Open-system Magmatic Process at Agung Volcano, Bali, Indonesia: Evidence from Phenocryst Compositions and Textures

Adzkia Noerma Arifa; I Gusti Bagus Eddy Sucipta; Idham Andri Kurniawan

Adzkia Noerma Arifa Geological Engineering Master Program, Faculty of Earth Sciences and Technology, Institut Teknologi Bandung
I Gusti Bagus Eddy Sucipta 2Petrology, Volcanology and Geochemistry Research Group, Geological Engineering, Faculty of Earth Sciences and Technology, Institut Teknologi Bandun
Idham Andri Kurniawan Petrology, Volcanology and Geochemistry Research Group, Geological Engineering, Faculty of Earth Sciences and Technology, Institut Teknologi Bandun

Keywords: Agung volcano, disequilibrium, magma recharge, open system

Abstract

Agung volcano is an active stratovolcano located in Bali, Indonesia. It is the highest volcano on Bali at 3142 m a.s.l. Samples are collected along the flanks and represent each lava unit. In term of petrology, samples are porphyritic high alumina basalt, basaltic andesite and andesite with inhomogeneous phenocrysts of plagioclase, clinopyroxene, orthopyroxene, olivine, amphibole and titano-magnetite. Plagioclase phenocrysts frequently show fine sieve, resorption surface, coarse sieve and oscillatory zoning texture. Augite and enstatite phenocrysts display texture less zone and monotonous crystal zoning. Olivine phenocrysts are less abundant and present reaction rims of Mg-rich of orthopyroxene and titano-magnetite or Ca-poor clinopyroxene. Hornblende micro-phenocrysts just found in AD 1963 andesitic lava. They have reaction rims composed of augite, feldspar and symplectitic intergrowth of titanomagnetite. Plagioclase, pyroxene and olivine phenocrysts frequently display increases in Mg or Ca contents within mid-crystals. Dissolution, resorption and reverse zoning suggest open system magmatic behavior of repeated mafic magma recharge and also mineral-magma disequilibrium.

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