Y-type granitic magmatism in the southwestern of the Alxa Block, Northwest China

    Origin of high Sr/Y-type granitic magmatism in the southwestern of the Alxa Block, Northwest China Xiao-Chun Zhou, Hong-Fei Zhang, Bi-Ji Luo, Fa-Bin Pan, Sha-Sha Zhang, Liang Guo PII: DOI: Reference:

S0024-4937(16)30056-1 doi: 10.1016/j.lithos.2016.04.021 LITHOS 3907

To appear in:

LITHOS

Received date: Accepted date:

20 November 2015 18 April 2016

Please cite this article as: Zhou, Xiao-Chun, Zhang, Hong-Fei, Luo, Bi-Ji, Pan, Fa-Bin, Zhang, Sha-Sha, Guo, Liang, Origin of high Sr/Y-type granitic magmatism in the southwestern of the Alxa Block, Northwest China, LITHOS (2016), doi: 10.1016/j.lithos.2016.04.021

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ACCEPTED MANUSCRIPT Origin of high Sr/Y-type granitic magmatism in the southwestern of the Alxa Block, Northwest China

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Xiao-Chun Zhoua, Hong-Fei Zhanga*, Bi-Ji Luoa, Fa-Bin Pana, Sha-Sha Zhanga, Liang Guoa

University of Geosciences, Wuhan 430074, P.R. China

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Abstract

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State Key Laboratory of Geological Process and Mineral Resources, and Faculty of Earth Science, China

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The petrogenesis of high Sr/Y-type magmas is still open to debate. Usually, such magmas could result from melting under high pressure settings (>12 kbar). In this paper, we gave an example that some high

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Sr/Y-type magmas could originate from melting of crustal materials at pressure of 10~12 kbar. We carried

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out a study of petrogenesis for Devonian high Sr/Y granites from the Beidashan batholith (397–411 Ma),

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southwestern Alxa Block, Northwest China. The Beidashan granites have SiO2 (69.21–74.60 wt.%) and Al2O3 (14.01–16.20 wt.%) with A/CNK ratios of 0.99–1.08. According to their trace element compositions

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and whole-rock zirconium saturation temperatures (TZr), the Beidashan granites can be divided into two groups: Group I ((Dy/Yb)N =1.2–3.0, Eu/Eu*=0.77–1.3, TZr=761–856 ℃), resulted from fluid-absent partial melting of mafic to intermediate crustal materials leaving garnet residuum at pressure of ~12 kbar; and Group II ((Dy/Yb)N =0.76–2.16, Eu/Eu*=1.7–5.3, TZr=651–785 ℃), formed by fluid-present melting of mafic to intermediate crustal materials with residual amphibole in the source at pressure of ~10 kbar. Both of the Group I and Group II show high Sr/Y and (La/Yb)N features. They show ISr=0.7134–0.7180, εNd(t)= -6.61 to -9.71, T2DM=1.7–1.9 Ga; εHf(t)= -5.6 to -9.9 and TDMC =1.7–2.0 Ga, indicating that the Beidashan high Sr/Y granites were derived from melting of crustal basement materials. Our results suggest that some high Sr/Y-type granites formed under relatively lower pressure conditions (~10–12kbar), and they could *

Corresponding author. e-mail: [email protected], phone number: +86 27 67883003, Fax: +86 27 67883002

ACCEPTED MANUSCRIPT not be an indicative of partial melting of thickened crust. Keywords: high Sr/Y-type granite; petrogenesis; fluid-present melting; fluid-absent melting; Alxa

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Block; Northwest China

1. Introduction

―Adakite‖ was initially used to describe a group of Cenozoic arc intermediate to felsic volcanic rocks,

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with high Sr/Y and (La/Yb)N ratios (Defant and Drummond, 1990). Martin et al. (2005) further classified

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them into two groups: high-SiO2 adakites (HSA) and low-SiO2 adakites (LSA). The geochemical composition of Archean tonalite-trondhjemite-granodiorites (TTGs) are closely similar with the HAS,

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indicating their petrogenetic analogy (Martin et al., 2005). They are generally thought to be derived from

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partial melting of basaltic rocks at high pressure (>12 kbar) with garnet amphibolite or eclogite as a residue

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(Moyen and Stevens, 2006; Qian and Hermann, 2013; Rapp and Watson, 1995; Rapp et al., 1991; Wolf and Wyllie, 1994). The term ―adakite‖ has been extendedly applied to many intermediate to felsic igneous

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rocks in continental settings (Atherton and Petford, 1993; Chung et al., 2003; He et al., 2011; Hou et al., 2004; Wang et al., 2007a; Wang et al., 2007b; Xu et al., 2002). These continental ―adakites‖ or high Sr/Y magmas are not consistent with the original definition of ―adakite‖ (Defant and Drummond, 1990), and their petrogenesis are still in hotly debated. Many studies indicated that melts with high Sr/Y and (La/Yb)N signatures were generated in high-pressure condition (Atherton and Petford, 1993; Gao et al., 2004; He et al., 2011; Hou et al., 2004; Wang et al., 2005). However, other mechanisms can also account for the magma generation of the high Sr/Y-type granites, such as partial melting of a high Sr/Y (and La/Yb) source (Moyen, 2009) and hornblende fractionation of hydrous mafic and intermediate magmas (Castillo et al., 1999). For the

ACCEPTED MANUSCRIPT hornblende fractionation model, fluids play an important role in the evolved high Sr/Y magmas (Davidson et al., 2007). Analogously, fluid-present melting may be also important for the high Sr/Y magma generation.

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Experimental studies on melting of mafic to intermediate rocks have proposed that fluid-present melting at

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low temperature would leave a residuum dominated by amphibole, clinopyroxene, Fe-Ti oxides and minor plagioclase, and produce melts with high SiO2 and Al2O3, and low FeOt and MgO contents (Beard and Lofgren, 1991; Moyen and Stevens, 2006). Field observations and geochemical studies also have suggested

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that fluid-present melting of mafic to intermediate source at relatively low pressure would have abundant

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amphiboles in the residual and form leucosomes with high Sr/Y and (La/Yb)N ratios similar to Archean TTGs and adakites (Reichardt and Weinberg, 2012). Therefore, the high Sr/Y igneous rocks can generate

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not only in high pressure condition but also in low pressure condition. For the high Sr/Y-type magma

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generation, we need to discuss the roles of source inheritance, melting conditions and magmatic

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differentiation.

In this contribution, we present in-situ zircon U–Pb age and geochemical and Sr–Nd–Hf isotope data

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for the Beidashan batholith in the southwest Alxa Block, Northwest China. Two groups of high Sr/Y granites were indentified. Our data indicate that the granites from the two groups were derived from the same source, but their magma generation mechanisms are distinct (fluid-present and fluid-absent, respectively) at 10–12 kbar. We also use these data to discuss the tectonic implications of the high Sr/Y-type magma generation.

