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 resorbedphenocgsts 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. 7 he 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.