2016 年 111 巻 3 号 p. 211-225
This contribution reports the metamorphic evolution of the high–pressure metamorphic rocks from the Bantimala Complex, South Sulawesi, Indonesia. Barroisite–bearing and barroisite–free eclogites were examined to assess their metamorphic evolutions, which have implications regarding the tectonic conditions in this region. The eclogites mainly consist of garnet, omphacite, phengite, rutile, and epidote, with or without barroisite. The variations in mineral assemblages are interpreted to depend upon local changes in the bulk chemical composition. The barroisite–bearing eclogites contain two types of euhedral garnet: coarse– (1–1.5 mm) and fine–grained (<0.5 mm). Mineral inclusions in the coarse–grained garnet core and mantle show epidote + titanite and glaucophane + epidote assemblages, that stabilized at 0.9–1.5 GPa and 350–550 °C within epidote blueschist–facies conditions. Mineral chemistry and chemical–mapping analyses indicate that both fine–grained garnet and the rim of coarse–grained garnet formed at peak P–T conditions, which were estimated as 2.3–2.7 GPa at 615–680 °C based on the garnet–omphacite–phengite–quartz equilibrium. Peak P–T conditions for barroisite–free eclogite were similar (2.5–2.7 GPa at 650–690 °C) to those for barroisite–bearing eclogite. Actinolite rims overgrowing matrix sodic–calcic amphiboles attest to retrogression at P < 0.5 GPa and T < 350 °C in a clockwise P–T path. The very low geothermal gradient experienced during the prograde path (~ 5 °C/km) likely suggests the subduction of an old and cold oceanic crust. The low geothermal gradient on the retrograde path suggests decompressional cooling during exhumation, possibly favored by a serpentinite–dominated matrix within a subduction channel environment.
