GEOLOGY & GEOPHYSICS, 2001, V 42, N 9, September.


PETROLOGY, GEOCHEMISTRY, AND MINERALOGY

Intrusive traps in the western Siberian Platform: petrochemistry and regional zoning of composition Sharapov V. N., Vasil'ev Yu. R., and Prusskaya S. N. 1291 Three petrochemical groups of basic rocks are statistically recognized among the intrusive traps of the Siberian Platform (SP) and Taimyr Peninsula. These are: (1) normal tholeiitic (predominant), (2) Ti- and Fe-enriched, with increased contents of alkalies (ferrodolerites), (3) low-Ti picritoid (of Noril'sk type), which are least abundant. The platform cover is most saturated with intrusions in the west, where in places up to 17 separate magmatic bodies occur and their total thickness may be more than 2000-2500 meters. In some sections two or three petrochemical groups of intrusions are present, whereas others are denominated by bodies of one type. There is distinct regional zoning in distribution of compositions of intrusive traps. In the southern and southwestern SP, the Noril'sk-type massifs are rare to absent. Only one deposit contained about 15 wt.% MgO, whereas the maximum contents of MgO at the bottom of differentiated sills are, as a rule, less than 11 wt.%. Striking differences are observed in occurrence of ferrobasites. In the western SP they make up no more than 11%, and in the southern SP, about one quarter of all rock groups. The rock enriched in TiO2 and FeO* (ferrodolerites) amount to about 70% in the south of the platform. The most basic, magnesian, and low-Ti melts occur within the Noril'sk-Kharaelakh district. South and north of it, the intrusive traps are, on average, more silicic, high-Ti, and alkaline. The molten intrusive basites in the north and south are richer in iron than those in the central part of the platform. In the east of the platform the melts are close to "normal tholeiitic". They contain less iron but are richer in Ca and Mg than the melts in the southwestern SP. The compositional zoning of intrusive traps is correlated with structure-formational zones and the structure of the Earth's crust: (1) magnesian and trachybasaltic differentiated intrusions are conjugate chiefly with the Yenisei-Khatanga depression, (2) Fe- and Ti-enriched basites are predominant in the Angara block, which is characterized by the thinnest consolidated Earth's crust in its central part, (3) normal tholeiitic basites seem to dominate in localities with the thick consolidated Earth' crust. Traps, petrochemistry, zoning
Melilitic and monticellite-bearing rocks of the Krestovskaya intrusion (northern Siberian Platform) and their genesis. Panina L. I., Sazonov A. M., and Usol'tseva L. M. 1306 The geologic structure of the Krestovskaya intrusion, discovered in 1966, is discussed. Petrographic characteristics of melilitic and monticellite-bearing rocks, their mineralogy, and composition of minerals, including rare ones (kalsilite, combeite, rankinite, larnite, and djerfisherite), are reported. Studies of melt inclusions have shown that melilitic and monticellite-bearing rocks crystallized at the magmatic stage in the following sequence: perovskite (1250-1230oC)...melilite (1235 1160oC)... monticellite (1090-990oC). The melt during crystallization of perovskite was lowly siliceous (30-33 wt.% SiO2), depleted in Al2O3 (5-6 wt.%) and MgO (6-8 wt.%), enriched in CaO (17-21 wt.%) and alkalies (totalling about 11 wt.