The Early Paleozoic Xitieshan syn-collisional granite in the North Qaidam ultrahigh-pressure metamorphic belt, NW China: Petrogenesis and implications for continental crust growth

    The Early Paleozoic Xitieshan syn-collisional granite in the North Qaidam ultrahigh-pressure metamorphic belt, NW China: Petrogenesis and implications for continental crust growth Zhixin Zhao, Junhao Wei, Lebing Fu, Shengnan Liang, Shaoqing Zhao PII: DOI: Reference:

S0024-4937(17)30034-8 doi:10.1016/j.lithos.2017.01.019 LITHOS 4217

To appear in:

LITHOS

Received date: Accepted date:

28 June 2016 30 January 2017

Please cite this article as: Zhao, Zhixin, Wei, Junhao, Fu, Lebing, Liang, Shengnan, Zhao, Shaoqing, The Early Paleozoic Xitieshan syn-collisional granite in the North Qaidam ultrahigh-pressure metamorphic belt, NW China: Petrogenesis and implications for continental crust growth, LITHOS (2017), doi:10.1016/j.lithos.2017.01.019

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ACCEPTED MANUSCRIPT The Early Paleozoic Xitieshan syn-collisional granite in the North Qaidam ultrahigh-pressure metamorphic belt, NW China: petrogenesis and implications for

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continental crust growth

Zhixin Zhaoa, Junhao Weia, Lebing Fua*, Shengnan Lianga, Shaoqing Zhaob

Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China

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School of Earth sciences, Yangtze University, Wuhan 430100, China

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*Corresponding author.

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Lebing Fu

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Faculty of Earth Resources, China University of Geosciences, Lumo Road No. 388, Hongshan District, Wuhan 430074,

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Hubei Province, China.

Tel: +86-13429802569, Fax: +86-27-67883053. E-mail addresses: [email protected] (Lebing Fu), [email protected] (Zhixin Zhao).

ACCEPTED MANUSCRIPT Abstract: Syn-collisional magmatism produced by partial melting of subducted oceanic and

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continental crust during the continental collision plays an important role in understanding the

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orogenic evolution and crustal growth. This contribution reports zircon LA-ICPMS U-Pb

of

syn-collisional

Xitieshan

intrusion

within

the

North

Qaidam

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compositions

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ages and Hf isotopic compositions, whole rock major and trace elements and Sr-Nd isotopic

ultrahigh-pressure metamorphic (UHPM) belt, to study its petrogenesis and contribution to

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continental crust growth. LA-ICPMS zircon U-Pb dating of the Xitieshan granite yields

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magmatic crystallization ages of 441 ± 2 Ma and 442 ± 2 Ma, which are consistent with the

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peak age of ultrahigh-pressure eclogite-facies metamorphism in the Xiteishan terrane. The

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temporal correlation between them confirms that the Xitieshan granite was products of syn-collisional magmatism during the continental collision between Qaidam and Qilian

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Blocks. These granites show high-K calc-alkaline and slightly peraluminous signature, low zirconium saturation temperatures and high contents of K2O with the S-type characteristics. They have rare earth element and trace element patterns resembling those of bulk continental crust, with strong fractionation of light and heavy rare earth elements ((La/Yb)N= 19–26), moderately Eu negative anomalies (δEu =0.65–0.71) and the obviously Nb, Ta, P and Ti negative anomalies. The Xitieshan granite also exhibits remarkable Sr-Nd isotopic differences (initial

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Sr/86Sr = 0.70920–0.71080 and εNd(t) = -4.54–4.11) from the

ACCEPTED MANUSCRIPT contemporaneous granites within North Qaidam UHPM belt. Combined with the positive εHf

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(t)(0.5–5.3) and ages of inherited zircons (475–518 Ma), the magmatism is best explained as

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resulting from melting of subducted oceanic and continental crust during continental collision.

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Isotopic mixing calculations suggest that ca. 28–35% ocean crust and ca. 65–72% continental

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materials contribute to the origin of the Xitieshan granite. Thus the syn-collisional felsic magmatism represents juvenile crust with input of oceanic crustal materials derived from the

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depleted mantle and the hypothesis “continental collision zones are primary sites for net

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continental crust growth” is applicable in the North Qaidam.

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Key words: Syn-collisional magmatism; Ultrahigh-pressure metamorphic belt; North Qaidam;

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Continental crust growth; Xitieshan; Magma mixing

ACCEPTED MANUSCRIPT 1. Introduction

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Granites are the major magmatic products of most collisional orogenic belts, and may be

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subdivided tectonically according to the temporal relationships with the major deformation

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events (Liégeois, 1998; Pearce et al., 1984). Among them, the syn-collisional granitic magmatism is well known to be closely related to the continental collision. During the

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continental collision, the thickened crust experiences temperature rise and evolves in accordance with clock-wise pressure-temperature (P-T) paths, where liquidus temperatures

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are reached and partial melting takes place (Brown, 1993, 2001). Magmas in this process would occur simultaneously with ultrahigh-pressure (UHP) metamorphism, which are

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commonly seen within different orogens, e.g. the Himalaya Orogen (Leech et al., 2005; Mo et al., 2008; Niu et al., 2013), the European Caledonian Orogenic Belt (Andersen, 1998;

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Atherton and Ghani, 2002; Hacker et al., 2010), and the Dabie Orogen (Xie et al., 2006). The products of syn-collisional magmatism are various, such as adakites, low Sr/Y sodic lavas,

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tonalites and S-type granites (Song et al., 2015). The increasing isotope-geochemical investigations of such magmatic rocks have shown the involvement of partial mantle melts in many cases due to the obvious mantle isotopic signatures they exhibit. However, recent studies have proved that prior subducted oceanic crust would be the best source candidate for these mantle isotopic signatures rather than the mantle peridotite or arc crust (e.g. Huang et al., 2014; Mo et al., 2008; Niu et al., 2013; Zhang et al., 2016a). As the oceanic crust was derived from the depleted mantle, the syn-collisional felsic magmatism with input of melts from oceanic crust could inherit the mantle isotopic signatures (Niu et al., 2013). Due to the participation of oceanic crust, syn-collisional magmatism in continental

ACCEPTED MANUSCRIPT collision zones recently has been considered to play an important role in the growth of continental crust (Mo et al., 2008; Niu et al., 2013; Song et al., 2014a). Continental crust

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growth was generally acknowledged to be derived from island arc magmatism (Arculus, 1981;

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McCulloch and Bennett, 1994; Taylor, 1967). However, Niu and his collaborators found that the “island arc” model for continental crust growth had many remarkable deficiencies (Niu et al., 2013; Niu and O'Hara, 2009) and proposed an alternative hypothesis “continental

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collision zones as primary sites for net continental crust growth” (Mo et al., 2008; Niu et al.,

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2013; Niu and O'Hara, 2009). This hypothesis has been tested through studies of syn-collisional granitoids from northwest China, such as the East Kunlun (Huang et al., 2014),

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West Kunlun (Zhang et al., 2016a), southern Tibet (Mo et al., 2008) and Qilian orogenic belts

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(Chen et al., 2016; Huang et al., 2016).

