Batur volcanic field (BVF), in Bali, Indonesia underwent a complex
evolution that comprised three periods of building and two major
caldera-forming eruptions. The magmas erupted show a broad range of
compositions from basaltic to rhyolitic (50-71 wt-% SiO2) and variable
phenocryst assemblages. Based on their mineralogy and geochemistry,
these magmas may be assigned to four distinct suites: (1)
plagioclase+olivine suite that comprises basalts only, (2)
plagioclase+orthopyroxene+clinopyroxene+amphibole suite that includes
andesites, (3) plagioclase+clinopyroxene+orthopyroxene suite that
consists of basalts to andesites and (4)
plagioclase+olivine+clinopyroxene suite with basaltic andesite to dacite
magmas. Modelling indicates that the compositional variations within the
various suites were produced by fractional crystallization of observed
phenocryst assemblages. It also suggests that the four suites were
produced by fractionation from basaltic parental magmas with similar
major element compositions. Incompatible trace element ratios, however,
indicate that there were significant differences in trace element
contents between the parental melts, likely to reflect variable degrees
of depletion of a MORB-like mantle wedge and variable degrees of
metasomatism by a slab-derived component enriched in large ion
lithophile elements (LILE) and light rare earth elements (LREE).
Additional variations in H2O (1-3 wt.%) within the parental melts are
necessary to account for the range of phenocryst assemblages that
produced the four suites. Initial variations in H2O content combined
with variable pressure of crystallization (possibly < 1-7 kbar) under
conditions varying from water-undersaturated to water-saturated may have
produced the observed range of phenocryst assemblages and ultimately
controlled the different fractionation trends. The repeated occurrence
of magmas from different suites during a single period of activity
suggests that the magmatic system at Batur comprised several independent
conduits and reservoirs dispersed at different levels in the crust and
near the mantle/crust interface. During the first building period, the
plagioclase+olivine+clinopyroxene suite became progressively predominant
to totally obscure the other suites during and after the caldera-forming
eruptions. This is thought to reflect progressive development of a
shallow magmatic system, conditions that ultimately resulted in the two
catastrophic caldera-forming eruptions. (C) 2004 Elsevier B.V. All
rights reserved.