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Research ArticleHf–Nd isotope constraints on the origin of Dehshir Ophiolite,

Central Iran

HADI SHAFAII MOGHADAM,1,* ROBERT J. STERN,2 JUN-ICHI KIMURA,3 YUKA HIRAHARA,3

RYOKO SENDA,3 AND TAKASHI MIYAZAKI3

1School of Earth Sciences, Damghan University, Damghan 36716-41167, Iran (email: [email protected]);2Geosciences Department, University of Texas at Dallas, Richardson, Texas 75083-0688, USA; and 3Institute

for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokosuka237-0061, Japan

Abstract The peri-Arabian ophiolite belt, from Cyprus in the west, eastward throughNorthwest Syria, Southeast Turkey, Northeast Iraq, Southwest Iran, and into Oman,marks a 3000 km-long convergent margin that formed during a Late Cretaceous (ca100 Ma) episode of subduction initiation on the north side of Neotethys. The Zagrosophiolites of Iran are part of this belt and are divided into Outer (OB) and Inner (IB)Ophiolitic Belts. We here report the first Nd–Hf isotopic study of this ophiolite belt,focusing on the Dehshir ophiolite (a part of IB). Our results confirm the Indian mid-oceanicridge basalt (MORB) mantle domain origin for the Dehshir mafic and felsic igneous rocks.All lavas have similar Hf isotopic compositions, but felsic dikes have significantly less-radiogenic Nd isotopic compositions compared to mafic lavas. Elevated Th/Nb and Th/Ybin felsic samples accompany nonradiogenic Nd, suggesting the involvement of sediments orcontinental crust.

Key words: Hf–Nd isotope, Late Cretaceous, ophiolites, subduction initiation, Zagros.

INTRODUCTION

Ophiolites are relicts of oceanic lithosphere thatdefine sutures between continental terranes. Theyare important markers of convergent margin pro-cesses and preserve the records of tectonic andmagmatic events from rifting–drifting throughsubduction, accretion, and collision stages of con-tinental margin evolution. Ophiolites form in avariety of plate-tectonic settings including oceanicspreading centers, back-arc basins, forearcs, andarcs and other extensional magmatic settingsincluding those formed by mantle plumes (e.g.Kusky 2004; Dilek et al. 2007; Santosh et al. 2009;Pearce & Robinson 2010). Among the variouscategories of ophiolites, the supra-subduction

zone (SSZ) types are dominant (Pearce et al.1984).

The origin and tectonic evolution of ophiolitesprovide important constraints on the evolution oforogenic belts. Understanding ophiolites in aregion such as Southwest Eurasia, the geologicalhistory of which is dominated by accretionarytectonics where tectonostratigraphic terranes ofdifferent origins are now juxtaposed (Ghazi et al.2003), is particularly important. The peri-Arabianregion from Cyprus in the west, eastward throughNorthwest Syria, Southeast Turkey, NortheastIraq, Southwest Iran, and into Oman marks a3000 km-long convergent margin, which formedduring a Late Cretaceous episode of subductioninitiation on the north side of Neotethys (ShafaiiMoghadam et al. 2010). This long-lived conver-gent margin is decorated by ophiolites thatformed in what became the forearc when a ca10-m.y. episode of subduction initiation began ca

*Correspondence.

Received 11 January 2012; accepted for publication 14 May 2012.

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Island Arc (2012) 21, 202–214

© 2012 Wiley Publishing Asia Pty Ltd doi:10.1111/j.1440-1738.2012.00815.x

100 Ma (e.g. Alabaster et al. 1982; Sengor 1990;Floyd et al. 1998; Robertson 1998, 2002; Godardet al. 2003, 2006; Garfunkel 2006; Robertson &Mountrakis 2006; Dilek et al. 2007; Dilek &Furnes 2009; Shafaii Moghadam & Stern 2011a).

