Tectonic generation is an important concept in geomechanics and tectonic geology.It refers to the sequence in which structural planes with different mechanical properties and orientations are formed during the same crustal movement.This occurs under the continuous action of the same period and dynamic action mode,or due to local changes in boundary conditions,which control the formation of structural features.Structural generation emphasizes the genetic relationship between structural features and serves as the basis for determining structural types and establishing structural systems.The structural generation relationship is common and distinct from the structural level.Different generation structures possess characteristics of time difference,derivation,absoluteness,and invisibility.Low-order structures are often distributed within the influence range of higher-order structures and are controlled by local stress fields.The study of ore-controlling structures should proceed from low order to high order.Starting with the investigation of ore-bearing structures,their mechanical properties and combination laws should be analyzed.This allows for the determination of ore-controlling structure types and the summarization of ore-controlling laws.Ore-prospecting predictions should follow a high-order to low-order approach.Based on the ore-controlling structure type and the control law of structure generation,the possible position and direction of low-order ore-bearing structures are analyzed,enabling ore-prospecting prediction.Through the structure generation analysis of ore-controlling structures,the Zoujiashan uranium deposit is considered to be controlled not by the NE-trending Zoujiashan-Shidong fault but by the NEE-trending hidden structural belt with a medium-low dip angle,dipping to NNW.The prospecting direction is along the SWW direction of the existing ore belt,and the deep part has a medium dip.In the Changjiang uranium ore field,uranium mineralization is not controlled by the Mianhuakeng fault and Youdongfault but by the NNW-trending structural belt with a steep dip angle.The shallow secondary fault is an ore-bearing(or ore-storing)structure,and the main structural belt,which merged from them and extended to the deep,plays the role of guiding mineralization and transporting ore-forming hydrothermal solutions(deposit distribution).The ore-prospecting direction is the extension and deepening part along the NNW-trending ore-bearing structure and the deep part of the surface mineral-free zone.When analyzing structural generation,it is essential to pay special attention to the genetic relationship of structural features along with structural periods.It is crucial to distinguish between structures formed before,during,and after mineralization.Within a specific ore field deposit or mineralization zone,the relationship between ore-guiding structures,ore-transporting(ore-matching)structures,and ore-storing(ore-bearing)structures(i.e.,the relationship between ore-controlling structures and ore-bearing structures)should be objectively analyzed.This analysis allows for the determination of an ore-controlling structural type,the construction of a structural ore-controlling model,and ultimatelyguides the practice of ore-prospecting and prediction.
Accurate evaluation of the distribution characteristics and physical properties of gas hydrate reservoirs is crucial for estimating gas hydrate resources,guiding development and utilization strategies,and understanding environmental impacts.Based on high-precision 3D seismic and well-logging data acquired in the Shenhu area,this paper presents a comprehensive method for discriminating between hydrate layers,free gas layers,and coexistence layers using integrated logging-seismic responses.Our results show that the gas hydrate layer and free gas layer at the W17 station can be accurately identified by combining post-stack seismic profiles with detailed analysis of logging curves.Above the BSR,a hydrate layer is characterized by relatively high seismic velocity,strong amplitude reflections,poor transverse continuity,and high resistivity and velocity values.In contrast,the underlying free gas layer exhibits low seismic velocity,low-frequency weak amplitude reflections,and low resistivity and velocity values.This study provides a robust basis and valuable reference for informed decision-making regarding gas hydrate reservoir identification,resource evaluation,exploration,and trial production activities.
The South-to-North Water Diversion Project Central Route (SNWDP-CR) is the largest water control project which has ever been built, and the aim of which is to optimize the reallocation of water resources from South China to North China. Since it was put into operation in December 2014, it has delivered more than 6 × 109 m3 of water to Beijing, which has changed the water supply pattern in Beijing and provided conditions for reducing groundwater extraction and controlling land subsidence. In this study, a variety of monitoring data are used to analyze the changes of the groundwater flow field, groundwater level, land subsidence, soil deformation, and hydrogeology parameters before and after the SNWDP-CR. The study showed that the groundwater level of the first to fourth aquifer groups rose on average by 2.72 m, 3.68 m, 3.31 m, and 1.91 m from 2015 to 2020. The average subsidence rate decreased from 18.8 mm/year in 2015 to 10.85 mm/year in 2020. The deformation characteristics of different lithological soil layers under different water level change modes can be summarized into 4 categories. The sand layer is mainly characteristic of elastic deformation. The cohesive soil layers of different depths have elastic, plastic, and creep deformation, and the viscoelastic-plastic characteristics are obvious. For different stages of soil deformation, the changes of elastic and inelastic skeletal specific storage rates are not constant. As the groundwater level decreases, the soil skeletal specific storage rate shows a decreasing trend.
