Sanandaj-Sirjan Metamorphic Zone Research Articles

What controls structural variations along the Zagros Collision Zone? Insights from geophysical observations and thermo-mechanical modelling

The Zagros Collision Zone is a complex tectonic region formed as a consequence of the collision between Arabia and Eurasia after the subduction of the Neo-Tethys ocean. The NW-SE striking Zagros orogen consists of the following parallel tectonic units (from SW to NE): Zagros Fold and Thrust Belt (ZFTB), Sanandaj–Sirjan Metamorphic Zone (SSZ), and Urumieh–Dokhtar Magmatic Arc (UDMA). In this study, we perform a combined analysis of recent geophysical data, revealing pronounced differences in the crustal and lithospheric structure along the Zagros Mountains. The northwestern sector shows a fairly uniform crustal thickening across the broad symmetric orogen from the ZFTB to the UDMA. In contrast, in the central Zagros, the transition from a relatively narrow zone of high elevations and high-frequency relief in the ZFTB to a smoother surface topography of the SSZ and UDMA occurs with an abrupt increase in Moho depth below the SSZ. The last observation has recently been interpreted as a result of “relamination” process, where the felsic upper crust of the Arabian plate underthrust the mafic crust of the Iranian plate. We present geodynamic numerical models of crustal relamination during continental collision and compute static gravity field of the resulting structures. We show that oblique closure of the Neo-Tethys affects lateral variations in the style and extent of crustal relamination, which control the observed along-strike changes in crustal configuration and orogen altitude. In particular, a narrow and higher orogen (as in the central Zagros) develops in the experiments with a young and wide oceanic plate, whereas an old and narrow subducting plate tends to form a broad and lower topography (as in the northwestern Zagros). This is geometrically consistent with the progressive closure of the Neo-Tethys from NW to SE during the oblique continental collision between Arabia and Eurasia.

Application of an approximate geostatistical simulation algorithm to delineate the gold mineralized zones characterized by fractal methodology

The Zarshuran Carlin-type gold deposit is located in the NW part of the Sanandaj-Sirjan metamorphic zone which hosts Neoproterozoic carbonate and black shale rock units. Concentration-Volume (C–V) fractal model and Turning Band Simulation method have been applied to effectively separate gold mineralized stages in the Zarshuran deposit. Three mineralized stages determined by the means of C–V fractal model and Turning Bands Simulation method were correlated with the mineralized stages considering geological studies. The results revealed the presence of enriched (>37 ppm) and Au high grade (5.5–37 ppm) mineralized zones within the deposit. A log ratio matrix was used to correlate the results obtained by the C–V fractal model and turning band simulation for Au and lithological units which showed that the main mineralized zones occur in the black gouges and Jasperoids.

Strain partitioning in the Kahdan shear zone: Microstructural and kinematic evidence from the SE Sanandaj-Sirjan zone, Zagros orogeny

Microstructural, vorticity and strain analyses, combined with geological field observations are fundamental tools for retrieving information about kinematics of deformation in crustal-scale shear zones. The Kahdan shear zone is located in the SE part of the Sanandaj-Sirjan metamorphic zone within the Zagros orogen, SW Iran. The Zagros Orogen that formed due to the oblique convergence between the Afro-Arabian plate and the central Iranian plateau. Three-dimension strain analyses show that finite strain ellipsoids shape is oblate (K = 0.38–0.89). Vorticity analysis on mylonites yielded arithmetic mean maximum (Wm max) and mean minimum values (Wm min) of 0.83 and 0.72, respectively (63–47% simple shear). Higher values of Wm and Rxz estimated adjacent to the shear zone boundaries and gradually decrease toward the central part of the zone. Our yielded results show a non-unified spatial variations of vorticity and strain values in the deformed metasandstone (Late Jurassic to Early Cretaceous) of the Kahdan shear zone. Vorticity analyses and noncoaxial indicators indicate that this zone experienced general shear deformation. The horizontal shortening of ∼13–27% measured perpendicular to the shear zone boundaries. Based on yielded results, this area provides a good example of strain partitioning in a ductile shear zone.

