NW Iran Research Articles

Mineral chemistry and alteration patterns of Cr-spinel in serpentinized peridotites from NW Iran

The Late Cretaceous ophiolite mélange in the Salmas area of NW Iran is a part of the Neotethys ophiolites. The mélange includes serpentinized harzburgite, serpentinites, mafic rocks, radiolarite, layered red pelagic limestones and grey and white marbles. Harzburgite main primary mineral phases are olivine, orthopyroxene, clinopyroxene and Cr-spinel. Cr-spinel has Cr2O3 contents of 21.13 to 30.18 wt% and high Al2O3 (38.67–48.52 wt%), FeO (15.18–18.13 wt%) and MgO (15.18–17.51 wt%) contents. The 100 × Cr/(Cr + Al) ratios of 23 to 34 indicate 9 to 13 % partial melting in the Mid Ocean Ridge (MOR) environment for the origin of the peridotites. An alteration zone is developed around the altered Cr-spinel. Fine-grained minerals assemblage at the spinel crystals’ margin includes Cr-rich chlorite, Cr-rich garnet and spinel-silicate mixture. A 2–5 μm wide transitional zone is developed between the chromite-silicate assemblage and the Cr-rich garnet zone. The chemical variations of major oxides across the alteration zone are mainly diffusion controlled. Al, Cr and Mg have diffused out from the primary spinel and Fe and Mn have diffused into the spinel. Cr-spinel is altered in two stages due to serpentinization. During the first stage and following hydration, spinel reacted with olivine and orthopyroxene to form Cr-rich chlorite and ferrian chromite. Silica formed at this stage. At the second stage, the reaction between the chromite-silicate assemblage and Cr-rich chlorite plus silica form the first stage and Ca2+ in the fluid (released from clinopyroxene alteration) produced Cr-rich garnet and H2O-rich fluid, at temperature between 400 and 600 °C.

Trace and minor elements in the ore minerals: An assessment of geochemical distribution in overprinting epithermal on porphyry mineralization at Chodarchay, NW Iran

The Chodarchay overprinting epithermal on porphyry mineralization occurs in the Tarom-Hashtjin subzone of the Alborz magmatic arc in northwestern Iran. Rock units include intrusions (phases I and II) and volcano-volcaniclastic rocks. The alteration types consist of potassic, phyllic, argillic, and propylitic. The porphyry type is mostly accompanied by potassic alteration. The Chodarchay Fault implies two extensional and compressional mechanisms. The porphyry type is associated with the extensional and the epithermal type with the compressional stage of the fault. Mineralization occurred mainly inside the Eocene volcanic and volcano-sedimentary rocks; some ores are associated with the Late Eocene quartz monzonite. The mineralization is divided into three zones from depth to surface: 1) Hypogene sulfide zone (deep); 2) Enriched supergene sulfide zone (near-surface); 3) Oxidized and leached zone (surface). The major sulfide minerals consist of chalcopyrite, pyrite, sphalerite, and galena. Based on the results of the new analyses, chalcopyrite hosts Ag as a solid-solution and contains Ag-rich Bi-sulfosalt micro-inclusions. Non-constant Cd/Zn ratio in the chalcopyrite can indicate varying physiochemical conditions during its crystallization. Low As contents in the pyrite caused relatively clean pyrite with very few elements. The Ni concentrations in the pyrite imply that it was sourced from mantle mafic–ultramafic rocks. Sphalerite is the only Au-host mineral among the sulfide minerals as a nano-mineral phase. Solid-solution Ag exists in the sphalerite. The sphalerite Fe/Zn ratios show that it was precipitated between 220 and 280 °C. Silver and Bi have the highest measured concentrations among all analyzed elements in the galena, mainly partitioning within the galena, but Hg and Cd are abundant in the sphalerite. Bornite is one of the major carriers of Ag and Bi. Silver and Bi exhibit considerable variations in the Cu-(Fe) sulfides. A new occurrence of sulfosalt minerals was identified at Chodarchay (N = 14). The discovered Bi-sulfosalts include cuprobismutite group minerals (cuprobismutite, hodrushite, and paderaite), aikinite-bismuthinite series (aikinite and friedrichite), emplectite, makovickyite, galenobismutite, gustavite, heyrovskyite, neyite, cupropavanite; and As-Sb-bearing Cu sulfosalts (watanabeite and colusite). This number of sulfosalts had not previously been reported from any of the Tarom or Alborz deposits or mines. The Au-Ag occurrence was reported from the neyite, emplectite, and gustavite. The Ag-rich minerals crystallize from fractionated fluids that cooled during rising. Silver-carrier sulfosalts may have been exsolution from pre-existing Ag-bearing chalcopyrite in the cooling system. Most of the Ag-bearing sulfosalt minerals formed at the margins and fractures of the chalcopyrite, indicating that the cooling started from these points. Thus, Ag at Chodarchay is hosted inside Cu-(Fe) sulfides, the significant part of them being chalcopyrite and Ag-rich sulfosalt micro-inclusions inside the chalcopyrite. The fluid temperature for the Ag and chalcopyrite precipitation is from 200 to 350 °C. Bismuth assemblages precipitated at > 350 °C. The cuprobismutite group crystallizes at temperatures > 300 °C. The supergene process and rare secondary enrichment of Ag and Au took place in the oxidation zone; this enrichment is dependent on the presence of manganese. Based on the fire-assay results, the concentration of gold is greater at shallower depths.

