Geological study has been carried out on the Adatara volcano, belonging to the Nasu volcanic zone in northeastern Japan, and petrological investigations have been made on the volcanic rocks from the volcano.
Adatara volcano consists of three cones; Maegatake, old Osyozan cones, and Adatara main cone. The sequence of the formation of the cones is as follows:
First stage: Maegatake cone consisting of calc-alkali andesite.
Second stage: Old Osyozan cone consisting of tholeiitic basalt and andesite.
Third stage: Adatara main cone consisting of cale-alkali andesite and its associated “tholeiitic” andesite which is subordinate tholeiite (Kawano et al., 1961).
Comparing calc-alkaline rock with tholeiitic rock whose SiO₂ content is nearly equal to that of the calc-alkaline rock, the calc-alkaline rock is enriched in K₂₀ more than that of the tholeiitic rock. This chemical difference cannot be explained by simple fractional crystallization from their common parental magma. The subordinate tholeiite erupting during the third stage might have been derived from the parental magma which formed the series of calc-alkaline rocks.