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Two Mesozoic oceanic phases recorded in the basic and metabasic rocks of the Nain and Ashin-Zavar ophiolitic mélanges (Isfahan province, Central Iran)

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Abstract

The Mesozoic ophiolitic mélanges of Nain and Ashin-Zavar are located in the western part of the Central-East Iranian microcontinent (CEIM), along the major faults of Nain-Baft and Dorouneh. They contain two different groups of highly metamorphosed rocks (amphibolitic rocks, schists, marbles and quartzites) formed through metamorphism of oceanic basaltic and sedimentary units, and also some less metamorphosed rocks (sheeted dikes, pillow lavas, limestones and radiolarian cherts), that were tectonically melanged. These features show that they formed in two distinct phases. Geochemical data point to an island arc tholeiitic affinity for the amphibolitic rocks, and to a MORB nature for the pillow lavas and sheeted dikes that are related to a back-arc basin. Accordingly, oceanic crust extensional processes should have been active during two phases: a- In Early Jurassic, the Nain and Ashin-Zavar oceanic crust segments started spreading and producing diabasic dikes and pillow lavas, covered by pelagic sediments, then they suffered a high-grade metamorphism during the closure of this oceanic sector around the Middle Jurassic. b- During Early-Late Cretaceous to Paleocene, oceanic spreading produced sheeted dikes, massive basalts, and basaltic pillow lavas throughout the Austrian orogenic phase. There is no evidence of high-grade metamorphism as amphibolitic rocks. Radiolarian cherts and Globotruncana limestones of Late Cretaceous age cover the basaltic rocks.
... Presence of Paleo-Tethys and Neo-Tethys related ophiolitic suits in Iran point to the spreading and closure of oceanic crusts during Paleozoic and Mesozoic eras [26,34,35,[38][39][40][41]. Plagiogranites are one of the important rock units of Iranian ophioites. ...
... The Ashin ophiolite mélange is situated in western part of the CEIM, west of the Anarak area, and Ashin and Zavar farms (Figs. 1, 2). It is considered as a remnant of the Neo-Tethys ocean [35]. Geological field investigations suggest that the Ashin ophiolite and associated sedimentary and metamorphic bodies have been emplaced into their present crustal level during the Paleocene to Early Eocene. ...
... All of the rock units mentioned above intermixed forming a colored ophiolitic mélange. Predominant rock type of mantle peridotites is harzburgite [35]. ...
Article
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.
... The closure of the western branch of the Neo-Tethys ocean between the Iranian and Arabian plates during the Upper Cretaceous resulted in formation of several ophiolites from Iran to Oman. In the Early Jurassic (?) to upper Early Cretaceous-Paleocene (Glennie 1992, Torabi 2004, Reichert 2007, Shirdashtzadeh et al. 2011, the eastern branch of the Neo-Tethys Ocean (or the so-called Nain-Baft Ocean) was another longlived ocean surrounding the Central-East Iranian Microcontinent (CEIM) (e.g., Muttoni et al. 2009, Ghasemi & Talbot 2006, Shirdashtzadeh et al. 2011. Nain ophiolitic mélange is part of the eastern branch of the Neo-Tethys Ocean (Fig. 1A). ...
... The closure of the western branch of the Neo-Tethys ocean between the Iranian and Arabian plates during the Upper Cretaceous resulted in formation of several ophiolites from Iran to Oman. In the Early Jurassic (?) to upper Early Cretaceous-Paleocene (Glennie 1992, Torabi 2004, Reichert 2007, Shirdashtzadeh et al. 2011, the eastern branch of the Neo-Tethys Ocean (or the so-called Nain-Baft Ocean) was another longlived ocean surrounding the Central-East Iranian Microcontinent (CEIM) (e.g., Muttoni et al. 2009, Ghasemi & Talbot 2006, Shirdashtzadeh et al. 2011. Nain ophiolitic mélange is part of the eastern branch of the Neo-Tethys Ocean (Fig. 1A). ...
