Sirjan Zone Research Articles

Calcite e-twins as a tectonic indicator, paleo stress pattern and structural evolution of the Zagros hinterland, SE Iran

Through the examination of calcite twins, this research outlines the tectonic development and paleo stress patterns of the Paleozoic Routshon complex situated in the southeastern segment of the Sanandaj–Sirjan zone, a hinterland region of the Zagros orogeny in southeastern Iran. The study of orogenic phase indicates that the deformation event affecting the southern sector of the Sanandaj-Sirjan zone aligns with the Cimmerian orogenic phase of the Late Triassic period. A variety of structural features at both map and outcrop scales highlight the importance of slip partitioning in the structural evolution of this region, driven by inclined transpression. Observations suggest that the deformation related to contractional components includes steeply to moderately plunging folds, dip-slip domain deformation primarily involving thrusts, and ongoing deformation by strike-slip component motion, which results in thrust-related ductile shear zones. The analysis of calcite c-axis fabrics from mylonite samples obtained from these shear zones indicates a low-temperature monoclinic pattern of non-coaxial deformation. This deformation type underscores the impact of the strike-slip component in the development of progressive simple shear within thrust-related shear zones in this segment of the Sanandaj-Sirjan zone. Dynamic analysis of c-axis fabric data reveals a NE-SW orientation for the principal compressive axes (σ1) in this area. This direction, corroborated by additional data such as fault surface, GPS, and earthquake focal mechanism data, confirms that the orientation of the compressive axes (σ1) has remained consistent from the Late Triassic to the present.

Petrography, U–Pb geochronology and geochemistry of Varcheh intrusions: Insight into younging trend of mid‐Cretaceous subduction in the northern Sanandaj–Sirjan Zone, western Iran

The Neotethyan Sanandaj–Sirjan Zone of western Iran has recorded major magmatic activities due to its continental arc tectonic setting during the Mesozoic. The Varcheh mafic intrusions were less‐studied plutons in the northern Sanandaj–Sirjan Zone (SSZ). Field evidence, petrography, geochemistry and U–Pb geochronological data were used to determine petrographic composition, geochemical nature, crystallization age and also to suggest a conceptual tectonomagmatic model for their emplacement. Small plutonic bodies are dominantly composed of monzogabbro that have intruded into the Cretaceous sedimentary rocks. Based on U–Pb zircon datings, these rocks have crystallized at 125–118 Ma in late Early Cretaceous (Barremian–Aptian), and are older than the supposed ages reported on geological maps. Varcheh rocks are not just typical calc‐alkaline rocks and some show alkaline affinity. Negative anomalies in Nb–Ta–Ti and enrichments in some large‐ion lithophile elements on spider diagrams are consistent with a subduction‐zone setting. Potential deep source for magma generation is partial melting of subcontinental lithospheric mantle wedge above a subducting slab of oceanic lithosphere. The spaces for the Varcheh mafic intrusions are accommodated by dominant dextral strike‐slip movement in a continental arc experiencing extension during late Early Cretaceous subduction. According to the zircon U–Pb geochronology results in this paper and previous U–Pb ages in the northern part of the SSZ, the mid‐Cretaceous magmatism reveals a significant NW‐ward younging trend and migration of the magmatic arc from the Barremian–Aptian in south‐east to the Albian–Cenomanian in the north‐west.

Geology and ore fluids geochemistry of the Hamzeh–Gharanin orogenic gold deposit, NW Iran

The Hamzeh–Gharanin gold deposit (HGD) is located in the Qolqoleh–Kasnazan shear zone in the northwest of Sanandaj–Sirjan Zone (SSZ) and 40 km SW of Saqez. The main rock units are an assemblage of the Precambrian metasedimentary (pelitic) rocks and the Cretaceous phyllite associated with marble interlayers. The post–Cretaceous intrusive granitoids with NW–SE trends intruded within these two units. Mineralization occurs mainly within the NE–SW extensional veines and veinlets in proto–mylonitic, mylonitic and ultra–mylonitic rocks. Hydrothermal alterations consist of sericitization, carbonatization, silicification and chloritization. The main ore minerals include pyrite, chalcopyrite, arsenopyrite, pyrrhotite, sphalerite, galena, gold, magnetite and hematite. The fluid inclusions study from quartz minerals associated with the mineralization shows four different phases: single vapor phase (V; type I), single liquid phase (L; type II), liquid–vapor biphasic (LV; type III) and three phases (LLV; type IV). Microthermometric studies were performed on types III and IV of fluid inclusions. Homogenization temperatures (Th) for Types III and IV inclusions vary between 138 and 310 °C (mean 230 °C) and 250–335 °C (mean 300 °C) and their salinity varies between 0.52 and 13.87 and 4.79 to 9.58 (wt% NaCl), respectively. Moreover, the density of Types III and IV inclusions vary from 0.71 to 0.927 g/cm3 and 0.700–0.870 g/cm3, respectively. The oxygen (in quartz: 4.58–7.80 ‰; mean 5.82 ‰) and δ34S (in pyrite: 5.54–6.28 ‰; mean 5.91‰) isotope compositions of the mineralizing fluids indicate an insignificant meteoric water input during hydrothermal evolution and a magmatic origin of sulfur in the deposit which is broadly similar to those of orogenic gold deposits worldwide.

