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Iran is a remarkable geoscientific laboratory where the full range of processes that form and modify the continental crust can be studied. Iran's crustal nucleus formed as a magmatic arc above an S-dipping subduction zone on the northern margin of Gondwana 600–500 Ma. This nucleus rifted and drifted north to be accreted to SW Eurasia ∼250 Ma. A new, N-dipping subduction zone formed ∼100 Ma along ∼3,000 km of the SW Eurasian margin, including Iran's southern flank; this is when most of Iran's many ophiolites formed. Iran evolved as an extensional continental arc in Paleogene time (66–23 Ma) and began colliding with Arabia ∼25 Ma. Today, Iran is an example of a convergent plate margin in the early stages of continent-continent collision, with a waning magmatic arc behind (north of) a large and growing accretionary prism, the Zagros Fold-and-Thrust Belt. Iran's crustal evolution resulted in both significant economic resources and earthquake hazards. Iran is a natural laboratory for studying how convergent plate margins form, evolve, and behave during the early stages of continental collision. Iran formed in the past 600 million years, originating on the northern flank of Gondwana, rifting away, and accreting to SW Eurasia. Iran is actively deforming as a result of collision with the Arabian plate, but earthquakes do not outline the position of the subducting slab. The Cenozoic evolution of Iran preserves the main elements of a convergent plate margin, including foredeep (trench), accretionary prism, and magmatic arc. [ABSTRACT FROM AUTHOR]

Jurassic igneous bodies of the Sanandaj–Sirjan zone (SaSZ) in SW Iran are generally considered as a magmatic arc but critical evaluation of modern geochronology, geochemistry and radiogenic isotopes challenges this conclusion. There is no evidence for sustained igneous activity along the ~1,200 km long SaSZ, as expected for a convergent plate margin; instead activity was brief at most sites and propagated NW at ~20 mm/a. Jurassic igneous rocks define a bimodal suite of gabbro‐diorite and granite. Chemical and isotopic compositions of mafic rocks indicate subcontinental lithospheric mantle sources that mostly lacked subduction‐related modifications. The arc‐like features of S‐type granites reflect massive involvement of Cadomian crust and younger sediments to generate felsic melts in response to mafic intrusions. We conclude that Jurassic SaSZ igneous activity occurred in a continental rift, not an arc. SaSZ igneous rocks do not indicate that subduction along the SW margin of Eurasia began in Jurassic time. [ABSTRACT FROM AUTHOR]

Subduction initiation (SI) requires the sinking of one plate beneath another and this exerts extensional stress on the overlying plate. How broad a region is affected by SI-related extension is unclear because most of the clearest SI examples– such as Izu-Bonin-Mariana arc– are deep under the ocean. A major SI event is recorded in the Late Cretaceous forearc ophiolites of Iran, related to the subduction of Neotethyan oceanic lithosphere beneath Eurasia. This caused extreme extension of the Iranian plate, up to ~1000 km away from the proto-trench and generated a series of back-arc oceanic basins, sedimentary basins, and core complexes and exhumed high-P rocks. The Late Cretaceous Iran example shows that SI can cause strong extension over a much wider region of the overriding plate than heretofore imagined and offers an accessible natural laboratory for studying SI processes. This understanding also provides an attractive new explanation for the origin of the South Caspian Sea. • Subduction initiation (SI) is a mechanism for forming Neotethyan ophiolites. • Ophiolites of Iran show SI exerts extensional stress on the overlying plate. • SI-related extension can affect broad regions of the overriding plate. [ABSTRACT FROM AUTHOR]

It is important to understand the Jurassic tectonic setting of SW Eurasia, and the igneous rocks of the Sanandaj-Sirjan zone in SW Iran provide critical evidence. Bimodal volcanic and hypabyssal rocks exposed around Songhor in the central part of the Sanandaj-Sirjan zone have zircon U-Pb crystallization ages of 156 Ma for felsic and 153 to 146 Ma for mafic rocks. Felsic volcanic and subvolcanic rocks with affinity to A-type granite have high SiO 2 (62.6 to 73.9 wt%) and total alkalis (K 2 O + Na 2 O = 5.1–9.8 wt%), low MgO (0.1–3.1 wt%) and slightly positive εNd(t) (+0.6 to +3.8) and moderate 87Sr/86Sr(i) ratios (0.7041 to 0.7061). The mafic rocks (basalt and micro gabbro/dolerite) with lower contents of SiO 2 (47.0 to 54.6 wt%), high MgO (4.1 to 10.7 wt%) with variable Na 2 O (2.0–5.9 wt%) and K 2 O (0.1–2.6 wt%) can be divided into two groups based on TiO 2 contents: 1) low-Ti and 2) high-Ti. The low-Ti group has 0.14 to 0.62 wt% TiO 2 with εNd(t) = +3.2 to +5.8 and 87Sr/86Sr(i) = 0.7032–0.7059. The high-Ti basalts have 1.4–1.6 wt% TiO 2 with εNd(t) = +1.2 to +4.2 and 87Sr/86Sr(i) =0.7039 to 0.7057. The low-Ti suite is less fractionated, with Mg# between 55 and 74 compared to 43 to 63 for the high-Ti suite. Felsic rocks may have formed by remelting of underplated mafic rocks which were derived from subcontinental lithospheric mantle along with minor assimilation of continental crust in the early stage of extension. Thinning of continental lithosphere continued with increasing partial melting to produce high-Ti mafic melts early and low-Ti mafic melts in the late stage. Similar Sr-Nd isotope ratios for the mafic and felsic rocks with an 8–10 Ma interval between these two groups hint at gradual thinning of the continental lithosphere. This is consistent with formation of transitional crust of a passive continental margin between Neo-Tethys and Iran continental crust in Jurassic time. [Display omitted] • Bimodal magmatism occurred from 156 to 144 Ma in west Iran (Songhor). • Felsic rocks were generated from the mantle with minor involvement of older crust. • Mafic rocks are divided into low-Ti (tholeiitic) and high-Ti (alkaline) groups. • Magmatic activity suggest that SW Iran was a Jurassic volcanic rifted margin. [ABSTRACT FROM AUTHOR]

Intra-plate basalts of ~ 35–0 Ma in East Eurasia formed in a broad backarc region above the stagnant Pacific Plate slab in the mantle transition zone. These basalts show regional-scale variations in Nd-Hf isotopes. The basalts with the most radiogenic Nd-Hf center on the Shandong Peninsula with intermediate Nd-Hf at Hainan and Datong. The least radiogenic basalts occur in the perimeters underlain by the thick continental lithosphere. Shandong basalts possess isotopic signatures of the young igneous oceanic crust of the subducted Pacific Plate. Hainan and Datong basalts have isotopic signatures of recycled subduction materials with billions of years of storage in the mantle. The perimeter basalts have isotopic signatures similar to pyroxenite xenoliths from the subcontinental lithospheric mantle beneath East Eurasia. Hainan basalts exhibit the highest mantle potential temperature ( T p ), while the Shandong basalts have the lowest T p . We infer that a deep high- T p plume interacted with the subducted Pacific Plate slab in the mantle transition zone to form a local low- T p plume by entraining colder igneous oceanic lithosphere. We infer that the subducted Izanagi Plate slab, once a part of the Pacific Plate mosaic, broke off from the Pacific Plate slab at ~ 35 Ma to sink into the lower mantle. The sinking Izanagi slab triggered the plume that interacted with the stagnant Pacific slab and caused subcontinental lithospheric melting. This coincided with formation of the western Pacific backarc marginal basins due to Pacific Plate slab rollback and stagnation. [ABSTRACT FROM AUTHOR]