Urumieh-Dokhtar Magmatic Arc Research Articles

Molybdenite Re–Os dating, petrology, and geochemistry of granitoids in the Bondar Hanza porphyry Cu deposit (Urumieh‐Dokhtar magmatic arc), Iran: Insight into petrogenesis, mineralization, and tectonic setting

The Bondar Hanza porphyry Cu deposit, hosted by a granitoid stock, is located 120 km south of Kerman city in the elongated NW–SE trending Urumieh‐Dokhtar magmatic arc (UDMA), Iran. The granitoid stock is a multiphase intrusive body, 2 km2 in surface area, which comprises microdiorite–microquartz diorite and granite–granodiorite. Petrological and geochemical analyses show that granitoids are peraluminous, magnesian, calcic to alkali‐calcic, and non‐adakitic intrusions characterized by negative Eu* anomalies. Thermobarometric and oxygen fugacity calculations indicate that magma emplacement occurred at 719–784°C, <100 MPa, and at a log fO2 of NNO +1.6 to +3.2. Sulphur isotopic composition of sulphide minerals (δ34S = 5.0–6.8 ‰) are similar to other porphyry Cu deposits worldwide. Molybdenite separates yield Re–Os ages of 28.22–28.03 Ma, indicating that the Cu mineralization and associated magmatism occurred during the Oligocene. The Bondar Hanza is one of the several granitoids from small porphyry Cu deposits in the UDMA that is related to island arc sub‐productive granitoids. It seems that the largest ore‐hosting porphyry systems within the UDMA are generally restricted to Miocene intrusions with adakitic affinity (e.g., Sarcheshmeh granitoids) and that the temporally discrete non‐adakitic magmatic systems are sub‐productive to barren. These data are interpreted to show that magmatism in the Bondar Hanza region was likely associated with partial melting of juvenile lower crust induced by north‐eastward subduction of the Neo‐Tethys oceanic lithosphere.

Geochemical and fluid inclusion studies of Kalchuyeh Cu-Au epithermal mineralization in the central part of the Urumieh-Dokhtar magmatic arc

Geochemical and fluid inclusion studies of Kalchuyeh Cu-Au epithermal mineralization in the central part of the Urumieh-Dokhtar magmatic arc

Petrology of Eocene Volcanic Rocks in Dastgerdo to Molla Ahmad Area (East of Isfahan); Middle Part of the Urumieh- Dokhtar Magmatic Arc (UDMA)

Petrology of Eocene Volcanic Rocks in Dastgerdo to Molla Ahmad Area (East of Isfahan); Middle Part of the Urumieh- Dokhtar Magmatic Arc (UDMA)

Zircon U-Pb Ages and Magmatic History of the Kashan Plutons in the Central Urumieh-Dokhtar Magmatic Arc, Iran: Evidence for Neotethyan Subduction during Paleogene-Neogene

The Kashan plutons are situated in the central part of Urumieh-Dokhtar magmatic arc recording subduction-related magmatism within the Alpine-Himalayan orogeny in Iran. These rocks consist of different calc-alkaline plutonic rocks including gabbro, gabbroic diorite, microdiorite, monzodiorite, tonalite, granodiorite, and granite. The plutons were emplaced into the Jurassic sedimentary units (Shemshak Formation) and the Eocene calc-alkaline volcanic and pyroclastic rocks. New U-Pb zircon ages show that the Kashan plutons formed during two main periods at 35.20±0.71 Ma in the Late Eocene (Priabonian) and at 18.90±0.84, 19.26±0.83, 19.30±1.2, and 17.3±1.8 Ma in the Early Miocene (Burdigalian). The reported events in the Kashan plutons imply the final phases of subductionrelated magmatism before the collision which happened between the Arabian and Iranian plates in the Middle Miocene. The plutonic activity in the Kashan region occurred during the transition from Eocene subduction-related setting to Middle Miocene collisional setting.

