Structure and Dynamics of a Silicic Magmatic System Associated with Caldera-Forming Eruptions at Batur Volcanic Field, Bali, Indonesia |
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Authors: | REUBI O; NICHOLLS I A |
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Institution: | VICTORIAN INSTITUTE OF EARTH AND PLANETARY SCIENCES, SCHOOL OF GEOSCIENCES, MONASH UNIVERSITY, MELBOURNE, VIC. 3800, AUSTRALIA |
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Abstract: | The Batur volcanic field (BVF), in Bali, Indonesia, underwenttwo successive caldera-forming eruptions that resulted in thedeposition of silicic ignimbrites. The magmas erupted duringand between these eruptions show a broad range of compositionsfrom low-SiO2 andesite to high-SiO2 dacite. On the basis oftheir geochemistry and mineralogy these magmas may be assignedto six groups: (1) homogeneous andesites with phenocryst compositionsessentially in equilibrium with the whole-rock composition;(2) remobilized crystal-rich low-SiO2 andesites with resorbedphenocrysts in equilibrium with the whole-rock composition;(3) mixed low-SiO2 dacite with a relatively large range of phenocrystcompositions, with most phenocrysts slightly too evolved tobe in equilibrium with the whole-rock; (4) extensively mixedlow-SiO2 dacites with a very large and discontinuous range ofphenocryst compositions, with most phenocrysts either more Mg-richor more evolved than the equilibrium compositions; (5) remobilizedcrystal-rich low-SiO2 dacites with resorbed and euhedral phenocrysts;(6) homogeneous high-SiO2 dacites lacking evidence for magmamixing and showing narrow ranges of phenocryst compositionsin equilibrium with the whole-rock composition. This range ofsilicic magmas is interpreted to reflect a combination of closed-and open-system fractional crystallization, magma mixing andremobilization of cumulate piles by heating. The variety ofmagmas erupted simultaneously during the caldera-forming eruptionssuggests that the magmatic system consisted of several independentreservoirs of variable composition and degree of crystallization.The magmatic evolution of individual reservoirs varied fromclosed-system fractional crystallization to fully open-systemevolution, thereby resulting in simultaneous production of magmaswith contrasted compositions and mineralogy. Extensive emptyingof the magmatic system during the caldera-forming eruptionsled to successive or simultaneous eruption of several reservoirs. KEY WORDS: caldera; ignimbrite; magmatic chambers; magma mixing; petrology; Sunda Arc |
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Keywords: | : caldera ignimbrite magmatic chambers magma mixing petrology Sunda Arc |
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