Supra-subduction and mid-ocean ridge peridotites from the Piranshahr area, NW Iran

Journal of Geodynamics 81 (2014) 41–55

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Supra-subduction and mid-ocean ridge peridotites from the Piranshahr area, NW Iran Robab Hajialioghli ∗ , Mohssen Moazzen Department of Earth Sciences, University of Tabriz, 51664 Tabriz, Iran

a r t i c l e

i n f o

Article history: Received 14 October 2013 Received in revised form 25 June 2014 Accepted 30 June 2014 Available online 22 July 2014 Keywords: Metaperidotites Supra-subduction zone Abyssal peridotite Piranshahr NW Iran Neotethys

a b s t r a c t The Piranshahr metaperidotites in the northwestern end of the Zagros orogen were emplaced following the closure of the Neotethys ocean. The ophiolitic rocks were emplaced onto the passive margin of the northern edge of the Arabian plate as a result of northeastward subduction and subsequent accretion of the continental fragments. The metaperidotites have compositions ranging from low-clinopyroxene lherzolite to harzburgite and dunite. They are mantle residues with distinct geochemical signatures of both mid-ocean ridge and supra subduction zone (SSZ) affinities. The abyssal peridotites are characterized by high Al2 O3 and Cr2 O3 contents and low Mg-number in pyroxenes. The Cr-number in the coexisting spinel is also low. The SSZ mantle peridotites are characterized by low Al2 O3 contents in pyroxenes as well as low Al2 O3 and high Cr-number in spinel. Mineral chemical data indicate that the MOR- and SSZ-type peridotites are the residues from ∼15–20% and ∼30–35% of mantle melting, respectively. Considering petrography, mineralogy and textural evidence, the petrological history of the Piranshahr metaperidotites can be interpreted in three stages: mantle stable stage, serpentinization and metamorphism. The temperature conditions in the mantle are estimated using the Ca-in-orthopyroxene thermometer as 1210 ± 26 ◦ C. The rocks have experienced serpentinization. Based on the textural observations, olivine and pyroxene transformed into lizardite and/or chrysotile with pseudomorphic textures at temperatures below 300 ◦ C during the initial stage of serpentinization. Subsequent orogenic metamorphism affected the rocks at temperatures lower than 600 ◦ C under lower-amphibolite facies metamorphism. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Ophiolites represent former fragments of oceanic lithosphere, now tectonically emplaced on land. Most Tethyan ophiolites occurring in the eastern Mediterranean region are interpreted as fragments of supra-subduction zone (SSZ)-generated oceanic lithosphere (Pearce et al., 1984; Robertson, 1994; Dilek et al., 1999, 2007; Parlak et al., 2002; Dilek and Flower, 2003; Saccani and Photiades, 2004; Dilek and Furnes, 2009). The Tethyan ophiolites with SSZ characteristics are widely interpreted to have formed in incipient arc–forearc settings shortly before their emplacement on continental margins. Shervais (2001) reviewed the petrological and geochemical signatures of SSZ ophiolites and suggested that these ophiolites experienced a sequence of events during their evolution in response to the change in tectonic setting from oceanic lithosphere formed at mid-ocean ridges to

∗ Corresponding author. Tel.: +98 411 339 2704. E-mail addresses: [email protected], r [email protected] (R. Hajialioghli). http://dx.doi.org/10.1016/j.jog.2014.06.003 0264-3707/© 2014 Elsevier Ltd. All rights reserved.

the initiation of subduction. Recent studies on many Tethyan ophiolites have shown that these ophiolites, particularly their upper mantle components, have records of a complex magmatic, geochemical and tectonic evolution, spanning different geodynamic settings within the same ocean basin. Some examples of ophiolites with both mid-ocean ridge (MOR)- and SSZ-type affinities include Pindos ophiolites in Greece (Saccani and Photiades, 2004), Troodos ophiolites in Cyprus (Portnyagin et al., 1997), Lycian and Antalya ophiolites in SW Turkey (Aldanmaz et al., 2009), Harmancik ophiolites in NW Turkey (Uysal et al., 2009, 2013), Kermanshah (Allahyari et al., 2010; Saccani et al., 2013) and Neyriz (Rajabzadeh and Nazari Dehkordi, 2012) ophiolites in the Main Zagros Fault Zone, Nain (Mehdipour Ghazi et al., 2010) and Oman ophiolites (Python and Ceuleneer, 2003; Yamasaki et al., 2006; Tamura and Arai, 2006; Dare et al., 2009; Clénet et al., 2010; Goodenough et al., 2010). The Iranian ophiolites of Zagros are part of a 3000 km long belt that extends from Troodos in Cyprus, to Smail in Oman (Fig. 1a). The ophiolites at either end of this belt are the best studied in the world (e.g. Pearce and Robinson, 2010; Dilek and Furnes, 2009; Dilek et al., 2007; Garfunkel, 2006; Godard et al., 2003, 2006; Robertson, 1998, 2002; Floyd et al., 1998; S¸engör, 1990; Albaster et al., 1982,

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R. Hajialioghli, M. Moazzen / Journal of Geodynamics 81 (2014) 41–55

Fig. 1. (a) Upper Cretaceous ophiolitic belt of SW Asia between Cyprus and Oman and location of the Zagros ophiolitic belt (modified after Blome and Irwin, 1985). (b) Distribution of the ophiolites in Iran. Kh: Khoy, Pi: Piranshahr, Rs: Rasht, Kr: Kermanshah, Ny: Neyriz, Bz: Bande-Zyarat, Ir: Iranshahr, Es: Esfandagheh, Ba: Baft, Sh, ShahreBabak, Na: Nain, Sb: Sabzevar, Ms: Mashhad, Bj: Birjand, Tk: Tchehel Kureh. (c) The Zagros orogen at the Iran and Iraq border consists of four main units including Urmia-Dokhtar magmatic arc, Sanandaj-Sirjan zone, Zagros fold-thrust belt and the Mesopotamian foreland basin.

among others). In contrast the Upper Cretaceous ophiolites of the Iranian Zagros are still poorly known (Fig. 1b). Ophiolites occur at the NW end of the Zagros orogen in the Kurdistan region at the Iran and Iraq border. The most important ophiolitic complexes at this part of the Zagros orogen are Penjwin and Mawat ophiolites in Iraq (Aswad et al., 2011; Aziz et al., 2011; Mohammad, 2011) and Kermanshah (Allahyari et al., 2010; Saccani et al., 2013), SarveAbad (Allahyari et al., 2013) and Piranshahr ophiolites (this study) in Iran. Aswad et al. (2011) concluded a forearc setting for the ophiolites in NE Iraq, based on chemistry of primary spinels in serpentinized peridotites. Aziz et al. (2011) reported Sr model ages of 80–110 Ma for ophiolite formation and/or serpentinization above an intra-oceanic supra-subduction zone. The ophiolites are metamorphosed at upper greenschist facies condition (Mohammad, 2011). Extensive studies are carried out on the Kermanshah ophiolite by Allahyari et al. (2010) and Saccani et al. (2013). Allahyari et al. (2010) preformed a detailed study on Kermanshah ophiolite and showed that foliated gabbros of this ophiolite represent a portion of MOR oceanic crust, while some lherzolites are residual MOR mantle, which are subsequently trapped in a supra-subduction zone. Based on these facts, they proposed a geodynamic model in which a MOR lithosphere was generated in the Triassic. An intra-oceanic island arc was formed in Lower Late Cretaceous and the residual MOR mantle (depleted lherzolite) was trapped in the supra-subduction zone. Eventually

ophiolitic rocks were obducted on the Arabian passive continental margin carbonates during Late Cretaceous. Saccani et al. (2013) suggested a model for the Kermanshah ophiolite similar to Ligurian Tethys, with exception that OIB-type components linked to the uprising of MORB-type depleted asthenospheric mantle had a pronounced influence on formation of Kermanshah ophiolite. The Piranshahr ophiolite located along the Main Zagros Thrust Zone is spatially close to the NE Iraq and Kermanshah ophiolites. There is no published account on this ophiolite. Detailed investigations on this ophiolite, which links the NE Iraq ophiolites to Iranian Zagros ophiolites provide information to interpret the evolution of the Neotethys in the Middle East. This paper presents the first petrological and mineralogical studies on the obscure Piranshahr ophiolite. The chemical compositions of the analysed minerals in the peridotites are used to determine the origin and evolution of the investigated rocks. The evolution of the Piranshahr ophiolite is studied in the framework of the model proposed by Allahyari et al. (2010) and Saccani et al. (2013). 2. Geological setting and field relations The Piranshahr ophiolite is located along the Zagros suture zone, between the Sanandai-Sirjan zone and the Zagros fold thrust belt (Fig. 1c) in NW Iran. The Zagros orogen at the Iran and Iraq

