Sr–Nd isotopes of Sabalan Volcano, NW Iran: insights into the origin of collisional adakites and geodynamic implications

Late Miocene-Quaternary adakitic rocks are widely distributed in northwestern Iran and are a key component of magmatism in the Turkish–Iranian Plateau of the collision zone between Eurasia and Arabia. Igneous rocks with adakitic affinity are distributed over large parts of northwestern Iran, eastern Turkey, and the Lesser Caucasus, and post-date a presumed slab break-off event at c. 10 Ma. Here, we present whole-rock Sr–Nd isotopic data for 4.5–0.149 Myr adakitic rocks of Sabalan volcano, NW Iran. These rocks are characterized by near-identical ⁸⁷Sr/⁸⁶Sr = 0.7044 to 0.7045 and ɛNd = + 2.24 to + 2.76. This is interpreted to indicate that Sabalan magmas were primarily generated by melting juvenile intrusions at the base of thickened lower crust and that assimilation of upper continental crustal rocks only played a minor role in their evolution relative to fractional crystallization. The delay between slab break-off beneath the Neo-Tethyan suture zone and magmatic activity at Sabalan is indicative for asthenospheric melting being triggered by small-scale mantle convection underneath the Turkish-Iranian Plateau. A preferred scenario is that the detached slab heated up in the mantle causing dehydration, and ascending fluids consequently lowered the viscosity of the mantle. Subsequently, the delaminated lower lithosphere was replaced by asthenospheric mantle which heated and partially melted lower crustal rocks to generate adakitic magmas. Collectively, these events occurred after a significant temporal hiatus that followed the earlier slab break-off event when remnants of subducted Neo-Tethyan lithosphere became detached.