Recycling of ancient subduction-modified mantle domains in the Purang ophiolite (southwestern Tibet)

    Recycling of ancient subduction-modified mantle domains in the Purang ophiolite (southwestern Tibet) Xiao-Han Gong, Ren-Deng Shi, W.L. Griffin, Qi-Shuai Huang, Qing Xiong, Sheng-sheng Chen, Ming Zhang, Suzanne Y. O’Reilly PII: DOI: Reference:

S0024-4937(16)30138-4 doi: 10.1016/j.lithos.2016.06.025 LITHOS 3971

To appear in:

LITHOS

Received date: Accepted date:

8 January 2016 23 June 2016

Please cite this article as: Gong, Xiao-Han, Shi, Ren-Deng, Griffin, W.L., Huang, QiShuai, Xiong, Qing, Chen, Sheng-sheng, Zhang, Ming, O’Reilly, Suzanne Y., Recycling of ancient subduction-modified mantle domains in the Purang ophiolite (southwestern Tibet), LITHOS (2016), doi: 10.1016/j.lithos.2016.06.025

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ACCEPTED MANUSCRIPT Recycling of ancient subduction-modified mantle

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domains in the Purang ophiolite (southwestern Tibet)

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Xiao-Han Gong*1,3, Ren-Deng Shi1,2,3, W.L. Griffin4, Qi-Shuai Huang1, Qing Xiong4,5, Sheng-sheng Chen1,3, Ming Zhang4 and Suzanne Y. O’Reilly4 1

Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan

CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, PR

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2

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Plateau Research, Chinese Academy of Sciences, Beijing 100101, China

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University of Chinese Academy of Sciences, Beijing 100049, China

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ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC,

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Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia

State Key Laboratory of Geological Processes and Mineral Resources, School of

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5

Earth Sciences, China University of Geosciences, Wuhan 430074, China

*Corresponding author: [email protected]

Abstract Ophiolites in the Indus-Yarlung Zangbo (IYZ) suture (southern Tibet) have been interpreted as remnants of the Neo-Tethyan lithosphere. However, the discovery of diamonds and super-reducing, ultra-high pressure (SuR-UHP) mineral assemblages

ACCEPTED MANUSCRIPT (e.g., coesite after stishovite, olivine after wadsleyite, native metals, alloys, and moissanite) in some of these massifs and associated chromitites requires a

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re-evaluation of their origin and evolution. A new petrological and geochemical study

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of the Purang ophiolite in the western IYZ suture sheds new lights on these issues. The depleted harzburgites of the Purang massif have low modal contents of clinopyroxene (<2%), and high Cr# [100*Cr3+/(Cr3++Al3+)] in spinel (>40~70) and

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pyroxenes (>16 in orthopyroxene, and >20 in clinopyroxene), suggesting high degrees

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of melt extraction (>20%). These features are not consistent with formation in a (ultra-) slow-spreading mid-ocean ridge. These peridotites have high modal contents

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of orthopyroxene; this, and the extremely high Cr# of spinels in these peridotites,

contain

rare

spinel-pyroxene

symplectites

after

garnet.

Their

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lherzolites

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suggests modification in a subduction zone. The clinopyroxene-rich harzburgites and

clinopyroxenes have low MREE to HREE ratios ((Sm/Yb)N <0.1) at relatively high

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HREE concentrations, and are Na-rich but Nd-poor. The relatively enrichment of Na but depletion of Nd in clinopyroxene cannot be explained by refertilization with MORB melts, but are consistent with an origin from Na-rich subcontinental lithospheric mantle (SCLM). All lines of evidence suggest that these peridotites underwent initial melting in the stability field of garnet-facies peridotites, followed by additional melting in the spinel-facies mantle. Whole-rock Os isotopic compositions of the Purang peridotites give ancient TRD model ages (up to 1.3 Ga), indicating that the formation of these ancient depletion residues predated the opening of Neo-Tethyan Ocean. These observations, together

ACCEPTED MANUSCRIPT with recent studies on other IYZ peridotites, suggest that the Purang peridotites are genetically unrelated to the associated mafic crust. Instead, they represent ancient

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SCLM domains, initially formed beneath a continental margin, and then modified by

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subduction, before they were incorporated into the Neo-Tethyan ocean basin. This model is consistent with the deep-mantle-recycling model for the presence of SuR-UHP phases in the IYZ ophiolites. The infiltration of MORB melts through these

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ancient depleted peridotites during their final exhumation in a (ultra-) slow-spreading

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centre may have refertilized them to produce the clinopyroxene-rich peridotites.

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Keyword: Purang ophiolite; sub-continental lithospheric mantle; mantle recycling;

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Indus-Yarlung Zangbo suture; Tibet

1. Introduction

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Ophiolites, fragments of ancient oceanic lithosphere that were tectonically emplaced onto continental margins, are the best archives of the evolution history of ocean basins from rifting, to drifting, to subduction initiation and final closure (review by Dilek and Furnes, 2014; Piccardo et al., 2014). Understanding of the origin, evolution and emplacement mechanism of slices of ancient oceanic lithosphere associated with continental sutures can advance our knowledge of plate tectonics and the Earth’s geodynamics (e.g., Nicolas and Boudier, 2003). For instance, most supra-subduction zone ophiolites are thought to form at spreading centers in forearcs during the subduction-initiation process (e.g., Stern, 2004; Whattam and Stern, 2011); analogues

ACCEPTED MANUSCRIPT are the modern Izu-Bonin-Mariana and Tonga-Kermadec arc-trench rollback systems (e.g., Ishizuka et al., 2014). Thus, the study of these ophiolites could provide

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significant constraints on the process and dynamics of intraoceanic subduction, which

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is a critical link in the plate tectonic cycle (e.g., Stern et al., 2012; Maffione et al., 2015).

In addition, ophiolites can provide unique opportunities to observe the diverse

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structure of oceanic crust and upper mantle and to study mantle processes including

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melt extraction (e.g., Kelemen et al., 1995; Le Me´e et al., 2004), melt migration and melt-rock interaction (e.g., Kelemen et al., 1992; Godard et al., 2000; Piccardo et al.,

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2014) as well as mantle heterogeneity (e.g., Shi et al., 2007; van Acken et al., 2008;

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Rampone and Hofmann, 2012). The geochemical and isotopic decoupling has led to

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reappraisal of the direct genetic relationship between ophiolitic peridotites and spatially associated crustal rocks (e.g., Rampone et al., 1998; Müntener et al., 2004;

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McCarthy and Müntener, 2015) and has heated the discussion about the geodynamic significance of isotopically ultradepleted domains in the convecting upper mantle (e.g., Rampone and Hofmann, 2012; Byerly et al., 2014; McCarthy and Müntener, 2015). The Indus-Yarlung Zangbo (IYZ) suture (South Tibet), separates the Indian continent to the south from the Asian continent to the north (e.g., Chang and Zheng, 1973; Yin and Harrison, 2000), and preserves abundant, large fresh peridotite massifs (e.g., Luobusa; Xigaze; Purang). They were originally accepted as remnants of oceanic lithosphere formed either in mid-ocean ridge settings (e.g., Nicolas et al., 1981; Girardeau et al., 1985; Miller et al., 2003; Liu et al., 2014) or in

ACCEPTED MANUSCRIPT supra-subduction zone settings (e.g., Zhou et al., 1996; 2014; Hébert et al., 2012) within the Jurassic-Cretaceous Neo-Tethys. However, their peculiar lithologic

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structures (e.g., Nicolas et al., 1981; Wu et al., 2014) distinguishes them from the

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traditional ophiolite sequences observed in the Semail ophiolite in Oman (e.g., Nicolas and Boudier, 1995) or Troodos in Cyprus (e.g., Moores and Vine, 1971). In addition, their compositional heterogeneity in terms of Sr-Nd (e.g., Wu et al., 2014)

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and Re-Os isotopes (e.g., Shi et al., 2012; Liu et al., 2012), as well as the occurrence

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of diamond and SuR-UHP mineral assemblages (e.g., Bai et al., 2001; Yang et al., 2007; Arai, 2013; Griffin et al., 2016) in some of the ophiolites make them different.

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These observations have raised debates about the origin and evolution of these IYZ

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ophiolitic massifs in recent years (e.g., Arai, 2010; Yang et al., 2014; Zhou et al., 2014;

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Wu et al., 2014; Griffin et al., 2016). The Purang ophiolite, located in the western part of the IYZ suture, represents an

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end-member type of the IYZ peridotite massifs in terms of field architecture (Wu et al., 2014). Though it was extensively studied during last decade, there is no consensus on its origin and evolution. Three models have been proposed: (1) It formed in a fossil slow-spreading mid-ocean ridge setting as an exhumed oceanic core complex (OCC); this model is mainly based on the geochemical studies of mafic rocks and mantle peridotites (e.g., Miller et al., 2003; Liu et al., 2014). (2) It was initially produced at a fossil (ultra-) slow-spreading mid-ocean ridge and subsequently was modified by melts/fluids in a supra-subduction zone; this model mainly emphasized the arc-related geochemical signature recorded in some Purang peridotites (e.g., Xu et al., 2008; Liu

ACCEPTED MANUSCRIPT et al., 2010; Li et al., 2015; Su et al., 2015). (3) It was generated in a rifted continental margin, based mainly on geochemical studies of spatially associated cherts (Huang et

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al., 2010) and doleritic dikes, and volcanic-sedimentary rocks (Liu et al., 2015).

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Although each model could explain part of the observed features of the Purang ophiolite, none of them can explain the presence of SuR-UHP mineral assemblages in this massif (Yang et al., 2011; 2014).

