Skublov S.G.,Russian Academy of Sciences |
Berezin A.V.,Russian Academy of Sciences |
Berezhnaya N.G.,Karpinskii All Russia Research Institute of Geology VSEGEI
Petrology | Year: 2012
Analysis of currently available data (877 individual high-precision zircon analyses) on the composition of zircons from eclogite complexes worldwide reveals general relations in the zircon composition: an anomalous decrease in the Th concentration (no higher than 3 ppm on average) and the Th/U ratio (0. 33 on average), a significant decrease in the concentrations of all REE (to 22 ppm) and particularly LREE (<2 ppm), and relatively low concentrations of Y (34 ppm), U (100 ppm), and P (41 ppm) at an elevated Hf concentration (11 400 ppm on average). The REE patterns of eclogitic zircons are noted for pronounced flat HREE patterns, poorly pronounced (if any) negative Eu anomalies, strongly reduced positive Ce anomalies (Ce/Ce* = 11 on average), and U-shaped configurations of LREE patterns up to the development of negative Nd anomalies. The relations detected in the distribution of trace elements and REE in eclogitic zircons are of universal nature and occur irrespective of the rock type (metabasites, metaultrabasites, or gneisses) and the metamorphic pressure (eclogites of high and ultrahigh pressure). The application of the aforementioned criteria makes it possible to reliable distinguish eclogitic zircons from those of magmatic or metamorphic genesis (not related to high-pressure metamorphism). Eclogites in the Belomorian Mobile Belt (in the Salma and Gridino areas) were determined to contain zircons in metagabbro eclogites; the cores of these zircons have an age of 2. 8-2.9 Ga and are of magmatic genesis, whereas their outer metamorphic zones have an age of 1.9 Ga and a trace-element composition typical of eclogitic zircons. Hence, the Belomorian Mobile Belt was affected only by single (Svecofennian, at ~1. 9 Ga) episode of eclogite metamorphism of Archean rocks. © 2012 Pleiades Publishing, Ltd.
Ronkin Yu.L.,Russian Academy of Sciences |
Efimov A.A.,Russian Academy of Sciences |
Lepikhina G.A.,Russian Academy of Sciences |
Rodionov N.V.,Karpinskii All Russia Research Institute of Geology VSEGEI |
Maslov A.V.,Russian Academy of Sciences
Doklady Earth Sciences | Year: 2013
The U-Pb age was obtained for the coexisting baddeleytte-zircon system from dunites of the Konder massif, Aldan shield. Four groups of zircons are heterogeneous by morphology, habit, age, and geochemistry in contrast to homogeneous baddeleytte. The studied zircon groups are characterized by several U-Pb age clusters in the range of 1895 ± 50 to 125.8 ± 3.8 Ma, which indicates their long evolution in Pt-bearing dunites. The young assemblage of baddeleytte and zircon (124.9 ± 1.9 and 125.8 ± 3.8 Ma, respectively) also differs from ancient zircons in the morphology and geochemistry and probably dates to a much later event of diapir evolution, which may be referred to the formation of apatite-phlogopite ore pyroxenites inside the dunite core and tectono-magmatic activation of the Aldan shield. © 2013 Pleiades Publishing, Ltd.
Vinogradov V.I.,Russian Academy of Sciences |
Belenitskaya G.A.,Karpinskii All Russia Research Institute of Geology VSEGEI |
Pokrovsky B.G.,Russian Academy of Sciences |
Bujakaite M.I.,Russian Academy of Sciences
Lithology and Mineral Resources | Year: 2011
Isotopic compositions of carbon and oxygen in carbonates and sulfur in sulfates of the Verkhnyaya Lena Formation (ε 2-ε 3), which terminates the Cambrian section of the Irkutsk Amphitheater of the Siberian Craton, are studied. Sulfates of the Verkhnyaya Lena Formation are marked by unusually low δ 34S values (4.6-12.0‰) relative to sulfates of the underlying Angara Formation. This is likely caused by variations in the facies-paleogeographic sedimentation at the transition of the Angara and Verkhnyaya Lena formations, as well as associated variations in the water and salt alimentation budget in sedimentation basins, due to their isolation from open sea and intensification of the continental and underground discharge. The δ 18O(PDB) value in carbonates decreases from -4.4‰ at bottom to -10.4‰ at top, reflecting variation in postsedimentary transformations and probable continuous freshening of sedimentation basin. Isotopic composition of carbon in most samples shows normal marine δ 13C values (0 ± 1‰). Only in some samples, does the δ 13C value increase up to -3.8 and 2.2‰ due to specific features of postsedimentary processes. The Rb-Sr systems of the clayey component of marls from the 500-m-thick section of the Angara Formation and bottom of the Verkhnyaya Lena Formation record an age of 512 ± 10 Ma, which is close to the assumed stratigraphic age of the Verkhnyaya Lena Formation. The 87Sr/ 86Sr initial ratio is 0. 7082 ± 0.0004. © 2011 Pleiades Publishing, Ltd.
