40 Ar/ 39 Ar mineral ages of eclogites from North Shahrekord in the Sanandaj–Sirjan Zone, Iran: Implications for the tectonic evolution of Zagros orogen

    40 Ar/39 Ar mineral ages of eclogites from North Shahrekord in the Sanandaj–Sirjan Zone, Iran: Implications for the tectonic evolution of Zagros orogen A.R. Davoudian, J. Genser, F. Neubauer, N. Shabanian PII: DOI: Reference: S1342-937X(16)30111-3 doi: 10.1016/j.gr.2016.05.013 GR 1637 To appear in: Gondwana Research Received date: Revised date: Accepted date: 18 November 2015 13 May 2016 18 May 2016 Please cite this article as: Davoudian, A.R., Genser, J., Neubauer, F., Shabanian, N., Ar/39 Ar mineral ages of eclogites from North Shahrekord in the Sanandaj–Sirjan Zone, Iran: Implications for the tectonic evolution of Zagros orogen, Gondwana Research (2016), doi: 10.1016/j.gr.2016.05.013 40 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT 40 Ar/39Ar mineral ages of eclogites from North Shahrekord in the Sanandaj–Sirjan T Zone, Iran: Implications for the tectonic evolution of Zagros orogen 1 RI P A.R. Davoudian1*, J. Genser2, F. Neubauer2 and N. Shabanian1 Faculty of Natural Resources and Earth Sciences, Shahrekord University, Shahrekord, Iran Department Geography and Geology, University of Salzburg, Salzburg, Austria NU 2 SC alireza.davoudian@gmail.com Abstract MA Eclogites are high-pressure/low-temperature metamorphic rocks and are regularly considered as an indicator of ancient subduction zones. Eclogites have recently been found in the North ED Shahrekord metamorphic complex (NSMC) of the Sanandaj-Sirjan zone and represent the PT only ones within the Zagros orogen. Their occurrence and timing are important for the reconstruction of convergence history and geodynamic evolution of the Neo-Tethys Ocean 40 AC CE and Zagros orogen. White mica from the eclogites and an associated paragneiss give Ar/39Ar ages ranging from 184.3 ± 0.9 to 172.5 ± 0.8 Ma and represent the age of cooling through the closure temperature for phengitic white mica. The NSMC also comprise the ductile NW-SE trending North Shahrekord Shear Zone (NSSZ), which is located in the northeast of the Main Zagros Reverse Fault. The NSMC consists mainly of various metasedimentary rocks, orthogneiss and small-sized bodies of metabasic rocks containing also the eclogites. Furthermore, pre-metamorphic granitoids represent part of the NSMC. The North Shahrekord eclogites are composed of garnet, omphacite, zoisite, Ca-Na amphibole, phengite and rutile. The highly deformed and metamorphosed granitoids yield hornblende and biotite 40Ar/39Ar ages 170.1± 0.9 Ma and 110.7 ± 0.3 Ma, respectively. According to the new age dating results of eclogites, the rocks are the oldest high-pressure metamorphic rocks in the Zagros orogenic belt testifying the Neo-Tethys Ocean subduction. Our new data 1 ACCEPTED MANUSCRIPT indicate that the eclogites formed during Early Jurassic subduction of a Panafrican microcontinental piece from the northern margin of the Neo-Tethyan Ocean under the Central T Iranian microplate. We suggest that initiation of subduction in Neo-Tethyan Ocean occurred RI P a few million years prior to 184 Ma (Pliensbachian stage). Introduction NU 1. SC Keywords: Zagros orogen, 40Ar/39Ar dating, Neo-Tethys, Sanandaj-Sirjan Zone, eclogite. MA The Alpine–Himalayan convergence belt, as one of the largest continuous orogenic belt in the world, represents a classic continental collision orogen including suture zones and ED micro-continental blocks, which extends from the eastern Mediterranean area to the Himalayas and formed as a consequence of closure of the Neo-Tethyan Ocean (Dercourt et PT al., 1986; Ricou, 1994; Stampfli and Borel, 2002; Rolland et al., 2009; Trifonov et al., 2012; Tian et al., 2015) (Fig. 1). The Zagros orogen is an important central part of the Alpine- AC CE Himalayan orogenic belt and includes the ophiolite-bearing Bitlis–Zagros Suture Zone as the main continental collision zone (Fig. 1). The remnants of the ophiolite can be traced from Oman to southwestern Iran and to southeastern Turkey over a distance more than 2500 km along the Bitlis-Zagros suture zone and the Semail-Oman ophiolite (Goffé et al., 1988; Dilek and Furnes, 2009; Okay et al., 2011; Ao et al., 2016). The formation of the Zagros orogen resulted from the long-standing convergence between the Iranian edge of Eurasia and Arabian plate as a part of Gondwana (Agard et al., 2011). The Neo-Tethyan oceanic gateway closure during the Arabia-Eurasia collision resulted in isolation of the Mediterranean Sea and Indian Ocean (Allen and Armstrong, 2008; Okay et al., 2011). The Zagros orogen consists of several parallel NW-SE trending tectonic units, which are from the northeast to southwest: (1) the ca. 150 km wide Urumieh-Dokhtar 2 ACCEPTED MANUSCRIPT Magmatic Arc forms a subduction-related Andean-type plutonic and volcanic arc during Eocene to Quaternary (Schröder, 1944; Förster, 1974; Berberian and King, 1981; Berberian T et al., 1982; Alavi, 2004), (2) the Sanandaj-Sirjan Zone (SSZ; Stöcklin, 1968) mainly RI P includes metamorphic complexes and granitic intrusions, the zone represents the internal magmatic and metamorphic part of the Zagros orogen (Agard et al., 2005; Mohajjel and SC Fergusson, 2014; Azizi et al., 2015; Shakerardakani et al., 2015; Shafaii Moghadam et al., 2015), (3) the Main Zagros Reverse fault (MZRF) or the Main Zagros Thrust (MZT), which NU is proposed to be the suture zone between the Arabian plate as a part of Gondwana and MA Eurasia, (4) the High Zagros (including the Crush zone) with imbricated tectonic slices comprising Mesozoic limestones, radiolarites, obducted ophiolite remnants (Agard et al., ED 2005), (5) the Zagros Simply Folded Belt, and (6) the Mesopotamian-Persian Gulf foreland basin (Berberian and King, 1981; Alavi, 1994; Mohajjel and Fergusson, 2000). East of the PT Hormuz Strait, the Main Zagros thrust is transformed through a major transform fault into a north-dipping active subduction zone where the oceanic crust of the Gulf of Oman is being AC CE subducted beneath the Makran (Farhoudi and Karig, 1977; Okay, 1989). High-pressure metamorphic rocks such as eclogites and blueschists are found in many orogenic belts and are generally considered as the product of subduction-related metamorphism (Agard et al., 2009; Smye et al., 2010; Ota and Kaneko, 2010; Cheng et al., 2015). Therefore, eclogites as useful resource provide key information for reconstructing the geodynamics of orogenic belts (Amaral et al., 2011; Liao et al., 2016), and may record subduction/continental collision and subsequent exhumation events and thus provide important information on the tectonothermal evolution of mountain chains (Liati et al., 2016; Klemd et al., 2015). Although a critical stage for understanding geodynamic processes, constraining the timing of high-pressure metamorphism, however, is known to be difficult. Rb-Sr and U-Pb zircon dating is often impossible in mafic eclogites because of the low Rb/Sr 3 ACCEPTED MANUSCRIPT ratios of phases suitable for dating (e.g. phengite) and the rare occurrence of zircon (e.g., Miller et al., 2005). Ar/39Ar geochronology is commonly applied to white micas in HP– LT metamorphic T 40 content is well suited to the 40 RI P rocks such as eclogites. White micas are often abundant in eclogite rocks, and their high-K Ar/39Ar technique (Sherlock and Kelley, 2002). On the other SC hand, as an important advantage, the Ar isotope system is less sensitive to modification during overprint than the Rb–Sr system (Bröcker et al., 2004). A disadvantage is that NU excess/extraneous Argon can enter the lattice of white mica, peculiarly in the case of MA polymetamorphic units (e.g., Sherlock and Arnaud, 1999; Baxter, 2003; Di Vicenzo et al., 2006; Qiu et al., 2008, 2010; Warren et al., 2012; Smye et al., 2013). This was recently ED explained as result of devolatilization of polymetamorphic complexes (Smye et al., 2013). Prior to the discovery of the eclogites in the North Shahrekord metamorphic complex PT (Davoudian et al., 2008), one of the most important enigmatic feature in Zagros orogen was the lack of the high-pressure metamorphic rocks related to the subduction of the Neo-Tethys AC CE Ocean. Only a few blueschists are found along the 1600-km long Main Zagros thrust, which forms the main Neo-Tethyan suture between Gondwana and Eurasia (Agard et al., 2006; Monié and Agard, 2009). The aim of this study is to present the first 40Ar/39Ar mineral ages from the eclogites and their associated paragneisses from the NSMC in the Sanandaj-Sirjan Zone (SSZ) as part of Zagros orogen. Finally, we consider the implications of our new findings for the tectonic evolution of the Zagros orogen and onset and mode of subduction of the Neo-Tethys Ocean under the SSZ during convergence between Arabia and Eurasia. 4 ACCEPTED MANUSCRIPT 2. Regional geology of Sanandaj-Sirjan Zone The study area is a part of the Sanandaj-Sirjan Zone (SSZ) (Stöcklin, 1968) or Imbricate T Zone of the Zagros Orogen (Alavi 1994, 2004) (Fig. 2). The SSZ is the tectono-magmatic and RI P metamorphic part of the Zagros orogeny (Stöcklin, 1968; Agard et al., 2005). After Late Triassic times, the Sanandaj-Sirjan Zone has been suggested to become an active margin SC associated with an accretionary prism (Sheikholeslami et al., 2008; Hassandzadeh and Wernicke, 2016). It is made of sedimentary Paleozoic to Cretaceous and both HP/LT and NU HT/LP metamorphic rocks formed in an accretionary prism located to the south of the Iranian MA microcontinent (Mouthereau, 2011). One of the main difficulties about the geological history of the Zagros Orogenic Belt is ED the tectonic setting of the SSZ (Ghasemi and Poor Kermani, 2009). The formation of Sanandaj-Sirjan Zone with the 1500 km length, 150 to 200 km width, was related to the PT Permian opening of the Neo-Tethys Ocean (Muttoni et al., 2003) and its subsequent subduction during Mesozoic to Cenozoic convergence and continental collision between the AC CE Afro-Arabian and the Eurasian plates (Mohajjel et al., 2003; Agard et al., 2005; Ghasemi and Talbot, 2006; Moritz et al., 2006). The timing of continental drift of the Central Iran and Arabian plates has been attributed to Permian to Late Triassic (e.g. Ghasemi and Talbot, 2006), Late Triassic to Early Jurassic (e.g. Agard et al., 2005; Davoudzadeh and Schmidt, 1984; Stampfli and Borel, 2002), or Middle Jurassic (Fazlnia et al., 2009). Okay and Tüysüz (1999) propose that the Neo-Tethys opened a separate ocean during the Early Triassic times. Therefore, most authors have suggested that opening of Neo-Tethys Ocean occurred in Late Triassic or Early Jurassic (e.g. Sengör, 1984; Stampfli, 2000; Stampfli and Borel, 2002). Some authors have proposed that the Sanandaj–Sirjan plate began to thrust over the Arabian Platform, forming the Zagros Mountains during Oligocene–Middle Miocene (e.g. Dercourt et al., 1993; Golonka, 2004). Recent models center the timing at around 30 ± 5 Ma, 5 ACCEPTED MANUSCRIPT Oligocene (François et al., 2014a). The main cause of thrusting in the Zagros Mountains, according to Sengör and Natalin (1996), was the closure of the Neo-Tethys Ocean and T counterclockwise rotation of the Arabian plate. RI P The Sanandaj-Sirjan Zone is composed of numerous imbricated tectonic slices of Panafrican, Paleozoic to Paleogene units, which were thrusted onto the High Zagros Belt SC (Agard et al., 2005; Nadimi and Konon, 2012; Shakerardakani et al., 2015). The southwestern edge of the zone is characterized by several ophiolitic units that record the geodynamic NU evolution of a Neo-Tethyan oceanic branch located between the Arabian shield (part of MA Gondwana) and the Sanandaj–Sirjan continental block of Iran (Saccani et al., 2013). Obduction of the Neyriz and Kermanshah ophiolites occurred along the southwestern margin ED during the Late Cretaceous (e.g. Babaie et al., 2006; Shahidi and Nazari, 1997; Whitechurch et al., 2013). PT In the Golpaygan area, the SSZ can be subdivided into the two parts (Eftekharnejad, 1981; Ghasemi and Talbot, 2006; Arfania and Shahriari, 2009): (1) the southern part consists AC CE of rocks deformed and metamorphosed in Middle to Late Triassic with granitoid complexes (such as, the Siah-Kuh granitoid stock; Arvin et al., 2007); (2) the northern part, deformed during the Jurassic to Late Cretaceous and contains many intrusive felsic rocks (such as Alvand, Boroujerd, Arak, Malayer, Azna, Boein and Golpaygan plutons mainly of Jurassic age). Jurassic to Cretaceous subduction-related, calc-alkaline volcanic and granitic rocks are reported from the Sanandaj–Sirjan Zone (e.g., Fazlnia et al., 2009; Shahbazi et al., 2010; Esna-Ashari et al., 2012; Mahmoudi et al., 2011). Berberian (1983) considered this zone to have been a Middle Jurassic to Cretaceous Andean-type magmatic margin and a Cenozoic fore-arc basin. The narrow arc-trench gap in this belt indicates a steep subduction zone (Isacks and Barazangi, 1977; Berberian and Berberian, 1981), which has metamorphosed parts of the SSZ (Fazlnia et al. 2009). Alternatively, Allen et al. (2013) pointed out that the 6 ACCEPTED MANUSCRIPT Mesozoic subduction-related magmatism is relatively small within the zone, perhaps implying periods of slow and/or flat slab subduction. T The SSZ mainly consists of metamorphic complexes and granitic intrusions (Jamshidi RI P Badr et al., 2013) in the complexly deformed sub-zone. With considering rare occurrences of high-pressure metamorphic rocks (Davoudian et al., 2008), the most of the metamorphic SC complexes shows a range of metamorphism