ORIGINAL PAPER

Investigation on mantle peridotites from Neyriz ophiolite,south of Iran: geodynamic signals

Mohammad Ali Rajabzadeh &

Teimoor Nazari Dehkordi

Received: 10 May 2012 /Accepted: 10 September 2012# Saudi Society for Geosciences 2012

Abstract Neyriz ophiolite in Abadeh Tashk area appears asfour major separated massifs in an area with 125 km2, southof Iran. Peridotites including harzburgite, dunite, and lesserlow-Cpx lherzolite are the major constituents of the ophio-lite with very minor mafic rocks. Usual gabbros of ophiolitecomplexes are virtually absent from the study area. Mineralmodality associated with bulk rock and mineral chemistry ofthe peridotites show a progression from fertile to ultra-refractory character, reflected by a progressive decrease inmodal pyroxenes and in Al2O3, CaO, SiO2, Sc, Ta, V, andGa values of the studied rocks by approaching chromitedeposits. The Neyriz peridotites vary from low-Cpx lherzo-lite (MgO, 41.97–43.1 wt.%; Al2O3, 0.8–1.3 wt.%) withlow content of Cr# spinel (36.7–37.6) and Fo olivine(90.79–91.5) to harzburgite (MgO, 44.31–45.25 wt.%;Al2O3, 0.29–0.45 wt.%; Cr# spinel, 58.2–73.45; Fo olivine,91.23–91.56), and then to dunite (MgO, 45.9–49.2 wt.%;Al2O3, 0.18–0.48 wt.%) with higher content of Cr# spinel(74.34–79.36) and Fo olivine (91.75–94.68). Compared tomodern oceanic settings, mineral and rock composition oflow-Cpx lherzolite plot within the field of mid-ocean-ridgeenvironment, whereas those of harzburgite and dunite fall inthe field of fore-arc peridotites. As a result of the studies onminerals and whole rock chemistry along with rock inter-relationships, we contend that the peridotites were subse-quently affected by percolating hydrous boninitic melt fromwhich the high-Cr–Mg, low-Ti chromitites were formedwithin mantle wedge above the supra-subduction zone in afore-arc setting.

Keywords Mantle peridotite . Ophiolite . Geodynamics .

Neyriz . Iran

Introduction

Ophiolites are regarded as fragments of upper mantle andoceanic lithosphere that obducted on continental margins,prior to or during an orogeny, i.e., continent–continent andarc–continent collisions (Dewey and Bird 1971; Coleman1977; Nicolas 1989; Bizimis et al. 2000; Pearce et al. 2000;Barth et al. 2003; Aldanmaz et al. 2009; Cawood et al.2009). They are generally found along suture zones, indi-cating ridge–trench interactions and/or subduction–accre-tion events in both collisional-type (i.e., Alpine,Himalayan, Appalachian) and accretionary-type (i.e.,North American Cordilleran) orogenic belts that mark majorboundaries between amalgamated plates or accreted terranes(Lister and Forster 2009; Dilek and Furnes 2011). However,it is accepted that they were formed in a variety of tectonicsettings, as indicated by the mineralogical and geochemicalcharacteristics. For example, layered ophiolite sequencesincluding sheeted dykes, fertile mantle character, and Al-rich spinels suggest a mid-ocean-ridge provenance (Dickand Bullen 1984; Dilek and Thy 1998; Morishita et al.2007; Uysal et al. 2007; Dare et al. 2009), whereas calc-alkaline magmatic affinity with refractory mantle characterand Cr-rich spinels imply subduction-related origin in an arcsetting (Pearce et al. 1984; Dick and Bullen 1984; Zhou etal. 1998; Uysal et al. 2007; Dare et al. 2009). To date,classification schemes have been mostly restricted to so-called mid-ocean-ridge basalt (MORB)-like and supra-subduction zone (SSZ) types (Pearce et al. 1984; Nicolasand Boudier 1991). According to petrological, geochemical,and mineralogical evidence from some ophiolites, it is statedthat there are examples of single ophiolites with both mid-

M. A. Rajabzadeh (*) : T. Nazari DehkordiDepartment of Earth Sciences, Faculty of Sciences,Shiraz University,71467-13565, Shiraz, Irane-mail: rajabzad@geology.susc.ac.ir

T. Nazari Dehkordie-mail: teimoornazari@yahoo.com

Arab J GeosciDOI 10.1007/s12517-012-0687-2

ocean ridge (MOR)- and SSZ-type affinities due to multi-stage histories of the ophiolite formation, i.e., Pindos inGreece (Saccani and Photiades 2004), Troodos in Cyprus(Portnyagin et al. 1997), and Semail in Oman (Python andCeuleneer 2003; Yamasaki et al. 2006; Clénet et al. 2010;Goodenough et al. 2010).

Investigations on geochemistry and mineralogy of mantleperidotites provide constraints on processes such as partialmelting, melt–rock interaction, and melt fractionation thatcontrol the evolution of the uppermost mantle and the over-lying crust (Kelemen et al. 1992; Parkinson and Pearce1998; Godard et al. 2000; Takazawa et al. 2003). The focusof this paper is to describe the geology, petrography, miner-alogy, and geochemistry of mantle section on a representa-tive sample set of rocks from Neyriz ophiolite in AbadehTashk area, south of Iran to recognize the tectonic environ-ment (e.g., supra-subduction or mid-ocean-ridge setting), inwhich the ophiolite was formed.

Geological setting

The most Iranian ophiolites are part of the Middle EasternNeo-Tethyan ocean of Mesozoic that geographically weredivided into four groups: (1) ophiolites of N Iran, along theAlborz range; (2) ophiolites of NW-SE Iran, along theZagros Suture Zone; (3) ophiolitic colored mélanges ofCentral Iran, and (4) ophiolites from SE Iran, in south ofJazmurian depression. They link to other Asian ophiolites,such as Pakistan in the east, or ophiolites in theMediterranean region, such as Turkish, Troodos, and EastEurope in the west (Arvin and Robinson 1994; Kamenetskyet al. 2001; Ghazi et al. 2004; Shaker Ardakani et al. 2009).

The Zagros collisional orogenic belt shows widespreadfolding and thrusting relative to crust thickening and uplift-ing. The southeastern part of the belt is located in Neyrizregion, indicating NW-striking thrust faults, ductile–brittleshear zones, folds, ophiolite, ophiolite mélange, and tectonicslices (Alavi 1980; 1994). Compositionally layered amphib-olites and metamorphosed sedimentary rocks as schists havebeen observed locally.

Neyriz ophiolite as one of the second mentioned groupthrust sheets cover about 1,500 km2 in this region, south ofIran. The Neyriz ophiolite is tectonically juxtaposed beneathcataclastically deformed island arc volcanic and volcanoclas-tic rocks (Babaie et al. 2000) and composed of three imbri-cated sheets, from bottom (SW) to top (NE) (Ricou 1968;1974). At the base, the ophiolite components were thrust ontothe Pichakun series (Ricou 1968) which consists of UpperTriassic limestone, Middle Jurassic oolitic limestone, andLower–Middle Cretaceous conglomeratic limestone, repre-senting Neo-Tethys sediments and which are overlain uncon-formably by anhydritic limestone of the Late Cretaceous

Tarbur Formation (Ricou 1974; Babaie et al. 2000). The initialemplacement as slivers of Neo-Tethyan oceanic crust over theAfro-Arabian continental shelf must have been aCenomanian–Maastrichtian event (Haynes and Reynolds1980; Lanphere and Pamic 1983; Alavi 1994). The south-western limit of the ophiolite thrust sheets distinguished by asystem of nappes and interesting folds in beds of pelagiclimestone and radiolarites, which has resulted in obductionof the ophiolite and associated sedimentary rocks over theMesozoic continental shelf sedimentary rocks of the Arabianplatform. The radiolarites and pelagic limestones have short-ened about 35–40% during folding, and the northeastern limitof the ophiolite thrust sheets distinguished by a system ofbreaching thrusts, which has resulted in transportation of theMesozoic continental shelf sedimentary rocks over the ophio-lites and severe crushing, and intermingling of various rockunits. On the basis of 40Ar/39Ar dating, this ophiolite complexformed between 96–98 Ma (Haynes and Reynolds 1980) andwas emplaced at 89 Ma (Lanphere and Pamic 1983).

