Iranian Microcontinent Research Articles

Geochemistry and petrogenesis of Raviz-Shanabad intrusions (SE UDMB): an evidence for Late Eocene magmatism

ABSTRACT The SE segment of the Urumieh-Dokhtar Magmatic Belt (UDMB), known as Dehaj-Sarduieh Magmatic Belt, includes a huge pile of Palaeogene volcano-sedimentary rocks and Eocene to Miocene mafic to felsic intrusions. In the middle of this belt (in Raviz-Shanabad, W Rafsanjan) mafic-intermediate intrusions (as stocks and/or dikes-sills) are abundant and have intruded into the volcano-sedimentary rocks of Dehaj-Sarduieh. The thickness of dikes range from 0.5 to >5 metres and the minimum diameter of the intrusions is ~10 m. These rocks are highly porphyritic and have gabbroic to dioritic composition. Petrographic data indicate that the main rock-forming minerals include plagioclase (labradorite to anorthite based on the new electron microprobe data), pyroxene (augite), and opaque (magnetite and titanomagnetite) minerals, set in a granular groundmass of the same minerals. New zircon U-Pb data show that the age of these rocks is 34.39 ± 0.31 Ma (Late Eocene). These rocks show calc-alkaline characteristics and their geochemical signatures are compatible with arcs with a derivation from subduction zones. Estimated temperature and pressure from Clinopyroxene of these rocks suggest a temperature in the range of 1050° to 1150°C and a pressure of >2 kbar. These data show the formation depth of these intrusions should be ~18 km. Initial 87Sr/86Sr ratio for these rocks varies from 0.70403 to 0.70409, whereas 143Nd/144Nd isotope ratio changes between 0.5128 and 0.5129, corresponding to εNd(i) values of +3.5 to +6.6. These geochemical and isotopic evidences show that the melts originated by 5% partial melting of amphibole spinel peridotite mantle source. Petrographic observations as well as mineral chemistry and isotopic studies indicate that assimilation-fractional crystallization (AFC) has probably played a dominant role in the evolution of the Raviz-Shanabad intrusions. We suggest these intrusions formed as a part of the Late Eocene high-flux magmatism in SE segment of the UDMB in an active continental margin, which was related to the subduction of Neo-Tethys oceanic lithosphere beneath the Central Iranian microcontinent.

Evolution of lithospheric mantle in the north of Nain‐Baft oceanic crust (Neo‐Tethyan ophiolite of Ashin, Central Iran)

AbstractThis 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%) confirms 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 northwest of Central‐East Iranian Microcontinent.

The age and geochemistry of the Bardkish syenite, northwest Iran: Syenite formation during Neo‐Tethyan subduction

AbstractSHRIMP U–Pb dating of zircon from two representative samples of the Bardkish syenitic pluton, Urumieh plutonic complex, northwestern Iran, yielded a Late Cretaceous weighted mean 206Pb/238U age of 91.1 ±0.8 Ma (Turonian). The pluton is characterized by high K2O content, Ti–Nb–Ta depletion, enrichment in light rare earth elements and large ion lithophile elements compared to high field strength element, Mg‐rich biotite and calcic amphibole, suggesting a shoshonitic affinity. Geochemical features of the Bardkish syenite are consistent with derivation by fractional crystallization of initial mafic parent magmas. It is most likely that primary magma of the syenite originated from a metasomatized subcontinental lithospheric mantle source with minor crustal component, and underwent a fractional crystallization process during its emplacement. The estimates of trapping pressures and temperatures suggest that crystallization began at the middle crustal level (less than 11 km) near 700–730 °C. The generation of the Bardkish syenite could be related to the subduction of Neo‐Tethyan oceanic crust beneath the Central Iranian microcontinent and it possibly marks a major magmatic episode prior to the final closure of the Neo‐Tethyan ocean in northwest Iran.

