Mineralogy and Geochemistry Variation of Igneous Rocks from Ungaran and Muria Volcano and Its Processes related to Subduction Zone Magmatism of Sunda Arc

Abstract

Sunda Arc represent Quaternary magmatism as one of the most significantly active volcanic arc in Indonesia resulted from subduction of oceanic plate beneath continental plate. Along this subduction system, there are systematic variations in subduction angle, depth, crust thickness, and composition contribute to geochemical characteristic of igneous rocks along and across the arc. This study compared mineralogy and geochemistry characteristics from two volcanoes which represent across-arc variation. Petrographic analysis have been conducted on rocks’ thin section samples from Ungaran and Muria Volcano to characterize mineralogy of rocks. Selected samples were analyzed using X-Ray Fluorescence (XRF) for geochemistry of major and trace elements. Mineralogy of Ungaran rocks are dominated by plagioclase, clinopyroxene, and hornblende phenocrysts with composition range from andesite to basaltic andesite of high-K calc alkaline series. Samples from Muria Volcano are from alkaline series characterized by feldspathoid and alkali feldspar rich rocks with calcic plagioclase and pyroxene phenocrysts. This mineral assemblages, supported by major elements data suggest fractionation process in magma. Geochemical trend is systematically consistent with other island arc setting, where there is significant increase of total alkali from Ungaran to Muria. Trace element data shows decreasing in LILE/HFSE ratio further from arc. This suggests that Ungaran has more crustal and subducted slab influence, while Muria has higher degree of partial melting from deeper Benioff Zone.