Northwestern part of Neyriz ophiolite in Abadeh Tashkarea (study area) is composed of four major separated mas-sifs in an area of 12.5 km long and 10 km wide between theBakhtegan depression to the southwest and high mountainsof Zagros Suture Zone (ZSZ) to the northeast. The studyarea as small mining district contains several high Cr oredeposits that are actively exploited. The ophiolite in thestudy area is thrusted over Bangestan Formation(limestone) of the Early Cretaceous along its western con-tact and is conformably covered by Tarbur Formationshallow-water marly limestone of the Late Cretaceous alongits northeastern border (Rajabzadeh 1998) (Fig. 1).

Ophiolite description

Harzburgite and dunite with lesser low-Cpx lherzolite arethe most ultramafic rocks of the Neyriz ophiolite at AbadehTashk area, displaying tectonite structure. A nearly homog-enous harzburgite with more than 2 km in thickness con-stitutes 80 % of the ophiolite complex at the base. Itbecomes highly depleted and interlayered with numerousdunite layers and dykes upward into the mantle–crust tran-sition zone similar to those of other ophiolites. The dunitelayers and dykes range from 2 to 40 m and from 20 cm to1 m wide, respectively, grading into the host peridotite overa few centimeters to a few tens of centimeters. Homogenousharzburgite at lower parts of the Neyriz ophiolite pile hostsminor low-Cpx lherzolite as patches with no distinct con-tact. The mantle peridotites record two successive episodesof plastic deformations, the first one is related to the igneousaccretion of the lithosphere in mid-oceanic-ridge settingsand the second one was developed during the first stage ofthe emplacement of the peridotites. These two events have

Arab J Geosci

been distinguished on the basis of microstructural criteria(Nadimi 2003).

Serpentinized cumulative dunite that grades progressive-ly into wehrlite as a thick massive rock occurs immediatelyon the mantle–crust transition zone (including depletedharzburgite and subordinate dunite) at the top of ophiolitepile (Fig. 2a). Some small bodies of disseminated chromititewith no economic value were formed along base of thedunite. Usual gabbros of the ophiolite complexes are virtu-ally absent from the ophiolite blocks in the study area, but atthe top of the mantle sequence, there are highly impregnatedperidotites that are cut by diabase (up to 10 m in thickness)(Fig. 2b) and orthopyroxenite cumulate sills and lenses (upto 3 m in thickness). These rocks show cumulate texture.Although intrusive contacts between the dykes and sills aredifficult to recognize owing to faulting, some intrusive con-tacts between dykes and dyke-in-dyke relationships areseen. At field observation, these are concordant with folia-tion of the ultramafic rocks and typically sharp contacts withhost rock. Mantle foliation which is clearly observed in the

basal harzburgite but discontinuously disappears upwardsstrikes 20–35° NW, dipping 35–44° to the southwest.

The most chromite deposits occurred in the mantle–crusttransition zone peridotites upwards the ophiolite column.The deposits are surrounded by residual dunite and thenharzburgite (in rare cases ended by lherzolite). The podiformdeposits are stretched and ruptured by deformation andsuboriented with their long axis parallel to the foliation ofhost rock. They are essentially concordant to subconcordantwith regard to their attitude in the surrounding rock, may beindicating that the ore bodies were emplaced early duringsolid-state flow. The largest one, Cheshmeh Bid deposit, hasa tabular shape with 0.5 to 8 m in thickness, 35–50 m inwidth, and up to 450 m in length (including 120,000 t ofore). The boundaries of the chromite deposits (generallymassive, nodular, banded, leopard) with enclosing duniteare generally sharp but diffuse in some deposits along azone of disseminated ore; meanwhile, dunite dyke marginsare gradational with the host residual dunite and harzburgite.Dunite envelopes can be distinguished from closed residual

Fig. 1 Distribution of ophiolites in Iran and location of study area in Zagros suture zones (after Stocklin 1977; Babazadeh and De Wever 2004)with simplified geological map of the Neyriz ophiolite in Abadeh Tashk area (modified from Rajabzadeh 1998)

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dunites by their sharp contacts with neighbor rocks, absenceof pyroxene, and euhedral to subhedral spinel grains. Whenchromitites of different textures are present, usually massiveones are in centers of the bodies, whereas the other is morecommon towards the rims.

The major structures in the study area are parallel toZSZ trending NW–SE. To the west and southwest, theophiolite, a widely extended colored mélange thatincludes pillow lava, radiolarian chert with accessorymanganese deposits, globotruncana limestone ofTuronian-Maastrichtian, and exotic blocs, was overlap-ped the ultramafics by local faults. Basalts were altered

in zeolite facies, and the colored mélange is coveredby younger sedimentary formations of Tertiary andQuaternary ages.

Analytical methods

One hundred twenty representative samples were systemati-cally selected from the ophiolitic rocks through a verticalsection towards ore–peridotite contacts. The polished thickand thin sections of the rocks were carefully studied usingconventional reflected and refracted light microscopy. Despite

Fig. 2 Microscopic and field photos from peridotites of Neyriz ophio-lite in Abadeh Tashk area. a Thrusting of harzburgite over cumulativerocks at the top of ophiolite pile, b diabase dyke in harzburgite, clherzolite with porphyroclastic texture, d kink-banded olivine in lher-zolite, e foliated harzburgite, f elongated olivine crystals in harzburgite

with protogranular texture, g mesh texture in highly serpentinizeddunite, h coarse-grained chromian spinels in massive chromitite, ipull-apart texture in banded chromitite. h–i were taken in reflectedlight. Olv olivine, Cpx clinopyroxene, Spl chromian spinel, Serpserpentine

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the inevitable difficulties caused by strong serpentinization, 25samples for major elements and some of trace element wereanalyzed using inductively coupled plasma-mass spectrome-try (ICP-MS) at ACME Analytical Laboratories, Ltd.,Canada, following fusion with lithium metaborate/tetraborateand digestion by nitric acid. Detection limits of ICP-MS lietypically between 0.04 and 0.01 % for the major elementsanalyzed. Owing to the low concentration of many of theelements of interest, special care is required to minimizesample contamination. Sample preparation was undertakenin clean-air laminar-flow hoods. Briefly, the procedure is asfollows. Into a Teflon vial, 4 ml HF and 1 ml HNO3 (SPA,ROMIL Cambridge) are added to 100 mg of powdered sam-ple, and the vial is sealed and left on a hot plate at 150 °C for48 h. The acid mixture is evaporated to near dryness; to themoist residue, 1 ml HNO3 is added; and the mixture is againevaporated to near dryness. A second 1 ml HNO3 is added andevaporated to near dryness. These steps convert insolublefluoride species into soluble nitrate species. To the sample,2.5 ml HNO3 is added and is diluted to 50ml after the additionof an internal standard, giving a final concentration of 20 ppbRe and Rh. The internal standard is used to compensate forany analytical drift and matrix suppression effects. The rela-tive error on these data is <1 % for elements that are substan-tially above their detection limits (0.005 wt.% for most majorelements; ∼ 4 ppm for Co, Cr, Ni, V, and W; and ∼1 ppm forCu, Sc, and Zn).

