Lower Cretaceous asthenospheric wedge-derived alkaline magmatism in the Zagros orogen (Iran) and its geodynamic significance

The complex Arabia−Eurasia plate interaction triggered Jurassic−Cretaceous orogenic magmatism within the Sanandaj−Sirjan Zone, the core of the Zagros orogen in Iran. While subduction-related magmatism is expected near convergent margins, the geodynamic mechanisms responsible for Early Cretaceous alkaline magmatism in the Sanandaj−Sirjan Zone remain unresolved. To address this gap, we present new geochronological and geochemical data from a unique suite of lower Cretaceous alkaline magmatic rocks within the composite Cheshmeh−Sefid syenite−gabbroic pluton, located in the southwestern Golpaygan Complex of the central Sanandaj−Sirjan Zone. High-precision zircon U-Pb geochronology using secondary ion mass spectrometry (SIMS) and laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) reveals emplacement ages of 128 ± 1 Ma for both syenite and gabbro, 127 ± 0.92 Ma for syenite, and 125 ± 0.9 Ma and 125 ± 1.3 Ma for isolated mafic and felsic dikes. Zircon Hf-O isotope compositions exhibit limited variability, with εHf(t) values of +4.1 to +11.4 and δ18O values averaging +4.93‰ to +5.44‰. Coupled trace element and isotopic data indicate that the parental magmas originated from low-degree partial melting of a metasomatized subcontinental lithospheric mantle (SCLM) source, characterized by spinel ± minor garnet, followed by fractional crystallization with negligible crustal assimilation. We propose that this distinct episode of Early Cretaceous alkaline magmatism across the Sanandaj−Sirjan Zone resulted from lithospheric extension following the cessation of flat-slab subduction, likely triggered by oceanward trench retreat that induced decompression melting above an upwelling asthenospheric mantle wedge.