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Abstract

Paleozoic of Jandaq and Posht-e-Badam ophiolites and Mesozoic ophiolitic mélanges of Nain and Ashin are located in the western part of the Central East Iranian Microcontinent. They are metamorphic relicts within the amphibolite facies of Paleo-Tethys and Neotethys Oceans. Petrological and geochemical studies of these Paleozoic and Mesozoic ophiolites suggest that they include oceanic basic dykes, pillow and massive lavas formed during mid-oceanic rifting processes. Parental basic rocks from Paleozoic ophiolites of Jandaq and Posht-e-Badam have tholeiitic to alkaline affinities. They are supposed to have been generated by lower partial melting of a relatively enriched mantle peridotite. Parental basic rocks from Mesozoic ophiolites have typical N-MORB affinity and were generated by partial melting of a depleted mantle source.
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... The Anarak ophiolite is one of the Paleozoic metaophiolites which are located in the western part of the CEIM and it is considered as a remnant of the Paleo-Tethys oceanic crust (Stampfli and Borel, 2004;Bagheri and Stampfli, 2008;Torabi, 2011). This ophiolite consists of mantle peridotite, serpentinized mantle peridotites, cumulates, gabbros, basic and ultrabasic dykes, pyroxenite, glaucophane-bearing metabasalts (blueschist), rodingite, listwaenite and trondhjemites (Sharkovski et al., 1984;Torabi et al., 2011). The Anarak ophiolite resemble the lherzolitic ophiolite type (Torabi, 2009). ...
... This ophiolite unconformably covered by Liassic continental molassic deposits, which later metamorphosed (Late Paleozoic metamorphic rocks) ( Figure 2). The mantle peridotites of Anarak ophiolite have undergone a high degree of serpentinization (Torabi, 2009;Torabi et al., 2011). ...
... The petrology and structural geology of the Anarak area has been studied by Sharkovski et al. (1984), Diefenbach et al. (1986), Stampfli and Borel (2004), Bagheri (2007), Bagheri and Stampfli. (2008) and Torabi et al. (2011). They have considered this ophiolite as belonging the Paleozoic ophiolites of Iran based on geological, geochemical and geodynamic investigations. ...
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The Anarak Paleozoic ophiolite with nearly east-west trend is located in the western part of the Central-East Iranian Microcontinent (CEIM). There are some leucocratic gabbros which are found as stocks and dykes that cross-cut the Anarak ophiolite. The studied gabbros are composed of clinopyroxene, amphibole, biotite, plagioclase, chlorite, epidote, garnet, sphene, apatite, prehnite, calcite, rutile, magnetite and ilmenite. Presence of a very high amount of hydrous minerals in the studied rocks reveal high activity of H2O in the involved fluids. Chemical characteristics of the Anarak gabbros show that residue minerals are garnet, amphibole and plagioclase. Major and trace element characteristics of the studied rocks are consistent with the magma that was formed by partial melting of a garnet-bearing amphibolite. These hydrous phases are possible products of the reaction between oceanic crust and the fluids of seawater derivation. Ingression of seawater through the oceanic crust and its downward penetration into the upper mantle caused the dissolution of a large amount of Ca and alkali components. At the lower parts of the oceanic crust, Ca added to the studied gabbros and produced new Ca-rich minerals. The Anarak samples show geochemical signatures of melts derived from a subducted oceanic slab. Distinct trace-element signatures show the similarity of the studied gabbros to adakite-like rocks. According to the age and location, it is suggested that the primary magma of the Anarak gabbros should be created by partial melting of Paleo-Tethys subducting oceanic crust
... Nain and Ashin ophiolites are two adjacent NeoTethyan ophiolitic mélange after closure of Nain-Baft oceanic crust in the northwest margin of CEIM (Central-East Iranian Microcontinent; Figure 1a). They are similar in most lithological and tectonomagmatic features (e.g., Shirdashtzadeh, Torabi, & Arai, 2011;Torabi, Shirdashtzadeh, Arai, & Koepke, 2011, etc.). Pirnia et al. (2010) introduced some plagioclase-bearing lherzolites in Nain ophiolite formed by melt impregnations. ...
... listwaenite, and rodingite; Shirdashtzadeh, 2014, Shirdashtzadeh et al., 2011, Shirdashtzadeh, Kachovich, Aitchison, & Samadi, 2015, mixed up in a matrix of serpentinite ( Figure 1b). 40 ...