There are relatively few sedimentological studies on the Cryogenian in the northwestern Yangtze Craton, especially in the Chengkou area of the Dabashan Mountain. Whether the Cryogenian Muzuo/Mingyue and Dai'anhe formations have glacial sedimentary characteristics is also uncertain due to lack of detailed sedimentological investigation, which limits the understanding of sedimentary evolution process of the Cryogenian in the Yangtze Craton. These lacunae are not conducive to the determination of the sedimentary stratigraphic framework and regional correlation of the Cryogenian. In this paper, four typical Cryogenian sections in the Chengkou area are selected for undertaking a detailed stratigraphic and sedimentological study, and the glacial sedimentary marks such as the gravels with glacial striae, dropstones, and subglacial soft sedimentary deformations are identified to prove the glacial genesis of the Dai'anhe and Muzuo/Mingyue formations. The facies of continental, periglacial and marine glacial are also discerned. On this basis, according to the rock assemblages, sedimentary characteristics, structure of stratigraphic succession, geochemical indices (such as chemical index of alteration) and detrital zircon UPb age, the Dai'anhe Formation is divided into two members. The ls' Member is correlated with the Gucheng Formation, the 2' Member is correlated with the Datangpo Formation, and the Muzuo/Mingyue Formation is correlated with the Nantuo Formation. Further, through regional correlation, the stratigraphic framework of the Cryogenian in the northwestern Yangtze Craton is re-evaluated, and correlated with that in the adjacent Shennongjia area and southeastern Yangtze Craton. The sedimentary co -evolution process of the Cryogenian in the Chengkou-Shennongjia areas is as follows: during the Stuartian glacial period, the glacial transgressive-regressive cycle from the glaciomarine to the land and then to the glaciomarine developed; subsequently extensive transgression of the interglacial period formed a lagoon environment represented by carbonaceous silty mudstone. With the global temperature falling again, the Marinoan glacial period was triggered, which was reflected in the multiple glacial dynamic evolution from the glaciomarine ice-proximal to the ice-distal environment, and accompanied by the opening of the ice sheet and finally to the end of the Cryogenian. This study has determined the correlation and subdivision of the Cryogenian in the northwestern Yangtze Craton, providing new basic data and important basis for the correlation of the Cryogenian in the Yangtze Craton.
Radon gas is the only radioactive inert gas, whose hazards for human have been confirmed by medical and health science. The WHO report revealed that the inhalation of radon and its decay products was carcinogenic for the human lungs among non-smokers. Approximate 3% similar to 15% human lung cancers are triggered by the radon gas among different countries all over the world. The synergistic effect of radon gas and smoke will significantly raise the possibility of lung cancer. In addition, excess radon inhalation will damage human nervous system, decrease the hemameba, and trigger the increasing of blood clotting and hyperglycemia. Radon is the decay product of 238U, whose one main source is the granite. Granite is the main constituent of continental crust of the Earth, occupying the 22% on volume. Lots of studies on granite origin have been implemented among geological community, however, the relationship of human lung cancer and the granite hasn't got enough attentions from geologists. In contrast, the medical and health community know much about the radon gas, but they don't understand the original source of radon gas and its closeness with granite. In the present study, we would like to address the linkage between granite and radon, shed light on the damage of radon gas for public.