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The North Qaidam Orogen is another orogenic belt in northwest China, and records the whole history of oceanic subduction followed by continental subduction, final collision,

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mountain building and collapse between the Qaidam and Qilian blocks during the Neoproterozoic to the Paleozoic (Song et al., 2006, 2014b; Zhang et al., 2005a, 2008a, 2009a, 2013a, 2016b). It is also one of the well-recognized ultrahigh-pressure metamorphic (UHPM) belts in the northwest China, and has been listed as the 22nd UHPM belt all over the world (Carswell and Compagnoni, 2003). Previous studies proposed that the continental collision between Qaidam and Qilian Blocks occurred during ca. 450–420 Ma after the closure of South Qilian Ocean according to the UHPM ages and P-T conditions (Chen et al., 2009; Mattinson et al., 2009; Meng and Zhang, 2009; Song et al., 2003a, 2005a, 2006; Xu et al., 2006; Yang et al., 2005; Yu et al., 2011; Zhang et al., 2008b, 2009a, b, 2010). Based on

ACCEPTED MANUSCRIPT detailed petrologic, geochronological and geochemical studies, the magmatism related to different orogenic processes have been identified within the North Qaidam, such as adakitic

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magmatism (437–410 Ma) and calc-alkaline granites (407–360 Ma) in Dulan (Song et al.,

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2014a; Wang et al., 2014; Wu et al., 2006, 2009, 2014; Yu et al., 2012), S-type and I-type granite (446–372 Ma) in Da Qaidam (Wu et al., 2007) and I-type granites (470–444 Ma) in Tuanyushan (Wu et al., 2009). The North Qaidam is another ideal test site for Niu and his

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collaborators‟ hypothesis, because: (1) it has the similar geodynamic setting to the orogenic

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belts in northwest China as discussed above; (2) the abundant UHPM ages from previous studies could provide accurate tectonic constraints for the granitic magmatism; (3) the

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wide-distributed granites related to the orogenic process could be used as perfect research

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objects. However, the studies about contributions of syn-collisional magmatism to continental

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crust growth in North Qaidam are rather limited. The Xitieshan pluton located in the central segment of the North Qaidam UHPM belt is

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the largest stock in the Xitieshan terrane. According to our detailed geochronological studies, we propose that the Xitieshan granite was the product of syn-collisional magmatism. A comprehensive study of this granite is thus significant for understanding the continental crust growth associated with the collision between the Qaidam and Qilian Blocks. In this paper, we report an integrated study of LA-ICP-MS zircon U–Pb ages and Hf isotopic compositions, whole-rock major and trace element and Sr-Nd isotopic compositions of Xitieshan granite with the aims of: (1) constraining the tectonic setting of the magmatism; (2) researching the origin and petrogenesis of the granites; and (3) understanding the mechanisms of crust growth in response to continental collision.

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2. Geological setting and samples

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The North Qaidam UHPM belt is located at the northeastern margin of the Tibet Plateau

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in NW China (Fig. 1a). It extends NWW direction for about 400 km and is bounded by the Qaidam block to the southwest and the Qilian block to the northeast; it is offset by the

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NEE-striking Altyn Tagh strike–slip fault in the northwest, and cut by a small NW-striking strike–slip fault in the east (Fig. 1b). To the south, the Qaidam basin is considered as a

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Cenozoic continental sedimentary basin discordantly underlain by the Precambrian crystalline basement and a Paleozoic fold belt (part of the Qaidam block). To the north is the

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Qilian block consisting predominantly of Precambrian gneiss, schist and marble, discordantly overlain by Paleozoic-Mesozoic sedimentary rocks and intruded by Paleozoic granitoids

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(Song et al., 2012; Tung et al., 2013; Zhang et al., 2015a). Further to the north, the Early Paleozoic Pacific-type North Qilian Suture is characterized by blueschist, eclogite, and

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ophiolite extending between the Alxa Block (the western part of the North China Craton) and the Qilian Block (Song et al., 2009a, b). The basement of the North Qaidam terrane is mainly composed of paragneiss, orthogneiss, marble, mafic granulite, amphibolite, local eclogite and garnet peridotite (Yang et al., 1998). According to their textures and mineral assemblages, the paragneisses here are subdivided into three types: (a) mica schist containing some garnet, kyanite and sillimanite and indicating an argillaceous protolith; (b) biotite ± muscovite gneiss derived from an arenaceous protolith; and (c) quartzites and other siliceous rocks (Song et al., 2003a; Wan et al., 2006; Zhang et al., 2004). These basement rocks are in fault contact with the overlying Early Paleozoic volcanic and sedimentary rocks, and intruded by the Paleozoic

ACCEPTED MANUSCRIPT granitoids (Fig. 1b). From Tuanyushan, Da Qaidam to Dulan, various scales of the Paleozoic granitoids exist along the North Qaidam (Fig. 1b). Based their geochronological and

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geochemical features, four episodes of the granitoid magmatism were recognized during the

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Paleozoic: 465–473, 440–446, 397–408 and 372–383 Ma (Wu et al., 2014). The UHP metamorphism in North Qaidam Orogen with subduction of crustal rocks to mantle depths of >80 km is indicated by occurrences of coesite inclusions within zircon in

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pelitic gneiss (Song et al., 2003a; Yang et al., 2001) and within garnet and omphacite in

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eclogite at Dulan, Xitieshan and Yuka terrane (Liu et al., 2012; Song et al., 2003b; Yang et al., 2002; Yu et al., 2013; Zhang et al., 2008a, 2009a, c, 2010). Diamond inclusions identified in

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zircon, and garnet exsolution structures in garnet peridotite from the Lüliangshan terrane

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suggest a deeper subduction depth of 100–200 km (Song et al., 2004, 2005a, b). In addition,

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the „„K-cymrite‟‟ and its pseudomorphs recognized in the Dulan eclogite indicate dehydration at rather high pressure of >2.5 Ga (Song et al., 2003a; Zhang et al., 2009d). Overall,

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numerous lines of petrographic evidence from the peridotites, eclogites and gneisses exhibit that the North Qaidam belt experienced UHP metamorphism (Zhang et al., 2013a). Based on rock associations, petrologic criteria and field relationships, four UHP terranes have been identified along the North Qaidam Orogen. From northwest to southeast, they are the Yuka eclogite-bearing terrane, the Lüliangshan garnet peridotite-bearing terrane, the Xitieshan eclogite-bearing terrane and the Dulan eclogite-bearing terrane (Fig. 1b). Every subunit is dominated by gneisses, with intercalated eclogites exposed in the Yuka, Xitieshan and Dulan terranes, and garnet peridotites in the Lüliangshan terrane. The Xitieshan terrane, located in the central segment of the North Qaidam UHPM belt,

