Abstract
The Asaji metamorphic rocks have previously been considered as a member of the Ryoke belt. However, the occurrence of ultramafic bodies interlayered in the Chokai Unit and the Hikata Unit has led to an alternative suggestion for the correlation of the Asaji metamorphic rocks with other terranes, such as the Sambagawa terrane or the Kurosegawa terrane. To discuss these possibilities, we examined the petrological characteristics of the ultramafic rocks and chemical compositions of chromian spinel.
The ultramafic body is composed mainly of pyroxenites and serpentinite. Spinel in the serpentinite has high Cr# (Cr/(Cr+Al) atomic ratio) of 0.7-0.9 and very low TiO2 content (<0.5wt%). Although the ultramafic body suffered weak contact metamorphism, chemical composition of chromian spinel has preserved those in the peridotite stage. The spinel chemistry suggests that the original peridotite is a refractory residue after the extraction of basaltic magma. The pyroxenites are subdivided into clinopyroxenite, olivine-clinopyroxenite and olivinewebsterite. Compositions of clinopyroxene from the pyroxenites are similar to those of arc cumulates. It suggests that the peridotites (now completely serpentinitized) and pyroxenites were formed in a subduction zone.
Chemical compositions of spinel in the serpentinites from the Saganoseki area (the Sambagawa metamorphic belt), the Sashu Fault on the southern margin of the Sambagawa metamorphic belt, and the Miemachi and Usuki area (the Kurosegawa belt) exposed near the Asaji metamorphic rocks were analyzed for comparison. Spinels from the Saganoseki area are clearly different in Cr# and TiO2 content from those from the Asaji area and also from the Sashu Fault. On the other hand, the chemical compositions of spinels from the Miemachi and Usuki area (the Kurosegawa belt) resemble to those from the Asaji area, suggesting a similar tectonic setting for the formation of those ultramafic rocks.
