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

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.