Lithos 124 (2011) 215–226

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Insight into the uppermost mantle section of a maturing arc: The Eastern Mirditaophiolite, Albania

Tomoaki Morishita a,b,⁎, Yildirim Dilek c, Minella Shallo d, Akihiro Tamura a, Shoji Arai e

a Frontier Science Organization, Kanazawa University, Kanazawa 920-1192, Japanb Department of Geology & Geophysics, University of Hawaii at Manoa, 1680 East-West Rd., Honolulu, HI 96822, USAc Department of Geology, Miami University, Oxford, OH 45056, USAd Fakulteti i Gjeologise dhe Minierave, Universiteti Politeknik, Tirana, Albaniae Department of Earth Sciences, Kanazawa University, Kanazawa 920-1192, Japan

⁎ Corresponding author. Frontier Science OrganizKanazawa 920-1192, Japan. Tel.: +81 76 264 6513; fax

E-mail address: moripta@kenroku.kanazawa-u.ac.jp

0024-4937/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.lithos.2010.10.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 March 2010Accepted 4 October 2010Available online 20 October 2010

Keywords:OphioliteMantle sectionAlbaniaMirditaHarzburgite–dunite–orthopyroxeniteIsland arcMid-ocean ridge

We examined peridotite massifs in the eastern part of theMirdita ophiolite (EMO), Albania, where arc-relatedmagmas are abundant in the upper volcanic sequences. Structurally, clinopyroxene porphyroclast-bearingharzburgites (Cpx-harzburgite hereafter) occur in the lower parts of the peridotite massifs, whereasharzburgites and dunites are more abundant towards the upper parts. Dunite is commonly associated withchromitite layers. Orthopyroxenite occurs as dikes and/or networks at all structural levels, although it is moreabundant in the uppermost sections. Orthopyroxenite commonly crosscuts the foliation of peridotites and thelithological boundaries between dunites (chromitite) and harzburgites, suggesting that it was formed in thelate stage. Major and trace element compositions of minerals in the Cpx-harzburgites indicate that they wereformed as the residue of less-fluxpartialmelting, and are similar to those in abyssal peridotites frommid-oceanridge systems. Harzburgites havemore depletedmajor element compositions than the Cpx-harzburgites. Lightrare earth element (LREE)-enrichment in clinopyroxene coupled with hydrous silicate mineral inclusions inspinels in harzburgites indicate that harzburgites were produced as a result of enhanced partial melting ofdepleted peridotites due to infiltration of hydrous LREE-enriched fluids/melts. Based on olivine and spinelchemistries, dunites are classified into two types: high-Cr# (=Cr/(Cr+Al) atomic ratio) spinel-bearing duniteandmedium-Cr# spinel-bearing dunite. Orthopyroxenites formed at the expense of the pre-existing peridotiteby reaction with hydrous orthopyroxene-saturated melts, which were produced by assimilation of dissolvedpyroxene during the formation of the dunite. Refractory harzburgite, high-Cr# spinel-bearing dunite, andorthopyroxenitemay have a genetic link to the late stage boniniticmagmas in the crustal section of the EMO. Incontrast, the Cpx-harzburgite was a residue related to mid-ocean ridge basalts (MORBs) or the “MORB-like”fore-arc basalt recently proposed by Reagan et al. (2010) from the Izu–Bonin–Mariana fore-arc. The medium-Cr# spinel-bearing dunite can be caused by interaction with a melt transitional between MORB-like andboninitic melts. The lithological variations and their relationships in the uppermantle section of the EMOwerecaused by changes inmagmatic compositions frommid-ocean ridge signatures to boninitic magmas, due to anincreasing contribution of slab-derived fluids in an island arc setting.

ation, Kanazawa University,: +81 76 264 6545.(T. Morishita).

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Ophiolites represent past fragments of oceanic lithosphere nowtectonically emplaced on land. Field relationships combined withgeochemical studies of volcanic sequences in ophiolites are commonlyapplied to reconstruct themagmatic and tectonic history of a part of theoceanic lithosphere (e.g. Dilek et al., 2008). A volcanic sequence in anophiolite sometimes shows varying chemical characteristics, indicatingspatial and/or temporal modification of the magmatic affinity in the

ophiolite (e.g. Pearce et al., 1984). In contrast to the studies of volcanicrocks in the ophiolite, the evolutionary history of the mantle sectionscorresponding to changes in tectonic settings is not yetwell understood.

Many ophiolites are now proposed to form in the “supra-subductionzone” (SSZ) setting (Beccaluva et al., 1984, 1994; Miyashiro, 1973;Pearce et al., 1984; Shervais, 2001 and references therein). Shervais(2001) reviewed the petrological and geochemical signatures of SSZophiolites and suggested that SSZ ophiolites experienced a sequence ofevents during their evolution in response to the change in tectonicsetting from oceanic lithosphere formed at mid-ocean ridges to theinitiation of subduction as follows: birth, youth, maturity, death andresurrection. Studies of mantle sections from SSZ ophiolites provideimportant information on the maturing of mantle wedges above

216 T. Morishita et al. / Lithos 124 (2011) 215–226

subduction zones, where continuous and/or episodic fluid fluxes areexpected from the subducting oceanic plate.

The Albanian ophiolites occur within the Dinaride–Hellenidesegment of the Alpine orogenic system and represent the remnants oftheMesozoicNeo-Tethyanocean (e.g., Shallo andDilek, 2003; Dilek andFurnes, 2009). The Albanian ophiolites are generally divided intowestern- and eastern-types based on petrological and mineralogicaldata, as discussed bymany authors (Bébien et al., 2000; Beccaluva et al.,1994; Bortolotti et al., 1996;Dilek et al., 2008;Hoeck et al., 2002;Nicolaset al., 1999; Shallo, 1990; Shallo et al., 1987, 1990 and referencestherein). The northern Albanian ophiolite belt, the Mirdita ophiolite,shows that MORB and SSZ affinities are dominant in the west and theeast, respectively (Beccaluva et al., 1994; Bortolotti et al., 1996, 2002;Dilek et al., 2008; Shallo, 1990; Shallo et al., 1987, 1990).

It should be emphasized that in this paper we specifically focus onthe eastern region of the Mirdita ophiolite (the Eastern MirditaOphiolite). Beccaluva et al. (1994) pointed out that the mantle sectionof the eastern Mirdita ophiolite is characterized by strongly depletedsignatures in melt components. Based on the petrology of the mantlesection combined with geochemical signatures of volcanic rocks in theeastern part, they concluded that the eastern part of the Mirditaophiolite was formed in supra-subduction settings. Bizimis et al. (2000)examined the trace element composition of clinopyroxene in someperidotites from the mantle sections of ophiolite complexes from theHellenic Peninsula including one peridotite body (Bulqiza) from theMirdita ophiolite. They suggested that the trace element composition ofclinopyroxene in these ophiolites is similar to modern arc peridotitesrecovered from Izu-Bonin-Mariana arc (Parkinson and Pearce, 1998;Zanetti et al., 2006). Detailed geochemical mapping in several supra-subduction zone ophiolites has recently revealed the presence of

Fig. 1. Simplified lithological map of the studied area showing sample localities discussed inBalkan Peninsula, with the Mirdita ophiolite in red.

distinct units within themantle section of an ophiolite (Arai et al., 2006;Batanova and Sobolev, 2000; Choi et al., 2008; Tamura and Arai, 2006;Uysal et al., 2009). We examine systematic petrological and mineral-ogical variations in the uppermostmantle section in the EasternMirditaophiolite in the context of a maturing of mantle wedge.