Sediment hosted stratiform barite – (Cu-Zn-Pb) deposits in the southwest Mahabad, Iran; implications for geology, ore textural, compositional, and S-O isotopes geochemistry

The southwestern part of Mahabad town in the northwestern part of the Sanandaj-Sirjan metamorphic zone (SSZ) of Iran hosts seven barite-base metal deposits. They have occurred within the Precambrian-aged metasedimentary succession including black color fine-grained shale, slate, and phyllite with minor calcareous intercalation. This investigation is focused on three main ore deposits including Meraneh, Sorkhab, and Shekarbeig. The mineralized units and host rocks have been metamorphosed to lower greenschist facies in the Post-Cretaceous.Five different stages of mineralization were distinguished based on the field observations, mineralogical and textural relationships, morphology of the orebodies, and mineral chemistry studies. The followings are the stages of evolution: (1) precipitation of framboidal, colloform, and monomineralic pyrite bands and fine-grained barite (2) stratiform accumulation of barite with sulfide and un-mineralized sediment strata (3) stockwork mineralization and associated alteration (4) formation of discrete vent complex associated with massive sulfides and barite in replacement zone, and (5) post-mineralization deformation and shearing process, contemporaneous with regional metamorphism.Pyrite, chalcopyrite, sphalerite, and galena are the major sulfide minerals, in order of abundance, found in the studied deposits, accompanied by the minor amounts of tetrahedrite and covellite. The stratiform, massive and stockwork ores are indicative of the main ore-forming event and are volumetrically the most important facies. The electron probe micro-analyzer (EPMA) data of sulfide minerals indicates a distinct variation in trace element concentrations during the ore formation. Besides, this study focuses on distributions of δ 34S value in sulfide minerals, as well as δ 34S and δ 18O value of barite mineralization in stratiform, stratabound, and stockwork ore zones. The paragenetic relationships, mineral chemistry and isotopic study of minerals reveal that the stages 1 and 2 can be considered as the pre-date hydrothermal input of the stage 3 in all the three ore deposits. In addition, the mineralization event during stage 1 of the ore formation was found out to have occurred in a low temperature environment within the unconsolidated sediments, instantly under the sea floor. During stage 2, the stratiform ore has been deposited due to the diagenetic processes. The up-flow exhalative hydrothermal fluids resulted in the thermal alteration of the organic-rich host rock during stage 3, and formation of the massive mineralized bodies in stage 4. Finally, the lower greenschist facies metamorphism affected both the ore and host rocks during stage 5.On the basis of mineral chemistry inspections, all types of pyrites have high Ni and low Co contents and great majority of them show the Co:Ni ratios of lower than 1. Stratiform pyrites were fine-grained and enriched in As, Au, and Co, in contrast to hydrothermally pyrites. Ni, Pb, Zn, Bi, Hg, and Cu were mostly concentrated in coarse-grained hydrothermally pyrits. Also, the temperature and sulfur fugacity increased from stratiform stage to hydrothermal stage. The Se content of chalcopyrite as well as Bi and Ag contents of Galena increased from feeder to massive and stratiform ore. Sphalerite was generally iron-poor and its Fe content decreases from feeder to massive and stratiform ore. The Co/Ni ratio of pyrite and Zn/Cd ratio of sphalerite can be used to discriminate the VMS, magmatic or SEDEX origins for ore deposits. The Zn/Cd ratio of the sphalerites and Co/Ni ratio of pyrites provided the evidence for SEDEX style mineralization in the studied deposits.Barites with various textures and morphologies show overlapped distributions of δ34S and δ18O values from the stages 2, 3, and 4, range from + 34.1 to + 41.3‰ and + 10.2 to + 12.6‰, respectively. The d34S versus d18O patterns for the stratiform barite resemble the patterns that indicate sulfate is consumed by AOM. Sulfides have different isotopic compositions: stratiform with markedly more positive δ 34S values of + 28‰ to + 33‰; stockwork with δ 34S values of + 8‰ to + 26‰; and massive pyrite with δ 34S between + 8‰ and + 26‰. We suggest that the isotopic relationships (δ34S pyrite ≈ δ34S barite) in the stratiform ore zones may be related to anaerobic oxidation of methane coupled to sulfate reduction (AOM-SR), which has been developed during open system diagenesis. Textural and isotopic evidence indicates that the stockwork and massive ores has grown by thermochemical sulfate reduction (TSR) and reductively dissolving barite. On the other hand, the trend between d18O and δ34S suggest that the growth of the stockwork and massive barite is related to fluid seep on modern continental margins. Textural evidence documented during the different mineralization stages together with concentration and the distribution pattern of trace elements and isotopic investigation in the stratiform, massive, and stockwork ore zones signify that the SEDEX-type mineralization has been occurred in this district.

