Abstract
Petrography (chemical analyses of rocks 15, clinopyroxenes 2, orthopyroxene 1, titanomagnetites 3; microscopic observation and compositional estimation of rock-forming minerals) of lavas of the Kirishima volcano are presented, with special reference to the distinguishable relation between olivine and pyroxene, which is able to make genetical classification of subalkaline rocks. Considering the thermodynamic knowledge and the data of olivines and pyroxenes obtained by Bowen and Schairer (1935) and Osborn and Muan (1956-59), the writer was induced to the following approach. If the oxygen partial pressures are sufficiently supplied, it may be tend to lower the temperature of crystallization. Mg-Fe distribution constant K(t) of olivine and pyroxene, and entropy changes _??_S of those mineral formation in magma.
From the theoretical consideration on the petrographic properties of the lavas, the author proposed a hypothetical phase diagram MgSiO3-Fe SiO3 join, by which are interpreted most facts; i.e. occurrence of myrmekitic magnetite in pyroxene, reverse zoning of orthopyroxene, abnormal relation of phenocryst pyroxene and groundmass one (Taneda 1943, 1947), and no pigeonite inverted from orthopyroxene, in calc alkaline rocks.
And by the petrochemical consideration, the writer agree with Yoder and Tilley's (1962) idea which the so-called “High alumina basalt magma” is not necessary as the original magma.