PETROLOGY, MINERALOGY, AND GEOCHEMISTRY OF VOLCANIC ROCKS IN THE GALUNGGUNG VOLCANO, TASIKMALAYA, WEST JAVA Fariha Elsyanaya Danti Putri (a*), I Gusti Bagus Eddy Sucipta (b), Idham Andri Kurniawan (b)
a) Laboratory of SEM EDS UPP Chevron Bandung Institute of Technology, Indonesia
b) Petrology, Volcanology, and Geochemistry Research Group, Geological Engineering, Faculty of Earth Science and Technology Bandung Institute of Technology, Indonesia
Abstract
Galunggung Volcano is one of the volcanoes in West Java with high eruption rate. This research aims to determine the petrological, mineralogical, and geochemical characteristics, rock-forming magma during 50.000 BP until recent. The primary data was used 29 samples for petrographically analysis and 5 samples for mineral chemistry. The secondary data is 53 data samples of geochemical and 5 data samples of mineral chemistry from the previous study.
The result show that the eruption of Galunggung Volcano is divided into 4 eruption periods, 50.000 to 10.000 BP, 4.200 more or less 150 BP, 1822 to 1894 to 1918, and 1982 to 1983 period consisting of 1, 2, and 3 phases. The 50.000 to 10.000 BP period produced basalt to basaltic andesite (SiO2 47 to 56 wt%) containing fosterite composition of olivine (Fo64 to 70) and An in plagioclase (An46 to 67). The 4.200 more or less 150 BP period produced basalt to basaltic andesite (SiO2 51 to 55 wt%) containing fosterite composition of olivine (Fo62 to 64) and An in plagioclase (An50 to 71). The 1822, 1894, and 1918 periods produced basalt to basaltic andesite (SiO2 51 to 55 wt%) containing An in plagioclase (An39 to 70). At the beginning of phase 1 in the 1982 to 1983 period produced basaltic andesite to andesite (SiO2 53 to 57 wt%), phase 2 produced basalt to basaltic andesite (SiO2 50 to 51 wt%) containing fosterite composition of olivine (Fo76 to 90) and An in plagioclase (An47 to 70), phase 3 produced basalt to basaltic andesite (SiO2 49 to 54 wt%) containing fosterite composition of olivine (Fo70 to 87) and An in plagioclase (An66 to 91).
The spider and tectonic discriminant diagrams show the tectonic setting of Galunggung Volcano related to the subduction process which is contaminated by continental and oceanic crust. The activity history of Galunggung Volcano was started by the initiation of basaltic magma injection (SiO2 47 wt%) with tholeiitic to calc alkaline affinity in differentiated magma at 50.000 to 10.000 BP period. In the 4.200 more or less 150 BP period, the vent moved to the southeast flank, named Galunggung Volcano and magma differentiation was still going on. In the 1822, 1894, and 1918 periods, the differentiation process named crystal fractionation continues simultaneously with injection of basaltic magma. In phase 1 of 1982 to 1983 period, a new generation of low Mg magma moved diapirically influenced by crystal fractionation en route to the surface. So, phase 1 was the most fractionated magma that erupted first, followed by the less developed magma in phase 2 and the unmodified high Mg magma that erupted in phase 3 in 1982 to 1983 period.
Keywords: volcanic activity, magma differentiation, Galunggung Volcano, eruption period