2. Geological backgrounds The Central Asian Orogenic Belt (CAOB) is regarded as one of the largest and longest-live accretionary orogenic collages, which extends from Urals through northern China Craton (NCC), Mongolia

ACCEPTED MANUSCRIPT to Okhotsk Sea in the east of Russia (Buchan et al., 2002; Jahn, 2004; Jahn et al., 2000; Khain et al., 2002; Kröner et al., 2014; Kröner et al., 2007; Şengör et al., 1988; Windley et al., 2007; Xiao et al., 2008; Xiao et

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al., 2009a). The Alxa Block, located in the southern CAOB, is usually considered as the western part of the

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NCC (Zhao et al., 2005), and connects with the Tianshan-Beishan orogenic belt and the Great Xinganling-Inner Mongolian orogenic belt, which represent the Paleo-Asian orogenic belt (Fig.1a). The Alxa Block could be compared with the Beishan orogenic belt westward and the Great Xinganling-Inner

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Mongolian orogenic belt to the eastward (Ao et al., 2012; Cocks and Torsvik, 2013; Feng et al., 2013; Xiao

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et al., 2010; Zheng et al., 2014). Thus, the Alxa Block is a key region in understanding the tectonic evolution of the southern CAOB (Ren et al., 2005a; Wang et al., 1994; Wu and He, 1993). Many studies

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have focused on the Tianshan-Beishan and Great Xinganling-Inner Mongolian orogenic belts (Ao et al.,

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2010; Ao et al., 2015; Ao et al., 2012; Chen et al., 2000; Jian et al., 2008; Jian et al., 2010; Liu et al., 2003;

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Mao et al., 2012; Wang et al., 2006b; Xiao et al., 2004; Xiao et al., 2009b; Xiao et al., 2003), while only few studies have been paid attention on the Alxa Block (Dan et al., 2014a; Dan et al., 2014b; Dan et al.,

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2015; Feng et al., 2013; Shi et al., 2014; Zheng et al., 2014). In the Alxa block, Precambrian crystalline basements are sporadically exposed. They are mostly covered by deserts. Only few locations, such as the Longshoushan, Beidashan and Diebusie-Mayatu areas, have been reported. The Beidashan crustal basement contains ~2.5 Ga TTG gneisses (Gong et al., 2012; Zhang et al., 2013a). The Longshoushan crustal basement is mainly composed of Paleoproterozoic (~2.3–1.9 Ga) amphibolite-facies meta-igneous and meta-sedimentary rocks (Dong et al., 2007; Gong et al., 2011). The Bayanwulashan Complex consists mainly of mafic and felsic metamorphic rocks (~2.34 Ga and ~2.32–2.26 Ga) (Dan et al., 2012; Shen et al., 2005), and the Diebusige Complex (~2.45–1.97 Ga) is composed of amphibolites, mafic gneisses and paragneisses (Dan et al., 2012; Geng et al., 2006; Shen et al.,

ACCEPTED MANUSCRIPT 2005). Both Bayanwulashan Complex and Diebusige Complex were overprinted by two metamorphic events at ca. 1.9 Ga and ca. 1.8 Ga (Dan et al., 2012; Geng et al., 2006, 2007; Geng et al., 2010; Shen et al.,

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2005). Neoproterozoic granitic gneisses (930–904 Ma) were emplaced in the central Alxa Block (Dan et al.,

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2014a; Geng and Zhou, 2010).

Three significant ophiolite belts have been reported in the eastern part of the Alxa Block from North to South: the Yagan ophiolite, the Engger Us ophiolite and the Quagan Qulu ophiolite (Fig.1b) (Feng et al.,

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2013; Wu and He, 1993; Zheng et al., 2014). Based on these ophiolitic belts, the eastern part of the Alxa

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Block can be divided into three tectonic subunits: the Zhusileng-Hangwula (ZHTZ), the Zongnaishan–Shalazhashan (ZSTZ) and the Nuru–Langshan tectonic zones (NLTZ) (Alxa Old Continent)

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(Fig. 1b) (Li, 2006; Wang et al., 1998; Wang et al., 1994; Wu and He, 1993; Zheng et al., 2014). The

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Engger Us ophiolite is mainly of ultramafic rocks, gabbros, basalts, and cherts. This belt is considered to be

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the suture between the North China Craton and the southern Central Asian Orogenic Belt (CAOB) (Li, 2006; Wang et al., 1998; Wu and He, 1993; Wu et al., 1998; Zheng et al., 2014). The Quagan Qulu

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ophiolite is exposed in the Badain Jaran fault belt, consisting of lenticular and striped ultramafic rocks, gabbros, cherts, and rare basalts (Zheng et al., 2014). The Quagan Qulu ophiolite belt represents a back-arc in north margin of the Alxa block that opened during the Early Paleozoic (Wang et al., 1994; Wu and He, 1993; Wu et al., 1998). These two ophiolite belts, together with the ZSTZ, formed an ―ocean-arc–back-arc basin‖ system (Feng et al., 2013; Li, 2006; Wang et al., 1994; Wu and He, 1993; Zheng et al., 2014). The NLTZ is characterized by extensively outcropped Precambrian basement rocks and Paleozoic intrusions (Feng et al., 2013; Li, 2006; Wang et al., 1994; Zheng et al., 2014). Phanerozoic granitoids are widely exposed in the Alxa Block (Resources, 1991). Most of them have magma crystallization ages from 320 Ma to 260 Ma (Feng et al., 2013; Li, 2006; Ren et al., 2005a; Ren et

ACCEPTED MANUSCRIPT al., 2005b; Shi et al., 2012; Shi et al., 2014; Su, 2012; Yang et al., 2014; Zhang et al., 2012; Zhang et al., 2013b; Zheng et al., 2014). These 320–260 Ma granitoids were suggested to result from the subduction and

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following closure of the Paleo-Asian Ocean (Feng et al., 2013; Li, 2006; Ren et al., 2005a; Ren et al.,

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2005b; Shi et al., 2012; Shi et al., 2014; Su, 2012; Yang et al., 2014; Zhang et al., 2012; Zhang et al., 2013b; Zheng et al., 2014). A few Early Paleozoic igneous rocks (~447 Ma intermediate-felsic volcanic rocks, 423Ma quartz diorite and ~394 Ma diorite) have also been found at the eastern margin of the Alxa Block

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(Dan et al., 2015; Li, 2006). Dan et al. (2015) suggests that the 423Ma quartz diorite at the eastern margin

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of the Alxa Block may record the collision between the Alxa Block and the North China Craton. However, early Paleozoic igneous rocks are widespread in the other part of the CAOB (Ao et al., 2012; Chen et al.,

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2010; Jahn, 2004; Jian et al., 2008; Kroener et al., 2008; Li et al., 2011; Xiao et al., 2004; Xiao et al., 2008;

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Xu et al., 2013). The early Paleozoic geodynamic setting of the Alxa Block and its tectonic connection with

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the other part of the CAOB are still poorly constrained (Li, 2006). In this paper, samples from the early Paleozoic Beidashan batholith in the southwest Alxa Block were

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collected. The Beidashan batholith, with an area of ~1000 km2, shows a sharp contact with the Archean TTG gneisses (Fig. 1c). Locally, this pluton is intruded by some mafic dykes with ~278 Ma (author unpublished data). The Beidashan batholith contains two type rocks: biotite granite and monzogranite. Both of them show variable textures from medium- to coarse-grained. In the field, contact interface between them is gradual, suggesting that they could be coevally emplaced. The biotite granite is consist of quartz (~30%), plagioclase (~35–40%), K-feldspar (20–25%) and biotite (~8%) (Fig. 2a-d). Accessory phases include apatite, zircon. Some microgranular mineral aggregates (biotite, garnet, and amphibole) are found (Fig. 2b). The monzogranite is composed of quartz (~30%), plagioclase (30–35%), K-feldspar (30–35%) and accessory minerals including apatite, sphene, and zircon (Fig. 2e-f).

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3. Analytical methods

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Major elements were measured by the conventional X-ray Fluorescence (XRF) at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geoscience, Wuhan. The analytical uncertainty is generally <5%. Trace elements and rare earth elements (REE) were measured

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using Agilent 7500a ICP–MS at the State Key Laboratory of Geological Processes and Mineral Resources

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(GPMR), China University of Geosciences, Wuhan. The analytical precision is better than 5–10% relative. The detailed sample-digesting procedure for ICP–MS analysis and accuracy are detailed by Liu et al.

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(2008).

