M. I. Kuz’min

Reconstruction of the Holocene Climate of Transbaikalia: Evidence from the Oxygen Isotope Analysis of Fossil Diatoms from Kotokel Lake

Fossil diatoms or Bacillariophyta are microscopicunicellular organisms with siliceous cell–frustule consisting of two separate valves that play an importantrole in marine and lake sedimentation. The data ofdiatom analysis provide reliable spatial and temporalreconstructions of the natural environment and theclimate of past geological epochs [1].The possibility of investigation of fossil Bacillariophyta by the oxygen isotope method in order to obtainpaleoinformation was originally demonstrated by L.Labeyrie [2]. The plotted isotope curves indicate variations in the temperature and isotope composition ofwater (δ

Absolute paleogeographic reconstructions of the Siberian Craton in the Phanerozoic: A problem of time estimation of superplumes

The intraplate activity within the Siberian Craton in the Phanerozoic is related to continental migration above the hot spot agglomeration compared to the African superplume. The continuity of intraplate activity within this superplume testifies to its age identity to the antipodal to the Rodinian superplume that destroyed the Rodinia supercontinent. This allowed us to conclude that the African superplume has existed for no less than 1 Ga. Because the Rodinian and Pacific superplumes are compared, it may be gathered that superplumes are the most long-lived deep-seated structures of the Earth. Their relation to the formation of supercontinents probably reflects the antiphased activity caused by the thermostating effect and energy accumulation by superplumes when being overlapped by supercontinents. When analyzing the evolution and generation of modern continents, it is necessary to consider both processes related to the plate boundaries and the activity of superplumes determining the intraplate magmatism therein.

Magmatism of the Khambin Graben and Early History of the Late Mesozoic Rift System Formation in the Western Transbaikal Region

The western Transbaikal region was repeatedly subjected to rifting during the Mesozoic. The Early Mesozoic was marked by the formation of a system of grabens filled in with Late Triassic‐Early Jurassic bimodal volcanic sequences of the Tsagan-Khurtei Group and Kunalei Complex, which is traced by massifs of alkalic granites [1‐5]. In the Late Mesozoic, a new rift system, generally conformable with the previous one, appeared [6‐9] and began to develop until the Late Cenozoic. Therefore, the grabens and horsts of the rift system are readily traceable in the present-day topography. The Late Mesozoic epoch commenced with the formation of the bimodal basalt‐trachybasaltic andesite‐trachydacite‐trachyrhyolite‐comendite volcanic association of the Ichetui Formation. Its compositional and structural similarity to the Tsagan-Khurtei Group served as the basis for determining the age of bimodal volcanic associations in some areas of the region, resulting in misleading interpretation of the structure, scale, and geodynamic settings of different-age rifting events. This problem can be exemplified by the Khambin volcanic field (Fig. 1), one of the largest in the region. The volcanic field is outlined as a ridge in geological maps, because it resembles the majority of outcrops of the Tsagan-Khuntei Formation in the topography [10, 11]. In such an interpretation, the field occupied the westernmost part of the Early Mesozoic rift system and, thus, governed its dimensions and structural peculiarities in the pinchout area. At the same time, the Khambin field, located between the western (Dzhida) and central (Khilok‐Tugnui) segments of the Late Mesozoic rift system (Fig. 1, inset), occupies a relatively large fragment of the rift system. In available maps, the fragment is shown as an anomalous zone because of the absence of Late Mesozoic magmatism. In this communication, we present systematic geological and geochronological (Rb‐Sr, K‐Ar) data, which point to the formation of Khambin field lava sequences as a result of Late Mesozoic rifting in several stages of volcanic activity. This conclusion is consistent with the multistage character of magmatic processes in other areas of the rift system. The materials obtained allow us to scrutinize specific features of the manifestation of early stages of Late Mesozoic rifting. The Khambin volcanic field extends in the NNW

Autonomous anorthosites of the Anabar Shield: Age, geochemistry, and formation mechanism

The new high-accuracy data on U–Pb zircon geochronology, Sm–Nd systematics, and geochemistry of anorthosites of the Anabar Shield are discussed. It is established that anorthosite massifs are composed of gabbro–anorthosites (1.96 Ga old) and oligoclasites (1.93 Ga old) in association with monzodiorites (1.84–1.90 Ga old) and porphyroblastic granites. These rocks were generated in the Archean (3.2–2.7 Ga ago) in the lower crust from quartz–diorite melts under the plume tectonics regime in line with the filterpressing mechanism. The rocks were successively exhumed to upper levels of the crust owing to the Paleoproterozoic impact-triggered process to form a tectonically juxtaposed complementary magmatic complex.

