Petrology of Chromitites in the Higashi-Akaishi Ultrahigh-Pressure (UHP) Peridotite Complex, Japan: Toward Understanding of General Features of the UHP Chromitites

minerals

Article Petrology of Chromitites in the Higashi-Akaishi Ultrahigh-Pressure (UHP) Peridotite Complex, Japan: Toward Understanding of General Features of the UHP Chromitites

Makoto Miura 1,2,*, Shoji Arai 1, Tomoyuki Mizukami 1, Vladimir R. Shmelev 3 and Satoko Ishimaru 4

1 Department of Earth Sciences, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan; [email protected] (S.A.); [email protected] (T.M.) 2 Gemological Institute of America (GIA) Tokyo Godo Kaisha, Tokyo 110-0016, Japan 3 Zavaritsky Institute of Geology and Geochemistry, Ural Branch, Russian Academy of Sciences, Yekaterinburg 620075, Russia; [email protected] 4 Department of Earth and Environmental Sciences, Faculty of Advanced Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-76-264-6513; Fax: +81-76-264-6545

Received: 9 October 2018; Accepted: 6 November 2018; Published: 11 November 2018

Abstract: Ultrahigh-pressure (UHP) chromitites containing UHP minerals such as coesite and diamond have been reported from some ophiolites in Tibet and the Polar Urals. Their nature, i.e., origin, P-T path and abundance, however, are still controversial and left unclear. Here we describe chromitites in the Higashi-akaishi (HA) ultramaﬁc complex in the Cretaceous Sanbagawa metamorphic belt, Japan, which experienced UHP condition (up to 3.8 GPa) at the peak metamorphism via subduction, in order to understand the nature of UHP chromitites. The HA peridotites typically contain garnets and are associated with eclogites, and their associated chromitites are expected to have experienced the UHP metamorphism. The Higashi-akaishi (HA) chromitites show banded to massive structures and are concordant to foliation of the surrounding peridotite. Chromian spinels in the chromitite and surrounding peridotites were sometimes fractured by deformation, and contain various inclusions, i.e., blade- and needle-like diopside lamellae, and minute inclusions of pyroxenes, olivine, and pargasite. The peculiar UHP minerals, such as coesite and diamond, have not been found under the microscope and the Raman spectrometer. Spinels in the HA chromitites show high Cr#s (0.7 to 0.85), and low Ti contents (<0.1 wt %), suggesting a genetic linkage to an arc magma. The HA chromitites share the basic petrographic and chemical features (i.e., diopside lamellae and arc-related spinel chemistry) with the UHP chromitites from Tibet and the Polar Urals. This suggests that some of the characteristics of the UHP chromitite can be obtained by compression, possibly via deep subduction, of low-P chromitite.

Keywords: chromitite; ultrahigh-pressure metamorphism; the Higashi-akaishi peridotite complex; diopside lamella; arc-related magmatism; subduction

1. Introduction Podiform chromitites, which are mainly composed of chromian spinels, have been interpreted as an igneous cumulate precipitated from a spinel-oversaturated melt formed by melt/peridotite reaction at low-pressure (low-P) conditions [1,2]. Podiform chromitites from ophiolites, such as the Oman ophiolite, commonly show various lines of evidence for low-P origin, such as the frequent presence of

Minerals 2018, 8, 525; doi:10.3390/min8110525 www.mdpi.com/journal/minerals Minerals 2018, 8, 525 2 of 18 Minerals 2018, 8, x FOR PEER REVIEW 2 of 18 presencepargasite of inclusions pargasite in inclusions chromian spinel in chromian [3,4]. However, spinel [3,4]. peculiar However, minerals peculiar indicating minerals ultrahigh-pressure indicating ultrahigh(UHP) and‐pressure highly reduced (UHP) and conditions, highly reduced such as diamond, conditions, moissanite, such as diamond, and alloys, moissanite, have been and found alloys, from havechromitites been found and surroundingfrom chromitites peridotites and surrounding in several ophiolites peridotites such in several as Luobusa ophiolites ophiolite such of as Tibet Luobusa [5–9] ophioliteand Ray-Iz of Tibet ophiolite [5–9] ofand the Ray Polar‐Iz ophiolite Urals [7 ].of Origin the Polar of suchUrals UHP [7]. Origin mineral-bearing of such UHP chromitites mineral‐bearing (UHP chromititeschromitites) (UHP is still chromitites) very controversial is still very because controversial their petrographic because andtheir petrologic petrographic features and arepetrologic unclear, featuresalthough are several unclear, models although were several proposed models [5,7,10 were–12]. proposed One of serious [5,7,10–12]. problems One in of constraining serious problems the origin in constrainingof UHP chromitites the origin is a of lack UHP of chromitites information is on a theirlack of P-T information trajectory. on their P‐T trajectory. ChromititesChromitites from from the the Higashi Higashi-akaishi‐akaishi complex complex will will give give us a us clue a clueto clear to clearthis problem. this problem. The HigashiThe Higashi-akaishi‐akaishi (HA) peridotite (HA) peridotite body in body the Sanbagawa in the Sanbagawa metamorphic metamorphic belt, Southwest belt, Southwest Japan (Figure Japan 1), has(Figure been1 ),interpreted has been interpreted to have originally to have formed originally at low formed‐P conditions at low-P conditions such as the such uppermost as the uppermost mantle in amantle mantle in wedge, a mantle and wedge, subsequently and subsequently experienced experienced UHP metamorphic UHP metamorphic condition condition up to 3.8 up GPa, to 3.8 due GPa, to downgoingdue to downgoing mantle mantle flow induced ﬂow induced by subduction by subduction [13–18]. [13– 18The]. The P‐T P-T trajectory trajectory has has been been established established basedbased on on petrological petrological and and petrofabric petrofabric studies studies of of peridotites peridotites and and related related rocks, rocks, although although UHP UHP minerals minerals havehave not not been been found found [15]. [15]. Clear Clear evidence evidence for for their their high high-P‐P conditions conditions is is the the existence existence of of pyrope pyrope-rich‐rich garnetgarnet in in peridotites peridotites [13]. [13]. We We expect expect that that the the HA HA chromitites chromitites also also have have experienced experienced the the UHP UHP metamorphismmetamorphism together together with with the the associated associated peridotites. peridotites. The The advantage advantage of the of HA the chromitite HA chromitite is that is itsthat P‐ itsT history P-T history has hasbeen been independently independently determined determined [13–18]. [13–18 The]. The HA HA chromitites chromitites will, will, therefore, therefore, provideprovide us us with with unrivaled unrivaled information information on on the the behavior behavior of of low low-P‐P chromitite chromitite upon upon compression compression via via UHPUHP metamorphism. metamorphism. We We present present here here petrographic petrographic and and petrological petrological features features of chromitites of chromitites from from the HAthe HAperidotite peridotite complex complex (Figure (Figure 1) in1) inorder order to tounderstand understand the the nature nature of of chromitites chromitites in in the the UHP metamorphicmetamorphic belt, belt, or or the the UHP UHP chromitites chromitites from from ophiolites. ophiolites.

FigureFigure 1. 1. LocalityLocality and and geological geological sketch sketch maps maps of of the the Higashi Higashi-akaishi‐akaishi ultramafic ultramafic complex complex located located on on thethe Sanbagawa Sanbagawa high high-P‐P metamorphic metamorphic zone. zone. (a) ( Locationa) Location of the of theHigashi Higashi-akaishi‐akaishi ultramafic ultramafic complex. complex. (b) General(b) General geological geological sketch sketch of the of the Higashi Higashi-akaishi‐akaishi ultramafic ultramafic complex complex and and surrounding surrounding metamorphic metamorphic rocks.rocks. Modified Modified from from Hattori Hattori et et al. al. [17]. [17]. Open Open star, star, sampling sampling localities. localities. 2. Geological Background 2. Geological Background The Sanbagawa belt, a Cretaceous regional metamorphic belt, extends over 800 km along The Sanbagawa belt, a Cretaceous regional metamorphic belt, extends over 800 km along southwest southwest Japan arc (Figure1a), and is mainly composed of pelitic and basic schists with small numbers Japan arc (Figure 1a), and is mainly composed of pelitic and basic schists with small numbers of of metagabbro and ultramafic bodies [14,19,20]. The Higashi-akaishi (HA) ultramafic complex is one of metagabbro and ultramafic bodies [14,19,20]. The Higashi‐akaishi (HA) ultramafic complex is one of the largest ultramafic complexes in the Sanbagawa metamorphic belt (Figure1). Ultramafic rocks from the largest ultramafic complexes in the Sanbagawa metamorphic belt (Figure 1). Ultramafic rocks the HA complex are dominated by dunites, which contain layers of clinopyroxenite and websterite from the HA complex are dominated by dunites, which contain layers of clinopyroxenite and websterite with small amounts of chromitite (Figure1). A unit of eclogite exists along the contact (the Gongen-goe with small amounts of chromitite (Figure 1). A unit of eclogite exists along the contact (the Gongen‐ goe area) between the HA peridotite and surrounding garnet amphibolite (Figure 1). Ultramafic rocks have experienced eclogite facies metamorphism because of the presence of pyrope‐rich garnet [14,19].