Geochemistry and mineral chemistry of gabbroic rocks from Horjand of Kerman province, Southeast of Iran: Implications for rifting along the northeastern margin of Gondwana

The Horjand area is part of the Kashmar-Kerman tectonic zone in the Central Iranian block, where the oldest continental crust of Iran can be found. Mafic igneous rocks associated with sedimentary units (gypsum, shale, limestone, dolomite and sandstone) belong to the magmatism of the Late Neoproterozoic–Early Cambrian Desu Series. Based on petrographic studies, the mafic igneous rocks comprise medium to coarse gabbro, micro-gabbro and hornblende-gabbro. The rocks mainly consist of plagioclase, olivine, clinopyroxene, biotite, amphibole and Fe-Ti oxides. The gabbroic rocks have a variable compositional range, with SiO2 (44.6–52.9 wt. %), MgO (2.94–13.83 %), CaO (5.56–15.23 wt. %) and TiO2 (1.19–2.68 wt. %) contents. Petrographic evidence and geochemical data suggest that fractional crystallization has played an important role in the evolution of the ascending magma of the gabbroic rocks. The chemical composition of the minerals is similar to those of gabbroic rocks that were formed in a non-orogenic environment. The crystallization of clinopyroxene of gabbros mainly occurred under temperatures ranging from 1045 to 1216 °C. The geochemical characteristics and mineral compositions of the gabbros show that the samples predominantly have alkaline to minor tholeiitic affinities. During magma ascent, crustal contamination was minor or absent. Rare earth element modeling demonstrates that the magma of the Horjand gabbros was produced from 10 to 15 percent partial melting of a spinel-garnet lherzolite mantle source. Chondrite-normalized rare earth element (REE) patterns are LREE-enriched with (La/Yb)N of 4.50-12.82. Both normalized REE and trace-element spidergrams are similar to patterns of HIMU-OIB (high mu (μ = U/Pb)-ocean island basalt) or intraplate alkaline magmatic rocks. The gabbros geochemically display an intra-plate tectonomagmatic setting without subduction geochemical characteristics. Based on the available data and mineral chemical composition the development of the Horjand gabbroic rocks could be related to an extensional phase, with upwelling melting of the asthenospheric mantle in the continental rift resulting in alkaline magmatism of Late Neoproterozoic–Early Cambrian age, in the Central Iranian micro-continent.

Lower Oligocene Calc-Alkaline Spessartitic Lamprophyres from Central Iran (East of Anarak Area); an Evidence from the Eastern Branch of Neotethys Subduction-Related Mantle Enrichment

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-e-kafi 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.

Geology, isotope geochemistry, and fluid inclusion investigation of the Robat Zn-Pb-Ba deposit, Malayer-Esfahan metallogenic belt, southwestern Iran

The Robat Zn-Pb-Ba deposit in the Arak mining district in the Malayer-Esfahan metallogenic belt (MEMB) has an estimated reserve of 5 million tonnes (Mt) ore grading 2.1 wt% Zn and 1.4 wt% Pb and 4 Mt barite ore. This deposit, hosted by Early Cretaceous carbonate rocks, formed in an extensional back-arc environment between the Sanandaj-Sirjan Zone and Central Iranian Microcontinent. Sulfide and barite ores display a strong lithological control and form stratabound stacked ore lenses, which are hosted in two horizons by silicified and dolomitized limestone. They are generally best developed below marl layers within intercalated shale, marl, and limestone.The textural and mineralogical data indicate two main stages of sulfide and barite mineralization at the Robat deposit. The first stage comprised deposition of volumetrically minor early fine-grained disseminated sulfides and barite near the seafloor in unconsolidated sediments. Subsequently, sub-seafloor replacement of barite, carbonate, and early sulfide laminae/bands occurred by precipitation of late sulfides and other hydrothermal minerals within the Early Cretaceous limestone-dominant sequence. The minerals in this later stage were deposited in a close relationship with quartz veins.Electron microprobe analyses revealed that sphalerite has a very high Fe content averaging about 1.23 wt% and is relatively enriched in As, Ga, Ge, Cd, Ag, and Bi, which is similar to Irish-type carbonate-hosted deposits. In addition, the Ag content of galena is comparatively low, although concentrations of Ga, Ge, Cd, Zn, and Sb are fairly high. Fluid inclusion microthermometric study of hydrothermal quartz revealed two types of primary two-phase (L + V) fluid inclusions in hydrothermal quartz. Type I fluid inclusions with irregular shapes were recorded along crystal growth zones, whereas type II inclusions are observed as isolated inclusions with relatively regular shapes that are less common than type I. Type I and II inclusions exhibit homogenization temperatures of 140–239 °C and 233–290 °C, with salinities of 2.2–11.8 and 5.5–11.5 wt% NaCl equiv., respectively. Fluid inclusions extracted from quartz displayed δDSMOW between −67‰ and −56‰ and measurements of hydrothermal quartz yielded δ18Ofluid (SMOW) of 12.9‰ to 23.5‰. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of galena samples fell in the ranges of 18.412–18.427, 15.642–15.647, and 38.556–38.580, respectively. The different ratios for the Pb isotopes suggest a dominant continental crust origin for Pb in the ores. Geological, textural, mineralogical, and isotopic data may be cumulatively interpreted to indicate that the Robat carbonate-hosted deposit formed from sub-seafloor replacement like commonly observed for Irish-type base metal deposits.

