Abstract
Lavas and pyroclastic rocks belonging to the Joboji Andesite Member of Itsukaichi Formation (Miocene) and to the Inaniwadake Formation (Pliocene) distribute in the Inaniwadake volcanic district, northern part of the volcanic front of NE Japan arc. The former are basaltic andesite to andesite and the latter basalt. All of the volcanic rocks are classified into extremely low-K tholeiitic series. Pigeonites occur as phenocrysts, microphenocrysts, groundmass minerals and reaction rims around orthopyroxene phenocrysts.
These tholeiites can be divided into two sub series (the Joboji and the Inaniwadake tholeiites). The Inaniwadake tholeiites are more typically tholeiitic than the Joboji and the Quaternary tholeiites on the volcanic front. Magnetite phenocrysts are rarely found in the Inaniwadake tholeiite but are common in the Joboji and the Quaternary tholeiites. The Inaniwadake magmas are characterized by lower oxygen fugacity and H2O content and by higher temperature (high-temperature type tholeiite) than the Joboji and the Quaterenary magmas (low-temperature type tholeiite) based on the whole-rock and mineral chemistry. The high-temperature type tholeiites of Pliocene age are not restricted to study area but also occur in Kurohanayama and Sasamoriyama, southern part of the volcanic front. Temporal decrease in temperature of magmas is consistent with an increase in volume of coexisting calc-alkaline magmas from Pliocene to Quaternary.
