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The siliciclastic sediments of Lower Triassic-Middle Jurassic Shemshak Group is located in the Central Alborz Mountain Range along the southern Caspian Sea shore. The analysis (CIA, ICV, PIA, CIW, Th/U ratio) indicates a moderate to intense weathering. The graph of Qp/F+RF versus Qt/F+RF revealed semi-humid climate. The petrography data reveal mainly a recycled orogeny provenance. Point counting data reveal mean value of Q40.9:F2.3:RF56.8 (Emarat section) and Q58.6:F8.2:RF33 (Emamzadeh Hashem section) for the dominant siliciclastic grains. The Th-Sc-Zr/10 and La-Th-Sc ternary diagrams indicate continental island arc setting for the source area of recycled sediments. Similar results were reported from the Middle Paleozoic-Triassic of the Turan plate and the southern border of the Scythian plate. New discrimination functions revealed active margin setting. Ratios or concentration of La/Sc, Th/Sc, La/Co, Eu/Eu*, Th/Co ,Th/Cr, Cr, Ni, Rb and bivariate plots of La/Sc- Co/Th and La/Sc-Th/Co testify for acidic-felsic source rocks. Ratios of Al2O3/Na2O, K2O/Na2O, and Th/U as well as variable compositions of conglomerate particles documented the recycling. Quartz pebble-cobbles and fine-grained quartz indicate mainly medium-high-grade metamorphic source rocks which has some contradict with Gorgan Schist (GS) with low-grade metamorphic as source remnant. High-grade metamorphic rock fragments increase towards the top of the section which is consistent with un-roofing in the source area. The results are mostly in the favor of a northern source from the Turan and/or Scythian plates near the Caucasus Mountains.

The Bangestan reservoir, which occurs in the Ahwaz oilfield, consists of the middle Cretaceous limestone Ilam and Sarvak Formations that were deposited in the Zagros Basin. The reservoir is divided into ten Zones (A to J) formed in the upper Albian-Santonian and contains considerable hydrocarbon accumulations. The limestones were deposited on an extensive shallow carbonate platform on a passive margin and are dominated by rudist biostrome and grainstone facies. Paleogeographical changes mean that identification of the facies is complex. Seismic stratigraphy and isotopic data are used to better understand the structural and geological setting and develop an understanding of the sedimentary environment. The results show that the rudist biostrome facies extends to a shallow interior shelf as a patch reef, indicating a regressive trend. Petrographic studies on 3174 thin sections identified twelve microfacies in the Bangestan reservoir. In the Sarvak Formation, the dominant microfacies consist of rudist biostrome, shoal bar, and lagoon facies, with a high sedimentation rate in the basin, whereas the Ilam Formation involves open and deep-marine facies. Seismic tectono-stratigraphy shows that there are seven sedimentary sequences with mostly regressive cycles and six maximum flooding surfaces from the upper Albian to the Santonian. In addition, the Ahwaz oilfield had experienced pre-tectonic extension before the Early Cretaceous and post-tectonic extension after the Late Cretaceous. The isotopic data from the Bangestan reservoir demonstrate two important disconformities (Cenomanian-Turonian and post-Turonian) that had an important influence on the evolution of the area evolution. Sr geochronology confirms long-term exposure to two main disconformities within the Ahwaz Bangestan reservoir.

We report evidence for possible Ediacaran (ca. 560 Ma) glaciation from two localities of the late Neoproterozoic Kahar Formation in the central Alborz Mountains of northern Iran. Here, the Kahar Formation consists of about 1000 m of siliciclastic sedimentary strata (sub-greenschist), with minor carbonate, which were deposited along the Peri-Gondwanan margin of Iran. The Kahar Formation represents some of Iran's oldest sedimentary basement exposures, but its base is not exposed anywhere. The lowest Kahar exposures in the study localities, which outcrop within anticlinorial cores ~ 120 km apart, offer insights to northern Iran's Neoproterozoic evolution. Distinct diamictite units occur near the base of both sections and are consistent with glaciogenic diamictites on the basis of the following: (1) geometry, (2) apparent widespread lateral continuity, (3) sharp erosional basal contact, (4) and occurrence of striated surfaces in one locality (Kahar Mountain). Their textural and compositional characteristics also provide less diagnostic features. Detrital zircon U–Pb ages from samples underlying and overlying the diamictite units indicate maximum depositional ages of about 560–550 Ma. Buff-gray microbial carbonate intervals of 9 to 17 m thick, with moderately negative δ13C compositions (− 5.4 to − 1‰) and 87Sr/86Sr values between 0.70867 and 0.70884 (Kahar Mountain locality), conformably overlie the diamictite units in each locality. These C and Sr isotopic compositions are consistent with the rapid increase in 87Sr/86Sr during the late Ediacaran recovery from the Shuram negative δ13C excursion. Our combined isotope stratigraphy and detrital zircon ages suggest that the Kahar and overlying Soltanieh formations in Iran correlate with the Buah and Ara formations (Nafun Group) in Oman.