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In this paper, we present new petrological/geochemical evidence from the Purang

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ophiolite (SW, Tibet), to show that these peridotites are probably not the simple resides complementary to spatially associated oceanic crust. We argue that the Purang

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peridotite massif represents ancient domains of subduction-modified SCLM, which

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may initially have formed beneath a continental margin and were subsequently

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recycled to become components of the Neo-Tethyan oceanic basin.

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2. Geological Setting

2.1. Architecture of the Purang ophiolitic sequence The Tibetan Plateau formed by the northward accretion of several terranes, now separated by several well-documented suture zones such as the Bangong-Nujiang suture (BNS), and the IYZ suture (Fig. 1). Ophiolites along these sutures directly record these tectonic events and thus are key elements in reconstructing the evolution of the Tethys ocean (e.g., Nicolas et al., 1981; Wang et al., 1987; Zhou et al., 1996; Hebert et al., 2012; Wu et al., 2014). The Purang peridotite massif, located in the western part of the IYZ suture, has a

ACCEPTED MANUSCRIPT roughly NW-SE direction and an area of about 700km2, and is the largest one along the whole suture. The architecture of the Purang ophiolite was first briefly described

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by Gansser (1964), then in more detail by Guo et al. (1991). During the past decade,

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extensive studies have been dedicated to different units of the Purang massif (e.g., Miller et al., 2003; Xu et al., 2008; Li et al., 2008; Huang et al., 2010; Liu et al., 2010; 2012; 2014; Yang et al., 2011; Liu et al., 2015; Li et al., 2015).

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The main characteristics of the Purang ophiolitic sequence can be summarized as

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follows: (1) the massif is in fault contact with its surrounding strata. (2) The predominant exposures are almost fresh mantle peridotite, which mainly consists of

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cpx-bearing harzburgites; partially-serpentinized spinel lherzolites in the northern part

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of the massif are in fault contact with the harzburgites. Occasionally meter-scale

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dunite bands and centimeter to decimeter-scale spinel pyroxenites and gabbronorites (Fig. S1) crosscut the harzburgite (Miller et al., 2003; Liu et al., 2014; Li et al., 2015).

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(3) Mafic plutonic rocks are very scarce. (4) There is no sheeted dike complex; instead, the rare dolerites and gabbros occur as dykes or veins (Fig. S1) intruding the peridotite. (5) Ophicalcites, with angular fragments of serpentine embedded in a matrix of large calcite crystals, occur locally (Gansser, 1964; Liu et al., 2015). (6) The Purang massif tectonically overlies various volcanic-sedimentary rocks including massive vesicular basaltic flows associated with radiolarian cherts, siliceous to oolitic limestones and silty shale interbedded with thin sandstone layers, and strongly deformed and hydrothermally altered silicic tuffaceous rocks (Huang et al., 2010; Liu et al., 2015).

ACCEPTED MANUSCRIPT Such architecture is generally comparable to that of the Alpine-Apennine ophiolites (e.g., Manatschal and Müntener, 2009; Piccardo et al., 2014), which were

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considered to represent either marginal domains of the Piedmontese-Ligurian basin,

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similar to the magma-poor ocean-continent transition of Western Iberia (e.g., Marroni et al., 1998; Manatschal and Müntener, 2009), or more oceanward domains of the Piedmontese–Ligurian basin, bearing remarkable structural and compositional

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similarities to modern (ultra-) slow spreading ridges (e.g. Lagabrielle and Cannat,

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1990; Rampone et al., 2008).

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2.2. Composition and age of the oceanic crust in the Purang massif

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Crustal rocks are generally rare in the Purang massif. They occur as gabbroic bodies a

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few meters wide and/or basaltic dikes crosscutting the peridotite, particularly at the southeastern end of the thrust sheet. They are undeformed and intruded after the

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plastic deformation of the ultramafic rocks (Miller et al., 2003). The basaltic dikes have typical N-MORB-like REE patterns and Sr-Nd isotope systematics indicative of an origin from a depleted upper mantle source, comparable to tholeiitic basalts from ridge-axis systems (Miller et al., 2003; Liu et al., 2015). The gabbronorites are characterized by the early crystallization of pyroxenes, in particular orthopyroxene, relative to plagioclase (Miller et al., 2003; Liu et al., 2014). They probably are cumulates crystallized from parent magmas with high-silica contents but are strongly depleted in Na, Ti, Zr, Sr and other incompatible trace elements (Miller et al., 2003; Liu et al., 2014); in this respect they are comparable to similar rocks from

ACCEPTED MANUSCRIPT the Alpine-Apennine ophiolites (e.g., Piccardo and Guarnieri, 2011). Previous dating of basaltic dikes from the Purang peridotite has yielded a Sm-Nd

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isochron age of 147 ± 25 Ma, which is identical to the Ar-Ar age (152 ± 33 Ma) of

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hornblende separated from a basaltic dyke (Miller et al., 2003). On the other hand, fine- to medium-grained gabbros and doleritic dikes with N-MORB-like affinity give zircon U-Pb ages of 120-130 Ma (e.g., Li et al., 2008; Chan et al., 2014; Liu et al.,

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2015).

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In conclusion, the rare but spatially associated N-MORB-like basaltic crust of the Purang ophiolitic massif was generated mainly within a short time span between

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130~120Ma, like similar rocks documented in other ophiolitic massif along the whole

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IYZ suture (e.g., Hebert et al., 2012; Chan et al., 2014; Wu et al., 2014).

3. Petrography

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The samples studied in detail here consist of 12 cpx-bearing harzburgites (the main lithology of the massif), 5 cpx-free harzburgites and 6 lherzolites. These samples, combined with those reported in the literature (Miller et al., 2003; Liu et al., 2012; 2014; Zhou et al., 2014; Li et al., 2015; Su et al., 2015), represent a comprehensive sampling of the whole massif and will be discussed together in this study. Generally, the Purang peridotites have coarse-granular, transitional to porphyroclastic textures. Olivine grains in all these samples represent at least two generations: (i) primary olivine grains with sizes ranging from 2-5mm, with evident undulose extinction or kink bands (Fig. 2a), which is a typical characteristic of high-T

ACCEPTED MANUSCRIPT ductile deformation under mantle conditions; (ii) recrystallized olivine grains (neoblasts) with much smaller size, filling embayments in coarse opx grains (Fig. 2b).

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Spinels in these samples are much smaller (0.5-1 mm) than coexisting silicates. They

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usually display two types of relationships with silicate minerals: (i) disseminated within the olivine matrix as relatively large, irregular grains, associated with pyroxenes or isolated grains with various size and shape, sometimes in chains; (ii) as

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fine to coarse-grained spinel-pyroxene micro-intergrowths (symplectites) either

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rimming coarser opx porphyroclasts (Fig. S2-d) or within the olivine matrix (Fig. 2f). Coarse to very coarse opx grains, with grain sizes similar to or larger than

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coexisting primary olivine, are heterogeneously distributed in these samples. They

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tend to occur as veins or clusters (Fig. 2e) in the olivine matrix and usually show

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diffuse or curvilinear boundaries. High-T plastic deformation textures including kink bands and intense elongation with a length: width up to 5:1 are well preserved in some

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coarse grains (Fig. 2b, 2c), especially from the cpx-harzburgites and lherzolite. Abundant exsolution lamellae of cpx (up to 60μm) or small cpx inclusions, with similar orientation and with shapes ranging from euhedral to lamellar, are common in these opx grains (Fig. S2-a). In contrast, small opx grains, without evident exsolution, occasionally occur as cpx ±opx ±ol assemblages along coarse pyroxene grains (Fig. 2d). Cpx in the Purang peridotites shows a semi-continuous grain size variation from very fine- to coarse-grained (0.2-2mm), much smaller than coexisting protogranular opx. Similarly, most coarser cpx grains in these samples occur as strings or clusters in

ACCEPTED MANUSCRIPT the olivine matrix (Fig. 2d; S2-c). Thin Opx exsolution lamellae (~10μm), usually occur in the cores of these coarse cpx grains (Fig. S2-c). On the other hand, very fine

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cpx grains frequently occur along kink-bands in opx (Fig. 2b), as cpx ±opx ±ol

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assemblages at opx grain boundaries, as part of typical spinel-pyroxene micro-intergrowths at opx grain boundaries (Fig. S2-d) or interstitial to coarse olivine

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grains.

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4. Analytical Methods

Modal mineralogy was calculated from major element bulk and mineral chemistry

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using PETMIX (Le Maitre, 1979). Bulk rock analyses used in the calculation were

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normalized to 100% anhydrous and all Fe is reported as FeO. Mineral compositions

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are averaged values for each sample. Whole-rock major element compositions were determined by X-ray fluorescence

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(XRF) at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). Precisions is 1–5% relative for elements present in concentrations higher than 1%, and about 10% for elements with concentrations less than 1%. Whole-rock trace elements (REE, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, Pb, Th and U) were analysed by ICP-MS after dissolution in Teflon vials using HF, HNO3 and HCL at the Institute of Tibetan Plateau Research, Chinese Academy of Sciences (ITPCAS). The relative standard deviation (RSD %) is less than 15% for most trace elements, while greater than 30% for MREEs (e.g., Sm, Eu, Gd, Tb) in some samples with ultra-low concentrations.

ACCEPTED MANUSCRIPT The major-element compositions of minerals were analyzed using JEOL JXA-8100 and JXA-8230 electron microprobes with an accelerating potential of 15 kV and

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Administration (Hangzhou) and ITPCAS, respectively.

