New age and geochemical constraints on the “Old Andesite” and middle miocene adakites from Pacitan, East Java, Indonesia

The subduction of the Indo-Australian Plate beneath the Eurasian Plate resulted in Cenozoic volcanism along the Sunda Arc, with Java Island located in its central portion. Pacitan is situated on the southern coast of East Java, where arc volcanic rocks are widely exposed and have traditionally been designated as Old Andesite, which erupted during the Eocene epoch. In this study, we present new whole-rock geochemical data, including major and trace elements, Sr[sbnd]Nd isotopes, and zircon U[sbnd]Pb ages and Lu[sbnd]Hf isotopes of volcanic rocks from Pacitan. Our age data indicate that magmatism in Pacitan occurred in three stages: Late Oligocene (26.7 to 26.0 Ma), Early Miocene (∼20 Ma), and Middle Miocene (13.4 to 11.4 Ma). The first two stages comprise various lithologies (basalt-andesite-dacite-rhyolite) exhibiting low-K to median-K calc-alkaline compositions, while the third stage includes an additional rock type with adakitic geochemical affinities. We propose that the Old Andesite in Pacitan is not as old as previously believed but was active from the Late Oligocene because of the Sunda subduction initiation in this region. The Middle Miocene adakitic volcanism was attributed to the onset of ridge subduction southwest of Java Island, a key tectonic process that we assert occurred earlier than previously suggested. Consequently, we propose a new three-stage tectonomagmatic evolution model for the central segment of the Tertiary Sunda subduction system.