Urumieh-Dokhtar Magmatic Assemblage Research Articles

Magma chamber evolution of the Ardestan pluton, Central Iran: evidence from mineral chemistry, zircon composition and crystal size distribution

AbstractThe Oligo–Miocene Ardestan quartz diorite to tonalite is part of widespread Cenozoic magmatism within the Urumieh–Dokhtar Magmatic Assemblage of Iran. The Ardestan pluton is composed mainly of varying proportions of plagioclase feldspar (normally zoned from bytownite to andesine), amphibole (magnesio-hornblende) and biotite. Biotite exhibits a range of Al values (~2–2.8 apfu) over very restricted Fe# ratios (0.42–0.56) which are characteristic of continental arc magmatic suites. High Ti2O contents of biotite (<6.1 wt.%) suggest a magmatic origin. Ti-in-biotite geothermometery gives a mean crystallisation temperature of 730 ± 56°C, slightly higher than calculated TZr.Ti°C (716 ± 50°C) and similar to the average TZr.sat°C (735 ± 26°C). These results are consistent with the low bulk-rock SiO2 contents, which provide minimum estimates of temperature and indicate zircon crystallised from a fractionated magma. Zircons from the Ardestan pluton have high (Sm/La)N (>10) ratios suggesting a magmatic origin. T–$f_{{\rm O}_{\rm 2}}$ calculations of oxygen fugacity between –13.6 to –16.9 indicate oxidising crystallisation conditions between the Ni–NiO (NNO) and Fe2O3–Fe3O4 (HM) buffers. Tight linear trends of log (XF/XOH), log (XCl/XOH) and log (XCl/XOH) vs. XMg represent a narrow range of $f_{{\rm H}_2O}$, fHF and fHCl, clearly indicating that physico-chemical conditions were essentially constant throughout the formation of magmatic biotite. The shape of crystal size distribution curves along with the medium Al and Mg contents in amphibole and biotite, respectively, are consistent with a history of magma mixing involving injections of basic magma into the evolving felsic chamber. Calculated residence time for Ardestan plagioclase crystals of ~630 years support field evidence that these plutons were emplaced at shallow depths.

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.

Basement characterization and crustal structure beneath the Arabia–Eurasia collision (Iran): A combined gravity and magnetic study

We present a study on the depth to basement and magnetic crustal domains beneath the Iranian Plateau by modeling aeromagnetic and gravity data. First, field processing of the aeromagnetic data was undertaken to estimate the general characteristics of the magnetic basement. Afterwards, inverse modeling of aeromagnetic data was carried out to estimate the depth to basement. The obtained model of basement was refined using combined gravity and magnetic forward modeling. Hereby, we were able to distinguish different magnetic domains in the uppermost crust (10–20 km depths) influencing the medium to long wavelength trends of the magnetic anomalies. Magnetic basement mapping shows that prominent shallow magnetic features are furthermore located in the volcanic areas, e.g. the Urumieh Dokhtar Magmatic Assemblage. The presence of ophiolite outcrops in SE Iran implies that shallow oceanic crust (with high magnetization) is the main source of one of the biggest magnetic anomalies in entire Iran area located north of the Makran.

Geochemical, Sr-Nd isotopic investigations and U-Pb zircon chronology of the Takht granodiorite, west Iran: Evidence for post-collisional magmatism in the northern part of the Urumieh-Dokhtar magmatic assemblage