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sample current of 20 nA at the Second Institute of Oceanography, State Oceanic

Mineral trace-element analyses were conducted by LA-ICP-MS at the Guangzhou Institute of Geochemistry, CAS. Detailed operating conditions for the laser ablation

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system and the ICP-MS instrument and data reduction are those described by Liu et al.

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(2008). Laser sampling was performed using a GeoLas 2005. An Agilent 7500a ICP-MS instrument was used to acquire ion-signal intensities. A “wire” signal

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smoothing device is included in this laser ablation system, by which smooth signals

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are produced even at very low laser repetition rates down to 1 Hz (Hu et al., 2012).

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Helium was used as the carrier gas. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. Nitrogen was added

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into the central gas flow (Ar +He) of the Ar plasma to lower the detection limit and improve precision (Hu et al., 2008). Each analysis incorporated a background acquisition of approximately 20-30 s (gas blank) followed by 50s of data acquisition from the sample. The Agilent Chemstation was used for the acquisition of each analysis. Element contents were calibrated against multiple reference materials (ML3B-G; T1-G) without applying internal standardization (Liu et al., 2008). The preferred values of element concentrations for the USGS reference glasses are from the GeoReM database (http://georem.mpch-mainz.gwdg.de/). Off-line selection and integration of background and analyte signals, and time-drift correction and

ACCEPTED MANUSCRIPT quantitative calibration were performed by ICPMSDataCal (Liu et al., 2008). Whole-rock Re-Os isotopes in peridotites were analyzed by isotope dilution at the

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Guangzhou Institute of Geochemistry, CAS. The experimental procedure is described

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by Li et al. (2010). About 1.5g powder was dissolved using inverse aquaregia (2.5 ml 10.8 mol/L HCl, 7.5 ml 14 mol/L HNO3) in sealed Carius tubes at 240 °C for 24 h (0.1 g and 48 h for spinel). Isotopic tracers (185Re and

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Os) were added to samples

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prior to dissolution. Os was purified using CCl4 and HBr, micro-distilled for 3 h

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(Birck et al., 1997) and dried to 3 μL. Os isotope measurements were carried out by N-TIMS on a Thermo Finnigan Triton (Creaser et al., 1991). Re was extracted and

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purified by anion exchange using AG1 × 8 resin. Re concentrations were measured

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using ICP-MS. Total blank levels were 4 to 11 pg and 0.3 to 2 pg for Re and Os, 187

Os/188Os of the blank was 0.14892. The contribution

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respectively, and the average

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of the blank to measured Os contents and 187Os/188Os of most samples was negligible.

5. Results

Based mainly on the microtextural setting of cpx and some basic petrographic/ geochemical indicators such as the modal contents of cpx, and Cr# in spinel and pyroxenes, the Purang peridotites can be roughly divided into two groups: (1) Depleted

harzburgites,

which

are

generally

characterized

by

absence

of

coarse-grained cpx, low cpx modes (<2 %), and high Cr# in spinel (> 40) and in pyroxenes (>16 in opx, >20 in cpx). (2) Cpx-harzburgites and lherzolites, which contain variable amounts of coarse-grained cpx, and lower Cr# in spinel (~18- <45)

ACCEPTED MANUSCRIPT and pyroxenes (<16 in opx, <20 in cpx).

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5.1. Whole-rock major and trace-element compositions

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Whole-rock major and trace-element compositions of the Purang peridotites are listed in Supplementary Table 1 and 2 (ST 1& 2). The peridotites are weakly-moderately serpentinized with loss on ignition (LOI) mainly from 2 to 7 wt %. They display

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overall refractory compositions (Fig. 3) with high MgO contents (39.9-45.2 wt %) but

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low contents of Al2O3 (0.34-2.50 wt %), CaO (0.45-2.63 wt %), Na2O (<0.02 wt %), and TiO2 (<0.05 wt %), relative to Primitive Upper Mantle (McDonough and Sun,

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1995). On the other hand, these peridotites have high SiO2 contents, ranging from

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44.2 to 47.3 wt %, and the harzburgite-orthopyroxenite mixed sample (PL1102G) has

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the highest SiO2 contents, up to 48.5 wt %. The depleted harzburgites overall have lower Al2O3 and CaO and higher MgO contents than the cpx-harzburgites and

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lherzolites, consistent with their low cpx contents. Primitive mantle-normalised REE and extended trace-element patterns of representative samples are presented in Fig 4. Most cpx-harzburgites and lherzolites show “spoon-shaped” REE patterns, defined by an increasing depletion from HREE to MREE (Lu to Nd), but a gradual enrichment in LREE (Pr to La). The depleted harzburgites have lower HREE concentrations and generally show “V-shaped” REE pattern, in which Eu and Gd have the lowest concentrations. In the extended trace-element diagram, all these peridotites generally show trends of increasing enrichment of highly incompatible trace elements (Nb to Rb) relative to LREE. Zr and

ACCEPTED MANUSCRIPT Hf are also weakly enriched relative to the neighbouring elements (Nd and Sm).

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Prominent positive anomalies of Ba, U, and Sr are observed in these samples.

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5.2. Mineral chemistry

Mineral compositions of the Purang peridotites are listed in Supplementary Table 3-7 and illustrated in Fig. 5-7. Olivine in the Purang peridotites has Mg# (=100*Mg/(Mg

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+ Fe)) mainly in the range 89.7-91.6 (Fig. 5). NiO contents range from 0.36 to 0.42

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wt %, and CaO contents are very low (<0.1 wt %) in all the samples. Spinel in the Purang peridotites shows continuous variation in Cr# (=100*Cr/(Cr+Al)) and Mg#,

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ranging from ~18 to 72 and from 49 to 72, respectively (Fig. 5). TiO2 contents of

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spinels are very low (<0.07 wt %).

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Opx displays the most significant inter-sample and within-sample compositional variations in the mineral assemblage. In a given sample, Al2O3 and Cr2O3 are

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positively correlated, decreasing from the core to rim of porphyroclasts and from large grains to small grains and opx within intergrowths. Al2O3, CaO and Cr2O3 contents of the interior of opx porphyroclasts in these samples vary from 1.1 to 5.1 wt %, from 0.96 to 2 wt % and from 0.53 to 0.9 wt %, respectively. Mg# and Cr# of opx in these samples range from 90-92 and 10-27, respectively. In Fig. 6, the literatures data that shows large variations of Mg# of opx; in contrast, our samples display good correlations among Al2O3 contents, Mg# and Cr# of opx. Cpx in the Purang peridotites generally shows inter- and within sample compositional variations similar to opx with Al2O3 and Cr2O3 decreasing from the

ACCEPTED MANUSCRIPT core to rim of porphyroclasts and from large grains to small grains and cpx within intergrowths. Their Mg# values vary from 91-94, Cr# values from 11-33 and CaO

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contents from 21.6-23.6 wt % (Fig. 7). TiO2 and Na2O contents of cpx in most

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samples are very low (<0.1 and < 0.25 wt %, respectively).

REE concentrations of cpx in representative samples are listed in Table 1 and illustrated in Fig. 8. Within-sample variability for REEs was investigated by analyzing

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cpx in several microstructural situations (porphyroclast core and rim, intergranular

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grains, symplectites). REE concentrations show no significant intra- or inter-grain variations at a sample scale, and thus no texture-related variation. Cpx REE patterns

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in these peridotites, on the whole, are strongly depleted in light REE (LREE) relative

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to middle (MREE) and heavy REE (HREE), similar to cpx in residual peridotites from

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mid-ocean ridges (Johnson et al., 1990; Johnson and Dick, 1992; Hellebrand et al., 2002; Brunelli et al., 2006). Yb(N) is 3-8 times chondritic (Anders & Grevesse, 1989),

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Dy/Yb(N) varies between 0.61 and 0.92, Sm/Yb(N) varies between 0.05 and 0.31 and Ce/Yb(N) ranges from 0.001 to 0.006.

5.3. Whole-rock Re-Os isotope The Os concentrations of the Purang peridotite vary from 2.38 to 7.50 ppb (Table 2), with most samples falling in the range between 3.0 and 5.5 ppb Os, as observed in other mantle peridotites (e.g., Meisel et al., 2001; Harvey et al., 2006; Becker et al., 2006). Re concentrations range from 0 to 0.48 ppb with an average of 0.27 ppb, similar to or lower than estimates for the upper mantle (0.3 ppb; Morgan, 1986).

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Re/188Os ranges from 0.001 to 0.940; most ratios are less than the average

chondritic value of 0.4.

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Os/188Os varies considerably from 0.1189 to 0.1410, well

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within the isotopic composition range of abyssal peridotites (review by Rampone and

mixed lithology, has the highest

187

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Hofmann, 2012). Sample PL1102G, which represents a harzburgite-orthopyroxenite Os/188Os (0.1410), while PL1208-1, representing

the most depleted sample，has the highest Os concentration (7.5 ppb) and lowest Os/188Os (0.1189). Re-depletion ages (TRD) calculated relative to an enstatite

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187

MA

chondrite reservoir (ECR; Walker et al., 2002) ranges from 0.1-1.3 Ga (Table 2), with several well-defined age clusters at ~240Ma, ~500Ma; and a broad span of ~700

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Ma-1.3 Ga, which is roughly consistent with recently reported data from other massifs

6. Discussion

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along the IYZ (see summary by Griffin et al., 2016).