This paper systematically sorts out the Meso-Neoproterozoic stratigraphic problems on the Stratigraphic Chart of China (2014), based on the achievements of relevant experts and the author's team in stratigraphic chronology in the past 10 years. By combining the results with the spatio-temporal development and correlation relationship of geological events, the study focuses on studying the definition of the bottom boundary of the Mesoproterozoic and the Mesoproterozoic Undefined System, the stratigraphic relationship in the Meso-Neoproterozoic transition period, and the division of the lower boundary of the Cryogenian, as well as the timing of the Tonian Luoquan glaciation. On this basis a subdivision and correlation framework of the Meso-Neoproterozoic strata of China are presented and the similarities and differences among the North China, Yangtze, and Tarim block discussed. The research indicates that it is more reasonable to set 1800 Ma (or 1780 Ma) as the lower boundary of the Mesoproterozoic than 1600 Ma. Although the Mesoproterozoic Undefined System is developed in both the northern and southern margins of North China and South China block, the Shennongjia Group in the northern margin of the Yangtze block is more favorable than others and as a result, suggested as the temporary stratotype of the Undefined System in China for focused research. The proto-Qingbaikou System in Jiao-Liao-Xuhuai-Ji-Yu of the North China block, is probably older than 1000 similar to 820 Ma), i. e., parts of it belong to the Mesoproterozoic. We propose that the proto-Nanhuan System corresponds to the international Cryogenian (720 similar to 635 Ma). The Qingbaikou System is bounded by a tectonic plane dated to 820 Ma-its lower part is named as the Lower Series of the Qingbaikou System (1000-820 Ma), and the upper part is named as the Upper Series of the Qingbaikou System (820 similar to 20 Ma). The decision on whether to divide these into two systems will be made when more evidence becomes available. The Luoquan Formation and coeval strata in the southern margin of North China block probably belong to the upper Sinian (Ediacaran). Our research provide data, evidence and the reference subdivision, to revise and improve for "the Meso-Neoproterozoic" division and correlation in the Stratigraphic Chart of China.
The Ediacaran glacial deposits are developed in the North China Craton, Tarim Craton and the microblocks among them, where they can extend for 3000 km from east to west. However, the chronology, scale of glaciation, paleogeographic reconstruction and tectonic setting are still controversial. Based on the spatial distribution, stratigraphy and sedimentary sequence, sedimentary environment and sedimentary facies of the Ediacaran diamictites in northern China, combined with previous literatures at home and abroad, the above issues related to Ediacaran glaciations are systematically analyzed. This paper revealed that during the Ediacaran, there should have been a glaciation of younger than 580 Ma (Gaskiers) all over the world. The age constraints of the Ediacaran glaciations in northern China were placed between 562. 5 Ma and 551 Ma, and the subglacial, periglacial, and proglacial sedimentary facies can be identified; these form the diamictite from subglacial to proglacial, followed by normal marine sedimentary sequences vertically (in ascending order), in line with the sedimentary response pattern of continental glacier (ice sheet). The Ediacaran diamictites and glacial erosional forms are widespread distributed globally, and the cap dolostones can be developed above the diamictites, exhibiting sedimentary structures similar to the Cryogenian cap dolostones, but the carbon isotope characteristics of them are not the same. This paper speculates that the Proto-Tethys Ocean and its surrounding continents may have developed continental ice sheets during the Ediacaran, but there was no affinity between the Asian blocks and the Gondwana supercontinent. The results provide geological evidences for the global paleogeographic reconstruction and tectonic setting restore of the Ediacaran period.
Audio magnetotelluric (AMT) is an effective exploration method to study the electrical structure of strata based on the electrical differences of rocks. The gas hydrate in the permafrost has the characteristic of high resistivity, which is different from the surrounding rock electrically. The AMT method can be used for the exploration and evaluation of gas hydrate in the permafrost. Based on the actual geological characteristics of gas hydrate reservoir in the Qilian Mountain permafrost, the geoelectric model of gas hydrate reservoir is established by combining with resistivity logging. The application range of AMT method for gas hydrate reservoir detection and the optimal acquisition parameter setting scheme were simulated by using finite element method and nonlinear conjugate gradient method. When porosity of gas hydrate reservoir is less than 5%, gas hydrate saturation is greater than 70%, occurrence scale is less than 50 m, or burial depth is greater than 500 m, AMT technique cannot identify and delineate the favorable gas hydrate reservoir. Survey line should be more than twice the length of probable occurrence scale, while tripling the length will make the best result. The number of stations should be no less than 6, and 11 stations are optimal. At the high frequency section (10 similar to 1000 Hz), there should be no less than 3 frequency points, 4 being the best number. The research results could provide important reference for gas hydrate reservoir electrical prospecting in the Qilian Mountain permafrost.
