Petrological and Geochemical Characteristics of Pumiceous Tephra Deposit from Slamet Stratovolcano, Central Java, Indonesia: Explosive Period of the Most Differentiated Magma of A Basaltic Stratovolcano

We reported detailed investigation of the pumiceous deposits reside in the western part of Slamet Volcano that are found as both primary and secondary product. Pyroclastic fall, flow and surge deposits represent those of the primary products, whereas lahar and fluvial deposits represent those of the secondary products. The tephra fall deposits are separated by pyroclastic flow and surge deposits as typical of sub-plinian to plinian eruption type. The secondary deposits of fluvial deposits occurred in between the primary deposits indicating repose time of the explosive events, whereas lahar deposits occurred as the youngest deposits concluding these pumiceous events. Petrographic observation show that pumice deposit is extremely vesicular with various bubble sizes, vitrophyric in texture, and contains subhedral phenocryst of plagioclase, orthopyroxene, clinopyroxene, opaque mineral and minor biotite. Chemical compositions of major and trace elements were obtained using ICP-MS-AES. SiO2 range from 58,7 to 63,9 wt.% and classified as andesite to dacite with LOI number range from -0,04 to 3,06. Ratios of incompatible elements (e.g. Ba/Zr, Rb/Zr, Zr/Nb, and Hf/Nb) consistently separate rocks of Old and Young Slamet. In these ratios, pumiceous samples are plotted within the range of Young Slamet implying for genetic relationship. These explosive events are "unique" considering typical recent basaltic activities of Slamet volcano and need to be carefully considered in the volcanic hazard mitigation.