Geology and geochemical evolution of volcanic rocks in Cibodas, West Java, Indonesia.

This study was carried out at Cibodas, West Java, Indonesia. The study area covered about 25km2 in which the landforms that was fonned in the Cibodas area are plains, hills and intrusion landforms. It was made up from volcanic rocks mostly and sedimentary rocks such as andesite, breccia, tuff and sandstone. The past environment for the Cibodas area are shallow marine based on the fossil type that has been found before it was deposited by the volcanic materials starting from Pliocene until Holocene eras. Volcanic rocks in Cibodas area is identified as low-K, calc- alkaline and high-K series of basalt, basaltic andesite and andesites suites which have 49.99-59.69 wt% of SiO2 and 0.0421-3.16 wt% K20. The volcanic rocks in the study area are under calc-alkaline which the rocks are fonned at the subduction zone. The rocks are classified fiom intermediate to acidic with the SiO2 concentration ranging from 51.98 — 61.82%. The volcanic rocks in the study area is believed to be fonned in convergent plates and this is suitable to use a classification based on the K20 vs SiO2. Petrographically, the magma is believed has became water undersaturated at the crystallization phase of hornblende and amphibole. The volcanic rocks in the study area are found more plagioclase and pyroxene due to crystallization in the magma chamber.