Mudstone lithogeochemistry and formation conditions of Vendian deposits in the Shkapovo-Shikhan Basin

Variation of geochemical modules and indices in mudstones from the Upper Vendian Kairovo and Shkapovo groups of the Shkapovo-Shikhan Basin provides the comprehensive information on changes in maturity of the fine aluminosiliciclastic material delivered in the basin, characterizes the redox environment in bottom water, and makes it possible to reconstruct the rock composition in provenance and its evolution through time. The generally moderate maturity of the fine terrigenous clastic material suggests that a nearly semiarid-semihumid climate dominated in paleodrainage area throughout the Late Vendian. It has been established that reducing environment did not exist in bottom water of the central Shkapovo-Shikhan Basin throughout the Late Vendian. Intermediate rocks prevailed in the paleodrainage area. More silicic rocks could occur only in the early Staropetrovo and late Salikhovo times. Data points of mudstones from the Kairovo and Shkapovo Groups plotted on the Cr-Ni, Co-V, Co/Hf-Ce/Cr, La-Th, and La/Sm-Sc/Th diagrams indicate that both Archean and more mature Paleoproterozoic crustal blocks existed in different proportions in the Late Vendian within source areas.     

P-T paths and problems of high-temperature polymetamorphism

Many Precambrian granulite-facies metamorphic complexes contain so-called straight gneisses, which are massive rocks with a clearly pronounced blastomylonitic texture, lineation, and gneissosity. These rocks occur exclusively in high-temperature ductile shear zones, which can develop either during the primary exhumation of rock complexes or during the overprinting by high-temperature dynamometamorphism. The main criterion for distinguishing between these two types of straight gneisses is the configuration of their P-T trajectories, which are recorded in the mineral assemblages in these rocks and their host gneisses. Ductile shear zones developed in Archean granulite gneisses simultaneously with their exhumation, and, hence, their P-T trajectories are segments of decompression and/or isobaric cooling paths. Straight gneisses in Proterozoic polymetamorphic complexes commonly compose high-temperature ductile shear zones overprinted on Archean granulite complexes, and the P-T paths of these rocks are Z-shaped. This means that, at a constant pressure in the middle part of the continental crust, the T _min of the older P-T trajectory corresponded to T _max of the younger trajectory, and often T _max–T _min > 100°C. Such ductile shear zones commonly have a strike-slip morphology and can be easily seen in aerial photographs and discerned during structural geological surveying. These zones can overprint older gneisses without any notable thermal effect on the latter. Relations of this type were identified in the granulite complexes of Limpopo in South Africa, Sharyzhalgai in the southwestern Baikal area, and Lapland in the Kola Peninsula. The results of our research propose a solution for the well-known problem of the significant discrepancies between the isotopic ages in high-temperature-high-pressure complexes and the partial or complete distortion of radiogenic isotopic systems under the effect of a newly inflowing metamorphic fluid. The application of geochronologic techniques to these situations is senseless, and only P-T trajectories provide insight into the actual age relations between the discrete tectono-metamorphic stages. It is thus expedient to conduct not only structural studies of metamorphic complexes but also their detailed petrological examination and the calculation of their P-T paths before geochronologic dating.     

Composition of sand storm particles in the Southern Far East

Lithostructural composition of the dust storm material (DMS) sampled in the southern Far East in the spring of 2002–2004 has been analyzed. Samples were taken near Vladivostok, where the eolian dust precipitation varied from 0.1 to 6.5 g/m² at that time. Pelitic and fine-silt particles predominate in eolian dust. Grain size, mineral, and chemical compositions of the DSM are compared to data on the DSM in adjacent territories of China, Korea, and Japan, as well as to background data on aerosol precipitation in southern Primorye. Possible mechanisms of the eolian dust transport from inner parts of Asia at different climatic variations in the Pleistocene-Holocene are discussed.     