Mineral chemistry using for evaluation of the Esmaeilabad granite generation in the Posht-e-Badam area (Central- East Iranian Microcontinent)

Mineral chemistry using for evaluation of the Esmaeilabad granite generation in the Posht-e-Badam area (Central- East Iranian Microcontinent)

Sandstone petrography and geochemistry of the Nayband Formation (Upper Triassic, Central Iran): Implications for sediment provenance and tectonic setting

Abstract The Upper Triassic (Norian–Rhaetian) Nayband Formation is situated at the southwestern margin of Central East Iranian Microcontinent and records Eo-Cimmerian events. The formation is composed of mixed carbonate-siliciclastic deposits. This study presents information on the tectonic reconstruction and palaeoclimate of the southwestern margin of Central East Iranian Microcontinent during the Late Triassic. Petrography and modal analyses of sandstones show a variety of quartz-rich petrofacies including subarkose, lithic arkose, sublitharenite, feldspathic litharenite and litharenite. The combined modal analysis and geochemical results of major and trace elements of the sandstone samples represents mixed sedimentary, intermediate, felsic igneous rocks and moderate- to high-grade metamorphic provenance areas. The major elements and modal analyses of the Nayband Formation sandstone samples suggest an active continental margin tec-tonic settings. The palaeoclimatic conditions were sub-humid to humid with relatively low to moderate weathering in the source area which is in agreement with the palaeogeography and palaeotectonic history of southwestern margin of Central East Iranian Microcontinent during the Late Triassic.

Cretaceous tectonic evolution of the Neo-Tethys in Central Iran: Evidence from petrology and age of the Nain-Ashin ophiolitic basalts

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.

Geochemistry and mineral composition of lamprophyre dikes, central Iran: implications for petrogenesis and mantle evolution

Late Proterozoic–Early Cambrian magmatic rocks that range in composition from mafic to felsic have intruded into the Hour region of the central Iranian micro-continent. The Hour lamprophyres are alkaline, being characterized by low contents of \(\hbox {SiO}_{2}\) and high \(\hbox {TiO}_{2}\), Mg# values, high contents of compatible elements, and are enriched in LREE and LILE but depleted in HFSE. Mineral chemistry studies reveal that the lamprophyres formed within a temperature range of \(\sim \)1200\(^{\circ }\) to \(1300^{\circ }\hbox {C}\) and relatively moderate pressure in subvolcanic levels. The Hour lamprophyres have experienced weak fractional crystallization and insignificant crustal contamination with more primitive mantle signatures. They were derived from low degree partial melting (1–5%) of the enriched mantle characterized by phlogopite/amphibole bearing lherzolite in the spinel-garnet transition zone at 75–85 km depth, and with an addition of the asthenospheric mantle materials. We infer the Hour lamprophyres to be part of the alkaline rock spectrum of the Tabas block and their emplacement, together with that of other alkaline complexes in the central Iran, was strongly controlled by pre-existing crustal weakness followed by the asthenosphere-lithospheric mantle interaction during the Early Cambrian.

