The Petrology of Eclogites from Bantimala Complex, South Sulawesi, Indonesia

Eclogites occur as tectonic blocks within blueschist in the Mesozoic Bantimala basement complex of South Sulawesi. The complex consists mainly of high-pressure Triassic-Jurassic metamorphic rocks and Cretaceous sediments and ultramafics. Whole-rock geochemistry suggests a wide range of origin for the eclogites. Three kinds of eclogite from eclogite-facies assemblages have been distinguished from this area; (a) apatite-bearing eclogite (AEC), (b) glaucophane eclogite (GEC) and (c) foliated epidote-quartz eclogite (FEC). All of these eclogites are alpine-type metamorphic rocks or “group C” eclogites in the classification of Coleman et al., (1965), from high- pressure, low temperature environments and associated with glaucophane schist. Typical mineral assemblages composed of garnet + clinopyroxene + phengite + rutile were identified as being equilibrium for different stages of metamorphism based on careful petrographic studies. The mineral assemblages and compositions preserve evidence of metamorphic evolution from prograde to peak metamorphic and retrograde stages. The prograde stage was indicated by preservation of prograde minerals within garnet porphyroblasts and the prograde growth zoning in garnet cores. Peak metamorphic stage was exhibited by the occurrences of the garnet + sodic pyroxene + glaucophane + epidote + phengite + chlorite + quartz + rutile mineral assemblages. Finally, the retrograde stage was evident by hydrous mineral reaction rims on garnet, particularly chlorite, epidote and glaucophane. P-T estimates using the garnet­clinopyroxene geothermobarometer for the eclogite facies metamorphism indicate minimum pressures of some 18-24.6 kbar and temperatures of approximately 540 – 615^oC.

Reference :
Adi Maulana, Andrew G. Christy, David J. Ellis, The Petrology of Eclogites from Bantimala Complex, South Sulawesi, Indonesia