melting of Amphibolite at 10 Kbar -the Effects of Temperature and Time. Contrib. Mineral. Petrol. 115 (4), 369–383. Wu, C.L., Yang, J.S., Wooden, J.L., Shi, R.D., Chen, S.Y., Meibom, A., Mattinson, C.G., 2004. Zircon U-Pb SHRIMP dating of the Yematan batholith in Dulan, North Qaidam, NW China. Chin. Sci. Bull. 49 (16), 1736–1740. Xia, Q.X., Zheng, Y.F., Zhou, L.G., 2008. Dehydration and melting during continental collision: constraints from element and isotope geochemistry of low-T/UHP granitic gneiss in the Dabie orogen. Chem. Geol. 247 (1–2), 36–65. Xiong, X.L., 2006. Trace element evidence for growth of early continental crust by melting of rutile-bearing hydrous eclogite. Geology 34, 945–948. Xiong, Q., Zheng, J.P., Griffin, W.L., O'Reilly, S.Y., Zhao, J.H., 2011. Zircons in the Shenglikou ultrahigh-pressure garnet peridotite massif and its country rocks from the North Qaidam terrane (western China): Meso-Neoproterozoic crust-mantle coupling and early Paleozoic convergent plate-margin processes. Precambrian Res. 187 (1–2), 33–57. Yakymchuk, C., Brown, M., Ivanic, T.J., Korhonen, F.J., 2013. Leucosome distribution in migmatitic paragneisses and orthogneisses: a record of self-organized melt migration and entrapment in a heterogeneous partially-molten crust. Tectonophysics 603, 136–154. Yang, J.J., Deng, J.F., 1994. Garnet Peridotite and Eclogites in the Northern Qaidam Mountains, Tibetan Plateau: A First Record on UHP Metamorphism and Tectonics. II.P Task Group III-6, Stanford (A-20). Yang, J.S., Xu, Z.Q., Song, S.G., Zhang, J.X., Wu, C.L., Shi, R.D., Li, H.B., Brunel, M., 2001. Discovery of coesite in the North Qaidam early Palaeozoic ultrahigh pressure (UHP) metamorphic belt, NW China. C. R. Acad. Sci. II Fasc. Sci. Terre Planet. 333 (11), 719–724. Yang, J.S., Xu, Z.Q., Zhang, J.X., Song, S.G., Wu, C.L., Shi, R.D., Li, H.B., Brunel, M., 2002. Early Paleozoic North Qaidam UHP metamorphic belt on the northeastern Tibetan plateau and a paired subduction model. Terra Nova 14, 397–404. Yang, J.S., Zhang, J.X., Meng, F.C., Shi, R.D., Wu, C.L., Xu, Z.Q., Li, H.B., Chen, N.S., 2003. Ultrahigh pressure eclogites of the North Qaidam and Altun Mountains, NW China, and their protoliths. Earth Sci. Front. 10, 291–314 (In Chinese with English abstract). Yang, J.S., Wu, C.L., Zhang, J.X., Shi, R.D., Meng, F.C., Wooden, J.L., Yang, H.Y., 2006. Protolith of eclogites in the north Qaidam and Altun UHP terrane, NW China: earlier oceanic crust? J. Asian Earth Sci. 28 (2–3), 185–204. Yin, A., Manning, C.E., Lovera, O., Menold, C.A., Chen, X.H., 2007. Early paleozoic tectonic and thermomechanical evolution of ultrahigh-pressure (UHP) metamorphic rocks in the northern Tibetan plateau, Northwest China. Int. Geol. Rev. 49 (8), 681–716. Yu, F.C., Wei, G.F., Sun, J.D., 1994. Metallogenic Modal for the Syntectonic Gold Deposits in Black Series-a Case Study of the Tanjianshan Deposit. Northwest University Press, Xi'an, pp. 1–130 (in Chinese). Yu, S.Y., Zhang, J.X., Li, J.P., 2009. Metamorphism history and dynamics of high-pressure granulites in the Dulan area of the North Qaidam Mountains, Northwest China. Acta Petrol. Sin. 25 (9), 2224–2234 (in Chinese with English abstract). Yu, S.Y., Zhang, J.X., Del Real, P.G., 2011a. Petrology and P-T path of high-pressure granulite from the Dulan area, North Qaidam Mountains, northwestern China. J. Asian Earth Sci. 42 (4), 641–660. Yu, S.Y., Zhang, J.X., Hou, K.J., 2011b. Two constrasting magmatic events in the Dulan UHP metamorphic terrane: implication for collisional orogeny. Acta Petrol. Sin. 27 (11), 3335–3349 (in Chinese with English abstract). Yu, S.Y., Zhang, J.X., Del Real, P.G., 2012. Geochemistry and zircon U-Pb ages of adakitic rocks from the Dulan area of the North Qaidam UHP terrane, North Tibet: constraints on the timing and nature of regional tectonothermal events associated with collisional orogeny. Gondwana Res. 21 (1), 167–179. Yu, S.Y., Zhang, J.X., Li, H.k., Hou, K.J., Mattinson, C.G., Gong, J.H., 2013. Geochemistry, zircon U Pb geochronology and Lu Hf isotopic composition of eclogites and their host gneisses in the Dulan area, North Qaidam UHP terrane: new evidence for deep continental subduction. Gondwana Res. 23 (3), 901–919. Yu, S.Y., Zhang, J.X., Mattinson, C.G., del Real, P.G., Li, Y.S., Gong, J.H., 2014. Paleozoic HP granulite-facies metamorphism and anatexis in the Dulan area of the North Qaidam UHP terrane, western China: constraints from petrology, zircon U-Pb and amphibole Ar-Ar geochronology. Lithos 198, 58–76. Yu, S.Y., Zhang, J.X., Sun, D.Y., Li, Y.S., Gong, J.H., 2015a. Anatexis of ultrahigh-pressure eclogite during exhumation in the North Qaidam ultrahigh-pressure terrane: constraints from petrology, zircon U-Pb dating, and geochemistry. Geol. Soc. Am. Bull. 127 (9–10), 1290–1312. Yu, S.Y., Zhang, J.X., Sun, D.Y., del Real, P.G., Li, Y.S., Zhao, X.L., Hou, K.J., 2015b. Petrology, geochemistry, zircon U-Pb dating and Lu-Hf isotope of granitic leucosomes within felsic gneiss from the North Qaidam UHP terrane: constraints on the timing and nature of partial melting. Lithos 218, 1–21. Yu, S.Y., Zhang, J.X., Li, S.Z., Sun, D.Y., Li, Y.S., Liu, X., Guo, L.L., Suo, Y.H., Peng, Y.B., Zhao, X.L., 2017. Paleoproterozoic granulite-facies metamorphism and anatexis in the Oulongbuluke Block, NW China: respond to assembly of the Columbia

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