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Литосфера

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Том 18, № 5A (2018)
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5-19 25
Аннотация

Earth has a number of differences from the planets of the Solar System, as well as other stellar-planetary systems, which were acquired during its formation and geological history. The early chaotian aeon was marked by Earth’s accretion, the differentiation of its primary material into a mantle and a core, as well as the by formation of its satellite (Moon). Earth’s geological history began 4500 million years ago in the Hadean aeon. At that time, the endogenous processes on Earth were largely controlled by meteorite and asteroid bombardments, which caused large-scale melting and differentiation of its upper layers. In magmatic chambers, differentiation proceeded until the appearance of granitoid melts. The Hadean continental crust was almost completely destroyed by meteorite bombardments, with the last heavy bombardment occurring at the end of the Hadean aeon (4000–3900 Ma). Conclusions about the geological situation of this aeon can be drawn only from the preserved Hadean zircons. In particular, their geochemical features indicate that Earth had an atmosphere. The Hadean aeon was replaced by the Archaean one, starting from which the processes of self-organisation were predominant on Earth. At that time, a crust composed of komatiite-basalt and tonalite-trondhjemite-granodiorite (TTG) rock series was forming. Its formation was driven by sagduction processes – vertical growth of the crust over rising mantle plumes. Thus, the lower basaltic crust subsided into the mantle, eclogitised and melted, which led to the appearance of sodium TTG rocks series. At the end of the Archaean aeon (3.1–3.0 Ga), lid tectonics, which determined the structure and development of the Archaean crust, was replaced by small-plate tectonics that later evolved into modern plate tectonics combined with mantle plume tectonics.

20-36 8
Аннотация

Murzinka massif constitutes an interformational sheet-like body reaching up 6 km in length steeply dipping to the east. Proterozoic metamorphic rocks of predominantly granulite facies (p = 5–6 kbar, T = 750–800℃) occur at the base of the massif, with Silurian-Devonian volcanic-sedimentary rocks metamorphosed in the epidote-amphibolite facies occurring in its roof. The petrogenic elements were determined at the Laboratory for Physicochemical Research Methods of the Zava ritsky Institute of Geology and Geochemistry, UB RAS. The content of trace elements was determined at the laboratories of the University of Granada in Spain and at the Institute of Geology and Geochemistry using the ICP-MS method. In the eastern direction, the rocks change their composition from predominantly basic to granitoid as they approach the massif. The gneisses of the granitoid composition experienced a high degree of melting; the anatectic melt formed the western part of the Murzinka massif. The granites form three complexes: 1) Yuzhakovo – veins of biotite orthoclase antiperthite gra nites varying in K2O content in the metamorphic rocks of the base of the massif; 2) Vatikha – biotite orthoclase antiperthite granites making up the western part of the murzinka massif; 3) Murzinka – two-mica predominantly microcline granites occurring in the eastern part of the massif. Vatikha and Murzinka granites have the same isotopic age (about 255 Ma). A clear geochemical zonation is revealed in the massif: from west to east (from the base to the roof), the contents of Rb, Li, Nb, Ta increase in the granites of the Vatikha and Murzinka complexes. In the same direction, the ratios K/Rb, Zr/Hf, Nb/Ta decrease, as well as the contents of Ba and Sr. Naturally, the compositions of such rock-forming minerals as plagioclase and biotite also change. The isotopic characteristics of the granites of the Vatikha (Sri = 0.70868–0.70923 and εNd255 from –8.9 to –11.9) and Murzinka (Sri = 0.70419–0.70549, εNd255 from –2.6 to +2.3) complexes suggest that the substratum of the former was represented by Proterozoic granite-gneisses, whereas the rocks of the newly formed crust, possibly similar to the Silurian-Devonian volcanogenic-sedimentary rocks, which are at contact with the Murzinka granites, served as the substrate for the latter.

