Formation process of olivine-clinopyroxene cumulates inferred from Takashima xenoliths, Southwest Japan arc

摘要

Rocks of the dunite-clinopyroxenite series (dunite, wehrlite, olivine clinopyroxenite, and clinopyroxenite) are common as cumulates formed around Moho in arc-related environments, but their formation processes remain unclear. They are common as xenoliths of Group I from Takashima in the Southwest Japan arc, and a description of their formation process is provided here. The rocks vary from dunite to clinopyroxenite via wehrlite and olivine clinopyroxenite, and all showing mosaic equigranular to weakly porphyroclastic textures. The rocks are completely free from plagioclase, and they contain ＜3 vol% chromian spinel. Some of them contain up to 3 vol% orthopyroxene; these are approximate mixtures of olivine and clinopyroxene. As the dunites change to clinopyroxenites, the Mg# [= Mg/(Mg + total Fe) atomic ratio] varies from 0.93 to 0.84 in olivine, and from 0.92 to 0.87 in clinopyroxene. The Cr# [= Cr/(Cr + Al) atomic ratio] of the chromian spinel varies from 0.8 to 0.2 with decreases in the Mg# of olivine and clinopyroxene. The Mg-Fe distribution relation between olivine and clinopyroxene suggests their subsolidus equilibration is around 800-900 ℃. Initial Mg#s expected at a magmatic temperature indicate that their formation proceeded from magma in the order of Mg-rich dunites followed by clinopyroxenites and then less Mg-rich dunite-wehrlite-olivine clinopyroxenite. This suggests a zigzag liquid path, starting from a mantle-derived olivine-oversaturated magma, around the olivine-clinopyroxene cotectic boundary. Continuous crystallization of olivine or clinopyroxene due to supersaturation could have enabled the magma to straddle the cotectic boundary to form alternately clinopyroxene- and olivine-oversaturated magmas.