ACCEPTED MANUSCRIPT predominantly consists of two-mica granitic gneisses of Neoproterozoic protolith and garnet–kyanite (±sillimanite)–biotite pelitic gneisses intercalated with minor eclogites (Chen

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et al., 2007; Fu et al., 2015). It was overthrust by the Early Paleozoic volcanic sedimentary

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rocks and locally intruded by granitic plutons (Fig. 1c). The gneisses are generally characterized by upper amphibolite to high-pressure granulite-facies mineral assemblages and extensive migmatization with nearly concordant leucosome pockets at the outcrop (Zhang et

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al., 2006, 2008b). Eclogites in this section occur as boudins and lenses within the gneisses,

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most of which were extensively retrograded to garnet amphibolites or amphibolites with the fresh eclogites only preserved in the center of large lenses. Peak metamorphic

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pressure-temperature (P–T) conditions in eclogites were estimated to be 2.71–3.17 GPa and

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751–791 °C (Zhang et al., 2011), followed by high-pressure (HP) granulite facies to upper

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amphibolite facies retrogression at ca. 2.0 GPa and 770–865 °C (Zhang et al., 2008b, 2009b). Zircon U–Pb dating of the Xitieshan eclogite and gneiss yielded eclogite-facies metamorphic

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age ranging from 422 to 480 Ma (Song et al., 2011; Xiong et al., 2012; Zhang et al., 2005b, 2008b, 2009b, 2011, 2015b). Slightly deformed felsic veins are widely distributed within both gneisses and retrograded eclogites, which were suggested to derive from partial melting of the host UHPM gneiss and eclogite (Chen et al., 2012; Yu et al., 2015a, b; Zhang et al., b). The Xitieshan granitic pluton is the largest stock in this area with length of ca. 16 km, width of 0.5–2 km and area of ca. 20 km2. It shows clear intrusive contact with wall-rock gneiss (Fig. 1c and 2a), and no xenoliths or enclaves were found within the pluton. This intrusion is pale-reddish in color (Fig. 2b) and has massive structure and medium to coarse -grained granitic texture with mineral grain sizes from 3 to 10 mm. Rock-forming minerals

ACCEPTED MANUSCRIPT comprise K-feldspar (40–50%), plagioclase (30–35 %), quartz (15–25%), biotite (5–8%), and accessory titanite, zircon and epidote (Fig. 2c-d). K-feldspar often occurs as subhedral grains

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between euhedral plagioclase crystals, and has biotite and plagioclase inclusions (Fig. 2d).

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Microcline is locally abundant (Fig. 2d), but orthoclase perthite is more common and occasionally displays Carlsbad twins. Plagioclase is often sericitized and occurs both as small euhedral laths and as larger mantled crystals. Quartz commonly displays xenomorphic

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granular texture with internal microfracture and undulatory extinction. Biotite is brown and

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euhedral to subhedral (Fig. 2c), partly replaced by chlorite in its edge and cleavage. Seven samples (B1307-1, B1308-1, B1309-1, B1309-2, B1310-1, B1310-2) were collected from the

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northern part of this pluton and six (B1403-1, B1403-2, B1404-1, B1404-2, B1405-1,

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B1405-2) from the southern part. Detailed sample locations are shown in Fig. 1c.

3. Analytical methods

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U-Pb dating and trace element analyses of zircon were conducted synchronously by LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR). Laser sampling was performed using a GeoLas 2005 and an Agilent 7500a ICP-MS instrument was used to acquire ion-signal intensities. After the LA-ICPMS analyses of U-Pb isotopes in zircon, the Lu-Hf isotopes were conducted by laser ablation of the same zircon domains on or close to the previously analyzed spots. Experiments were conducted using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) in combination with a Geolas 2005 excimer ArF laser ablation system that were hosted at the GPMR. Off-line selection and integration of analytical signals, and mass bias calibrations were performed

ACCEPTED MANUSCRIPT using ICPMSDataCal (Liu et al., 2010a). Whole-rock samples were crushed in a corundum jaw crusher (to 60 meshes). About 60

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g was powdered in an agate ring mill to less than 200 meshes. Major elements geochemical

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analyses were undertaken at ALS Mineral/ALS Chemex (Guangzhou) Co. Ltd at Guangzhou, China. Major oxide concentrations were measured by XRF spectrometer. The analytical precisions were better than 0.01%. Trace elements of the samples were analyzed at the

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GPMR by using an Agilent 7500a ICP-MS. Sr and Nd isotope compositions were measured

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on MAT 261 and Triton thermal ionization mass spectrometer respectively at the Wuhan Institute of Geology and Mineral Resources in China. The precision for

Rb/86Sr and

Sm/144Nd are better than 1% and 0.5%, respectively.

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4. Results

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The detailed analytical conditions and procedures are described in the Appendix A.

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4.1 LA-ICP-MS U-Pb zircon dating Zircon U–Pb isotope and trace element data determined by LA-ICP-MS are listed in Table S1 and Table S2 respectively, and representative CL images of analyzed crystals are shown in Fig. 3. As CL images indicated, zircon grains have typical oscillatory magmatic zoning with prismatic, colorless, columnar crystals without inclusions. Most of the grains are 100–200 μm long, with width/length ratios of about 1–4. Th and U contents range from 143–1751 ppm and 422–1937 ppm respectively, with high ratios of Th/U (0.31–1.07), which illustrate their magmatic origin. Furthermore, the chondrite-normalized rare earth element (REE) patterns of zircon show moderate negative Eu anomalies, positive Ce anomalies and

ACCEPTED MANUSCRIPT steep heavy rare earth element (HREE) patterns (Fig. 3c) as most magmatic zircons do (Wu and Zheng, 2004).

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Eighteen spots were analyzed on the zircons from B1310-1, and most of which yielded

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concordant 206Pb/238U ages ranging from 438 Ma to 443 Ma (Table S1). These analyses show a weighted mean age of 441 ± 2 Ma (MSWD = 0.063, 1ζ for errors), excluding one inherited zircons showing

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Pb/238U ages of 475± 7 Ma (Fig. 3a). Twenty spots were analyzed on the 206

Pb/238U ages ranging from 440 Ma to

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zircons from B1403-1, and most of the zircons have

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444 Ma with a weighted mean of 442 ± 2 Ma (MSWD = 0.084, 1ζ for errors), which is similar to the data from B1310-1 within error. Nevertheless, two inherited zircons from 206

Pb/238U ages of 480 and 518 Ma (Fig. 3b). Thus, the best estimate for the

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B1403-1 show

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crystallization age of the Xitieshan granite should be ca. 442 Ma.