from greenschist to amphibolite facies (Mohajjel and Fergusson 2000; Ghasemi and Poor Kermani, 2009). The amphibolite-facies rocks of the NU metamorphic complexes have mainly undergone a retrograde metamorphism and deformation MA under greenschist facies conditions (e.g. Alizadeh et al., 2010 for the Tutak complex; Sheikholeslami et al., 2008 for the Neyriz metamorphic complex; Moritz et al., 2006 for the ED metamorphic Muteh complex; Davoudian et al., 2008 for the NSMC). PT 3. Geological setting In the study area, several major NW–SE trending faults are parallel to the Main Reverse AC CE Zagros Fault and include the Dalan, Sheida, and Ben Faults (Fig. 3). These faults exhibit reverse displacement with a dextral strike-slip component and dip toward the NE, excluding the SW-dipping Dalan Fault (Babaahmadi et al., 2012). According to the array of faults, geological characteristics and stratigraphic similarities, the area is divided into three marginal zones, the Northern, Central and Southern zones (Ghasemi et al., 2005) (Fig. 3), all representing parts of SSZ. The Southern zone is between the Main Reverse Zagros and Ben Faults, while the Northern zone is located in the northeast of the Dalan Fault. Both marginal Southern and Northern zones consist of sedimentary rocks mainly including Cretaceous limestones and Jurassic units (sandstone, siltstone, conglomerate and limestone) and Eocene conglomerate. The Central zone includes high- and low-grade metamorphic rocks and plutonic, volcanic and volcano-sedimentary rocks and is bordered by the Dalan and Ben 7 ACCEPTED MANUSCRIPT Faults in the northeast and southwest, respectively. The oldest sedimentary rocks of the zone are Permian limestones, with Late Permian fossils (Ghasemi el al., 2005), which are thrusted T over Jurassic units by the Sheida Fault (Babaahmadi et al., 2012). The Jurassic units include RI P volcanic-sedimentary strata, basalt, andesite, pyroclastic rocks, shale, sandstone and limestone. The volcanic rocks formed in Late Jurassic time (148.2 ± 0.9 Ma, 40 Ar/39Ar SC hornblende; Emami, 2008). Most volcanic rocks show evidence of low-temperature metamorphism (prehnite–pumpellyite facies), but locally lower greenschist facies mineral NU assemblages including actinolite, epidote and chlorite were observed especially in basaltic MA rocks. The NSMC is characterized by the extensive effects of a WNW–ESE trending large-scale ED ductile North Shahrekord Shear Zone (NSSZ). The exposures of the shear zone extend along Zayandeh-Rood River, from Sadegh-Abad village toward west-northwest to Chadegan town PT (Fig. 4). The shear zone is sub-parallel to the Main Zagros Reverse Fault. The cataclastic rocks along reverse faults define the borders of the shear zone. The shear zone's southern AC CE boundary is defined by the NW-striking Yancheshme Fault (Fig. 4), which separates the metamorphic rocks from the Late Jurassic volcanic and sedimentary units at near SadeghAbad village. The NSMC is made up of three main units including a high-grade and a low-grade metamorphic unit and a deformed metagranitoid unit. Permian limestone are thrusted over these units (Fig. 4). The first unit contains the highest grade metamorphic rocks of the complex and can be subdivided into three sub-units: (1) one comprising paragneisses, calcschists and marbles with blocks or lenses of more and less retrogressed eclogites and amphibolites, (2) another one contains quartzofeldspathic schists, amphibolites, garnet amphibolites, calc-schists, marbles, graphitic schists and mica-schists, and (3) Panafrican orthogneisses (U-Pb zircon age of 569 ±13 Ma; Davoudian et al., manuscript in preparation) 8 ACCEPTED MANUSCRIPT with lenses of amphibolites. In fact, the sub-units 1 and 2, including high-grade metamorphic rocks, are intruded by the Pan-African orthogneisses (sub-unit 3). Therefore, we consider that T these three sub-units of the NSMC represent Pan-African basement. RI P The low-grade unit comprises micaschists, metapsammites, phyllites, marbles and metadolerites. The highly deformed metagranitoids unit consists of many small and medium- SC sized granitoid plutons that have intruded into the other metamorphic rocks (Fig. 4). The metagranitoids mainly contain the assemblage quartz + K-feldspar + plagioclase + biotite + NU hornblende + allanite + sphene + magnetite + epidote + apatite + zircon ± garnet (Davoudian, MA 2010). They are mostly exposed south of Zayandeh-Rood Lake, forming elongate bodies parallel to the WNW-ESE-trend of the NSSZ. Most metagranitoid bodies and other ED metamorphic rocks are more or less subjected to ductile deformation and are highly folded (Fig. 5a); therefore, most of eclogites and associated rocks have undergone mylonitization PT causing a mylonitic foliation and lineation (Davoudian et al., 2008). The eclogitic metabasites are the only known eclogites of the Zagros orogen. The AC CE presence of these high-grade metamorphic complex juxtaposed against low-grade metamorphic rocks could be related to the deep shear zone that was active and was exhumed during an initial stage of orogeny during Jurassic or Cretaceous times. Eclogites occur as numerous meter- to decameter-sized lenses with paragneisses and marbles (Fig. 5b, c). A few eclogites occur as large bodies several tens of square meters in size (Fig. 5d). Sometimes, the eclogites show pillow-shaped structures (Fig. 5f) indicating sea floor volcanism. Generally, the foliation of eclogites, retrogressed eclogites and paragneisses is folded and eclogites have a well-defined foliation and mineral lineation defined by high-P minerals (e.g. omphacite, zoisite, phengite and amphibole) (Fig. 5f) defining an L>S fabric. A penetrative stretching lineation also occurs in paragneisses encasing the eclogites (Fig. 5g). Lineations measured in the paragneisses outside the eclogite bodies are parallel to those 9 ACCEPTED MANUSCRIPT inside the eclogite bodies, and are well-defined by arrangements of quartz or feldspar grains. The foliation of paragneiss is defined by minerals of phengite and biotite. In some cases, the T main foliation of eclogites is cross-cut by veins of plagioclase or fibrous calcite (Fig. 5h) due RI P to extensive propagation of fluids into the eclogites during exhumation events. SC 4. Eclogites in the Northern Shahrekord 4.1 Petrography NU In order to determine the age of high-pressure metamorphism, two fresh eclogites MA (samples M40-14 and M40-57), four slightly retrogressed eclogites (samples M38-5, M3811, M40-13 and M40-2) and a paragneiss associated with the eclogites (sample S19-2) from 40 Ar/39Ar geochronology. A detailed petrological ED the northern Shahrekord were chosen for discussion of the eclogites from the study area was presented in Davoudian et al. (2008). We by the 40 PT will limit our discussion here to the seven samples, from which white mica grains were dated Ar/39Ar technique. The observed textures and mineral assemblages of the studied AC CE eclogites display no pervasive greenschist-facies overprint. Sample M40-14 is a fresh mafic eclogite with no secondary alteration products especially no plagioclase. The peak pressure assemblage includes garnet, omphacite, zoisite, phengite, sodic-calcic amphibole (barroisite and alumino-barroisite), quartz and rutile. The omphacite grains as large as 700 µm in size are abundant and have a Jd37-47Ae0-2 Q53-61 composition (Fig. 6a), the maximum jadeite content in omphacite from the eclogites of North Shahrekord was observed in this sample. Garnets with an idiomorphic shape (Fig. 6a) are abundant and generally inclusion-free but when present the inclusion population consists of rutile, quartz and white mica. The sample abundantly has up to 2 mm large zoisite (epidote) crystals, so that this sample M40-14-2 is an epidote-(zoisite-)eclogite. The foliation is defined by coarsegrained zoisite grains, up to 800 µm long phengite flakes and omphacite with rutile (Fig. 6b). 10 ACCEPTED MANUSCRIPT Most rutile grains are mantled by sphene (Fig. 6c). In general, phengites are classified as two groups, I and II. Phengites I are mostly fresh without alteration, and symplectite aggregates T around the group I phengites are rare (Fig. 6d). Phengites II are in part slightly affected by RI P symplectic intergrowth with biotite and feldspar. Sample M40-57 is a fine- to medium-grained eclogite with a granoblastic texture. The SC eclogite consists of garnet (abundant, up to 400 µm), omphacitic clinopyroxene (up to 300 µm), phengitic mica, sodic-calcic amphibole, quartz, zoisite, as well as rutile, sphene and NU carbonate. Rarely, plagioclase, calcic-amphibole and biotite occur as secondary alteration MA products along grain boundaries. Plagioclase grains enclose inclusions such as amphibole, garnet, omphacite and rutile mantled by sphene. The phengitic mica grains with a size up to ED 300 µm display no alteration and are without symplectitic texture, only they are slightly replaced by minor biotites along rims and cleavage (Fig. 6e). PT Sample M40-2 is a weakly foliated eclogite with slight retrogression and contains sodiccalcic amphiboles (barroisite, katophorite, taramite), omphacite with maximum 46 mol% AC CE jadeite, paragonitic white mica, garnet, zoisite, dolomite, and minor sphene and rutile. Calcite and albite with calcic-amphibole (edenite) are present as secondary phases. Albite forms up to 1 mm large poikiloblasts containing numerous inclusions of garnet, sodic-calcic amphibole, carbonate, paragonite, omphacite, zoisite and minor amounts of rutile mantled by sphene (Fig. 6f). White mica flakes of 300 µm in size are euhedral to subhedral and some of them are enclosed by plagioclase. Coarse sphene grains are abundant as individual phase or form a mantle around rutile. In the sample, the main foliation is sometimes cross-cut by veins of fibrous calcite. Sample M40-13 is an eclogite, in which the high-pressure assemblage including omphacite, garnet and phengitic white mica is accompanied by symplectite rims and other replacement products along grain boundaries of the eclogite facies minerals. The rock is 11 ACCEPTED MANUSCRIPT foliated and sodic-calcic amphiboles and zoisite form the main foliation. Phengite grains are small (300 µm in size). Garnet grains are mostly free of inclusions and are partly replaced by T zoisite (clinozoisite) and calcic amphibole. In some cases, zoisite forms isolated aggregates. RI P The sample shows an ESE-trending mineral respectively stretching lineation. Sample M38-5 is an epidote eclogite consisting of garnet, zoisite, sodic-calcic amphibole, SC omphacite, phengitic white mica, dolomite as well as rutile, quartz and calcite. Plagioclase, calcic amphibole, biotite and sphene occur as secondary phases. The sample shows a slight NU retrogression to amphibolite facies, in which the overprinting amphibolite facies mineralogy MA is limited to plagioclase and calcic amphibole and minor biotite. Most phengitic white mica grains (with a size of 0.5 to 0.7 mm) are inclusion-free, but phengites contain minor ED inclusions of rutile, zoisite and very fine zircon, also fine-grained, up to 100 µm long phengites are enclosed by plagioclase. Biotite as secondary phase is observed along grain PT boundaries and cleavages of some phengite grains. The fresh phengites without replacement are enclosed by other eclogite facies minerals especially within zoisite. In the sample, foliation. AC CE elongated and zoned zoisite grains with sodic-calcic amphibole form a weakly developed The sample M38-11a is a strongly foliated eclogite. The mylonitic foliation is mostly defined by medium- to coarse-grained sodic-calcic amphibole (kataphorite and barroisite), zoisite and partly phengite. Euhedral to subhedral garnet occurs as small- to medium-sized grains that partially are fractured and replaced by clinozoisite. Calcic amphibole is formed around sodic-calcic amphiboles and garnets. Zoisite occurs as (1) isolated and lenticular aggregates of fine-grained un-oriented crystals, and (2) coarse to medium-grained zoned crystals parallel to the foliation. In general, three phengite groups can be distinguished in this sample. Phengite I grains are mostly subhedral to euhedral, unstrained crystals with a size of 250 to up to 400 µm, which display no evidence for recrystallization or deformation and 12 ACCEPTED MANUSCRIPT which are in textural equilibrium with the other eclogite facies minerals (especially zoisite and omphacite) and may contain minor inclusions of rutile. Phengite II grains have a light T green color and occur in the matrix showing partially recrystallization. The phengites are RI P mostly enclosed by symplectite textures including biotite and albite (Fig. 6g). Phengite III grains with a size of 300 to 400 µm partially show deformation features such as kink-bands SC and wavy extinction. Chlorite is observed along grain boundaries and cleavage planes of phengites. The phengites with pale-green color display partly an orientation, which is not NU parallel with the main foliation of the eclogite. Albite and calcite are present as retrogression MA products and occur as poikiloblasts with numerous inclusions of other minerals, i.e. garnet, omphacite and phengite. ED In summary, most of eclogite samples show two or three groups of phengitic white mica, which have been selected for age dating. PT S19-2 is a paragneiss that is associated with eclogites. The peak pressure assemblage includes quartz, feldspar, garnet, phengite, biotite, tourmaline and rutile. The rocks were AC CE strongly mylonitized during subsequent deformation and display S-C shear band fabrics. Up to 5 mm large albite poikiloblasts contain numerous inclusions including quartz, garnet, phengite, iron-oxide and rutile. Elongate >1 mm-long phengite crystals with biotites define a foliation, which wraps around rotated inclusion-rich albite grains (Fig. 6h). Phengite inclusions in albite are fine- to medium-grained with a size up to 150 µm. The garnet grains present as inclusion in albite poikiloblasts mostly display no alteration to chlorite, whereas garnet crystals in the matrix are mainly replaced by chlorite. This sample shows partly retrogression with albite and chlorite, which replaced garnet and biotite. 