The rock-forming minerals were analyzed quantitativelyby a four-channel CAMEBAX SX50 electron microprobe atthe Service Communications of the University of Nancy I,France. The smaller grains (<1 mm) were checked semi-quantitatively by EDS. Analytical conditions for quantita-tive analyses were: 20 kV accelerating voltage, 10 nA probecurrent, and a beam diameter of 1 μm. Counting times of8 and 4 s for peak and background, respectively, were usedin all analytical runs. Raw data were revised by a PAP(Pouchou and Pichoir 1984, 1991) matrix correction. Tenelements were determined automatically at silicate–spinelanalyses. Calibrations were performed using natural andsynthetic standards. Spinel and silicate analyses were per-formed by using albite (for Al, Na), olivine (for Mg), ortho-clase (for Si, K), wollastonite (for Ca), hematite (for Fe),MnTiO3 (for Mn, Ti), Cr2O3 (for Cr), NiO (for Ni), ZnO (forZn), and metallic V (for V). The chemical data on spinelwere stoichiometrically recalculated in order to distinguishFeO from Fe2O3 according to Carmichael’s procedure(Carmichael 1967). In this second routine, the X-ray linesmeasured were Kα for all elements at spinel and silicateanalyses.

The modal proportions in the rock samples were deter-mined by calculations based on the whole rock major ele-ments, and then, the results were compared with microscopicobservations.

Petrography

Based on modal abundances of principal mineral phases, thestudied peridotites can be divided into three main groups:(1) low-Cpx lherzolites, (2) harzburgites, and (3) dunites.

Low-Cpx lherzolite of Neyriz peridotites with protogra-nular and medium- to coarse-grained porphyroclastic tex-tures contains 60–65 % olivine, 25–30 % orthopyroxene, 5–7 % clinopyroxene, and 1–3 % Cr-bearing spinel by volume.Protogranular types have weakly deformed textures of oliv-ine and orthopyroxene, such as kink bands in olivine andkinked or distorted lamellae in pyroxenes, and porphyro-clastic rocks are characterized by millimeter-sized porphyr-oclasts of olivine and orthopyroxene, varying from anhedralto subhedral, embedded in a fine-grained neoblastic matrixof olivine (Fig. 2c). Both olivine and orthopyroxene showinternal deformation as deformation lamellae along the slipplanes, kink banding, and wavy extinction (Fig. 2d).Clinopyroxene is present as interstitial small crystals or asexsolution lamellae in orthopyroxene. Chromian spinels aretypically anhedral to amoeboid reddish brown crystals.

Harzburgite is the most abundant ultramafic rock ofNeyriz ophiolite that is made of 70–80 % olivine, 15–25 % orthopyroxene, 1–2 % clinopyroxene, and 3–5 %Cr-bearing spinel. Harzburgite displays protogranular,granoblastic, and porphyroblastic textures with millimeter-sized porphyroclasts of brecciated olivine, and kink-bandedand plastically deformed orthopyroxene. This rock is some-times cataclastic to mylonitic at the contact with sedimenta-ry Mesozoic rocks. Olivine and orthopyroxene grains arecommonly elongated coarse-grained (up to a few centi-meters), giving the rock a well-developed foliation(Fig. 2e–f). Serpentinization has mainly been developedalong fractures of the rock. It appears that orthopyroxenewas less serpentinized than olivine, but in highly serpenti-nized harzburgite, it was altered to bastite. Clinopyroxene ispresent as intercumulus small crystals or as exsolution lamel-lae in orthopyroxene. Cr-bearing spinels with brown to red-dish brown color under the microscope are anhedral oramoeboid. The spinels occur interstitially to or within olivine;the rims are sometimes opaque due to replacement by ferrit-chromite or magnetite. Clinopyroxene is also as interstitialsmall crystals or as exsolution lamellae in orthopyroxene.

Dunite often displays equigranular texture but cataclastictexture at the margin of the fault zones. The mineralogy ofresidual dunite in the studied ophiolite is more or lessconstant with olivine (more than 90 %) with minor ortho-pyroxene (2–4 %) and Cr-bearing spinel (1–2 %) with noclinopyroxene. Olivine locally displays kink banding andwavy extinction by plastic deformation. Dunite from chro-mitite envelope is the most serpentinized rock among themantle peridotites; sometimes the degree of serpentinizationreaches 90 % of the rock. Alteration is marked by growth of

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secondary minerals along cracks and grain boundaries, lead-ing eventually to small islands of fresh olivine in serpentinematrix (mesh texture) (Fig. 2g). The dunite includes moreabundant euhedral to subhedral chromian spinel with a fewthin bands of disseminated chromitite.

At the top of the mantle sequence, there are highlyimpregnated peridotites including microleucogabbro andpyroxenite cumulate sills and lenses. These rocks consistof serpentinized olivine, plagioclase, and clinopyroxeneshowing cumulate texture. Although intrusive contacts be-tween dykes are difficult to recognize owing to faulting,some intrusive contacts between dykes and dyke-in-dykerelationships are seen.

Orthopyroxenite consists of orthopyroxene (up to 95 %),olivine (0–3 %), clinopyroxene (0–2 %), and Cr-bearingspinel (1–2 %). Crystals of orthopyroxene, reaching to3 cm in size, form 120° triple-grain junctions with variableintensity of deformation, and show warped cleavages andkink bands. This appearance is consistent with that theorthopyroxenite formation is predated or is synchronouswith development of the rock foliation (Caran et al. 2010).

Diabases from dykes display intergranular and non-vesicular textures that consist mainly of plagioclase and cli-nopyroxene, with a grain size in the range between 0.2 and0.8 mm. The color index of the diabase dykes varies in anarrow range from 30 to 40. The center of the plagioclasecrystals (usually phenocrysts) was slightly altered to saussur-ite, and the pyroxene has been partly altered to chlorite espe-cially at the dyke margins. The diabase dykes margins arecharacterized as fine grains, but the intruded rocks werecoarse-grained in the contact with dykes by recrystallization.

Coarse-grained massive chromitites (up to 2 cm) are gen-erally composed of more than 85 % chromian spinel crystalswith serpentinized olivine and chlorite as gangue minerals(Fig. 2h). In contrast, disseminated ores contain smaller(0.2–1 cm) and more euhedral grains in which olivine andserpentine are the principal interstitial minerals. The effects oftectonic activities are recorded as cataclastic and pull-aparttextures in some chromitites samples. In the nodular ores, thechromian spinel grains constitute aggregates of 1–3 cm nod-ules within olivine-rich matrix. Banded chromitites areformed by alternating chromian spinel-rich (mean 1 cm inthickness) and serpentine-rich bands (up to 5 cm in thickness),exhibiting pull-apart and cataclastic textures (Fig. 2i).

Mineral chemistry

Many points were measured in all available fresh minerals toget detailed chemical characteristics of the samples and to testthe degree of equilibration between and within the mineralgrains. Representative microprobe analyses on mineral phasesof mantle peridotites are listed in Tables 1, 2, and 3.

Fo content (forsterite) of olivine varies in the range of91.75–94.68 in dunite, 91.23–91.56 in harzburgite, and90.79–91.5 in lherzolite. NiO in this mineral ranges between0.27 and 0.66 wt.% in dunite, 0.42 and 0.57 wt.% in harz-burgite, and 0.25 and 0.42 wt.% in lherzolite; thus, Fo andNiO values of olivine in dunite are the highest in compari-son with those in other peridotites. The concentration of Mn,Al, Ca, and Ti is negligible (Table 1).

Orthopyroxenes are generally enstatite in composition(average En90.97) with low contents in trace elements andCr2O3 (less than 0.46 wt.%) than those of harzburgite anddunite from the study area. The same results were reportedfrom other ophiolites, i.e., Antalya ophiolite in Turkey(Caran et al. 2010) and Semail ophiolite in Oman (Tamuraand Arai 2006). As shown in Table 2, lherzolite has higherAl contents in orthopyroxene and clinopyroxene than thoseof harzburgite and orthopyroxene of dunite.