New detrital zircon U–Pb insights on the palaeogeographic origin of the central Sanandaj–Sirjan zone, Iran

AbstractNew detrital U–Pb zircon ages from the Sanandaj–Sirjan metamorphic zone in the Zagros orogenic belt allow discussion of models of the late Neoproterozoic to early Palaeozoic plate tectonic evolution and position of the Iranian microcontinent within a global framework. A total of 194 valid age values from 362 zircon grains were obtained from three garnet-micaschist samples. The most abundant detrital zircon population included Ediacaran ages, with the main age peak at 0.60 Ga. Other significant age peaks are atc. 0.64–0.78 Ga, 0.80–0.91 Ga, 0.94–1.1 Ga, 1.8–2.0 Ga and 2.1–2.5 Ga. The various Palaeozoic zircon age peaks could be explained by sediment supply from sources within the Iranian microcontinent. However, Precambrian ages were found, implying a non-Iranian provenance or recycling of upper Ediacaran–Palaeozoic clastic rocks. Trace-element geochemical fingerprints show that most detrital zircons were sourced from continental magmatic settings. In this study, the late Grenvillian age population atc. 0.94–1.1 Ga is used to unravel the palaeogeographic origin of the Sanandaj–Sirjan metamorphic zone. This Grenvillian detrital age population relates to the ‘Gondwana superfan’ sediments, as found in many Gondwana-derived terranes within the European Variscides and Turkish terranes, but also to units further east, e.g. in the South China block. Biogeographic evidence proves that the Iranian microcontinent developed on the same North Gondwana margin extending from the South China block via Iran further to the west.

Seismic images of crust and upper mantle in Central Iran (from Jiroft to Ashtian) via teleseismic waves

A detailed knowledge of the thickness of crust and upper mantle structure is important for understanding a plate tectonics and geodynamics in the region. We use body wave for detecting details of the subsurface structure. The information in this research is collected from a seismic linear profile that extends across the Sanandaj-Sirjan metamorphic zone in seismic states of Central Iran and Zagros. We compute P receiver functions to investigate crustal and upper mantle discontinuities. We use teleseismic events (mb ≥ 5.5, 30° < Δ < 95°) registered between 1996 and 2018 and recorded at 10 short-period stations with 3 components and 17 broadband stations with high signal to noise ratio. The observed depth of Moho in the study area is approximately 50 km and rises to 70 km at the end of the seismic linear profile beneath Sanandaj-Sirjan zone. In Central Iran, depths discontinuities in the transition zone are shown by the reference model of deviation, which can be attributed to the convergence of Arabian plate with the Central Iran plateau. Also, the study area was identified as geothermal susceptibility by SUNA and this observation was confirmed.

Sub-pixel Mapping of Copper- and Iron-Bearing Metamorphic Rocks Using ASTER Data: A Case Study of Toutak and Surian Complexes, NE Shiraz, Iran

Metamorphic rock units of Toutak and Surian complexes located in the Sanandaj–Sirjan metamorphic zone, NE Shiraz, Iran, were mapped using the matched filtering (MF) method on the visible near-infrared (VNIR) and shortwave infrared (SWIR) bands of advanced spaceborne thermal emission and reflection radiometer (ASTER) sensor. These complexes host magnetite and Cu–Zn–Ag volcanogenic massive sulfide (VMS) deposits. Mixed pixels of lithologic–mineralogic indicators of these deposits were detected to identify the prospecting areas. The spectra of end-members were extracted from imagery using sequential maximum angle convex cone (SMACC) model and were compared to the field and laboratory spectra to examine their characteristics. The key minerals including chlorite + epidote (greenschist), muscovite + biotite (mica schist), calcite + dolomite (marble), and muscovite + oligoclase (gneiss) were identified and were input to the algorithm for estimating the sub-pixel fractions of desired minerals. A generalized map of lithologies was then produced by combining the fraction maps of greenschist, mica schist, marble, and gneiss using a GIS. Finally, this lithologic association was applied to define the prospecting areas. The accuracy of results was assessed using R2 coefficient, field observations, and XRD analysis. The R2 coefficient was 0.74 for greenschist, 0.63 for mica schist, 0.65 for marble, and 0.53 for gneiss. The results also showed good agreement with field evidence, and thus, they can be used for finding potential targets for copper and iron deposits.

Crustal and uppermost mantle shear wave velocity structure beneath the Middle East from surface wave tomography

