Home – Home – Jornals – Russian Geology and Geophysics 2005 number 5

Mantle-crust fluid systems are associated with zones of decompression melting above hot spots. The place and duration of their development are different at the progressive and regressive stages of the related melting zone (asthenospheric lens or plume). Mass exchange and metasomatism in lithospheric rocks (structure of metasomatic column), including the degree of granitification, are controlled by the structural pattern of permeable zones above the fluid systems. Strong metasomatic alteration of mantle rocks can change their density as much as to make the anomalous high and low densities in the temperature field different from the ρ = f(T, P) functions used in tomographic interpretations. Local melting of metasomatized lithospheric rocks above melting zones (plumes) appears to be quite common to the mantle-crust fluid systems.

We studied multiphase symmetrical dikes that cross-cut Triassic tuff lavas in the Kamen' alkali mafic province in the northern Siberian craton, especially, the structure of vitreous and crystalline dikes, their petrography, major-element compositions, and mineralogy. The dike swarm has a symmetrical structure produced by compositionally similar left-hand and right-hand half-dikes. The dikes show five phases of melt injection. Vitreous dikes are composed of olivine-clinopyroxene or occasionally picritic (in the center) porphyry. Each half-dike consists of a quench contact with spherulites of silicic glass and carbonate and an inner zone with feldspar, and feldspar with kaersutite and/or mica. Early phases in crystalline dikes are variolitic clinopyroxenites and late phases are picritic porphyry. The rock chemistry and mineralogy indicate presumably limburgite composition of the parent melt. The multiphase dikes originated by repeated melt injection into pulse-like opening fractures whereby the melt moving along the magma conduit underwent differentiation with separation of magnesian fluid. Overcooling of the limburgite melt in the dikes provided separation of carbonatite and alkali-silicic fluids and heteromorphic crystallization of variolitic clinopyroxenites.

Granites of the Late Mesozoic Kukul'bei ore-bearing complex in the Aga structure-formational zone of eastern Transbaikalia are studied. It is shown that the concentrations of incompatible trace elements in them are correlated with the domal morphology of the roof of granite intrusive systems of the Kukul'bei complex. Massifs of biotite granites of the major intrusive phase (MIP) are localized in the centers of domes formed in enclosing sand-shaly rocks, and younger ore-bearing leucogranite (usually muscovite) differentiates occur on the flanks of the axial, most uplifted, zones of the domes. The studied granites are highly aluminous and potassic. The MIP granites are enriched in granitophile trace elements, including Sn, W, Be, Ta, and volatiles, which are of plutonic genesis. The enrichment is shown to be of local character: The most enriched granite bodies occur in the axial zones of domes, whereas granites with lower concentrations of trace elements (as low as their clarkes), in the peripheral, least uplifted, zones of the domes. Rare-metal mineralization is localized in leucogranites drastically depleted in Sr and Ba; its productivity is directly correlated with the concentrations of trace elements in the leucogranites.
The granites of intrusive systems of the Kukul'bei complex, localized in deep-fault zones, might be enriched in trace elements (including volatiles F and B) as a result of their supply with mantle fluids. This process led to the formation of crustal granite magma chambers of ore-bearing rare-metal intrusions. Their subsequent evolution during the fluid supply resulted in leucogranite differentiates enriched in trace, ore-forming, and volatile elements, which were the major source of ore-bearing fluids and hydrotherms at the postmagmatic stage.

Study is given to the petrography, geochemistry, ⁴⁰Ar/³⁹Ar age, and geodynamic settings of the formation of volcanics from boreholes drilled in the Uralian part of the West Siberian syncline (upper reaches of the Severnaya Sos'va River) in recent years. Carboniferous and Permo-Triassic basalts and scarcer basaltic andesites, trachybasalts, and basaltic tuffs have been recognized here. Heterochronous basalts differ in the degree of paleohypergene and greenstone alteration and in geochemical parameters.
The Carboniferous and Permo-Triassic basalts are rocks of calc-alkalic series with normal or, less often, moderate alkalinity. They have moderate contents of K, but some Carboniferous samples are poor and some Permo-Triassic samples are rich in it. Both types of basalts, particularly the Permo-Triassic ones, are enriched in incompatible elements relative to N-MORB.
According to geochemistry, the Carboniferous basalts are island-arc volcanics, and the Permo-Triassic ones are rift trap rocks, which are widespread in the Upper Permian and Triassic strata of the West Siberian syncline.

