The Dehshir ophiolite (central Iran): Geochemical ...rjstern/pdfs/DehshirGSAB10.pdf · “Penrose ophiolite,” including tectonized ... Shahid Beheshti University, Tehran, Iran

1516

ABSTRACT

The Late Cretaceous Dehshir ophio lite is an important element within the Inner Zagros (Nain-Baft) ophiolite belt and contains all components of a complete “Penrose ophiolite,” including tectonized harz burgites, gabbros, sheeted dike com-plexes, pillowed basalts, and rare ultra-mafi c-mafi c cumulates. The cumulate rocks of this ophio lite are composed of plagioclase lherzo lite, clinopyroxenite, leucogabbro, and pegmatite gabbro. All the massifs in the Inner Zagros ophiolite belt are overlain by Turonian-Maastrichtian pelagic limestones (93.5–65.5 Ma). Clinopyroxene compositions of Dehshir mafi c rocks are similar to those of both boninites and island-arc tholeiites. Nearly all spinels from the inner ophiolite belt are similar to those of highly depleted harzburgites from intra-oceanic forearcs, although some Dehshir harzburgite spinels plot within the fi eld for abyssal (mid-ocean-ridge basalt) peridotites. All components of the Dehshir and other ophiolites of this belt show strong suprasubduction-zone affi ni ties, from harzburgitic mantle to ophiolitic lavas. Volcanic rocks have a mixture of dominantly arc-like (island-arc tholeiite, boninite, and calc-alkaline) and subordinate mid-ocean-ridge basalt–like compositional features, usually with mid-ocean-ridge basalt–like rocks at the base and arc-like rocks at the top. Our data for the Dehshir ophiolite and the similarity of these results to those for Iranian inner and outer belt ophiolites compel the conclusion that a geographically long, broad, and continuous tract of forearc lithosphere was created at about the same time during the earliest stages of subduc-tion along the southern margin of Eurasia in Late Cretaceous time.

INTRODUCTION

Late Cretaceous ophiolites in the Mediter-ranean eastward through Turkey, Syria, Iran, Oman, Afghanistan, and into Pakistan are fos-sil slices of Neotethyan oceanic lithosphere (Robert son, 2002). In the last few decades, many studies have focused on geochemistry and genesis of crustal as well as mantle sequences from Late Cretaceous ophiolites, especially in Oman, Cyprus, and Turkey (e.g., Dilek et al., 2007; Garfunkel, 2006; Godard et al., 2003, 2006; Robertson, 2002; Floyd et al., 1998; Ala-baster et al., 1982; among others). Despite broad affi nities to other ophiolites in the Tethyan Mediterranean-Oman ophiolite belt, the Late Cretaceous ophiolites of south-central Iran re-main poorly known in terms of geochemistry, petrogenesis, and tectono-magmatic evolution.

The Iranian ophiolite belt, of central interest to this study, lies along the NE fl ank of the Zagros fold-and-thrust belt, which fl anks the Persian Gulf on the southern margin of Eurasia. The Ara-bian plate has been moving north since Late Cre-taceous time as Neotethys was consumed, fi rst as part of Africa, and more recently as a separate plate. The Zagros orogenic belt—in the position of an accretionary prism—manifests this ongo-ing plate convergence, which is in transition now from subduction to continental collision.

Abundant ophiolites in the Zagros orogenic belt define the suture between Arabia and Eurasia, and comprise two parallel belts. Fol-lowing Stocklin (1977), we use “Outer Zagros ophiolite belt” and “Inner Zagros ophiolite belt” to describe the belts containing the Neyriz and Nain-Baft (Dehshir) ophiolites, re-spectively (Fig. 1).

The Neyriz ophiolite lies immediately south-southwest of the Main Zagros Thrust (MZT) and is the best-studied ophiolite of the Outer Zagros ophiolite belt. There is a broad consensus that this ophiolite formed in a suprasubduction-zone environment (e.g., Arvin, 1982; Babaie et al.,

2001, 2005, 2006; Ghazi and Hassanipak, 2000; Ghazi et al., 2003). The formation and tectono-magmatic evolution of Inner Zagros ophiolite belt, including the Nain-Baft ophio-lites and the Dehshir ophiolite of interest here, are controversial and have been variously in-terpreted as: (1) representing a narrow, Red Sea–like ocean, created at a slow spreading center (e.g., Davoudzadeh , 1972; Berberian and King, 1981; McCall and Kidd, 1981; Desmons and Beccaluva , 1983; Şengor, 1990; Arvin and Robin son, 1994; Arvin and Shokri, 1997; Babaie et al., 2001); (2) an arc basin related to Tethyan subduction (e.g., Delaloye and Desmons, 1980; Ghazi and Hassanipak, 2000); and (3) a back-arc basin (Shahabpour, 2005; Agard et al., 2006).

Limited geochronology indicates that inner and outer belt ophiolites formed at the same time during the Late Cretaceous and may have originally been a continuous sheet. Horn-blende gabbros from the inner belt yield a K/Ar age of 93 Ma (Shafaii Moghadam et al., 2007), whereas outer belt Neyriz diabases and hornblende gabbros yield 40Ar/39Ar ages of 86 and 93 Ma, respectively (Lanphere and Pamic, 1983), and Neyriz hornblende gab-bros yield 40Ar/39Ar ages of 92.1 ± 1.7 Ma and 93.2 ± 2.5 Ma (Babaie et al., 2006). These are broadly similar to ages of other ophiolites of the Mediterranean-Oman belt, which get younger toward the west, from ca. 95 Ma for Oman (Hacker et al., 1996) to 90–94 Ma for Cyprus (Mukasa and Ludden, 1987). This similarity in ophiolite ages and suprasubduction-zone chem-istry led Mukasa and Ludden (1987) to iden-tify the Mediterranean-Oman ophiolite belt as “…a[n] ~3000 km long axis of rift-related is-land arcs” (p. 825). The similarity of ages for inner and outer belt ophiolites and their parallel alignment between the Urumieh-Dokhtar arc and the Zagros fold-and-thrust belt suggest that they may be exposed limbs of a deformed and eroded anticlinoria, and may represent forearc basement between the Urumieh-Dokhtar arc

For permission to copy, contact [email protected]© 2010 Geological Society of America

GSA Bulletin; September/October 2010; v. 122; no. 9/10; p. 1516–1547; doi: 10.1130/B30066.1; 16 fi gures; 7 tables.

†E-mail: [email protected]

The Dehshir ophiolite (central Iran): Geochemical constraints on the origin and evolution of the Inner Zagros ophiolite belt

H. Shafaii Moghadam1,†, Robert J. Stern2, and M. Rahgoshay3

1School of Earth Sciences, Damghan University of Basic Sciences, Damghan, Iran2Geosciences Department, University of Texas at Dallas, Richardson, Texas 75083-0688, USA3Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran

The Dehshir ophiolite

Geological Society of America Bulletin, September/October 2010 1517

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Figure 1. (A) Map showing the distribution of the Nain-Baft (inner ) Zagros ophiolitic belt, the Kermanshah-Neyriz (outer) Zagros ophiolitic belt, and the location of the Urumieh-Dokhtar mag-matic arc (Eocene-Quaternary) and Main Zagros thrust (MZT). (B) Schematic cross section show-ing the relations between the outer and the inner Zagros ophio-litic belts and the Zagros fold-and-thrust belt.

Shafaii Moghadam et al.

1518 Geological Society of America Bulletin, September/October 2010

and the Zagros accretionary prism, developed over a N-dipping subduction zone on the south-ern Eurasian margin. The forearc interpretation is further supported by the ~150 km width of the region encompassed by inner and outer ophio-lites, which is about that of typical forearcs (Stern, 2002). This may be termed the “Iranian forearc hypothesis.”

In this paper, we present fi eld, petrologic, and geochemical data for different lithological units of the Dehshir ophiolite, part of the Inner Zagros ophiolite belt, which is exposed south-southwest of the Urumieh-Dokhtar magmatic arc (Fig. 1). To our knowledge, this is the fi rst modern petro-logic and geochemical study of the Dehshir ophiolite. We use this information and our inter-pretations to test the Iranian forearc hypothesis , i.e., those Zagros ophiolites defi ne the disrupted remnants of a huge (3000-km-long) tract of forearc basement that formed during an episode of subduction initiation along the southern mar-gin of Eurasia in Late Cretaceous time.

GEOLOGICAL OUTLINE

The Zagros orogenic belt consists of fi ve trench-parallel tectonic subdivisions, from un-deformed trench fi ll (SW) to magmatic arc (NE; Alavi, 1994): (1) Mesopotamian foredeep–Persian Gulf; (2) Zagros fold-and-thrust belt; (3) outer ophiolite belt; (4) Sanandaj-Sirjan zone; (5) inner ophiolite belt; and (6) Urumieh-Dokhtar magmatic arc (Fig. 1B). The Zagros orogenic belt plus foredeep defi nes a classic convergent plate margin (Farhoudi and Karig, 1977; Farhoudi, 1978). The foredeep represents a fi lled trench, the Zagros fold-and-thrust belt represents an accretionary prism, the ophiolite belts represent disrupted forearc crust, and the Urumieh-Dokhtar magmatic arc is, of course, the magmatic arc. The last four of these sub-divisions and the Sanandaj-Sirjan zone are de-scribed further next.

Zagros Fold-and-Thrust Belt

The Zagros fold-and-thrust belt (Fig. 1B) refl ects the shortening and offscraping of thick sediments from the northeastern margin of the Arabian platform, essentially behaving as the ac-cretionary prism for the Iranian convergent mar-gin (Farhoudi and Karig, 1977; Farhoudi, 1978). The Zagros Fold-and-Thrust Belt is bounded on the northeast by the Main Zagros thrust (Ber-berian and King, 1981; Berberian, 1995).

Outer Zagros Ophiolitic Belt

Geologic relations observed at the Neyriz ophiolite refl ect those of typical outer belt ophio lites. Three imbricated sheets are recog-

nized in the Neyriz ophiolite, from SW to NE, (Ricou, 1971, 1974, 1976; Ricou et al., 1977). (1) The base of the complex includes abyssal sediments of the Neotethys Ocean as slices of Late Triassic limestone, Middle Jurassic oolitic limestone, and Lower-Middle Cretaceous con-glomeratic limestone, known as the Pichakun Series (Ricou, 1971). (2) Two mélange units consist of exotic blocks of Permian-Triassic limestones asso ciated with radiolarites, alkaline and tholeiitic pillow lavas, serpentinites, alkali gabbros, sandstones, tuffi tes, and metamorphic rocks. This sequence (whole package) is similar to the Hawasina Group of Oman (Bechennec et al., 1990). (3) Tectonic slices of oceanic mantle and crust (the ophiolite component) thrust SW over the Pichakun and mélange series.

Sanandaj-Sirjan Zone

The Sanandaj-Sirjan zone separates the Inner Zagros ophiolite belt from Outer Zagros ophio-lite belt. Rocks of the Sanandaj-Sirjan zone make up the metamorphic core of Zagros belt (Mohajjel et al., 2003). Rocks of the Sanandaj-Sirjan zone span most of Phanerozoic time, including imbricated slices of marine and con-tinental siliciclastic sediments metamorphosed under low- and medium-grade greenschist conditions (Alavi, 1994). Lavas and intrusive bodies accompany these sediments. Because it separates inner and outer belt ophiolites, the na-ture of this zone is a critical test of the Iranian forearc hypothesis. This zone is generally con-sidered to be either an accreted microcontinent that separated from Gondwanaland during the Early Jurassic (e.g., Stocklin, 1968; Golonka, 2004; among others) or a once-active margin of the Central Iranian block (e.g., Berberian and King, 1981; Sheikholeslami et al., 2008; Fazlnia et al., 2009). We think that the nature of Sanandaj-Sirjan zone sediments, their meta-morphism, and their tectonic setting suggest that they are exhumed offscraped sediments of the Arabian plate, which now defi nes an anticlino-rium structurally beneath the forearc ophiolites (Fig. 1B). The presence of such an anticlinorium was fi rst proposed by Alavi (2004, his fi g. 5).

Inner Zagros Ophiolite Belt

The Inner Zagros ophiolite belt (also known as the Nain-Baft ophiolite belt) occurs in several massifs named after nearby towns. These ophio-lites can be traced along strike for ~500–600 km and are, from northwest to southeast, the Nain, Dehshir, Shahr-e-Babak, and Balvard-Baft ophiolites (Fig. 1A). These massifs contain the components of complete “Penrose ophiolites,” including tectonized harzburgites, gabbros,

and pillowed basalts. Sheeted dike complexes and ultramafi c-mafi c cumulates are rare. All the massifs in the Inner Zagros ophiolite belt are overlain by Turonian-Maastrichtian pelagic limestones (93.5–65.5 Ma), consistent with the limited geochronologic evidence that these ophiolites formed by seafl oor spreading ca. 90–95 Ma. Several lines of evidence, such as high Cr content (Cr# > 60) of spinel in harz-burgite, fl at rare earth element (REE) patterns of the lavas, Nb-Ta negative anomalies and large ion lithophile element (LILE) positive anomalies, and the presence of low-Ti clino-pyroxenes in the lavas , indicate that the Inner Zagros ophiolite belt formed above a subduction zone (Delaloye and Desmons, 1980; Ghazi and Hassanipak, 2000; Shafaii Moghadam et al., 2007; Shafaii Moghadam, 2009).

The Urumieh-Dokhtar Magmatic Assemblage

The Urumieh-Dokhtar magmatic assemblage forms an Andean-type magmatic arc associated with subduction of Neotethyan oceanic fl oor (Berberian et al., 1982; Berberian and Ber-berian, 1981; Shahabpour, 2007). Magmatic activity continued from Cretaceous to Pliocene-Quaternary time, but peak activity was in late Eocene time (Farhoudi, 1978; Amidi et al., 1984; Berberian and King, 1981). Early igneous rocks were generally calc-alkaline, changing in Oligocene-Miocene time into shoshonitic and alkaline igneous rocks (Amidi et al., 1984).

FIELD OBSERVATIONS OF THE DEHSHIR OPHIOLITE

The Dehshir ophiolite is exposed discontinu-ously over ~150 km2 (Fig. 2), near the center of the Inner Zagros ophiolite belt. It consists of oceanic mantle and crustal sequences that in-clude harzburgite, gabbro, plagiogranite, sheeted dikes, and pillow lava capped by Turonian-Maastrichtian (93.5–65.5 Ma) Globotrunca-bearing pelagic limestone that rests conformably on the ophiolite (Fig. 3; Sabzehei, 1997). The Dehshir ophiolite has been previously described mainly as part of regional geological mapping (Sabzehei, 1997). No petrologic or geochemical studies have been reported for this ophiolite.

No complete lithostratigraphic succession of the ophiolite is preserved, but a tentative re-construction is presented in Figure 3, based on several sections evaluated in the fi eld (shown on right side of fi gure). The mantle sequence is exclusively harzburgite with minor leucogabbro pockets (Fig. 3). The harzburgite exhibits plas-tic deformation and high-temperature foliation, marked by stretched orthopyroxene and spinel



(e.g., Nicolas et al., 1971). Diabasic dikes (<2 m thick) are abundant in the mantle sequence. Intru-sive contacts with the serpentinized peridotite are sharp and regular, suggesting that the harzburgite had cooled at the time of mafi c injection. Some diabase dikes are rooted in small pegmatite gab-bro pods. Rodingitized gabbro dikes, recognized by both their whitish weathered surface and secondary minerals such as pectolite (hydrated Ca-Na silicate, Ca

2NaHSi

3O

9) and wollastonite,

are observed throughout the exposed mantle se-

quence (Fig. 4A). Pegmatite and isotropic gab-bros are common within the mantle sequence (Fig. 4D). Truncation of gabbro-diabase dikes by serpentinized shear zones is common. No relation between the mafi c dikes injected in the peridotite and those in the sheeted dike complex was observed in the fi eld.

The cumulate ultramafi c-mafi c rocks of the Dehshir ophiolite are characterized by inter-layered leucogabbro, clinopyroxenite, and plagio-clase peridotite, as pockets within the peridotites

(Figs. 3 and 4C). These rocks grade upward into unlayered, massive gabbros. Slices of am-phibole gabbro with plagiogranite dikelets can be observed near Aziz-Abad village (Fig. 2), showing faulted contacts with the ophio litic vol-canics. Rare andesitic dikes intrude the amphi-bole gabbro.

Dehshir ophiolite volcanics consist of pillow basalts and massive basalt and andesite lava fl ows (max. 100–200 m thick). The base of the volcanic pile is intruded by compositionally bi-modal dikes that grade down into a sheeted dike complex. In the sheeted dike complex, each dike (<0.5 m thick) has been injected into others ; locally, rhyolite patches are observed as screens between dikes. Most dikes are hydrothermally altered, especially at contacts. Less altered dikes show a chilled margin on one side and a brec-ciated margin on the other side. Dikes are both felsic and mafi c; the felsic dikes are younger because they cut the mafi c dikes. Felsic dikes are about three times more abundant than mafi c dikes. The felsic dikes are not associated with felsic lavas, and there is no clear relationship between felsic dikes and granitic plugs.

Dehshir ophiolite volcanics are stratigraphi-cally overlain by Turonian-Maastrichtian (93.5–65.5 Ma) pelagic limestones and cherts (Fig. 4B). Eocene fl ysch (Qom Formation) and Miocene molasse (red sandstones of the lower Red Formation; Sabzehei, 1997) also rest un-conformably on the ophiolite and Cretaceous pelagic sediments.

The metamorphic sole of the Dehshir ophiolite is dominated by alternating actinolite- chlorite-garnet-muscovite schists and amphibolites, showing faulted contact with other ophiolite rock units near Zoolouzar village (Fig. 2). Small biotite-granite plugs, clearly younger than the metamorphic rocks, crosscut these metamor-phic rocks. The protolith of the metamorphic rocks appears to have been pyroclastic rocks (tuffs and tuffi tes), evidenced by bedding.

Petrography and Mineral Composition of the Dehshir Ophiolite

About 100 thin sections from all rock units of the Dehshir ophiolite were studied. From these, we selected 17 samples for microprobe analyses, including seven peridotites, two leuco-gabbros, three gabbroic rocks, two pillow lavas, one amphibolite, and two samples of diabasic dikes injected into harzburgite. Electron micro-probe analyses were performed on a Cameca SX-50 equipped with four spectrometers and wavelength dispersive system at the University of Paris VI, France. Operating conditions were 20 nA and a focused beam at 15 kV accelerating voltage, using natural standards.

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Figure 2. Simplifi ed geological map of the Dehshir ophiolite (compiled after Sabzehei, 1997). Location of samples is also shown.



Dehshir ophiolite Pelagic limestone

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Figure 3. Simplifi ed stratigraphic column displaying idealized internal lithologic suc-cessions in the Dehshir ophiolite. Approxi-mate stratigraphic positions of samples are also shown, especially for lavas (small column on right). Position of photographs shown in Figure 4 is also shown.

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Figure 4. Field photographs and photomicrographs of the Dehshir ophiolite (approximate positions shown in Fig. 3). (A) Injection of gabbroic dike in harzburgite. The intrusive contacts of the dike are sharp and regular. (B) Stratigraphic contact of pillow lava with overly-ing chert nearing Ardan village (Fig. 2). (C) Layered leucogabbro as magmatic pocket within the harzburgite. (D) Small pod of pegma-tite gabbro with sharp to gradational contact within harzburgite. Photomicrographs (cross-polarized light): (E) leucogabbro (G06–23), altered coarse plagioclase (Plag), and clinopyroxenes (Cpx), showing intergranular texture, and (F) plagioclase lherzolite (G06–20), show-ing altered cumulate olivine and plagioclase.