Petrologic evolution of the Gysian ophiolitic serpentinites, NW Iran

Petrologic evolution of the Gysian ophiolitic serpentinites, NW Iran

The P–T–X conditions of staurolite-garnet metabasites, NW Iran: Implications for metamorphism during Arabia-Eurasia collision

The staurolite-bearing Avajiq metabasites (AMB) crop out near the NW border of Iran with Turkey and contain the peak assemblage garnet + Ca-amphibole + staurolite + biotite + chlorite1 + ilmenite. The AMB record peak pressure–temperature (P-T) conditions of >1 GPa at ∼650 °C; the moderately high pressure is consistent with the presence of staurolite in metabasalt. Garnets exhibit a weak prograde zoning, with relatively homogenous cores and a slight increase in Mg and decrease in Ca, Fe, and Mn toward the rim, consistent with garnet growth during a clockwise P-T path. Texturally late margarite + biotite + muscovite correspond to retrograde P-T conditions of ∼0.5 GPa at ∼500 °C. This stage was followed by further retrogression under greenschist facies conditions recorded by late pumpellyite + paragonite ± calcite. Based on the MORB-like composition of the AMB, it has been unclear whether the metabasites are associated with Neo-Tethyan ophiolites in the area or formed within a continental setting. However, the petrologic association of metabasites with micaschist, gneiss and marble and upper amphibolite facies conditions of metamorphism indicate that the basaltic rocks likely metamorphosed during Arabia-Eurasia continental collision, following consumption of the intervening Neo-Tethys Ocean and emplacement of the ophiolitic rocks in the region.

Eocene high-K magmatic flare-up in a context of south dipping subduction and strike-slip tectonics: Insights from the Talysh Massif, NW Iran

The geodynamic significance of Cenozoic magmatism of the Central Tethyan orogenic belt linking the tectonic zones of Iran, South Armenian Block (Lesser Caucasus) and Turkey remains a subject of debate. This study is focused on the thick geological succession of high potassic magmatic rocks erupted in Talysh Massif, NW Iran. We report a comprehensive geochemical study that demonstrates that this succession is comprised of shoshonitic mostly basaltic and some rhyolitic pyroclastic rocks emplaced during an Eocene magmatic flare-up. Forsterite rich olivine (Fo 70–93), combined to high-Mg clinopyroxene (Mg# 0.6–0.8) and high-Ti calcic amphibole (magnesiohastingsite) are the dominant mafic minerals in these rocks. Mafic lavas are enriched in LILE and depleted in HFSE, with high Th/Yb, Ba/Th and Nb/Zr ratios, suggestive of an enriched mantle source following Tethys subduction history. Interlayered felsic lavas allowed to date the series by U-Pb on zircon at 43.17 ± 0.40 Ma, with one relict core displaying an inherited age of 580 Ma. eHfi CHUR of Eocene zircons range from 1.7 to −0.9 (with corresponding TDM ages around 1.0 Ga). The latter, along with the detection of a Neoproterozoic zircon xenocryst, indicates a crustal contamination by a Neoproterozoic (and older) lower crust. These data are in agreement with the partial melting of a subduction-modified subcontinental mantle. This magmatic flare-up could have been triggered by an Asthenosphere upwelling related to the onset of south-dipping subduction in the Transcaucasus region and transcrustal strike-slip faults with a more significant crustal contamination than in the Lesser Caucasus region.