Minerals 2018, 8, 525 3 of 18 area) between the HA peridotite and surrounding garnet amphibolite (Figure1). Ultramaﬁc rocks haveMinerals experienced 2018, 8, x FOR eclogite PEER REVIEW facies metamorphism because of the presence of pyrope-rich garnet3 [of14 18,19 ]. Chromitites occur as sparse concordant to subconcordant lenses and layers within dunites (Figure2). TheChromitites HA chromitites occur varyas sparse from concordant banded to to massive subconcordant types with lenses an increaseand layers in within the degree dunites of deformation(Figure 2). The HA chromitites vary from banded to massive types with an increase in the degree of deformation and chromite modal amount [21] (Figure2b,c). Thick chromitite lenses have been almost mined out, but and chromite modal amount [21] (Figure 2b,c). Thick chromitite lenses have been almost mined out, a few small/thin lenses still remain in dunite-dominant portion of the complex [22]. The HA peridotites but a few small/thin lenses still remain in dunite‐dominant portion of the complex [22]. The HA peridotites and included chromitite lenses are strongly deformed and folded near the garnet amphibolite body of and included chromitite lenses are strongly deformed and folded near the garnet amphibolite body the Gongen-goe area (Figure2b). Chromitite samples from the central part of the HA complex are far of the Gongen‐goe area (Figure 2b). Chromitite samples from the central part of the HA complex are lessfar deformed less deformed than thethan banded the banded chromitites chromitites in the in Gongen-goethe Gongen‐goe area area (Figure (Figure2c). 2c).

FigureFigure 2. 2.Chromitites Chromitites from from the the Higashi-akaishi Higashi‐akaishi ultramaﬁcultramafic complex. (a (a) )Banded Banded chromitite chromitite pods pods in in dunitedunite closed closed to to thethe contact contact with with eclogite eclogite-facies‐facies rocks rocks (Figure (Figure 1). Chromitite1). Chromitite is concordant is concordant to the foliation to the foliationof the surrounding of the surrounding dunite (black dunite arrow). (black (b arrow).) A folded (b) banded A folded chromitite banded in chromitite a boulder in from a boulder scree near from screethe nearGongen the‐goe. Gongen-goe. Note the strongly Note the deformed strongly nature. deformed (c) A massive nature. chromitite (c) A massive sample chromitite (boulder) samplefrom (boulder)sampling from site sampling 2. Yellowish site part 2. Yellowish (silicate matrix) part (silicate is composed matrix) of isolivine. composed of olivine.

3. Materials3. Materials and and Methods Methods ChromititeChromitite samples samples were were collected collected from from outcropsoutcrops and boulders at at two two sites sites in in the the Higashi Higashi-akaishi‐akaishi ultramaﬁcultramafic complex complex (Figure (Figure1 1).). Polished Polished thin sections sections of of chromitites chromitites were were examined examined with with microscope microscope for forpetrographic petrographic observation observation (Figure (Figure 3), and3), tested and testedwith Raman with spectroscopy Raman spectroscopy and a microprobe and a microprobeat Kanazawa at KanazawaUniversity, University, Japan. Japan. Laser-RamanLaser‐Raman spectroscopy spectroscopy (LabRAM (LabRAM HR800, HR800, HORIBA Jobin Jobin Yvon) Yvon) with with a a532 532 nm nm Nd:YAG Nd:YAG laser laser (J100GS-16,(J100GS‐16, Showa Showa Optronics Optronics Co., Co., Ltd.) Ltd.) of of Department Department of Earth Sciences Sciences at at Kanazawa Kanazawa University University was was usedused to to identify identify minute minute silicate silicate lamellae lamellae andand inclusionsinclusions within chromite. chromite. The The Nd:YAG Nd:YAG laser laser has has an an irradiationirradiation power power at at 1.6 1.6 mW mW with with a spectrala spectral resolution resolution of of about about± ±2.52.5 toto ±±3.53.5 cm cm−−1. 1For. For tiny tiny inclusions inclusions andand exsolutions exsolutions in in host host chromites, chromites, and and the the gained gained Raman spectra spectra were were processed processed to to remove remove the the signal signal of hostof host chromite chromite using using LabSpec LabSpec software software (Version (ver. 5) as 5) shown as shown in Figure in Figure 4. 4. MineralsMinerals were were analyzed analyzed by by a a wave-length wave‐length dispersivedispersive electron electron microprobe microprobe (JXA8800R, (JXA8800R, JEOL) JEOL) at at KanazawaKanazawa University. University. Analytical Analytical conditions conditions were 20 kV accelerating accelerating voltage, voltage, 20 20 nA nA probe probe current, current, andand 3 or 3 or 0 µ 0m μm probe probe diameter diameter for for quantitative quantitative spotspot analysis (Figure (Figure 5,5, Table Table 11).). A 20 kV kV accelerating accelerating voltage and a 100 nA probe current with a beam diameter of <1 μm were used for elemental voltage and a 100 nA probe current with a beam diameter of <1 µm were used for elemental distribution distribution maps (Figure 6). Natural and synthetic minerals were used as standards, and the ZAF maps (Figure6). Natural and synthetic minerals were used as standards, and the ZAF matrix correction matrix correction algorithm was used during data reduction. Ferrous and ferric iron contents of algorithm was used during data reduction. Ferrous and ferric iron contents of chromian spinel were chromian spinel were calculated assuming spinel stoichiometry, while all iron was assumed to be calculated assuming spinel stoichiometry, while all iron was assumed to be Fe2+ in silicates. Mg# is Fe2+ in silicates. Mg# is Mg/(Mg + Fe2+) atomic ratio, and Cr# is Cr/(Cr + Al) atomic ratio. 2+ Mg/(MgThree + Fe chromitite) atomic samples ratio, and were Cr# analyzed is Cr/(Cr for + all Al) Platinum atomic‐ ratio.group elements (PGEs; Os, Ir, Ru, Rh, Pt and Pd) (Figure 2c; Table 2). The whole‐rock PGE contents in chromitite samples were determined by using ICP‐MS after Ni‐sulfide fire assay collection at the Genalysis Perth Laboratory Services, Australia. The detection limits are 1 ppb for all PGEs.