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4.2 Major and trace element geochemistry Thirteen samples were analyzed for major and trace element compositions of Xitieshan

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granite and the results are summarized in Table S3. Additionally, analysis results for the international reference materials are listed in the Table S4. The granites are characterized by a narrow range of SiO2 (68.30–70.60 wt. %), low contents of FeOT (total Fe as FeOT; 2.00–2.57 wt. %) and MgO (0.80–1.34 wt. %). In the Q'-ANOR diagram, the Xitieshan granite mostly are plotted in the monzogranite field (Fig. 4a). The samples also have relatively high concentrations of Na2O (3.50–3.91 wt. %), K2O (3.13–4.00 wt. %) and Al2O3 (14.35–15.40 wt. %), defining weakly-peraluminous characteristics (A/CNK=1.00 –1.13, A/NK=1.41–1.60) (Fig. 4b), and belong to the high-K calc-alkaline series (Fig. 4c). The differentiation index of Xitieshan granite varies from 82.90 to 86.11. Furthermore, in (K2O +

ACCEPTED MANUSCRIPT Na2O)/CaO vs. (Zr + Nb + Ce + Y) diagram (Fig. 4d), Xitieshan granite plots in the unfractionated granites field.

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The total REE contents of Xitieshan granite range from 129 to 202 ppm. In the

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chondrite-normalized REE patterns, the samples are enriched in light rare earth elements (LREEs) and depleted in HREEs, with (La/Yb)N ratios ranging from 19 to 26. These (La/Yb)N ratios are higher than those of upper (15.5) and lower continental crust (5.3)

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(Rudnick and Gao, 2003), illustrating strong REE fractionation of the Xitieshan intrusion.

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The granites show moderately negative Eu anomalies with δEu value of 0.65–0.71 (Fig. 5a and Table S3). In primitive mantle-normalized spider diagrams, Xitieshan granite samples

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show coherent patterns and enrichments in large-ion lithophile elements (LILEs), such as Rb,

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K, Th and U. In contrast, they are depleted in high-field-strength elements (HFSEs), such as

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Nb, Ta, Ti and P (Fig. 5b), and have low concentrations of Cr (8.6–20.9 ppm), Ni (4.6–24.1 ppm) and Y (11–17 ppm) (Fig. 5b and Table S3). Furthermore, the Nb–Ta–Ti depletion and

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slight Sr–Eu depletions of these samples are identical to those of the bulk continental crust (BCC)(Fig. 5).

4.3 Whole-rock Sr–Nd and Zircon Hf isotopic compositions The whole-rock Sr–Nd isotope data are presented in Table S5 and plotted in Fig. 6a. Initial

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Sr/86Sr and εNd(t) values are calculated using zircon U–Pb age of 442 Ma. The

Xitieshan granite samples show uniform Sr–Nd isotopic compositions with whole-rock initial 87

Sr/86Sr ratios of 0.70920 to 0.71080, εNd(t) of −4.61 to −3.75, which are considerably

intermediate between those of contemporaneous Tuanyushan I-type and Da Qaidam S-type granites (Fig. 6a). The two-stage Nd model ages (TDM2(Nd)) are also homogeneous, ranging

ACCEPTED MANUSCRIPT from 1555 to 1486 Ma. The analytical results of Lu-Hf isotope components are listed in Table S6. Sixteen and

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Lu/177Hf ratios are less than 0.0012 (ranging from 0.000603 to 0.001636 with an

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the

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eighteen spots were analyzed on the zircons from B1310-1 and B1403-1 respectively. Most of

average of 0.001103), indicating the negligible amount of radiogenic 177Hf. 176Hf/177Hf values vary from 0.282517 to 0.282662 and the average is 0.282574. The corresponding εHf(t) values

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are from 0.5 to 5.3 (Fig. 6b and Table S6) and the two-stage Hf model ages (TDM2(Hf)) range

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between 1002–1269 Ma.

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5. Discussion

5. 1 Geochronology and tectonic setting

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The North Qaidam had experienced UHP metamorphism resulted from collision and subduction, similar to Western Gneiss Region (WGR) in Norway and Dabie-Sulu UHPM

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terranes in Eastern China (Song et al., 2014b; Zhang et al., 2013a). For North Qaidam UHPM belt, the generally accepted subduction process can be summarized as: after South Qilian Ocean closed, the continental portion of the Qaidam block continued to subduct beneath the Qilian block along the subduction zone by the drag force of subducted oceanic lithosphere, leading to significant crustal thickening (Wu et al., 2014; Zhang et al., 2013a). This subduction process is well constrained by the geochronological studies on UHPM rocks. Recent years, a large number of reliable zircon U–Pb ages for the North Qaidam eclogite, gneiss and granulite have been conducted as shown in Table S7. The ages of UHP metamorphism for the 4 sub-metamorphic terranes are broadly similar in spite of different

ACCEPTED MANUSCRIPT age peaks (Table S7 and Fig. 7). Thus, the four UHPM terranes might have suffered a coherent UHP metamorphism, and the UHP rocks have experienced prolonged eclogite-facies

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metamorphism (Mattinson et al., 2006; Zhang et al., 2010), which might last from 460 to 420

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Ma. However, the duration of UHP metamorphism in most UHPM belts is identified to be less than 20 million year (Myr) (Zheng, 2012; Zheng et al., 2013) and a period of ca. 40 Myr that might be too long for a single UHPM event of continental subduction (Song et al., 2014b;

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Zhang et al., 2013a). For instance, the UHP metamorphism in the Dabie-Sulu UHPM belts

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lasted for only 15–20 Myr (Liu and Liou, 2011; Zheng et al., 2009); WGR experienced HP-UHP metamorphism between 425 Ma and 400 Ma (Hacker, 2007), and duration of

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eclogitization is 10 Myr from 55 to 45 Ma in the Himalaya orogen (Guillot et al., 2008; Mo et

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al., 2008). Recently, the prolonged eclogite-facies metamorphism in Dulan terrane was

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subdivided into two stages based on the investigations of petrology and U-Pb zircon geochronology: the earlier stage (445–460 Ma, peaked at 447 Ma) eclogite-facies

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metamorphism related to oceanic subduction and the later stage (435–420 Ma, peaked at 435 Ma) eclogite-facies metamorphism related to continental subduction (Song et al., 2014b, 2015). In other terranes, except for Yuka terrane, although multi-stage metamorphism is also unambiguous, the peak ages of different stages of metamorphism are various (Fig. 7). These differences were interpreted to be due to the irregularities of the subduction plate, thus continental subduction and oceanic subduction may occur at the same time along the four different terranes (Wu et al., 2009, 2014). In Xitieshan terrane, it is well acknowledged that the later stage (peaked at 442 Ma) eclogite-facies metamorphism was related to continental collision, which is indicated by rock assemblages and occurrence of coesite pseudomorph and

ACCEPTED MANUSCRIPT inclusion (Liu et al., 2012; Zhang et al., 2009e). Meng et al (2005) obtained concordant age of 428 ± 1 Ma for the Xitieshan granite using

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TIMS zircon U-Pb dating on five zircon grains, thus claimed the Xitieshan granite was

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formed in post-collisional setting at early Paleozoic. However, according to our detailed geochronological studies, the zircons from Xitieshan granite (B1310-1 and B1403-1) yield weighted mean ages of 441 ± 2Ma and 442 ± 2 Ma respectively, which are considerably

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consistent with the peak age (442 Ma) of UHP eclogite-facies metamorphism related to

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continental collision in the Xiteishan terrane (Fig. 1c and 7d). The identical ages demonstrate the remarkable synchronism of granitic magmatism and UHP eclogite-facies metamorphism.

