Abstract
Three main episodes of volcanic activities are recognized in the Mt. Sunda volcanic complex eruptions of the pre-, syn- and post-caldera units named Pre-Sunda Volcanics, Sunda Volcanics and Tangkuban Parahu Volcanics. The first episode, is represented by the Batunyusun Andesite (1.1 Ma) which unconformably overlies sedimentary deposits. The second episode, produced the Sunda Volcanics having ages between 0.56 and 0.180 Ma. They consist of the Sunda Andesite with ages of 0.56-0.205 Ma and the huge volume of Sunda Pyroclastics covering an area of 200 km2, associated with a caldera-forming eruption of Sunda caldera between 0.205-0.18 Ma. The post-caldera Tangkuban Parahu Volcanics at the third episode consist of Tangkuban Parahu Andesite and Tangkuban Parahu Pyroclastics with ages of 0.062 - 0.022 Ma. Carbon-14 dating of younger craters reveals an age of 0.0099-0.0014 Ma. They are assumed to be associated with phreatic or hydrothermal eruptions of the Domas, Ratu, Siluman, and Ciater craters. The thermal discharges are mostly bicarbonate waters with subordinate sulphate and chloride type waters. The sulphate waters of Domas and Kancah are associated with high volcanic terrain (900-1, 700 m a.s.l.) . Bicarbonate waters are located at moderate slopes of the volcanoes, and chloride waters are generally located at lower elevations. The Ciater thermal discharge (1, 000 m a.s.l.) has a low pH, high chloride, high fluoride, and relatively high arsenic concentrations. It is linked with active volcanic gases from the Tangkuban Parahu or the Domas crater. The SiO2 and Na/K geothermometry represents a subsurface temperature as 120-310°C. The chemical composition of fumarolic gases from the Ratu crater consists much more of hydrous gases, H2O, CO2, H2S and small amount of SO2. The calculated subsurface temperatures based on gas geothermometry were approximately 280-400°C, probably reflecting the high temperature of fluids. in the depth.
