correlation of certain stratigraphic levels in various deposiDetailed tephrochronological studies in Kamchatka Peninsula, tional successions. Their ages being determined, they beRussia, permitted documentation of 24 Holocene key-marker come excellent time markers and can be used in geochronotephra layers related to the largest explosive eruptions from 11 logical investigations. Tephra (volcanic ash) horizons are volcanic centers. Each layer was traced for tens to hundreds of among the best marker beds of this kind. C yr B.P.) SH 2 (É1000 yr B.P.) and SH 3 (É1400 yr B.P.) from is a few tens of centimeters thick in areas distant from the Shiveluch volcano; KZ (É7500 yr B.P.) from Kizimen volcano; active volcanoes and increases up to several meters at their KRM (É7900 yr B.P.) from Karymsky caldera; KHG (É7000 yr foot. This cover provides a continuous record of the explo-B.P.) from Khangar volcano; AV 1 (É3500 yr B.P.), AV 2 (É4000 sive eruptions during the Holocene, while earlier ash layers yr B.P.), AV 4 (É5500 yr B.P.), and AV 5 (É5600 yr B.P.) from in Kamchatka were almost everywhere destroyed during Avachinsky volcano; OP (É1500 yr B.P.) from the Baraniy Amfi-Late Pleistocene glaciation and occur as isolated beds. stratigraphic value and 9 important local marker ash layers.from Shtyubel cone in Ksudach volcanic massif; and KO (É7700 yr B.P.) from the Kuril Lake-Iliinsky caldera. Tephra layers SH 5 Identification of tephra sources was possible due to prelimi-(É2600 yr B.P.) from Shiveluch volcano, AV 3 (É4500 yr B.P.) nary studies that included detailed mapping of the Holocene from Avachinsky volcano, OP tr (É4600 yr B.P.) from Opala vol-eruptive centers (Melekestsev et al., 1974) and investigations cano, KS 3 (É6100 yr B.P.) and KS 4 (É8800 yr B.P.) from Ksudach of geochemical types and petrology of Late Cenozoic volcacalderas, KSht 1 (É1100 yr B.P.) from Shtyubel cone, and ZLT nic rocks (Volynets, 1994). These data allowed us to identify

The ages of most of calderas,

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 SiO 2 >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 87 Sr/ 86 Sr ratios from 0.70334 to 0.70366, but have almost constant Nd isotopic ratios ( 143 Nd/ 144 Nd 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 14 C dating that permits analysis of time-space patterns of volcanic activity, evolution of the erupted products, and volcanic hazards.

The ages of most of calderas,