SUMMARY We have constructed a 3-D shear wave velocity (Vs) model for the crust and uppermost mantle beneath the Middle East using Rayleigh wave records obtained from ambient-noise cross-correlations and regional earthquakes. We combined one decade of data collected from 852 permanent and temporary broad-band stations in the region to calculate group-velocity dispersion curves. A compilation of >54 000 ray paths provides reliable group-velocity measurements for periods between 2 and 150 s. Path-averaged group velocities calculated at different periods were inverted for 2-D group-velocity maps. To overcome the problem of heterogeneous ray coverage, we used an adaptive grid parametrization for the group-velocity tomographic inversion. We then sample the period-dependent group-velocity field at each cell of a predefined grid to generate 1-D group-velocity dispersion curves, which are subsequently inverted for 1-D Vs models beneath each cell and combined to approximate the 3-D Vs structure of the area. The Vs model shows low velocities at shallow depths (5–10 km) beneath the Mesopotamian foredeep, South Caspian Basin, eastern Mediterranean and the Black Sea, in coincidence with deep sedimentary basins. Shallow high-velocity anomalies are observed in regions such as the Arabian Shield, Anatolian Plateau and Central Iran, which are dominated by widespread magmatic exposures. In the 10–20 km depth range, we find evidence for a band of high velocities (>4.0 km s–1) along the southern Red Sea and Arabian Shield, indicating the presence of upper mantle rocks. Our 3-D velocity model exhibits high velocities in the depth range of 30–50 km beneath western Arabia, eastern Mediterranean, Central Iranian Block, South Caspian Basin and the Black Sea, possibly indicating a relatively thin crust. In contrast, the Zagros mountain range, the Sanandaj-Sirjan metamorphic zone in western central Iran, the easternmost Anatolian plateau and Lesser Caucasus are characterized by low velocities at these depths. Some of these anomalies may be related to thick crustal roots that support the high topography of these regions. In the upper mantle depth range, high-velocity anomalies are obtained beneath the Arabian Platform, southern Zagros, Persian Gulf and the eastern Mediterranean, in contrast to low velocities beneath the Red Sea, Arabian Shield, Afar depression, eastern Turkey and Lut Block in eastern Iran. Our Vs model may be used as a new reference crustal model for the Middle East in a broad range of future studies.

The Barika gold-bearing Kuroko-type volcanogenic massive sulfide (VMS) deposit, Sanandaj-Sirjan zone, Iran

The Barika deposit is the first documented auriferous Kuroko-type volcanogenic massive sulfide (VMS) deposit in Iran. It is hosted in meta-andesites of the Sanandaj-Sirjan metamorphic Zone (SSZ). The mineralization and host rocks have been metamorphosed to lower greenschist facies. The deposit has Au and Ag grades in the stratiform lens averaging 3.5 and 175 g/t, respectively, and contains 2.3 tonnes of Au and 143 tonnes of Ag. The base metal (Pb + Zn + Cu) content of the deposit is low (<1 wt%). The stratiform part of the Barika deposit is approximately 150 m long and up to 20 m thick, and consists of massive to semi-massive sulfides, some of which are banded, which are overlain by barite and siliceous exhalites. The >200 m long and up to 500 m wide stringer zone immediately stratigraphically underlies the stratiform mineralization. Veins within the stringer zone are composed of quartz, pyrite, galena, sphalerite, tetrahedrite-tennantite, and rare chalcopyrite. Stratiform mineralization is composed of pyrite, sphalerite, galena, stibnite, tetrahedrite-tennantite, several Ag-(As-Sb-Pb) sulfosalt minerals (guettardite, ferdowsiite, barikaite, stephanite, pyrargyrite, trechmannite, miargyrite, As-rich andorite, As-rich geocronite, Sb-rich seligmanite and smithite), realgar, and electrum.Based on mineral assemblages and textures, four mineralization stages are recognized. During stage 1, pyrite, sphalerite, galena, tetrahedrite-tennantite, and stibnite were deposited during synvolcanic hydrothermal activity. During stages 2 and 3, post-mineralization deformation and shearing contemporaneous with regional metamorphism modified most of the primary textures in the stratiform mineralization. Stage 2 mineralization represents Stage 1 mineralization that was remobilized during regional metamorphism, and this mineralization is characterized by gold as submicroscopic inclusions or as “invisible” gold locked in pyrite, as well as electrum in the barite mineralization. Regional metamorphism and deformation (D1) caused recrystallization of primary mineralization (mainly pyrite) and migration of submicroscopic gold via solution-precipitation creep to the grain boundaries to form microscopic electrum. Stage 3 mineralization is attendant with D2 movement along the Barika shear zone that controlled the overall geometry of the deposit and resulted in the development of ductile-brittle deformation textures and the formation of several Ag(As-Sb-Pb)-rich sulfosalt minerals and electrum. Most of the electrum grains occur together with sulfosalt minerals that fill open spaces formed during shearing. Stage 4 mineralization is marked by the formation of white, barren quartz veins and high-angle normal faults that were developed during D3 extension-related uplift.Primary two-phase fluid inclusions in quartz of the stringer zone and barite from the stratiform mineralization homogenize between 132° and 283 °C (average ~ 200 °C). Salinities of the primary fluid inclusions range from 1.4 to 9.6 wt% NaCl eq. The sulfur isotope compositions of sulfide minerals from stratiform and stockwork mineralization in the Barika deposit range from δ34S −0.8 to +5.6‰. This relatively narrow range suggests that the sulfur in the sulfide mineralization was derived from the leaching of igneous sulfur from the underlying andesitic rocks, similar to most Japanese Kuroko VMS deposits. Temperatures of formation for Barika sulfides estimated based on sulfur isotope values for coexisting galena-sphalerite (Δ = 2–4.8‰) and galena-pyrite (Δ = −3.3 to −5.4‰) pairs range from 146 to 293 °C, and these are consistent with fluid inclusion homogenization temperatures. The primary mineralizing fluids have a calculated δ18O value of between +4.3and +6.3‰, based on the δ18O compositions of quartz from the stringer zone and assuming a temperature of 200 °C and that isotopic equilibrium was attained.

