2019年 30卷 第2期
A comprehensive geophysical profile stretching from Qingyijing at the southern edge of the Junggar Basin to Ubara on the northern margin of the Junggar Basin was conducted in an attempt to probe the crustal structure of the western Junggar Basin (hereafter referred to simply as nthe Basino), and, in particular, the structure and property of the Basin's crystalline basement. A survey using seismically converted waves was conducted along this profile to determine the characteristics of the P- and Swave velocities typical of the crust and uppermost mantle. A joint inversion of gravitation and aeromagnetic data was also performed to acquire the density and magnetization intensity values found beneath the western Basin. This research revealed that the Basin is composed of the so-called Manasi terrain in the south, and the Wulungu terrain in the north. Their boundary is located along the WNW-trending Dishuiquan-Sangequan suture, linking the NE-striking Da'erbute suture (DS) in the west, and the WNWtrending Kalameili suture (KS) in the east. In its northern part, the Wulungu-type terrain has a doublelayered basement, of which the upper layer is a folded basement of Hercynian orogenic origin, and the lower layer is a crystalline basement of Middle-Upper Proterozoic age. The southern part of the Basin, i.e., the Manasi terrain, has a single-layered crystalline basement. The folded basement here is too thin to be clearly distinguished.
Ocean temperature and dissolved oxygen concentrations are critical factors that control ocean productivity, carbon and nutrient cycles, and marine habitat. However, the evolution of these two factors in the geologic past are still unclear. Here, we use a new oxygen isotope database to establish the sea surface temperature (SST) curve in the past 500 million years. The database is composed of 22 796 oxygen isotope values of phosphatic and calcareous fossils. The result shows two prolonged cooling events happened in the Late Paleozoic and Late Cenozoic, coinciding with two major ice ages indicated by continental glaciation data, and seven global warming events that happened in the Late Cambrian, Silurian- Devonian transition, Late Devonian, Early Triassic, Toarcian, Late Cretaceous, and Paleocene-Eocene transition. The SSTs during these warming periods are about 5-30¦ higher than the present-day level. Oxygen contents of shallow seawater are calculated from temperature, salinity, and atmospheric oxygen. The results show that major dissolved oxygen valleys of surface seawater coincide with global warming events and ocean anoxic events. We propose that the combined effect of temperature and dissolved oxygen account for the long-term evolution of global oceanic redox state during the Phanerozoic.
Two Permian-Triassic boundary (PTB) sections (Pojue and Dala) are well exposed in an isolated carbonate platform (Napo Platform) on the southwestern margin of the Nanpanjiang Basin, South China. These sections provide an insight into the transition across the PTB and a detailed investigation of the conodont biostratigraphy and inorganic carbon isotopes is presented. The PTB at the Pojue Section is placed at the base of Bed 10B (a dolomitized mudstone found below a microbialite horizon), defined by the first occurrence of Hindeodus parvus. At the Dala Section, four conodont zones occur. They are, in ascending order, the Hindeodus parvus Zone, Isarcicella staeschei Zone, Isarcicella isarcica Zone and Clarkina planata Zone. Comparison with the Pojue Section suggests the PTB at Dala also occurs at the base of dolomitized mudstone below a microbialite horizon, although the first occurrence of Hindeodus parvus is near the top of a microbialite bed:an occurrence that is also seen in other platform sections. The succeeding microbialite beds developed during the ongoing PTB mass extinction phase. This time was characterized by low carbon isotope values, and a microbialite ecosystem that provided a refuge for selected groups (bivalves, ostracods and microgastropods) that were likely tolerant of extremely high temperatures.