Geochronology, petrogenesis, and tectonic setting of amphibolitic rocks from the Tutak metamorphic Complex, Sanandaj-Sirjan Zone, Iran

Amphibolitic rocks of the Tutak Metamorphic Complex (TMC), in the southern part of the Sanandaj Sirjan Zone (SaSZ), are associated with meta-dolerites, felsic intrusions, mica-schists, and marbles as bodies and interlayers. Rutile grains from an amphibolite (ES-18-1) yield a U-Pb age of 56.08 ± 1.13 Ma (Late Paleocene-Early Eocene) while Zr-in-rutile thermometry indicates peak temperatures ∼ 616 to 674 °C. These data are inferred to reflect the timing of medium-pressure/medium-temperature (MP/MT) amphibolite facies metamorphism in Late Paleocene-Early Eocene rather than upper Cretaceous time in the TMC complex. Geochemically, the amphibolites are from an Nb-enriched basalt-andesite to basalt protolith with sub-alkaline to alkaline affinity. The amphibolites have moderate 87Sr/86Sr(t) ratios (0.702451 to 0.712720), 143Nd/144Nd(t) ratios (0.512122 to 0.512209), positive ƐNd(535 Ma) (3.38 to 5.08) values and two-stage Nd model ages (TDM) between 0.84 and 1.3 Ga. The geochemical trends and (La/Yb)cn> 3, Nb/La> 0.6, Sm/Nd< 0.3, Zr/Y (4.42 to 7.47, avg. 5.72) ratios with moderate 87Sr/86Sr(535) ratios and positive ƐNd(535 Ma) values indicate that the mafic protoliths of the amphibolites are probably derived from the low partial melting of subcontinental lithospheric mantle in a spinel lherzolite field, with enriched mantle (E-MORB/OIB-like) nature. Subsequently, they experienced minor crustal contamination in the early stages of the development of intra-continental-type back-arc basin (BABB). Lack of subduction signals in the amphibolites with E-MORB/OIB-like features may result from the roll-back of subducting Proto-Tethys oceanic slab during the Cambrian similar to other peri-Gondwana terranes.

Mode and timing of the Early Cretaceous transgression in Iran: Insights from the stratigraphic evidence of sea-level changes and geodynamic events

The study of selected logs from the Central-Eastern Iran Microplate (CEIM; area of Arusan) and Sanandaj–Sirjan Zone (SSZ; areas of Esfahan east and Tiran) constrains timing and evolution of the Cretaceous successions, resting unconformably on older sediments or non-conformably on the metamorphic basement. The Cretaceous marine transgression occurred almost synchronously on a levelled topography, documenting a previous deep erosion. Above a basal siliciclastic unit, rudist- and orbitolinid-rich lower Aptian limestone (“Orbitolina Limestone”, Shak Kuh Formation, Taft Formation) were deposited. Later, the stratigraphic evolution of the three studied areas differentiates, documenting changes in the depositional environments, reflecting different subsidence rates. In the Tiran area, marls and cherty limestones covering shallow-water limestones record a deepening trend. A differentiation of depositional environment is observed at Arusan, where conglomerates to the west pass to basinal sandstone and marls to the east. Here, a Coniacian-Campanian reprise of the carbonate production (with skeletal limestones rich in rudists, ostreids, and abundant orbitoidids) is covered by cherty marls. In the Esfahan north area, glauconitic hardground marls covered by chert record important deepening during the early Cenomanian, followed by the reprise of shallow-water carbonate production (middle Cenomanian). The synchronous deposition of Aptian carbonate successions on large parts of the CEIM, SSZ and North-Iranian domains suggests a rapid regional transgression controlled by a sea-level rise. Stratigraphic differentiation occurring in late Aptian–Cenomanian times, recording the onset of tectonic events differently affecting the aforementioned domains, likely related to the opening of oceanic seaways in the CEIM and the SSZ.