NORIAN AMMONOIDS FROM THE NAYBAND FORMATION (IRAN) AND THEIR BEARING ON LATE TRIASSIC SEDIMENTARY AND GEODYNAMIC HISTORY OF THE IRAN PLATE

A Middle Norian (Alaunian 2) ammonoid assemblage from north of Esfahan (Central Iran) is described and chronostratigraphically evaluated. Formerly known as Distichites fauna, it represents a geographically widely distributed and stratigraphically important fossil level in the lower part of the Upper Triassic Nayband Formation. The new distichitid ammonoid genus Mesodistichites with the new species M. evolutus are introduced; additional faunal members are Noridiscites nodosus n. sp. and the leiostracean Stenarcestes diogenis and Pinacoceras cf. imperator. The Nayband Formation of the Esfahan region, belonging to the Zefreh-Soh Facies, is lithostratigraphically emended to contain three formally introduced members (Parsefid, Venher and Niazmargh members), which are all of Norian age, whereas Rhaetian sediments are missing. These members are correlated with Norian lithostratigraphic units of the type sequence of the formation in Nayband, eastern Iran. Integration of all Iranian Nayband data allows the recognition of three 3rd order sequences within this formation and the proof of a major pre- or syn-Jurassic unconformity across Central Iran and the Central-East Iranian Microcontinent (CEIM) related to the Main-Cimmerian event. Because of the latter and of major lithostratigraphic and facial differences, we propose to exclude the Nayband Formation from the Shemshak Group. A careful review of the available biostratigraphic data from northern and southern Iran let us further assume that the collision of the Iran Plate with Eurasia occurred in the later Early Carnian and was concurrent to the onset of the Carnian Humid Episode.

Chemical composition of rock-forming minerals and crystallization physicochemical conditions of the Middle Eocene I-type Haji Abad pluton, SW Buin-Zahra, Iran

The Haji Abad intrusion is a well-exposed Middle Eocene I-type granodioritc pluton in the Urumieh–Dokhtar magmatic assemblage (UDMA). The major constituents of the investigated rocks are K-feldspar, quartz, plagioclase, pyroxene, and minor Fe–Ti oxide and hornblende. The plagioclase compositions fall in the labradorite, andesine, and oligoclase fields. The amphiboles range in composition from magnesio-hornblende to tremolite–hornblende of the calcic-amphibole group. Most pyroxenes principally plot in the field of diopside. The calculated average pressure of emplacement is 1.9 kbar for the granodioritic rocks, crystallizing at depths of about 6.7 km. The highest pressure estimated from clinopyroxene geobarometry (5 kbar) reflects initial pyroxene crystallization pressure, indicating initial crystallization depth (17.5 km) in the Haji Abad granodiorite. The estimated temperatures using two-feldspar thermometry give an average 724 °C. The calculated average temperature for clinopyroxene crystallization is 1090 °C. The pyroxene temperatures are higher than the estimated temperature by feldspar thermometry, indicating that the pyroxene and feldspar temperatures represent the first and late stages of magmatic crystallization of Haji Abad granodiorite, respectively. Most pyroxenes plot above the line of Fe3+ = 0, indicating they crystallized under relatively high oxygen fugacity or oxidized conditions. Furthermore, the results show that the Middle Eocene granitoids crystallized from magmas with H2O content about 3.2 wt%. The relatively high water content is consistent with the generation environment of HAG rocks in an active continental margin and has allowed the magma to reach shallower crustal levels. The MMEs with ellipsoidal and spherical shapes show igneous microgranular textures and chilled margins, probably indicating the presence of magma mixing. Besides, core to rim compositional oscillations (An and FeO) for the plagioclase crystals serve as robust evidence to support magma mixing. The studied amphiboles and pyroxenes are grouped in the subalkaline fields that are consistent with crystallization from I-type calc-alkaine magma in the subduction environment related to active continental margin. Mineral chemistry data indicate that Haji Abad granodiorites were generated in an orogenic belt related to the volcanic arc setting consistent with the subduction of Neo-Tethyan oceanic crust beneath the central Iranian microcontinent.