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This study is focused on a plagioclase-bearing spinel lherzolite from Chah Loqeh area in the Neo-Tethyan Ashin Ophiolite. It is exposed along the west of left-lateral strike-slip Dorouneh fault in the northwest of Central-East Iranian Microcontinent. Mineral chemistry (Mg#olivine < ~ 90, Cr#clinopyroxene < ~ 0.2, Cr#spinel < ~ 0.5, Al2O3Orthopyroxene > ~ 2.5 wt%, Al2O3Clinopyroxene > ~ 4.5 wt%, Al2O3Spinel > ~ 41.5 wt%, Na2Oclinopyroxene > ~ 0.11 wt%, and TiO2clinopyroxene > ~ 0.04 wt%) confirm Ashin lherzolite was originally a mid-oceanic ridge peridotite with low degrees of partial melting at spinel-peridotite facies in a lithospheric mantle level. However, some Ashin lherzolites record mantle upwelling and tectonic exhumation at plagioclase-peridotite facies during oceanic extension and diapiric motion of mantle along Nain-Baft suture zone. This mantle upwelling is evidenced by some modifications in the modal composition (i.e. subsolidus recrystallization of plagioclase and olivine between pyroxene and spinel) and mineral chemistry (e.g., increase in TiO2 and Na2O of clinopyroxene, and TiO2 and Cr# of spinel and decrease in Mg# of olivine), as a consequence of decompression during a progressive upwelling of mantle. Previous geochronological and geochemical data and increasing the depth of subsolidus plagioclase formation at plagioclase-peridotite facies from Nain ophiolite (~ 16 km) to Ashin ophiolite (~ 35 km) suggest a south to north closure for the Nain-Baft oceanic crust in the northwestern of Central-East Iranian Microcontinent.
... The Central -East Iranian Microcontinent located between the Arabian and Eurasian plates (Fig. 1A). This microcontinent is surrounded by faults and fold-and-thrust belts, and the remnants of the Neotethys Ocean (ophiolites) in the west, including Naein, Ashin, Jandaq, Mashhad, Tchehel Kureh, Iranshahr, Fanuj-Maskutan, Band-e-Ziarat, Esfandagheh, Baft, and Surk ophiolites (e.g., Torabi, 2009;Rajabi and Torabi, 2012;Torabi et al., 2011;Shafaii Moghadam and Stern, 2014;Shirdashtzadeh et al., 2010Shirdashtzadeh et al., , 2011Shirdashtzadeh et al., , 2014. From east to west, CEIM consists of four major crustal domains of Lut Block, Tabas (Kerman-Tabas) Block, Posht-e-Badam block and Yazd (Naein) Block (Alavi, 1991) which are separated by a series of intersecting faults with nearly north to south direction (Fig. 1A). ...
... By Alpine-Himalayan orogeny in Cenozoic, closure of Neo-Tethys Ocean occurred along the suture zones and defined by ophiolitic outcrops from Baft, Naein, Ashin and Jandaq (e.g., Torabi, 2009Torabi, , 2010Rajabi and Torabi, 2012;Torabi et al., 2011;Shafaii Moghadam and Stern, 2014;Shirdashtzadeh et al., 2010Shirdashtzadeh et al., , 2011Shirdashtzadeh et al., , 2014etc.) (Fig. 7D). ...
... (1) Northeastward subduction of the Neotethyan oceanic crust along the Zagros Thrust Zone (ZTZ) beneath Central Iran from the Triassic to the Eocene [71] at distance of about 400 km from the study area. (2) Eastward subduction of the Eastern Neotethys branch from Mesozoic to Cenozoic (Naein and Ashin ophiolites) [72,76]. ...
Article
The Lower Oligocene Kal-e-kafi (East of Anarak, Central Iran) lamprophyres occur as stocks and dikes, which cross-cut the Eocene volcanic and Cretaceous sedimentary rocks. The predominant minerals of these lamprophyres are hornblende (magnesiohastingsite) and clinopyroxene (diopside) phenocrysts set in a fine- to medium-grained matrix of the same minerals plus plagioclase (labradorite to bytownite), sanidine, apatite, and magnetite. Secondary minerals are chlorite, magnetite, calcite, and epidote. Petrography, mineral chemistry, and whole rock compositions classify these rocks as calc-alkaline lamprophyre, in general, and spessartite in particular. These samples have intermediate compositions (SiO2 ~ 58 wt %). The chondrite- normalized REE patterns and primitive mantle-normalized multi-element spider diagram of Kal-ekafi lamprophyres are remarkably parallel and suggest that these dikes and stocks were derived from the same parental magma and underwent similar melt extraction. These rocks are enriched in alkalies, large-ion lithophile elements (e.g., Rb, Ba, K), and light rare-earth elements (e.g., La, Ce), and exhibit moderate to high fractionation in LREE patterns, with an average La/Lu ratio of 112. The large amount of hydrous fluids coming from the subducted slab rather than sediments caused to the enrichment and metasomatism of subcontinental lithospheric mantle source. Crustal contamination and assimilation of host rocks also played role in the genesis of these lamprophyres. Geochemical characteristics of the studied rocks suggest that parental magma have been derived from partial melting of a metasomatized amphibole-bearing spinel lherzolite of lithospheric mantle, which was previously modified by dehydration of a subducting slab. Subduction of oceanic crust around the Central-East Iranian Microcontinent (CEIM) is the most reasonable mechanism to explain enrichment in volatiles of the mantle, and the lamprophyric magmatism of the Kal-e-kafi area in Lower Oligocene times. Several tectonomagmatic discrimination diagrams indicate that the Kal-e-kafi lamprophyres occurred during postcollisional period of lithospheric extension.