R. Hajialioghli, M. Moazzen / Journal of Geodynamics 81 (2014) 41–55

border consists of four main units including Urmia-Dokhtar magmatic arc, Sanandaj-Sirjan zone, Zagros fold-thrust belt and the Mesopotamian foreland basin, all having a NW–SE trend (Fig. 1c). The Urmia-Dokhtar magmatic belt (Schröder, 1944) is consisted mainly of Tertiary calc-alkaline volcanic and plutonic rocks with a magmatic climax during Oligo-Miocene (Berberian and King, 1981; Bina et al., 1986). The Sanandaj-Sirjan zone is a complex of mainly Jurassic metamorphic rocks intruded by several plutons (Berberian and King, 1981). This zone is considered to be an active Andean-type margin during Middle Jurassic to Cretaceous with calc-alkaline plutonism progressively shifted northward (S¸engör, 1990; Agard et al., 2005). The Zagros fold-thrust belt is composed predominantly of thick carbonates shelf deposits with a Permo-Triassic to Late Cretaceous age, followed by a Palaeocene to Pliocene sedimentary succession (James and Wind, 1965; Stöcklin, 1974). The suture zone between the Arabian plate and the Central Iran micro-continent is made of tectonically imbricated slices of shelf carbonates with a Mesozoic age, remnants of the Neotethys oceanic crust represented as ophiolitic complexes and Tertiary volcanic rocks and flysch-type deposits (Berberian and King, 1981; Ghasemi and Talbot, 2006; Saccani et al., 2013). Two main rock associations can be found in the Piranshahr area. The first one comprises rock units belonging to the Sanandaj-Sirjan zone and the second one includes rock units of the Piranshahr ophiolitic complex (Fig. 2). The Sanandaj-Sirjan rock units are the oldest rocks in the Piranshahr area, which comprise pelitic and metabasic schists exposed at the east of the Ghardsur village (Fig. 2). Amphibolite, granitoid and metagranitoid accompany the schists. A Precambrian age for these rocks is proposed on the basis of stratigraphy and lithological features (Khodabandeh, 2004). Palaeozoic slaty shales containing cherts as nodules and bands are interlayered with dolomite and calcareous rocks (Barut and Mila Formations, Khodabandeh, 2004). The Permian rocks including crystalline

43

limestone and dolomite with significant thickness, crop out mainly in the north western and eastern parts of the study area (Fig. 2). The Piranshahr ophiolite appears as an ophiolitic tectonic mélange. The rocks are strongly faulted with NW–SE trending faults parallel to the Sanandaj-Sirjan Zone trend. The mélange contains different rock types including serpentinized mantle peridotites, diabasic rocks, more likely representing the dyke complex, basalts and spilitic basalts occasionally with pillow structures and grey microcrystalline (pelagic) limestone. The metamorphic rocks within the ophiolitic mélange are metabasic schists, metapelites, calc-silicate rocks, amphibolites, metaperidotites and serpentinites (Fig. 2). Mantle peridotites are the most abundant rock types and form more than 70% of the ophiolitic outcrop. They occur as discontinuous bodies within the mélange. These rocks occur mainly in the western part of the area. Low-clinopyroxene lherzolite, harzburgite and dunite patches are the main rock types of the peridotites. Due to mélange nature of the ophiolitic complex, no correlation and field lithological relations can be made for different rock types. The peridotites studied here are significantly serpentinized, but the degree of serpentinization is various in different rocks. Metaperidotites can be classified into two groups on the basis of degree of serpentinization: (1) metaperidotites and (2) serpentinites. Metaperidotites are massive and are composed of relicts of olivine, orthopyroxene, clinopyroxene and spinel together with metamorphic phases such as serpentine, clinochlore and tremolite. The serpentinites contain >90 vol.% serpentine. They vary in colour from light green through brown to black. Tremolite/actinolite-rich rocks occur as dykes within the serpentinites and consist almost entirely (>95 vol.%) of coarse grained tremolite/actinolite (up to 2 cm in size). These rocks are probably originated as pyroxenite dykes, but no primary pyroxenes are preserved to confirm this.

Fig. 2. Geological map of the Piranshahr area. Adapted from Khodabandeh (2004).

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R. Hajialioghli, M. Moazzen / Journal of Geodynamics 81 (2014) 41–55

3. Analytical method Primary magmatic and metamorphic minerals from low-Cpx lherzolites and harzburgites were analysed for major element oxides by wavelength-dispersive spectrometry using a JEOL JXA8200 microprobe at the Freie Universität Berlin. The measuring conditions were 15 kV accelerating voltage and 10–20 nA beam current. The spot size was between 3 and 5 ␮m. A ZAF correction procedure was applied. Natural and synthetic standards were used for the calibration of the machine. Representative mineral analyses are given in Tables 1–5. Micro-Raman system (HORIBA JObin Yvon, LabRam HR800) and an optical microscope (Olympus, BX41) used to determine serpentine polymorphs. Fig. 3 shows Raman spectra of the antigorite, lizardite and chrysotile in the Piranshahr metaperidotites. 4. Petrography The mineral composition and textural relations in the studied serpentinites and metaperidotites are discussed below. The mineral name abbreviations used are from Kretz (1983). 4.1. Serpentinites Serpentine (>90 vol.%) is the main constituent of the serpentinites. Mesh texture after olivine and bastite after pyroxene are the typical features of the serpentinites (Fig. 4a and b). Lizardite surrounds olivine relicts at the core of mesh. Iron oxide (probably magnetite) is formed at the core of mesh texture. It is formed from partial or complete serpentinization of olivine. It also occurs as fine-grained minerals along margins of the mesh texture. The serpentines also show hourglass texture characterized by lizardite at the core surrounded by chrysotile at the rim (Fig. 4c). Lizardite and chrysotile are not readily distinguishable under the microscope, but they show relatively different birefringence. Hourglass texture is interpreted to be related to the serpentine recrystallization at the stability limit of chrysotile (Wicks and Whittaker, 1977). O’Hanley

(1996) considered hourglass texture in serpentinites in association with extensive fracturing in olivines. Antigorite is formed at the expense of former lizardite and chrysotile in some of the samples. Interpenetrative texture in the serpentinites (Fig. 4d) can occur due to fluid-assisted prograde metamorphism of the serpentine pseudomorphs (e.g. Li et al., 2004). Amphibole forms as relatively coarse-grained crystals (Fig. 4b). It is difficult to define the primary mineral assemblage of the serpentinites since dominant primary minerals except for spinel are mainly changed into serpentine. However the textures of the secondary minerals suggest that they are formed dominantly from olivine and orthopyroxene. 4.2. Metaperidotites The metaperidotites in the Piranshahr area are serpentinized partially (about 20 vol.%). Orthopyroxene and clinopyroxene are subhedral and occur with blastoporphyritic texture (Fig. 5a–d). Some clinopyroxene crystals occur as relicts in tremolite. Olivine is partially replaced by serpentine, and shows typical mesh texture. Reddish brown spinel, 3 mm across, is present in partially serpentinized metaperidotites and shows black colour along rims and fractures (Fig. 5e and f). Fig. 6 shows a SEM image of spinel in which secondary ferrite–chromite is formed along the fractures and at the rims of the primary spinel. Chlorite surrounds spinel in places (Fig. 5e and f). Antigorite is further overprinted by secondary lizardite and chrysotile which probably has been formed due to late alteration process. The proportions of orthopyroxene, clinopyroxene and olivine relicts and pseudomorphs (mesh texture and bastite after olivine and pyroxene, relatively) in the Piranshahr metaperidotites provide information on the nature of the original peridotite. The relative abundance of relict and pseudomorphed olivine (50–60 vol.%) and orthopyroxene (20–35 vol.%) and the rarity of relict and pseudomorphed clinopyroxene (∼10 vol.%) suggest that most of the investigated metaperidotites originated as low-Cpx lherzolite and harzburgite. Dunite is relatively rare.

