Fabrics and water contents of peridotites in the NeotethyanLuobusa ophiolite, southern Tibet: implications for mantlerecycling in supra-subduction zones

Meng Yu1, Qin Wang1,* & Jingsui Yang21 State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University,Nanjing 210046, China

2 Institute of Geology, Chinese Academy of Geological Sciences, Beijing, ChinaMY, 0000-0001-7769-6075; QW, 0000-0001-7236-5525

*Correspondence: qwang@nju.edu.cn

Abstract: The preservation of ultra-high-pressure and super-reducing phases in the Neotethyan Luobusa ophiolite in Tibetsuggests their deep origin near themantle transition zone. Dunite and harzburgite core samples from the Luobusa ScientificDrillingProject show supra-subduction zone geochemical signatures and equilibration temperatures of c. 950–1080°C. Olivine shows A-,B-, C- and E-type fabrics, and combinations of A- and E-type or B- and E-type fabrics. Transmission electron microscopyobservations show straight dislocations and the activation of multiple slip systems [100](010), [001](010), [001](100) and [100](001) in olivine. The mean water content in olivine, orthopyroxene (Opx) and clinopyroxene (Cpx) from 24 peridotite samples was16 ± 5, 90 ± 21 and 492 ± 64 ppm, respectively, which is different from the water content of hydrated peridotites above the mantlewedge. The trace element compositions of Cpx exclude significant metasomatism after melt extraction. The high hydrogen partitioncoefficient between Cpx and Opx (DH

Cpx/Opx = 5.56 ± 0.96) implies equilibrium at high pressures and rapid exhumation. Basedon deformation experiments, the B- and C-type fabrics could be formed in a subduction zone at depths >200 km, whereas theA- and E-type fabrics were produced in the shallow mantle. In a process triggered by slab rollback, the Luobusa peridotites mayhave been rapidly exhumed within a subduction channel and mixed with the lithospheric mantle of the forearc.

Supplementary material:Major oxide contents in Opx, Cpx and spinel, trace element concentration in Cpx, micrographs andTEM images of peridotite are available at https://doi.org/10.6084/m9.figshare.c.4307828

Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Received 6 August 2018; revised 15 November 2018; accepted 16 November 2018

The fabrics of ophiolitic peridotites record thermodynamic processesfrom the mid-ocean ridge to the subduction zone, followed by theirfinal emplacement at a convergent boundary (Dilek & Robinson2003 and references cited therein). The lattice-preferred orientations(LPOs) of olivine and orthopyroxene (Opx) from both a fast-spreading ridge, such as the Oman ophiolite (Boudier & Nicolas1995), and a slow-spreading ridge, such as the Lanzo massif in theAlps (Boudier 1978), indicate that dislocation creep is the dominantdeformation mechanism in the oceanic lithosphere. The LPO ofolivine in naturally deformed peridotites is generally characterizedby the [100] axis parallel to the lineation and the (010) plane parallelto the foliation, i.e. the A-type fabric (Ben Ismaıl̈ &Mainprice 1998;Tommasi & Vauchez 2015; Michibayashi et al. 2016; Tommasiet al. 2016), which can be attributed to the dominant activation of theslip system [100](010) at high temperatures and low strain ratesunder upper mantle conditions (e.g. Carter & Avé Lallemant 1970;Zhang et al. 2000; Hirth & Kohlstedt 2003). Because olivine is themost abundant mineral in the upper mantle, the A-type olivine fabricis used to infer the direction of mantle flow from seismic anisotropy(e.g. Blackman & Kendall 2002; Park & Levin 2002 ).

Deformation experiments investigating the effects of water, stress,pressure and temperature have found a number of different fabrictypes in olivine (e.g. Carter & Avé Lallemant 1970; Jung & Karato2001; Jung et al. 2006;Ohuchi et al. 2011;Ohuchi& Irifune 2013). Inaddition to the A-type fabric, B- and E-type olivine fabrics, producedby the predominance of the [001](010) and [100](001) slip systems,respectively, have been observed in peridotites from supra-subductionzones (SSZ) – for example, the B-type fabric was seen in peridotites

from the Sanbagawa metamorphic belt of SW Japan (Mizukami et al.2004; Tasaka et al. 2008), the E-type fabric was seen in peridotitesfrom the accreted Talkeetna arc in south-central Alaska (Mehl et al.2003) and theHidakametamorphic belt in northern Japan (Sawaguchi2004). It is still not clear whether the trench-parallel seismicanisotropy observed in forearc regions is caused by the B-typeolivine fabric in the water-rich mantle wedge or the A-type olivinefabric resulting from trench-parallel flow (Long & Becker 2010).

Based on the water-induced fabric transition in olivine indeformation experiments (Jung & Karato 2001), the C-type fabric ingarnet peridotites from ultra-high-pressure (UHP) metamorphicterranes is often attributed to ductile deformation in the hydrousdeep mantle, e.g. the Cima Di Gagnone of the Central Alps (Freseet al. 2003; Skemer et al. 2006), the Norwegian Caledonides(Katayama et al. 2005) and the North Qaidam UHP belt (Jung et al.2013). However, Xu et al. (2006) reported that the C-type fabricfrom the Zhimafang garnet peridotites in the Sulu UHP terrane wasformed under 4–7 GPa, 750–950°C and water-poor conditions in acontinental subduction zone. This interpretationwas confirmed by theC-type fabric in water-poor olivine from the Xugou peridotites in theSuluUHP terrane (Wang et al. 2013b). In addition, water-poor olivinefrom the Western Gneiss Region (Norway) has developed the B-typefabric in strongly sheared peridotites and the C-type fabric in garnetperidotites that had undergone UHP metamorphism at P > 6 GPa and850–950°C, implying a water-independent fabric transition in a coldand dry continental subduction zone (Wang et al. 2013a).

The Luobusa ophiolite, located in the eastern part of the Yarlung–Zangbo Suture Zone (YZSZ) in southern Tibet, represents a

© 2019 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

Research article Journal of the Geological Society

Published online January 10, 2019 https://doi.org/10.1144/jgs2018-152 | Vol. 176 | 2019 | pp. 975–991

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remnant of the Neotethyan oceanic lithosphere in an SSZenvironment (Zhou et al. 1996, 2005, 2014; Xu et al. 2015a;Dilek & Yang 2018). Different models, such as a mantle plume(Yang et al. 2014; Xiong et al. 2015), channelized mantle upwellingfrom the transition zone (Griffin et al. 2016) or slab rollback-induced channel flow from the transition zone (Dilek & Yang 2018)have been proposed to interpret the occurrence of UHP and super-reducing phases (e.g. in situ diamond, moissanite and native Fe) inthe Luobusa peridotites and chromitites. Only the A-type olivinefabric has so far been observed in the Luobusa peridotites at outcropand without measurements of the water content (Xu & Jin 2010;Sun et al. 2016), which casts doubt on the implications of theperidotite microstructure for the interpretation of deep mantleprocesses.