2. Geological outline and sample descriptions

The Mirdita ophiolite is located in the northern ophiolite belt ofAlbania (Fig. 1). Based on differences in the internal stratigraphy andchemical composition of the crustal unit, two types of ophiolites havebeen recognized in the Mirdita ophiolite, namely the Western MirditaOphiolite (WMO) and the Eastern Mirdita Ophiolite (EMO) (Beccaluvaet al., 1994; Bortolotti et al., 1996; Dilek et al., 2008; Shallo, 1990; Shalloet al., 1987, 1990) Boninitic dikes and lavas crosscut and/or overlieearlier extrusive rocks in the EMO (Beccaluva et al., 1994; Dilek et al.,2008; Shallo et al., 1987). The crustal section of the WMO has MORBaffinities, whereas that of the EMO shows predominantly SSZgeochemical affinities. The extrusive sequence in the EMO consists ofpillowed to massive flows ranging in composition from basalt andbasaltic andesite in the lower section to andesite, dacite, and rhyodacitein the upper part (Bortolotti et al., 1996; Dilek et al., 2008). Largeperidotite massifs are exposed at the western and eastern ends of theMirdita ophiolite. Plagioclase-bearing peridotites are frequently ob-served in the WMO, whereas harzburgite is dominant in the EMO(Beccaluva et al., 1994, 1998; Beqiraj et al., 2000; Hoxha and Boullier,1995). In this paper, we focus on three peridotite massifs (Bulqiza, Lureand Kukësi massifs) located in the EMO (Fig. 1). All massifs havebasically the same petrological and mineralogical characteristics. The

this study. Inset map shows the distribution of a part of the Tethyan ophiolites in the

217T. Morishita et al. / Lithos 124 (2011) 215–226

Bulqiza massif has economically important high-Cr/Al chromitite ores(Beccaluva et al., 1998; Beqiraj et al., 2000).

Systematic lithological variations in the mantle section withproximity to the crustal section have previously been recognized(Beccaluva et al., 1998; Beqiraj et al., 2000; Hoxha and Boullier, 1995).We also confirmed systematic lithological variations in the mantlesection: clinopyroxene porphyroclast-bearing harzburgites (Cpx-harzburgites hereafter, Fig. 2a) are sometimes observed in the easternmargin of massifs (01Blq, 02Lur), i.e. the basal part of the mantlesection, whereas harzburgite and dunite are dominant in the upperparts of the mantle section (Beccaluva et al., 1998; Beqiraj et al., 2000;Dilek andMorishita, 2009; Hoxha and Boullier, 1995). Cpx-harzburgiteshave a porphyroclastic texture. Clinopyroxene occurs as both porphyr-oclastic grains and their recrystallized fine grains. The relationshipsbetween Cpx-harzburgite and other lithologies are not well observed inthis study. The lithological boundary between dunites and harzburgitesis usually sharp and is sometimes nearly parallel to the foliation planedefined by mineral orientations. Dunite also frequently occurs as smallbodies with complicated irregular boundaries with harzburgites(Fig. 2b). Harzburgite shows granular to porphyroclastic textures.Chromitite layers a few cm thick are frequently observed in dunite andusually occur parallel to each other and to the lithological boundarybetween dunite and harzburgite (Fig. 2c). Chromitite layers areoccasionally tightly folded in dunites (Fig. 2c). It is interesting to notethat inclusions of silicate minerals, such as amphibole, orthopyroxene,clinopyroxene, and their secondary minerals (e.g., chlorite andserpentine), are commonly found within chromian spinels in harzbur-gites near dunite (Fig. 3a).

Fig. 2. (a) Polished surfaceof a clinopyroxeneporphyroclast-bearingharzburgite (01Blq). (b) Fieof chromitite layers in dunite (03 Kuk) Chromitite is sometimes tightly folded (arrow). (d) Orclinopyroxene, D = dunite, H or Harz = harzburgite.

Orthopyroxenite dikes/layers a few cm to 3 m wide are frequentlyobserved in the uppermost section of the mantle sequence (Beccaluvaet al., 1998; Dilek and Morishita, 2009) (Fig. 2d). They rarely occur aslayers nearly parallel to the foliation and lithological boundaries in thehost peridotites, and more frequently occur as dike-like featurescutting all lithological boundaries at high angles (Fig. 2b), indicatingthat they are related to late melt migration through the mantlesection. Fewer deformation textures are observed in orthopyroxe-nites. Orthopyroxenites mainly consist of coarse-grained orthopyr-oxene (up to 10 cm across) with small amounts of spinel and olivine.Olivines sometimes show resorbed textures in large orthopyroxenegrains (Fig. 3b). Large orthopyroxenes have many clinopyroxeneexsolution lamellae. Dark brown spinel is commonly included in largeorthopyroxene grains. Orthopyroxenites locally contain amphiboleand/or clinopyroxene. Clinopyroxene sometimes occurs as veinsalong orthopyroxenites. Amphibole occurs as an interstitial phasealong the grain boundaries of orthopyroxene and also as poikiliticphases including orthopyroxene grains (Fig. 3c).

3. Mineral chemistry

3.1. Analytical methods

Major-element compositions of minerals were analyzed using anelectron probe micro-analyzer (JEOL JXA-8800 Superprobe) at Kana-zawa University. The analyses were performed under an acceleratingvoltage of 15 kV andbeamcurrent of 20 nA, usinga 3 μmdiameter beam.Natural and syntheticmineral standardswere employed for allminerals.

ld relationshipsbetweendunite, harzburgites andorthopyroxenite (01 Lur). (c)Occurrencethopyroxenite network (yellow arrows) in harzburgites (04 Blq). C = chromitite, cpx =

Fig. 3. (a) Back-scattered electron image of a spinel grain with silicate mineral inclusions in harzburgites near a dunite. (b) Resorbed olivine in a large orthopyroxene grain in anorthopyroxenite (04Blq). (c) Poikilitic amphibole (light green phase) in orthopyroxenite. amph = amphibole, ol = olivine, OPX = orthopyroxene, spl = spinel.

218 T. Morishita et al. / Lithos 124 (2011) 215–226

JEOL software using ZAF corrections was employed. Details of EPMAwere described inMorishita et al. (2003). Representativemajor elementcompositions ofminerals are shown in Table 1. Rare earth element (REE)and trace element (Li, Ti, Sr, Y, Zr, andNb) compositionsofmineralsweredetermined by 193 nm ArF Excimer laser ablation-inductively coupledplasma-mass spectrometry (LA-ICP-MS) at Kanazawa University (Agi-lent 7500S equippedwithMicroLasGeoLasQ-plus+; Ishida et al., 2004).Clinopyroxeneswere analyzed by ablating 30–80 μmspot diameter at 5–10 Hz, depending on the size and trace element abundances of analyzedminerals. TheNISTSRM612wasusedas theprimary calibration standardandwas analyzed at the beginning of each batch of b8 unknowns, with alinear drift correction applied between each calibration. The elementconcentrations of NIST SRM 612 for the calibration are selected from thepreferred values of Pearce et al. (1997). Data reduction was facilitatedusing 29Si and 42Ca as internal standard elements, based on Si and Cacontents obtained by EPMA following a protocol essentially identical tothat outlined by Longerich et al. (1996). Details of the analytical methodand data quality were described in Morishita et al. (2005a,b).Representative analyses in trace element compositions of minerals areshown in Table 2.