4.2. Mineral chemistry 13 ACCEPTED MANUSCRIPT Electron-microprobe analyses were performed at the Department of Geography and Geology, University of Salzburg. White micas from five of these samples were analyzed T using a wave-length dispersive JEOL Superprobe JXA-8600 system (4 spectrometers, LiF, RI P PET, TAP crystals) under operating conditions of 15 kV and 40 nA beam current, with a beam diameter of 5 µm. 40 SC White mica grains from the eclogite and associated paragneiss samples dated by the Ar/39Ar method were investigated. White mica in all of samples displays phengitic NU composition, with the exception of M40-2 sample, which has a paragonite composition MA (Table 1, Fig. 7a). Phengite is high-silica mica and a solid-solution between muscovite and celadonite. In most of the eclogite and paragneiss samples, the compositional variation of ED phengite crystals is relatively limited, but the compositions of some phengites that have high Cr in sample M40-14 plot far below the ideal Tschermak substitution line in Si–Al diagram PT and have there a slight deviation from the ideal composition (Fig. 7b), probably due to a substantial Al–Fe3+ substitution (Liu et al., 2008). Si-contents of phengites vary from 3.23 up AC CE to 3.37 and from 3.28 to 3.37 Si atoms per formula unit (a.p.f.u) based on a stoichiometry of 11 oxygen atoms (Table 1) in the paragneiss and the eclogites, respectively. Large phengite crystals generally contain Si-contents similar with small phengite flakes and their cores and rim compositions of some phengites show a weak zoning (Fig. 7c). Sample M40-14 has the maximum Si-contents of phengites within eclogites and slightly retrogressed eclogites. MgO concentrations range between 2.07 and 3.36 wt% and FeO concentrations from 1.48 wt% to 2.65%. Thus, the XMg values are >0.66 and reach up to 0.75 in paragneiss and to 0.8 in the eclogites. The paragonite component [Na/ (Na + K)] in the phengite is below 0.17 in paragneiss and eclogites. Some of white mica grains in eclogite sample M40-2 represent paragonite ([Na/ (Na+K)] = 0.90–0.92) with an average K2O concentration of ~1 wt. % and XMg [Mg/ (Mg+Fe)] values in the range between 0.51 and 14 ACCEPTED MANUSCRIPT 0.58. In eclogite sample 40-14, some phengite grains show higher Cr-contents in the core (0.11 up to 0.23 a.p.f.u) than in their rims (< 0.2 a.p.f.u). Because of the presence of T paragonite in the eclogite facies mineral assemblage, sample M40-2 is, therefore, a RI P paragonite eclogite (e.g., Ahn et al., 1985). White mica has long been recognized to become progressively phengitic with increasing SC pressure in regional metamorphism (Ernst, 1963; Massonne and Schreyer, 1987; Coggon and Holland, 2002). The Si content of phengite is positively correlated with pressure (Velde, NU 1965; Massonne and Schreyer, 1987; Warren et al., 2011). The Si-contents of all of the MA analyzed white mica grains indicate that white mica crystallization took place at high pressure. Phengite coexisting with rutile and quartz/coesite is a valuable tool to determine the ED pressure of crystallization of LT-(U)HP metamorphic rocks (Auzanneau et al., 2010). Overall, no significant variations in phengite compositions were observed for a given sample PT nor between the studied samples. AC CE 4.3. Thermobarometry and PT path The petrographic features and reaction textures of the investigated eclogites display two main metamorphic stages: (1) a peak eclogite facies stage, and (2) a subsequent amphibolite facies stage (Davoudian et al., 2008). None of the investigated eclogites show greenschist overprint, due to the lack of textural relationships and mineralogical evidence like the formation of retrogressive chlorite and actinolite. The mineral assemblage of the eclogite facies consists of garnet + omphacite + sodiccalcic amphiboles + zoisite + phengite + rutile + quartz ± dolomite with minor pyrite and magnetite. Peak metamorphic conditions for the formation of the eclogites have been calculated using the Krogh Ravna and Terry (2004) calibration for the garnet + clinopyroxene + phengite ± kyanite ± quartz/coesite assemblage. Peak metamorphic conditions for a 15 ACCEPTED MANUSCRIPT particular eclogite sample were calculated using garnet with maximum (maximum Si- T omphacite with maximum jadeite content and phengite with maximum , RI P content). This thermobarometry yields a maximum pressure of 25 kbar at T=600 ºC (Davoudian et al., 2008) that obtained from the best-preserved eclogite sample M40-14. The SC calculations with THERMOCALC (Holland and Powell, 1998) gave a maximum pressure 23.5 kbar at 520 ºC. The obtained pressure is similar to the calculated results using the NU calibration of Krogh Ravna and Terry (2004), but the temperature is significantly less (about MA 80 ºC). The relatively similar temperature is also obtained by the new formulation of garnet– clinopyroxene geothermometer (Nakamura, 2009), which resulted in 527 to 573 ºC at P=24 Kbar. ED Zirconium contents of rutile can be used to estimate equilibration temperatures in the PT assemblage rutile-quartz (Zack et al., 2004; Ferry and Watson, 2007; Tomkins et al., 2007). Zr-in-rutile thermometry have been successfully used in recent years to evaluate the peak AC CE temperatures of high-pressure rocks, which underwent complex metamorphic evolution (Liu et al., 2015). Zr-in-rutile thermometry on the samples of the eclogites yields temperatures that reflect progressive crystallization of rutile at an average of 565 ºC, which is close to the obtained temperatures using the calibration of Nakamura (2009). Accordingly, the 24 kbar average pressure, corresponding to a depth about 80 km, and 560 °C average temperature conditions reflect an anomalously low geothermal gradient (7 °C/km), which is representative for subduction zone environments in convergent margins (e.g. Hacker and Peacock, 1995; Aoya and Wallis, 1999). The mineralogical evidence and textural features show that the investigated eclogites did not undergo extensive amphibolite facies metamorphic overprint after the eclogite metamorphic stage. The high-pressure minerals including omphacite, phengitic white mica, 16 ACCEPTED MANUSCRIPT sodic-calcic amphibole and rutile slightly retrograded to the mineral assemblage indicative of amphibolite facies conditions such as calcic amphibole (edenite, pargasite and T magnesiohornblende), plagioclase, biotite and sphene. In some cases, phengites are replaced RI P by symplectic intergrowth of biotite and feldspar. The growth of symplectite textures around phengites can be referred to the medium-pressure amphibolite stage (Di Vincenzo et al., SC 2001). The P–T conditions of amphibolite facies overprint- have been determined by amphibole- NU plagioclase thermometry (Holland and Blundy, 1994) in combination with Al-in-amphibole MA barometry (Anderson and Smith, 1995) giving values of 9.8 kbar at 555 ºC. The temperature for the amphibolite facies overprint in comparison with that of predating eclogite facies ED indicates isothermal exhumation of the eclogites (Fig. 8), whereas the strongly retrogressed eclogites and the amphibolites derived from the eclogites especially in the southern part of PT studied area show heating of approximately 100 ºC during decompression (Davoudian et al., 2008). The event of retrograde amphibolite facies conditions may be inferred in terms of AC CE exhumation, directly after the high-pressure stage, without the occurrence of a further discrete metamorphic event. 4.4. Origin and age of protolith Based on the major and minor element chemistry of all six samples, the eclogites are similar with a basalt to basaltic andesite composition. The whole rock analyses of eclogites and other associated metabasites, including amphibolite and garnet amphibolites, have shown that their protolith was a quartz-normative tholeiitic basalt. Trace-element characteristics of the further suggest that they resemble tholeiites such as mid-ocean ridge (MORB) type basalts (Davoudian et al., 2006). The age of these mid-oceanic ridge basalts, as protolith of eclogites and amphibolites, cannot be exactly established but the enclosing orthogneiss yields 17 ACCEPTED MANUSCRIPT a late Neoproterozoic LA-ICP-MS zircon U-Pb age of 569±13 Ma, with Th/U ratios mostly 0.15 to 0.59 (Davoudian et al., in preparation). Actually, the Late Neoproterozoic age T constrains the protolith age of orthogneiss that experienced a HP-LT metamorphic event at RI P Early Jurassic (see below). Furthermore, the protoliths of the eclogites are Panafrican basic igneous rocks, and in general the NSMC is a Panafrican basement, which consists of various SC metamorphic rocks of mixed continental (e.g., paragneiss, orthogneiss and schists) and oceanic (e.g., eclogites and amphibolites) origin and which are intruded by the pre- NU metamorphic granitoids. The Panafrican protolith age of the orthogneiss from the NSMC is MA similar to ages obtained from other Pan-African metamorphic/magmatic complexes of the SSZ and Central Iran (e.g. Ramezani and Tucker, 2003; Hassanzadeh et al., 2008; Rahmati- ED Ilkhchi et al., 2011; Shakerardakani et al., 2015). PT 5. Geochronological data form Sanandaj-Sirjan Zone Results of previous geochronology of metamorphic and igneous rocks of the Sanandaj- AC CE Sirjan Zone (SSZ) are compiled in Table 2. Reliable geochronological ages are mainly available from recent age dating by U-Pb and 40Ar/39Ar techniques. Recently, zircon U–Pb geochronological data are available from a significant number of the subduction-related granitoids in the SSZ, especially in the northern part of the zone (e.g. Alvand, Boroujerd, Aligudarz, Astaneh and Kolah Ghazi). The dating results show that the granitoids were emplaced within a short period of time during middle Jurassic from 161 up to 172 Ma as the sign of magmatic arc formation (Khalaji, 2007; Shahbazi et al., 2010; Mahmoudi et al., 2011; Esna-Ashari et al., 2012; Chiu et al., 2013; Shakerardakani et al., 2015). Emami (2008) reported 40 Ar/39Ar incremental heating ages in the ranges 148–170 Ma for hornblende from two andesite and diorite samples respectively, in the volcano-sedimentary complex exposed in the north of Shahrekord. 18 ACCEPTED MANUSCRIPT In addition, some data have been reported for the metamorphic rocks in the SSZ. Rashidnejad-Omran et al. (2002) obtained amphibole K-Ar age 174.5 Ma for amphibole of an 40 Ar/39Ar age for hornblende from an amphibolite sample in RI P (1980) also reported a 170 Ma T amphibolite located in the metamorphic complex north of Golpayegan. Haynes and Reynolds the Neyriz ophiolite. The 40Ar/39Ar muscovite age of about 180 Ma of a granite sample from SC the Tutak gneiss dome indicates the time of deformation of the granite (Alizadeh et al., 2010). In general, very few age data have been reported for high-pressure metamorphic rocks from NU the SSZ and Zagros orogenic belt. Agard et al. (2006) reported an age range of 85 to 95 Ma MA of white mica from the only known blueschist facies rocks present in the footwall of the SSZ (Hajiabad area), a unit sandwiched between the High Zagros and the SSZ. These ages are ED coincident with obduction processes in the region. PT 6. 40Ar/39Ar dating For the last three decades, the 40Ar/39Ar dating technique is used to constrain the timing AC CE of UHP and HP conditions (e.g. Altherr et al., 1979; Sherlock et al., 1999; El-Shazly et al., 2001; Putlitz et al., 2005; Liu et al., 2008; Warren et al., 2011). The 40Ar/39Ar dating method of white mica is probably the most widely used isotopic method for obtaining geochronological data on high-pressure grade metamorphic terrains (Wijbrans et al., 1990). In general, analyzed white mica grains are phengite and paragonite in composition. Dating of phengitic white mica by the 40 Ar/39Ar method mostly has been a useful tool for understanding the high-pressure evolution within an evolving orogen (e.g., Scaillet, 1998; Di Vincenzo et al., 2006; Kurz et al., 2008) because other systems including the Rb–Sr system in white micas are more sensitive to modification during subsequent overprint than the Ar isotope system (Bröcker et al., 2004). 19 ACCEPTED MANUSCRIPT 6.1. Analytical method The 40 Ar/39Ar analytical techniques follow that described by Handler et al. (2004). 