Gold metallogeny in Iran; implications for gold exploration and conceptual modelling

Significant progress in the classification, definition and understanding of the main Au deposit types could significantly aid improvements in Au exploration. Because of the wide occurrence of Au in the central part of the Tethyan Eurasian Metallogenic Belt, Iranian structures composed of more than 17 zones (arcs and blocks) are considered as having one of the largest Au reserves in the Middle East. Without attempts at understanding the tectono-magmatic evolution of Iran and the geodynamic settings of Au deposition, the establishment of a reliable predictive exploration model for Au-type deposits in Iran and other parts of the world will be unsuccessful. By considering a total of 33 Au deposits and prospects in Iran, a mineralization sequence is revealed from orogenic and volcanogenic massive sulfide (VMS), orogenic, Carlin-type, epithermal/porphyry Cu–Au/skarn, epithermal/iron oxide–copper–gold (IOCG). The trend of deposition gradually changes in the SW–NE axis to intrusion-related, epithermal and porphyry Cu–Au deposits at the Urumieh–Dokhtar Magmatic Arc (UDMA) and post-arc magmatism. VMS deposits occur adjacent to the NE Zagros Fold and Thrust Belt, at the Sanandaj–Sirjan Zone. The Zagros Orogeny and associated post-collisional magmatism at UDMA host many porphyry, epithermal and intrusion-related Au deposits, with a major magmatism peak in the Miocene. This work reveals that orogenic and Carlin-type Au mineralization are linked genetically. After each associated subduction for Palaeotethys (286–215 Ma) and Neotethys (210–68 Ma) in Iran, VMS and orogenic Au-deposits are formed in the border of the subduction (±obduction) zone. The porphyry, intrusion-related, epithermal and IOCG mineralization are emplaced in appropriate formations and structures during collision and post-collision processes.

Geochemistry and Sr–Nd isotopic characteristics of ferroan-magnesian metaluminous granites of the NW Sanandaj–Sirjan zone, Iran: granite formation in a compressional–extensional setting during Late Jurassic time

AbstractThe Almogholagh–Dehgolan region is in the North Sanandaj–Sirjan zone of NW Iran. The granites of the region are metaluminous and display geochemical and textural characteristics of transitional granites between ferroan (A-type) and I-type granites. In geotectonic discrimination diagrams, the Almogholagh–Dehgolan granites plot mainly in the fields of within-plate granites and volcanic arc granites. With the exception of the Qalaylan granites, parts of other granites resemble A2-type granites. Granites of the Qalaylan intrusive body have petrographic and geochemical features close to I-type granites and are not A-type. Primary mantle and chondrite-normalized spider diagrams show enrichments in light rare earth elements relative to heavy rare earth elements. For an age of 150 Ma, the initial87Sr/86Sr and143Nd/144Nd ratios vary from 0.702769 to 0.706545 and from 0.512431 to 0.512558, respectively. Epsilon Nd values vary in a relatively limited range between −0.3 and +2.2, which corresponds to a mixed mantle–crustal source. On the basis of new geochemical and isotopic data, we suggest a geodynamic model involving partial melting of lower crustal rocks with the contribution of mantle magmas in a weakly extensional tectonic setting for the generation of the A-type granites of the region. The occurrence of ferroan (A-type) granites in this region of the Sanandaj–Sirjan zone indicates the existence of a partly extensional tectonic environment in a mainly compressional subduction-related regime in Late Jurassic time.

The coherence function and lithospheric elastic thickness of the Zagros fold and thrust belt

SUMMARY This study derives the spatial variation of the elastic thickness (Te) and its implications for understanding the structure, geodynamic and seismicity of the lithosphere for the Zagros fold and thrust belt region of the Arabia–Eurasia collision zone. Te is calculated using the coherence function in the fan wavelet domain based on recent terrestrial Bouguer gravity and topography data as input signals. Utilizing the load deconvolution method and Brent's method of 1-D minimization, the final Te for the survey region is estimated for each grid node of the studied area. To illustrate the mass distribution in the studied area, the subsurface loading fraction (F) is calculated simultaneously with Te in the inversion. The crust thickness and density from three different global crustal models are tested and the results obtained for these input models do not yield substantially different Te patterns. The final results are in accord with the global Te models as well as previous rheological, geodynamical and flexural studies, however, this study establishes much more detailed regional information. The calculations yield a mean value of Te of 61 km for the Zagros, with a mean estimated error of about 5 km. The high-Te values (&amp;gt;70 km) are observed in the southeast of the studied area (some parts of the Sanandaj–Sirjan zone, Urumieh–Dokhtar magmatic arc and most of the Central Iranian blocks); while over most of the northwest of the studied area, the value of Te is about 58 km. The Te results are consistent with the lithospheric structure of the study area and also support the idea of the crust–mantle decoupling. Further, there is a positive and negative correlation between the surface wave velocity and surface heat flow, respectively. The mean value estimated for the internal loading friction (F) of 0.4 means in most of the studied areas we may consider that the surface loading is dominant, or at least the ratio of the surface and subsurface loading can be assumed equal. Based on earthquake distribution in the period 1900–2020, seismicity is more likely to occur in areas with a relatively low value of Te.

