Origin of three peridotite suites from Horoman peridotite complex, Hokkaido, Japan; Melting, melt segregation and solidification processes in the upper mantle.

Abstract

The Horoman peridotite complex, Hidaka belt, Hokkaido, northern Japan, is composed of three peridotite suites which have distinct petrographic characteristics. The first is the Main three peridotite suites which have distinct petrographic characteristics. The first is the Main Harzburgite-Lherzolite (MHL) suite, which occupies the most part of the Horoman peridotite complex. The MHL suite consists of harzburgite, spinel lherzolite and plagiocalse lherzolite, which form a layered complex with gradational layer boundaries. Two-pyroxene spinel symplectite, which is interpreted as a chemically modified garnet pseudomorph, occurs in spinel and plagioclase lherzolites. The MHL suite is of typical residual origin after various degree of magma extraction, which resulted in wide chemical variations. The second, the Banded Dunite-Harzburgite (BDH) sutie, consists of conspicuously layered dunite, harzburgite and olivine orthopyroxenite with sharp layer boundaries. The BDH peridoties are pobably exotic blocks within the MHL suite. Mineralogical characteristics of the BDH suite suggests that it is a cumulate from a high-Mg magma (e. g., high-magnesian andesite). The third, the Spinel-rich Dunite-Wehrlit (SDW) suite, consists of dunites containing abundant large oblate chromian spinel grains. The dunite becomes locally wehrlitic and contains clinopyroxene up to 8%. The SDW occurs in the MHL harzburgite zones as layers with maximum thickness of 15m. The layer boundaries with the harzburgite of the MHL suite are generally sharp. The petrographical and mineralogical characteristics show that the SDW is a cumulate from a magma segregated from the surrounding MHL suite. The melt segregation may be spreading away from the upwelling center as a corner flow. A series of layered residual peridotite (MHL suite) zones, of which degree of melt extraction increases inwards, formed around the SDW dikes.