40 Ar/39Ar analyses, and the age T Preparation of the samples before and after irradiation, the RI P calculations were carried out at the ARGONAUT Laboratory of the Geology Division at the University Salzburg. Mineral concentrates were prepared by crushing, sieving, and hand- SC picking of the grain sizes of 250–355 μm and 355–500 μm. They were further purified by washing with deionized water. The grains were separated by accurate selection by hand- NU picking under a binocular microscope to avoid the presence of altered and inclusion bearing MA grains, so that the mineral separates were at least 99% pure. For measurements, 10–20 carefully selected grains with similar properties (e.g. color and grain size) were finally used. ED Phengite occurs mostly as colorless and light green to very light green grains. These color groups were analyzed separately. Mineral concentrates were packed in aluminium foil and PT placed in quartz vials. For calculation of the J-values, flux-monitors were placed between each 4 and 5 unknown samples, which yield a distance of ca. 5 mm between adjacent flux- AC CE monitors. The sealed quartz vials were irradiated in the MTA KFKI reactor (Budapest, Hungary) for 16 h. Correction factors for interfering isotopes were calculated from 10 analyses of two Ca-glass samples and 22 analyses of two pure K-glass samples, and are: 36 Ar/37Ar(Ca) = 0.00022500, 39 Ar/37Ar(Ca) = 0.000614 and 40 Ar/39Ar(K) = 0.0266. Variation in the flux of neutrons was monitored with the DRA1 sanidine standard for which a 40 Ar/39Ar plateau age of 25.03 ± 0.05 Ma was originally reported (Wijbrans et al., 1995). Here we use the revised value of 25.26 ± 0.05 Ma (van Hinsbergen et al., 2008). 40Ar/39Ar analyses were carried out using a UHV Ar-extraction line equipped with a combined MERCHANTEK™ UV/IR laser system, and a VG-ISO-TECHTM NG3600 mass spectrometer. Stepwise heating analyses of samples were performed using a defocused (~1.5 mm diameter) 25W CO2-IR laser operating in Tem00 mode at wavelengths between 10.57 and 20 ACCEPTED MANUSCRIPT 10.63 µm. Gas admittance and pumping of the mass spectrometer and the Ar-extraction line are computer controlled using pneumatic valves. The NG3600 is an 18 cm radius 60º T extended geometry instrument, equipped with a bright Nier-type source operated at 4.5 kV. RI P Measurements are performed on an axial electron multiplier in static mode, peak jumping and stability of the magnet is controlled by a Hall-probe. For each increment the intensities of Ar, 37 Ar, 38 Ar, 39 Ar and 40 Ar are measured, the baseline readings on mass 34.5 are SC 36 automatically subtracted. Intensities of the peaks are back-extrapolated over 16 measured NU intensities to the time of gas admittance either by a straight line or a curved fit, depending on MA intensity and type of pattern of the evolving gas. Intensities are corrected for system blanks, background, post-irradiation decay of 37Ar, and interfering isotopes. Isotopic ratios, ages and ED errors for individual steps are calculated following suggestions by McDougall and Harrison (1999) and Scaillet (2000) using decay factors reported by Steiger and Jäger (1977). AC CE 2001). PT Definition and calculation of plateau ages were carried out using ISOPLOT/EX (Ludwig, 6.2. 40Ar/39Ar dating results As mentioned above, 40Ar/39Ar mineral dating was performed on 12 mineral concentrates from 7 samples. A summary of the analyzed samples and calculated ages are given in Table 3. Analytical results are graphically displayed as age spectra in Fig. 9 together with 36Ar/40Ar vs. 39Ar/40Ar isotope correlation plots. For age discussion, we use the time-scale calibrations of Gradstein and Ogg (2004). From sample M40-14, two color fractions (M40-14A: colorless, and M40-14B: light green) with same size have been analyzed. The Ar-release spectrum of sample M40-14A displays a perfectly flat pattern, representing a homogeneous undisturbed Ar-isotopic composition released through the experiment. Age calculation excluding the first and the two 21 ACCEPTED MANUSCRIPT last steps yield an age 184.30 ± 0.90 Ma (steps 2–11 including 87.1% of 39 Ar released, Fig. 9a). The low laser energy step 1 shows some younger age of 149.4 ±3.9 Ma, probably due to T loss of radiogenic 40Ar. The 36Ar/40Ar vs. 39Ar/40Ar isotope correlation plot yields a 40Ar/36Ar RI P = 300 ± 43 with a MSWD (Mean Square of Weighted Deviates) of 3.5 (Fig. 9b). Because this value is very close to the present-day atmospheric composition of Ar (40Ar/36Aratm = 295.5), SC we conclude that no excess Ar has been incorporated at or after the time of initial closure of the isotopic system in the phengitic white mica and that the age is, therefore, geologically NU significant. (steps 1–18: 100% 39 MA The light green phengite grains of the concentrate M40-14B yield a nearly flat plateau Ar released) recording an age of 180.0 ± 0.40 Ma (Fig. 9c), indicating of the radiogenic 36 Ar-components. Minor loss of excess Ar/40Ar vs. 39 40 Ar-components is also indicated Ar/40Ar isotope correlation plot, where isotope correlation analysis of PT by the 40 ED slightly younger ages than sample M40-14A (about 4 Ma), probably as a result of minor loss all steps yield an intercept of 40 Ar/36Ar = 371 ± 84 (MSWD = 0.47), being higher than AC CE atmospheric composition (Fig. 9d). Phengitic white mica of sample M40-57 has the same grain size (250–355 μm) and is colorless. The Ar-release plot displays fairly consistent ages of individual release steps, except for the two first increments (comprising only 1% 39 Ar released). Calculation over steps 3–11, together comprising 98.5% of the total 39Ar released yield an age of 179.9 ± 0.39 Ma (Fig. 9e). Regression analysis over these steps within the 36Ar/40Ar vs. 39Ar/49Ar isotope correlation plot yields a y-axis intercept of 40Ar/36Ar = 338 ± 110 with a MSWD of 1.04 (Fig. 9f). White mica from sample M40-2, an eclogite bearing paragonite, yields a flat plateau age of 178.1 ± 0.86 Ma, and comprising 99.82 percent of 39 Ar released (Fig. 9g). The first step records an age of 10.8 ± 98.8 that the age is geologically meaningless. The regression of the 22 ACCEPTED MANUSCRIPT 36 Ar/40Ar vs. 39Ar/49Ar isotope correlation plot yields a y-axis intercept of 40Ar/36Ar = 372 ± 92 and MSWD = 1.15 (Fig. 9h). T The colorless white mica with phengitic composition from sample M40-13 yields a flat RI P plateau age of 175.6 ± 0.38 Ma (steps 3–11 including 92.6 % of 39Ar released, Fig. 9i). The Ar-release pattern of the sample shows minor fluctuations in the low-temperature gas release 39 Ar released), which SC steps. The first step records a younger age of 148.9 ± 12 Ma (0.5% point to some loss of 40Ar-components, the age is same to the first step in sample M40-14A. 39 Ar released) display increasing ages of 189.1 ± 3.2 Ma and NU Also, the steps 2 and 3 (6.9% MA 185.1 ± 1.6 Ma. The sample displays evidence of ductile deformation such as a mineral respectively stretching lineation. The deformation probably allowed incorporation of minor excess 40 Ar-components. The 36 Ar/40Ar vs. 39 Ar/40Ar isotope correlation plot resulted in an ED age of 172.2± 2.1 Ma (with a 40Ar/36Ar initial value of 506 ± 100, MSDW=2.1, Fig. 9j). PT From sample M38-5 two fractions according to their color (with same size) have been analyzed (M38-5A: colorless, and M38-5B: light green). The concentrate M38-5A yield a flat AC CE plateau age of 182.90 ± 0.36 Ma comprising 100 percent of 39Ar released (Fig. 9k). The age ca. 183 Ma is similar with age of sample M40-14A. The 36 Ar/40Ar vs. correlation plot resulted in an age of 182.3 ± 2.0 Ma (with a 40 39 Ar/40Ar isotope Ar/36Ar initial value of 364 ± 110, MSDW=0.14, Fig. 9l). The phengitic white mica with the light green color from M385B yields a flat plateau (steps 2–14: 88.8% 39 Ar released) recording an age of 173.6 ± 0.28 Ma (Fig. 9m), which is about 9.5 Ma younger than the age of colorless phengites for sample M38-5A. The 36Ar/40Ar vs. 39Ar/40Ar isotope correlation plot resulted in an age of 176.0 ± 2.1 Ma (with a 40Ar/36Ar initial value of 234 ± 25, MSDW=4.4, Fig. 9n). According to the color of phengitic mica grains from sample M38-11, which is a mylonitic eclogite, three fraction (all with same size 250–355 μm) have been separated and individually analyzed (M38-11A: colorless, M38-11B: light green and M38-11C: very light 23 ACCEPTED MANUSCRIPT green). The colorless white mica concentrate of sample M38-11A, indicates a non-flat pattern with ages from 241.5 Ma to 150.9 Ma (Fig. 9o). The third step only yields an age of 181.5 ± 40 Ar/36Ar initial value of 847 ± 490, MSDW=1.5, Fig. RI P in an age of 180.8 ± 3.8 Ma (with a T 0.69 Ma (78.0 % 39Ar released). The 36Ar/40Ar vs. 39Ar/40Ar isotope correlation plot resulted 9p). The concentrate shows evidence of loss and excess of radiogenic Ar, resulting in the SC effects of deformation during ductile shearing. 40 The white mica concentrate with a light green color of sample M38-11B, yield a slightly 39 Ar released) have ages between 362.3 to 212.0 Ma, which MA (together comprising 1.9 % NU disturbed age pattern with minor fluctuations in the first six release steps. The first three steps indicate excess 40Ar components. Then, the Ar-release plot indicates a U-shaped pattern with 39 ED decreasing ages from 195.0 Ma to 184.7 Ma, and finally defines a plateau (steps 7–10: 79.4% Ar released) with an age of 175.6 ± 0.94 Ma (Fig. 9q). Isotope inversion resulted in an age PT of 187 ± 19 Ma with a 40Ar/36Ar initial value of 406 ± 110, MSWD of 2.1 (Fig. 9r). Phengite concentrate of sample M38-11C yields a disturbed age spectrum with excess Ar in the steps AC CE 1–7 and an age of 172.5 ± 0.81 Ma from steps 8–20, comprising 86.5% together of released (Fig. 9s). Isotope inversion resulted in an age of 172.9 ± 2.2 Ma with a 40 39 Ar Ar/36Ar initial value of 308 ± 65, MSWD of 1.9 (Fig. 9t). This isotope correlation age is well in agreement with the plateau age and is, therefore, considered to be geologically significant representing the youngest age of all ages of this study. From sample S19-2, a paragneiss associated with eclogites, two size fractions (S19-2A: 250–355 μm, and S19-2B: 355–500 μm) have been analyzed. The Ar-release spectrum of large white mica grains from sample S19-2A displays a slightly disturbed age pattern with excess Ar in steps 2 and 3 and a plateau age of 175.31 ± 0.83 Ma from steps 4 to 8, comprising 74.0% together of 39Ar released (Fig. 9u). Older ages from 178.05 to 182.23 Ma have been found in the remaining high-laser energy steps (9–14). The ages are similar with 24 ACCEPTED MANUSCRIPT the ages obtained for the eclogites, especially samples M40-14B and M40-57. Isotope inversion resulted in an age of 174.6 ± 1.8 Ma with a 40 Ar/36Ar initial value of 396 ± 110, T MSWD of 0.67 (Fig. 9v). White mica with a smaller grain size from sample S19-2B yields a 39 Ar released, Fig. 9w). RI P flat plateau age of 175.3 ± 0.29 Ma (steps 4–13; 95.4 percent of Isotope inversion resulted in a same age of 175.4 ± 1.7 Ma with a 40 Ar/36Ar initial value of SC 311 ± 17, MSWD of 2.3 (Fig. 9x). Since the initial value is comparable to the present day atmospheric composition of Ar (40Ar/36Aratm = 295.5) and good correlation of the isotope NU inversion age with the plateau age, we conclude that no excess radiogenic 40 Ar has been MA incorporated in the small-sized phengite grains. In addition to white mica, hornblende and biotite with the same 250–355 μm size fraction ED were separated from a deformed metagranite, which intruded the metamorphic rocks before the eclogite-facies metamorphism event. The hornblende grains of the metagranite of sample 39 PT M48-2, from the southern part of the NSMC, yield a nearly flat plateau (steps 3–8: 96.4% Ar released) recording an age of 170.1 ± 0.95 Ma (Table 3; Fig.10a), indicating a younger AC CE age than the high-pressure metamorphic rocks. Isotope inversion resulted in an age of 169.9 ± 2.4 Ma with a 40 Ar/36Ar initial value of 294 ± 12, MSWD of 0.80 (Fig.10b). Because this value is statistically equal to the present day atmospheric composition of Ar, we conclude that no excess Ar has been incorporated at or after the time of initial closure of the isotopic system in the hornblende and so that the age is geologically significant. This age could be interpreted as the cooling age of hornblende after peak conditions of metamorphism, and the age slightly postdate the obtained white mica ages from eclogites. Biotite from the metagranite yields a nearly flat plateau age of 110.7 ±0.32 Ma comprising 75 percent of 39Ar released (Table 3; Fig. 10c). The first step records an age of 41.3 ± 0.8 Ma, an age, which is likely geologically significant. The regression of the 36 Ar/40Ar vs. 39 Ar/49Ar yields a y-axis intercept of 40Ar/36Ar = 350 ± 48 and MSWD = 0.53 (Fig. 10d). Therefore, the cooling ages 25 ACCEPTED MANUSCRIPT of the metamorphic hornblende and biotite from the metagranitoid are considered to record the passage of the minerals through their closure temperatures of 500–550 ºC for hornblende 40 Ar/39Ar mineral ages of the metagranite constrain Middle Jurassic RI P about 4 ºC/Myr. The T (Harrison, 1982), and 275–300 ºC for biotite (Harrison et al., 1985), indicating a cooling rate (170.1 Ma) to Early Cretaceous (110.7 Ma) cooling to ~300 °C following the high-pressure SC metamorphism event. According to this biotite age, we propose that maximum Cretaceous metamorphic conditions reached lower greenschist facies. Clearly, our new data indicate that NU the temperature of post-Jurassic metamorphic and associated deformation phases in part of MA the Sanandaj-Sirjan metamorphic belt was too low to reset the Early Jurassic ages. The first step age of 41.3 ± 0.8 Ma of biotite from the metagranite is synchronous with ED the Eocene U–Pb zircon ages were obtained for magmatic rocks in the Urumieh–Dokhtar magmatic arc (e.g. Verdel et al., 2011; Chiu et al., 2013) and SSZ (Azizi et al., 2011b; PT Hassandzadeh and Wernicke, 2016) and close to zircon and fission track and (U-Th)/He ages AC CE from the SSZ (François et al., 2014b). 