Mantle Sequence

The mantle sequence is dominated by perido-tite residues after melting. These ultramafi c rocks are mostly altered to serpentinite, but relict oli-vines and pyroxenes are common. The intensity of serpentinization ranges from moderate (<40%) to high (>60%). Reconstructed perido tites were mainly harzburgite (<3% modal clinopyroxene) and clinopyroxene-bearing harz burgite (3%–5% clinopyroxene). There is more harzburgite than clinopyroxene-bearing harzburgite in the Dehshir mantle sequence. Textural observa-tions show that clinopyroxenes are magmatic but formed late, perhaps refl ecting melt infi ltra-tion. Harzburgite and clinopyroxene-harzburgite mainly show porphyroclastic texture, with coarse-grained orthopyroxene.

Olivine relicts in harzburgites are homo-geneous, Fo

90 to Fo

91 (Table 1; Table 2), and are

often kink-banded; NiO contents range from 0.23% to 0.44%, typical of mantle olivines (Sobolev et al., 2005). Orthopyroxene por phyro-clasts have compositions between Wo

2En

89Fs

9

and Wo3En

83Fs

14.

Figure 5A shows some variations in Dehshir ophiolite orthopyroxene (Opx) Mg#. Dehshir perido tite orthopyroxenes have low Al

2O

3 (0.5%–

1.6%) and CaO (0.8%–2.2%) contents (Table 3), resembling depleted harzburgites from mod-ern forearcs (Fig. 5A). Clinopyroxene-bearing harz burgite contains porphyroclasts of diopside to augite, from Wo

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52Fs

8 to Wo

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40Fs

11.

Al2O

3 and TiO

2 contents of clinopyroxenes

( Table 3) are similar to the very low values found in clinopyroxenes from highly depleted forearc peridotites (Ishii et al., 1992) (Fig. 5B). Dehshir harzburgite clinopyroxenes are chemically dis-tinct from those of moderately depleted abys-sal peridotites, which have signifi cantly higher

Al2O

3 and TiO

2 contents (Hébert et al., 1990;

Johnson et al., 1990) (Fig. 5B).Lobate or vermicular Cr-spinel is the main

accessory phase in Dehshir ophiolite perido-tites. Spinel in clinopyroxene-harzburgites is light brown and Al-rich, with low Cr# (= [100Cr/Cr + Al] = 21–22), whereas depleted harzburgites contain dark brown spinel with high Cr# (61–82; Table 4). These Cr-spinels show chemical variations following both mid-ocean-ridge basalt (MORB) and forearc trends of Dick and Bullen (1984) (Fig. 6A). Spinel compositions are plotted in the discrimination diagram (Fig. 6B; Kamenetsky et al., 2001), where peridotite spinels can be divided into two distinct types: (1) those with low Al

2O

3 and

low TiO2 contents, consistent with the fi eld of

suprasubduction-zone peridotite spinels; and (2) those with more Al

2O

3 and TiO

2, similar to

MORB peridotites. Some low-Al spinels have

TABLE 1. REPRESENTATIVE WHOLE-ROCK ANALYSES OF THE ROCK UNITS FROM THE DEHSHIR OPHIOLITE

Rock: d.dike d.dike d.dike d.dike d.dike d.dike p. lava p. lava p. lava p. lava basaltSample: A06-1 AZ06-29 G06-16 DAR05-3 DAR05-4 DZ05-4 A06-2 A06-3 DAR05-6 AZ06-32 DR05-2ALocation: 31°23.77N 31°17.08N 31°07.55N 31°23.33N 31°23.33N 31°17.20N 31°23.77N 31°23.77N 31°23.43N 31°17.08N 31°16.41N

53°50.05E 53°57.09E 54°00.49E 53°51.04E 53°51.04E 53°53°49E 53°50.05E 53°50.05E 53°51.08E 53°57.11E 53°57.02ESiO2 51.68 48.73 52.24 52.20 51.10 51.80 46.87 45.42 50.40 61.50 51.00Al2O3 15.51 14.12 15.01 13.90 14.30 14.60 10.78 13.86 13.70 15.48 12.20MgO 7.20 9.64 4.75 5.45 4.26 4.39 6.06 2.59 2.90 1.53 6.82CaO 4.99 12.68 8.22 9.76 8.35 9.27 13.94 12.69 8.23 6.76 6.98FeOt 10.03 9.35 10.65 10.20 12.50 9.87 9.15 10.45 11.80 6.84 13.10MnO 0.16 0.11 0.17 0.16 0.19 0.14 0.15 0.15 0.17 0.20 0.24TiO2 0.93 0.28 0.82 0.78 1.37 0.69 0.44 0.62 1.94 0.50 1.81Na2O 4.40 1.38 5.20 3.79 4.28 5.23 4.60 5.88 5.46 4.58 3.73K2O 1.39 0.06 0.82 0.58 0.17 0.82 0.71 0.15 0.55 0.04 0.88P2O5 0.10 0.02 0.04 0.16 0.11 0.12 0.14 0.30 0.36 0.09 0.24LOI 2.41 2.73 1.03 2.21 1.79 1.96 5.85 7.13 3.32 1.66 1.47Total 98.80 99.10 98.95 99.24 98.45 98.98 98.68 99.24 98.82 99.17 98.50Mg# 56.13 64.78 44.29 48.79 37.80 44.23 54.14 30.61 30.47 28.49 48.14Sc 36.88 46.98 24.49 34.00 36.60 31.00 49.38 37.91 38.00 16.94 43.00V 281.19 236.01 297.23 237.00 381.00 293.00 360.50 290.53 270.00 98.13 363.00Cr 109.90 299.41 14.11 82.00 22.10 62.00 580.29 38.12 161.00 6.12 79.00Ni 58.70 94.63 36.01 32.00 16.80 41.00 211.20 27.34 118.00 4.89 51.00Rb 15.20 0.44 7.51 8.00 2.47 12.60 12.07 1.68 11.10 0.31 9.34Sr 868.73 272.42 210.61 279.00 150.00 701.00 203.72 187.28 240.00 287.90 143.00Y 22.64 8.15 21.83 25.00 29.00 19.90 11.79 15.99 51.60 23.39 56.00Zr 52.49 10.38 33.24 68.20 65.90 49.30 25.33 33.92 163.00 50.15 185.00Nb 1.35 0.29 0.35 1.65 1.02 0.76 0.66 0.91 5.33 1.29 4.84Cs 7.43 0.02 0.11 0.03 0.15 0.53 0.63 0.29 0.43 0.08 0.10Ba 192.01 13.64 27.29 143.00 18.30 15.35 81.81 42.27 74.60 13.43 236.00Hf 1.59 0.37 1.18 1.98 1.98 1.60 0.75 1.03 4.19 1.64 4.81Ta 0.08 0.02 0.03 0.10 0.18 0.05 0.04 0.06 1.07 0.09 0.38Pb 1.32 0.43 0.40 1.84 4.53 4.16 13.68 3.00 3.50 1.53 2.52Th 0.77 0.12 0.08 0.76 0.23 0.50 0.52 0.77 0.54 0.61 0.47U 1.13 0.08 0.05 0.22 0.20 0.16 0.41 0.38 0.24 0.42 0.21La 4.89 0.57 1.04 5.36 2.37 2.99 4.57 5.57 7.83 3.78 7.48Ce 10.63 1.41 3.64 12.10 7.36 7.24 8.01 11.17 20.40 9.03 21.40Pr 1.55 0.24 0.69 1.78 1.33 1.14 1.14 1.59 3.24 1.45 3.44Nd 7.54 1.36 4.01 8.91 7.51 6.08 5.39 7.33 16.80 7.33 17.80Sm 2.31 0.57 1.65 2.86 2.80 2.14 1.47 1.94 5.34 2.30 6.09Eu 0.80 0.21 0.60 1.05 1.15 0.74 0.49 0.64 1.88 0.87 1.80Gd 2.48 0.74 2.09 3.38 3.61 2.53 1.42 1.89 6.76 2.59 6.99Tb 0.52 0.16 0.47 0.60 0.71 0.47 0.27 0.36 1.22 0.53 1.29Dy 3.35 1.11 3.17 4.33 4.82 3.39 1.63 2.25 8.28 3.42 9.14Ho 0.79 0.28 0.77 1.02 1.17 0.78 0.38 0.53 1.94 0.86 2.12Er 2.16 0.79 2.12 2.52 3.10 2.11 1.04 1.41 5.04 2.34 5.66Tm 0.38 0.14 0.38 0.38 0.46 0.34 0.18 0.25 0.77 0.42 0.88Yb 2.15 0.83 2.23 2.66 2.91 2.23 0.99 1.47 4.65 2.41 5.50Lu 0.34 0.12 0.34 0.43 0.51 0.36 0.15 0.22 0.76 0.38 0.87

(continued)



higher TiO2 (0.1% < TiO

2) (Table 4), and these

values are interpreted to be the result of magma-mantle interaction (Cannat et al., 1997).

Cumulate Rocks

Dehshir ophiolite cumulate rocks are plagio-clase lherzolite, clinopyroxenite, leucogabbro, and pegmatitic gabbro. Plagioclase lherzolites have fractionated olivine with Fo

83 to Fo

84.

Oli vine NiO content is lower than that of harz burgite olivines (0.20%–0.22% vs. 0.23%–0.44%), refl ecting magmatic versus residual nature. Plagioclase is bytownite (An

87–88) to

nearly pure anorthite (An99

) in these lherzolites (Fig. 4F). Plagioclase and clinopyroxene are the predominant cumulus phases in pegmatitic gab-bros. Clinopyroxene has been partly converted into tremolite, whereas plagioclase is variably altered into sericite and clay minerals. Plagio-

clase occurs as large euhedral-subhedral cumu-lus grains in leucogabbros and, together with clinopyroxene, displays coarse-grained granular texture (Fig. 4E). Plagioclase laths are aligned to defi ne a magmatic foliation that is parallel with cumulate layering. Olivine is interstitial between plagioclase. Accessory titanomagnetite and ilmenite also occur as intercumulus grains. Colorless clinopyroxene is diopside, ranging from Wo

50En

45Fs

5 to Wo

47En

47Fs

6. The crystal-

lization order in Dehshir leucogabbros is plagio-clase + plagioclase-clinopyroxene + olivine + titanomagnetite-ilmenite.

Mafi c Magmatic Rocks

Fine-grained isotropic gabbros, pillow ba-salts, basaltic lava fl ows, and basaltic-andesitic dikes (in sheeted dike complex and intruding amphibole gabbros) are the mafi c magmatic

rocks of the Dehshir ophiolite. Plagioclase and clinopyroxene are the main constituents of the isotropic gabbros, defi ning intergranular to subophitic texture. Green amphibole ( magnesio-hornblende) and disseminated ilmenite are ac-cessory phases. Common secondary minerals are albite, prehnite, calcite, epidote, chlorite, and actinolite. Clinopyroxene is diopside to augite, Wo

35En

43Fs

22 to Wo

47En

43Fs

10.

Pillowed and massive lavas are porphyritic, with clinopyroxene (<5% modal) and plagio-clase (15%–20%) phenocrysts in an altered glassy groundmass with plagioclase micro-lites. All rock types are metamorphosed to lower greenschist facies; common metamor-phic phases are sericite, chlorite, fi ne-grained polycrystalline quartz, calcite, epidote, and prehnite. Clinopyroxene phenocrysts (augite) are colorless, ranging from Wo

35En

52Fs

13 to

Wo45

En41

Fs14

.

TABLE 1. REPRESENTATIVE WHOLE-ROCK ANALYSES OF THE ROCK UNITS FROM THE DEHSHIR OPHIOLITE (continued)

Rock: basalt b.dike an.dike basalt gabbro gabbro gabbro am.gab. am.gab. peg.gab leucog.Sample: DAR05-5 AZ06-38 DZ05-1D D06-10 R12 DI05-12 DR05-3 DZ05-1H D06-3 DR05-1 G06-17Location: 31°23.33N 31°16.97N 31°17.20N 31°21.94N 31°16.41N 31°21.94N 31°16.54N 31°18.34N 31°20.12N 31°17.03N 31°08.34N

53°51.04E 53°57.28E 53°53°49E 53°54.44E 53°57.02E 53°54.44E 53°57.47E 53°53.14E 53°53.32E 53°56.42E 54°01.38ESiO2 48.80 44.91 57.40 50.48 50.10 49.10 38.20 54.40 45.92 43.30 44.68Al2O3 13.10 14.40 13.10 14.60 14.80 14.00 10.40 14.70 16.76 20.20 14.53MgO 3.76 6.94 7.47 9.80 9.42 11.10 4.91 8.31 8.13 8.63 10.90CaO 14.60 22.69 5.98 9.57 5.99 10.40 21.40 5.75 10.64 15.00 21.82FeOt 11.70 4.22 7.62 8.87 9.99 9.29 12.10 7.70 10.59 5.59 2.72MnO 0.19 0.05 0.13 0.15 0.15 0.14 0.19 0.13 0.17 0.10 0.06TiO2 1.26 1.82 0.39 0.32 0.38 0.32 1.82 0.52 0.51 0.07 0.11Na2O 3.10 0.06 5.26 2.14 3.41 1.53 3.60 5.84 2.00 0.15 0.04K2O 0.00 0.97 0.55 0.22 2.14 0.99 0.15 0.08 0.85 1.29 0.02P2O5 0.11 0.21 0.05 0.02 0.05 0.05 0.26 0.05 0.04 0.02 0.00LOI 2.23 2.64 1.20 2.72 2.97 2.30 5.68 2.01 3.04 4.69 3.63Total 98.89 98.90 99.17 98.90 99.31 99.14 98.70 99.47 98.63 99.09 98.51Mg# 36.43 74.58 63.61 66.33 62.70 68.05 41.98 65.80 57.78 73.35 87.70Sc 34.00 26.64 34.00 40.60 48.00 45.00 33.00 31.00 53.20 24.00 38.61V 359.00 387.23 223.00 198.74 283.00 236.00 274.00 199.00 326.37 102.00 109.98Cr 32.80 214.13 195.00 530.06 263.00 510.00 99.00 295.00 116.74 101.00 1083.04Ni 16.10 90.63 84.00 108.79 65.00 152.00 44.00 157.00 55.84 154.00 194.80Rb 0.56 10.59 11.50 2.79 25.30 11.80 9.97 1.15 11.46 17.40 0.57Sr 85.40 517.44 106.00 208.65 362.00 719.00 267.00 74.00 929.82 362.00 627.93Y 28.40 41.98 13.00 8.85 11.00 8.64 41.00 31.00 10.68 2.28 3.82Zr 65.90 94.31 20.50 16.83 30.00 17.30 139.00 44.00 13.06 2.71 2.38Nb 0.89 4.41 0.31 0.28 0.63 0.25 6.15 0.81 0.43 0.40 0.01Cs 0.36 0.23 ND 0.18 1.51 3.81 12.00 0.09 0.19 ND 0.09Ba 14.50 358.70 41.00 58.97 216.00 258.00 18.00 31.00 323.27 2457.00 59.14Hf 1.99 2.88 0.66 0.51 0.88 0.62 3.68 1.61 0.46 0.08 0.09Ta 0.08 0.27 0.03 0.02 0.05 0.01 0.50 0.07 0.03 0.11 0.00Pb 3.81 0.23 2.08 0.25 4.14 2.14 2.12 2.34 0.96 0.92 0.00Th 0.14 0.16 0.17 0.12 0.39 0.06 0.60 0.27 0.12 0.04 0.02U 0.06 0.10 0.07 0.09 0.14 0.10 0.22 0.16 0.06 0.02 0.02La 2.32 4.39 0.83 1.27 1.26 1.36 8.51 1.80 2.39 0.38 0.03Ce 7.15 13.15 2.16 2.00 2.96 3.41 21.80 5.34 2.74 0.45 0.13Pr 1.26 2.32 0.36 0.33 0.44 0.53 3.30 0.96 0.51 0.05 0.05Nd 7.14 12.56 1.98 1.68 2.24 2.50 16.00 5.60 2.76 0.26 0.39Sm 2.76 4.35 0.84 0.67 0.86 0.80 5.20 2.29 0.96 0.08 0.22Eu 0.98 1.10 0.36 0.30 0.37 0.33 1.74 0.59 0.39 0.31 0.15Gd 3.39 4.94 1.28 1.01 1.11 1.07 5.72 3.15 1.09 0.10 0.33Tb 0.66 1.04 0.26 0.17 0.22 0.19 1.02 0.66 0.24 0.02 0.08Dy 4.64 6.70 2.04 1.31 1.70 1.49 7.19 4.76 1.52 0.28 0.53Ho 1.07 1.55 0.51 0.31 0.38 0.35 1.64 1.15 0.37 0.07 0.13Er 2.80 4.18 1.36 0.83 1.10 0.93 4.29 3.24 0.99 0.23 0.32Tm 0.44 0.71 0.21 0.14 0.18 0.16 0.65 0.51 0.17 0.04 0.05Yb 2.90 4.12 1.49 0.90 1.21 1.00 4.02 3.31 1.00 0.29 0.29Lu 0.48 0.60 0.24 0.13 0.20 0.17 0.63 0.54 0.15 0.06 0.04

(continued)



Diabasic Dikes in Mantle Harzburgites

Diabasic dikes intrude Dehshir mantle harz-burgites. The texture of these rocks ranges from fi ne to medium grained, aphyric to moderately porphyritic, with clinopyroxene (Wo

38En

47Fs

15

to Wo43

En44

Fs13

; <5% modal) and plagioclase (5%–15%) phenocrysts set in an intergranular groundmass. Accessory phases are interstitial amphibole (both hornblende and secondary actino lite) and titanomagnetite.

Acidic Rocks

Dacite dikes are common in the sheeted dike complex. Albite microlites, K-feldspar, and quartz are the main constituents. Plagio-granites occur both as dikelets injected into isotropic gabbros and as small plugs emplaced in metamorphic rocks. Plagioclase, ortho clase,

and anhedral quartz grains are found in these rocks. Accessory phases are amphibole and biotite.

Metamorphic Rocks

The metamorphic sole of the Dehshir ophio-lite is a zone (~50 m thick) of alternating actinolite-chlorite-garnet-muscovite schists and amphibolites. The metamorphic grade varies from greenschist to lower amphibolite facies. These metamorphic rocks show granoblastic to nematoblastic texture with amphibole, feld-spar, quartz, muscovite, biotite, chlorite, garnet, epidote, sphene, and prehnite as predominant phases. Plagioclase compositions range from al-most pure albite to An

36 (andesine). K-feldspar is

almost pure orthoclase (Or98

) (Table 6). Amphi-bole occurs as green hornblende and secondary pale-green actinolite. The most abundant type

is fi brous actinolite (Aliv < 0.5; TiO2 < 0.5%)

( Table 7). Moreover, amphibole compositions de-fi ne a restricted trend from magnesio-hornblende to actinolite in the Mg/Mg + Fe+2 versus Si (in tetrahedral site) diagram (Fig. 7). These amphi-boles plot far from the representative magmatic amphibole compositions for oceanic gabbros defi ned by Coogan et al. (2001), indicating that Dehshir amphiboles are metamorphic.