37-47 18
Аннотация

Plume-related magmatism is widespread and its existence is well-founded. Mostly, plume-related magmatism is represented by trap rocks, oceanic island basalts (OIB) and oceanic plateau basalts (OPB), although the composition of plume-related igneous products is very diverse. Among others, silicic igneous rocks – rhyolites and granites – play a prominent role. Two main types of plume magmatism are recognised. The former comprises Large Igneous Provinces (LIP) and is thought to be born at the core-mantle boundary within structures called superswells, which produce giant, short-lived mantle upwellings resulting in abundant magmatism on the earth’s surface. The latter is represented by time-progressive linear volcanic chains formed by single plumes – thin upward mantle flows being continuously active during longer periods of time. It is shown that the relative volume of silicic magmatism strongly depends on the type of the earth’s crust. Among continental trap basalts, silicic magmatism is usually present, being subordinate to the basalts in volume, and belongs to the bimodal type. However, in some cases, continental LIPs are formed predominantly by silicic rocks (silicic LIPS, or SLIPS). Oceanic LIPs are mainly basaltic comprising an insignificant or no amount of silicic rocks. Time-progressive volcanic chains are rarely found on the continents and, as a rule, include a significant silicic component. Oceanic chains are comprised mostly of basalts (OIB), although at the top of volcanoes there are more acid and alkaline differentiates, which, howe ver, usually lack rhyolites and granites, except for the cases when the relics of the continental crust or anomalously thick mafic crust are present. The analysis suggests that the melting of continental crust plays an important role in the formation of plume-related rhyolite-granite magmatism. As for the Urals, the presence of plume-related magmatism in its history has been pro ven relatively recently. Plume events characterised by the presence of (rhyolite)-granite components include mashak (1380–1385 Ma), Igonino (707–732 Ma), Mankhambo (mainly Cambrian), Ordovician Kidryasovo, Stepninsky (Permian) and Urals-Siberian (Triassic).

48-56 10
Аннотация

The determination of ancient ages of zircons in dunites from the orogenic regions and central-type platform massifs raised a number of issues: 1) the equilibrium of zircon with dunite material and, as a result, the possibility of determining the age of dunite using zircon; 2) polychronic zircons in dunites and the mechanism for the formation of zoned zircon crystals; 3) the origin of the oldest dunite material dated at more than 2500 Ma; 4) mechanism for the formation of zoned zircon crystals in dunite. The article presents results of studying phase equilibrium in the system MgO–SiO2–ZrO2, which confirmed the possibility of zircon crystallisation in equilibrium with olivine and pyroxene. It was found that zircon is stable in dunite up to 1450°C, whereas at higher temperatures zircon is replaced by baddeleyite. It is shown that zoned zircon crystals can be formed in dunite as a result of the gradual transformation of zircon into baddeleyite and vice versa. Drawing on the experimental data, the authors proposed a mechanism for the accumulation of dunite material in the form of restite, which forms during the partial melting of mantle peridotite, as well as the possible way for dunite resitite to raise to the surface in the form of diapir. The difference between the Ural alpine-type ultrabasites and the ultrabasites of the Platiniferous Belt is discussed. It is proposed that the alpine-type ultrabasite occur сloser to the surface where they actively interact with water.

