Post-mid-Cretaceous eastern North Sea evolution inferred from 3D thermo-mechanical modelling

The Late Cretaceous–Cenozoic evolution of the eastern North Sea region is investigated by 3D thermo-mechanical modelling. The model quantifies the integrated effects on basin evolution of large-scale lithospheric processes, rheology, strength heterogeneities, tectonics, eustasy, sedimentation and erosion.

The evolution of the area is influenced by a number of factors: (1) thermal subsidence centred in the central North Sea providing accommodation space for thick sediment deposits; (2) 250-m eustatic fall from the Late Cretaceous to present, which causes exhumation of the North Sea Basin margins; (3) varying sediment supply; (4) isostatic adjustments following erosion and sedimentation; (5) Late Cretaceous–early Cenozoic Alpine compressional phases causing tectonic inversion of the Sorgenfrei–Tornquist Zone (STZ) and other weak zones.

The stress field and the lateral variations in lithospheric strength control lithospheric deformation under compression. The lithosphere is relatively weak in areas where Moho is deep and the upper mantle warm and weak. In these areas the lithosphere is thickened during compression producing surface uplift and erosion (e.g., at the Ringkøbing–Fyn High and in the southern part of Sweden). Observed late Cretaceous–early Cenozoic shallow water depths at the Ringkøbing–Fyn High as well as Cenozoic surface uplift in southern Sweden (the South Swedish Dome (SSD)) are explained by this mechanism.

The STZ is a prominent crustal structural weakness zone. Under compression, this zone is inverted and its surface uplifted and eroded. Contemporaneously, marginal depositional troughs develop. Post-compressional relaxation causes a regional uplift of this zone.

The model predicts sediment distributions and paleo-water depths in accordance with observations. Sediment truncation and exhumation at the North Sea Basin margins are explained by fall in global sea level, isostatic adjustments to exhumation, and uplift of the inverted STZ. This underlines the importance of the mechanisms dealt with in this paper for the evolution of intra-cratonic sedimentary basins.     

Hydrothermal Redistribution of Rare—Earth Elements in Pingxiang Dacite,Guangxi

Distribution of the rare-earth elements (REE) in dacite has been studied so as to get a better understanding of the migration behavior of REE during alteration. Both unaltered and altered samples were collected in an unpolluted area of Guangxi Zhuang Autonomous Region, southwest China. The REE concentrations were analyzed by ICP-MS. It is concluded that the REE were enriched during dacite alteration in varying degrees. The chondrite-normalized REE patterns of altered samples approximately maintain the characteristics of unaltered samples. However, if we normalize the REE concentrations of altered samples with unaltered dacite, fractionation of REE will appear. The LREE are more enriched than HREE in all altered samples with the LREE possibly precipitated as carbonate minerals. Both positive and negative Eu anomalies exist. Enrichment, immobility and depletion are noticed for the element Lu. Heavy mineral alteration, difference in stability constant between carbonate LREE and HREE complexes, downward migration of weathering fluid and microenvironment change may be responsible for the fractionation of REE in the altered dacite.     

Production of selected cosmogenic radionuclides by muons: 2. Capture of negative muons

We have determined the production yields for radionuclides in Al₂O₃, SiO₂, S, Ar, K₂SO₄, CaCO₃, Fe, Ni and Cu targets, which were irradiated with slow negative muons at the Paul Scherrer Institute in Villigen (Switzerland). The fluences of the stopped negative muons were determined by measuring the muonic X-rays. The concentrations of the long-lived and short-lived radionuclides were measured with accelerator mass spectrometry (AMS) and γ-spectroscopy, respectively. Special emphasis was put on the radionuclides ¹⁰Be, ¹⁴C and ²⁶Al produced in quartz targets, ²⁶Al in Al₂O₃ and S targets, ³⁶Cl in K₂SO₄ and CaCO₃ targets, and ⁵³Mn in Fe₂O₃ targets. These targets were selected because they are also the naturally occurring target minerals for cosmic ray interactions in typical rocks. We also present results of calculations for depth-dependent production rates of radionuclides produced after cosmic ray μ⁻ capture, as well as cosmic ray-induced production rates of geologically relevant radionuclides produced by the nucleonic component, by μ⁻ capture, by fast muons and by neutron capture.     

Gravity and magnetic signatures of volcanic plugs related to Deccan volcanism in Saurashtra, India and their physical and geochemical properties

