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Late Eocene – Oligocene volcanic rocks in the eastern Saveh region of the Urumieh-Dokhtar magmatic arc (UDMA) are representative of Paleogene magmatic activity in Iran. They show a wide range of silica-undersaturated to silica-oversaturated compositions, from basalt-trachy basalt and tephri-phonolite to trachyte-latite. Whole rock chemical compositions define a continuous assemblage of mafic and felsic rocks in terms of SiO2 (46.3 to 71.1 wt.%), Al2O3 (12.5–19.2 wt.%), K2O (2.06–12.23 wt.%) and TiO2 (0.41–1.75 wt.%) contents, Mg number (6.8–51) and K2O/Na2O ratios (0.53–30.2). Abundances of rare earth elements in the tephri-phonolite group are much higher than in the trachyte-latite and basalt-trachy basalt groups. Elevated contents of large ion lithophile elements such as Ba (191–7311 ppm) and Pb (6.49–118 ppm) as well as depletions in high field strength elements such as Nb (2.43–40.6 ppm), Ta (0.15–2.24 ppm), Zr (40.8–258 ppm) and TiO2 (0.40–1.77 wt.%) characterize these rocks. Initial ratios of 87Sr/86Sr from 0.7046 to 0.7074 and ɛNd(t) from +0.9 to +3.3 show that these were derived from parental magma extracted from partial melting of metasomatized mantle, namely amphibole- and/or phlogopite-bearing peridotite. Fractionation of feldspar, clinopyroxene, amphibole and phlogopite, and minor contamination with upper crust was responsible for the large variation of the rock types observed. We explain contemporaneous late Eocene – Oligocene calc-alkaline and high-K magmatic activity in the UDMA as due to the partial melting of both hot metasomatized asthenospheric and subcontinental lithospheric mantle in an extensional tectonic regime. This extensional regime probably developed due to the rollback of the subducted Neo-Tethys oceanic plate. [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]

Late Paleocene adakitic granitoids are rare in Iran except in the northwest. We focused on the adakitic Saqqez-Takab pluton, which occupies an area of ~600 km2. New U-Pb zircon dating yield crystallization ages of 58–56 Ma (Thanetian). These granitoids comprises granite, quartz monzonite and monzogranite with granular to mylonitic textures. They have high SiO 2 abundances (59.1–78.6 wt%), high ratios of Sr/Y (50–490) and La/Yb ratios (19–237), along with high Ba (779–2466 ppm) and light rare earth element contents (e.g., La = 11–70 ppm). Saqqez-Takab granitoids have compositions that are similar to high-silica adakites, including low Mg- number (Mg# < 0.5), TiO 2 (<0.93 wt%), Cr (<38 ppm) and Ni (<55 ppm). K 2 O/Na 2 O ratios of these granitoids are high (>0.7), resembling K-rich adakitic granitoids. Initial ratios of 87Sr/86Sr and ε Nd (t) vary from 0.7044 to 0.7053 and −2.8 to +2.0 respectively. These isotopic signatures differ from those of typical subducting slab-derived adakites (O-type adakite) with high positive ε Nd (t) and from collision-related adakites (C-type adakite) with negative ε Nd (t). The geochemical and isotopic characteristics of Saqqez-Takab granitoids are most consistent with magma being produced by amphibole-dominated fractionation of hydrous melts of subcontinental lithospheric mantle accompanied by minor assimilation of lower mafic calc alkaline continental crust. Distribution of similar-ages adakites in the NW Iran and E Pontides of Turkey suggests that these melts formed similarly. Upwelling of hot asthenosphere due to Neotethys slab rollback led to partial melting of lower continental crust calc alkaline mafic rocks and/or amphibole fractionation from mafic magma produced K-rich adakitic rocks in eastern Turkey and NW Iran during the late Paleocene. • High-K adakitic granites from NW Iran have zircon U-Pb ages of 58–57 Ma. • Partial melting of subcontinental lithosphere was the source of mafic precursors. • Amphibole fractionation played the main role in generating Paleocene adakites. • These adakites were produced in continental crust of normal thickness. [ABSTRACT FROM AUTHOR]

Subduction initiation is a key process in the operation of plate tectonics. Our understanding of melting processes and magmatic evolution during subduction initiation has largely been developed from studies of the Izu-Bonin-Mariana forearc. Many suprasubduction zone ophiolites are analogous to the Izu-Bonin-Mariana forearc sequence. However, whether there are differences between Izu-Bonin-Mariana subduction initiation sequences and suprasubduction zone ophiolites remains unclear. Here, we report field geological, geochemical, and geochronological data from mafic and felsic rocks in the Xigaze ophiolite (southern Tibet) mantle and crustal section; the same types of published data from both this ophiolite and the Izu-Bonin-Mariana forearc are compiled for comparison. The ophiolite section is intruded by various late-stage dikes, including gabbroic pegmatite, diabase, basalt, and plagiogranite. The compositions of clinopyroxene and amphibole suggest that gabbroic pegmatite formed from hydrous high-SiO2 depleted melts, while whole-rock compositions of basaltic and diabase dikes show negative Nb and Ta anomalies, suggesting flux melting of depleted mantle. Along with the mafic rocks, plagiogranite has a roughly constant content of La and Yb with increasing SiO2 contents, implying hydrous melting of mafic amphibolite. Early-stage pillow basalts exhibit geochemical affinities with Izu-Bonin-Mariana forearc basalts, but they are slightly enriched. Synthesized with the regional geological setting and compared with Izu-Bonin-Mariana forearc magmatism, we propose that the transition from mid-ocean ridge basalt-like lavas to subduction-related mafic and felsic dikes records an Early Cretaceous subduction initiation event on the southern flank of the Lhasa terrane. However, the mantle sources and the magmatic evolution in the Xigaze ophiolite are more variable than those for the Izu-Bonin-Mariana forearc. [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]