Here we present an integrated study of harzburgite and dunitesamples from two boreholes of the Luobusa Scientific DrillingProject. The A-, B-, C- and E-type olivine fabrics, as well asdifferent slip systems in olivine, were recognized in the coresamples. Combined with the chemical composition and watercontent of olivine, Opx and clinopyroxene (Cpx), we propose thatthe Luobusha ophiolite presents a mixture of peridotites fromdifferent depths in a subduction channel. Channel flow from thetransition zone to shallow depths induced by slab rollback providesan important pathway for the incorporation of subducted materialinto the oceanic lithosphere and a connection with plate tectonicprocesses.

Geological setting

As the tectonic boundary between the Eurasian and Indian plates,the nearly east–west-striking YZSZ extends for >2000 km andseparates the Lhasa Terrane in the north from the Himalayan Orogenin the south (Fig. 1a). The onset of India–Asia continental collisionhas been constrained to 59 ± 1 Ma by stratigraphic dating (Hu et al.2016). The Gangdese arc along the southern margin of the LhasaTerrane is an Andean-type continental margin formed by northwardsubduction of the Neotethyan oceanic lithosphere from the LateTriassic to the Paleocene and has been overprinted by post-collisional magmatism in the thickened crust. The Gangdesebatholith is mainly composed of gabbros, diorites, granodiorites,granites and leucogranites, with four discrete stages of magmatismat 205–152, 109–80, 65–41 and 33–13 Ma (e.g. Mo et al. 2007;Wen et al. 2008; Chung et al. 2009; Ji et al. 2009; Zhu et al. 2011;Zhang et al. 2014).

The ophiolite massifs and ophiolitic mélanges along the YZSZpreserve two age peaks of generation: 180–150 and 130–110 Ma(Hébert et al. 2012; Xu et al. 2015b). All the ophiolites showgeochemical features of an SSZ environment, particularly arc andback-arc settings (Hébert et al. 2012). The Luobusa ophiolite in theeastern YZSZ is the largest chromite mine in China and extends c.42 km along-strike and covers an area of c. 70 km2. It mainlyconsists of harzburgites, dunites, sparse podiform chromitites andmafic cumulates (Zhou et al. 1996; Xiong et al. 2015). The Luobusaophiolite was formed at a mid-ocean ridge in the Mid-Jurassic, asconstrained by a whole-rock Sm–Nd isochron age of 177 ± 31 Mafor gabbro dykes (Zhou et al. 2002) and a sensitive high-resolutionion microprobe U–Pb age of 163 ± 3 Ma for mid-ocean ridge basalt(MORB)-type diabase (Zhong et al. 2006). It was then re-fertilizedby boninitic melts in a mantlewedge above the subduction zone at c.126 Ma (Malpas et al. 2003; Zhou et al. 2005, 2014).

Bounded by two south-dipping thrust faults, the Luobusaophiolite was thrust over the upper Oligocene–lower Mioceneconglomerate of the Luobusa Formation to the north and isunderlain by Upper Triassic flysch to the south (Fig. 1b)(Yamamoto et al. 2007; Liang et al 2011; Xu et al. 2015a).Although all the ultramafic rocks in the Luobusa ophiolite appear to

have equilibrated at relatively shallow depths as spinel- orplagioclase-bearing peridotites (Hébert et al. 2003; Xiong et al.2015), the occurrence of UHP (diamond, TiO2 II, coesite, stishovitepseudomorph, phase BMJ) and super-reducing phases (nativeelements, alloys, carbides, nitrides and moissanite) in the Luobusaperidotites and chromitites shows that they originated from the deepupper mantle or the mantle transition zone under conditions of verylow oxygen fugacity (Bai et al. 1993; Robinson et al. 2004; Yanget al. 2007, 2014; Dobrzhinetskaya et al. 2009; Yamamoto et al.2009; Xu et al. 2015b; Griffin et al. 2016; R.Y. Zhang et al. 2016).The abundant clinoenstatite lamellae in enstatite from lherzolite andharzburgite core samples of the Luobusa Scientific Drilling Projectsuggest that the Luobusa peridotites originated at P > 7 GPa (R.Y.Zhang et al. 2016). In addition, crustal minerals (e.g. zircon, quartz,corundum, K-feldspar, plagioclase, apatite and amphibole) havebeen found as mineral inclusions in the Luobusa peridotites andpodiform chromitites (Robinson et al. 2015). These findings haveinitiated a heated debate about the formation mechanisms of theLuobusa ophiolite, as well as the recycling of the subductedcontinental and oceanic materials.

Sample description

To constrain the petrological and structural variations of theLuobusa ophiolite, the Luobusa Scientific Drilling Project drilledtwo boreholes (29° 13′ 24.19″ N, 92° 11′ 36.33″ E) at an altitude of4378 m: the 1478.8 m deep LBSD-1 borehole and the 1853.8 mdeep LBSD-2 borehole (Fig. 1b). Serpentinization is common in theLuobusa peridotites and some of the core samples are stronglyserpentinized and fragile, which makes it difficult to identify thelineation and foliation of the peridotites.

We selected 17 relatively fresh harzburgite and seven relativelyfresh dunite samples from the LBSD-1 and LBSD-2 boreholes(Fig. 1c; Table 1). Most of the samples show a degree ofserpentinization <5%, although samples B186, B199, B281,B329, B137, B155, B226, B368 and B536 have a degree ofserpentinization of 5–10%. The serpentine minerals are lizarditeand/or chrysotile aggregates and appear along grain boundaries orfractures. Theses samples are coarse grained and show granoblasticor porphyroclastic textures.

The harzburgite samples contain olivine (65–85 vol.%), Opx (10–28 vol.%) and Cpx (1–5 vol.%), with minor spinel and serpentine(Fig. S1). Cpx occurs as either small interstitial grains or exsolutionlamellae in Opx grains. The dunite samples consist of 85–95 vol.%olivine, minor Opx, and interstitial Cpx and spinel. The grain size ofolivine is usually 1–3 mm, although it sometimes reaches 8 mm.Both olivine and Opx show plastic deformation features, such asirregular grain boundaries (Fig. 2a), kink bands in olivine (Fig. 2b),the undulose extinction of coarse-grained olivine (Fig. 2c–d) and thedynamic recrystallization of fine-grained olivine (0.1–0.5 mm grainsize) (Fig. 2e–f ). We also observed abundant exsolution lamellae ofCpx in Opx grains (Fig. 2g–h), which has previously been reportedin harzburgite core samples from the LBSD-1 borehole andattributed to the transformation of clinoenstatite from high-temperature Ca-bearing orthoenstatite at pressures >7 GPa (Zhanget al. 2017). Some harzburgite samples show coarsely vermicularsymplectites of Opx + Cr–Al spinel ± Cpx (Fig. 3), which have beeninterpreted as the breakdown products of majoritic garnet, withestimated minimum pressures >13 GPa (Griffin et al. 2016).

Methods

Mineral composition analyses

The major element compositions of olivine, Opx, Cpx and spinelfrom 23 peridotite samples were determined using a Shimadzu

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EPMA1600 electron microprobe at the Chinese Academy ofSciences Key Laboratory of Crust–Mantle Materials andEnvironments at the University of Science and Technology ofChina and a JEOL JXA-8100 electron microprobe at the State KeyLaboratory for Mineral Deposits Research at Nanjing University.