57-77 16
Аннотация

The article considers xenocrysts and megacrysts hosted in the rocks of Early Cretaceous olivine-basalt-basanite-nephelinite association that outcropped in the erosion crater of Makhtesh Ramon (Natural Reserve of Mishmar ha-Nagev, Israel). This magmatic rock association contains a wide spectrum of xenoliths trapped at different crustal levels. These are upper mantle, lower and upper crustal xenoliths. Mantle xenoliths are represented by peridotites, olivine clinopyroxenites, clinopyroxenites, olivine websterites, websterites and their amphibole-bearing analogues. Lower crustal xenoliths are mafic granulites, such as metagabbros and plagioclasites, whereas upper crustal xenoliths are the fragments of Neoproterozoic tuffs. Xenocrysts and megacrysts are the fragments of xenoliths that chipped from them on their way to the surface. Alterations in xenoliths, xenocrysts and megacrysts caused by the host melt constitute a common petrographic feature. Xenocrysts and megacrysts are mainly represented by minerals that are compatible with the magmatic rock association. These are olivine, clinopyroxene, amphibole, nepheline, plagioclase, anorthoclase, apatite, magnetite and spinel. The xenocrysts of quartz and orthopyroxene are incompatible with the SiO2-undersaturated host rock of this magmatic association. Main reasons determining the interaction between magma and xenoliths include rapid decompression, metamorphism and metasomatism. Xenocrysts are subjected to metamorphism that corresponds to high-temperature facies of contact metamorphism, up to the partial melting of xenocrysts. Metasomatism is directed at equalising the compositions of xenocrysts and eponymous minerals that crystallised from the host melt. There are several important criteria adopted to identify xenocrysts and distinguish them from phenocrysts. These are partial melting, solid-phase decomposition, decrystallisation of primary (before-trapping) textures, recrystallisation and self-faceting of initially xenomorphic grains into the crystals with perfect habits. The chemical composition of xenocrysts has mineral and geochemical indications of xenogenic origin, as well as the signs of a newly-formed substance.

78-89 7
Аннотация

The geo-petrological model of diamond-bearing fluid-explosive breccia formations constitutes a well-structured system of features that are typical of several similar formations in the Cis-Ural and West Ural areas of the Perm Territory. The model reflects a number of basic common factors associated with the morphology of these structures, their rock composition and the conditions for their formation. In this paper, the authors characterise regional and local geological positions featuring diamond-bearing formations, as well as the parameters common for the areas of their development. The necessity of mineralogical and geochemical studies of black sand, while prospecting for diamond-bearing targets is highlighted. This will help identify specific mineral associations and geochemical anomalies typical of these widespread formation areas. The description of a geological structure associated with the best-studied deposit (Efimovsky) is given in detail. The description of this deposit is used as an example to illustrate the shape of breccia bodies and their polyphase structure, as well as show their texture and rock structure specifics. Special attention is paid to the petrographic characteristics of all kinds of fluid-explosive breccias, which to a different extent contain clastic, protomagmatic and newly formed fluidogenic material. The paper gives the characteristics and specifics of mineral grains of various origin, many of which are abundant in gas-liquid inclusions characterised by block extinction, while quartz possess planar elements. The authors examined the differences in the diamond potential of rocks belonging to different successive evolution phases of fluidogenic breccia formations. When studying newly discovered breccia structures that have a limited number of features, the model considered in the paper will help to predict the missing features and assessment criteria for diamond potential.

90-101 5
Аннотация

The article covers the U-Pb dating of minerals belonging to the pyrochlore group from the rare-metal ore deposits of Ilmeny-Vishnevogorsky carbonatite-miaskite complex (South Urals). Individual pyrochlore crystals were dated through a new technique of in-situ U-Pb dating using SHRIMP-II, which was developed at VSEGEI (St.Petersburg). The U-Pb dating of high-uranium pyrochlore (more than 2.5 wt % UO2) was carried out employing laser ablation and ICP-MS. The U-Pb systems of studied pyrochlore samples indicate a multi-stage formation of rare-metal niobium mineralisation. The earliest age of ore formation (378 ± 4.9 ma) is yielded by the U-Pb systems of U-bearing pyrochlores from the carbonatites of the Potanino deposit. This period of ore formation is probably associated with the final stages in the crystallisation of the alkaline-carbonatite magmatic system. The next periods of ore formation (230 ± 1.5 Ma) are widely manifested in the Vishnevogorsky and later in the Potanino deposit (217.2 ± 1.9 Ma), which is probably associated with the remobilisation and redeposition of alkaline-carbonatite and rare-metal substances at the post-collision stage in the evolution of Ural carbonatite complexes.



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ISSN 1681-9004 (Print)
ISSN 2500-302X (Online)