Holocene eruptive history of Ksudach volcanic massif, South Kamchatka: evolution of a large magmatic chamber

The combination of geological, tephrochronological and geochemical studies is used to reconstruct the Holocene eruptive history of Ksudach volcanic ma ssif, South Kamchatka and to trace the evolution of its magma. Ksudach is l ocated in the frontal volcanic zone of Kamchatka. From Early Holocene till AD 240, the volcano had repetitive voluminous caldera-forming eruptions. La ter they gave way to frequent moderate explosive-effusive eruptions that fo rmed the Shtyubel' stratovolcano inside the nested calderas, and then to fr equent larger explosive eruptions. Holocene eruptive products are low-K2O t wo pyroxene-plagioclase basaltic andesite to rhyodacite, Mineralogical, geo chemical and isotopic data suggest that all the rock varieties originated a s a result of fractionation of an initial mafic melt, with insignificant co ntamination and assimilation. Intensive mixing of the fractionating melts p rior to, and during the course of the eruptions, is ubiquitous. The eruptio ns might have been triggered by repetitive injections of new mafic melt int o the silicic chamber. Crystallization of the andesitic and rhyodacitic mel ts is estimated to have occurred at temperatures of 970-1010 degrees C and 890-910 degrees C, respectively, P-H2O 1.5-2.0 kbar and f(O2) close to the NNO buffer. According to the experimental data, such P-H2O corresponds to 4 .5%-5.5% of water in the melt, that is close to the content of water in the silicic hornblende-bearing magmas of the rear zone of the Kuril-Kamchatka are. Hence, we suggest that the transition from pyroxene phenocryst associa tions of the frontal zone to the hornblende-bearing ones of the rear zone m ight be interpreted as reflecting higher temperatures of crystallization of the melts from the frontal zone rather than increasing water content in th e rear zone magmas. (C) 1999 Elsevier Science B.V. All rights reserved.