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FigureFigure 3. 3. Photomicrographs of ofchromitites chromitites and surrounding and surrounding garnet‐bearing garnet-bearing wehrlite from wehrlite the Higashi from‐ the Higashi-akaishiakaishi ultramafic ultramafic complex. Plane complex.‐polarized Plane-polarized light (a–f), backscattered light (a–f ),electron backscattered (g–h) and electroncrossed‐polarized (g–h) and crossed-polarizedlight (i) images. light(a) Banded (i) images. chromitite (a) Banded (HC123) chromitite from the (HC123) Gongen from‐goe the(sampling Gongen-goe site in (sampling Figure 1b). site in FigureNote that1b). spinels Note that are spinels strongly are deformed. strongly deformed. (b) Massive ( b )chromitite Massive chromitite (HA‐1) from (HA-1) western from part western of the part of theHigashi Higashi-akaishi‐akaishi ultramafic ultramafic complex complex (sampling (sampling site 2 site in Figure 2 in Figure 1b). 1Noteb). Note the olivine the olivine-filled‐filled fractures fractures in spinel. (c) Close‐up image of a fine‐grained part in banded chromitite (HC‐123). (d) Close‐up image in spinel. (c) Close-up image of a fine-grained part in banded chromitite (HC-123). (d) Close-up image of a coarse spinel grain in banded chromitite (HC‐123). Note that the only core part contains numerous of a coarse spinel grain in banded chromitite (HC-123). Note that the only core part contains numerous needle‐ and blade‐like silicate lamellae. (e) Spinel of the massive chromitite (HA‐1) containing silicate needle- and blade-like silicate lamellae. (e) Spinel of the massive chromitite (HA-1) containing silicate lamellae (yellow arrow). Several silicate lamellae show parallelism. (f) Close‐up image of a blade‐like lamellae (yellow arrow). Several silicate lamellae show parallelism. (f) Close-up image of a blade-like silicate lamella. (g) Back‐scattered electron image of primary silicate mineral inclusions (pyroxenes silicate lamella. (g) Back-scattered electron image of primary silicate mineral inclusions (pyroxenes and amphibole) (white arrows) and silicate exsolution. (h) Back‐scattered electron image of a primary and amphibole) (white arrows) and silicate exsolution. (h) Back-scattered electron image of a primary inclusion composed of diopside + K‐phlogopite + pentlandite in spinel. (i) Surrounding garnet‐bearing inclusion composed of diopside + K-phlogopite + pentlandite in spinel. (i) Surrounding garnet-bearing wehrlite from the Gongen‐goe mountain path. wehrlite from the Gongen-goe mountain path.

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Figure 4. A typical Raman spectrum of diopside exsolution lamella in chromite from massive chromitite. Figure 4. A typical Raman spectrum of diopside exsolution lamella in chromite from massive chromitite. Effect from host chromite was removed from the original spectrum by using LabRam Version 5 software Effect from host chromite was removed from the original spectrum by using LabRam ver.5 software (HORIBA) at Kanazawa University. (HORIBA) at Kanazawa University. Table 1. Selected microprobe analyses of spinel and associated silicate minerals from the HA chromitites.

Texture Massive Banded Mineral Chr. Olv. Chl. Dio. In Chl. In Chr. Olv. Dio. In Par. In Phl. In Sample No. 01a-1 01a-6 02-06 02-10 02-05 123-15 123-19 123-23 123-14 123-22

SiO2 - 42.41 34.88 55.70 34.11 - 42.96 56.11 45.88 41.90 TiO2 0.12 - - 0.02 - 0.24 - 0.06 0.22 0.08 Al2O3 9.08 - 9.33 0.48 12.99 12.10 - 0.56 11.75 14.38 Cr2O3 59.95 - 6.62 1.63 3.64 57.04 - 0.94 2.93 1.38 FeO* 22.28 5.42 2.17 1.24 1.90 20.55 4.55 1.32 2.37 1.33 MnO 0.41 0.08 0.03 0.05 0.02 0.38 0.06 0.05 0.03 - MgO 9.23 54.02 33.42 17.99 34.48 11.75 54.17 18.10 19.95 27.56 CaO - - - 24.39 - - 0.01 24.29 12.13 0.10 Na2O - - - 0.33 - - - 0.35 3.25 0.45 K2O - - - 0.01 - - - 0.01 0.92 8.89 NiO 0.03 0.37 0.21 0.03 0.22 0.06 0.35 0.04 0.09 0.20 Total 101.09 102.3 86.68 101.87 87.38 102.12 102.10 101.83 99.51 96.25 Mg# 0.455 0.947 0.965 0.963 0.970 0.555 0.955 0.961 0.938 0.974 Cr# 0.816 0.760 YCr 0.787 0.721 YAl 0.178 0.228 YFe3+ 0.035 0.051 Abbreviations are as follows: Chr., chromite; Olv., olivine; Chl., chlorite; Dio. In, diopside inclusion in chromite; Chl. In, chlorite inclusion; Par. In, pargasite inclusion; Phl. In, phlogopite inclusion. FeO*, total iron as FeO; Mg#, 2+ 3+ Mg/(Mg+Fe ) atomic ratio; Cr#, Cr/(Al + Cr) atomic ratio; YCr, Cr/(Al + Cr + Fe ) atomic ratio; YAl, Al/(Al + Cr 3+ 3+ 3+ + Fe ) atomic ratio; YFe3+, Fe /(Al + Cr + Fe ) atomic ratio. Horizontal line (-) means below the detection limits.

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Figure 5. Chemical characteristics of chromian spinel from the Higashi akaishi chromitites and Figure 5. Chemical characteristics of chromian spinel from the Higashi akaishi chromitites and ultrahigh-pressure chromitites from Tibet and the Polar Urals. (a) Trivalent cation (Cr, Al, and Fe3+) ultrahigh‐pressure chromitites from Tibet and the Polar Urals. (a) Trivalent cation (Cr, Al, and Fe3+) diagrams.Figure (b) Relationships 5. Chemical characteristics between Mg# of (Mg/(Mgchromian spinel + Fe 2+from) atomic the Higashi ratio) akaishi and Cr# chromitites (Cr/(Cr and + Al) atomic diagrams.ultrahigh (b) Relationships‐pressure chromitites between from Mg# Tibet (Mg/(Mg and the Polar + Fe 2+Urals.) atomic (a) Trivalent ratio) and cation Cr# (Cr,(Cr/(Cr Al, and + Al) Fe 3+atomic) ratio).ratio). (c) diagrams.( Relationshipsc) Relationships (b) Relationships between betweenCr# Cr# and and Mg#Ti content Ti(Mg/(Mg content in spinel.+ Fe in2+) atomic spinel.Note that ratio) Note rim and of Cr# that spinel (Cr/(Cr rim in banded of+ Al) spinel atomic chromitite in banded 3+ 3+ chromititeis richratio). isin richTi. (c ()d inRelationships) Relationships Ti. (d) Relationships between between Cr# and TiO between Ti content2 content TiOin spinel. and2 content Fe Note3+/(Cr that and + rim Al Fe of+ spinelFe/(Cr3+) atomicin +banded Al ratio + chromitite Fe for) atomicthe HA ratio for thechromitites. HAis chromitites. rich in Fields Ti. (d) forRelationships Fields MOR for plutonics MOR between plutonics TiOand2 contenthot and‐spot and hot-spot plutonics Fe3+/(Cr plutonics + Alare + Fequoted3+ are) atomic quoted from ratio Arai fromfor the et Arai HAal. [23]. et al. [23]. CompositionalCompositionalchromitites. ﬁelds fields Fields of of concordant for concordant MOR plutonics (CCh) (CCh) and andand hot discordant‐spot plutonics (DCh) (DCh) are chromitites quoted chromitites from from Arai from Northern et Northernal. [23]. Oman Oman Compositional fields of concordant (CCh) and discordant (DCh) chromitites from Northern Oman ophioliteophiolite are plottedare plotted in (ina– (ca)–c for) for comparison comparison [[24].24]. Note that that spinels spinels from from the the HA HA chromitites chromitites show show a a ophiolite are plotted in (a–c) for comparison [24]. Note that spinels from the HA chromitites show a sub‐arc trend. sub-arc trend.sub‐arc trend.

Figure 6. Backscattered electron image and elemental distribution map of a diopside lamellae‐bearing Figure 6. Backscattered electron image and elemental distribution map of a diopside lamellae-bearing spinel grain (center) in a banded chromitite sample (HC123 from sampling site 1 in Figure 1). Rim of spinelFigure grain 6. (center) Backscattered in a banded electron chromitite image and elemental sample (HC123 distribution from map sampling of a diopside site 1 lamellae in Figure‐bearing1). Rim of

the spinelspinel grain grain shows(center) high in a banded total Fe chromitite and slightly sample low (HC123 Al contents from sampling relative tosite the 1 in core Figure of spinel.1). Rim (ofa) BSE image. (b) Fe distribution map. (c) Al distribution map. Al-rich mineral surrounding the spinel grain is chlorite. Minerals 2018, 8, 525 7 of 18

Three chromitite samples were analyzed for all Platinum-group elements (PGEs; Os, Ir, Ru, Rh, Pt and Pd) (Figure2c; Table2). The whole-rock PGE contents in chromitite samples were determined by using ICP-MS after Ni-sulﬁde ﬁre assay collection at the Genalysis Perth Laboratory Services, Australia. The detection limits are 1 ppb for all PGEs.