The Asara Shale Member of the Karaj Formation, Middle to Upper Eocene in age, includes mixed volcaniclastic, siliciclastic and carbonate strata, in the Alborz Mountains, northern Iran. These strata have been geochemically analyzed to constrain their source area provenance. Most of the major and trace element contents of Asara Shale samples are generally similar to the upper continental crust (UCC) values and their chemical index of alteration (CIA) values (31.7–50.9), suggesting a semi-arid climate in a weathering-limited source area. Moreover, correlation of major element oxides suggests the controlling effect of plagioclase and clay minerals on these data when XRD analysis and SEM images indicate clastic sourced illite and chlorite in the samples, probably due to the weathering of volcanic ash and glass in an aqueous transport and depositional environment. However, the chondrite-normalized REE plot of mudrocks of the Asara Shale show a similarity with the UCC composition. Mineral composition and geochemistry of the Asara Shale mudrocks reveal the rapid reworking of penecontemporaneous volcaniclastic deposits, which had covered the surrounding areas, instead of their epiclastic genesis related to erosion and transport. Major, trace, and rare earth element (REE) data, imply a felsic composition of the volcanic source, related to an active continental margin/continental island arc and most probably back-arc tectonic setting for the depositional basin, inferring a mid-upper Eocene extensional volcanism in a subduction complex.

This article presents new data to understand the nature of the hidden crystalline basement of northern Iran and the tectonic setting of Iran during late Neoproterozoic time. The siliciclastic-dominated Kahar Formation represents the oldest known exposures of northern Iran and comprises late Ediacaran (ca. 560–550 Ma) compositionally immature sediments including mudrocks, sandstones, and conglomerates. This work focuses on provenance of three well preserved outcrops of this formation in Alborz Mountains: Kahar Mountain, Sarbandan, and Chalus Road, through petrographic and geochemical methods. Mineralogical Index of Alteration (MIA) and Chemical Index of Alteration (CIA-after correction for K-metasomatism) values combined with A–CN–K relations suggest moderate weathering in the source areas. The polymictic nature of Kahar conglomerates indicates a mixed provenance for them. However, modal analysis of Kahar sandstones (volcanic to plagioclase-rich lithic arkose) and whole rock geochemistry of mudrocks suggest that they are largely first-cycle sediments and that their sources were remarkably late Ediacaran, intermediate-felsic igneous rocks from proximal arc settings. Tectonic setting discrimination diagrams also indicate a convergent plate margin and continental arc related basin for Kahar sediments. This interpretation is supported by the phyllo-tectic to tectic composition and geochemistry of mudrocks. These results reveal the presence of a felsic/intermediate subduction-related basement (∼600–550 Ma) in this region, which provides new constraints on subduction scenario during this time interval in Iran, as a part of the Peri-Gondwanan terranes.

Petrography and geochemistry (major, trace and rare earth elements) of clastic rocks from the Lower Cambrian Lalun Formation, in the Posht-e-badam block, Central Iran, have been investigated to understand their provenance. Petrographical analysis suggests that the Lalun conglomerates are dominantly with chert clasts derived from a proximal source, probably chert bearing Precambrian Formations. Similarly, purple sandstones are classified as litharenite (chertarenite) and white sandstones as quartzarenite types. The detrital modes of purple and white sandstones indicate that they were derived from recycled orogen (uplifted shoulders of rift) and stable cratonic source. Most major and trace element contents of purple sandstones are generally similar to upper continental crust (UCC) values. However, white sandstones are depleted in major and trace elements (except SiO2, Zr and Co) relative to UCC, which is mainly due to the presence of quartz and absence of other Al-bearing minerals. Shale samples have considerably lower content in most of the major and trace elements concentration than purple sandstones, which is possibly due to intense weathering and recycling. Modal composition (e.g., quartz, feldspar, lithic fragments) and geochemical indices (Th/Sc, La/Sc, Co/Th, Cr/Th, Cr/V and V/Ni ratios) of sandstones, and shales (La/Sc and La/Cr ratios) indicate that they were derived from felsic source rocks and deposited in a passive continental margin. The chondrite-normalized rare earth element (REE) patterns of the studied samples are characterized by LREE enrichment, negative Eu anomaly and flat HREE similar to an old upper continental crust composed chiefly of felsic components in the source area. The study of paleoweathering conditions based on modal composition, chemical index of alteration (CIA), plagioclase index of alteration (PIA) and A–CN–K (Al2O3 − CaO + Na2O − K2O) relationships indicate that probably chemical weathering in the source area and recycling processes have been more important in shale and white sandstones relative to purple sandstones. The results of this study suggest that the main source for the Lalun Formation was likely located in uplifted shoulders of a rifted basin (probably a pull-apart basin) in its post-rift stage (Pan-African basement of the Posht-e-badam block).

Geoscientists have always considered the Neyriz region, located along the Zagros Suture Zone, an important area of interest because of the outcrops of Neotethys ophiolitic rocks. We carried out a modal analysis of the Cenozoic sandstones and geochemistry of the detrital Cr-spinels at Neyriz region in order to determine their provenance and tectonic evolution in the proximal part of Zagros Basin. Our data shows a clear change in provenance from the Late Cretaceous onwards. As from the Late Cretaceous to Eocene, lithic grains are mostly chert and serpentinite; and higher Cr# values of the detrital Cr-spinel compositions indicate that they originate from the fore-arc peridotites and deposited in an accretionary prism setting during this period. From the Late Oligocene to the Miocene periods, volcaniclastic and carbonate lithic grains show an increasing trend, and in the Miocene, metasedimentary lithic grains appear in the sediments. Ophiolite obduction caused a narrow trough sub-basin to be formed parallel to the general trend of the Zagros Orogeny between the Arabian and Iranian plates in Oligocene. From the Miocene onwards, the axial metamorphic complex belt was uplifted in the upper plate. Therefore, the collision along the Zagros Suture Zone must have occurred in the Late Oligocene.