CLINOPYROXENE COMPOSITIONS OF MAFIC MAGMATIC ROCKS

It is widely accepted that clinopyroxene com-positions in basalts are a useful indicator of dif-ferent magma compositions, and thus tectonic settings (Leterrier et al., 1982), and of differ-ent ophiolite types (Huot et al., 2002). Com-positional contrasts between clino pyroxenes from different mafi c rock units of the Dehshir


Rock: leucog. leucog. leucog. leucog. dc.dike dc.dike dc.dike dc.dike granite granite amphib.Sample: G06-18 G06-11 G06-23 G06-6 AZ06-8 AZ06-11 AZ06-36 AZ06-31 AZ06-25 DZ05-6 DZ05-9Location: 31°08.34N 31°08.34N 31°07.12N 31°08.34N 31°17.03N 31°17.03N 31°17.05N 31°17.05N 31°16.23N 31°17.32N 31°17.32N

54°01.38E 54°01.38E 54°02.01E 54°01.38E 53°57.79E 53°57.79E 53°57.79E 53°57.79E 53°56.43E 53°53.67E 53°53.67ESiO2 39.73 43.21 43.58 39.38 71.20 63.84 71.26 68.36 76.45 76.60 42.50Al2O3 23.33 22.98 18.07 2.15 12.78 14.52 10.82 14.20 11.36 12.80 14.70MgO 4.17 6.06 10.73 37.61 0.96 2.32 0.10 1.42 0.83 0.77 12.90CaO 24.74 17.33 12.17 0.75 2.57 5.43 11.11 1.53 3.74 1.13 15.10FeOt 1.29 3.12 7.49 8.00 3.98 5.66 3.01 4.75 1.49 1.17 9.28MnO 0.03 0.06 0.11 0.12 0.10 0.12 0.07 0.11 0.02 0.02 0.15TiO2 0.04 0.09 0.20 0.02 0.32 0.35 0.31 0.41 0.11 0.21 0.32Na2O 0.00 0.81 0.37 0.00 5.93 4.70 0.23 6.89 3.80 5.78 0.62K2O 0.00 1.23 2.46 0.00 0.02 0.05 0.00 0.09 0.09 0.41 0.29P2O5 0.01 – 0.00 0.01 0.03 0.04 0.03 0.07 0.00 0.07 0.05LOI 5.58 4.11 3.36 11.23 1.21 2.05 2.10 1.23 1.07 0.87 3.30Total 98.92 99.00 98.56 99.27 99.10 99.08 99.05 99.07 98.97 99.89 99.15Mg# 85.18 77.60 71.86 89.34 30.19 42.28 5.73 34.76 49.68 53.89 71.25Sc 9.49 23.39 46.79 8.92 15.02 23.74 12.58 12.22 2.34 2.00 49.00V 31.93 64.01 168.20 62.44 34.90 102.64 53.69 56.91 33.64 15.00 162.00Cr 734.42 237.71 352.22 2585.78 2.24 42.18 26.56 3.64 6.13 14.00 719.00Ni 132.07 60.46 130.23 1934.00 5.26 18.74 8.85 3.57 16.11 9.00 162.00Rb 0.29 22.85 48.92 0.08 0.41 0.33 0.19 0.71 1.00 7.96 1.72Sr 30.11 407.24 425.71 6.38 88.83 111.28 320.74 121.26 170.50 94.00 55.00Y 1.37 2.46 3.22 1.62 23.03 18.48 32.02 25.20 2.85 9.33 10.00Zr 1.30 1.04 0.98 0.67 57.81 45.02 73.26 61.79 50.25 77.40 13.00Nb 0.00 0.00 0.05 0.00 1.35 1.09 1.97 1.55 0.41 1.35 0.39Cs 0.03 0.84 1.95 0.08 0.02 0.02 0.06 0.12 0.00 ND 0.05Ba 2.28 103.49 147.39 0.34 8.46 6.72 12.76 42.63 22.87 124.00 34.00Hf 0.03 0.04 0.05 0.01 1.84 1.40 2.32 2.00 1.20 1.95 0.46Ta 0.00 0.00 0.00 0.00 0.09 0.07 0.12 0.10 0.03 0.10 0.03Pb 0.42 – 0.99 0.12 1.85 1.49 1.24 1.30 3.00 3.95 17.80Th 0.01 0.00 0.00 0.04 0.70 0.56 1.00 0.78 0.41 0.89 0.29U 0.03 0.02 0.02 0.05 0.33 0.24 0.36 0.40 0.20 0.39 0.14La 0.21 0.25 0.28 0.02 4.16 3.46 6.13 4.74 1.93 3.46 1.00Ce 0.12 0.18 0.20 0.01 9.59 7.87 13.38 11.02 2.16 5.67 2.19Pr 0.02 0.04 0.04 0.00 1.48 1.22 2.02 1.69 0.25 0.70 0.30Nd 0.15 0.24 0.32 0.01 7.23 5.85 9.81 8.32 0.94 2.75 1.55Sm 0.06 0.14 0.20 0.02 2.29 1.84 2.94 2.58 0.23 0.69 0.62Eu 0.14 0.11 0.14 0.01 0.73 0.62 1.16 0.78 0.17 0.25 0.28Gd 0.11 0.24 0.28 0.06 2.42 1.96 3.25 2.77 0.23 0.79 0.82Tb 0.02 0.04 0.05 0.02 0.50 0.40 0.71 0.58 0.05 0.14 0.19Dy 0.15 0.29 0.40 0.13 3.43 2.69 4.74 3.83 0.31 1.30 1.50Ho 0.03 0.07 0.09 0.03 0.82 0.65 1.13 0.91 0.08 0.29 0.36Er 0.09 0.18 0.22 0.10 2.26 1.77 3.16 2.55 0.26 0.87 1.08Tm 0.01 0.03 0.04 0.02 0.40 0.31 0.56 0.44 0.04 0.17 0.18Yb 0.08 0.15 0.21 0.13 2.46 1.90 3.28 2.73 0.34 1.21 1.21Lu 0.01 0.02 0.03 0.02 0.38 0.29 0.48 0.41 0.06 0.22 0.20

(continued)



ophiolite are revealed by diagrams that plot TiO

2 and Al

2O

3 contents versus Mg# and Cr

2O

3

+ TiO2, respectively (Fig. 8). Although Dehshir

clinopyroxenes have Mg# values ranging from 66.9 to 90.5, only clinopyroxene from leuco-gabbros shows high values (Fig. 8A). The high Mg# of clinopyroxenes is consistent with py-roxene composition in other ophiolites and is-land arcs (De Bari and Coleman, 1989). TiO

2

(<0.8%), Cr2O

3 (<0.7%), and Al

2O

3 (0.5%–4%)

abundances (Table 5) are similar to clinopyrox-enes of both island-arc tholeiites and boninites (Fig. 8B), and the data defi ne a compositional array that is typical of suprasubduction-zone ba-salts. The low TiO

2 contents of Dehshir mafi c

clinopyroxenes likely refl ect a depleted mantle source (e.g., Pearce and Norry, 1979). The low

Na2O content (<0.5%) of Dehshir mafi c pyrox-

enes indicates low-pressure crystallization and the depleted tholeiitic nature of the parental magma (e.g., Bonev and Stampfl i, 2009). The low Ti/Altot ratio places clinopyroxenes in the arc tholeiitic–boninitic fi eld (Leterrier et al., 1982), further indicating formation in a supra-subduction-zone environment.

WHOLE-ROCK GEOCHEMISTRY

Analytical Methods

Major- and trace-element analyses were car-ried out using inductively coupled plasma–mass spectrometry (ICP-MS) and ICP–atomic emis-sion spectrometry (AES) at the Centre de Géo-

chimie de la Surface, Strasbourg (France), using standard methods (Govindaradju, 1994). For analyses using ICP-AES, the relative precision is ±10%, while ICP-MS analyses have a preci-sion of ±5%. Table 1 presents chemical data for 37 whole-rock samples, including peridotites and pyroxenites (fi ve samples), leuco gabbros (four), gabbroic rocks (six), basaltic lavas and pillow lavas (nine), felsic rocks including plagio granites and dacitic dikes in the sheeted dike complex (six), amphibolite (one), and six samples from diabasic dikes that intrude harz-burgite. Samples have variable loss on ignition (LOI; 0.9%–7.1%, except peridotites, which have up to 11.66% LOI), which attests to the variable degree of alteration of these rocks. We therefore use immobile trace elements and rare earth elements (REEs) for the evaluation of geo-chemical signatures and the tectono-magmatic environment of the rocks.

Mantle Harzburgites

Harzburgites and clinopyroxene-bearing harz-burgites are characterized by Mg# (100Mg/Mg + Fet) values ranging from 87.9 to 90.4. Major oxides such as Al

2O

3 and CaO are useful

for evaluating the degree of peridotite deple-tion (Ishii et al., 1992; Pearce et al., 1992). The relatively high modal abundance of pyroxene in clinopyroxene-bearing harzburgite from the Dehshir ophiolite leads to moderate Al

2O

3

(1.1%–3.6%; Table 1). This is signifi cantly higher than Al

2O

3 contents of highly depleted

peridotites from the Izu-Bonin-Mariana forearc (Ishii et al., 1992), but it is similar to Al

2O

3

contents of South Sandwich forearc peridotite (Pearce et al., 2000). In spite of some scatter in CaO contents (0.7%–1.2%; Table 1), the Dehshir harzburgites have CaO/Al

2O

3 values close to de-

pleted MORB mantle values, 0.7–0.9 (Jagoutz et al., 1979; Hart and Zindler, 1986; Workman and Hart, 2005), although ratios as low as 0.2 for clinopyroxene-harzburgite may refl ect CaO loss. Harzburgites (AZ06–40, G06–13) are character-ized by lower TiO

2 (<0.01%) and Al

2O

3 (1.12%–

1.68%), and rare earth element (REE) contents refl ect a more refractory nature. REE abun-dances are generally very low but show light (L) REE enrichment (Fig. 9). Their U-shaped pattern resembles the most refractory perido-tites sampled in orogenic and ophiolitic massifs (Godard et al., 2008). Such selective enrichment in LREE cannot be explained as residues after partial melting of primitive mantle. Three dis-tinct mechanisms have been proposed to account for this enrichment: (1) mantle metasomatism by slab-derived fl uids; (2) contamination by conti-nental sources; and (3) alteration during ophio-lite obduction (Prinzhofer and Allègre, 1985;


Rock pyrox. peridotite peridotite peridotite peridotiteSample DZ05-8 G06-20 AZ06-40 G06-13 AZ06-26Location 31°17.32N 31°07.12N 31°17.83N 31°06.16N 31°17.48N

53°53.67E 54°02.01E 53°56.37E 54°00.32E 53°56.73ESiO2 50.70 40.32 38.39 40.05 35.63Al2O3 2.68 8.72 1.12 1.68 3.55MgO 21.00 25.45 39.73 39.13 37.65CaO 15.60 9.39 0.96 1.24 0.68FeOt 6.24 9.41 7.52 7.64 9.24MnO 0.12 0.12 0.11 0.11 0.11TiO2 0.16 0.10 0.01 0.01 0.05Na2O 0.05 0.16 0.00 0.00 0.00K2O 0.05 0.06 0.01 0.01 0.00P2O5 0.05 – 0.00 0.00 0.01LOI 2.58 5.57 11.08 9.32 11.66Total 99.04 99.31 98.93 99.19 98.59Mg# 85.71 82.82 90.41 90.14 87.90Sc 84.00 24.57 7.05 11.05 7.03V 208.00 91.38 43.46 55.94 60.82Cr 2213.00 2574.57 2604.01 2673.83 4228.47Ni 249.00 835.78 2033.00 2047.00 1691.17Rb 0.31 0.33 0.18 0.36 0.14Sr 18.00 78.34 7.00 33.20 13.40

32.262.140.178.300.5YZr 2.00 1.99 1.49 1.14 1.35Nb 0.02 0.18 0.02 0.01 0.01Cs 0.04 – 0.20 0.07 0.03Ba 7.00 9.72 0.54 0.60 3.87Hf 0.09 0.07 0.02 0.01 0.03

10.000.000.010.0DNaTPb 2.11 – 0.09 0.93 0.39Th 0.02 0.01 0.01 0.01 0.01

40.050.010.020.020.0ULa 0.01 0.66 0.01 0.29 0.09Ce 0.30 0.19 0.02 0.05 0.07Pr 0.09 0.04 0.00 0.00 0.02Nd 0.55 0.29 0.02 0.01 0.10Sm 0.33 0.18 0.01 0.01 0.06Eu 0.13 0.08 0.00 0.00 0.04Gd 0.48 0.33 0.01 0.01 0.11Tb 0.10 0.07 0.01 0.01 0.03Dy 0.75 0.50 0.02 0.04 0.18Ho 0.18 0.11 0.00 0.01 0.04Er 0.48 0.30 0.02 0.04 0.12Tm 0.08 0.05 0.00 0.01 0.02Yb 0.47 0.29 0.03 0.07 0.13Lu 0.08 0.04 0.01 0.01 0.02

Note: Abbreviations are as follows: d. dike—diabasic dike; p. lava—pillow lava; b. dike—basaltic dike; an. dike—andesitic dike; am. gab.—amphibole gabbro; peg. gab.—pegmatitic gabbro; leucog.—leucogabbro; dc. dike—dacitic dike in sheeted dike complex; amphib.—amphibolite; pyrox.—pyroxenite; Pl. lherz.—plagioclase lherzolite; harzbrg.—harzburgite; Cpx-harz.—clinopyroxene-bearing harzburgite. LOI—loss on ignition.



Bodinier et al., 1990; Gruau et al., 1998). In contrast, the clinopyroxene-bearing harzburgite is less refractory, as testifi ed by slightly higher TiO

2 (>0.05%), Al

2O

3 (3.5%), and REE contents.

The REE pattern (Fig. 9) is characterized by bulk REE enrichment (relative to the harzburgite) and

by LREE depletion with respect to heavy (H) REEs. The REE patterns of Dehshir depleted harzburgites (AZ06–40, G06–13) are similar to both Izu-Bonin-Mariana forearc (Pearce et al., 1992) and Thetford Mines ophiolite (Pagé et al., 2009) harzburgites (Fig. 9).

Cumulate Rocks

Cumulate rocks include plagioclase lherzo-lite, clinopyroxenite, pegmatite gabbro, and leucogabbro; the chemical compositions of these rocks are clearly related to their modal mineralogy. Plagioclase lherzolite (G06–20) is characterized by much higher contents of Al

2O

3

(8.7%) and CaO (9.4%), higher CaO/Al2O

3

(1.1), and lower Mg# (82.8) than expected for residual peridotites, and these values are con-sistent with a cumulate origin. Similarly, the clinopyroxenite has high concentrations of CaO (15.6%) and bulk REEs (Fig. 9) and low Mg# (85.7), consistent with a magmatic origin. The origin of mantle pyroxenite is controversial. The major processes that have been suggested for the occurrence of ophiolitic pyroxenite in-clude: in situ partial melting of host peridotite (e.g., Chen et al., 2001), interaction between melt and host peridotite (e.g., Garrido and Bodinier, 1999), and crystal segregation along magma conduits (Bodinier et al., 1987). Clino-pyroxenite is characterized by a LREE-depleted REE pattern, signifying that these rocks were neither affected by a metasomatic phase, nor do they represent simple partial melt of surround-ing peridotites, but they may be crystal mag-matic segregates (Fig. 9).

Leucogabbros cover a wide compositional range (Table 1), reflecting variable modal compositions due to their cumulate nature.

Mariana TroughIBM & S. Sd Forearc

Dehshir

Baft

Shahr-e-Babak

Nain

NeyrizOuter Belt

Inner Belt

80 90 1000

1

2

3

4

5

6

100Mg/(Mg+Fe)

Abyssal peridotite Forearc

peridotite

A

0 1 2 3 4 5 6 7

0

0.1

0.2

0.3

0.4

0.5

etitodirepcraeroF

Melt percolation

BA

TiO

2 (w

t%)

Al2O3 (wt%)

Al 2

O3

(wt%

)

Abyssalperid. (1)

Abyssalperid. (2)

OPX CPX

Figure 5. Composition of Dehshir ophiolite harzburgite pyroxenes. (A) Orthopyroxene Al2O3 versus Mg# composition diagram. Fields for abyssal peridotites (Johnson et al., 1990) and forearc peridotites (Ishii et al., 1992) are shown for comparison. (B) Clinopyroxene TiO2 against Al2O3 compositions for Dehshir harzburgites. Comparative fi elds are shown for abyssal peridotites (Abyssal peridotite 1, Hébert et al., 1990; Abyssal peridotite 2, Johnson et al., 1990) and forearc peridotites (Ishii et al., 1992). The long, thin arrow shows changes ex-pected for residual clinopyroxene due to progressive partial melting of peridotite. Short thick arrow shows compositional changes due to reequilibration of clinopyroxene with percolating melt. Izu-Bonin-Mariana (IBM) and South Sandwich forearc harzburgite data are from Parkinson and Pearce (1998) and Pearce et al. (2000), respectively; Mariana Trough mantle data are from Ohara et al. (2002).

TABLE 2. REPRESENTATIVE COMPOSITION OF OLIVINE PORPHYROCLASTS IN DEHSHIR PERIDOTITES

Sample: AZ06-40 AZ06-40 AZ06-40 DZ05-3 DZ05-3 G06-20 G06-2 G06-2 G06-2Rock type: harzb. harzb. harzb. cpx-harz. cpx-harz. plg. lherz plg. lherz plg. lherz plg. lherzSiO2 41.52 42.23 41.50 41.21 41.68 40.05 39.81 38.95 38.96TiO2 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00Al2O3 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.01 0.02FeO 8.82 9.17 8.79 9.84 8.78 15.02 15.58 15.66 15.01MnO 0.16 0.02 0.09 0.05 0.15 0.20 0.15 0.24 0.21MgO 49.74 49.49 49.46 49.91 48.95 43.56 43.52 43.95 43.93CaO 0.04 0.02 0.00 0.04 0.03 0.02 0.03 0.05 0.01Na2O 0.01 0.04 0.09 0.06 0.07 0.02 0.01 0.00 0.00K2O 0.02 0.04 0.03 0.06 0.01 0.00 0.00 0.01 0.00NiO 0.44 0.23 0.42 0.32 0.37 0.21 0.22 0.22 0.20Cr2O3 0.00 0.00 0.00 0.01 0.00 0.02 0.00 0.00 0.02Total 100.76 101.25 100.38 101.49 100.05 99.10 99.31 99.10 98.34Si 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00Al 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Fe2 0.18 0.18 0.18 0.20 0.18 0.31 0.33 0.34 0.32Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01Mg 1.79 1.75 1.78 1.81 1.75 1.62 1.63 1.68 1.68Ca 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Na 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ni 0.01 0.00 0.01 0.01 0.01 0.00 0.00 0.01 0.00Cations 2.98 2.94 2.97 3.02 2.94 2.95 2.96 3.03 3.01Fa% 0.09 0.09 0.09 0.10 0.09 0.16 0.17 0.17 0.16Fo% 0.91 0.91 0.91 0.90 0.91 0.84 0.83 0.83 0.84

Note: Abbreviations: plg. lherz.—plagioclase lherzolite; harzb.—harzburgite; cpx-harz.—clinopyroxene-bearing harzburgite.



TAB

LE 3

. RE

PR

ES

EN

TAT

IVE

CO

MP

OS

ITIO

N O

F P

YR

OX

EN

ES

(O

RT

HO

PY

RO

XE

NE

AN

D C

LIN

OP

YR

OX

EN

E)

IN D

EH

SH

IR P

ER

IDO

TIT

ES

Sam

ple:

AZ

06-4

0A

Z06

-40

AZ

06-4

0A

Z06

-40

AZ

06-4

0G

06-1

3G

06-1

3G

06-1

3G

06-1

3G

06-1

3G

06-1

3G

06-1

3G

06-1

3G

06-1

3G

06-1

3D

Z05

-3D

Z05

-3D

Z05

-3R

ock

type

:ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

cpx-

harz

.cp

x-ha

rz.

cpx-

harz

.S

iO2

55.8

356

.79

57.1

156

.96

56.8

755

.79

55.6

655

.73

56.7

656

.55

56.5

457

.06

56.7

453

.45

56.4

855

.33

52.7

256

.48

TiO

20.

000.

030.

010.

000.

040.

000.

000.

010.

030.

090.

000.

000.

000.

070.

000.

030.

070.