Spinel in peridotites from the Neyriz ophiolite is Cr rich.Cr# [100Cr/(Cr+Al)] varies between 36.7–37.6, 58.2–73.45,and 74.34–79.36 in lherzolite, harzburgite, and dunite, respec-tively. Mg# [100 Mg/(Mg+Fe)] in the spinels changes ininverse direction with Cr#. It is higher in spinels of lherzolite(58.4–59.2) than those of dunite (32.56–49.09). The TiO2

contents of spinels are up to 0.11 wt.% (Table 3). The coresof spinel grains are usually not affected by alteration and showlow Fe3+ contents, but in highly serpentinized samples, spinelshave been completely altered to ferrian chromite or chromianmagnetite. The spinel compositional relationships are consis-tent with those of podiform chromitites in ophiolites else-where, i.e., Semail ophiolte in Oman (Le Mée et al. 2004;Arai et al. 2006; Monnier et al. 2006; Python et al. 2008). Therelationships were also well described from podiform chromi-tites in ophiolites by Barnes and Roeder (2001), and those ofboninitic magmas (Van der laan et al. 1992).

Results of more than 40 analyses from cores of chromianspinel in chromite deposits yield variable composition withCr# values from 73.32 to 81.82 and Mg# from 62.83 to71.47. The Al2O3 content in this mineral varies in the rangeof 9.12–13.85 wt.% with minor amounts of Ti (<0.08 wt.%TiO2), Mn (0.07–0.28 wt.% MnO), Ni (<0.1 wt.% NiO), andV (<0.29 wt.% V2O3) (Table 4). Interstingly, chromianspinel from the ore deposits located in the uppermost partof the mantle section shows lower Cr# which is accompa-nied by higher iron and Ti contents. There is positive cor-relation between the composition of chromian spinel (Cr#)in individual ore body and the thickness of dunite envelope.

Whole peridotite chemistry

Low to medium serpentinization of studied peridotites isindicated by loss-on-ignition (LOI) values of the rocks(3.8–7.4 %). To reduce the effects of the phenomenon, the

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Tab

le1

Com

positio

nof

olivinein

theAbadehTashk

area

perido

tites

(selectedsamples)

Lherzolite

Harzburgite

Dunite

Sam

ple

1-L

2-L

4-L

8-H

9-H

10-H

11-H

20-D

21-D

22-D

23-D

24-D

25-D

26-D

27-D

SiO

242.05

38.99

41.26

42.63

42.37

40.19

41.86

41.99

42.37

40.81

41.67

41.51

40.72

34.84

41.99

TiO

20.00

0.01

0.00

0.00

0.00

0.04

0.00

0.11

0.00

0.02

0.02

0.00

0.00

0.02

0.00

Al 2O3

0.00

0.02

0.19

0.00

0.00

0.03

0.00

0.00

0.00

0.00

0.03

0.00

0.00

0.00

0.00

Cr 2O3

0.00

0.00

0.59

0.02

0.00

0.00

0.00

0.00

0.00

0.01

0.02

0.17

0.00

0.00

0.00

FeO

8.44

8.44

9.08

8.41

8.05

8.55

8.54

5.36

8.05

8.26

7.83

7.89

8.66

8.00

5.83

MnO

0.01

0.00

0.04

0.13

0.09

0.07

0.11

0.00

0.09

0.00

0.11

0.00

0.13

0.13

0.00

MgO

47.63

50.97

50.21

50.89

50.27

49.9

50.45

53.52

50.27

51.53

50.52

51.71

52.73

55.45

49.73

CaO

0.06

0.06

0.12

0.00

0.03

0.01

0.08

0.08

0.03

0.01

0.06

0.07

0.13

0.00

0.06

NiO

0.28

0.25

0.42

0.42

0.47

0.57

0.46

0.66

0.47

0.41

0.27

0.34

0.31

0.38

0.51

Total

98.48

98.74

101.91

102.51

101.27

99.35

101.51

101.72

101.27

101.04

100.56

101.69

102.68

98.81

98.13

Si

1.035

0.967

0.991

1.011

1.015

0.989

1.005

0.994

1.015

0.985

1.006

0.993

0.971

0.874

1.027

Ti

0.000

0.000

0.000

0.000

0.000

0.001

0.000

0.002

0.000

0.000

0.000

0.000

0.000

0.000

0.000

Al

0.000

0.000

0.000

0.000

0.000

0.001

0.000

0.002

0.000

0.000

0.000

0.000

0.000

0.000

0.000

Cr

0.000

0.000

0.011

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.003

0.000

0.000

0.000

Fe2

+0.174

0.175

0.182

0.167

0.161

0.176

0.171

0.106

0.161

0.167

0.159

0.158

0.173

0.168

0.119

Mn

0.000

0.000

0.001

0.003

0.002

0.002

0.002

0.000

0.002

0.000

0.002

0.000

0.003

0.003

0.000

Mg

1.748

1.884

1.798

1.799

1.796

1.831

1.805

1.888

1.796

1.854

1.818

1.843

1.874

2.073

1.814

Ca

0.001

0.002

0.003

0.000

0.001

0.000

0.002

0.002

0.001

0.000

0.002

0.002

0.003

0.000

0.002

Ni

0.006

0.005

0.008

0.008

0.009

0.011

0.009

0.012

0.009

0.008

0.005

0.007

0.006

0.008

0.01

Total

2.965

3.033

2.995

2.988

2.984

3.01

2.995

3.006

2.984

3.015

2.993

0.006

3.029

3.126

2.972

Fa(Fe2

+)

9.05

8.51

9.21

8.48

8.24

8.77

8.67

5.32

8.24

8.25

8.05

7.89

8.44

7.49

6.17

Fo(Mg)

90.95

91.5

90.79

91.52

91.56

91.23

91.33

94.68

91.76

91.75

91.85

92.11

91.95

92.51

93.83

Arab J Geosci

Tab

le2

Com

positio

nof

pyroxenesin

theAbadehTashkarea

perido

tites

(selectedsamples)