Imaging the 2–D crust and upper mantle structure of the Iranian plateau resolved by potential field and seismic data

In this study, the crust and upper mantle density distribution across the Iranian plateau along a ~1500 km profile are investigated using an integrated modeling of potential field data (gravity, geoid and topography) and by assuming local isostatic equilibrium. Our profile is initiated in Arabian platform and running through the main geological structures of Iran including Zagros orogen, Sanandaj – Sirjan Zone (SSZ), Urumieh – Dokhtar Magmatic Arc (UDMA), Central Iran (CI) block, Tabas/Lut blocks, and East Iranian Range (EIR). In the second step, in order to verify the integrated model, the 1–D S–wave absolute velocities for the crust and upper mantle beneath 21 seismic stations which partially covered the density profile are obtained using the joint inversion of P–receiver function (PRF) and surface–wave dispersion data. Our results indicate a thick crust (~60 km) beneath the SSZ and UDMA which thinning toward the east and experienced its minimum value (~40 km) beneath the central Iran, as well as Tabas/Lut blocks. Moreover, a thick lithosphere (>150 km) is observed under the Zagros, SSZ and parts of the UDMA, whereas a thinner lithosphere (~120 km on average in velocity model) is presented under the most of our profile. Both density and velocity models indicate a lithospheric thinning toward the CI block which imply the slab breakoff or lithospheric delamination followed by the upwelling of the asthenosphere in the Neogene. The presence of a relative fast structure in the crust beneath the Tabas block which is coinciding with a shallow high velocity body in the uppermost mantle elucidates a signature of former oceanic crust, which disappeared after the collision of Lut and Afghan continental blocks and E-directed subduction of Neotethys under the Afghan microcontinent either in post-Albian or middle Eocene times.

A new perspective on Cenozoic calc-alkaline and shoshonitic volcanic rocks, eastern Saveh (central Iran)

ABSTRACT Late Eocene – Oligocene volcanic rocks in the eastern Saveh region of the Urumieh-Dokhtar magmatic arc (UDMA) are representative of Paleogene magmatic activity in Iran. They show a wide range of silica-undersaturated to silica-oversaturated compositions, from basalt-trachy basalt and tephri-phonolite to trachyte-latite. Whole rock chemical compositions define a continuous assemblage of mafic and felsic rocks in terms of SiO2 (46.3 to 71.1 wt.%), Al2O3 (12.5–19.2 wt.%), K2O (2.06–12.23 wt.%) and TiO2 (0.41–1.75 wt.%) contents, Mg number (6.8–51) and K2O/Na2O ratios (0.53–30.2). Abundances of rare earth elements in the tephri-phonolite group are much higher than in the trachyte-latite and basalt-trachy basalt groups. Elevated contents of large ion lithophile elements such as Ba (191–7311 ppm) and Pb (6.49–118 ppm) as well as depletions in high field strength elements such as Nb (2.43–40.6 ppm), Ta (0.15–2.24 ppm), Zr (40.8–258 ppm) and TiO2 (0.40–1.77 wt.%) characterize these rocks. Initial ratios of 87Sr/86Sr from 0.7046 to 0.7074 and ɛNd(t) from +0.9 to +3.3 show that these were derived from parental magma extracted from partial melting of metasomatized mantle, namely amphibole- and/or phlogopite-bearing peridotite. Fractionation of feldspar, clinopyroxene, amphibole and phlogopite, and minor contamination with upper crust was responsible for the large variation of the rock types observed. We explain contemporaneous late Eocene – Oligocene calc-alkaline and high-K magmatic activity in the UDMA as due to the partial melting of both hot metasomatized asthenospheric and subcontinental lithospheric mantle in an extensional tectonic regime. This extensional regime probably developed due to the rollback of the subducted Neo-Tethys oceanic plate.

Analysis of the infiltrative metasomatic relationships controlling skarn mineralization at the Abbas-Abad Fe-Cu Deposit, Isfahan, north Zefreh Fault, Central Iran

