Petrochemical studies of the Cenozoic basaltic rocks from the Sera plateau, Hiroshima Prefecture

Abstract

In the Sera plateau there exist many domes of basaltic rocks, which belong mostly to the alkalic rock series and partly to the calc-alkalic rock series. They are believed to have been erupted from the Pleistocene to the Recent. From petrography and petrochemistry, basaltic rocks are classified as follows: picrite-basalt, olivine-basalt, augiteolivine-basalt, olivine-augite-hornblende-basalt, andesitic basalt, andesite and hypersthene-augite-basalt. Volcanic activity began with the eruption of talc-alkalic rocks, which appeared sporadically. After this minor activity, large amounts of alkalic and calc-alkalic rocks were erupted over the whole area. Among them, hypersthene-augite-basalt contains many granitic xenoliths and xenocrystic plagioclase, quartz, pyroxene and hornblende. It seems likely that the rock was produced by the contamination of alkali olivine basalt magma. Chemically, these basaltic rocks are characterized by high content of K2O and the rocks of the alkalic series contain normative nepheline, and those of the talc-alkalic series have normative quartz when one calculates the normative composition.