Table 2. Whole-rock analyses of bulk-rock platinum group element compositions (ppb) in the HA chromitites.

Sample Os Ir Ru Rh Pt Pd Total (ppb) Pd/Ir Ru/Pt HA-1 28 29 100 11 3 3 174 0.10 33.33 HA-2 9 17 67 8 8 5 114 0.29 8.38 HA-3 23 45 115 19 48 104 354 2.31 2.40

4. Results

4.1. Petrography of the Higashi-Akaishi Chromitites Chromitites comprise euhedral to subhedral chromian spinel, olivine, serpentine, chlorite with minor amounts of amphibole (Figure3). Some chromitite samples show a severe hydrothermal alteration, and typically contain kämmereite, chlorite, serpentine and a small amount of phlogopite, uvarovite and ehimeite, which is a Cr-dominant member of Ca amphibole [25]. Chromian spinels in the chromitites are reddish to dark brown in thin section, indicating their Cr-rich character (Figure3). Spinels in the banded chromitites are highly deformed and fractured (Figure3a). Some of spinels in the chromitites and associated dunite have thin cracks ﬁlled by olivine, suggesting brittle and ductile behaviors of spinel and olivine, respectively, at the high-T deformation stage (Figure3b). Grain boundaries of olivines in dunite surrounding the banded chromitites show a triple junction, suggesting its textural equilibrium via recrystallization. The core of coarse spinel grains from the massive and banded chromitites typically contains numerous needle- and rod-shaped silicate lamellae, showing parallelism (Figure3d–f). The silicate lamellae are especially prominent in the banded chromitites. The amount and frequency of lamellae decreases towards the marginal part of the grain (Figure3d). This is basically similar to the distribution of clinopyroxene exsolution lamellae in orthopyroxene grains commonly observed in peridotites, which experienced cooling and decompression (Figure7). Surrounding ﬁne spinel grains do not contain silicate lamellae (Figure3c). Minute (several tens microns) orbicular inclusions composed of clinopyroxene, amphibole, olivine, chlorite, and K-phlogopite are found from some of spinel grains in the HA chromitites (Figure3g). They usually show sparse non-linear distributions in thin section indicating a primary origin. Solid-phase secondary inclusions are mostly composed of chlorite and antigorite. Platinum-group element minerals (PGM) have not been found from the HA chromitites. Dunites comprise mosaic olivine, euhedral to subhedral spinel, antigorite and a small amount of clinopyroxene. Some of chromian spinels show strong deformation textures such as fracturing. The peridotites adjacent to the banded chromitite bodies typically contain clinopyroxene, garnet, and subhedral to anhedral spinel (Figure3i). Minerals 2018, 8, x FOR PEER REVIEW 8 of 18 do not contain silicate lamellae (Figure 3c). Minute (several tens microns) orbicular inclusions composed of clinopyroxene, amphibole, olivine, chlorite, and K‐phlogopite are found from some of spinel grains in the HA chromitites (Figure 3g). They usually show sparse non‐linear distributions in thin section indicating a primary origin. Solid‐phase secondary inclusions are mostly composed of chlorite and antigorite. Platinum‐group element minerals (PGM) have not been found from the HA chromitites. Dunites comprise mosaic olivine, euhedral to subhedral spinel, antigorite and a small amount of clinopyroxene. Some of chromian spinels show strong deformation textures such as fracturing. The peridotitesMinerals 2018 adjacent, 8, 525 to the banded chromitite bodies typically contain clinopyroxene, garnet,8 of 18 and subhedral to anhedral spinel (Figure 3i).

FigureFigure 7. Photomicrographs 7. Photomicrographs showing showing a typical a typical exsolution exsolution texture texture in orthopyroxene porphyroclast porphyroclast in mantlein mantleharzburgite harzburgite (Wadi (WadiRajmi area, Rajmi northern area, northern Oman Omanophiolite) ophiolite) due to due subsolidus to subsolidus cooling. cooling. (a) Plane‐ polarized(a) Plane-polarized image; (b) image; Crossed (b)‐ Crossed-polarizedpolarized image. image. Note Note that that clinopyroxene clinopyroxene lamellaelamellae decrease decrease in in abundanceabundance towards towards the the marginal marginal part part in in the the orthopyroxene grain grain (white (white dotted dotted line). line). Compare Compare with with FigureFigure 3d. 3d. 4.2. Raman Spectroscopic Features of Inclusions 4.2. Raman Spectroscopic Features of Inclusions Raman spectroscopy revealed that the exsolution lamellae within chromites from massive and bandedRaman HA spectroscopy chromitites arerevealed mainly that diopside the exsolution (Figure4). A lamellae typical exsolution within chromites lamella (Figure from4 )massive shows and bandedRaman HA spectroscopic chromitites featuresare mainly of diopside diopside [25 (Figure]. The minute 4). A typical orbicular exsolution silicate inclusions lamella (Figure(Figure3g) 4) in shows Ramanspinels spectroscopic from chromitites features are composedof diopside of clinopyroxene,[25]. The minute amphibole, orbicular olivine, silicate chlorite, inclusions and phlogopite (Figure 3g) in spinelsas described from chromitites above. Microprobe are composed analyses of clinopyroxene, suggest that the amphiboleamphibole, and olivine, mica phases chlorite, in the and orbicular phlogopite as describedinclusions above. are pargasite Microprobe and K-phlogopite, analyses suggest respectively, that the as amphibole described below and mica (Table phases1). in the orbicular inclusions4.3. Mineral are pargasite Chemistry and K‐phlogopite, respectively, as described below (Table 1).

4.3. MineralCore Chemistry of chromian spinels from the HA chromitites shows high Cr#s, 0.65 to 0.82, and low TiO2 contents, 0.1 to 0.5 (Figure5). Chromian spinels are slightly higher in Cr# in the massive HA chromitites thanCore in of the chromian banded HAspinels chromitites from the (Figure HA chromitites5a). The Cr# shows of spinel high ranges Cr#s, from 0.65 0.72 to 0.82, to 0.82 and for low the TiO2 contents,massive 0.1 chromititeto 0.5 (Figure pods, 5). andChromian 0.65 to 0.78spinels in the are banded slightly chromitite higher in podsCr# in (Figure the massive5b). Spinels HA chromitites show thanzonation in the banded in terms HA of Fe chromitites3+, Al and Ti (Figure [26] in the 5a). banded The Cr# chromitites of spinel (Figures ranges5 andfrom6), 0.72 where to the 0.82 rim for the 3+ massiveof spinel chromitite grains contrastingly pods, and 0.65 shows to a0.78 relatively in the high banded Fe content,chromitite suggesting pods (Figure chemical 5b). modiﬁcation Spinels show zonationof spinel in terms in low-T of Fe alteration3+, Al and [27 Ti] (Figure [26] in5 thea,c). banded Core of chromitites the coarse spinel (Figures grains 5 and containing 6), where diopside the rim of lamellae (Figure3d) shows a low Fe (total) content and a slightly high Al content as compared with spinel grains contrastingly shows a relatively high Fe3+ content, suggesting chemical modification of the rim (Figure6). Surrounding ﬁne spinel grains free of diopside lamellae (Figure3c) show higher Fe spinel in low‐T alteration [27] (Figure 5a,c). Core of the coarse spinel grains containing diopside lamellae (total) and lower Al contents. (Figure 3d)Olivines shows show a low slightly Fe (total) higher content Fo (=100 and Mg#) a values,slightly 94 high to 96, Al in content the massive as compared HA chromitites with than the rim (Figure 6). Surrounding fine spinel grains free of diopside lamellae (Figure 3c) show higher Fe (total) in the banded ones (Fo92–95) (Figure8), described by Hattori et al. [ 17]. The NiO content of olivines and varieslower fromAl contents. 0.30 to 0.42 wt % in the HA chromitites, being clearly lower than in ordinary podiform chromititesOlivines show from slightly ophiolites higher and almost Fo (=100 equivalent Mg#) values, to the value 94 to in 96, residual in the mantle massive peridotites HA chromitites (Figure8). than in the banded ones (Fo92–95) (Figure 8), described by Hattori et al. [17]. The NiO content of olivines varies from 0.30 to 0.42 wt % in the HA chromitites, being clearly lower than in ordinary podiform chromitites from ophiolites and almost equivalent to the value in residual mantle peridotites (Figure 8).