Structural Control on the Salmas Geothermal Region, Northwest Iran, from Fractal Analysis and Paleostress Data

AbstractThe Salmas geothermal field is located in NW Iran. Subduction of Neo‐Tethys oceanic crust beneath the Iranian microcontinent caused to propagation of the magmatic‐Arc. Fractures and faults in the convergent zone have created path‐ways for the circulation of geothermal fluid. Fracture concentration in the Salmas geothermal field has been characterized using of the fractal method and creation of a fracture density map that shows the highest concentration in the central part of the study area. The permeability of fractures has been evaluated by analyzing their orientation in respect to the paleostress axes. Also, the fractal analyzing result indicates the maximum fractal dimension (1.96) is around the thermal spring outlet. Paleostress analyzing revealed that in the central part of the study area, σ1 axes orientation is S90°W/10° and the σ2 dip is near to the vertical in this stress field, where strike slip faults can be propagated. In the SE part near the recharge of the thermal springs, the σ3 plunge increases to 70° and σ1 orientation is N15°E/20°, in this local tectonic regime thrust fault developed. Fractures have an important role in the circulation of fluid and the fractal dimension increases near the thermal springs in the Salmas geothermal field. Regarding the paleostress data fracture with N‐S direction such as the F1 fault zone (parallel to the σ1 axes), a suitable pathway for deep circulation of geothermal fluid flow has been created.

U-Pb zircon dating, Sr-Nd isotope and petrogenesis of Sarduiyeh granitoid in SE of the UDMA, Iran: implication for the source origin and magmatic evolution

ABSTRACT The Sarduiyeh granitoid (SG) is intruded in the southeastern part of the Dehaj-Sarduiyeh volcano-sedimentary belt in the southeastern end of the Urumieh-Dokhtar Magmatic Arc (UDMA) in Iran. The medium-to-coarse-grained granitoid unit, with granular texture consists mainly of diorite, tonalite, granodiorite and monzogranitic rocks. Mineralogically, these rocks consist mainly of plagioclase, K-feldspar, quartz, biotite and hornblende. The whole rock geochemical analyses indicates that the SG is calc-alkaline, I-type, metaluminous, enriched in large ion lithophile elements (LILE; such as K, Cs, Pb) and depleted in high field strength elements (HFSE; such as Ti, Nb, Ta, Zr). Chondrite normalized plot of SG rare earth elements (REE) show light rare earth element enrichments with (LaN/YbN = 2.44–8.68) and flat heavy rare earth element patterns with (GdN/YbN = 1.02–1.36). The rather high Y (av. 19.35 ppm), low Sr content (av. 293.76 ppm) and low Cr and Ni contents (av. 20.1 and 4.69 ppm, respectively) of the SG demonstrate its normal calc-alkaline and non-adakitic nature, the features of Jebal Barez-type granitoids. The geochemical characteristics and isotopic composition, low ISr (0.7046–0.7049) and positive ɛtNd (+3.4 to +4.03) values, of the SG suggest that its parental magma formed as a result of partial melting from metabasic rocks of lower crust in a subduction-related arc setting. Fractionation of an assemblage dominated by plagioclase, K-feldspar, amphibole and magnetite may have been responsible for the evolution of the SG magma. U-Pb zircon geochronology gives an age of 27.95 ± 0.27 Ma for the SG, suggesting that the final collision between the Arabian plate and Central Iranian microcontinent may have happened in the Late Oligocene.