00A

l 2O3

1.19

1.25

1.16

1.20

1.18

1.20

1.12

1.14

1.07

1.15

1.11

1.14

1.19

1.57

1.13

1.13

1.54

1.41

FeO

8.78

9.09

8.94

8.82

9.42

8.95

8.87

9.20

8.62

8.55

8.80

8.84

8.92

5.06

9.32

9.29

2.28

6.30

Cr 2

O3

0.37

0.26

0.35

0.22

0.32

0.32

0.21

0.30

0.38

0.33

0.22

0.30

0.26

0.61

0.38

0.26

0.68

0.47

MnO

0.24

0.16

0.13

0.13

0.33

0.23

0.11

0.17

0.14

0.40

0.25

0.21

0.22

0.05

0.12

0.22

0.11

0.16

NiO

0.00

0.16

0.00

0.06

0.05

0.05

0.08

0.18

0.00

0.08

0.08

0.11

0.04

0.14

0.05

0.12

0.01

0.07

MgO

31.8

231

.86

32.0

531

.79

31.5

631

.75

32.3

831

.36

31.5

731

.98

31.8

431

.75

31.6

018

.58

31.5

831

.24

17.9

834

.16

CaO

1.40

1.28

1.30

1.44

1.43

1.25

0.90

1.39

1.46

1.43

1.38

1.38

1.36

20.0

71.

361.

2723

.14

1.23

Na 2

O0.

040.

100.

090.

120.

020.

110.

050.

130.

000.

050.

050.

020.

040.

150.

040.

130.

170.

03K

2O

0.07

0.00

0.03

0.03

0.03

0.04

0.03

0.02

0.00

0.00

0.02

0.00

0.01

0.04

0.02

0.06

0.00

0.05

Tota

l99

.75

100.

9710

1.16

100.

7610

1.25

99.6

999

.41

99.6

410

0.03

100.

6210

0.29

100.

8110

0.37

99.7

910

0.48

99.0

898

.70

100.

36T

Si

1.96

1.97

1.97

1.98

1.97

1.96

1.95

1.96

1.99

1.97

1.97

1.98

1.98

1.95

1.97

1.96

1.93

1.94

TAl

0.04

0.03

0.03

0.02

0.03

0.04

0.05

0.04

0.01

0.04

0.03

0.02

0.02

0.05

0.03

0.05

0.07

0.06

TF

e30.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00M

1Al

0.01

0.02

0.02

0.03

0.02

0.01

0.00

0.01

0.03

0.01

0.02

0.03

0.03

0.02

0.02

0.00

0.00

0.00

M1T

i0.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00M

1Fe3

0.03

0.01

0.00

0.00

0.00

0.04

0.05

0.04

0.00

0.01

0.01

0.00

0.00

0.03

0.01

0.05

0.05

0.05

M1F

e20.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00M

1Cr

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.02

0.01

0.01

0.02

0.01

M1M

g0.

950.

960.

970.

970.

970.

950.

950.

940.

960.

960.

970.

960.

960.

930.

970.

940.

920.

94M

1Ni

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

M2M

g0.

710.

690.

690.

680.

660.

710.

750.

700.

690.

700.

690.

680.

680.

080.

680.

710.

060.

82M

2Fe2

0.22

0.25

0.26

0.25

0.27

0.22

0.21

0.23

0.25

0.24

0.25

0.26

0.26

0.13

0.27

0.23

0.02

0.13

M2M

n0.

010.

010.

000.

000.

010.

010.

000.

010.

000.

010.

010.

010.

010.

000.

000.

010.

000.

01M

2Ca

0.05

0.05

0.05

0.05

0.05

0.05

0.03

0.05

0.06

0.05

0.05

0.05

0.05

0.78

0.05

0.05

0.91

0.05

M2N

a0.

000.

010.

010.

010.

000.

010.

000.

010.

000.

000.

000.

000.

000.

010.

000.

010.

010.

00M

2K0.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00S

um_c

at4.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

00W

O2.

662.

422.

462.

732.

692.

381.

702.

662.

802.

702.

612.

632.

5940

.21

2.58

2.44

46.2

62.

29E

N83

.98

83.9

284

.18

84.0

182

.93

83.9

885

.06

83.3

684

.11

84.0

983

.99

83.9

583

.81

51.8

083

.43

83.3

350

.01

88.3

4F

S13

.36

13.6

613

.36

13.2

614

.38

13.6

313

.24

13.9

913

.10

13.2

113

.40

13.4

313

.60

7.99

13.9

914

.24

3.73

9.38

Min

eral

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Cpx

Opx

Opx

Cpx

Opx

( con

tinue

d)



TAB

LE 3

. RE

PR

ES

EN

TAT

IVE

CO

MP

OS

ITIO

N O

F P

YR

OX

EN

ES

(O

RT

HO

PY

RO

XE

NE

AN

D C

LIN

OP

YR

OX

EN

E)

IN D

EH

SH

IR P

ER

IDO

TIT

ES

(con

tinue

d)

Sam

ple:

DZ

05-3

DZ

05-3

DZ

05-3

DZ

05-3

DZ

05-3

DZ

05-3

DZ

05-3

DZ

05-3

AZ

06-2

6A

Z06

-26

AZ

06-2

6A

Z06

-26

AZ

06-2

6A

Z06

-26

AZ

06-2

6A

Z06

-26

AZ

06-2

6A

Z06

-26

Roc

k ty

pe:

cpx-

harz

.cp

x-ha

rz.

cpx-

harz

.cp

x-ha

rz.

cpx-

harz

.cp

x-ha

rz.

cpx-

harz

.cp

x-ha

rz.

cpx-

harz

.cp

x-ha

rz.

cpx-

harz

.cp

x-ha

rz.

cpx-

harz

.cp

x-ha

rz.

cpx-

harz

.cp

x-ha

rz.

cpx-

harz

.cp

x-ha

rz.

SiO

256

.64

53.5

656

.19

56.8

456

.42

56.4

653

.60

53.4

556

.79

56.4

855

.83

56.7

756

.01

56.0

256

.60

56.2

956

.20

51.7

6Ti

O2

0.00

0.03

0.07

0.00

0.00

0.04

0.01

0.11

0.00

0.05

0.06

0.06

0.05

0.00

0.00

0.06

0.03

0.05

Al 2O

31.

261.

511.

310.

820.

580.

660.

870.

950.

690.

661.

180.

750.

720.

750.

730.

660.

850.

12F

eO6.

402.

565.

828.

517.

368.

253.

593.

108.

298.

629.

347.

567.

208.

357.

287.

337.

356.

73C

r 2O

30.

400.

760.

330.

180.

350.

230.

670.

790.

350.

360.

390.

360.

370.

320.

330.

300.

350.

00M

nO0.

150.

240.

140.

080.

190.

250.

210.

180.

160.

240.

420.

000.

230.

240.

390.

080.

190.

39N

iO0.

210.

060.

250.

160.

010.

020.

000.

000.

130.

060.

120.

000.

160.

060.

000.

000.

290.

00M

gO33

.94

17.5

633

.66

32.9

432

.64

32.8

517

.98

17.4

433

.11

32.7

031

.37

33.1

232

.88

32.4

932

.02

31.4

232

.19

14.6

3C

aO0.

9823

.20

1.16

1.22

1.25

1.22

22.0

722

.88

1.36

1.37

1.50

1.28

1.23

1.23

1.22

2.23

1.15

24.7

5N

a 2O

0.09

0.25

0.10

0.13

0.10

0.09

0.21

0.23

0.06

0.00

0.04

0.04

0.04

0.12

0.20

0.13

0.17

0.07

K2O

0.04

0.04

0.00

0.00

0.00

0.01

0.00

0.05

0.00

0.00

0.01

0.03

0.03

0.02

0.06

0.03

0.00

0.01

Tota

l10

0.11

99.7

799

.03

100.

8898

.90

100.

0899

.21

99.1

810

0.94

100.

5410

0.26

99.9

798

.94

99.6

098

.83

98.5

598

.77

98.5

0T

Si

1.95

1.95

1.96

1.96

1.98

1.96

1.96

1.96

1.96

1.96

1.95

1.97

1.97

1.96

1.99

1.99

1.98

1.95

TAl

0.05

0.05

0.04

0.03

0.02

0.03

0.04

0.04

0.03

0.03

0.05

0.03

0.03

0.03

0.01

0.01

0.02

0.01

TF

e30.

000.

000.

000.

010.

000.

010.

000.

000.

010.

010.

000.

000.

010.

010.

000.

000.

000.

05M

1Al

0.01

0.01

0.01

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.02

0.02

0.01

0.00

M1T

i0.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00M

1Fe3

0.04

0.03

0.03

0.04

0.01

0.04

0.03

0.03

0.04

0.03

0.03

0.02

0.03

0.04

0.00

0.00

0.01

0.06

M1F

e20.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

11M

1Cr

0.01

0.02

0.01

0.01

0.01

0.01

0.02

0.02

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.00

M1M

g0.

940.

930.

950.

950.

970.

960.

950.

950.

950.

960.

950.

970.

960.

950.

970.

970.

960.

82M

1Ni

0.01

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.00

0.00

0.00

0.01

0.00

M2M

g0.

810.

020.

800.

750.

730.

750.

030.

010.

750.

730.

690.

750.

760.

750.

710.

690.

730.

00M

2Fe2

0.15

0.05

0.14

0.20

0.21

0.20

0.08

0.07

0.19

0.21

0.24

0.20

0.18

0.19

0.21

0.22

0.21

0.00

M2M

n0.

000.

010.

000.

000.

010.

010.

010.

010.

010.

010.

010.

000.

010.

010.

010.

000.

010.

01M

2Ca

0.04

0.91

0.04

0.05

0.05

0.05

0.87

0.90

0.05

0.05

0.06

0.05

0.05

0.05

0.05

0.08

0.04

1.00

M2N

a0.

010.

020.

010.

010.

010.

010.

020.

020.

000.

000.

000.

000.

000.

010.

010.

010.

010.

01M

2K0.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00S

um_c

at4.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

00W

O1.

8446

.57

2.20

2.27

2.38

2.28

44.0

946

.03

2.52

2.55

2.84

2.40

2.34

2.31

2.36

4.30

2.23

48.8

5E

N88

.57

49.0

488

.96

85.2

686

.41

85.3

349

.98

48.8

285

.27

84.5

982

.71

86.5

286

.67

85.0

686

.07

84.5

186

.41

40.1

8F

S9.

594.

398.

8412

.47

11.2

212

.39

5.93

5.15

12.2

112

.86

14.4

411

.08

10.9

912

.62

11.5

711

.19

11.3

610

.97

Min

eral

Opx

Cpx

Opx

Opx

Opx

Opx

Cpx

Cpx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Cpx

(con

tinue

d)



TAB

LE 3

. RE

PR

ES

EN

TAT

IVE

CO

MP

OS

ITIO

N O

F P

YR

OX

EN

ES

(O

RT

HO

PY

RO

XE

NE

AN

D C

LIN

OP

YR

OX

EN

E)

IN D

EH

SH

IR P

ER

IDO

TIT

ES

(con

tinue

d)

Sam

ple:

AZ

06-2

6D

A-2

4D

A-2

4D

A-2

4D

A-2

4D

A-2

4D

A-2

4D

A-2

4D

A-2

4D

A-2

4D

A-2

4D

A-2

4D

A-2

1D

A-2

1D

A-2

1D

A-2

1D

A-2

1D

A-2

1R

ock

type

:cx

p-ha

rz.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.S

iO2

51.2

755

.22

56.3

355

.21

50.6

250

.77

54.5

956

.54

55.4

855

.96

55.1

655

.88

50.0

551

.51

52.3

654

.84

55.9

255

.09

TiO

20.

000.

010.

000.

000.

000.

010.

090.

000.

040.

030.

000.

000.

000.

010.

020.

050.

020.

03A

l 2O3

0.03

0.65

0.69

0.60

0.86

0.80

0.66

0.84

0.67

0.64

0.65

0.67

0.84

0.81

0.54

0.67

0.73

0.66

FeO

7.33

7.38

7.32

7.47

4.01

3.94

7.48

8.92

7.06

7.46

7.92

6.97

3.84

3.36

7.12

7.48

7.28

7.16

Cr 2

O3

0.00

0.37

0.36

0.41

0.61

0.55

0.34

0.31

0.31

0.30

0.20

0.30

0.62

0.63

0.28

0.26

0.35

0.43

MnO

0.38

0.23

0.38

0.26

0.12

0.18

0.08

0.22

0.07

0.05

0.23

0.12

0.07

0.25

0.16

0.20

0.20

0.03

NiO

0.03

0.00

0.00

0.16

0.00

0.00

0.00

0.06

0.00

0.06

0.14

0.09

0.06

0.04

0.17

0.02

0.00

0.00

MgO

14.5

033

.92

32.0

632

.49

17.1

817

.93

33.9

233

.47

33.1

533

.60

33.0

133

.23

18.9

518

.73

28.0

333

.69

34.1

333

.69

CaO

24.7

61.

121.

131.

5624

.55

23.9

11.

400.

781.

261.

511.

401.

4423

.35

23.2

79.

771.

551.

361.

38N

a 2O

0.17

0.03

0.02

0.04

0.23

0.21

0.07

0.02

0.02

0.06

0.04

0.01

0.23

0.27

0.06

0.02

0.09

0.03

K2O

0.03

0.04

0.01

0.00

0.01

0.03

0.00

0.00

0.01

0.01

0.00

0.00

0.01

0.00

0.02

0.01

0.06

0.00

Tota

l98

.49

98.9

798

.29

98.2

298

.18

98.3

298

.63

101.

1698

.07

99.6

998

.75

98.7

398

.02

98.8

898

.52

98.7

910

0.15

98.4

9T

Si

1.93

1.93

2.00

1.95

1.88

1.87

1.91

1.95

1.96

1.94

1.94

1.96

1.84

1.88

1.87

1.92

1.93

1.93

TAl

0.00

0.03

0.00

0.03

0.04

0.04

0.03

0.03

0.03

0.03

0.03

0.03

0.04

0.04

0.02

0.03

0.03

0.03

TF

e30.

070.

050.

000.

020.

000.

000.

060.

020.

010.

030.

030.

010.

000.

000.

000.

050.

040.

04M

1Al

0.00

0.00

0.02

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

M1T

i0.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00M

1Fe3

0.08

0.06

0.00

0.04

0.00

0.00

0.08

0.05

0.03

0.05

0.06

0.03

0.00

0.00

0.00

0.07

0.07

0.06

M1F

e20.

080.

000.

000.

000.

030.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00M

1Cr

0.00

0.01

0.01

0.01

0.02

0.02

0.01

0.01

0.01

0.01

0.01

0.01

0.02

0.02

0.01

0.01

0.01

0.01

M1M

g0.

810.

930.

970.

950.

950.

980.

910.

940.

960.

940.

930.

960.

980.

980.

990.

920.

920.

93M

1Ni

0.00

0.00

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.00

0.00

0.00

M2M

g0.

000.

840.

730.

770.

000.

000.

860.

770.

790.

800.

800.

780.

060.

040.

510.

840.

830.

83M

2Fe2

0.00

0.11

0.22

0.16

0.09

0.12

0.08

0.19

0.16

0.14

0.14

0.16

0.12

0.10

0.21

0.10

0.10

0.12

M2M

n0.

010.

010.

010.

010.

000.

010.

000.

010.

000.

000.

010.

000.

000.

010.

010.

010.

010.

00M

2Ca

1.00

0.04

0.04

0.06

0.98

0.95

0.05

0.03

0.05

0.06

0.05

0.05

0.92

0.91

0.37

0.06

0.05

0.05

M2N

a0.

010.

000.

000.

000.

020.

020.

010.

000.

000.

000.

000.

000.

020.

020.

000.

000.

010.

00M

2K0.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00S

um_c

at4.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

004.

00W

O48

.60

2.07

2.19

2.96

47.5

045

.92

2.57

1.43

2.38

2.79

2.61

2.71

44.2

644

.62

17.9

42.

842.

492.

56E

N39

.58

86.9

886

.19

85.6

146

.26

47.9

186

.61

85.4

787

.11

86.3

785

.53

86.8

849

.96

49.9

671

.62

86.1

586

.82

87.0

3F

S11

.81

10.9

511

.62

11.4

36.

246.

1710

.82

13.1

010

.51

10.8

411

.86

10.4

15.

785.

4110

.44

11.0

110

.68

10.4

1M

iner

alC

pxO

pxO

pxO

pxC

pxC

pxO

pxO

pxO

pxO

pxO

pxO

pxC

pxC

pxP

igeo

nite

Opx

Opx

Opx

( con

tinue

d)



TAB

LE 3

. RE

PR

ES

EN

TAT

IVE

CO

MP

OS

ITIO

N O

F P

YR

OX

EN

ES

(O

RT

HO

PY

RO

XE

NE

AN

D C

LIN

OP

YR

OX

EN

E)

IN D

EH

SH

IR P

ER

IDO

TIT

ES

(con

tinue

d)

Sam

ple:

DA

-21

DA

-21

DA

-21

DA

-21

DA

-21

DA

-7D

A-7

DA

-7D

A-7

DA

-7D

A-7

DA

-7D

A-7

DA

-7D

A-7

DA

-7D

A-7

Roc

k ty

pe:

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.ha

rzb.

harz

b.S

iO2

56.4

854

.76

55.4

854

.33

55.1

055

.00

54.8

155

.25

53.4

255

.22

54.4

354

.82

54.7

054

.49

53.6

653

.73

54.0

8Ti

O2

0.03

0.00

0.05

0.06

0.02

0.00

0.01

0.00

0.04

0.00

0.01

0.03

0.03

0.00

0.02

0.03

0.00

Al 2O

30.

490.

640.

640.

630.

590.

640.

560.

670.

660.

580.

630.

620.

570.

600.

740.

620.

92F

eO6.

927.

147.

407.

266.

977.

747.

427.

296.

887.

096.

937.

217.

237.

187.

577.

463.

44C

r 2O

30.

150.

330.

300.

400.

210.

400.

300.

230.

410.

320.

290.

250.

340.

310.

340.

250.

69M

nO0.

160.

110.

260.

330.

210.

220.

230.

180.

200.

100.

230.

200.

150.

190.

000.

190.

12N

iO0.

030.

000.

090.

070.

060.

000.

000.

000.

160.

090.

170.

000.

260.

000.

080.

020.

00M

gO34

.01

34.0

033

.18

34.0

034

.05

32.6

633

.74

33.2

434

.76

34.1

734

.10

33.6

333

.37

34.3

135

.13

33.9

318

.17

CaO

0.83

1.34

1.35

1.43

1.43

1.57

1.33

1.42

1.41

1.32

1.49

1.50

1.30

1.16

1.04

1.78

21.5

2N

a 2O

0.01

0.00

0.00

0.06

0.05

0.05

0.06

0.02

0.05

0.04

0.02

0.05

0.03

0.05

0.02

0.08

0.29

K2O

0.00

0.00

0.00

0.00

0.00

0.00

0.04

0.01

0.00

0.00

0.01

0.00

0.02

0.00

0.01

0.05

0.08

Tota

l99

.11

98.3

198

.75

98.5

698

.69

98.2

798

.49

98.3

198

.00

98.9

398

.32

98.3

198

.01

98.2

798

.61

98.1

399

.31

TS

i1.

971.

921.

951.

901.

931.

941.

921.

951.

871.

931.

911.

931.

931.

911.

871.

891.

98TA

l0.

020.

030.

030.

030.

020.

030.

020.

030.

030.

020.

030.

030.

020.

030.

030.

030.

02T

Fe3

0.01

0.05

0.03

0.07

0.05

0.03

0.05

0.03

0.00

0.05

0.07

0.05

0.05

0.07

0.10

0.09

0.00

M1A

l0.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

02M

1Ti

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

M1F

e30.

030.

070.