Minerals 2018Minerals, 8, 5252018, 8, x FOR PEER REVIEW 9 of 18 9 of 18

Minerals 2018, 8, x FOR PEER REVIEW 9 of 18

Figure 8. RelationshipsFigure 8. Relationships between between Fo mol Fo mol% % and and NiO NiO content content of olivines from of olivines the Higashi akaishi from chromitites. the Higashi akaishi Chemical ranges for chromitites from Oman, Tibet, and the Polar Urals, are from Arai and Miura [28]. chromitites. ChemicalCompositional ranges range for of Olivine chromitites Mantle Array from is from Oman, Takahashi Tibet, et al. and [29]. theNote Polarthat olivines Urals, in the are from Arai

and Miura [28].HA Compositional chromitites show lower range Ni contents of Olivine than in Mantle podiform chromitites Array is from from Oman Takahashi ophiolite. On et the al. [29]. Note Figure 8. Relationshipsother hand, olivines between in UHP Fo mol% chromitites and NiO from content Tibet and of the olivines Polar Urals from show the Higashiextraordinary akaishi higher chromitites. Ni that olivinesChemical in therangesand HA Mg for contents chromitites chromitites than in thefrom show HA Oman, chromitites. lower Tibet, Ni contentsand the Polar than Urals, in podiform are from Arai chromitites and Miura from[28]. Oman ophiolite.Compositional On the other range hand,of Olivine olivines Mantle in Array UHP is from chromitites Takahashi from et al. [29]. Tibet Note and that the olivines Polar in Urals the show 4.4. Platinum‐Group Element Chemistry extraordinaryHA chromitites higher show Ni and lower Mg Ni contents contents than than in podiform the HA chromitites.chromitites from Oman ophiolite. On the other hand,Massive olivines chromitite in UHP chromitites samples (HA from‐1, 2, Tibet and 3and in Table the Polar 2) are Urals high showin total extraordinary PGE, 174, 114 higherand 354 Ni ppb, respectively (Figure 9a; Table 2). Two chromitite samples (HA‐1 and 2) are distinctly enriched 4.4. Platinum-Groupand Mg contents Element than Chemistry in the HA chromitites. in Ir subgroup PGE (IPGE: Os, Ir, Ru) in preference to Pd subgroup PGE (PPGE: Rh, Pt, Pd) and show Massive chromititelow ratios samplesof Pd/Ir (around (HA-1, 0.1 to 2, 0.3) and (Figure 3 in 9b; Table Table 22).) areThey high show negative in total slopes PGE, from 174, Ru 114to Pt and 354 ppb, 4.4. Platinumin‐ theirGroup chondrite Element‐normalized Chemistry PGE patterns (Figure 9b) as in typical ophiolitic chromitites [30]. Their respectively (Figureenrichment9a; Table in Ir and2). Ru Two is similar chromitite to that of samplesthe Oman arc (HA-1‐related discordant and 2) are chromitites distinctly [30]. One enriched in Ir Massive chromitite samples (HA‐1, 2, and 3 in Table 2) are high in total PGE, 174, 114 and 354 subgroup PGE (IPGE:chromitite Os, sample Ir, (HA Ru)‐3) in showing preference the features to of Pd some subgroup high‐degree PGEmodification (PPGE: at low Rh, T, e.g., Pt, Fe Pd)3+ and show ppb, respectivelyenrichment (Figure and formation 9a; Table of chlorite2). Two is chromititeclearly enriched samples in PPGE (HA (Figure‐1 and 9b). 2) are distinctly enriched low ratiosin Ir of subgroup Pd/Ir PGE (around (IPGE: 0.1 Os, to Ir, 0.3) Ru) in (Figure preference9b; Tableto Pd subgroup2). They PGE show (PPGE: negative Rh, Pt, slopesPd) and fromshow Ru to Pt in theirlow chondrite-normalizedratios of Pd/Ir (around 0.1 to PGE 0.3) patterns(Figure 9b; (Figure Table 2).9 b)They as show in typical negative ophiolitic slopes from chromitites Ru to Pt [ 30]. Their enrichmentin their chondrite in Ir‐normalized and Ru is PGE similar patterns to that (Figure of the 9b) Omanas in typical arc-related ophiolitic discordant chromitites [30]. chromitites Their [30]. One chromititeenrichment sample in Ir and (HA-3) Ru is similar showing to that the of features the Oman of arc some‐related high-degree discordant modiﬁcation chromitites [30]. at One low T, e.g., chromitite sample (HA‐3) showing the features of some high‐degree modification at low T, e.g., Fe3+ Fe3+ enrichment and formation of chlorite is clearly enriched in PPGE (Figure9b). enrichment and formation of chlorite is clearly enriched in PPGE (Figure 9b).

Figure 9. Bulk-rock PGE characteristics of massive chromiitte samples from the Higashi akaishi chromitite samples. Fields of chromitites from Oman were quoted from Ahmed and Arai [30] and Miura and Arai (unpublished data). Compositional ﬁelds of UHP chromitites from Tibet and the Polar Urals are shown for comparison [24]. (a) Relationships between the total bulk PGE content in chromitites and Cr# of their spinels. (b) PGE patterns of the Higashi-akaishi massive chromitites. One sample enriched in Pt and Pd is strongly hydrated at low-T conditions. Minerals 2018, 8, 525 10 of 18

5. Discussion

5.1. P-T History of the Higashi-Akaishi Chromitite The HA complex shows a counterclockwise P-T trajectory, which was caused by subduction and exhumation, are divided into four deformation stages, D1,D2,D3, and D4 [31] in terms of microstructure of minerals. Petrological and geochemical studies suggest that the HA peridotite complex originated as plutonics in an uppermost part of the mantle wedge [14,17,31,32]. After the igneous formation, the proto-HA peridotite complex subsided via the subduction channel by an active mantle flow (D1 and D2 stage) [14,15]. The coexistence of forsterite-rich olivine and pyrope in ultramafics indicates that the HA body underwent high-P conditions (>1.8 GPa) [19,33]. Enami et al. [14] examined the olivine-pyroxenes-garnet assemblage in garnet clinopyroxenite, websterite, and wehrlite from the Gongen-goe area (Figure1), and obtained a series of P-T conditions of the prograde metamorphism conducted by the subduction. They used Al solubility in orthopyroxene in equilibrium with garnet in the HA garnet peridotite, and delineated a P-T trajectory with a high P/T gradient (>3.1 GPa/100 ◦C) from 1.5–2.4 GPa at 700–800 ◦C to 2.9–3.8 GPa at 700–810 ◦C (Figure 10). Such P-T conditions are consistent with features of crystallographic preferred orientations (CPO) of olivines [16]. Mizukami et al. [16] reported B-type CPO in olivines [34] from HA peridotites. This strongly suggests that the olivines were deformed at hydrous and high-stress conditions such as in the supra-subduction zone mantle during the prograde metamorphism (D2 stage). The coexistence of olivine and antigorite in strongly foliated peridotites in the marginal part of the HA complex provides ◦ us with a possible uplift path (D3) from the peak of prograde metamorphism, i.e., 3.8 GPa/700 C to ◦ around 1.0 GPa/500 C[31] (Figure 10). After the D3 stage, the HA complex followed an exhumation path (D4) similar to the clockwise P-T trajectory of the surrounding Besshi unit, which mainly consists of pelitic and basic schists, and that path (D4) is related to the regional exhumation of the Sanbagawa belt (Figure1). Then, retrograde amphiboles in garnet peridotite were probably formed at around 1.0 GPa and 600–700 ◦C[14,31] (Figure9). In short, the HA complex was originally formed at an uppermost part of a mantle wedge and subsequently experienced UHP conditions, up to 3.8 GPa, before exhumation to the surface: it is of recycling origin along the boundary of the mantle wedge (Figure 10). The HA chromitites are coherent to the surrounding peridotites (Figure2) and they altogether share the same P-T trajectory (Figure 10). The chemical characteristics of the HA chromitites are consistent with this recycling model. Spinels from the HA peridotites show high Cr#s (>0.6), suggesting that the arc-related magma formed by high degree of partial melting was involved in formation of the HA ultramafic complex [17,32]. The spinel and PGE chemistries of the HA chromitites are also consistent with this interpretation (Figures5 and9). The core of chromian spinels in the HA chromitites shows high Cr#s (0.65 to 0.85) and low TiO2 contents (<0.5 wt %) (Figure5), suggesting the arc-magma parentage [31]. The HA chromitite spinels show the depletion in TiO2 content at a given Fe3+/(Cr + Al + Fe3+) atomic ratio of spinels (Figure5c), which is characteristic of plutonics of a sub-arc origin [23] (Figure5d). The enrichment of IPGE, such as Ir and Ru, and the negative slope from Ru to Pd in the PGE patterns (Figure9b) suggest that the magma responsible for the formation of the HA chromitites had a highly depleted character [30]. The HA chromitites are also similar in the spinel chemistry and PGE chemistry to arc-related discordant chromitites from northern Oman ophiolite (Figures5 and9). The Fo and NiO contents of olivine in chromitites have been enhanced through subsolidus re-equilibration with chromian spinel [35,36]. Original igneous olivines in the HA chromitites were lower in Fo and Ni contents than the current values. The HA chromitite olivines (Figure8) indicate an evolved nature of the involved magma, a relationship which possibly means a strong action of olivine fractionation during the HA chromitite formation. Minerals 2018, 8, x FOR PEER REVIEW 11 of 18