Thermodynamics of arsenates,selenites, and sulfates in the oxidation zone of sulfide ores. X. thermal stability and dehydration features of synthetic analogs of the cobaltomenite–ahlfeldite solid solution series

The aim of this study is the experimental investigation of the synthetic analogs of cobaltomenite, CoSeO₃ ? 2H₂O, ahlfeldite, NiSeO₃ ? 2H₂O, members of the cobaltomenite–ahlfeldite solid solution series (Ni _x Co_1–x )SeO₃ ? 2H₂O, and singularities of their dehydration and dissociation. The intermediate members of the cobaltomenite (CoSeO₃ ? 2H₂O)–ahlfeldite (NiSeO₃ ? 2H₂O) series have been synthesized and studied with a combination of X-ray diffraction, scanning electron microscopy, and the simultaneous application of thermogravimetry (TG) and differential scanning calorimetry (DSC) within the temperature range from 25 to 640°C. The complete solid solution series corresponds to the monoclinic space group P2₁/n. Unit-cell dimensions decrease in all crystallographic directions as the amount of Ni increases. The angle β increases from 98.82(1) (cobaltomenite) to 99.05(1)° (ahlfeldite). It has been established that CoSeO₃ ? 2H₂O and NiSeO₃ ? 2H₂O dehydrate at 120–340°C through two stages apparently corresponding, to the formation of intermediate hydrated species CoSeO₃ ? H₂O and NiSeO₃ ? 1/3H₂O. The reaction enthalpies for each dehydration stage of CoSeO₃ ? 2H₂O and NiSeO₃ ? 2H₂O have been determined. Changes of the unit-cell dimensions and dehydration temperatures are rationalized in terms of the Co and Ni site occupancy in the structure of the cobaltomenite–ahlfeldite solid-solution series members.     

压实黄土非饱和渗透系数试验研究

压实黄土非饱和渗透特性具有重要的工程意义。对不同干密度的压实黄土试样,采用非饱和导水率测定系统进行非饱和渗透系数量测,得到不同干密度压实黄土的渗透系数与吸力的关系。试验结果表明：渗透系数随吸力的增大而成非线性减小,随干密度的减小而增大;在低吸力时,干密度对渗透系数的影响较大;在较高吸力时,干密度对渗透系数的影响较小。进一步给出了压实黄土的渗透系数模型并进行回归分析,得到各Wind幂函数模型参数与干密度的关系,建立了基于干密度的压实黄土渗透系数的确定方法。     

龙门山及其邻区的地壳厚度和泊松比

根据龙门山及其周边地区(26°～35°N,98°～109°E)的132个台站的宽频带远震记录,使用H-k叠加方法计算地壳厚度和波速比.结果表明该区域的地壳厚度总体变化是:从东向西增加,东部的最小厚度为37.8km,西部的最大厚度是68.1 km,其中横跨龙门山断裂带的地壳厚度变化最大,从东南的41.5km增加到西北的52.5km.根据Airy均衡理论,用台站的高程和观测地壳厚度数据求得最小二乘意义下的壳幔密度差为0.649g/cm3,平均地壳厚度为37.9km.龙门山及其邻近地区基本上处于均衡状态.松潘-甘孜地体北部和西秦岭造山带具有低泊松比(v<0.26),扬子地台的西南部具有低一中泊松比(v<0.27),松潘-甘孜地体南部、三江褶皱带和四川盆地具有中一高泊松比(0.26≤P≤0.29).该地区的泊松比空间分布不支持青藏高原东部广泛分布的下地壳流的假说.龙门山断裂带南段及其附近地区的高泊松比(v≥0.30)可以看成是地壳具有较高的铁镁质组分和/或存在部分熔融.该地区下地壳可能是处于富含流体和温度较高的部分熔融状态.松潘-甘孜块体南部的上地壳物质向东运动,受刚性强度较大的扬子地台的阻挡,导致沿龙门山断裂带产生应变积累.当断层被地壳流体弱化,积累的应变能量快速释放,产生汶川Ms8.0地震.