... This massif has undergone serpentinization and a pervasive high-grade metamorphism at amphibolite facies at ~630 -700 °C/ 7-15 kbar (Shirdashtzadeh et al. 2014) similar to the one in associated metamorphosed mafic rocks from the same ophiolite (i.e. amphibolites at 650 -800 °C/7-8 kbar; Shirdashtzadeh et al. 2010, Shirdashtzadeh et al. 2011. Throughout the Nain Ophiolite, rodingite was formed in association with gabbroic dikes intruded into the peridotites ( Fig. 2A), but in Darreh-Deh massif (Fig. 2B), rodingites occur locally as pale-green veins within the serpentinized peridotites (Fig. 2C). ...
Article
The petrography and mineral chemistry of the metamorphosed lherzolite in Darreh-Deh massif (east of Nain Ophiolite, Central Iran) is investigated in order to find the calcium source for rodingitization and tremolitization. In comparison with olivine and orthopyroxene, the clinopyroxene has lower modal content and is more alteration-resistant. The microprobe data and petrography of these lherzolites indicate that Ca2+ cations can be released during serpentinization of orthopyroxene (with ~18 vol% and CaO~2.7 wt%) and clinopyroxene (with ~6 vol% and CaO~ > 20 wt%). In contrast, pervasive serpentinization of mantle olivine with ~70 vol% and CaO~0.02 – 0.07 wt% is another expected source for producing Ca2+ rather than metamorphic olivine with CaO~ < 0.02 wt%. The released Ca2+ cannot be completely accommodated in crystal lattice of produced serpentine (with CaO~0.02 – 0.06 wt%), talc and chlorite (with CaO~0.015 wt%), but it can participate in formation of Ca-bearing tremolite (CaO~13 wt%), as a result of serpentinization of clinopyroxenes or subsequent metamorphism of peridotites at amphibolite facies and in formation of coarse-grained clinopyroxene blades and tremolite during rodingitization. Therefore, the calcium content in clinopyroxene, orthopyroxene and olivine of a plagioclase–free peridotite is a potential source of Ca2+, depending on the degree of serpentinization or chloritization.
... In the Yazd Block (Figure 1a), the Nain Mesozoic ophiolitic zone is a remnant of Neo-Tethys oceanic crust, occurring between Tertiary volcanic and sedimentary rocks to the north of Nain city (e.g., Davoudzadeh 1972) (Figure 1b). This ophiolitic mélange was formed along Nain-Baft fault, between the Sanandaj-Sirjan Zone and the CEIM, some time in late Lower Cretaceous-Palaeocene (e.g., Glennie 1992;Torabi 2004;Reichert 2007;Shirdashtzadeh et al. 2010Shirdashtzadeh et al. , 2011Shirdashtzadeh et al. , 2014. The Nain ophiolitic mélange (Figure 1c) is a mixture of various oceanic sedimentary, igneous and metamorphic lithologies, obducted onto the northwestern border of Yazd Block (e.g., Shirdashtzadeh et al. 2010Shirdashtzadeh et al. , 2014Shirdashtzadeh et al. , 2015Shirdashtzadeh et al. , 2020b. ...
... This is because its U-Pb zircon age (448 Ma, Ordovician) is much older than Mesozoic-Cenozoic ages (~ Early Jurassic to Eocene; e.g. Glennie 1992;Torabi 2004;Reichert 2007;Shirdashtzadeh et al. 2010Shirdashtzadeh et al. , 2011Shirdashtzadeh et al. , 2014Shirdashtzadeh et al. , 2020b suggested for subduction, closure and obduction of Neo-Tethys oceanic crust. Regarding the mélanges classification on the basis of processes and tectonic settings of their formation (Festa et al. 2010), then the Nain ophiolitic mélange is analogous to subduction and collision mélanges, generated by tectonic and gravitational processes during which exotic blocks are recycled from other previously formed mélanges. ...
Article
This study investigates relicts of some granitic Gondwanan basement unexpectedly outcropping in the northwest of Central-East Iranian Microcontinent (CEIM) and incorporated into an ophiolitic mélange. Based on petrographical (e.g. high modal content of muscovite (~10 vol.%), absence of hornblende, inherited zircons (>541 Ma)), geochemical (peraluminous and calc-alkaline S-type affinity, high silica, high ‘light rare earth element (LREE)/heavy rare earth element (HREE)’ ratios, negative Nb and Ti anomalies), and geochronological (magmatic zircon age ~448 Ma) results, it is an Ordovician anatectic granite formed from a sedimentary source during crustal thickening in a syn-collisional setting. It shows some signatures of metamorphic deformation (cataclastic fabric, quartz bulging recrystallization, and foliation) likely developed in the Devonian (~410 Ma). The U-Pb zircon ages from this granite are analogous to the other Ordovician collision-related magmatic events in the CEIM (Chahak to Airekan, Balvard). Our results confirm that Cadomian subduction and closure of the Proto-Tethys Ocean to the north of the Gondwana supercontinent resulted in crustal thickening during Ordovician collision-related magmatism and Devonian-Carboniferous regional metamorphism in the CEIM.