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Whole-rock Sr and Nd isotopic ratios were measured by a Triton thermal ionization mass spectrometer

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at GPMR. 87Rb/86Sr ratios were calculated from measured Rb and Sr contents by TIMS. 147Sm/144Nd ratios were calculated from measured Sm and Nd contents by ICP–MS. The measured Sr and Nd isotopic ratios

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were normalized to 86Sr/88Sr=0.1194 and 146Nd/144Nd=0.7219, respectively. For further details of the Sr and Nd isotopic analytical procedures see (Gao et al., 2004). Zircons were separated from whole-rock samples by heavy-liquid and magnetic methods and then purified by hand picking under a binocular microscope. Zircon U–Pb zircon dating was also conducted by LA–ICP–MS at GPMR. Zircon 91500 was used as external standard for U–Pb dating, and was repeatedly analyzed every 5 analyses. Preferred U–Th–Pb isotopic ratios used for zircon 91500 are from Wiedenbeck et al. (1995). The uncertainties of preferred values for the external standard 91500 were propagated to the combined errors of the samples. Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_ver3 (Ludwig, 2003).

ACCEPTED MANUSCRIPT Zircon Hf isotope analysis was carried out in situ using a Neptune MC–ICP–MS at GPMR. Analytical spots were located close to or on the top of LA–ICP–MS spots or in the same growth domain as

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inferred from CL images. The instrumental conditions and data acquisition were described by Hu et al.

60 mJ. Measured

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Hf/177Hf ratios were normalized to

standard zircons gave

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(2012). The analyses were conducted with a beam diameter of 44 lm, a hit rate of 8 Hz and laser energy of 179

Hf/177Hf =0.7325. During the analysis, the

Hf/177Hf of 0.282308 ± 40 for 91500, 0.282013 ± 40 for GJ-1 and 0.282677± 40

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for Temora. Off-line selection and integration analyte signals, and isobaric interference and mass

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fractionation correction of Lu–Hf isotopic ratios were also performed by ICPMSDataCal (Liu et al., 2010).

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5.1. U–Pb zircon ages

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5. Results

Four samples (A29, A50, A56 and A74) from the Beidashan batholith were selected for LA–ICP–MS

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zircon U–Pb dating. The results are listed in the Supplementary Table 1. Representative zircon CL images and their U–Pb concordia plots are shown in Fig. 3 and Fig. 4, respectively. Zircons from all the samples are euhedral, short to long prismatic and 50–200 μm in length with aspect ratios of 1:1–4:1 (Fig. 3). Most of zircons exhibit clear oscillatory zoning (Fig. 3), suggesting magmatic origin (Corfu et al., 2003). They show variable U (34.9–3971 ppm) and Th (17.3–2408 ppm) contents, and Th/U ratios (0.06–1.35) (Supplementary Table 1). Some zircons show clearly core-rim textures (Fig. 3). For sample A29 (medium-coarse-grained biotite granite), twenty analyses yield

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Pb/238U ages

between 395 and 417 Ma, with a weighted mean of 408±3 Ma (MSWD=0.99) (Fig. 4a). Four other analyses give slightly old 206Pb/238U ages between 439 and 448 Ma, which could be xenocrysts.

ACCEPTED MANUSCRIPT Ten analyses from samples A50 (medium-coarse-grained monzogranite) yield

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Pb/238U ages from

406 to 416 Ma, with a weighted mean of 411±3 Ma (MSWD=0.52) (Fig. 4b), One spot gives a

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Pb/238U

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age of 472 Ma. One spot shows a younger age (366±4 Ma), probably due to lead loss. 206

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Twelve analyses from sample A56 (porphyritic monzogranite) yield

Pb/238U ages from 388 to 410

Ma, with a weighted mean of 397±4 Ma (MSWD=0.52) (Fig. 4c). Two spots give and 439 Ma (Fig.4c).

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Pb/238U ages of 421

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For sample A74 (coarse-grained biotite granite), fifteen analyses yield 206Pb/238U ages from 393 to 418

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Ma, with a weighted mean of 409±5 Ma (MSWD=2.8) (Fig. 4d). Three analyses on inherited cores give 206

Pb/238U ages from 429 to 444 Ma (Fig. 4d). Two additional analyses show relatively young

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Pb/238U

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ages 387 and 388 Ma due to lead loss (Fig. 4d).

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Our U–Pb zircon dating for above four samples from the Beidashan batholith shows the magma

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crystallization age ranging from 397 to 411 Ma.

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5.2. Major and trace elements

Major and trace element data of all samples are given in Table 1. All analyzed samples from the Beidashan granites are high SiO2 (mostly >71 wt.%) and metaluminous to weakly peraluminous (A/CNK= 0.99–1.12) (Table 1). Although the Beidashan granites have variable mineral assemblages and textures, they can be divided into two groups based on their geochemical features. The Group I has slightly low SiO2 contents (69.21–72.79 wt.%) and high MgO, TFeO, TiO2, and P2O5 contents (Table 1 and Fig. 5d-f), high Zr (112–336 ppm), Th (5.97–22.14 ppm), Cr (0.75–5.20 ppm) and LREE contents (59.41–283.43 ppm); and the Group II is characterized by relatively high SiO2 contents (72.08–74.60 wt.%), low Zr (25.74–17.07 ppm), Th (0.82–4.36 ppm), Cr (0.42–3.06 ppm) and LREE

ACCEPTED MANUSCRIPT contents (16.34–57.51 ppm) (Table 1 and Fig. 5g-h). Both of the Groups I and II show similar primitive-mantle-normalized trace element patterns with

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positive anomalies of Rb, Ba and Th, and negative anomalies of high-field strength elements (HFSEs, e.g.

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Nb, Ta, P and Ti) (Fig. 6a and 6c). The Group I displays more enriched LREE and stronger fractionated REE patterns ((La)N =62–175 and (La/Yb)N =16–299, mostly>75) than those of the Group II ((La)N =15–60 and (La/Yb)N =12–39) (Fig. 5i). The Group I shows slightly negative or positive Eu anomaly (0.77–1.3),

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but the Group II has obviously positive Eu anomaly (1.7–5.3) (Table 2 and Fig. 6). All samples exhibit high

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Sr (>426 ppm) and La (4.39–64.27 ppm) and low Y (1.44–9.53 ppm) and Yb (0.11–0.93 ppm), with high Sr/Y (>45) and (La/Yb)N (21–214) ratios, indicating geochemical features similar to adakites or high Sr/Y

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5.3. Isotope geochemistry

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magmas (Fig. 7a and 7b).

Whole-rock Sr and Nd isotopic data are listed in Table 2 and shown in Fig. 8. Zircon Lu–Hf isotopic

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data are presented in Supplementary Table 2 and plotted in Fig. 9. Their initial 87Sr/86Sr isotopic ratio (ISr), εNd(t) and zircon εHf(t) values are calculated at their emplacement ages (Table 2). The Beidashan Group I has a restricted range of Sr–Nd isotopic compositions with ISr = 0.7134 to 0.7185, εNd(t) = -6.6 to -9.1, and two-stages crustal model ages (T2DM) of 1.7 to 1.9 Ga (Fig. 8). One sample (A74) from the Group I shows εHf(t) values of -5.7 to -7.7 with a weighted mean of -6.4±0.6 (MSWD=0.5) and TDMC of 1.7–1.9 Ga (Fig. 9). The Beidashan Group II has comparable Sr–Nd–Hf isotopic compositions with those of the Beidashan Group I. Samples from the Group II exhibit ISr = 0.7136 to 0.7179 and εNd(t) = -7.7 to -9.3, and T2DM = 1.8 to 1.9 Ga (Fig. 8). Three samples (A29, A50, A56) from the Group II have εHf(t) values of -5.7 to -9.9 and TDMC of 1.8–2.0 Ga (Fig. 9).