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Hence, we proposed the Xitieshan granite to be products of syn-collisional magmatism,

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which is also indicated on a tectonic discrimination diagram (Fig. 8).

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Contemporaneous magmatic irruption occurred in other terranes within the Qaidam UHPM belt. For instance, zircon SHRIMP dating for Tuanyushan granitoids in the western

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segment of North Qaidam yields an age of 444 ± 4 Ma (Wu et al., 2009); one of the three groups of zircon SHRIMP U-Pb ages of the granites in Da Qaidam area was reported as 446 ± 4 Ma (Wu et al., 2007); SHRIMP U-Pb dating of zircons from the Lüliangshan granites in Lüliangshan terrane yielded age of 430 ± 8 Ma (Meng and Zhang, 2008). These ages are consistent with that of Xitieshan granite within error, indicating that this episode of syn-collisional magmatism did not only activate in Xitieshan terrane, but along the North Qaidam UHPM belt. 5. 2 Petrogenesis of the Xitieshan granite The Xitieshan granite shows S-type characteristics based on their petrographic and

ACCEPTED MANUSCRIPT geochemical features. Firstly, all samples exhibit high contents of K2O, strong fractionation of LREE and HREE, moderately negative Eu anomalies (Fig. 5a and Table S3), indicating

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their high-K calc-alkaline and peraluminous nature. Secondly, the Th and Y contents

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decreasing with increasing Rb contents are similar to those of S-type granites (Fig. 9)(Chappell, 1999). Thirdly, they exhibit the characteristic of S-type granite by the high (87Sr/86Sr)i (>0.709) values (Chappell and White, 1992) and relatively low zirconium

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saturation temperatures (760 °C to 799 °C, a mean of 779 °C) (Guo et al., 2012; Kalsbeek et

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al., 2001; Miller et al., 2003). Additionally, the granites show comparable major element compositions with unfractionated S-type granites from the Lachlan Fold Belt (Chappell and

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White, 1992). They are low in Zr + Nb + Y + Ce contents (209.6–298.4 ppm) and (K2O +

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Na2O)/CaO ratios (2.74–4.60 wt.%), lying within the unfractionated granite field (Fig. 4d).

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Furthermore, they have very low FeOT/MgO (1.82–2.63) and Sm/Nd ratios (0.17–0.19), also precludeing the possibility of intensive fractionation (Li et al., 2007; Siebel et al., 1995; Thuy

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et al., 2004).

The genesis of S-type granite can be attributed to either pure crustal melting, or hybridization of multiple magmas (Barbarin, 1999; Douce, 1995, 1999; Douce and Beard, 1995). The high-silica and low-grade differentiation characteristics of the Xitieshan granite with the absence of mafic magmatic enclaves (MMEs) or xenoliths, illustrate that (assimilation) fractional crystallization, if occurrd, cannot be the dominant processes. In order to better constrain the petrogenesis of the magmatic precursors of the Xitieshan granite, incompatible (CI) and compatible (CC) elements are chosen to determine the different magmatic processes, including the partial melting, fractional crystallization and mixing

ACCEPTED MANUSCRIPT (Schiano et al., 2010). As illustrated in Fig. 10, the Xitieshan granite produces a hyperbolic curve on the Rb versus Rb/V discrimination plot and a linear trend on the companion plot of

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1/V versus Rb/V, which are obviously consistent with the mixing trend. In addition, the

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mixing processes shown in Fig. 10 must occur between magmas after their segregation from the solid source, rather than between their solid sources (Schiano et al., 2010). The Sr-Nd isotope compositions can be used to trace the sources of the granite. The

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Xitieshan granite has a narrow range of εNd(t) values of -4.6 to -3.8 and (87Sr/86Sr)i ratios of

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0.70920 to 0.71080, which are intermediate between those of the mantle-derived rocks and continental crust-derived rocks in the North Qaidam UHPM belt (Fig. 6a). This feature also

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implies a mixed origin for the Xitieshan granite. Specifically, the protolith of early Paleozoic

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Tuanyushan I-type granites (εNd(t)>0) and the Dulan adakitic veins and intrusions has been

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interpreted as tholeiitic oceanic crust derived from the depleted mantle (Song et al., 2014a; Wu et al., 2009). Analogously, the Dulan oceanic-type eclogite with positive εNd(t) values

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was metamorphosed from oceanic mafic rocks (Zhang et al., 2008a, 2016b). The Xitieshan continental-type eclogite has Neoproterozoic protolith ages (750–870 Ma) and wide range of εNd(t) values and (87Sr/86Sr)i ratios, which was interpreted to be metamorphosed from Neoproterozoic mafic rocks derived from the depleted mantle with input of continental crustal melt (Xiong et al., 2012; Zhang et al., 2005b, 2011, 2016b). Felsic veins hosted in the Xitieshan eclogite have relatively wide range of εNd(t) values suggesting a mixed source between the melts from the eclogite and adjacent paragneiss (Yu et al., 2015a; Zhang et al., 2016b). In contrast, the εNd(t) values and (87Sr/86Sr)i ratios of felsic veins hosted in and derived from the Xitieshan paragneiss are similar to the S-type granite in Da Qaidam. Both of

ACCEPTED MANUSCRIPT them were reported to be derived from the partial melting of continental crust (Wu et al., 2007, 2009; Zhang et al., 2015b).

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The Hf isotope compositions have been thought to be effective tracers of parental

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reservoirs of granites as well. The zircons from Xitieshan granite have positive εHf(t) values, which are lower than those of the Xitieshan continental-type eclogite, Dulan oceanic-type eclogite and oceanic crust-derived adakitic intrusions, but remarkably higher than those of the

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North Qaidam basement (represented by Dulan granet-bearing mica-schist; Table S6 and Fig.