3.2. Olivine

The forsterite and NiO contents of olivine increase from Cpx-harzburgite (91.1±0.1, 0.38±0.02 wt.%) to dunite-associated chromi-tite layers (94–95, 0.46–0.53 wt.%) through harzburgites (91–92, 0.38–0.4 wt.%) (Fig. 4). The forsterite and NiO contents of olivine in a dunitedirectly contacting with harzburgites (91.3±0.1, 0.31±0.04 wt.%) areidentical to those in the host harzburgite (91.4±0.2, 0.30±0.03 wt.%).The forsterite andNiO contents of olivine in chromitite layers are higherthan those in the host dunites (96, 0.5–0.58 wt.%), because ofequilibration with chromian spinel at low temperature conditions. TheNiO contents of olivines with ragged rims enclosed by large orthopyr-oxene grains in orthopyroxenites are likely to be slightly higher (up to0.47 wt.%) than the host olivines (usually less than 0.4 wt.%) (Fig. 4).

3.3. Orthopyroxene

The Mg# (= 100 Mg/(Mg+Fe) atomic ratio) of orthopyroxeneincreases from Cpx-harzburgite to harzburgite. The Mg-number of largeorthopyroxene grains in orthopyroxenites is lower in a cpx-bearing layer(90) than others (92). The Al2O3 content of orthopyroxene usuallydecreases from core to rim (from 2.7 to 1.8 wt.% for Cpx-harzburgite) inboth peridotites andorthopyroxenites. TheAl2O3 contents of the cores of

orthopyroxene porphyroclasts decrease from Cpx-harzburgite (3 wt.%)to harzburgite (1.3 wt.%) (Fig. 5). The Al2O3 content of the cores of largeorthopyroxene grains in orthopyroxeniteswithout cpx and amphibole isusually 1.5 wt.%. Those surrounded by amphibole in amphibole-bearingorthopyroxenites reached 2 wt.%. The Al2O3 content of orthopyroxenitesurrounded by amphibole in orthopyroxenites is higher (N2 wt.%) thanothers (1.5 wt.%). The TiO2 content of orthopyroxene is usually lowerthan the detection limit of the EPMA (b0.04 wt.%). Orthopyroxeneinclusions within spinel in harzburgites are low in TiO2 (lower than thedetection limit of the EPMA, b0.04 wt.%) and Al2O3 (0.3 wt.%) (Fig. 6a).

3.4. Clinopyroxene

TheAl2O3 content of clinopyroxene is 2 wt.% for Cpx-harzburgite, c.a.1 wt.% for harzburgite and dunite, and 1.5–1.8 wt.% for cpx-bearingorthopyroxenite. The TiO2 and Na2O contents of clinopyroxene areusually lower than the detection limit (b0.04 wt.%) of the EPMA exceptfor the TiO2 content of dunite and the Na2O content of harzburgite.Clinopyroxene in the cpx-bearing orthopyroxenite is slightly higher inNa2O (c.a. 0.15 wt.%) than in other samples (usually b0.05 wt.%).Clinopyroxene inclusions within spinel in harzburgites are low in TiO2

(b0.04 wt.%), Al2O3 (0.4 wt.%) and Na2O (b0.03 wt.%). Light REE (LREE)contents of clinopyroxene in the cpx porphyroclast-bearing harzburgiteand oneharzburgite are lower than the detection limit (0.01–0.02 ppm)of the analyses. Their chondrite (CH)-normalized REE patterns are,therefore, fractionated in LREEs (Fig. 7a). The CH-normalized REEpatterns of clinopyroxene in other harzburgites are enriched in LREEcompositions although middle REEs are lower than the detection limit(0.02 ppm) of the analyses. Their heavy REE (HREE) compositions aremore depleted than those in the other harzburgites (Fig. 7b). Sr contentis lower in the Cpx-harzburgite than others (Fig. 7b). Primitive mantle-normalized trace element patterns of clinopyroxene in dunites andharzburgites show a positive Ti anomaly (Fig. 7b).

3.5. Spinel

It is noted that the Cr# [= Cr/(Cr+Al) atomic ratio] of spinel withsilicate inclusions in harzburgites is heterogeneouswithin each grain, aswell as among different grains (0.4–0.7). Conversely, those in othersamples are homogeneous. Despite the heterogeneities in somesamples, the Cr# of spinel increases from Cpx-harzburgite (0.48) todunite (0.78–0.82) through harzburgites (Fig. 8). The Cr-number ofspinel in orthopyroxenite ranges from 0.5 to 0.7. The TiO2 content ofspinel in clinopyroxene porphyroclast-bearing harzburgites,

Table 1Representative major element compositions of minerals.

Sample Lithology Mineral Anal# SiO2 TiO2 Al2O3 Cr2O3 FeO* MnO MgO CaO Na2O K2O NiO Total Mg# Cr#

01Blq Cpx-harz Olivine N=7 41.0 b0.04 b0.03 b0.1 8.7 0.11 50.2 b0.04 b0.03 b0.03 0.38 100.4 0.911Std 0.2 0.1 0.03 0.3 0.02 0.5 0.001Spinel N=6 b0.03 0.04 29.2 40.5 15.9 0.24 14.1 b0.04 b0.03 b0.03 0.10 100.1 0.626 0.482Std 0.01 1.7 1.7 0.4 0.02 0.3 0.02 0.4 0.007 0.024Opx core 21–7 56.4 b0.04 2.7 0.70 5.5 0.14 34.3 0.68 b0.03 b0.03 0.12 100.6 0.917 0.150Opx rim 24–7 57.6 b0.04 1.8 0.29 5.9 0.15 34.8 0.32 b0.03 b0.03 b0.1 101.0 0.913 0.094Cpx core 42–13 54.2 0.04 2.0 0.71 1.9 b0.1 17.7 23.9 0.03 0.00 b0.1 100.7 0.942 0.190

03Blq-L Du/Chr (Chr) Olivine N=4 41.4 b0.04 b0.03 b0.1 4.3 b0.1 53.9 b0.04 b0.03 b0.03 0.51 100.1 0.957 0.845Std 0.09 0.29 0.45 0.00 0.3Spinel N=8 b0.03 0.15 11.0 58.7 15.2 0.28 13.7 b0.04 b0.03 b0.03 99.2 0.660 0.782Std 0.02 0.11 0.35 0.17 0.03 0.21 0.3 0.009 0.002

Du/Chr (Du) Olivine N=6 41.5 b0.04 b0.03 b0.1 5.7 0.10 52.4 0.09 b0.03 b0.03 0.46 100.2 0.942 0.639Std 0.16 0.31 0.01 0.34 0.02 0.01 0.2Spinel N=6 b0.03 0.15 10.6 57.2 19.2 0.32 11.8 b0.04 b0.03 b0.03 b0.1 99.2 0.573 0.784Std 0.01 0.40 0.58 0.93 0.04 0.55 0.3 0.023 0.007Cpx 74–7 54.4 0.05 1.0 0.59 1.05 b0.1 17.1 25.5 0.05 b0.03 b0.1 99.7 0.967 0.272

04Blq-F Chromitite Olivine N=7 41.4 b0.04 b0.03 b0.1 3.8 0.05 53.5 b0.04 b0.03 b0.03 0.58 99.3 0.962Std 0.31 0.29 0.02 0.39 0.03 0.7 0.003Spinel N=11 b0.03 0.15 9.6 61.1 14.1 0.29 13.9 b0.04 b0.03 b0.03 b0.1 99.2 0.670 0.811Std 0.01 0.25 0.48 0.39 0.02 0.30 0.7 0.011 0.004

