Relict clinopyroxenes of Mikabu Green Rocks of the Ogose area and of greenstones of the Chichibu terrain of the Agano area, Kanto Mountains

Abstract

The original igneous rocks of weakly metamorphosed greenstones are basalt-dolerite and limburgite in Mikabu Green Rocks of the Ogose area, and basalt-dolerite-bostonite and limburgite in the Chichibu melange zone of the Agano area, both of Kanto Mountains. Relict phenocrystic clinopyroxenes of basaltic and limburgitic rocks of both areas are chromian augite (Cr₂O₃ 0.4-0.8 wt.%) to chrome augite (Cr₂O₃ more than 0.8 wt.%), but, in general, comparatively poor in TiO₂ (less than 0.1 wt.%). However, limburgitic pyroxenes of the Chichibu melange zone are slightly richer in TiO₂ and Cr₂O₃ and higher in Mg/(Fe+Mg) than those of the Mikabu rocks. The phenocrystic clinopyroxenes are generally similar in Cr₂O₃/TiO₂ ratios to clinopyroxenes of ocean-floor basalts and of peridotite xenoliths derived from the mantle.