Evidence for Archean crust in Iran provided by ca 2.7 Ga zircon xenocrysts within amphibolites from the Sanandaj–Sirjan zone, Zagros orogen

High-precision secondary-ion mass spectrometry (SIMS) U–Pb and Hf–O isotopic microanalyses of zircons from the central Sanandaj–Sirjan metamorphic zone (SSMZ) of the Zagros orogen, Iran, provide evidence of the presence of 2.7 Ga xenocrystic zircons, suggesting that the SSMZ records Neoarchean crustal components that have not previously been reported in Iran. The two amphibolite samples analyzed from the Muteh–Golpaygan metamorphic complex during this study include one sample (GQ-10) that yielded xenocrystic zircons with ages clustered around ca 2.7 Ga. The second sample (GC-15) contains zircons that show two distinct xenocrystic zircon domains with ages of (i) ca 2.7 and 2.5 Ga, and (ii) ca 52 Ma. The ca 2.7 Ga zircons and zircon domains have homogeneous, mantle-like δ18O values with a mean of 6.10 ± 0.69‰. They have εHf(t) values that vary between +0.2 to +6.8 with TDMC ages of 3.1–2.7 Ga, suggesting they crystallized from granitoid melts derived from juvenile components. However, the younger ca 2.5 Ga zircons yield an average δ18O value of 6.17 ± 0.24‰ and have evolved Hf isotopic compositions (εHf(t) −0.8 to −3.4; TDMC = 3.2–3.1 Ga), indicating that the older crust in this region underwent more intense later reworking than is the case for crustal material formed at ca 2.7 Ga. These data provide evidence of Neoarchean basement rocks in this region, indicating that the SSMZ records ancient crustal growth events. The youngest ca 52 Ma xenocrystic zircons within these samples most likely reflect the late Paleocene–Eocene magmatic ‘flare-up’ within Iran. These zircons have slightly elevated δ18O values (7.69 ± 0.21‰) and negative Hf isotopic compositions (εHf(t) −8.3 to −12.1; TDMC = 1.9–1.6 Ga), suggesting intensive crustal reworking at ca 50 Ma.

Lateral variation of crust and upper mantle structures in NW Iran derived from surface wave analysis

To obtain the shear velocity structure across North-West of Iran and surrounding areas to a depth of 160 km, we performed a namely Hedgehog nonlinear inversion on Rayleigh wave group velocity dispersion curves in the period range from 7 to 60 s. The distributed dispersion curves are the results of our surface wave dispersion tomography using the data of 280 local and regional seismic events, recorded by the medium- and broad-band seismic stations in the region. We outline different crust and upper mantle structures for the study area based on calculated group and shear velocities. Our results reveal relatively low velocities at the shorter periods (7–10 s) in the presence of sedimentary basins (e.g., South Caspian Basin) and for eastern Anatolia and relatively high velocities along the Sanandaj–Sirjan Metamorphic zone, Alborz, Talesh, and the Lesser Caucasus Mountains. By depth inversion of group velocities, we observed 14-km-thick sediments in South Caspian Basin and Kura Depression. Based on our maps at 20 s, we outline different crustal models for the region and highlight the differences between South Caspian Basin and NW Iran, on one side, and the similarities between the South Caspian Basin and Kura Depression that extend beneath Talesh, Alborz, and Lesser Caucasus, on the other. Comparing the shear velocity of lower crust in South Caspian Basin and Kura Depression with that of NW Iran proves different origination of lower crust in the basin, probably oceanic source, because of its significant higher shear velocity rather than NW Iran. In Talesh, we observe indications of an under-thrusting of the lower crust of SCB beneath NW Iran while the middle crust is locked. The analysis of group velocities at longer periods (≥ 35 s) and obtained shear velocity models allows us to outline different lithospheric structures and crustal depth in the region. The high group velocities in Talesh, South Caspian Sea, and Lesser Caucasus on one side and Zagros Folding and Thrust Belt on the other, beside the result of shear velocity models, suggest the presence of a stable and thick mantle lid that seems to be thin or absent in the eastern Anatolia and much of NW Iran. The shallowest Moho and Lithosphere Asthenosphere boundary depths of 37 and 63 km were observed in Eastern Anatolian Accretionary Complex. The thin mantle lid in this region has affected the whole crust in such a way that we observed the lowest shear velocities inside the crust in this region. We observed a significant thickening of both crust and lithosphere in Sanandaj–Sirjan Metamorphic zone comparing to Urmieh Dokhtar Magmatic Arc and Zagros Folding and Thrust Belt on its two sides.