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6.1. Melt percolation and melt-rock reactions in the Purang peridotites Field and microstructural observations confirm previous observations (e.g., Miller et al., 2003; Liu et al., 2014; Li et al., 2015) that the Purang peridotites have undergone various degrees of melt percolation and melt-rock interactions, witnessed by the local occurrence of decimeter- to meter-scale gabbroic veins or dikes intruding the peridotites (Fig. S1). These observations suggest that these melts traversed the peridotites in a localized way. On a broader scale, the occurrence of olivine embayments within exsolved opx porphyroclasts in the peridotites (Fig. 2b), is generally considered as reflecting reaction of silica-undersaturated fluids with the

ACCEPTED MANUSCRIPT surrounding mantle during melt percolation (e.g., Kelemen et al., 1995; Piccardo et al., 2007; Rampone et al., 2008), and the occurrence of cpx selvages and small cpx ±opx

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±ol pockets at the grain boundaries of coarse pyroxenes (Fig. 2d; S2-b), suggests

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either crystallization from percolating melts or reaction of such melts with mantle minerals (e.g., Seyler et al., 2003; Piccardo et al., 2007; Suhr et al., 2008). The effect of melt percolation and melt-rock reactions are exhibited by various

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degrees of modification in terms of modal compositions, whole-rock major and trace

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element compositions and mineral chemistry of the Purang peridotites. Modal compositional modification is evident in the cpx-harzburgites and

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lherzolites. Most lherzolites (cpx >5 vol. %) are virtually indistinguishable from some

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cpx-harzburgites (cpx <5 vol. %) with respect to their Mg# ratios, HREEs

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concentrations and mineral chemistry. Their more ‘fertile’ character is mainly reflected in higher cpx contents (Fig. S3) and thus higher whole-rock CaO contents,

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suggesting modal metasomatism involving crystallization of secondary cpx from percolating melts; this is consistent with the occurrence of small cpx grains along the boundaries of coarse pyroxene grains in these peridotites (Fig. 2b; 2d). Whole-rock LREEs (e.g., La, Ce) in all the analysed samples are enriched, relative to expected residues of partial melting (Fig. 4a). This, coupled with the overall enrichment of LILE (e.g., Rb, Ba, Th, U) and HFSE (e.g., Nb, Zr) (Fig. 4b), may reflect post-melting refertilization (e.g., Niu, 2004). Modifications

of

mineral

compositions

are

most

evident

in

some

cpx-harzburgites (especially those from published data) and depleted harzburgites,

ACCEPTED MANUSCRIPT which have olivines and pyroxenes with lower Mg# (Fig. 5-7), suggesting re-equilibration with Fe-rich melts. TiO2 contents of spinel in some cpx-harzburgites

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(literatures data) show mild enrichment (Fig. 5d), which indicates reaction with

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MORB melts (e.g., Pearce et al., 2000).

Though melt percolation and melt-rock interaction could result in variable degrees of compositional modification of the Purang peridotites, some reliable

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indicators of melt depletion might survive these modifications, and could be used to

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deduce the melting history.

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6.2. Degree of melting and ultra-refractory character of the Purang peridotites

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Spinel Cr# serves as a good qualitative indicator for the degree of melting experienced

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by plagioclase-free and vein-free peridotitic residues (Dick and Bullen, 1984; Arai, 1994). The good correlation between HREE concentrations in cpx and Cr# in

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associated spinels (Fig. 9a) indicates that this approach can be used quantitatively to estimate the degree of melting (Hellebrand et al., 2001). Overall, the Purang peridotites have spinels with TiO2 less than 0.15 wt %, which suggests limited influence of late-stage melt-impregnation or re-equilibration in the plagioclase stability field (e.g., Dick and Bullen, 1984; Seyler et al., 2003). Using the equation described by Hellebrand et al. (2001), the average degrees of pure fractional melting of the cpx-harzburgite-lherzolite group and the depleted harzburgite groups range from ~8% to ~16% and from ~16% to >20%, respectively. The Al2O3 contents of opx also can be used to infer the relative degree of melt

ACCEPTED MANUSCRIPT extraction (Bonatti and Michael, 1989; Simon et al., 2008). As shown in Fig. 6b, opx in the Purang peridotites shows positive co-variation between Al2O3 and Cr2O3

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contents, similar to those documented in modern oceanic peridotites (e.g., Parkinson

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and Pearce, 1998; Seyler et al., 2007; Dijkstra et al., 2010), This correlation has generally been interpreted as the results of reaction of Mg-Tschermak’s components (in opx) plus forsterite to spinel and enstatite with decreasing temperature

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(Witt-Eickschen and Seck, 1991). On the other hand, Cr# of opx, which shows no

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significant modification due to the coupled variation of Al2O3 and Cr2O3, should reliably record the degree of melt extraction prior to subsolidus re-equilibruim.

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Overall, the Purang peridotites display a good correlation between Cr# of opx and Cr#

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of spinels (Fig. 6d), which is consistent with their being residues after variable

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degrees of melt extraction. The main composition range of cpx-harzburgites and lherzolites is comparable to those from the most depleted endmember of the SWIR

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and CIR system (Hellebrand et al., 2002; Seyler et al., 2003), while the main compositional range of the depleted harzburgites is comparable to those from the most depleted endmember of the MAR system (Seyler et al., 2007; Suhr et al., 2008) and to peridotites from forearc settings (Ishii et al., 1992; Parkinson and Pearce, 1998). The depleted harzburgites are characterized by high Cr# in pyroxenes and spinel (>40-70), extremely low Al2O3 in opx (1.1-2.1 wt %) and low HREE contents in cpx. The Cr# of spinels in these samples extends beyond the compositional range found in peridotites from the fast-spreading East-Pacific Rise (Dick and Natland, 1996) and from the Fifteen-Twenty Fracture zone on the slow-spreading Mid-Atlantic Ridge

ACCEPTED MANUSCRIPT (Seyler et al., 2007), which represent the most refractory residua documented in modern oceanic ridges systems so far (Dick and Bullen, 1984; Dick and Natland,

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1996; Hellebrand et al., 2001; Suhr et al., 2008). There are two possible processes that

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could produce Cr# of spinels ≥60: (i) melting in a cratonic setting due to high mantle temperatures, or (ii) melting in a sub-arc setting due to fluxing of hydrous fluids (e.g., Parkinson and Pearce, 1998). The first possibility is less likely for these depleted

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harzburgites because they have overall lower Mg# in spinel at a given spinel Cr#, and

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higher whole-rock FeOT contents, compared with typical cratonic peridotites (e.g., East Greenland, Bernstein et al., 1998). The high SiO2 and high FeO contents in these

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peridotites might suggest reaction with a Fe2O3-rich hydrous fluid originating from

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the slab and mantle wedge (Herzberg, 2004). The lower Mg# in spinel at a given

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spinel Cr# and the low Al2O3 in opx (Fig. 6) are more similar to peridotites from modern arc-related settings (e.g., Ishii et al., 1992; Parkinson and Pearce, 1998). This

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supports previous arguments that the Purang massif has undergone subduction-related modification (e.g., Xu et al., 2008; Liu et al., 2010; Li et al., 2014; Su et al., 2015).

6.3. Garnet signature and Na enrichment in Purang peridotite cpx Cpx in some cpx-harzburgites and lherzolites show fractionated M-HREE pattern, i.e. low (M/HREE)N (<0.1) at relatively high (HREE)N concentrations (Fig. 8; 9b), similar to those documented in some peridotites from the SWIR (Seyler et al., 2011) and the CIR (Hellebrand et al., 2002). Such fractionation cannot be generated by fractional melting in the stability field of spinel peridotite, but requires initial melting of a garnet

ACCEPTED MANUSCRIPT peridotite followed by spinel-facies re-equilibration and subsequent melting of the spinel peridotite (Hellebrand et al., 2002). Fig. 9b models the results of such a

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polybaric, near-fractional melting process, in which melting trajectories for both cpx

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in equilibruim with residual garnet, and spinel-facies-projected cpx compositions are displayed. The projection simulates cessation of melting under garnet-facies conditions, followed by continuous transition from a garnet- to a spinel-peridotite

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assemblage. It presents the equilibruim cpx compositions after ascent into the spinel

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peridotite stability field. Compared with the modelling results, the Purang peridotites can be explained by 4-8% melting in the stability of garnet field peridotite, followed

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by a further 5-14% melting under spinel-facies conditions. This explanation is

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supported by the recent discovery of rare coarse-grained spinel-pyroxene symplectites

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in some of these cpx-harzburgites and lherzolites (Fig. 2f). These structures are similar to those observed in peridotite massifs from Horoman (e.g., Morishita and

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Arai, 2003), Alpine-Appenine systems (e.g., Piccardo et al., 2007) and peridotite xenoliths (e.g., Shimizu et al., 2008), and would appear to represent the decomposition products of mantle garnets under spinel-facies conditions (Gong et al., submitted). The Na contents of cpx from the Purang peridotites provide further constraints on the conditions of partial melting and post-melting metasomatism of these rocks. The incompatibility of Na is considered to be similar to that of Nd at low pressure, but at high pressures sodium becomes more compatible (Blundy et al., 1995) whereas Nd is more incompatible (Salters et al., 2002). Partial melting at high pressures in the

ACCEPTED MANUSCRIPT garnet peridotite stability field should quickly decrease the Nd/Na ratio in residual cpx, whereas this ratio remains approximately constant during melting in the spinel

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stability field. Compared with the modeling results by Hellebrand and Snow. (2003)

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and Müntener et al. (2010), some Purang peridotites (e.g., sample PL1202) show relatively high Na, but low Nd contents in cpx, consistent with high-pressure (garnet-facies) partial melting of Na-rich SCLM-cpx source (Fig. 10).

