Abstract
Picrite basalt was found in the Paleogene Kobotoke Group, central Japan. The picrite basalt occurs as a small mass enclosed by greenish sandstone and mudstone having clasts derived from picrite basalt and related rocks. The picrite basalt mass is a large boulder emplaced in a similar stratigraphical position to serpentinite masses in the Kobotoke Group. Phenocrysts of magnesian olivine (Fo89-90) with opaque spinel inclusions occupies more than 50 volume % of the rock. The spinel is Cr-rich, having the Cr/(Cr+Al) atomic ratio of around 0.7, and is also Ti-rich, having 1 to 3 wt % of TiO2. The rock is characterized by high Ti/(Na+K) and Na/(Na+K) ratios. The picrite basalt from the Kobotoke belt is quite similar to those from other Circum-Izu massif areas (the Mineoka and Setogawa belts) in terms both of the mineral chemistry and of the wholerock chemistry. It is noteworthy that the petrological characteristics of the Circum-Izu massif picrite basalts are identical to those of the Hawaiian tholejites.