% with predominance of K over Na), and contained 2-3% SO3 and, perhaps, some quantities of Cl and CO2, i. e., was alkaline-ultrabasic, enriched in Ca, and had K-Na or even Na-K type of alkalinity. During crystallization of perovskite, liquid silicate-salt immiscibility manifested itself. Crystallization of melilite started from the same melt and proceeded under the conditions of ongoing two-phase liquid immiscibility with a drastically changing oxygen fugacity. During crystallization and fractionation of melilite, the contents of Si and Al in the melt strongly increased, the content of Fe sharply decreased, the amount of Na in the total of alkalies diminished and that of K grew, and the quantities of Ca and Mg remained the same. Subsequently the high content of K favored binding of Al-Si radicals into molecules of phlogopite and kalsilite and crystallization of djerfisherite under reducing conditions. The extremely high activity of Ca favored crystallization of rather rare calcium silicates, larnite and rankinite, and binding of the rest amount of Na into combeite and pectolite. The silicate- salt melt, spatially separated as a result of liquid immiscibility, was compositionally close to highly siliceous carbonatitic alkaline melt of Fort Portal in Uganda. It was responsible for autometasomatic transformation of olivine and pyroxene rocks, which resulted in monticellite and garnet at the magmatic stage. Among volatiles, CO2 and H2O were predominant in silicate-salt melts, while CO was present in minor amount. Melilitic and monticellite-bearing rocks, melt inclusions, two-phase liquid immiscibility, highly siliceous carbonatitic melts, autometasomatism of magmatic stage
Middle Paleozoic age of metamorphism in the South-Chuya complex in Gorny Altai (results of Ar-Ar, Rb-Sr, and U-Pb isotope dating) Plotnikov A. V., Titov A. V., Kruk N. N., Ota T., Kabashima T., and Hirata T. 1325 The South-Chuya metamorphic complex, Gorny Altai, lies on the northeastern margin of the Altai-Mongolian terrain. Metamorphic rocks of the kyanite- sillimanite type occur in the western part of the complex, whereas higher- temperature and shallower rocks (andalusite-sillimanite type) exist in the center and in the east. In our preceding works, polymetamorphic nature of the South-Chuya complex was established and Early Cambrian age of the first stage of metamorphism of the kyanite-sillimanite type was substantiated. This paper reports results of complex isotope dating of metamorphic rocks and migmatites of the superposed step of metamorphism of the andalusite-sillimanite type. Geochronological study was carried out at three reference localities in the central and eastern South-Chuya complex. As a result, only Middle Paleozoic ages were obtained (from the Late Silurian through the Early Carboniferous). Comparison and analysis of the geochronological data obtained by different methods permit us to recognize the following stages in the history of the South- Chuya complex: (1) Early Cambrian metamorphism of the kyanite-sillimanite type; (2) Late Silurian superposed high-temperature metamorphism and ultrametamorphism of the andalusite-sillimanite type; (3) completing Middle-Late Devonian and Early Carboniferous thermal events, which manifested themselves in disturbance of K-Ar isotope systems of biotites and white micas of some localities. The last thermal event was involved with the intrusion of veins of leucogranites and was accompanied by brittle and brittle-plastic deformations. Polymetamorphism, ultrametamorphism, geochronology, Middle Paleozoic, Gorny Altai
The role of gravity in supramolecular crystallization during formation of precious opal Serdobintseva V. V. and Kalinin D. V. 1340 Experimental studies have revealed a dependence of the rates of sedimentation and growth of supramolecular crystals on the degree of deionization of suspensions of monodisperse silica spheres. At low pH, the rate of crystallization is determined by sedimentation of structural units. The calculated energy of interaction of two particles shows a drastic predominance of electrostatic repulsive forces over molecular attractive ones. On crystallization during sedimentation, the repulsion of structural units is balanced out by gravitational forces, an equivalent of molecular attractive forces. Silica, monodisperse system, sedimentation, supramolecular crystallization, gravitational forces