7. Discussion Ages decrease from 184.3 ± 0.90 Ma in the fresh eclogites to 172.5 ± 0.93 Ma in those that more severely experienced either amphibolite-facies overprint or mylonitization due to ductile deformation. Since the thermobarometric calculations from the Northern Shahrekord eclogites show nearly isothermal decompression, the variation of ages seems to result rather from ductile deformation than temperature. Some authors have proposed that ductile deformation plays a more important role in the resetting of Ar-isotopic systems than temperature (e.g. West and Lux, 1993; Kurz et al., 2008; Shi et al., 2014). It seems also possible that complete resetting of older micas at amphibolite to greenschist overprinting grade does not occur (e.g. Wijbrans et al., 1990; Kurz et al., 2008), so different ages in 26 ACCEPTED MANUSCRIPT multiple deformed and poly-metamorphic terrains, such as Sanandaj-Sirjan Zone, could be a common phenomenon. An age of ca. 184 Ma is only observed in eclogite samples not T affected by subsequent deformation (e.g. sample M40-14 sample) and it could be interpreted RI P as the potential age of eclogite facies metamorphism. Kurz et al. (2008) have indicated that phengites formed under eclogite-facies metamorphic conditions, retain their initial isotopic SC signature, even when associated lithologies were overprinted by short-living greenschist- to amphibolite-facies grade metamorphism. The eclogites and the associated assemblages were NU more or less subjected to deformation. Therefore, most of the eclogites and the associated MA rocks have undergone localized mylonitization causing the formation of the mylonitic foliation and lineation. The more deformed samples (e.g. M38-11) are characterized by the ED growth and recrystallization of most minerals along the mylonitic foliation and the mineral stretching lineation, contrasting with the more equidimensional crystal habit of relatively less PT deformed samples (e.g. M40-14). The phengites from the paragneiss (sample S19-2) show a ductile deformation as S/C fabrics and yield an age about 175 Ma in both grain sizes of 200– AC CE 355 µm and 355–500 µm. Since, deformation resulted in the resetting of the Ar isotopic system within the recrystallized white mica (e.g. Villa, 1998; Mulch et al., 2002; Kurz et al., 2008; Shi et al., 2014) it seems that the phengites, during ductile deformation, have been recrystallized to form a mylonitic foliation. Overall, these age data point to the presence of continuous deformation stages during exhumation of the high-pressure rocks, since there is appropriate overlap between ages obtained from all of samples. We conclude that the age variation from 184.3 ± 0.90 Ma in the fresh eclogites to 172.5 ± 0.93 Ma in eclogites more affected by subsequent deformation during exhumation resulted in partial resetting of the argon isotopic system in white mica. 7.1 Age of high-pressure metamorphism 27 ACCEPTED MANUSCRIPT In most samples, no evidence exists for extraneous argon as the isotope inversion of most samples shows, within error, air argon isotopic values and excess argon in the first analytical T steps is limited to three samples only M38-11A, M-38-11C and S19-2A. These patterns RI P argue, therefore, for a geological significance of white mica ages. The white mica ages reported here are interpreted as cooling ages and as a result, the high-pressure metamorphism SC due to subduction of a marginal part of the Neo-Tethys Ocean beneath Sanandaj-Sirjan Zone is slightly older than these ages. The age difference between the cooling ages and the age of NU high-pressure metamorphic event mostly depends on the closure temperatures or blocking MA temperature for Ar within white mica. In previous studies, a key point an Ar retention temperature of white mica was found to be significantly variable from 350–450 °C (e.g. ED Jäger, 1979; Harrison et al., 2009) to 550–600 °C particularly for phengite (e.g. Villa 1998; Di Vincenzo et al. 2004; Oberhänsli et al., 2012). The results obtained by Di Vincenzo et al. PT (2001) have shown that in the lack of recrystallization, white mica preserves argon isotope records relating to the high-pressure stage, which survived amphibolite retrogression at AC CE temperatures of 550 to 650 ºC if the duration is short (see also Qiu et al., 2010). Phengitic white mica is generally predicted to have a closure temperature of about 500–550 °C, i.e. slightly or significantly higher than that of muscovite (Lister & Baldwin 1996; Stuart 2002; Kurz et al., 2008; Harrison et al., 2008). Furthermore, several studies (e.g. Bosse et al., 2000; Kühn et al., 2000; Giorgis et al., 2000; Di Vincenzo et al., 2001; Putlitz et al., 2005) revealed, that older micas ages can be preserved, despite subsequent re-equilibration at temperatures as high as 600 °C. Gouzu et al. (2006) have indicated that phengite inclusions in the garnet crystals from the eclogitic rocks in the Tso Morari Complex (western Himalaya, India) give ages which are significantly older than the spot ages from the matrix phengites and the inclusion ages are consistent with a zircon SHRIMP age. Tomaschek et al. (2003) and Putlitz et al. (2005) have shown that the oldest 40 Ar/39Ar mica ages are in good agreement with 28 ACCEPTED MANUSCRIPT SHRIMP U–Pb age for metamorphic zircons. Recent studies (e.g. Baldwin et al., 2004; Monteleone et al., 2007; Fotoohi Rad et al., 2009) indicate that geologically time spans T between the peak of eclogite-facies metamorphism and subsequent exhumation to upper- RI P crustal levels is short (<10 Myr). The oldest age of ca. 184 Ma is only observed in eclogites (e.g. sample M40-14) not content. It is suggested that oldest phengite 40 SC affected by subsequent deformation and the rock have omphacite with the highest jadeite Ar/39Ar ages represent the best estimate of a NU minimum crystallization age (Putlitz et al., 2005). The phengitic white micas from sample MA M40-14 with the oldest age of ca. 184 Ma, formed during peak metamorphism (T= ~560 °C and P= ~24 Kbar). The peak temperature is equal to or little top the closure temperature of ED phengitic white mica (550–600°C, Villa, 1998; Di Vincenzo et al., 2003). Therefore, considering the above mentioned points, we suggest that an age 184 Ma could be interpreted this age. PT to be close to the burial age of the eclogites, which is probably a few million years older than AC CE White mica in sample M40-2 is paragonite and yields an age ~178 Ma that is a little (6 Ma) younger than the oldest age of ca. 184 Ma. This could be attributed to the lower closure temperature of paragonite compared with phengite as already stated by Blanckenburg et al. (1989). Overall, there are rare published experimental or field data on Ar closure in paragonite. Some authors have reported younger K-Ar and 40Ar/39Ar ages for paragonite than coexisting phengite, which implies a lower blocking temperature for paragonite compared to phengite, assuming that both minerals crystallized during the same metamorphic event (e.g. Peucat, 1986; Dallmeyer et al., 1989; El-Shazly and Lanphere, 1992). Based on the structural similarity of both muscovite and paragonite is assumed that the closure temperatures for Ar in paragonite and muscovite are the same (e.g. Konzett and Hoinkes, 1996; Tsujimori et al., 2006, Schneider et al., 2004; Fotoohi Rad et al., 2009). 29 ACCEPTED MANUSCRIPT The textures of the minerals of the eclogite facies in the two rock types including eclogites and paragneiss show that deformation started at the pressure peak within eclogites T facies metamorphic conditions. Some eclogites show an eclogite facies mineral lineation RI P made by omphacite. The deformation have been continued along the exhumation path from the eclogite facies to amphibolite facies and finally to green-schist facies. It could be now SC accepted that younger ages lower than 184 Ma probably represent partial resetting by the later amphibolite facies overprint and deformation during exhumation. NU In some cases, the deformation has caused relatively extensive propagation of fluids into MA the eclogites, particularity in M38-11 sample, which yields the youngest age of 172.5 Ma. The partial resetting and propagation of fluids probably are resulted in loss of 40 Ar in some ED samples. The thermobarometric calculations from the North Shahrekord eclogites show nearly isothermal decompression from 24 to 10 kbar. The results show that there are PT systematic differences in ages of the different of phengite types, as the colorless phengite grains show the older ages than light green ones in every given sample. We found no AC CE convincing relationship between the optical properties of the two phengitic white mica types with their oxide-contents according to the studies of mineral chemistry. It seems that the classification of white micas into color groups are related more to the very small structural differences than to compositional differences (e.g., Finch, 1963; Fleet, 2003). Minor structural differences have been caused by deformation. In most cases, the investigated rocks have been experienced more and less deformation within the ductile shear zone of North Shahrekord during exhumation of the high-pressure eclogite facies rocks. Therefore, deformation can play a more important role than temperature in the resetting of isotopic system (e.g. Kurz et al., 2008). On the other hand, in contrast to other parts of SSZ, the NSMC does not include the granitic bodies, which intruded after peak conditions of high-pressure metamorphism, while 30 ACCEPTED MANUSCRIPT there are lots of granitoid plutons in most areas of SSZ and sometimes the plutons are as large as batholith, for example Alavand and Boroujerd, which were mostly emplaced during T Middle Jurassic time (e.g. Shahbazi et al., 2010; Mahmoudi et al., 2011). However, no such SC eclogites and paragneiss by intrusions is highly unlikely. RI P post-eclogite pluton is near to the study area. Therefore, resetting of white micas from 7.2. Are the new data real? NU The data are consistent with new recent data from other parts of Sanandaj-Sirjan Zone. MA Alirezaei and Hassanzadeh (2012) have reported ion microprobe analyses of zircon grains from Hasanrobat granite in central Sanandaj-Sirjan Zone that yield U-Pb ages about 288.3 ± ED 3.6 Ma. They have suggested a major extension in Lower Permian that finally led to the opening of Neo-Tethys Ocean. According to the onset of magmatism along its northern PT Iranian margin, Stampfli and Borel (2002) attributed the initiation of Neo-Tethys subduction to Early Jurassic times. The 40 Ar /39Ar amphibole age of 169.9 Ma obtained for a diorite AC CE (Emami et al., 2008) from the south of the study area is similar to many ages of ca. 171– 165 Ma that are reported for a significant number of the subduction-related granitoids in Sanandaj-Sirjan Zone, especially in the northern part of the zone (e.g. Shahbazi et al., 2010; Mahmoudi et al., 2011; Esna-Ashari et al., 2012; Chiu et al., 2013; Shakerardakani et al., 2015; Table 2). An important observation is that intrusions of Middle Jurassic gabbro, diorite and granite post-date amphibolite-grade metamorphism within the SSZ. The new recent age data of the granitoids shows an important magmatic event along the Sanandaj-Sirjan Zone during middle Jurassic period as the sign of magmatic arc formation (e.g. Mahmoudi et al., 2011; Shakerardakani et al., 2015). Overall, our new age data are compatible with other age dating results from the SSZ and our new data are consistent with other evidence of subduction within the SSZ. In addition to 31 ACCEPTED MANUSCRIPT Iran, the Jurassic subduction-related magmatism is widespread in northern Turkey, the Caucasus, and Greece, indicating continuous arc magmatic activity along the Eastern T Pontides, the Lesser Caucasus and the Sanandaj-Sirjan Zone (e.g. Çelik et al., 2013; Mederer RI P et al., 2013; Topuz et al., 2013). SC 7.3. Age of onset of Neo-Tethys subduction The other problematic subject is the age of onset of the subduction of Neo-Tethys Ocean NU under the Iranian plate. Some authors (e.g. Mohajjel et al., 2003; Ghasemi and Talbot, 2006 MA and references therein) have assumed that subduction of Neo-Tethys may have started beneath the South of Sanandaj-Sirjan Zone by Late Jurassic–Early Cretaceous. Fazlnia et al. ED (2009), according to the Middle Jurassic age (173 ± 1.6 Ma) for the anorthositic rocks that formed in an anorogenic setting, have suggested that no true oceanic crust (Neo-Tethys PT Ocean) developed during the Middle Jurassic. Mohajjel et al. (2003) proposed a Late Jurassic age for initiation of subduction of the Neo-Tethyan sea floor. These suggestions are not AC CE compatible with our results. We propose that the oldest 40 Ar/39Ar phengite ages show the minimum age for the onset of Neo-Tethys subduction under the Iranian edge of the Eurasia plate, meaning Pliensbachian during Early Jurassic. Two alternative hypotheses could be proposed: (1) subduction of a marginal part of the Neo-Tethyan Ocean, or (2) subduction of a continental piece if the orthogneiss is part of an older, potentially rifted passive continental margin (Fig. 11). We conclude that the initiation of subduction of Neo-Tethys Ocean under the Iranian microcontinent along the Main Zagros Thrust, took place during the Early Jurassic. This is in agreement with Berberian (1983) and Stampfli and Borel (2002) who proposed an age about 200 Ma (beginning of Early Jurassic). Agard et al. (2011) have proposed that subduction of the Neo-Tethys was active by 180–170 Ma. In addition, the results support recent data 32 ACCEPTED MANUSCRIPT indicating Early Jurassic subduction along the İzmır-Ankara-Zagros suture (e.g., Topuz et al., 2013). T Nevertheless, the new age data is in contrast to the Middle and Late