Geochemistry and Petrogenesis of metabasites from the Gol-e-Gohar Complex in southern Sanandaj-Sirjan metamorphic zone, South of Iran; Evidences for crustal extension and magmatism at early Palaeozoic

In Gol-e-Gohar metamorphic Complex from south-eastern Sanandaj-Sirjan metamorphic zone (Kerman province, Iran), there are two types of metabasites contain layered metamorphosed lava flows and the younger meta-gabbros. The protoliths formed in the Paleozoic era and were metamorphosed during the early Cimmerian orogenic phase in the late Triassic, under temperatures of 640–680oC and pressures of ~7–10.5kbar (amphibolite facies). These rocks are garnet-bearing amphibolites, garnet free amphibolites and metamorphosed gabbros. Many mineralogical and chemical aspects of these metabasites are similar, although the layered metabasites show tholeiitic and the meta-gabbros depict alkaline affinities. Evidences such as whole rock geochemical characteristics, Sr and Nd isotopic data, ( 143 Nd/ 144 Nd initial =0.511913–0.512067; eNd 550Ma =-0.31–2.68), relatively flat patterns of chondritenormalized Rare Earth Elements and multi-elemental diagrams, the enrichment in TiO 2 (average content ~2.16) and high Zr/Y ratios (3–8), indicate that all of Gol-e-Gohar metabasites are formed in an extentional intracontinental rift zone from tholeiitic to alkaline magmas. The data suggest that the paren magmas could derived by low degrees of partial melting of spinel-lherzolite sources in subcontinental lithospheric mantle. These evidences confirm the existence of extentional environments in southern part of the Sanandaj-Sirjan metamorphic zone in the Paleozoic era, when large extensional depressions developed in the Sanandaj-Sirjan metamorphic zone and underlying asthenosphere ascent and partially melted during this time. Gradually, thick sequences of continental detritic sediments and tholeiitic lava flows accumulated in these troughs. Subsequent magmatic event in the area characterizes by emplacement of alkaline gabbro intrusions. At the early Cimmerian orogeny, these sedimentaryigneous rocks associations metamorphosed and the Gol-e Gohar metabasites formed.

Feather-like hornblende aggregates in the phyllites from the southern Sanandaj–Sirjan zone, Iran; their origin and mode of formation

Several outcrops of spectacular feather-like hornblende aggregates occur in the phyllites of the Gol-e-Gohar complex (southeast of Iran) and form special Garbenschiefer rock types. The Gol-e-Gohar complex, as a part of the southern Sanandaj-Sirjan metamorphic zone, contains a succession of metabasites, phyllites, and slates intruded by dioritic intrusions. There are two types of hornblendes in the phyllites; the first one is concentrated around the fractures and the second one is randomly distributed in the rocks and forms radial feather-like hornblende aggregates. Petrographical and chemical characteristics of these two shapes of hornblendes are the same, except the latter is developed parallel to the foliation planes. The hornblendes occur as unstrained needle-shaped porphyroblasts with oriented quartz and feldspar inclusions as well as polygonal grains in the matrix with mosaic texture, which implies the matrix has been recrystallized. On the basis of field observations, petrography, and chemical compositions of the hornblendes, we inferred that the Garbenschiefer phyllites formed during hydrothermal metamorphism in association with penetration of hot fluids. The compositions of hornblende aggregates are similar to those formed in hydrothermal systems and differ from regional and thermal metamorphic amphiboles. All evidence shows that, in the studied area, ascending of dioritic intrusions increases fluid temperature, the hot fluids leach some elements from the metabasites, and finally the enriched fluids flow upward via the fractures. In the upper levels, the fluids penetrate into the phyllites along their foliation planes and, with a decrease in temperature and pressure, they crystallize hornblende aggregates under static hydrothermal conditions.

The south Zagros suture zone in teleseismic images

The geometry of intra-continental lithosphere boundaries along the Zagros orogenic belt in the Arabia-Eurasia collision is investigated by means of teleseismic data. The data are gathered over a seismic linear profile extending across south Zagros, the Sanandaj–Sirjan metamorphic zone, the Urumieh–Dokhtar magmatic arc, Central Iran, and the Kopeh Dagh - Binalud mountains. We exploit the P and S receiver functions leading to map the geometry of the crustal and subcrustal interfaces. The migrated depth sections reveal an abrupt crustal thickening and a gentle crustal thinning ~60km north and ~30km south of the Zagros suture, respectively. Associated to the buckled antiformal Moho south of the suture, a deeper synform in the lithospheric lid of the lower Arabia plate is shown by migrated depth sections affecting the lithospheric mantle of the Arabia plate beneath the suture zone. This geometry implies an unexpected intra-lid decoupling. These features imply that the Central Iran lithosphere acts as a relatively strong backstop producing significant internal deformation expressed by shortening and thickening at the edge of the Arabian lithosphere. The 410km and 660km transition zones are imaged by P to S converted phases and showed lateral continuity implying an originally low dip angle subduction of the oceanic Arabian plate beneath Central Iran.

