西至中爪哇第四紀火山岩之鋯石年代學與地球化學研究

Abstract

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The Sunda Arc is located on the southeastern edge of Sundaland and was produced by the Indo-Australian Plate subduction of beneath the Eurasian Plate. Java Island is belonging to the eastern Sunda Arc, where arc magmatism began in the early Cenozoic and continued to Recent on this island. Previous studies used K-Ar methods for dating igneous rocks, which may easily to be affected by sample weathering and alteration. Furthermore, their geochemical analyses are limited to some prevalent volcanoes. In this work, I select six volcanic bodies in West Java and Central Java to study the zircon U-Pb geochronology and whole-rock geochemistry. A total of thirty-two volcanic rock samples were collected from West Java (Mount Sunda, Mount Malabar, Mount Guntur, and Mount Tangkuban) and Central Java (Mount Slamet and Mount Merapi). Geochemical compositions of these samples from West to Central Java range from calc-alkaline to high-K calc-alkaline with lithologies varying from basalt to andesite, and dacite. There are no significant variations among their Spidergram and Rare Earth Element (REE) patterns. They show the typical subduction signature which is enriched in Large Ion Lithophile Elements (LILE, Rb, Ba, Th, U, and K) and depleted in High Field Strength Elements (HFSE, Nb, Ta, and Ti) with negative Eu anomaly in some samples. Zircon U-Pb ages show that Mount Sunda erupted andesite (18JA03-1) and dacite (18JA03-4) around 4.0 Ma and 4.0 Ma, respectively. Mount Malabar erupted basaltic andesite (18JA04-1) and dacite (18JA05-1) yielding a mean age of 6.8 Ma and 4.4 Ma. Moreover, Mount Guntur basaltic andesite (17JA56) yield a mean age of 0.4 Ma. Both Mt. Slamet and Mt. Merapi samples are mafic and did not yield any zircon U-Pb age. According to the geochemical binary diagrams of major and trace elements from West Java, Mount Sunda and Tangkuban show characteristics of Rear-Arc while Mount Malabar is related to the Volcanic Front. Mount Guntur which shows transitional geochemical patterns indicate forming on the boundary between the Front Volcanic and Rear-Arc.