The Abbas-Abad volcano-sedimentary-hosted Fe-Cu skarn deposit, NE Isfahan, is one of the most important skarns in the central Urumieh-Dokhtar magmatic arc. It was formed along the contact with the Dorojin granitoid massif next to the Zefreh Fault. The plutonic rocks are of I-type, volcanic arc affinity with normal-K, metaluminous, calc-alkaline to calcic quartz diorite, tonalite, and granodiorite, similar to many other Fe-type skarn-related granitoids worldwide. The parent magma involved in this skarn system is high temperature and relatively oxidized. Amphibole geobarometery in quartz diorite yielded a crystallization pressure lower than 200 MPa (2 kb, ~7 km) at 724° to 785 °C. Granitoid compositions around the ore deposit are mainly granodiorite. U-Pb zircon dating yields Early Miocene ages of 23.0 ± 1.6 Ma for a quartz dioritic rock and 21.3 ± 1.5 Ma for a tonalitic sample. The injection and cooling of the granodiorite produced a hornblende hornfels aureole with endoskarn.Paragenetic relationships and microprobe data indicate that Abbas-Abad calcic skarn evolution can be subdivided into three stages as follow: (I) Prograde skarn associated andradite-rich garnet (Adr93-98Grs0-4Spe1-2) and pyroxene, (II) Retrograde skarn starting with garnet (Adr53-69Grs28-43Spe2-4), magnetite, and sulfide minerals associated with calcic-alteration, and (III) Post-ore with pyrite, chalcedony, epidote, quartz, calcite, and zeolite veinlets. Textural and compositional studies of garnet and magnetite from the garnet-bearing exoskarn zone reveal the multiple events associated with skarn formation. Garnets are characterized by low TiO2 and relatively high CaO that are indicative of a calcareous wall-rock among Eocene volcaniclastic rocks. They are grouped into garnet-1 (low w/r) and garnet-2 (inverse zoning at high w/r) with notable Cu-contents (up to 743 ppm). Petrographically, magnetite morphology is divided into fine-grained granular, needle-like, and polycrystalline aggregates. Mineral chemistry of needle-like type reveals impure components (Al2O3, CaO, and SiO2). This type formed from dissolution-reprecipitation processes during a stage of reequilibration in the skarn system. Mixing with cooler external fluid (rich in oxygen and poor in Fe2+) is reflected in individual features during infiltration metasomatism during garnet and magnetite growth, such as oscillatory zoning and needle-like textures. Thus, we infer increasing pH (decreasing acidity) and decreasing T related to carbonate neutralization reactions affecting Fe- and Cu-chloride complexing as the main controls on mineralization.The structural studies of the area show that movement of the dextral transtensional Zefreh Fault provide local zones for emplacement of Dorojin granitoid during the Early Miocene. Consequently, the dextral transtensional Zefreh Fault and dextral transpression associated with the Marbin-Rangan Fault uplifted the skarn and host units and Dorojin body under the roughly N-S directed maximum compression direction. Furthermore, the interplay of Zefreh and Marbin-Rangan faults within the N-S regional compressional regime formed an anticlinal structure that exposed the Dorojin body within the core.

Identification of Geochemical Anomalies Using Fractal and LOLIMOT Neuro-Fuzzy modeling in Mial Area, Central Iran

The Urumieh-Dokhtar Magmatic Arc (UDMA) is recognized as an important porphyry, disseminated, vein-type and polymetallic mineralization arc. The aim of this study is to identify and subsequently determine geochemical anomalies for exploration of Pb, Zn and Cu mineralization in Mial district situated in UDMA. Factor analysis, Concentration-Number (C-N) fractal model and Local Linear Model Tree (LOLIMOT) algorithm used for this purpose. Factor analysis utilized in recognition of the correlation between elements and their classification. This classified data used for training the LOLIMOT algorithm based on relevant elements. The results of the LOLIMOT algorithm represent anomalies in areas with no lithogeochemical samples. Although, the C-N log-log plot for target elements were generated based on stream sediment and lithogeochemical samples which could be delineated mineral potential maps of the target elements. Results obtained by the LOLIMOT and fractal modeling show that the SW and the Eastern parts of the area are proper for further exploration of Cu, Pb, and Zn.