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6. Discussion

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6.1. Petrogenesis

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Both of the two groups of the Beidashan granites are characterized by high Sr, Sr/Y and (La/Yb)N and low Y and Yb values, showing geochemical signatures of high Sr/Y-type granite (Fig. 7). Several hypotheses have been proposed for the origin of igneous rocks with high Sr/Y geochemical signature:

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partial melting of subducted oceanic slab (Defant and Drummond, 1990; Kay and Marquez, 1993; Rapp et

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al., 1999; Rapp et al., 1991), assimilation and fractional crystallization (AFC) processes of mantle-derived basaltic magma (Castillo et al., 1999), partial melting of thickened lower crust or delaminated lower crust

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(Atherton and Petford, 1993; Chung et al., 2003; Guo et al., 2007; He et al., 2011; Hou et al., 2004; Rapp et

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Qian and Hermann, 2013).

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al., 2002; Xu et al., 2007), and partial melting of high Sr/Y crustal source (Ma et al., 2015; Moyen, 2009;

The granites from the two groups show high SiO2 and variable K2O/Na2O ratios, with more evolved

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Sr–Nd–Hf isotope compositions. These features rule out an origin from partial melting of a subducted oceanic slab. The AFC process could account for the high K2O content and evolved Sr–Nd isotope compositions. However, such an origin requires significant volumes of mafic melts (Castillo, 2006; Castillo et al., 1999). In the field, no coeval mafic rocks have been found in the study area. Thus, it is unlikely that the Beidashan high Sr/Y magmas formed by AFC process. The two groups of the Beidashan granite show similar Sr–Nd–Hf isotopic compositions (Fig. 8 and 9), indicating that they share the same magma source. Both of them have high ISr (~0.713 to 0.718) and negative εNd(t) (-9.6 to -5.7) and εHf(t) values (-9.9 to -5.7), with old crustal model ages (1.7 to 2.0 Ga), suggesting that their magma sources mostly involved reworking of the Mesoproterozoic crustal materials. However, the two groups have distinct geochemical

ACCEPTED MANUSCRIPT features, indicating that they have distinct petrogenesis. We discuss two different origins for these high

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Sr/Y granites based on their melting conditions (e.g. pressure/depth, temperature and water).

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6.1.1 The Group I: partial melting of crustal materials under fluid-absent condition The Group I has high Sr/Y and (La/Yb)N ratios, with slightly negative or positive Eu anomalies (0.77–1.3) (Fig. 6), suggesting that plagioclase is poor or absent in their source residual. Many studies have

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demonstrated that both garnet and amphibole as source residual can account for high Sr/Y and La/Yb melts

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(Davidson et al., 2007; Müntener et al., 2008; Macpherson et al., 2006). Amphibole preferentially incorporates MREE over HREE, with partition coefficient Kd >1 for MREE, while garnet preferentially

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incorporates HREE (Davidson et al., 2007). Therefore, abundant garnet in the residual would have high

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Dy/Yb ratio in melts, but amphibole in the residual would have relatively low Dy/Yb ratio in melts

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(Davidson et al., 2007; Macpherson et al., 2006). The Group I with high (La/Yb)N (16–299) and (Dy/Yb)N (1.2–3.0) ratios (Fig. 10) is consistent with abundant garnet in the source residual (Davidson et al., 2007;

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Macpherson et al., 2006). Experimental studies have indicated that melts generated by dehydration melting of amphibolites at pressure > 12 kbar under fluid-absent conditions have high Sr/Y and La/Yb ratios, and coexist with a residuum of garnet, amphibole and minor pyroxene without plagioclase (Moyen and Stevens, 2006; Qian and Hermann, 2013; Rapp et al., 1991). Thus, we suggest that the Group I was generated by fluid-absent partial melting of mafic to intermediate rocks. Commonly, for granitic melts formed by increasing degree of partial melting of a common source, their compatible elements would increase and incompatible elements would decrease with decreasing SiO2. The increasing TiO2, MgO, P2O5, Zr, and Cr contents and decreasing K2O (Rb) with decreasing SiO2 (Fig. 5c-i) suggest that the Group I was generated by increasing degree of partial melting of a common source.

ACCEPTED MANUSCRIPT The high SiO2 contents (69.21–72.79 wt.%) of the Group I imply a low degree of partial melting (F) (Moyen and Stevens, 2006; Rapp, 1995; Zamora, 2000), corresponding to a relative low temperature.

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Recently, zircon (TZr) and monazite (TLREE) dissolution in granitic melts has been investigated

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experimentally over a range of temperatures, melt H2O contents and melt compositions (Montel, 1993; Watson and Harrison, 1983). The increase of MgO and LREE contents and (La/Yb)N and (Dy/Yb)N ratios with increasing TZr (Fig. 10) indicates increasing garnet and consuming accessory minerals (e.g. zircon,

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monazite and/or apatite) in the source regions with the degree of partial melting increasing. The Eu

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anomalies of the Group I show no obvious change with TZr, implying that plagioclase was poor before melting in the residual. Our calculated results show that the Group I has TZr of 761–856 °C and TLREE of

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794–887 °C (Fig. 11), respectively. Both of the minimum values of the TZr and TLREE for the Group I are

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slightly lower than the solidus (~820–900 °C) for amphibolite dehydration melting at low pressure. But, at

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higher pressure (~10–15kbar), the solidus shows a sharp decline at temperatures close to those of the water-saturated case (Moyen and Stevens, 2006; Qian and Hermann, 2013; Rapp, 1995; Wyllie and Wolf,

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1993). All these characteristics support a derivation of the Group I from low degree partial melting of K-rich mafic-intermediate rocks at T=761–887 °C and at pressures of ~12 kbar equivalent to a crustal thickness of ~40 km.

6.1.2 The Group II: water-fluxed melting of crustal materials at relative low pressure Compared with the Group I, Group II has slightly higher SiO2 contents (72.08–74.60 wt.%), lower Zr (25.74–17.07 ppm), Th (0.82–4.36 ppm), Cr (0.42–3.06 ppm), LREE contents (16.34–57.51 ppm) (Table 1 and Fig. 5) and lower (Dy/Yb)N ratios (0.76–2.16) (Fig. 10), with obviously positive Eu anomalies (1.7–5.3) (Table 1 and Fig. 6). The Eu anomalies of the melts could constrain the role of feldspar in source melting and following magma emplacement processes (Cowie and Scholz, 1992; de Wit, 1998; Frost et al., 2001;

ACCEPTED MANUSCRIPT Green, 1995). The obviously positive Eu anomalies in the Group II could be related to plagioclase accumulation in the magma reservoir or plagioclase consumption in the source. As pointed out above, both

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of the Group I and II granites from the Beidashan batholith were derived from the same magma sources. If

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the Group II was formed by Group I through plagioclase accumulation, the Group II should have the higher CaO, Na2O, and Sr than those of the Group I, and the evolved melt from the Group I should exhibit obvious negative Eu anomalies. In fact, the Group I have CaO, Na2O and Sr contents similar to those of the

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Group II. The Group I does not have obvious negative Eu anomalies in its evolved melts (high SiO2). In

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addition, there is no positive correlation between the Na2O, K2O, CaO and Sr contents and Eu anomalies for the Group II (Fig. 12). These observations suggest that the positive Eu anomalies for the Group II is

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most likely to be inherited from a source region rather than acquired by plagioclase accumulation during

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magma emplacement. The consumption of plagioclase in the source would also result in high Sr/Y and

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high Eu/Eu* ratios (>1) in the magma.