Jurassic ages RI P suggested by previous workers (e.g. Sengör, 1990; Mohajjel and Fergusson, 2000; Mohajjel et al., 2003; Fazlnia et al., 2009) or the Middle Cretaceous age proposed by Ghasemi and SC Talbot (2006). On the other hand, some of researchers proposed that the continental drift and inception NU of Neo-Tethys occurred during the Late Jurassic (Berberian and Berberian, 1981; Dercourt et MA al., 1986; Golonka, 2004), whereas our age data show that opening of Neo-Tethys must have occurred at least before late Early Jurassic and probably at Triassic times. ED Also, the existence of the eclogites, which formed in relation to subduction of NeoTethys, clearly shows that the suture zone of Arabia and Eurasia is located between the PT Sanandaj-Sirjan Zone and the High Zagros (the Crush zone or the Imbricated zone). This new finding is contrary to suggestions of Alavi (2004), who proposed that subduction and suturing AC CE occurred between the Sanandaj-Sirjan Zone and the central Iran along Urumieh-Dokhtar magmatic belt. We also note that major portions of the Central Iranian basement were affected by Triassic and Jurassic amphibolite and greenschist facies metamorphic conditions (Fig. 11; Bagheri and Stampfli, 2008; Rahmati-Ilkhchi et al., 2011; Kargaranbafghi et al., 2012). The Pan-African basement of Iran mainly occurs in structural zones of the SSZ and Central Iran. These plutono-metamorphic basement terranes mostly contain metagranite, orthogneiss (metagranitoid), paragneiss, schist and amphibolite, in which Panafrican granitoids (now orthogneiss) commonly intruded the meta-sedimentary rocks and amphibolites (e.g. Shafaii Moghadam et al., 2015). The basement rocks have mainly experienced a metamorphic 33 ACCEPTED MANUSCRIPT overprint during Jurassic times (e.g. Rachidnejad-Omran et al., 2002; Hassanzadeh et al., 2008; Rahmati-Ilkhchi et al., 2011, Masoodi et al., 2013). T The occurrence of eclogites only in the NSMC of the SSZ and their apparent absence in RI P the metamorphic complexes of the Central Iranian basement, suggest a different metamorphic evolution in these two important zones of Iran. In the SSZ, on the other hand, the basement SC rocks were metamorphosed under high pressure-low temperature conditions, with a low geothermal gradient (7 °C/km) in early Jurassic time and were exhumed along a cooling path NU in middle Jurassic to early Cretaceous. In contrast, in the Central Iran (e.g. the Deh-Salm MA metamorphic Complex: Mahmoudi et al., 2010; the Shotur Kuh metamorphic complex: Rahmati-Ilkhchi et al., 2011; the Anjul metamorphic complex: Bröcker et al., 2014), the ED basement rocks mostly show high- to medium-temperature and medium-pressure Barroviantype metamorphism, with a high geothermal gradient, synchronous with the intrusive activity PT mainly at the Middle Jurassic (e.g. Rahmati-Ilkhchi et al., 2011).The high geothermal AC CE gradient suggests a collisional type of metamorphism. 7.4. Ages of other blueschists and eclogites related to Neo-Tethys subduction Most of existing dating results of blueschists and eclogites related to subduction and closure of the Neo-Tethys Ocean along the suture zones range within Late Cretaceous (e.g. Oberhänsli et al., 2012; Sherlock et al., 1999; Rolland et al., 2009; Bröcker et al., 2013; Baroz et al., 1984; Agard et al., 2006). Oberhänsli et al. (2012) considered LT-HP metamorphic rocks from the Bitlis complex that were subducted and stacked to form a nappe complex during the closure of the Neo-Tethys (HP Bit, Fig. 1). 39 Ar/40Ar dating of white mica in different parageneses from the Bitlis complex display ages of 74–79 Ma (Campanian, Late Cretaceous) dating peak metamorphism and rapid exhumation to an almost isothermal greenschist stage at 67–70 Ma (Maastrichtian, Late Cretaceous). Similar Rb-Sr white mica 34 ACCEPTED MANUSCRIPT crystallization ages of 79.7 Ma to 82.8 Ma were reported from a belt of high-pressure lowtemperature rocks in the Tavsanli Zone in NW Turkey (EC Tav, Fig. 1) (Sherlock et al., 40 Ar/39Ar phengite data, blueschists of the Amassia- T 1999). Based on geological, P-T and (2009). The 40 RI P Stepanavan Suture Zone in Armenia (Bs Arm, Fig. 1), have been evaluated by Rolland et al. Ar/39Ar phengite ages obtained for the high-P assemblages range between 95 SC and 90 Ma, while the ages of epidote-amphibolite retrogression assemblages range within 73.5–71 Ma (Rolland et al., 2009). Recently, the metamorphic rocks including eclogites, NU blueschists and retrograde assemblages have dated by Bröcker et al. (2013) using 40 Ar/39Ar, MA U-Pb and Rb-Sr dating in the Sistan Suture Zone, eastern Iran (Ec Sis, Fig. 1), that formed as a result of eastward-directed subduction of a Neo-Tethyan Ocean basin beneath the Afghan ED block. The new Rb-Sr, 40Ar/39Ar and U-Pb ages for HP rocks and epidote amphibolites show the ages of 85–87 Ma, that have interpreted as the documentation of Late Cretaceous PT subduction zone processes for the geodynamic evolution of the Sistan Suture Zone (Bröcker et al., 2013). Baroz et al. (1984) reported a Late Cretaceous of 84 Ma (Campanian) for the AC CE blueschists from the ophiolitic tectonic mélange of the Sabzevar in NE Iran (Bs Sab, Fig. 1). The Sabzevar ophiolites mark the Neo-Tethys suture in east-north-central Iran and the formation of blueschists are attributed to north-east dipping subduction of the Neo-Tethys oceanic crust (Omrani et al., 2013). Another high-pressure assemblage related to Neo-Tethys subduction is the blueschists of the Deyader complex in the Makran accretionary prism in SE Iran (Bs Mak, Fig. 1). The probable age of the blueschist metamorphism is proposed postLate Cretaceous, as indicated by the presence of partially recrystallized Globotruncanabearing limestones (McCall, 1985; Maruyama et al., 1996). The blueschists represent exotic tectonic blocks within accretionary complexes and are similar to the Cretaceous blueschists in other segments of the Alpine-Himalayan orogen (Maruyama et al., 1996). The mineralogy of 35 ACCEPTED MANUSCRIPT the metamorphic rocks confirms low-temperature blueschist facies metamorphic grade (Hunziker and Burg, 2009). T One of the rare occurrences of high-pressure metamorphism along the Zagros orogen is RI P the blueschists of the Hajiabad area (Zagros blueschists), which are located in the southeastern most part of Zagros (BS Zag, Fig. 1). There, the blueschist exposures are found SC sandwiched between the High Zagros and the Sanandaj-Sirjan Zones (Agard et al., 2006). The blueschists are associated with serpentinites, radiolarites and lower greenschist facies NU metabasalts as tectonic slices within the ophiolitic mélanges. They yield 40Ar/39Ar ages of 93 MA and 106 Ma (Monié and Agard, 2009) coincident with obduction processes in the region (Agard et al., 2011). The eclogite facies samples from lithologically different units, which 40 Ar/39Ar and Rb–Sr methods ED structurally underlie the Semail ophiolite were dated by the (e.g., El-Shazly et al., 2001) (EC Oman, Fig. 1). Clinopyroxene–phengite, epidote–phengite PT and whole-rock–phengite Rb–Sr isochrons for the eclogite samples yield ages of ~78 Ma. The results have shown that high-pressure metamorphism of the Oman margin took place in 40 AC CE the Late Cretaceous contemporaneously with ophiolite emplacement (El-Shazly et al., 2001). Ar/39Ar ages of phengite schists hosted within the eclogite yield ages of 77–89 Ma (Warren et al., 2011). Most of the mentioned HP-LT metamorphic rocks are closely associated with ophiolites and appear as tectonic blocks in serpentinite mélanges (e.g. the Zagros blueschists in the Hajiabad area, Agard et al., 2006). The new age data of the eclogites of North Shahrekord (Zagros eclogites) show that they are about 100 Ma older than other the ages of the blueschists and eclogites related to subduction and closure Neo-Tethys Ocean from Iran and neighboring countries. 36 ACCEPTED MANUSCRIPT 8. Conclusions Our new 40Ar/39Ar mineral data give evidence for the following major conclusions: T (1) The age of the high-pressure eclogite facies metamorphism in the Sanandaj-Sirjan Zone RI P of the Zagros orogen is Early Jurassic. The phengitic white mica grains with higher Sicontents and from eclogite samples with the least retrogression yield older ages than SC other phengitic white micas. Moreover, our results show that as the eclogite facies metamorphic rocks with the highest jadeite content in omphacites and the highest Si NU contents in phengites are selected for calculating the peak pressure, these rocks are the MA most suitable eclogite rock for 40Ar/39Ar dating. (2) The timing of initiation of subduction the Neo-Tethys is close or a few million years ED prior to 184 Ma, probably about the beginning of Early Jurassic. The new age data confirm that the long-standing convergence history between the Iranian microplate as a PT part of Eurasia and Arabia (as a part of Gondwana), that was previously suggested by Agard et al. (2011), have started before 184 Ma. AC CE (3) The ages of the eclogite-facies event and the subsequent amphibolite-facies retrogression have a little difference, about 10 Ma. Thus, exhumation of the subducted crust from mantle to mid-crustal depth was relatively rapid, taking few million years. (4) The Zagros eclogites are related to subduction whereas other high-pressure assemblages such as Sai Hatat in Oman and Hajiabad blueschists in SE Iran are associated with the ophiolites and colored mélange and are related to obduction/collision. (5) According to the new age dating results of eclogites, the rocks are the oldest highpressure metamorphic rocks in the Zagros orogenic belt and of the Neo-Tethys Ocean subduction from Iran and neighboring countries. Acknowledgments 37 ACCEPTED MANUSCRIPT We are grateful to Robert Handler, Gertrude Friedl and Fritz Finger for their valuable help at Salzburg University. Discussions with G. A. Ahmadi and his company during field T trips are also greatly acknowledged. Suggestions by Philippe Agard and Sasan Sedighi to an RI P earlier version of the manuscript are gratefully acknowledged. We also acknowledge constructive remarks by Songjian Ao and two anonymous journal reviewers. We would like SC to thank associate editor Prof. Yunpeng Dong and editor-in-chief Prof. M. Santosh for their NU encouragement. MA References Agard, P., Monié, P., Gerber, W., Omrani, J., Molinaro, M., Meyer, B., Labrousse, L., ED Vrielynck, B., Jolivet, L., Yamato, P., 2006. Transient, synobduction exhumation of Zagros blueschists inferred from P‐T, deformation, time, and kinematic constraints: PT Implications for Neotethyan wedge dynamics. 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W., 2010. Abbreviations for names of rock-forming minerals. PT American Mineralogist 95,185–187. Wiesinger M., Neubauer F., Handler R., 2006. Exhumation of the Saualpe eclogite unit, AC CE Eastern Alps: constraints from 40 Ar/39Ar ages and structural investigations. Mineralogy and Petrology 88, 149–180. Wijbrans, J. R., Pringle, M. S., Koppers, A. A. P., Scheveers, R., 1995. Argon geochronology of small samples using the Vulkaan argon laserprobe. Proceedings of the Royal Netherlands Academy of Arts and Sciences 98, 185–218. Wijbrans, J.R., Schliestedt, M., York, D., 1990. Single grain argon laser probe dating of phengites from the blueschist to greenschist transition on Sifnos (Cyclades, Greece). Contributions to Mineralogy and Petrology 104, 582–593. Winchester, J., Floyd, P., 1977. Geochemical discrimination of different magma series and their differentiation products using immobile elements. Chemical Geology 20, 325– 343. 62 ACCEPTED MANUSCRIPT Zack, T., Moraes, R., Kronz, A., 2004. Temperature dependence of Zr in rutile: empirical calibration of a rutile thermometer. Contributions to Mineralogy and Petrology 148, T 471–488. RI P Zhai, Q., Zhang, R. Jahn, B., Li, C., Song, S., Wang. J., 2011. Triassic eclogites from central Qiangtang, northern Tibet, China: Petrology, geochronology and metamorphic P–T AC CE PT ED MA NU SC path. Lithos 125, 173–189 Figure 1. Structural frame of the Middle-East portion of the Alpine-Himalayan convergence belt (modified and read apted after; Authemayou et al., 2005; Baker and Jackson, 1993) with locations of LT-HP assemblages. EC Tav: blueschist and eclogite-facies metamorphic rocks, Turkey, the Tavsanli Zone (Sherlock et al., 1999); HP Bit: Bitlis complex with LT-HP metamorphism (Oberhänsli et al., 2012); BS Arm: Blueschists, Armenia of the Amassia63 ACCEPTED MANUSCRIPT Stepanavan Suture Zone (Rolland et al., 2009); EC Sis: Eclogites, E Iran, the Sistan Suture Zone, (Fotoohi Rad et al., 2009; Bröcker et al., 2013); Bs Sab: Blueschists, NE Iran, T Sabzevar structural zone (Baroz et al., 1984); BS Zag: Blueschists, SE Iran, the Zagros RI P Hajiabad area (Agrad et al., 2006); EC Oman: Eclogites, NE Oman, Saih Hatat (Warren et al., 2011); Bs Mak: Blueschists of the Deyader Complex, Makran, SE Iran (Hunziker and SC Burg, 2009; McCall, 1985); EC Zag: Eclogites, N Shahrekord, SW Iran (Davoudian et al., AC CE PT ED MA NU 2008). 64 AC CE PT ED MA NU SC RI P T ACCEPTED MANUSCRIPT Fig. 2. Simplified structural map of Iran (Stöcklin, 1968) showing the position of the Sanandaj-Sirjan Zone. The box indicates the location of Fig. 3. 65 AC CE PT ED MA NU SC RI P T ACCEPTED MANUSCRIPT Fig. 3. The major faults in the studied area on DEM image from SRTM data (ftp://ftp.glcf.umiacs.umd.edu/glcf/SRTM). The box indicates the location of Fig. 4. 66 AC CE P TE D MA N US CR IP T ACCEPTED MANUSCRIPT Fig. 4. Geological map of the studied area. Star locates the eclogite samples studied in the paper (modified after Davoudian el al., 2008; Babaahmadi et al., 2012; Ghasemi et al., 2005). 