Xenolith petrography for identification of substratum lithology of an active volcano: a case study of Lava Kekep, Sundoro Volcano, Central Java

Abstract

Abstract Sundoro volcano is an active Quaternary stratovolcano in Central Java, located approximately 50 km west of the Merapi volcano. It is attributed to parasitic cones producing basaltic lava flow. We present our findings of sedimentary and igneous xenoliths embedded in the basaltic lava of the Kekep parasitic cone. Xenolith is one of the key materials to help identify various lithologies composing the substratum of a volcano. Twelve thin sections were analysed through detailed petrographic observation. We identified variations of xenolith lithology consisting of sedimentary rock, particularly siltstone, and igneous rocks such as dunite, pyroxenite and lherzolite. Detailed accounts of substratum lithologies comprises of (a) siltstone composed of >0.125-0.5 mm size plagioclase crystals and silicic fine-grained matrix, (b) pyroxenite consists of massive interlocking 1-5 mm size clinopyroxene and few <1 – 1 mm orthopyroxene, (c) dunite consists of 1 - 2 mm phenocryst of plagioclase, pyroxene, and olivine, embedded in a minor groundmass of plagioclase and pyroxene, (d) lherzolite consists of massive interlocking olivine crystals 1-5 mm in size. The analysis results identify gradual or groundmass and sharp contact between basaltic host rock and xenoliths (pyroxenite, lherzolite, dunite and siltstone). Other characteristics of the twelve samples includes (a) <1 mm minerals fragments embedded in silt to clay-grained lithic sandstone, (b) siltstone showing chilled-baked margin in hand sample specimen, (c) identified disequilibrium textures in basaltic lava, and contact between xenolith and host rock. Substratum lithology along the Java arc is likely composed of Tertiary sedimentary rock in the upper part, transitions of granitoid – gabbroic diorite crustal material in the lower part, with ultramafic crustal material under Sundoro.