Ocellar-porphyroblastic granitoids of the western part of the Aldan Shield: Geochemistry, age, and mechanism of formation

462 The western part of the Aldan Shield occupies the whole area of the Olekma–Vitim Highland. In this area the Charskaya ring structure with a diameter of 260–280 km [1, 2] includes almost all structural– material complexes of the Early Precambrian: from crystalline infra and supracrustal rocks of Pale oarchean and Mesoarchean metavolcanogenic–sedi mentary formations of greenstone suture zones (troughs) to sedimentary rocks of the Udokan proto platform trough. Different magmatic and palingene– metasomatic rocks of the Archean and Paleoprotero zoic are widely abundant in the region (Fig. 1). Because of good outcropping, we may clearly observe the geological relationships between these polychro nous formations, the age of which in most cases was determined by U–Pb and, rarely, Sm–Nd methods (see [1] for references).

New Data on Vegetation and Climate Reconstruction in the Baikal-Patom Highland (Eastern Siberia) in the Last Glacial Maximum and Early Holocene

The first results of anthracological investigation for Eastern Siberia on the carbonaceous remains of woody and shrubby plants at the archaeological sites Kovrizhka III and IV in the lower reaches of the Vitim River are presented. The results of anthracological studies enabled us to obtain new data on changes in vegetation and climate along the lower reaches of the Vitim River. As a result, new data on human habitation in the lower reaches of the Vitim River in the last glacial maximum and early Holocene were obtained.

The first discovery of Hadean zircon in garnet granulites from the Sutam River (Aldan Shield)

For the first time in Russia, a Hadean zircon grain with an age of 3.94 Ga (ID-TIMS) has been discovered in high-aluminous garnet granulites of the Aldan Shield among the U–Pb zircons with an age from 1.92 Ga. In this connection, the problems of its parental source, the petrogenesis of granulites that captured this zircon, and the mechanism of occurrence of these deep rocks in the upper horizons of the crust have been solved. The comparison of the geochemistry of garnet granulites and the middle crust has shown that the granulites are enriched in the entire range of rare-earth elements (except for the Eu minimum), as well as in Al2O3, U, and Th and are depleted in the most mobile elements (Na, Ca, Sr). In the upper part of the allitic weathering zone of the middle crust, which formed under conditions of arid climate, this zircon grain was originated from the weathered granites from the middle crust. In the latter case, they were empleced discretely in the upper granite–gneiss crust under high pressure conditions (the rutile age is 1.83–1.82 Ga). The zircon with an age of 3.94 Ga is comparable to the Hadean zircons from orthogneisses of the Acasta region (Canadian Shield, 4.03–3.94 Ga).

The substance of the Chelyabinsk meteorite: Results of geochemical and thermomagnetic studies

The fragments of the Chelyabinsk meteorite studied are represented by light-gray granular rock of chondritic structure. The chondrules and their cementing matter are mainly constituted by olivine and orthopyroxene. The matrix consists of a pyroxene-olivine aggregate with plagioclase, apatite, melted glass, and the inclusions of ore minerals: taenite, kamacite, troilite, pyrrhotite and pentlandite (more rarely), and individual grains of chromite and ilmenite. The comparison of the composition of the Chelyabinsk meteorite to the average composition of LL chondrites had shown their complete convergence. The concentrations of sidero- and chalcophile rare elements in the meteorite, normalized to CI chondrites, are much close to the values for LL chondrites and almost reproduce the character of their distribution in the spider diagram. However, some high-charged and lithophile elements (Nb, Zr, Hf, Sr, Ba, Th, and U) not belonging to the mentioned groups are characterized by somewhat increased contents. The enrichment of the samples of the Chelyabinsk meteorite in rare-earth elements compared to LL chondrite (5.18 against 3.58 ppm) is also revealed. This is related to the higher concentrations of light lanthanides in the meteorite samples, which is seen from the increased La/Yb ratio compared to the value for LL chondrite (1.9–2.3 and 1.4, respectively). Iron-nickel alloys are the main magnetism carriers in the Chelyabinsk meteorite. The compositions of kamacite, taenite, chromite, and Fe-sulfides are not much different. The optical and microprobe data are confirmed by the thermomagnetic parameters as well: (1) The specific magnetization of 4–6 Am2/kg points to small variations in the concentrations of magnetic minerals. (2) The M(T) curves for all the samples nearly repeat each other, and the Curie temperatures of 490–520 and 740–770°C are registered in the curves of the first and second heating, hence, these curves correspond to kamacite of various composition, right up to pure iron. (3) The monocline ferrimagnetic pyrrhotite of TC = 320–340°C is registered in the treated fragments in both the M(T) curves of heating and cooling. (4) The concentrations by thermomagnetic analysis amount to 0.6–1.6% (0.9% average) for kamacite, 0.7–1.5% (1.1% average) for taenite, and 0–1.5% (0.4% average) for monocline pyrrhotite. (5) No magnetite was found in the M(T) curve during the first heating of the samples. Hence, the content of magnetite is much below 0.1.