Lateral and depth variations of coda Q in the Zagros region of Iran

We have analyzed more than 2800 local earthquakes recorded by the Iranian National Seismic Network (INSN) and the Iranian Seismological Center (IRSC) to estimate coda wave quality factor, Q c , in the Zagros fold and thrust belt and the Sanandaj-Sirjan metamorphic zone in Iran. We used the single backscattering model to investigate lateral and depth variations of Q c in the study region. In the interior of Zagros, no strong lateral variation in attenuation parameters is observed. In SE Zagros (the Bandar-Abbas region) where transition to the Makran subduction setting begins, the medium shows lower attenuation. The average frequency relations for the SSZ, the Bandar-Abbas region, and the Zagros are Q c = (124 ± 11)f 0.82 ± 0.04, Q c = (109 ± 2)f 0.99 ± 0.01, and Q c = (85 ± 5)f 1.06 ± 0.03, respectively. To investigate the depth variation of Q c , 18 time windows between 5 and 90 s and at two epicentral distance ranges of R < 100 km and 100 < R < 200 km were considered. It was observed that with increasing coda lapse time, Q 0 (Q c at 1 Hz) and n (frequency dependence factor) show increasing and decreasing trends, respectively. Beneath the SSZ and at depths of about 50 to 80 km, there is a correlation between the reported low velocity medium and the observed sharp change in the trend of Q 0 and n curves. In comparison with results obtained in other regions of the Iranian plateau, the Zagros along with the Alborz Mountains in the north show highest attenuation of coda wave and strongest frequency dependence, an observation that reflects the intense seismicity and active faulting in these mountain ranges. We also observe a stronger depth dependence of attenuation in the Zagros and SSZ compared to central Iran, indicating a thicker lithosphere in the Zagros region than in central Iran.

Panafrican basement and Mesozoic gabbro in the Zagros orogenic belt in the Dorud–Azna region (NW Iran): Laser-ablation ICP–MS zircon ages and geochemistry

The Dorud–Azna region of the Sanandaj–Sirjan metamorphic zone (SSMZ) in NW Iran is part of the Zagros orogenic belt and exposes an amphibolite-grade metamorphic succession of mixed continental (e.g., the granitic Galeh–Doz orthogneiss) and oceanic or rift (e.g., various amphibolites) origin, which is intruded by the Darijune gabbro. Laser-ablation ICP–MS U–Pb zircon ages of 608±18Ma and 588±41Ma of the granitic Galeh–Doz orthogneiss indicate a Panafrican basement similar to that of the Yazd block of Central Iran. We therefore interpret this part of the SSMZ to represent a previously subducted part of the Panafrican basement of the Iranian microcontinent. Based on geochemistry, amphibolites and metagabbros from the Amphibolite unit representially potentially the cover on the Panafrican Galeh–Doz orthogneiss are interpreted to have their origin in a likely Upper Paleozoic to Mesozoic rift or E-MORB ophiolite succession. One of the metagabbros, here termed Dare-Hedavand metagabbro yield a 206Pb/238U mean age of 314.6±3.7Ma. Talc-bearing greenschists to epidote amphibolites of the Triassic June complex formed in a similar, but younger environment. Furthermore, the metamorphic complex is intruded by the Darijune gabbro, which yields a mean ICP–MS U–Pb zircon age of 170.2±3.1Ma. The final stage of likely Jurassic greenschist facies-grade metamorphism postdates the gabbro intrusion. We interpret the Darijune gabbro to have resulted from the initial subduction of Neotethyan oceanic lithosphere in a continental arc setting. The new data combined with data from literature indicate that the SSMZ represents a tectonic element of a mixed continental and oceanic origin, likely accreted to Central Iran during a Jurassic tectonic process. The new data also constrains thick-skinned nappe stacking within the Sanandaj–Sirjan zone.

