[S-VC28] International Volcanology

[SVC28-P03] Petrological and geochemical characteristics of volcanic rocks from Raung volcano, East Java, Indonesia

*Mradipta Lintang Alifcanta Moktikanana¹、Agung Harijoko²、Tsukasa Ohba¹、Haryo Edi Wibowo² (1.Akita University、2.Gadjah Mada University)

キーワード：caldera, basaltic, magma evolution, magma mixing

Raung volcano in East Java is located in the triple junction of big cities: Jember, Bondowoso and Banyuwangi. It consists of two volcanic centers – the currently active Raung cone and the older edifice Gadung cone, a remnant of volcanic sector collapse phenomena. The historical activity of this caldera-topped stratovolcano is represented by basaltic to andesitic Strombolian eruption that occurs every one to five years. Previous report states the presence of pumice fall deposit that may indicate the most evolved magma composition from pre-historical activity. Despite the fact that it exposes a considerable risk of volcanic hazard, there are very limited reports on the petrological study of Raung volcanic products. We study the petrology and geochemistry characteristics of Raung products that are varies from lava flow, block and ash flow, scoria fall, and pumice fall, on the basis of petrography observation and whole rock geochemistry (ICP-AES & ICP-MS). The magma composition of Raung volcano ranges from basaltic to dacitic (SiO₂ 48 – 64 wt%). The most primitive product is represented by basaltic lava from Gadung cone, that may be related to the re-establishment of magma system following sector collapse through mafic volcanism. Lava, lithic blocks from block and ash flow deposit, and scoria fall from Raung cone reflects intermediate composition, while dacitic pumice fall confirms the most evolved composition. Moreover, change in SiO₂ content is well represented in the mineralogy. Mafic minerals such as olivine, clinopyroxene, and orthopyroxene are present in the lower SiO₂ content (<60 wt%), while hornblende and biotite are present in higher SiO₂ content (>60 wt%). Raung magma can be classified into two types based on the mineralogy, element ratio, and Eu content. Type 1 magma is more crystal-rich, depleted in trace elements & REE, and not showing Eu anomaly. Meanwhile, Type 2 magma is enriched in trace elements & REE, and more crystal-poor with negative Eu anomaly. Both magma types derived from separated magma differentiation mechanism controlled mostly by plagioclase crystallization during fractional crystallization. Mingling texture, disequilibrium feature in the rock micro textures (reverse zoning, sieve, resorbed, orthopyroxene mantled by clinopyroxene), along with geochemical feature indicate magma mixing and multi-episode of primitive magma replenishment. This study may provide a clue about the presence of heterogeneous magma under Raung volcano and its potential of evolving into dacitic composition that may results in more explosive behaviour compared to the present-day andesitic volcanism.