The article presents new data on isotopic-geochemical composition of igneous rocks from Kumroch segment of Achayvayam-Valagin Late Mezosoic ― Early aleozoic volcanic arc. It is shown that three series of rocks may be distinguished in composition of island-arc basalt-andesite lava of volcanogenic-tuffaceous unit of Khapitskaya suite: low-potassium (tholeiitic), medium-potassium (calc-alkalic) and high-potassium (alkalic). These rock series differ in K2O content and proportion of femic components and SiO2. Compositions of the studied rocks were compared with composition of basalts from the Idzu-Bonin Island Arc. The obtained results suggest that the samples of medium-potassium and high-potassium groups were formed in back arc zone of the volcanic arc and low-potassium tholeiites were possibly formed in the inter-arc rift.

The peculiarity of extrusive rocks of Mendeleev Volcano draws attention of researchers. According to previous research, the Mendeleev Volcano is telescoping caldera, composed of two shafts, the central cone and final extrusion. However, there are no any cuts that characterize the morphological structure of these elements, in the literature. Conducted in 1991, field studies have allowed us to design a more simple circuit of Mendeleev Volcano structure, as well as the study of the cut of Mechnikov Mountain's caldera shaft enabled us to offer the following explanation of the genesis of the extrusion of the Mendeleev Mountain.
Basalts that filled the peripheral focus of Mendeleev Volcano evolved till andesite-dacite by fractional crystallization. Fluid accumulation in the chamber has led to liquid immiscibility of residual magma to dacitic and andesitic compositions. Being thrown out with final explosion they formed a patch of contrast rocks: white dacitic pumices and black andesitic slags. Magma remnants of contrast compositions, losing fluids, were pressed out as andesite-dacite extrusions containing anomalous phenocrysts parageneses.

Data are presented about the chemical and isotopic composition of fumarolic gases and thermal water collected in the crater of the Mutnovskiy volcano from 1963 to 1989. A thermodynamic approach, balance estimates and information on the hydrogeological characteristics of the Mutnovskiy geothermal field and volcanic structure were used in the interpretation of the data. Three groups of fumarolic vents in the craters of the volcano have been active for many years (the Aktivnaya cone, the Verkhneye field and the Donnoye field) and differ in the composition of their gases, the isotopic composition of the water, a activity and He ratios. It was shown that the composition of the fumarolic gases is formed as a result of the interaction between the magmatic fluid, the surface and thermal waters and also the boiling of ultra-acidic brines-the mixed waters that form the local hydrothermal system of the Donnoye field in the crater of the Mutnovskiy volcano.