Dunite layer Olivine N=10 41.4 b0.04 b0.03 b0.1 5.0 b0.1 52.7 0.07 b0.03 b0.03 0.53 99.8 0.950Std 0.16 0.16 0.27 0.03 0.01 0.3 0.002Spinel N=13 b0.03 0.15 8.60 60.4 18.6 0.35 11.2 b0.04 b0.03 b0.03 b0.1 99.4 0.554 0.825Std 0.01 0.37 0.90 1.1 0.03 0.60 0.5 0.027 0.008

04Blq-B Harzburgite Olivine N=7 41.0 b0.04 b0.03 b0.1 8.5 0.12 50.4 b0.04 b0.03 b0.03 0.38 100.4 0.913Std 0.05 0.09 0.02 0.17 0.02 0.2 0.001Spinel bright 82–8 b0.03 b0.04 15.5 53.0 19.6 0.39 10.4 b0.04 b0.03 b0.03 b0.1 98.9 0.503 0.696Spinel normal 84–8 b0.03 b0.04 18.2 50.7 18.4 0.29 10.9 b0.04 b0.03 b0.03 b0.1 98.6 0.524 0.652Spinel dark 86–8 b0.03 b0.04 23.7 44.7 18.2 0.26 11.9 b0.04 b0.03 b0.03 b0.1 98.8 0.551 0.558Amph inc 70–7 52.8 0.08 5.6 2.1 2.1 0.05 21.2 12.6 0.52 0.20 b0.1 97.4 0.947 0.201Opx inc 71–7 58.2 b0.04 0.53 0.84 5.2 0.17 36.3 0.26 b0.03 b0.03 b0.1 101.5 0.925 0.516Cpx inc 32–4 54.6 b0.04 0.41 0.78 1.6 b0.1 17.9 24.4 b0.03 b0.03 b0.1 99.8 0.952 0.558Cpx 46–5 54.3 b0.04 1.2 0.54 2.0 b0.1 17.8 23.7 0.03 b0.03 b0.1 99.7 0.941 0.233Cpx 64–7 54.6 b0.04 1.2 0.48 1.8 b0.1 18.1 23.8 0.03 b0.03 b0.1 100.1 0.947 0.207

03Blq-B Harzburgite Olivine N=16 40.9 b0.04 b0.03 b0.1 8.0 0.12 51.1 b0.04 b0.03 b0.03 0.40 100.5 0.919Std 0.20 0.15 0.02 0.39 0.03 0.6 0.001Spinel normal 45–4 b0.03 b0.04 16.4 52.5 18.4 0.27 11.3 b0.04 b0.03 b0.03 b0.1 98.9 0.542 0.682Spinel darc 49–4 b0.03 b0.04 24.5 44.0 17.2 0.29 13.4 b0.04 b0.03 b0.03 b0.1 99.4 0.610 0.547Amph inc 87–10 52.0 0.07 6.8 2.1 2.3 b0.1 21.0 12.7 0.701 0.23 b0.1 98.0 0.943 0.171Opx inc 102–11 58.3 b0.04 0.54 0.77 4.6 0.14 36.5 0.35 b0.03 b0.03 b0.1 101.2 0.934 0.489Cpx inc 103–11 54.3 b0.04 0.86 0.94 1.7 b0.1 18.1 24.0 b0.03 b0.03 b0.1 100.0 0.950 0.424Cpx 31–2 54.4 b0.04 1.2 0.62 1.7 b0.1 17.8 24.2 0.06 b0.03 b0.1 99.9 0.950 0.251Opx core 41–3 57.0 b0.04 1.4 0.57 5.1 0.12 34.6 0.53 b0.03 b0.03 b0.1 99.3 0.923 0.215Opxc rim 26–3 57.2 b0.04 1.1 0.41 5.3 0.14 35.1 0.71 b0.03 b0.03 b0.1 100.0 0.922 0.197

01Kuk-D Dun/Harz (Dun) Olivine N=4 41.0 b0.04 b0.03 b0.1 8.7 0.16 50.9 b0.04 b0.03 b0.03 0.31 101.1 0.913 0.684Std 0.24 0.15 0.05 0.17 0.04 0.4Spinel N=6 b0.03 0.04 16.6 51.4 20.8 0.33 10.3 b0.04 b0.03 b0.03 b0.1 99.6 0.495 0.675Std 0.02 0.28 0.50 0.73 0.03 0.41 0.4 0.017 0.005

Dun/Harz (Harz) Olivine N=9 41.1 b0.04 b0.03 b0.1 8.5 0.14 50.8 b0.04 b0.03 b0.03 0.30 100.9 0.914 0.774Std 0.26 0.19 0.05 0.23 0.03 0.4Spinel N=8 b0.03 0.05 21.3 47.6 18.5 0.31 11.6 b0.04 b0.03 b0.03 b0.1 99.4 0.541 0.600Std 0.03 1.3 1.5 0.90 0.06 0.54 0.4 0.026 0.022Opx core 100–6 57.3 b0.04 1.3 0.53 5.5 0.13 34.8 0.76 b0.03 b0.03 0.04 100.4 0.918 0.217Opx rim 102–6 57.5 b0.04 0.98 0.25 5.6 0.14 35.1 0.50 b0.03 b0.03 0.02 100.1 0.918 0.147Cpx 103–9 52.9 b0.04 1.1 0.59 1.6 b0.1 17.7 23.9 0.05 b0.03 0.07 97.9 0.953 0.261

03Blq Orthopyroxenite Olivine-host 80 40.5 b0.04 0.00 0.00 8.3 0.15 50.7 b0.04 b0.03 b0.03 0.34 99.9 0.92Olivine-fine 179–18 40.4 b0.04 0.00 0.00 8.3 0.15 50.6 b0.04 b0.03 b0.03 0.46 100.0 0.92Spinel N=6 b0.03 b0.04 18.0 52.2 19.1 0.3 10.9 b0.04 b0.03 b0.03 b0.1 100.7 0.52 0.66Std b0.04 3.0 3.1 2.0 0.05 1.2 0.2 0.05 0.05Opx core 21–4 56.6 b0.04 1.2 0.19 5.5 0.13 34.9 0.69 b0.03 b0.03 b0.1 99.3 0.92 0.10Opx rim 24–4 57.0 b0.04 0.48 0.09 5.6 0.13 35.1 0.30 b0.03 b0.03 b0.1 98.8 0.92 0.12Opx* 16 55.8 b0.04 2.2 0.42 5.5 0.10 34.5 0.64 b0.03 b0.03 b0.1 99.4 0.92 0.11Amphibole 133 46.9 0.10 11.0 1.4 2.4 0.02 19.2 12.5 1.7 0.66 0.12 96.0 0.93 0.08

FeO* is total iron. Mg#=Mg/(Mg+Fetotal) atomic ratio except for spinel (=Mg/(Mg+Fe2+). Fe2+ in spinel was calculated from spinel stoichiometry). Cr#=Cr/(Cr+Al) atomic ratio.Anal#=nameof analytical point oranalytical numbers (N) for average compositions.Harz=harzburgite,Du=dunite, Chr=chromitite, std=standarddeviation, opx=orthopyroxene,Opx*=orthopyroxene included in amphibole, cpx=clinopyroxene. Spinel bright, normal, anddark=bright, normal anddarkareawithinheterogeneous spinel grain, respectively; inc=inclusion within spinel; amph = amphibole.

219T. Morishita et al. / Lithos 124 (2011) 215–226

harzburgites and orthopyroxenites is generally low (b0.05 wt.%)whereas that of dunite or chromitite is around 0.15 wt.%. The YCr [=100 Cr/(Cr+Al+Fe3+) atomic ratio] of spinel is slightly higher indunites and chromitites (3–6) than in Cpx-harzburgites, harzburgitesand orthopyroxenites (0.5–2.5). Spinels with silicate mineral inclusionsin harzburgites are also heterogeneous in YCr (1–6).

