A.I. Kiselev, I.V. Gordienko, V.V. Lashkevich
Petrological aspects of gravitational instability of the tectonically thickened lithosphere

The lower part of the lithosphere in collisional orogens can be delaminated due to density inversion between the asthenosphere and the colder thickened lithospheric mantle. Density changes within the crust and lithospheric mantle resultant from phase transition and compositional variability with the change of P-T conditions are not usually considered in delamination models. We emphasize that these effects can be very important, probably dominant in regard to the effect of simple change of the mantle thermal structure. This paper presents results of numerical modeling using the “Selector” program for eclogitization of basalts of the lower crust as well as changes of phase composition and density of underlying peridotite resultant from tectonic thickening of the lithosphere and its dipping into the asthenosphere. As the depth of the lower crust increases, the basic granulites (basalts) pass into eclogites. Incidentally, at the crust-mantle boundary (P=20 kbar) density inversion is noted, since the newly formed eclogite is heavier than the underlying peridotite by 6%. The difference in densities is the potential energy for delamination of the eclogitic part of the crust. P=70 kbar and T=1300°C correspond to the lower boundary of the lithosphere according to the model. Asthenospheric temperature is equal to 1400°C. Density inversion is only attained, given the isochemical composition of the lithosphere and asthenosphere at the expense of 100°С temperature difference, and constitutes 0.0022%. Compositional differences of two other model-derived asthenospheric compositions with regard to the lithosphere are not compensated by its higher temperature. Asthenospheric density is greater than that of the lithosphere base. Density inversion occurs if one assumes the presence in asthenosphere composition comparable to primitive mantle or KH-lherzolite no less than 1.40 and 0.83 wt. % of conventionally neutral fluid, respectively. Such amount of the fluid is clearly overstated and is not completely consistent with contemporary estimates of fluid content in the mantle. Therefore, only a fluid-bearing asthenosphere corresponding to the composition of depleted mantle of mid-ocean ridges (DMM) i.e. a reservoir existing from the Precambrian, is the most appropriate material for lithosphere delamination. In this model, abyssal peridotite is most closely corresponds to DMM with respect to other more fertile compositions of the asthenosphere. Heat advection related to the uplift of fluid-bearing plumes distant from collision events can initiate repeated delamination of gravitationally unstable lithospheric parts.

 V.P. Simanenko, A.I. Malinovsky, V.V. Golozubov
Early Cretaceous basalts of the Kema terrane, a fragment of the Moneron-Samarga island arc system

Data on geology, petrology, chemical composition of rock-forming minerals, petrochemistry, and geochemistry of the Early Cretaceous basalts of the Kema Formation exposed in the Kema River drainage area (North Primorye) are given. The Kema Formation is a major structural component of the Kema Terrane interpreted as the Sikhote-Alin part of a larger Moneron-Samarga island arc system. The views on the subject of basalt petrogenesis and geodynamic conditions of the Early Cretaceous magmatism are presented. It is shown that the main petrological and geochemical characteristics of the Early Cretaceous (Aptian to Albian) volcanic rocks show affinities to basalts of K-rich calc-alkaline and shoshonite series from the back-arc zones of mature island arcs. It is suggested that basaltic melts were derived from the mantle; the latter being geochemically similar to the intraplate Hawaii -type source enriched in fluid that arrived from the subducting pelagic deposits. Low concentrations of femic elements and the values of some indicative trace element ratios suggest that the studied basalts formed from residual magmas produced by olivine and pyroxene fractionation as well as by early separation of plagioclase from primary picrobasaltic mantle-derived magmas.

