Home – Home – Jornals – Russian Geology and Geophysics 2003 number 11

The marine biota of the Arctic paleobiogeographical realm differed from the biota of the Boreal realm in being taxonomically depauperated rather than in having endemics of a high rank (superrealm). The biota of the West Siberian province is typical of the Arctic. The Upper Cretaceous composite section in northern West Siberia contains traces of biotic and abiotic events restricted to the boundaries of stages or substages: Cenomanian-Turonian, Middle-Upper Turonian, Turonian-Coniacian, Lower-Upper Coniacian, Coniacian-Santonian, Santonian-Campanian, and Maastrichtian-Danian. There are two levels at which the maximum diversity of bottom mollusks, foraminifers, and dinocysts coincides and the taxonomic composition of the same groups becomes most uniform: at the Cenomanian-Turonian boundary and in the Late Santonian. They coincide with the warmest episodes in the Late Cretaceous of northern Siberia. In time, the two bioevents correspond to the period of accumulation of

Seven cores from the central part of the Sea of Okhotsk were subjected to a complex analysis by paleomagnetic, diatomic, palynological, geochemical, and isotope-oxygen methods, and the effect of climate fluctuations on some characteristics of bottom sediments have been estimated. Groups of biogeochemical and physical parameters of deposits were specially normalized and summarized for each of the cores. The data obtained were compared with a standard curve and mutually with each other, with various chronostratigraphic benchmarks taken into account. To model generalized curves of uniform parameters, a scheme of correlation has been devised. A spectral analysis of summarized records has revealed some apparent harmonic components. An attempt is made to relate frequencies of these components to the known periods of variation of climatic processes as well as to variations of parameters of the Earth's orbit.

Magmatic and ore-metasomatic processes in the Aksug Cu-Mo ore cluster were dated by the ⁴⁰Ar/³⁹Ar age method. It has been established that the formation of the Aksug porphyry Cu-Mo deposit was related to the origin of an ore-bearing porphyry complex and followed a long multistage granitoid magmatism (532-522; 497 1; 490-488; and 462 Ma). Dating of differently metasomatized ore-bearing porphyries (sericite and sericitized plagioclase) revealed three periods of endogenous events (404-401; 364-354; and 331-324 Ma) obviously corresponding to the periods of various ore-metasomatic processes: formation of stockwork pyrite-molybdenite-chalcopyrite mineralization and mineral association with native copper and chalcocite. The final stage was related to the formation of aplite-granite association (336-324 Ma), in which minor veinlet and nest pyrite-chalcopyrite mineralization was found. Despite the long overall period of endogenous processes, the ⁸⁷Sr/⁸⁶Sr values of accessory apatite in all studied Aksug rocks vary over a narrow range, 0.70427-0.70496, close to those in the mantle, which suggests that the parental magmas were generated at the same depths. This is consistent with the high activity of Cl, specific for all heterochronous magmatic processes, which might have been related to the dehydration of submerging oceanic plate. The expansive metasomatic transformation of country rocks at the deposit was accompanied by release of much Cl, which might seriously affect the overall balance of halogens in the ore-magmatic system. The ³⁴S values of sulfides (0.6-3.1<194>) from ore zones are close to those of meteorites.

Late Paleozoic granitoid magmatism in the southeastern East Sayans was focussed in the eastern Oka zone adjacent to the Sharyzhalgai basement uplift of the Siberian craton. The Rb-Sr systematics of the rocks and minerals indicates three magmatic episodes of 307-304 (1), 283-282 (2), and ca. 261 (3) Ma. The intrusion pulses of the Samsal subalkaline leucogranites and the Khon'chin rare-metal Li-F alkaline granites and syenites (episode 1) were followed by the formation of the Shagaite-Gol fluid-explosive rocks and amazonite-bearing pegmatite veins (episode 2) and then veins of fine-grained subalkaline leucogranites found within the Samsal intrusion (episode 3). The granitoids are mostly A-type. The Samsal pluton shows postcollisional trace element signature unlike the Khon'chin dikes and the Shagaite-Gol rocks with syncollisional and within-plate characteristics. The Late Carboniferous-Early Permian plutonism in the East Sayan region was roughly coeval with the intrusion of the Angara-Vitim batholith in western Transbaikalia (310 to 280 Ma) and Kalba intrusion in southwestern Altai (290-270 Ma).