Recognition of REEs anomalies using an image Fusion fractal-wavelet model in Tarom metallogenic zone, NW Iran

This study aims to detect REE geochemical anomalies in relationship to Iron-apatite ores utilizing an image Fusion based on Deep Learning (FDL). The geochemical data was modeled for elements related to Iron-apatite mineralization using multi b Spline B. The results were fusioned in applying the Deep learning method based on pre-trained networks. Wavelet-Number (WN) fractal model classified the best results based on the combination of a two-dimensional Discrete Wavelet Transformation (DWT) signal analysis and a Concentration-Area (C-A) fractal modeling. Sym8 carried the DWT as a selected wavelet pattern for REE based on Stream sediment samples collected from the Tarom region (NW Iran). In addition, the DWT was decomposed by wavelet coefficients at five levels. Furthermore, the DWT data were classified using a fractal-wavelet model to delineate REE anomalies from background levels in this region. Overlayed with the catchment basins model and weighted using the upstream and downstream parts. As a result, the prominent REE source anomalies are located in the southern parts of the study area. The results obtained by the proposed fractal-wavelet modeling are in connection with field check anomaly samples and the rock samples collected from the Iron-Apatite ore deposits.

Mineral chemistry of gabbro- dioritic xenoliths in Monavvar volcanic rocks (Northwest of Tabriz- NW Iran)

Mineral chemistry of gabbro- dioritic xenoliths in Monavvar volcanic rocks (Northwest of Tabriz- NW Iran)

Aftershock Analyses of the Qotur Doublet-Earthquakes on 23 February 2020 in West-Azarbaijan Province, NW Iran

Aftershock Analyses of the Qotur Doublet-Earthquakes on 23 February 2020 in West-Azarbaijan Province, NW Iran

Mineralogy and geochemistry of the Mazraeh manganese deposit, southwest of Maku, NW Iran

Mineralogy and geochemistry of the Mazraeh manganese deposit, southwest of Maku, NW Iran

Grade estimation and 3D modelling of epithermal gold deposits using an improved integrated SVM-metaheuristic method in Zailik, NW of Iran

Grade estimation and 3D modelling of epithermal gold deposits using an improved integrated SVM-metaheuristic method in Zailik, NW of Iran

Shoshonitic pluton and an associated base-metal deposit in the Tarom–Hashtjin metallogenic zone, NW Iran: implication for tectono-magmatic evolution and metallogenic considerations

Petrological, geochemical and zircon U–Pb and Hf isotope studies were carried out on the Hajseyran pluton, along the Tarom–Hashtjin metallogenic zone, as part of the Alborz–Azerbaijan magmatic belt, which is located in the central part of the Alpine–Himalayan orogenic belt. The Hajseyran pluton, which is made up of syenite, monzonite, quartz-monzonite and granite, yields a zircon U–Pb age of 39.20 ± 0.56 Ma (for a quartz-monzonite sample) with shoshonitic features. Enrichment in large ion lithophile elements, depletion in high-field-strength elements and negative anomalies of Nb, Ta and Ti in Chomalou samples suggest a subduction-related arc magmatic setting. The Chomalou epithermal Pb–Zn–Cu deposit occurs close to the Hajseyran pluton with the quartz–base metal sulfide veins predominantly hosted by Eocene volcanic rocks. The ore-forming processes include chalcopyrite and sphalerite–galena formation, with the latter the major mineralisation in this region. Microthermometric results from the liquid–vapour fluid-inclusion assemblages show that the homogenisation temperatures in the first-stage quartz range from 230 to 310 °C, in the sphalerite and second-stage quartz from 161 to 293 °C and in the late-stage quartz from 175 to 262 °C. The salinities of fluids decrease from early-stage quartz to late-stage quartz. The calculated stable isotope values of ore–fluid components (δ18Owater = +3.28‰ to −3.68‰; δ34SH2S = +5.5‰ to +7.8‰) show that magmatic fluids have played a significant role during mineralisation. The Chomalou deposit is interpreted as a volcanic-hosted intermediate-sulfidation type of epithermal mineralisation formed through shallow-level emplacement of the Hajseyran pluton, which formed via partial melting of phlogopite–amphibole-bearing sub-continental lithospheric mantle. The Neotethyan slab roll-back and concomitant asthenospheric upwelling led to the decompression melting of metasomatised sub-continental lithospheric mantle and the formation of several K-rich plutons and associated epithermal deposits in the Alborz–Azerbaijan magmatic belt.