The Zagros fold-and-thrust belt of SouthwestIran defines the central part of the peri-Arabianconvergent margin and reflects the Oligo-Mioceneclosure of Neotethys. The growth of this belt con-tinues with subduction–convergence of Arabiaunder Eurasia at about 25 mm/year (Reilingeret al. 2006), commencing at 10–20 Ma (McQuarrieet al. 2003). Many ophiolites define the evolvingperi-Arabian suture between Arabia and Eurasia.Isotopic studies provide important insights onthese ophiolites. In recent decades, several studiesof the Nd–Sr isotopic signatures of Oman andTroodos ophiolitic lavas used these data to tracethe geochemical properties of the mantle source(including mantle domain; Pacific and/or Indian;e.g. Zhang et al. 2005) and to elucidate theirtectono-magmatic setting, but no Hf isotopicstudies have yet been reported. The aim of thispaper is to present Nd and Hf isotopic composi-tions of lavas from the Dehshir ophiolite of South-west Iran, and to use these results to betterunderstand the nature and origin of this ophiolitebelt. These are the first Nd isotopic results forZagros inner belt ophiolites and the first Hf isoto-pic results for any of the Late Cretaceous peri-Arabian ophiolites.

GEOLOGIC SETTING

The Zagros Ophiolite Belt lies along the Northeastflank of the Zagros fold-thrust belt (Fig. 1; seeShafaii Moghadam & Stern 2011a,b for a detailedreview of Zagros ophiolites, and ShafaiiMoghadam & Stern 2011b for a brief overview).Zagros ophiolites can be subdivided into an outerbelt (OB) and an inner belt (IB), south of the MainZagros Thrust Fault (MZT) and along the south-west periphery of the Central Iranian Block(Stocklin 1977). OB includes the Kermanshah,Neyriz, and Haji–Abad ophiolites whereas IBcomprises the Nain, Dehshir, and Balvard–Baftophiolites (Fig. 1). The Nain–Baft ophiolites aredistributed along the Nain–Dehshir–Baft strike-slip fault, which has fragmented and disruptedthese ophiolites. The Dehshir ophiolite is exposeddiscontinuously over about 150 km2 near thecenter of the IB. Despite shearing and fragmenta-

tion of the Dehshir ophiolite as a result of strike-slip movements of the Nain–Dehshir–Baft Fault,stratigraphic contacts between rock units are pre-served in many localities.

The Dehshir ophiolite lava sequence is con-formably capped by Turonian–Maastrichtian(93.5–65.5 Ma) Globotruncana-bearing pelagiclimestones (Fig. 2). The Dehshir ophiolite isdescribed in detail by Shafaii Moghadam et al.(2010). The mantle sequence is composed ofharzburgite with minor cumulate and intrusiverocks including plagioclase lherzolite, clinopyrox-enite, leucogabbro, pegmatitic and isotropicgabbro, and diabasic dikes (Fig. 3c).

The Dehshir ophiolite crustal section includesbasaltic to andesitic massive flows (Fig. 3a), pillowbasalts (Fig. 3d), and a basaltic–dacitic sheeteddike complex (SDC, Figs 3b,4), but amphibolegabbro and diorite make up most of the crustalsequence. Plagiogranite occurs as dikelets injectedinto amphibole-bearing isotropic gabbro anddiorite. Metamorphic rocks (metamorphosed lavasand their pyroclastic equivalents) include fine-grained amphibolite and greenschist. Graniticplutons intrude metamorphic rocks and rarelythe ophiolite. Most of Dehshir ophiolite magmaticrocks show Nb depletions on spider diagrams,indicating that mafic melts were generated froma subduction-modified mantle source (Fig. 4b,c;modified from Shafaii Moghadam et al. 2010).Dehshir ophiolitic lavas evolved from earlynormal-type mid-oceanic ridge basalt (NMORB)type lavas to late calc-alkaline–boninitic lavaswithout any stratigraphic hiatus such as uncon-formity or sediment deposition, as expected forforearc ophiolites associated with subduction ini-tiation (Whattam & Stern 2011). This progressionof forearc ophiolite chemistry from MORB toisland-arc tholeiite (IAT) and boninite is also welldocumented and discussed in detail for Mediterra-nean ophiolites (Dilek et al. 2008; Dilek & Furnes2009, 2011; Dilek & Thy 2009).

