Abstract The study area is located in the north of Doroh and northern part the Sistan suture zone. Volcanic rocks such as basalt, Basaltic andesite, trachyandesite and andesite  with Cretaceous age are interbeded with  sedimentary rocks. Main textures in these rocks are porphyric, glomeroporphyric, intergranular, vesicular and amygdaloidal. Plagioclase and pyroxene are main minerals in basalt and basaltic andesite and main phenocrysts in andesite and trachyandesite are plagioclase and hornblende with minor of alkali feldspar. Geochemically, these rocks have calc alkaline and tholeiitic nature and show enrichment of LREE, LILE and depletion of Ti,Nb and P that support their relation to subduction zone. Enrichment  in U, Cs, K, Sr, Zr, Th can be result of crustal contamination. This magmatism can support  subduction of Sistan oceanic lithosphere in east of Iran.

The Eocene dykes in the north and the west of Zahedan exposed into the flysch and Zahedan granitoid rocks in the Sistan suture zone. These rocks, including mafic dykes, ultramafic and felsic are mostly alkaline in nature and sometimes are contaminated. The dykes in discussion, display mostly porphyritic and glomeroporhyric textures. Plagioclase, hornblende and biotite are the main phases as well as the matrix of the rocks. The presence of rounded plagioclase and corrosion Gulf of quartz, normative changes and variation in minor and trace elements are indicative of contamination with continental crust. Enrichment of LREE relative to HREE and the lack of Eu anomaly are important characteristics of all of the studied rocks. High ratios (La/Sm)N, and (La/Yb)N and(Sm/Yb)N and the alkaline nature of most of the samples, indicate a melting process about 1 percent of a mantle with garnet lherzolite composition. Such conditions may be completion of subduction of Sistan oceanic lithosphere beneath the Afghan block ended. Slab sinking to a considerable down, fluid released from the edge caused some chemical and mineralogical changes of mantle wedge, and to facilitate partial melting of the rocks. The fluid migration to higher levels due to the lower parts of the upper crust and mantle wedge are batch melting. As the molten materials increased, they were transferred to higher levels in the Oligo-Miocene period and have been emplaced in the upper parts of the crust. It seems that after the Oligo-Miocene time and as subduction continued, fractures developed and the ascending magma emplaced into these fractures, which finally gave rise to the formation of the study dykes.

Sistan suture zone ophiolitic rocks are remnants of Tethyan oceanic lithosphere between Lut and Afghan continental blocks. This oceanic basin has been existed at least during the Cretaceous. Mineral chemistry from crustal sequence ultramafic-mafic associations and petrographical evidences from parts of these ophiolitic complexes (the ophiolitic rocks located between the south of Birjand and Tchehel Kureh ophiolites) suggest distinct geodynamic setting for the formation of oceanic lithosphere. Plutonic rocks from crustal sequence mainly include ultramafic cumulates (dunite and pyroxenite)، cumulate gabbro، gabbronorite and isotropic gabbro. The crystallization trend in these rocks is variable including earlier crystallization of pyroxene relative to plagioclase and vice versa which implies SSZ (supra-subduction zone) and MORB (Mid-Oceanic Ridge Basalt) - type trends، respectively. The composition of plagioclase in cumulate gabbros and gabbronorite is bytownite and anorthite، respectively. Moreover، the contents of compatible and incompatible elements of clinopyroxene and orthopyroxene in ultramafic cumulates، cumulate gabbros and gabbronorite represent significant differences. The chemical composition of these minerals and crystallization trends indicate that pyroxenites-gabbronorite and troctolite-cumulate gabbros-isotropic gabbros have been formed from two different magma series with SSZ and MORB affinities، respectively. In the study area، it seems that the MORB-type magmatism resulted from partial melting beneath mid ocean ridge produced a depleted mantle. Subsequently، consequence of intra-oceanic subduction the MORB mantle has been suffered higher depletion due to fluid-induced melting in supra subduction zone setting and produced SSZ-type magmatism.

