The circum-Izu Massif peridotite, central Japan, as back-arc mantle fragments of the Izu-Bonin Arc system

Abstract

The Circum-Izu Massif peridotite, has been uplifted along a transcurrent plate boundary during the opening of the Shikoku Basin. The peridotite was investigated in order to understand petrological characteristics of the upper mantle beneath the back-arc (or inter-arc) basin. It is characterized by harzburgite or clinopyroxene-poor lherzolite (Fo90-92) frequently with calcic plagioclase (An88-96). Chromian spinel (Cr/(Cr+Al) ratio, 0.4-0.6) often has orbicular primary inclusions of pargasite +K- (or Na-) phlogopite + orthopyroxene. The back-arc mantle peridotite resembles some refractory ocean-floor peridotite but generally has lower-Ti mineral chemistry. Unlike the ocean-floor peridotite the back-arc mantle peridotite could have primary hydrous minerals. -from Author