Magnesian Basalts of Shiveluch Andesite Volcano, Kamchatka

Abstract

The eruptive history of the Shiveluch andesite volcano included two Holocene events, during whichthe volcano erupted unusual rocks: medium-potassium, amphibole-bearing magnesian basalts (7600 years ago) and high-potassium magnesian basalts with phlogopite and amphibole (3600 years ago). The volumes of tephrawere approximately 0.1 and 0.3 km3, respectively. Some of the mineralogical and geochemical features of the Holocene basalts were inherited by the subsequent basaltic andesites and andesites. These are similar in Mgvariation ranges of olivine, clinopyroxene, and amphibole phenocrysts, high Mg contents, and high Cr and Ni concentrations. This and the results of mass-balance calculations do not contradict the view that the Shiveluchvolcanic rocks originated during the crystal fractionation of Holocene basalt melts. However, the other geochemical features of the Shiveluch rocks, e.g., their similar REE contents, cast doubt on the formation ofthe magnesian basaltic andesites through fractional crystallization of magnesian basalt magma and suggest that they originated as a result of interaction between magnesian basalt magma and a depleted mantle material at ashallow depth. At the same time, the different mineral compositions of the Holocene medium- and high-potassium basalts and the results of mass-balance calculations indicate that their parental magmas might be producedby the melting of different rocks.