3-D crustal structure of the Iran plateau using phase velocity ambient noise tomography

SUMMARY More accurate crustal structure models will help us to better understand the tectonic convergence between Arabian and Eurasian plates in the Iran plateau. In this study, the crustal and uppermost mantle velocity structure of the Iran plateau is investigated using ambient noise tomography. Three years of continuous data are correlated to retrieve Rayleigh wave empirical Green's functions, and phase velocity dispersion curves are extracted using the spectral method. High-resolution Rayleigh wave phase velocity maps are presented at periods of 8–60 s. The tomographic maps show a clear consistency with geological structures such as sedimentary basins and seismotectonic zones, especially at short periods. A quasi-3-D shear wave velocity model is determined from the surface down to 100 km beneath the Iran plateau. A transect of the shear wave velocity model has been considered along with a profile extending across the southern Zagros, the Sanandaj-Sirjan Zone (SSZ), the Urumieh-Dokhtar Magmatic Arc (UDMA) and Central Iran and Kopeh-Dagh (KD). Obvious crustal thinning and thickening are observable along the transect of the shear wave velocity model beneath Central Iran and the SSZ, respectively. The observed shear wave velocities beneath the Iran plateau, specifically Central Iran, support the slab break-off idea in which low density asthenospheric materials drive towards the upper layers, replacing materials in the subcrustal lithosphere.

Crystallization Conditions and Mineral Chemistry in the East of Tafresh, Central Iran, with Insights into Magmatic Processes

The Tafresh granitoids are located at the central part of the Urumieh‐Dokhtar Magmatic Arc (UDMA) in Iran. These rocks, mainly consisting of diorite and granodiorite, were emplaced during the Early Miocene. They are composed of varying proportions of plagioclase + K‐feldspar + hornblende ± quartz ± biotite. Discrimination diagrams and chemical indices of amphibole phases reveal a calc‐alkaline affinity and fall clearly in the crust‐mantle mixed source field. The estimated pressure, derived from Al in amphibole barometry, is approximately 3 Kb. The granitoids are I‐type, metaluminous and belong to the calc‐alkaline series. They are all enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements and display geochemical features typical of subduction‐related calc‐alkaline arc magmas. Most crystal size distribution (CSD) line patterns from the granitoids show a non‐straight trend which points to the effect of physical processes during petrogenesis. The presence of numerous mafic enclaves, sieve texture and oscillatory zoning along with the CSD results show that magma mixing in the magma chamber had an important role in the petrogenesis of Tafresh granitoids. Moreover, the CSD analysis suggests that the plagioclase crystals were crystallized in a time span of less than 1000 years, which is indicative of shallow depth magma crystallization.

Mineralization, alteration and geochemical characteristics of host rocks in the Rangraz iron oxide-copper mineralization area, north Saveh, central part of Urumieh–Dokhtar magmatic arc

Mineralization, alteration and geochemical characteristics of host rocks in the Rangraz iron oxide-copper mineralization area, north Saveh, central part of Urumieh–Dokhtar magmatic arc

Geology, mineralogy, ore fluid characteristics, and 40Ar/39Ar geochronology of the Kahang Cu-(Mo) porphyry deposit, Urumieh-Dokhtar Magmatic Arc, Central Iran