Evaluation of structural and geological factors in orogenic gold type mineralisation in the Kervian area, north-west Iran, using airborne geophysical data

The Piranshahr-Sardasht-Saqqez Zone (PSSZ) in the north-west of the Sanandaj-Sirjan metamorphic zone (SSZ) hosts some major Iranian gold deposits. In the south-east of PSSZ, there is a north-east trending orogenic gold belt which contains three gold deposits/occurrences (Qolqoleh, Kervian and Ghabaghloujeh). In this research, studies are focused on processing and analysing airborne magnetic and radiometric data in order to find applicable indicators for prospecting gold in this area.Former studies on the gold deposits/occurrences in the study area suggest three essential factors in local orogenic gold mineralisation: (1) intersecting deep bending structures/shear zones, (2) Fe-rich mafic meta-volcanic lithologies (primary source and host rocks) and (3) altered mylonitic granites (secondary host rock). Geological structures and lithological contacts can be mapped based on locating edges in the magnetic field at different depths.In this study, we extracted the structure from aeromagnetic data by reduction to the pole, upward continuation and applying a tilt derivative filter to the horizontal derivative of the upward continued data. Upward continuation was to several levels from 500 to 4000 m. Afterwards, a 3D architecture was built based on extracted subsurface lineaments in different levels. This 3D model can assist in the visualisation of the underground shape of structures that may influence gold mineralisation.Moreover, mafic meta-volcanic rocks in the study area, which contain magnetic minerals such as magnetite, titanomagnetite and ilmenite, can be mapped using aeromagnetic data. Mylonitic granites, which are the other host rock in the deposits, were mapped using airborne radiometric data.

Geochemistry, Paragenesis, and Wall-Rock Alteration of the Qatruyeh Iron Deposits, Southwest of Iran: Implications for a Hydrothermal-Metasomatic Genetic Model

The Qatruyeh iron deposits, located on the eastern border of the NW-SE trending Sanandaj-Sirjan metamorphic zone, southwest of Iran, are hosted by a late Proterozoic to early Paleozoic sequence dominated by metamorphosed carbonate rocks. The magnetite ores occurred as layered to massive bodies, with lesser amounts of disseminated magnetite and hematite-bearing veins. Textural evidences, along with geochemical analyses of the high field strengths (HFSEs), large ion lithophiles (LILEs), and rare earth elements (REEs), indicate that the main mineralization stage occurred as low-grade layered magnetite ores due to high-temperature hydrothermal fluids accompanied by Na-Ca alteration. Most of the main ore-stage minerals precipitated from an aqueous-carbonic fluid (3.5–15 wt.% NaCl equiv.) at temperatures ranging between 300° and 410°C during fluid mixing process, CO2 effervescence, cooling, and increasing of pH. Low-temperature hydrothermal activity subsequently produced hematite ores associated with propylitic alteration. The metacarbonate host rocks are LILE-depleted and HFSE-enriched due to metasomatic alteration.

A Magnetotelluric Survey of Ophiolites in the Neyriz area of southwestern Iran

A wide band magnetotelluric study of the ophiolitic zone of the Zagros orogenic belt was conducted in the Neyriz area of southwestern Iran. The purpose of the study was to image subsurface structures electrically and relocate the main Zagros thrust fault in the region. The thrust fault has a complex structure with obscure behavior and is believed to be located within a zone of ongoing continental plate convergence. The fault zone with a NW–SE geological trend is parallel to the Zagros orogenic belt and separates the Neyriz ophiolite assemblage from the adjacent Sanandaj-Sirjan metamorphic zone. Magnetotelluric data were collected along a SW–NE profile across the geologic strike; the study included 18 stations and modeling was performed using a 2-D inversion scheme. Analysis of both modes of magnetotelluric data (TE and TM) clarifies the signatures of large resistivity variation in the study area. Due to the presence of a high contrast in resistivity between the ophiolites and neighboring rocks, we are able to discern two sharp boundaries as faulting planes and borders of the ophiolite–radiolarite zone in the north-eastern and southwestern parts of the 2-D resistivity models, respectively.

Petrological Evolution of the Upper Mantle Beneath the Southern Sanandaj-Sirjan Zone: Evidence from Kuhshah Peridotite Massif, Southeast Iran

The Kuhshah ultramafic complex is located in the south-east of Sanandaj-Sirjan metamorphic zone, near the probable remnants of Neotethys plate in Iran. It consists of highly depleted harzburgites, dunites, chromitite bands and altered gabbros. The ultramafic parts have been intruded by numerous clinopyroxenite dykes and veins. In the harzburgites, there are different generations of olivine, orthopyroxene and spinel. The chemical composition of the first generation minerals indicates that the harzburgites are depleted in incompatible elements, but the other generations show melt/rock interaction features which can be found in a suprasubduction zone setting. During subduction of the Neotethys plate beneath the Sanandaj-Sirjan zone, a back arc basin developed between the Sanandaj-Sirjan arc and the central Iranian microcontinent. Whole rock and mineral chemistry, specially, cromespinels, show that the harzburgites formed underneath this basin. After depletion, ascending melts with boninitic compositions reacted with these peridotites to form dunites and chromitite bands. The updoming and decompression of the mantle in the next stage, led to remelting at deeper levels and development of clinopyroxenite parental melts which intruded into the uppermost mantle. Then, the new basin closed and the Kuhshah peridotites were emplaced in present position.