Not only hydrodynamic but also eolian processes have an effect on placer formation. Our experimental, mineralogical, and field studies as well as analysis of recent literature data permitted us to recognize a new morphologic type, eolian gold, and a new genetic type of gold deposits, eolian placers. Eolian gold is represented by gold flakes with ridgelike edges, toroidal grains and hollow spheroids 0.1 to 0.25 mm in size, as well as by compact disc-shaped, flat gold particles with ridgelike edges, and lump-shaped ones covered with a specific fibrous membrane measuring 0.25 mm and more. In addition to gold particles characterized by eolian features, gold-crustated quartz occurs. Analysis of distribution of eolian gold shows that the above-mentioned shapes of gold are widespread in Proterozoic-Cenozoic deposits almost in all platforms of the world. The eolian gold occurrences are well correlated with fragmentally developed surfaces of deflation paleodeserts, the halo of which can be reconstructed from findings of ventifacts. Both arid and nival climates are favorable for eolian placer formation because the epoch of glaciation is characterized by intense eolian processes that gave rise to placer formation. Eolian gold placers may be formed as the result of long-lasting activity of unidirectional winds owing to the deflation of a primary source itself, gold-bearing crusts of weathering or previously generated alluvial, beach, and other placers of varying genesis. We suggest to divide them into eolian gold placers and heterogeneous ones, i.e., eolian-proluvial, eolian-alluvial, eolian-marine, etc. Eolian gold placers have a specific structure: The producing horizon overlies the deflation surface like a blanket, with its thickness extremely small (tens of centimeters), and is complicated by jet series. This horizon is usually composed of sandstones and gravel with a low content of argillaceous matter containing ventifacts and wind-worn gold particles. The eolian gold placers are divided into autochthonous and allochthonous. Autochthonous deflation placers are generated by denudation of a primary source matrix either with deflation of the crust of weathering formed in the primary source or with deflation of the placer previously formed. Allochthonous eolian placers are basal (transit) and dune ones. Basal eolian placers are confined to the eolian denudation surface and are mainly developed in deflation basins and grooves. Allochthonous dune placers of gold occur between denudation and accumulation zones. Lying far from the primary source, they have no economical value because the metal particles that migrated together with blown sands are very small, dispersed, with low contents of gold. Not only remarkable shapes of gold particles and character of their surface, but also ventifacts, specific structure of producing bed, deflation relief structures, i.e., grooves and basins, and typical lithology of sedimentary deposits are exploration tools for revealing eolian gold placers. As far as wind-worn gold particles are not only of mineralogical interest but also form high concentrations of the metal, e.g., in the Witwatersrand gold placer, we believe that the discovery of eolian gold placers of varying age in platforms all over the world is quite promising, in particular, in the eastern European, Siberian, North American, South American, African, and Australian Platforms as well as in Tuva and Mongolia.