The Kahang Cu-Mo porphyry deposit is located in the middle part of the Urumieh-Dokhtar Magmatic Arc, Central Iran, and is hosted by middle to late Miocene (~14 Ma) porphyritic quartz-diorite and granodiorite stocks intruded into the Eocene tuff, basaltic andesite, and andesitic flow sequences. The Miocene intrusions are calc-alkaline in composition and reflect arc-type to collisional environments (Nb/Y ~ 0.6) based on Nb-Y, Sr/Y-Y, and La/Yb-Yb discrimination diagrams.Copper mineralization occurs within the quartz diorite and granodiorite porphyries and volcanic rock sequences, which have been extensively altered to a central potassic zone (biotite-rich-K-metasomatism), a medial quartz-sericite-pyrite (phyllic) zone, an outer argillic alteration zone, and a peripheral propylitic alteration zone. Hypogene mineralization predominantly occurs within the central potassic zone within veins and breccias. The grade of the mineralization (avgs. 0.6% Cu with 120 million metric tons (Mt) ore reserve and decreases outward from the potassic zone, through a transitional potassic-phyllic zone, to the phyllic zone. Supergene mineralization (5 Mt at 0.3% Cu) that is partially developed in upper parts of the system forms <10 million metric tons (Mt) of the total resource of the Kahang deposit.Three stages of mineralization are recognized based on vein geometry, mineral assemblages, and ore textures. Early-stage mineralization is characterized by the occurrence of quartz-magnetite-chalcopyrite ± molybdenite-pyrite assemblages in veins, veinlets, and breccias with biotite-K-feldspar envelopes. Main-stage Cu- (Mo) mineralization is represented by quartz-chalcopyrite ± molybdenite ± pyrite-gold assemblages in disseminations within biotite-K-feldspar-sericite-altered rocks, and is responsible for the bulk of economic Cu-(Mo) mineralization. Late-stage mineralization, characterized by quartz-pyrite-sphalerite-galena assemblages in veinlets, voids, and fractures with kaolinite-illite envelopes occurs in upper parts of the deposit, and overprints previous alteration and mineralization.Four main types of fluid inclusion are distinguished in quartz from quartz-sulfide veins and veinlets: type I vapor-rich (V) two-phase inclusions with vapor/liquid ratios of 60 to 90; type II liquid-rich (L-V) inclusions with vapor/liquid ratios of < 50; type IIIa and IIIb (L-V-S) hypersaline inclusions with vapor/liquid ratios of 45 to 85 and solid phase of < 30; and type IV (L-V-S-H) multiphase inclusions. Ore fluid salinities during the main-stage mineralization were approximately 35 wt% NaCl equiv at temperatures between 220 and 450 °C and pressures between 500 and 1000 bar, corresponding to depths between 1 and 3 km.The calculated δ18Ofluid and δDfluid values for quartz-biotite veins associated with potassic alteration range between 9.7 and 3.1‰ and − 52 to − 49.8‰, respectively, consistent with precipitation from a high-temperature primary magmatic fluid, which gradually mixed with meteoric fluids in later stages. The δ34Ssulfide isotope compositions of stages II and III-chalcopyrite and pyrite range from 3.6 to 8.6‰ consistent with deposition from an oxidized, sulfate-dominant, high-temperature magmatic-hydrothermal fluid.The 40Ar/39Ar ages on hydrothermal biotite from porphyry stocks and potassic alteration zones indicate that the hydrothermal activity and subsequent porphyry Cu- (Mo) mineralization occurred between 14.73 ± 0.05 and 14.87 ± 0.06 Ma.

A hidden crust beneath the central Urumieh‐Dokhtar Magmatic Arc revealed by inherited zircon ages, Tafresh, Iran

Laser ablation ICP‐MS U–Pb dating on inherited zircons from Neogene volcanic rocks of the Tafresh area in the central part of Urumieh‐Dokhtar Magmatic Arc (UDMA) yielded Paleoproterozoic, Late Neoproterozoic–Early Cambrian, Carboniferous, and Cretaceous ages. These ages improve our understanding of the geodynamical evolution of central UDMA. The 2,379 Ma age (i.e., Paleoproterozoic) is the oldest age reported in the central UDMA and is similar to the age of detrital zircons from Central Iran. The Paleoproterozoic zircons may come from the African Craton and fed from Central Iran sedimentary sequences. The Late Neoproterozoic–Early Cambrian ages (605 ± 11 Ma to 533 ± 9.2 Ma) indicate a basement for the UDMA, which is similar to that of the Sanandaj‐Sirjan Zone, Central Iran, and northwest Iran. The Carboniferous zircon age (328.4 Ma) is interpreted as being derived from the Eurasian arc produced by northward subduction of the Paleo‐Tethys Oceanic Plate. The Late Cretaceous zircon ages (106 to 71.2 Ma) might be regarded as indicating the Late Cretaceous magmatic episode, marking the subduction of oceanic crust that followed the Neo‐Tethys subduction.

