A 3D velocity model of the Earth's crust beneath the Klyuchevskoy volcanic group has been constructed using the seismic tomography method. Anomalies of the velocity parameters related to the zones of magma supply to active volcanoes have been distinguished. Petrological data on the composition, temperature, and pressure of generation and crystallization of primary melts of Klyuchevskoy volcano magnesian basalts have been obtained. The primary melt corresponds to picrite (MgO = 13-14 wt %) with an ultimate saturation of SiO2 (49-50 wt %), a high H2O content (2.2-2.9%), and incompatible elements (Sr, Rb, Ba, Hf). This melt is formed at pressures of 15-20 kbar and temperatures of 1280--1320С . Its further crystallization proceeds in intermediate magma chambers at two discrete pressure levels (i.e., greater than 6, and 1-2 kbar). The results of the petrological studies are in good agreement with the seismotomographic model.

Numerous summit and parasitic eruptions of moderate potassium magnesian and high-alumina basalts and basaltic andesites, their mineralogic and geochemical features, and the composition of in situ chilled melt inclusions in the olivine of cinder lapilli discharged by Klyuchevskoi Volcano all provide evidence of the presence of magma chambers beneath the volcano. This is also supported by a dualism in the variation of CaO and A1203 concentrations in olivine and clinopyroxene during crystallization. The mineralogic features in the high-alumina basalts that were discharged by all parasitic eruptions of Klyuchevskoi provide evidence of magnesian magma being emplaced from a deeper chamber into a shallow high-alumina chamber. The distribution of incoherent elements in the volcano's magnesian and aluminiferous rocks shows that they came from a single mantle source. The geochemical and mineralogic data are in good agreement with the results of geophysical surveys that concern the structure and properties of the lithosphere beneath Klyuchevskoi.

The article presents new data on magmatic rock composition from the Stolbovaya formation, Kamchatsky Mys Peninsula. The studied rocks belong to high alumina tholeiitic island arc series and their geochemical characteristics are identical to those from Kronotskaya formations in Kronotsky Peninsula. Magmanicsedimentary complexes of the Stolbovskaya formations were deposited within the volcanic archipelago of the Kronotsky volcanic arc. Geochemical and isotopic data from the studied rocks give evidence that they were formed from a depleted mantle source. The certain observed differences in Eocene rock composition from Kamchatsky Mys and Kronotsky peninsulas are likely caused by differences in fluid regime.