Geochemistry of the Adang Volcanics in Western Sulawesi: Unveiling the tectonic evolution of the opening of the Makassar Strait

Abstract

The Makassar Strait, located in central Indonesia, serves as a divider between Western and Eastern Indonesia and remains a subject of ongoing debate regarding its formation. Two primary theories have been proposed: (1) the Makassar Strait opening established a mid-ocean ridge (MOR) and led to a double subduction, and (2) it was formed through intracontinental rifting accompanied by mantle-plume magmatism. In this study, we present findings on the Adang Volcanic Complexes in Western Sulawesi, situated on the eastern side of the Makassar Strait, which have exposures of the Middle to Late Miocene age interspersed with crystalline limestone and carbonate rocks, indicating a correlation with a marine environment. The rocks investigated in this study are predominantly low-silica mafic rock, mainly composed of leucite/pseudoleucite-bearing trachytic tuff, agglomerate volcanic breccia, lavas, peralkaline dykes, and multiple mafic and leucitic intrusions. Geochemical analysis of the 12 drill core samples of leucite-bearing Adang Volcanics reveals their alkaline nature, characterized by high TiO₂ relative to Al₂O₃ [TiO₂ > (−1.1610 + 0.1935 × Al₂O₃)]. The samples also display geochemical signatures of Nb/Zr > 0.0627, Th/Nb > 0.67, a high ratio of Nb/Y (>0.6561) with an enrichment of light rare earth elements (LREE; La/Yb > 20), low Nb/La <0.6, and a low negative Tantalum anomaly (δTa 0.36–0.41), suggesting the magmatism was generated from the tectonics of continental extension stretching rather than subduction arc-related or mature continental rifts, where the magmatism was triggered by decompression melting during the extensional processes. Furthermore, the magmatism is thought to originate from partial melting of enriched subcontinental lithospheric mantle (SCLM), which is uncorrelated with the source of asthenospheric mantle. We propose that the formation process of the Adang Volcano is a product of the tectonic processes of the Makassar Strait opening in Central Indonesia during the Paleogene to the Early Neogene era.