Abstract
Volcanic activity in the Akan volcanic field in the southwestern end of the Kurile arc began in early Pleistocene and formed stratovolcanoes. The somma lavas are tholeiite series and range from 51-60 wt% SiO2. The eruption of voluminous calc-alkali rhyolitic magma (68-73 wt% SiO2) with caldera formation took place at middle Pleistocene. The trace-element characteristics of the volcanic rocks from the Akan volcanic field suggest that the felsic magma implies a difficulty for its derivation from basaltic magma such as the somma lava. Modeling the trace element processes in volcanic rocks indicate that the most basic rock of the somma plots near partial melting 20% of the mantle source with the fractional crystallization subsequent to the partial melting. On the other hand, the Akan pyroclastic flow deposits may be produced by variable degree of partial melting of an andesitic crust source with local heterogeneity of the source material. The most plausible heat source for the genesis of the felsic magma may be latent heat from the mantle-derived basaltic magma such as the somma lava.