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6b). This signature was commonly interpreted to be the significant input of juvenile mantle materials, either directly by mantle-derived mafic melts or by remelting of juvenile

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mantle-derived mafic crust (Wong et al., 2009). The TDM2(Hf) ages of the Xitieshan granite

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range from 992 to 1269 Ma, considerably younger than TDM(Nd) and TDM2(Hf) ages of the

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host rocks (1.9–2.2 Ga and 1.5–2.8 Ga respectively) (Meng et al., 2005; Wan et al., 2006; Yu et al., 2013). This further implies that a mantle-derived end member was required to form the

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granites with S-type signatures in Xitieshan except for the continental crustal end member. However, there are no contemporaneous basites within North Qaidam UHPM belt, illustrating the absence of direct materials from the mantle. Definitely, the juvenile crust, such as island arc volcanic rocks, can also produce the “continental signature” (e.g., enriched in LREE and LILE, depleted in HFSE) and mantle-like isotopes (e.g. εNd(t)>0 and εHf(t)>0 ). However, the bulk arc crust has been to be too mafic to produce the more felsic melts (Niu et al., 2013). In addition, the negative εNd(t) and TDM2(Nd) ranging between 1555 to 1486 Ma are not indicative of arc volcanic rocks origin. Previous studies proposed the existence of the South Paleo Qilian Ocean between the Qaidam and Qilian block (Xu et al., 2006; Zhang et al.,

ACCEPTED MANUSCRIPT 2016b; Zhu et al., 2014). Zircons from the oceanic metagabbro within the Dulan terrane yielded magmatic ages of 480–544 Ma (Zhang et al., 2008a), and a Wanggaxiu arc-type

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gabbro outcrop adjacent to Dulan terrane gave a similar magmatic age of 468–522 Ma (Zhu

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et al., 2010). These ages are obviously accordant with those of inherited zircons in the Xitieshan granite (475±7, 480±3 and 518±6 Ma), implying the materials from Early Paleozoic oceanic crust may be involved in the generation of the Xitieshan granite. This

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hypothesis is further supported by REE and trace element geochemistry evidence. As

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illustrated in Fig. 5, the Xitieshan granite samples have REE and trace element patterns resembling those of mixing between the North Qaidam basement metasedimentary rocks

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(represented by the Xitieshan paragneiss) and oceanic crust-derived melts (represented by the

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Dulan adakitic tonalite), because of the significant complementarities between the two end

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members (Meng et al., 2005; Song et al., 2014a; Wan et al., 2006; Zhang et al., 2015b). Thus, these evidences suggest that the Xitieshan granite may be produced by mixing of magma

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derived from continental and oceanic crust. We performed a binary-mixing calculation to test the possibility of magma mixing origin of the Xitieshan granite. On the one hand, we used the Dulan adakitic melt as the mantle end member because: (1) the partial melting of the Early Paleozoic South Paleo Qilian oceanic crust might occur exactly right under eclogite-facies P-T conditions because of the temporal and spatial consistency with the UHP metamorphism as described above; (2) the composition of the melts would be similar to adakites, when the underthrusting ocean crust melted under eclogite-facies conditions with the presence of garnet as a residual phase (Niu et al., 2013); (3) these adakitic tonalites and trondhjemites in Dulan terrane have been proved to be typical

ACCEPTED MANUSCRIPT products of melting of the Early Paleozoic South Paleo Qilian oceanic crust derived from the depleted mantle (Song et al., 2014a). On the other hand, the felsic veins hosted in Xitieshan

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paragneiss are used as the continental crustal end member. These felsic veins were shown to

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have been derived from the partial melting of the host paragneiss during the exhumation of subducted plate (Zhang et al., 2015b). Therefore, the felsic veins within paragneiss could be used to estimate the isotopic signatures of melts derived from partial melting of the same

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paragneiss under similar P-T conditions (Douce and McCarthy, 1998; Zhang et al., 2008b,

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2009b). Most importantly, the both selected end members have clear oceanic and continental crustal features respectively and occurred in the same orogenic system with the Xitieshan

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granite. As shown in Fig. 6a, the Xitieshan granite could well achieve the isotopic signature

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of magma mixing between melts derived from oceanic and continental crust. The proportion

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of continental crustal materials would be 65%–72%, which is also compatible with the S-type characteristics exhibited by the Xitieshan granite. Moreover, the positive εHf(t) values are

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apparently inconsistent with the suggestion of negative εNd(t), although Hf and Nd isotopes are expected to correlate with each other theoretically. This apparent Nd-Hf isotopic decoupling (negative εNd(t) with positive εHf(t) values) is commonly seen in syn-collisional granitoids, such as Baojishan pluton in the North Qilian Orogen and the Permian–Triassic batholiths in the East Kunlun Orogenic belt. The large difference in Nd/Hf ratios between the recycled terrigenous sediments and the oceanic crust is explained to account for this signature (Chen et al., 2015; Huang et al., 2014), which is appropriate for the Xitieshan granite as well. According to the zircon saturation temperatures, the Xitieshan granite formed at relatively low temperatures (<800 °C), which are too low for dehydration melting involving

ACCEPTED MANUSCRIPT biotite or hornblende, and the partial melting probably require fluid influx (Douce and McCarthy, 1998; Miller et al., 2003). It is well acknowledged that the oceanic crust and

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overlying sediments significantly release aqueous fluid during subduction (Bebout, 2007;

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Jarrard, 2003; Peacock, 1990). Therefore, the aqueous fluid derived from prior subducted oceanic crust and overlying sediments may contribute to the melting of continental crust. Compared with the typical Lachlan S-type granites (Chappell, 1999; Chappell and White,

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1992), the Xitieshan granite has lower aluminum saturation index values (<1.1) and higher

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Na2O (>3.2 wt. %) contents (Fig. 4b and Table S3). These geochemical features have also been found in other S-type granites, such as Piaochi intrusion in Qinling orogen, central

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China (Qin et al., 2014) and granites from Dalat zone, southern Vietnam (Thuy et al., 2004).

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According to Douce (1999), partial melting at relatively high pressure may yield the less

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peraluminous granite. Equally important, the lower the temperature and the higher the pressure at which H2O-fluxing induces melting in the thickened continental crust, the more

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sodic are the melts (Douce and McCarthy, 1998). We propose the generation processes of the Xitieshan granite as follows: (1) during the late continental collision and subduction, the early subducted continental and oceanic crust experienced UHP metamorphism because of the high temperature and ultra-high pressure (eclogite-facies conditions); Simultaneously, they experienced partial melting through temperature rise and fluid influx in the processes of crust thickening, producing Na-rich melt and adakitic melt respectively under such P-T conditions (Douce and McCarthy, 1998; Niu et al., 2013); (2) melts from the two end members mixed adequately at the initial stage of crystallization because the products were calc-alkline granites and there are few MMEs

ACCEPTED MANUSCRIPT within the granites (Fernandez and Barbarin, 1991); The mixed melts experienced ascent with limited fractional crystallization of mafic minerals and emplaced in the continental crust at ca.

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5. 3 Implications for continental crust growth

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442 Ma.