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A Na-rich cpx source is also suggested by a plot of Na2O in cpx vs. Cr# in

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associated spinel (Fig. 7b); some peridotites (both from this study and literature data) show variable degrees of Na enrichments at given Cr#, relative to expected residues

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of near-fractional spinel-field melting. The high Na/Ti in these cpx (Fig. 7c),

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reflecting both Na enrichment and Ti depletion, cannot be explained by refertilization

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with spatially associated N-MORB-like melts as it fails to account for the extremely low TiO2 contents of the cpx and associated spinel. However, it could be explained if

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the cpx from the protoliths were low in TiO2 relative to Na2O, like pyroxene in some peridotites from subcontinental mantle (e.g., Seyler and Bonatti, 1994). This fractionation of Na relatively to Ti in protoliths cpx could resulted either from partial melting at high pressure (e.g., Putirka, 1999) or from post-melting metasomatism. Similar enrichment of Na vs Ti in cpx has also been observed in some peridotites from modern (ultra) slow-spreading ridge system (e.g., Seyler et al., 2003; 2007; Hellebrand and Snow, 2003) as well as ocean-continent transition zones (OCT) (e.g., Müntener and Manatschal, 2006). In these instances, it has generally been explained as a signature inherited from earlier melting/metasomatism events probably related to

ACCEPTED MANUSCRIPT sub-continental lithospheric mantle that survived the low degree melting and melt

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6.4. Orthopyroxene enrichment in the Purang peridotites

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refertilization beneath present-day ridges.

One important petrological character of the IYZ peridotites is that they overall have high modal opx contents, up to 35 vol. % (opx-rich harzburgites of e.g., Wang et al.,

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1987). The Purang peridotites display a combination of variable but commonly high

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modal opx (20~35%) with very low modal cpx (5~9% in lherzolites, 2~5% in cpx-harzburgites; 1~3% in depleted harzburgites) (Fig. S3). The whole-rock

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major-element compositions essentially reflect these variations in the relative

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proportions of opx and ol. In a plot of MgO/SiO2 vs Al2O3/SiO2 (Fig. 3a), the high

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SiO2 contents in these peridotites result in low MgO/SiO2, compared with the ‘terrestrial array’ (Jagoutz et al., 1979; Hart and Zindler, 1986), which in fact is a

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trend of magmatic depletion (or enrichment) relative to a Primitive Mantle source. Similar low MgO/SiO2 ratios are commonly observed in abyssal peridotites, and generally have been ascribed to seafloor weathering (e.g., Snow and Dick, 1995). However, the high opx contents in the Purang peridotites are obvious both in the field as veinlets (Fig. S1) and in thin sections as clusters (Fig. 2e). Besids, most samples (both from literature and this study) are only weakly-moderately serpentinized and their low MgO/SiO2 must be ascribed to other factors. There are several possible processes that could explain the high modal opx in the Purang peridotites. The first is high-pressure partial melting in the garnet-peridotite facies. The

ACCEPTED MANUSCRIPT overall high proportion of opx in the Purang peridotites cannot be explained by any low pressure (spinel-facies) melting of Primitive or Depleted Mantle (Kinzler and

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Grove, 1992; Baker and Stopher, 1994). However, if partial melting occurred at ~20

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kbar, close to the transition between the garnet and spinel lherzolite stability fields, residues will be enriched in opx up to the point at which cpx and garnet are exhausted, where modal opx is 30~35 wt %; after this point opx will be progressively removed

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by continued melting (e.g., Kinzler, 1997; Walter, 1998). The preservation of rare

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coarse-grained spinel-pyroxene symplectites after garnet (Gong et al., submitted), and the garnet signature of cpx in some cpx-harzburgites and lherzolites documented

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above suggest that the Purang peridotites underwent initial melting in the

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garnet-peridotite facies. A similar mechanism has also been invoked to explain some

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least-reacted peridotites with similar high modal opx, garnet signatures in cpx and mineral chemistry, from the Eastern Central Alps (Müntener et al., 2010), the north

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Lanzo massif (Guarnieri et al., 2012) and in the 1274A MAR (Seyler et al., 2007). Thus, we argue that some of the observed high opx modes in the Purang peridotites could be ascribed to high-pressure melting in the garnet-peridotite stability field. Another process involves reaction between silica-rich fluids/melts and variably depleted peridotites. Two possible scenarios are indicated in the Purang peridotites: (i) reaction with silica-rich fluids/melts presumably derived from subducting slabs in a subduction zone. This process has been proposed to explain high opx contents observed in the mantle sections of SSZ-type ophiolites (e.g., Kelemen et al., 1992; Morishita et al., 2011) as well as in sub-arc xenoliths (e.g., Parkinson and Pearce,

ACCEPTED MANUSCRIPT 1998; McInnes et al., 2001; Arai et al., 2004; Ishimaru et al., 2007). Thus, the high opx modes in some depleted harzburgites, which show overall high whole-rock FeOT

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(Fig. 3c) and ultra-refractory mineral compositions (Fig. 5-7), might suggest reaction

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with silica-rich fluids/melts in a subduction zone. (ii) Reaction with derivative, silica-saturated melts in an ultra-slow spreading setting. Gabbro-norite cumulates, widely documented among the Alpine-Apennine ophiolites (e.g., Erro-Tobbio;

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Internal Ligurides and Monte Maggiore), have been generally interpreted as

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crystallized from derivative liquids that attained silica-saturation via melt-peridotite interaction, from primary silica-undersaturated single melt fractions that formed after

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fractional melting of spinel-facies DMM asthenospheric source (e.g., Piccardo et al.,

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2007; Piccardo and Guarnieri, 2011). The presence of gabbronorite dikes in the

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Purang massif (Miller et al., 2003; Liu et al., 2014) opens the possibility that high modal orthopyroxene in the Purang peridotites can partly be ascribed to reaction with

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such derivative, silica-saturated melts during their final exhumation in a (ultra-) slow-spreading setting. A third process is mechanical mixing of residual peridotites with pyroxene-rich veins (e.g., gabbronorites), which is suggested by a few peridotites with extremely high modal opx up to 40~45 %. High modal opx in these samples is consistent with the occurrence of pyroxenes as veinlets or as clusters. This process is best illustrated in sample PL1102G, which represents a harzburgite-orthopyroxenite mixed lithology with highest SiO2 (modal opx up to 54%). Mechanical mixing of peridotites and pyroxenites by ductile stretching, as suggested for Totalp massifs in eastern Swiss

ACCEPTED MANUSCRIPT Alps (e.g., van Acken et al., 2008), result in mixed lithologies with extremely high

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extremely “enriched” Os isotope signatures in these samples.

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modal opx, and “atypical” whole-rock compositions, but also would result in

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We conclude that high modal opx content in the Purang peridotites is the combined results of different mantle processes during their multi-stages evolution.

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6.5. Inherited melting/metasomatic signature: Possible origin as ancient SCLM

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The Purang peridotite massif has been generally accepted as relict lower Cretaceous Neo-Tethys oceanic lithosphere (e.g., Miller et al., 2003; Liu et al., 2014; Su et al.,

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2015). However, as shown above, they preserved several petrological features distinct

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from typical residual abyssal peridotites (i.e., oceanic lithosphere mantle). These

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include: (i) cpx with garnet-field signatures (low MREE to HREE ratios with relatively high YbN) and high modal opx in some least reacted peridotites, suggesting

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initial melting in the stability field of garnet-facies peridotite followed by further melting in the spinel peridotite field; (ii) Na enrichment in cpx (Na-rich, but relatively Nd-poor cpx) of some peridotites (e.g., PL1202) is consistent with high-pressure (garnet-field) partial melting of a Na-rich SCLM-cpx source. A high degree of melting of peridotite, starting in the garnet stability field, requires relatively high mantle potential temperature (Robinson and Wood, 1998). Johnson et al. (1990) suggested that the conditions of such a melting process for abyssal peridotites can be partly explained by the involvement of hotspot magmatism. However, this is inconsistent with the normal mantle temperature inferred from the

ACCEPTED MANUSCRIPT chemistry of the basalts and gabbros from the Purang massif, which have normal MORB compositions (e.g., Miller et al., 2003; Liu et al., 2015). A Na-rich SCLM

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source is also inconsistent with generally accepted DMM source for N-MORB melts

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and abyssal peridotites. These, together with the apparent contrast between overall high degrees of partial melting inferred from peridotites and very low degree of melt extraction deduced from the essential absence of a crustal sequence in the Purang

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massif, indicate that these peridotites are unlikely to be the complementary residue of

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the spatially associated MORBs. Similar situations have been documented in peridotite massifs from the Alpine-Appenine system such as the Eastern Central Alps

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(Müntener et al., 2010) and the Civrari massif (McCarthy and Müntener, 2015), as

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well as in modern oceanic ridge systems such as the SWIR (e.g., Salter and Dick,

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2002; Seyler, 2003) and Arctic Ocean (Hellebrand and Snow, 2003), in which these distinct features in peridotites are interpreted as inherited signatures from ancient

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SCLM.

An origin as ancient SCLM for the Purang peridotites is consistent with the age contrast between one peridotite cpx with an TRD Nd model age of 187 Ma (Miller et al., 2003) and well-constrained U-Pb zircon ages (130~120Ma) of the basaltic and gabbroic crust (Li et al., 2008; Liu et al., 2015). It is also consistent with recent Sr-Nd isotopic studies on several IYZ peridotites massifs by Wu et al. (2014), which show restricted Nd isotopic compositions in the gabbroic rocks, consistent with an origin from DMM source, but large variations of Nd isotopic compositions in the peridotites, suggestive of complex melt metasomatism, as commonly observed in SCLM.

