印尼東爪哇巴契丹地區中新世中期火山岩的年代與地球化學特徵

Abstract

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Cenozoic volcanoes on Java, located at SE Sundaland were produced by the subduction of Indian-Australian oceanic crust beneath the Eurasian Plate. Pacitan is located on the southern coast of East Java. In addition, Pacitan is also part of the Southern Mountains range, where some volcanic rocks are exposed. The volcanic rocks are known as Old Andesite, which was formed in the Eocene. This study focuses on the volcanic rocks in this area to identify the geochemical variations from different magmatic stages. The methods used are major elements, trace elements, Sr-Nd isotopes, Zircon U-Pb ages, and zircon Lu-Hf isotopes in volcanic rocks. Based on the data obtained, the Pacitan area shows active volcanism three periods: starting from the Late Oligocene (26.7 to 26.0 Ma), followed by the Early Miocene (~20 Ma), and Middle Miocene (13.4 to 11.4 Ma). The Late Oligocene volcanic rocks are composed of medium- to low-K calc-alkaline andesite to dacite. Early Miocene volcanic rocks are composed of medium-K calc-alkaline series to low-K calc-alkaline dacite to rhyolite while, the Middle Miocene volcanic rocks are composed of high-K calc-alkaline to tholeiitic basalt to dacite. Based on the geochemical approach, in the Middle Miocene, adakite was found. Besides that, Nb/U–Nb, Ce/Pb–Ce diagrams, and the depletion of HFSEs (Nb, Ta, and Ti) show arc characteristics in these volcanic rocks. The Old Andesite, based on the data obtained, is believed to be not as old as previously thought, due to the Sunda subduction inquisition that occurred in Pacitan, and the Old Andesite is estimated to have been active since the Oligocene. The adakite process that occurred in Pacitan is associated with the beginning of the subduction ridge in the southwest of Java, where the tectonic process occurred earlier than previously thought. From this, the tectonic evolution model that occurred in Pacitan is the latest three-stage tectonomagnetic evolution model for Tertiary subduction systems.