Solid Earth Sciences最新文献

The Physico-chemical and mineralogical properties of the Cretaceous shales of Calabar Flank southeastern Nigeria were examined for their Provenance, Paleo-weathering Conditions and Industrial Applications. Twenty samples of shale from Ekekpon and Nkporo Formations were analysed using X-Ray Fluorescence, X-ray diffraction (XRD), scanning electron microscopy (SEM) and geotechnical techniques. Results from the major oxides of elements revealed that the Nkporo Shale is enriched with SiO₂ > Al₂O₃ > LOI > Fe₂O₃ > SO₃ while the Ekepkon shale is also enriched in SiO₂ > Al₂O₃ > LOI > Fe₂O₃ > CaO > K₂O. The XRD revealed kaolinite as the major clay mineral, while other non-clay minerals are quartz, vallerite, mica, nacaphite, sodalite. The SEM shows that the Ekepkon Shale is enriched in calcium. The correlation matrix revealed SiO₂ resides more in the quartz phase of the Nkporo Shales, while in the Ekekpon Shale the SiO₂ is associated with the clay. The element ratios and discrimination diagrams revealed that the provenance of the shales is of intermediate to mafic igneous rocks and a tectonic setting of continental to oceanic arcs. The presence of kaolinite in the Nkporo Shale with the chemical index of alteration (CIA) values ranging from 83.58 to 96.93, indicated that the shale is derived from intensive chemical weathering in the source area. The low CIA values (37.90–76.96) in Ekenkpon Shale is attributed to CaO and calcium enrichment in the shale and the shale contains high quartz and kaolinite therefore the shale is suggested to have been derived from an intermediate to intense weathering activities in the source area. The geotechnical properties revealed that both shales are suitable for ceramic and refractory industries. The Ekenkpon Shale will be suitable for use as filling material and as raw material in cement industries. However, the shales do not meet the specifications for paper and rubber production due to the high contents of iron oxide_, lime, magnesia, potash and soda.

The Sanmenxia Basin, located on the southeastern margin of the Shanxi rift and filled with Cretaceous-Paleogene fluvial and lacustrine sediments, is a faulted basin bounded by a series of normal strike-slip faults. This provides a valuable opportunity to investigate the early tectonic deformation in the Fenwei graben. In this study, we report an integrated rock magnetism and AMS analysis of two sections from the Sanmenxia Basin spanning an interval from the Late Cretaceous to the Eocene. The rock magnetic results demonstrate that magnetite and hematite are the main magnetic carriers of remanence, and paramagnetic minerals and hematite are major contributors to the AMS in both sections of the Sanmenxia Basin. Together with the relatively low-corrected anisotropy values, the tightly grouped minimum principal axes are almost perpendicular to the bedding plane, and the well-defined magnetic lineation is generally parallel to the bedding trend, indicating that the primary sedimentary fabrics in the Sanmenxia Basin were overprinted by the initial deformation. The NW–SE magnetic lineation denotes the NW–SE stretching during the Late Cretaceous-Eocene. The stretching process may have been controlled by the Indian-Eurasian convergence and/or the subduction of the western Pacific plate.