3.6. Amphibole

Following the classification of Leake et al. (1997), amphibole in theorthopyroxenites are edenite, edenitic hornblende, magnesio horn-blende and tremolitic hornbrende. Amphibole inclusions withinchromian spinel in harzburgites are magnesio hornblende, tremolitic

Table 2Representative trace element compositions of clinopyroxene.

Sample # 01Blq 04Blq-B 03Blq-B 03Blq-LLithology Cpx-Harz harzburgite Harzburgite DuniteAnal # 42-13 46-5 64-7 31-2 74-7

Li 10 4 5 4 0.61Ti 159 89 86 52 296Sr 0.16 0.12 0.15 0.25 0.86Y 1.02 0.40 0.37 0.18 0.56Zr 0.05 0.06 0.06 0.07 0.08Nb 0.05 0.05 0.04 0.07 0.06Ba 0.21 b0.05 b0.05 0.20 0.03La b0.01 b0.01 b0.01 0.29 0.08Ce b0.01 b0.01 b0.01 0.01 b0.01Pr b0.01 b0.01 b0.01 b0.01 b0.01Nd b0.02 b0.02 b0.02 b0.02 b0.02Sm b0.02 b0.02 b0.02 b0.02 b0.02Eu b0.01 b0.01 b0.01 b0.01 b0.01Gd 0.015 b0.02 b0.02 b0.02 b0.02Tb 0.01 b0.01 b0.01 b0.01 b0.01Dy 0.12 0.06 0.04 0.01 0.07Ho 0.03 0.01 0.01 0.01 0.02Er 0.16 0.06 0.06 0.04 0.09Tm 0.03 0.01 0.01 0.01 0.02Yb 0.22 0.09 0.10 0.08 0.11Lu 0.03 0.02 0.02 0.02 0.02

Anal# = name of analytical point (the same as Table 1). Harz = harzburgite.

0

1

2

3

4

5

6

0 0.5 1Cr# (Cr/(Cr+Al) atomic ratio, Spinel

Al 2

O3

wt.

%, O

rtho

pyro

xene

SWIR

CIR

Hess Deep

Garret

IBM

MAR OCC: Atlantis Massif

01-Blq 04B-Blq

03B-Blq

01D-KUK

02-Lur

cpx-Harz Harz

MAR 1274A

Fig. 5. Correlations between Al2O3 content (wt.%) of orthopyroxene porphyroclasts andthe Cr# of spinels. Data source for the mid-ocean ridge systems: South Indian Ridge(SWIR), Atlantis II Fracture Zone, Dick (1989) and Seyler et al. (2003); Central IndianRidge (CIR), Hellebrand et al. (2002), Morishita et al. (2009); Mid-Atlantic Ridge(MAR), Atlantis Massif (oceanic core complex; OCC), Tamura et al. (2008) and 1274A,Seyler et al. (2007); Garrett and Hess Deep, East Pacific Rise (EPR), Constantin (1999)and Dick and Natland (1996). Data source for the Izu–Bonin–Mariana forearc (IBM),Parkinson and Pearce (1998) and Zanetti et al. (2006).

220 T. Morishita et al. / Lithos 124 (2011) 215–226

hornblende and tremolite. The TiO2, Na2O, and K2O contents ofamphiboles are usually low, b0.1 wt.%, b2 wt.% and b1 wt.%, respec-tively, and the K/(K+Na) atomic ratio is b0.25. The Cr2O3 content isaround 1.5 wt.%. Amphibole inclusions within spinel in harzburgite arelow in Al2O3 (b7 wt.%), TiO2 (b0.1 wt.%) (Fig. 6b), Na2O (b1 wt.%) andK2O (b0.2 wt.%).

4. Discussion

The petrology of the mantle section of the Eastern Mirdita ophioliteis characterized as follows (Dilek and Morishita, 2009). Structurally,cpx-harzburgites occur lower in the mantle section, while harzburgiteand dunite aremore abundant higher in the section.Dunite is associatedwith chromitite layers. Orthopyroxenite crosscuts all lithologies ofdunite (chromitite)-harzburgite in the late stage. We discuss petrogen-esis of (1) Cpx-harzburgites, (2) harzburgite-dunites (chromitites) and(3) orthopyroxenites.

0

0.1

0.2

0.3

0.4

0.5

0.6

90 91 92 93 94 95 96

NiO

wt.

%

cpx-Harz 01-Blq

02-Lur

Harz

04F-Blq

03L-Blq

alternating with chromitite layers

01D-Kuk01D-Kuk

Dunite

03B-Blq

04B-BlqOrthopyroxenite

Fine grains

Coarse-grains& host grains

Fo olivine

Fig. 4. Relationships between Fo content and NiO wt.% of olivine in peridotites,orthopyroxenites and their host peridotites. Harz = harzburgite.

4.1. Origin of Cpx-harzburgites in the lower part of the mantle section: aresidue of less-fluxed partial melting related to mid-ocean ridge basaltsor fore-arc basalts?

Clinopyroxenes in Cpx-harzburgites are characteristically depletedin LREEs and other incompatible elements, indicating that they are asimple residue of partial melting and melt extraction from mantlematerials rising adiabatically beneath ocean ridges (Hellebrand et al.,2002; Johnson et al., 1990). Based on the method of Hellebrand et al.(2001), the Cpx-harzburgites are the residue after 17% partial melting.This is consistent with the fractional melting model (Ozawa, 2001) ofdepleted MORB mantle (DMM: Workman and Hart, 2005) at spinel-peridotite stability conditions (Fig. 9a).

Cpx-harzburgite mineral compositions plot within the composi-tional range of depleted abyssal peridotites in mid-ocean ridgesystems (Figs. 5, 7, 8), with the exception of the Mg# of clinopyroxene(Fig. 10). The Mg# of clinopyroxene is higher in the Cpx-harzburgite(and other samples) than in abyssal peridotites frommid-ocean ridgesystems, when compared at the same Cr# of clinopyroxene and Focontent of olivine (Fig. 10b). The high-Mg# of clinopyroxene in thestudied samples can be explained by a temperature-dependent Fe–Mg exchange reaction between olivine and clinopyroxene undersubsolidus conditions (Kawasaki and Ito, 1994) (i.e., clinopyroxenesin the studied samples equilibrated at lower temperature than thosein the abyssal peridotites from mid-ocean ridge systems) because ofthe differences in exhumation history near mid-ocean ridges and atconvergent margins. We conclude that geochemical characteristics ofthe Cpx-harzburgite in the EMO are very similar to those of depletedabyssal peridotites in mid-ocean ridge systems.

It is, however, not necessary that the Cpx-harzburgite formedbeneath a mid-ocean ridge; Reagan et al. (2010) recently reportedMORB-like basalticmagmatism followed by boniniticmagmatism in theIzu–Bonin–Mariana fore-arc regions. They postulated that the MORB-like tholeiitic basalts were the first lavas to erupt after the oceanic platebegan to subduct and termed them “fore-arc basalt (FAB)”. Stern andBloomer (1992) predicted a localized extensional zone directly abovethe shallow dipping slab in a compressional setting. This conceptualmodel was examined with visco-elastoplastic models by Hall et al.(2003) and a following paper (Gurnis et al., 2004). According to thismodel, adiabatic decompressional melting for the MORB-like melts

0

0.1

0.2

0 0.5 1 1.5 2 2.5

0

1

2

3

4

5

0 5 10 15

TiO

2 w

t. %

Al2O3 wt. %

TiO

2 w

t. %

a: Orthopyroxene

b: Amphibole

Chromitite (Hess Deep)

Troctolite (Hess Deep)

Harz (Atlantis Massif)

Detection limit

This study

Chromitite (Mid-Cayman)

Troctolite (Mid-Cayman)

Troctolite (Hess Deep)

Chromitite (Hess Deep)

Harz (Atlantis Massif)03B-Blq04B-Blq

This study

Al2O3 wt. %

Fig. 6. Compositional relationshipbetweenTiO2wt.% andAl2O3wt.% oforthopyroxeneandamphibole inclusionswithin spinel in harzburgites from this study and the AtlantisMassifof the Mid-Atlantic Ridge (Tamura et al., 2008). Those in chromitites and associatedtroctolite from ocean floor (Arai and Matsukage, 1998) are also shown. Harz =harzburgite.