Recent U–Pb zircon ages for Dehshir plagio-granites indicates ophiolite formation of ca 100 Ma(H Shafaii Moghadam, F Corfu & RJ Stern unpubl.data, 2012) (Fig. 4). This is similar to radiometricages of other IB ophiolites including Nain ophiolite(ca 101 Ma; H Shafaii Moghadam, F Corfu & RJStern unpubl. data, 2012) but is a little is older thanages of other Late Cretaceous ophiolites of South-west Eurasia, which show U–Pb zircon ages of93–98 Ma for Semail, Oman (Tilton et al. 1981) and92 Ma for Troodos, Cyprus (Mukasa & Ludden1987).

Hf–Nd isotopes 203

© 2012 Wiley Publishing Asia Pty Ltd

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Fig. 1 (a) Map showing the distribution of the inner (Nain–Dehshir–Baft–Shahr-e-Babak) and outer (Kermanshah–Neyriz–Haji–Abad) Zagros ophioliticbelts, the location of the Urumieh–Dokhtar magmatic arc (Eocene–Quaternary), and main Zagros thrust (MZT). (b) Schematic cross section (A-B in Fig. 1a)showing the relationship between the outer and the inner Zagros ophiolitic belts and the Zagros thrust-fold belt (after Shafaii Moghadam et al. 2010).

204 H. Shafaii Moghadam et al.


ANALYTICAL METHODS

Samples were previously studied for petrology andchemical composition (Shafaii Moghadam et al.2010), so only Sm, Nd, Lu, and Hf concentrationsand Nd and Hf isotopic compositions were deter-mined in this study. Prior to digestion, powderswere leached with 6N HCl at 80°C for two hours,rinsed with de-ionized water, and then dried. Thesamples were weighed and digested with HF andHClO4, then dissolved in HCl. A part of the aliquotwas taken from the solution by a micropipette fortrace element analysis after dilution by nitric acid.

Concentrations of Sm, Nd, Lu, and Hf wereanalyzed by inductively coupled plasma massspectrometry (ICP-MS) using an Agilent 7500cesystem (Agilent, Santa Clara, CA, USA) fittedwith PFA sample introduction and a Pt-injecttorch system. The ICP-MS system was operatedin no-collision-gas and multi-tune acquisitionmode. This combination allowed a wide range ofelements to be precisely determined using pulsecounting detection with the HF-containing samplesolution delivered directly into the plasma.Methods for sample dissolution, preparation, andmeasurement have been described by Chang et al.

Fig. 2 Simplified geologic map of theDehshir ophiolite (compiled after Sabze-hei 1997). U–Pb age data (H ShafaiiMoghadam, F Corfu, RJ Stern, unpubl.data, 2012). The location of samples(filled circle) for Hf–Nd isotope study isalso shown on the map.

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(2003) and Nakamura and Chang (2007). Theexternal reproducibility and the internal precisionfor ICP-MS analyses, estimated from repeatedmeasurements of international reference samples(JB-1a of GSJ, BCR-2 and BIR-1 of USGS) weremostly better than 5% and 3% (relative standarddeviation, RSD), respectively.

For isotopic analyses, Hf was separated by asingle-column method using Ln-Spec resin(Eichrom Technologies, Lisle, IL, USA) followingMünker et al. (2001). The total procedural blankfor Hf was less than 25 pg. Hf isotope ratios weremeasured on a sector-type multi-collector ICP-MS(Neptune; Thermo Fischer Scientific, Bremen,Germany) at the Institute for Research on EarthEvolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC).Approximately 100–200 ng of each Hf sample wasdiluted with 10 g of a 3% HNO3 solution and ana-lyzed through a desolvating nebulizer (Aridus II;

CETAC Technologies, Omaha, NB, USA). Themass fractionation factor was determined frommeasured 179Hf/177Hf, and other isotope ratios werenormalized to 179Hf/177Hf = 0.7325 using an expo-nential law. 173Yb and 175Lu peaks were monitored tocorrect for interference of 176Yb and 176Lu on 176Hf.Repeated measurement of the standard JMC475provided 176Hf/177Hf = 0.282141 � 0.000007 (2s,n = 8) during this study. The measured 176Hf/177Hfratios for the samples are normalized to a JMC475value of 176Hf/177Hf = 0.28216.