67 AC CE P TE D MA NU SC RI PT ACCEPTED MANUSCRIPT 68 ACCEPTED MANUSCRIPT Fig. 5. Field photographs of eclogite and associated paragneiss in North Shahrekord. (a) and (b) Lenses of eclogite enclosed within paragneiss showing the close association eclogites with PT paragneiss. Length of hammer, ca. 35 cm. (c) Contact between eclogites and marble, length of RI hammer, ca. 45 cm. (d) A large outcrop of the eclogites with desert varnish. (e) Pillow-shaped structure in a lens of eclogites, which are enclosed within paragneiss. (f) Close-up view of SC structural feature in an eclogite body, the white arrow indicates orientation of foliation within the NU eclogite (sample M40-14) and black marker show mineral lineation. Length of paper clip is 3 cm. (g) WNW-trending stretching lineation on foliation surface are strongly developed in MA paragneiss that commonly defined by a grain shape fabric. Length of paper clip is 3 cm. (h) Vein AC CE P TE D with fibrous calcite cross-cuts the main foliation of eclogite. 69 AC CE P TE D MA NU SC RI PT ACCEPTED MANUSCRIPT 70 ACCEPTED MANUSCRIPT Fig. 6. Photomicrographs showing typical mineralogy and textures in rocks from the North PT Shahrekord eclogites and paragneiss. (a) Omphacite with garnet in fresh eclogite (sample M40- Eclogite shows foliation with RI 14) viewed in cross-polarized light (XPL – crossed polarizers); width of view is 1 mm. (b) shape-preferred orientation of omphacite grains. Crossed SC polarizers (XPL), width of view is 1 mm. (c) Rutile mantled by sphene with garnet and quartz. NU XPL, width of view is 0.7 mm. (d) Undeformed phengite flakes without alteration are enclosed by high-pressure assemblage including garnet and zoisite showing no preferred orientation. View MA in plane-polarized light (parallel polarizers - PPL), width of view is 1 mm. (e) Rutile inclusion in phengite which are slightly replaced by biotites along cleavage. PPL, width of view is 0.4 mm. TE D (f) Albite containing numerous inclusions of garnet, carbonate, white mica, omphacite and phengite. XPL, width of view is 1.5 mm. (g) Symplectite textures (Sym) including biotite and AC CE P albite around phengites in sample M38-11; PPL, width of view is 0.8 mm. (h) Mylonitic foliation, which is defined by phengite and biotite in paragneiss (sample S19-2). Note phengite flakes wrapping around albite crystals; XPL, width of view is 8 mm. Mineral abbreviations after Whitney and Evans (2010). 71 TE D MA NU SC RI PT ACCEPTED MANUSCRIPT AC CE P Fig. 7. Phengite compositions in eclogite and paragneiss samples. (a) Muscovite (Ms) – 50% celadonite (Cel) – paragonite (Prg) triangle plot of chemical composition of phengite and paragonite analyzed by electron microprobe. (b) Chemical variation diagrams (Si vs Al and Si-1 vs Fe + Mg cations per formula unit). Lines correspond to the ideal muscovite–celadonite join. (c) Weak zoning of Si-content in phengite grains from the Shahrekord eclogite. Red circles mark the location of the spots analysed. The numbers give Si-content (p.f.u) for the indicated spots. 72 AC CE P TE D MA NU SC RI PT ACCEPTED MANUSCRIPT Fig. 8. Exhumation P–T path for the investigated eclogite rocks from North Shahrekord. Geotherms of 5°C/km, 7 °C/km, 10 °C/km, and 20 °C/km are indicated. The main metamorphic facies field boundaries and subdivision of the eclogite facies field are based on Liou et al. (2004). GS–greenschist; EA–epidote amphibolite; AM–amphibolite; GR–granulite; HGR–high pressure granulite; BS–blueschist; Amp EC–amphibole eclogite, Ep-EC–epidote eclogite, LW-EC– lawsonite eclogite. The stability fields of diamond (Bundy, 1980) and coesite (Hemingway et al., 1998) are shown. 73 AC CE P TE D MA NU SC RI PT ACCEPTED MANUSCRIPT 74 AC CE P TE D MA NU SC RI PT ACCEPTED MANUSCRIPT 75 ACCEPTED MANUSCRIPT Fig. 9. Results of laser-ablation isotope inversion diagrams) 40 Ar/39Ar step-heating experiments (Ar release patterns and of phengite (Ph) and paragonite (Pg) from the eclogites and PT paragneiss of the North Shahrekord metamorphic complex, Mineral abbreviations after Whitney AC CE P TE D MA NU SC RI and Evans (2010). 76 40 Ar/39Ar step-heating experiments (Ar release patterns and MA Figure 10. Results of laser-ablation NU SC RI PT ACCEPTED MANUSCRIPT isotope inversion diagrams) of amphibole (Hbl) and biotite (Bt) from the deformed metagranite AC CE P TE D of the North Shahrekord metamorphic complex. Figure 11. Tectonic model for Early Jurassic onset of subduction and exhumation of the NeoTethyan continental margin. 77 ACCEPTED MANUSCRIPT Table 1. Representative electron microprobe analyses of white mica (phengite and paragonite) from eclogite and paragneiss samples of North Shahrekord, Iran. SiO2 50.11 48.91 47.89 49.68 49.49 49.89 48.91 49.65 48.26 47.36 49.32 49.05 TiO2 0.59 0.60 0.64 0.71 0.69 0.42 0.49 0.53 0.41 0.52 0.57 0.74 Al2O3 27.90 28.74 30.59 28.60 29.17 28.53 26.75 28.18 26.88 25.42 28.12 28.63 Cr2O3 0.03 0.01 0.02 0.03 0.01 0.00 2.70 0.27 0.06 2.35 0.01 FeO 1.92 2.35 2.07 2.26 2.17 1.68 1.80 1.72 1.52 1.51 2.05 MnO 0.00 0.00 0.03 0.03 0.02 0.00 0.00 0.02 0.01 0.00 MgO 3.25 3.06 2.35 3.09 2.93 3.18 2.94 3.11 3.19 3.09 CaO 0.05 0.00 0.00 0.00 0.00 0.01 0.01 0.03 0.03 Na2O 0.65 0.67 1.10 0.93 0.90 0.90 0.67 0.88 0.88 K2O 9.14 9.23 9.33 9.11 9.06 9.21 9.40 9.29 9.83 Total 93.64 93.57 94.02 94.44 94.44 93.82 93.67 93.68 91.07 Si 3.37 3.31 3.23 3.33 3.31 3.35 3.33 3.35 Ti 0.03 0.03 0.03 0.04 0.04 0.02 0.03 Al 2.21 2.29 2.43 2.26 2.30 2.26 2.15 0.00 0.00 0.00 0.00 0.00 0.00 0.11 0.13 0.12 0.13 0.12 0.09 Mn 0.00 0.00 0.00 Mg 0.33 0.31 0.24 Sum 6.05 6.07 6.05 Ca 0.00 0.00 0.00 Na 0.09 0.09 0.14 K 0.79 0.80 0.80 Sum 0.87 0.89 Al(IV) 0.63 Al(VI) M40-2 M38-11 45.53 46.31 51.53 51.38 51.09 51.78 0.50 0.17 0.16 0.66 0.52 0.56 0.65 27.98 37.90 38.29 31.26 30.66 29.58 30.60 0.01 0.18 0.00 0.02 0.00 0.00 0.00 0.00 2.28 2.16 1.19 0.33 0.00 0.00 0.00 0.00 0.00 0.02 0.04 0.00 0.00 1.99 1.69 1.81 1.79 3.09 3.15 3.14 0.93 0.20 0.00 0.00 0.01 0.02 SC RI 49.02 0.00 0.00 0.02 0.33 0.31 2.97 2.97 3.00 3.05 0.81 0.75 0.79 0.68 6.59 6.63 0.07 0.06 0.04 0.10 9.84 9.93 9.82 10.01 0.89 1.08 0.81 0.94 0.83 0.84 90.93 93.84 94.49 93.73 93.55 93.33 10.08 9.94 10.03 9.77 3.36 3.34 3.34 3.30 3.33 2.98 3.02 3.28 3.30 3.33 3.31 0.03 0.03 0.03 0.04 0.03 0.01 0.01 0.03 0.03 0.03 0.03 2.24 2.21 2.11 2.24 2.27 2.24 2.93 2.94 2.35 2.32 2.27 2.31 D MA 0.03 0.02 0.15 0.01 0.00 0.13 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.10 0.10 0.09 0.09 0.12 0.13 0.12 0.07 0.02 0.11 0.09 0.10 0.10 TE Fe 2+a M40-13 AC CE P Cr M40-14 PT S19-2 NU Sample 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 0.31 0.29 0.32 0.30 0.31 0.33 0.32 0.31 0.32 0.32 0.09 0.02 0.28 0.29 0.29 0.29 6.06 6.06 6.04 6.05 6.04 6.01 6.02 6.04 6.05 6.04 6.07 6.01 6.05 6.03 6.02 6.04 0.00 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 0.00 0.01 0.12 0.12 0.12 0.09 0.12 0.12 0.11 0.10 0.10 0.09 0.84 0.84 0.10 0.12 0.11 0.10 0.78 0.77 0.79 0.82 0.80 0.87 0.88 0.86 0.84 0.87 0.07 0.09 0.82 0.82 0.83 0.80 0.95 0.90 0.89 0.91 0.91 0.92 1.00 1.00 0.96 0.95 0.96 0.93 0.95 0.92 0.94 0.94 0.91 0.69 0.77 0.67 0.69 0.65 0.67 0.65 0.64 0.66 0.66 0.70 0.67 1.02 0.98 0.72 0.70 0.67 0.69 1.59 1.60 1.66 1.58 1.61 1.61 1.48 1.59 1.57 1.45 1.58 1.57 1.57 1.91 1.97 1.62 1.63 1.61 1.62 Na/(Na+K) 0.10 0.10 0.15 0.13 0.13 0.13 0.10 0.13 0.12 0.11 0.10 0.11 0.09 0.92 0.90 0.11 0.13 0.11 0.12 Mg/(Fe+Mg) 0.75 0.70 0.67 0.71 0.71 0.77 0.75 0.76 0.79 0.78 0.73 0.71 0.72 0.58 0.51 0.73 0.76 0.75 0.75 a All FeO calculated as Fe2+. 78 ACCEPTED MANUSCRIPT Table 2. Compilation of geochronological age data from the Sanandaj-Sirjan Zone and other tectonic units related with Zagros orogeny. granitoid granitoid zircon zircon Method Age (Ma) Interpretation Author(s) U–Pb 165±5.0 crystallization Esna-Ashari et al., U–Pb Chah-Ghand 166.5±1.9 crystallization Shahbazi et al., 2010 granite zircon U–Pb 163.9–161.7 crystallization Shahbazi et al., 2010 leucocratic zircon U–Pb 154.4±1.3– crystallization Shahbazi et al., 2010 167.5±1.0 crystallization Mahmoudi et al., U–Pb zircon U–Pb granitoids zircon granodiorite amphibole AC CE P Hajiabad blueschist Gosheh– MA granitoid zircon D granitoid 153.3±2.7 Tavandasht Gorveh 2011 U–Pb and gabbro Golpayegan Mahmoudi et al., zircon TE Boroujerd crystallization gabbro granitoid Astaneh 165.2±0.2 2012 NU Alvand mineral PT Aligoodarz Dated RI Rock type SC Area amphibolite granitoid granitoid amphibole zircon zircon phengite 169.0±1.0 2011 crystallization Mahmoudi et al., 2011 U–Pb 169–172 crystallization Khalaji et al., 2007 K-Ar 159–167±5 exhumation Sheikholeslami et al. 2008 K-Ar 174.5±2.9 cooling age Rashidnejad-Omran et al., 2002 U–Pb 156.5±0.6– crystallization 149.3±0.2 U–Pb 34.9±0.1 Mahmoudi et al., 2011 crystallization Mahmoudi et al., 2011 Ar-Ar 84.03±0.81– exhumation Agard et al., 2006 crystallization Mahmoudi et al., 95.60±1.07 Hasan granitoid zircon U–Pb 108.8±0.3 Salary, 2011 Saqqez Hassan - granitoid zircon U–Pb 288.3±3.6 crystallization Robat Alirezaei and Hassanzadeh, 2012 Kolah Ghazi granite zircon U–Pb 164.6±2.1 crystallization Chiu et al., 2013 Makran trondhjemite Zircon U–Pb 156.4±1.7– crystallization Hunziker et al., 2011 formation Ghazi et al., 2004 160.5±1.4 Makran ophiolite hornblende Ar-Ar 140.7±2.2– 79 ACCEPTED MANUSCRIPT 142.9±3.5 amphibolite amphibole Ar-Ar 109.65±2.04 cooling age Moritz et al., 2006 Muteh metapelite biotite Ar-Ar 108.29±0.92 cooling age Moritz et al., 2006 Naqadeh granite zircon U–Pb 98.5±1.7 crystallization Mazhari et al., 2011 Neyriz area garnet amphibole Ar-Ar 170 tectonic Haynes and emplacement Reynolds, 1980 cooling Babaie et al., 2006 cooling age Emami, 2008 cooling age Emami, 2008 crystallization Fazlnia et al., 2007 Fazlnia et al., 2007 plagiogranite hornblende Ar-Ar ophiolite North 92.07±1.6993.19±2.48 diorite amphibole Ar-Ar 169.91±0.97 andesite amphibole Ar-Ar 148.19±0.89 anorthosite zircon U-Th SC Neyriz Shahrekord Qori NU Shahrekord North RI amphibolite PT Muteh 170.5±1.9- Qori metapelitic MA 173.0±1.6 crystallization U-Th 147.4±0.76 crystallization Fazlnia et al., 2009 U–Pb 175.2±1.6 crystallization Chiu et al., 2013 whole rock Sm-Nd 199±30 crystallization Arvin et al., 2007 zircon U–Pb 143.5±2.3- crystallization Azizi et al., 2011a zircon U-Th 167.0±3.1 trondhjemites zircon Sargaz granite zircon Siah-Kuh granitoid Suffi Abbad granite AC CE P Tutak TE Qori Deformed- D xenoliths 164.3±8.1- 147.5±1.3 muscovite Ar-Ar 179.5±1.7 deformation Alizadeh et al., 2010 biotite Ar-Ar 76.8±0.2 deformation Alizadeh et al., 2010 zircon U–Pb 170.2±3.1 crystallization Shakerardakani et al., granite Tutak Deformedgranite Dorud–Azna Gabbro 2015 80 ACCEPTED MANUSCRIPT Table 3. 40Ar/39Ar analytical results from Eclogite, paragneiss and metagranite samples of the North Shahrekord metamorphic complex, Iran. PT Eclogite sample: M40-14A Phengite (250–355 μm) J-Value: 0.00552 +/- 0.00003 Ar/39Ar +/- 37 Ar/39Ar +/- 40 Ar/39Ar %40Ar* +/- RI 36 %39Ar age [Ma] +/- 0.01912 0.00125 0.03516 0.03973 21.30731 0.27605 73.5 1.4 149.4 3.9 2 0.01218 0.00087 0.06684 0.02165 22.99559 0.13088 84.4 2.7 183.4 2.6 3 0.00243 0.00030 0.01595 0.00770 20.18766 0.03829 96.4 7.5 184 1.2 4 0.00145 0.00124 0.01088 0.02080 20.21840 0.08690 97.9 3.4 186.9 3.5 5 0.00064 0.00039 -0.01398 -0.00607 19.69596 0.07331 99 11.4 184.3 1.5 6 0.00075 0.00025 -0.00892 -0.00524 19.71502 0.05573 98.9 13.6 184.2 1.2 7 0.00027 0.00012 -0.01433 -0.00287 19.64145 0.02891 99.6 17.5 184.8 0.9 8 0.00093 0.00047 -0.00084 -0.00859 19.75244 0.06983 98.6 5.9 184.1 1.6 9 0.00064 0.00018 -0.00248 -0.00369 19.62230 0.03843 99 14.8 183.7 1.0 10 0.00170 0.00047 0.03556 0.01082 19.84647 0.08134 97.5 4.9 182.9 1.6 11 0.00100 0.00046 0.02120 0.01091 19.79302 0.05513 98.5 5.3 184.3 1.6 12 0.00005 0.00013 0.03916 0.00643 19.15410 0.03109 99.9 10.8 181 0.9 13 0.00135 0.00217 0.41042 0.08310 19.51632 0.17736 98.1 0.8 181.2 6.0 Steps 1–13 =plateau age: 100.0 184.3 0.9 %40Ar* %39Ar age [Ma] +/- NU MA TE AC CE P SC 1 D step Eclogite sample: M40-14B Phengite (250–355 μm) J-Value: 0.0055 +/- 0.00003 step 36 Ar/39Ar +/- 37 Ar/39Ar +/- 40 Ar/39Ar +/- 1 0.00492 0.00083 0.00245 0.01002 20.65221 0.05558 93 4.9 181 2.4 2 0.00210 0.00444 0.07197 0.09702 20.04985 0.32987 96.9 0.5 183.1 12.1 3 0.00209 0.00080 0.06715 0.01328 19.91528 0.09831 96.9 3.2 181.9 2.4 4 0.00773 0.00122 0.09779 0.01304 21.48431 0.10374 89.4 2.8 181.1 3.4 5 0.00033 0.00010 -0.00514 -0.00211 19.07495 0.03702 99.5 23.5 179 0.9 6 0.00021 0.00017 -0.00220 -0.00300 19.06824 0.04045 99.7 16.4 179.3 1.0 7 0.00032 0.00052 0.01416 0.00829 19.30932 0.04019 99.5 6 181.2 1.6 8 0.00095 0.00032 0.00996 0.00463 19.31637 0.04432 98.6 9.5 179.6 1.2 9 0.00018 0.00028 -0.00597 -0.00502 19.21753 0.04147 99.7 8.6 180.7 1.1 10 0.00086 0.00066 0.01453 0.01576 19.53799 0.10873 98.7 2.6 181.8 2.2 11 0.00409 0.00214 0.07988 0.06748 20.50400 0.37173 94.1 0.9 181.9 6.5 12 0.00079 0.00421 0.10890 0.12942 19.56665 0.26322 98.9 0.5 182.3 11.4 13 0.00152 0.00298 0.07004 0.07867 20.02640 0.25952 97.8 0.8 184.4 8.3 14 0.00098 0.00069 0.08797 0.02122 19.71304 0.11079 98.6 3.7 183.1 2.2 15 0.00259 0.00088 0.01855 0.02569 19.97156 0.10569 96.2 2.9 181.1 2.6 16 0.00006 0.00048 0.01185 0.01249 19.01208 0.04852 99.9 5.3 179.2 1.6 81 ACCEPTED MANUSCRIPT 17 0.00304 0.00178 0.00742 0.03610 19.98134 0.20423 95.5 18 0.00008 0.00036 0.06078 0.01464 19.02252 0.02415 99.9 Steps 1–18 =plateau age: J-Value: 0.00557 +/- 0.00003 Ar/39Ar +/- 37 Ar/39Ar +/- 40 Ar/39Ar %40Ar* +/- RI 36 step 180.0 5.1 5.9 179.3 1.2 100.0 179.6 0.9 %39Ar age [Ma] +/- PT