Experimental studies have suggested that fluid fugacity strongly influences melt composition and

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residuum mineralogy during partial melting of basaltic compositions at mid- to lower-crustal pressures (Clemens and Vielzeuf, 1987; Douce, 2005; Douce and Beard, 1995; Le Breton and Thompson, 1988; Moyen and Stevens, 2006; Qian and Hermann, 2013; Thompson, 1983; Wolf and Wyllie, 1994). Breakdown of amphibole would trigger partial melting under fluid-absent conditions in mafic source, while plagioclase would firstly be melted in fluid-present melting reaction. Fluid-present reaction would leave a residuum dominated by amphibole, clinopyroxene, Fe-Ti oxides and minor plagioclase, and would lead to positive Eu anomaly in the melts (Moyen and Stevens, 2006). Thus, the positive Eu anomalies of the Group II granites are probably an indicative of fluid-present melting. The Group II shows high Sr/Y and (La/Yb)N ratios, but with distinctive concave upward REE patterns

ACCEPTED MANUSCRIPT (Fig. 6d), reflecting that plagioclase had been consumed and a significant proportion of amphibole had been leaved in the residuum. Most samples have (Dy/Yb)N<1.1 (Fig. 10), also suggesting that amphibole is

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a dominate residual phase in the source. A few samples show (Dy/Yb)N>1.1, suggesting that garnet is

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required in the residue. The microgranular mineral aggregate (biotite, garnet, and amphibole) found in sample A36 also agrees with garnet and amphibole as the residual phases.

The Group II has TZr ranging from 651 to 785 °C (Table 1 and Fig. 11). Meanwhile, LREE saturation

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temperature estimates exhibit 661–737°C (Table 1 and Fig. 11). The fluid-present solidus is subhorizontal

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for low pressures, then progressive turns upwards and becomes subvertical above 10 kbar. The minimum temperature is about 650–750°C at 6–10 kbar (Beard and Lofgren, 1991; Moyen and Stevens, 2006). The

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garnet as the residual phases would occur above 8 kbar (Holland, 1980). The relatively low TZr and TLREE

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of the Group II granites are comparable with the fluid-present melting temperature (651 to 785 °C) at the

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low-medium pressures. The Group II granites show lower LREE contents (Fig. 6d), Zr, Th and La/Yb than those of the Group I, suggesting that some accessory minerals were retained in the residual at relatively

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low temperature melting. Thus, we suggest that the Group II granites were derived from fluid-present melting under higher fH2O at pressure of ~10 kbar.

6.2 Trace element modeling Model calculations assume non-modal batch melting. Because the stoichiometries of the melting reactions are not well known, we used a modal batch melting equation and varied mode as a function of melt fraction. The Nernst partition coefficients (D) between minerals and melts were calculated using the min eral / melt

equation: Di

min eral

= Ci

melt

/ Ci

min eral

, in which Ci

melt

and Ci

represent the concentrations of

element i in the mineral and melt, respectively. By using experimentally determined partition coefficients,

ACCEPTED MANUSCRIPT it is possible to calculate trace element fractionation between melt and the residual phases for predicted assemblages (Fig. 13). Applied partition coefficients (Table 3) are from http://earthref.org/KDD. As stated

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above, the geochemical and isotopic data have suggested that the magma source for the Beidashan granites

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could be the Mesoproterozoic continental crustal materials beneath the Alxa block. We choose the composition of the average lower continent crust (LCC) to represent the composition of the parent rock (Rudnick and Gao, 2003). The Beidashan granites could be derived from the Mesoproterozoic amphibolitic

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source, in which the Group I granites were originated by dehydration melting and the Group II granites were derived from fluid-present melting under higher fH O. The melting reaction for the Beidashan Group I

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2

granite would be Hbl + Pl = Grt+ Hb2 +Cpx +melt under water-absent condition (Moyen and Stevens, 2006;

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Thompson, 1983; Wolf and Wyllie, 1994). In contrast, the Beidashan Group II granite would be consistent

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with a water-present melting reaction: Hbl + Pl(An40) + Bt +H2O = Hbl2+ (Grt) +melt (Kenah and

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Hollister, 1983; Lappin and Hollister, 1980).We are still far from clear about the proportions of the residual mineral phases, so we tested the effects of these minerals in the residuum by variously considering the

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proportions of clinopyroxene, amphibole, plagioclase and garnet constrained by Wyllie and Wolf (1993) and Qian and Hermann (2013). We assumed that the residue for Group I generated by partial-melting of mafic-intermediate rocks under fluid-absent condition is Grt:Am:Cpx:Pl=45:40:10:5 and the residues for the Group II from water-present melting of mafic-intermediate rocks is Am:Pl:Grt=85:10:5. Our modeling results indicate that the trace element compositions of ～1% to 10% batch melting for the Group I and ~1% to 20% batch melting for the Group II are strikingly close to their actual trace element compositions (Fig. 13), respectively. Our modeling shows that both of fluid-absent partial melting with dominantly residual garnet and clinopyroxene and fluid-present partial melting with mainly residual amphibole would generate high Sr/Y and La/Yb melts, which fully supports our above conclusions.

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6. 3. Implication for the formation of high Sr/Y-type magma

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The two groups of the Beidashan granitic magma with high Sr/Y and (La/Yb)N are not adakites that

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generated by partial melting of young subducted basaltic ocean crust under eclogite-face metamorphism (Defant and Drummond 1990). The Beidashan high Sr/Y granitic magma with high K2O/Na2O and evolved Sr-Nd-Hf isotopic features is more compared to continental adakitic rocks (Moyen, 2009). The high

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Sr/Y-type granitoids are abundant in many orogens, and play an important role in discussing

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post-collisional geodynamic processes (Chung et al., 2003; Xu et al., 2007). Chung et al. (2003) reported such magmas from southern Tibetan plateau, and argued for crustal thickness of ≥50 km in the

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post-collisional setting. This suggestion was widely accepted during the following few years (Chung et al.,

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2003; Wang et al., 2006a; Wang et al., 2007a; Wang et al., 2007b; Xu et al., 2002). However, recent

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investigations found that the petrogenesis of high Sr/Y granitoids in orogens is multifarious. It was found that breakdown of amphibole would trigger partial melting and produce peritectic Opx, Cpx and/or Grt

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with pressure of ~12 kbar under fluid-absent conditions (Moyen and Stevens, 2006; Qian and Hermann, 2013), resulting in the melts with high Sr/Y signature. The petrogenesis of the Beidashan Group I is consistent with melting at pressure of ~12 kbar. Thus, high Sr/Y granitoids may not be an indicative of high pressure melting condition, especially for high Sr/Y igneous rocks derived from continental crustal materials (Barbarin, 1999; Moyen, 2009). We suggest that the Group II granites were formed under fluid-present condition. The importance of fluid-present melting was disregardful in the past twenty years. Small volumes of water could store in stable lower or middle crust (Ramberg, 1952; Yardley, 2009; Yardley and Valley, 1997) and low porosity of high-grade rocks (Collins et al., 1989; Finger and Clemens, 1995; Wickham, 1987). Recent investigation

ACCEPTED MANUSCRIPT indicates that active tectonic processes can also create conditions for the release of free fluids and high permeability channels, which may trigger voluminous melting (Jung et al., 2000; Prince et al., 2001;

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Sawyer, 1998; Weinberg and Hasalova, 2015; Weinberg and Mark, 2008). Fluid-present incongruent

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melting reactions commonly form peritectic mineral assemblage of amphibole ± garnet/cordierite ± pyroxene at low pressure (Weinberg and Hasalova, 2015; and references therein). Our modeling results show that both of fluid-absent partial melting with dominantly residual garnet and clinopyroxene and

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fluid-present partial melting with mainly residual amphibole would generate high Sr/Y and La/Yb melts.