040.

090.

070.

050.

070.

050.

000.

070.

080.

070.

060.

080.

120.

110.

01M

1Fe2

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

M1C

r0.

000.

010.

010.

010.

010.

010.

010.

010.

010.

010.

010.

010.

010.

010.

010.

010.

02M

1Mg

0.97

0.92

0.95

0.90

0.92

0.94

0.92

0.94

0.98

0.92

0.90

0.92

0.92

0.91

0.87

0.88

0.95

M1N

i0.

000.

000.

000.

000.

000.

000.

000.

000.

010.

000.

010.

000.

010.

000.

000.

000.

00M

2Mg

0.80

0.86

0.79

0.88

0.85

0.78

0.85

0.80

0.84

0.86

0.88

0.84

0.84

0.89

0.96

0.90

0.04

M2F

e20.

160.

090.

150.

060.

080.

150.

090.

140.

200.

090.

060.

100.

110.

060.

000.

020.

09M

2Mn

0.01

0.00

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.00

0.01

0.01

0.01

0.01

0.00

0.01

0.00

M2C

a0.

030.

050.

050.

050.

050.

060.

050.

050.

050.

050.

060.

060.

050.

040.

040.

070.

84M

2Na

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.02

M2K

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Sum

_cat

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

WO

1.55

2.46

2.53

2.62

2.62

2.94

2.46

2.65

2.55

2.43

2.74

2.77

2.43

2.12

1.87

3.24

43.4

0E

N88

.16

87.1

286

.29

86.5

487

.07

85.3

986

.54

86.4

587

.45

87.2

787

.01

86.5

386

.79

87.3

687

.55

85.8

950

.99

FS

10.3

010

.42

11.1

810

.84

10.3

111

.67

11.0

010

.90

10.0

010

.30

10.2

510

.71

10.7

710

.53

10.5

810

.87

5.61

Min

eral

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Opx

Cpx

Not

e: A

bbre

viat

ions

are

as

follo

ws:

har

zb.—

harz

burg

ite; c

px-h

arz.

—cl

inop

yrox

ene-

bear

ing

harz

burg

ite; o

px—

orth

opyr

oxen

e.



TAB

LE 4

. CO

MP

OS

ITIO

N O

F S

PIN

ELS

IN D

EH

SH

IR H

AR

ZB

UR

GIT

ES

Sam

ple

AZ

06-4

0A

Z06

-40

AZ

06-4

0A

Z06

-40

AZ

06-4

0A

Z06

-40

AZ

06-4

0A

Z06

-40

DZ

05-3

DZ

05-3

DZ

05-3

DZ

05-3

DZ

05-3

DZ

05-3

G06

-13

G06

-13

G06

-13

DA

-24

DA

-24

DA

-24

SiO

20.

000.

000.

050.

000.

000.

030.

000.

000.

000.

040.

030.

050.

060.

050.

090.

050.

000.

100.

000.

00Ti

O2

0.03

0.01

0.02

0.05

0.02

0.02

0.08

0.10

0.13

0.08

0.10

0.17

0.18

0.23

0.22

0.01

0.01

0.09

0.04

0.03

Al 2O

348

.18

48.1

648

.27

48.1

248

.98

17.7

417

.15

17.0

417

.36

10.5

610

.49

8.76

8.97

9.17

8.46

17.8

719

.25

17.6

117

.04

18.1

7F

eO(T

)13

.63

13.7

113

.47

13.0

213

.19

21.3

321

.80

21.4

021

.17

24.3

225

.17

27.2

527

.14

25.5

725

.54

20.8

422

.84

21.9

125

.80

21.6

9M

nO0.

200.

230.

100.

040.

250.

400.

310.

170.

160.

120.

280.

310.

300.

170.

280.

120.

310.

180.

270.

18M

gO18

.89

17.9

117

.80

17.5

917

.72

11.5

311

.44

11.1

711

.30

9.63

9.56

7.35

7.58

8.85

7.79

12.3

511

.51

11.5

39.

4511

.68

CaO

0.03

0.00

0.04

0.01

0.01

0.00

0.00

0.05

0.03

0.09

0.05

0.00

0.00

0.00

0.00

0.02

0.02

0.06

0.00

0.00

Na 2

O0.

040.

050.

040.

050.

040.

040.

190.

000.

030.

130.

000.

100.

100.

240.

130.

060.

160.

280.

060.

02K

2O

0.00

0.04

0.01

0.00

0.02

0.02

0.00

0.00

0.00

0.03

0.03

0.01

0.01

0.06

0.04

0.01

0.03

0.07

0.00

0.00

Cr 2

O3

18.9

118

.66

19.8

819

.25

18.7

947

.66

47.9

148

.54

48.4

554

.32

53.7

555

.59

55.0

355

.08

55.4

448

.70

45.6

847

.85

46.6

947

.77

NiO

0.14

0.27

0.29

0.27

0.26

0.16

0.04

0.13

0.01

0.11

0.12

0.04

0.00

0.00

0.13

0.02

0.00

0.11

0.18

0.14

ZnO

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

0.00

0.00

0.02

0.11

0.35

0.00

0.16

0.44

0.14

V2O

5N

DN

DN

DN

DN

DN

DN

DN

DN

DN

DN

D0.

270.

140.

000.

040.

350.

360.

280.

210.

41S

um:

100.

0499

.05

99.9

698

.41

99.2

998

.93

98.9

398

.61

98.6

599

.44

99.5

899

.89

99.5

299

.44

98.2

810

0.75

100.

1610

0.21

100.

1810

0.22

Fe 2

O3

3.40

2.30

1.30

1.00

1.20

5.30

5.70

4.80

4.60

6.20

7.00

3.70

4.00

4.40

3.30

3.90

4.50

3.80

4.80

3.70

FeO

10

.60

11.7

012

.30

12.1

012

.10

16.6

016

.70

17.1

017

.10

18.7

018

.80

21.2

020

.80

19.0

020

.00

15.2

016

.50

16.3

018

.90

16.2

0S

i0.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00Ti

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.01

0.00

0.00

0.00

0.00

0.00

Al

1.53

1.55

1.54

1.56

1.57

0.67

0.65

0.65

0.66

0.41

0.41

0.36

0.37

0.37

0.35

0.67

0.73

0.67

0.66

0.69

Fe3+

0.07

0.05

0.03

0.02

0.02

0.13

0.14

0.12

0.11

0.15

0.18

0.10

0.10

0.11

0.09

0.09

0.11

0.09

0.12

0.09

Fe2+

0.24

0.27

0.28

0.28

0.27

0.44

0.45

0.46

0.46

0.52

0.52

0.62

0.60

0.55

0.59

0.40

0.44

0.44

0.52

0.43

Mn

0.00

0.01

0.00

0.00

0.01

0.01

0.01

0.00

0.00

0.00

0.01

0.01

0.01

0.00

0.01

0.00

0.01

0.00

0.01

0.00

Mg

0.76

0.73

0.72

0.72

0.72

0.55

0.54

0.54

0.54

0.48

0.47

0.38

0.39

0.45

0.41

0.58

0.55

0.55

0.46

0.56

Ca

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Na

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.00

0.00

0.01

0.00

0.01

0.01

0.02

0.01

0.00

0.01

0.02

0.00

0.00

K0.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00C

r0.

400.

400.

430.

420.

401.

201.

211.

231.

231.

421.

411.

521.

511.

501.

541.

221.

151.

221.

211.

21N

i0.

000.

010.

010.

010.

010.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00V

N

DN

DN

DN

DN

DN

DN

DN

DN

DN

DN

D0.

010.

000.

000.

000.

010.

010.

010.

010.

01Z

n N

DN

DN

DN

DN

DN

DN

DN

DN

DN

DN

D0.

000.

000.

000.

000.

010.

000.

000.

010.

00To

tal:

3.00

3.00

3.00

3.00

3.00

3.00

3.01

3.00

3.00

3.01

3.00

3.00

3.00

3.01

3.01

3.00

3.01

3.01

3.00

3.00

Cr#

20.8

420

.63

21.6

521

.16

20.4

764

.31

65.2

065

.65

65.1

977

.53

77.4

780

.98

80.4

580

.11

81.4

764

.64

61.4

264

.57

64.7

763

.82

Mg#

76.1

373

.26

72.1

272

.12

72.2

955

.38

54.9

553

.78

54.1

447

.80

47.4

838

.16

39.3

245

.30

40.9

659

.13

55.4

155

.76

47.1

356

.27



These rocks are characterized by variable Mg# (71.9–87.7), CaO (12.2%–24.7%), Al

2O

3

(14.5%–23.3%), TiO2 (0.04%–0.2%), and CaO/

Al2O

3 (0.67–1.5), except sample G06–6, which

has the lowest CaO (0.75%), Al2O

3 (2.15%),

TiO2 (0.02%), and CaO/Al

2O

3 (0.35) and higher

Mg# (89.3). This sample has also high contents of Cr (2586 ppm) and Ni (1934 ppm).

In general, incompatible element contents are very low in the cumulate rocks because they do not fractionate into the common cumu-lus phases (plag + cpx + ol). Leucogabbros are strongly depleted in REE. They display slightly LREE-depleted patterns (Fig. 10A), character-ized by an increase from heavy REE (HREE) to middle REE (MREE) (Dy/Yb ~ 1.2 × chon-drite) and lower LREE/MREE (Ce/Gd ~ 0.2 × chondrite). The exception is sample G06–6, which is characterized by a steady decrease from HREE to LREE (Yb/Ce ~ 37 × chondrite). Most leuco gabbros show a positive Eu anomaly due to plagio clase accumulation (except sample G06–6). The normal (N)-MORB–normalized trace-element patterns for leucogabbros (Fig. 10B) show strong depletions in Nb relative to LREE (Nb/La = 0.003–0.2 × N-MORB). The

leucogabbros are enriched in highly incompati-ble LILEs (e.g., Rb, Ba, and K) relative to LREE (e.g., Ba/La = 9–675 × N-MORB). They also show prominent spikes in Sr and Pb (Sr/La = 4–502 × N-MORB and Pb/La = 17–67 × N-MORB) and selective enrichment in U rela-tive to the neighboring elements Th and LREE (U/Th = 3–18). The LILE and U abundances could have been affected by alteration.

Pegmatite gabbro is similar in terms of Al2O

3

(20.2%), CaO (15%), TiO2 (0.07%), Mg#

(73.4), and CaO/Al2O

3 (0.74) to the magmatic

rocks containing cumulate plagioclase and clinopyroxene (e.g., Pearce et al., 1995). Peg-matite gabbro shows a highly depleted REE pattern (Fig. 10C), consistent with its cumulate nature. A positive Eu anomaly (Eu/Eu* = 10.5) associated with high Al

2O

3, Sr, K, and Ba con-

tents is evidence of plagioclase accumulation.

Mafi c Rocks

The SiO2 abundances in gabbros cover a

broad range (38.2%–54.4%), indicating mag-matic fractionation and alteration. Gabbros are characterized by low TiO

2 (0.3%–0.5%) and

Zr (13–44 ppm) and high Mg# (58–68), show-ing a boninitic affi nity (Crawford et al., 1989). The exception is sample DR05–3, which has higher TiO

2 (1.8%) and Zr (139 ppm) and

lower Mg# (42), consistent with tholeiitic af-fi nity. Compositional data for mafi c igneous rocks plotted on a revised FeOt/MgO versus SiO

2 diagram (Fig. 11) are bimodal, with sub-

equal tholeiitic and calc-alkalic samples. The pillow lavas are tholeiitic, dacite dikes are calc-alkalic, and lava fl ows and dikes scatter in both fi elds.

The gabbros have low Ti/V (8.1–15.8) and fall into boninitic–island-arc tholeiitic (IAT) fi elds of Shervais (1982), except sample DR05–3, which has higher Ti/V (39.8) and MORB affi n-ity (Fig. 12). Dehshir gabbros show three kinds of REE patterns (Fig. 10C): (1) fl at (D05–12, R-12, D06–3); (2) LREE depleted (DZ05–1H); and (3) slightly LREE enriched (DR05–3). Dehshir gabbros are depleted in Nb and Ta and enriched in Sr (except sample DZ05–1H), U, Pb, K, Ba, and Rb relative to N-MORB (Fig. 10D). Sample DR05–3 is slightly enriched in LREE (La(N)/Yb(N) = 1.5) and slightly depleted in Nb relative to LREE (Nb/La ~ 0.8 × N-MORB).

The pillow lavas range from basalt to andesite (45.4%–61.5% SiO

2) and are fractionated (Mg#

= 28.5–54.1), with variable TiO2 (0.4%–1.9%),

Zr (25.3–163 ppm), and Ti/V (7.3–43.1) ratios. Pillow lavas are tholeiitic, with FeOt/MgO be-tween 1.5 and 4.5, and plot in the high-Fe fi eld of Figure 11. On Ti/V diagram (Fig. 12), they plot in the island-arc tholeiite (IAT)–boninitic fi elds of Shervais (1982), except samples DAR05–6 and A06–2, which have respectively higher and lower Ti/V ratios, showing MORB and calc-alkaline affi nities. In contrast to the pillow lavas, the basaltic fl ows are mostly iron-poor, plotting in the low- to medium-Fe fi elds of the FeOt/MgO versus SiO

2 diagram (Fig.

11). Pillow lavas show either fl at REE patterns (La(N)/Yb(N) = 1.1–1.2) or fractionated LREE-enriched patterns (LaN/YbN = 2.7–3.3) typical of tholeiitic (IAT) and calc-alkaline series, respec-tively (Fig. 10E). Pillowed basalts are depleted in Nb (Nb/La ~ 0.2–0.7 × N-MORB) and en-riched in Pb, U, Ba, and Th (e.g., Th/La ~ 1–3 × N-MORB) relative to LREE in N-MORB nor-malized diagram (Fig. 10F). Basaltic lava fl ows (samples DR05–2A, DAR05–5) and basaltic dike in sheeted dike complex (AZ06–38) are tholeiitic with 44.9%–51% SiO

2, 1.3%–1.8%

TiO2, 66–185 ppm Zr, 3.8%–6.9% MgO, and

Ti/V = 21–30. The andesitic dike in amphi-bole gabbros (DZ05–1D) and a basaltic lava (D06–10) show boninitic affi nities, with higher MgO (7.5%–9.8%) and SiO

2 (50.4%–57.4%)

and very low TiO2 (0.3%–0.4%), Zr (16.8–

20.5 ppm), and Ti/V (9.6–10.5).

0.8 0.6 0.4 0.2 0.01.0

Mg# Mg/(Mg+Fe+2)

0.0

0.2

0.4

0.6

1.0

0.8

Cr#

(Cr/

Cr+

Al)

Boninite

MORB

Back-arc basin

Forearc

A

TiO

2 (w

t%)

MORBperidotite

SSZ peridotite

MORB

10

1

0.1

0.01

0 10 20 30 40 50

Al2O3 (wt%)

Arc

B

Dehshir

Baft

Shahr-e-Babak

Nain

NeyrizOuter Belt

Inner Belt

Mariana Trough

IBM & S. Sd Forearc

Figure 6. (A) Cr# versus Mg# diagram (modifi ed after Dick and Bullen, 1984) for spinels in harzburgite. For comparison, we also plot the data from the Inner Zagros ophiolite belt, including Nain, Shahr-e-Babak, and Baft (data from Shafaii Moghadam, 2009) and from the Outer Zagros ophiolitic belt (Neyriz; data from Jannessary, 2003). Note complete overlap between inner and outer belt data, although outer belt is overall more depleted. (B) Diagram (after Kamenetsky et al., 2001) showing TiO2 against Al2O3 contents of spinels, in order to discriminate between various types of peridotites (fi elds outlined by black lines) and lavas (fi elds outlined by gray lines). Note that spinels plot in fi elds for both mid-ocean-ridge basalt (MORB) peridotites and suprasubduction-zone (SSZ) peridotites. Izu-Bonin-Mariana (IBM) and South Sandwich forearc harzburgite data are from Parkinson and Pearce (1998) and Pearce et al. (2000), respectively; Mariana Trough mantle data are from Ohara et al. (2002).



TAB

LE 5

. CLI

NO

PY

RO

XE

NE

CO

MP

OS

ITIO

NS

IN T

HE

DE

HS

HIR

MA

FIC

RO

CK

S IN

CLU

DIN

G G

AB

BR

OS

, LE

UC

OG

AB

BR

OS

, PIL

LOW

LA

VA

S, A

ND

DIA

BA

SIC

DIK

E S

Sam

ple:

DI0

5-12

DI0

5-12

DI0

5-12

DI0

5-12

DI0

5-12

DI0

5-12

D06

-3D

06-3

D06

-3D

06-3

D06

-3R

-12

R-1

2R

-12

R-1

2G

06-1

6G

06-1

6G

06-1

6R

ock

type

:ga

bbro

gabb

roga

bbro

gabb

roga

bbro

gabb

roga

bbro

gabb

roga

bbro

gabb

roga

bbro

gabb

roga

bbro

gabb

roga

bbro

diab

ase

diab

ase

diab

ase

SiO

249

.62

53.5

752

.49

52.4

752

.93

56.1

252

.33

53.1

553

.93

54.4

854

.48

54.3

853

.13

53.8

653

.68

53.2

951

.59

50.8

1Ti

O2

0.72

0.03

0.30

0.23

0.13

0.05

0.40

0.08

0.03

0.09

0.10

0.00

0.07

0.17

0.23

0.25

0.68

0.67

Al 2O

33.

271.

171.

581.

731.

630.

651.

851.

740.

590.

620.

660.

720.

661.

161.

521.

243.

643.

40F

eO12

.92

5.88

7.58

7.73

5.87

12.6

67.

375.

635.

945.

855.

425.

755.

376.

295.

647.

938.

798.

25C

r 2O

30.

000.

060.

090.

190.

160.

080.

120.

000.

080.

130.

090.

060.

000.

020.

090.

080.

080.

07M

nO0.

410.

000.

330.

250.

110.

270.

190.

230.

180.

150.

260.

260.

160.

130.

190.

160.

270.

36N

iO0.

210.

070.

140.

080.

000.

000.

000.

000.

110.

000.

000.

060.

200.

100.

160.

000.

030.

00M

gO14

.69

15.0

215

.78

15.7

515

.33

27.4

213

.91

15.6

115

.47

15.4

315

.54

15.4

915

.88

14.8

715

.31

15.6

316

.04

15.9

1C

aO16

.83

22.9

719

.83

20.3

522

.40

1.69

22.4

621

.96

23.0

623

.08

23.0

823

.16

22.8

922

.50

22.9

121

.05

18.8

818

.91

Na 2

O0.

450.

270.

290.

170.

120.

050.

260.

250.

080.

090.

150.

060.

160.

010.

180.

190.

320.

30K

2O

0.00

0.05

0.06

0.07

0.03

0.02

0.03

0.01

0.00

0.00

0.00

0.00

0.03

0.00

0.00

0.00

0.00

0.03

Tota

l99

.13

99.1

198

.47

99.0

398

.70

99.0

098

.91

98.6

599

.48

99.9

099

.78

99.9

498

.56

99.0

999

.89

99.8

410

0.32

98.7

0T

Si

1.87

1.99

1.97

1.96

1.98

2.03

1.96

1.98

2.00

2.01

2.01

2.01

1.98

2.01

1.98

1.97

1.90

1.90

TAl

0.13

0.01

0.03

0.04

0.03

0.00

0.04

0.02

0.00

0.00

0.00

0.00

0.02

0.00

0.02

0.03

0.10

0.10

M1A

l0.

020.

040.

040.

030.

050.

030.

050.

060.

030.

030.

030.

030.

010.

050.

050.

030.

050.

05M

1Ti

0.02

0.00

0.01

0.01

0.00

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.01

0.01

0.02

0.02

M1F

e30.

110.

000.

000.