The Fanjingshan mafic-ultramafic rocks in the west Jiangnan Orogen of South China are considered to be a potential target for mineral exploration. However, the petrogenesis and magma evolution of these rocks are not yet clearly constrained, let along their economic significance. The compositions of platinum group elements (PGE) in the Fanjingshan mafic-ultramafic rocks can provide particular insight into the generation and evolution of the mantle-derived magma and thus the potential of Cu-Ni-PGE sulphide mineralization. The Fanjingshan mafic-ultramafic rocks have relatively high Pd-subgroup PGE (PPGE) relative to Ir-subgroup PGE (IPGE) in the primitive mantle-normalized diagrams. Meanwhile, the Fanjingshan mafic-ultramafic rocks have low Pd/Ir (11-28) ratios, implying relatively low degree of partial melting in the mantle. Low Cu/Pd ratios (545-5 216) and high Cu/Zr ratios (0.4-5.8 with the majority greater than 1) of Fanjingshan ultramafic rocks indicate that the S-undersaturated parental magma with relatively high PGE was formed. Although the Fanjingshan mafic rocks have remarkably higher Cu/Pd ratios (8 913-107 016) likely resulting from sulphide segregation, the degree of sulphide removal is insignificant. Fractionation of olivine rather than chromite and platinum group minerals or alloys governed the fractionation of PGE and produced depletion of IPGE (Os, Ir and Ru) relative to PPGE (Rh, Pt and Pd), as supported by the positive correlation between Pd/Ir and V, Y and REE. Collectively, original S-undersaturated magma and insignificant crustal contamination during magma ascent and emplacement result in the separation of immiscible sulphide impossible and thus impede the formation of economic CuNi-PGE sulphide mineralization within the Fanjingshan mafic-ultramafic rocks.
The Bainaimiao Group, which crops out in the Sonid Right Banner area of Inner Mongolia, China, comprises mainly metamorphosed volcano-sedimentary rocks. This group can be divided into two formations:a lower formation characterized by intermediate-felsic volcanic rocks, and an upper formation of intermediate-mafic volcanic rocks. Zircon dating indicates that biotite leptynite from the lower formation and chlorite-sericite schist from the upper formation crystallized at 499±2 and 478±2 Ma, respectively, corresponding to different volcanic events. Meta-volcanic rocks of the Bainaimiao Group belong to the calcalkaline series, and the SiO2 concentrations suggest that their protoliths were mainly basalts and rhyolites. Greenschist rocks of the group are enriched in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), with (La/Yb)N ratios of 3.08-10.9. In addition, they are enriched in large-ion lithophile elements, including Rb, Ba and K, and depleted in the high-field-strength elements Nb, Ta and Ti. The felsic meta-volcanic rocks also exhibit relative enrichments in LREEs compared with HREEs, with high (La/Yb)N ratios of 2.92-9.89, and are enriched in Rb, Ba, Zr and Hf, and depleted in Sr, Nb and Ta. These geochemical characteristics indicate that the meta-basic volcanic rocks were originated from partial melting of sub-arc mantle wedge that had been previously metasomatized by subducted-slab-derived fluids, whereas the meta-felsic volcanic rocks were generated by partial melting of continental crust. These results suggest that the meta-volcanic rocks of the Bainaimiao Group are the products of oceanic plate subduction magmatism along an active continental margin, which can be attributed to Early Paleozoic subduction of the PaleoAsian Ocean plate beneath the northern margin of the North China Craton.
In this study, we report zircon U-Pb age and geochemical data on the Middle Permian siliceous clastics and basalt samples of Lugu Formation collected in the Yaqu region from central Qiangtang. Combined with the published data, we establish the spatial and temporal evolution of the rift in central Qiangtang from the Early to Late Permian. Zircon U-Pb dating by LA-ICP-MS yields a concordant age with a weighted mean 206Pb/238U age of 266.6±2.8 Ma (n = 6, MSWD = 0.55) for the basalt. The results of detrital zircons from the siliceous clastics exhibit a prominent population of 257-270 Ma with a maximum depositional age of 265.4±2.6 Ma (n = 19, MSWD = 2.7) and three minor populations with peak ages of 450,700-800, 1 800 Ma, as well as one older age of 2 039 Ma, generally coinciding with the geological events that occurred at different epochs in Qiangtang. The basalts display enrichments in highfield-strength elements (HFSE) such as Th, Ta and Hf, but show relative depletion in large-ion-lithophile elements (LILE) such as Ba, K and Sr, as well as slight depletion in Nb and exhibiting no Eu anomalies. All the samples are distributed in the within-plate setting on the Zr vs. Zr/Y and Th/Hf vs. Ta/Hf discrimination diagrams. The integration of these new data together with the regional geological background indicates that the Lugu Formation was formed in a continental rift-related setting of the central Qiangtang terrane during the Middle Permian. We propose a temporal and spatial framework that the continental rift opened as a result of the break-up of Gondwana during the Early Permian, ran to its peak in the Middle Permian and closed in the Late Permian (290-257 Ma), which could be a key constraint on the Permian evolution of Qiangtang.
