Geochemistry of eclogite- and blueschist-facies rocks from the Bantimala Complex, South Sulawesi, Indonesia: Protolith origin and tectonic setting

摘要

We present the first data on bulk-rock major and trace element compositions foruda suite of eclogite- and blueschist-facies rocks from the Bantimala Complex, Indonesia,udwith the aim of better constraining the protolith origins and nature of the subducted crust.udThe eclogites can be classified into two groups: glaucophane-rich eclogite and glaucophanefreeudeclogite, whereas the blueschists are divided into albite???epidote glaucophanite andudquartz???glaucophane schists. SiO2 contents of the eclogites are 43.3???49.6 wt%, withudNa2O + K2O contents 3.7???4.7 wt%. The blueschists show a wider range of compositions,udwith SiO2 = 40.7???63.8 wt% and Na2O + K2O = 2.7???4.5 wt%. Trace element data suggest thatudthe eclogite protoliths include both enriched and normal mid-oceanic ridge basaltud(E-MORB and N-MORB) and also gabbroic cumulates. The blueschists show more variationudin protoliths, which include N-MORB, Oceanic Island Basalt (OIB) and Island ArcudBasalt (IAB). Plots of element concentrations against the immobile Zr show considerableudmobility of large ion lithophiles but not of high field-strength elements during highpressureudmetamorphism, and indicate that the high SiO2 content of some blueschists isudprobably due to metasomatism by a LILE-rich siliceous aqueous fluid. Strong correlationsudbetween K, Rb, Ba and Cs suggests that enrichment of these elements occurred by a singleudprocess. All the protoliths were subducted, metamorphosed to blueschist/eclogite-faciesudand subsequently exhumed. It is noteworthy that the samples deduced to have come fromudthicker-crust environments (OIB, IAB) were subducted to shallower depths (blueschistfacies)udthan MORB-derived samples, all except one of which reached eclogite-facies conditions.udThe geochemical data of this study demonstrate the variety of ocean floor typesudthat were subducted under the southeast margin of Sundaland in the late Jurassic period.