Detrital chromian spinels from the Miocene Haccho Formation of the Mineoka Group and the Sakuma Group, Boso Peninsula, central Japan

Abstract

Detrital chromian spinels were newly found from the Early Miocene Haccho Formation of the Mineoka Group and the early Middle Miocene Sakuma Group in the southern part of the Boso Peninsula, central Japan. The detrital chromian spinels from those beds are similar in chemistry, and they are divided into two groups in terms of Ti content, namely low Ti (TiO₂ < 1 wt%) and high Ti (TiO₂ > 1 wt%) groups. Most of the low Ti group spinels chemically correspond to those of the Mineoka serpentinites, which seem to have been derived from the backarc peridotites. However, some of them have higher Cr content, and have been probably derived from the forearc peridotites or boninite. The high Ti group spinels chemically resemble those of alkaline and picritic basalts in the Mineoka, Hayama, Kobotoke and Setogawa Belts. The occurrence of these chromian spinels suggests that the ophiolitic rocks in the Mineoka Belt were emplaced in the Early Miocene or earlier times.