Nd in the sample was separated from thefirst eluent of the Hf column and further separatedby cation exchange resin (AG50W-X8) usingHCl for rare earth elements (REEs). A secondseparation of Nd was made using a miniaturecation exchange resin (AG50W-X8) column with2-hydroxyisobutyric acid (HIBA) (Hirahara et al.2009). The collected Nd fractions were dissolvedin 1 M HNO3. Approximately 50–100 ng of each Nd

sheeted dike complex

late dacite dike

early diabase dike

lava flow sequence

(b)

diabase dike within mantle harzburgite pillow lava sequence

(a)

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Fig. 3 Field photographs of Dehshir ophiolite. (a) Outcrop of massive lava flows, west of Aziz-Abad village. (b) Late dacite dikes injected into earlydiabase dikes in sheeted dike complex. (c) Isolated diabase dike in mantle sequence. (d) Pillow lava sequence, south of Aziz-Abad village.



sample was loaded onto an outgassed Re filamentand dried at 0.6 A, and then introduced into athermal ionization mass spectrometer (TRITON;Thermo Fisher Scientific) equipped with nineFaraday cups at IFREE, JAMSTEC and analyzedby a normal double filament method. The normal-ization factor used to correct for Nd isotopicfractionation was 146Nd/144Nd = 0.7219, and themeasured isotope ratios were further correctedusing the exponential law.

A standard JNd-1 was analyzed during analysesand yielded 143Nd/144Nd = 0.512096 � 0.000009 (2s,n = 4). eHf(t) and eNd(t) values were calculatedassuming an age of 100 Ma and relative to chon-dritic uniform reservoir (CHUR) values of 0.282785for 176Hf/177Hf and 0.512630 for 143Nd/144Nd.

RESULTS

In this section we focus on Hf and Nd isotopecompositions of six samples from the Dehshirophiolite (Table 1). These are three mafic and threefelsic samples including a basalt pillow lava (AZ06-32), basalt lava flow (DR05-2A), diabase dike

intruding the mantle (G06-16), and three felsicdikes in the SDC (AZ06-8, AZ06-11, AZ06-31)(Fig. 4a). These rocks were previously studied formajor, trace, and REE concentrations (ShafaiiMoghadam et al. 2010) and the results reproducedin Figure 4b,c.

Briefly, the basalt lava (DR05-2A) is slightlylight REE (LREE)-depleted. It lacks Nb and Taanomalies relative to La and thus mimics NMORB(Fig. 4c). G06-16 (diabase dike) is LREE-depletedand strongly depleted in Nb and Ta, similar to IAT.

REE and trace element patterns for felsic dikesamples (AZ06-8, AZ06-11, AZ06-31) as well as acalc-alkaline mafic pillow lava sample (AZ06-32)show similar flat REE patterns, with no or weakEu anomalies, and trace element patterns withnegative Nb and Ta anomalies relative to La,which are common in subduction zone lavas (e.g.Pearce et al. 2005). Felsic lavas have low Sr/Y (3.9–6.0), indicating that they formed by low-P pro-cesses and are not adakites (slab melts) (e.g.Martin et al. 2005).

It is also notable that the MORB-like basalt lava(DR05-2A) does not show an elevated Th/Nb(about 0.1) ratio, whereas other samples with arc

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

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Fig. 4 (a) Simplified lithological succession in the Dehshir ophiolite. Approximate positions of samples analyzed for Hf and Nd isotopic compositionsare shown. U–Pb age data (H Shafaii Moghadam, F Corfu, RJ Stern, unpubl. data, 2012). (b) Chondrite-normalized REE and (c) MORB-normalized traceelements patterns are also shown for the Dehshir lavas. Data are from Shafaii Moghadam et al. (2010).



signatures (AZ06-8, AZ06-11, AZ06-31, AZ06-32)have high Th/Nb (0.47–0.52), suggesting involve-ment of subducted sediments in the magma sourceor crustal contamination, although the extent ofTh enrichment is smaller for IAT diabase (0.23)(G06-16).

