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

Abstract

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 glaucophanefree eclogite, whereas the blueschists are divided into albite-epidote glaucophanite and quartz-glaucophane schists. SiO 2 contents of the eclogites are 43.3-49.6 wt%, with Na 2 O + K 2 O contents 3.7-4.7 wt%. The blueschists show a wider range of compositions, with SiO 2 = 40.7-63.8 wt% and Na 2 O + K 2 O = 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 highpressure metamorphism, and indicate that the high SiO 2 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 (blueschistfacies) 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.

The distinctive tectonic and metamorphic history of the Barru Block, South Sulawesi, Indonesia: Petrological, geochemical and geochronological evidence

“…F and Cl are below detection limit. Zircon from the metamorphic rocks in Barru Block has similar Hf content (up to 2.08wt% HfO 2 = 1.83 mol% HfSiO 4 ) to that from the Bantimala Block (Maulana, 2009;Maulana, et al 2013) ( Table 7).…”

“…In contrast to the Bantimala Block (Maulana et al, 2013), metamorphic rocks in the Barru area have only experienced greenschist-and/or amphibolite-facies metamorphism. These rocks comprise a diverse range of lithologies including chloritic phyllites, quartzite, chlorite schists, garnet-mica schists, quartz-mica schists, amphibolites and quartzofeldspathic gneisses.…”

“…All the garnets plotted in the 'C' field of Coleman et al (1965) (Fig. 3.a) and are significantly different in pyrope content from those of the blueschist and eclogite in the Bantimala Block (Maulana et al, 2013).…”

“…In contrast to the Bantimala area where the presence of eclogite-and blueschist-facies rocks documents exhumation of deeply-subducted oceanic crust, metamorphic rocks from the Barru Block were returned to the surface from a much shallower depth. This may result from a shallower angle of subduction and/or that the subducted Barru crust was younger, warmer and thus more buoyant (Maulana et al, 2013). Metabasic rocks in the Barru Block are amphibolites, which are completely absent in the Bantimala Block.…”

“…In contrast to the Bantimala Block which has received much attention (e.g. Sukamto, 1982;Wakita et al, 1996;Miyazaki et al, 1996;Parkinson et.al., 1998;Parkinson, 1998;Maulana et al, 2013;Setiawan et al, 2013), the Barru Block still remains poorly studied, despite its potential significance for understanding the geological processes operating at the eastern margin of Sundaland in the Late Mesozoic.…”