MAGNETIC EVOLUTION OF THE DIENG VOLCANIC COMPLEX, CENTRAL JAVA

PROCEEDINGS JOINT CONVENTION BALI 2007
The 32^nd HAGI, The 36^th IAGI, and The 29^th IATMI Annual Conference and Exhibition

MAGNETIC EVOLUTION OF THE DIENG VOLCANIC COMPLEX, CENTRAL JAVA

Haryo Edi Wibowo^1
1Department of Geological Engineering Gajah Mada University, Yogyakarta
ABSTRAK

The Dieng volcanic complex (DVC) that is situated in Central Java, consists of many volcanic edifices inside and around the caldera structure representing a series of island arc magmatism activities. Radiometric dating reveal that the activities take place from 3.60 Ma to 0.06 Ma or from Pliocene to Recent. This enables us to study evolution of magma in a volcanic complex temporally. This study would be useful to enhance knowledge about volcanic related resources exploration such as gold deposit which quite common in tertiary volcanism.

In order to understood magma evolution in DVC therefore, field geologic observation and rock sample collection from different volcanic edifices of different periods were conducted. In addition, satellite image analysis was conducted to generate a volcanic edifice distribution map. Furthermore, rock samples were analyzed using thin section observation and whole-rock petrochemical compositions analyses to study petrogenesis of magmas.

The analyses result in the spatial and temporal geochemical evolution of volcanic edifices within the DVC. Generally, the volcanic rocks alkalinity increases gradually from the oldest edifice (i.e. G. Kendang-Prahu; 3.6 to ~2.5 Ma) to the youngest ones (i.e. G. Kendil-Sikunir). From the oldest to the youngest volcanoes, the magmatic affinities change from the medium K into the upper limit of high K class. K2O and Rb data show that DVC, except the Prahu volcano, gradually change its status from the trench-side volcanism during the Pliocene-Pleistocene into the backarc-side volcanism during the Pleistocene-Recent. The trends of major oxides versus silica contents show that magmas underneath DVC experienced fractional crystallization and crust contamination for each volcanism event, especially advanced differentiation among the pre caldera volcanism. The presence of large eruption crater within most of the pre caldera volcanoes and dacitic volcanism among them are identical with explosive eruption events that most likely had caused the caldera collapse. Following major magmatic evolution events are identified. First, the source of DVC magmas are generally from the partial melting of peridotite mantle which is represented by basalt as primitive magma. The low Ni content indicates that the basalt as primitive magma already experienced contamination. Second, the geologic setting of DVC changes from a trench-side magmatism during the Pliocene into a backarc-side magmatism during the Quaternary. Third, caldera-forming eruptions took place when the DVC setting was on the trench-side and when the advanced differentiation process took place.