Potassium alkaline volcanism of Alaid volcano, Kuril Islands: the role of subduction melange in magmogenesis

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New data are presented on the contents of major elements and trace elements, Sr-Nd-Pb isotopes in the Holocene high-potassium basic lavas of the Alaid volcano, located in the north of the Kuril island arc in the junction zone with the Kamchatka volcanic segment. According to petrochemical criteria, two groups of coeval rocks are distinguished: Ne-normative shoshonites and high-potassium subalkaline basalts, which are similar to each other in a number of geochemical characteristics. Chondrite-normalized REE distribution spectra show enrichment in LREE, with flat HREE distribution spectra, and the absence of Eu and Ce anomalies. MORB-normalized incoherent element concentrations show LILE enrichment and a well-defined negative Ta-Nb-Ti anomaly typical of suprasubduction volcanics. High K2O/Rb and Rb/Sr ratios indicate the presence of biotite and amphibole in the magmatic source, while low Sr/Y ratios and flat distribution spectra of medium and heavy lanthanides indicate the absence of garnet in the restite paragenesis. Significant variations in the contents of macro- and microcomponents at similar MgO concentrations indicate a heterogeneous magmatic source, and taking into account linear mixing trends in isotope and discrimination diagrams, experimental data, suggest the involvement in magmogenesis of not only the peridotite mantle, but also amphibole-clinopyroxene mineral paragenesis. An analysis of the literature data shows that in «cold» island arcs, manifestations of potassium alkaline magmatism are often, if not in all cases, associated with local extension zones. Since such zones are associated with the adiabatic rise of a hot and plastic asthenosphere, it can be assumed that subduction melange formed along the boundary of the slab and supra-subduction mantle, consisting of hydrated fragments of ultrabasites and metamorphosed oceanic crust transformed into amphibole-bearing pyroxenites, was involved in the melting. This mechanism makes it possible to logically explain the geochemical and isotopic features of the anomalous alkaline magmatism of the Kuril island arc and the connection with the anomalous tectonics of its northern segment. The results obtained may be important in discussing the genesis of potassium alkaline magmas manifested in subduction geodynamic settings.