Long-Term Volcanic Evolution Surrounding Dieng Geothermal Area, Indonesia

Abstract

The Dieng Volcanic Complex (DVC), in the Central part of Java Island, is characterized by a collapse structure containing 17 post intra-caldera eruptive centers. This volcanic complex shows long-term volcanic activity of about 3 M.y. and is possibly record the long-term magma evolution at a single volcanic complex. The volcanic edifices in DVC can be grouped into three stages, namely pre-caldera (~3 Ma), post-caldera I (~2 to 1 Ma) and post-caldera II (< 1 Ma). Major element rock compositions suggest that the DVC magma cyclically evolved from basaltic to dacitic composition. Both pre-caldera and post-caldera I have a wide range of composition from basalt to dacite, in contrast the post-caldera II ranges from andesite to dacite. Phenocryst assemblage of all the stage show similar composition including plagioclase, clinopyroxene, orthopyroxene, olivine and magnetite. The post-caldera II lava also contain biotite phenocrysts and are richer in groundmass glass. The increase of silica content is followed by increasing potassium content (K57.5) so that we can distinguish medium-K and high-K magmatism in early and late stage of each group in pre- and post-caldera.
Harker diagrams indicate that magma in DVC is differentiating from the same magma source. Chondrite normalized incompatible element plots show similar patterns among for all edifices, and are typical of the island arc compositions, with enrichment of LILE and LREE compared to HSFE and HREE. Ta/Nb and Zr/Nb ratios of the lava from all stages are similar and range from 0.04 to 0.1 and 16 to 37, respectively, indicating that the mantle composition beneath DVC is still the same and resembles the ratio of Indian MORB. Ce/Pb and Th/Yb ratios indicate the contribution of continental material either as crustal contamination during the passage of magma to surface or sediment influx during partial melting. There are no significant geochemical differences among magmas at DVC.