Solid Earth Sciences最新文献

Evolution of the late Ordovician–Silurian Caledonian orogeny (ca. 460–420 Ma) in the South China Block (SCB) remains controversial due to the overprinting Triassic (Indosinian) and Jurassic-Cretaceous (Yanshanian) orogenic tectonics and the absence of enough petrologic records. This work reported a systematic study of whole-rock geochemistry, zircon U–Pb geochronology and Hf isotopes of the first Caledonian A-type granitoids (Dishui) in Jinxiu county, Guangxi (SW China). The Dishui granitoids are tectonically located in the southwestern Qin-Hang suture belt, which is the largest ancient orogenic belt formed by the collision of the Yangtze and Cathaysia blocks in South China. The Dishui granitoids consist primarily of a syenogranite pluton with a small-scale granodiorite porphyry dike. The Dishui syenogranite and granodiorite porphyry have LA-ICP-MS zircon U–Pb ages of 437.9 ± 1.0 Ma (MSWD = 0.77) and 436.6 ± 0.8 Ma (MSWD = 1.7), respectively. The syenogranite has high contents of SiO₂ (73.20–77.35 wt.%), total alkalis (Na₂O + K₂O = 7.42–8.99 wt.%), total REE (198–445 ppm), high field strength elements (HFSEs: Zr + Nb + Ce + Y = 253–520 ppm), and 10,000∗Ga/Al (2.93–3.11) and FeO_t/(FeO_t + MgO) (0.77–0.96) ratios and high F concentration, resembling highly-fractionated A-type granite. The granodiorite porphyry displays similar A-type granite affinities, including high (Zr + Nb + Ce + Y) (410–485 ppm), 10,000∗Ga/Al (2.47–4.59), and zircon saturation temperature (T_Zr = 908–927 °C), medium to high SiO₂ (64.56–64.95 wt.%), Fe₂O₃ (7.70–8.11 wt.%), and MgO (3.69–4.09 wt.%), and low FeO_t/(FeO_t + MgO) (0.61–0.64), resembling magnesian unfractionated A-type granitoid. The Dishui syenogranite and granodiorite porphyry dike have zircon ε_Hf(t) value and two-stage model age (T_DM2) of −3.9 to +1.5 and 1.32–1.66 Ga, and −1.7 to +11.8 and 0.94–1.53 Ga, respectively, both much more depleted than many Caledonian gneissic/massive granites and the Devonian A-type granites (ε_Hf(t) = −14.9 to −3.5) in South China. All these data suggest that the Dishui syenogranite was derived from partial melting of the Proterozoic metamorphic basement with mantle-derived melt input, and the magma subsequently underwent extensive fractionation. Meanwhile, the Dishui granodiorite porphyry was likely derived from the same crustal melt with more mantle-derived input. New ages of the Dishui A-type granitoids (ca. 436–437 Ma) in the Qin-Hang belt indicate a tectonic transition from syn-collisional crustal thickening to post-collisional extension at ∼437 Ma in South China.

Earth and planetary sciences require extensive microanalyses to quantify most common major and minor elements O, Na, Mg, Al, Si, K, Ca, Ti, Cr, Mn, Fe and Ni in minerals/rocks. With O usually calculated, this is frequently done with expensive electron probe X-ray microanalyzer using a wavelength dispersive method (WDS), and much less so with expensive ground-based scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). Both instruments are not readily accessible to many scientists though. Here we selected eight natural minerals, containing those elements to various amounts, to test the performance of an economical desktop SEM attached with an EDS consisting of just one 10 mm² silicon drift detector (SDD). The compositions of the minerals were established by extensive electron probe X-ray microanalyses (EPMA)-WDS conducted under routine analytical conditions. They were used to evaluate the performance of the desktop SEM/SDD-EDS system. The examination shows that under modest analytical conditions it can generate accurate results for those elements, with detection limits (∼0.1 wt%) much comparable to routine WDS analyses. Therefore, economical desktop SEM/SDD-EDS system can be an affordable and widely-accessible instrument for extensive and accurate quantification of those most common major and minor elements in minerals/rocks.