Prediction of tunnel squeezing in soft sedimentary rocks by geoelectrical data

Squeezing is time-dependent deformation that can cause technical difficulties and financial consequences in underground structures. This study employs electrical resistivity data to predict squeezing intensity along the Beheshtabad tunnel in the Sanandaj–Sirjan zone in Iran. For comparison analysis, a semi-empirical approach was correlated with numerical modeling to predict tunnel squeezing at the design stage. The squeezing intensity obtained for the Beheshtabad tunnel was then compared with the instability observations along the Golab tunnel excavated in sedimentary rocks of the same zone. We developed a relationship between electrical resistivity and strains and provided a new strain-based squeezing classification system. The calibrated electrical data produced more accurate results for predicting tunnel squeezing than the conventional methods. The results show that rock type, joint properties, and water saturation impact squeezing.

Petrogenesis and geochemical characteristics of Plio-Quaternary alkali basalts from the Qorveh–Bijar volcanic belt, Kurdistan Province, NW Iran

AbstractThe Pliocene–Quaternary volcanic rocks which outcrop between Qorveh and Bijar are part of post-collisional within-plate volcanic activity in northern Iran. These mafic alkaline rocks form part of the northern arm of the Sanandaj–Sirjan (Hamedan–Tabriz) zone. Thermobarometry on equilibrium clinopyroxene – whole-rock pairs yields pressures and temperatures of 4–6 (±1.8) kbar and 1182–1213 (±27) °C, respectively; olivine – whole-rock (melt) equilibrium thermometry yields crystallization temperatures of 1212–1264 (±27) °C. Field relationships, including the presence of pyroxenitic xenoliths, and geochemical evidence (e.g. high FeO/MnO, and low CaO compared to lavas derived from peridotite sources) suggest a pyroxenitic mantle source for the studied rocks. Variation of trace elements and isotopic ratios (i.e. Ce/Pb, Ba/La, 87Sr/86Sr) indicate that this pyroxenite mantle source was generated by interaction between melted sediments of the subducted Neo-Tethys slab with ambient peridotitic lithospheric mantle. The resulting metasomatized lithosphere is denser and has a lower viscosity than the peridotitic mantle, and tectonic disturbance can cause it to fall into the depths of the mantle. The descending volatile-rich material starts to melt with increasing temperature. Modelling of rare earth element (REE) abundances suggests that &lt;1 % partial melting of the descending pyroxenite could create the Plio-Quaternary alkali basaltic magma of the Qorveh–Bijar. The geochemical evidence for lithospheric foundering, and hence drip magmatism, in the Qorveh–Bijar volcanic belt is supported by seismographic studies indicating thinned lithosphere beneath the study area.

A Jurassic volcanic passive margin in Iran and Turkey

AbstractBroadly similar Early to Middle Jurassic stratigraphic sequences including bimodal igneous rocks of the Sanandaj–Sirjan Zone of Iran and the Sakarya Zone of Turkey suggest that these formed in a common tectonic setting in an extensional basin that evolved from a terrestrial magmatic rift to a marine shelf and passive continental margin. Whole‐rock chemistry and Sr–Nd isotope signatures indicate derivation of mafic melts from partial melting of the subcontinental lithosphere. Decompression associated with extension led to 5%–30% partial melting of spinel–garnet lherzolite with minor involvement of continental crust, producing tholeiitic to transitional basaltic magma. Extensional basins inverted during the Mid‐Late Jurassic. These relationships suggest the Early to Middle Jurassic formation of a volcanic rifted margin on the SW Eurasian margin, similar to that of offshore Norway.