Salt wedges and trapped brines of low-latitude endoreic saline lakes as potential modulators of GHG emission

Large salt lakes are long-term witnesses to climatic conditions and land use in their basins. The majority are experiencing a drastic drop in water levels due to climate change and human impact. Endoreic Lake Urmia (NW Iran), the sixth largest salt lake worldwide, is a striking example of this decline. Quantification of the relative contributions of natural variability and human impact on the lake's water supply is therefore essential. Here we present isotopic and radiocarbon analyses of surface and groundwater from the Shahr Chay River catchment, entering Lake Urmia on its western shore, and radiocarbon dating of a sedimentary core. Lake Urmia behaves like a large saltwater wedge almost entirely fed by the river and shallow groundwater. This leads to trapping of residual brines and formation of CH4 and secondary CO2 greenhouse gases, impacting sediment geochemical records and corresponding time scales for paleoenvironmental reconstructions. We conclude that (1) salt lakes functioning like a saline wedge, allowing organic matter oxidation, could contribute to increasing methane sources or reducing carbon sinks globally, and (2) endoreic basins worldwide need to be monitored before aridification-related salinization leads to the establishment of a saline wedge precluding any possibility of return to an equilibrium state.

Revisiting the geodynamics of the Middle East region from an integrated geophysical perspective

A long-standing question in geodynamics is whether mantle flow is driven by the plate motion alone, or mantle upwelling makes a significant contribution to it. Subducting slabs and lateral variations of the continental lithosphere can further influence the asthenospheric flow and control its direction. The Middle East region (MER) is a complex continental setting where different processes such as rifting, break-up, plate collision, and tectonic escape kinematically interact with each other. In this context, the role that lithospheric structure, mantle flow, and active upwellings may play is debated. Tomographic images provide a snapshot of the current thermal conditions of a region and seismic anisotropy can also help resolve mantle convection. Here, we synthesize shear-wave splitting observations together with up-to-date tomography models of the mantle structure beneath the MER and other geophysical data. Low-velocity anomalies are seen at asthenospheric depths beneath W Arabia, NW Iran, and Anatolia, suggesting a spreading zone of warm mantle. Two deep low-velocity bodies in Afar and Levant –interpreted as hot mantle plumes– are the sources of this shallower mantle flow. Where low velocities are imaged, we observe predominantly NE–SW oriented anisotropy, anomalously high topography, and abundant basaltic volcanism. The integrated analysis suggests that a horizontal component associated with active upwelling is present in the upper-mantle flow field. The large-scale circulation flow fed by the Afar and Levant Plumes, aided by the subduction-induced forces, facilitates the lateral motion of the Anatolian microplate and affects the dynamic evolution of the Zagros orogen. The proposed scenario demonstrates that the interplay between plate-tectonic events and mantle dynamics controls the kinematics of the region and can explain the general patterns of deformation observed at the surface.

Developing a spatio-temporal interactions model for car crashes using a novel data-driven AHP-TOPSIS

We have proposed a data-driven method for spatio-temporal analysis of car crashes based on the Multi Criteria Decision Making (MCDM) procedure in the Zanjan city, NW Iran which are recorded in the period 2019–2020. A combination of Analytic Hierarchy Process (AHP) and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) is used in this paper to accurately identify the spatio-temporal interactions exist in car crashes. A data-driven AHP-TOPSIS procedure is arranged based on assigning proper weights to the time series related to the peak of accidents. On the other hand, for spatial analysis, the Kernel Density Estimation method was used to create the continuous-value maps of different traffic accidents and then classified using Natural Breaks Classifier. In fact, the proposed methodology can be used to identify car crash hotspots by considering spatio-temporal interactions as well as addressing exaggerated weightings arising from knowledge-driven modeling. By using the spatio-temporal interaction maps in which the location and time of crashes are considered, simultaneously, it is possible to provide a new scientific strategy for identifying car crash hotspots which can lead to better traffic management, improved allocation of resources, and enhanced prevention regulations.