Hf is a high field strength element (HFSE) andis essentially immobile via aqueous fluids releasedfrom the subducted slab. Thus, it should not beadded to the mantle wedge if the slab flux isaqueous fluid (Handley et al. 2011). In contrast,melting of subducted sediments or crustal con-tamination will add both Nd and Hf to the magma.Sediment melts will change isotopic compositionsof the magmas towards less radiogenic Nd and Hfthan melts from ambient mantle. Hf–Nd isotopesystematics thus provide useful perspectives forinferring mantle source characteristics and therole of sediments in generating arc magmas (e.g.Pearce et al. 1999; Vervoort et al. 1999, 2011;Woodhead et al. 2001; Yogodzinski et al. 2010).

Figure 5 summarizes Dehshir Hf–Nd isotopicdata (corrected for 100 Ma of radiogenic growth).This figure shows that the six samples clustertightly, with eNd(t) = +6.4 to +8.3 and eHf(t) = +16.3to +17.4. Both mafic and felsic Dehshir lavas showIndian mantle domain affinity (e.g. Pearce et al.2007) (Fig. 5a) and so plot slightly above the globalmantle and crustal arrays defined by Vervoort et al.(1999) (Fig. 5b). This feature does not seem to begreatly affected by sediment melt addition orcrustal sediment assimilation because similar Nd–Hf isotopic compositions are seen for MORB andIAT-like mafic rocks lacking significant Th enrich-ment (G06-16 and DR05-2A) as for the mafic sample

with Th enrichment and arc signatures (AZ06-32),although the latter has slightly lower eNd(t).

Contamination of mantle wedge by slab-derived fluids released from dehydration ofaltered oceanic crust is undetectable on the eHf(t)vs eNd(t) plot (e.g. Handley et al. 2007, 2011).However, the mantle source of the Dehshir lavascould have been affected by subducted pelagicsediments and/or the mantle-derived melt, or byassimilation of sediments in the continental crust,because these lavas have considerable variationsin eNd(t) (Fig. 5).

The felsic dikes of this study may reflect theinvolvement of subducted sediments or crust inthe mantle sources, which have either highNd/Hf (pelagic sediment) or low Nd/Hf (continen-tal crust) (e.g. Gasparon & Varne 1998; Plank &Langmuir 1998; Carpentier et al. 2009; Handleyet al. 2011). Figure 6 shows the results of bulk-mixing calculations between mantle-derived meltand sediments and continental crust contaminants.Involvement of subducted sediments in the mantlesource affects Nd more than Hf isotopic com-positions (Fig. 6). In contrast, involvement of con-tinental crust affects Hf as well as Nd isotopiccompositions (Fig. 6).

The Dehshir felsic samples have similar eHf(t)(average eHf(t) = 16.6) to the MORB-like basalt(average eHf(t) = 17.0) but significantly less radio-genic Nd isotopic compositions than the maficsamples (average felsic eNd(t) = 6.64 in contrastto average mafic eNd(t) = 7.33). The horizontaloffsets to slightly lower eNd for the Dehshir felsiclavas (Fig. 6) may reflect greater involvement ofpelagic sediments, which would have high Nd/Hf

Table 1 Sm–Nd and Lu–Hf concentrations and Nd–Hf isotopic data of the Dehshir volcanic rocks

Dehshir ophiolite Diabase dike Pillow basalt Lava flow Dacite dike Dacite dike Dacite dike

G06-16 AZ06-32 DR05-2A AZ06-8 AZ06-11 AZ06-31Nd (ppm) 3.342 6.787 15.377 7.752 5.633 5.544Sm (ppm) 1.446 2.047 5.081 2.318 1.728 1.668Lu (ppm) 0.337 0.382 0.792 0.425 0.348 0.381Hf (ppm) 1.040 1.529 1.110 1.879 1.370 1.966143Nd/144Nd 0.5131041 0.5129595 0.5130113 0.5129697 0.5129660 0.51297222ss 0.0000055 0.0000038 0.0000051 0.0000055 0.0000043 0.0000067147Sm/144Nd 0.261 0.182 0.200 0.181 0.185 0.1822s 0.007 0.005 0.002 0.005 0.007 0.004176Hf/177Hf 0.2832863 0.2832513 0.2833698 0.2832477 0.2832360 0.28323302s 0.0000077 0.0000066 0.0000058 0.0000078 0.0000079 0.0000069176Lu/177Hf 0.046 0.035 0.101 0.032 0.036 0.0282s 0.001 0.000 0.001 0.001 0.001 0.000eNd(t)† 8.31 6.44 7.24 6.67 6.54 6.7eHf(t)‡ 17.39 16.82 16.9 16.92 16.25 16.68