The analytical conditions were a 20 nA beam current, a 15 kVaccelerating voltage and a 1 µm spot diameter. For each sample, themajor element concentrations in each mineral were the mean valueof five to eight grains. The trace element concentrations in Cpx from11 harzburgite samples was acquired using an Agilent 7700× laser

Fig. 1. (a) Tectonic sketch of the Tibetan Plateau. (b) Simplified geological map of the Luobusa ophiolite in southern Tibet. (c) Lithological profile of theLBSD-1 and LBSD-2 boreholes and location of samples (modified from Xu et al. 2015a). EBSD, electron back-scattered diffraction.

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Table 1. Major oxide concentrations in olivine from peridotite core samples in the Luobusa ophiolite

Sample Lithology Depth (m) SiO2 TiO2 Al2O3 FeOT MnO MgO CaO NiO Cr2O3 Total Mg#

LBSD-1 boreholeB98 Hz 164 41.10 0.02 0.01 9.38 0.10 48.38 0.01 0.34 0.01 99.34 90.2B106 Hz 188 41.12 0.00 0.01 9.20 0.11 48.73 0.01 0.34 0.02 99.53 90.4B143 Hz 284 41.41 0.01 0.00 9.06 0.10 48.70 0.02 0.37 0.02 99.69 90.6B153 Hz 303 41.09 0.01 0.00 9.48 0.09 48.52 0.01 0.32 0.01 99.53 90.1B169 Hz 342 41.04 0.01 0.00 9.17 0.11 48.82 0.01 0.40 0.01 99.56 90.5B180 Hz 367 41.49 0.01 0.01 9.84 0.15 48.17 0.01 0.28 0.00 99.98 89.7B186 Hz 379 41.36 0.01 0.00 9.46 0.10 48.47 0.01 0.41 0.00 99.82 90.1B199 Hz 407 41.06 0.01 0.00 9.56 0.14 48.49 0.02 0.36 0.02 99.65 90.0B245 Hz 524 41.22 0.01 0.00 9.16 0.11 48.87 0.02 0.40 0.01 99.79 90.5B256 Dun 558 41.43 0.01 0.00 8.70 0.10 48.88 0.02 0.33 0.00 99.45 90.9B281 Hz 627 41.30 0.01 0.00 9.87 0.13 48.47 0.02 0.42 0.01 100.23 89.7B300 Dun 682 41.33 0.01 0.00 7.29 0.09 50.15 0.07 0.35 0.01 99.31 92.5B329 Hz 790 41.03 0.02 0.01 9.84 0.11 46.31 0.02 0.36 0.20 99.49 89.7B520 Dun 1370 40.48 0.00 0.00 11.61 0.16 46.31 0.22 0.14 0.01 98.93 87.7

LBSD-2 boreholeB120 Hz 316 41.05 0.01 0.01 9.61 0.11 48.72 0.02 0.36 0.01 99.88 90.0B137 Hz 354 41.66 0.01 0.00 9.67 0.15 48.27 0.00 0.26 0.01 100.06 89.9B155 Dun 398 40.94 0.00 0.01 9.29 0.11 49.14 0.01 0.33 0.01 99.83 90.4B196 Hz 504 41.26 0.01 0.00 9.81 0.10 48.57 0.02 0.39 0.01 100.16 89.8B226 Hz 572 41.21 0.01 0.01 9.55 0.13 48.32 0.02 0.35 0.01 99.62 90.0B269 Dun 673 40.85 0.00 0.01 8.07 0.09 49.88 0.02 0.35 0.00 99.27 91.7B368 Hz 897 41.39 0.01 0.00 9.74 0.13 48.13 0.02 0.39 0.01 99.81 89.8B512 Dun 1375 41.10 0.01 0.00 9.23 0.11 48.85 0.13 0.25 0.02 99.70 90.4B536 Dun 1450 41.01 0.01 0.01 9.42 0.15 48.56 0.19 0.25 0.01 99.61 90.2

Dun, dunite; Hz, harzburgite; FeOT, total Fe in FeO.

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ablation inductively coupled plasma mass spectrometer with anenergy density of 10 J cm−2, a spot diameter of 44 µm, a frequencyof 4 Hz and ablation time of 40 s. For each sample, four to ten Cpxgrains were analysed to give a mean value.

Microstructural analyses

The LPOs of olivine and Opx from nine harzburgite samples weremeasured using the electron back-scattered diffraction (EBSD)

technique. We used a JEOL JSM-6490 scanning electronmicroscope equipped with an Oxford Nordlys-S EBSD detectorand Channel 5+ software at the State Key Laboratory for MineralDeposits Research. The highly polished thin sections were tiltedby 70° and we used an accelerating voltage of 20 kV, 200×magnification and a working distance of 14–23 mm. Samples B106and B180 were measured by automatic mapping, whereas the otherseven samples were measured by manual indexing. The grain size ofOpx in harzburgite B143 was so large that we could not find enough

Fig. 2. Photomicrographs (cross-polarized light) of representative core samples. (a) Slight serpentinization of olivine in harzburgite. (b) Kink bands of olivineand Opx in Cpx-bearing harzburgite. (c, d) Undulose extinction of coarse-grained olivine in harzburgite and dunite. (e, f ) Dynamic recrystallization of olivinein foliated harzburgites. (g, h) Exsolution lamellae of Cpx in Opx in harzburgites. Cpx, clinopyroxene; Ol, olivine; Opx, orthopyroxene; Srp, serpentine.

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grains for plotting. Excluding sample B143, at least 100 grains ofolivine and 50 grains of Opx were analysed for each sample toensure the reliability of the statistical analyses.

For samples without evident lineation and foliation, non-orientedthin sections were prepared for microstructural analyses. Despitevariations in composition and water content, Opx in peridotitesdevelops a stable LPO characterized by the [001] axis parallel tothe lineation and the (100) plane parallel to the foliation (e.g.Manthilake et al. 2013; Bystricky et al. 2016; Soustelle &Manthilake 2017; Jung 2017). Therefore the [001](100) fabric ofOpx was used as a coordinate reference to rotate the EBSD data,

i.e. the point maxima of the [001] axis parallel to the X direction andthe point maxima of the [100] axis parallel to the Z direction. Thepole figures of olivine and Opx were plotted using the Petrophysicsprogram of Mainprice (1990).

To examine the slip systems of olivine, we carried outtransmission electron microscopy (TEM) observations of olivineat the Institute of Geology and Geophysics, Chinese Academy ofSciences. The samples for the TEM imaging were prepared byfocused ion beam milling using a Carl Zeiss Auriga Compactsystem. The acceleration voltage was 0.1–30 kV and the beamcurrent was 1 pA–50 nA. The resolution ratio was 5 nm (30 kV,

Fig. 3. Coarse symplectites of Cr–Al spinel and orthopyroxene from harzburgite samples: (a, c) plane polarized and (b, d) crossed-polarizedphotomicrographs. Ol, olivine; Opx, orthopyroxene; Spl, spinel.