Table 1 Representative mineral chemistry of olivine in the Piranshahr metaperidotites. Low-Cpx lherzolites

Harzburgites

No.

Na

Nb

87

62

41

51

60

61

66

19

20

21

22

SiO2 TiO2 Al2 O3 Cr2 O3 FeO* MnO MgO NiO CaO Total (O) Si Ti Al Cr Fe(II) Mn Mg Ni Ca Total Mg# Fo Fa Tp

40.50 0.00 0.00 0.02 12.65 0.30 45.79 – 0.00 99.26 4 1.02 0.00 0.00 0.00 0.27 0.01 1.71 – 0.00 3.01 0.86 86.31 13.37 0.32

40.40 0.00 0.00 0.01 12.40 0.32 45.00 – 0.00 98.13 4 1.03 0.00 0.00 0.00 0.26 0.01 1.70 – 0.00 3.00 0.87 86.31 13.34 0.35

40.42 0.00 0.00 0.01 11.55 0.17 49.10 – 0.06 101.30 4 0.99 0.00 0.00 0.00 0.24 0.00 1.79 – 0.00 3.02 0.88 88.2 11.63 0.17

41.47 0.00 0.00 0.00 8.72 0.07 51.05 0.38 0.03 101.72 4 0.99 0.00 0.00 0.00 0.17 0.00 1.83 0.01 0.00 3.00 0.91 91.19 8.74 0.07

41.62 0.00 0.00 0.00 8.49 0.09 50.95 0.38 0.00 101.53 4 1.00 0.00 0.00 0.00 0.17 0.00 1.82 0.01 0.00 3.00 0.91 91.36 8.54 0.10

40.31 0.00 0.00 0.00 8.26 0.11 50.92 0.31 0.01 99.92 4 0.98 0.00 0.00 0.00 0.17 0.00 1.85 0.00 0.00 3.00 0.92 91.55 8.33 0.12

40.06 0.00 0.00 0.00 8.60 0.11 50.55 0.38 0.00 99.70 4 0.98 0.00 0.00 0.00 0.18 0.00 1.85 0.01 0.00 3.02 0.91 91.18 8.70 0.12

41.31 0.00 0.00 0.02 8.85 0.16 50.92 0.37 0.00 101.63 4 0.99 0.00 0.00 0.00 0.17 0.00 1.83 0.01 0.00 3.00 0.91 90.96 8.87 0.17

40.10 0.00 0.00 0.00 8.37 0.14 50.17 0.35 0.00 99.13 4 0.99 0.00 0.00 0.00 0.17 0.00 1.84 0.01 0.00 3.01 0.91 91.3 8.54 0.15

40.61 0.01 0.00 0.00 9.41 0.19 49.99 0.37 0.01 100.59 4 0.99 0.00 0.00 0.00 0.19 0.00 1.82 0.01 0.00 3.01 0.90 90.26 9.53 0.20

40.59 0.00 0.00 0.00 9.68 0.18 49.40 0.40 0.02 100.27 4 0.99 0.00 0.00 0.00 0.20 0.00 1.80 0.01 0.00 3.00 0.90 89.93 9.88 0.19

40.94 0.00 0.00 0.00 9.86 0.12 49.93 0.39 0.01 101.25 4 0.99 0.00 0.00 0.00 0.20 0.00 1.80 0.01 0.00 3.00 0.90 89.91 9.96 0.13

40.89 0.00 0.005 0.00 10.08 0.13 49.67 0.37 0.00 101.14 4 0.99 0.00 0.00 0.00 0.21 0.00 1.80 0.01 0.00 3.01 0.90 89.66 10.21 0.14

*

Fe total as FeO.

R. Hajialioghli, M. Moazzen / Journal of Geodynamics 81 (2014) 41–55

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Table 2 Representative mineral chemistry of orthopyroxene in the Piranshahr metaperidotites. Low-Cpx lherzolites

Harzburgites

No.

110

60

61

71

72

73

76

7

8

16

17

21

46

SiO2 TiO2 Al2 O3 Cr2 O3 FeO* MnO MgO NiO CaO Na2 O K2 O Total (O) Si Ti Al Cr Fe(II) Mn Mg Ni Ca Na K Total Mg# Wo% En% Fs%

55.89 0.01 2.64 0.62 5.88 0.14 31.96 0.00 1.06 0.03 0.00 98.23 6 1.96 0.00 0.11 0.02 0.17 0.00 1.67 0.00 0.04 0.00 0.00 3.97 0.91 2.11 88.73 9.16

56.54 0.03 2.83 0.63 5.81 0.18 35.05 0.09 0.55 0.00 0.00 101.71 6 1.92 0.00 0.11 0.02 0.16 0.01 1.77 0.00 0.02 0.00 0.00 4.01 0.91 1.02 90.55 8.43

57.01 0.05 3.01 0.75 5.58 0.12 34.41 0.07 0.80 0.00 0.01 101.81 6 1.93 0.00 0.12 0.02 0.16 0.00 1.74 0.00 0.03 0.00 0.00 4.00 0.92 1.51 90.28 8.21

55.68 0.00 2.80 0.57 5.80 0.10 33.61 0.05 1.01 0.00 0.02 99.64 6 1.93 0.00 0.11 0.02 0.17 0.00 1.74 0.00 0.04 0.00 0.00 4.01 0.91 1.93 89.42 8.65

56.17 0.00 2.87 0.63 5.54 0.09 33.80 0.05 1.10 0.02 0.01 100.28 6 1.93 0.00 0.12 0.02 0.16 0.00 1.73 0.00 0.04 0.00 0.00 4.00 0.92 2.10 89.66 8.24

56.14 0.01 2.86 0.66 5.21 0.13 33.44 0.08 1.28 0.00 0.00 99.81 6 1.94 0.00 0.12 0.02 0.15 0.00 1.72 0.00 0.05 0.00 0.00 4.00 0.92 2.47 89.68 7.85

55.95 0.02 2.97 0.73 5.22 0.15 33.90 0.09 1.00 0.01 0.01 100.05 6 1.93 0.00 0.12 0.02 0.15 0.00 1.74 0.00 0.04 0.00 0.00 4.00 0.92 1.92 90.28 7.80

57.79 0.00 1.20 0.43 5.88 0.08 35.18 0.09 0.76 0.02 0.01 101.43 6 1.96 0.00 0.05 0.01 0.17 0.00 1.78 0.00 0.03 0.00 0.00 4.00 0.92 1.01 91.91 7.07

57.23 0.01 1.13 0.47 5.33 0.20 34.96 0.04 0.80 0.00 0.02 100.19 6 1.96 0.00 0.05 0.01 0.16 0.01 1.79 0.00 0.03 0.00 0.00 4.00 0.93 2.00 91.00 7.00

57.63 0.02 1.21 0.51 5.52 0.12 34.83 0.14 0.78 0.01 0.00 100.76 6 1.97 0.00 0.05 0.01 0.16 0.00 1.77 0.00 0.03 0.00 0.00 4.00 0.92 1.01 90.91 8.08

57.62 0.00 1.17 0.42 5.71 0.12 34.94 0.04 0.96 0.00 0.01 100.98 6 1.96 0.00 0.05 0.01 0.17 0.00 1.78 0.00 0.04 0.00 0.00 4.00 0.92 1.98 90.10 7.92

57.70 0.01 1.14 0.50 5.40 0.14 34.48 0.09 1.23 0.00 0.00 100.70 6 1.97 0.00 0.05 0.01 0.16 0.00 1.76 0.00 0.05 0.00 0.00 4.00 0.91 2.02 89.90 8.08

57.16 0.04 1.18 0.49 5.66 0.14 35.05 0.10 0.88 0.00 0.02 100.70 6 1.95 0.00 0.05 0.01 0.16 0.00 1.78 0.00 0.03 0.00 0.00 4.00 0.93 1.98 91.09 6.93

*

Fe total as FeO.

