First Data on Lamprophyre-Like, Amphibole-Rich Basaltic Andesites Erupted in 2024 on the Western Slope of Shiveluch Volcanic Massif, Kamchatka

Abstract

In April 2024, a new lava dome named “300 Years of the Russian Academy of Sciences” (“300 Years of RAS”) began to grow on the western slope of the Shiveluch volcanic massif in Kamchatka. The first lava and pyroclastic samples, collected shortly after dome initiation in May 2024 and following a brief explosive episode in August 2024, are composed of amphibole-rich basaltic andesites with calc-alkaline, moderate-K, high-magnesian composition (SiO₂ = 55.7–56.3 wt %; K₂O = 1.4–1.5 wt %; MgO = 6.4–7.8 wt %; Mg# = 61–64 mol %). These rocks are mineralogically unusual for Shiveluch modern eruptions. They contain high modal amount of amphibole (up to 30 vol %), lack plagioclase phenocrysts, and contain sporadic phenocrysts of pyroxenes, and olivine grains mantled by reaction rims. These characteristics are typical of lamprophyric rocks, particularly spessartites. Petrographic and mineralogical evidence suggests that the lamprohyre-like basaltic andesites were derived from hydrous magmas ascending from mid-to-lower crustal levels (e.g. 16–24 km) of Shiveluch plumbing system. During ascent, these magmas likely interacted with andesitic magma, as indicated by amphibole–plagioclase-phyric andesite xenoliths found in early extrusive products. The eruption of rare, lamprophyre-like magmas provides new insights into deep crustal magma evolution beneath Shiveluch massif and has important implications for understanding magma transfer mechanisms and the activation of lower crustal reservoirs beneath arc volcanoes.