ACCEPTED MANUSCRIPT The Purang peridotites give Re-depletion model ages ranging from 0.1 to 1.3 Ga (Table 2) with several well-defined age clusters of ~240Ma, ~500Ma and a broad span

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of ~700Ma-1.3Ga (Fig. 11a). Although the Os isotopic composition of any mantle

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peridotite may represent a mixture of several generations of PGMs and sulfides, such clusters have been interpreted as reflecting discrete melt-extraction events in the mantle (e.g., Rudnick and Walker, 2009). A recent revisit of the Alpine-Apennine

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ophiolites have shown that these peridotite massifs broadly record three melting

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“events”, which include: (1) Proterozoic events, representing the preexisting, heterogeneous SCLM affected by ancient (2-1 Ga) melting events; (2) Permian events

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(300-230 Ma), related to widespread Permian mafic magmatic activity in Western

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Europe, associated with post-Variscan extension and the reorganization of Pangea;

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and (3) Jurassic events, related to the formation of Jurassic MORBs and “rejuvenation” of isotopically and chemically heterogeneous SCLM during the opening of the

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Ligurian Tethys. These results provide compelling evidence that peridotites with depleted isotopic compositions (indicating ancient melting events) from (ultra)slow spreading environments are related to the exhumation of older and variably depleted rafts of subcontinental lithospheric mantle during continental breakup and rifting (McCarthy and Müntener, 2015). Here we invoke a similar interpretation, and argue that the Purang peridotites represent older and variably depleted domains of subcontinental lithospheric mantle, which may have been accreted to the SCLM and isolated from the convective upper mantle during conductive cooling, while the relatively younger ages (~240Ma) might represent melt extraction and/or melt

ACCEPTED MANUSCRIPT impregnation events related to the opening and evolution of the Neo-tethys ocean. We conclude that these distinct petrologic features together with the ancient

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depletion ages suggest that the Purang peridotites represent fragments of ancient

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metasomatised SCLM domains. These distinct characteristics in the Purang peridotites are unrelated to late-stage decompression melting and related

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refertilization, but are inherited from metasomatised SCLM.

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6.6. Evolution of the Purang peridotite: A possible recycling model? Previous studies have argued that the evolution of the Purang peridotites either

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involves a single-stage exhumation at a slow-spreading mid-ocean ridge to form an

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oceanic-core complex (OCC), followed by emplacement into the continental margin

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(e.g., Liu et al., 2014), or a two-stage process in which the Purang peridotite initially formed at a fossil (ultra-) slow-spreading mid-ocean ridge and subsequently was

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modified by melts/fluids at a supra-subduction zone setting, before incorporation into the continental margin (e.g., Xu et al., 2008; Liu et al., 2010; 2012; Li et al., 2015; Su et al., 2015). However, neither model provides an explanation for the presence of SuR-UHPs in the Purang massif (Yang et al., 2011; 2014) and both are inconsistent with some of the new petrological observations presented in this study. Recent work on mafic dikes intruding the Purang peridotites suggests formation in a rifted continental-margin setting (Liu et al., 2015) rather than in a slow-spreading oceanic ridge (e.g., Miller et al., 2003). This, together with the similarity of spatially associated volcanic-sedimentary cover sequences on the Purang massif to cover

ACCEPTED MANUSCRIPT sequences from ocean-continent transition zones (OCT) in rifted continental margins (e.g., Manatschal and Müntener, 2009), led Liu et al. (2015) infer that the Purang

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peridotites represent fragments of SCLM exhumed from beneath the northern margin

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of the Gondwana continent, similar to the well-established evolution model for OCT sequences from the Alpine-Apennine system (review by Manatschal and Müntener, 2009).

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The new petrological and geochemical evidence presented in this study provide

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strong arguments for an origin of the Purang peridotites as ancient SCLM, initially formed beneath a continental margin, as suggested by Liu et al. (2015). However,

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there are several obvious differences between the Purang sequence and the

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well-documented OCT sequences from Alpine-Apennine system (e.g., Lanzo, East

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Central Alps). These include: (1) the absence of pre-rift contacts between subcontinental mantle and continental crust and the association of top-basement

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detachment faults with continent-derived blocks (extensional allochthons) typically observed in these Alpine-Apennine ophiolitic massifs (e.g., Manatschal and Müntener, 2009). (2) Lack of pl-bearing peridotites in the Purang massif, while these are commonly observed in these Alpine-Apennine ophiolitic massifs and ascribed to pervasive shallow melt impregnation (e.g., Müntener et al., 2004; Rampone et al., 2008; Guarnieri et al., 2012). (3) The depleted harzburgites in the Purang massif are much more refractory than any known Alpine-Apennine ophiolitic massifs (e.g., Lanzo; Ligurides; Eastern Central Alps), and show geochemical affinities to forearc peridotites, suggesting subduction-related modification. (4) The discovery of

ACCEPTED MANUSCRIPT SuR-UHPs assemblages in the Purang peridotite massif (Yang et al., 2011; 2014) like those detailed documented in the Luobusa massifs to the eastern part of IYZ suture

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(e.g., Bai et al., 2001; Arai, 2013; Yang et al., 2014; Griffin et al., 2016), suggesting

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metamorphism in the deep mantle, probably in the Mantle Transition zone (e.g., Yang et al., 2014). The lack of pl-bearing peridotites could be ascribed to the occurrence of late-stage melt impregnation in the spinel-peridotite stability field, and the absence of

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spatially-associated continental derived extensional allochlthons could be explained

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by locating the Purang massif more oceanward relative to the ancient continental margin, like the Mt. Maggiore (Corsica, e.g., Rampone et al., 2008) and south Lanzo

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massifs (e.g., Piccardo et al., 2007) in the Alpine-Apennine system. However, the

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presence of some depleted harzburgites in the Purang peridotites with arc signatures,

evolution.

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and the discovery of SuR-UHPs in the Purang massif suggest a more complex

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The presence of depleted harzburgites in the Purang massif with arc signatures suggests that these peridotites had gone through additional depletion in an arc-related setting. However, in contrast with previous models in which the Purang peridotites underwent subduction-related modification after their initial formation in a mid-oceanic ridge (e.g., Liu et al., 2010; 2012; Su et al., 2015), we argue that these subduction-related modifications took place long before their exhumation and exposure as an OCC on the ocean floor. This is consistent with the ancient TRD ages recorded in these peridotites and field observation that there is no arc-related magmatism recorded in the crustal section of the Purang sequence. Other arc-related

ACCEPTED MANUSCRIPT signatures such as the crystallization of hydrous phases (e.g., pargasitic amphibole) also are scarce in the Purang peridotites (Liu et al., 2010).

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Recent evaluations of the existing models (e.g., Arai, 2013; Yang et al., 2014;

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Zhou et al., 2014; McGowan et al., 2015) have tried to reconcile the occurrence of SuR-UHPs mineral assemblages both in these IYZ massifs and associated chromitites, and concluded that the most likely explanation is that these massifs/chromitites have

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experienced deep recycling. They originally formed in a continental-margin

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suprasubduction-zone environment before being carried down into the deep mantle and brought up again to the shallow mantle by convection (e.g., Griffin et al., 2016).

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An origin for the Purang peridotites as ancient subduction-modified SCLM might

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further support the proposed deep recycling model for the IYZ peridotite massifs

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(Griffin et al., 2016). The possible evolution of the Purang massif can be outlined as follows: (i) The Purang peridotites represent variably depleted domains of SCLM

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beneath a continental margin, which have been affected by ancient (Proterozoic?) melting events; (ii) Subsequently, some of these peridotites might have been modified (with additional depletion and metasomatism) in an arc setting when a subduction zone was initiated along or adjacent to the continental margin; (iii) the metasomatised SCLM was removed and carried to the deep mantle with a subducting slab, where they gained their SuR-UHP signatures; (iv) these ancient SCLM domains were infiltrated and rejuvenated by N-MORB-like melts during their exhumation at a marginal basin. The best modern analogue is the case documented in Newfoundland margin.

ACCEPTED MANUSCRIPT Müntener and Manatschal. (2006) argued that the refractory spinel peridotites drilled at Site 1277 (ODP Leg 210) represent inherited (Caledonian or older) subarc mantle.

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It is possible that these refractory peridotites represent recycled ancient subcontinental

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lithospheric mantle derived from continental margins, that later was modified (additional depletion) and delaminated along with a subducting slab. In fact, recycling of ancient, highly refractory mantle domains with arc-related affinity has also been

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documented in peridotites xenoliths from various modern oceanic settings (e.g.,

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Simon et al., 2008). The presence of the distinctive SuR-UHP assemblages in the IYZ massifs simply suggests a much deeper recycling.

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The compositional similarity between most Purang peridotites (cpx-harzburgites

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and lherzolites) and abyssal peridotites, and their high equilibration temperatures (Liu

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et al., 2012) suggest that these ancient SCLM domains have been rejuvenated by late-stage percolating MORB melts. Similar modifications of the composition and

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thermal state of the mantle lithosphere by younger percolation melts, resulting in the rejuvenation of older mantle lithosphere domains, has long been recognized as an important mantle process during the evolution of mantle lithosphere (e.g., Müntener et al., 2004; Griffin et al., 2009; O’Reilly and Griffin, 2013; Piccardo et al., 2014).

7. Conclusions (1) The depleted harzburgites in the Purang ophiolite represent residues of high degrees (>20%) of melt extraction, which is not consistent with formation in a (ultra-) slow-spreading mid-ocean ridge. The extremely high Cr# in spinels and enrichment of

ACCEPTED MANUSCRIPT modal orthopyroxene in some peridotites suggest modification in a subduction zone. (2) The preservation of rare spinel-pyroxene symplectites after garnet, the garnet REE

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signature in clinopyroxene, and the high modal orthopyroxene in cpx-harzburgites and

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lherzolites suggest high-pressure melting in the garnet-stability field; Na enrichment in peridotitic clinopyroxene is consistent with derivation from a Na-rich SCLM-cpx protolith.

