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Journal of Petrology Advance Access published online on March 11, 2005
Journal of Petrology, doi:10.1093/petrology/egi019
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oupjournals.org
Received November 18, 2003
Accepted January 27, 2005

Article

Structure and Dynamics of a Silicic Magmatic System Associated with Caldera-Forming Eruptions at Batur Volcanic Field, Bali, Indonesia

O. REUBI 1* and I. A. NICHOLLS 1

1 VICTORIAN INSTITUTE OF EARTH AND PLANETARY SCIENCES, SCHOOL OF GEOSCIENCES, MONASH UNIVERSITY, MELBOURNE, VIC. 3800, AUSTRALIA

* To whom correspondence should be addressed.
O. REUBI, E-mail: olivier.reubi{at}okeano.org


  Abstract

The Batur volcanic field (BVF), in Bali, Indonesia, underwent two successive caldera-forming eruptions that resulted in the deposition of silicic ignimbrites. The magmas erupted during and between these eruptions show a broad range of compositions from low-SiO2 andesite to high-SiO2 dacite. On the basis of their geochemistry and mineralogy these magmas may be assigned to six groups: (1) homogeneous andesites with phenocryst compositions essentially in equilibrium with the whole-rock composition; (2) remobilized crystal-rich low-SiO2 andesites with resorbed phenocrysts in equilibrium with the whole-rock composition; (3) mixed low-SiO2 dacite with a relatively large range of phenocryst compositions, with most phenocrysts slightly too evolved to be in equilibrium with the whole-rock; (4) extensively mixed low-SiO2 dacites with a very large and discontinuous range of phenocryst compositions, with most phenocrysts either more Mg-rich or more evolved than the equilibrium compositions; (5) remobilized crystal-rich low-SiO2 dacites with resorbed and euhedral phenocrysts; (6) homogeneous high-SiO2 dacites lacking evidence for magma mixing and showing narrow ranges of phenocryst compositions in equilibrium with the whole-rock composition. This range of silicic magmas is interpreted to reflect a combination of closed- and open-system fractional crystallization, magma mixing and remobilization of cumulate piles by heating. The variety of magmas erupted simultaneously during the caldera-forming eruptions suggests that the magmatic system consisted of several independent reservoirs of variable composition and degree of crystallization. The magmatic evolution of individual reservoirs varied from closed-system fractional crystallization to fully open-system evolution, thereby resulting in simultaneous production of magmas with contrasted compositions and mineralogy. Extensive emptying of the magmatic system during the caldera-forming eruptions led to successive or simultaneous eruption of several reservoirs.

Keywords: caldera; ignimbrite; magmatic chambers; magma mixing; petrology; Sunda Arc.
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