involved magma, a relationship which possibly means a strong action of olivine fractionation during the HA chromitite formation. The presence of possible primary inclusions of pargasite in spinels (Figure 3g) indicates that the HA chromitites were formed within the stability field of pargasite, i.e., at a relatively low‐P condition (<3 GPa) (Figure 10). The inclusion pargasite is significantly different in the mode of occurrence and chemistry from the ehimeite, a Cr‐dominant Ca amphibole, formed by reaction between chromitite and the metamorphic fluid at a retrograde stage of the Sanbagawa metamorphism during the exhumation stage [22]. The pargasitic amphibole in spinel may have been unstable at the peak metamorphic condition that the HA complex has experienced (>3 GPa, 700–800 °C to <3.8 GPa, 500–700 °C) (Figure 10), based on experimental data so far obtained [37–41]. Recent experimental study of alkali amphiboles under hydrous conditions, however, indicates that some alkali amphiboles are potentially stable under such a P‐T condition [42] (Figure 10). In summary, the HA chromitites have probably experienced a UHP Mineralscondition,2018, 8, 525 within the stability field of coesite but slightly lower than the diamond stability field, although11 of 18 no UHP minerals have ever been found.

FigureFigure 10. P-T 10. diagram P‐T diagram for for the the stability stability fields fields of of minerals minerals and facies facies transitions transitions of ofperidotite. peridotite. The TheP‐T P-T trajectorytrajectory of the of Higashi-akaishi the Higashi‐akaishi peridotite peridotite body body isis fromfrom Mizukami Mizukami and and Wallis Wallis [27] [27 and] and Hattori Hattori [28]. [28]. Geotherms,Geotherms, diamond-graphite diamond‐graphite and and coesite-quartz coesite‐quartz transition transition lines lines are are from from Green Green and and Ringwood Ringwood [43], [43], Naemura et al. [44] and Mirward and Massone [45], respectively. The pargasite stability limit was Naemura et al. [44] and Mirward and Massone [45], respectively. The pargasite stability limit was compiled from Foley [37], Fumagalli and Poli [39], Fumagalli et al. [40], Niida and Green [38], and compiled from Foley [37], Fumagalli and Poli [39], Fumagalli et al. [40], Niida and Green [38], Frost [41], and the alkali amphibole stability limit is from Pirard and Hermann [42]. and Frost [41], and the alkali amphibole stability limit is from Pirard and Hermann [42]. Presence of the diopside lamellae in spinel from the HA chromitites (Figures 3 and 4) suggests Thesome presence long duration of possible of subsolidus primary cooling inclusions and/or decompression of pargasite inof the spinels chromitite (Figure [4,9].3g) The indicates exsolved that the HAsilicate chromitites lamellae in were the HA formed chromitites within (Figures the 3 stability and 5) are field possibly of pargasite, originated from i.e., silicate at a relatively components low-P conditionin the (<3 primary GPa) magmatic (Figure chromian10). The spinel; inclusion natural pargasite chromitite is spinels significantly equilibrated different at low temperatures in the mode of occurrenceare very and poor chemistry in Si and Ca from [1]. The the diopside ehimeite, lamellae a Cr-dominant are dominant Ca in amphibole,the central part formed of coarse by spinel reaction betweengrains, chromitite which is and relatively the metamorphic low in Fe3+ and fluid high at in a Al retrograde contents, stageand decrease of the Sanbagawa sharply in abundance metamorphism to duringtheir the marginal exhumation part in stage banded [22 chromitites]. The pargasitic (thin chromitite amphibole bands) in (Figures spinel 3d may and have 7). Such been distribution unstable of at the peak metamorphicsilicate lamellae condition is possibly that due the to high HA mobility complex of has the experiencedcomponents, which (>3 GPa, have 700–800 been diffused◦C to <3.8away GPa, outside the spinel grains to join the silicate matrix during cooling. This is supported by the similarity 500–700 ◦C) (Figure 10), based on experimental data so far obtained [37–41]. Recent experimental with the distribution pattern of diopsidic clinopyroxene exsolution lamellae in cooled mantle study of alkali amphiboles under hydrous conditions, however, indicates that some alkali amphiboles orthopyroxene porphyroclasts in peridotites (Figure 7), where the high‐T diopsidic component was are potentiallyexsolved as stable lamellae under in the such central a P-T part condition and diffused [42 out] (Figure from the 10 marginal). In summary, part of the the orthopyroxene HA chromitites have probablygrain (Figure experienced 7). a UHP condition, within the stability field of coesite but slightly lower than the diamond stability field, although no UHP minerals have ever been found. Presence of the diopside lamellae in spinel from the HA chromitites (Figures3 and4) suggests some long duration of subsolidus cooling and/or decompression of the chromitite [4,9]. The exsolved silicate lamellae in the HA chromitites (Figures3 and5) are possibly originated from silicate components in the primary magmatic chromian spinel; natural chromitite spinels equilibrated at low temperatures are very poor in Si and Ca [1]. The diopside lamellae are dominant in the central part of coarse spinel grains, which is relatively low in Fe3+ and high in Al contents, and decrease sharply in abundance to their marginal part in banded chromitites (thin chromitite bands) (Figures3 and7) . Such distribution of silicate lamellae is possibly due to high mobility of the components, which have been diffused away outside the spinel grains to join the silicate matrix during cooling. This is supported by the similarity with the distribution pattern of diopsidic clinopyroxene exsolution lamellae in cooled mantle orthopyroxene porphyroclasts in peridotites (Figure7), where the high-T diopsidic component was exsolved as lamellae in the central part and diffused out from the marginal part of the orthopyroxene grain (Figure7). The HA chromitites are different in geological and petrographic characteristics from typical podiform chromitites from ophiolites, such as those from the Oman ophiolite [4,30]. Mantle harzburgites, which usually host podiform chromitites with a dunite envelope [1], are completely missing in the HA Minerals 2018, 8, 525 12 of 18