... In both ophiolitic mélanges, metaophiolitic rocks include metagabbros, metaplagiogranites, amphibolites, banded metacherts, and successions of marbles and schists (Sharkovski et al., 1984;Shirdashtzadeh et al., 2010). Shirdashtzadeh et al. (2010) and Torabi et al. (2011b) suggested that the protholiths of the amphibolitic rocks from both Nain and Ashin ophiolites were represented by diabasic dykes and basaltic pillow lavas both showing normal mid-ocean ridge affinity (N-MORB). However, Shirdashtzadeh et al. (2011) also proposed an IAT affinity for the amphibolites. ...
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The Nain and Ashin ophiolites consist of Mesozoic mélange units that were emplaced in the Late Cretaceous onto the continental basement of the Central-East Iran microcontinent (CEIM). They largely consist of serpentinized peridotites slices; nonetheless, minor tectonic slices of sheeted dykes and pillow lavas - locally stratigraphically associated with radiolarian cherts - can be found in these ophiolitic mélanges. Based on their whole rock geochemistry and mineral chemistry, these rocks can be divided into two geochemical groups. The sheeted dykes and most of the pillow lavas show island arc tholeiitic (IAT) affinity, whereas a few pillow lavas from the Nain ophiolites show calc-alkaline (CA) affinity. Petrogenetic modeling based on trace elements composition indicates that both IAT and CA rocks derived from partial melting of depleted mantle sources that underwent enrichment in subduction-derived components prior to melting. Petrogenetic modeling shows that these components were represented by pure aqueous fluids, or sediment melts, or a combination of both, suggesting that the studied rocks were formed in an arc-forearc tectonic setting. Our new biostratigraphic data indicate this arc-forearc setting was active in the Early Cretaceous. Previous tectonic interpretations suggested that the Nain ophiolites formed, in a Late Cretaceous backarc basin located in the south of the CEIM (the so-called Nain-Baft basin). However, recent studies showed that the CEIM underwent a counter-clockwise rotation in the Cenozoic, which displaced the Nain and Ashin ophiolites in their present day position from an original northeastward location. This evidence combined with our new data and a comparison of the chemical features of volcanic rocks from different ophiolites around the CEIM allow us to suggest that the Nain-Ashin volcanic rocks and dykes were formed in a volcanic arc that developed on the northern margin of the CEIM during the Early Cretaceous in association with the subduction, below the CEIM, of a Neo-Tethys oceanic branch that was existing between the CEIM and the southern margin of Eurasia. As a major conclusion of this paper, a new geodynamic model for the Cretaceous evolution of the CEIM and surrounding Neo-Tethyan oceanic basins is proposed. © 2019 China University of Geosciences (Beijing) and Peking University
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The Ashin ophiolite is situated in the western part of Central Iran and presents two stages of Jurassic and Cretaceous spreading. The Ashin ophiolite represents fragments of the Neo-Tethys oceanic lithosphere. Plagiogranite intrusions of this ophiolite have good exposures. Plagiogranites of Cretaceous are more fresh than the metamorphosed samples of Jurassic. The main minerals of plagiogranites from the Ashin ophiolite are plagioclase, quartz and amphibole. Plagiogranites of the Jurassic have tholeitic nature with higher amounts of amphibole, \({\text{F}}{{{\text{e}}}_{2}}{\text{O}}_{3}^{*},\) TiO2, Co and lower values of Mg#, Th and Sr than the Cretaceous calc-alkaline plagiogranites. The chondrite-normalized REE patterns of these plagiogranites are characterized by higher values of REEs and negative Eu anomalies for the Jurassic samples and low values of REEs and positive Eu anomalies for the Cretaceous ones. Very low values of HREEs in the Cretaceous plagiogranites indicates a non-peridotitic source rock. We suggest that the Jurassic plagiogranites are formed by fractional crystallization of a low-K tholeitic magma; and the adakitic Cretaceous plagiogranites are formed by partial melting of an amphibolite in the subducting slab. Geochemical criteria of the Ashin plagiogranites indicate changing the Ashin ophiolite tectonic setting from a mid-ocean ridge system in the Jurassic to a supra-subduction zone in the Cretaceous.