Subduction of Neo-Tethys lithosphere beneath the Iranian plateau during Neogene led to the formation of a NW-SE trending volcano-plutonic zone called Urumieh-Dokhtar magmatic assemblage (UDMA). The Takht granodiorite (NE of Hamedan Province, western Iran) belongs to the UDMA and has geochemical properties of post-collisional granitoids that was formed after the collision of Arabian and Iranian plateaus. This body contains rounded mafic micro-granular enclaves with relatively gradational rims indicating the effect of magma mixing/mingling in formation of the granodiorite body. The determination of U-Pb zircon age proved the Takht granodiorite was formed at Miocene (16.8 ± 0.24 Ma). The Nd-Sr isotope ratios and Sr/Nd, Nb/La and Th/U ratios of the granodiorite confirmed the magma was formed mainly by melting of continental crust, and its enclaves originated from a mantle derived mafic magma. Samples show negative anomalies in Nb, Sr, Ti, P and Eu, whereas positive anomalies in Th, K, Zr, Yb and Rb that reveals contribution of mantle and crustal materials in their generation. The Takht granodiorite has geochemical features of A2-type granites and also shows properties of both the volcanic arc and within plate magmatism association granitoids (high levels of LILEs and HFSEs). Regarding this interpretation and also post-collisional tectonic regime, it can be concluded that post-collision extensions caused deep faults in the UDMA that let mantle derived magmas rise up to the thicken crust. Such magma triggered melting in the middle crustal levels and was contaminated with crustal materials to generate granodiorite and enclave magmas respectively. The results of the current study decipher collision between the Arabian and the Iranian plateaus occurred before Miocene and the magmatism in the UDMA continued after closure of Neo-Tethys.

Potential field signatures along the Zagros collision zone in Iran

The Zagros orogenic belt, known as an active fold-thrust belt, was formed in southwestern Iran due to the convergence of the Arabian and Eurasian plates. In this study, potential field data are inverted in 3D to image the variations of magnetic susceptibility and density contrast along the collision zone, resulting in better tectonic understanding of the studied region. Geophysical data measured by airborne magnetic and ground-based gravity systems are used to construct an integrated model that facilitates the interpretations of various tectonic zones across a 450-km line. This line intersects the main structural units from the SW portion of the Zagros belt. The constructed model reveals a contrast that indicates the transition between the two continental plates coinciding with the western boundaries of the Sanandaj-Sirjan Zone (SSZ) at the Main Zagros Thrust (MZT) fault. The subduction of the Arabian continental crust below the Iranian one is evident because of its lower susceptibility property and alternating sequence of high and low density regions. Higher susceptibility, magnetic remanence and density are the mainstays of the Urumieh-Dokhtar Magmatic Assemblage (UDMA) zone at the NE of the studied route, whereas lower values of these properties correspond to (1) the thin massive Tertiary-Neogene and Quaternary sediments of the central domain (CD) zone, and (2) the thick sedimentary and salt intrusion cover over the Zagros Fold-and-Thrust belt (ZFTB). Higher density of regions in the Arabian crust below the ZFTB implies that fault activities have caused significant vertical displacement of the basement. Finally, a simplified geological model is presented based upon the inversions of the geophysical data, in which the main geological units are divided along the studied route.

Alteration Mapping in Central Urumieh-Dokhtar Magmatic Assemblage of Iran Using ASTER and OLI Data

The study area is located on the Urumieh-Dokhtar Magmatic Assemblage of Iran, in the west of Ardestan between the East longitude 52°1' to 52°18' and the North latitude 33°17' to 33°27'. Remote sensing techniques are suitable for studying the alterations occurring in the igneous terranes. The alteration zones are well illustrated by implementation of the principal component analysis and the Crosta methods and Spectral Feature Fitting on ASTER data. In order to identify the lineaments, both Landsat-8 satellite imagery and GDEM-ASTER data are used in spatial processing. Using directional filtering and automatic extraction of lineaments, a tectonic lineaments map is prepared. Then alteration maps, tectonic lineaments map and 1:100,000 geology map are used to identify areas with high potential of Cu mineralization.

