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 SiO2 (62.6 to 73.9 wt%) and total alkalis (K2O + Na2O = 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 SiO2 (47.0 to 54.6 wt%), high MgO (4.1 to 10.7 wt%) with variable Na2O (2.0–5.9 wt%) and K2O (0.1–2.6 wt%) can be divided into two groups based on TiO2 contents: 1) low-Ti and 2) high-Ti. The low-Ti group has 0.14 to 0.62 wt% TiO2 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% TiO2 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 sub- continental 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.