Low metamorphic argillaceous rocks and carbonate rocks are widely distributed in the Meso-Neoproterozoic stratigraphic framework of the three continental blocks in China. Its geochemistry characteristics of elements provide important basic data for us to study the history evolution of the lithosphere. In this paper, we collected nearly 2000 sets of geochemistry data in argillaceous rocks/carbonate rocks that have been published in China and abroad in the past decade, including 40 major and trace element content compositions, involving the Changcheng System, Jixian System, Undefined System and Qingbaikou System of the North China block, the Nanhua System and the Sinian System of the Yangtze and Tarim blocks. In combination with the lithology, experimental methods and geological background characteristics of the samples in the literature, we mainly draw the following conclusions: (1) The average content of major elements in argillaceous rocks is similar to that in unmetamorphosed pelite in eastern China. But it was influenced overall by the Neoproterozoic ice age and has a relatively high content of Na, K and Ca elements that are easily lost by chemical weathering. The content composition of major elements in carbonate rocks can be compared with that of unmetamorphosed mud-rich carbonate rocks in eastern China. There is a small amount of terrigenous detrital input, mainly dolomite, and the Mg content is high. (2) The high Mn/Sr ratio in carbonate rocks should be related to diagenetic alteration. Although silicate minerals in carbonate rocks are less than those in argillaceous rocks, Sc, REE + Y and redox sensitive elements are more enriched. Sc and REE + Y elements are related to the hybridization of terrigenous clasts, and the latter is related to the oxidized marine environment. (3) In the early Mesoproterozoic, the negative anomaly of Ce element and the increase of redox sensitive element content were recorded in the sediments of North China, which responded to a shallow sea oxidation event and promoted the biological evolution. (4) In the Neoproterozoic, the Tarim argillaceous rocks had a wide range of chemical alteration indexes, which recorded the "Snowball Earth" event. With the increase of ocean oxygen during interglacial and post glacial periods, the content of some trace elements in Neoproterozoic sediments of the Yangtze block increased again, which corresponded to the emergence of Ediacara biota.
The Late Triassic Fanshan ultrapotassic alkaline complex in the northern margin of the North China Craton is mainly composed of clinopyroxene and pyroxene syenite, and is famous for the development of endogenetic apatite-magnetite deposits. Apatites in the Fanshan complex were selected as the research object in this paper to clarify the oxygen fugacity and water content of the parent magmas by measuring their major-and trace-elements and in-situ Sr-Nd isotopic compositions. The study shows that the Fanshan apatites are characterized by enrichment of light rare earth elements and depletion of heavy rare earth elements. The distribution patterns of rare earth elements exhibit the characteristics of right-deviation with low Ga levels and high 6Eu values. These characteristics indicate that the complex has high magmatic oxygen fugacity, which is also supported by apatite Mn oxygen fugacity meter. All apatites are characterized by high Sr and low Y, and their high ratios of Sr/Y (>30) and La/Yb (>90) exhibit an obvious positive correlation, suggesting a high water content for the Fanshan ultrapotassic magmas. The in-situ Sr-Nd isotopic compositions of the Fanshan apatites plot in the mantle field, obviously deviating from the crustal trend. The characteristics of "oxidation and water-rich" of the parent magmas is an important factor for the apatite-magnetite mineralization in the Fanshan complex, which is probably related to the metasomatism of the altered oceanic crust on the lithospheric mantle during the plate subduction. Comparing the composition data of apatites in the Fanshan complex with those in other geological settings worldwide, it is considered that the apatites with the characteristics of "high Sr, low F and little Cl" can serve as a characteristic mineral for identifying ultrapotassic alkaline rock complexes (and their related metal deposits).