6.4. Tectonic implications

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The tectonic evolution of the Alxa Block was considered to be comparable with the history of active

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continental margins of the Paleo-Asian Ocean from the late Neoproterozoic to Triassic, beginning with

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fragmentation of the Rodinia supercontinent (Sengor et al., 1993). In the Early Paleozoic, siliceous limestones and marbles containing trilobites graptolites and Dalmanites deposited in the ZHTZ, which

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possibly was a passive non-magmatic continental margin (Feng et al., 2013; Li, 2006; Resources, 1991). During the Devonian, the ZHTZ was transferred from a passive continental margin to an active margin with volcanic activity (Li, 2006). The Cambrian to Early Carboniferous sediments are widely missing in the ZSTZ and the NLTZ, indicating a lose of sedimentation on the southern side of the Ocean (Resources, 1991). The absence of the Early Paleozoic strata in Alxa Block provides evidence that these zones had been uplifted at that time. The uplift event in the Alxa block is most likely attributed to the Paleo-Asian ocean subduction southward along the northern margin of the Alxa block (Feng et al., 2013), which has been proposed in Northern Xinjiang in the west and Inner Mongolia in the east (Ao et al., 2015; Ao et al., 2012; Gao et al., 2009; Jian et al., 2008; Jian et al., 2010; Liu et al., 2003; Mao et al., 2012). As a result of

ACCEPTED MANUSCRIPT south-dipping subduction during Early Paleozoic, an island arc in the ZSTZ and a back-arc basin in Quagan Qulu formed along the north of the Nuru-Langshan Zone (Li, 2006; Wang et al., 1994; Wang, 2012;

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Wu and He, 1993).

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Our work shows that part of the mid- to lower crust had been melted under water-present conditions during 397-411 Ma. Fluid-induced melting generally requires an external fluid source (White et al., 2007). The free water may resulted from active tectonics and magmatism, such as subduction-shortening events,

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regional metamorphism and underplating of mafic magmas (Weinberg and Hasalova, 2015). The regional

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metamorphic fluids commonly lead to form migmatites (e.g. metatexites, diatexites, melt in shear zones), and fluids released by the underplating mafic magmas would associate with mafic intrusions in the studied

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area. The field observation in the Alxa block can rule out that the fluids were originated from the above two

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processes. Thus, the free water in the mid- to lower crust is most likely related to the Quagan Qulu

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back-arc extension in Early Paleozoic. Continental rifting and breakup is widely thought to be either associated with Plume-type thermal weakening or serpentinization resulting from water percolation into the

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sub-continental lithospheric mantle (Barckhausen et al., 2014; Franke, 2012; Pérez-Gussinyé et al., 2006; Perez-Gussinye and Reston, 2001; Reston, 2009; Sutra and Manatschal, 2011; White and Dan, 1989; Whitmarsh et al., 2001). The lower crust and the sub-continental lithospheric mantle should experience partial melting and serpentinization, if free water migrates from the surface through back-arc rift. Our current study on the petrogenesis of Early Paleozoic Alxa granite has revealed that hydration melting occurred in the deep crust in the rifting of the Quagan Qulu back-arc basin. Therefore, we propose that the Devonian Beidashan granites might witness the back-arc rifting of the Quagan Qulu back-arc basin.

7. Conclusions

ACCEPTED MANUSCRIPT The Beidashan granites (397–411 Ma) from the Alxa block, northwest China can be divided into two groups: Group I (Eu/Eu*=0.77–1.3, TZr=761–856 ℃) and Group II (Eu/Eu*=1.7–5.3, TZr=651–785 ℃).

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Both of the Beidashan Group I and Group II granites show high Sr/Y geochemical signatures. The Group I

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and Group II granites were derived from partial melting of the crustal basement materials under fluid-absent and fluid-present conditions, respectively, at pressure of 10~12 kbar. The presence of a separate aqueous phase in the lower crust of the southwest Alxa Block may probably be resulted from

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regional active tectonics related to the Quagan Qulu back-arc rifting. Our results suggest that some high

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Sr/Y-type granites are not taken as an indicative of partial melting of thickened crust.

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Acknowledgments

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This research is supported by the Natural Science Foundation of China (grants: 41573021 and

41373040). We sincerely thank Dr. Yong–Sheng Liu for LA–ICP–MS zircon U–Pb dating and Dr.

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Zhao–Chu Hu for LA–MC–ICM–MS Zircon Hf isotopic analyses. We thank two reviewers for their constructive comments and suggestions that helpfully improve this manuscript.

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I 72.80 0.21 14.10 1.37 0.03 0.35 1.91 3.72 3.59 0.03 0.63 98.7 34 0.97 1.05 0.77 2.12 123.9 431 8.92 120 19.82 1106 0.77 0.75 23.44 11.99 0.68 26.66 63.26 5.20 18.19 3.21 1.17 2.33 0.33 1.61 0.29 0.74 0.10 0.67 0.10 48 763 809 1.25 28 1.60

A114 A115 A129 porphyritic biotite granite II I I 72.08 69.21 72.50 0.18 0.43 0.17 14.48 15.03 14.45 1.87 3.40 1.63 0.01 0.02 0.01 0.54 0.95 0.54 2.13 3.28 2.01 3.65 3.46 3.11 4.43 2.94 5.02 0.05 0.15 0.06 0.51 1.02 0.62 99.9 99.9 100.1 36 36 39 1.71 1.06 1.54 0.99 1.01 1.02 2.63 2.49 2.25 1.98 2.19 0.60 95.2 44.9 100.0 435 444 505 3.75 4.52 3.13 171 202 153 5.69 7.46 3.89 1526 1066 1590 4.95 5.58 4.19 0.32 0.41 0.06 28.82 19.04 29.32 4.36 10.66 13.25 0.51 0.76 0.49 14.29 27.07 31.61 26.74 65.41 67.84 2.85 6.79 6.93 10.67 24.46 25.16 1.74 4.29 4.39 1.23 1.31 1.02 1.26 3.05 2.45 0.15 0.27 0.22 0.68 1.11 0.77 0.13 0.17 0.12 0.31 0.36 0.26 0.06 0.05 0.04 0.34 0.26 0.28 0.06 0.05 0.05 116 98 161 785 799 779 693 802 813 2.41 1.06 0.86 30 74 81 1.34 2.84 1.84

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II 72.62 0.14 15.51 1.75 0.02 0.28 2.57 4.04 2.99 0.04 0.58 100.5 24 0.74 1.07 3.06 68.8 474 3.80 67 9.06 1043 2.00 0.24 20.85 2.16 0.24 7.30 13.72 1.49 5.45 1.04 0.60 0.93 0.13 0.69 0.12 0.27 0.04 0.21 0.04 125 717 665 1.84 25 2.16

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I 72.20 0.26 15.47 1.82 0.01 0.51 3.74 4.20 0.88 0.02 0.35 99.5 36 0.21 1.06 1.76 28.8 866 1.63 141 1.94 606 4.10 0.33 7.68 7.36 0.37 22.34 49.61 4.72 16.80 2.33 0.79 1.16 0.10 0.40 0.06 0.17 0.03 0.18 0.03 530 776 794 1.31 89 1.49

A74 biotite granite I 72.79 0.15 14.56 1.33 0.02 0.36 2.18 3.63 3.45 0.04 0.56 99.1 35 0.95 1.06 0.89 2.28 119.1 474 5.66 115 7.01 1211 0.89 0.54 27.45 10.80 0.61 30.42 61.21 6.07 21.14 3.60 0.83 2.54 0.33 1.28 0.19 0.41 0.05 0.29 0.05 84 761 818 0.79 74 2.90