The Gebunongba iron polymetallic deposit is a typical skarn deposit located at the westernmost end of the discovered iron polymetallic deposits in the Gangdese metallogenic belt. Major and trace elements of the monzonite granite related to mineralization show that these rocks are peraluminous (ACNK=1.15-1.21) and are rich in Si (76.63 wt.%-76.93 wt.% SiO2), K (K2O/Na2O ratio of 1.80-2.30), LREE and LILEs (Rb, Ba, Th, U, Pb), but is depleted in high field strength elements (Nb, Ta, Ti and HREE). In addition, these rocks show obviously negative Eu anomalies (δEu=0.48-0.55). The LA-ICPMS U-Pb age of zircons in monzonite granite is 59.72±0.55 Ma (MSWD=0.79), whereas the 40Ar/39Ar age of muscovite in iron ores is 59.22±0.61 Ma (MSWD=16.20). This indicates that the deposit formed at the syn-collision stage of Lhasa-India terrane is later than the northward subduction of the Yajiang crust. The monzonite granite has been probably derived from the partial melting of ancient lower crustal materials, which is probably resulted from the underplating of mantle-derived magmas. It is favorable for the formation of iron polymetallic deposit. Iron polymetallic mineralization is prevalent in Gangdese metallogenic belt at syn-collision stage. Therefore, syn-collision stage is an important mineralization stage for iron polymetallic deposits. The results of this study proved that iron polymetallic mineralization still took place in the western segment of Gangdese metallogenic belt and provided basis for further prospecting the deposits of the same type.
The gold mineralization in the Longtoushan hydrothermal gold deposit is concentrated within the contact zone of the granitic complex. Whole rock geochemistry and in-situ U-Pb and Hf isotopic data were used to constrain the genesis and age of the granites and related Cu-Au mineralization in the Longtoushan Deposit. The granites mainly consist of the granite porphyry, rhyolite porphyry, porphyritic granite and quartz porphyry. LA-ICP-MS U-Pb dating of zircons from the granite porphyry, rhyolite porphyry and quartz porphyry indicates that they intruded from ca. 94 to 97 Ma. These intrusions exhibit similar trace element characteristics, i.e., right-dipping REE patterns, depletion of Ba, Sr, P and Ti, and enrichment of Th, U, Nd, Zr and Hf. The εHf(t) values of zircons from the granite porphyry, rhyolite porphyry and quartz porphyry range from -26.81 to -8.19, -8.12 to -5.33, and -8.99 to -5.83, respectively, suggesting that they were mainly derived from the partial melting of the Proterozoic crust. The Cu-Au mineralization is mainly related to the rhyolite porphyry and porphyritic granite, respectively. The Longtoushan granites were most likely formed in a post-collisional extensional environment, and the deposit is a part of the Late Yanshanian magmatism related mineralization in the Dayaoshan area and its adjacent areas.
Late Paleozoic mafic-intermediate intrusions are widespread in the central Tianshan belt, but their tectonic settings remain controversial. Petrogenesis of these intrusions can provide insights into the tectonic evolution of the belt and its adjacent areas. This study presents new whole-rock geochemical and zircon U-Pb geochronology and Hf isotopic data for the Yaxi diorite and Qianzishan gabbro on the northern margin of the central Tianshan (NW China). Zircon U-Pb dating yielded the magma crystallization ages of 313±4 and 295±4 Ma for the Yaxi diorite and Qianzishan gabbro, respectively. They have lower Mg# values, Ni and Cr contents than typical mantle-derived primary melt, with negative correlations between MgO, TFeO and SiO2 contents, indicating clinopyroxene and olivine fractionation during magma evolution. They are characterized by enrichment of large ion lithophile elements (e.g., Rb, Ba and Sr) and depletion of high field strength elements (e.g., Nb, Ta and Ti) with high Ba/Th and Rb/Y, suggesting that their mantle sources had been metasomatized by slab-derived fluids. In addition, the Qianzishan gabbro has high Al2O3 contents (19.54 wt.%-20.88 wt.%) and positive Eu anomalies (Eu/Eu*=1.09-1.42), which can be attributed to accumulation of plagioclase. Geochemical and zircon Hf isotopic compositions reveal that both the Yaxi diorite and Qianzishan gabbro were derived from depleted lithospheric mantle in the spinel stability field with insignificant crustal contamination. In association with previous investigations, we suggest that the Yaxi and Qianzishan intrusions were emplaced in a subduction-related environment, which means that the subduction of the Junggar Ocean lasted at least to the earliest Permian.