Tectonic and eustatic controls on the spatial distribution and stratigraphic architecture of late early Cambrian successions at the northern Gondwana margin: The siliciclastic-carbonate successions of the Lalun Formation in central Iran

Abstract The lower Cambrian Lalun Formation of the Tabas Block, Central Iran, allows to generate a sequence stratigraphic model which characterises the sedimentary evolution and improves palaeoenvironmental interpretation of sedimentary successions along the northern margin of the Prototethys. This formation provides a key for understanding processes that took place during the late early Cambrian in Iran and is the best representative for the complex mosaic of mixed siliciclastic-carbonate systems which illustrate the development and evolution of fluvial and tide-dominated estuarine systems adjacent to an open coast during this interval in the region. Detailed facies analysis allows to differentiate five vertically-stacked groups of facies associations (fluvial facies associations; estuarine facies association; open coast tidal flat facies association; carbonate ramp facies association; and delta facies association) in the uppermost interval of the Lalun Formation. The conceptual depositional models for the siliciclastic-carbonate successions of the Lalun Formation are summarised in a sequence stratigraphic framework, including three third-order depositional sequences (braided fluvial systems; incised-valley fill; and carbonate ramp and tide-influenced delta systems). The data suggest that the Central Iranian Microcontinent broke up during latest Neoproterozoic–early Cambrian times, resulting in the Proto-/Palaeotethys rifting, with considerable differential subsidence recognizable from the reactivation of faults in the early Cambrian which led to new syn-rift successions. The mosaic pattern of deposition observed among the studied sections suggests a compartmentalization of the depositional setting as a result of the interplay between tectonics, sediment supply, and eustasy. Abrupt lateral changes in facies and thickness within the sequences depositional sequences suggest that deposition was controlled by repeated tectonic downdrop and uplift as a consequence of local syndepositional, extensional fault movements. Tectonic activities in the studied region controlled sedimentation and accommodation as well as the position of depocentres within the basin, and the stratal geometries within the sequences. However, the stratigraphic architectural style of the depositional sequences and the asymmetrical trend of the transgressive and regressive second-order sequences imply that the deposition of the late early Cambrian successions is at least partly a result of eustatic sea level changes and cannot be linked exclusively to differential subsidence due to fault movements. Our study indicates that stratigraphic relationship between carbonate and siliciclastic facies in the depositional sequences was also controlled by subsidence/uplift in the hinterland and the areas of siliciclastic input rather than climatic changes. The late early Cambrian depositional environments evolved from braided rivers and an incised valley system (lowstand) to transgressive incised-valley-fill successions which are capped by transgressive open coast tidal flat and carbonate open marine facies. Depositional history of the formation terminated with a regression that ceased with progradation of tide-influenced delta systems. The stratigraphic evolution described in this study documents an eustatic sea-level fall interposed between the top of the Lalun Formation and the base of transgressive tidal bars of the overlying Fasham Formation reflecting the complex (sub-)globally recognizable regressive–transgressive event at the lower–middle Cambrian boundary interval, which led to the establishment of an extensive middle–late Cambrian carbonate platform in present-day northern and central Iran.

Gabbroic–dioritic dykes from the Sanandaj–Sirjan Zone: windows on Jurassic and Eocene geodynamic processes in the Zagros Orogen, western Iran

The Sanandaj–Sirjan Zone (SaSiZ) is a magmatic terrane within the Zagros Orogen, western Iran, marking the Tethyan suture zone between the Afro-Arabian Plate and the Central Iran Micro-Continent. Mafic–intermediate dyke swarms with Middle Jurassic (Group 1: hornblende gabbro and diorite) and Late Eocene (Group 2: hornblende–pyroxene gabbro) ages are recognized in the Malayer–Boroujerd Plutonic Complex of the northern SaSiZ. Group 1 dykes have elemental and isotopic signatures consistent with melting of a mantle source modified during Neo-Tethyan subduction. Some Group 1 magmas evolved to intermediate compositions through assimilation and fractional crystallization. Group 2 dykes have within-plate trace element geochemical signatures, modelled as deriving from low-degree melting of asthenospheric mantle without a subduction influence. Published models postulate either a Cretaceous–Eocene Neo-Tethyan flat-slab scenario or a Latest Cretaceous–Paleogene Neo-Tethyan break-off event beneath the SaSiZ. Such models do not reconcile with the Late Eocene presence of within-plate magmatism in westernmost Iran, very close to the Zagros Suture. We argue that a period of flat-slab subduction concluded with sub-parallel subduction of a Neo-Tethyan ridge to the trench. The resulting slab break-off event in the Late Eocene is responsible for generation of the distinct Mesopotamia and Zagros slabs in mantle tomography models. Break-off was followed by small-volume within-plate type magmatism before short-lived re-establishment of Tethyan subduction prior to the final Arabia–Eurasia collision. Supplementary material: Field photographs, photomicrographs, additional geochemical plots, descriptions of analytical methods and tables of geochemical modelling parameters are available at https://doi.org/10.6084/m9.figshare.c.4126196