A.A. Sorokin, V.A. Ponomarchuk, I.M. Derbeko, A.P. Sorokin
New data on geochronology of magmatic assemblages in the Khingan-Olonoy volcanic zone (Russian Far East)

The paper features new isotopic and geochronological data for the Mesozoic magmatic assemblages of the Khingan-Olonoy volcanic zone of the Khingan-Okhotsk volcanic belt obtained by ⁴⁰Ar/³⁹Ar technique as well as the first results of geochemical studies of rocks conducted with the use of precision analytical devices. Two discrete stages of magmatic activity (111-105 Ma and 101-99 Ma) are distinguished in the history of formation of the volcanic zone under study. The former stage corresponds to eruption of the differential basalt-andesite-dacite association (with predominance of andesites), and the latter stage corresponds to manifestation of essentially acid magmatism (normal, subalkali, and alkali compositions). In spite of the fact that the collection of the analyzed samples covers all the already known rock complexes of the region, the traditional concept of the presence of volcanics of a wide age range (from the Early Cretaceous to the Cretaceous- Paleogene boundary) within the Khingan-Olonoy zone is not confirmed at the present stage of investigations. Geochemical peculiarities of rocks in this zone do not allow us to suggest their origin in a subduction environment; however, they allow the formation of the rocks in an environment of the transform continental margin.

A.V .Okrugin
Crystallization-liquation model of the formation of platinum-group element-chromitite ores in mafic-ultramafic complexes

An analysis was made of phase diagrams of petrogenic oxides and mineralogical-petrological features of the rocks of mafic-ultramafic complexes, which suggests liquation formation of platinum-group element-chromitite ores. The author constructed a generalized hypothetic SiO2–(Cr2O3 + Fe₂O₃ + Al₂O₃)–(MgO + FeO + CaO) ternary diagram reflecting crystallization-and-liquation differentiation of picritic magma resulting in segregation of the melt into immiscible silicate and high-Cr liquids. It is shown that during gravitational fractionation the liquation oxide-ore liquid passes through large volumes of enclosing magma, adsorbing and accumulating platinum-group elements. The low solubility of platinum-group elements in chrome-spinellids conditions segregation in the course of crystallization of ore liquates, of discrete grains of platinum-group minerals forming both lode and placer deposits.

 M.V. Goroshko, V.A. Guroyanov
Conditions of location of complex uranium-rare metal mineralization in the massifs of ultrabasic alkali rocks, south-eastern part of the Siberian platform

This paper gives characteristics of the geological position, composition and geochemical features of central-type intrusive massifs holding carbonatites which are developed in the south-eastern part of the Siberian platform, including those that have been mapped personally by one of the authors.
Using new data obtained by production geological organizations during prospecting, the features of location of uranium-rare metal mineralization in different massifs of ultrabasic alkali rocks are characterized. It is shown that uranium-rare metal mineralization belongs to three types of ore-bearing rocks: carbonatites, metasomatites of calcite-magnesium-fluorine-ferruginous composition replacing pyroxenites, and albitites in fault zones at massifs’ exocontacts.
Practical implications of uranium-rare metal mineralization in central-type massifs are considered in case of recognition in them crusts of weathering and alluvial placers.

T.D. Zonova, EA. Yazykova
Cretaceous Aucellina from the Sikhote- Alin.

The results of the study of representatives of the Aucellina genus from the southern part of the Pacific Coast Area (Sikhote-Alin and the Lower Amur Region) are given in this paper. Seven species were determined, including five endemics. The detailed faunal locations are shown. The representatives of other attendant faunal groups are also referred to. Different evidence for the stratigraphic range of the investigated taxa is discussed. The Aptian-Albian interval is proposed as an optimum for these species.

V.I. Dzhurik, S.P. Serebrennikov, A.D. Drennov
The study of dynamics of seismic risk in conditions of the evolving cryolite zone

Data on the analysis of seismic effects during large earthquakes are offered for the territories located in different climatic zones of Siberia and northeastern Asia. The trends dominating the change of macroseismic development are indicative of the fact that damping of seismic intensity with distance is different for continuous and insular permafrost zones. Physical and geological models characterizing the most probable types of near-surface and deep-seated inhomogeneities of the cryolite zone are constructed based on experimental research data. Their influence on the initial seismic signals is determined in the course of probable temperature variations and permafrost degradation under the effect of simulated and man-made factors. As a first approximation, the dynamics of seismic risk at probable temperature variations in the cryolite zone is studied based on this determination.