We have studied the geologotectonic and petrochemical peculiarities of the occurrences of subalkaline ultrabasic lavas of the northern Siberian Platform. The constructed petrochemical diagrams evidence the existence of compact rock series from meymechites through porphyritic peridotites, picrite porphyries, picrites, and picrite basalts to plateau basalts. The TiO₂-MgO and K₂O/(K₂O + Na₂O)-MgO diagrams show a broad scatter of composition points. The K₂O-MgO diagram supports the existence of low-, medium-, high-, and superhigh-potassium rock series. We suggest that K serves as an indicator of the genetic relationships between the rocks from meymechites to plateau basalts (traps). Formation of K-rich magmas begins with metasomatism of the mantle substratum interacting with deep-seated K-enriched fluids. Then it gives way to potassic magma metasomatism of ultrabasic and daughter basic melts. Potassic magma metasomatism is distinguished by increased K₂O/(K₂O + Na₂O) of the melt with an increase in its Mg content.

Five age stages of xenolith-bearing basalts of the Vitim and Khamar-Daban lava plateaus have been recognized by K-Ar dating. Thermobarometry of the basalt melts suggests deep levels of magma derivation at the initial and final stages. The PT paths of the melt upwelling in the period of generation of lava plateau are steeper than the PT paths of hydrous lavas during the initial and final periods of volcanic activity. Temperature sections for a mineral (rock) vary quickly in time. For the Vitim Plateau, three zones of melt concentration have been expressed at Stage I (18-16 Ma): 25-27 kbar, 1300-1100

Unique diamond placers have formed in the north of the Sakha Republic (Yakutia). They abound in diamonds with light carbon isotope composition, ¹³C = -22.5 % (northern (Ebelyakh) type). Such diamonds were not found in kimberlites of the Yakutian diamondiferous province, but they were discovered in the well-known primary deposits of the Arkhangel'sk province. The primary sources of Ebelyakh diamonds are supposedly of Triassic age. Primary aureole of Ebelyakh diamonds has been revealed in Carnian (Upper Triassic) littoral sediments. Primary sources of these diamonds are predicted in the northeast of the Siberian craton in the zone of its junction with a mobile belt.

Late Cenozoic kinematics and geometry of regional and local active faults in the northern and western sides of the Hovsgol basin have been studied on the basis of satellite imagery and field measurements. W-E faults show reverse-oblique motion with a left-lateral strike slip component; strike-slip components in diagonal faults are right-lateral on NW planes and left-lateral in NE faults. This fault geometry corresponds to transpression with NEN principal compression.

We report results from a GPS network installed following special criteria as part of an international project and implications for recent crustal movements in the western Altai-Sayan region. The network includes one permanent (NVSK, near Novosibirsk) and 17 campaign sites resurveyed yearly in the Altai mountains and their northern surroundings. The sites are operated with three Trimble 4700 GPS receivers and hard-mounted antennas at geodetic marks, in 2-3 day sessions at each site. The experience of seasonal and permanent monitoring along the ~76 km Talaya-Irkutsk baseline was used to analyze some methodological issues of GPS measurements.

Earthquakes are an important source of knowledge on mode of deformation and strength of rocks. According to the available hypocenter locations and recent relocations, most of seismicity in the Baikal rift is confined to the upper 26-27 km of the crust. The base of seismogenic crust corresponds to transition from brittle failure to plastic flow. The 0-27 km depth range of brittle failure in rifts is consistent with mostly mafic crust, high pore pressure, and an

A 2D solution for the TEM field has been obtained by introducing additional perturbation eddy currents into Maxwell's equation. The numerical results have been tested against physical modeling and measurements in a well.