Tectonism often plays an important role in the mineralization process, which is generally thought to be the main controlling factor in the accumulation of economic materials (e.g., gold, coal, oil and gas) through deformation. However, numerous experimental and theoretical studies have suggested that tectonic stress not only causes deformation (physical changes) in rocks and minerals but also promotes their chemical changes by acting directly on chemical bonds and causing bond scission or regeneration, called tectonic stress chemistry (TSC). In recent years, TSC actions caused by tectonic activities have provided new ideas and evidence for explaining the chemical structural evolution of coal, hydrocarbon formation, organic (coal-derived) and inorganic graphitization and hydrothermal mineralization under shear stress. These background studies have provided incentives and insights into how tectonic stress affects the chemical structures of minerals, rocks and even ore-forming fluids in the process of mineralization. In this paper, we briefly review: (1) the concept of TSC; (2) the TSC process in the formation of shear zone type gold deposits from stress concentration, brittle fracturing, sudden reduction of fluid pressure, and flash vaporization to gold precipitation; (3) mechanisms of the macromolecular structural evolution of coal and gas generation under shear stress from deformation experiments and molecular dynamic simulations; (4) coal-derived graphitization caused by preferred orientation and extension of the basic structural units (BSUs) under shear stress; and (5) some preliminary experimental explorations on inorganic graphitization in carbonate-hosted shear zones. In addition, some existing problems and possible solutions for these processes are also discussed. Finally, we propose additional potential TSC processes in extensive geological processes, e.g., the relationship between deformation and metamorphism and trigger mechanisms of slow-slip earthquakes. To further explore these processes, a combination of experiments and molecular dynamic simulations should be undertaken by researchers.

In present study three sections of early Eocene Chorgali Formation were measured from Pail, Wanhar and Sar Kalan area, Central Salt Range, Punjab, Pakistan. At these localities the Chorgali Formation is about 12m, 8.8m and 15.1m thick, respectively. A total number of 53 samples were collected, 18 from Pail section, 18 from Wanhar section and 17 from Sar Kalan section. Eocene Chorgali Formation in this area is consisted of grey to pale grey limestone, greenish grey shale intercalations and argillaceous limestone. At Pail section nine various microfacies were recorded i.e., Mudstone microfacies (CGP1), Bioclastic mudstone microfacies (CGP2), Nummulites-Lockhartia wackestone microfacies (CGP3), Nummulitidae wackestone microfacies (CGP4), Bioclastic wackestone microfacies (CGP5), Alveolina-Nummulites wackestone microfacies (CGP6), Alveolina wackestone microfacies (CGP7), Nummulites-Alveolina wackestone microfacies (CGP8) and Intraclastic-peloidal packstone microfacies (CGP9).At Wanhar section five facies were recorded i.e., Rotaliidae wackestone microfacies (CGW1), Nummulitidae Wackestone microfacies (CGW2), Nummulites- Lockhartia wackestone microfacies (CGW3), Nummulites-Assilina wackestone microfacies (CGW4), Intraclastic-peloidal packstone microfacies (CGW5).At Sar Kalan section total four facies were recorded i.e., Bioclastic wackestone microfacies (CGSK1), Nummulites-Assilina wackestone microfacies (CGSK2), Nummulitidae wackestone microfacies (CGSK3), Intraclastic-peloidal packstone microfacies (CGSK4).The assemblage of larger foraminifera were recorded to describe the biota of the formation and to interpret the paleo-environments with implications of sequence stratigraphy. Field observations and microfacies analysis suggest that the deposition of Chorgali Formation at these localities probably took place in inner shelf conditions. Presence of shallow water benthic larger foraminifer's further support lagoon to bay environment of the genesis of the formation. The formation might have been deposited because of falling stage system tract (FSST), showing a progradational pattern of deposition. The basin ward shift of deposition indicates the regressive sequence.