† eNd(t) = [(143Nd/144Nd) sample/(143Nd/144Nd)CHUR - 1] ¥ 10 000‡ eHf (t) = [(176Hf/177Hf) sample/(176Hf/177Hf)CHUR - 1] ¥ 10 000



ratios. Similarly, slightly lower eNd(t) for the arc-type mafic lava with a high Th/Nb (AZ06-32,eNd(t) = 6.44) is also attributable to mixing sedi-ments with the source mantle (Fig. 6).

DISCUSSION

SIGNIFICANCE OF INDIAN MANTLE DOMAIN SIGNATUREIN TETHYAN OPHIOLITES

The Indian mantle domain signature is knownfrom south of Australia along the SoutheastIndian Ridge to south of Africa on the southwestIndian ridge (Mahoney et al. 1992). Isotopicallythis mantle domain signature is characteristic ofseveral back-arc basins in the western Pacific(e.g. Stern et al. 1993; Hickey-Vargas 1998). Thepresence of this mantle domain is inferred forTethyan seafloor in southern Eurasia because thenow-vanished Tethys Ocean covered most of theregion currently occupied by Indian Ocean(Zhang et al. 2005). Nd–Sr–Pb isotope data onrocks from Tethyan Jurassic to Late Cretaceousophiolites, including Oman (Benoit 1997), IranianNeyriz and Band-e-Zeyarat, and Tibet ophiolites(Zhang et al. 2005) also show signatures forIndian Ocean-type mantle source. The Jurassic –Early Cretaceous Masirah ophiolite does notshow such a signature (Mahoney et al. 1998). Allthe cited studies use Nd, Pb, and alteration-modified Sr isotopes. Seafloor alteration usuallycan greatly affect Sr isotope composition of therocks, so this measurement is less useful than Ndand Hf isotope data. New data reported hereusing fluid-immobile Hf–Nd isotopes confirm anIndian mantle domain for the Late CretaceousDehshir ophiolite.

Fig. 5 eHf(t) vs eNd(t) diagram forDehshir igneous rocks. Note thatDehshir igneous rocks have IndianOcean-type mantle affinity (a) and plotabove the global arrays (b) of Vervoortet al. (1999).

Mantle ArrayeHf = 1.33eNd + 3.19

Crustal ArrayeHf = 1.35eNd + 2.82

Terrestrial ArrayeHf = 1.36eNd + 2.95

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+

1

2

5

10

0.51

3

10

Fig. 6 eHf(t) vs eNd(t) diagram showing how Dehshir lavas are domi-nated by melt sources similar to Indian Ocean MORB (IMORB). Data offelsic samples can be explained by participation addition of <<0.5%pelagic sediments. Models of bulk mixing between IMORB and highNd/Hf pelagic clays and low Nd/Hf continental crust are from Handleyet al. (2011). Assumed values for end-components are: IMORB melt,Nd = 0.97 ppm, Hf = 0.25 ppm, 143Nd/144Nd = 0.513042, 176Hf/177Hf = 0.283211; pelagic clay, Nd = 187.9 ppm, Hf = 5.73 ppm, 143Nd/144Nd = 0.512236, 176Hf/177Hf = 0.282828; and sand-rich sediments(continental crust), Nd = 31.3 ppm, Hf = 5.58 ppm, 143Nd/144Nd = 0.511910, 176Hf/177Hf = 0.282230.



