PETROLOGICAL FEATURES OF MODERN ARC VOLCANISM ON THE EXAMPLE OF BATUR VOLCANO LAVAS (BALI ISLAND, INDONESIA)

Abstract

The relevance of the modern volcanism study is to understand the complex processes of the magma differentiation, assimilation and mixing that recorded in the mineralogical, structural and petrochemical features of volcanic rock units. On the other hand, economically significant deposit of non-ferrous and noble metals is often associated with basic volcanism of increased alkalinity in modern island-arc systems. The main aim of this work is to study and interpret the mineralogical and petrochemical features of the lavas from the active Batur volcano (Bali Island, Indonesia). Objects: samples of lava from the 1964 Batur eruption. Methods. The rock units of modern island arc lavas and the minerals found in them have been studied by optical and electron microscopy using energy dispersive spectroscopy analysis. The major elements of the rock units were obtained using silicate analysis, while trace element composition – using inductively coupled plasma mass spectrometry analysis. As a result of the conducted studies it was found that all samples of the lavas from Batur volcano have porphyry structure and contain phenocrysts of plagioclase (17...23 vol. %), olivine (3 vol. %) and pyroxene (2 vol. %) located in the fine-grained ground mass containing plagioclase, titanomagnetite, pyroxene and volcanic glass. The phenocrysts of plagioclase are characterized by a «sieve-textured domains» structure, with oriented melt inclusions (~0,05 mm) containing primary silicate and sulfide melts. The petrochemical parameters of the Batur lavas suggest that they correspond to high-alumina basalts (Al2O3≥18,0). The complex structure of zonal plagioclase phenocrysts is consistent with the rapid cooling of the initial melts, when the released plagioclase cannot come into equilibrium with the remaining liquid magma. Its bulk chemical composition corresponds to labrador-bytownite. Olivine reveals several variants for iron redistribution in its composition including zonal crystals with iron accumulation in the marginal parts of grains, specific «excolutions structures» and homogeneous olivine crystals with Mg#(Fo) [=Mg/(Fe2++Mg)*100 % atomic ratio] component ~66,0. The increased iron content and manganese oxide alloy (Mn2+) in olivine may indicate that the Batur lavas are derivatives from the olivine basalt with an alkaline bias. Presumably, during the period of intense volcanism Batur volcano eruption in 1964, the magma chamber was periodically enriched with olivine basalt at some regularity. Further, this melt rose to the upper part of the chamber, followed by fractionation of olivine and clinopyroxene during crystallization of plagioclase and their accumulation in the upper part of the magma chamber. During the period of the input of a new portion of the initial melt, the existing mixture had not come into equilibrium with the incoming portion of the melt, has erupted to the surface and formed the lavas of high-alumina basalt. The presence of covellite and chalcopyrite globules in melt inclusions in the phenocrysts of plagioclase may be the evidence of the existence of a highly fractionated sulfide melt at a deeper level.

Keywords

• modern island-arc volcanism
• high-alumina basalts
• plagioclase
• olivine
• melt inclusions
• sulfide globules