Lherzolite

Harzburgite

Dunite

Sam

ple

1-L

2-L

3-L

4-L

8-H

9-H

10-H

11-H

12-H

13-H

14-H

20-D

21-D

22-D

SiO

254.18

53.11

58.49

58.44

53.13

52.73

57.64

57.83

58.22

60.11

58.31

56.92

57.72

55.56

TiO

20.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.02

0.00

Al 2O3

2.68

3.34

2.51

1.81

1.61

1.51

0.18

0.73

0.93

0.51

1.00

0.02

0.04

0.03

Cr 2O3

0.73

0.52

0.00

0.46

0.71

0.81

0.01

0.27

0.33

0.00

0.41

0.00

0.00

0.00

Fe 2O3

0.07

0.44

0.21

0.00

2.32

2.36

0.00

1.15

0.00

0.27

0.00

0.93

1.79

2.71

FeO

1.74

3.29

5.69

5.61

1.71

0.72

7.62

4.63

5.51

4.14

5.54

5.94

3.82

4.14

MnO

0.05

0.28

0.08

0.21

0.05

0.06

0.00

0.21

0.14

0.05

0.05

0.17

0.31

0.06

MgO

17.58

15.83

33.64

33.34

17.58

17.54

33.93

35.46

33.73

34.11

34.03

34.6

36.26

34.78

CaO

24.42

23.71

0.42

0.76

23.72

24.01

0.00

0.84

0.91

0.41

1.15

0.07

0.15

0.17

Na 2O

0.08

0.23

0.02

0.00

0.05

0.05

0.02

0.00

0.21

0.02

0.24

0.04

0.01

0.00

K2O

0.00

0.09

0.00

0.01

0.01

0.01

0.00

0.01

0.01

0.00

0.05

0.00

0.01

0.00

Total

100.53

99.84

101.05

100.62

100.81

99.79

99.41

101.13

99.96

99.58

100.77

98.69

100.13

97.45

Si

1.956

1.946

1.988

1.995

1.941

1.925

2.004

1.967

2.000

1.988

1.991

1.989

1.976

1.964

Ti

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.001

0.000

Aliv

0.044

0.054

0.012

0.005

0.065

0.065

0.000

0.029

0.000

0.012

0.011

0.001

0.002

0.001

Alvi

0.027

0.047

0.008

0.028

0.000

0.000

0.007

0.000

0.038

0.008

0.032

0.000

0.000

0.000

Cr

0.021

0.015

0.000

0.012

0.021

0.023

0.000

0.007

0.009

0.000

0.011

0.000

0.000

0.000

Fe3

+0.002

0.012

0.005

0.000

0.061

0.065

0.000

0.029

0.000

0.005

0.000

0.024

0.046

0.072

Fe2

+0.052

0.101

0.162

0.161

0.021

0.022

0.222

0.132

0.158

0.162

0.015

0.173

0.109

0.122

Mn

0.001

0.009

0.021

0.006

0.001

0.002

0.000

0.006

0.004

0.021

0.002

0.005

0.009

0.002

Mg

0.946

0.865

1.806

1.748

0.951

0.955

1.758

1.798

1.272

1.806

1.732

1.802

1.851

1.832

Ca

0.944

0.931

0.015

0.028

0.932

0.939

0.000

0.031

0.033

0.015

0.042

0.003

0.006

0.006

Na

0.006

0.017

0.001

0.000

0.003

0.003

0.001

0.000

0.013

0.001

0.016

0.003

0.001

0.000

K0.000

0.004

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.002

0.000

0.000

0.000

Total

4.000

4.000

4.000

3.982

3.981

4.000

3.993

4.000

3.983

4.000

3.993

4.000

4.000

4.000

Fa

2.71

5.31

8.16

8.27

1.15

1.15

11.19

6.72

8.25

8.16

8.19

8.77

5.56

6.24

En

48.71

45.61

91.07

90.32

49.82

49.82

88.81

91.72

90.03

91.07

89.63

91.12

94.16

93.43

Wo

48.62

49.09

0.77

1.44

49.02

49.02

0.00

1.57

1.72

0.77

2.18

0.13

0.28

0.33

Arab J Geosci

Tab

le3

Com

positio

nof

Cr-bearingspinelsin

theAbadehTashk

area

perido

tites

(selectedsamples)

Lherzolite

Harzburgite

Sam

ple

1-L

2-L

3-L

4-L

8-H

9-H

10-H

11-H

12-H

13-H

14-H

15-H

16-H

17-H

SiO

20.02

0.05

0.00

0.01

0.11

0.11

0.00

0.00

0.66

0.00

0.03

0.01

0.00

0.00

TiO

20.03

0.05

0.05

0.06

0.00

0.04

0.06

0.07

0.00

0.00

0.00

0.03

0.04

0.08

Al 2O3

35.83

36.31

35.22

34.41

17.19

19.11

23.75

22.83

21.55

20.49

19.04

13.64

16.17

16.27

Cr 2O3

32.11

30.11

30.52

30.22

52.65

50.32

45.69

47.39

50.72

48.44

50.84

56.27

52.41

53.17

V2O5

0.33

0.26

0.21

0.06

0.19

0.33

0.26

0.07

0.16

0.18

0.38

0.18

0.06

0.11

Fe 2O3

2.02

1.98

1.91

1.82

1.61

1.59

1.32

0.00

0.00

1.99

0.53

1.51

3.97

3.62

FeO

15.61

15.82

16.72

16.91

16.94

17.63

14.81

16.86

18.55

18.14

18.12

17.57

12.82

12.32

MnO

0.15

0.13

0.14

0.13

0.16

0.22

0.17

0.22

0.00

0.24

0.31

0.14

0.27

3.27

MgO

14.22

13.92

14.11

14.43

11.72

11.45

13.39

11.44

7.24

11.07

10.92

10.73

14.24

14.46

ZnO

0.12

0.00

0.09

0.04

0.00

0.12

0.13

0.00

0.00

0.09

0.00

0.04

0.00

0.00

NiO

0.13

0.00

0.08

0.14

0.12

0.13

0.19

0.00

0.23

0.15

0.08

0.18

0.00

0.04

Total

100.48

98.57

98.97

98.14

100.64

101.01

99.75

98.86

99.11

100.79

100.23

100.31

100.58

100.32

Cr#

37.61

36.71

36.77

37.21

67.27

63.84

56.34

58.2

61.22

61.32

64.17

73.45

67.7

68.68

Mg#

59.12

58.82

59.22

58.42

55.11

55.68

61.71

52.77

41.05

52.12

51.25

52.11

66.46

67.67

Harzburgite

Dunite

(contin

ued)

Sam

ple

18-H

20-D

21-D

22-D

23-D

24-D

25-D

26-D

27-D

28-D

29-D

30-D

31-D

32-D

SiO

20.01

0.38

0.46

0.08

0.21

0.12

0.22

0.00

0.00

0.00

0.09

0.12

0.42

0.14

TiO

20.05

0.02

0.11

0.08

0.06

0.06

0.00

0.08

0.02

0.07

0.01

0.12

0.03

0.00

Al 2O3

21.12

13.06

12.54

11.21

10.96

10.97

10.01

9.58

10.71

10.51

10.97

10.65

12.17

12.77

Cr 2O3

48.32

56.37

55.91

55.11

54.59

55.61

55.05

54.92

56.73

55.52

57.28

52.54

57.49

57.15

V2O5

0.31

0.32

0.36

0.11

0.31

0.33

0.18

0.31

0.49

0.07

0.07

0.07

0.29

0.38

Fe 2O3

0.26

0.00

0.35

3.99

4.53

3.65

6.03

7.21

2.99

2.78

2.92

7.39

0.00

0.29

FeO

19.57

19.43

19.43

19.77

20.35

18.21

18.74

19.52

20.36

19.62

18.43

20.96

20.25

19.71

MnO

0.24

0.31

0.14

0.12

0.09

0.42

0.21

0.16

0.55

0.52

0.26

0.33

0.15

0.28

MgO

10.21

9.47

9.62

9.12

8.81

9.87

9.76

9.19

8.35

8.32

9.89

8.28

9.26

9.32

ZnO

0.00

0.00

0.22

0.00

0.01

0.00

0.00

0.04

0.00

0.00

0.00

0.00

0.00

0.12

NiO

0.07

0.13

0.00

0.00

0.19

0.00

0.05

0.17

0.18

0.14

0.07

0.06

0.00

0.00

Total

100.11

99.47

99.12

99.57

100.11

99.22

100.25

101.18

100.37

97.48

99.99

100.48

100.06

100.12

Cr#

60.56

74.34

74.95

76.73

76.97

77.28

78.67

79.36

78.05

78.00

77.79

76.81

76.01

75.02

Mg#

48.13

46.43

46.87

45.05

43.55

49.09

48.11

45.66

42.24

43.03

48.89

41.31

44.89

32.56

Arab J Geosci

Tab

le4

Com

positio

nof

chromianspinel

inchromititesfrom

AbadehTashk

area

(selectedsamples)