... This type of volcanism usually is associated with strike-slip faulting to allow the rapid lava ascending [36]. Subduction of the Neo-Tethys oceanic crust in this region commenced in the Triassic and terminated in the Eocene [49,67,78]. Geological study of the Naein and Ashin ophiolites concluded that in the Triassic the continental crust was disrupted along the Great Kavir fault [67], and spreading of the oceanic crust was started. ...
... Subduction of the Neo-Tethys oceanic crust in this region commenced in the Triassic and terminated in the Eocene [49,67,78]. Geological study of the Naein and Ashin ophiolites concluded that in the Triassic the continental crust was disrupted along the Great Kavir fault [67], and spreading of the oceanic crust was started. In the Early Eocene, tensional stresses gradually changed into compression, which was the beginning of the closure of the oceanic basin [58,75]. ...
... This tectonized ophiolite is an ophiolitic mélange, composed of a wide variety of tectonized ophiolitic igneous, sedimentary, and metamorphic rocks (e.g. pelagic limestones, radiolarian cherts, basaltic lava flows and pillow lavas, diabasic dikes, plagiogranites, gabbros, pyroxenites, chromitite, peridotites, marbles, schists, quartzites, skarns, banded metacherts, metagabbros, orthoamphibolites, metaperidotites, spilites, serpentinites, listwaenite, and rodingite; Shirdashtzadeh, 2014;Shirdashtzadeh et al., 2015;Shirdashtzadeh et al., 2011), mixed up in a matrix of serpentinite ( Figure 1b). 40 K-40 Ar dating of Sharkovski et al. (1984), and geological interpretations of Shirdashtzadeh et al. (2011) and Shirdashtzadeh, Torabi, Meisel, et al. (2014) suggested Jurassic to early Late Cretaceous ages for the spreading magmatism in this ancient oceanic realm. ...
... pelagic limestones, radiolarian cherts, basaltic lava flows and pillow lavas, diabasic dikes, plagiogranites, gabbros, pyroxenites, chromitite, peridotites, marbles, schists, quartzites, skarns, banded metacherts, metagabbros, orthoamphibolites, metaperidotites, spilites, serpentinites, listwaenite, and rodingite; Shirdashtzadeh, 2014;Shirdashtzadeh et al., 2015;Shirdashtzadeh et al., 2011), mixed up in a matrix of serpentinite ( Figure 1b). 40 K-40 Ar dating of Sharkovski et al. (1984), and geological interpretations of Shirdashtzadeh et al. (2011) and Shirdashtzadeh, Torabi, Meisel, et al. (2014) suggested Jurassic to early Late Cretaceous ages for the spreading magmatism in this ancient oceanic realm. Two distinct mid-Cretaceous radiolarian faunas, one mid-Albian (~107 Ma) and the other Turonian (~94 Ma) accumulated in a deep marine setting within the eastern branch of the Neo-Tethyan in Iran (Shirdashtzadeh et al., 2015). ...
Article
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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.
... 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. According to these authors, metacherts and marbleschists successions can be regarded as the original sedimentary cover of the pillow lava basalts. ...
... In fact, the estimated crystallization depths increase from IAT pillow lavas (8e19 km) to CA pillow lavas (w23 km). The Mesozoic ophiolites of Nain-Ashin were emplaced before the Paleocene, since they are covered by PaleoceneeEocene sedimentary rocks (Shirdashtzadeh et al., 2011, and references therein). ...
Article
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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
... 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). ...
... Central Iran from Late Triassic to Early Eocene [24,29,36,37,39,40]. Naein and Ashin ophiolites are remnants of this CEIM surrounding oceanic crust [27,32] (Figs. 1 and 2). ...