This work studies the mineralogical and geochemical features of ooidal ironstone formation of the Upper Cretaceous Ayat Formation located in the Turgay depression (Kazakhstan). The origin of ooidal ironstone deposits has been the subject of many discussions for a long time. These results present diagenetic conditions of the marine ironstone precipitation in the Ayat Formation. Various authigenic minerals, such as siderite, glauconite, goethite, pyrite, hydroxylapatite, wurtzite, and barite indicate fluctuations in the different geochemical conditions during the diagenesis of marine sediments of the Ayat Formation. The precipitation of in situ pyrite and wurtzite was controlled by an oxygen-deficient environment at the interface between water and sediment, which was accompanied by bacterial sulphate reduction under seabed conditions and, in consequence, by the concentration of sulfides. The authigenic assemblage of siderite, wurtzite, and barite indicates an integrated process of their input to site the sedimentation. The stable isotopic composition of siderite supports the microbial carbonate origin by decomposing the organic matter. However, geochemical features (the pattern Co/Zn vs (Co + Ni + Cu)) evidence the hydrogenous precipitation of iron. It involves the interaction of two central in situ processes to form Ayat ooidal ironstones. One of them is the iron precipitation from the bottom water, and the second is the microbial generation of hydrocarbons. Marine Ayat and channel Lisakovsk ironstones have similar features of bulk rock geochemistry. It reflects the exact environments of the ooidal iron ore formation, while the wall rocks have distinctive facies conditions.

Parts of A-type granites are closely related to some critical mineral resources. The study of origin and geochemical properties of those A-type granites are of great significance in revealing the metallogenic potential. The Early Cretaceous Huangmeijian (HMJ) complex is located in the Lower Yangtze River Belt (LYRB), eastern China, which is proposed to be associated with uranium deposits. Here, we present whole-rock major and trace elements, and zircon U–Pb geochronology to reveal the formation of the two-stage, uranium mineralization of A-type granites. The high Zr + Nb + Ce + Y concentrations (598−1181 ppm) and Ga/Al ratios (2.7−4.2) suggest an A-type geochemical affinity. On the ternary discriminant diagrams, they can be further divided into A₁-type. In-situ LA-ICP-MS U–Pb dating on the zircon grains from the HMJ complex yielded weighted mean ages of 129.7 ± 1.6 Ma (quartz alkali feldspar syenite), 131.3 ± 1.0 Ma (quartz alkali feldspar syenite), and 118.7 ± 0.7 Ma (alkali feldspar granite), respectively. The former two age results are consistent within error, and similar to other A-type granites in the LYRB. The latter one is newly identified, and is obviously younger than most A-type granites developed in the LYRB. Geochemical data suggest that these A-type granites are formed under high temperature and low oxygen fugacity conditions in an extensional setting. The presence of huttonite favors for the hydrothermal event and the uranium mineralization. It is important to note that, compared to the ca. 130 Ma quartz alkali feldspar syenites, the ca. 118 Ma alkali feldspar granites have obviously higher U (21.3–79.2 ppm) and Th (81.2–104 ppm) concentrations, which are also higher than those of other A-type granites developed in the LYRB. Moreover, the U concentration is even higher than some uranium mineralized granites of South China. Thus, besides the previously reported ca. 130 Ma A-type granite, the ca. 118 Ma A-type granite is of great probability to be a main source of uranium for the formation of uranium deposits in the LYRB, eastern China.