Lithophysae is a special primary structure in acid volcanic rocks, commonly found in rhyolite. A study of genesis of lithophysa structure is significant for understanding the properties and eruptive environment of rhyolitic magma. However, research on the magmatic properties, formation mechanism and influencing factors of lithophysae formation are still limited. In this paper, considering the lithophysae of Cretaceous Qingshan Group rhyolite of Sulu orogenic belt as the research object, the development law of the lithophysae, the mineral composition and element changes of the lithophysae and the cementation slurry between the lithophysae have been systematically studied through mineralogy, petrology, geochemistry and other methods. The results indicate that the lithophysa rhyolite mainly develops in the upper subfacies of the overflow facies, and lithophysae can be divided into two types: solid type and hollow type. The geochemical characteristics show that lithophysa rhyolite and fluidal rhyolite belong to homologous magma, and lithophysa rhyolite has experienced a higher degree of differentiation. The water content in the cementation slurry of lithophysa rhyolite is higher than that of the glassy in the lower fluidal rhyolite, resulting in a relatively obvious trend of escape of volatiles in the upper subphase of the overflow phase. Due to the high viscosity of silicon-rich magma, volatiles fail to escape smoothly, and a sudden cooling occurs at the escape point to form the lithophysa wall. Affected by instantaneous stress and degree of undercooling, various cavities are formed. As the crystallization temperature decreases, the residual magma inside the cavity condenses and crystallizes successively to form chalcedony and quartz. The formation of the lithophysae means the addition of water in the process of magmatism or eruption environment, which is of great significance for the study of the evolution of acid magma and the mechanism of volcanic eruption.
The Weng'an biota in the Ediacaran Doushantuo Formation phosphorus block in Weng'an, Guizhou is an important window to understand the origin and evolution of early life. Because of their special phosphosalination, globular fossils preserve a great deal of fine cellular and even subcellular structures. However, the mechanism by which these fossils are preserved is unclear. Therefore, in this paper, Volvox and Eudorina were used as experimental objects to explore the preservation status of algae under different burial environments by changing the pH, temperature and oxygen availability, to simulate the burial environment of early phosphate fossils. The results showed that there was a large difference in the conservation potential of algae under different experimental conditions. Though the conservation potential of the two species of algae under the same environment was not consistent, both had sufficient conservation potential. During the burial process, the cells of Chlorella were folded and deformed, but the morphological changes were not obvious under different conditions. The morphology of Volvox blooms is very similar to the cell division form of a kind of multicellular globular fossil in the Weng'an biota. Under the same or different experimental conditions, the individual diameter of the same species of algae with the same early state showed a strong difference. The experimental conditions were set to simulate the natural burial environment, so the experimental phenomenon may be reproduced in the real burial process. The experimental results show that the algae in the phosphate burial environment also have sufficient preservation potential. Phosphorylation can lead to the acquisition of morphological information outside the organism itself. We should be careful to classify the properties of phosphate globular fossils because the classification based on the diameter may not be reliable. Embryoid fossils are not directly equivalent to phosphatized algae, and their similarities may result from convergent evolution.
The Shennongjia Group is considered to be the best example of Mesoproterozoic in China with the exception of the Jixian section in Tianjin. Since it was formed roughly at 1400-1000 Ma, it is generally regarded as a potential candidate for missing stratigraphic records from 1320-1000 Ma in the stratigraphic table of China. Although the studies on chronostratigraphy, sedimentology, paleontology (stromatolite formation and distribution) and paleomarine geochemistry of the Shennongjia Group have made remarkable progress in recent years, there is still no consensus on the division of this group. Based on the study of the Upper Precambrian in the Shennongjia and the Shennongjia 1 200000 mapping scheme, this paper redefines the sequence of the Shennongjia Group through systematic studies undertaken in the past 10 years. It is concluded that the Shennongjia Group can be divided into three subgroups from the bottom to the top: the lower subgroup, the middle subgroup and the upper subgroup. The lower subgroup from the bottom to the top includes the Yingwodong Formation, the Dayanping Formation, the Macaoyuan Formation, the Luanshigou Formation, the Dawokeng Formation and the Kuangshishan Formation. The middle subgroup includes the Taizi Formation, the Yemahe Formation, the Wenshuihe Formation and the Shicaohe Formation. The upper subgroup includes the lower Songziyuan Formation and the upper Wagangxi Formation. The above sequence determination of the Shennongjia Group fully combine the characteristics and development laws of isotope chronology, sedimentary geological evolution, stromatolite morphological types and cyclic development, isotope and element geochemical anomalies. The results of this study are a summary of the research progress on the Shennongjia Group in the past 10 years. Although the basic conditions for establishing candidate stratotypes have been met, the Shennongjia Group not only lacks the stratigraphic ages of the top and bottom boundaries, but also fails to locate the underlying basement. Further comprehensive studies are needed in the future.