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Group SiO2 TiO2 Al2O3 TFeO MnO MgO CaO Na2O K2O P2O5 LOI total Mg# K2O/Na2O A/CNK Cr Ni Rb Sr Y Zr Nb Ba Hf Ta Pb Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sr/Y TZr TLREE Eu/Eu* (La/Yb)N (Dy/Yb)N

A29 biotite granite II 72.12 0.25 16.20 1.77 0.00 0.56 4.02 4.19 0.88 0.02 0.52 100.5 39 0.21 1.07 1.63 28.7 927 1.57 142 1.13 533 4.10 0.02 6.43 0.98 0.33 3.59 8.05 0.74 2.72 0.48 0.76 0.31 0.04 0.22 0.04 0.15 0.03 0.19 0.03 589 777 626 5.68 13 0.76

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A111 A119 A50 A53 A92 A52 A54 A56 A57 biotite granite monzogranite porphyritic monzogranite Group I I II II II II II II II SiO2 71.78 72.69 72.98 74.60 73.02 72.21 73.50 73.97 74.10 TiO2 0.44 0.21 0.11 0.04 0.11 0.17 0.21 0.04 0.13 Al2O3 14.10 14.52 15.73 14.01 14.41 14.30 14.18 14.92 14.87 TFeO 2.77 1.72 1.03 0.44 0.95 1.49 1.50 0.54 1.24 MnO 0.02 0.01 0.01 0.01 0.01 0.02 0.03 0.01 0.02 MgO 1.03 0.52 0.26 0.10 0.23 0.43 0.27 0.08 0.27 CaO 3.74 2.50 2.79 1.82 2.34 2.28 0.98 2.25 2.59 Na2O 3.17 3.45 3.66 2.80 3.35 3.32 2.97 3.70 3.48 K2O 1.51 3.79 3.27 5.09 3.46 3.51 5.52 3.51 3.23 P2O5 0.11 0.04 0.03 0.01 0.01 0.04 0.06 0.02 0.01 LOI 1.33 0.55 0.67 0.52 0.42 0.82 0.67 0.65 0.58 total 100.0 100.0 100.5 99.4 98.3 98.6 99.9 99.7 100.5 Mg# 42 37 33 31 32 36 26 23 30 K2O/Na2O 0.85 1.38 0.89 1.82 1.03 1.06 1.86 0.95 0.93 A/CNK 1.03 1.01 1.07 1.04 1.07 1.07 1.12 1.07 1.07 Cr 3.61 1.09 1.40 0.42 0.88 2.91 1.39 0.62 1.13 Ni 1.18 1.81 2.13 1.85 1.86 Cu 0.95 0.57 2.17 0.61 2.71 1.16 2.07 1.34 2.78 Zn 58.2 40.8 15.8 10.1 26.6 40.9 43.4 12.8 27.3 Rb 32.8 71.1 66.5 101.0 73.5 80.9 81.4 77.7 82.5 Sr 710 484 567 406 613 614 638 571 513 Y 4.82 3.06 2.34 2.46 1.44 2.38 3.73 1.81 2.37 Zr 215 177 61 38 60 92 53 26 72 Nb 2.85 4.71 2.81 1.83 3.82 4.74 3.83 3.37 6.88 Ba 1103 1408 1326 1337 1778 1060 1628 769 958 Hf 5.73 4.93 1.40 0.42 0.88 2.91 1.39 0.62 1.13 Ta 0.07 0.32 0.09 0.34 0.36 0.25 0.46 0.13 0.54 Pb 10.43 25.73 22.77 28.62 26.40 20.10 32.13 24.13 35.27 Th 21.04 22.14 1.33 1.79 0.82 3.91 2.92 1.26 1.24 U 0.81 0.81 0.16 0.53 0.20 0.34 0.55 0.21 0.37 La 64.27 65.10 5.57 4.95 4.39 9.04 9.00 6.09 5.46 Ce 146.21 131.11 11.48 9.11 7.77 25.06 20.29 9.49 8.36 Pr 14.34 13.73 1.16 0.99 0.85 2.01 2.03 1.19 1.11 Nd 49.97 48.40 4.30 3.56 2.95 7.65 7.48 4.40 4.15 Sm 6.97 7.08 0.89 0.58 0.57 1.50 1.43 0.83 0.75 Eu 1.66 1.42 0.84 0.92 0.68 0.71 0.76 0.58 0.81 Gd 3.50 3.52 0.63 0.45 0.37 1.00 0.90 0.48 0.53 Tb 0.33 0.26 0.08 0.06 0.05 0.11 0.13 0.06 0.07 Dy 1.36 0.89 0.41 0.41 0.23 0.45 0.67 0.29 0.38 Ho 0.18 0.12 0.06 0.07 0.05 0.08 0.09 0.05 0.08 Er 0.39 0.24 0.19 0.22 0.14 0.21 0.32 0.14 0.24 Tm 0.05 0.03 0.02 0.05 0.02 0.03 0.03 0.02 0.05 Yb 0.31 0.22 0.19 0.30 0.14 0.16 0.25 0.11 0.25 Lu 0.05 0.04 0.04 0.05 0.03 0.03 0.04 0.02 0.05 Sr/Y 147 158 242 165 425 258 171 316 216 TZr 812 792 710 677 711 743 705 651 724 TLREE 887 876 653 637 631 696 696 649 642 Eu/Eu* 0.91 0.77 3.28 5.30 4.27 1.68 1.89 2.56 3.73 (La/Yb)N 151 215 21 12 23 41 26 39 16 (Dy/Yb)N 2.98 2.75 1.41 0.91 1.11 1.91 1.78 1.73 1.02 Notes: TFeO = All Fe calculated as Fe2O3; Mg#= molar (MgO / (MgO + FeOt))*100; A/CNK = molar Al2O3/ Na2O+K2O).

A89 II 73.09 0.14 14.32 1.36 0.02 0.32 2.71 3.47 2.52 0.01 0.93 98.9 32 0.73 1.07 0.97 1.53 3.11 40.4 69.3 540 3.79 76 7.45 766 0.97 0.43 32.27 3.68 0.43 9.94 21.22 2.28 8.14 1.49 0.78 0.93 0.13 0.66 0.12 0.31 0.05 0.33 0.05 142 730 695 1.90 21 1.33 (CaO+

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Table 2 Sr-Nd isotopic data

2σ

(87Sr/86Sr)0

0.713493 0.713449 0.717545 0.718151 0.718500 0.717646

0.000008 0.000004 0.000004 0.000004 0.000007 0.000007

0.7128 0.7129 0.7133 0.7124 0.7178 0.7145

0.713578 0.715112 0.716540 0.716512 0.716677 0.716244 0.717909 0.715401

0.000004 0.000007 0.000006 0.000005 0.000006 0.000005 0.000007 0.000004

0.7131 0.7127 0.7144 0.7143 0.7146 0.7143 0.7137 0.7134

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Sm/144Nd

143

Nd/144Nd

2σ

(143Nd/144Nd)t εNd(t) TDM(Ga) T2DM(Ga)

0.000007 0.000008 0.000003 0.000004 0.000006 0.000005

0.511701 0.511647 0.511721 0.511774 0.511698 0.511725

-8.0 -9.1 -7.6 -6.6 -8.4 -7.9

1.4 1.5 1.6 1.7 1.5 1.6

1.8 1.9 1.8 1.7 1.8 1.8

0.511996 0.511931 0.511944 0.511955 0.512000 0.511982 0.511946

0.000006 0.000007 0.000015 0.000003 0.000014 0.000008 0.000007

0.511688 0.511630 0.511648 0.511668 0.511664 0.511717 0.511633

-8.3 -9.7 -9.4 -9.0 -8.7 -7.7 -9.3

1.8 1.9 1.8 1.8 2.0 1.5 1.9

1.8 1.9 1.9 1.9 1.9 1.8 1.9

CR

0.511897 0.511871 0.511996 0.512093 0.511908 0.511988

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0.0732 0.0838 0.1030 0.1193 0.0843 0.1056

