Evolution of the Isotopic-Geochemical Composition of Rocks of Uksichan Volcano, Sredinnyi Range, Kamchatka, and Its Relations to the Tectonic Restyling of Kamchatka in the Neogene

The paper presents newly acquired data on concentrations of major and trace elements and on Sr, Nd, and Pb isotope composition in Pliocene and Late Pleistocene-Holocene mafic volcanic rocks of the Uksichan volcanic center, one of the largest in the Sredinnyi Range of Kamchatka. Based on these data, the mafic Pliocene volcanics are thought to be produced by the melting of heterogenized mantle, which had been hybridized by subduction and asthenospheric processes. The behavior of HFSE and Pb isotopic systematics provide evidence of the melting of subducted sediment and origin of pyroxenite segregations in the peridotite matrix. The low 8/4Pb values of the Pliocene lavas of Uksichan shield volcano and in modern large volcanic centers in the Central Kamchatka Depression are correlated with the magmatic productivity, which indicates, when considered together with HFSE and HREE behavior, that the Pacific asthenosphere was involved in the magma-generating processes. The Late Pleistocene-Holocene basalt volcanism, which was distributed over the peripheries of the Pliocene shield edifice, developed in an extensional environment as a result of the melting of an enriched mantle source. The attenuation and then complete termination of volcanic activity in the Sredinnyi Range in the Late Pleistocene-Holocene were associated with an increase in the 8/4Pb of the mafic lavas, which indicates that the center of the activity of the oceanic asthenosphere shifted eastward toward the Central Kamchatka Depression. The influence of the oceanic asthenosphere on subduction-related magmatism is not unique for convergence zones and should be taken into consideration when models are constructed for the origin of juvenile continental crust.