The mechanism of continental crust growth has long been the focus of discussion and continues to be a controversial issue (Allègre and Rousseau, 1984; Armstrong and Harmon,

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1981; Niu et al., 2013). It was widely believed that arc magmatism is the main mechanism for

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the continental crust growth (Arculus, 1981; McCulloch and Bennett, 1994; Taylor, 1967). However, the “island arc” model for continental crust growth has major difficulties in

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explaining: (a) the componential and isotopic differences between primary arc magmas and

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BBC and (b) that arc crust production contributes almost no net mass to continental crust

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growth because it is mass balanced by subduction erosion and sediments recycling (Niu et al., 2013; Niu and O'Hara, 2009; Scholl and von Huene, 2007; Sterna and Scholl, 2010).

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Consequently, Niu and his collaborators proposed that the arc magmatism was not an ideal model for the continental crust growth and stated continental collision zones are primary sites for net continental crust growth in Phanerozoic (Mo et al., 2008; Niu et al., 2013; Niu and O'Hara, 2009; Song et al., 2014a). As discussed above, the Xitieshan granite formed in continental collision zones between Qaidam and Qilian block and has REE and trace element patterns resembling those of BCC. Their mantle–like Sr-Nd-Hf isotopic signatures emphatically indicate the magmas were derived from continental crustal components with input of oceanic crust-derived melt. The contemporaneous Tuanyushan I-type granites in the western segment of North Qaidam were

ACCEPTED MANUSCRIPT totally derived from the subducted oceanic crust (Wu et al., 2007, 2009). In the Dulan terrane, partial melting of the oceanic-type eclogite produced the adakitic tonalite and trondhjemite,

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which occurred at ca. 437–410 Ma, overlapping the UHPM ages (435–420 Ma) (Song et al.,

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2014a, 2015; Yu et al., 2013). As the ocean crust was derived from the depleted MORB mantle at an ocean ridge, we proposed that the generation process of Xitieshan and Tuanyushan granites, and the Dulan adakitic tonalite and trondhjemite added mantle-derived

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materials to form juvenile continental crust. This indicates continental collision produced the

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juvenile crust at 446–410 Ma in North Qaidam, which was highly comparable to the scenarios that happened in South Tibet at 45–65 Ma (Mo et al., 2008; Niu et al., 2013), East

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Kunlun Orogen at ca. 250 Ma (Huang et al., 2014) and West Kunlun Orogen at ca. 225 Ma

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(Zhang et al., 2016a). Therefore, syn-collisional felsic magmatism associated with continental

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collision in North Qaidam remarkably contributed to continental crust growth. This also proves that a continental collision zone would be the ideal site for continental crust growth

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(Mo et al., 2008; Niu et al., 2013; Niu and O'Hara, 2009; Song et al., 2014a).

6. Conclusion

In-situ zircon U–Pb ages and Hf isotopic ratios, whole-rock major and trace element and the Sr-Nd isotopic compositions of the Xitieshan granite from the North Qaidam UHPM belt indicate that: (1) The Xitieshan granite formed at 442 Ma, synchronously with peak UHP metamorphism in North Qaidam. (2) The Xitieshan granite is identified to be the product of syn-collisional magmatism

ACCEPTED MANUSCRIPT formed during the continental subduction between Qaidam block and Qilian Block.

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mixing of melts derived from continental and the oceanic crust.

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(3) The Xitieshan granite is unfractionated granite with S-type signatures, resulting from

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(4) Continental crust growth event occurred at 446–410 Ma in North Qaidam by the

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contribution of syn-collisional felsic magmatism associated with continental collision.

Acknowledgement

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This work was jointly supported by the National Natural Science Foundation of China (41302065 and 41202054), the Fundamental Research Founds for National University, China

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University of Geosciences (Wuhan) (CUG120702 and CUG120842), and China Postdoctoral Science Foundation (2012M521493). We thank the editor Sun-Lin Chung and reviewers for

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their detailed and constructive comments. We are also grateful to David Leach, Jun Tan and Yanjun Li for their helpful discussions. We thank Yongsheng Zhong and Li Sun for their field

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assistance. Xiang Li is thanked for his help with sample preparation.

Appendix A. Analytical methods Zircons were extracted from the whole-rock sample of B1310-1 and B1403-1, using the density and magnetic separation techniques in the laboratory of Langfang Regional Geological Survey Institute, Hebei Province, China. Cathodoluminescence (CL), reflected and transmitted light images were obtained at the electron microprobe laboratory in the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan. U-Pb dating and trace element analyses of zircon were conducted

ACCEPTED MANUSCRIPT synchronously by LA-ICP-MS at the GPMR. Laser sampling was performed using a GeoLas 2005 and an Agilent 7500a ICP-MS instrument was used to acquire ion-signal intensities. The

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diameter of the laser ablation craters was 32 μm. Zircon 91500 was used as external standard

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for U-Pb dating, and was analyzed twice every 5 analyses. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as description by Liu et al. (2008a, 2010a, b). Off-line selection and integration of background

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and analytical signals, and time-drift correction and quantitative calibration for trace

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element analyses and U-Pb dating were performed by ICPMSDataCal (Liu et al., 2008a, 2010b). Concordia diagrams and weighted mean calculations were made using Isoplot/Ex

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ver3 (Ludwig, 2003).

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After the LA-ICPMS analyses of U-Pb isotopes in zircon, the Lu-Hf isotopes were

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conducted by laser ablation of the same zircon domains on or close to the previously analyzed spots. Experiments were conducted using a Neptune Plus MC-ICP-MS (Thermo

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Fisher Scientific, Germany) in combination with a Geolas 2005 excimer ArF laser ablation system that were hosted at the GPMR. The energy density of laser ablation used in this study was 5.3 J cm-2. All data were acquired on zircon in single spot ablation mode at a spot size of 44 μm. Detailed operating conditions for the laser ablation system and the MC-ICP-MS instrument and analytical method are the same as description by Hu et al. (2012). Off-line selection and integration of analytical signals, and mass bias calibrations were performed using ICPMSDataCal (Liu et al., 2010a). Whole-rock samples were crushed in a corundum jaw crusher (to 60 meshes). About 60 g was powdered in an agate ring mill to less than 200 meshes. Major elements geochemical

ACCEPTED MANUSCRIPT analyses were undertaken at ALS Mineral/ALS Chemex (Guangzhou) Co. Ltd at Guangzhou, China. Major oxide concentrations were measured by XRF spectrometer. Fused glass disks

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with Lithium Borate were used and the analytical precisions were better than 0.01%,

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estimated from repeated analyses of the standards LAT-CS9, NCSDC73303, SARM-3, SARM-32 and SARM-45. Trace elements of the samples were analyzed at the GPMR. The samples were digested by HF + HNO3 in Teflon bombs and analyzed with an Agilent 7500a

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ICP-MS. During the determination, AGV-2, BHVO-2, BCR-2 and RGM-2 are used as

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reference materials. The detailed sample-digesting procedure for ICP-MS analyses and analytical precision and accuracy for trace elements are the same as description by Liu et al

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(2008b).