This paper presents zircon U-Pb-Hf isotopic compositions and whole-rock geochemical data for monzogranites and mafic-ultramafic complexes of the Maxingdawannan area in the western end of the east Kunlun orogenic belt, western China. The data are used to determine the ages, petrogenesis, magma sources, and geodynamic setting of the studied rocks. U-Pb zircon dating indicates that monzogranites and gabbros of the complexes were emplaced at 399 and 397 Ma, respectively. The monzogranites are shoshonitic, with high SiO2, Al2O3 and total-alkali contents, and low TFeO, MgO, TiO2 and P2O5 contents. The mafic-ultramafic complexes are characterized by low SiO2 contents. The monzogranites display enrichment in light rare-earth elements (LREE) and large-ion lithophile elements (LILE), depletion in heavy REEs (HREE) and high-field-strength elements (HFSE), and negative Eu anomalies (Eu/Eu*=0.36-0.48). The mafic-ultramafic complexes are also enriched in LREEs and LILEs, and depleted in HREEs and HFSEs, with weak Eu anomalies (Eu/Eu*=0.84-1.16). Zircon εHf(t) values for the monzogranites and mafic-ultramafic complexes range from -6.68 to 1.11 and -1.81 to 6.29, with zircon model ages of 1 812-1 319 Ma (TDM2) and 1 087-769 Ma (TDM1), respectively. Hf isotopic data indicate that primary magmas of the monzogranites are originated from partial melting of ancient lower crust during the Paleo-Mesoproterozoic, with a juvenile-crust component. Primitive magmas of the mafic-ultramafic complexes are likely originated from a depleted-mantle source modified by slab-derived fluids and contaminated by crustal components. Geochemical data and the geological setting indicate that Devonian intrusions in the Maxingdawannan area are related to northward subduction of the ProtoTethys oceanic lithosphere.
The Weiyuan Structure is the largest surface structure in the Sichuan Basin. However, the abundance of the Dengying Formation gas reservoir in the Weiyuan Structure is low. The height of the gas column is 244 m, but the integrated abundance is only 26.4%. After nearly 40 years of exploration, the Gaoshi1 Well and Moxi8 Well yielded gas flows that marked an important exploration success after the discovery of the Sinian Dengying Formation gas reservoir in the Weiyuan Structure, Sichuan Basin, Lower-Paleozoic in 1964. Combined with research examples of oil and gas migration and gas chimneys around the world, the authors used comprehensive geological-geophysical-geochemical research methods to provide a reasonable explanation of the low abundance of the gas reservoir in the Weiyuan Structure based on the surface and subsurface data. The latest research results show that (1) currently, the Weiyuan Structure is the apex of the Dengying Formation in the Mid-Sichuan Basin. The Guang'an, Longnüsi, Gaoshiti-Moxi, and Weiyuan structures are a series of traps in the Dengying Formation with gradual uplifting spill and closure points during the regional uplift of the Himalayan period. The natural gas of the Dengying Formation accumulated in different ways over a wide range and long distance in the Sichuan Basin. (2) At approximately 40 Ma, the Weiyuan area started to uplift and form the present structure, and it is the only outcropped area with the Triassic Jialingjiang Formation and Leikoupo Formation in the surface of the Sichuan Basin (except the steep structural belt in East Sichuan). Caused by the uplift and denudation, the core of the Weiyuan Structure has formed an escaping "skylight" for natural gas. The evidence of a gas chimney includes (1) the component percentage of non-hydrocarbon gas, which decreased from the bottom to the top, (2) the pressure coefficient is normal because the gas reservoir from the Upper Sinian to the Lower Permian commonly have a normal pressure coefficient (an average of 1.0), and (3) the isotope geochemistry of the argon mostly represents abiogenic characteristics of a deep source, and the 40Ar/36Ar ratio is as high as 2 855-5 222 in the Upper Permian. All of these characteristics provide sufficient evidence for a gas chimney effect. The characteristics of low abundance in the Weiyuan Structure can be a reference example for studying the late reconstruction of deep oil and gas reservoirs in the superimposed basins of western China.