Post-collisional Plio-Pleistocene Anar - Dehaj adakitic subvolcanic domes in the central volcanic belt of Iran: geochemical characteristics and tectonic implications

In the area between Anar and Dehaj, located in the Urumieh–Dokhtar magmatic assemblage (UDMA), there are some Plio-Pleistocenic subvolcanic porphyritic andesitic-dacitic domes. The subvolcanic domes, as a part of Dehaj-Sarduieh belt in Kerman province, show tafoni erosion and magma mingling evidences. Petrographically, the domes are andesite and trachyandesite in composition and are characterized by microlitic-porphyritic, glomeroporphyritic, trachytic and aphanitic textures. Mineralogically, the domes consist of plagioclase, amphibole, clinopyroxene and opaque minerals. Geochemical studies reveal that Anar-Dehaj subvolcanic domes can be divided two groups, named group 1 and group 2, on the basis of the silica content. Group 1 shows high-K calc- alkaline andesite- trachyandesite composition; whereas group 2 is medium-K calc-alkaline dacite. The absence of a distinct Eu anomaly in all samples suggests contemporaneous crystallization of plagioclase and amphibole, and/or oxidation state of magma. Trace element discrimination diagrams along with chondrite-normalized rare earth element patterns show that the Anar- Dehaj subvolcanic rocks formed in a subduction related environment. Moreover, high Sr/Y and La/Yb ratios along with low Y and HREE contents are consistent with adakitic natures. It is suggested that the Anar- Dehaj subvolcanic domes formed in a post-collisional setting due to slab melting or underplating of basaltic magmas under thick Plio-Pleistocine continental crust.

A general framework of TOPSIS method for integration of airborne geophysics, satellite imagery, geochemical and geological data

This work presents the promising application of three variants of TOPSIS method (namely the conventional, adjusted and modified versions) as a straightforward knowledge-driven technique in multi criteria decision making processes for data fusion of a broad exploratory geo-dataset in mineral potential/prospectivity mapping. The method is implemented to airborne geophysical data (e.g. potassium radiometry, aeromagnetic and frequency domain electromagnetic data), surface geological layers (fault and host rock zones), extracted alteration layers from remote sensing satellite imagery data, and five evidential attributes from stream sediment geochemical data. The central Iranian volcanic-sedimentary belt in Kerman province at the SE of Iran that is embedded in the Urumieh–Dokhtar Magmatic Assemblage arc (UDMA) is chosen to integrate broad evidential layers in the region of prospect. The studied area has high potential of ore mineral occurrences especially porphyry copper/molybdenum and the generated mineral potential maps aim to outline new prospect zones for further investigation in future. Two evidential layers of the downward continued aeromagnetic data and its analytic signal filter are prepared to be incorporated in fusion process as geophysical plausible footprints of the porphyry type mineralization. The low values of the apparent resistivity layer calculated from the airborne frequency domain electromagnetic data are also used as an electrical criterion in this investigation. Four remote sensing evidential layers of argillic, phyllic, propylitic and hydroxyl alterations were extracted from ASTER images in order to map the altered areas associated with porphyry type deposits, whilst the ETM+ satellite imagery data were used as well to map iron oxide layer. Since potassium alteration is generally the mainstay of porphyry ore mineralization, the airborne potassium radiometry data was used. The geochemical layers of Cu/B/Pb/Zn elements and the first component of PCA analysis were considered as powerful traces to prepare final maps. The conventional, adjusted and modified variants of the TOPSIS method produced three mineral potential maps, in which the outputs indicate adequately matching of high potential zones with previous working and active mines in the region.

A combined magnetometry and gravity study across Zagros orogeny in Iran

In this work, the structural geology and the tectonic conditions of the Zagros orogeny along the route of Qom to Kermanshah cities were investigated using the combined geophysical methods of the airborne magnetometry and the ground-based gravity data. Airborne magnetometry data of Iran with a line space of survey, 7.5km, were used to model the magnetic susceptibility property along the route. At first, the airborne magnetic data were stably 500-m downward continued to the ground surface in order to enhance minor changes of the Earth’s magnetic field over the studied region. Afterward, 3D inverse modeling of the magnetic data was implemented to the downward continued data, and subsequently the section of magnetic susceptibility variation along the desired route was extracted and imaged at depth. The acquired model could appropriately predict the observed magnetic data, showing low misfit values between the observation and the predicted data. The analytic signal filter was applied to the reduced-to-pole (RTP) magnetic data leading to the determination of the active and probable hidden faults in the structural zones of the Zagros, such as Sanandaj-Sirjan, Central Domain (CD) and Urumieh-Dokhtar based upon the generated peaks along the profile of analytic signal filter. In addition, the density variations of the subsurface geological layers were determined by 3D inverting of the ground-based gravity data over the whole study area, and extracting this property along the route. The joint models of magnetic susceptibility and density variation could appropriately localize the traces of faults along with the geologically and tectonically structural boundaries in the region. The locations of faults correspond well to the variation of geophysical parameters on the inverted sections. Probable direction, slope and extension at depth of these faults were also determined on the sections, indicating a high tectonized zone of the Sanandaj-Sirjan Zone (SSZ) parallel to the zone of the Urumieh-Dokhtar Magmatic Assemblage (UDMA). The UDMA zone increases the magnetic and the Bouguer anomalies by intruding into the CD zone as well.