0.01

0.1

1

10

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

0.001

0.01

0.1

1

Nb La Ce Sr Nd Zr Sm Eu Gd Ti Dy Y Er Yb Lu

Cho

ndrit

e-no

rmal

ized

Prim

itive

man

tle-n

orm

aliz

ed

cpx-Harz (01-Blq)

Harz (04B-Blq)

Harz (03B-Blq)

Dunite (03L-Blq)

cpx-Harz (01-Blq)

Harz (04B-Blq)

Harz (03B-Blq)

Dunite (03L-Blq)

a

b

Fig. 7. Chondrite-normalized REE patterns (a) and primitive mantle-normalized traceelement patterns (b) of clinopyroxenes in cpx porphyroclast-bearing harzburgites(cpx-Harz), harzburgites (Harz) and dunite (Dunite).

221T. Morishita et al. / Lithos 124 (2011) 215–226

occurred due to infiltration of hot mantle into the extensional region.Geochemically, no significant differences in major and trace elementcompositions of residues after partial melting are expected betweenmid-ocean ridges and the extensional zone at the fore-arc setting. Infact, Reagan et al. (2010) pointed out that someophiolites, including theMirdita amphibolites, were originated by near-trench volcanism atsubduction initiation. We cannot say definitively whether the Cpx-harzburgite is a residue related to MORB or FAB, and further studies arerequired as more generally discussed later. In the next section, we alsodiscuss a possibility that some dunites (chromitites) in this study wererelated to the FAB.

In addition to the discussion above, it is worthwhile to compare theEMO and WMO mantle sections because MORB-like igneous rocksdominate in the crustal section of the WMO, although the relationshipbetween EMO and WMO has been a matter of debate (Bébien et al.,1998, 2000; Bortolotti et al., 2002; Hoeck et al., 2002;Meshi et al., 2010;Nicolas et al., 1999; Shallo and Dilek, 2003; Tremblay et al., 2009). In theShebenikmassif, located southof theBulqizamassif (Fig. 1), Bébienet al.(1998) suggested that plagioclase wehrlites, troctolites and gabbrosabove highly depleted harzburgites and dunites were products of the

crystallization of MORB affinity melts. Hoeck et al. (2002) proposed asystematic variation in petrography and geochemistry from north tosouth in the western belt, with increasingly SSZ signatures towards thesouth, based on studies in the western-type of the southern Albanianophiolites (Voskopoja ophiolites) combined with results from thePindos ophiolites. On the other hand, Nicolas et al. (1999) and Meshi etal. (2010) suggested that the differences between the WMO and EMOwere due to successive episodes of magmatic and amagmatic extensionin a slow-spreading mid-ocean ridge environment. Further studies onpetrology andmineralogy in themantle sectionof theWMOcanprovideuseful lines of information about the variation of themantle sectionwithrespect to differences in the crustal section.

4.2. Origin of refractory harzburgites and dunites including chromitites:implications for influx melting beneath island arc setting

The presence of refractory peridotites consisting of harzburgites anddunites (including chromitites) was previously recognized as adominant characteristic of the uppermost section in the EMO (Beccaluvaet al., 1994, 1998; Bizimis et al., 2000). Mineral compositions inharzburgites and dunites (+ chromitites) are generally outside of thecompositional rangeof those in abyssal peridotites frommid-ocean ridgesystems, but are similar to those of fore-arc peridotites (Figs. 5, 8 and10).Clinopyroxenes from harzburgites and dunites have lower heavy REEconcentrations and extremely low middle REE concentrations (usuallylower than the detection limit of the analyses), whereas LREE (+ Sr) are

0

10

20

30

40

50

60

70

80

90

100

050100 100

Opxnite

cpx-Harz

Dunite

Chr

Boninite

Harz

Mg#

Cr#

0

10

20

30

40

50

60

70

80

90

100

050Mg#

Cr#

Dunite(01D-Kuk)

Abyssal peridotites

Fore-arc peridotites

a

IBM Dunite

P & P 98

Ishii et al (averaged)

Fore-arc

Abyssal

b

High-Cr du. (this study)

Med.-Cr du. (this study)

Boninite-IBM & Others

Fig. 8. Compositional relationship between Cr# (=Cr/(Cr+Al) atomic ratio) and Mg#(=100Mg/(Mg+Fe2+) atomic ratio) of spinel. (a) Compositional variation of Cpx-harzburgite (cpx-Harz), harzburgite (Harz), dunite (Dunite), chromitite (Chr), andorthopyroxenite (Opxnite) in the mantle section of the EMO. Compositional range of aboninitic dike from the Mirdita ophiolite is also shown (Boninite) (unpublished data).(b) Comparison between the studied dunite, the Izu–Bonin–Mariana (IBM) dunite, andboninites from the IBM and other localities. Data for abyssal peridotites from Dick(1989), Arai and Matsukage (1998), plagioclase-poor samples of Constantin (1999),Hellebrand et al. (2002), harzburgites of Seyler et al. (2003); for fore-arc peridotitesfrom Ishii et al. (1992) and Parkinson and Pearce (1998); and for boninites from Kurodaet al. (1978), Walker and Cameron (1983), Bloomer and Hawkins (1987), Falloon et al.(1989), van der Laan et al. (1992) and Sobolev and Danyushevsky (1994). The IBMdunite of Ishii et al. (1992) in (b) was averaged data.

0.001

0.01

0.1

1

10

La Ce Sr Nd Zr Sm Eu Ti Dy Y Er Yb

Start

5%

10%

15%(2nd stage)

20%

17%

1st stage: less flux

0.001

0.01

0.1

1

10

La Ce Sr Nd Zr Sm Eu Ti Dy Y Er Yb

Influx

Influx

Start

Calc-melt (dunite-cpx)

total melt segregated from the system (melting model)

Calc-cpx(melting model)

dunite cpx

2nd stage: infulx melting

cpx-Harz (01-Blq)

Harz (04B-Blq)Harz (03B-Blq)

Dunite (03L-Blq)

Prim

itive

man

toe-

norm

aliz

ed

Prim

itive

man

toe-

norm

aliz

ed

Fig. 9. Observed and modeled primitive mantle-normalized trace element patterns forclinopyroxene. Initial bulk compositions and initial mineral mode for the 1st stage are fromWorkman andHart (2005), while those for the 2nd stage are the result after 15% fractionalmelting of the 1st stage. Melting stoichiometries for the 1st and 2nd stages at spinel-peridotite conditions areolivine0.05, spinel 0.05, orthopyroxene0.3, clinopyroxene0.6; andolivine — 0.1, spinel 0.1, orthopyroxene 0.4, clinopyroxene 0.6, respectively. Mineral/meltpartition coefficients are listed by Ozawa and Shimizu (1995) and Ozawa (2001). Thecalculated melt compositions equilibrated with clinopyroxene (calc-melt, dunite-cpx) andtotal melt segregated from the system using this model are also shown in (b). Influxcomposition for La, Ce, Sr and Zr was optimized as follows: critical melt fraction α=0.01;influx rate β=1;melt separation rate after trappedmelt τ=0; trappedmelt crystallizationreaction stoichiometry = olivine 0.4, spinel 0.05, orthopyroxene 0.2, clinopyroxene 0.35.Influxcompositions forHREEs+YandTiwere assumed tobe0.01 timesand0.005 times theprimitive mantle values, respectively.