The petrophysical property of mudstone often transforms from ductile to brittle in the process of burial-uplift. The deformation mechanism of fault in brittle and ductile mudstone caprock is different, which leads to the formation of different types of fault zone structure. Different methods are required to evaluate the sealing mechanism of those fault zones. Based on the caprock deformation mechanism, fault sealing mechanism, quantitative evaluation method of vertical fault sealing capacity is put forward in this study. Clay smear is formed in the process of plastic deformation and its continuity controls the sealing capacity of fault. The outcrop and oil field data have confirmed that when sealing parameter SSF is less than 4-7, the clay smear becomes discontinuous and then oil and gas go through the caprock and migrate vertically. Quantities of fractures are formed in mudstone in the process of brittle deformation. The fracture density increases with the increase of the fault displacement. When the fractures are connected, oil and gas go through the caprock and migrate vertically. The connectivity of fault depends on the displacement and the thickness of caprock. On the basis of the above, a method is put forward to quantify the connectivity of fault with the juxtaposition thickness of caprock after faulting. The research on the juxtaposition thickness of caprock after faulting of the member Ⅱ of Dongying Formation in Nanpu depression and the distribution of oil and gas indicates when the juxtaposition thickness of caprock is less than 96.2 m, the fault becomes leaking vertically. In the lifting stage, with the releasing and unloading of the stress, the caprock becomes brittle generally and then forms through going fault which will lead to a large quantity of oil and gas migrate vertically.
In this study, biomarkers, together with stable carbon (δ13C) and hydrogen (δD) isotopic compositions of n-alkanes have been examined in a suite of condensates collected from the East China Sea Shelf Basin (ECSSB) in order to delineate their source organic matter input, depositional conditions and evaluate their thermal maturity. Previously, GC-MS analyses have shown that all the condensates are formed in oxidizing environment with terrestrial plants as their main source input. No significant differences were apparent for biomarker parameters, likely due to the low biomarker content and high maturity of these condensates. Conventional GC-MS analysis however, may provides limited information on the sources and thermal maturity of complex mixtures due to insufficient component resolution. In the current study, we used comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC×GC-TOFMS) to increase the chromatographic resolution. Compounds such as alkyl cyclohexanes, alkyl cyclopentanes and diamondoids, which can be difficult to identify using conventional GC-MS analysis, were successfully identified using GC×GC-TOFMS. From our analyses we propose two possibly unreported indicators, including one maturity indicator (C5--cyclohexane/C5+-cyclohexane) and one oxidation-reduction environment indicator (alkyl-cyclohexane/alkyl-cyclopentane). Multiple petroleum charging events were proposed as an explanation for the maturity indicators indexes discrepancy between methyl-phenanthrene index (MPI) and methyl-adamantane index (MDI). In addition, the stable isotopic results show that condensates from the Paleogene have significantly higher positive δ13C values of individual n-alkanes than the Neogene samples. Based on δD values, the samples can be divided into two groups, the differences between which are likely to be attributed to different biosynthetic precursors. Variation within each group can likely be attributed to vaporization.