Petrology and geochemistry of the Karaj Dam basement sill: Implications for geodynamic evolution of the Alborz magmatic belt

The northeastward subduction of the Neo-Tethyan oceanic lithosphere beneath the Iranian block produced vast volcanic and plutonic rocks that now outcrop in central (Urumieh–Dokhtar magmatic assemblage) and north–northeastern Iran (Alborz Magmatic Belt), with peak magmatism occurring during the Eocene. The Karaj Dam basement sill (KDBS), situated in the Alborz Magmatic Belt, comprises gabbro, monzogabbro, monzodiorite, and monzonite with a shoshonitic affinity. These plutonic rocks are intruded into the Karaj Formation, which comprise pyroclastic rocks dating to the lower–upper Eocene. The geochemical and isotopic signatures of the KDBS rocks indicate that they are cogenetic and evolved through fractional crystallization. They are characterized by an enrichment in LREEs relative to HREEs, with negative Nb–Ta anomalies. Geochemical modeling using Sm/Yb versus La/Yb and La/Sm ratios suggests a low-degree of partial melting of a phlogopite–spinel peridotite source to generate the KDBS rocks. Their low ISr=0.70453–0.70535, ɛNd (37.2Ma)=1.54–1.9, and TDM ages ranging from 0.65 to 0.86Ga are consistent with the melting of a Cadomian enriched lithospheric mantle source, metasomatized by fluids derived from the subducted slab or sediments during magma generation. These interpretations are consistent with high ratios of 206Pb/204Pb=18.43–18.67, 207Pb/204Pb=15.59, and 208Pb/204Pb=38.42–38.71, indicating the involvement of subducted sediments or continental crust. The sill is considered to have been emplaced in an environment of lithospheric extension due to the slab rollback in the lower Eocene. This extension led to localized upwelling of the asthenosphere, providing the heat required for partial melting of the subduction-contaminated subcontinental lithospheric mantle beneath the Alborz magmatic belt. Then, the shoshonitic melt generates the entire spectrum of KDBS rocks through assimilation and fractional crystallization during the ascent of the magma.

Oligocene–Miocene geodynamic evolution of the central part of Urumieh-Dokhtar Arc of Iran

Basic volcanic rocks from Tafresh, west Kashan, and west Nain volcanic successions in the central part of Urumieh-Dokhtar Magmatic Assemblage (UDMA) of Iran yield K–Ar ages ranging from 26.8 to 18.2 Ma. These ages indicate significant Late Oligocene–Early Miocene basic volcanism in the UDMA. These ages, combined with K–Ar ages of 26.0 and 14.1 Ma, respectively, for associated low-silica and high-silica adakites, help constrain reconstructions of the UDMA geodynamic evolution. Late Oligocene–Early Miocene slab roll-back associated with an asthenospheric mantle influx are suggested as the major processes responsible for concurrent volcanism showing Nb–Ta-depleted, Nb–Ta-enriched and low-silica adakite signatures. Slab roll-back, the likely consequence of a decrease in subduction velocity, led to partial melting of the subducted slab and produced Early–Middle Miocene high-silica (dacitic) adakites. Oligocene to Miocene volcanic rocks do not conform to the Oligocene continental collisional model for the UDMA, rather they suggest a decrease in the subduction rate that prompted the asthenospheric mantle influx.

