The main effusives of the ancient and present cones of Tyatya Volcano differ in the content of CaO, Al2O3, Sr, K2O and most incoherent elements. Geochemical evidence and also results of numerical modeling suggest that basalts of the initial stage of formation of the volcano were the result of the relatively high degree of melting of mantle source and subsequent deep fractionation in its rise to the surface accompanied with plagioclase accumulation. Considering that the lavas of the ancient cone of Tyatya Volcano are in many respects petrologically close to basic volcanics of the Miocene stage of Kunashir Island formation, the results of the relevant investigations can have implications for understanding the features of the evolution of volcanism in the Kuril island arc on the whole.

New data on petrography, chemical and mineral composition of Pleistocene- Holocene basaltic lavas from shield volcanoes and scoria cones of Uksichan volcanic field (the Central Kamchatka volcanic belt, Kamchatka peninsula) show that they originate from a common magma source and parental magma, but with different degree and types of fractional crystallization. The computer simulation evidences that the lavas from late shield volcanoes were formed during isobaric crystallization (H2O ~ 2 wet %, ƒО2 – +1.2 NNO) under a pressure of 5 to 1 kbar with a step of 1 kbar. Such physical conditions correspond to relatively slow rising of magma chambers from depth of ~ 15 km to ~ 3 km. Scoria cones were generated during polibaric fractional crystallization (H2O ~ 2.6 wet. %, ƒО2 – +1.1 NNO) with decompression speed of 0.25 kbar / % crystallization. This type of differentiation assumes the relatively fast melt transport without a long storage in the crust magma chambers. Petrochemical indicators of two differentiation types are behavior of CaO and Al2O3.