Description: Most arcs show systematic temporal and spatial variations in magmatism with clear shifts in igneous rock compositions between those of the magmatic front (MF) and those in the backarc (BA). It is unclear if similar magmatic polarity is seen for extensional continental arcs. Herein, we use geochemical and isotopic characteristics coupled with zircon U‐Pb geochronology to identify the different magmatic style of the Iran convergent margin, an extensional system that evolved over 100 Myr. Our new and compiled U‐Pb ages indicate that major magmatic episodes for the NE Iran BA occurred at 110–80, 75–50, 50–35, 35–20, and 15–10 Ma. In contrast to NE Iran BA magmatic episodes, compiled data from MF display two main magmatic episodes at 95–75 and 55–5 Ma, indicating more continuous magmatism for the MF than for the BA. We show that Paleogene Iran serves as a useful example of a continental arc under extension. Our data also suggest that there is not a clear relationship between the subduction velocity of Neotethyan Ocean beneath Iran and magmatic activity in Iran. Our results imply that the isotopic compositions of Iran BA igneous rocks do not directly correspond to the changes in tectonic processes or geodynamics, but other parameters such as the composition of lithosphere and melt source(s) should be considered. In addition, changes in subduction zone dynamics and contractional versus extensional tectonic regimes influenced the composition of MF and BA magmatic rocks. These controls diminished the geochemical and isotopic variations between the magmatic front and backarc.

Description: Highlights • Cadomian continental arc crust of NE Iran was built during ∼15 Myr of magmatism. • Magmatic flare-up in Iran Cadomia occurred over ∼45 Myr; 570 to 525 Ma. • Geochemical differentiation in “hot zones” built the stratified continental crust of Iran. Abstract The generation and differentiation of continental crust by arc magmatism is strongly influenced by episodes of high magmatic flux (“flare-ups”). Magmatic flare-ups encourage the development of deep crustal hot zones where magmatic differentiation and density stratification combine to form the upper felsic and lower mafic continental crust. Such processes, which are responsible for the construction of continental arc crust, are prolonged events, which build a ∼30-40 km arc crust over tens of million years (∼100 Myr). New zircon U-Pb data reveal that the construction of Cadomian crust from NE Iran occurred over ∼15 ± 0.3 Myr. However, compiled zircon U-Pb ages reveal a prolonged magmatic flare-up of ∼45 Myr; ∼570 to 525 Ma. Basement outcrops in NE Iran expose lower- and upper crust that show how magmatic-geochemical differentiation occurred deep beneath a Cadomian continental arc in a crustal hot zone. Isotopic data for igneous rocks produced during this 45 Myr episode reveal interactions between mantle-derived melts and old continental crust. Synthesis of new and published data indicates that this type of interaction is common during periods of high magmatic fluxes. Our results indicate that differentiation of mafic melts in the lower crust during prolonged magmatic flare-ups plays a key role in building a stratified continental crust.

Description: Most continental arcs are built up over a long time (≥100 myr), and while subduction may be ongoing throughout this interval, magmatism appears to be highly episodic. This episodic behaviour is characterized by high-flux magmatic events but an overall low rate of magmatism. The causes of high-flux magmatic events (“flare-ups”) are enigmatic in many continental arcs. Bulk-rock Sr, Nd, and Pb isotopes, as well as zircon O and Hf isotopes, imply that the mantle and the continental crust can be involved in magmatic flare-ups. However, the relative contributions of mantle vs. crust with changes in eruption rates can differ from arc to arc. The Cenozoic magmatic arcs of Iran, built on mature continental crust, are an excellent candidate for studying the geochemical-isotopic feedback of magmatic pulses to understand the triggers for a flare-up. Our new data constrain the timing of the flare-up in NE Iran to the Early to Middle Eocene (51–43 Ma). This flare-up is characterized by the outpouring of high-K calc-alkalic to shoshonitic magmas at ∼110 ± 8 km3/myr - km. Geochemical modelling using the “Arc Basalt Simulator version 3″ shows that the high-K trachybasalts, moderately to extremely depleted in high-field strength elements, can be derived from the shallower (3.0 GPa; 870 °C) to deeper parts (5.0–5.4 GPa; 965–980 °C) of a subducting slab with ∼1.0 to 5.5 % slab melt flux. Mixing modelling using Sr, Nd, and Pb isotope data indicates that the Torud mafic-intermediate magmatic rocks can be generated by adding ∼ 1% to 〈6% of slab components (50% AOC: 50% sediment) to an Indian MORB-like mantle. Our results suggest that the high magmatic fluxes in NE Iran were instigated mainly by Eocene slab steepening after Paleocene flat-slab subduction, resulting in enhanced upwelling and melting of a volatile-enriched asthenospheric mantle.