HF–ND ISOTOPE CONSTRAINTS ON SLAB INPUTS

There are no other Hf isotopic data for Late Cre-taceous ophiolites in the 3000 km-long suture zonethat stretches from Troodos to Semail. There issome Nd isotope data on lavas from Neyriz, Iran(Zhang et al. 2005), Oman (Godard et al. 2006), andTroodos (McCulloch & Cameron 1983) ophiolites.These data are summarized on a eNd vs Th/Ybdiagram, combining all data from the literature onthe Oman and Neyriz ophiolites along with ourDehshir data (Fig. 7).

The Dehshir ophiolitic rocks have broadlysimilar but variable Nd isotopic compositions(eNd = +6 to +8) reflecting different slab inputs, asimplied by variable Th/Yb ratios (Fig. 7). TheDehshir felsic dikes (AZ06-8, AZ06-11, AZ06-31)and the calc-alkaline mafic pillow lava (AZ06-32)are characterized by high Th/Yb ratios and slightlyless radiogenic Nd isotopic compositions. Thesecontrast with the two other MORB- and IAT-typeDehshir mafic samples (G06-16 and DR05-2A) withlower Th/Yb and more radiogenic Nd (Fig. 7).

The Oman lower lavas (Geotimes or V1 unit)along with Neyriz and Dehshir (DR05-2A) MORB-type lavas have radiogenic Nd with moderately lowTh/Yb (Fig. 7). The upper boninite-type/highlydepleted tholeiitic lavas from Oman (Lasail orV2 unit), one Neyriz lava and one Dehshir IAT-type lava (G06-16) are characterized by radiogenicNd with the lowest Th/Yb (Fig. 7).

From Figure 7, a few important aspects canbe seen. First, it is clear that all the Dehshirlavas with arc signatures (e.g. samples G06-16and DR05-2A with low Th/Nb = 0.1–0.23 and

Th/Yb = 0.04–0.09 respectively) are clearly differ-ent from other lava suites, indicating stronginvolvement of sediment components in the meltsource (e.g. samples AZ06-32, AZ06-8, AZ06-11,and AZ06-31 with higher Th/Nb = 0.47–0.52 andTh/Yb = 0.25–0.29). Whether this is from shallowIran crust or subducted sediments is not clear.

Second, the MORB-type lavas (Geotimes) andboninite-type/highly depleted tholeiitic lavas(Lasail) from Oman plot in different fieldsalthough Nd isotope compositions are almost iden-tical. The Dehshir lava with MORB-type affinity(DR05-2A) falls in the MORB-type lava field fromOman, while IAT-type lava (G06-16) shows affinitywith the boninite-type/highly depleted tholeiiticlava field from Oman (Fig. 7). Therefore, theMORB-like mafic lavas still largely reflect themantle source in terms of Nd and Hf isotopes (seeabove section).

IMPLICATION FOR TECTONIC SETTING

The eNd(t) data available for a 3000 km-longTethyan belt including the Oman, Dehshir, Neyriz,and Troodos ophiolites are shown in Figure 8. Notethat most Troodos samples are less radiogenicthan those of the Dehshir, Neyriz, and Oman,which plot between eNd(t) = +6.5 and +9. It isnotable that the samples with eNd(t) > +7 includethe MORB-type and boninite-type/highly depletedtholeiitic lavas from the Dehshir, Oman, andNeyriz ophiolites, whereas those with eNd(t) < +7are for sediment-affected arc-type lavas in the caseof Dehshir. Therefore, the Troodos lavas witheNd(t) = 0 to +6 can be highly affected by sedimentcomponents. These less radiogenic Troodos lavasare upper lavas of boninitic affinity.

The nearly simultaneous formation of supra-subduction zone Tethyan ophiolites forming a beltfrom Oman in the east through Iran to SouthwestTurkey and Troodos (Cyprus) in the west, overthousands of kilometers, provides the best geologi-cal evidence that this ophiolite belt reflects a majorsubduction initiation event (Pearce & Robinson2010). For this setting, Ishizuka et al. (2006) andReagan et al. (2010) suggested models in whichsubduction initiation was accompanied by spread-ing to produce forearc basalts with no signature ofslab-derived components and then by boniniticprotoarc volcanism with more and various sub-ducted slab signatures, reflected in the relatederuption of tholeiitic and boninitic lavas.