Ophiolites are a set of oceanic rocks with different appearance and mineralogy in the world's largest orogenic belt, from the Alpine to the Himalayas. The ophiolites of Iran are also located in this belt. Among ophiolites in Iran, the Nehbandan Ophiolitic Complex in the east of the country is of great importance. The complete ophiolitic sequence consists of two sets. The first is the crust sequence, gabbro, diabase and basalt, and the second is a mantle sequence or peridotites, both of which are sequences in the Nehbandan Ophiolitic Complex. The main purpose of this research study is mantle section. There are three study areas, located near the city of Nehbandan: Kalateh-Shahpori, Qadam Gah peridotites that are about 30 km northwest of the city of Nehbandan near the Chahar Farsang village and the third area is located between the Khansharaf village and Nasfandeh Kuh area that is 10 km east of Nehbandan.

In this lithological and mineralogical research study, thin and polished sections were prepared from samples. The thin sections were analyzed by polarizing OLYMPUS microscope BH-2 and the polished sections were analyzed by the OLYMPUS BX-60 reflecting microscope. A CAMECA SX100 electron probe microanalyzer was used to determine the chemical composition of the minerals in samples. The analytical condition include 15 kV and 20 nA rays with periods of 10 to 30 seconds at peaks for different minerals that are analyzed at the electron probe microanalysis center in the University Of Toulouse, France. The stoichiometry of minerals was used to calculate the amount of Fe3+ for access to the structural formula of minerals (Droop, 1987.

In terms of petrography, the Kalateh Shahpori peridotites are of the Harzburgite type and the Nasfandeh Kuh peridotites are of the Lherzolite type. The Qadam Gah peridotites are both geographically and petrographically indicative of the state of transition between the two other regions. The mineralogy of the Kalateh Shahpori peridotites is composed of olivine (Fo91), orthopyroxene (En90 Fs9), and the Cr-spinel is of the high Cr type. The Nasfandeh Kuh peridotites have olivine minerals that are Chrysolite (Fo89), orthopyroxene (En89 Fs9) and (En86 Fs10), clinopyroxene (En46 Fs5 Wo49) and, the Cr-spinel is of the high Al type. The Qadam Gah peridotites are composed of olivine (Fo90), orthopyroxene (En89 Fs9.5), clinopyroxene (En47 Fs3 Wo50) and, the Cr-spinel is of the medium Cr type.According to geochemical data and petrogenesis, the Kalateh Shahpori harzburgites are of the supra-subduction zone type in the forearc basin. The Nasfandeh Kuh Lherzolites are of the middle-oceanic type. The Lherzolites of Qadam Gah have the same characteristics of both regions in terms of the formation environment. However, they are much more similar to the middle-oceanic peridotites. The degree of partial melting of the peridotite has a direct relationship with the Cr content and it has an inverse relationship with the Al2O3 content in the chromium-spinel of the peridotite (Hellebrand et al., 2001). Probably, these lherzolites formed due to the re-fertilization of harzburgites (Monsef et al., 2018). Accordingly, Kalateh-Shahpori harzburgites with 20% partial melting are of high-grade, and the Nasfandeh Kuh Lherzolites with 5% partial melting are of the low grade type. The herzolites of the Qadam Gah are approximately 11% partial melting and are located between the Kalateh Shahpori peridotites and the Nasfandeh Kuh peridotites. The high degree of melting in the Harzburgites may indicate their remelting in the fluid environment because the hydrosis condition increases the degree of partial melting of peridotite (Hirose and Kawamoto, 1995). The Cr# in Cr-spinel, and the Mg# in olivine of the peridotites indicate the presence of at least 3 types of peridotites in the Nehbandan Ophiolitic Complex. According to mineralogy, petrography, geochemistry, and petrogenesis studies of the peridotites in the Nehbandan ophiolitic complex, it is recommended to explore possible chromite deposits, high melting and supra-subduction harzburgite zones such as Kalateh Shahpori harzburgites which should be considered to be the first priority. Then the peridotites of transition regions such as Qadam Gah should be at second priority and finally the low melting middle-oceanic lherzolites such as the Nasfandeh Kuh shuld be considered to be the third priority.