010.

000.

000.

000.

000.

000.

000.

000.

000.

020.

000.

000.

000.

030.

04M

1Fe2

0.03

0.12

0.07

0.07

0.09

0.00

0.16

0.08

0.11

0.12

0.11

0.11

0.08

0.11

0.10

0.10

0.01

0.01

M1C

r0.

000.

000.

000.

010.

010.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00M

1Mg

0.83

0.83

0.88

0.88

0.85

0.97

0.78

0.87

0.86

0.85

0.86

0.85

0.88

0.83

0.84

0.86

0.88

0.89

M1N

i0.

010.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

010.

000.

010.

000.

000.

00M

2Mg

0.00

0.00

0.00

0.00

0.00

0.51

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

M2F

e20.

270.

060.

170.

160.

090.

380.

070.

100.

070.

060.

060.

070.

070.

080.

080.

150.

230.

21M

2Mn

0.01

0.00

0.01

0.01

0.00

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.00

0.01

0.01

0.01

0.01

M2C

a0.

680.

920.

800.

810.

900.

070.

900.

880.

920.

910.

910.

920.

910.

900.

910.

840.

740.

76M

2Na

0.03

0.02

0.02

0.01

0.01

0.00

0.02

0.02

0.01

0.01

0.01

0.00

0.01

0.00

0.01

0.01

0.02

0.02

M2K

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Sum

_cat

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

WO

35.3

147

.40

41.3

441

.96

46.2

83.

3947

.08

45.5

146

.71

46.8

846

.97

46.8

846

.43

46.6

846

.98

42.8

539

.12

39.5

9E

N42

.86

43.1

345

.77

45.1

944

.08

76.4

040

.55

45.0

143

.61

43.6

044

.00

43.6

244

.82

42.9

343

.69

44.2

946

.22

46.3

4F

S21

.83

9.47

12.8

912

.85

9.64

20.2

112

.37

9.48

9.68

9.52

9.02

9.50

8.76

10.3

99.

3312

.86

14.6

614

.08

Mg#

66.9

481

.99

78.7

578

.40

82.3

179

.42

77.0

783

.16

82.2

782

.45

83.6

382

.75

84.0

680

.82

82.8

877

.85

76.4

777

.46

(con

tinue

d)



TAB

LE 5

. CLI

NO

PY

RO

XE

NE

CO

MP

OS

ITIO

NS

IN T

HE

DE

HS

HIR

MA

FIC

RO

CK

S IN

CLU

DIN

G G

AB

BR

OS

, LE

UC

OG

AB

BR

OS

, PIL

LOW

LA

VA

S, A

ND

DIA

BA

SIC

DIK

ES (

cont

inue

d)

Sam

ple:

G06

-16

G06

-16

DZ

05-4

DZ

05-4

DZ

05-4

DZ

05-4

DZ

05-4

DZ

05-4

DA

R05

-6D

AR

05-6

DA

R05

-6D

AR

05-6

DA

R05

-6D

AR

05-6

DA

R05

-6D

AR

05-6

DA

R05

-6D

AR

05-6

Roc

k ty

pe:

diab

ase

diab

ase

diab

ase

diab

ase

diab

ase

diab

ase

diab

ase

diab

ase

pillo

wpi

llow

pillo

wpi

llow

pillo

wpi

llow

pillo

wpi

llow

pillo

wpi

llow

SiO

251

.99

51.2

651

.75

51.8

151

.17

51.2

852

.28

52.1

451

.46

52.2

051

.85

51.0

351

.92

51.6

750

.71

51.8

551

.97

51.4

2Ti

O2

0.31

0.44

0.48

0.39

0.49

0.60

0.36

0.42

0.73

0.38

0.25

0.41

0.32

0.27

0.32

0.37

0.32

0.32

Al 2O

31.

541.

801.

791.

933.

583.

541.

991.

923.

252.

692.

572.

502.

562.

892.

552.

582.

662.

23F

eO7.

339.

229.

609.

028.

818.

797.

957.

997.

948.

208.

447.

988.

077.

827.

647.

566.

868.

10C

r 2O

30.

060.

060.

040.

130.

090.

050.

100.

120.

000.

320.

310.

220.

280.

280.

370.

290.

380.

12M

nO0.

320.

320.

280.

280.

120.

290.

200.

280.

190.

090.

190.

220.

240.

310.

230.

270.

050.

13N

iO0.

000.

090.

070.

110.

000.

000.

160.

000.

090.

000.

090.

010.

000.

010.

030.

000.

000.

03M

gO15

.38

16.4

315

.99

16.0

815

.93

15.7

516

.85

16.2

513

.86

16.1

516

.56

16.4

416

.24

16.1

816

.51

16.3

816

.33

17.2

7C

aO20

.89

18.1

818

.68

18.3

918

.57

18.7

918

.45

19.0

420

.98

18.6

219

.14

19.1

918

.80

19.0

919

.62

19.5

519

.49

18.1

6N

a 2O

0.20

0.21

0.27

0.18

0.28

0.36

0.29

0.22

0.33

0.44

0.32

0.23

0.34

0.23

0.27

0.17

0.25

0.25

K2O

0.00

0.01

0.01

0.00

0.00

0.00

0.02

0.02

0.01

0.00

0.03

0.01

0.01

0.00

0.00

0.01

0.04

0.00

Tota

l98

.02

98.0

398

.95

98.3

299

.04

99.4

398

.65

98.3

998

.84

99.0

999

.75

98.2

498

.77

98.7

598

.24

99.0

398

.34

98.0

2T

Si

1.96

1.93

1.94

1.95

1.91

1.90

1.95

1.95

1.93

1.94

1.91

1.91

1.93

1.93

1.90

1.93

1.94

1.92

TAl

0.04

0.07

0.07

0.05

0.10

0.10

0.05

0.05

0.07

0.06

0.09

0.09

0.07

0.07

0.10

0.07

0.06

0.08

M1A

l0.

030.

010.

010.

030.

060.

060.

040.

040.

080.

060.

020.

020.

050.

050.

010.

040.

060.

02M

1Ti

0.01

0.01

0.01

0.01

0.01

0.02

0.01

0.01

0.02

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

M1F

e30.

010.

050.

040.

000.

020.

030.

020.

000.

000.

010.

060.

060.

020.

010.

090.

020.

000.

05M

1Fe2

0.09

0.00

0.04

0.04

0.01

0.02

0.00

0.04

0.12

0.02

0.00

0.00

0.02

0.02

0.00

0.02

0.02

0.00

M1C

r0.

000.

000.

000.

000.

000.

000.

000.

000.

000.

010.

010.

010.

010.

010.

010.

010.

010.

00M

1Mg

0.86

0.92

0.89

0.90

0.88

0.87

0.93

0.91

0.78

0.89

0.89

0.91

0.90

0.90

0.89

0.91

0.91

0.91

M1N

i0.

000.

000.

000.

000.

000.

000.

010.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00M

2Mg

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.02

0.01

0.00

0.00

0.04

0.00

0.00

0.05

M2F

e20.

130.

240.

220.

240.

240.

220.

230.

210.

130.

220.

200.

190.

220.

210.

150.

200.

200.

20M

2Mn

0.01

0.01

0.01

0.01

0.00

0.01

0.01

0.01

0.01

0.00

0.01

0.01

0.01

0.01

0.01

0.01

0.00

0.00

M2C

a0.

840.

730.

750.

740.

740.

750.

740.

760.

840.

740.

760.

770.

750.

760.

790.

780.

780.

73M

2Na

0.02

0.02

0.02

0.01

0.02

0.03

0.02

0.02

0.02

0.03

0.02

0.02

0.03

0.02

0.02

0.01

0.02

0.02

M2K

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Sum

_cat

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

WO

43.2

737

.49

38.4

138

.28

38.9

239

.31

38.2

439

.59

45.0

139

.15

39.1

239

.59

39.2

739

.81

40.2

640

.35

40.9

337

.36

EN

44.3

547

.15

45.7

446

.60

46.4

745

.85

48.5

746

.99

41.3

847

.24

47.1

147

.19

47.1

946

.96

47.1

447

.03

47.7

449

.44

FS

12.3

815

.36

15.8

515

.12

14.6

214

.84

13.1

913

.42

13.6

113

.61

13.7

713

.22

13.5

513

.23

12.6

012

.62

11.3

313

.21

Mg#

78.9

176

.05

74.8

076

.05

76.3

276

.15

79.0

778

.38

75.6

877

.81

77.7

678

.58

78.1

978

.67

79.3

979

.42

80.9

379

.17

( con

tinue

d)



TAB

LE 5

. CLI

NO

PY

RO

XE

NE

CO

MP

OS

ITIO

NS

IN T

HE

DE

HS

HIR

MA

FIC

RO

CK

S IN

CLU

DIN

G G

AB

BR

OS

, LE

UC

OG

AB

BR

OS

, PIL

LOW

LA

VA

S, A

ND

DIA

BA

SIC

DIK

ES (

cont

inue

d)

Sam

ple:

DA

R05

-6D

AR

05-6

A06

-2A

06-2

A06

-2A

06-2

A06

-2A

06-2

A06

-2A

06-2

A06

-2A

06-2

A06

-2A

06-2

G06

-23

G06

-23

G06

-23

G06

-23

Roc

k ty

pe:

pillo

wpi

llow

pillo

wpi

llow

pillo

wpi

llow

pillo

wpi

llow

pillo

wpi

llow

pillo

wpi

llow

pillo

wpi

llow

leuc

og.

leuc

og.

leuc

og.

leuc

og.

SiO

251

.68

49.9

351

.28

51.8

951

.69

51.1

651

.45

50.1

250

.70

51.8

551

.85

50.3

650

.59

50.6

051

.98

53.0

152

.80

53.0

9Ti

O2

0.35

0.23

0.23

0.22

0.22

0.18

0.23

0.38

0.29

0.10

0.10

0.22

0.36

0.34

0.25

0.17

0.23

0.25

Al 2O

32.

142.

492.

422.

352.

091.

972.

072.

602.

261.

742.

522.

722.

592.

602.

872.

092.

992.

02F

eO8.

317.

878.

048.

018.

488.

608.

988.

178.

115.

566.

127.

838.

108.

153.

673.

103.

283.

94C

r 2O

30.

240.

290.

220.

240.

170.

210.

200.

280.

280.

380.

720.

250.

270.

320.

470.

300.

460.

23M

nO0.

210.

390.

230.

200.

340.

180.

240.

100.

380.

100.

100.

280.

340.

210.

000.

030.

120.

13N

iO0.

030.

030.

000.

000.

000.

080.

000.

040.

000.

000.

000.

010.

010.

000.

000.

000.

000.

04M

gO17

.00

18.9

017

.02

16.8

417

.33

17.7

317

.51

18.4

018

.79

17.2

317

.21

17.2

616

.78

15.8

916

.25

16.5

316

.30

16.3

1C

aO18

.04

18.7

118

.40

18.0

717

.54

17.6

317

.39

18.7

117

.96

21.0

020

.62

19.2

119

.22

19.6

823

.58

23.3

122

.61

23.2

9N

a 2O

0.27

0.30

0.26

0.27

0.26

0.24

0.21

0.37

0.27

0.32

0.15

0.26

0.24

0.37

0.31

0.29

0.25

0.22

K2O

0.00

0.00

0.00

0.02

0.01

0.04

0.00

0.00

0.00

0.01

0.03

0.00

0.03

0.02

0.01

0.00

0.00

0.00

Tota

l98

.25

99.1

398

.10

98.0

998

.14

98.0

298

.26

99.1

999

.03

98.2

899

.41

98.3

998

.53

98.1

899

.39

98.8

499

.04

99.5

1T

Si

1.93

1.83

1.92

1.94

1.93

1.91

1.92

1.84

1.87

1.92

1.91

1.87

1.89

1.90

1.91

1.95

1.94

1.95

TAl

0.07

0.11

0.08

0.06

0.07

0.09

0.08

0.11

0.10

0.08

0.09

0.12

0.11

0.10

0.09

0.05

0.06

0.05

M1A

l0.

030.

000.

020.

050.

020.

000.

010.

000.

000.

000.

020.

000.

000.

010.

030.

040.

070.

04M

1Ti

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.00

0.00

0.01

0.01

0.01

0.01

0.01

0.01

0.01

M1F

e30.

030.

170.

060.

010.

040.

090.

060.

160.

130.

080.

060.

130.

100.

090.

060.

010.

000.

01M

1Fe2

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.03

0.01

0.05

M1C

r0.

010.

010.

010.

010.

010.

010.

010.

010.

010.

010.

020.

010.

010.

010.

010.

010.

010.

01M

1Mg

0.92

0.82

0.91

0.93

0.92

0.90

0.91

0.83

0.86

0.90

0.90

0.86

0.88

0.88

0.89

0.91

0.89

0.89

M1N

i0.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00M

2Mg

0.03

0.22

0.04

0.01

0.05

0.09

0.06

0.18

0.18

0.05

0.05

0.10

0.06

0.01

0.00

0.00

0.00

0.00

M2F

e20.

230.

010.

190.

240.

220.

180.

220.

050.

090.

090.

130.

110.

150.

170.

050.

060.

090.

06M

2Mn

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.00

0.01

0.00

0.00

0.01

0.01

0.01

0.00

0.00

0.00

0.00

M2C

a0.

720.

740.

740.

720.

700.

710.

700.

740.

710.

840.

810.

770.

770.

790.

930.

920.

890.

92M

2Na

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.03

0.02

0.02

0.01

0.02

0.02

0.03

0.02

0.02

0.02

0.02

M2K

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Sum

_cat

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

WO

37.3

236

.37

37.9

137

.71

36.1

435

.87

35.5

236

.85

35.4

042

.52

41.7

138

.76

39.1

040

.73

48.0

747

.81

47.1

647

.37

EN

48.9

251

.10

48.7

948

.92

49.6

750

.19

49.7

750

.43

51.5

348

.54

48.4

648

.46

47.4

945

.76

46.0

947

.18

47.3

246

.16

FS

13.7

612

.53

13.3

013

.37

14.1

913

.95

14.7

112

.72

13.0

68.

949.

8312

.79

13.4

113

.51

5.84

5.01

5.52

6.46

Mg#

78.4

781

.06

79.0

478

.94

78.4

578

.59

77.6

580

.05

80.5

184

.66

83.3

679

.70

78.6

877

.65

88.7

590

.48

89.8

788

.06

(con

tinue

d)



Geochemical results thus indicate that there are two types of basaltic-andesitic fl ows and dikes: (1) calc-alkaline basalt and andesite that are depleted relative to N-MORB (e.g., Yb = 0.9–1.5 ppm versus N-MORB = 3.05 ppm Yb; Sun and McDonough, 1989), similar to boninite, and (2) tholeiitic basalts with bulk REE con-tents that are similar or slightly enriched rela-tive to N-MORB (e.g., Yb = 2.9–5.5 ppm; Fig. 10G). Most basalt lavas are LREE-depleted (LaN/YbN of 0.4–1). They are enriched in LILE and depleted in high fi eld strength ele-ments (HFSE), so that LILE/HFSE ratios are higher than N-MORB (e.g., Nb/La ~ 0.2–0.7 × N-MORB; Fig. 10H). The exception is sample AZ06–38, which is not so depleted in HFSE (Nb/La ~ 1.1 × N-MORB).

Disruption of rock units during ophio-lite emplacement prohibits reconstruction of ophio litic lava chemostratigraphy in detail, but our fi eld observations in conjunction with results discussed here indicate that basalt lava compositions vary from MORB and/or island-arc tholeiitic (IAT) to both calc-alkaline and boninitic with time (Fig. 3). For example, the base of pillow lavas in Aziz-Abad, Figure 3 right, with island-arc tholeiitic affi nity (sam-ple AZ06–32), is intruded by basaltic-dacitic dike swarms. The fi rst basaltic dikes (sample AZ06–38) are N-MORB, intruded by late

dacitic dikes with a suprasubduction-zone chem-ical signature. There is no fi eld evidence for a break between the MORB-like (lower) and the upper (island-arc tholeiitic/boninite) sequences in the form of an unconformity or intervening sedimentary sequence.

Diabasic Dikes in Mantle Harzburgite

Diabasic dikes intruding harzburgite have a restricted range in silica (48.7%–52.2%). These are fractionated (Mg# = 37.8–64.8) and contain variable TiO

2 (0.3%–1.4%), Zr (10.4–68.2 ppm),

and Ti/V (7.2–21.5). Based on FeOt/MgO, they are more like basaltic lava fl ows than pillow lavas , characterized mostly by low- to medium-Fe contents (Fig. 11), perhaps indicating an H

2O-

rich magma (Arculus, 2003). The dikes fall into island-arc tholeiitic and MORB fi elds of Shervais (1982), while sample AZ06–29 shows boninitic affi nity (Fig. 12) with low abundances of Zr, TiO

2, and Ti/V ratio and higher Mg#. These dikes

have REE contents close to N-MORB (e.g., Yb = 2.2–2.9 ppm). Sample AZ06–29 is distinguished by its lower REE content (e.g., Yb = 0.8 ppm), similar to boninite and to basaltic lavas D06–10 and DZ05–1D. Low REE contents associated with high abundances of compatible elements and Mg# of these samples refl ect melt issued from more refractory mantle source, perhaps as a

result of two-stage melting (e.g., Ernewein et al., 1988; Dilek et al., 2007).

Similar to other Dehshir ophiolite mafi c magmatic rocks, diabasic dikes in harzburgite are characterized by two types of REE pat-terns: (1) some samples (DAR05–4, G06–16, AZ06–29, and DZ05–4) are depleted in LREE relative to HREE (Fig. 10I), with LaN/YbN rang-ing from 0.3 to 0.9 (LaN/YbN = 0.59 for aver-age N-MORB; Sun and McDonough, 1989). Relative to N-MORB, these dikes are enriched in U, Pb, K, Ba, and Sr (e.g., K/La ~ 2–27 × N-MORB) and depleted in Nb and Ta (e.g., Nb/La ~ 0.3–0.5 × N-MORB; Fig. 10J), with high LILE/HFSE ratios. The REE profi le of these dikes is similar to those of island-arc tholeiite. If not refl ecting alteration, their high LILE/HFSE values are most simply interpreted as an island-arc signature, where the LILE-enriched aqueous fl uid derived from the subducted slab has modi-fi ed the mantle wedge, which then has melted to generate basaltic melts with these signatures (e.g., Tatsumi et al., 1986; Stern, 2002). (2) Dia-basic dikes (DAR05–3 and A06–1) have slight LREE enrichment (LaN/YbN = 1.4–1.6) and also are relatively depleted in Nb and Ta (e.g., Nb/La ~ 0.3 × N-MORB), typical of calc-alkaline series. Sample A06–1 is compositionally similar to overlying pillow lavas (which are in faulted contact with peridotite; Fig. 3). All dia basic dikes have high Th/Yb (Fig. 13) distinct from MORB, confi rming their arc-related affi nity.

Metamorphic Rocks

Amphibolite (sample DZ05–9) shows a depleted composition, with SiO

2 (42.5%),

MgO (12.9%), TiO2 (0.32%), Zr (13 ppm),

and Mg# (71.3) abundances similar to de-pleted basalts and gabbros. The amphibolite falls into island-arc tholeiitic–boninite fi eld on the Ti-V diagram (Fig. 12). The rock displays LREE-depleted REE patterns (LaN/YbN = 0.6) (Fig. 10K). Relative to N-MORB, the rock shows negative Nb and Ta anomalies (e.g., Nb/La ~ 0.4 × N-MORB) and positive LILE spikes (U, Pb, Th, Ba; e.g., U/La ~ 8 × N-MORB) (Fig. 10L). Geochemically, amphibolite is simi-lar to depleted basaltic rocks (DZ05–1D or D06–10), and, therefore, its protolith is thought to have been depleted mafi c rocks.