This work presents the results of a study of radionuclide activity in cryoconite from glacier and glacial soil of the Central Caucasus. Cryoconite were sampled from the surface of the Garabashi glacier, and soil samples were taken from the humus horizon of periglacial mountain forest-meadow soil. Measurements were performed with low-background germanium gamma spectrometers located inside a passive shield consisting of ∼20 cm of copper, ∼15 cm of lead, and ∼8 cm of borated polyethylene. The specific activity of radionuclides (Be-7, K-40, Th-232, U-235, U-238, Cs-137) was established. It was revealed that all measured spectra contain γ-lines from decays of K-40, decay chains of U-238, U-235 and Th-232. In addition, the spectra of cryoconite samples from the Garabashi glacier show a 477.6 keV line from the decay of the cosmogenic isotope Be-7, and in the soil sample a 661.7 keV line from the radionuclide Cs-137. No radionuclide Be-7 was detected in the mountain forest-meadow soil. Radionuclide Cs-137 is present in the soil sample, while in cryoconite, is not detected. Radioisotope activities in the Garabashi glacier cryoconite, except for the cosmogenic isotope Be-7, do not differ significantly in terms of mass, i.e., the content of K-40, U-238, U-235 and Th-232 in them is approximately the same. The activity of all the studied radionuclides in the soil sample compared to cryoconite samples is lower, although the differences are not significant, except for Th-232, whose activity in soil is almost two times lower.

Buckling instability has been identified as a possible mechanism of pile failure in liquefiable ground and this failure mechanism is not explicitly mentioned in most of the design codes. Pile buckling would be affected, however, by various factors including liquefaction zone of foundations soils, axial loads of piles, geometry as well as arrangement of pile foundation, etc. An efficient approach using deterministic by Bhattacharya is proposed to compute the buckling instability in pile. This method is verified and validated using 3D finite-element simulation through OpenSeesPL. A more comprehensive study of numerical simulation would include the effects of various factors on the responses of piles and foundations soils due to seismic loading. The findings reported that an increase in axial loading would generally increase the excess pore pressure in soils and would generally increase the deflection and bending moment in piles and acceleration responses in soils. An increase in pile spacing would generally increase the deflection and bending moment in piles, as a result of more soil volume among the piles. An increase in diameter of pile would increase in rigidity and maximum bending capacity of piles and thus would resist more energy released in liquefiable ground that amplifies the deflection (curvature) of pile. A comparison of two approaches confirms the pile would be safe from buckling failure against soil liquefaction during seismic loading. Finally, this study would provide for predicting pile buckling instability and the behaviors of piles and foundation soils due to seismic shaking and liquefied ground.

The Xing–Meng Orogenic Belt hosts an amount of low-sulfidation epithermal gold deposits. However, the tectonic-magmatic setting remains problematic, which hinders understanding the factors that control the gold endowment in the region. Wulaga deposit is the largest low-sulfidation epithermal gold deposit in the northeastern Xing–Meng Orogenic Belt. Gold mineralization occurs in the crypto-explosive breccia zone of subvolcanic granodiorite porphyry. Zircon U–Pb ages of three granodiorite apophyses and previous pyrite Rb–Sr dating (113.8 ± 4.4 Ma) indicate granodiorite porphyry and gold mineralization was coeval. The ore-related granodiorite porphyry is moderate SiO₂, high-K calc-alkaline, and metaluminous, suggesting an I-type granite. Moreover, Wulaga granodiorite porphyry displays low initial ⁸⁷Sr/⁸⁶Sr ratios and positive ε_Nd(t) values with T_DM2(Nd) of 799.7–897.4 Ma and ε_Hf(t) values with T_DM2(Hf) of 652–785 Ma, indicating that it was derived from partial melting of the Neoproterozoic juvenile mafic lower crust. Ti-in-zircon thermometry, medium–high Sr/Y ratio, high Ba/La, and f_O2 value indicate that Wulaga granodiorite porphyry formed at relatively low temperatures (∼700 °C), rich water, and high fugacity within the stability field of garnet in the juvenile lower crust. Combined with ore-related tectonic-magmatic activities in the Wulaga, Dong'an, and Sandaowanzi gold deposits, the Early Cretaceous low-sulfidation epithermal gold deposits in the Xing–Meng Orogenic Belt were formed from magmatic-hydrothermal events triggered by mutual interaction between post-orogenic lithospheric extension related to the closure of the Mongol-Okhotsk Ocean and arc-back extension associated with rollback of the Paleo-Pacific Ocean plate. These data indicate the potential existence of the Early Cretaceous epithermal gold deposits that are related to the contemporaneous igneous activity.