Mineralization and structural features of Kalchouyeh copper-gold deposit (west-central Iran)

The Kalchouyeh copper deposit is located about 110 km east of Isfahan, in the Urumieh-Dokhtar magmatic assemblage (UDMA) arc of Iran. Mineralization is closely associated with the calc-alkaline volcanism of the UDMA arc and controlled by northwest–southeast trending strike-slip faults (Qom-Zefreh and Nain-Baft faults). A pull-apart basin along these strike-slip faults has facilitated the localization of magma and related hydrothermal systems and eventually the development of Kalchouyeh mineralization. Copper mineralization occurs mainly as veins and veinlets and is related to two distinct main stages of hypogene and supergene. Hypogene mineralization includes Cu and Pb sulfides (chalcopyrite, pyrite, galena) and Fe oxide (magnetite). Chalcocite and covellite are found in supergene mineralization. Microthermometric measurements of fluid inclusions on two mineralized veins show the presence of two different kinds of ore fluids: (1) low temperature and salinity in relation to vein A (homogenization temperature 175–252 °C, salinity 0.82–4.23 wt.% NaCl) and (2) low to moderate temperature and salinity in relation to vein B (homogenization temperature 255–324 °C, salinity 0.16–9.32 wt.% NaCl). Copper mineralization at Kalchouyeh appears to be epithermal and low-sulfidation in style, and ore deposition was the result of cooling and surface fluid dilution.

Presence of a layered lithosphere beneath the Zagros collision zone

The continental collision between the Arabian and Eurasian plates following the subduction of the Neo-Tethys ocean beneath Eurasia resulted in a complex deformation across the Zagros Mountains. We used more than 700 S receiver functions obtained from 61 permanent stations to provide a detailed image of the lithospheric interaction between the Eurasian and Arabian plates. Our results suggest the existence of two different lithospheric blocks. A 200 km thick lithosphere was observed beneath the Zagros collision zone and most likely represents the Arabian lithosphere, which has been strongly deformed, thickened and depleted. Based on our results, shortening due to the collisional process is differently accommodated by the crust and mantle lithosphere of the Zagros collision zone. We localized the thick Arabian lithosphere beneath the Zagros, Sanandaj–Sirjan Zone (SSZ) and Urumieh–Dokhtar Magmatic Assemblage (UDMA), whereas the crustal thickening (~ 70 km) seems to be concentrated just beneath the SSZ. Furthermore, we found a thin lithosphere of about 80–90 km that we interpret here as the Iranian lithosphere, beneath Central Iran and Alborz. Additionally, our results suggest the presence of remnants of the fossil Neo-Tethys subduction at depths ranging between 80 and 150 km within the Arabian lithosphere. This dipping structure can be seen beneath the Zagros, SSZ and UDMA and seems to disappear towards the northeast beneath Central Iran and Alborz. These findings may support the idea of a breakoff of the oceanic Neo-Tethyan slab beneath Central Iran, which results in an asthenospheric upwelling and thinning of the Iranian lithosphere beneath Central Iran and Alborz. The boundary between the Arabian and Iranian plates seems to be located northeast of the UDMA in the northwest of Zagros and southwest of the UDMA in the Central Zagros region.

Crustal thickness variations in the Zagros continental collision zone (Iran) from joint inversion of receiver functions and surface wave dispersion