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The Naein ophiolite is the most complete ophiolitic exposure in Cental Iran and considered as a remnant of the Mesozoic Central East Iranian microcontinent (CEIM) confining oceanic crust. In the northeastern part of this ophiolite (Darreh Deh area) within the mantle peridotites, a few hundred meters below the top of the Moho transition zone (MTZ), the hornblendites are present as dykes (former cracks and joints) from a few millimeters to nearly 50 cm wide. They have sharp boundaries with the surrounding mantle harzburgites and dunites. These hornblendites are pale green and coarse-grained in hand specimen and composed of magnesio-hornblende (Mg# = 0.93), chlorite (penninite and clinochlore, Mg# = 0.95), Cr-spinel (chromite, Cr# = 0.67 and Mg# = 0.55), tremolite, calcite and dolomite. Tremolites were formed by retrograde metamorphism of hornblendes. Calcite and dolomite occur as late-stage veins. Very high amount of primary hydrous phases (~94 vol % hornblende and chlorite), as well as peculiar mineralogical and chemical characteristics of the Naein ophiolite mantle hornblendites, do not match a magmatic origin. They are possibly products of the reaction between mantle peridotites and seawater-originated supercritical fluids, rich in silicate components. The presence of primary hydrous phases (hornblende and chlorite) may reveal high activity of H2O in the involved solution. The chemical composition of chromite in the hornblendites is near to the average chromite composition from the surrounding harzburgite and dunite. This suggests that the main source of Cr should be chromites of nearby peridotites, which were totally or partly dissolved by hydrothermal fluids. The positive anomaly of Eu in the chondrite-normalized REE patterns of hornblendes, high modal abundance of Ca-rich hornblende, as well as presence of calcite and dolomite, point to seawater ingression through the gabbros in to the uppermost mantle peridotites. The higher value of MgO than CaO, presence of high-Cr chromite and Cr-enrichment of hornblendes and chlorites indicate a higher contribution of peridotites rather than gabbros to the chemical characteristics of the involved fluids. This study shows that circulation of possibly seawater-derived high temperature hydrous fluids in the upper mantle can leach and provide necessary elements to form hornblendite in joints and cracks of the uppermost mantle.
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We present mineral chemistry data and petrological evidence from the Yarlung Zangbo suture zone ophiolites (Southern Tibet) suggesting that they represent a collage of heterogeneous massifs. Mantle sections in these ophiolites consist of harzburgite and lherzolite cut by several generations of gabbroic to diabasic intrusions, all affected by high-temperature deformation. Pyroxenitic bands are parallel to the mantle foliation. Crustal plutonic sections, consisting of dunite, wehrlite and gabbro, are thin or absent and have been observed only in the Dazhuqu massif Plagioclase is an additional phase associated with crustal peridotites. The mineral chemistry of silicate minerals and spinel in the mantle and crustal rocks varies widely and is believed to reflect complex melt percolation and reaction. The massifs record polybaric exhumation steps from at least 50 km depth to the plagioclase stability field. Pyroxene has re-equilibrated compositions from 1200 degrees C down to medium-grade metamorphic conditions. The mantle peridotites are interpreted as the residues of 10-40% partial melting of a fertile lherzolitic source. High Cr number, low TiO(2) content and relatively high Fe(3+) number of spinels suggest that the ophiolitic massifs were generated in a suprasubduction zone (arc or back-arc) environment.
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Plagioclase lherzolites of Nain mélange, Iran, show peculiar textures that indicate melt impregnation: (1) droplet or bleb-like grains of plagioclase distributed in the peridotite matrix, (2) plagioclase-bearing clinopyroxenite seams, and (3) trails of plagioclase crosscutting pyroxene porphyroclasts. The textural characteristics show post-deformational igneous formation of plagioclase, and possibly, associated clinopyroxene, from the impregnating melt. The melt has precipitated the clinopyroxenite seams and chemically modified all the peridotite minerals. Highly refractory compositions of the precipitated minerals suggest involvement of a highly depleted MORB-like melt. The melt was an increment of partial melt produced by 8% to 10% fractional melting from the MORB source. This is in contrast to the involvement of ordinary MORB in melt impregnation in abyssal plagioclase peridotites. Integration of increments of mantle partial melts to form MORB was possibly incomplete in the very incipient mid-ocean ridge as in the short-lived Nain back-arc basin.