peridotiteMinerals complex. 2018, 8, Thex FOR PEER thick REVIEW dunite-wherlite-clinopyroxenite suite hosting chromitites12 of 18 in the HA complex is equivalent to a cumulus mantle [17,32] or a kind of Moho transition zone (MTZ) in the The HA chromitites are different in geological and petrographic characteristics from typical sub-arc mantle [46–49]. We interpret that the dunite-dominant HA complex itself is equivalent to the podiform chromitites from ophiolites, such as those from the Oman ophiolite [4,30]. Mantle harzburgites, dunite envelopewhich usually of podiform host podiform chromitites, chromitites as with discussed a dunite envelope by Arai [1], and are completely Abe [48] missing on sub-arc in the podiform chromititesHA beneath peridotite the complex. Southwest The thick Japan dunite arc.‐wherlite‐clinopyroxenite suite hosting chromitites in the HA complex is equivalent to a cumulus mantle [17,32] or a kind of Moho transition zone (MTZ) in 5.2. Comparisonthe sub with‐arc mantle the UHP [46–49]. Chromitites We interpret from that Tibet the dunite and‐ thedominant Polar HA Urals complex itself is equivalent to the dunite envelope of podiform chromitites, as discussed by Arai and Abe [48] on sub‐arc podiform The HAchromitites chromitites beneath are the similar Southwest in petrographicJapan arc. feature to UHP chromitites, except for the absence of the peculiar UHP minerals, from Tibet and the Polar Urals. The Lubusa ophiolite, one of the 5.2. Comparison with the UHP Chromitites from Tibet and the Polar Urals ophiolites that contain UHP chromitites, on the east-end of the Indus-Yarlung-Zangbo suture zone, Tibet (Figure 11Thea,b) HA has chromitites been interpreted are similar in to petrographic have originated feature to at UHP a mid-ocean chromitites, except ridge for between the absence the of Indian and the peculiar UHP minerals, from Tibet and the Polar Urals. The Lubusa ophiolite, one of the ophiolites the Asian continentsthat contain [UHP50]. Thischromitites, ophiolite on the was east obducted‐end of the as Indus a slice‐Yarlung of the‐Zangbo Neo-Tethyan suture zone, oceanic Tibet lithosphere due to collision(Figure of11a,b) the has two been continents interpreted into have the Earlyoriginated Paleogene at a mid‐ocean (around ridge 65between Ma) the [51 Indian], and and experienced later modificationthe Asian continents at a subduction [50]. This ophiolite zone environment was obducted as before a slice of its the obduction Neo‐Tethyan [ 50oceanic]. The lithosphere Ray-Iz ophiolite, another exampledue to collision of the UHPof the chromititetwo continents occurrence in the Early [ 7Paleogene], is located (around in the 65 Ma) Polar [51], Ural and region experienced (Figure 11c–e), later modification at a subduction zone environment before its obduction [50]. The Ray‐Iz ophiolite, and was emplacedanother example by collisionof the UHP ofchromitite the Magnitogosrk occurrence [7], is arclocated and in the the Polar East Ural Europian region (Figure continent 11c–e), block in the mid-Paleozoicand was emplaced around by 400 collision Ma of [52 the]. Magnitogosrk Ultramafic arc and and mafic the East rocks Europian in this continent region block reflect in the a series of westward-directedmid‐Paleozoic thrust around stacks 400 inMa which [52]. Ultramafic the oceanic and andmafic arc rocks sequences in this region lie on reflect the continentala series of margin of the Europeanwestward plate‐directed [53]. thrust stacks in which the oceanic and arc sequences lie on the continental margin of the European plate [53].

Figure 11.FigureLocality 11. Locality and geological and geological sketch sketch maps maps ofof the Luobusa Luobusa ophiolite, ophiolite, Tibet, Tibet,and Ray and‐Iz ophiolite, Ray-Iz ophiolite, the Polar Urals. (a) Location of the Luobusa ophiolite. (b) General geological sketch of the Luobusa the Polar Urals. (a) Location of the Luobusa ophiolite. (b) General geological sketch of the Luobusa ophiolite, Tibet. Modified from Zhou et al. [50] and Yamamoto et al. [9]. (c) Location of the Ray‐Iz ophiolite,ophiolite. Tibet. Modiﬁed (d) General from geological Zhou sketch et al. of [50 the] and Ray‐ YamamotoIz ophiolite, the et al.Polar [9 ].Urals. (c) LocationModified from of the Ray-Iz ophiolite.Shmelev (d) General [54] and geological Yang et al. [7]. sketch (e) Geological of the Ray-Iz sketch of ophiolite, the no. 384 the chromitite Polar body Urals. (diamond Modiﬁed‐ from Shmelev [54bearing)] and in Yang the central et al. [area7]. (ofe )the Geological Ray‐Iz massif. sketch Star indicates of the no. sampling 384 chromitite locality. body (diamond-bearing) in the central area of the Ray-Iz massif. Star indicates sampling locality.

As previously mentioned, the spinels from the HA chromitites have numerous diopside lamellae (Figure 3). This feature is shared with the UHP chromitites from Tibet and the Polar Urals [9,24] (Figure 12). Yamamoto et al. [9] and Miura [24] reported silicate lamellae from spinels in the UHP chromitites from Tibet and the Polar Urals, respectively. Such silicate lamellae have been also observed from Mineralsspinels 2018in UHP, 8, 525 metamorphic spinel‐garnet peridotites from the Bohemian Massif, Czech Republic13 [44]. of 18 Additionally, pods of the UHP chromitites from both the Tibetan and the Polar Ural ophiolites are mostly concordant to the foliation of surrounding mantle peridotite. The structural characteristics of Uralthe mantle ophiolites peridotites are mostly suggest concordant that to they the foliation have experienced of surrounding deformation mantle peridotite. possibly The via structural mantle characteristicsconvection flow of [55]. the mantle Arai et peridotites al. [56] observed suggest pull that‐apart they havecracks experienced in chromite deformation grains filled possibly by olivine via mantle(PACO convectiontexture) in the ﬂow Tibetan [55]. Arai UHP et chromitites, al. [56] observed and suggested pull-apart the cracks PACO in texture chromite possibly grains indicates ﬁlled by olivinebrittle deformation (PACO texture) of spinels in the Tibetanand ductile UHP behavior chromitites, of olivine and suggested at a UHP thecondition PACO texture[28]. The possibly PACO indicatestexture was brittle also deformationdescribed from of the spinels Polar and Urals ductile UHP behaviorchromitites of [24]. olivine As atdescribed a UHP above, condition the [HA28]. Thechromitite PACO share texture the was PACO also texture described (Figure from 3b) the with Polar the Urals UHP UHP Tibetan chromitites and Polar [24 ].Ural As chromitites. described above, This thesuggests HA chromitite that they share share the the PACO similar texture geotectonic (Figure3 historyb) with thetoo. UHP In addition, Tibetan andthe Polarspinels Ural from chromitites. both the This suggests that they share the similar geotectonic history too. In addition, the spinels from both the Tibetan and Polar Ural UHP chromitites show high Cr#s (around 0.8 to 0.85) and low TiO2 contents Tibetan(<0.2 wt and %), Polarindicating Ural UHPtheir chromititesarc‐related magmatic show high origin Cr#s (around [24,57] (Figure 0.8 to 0.85) 6). This and lowis consistent TiO2 contents with (<0.2their wtPGE %), chemistries, indicating their which arc-related result favors magmatic the originhighly [24 depleted,57] (Figure magma6). This parentage is consistent for withthe theirUHP PGEchromitites chemistries, (Figure which 9). Again, result favors the UHP the highlychromitites depleted share magma the similar parentage spinel for thechemistry UHP chromitites and PGE (Figurecharacteristics9). Again, with the the UHP HA chromitites chromitites; share arc the‐related similar depleted spinel chemistry magmas and formed PGE characteristicsprotoliths of these with thechromitites. HA chromitites; After the arc-related igneous formation, depleted the magmas HA chromitites formed protoliths as well as the of these other chromitites.UHP chromitites After were the igneouspossibly formation,experienced the deep HA recycling chromitites at assubduction well as the zones. other The UHP deep chromitites subduction were can possibly produce experienced the UHP deepchromitites recycling from at subductionthe sub‐arc zones.low‐P chromitites The deep subduction (Figure 13). can We produce suggest thethat UHP the chromititesHA chromitite from is thepotentially sub-arc a low-P UHP chromitite chromitites although (Figure now 13). UHP We suggest minerals that such the as HA coesite chromitite and diamond is potentially have not a UHPbeen chromititefound (Figure although 10). now UHP minerals such as coesite and diamond have not been found (Figure 10).

