M. Yu. Puzankov

Evolution stages and petrology of the Kekuknai volcanic massif as reflecting the magmatism in Backarc zone of Kuril-Kamchatka island arc system. Part 1. Geological position and geochemistry of volcanic rocks

The evolution of the Quaternary Kekuknai volcanic massif (the western flank of the Sredinnyi Range in Kamchatka) has been subdivided into five stages: (1) the pre-caldera trachybasaltbasaltic andesite, (2) the extrusive trachyandesite-trachydacite, (3) the early trachybasalt, (4) the middle hawaiitemugearite (with occasional occurrences of basaltic andesites), and (5) the late trachybasalt-hawaiitemugearite (with occasional andesites) of areal volcanism. On the basis of petrologic data we identified the island arc and the intraplate geochemical types of rocks in the massif. The leading part in petrogenesis was played by dynamics of the fluid phase with a subordinated role of fractional crystallization and hybridism. Successive saturation of rocks with the fluid phase in the course of melt evolution stopped at the time of caldera generation when most fluid mobile elements and silica had been extracted. The geological and petrologic data attest to the formation of the massif in the environment of a backarc volcanic basin during the beginning of rifting with active participation of mantle plume components.

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.

The evolutionary stages and petrology of the kekuknai volcanic massif reflecting the magmatism in the backarc zone of the kuril-kamchatka island arc system. Part II. petrologic and mineralogical features, petrogenesis model

The Kekuknai massif was formed in the course of tectono-magmatic activity that involved the origin of a shield volcano and a caldera depression with associated emplacement of extrusions that terminated in intense post-caldera areal volcanism. The mineralogical compositions of the massif’s rocks have been considered in detail. The use of previously known and newly developed indicator properties of rock-forming minerals allowed the reconstruction of the general picture of the magmatic melt evolution and conditions of rock crystallization (various fluid and water saturation levels, as well as the oxidation state of the system). Essentially island-arc or intraplate characteristics of the massif’s rock compositions are found at different stages of development of a single fluid-magmatic system. Decompression evolution of the parent deep-seated basanitic magma occurred via occurrence in intermediate magma chambers of daughter magmas of trachybasalt (pre-caldera stage) or hawaiite (areal volcanism) composition. Subsequent emanate-magmatic differentiation of these melts, combined with crystallization differentiation under changing P-T-conditions, resulted in the formation of the entire diversity of the Kekuknai rocks.

Neogene basanites in western Kamchatka: Mineralogy, geochemistry, and geodynamic setting

Neogene (N12-N21?) K-Na alkaline rocks were found in western Kamchatka as a subvolcanic basanite body at Mount Khukhch. The basanites have a microphyric texture with olivine phenocrysts in a fine-grained doleritic groundmass. The olivine contains inclusions of Al-Cr spinel. The microlites consist of clinopyroxene, plagioclase, magnetite, and apatite, and the interstitial phases are leucite, nepheline, and analcime. The Mount Khukhch basanites are characterized by elevated concentrations of MgO, TiO2, Na2O, and K2O, high concentrations of Co, Ni, Cr, Nb, Ta, Th, U, LREE (LaN/YbN = 10.8−12.6, DyN/YbN = 1.4−1.6) at moderate concentrations of Zr, Hf, Rb, Ba, Sr, Pb, and Cu. The values of indicator trace-element ratios suggest that basanites in western Kamchatka affiliate with the group of basaltoids of the within-plate geochemical type: Ba/Nb = 10−12, Sr/Nb = 17−18, Ta/Yb = 1.3−1.6. The basanites of western Kamchatka show many compositional similarities with the Miocene basanites of eastern Kamchatka, basanites of some continental rifts, and basalts of oceanic islands (OIB). The geochemistry of these rocks suggests that the basanite magma was derived via the ∼6% partial melting of garnet-bearing peridotite source material. The crystallization temperatures of the first liquidus phases (olivine and spinel) in the parental basanite melt (1372–1369°C) and pressures determined for the conditions of the “mantle” equilibrium of the melt (25–26 kbar) are consistent with the model for the derivation of basanite magma at the garnet depth facies in the mantle. The geodynamic environment in which Neogene alkaline basaltic magmas occur in western Kamchatka was controlled by the termination of the Oligocene—Early Miocene subduction of the Kula oceanic plate beneath the continental margin of Kamchatka and the development of rifting processes in its rear zone. The deep faulting of the lithosphere and decompression-induced magma generation simultaneous with mantle heating at that time could be favorable for the derivation of mantle basite magmas.

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.

Modeling the thermohydrogeochemical conditions for the generation of productive reservoirs in volcanogenic rocks

This paper considers the role of hydrothermal processes in the generation of porous and permeable reservoirs in volcanogenic rocks, their boundedness with low-permeability interfaces, and the accumulation of fluids of various origins and phase states in the reservoirs. The Rogozhnikovskii productive reservoir is an example of a volcanogenic oil reservoir in western Siberia, it is confined to fluid upflows that are marked by positive anomalies in temperature and pressure. The Mutnovskii productive reservoir is an example of a high-temperature two-phase (water + steam) reservoir in Kamchatka that is supplied with deep heat-carrier upflows that are also associated with positive anomalies in temperature and pressure. The iTOUGH2 inversion simulation is used to estimate the discharge of deep upflows and then to represent a possible mechanism for the evolution of permeability-porosity and self-sealing of such reservoirs that result from water-rock chemical interaction. These methods are applied to the Rogozhnikovskii and Mutnovskii reservoirs. Both of these scenarios demonstrate the possible generation of productive reservoirs by hydrothermal circulation and show a short-lived drop in pressure during the earlier phase (which favors the inflow of fluids into the reservoir) and self-sealing with low-permeability interfaces during the later phase of hydrothermal circulation (which favors long-term storage of fluids in reservoirs).

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.

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

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

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