The origin of calc-alkaline high-alumina basalts (HAB) of the Klyuchevskoy volcano, Kamchatka, was examined using electron microprobe analyses of phenocrysts and mineral phases included in the phenocrysts. Continuous trends on major-element variation diagrams suggest the HAB were derived from high-magnesia basalt (HMB) by fractional crystallization. Phenocrysts in the HAB are strongly zoned: olivine (Mg# 91–64), clinopyroxene (Wo45–38En40–51Fs5–20) and chrome—spinel/magnetite inclusions in them (Cr2O3 45–0 wt.%, TiO2 0.5–11%). Microprobe analyses of minerals included in the phenocrysts provide additional constraints on the mineral crystallization trends in the HAB. Fe/Mg partitioning data, when applied to the phenocrysts cores, show they crystallized from a HMB. The similarity of phenocryst core compositions in HAB with those in HMB strongly suggests a genetic relationship between the two magma types.

Late Pleistocene-Holocene volcanism in Kamchatka results from the subduction of the
Pacific Plate under the peninsula and forms three volcanic belts arranged in en echelon manner
from southeast to northwest. The cross-arc extent of recent volcanism exceeds 250 km and
is one of the widest worldwide. All the belts are dominated by mafic rocks. Eruptives with
SiO2>57% constitute ~25% of the most productive Central Kamchatka Depression belt and
~30% of the Eastern volcanic front, but <10% of the least productive Sredinny Range belt.
All the Kamchatka volcanic rocks exhibit typical arc-type signatures and are represented
by basalt-rhyolite series differing in alkalis. Typical Kamchatka arc basalts display a strong
increase in LILE, LREE and HFSE from the front to the back-arc. La/Yb and Nb/Zr increase
from the arc front to the back arc while B/Li and As, Sb, B, Cl and S concentrations decrease.
The initial mantle source below Kamchatka ranges from N-MORB-like in the volcanic front
and Central Kamchatka Depression to more enriched in the back arc. Rocks from the Central
Kamchatka Depression range in 87Sr/86Sr ratios from 0.70334 to 0.70366, but have almost
constant Nd isotopic ratios (143Nd/144Nd 0.51307–0.51312). This correlates with the highest
U/Th ratios in these rocks and suggest the highest fluid-flux in the source region.
Holocene large eruptions and eruptive histories of individual Holocene volcanoes have been
studied with the help of tephrochronology and 14C dating that permits analysis of time-space
patterns of volcanic activity, evolution of the erupted products, and volcanic hazards.

Volcanic eruptions from Kamchatka have deposited many unique tephra layers over a large region within the North Pacific, providing important isochrons between key sites such as marine ODP core 883 (Pacific Ocean, Detroit Seamount) and Elgygytgyn Lake (Chukotka, eastern Siberia). Here we present a compilation of C14 dates on major Holocene tephras from the volcanically highly active region, based on decades of detailed stratigraphical fieldwork on Shiveluch, Kliuchevskoy, and other volcanoes.The 12-m thick tephra sequence at the Kliuchevskoy slope has been continuously accumulating during the last ∼11 ka. It contains over 200 visible individual tephra layers and no datable organic material. The section is dominated by dark-gray mafic cinders related to Kliuchevskoy activity. In addition, it contains 30 light-colored thin layers of silicic tephra from distant volcanoes including 11 layers from Shiveluch volcano located only 65 km to the north. We have used EMPA glass analysis to correlate most of the marker tephra layers to their source eruptions dated earlier by C14 (Braitseva et al., 1997; Ponomareva et al., 2007), and in this way linked Kliuchevskoy tephra sequence to sequences at other volcanoes including Shiveluch. The C14 dates and tephras from the northern Kamchatka are then combined into a single Bayesian framework taking into account stratigraphical ordering within and between the sites. This approach has allowed us to enhance the reliability and precision of the estimated ages for the eruptions. Age-depth models are constructed to analyse changes in deposition rates and volcanic activity throughout the Holocene. This detailed chronology of the eruptions serves as a basis for understanding temporal patterns in the eruption sequence and geochemical variations of magmas. This research could prove important for the long-term forecast of eruptions and volcanic hazards.