2006 年 35 巻 5 号 p. 231-243
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.