Dating of Oligocene granitoids in the Khak-Sorkh area, Central Urumieh-Dokhtar arc, Iran, and a genetic linkage with the associated skarn iron deposit

Calc-silicate assemblages and skarn-type iron ores are associated with granite and granodiorite bodies intruding Triassic-Jurassic sedimentary strata in the Khak-Sorkh area in the central Urumieh-Dokhtar magmatic arc (UDMA), Iran. The intrusive bodies are metaluminous to slightly peraluminous, high-K calc-alkaline, and of I-type affinity. The skarn-type iron ore at Khak-Sorkh occurs in several ore bodies of variable sizes consisting of magnetite, subordinate hematite, and minor sulfides. The prograde assemblage of diopsidic pyroxene, andraditic garnet, and olivine is locally preserved, but the common intimate association of the iron ore with serpentine and lesser phlogopite suggests that ore formation occurred during retrograde hydrous reactions. Zircon dated using the isotope dilution TIMS U-Pb technique yields overlapping ages of 30.05 ± 0.06 Ma and 29.94 ± 0.09 Ma for two granite samples, consistent with the age of 29.51 ± 0.13 Ma for coexisting monazite, constraining the time of magma emplacement. A similar age of 29.53 ± 0.11 Ma was obtained for zircon grains from a highly epidotized granite offshoot enclosed in dolomitic limestones. Zircon from granodiorite provides a maximum age of 31.23 ± 0.16 Ma, with coexisting xenotime and titanite yielding 29.49 ± 0.10 M 29.67 ± 0.42 Ma, respectively. The 207Pb/204Pb ratios for epidote from both endoskarn and exoskarn (15.584–15.669) are identical implying a common fluid source. They are distinguished from those for calcite from the host limestone (15.774–15.779), the latter consistent with upper crustal sources. The new results confirm that significant arc-type plutonism occurred in the central UDMA during the Oligocene, consistent with the widespread I-type plutons in southern and northern sections of the UDMA.

Pb isotope geochemistry of the late Miocene–Pliocene volcanic rocks from Todeshk, the central part of the Urumieh–Dokhtar magmatic arc, Iran: Evidence of an enriched mantle source

The late Miocene–Pliocene volcanic rocks from Todeshk, south-east of Isfahan, are located in the middle of the Urumieh–Dokhtar magmatic belt. The belt is considered the subduction-related magmatic arc. The late Miocene–Pliocene calc-alkaline volcanic rocks are mainly andesite and dacite. The rocks have been formed during the post-collisional stage of the Zagros orogen. Geochemical data show the enrichment of light rare-earth elements and large ion lithophile elements such as Cs, Rb, K, Pb, Ba and Th as well as the depletion of elements with high field strength such as Nb, Ta and Ti. The Pb–Sr–Nd isotopic ratios of the studied rocks are characterised by \({}^{206}\hbox {Pb}/{}^{204}\hbox {Pb} =18.41\)–18.72; \({}^{207}\hbox {Pb}/{}^{204}\hbox {Pb} =15.64\)–15.67; \({}^{208}\hbox {Pb}/{}^{204}\hbox {Pb} = 38.49\)–38.83; \({}^{207}\hbox {Pb}/{}^{206}\hbox {Pb} = 0.8372\)–0.8496; \({}^{208}\hbox {Pb}/{}^{206}\hbox {Pb} = 2.0743\)–2.0905; \({}^{87}\hbox {Sr}/{}^{86}\hbox {Sr} = 0.7051\)–0.7068 and \({}^{143}\hbox {Nd}{/}{}^{144}\hbox {Nd} = 0.5125\). The rocks have \({\Delta }7/4\hbox {Pb} = 15.44\)–15.82 and \({\Delta }8/4\hbox {Pb} = 57.26\)–60.44. Based on petrological studies and the whole rock Pb, Sr and Nd isotopes data, the late Miocene–Pliocene calc-alkaline volcanic rocks have been generated from the partial melting of the subduction-related metasomatised mantle. Additionally, the slab-derived melts and fluids were recycled into the mantle source. The data demonstrate that terrigenous sediments accompanied by a subducted slab play an important role in the formation of the enriched mantle as the source of volcanic rocks.