Felsic Rocks

Dacites in the sheeted dike complex have high SiO

2 (63.8%–71.3%), and low K

2O (0%–

0.09%), TiO2 (0.32%–0.41%), Zr (45–73 ppm),

and Mg# (5.7–42.3), typical of suprasubduction zone–related felsites. Na

2O abundances are vari-

able (0.2%–6.9%), probably due to alteration.

TABLE 5. CLINOPYROXENE COMPOSITIONS IN THE DEHSHIR MAFIC ROCKS INCLUDING GABBROS, LEUCOGABBROS, PILLOW LAVAS, AND DIABASIC DIKES (continued)

Sample: G06-23 G06-23 G06-17 G06-17 G06-17 G06-17 G06-17 G06-17Rock type: leucog. leucog. leucog. leucog. leucog. leucog. leucog. leucog.SiO2 52.08 52.68 53.23 51.73 51.84 52.14 51.86 52.71TiO2 0.33 0.22 0.24 0.27 0.21 0.14 0.33 0.28Al2O3 2.73 2.63 2.63 3.64 3.21 3.27 3.29 2.66FeO 3.32 3.01 3.18 3.44 3.09 3.43 3.18 3.57Cr2O3 0.47 0.38 0.43 0.40 0.38 0.30 0.39 0.38MnO 0.03 0.12 0.03 0.12 0.13 0.17 0.12 0.09NiO 0.00 0.08 0.11 0.01 0.11 0.01 0.01 0.01MgO 16.17 15.92 16.44 16.22 16.33 16.19 16.13 16.54CaO 23.38 24.22 23.00 22.26 22.70 22.77 23.24 22.84Na2O 0.30 0.29 0.23 0.29 0.22 0.33 0.30 0.30K2O 0.00 0.03 0.00 0.01 0.02 0.00 0.00 0.01Total 98.80 99.57 99.51 98.40 98.24 98.75 98.87 99.39TSi 1.92 1.93 1.95 1.91 1.92 1.92 1.91 1.93TAl 0.08 0.07 0.05 0.09 0.08 0.08 0.09 0.07M1Al 0.04 0.04 0.06 0.07 0.06 0.06 0.05 0.05M1Ti 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.01M1Fe3 0.03 0.03 0.00 0.01 0.01 0.02 0.03 0.02M1Fe2 0.02 0.04 0.02 0.01 0.01 0.01 0.01 0.01M1Cr 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01M1Mg 0.89 0.87 0.90 0.90 0.90 0.89 0.89 0.90M1Ni 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00M2Mg 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00M2Fe2 0.05 0.02 0.08 0.09 0.08 0.07 0.06 0.08M2Mn 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00M2Ca 0.92 0.95 0.90 0.88 0.90 0.90 0.92 0.90M2Na 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02M2K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sum_cat 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00WO 48.22 49.62 47.54 46.75 47.36 47.33 48.17 46.89EN 46.40 45.37 47.28 47.41 47.39 46.82 46.50 47.25FS 5.39 5.01 5.17 5.84 5.25 5.85 5.34 5.87Mg# 89.67 90.39 90.21 89.36 90.40 89.37 90.02 89.20



TAB

LE 6

. RE

PR

ES

EN

TAT

IVE

CO

MP

OS

ITIO

N O

F P

LAG

IOC

LAS

E IN

DE

HS

HIR

AM

PH

IBO

LIT

ES

AN

D P

LAG

IOC

LAS

E L

HE

RZ

OLI

TE

S

Roc

k ty

pe:

Am

phib

Am

phib

Am

phib

Am

phib

Am

phib

Am

phib

Am

phib

Am

phib

Am

phib

Am

phib

Am

phib

Am

phib

Am

phib

Am

phib

Am

phib

Am

phib

plg

lher

zpl

g lh

erz

plg

lher

zpl

g lh

erz

Sam

ple:

DZ

05-9

DZ

05-9

DZ

05-9

DZ

05-9

DZ

05-9

DZ

05-9

DZ

05-9

DZ

05-9

DZ

05-9

DZ

05-9

DZ

05-9

DZ

05-9

DZ

05-9

DZ

05-9

DZ

05-9

DZ

05-9

G06

-20

G06

-20

G06

-20

G06

-20

SiO

269

.11

65.3

857

.11

69.4

468

.44

59.2

861

.25

66.5

865

.31

58.3

869

.00

69.3

363

.78

63.8

869

.07

69.1

541

.02

46.4

546

.44

46.0

5Ti

O2

0.00

0.02

0.00

0.05

0.05

0.08

0.00

0.09

0.07

0.06

0.03

0.00

0.05

0.00

0.00

0.03

0.04

0.01

0.01

0.04

Al 2O

319

.55

19.5

926

.49

19.6

819

.58

25.9

024

.39

20.0

920

.54

25.1

219

.64

19.8

318

.38

18.8

819

.81

19.4

431

.75

34.2

934

.49

34.9

4F

eO0.

020.

000.

140.

170.

000.

050.

080.

240.

140.

030.

000.

060.

190.

190.

120.

001.

680.

270.

280.

28M

nO0.

000.

000.

000.

000.

040.

140.

000.

020.

000.

000.

000.

040.

070.

000.

000.

000.

100.

060.

030.

02M

gO0.

000.

020.

000.

000.

000.

010.

020.

010.

050.

000.

010.

000.

010.

000.

010.

000.

130.

010.

000.

00C

aO0.

100.

336.

580.

020.

057.

446.

340.

220.

256.

210.

050.

100.

000.

030.

120.

0523

.49

17.8

918

.02

18.0

7N

a 2O

11.4

412

.61

8.51

12.0

711

.70

7.44

8.25

11.9

611

.69

8.14

11.2

311

.54

0.11

0.17

11.5

711

.68

0.10

1.36

1.38

1.48

K2O

0.04

0.34

0.11

0.02

0.07

0.06

0.15

0.08

0.13

0.11

0.04

0.11

15.6

615

.86

0.05

0.06

0.01

0.01

0.01

0.00

Cl

0.06

0.81

0.02

0.02

0.08

0.00

0.00

0.00

0.17

0.02

0.02

0.02

0.02

0.03

0.00

0.00

0.02

0.00

0.00

0.01

Cr 2

O3

0.05

0.00

0.05

0.04

0.00

0.13

0.07

0.00

0.00

0.00

0.06

0.00

0.00

0.00

0.00

0.04

0.01

0.00

0.00

0.00

Tota

l10

0.38

99.1

099

.01

101.

5110

0.00

100.

5210

0.55

99.2

798

.35

98.0

710

0.07

101.

0298

.25

99.0

410

0.75

100.

4498

.32

100.

3510

0.66

100.

89S

i6.

005.

915.

176.

005.

985.

285.

455.

905.

845.

315.

995.

995.

975.

945.

986.

014.

194.

284.

274.

23A

l2.

002.

092.

832.

002.

022.

722.

552.

102.

162.

692.

012.

022.

032.

072.

021.

993.

823.

723.

733.

78Ti

0.00

0.00

0.00

0.00

0.00

0.01

0.00

0.01

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Fe2

0.00

0.00

0.01

0.01

0.00

0.00

0.01

0.02

0.01

0.00

0.00

0.00

0.02

0.02

0.01

0.00

0.14

0.02

0.02

0.02

Mn

0.00

0.00

0.00

0.00

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.00

0.00

0.00

0.01

0.00

0.00

0.00

Mg

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.02

0.00

0.00

0.00

Ca

0.01

0.03

0.64

0.00

0.00

0.71

0.60

0.02

0.02

0.61

0.01

0.01

0.00

0.00

0.01

0.00

2.57

1.77

1.77

1.78

Na

1.93

2.21

1.50

2.02

1.98

1.29

1.42

2.06

2.03

1.44

1.89

1.93

0.02

0.03

1.94

1.97

0.02

0.24

0.25

0.26

K0.

010.

040.

010.

000.

010.

010.

020.

010.

020.

010.

010.

011.

871.

880.

010.

010.

000.

000.

000.

00C

atio

ns9.

9410

.29

10.1

610

.04

10.0

010

.02

10.0

510

.11

10.0

910

.06

9.91

9.96

9.92

9.93

9.97

9.98

10.7

610

.04

10.0

510

.07

X8.

008.

008.

008.

008.

008.

018.

008.

018.

018.

008.

008.

008.

008.

008.

008.

008.

008.

008.

008.

00Z

1.94

2.29

2.16

2.04

2.00

2.02

2.05

2.11

2.08

2.06

1.90

1.96

1.91

1.93

1.97

1.98

2.76

2.04

2.04

2.06

Ab

99.2

096

.90

69.6

099

.80

99.4

064

.20

69.6

098

.60

98.1

069

.90

99.5

098

.90

1.10

1.60

99.1

099

.40

0.80

12.1

012

.20

12.9

0A

n0.

501.

4029

.80

0.10

0.20

35.5

029

.50

1.00

1.20

29.5

00.

300.

500.

000.

200.

600.

2099

.20

87.8

087

.80

87.1

0O

r0.

301.

700.

600.

100.

400.

300.

800.

400.

700.

600.

300.

6098

.90

98.3

00.

300.

400.

000.

100.

000.

00



They are calc-alkaline on the FeOt/MgO versus SiO

2 diagram (Fig. 11). Relative to N-MORB,

the rocks are depleted in Nb, Ta, P, and Ti, whereas U, Th, and Pb show positive anoma-lies (Fig. 10L). These rocks show trace-element similarities to low-K tholeiitic or calc-alkaline rock series. Dacitic dikes are distinguished by very low Sr/Y and LaN/YbN (3.9–10 and 1.2–

1.3, respectively), and fl at REE patterns (Fig. 10K), typical of low-pressure fractionates of basalt or melts of amphibolite.

Plagiogranites have uniformly high SiO2

(>76%) and Na2O (>3.8%) and low TiO

2

(<0.22%), K2O (<0.41%), and Zr (<78 ppm)

contents. They are depleted in total REE and display concave-up REE patterns (LaN/YbN =

2.1–4.1; Fig. 10K). In spite of the more evolved nature of these rocks, HFSEs are depleted rela-tive to N-MORB. Rb, Ba, Th, U, Pb, K, and Sr display positive anomalies (Fig. 10L). The pres-ence of a positive Eu anomaly (Eu/Eu* ~ 1–2) may refl ect feldspar accumulation. The plagio-granites and dacite dikes are compositionally distinct, with completely different REE patterns ,

Si (Tetrahedral site)

Mg

/(M

g+

Fe+

2 )

TschermakiteTschHblMagnesio-

hornblende

ActHbl

HblTrTremolite

Actinolite

Ferro-actinolite

Ferro-hornblende

FeTschHbl

TschermakiteFerro-Act

Hbl

Fe

8.0 7.5 7.0 6.5 6.0 5.5 0

1

A Al (IV)

Ti (c

.f.u

.)

0

0.1

0.2

0.3

0.4

0 0.4 0.8 1.2 1.6 2.0

Magmatic

B

Figure 7. Compositional varia-tions of amphibole from Dehshir ophiolite amphibolites. (A) Mg/Mg+Fe+2 against Si (in tetra-hedral site) diagram (Leake et al., 1997) showing actino-lite, actinolitic hornblende, and magnesio-hornblende compo-sitional affi nity for amphibole grains in amphibolites. (B) Ti versus Aliv diagram for green am-phiboles of amphibolites (c.f.u.—cation per formula unit). Field outlines magmatic amphibole compositions of oceanic gabbros, with main compositions shaded in gray (Coogan et al., 2001).

TABLE 7. REPRESENTATIVE COMPOSITION OF AMPHIBOLE IN DEHSHIR AMPHIBOLITES

Rock type: amph. amph. amph. amph. amph. amph. amph. amph. amph. amph. amph.Sample: DZ05-9 DZ05-9 DZ05-9 DZ05-9 DZ05-9 DZ05-9 DZ05-9 DZ05-9 DZ05-9 DZ05-9 DZ05-9SiO2 52.13 49.26 48.50 49.27 50.93 50.60 50.66 44.91 54.75 55.10 49.65TiO2 0.13 0.59 0.88 0.76 0.17 0.17 0.15 0.30 0.09 0.15 0.31Al2O3 4.66 6.08 6.99 5.89 2.36 3.10 3.82 11.17 2.99 2.66 9.47FeO 13.40 17.37 17.01 16.27 12.20 12.90 14.18 14.82 10.91 10.22 9.35Cr2O3 0.01 0.04 0.00 0.09 0.03 0.29 0.16 0.03 0.03 0.02 0.28MnO 0.29 0.28 0.27 0.31 0.18 0.22 0.20 0.20 0.41 0.31 0.26MgO 14.95 12.20 11.87 13.13 17.05 15.51 14.66 11.31 16.80 16.99 16.87CaO 11.64 10.03 10.67 11.16 11.90 11.73 11.59 12.05 12.05 12.33 9.00Na2O 0.54 0.75 0.69 0.56 0.49 0.42 0.68 1.01 0.34 0.21 1.52K2O 0.16 0.13 0.13 0.22 0.14 0.08 0.15 0.42 0.05 0.07 0.23

00.040.030.030.050.050.041.090.001.050.030.0lCTotal 97.94 96.73 97.10 97.66 95.55 94.78 96.13 96.22 98.41 98.07 96.66

61.728.767.796.674.705.764.732.712.753.725.7iST48.081.042.013.135.015.014.077.008.066.084.0lAT

TFe3 0.00 0.00 0.00 0.00 0.13 0.00 0.00 0.00 0.00 0.00 0.0000.000.000.000.000.000.000.000.000.000.000.0iTT

Sum_T 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.0067.062.062.056.041.040.000.052.034.014.023.0lAC

CCr 0.00 0.01 0.00 0.01 0.00 0.03 0.02 0.00 0.00 0.00 0.03CFe3 0.00 0.00 0.00 0.15 0.33 0.26 0.12 0.21 0.00 0.00 0.00

30.020.010.030.020.020.020.080.001.070.010.0iTCCMg 3.22 2.71 2.63 2.87 3.72 3.43 3.22 2.51 3.55 3.59 3.62CFe2 1.45 1.81 1.85 1.64 0.92 1.22 1.49 1.59 1.17 1.13 0.55CMn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00CCa 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sum_C 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00BMg 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00BFe2 0.16 0.36 0.27 0.21 0.11 0.11 0.14 0.05 0.12 0.09 0.58BMn 0.04 0.04 0.03 0.04 0.02 0.03 0.03 0.03 0.05 0.04 0.03BCa 1.80 1.60 1.70 1.75 1.87 1.86 1.83 1.93 1.83 1.88 1.39BNa 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sum_B 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00ACa 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00ANa 0.15 0.22 0.20 0.16 0.14 0.12 0.19 0.29 0.10 0.06 0.43

40.010.010.080.030.010.030.040.030.020.030.0KASum_A 0.18 0.24 0.23 0.20 0.16 0.14 0.22 0.37 0.10 0.07 0.47Sum_cat 15.18 15.24 15.23 15.20 15.16 15.14 15.22 15.37 15.10 15.07 15.47

10.0lCC 0.01 0.02 0.02 0.04 0.01 0.01 0.01 0.01 0.01 0.00



suggesting no relationship. Plagiogranite also has variable Sr/Y and LaN/YbN ratios (10–60 and 2.1–4.1, respectively), resembling low-pressure basaltic fractionates or anatectic melts of amphibolite, although one sample (AZ06–25; plagiogranitic veins in amphibole gabbros) is adakite-like. The geochemical signatures of these rocks, such as low contents of Zr, Ti, and bulk REE, are also similar to the dacite dikes.

We use a Th/Yb versus Nb/Yb systematic dia-gram (Fig. 13) to explore the petrogenesis and tectonic setting of Dehshir igneous rocks. In this diagram, Yb is used as a normalizing factor for Nb and Th to remove the effects of partial melting, fractional crystallization, and crystal accumulation (Pearce and Peate, 1995; Pearce et al., 1995). Th and Nb have similar melt-crystal distribution coeffi cients (Leat et al., 2004), but Th is added in subduction zones, whereas Nb is not. Most Dehshir mafi c rocks (except for fi ve samples) are displaced from the MORB–ocean-island basalt (OIB) array due to higher concentrations of the subduction-mobile element Th. These relations reinforce the con-clusion that most Dehshir melts were derived from subduction-modifi ed MORB-like astheno-spheric mantle.

COMPARISON WITH OTHER ZAGROS AND OMAN OPHIOLITES AND FOREARCS

This section compares the geochemical fea-tures of Dehshir peridotites and lavas with those of other Zagros inner and outer belt ophiolites. This type of comparison better allows us to

evaluate whether or not the two ophiolite belts represent inner and outer portions of a single disrupted ophiolite sheet, and thus test the Iranian forearc hypothesis. We also compare the Iranian ophiolites to the well-studied Oman ophiolite. The Oman ophiolite is the on-strike

continuation of the Iranian ophiolites, is the same age, and could be part of the same Late Cretaceous forearc as that represented by Zagros ophio lites. We also compare our data with present-day forearcs and ancient equivalents such as the Betts Cove (e.g., Bédard, 1999) ophiolite.

60 70 80 90 100100Mg/(Mg+Fe)

0

0.2

0.4

0.6

0.8

TiO

2 (w

t%)

Leucogabbro

Pillow lava

Diabasic dike

Isotropic gabbro

0 0.5 1 1.5 2 2.5 3

TiO2+Cr2O3 (wt%)

Al 2

O3

(wt%

)

0

2

4

6

8

N-MORB

Back-arc basin basalt

Arc tholeiite

Boninite

A B

Figure 8. Compositional variations of clinopyroxene from different mafi c rock units of the Dehshir ophiolite. (A) TiO2 versus Mg#; (B) Al2O3 versus TiO2 + Cr2O3 diagram, adapted after Huot et al. (2002). Fields outline clinopyroxene compositions in boninites (Van der Laan et al., 1992), island-arc tholeiites, and back-arc basin basalts (Hawkins and Allan, 1994), and normal mid-ocean-ridge basalt (N-MORB) (Stakes and Franklin, 1994).

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

0.01

0.1

1.0

10.0

Sam

ple/

Chon

drite

Clinopyroxenite (DZ05-8)

Plagioclase lherzolite (G06-20)

Harzburgite(AZ06-40)

(G06-13)

Cpx-harzburgite(AZ06-26)

0.001

Av. Abyssal peridotite

Leg 125 IBM Forearc harzburgite TMO harz.

Figure 9. Chondrite-normalized rare earth element (REE) patterns of harzburgite, clino-pyroxene (Cpx)-bearing harzburgite, plagioclase lherzolite, and clinopyroxenite from the Dehshir ophiolite (chondrite concentrations are from McDonough and Sun, 1995). Fields for Izu-Bonin-Mariana forearc harzburgites (Pearce et al., 1992) and Thetford Mines ophio-lite (TMO; Pagé et al., 2009) harzburgites are shown for comparison.



