A. B. Perepelov

The space-time relationships between volcanic associations of different alkalinities: The Belogolovskii massif in Kamchatka’s Sredinnyi Range. Part 1. The geology, mineralogy, and petrology of volcanic rocks

We proposed a geological and petrologic model for the generation of the Belogolovskii Late Pliocene to Early Pleistocene volcanic massif. We identified two petrochemical series of rocks with varying alkalinities, viz., normal and moderate. The evolution of volcanic products and the mineralogic composition of rocks of varying alkalinities provide evidence that the sources of parent magmas are spatially independent and reside at different depths. Crystallization differentiation is the leading process that is responsible for the generation of the initial melts that give rise to the range of rocks within a series. The evolution of the alkaline basaltic magma occurred stepwise, producing autonomous daughter melts with the following compositions: trachybasalt-trachyandesite-trachyte-trachyrhyolite and comendite. These melts were localized in inter-mediate magma chambers at different depths.

Petrology of mafic enclaves in the 2006–2012 eruptive products of Bezymianny Volcano, Kamchatka

This paper reports the results of the first comprehensive petrological study of mafic enclaves widespread in the products of recent (2006–2012) eruptions of Bezymianny Volcano, Kamchatka. Four types of mafic enclaves were distinguished on the basis of the composition and morphology of minerals, P–T conditions of formation of mineral assemblages, and structural and textural characteristics of the rocks. Disequilibrium assemblages of mafic enclaves indicate a complex structure of the magmatic plumbing system of the volcano, including a shallow chamber with andesite–basaltic andesite magmas and a deep reservoir filled in part with plagioclase–hornblende cumulates and fed by basic magmas with mantle harzburgite xenoliths. The mafic enclaves were formed at different levels of the magmatic plumbing system of the volcano and correspond to different degrees of mixing of interacting magmas. The most abundant enclaves were formed during magma ascent from the deep reservoir (960–1040°C, 5–9 kbar) into the shallow andesitic chamber (940–980°C). Enclaves of plagioclase–hornblende cumulates from the basic magmas feeding the deep reservoir (T > 1090°C and P > 9 kbar) are much less common.

Space–time relationships between volcanic associations of different alkalinities: The Belogolovskii Massif, Sredinnyi Range, Kamchatka. Part II. Geochemistry of volcanic rocks and magma sources

Data are presented relating to volcanic series in the Belogolovskii Massif, Sredinnyi Range, Kamchatka. We discuss new geochronologic data, the distributions of rare elements and platinum elements in the rocks, and list the isotope characteristics of volcanic series with normal and moderate alkalinities. We show that the Late Pliocene to Early Pleistocene rocks that belong to the moderate alkaline series of the Belogolovskii volcanic massif are different from rocks in the normally alkaline series of the Late Miocene to Middle Pliocene volcanogenic basement in having higher concentrations of the HFSE and LILE components. We propose a model for the generation of moderate alkaline magmas involving a heterogeneous depleted and a heterogeneous enriched source of material. According to the isotope data, one of these sources may be the subducted oceanic lithosphere of the Pacific and the Commander-Islands type, while the other source was recycled material of the Indian MORB type.

Eclogite trace in evolution of Late Cenozoic alkaline basalt volcanism on the southwestern flank of the Baikal Rift Zone: Geochemical features and geodynamic consequences

Eclogitized material from the oceanic lithosphere are the most likely source of alkaline basalt magmas in the formation of Late Cenozoic volcanic areas on the southwestern flank of the Baikal Rift Zone. Basaltic trachyandesites of the early stage of volcanism (Pg32 ~ 28–23 Ma) are rich in high field strength elements (HFSE), P2O5, F, Zn, Ga, Sr, Sn, and light rare earth elements (LREE); they are characterized by high values of the following ratios: Fe/Mn = 72–77, Sm/Yb = 7.7–8.5, Sr/Y = 57–63, and Ga/Sc = 2.1–2.3. At this stage, magmas are formed under conditions with a 2–8% degree of partial melting of the mantle substrate enriched with the material of the eclogite source (50–70%) (Cpx/Grt = 1.5–1.7). Basaltoid magmas of the final stage of volcanism (N13–N21 ~ 6–4 Ma) are formed from melting (1.5–4%) of a less fertilized mantle (Cpx/Grt = 2.1–3.1, Fe/Mn = 62–71, Sm/Yb = 3.5–4.6, Sr/Y = 29–44, Ga/Sc = 1.0–1.4). The directed variations of the compositions of the successive basaltoid magmas, which were formed in the Late Cenozoic, create an “eclogite trace” in this area.

