Tracing young subduction beneath northwest Iran: Insights from cosmogenic 10Be recycling and Sr-Nd-Hf isotopes in late Miocene volcanoes

Abstract Late Miocene–Quaternary volcanoes in northwestern Iran, characterized by andesitic to dacitic compositions including adakitic rocks with high Sr/Y and La/Yb ratios, exhibit zircon U-Pb ages ranging from 12 to 2 Ma. The isotopic compositions of whole-rock samples (εNd = +0.1 to +4.4; εHf = +2.8 to +11.8; 87Sr/86Sr < 0.705) indicate that the primary magma originated from partial melting of subcontinental lithospheric mantle. The whole-rock chemistry supports an arc magmatic signature, which is consistent with an active continental margin setting. To investigate sediment recycling, cosmogenic beryllium-10 (10Be) isotopes, with a short half-life (1.39 m.y.), were measured in 40 fresh whole-rock samples. Elevated 10Be concentrations (>106 atoms/g) in certain volcanoes confirm a young metasomatized mantle beneath the northwest Iran block. Variations in 10Be concentrations that increase from the front-arc to the rear-arc magmatic rocks suggest greater incorporation of pelagic sediments (rich in 10Be) away from the trench zone. These findings indicate that late Miocene–Quaternary magmatic activity coincided with or followed the formation of an active margin linked to the subduction of the Khoy-Oshnavieh oceanic branch of the northern Neo-Tethys Ocean beneath northwest Iran. Evidence suggests that this branch remained active until the late Miocene. This study highlights the significant role of subducted terrestrial and pelagic sediments in Pliocene–Quaternary magmatism, as supported by 10Be data. This study underscores the utility of cosmogenic 10Be as a reliable tracer for sediment incorporation in young magmatic systems along active margins, providing insights into the geodynamic processes shaping northwestern Iran.