Concurrent adakitic and non-adakitic Late Miocene-Quaternary magmatism at the Sahand volcano, Urumieh-Dokhtar Magmatic Arc (NW Iran)

The Sahand volcano in NW Iran is characterised by voluminous Late Miocene to Quaternary magmatic activity. Petrographic, mineral chemical, whole-rock geochemical and Sr-Nd-Pb isotope data allowed to identify two rock groups, continuously emplaced from the ancient Paleo-Sahand to the later Neo-Sahand stages. These are represented by abundant evolved rocks, with dacitic to rhyolitic compositions and less common intermediate (andesitic) rocks. Both rock types have the typical LILE-enriched and HFSE-depleted fingerprints of magmas originated from subduction-modified mantle sources, as well as radiogenic Sr- and Pb- (18.8–18.9 206Pb/204Pb, 15.6–15.7 207Pb/204Pb and 38.8–39.2 208Pb/204Pb) isotope compositions. Nevertheless, the evolved rocks have lower 87Sr/86Sr (0.70437–0.70491 vs 0.70494–0.70585) and display an evident high Sr/Y vs Y and high La/Yb vs Yb “adakitic” signature, suggesting derivation from the partial melting of lower continental crust rocks, rather than from the differentiation of the non-adakitic intermediate rocks. The latter would be the derivative melts of magmas generated by the partial melting of lithospheric mantle sources modified by metasomatic fluids released by the subducted Neotethyan oceanic lithosphere. During their ascent and differentiation, these magmas likely provided the heat necessary to induce partial melting of the lower continental crust, eventually producing adakitic evolved magmas. Quantitative models and geothermobarometric calculations suggest that differentiation processes for both rock groups occurred mainly by polybaric fractional crystallisation (in the 29–4 km depth range), possibly coupled with minor assimilation of local crustal rocks. The peculiar style of the Sahand magmatic activity (with both adakitic and non-adakitic products) and its temporal evolution (with an overall increase of the degree of evolution and a decrease of the non-adakitic melts) are somewhat different with respect to those of the coeval neighbouring districts of Saray, Sabalan and Bijar-Qorveh. These can be reconciled within a general model for NW Iran where magmatism is triggered by upwelling of the sub-lithospheric mantle as a consequence of slab break-off (or the opening of a slab tear). The progressive propagation (and subsequent exhaustion) of the thermal pulse and the increasing thickness of the continental crust played a main role in determining both the progressively later inception of active magmatism (from ∼11 to 4.5 Ma) and the change in magma types (from largely variable to adakitic only) at increasing distance from the Iran-Arabia suture.