Petrology and geochemistry of sheeted dykes and pillow lavas from the Sabzevar Ophiolitic Mélange (Northeast Iran): new Constraints for the Late Cretaceous evolution of the Neo-Tethys Oceanic Basin between the Central Iranian Microcontinent and Eurasia

TheSabzevarophiolite-bearingmelangemainlyincludestectonic slicesofophiolitesregardedas remnants of the Late Cretaceous oceanic basin located between the northern margin of the Central Iran Microcontinent and the southern margin of the Turan (Eurasia) plate, as well as remnants of its northern continental margin. We present here new mineral chemistry and whole rock chemical data on the sheeted dyke complex and pillow lavas series from this ophiolite. The sheeted dykes consist of basalts and basaltic andesites showing island arc tholeiitic (IAT) affinity (Group 1). They display low TiO 2 (0.37-0.81 wt%) contents and N-MORB normalized incompatible element patterns featuring Th enrichment and Ta and Nb depletion. The pillow lavas show different chemical features compared to the sheeted dykes and can be subdivided in two distinct geochemical groups. The first Group of pillow lavas (Group 2) mainly consists of andesites showing a clear calc-alkaline (CA) affinity with low TiO 2 (0.71-0.81 wt%) contents. Their N-MORB normalized incompatible element patterns display marked enrichments in Th, U, La, and Ce, and depletion in Ta, Nb, and Ti. The second Group of pillow lavas (Group 3) is mainly represented by alkaline basalts with high TiO 2 (1.54-3.45 wt%) contents. N-MORB normalized incompatible element patterns show significant Th, Ta, and Nb enrichments. Trace element modeling shows that CA primary melts were generated by 20% partial melting of a depleted lherzolite mantle source that had been enriched by continental crust chemical components. IAT basalts were generated from various degrees of partial melting of either depleted lherzolitic or harzburgitic mantle sources bearing no or negligible influence from continental crust components. Alkaline basalts were generated by polybaric partial melting of an enriched within-plate oceanic mantle source. A new tectono-magmatic model that can explain the different rock association cropping out in the Sabzevar ophiolite is proposed. This model implies the existence, on the southern margin of the Turan plate during the Late Cretaceous, of a continental arc-forearc system where CA and IAT rocks were erupted, respectively. Alkaline basalts were erupted in a subduction-unrelated oceanic basin and then tectonically accreted to the forearc as a consequence of the collision between the seamount and the arc-forearc system.

Geochemistry and source characteristics of Dehsard mafic volcanic rocks in the southeast of the Sanandaj–Sirjan zone, Iran: implications for the evolution of the Neo-Tethys Ocean