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(3) Whole-rock Os isotopic compositions of the Purang peridotites give ancient TRD

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model ages (up to 1.3Ga), indicating that they existed significantly earlier than the opening of the Neo-Tethys Ocean. They may represent ancient subduction-modified

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SCLM domains, probably initially formed beneath a continental margin.

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(4) Infiltration of late-stage N-MORB melts refertilized these ancient SCLM domains

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during their final emplacement, resulting in rare preservation of original signatures. (5) A deep recycling model is preferred to reconcile the presence of SuR-UHPs

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mineral assemblages.

Acknowledgements This work was funded by Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB03010203), the National Natural Science Foundation of China (41372063 and 41172059), and the ARC Centre of Excellence for Core to Crust Fluid Systems of Excellence for Core to Crust Fluid Systems. This is contribution XXX from the ARC Centre of Excellence for Core to Crust Fluid Systems (www.CCFS.mq.edu.au) and contribution XXX from the GEMOC National Key

ACCEPTED MANUSCRIPT Centre (www.GEMOC.mq.edu.au).

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Tibet. Earth and Planetary Science Letters 261, 33-48 Shi, R.D., Huang Q.S., Liu DL., Fan SQ., Zhang XR., Ding L., et al., 2012. Recycling of ancient sub-continental lithospheric mantle constraints on the genesis of the ophiolitic podiform chromitites. Geological Review 58, 643-652 (in Chinese with English abstract) Simon, N.S.C., Neumann, E.R., 2008. Ultra-refractory domains in the oceanic mantle lithosphere sampled as mantle xenoliths at ocean islands. Journal of Petrology 49: 1223-1251 Snow, J.E., Dick, H.J.B., 1995. Pervasive magnesium loss by marine weathering of

ACCEPTED MANUSCRIPT peridotite. Geochimica et Cosmochimica Acta 59, 4219-4235. Stern, R.J., 2004. Subduction initiation: Spontaneous and induced, Earth and

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Planetary Science Letters 226, 275-292.

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Stern, R.J., Reagan, M., Ishizuka, O., Ohara, Y., Whattam, S., 2012. To understand subduction initiation, study forearc crust: To understand forearc crust, study ophiolites. Lithosphere 4, 469-483

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Suhr, G., Kelemen, P., Paulick, H., 2008. Microstructures in Hole 1274A peridotites,

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ODP Leg 209, Mid-Atlantic Ridge: Tracking the fate of melts percolating in peridotites as the lithosphere is intercepted. Geochemistry, Geophysics,

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Geosystems 9(3), doi:10.1029/ 2007GC001726.

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Su, B.X., Teng, F.Z., Hu, Y., Shi, R.D., Zhou, M.F., Zhu, B., et al., 2015. Iron and

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magnesium isotope fractionation in oceanic lithosphere and sub-arc mantle: perspectives from ophiolites. Earth and Planetary Science Letters 430, 523-532.

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Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic systematics in ocean basalt: implications for mantle composition and processes. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publications 42, 313-345. van Acken, D., Becker, H., Walker, R.J., 2008. Refertilization of Jurassic oceanic peridotites from the Tethys Ocean - Implications for the Re-Os systematics of the upper mantle. Earth and Planetary Science Letters 268, 171-181. Wang, X.B., Bao, P.S., Xiao, X.C., 1987. Ophiolites of the Yarlung Zangbo (Tsangbo) river, Xizang (Tibet). Beijing: Publication House Survey Map. 1-118 (in Chinese

ACCEPTED MANUSCRIPT with English abstract).

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Depleted Lithosphere. Journal of Petrology 39, 29-60.

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Walker, M.J., 1998. Melting of Garnet Peridotite and the Origin of Komatiite and

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Walker, R.J., Horan, M.F., Morgan, J.W., Becker, H., Grossman, J.N., Rubin, A.E., 2002. Comparative 187Re–187Os systematics of chondrites: implications regarding early solar system processes. Geochimica et Cosmochimica Acta 66, 4187-4201.

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Whattam, S.A., and Stern, R.J., 2011, The ‘subduction-initiation rule’: A key for

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linking ophiolites, intra-oceanic forearcs and subduction initiation: Contributions to Mineralogy and Petrology 162, 1031-1045.

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Witt-Eickschen, G., Seck, H. A., 1991. Solubility of Ca and Al in orthopyroxene from

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spinel peridotites: an improved version of an empirical geothermometer.

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Contributions to Mineralogy and Petrology 106, 431-439. Workman, R.K., Hart, S.R., 2005. Major and trace element compositions of the

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depleted MORB mantle (DMM). Earth and Planetary Science Letters 231, 53-72. Wu, F.Y., Liu, C.Z., Zhang, L.L., Zhang, C., Wang, J.G., Ji, W.Q., Liu X.C., 2014. Yarlung Zangbo ophiolite: A critical updated view. Acta Petrologica Sinica 30, 293-325 (in Chinese with English abstract). Xu, D.M., Huang, G.C., Lei, Y.J., 2008. Geochemistry and tectonic significance of mantle peridotites form the Laangcuo ophiolite massif，southwest Tibet. Acta Petrologica Et Mineralogica 27, 1-13(in Chinese with English abstract). Yang, J.S., Dobrzhinetskaya, L., Bai, W.J., Fang, Q.-S., Robinson, P.T., Zhang, J.F., Green II, H.W., 2007. Diamond- and coesite-bearing chromitites from the

ACCEPTED MANUSCRIPT Luobusa ophiolite, Tibet. Geology 35, 875-878. Yang, J.S., Xu, X.Z., Li, Y., J.Y., L., Ba, D.Z., Rong, H., et al., 2011. Diamonds

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recovered from peridotite of the Purang ophiolite in the Yarlung-Zangbo suture

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of Tibet: A proposal for a new type of diamond occurrence. Acta Petrologica Sinica 27, 3171-3178 (in Chinese with English abstract).

Yang, J.S., Robinson, P.T., Dilek, Y., 2014. Diamonds in ophiolites. Elements 10,

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127-130.

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Yin, A., Harrison, T.M., 2000. Geological evolution of Tibetan-Himalayan orogeny. Annual Review of Earth and Planetary Sciences 28, 211-280.

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Zhou, M.F., Robinson, P.T., Malpas, J., Li, Z.J., 1996. Podiform chromitites in the

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Luobusa ophiolite (southern Tibet): implications for melt–rock interaction and

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chromite segregation in the upper mantle. Journal of Petrology 37, 3-21 Zhou, M.F., Robinson, P.T., Su, B.X., Gao, J.F., Li, J.W., Yang, J.S., Malpas, J., 2014.

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Compositions of chromite, associated minerals, and parental magmas of podiform chromite deposits: the role of slab contamination of asthenospheric melts in SSZ environments. Gondwana Research 26, 262-283. Zhou, W.D., Yang, J.S., Zhao, J.H., Xiong, F.H., Ma, C.Q., Xu, X.Z., Liang, F.H., Liu, F., 2014. Mineralogical study and the origin discussion of Purang ophiolite peridotites, western part of Yarlung-Zangbo Suture Zone (YZSZ), Southern Tibet. Acta Petrologica Sinica 30, 2185-2203 (in Chinese with English Abstract).

Figure captions Fig. 1 Geological sketch maps of the Purang peridotite massif (modified after Liu et al.

ACCEPTED MANUSCRIPT 2014). Insert shows major tectonic units of the Tibetan Plateau. IYZ: Indus-Yarlung Zangbo suture; BNS: Bangong-Nujiang suture; JS: Jinshajiang suture. Yellow

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diamonds are localities of diamondiferous peridotites (after Yang et al., 2014). White

(2014); Li et al. (2015) and Su et al. (2015).

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stars are sample locations reported from literature: Liu et al. (2010; 2014); Zhou et al.

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Fig. 2 Photomicrographs showing representative microstructures and mineral

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associations in the Purang peridotites. (a) Coarse-grained olivine with kink bands. (b) Coarse-grained opx with kink bands, exsolution of cpx lamellae, embayment of

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olivine, crystallization of cpx along the fractures and boundaries. (c) Elongation of

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exsolved coarse-grained opx with a length: width of 3:1. (d) Recrystallized

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fine-grained cpx +opx ±ol along the boundary of coarse cpx. (e) Patches or clusters of opx ±cpx grains within olivine matrix. (f) Coarse-grained spinel-pyroxene symplectite

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within olivine matrix.

Fig. 3 Whole-rock variations of MgO/SiO2 vs Al2O3/SiO2 (a), MgO vs Al2O3 (b), MgO vs FeOT (c), and Al2O3 vs CaO (d) in the Purang peridotites. Literature data for the Purang peridotites are from Miller et al. (2003); Liu et al. (2012; 2014), Zhou et al. 2014; Li et al. (2015) and Su et al., (2015). All compositions are recalculated on a volatile free basis. The thick grey line on Fig. 3a represents the bulk silicate earth evolution (“terrestrial array” after Jagoutz et al. (1979) and Hart and Zindler (1986)). Melting trends in Fig. 3b are modeled by polybaric near-fractional (1% melt porosity)

ACCEPTED MANUSCRIPT melting of a fertile mantle source with initial depths of Po =25 kbar, and of isobaric batch melting at P =20 kbar (cf. Niu, 1997). On Fig. 3c, dashed grey lines are contours

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of Mg# (=100×Mg/(Mg+Fe)) ranging from 87 to 93. Estimated compositions of

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Primitive Mantle (PM, McDonough and Sun, 1995) and Depleted MORB Mantle (DMM, Workman and Hart, 2005) are shown for comparison, as are the ranges for abyssal peridotites: SWIR: Southwest Indian oceanic ridge (Seyler et al., 2003; Niu,

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2004); CIR: Central Indian oceanic ridge (Niu, 2004); MAR: ODP Sites 1274,

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Mid-Atlantic oceanic ridge (Harvey et al., 2006; Paulick et al., 2006; Godard et al., 2008); IBM: Izu-Bonin-Mariana forearc peridotites (Parkinson and Pearce, 1998; Ishii

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et al., 1992).