Sam

ple

1-C

2-C

3-C

4-C

5-C

6-C

7-C

8-C

9-C

10-C

11-C

12-C

13-C

14-C

15-C

16-C

17-C

SiO

20.09

0.00

0.04

0.08

0.03

0.04

0.01

0.00

0.00

0.02

0.00

0.00

0.00

0.00

0.03

0.00

0.00

TiO

20.07

0.02

0.06

0.00

0.00

0.06

0.04

0.06

0.07

0.06

0.00

0.06

0.00

0.08

0.00

0.07

0.02

Al 2O3

13.85

12.95

13.34

12.44

12.02

13.34

12.41

12.45

12.19

12.02

11.96

12.46

12.22

11.62

11.87

9.12

9.39

Cr 2O3

56.72

58.07

58.47

58.98

59.39

58.47

58.83

57.96

57.97

57.47

56.86

57.54

57.18

59.04

58.45

61.22

60.8

V2O3

0.11

0.08

0.22

0.25

0.13

0.21

0.11

0.07

0.19

0.05

0.12

0.25

0.21

0.29

0.18

0.00

0.05

Fe 2O3

2.08

2.22

1.26

2.07

1.68

1.26

2.12

1.93

1.87

2.68

2.21

2.28

2.57

2.29

2.25

4.12

3.38

FeO

12.82

11.88

13.24

13.01

13.79

13.24

12.37

12.32

13.45

12.97

12.91

13.28

12.94

13.77

13.42

11.08

10.43

MnO

0.13

0.25

0.12

0.22

0.25

0.12

0.11

0.17

0.14

0.25

0.24

0.28

0.21

0.14

0.07

0.28

0.23

MgO

13.92

14.28

13.76

13.81

13.09

13.76

14.08

13.83

13.05

13.29

12.92

13.16

13.25

13.13

13.17

14.57

14.69

ZnO

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

1.00

0.00

0.00

0.11

0.00

0.00

NiO

0.16

0.06

0.00

0.08

0.06

0.00

0.05

0.04

0.22

0.09

0.22

0.14

0.14

0.09

0.12

0.03

0.12

Total

99.92

99.81

100.49

100.92

100.44

100.49

100.11

98.82

99.13

98.91

97.41

99.55

98.72

100.45

99.64

100.49

99.12

Si

0.003

0.000

0.001

0.003

0.001

0.001

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.001

0.000

0.000

Ti

0.002

0.000

0.001

0.000

0.000

0.001

0.001

0.001

0.002

0.001

0.000

0.002

0.000

0.002

0.000

0.002

0.000

Al

0.517

0.458

0.497

0.464

0.453

0.497

0.465

0.473

0.464

0.458

0.463

0.472

0.467

0.438

0.451

0.345

0.358

Cr

1.422

1.459

1.463

1.475

1.501

1.463

1.482

1.476

1.482

1.471

1.478

1.463

1.465

1.495

1.488

1.552

1.557

V0.003

0.002

0.005

0.006

0.003

0.005

0.003

0.002

0.005

0.001

0.003

0.006

0.006

0.008

0.005

0.000

0.001

Fe3

+0.052

0.053

0.031

0.049

0.042

0.031

0.052

0.047

0.045

0.065

0.055

0.055

0.063

0.055

0.055

0.099

0.082

Fe2

+0.342

0.316

0.351

0.344

0.369

0.352

0.329

0.332

0.364

0.351

0.355

0.357

0.351

0.369

0.362

0.297

0.283

Mn

0.003

0.007

0.003

0.005

0.007

0.003

0.003

0.005

0.004

0.007

0.007

0.008

0.006

0.004

0.002

0.007

0.006

Mg

0.657

0.677

0.649

0.651

0.624

0.649

0.668

0.664

0.629

0.642

0.633

0.631

0.642

0.627

0.632

0.696

0.709

Zn

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.002

0.000

0.000

0.003

0.000

0.000

Ni

0.004

0.002

0.000

0.002

0.002

0.000

0.001

0.001

0.005

0.002

0.005

0.004

0.004

0.002

0.003

0.001

0.003

Total

3.001

3.001

2.999

2.999

3.000

2.999

3.000

3.000

3.000

2.998

2.999

3.000

3.002

3.000

3.000

2.999

2.999

FeO

/MgO

0.92

0.83

0.96

0.94

0.99

0.96

0.87

0.89

1.03

0.97

0.99

1.00

0.97

1.04

1.01

0.76

0.71

XAla

0.26

0.23

0.24

0.23

0.22

0.24

0.23

0.23

0.23

0.23

0.23

0.23

0.23

0.22

0.22

0.17

0.17

XFe3

+a

0.02

0.02

0.01

0.02

0.02

0.01

0.02

0.02

0.02

0.03

0.02

0.02

0.03

0.02

0.02

0.04

0.04

Cr#

73.32

75.05

74.62

76.08

76.82

74.62

76.07

75.74

76.13

76.24

76.14

75.6

75.83

77.32

76.77

81.82

81.28

Mg#

65.89

68.17

64.96

65.42

62.83

64.96

67.01

66.66

63.34

64.65

64.06

63.86

64.58

62.95

63.58

70.09

71.47

aXAl0

Al/(Al+

Cr+

Fe3

+)andXFe3

+0Fe3

+/(Al+

Cr+

Fe3

+)

Arab J Geosci

major element contents were normalized on a volatile-freebasis for better comparison the samples. By using thismethod, very good correlations were observed amongMgO, other major oxides, and some trace elements forrelatively robust petrogenetic interpretations. Although theaverage MgO content in all peridotites is greater than41 wt.% which shows that these rocks are highly depletedupper mantle peridotites, there is a systematic increase inMgO value from low-Cpx lherzolite to harzburgite and thento dunite.

Geochemical data pertaining to the pyroxene content ofthe rocks in mantle peridotite section at Neyriz ophiolitedemonstrate a large-scale compositional heterogeneity. Thethree distinct peridotites are characterized based on geo-chemistry of the rocks as: (a) low-Cpx lherzolite that reflectsless depletion in elements, e.g., Al2O3 (0.8–1.3 wt.%), CaO(1.4–1.67 wt.%), SiO2 (44.5–46.64 wt.%), Sc (9–13 ppm),V (45–52 ppm), Ga (1.6–1.8 ppm), and Ta (0.4–0.7 ppm)along with lower Ni (1,406–1,762 ppm) and Co (115.2–122.4 ppm) contents, for equivalent MgO (41.97–43.1 wt.%); (b) harzburgite which shows more depletedcharacter than lherzolite by lower contents of Al2O3 (0.29–0.45 wt.%), CaO (0.34–0.47 wt.%), SiO2 (44.63–45.51 wt.%), Sc (5–9 ppm), V (27–35 ppm), Ta (0.2–0.5 ppm), and Ga (0.9–1.4 ppm), and higher Ni (2337–2456 ppm) and Co (124.3–154.9 ppm) values, for equiva-lent MgO (44.31–45.25 wt.%); and (c) residual dunite, themost depleted rock with the lowest SiO2 (40.5–43.1 wt.%),Al2O3 (0.2–0.48 wt.%), CaO (0.06–0.5 wt.%), Sc (4–5 ppm),V (19–27 ppm), Ga (0.9–1 ppm), and Ta (0.1–0.4 ppm), and the highest MgO (45.9–49.2 wt.%), Ni(2550–2731 ppm), and Co (138.7–160.5 ppm) contents(Table 5). Incompatible elements, e.g., Al, Ca, Si, Sc, Ga,Ta, and V, show negative covariance with MgO in all threerock types, but the compatibles such as Ni and Co increasewith decrease in modal Opx and Cpx at the expense ofolivine (increase of Ol/(Opx+Cpx)) (Fig. 3). Not surpris-ingly, mineral phase compositions show analogous changesthat match those of their host whole-rock chemistry. It isworthy to note here that there are enrichments in W (275–1276 ppm) and Ba (10–105 ppm) and depletion in Y(<1 ppm) in Neyriz peridotites.

Discussion

Partial melting and melt–peridotite interaction

Chemical and mineral phase composition in upper mantleperidotites are considered to be reliable petrogenetic indica-tors that are dependent on the degree and/or conditions ofpartial melting in addition to the effects of melt–rock inter-action, e.g., partial melting in lherzolite forms a basaltic melt

with residual mantle peridotite such as harzburgite (Dickand Bullen 1984; Parkinson and Pearce 1998; Ahmed et al.2001; Piccardo et al. 2007; Uysal et al. 2007). By thismanner, the modal abundance and mineral composition ofthe upper mantle peridotites in Neyriz ophiolite reflect theextent of prior depletion in mineral phases and fusibleelements such as Ca, Al, and Ti of preexisting fertile mantle.

