Magmatic evolution at Batur volcanic field, Bali, Indonesia: petrological evidence for polybaric fractional crystallization and implications for caldera-forming eruptions

摘要

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.% SiO_2) 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+cli-nopyroxene+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 H_2O (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 H_2O 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.