ИСТИНА

Geochemical variations in an E-W transect across the Klyuchevskaya Volcanic Group (KVG), Kamchatka, are constrained by a compilation of published data complemented by new major and trace element data on rocks from five previously poorly-studied volcanoes: Bolshaya Udina, Malaya Udina (Udina massif), Ovalnaya Zimina, Ostraya Zimina (Zimina massif) and Gorny Zub. Laves erupted from these volcanoes are medium-K calcalkaline basaltic andesites to dacites except few low-Mg basalts. On K O vs SiO diagram three trends are distinguished: Trend 1 with low potassium includes lavas from Gorny Zub, Bolshaya Udina, and few samples from Zimina massif; Trend 2 with elevated potassium consists of lavas from Malaya Udina and partly lavas from Zimina massif; Trend 3 with highest potassium is formed by Ovalnaya Zimina lavas. Older basement lavas all belong to the high-K basalt-andesite rocks, suggesting a period of variable in silica high-K volcanism in the KGV during the Middle Pleistocene. Estimated degrees of mantle melting for different trends are similar and cannot explain the large variation of K O in the lavas. However, mixing of melts derived from variably enriched/depleted mantle sources and subsequent fractional crystallization can explain all rock variations. Compared to other KVG lavas most of our samples form narrow trends that only show minor overlap with the compositions of other KVG volcanoes, except Bezymianny. For the same MgO, these lavas are poorer in alkalis, TiO , P O , FeO, and higher in SiO compared to other KVG volcanics. The (MgO) - corrected major, trace elements and trace element ratios in middle-K series show a systematic increase in incompatible, fluid-mobile elements with slab depth from 160 km in the E to 200 km in the W. This zonation is also expressed in increasing LILE, LREE, lower LILE/HFSE ratios from E to W at nearly constant MREE and HREE, which suggests variable contribution of slab components to the mantle source. The recent increase in seismicity below Udina massif could herald future eruptions in this area. The compositional similarity between Udina lavas and rocks from Bezymianny volcano suggests the possibility of similar explosive eruptions from Udina. This research was supported by grants DFG Wo 362/51-1, and RFBR 16-55-12040 and 17-55-50005.