Baikal–Vilyui Paleoproterozoic Belt of The Siberian Platform: Regional Gold-Controlling Structure

The article demonstrates that the placer gold content of the Vilyui Syneclise is governed by the regional structure of the crystalline basement of the Siberian Platform—the Baikal–Vilyui Paleoproterozoic belt, the boundaries and tectonics of which are substantiated by analysis of the gravitational field structure. The belt includes a system of basement blocks with a common northeastern strike, which form horsts (of the Suntar type) and grabens (of the Kempendyai type). The gold-bearing placers of the Vilyui Syneclise are mainly confined to the Suntar, Tyukyan, and Chybyda blocks of the belt, the metamorphic and igneous rocks of which were the primary gold sources in the sedimentary cover. The suture zone of the Baikal–Vilyui belt was very permeable to deep fluids responsible for gold migration. The types of possible primary gold sources (gold–platinoid, low-sulfidation gold-quartz, and gold–silver) reflect the peculiarities in the evolution of Early Precambrian gold during sedimentation in the Vilyui Syneclise. The areas controlled by the Suntar, Kempendyai, Tyukyan, Chybyda, and Khapchagai blocks of Early Precambrian rocks, which may contain gold objects, should be considered promising for buried gold placers in the Vilyui Syneclise.

К вопросу о рудных формациях благородных и цветных металлов Омолонского террейна (Cеверо-Восток Азии)

The author characterizes gold, silver, copper, lead, and zinc deposits developed within the Omolon craton terrane. Outlined are ore formations (genetic types) including the deposits. The principal features of the terrane metallogeny terrane are shown as determined by the stable structure of the crystalline basement, consolidated in Early Precambrian, which resulted in contraction of the range of mineralization genetic types and in formation of deep ore- and magma-controlling faults and large deposits.

Генезис магм с позиции горячей гетерогенной аккреции Земли

The obtained numerous proofs of hot heterogeneous accretion of the Earth lead to a fundamentally new solution of the magma genesis problem. According to these data, in the course of the silicate mantle accretion, the global magmatic ocean emerged under the impact heat emission. Its bottom part crystallized and fractionated as a result of the pressure increase of the upper parts being formed. Cumulates formed the ultrabasic mantle; residual melts, the magmatic ocean. The increase in ocean temperature and depth caused the evolution of bottom residual melts from acidic to ultrabasic, the appearance of corresponding layers in the ocean, and the reverse geothermal gradient in the mantle. The top-down cooling and crystallization of the ocean, 3.8 billion years ago, Early Precambrian crystal complexes, acidic crust, and the lithosphere of ancient platforms were formed. The separation of residual melts from various layers determined the evolution of magmatism from acidic to alkaline-ultramafic and kimberlite. Heating of the mantle by a high-temperature core resulted in the appearance of a direct geothermal gradient at the end of the Proterozoic, convection in the mantle, and modern geodynamic environments. In the latter, magmas are formed by the frictional and decompression remelting of the magmatic ocean differentiates.

SOME PROBLEMS GEOCHEMISTRY OF VOLCANITES OF THE PECHENGA STRUCTURE (KOLA PENINSULA)

Some problems of dating the most important stratigraphic units of the Early Proterozoic Pechenga Structure are considered. The structure is composed of rocks of the Pechenga Complex, which in turn is divided into well-studied formations of the North-Pechenga subcomplex and poorly studied formations – the South-Pechenga subcomplex. This structure is one of the most studied Early Precambrian structures in the world. Volcanites of the Pechenga Structure have been studied for nearly half a century. However, many determinations of the isotopic ages of reference levels of geological crossection were carried out by insufficiently accurate Rb-Sr, Sm-Nd, and other methods. Nevertheless, in recent years, using the modern U-Pb method, age characteristics of the most important stratigraphic units of the North-Pechenga subcomplex have been obtained.

