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

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.