Stratigraphy of Kurohana-Yama basalt, Sendai: Pigeonite bearing basalts erupted in the period between late Miocene and early Pliocene

Abstract

Detailed stratigraphical investigation revealed that pigeonite-phyric basalts from Kurohana-Yama, Sendai, Japan erupted at late Miocene or early Pliocene, with the age of the eruption is younger than 8 Ma. It is because K-Ar ages of dikes which are stratigraphicaly older than Kurohana-Yama lavas are about 8 Ma (Tsunakawa et al., 1983).
     The pigeonite basalts from Kurohana-Yama are chemically devided into two types; type A basalts contain higher MgO (7 wt%) and lower Al₂O₃ (17-18 wt%) than type B basalts (MgO 5.2-5.5 wt% and Al₂O₃ 19-20 wt%). All these basalts contain plagioclase and olivine phenocrysts .Assemblages of phenocryst pyroxenes are various and most of the basalts contain large pigeonite phenocrysts.