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Kolah-Ghazi granitoid (KGG), situated in the southern part of the Sanandaj–Sirjan Zone (SNSZ), Iran, is a peraluminous, high K calc-alkaline, cordierite-bearing S-type body that is mainly composed of monzogranite, granodorite and syenogranite. Zircon U–Pb ages indicate that the crystallization of the main body occurred from 175 Ma to 167 Ma. Two kinds of xenoliths are found in KKG rocks: (i) xenoliths of partially melted pelites including cordierite xenocrysts and aluminoslicates, and (ii) mafic microgranular enclaves that reflect the input of mantle-derived mafic magmas. Field observations and geochemical data of KGG rocks are consistent with their derivation from a multiple sources including melts of metasediments and mantle-derived melts. We infer that these magmas originated by the anatexis of a metasedimentary source (mixture of metapelite and metagreywacke) in the mid- to lower-crust under low water-vapor pressures (0.5-1 Kbar) and temperature of ∼800 °C. KGG is the product of biotite incongruent melting of this metasedimentary source. S-type granites are commonly thought to be produced in continent-continent collision tectonic environment. However, trace element discrimination diagrams show that S-type KGG rocks formed in an arc-related environment. The roll-back of Neo- Tethyan subducting slab accompanying oblique subduction in Late Triassic to Early Jurassic time induced trench rollback, back arc basin opening and filling with turbidite flysch and molasse- type siliciclastic sediments of the Shemshak Group on the overriding plate. Further changes in the subducting slab to flat subduction in Middle Jurassic time, the time of peak magmatism in the SNSZ, led to thickening and high temperature-low pressure metamorphism of the backarc turbidite deposits and consequent anatexis of the metasedimentary source to produce the KGG S- type rocks along with several other I-type granitoids in the SNSZ. [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]