Volcanic structure and composition of Old Shiveluch volcano, Kamchatka

This paper reports results of a new comprehensive geological mapping of the Late Pleistocene Old Shiveluch volcano. The mapping results and geochemical data on major and trace element composition of the volcanic rocks are used to characterize spatial distribution, eruptive sequence and volumetric relationships between different rock types of the volcano. Old Shiveluch volcano had been constructed during two main stages: initial explosive and subsequent effusive ones. Pyroclastic deposits of the initial stage are represented by agglomerate and psephytic tuffs with very few lava flows and form at least 60% of volume of the Old Shiveluch edifice. The deposits of the second stage are dominantly lava flows erupted from four vents: Central, Western, Baidarny and Southern, reconstructed from the field relationships of their lava flows. About 75% of the Old Shiveluch edifice, both pyroclastic deposits and lava, are composed of magnesian andesites (SiO2=57.3-63.8 wt.%, Mg# = 0.53-0.57). The most abundant andesitic lavas were coevally erupted from the Central and Western vents in the central part of the edifice. Less voluminous high-Al basaltic andesites (SiO2=53.5-55.7 wt.%, Mg# = 0.52-0.56) were produced by the Western, Baidarny and Southern vents situated in the south-western sector. Small volume high-Mg basaltic andesites (SiO2=53.9-55.0 wt.%, Mg# = 0.59-0.64) occur in the upper part of the pyroclastic deposits. Andesites of Old and Young Shiveluch Volcanoes have similar compositions, whereas Old Shiveluch basaltic andesites are compositionally distinctive from those of the Young Shiveluch by having lower Mg#, SiO2, Cr and Ni, and higher Al2O3, Febr, CaO, TiO2, and V contents at given MgO. Geochemical modeling suggests that the compositions of the intermediate Old Shiveluch magmas can be reasonably explained by simple fractional crystallization of olivine, clinopyroxene, plagioclase and magnetite (+/- hornblende) from water-bearing (similar to 3 wt% H2O) high-Mg# basaltic parental magma at intermediate to shallow crustal depths (<15 km). Mixing of evolved (SiO2>60 wt.%, Mg#<0.55) and primitive magmas (SiO2<54 wt.%, Mg#>0.65) played a relatively minor role in creating the compositional diversity of the Old Shiveluch magmas compared to the Young Shiveluch ones. The pronounced change in the Shiveluch magma compositions could have been related to adjustments of the magma plumbing system beneath Old Shiveluch following the large scale sector collapse in the Late Pleistocene that enabled a common mixing of evolved and primitive magmas on the later, Holocene stage of the volcano evolution. (C) 2012 Elsevier B.V. All rights reserved.