The genetic classification of ferromanganese nodules is an important geological problem in characterizing seabed mineral resources. The discrimination of nodules by geochemical characteristics has been a challenge since the discovery of nodules in the seabed. In recent years, with the application of Big Earth Data mining and the application of machine learning technologies, creative ideas and methods has resulted in the discrimination of nodules using geochemical characteristics. Based on the geochemical data of ferromanganese nodules obtained in Pacific Ocean over the years, the four types of nodules in deep-sea basin, the diagenetic, mixed, hydrogenetic-I and hydrogenetic-II types, have been identified successfully by Gaussian mixture model cluster analysis technology. Furthermore, the spatial prediction results of different genetic types of nodules show that the northwest Pacific Ocean is one of the main distribution areas of hydrographic-I cobalt-rich nodules. This genetic type is mainly distributed in the intermountain basins of Marcus-Wake seamounts, north of the Magellan seamounts, the Marshall seamounts and the southwest of the Central Pacific seamount group, as well as the nearby Pigafetta basin and the northwest of central Pacific basin. These basins are prospective areas for the future exploration of cobalt-rich ferromanganese nodules in the northwest Pacific Ocean.
The Tonian period marks one of the most critical time intervals in the evolutionary history of the early life and environment. It follows a golden age of cyanobacteria in the Mesoproterozoic and precedes a global biological collapse in the Cryogenian. The southern Jilin Province, situated on the northeastern margin of the North China block, offers an excellent opportunity to study paleontology, biostratigraphy and paleoenvironment during the Tonian period. This region boasts complete sedimentary successions of strata, well-exposed outcrops, and the presence of various well-preserved microfossils within chert bands, nodules, and shales. In this study, we present an assemblage of microfossils discovered in cherts from the uppermost sediments of the Wanlong Formation in the Erdaojiang area of south Jilin Province. We have identified a total of nine species belonging to six genera, including filamentous cyanobacteria such as Siphonophycus robustum, S. typicum, S. kestron, S. solidum, spheroidal cyanobacteria such as Eoentophysalis belcherensis, Gloeodiniopsis lamellosa Scissilisphaera bistratosa and incertae sedis species like Glenobotrydion majorinum Globophycus rugosum. These newly documented microfossils provide valuable insights into the paleoenvironment of the Wanlong Formation and facilitate the regional correlations of biostratigraphy. Furthermore, the microbiota is predominantly composed of cyanobacteria filaments, particularly Siphonophycus mat, with horizontally oriented filaments typically exceeding 400 rim in length, indicating taphonomic characteristics of autochthonous or near-autochthonous preservation. The comprehensive analysis of the paleoenvironment suggests that the uppermost sediments of the Wanlong Formation in the Erdaojiang area of south Jilin Province represent a shallow, low intertidal or subtidal environment with a low sedimentation rate.
The acritarch microfossils arediscovered from the chert nodules and bands in the Member II of the Ediacaran Doushantuo Formation at Xiangerwan section in the western flank of Huangling anticline in the Xingshan County of Yichang, Hubei Province, South China. They are represented taxonomically by 8 species of 8 genera, including Appendisphaera grandis, Crassimembrana multitunica Leiosphaeridia minutissima Megasphaera inornata, Schizofusa zangwenlongii, Symphysosphaera basimembrana Urasphaera nupta, Variomargosphaeridium gracile. Since the existence of Schizofusa zangwenlongii it might be tentatively correlated with the second assemblage zone (Tanarium tuberosum-Schizofusa zangwenlongii) proposed by Liu and Moczydiowska (2019). However, it does not contain genus Tianzhushania which is common in the Member 11 of the Doushantuo Formation in the southeastern flank of Huangling anticline, there might exist environmental variations between the two flanks of the Huangling anticline.