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Fig. 4 Primitive mantle-normalized rare earth element patterns (a) and extended trace element patterns (b) for the Purang peridotites. Fractional melting models in Fig. 4a

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are from Niu. (2004). Primitive mantle-normalized values are from Sun and McDonough. (1989).

Fig. 5 (a) Mg# (= 100*Mg/(Mg+Fe)) vs NiO of olivine; (b) Mg# of olivine vs Cr# (=100*Cr/(Cr+Al)) of associated spinel; (c) Mg# vs Cr#, and (d) TiO2 vs Cr# of spinel of the Purang peridotites. Also shown for comparison are the ranges for abyssal peridotites: SWIR: Southwest Indian oceanic ridge (Seyler et al., 2003); CIR: Central Indian oceanic ridge (Hellebrand et al., 2002); MAR: ODP Sites 1274, Mid-Atlantic oceanic ridge (Seyler et al., 2007; Suhr et al., 2008); IBM: Izu-Bonin-Mariana forearc

ACCEPTED MANUSCRIPT peridotites (Parkinson and Pearce, 1998; Ishii et al., 1992). Data for Tonga-IAT (island-arc tholeiite), IBM-IAT and boninite, and abyssal basalt (MORB) shown in

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Fig. 5d are from Peace et al. (2000).

Fig. 6 (a) Mg# vs Al2O3, (b) Cr2O3 vs Al2O3, and (c) Mg# vs Cr# in opx, (d) Cr# of

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opx vs Cr# of associated spinel of the Purang peridotites. Symbols are as in Fig. 5.

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Fig. 7 (a) Na2O vs TiO2 of cpx, (b) Na2O vs Cr# of associated spinel, and (c) Na/Ti (apfu) vs Cr# of cpx, (d) Cr# of cpx vs Cr# of associated spinel in the Purang

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peridotites. Symbols are as in Fig. 5.

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Fig. 8 Chondrite-normalized REE concentrations of clinopyroxene in the Purang peridotites. Bars represent 1 standard deviation of averaged concentration of cpx cores

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from each sample. Also shown for comparison are the ranges for abyssal peridotites after Johnson et al. (1990), Hellebrand et al. (2002) and Brunelli et al. (2006). Chondrite-normalized values are from Anders and Grevesse (1989).

Fig. 9 (a) Chondrites-normalized Yb contents of cpx vs Cr# of associated spinel. (b) Chondrites-normalized Sm/Yb vs Yb contents of cpx. In Fig. 9a, the ranges for abyssal peridotites are shown for comparison: MAR: Vema Fracture Zone (VFZ) on the Mid-Atlantic ridge, Brunelli et al. (2006); EPR: Garrett and Hess Deep, East Pacific Rise, Dick and Natland. (1996). Other symbols are as in Fig. 5. Also shown in

ACCEPTED MANUSCRIPT Fig.9b are fractional melting models of spinel- and garnet-peridotite sources, as well as garnet-facies melting followed by spinel-facies melting (after Hellebrand and Snow,

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2003). Chondrite-normalized values are from Anders and Grevesse (1989).

Fig. 10 Na2O contents vs chondrite-normalized Nd concentrations of cpx in the Purang peridotites. Also shown are modelled melting results after Hellebrand and

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Snow. (2003) and Müntener et al. (2010). (1) Fractional melting of depleted mantle

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source (Dcpx/l Na =0.2); (2) critical melting (1% residual melt porosity) of a Na-rich subcontinental mantle source with Dcpx/l Na =0.2; (3) fractional melting of a

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spinel-peridotite facies Na-rich subcontinental mantle source with Dcpx/l Na =0.3; (4)

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fractional melting of a garnet-peridotite facies Na-rich subcontinental mantle source

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with Dcpx/l Na of 0.5; (5) polybaric melting model with 4% melting of a garnet-peridotite (Dcpx/l Na =0.5) followed by additional melting in the spinel

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peridotite field (Dcpx/l Na =0.3).

Fig. 11 Whole-rock

187

Os/188Os vs

187

Re/188Os (a), and Cr# of spinel (b) from the

Purang peridotites. Inset in Fig. 11a is a histogram of rhenium-depletion model ages (TRD) for whole-rock samples from the Purang and Dongbo massif, calculated following Shirey and Walker. (1998); model ages are based on the enstatite chondrite reservoir (ECR; Walker et al., 2002). Literature data are from Purang (Liu et al., 2012) and the nearby Dongbo massif (Niu et al., 2015).

ACCEPTED MANUSCRIPT Tables Table 1 Trace-element concentrations of clinopyroxenes in the Purang peridotites.

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Table 2 Whole-rock Re–Os isotopic compositions of the Purang peridotites. Model

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ages are based on the enstatite chondrite reservoir (ECR; Walker et al., 2002)

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CE P

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D

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Table 1. Whole-rock Re-Os isotopic compositions of the Purang peridotites. 187

Re/188

Re

Os

(ppb)

(ppb)

Os

PL1104B

0.29

3.18

0.437

PL1104F

0.37

3.05

0.581

PL1202-2

0.11

4.98

0.106

PL1210-1

0.38

5.09

0.356

PL1102A

0.19

3.74

0.241

PL1102G

0.38

4.46

PL1102H

0.29

3.48

PL1103A

0.21

3.66

0.270

PL1104A

0.33

4.28

0.367

0.20

3.21

0.296

0.10

3.59

0.129

PL1104E

0.22

3.06

0.345

PL1204-9

0.46

2.38

0.940

PL1102E

0.32

3.73

0.410

PL1208-1

0.00

7.50

0.001

PL1209-5

0.48

5.50

0.422

Sample

187

Os/188

2SE

Os

2SE

TRD

1σ

(130Ma)

(Ga)

AC

PL1104D

0.00 3

0.00 9

0.398

29

IP

0

0.000

0.12768

0.000

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0.02

0.12813

0.13023 0.12797

23

0.000 27

0.000 25

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0.410

D

TE

CE P

PL1104C

6

0.000

0.02

0.000

0.01

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Cpx-Harzburgite

0.01

T

Lherzolite

2 1

0.01 3 0.00 9 0.02 9 0.01 1 0.00 9 0.00 9 0.01 7

0.12698 0.14099 0.12674 0.12613 0.12530 0.12917 0.12181 0.12521 0.13203

24 20 0.000 29 0.000 26 0.000 22 0.000 23 0.000 25 0.000 25 0.000 24

0.13

0.02

0.24

0.02

-

-

0.13

0.02

0.23

0.02

-

-

0.32

0.02

0.36

0.02

0.51

0.02

-

-

0.93

0.02

0.52

0.02

-

-

0.07

0.02

1.30

0.01

-

-

Depleted Harzburgite 0.01 4 0.00 6 0.01 7

0.12853 0.11886 0.13203

0.000 29 0.000 18 0.000 39

TRD, Re-depleted model ages calculated following Shirey & Walker (1998); Model ages are based on the enstatite chondrite reservoir (ECR; Walker et al., 2002).

ACCEPTED MANUSCRIPT Table 2. Trace-element compositions of clinopyroxene in the Purang peridotites. Sam ple

PL1104B

PL1202-2

PL1210-1

PL1211-3

Occu Pc

Pr

Pc

Pr

Pc

Pr

Pc

T

rrenc e

6

1σ

4

1σ

8

1σ

3

1σ

6

1σ

59

2

65

4

56

2

58

3

54

2

(ppm )

La

Ce

Pr

Nd

Sm

Eu

Gd

13

4

8

5.

0.

95

39

0

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0.

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05

9

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0.

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04

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3

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4

4

0.

9.

0.

9.

31

87

36

91

15

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0

2

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11

40

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24

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Zr

0.

25

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22

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Sr

25

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1σ

56

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0.0 66

5.0 20 0.0 32

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0.

0.

0.

00

01

00

00

00

5

3

7

9

6

0.

00 5

6

4

0

3

0.

0.

0.

0.

00

10

01

10

01

7

5

9

4

1

0.

0.0

9

73 0.1 98

0.6 06

0.1 04

0.6 61 0.0 97 0.0 11

N: number of analysis; Pc: porphyroclast core; Pr: porphyroclast rim.

AC

IP

4

0.

9

SC R

5

4

T

Yb

00

8

NU

Tm

09

0.1

5

MA

Er

0.

7

D

Ho

0.

7

TE

Dy

1

CE P

Tb

9

0.0

0.0

42

01

0.4

0.0

59

26

0.1

0.0

14

26

0.4

0.0

40

68

0.0

0.0

66

04

0.4

0.0

92

73

0.0

0.0

87

16

0.0

0.0

05

03

ACCEPTED MANUSCRIPT Highlights 1) The Purang peridotites are not the complementary mantle residues of spatially

IP

T

associated oceanic crust.

SC R

2) The Purang peridotites may represent ancient subduction-modified SCLM domains, probably initially formed beneath a continental margin.

3) The Purang peridotites have been thermally and chemically modified by late-stage

NU

percolating N-MORB melts.

MA

4) A deep mantle recycling model is preferred to reconcile the presence of SuR-UHP

AC

CE P

TE

D

mineral assemblages.