Table 3 Representative mineral chemistry of clinopyroxene in the Piranshahr metaperidotites. Low-Cpx lherzolites

Harzburgites

No.

113

97

98

80

82

69

67

65

38

63

27

34

35

SiO2 TiO2 Al2 O3 Cr2 O3 FeO* MnO MgO CaO Na2 O K2 O Total (O) Si Ti Al Cr Fe(II) Mn Mg Ca Na K Total Mg# Cr# Quad Jd Ae

52.31 0.05 2.99 0.98 2.00 0.05 15.66 25.24 0.12 0.00 99.40 6 1.96 0.00 0.13 0.03 0.06 0.00 0.87 1.01 0.01 0.00 4.07 0.93 0.18 99.06 0.53 0.41

52.71 0.07 2.83 1.06 2.07 0.07 16.22 25.03 0.14 0.01 100.21 6 1.96 0.00 0.12 0.03 0.06 0.00 0.90 0.99 0.01 0.00 4.07 0.93 0.20 98.95 0.43 0.61

52.49 0.08 3.14 1.10 2.06 0.11 15.79 25.42 0.13 0.01 100.33 6 1.95 0.00 0.14 0.03 0.06 0.00 0.87 1.01 0.01 0.00 4.07 0.93 0.19 99.01 0.44 0.54

52.77 0.01 3.72 1.27 2.29 0.03 16.71 24.03 0.14 0.01 100.98 6 1.95 0.00 0.16 0.04 0.07 0.00 0.92 0.95 0.01 0.00 4.10 0.93 0.19 98.94 0.57 0.48

53.44 0.04 3.23 0.96 1.92 0.08 17.14 24.25 0.11 0.00 101.17 6 1.96 0.00 0.14 0.03 0.06 0.00 0.94 0.95 0.01 0.00 4.09 0.94 0.17 99.15 0.52 0.33

52.81 0.07 3.04 1.02 1.91 0.06 17.13 23.95 0.14 0.00 100.13 6 2.04 0.00 0.00 0.03 0.03 0.00 0.93 0.94 0.01 0.00 3.98 0.97 0.18 98.96 0.05 0.99

55.12 0.04 0.99 0.29 1.68 0.07 18.17 25.18 0.06 0.00 101.6 6 1.98 0.00 0.04 0.01 0.05 0.00 0.97 0.97 0.00 0.00 4.02 0.95 0.17 99.60 0.08 0.33

54.84 0.00 0.23 0.36 0.95 0.03 18.46 24.45 0.05 0.00 99.37 6 2.00 0.00 0.01 0.01 0.03 0.00 1.00 0.95 0.00 0.00 4.00 0.97 0.50 99.62 0.14 0.24

54.56 0.00 1.19 0.58 2.64 0.15 17.99 23.58 0.09 0.00 100.78 6 1.98 0.00 0.05 0.02 0.08 0.00 0.97 0.92 0.01 0.00 4.03 0.92 0.25 99.36 0.26 0.38

54.30 0.00 0.61 0.54 0.99 0.01 19.17 22.99 0.05 0.04 98.70 6 1.99 0.00 0.03 0.02 0.03 0.00 1.05 0.90 0.00 0.00 4.02 0.97 0.37 99.65 0.11 0.23

55.85 0.00 0.07 0.09 0.91 0.03 18.50 25.17 0.04 0.02 100.68 6 2.00 0.00 0.00 0.00 0.03 0.00 0.99 0.97 0.00 0.00 3.99 0.97 0.44 99.72 0.28 0.00

55.39 0.00 0.05 0.20 0.88 0.01 18.24 24.85 0.06 0.00 99.68 6 2.01 0.00 0.00 0.01 0.03 0.00 0.98 0.96 0.00 0.00 3.99 0.97 0.72 99.54 0.22 0.24

55.83 0.00 0.01 0.15 0.91 0.01 18.18 25.14 0.08 0.01 100.32 6 2.01 0.00 0.00 0.00 0.03 0.00 0.98 0.97 0.01 0.00 4.00 0.97 0.90 99.43 0.05 0.52

*

Fe total as FeO.

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Table 4 Representative mineral chemistry of original and secondary spinels in the Piranshahr metaperidotites. Magmatic spinel Low-Cpx lherzolites

Harzburgites

No.

Na

105

51

52

54

57

66

11

12

1

2

3

10

4

SiO2 TiO2 Al2 O3 Cr2 O3 FeO* MnO MgO CaO Total (O) Si Ti Al Cr Fe(II) Fe(III) Mn Mg Ca Total Mg# Cr#

0.04 0.05 33.39 34.42 16.00 0.46 14.01 0.00 98.37 4 0.00 0.00 1.16 0.80 0.37 0.03 0.01 0.61 0.00 2.98 0.60 0.41

0.04 0.05 33.39 34.45 15.60 0.46 14.01 0.00 98.00 4 0.00 0.00 1.16 0.81 0.37 0.02 0.01 0.62 0.00 2.99 0.61 0.41

0.00 0.01 34.39 34.00 14.97 0.18 15.67 0.00 99.22 4 0.00 0.00 1.17 0.78 0.32 0.04 0.00 0.68 0.00 2.99 0.65 0.40

0.03 0.04 34.76 34.39 14.86 0.23 15.95 0.00 100.26 4 0.00 0.00 1.17 0.78 0.31 0.04 0.01 0.68 0.00 2.99 0.66 0.40

0.00 0.03 35.19 33.04 15.44 0.17 16.15 0.01 100.03 4 0.00 0.00 1.19 0.75 0.31 0.06 0.00 0.69 0.00 3.00 0.65 0.39

0.00 0.01 34.44 34.48 14.80 0.19 16.32 0.01 100.25 4 0.00 0.00 1.16 0.78 0.30 0.05 0.00 0.70 0.00 2.99 0.66 0.40

0.00 0.05 34.96 34.15 14.60 0.26 16.11 0.02 100.15 4 0.00 0.00 1.18 0.77 0.30 0.04 0.01 0.69 0.00 2.99 0.66 0.40

0.00 0.07 22.36 46.35 18.87 0.37 12.77 0.00 100.79 4 0.00 0.00 0.80 1.12 0.41 0.07 0.01 0.58 0.00 2.99 0.55 0.58

0.01 0.02 22.34 45.35 20.00 0.31 11.46 0.00 99.49 4 0.00 0.00 0.82 1.12 0.46 0.06 0.01 0.53 0.00 3.00 0.51 0.58

0.01 0.05 21.46 47.12 19.15 0.30 12.40 0.00 100.49 4 0.00 0.00 0.78 1.15 0.42 0.07 0.01 0.57 0.00 3.00 0.54 0.60

0.00 0.07 21.07 48.01 19.11 0.28 12.29 0.01 100.84 4 0.00 0.00 0.76 1.17 0.43 0.06 0.01 0.56 0.00 2.99 0.53 0.60

0.01 0.06 20.98 47.76 19.13 0.29 12.55 0.00 100.78 4 0.00 0.00 0.76 1.16 0.42 0.07 0.00 0.57 0.00 2.98 0.54 0.60

0.00 0.08 20.99 47.47 19.05 0.26 12.46 0.00 100.31 4 0.00 0.00 0.76 1.16 0.42 0.07 0.01 0.57 0.00 2.99 0.54 0.60

0.03 0.09 20.97 48.07 18.84 0.27 12.46 0.00 100.73 4 0.00 0.00 0.76 1.17 0.42 0.06 0.01 0.57 0.00 2.99 0.54 0.61

Min.

Metamorphic spinels

No.

