[SVC39-P12] MAGMA GENESIS OF SLAMET VOLCANO, CENTRAL JAVA, INDONESIA
Keywords:Slamet Volcano, Magma Mixing, Magma Genesis, Petrogenesis
Several researchers have studied Slamet Volcano and gave a general process in Old Slamet and Young Slamet. Vukadinovic and Sutawidjaja (1995) discuss that Old Slamet had more fractionation crystal and there was mafic, hot, fluidal magma recharge in Young Slamet. In the present study, we focus on Young Slamet Complex and will use more samples from different rock units based on Young Slamet Stratigraphy to conclude the magma genesis process of Young Slamet Volcano in detail with calculating magma pressure, temperature and oxygen fugacity.
Six different units rock samples, including lava and scoria samples, have been collected for the stratigraphic units: the oldest rock unit is Slamet Lava 1 (SL1), above it then deposited Slamet Lava 2 (SL2), Scoria Fall 1 (SJP1), Scoria Cones (KSL), Slamet Lava 3 (SL3) and the youngest is Scoria fall 2 (SJP2). Those samples were analyzed using an optical microscope, XRF, SEM-EDS, and EPMA.
The phenocrysts are plagioclase, pyroxene, olivine and amphibole with textures such as zoning, glomerocrysts, synneusis and sieve texture. Young Slamet bulk compositions for 25 samples show SiO2 and K2O contents from 46.7 – 59.12 wt.% and 1.06 – 2.47 wt.%, respectively. The rock samples are classified as calc-alkaline to high-K calc-alkaline series and classified from basalt, trachybasalt to andesite. The widest ranges of the anorthite (An) content of plagioclase from 51.06 to 86.81 mol %, magnesium content of pyroxene from 73.64 to 89.81 mol % and forsterite (Fo) content of olivine from 58.7 to 86.5 mol % are observed in SL1, and gradually decrease in SL2 and then increase in SJP1. Bimodal distribution of anorthite and forsterite were observed in SJP1 samples. Based on these data we suggest that several times of magma mixing occurred in Young Slamet Complex. Magma pressure, temperature and oxygen fugacity will be determined from iron and titanium oxide minerals and two pyroxenes to quantitatively know the physical condition of magmatic processes.