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Sr/86Sr

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Beidashan Group I granite A25 408 0.1170 A30 408 0.0965 A74 408 0.7282 A76 408 0.9920 A111 408 0.1341 A129 408 0.5739 Beidashan Group II granite A29 408 0.0898 A36 408 0.4205 A54 397 0.3698 A56 397 0.3946 A89 397 0.3722 A50 411 0.3399 A53 411 0.7218 A92 411 0.3473

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0.1060 0.1151 0.1159 0.1140 0.1104 0.1248 0.0986 0.1165

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Rb/86Sr

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87

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Sample Age(Ma)

Notes: 87Rb/86Sr and 147Sm/144Nd ratios are calculated using Rb, Sr, Sm and Nd contents (Table 1), measured by ICP-MS; εNd(t) values are calculated using present-day (147Sm/144Nd)CHUR=0.1967 and (143Nd/144Nd)CHUR=0.512638; TDM values are calculated using present-day (147Sm/144Nd)DM=0.2137 and (143Nd/144Nd)DM=0.51315.

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Table 3 Partition coefficients and assumed source rocks for trace element modeling

presumed parent rock source Partition coefficients (LCC) (ppm) Cpx Amp Pl Grt Rb 11 0.03 0.4 0.3 0.9 Sr 348 0.5 0.01 4.4 0.02 Y 16 2.6 11.3 0.04 130 Zr 68 0.33 0.5 0.2 0.8 Nb 5 2.1 0.2 1.3 0.004 Ba 259 0.1 0.387 0.48 0.002 La 8 2.8 0.86 0.3 0.37 Ce 20 1.54 0.68 0.032 0.79 Nd 11 2.3 1.6 0.19 0.4 Sm 2.8 2.9 2.3 0.12 0.84 Gd 3.1 1.41 2 0.09 13.6 Dy 3.1 1.22 10.2 0.07 116 Yb 1.5 1.14 4.41 0.06 54 Lu 0.25 3 1.8 0.1 47 Th 1.2 0.1 0.16 0.01 0.0075 U 0.2 0.12 0.45 0.01 0 Note: LCC: lower continental crust of Rudnick and Gao, 2003; Cpx: clinopyroxene; Amp: amphibole; Pl: plagioclase; Grt: garnet.

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ACCEPTED MANUSCRIPT Figure captions Fig. 1. (a): Simplified geological map of the Alxa Block, northwest China; (b): Simplified geological map

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of the Alxa Block Phanerozoic granitoids, SFL, southern margin fault of Longshoushan; WFB, western

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margin fault of Bayanwulashan; EUOB, Enger Us Ophiolite Belt. (Age data sources: Dan et al., 2014a; Dan et al., 2014b; Feng et al., 2013; Ren et al., 2005a; Ren et al., 2005b; Shi et al., 2012a; Su, 2012; Yang, 2014; Zhang et al., 2012; Zhang et al., 2013b; Zheng et al., 2014); (c): Geological sketch-map of the

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Beidashan area (NMBGMR(NeiMongol Bureau of Geology and Mineral Resources), 1991), with sample

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locations.

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Fig. 2. Microphotographs of representative samples of the Beidashan Devonian granite. (a) and (b) :

monzogranites.

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medium-coarse-grained biotite granites; (c) and (d): porphyritic biotite granites; (e) and (f): porphyritic

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Fig. 3. Representative zircon CL images. (a) medium-coarse-grained biotite granite (A29); (b) medium-coarse-grained monzogranite (A50); (c) porphyritic monzogranite (A56); (d) coarse-grained biotite granite (A74). The solid line circles show LA-ICPMS dating spots and corresponding

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Pb/238U

apparent ages. The dash line circles show Lu-Hf isotope analysis and corresponding εHf(t) values. White bars represent 100 µm.

Fig. 4. Zircon U-Pb Concordia diagrams. (a) medium-coarse-grained biotite granite (A29); (b) medium-coarse-grained monzogranite (A50); (c) porphyritic monzogranite (A56); (d) coarse-grained

ACCEPTED MANUSCRIPT biotite granite (A74). Inserts in (a), (b), (c) and (d) show weighted mean ages. Dashed ellipses are not

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included in the age calculation. Ellipses represent 1-sigma uncertainty for individual analyses.

Fig. 5. (a): K2O + Na2O versus SiO2 diagram; (b): A/NK [molar ratio Al2O3/(Na2O + K2O)] versus A/CNK [molar ratio Al2O3/(CaO + Na2O + K2O)] diagram for the Devonian granite of the Alxa Block

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(Maniar & Piccoli, 1989); (c): K2O versus SiO2 diagram (Peccerillo & Taylor, 1976); (d): TiO2 versus SiO2;

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(e): MgO versus SiO2; (f): P2O5 versus SiO2; (g): Cr versus SiO2; (h): Zr versus SiO2; (i): LaN versus SiO2.

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Fig. 6. Primitive mantle-normalized trace element spider diagrams (a and c) and chondrite normalized rare

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McDonough, 1989.

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earth element patterns (b and d) for the Beidashan batholith. Normalizing values are from Sun and

Fig. 7. (a): Sr/Y versus Y diagram. Field of adakite is from Defant and Drummond, 1990. (b): (La/Yb)N versus YbN diagram. Field of adakite is from Defant and Drummond, 1990. Symbols as in Fig. 5.

Fig. 8. εNd(t) versus ISr diagram. Symbols as in Fig. 5.

Fig. 9. Plot of zircons εHf(t) versus zircon U-Pb age.

Fig. 10. Variation of (a) Diagram of MgO versus Zircon saturation temperature (TZr); (b) Diagram of MgO

ACCEPTED MANUSCRIPT versus Eu/Eu*; (c) Diagram of Eu/Eu* versus TZr; (d) Diagram of LREE versus TZr; (e) Diagram of (La/Yb)N versus TZr; (f) Diagram of (Dy/Yb)N versus TZr. Zircon saturation temperature was shown for

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comparison (calculated by Watson and Harrison, 1983). Symbols as in Fig. 5.

Fig. 11. Diagram of the LREE saturation temperature (TLREE) versus zircon saturation temperature (TZr).

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The LREE saturation temperature was calculated by Montel, 1993. Zircon saturation temperature was

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calculated by Watson and Harrison, 1983. Symbols as in Fig. 5.

Fig. 12. Variation of selected major and trace elements versus Eu/Eu* for the Beidashan batholith: (a)

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Na2O; (b) CaO; (c) K2O; (d) Sr. Symbols as in Fig. 5.

Fig. 13. Trace element modeling for two groups of high Sr/Y granites: Data source for the composition of

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the parent rock are from Rudnick and Gao (2003); data source for the mineral assemblage of the amphibolitic source dehydration melting (Group I) is from Wyllie and Wolf (1993); data source for the mineral assemblage of the amphibolitic source derived from fluid-melting (Group II) is from Qian and Hermann (2013); data sources for the partition coefficient are from http://earthref.org/KDD/.

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1. High Sr/Y-type magmas derived from fluid-present melting of mafic to intermediate crustal materials

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with residual amphibole

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2. High Sr/Y-type magmas derived from fluid-absent partial melting of mafic to intermediate crustal materials leaving garnet residuum

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3. Some high Sr/Y-type granites may not be an indicative of partial melting of thickened crust