Petrological studies of the alkali basalts from eastern San-in (1)

Basalts of Kibi-plateau

Abstract

Basalts of Kibi plateau are exposed as many domes of small scale and a few lava flows as well which were erupted in Pleistocene age. They all belong to the alkali rock series and are classified into olivine or titanaugite olivine trachybasalt, titanaugite olivine trachydolerite and titanaugite olivine trachyandesite. Eleven of them are chemically analysed. The distinct characteristics of the rocks of this province are as follows : 1) They are chemically richer in MgO and poorer in Al₂O₃ because of the abundance of olivines, titanaugites and xenocrystic clinopyroxenes. 2) Some have properties which closely resemble those of limburgite in both mineral and chemical constituents. 3) Olivine and pyroxene rich inclusions are contained in the rocks of this province, and especially in Arato-yama and Yunodan-yama are found the largest occurrence of the inclusions in Chugoku area. 4) Zeolite like mineral occurs frequently in the marginal part of the inclusions.