It has been documented that the clinopyroxene content ofperidotites reflects the degree of depletion, whereas theforsterite content of olivine is a measure of the total degreeof melting, as olivine–melt equilibrium is not changed sub-stantially by H2O (Gaetani and Grove 1998). On the basis ofthese criteria, Neyriz peridotites from Abadeh Tashk areawith very low modal amount of clinopyroxene and a highlymagnesian olivine are highly depleted rocks that have un-dergone high degrees of partial melting. Chromian spinel isalso regarded as one of the best indicators of the partialmelting process in mantle peridotites (Matsukage and Kubo2003; Tamura and Arai 2006; Uysal et al. 2007). The Cr#versus Mg# of spinel-group minerals in the studied rocks isinversely correlated, consistent with an increasing degree ofpartial melting (Fig. 4). Compared to modern oceanic set-tings, most of the chromian spinels of low-Cpx lherzoliteplot within the field of MOR environment, whereas those ofharzburgite and dunite fall in the field of fore-arc residualperidotites. Cr# (up to 79) and lowest TiO2 values of spinelin dunite suggest a linkage to a boninitic melt (Fig. 5)(Caran et al. 2010). In this regard, Fo content of olivineand Cr# of spinel from all rock types fall into the olivine–spinel mantle array of Arai (1994), indicating their residualorigin and showing a trend of partial melting. Among theserocks, dunite includes a very refractory olivine–spinel as-semblage (Fo, 90.79–94.68 and Cr#, 58.2–79.36), indicat-ing 40 % of partial melting (Fig. 6). The ultra-refractorycharacter of this rock is common to dunites in severalmodern SSZ and thus could hypothetically result either fromrelatively large degrees of partial melting of depleted MOR-type mantle in a SSZ or an interaction between highlymagnesian melt (boninitic magma) and depleted oldMORB-type peridotite, which produces residue of extreme-ly high degrees of partial melting (Parkinson and Pearce1998; Pearce et al. 2000). Alumina contents in pyroxenesare known to be sensitive to the degree of mantle melting,decreasing with increasing depletion of peridotites in pyrox-enes (Ishii et al. 1992; Uysal et al. 2007). As shown inTable 2, lherzolite has higher Al contents in orthopyroxeneand clinopyroxene than those of harzburgite and dunite,clearly following a depletion trend. The TiO2 versus Al2O3

content in the clinopyroxene from lherzolite to refractoryharzburgite shows a compositional variation with a progres-sive increase in partial melting of the host rock (Fig. 7). Inthis case, grains of high-Al spinel in peridotites were mod-ified by reaction with boninitic melts from which high-Cr

Arab J Geosci

Tab

le5

Mod

alandchem

ical

(major

elem

entas

weigh

tpercentandtraceelem

entas

partspermillion)

compo

sitio

nsof

perido

tites

intheAbadehTashk

area

(selectedsamples)

Lherzolite

Harzburgite

Dunite

Sam

ple

1-L

2-L

3-L

8-H

9-H

10-H

11-H

12-H

13-H

14-H

15-H

16-H

20-D

21-D

22-D

Olv

6562

6678

7679

7973

7280

8178

9796

94

Opx

2631

2517

2015

1622

2415

1618

23

4

Cpx

76

62

12

22

22

12

00

0

Spl

21

33

34

33

23

22

11

2

SiO

244.81

46.64

44.52

45.51

45.81

44.94

45.15

45.23

44.9

44.82

44.72

44.63

41.49

40.52

43.11

Al 2O3

1.31

0.81

1.11

0.37

0.45

0.29

0.42

0.42

0.38

0.37

0.35

0.35

0.22

0.18

0.48

Fe 2O3

8.92

7.51

8.81

8.61

8.72

8.77

8.41

8.54

8.81

8.62

8.91

8.73

9.29

9.15

8.81

MnO

0.13

0.14

0.14

0.12

0.14

0.13

0.12

0.14

0.14

0.13

0.14

0.14

0.14

0.12

0.11

MgO

42.77

41.97

43.11

44.47

44.31

45.06

44.97

44.75

44.86

44.81

45.13

45.25

48.49

49.21

45.92

CaO

1.52

1.67

1.41

0.41

0.43

0.34

0.42

0.46

0.46

0.42

0.39

0.47

0.06

0.31

0.51

P2O5

bdl

0.01

bdl

0.01

0.01

bdl

0.01

bdl

bdl

bdl

bdl

bdl

bdl

0.01

bdl

Na 2O

bdl

bdl

bdl

bdl

bdl

bdl

bdl

0.01

bdl

bdl

bdl

bdl

0.01

bdl

bdl

TiO

2bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

K2O

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Total

99.41

98.73

99.04

99.95

100.32

99.98

99.94

99.99

99.97

99.57

100.01

99.99

100.21

99.41

98.89

LOI

4.42

3.82

4.52

7.31

4.91

7.11

7.31

4.92

4.51

4.21

4.12

4.51

7.42

4.75

4.41

Serp

24.55

21.22

25.11

40.61

27.27

39.49

40.61

27.33

25.05

23.38

22.88

25.05

41.22

26.38

24.49

Sc

9.00

13.00

10.00

8.00

9.00

7.00

8.00

8.00

9.00

8.00

6.00

5.00

5.00

4.00

5.00

V45.00

52.00

48.00

30.00

32.00

27.00

32.00

35.00

32.00

31.00

29.00

28.00

19.00

27.00

26.00

Cr

1026

1094

1026

1026

1094

1026

1026

1026

957

957

889

957

1436

1026

889

Co

115.22

122.41

118.21

142.92

137.11

138.81

154.92

124.31

140.12

142.21

147.52

150.21

138.72

160.15

147.12

Ni

1406

1491

1762

2337

2422

2456

2349

2416

2423

2421

2424

2391

2731

2611

2550

Cu

1.72

6.52

14.73

2.72

5.51

2.92

3.51

6.82

9.33

3.82

8.21

7.72

1.41

3.92

2.43

Zn

27.00

27.00

27.00

28.00

28.00

30.00

27.00

26.00

28.00

27.00

29.00

29.00

32.00

29.00

30.00

Ga

1.71

1.62

1.83

1.06

1.41

1.11

1.23

1.00

1.23

1.12

1.30

0.92

1.00

0.92

1.00

Rb

0.21

0.32

0.13

0.23

0.12

bdl

0.22

bdl

0.12

bdl

bdl

0.11

bdl

0.23

bdl

Sr

0.72

0.52

1.31

0.93

0.73

0.81

1.32

0.62

1.33

0.71

bdl

2.23

bdl

0.22

bdl

Ybdl

bdl

bdl

bdl

bdl

bdl

bdl

0.11

0.12

bdl

0.21

bdl

bdl

bdl

bdl

Nb

bdl

0.22

bdl

0.62

0.41

bdl

0.62

bdl

0.23

bdl

0.21

0.32

bdl

0.23

bdl

Cs

bdl

bdl

bdl

0.12

bdl

bdl

0.13

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Ba

22.00

10.00

59.00

33.00

41.00

22.00

56.00

12.00

50.00

52.00

27.00

105.00

10.00

10.00

17.00

Ta0.62

0.71

0.42

0.62

0.62

0.43

0.43

0.31

0.51

0.31

0.41

0.32

0.32

0.11

0.22

W776.65

588.29

637.28

872.23

684.12

564.00

1276.00

429.12

722.00

688.14

720.84

1103.00

275.12

700.62

480.11

REEtot

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Olv,O

px,C

pxandSp

lrepresentsthemodalabundances

(volum

epercent)of

olivine,orthopyroxene,clinopyroxene,andspinelinperidotitesamples,respectively.Detectio

nlim

itof

major

oxides

andtrace

elem

entswas

0.01

%and0.1ppm,respectively.The

degree

ofserpantin

izationisbasedon

estim

ated

LOIforasimpleserpantin

izationreactio

nusingtherelatio

nshipSerp(w

t.%)0

(100/18)LOI(w

t.%)

LOIloss

onignitio

n,bd

lbelow

detectionlim

it.