150

151

152

153

154

5

9

108

SiO2 TiO2 Al2 O3 Cr2 O3 FeO* MnO MgO CaO Total (O) Si Ti Al Cr Fe(II) Fe(III) Mn Mg Ca Total Mg# Cr#

2.15 0.09 8.54 32.54 49.87 0.77 4.39 0.00 98.35 4 0.07 0.00 0.35 0.89 0.83 0.61 0.02 0.22 0.00 2.99 0.13 0.72

0.40 0.19 10.33 30.46 52.93 0.73 3.22 0.00 98.26 4 0.01 0.00 0.42 0.84 0.83 0.70 0.02 0.17 0.00 2.99 0.10 0.66

0.59 0.21 4.44 30.32 59.78 0.83 1.93 0.02 98.12 4 0.02 0.00 0.19 0.86 0.90 0.90 0.02 0.10 0.00 2.99 0.05 0.82

1.77 0.29 2.82 29.74 59.81 0.84 2.76 0.00 98.03 4 0.06 0.01 0.12 0.84 0.90 0.89 0.02 0.15 0.00 2.99 0.08 0.88

0.60 0.29 3.03 31.20 61.00 0.77 1.49 0.00 98.38 4 0.02 0.01 0.13 0.89 0.93 0.92 0.02 0.08 0.00 2.99 0.04 0.87

0.31 0.04 12.31 29.57 49.34 0.16 6.56 0.01 98.30 4 0.01 0.00 0.49 0.78 0.68 0.71 0.00 0.33 0.00 3.00 0.19 0.62

0.11 0.05 16.64 41.96 31.99 0.43 6.82 0.00 98.00 4 0.00 0.00 0.65 1.10 0.65 0.24 0.01 0.34 0.00 3.00 0.28 0.63

0.73 0.04 13.56 35.34 42.71 1.02 3.88 0.03 98.04 4 0.02 0.00 0.55 0.96 0.78 0.44 0.03 0.20 0.00 2.98 0.14 0.64

5. Mineral chemistry

and Al2 O3 contents in orthopyroxene in the harzburgites are low as 0.42–0.51 wt% and 1.13–1.21 wt%, respectively.

5.1. Olivine 5.3. Clinopyroxene The chemical composition of olivine is given in Table 1. The formula is calculated using regular solution in a two-site model and three cations for four oxygens. Olivine is mainly forsterite in composition with a Mg# [(Mg)/(Mg + Fe2+ )] of 0.86–0.88 and 0.90–0.92 in low-Cpx lherzolites and harzburgites, respectively.

5.2. Orthopyroxene The structural formula of orthopyroxene is calculated assuming a total of four cations per six oxygen atoms (Table 2). The analysed orthopyroxenes are enstatite (En88.4–90.6 ) with relatively invariable MgO (31.96–35.05 wt%) contents. Orthopyroxene in the low-clinopyroxene lherzolites has high Cr2 O3 and Al2 O3 contents of 0.56–0.75 wt% and 2.60–3.01 wt%, respectively, whereas Cr2 O3

The chemical composition of clinopyroxene is given in Table 3. The studied clinopyroxenes plot in the diopside field of pyroxene nomenclature diagram after Morimoto et al. (1988). The TiO2 content of the analysed clinopyroxenes is inconsiderable (up to 0.08). Clinopyroxene in lherzolites is characterized by the high Al2 O3 (2.96–3.72 wt%), low Cr2 O3 (0.91–1.27 wt%) and Na2 O (0.11–0.15 wt%) contents. Clinopyroxene MgO values (17.13–17.97 wt%) in these rocks are low. Al2 O3 and Cr2 O3 contents in clinopyroxene in the lherzolite are low as 0.01–1.19 wt% and 0.04–0.58 wt%, respectively. Clinopyroxene in harzburgites has MgO content of 17.99–19.57 wt% and Na2 O content reaches up to 0.09 wt%. The mantle-derived clinopyroxenes plot in the field corresponding to suboceanic peridotites in the Na vs. Cr diagram of Kornprobst et al. (1981) (Fig. 7).

R. Hajialioghli, M. Moazzen / Journal of Geodynamics 81 (2014) 41–55

5.4. Spinel Spinel is an important accessory mineral in the Piranshahr metaperidotites. Chemically and texturally (see Fig. 6), the investigated spinels can be divided into two categories, primary and secondary spinels (Table 4). The primary spinels in lherzolites have Cr# (Cr/Cr + Al) of 0.39–0.41 which is different from Cr# of spinel in the harzburgites (0.58–0.61). On the Mg# vs. Cr# diagram (Fig. 8a), the primary spinels plot in the field for alpine peridotites, whereas secondary spinels plot in the field for metamorphic ones. The Al2 O3 contents in the primary spinel from lherzolites and harzburgites are 33.39–35.69 wt% and 20.97–22.36 wt%, respectively. Cr2 O3 content in primary spinel of the lherzolites is 33.04–34.48 wt% which considerably differs from its value in the harzburgites (45.35–48.07 wt%). The secondary spinel is Ferich (FeO up 61 wt%) with low Al2 O3 (1.11–16.63 wt%) content. The Al–Cr–Fe trivalent cation diagram shows primary chromian spinel and its alteration products in the Piranshahr metaperidotites (Fig. 8b). The primary spinels show the mantle array affinity on the Al2 O3 vs. Cr2 O3 diagram (after Franz and Wirth, 2000) (Fig. 9a). On diagram of [Mg/(Mg + Fe)] vs. (Cr/Cr + Al) (after Arai, 1994; Barnes and Roeder, 2001), the spinels plot within the ophiolite field (Fig. 9b). The compositional difference between Fe-rich rim and Alrich core suggest that Fe-rich spinel may be formed by Mg–Fe2+ and Al (Cr)–Fe3+ exchanges between spinel and the surrounding silicate minerals during amphibolite facies metamorphism (e.g. Farahat, 2008). Fig. 10 shows clearly replacement of (Al3+ + Mg2+ ) vs. (Cr3+ + Fe2+ ) in the investigated primary spinels. 5.5. Amphibole Amphibole formulae are calculated assuming a total of 16 cations per 23 oxygens (Table 5). The compositional variations in the amphibole are small. Mg and Ca contents are high, up to 5.02 a.p.f.u. and 1.98 a.p.f.u., respectively. The analysed amphiboles can be ascribed to the calcic-amphibole group (Leake et al., 1997) and are tremolite (Leake, 1978).

47

Table 5 Representative mineral chemistry of amphibole (Amp), chlorite (Chl) and antigorite (Atg) in the Piranshahr metaperidotites. Min.

Amp

Chl

Atg

No.

85

156

157

159

Rh

1

SiO2 TiO2 Al2 O3 Cr2 O3 FeO* MnO MgO CaO Na2 O K2 O Total (O) Si Ti Al Cr Fe(II) Mn Mg Ca Na K Total

57.64 0.01 1.11 0.73 2.04 0.03 23.34 13.59 0.29 0.04 98.82 23 7.82 0.00 0.18 0.08 0.23 0.00 4.72 1.98 0.08 0.01 15.10

57.20 0.01 1.41 0.44 1.92 0.02 24.37 12.66 0.41 0.10 98.54 23 7.77 0.00 0.23 0.05 0.22 0.00 4.93 1.84 0.11 0.02 15.17

56.95 0.00 1.38 0.43 1.77 0.09 24.83 12.22 0.26 0.14 98.07 23 7.72 0.00 0.22 0.05 0.20 0.01 5.02 1.78 0.07 0.02 15.09

57.07 0.00 1.38 0.43 1.75 0.05 24.32 12.68 0.40 0.11 98.19 23 7.77 0.00 0.22 0.05 0.20 0.01 4.94 1.85 0.11 0.18 15.33

31.38 0.08 13.59 0.01 14.43 0.27 27.65 0.03 0.04 0.02 87.50 14 2.89 0.00 2.10 0.00 0.22 0.01 4.79 0.00 0.00 0.00 10.01

42.24 0.00 3.72 0.89 4.43 0.07 36.31 0.10 0.03 0.07 87.86 14 1.90 0.00 0.40 0.06 0.16 0.00 2.44 0.00 0.00 0.01 4.97

5.6. Chlorite The analysed chlorites in the serpentinized harzburgites are clinochlore on the basis of classification of Hey (1954) and show small compositional variation. Magnesium is the main occupant of the octahedral site and the concentration of Fe2+ is very small. MnO (<0.27 wt.%), TiO2 (<0.08 wt.%) and Cr2 O3 (<0.01 wt.%) all have low contents (Table 5). 5.7. Antigorite Table 5 contains representative analyses of antigorite in the studied metaperidotites. Relatively high Al2 O3 content of 2.73 wt% can be considered as a result of Tschermak substitution, favoured by

Fig. 3. Raman spectra of (a) antigorite, (b) lizardite and (c) chrysotile in the Piranshahr metaperidotites.