Fig. 4. Compositional relationships of (a)NiO content v. Mg# in olivine, Al2O3

content v. Mg# in (b) orthopyroxene and(c) clinopyroxene, and (d) TiO2 contentv. Cr# in spinel for harzburgite and dunitesamples from the LBSD-1 and LBSD-2boreholes.

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1 pA). Imaging and electron diffraction were performed on a JEOLJEM-2100 transmission electron microscope using an LaB6electron shooter and an accelerating voltage of 200 kV.

Water content measurements

Fourier transform infrared spectrometry (FTIR) was used todetermine the water content of olivine, Opx and Cpx from 17harzburgites and seven dunites. A Vertex 70 spectrometer coupledwith a Hyperion 2000 microscope at the State Key Laboratory forMineral Deposits Research and a Nicolet 5700 spectrometercoupled with a Continuμm microscope at the School of Earth andSpace Sciences, University of Science and Technology of China,were used for comparison. Both instruments have a KBr beamsplitter and a mercury cadmium telluride detector cooled by liquidnitrogen. The thickness of the double-polished thin sectionsused for the water content measurements varied between 150 and300 µm. Prior to analysis, the thin sections were placed in an oven at120°C for at least 8 h to remove any free water from the surface andfractures. The unpolarized FTIR spectra were collected in the range4000–2800 cm−1 at room temperature, with 128 or 256 scan timesand a resolution of 8 cm−1. Depending on the grain size, aperturesof 50 × 50 or 25 × 25 µm were used for optically clean, crack- andinclusion-free regions. For each sample, 4–27 grains of each mineralwere measured to obtain the mean water content.

A modified form of the Beer–Lambert law was used to calculatethe hydrogen concentration:

CH2O ¼ A=(I � t) (1)

where CH2O is the content of hydrogen species in ppm H2O, A is theintegrated area (cm−1) of the absorption bands in the region ofinterest, I is the integral specific absorption coefficient (ppm−1·cm−2)and t is the sample thickness (cm). The infrared spectra wereintegrated from 3325 to 3650 cm−1, the region dominated by thestretching vibrations of OH bonds. The integral specific coefficientswere 14.84 ppm−1 cm−2 forOpx and 7.09 ppm−1·cm−2 for Cpx (Bellet al. 1995). For olivine, 1/0.188 ppm−1·cm−2 was adopted as theintegral specific coefficient (Bell et al. 2003), which could be 3.5times the concentration derived from the Paterson (1982) calibrationfor unpolarized samples.

Results

Mineral chemistry and equilibration temperatures

Table 1 lists the mean major element compositions of olivine fromthe Luobusa peridotite samples. The Mg# values, defined as theMg/(Mg + Fe) atomic ratio, vary between 89.6 and 90.6 for olivinein the harzburgites and between 87.7 and 92.5 for olivine in thedunites. Except for dunite B520, with an Mg# for olivine as low as87.7, the mean Mg# of olivine in the dunites is 91.0, higher than themean value of 90.1 for the harzburgites (Fig. 4a). The Opx and Cpxgrains fall into the composition ranges of enstatite and diopside,respectively (Table S1). The Mg# values of Opx are 89.8–90.8 inthe harzburgite samples and 91.4–92.2 in the dunite samples,whereas those of Cpx are in the range 92.4–94.1 in the harzburgitesamples and 94.2–95.0 in the dunite samples (Table S1). The Mg#values of Opx and Cpx in the harzburgites and dunites show anegative correlation with the Al2O3 content (Fig. 4b, c). The Cr#values in spinel, defined as the Cr/(Cr + Al) atomic ratio, are20.4–37.1 for the harzburgite samples and 54.9–78.1 for thedunite samples (Table S1 and Fig. 4d). The spinel grains inthe harzburgites are characterized by a higher Al2O3 content(35.56–48.65 wt%), a lower Cr2O3 content (18.57–31.29 wt%) anda higher MgO content (13.84–17.31 wt%) than those in the dunites

(Table S2). The Mg# values in spinel are 25.3–46.9 for theharzburgite samples and 58.0–68.9 for the dunite samples.

The relationships between the Mg# in olivine and the Cr# inspinel (Fig. 5a) and between the Mg# and Cr# in spinel (Fig. 5b)indicate that the Luobusa harzburgites are abyssal peridotitessubjected to 10–20% partial melting, whereas the more depleteddunites are SSZ peridotites that experienced a high degree of partialmelting and were then modified by melt–rock interactions in amantle wedge, as suggested by previous studies (Zhou et al. 1996,

Fig. 5. Chemical variations of olivine, spinel and orthopyroxene inharzburgite and dunite samples from the LBSD-1 and LBSD-2 boreholes.(a) Compositional relationship between Cr# in spinel and Mg# in olivine.Fields for abyssal peridotites (Dick & Bullen 1984) and supra-subductionzone peridotites (Pearce et al. 2000) are shown within dotted lines. Partialmelting and fractional crystallization trend (Arai 1994) and the degree ofpartial melting (Jaques & Green 1980) are shown by black arrows.(b) Compositional variations of Cr# v. Mg# in spinel. (c) Compositionalrelationship of Al2O3 content in orthopyroxene v. Cr# in spinel. The fieldsfor abyssal and supra-subduction zone peridotites are from Bonatti &Michael (1989). FMM, fertile mid-ocean ridge basalt mantle; OSMA,olivine–spinel mantle array, which is a spinel peridotite restite trend(Arai 1994).

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2005; Xiong et al. 2015). The negative correlation between theAl2O3 content in Opx and the Cr# in spinel (Fig. 5c) confirmsthe different origins and different degrees of partial melting of theLuobusa harzburgites and dunites.

Assuming a pressure of 1.5 GPa, which corresponds toequilibration in the spinel stability field, the equilibrationtemperatures of our samples were calculated using the Ca-in-Opxthermometer (Brey & Köhler 1990), the Al-in-Opx thermometer(Witt-Eickschen & Seck 1991) and the two-pyroxene thermometer(Taylor 1998) (Table 2). There is no data for dunites B269, B512,B520 and B536 due to their lack of Opx porphyroblasts in thinsection. Orthopyroxene porphyroblasts in the harzburgites anddunites are very uniform in their CaO content (0.44–1.11 wt%), butvariable in their Al2O3 content (2.96–4.24 wt% for the harzburgitesand 0.68–1.83 wt% for the dunites). For dunites B155 and B256,the Al-in-Opx thermometer yields equilibration temperatures of776–896°C, c. 100–250°C lower than the values from the Ca-in-Opx and two-pyroxene thermometers (Table 2). The low-Al Opx inthe Luobasa dunites has been attributed to the reaction of boniniticmelts with the surrounding peridotites (Zhou et al. 2005). Given thebetter performance of the Taylor (1998) two-pyroxene thermometerfor peridotites (Nimis & Grütter 2010), the equilibration tempera-tures of the studied harzburgites and dunites are c. 950–1080°C.