GEOLOGICAL AND GEOMORPHOLOGICAL ANALYSIS AND HYDROGEOCHEMICAL ENVIRONMENT OF AQUIFERS OF THE SOUTHERN WEST COAST OF KARNATAKA, INDIA

Dakshina kannada district is situated in peninsular region. The peninsula is composed of geologically ancient rocks of diverse original and most of them have undergone metamorphism. The early Precambrian tonalitic gneisses invaded by granites, granulites and dolerite dykes. Granulites are mostly restricted to areas south of Mangalore. High grade alumina rich (corundum bearing) metamorphic schists have been encountered and younger alkaline intrusive rocks like Aegerine syenites have been reported. There are ve rivers and estuaries. Number of lineaments cut across each other and some lineaments are parallel to each other. The Arabian sea class is the largest among other land cover features in the study area. The river/tidal creek land cover appear as long irregular and sinous in outline. Mulki river, Netravati river, Gurupur river at southern terrain. The qualities of groundwater at sandy aquifer are good, lateritic/weathered gneissic rocks it is sweet.

Genesis of Early Precambrian crystalline complexes

The obtained evidence of hot heterogeneous accretion of the Earth leads to a fundamentally new solution of the problem of the genesis of Early Precambrian crystal complexes. According to this approach, a powerful impact heat release during accretion resulted in the formation of a layered global magmatic ocean. Its upper acidic layer arose from low-pressure residual melts of the bottom parts of the still shallow early ocean, fractionated under the influence of an increase in the load pressure of the forming upper layers. The solidification of the uppermost parts of the acidic layer led to the formation of the most ancient tonalite-trondyemite complexes. Gneiss-crystalshale complexes were formed from its deeper parts by lowering the crystallized near-surface areas together with sediments accumulated on them and lifting the underlying magmas of often more mafic composition in their place. Leaching of the near-surface parts of the solidified rocks under the influence of acidic emanations of the magmatic ocean caused the predominance of quartzites and high-alumina gneisses among the oldest pararocks. Due to the solidification of the magmatic ocean from top to bottom, the isotopic age of gneiss decreases on average with depth. The surfacing of residual melts of its various layers led to the evolution of magmatism of ancient platforms from acidic to alkaline-ultrabasic and kimberlite. The separation of ore-bearing emanations of the magmatic ocean caused the formation of numerous often unique deposits.

Juxtaposition of diverse, subduction-related tectonic blocks with contrasting metamorphic features and ages in the Paleoproterozoic Aketashitage orogen, NW China: Implications for Precambrian orogeny

The comprehensive investigation of orogenic-related litho-structural assemblages, metamorphism, and geochronology in early Precambrian orogens can help us better understand the features of plate tectonics in early Earth. The Paleoproterozoic Aketashitage orogenic belt is located at a key position in northwestern China and connects the North China craton, Tarim craton, Altaids orogen, and Tethys orogen. Garnet-bearing mafic and paragneissic granulite occur as interlayers or blocks preserved within paragneissic matrix, and two to three generations of metamorphic mineral assemblages were identified. Geothermobarometry and pseudosection modeling yielded clockwise metamorphic P-T paths passing from 7.5‒8.6 kbar/575‒715 °C (M1) through 7.4‒12.2 kbar/715‒895 °C (M2) and finally to 5.2‒7.3 kbar/710‒800 °C (M3) for the mafic and paragneissic granulite as well as amphibolite, which is indicative of metamorphic features of subduction/collision zones. Peak metamorphic P-T conditions of all the samples lie in the medium P/T facies series, suggesting that the thermal gradient (∼20‒31 °C/km) of this Paleoproterozoic orogenic belt was obviously higher than most of the Phanerozoic subduction zones. Secondary ion mass spectrometry (SIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of zircon and monazite yielded metamorphic ages of ca. 1.98−1.96 Ga in the eastern part of the orogen, ca. 1.86−1.85 Ga in the western part, and a maximum depositional age of ca. 2.06 Ga for paragneiss. Compared with previous studies, the Aketashitage orogen is composed of unordered juxtaposition of diverse, subduction-related tectono-metamorphic blocks with different protoliths, metamorphic grades, and ages preserved within the paragneissic matrix deposited in the Paleoproterozoic, which is highly similar to Phanerozoic mélange. A Paleoproterozoic subduction-metamorphic-exhumation-accretionary process was deciphered, similar to that found in accretionary/orogenic wedge in Phanerozoic orogens. The juxtaposition of diverse, subduction-related tectonic blocks with contrasting ages and metamorphic features can serve as a marker of early Precambrian orogens and plate tectonics.