PETROLOGY OF EJECTA PRODUCED BY THE AKADEMII NAUK CALDERA ERUPTION OF JANUARY 2-3, 1996

Abstract

The subaqueous explosive eruption on January 2-3, 1996, produced tephra of predominantly aluminous calc-alkaline basalt composition. The rocks have a moderate alkalinity and an essential mineral assemblage of olivine, plagioclase, and clinopyroxene. The pumice bombs ejected during the closing phase of the eruption account for 2-3% of the overall volume of the pyroclastic material. The morphology of the bombs and their relationships with the basalts indicate that silicic and basic magmas were erupted simultaneously. The composition of the pumice bombs varies from andesite to rhyolite, with predominant rhyodacite. The occasional bombs consist of highly potassic material (3.0-4.8%), a feature that is generally atypical of the Eastern Volcanic Belt rocks. Comparative analysis of the products of the 1996 eruption with the older volcanic rocks of the Karymsky volcanic center revealed that the aluminous basalts of Late Pleistocene age, which have analogous mineral assemblages were erupted in the north of the structure. This paper reports microprobe analyses of minerals and glasses of the basalts, pumice bombs, and a partly fused granodiorite xenolith from the basalt. The highly potassic composition of the glass matrix in most of the bombs and its similarity to the composition of the glass in the fused granodiorite xenolith suggest that the bombs were produced by a melt mobilized by the effect of the high-temperature basalt on the granite material (occurring in the subsolidus state) of upper crustal magma chamber.