The Late Jurassic-Early Cretaceous Dehsard mafic volcanic rocks crop out in the southeastern Sanandaj-Sirjan Zone (SSZ), composed primarily of basalts and basaltic andesite with subordinate dolerite. They are influenced to some degree by hydrothermal alteration under zeolite-greenschist facies. Using fairly immobile trace elements, the mafic volcanic rocks show subalkaline (tholeiitic) affinities. They commonly have similar designs with somewhat strong enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs; e.g., Nb, Ta, Ti) and nearly flat heavy rare earth element (HREE) patterns. The negligible or absence of negative Eu anomalies indicate that plagioclase played an insignificant role during magma evolution. The low La/Nb (1.03-2.31) and Nb/Y (0.12-0.46) ratios, relatively high Zr/Y (4.03-8.18) and Th/Ta (2.25- 9.64) ratios, steady enhanced normalized patterns, and moderate La/Nb ratios hint at an island arc and most likely a back-arc basin environment for the formation of Dehsard mafic volcanic rocks. The arc magma resulted from partial melting of depleted mantle source that experienced assimilation and fractional crystallization and was enhanced by melts of subducted sediments or contribution of slabderived fluids in an intraoceanic subduction environment in the Neo-Tethyan Ocean. Therefore, the presence of an island arc setting (Dehsard island arc) must be investigated in the south of the SSZ prior to the Late Jurassic-Early Cretaceous as the Neo-Tethys oceanic crust was subducting north beneath the southern margin of the Central Iranian Microcontinents. The later collision of the arc with SSZ led to tectonic proximity of the Dehsard mafic volcanic rocks to SSZ components.

A magmatic record of Neoproterozoic to Paleozoic convergence between Gondwana and Laurasia in the northwest margin of the Central-East Iranian Microcontinent

This paper presents petrographic and U-Pb LA-ICP-MS isotopic data of a single peraluminous intrusion (alkali-feldspar granite of Airekan) with partial gneissic foliation on the north western margin of the Yazd Block in the west of the Central-East Iranian Microcontinent (CEIM) to discuss the evolution of this terrain. This intrusion typically suggests a continental collision orogenesis along the northern frontier of Gondwana in the CEIM. The U-Pb dating of zircons indicates that Airekan granite has a distinctive inherited magmatic component at ∼518.2 ± 4.9 Ma but zircons with 892, 679, 614 and 554 Ma ages. These zircons were likely to be ultimately sourced from neighboring Neoproterozoic – Cambrian leucogranite terrains, similar to the other localities in the northern margin of Gondwana, along the Alpine-Himalayan belt from Europe, Turkey, NE Iran, CEIM, Pakistan, India to southern Tibet. The zircons and overgrowths around the inherited components with igneous morphology indicate a plutonism at Lower Ordovician (∼483 ± 2.9 Ma). Petrographic features and morphology of zircon crystals indicate that partial resetting of the U-Pb system in zircons have occurred at ∼382.6 ± 3.2 Ma in Devonian, during the Paleo-Tethys evolutions in Khur region (Anarak-Jandaq terrane) in the southern active margin of Laurasia.

Petrology and Rare Earth Elements Mineral Chemistry of Chadegan Metabasites (Sanandaj-Sirjan Zone, Iran): Evidence for Eclogite-Facies Metamorphism during Neotethyan Subduction

The metabasites of Chadegan, including eclogite, garnet amphibolite and amphibolite, are forming a part of Sanandaj–Sirjan Zone. These rocks have formed during the subduction of the Neo–Tethys ocean crust under Iranian plate. This subduction resulted in a subduction metamorphism under high pressuremedium temperature of eclogite and amphibolites facies condition. Then the metamorphic rocks were exhumed during the continental collision between the Afro–Arabian continent and the Iranian microcontinent. In the metabasite rocks, with typical MORB composition, garnet preserved a compositional zoning occurred during metamorphism. The magnesium (XMg) gradually increases from core to rim of garnets, while the manganese (XMn) decreases towards the rim. Chondrite–normalized Rare Earth Element patterns for these garnets exhibit core–to–rim increases in Light Rare Earth Elements. The chondrite–normalized REE patterns of garnets, amphiboles and pyroxenes display positive trend from LREEs to Heavy Rare Earth Elements (especially in garnet), which suggests the role of these minerals as the major controller of HREE distribution. The geochemical features show that the studied eclogite and associated rocks have a MORB origin, and probably formed in a deep–seated subduction channel environment. The geothermometry estimation yields average pressure of ~22 kbar and temperature of 470–520°C for eclogite fomation. The thermobarometry results gave T = 650–700°C and P ≈ 10–11 kbar for amphibolite facies.