Raman spectroscopy, an ideal technique to quantify the Mg–Al cation distribution on the tetrahedral T-sites and octahedral M-sites of MgAl₂O₄-spinel, was not calibrated before. By performing 16 annealing experiments on some MgAl₂O₄-spinel crystal plates at P from 1 atm to 5.0 GPa and T from 823 to 1873 K, a series of samples with different magnitudes of cation disorder (characterized by the inversion parameter x, i.e., the molar fraction of Al cations on the T-sites) have been generated. Single-crystal X-ray diffraction analyses have been performed to all these samples, and found the x values varying from 0.146 (15) to 0.362 (17). Multiple Raman spectra have been collected and analyzed for every sample. The Raman results show that the Raman scattering capabilities of the Al cations and the Mg cations on the T-sites are different, and their ratios are dependent on the Mg–Al cation distributions (i.e., x). With our extensive single-crystal X-ray diffraction data and Raman data, correlations between the x values and the relative Raman intensities of the ∼766 and ∼722 cm⁻¹ peaks (I_∼766/I_∼722 ratio), respectively caused by the internal vibrational modes of the MgO₄ and AlO₄ groups, have been established: the equations are x = 0.121 × log₁₀²(I_∼766/I_∼722) − 0.344 × log₁₀(I_∼766/I_∼722) + 0.392 (R² = 0.927; Raman peak height data used) and x = 0.069 × log₁₀²(I_∼766/I_∼722) − 0.253 × log₁₀(I_∼766/I_∼722) + 0.379 (R² = 0.928; Raman peak area data used). With this calibration, Raman spectroscopy can now be conveniently used to determine the x values of the MgAl₂O₄-spinel of different geological origins, significantly facilitating the inference of the thermal history of relevant geological bodies.

The evolution of the U-Pb decay system is determined by their initial isotopic composition in the proto-Earth and the subsequent global differentiation. The differentiation is highly complicated because of large-scale evaporation and multi-stage core formation in Earth accretion. We statistically rebuild the accretional history of Earth using a series of N-body simulations. This provides us with an estimation of the amount of silicate melting and thus temperature and pressure at the bottom of the magma oceans driven by continuous planetesimal impacts. We further assumed different evolutionary paths of the redox state and found a reduced process from an oxidized state consistent with the current value of Pb content and μ value (²³⁸U/²⁰⁴Pb) in the bulk silicate Earth. Meanwhile, the fraction of the impactor's core that participates in the re-equilibration is around 0.2–0.7. Our model predicts the final μ value equals the observed value, 8.25, regardless of the minor contribution of the late veneer (0.2). The evolution of μ determines the growth rate of radiogenic Pb isotopes. The episodic increase of μ in multi-stage core formation accelerates the growth of radiogenic Pb isotopes (²⁰⁶Pb and ²⁰⁷Pb) and finally causes a slight deviation of the composition of Pb isotopes (²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb) to the right of 4.567-Ga Earth Geochron. A multi-stage evolution model for U–Pb system can explain the modern terrestrial μ value, but has little influence on the puzzle of “the first Pb paradox”.

Combined use of trace elements, rare earth elements (REEs) with hydrogen (δH) and oxygen (δ¹⁸O) stable isotope composition to elucidate origin and paleo-environment-conditions and reconstruction is a contemporary trend in the field of geochemistry. Geochemical investigations were carried out on the shale deposit from five wells within the Asu River Group Formation, exposed at Ikwo, Lower Benue Trough (LBT). Results of trace elements for the shale deposits examined showed averages of Co (19.10 ppm), Th (16.50 ppm), Zn (103.73 ppm), Sr (203.71 ppm) and Zr (292.80 ppm) compared to PAAS and UCC. Observed negative Eu anomalies, enriched LREEs and depleted HREEs patterns have shown that the shales are from rocks of continental origin. This is provided by Al/(Al + Fe + Mn) with a value > 0.2. Plots of Zr/Sc vs Th/Sc, La/Th vs. Hf and Cr/V vs. Y/Ni indicated felsic igneous rock precursors for the shale samples. C-parameter, ratios of Rb/Sr and Sr/Cu signify paleo-climatic conditions of semi-humid to arid, Ba/Al showed low paleo-productivity of the basin during shale deposition, largely within the freshwater setting according to the ratio of Sr/Ba. V/(V + Ni), U/Th, Ni/Co ratios and Ce/Ce∗ anomalies have revealed an oxic depositional environment with Fe/Ti > 20 suggesting hydrothermal activity. The temperature of formation (120 °C–∼195 °C) coupled with δ¹⁸O (+17 to + 23‰) and δH (−46.90 to −38.80‰) is consistent with materials of sedimentary origin from chemically weathered felsic precursors under humid climatic conditions with an influence of hydrothermal activity.