It is still in question whether or not the loweNd(t) is totally caused by the involvement of

0.00 0.05 0.10 0.15 0.20 0.25 0.30 Th/Yb

6.00

6.50

7.00

7.50

8.00

8.50

9.00

εNd (t)

Dehshir felsic lava

Neyriz lava

Oman lava (Geotimes)

Oman lava (Lasail)

MORB-type lavas

Boninite-type/highly

Dehshir mafic lava

G06-16

DR05-2A

AZ06-32

depleted tholeiitic lavas

Fig. 7 eHf vs Th/Yb for Dehshir lavas and comparison with lavas fromother late Cretaceous ophiolites including Neyriz and Oman ophiolites.Data sources are Zhang et al. (2005) for Neyriz lavas and Godard et al.(2006) for Oman lavas.



sediments with the supra-subduction zone mantle.Fertile Indian domain mantle source also accountsfor this. Another question is what is regional iso-topic heterogeneity and whether or not there is analong-strike gradient in isotopic composition forthis ophiolite belt. More Nd–Hf isotope data alongwith trace element compositions for other LateCretaceous Tethyan ophiolites are needed to testthis idea.

CONCLUSIONS

We have highlighted the importance of Hf and Ndisotopes as a petrogenetic tool for characterizingthe mantle source and sediment contributions,either subducted or upper plate, for magmasources of the Dehshir supra-subduction zoneophiolite. In a eHf(t) vs eNd(t) plot, all Dehshir

volcanic rocks show derivation from an IndianMORB-type mantle domain. The Dehshir felsicrocks have similar eHf(t) ratios to the MORB-likemafic lavas but have less radiogenic Nd isotopiccompositions than the mafic samples. This maymean that felsic lavas reflect greater involvementof crustal sediments in the source region. Theavailable eNd(t) data for the Oman, Dehshir,Neyriz, and Troodos ophiolites indicate thatTroodos lavas are the least radiogenic of the LateCretaceous Peri-Arabian Tethyan ophiolites.

ACKNOWLEDGEMENTS

We are very grateful to Yildrim Dilek for a con-structive review of the manuscript. Editorial sug-gestions by Tsuyoshi Ishikawa and Susumu Uminoare highly appreciated.

aeS

naips

aC

fluGnais

reP

Oman

Tehran

susacuaC

Kermanshah

Haji-Abad

Neyriz

Nain

Dehshir

Baft

Makran Kahnuj

Birjand

Sabzevar

naruT

enoZnois

illoCsor

gaZ-sil

tiB

AIBARA

EURASIA

Black Sea

Istanbul

AFRICA

Kizildag

FAE

NAF

Ankara

TAURIDES

Troodos Baer-Bassit

Pozanti-Karsanti

aeS

deR

Ionian Sea

hcnerTcinelleH

Mediterranean Sea

Aegean Sea

Mirdita

Budapest

Zagreb

enoZ

radr

aV

enoZ

sodn

iP

seni

nnep

pA

aeS

citair

dA

Moesian Platform

10º E 20º E 30º E 40º E 50º E 60º E

20º N

30º N

40º N

Tethyan OphiolitesTethyan Suture Zones

Lycian

Antalya Mersin

Beysehir

~101 Ma

~100 Ma

~95 Ma

~92 Ma

90-92 Ma

0 1 2 3 4 5 6 7 8 9 10

eNd

Freq

uenc

y

0

2

4

6

8

10

12

Dehshir lavasNeyrizlavas

Troodos upper lavas

Troodos lower lavas

Oman lavas

(b)

(a)

Fig. 8 (a) Simplified tectonic map of the eastern Mediterranean–Zagros region showing the distribution of the Neotethyan ophiolites and suture zones(modified after Dilek et al. 2007). For clarity we have shown the U-Pb ages of the Neo-Tethyan ophiolites. (b) Histogram compares available eNd(t) datafor Oman, Dehshir, Neyriz, and Troodos ophiolites along the 3000 km-long Tethyan ophiolite belt. Data are from Zhang et al. (2005), Godard et al. (2006)and McCulloch and Cameron (1983) for Neyriz, Oman and Troodos ophiolites, respectively.



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