Golpayegan Metamorphic Complex (Sanandaj–Sirjan Zone, Iran) as Evidence for Cadomian Back-Arc Magmatism: Structure, Geochemistry and Isotopic Data

Golpayegan Metamorphic Complex (Sanandaj–Sirjan Zone, Iran) as Evidence for Cadomian Back-Arc Magmatism: Structure, Geochemistry and Isotopic Data

Zircon U–Pb and titanite U–Th–Pb ages of the Ghorveh mixed granitoid pluton: Implications for the Late Jurassic supra‐subduction extension of the Sanandaj–Sirjan Zone, Iran

The Ghorveh batholith, located in the central part of the Sanandaj–Sirjan Zone (SSZ), Iran, preserves several sequences of Mesozoic plutonic and volcanic rocks. This study presents systematic analyses of the calc‐alkaline granites and monzodiorites from the southern part of the Ghorveh pluton. The granites are metaluminous to slightly peraluminous with A/CNK ratios of 0.97–1.02, indicating an A‐type granitic affinity. High Rb, Th, and K and low Sr, P, and Ti concentrations indicate an involvement of crustal materials. Whole‐rock negative εNd(t) values and relatively high TDM1 model ages support a continental crustal source. Thus, the granites are of fractionated A‐type in composition produced by partial melting of crustal materials. The monzodiorites have incompatible trace element patterns with enrichment in light rare earth elements (LREEs) and large‐ion lithophile elements (LILEs) relative to heavy rare earth elements and high‐field strength elements. This combined with the high Mg# values and the positive εNd and εHf(t) values, indicate a mantle‐derived origin. Dating results show that the granites and monzodiorites have uniform crystallization ages of ~150 Ma. Previous results and our new data suggest that the Jurassic subduction of Neo‐Tethyan Oceanic crust under the Eurasian continent was followed by slab rollback that induced back‐arc extension. Heat transfer from arc magmas increased the degree of decompressional melting in the thinned lithosphere. Melting of the presumably homogeneous crust was concurrent with emplacement of the intermediate composition magmas. Consequently, we propose that the Ghorveh area of the central SSZ developed in a supra‐subduction extensional phase during the Late Jurassic.

Geochemical and geochronological constraints on origin of the Sawlava ophiolite (NW Iran): Evidence for oceanic mantle evolution beneath Iran-Iraq border

The Sawlava ophiolite is situated on the Iran–Iraq border within the Walash-Kermanshah ophiolitic belt and represents remnants of the Cenozoic southern Neo-Tethys Ocean, which was located between the Arabian plate and the Sanandaj–Sirjan zone. It contains a complete ophiolitic sequence started from harzburgites with foliated/discordant dunites, gabbroic bodies, a dyke complex, and pillow as well as massive basaltic lavas. The whole-rock and mineral chemistry of the Sawlava peridotites suggest that they are a mantle residuum after ca. 20–30% partial melting and melt extraction in the suprasubduction zone (SSZ) setting. The volcanic rocks generally range from island arc tholeiite to calc-alkaline affinities. Whole-rock chemistry indicates that volcanic units have compositions akin to depleted- and enriched-types mid-ocean ridge basalts (E-MORB). Layered- and isotropic gabbros show geochemical affinity similar to the normal mid-ocean ridge basalt (N-MORB) and E-MORB compositions, respectively. Their llight rare earth elements (LREE) observed in both N-MORB-like and E-MORB-like rocks, define two main basic geochemical types (LREE-enriched and LREE-depleted), suggesting several magmatic pulses from different mantle compositions that reveal mantle evolution and heterogeneity across the study area. Basaltic lavas weakly depleted in LREE show relatively flat REE patterns and (La/Yb)N values of 0.77 to 1.24, whereas LREE-enriched rocks have a (La/Yb)N mean values of 1.7, and lack an Eu anomaly. These subgroups of basaltic rocks show different extrusion ages – Paleocene–Eocene (59–50 Ma) and Oligocene (29 Ma), respectively. Petrogenetic modeling shows that mafic N- and E-MORB type rocks of Sawlava, which have (La/Yb)N = 0.97 and 1.7 and (Dy/Yb)N = 1.13 and 1.1, respectively, formed due to partial melting of a mantle source less enriched in LREE than an garnet lherzolite ((La/Yb)N = 0.88 and (Dy/Yb)N = 0.94) source or, alternatively, from very low degree (<2%) partial melting of a spinel-bearing DMM. The La/Yb–Dy/Yb systematics of these rocks are interpreted to be compatible with ~2 to 10% and ~ 20% of partial melting of a garnet bearing spinel lherzolite source for the N- and E-MORB type rocks, respectively. The geochemical data and results reveal that magmatic-originated rocks from both Paleocene-Eocene and Oligocene lavas seem to be less heterogeneous compared with the mantle sources of the Walash-Kermanshah ophiolites, likely due to different slab-fluid components and variations of partial melting degrees. The studied volcanic rocks therefore possibly record chemical heterogeneity due to magmatic activity at an oceanic spreading center alongside slab rollback and back-arc extension at the Eurasian continental margin.