The Wujiaping Sn deposit is located at the northern Dayishan ore field, South China, whose ore veins are mainly hosted in the Dayishan pluton. LA–ICP–MS zircon U–Pb dating for the medium-fine grained- and medium-coarse grained-biotite monzogranite of Dayishan pluton yields emplacement ages of 154.5 ± 1.6 Ma (MSWD = 2.0) and 155.5 ± 0.5 Ma (MSWD = 1.9), respectively, which are consistent with the muscovite ⁴⁰Ar-³⁹Ar plateau age of 150.4 ± 0.9 Ma for the quartz vein type ore veins. It is indicated that the Sn mineralization in the Wujiaping deposit is related to the Late Jurassic granitic magmatism. These granites show the geochemical features of highly fractionated S-type granite: 1) low Zr + Nb + Ce + Y contents (<209 ppm); 2) high A/CNK ratios (>1.1); 3) low crystallization temperature (mean = 688 °C); 4) high Rb/Sr ratios (mostly > 48); 5) high differentiation index (DI > 90); and 6) low CaO, P₂O₅, Sr, and Eu contents. Whole rocks isotopes show that these granites shows variable initial ⁸⁷Sr/⁸⁶Sr ratios (0.70251–0.71208), negative ε_Nd(t) values (−7.94–5.62) and old two stage Nd model ages of 1491–1563 Ma. LA–MC–ICP–MS zircon Lu–Hf isotopes show that the medium-fine-grained monzogranite have positive ε_Hf(t) values of 0.37–8.5 and two stage Hf model ages of 662–1180 Ma, whereas the medium-coarse grained-biotite monzogranite have negative ε_Hf(t) values of −6.42–3.55 and two stage Hf model ages of 1431–1599 Ma. It is proposed of that these granites are originated from melts mixed by crustal- and mantle-constituents and are formed in an extensional setting caused by the subduction of the Palaeo-pacific plate. The low LogfO₂ values calculated through zircons (−21.2–13.1) and high F contents (3630–5120 ppm) indicate the granites derived from reduced and F-rich melts. Therefore, the reduced melt is highly fractionated and enriched in Sn and F, resulting in the large scale Sn mineralization in the Dayishan ore field.

How the Dabie Orogenic Belt (DOB) and the Tan-Lu Fault Zone (TLFZ) transformed during the early Indosinian period is the key to reveal the convergence process between the North China Block (NCB) and the South China Block (SCB). Tongcheng area in the eastern margin of northern DOB is the tectonic node that connects the WNW-trending DOB and the NE-trending TLFZ. The typical ENE-NE-trending gneiss belts penetrably developed in Tongcheng area provide an ideal natural lab to decipher the transformation between the DOB and the TLFZ. Here we conduct an integrated structural and geochronological research on the ENE-NE-trending gneiss. Zircon U–Pb dating on the ENE-NE-trending gneiss yielded metamorphic ages ranging from 255 ± 9 Ma to 203 ± 10 Ma. The weakly deformed veins which intruded the surrounding gneiss yielded two groups of ages, 825 ± 29 Ma to 713 ± 7 Ma and 125 ± 2.6 Ma (weighted mean age), which indicate the protolith age of surrounding rocks and the intrusive timing of the vein, respectively. Integrated structural, microstructural and kinematic analysis indicate that no lateral structural superposed on the gneiss or veins. Therefore, it can be speculated that the deformation and metamorphism of the gneiss should simultaneously formed during the Early Triassic, as a result of continuous tearing from the DOB to the northeastern Sulu Orogenic Belt (SOB) during subduction of the Yangtze block beneath the NCB. Formation of the tearing belts, i.e., the ENE-NE-trending gneiss belts, accomplished the tectonic transformation between the DOB and the TLFZ. They could be regarded as an embryonic form of the TLFZ, which are also apparently different from the TLFZ by characteristics of non-strike slipping.