It is noteworthy that some Opx grains in the harzburgites have avery high CaO content, e.g. sample B120. Such samples showabundant exsolution lamellae of Cpx in Opx grains (Fig. 2h), whichhas been interpreted as the transformation of clinoenstatite fromorthoenstatite at pressures >7 GPa (Zhang et al. 2017). The deeporigin of sample B120 is consistent with the coarse vermicularsymplectites of Opx + Cr–Al spinel ± Cpx (Fig. 3), which has beenattributed to the breakdown of majoritic garnets in Luobusaharzburgites with estimated minimum pressures >13 GPa (Griffinet al. 2016). Therefore the calculated temperature, assuming P =1.5 GPa, only yields constraints on the temperature of their lastequilibrium stage at shallow mantle depths.

Trace element composition of Cpx

The Cpx from the 11 Luobusa harzburgites show consistent rareearth element (REE) patterns, with a slow decrease in heavy to

medium REEs from Yb to Eu and a rapid decrease in light REEsfrom Sm to Ce (Fig. 6a). Cpx from the harzburgites containsextremely low concentrations of the more incompatible lithophileelements (Table S3) and has negative anomalies in Sr, Zr and Cerelative to neighbouring REEs in the trace element patterns

Table 2. Estimated temperatures from peridotite core samples in the Luobusa ophiolite

Sample Lithology Depth (m)

Temperatures (°C)

Olivine fabricsCa-in-Opx Al-in-Opx Two-pyroxene

B98 Hz 164 932 1034 956 B + EB106 Hz 188 1077 968 1021 BB143 Hz 284 1044 993 1080 AB153 Hz 303 1135 1028 1011 EB120 Hz 316 998 1035 984B169 Hz 342 1041 995 1059 AB137 Hz 354 940 921 1061 B + EB180 Hz 367 1032 891 1022 B + EB186 Hz 379 971 1022 999B199 Hz 407 872 1024 973B196 Hz 504 1039 1046 1012B245 Hz 524 1148 990 1082 CB226 Hz 572 915 1020 1002B281 Hz 627 957 1021 1052B329 Hz 790 1017 1051 963B368 Hz 897 990 1032 982B155 Dun 389 1128 896 975B256 Dun 558 1067 776 987 A + E

Cpx, clinopyroxene; Dun, dunite; Hz, harzburgite; Opx, orthopyroxene.Thermometers: Ca-in-Opx thermometer (Brey & Köhler 1990), Al-in-Opx thermometer (Witt-Eickschen &Seck 1991), two-pyroxene thermometer (Taylor 1998).

Fig. 6. (a) Chondrite-normalized rare earth element and (b) primitivemantle-normalized trace element patterns of clinopyroxene from theLuobusa harzburgites. Normalizing values taken from McDonough & Sun(1995). Cpx, clinopyroxene.

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(Fig. 6b). These features are similar to the Cpx found in abyssalperidotites (Johnson et al. 1990) and agree with previous studies ofthe Luobusa harzburgites (Zhou et al. 1996, 2005). Therefore theLuobusa harzburgites did not experience pervasive metasomatismafter melt extraction.

Fabrics of olivine and pyroxene

After rotation of the EBSD data, Opx from eight harzburgitesamples and one dunite sample show the concentration of grainswith the [001] axis sub-parallel to the lineation and the (100) planesub-parallel to the foliation (Fig. 7). This fabric pattern agrees withthe typical LPO of Opx in peridotites (e.g. Tommasi et al. 2016;Jung 2017). Therefore the structural coordinate system is estab-lished with X parallel to the lineation, Z normal to the foliation andY normal to the lineation in the foliation plane. The pole figures ofolivine can be plotted using the rotated EBSD data.

The olivine in our samples shows complex fabrics (Fig. 8;Table 3). Harzburgites B143 and B169 have developed a typicalA-type fabric with the maximum concentration of the [100], [010]

and [001] axes sub-parallel to the X, Z and Y directions,respectively, implying dominant activation of the [100](010) slipsystem. By contrast, harzburgite B106 shows a B-type fabriccharacterized by the maximum concentration of the [001] axisparallel to the lineation and the [010] axis perpendicular to thefoliation, suggesting the predominance of the [001](010) slipsystem. Harzburgite B153 has developed an E-type fabric due tothe activation of the [100](001) slip system. Harzburgite B245 showa C-type fabric with the point maxima of the [001] axis parallelto the lineation and those of the [100] axis perpendicular tothe foliation, reflecting the dominant activation of the [001](100)slip system.

It is noteworthy that olivine from other samples shows maximumconcentrations of the [100] axis and the [010] axis sub-parallel tothe lineation and perpendicular to the foliation, respectively, but the[001] axis does not concentrate along the Y direction as in thetypical A-type fabric. For dunite B256, the [001] axis of olivineconcentrates sub-perpendicular to the foliation, suggesting theactivation of both the [100](010) and [100](001) slip systems, i.e. acombination of A- and E-type fabrics. For harzburgites B98, B137

Fig. 7. Pole figures of orthopyroxene in the peridotite core samples from the Luobusa ophiolite. Crystallographic orientations are plotted as one point pergrain in the lower hemisphere equal-area projection. J-index, fabric strength of orthopyroxene; n, number of grains analysed; pfJ, texture index for the fabricstrength of each axis; X, parallel to the lineation; Z, normal to the foliation.

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and B180, the [001] axis of olivine forms two-point maxima sub-parallel to the lineation and sub-perpendicular to the foliation,respectively. Given the orthorhombic crystallography of olivine, theasymmetrical pole figures allow us to distinguish the correspondingslip direction and the slip plane of olivine. For example, the two-point maxima of the olivine [001] axis from sample B98 are sub-parallel to those of the [100] and [010] axes, respectively,suggesting the activation of both the [001](010) and [100](001)slip systems. Therefore the three harzburgite samples record acombination of B- and E-type fabrics.

The bright-field TEM images of olivine grains were used to inferthe dominant slip systems in olivine and compared with the EBSD-derived LPO of olivine (Fig. 9). The Burgers vectors weredetermined using the g · b = 0 and g · (b Ʌu) = 0 criteria, where bis the Burgers vector, g is the diffraction vector and u is a unit vectoralong the dislocation line. The dislocation direction was obtainedthrough the corresponding selected-area electron diffraction(SAED) pattern. The index of the point in the SAED patternrepresents the plane of the corresponding index in reciprocal space.Figure S2 shows the bright-field images of olivine and thecorresponding SAED patterns of the studied samples. The observedslip systems in olivine grains from the Luobusa peridotites aresummarized in Table 3.

Almost all the dislocations in the bright-field images of olivinefrom the Luobusa peridotites are straight (Fig. 9a–c), suggestingthat the dominant deformation mechanism is dislocation creep atrelatively low temperatures (either low temperature and lowpressure, or high temperature and UHP). Dislocation loops(Fig. 9a), which are formed at high temperature or throughdislocation cross-slipping (Raterron et al. 2007, 2009), are rare.The occasionally observed square-like figures (Fig. 9c) anddislocation tangles (Fig. 9d) are characteristic of low-temperaturedeformation at high pressure (Raterron et al. 2004). Dislocations arealigned along a particular direction and form sub-grain boundaries.The preservation of straight dislocations and sub-grains in olivinesuggests that the Luobusa peridotites experienced limited annealing.