Inner Zagros Ophiolite Belt

Recent studies of other inner belt ophio-lites such as Nain, Shahr-e-Babak, and Baft complexes (Shafaii Moghadam, 2009) sug-gest that these harzburgites, like the Dehshir harz burgite, are very depleted, characterized by low CaO and Al

2O

3, and show geochemi-

cal similarities to both forearc harzburgites and abyssal peridotites. The higher content of CaO and Al

2O

3 in some clinopyroxene-bearing harz-

burgites may be due to the percolation of and reaction with basaltic melt, as is seen for some abyssal peridotites (e.g., Dijkstra et al., 2006; Pagé et al., 2008). Because spinel composition is widely accepted for characterizing the geo-

tectonic setting and petrogenesis of peridotites (e.g., Dick and Bullen, 1984; Rahgo shay, 1986; Rollinson, 2005), we compared peridotite spinel compositions (Cr# = [100Cr/Cr + Al] versus Mg#) from both inner and typical outer (Neyriz) ophiolite belts (Fig. 6). Nearly all spinels from inner ophiolite belt perido-tites are similar to those of harzburgite from

0.01

0.1

1.0

10.0

Leucogabbro

G06-6

G06-17

G06-18

G06-23G06-11

0.0001

0.001

0.01

0.1

1

10

100

Leucogabbro

0.1

1.0

10.0

100.0

+++++++++++++

++

Gabbro

pegmatite gabbroDR05-1

+

DZ05-1H (IAT)

DI05-12 (IAT)D06-3 (IAT)R-12 (IAT)DR05-3 (T-MORB)

+ + + + +++

+

+

+

++

+

++

++

++

Gabbro

0.01

0.1

1.0

10.0

100.0

1000.0

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

1

10

100Pillow lava

A06-2A06-3Calc-alkaline IAT AZ06-32

DAR05-6

Rb Th Nb K Ce Pr P Zr Ti YbBa U Ta La Pb Sr Nd Hf Y

0.1

1

10

100Pillow lava

AB

C D

E F

Sam

ple

/Ch

on

dri

teSa

mp

le/C

ho

nd

rite

Sam

ple

/MO

RB

Figure 10 (on this and following page). Chondrite-normalized rare earth element (REE) patterns (chondrite abundances are from McDonough and Sun, 1995) and normal mid-ocean-ridge basalt (N-MORB)–normalized multi-elements patterns (N-MORB concentrations are from Sun and McDonough, 1989) for rock units of the Dehshir ophiolite.



intraoceanic forearcs, such as Izu-Bonin-Mariana and South Sandwich forearcs (Fig. 6), although some Dehshir harzburgite spinels plot with abyssal (MORB) peridotite.

It has been demonstrated that Al2O

3 content

in orthopyroxene and clinopyroxene of residual harzburgite is sensitive to the extent of partial melting and thus tectonic environment (e.g.,

Dick and Bullen, 1984; Michael and Bonatti, 1985), because Al behaves as a moderately in-compatible element during mantle melting and is progressively depleted during partial melting (Niu and Hékinian, 1997). Orthopyroxene and clinopyroxene porphyroclasts of other inner ophiolite belt harzburgites are also depleted in Al

2O

3, resembling those of harzburgites from

Izu-Bonin-Mariana and South Sandwich fore-arcs (Fig. 5A).

Dehshir lavas have geochemical signatures similar to those of other inner ophiolite belt lavas . Nearly all of these fall into island-arc tholeiitic and boninite fi elds on a Ti-V diagram, similar to both depleted Lasail lavas (V

2 lavas)

of Oman and as Izu-Bonin-Mariana forearc

Figure 10 (continued).

0.01

0.1

1

10

100Basaltic-andesitic rocks

1

10

100Basaltic-andesitic rocks

DR05-2A lava

AZ06-38 basaltic dikesheeted dike complex

DAR05-5 lava

DZ05-1D andesitic dikein amphibole gabbro

D06-10 lava

0.1

1

10

100

Sam

ple

/N-M

OR

B

Diabasic dikes

1

10

100

Sam

ple

/Ch

on

dri

te

Diabasic dikesA06-1

DAR05-3Calc-alkaline

G06-16DARo5-4

DZ05-4IAT

AZ06-29Boninite


1

10

100

AZ06-8AZ06-11

AZ06-31AZ06-36

Dacitic dikesSheeted dike cmplx

0.01

0.1

1

10

100


Amphibolite (DZ05-9)

plagiogranite (DZ05-6)plagiogranite (AZ06-25)

G H

I J

K L



boninites , with some exceptions showing af-fi nity to MORB (Figs. 14A and 14C). Dehshir lavas also are similar to other inner belt ophi-olitic lavas in terms of nearly fl at REE patterns and trace-elements patterns with similar spikes in LILEs, Pb, and Sr and depletions in Nb and Ta (Figs. 15A and 15B). The lavas with low REE abundances are also distinguished by very low HFSE contents relative to N-MORB, sug-gesting similarity to boninitic melts produced by melting of highly depleted mantle. Barring unusually systematic alteration, the similarity in positive spikes for both Dehshir lavas and other inner belt ophiolites shows participation of slab-derived fl uids in the mantle wedge (Fig. 15B).

Similar to Oman ophiolitic lavas, Dehshir lavas show two trends: (1) some have REE pat-terns similar to Geotimes (V1; lower sequence) lavas, consistent with tholeiitic affi nities (Fig. 15E), and (2) others have depleted REE pat-terns similar to Lasail (V2; upper sequence units) lavas, showing a more arc-like source (Fig. 15E). However, Dehshir depleted lavas are more depleted in HREE and HFSE compared to Lasail lavas, suggesting a highly refractory source, such as that responsible for Izu-Bonin-Mariana and Betts Cove boninites (Fig. 15G). A highly depleted residue was also suggested for generation of Izu-Bonin-Mariana and Betts Cove fore-arc boninites (e.g., Pearce et al., 1992; Bédard, 1999).

Outer Zagros Ophiolite Belt

Among outer belt ophiolites, only the Neyriz ophiolite has been studied in suffi cient detail to be useful here. Whole-rock compositions of

40 45 50 55 60 65 70 75

SiO2 (wt%)

0

1

2

3

4

5

FeO

t /M

gO

high-Fe

calc-alkalic

tholeiitic

low-Fe

medium-Fe

Diabase dike

Pillow lava

Lava flow

Dacitic dike

Figure 11. FeOt/MgO versus SiO2 projection (volatile free, normalized to 100% total) for the Dehshir mafi c rocks and dacitic dikes. Boundaries between tholeiitic and calc-alkaline suites (thin solid line) and between low-, medium-, and high-Fe suites (thick, dashed lines) are from Miyashiro (1974) and Arculus (2003), respectively.

Oman V1

Oman V2

0 5 10 15 20 25

Ti (ppm)/1000

0

100

200

300

400

500

600

)m

pp(

V

Arc tholeiites MORB & BABB

etini

no

B

Diabase dike

Pillow lava

Lava flow

Gabbro

Amphibolite

Figure 12. Ti versus V diagram (after Shervais, 1982) for Dehshir mafi c rocks and amphibolite. Most Dehshir mafi c rocks occupy island-arc tholeiitic (IAT) and boninite (Ti/V < 10) fi elds, although some mafi c rocks fall into MORB/BABB fi eld. For comparison, fi elds for the V1 (lower, Geotimes unit) and V2 (upper, Lasail unit) lavas of the Oman ophiolite (data from Alabaster et al., 1982) are shown.

0.1 1 10 100 Nb/Yb

0.01

0.1

1.0

bY/hT

10.0

volcanic arc array yarraBIO-BROM

N-MORB

E-MORB

OIB

Diabase dike

Pillow lava

Lava flow

Gabbro

noitc

ud

buS

tne

no

pm

oc

Figure 13. Th/Yb versus Nb/Yb diagram (after Leat et al., 2004) for the Dehshir ophio lite. Most mafic rocks from the Dehshir ophiolite have high Th/Yb and plot above the mantle array near the volcanic arc array, with some exceptions that are mid-ocean-ridge basalt (MORB)–like.



Neyriz ophiolite peridotites are highly depleted in both CaO (0.25%–0.45%) and Al

2O

3 (0.18%–

0.43%) (Jannessary, 2003). Orthopyroxenes in both porphyroclastic and granoblastic harz-burgites are also depleted in Al

2O

3. Neyriz

harzburgite spinel is characterized by high Cr#, like those from Mariana trench and other forearc peridotites (Fig. 6). All of these characteristics are similar to those of modern (e.g., Izu-Bonin-Mariana ; Pearce et al., 1992) or ancient (e.g., Thetford Mines ophiolite; Pagé et al., 2009) forearc harzburgite. Like inner belt ophiolites,

Neyriz ophiolite basalts, diabasic dikes, andesites , dacites, and rhyolites show spikes in LILEs and depletions in HFSEs and HREEs (Fig. 15D). Neyriz ophiolite lavas generally have higher Ti/V than Dehshir lavas, falling mostly in the MORB-BABB (Back-Arc Basin Basalt) fi eld (Fig. 14B). Many Dehshir lavas are more depleted in HREE and HFSE relative to Neyriz lavas (Fig. 15D). Compositionally, the Neyriz lavas are interme-diate between tholeiitic lavas of the Oman Geo-times unit and depleted lavas of the Lasail unit (Figs. 14B, 15E, and 15F).

TECTONIC SETTING OF DEHSHIR AND OTHER SW IRANIAN OPHIOLITES

Considered together, our data for the Dehshir ophiolite, and the similarity of these results to those for Iranian inner and outer belt ophio-lites along with the better studied Troodos and Oman ophiolites, compel the conclusion that a long, broad, and continuous tract of oceanic lithosphere was created at about the same time during the Late Cretaceous along the southern

B

0 5 10 15 20 250

100

200

300

400

500

600

etininoB

Nain lavas

Dehshir lavas

Shahr-e-Babak lavas

Baft lavas

Oman V1

Oman V2

0 5 10 15 20 250

100

200

300

400

500

600 Neyriz lavas with 47%<SiO2<78%

Dehshir mafic rocks

A Arc tholeiites

)mpp(

V

MORB & BABB

0 5 10 15 20 250

100

200

300

400

500

600 IBM forearc boninite

Dehshir mafic rocks

C

Ti (ppm)/1000

)mpp(

V

Betts Cove boninite

Figure 14. (A) Comparison of Ti/V for Dehshir ophiolite lavas with other inner belt ophiolitic lavas (data from Shafaii Moghadam, 2009), (B) with lavas from a typical outer belt ophiolite, the Neyriz ophiolite (data from Jannessary, 2003), and (C) with boninites from Izu-Bonin-Mariana forearc (Pearce et al., 1992) and Betts Cove ophiolites (Bédard, 1999). Nearly all lavas from the inner ophiolitic belt plot in island-arc tholeiitic (IAT) and boninite fi elds, while the Neyriz lavas are distinguished by higher, mid-ocean-ridge basalt (MORB)–like Ti/V.



Neyriz lavas

1

10

100

etirdnohC/elpmaS

Comparison of Dehshir lavas with lavas from the outer Zagros ophiolitic belt

1

10

100 Comparison of Dehshir lavas with lavas from the inner Zagros ophiolitic belt

Nain pillow lavasShahr-e-Babak pillow lavas

Baft basaltic rocks

0.01

0.1

1

10

100

BRO

M-N/elpmaS

0.01

0.1

1

10

100

A B

C D

0.01

0.1

1

10

100

E FGeotimes lavas (lower)

Lasail lavas (upper)

1

10

100 Comparison of Dehshir lavas with Oman ophiolite lavas

0.01

0.1

1

10

100


G H


1

10

100 Comparison of Dehshir lavas with IBM and Betts Cove

forearc/ophiolite boninitesIBM forearc boninites

Betts Cove boninites

Av. BC boninites

Figure 15. Chondrite-normalized rare earth element (REE; left) and normal (N) mid-ocean-ridge basalt–normalized (MORB; right) trace-elements patterns of Dehshir ophiolitic lavas and diabasic dikes injected into peridotite, compared with lavas from other inner ophiolite belt (A, B), outer ophiolite belt (C, D), Geotimes and Lasail units of Oman ophiolite (E, F) and Izu-Bonin-Mariana (IBM) forearc and Betts Cove ophiolite boninites (G, H; see text for more detail). Data are after Jannessary (2003) for the Neyriz ophiolite, after Alabaster et al. (1982) and Godard et al. (2003) for Oman lavas, and from Pearce et al. (1992) and Pagé et al. (2009) for Izu-Bonin-Mariana forearc and Betts Cove (BC) ophiolite boninites, respectively.



margin of Eurasia. This tract extended beyond Iran through Syria and Turkey as far as Cyprus to the west and Oman and perhaps Pakistan to the east. The similarity of ages for igneous rocks and overlying sediments, both along (from Cy-prus to Oman) and across (inner to outer belt) the ophiolite belt indicates that a successful model should explain all of these occurrences.

All components of the Dehshir and other ophio-lites of this belt show strong suprasubduction-zone affi nities, from harzburgitic mantle to lavas . Most volcanic sections have mixtures of arc-like and MORB-like compositions, usually MORB-like at the base and arc-like at the top. Mid-ocean-ridge–like basalts do not require a mid-ocean-ridge tectonic environment be-cause such lavas are not diagnostic of tectonic environment, only that asthenospheric mantle melted to ~10%–20% due to decompression. A two-phase tectonic model would be required if there was evidence for a signifi cant hiatus between MOR-like lower basalts and arc-like upper basalts, but this is nowhere reported. In-stead, the volcanic succession appears to refl ect progressive depletion of originally fertile mantle due to increased water fl ux when the founder-ing lithosphere reached a certain depth and true subduction started. This interpretation is consis-tent with results of recent Shinkai 6500 diving on the inner trench wall of the Mariana Trench, which found MORB-like basalts, interpreted as fi rst-stage melts of asthenospheric mantle when subduction started there in Eocene time (Ohara et al., 2008). This inference precludes models calling for the Dehshir and other SW Iranian ophiolites to have formed at a normal mid-ocean ridge, a small Red Sea–like basin, or anywhere except at a strongly extensional, incipient con-vergent margin. This is consistent with the ob-servation that Zagros ophiolites formed when relative plate motions in the region changed dra-matically (Garfunkel, 2006).

The important remaining questions become: (1) where exactly in an intraoceanic arc system did these ophiolites form, arc, back-arc basin, or forearc? (2) Did these ophiolites largely form in place adjacent to the southern margin of Eurasia (autochthonous or para-autochthonous) or are they far-traveled, exotic fragments of oceanic lithosphere (allochthonous)? Given that supra-subduction-zone ophiolites form by seafl oor spreading at a convergent margin, this could be either a back-arc basin or a forearc during the earliest stages of subduction (Pearce, 2003). Lava compositions do not distinguish between these options, although boninites are rare in back-arc basins (Bédard et al., 1998). Peridotite compositions do serve to distinguish between forearc and back-arc settings. Dehshir and other SW Iranian ophiolitic peridotites include ultra-

depleted harzburgites that today are only found in modern intraoceanic forearcs. At some point in their evolution, the shallow upper mantle beneath the forearcs experienced much more extensive melting—to the point of clinopyrox-ene exhaustion—than is found anywhere else in present-day Earth (Bonatti and Michael, 1989; Neumann and Simon, 2009). Because modern forearcs are cold and incapable of generat-ing melts, this catastrophic melting must have occurred under very special conditions, most likely when subduction began. A much larger volume of magma is produced when a subduc-tion zone fi rst forms than for mature arcs (Stern and Bloomer, 1992; Jicha et al., 2006). Such large volumes of melt generation require re-sidual peridotites to be unusually depleted. This is consistent with the observation that boninites and very depleted tholeiitic basalts are produced during subduction initiation and are common in the well-exposed Izu-Bonin-Mariana forearc (Stern, 2004).

The interpretation that Dehshir and other Zagros ophiolites formed in a back-arc basin also meets tectonic diffi culties because there is no obvious associated arc. Any such arc to the north requires S-dipping subduction, collision with Eurasia sometime after 90 Ma, and reversal of subduction polarity, for which there is no evi-dence. Alternatively, an arc to the SW, overlying a N-dipping subduction zone, must have been completely obliterated. There is no evidence that anything like either of these scenarios hap-pened. Instead, the SW Iranian ophiolites today lie in a forearc position, which is most simply explained as being forearc crust preserved in situ since Late Cretaceous time.

The preferred scenario, whereby a broad tract of ophiolites formed along the southern mar-gin of Eurasia during a subduction initiation event, is summarized for Dehshir, SW Iranian ophiolites, and other Late Cretaceous ophiolites from Cyprus to Oman in Figure 16. This inter-pretation answers the second question as well:

Figure 16. Generation of SW Iranian ophio lites as part of a forearc formed during subduc-tion initiation, modifi ed for the Late Cretaceous evolution of southern Eurasia after Stern (2004). Left panels are sections perpendicular to the evolv-ing plate boundary, and right panels are map views. (A–B) Initial (>95 Ma) confi guration. Two lithospheres of differing density (buoyant Eurasia, dense Tethys) are juxtaposed across a transform fault or fracture zone. (C–D) Old, dense lithosphere sinks asymmetrically, with maxi mum subsidence nearest the transform fault or fracture zone. Asthenosphere migrates over the sinking lithosphere and propagates in directions that are both orthogonal to the original trend of the transform/fracture zone as well as in both directions parallel to it. Strong extension in the region above the sinking lithosphere leads to seafl oor spreading, forming infant arc crust of the proto-forearc. This is when the Dehshir ophiolite and other SW Iranian ophiolites form, ca. 93 Ma. (D) This panel shows that this process is asynchronous along the margin; ophiolite ages indicate that subduction propagated from E (Oman) to W (Cyprus) between 95 and 90 Ma. (E–F) Beginning of downdip motion of the lithosphere marks the beginning of true subduction (<90 Ma). Strong extension in the forearc and trenchward migration of asthenosphere ends, cooling the subforearc mantle, which becomes forearc lithosphere; further addition of water leads only to serpentinization. The locus of igneous activity retreats ~200 km away from the trench, to the region where asthenospheric advection continues, forming the Urumieh-Dokhtar arc. This forearc today defi nes the Oman (Om), SW Iran (SI), and Cyprus (Cy) ophiolites.

Eurasia(buoyant)

Transform or fracture zone

Spreading(proto-forearc)

Retreatinghinge

Urumieh-DokhtarArc

SW Iran ophiolites (forearc)

OB

Sinking

True subduction

A

C

>95 Ma

90–95 Ma

E<90 Ma

Tethys

B

D

F

Eurasia

Tethys(dense)

OBIB

Om

SI

Cy

N S



these ophiolites formed essentially where they are today, attached to the southern margins of Eurasia. They are not far-traveled, exotic frag-ments of oceanic lithosphere but are partially preserved fragments of an oceanic forearc that formed when subduction began along the south-ern margin of Eurasia, ca. 95 Ma.

CONCLUSIONS

The Dehshir ophiolite is an important ele-ment within the Inner Zagros ophiolite belt. Along with other Zagros ophiolites, Dehshir represents autochthonous forearc lithosphere that formed by seafl oor spreading along the southern margin of Eurasia during Late Creta-ceous time. This forearc crust was once continu-ous, both across the two belts as well as from Cyprus to Oman. This forearc has been dis-rupted by continued shortening, especially since collision began with Arabia in Miocene time, as well as by underplating beneath the forearc and subsequent exhumation of the underplate to form the Sanandaj-Sirjan zone. Zagros ophio-lites and correlatives from Cyprus to Oman con-stitute a tract of oceanic lithosphere that formed along the southern margin of Eurasia during a subduction initiation event. Since that event, the Zagros orogen has evolved as a convergent plate margin, with the Persian Gulf representing a fi lled trench, the Zagros fold-and-thrust belt rep-resenting an accretionary prism that continues to grow, and the Urumeih-Dokhtar magmatic complex representing a waning magmatic arc. This interpretation provides a robust and simple framework for understanding the tectonic evolu-tion of the region.

ACKNOWLEDGMENTS

This study was supported fi nancially by the French Centre National de la Recherche Scientifi que (CNRS) (contribution of the UMR 7516 “Institut de Physique du Globe,” Strasbourg, France), and the Cultural Service of the French Embassy at Tehran. We are deeply indebted to Sixte Blanchy, Christian Duhamel, and Hubert Whitechurch for their assistance. R. Boutin at Universite de Louis Pasteur greatly contributed to the whole-rock analyses. Finally, we are very grateful to Réjean Hébert and Jean Bédard for their construc-tive reviews of the manuscript. Editorial suggestions by J. Brendan Murphy and Yildirim Dilek are highly appreciated.

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