Petrology of exotic high-Mg, Cr peridotite bodies in the Horoman Peridotite Complex, Japan

Abstract

We investigated exotic peridotite bodies, which are characterized by high Mg, Cr minerals, in the Horoman Peridotite Complex, Japan. The high Mg, Cr peridotites appear to be blocks in specific horizons of a layered structure in the main peridotite body. Foliations and lineations in the high Mg, Cr peridotite are concordant with those in the surrounding peridotites. A clinopyroxene rich high Mg, Cr peridotite (called cpx bearing high Mg, Cr peridotite) was newly discovered. Modal variations and chemical compositions of minerals in the high Mg, Cr peridotites suggest their crystal cumulus origin from a melt rather than residual origin after high degree of partial melting. Trace element characteristics of clinopyroxene in the cpx bearing high Mg, Cr peridotite are similar to those of clinopyroxene related to the formation of a high Mg andesite. Based on results in this study combined with the previous work, the evolutionary history of the Horoman Complex including the high Mg, Cr peridotites is as follows. The high Mg, Cr blocks and the main body of the Horoman were formed at different tectonic settings, a subduction related environment and a mid ocean spreading ridge, respectively. As the tectonic setting of the main body was changed from an ocean floor to a subducting oceanic lithosphere, the high Mg, Cr peridotites could be incorporated into the main body as tectonic blocks during the subduction of the main body. Then these diverse rocks were simultaneously deformed during their exhumation from the upper mantle conditions to the surface.