Behavior of PGM impurities on crystallization of melts of the Fe-Ni-S system (FexNi0.49-xS0.51 section) Sinyakova E. F., Kosyakov V. I., and Kolonin G. R. 1346 Experiments have revealed the main tendencies of the physicochemical behavior of Ni, Fe, and PGM impurities at an early stage of crystallization of Fe-Ni sulfide melts with a small surplus of sulfur relative to the total of metals. It has been shown that crystallization of sulfide melts of the section FexNi0.49-xS0.51 occurs at 900 oC throughout the interval of ratios Ni/(Ni + Fe) with the formation of a monosulfide solid solution (mss) as the main phase, Fe being fractionated chiefly into mss and Ni, into the melt. Against this background, under certain conditions PGE-bearing phases can form - (Pt, Fe, Ir)-alloy (at initial >> Ni), RuS2 (at initial Ni >> Fe), (Pt, Ni)S - in a specimen of pure Ni initial composition, i.e., at the ultimately high value of lg(<$Ef sub {roman {S sub 2}}>) = -1.2. It has been shown that on mss crystallization considerable amounts of Pd accumulate in the residual melt (then Pt not separated in the form of alloy with Fe and Ir join it), whereas Ir and Ru pass predominantly into mss. With an excess of Fe in initial compositions (low values of <$Ef sub {roman S sub 2}>), most Rh passes into the melt, but an increase in the initial content of Ni leads to predominant distribution of Rh in mss. Special experiments with Pd-bearing specimens of the explored section suggest a drastic (to about 600 oC) increase in temperature of existence of the last drops of a sulfide melt enriched in Pd (up to 8 at.%). At least, it yields crystals of Pd-pentlandite (3.4 at.% Pd), a synthetic analog of vasilite Pd16S7, and a phase of the type of Pd-bearing heazlewoodite (with 19.3 at.% Pd). Fe-Ni-S system, platinum, iridium, light platinoids, monosulfide solid solution, sulfide melt, partition coefficients
Garnet ultramafites and associated metabasic rocks of the Biryusa block (southern Siberia) Kolotilina T. B. and Mekhonoshin A. S. 1213 A detailed petrologo-geochemical study of basic and ultrabasic rocks of the Biryusa block has shown that they resulted from metamorphism of weakly differentiated rocks of lhersolite-olivine websterite series and tholeiitic basalts. The temperatures of formation of igneous pyroxenes of ultrabasic rocks are estimated at 1100-1200-C. The subsequent high-pressure metamorphism at 800- 900-C and 16-20 kbar produced garnet parageneses in the most ferruginous derivates of ultrabasic magma and in basic rocks. Garnet ultramafites, lherzolites, metabasic rocks, high-pressure metamorphism
Dynamics of melting in the oceanic upper mantle. Istomin V. E., Vasil'ev Yu. R., and Mazurov M. P. 1362 EPR spectrscopy was used to study specific features of distribution of paramagnetic admixtures in endogenous carbonates formed in the zones of contact of trap complex with deposits of the Siberian Platform cover. It was established that the main paramagnetic centers in the studied samples are: a) ions Mn2+, isomorphously replacing Ca2+ and Mg2+, b) ions Fe3+, forming oxide or hydroxide clusters 100-1000 A and more in size, and c) radiation centers, originated from natural ionizing radiation. Using indicator features of spectra, it was established that carbonates from magnesian-skarn associations are a solid solution of calcite with dolomite and ankerite. Compared with carbonatites, calcites from the contacts with traps have a lower Fe content and low concentrations of Fe3+ clusters. Radiation centers were detected in Fe-free calcites accompanying apatite. The number of paramagnetic ions Mn2+ decreases in low-temperature generations of calcite. All genetic groups have a tendency to replacement of structurally inhomogeneous high-temperature solid solutions of carbonates by stoichiometric calcites, dolomites, ankerites, and siderites. Carbonates, electronic paramagnetic resonance, trap, metasomatism.