The Bouguer anomaly and the total intensity magnetic maps of Saurashtra have delineated six circular gravity highs and magnetic anomalies of 40-60 mGal (10⁻⁵m/s²) and 800-1000 nT, respectively. Three of them in western Saurashtra coincide with known volcanic plugs associated with Deccan Volcanic Province (DVP), while the other three in SE Saurashtra coincide with rather concealed plugs exposed partially. The DVP represents different phases of eruption during 65.5±2.5 Ma from the Reunion plume. The geochemical data of the exposed rock samples from these plugs exhibit a wide variation in source composition, which varies from ultramafic/mafic to felsic composition of volcanic plugs in western Saurashtra and an alkaline composition for those in SE Saurashtra. Detailed studies of granophyres and alkaline rocks from these volcanic plugs reveal a calc-alkaline differentiation trend and a continental tectonic setting of emplacement. The alkaline plugs of SE Saurashtra are associated with NE-SW oriented structural trends, related to the Gulf of Cambay and the Cambay rift basin along the track of the Reunion plume. This indicates a deeper source for these plugs compared to those in the western part and may represent the primary source magma. The Junagadh plug with well differentiated ring complexes in western Saurashtra shows well defined centers of magnetic anomaly while the magnetic anomalies due to other plugs are diffused though of the same amplitude. This implies that other plugs are also associated with mafic/ultramafic components, which may not be differentiated and may be present at subsurface levels. Paleomagnetic measurements on surface rock samples from DVP in Saurashtra suggest a susceptibility of 5.5×10⁻² SI units with an average Koenigsberger ratio (Q_n) of almost one and average direction of remanent magnetization of D=147.4° and I=+56.1°. The virtual geomagnetic pole (VGP) position computed from the mean direction of magnetization for the volcanic plugs and Deccan basalt of Saurashtra is 30°N and 74°W, which is close to the VGP position corresponding to the early phases of Deccan eruption. Modeling of gravity and magnetic anomalies along two representative profiles across Junagadh and Barda volcanic plugs suggest a bulk density of 2900 and 2880 kg/m³, respectively and susceptibility of 3.14×10⁻² SI units with a Q_n ratio of 0.56 which are within the range of their values obtained from laboratory measurements on exposed rock samples. The same order of gravity and magnetic anomalies observed over the volcanic plugs of Saurashtra indicates almost similar bulk physical properties for them. The inferred directions of magnetization from magnetic anomalies, however, are D=337° and 340° and I=−38° and −50° which represent the bulk direction of magnetization and also indicate a reversal of the magnetic field during the eruption of these plugs. Some of these plugs are associated with seismic activities of magnitude ≤4 at their contacts. Based on this analysis, other circular/semi-circular gravity highs of NW India can be qualitatively attributed to similar subsurface volcanic plugs.     

Sedimentary cycles of trace elements in Salawusu River Valley since 150 ka BP

The paper makes some analyses on 11 trace elements in the Milanggouwan stratigraphical section in the Salawusu River valley, which is regarded as a prototype geology-palaeoclimate record since 150 ka BP. The results show that the content and variation of trace elements has experienced remarkably regular changes in the pace with coarse and fine sedimentary cycles of palaeo-aeolian sands to its overlying fluvio-lacustrine facies or/and palaeosols. The trace elements with chemical properties of relatively active (V, Sr, Cu, Ni, As) and relatively stable (P, Pb, Rb, Mn, Nb, Zr) are a manifestation of the corresponding 27 changeable cycles between peak and valley values, appearing a multi-fiuctuational process line of relative gathering and migration since then. The low numerical value distribution of these two types of trace elements in the aeolian sand facies represents erosion and accumulation under wind force during the cold-dry climate. Whereas their enrichments in both fluvio-lacustrine facies and palaeosols are related to the valley’s special low-lying physiognomic position between the Ordos Plateau and the Loess Plateau under the warm and humid climate conditions. The above relatively migrated and gathered change of the trace elements is the result of 27 climatic cycles of cold-dry and warm-humid, which is probably caused by repeated alternations of winter monsoon and summer monsoon in the Mu Us Sandy Land influenced by the climate vicissitudes in northern hemisphere during glacial and interglacial periods since 150 ka BP.     

塔里木河下游地下水位与水化学成分关联度排序分析

根据2001年塔里木河下游第四次生态输水前后地下水位、水质变化的监测和取样分析资料。运用灰色关联分析法。对塔里木河下游地表生态输水前、后的地下水位与地下水化学成分进行了关联分析。分析研究表明，生态输水对地下水质的影响比较明显。在关联排序中。阴离子的SO4^2-、Cl^-以及阳离子的Ca^2 、Mg^2 与地下水位变化的关联性较强；pH值与地下水位的关联度最低。仅为0．534；矿化度与地下水位关联度最高,达到0．863。     

斑岩铜矿中铂族元素的研究现状及展望

文章对当前国内外斑岩铜矿中铂族元素的研究现状进行了较为详细的阐述，指出研究与铂族元素矿化有关的斑岩铜矿的流体包裹体及其富集成矿过程，以及铂族元素对斑岩铜矿成矿环境，成岩岩浆的起源和成矿物质来源示踪是该领域的发展方向和前沿。     

青海锡铁山热水沉积型铅锌矿床的地球化学特征

通过对锡铁山铅锌矿床的围岩(矿石)化学成分、稀土及微量元素特征的研究，发现矿区地层普遍具有铈负异常(δCe=0．47～0．90)，为海相沉积岩，近矿围岩中Ag、Hg、As、Sb、Ba等热水沉积指示元素偏高，显示成矿流体性质为热卤水，矿区火山岩不发育，矿床类型可以划为Sedex型。     

Analysis of Karst Tufa from Guangxi, China

The paper presents an analysis of characteristic karst tufa from Guangxi, China, which has not been studied before. A comparison with tufa from Dinaric Karst of Croatia is discussed in view of the C-type climate. The major mineral is calcite. Minor minerals are quartz and dolomite, depending on location. The content of calcium carbonate varies from 65% to 92%, and that of magnesium carbonate from 0.03% to 1.77%. Among other elements, the most abundant are Fe, from 0.02% to 1.50%, and Ti, from 0.15% to 0.27%. Many other trace elements (V, Cr, Mn, Ni, Cu, Zn, As, Se, Br, Rb, Sr, Y, Zr, Hg and Pb) are also present. Specific activity of radionuclides 40K, 232Th, 137Cs, 226Ra and 238U varies from sample to sample. Concentration of U in tufa is close to that reported for sedimentary carbonate. Low concentration of 137Cs indicates that this part of the world was not exposed to nuclear explosions. The concentration of 226Ra is the highest in Mashan County. The ratio 238U/226Ra (0.21-0.71) in tufa from Mashan Co