LAUSR

We present new geological and isotopic geochemical data on rock compositions at the earlier stage (basement, the shield edifice) in the evolution of the Tolbachik Volcanic Center and the basement plateau of Ushkovskii Volcano. We used additional material from the literature to get a general picture of the evolution of both centers of volcanism. The increasing concentrations of nearly all trace components (except Sr) combined with decreasing degrees of differentiation in most major components and Sr isotope ratios in rocks of the Tolbachik Center are controlled by the change from a lithospheric mantle reservoir to an asthenosheric one and by a decreasing degree of melting in the reservoirs (25–20% → 15–10% → <5%). The change of mantle sources and the diminution of their endogenous activity may indicate a decay of the Tolbachik Center volcanism. Ushkovskii Volcano has no shield edifice, but the composition of this stratovolcano itself shows many signs indicating rocks of a basement plateau. Here, an asthenospheric mantle reservoir was evident at the very beginning, although the rocks show both island-arc and intraplate geochemical characteristics.