Figure 12. Photomicrographs of micro inclusions in chromian spinel from UHP chromitites from the Luobusa (a,,c) and Ray-IzRay‐Iz (b,,d)) ophiolites. (a) Spinel of the chromitite (Cr-11(Cr‐11 body) in the eastern part of the Luobusa ophiolite. Yellow arrows indicateindicate silicatesilicate lamellae.lamellae. ((bb)) Spinel of thethe chromitite (no.(no. 384 body) in the central area area of of the the Ray Ray-Iz‐Iz ophiolite. ophiolite. Yellow Yellow arrows arrows indicate indicate silicate silicate lamellae. lamellae. (c) ( cSpinel) Spinel of ofthe the coesite coesite and and amphibole amphibole bearing bearing chromitite chromitite (LA326) (LA326) in in the the central central part part of of the the Luobusa Luobusa ophiolite. White arrows indicate amphibole inclusions. (d) Orbicular diopside inclusions (white arrows) in spinel in a diamond-bearing UHP chromitite (no. 384). Note that amphiboles are totally absent in primary inclusions. Minerals 2018, 8, x FOR PEER REVIEW 14 of 18

White arrows indicate amphibole inclusions. (d) Orbicular diopside inclusions (white arrows) in spinel Mineralsin2018 a diamond, 8, 525 ‐bearing UHP chromitite (no. 384). Note that amphiboles are totally absent in primary14 of 18 inclusions.

FigureFigure 13.13. Catoons for for genesis genesis of of the the Higashi Higashi-akaishi‐akaishi chromitite chromitite and and inclusion inclusion in inchromian chromian spinel. spinel. (a) (Aa) petrologic A petrologic model model for for the the HA HA peridotite peridotite body body containing containing chromitite. chromitite. The The igneous igneous formation formation of the of theprotolith protolith of HA of HA peridotite peridotite body body at Stage at Stage (a). (a). The The proto proto-HA‐HA peridotite peridotite body body was was dragged dragged down down via viathe the subduction subduction channel channel by by active active mantle mantle flow flow in in a a supra supra-subduction‐subduction zone, and experiencedexperienced UHPUHP metamorphismmetamorphism (Stage(Stage (b)).(b)). After the progradeprograde metamorphism,metamorphism, thethe bodybody experiencedexperienced retrograderetrograde metamorphismmetamorphism due due to to its its uplift uplift (Stage (Stage (c)). (c)). (b) An(b) illustration An illustration showing showing the evolution the evolution of primary of primary silicate inclusionssilicate inclusions and diopside and diopside exsolution exsolution lamellae lamellae in chromian in chromian spinel during spinel subsolidus during subsolidus cooling. cooling. 6. Implications for Origin of UHP Chromitites 6. Implications for Origin of UHP Chromitites The HA chromitites were formed in a thick MTZ composed of a dunite-wehrlite-clinopyroxenite The HA chromitites were formed in a thick MTZ composed of a dunite‐wehrlite‐clinopyroxenite suite by sub-arc magma, which had a highly depleted feature (Figure 13a) as in a sub-arc lithosphere suite by sub‐arc magma, which had a highly depleted feature (Figure 13a) as in a sub‐arc lithosphere model based on xenoliths by Arai et al. [58]. After the igneous and subsequent in situ cooling stage, model based on xenoliths by Arai et al. [58]. After the igneous and subsequent in situ cooling stage, they were transported via subduction channel by active mantle flow within a mantle wedge (a path they were transported via subduction channel by active mantle flow within a mantle wedge (a path from (a) to (b) in Figure 12a). Then, the long duration of subsolidus cooling by subducted slab probably from (a) to (b) in Figure 12a). Then, the long duration of subsolidus cooling by subducted slab probably induced the exsolution of diopside lamellae in chromian spinel (Figure 13b). The chromitites were induced the exsolution of diopside lamellae in chromian spinel (Figure 13b). The chromitites were deformed under UHP conditions (up to 3.8 GPa) during the prograde metamorphism of the HA deformed under UHP conditions (up to 3.8 GPa) during the prograde metamorphism of the HA complex, complex, and then the PACO texture was produced in spinel grains (Figure3b). After the modification and then the PACO texture was produced in spinel grains (Figure 3b). After the modification at UHP at UHP conditions, they were exhumed together with the surrounding Sanbagawa metamorphic rocks conditions, they were exhumed together with the surrounding Sanbagawa metamorphic rocks (Figures (Figures 10 and 13). 10 and 13). Several models for the formation of UHP chromitites were proposed. Ruskov et al. [10] and Several models for the formation of UHP chromitites were proposed. Ruskov et al. [10] and Yang Yang et al. [7] interpreted these UHP chromitites as deep-mantle igneous products to explain the et al. [7] interpreted these UHP chromitites as deep‐mantle igneous products to explain the presence presence of some UHP minerals, such as euhedral diamonds and possible coesite pseudomorphs after of some UHP minerals, such as euhedral diamonds and possible coesite pseudomorphs after stishovite. stishovite. They cannot explain, however, geological and petrological features of the UHP chromitites, which are shared with low-P chromitites. Arai [11,12] instead proposed a deep recycling model, where Minerals 2018, 8, 525 15 of 18 the UHP chromitites can be formed from low-P chromitites by UHP metamorphism (deep recycling) via mantle convection flow. This model explains many features of UHP chromitites. UHP chromitites from both the Tibetan and Polar Ural ophiolites are divided into two types in terms of mineral species as inclusions in spinel [24]. One (e.g., LA326 and LT69 in Figure 10b) is characterized by the coexistence of diopside (and rarely coesite described by Yamamoto et al. [9]) lamellae and primary Na amphibole inclusion in spinel [24] (Figure 11a,c). The other contains diamonds (e.g., Cr-11 body in Figure 10b and No. 384 body in Figure 10e) [7,8], and is free of the primary Na amphibole inclusion (Figure9b,d) [ 24]. Coexistence of coesite and Na amphibole inclusions in spinel in the former indicates a conditions of relatively low temperature (around 700 to 800 ◦C) and high pressure (around 3 GPa) (Figure9), which is possibly available from the mantle wedge. UHP metamorphism in a subduction zone environment is clearly more favorable than the deep-mantle igneous origin beneath mid-ocean ridge [8,10,59] for this type of UHP chromitite (Figure9). This is concordant with a model of Arai [ 11,12] where deep recycling to the base of the upper mantle (<12–16 GPa [60–63]) produces UHP chromitites from low-P igneous ones. Current high-P experimental studies in the systems of MgCr2O4-FeCr2O4 and MgCr2O4-Mg2SiO4 suggested that UHP chromitites were possibly formed at shallower parts of mantle than the mantle transition zone [60–63]. Some petrographic and isotopic characteristics of UHP chromitites and surrounding peridotites from Tibet support the UHP metamorphism model in a subduction zone above [64,65]. The latter diamond-bearing UHP chromitites are possibly of deeper mantle origin than the former amphibole-bearing UHP chromitites because of the absence of primary amphibole inclusion in spinel (Figure9). The HA chromitites are similar to the amphibole-bearing type of UHP chromitites in petrographic and chemical characteristics. The UHP chromitites, at least in part, can be formed by UHP metamorphism from low-P chromitites.

Author Contributions: M.M., S.A., T.M., V.R.S., and S.I. collected samples; M.M. prepared samples and performed the Raman spectroscopic and EPMA analyses; M.M., and S.A. wrote manuscript; M.M. designed the figures and tables. Funding: This study was partly supported by JSPS KAKENHI (grant number 25.8426) and Fukada Grant-in-Aid. Acknowledgments: We are grateful to E. Pushkarev, D. Kuznetsov, and D. Dyuryagina for their assistance during our fieldwork in the Polar Urals, and to S. Ishigami for his assistance in fieldworks in the Higashi-akaishi peridotite body. S. Yamamoto kindly provided us UHP chromitite samples from Tibet. We appreciate S. Umino, T. Morishita, A. Tamura, and Y. Soda for their discussions. Comments from three anonymous reviewers improved our manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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