Eclogite sample: M40-57 Phengite (250–355 μm) 1.9 0.07632 0.03919 0.47284 0.65128 36.64878 1.70845 38.5 0.2 136.5 107.6 2 0.02320 0.00822 0.10532 0.11217 26.78140 0.55189 74.4 0.8 189.8 22.3 3 0.01291 0.00261 0.29603 0.08280 22.06891 0.36356 82.8 1.1 174.7 7.6 4 0.00060 0.00015 -0.00959 -0.00765 18.85135 0.03595 99.1 26.1 178.3 1.0 5 0.00007 0.00014 0.00027 0.00647 18.80477 0.03560 99.9 34.4 179.3 0.9 6 0.00098 0.00054 0.00694 0.01928 19.26090 0.10463 98.5 5.7 181 1.9 7 0.00028 0.00025 0.00038 0.00527 19.00866 0.06392 99.6 17.1 180.6 1.2 8 0.00175 0.00088 0.06814 0.02620 19.52981 0.07839 97.4 3.3 181.5 2.6 9 0.00073 0.00039 0.01956 0.00988 19.14105 0.06673 98.9 8.3 180.6 1.5 10 0.00261 0.00186 0.23084 0.05020 19.49722 0.16400 96.1 1.5 179 5.3 0.00052 0.00220 0.00972 0.04879 19.45055 0.25800 99.2 NU MA D =plateau age: 1.5 184 6.4 100.0 179.9 0.9 TE 11 Steps 1–11 SC 1 Eclogite sample: M40-2 Phengite (250–355 μm) step AC CE P J-Value: 0.00556 +/- 0.000026 36 Ar/39Ar +/- 37 Ar/39Ar +/- 40 Ar/39Ar +/- %40Ar* %39Ar age [Ma] +/- 1 0.13092 0.03387 0.32827 0.63706 39.76185 1.46682 2.7 0.2 10.8 98.8 2 0.02010 0.00481 0.20148 0.15219 21.96616 0.33463 73.0 0.9 154.0 13.3 3 0.01090 0.00206 0.15511 0.05007 21.88705 0.20656 85.3 2.3 178.1 5.8 4 0.00158 0.00014 -0.00153 -0.00528 19.18610 0.03771 97.6 20.8 178.5 1.0 5 0.00048 0.00009 -0.01091 -0.00312 18.83080 0.03633 99.2 39.4 178.2 0.9 6 0.00095 0.00021 0.02293 0.00783 19.04669 0.03337 98.5 13.0 178.9 1.0 7 0.00015 0.00015 0.01668 0.00408 18.61081 0.02016 99.8 23.4 177.1 0.9 100.0 178.1 1.0 Steps 1–7 = plateau age: Eclogite sample: M40-13 Phengite (250–355 μm) J-Value: 0.0055 +/- 0.00003 step 36 Ar/39Ar +/- 37 Ar/39Ar +/- 40 Ar/39Ar +/- %40Ar* %39Ar age [Ma] +/- 1 0.01875 0.00436 0.62417 0.97139 21.04472 0.30713 73.9 0.5 148.9 12.0 2 0.00543 0.00113 0.16687 0.53702 21.57549 0.09712 92.6 1.7 189.1 3.2 3 0.00383 0.00049 0.03322 0.12077 20.67332 0.06541 94.5 5.2 185.1 1.6 4 0.00067 0.00009 0.03367 0.06424 18.64208 0.02011 99.0 28.7 175.2 0.8 82 ACCEPTED MANUSCRIPT 0.00093 0.00016 0.00349 0.05480 18.67155 0.01754 98.5 17.3 174.8 0.9 6 0.00012 0.00011 0.02254 0.05666 18.42971 0.02946 99.8 16.0 174.8 0.9 7 0.00088 0.00248 0.06024 0.89002 19.83448 0.16592 98.7 0.8 185.5 6.8 8 0.00085 0.00025 0.01475 0.09226 18.85338 0.05750 98.7 6.9 176.7 1.2 9 0.00090 0.00022 0.00526 0.07656 18.74613 0.02875 98.6 9.3 175.5 1.0 10 0.00124 0.00092 -0.00241 -0.21725 19.15105 0.08037 98.1 2.9 178.3 2.7 11 0.00034 0.00020 0.00188 0.05253 18.71282 0.03335 99.5 10.8 176.7 1.0 Steps 1–11 =plateau age: 100.0 175.2 0.9 %40Ar* %39Ar age [Ma] +/- RI J-Value: 0.00548 +/- 0.00003 Ar/39Ar 37 Ar/39Ar +/- 40 Ar/39Ar +/- 0.01406 0.04716 0.12844 0.99458 23.27225 2.96535 82.2 0.1 179.8 126.6 2 0.01267 0.00632 0.11759 0.11336 25.02965 0.41188 85.1 0.9 199.0 16.9 3 0.00151 0.00081 0.03009 0.02112 19.86643 0.10261 97.8 5.9 182.4 2.4 4 0.00305 0.00038 0.03279 0.01449 20.44380 0.06792 95.6 12.1 183.5 1.4 5 0.00126 0.00028 0.01235 0.01360 19.85823 0.08267 98.1 12.2 183.0 1.3 6 0.00055 0.00017 -0.00015 -0.00780 19.60749 0.02957 99.2 29.8 182.6 0.9 7 0.00144 0.00033 0.01074 20.01286 0.03776 97.9 11.6 183.8 1.2 8 0.00505 0.00395 0.23937 0.14054 21.37801 0.22550 93.1 1.3 186.7 10.6 9 0.00011 0.00062 0.01278 0.01370 19.46703 0.07545 99.8 7.2 182.5 1.9 10 0.00053 0.00126 0.01418 0.03050 19.49083 0.10745 99.2 4.1 181.6 3.5 11 0.00026 0.00086 0.01281 0.02551 19.69418 0.11573 99.6 5.1 184.1 2.6 12 0.00002 0.00055 0.05896 0.01199 19.41180 0.03420 100.0 9.7 182.3 1.7 Steps 1–12 =plateau age: 100.0 182.9 0.9 MA 1 D +/- NU 36 SC Eclogite sample: M38-5A Phengite (250–355 μm) step PT 5 AC CE P TE 0.01274 Eclogite sample: M38-5B Phengite (250–355 μm) J-Value: 0.00546 +/- 0.00002 step 36 Ar/39Ar +/- 37 Ar/39Ar +/- 40 Ar/39Ar +/- %40Ar* %39Ar age [Ma] +/- 1 0.03440 0.00282 0.02516 0.02204 26.03587 0.34670 60.9 0.3 149.7 7.7 2 0.01633 0.00441 0.02101 0.03256 23.59212 0.23787 79.5 0.3 175.8 11.8 3 0.00664 0.00044 0.00190 0.00320 20.57041 0.05465 90.5 2.7 174.4 1.4 4 0.00611 0.00114 0.00208 0.01270 20.29473 0.15290 91.1 1.0 173.3 3.3 5 0.00176 0.00035 0.00084 0.00179 19.24339 0.03867 97.3 4.2 175.4 1.2 6 0.00120 0.00008 0.00015 0.00168 18.82925 0.03002 98.1 7.1 173.2 0.8 7 0.00176 0.00005 0.00013 0.00065 18.99354 0.03368 97.3 17.1 173.2 0.8 8 0.00069 0.00005 -0.00013 -0.00063 18.68412 0.05043 98.9 15.5 173.2 0.9 9 0.00127 0.00013 0.00319 0.00121 18.91666 0.03005 98.0 6.0 173.8 0.8 10 0.00123 0.00007 0.00592 0.00034 18.84722 0.03375 98.1 7.1 173.3 0.8 11 0.00039 0.00005 0.00157 0.00052 18.58180 0.04245 99.4 16.8 173.1 0.8 83 ACCEPTED MANUSCRIPT 0.00082 0.00011 0.00482 0.00125 18.69272 0.02934 98.7 7.1 173.0 0.8 13 0.00011 0.00055 0.01566 0.00675 19.02142 0.10135 99.8 1.6 177.8 1.9 14 0.00071 0.00041 0.00953 0.00451 18.97876 0.05164 98.9 2.4 175.8 1.4 15 0.00057 0.00020 0.00006 0.00283 19.15333 0.05251 99.1 4.0 177.7 1.0 16 0.00286 0.00060 0.01463 0.00779 19.60497 0.07061 95.7 1.3 175.7 1.8 17 0.00037 0.00080 0.01442 0.00914 18.92688 0.08753 99.4 1.4 176.3 2.4 18 0.00133 0.00093 0.02269 0.01205 19.53503 0.13591 98.0 1.0 179.2 2.8 19 0.00096 0.00028 0.02623 0.00357 18.94304 0.07845 98.5 2.0 174.9 1.2 20 0.00164 0.00062 0.04477 0.00684 19.32801 0.08869 97.5 1.3 176.5 2.0 Steps 1–20 =plateau age: 100.0 173.3 0.8 Ar/39Ar 37 Ar/39Ar +/- 40 Ar/39Ar +/- %40Ar* %39Ar age [Ma] +/- 1 0.01092 0.00515 0.17523 0.18168 29.73428 0.31536 89.2 2.6 241.5 13.2 2 0.00440 0.00170 0.07167 0.04271 23.19963 0.19367 94.4 9.5 201.7 4.7 3 0.00007 0.00023 -0.01055 -0.00706 19.62253 0.04045 99.9 78.0 181.5 1.0 4 0.01152 0.01754 0.00538 0.42478 16.62828 0.65111 79.5 0.9 124.4 47.4 5 0.00364 0.00996 0.21672 D +/- MA 36 RI SC J-Value: 0.00540 +/- 0.00002 NU Eclogite sample: M38-11A Phengite (250–355 μm) step PT 12 17.22163 0.31169 93.8 1.6 150.9 26.5 6 0.00306 0.00880 1.21387 0.25870 19.90443 0.26553 95.9 1.7 177.2 23.1 7 0.00928 0.00695 0.01325 0.14636 20.26798 0.23294 86.5 2.7 163.1 18.4 0.01212 0.00431 0.09863 0.10373 20.54044 0.31921 82.6 TE 3.0 158.1 11.6 100.0 181.5 1.0 1 AC CE P 8 0.24824 0.29516 0.46154 8.11643 12.25325 127.79072 68.36081 32.2 0.0 362.3 1036.9 2 0.02920 0.07148 1.57559 1.39116 43.87652 4.70602 80.6 0.1 314.0 174.8 3 0.00610 0.00218 0.00723 0.06740 25.03377 0.41101 92.8 1.7 212.0 6.5 4 0.00252 0.01271 0.01872 0.21638 22.01010 0.45103 96.6 0.5 195.0 32.9 5 0.00224 0.00047 0.00116 0.00594 21.29292 0.03776 96.9 7.4 189.5 1.5 6 0.00054 0.00049 -0.00400 -0.02107 20.23916 0.03471 99.2 11.0 184.7 1.5 7 0.00048 0.00007 -0.00189 -0.00222 18.98867 0.01349 99.3 44.4 173.9 0.8 8 0.00071 0.00017 0.00170 0.00460 19.44125 0.02910 98.9 22.0 177.2 0.9 9 0.00248 0.00036 0.02721 0.01305 19.89737 0.05620 96.3 8.9 176.7 1.3 10 0.00191 0.00125 0.05267 0.02201 20.14743 0.14678 97.2 4.1 180.4 3.6 Steps 1–10 =plateau age: 100.0 175.6 1.3 Steps 1–8 =plateau age: Eclogite sample: M38-11B Phengite (250–355 μm) J-Value: 0.00537 +/- 0.00002 step 36 Ar/39Ar +/- 37 Ar/39Ar +/- 40 Ar/39Ar Eclogite sample: M38-11C Phengite (250–355 μm) 84 +/- %40Ar* %39Ar age [Ma] +/- ACCEPTED MANUSCRIPT J-Value: 0.0054 +/- 0.00002 36 Ar/39Ar +/- 37 Ar/39Ar +/- 40 Ar/39Ar +/- %40Ar* %39Ar age [Ma] +/- 0.02282 0.00684 0.00724 0.05115 30.69912 0.52460 78.0 0.2 219.5 17.8 2 0.01675 0.00249 0.00413 0.01455 24.25629 0.25254 79.6 0.4 178.9 6.8 3 0.00262 0.00035 -0.00137 -0.00245 19.68813 0.05901 96.1 2.5 175.4 1.3 4 0.00137 0.00024 0.00044 0.00260 19.59080 0.04168 97.9 4.0 177.8 1.0 5 0.00050 0.00087 0.00227 0.00467 19.81753 0.11256 99.3 1.2 182.1 2.6 6 0.00025 0.00041 0.00012 0.00139 19.67194 0.06135 99.6 3.2 181.5 1.4 7 0.00021 0.00069 0.00127 0.00407 20.00675 0.03488 99.7 2.0 184.5 2.0 8 0.00036 0.00015 0.00031 0.00113 18.69121 0.02055 99.4 8.9 172.5 0.8 9 0.00132 0.00027 0.00056 0.00157 19.07941 0.02646 97.9 5.0 173.4 1.0 10 0.00063 0.00007 -0.00031 -0.00035 18.86438 0.03057 99.0 21.4 173.3 0.8 11 0.00057 0.00011 0.00021 0.00061 18.68815 0.03838 99.1 12.1 171.9 0.8 12 0.00028 0.00005 0.00299 0.00047 18.60676 0.03586 99.6 14.7 172.0 0.8 13 0.00091 0.00014 0.00875 0.00109 18.73030 0.03765 98.6 6.2 171.4 0.9 14 0.00130 0.00064 0.04899 0.00541 18.73302 0.12603 98.0 1.5 170.5 2.1 15 0.00516 0.00141 0.02519 0.01795 20.35794 0.14885 92.5 0.7 174.7 4.0 16 0.00821 0.00174 0.04262 0.01638 20.76419 0.19396 88.3 0.5 170.4 4.9 17 0.00278 0.00260 0.00550 0.03182 19.74711 0.37121 95.8 0.4 175.5 7.5 18 0.00009 0.00048 0.00211 0.00846 18.97679 0.06692 99.9 1.7 175.8 1.6 19 0.00379 0.00228 0.03836 0.01569 20.14759 0.16833 94.5 0.6 176.5 6.2 20 0.00002 0.00009 -0.00008 -0.00108 18.53765 0.02676 100.0 13.0 172.1 0.8 Steps 1-20 =plateau age: 100.0 172. 5 0.8 RI SC NU MA TE AC CE P PT 1 D step Paragneiss sample: S19-2A Phengite (355–500 μm) J-Value: 0.00558 +/- 0.00003 step 36 Ar/39Ar +/- 37 Ar/39Ar +/- 40 Ar/39Ar +/- %40Ar* %39Ar age [Ma] +/- 1 0.02866 0.00827 0.09558 0.11018 24.08808 64.80000 0.7 150.6 22.8 1.0 2 0.00555 0.00171 0.01029 0.04672 22.10799 92.60000 2.1 194.8 4.8 2.0 3 0.00093 0.00038 0.00554 0.00698 19.36802 98.60000 8.6 182.4 1.4 3.0 4 0.00063 0.00007 0.00091 0.00251 18.47913 99.00000 32.6 175.1 0.8 4.0 5 0.00091 0.00009 0.00102 0.00716 18.62702 98.60000 30.5 175.7 0.8 5.0 6 0.00283 0.00045 0.01940 0.01565 19.04891 95.60000 6.0 174.4 1.5 6.0 7 0.00257 0.00083 0.10376 0.02833 19.17359 96.10000 3.0 176.3 2.5 7.0 8 0.00637 0.00145 0.21019 0.04672 19.99139 90.70000 1.9 173.6 4.4 8.0 9 0.00230 0.00097 0.00215 0.03380 19.51689 96.50000 2.2 180.1 3.0 9.0 10 0.00312 0.00097 0.01610 0.03783 19.82013 95.30000 2.5 180.6 3.0 10.0 11 0.00494 0.00361 0.03652 0.07496 20.35420 92.80000 1.1 180.6 10.2 11.0 12 0.00389 0.00209 0.00841 0.03344 20.22387 94.30000 2.2 182.2 6.1 12.0 13 0.00251 0.00212 0.01311 0.01746 19.35900 96.20000 5.3 178.1 5.8 13.0 14 0.00557 0.00288 0.15985 0.09668 20.59772 92.10000 0.9 181.2 8.3 14.0 85 ACCEPTED MANUSCRIPT 15 0.00640 Steps 1-15 =plateau age: 0.01178 0.20088 0.35076 21.39059 91.20000 0.3 186.2 31.7 15.0 100.0 175.3 0.8 Paragneiss sample: S19-2B Phengite (250–355 μm) 36 Ar/39Ar +/- 37 Ar/39Ar +/- 40 Ar/39Ar +/- %40Ar* %39Ar age [Ma] +/- 76.8 0.3 178.3 10.1 0.01903 0.00369 -0.00129 -0.02610 24.27115 0.25956 2 0.00615 0.00058 0.00573 0.00446 20.76938 0.06119 91.2 1.6 181.1 1.8 3 0.00502 0.00083 0.00706 0.00490 20.40860 0.12111 92.7 1.2 180.9 2.6 4 0.00143 0.00010 0.00010 0.00050 18.84799 0.03370 97.8 9.6 176.3 0.9 5 0.00083 0.00010 0.00014 0.00084 18.56274 0.02937 98.7 9.0 175.3 0.9 6 0.00088 0.00014 0.00208 0.00079 18.56528 0.03842 98.6 5.4 175.2 0.9 7 0.00071 0.00020 0.00004 0.00104 18.59428 0.03964 98.9 5.8 175.9 1.0 8 0.00063 0.00012 0.00007 0.00053 18.51848 0.03163 99.0 11.0 175.4 0.9 9 0.00080 0.00004 0.00120 0.00041 18.50843 0.04904 98.7 23.4 174.9 0.9 10 0.00067 0.00050 0.02866 0.00558 18.57909 0.04683 98.9 2.0 175.9 1.6 11 0.00023 0.00007 0.00377 0.00084 18.25683 0.03920 99.6 12.4 174.1 0.9 12 0.00411 0.00065 0.05460 0.01067 19.48101 0.08997 93.8 0.9 174.9 2.1 13 0.00027 0.00013 0.00149 0.00140 18.22708 0.02374 99.6 6.6 173.8 0.9 14 0.00000 0.00024 0.00641 0.00243 18.33764 0.04495 100.0 3.3 175.5 1.1 15 0.00016 0.00025 0.00399 0.00203 18.55409 0.03869 99.8 3.1 177.1 1.1 16 0.00002 0.00019 0.00373 0.00123 18.51014 0.04617 100.0 4.4 177.0 1.0 Steps 1–16 =plateau age: 100.0 175.3 0.9 SC NU MA TE AC CE P RI 1 D step PT J-Value: 0.00558 +/- 0.00002 Metagranite sample: M48-2 Amphibole (200–250 μm) J-Value: 0.005257 +/- 0.000032 step 36 Ar/39Ar +/- 37 Ar/39Ar +/- 40 Ar/39Ar +/- %40Ar* %39Ar age [Ma] +/- 1 0.68544 0.01305 2.00497 0.05328 251.90916 4.51966 19.7 0.7 417.8 12.8 2 0.09938 0.00163 2.89712 0.02899 46.12903 0.28054 36.8 2.9 154.4 4.2 3 0.01565 0.00024 3.61877 0.00821 23.16837 0.03116 81.2 16.5 170.5 1.2 4 0.00980 0.00012 3.68215 0.00765 21.35404 0.03525 87.8 39.1 169.8 1.1 5 0.01374 0.00017 3.71913 0.01304 22.45173 0.04719 83.2 15.7 169.2 1.1 6 0.06337 0.00137 3.62754 0.02566 36.94384 0.12880 50.0 2.2 167.7 3.7 7 0.02109 0.00041 3.74856 0.01789 24.69091 0.10528 75.9 8.9 169.8 1.6 8 0.00693 0.00045 3.67426 0.02528 20.86523 0.05195 91.5 14.0 172.9 1.6 Steps 1–8 =plateau age: 100.0 170.1 1.0 Metagranite sample: M48-2 Biotite (200–250 μm) J-Value: 0.005299 +/- 0.000028 step 36 Ar/39Ar +/- 37 Ar/39Ar +/- 40 Ar/39Ar +/- %40Ar* %39Ar age [Ma] +/- 1 0.03745 0.00022 0.04279 0.00114 15.44515 0.06732 28.3 10.6 41.3 0.8 2 0.01292 0.00035 0.00274 0.00244 13.02351 0.03078 70.7 6.1 85.8 1.1 86 ACCEPTED MANUSCRIPT 0.00995 0.00029 0.00344 0.00208 13.58135 0.08091 78.3 6.1 98.8 1.1 4 0.00519 0.00012 0.00756 0.00208 13.51392 0.01621 5 0.00426 0.00006 0.03805 0.00042 13.20037 0.01546 88.7 8.1 110.9 0.7 90.5 33.4 110.6 0.6 6 0.00423 0.00018 0.03947 0.00166 13.19980 0.02836 90.5 8.4 110.7 0.8 7 0.00408 0.00066 0.00043 0.01091 13.14830 0.04750 90.8 2.6 110.6 1.9 8 0.00364 0.00013 0.04980 9 0.00365 0.00036 0.04290 0.00156 13.04781 0.02045 91.8 8.4 110.9 0.7 0.00358 12.95678 0.03750 91.7 3.5 110.0 1.2 10 0.00372 0.00021 0.22928 0.00233 12.93946 0.02740 11 0.00433 0.00067 0.11589 0.00723 13.53526 0.04930 12 13 0.00589 0.00075 0.32145 0.00900 13.96322 0.01336 0.00115 0.33785 0.01040 16.44504 14 0.02403 0.00256 0.22744 0.02030 19.89035 Steps 1–14 =plateau age: RI 6.4 109.8 0.8 90.6 1.9 113.5 1.9 SC 91.6 0.06162 87.7 2.3 113.3 2.1 0.10599 76.1 1.5 115.8 3.2 0.11633 64.4 0.7 118.3 6.8 100.0 101.7 0.3 NU MA D TE AC CE P 87 PT 3 NU SC RI PT ACCEPTED MANUSCRIPT AC CE P TE D MA Graphical abstract 88 ACCEPTED MANUSCRIPT AC CE P TE D MA NU SC RI PT Highlights This work reports, for the first time, age of eclogites in the Sanandaj-Sirjan zone. The Shahrekord eclogites are the oldest HP metamorphic rocks in Zagros orogenic belt. We suggest subduction of Neo-Tethyan occurred about the beginning of Early Jurassic. 89