Middle-to-Late Eocene shoshonitic basaltic volcanism in NW Iran (Kivi region): implications for a buried Neo-Tethyan suture

In Iran, Eocene igneous rocks form two main magmatic belts: the Azerbaijan-Alborz and Urumieh-Dokhtar. This paper examines the petrogenesis of Middle–Late Eocene andesitic to basaltic rocks and dykes (microgabbro, dolerite, lamprophyre) in the western Azerbaijan-Alborz belt, focusing on their geodynamic context. The volcanic rocks in the Kivi area show shoshonitic affinity, with large‐ion lithophile element enrichment and high-field strength element depletion, indicating a subduction-related origin. Despite variations in microtextures, all rock types exhibit similar initial ⁸⁷Sr/⁸⁶Sr and εNd values (0.7050 to 0.7068 and − 0.18 to 1.20). Chondrite-normalized rare-earth element patterns are strongly fractionated, spoon-shaped, and devoid of any negative Eu anomaly, ruling out any significant low-pressure fractionation. The basaltic-to-andesitic country rocks are more fractionated (Mg# 42–52) compared to the dykes (Mg# 49–66). The geochemical features are consistent with fractionation of olivine, clinopyroxene, and hornblende. All the different dykes are likely linked to different batches of similar magma sources, with lamprophyre differing due to hydrous phases like hornblende and biotite. Magmas of the Kivi volcanic rocks were derived from partial melting of a spinel- and garnet-bearing lithospheric mantle, metasomatized by slab-related fluids. Regional geological evidence such as (i) the submergence below sea level during the Early Eocene, and (ii) the formation of roughly N–S striking dyke swarms suggest that the Eocene magmatism occurred in an extensional setting, and compressive tectonics began by the latest Eocene. Given the elongation of the Azerbaijan-Alborz magmatic belt, its distance from the Zagros Neo-Tethyan seaway (up to 500–550 km), the occurrence of Eocene magmatism in shallow-marine settings in both belts, and the lack of magmatism between the two zones, we argue that the Eocene magmatism in the Azerbaijan-Alborz belt cannot be explained by flat subduction and slab rollback of the subducting oceanic lithosphere along the Zagros Neo-Tethyan seaway. Instead, it is likely linked to lithospheric delamination along a hidden Neo-Tethyan suture beneath young sedimentary cover, connecting the Sevan-Akera and Sebzevar Neo-Tethyan sutures.