Metamorphic stages in mountain belts during a Wilson cycle: A case study in the central Sanandaj–Sirjan zone (Zagros Mountains, Iran)

Polymetamorphic units are important constituents of continent–continent collisional orogens, and rift metamorphic assemblages are often overprinted by subsequent metamorphism during subduction and collision. This study reports the metamorphic conditions and evolution of the Dorud–Azna metamorphic units in the central part of the Sanandaj–Sirjan zone (SSZ), Iran. Here, new geothermobarometry results are integrated with 40Ar/39Ar mineral and Th–U–Pb monazite and thorite ages to provide new insight of polyphase metamorphism in the two different basement units of the SSZ, the lower Galeh-Doz orthogneiss and higher Amphibolite-Metagabbro units. In the Amphibolite-Metagabbro unit, staurolite micaschist underwent a prograde P–T evolution from 640 ± 20 °C/6.2 ± 0.8 kbar in garnet cores (M1) to 680 ± 20 °C/7.2 ± 1.0 kbar in garnet rims (M2). Three Th–U–Pb monazite ages of 306 ± 5 Ma, 322 ± 28 Ma and 336 ± 39 Ma from the garnet-micaschists testify the Carboniferous age of M1 metamorphism. In the same unit, the metagabbro records P–T conditions of 4.0 ± 0.8 kbar and 580 ± 50 °C in the (magmatic) amphibole core (Late Carboniferous intrusion) to 7.5 ± 0.7 kbar and 700 ± 20 °C in the amphibole rim indicating a prograde P–T path during subsequent burial (M1). New 40Ar/39Ar dating of white mica from the staurolite micaschist yielded a staircase pattern ranging from 36 ± 12 Ma to 170 ± 2 Ma. This implies polymetamorphism with a minimum Late Jurassic cooling age through the Ar retention temperature of ca. 425 ± 25 °C after M2 metamorphism and a Paleogene low-grade metamorphic overprint (M3), while 40Ar/39Ar white mica dating of garnet micaschist yielded a plateau age of 137.84 ± 0.65 Ma. We therefore interpret the amphibolite-grade metamorphism M2 to have predated 170 Ma and is likely between 180 and 200 Ma. Furthermore, it is overprinted at about 36 Ma under retrogressive low-grade M3 metamorphism (at temperatures of ~350–240 °C) during final shortening and exhumation. In the underlying Galeh-Doz unit, the Panafrican granitic orthogneiss intruded at P–T conditions of 3.2 ± 4 kbar and 700 ± 20 °C, then it was metamorphosed and deformed at 600 ± 50 °C and 2.0 ± 0.8 kbar (metamorphic stage M1) prior to Late Carboniferous intrusion of mafic dikes. 40Ar/39Ar dating of amphibole from the Galeh-Doz orthogneiss gave plateau-like steps between 260 and 270 Ma, representing the age of cooling through ca. 500 °C after the M1 metamorphic event. Interestingly, the results of this study demonstrate polyphase metamorphic histories in both the Galeh-Doz orthogneiss and Amphibolite-Metagabbro units at different P–T conditions and final thick-skinned Paleogene emplacement of these units over the underlying low-grade metamorphic June Complex. Our findings suggest that both units are affected by high-T/low-P Late Carboniferous orogenic metamorphism along with the bimodal magmatism, as result of rifting. We propose that the Early Jurassic amphibolite-grade M2 metamorphism of the SSZ is correlated with the initial subduction of the Neotethyan Ocean. Eventually, the investigated units reflect various stages of a Wilson cycle, from rifting to initiation of the subduction in final plate collision.

A report from the Oligocene Qom Formation benthic foraminifera; northwest of the Sanandaj–Sirjan Zone; Western Iran

A report from the Oligocene Qom Formation benthic foraminifera; northwest of the Sanandaj–Sirjan Zone; Western Iran

گرانیتوئیدهای پهنه سنندج-سیرجان متعلق به سری ایلمینیت (نوع S)، همزاد با کوه زایی سیمیرین (178-160 میلیون سال پیش): بررسی علت عدم تشکیل کانی سازی قلع پورفیری