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R. Hajialioghli, M. Moazzen / Journal of Geodynamics 81 (2014) 41–55

Fig. 4. Microphotographs of the serpentinites: (a) lizardite and olivine relics in mesh texture, Plane Polarized Light; (b) bastite after pyroxene, Plane Polarized Light; (c) hourglass texture containing lizardite at the core surrounded by chrysotile at the rim, Cross Polarized Light; (d) antigorite with interpenetrative texture, Cross Polarized Light.

increasing temperatures in this mineral (Li et al., 2004). Cr2 O3 is low (∼0.89 wt%). This can be attributed to the small amounts of clinopyroxene and the olivine-rich nature of the protoliths. Li et al. (2004) noted that serpentine pseudomorphs after olivine contain small amounts of Cr2 O3 . However the composition of the unchanged original olivine shows that the Cr2 O3 content is low. 6. Discussion and conclusions

crystallization for these rocks. It appears that olivine has experienced subsolidus changes more than other mineral phases during peridotites exhumation within the ophiolitic mélange. The highest estimated temperatures lie in the stability field of spinel peridotite defined by Herzberg and Chapman (1976), Akella (1976) and Presnall (1976). Therefore the studied peridotites were stable in the spinel-peridotite condition before the late changes. Estimating the equilibration pressure is problematic due to the lack of suitable mineral paragenesis.

6.1. The peridotites evolution Considering petrography, mineralogy and textural evidence, the crystallization history of the Piranshahr metaperidotites can be discussed in three stages. 6.1.1. Mantle stage (stage 1) Temperature constraints of the harzburgites were estimated using the Ca-in orthopyroxene, olivine–clinopyroxene, olivine–orthopyroxene–spinel, olivine–spinel and orthopyroxene– clinopyroxene thermometers using the PTMAFIC program (Soto and Soto, 1995). In order to avoid effects of disequilibrium due to subsequent metamorphic and alteration processes, only the core compositions of analysed relict minerals were used. The textural relations of the spinel cores and other primary minerals is not clear due to serpentinization, however unaltered spinel cores are taken to be in equilibrium with other primary phase (i.e. olivine and pyroxene). The Ca-in-orthopyroxene thermometer by Brey and Köhler (1990) gives a temperature of 1210 ± 26 ◦ C. The Al/Cr in orthopyroxene (Witt-Eickschen and Seck, 1991) and clinopyroxene (Bertrand and Mercier, 1985) thermometers yield temperatures of 1100 ◦ C. Thermometers involving olivine (e.g. Fabries, 1979; Ballhaus et al., 1991; Gasparik and Newton, 1984) give lower temperature (900–1000 ◦ C). All estimated equilibration temperatures for minerals in the studied peridotites show subsolidus

6.1.2. Serpentinization (stage 2) The Piranshahr peridotites underwent serpentinization. Based on textural observations, olivine and pyroxene are first transformed into lizardite and/or chrysotile with pseudomorphic textures during initial stage of serpentinization (e.g. Viti and Mellini, 1998). They exhibit a pseudomorphic texture, preserving the outlines of fracture patterns and cleavages of olivine and pyroxene. Olivine and pyroxene transformation into lizardite and chrysotile can be explained by the following reaction: Opx + Ol + H2 O + O2 = Lz/Ctl + Mgt

(1)

Also forsterite component of olivine may combine with aqueous SiO2 to form lizardite/chrysotile: 3Fe2 SiO4 + O2 = 2Fe3 O4 + 3SiO2 Fa

fluid

Mgt

(2)

fluid

3Mg2 SiO4 + SiO2 + 4H2 O = 2Mg3 Si2 O5 (OH)4 Fo

fluid

fluid

(3)

Lz/Ctl

Lizardite and chrysotile form directly from olivine at temperatures well below 300 ◦ C (O’Hanley and Wicks, 1995). According to Auzende et al. (2006), chrysotile itself is stable with respect to antigorite only at temperatures <250–280 ◦ C (depending on pressure). Furthermore, chrysotile is only stable in the presence of fluids with very low XCO2 (Hajialioghli et al., 2007).

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49

Fig. 5. Microphotographs of the metaperidotites. (a) Orthopyroxene and clinopyroxene in the low-Cpx lherzolite, Plane Plorized Light; (b and c) clinopyroxene, orthopyroxene and olivine in the harzburgite, Plane Plorized Light; (d) mesh texture of olivine in dunite, Plane Plorized Light; (e and f) reddish brown spinel exhibits dark colour at the rim and along the fractures in the low-Cpx lherzolite and harzburgite. Antigorite occurs around spinel porphyroblasts, Plane Plorized Light.

Antigorite replaces the original lizardite and/or chrysotile to form pseudomorphs during subsequent prograde metamorphism of the Piranshahr metaperidotites. Accurate P–T condition of alteration in the investigated metaperidotites cannot be determined because of lacking of proper mineral assemblages in the low-grade serpentinized metaperidotites. However Auzende et al. (2006) showed that chrysotile is stable with respect to antigorite at temperatures <250–280 ◦ C (depending on pressure and XCO2 ). 6.1.3. Metamorphic stage (stage 3) The peak-metamorphic mineral assemblage formed at this stage includes tremolite, clinochlore, calcite/dolomite and the high-T serpentine polymorph, antigorite. Rare orthopyroxene and small relicts of olivine and spinel have been preserved in these rocks. The spinel composition of the serpentinized ultramafic rocks is a clue to evaluate metamorphic effects (e.g. Kimball, 1990; Burkhard, 1993; Oh et al., 2010). The Mg/(Mg + Fe2+ ) values in the metamorphic spinel-group minerals are dominantly controlled by the metamorphic grade (Barnes, 2000). Spinels metamorphosed under greenschist facies display Mg/(Mg + Fe2+ ) values ranging from 0.4

to 0.7 while those metamorphosed under amphibolite facies have Mg/(Mg + Fe2+ ) values lower than 0.35 (Barnes, 2000). Fig. 8b indicates lower amphibolite facies metamorphism of the Piranshahr metaperidotites on Cr–Al–Fe3+ trivalent cation diagram. Spinel compositions of the investigated metaperidotites at the rim and near to the fractures (Fe-rich) are different from the core compositions (Al-rich) (Fig. 6). The low Mg/(Mg + Fe2+ ) values, up to 0.28 in the Fe-rich spinel rims of the Piranshahr metaperidotites, presumably suggest that the rocks are metamorphosed under amphibolites facies. The relatively higher Fe3+ contents of the Fe-rich spinel (Fe3+ /Fe2+ = 0.27–0.51) indicate that the last stage metamorphism occurred under higher oxidation conditions than the magmatic stages (Fe3+ /Fe2+ = 0.05–0.17). The Al-rich serpentine polymorph, antigorite in the mineral assemblages of the Piranshahr metaperidotites could not have been formed by metamorphism of olivine, pyroxene or even serpentine implying that Al-rich spinel is the only mineral that could have supplied Al, necessary for their formation (e.g. Oh et al., 2010). It is likely that metamorphic Fe-rich spinel rims formed during the metamorphic stage after the serpentinization. During this process, olivines

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R. Hajialioghli, M. Moazzen / Journal of Geodynamics 81 (2014) 41–55

broke down to serpentine and magnetite supplied Fe into spinel by diffusion while Mg and Al in spinel diffused into Al-rich serpentine and chlorite, resulting Fe-rich spinel (e.g. Oh et al., 2010). Antigorite in the Piranshahr metaperidotites also can be formed by transformation of low-T serpentine polymorphs lizardite and chrysotile. Antigorite is stable at higher temperatures (∼250–600 ◦ C; Evans, 2004). Antigorite co-existing with tremolite and clinochlore in mineral assemblages of the investigated metaperidotites as well as the compositional changes at the rims and along fractures of spinels are the supporting evidence of lower amphibolites facies metamorphism.