222 T. Morishita et al. / Lithos 124 (2011) 215–226

enriched in some samples, as already suggested by Bizimis et al. (2000).HREE concentrations aremost affected at high-degree of partial melting,whereas LREEs are likely to be susceptible to changes during/aftermelting events.

Chromian spinels in some harzburgites contain hydrous silicateinclusions. The presence of hydrous silicate inclusions in the spinel doesnot directly suggest the infiltration of the hydrous fluids/melts sincehydrous silicate inclusions in chromian spinel from “harzburgites”werealso reported from the Atlantis Massif along the Mid-Atlantic Ridgewhere reactions between peridotites and highly evolved melts fromMORB compositions were expected (Tamura et al., 2008). However,gabbroic rocks donot always occur close to the harzburgites in the EMO.Hydrous silicate inclusions within spinels were also reported in oceanicchromitite–dunite (+ troctolites) (Arai et al., 1997). Arai et al. (1997)suggested that the hydrous silicate inclusions crystallized from highlyevolved melts after melt-peridotite interactions coupled with zone-refining effects (Harris, 1957; Kushiro, 1968) in a stagnant or failedmeltconduit. Although chemical compositions of hydrous silicate inclusionswithin spinel do not directly reflect a parent melt composition whichreacted with the host peridotites to result in the formation of dunite,occurrences and main mineral assemblages are similar between themid-ocean ridge samples and the studied samples. Irrespective of thesimilarities in occurrences of silicate inclusions and their host rocks,chemical characteristics of silicate inclusions, such as TiO2 content, areapparently distinctive from those in the mid-ocean ridge samples(Fig. 6). These chemical and petrological features in harzburgites can beproduced as a result of enhanced partial melting of depleted peridotitescaused by the infiltration of a hydrous LREE-enriched, TiO2-poor flux,which is different from MORB-related melts.

We initially applied an open system melting model using thespreadsheet of Ozawa (2001) in order to reveal chemical signatures ofinflux fluids/melts. We assumed that the starting composition was theresidue after 15% fractional melting from DMM and that the degree of

partial melting was 5% for the 2nd stage of melting (Fig. 9b). Otherparameters are shown in the caption of Fig. 8. We optimized influxfluid/melt compositions for La, Ce, Sr and Zr under a critical opensystem melting to fit their contents to observed clinopyroxene inharzburgite (Fig. 8b), whereas HREE and Ti contents of the influxfluids/melts were simply assumed to be low, 0.01 and 0.005 times theprimitive mantle values of McDonough and Sun (1995), respectively.An estimated influx fluid/melt has high LREE/HREE ratios as expected.Regardless of our preliminary estimation, total melt compositionsfrom the systems using the model show some similarities to thecalculated melt equilibrated with clinopyroxene in dunite, such ashigh LREE/HREE ratio and positive Ti anomaly, but large differences inSr content. Both calculated melts and simulated melts are differentfrom volcanic rocks in the crustal section of the EMO, probablyindicating that magma compositions changed during transport to theupper crustal level as a result of crystallization and/or interactionswith lower-crustal materials. Since compositions of influx fluids/melts

0.0

0.1

0.2

0.3

0.4

0.88 0.90 0.92 0.94 0.96 0.98

0.9

0.91

0.92

0.93

0.94

0.9 0.92 0.94 0.96 0.98

cpx-Harz

04B-Blq01D-KUK03B-Blq

Mg/(Mg+Fe) atomic ratio, Clinopyroxene

Mg/(Mg+Fe) atomic ratio, Clinopyroxene

Cr/

(Cr+

Al)

atom

ic r

atio

n, C

linop

yrox

ene

Mg/

(Mg+

Fe)

ato

mic

rat

io, O

livin

e

1:1

IBM

IBM

MAR 1274

Hess Deep

Garret

CIRSWIR

MAR 1274

Hess Deep

01-Blq02-Lur

Harz

MAR Atlantis M

cpx-Harz01-Blq02-Lur

Harz04B-Blq

01D-KUK03B-Blq

a

b

Fig. 10. Compositional relationships between Mg# and Cr# in clinopyroxene (a), andMg# of coexisting clinopyroxene and olivine (b). Only data with Mg#N0.9 and Cr#b0.4are shown. Data sources are the same as Fig. 8.

223T. Morishita et al. / Lithos 124 (2011) 215–226

are also expected to change during interactions with the host rocks(Navon and Stolper, 1987), we need more systematic sampling fromseveral localities including crustal sections to reveal the quantitativeevolution of melt compositions though melt-rock interactions.

Some dunites, particularly those associated with chromitites, havehigh-Fo and NiO contents of olivine, and high-Cr# of spinel. Thesecharacteristics might be explained if they are the residues afterextremely high-degrees of partial melting (Kubo, 2002). However, thedunite with low-NiO and low-Fo olivine (Figs. 4 and 8) and the fact thatthe boundaries between dunite and harzburgite are usually very sharpsuggest an origin as reaction products rather than residues afterextremely low degree partial melting. Partial melts formed at highpressure in the mantle are saturated only in olivine at lower pressure,and will dissolve pyroxenes during melt/rock reaction resulting indunite (Kelemen, 1990; Kelemen et al., 1995). Chromitites in uppermantle peridotites are generally interpreted to be formed by melt–mantle interactions (Arai, 1995, 1997a,b; Arai and Yurimoto, 1994;

Zhou, 1994). It is interesting to note that there are large differences inthe Fo content of olivine and the Cr# of spinel in dunite samples: onegroup is characterized by high-Cr# spinel and high-Fo olivine, and theother by medium-Cr# spinel and low-Fo olivine (Figs. 4 and 8). Twodistinctive melts, at least, were required for the formation of thesedunites. The Cr# of spinel in a boninitic dike in the EMO is the same asthat in the high-Cr# spinel-bearing dunite (Fig. 8). Onemelt responsiblefor the high-Cr# spinel-bearing dunite (and refractory harzburgite aswell) can, therefore, be assumed for magmas related to boniniteactivities that were recorded in the crustal section of the EMO (Dilek etal., 2008). On the other hand, the Cr# of spinel in the other group (Cr#0.67) is higher than those in typical abyssal peridotites in themid-oceanridge systems, and is generally lower than the majority of those inboninitic magmas (Fig. 8b). Reagan et al. (2010) reported a transitionmagma between the MORB-like FAB and boninitic compositions in theIBM fore-arc. The medium-Cr# spinel-bearing dunite could be a meltconduit for transitionmagmas. Thewide range of variation in the Cr# ofspinel in dunite from the IBM (Fig. 8b)was probably caused by changingmelt compositions in the fore-arc setting throughout this time.