Parental melts of the last volcanic pulse in the Sedanka field, Sredinny Range, Kamchatka

Compositions of melts for the youngest and located hypsometrically below others eruption centers of Sedankinskiy field of areal volcanics were studied by the olivine-hosted melt inclusions method. The traceelement content in the melts is reported for the first time. It is argued that the previously described low-potassium melt group is a melting product of heterogeneous inclusions. The melts of these eruptions differ from those of the Kekunai field of areal volcanic rocks in trace element content within the measurement uncertainty. Having similar geochemical characteristics, they are in equilibrium with more magnesian olivine and, therefore, are the most primitive for the Holocene volcanic rocks of the Sredinny Range.The melts of the studied eruptions, which are in equilibrium with the most magnesian olivine, fall in the field of pyroxenite melting under hydrous conditions in the CMAS diagram and may be the result of nearly pure pyroxenite melting.

The Bol’shoi Semyachik volcanic massif, Kamchatka: Composition of the rocks and minerals, and petrogenesis

This paper is the first attempt at a detailed description of petrographic, mineralogical, and geochemical features for the rock complex that makes the Bol’shoi Semyachik volcanic massif (BSVM). The massif was formed in three phases: the pre-caldera phase, the caldera-generating phase, and the post-caldera phase. The rocks vary in composition from moderately magnesian basalts (48.86–51.87 wt %, SiO2, 7.4–8.3 wt % MgO) to rhyolites (75.12 wt % SiO2, 3.86 wt % K2O). The pre-caldera phase was dominated by basaltic andesites and andesites. The composition of the pyroclastic deposits varies from rhyolite to andesite, thus providing evidence of a zonal structure of the upper crustal magma chamber beneath the caldera. The upper crustal chamber received more basalts after the eruption of ignimbrites. Practically all BSVM rocks are of hybrid derivation, as suggested by non-equilibrium mineral associations that are present in the rocks and by a complex zonality of phenocrysts. The variation in the major oxides and trace elements in volcanic rocks indicate a leading part played by fractional crystallization in the origin of the entire range of rocks in the area. The BSVM magnesian basalts have low concentrations of REEs and are depleted in high-charge elements, thus suggesting a relationship between the primary melts and a depleted mantle source of the N-MORB type. High concentrations of fluid-mobile large-ion elements (Cs, Rb, Ba, K, Pb, and Sr) in these shows that the magma generation involved fluids that separated from the subducted oceanic plate. We describe a conceptual model of the associated magmatic system.

First data on REE-bearing silicates from volcanic rocks of Kamchatka

The first finds of the REE-bearing silicates in the Kamchatka island-arc system were reported from K-Na alkaline trachytes from a large volcanic body of Bolshoi volcano in the back-arc zone of the Sredinny Ridge volcanic belt (56°24′10″ N and 157°56′45″ E) of Kamchatka. Chevkinite microlites are found in a trachytic groundmass in association with microlites of ferropigeonite, ferroaugite, anorthoclase, Na-sanidine, richterite, apatite, Nb-bearing ilmenite, zirkelite, baddeleyite, and zircon. The chevkinites from the Bolshoi volcano trachytes have higher Nb and Zr and similar REE contents compared to the chevkinite-group minerals from igneous rocks formed in different geodynamic settings. The crystallization conditions for chevkinites from Bolshoi volcano were determined by comparing the compositions of trachytes and their mineral assemblages with those of trachytes from Belogolovskii volcano of Sredinny Ridge, Kamchatka. The results show that chevkinites were formed during prolonged crystallization of a trachytic magma under reducing conditions at

Volcanism in the Baikal rift: 40 years of active-versus-passive model discussion

Delta log f_{O_2 }

Origin and evolution of Neogene alkali-basaltic magmas in the southwestern flank of the Baikal rift system (Heven lava plateau, northern Mongolia)

ranging from −1.36 to −1.55 NNO, relatively low temperatures of 794–741°C, and moderate HF concentrations of magmatic fluids (0.003–0.0029 mol/dm3). These results, combined with the other data, are of crucial interest for studies of the REE budget during the evolution of high-alkaline magmas and can be used as indicators for determining the nature and role of different magma sources in the active continental margin setting of Kamchatka.