Variations in crustal thickness in the Zagros determined by joint inversion of P wave receiver func- tions (RFs) and Rayleigh wave group and phase velocity dispersion. The time domain iterative deconvolution pro- cedure was employed to compute RFs from teleseismic recordings at seven broadband stations of INSN network. Rayleigh wave phase velocity dispersion curves were estimated employing two-station method. Fundamental mode Rayleigh wave group velocities for each station is taken from a regional scale surface wave tomographic imaging. The main variations in crustal thickness that we observe are between stations located in the Zagros fold and thrust belt with those located in the Sanandaj-Sirjan zone (SSZ) and Urumieh-Dokhtar magmatic assemblage (UDMA). Our results indicate that the average crustal thickness beneath the Zagros Mountain Range varies from ∼46 km in Western and Central Zagros beneath SHGR and GHIR up to ∼50 km beneath BNDS located in easternmost of the Zagros. Toward NE, we observe an increase in Moho depth where it reaches ∼58 km beneath SNGE located in the SSZ. Average crustal thickness also varies beneath the UDMA from ∼50 km in western parts below ASAO to ∼58 in central parts below NASN. The observed variation along the SSZ and UDMA may be associated to ongoing slab steepening or break off in the NW Zagros, comparing under thrusting of the Arabian plate beneath Central Zagros. The results show that in Central Iran, the crustal thickness decrease again to ∼47 km below KRBR. There is not a significant crustal thickness difference along the Zagros fold and thrust belt. We found the same crystalline crust of ∼34 km thick beneath the different parts of the Zagros fold and thrust belt. The similarity of crustal structure suggests that the crust of the Zagros fold and thrust belt was uniform before subsidence and deposition of the sediments. Our results confirm that the shortening of the western and eastern parts of the Zagros basement is small and has only started recently.

Magmatic and geodynamic evolution of Urumieh–Dokhtar basic volcanism, Central Iran: major, trace element, isotopic, and geochronologic implications

Basic volcanic rocks from the West Nain area of the Urumieh–Dokhtar Magmatic Assemblage demonstrate significant subduction-related geochemical characteristics; these along with the new age data obtained for the volcanic rocks shed new light on the geodynamic evolution of the Iranian segment of Alpine–Himalayan orogeny. The late Oligocene (26.5 Ma) high-Nb basic volcanic rocks are likely to represent a transient rather enriched asthenospheric mantle underlying the otherwise dominantly Eocene–early Oligocene West Nain island arc. Lithospheric mantle geochemical signatures of the low-Zr volcanic rocks (20.6 Ma) and high-Th volcanic rocks (19.7 Ma) imply replacement of the underlying mantle. The substitution of asthenospheric mantle by a lithospheric mantle wedge might have been associated with – or perhaps caused by – an increase in the subduction rate. Culmination of the West Nain magmatism into slab melting that produced the early Miocene (18.7 Ma) adakitic rocks is compatible with subsequent ascent that triggered slab decompression melting.

The Origin of Co-Magmatic Andesite Enclaves in the Rabor Miocene Pyroxene Andesites, SE Kerman, Iran: Insights from Textural and Mineralogical Composition

The Miocene pyroxene andesite lava flows are exposed in southeastern edge of Urumieh Dokhtar Magmatic assemblage in Iran. The hypocrystalline andesite in parts contain conspicuous co-magmatic igneous enclaves which are dark grey and occur mostly as spherical and occasionally as ribbon shapes with some showing chilled margins. Petrographic study shows that the ribbon type enclaves have been formed by merging of rounded blobs. Mineralogically the enclaves consist of plagioclase, clinopyroxene, orthopyroxene and magnetite as phenocrysts and microphenocrysts in a glassy microlitic groundmass of the same minerals. The host andesite contains the same mineral assemblage, but with high amount of glassy groundmass. The plagioclase shows a relatively large range of anorthite content (An43-91), whereas pyroxenes generally show a uniform chemical composition and are classified mainly as hypersthene (En59.67 to En73.30) and augite (Wo40.88-46.00, Fs11.08-17.07, En36.69-47.85). Enclaves are cogenetic with the host andesites and reveal the existence of disequlibrium crystallization in the magma chamber. The enclaves are resulted from a magmatic emulsion with suspended droplets of one magma in another. The rounded enclaves are likely to represent quenched blebs of intruding new andesite magma with higher liquidus that have been incorporated into the host andesitic magma in a liquid or near liquid state. Their small sizes reflect a small viscosity contrast and some turbulence during mixing. The intrusion of fresh magma also resulted convection in the magma chamber which along with the temperature differences has permitted buoyant uprising of enclaves and incorporating some of the phenocrysts from the host andesitic magma.