Velikoslavinskii S.D.,Russian Academy of Sciences |
Kotov A.B.,Russian Academy of Sciences |
Tolmacheva E.V.,Karpinskii All Russia Research Institute of Geology VSEGEI |
Sal'nikova E.B.,Russian Academy of Sciences |
And 2 more authors.
Petrology | Year: 2011
The central portion of the Aldan Shield hosts very widely spread Archean and Early Proterozoic granitoids, much of which are granite-gneisses. Geochemical lines of evidence, data on inclusions in minerals, and Sm-Nd isotopic geochemical data suggest that the protoliths of granite-gneisses in the central part of the Aldan Shield were granitoids that had various composition, age, and were derived from distinct sources and under different parameters and were then emplaced in different geodynamic environments. The granitoids belong to at least two types of different composition that occur within spatially separated areas. The protoliths of granite-gneisses in the western part of the Western Aldan Megablock and the junction zone of the Chara-Olekma and Aldan geoblocks (granite-gneisses of type I) had the same age and affiliated to the same associations as the within-plate granitoids of the Nelyukinskii Complex. Their parental melts were derived at 2.4-2.5 Ga by the melting of Archean tonalite-trondhjemite orthogneisses of the Olekma and Aldan complexes. The protolith of granite-gneisses in the eastern portion of the Western Aldan Megablock (granite-gneisses of type II) can be subdivided into two groups according to their composition: granitoids with geochemical characteristics of subduction- and collision-related rocks. The protoliths of the type-II granite-gneisses with geochemical characteristics of subduction granitoids were produced simultaneously with the development of the Fedorovskaya island arc (at 2003-2013 Ma), whereas the protoliths of the type-II granite-gneisses with geochemical characteristics of collision granitoids were formed in the course of accretion of the Fedorovskaya island arc and the Olekma-Aldan continental microplate at 1962-2003 Ma, via the melting of magmatic rocks of the Fedorovskaya unit and older continental crustal material. © 2011 Pleiades Publishing, Ltd.
Petrov O.V.,Karpinskii All Russia Research Institute of Geology VSEGEI |
Bogatikov O.A.,Russian Academy of Sciences |
Sharpenok L.N.,Karpinskii All Russia Research Institute of Geology VSEGEI |
Kurchavov A.M.,Russian Academy of Sciences
Petrology | Year: 2010
The Petrographic Code of Russia is a body of major rules and recommendations that establish a unified and standardized petrographic terminology and nomenclature for endogenic and impact rocks and the taxonomy of petrographic units. The single system of terms and concepts underlying the petrographic code shall be obligatory for all departments and organizations in conducting geological operations in the territory of Russia. This system shall be applied when state geological maps of various scales and series legends for them are prepared and when maps are created under international projects. The structure of the second and third editions of the Code was modified compared with that of the first edition, and this edition includes new appendices. With regard for new standards and requirement of geological practice, additional sections concerning sedimentary-volcanic, migmatite, and some other rocks were introduced into the Code. The sections devoted to metamorphic and metasomatic rocks were revised and amended, and arguments were proposed justifying the recognition (as an individual genetic type) of fluid-explosion rocks, which can be accompanied by ore mineralization of various types. Most definitions are revised, many entries are reworked and or abridged, and new entries are added. The revised wording of the Code involves recommendations from the International Commission on Systematics in Petrology of the International Unit of Geological Sciences. © 2010 Pleiades Publishing, Ltd.