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Geochemistry of eclogite- and blueschist-facies rocks from the bantimala complex, south sulawesi, indonesia: Protolith origin and tectonic setting
Maulana A.
Island Arc
Q2Abstract
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 B antimala C omplex, I ndonesia, 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. 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 , O ceanic I sland B asalt ( OIB ) and I sland A rc B asalt ( IAB ). Plots of element concentrations against the immobile Z r show considerable mobility of large ion lithophiles but not of high field‐strength elements during high‐pressure 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 , R b, B a and C s 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 J urassic period.