Potassium Alkaline Volcanism of Alaid Volcano, Kuril Islands: the Role of Subduction Melange in Magma Genesis

New major-, trace-element and Sr-Nd-Pb isotope data are presented on the Holocene high-potassium basic lavas of Alaid volcano, which is located in the north of the Kuril island arc, in the junction zone with the Kamchatka volcanic segment. According to the petrochemical criteria, two groups of coeval rocks are distinguished: Ne-normative shoshonites and high-potassium subalkaline basalts, which have many similar geochemical characteristics. Chondrite-normalized REE distribution patterns show LREE enrichment, with flat HREE pattern, and the absence of Eu and Ce anomalies. MORB-normalized incompatible element patterns show LILE enrichment and a well-defined negative Ta–Nb–Ti anomaly typical of suprasubduction volcanics. The high K2O/Rb and Rb/Sr ratios indicate the presence of biotite and amphibole in the magmatic source, while the low Sr/Y ratios and flat MREE and HREE distribution patterns indicate the absence of residual garnet. Significant variations in the contents of major- and trace elements at similar MgO concentrations indicate a heterogeneous magma source, while linear mixing trends in isotope and discrimination diagrams, as well as experimental data, suggest the involvement in magmogenesis of not only peridotite mantle, but also amphibole–clinopyroxene mineral paragenesis. An analysis of literature data shows that the manifestations of potassium alkaline magmatism in “cold” island arcs are frequently, if not always, confined to local extension zones. Since such zones are associated with the adiabatic rise of a hot and ductile asthenosphere, it can be assumed that melting involved subduction mélange, which is formed along the boundary of the slab and supra-subduction mantle and consists of hydrated fragments of ultrabasites and metamorphosed oceanic crust transformed into amphibole-bearing pyroxenites. 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 relation of its northern segment with anomalous tectonics. The results obtained may be important in discussing the genesis of potassium alkaline magmas occurred in subduction geodynamic settings.