A comprehensive investigation of the causes of drying and increasing saline dust in the Urmia Lake, northwest Iran, via ground and satellite observations, synoptic analysis and machine learning models

Nowadays, dried lakes have turned into important dust sources with serious environmental, climatic and socio-economic impacts. In this study, climatic, terrestrial and anthropogenic effects that caused desiccation of the Urmia Lake, NW Iran are investigated. The main objective is to assess the influence of climatic and regional factors against anthropogenic and local ones in the dryness of the lake during the last two decades. Annual temperature, precipitation and wind speed data measured at nearby meteorological stations, along with water discharge of eight main rivers flowing into the lake are analyzed. The results reveal that the main cause for turning Urmia into a dried lake bed was human interference, since climatic factors contributed positively but to a lesser degree, while terrestrial factors may also play a crucial role. Among anthropogenic interferences, the construction of a causeway had a weak effect in the lake's dryness, while construction of many river dams highly contributed to this disaster. Furthermore, four machine learning models showed that groundwater decline is also an important factor for lake's desiccation. Analysis of dusty days at stations in the Urmia Basin showed similar inter-annual and seasonal dust variability over the whole western Iran. This indicates that dust over the Urmia Basin is mainly affected by the regional dust activity, while the dried lake beds contribute to a lesser degree to the enhanced dust loading. The dominant wind flow is mostly southwesterly, carrying Urmia saline dust toward NE directions, affecting Tabriz city. Three main synoptic clusters, with dipole features consisted of high-pressure systems over Europe, Central Mediterranean and North Africa and low pressure over SW Asia, facilitated strong westerly winds from arid, desert areas of the Middle East toward the Urmia Basin during 118 dust storms in the region.

Geobody estimation by Bhattacharyya method utilizing nonlinear inverse modeling of magnetic data in Baba-Ali iron deposit, NW Iran

One of the essential geophysical concerns is the estimation of the physical and geometrical parameters of the reserve (geobody), which is done by exploiting the nonlinear inverse modeling of magnetic data. The present study includes preparing and modeling magnetic data to suggest the Baba Ali Iron ore deposit's drilling locations in NW Iran. The area is covered with 1000 points of geomagnetic reading with an almost 5 × 10 m2 regularly spaces grid trending WE. The areal and depth extent of the iron ore geobody was unknown. The Bhattacharyya method by MATLAB software coding was employed to underestimate the misfit function and re-construct potential field data, providing the most suitable fitting with measured magnetic data. In this order, the residual calculated anomaly exhibited an excellent two-dimensional conformation with forward modeling. Furthermore, 3D modeling correctly reconstructs the anomalies' productive resources' properties. After preparing full magnetic maps, the magnetic lenses distinguished in four anomalies of surface depths, 20, 50, and deeper than 50 m for this zone. This magnetite lens for the first zone was estimated based on analytical signal filters applied on the entire magnetic map so that the lens's depth is trivial and almost zero. Due to specific gravity calculated as 4.77 t/m3, initial storage capacity is suggested to be about 95,400 tons of magnetite, pyrite, and hematite minerals at most in an area about 6 Km2. Finally, to complete the preliminary explorations of the specified area, exploratory drilling is suggested for three points by inverse modeling. Regarding this study as the first try in magnetic reconnaissance step of Iron mineral exploration in the study area, there is no geological constraints available based on drilling evidences. However, the model is well satisfies the surface anomalies considering residual magnetic property.

Mineralogy, geochemistry, and fluid inclusion of the Balestan Iron deposit, Southeast of Urmia, NW Iran

Mineralogy, geochemistry, and fluid inclusion of the Balestan Iron deposit, Southeast of Urmia, NW Iran

Post-collisional ultrapotassic volcanic rocks and ultramafic xenoliths in the Eslamieh Peninsula, NW Iran: Petrological and geochemical constraints on mantle source and metasomatism

The Eslamieh Peninsula in NW Iran exposes Miocene potassic to ultrapotassic volcanic rocks that are often associated with a variety of ultramafic xenoliths. Based on geochemical features, the rocks investigated in this study can be subdivided into two main groups: 1) MgO-rich (8.0–13.9 wt%) basic (SiO2 = 46.1–50.9 wt%) rocks, which comprise mafic lamprophyres and pyroxenite xenoliths, and 2) MgO-poor (1.5–6.4 wt%) basic to intermediate (SiO2 = 48.2–61.2 wt%) rocks, mainly consisting of trachytes, felsic lamprophyres, and analcime-bearing tephriphonolites and phonotephrites. Rocks in both groups have variable but generally low Ni (10–115 ppm; mostly <60 ppm) and Cr (6–628 ppm; mostly <200 ppm), and display enrichment in LREE over HREE (e.g., La/Yb = 17–55), with a large overlap between the MgO-rich and MgO-poor types. These features are coupled with enriched LILE/HFSE ratios (e.g., Ba/Nb mostly 16–116), negative Nb-Ta-Ti anomalies and positive Pb peaks in primitive mantle-normalised diagrams (e.g., Nb/Nb* mostly <0.7). Both high-and poor-MgO samples have strongly radiogenic (87Sr/86Sr)i ratios (0.7078–0.7086), (143Nd/144Nd)i below the Chondritic Uniform Reservoir (CHUR) = 0.51235–0.51244, and ɛHfi ranging from −3.1 to −11.1‰, suggesting involvement of a lower continental crust component in their genesis. The major oxide and trace element contents the high-MgO and high-CaO rocks indicate derivation from a subcontinental lithospheric mantle source, variably metasomatised by fluids and melts released by a subducted slab in a mantle wedge. Primitive melts evolved via mafic mineral fractionation and possible assimilation of lower crust, generating the SiO2 higher and CaO-MgO-poorer group. We suggest that small degree partial melting of the enriched mantle sources could have been triggered by Neo-Tethys slab roll-back during the Arabian-Iranian collision.