The Late Precambrian Uchur-Yudoma hypostratotype includes the Uchur-Maya plate (a depression in the southeast of the Siberian Platform), the Yudoma-Maya pericratonic trough, and the Okhotsk microcontinent. The sequence of stratigraphic units ranked groups and formations has surely been established for all these structures, but the location and nature of boundaries between them as well as their age are still debatable. The information available casts doubt upon many universally accepted concepts. It is proven that the Maya Group giving rise to the Mayanian, the lower Neoproterozoic unit, overlaps the entire region under study, being in contact with diverse older units. However, its starting Kerpyl Subgroup is not coeval with the entire Ennin Formation completing the Lower Riphean in the western Uchur-Maya plate but only with a separate member of deposits erroneously considered its part. The pre-Kerpylian tectonic rearrangement contributed much to the specific paleogeography of the region, which is also commonly associated with the pre-Vendian events alone. The Kerpylian was preceded by an accretion to the Siberian craton of its surrounding microcontinents while the supercontinent Rodinia was completing its formation. Nothing of the kind occurred prior to the formation of the Lakhanda Subgroup closely connected with the Kerpyl one. The pre-Kerpylian changes in phytolith and microfossil biotas served as a base for the paleontological substantiation of the lower boundary of the Upper Riphean. Their specific character that appeared in the Kerpylian continued to develop in the Lakhanda Subgroup. Therefore, there is no need to distinguish the Lakhandinian as a separate group to oppose it to the Kerpylian. The data reported show that the lower boundary of the Neoproterozoic is not younger than 1100 Ma.
A persistent sequence of K-Ar and Rb-Sr ages corresponding to the available paleontological data shows that the Ui Group formed in the range 750-650 Ma. This does not contradict U-Pb and Sm-Nd ages (about 1000 Ma) obtained from its cutting dikes. These figures indicate the age of the magma chamber rather than the time of their intrusion. The Ui Group dated in this way and its analogs in other regions correspond only to the upper half of the Baikalian and Cryogenian of the International Scale (850-650 Ma). This conclusion is in discrepancy with the common opinion that the Ui Group is closely connected with the Lakhandinian. In the same way, a number of formations disappear from the Ui Group leaving no trace, while on the eastern slope of the Omnya rise, the Ust'-Kirbi Subgroup completing the Ui Group rests immediately upon the Lakhandinian. These relationships are due to the regressive structure of the Ui deposits, where the event nature marking the beginning of a new group is masked by the considerable subsidence of the Yudoma-Maya trough. It is related to the extension that accompanied the breakup of the supercontinent Rodinia. In other parts of the Siberian craton where this process was accompanied by intense compression, the beginning of the Baikalian has a distinct event basis and an appropriate complex of basal deposits. Because of the complex event nature of the Baikalian in Siberia some geologists refer it partly to the Middle Riphean using U-Pb ages of dikes, whereas the others date some part of the Baikalian by the Vendian. Neither thinking is true because of the Late Riphean age of the paleontological remains. The event nature of the Vendian, the upper unit of the Neoproterozoic, is due to the repeated accretion to the Siberian craton of its surrounding microcontinents, which triggered the Caledonian tectonic activity. The principal character of the events limiting the main Neoproterozoic stratons in Siberia makes them promising as units of the General Stratigraphic Scale of Precambrian.

Callovian and Late Jurassic zonal biostratigraphy and evolution of foraminifers in West Siberia have received better constraints from the foraminiferal taxonomy in newly studied sections and from revised earlier data. The evolution stages of foraminifers, each beginning at the onset of a Boreal transgression, were inferred from species diversity within zones and from changes in their coeval assemblages in different geographic regions of West Siberia. The boundaries between stages are marked by migration of generic and specific taxa into the West Siberian sea. Taxonomic diversity was used as an additional criterion in division and correlation of sections. Generic variations within the studied time span were associated with dramatic abiotic events which were apparently controlled by eustatic transgressions and regressions. Invasion of species- and genus-ranked migrants into the West Siberian Sea marks the borderlines between the stages. The peaks and dips of taxonomic diversity curves for the studied benthos assemblages show good correlation among different regions of West Siberia and with their counterparts in the Russian Platform and northern Central Siberia.

Simulation of the global electromagnetic field as a sum of related electric and magnetic modes simplifies the solutions to both forward and inverse global EM problems. The forward problem (for EM field) is solved with only horizontal or only vertical field components measured on the Earth's surface, and the inverse problem (for conductivity inside the Earth) is solved by fitting of the correlated surface horizontal and vertical components. The component correlation approach to lateral conductivity variations inside the Earth requires neither source modeling nor computing source dependences of impedances and tippers. Processing measured global Sq variations with the new method tentatively indicates the presence of lateral variations in diurnal-period skin depth and in skin-depth average conductivity.

New data on the age of shear deformations in the Ol'khon region (western Cisbaikalia) are reported. The Ol'khon collisional system has an intricate folded structure with three successively formed parageneses: nappe, domal, and shear. Shear deformations appeared at the final stages of the formation of this region and determined its general structure. The age of the deformations has been first estimated by ⁴⁰Ar/³⁹Ar dating of minerals with indications of shearing genesis. It has been established that the shearing took place 445-434 Myr ago, i.e., much later than the peak of metamorphism of granulite facies (480-500 Myr ago). The data obtained also suggest the existence of a thermal event (no earlier than 415 Myr ago), which led to a partial re-equilibration of the isotopic system.