Description: Highlights • Alkaline rocks including nepheline syenites, monzogabbros and nepheline-monzosyenites are abundant in NW Iran. • 40Arsingle bond39Ar data show ages of 39.2–43.6 Ma for plutonic rocks and 41.8 Ma for crosscutting trachy-andesitic dikes. • Isotope modelling suggests mixing between depleted mantle and sediment melts for the formation of these rocks. Abstract High Na- to K- alkaline magmatism is common in the rear-arc region of the Cenozoic Urumieh-Dokhtar Magmatic Belt of Iran, but their geochemical signatures, as well as their formation mechanisms, have been poorly studied thus far. In the Kleybar area of NW Iran, Middle Eocene magmatic activity comprises silica-undersaturated to -saturated gabbros, monzogabbros, monzosyenites to nepheline-monzosyenites, and nepheline syenite as well as tephritic to trachy-basaltic and trachy-andesitic dikes. New 40Arsingle bond39Ar results show a restricted age range (39.2–43.6 Ma) for the intrusive rocks (43.16 ± 0.43 and 43.34 ± 0.43 Ma for gabbros, 43.56 ± 0.44 for monzogabbros, 39.22 ± 0.48 and 42.09 ± 0.42 Ma for nepheline syenites, 42.17 ± 0.42 Ma for nepheline-monzosyenites), whereas a cross-cutting trachy-andesitic dike yielded an 40Arsingle bond39Ar age of 41.78 ± 0.42 Ma. The Kleybar rocks are enriched in alkalis (K2O and Na2O) with variable K2O/Na2O ratios (0.14 to 2.93), light rare earth elements and large ion lithophile elements such as Th, Rb, K, U and Pb. High field strength elements such as Nbsingle bondTa are depleted in these rocks. The isotopic compositions of the different rock types are quite variable: gabbros and monzogabbros have 87Sr/86Sr(t) = 0.70419–0.70436, εNd(t) = +3.1 to +3.8 and εHf(t) = + 7.8 to +10.1; nepheline monzosyenites and nepheline syenites have 87Sr/86Sr(t) = 0.70359–0.70566 (except a nepheline syenite with a ratio of 0.74833 and very high Rb/Sr), εNd(t) = +1.5 to +4.2 and εHf(t) = +4.6 to +8.8; and trachy-basaltic to trachy-andesitic to tephritic dikes have 87Sr/86Sr(t) = 0.70437–0.70469, εNd(t) = +2.8 to +4.2 and εHf(t) = +8.3 to +10. In the thorogenic-Pb isotope diagram, the Kleyber igneous rocks define an array above the Northern Hemisphere Reference Line (NHRL), with ∆8/4 (deviation from the NHRL) of ~40–60. These samples also plot above the NHRL in uranogenic Pb space (∆7/4–5-10). The positive ∆8/4 Pb and ∆7/4 Pb may reflect the involvement of subducted terrigenous sediments in their mantle source during the subduction of the Neotethyan oceanic lithosphere. Two nepheline syenite samples have significantly higher thorogenic and uranogenic Pb isotopic compositions, that may reflect assimilation of surrounding clay-rich sedimentary rocks. Modelling of trace elements compositions using less fractionated Kleybar trachybasalt and fine-grained monzogabbro samples indicate that a 96:4 mixture of the depleted mantle and subducting (trench)-sediment melts with 6% aggregated fractional melting closely matches the trace-element abundances of the Kleybar trachybasalt and monzogabbro. Together with previous studies on high-K volcanic rocks from NW Iran, our results indicate that Neotethyan slab retreat and related extension of the Iranian continental lithosphere in the rear-arc region of the Urumieh-Dokhtar Magmatic Belt generated alkali-rich magmatic rocks throughout the NW Iran rear-arc during Middle-Late Eocene.

Description: The Zagros fold-and-thrust belt of SW Iran is a young continental convergence zone, extending NW-SE from eastern Turkey through northern Iraq and the length of Iran to the Strait of Hormuz and into northern Oman. This belt reflects the shortening and off-scraping of thick sediments from the northern margin of the Arabian platform, essentially behaving as the accretionary prism for the Iranian convergent margin. Distribution of Upper Cretaceous ophiolites in the Zagros orogenic belt defines the northern limit of the evolving suture between Arabia and Eurasia and comprises two parallel belts: (1) Outer Zagros Ophiolitic Belt (OB) and (2) Inner Zagros Ophiolitic Belt (IB). These belts contain complete (if disrupted) ophiolites with well-preserved mantle and crustal sequences. Mantle sequences include tectonized harzburgite and rare ultramafic-mafic cumulates as well as isotropic gabbro lenses and isolated dykes within the harzburgite. Crustal sequences include rare gabbros (mostly in IB ophiolites), sheeted dyke complexes, pillowed lavas and felsic rocks. All Zagros ophiolites are overlain by Upper Cretaceous pelagic limestone. Limited radiometric dating indicates that the OB and IB formed at the same time during Late Cretaceous time. IB and OB components show strong suprasubduction zone affinities, from mantle harzburgite to lavas. This is shown by low whole-rock Al2O3 and CaO contents and spinel and orthopyroxene compositions of mantle peridotites as well as by the abundance of felsic rocks and the trace element characteristics of the lavas. Similarly ages, suprasubduction zone affinities and fore-arc setting suggest that the IB and OB once defined a single tract of fore-arc lithosphere that was disrupted by exhumation of subducted Sanandaj-Sirjan Zone metamorphic rocks. Our data for the OB and IB along with better-studied ophiolites in Cyprus, Turkey and Oman compel the conclusion that a broad and continuous tract of fore-arc lithosphere was created during Late Cretaceous time as the magmatic expression of a newly formed subduction zone developed along the SW margin of Eurasia.