P. A. Tishin

Neoproterozoic carbonatite magmatism of the Yenisei Ridge, Central Siberia: 40Ar/39Ar geochronology of the Penchenga rock complex

Abstract40Ar/39Ar isotopic ages of magnesioarfvedsonite (725.9 ± 6.3 myr) and phlogopite (637.6 ± 5.7 myr) from carbonatites of the Penchenga rock complex located in the South Trans Angara segment of the Yenisei Ridge have been determined. Based on this, the formation of the rock complex is associated with the start of the period of Neoproterozoic (725–700 Ma) rifting and alkaline magmatism in the region, the latter being associated with the former. The appearance of zones penetrable for magma of the “disseminated” rifting type is associated with the development of late collision shear dislocations in a consolidated marginal continental crust. The primary melts had a plume nature, but matter from different mantle sources intermixed in the environment of the changing geodynamic regime. The younger age of the mica was apparently caused by partial remelting of carbonatites under the influence of heat and fluids generated during the formation of the adjacent large granite massif of a younger age.

Zircon age range and sources of alkaline rocks from the Kurgusul intrusion, Kuznetsk Alatau: The first U-Pb (SHRIMP II) and Sm-Nd isotope data

U-Pb isotope analysis of basic feldspathoid rocks (juvites) of the Kurgusul intrusion, NE Kuznetsk Alatau, revealed three generations of zircons of various ages (∼1.3–1.5 Ga; 484.3 ± 5.5 and 393.6 ± 9.2Ma). This suggests several stages of regional alkaline basic magmatism in the Middle Cambrian-Early Ordovician and Early-Middle Devonian and melting of the Mesoproterozoic continental crust, which form part of the basement of the Kuznetsk Alatau terrane. The trace element geochemical data indicate a continental margin setting characterized by the complex interaction of a plume-related, supra-subduction zone and crustal materials. A possible model of the magmatic evolution assumes that the ascent of a plume at the Cambrian-Ordovician boundary induced generation of the initial alkaline basaltic magmas and metasomatism and erosion of the lower lithosphere. Repeated partial melting of the lower crust after a 100 Myr period produced new magma batches of alkaline composition, which inherited zircons from the preceding episode of magma generation. The inherited juvenile magmatic source (PREMA + E-type MORB + EM) is confirmed by similarities in the Sm-Nd isotopic signatures (ɛNd(T) ≈ +4.5 to +5.7, T(Nd)DM ≈ 0.8–0.9 Ga) of derivatives of regional alkaline basic complexes of different ages.

Age and source material of the Kingash ultramafic-mafic massif, East Sayan

The REE distribution patterns and Nd whole-rock and mineral isotope ratios of the Kingash ultramafic-mafic massif enabled us to propose a multistage history for its evolution at 1410 and 875 Ma. These stages reflect the magmatic evolution of the Siberian paleocontinent margin during the Late Precambrian. The age of metamorphism of the massif during collision and accretion in the Early Paleozoic (∼500 Ma) was obtained based on a Sm-Nd mineral isochron from rheomorphic veined albitite. The Nd and Sr isotopic compositions of rocks from the Kingash massif suggest mantle sources for picritic and basic magmas, which are thought to have originated by mixing of different proportions of depleted (PREMA or DM) and enriched (EM) melts. The initial isotope ratios of the parental melts transformed during interaction with Sr-rich material from the host metasedimentary complexes.

Isotope composition and magma sources of the Kresty volcanopluton in Polar Siberia

The first data are presented on the distribution of rare elements and isotope composition of neodymium and strontium in the rocks of the alkaline-ultramafitic association of the Kresty volcanoplutonic structure (one of the probable satellites of the Gulinian massif being the largest in the Maimecha-Kotui alkaline province in Polar Siberia). On the basis of the results of geochemical studies performed and formerly obtained geophysical and isotope data, a new model is proposed for the plume nature of magmatism in the region, based on the action of a single mantle source of the PREMA type and its active interaction with the matter of the Earth’s crust.

GEOCHRONOLOGY OF GRANITES, SURROUNDING SCHISTS AND ORES OF THE PANIMBA GOLD DEPOSIT (THE YENISEY RIDGE)

~ ~ ~ Data of isotope geochronological researches of zircons by U-Pb method, also results of Ar-Ar method researches of muscovites from the gold-bearing ores, surrounding schists and granites of the Panimba gold deposit in the Yenisey Ridge are presented. Regional metamorphism of the lower Riphean Kordinskaya suite terrigene sediments, containing ore bodies, took place in the interval of 996,0±32 - 889,0±26,6 Ma. The graniteforming and contact metamorphism of schists in the ore field of the mineral deposit have occurred on a boundary of 868,9±6,5 Ma. The absolute age of the quartz gold sulphide orebearing veins lies in the interval of 817,2±5,3 - 744±17 Ma (by muscovite with Ar-Ar method) and 815,0±37,6 - 762,3±33,3 Ma (by zircon from ore schists with U-Pb method). The long history of development of the Yenisei Range from Upper Archean to Upper Paleozoic was reflected in an accessory zircon of ore schists.

Age of overthrust-type granites in the accretionary‒collisional system of the early Caledonides (western Baikal region)

The western Baikal region (Ol’khon region, Nutgei zone, Kharikta area) is characterized by the development of packets of thrust-type folds and synkinematic granites corresponding to the stage of thrust deformations (overthrust-type granites). Two stages in the formation of thrust-type folds separated by episodes of granite emplacement are definable. The data obtained make it possible to estimate both the time of transition from thrust to strike-slip deformations and the duration of accretionary‒collisional events that resulted in regional strike-slip deformations, which terminated in the western Baikal region 460‒455 Ma ago.

In-bearing potential of tin‒sulfide mineralization in ore deposits of the Russian Far East

The increased demand for indium has made it necessary to revise prospects of In-bearing tin ore deposits in the Russian Far East on the basis of geological data and results of recent analytical methods (X-ray fluorescence with synchrotron radiation, atomic absorption, and ICP-MS). The average In contents in ores of the Tigrinoe and Pravourmiiskoe deposits vary from 55 to 70 ppm, which allows tin ore deposits with Sn‒sulfide mineralization to be considered as quite promising with respect to In production from ores of Russian deposits. By their estimated In reserves, the Tigrinoe and Pravourmiiskoe deposits may be attributed to large ore objects.

Effect of the Addition of Electrocorundum on Structural-Phase Transformations in the Quartz Lining of an Industrial-Frequency Crucible-Type Induction Furnace

Results are presented from studies of the phase interactions that take place between the metal, lining, slag, and gas inside the lining of an industrial-frequency crucible-type induction furnace (CIF). The lining is composed of quartzite, boric acid, and electrocorundum. The phase composition of the lining is determined. The lining lasted 295 heats in the production of ferrous-metal alloys at temperatures up to 1570°C. The chemical and phase compositions of the slag-covered layer and the sintered layer of the lining are also determined.

Preferred Orientation Evolution of Olivine Grains as an Indicator of Change in the Deformation Mechanism

The paper presents the results of investigations of deformed natural polycrystalline olivine. The relationship of the structure of polycrystalline olivine grains to three modal size distributions has been revealed. Grains of different size were observed to be strained at threshold temperatures of 950, 775, and 650?C. It has been demonstrated that the microstructure develops as the dislocation mechanism changes from diffusion creep to grain boundary sliding. The changes in deformation mechanisms promote the change in the preferred crystallographic orientations of olivine from type A to type D and then to type B. The relation of the transitions between different types of orientations to the conditions of deformation in the lower layers of the lithosphere at the plate boundaries is discussed.