Article
The Early Oligocene alkalibasalts exposed in the Central Toveireh area located in the southwest of Jandaq city in Isfahan Province (Iran) and northwest of the Central-East Iranian Microcontinent (CEIM). Field studies reveal that these alkalibasalts crosscut the Eocene calc-alkaline volcanic rocks and granitoids and covered by the Miocene sedimentary rocks. The basaltic magma rose to the surface along the local faults. Based on the petrography, these alkalibasalts are composed of major minerals of olivine and plagioclase, and minor minerals of clinopyroxene, sanidine, Cr-spinel, and magnetite. The microscopic textures are porphyritic, microlithic porphyritic, trachytic, anti-rapakivi, corona, sieved texture, and poikilitic. Olivines are forsterite and chrysolite (Fo 0.90–0.75), plagioclases are labradorite to oligoclase (An63.1–20.7), alkali-feldspars are sanidine (Or50.3–61.5), clinopyroxenes are diopside and augite (Mg# 0.79 to 0.87), and Cr-spinels are hercynite (Cr# 0.24 to 0.25) in chemical composition, spinels (Mg# 0.72 to 0.76) are present as xenocrysts in some samples. The average contents of SiO2 and TiO2 of these rocks are 48.96 and 1.54 (wt %), respectively. The chondrite and primitive mantle-normalized diagrams characterized by enriched LREE relative to HREE, LILE enrichment, and absence of evident Eu anomaly. The normative content of nepheline reaches up to 14.6%. Modal and normative mineralogy, as well as geochemical data of minerals and whole rocks revealed that these rocks are sodic to highly sodic alkalibasalts formed in a within-plate continental tectonomagmatic setting after the cessation of subduction. The whole-rock chemical data indicate that the Central Toveireh alkalibasalts probably formed by relatively medium degrees of partial melting of an amphibole-bearing garnet lherzolite from the asthenospheric mantle of about 105 km in depth, which was previously enriched by subduction of the Neo-Thetyan slab.
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[PUBLISHED IN: NEW FINDINGS IN APPLIED GEOLOGY] ***** The study of petrological findings and geochemical data of mafic and ultramafic rocks and related petrographic units in the ophiolites can be an introduction to environmental geology studies, agriculture and natural resources. Regarding the urban and rural areas of Nain to Ashin in the vicinity of ophiolitic zone (Central Iran), geochemical investigations of mafic and ultramafic rocks of these ophiolites have very important applications in the field of environmental geology of these areas. Therefore, available geochemical data of basic rocks (pillow lavas and basalts), metabasic rocks (amphibolites) and metamorphosed and altered peridotites are considered based on environmental factors. For examples, in the peridotites which are one of the most abundant rock units in these ophiolites, the enrichment factor (Ef) for Ni is extremely high, for Cr is very high, and for Co is high. In addition, the geo-accumulation index or Igeo for Ni (~4) and Ni (>5) in peridotites is heavily to extremely high. therefore, high Ef and Igeo factors of the studied heavy metals (e.g., chrome, nickel, cobalt, arsenic, vanadium) and asbestose minerals (hornblende, tremolite, talc and chrysotile) in the mafic and ultramafic rocks of Ophiolites can be known as some dangerous environmental pollutants. Thus, investigatation of the volume of such elements penetrated into the surface and underground waters and soils in the villages and cities at the foot of these ophiolites could be a theme for ongoing studies in environmental geology of these areas. بررسی یافته‌های سنگ‌شناسی و داده‌های زمین‌شیمبایی سنگ‌های مافیک و الترامافیک و واحدهای سنگ‌شناسی مرتبط با آنها در افیولیت‌ها، مقدمه‌ای برای بررسی‌های زمین‌شناسی زیست‌محیطی، کشاورزی و منابع طبیعی است. با توجه به قرارگیری مناطق شهری و روستایی نایین تا عشین در نزدیکی مناطق افیولیتی (شمال‌غرب خردقاره مرکز – شرق ایران)، بررسی زمین‌شیمی واحدهای مافیک