Evolutional History of the Uppermost Mantle of an Arc System: Petrology of the Horoman Peridotite Massif, Japan

Abstract

The Horoman peridotite massif, Hidaka belt, Hokkaido, northern Japan, is composed of three peridotite suites each of which has different age and origin. The first is the Main Harzburgite–Lherzolite Suite (MHLS), which occupies most of the Horoman massif. The MHLS forms a layered complex of harzburgite, spinel lherzolite and plagioclase lherzolite with gradual litho-logical boundaries. Characteristics of mineral chemistry of the MHLS show a residual origin after arc magma extraction. The second is the Banded Dunite—Harzburgite Suite (BDHS), which is characterized by a conspicuous layering of dunite, harzburgite and olivine orthopyroxenite with sharp boundaries. The BDHS peridotites probably make exotic blocks within the MHLS. The BDHS peridotites are cumulates from some high-Mg magma (i.e. high magnesian andesite or boninite), deduced from mineralogical characteristics; high Cr/(Cr + Al) atomic ratio of spinel, high Fo content and low Ni content of olivine. They were formed in a fore-arc setting. The third is the Spinelrich Dunite—Wehrlite Suite (SDWS), which always occurs as layers in the harzburgite of the MHLS. The SDWS is mainly composed of dunite enriched with chromian spinel and sometimes with clinopyroxene. The mineral chemistry indicates that the SDWS is of cumulus origin from the magma which had been released from (or coexisted with) the surrounding MHLS harzburgite. The Horoman peridotite massif is, therefore, a fragment of arc mantle which had evolved from a fore-arc mantle. Tectonics and geology of Hokkaido indicate that the Horoman massif had been derived from the southwestern margin of the Okhotsk (or North America) plate.