Moho depth variations and Vp/Vs ratios in the seismotectonic zones of Central Iran, Eastern Iran, and Makran: using a modified Zhu and Kanamori method

Variations of the Moho depth and Vp/Vs ratio beneath the Central Iran, Eastern Iran, and Makran regions using a modified Zhu and Kanamori method are presented in this paper. Receiver functions have been calculated from 3 years of teleseismic recordings at seventeen broadband stations using a time-domain iterative deconvolution method. Our results indicate that Moho depth changes from 56 km beneath the western part of Central Iran to 40 km in Central Iran and 38 km in Eastern Iran. The relatively thick crust beneath western margin of Central Iran is due to the close proximity of this region to the Sanandaj-Sirjan Zone and the Urumieh-Dokhtar magmatic arc that are associated with underthrusting of the Arabian plate beneath Central Zagros along the main Zagros reverse fault. Also, crustal thickening of this region would be due to volcanic intrusions, magmatic. The reduction of crustal thickness beneath the eastern stations in this region could be due to the absence of a collisional zone in the tectonic evolution of the region. The results beneath the CHBR station located in Makran indicate a value of 31 km for crustal thickness and 1.70 for the Vp/Vs ratio. It is associated to subduction of the oceanic crust of the Arabian plates, beneath the southern part of the Makran crust with a very low slope. The crust beneath Central and Eastern Iran has a moderate thickness which decreases from west to east.

Geochemistry and zircon U-Pb geochronology of Miocene plutons in the Urumieh-Dokhtar magmatic arc, east Tafresh, Central Iran

ABSTRACT The Tafresh plutons that include Ahmadabab diorite, Vasfonjerd monzonite, Mehrezamin diorite and Chahak diorite, located to the east of Tafresh city, north-central Iran, are part of Urumieh-Dokhtar magmatic arc. U-Pb dating of zircon grains provides emplacement ages of 22.3 ± 1 Ma for the Ahmadabad diorite, and tightly clustered ages of 22.2 ± 0.2 Ma, 21.3 ± 0.2 Ma, and 21.7 ± 0.4 Ma for Vasfonjerd monzodiorite, Mehrezamin diorite-monzonite, and Chahak diorite-monzonite plutons, respectively. These rocks are metaluminous to weakly peraluminous, calc-alkaline, and characterized by enrichment in light rare earth elements, Nb-Ta negative anomalies, and high LILE/HFSE ratios. Tafresh plutonic rocks originated from a parental magma source and experienced different degrees of partial melting. Geochemical signatures of Tafresh plutonic rocks, such as a wide range of Y/Nb (2.7–8.4) and low Zr/Nb (19.5–35.) ratios, Nb/Ta (11.46–18.15), argue for mantle–crust interaction during generation of Tafresh magmas. Relatively low Nb/La ratios further indicate that the lithospheric mantle played a significant role in melt generation. HREE signatures (i.e. decrease Dy/Yb with increasing SiO2) preclude substantial involvement of garnet either in the residue, both during partial melting and fractionation of the magma. The plutons are a product of final stages of subduction-related magmatism prior to the collision between the Arabian and Eurasian tectonic plates.

Extensional magmatism in a continental collision zone,Tafresh area, western central Iran: structural, geochemical and mineralogical considerations

The Tafresh area, located on the middle part of Urumieh-Dokhtar magmatic arc, is a suitable area for considering the effects of tectonic structures on development of the volcanic facies in the magmatic provinces. The main structure in the area is a dextral fault (so called Rahjerd Fault) that has produced the local extensional basins and also the parallel dyke swarms (or feeder dykes) on its both sides. So, the pyroclastic deposits, basaltic-andesitic lava flows, parallel dyke swarms were appeared by explosive subaqueous and then effusive subaerial eruptions and eventually a dioritic stock intruded in the volcanic pile. Petrologically, the magmatic rocks are belonged to the calcalkaline suite that had been changed and evolved in the magma chambers of local extensional basins in a collisional tectonic setting. The mineral chemistry and geochemical modeling as well as the coexistence of different mineral paragenesis of plagioclases (An47-72 and An17-37), pyroxenes (diopside and pigeonite) and amphiboles (magnesiohastingsite and magnesiohornblende) reveal that the AFC process is a dominant process in the magma chamber. Also, according to geothermobarometric calculations, the investigated volcanic rocks of the area could be grouped into two types: one type with higher P-T (>6Kbar and about 900 °C) including Eocene pigeonite-magnesiohastingsite bearing andesites (PHA) and Miocene andesites and other with lower P-T (1-4 Kbar and 600- 800 °C) including the Eocene diopside-magnesiohornblende bearing andesites (DHA) and the dioritic stock.