Introduction The granitic rocks are divided into magnetite and ilmenite series (Ishihara 1977), coinciding spatially with the I-type (eNdi0; (87Sr/86Sr)i~0.704-0.706) and S-type granites (eNdi 0.707, N-MORB normalized patterns with enrichment in Low- and High Field Strength Elements (LFSE and HFSE) and depletion in P and Ti contents, chondrite-normalized patterns with flat heavy rare earth elements (HREE) patterns, magnetic susceptibility 4 km, regional metamorphism at ​​green schist facies (and amphibolite) following Cimmerian orogeny, low (Eu/Eu)N (reducing conditions), magnetic susceptibility 0.707) show that, unlike previous studies, these granitoids are S-type granitoids formed by melting the continental crust in a collisional zone. Therefore, tin mineralization might probably occurred in connection with them. However, there is ample evidence of the absence of tin mineralization by the magma that forms these S-type granitoids, that are including the lack of hydrothermal fluids and consequently mineralization potential (due to the absence of alteration minerals in ASTER satellite images), low content of tin, copper, lead and zinc elements in these granitoids Sanandaj-Sirjan Zone and associated river sediments, (Eu/Eu)N value > 0.2, Rb/Sr 200 ppm, as well as the geological, geophysical and geochemical similarities to barren S-type (ilmenite series) granitoids in Lut block (in Najmabad, Sorkh kuh to Shah kuh areas) which have formed in a continental collision system during the Cimmerian orogeny. References Ahadnejad, V., Valizadeh, M., Deevsalar, R. and Rezaei-kahkhaei, M., 2011. Age and geotectonic position of the Malayer granitoids: Implication for plutonism in the Sanandaj-Sirjan Zone, W Iran. Neues Jahrbuch fur Geologie und Palaontologie - Abhandlungen, 261(1): 61–75. https://doi.org/10.1127/0077-7749/2011/0149 Bayati, M., Esmaeily, D., Maghdour-mashhour, R., Li, X. and Stern, R.J., 2017. Geochemistry and petrogenesis of Kolah-Ghazi granitoids of Iran: Insights into the Jurassic Sanandaj-Sirjan magmatic arc. Chemie der Erde- Geochemistry, 77(2): 281–302. https://doi.org/10.1016/j.chemer.2017.02.003 Esna-Ashari, A., Tiepolo, M., Valizadeh, M., Hassanzadeh, J. and Sepahi, A., 2012. Geochemistry and zircon U–Pb geochronology of Aligoodarz granitoid complex, Sanandaj-Sirjan Zone, Iran. Journal of Asian Earth Sciences, 43(1): 11–22. https://doi.org/10.1016/j.jseaes.2011.09.001 Ishihara, S., 1977. The Magnetite-series and Ilmenite-series Granitic Rocks. Mining Geology, 27(145): 293–305. https://doi.org/10.11456/shigenchishitsu1951.27.293 Khalaji, A.A., Esmaeily, D. and Valizadeh, M. V., 2007. Petrology and geochemistry of the granitoid complex of Boroujerd, Sanandaj-Sirjan Zone, Western Iran. Journal of Asian Earth Sciences, 29(5-6): 859–877. https://doi.org/10.1016/j.jseaes.2006.06.005 Neiva, A.M.R., 2002. Portuguese granites associated with Sn-W and Au mineralizations. Bulletin of the Geological Society of Finland, 74: 79–101. https://doi.org/10.17741/bgsf/74.1-2.003 Shahbazi, H., Siebel, W., Pourmoafee, M., Ghorbani, M., Sepahi, A.A., Shang, C.K. and Abedini, M.V., 2010. Geochemistry and U – Pb zircon geochronology of the Alvand plutonic complex in Sanandaj – Sirjan Zone (Iran): New evidence for Jurassic magmatism. Journal of Asian Earth Sciences, 39(6): 668–683. https://doi.org/10.1016/j.jseaes.2010.04.014 Takahashi, M., Aramaki, S. and Ishihara, S., 1980. Magnetite-Series/Ilmenite Series vs. I-type/S-type granitoids, Granitic Magmatism and related mineralization. Mining Geology, Special Issue, 8: 13–28. Tahmasbi, Z., Castro, A., Khalili, M., Khalaji, A.A. and De, J., 2010. Petrologic and geochemical constraints on the origin of Astaneh pluton, Zagros. Journal of Asian Earth Sciences, 39(3): 81–96. https://doi.org/10.1016/j.jseaes.2010.03.001 Zheng, W., Mao, J., Zhao, C., Zhang, Z., Xiao, W., Ji, W., Majidifard, M.R., Rezaeian, M., Talebian, M., Xiang, D., Chen, L., Wan, B., Ao, S. and Esmaeili, R., 2018. Geochemistry, zircon U-Pb and Hf isotope for granitoids, NW Sanandaj-Sirjan zone, Iran: Implications for Mesozoic-Cenozoic episodic magmatism during Neo-Tethyan lithospheric subduction. Gondwana Research, 62: 227–245. https://doi.org/10.1016/j.gr.2018.04.002

Zircon U–Pb ages and petrogenesis of late Miocene adakitic rocks from the Sari Gunay gold deposit, NW Iran