Foraminiferal distribution and biostratigraphy of the Lower Miocene Qom formation, north of Bijar, NW Iran

This study is based on larger benthic foraminifera that dominate the Lower Miocene shelf carbonates of the Qamcheqai section, NW Iran. During the time of deposition, the investigated area was positioned on the Eurasian side of the Tethyan Seaway between the Eastern and Western Tethys. Seven porcelaneous and hyaline perforated species were determined, including Praebullalveolina curdica, Peneroplis evolutus, Androsina cf. diyarbakirensis, Sivasina egribucakensis, Miogypsina cf. intermedia, Operculina complanata, and Amphistegina bohdanowiczi. The presence of Miogypsina cf. intermedia, and Praebullalveolina curdica allowed to correlate the assemblage with SBZ 25 Zone, indicating a Burdigalian age. The results of this study extend the paleogeographic distribution of the endemic taxa Androsina and Sivasina eastwards as far as Central Iran.

Concurrent adakitic and non-adakitic Late Miocene-Quaternary magmatism at the Sahand volcano, Urumieh-Dokhtar Magmatic Arc (NW Iran)

The Sahand volcano in NW Iran is characterised by voluminous Late Miocene to Quaternary magmatic activity. Petrographic, mineral chemical, whole-rock geochemical and Sr-Nd-Pb isotope data allowed to identify two rock groups, continuously emplaced from the ancient Paleo-Sahand to the later Neo-Sahand stages. These are represented by abundant evolved rocks, with dacitic to rhyolitic compositions and less common intermediate (andesitic) rocks. Both rock types have the typical LILE-enriched and HFSE-depleted fingerprints of magmas originated from subduction-modified mantle sources, as well as radiogenic Sr- and Pb- (18.8–18.9 206Pb/204Pb, 15.6–15.7 207Pb/204Pb and 38.8–39.2 208Pb/204Pb) isotope compositions. Nevertheless, the evolved rocks have lower 87Sr/86Sr (0.70437–0.70491 vs 0.70494–0.70585) and display an evident high Sr/Y vs Y and high La/Yb vs Yb “adakitic” signature, suggesting derivation from the partial melting of lower continental crust rocks, rather than from the differentiation of the non-adakitic intermediate rocks. The latter would be the derivative melts of magmas generated by the partial melting of lithospheric mantle sources modified by metasomatic fluids released by the subducted Neotethyan oceanic lithosphere. During their ascent and differentiation, these magmas likely provided the heat necessary to induce partial melting of the lower continental crust, eventually producing adakitic evolved magmas. Quantitative models and geothermobarometric calculations suggest that differentiation processes for both rock groups occurred mainly by polybaric fractional crystallisation (in the 29–4 km depth range), possibly coupled with minor assimilation of local crustal rocks.The peculiar style of the Sahand magmatic activity (with both adakitic and non-adakitic products) and its temporal evolution (with an overall increase of the degree of evolution and a decrease of the non-adakitic melts) are somewhat different with respect to those of the coeval neighbouring districts of Saray, Sabalan and Bijar-Qorveh. These can be reconciled within a general model for NW Iran where magmatism is triggered by upwelling of the sub-lithospheric mantle as a consequence of slab break-off (or the opening of a slab tear). The progressive propagation (and subsequent exhaustion) of the thermal pulse and the increasing thickness of the continental crust played a main role in determining both the progressively later inception of active magmatism (from ∼11 to 4.5 Ma) and the change in magma types (from largely variable to adakitic only) at increasing distance from the Iran-Arabia suture.