Geochemistry of eclogite- and blueschist-facies rocks from the bantimala complex, south sulawesi, indonesia: Protolith origin and tectonic setting

We present the first data on bulk-rock major and trace element compositions for a suite of eclogite- and blueschist-facies rocks from the Bantimala Complex, Indonesia, with the aim of better constraining the protolith origins and nature of the subducted crust. The eclogites can be classified into two groups: glaucophane-rich eclogite and glaucophane-free eclogite, whereas the blueschists are divided into albite-epidote glaucophanite and quartz-glaucophane schists. SiO₂ contents of the eclogites are 43.3-49.6 wt%, with Na₂O+K₂O contents 3.7-4.7 wt%. The blueschists show a wider range of compositions, with SiO₂=40.7-63.8 wt% and Na₂O+K₂O=2.7-4.5 wt%. Trace element data suggest that the eclogite protoliths include both enriched and normal mid-oceanic ridge basalt (E-MORB and N-MORB) and also gabbroic cumulates. The blueschists show more variation in protoliths, which include N-MORB, Oceanic Island Basalt (OIB) and Island Arc Basalt (IAB). Plots of element concentrations against the immobile Zr show considerable mobility of large ion lithophiles but not of high field-strength elements during high-pressure metamorphism, and indicate that the high SiO₂ content of some blueschists is probably due to metasomatism by a LILE-rich siliceous aqueous fluid. Strong correlations between K, Rb, Ba and Cs suggests that enrichment of these elements occurred by a single process. All the protoliths were subducted, metamorphosed to blueschist/eclogite-facies and subsequently exhumed. It is noteworthy that the samples deduced to have come from thicker-crust environments (OIB, IAB) were subducted to shallower depths (blueschist-facies) than MORB-derived samples, all except one of which reached eclogite-facies conditions. The geochemical data of this study demonstrate the variety of ocean floor types that were subducted under the southeast margin of Sundaland in the late Jurassic period.