4.3. Origin of the orthopyroxenites: Silica-enrichment in the uppermostmantle section beneath island arc setting

Small ragged olivine grains in large orthopyroxene grains inorthopyroxenites are higher in NiO than other olivine grains in the hostperidotites. Early crystallization of orthopyroxene implies that the meltsfor the formation of orthopyroxenites were saturated in orthopyroxenecomponents, i.e., high-SiO2 content. This texture combined with thepresence of amphibole indicates that the orthopyroxenites formed atthe expense of the pre-existing olivine by reaction with hydrousorthopyroxene-saturated melts, resulting in an increasing NiO contentin the residual olivine as the volume of the olivine was decreased (e.g.,Kelemen et al., 1998). Assimilation of dissolved pyroxene for theformation of the dunite in the studied area, moving it towards opxsaturation, may be a major mechanism for the formation of meltssaturated in orthopyroxene, and could be the source for the orthopyr-oxenites (Kelemen, 1990). Furthermore, the effect of H2O substantiallyincreases themaximumSiO2 content of olivine-saturated liquids. This isconsistent with the orthopyroxenites being products of hydrous partialmelting in the EMOuppermostmantle sections. This combinedwith thelate formation and high-Cr# spinel (Fig. 8) indicates a genetic link to thelate stage boninitic magmas in the crustal section of the EMO. If this istrue, it is noted that geochemical signatures of boniniticmeltsmay havebeen acquired during the interaction with surrounding depletedperidotites during the migration of melts through the mantle (Varfalvyet al., 1997). Further systematic studies on orthopyroxenites are neededto understand the origin of high-Mg andesite, including boniniticmagmas. Progressive CaO enrichment due to the crystallization oforthopyroxene (and spinel) from orthopyroxene-saturated parentmelts lead to the crystallization of clinopyroxene, and amphibole-bearing orthopyroxenites were finally formed from the most evolvedmembers of the melts related to the orthopyroxenites.

Orthopyroxeniteswere commonly observed in theperidotite sectioninmanyophiolites (e.g.,Miyamori ophiolite of Japan, Ozawa, 1988; Lekaophiolite of the Norwegian Caledonides, Furnes et al., 1992; Maaløe,2005; Bay of Island ophiolite of Newdoundland, Varfalvy et al., 1996,1997; Josephine peirdotite of dismembered ophiolite in the KlamathMountains, Kelemen and Dick, 1995; Oman ophiolite, Tamura and Arai,2006; Coast Range ophiolite of California, Choi et al., 2008). Tamura andArai (2006) examined orthopyroxenites with their related harzburgitesanddunites from thenorthernpart of theOmanophiolite. They revealedthat their suites are characterized by high-Cr# spinel and clinopyroxenewith low abundances of middle-heavy REEs and LREE-enrichments, ascomparedwith other samples, suggesting that the calculatedmelts fromclinopyroxene compositions are similar to boninitic melts. This isconsistent with the fact that boninitic magmas and their relatives were

224 T. Morishita et al. / Lithos 124 (2011) 215–226

recently reported in the crustal sequence of the Oman ophiolite(Ishikawa et al., 2002; Yamasaki et al., 2006).

Silica-enrichment of mantle peridotites, i.e., the formation oforthopyroxene, has also been reported in several sub-arc xenoliths(Arai and Kida, 2000; Arai et al., 2003, 2004; Ishimaru et al., 2007;McInnes et al., 2001). We would like to emphasize that silica-enrichment in the uppermost mantle section under island arcs is aubiquitous phenomena caused by infiltration of silica-rich hydrousfluids/melts derived from subducting slabs. Orthopyroxene in somesub-arc xenoliths is, however, characterized by extremely low Al2O3,Cr2O3 and CaO contents, and is thought to have a metasomatic originrelated to interaction with slab-derived fluids. Silica-enrichmentobserved in the uppermost section of the peridotite massifs in theEMO as well as other ophiolites may be applied more generally tospeculate on subduction zone magmatism rather than simple metaso-matic reactions due to infiltration of slab-derived fluids.

5. Concluding remarks

This study revealed the presence of geochemically distinct peridotitesin the mantle section of the EMO. One is the Cpx-harzburgite which is aresidue of less-flux melting. The other is highly refractory harzburgite–dunite (+ chromitite) which was mainly formed by a high-degree ofpartial melting with LREE/HREE flux components, most likely related toboniniticmagmatism in themantlewedge. The latter ismore common inthe upper section of the EMO peridotite massifs. Orthopyroxenite is thelater product of melt-rock reactions. These lithological variations andtheir relationships observed in peridotite massifs from the EMO can beexplained by changes in magmatic compositions from mid-ocean ridgebasalt signatures to arc-relatedmagmas includingboniniticmagmas. Thecontributions of arc-related components increased, modifying the pre-existing “mid-ocean ridge-like” mantles, and resulting in the formationof harzburgite–dunite (+ chromitite)–orthopyroxenite suites. It is notyet clear whether the pre-existing mantle is of true mid-ocean ridgeorigin or FAB-related. This scenario is basically consistent with the onethatwas speculated fromvolcanic sequences in the EMO (e.g. Dilek et al.,2008; Reagan et al., 2010).

Detailed geological and geochemical studies in several ophioliteshave revealed the presence of distinct peridotite unitswithin themantlesection of ophiolites (Ahmed and Arai, 2002; Aldanmaz et al., 2009; Araiet al., 2006; Batanova and Sobolev, 2000; Caran et al., 2010; Choi et al.,2008; Ghazi et al., 2010; Melcher et al., 1997; Saccani et al., 2010;Tamura and Arai, 2006; Uysal et al., 2009). In the case of the Troodosophiolite, which contains arc-signatured magmatic rocks in the crustalsection (Miyashiro, 1973; Pearce and Robinson, 2010; Robinson andMalpas, 1990), Batanova and Sobolev (2000) reported that abyssal-typeperidotites were distributed in the lower part of the peridotite bodywhereas highly refractory harzburgites and dunite are dominant in theupper part of the body. These lithological variations and their relation-ships in ophiolites can be explained by a shift in tectonic setting frommid-ocean ridge to island arc. If, however, the “MORB-like” FAB is aubiquitous phenomenon at the initiation of subduction, we shouldreconsider our interpretation of ophiolite mantle sections. FAB magma-tism at the earliest stage of subduction initiation is a new concept. Morecareful investigations of fore-arc peridotites and the mantle section ofophiolites are required to determine the differences betweenMORB andFAB, and whether FAB magmatism at subduction initiation is present atother locations.

The frequencyof highly refractoryharzburgite (+dunite, chromitite)and orthopyroxenite dikes in the mantle section can be a good indicatorof arc-related magmatic modifications in the mantle, and is expected tovary from locality to locality. For instance, these lithologies are morecommon in the EMO than in theOman ophiolite. This combinedwith thedifferences in crustal sections between ophiolitesmay reflect differencesin the stage of life of the subduction zone: i.e., the Oman and the EMOmay be snapshots of “baby” and “teenager” stages, respectively. The

Tethyan ophiolites will give us an opportunity to investigate thematuring processes of crust–mantle sections in subduction zones.

Acknowledgements

We are grateful to Prof. Adil Neziraj of the Geological Survey ofAlbania, who supported to the field works in Albania. The constructivereviews by John Shervais and Alberto Zanetti much improved themanuscript. T.M. would like to thank Eric Hellebrand for thediscussions on abyssal peridotites, to Kazuhito Ozawa for providingthe spread sheet, and to Alice Colman for the improvement of Englishin the manuscript. This study was financially supported by a Grant-in-Aid for Scientific Research of theMinistry of Education Culture, Sports,Science and Technology of Japan (No. 21403010), the Excellent YoungResearchers Overseas Visit Program from the Japanese Society for thePromotion of Science (JSPS) and Special Coordination Funds forPromoting Science and Technology (JST) for Program for improve-ment of Research Environment for Young Researchers to T.M.

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