Capabilities and constraints of the Pb-Pb dating of metamorphogenic minerals using the step-leaching method

Based on particular examples, this paper considers the capabilities and constraints of the step-leaching Pb-Pb dating (PbSL) of metamorphogenic minerals. It was shown that stepwise leaching allows the separation of fractions enriched in uranogenic and thorogenic Pb isotopes, which can be used for the determination of mineral ages and the time of crystallization of coexisting equilibrium and, occasionally, disequilibrium monazites. The presence of monazite in a mineral is indicated by a high Th/U ratio similar to that of monazite. The main limitation of the method is related to the presence in minerals of both disequilibrium domains of the mineral matrix and disequilibrium monazite microinclusions. By the example of minerals studied, we discussed three scenarios for the development of the U-Th-Pb isotopic systems of metamorphogenic minerals.     

The eastern boundary of the Baikal collisional belt: Geological,geochronological, and Nd isotopic evidence

New geological. geochronological, and Nd isotopic data are reported for the rocks occurring at the interfluve of the Barguzin, Nomama, and Katera rivers, where the main structural elements of the Early Paleozoic collisional system have been established. The strike-slip and thrust Tompuda-Nomama and Barguzin boundary sutures separate the Svetlaya and the Katera zones of the Baikal-Muya Belt from the Barguzin terrigenous-carbonate terrane. The age estimates of syntectonic (prebatholithic) gneissic granite and gabbrodiorite intrusive bodies (469 ± 4 and 468 ± 8 Ma, respectively) coincide with the age of collisional events in the Ol’khon, Southwest Baikal, and Sayan regions (480–470 Ma). A linear zone with zonal metamorphism and granite-gneiss domes dated at 470 Ma is revealed in the allochthonous fold-nappe packet of the Upper Riphean Barguzin Formation. This zone of Caledonian remobilization marks the collisional front between the Riphean structural units of the Barguzin Terrane consolidated 0.60–0.55 Ga ago and the Baikal-Muya Belt. New data allow us to recognize this zone as the northeastern flank of the Baikal Collisional Belt. The Nd isotopic data for the reference igneous complexes of the collisional zone indicate that the Late Riphean juvenile crust was involved in the Ordovician remobilization in the zone of conjugation of the consolidated Baikalian structural elements at the northeastern flank of the Baikal Belt and likely was a basement of the entire Barguzin Terrane or, at least, its frontal portion. The lateral displacements of the terranes to the northeast during the Early Ordovician collision were constrained by the rigid structural framework of the Baikalides in the Muya segment of the Baikal-Muya Belt, where the Riphean blocks were involved in strike-slip faulting and the Vendian-Cambrian superimposed basin underwent deformation. Finally, it may be concluded that the Early Ordovician was an epoch of collision, complex in kinematics, between heterogeneous blocks of the continental crust: the Baikalides of the Baikal-Muya Belt and polycyclic Barguzin-Vitim Superterrane.     

Age of granitoids from the conjunction zone of the Baikal-Muya foldbelt and the Kalar metamorphic terrane

One of the key segments in the conjunction zone between the Baikal folded area of Baikalides, the Early Precambrian Aldan-Stanovoi shield, and the Barguzin-Vitim superterrane involving fragments of the Early Precambrian, Baikalian, and Paleozoic folded complexes is discussed. Within this segment, complicated tectonic contacts between the Late Riphean complexes of the Param-Shaman paleotrough zone in the Baikal-Muya foldbelt of Baikalides and Lower Precambrian complexes of the Kalar metamorphic terrane are mapped. The results of the U-Pb zircon isotopic dating (TIMS and SHRIMP-II) of gneisses-syenites from the Burgai Complex and gneissoid granites of the Drevnestanovoi Complex of the Early Precambrian age, as well as results of the Nd-isotope study of reference magmatic and stratified complexes of the region are presented. The ages of the oldest gneiss-syenites from the Burgai Complex and overlying plagiomigmatites in the conjunction zone have been established to differ by less than 1 Ma, making up 601 ± 5 Ma. Drevnestanovoi gneissoid granites in the conjunction zone are of the Late Paleozoic age (325–270 Ma). According to Nd isotopic data, the age of the source, from which Vendian gneisses-syenites and granites were melted, was established to be not older than the Riphean, and the material of the old continental crust to be the protolith of the upper Paleozoic granites. It has been inferred that the collision junction of Baikalian and Early Precambrian structures of the Baikal folded area and the Aldan-Stanovoi Shield into a single block took place 600 Ma ago.     