و الترامافیک این افیولیت‌ها از دیدگاه زمین‌شناسی زیست‌محیطی اهمیت بالایی دارد. برای این منظور، داده‌های زمین‌شیمیایی موجود برای سنگ‌های بازیک (گدازه‌های بالشی و دایک‌های دیابازی)، متابازیک (آمفیبولیت‌ها)، پریدوتیت‌ها ی دگرگون و دگرسان‌شدة این دو افیولیت از دیدگاه فاکتورهای زیست‌محیطی بررسی شدند. برای نمونه، در پریدوتیت‌ها که از فراوان‌ترین گروه‌های سنگی این افیولیت‌ها به‌شمار می‌روند، مقدار غنی‌شدگی (Ef) برای عنصر نیکل غنی‌شدگی بسیار بسیار بالا، برای کروم غنی‌شدگی بسیار بالا و برای کبالت غنی‌شدگی بالا را نشان می‌دهد. همچنین، مقدار شاخص تجمع زمین (Igeo) برای عنصرهای کروم (4 Igeo ~ ) و نیکل (5Igeo > ) در پریدوتیت‌ها به شدت بسیار بالاست. ازاین‌رو، مقدار بالای فاکتورهای Ef و Igeo فلزات سنگین بررسی‌شده (مانند: کروم، نیکل، کبالت، آرسنیک و وانادیم) و کانی‌های آزبستوزی (مانند هورنبلند، ترمولیت، تالک و کریزوتیل) در سنگ‌های مافیک و الترامافیک افیولیت‌ها می‌توانند از آلاینده‌های خطرناک زیست‌محیطی در این افیولیت‌ها به‌شمار بروند. بنابراین، بررسی میزان ورود این عنصرهای به آب‌های زیرزمینی و زنجیره غذایی ساکنین روستاها و شهرهای دامنه مناطق افیولیتی می‌تواند موضوع بررسی‌های زیست‌محیطی در این مناطق باشد.
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The early Oligocene alkali basalts are cropped out in the Central Toveireh area in the southwest of Jandaq and northwest of Central-East Iranian Microcontinent (CEIM). Field studies show that in this region, the faults’ function, especially the Great-Kavir fault, and occurrence of an extension zone have caused rapid ascent of magma, and therefore, the alkali basalts of Central Toveireh reached the surface through and along the faults. Based on the petrography studies, microscopic textures are including porphyritic, poikilitic, microlitic porphyritic, sieve texture, Trachyte, anti-rpakivi and corona.These basalts are composed of olivine (forsterite and chrysolite), clinopyroxene (diopside and augite), plagioclase (labradorite-andesine) and spinel (spinel-hercynite). Chemical composition of clinopyroxenes indicate that these rocks are similar to the within-plate continental basalts.
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The Nain-Baft ophiolitic suture, along the active margin of the Central Iranian continental block (the Sanandaj-Sirjan zone) features back-arc extension during the Late Mesozoic. This ophiolitic belt is characterized by occurrence of mafic lavas including pillow lavas, diabasic dikes and layers, massive basaltic lavas and basaltic-andesitic rock fragments in the volcanic breccias. These mafic lavas display both calc-alkaline and island-arc tholeiitic affinities with enrichment in LILE and depletion in HFSE. Conventional K-Ar measurements on amphibole indicate the Middle Cretaceous for the creation and evolution of the Nain-Baft back-arc basins. As a result of oblique subduction of the Tethyan Ocean, a narrow transtensional back-arc basin could start to open along large transcurrent faults in the active margin of the Iranian continental block.
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The fifth phase of Eocene volcanism in the central part of the Urumieh-Dokhtar magmatic arc (UDMA), comprises lavas with shoshonitic characteristics, ranging from trachybasalt to trachydacite. In shoshonites of the Qaleh-Khargooshi area, leucite changes to analcime, analcime to albite, sanidine to albite and anorthoclase, and clinopyroxene to albite. These reactions, together with other field, petrographic and geochemical features, are evidence of an original absarokitic magma which was changing to generate the different rock units. The parental magma of these shoshonites was produced by a low degree of partial melting from a metasomatized enriched mantle source. During the ascent of shoshonitic magma through the continental crust, crustal contamination, chemical changes and mineralogical transformations took place. The geochemistry of these shoshonitic rocks is similar to potassic volcanic rocks of continental arcs or convergent margins.