Structural and petrological evidence for the continuation of the Isfahan fault system across the Urumieh-Dokhtar zone of central Iran

The Isfahan fault system, a north-trending, dextral strike-slip fault across the Sanandaj-Sirjan zone, represents the boundary between the northwestern and the southeastern parts of the Sanandaj-Sirjan zone and it terminates in the north at the southern boundary of the Urumieh-Dokhtar zone. This paper focuses on the continuation of the Isfahan fault system across the Urumieh-Dokhtar zone north of Isfahan city. The Urumieh-Dokhtar magmatic assemblage belt is located along the active margin of the Iranian plate and the Arabian plate. The Karkas fault strikes nearly north-south, has a length of about 40 km, a normal component of movement, and it truncates the Urumieh-Dokhtar zone. Due to the location of this fault and the mechanism similar to the Isfahan fault system, the Karkas fault can be considered a continuation of the Isfahan fault system that has been displaced dextrally by the southwestern bordering faults of the Urumieh-Dokhtar zone. The unique juxtaposition association of the Silurian volcanic rocks in the Urumieh-Dokhtar zone, near the Karkas fault, provides an important evidence regarding the major role of this fault in the geological evolution of the region. The Silurian volcanic rocks outcrop in two districts of the study area and generally are composed of basalts. The alkaline basalt composition is determined from mineralogy and immobile elements geochemistry. The geotectonic setting diagrams classified the Silurian volcanic rocks as the within plate basalts. Thus, an intracontinental rifting under extensional tectonic regime can be inferred as the setting that controlled formation of these volcanic rocks. They were created by an alkaline to transitional magmas generated due to low partial melting at depth. The alkaline basalts were most likely derived from an asthenosphere-dominated mantle source due to extension and partial melting. The north trending extensional faults affected thinned overlying continental lithosphere in the Paleozoic era, facilitating magma penetration and eruption.

Slab partial melts from the metasomatizing agent to adakite, Tafresh Eocene volcanic rocks, Iran

AbstractGeochemical and mineralogical characteristics of the Eocene volcanic succession in Tafresh area of the Urumieh–Dokhtar Magmatic Assemblage (UDMA) are unique in the 2000‐km‐length assemblage. Demonstrating rather steep rare earth element (REE) patterns and the widespread presence of amphibole (+biotite) phenocrysts are two distinct characters that dominate the Eocene volcanic succession of mainly andesitic composition. Coincidence of the geochemical and mineralogical characteristics of the whole volcanic succession with adakites, rather amphibole‐ (+biotite) rich dacitic (with 61–64 wt% SiO2) stocks and dykes, is considered as the key in unraveling the role of ‘slab‐derived melt contribution’ in petrogenesis of the volcanic succession. Slab‐derived melting has been an ongoing process that metasomatized some parts of the mantle wedge from which hybrid rocks (andesites) are derived. Basalts with distinct signatures of slab melt metasomatism are yet another support for the occurrence of slab melting. Interlayering of normal, island‐arc‐type calc‐alkaline volcanic rocks with the slab‐melt metasomatized basalts and hybrid andesites suggests that the slab melting has been motivated by the subduction. Formation of the Tafresh Caldera, the likely consequence of an explosive eruption, is compatible with the volatile‐bearing nature of the adakitic volcanism in the study area. It is indicated by the ubiquitous presence of the hydrous minerals. Beneath the Tafresh area, in Eocene time, the subducting slab seems to have reached a critical high depth that is enough for the development of amphibolite–eclogite. The slab deformation, motivated by the geometry of subduction and/or the underlying mantle's steeper geotherms, is suggested to have resulted in the slab melting that helped develop a rock assemblage unique to the UDMA.

