A study of the stratigraphy, volcanology and geochemistry of pyroclastic rocks from the Ijen caldera complex, East Java, Indonesia

Abstract

The Ijen caldera complex is a large volcano caldera depression, and is located on the Banyuwangi Peninsula in East Java (7°55'36" - 8°11'24" S latitude and 114°04'18" - 114°18'44 E longitude). The diameter of the caldera ranges from 22 km by 25 km at the rim to 19 by 21 km at the floor. The Ijen caldera complex is located in an active tectonic region, where the Indian plate is now sliding approximately northward beneath Java at about 6 - 7 cm/yr. This study is in two parts. The first is detailed field mapping and includes a geological map and stratigraphy, which forms the basis for other work. The second concentrates on the post - caldera pyroclastic deposits. In conjunction with the field nature, grain size analyses and Scanning Electron Microscope (SEM) images, representative pyroclastic materials are examined and the main eruption mechanisms determined. In the late Pleistocene or early Holocene, the ancestral Mt. Ijen (± 3500 m asl) erupted violently and produced andesitic ignimbrites and andesite - dacite plinian pumice totaling about 466 km3. This material blankets approximately 50 X 50 km2 and was mostly dispersed to the North. These eruptions led to the formation of a large caldera structure in the summit of Mt. Ijen. Post - caldera products include the large andesite cones of Ijen, Marapi, Suket, Ringgih, Rante and Mt. Jampit - Pendil sited around the caldera rim, and basalt lavas and scoria cones on the caldera floor. The pyroclastic rocks are described petrographically and major and trace elements analyses presented. The minerals (especially phenocrysts) observed are: plagioclase, pyroxene, olivine and Ti - magnetite. Representative samples were analyzed by microprobe. The relationship of these is illustrated and together with whole rock chemistry, the origin and the nature of the melt that may have been involved in the formation of the pyroclastic rocks in the Ijen complex is discussed. All pyroclastic rocks are calc - alkaline type. They are characterized by high alumina and high K (especially basaltic - andesite), all are quartz - hypersthene normative. Volcanic rocks in the Ijen caldera area are divided into pre - caldera, caldera and post - caldera. Rocks from each period evolved by fractionation from more mafic to more potassic types. Repetition in these trends is considered to result from fractionation of successive magma batches. Rock compositions trend from basalt to andesite to dacite, but the paroxysmal caldera forming eruption is mostly andesite. The more calcic of the pre - caldera rocks is basalt - basaltic andesite. This rock typically has plagioclase, orthopyroxene, and clinopyroxene phenocrysts. The dacite and the andesites of the caldera forming phase typically have plagioclase and orthopyroxene phenocrysts. The post - caldera rocks are basalt to basaltic andesite. The basalt typically contains phenocrysts of olivine, clinopyroxene and plagioclase (An50). The basaltic andesite mostly forms cinder cones and typically has orthopyroxene, olivine and plagioclase bytownite phenocrysts. Geochemically the rocks range from basalt to dacite and on standard Harker type diagrams show the expected relationships between SiO2 and major oxides. The rocks are characterized by rather high Al2O3 content, low MgO and TiO2 concentrations and relatively high Fe2O3. All rocks contain less than 1.2 wt % TiO2, typical of calc - alkaline island arcs. The LIL Elements (Sr, K, Rb, Ba, Th) are enriched and the heavy (HFSE) elements (Nb, Ce, P, Zr, Ti, Y) are depleted relative to N - type MORB. There is no indication of primary magma being erupted. All rocks have undergone secondary processes of fractional crystallization, contamination and magma mixing. Magma mixing is evidenced by xenocrysts, reverse zoned plagioclase rims and more calsic groundmass plagioclase, as seen in microprobe analyses. Geochemical characteristics indicate that parental magmas of the volcanic rocks in the Ijen complex were probably produced by partial melting of the convecting mantle wedge above subducting oceanic slab. In the Ijen caldera area, many eruption types have been experienced. These include air fall tephra (pumice and scoria), pyroclastic flows, surge, ignimbrite, debris avalanche and lava flows. These eruptions might be expected again in the future. The scoria cones which are aligned along the NW - SE fissures in the caldera floor are monogenetic, and are likely to occur on the caldera floor in the future. In the near future the most probable eruption from the Ijen caldera complex is considered to be a phreatic eruption from Mt. Ijen. This volcano has intervals between historical eruptions of 21 - 100 years. This eruption may produce lahars that are most likely to flow down the Kali Banyupahit river.