Microorganisms are unique among all of the living organisms because of their high population size, advanced genetic diversity, short generation time, and quick response to the small change in environmental conditions. Remote alpine lakes of the Third Pole region provide the unique habitat for microorganisms acting as a natural laboratory and offering the information about the ecological roles of microorganisms. Many researchers focused on microbial communities as well as the impact of physicochemical, biological and hydrological parameters in lakes of this region since decades but the comprehensive review focusing on bacterial diversity and the role of environmental parameters still lacks. Here we reviewed bacterial diversity in lakes of the Third Pole region by analyzing 16S rRNA clone libraries accessed from previous research findings. A total of 5 388 bacterial 16S rRNA gene sequences were analyzed and classified into different phylogenetic groups. The average relative abundance of dominant taxa includes Betaproteobacteria (19%), Bacteroidetes (18%), Gammaproteobacteria (16%), Actinobacteria (15%), Alphaproteobacteria (14%), Cyanobacteria (7%), and Firmicutes (5%). Several adaptational strategies were adopted by these dominant bacterial groups in order to accommodate in the respective habitat. Nevertheless, lake water properties like temperature, pH, salinity, incident UV radiation, turbidity, and nutrients also played role in bacterial diversity.
To investigate the vertical variation of microbial functional groups particular nitrogen fixers and ammonia oxidizers, sediment samples from a 155 cm deep peat profile were collected from the acidic Dajiuhu Peatland and subsequently subjected to clone library construction and quantification. Results showed that nifH gene abundance varied between 105-108 copies per gram dry sediment and reduced gradually with depth. The abundances of ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) abundance were generally comparable in each sample. More AOA was observed with a depth ≤55 cm, whereas AOB was more abundant with a depth >55 cm. Phylogenetically nifH could be divided into 94 OTUs which mainly affiliated with α-Proteobacteria. AOA were affiliated with Nitrosotalea from Group 1.1a associated (nearly 90%) and Nitrososphaera from Group I.1b. All AOB belonged to Nitrosospira. Notably, DJH11 with the depth of 50-55 cm was observed to have the highest abundance and the highest diversity of nitrogen fixers and ammonia oxidizers among all the samples. Pearson's correlation analysis showed a positive relationship between water content and pH with the nifH gene abundance. Our results offer the first insight about the microbial community composition and diversity involved in nitrogen cycles in the Dajiuhu Peatland.
The microstructural analysis of muds and mudrocks requires very high-resolution measurement. Recent advances in electron microscopy have contributed significantly to the improved characterisation of mudrock microstructures and their consequent petrophysical properties. However, imaging through electron microscopy is limited to small areas of coverage such that upscaling of these properties is a great challenge. In this paper, we develop a new methodology for multiple large-area imaging using scanning electron microscopy through automated acquisition and stitching from polished thin-sections and ion-milled samples. The process is fast, efficient and minimises user-input and bias. It can provide reliable, quantifiable data on sediment grain size, grain orientation, pore size and porosity. Limitations include the time involved for individual runs and manual segmentation, the large amount of computer memory required, and instrument resolution at the nano-scale. This method is applied to selected samples of Quaternary muddy sediments from the Iberian margin at IODP Site 1385. The section comprises finegrained (very fine clayey silts), mixed-composition, biogenic-terrigenous hemipelagites, with a pronounced but non-regular colour cyclicity. There is a multi-tiered and diverse trace fossil assemblage of the deep-water Zoophycos ichnofacies. The sediment microstructures show small-scale heterogeneity in all properties, and an overall random fabric with secondary preferred grain-alignment. These results on the fabric differ, in part, from previous studies of hemipelagic muds. Further work is underway on their comparison with other deep-water sediment facies.
Based on gravity frequency shift effect predicted by general relativity theory, this study discusses an approach for determining the gravity potential (geopotential) difference between arbitrary two points P and Q by remote comparison of two precise optical clocks via optical fiber frequency transfer. After synchronization, by measuring the signal's frequency shift based upon the comparison of bidirectional frequency signals from P and Q oscillators connected with two optical atomic clocks via remote optical fiber frequency transfer technique, the geopotential difference between the two points could be determined, and its accuracy depends on the stabilities of the optical clocks and the frequency transfer comparison technique. Due to the fact that the present stability of optical clocks achieves 1.6×10-18 and the present frequency transfer comparison via optical fiber provides stabilities as high as 10-19 level, this approach is prospective to determine geopotential difference with an equivalent accuracy of 1.5 cm. In addition, since points P and Q are quite arbitrary, this approach may provide an alternative way to determine the geopotential over a continent, and prospective potential to unify a regional height datum system.