Petrogenesis of post-collisional A-type granitoids from the Urumieh–Dokhtar magmatic assemblage, Southwestern Kerman, Iran: Constraints on the Arabian–Eurasian continental collision

Three plutons (Deh-Siahan, Bande-Bagh and Baghe-Khoshk Sharghi, collectively referred to as the DBB hereafter) in southwestern Kerman, in the southeastern part of the Urumieh–Dokhtar magmatic assemblage (UDMA) of the Zagros orogenic belt differ from the typical calc-alkaline metaluminous, I-type intrusions of the region. The DBB intrusions have a distinct lithological assemblage varying from diorite through monzogranite and monzonite to alkali feldspar syenite and alkali granite. The DBB granitoids are metaluminous to slightly peraluminous, alkaline to shoshonitic in composition and have high total alkali contents with K 2O > Na 2O, high FeO T /MgO values, and low CaO and MgO contents. They are enriched in some LILEs (such as Rb and Th) and HFSEs (such as Zr, Y and REEs except Eu) and depleted in Sr and Ba relative to primordial mantle, and have low concentrations of transitional metals. These features along with various geochemical discriminant diagrams suggest that the DBB granitoids are post-collisional A-type granitoids, which had not been recognized previously in the UDMA. The chondrite-normalized REE patterns of the DBB granitoids show slightly enriched light REEs [(La/Sm) N = 2.26–4.13], negative Eu anomalies [(Eu/Eu*) N = 0.19–0.74] and flat heavy REE patterns [(Gd/Yb) N = 0.80–1.87]. The negative Eu anomaly indicates an important role for plagioclase and/or K-feldspar during fractional crystallization. Whole-rock Rb–Sr isotope analysis yields an isochron age of 33 ± 1 Ma with an initial 87Sr/ 86Sr value of 0.7049 ± 0.0001. Whole-rock Sm–Nd isotope analysis gives ε Nd t values from + 2.56 to + 3.62 at 33 Ma. The positive ε Nd t and low I Sr values of the DBB granitoids together with their T DM of 0.6–0.7 Ga suggest their formation from partial melting of a lithospheric mantle source, modified by fluids or melts from earlier subduction processes. Melting of lithospheric mantle occurred via a dehydration melting process at pressures below the garnet stability field, as a consequence of lithospheric mantle delamination or break-off of a subducted slab and melting of the lithospheric mantle by upwelling of hot asthenosphere. On the basis of Rb/Sr age dating and the post-collisional geochemical signatures of the DBB granitoids, along with extensive pre-collisional volcanic eruptions in Middle Eocene, we suggest Late Eocene for the time of collision between the Arabian and Central Iranian plates. This also implies that the calc-alkaline I-type intrusions in the southwestern Kerman and in other parts of the UDMA may have formed in a post-collisional context.

Seismological evidence for crustal-scale thrusting in the Zagros mountain belt (Iran)

SUMMARY Crustal receiver functions (RFs) computed from the records of 45 temporary seismological stations installed on a 620-km-long profile across central Zagros provide the first direct evidence for crustal thickening in this mountain belt. Due to a rather short 14 km average station spacing, the migrated section computed from radial RFs displays the Moho depth variations across the belt with good spatial resolution. From the coast of the Persian Gulf to 25 km southwest of the Main Zagros Thrust (MZT), the Moho is almost horizontal with slight depth variations around 45 km. Crustal thickness then increases abruptly to a maximum of ∼70 km beneath the Sanandaj‐Sirjan metamorphic zone, between 50 and 90 km northeast of the surface exposure of the MZT. Further northeast, the Moho depth decreases to ∼42 km beneath the UrumiehDokhtar magmatic assemblage and the southern part of the Central Iranian microcontinent. The region of thickest crust is located ∼75 km to the northeast of the Bouguer anomaly low at −220 mGals. Gravity modelling shows that the measured Moho depth variations can be reconciled with gravity observations by assuming that the crust of Zagros underthrusts the crust of central Iran along the MZT considered as a crustal-scale structure. This hypothesis is compatible with shortening estimates by balanced cross-sections of the Zagros folded belt, as well as with structural and petrological studies of the metamorphic Sanandaj‐Sirjan zone.