Home – Home – Jornals – Russian Geology and Geophysics 2008 number 12

The Borisikha ophiolite, composed of serpentinites and amphibolites, is an extension of the Isakovka ophiolite belt in the near-Yenisei part of the Yenisei Ridge. It is a slightly disturbed ~3.5 km thick fragment of the ophiolite association; its crustal part is composed of metamorphosed wehrlite, troctolite, and gabbro cumulates. The cumulate composition and the geochemical features, which do not show any suprasubductional overprinting, indicate that the ophiolite is a normal oceanic lithosphere. Zircons extracted from pegmatoid amphibolite show sectorial magmatic zoning, low U contents, and magmatic Th/U values. The zircon age determined on a SHRIMP-II mass spectrometer is 682±13 Ma, which corresponds to the time of formation of oceanic crust. In the second half of the Late Riphean, the oceanic lithosphere that included the Borisikha ophiolite was subducted beneath the margin of the Siberian continent. At the same time, rifting, formation of half-grabens, and alkaline magmatism took place at the rear of the Yenisei Ridge. Similar events took place in southeastern East Sayan. Correlation between the coeval Late Baikalian structures along the southern and western margins of the Siberian Platform calls for special study.

The intensity of postmagmatic processes in the Botuobinskaya pipe kimberlites was estimated from the calculated content of normative secondary quartz (Q). Several simple algorithms are proposed to calculate the Q content from chemical analyses of kimberlites. Ten groups of altered kimberlites have been recognized from the Q contents. The contents of MgO, some trace elements, and LREE in the groups, the contents of Cr and Ca of crimson garnets, the diamond contents of kimberlites, and the average weight of diamonds decrease as the Q content increases. It is shown that the negative SiO₂-MgO correlation is the most effective indicator of the postmagmatic alteration of kimberlites. As the degree of their secondary alteration increases, the kimberlites transform into an assemblage of quartz and clay minerals enriched in some trace elements and almost completely lacking REE and diamonds.

We consider the applications of existing mathematical models to heat and mass transfer dynamics in different zones of mantle-crust magmatic systems and discuss problems concerning quantitative modeling of mineralization associated with fractional crystallization of mafic melts in magma chambers, with or without crustal contamination.

Gold mineralization of the Tardan deposit is of different spatial occurrences and is related to different hydrothermal-metasomatic formations, the main ones being skarn-magnetite bodies, metasomatites of mineralized crush zones, and metasomatites of argillizitic-rock association. The formation of gold mineralization was a multistage process related to the repeated magmatism of the Tannu-Ola complex. It took place in a wide temperature range (400-150

The Borehole Vostok 3 drilled in the east of the West Siberian Plate (Tomsk Region) revealed a Vendian section in the depth range 5002-3870 m, which was subdivided into the Poiga, Kotodzha, and Raiga Formations based on geological, geophysical, and paleontological data. In the Kotodzha and Raiga Formations, typical Upper Vendian fossils of Cloudina hartmanae and Namacalathus sp. were found along with diverse Platysolenites, which are commonly considered to be of zonal significance in Lower Cambrian strata. Hence, the stratigraphic interval with abundant diverse Platysolenites has a wider stratigraphic range than it was believed earlier and seems to cover the Upper Vendian and Lower Cambrian deposits. The Borehole Vostok 3 is the first Siberian occurrence of the fossils Namacalathus , the world's fourth occurrence of the Cloudina-Namacalathus association, and the first site where coexisting Platysolenites and typical Vendian organisms have been found. Therefore, the borehole provides one of the most informative (in paleontological context) Upper Vendian sections.

Eleven new estimates of heat flow ( q ) from the southern Altai-Sayan Folded Area (ASFA) have provided update to the heat flow map of Gorny Altai. Measured heat flow in the area varies from 33 to 90 mW/m², with abnormal values of >70 mW/m² at four sites. The anomalies may have a deep source only at the Aryskan site in the East Sayan (q = 77 mW/m²) while high heat flows of 75-90 mW/m² obtained for the Mesozoic Belokurikha and Kalguty plutons appear rather to result from high radiogenic heat production in granite, which adds a 25-30 W/m² radiogenic component to a deep component of 50-60 mW/m². The latter value is consistent with heat flow estimates derived from helium isotope ratios (54 mW/m² in both plutons). Heat flow variations at other sites are in the range from 33 to 60 mW/m². The new data support the earlier inferences of a generally low heat flow over most of ASFA (average of 45-50 mW/m²) and of a

We have discovered a 2-4 year periodicity in geomagnetic secular variation (SV) from data of 110 world magnetic observatories. The periodicity in the horizontal component ( H ) is most prominent and appears to be globally uniform in different regions, on all continents, and in both hemispheres. The quasi-periodic short-wavelength variations show up in the vertical component ( Z ) as well but locally superpose on long-wavelength regional anomalies. We presume that the short-period fluctuations may be produced by instability of the eccentric dipole (ED) axis proceeding from the analysis of the SV field and optimization modeling of the dipole field with varied ED parameters.

An improved technique is suggested for quantifying seismic activity over averaging areas of an arbitrary size. The example of the Altai-Sayan seismic zone is used to substantiate the choice of a 1

Relations between indoor and soil gas radon were experimentally studied in villages of the Baikal region. On the basis of the obtained data, the soil-to-indoor radon ratio was calculated, which can be used for prediction of radon pollution in buildings of the same kind.

Climatic changes in southeastern Transbaikalia in the Middle and Late Holocene and their influence on alluvial sedimentation environments are reconstructed from the results of study of the Ilya floodplain sediments (Alkhanai National Park). At the beginning of the Subboreal period, the regional climate became more arid, which led to a significant increase in steppe species communities in the landscapes. Intense climate aridization also took place at the beginning and at the end of the Subatlantic period of the Holocene. The alluvial-sedimentation rate increased during the weakening of aridization and decreased during cooling and the intensification of aridization. The obtained regional data are compared with data on the adjacent areas and the global climatic changes in the Northern Hemisphere.