Ophiolitic outcrops of Sistan zone are remnants of Neo-Tethyan oceanic lithosphere, once existed between Lut and Afghan continental blocks. Mantle peridotites including harzburgites and cpx-harzburgites are dominant ophiolitic outcrops. Petrographical evidence together with geochemical characteristics such as CaO (0.88 – 2.94 wt.%), Al2O3 (0.76- 3.51 wt.%) and MgO (38.79- 45.25 wt.%) variations suggest different degrees of mantle depletion. The study of chemical composition of Cr-spinel shows it’s highly sensitive nature to alteration. During alteration, Cr-spinel commonly displays increasing Fe, Mn, Cr# and in some cases Si and decreasing Al, Mg and Mg# values. Since the chemistry of Cr-spinel is commonly used as a petrogenetic tool so it should be regarded with caution. Based on the chemistry of fresh Cr-spinels of a cpx-harzburgite sample, the melting percent of about 11 to 11.5% has been obtained. Moreover, the Cr# (26.5 to 27.5), Mg# (64.9 to 65.8) and Al2O3 (42.3 to 43.4 wt.%) of Cr-spinels suggest abyssal peridotite or MORB-like geodynamic setting. Although, based on the whole rock chemistry, the mantle peridotites display various depletion degrees which may imply various mid-ocean ridge to supra-subduction zone tectonic settings of ophiolite generation.

The post-ophiolite igneous rocks with dacite/rhyolite composition outcropped in the ophiolitic complex of Sabzevar zone. The calc-alkaline and high silica adakitic nature of these rocks has been proved previously. The rocks display initial 87Sr/86Sr ratios and εNd(t) values of 0.70444 to 0.70382 and +5.30 to +6.18 respectively, as same as these values in MORB compositions. These isotopic signatures are corresponding to adakitic compositions derived from melting of the Cenozoic oceanic subducted slabs. The emplacement age of these rocks into the Sabzevar ophiolitic zone obtained about 48 Ma by SHRIMP II U–Pb method on separated zircons. The results show that parental magma of the Sabzevar adakitic rocks generated by partial melting of a garnet-amphibolite or eclogite source derived from metamorphism of Sabzevar subducted oceanic lithosphere in early Eocene time. Magma differentiation via amphibole fractionation was the main factor in the manifestation of more adakitic signatures in more-evolved rocks (rhyolites).

We performed a 3D teleseismic tomography to image the lithosphere and upper mantle structures in the northwest of the Zagros collision zone and the Iranian plateau. The Iranian Plateau is a high relief region that has formed as a result of continental collision between the Arabian plate and Eurasia in the latter part of the Cenozoic. The Zagros and Alborz active tectonic belts are situated on the southwestern and northern margins of the Plateau, respectively. Our aim was to investigate the lithospheric structure and the geometry of the subducted oceanic slab in a region encompassing the Zagros and Alborz mountain ranges in NW Iran. For this purpose teleseismic data recorded at two temporary networks in NW Iran and several stations of the Iranian permanent networks were used in the ACH tomography scheme developed by Aki et al. (1977). A total number of 8164 seismic rays where used in the tomography. Checkerboard synthetic tests were performed to insure that the tomography had adequate resolution power in order to have reliable interpretation of the results. Our seismic tomograms distinguish three major anomalous regions in terms of velocity variation with adequate resolution in the study area. They are: 1) a high velocity anomaly corresponding to the Zagros lithosphere, 2) two low velocity anomalies underneath the Sahand and Sabalan volcanos in NW Iran, 3) and a deep high velocity perturbation delineating position and geometry of the subducted oceanic slab in the upper mantle. Our results show the lithosphere beneath the Zagros Mountains has a thickness almost twice as that in central Iran and the Alborz Mountains. NW Iran shows no high velocity character at shallow depths, indicating a thin and possibly warm lithosphere. In NW Iran the lithosphere reaches its minimum thickness anywhere throughout the Iranian Plateau. Two low velocities in our model indicate anomalously warm crust beneath the Sahand and Sabalan volcanos. These crustal anomalies link with deeper low velocity regions in the lithosphere and shallow upper mantle. The presence of low velocities at this depth range in NW Iran can either be related to partial melting associated with the mantle wedge region above the subducted slab, or to the possibility of lithospheric delamination. We have traced the Tethyan slab down to 650 km depth in the central part of the model. A discontinuity in the structure of the subducted slab has been mapped in the depth range of 250 km at the base of the Zagros lithosphere, which can be the location of a slab detachment in the central part of the model. The location and depth of the velocity discontinuity point to a post-collisional and relatively young slab breakoff of 10-5 Ma age in NW Zagros