Water content

The representative FTIR spectra of olivine, Opx and Cpx from 17harzburgite samples and seven dunite samples are given inFigure 10. The absorption bands of structural hydroxyl appear inthe range 3400–3570 cm−1 for olivine (Fig. 10a). The infraredspectra of Opx are characterized by three hydroxyl absorption bandsin the ranges 3410–3420, 3515–3525 and 3560–3570 cm−1,whereas those of Cpx are in the ranges 3450–3465, 3530–3535

Fig. 8. Pole figures of olivine in the peridotite core samples from the Luobusa ophiolite. Crystallographic orientations are plotted as one point per grain inthe lower hemisphere equal-area projection. J-index, fabric strength of orthopyroxene; n, number of grains analysed; pfJ, texture index for the fabric strengthof each axis; X, parallel to the lineation; Z, normal to the foliation.

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and 3645–3648 cm−1 (Fig. 10b). The broad absorption band near3400 cm−1 has been attributed to the inclusion of molecular water,whereas the absorption peaks near 3690 and 3717 cm−1 are causedby the OH stretching bands of serpentine. The absorption band at3594 cm−1 matches the OH stretching bands of hydrous minerals,such as vermiculite group minerals (e.g. Khisina et al. 2001;Johnson et al. 2002; Beran & Libowitzky 2006; Aubaud et al.2007). The very high absorptions between 3720 and 3750 cm−1

occur in some olivine grains (Fig. 10a) and may be caused by non-hydrogen-bonded OH, such as inclusions of a sheet silicate (Bellet al. 2003). The contribution of these extrinsic hydrogens wasexcluded in the baseline correction used to obtain the water contentof olivine and pyroxene.

Table 3 shows that the water content of olivine is very low(6–25 ppm) with a mean of 16 ± 5 ppm. By contrast, both Opx andCpx have high water contents. The water content of Opx is56–141 ppmwith amean of 90 ± 21 ppm, whereas thewater contentof Cpx varies from 373 to 1273 ppm. If we exclude the extremelywater-rich Cpx with 1273 ppm in harzburgite sample B153, themean water content of Cpx is 492 ± 64 ppm.

Discussion

Implications of water content in Luobusa peridotites

Consistent with previous studies on the Luobusa ophiolite (Zhouet al. 1996, 2005; Dilek & Furnes 2011, 2014; Xiong et al. 2015),the studied harzburgites are abyssal peridotites subjected to 10–20%partial melting, whereas the more depleted dunites are SSZperidotites that have been modified by melt–rock interactions in amantle wedge (Fig. 5). The subducted oceanic lithosphere has lostmost of its water at depths >200 km due to the continuousbreakdown of hydrous minerals such as serpentine, talc and chlorite(Hacker et al. 2003). The mantle wedge above the subducted

oceanic lithosphere generally has a high water content as a result ofmetasomatism and fluid infiltration, as evidenced by arc magma-tism. However, olivine in 24 peridotite core samples from theLuobusa ophiolite is very poor in water, whereas Cpx is extremelyrich in water relative to olivine and Opx (Table 3). The REE patternsof Cpx indicate that the Luobusa peridotites did not experiencepervasive metasomatism after partial melting (Fig. 6).

Figure 11a shows that the diffusion of hydrogen in pyroxene isabout 500 times slower than in olivine (Mackwell & Kohlstedt1990; Ingrin et al. 1995; Stalder & Skogby 2003). Given thetemperature dependence of hydrogen diffusion, the very low watercontent in olivine shows that widespread serpentinization of theLuobusa peridotites occurred at shallow depths. The combined δ13Cand nitrogen data of microdiamonds from the Luobusa peridotitesindicate that these microdiamonds were formed over a narrow andcold temperature range (<950°C) and were incorporated into thechromitites and peridotites near the mantle transition zone during ashort residence time (i.e. within several million years) at hightemperature in the deep mantle (Xu et al. 2018). Assuming 5 myrfor the exhumation of microdiamonds and their host rocks from thetransition zone to the shallow mantle depths, this implies a very fastexhumation rate of 6–8 cm a−1. Based on the hydrogen diffusioncoefficients and fast exhumation rates, Cpx and Opx in the Luobusaperidotites have been subjected to limited hydrogen loss duringexhumation, whereas olivine may have been subjected to significanthydrogen loss when temperatures exceeded 1000°C (Fig. 11b). Thisimplies that olivine was strongly dehydrated when the peridotiteswere subducted to depths >200 km (i.e. the geothermal gradient is>5°C km−1) or when they entered the mantle transition zone.

When the slab reaches the mantle transition zone, the melt–rockinteractions could significantly dehydrate olivine given the partitioncoefficient of hydrogen between the melt and olivine (Hirschmannet al. 2009). The partition coefficient of hydrogen between Cpx andOpx (DH

Cpx/Opx) is 5.56 ± 0.96 for the Luobusa peridotites. This

Table 3. Water contents of olivine, Opx and Cpx, and fabrics and slip systemts in olivine from the Luobusa peridotites

SampleDepth(m) Lithology

Olivine Orthopyroxene Clinopyroxene

Olivinefabrics Slip systems

No. ofgrains

Water content(ppm)

No. ofgrains

Water content(ppm)

No. ofgrains

Water content(ppm)

B98 164 Hz 10 15 13 114 B + E [001](100), [001](010)B106 188 Hz 14 19 25 99 8 563 B [001](100)B143 284 Hz 13 13 24 77 4 500 A [100](010)B153 303 Hz 10 23 26 94 8 1273 E [001](100), [001](010)B169 342 Hz 11 22 32 96 A [001](hk0), [100](021)B180 367 Hz 12 13 17 111 10 467 B + E [100](010), [100](011)B186 379 Hz 8 6 10 56 0B199 407 Hz 18 23 22 84 13 373B245 524 Hz 11 15 37 72 12 507 C [001](010), [001](100)B252 546 Hz 13 15 8 63B256 558 Dun 10 21 20 84 A + E [100](001)B281 627 Hz 4 15 11 69B300 682 Dun 14 18B329 790 Hz 9 14 12 75B520 1370 Dun 7 18B120 316 Hz 11 8 20 93 12 479B137 354 Hz 9 25 10 141 B + E [001](hk0), [001](010)B155 398 Dun 10 23 19 95 2 557B196 504 Hz 10 8 11 70B226 572 Hz 7 21 10 90B368 897 Hz 7 16 10 122B269 673 Dun 3 13B512 1375 Dun 4 8B536 1450 Dun 10 14

Water contents estimated using the calibration of Bell et al. (2003) for olivine and Bell et al. (1995) for orthopyroxene and clinopyroxene.