Geochemical and Sr–Nd isotopic constraints on the genesis of the Soheyle-PaKuh granitoid rocks (central Urumieh-Dokhtar magmatic belt, Iran)

ABSTRACT Soheyle-Pakuh granitoid rocks, with a variety of quartz diorite, quartz monzodiorite, granodiorite, tonalite, and granite, have been emplaced into the Tertiary volcanic rocks in the Urumieh-Dokhtar magmatic arc in central Iran. Zircon U–Pb dating yields an age of 39.63 ± 0.93 Ma for the crystallization of this body. Whole-rock compositions show that SiO2 changes from 52.31 to 65.78 wt.% and Al2O3 varies from 15.54 to 18.24 wt.%, as well as high concentrations of large-ion lithophile elements (LILE, e.g. Cs, Rb, Ba, and K) and quite low contents of high field strength elements (HFSE, e.g. Nb, Ti, P), as expected in I-type arc granitoids formed in an active continental margin setting. Initial ratios of 87Sr/86Sr and 143Nd/144Nd exhibit ranges 0.7043–0.7047 and 0.51284 to 0.51287, respectively, with positive εNd(t) from +4.9 to +5.5 with a young TDM1 age (483–674 Ma); this tracer isotopic data suggesting that the SPG originated from juvenile basaltic crust derived from depleted mantle (~90%) with variable contributions from undepleted mantle and approximately 10% old lower crust, despite diverse processes (e.g. magma mixing and fractional crystallization) during their evolution and emplacement into a local extensional setting within the continental margin arc. The isotopic data are similar to those of other Phanerozoic granitoids of the Central Asian Orogenic Belt and corroborate melting of predominantly mantle-derived juvenile crustal protoliths and indicating extensive addition of new continental crust, during Cambrian-Neoproterozoic time, in the suprasubduction zone beneath the central Urumieh-Dokhtar magmatic arc. Generation of these types of granitoids favours a model whereby rollback and (or) break-off of a subducted slab with subsequent lithospheric extension triggered by mantle upwelling, heat advection, and underplating resulting in melting of the central UDMA mantle-derived juvenile lower continental crust in the Late Eocene.

Geochemical and tectonic significance of Arbat alkali gabbro-monzonite-syenite intrusions, Urumieh–Dokhtar Magmatic Arc, Iran

The Oligocene Arbat alkali intrusions of the Eastern Miandoab are located in the northwestern part of Iran and belong to the Urumieh–Dokhtar Magmatic Arc (UDMA). The intrusions show a ring structure with gabbro-monzogabbro-monzodiorite (mafic units) on the edges, with monzonite-monzosyenite-syenite (felsic units) gradually going towards the central parts. The textures in different rock types are cumulate, granular and laminated. The high values of (La/Sm)n and (La/Yb)n, contents of K, Rb and Cs (positive anomalies normalized on the basis of the primitive mantle), low concentrations of Hf, Nb, Zr and Ta (negative anomalies), and the changes in Th/Nb, Th/Ta, La/Nb and Ce/Pb ratios along with the geochemical and tectonic setting evidence exhibit a subduction-modified mantle origin for the formation of these rocks. Accordingly, the intrusions were formed between the Central Iran and the Arabian plates as a result of the partial melting of a mantle wedge at a syn-collision or post-collision arc-related environment. Our data suggested that, after the end of the oblique Neotethys subduction and during/after the continental collision, the break-off or rollback of the Neotethys slab beneath western Iran, in the Oligocene, might have occurred. Such a process led to the change in the geothermal gradient of the mantle wedge because of the subduction fluids, transtension, pressure reduction along the SE-trending lateral depth strike-slip fault zones in the upper part of the mantle wedge, decompression partial melting at the mantle, and the resulting formation of a mafic potassium-rich melt. The mafic magma was injected into crustal magma chambers; probably, the fractional crystallization and partial contamination occurred with crustal components, forming the intermediate and felsic rocks in the intrusions. Geochemical evidence related to the variations in the ratios of Th/Yb, Ta/Yb, Rb/Y, and Nb/Y and Harker variation diagrams along with the spider diagrams confirmed fractional crystallization and partial FC (fractional crystallization) and AFC (assimilation and fractional crystallization) in the intrusions.