Arab J Geosci

chromitites crystallized. During this interaction, the Cr/Alvalue of spinel increased in the direction of the ore bodies. Itis believed that boninitic melts can produced in arc settingsabove supra-subduction zones (SSZ) where the presence ofwater, released after dehydration of the subducting slab,may depress melting point, producing great degrees of par-tial melting of variably depleted peridotite (Zhou andRobinson 1997; Edwards et al. 2000; Gervilla et al. 2005;Prichard et al. 2008; Uysal et al. 2009). The presence ofaqueous fluids at formation of peridotites in Neyriz ophioliteis well explained by primary hydrosilicates found as inclu-sions in chromite deposits (Rajabzadeh 1998). Al, Ca, Si, V,Ta, Sc, and Ga are preferentially enriched in melts resulting

from the interaction of primitive basaltic melts with refrac-tory peridotites; thus, progressive increase in whole-rockrefractory character is accompanied by higher Mg# in oliv-ine, higher Cr# in spinel, lower clinopyroxene, and higherolivine beside decrease in incompatible elements in respec-tive bulk rock chemistry of the rocks. Accordingly, lessdepleted lherzolites give way to more depleted harzburgiteand then highly depleted dunite.

The mechanism of chromian spinel concentration inpodiform chromitites has been discussed in many papers,but the processes that concentrate large amounts of chromitedeposits are still a matter of debate (Zhou et al. 1994; Dickand Natland 1996; Arai 1997; Uysal et al. 2007). Most

Fig. 3 Abundance of MgO versus that of various major and traceelements for the peridotites of Abadeh Tashk area. SiO2, Al2O3, andCaO are in weight percent and all the trace elements are in parts per

million. Field of MOR peridotite is from Niu (2004). Fields of fore-archarzburgites and SSZ dunites are from Parkinson and Pearce (1998)

Arab J Geosci

chromite deposits in the Neyriz ophiolite are surrounded bydunite envelope. Considering that harzburgite is residue ofthe first-stage of partial melting and that chromites formedfrom boninitic melt (Uysal et al. 2005), it is most likely thatinteraction between the melt with harzburgite and lherzoliteinvolved in the removal of orthopyroxene by incongruentmelting in these rocks results in the formation of residualdunite around most of the chromitite bodies (Kelemen et al.1992; Zhou et al. 1996; Kubo 2002; Zhou et al. 2005). Asthe initial source of chromium is contained in clinopyroxene(diopside) (Boudier and Nicolas 1985), the intensity ofinteraction between boninitic melt and MORB-type perido-tite plays an essential role for the eventual formation ofchromite deposits. The mantle wedge above a subductionzone is the most likely tectonic setting where hydrous boni-nitic melts can be formed and upwell. Orthopyroxenite veinsin the study area are the evidence of migrating Mg–Si-rich/

Al-poor melt through the peridotites. At the presence ofH2O, the melt crystallizes orthopyroxene, as water assistednot only to enhance the degree of melting of the mantle butalso to form the Si-rich melt (Tamura and Arai 2006).

Tectonic setting and formation of the Neyriz peridotites

A model is proposed here to explain the formation of Neyrizperidotites that represent the fragments of Neo-Tethyan oce-anic lithosphere trapped in the mantle wedge above subduc-tion zone. At the first-stage of partial melting of fertileperidotite in an extensional tectonic setting, Al-rich melt wasproduced, leaving behind a mantle residue depleted to alimited extent and carrying spinel with a low Cr#. These rockswere essentially residual left after extraction of magma frommid-ocean-ridge basalt (MORB) type in Jurassic. Followingperiods of extension, the environment underwent a compres-sional event in Upper Cretaceous accompanied by subduction

Fig. 4 Compositional variations of Cr# versus Mg# in spinel fromAbadeh Tashk peridotites. Field of MOR peridotites is taken from Dickand Bullen (1984), that for fore-arc peridotites is from Ishii et al.(1992) and that of boninite is taken from Van der Laan et al. (1992)

Fig. 5 Compositionalvariations of spinel fromAbadeh Tashk peridotites. aTiO2 versus Al2O3 and b TiO2

versus Cr#. Fields of MORBand Boninite are from Dick andBullen (1984). Field of SSZperidotites is from Kamenetskyet al. (2001)

Fig. 6 Compositional relationship between Cr# of spinel and Focontent of olivine from Abadeh Tashk area peridotites. Fields ofMOR and SSZ peridotites are from Pearce et al. (2000)

Arab J Geosci

processes. Fluids liberated from the subducted slab metasom-atized the already depleted mantle. Metasomatic alteration ofresidual orthopyroxene and clinopyroxene in a low-degree-depleted harzburgite by Na- and K-rich fluids produced asecond-stage fluid-rich melt (Plank and Langmuir 1998).The reported primary Na-rich amphiboles hosted by chromianspinels in chromitites from Neyriz ophiolite (Rajabzadeh1998; Rajabzadeh and Moosavinasab 2012) are consistentwith the scheme. This melt was rich in Mg, leaving behindmore depleted mantle and containing spinel with higher Cr#.During migration of the hydrous melts, chemical disequilibri-um took place between melt and the country rocks. Thesemelts were out of equilibrium with the surrounding perido-tites, leading to dissolution of pyroxene and precipitation ofolivine and forming dunite. This depletion trend, which isaccompanied by decrease of the modal abundance of clino-pyroxene and increase of olivine, is consistent with the for-mation of the peridotites as mantle residues from variableextent of basaltic melt extraction (Aldanmaz et al. 2009).The Mg number of olivine and Cr number of chromian spinelin the resulting rocks increased as the melt fraction progres-sively developed by incongruent melting. When these meltsreached the upper part of the mantle sequence, they ponded insmall pockets, where they continued to react with the hostharzburgite and increased the silica contents of the melts,leading to precipitation of chromite with dunite envelopes.The sharp boundaries between the chromitite pods and duniteenvelopes, by one hand, and the gradational margins of thedunite with the host harzburgite, on the other hand, support theformation of chromitites and their dunite envelopes by melt–rock interaction rather than injection of an ore magma into afracture and subsequent crystallization, in which sharp con-tacts between ore deposits and dunite envelopes are produced

(Zhou et al. 2005). The predominant high Cr# is evidencedthat the chromitites were formed in equilibrium with melts ofboninitic affinity in SSZ setting (Zhou et al. 1996; Melcher etal. 1997; Malpas et al. 2003; Uysal et al. 2005: Mukherjee etal. 2010).

Conclusions

1. The peridotites of Neyriz ophiolite in Abadeh Tashkarea are divided into three groups including low-Cpxlherzolites, harzburgites, and dunites that indicate vary-ing degree of partial melting (from 12 % in lherzolite to40 % in dunite).

2. According to phase modality, mineral chemistry, andwhole-rock composition, lherzolite was formed at thefirst stage in an extensional regime in MOR environ-ment, whereas harzburgite and dunite were formed byhigh degree of partial melting and extraction of magmasassisted by H2O-rich fluid influx derived from the sub-ducted oceanic slabs in arc-related settings at the secondstage. The formation of harzburgite and dunite isexplained by incongruent melting of pyroxenes trig-gered by injection of a hydrous boninitic melt intoprimary fertile mantle rocks.

3. Interaction between the hydrous boninitic melt andcountry rock results in the formation of high-Cr chro-mite deposit surrounded by dunite envelope in hostclinopyroxene-bearing harzburgite in SSZ setting.

Acknowledgments M. Ohnenstetter and D. Ohnenstetter (CNRS,Nancy, France) are greatly acknowledged who kindly provided helpfulcomments during this study. The authors wish thanks the personnel atService Communications, University Nancy II, France for their helpwith microprobe analyses. The authors would like to thank the Re-search Council of Shiraz University for financing this research. Weexpress gratitude to Engineer Parsaei from Fars Chromite IndustrialMining Company, who provided field work possibilities.

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