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Fig. 9. Representative bright-field transmission electron microscopy images of olivine from the Luobusa peridotites and corresponding diffraction patterns(insets). (a) Straight dislocations in olivine from sample B106; dislocation loops are visible. (b) Short-line dislocations in olivine from sample B143.(c) Sub-grain boundaries and straight dislocations in olivine from sample B153. (d) Straight dislocations in the paper plane and dot-like dislocations normalto the paper plane in olivine from sample B245; dislocation tangle is visible.

Fig. 10. Representative Fourier transform infrared spectra and water content of (a) olivine and (b) orthopyroxene and clinopyroxene from the Luobusaperidotites. The water contents were obtained using the Bell et al. (2003) integral specific coefficient for olivine and the Bell et al. (1995) integral specificcoefficient for orthopyroxene and clinopyroxene. The green lines represent the spectra after subtraction of the background. All spectra are normalized to1 cm thickness and shifted vertically for clarity. Cpx, clinopyroxene; Ol, olivine; Opx, orthopyroxene.

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value is much higher than DHCpx/Opx of 1.2–3.5 from previous

measurements on natural peridotites (Bell & Rossman 1992; Bell &Ihinger 2000; Peslier et al. 2002, 2007, 2008, 2010, 2012; Peslier &Luhr 2006; Grant et al. 2007; Li et al. 2008; Yang et al. 2008)(Fig. 12). The experiments on hydrogen partitioning in mantleminerals show a trend of increasing DH

Cpx/Opx with pressures up to4 GPa (Aubaud et al. 2004; Hauri et al. 2006; Tenner et al. 2009;Kovács et al. 2012; Rosenthal et al. 2015; Demouchy et al. 2017).The abundant exsolution lamellae of Cpx in Opx grains (Fig. 2g, h)from the Luobusa peridotite core samples suggest an origin atP > 7 GPa (Zhang et al. 2017). This is consistent with the estimationof P > 13 GPa from the coarse vermicular symplectites of Opx +Cr–Al spinel ± Cpx as breakdown products of majoritic garnet inharzburgite samples (Fig. 3) (Griffin et al. 2016). Therefore theextremely highDH

Cpx/Opx in the Luobusa peridotites probably benefitsfrom a higherDH

Cpx/Opx at pressures >7 GPa and less hydrogen loss inCpx than in Opx during rapid exhumation. It is worth noting the highwater content of high-pressure phases of olivine, i.e. wadsleyite andringwoodite, from volatile-rich kimberlite xenoliths (Pearson et al.2014). Given the localized non-equilibrium phases in the Luobusaperidotites and chromitites, the transition zone could be heteroge-neous in both composition and water content (Dilek & Yang 2018).The highwater content of Cpx andOpx from the Luobusa peridotitesshow that the subducted oceanic lithosphere can transport a lot ofwater into the mantle transition zone via pyroxene.

Exhumation of Luobusa peridotites in asubduction channel

It is still unclear how the UHP and super-reducing phases wereformed and preserved in the Luobusa peridotites and chromitites,which are generally regarded as relicts of the Neotethyan oceaniclithosphere and are characterized by the geochemical signatures ofan SSZ environment (Griffin et al. 2016; Dilek & Yang 2018 andreferences cited therein). Although the mantle plume model (Yanget al. 2007) and the rapid channelized mantle upwelling model(Griffin et al. 2016) can explain the coexistence of shallow pre-metamorphic chromitites and peridotites and their metamorphismin the mantle transition zone at pressures >13 GPa, it is difficult toexplain thewidespread straight dislocations and sub-grains in olivineand the complex olivine fabrics in the Luobusa peridotites, whichare easily modified at high temperature of mantle upwelling.In addition, the preservation of crustal minerals (e.g. zircon, quartz,corundum and K-feldspar) in ophiolitic peridotites and chromititessuggests the mixing of subducted continental crustal materials withoceanic lithosphere in a subduction channel (Yang et al. 2014;Robinson et al. 2015; Dilek & Yang 2018).

All the studied peridotites are coarse grained with granoblastic orporphyroclastic textures, which excludes stress-induced B-typefabrics in mylonitic peridotites (Wang et al. 2013a) or E-typefabrics in a hydrated ductile shear zone at the Moho transitionzone (Michibayashi & Oohara 2013). Deformation experiments onolivine aggregates show different fabric types with increasingtemperature and pressure. Carter & Avé Lallemant (1970) deformeddunites and peridotites under conditions of 0.5–3 GPa and300–1400°C in the presence or absence of water. They found thatthe predominant slip systems in olivine changed from [001]{110} atlow temperatures to [100]{0kl}(D-type fabric) and then [100](010)(A-type fabric) with increasing temperatures. Their observationswere confirmed by the A-type fabric of olivine aggregates in simpleshear deformation experiments at 300 MPa and 1300°C (Zhanget al. 2000). However, deformation experiments on single crystalsof forsterite using a multi-anvil apparatus showed that UHP willfavour [001] slip in olivine at pressures >7 GPa temperatures of1200–1400°C (Couvy et al. 2004; Raterron et al. 2007, 2009). Morerecent deformation experiments have found that the A-type fabric in

Fig. 11. (a) Arrhenius diagram of various hydrogen diffusivities and (b)the effective hydrogen diffusion distance in olivine, orthopyroxene andclinopyroxene for 1, 5 and 10 Ma using a grain size of 1 mm. Hydrogendiffusion along [100] is faster than that along [010] and [001] in olivine(Mackwell & Kohlstedt 1990). The effective diffusion distance ofhydrogen in olivine takes into account both lattice diffusion along [100](Mackwell & Kohlstedt 1990) and diffusion in grain boundaries assuminga grain size of 1 mm and a grain boundary width of 0.75 nm (Demouchy2010, 2012). The hydrogen diffusion coefficients of pyroxene are fromdehydration experiments (Ingrin et al. 1995; Stalder & Skogby 2003).Cpx, clinopyroxene; Ol, olivine; Opx, orthopyroxene.

Fig. 12. Water concentration in clinopyroxene v. water concentration inorthopyroxene in Luobusa peridotite core samples. The results fromprevious studies on natural peridotites (Bell & Rossman 1992; Bell &Ihinger 2000; Peslier et al. 2002, 2007, 2008, 2010, 2012; Peslier & Luhr2006; Grant et al. 2007; Li et al. 2008; Yang et al. 2008) are shown asgrey dots. The inset shows the correlation between DCpx/Opx (the partitioncoefficient of hydrogen between clinopyroxene and orthopyroxene) withpressure in experimental studies (Aubaud et al. 2004; Hauri et al. 2006;Tenner et al. 2009; Kovács et al. 2012; Rosenthal et al. 2015; Demouchyet al. 2017) (modified after Xia et al. 2017). Cpx, clinopyroxene; Opx,orthopyroxene.