Pedogenic alteration of aeolian sediments in the upper loess mantles of the Russian Plain

Study of the upper loess strata within the profile of surface soils highlighted the role of pedogenesis in the formation of characteristic features of loess. Loess-paleosol sequences within the study area are influenced by their position in paleocryogenic microrelief. Clear evidence of sequential loess sedimentation, accompanied by slope processes and pedogenesis, is present in soil profiles within former thermokarst depressions. Different stages of loess sedimentation are marked by cryomorphic features, solifluction stripes and buried humus horizons. The balance between the rate of sedimentation, intensity of slope processes and pedogenesis changed within the upper 3 m of loess strata. Corresponding loess strata in inter-depression areas were also formed by sequential accumulation of aeolian dust, gradually altered by initial pedogenesis that left weakly developed soil profiles without clear horizonation. Pedogenesis resulted in diverse complexes of secondary carbonates, loose soil fabric and microfabric with abundant pores of biogenic and cryogenic nature, as well as other features, characteristic of soils of cold arid environments. The uniformity of these features throughout the upper loess strata confirms the synlithogenic nature of pedogenesis that accompanied loess accumulation.     

The variation of silver content in native gold particles as revealed by ESCA

Depth profiling analysis of native gold samples using X-ray photoelectron spectroscopy (XPS) shows the surface layers of lode gold to be significantly enriched with silver to a depth of 0.02–0.04 microns and the surface layers of placer gold to be depleted of silver up to a depth of several microns. The change in the surface composition of gold samples taken from oxidation zones is of more complicated character. Model experiments suggest the surface segregation of silver in lode gold samples to be due to outward diffusion and oxidation of silver at the surface. In placer gold the depletion of silver is caused by leaching of the latter from the surface layers.     

Ultrahigh-Pressure Liquation of an Impact Melt

Liquation structures were described in ultrahigh-pressure impact glasses of the Kara astrobleme (Pay-Khoy) with differentiation into the bisilica, aluminosilicate, and ore components for the first time. The sequence of differentiation of mineral phases upon solidification of an ultrahigh-pressure impact melt was established: coesite, silicate glass, augite, aluminosilicate glass of albite composition, and pyrite. The discovered impact glasses are highly resistant to postimpact alterations.     

Archeological Monuments: Evidence of Strong Earthquakes in the Past (Using the Example of the Southwestern Part of the Issyk-Kul Lake Region,the Tien Shan)

Izvestiya, Atmospheric and Oceanic Physics - This paper presents new materials and summarizes existing ones on archeoseismology studies of stony walls conducted in the Alabash-Konurolyong...     

Role of microstructure in clay emanation

The influence of clay microstructure on radon emanation has been studied based on the analysis of literary data and direct measurements of the emanation coefficient of different types of clay rocks. Experiments have shown that modern and Holocene clay and silt with a cellular microstructure have the highest emanation coefficient (more than 60%). The majority of clay rocks transformed by diagenesis (showing either matrix or turbulent microstructure), have the emanation coefficient of about 50%. The emanation coefficients of lithified clay and argillite (laminar microstructure) do not exceed 10%. Thus, the emanation capacity of clay depends on clay microstructure and regularly decreases in the following row: cellular > matrix or turbulent > laminar microstructure.