isotopes, and U–Pb geochronology are presented for three alkaline intrusions (Hashroud, Sisan, and Sarab) in the Zanjan-Takab complex in northwestern Iran, to investigate their sources, petrogenesis, and tectonic implications of emplacement. The Hashroud and Sisan plutons are mainly composed of silica-saturated granite and syenogranite, respectively, whereas the Sarab region consists of nepheline- bearing syenite, and associated undersaturated lavas ranging in composition from tephriphonolite to trachy-basalt/andesite. Zircon U–Pb geochronology gives emplacement ages of 38 and 36 Ma for the Hashroud and Sisan silica-saturated rocks, which are slightly younger than the Sarab silica-undersaturated rocks (40 Ma). Silica-undersaturated and silica-saturated rocks differ from each other mostly in terms of their silica content, but show comparable incompatible trace element distributions, typical of subduction-related magmatic rocks. The undersaturated rocks display higher LILE/HFSE (Ba/Th: up to 365), but relatively similar LILE/LREE (Ba/La: 13–44) values to those of saturated rocks (Ba/Th: up to 38; Ba/La: up to 27). Silica-undersaturated and -saturated rocks also show distinct Sr–Nd–Pb isotopic compositions, with the former having less radiogenic Sr (87Sr/86Sr, 0.70452–0.70510) and Pb (206Pb/204Pb, 18.61–18.67; 207Pb/204Pb, 15.57–15.60; 208Pb/204Pb, 38.64–38.74) and more radiogenic Nd (143Nd/144Nd, 0.512648–0.512674) than the latter (87Sr/86Sr, 0.70619– 0.71409; 206Pb/204Pb, 18.78–18.82; 207Pb/204Pb, 15.60–15.63; 208Pb/204Pb, 38.78–38.82; 143Nd/144Nd, 0.512613–0.512620). Geochemical and isotopic compositions of the silica-undersaturated rocks suggest the involvement of fluids derived from subducted oceanic crust and subordinate sediment as metasomatizing agents in their mantle source. On the other hand, the studied silica-saturated units, which crystallized from mafic parental melts modified by crustal assimilation and fractional crystallization, originated from a lithospheric mantle source that had been metasomatized via a relatively high volume of terrigenous subducted sediment melts. Both the Eocene saturated and undersaturated igneous rocks formed in a post-collisional extensional setting. Our study, along with compiled data, also finally settles the long-standing debate about the geodynamic evolution of the NW Iranian belt, showing that a compressive to extensional tectonic regime existed during the Arabia–Eurasia collision. This was related to the Neo-Tethyan slab roll-back, which generated alkalirich magmatic rocks throughout the NW Iranian belt

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