Kronotsky Volcano – A low-K end-member frontal volcano in Kamchatka: Geological structure and composition of rocks and minerals

Kronotsky volcano is the largest but poorly studied stratovolcano in the frontal zone of the Kamchatka arc. Here we present the first systematic data on the geological structure of this volcano and the composition of its rocks and minerals. Kronotsky lavas are predominantly low-K, high-Fe tholeiitic basalts to basaltic andesites with SiO₂ = 47.0–53.2 wt%, K₂O = 0.24–0.65 wt%, FeO^⁎ = 9.1–13.4 wt% and MgO = 3.7–11.2 wt%. Andesites (SiO₂ = 59.9–60.5 wt%, K₂O = 0.65 wt%, FeO^⁎ = 7.7–7.6 wt%, MgO = 2.4–2.6 wt%) are rare and occur among the lava cones and necks from the latest stage of activity. The major phenocrysts in Kronotsky basalts are olivine (Fo_91.9–67), plagioclase (An_95–40) and high-Ca pyroxene (Mg#_89.3–50). Orthopyroxene occurs as phenocrysts (Mg#_61–67) in the andesites and rarely in the basaltic andesites, and as inclusions (Mg#₈₃) in the high Ca-pyroxene glomerocrysts of the most mafic varieties of basalts. Ti-magnetite (Fe²⁺/Fe³⁺ = 0.98–1.51, TiO₂ = 11.0–16.5 wt%) forms subphenocrysts in andesites and microlites in the groundmass of basalts. Inclusions of Cr-spinel (Cr/Cr + Al = 0.19–0.79, TiO₂ = 0.18–2.81 wt% and Al₂O₃ = 8.7–33.2 wt%) are found in olivine phenocrysts. Variations of whole rock and mineral compositions reflect the existence of long-lived magma chamber(s) under the volcano and the processes of crystal differentiation, accumulation and magma mixing as well as crystallization associated with magma degassing during ascent and decompression.

Compared to all Quaternary volcanoes on Kamchatka, Kronotsky basalts are the most depleted in K₂O (down to 0.24 wt%) and a number of other incompatible elements (Ti, Zr, Nb, LREE), and are characterized by the lowest Nb/Zr (≤0.02) and Nb/Yb (≤0.5) ratios. In contrast, Ba/Nb (up to 280) and especially Ba/Th (up to 938) ratios are relatively high compared to other Kamchatka volcanoes. The composition of high-Mg olivine (Fo ≥ 87.5 mol%, Ni ≤2500 ppm, Fe/Mn = 56–71) indicates that the primary magmas of Kronotsky volcano originated from a peridotite mantle source. Trace element modeling suggests that the mantle was melted to a high degree (up to 25 %) and was more depleted than the source of typical MORB and of most parental magmas in Kamchatka. The exceptionally high degree of melting, combined with a strong relative enrichment in fluid mobile elements, suggests an enhanced fluid flux from the subducting Pacific plate beneath Kronotsky volcano, which is responsible for the extensive mantle melting. The Krusenstern fracture zone on the Pacific plate, subducting beneath central Kamchatka, may supply abundant hydrous fluids to the mantle wedge beneath Kronotsky volcano, contributing to its distinctive character within the Kamchatka arc.