Petrogenesis study of Anak Krakatau volcanic rock.

Krakatau is one of Quaternary volcanoes that has catastrophic eruption histories including the eruption in 1883 causing fatalities about 36.500 people. Such an event has led the volcano to be interestingly investigated as an attempt for volcanic disaster mitigation. We conduct petrogenesis studies through secondary data from [1,2]. The secondary data used in this study includes the geochemistry of lava samples from the Anak Krakatau lava unit and lava samples from Young Krakatau. Analysis of the geochemistry data aims to identify the magma affinity, type of the rocks, magma differentiation pattern, and magma association with the tectonic setting. We also characterized our 13 lava samples from Anak Krakatau for their mineral abundance and textures. In general, the phenocrysts of all lava samples consist of plagioclase, orthopyroxene, clinopyroxene, olivine, amphibole, and opaque minerals. We observed microtextures of porphyritic and textures of individual crystals such as sieve texture and oscillatory zoning in plagioclase and reaction rims in mafic minerals. The presence of the sieve texture and crystal zoning indicated the disequilibrium in the magmatic processes during mineral formation. In addition, a wide range of An content suggested that there were disequilibrium processes such as magma mixing and/or assimilation. The plot of SiO2 against K2O shows that all the samples belong to the medium-K calc-alkaline magma series. Besides that, the TiO2 content of the 1883 product in the Young Krakatau phase and the andesitic lava product of 1981 in the Anak Krakatau phase are smaller than 1.4 wt%. Such low values indicated that the magma beneath the volcano is associated with partial melting in the subduction zone, where the crustal assimilation in the Krakatau volcanic complex is subtle as indicated by a relatively low isotope ratio of 87Sr/86Sr of 0.704393 (1883-Young Krakatau) and 0.704428 (1981-Anak Krakatau). [ABSTRACT FROM AUTHOR]