6.2. Partial melting Mineralogy of the Piranshahr metaperidotites reveals the various degrees of partial melting experienced by the rocks. Low amounts of Al2 O3 in the harzburgites provide strong evidence for high degrees of melt extraction during partial melting (Fig. 11 ). Cr# of the spinel increases with progressive degrees of partial melting, which reduces the Al contents of pyroxene and the host rock (Jaques and Green, 1980; Ohara and Ishi, 1998). The association of these characteristics with high degrees of partial melting is confirmed by higher Cr# in spinels of the harzburgites, which are highly depleted when compared to the low-Cpx lherzolites. Partial melting degree of the Piranshahr metaperidotites can be calculated by using their residual minerals. The composition of primary spinels in upper mantle peridotites is generally highly informative. Cr# of spinel can be used to determine the degree of partial melting experienced by spinel-bearing peridotites (e.g. Dick and Bullen, 1984; Michael and Bonatti, 1985; Arai, 1994; Hellebrand et al., 2001, 2002). Compositions of the low-Cpx lherzolite and harzburgite samples imply that these rocks underwent ∼15–20% to ∼30–35% degrees of partial melting, respectively (Fig. 12a and b). We also calculated the degrees of partial melting based on an equation proposed by Hellebrand et al. (2001) for spinel Cr# values between 0.39 and 0.61. The calculated values correlate more or less with those deduced above. The TiO2 content of residual spinel is expected to decrease with an increase in the Cr# of spinel, as the degree of partial

Fig. 6. SEM image of spinel. Fe-rich spinel at the rim and along the fractures shows light colour.

Fig. 7. Na*1000 a.p.f.u. vs. Cr*1000 a.p.f.u. plot for the mantle-derived clinopyroxenes (Kornprobst et al., 1981). The clinopyroxenes plot in the field corresponding to an oceanic mantle.

Fig. 8. (a) Cr# vs. Mg# diagram showing the chemical composition of primary (magmatic) and secondary (metamorphic) spinels (harzburgite and lherzolite fields after Stevens, 1944; alpine and stratiform fields after Irvine, 1967), (b) Al–Cr–Fe3+ trivalent cation diagrams showing primary chromian spinel and its alteration products. Fields of spinels in forearc peridotites are after Ishii et al. (1992) and podiform chromitites are after Arai and Yurimoto (1994). Thick grey arrowed line indicates a trend of spinel composition change during retrograde transition from upper amphibolite-, lower amphibolite-, to greenschist facies (Müntener et al., 2000).

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51

Fig. 9. (a) Al2 O3 vs. Cr2 O3 (wt.%) diagram of spinel (after Franz and Wirth, 2000) and (b) Mg/(Mg + Fe2+ ) vs. Cr/(Cr + Al) diagram of spinel (after Arai, 1994; Barnes and Roeder, 2001).

6.3. Geodynamic interpretation

Fig. 10. Cation plot for Cr-spinel, which clearly shows (Al3+ + Mg2+ ) for (Cr3+ + FeTotal ) exchange.

Abyssal and SSZ peridotites show distinct mineralogical and geochemical characteristics. Abyssal peridotites generally comprise lherzolites and Cpx-rich harzburgites, formed by MORB-type melt extraction as a result of partial melting of fertile mantle under dry conditions (e.g., Dick and Bullen, 1984; Johnson and Dick, 1992). SSZ peridotites are the remnants of much higher degrees of partial melting of the upper mantle above a subducting slab, containing spinel with higher Cr# (e.g. Aldanmaz et al., 2009). The Piranshahr harzburgites with high Cr# spinel are SSZ harzburgites whereas the low-Cpx lherzolites having relatively low Cr#, correspond to abyssal peridotites (Fig. 12). Olivine in these harzburgites also is characterized by higher forsterite content. Pyroxenes in SSZ-type peridotites are also expected to be depleted in the elements such as Al, Cr and Na due to the higher degrees of partial melting. Orthopyroxene and clinopyroxene in the Piranshahr harzburgites are characterized by considerable low Al2 O3 and Cr2 O3 but higher Mg# (Tables 2 and 3). Based on mineral chemistry of the analysed pyroxenes, the harzburgites clearly plot in the field of forearc peridotites, whereas the low-Cpx lherzolites are abyssal peridotite (Fig. 13). Hence, we infer that the Piranshahr peridotites show the characteristics of both abyssal and SSZ type peridotites, which show many similarities with equivalent rocks from the Kermanshah (Allahyari et al., 2010; Saccani et al., 2013), Neyriz (Rajabzadeh and Nazari Dehkordi, 2012) and Nain (Mehdipour Ghazi et al., 2010) ophiolites (see Fig. 12).

6.4. Tectonic model for the Piranshahr ophiolites

Fig. 11. TiO2 (wt%) vs. Al2 O3 (wt%) for the clinopyroxene from the Piranshahr metaperidotites. Fields of MOR and SSZ peridotites are from Johnson et al. (1990) and Ishii et al. (1992), respectively.

melting increases (e.g. Pearce et al., 2000). TiO2 contents of spinel in the Piranshahr harzburgites do not show enrichment with increasing Cr#. This can be interpreted by subsequent reaction of peridotites with island arc tholeiite (IAT) melts (e.g. Choi et al., 2008) during subduction process. The low Fe3+ contents of the Cr-spinels in the investigated rocks indicate relatively low oxygen fugacity conditions for their primary source.

Formation of SSZ ophiolites is commonly attributed to further melting of depleted mantle by injection of hydrous melts above subduction zones (e.g. Kubo, 2002). SSZ complexes are far better represented and preserved than MORB ophiolites in the orogenic belts. Many of the Late Cretaceous ophiolites are located along the bordering Northern Arabian margin, through Iran, eastern Turkey and northern Syria (Ricou, 1971; Glennie, 2000), up to, but not including, Cyprus (Robertson, 2004). Parkinson et al. (1992), Parkinson and Pearce (1998), and Pearce et al. (2000) have shown that both SSZ and abyssal peridotites may occur in forearc tectonic settings although the former are typically dominant. Robertson (2004) considered the major Cretaceous ophiolites of the Mediterranean and Middle East as intra-oceanic subduction zones. Recent studies show that most of the eastern Mediterranean ophiolites provide the best geological evidence for the both abyssal and supra-subduction nature of the Tethyan ophiolitic belt by a major subduction initiation event. Some of examples include the Albania and Greece (Dinaric and Hellenic ophiolitic belt; Bortolotti et al., 2013), SW Turkey (Lycian

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R. Hajialioghli, M. Moazzen / Journal of Geodynamics 81 (2014) 41–55

Fig. 12. (a) Plots of Cr# of spinel vs. Fo content of coexisting olivine. Olivine–spinel mantle array and melting trend are from Arai (1994). Abyssal peridotite field from Dick and Bullen (1984), oceanic SSZ peridotite field from Pearce et al. (2000). (b) Spinel compositions plotted on a Mg# vs. Cr# diagram. Spinel fields of abyssal (Dick and Bullen, 1984; Arai, 1994) and forearc peridotites (Ishii et al., 1992; Parkinson and Pearce, 1998) are shown for comparison. Partial melting trend (arrow) is from Arai (1992).

and Antalya ophiolites; Aldanmaz et al., 2009), NW Turkey (Harmancik ophiolites; Uysal et al., 2009, 2013), Zagros Zone of Iran (Kermanshah and Neyriz ophiolites; Allahyari et al., 2010; Saccani et al., 2013; Rajabzadeh and Nazari Dehkordi, 2012) and Oman ophiolites (Tamura and Arai, 2006; Dare et al., 2009; Goodenough et al., 2010).