TECTONICS

Cenozoic strike-slip tectonics in Altai. Novikov I. S. 1368 The geodynamic framework of the Altai mountainous province has been modeled on the basis of structural data. The Altai mountains formed by brittle deformation and plastic flow of crustal material at the southern and western boundaries of the Mongolian microplate in the field of regional compression caused by the India-Eurasia collision. Two principal flows were directed northwestward and eastward. Crustal shortening occurred in elongate strike-slip zones and was accompanied by subsidence of the Moho and growth of mountains. The latter occurred along reverse faults (or, less often, along thrusts) or was associated with large-magnitude motions on major strike-slip faults. Cenozoic vertical offset along individual faults has been 3-4 km, and the amount of horizontal displacement attains 40 km. In the northwestern segment of the province within the Russian Altai, the amount of horizontal displacement along strike-slip faults has been partially reduced as the major faults split into north-striking tension faults and west-striking reverse faults. The area of mountains increased at the account of marginal parts of stable surrounding blocks involved in the uplifting. As evidenced by the modern seismicity mostly concentrated along the main fault boundaries of Altai, the process that started in the Oligocene and became more active in the latest Neogene continues at present. Cenozoic orogeny, strike-slip faults, neotectonics, Altai
GEOPHYSICS

Lithospheric stress field as a control over seismogenic fault parameters and earthquake magnitudes. Lunina O. V. 1381 The paper considers quantitative relationships between the length of seismogenic rupture, displacement, and magnitude of earthquakes as a function of slip geometry and regional stress field. These relationships differ for faults of the same geometry (normal, reverse, or strike-slip) formed in different lithospheric stress fields. Strike-slip faults are the most universal seismogenic ruptures. Their parameters show a close relationship with earthquake magnitudes in any stress field. Normal and reverse faults formed under extension and compression, correspondingly, or in transitional fields show most regular relationships of their parameters. Zones of compression are the most complicated settings for faulting with any slip type. The results of these studies are important for reconstructions of paleoseismicity. Seismogenic rupture, parameters, stress field, slip type
Deep structure of the Chuya basin (Gorny Altai), as imaged by TEM soundings. Nevedrova N. N., Epov M. I., Antonov E. Yu., Dashevskii Yu. A.,and Duchkov A. D. 1391 A geoelectric model of the Chuya basin has been constructed on the basis of an extensive collection of TEM data obtained in the 1970-80s. The experimental data were interpreted by solving inverse problems using the ERA and SONET computer systems. The modeling imaged the deep structure of the basin and provided a more exact idea of the thickness of its Cenozoic fill. Moreover, it revealed main features of the regional seismicity pattern. Transient electromagnetic soundings (TEM), inverse geoelectric problem, seismicity, induced polarization (IP), Chuya basin, Gorny Altai
BRIEF COMMUNICATIONS

Stromatolites and oncolites in Mesozoic deposits of the West Siberian Plate Zanin Yu. N., Luchinina V. A., Levchuk M. A., and Pertseva G. M. 1409 We describe stromatolites and oncolites found in Mesozoic strata of the West Siberian Plate. They occur in the Upper Jurassic terrigenous-carbonate deposits of the upper member of the Abalak Formation in the west of the plate. This formation is an analog of the Georgievka Formation widespread in the region. The stromatolites are small-columnar, with the columns 1.5-2.5 cm wide and up to 4.5 cm high. The oncolites reach 4-5 cm in size. Along with stromatolites and oncolites, there are also stromatolite concretions measuring up to 10 cm and microstromatolitic and microoncolitic bodies as small as few millimeters. Stromatolites, oncolites, Upper Jurassic, West Siberia
First data on Early Cretaceous pegmatites in Western Transbaikalia (Rb-Sr dating) Shadaev M. G., Posokhov V. F., and Ripp G. S. 1413 We performed Rb-Sr isotope dating of dikes of granite pegmatites breaking through the alkali gabbroids and syenites of the Oshurkovsky Massif. The refined age of the massif is 125 Ma. Pegmatites form a field including several tens of bodies. We have established that the pegmatites formed in two stages: 113<$E+->2 Ma and 120<$E+->2.4 Ma ago. Pegmatites of these intrusion stages differ in geochemical features. The close ratios of Sr isotopes in the pegmatites of the first stage and in the massif gabbroids suggest participation of alkaline- basaltic magmas in granitoid pegmatite formation. Rb-Sr isotope dates, granite pegmatites, Early Cretaceous, Transbaikalia