Miocene tectonic of the Southeast Arm of Sulawesi, Indonesia: Based on petrology data, geochemistry, and ⁴⁰ Ar/ ³⁹ Ar geochronology of metamorphic rocks from Rumbia Complex

Abstract

The Rumbia Complex or the Rumbia Mountains is more popularly referred to as Rumbia Schist Complex, located in the Southern Part of the Southeast Arm of Sulawesi. It is one of the regional metamorphic rock complexes in Sulawesi. This area became popular when the discovery placer gold deposit in Bombana area around Rumbia Mountains, in 2007. This study aims to identify metamorphic rocks of the Rumbia Complex including petrology, geochemistry, and radiometric age dating to determine the evolution of the rock, as well as its relation to the tectonic evolution of the Southeast Arm of Sulawesi. The research methodology includes a geological survey to see the distribution of rock units as well as representative sampling and create a geological map of the Rumbia Complex. Furthermore, thin section analyses with polarization microscope for rock identification, as well as geochemical analysis to obtain Rare Earth Element (REE) usingLA-ICP-MS to determine rock protoliths. While radiometric age dating used 40 Ar/ 39 Ar geochronology to determine rocks evolution, and the interpretation of geological events in the Rumbia Complex, and or in the Southeast Arm of Sulawesi. The research results showed that Rumbia Mountains is dominated by metamorphic rocks (Mica Schist, Chlorite Schist, and Glaucophane Schist). The result of thin section analysis it is known that some of Glaucophane Schist have undergone retrogradation into Chlorite Schist. While consistent of the Rare Earth Element (REE), it is known that the metamorphic rock protolith of Glaucophane Schist, and Chlorite Schist are "Oceanic Island Basalt" (OIB). Whereas based on radiometric age dating results it is known to have occurred three times large geological events in the Rumbia Complex during Miocene (23 Ma, 20 Ma, and 17 Ma). It is suspected to be related to the amalgamation events of the three microcontinents (Rumbia, Mekongga, and Meluhu Microcontinents) formed the Southeast Arm of Sulawesi as we know today.