The Piranshahr peridotites in northwestern end of the Zagros orogen have distinct mineral chemical compositions, suggesting their formation in two distinct stages. In the first stage, the low-Cpx lherzolites were produced as the residues of anhydrous MOR-type melting beneath the mid-ocean ridge system within the southern branch of Neotethys. This seafloor system evolved between

Fig. 13. (a and b) Cr2 O3 and Al2 O3 vs. Mg# in orthopyroxene and (c and d) Cr2 O3 and Al2 O3 vs. Mg# in clinopyroxene. Low-Cpx lherzolites and harzburgites plot in abyssal and forearc peridotite fields, respectively. The fields for abyssal and forearc peridotites are from Johnson et al. (1990) and Ishii et al. (1992).

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Fig. 14. (a–c) Petrogenetic–tectonic model for the Pirnshahr ophiolite in the southern branch of Neotethys (see Section 6).

the Arabian Plate and Central Iran microcontinent during the early Mesozoic (Fig. 14a). Nearly 15–20% anhydrous, decompressional partial melting of a MOR mantle at the spreading centre formed the low-Cpx lherzolites at this stage. According to Gealey (1988), subduction of Neotethys between the Arabian plate and Iranian microplate started from Late Jurassic (?)-Early Cretaceous caused regional metamorphism and associated magmatism in the Sanandaj-Sirjan Zone. The previously formed MOR oceanic lithosphere was subsequently emplaced in the upper plate of a NEdipping subduction zone (Fig. 14b). In this stage, more refractory harzburgites formed as residues after ∼10–15% partial melting of a depleted MORB source as a result of subduction initiated magmatism. The evidence for a supra-subduction zone origin of these harzburgites is largely based on mineral chemistry of the samples, including the high Cr# of the spinels and high Mg# but low Al2 O3 and Na2 O content of pyroxenes in the investigated harzburgites. The low-Cpx lherzolites in our study area are thus genetically related to previously formed abyssal peridotites subsequently emplaced in a forearc region. Slab-derived fluids, particularly originated from the dehydrated serpentinites in the downgoing slab

(inset in Fig. 14b) percolated through the peridotites in the mantle wedge and affected their host mantle rocks heterogeneously at all scales. Some of the abyssal peridotites trapped in this wedge, preserving their original geochemical fingerprints (Fig. 14b). During the Late Cretaceous, obduction occurred on the northeastern margins of the Arabian plate (Berberian and King, 1981; Stöcklin, 1977; Golonka, 2004). Terminal closure and continental collision of Arabia/Central Iran micro-continent were later, as late as Miocene (Guest, 2004; Homke et al., 2004; Allen et al., 2004; S¸engör et al., 2008) or Mio-Peliocene (Agard et al., 2005) (Fig. 14c). The model proposed here for the Piranshahr ophiolites is in accordance with the model proposed by Allahyari et al. (2010) and Saccani et al. (2013) for the geodynamic evolution of the Kermanshah ophiolite. Concerning metamorphic evolutions of the Piranshahr metaperidotites, we interpret the development of pseudomorphic textures as mesh texture after olivine and bastites after pyroxene consistent with ridge-related ocean-floor serpentinization. The subsequent metamorphic stage reflects a re-heating of the hydrated rocks to lower amphibolite facies, resulting in crystallization of the minerals formed at peak-metamorphic conditions.

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The emplacement of ophiolites formed in the intra-oceanic setting onto the Arabian margin is believed to take place well before the complete ocean closure and continental collision (Dilek et al., 2010). However, no general consensus exists on this model. Three possible mechanisms can be postulated for the metamorphism of the ophiolitic complex: metamorphism during downgoing plate (which would affect the low-Cpx-lherzolites only), metamorphism during ophiolite obduction onto the north margin of the Arabian plate and metamorphism during continental collision and emplacement of the Sanandaj-Sirjan domain over the ophiolite nappe. Since metamorphic phases such as antigorite, metamorphosed spinel, tremolite and clinochlore appear in both harzburgite and lherzolite, it can be deduced that the prograde regional metamorphism presumably occurred in an orogeny in which the Piranshahr metamorphic complex was assembled. It is likely that continental collision of this orogenic phase caused the lower amphibolite-facies metamorphism of the investigated metaperidotites. The age of the orogeny responsible for this metamorphism is not known. Acknowledgments We thank Dr. R. Milke from Freie Universität Berlin for his assistance with microprobe analysis. A.A. Khodabandeh from Geological Survey of Iran helped with the field works. Thoughtful review of the manuscript by two anonymous referees of the journal is highly appreciated. We are grateful to Dr. Schellart for his generous helps. This research is supported by the University of Tabriz through the grant No. 12/1237. References Agard, P., Omrani, J., Jolivet, L., Mouthereau, F., 2005. Convergence history across Zagros (Iran): constraints from collisional and earlier deformation. Int. J. Earth Sci. 94, 401–419. Akella, J., 1976. Garnet pyroxene equilibria in the system CaSiO3 –MgSiO3 –Al2 O3 and in a natural mineral mixture. Am. Mineral. 61, 589–598. Albaster, T., Pearce, J.A., Malpas, J., 1982. The volcanic stratigraphy and petrogenesis of the Oman ophiolite. Contrib. Mineral. Petrol. 81, 168–183. Aldanmaz, E., Schmidt, M.W., Gourgaud, A., Meisel, T., 2009. Mid-ocean ridge and supra-subduction geochemical signatures in spinel-peridotites from the Neotethyan ophiolites in SW Turkey: implications for upper mantle melting processes. Lithos 113, 691–708. Allahyari, K., Saccani, E., Pourmoafi, M., Beccaluva, L., Masoudi, F., 2010. Petrology of mantle peridotites and intrusive mafic rocks from the Kermanshah ophiolitic complex (Zagros belt, Iran): implications for the geodynamic evolution of the Neo-Tethyan oceanic branch between Arabia and Iran. Ofioliti 35, 71–90. Allahyari, K., Saccani, E., Rahimzadeh, B., Zeda, O., 2013. Mineral chemistry and petrology of highly magnesian ultramafic cumulates from the Sarve-Abad (Sawlava) ophiolites (Kurdistan, NW Iran): new evidence for boninitic magmatism in intraoceanic fore-arc setting in the Neo-Tethys between Arabia and Iran. J. Asian Earth Sci. 79, 3120328. Allen, M., Jackson, J., Walker, R., 2004. Late Cenozoic reorganization of the Arabia–Eurasia collision and the comparison of short-term and long-term deformation rates. Tectonics 23, http://dx.doi.org/10.1029/2003TC001530, 16 pp. Arai, S., 1992. Chemistry of chromian spinel in volcanic rocks as a potential guide to magma chemistry. Mineral. Mag. 56, 173–184. Arai, S., 1994. Characterization of spinel peridotites by olivine–spinel compositional relationships: review and interpretation. Chem. Geol. 113, 191–204. Arai, S., Yurimoto, H., 1994. Podiform chromitites of the Tari–Misaka ultramafic complex, southwestern Japan, as mantle–melt interaction products. Econ. Geol. 89, 1279–1288. Aswad, K.J.A., Aziz, N.R.H.A., Koyi, H.A., 2011. Cr-spinel compositions in serpentinites and their implications for the petrotectonic history of the Zagros Suture Zone, Kurdistan Region, Iraq. Geol. Mag. 148 (5/6), 802–818. Auzende, A.L., Guillot, G., Devouard, B., Baronnet, A., 2006. Behavior of serpentinites in convergent context: microstructural evidence. Eur. J. Mineral. 18, 21–33. Aziz, N.R.H., Elias, E.M., Aswad, K.J., 2011. Rb–Sr and Sm–Nd isotope study of serpentinites and their impact on the tectonic setting of Zagros Suture Zone, NE-Iraq. Iraqi Bull. Geol. Min. 7, 67–75. Ballhaus, C., Berry, R.F., Green, D.H., 1991. High pressure experimental calibration of the olivine–orthopyroxene–spinel oxygen barometer: implications for the oxidation of the mantle. Contrib. Mineral. Petrol. 107, 27–40. Barnes, S.J., 2000. Chromite in komatiites. II. Modification during greenschist to midamphibolite facies metamorphism. J. Petrol. 41, 387–409. Barnes, S.J., Roeder, P.L., 2001. The range of spinel compositions in terrestrial mafic and ultramafic rocks. J. Petrol. 42, 2279–2302.

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