Major and trace element and Sr–Nd–Pb isotopic variations in mafic volcanic rocks hve been studied in a 220 km transect across the Kamchatka arc from the Eastern Volcanic Front, over the Central Kamchatka Depression to the Sredinny Ridge in the back-arc. Thirteen volcanoes and lava fields, from 110 to 400 km above the subducted slab, were sampled. This allows us to characterize spatial variations and the relative amount and composition of the slab fluid involved in magma genesis. Typical Kamchatka arc basalts, normalized for fractionation to 6% MgO, display a strong increase in large ion lithophile, light rare earth and high field strength elements from the arc front to the back-arc. Ba/Zr and Ce/Pb ratios, however, are nearly constant across the arc, which suggests a similar fluid input for Ba and Pb. La/Yb and Nb/Zr increase from the arc front to the back-arc. Rocks from the Central Kamchatka Depression range in 87Sr/86Sr from 0·70334 to 0·70366, but have almost constant Nd isotopic compositions (143Nd/144Nd 0·51307–0·51312). This correlates with the highest U/Th ratios in these rocks. Pb-isotopic ratios are mid-ocean ridge basalt (MORB)-like but decrease slightly from the volcanic front to the back-arc. The initial mantle source ranged from N-MORB-like in the volcanic front and Central Kamchatka Depression to more enriched in the back-arc. This enriched component is similar to an ocean-island basalt (OIB) source. Variations in (CaO)6·0–(Na2O)6·0 show that degree of melting decreases from the arc front to the Central Kamchatka Depression and remains constant from there to the Sredinny Ridge. Calculated fluid compositions have a similar trace element pattern across the arc, although minor differences are implied. A model is presented that quantifies the various mantle components (variably depleted N-MORB-mantle and enriched OIB-mantle) and the fluid compositions added to this mantle wedge. The amount of fluid added ranges from 0·7 to 2·1%. The degree of melting changes from ∼20% at the arc front to <10% below the back-arc region. The rocks from volcanoes of the northern part of the Central Kamchatka Depression—to the north of the transect considered in this study—are significantly different in their trace element compositions compared with the other rocks of the transect and their source appears to have been enriched by a component derived from melting of the edge of the ruptured slab.

Data on the geology, petrography, and geochemistry of previously geochemically unstudied Middle-Late-Pleistocene rocks from Tolbachik volcanic massif (Central Kamchatka Depression, CKD) are presented. Two volcanic series – middle-K and high-K were erupted. The geochemical history of the massif was started earlier 86 ka (K-Ar dating) with the formation of the Tolbachik pedestal presented by middle-K series. During stratovolcanoes formation both series occur and the role of high-K melts was increasing with time. In Holocene high-K rocks are dominated but some cinder cone lavas are presented by middle-K high-Mg melts which suggest that both volcanic series are still exists. The computer modeling show that both series can be explained by the process of crystal fractionation at different water content from nearly or the same mantle source similar to high-Mg basalts of 1975 Northern Breakthrough. Middle-K rocks could crystallize at water-rich conditions (more than 2% of H2O) while the high-K rock could crystallize at dry conditions at the same pressure. However the existence of different mantle sources and possible magma mixing cannot be excluded. Our data show that fractional crystallization at different P-T-H2O-fO2 conditions can be one of the main processes responsible for rock variations at CKD. Sr-Nd-Pb isotopes suggest 2-4% of crustal assimilation to the magma chamber during pedestal and stratovolcanoes formation while lava-cinder cones are not show evidences of crustal assimilation. Major and trace element data coupled with K-Ar dating provide strong evidence that Povorotnaya mount located in 8 km NE of Plosky Tolbachik is the old block of the Tolbachik massif pedestal and for the moment the oldest known object (306 ka by K-Ar dating) in Klyuchevskaya group.

Geology, petrology and geochemistry of the Tolbachik volcanic massif, Kamchatka, Russia. Available from: https://www.researchgate.net/publication/282656425_Geology_petrology_and_geochemistry_of_the_Tolbachik_volcanic_massif_Kamchatka_Russia [accessed Jun 19, 2017].

Kamen volcano is an extinct volcanic complex located in the central part of the Klyuchevskaya group of volcanoes (KGV) between active Klyuchevskoy, Bezymianny, and Ploskie Sopky volcanoes. Kamen volcano was mapped by V.A. Ermakov only in the 1970s. However the modern geochemical studies of Kamen volcano have not been previously carried out and its relationship and petrogenesis in comparison to other active neighbors are unknown. A modern geochemical study of Kamen volcano is needed because it will shed light not only on the history of the volcano itself and its closest neighbors, but also on the history and magmatic evolution of the KGV melts in general. The distance between the summits of Kamen and Klyuchevskoy is only 5 km, the same as between Kamen and Bezymianny. The close relationship in space and time of the KGV and the common zone of seismicity below them suggests a common source and a possible genetic relationship between their magmas. However, the Late-Pleistocene-Holocene lavas of all these neighboring volcanoes are very different: high-Mg and high-Al Ol-Cpx-Pl basalts and basaltic andesites occur at Klyuchevskoy volcano, and Hbl-bearing andesites and dаcites dominate at Bezymianny volcano. The rocks of Ploskie Sopky volcano, situated only 10 km NW of Kamen, are represented by medium-high-K subalkaline lavas.

