GEOCHEMICAL CHARACTERISTIC OF METAMORPHIC ROCKS FROM SOUTH SULAWESI, CENTRAL JAVA, SOUTH AND WEST KALIMANTAN IN INDONESIA

Abstract

Various metamorphic rocks are exposed in the central part of Indonesia, including the islands of Java, Kalimantan and Sulawesi. Metamorphic complexes, regarded as products of Cretaceous subduction of Indonesia outcrop in Central Java, South Kalimantan, and South Sulawesi. The lithology in these locations is predominantly composed of high-pressure metamorphic rocks. Moreover, metatonalite is exposed in the Schwaner Mountains of West Kalimantan. This study describes the geochemical character of the metamorphic rocks to consider the original rock composition and ascertain their relationship to the tectonic environments in the area. The geochemical characteristics suggest that protolith of the metamorphic rocks from South Sulawesi, Central Java and South Kalimantan can be categorized as metabasic and metamorphosed sedimentary rocks (pelite to greywacke). The origin of the metabasic rocks from South Sulawesi and Central Java is considered as alkali basalt or sub-alkali andesite to basalt. The protolith is characterized by signatures pointing to N-MORB, E-MORB and within-plate basalt settings with a tholeiitic nature. Eclogites and blueschists from South Sulawesi contain both MORB and withinplate basalt signatures that might indicate several ocean islands existed and subducted together with oceanic floor composed of MORB during the Cretaceous. Meanwhile, eclogites and blueschists from Central Java mostly show within-plate basalt signatures, whereas amphibolites and garnet amphibolites are characterized by MORB. These results suggest several possibilities: a different component between the upper- and lower-oceanic crusts; a difference in the metamorphic ages between eclogite- and amphibolite-facies metamorphism; and a change of the subduction angle between the two metamorphisms. Metatonalites from the Schwaner Mountains are calc-alkaline rocks derived from volcanic-arc tectonic environments. Two samples indicate adakitic nature. One sample of adakitic metatonalite has the age of 233 ± 3 Ma (Late Triassic) that could be divided from the major Cretaceous non-metamorphosed granite event in the same region. These might imply that the subduction mechanism and felsic magma genesis changed between the Early Triassic and the Cretaceous. Some metatonalites show similar signature to the Cretaceous granite, indicating that subsequent metamorphism occurred during the Cretaceous subduction system.