Summary For the present two-dimensional (2-D) lithospheric density-temperature distribution modeling, we have selected a north-south geo-transect cut in east of Iran from the Oman Sea to the Lut block with a length of about 800 km. As the structural domains, investigated in this paper, have mainly been created under the effect of the north-south subduction of the Arabian Oceanic Plate under the Eurasia, thus, most of these structures are approximately perpendicular to this direction. As a result, in order to avoid three-dimensional (3-D) structural effects in our 2D modeling, we have selected a profile perpendicular to the strike, which is roughly the north-south direction.
Introduction In order to avoid interpreting isolated anomaly features being located accidentally on the profile of the geoid, free air and topography data, we average the data within a width of 50 km to both sides of the profile and calculate the variance within this stripe using 5 km long steps along the profile. The resulting standard deviation is plotted as uncertainty bars. Since the lateral resolution of our data is of the order of several kilometers, therefore, we cannot reproduce small‐scale details. The generated model consists of a sedimentary layer, the upper, middle and lower crust and a lithospheric mantle layer. In order to constrain the various thicknesses, the results of previous works carried out in the study region have been used.
Methodology and Approaches Modeling carried out in this research is based on the assumption of local isostatic equilibrium and joint interpretation of gravity, geoid and topography data with temperature-dependent density. Bouguer anomaly data are mainly sensitive to crustal density distribution and decrease proportionally to the squared distance (r) from an object, whereas elevation is very sensitive to changes in density and thickness of the lithospheric mantle and the geoid undulations diminish proportionally to r−1. The data different sensitivity on distance from the object can be a good constraint on modeling lithospheric density-temperature distribution.
Results and Conclusions Because of subducting Arabian plate in Makran subduction zone, thickness of the lithosphere increase from the Oman Sea (90 km) to the Taftan-Bazman volcanic arc, with a maximum thickness (290 km) under the volcanic arc. Based on our modeling results, subduction of the oceanic crust of Arabian plate under the Makran belt starts with a very low dip angle and bends under the Taftan-Bazman volcanic arc with a relatively higher dip. There is a logical correlation between the subducted slab and the seismicity of Makran subduction zone. Because of the low dip subducting slab, there is a long distance between volcanic arc and forearc setting (~450 km). Further to the north, lithospheric thickness is reduced to the ~200 km under the Lut block which is in good agreement with earlier studies. Our integrated modelling approach reveals a crustal configuration with the Moho depth varying from ∼21 km beneath the Oman Sea, and ∼47 km beneath the Taftan-Bazman volcanic arc to about 37 km beneath the Lut block. Across the Makran subduction zone, Moho depth is increasing from the Oman seafloor and Makran forearc setting (~21 km) to the volcanic arc (∼45–50 km). Furthermore, we have found that the oceanic crust is a two layered plate and there is a thick sedimentation there to about 8-9 km.

The study rocks outcropped in the northeast Semnan area. These rocks, composed of monzogabbro-monzodiorite, was intruded in the Eocene sedimentary and volcanic rocks. Plagioclase and clinopyroxene are the main constituent minerals of these rocks. The study rocks represent granular and poiiklitic textures. Based on EMPA data, clinopyroxenes, and plagioclases are diopside and augite and andesine, labradorite and bitonite in compositions and formed in temperatures ranged from 1104 to 1168 and less than 700 °C respectively. On the basis of tectonic discrimination diagrams, the investigated samples fall into volcanic arc domain resulted in subduction of Neothetian oceanic lithosphere beneath Central Iran block.

Volcanic and volcaniclastic rocks with Cenozoic age are widespread at Bozghush Mountain, south of Sarab, northwest of Iran. The composition of the studied rocks is trachybasalts, trachyandesites and megaporphyritic andesites with probable middle Eocene age(?). The main texture of these rocks is porphyritic. Plagioclase, clino-pyroxene and sanidine are the main minerals in these volcanic rocks. All volcanic sequences show similar trace element and REE patterns, with troughs in Nb, Zr, strong enrichments in Rb, Ba, Th, La and depletions in Ti, Yb, Y, relative to N-MORB, indicating a subduction-metasomatized lithospheric mantle as their melt source. On the basis of Sm/La versus Th/La diagram and Ba/Nb (>49) and Ba/Ta (>900) ratios, the influence of slab-derived sediments and fluids on the genesis of these rocks is obvious. It seems that the lithospheric mantle source beneath the Bozghush Mountain was metasomatized by subducted NeoTethys oceanic lithosphere derived fluids and subsequent slab break, delamination of the mantle lithosphere and tectonic extension due to the roll-back of the subducting Tethyan lithosphere which gave rise to decompression melting of the subduction-metasomatized lithospheric mantle and the generation of the studied rocks at Bozghush Mountain.