The primary goal of this paper is to summarize all of the published data on the Tolbachik volcanic massif in order to provide a clear framework for the geochronologic, petrologic, geochemical and to a lesser extent the geophysical and tectonic characteristics of the Tolbachik system established prior to the 2012–2013 eruption. The Tolbachik massif forms the southwestern part of the voluminous Klyuchevskoy volcanic group in Kamchatka. The massif includes two large stratovolcanoes, Ostry (“Sharp”) Tolbachik and Plosky (“Flat”) Tolbachik, and a 70 km long zone of the basaltic monogenetic cones that form an arcuate rift-like structure running across the Plosky Tolbachik summit. The Tolbachik massif gained international attention after the 1975–1976 Great Tolbachik Fissure Eruption (GTFE), which was one of the largest eruptions of the 20th century and one of the six largest basaltic fissure eruptions in historical time. By the end of the GTFE, 2.2 km3 of volcanic products of variable basaltic compositions with MORB-like isotopic characteristics covered an area of > 1000 km2. During the following three decades more than 700 papers on various aspects of this eruption have been published both in national and international journals. Although the recent 2012–2013 eruption, which is the main topic of this volume, was not as long as the {GTFE} in duration or as large in area and volume of the erupted deposits, it brought to the surface a unique volcanic material never found before. In order to understand the data from new eruptions and make significant progress towards a better understanding of the Tolbachik magmatic system it is important to be able to put the new results into the historic context of previous research.

The data on the geochemistry of the rocks of Kizimen Volcano and results of microprobe studies of major and trace elements in plagioclase grains from acid lavas and basalt inclusions are presented. The characteristics of the Kizimen Volcano are the following: (1) basalt inclusions are abundant in acid lavas; (2) banded, mixed lavas occur; (3) the distribution curves of rare earth elements of acidic lavas and basalt inclusions intersect; (4) Sr–Nd isotope systematics of the rocks and inclusions do not indicate mixture with crustal material; (5) plagioclase phenocrysts are of direct and reverse zonation; (6) olivine and hornblende, as well as acid and mafic plagioclases, coexist in the rocks. The studies revealed that the rocks are of a hybrid nature and originated in the course of repeated mixture of acid and mafic melts either with chemical and ther mal interaction of melts or exclusively thermal ones. Study of the major and trace element distribution in zonal minerals provides an informative tool for understanding the history of the generation and evolution of melts in a magma chamber

Abstract Data on the geology, petrography, mineralogy, and geochemistry of rocks from Kamen Volcano (Central Kamchatka Depression) are presented and compared with rocks from the neighbouring active volcanoes. The rocks from Kamen and Ploskie Sopky volcanoes differ systematically in major elemental and mineral compositions and could not have been produced from the same primary melts. The compositional trends of Kamen stratovolcano lavas and dikes are clearly distinct from those of Klyuchevskoy lavas in all major and trace element diagrams as well as in mineral composition. However, lavas of the monogenetic cones on the southwestern slope of Kamen Volcano are similar to the moderately high-Mg basalts from Klyuchevskoy and may have been derived from the same primary melts. This means that the monogenetic cones of Kamen Volcano represent the feeding magma for Klyuchevskoy Volcano. Rocks from Kamen stratovolcano and Bezymianny form a common trend on all major element diagrams, indicating their genetic proximity. This suggests that Bezymianny Volcano inherited the feeding magma system of extinct Kamen Volcano. The observed geochemical diversity of rocks from the Klyuchevskaya group of volcanoes can be explained as the result of both gradual depletion over time of the mantle N-MORB-type source due to the intense previous magmatic events in this area, and the addition of distinct fluids to this mantle source.

Data on the geology, petrography, and geochemistry of Middle–Late-Pleistocene rocks from the Tolbachik volcanic massif (Kamchatka, Klyuchevskaya group of volcanoes) are presented and compared with rocks from the neighboring Mount Povorotnaya, Klyuchevskaya group basement, and Holocene–historical Tolbachik monogenetic cones. Two volcanic series of lavas, middle-K and high-K, are found in the Tolbachik massif. The results of our data analysis and computer modeling of crystallization at different P–T–H2O–fO2 conditions allow us to reconstruct the geochemical history of the massif. The Tolbachik volcanic massif started to form earlier than 86 ka based on K–Ar dating. During the formation of the pedestal and the lower parts of the stratovolcanoes, the middle-K melts, depleted relative to NMORB, fractionated in water-rich conditions (about 3 of H2O). At the Late Pleistocene–Holocene boundary, a large fissure zone was initiated and the geodynamical regime changed. Upwelling associated with intra-arc rifting generated melting from the same mantle source that produced magmas more enriched in incompatible trace elements and subduction components; these magmas are high-K, not depleted relative to N-MORB melts with island arc signatures and rift-like characteristics. The fissure opening caused degassing during magma ascent, and the high-K melts fractionated at anhydrous conditions. These high-K rocks contributed to the formation of the upper parts of stratovolcanoes. At the beginning of Holocene, the high-K rocks became prevalent and formed cinder cones and associated lava fields along the fissure zone. However, some features, including 1975–1976 Northern Breakthrough, are represented by middle-K high-Mg rocks, suggesting that both middle-K and high-K melts still exist in the Tolbachik system. Our results show that fractional crystallization at different water conditions and a variably depleted upper mantle source are responsible for all observed variations in rocks within the Tolbachik volcanic massif. Sr–Nd isotopes are consistent with 2–4 crustal assimilation during formation of the pedestal and stratovolcanoes, while the young lava fields do not show evidence of crustal assimilation. Major and trace element data coupled with K–Ar dating provide strong evidence that Mount Povorotnaya, located in 8 km northeast of Plosky Tolbachik, is an old block of the Tolbachik massif pedestal and for the moment it is the oldest (306 ka) known object in Klyuchevskaya group of volcanoes.