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dry olivine aggregates changes to a B-/C-type fabric at 7.6 GPa and1400°C (Ohuchi et al. 2011). However, at 7.2–11.1 GPa and 1127–1500°C, olivinewill develop A- and B-type fabrics under water-richand moderately wet conditions, respectively, whereas the C-typefabric is dominant under water-poor conditions at pressure>9.6 GPa and 1400°C (Ohuchi & Irifune 2013). Although wecannot exclude the possibility that different water contents in olivineaffected the development of fabrics during exhumation, thepreservation of straight dislocations (Fig. 9) and the generalconsistency between olivine fabrics and slip systems (Table 3)suggest the deformation of olivine under water-poor and relativelylow-temperature conditions because a high water content and hightemperature will facilitate the modification of dislocations.Compared with deformation experiments (Couvy et al. 2004;Raterron et al. 2007, 2009; Ohuchi et al. 2011; Ohuchi & Irifune2013), the B- and C-type olivine fabrics in the Luobusa peridotitescould be formed at pressures >7.6 and >9.6 GPa, respectively,whereas the A- and E-type olivine fabrics could be formed bydynamic recrystallization in the shallow mantle.

The ophiolites along the YZSZ were first obducted onto thecontinental margin of proto-India or onto a series of intra-oceanicisland arcs (Aitchison et al. 2007; Hébert et al. 2012). The Luobusaophiolite was formed at a mid-ocean ridge in the Mid-Jurassic(Zhou et al. 2002; Zhong et al. 2006) and then re-fertilized byboninitic melts in a mantle wedge above the subduction zone at c.126 Ma (Malpas et al. 2003; Zhou et al. 2005, 2014). The 40Ar/39Ardating of amphiboles from the Luobusa amphibolites yieldsamphibolite facies metamorphism at 90–80 Ma, which wasinterpreted as the initial tectonic displacement of the Luobusaophiolite (Malpas et al. 2003). By contrast, zircon U-Pb ages ofgabbro and amphibolite from the Luobusa ophiolite yield 128–131 Ma (C. Zhang et al. 2016), comparable with the 40Ar/39Arcooling ages of amphiboles at 127–124 Ma for strongly foliatedamphibolites in the Xigaze ophiolite in the central YZSZ (Guilmetteet al. 2009). This implies multiple emplacement events before theclosure of the Neotethyan Ocean along this suture zone. Slabrollback and southwards trench migration at c. 130–120 Ma hasbeen proposed to explain the accretion of the Neotethyan forearclithosphere in Tibetan ophiolites (Xiong et al. 2016).

Based on these geochronological constraints and the multiplemagmatic events of the Gangdese arc, we propose a model in whichchannel flow induced by slab rollback can be used to explain themixture of peridotites with different olivine fabrics and UHP

minerals in the Luobusa ophiolite. As shown in Figure 13a, theLuobusa ophiolite probably represents the remnants of forearcoceanic lithosphere above a north-dipping subduction zone. TheNeotethyan oceanic lithosphere was subducted to the transitionzone in the Early Cretaceous. The slab rollback at 130–120 Maformed a new subduction system to the south of the original systemand triggered very rapid exhumation of the Luobusa peridotites andchromitites along the subduction channel between the subductingoceanic crust and the big mantle wedge. The return flow in theoceanic subduction channel resulted in a mixture of peridotites withdifferent origins and fabrics, including peridotites and chromititesderived from the transition zone, UHP phases bearing SSZperidotites and chromitites from the big mantle wedge, and SSZperidotites that had been trapped in the subduction channel atshallow depths (Fig. 13b). This scenario is comparable with the dualorigin of the Dabie–Sulu peridotites (Liou et al. 2000) and theexhumation of UHP rocks in continental subduction zones (Zheng2012), but the return depth of the subducted materials is deeper thanexpected.

Compared with the mantle plume model, the exhumation ofperidotites and chromitites in an oceanic subduction channeloccurred at relatively low temperatures and a high exhumation rate.This model allows the addition of crustal minerals to peridotites andchromitites, the preservation of fabrics and dislocations in olivine,and the preservation of the δ13C and N signatures of microdiamonds.The subduction channel provides an important pathway for thetransportation of subducted oceanic and continental materials fromthe mantle transition zone to shallow depths. In addition, due to thevery low water content in olivine and the lack of hydrous minerals atdepths >200 km, the water released from the subduction channel tothe mantle wedge was limited during exhumation of the UHP rocks,which explains the interruption of magmatism in the Gangdese arcbetween 130 and 120 Ma. Therefore our model supports theexhumation of diamonds and UHP minerals by channel flowdriven by slab rollback in forearc settings (Dilek & Yang 2018).

In a similar manner to the different fabrics seen in water-poorolivine from peridotites in the Western Gneiss Region (Norway)(Wang et al. 2013a), the Luobusa peridotites provide evidence forthe water-independent development of fabrics in a subductionchannel. Given the recent discovery of microdiamonds and UHPminerals in other ophiolites (Yang et al. 2014; Dilek & Yang 2018),it appears that the oceanic lithosphere has a more complexdeformation history than that predicted based on plate tectonics

Fig. 13. (a) Model of channel flow triggered by slab rollback for the rapid exhumation of supra-subduction zone peridotites in a subduction channel. See textfor description. (b) Structure of the subduction channel shown as the dashed black rectangle in part (a). SSZ, supra-subduction zone; UHP, ultra-high pressure.

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theory. Further investigations of microstructure of ophiolites fromdifferent tectonic settings is required to investigate the heterogeneityof the oceanic lithosphere and the recycling of material throughsubduction channels.

Conclusions

We studied the composition, microstructure and water content ofdunite and harzburgite core samples from the Luobusa ScientificDrilling Project to explore the complex deformation history ofthe diamond-bearing Luobusa ophiolite. All the peridotite samplesshow an SSZ signature and equilibration temperatures at c. 950–1080°C. The development of A-, B-, C- and E-type fabrics of olivineand the preservation of straight dislocations in olivine suggest theirdeformation at relatively low temperatures. Olivine is water-poor,with a meanwater content of 16 ± 5 ppm due to the loss of hydrogen,whereas Opx and Cpx preserve high water contents and highhydrogen partition coefficients (DH

Cpx/Opx= 5.56 ± 0.96). The REEpatterns of Cpx exclude the possible hydration of the Luobusaperidotites by mantle metasomatism. The subducted oceaniclithosphere can therefore transport a lot of water into the mantletransition zone via pyroxene. The B- and C-type fabrics in water-poor olivine represent a fossil fabric formed at great depths, whereasthe A- and E-type fabrics were formed at shallow depths duringexhumation. Our results, combined with previous studies, wepropose channel flow driven by slab rollback to explain themixture of different olivine fabrics, the extremely high DH

Cpx/Opx,and the occurrence of UHP and super-reducing mineral inclusions inthe Luobusa peridotites and chromitites.

Acknowledgements We are grateful to Y. Dilek for his invitation andencouragement to submit this paper, and for discussions with Y. Dilek, R. Wirth,W.L. Griffin and S.Y.O’Reilly. Constructive comments from T. Tsujimori and ananonymous reviewer were very helpful in improving this paper.

Funding This research was supported by the NSFC project (41590623) andthe National Key R & D Plan of China (Grant No. 2017YFC0601406).

Scientific editing by Yildirim Dilek

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991Fabrics of the Luobusa peridotites

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