Petrogenesis of Kalisonggo tertiary lava, Girimulyo Kulonprogo, Yogyakarta based on mineralogy and geochemical analysis

Abstract

The andesite lava that exposed in Kalisonggo, Girimulyo, Kulonprogo, Yogyakarta is the product of Kulonprogo Volcano activity. Kulonprogo Volcano is an ancient volcano that has three eruption centers. This study was conducted to determine the petrogenesis of Kalisonggo lava including sources, tectonic setting and other processes that controlled the formation of the lava. The methods used in this research are petrographic analysis (6 rock samples) and geochemical analysis (major oxides and trace elements) by XRF. Petrographic analysis shows that in general, all samples are composed by plagioclase, clinopyroxene, orthopyroxene, opaque mineral, and glass. Hornblende occured in three samples from the south area only and no hornblende found in the sample from north area. The rocks have porphyritic, ophitic, glomeroporphyritic texture, and some plagioclase zoning. The XRF geochemical analysis result show that the rocks composition are andesitic-basaltic andesite. From SiO2 vs K2O + Na2O diagrams and Co vs Th diagrams, the rocks is interpreted as product of Gadjah volcano eruption. This product characterized by calc-alkaline magma series. The magma affinity indicates that the magma generated in subduction zone or island arc.