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Glebovitskii V.A.,Russian Academy of Sciences | Sedova I.S.,Russian Academy of Sciences | Berezhnaya N.G.,All Russia Geological Research Institute VSEGEI | Larionov A.N.,All Russia Geological Research Institute VSEGEI | Samorukova L.M.,Russian Academy of Sciences
Stratigraphy and Geological Correlation | Year: 2012

This work presents new U-Pb data (SHRIMP-II) for zircons from products of granitization and leucosomes of migmatites from amphibolite and granulite-facies zones developed on rocks of the tonalite-trondhjemite group of the unstratified basement and supracrustal formations of the western part of the Aldan granulite area. The age data obtained were interpreted using the data available on the U and Th geochemistry. The main geochemical trend of transition from primary zircons, crystallizing from the melt to the later metamorphic zircons is manifested in increasing U and Th concentrations in zircons. In this case, the Th/U ratio decreases, as do the values of the Ce anomaly and LuN/LaN ratio. By studying the sequence of autochthonous and paraautochthonous granite formation in the amphibolite-facies zone the ancient (3222-3226 Ma) metamorphic event in the Aldan Shield (a manifestation of the ultrametamorphic processes (granitization and migmatization), superimposed on rocks of an ancient infracomplex (3. 3-3. 4 Ga) and gneisses and schists of supracrustal formations) was established. The data obtained indicate the Middle Archean age of both metamorphosed rock complexes. The ancient period of evolutionary development of the Aldan shield was followed by development of diatectic granitoids with an age of 2450 Ma, which is correlated well with Proterozoic granitoids from the conjunction zone between the Aldan granulite area and Olekma granite-greenstone terrain. The study of similar granitoids from the granulite-facies zone allowed us to determine the age of the last granulite metamorphism as 2030-2100 Ma, which corresponds preliminary to the time of formation of the Fedorovka island arc. According to this, the conclusion has been made that high-temperature and high-gradient metamorphism is the suprasubduction process manifested in the continental margin back-arc. At the final stage of the Paleoproterozoic granite formation the complex of diatectic intrusive chambers (1960 Ma) formed. This resulted in granitoid magmatism, manifested in the central part of the Aldan granulite area simultaneously with the island arc-continental margin collision or at the post-collision stage. The diatectic granitoids contain inherited zircons with an age of over 2677 Ma, indicating the manifestation of a high-gradient metamorphic event in this time, as in adjacent territories. © 2012 Pleiades Publishing, Ltd. Source

Pasava J.,Czech Geological Survey | Knesl I.,Czech Geological Survey | Vymazalova A.,Czech Geological Survey | Vavrin I.,Czech Geological Survey | And 2 more authors.
Geoscience Frontiers | Year: 2011

The Polar Urals region of northern Russia is well known for large chromium (Cr)-bearing massifs with major chromite orebodies, including the Centralnoye I deposit in the Ray-Iz ultramafic massif of the Ural ophiolite belt. New data on platinum (Pt)-group elements (PGE), geochemistry and mineralogy of the host dunite shows that the deposit has anomalous iridium (Ir) values. These values indicate the predominance of ruthenium-osmium-iridium (Ru-Os-Ir)-bearing phases among the platinum-group mineral (PGM) assemblage that is typical of mantle-hosted chromite ores. Low Pt values in chromites and increased Pt values in host dunites might reflect the presence of cumulus PGM grains. The most abundant PGM found in the chromite is erlichmanite (up to 15 μm). Less common are cuproiridsite (up to 5 μm), irarsite (up to 4-5 μm), and laurite (up to 4 μm). The predominant sulfide is heazlewoodite, in intergrowth with Ni-Fe alloys, sporadically with pentlandite, and rarely with pure nickel. Based on the average PGE values and estimated Cr-ore resources, the Centralnoye I deposit can be considered as an important resource of PGE. © 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. Source

Mikhalsky E.V.,VNIIOkeangeologia | Belyatsky B.V.,All Russia Geological Research Institute VSEGEI | Presnyakov S.L.,All Russia Geological Research Institute VSEGEI | Skublov S.G.,Russian Academy of Sciences | And 5 more authors.
Precambrian Research | Year: 2015

In this paper we present new U-Pb zircon age, Sm-Nd isotopic and chemical composition data for rocks cropping out in a few isolated nunataks in Wilhelm II Land in East Antarctica, namely Mirny oasis, Mt Brown and Gaussberg volcano, which contains xenogenic crustal material. These outcrops were subjects of geological investigations during the Soviet Antarctic Expedition of 1956-1957. Our data show that this region is underlain by a uniform crust which experienced a high-grade metamorphic event at ca 980-920Ma, co-eval with the Rayner Orogeny in Kemp Land and the northern Prince Charles Mountains. Extensive indications of a ca 500Ma event in coastal areas (granitoid intrusions in Mirny oasis and inherited zircons found in Gaussberg volcano), together with the lack of indications of this age in Mt Brown, point to a concentration of ca 500Ma processes (roughly co-eval with the Prydz Orogeny) in the coastal part of Wilhelm II Land and their attenuation inland. We also determined a ca 1480Ma age for a mafic magmatic protolith in Mt Brown which may be correlated with roughly co-eval orthogneiss in the Bunger Hills area. These observations suggest the conjugate positions of these crustal blocks in the early Mesoproterozoic and argue against a Cambrian suture running between them. In Gaussberg volcano a range of zircon 206Pb/238U ages of ca 320Ma, ca 500Ma, ca 980Ma, and ca 2000-1800Ma has been determined. The presence of ca 2000-1800Ma zircons indicates involvement of mid-Palaeoproterozoic rocks in the structure of Wilhelm II Land. This argues for possible conjugation of this region with other East Antarctic blocks experienced the Palaeoproterozoic tectonic evolution and which have been considered to comprise the Mawson palaeocontinent. © 2014 Elsevier B.V. Source

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