AbstractLate Miocene volcanic rocks host the Sari Gunay epithermal gold deposit in NW Iran. These rocks are located within the Hamedan–Tabriz volcanic belt and occupy the northwestern part of the Sanandaj–Sirjan zone (SaSZ). The volcanic rocks span in composition from latite to dacite and rhyolite. Plagioclase, hornblende, biotite and quartz are the main phenocrysts in a fine-grained and glassy matrix. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating yielded crystallization ages of 10.10 ± 0.01 Ma and 11.18 ± 0.14 Ma for rhyolite and dacite, respectively. High ratios of Sr/Y (&gt; 20) and La/Yb (&gt; 20), high contents of Sr (≥ 400 ppm), low contents of MgO (≤ 6 wt%), Y ≤ 18 ppm (c. 16.5 ppm), Yb ≤ 1.9 ppm (c. 1.53 ppm) and weak negative Eu anomalies (Eu*/Eu c. 0.81) are compatible with a high-silica adakitic signature of the rocks. Regarding the location of the study area nearly 100 km from the Zagros suture zone, we argue that delamination of lithospheric mantle beneath the SaSZ has played a key role in the development of the adakitic rocks in a post-collision tectonic regime. The adakitic melts are suggested to have formed by partial melting of delaminated continental lithosphere and/or lower crustal amphibolite following the collision of the Arabian and Iranian plates.

Tectonic significance of the late Eocene (Bartonian) calc-alkaline granitoid body in the Marivan area, Zagros suture zone, northwest Iran

ABSTRACT The Marivan granitoid body of the northern Sanandaj–Sirjan Zone (N-SaSZ) of northwest Iran was emplaced into Cretaceous sediments, and is associated with minor amounts of iron skarn-type mineralization along the contact between the intrusion and surrounding country rocks. The granitoids within the intrusion include (hornblende-rich) diorite, granodiorite, and granite phases, and zircon U–Pb dating of four samples from this intrusion indicate they were emplaced during the late Eocene (Bartonian; 37.7 ± 1.0 Ma). These plutonic rocks can be classified into two main types, granitic and dioritic rocks, according to petrography and whole-rock major element geochemistry. Both the granitic and dioritic rocks are high-K, and some of the granitic rocks have an I-type affinity and peraluminous to metaluminous compositions. Both the granitic and dioritic rocks have εNd(t) values that cluster around zero (–3.3 to +3.0) and low 87Sr/86Sr(i) values (0.7040–0.7076), suggesting derivation from a mafic juvenile crustal or depleted mantle source, which was possibly mafic calc-alkaline rocks or amphibolites within the lower crust. The new zircon U–Pb ages and petrological and geochemical data presented in this study suggest that the Palaeocene–early Eocene collision of the Arabian and Iranian plates caused the ascent and emplacement of hot mafic magma through a slab window into the crust. This increased the geothermal gradient within the root zone of the developing magmatic system at the base of the continental crust, causing the partial melting of lower crust in this region. This generated mafic to intermediate magmas that were either directly emplaced into the upper crust or mixed with varying amounts of upper crustal material prior to emplacement, which formed the diorite to leucogranite range of compositions within the Marivan granitoid intrusion.

P–T–t path of the Boroujerd Complex, north‐west Sanandaj–Sirjan Zone, western Iran: Insights from phase equilibrium modelling and thermobarometry

Variations in the pressures and temperatures of metamorphism reflect the complexity of collisional orogenic systems. In the north‐west Sanandaj–Sirjan Zone of western Iran, migmatites in the Boroujerd area record earlier dynamothermal metamorphism followed by the development of a contact metamorphic aureole induced by the Jurassic Boroujerd Batholith. Integrated micro‐textural observations, mineral compositions, and the results of phase equilibrium modelling indicate that peak metamorphic temperatures reached 640–660°C in andalusite schists and ~700°C in cordierite–sillimanite migmatites. The pressure does not exceed 3.5 kbar, implying a maximum depth of ~13 km. A hairpin clockwise P–T path in the Boroujerd region was inferred from mineral relations (e.g., replacement of andalusite1 by sillimanite in the migmatites in the heating stage, which was followed by a retrograde episode with growth of andalusite2) and P–T pseudosections. Metamorphism of the Boroujerd area was coeval with voluminous intermediate–mafic magmatism, which provided the heat necessary for partial melting. This study demonstrates that metamorphic sequences of the north‐west Sanandaj–Sirjan Zone developed under similar metamorphic conditions in the Lower to Middle Jurassic and record an extensional back‐arc setting related to the subduction of Neo‐Tethys.