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Sr and Nd isotope compositions were measured on MAT 261 and Triton thermal

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ionization mass spectrometer respectively at the Wuhan Institute of Geology and Mineral Resources in China. Procedural blanks were 1×10−9 g and 1.4×10−9 g for Rb and Sr

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respectively; 9.6×10−11 g and 5.4×10−11 g for Nd and Sm respectively. The work conditions of the instrument were controlled by international reference materials NBS987 (Sr), NBS607 (Sr), GBW0411 (Sr), GBW04419 (Nd), BCR-2 (Nd) and JMC (Nd). The measured values for the NBS-987, NBS607 and GBW0411, were

87

Sr/86Sr=0.71021 ± 0.00008 (2ζ), 1.20046 ±

0.00006 (2ζ) and 0.75986 ± 0.00007 (2ζ) respectively; values for the GBW04419, BCR-2 and JMC were

143

Nd/144Nd=0.512717 ± 0.000005, 0.512633± 0.000003 and 0.511554±

0.000003, respectively, during the period of data acquisition, which were in agreement with the recommended values within the limit of error. 143Nd/144Nd values were corrected for mass fractionation by normalization to 146Nd/144Nd=0.7219, and 88Sr/86Sr ratios were normalized to

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Sr/86Sr=8.3752. The precision for

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Rb/86Sr and

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Sm/144Nd are better than 1% and 0.5%,

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respectively.

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Figure captions

Fig. 1 Location of the Qaidam-Qilian mountain-basin system in China (a), the

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Qilian–Qaidam–Altyn region in NE Tibet (b, modified after Zhang et al. (2015a)) and geological sketch map of the Xitieshan UHP terrane showing sampling localities (c, modified

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[7] Chen et al.(2012); [8] Liu et al. (2012); [9] Xiong et al. (2012); [10] Zhang et al. (2012); [11] Yu et al. (2015a).

Fig. 2 (a) Field outcrop of the Xitieshan granite; (b) Photo of the Xitieshan granite sample; (c-d) Photomicrographs of the Xitieshan granite (cross-polarized light). Qz: quartz; Pl: plagioclase; Kf: K-feldspar; Bt: biotite.

Fig. 3 Zircon U–Pb concordia diagrams of the Xitieshan granites and representative CL images of analyzed zircon crystals (a: B1310-1 and b: B1403-1); chondrite-normalized REE

ACCEPTED MANUSCRIPT patterns of zircons from the Xitieshan granites (c), normalizing values are from Sun and

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McDonough (1989).

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Fig. 4 Geochemical classification of the Xitieshan granites: (a) Q'-ANOR normative composition diagram, where Q‟ = 100 × Q/(Q + Or + Ab + An) and ANOR= 100 × An/(Or + An)(after Streckeisen and Le Maitre, 1979); (b) A/CNK ((Al2O3/CaO+Na2O+K2O)molar) vs.

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A/NK ((Al2O3/Na2O+K2O)molar) (after Maniar and Piccoli, 1989); (c) K2O vs. SiO2 diagram

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diagram(after Whalen et al., 1987).

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(after Rickwood, 1989) and (d) (K2O+Na2O)/CaO vs. (Ce+Nb+Zr+Y) discrimination

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Fig. 5 (a) Chondrite-normalized REE patterns of the Xitieshan granites; (b) primitive

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mantle-normalized trace elements spider diagrams of the Xitieshan granites. Normalizing values are from Sun and McDonough (1989). The BCC (red solid line) composition is also

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plotted for comparison. Data of the Xitieshan paragneiss is after Zhang et al. (2015b), Meng et al.(2005) and Wan et al. (2006). Data of the Dulan oceanic crust-derived tonalite is from Song et al.(2014a). The BCC values are from Rudnick and Gao (2003).

Fig. 6 (a) Plots of whole rock (87Sr/86Sr)i vs. εNd(t) and (b) zircon εHf(t) vs. U–Pb age. Blue line represents the mixing calculation between North Qaidam oceanic crust-derived adakitic magma and the North Qaidam basement-derived melt. The compositions of end-members used for mixing calculations are: North Qaidam oceanic crust-derived adakitic magma represented by Dulan adakitic tonalite samples (average composition: Sr= 1173 ppm, Nd =

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(average composition: Sr = 331 ppm, Nd = 7.13 ppm , (87Sr/86Sr)i = 0.71989 and εNd(t) =

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−8.03)(Zhang et al., 2015b). Data source of whole-rock Sr-Nd isotope composition: the Xitieshan continental-type eclogite (Yu et al., 2015b; Zhang et al., 2013b), felsic veins hosted in the Xitieshan amphibolite (Yu et al., 2015b; Zhang et al., b), veins hosted in the Xitieshan

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paragneiss (Zhang et al., 2015b), Dulan oceanic-type ecologite (Zhang et al., 2008a), Dulan

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adakitic veins and intrusions (Song et al., 2014a), the Tuanyushan early Paleozoic granite from the west segment of the North Qaidam (Wu et al., 2009) and Da Qaidam early Paleozoic

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S-type granite (Wu et al., 2007). K = [(Sr/Nd)oceanic crust-derived melt] / [(Sr/Nd)basement-derived melt],

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where Kmax, Kmin, and Kaverage are the maximum, minimum and average values respectively.

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Data source of zircon Hf isotope composition: the Xitieshan continental-type eclogite (Liu et al., 2014; Zhang et al., 2016b), Dulan oceanic-type ecologite (Zhang et al., 2016b), Dulan

2013).

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adakitic veins and intrusions (Song et al., 2014a), Dulan garnet-bearing mica-schist (Yu et al.,

Fig. 7 Distributions of metamorphic ages for the four terranes in the North Qaidam UHPM belt (detailed data and references are listed in Supplementary Table)

Fig. 8 R1 vs. R2 diagram for the Xitieshan granites (after Batchelor and Bowden, 1985). R1 = 4Si − 11(Na + K) − 2(Fe + Ti), R2 =6Ca + 2Mg + Al.

ACCEPTED MANUSCRIPT Fig. 9 Th vs. Rb diagram (a) Y vs. Rb diagram (b) for identifying I-S type granites (after

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Chappell, 1999)

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Fig. 10 Plots of Rb versus Rb/V (a) and 1/V versus Rb (b) for the Xitieshan granites with mixing curves. Also shown are schematic CI versus CI/CC and 1/CC versus CI/CC diagrams with curves showing melt compositions produced by mixing, fractional crystallization and

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partial melting processes, where CI is content of an incompatible element, and CC content of a

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ACCEPTED MANUSCRIPT Highlights The Xitieshan syn-collisional granite in North Qaidam UHPM belt formed at 442 Ma.

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Geochronology and isotope features suggest contributions from subducted ocean crust.

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The Xitieshan granite is produced by mixing melts from continental and ocean crust.

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Syn-collisional magmatism contributes to continental crust growth in collision zones.