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The Nain ophiolite is a highly dismembered ophiolite complex cropping out at the north of the Nain town to the west of central Iran. The igneous rocks of this complex consist of both mantle and crustal suites and include serpentinized peridotites, peridotites, harzburgites associated with dunite and lherzolite, pegmatitic and isotropic gabbros, plagiogranites, sheeted dikes and pillow lavas. Several pyroxenite, wehrlite and rodingite dikes are present in the ultrabasic rocks. The sheeted dikes include subalkaline basalts, basaltic andesites and andesites. Their magma was of sub-alkaline (low potassium tholeiite) type and they are chemically similar to island arc tholeiitic basalts. The N-MORB-normalized incompatible elements for the sheeted dike samples indicate depletion in most of the high field strength elements (HFSE). The concentrations of the large ion lithophile elements (LILE) in these rocks are all greater than those in the N-MORB. Significant chemical characteristics of the these rocks are the positive anomaly for Th and negative anomaly of Nb that are considered to represent a subduction zone component. The chondrite-normalized rare earth element (REE) patterns of these rocks show HREE enrichment and LREE depletion [(LaN/SmN)ave = 0.63]. Their geochmistry also shows that the primary melt derived from high degrees of partial melting of a mantle source previously depleted with respect to the source of mid-ocean ridge basalts, and were subsequently enriched by aqueous fluids driven off subducted oceanic lithosphere in an arcbasin setting. We conclude that the Nain ophiolite is a supra-subduction zone type ophiolite.
Article
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.
Article
A simple general equation is presented for estimating the Fe ³⁺ concentrations in ferromagnesian oxide and silicate minerals from microprobe analyses. The equation has been derived using stoichiometric criteria assuming that iron is the only element present with variable valency and that oxygen is the only anion. In general, the number of Fe ³⁺ ions per X oxygens in the mineral formula, F , is given by; where T is the ideal number of cations per formula unit, and S is the observed cation total per X oxygens calculated assuming all iron to be Fe ²⁺ . Minerals for which this equation is appropriate include pyralspite and ugrandite garnet, aluminate spinel, magnetite, pyroxene, sapphirine and ilmenite. The equation cannot be used for minerals with cation vacancies (e.g. micas, maghemite) unless, as in the case of amphiboles, the number of ions of a subset of elements in the formula can be fixed. Variants of the above equation are presented for some of the numerous published schemes for the recalculation of amphibole formulae. The equation is also inappropriate for minerals showing Si ⁴⁺ = 4H ⁺ substitution (e.g. staurolite, hydrogarnet), minerals containing an unknown proportion of an unanalysed element other than oxygen (e.g. boron-bearing kornerupine) and minerals containing two or more elements with variable valency.
Article
It has been suggested that in a number of places such as Greece, Turkey, the Central Pamirs, and Thailand Neo-Tethys may have opened as a back-arc basin above a Palaeo-Tethyan subduction zone. The purpose of this paper is to test a similar suggestion for Oman. I review the late Palaeozoic to end-Mesozoic tectonic evolution of the Middle Eastern Tethysides in terms of a new tectonic model, whose main tenet is to regard the late Palaeozoic to Late Triassic basements of the Pontide/Dzirula/Adzharia-Trialeti/Arvin-Karabagh/Sanandaj - Sirjan zones collectively as a NNE-facing Palaeo-Tethyan magmatic arc, here named the 'Podataksasi arc' (or 'zone') whose Jurassic - Cretaceous movement with respect to Eurasia was responsible for much of the coeval deformation in Iran and Transcaucasia. The late Palaeozoic deformation of the Omani basement is regarded as a part of the retroarc fold and thrust belt of this arc and therefore independent of the Hercynides in Europe and NW Africa. The arc may have been compressional in late Carboniferous to possibly early Permian time and turned extensional in the earlier middle Permian. As a result, it rifted from NE Gondwana-Land, opening, successively, the Hawasina basin in the middle Permian and the main Neo-Tethyan ocean in the Triassic, together forming the 'Omani Neo-Tethyan back-arc basin complex'. Nearly all of the tectonic and magmatic characteristics of this basin complex are compatible with a back-arc basin interpretation, except perhaps the anomalously far inland location of the initial rift axis. A rather complete Mesozoic tectonic history of the entire Middle Eastern Tethysides is given to justify the model presented.