The unique clastic formation preserved in the Yangtze Craton provides excellent material for studying the mid-Neoproterozoic evolution, and the abundantzircon U-Pb ages of tuff interlayers within also provide effective geochronological constraints for the mid-Neoproterozoic evolution of the Yangtze Craton. However, the evolution of the Liantuo Formation in the Yangtze Craton still lacks geochronological constraints, which restricts study of sedimentary evolution of the Yangtze Craton in the late Tonian. In this study, field geological investigation and LA-ICP-MS technology were used to obtain 763. 1 +/- 6. 2 Ma (Shennongjia) tuff zircon U-Pb concordant age at the bottom boundary of the Liantuo Formation, 761. 8 +/- 7. 1 Ma (Hefeng) tuff zircon U-Pb concordant age in lower part of the Liantuo Formation, 764. 1 +/- 3. 5 Ma (Tongshan), 751. 5 +/- 6. 3 Ma (Changyang), 752. 1 +/- 6. 5 Ma (Shennongjia) of tuff zircon U-Pb concordant ages in marine-terrestrial strata of middle and lower part of the Liantuo Formation, 729. 6 +/- 9. 2 Ma (Xiuning Formation), 722. 4 +/- 4. 5 Ma (Shennongjia) tuff zircon U-Pb concordant ages in marine strata of upper part of Liantuo Formation, and 712. 4 +/- 6. 4 Ma tuff zircon U-Pb concordant age in middle part of the Longtanhe Formation. These data and stratigraphic sequence show that: 820-770 Ma Yangtze Craton was exposed to erosional environment, terrestrial deposit began 770 Ma, after 750 Ma Yangtze Craton completely transitioned to marine deposit that continued until Cryogenian. The strata framework during the late Tonian shows that the evolution of Yangtze was controlled by Rodinia supercontinent breakup process, and may be closely related to deep mantle plume induced extensional rift, magmatic mantle plume cooling. This study provides effective geochronological constraints for the late Tonian strata in the Yangtze Craton, reveals synergistic relationship between sedimentary evolution and tectonic background of Yangtze Craton during the mid-Neoproterozoic, provides effective data support for evolution of Yangtze Craton during Rodinia breakup.
The South Altun orogenic belt is an important subduction-collision complex belt in northwest China, located between the Tarim and the Qaidam basins. Its early Paleozoic tectonic evolution has been one of the research hotspots in recent years. However, the timing of oceanic crust subduction has been controversial. In this paper, we study the petrology, geochemistry, zircon U-Pb chronology and zircon Lu-Hf isotope geochemistry of the Manya quartz monzonite, and discuss its petrogenesis and the tectonic environment. The samples show high alkali, potassium-rich, low-titanium, iron-poor and Nd-Ta-Ti anomalies similar to shoshonitic rocks. The quartz monzonites were generated between 511 Ma to 495 Ma, and the values of epsilon(Hf) (t) mainly range from -3. 51 to -0. 08, with some positive values ranging from 0. 04 to 1. 69. Based on our research, we infer that the subducting oceanic crust released large amounts of water when arriving at amphibolite facies boundaries, and triggered the mantle wedge peridotite hornblende metasomatism. Due to the dragging of the subducting oceanic crust, the temperature of the mantle wedge rose, causing the melting of the hornblende metasomatized peridotite, which triggered the partial melting of upper crust materials. Finally, crustal melts mixed with smaller mantle melts to form quartz monzonite. The similar to 500 Ma granites are petrological response to transition from oceanic island arc environment to active continental margin. Thus, the southern Altun oceanic crust may have begun subducting at around 517 Ma.
The source rocks of the Paleogene Wenchang Formation and the Enping Formation in the Baiyun sag are complex. Although the semi-deep lacustrine, shallow lacustrine, terrigenous marine, and deltaic facies have been identified, the mechanism of organic matter enrichment is unknown, leading to long-standing disagreement about the main source rocks in the area and their potential. This study analyzed the main trace elements and the characteristics of strawberry-shaped pyrite grain size in different types of source rocks that have been drilled and analyzed the development and preservation conditions of organic matter in different types of source rocks. It revealed the organic matter enrichment mechanism of different types of source rocks in the Baiyun sag for the first time. The results show that the primary productivity level of shallow lacustrine source rocks in the Baiyun sag is high. The distribution range of strawberry-shaped pyrite grain size in the Wenchang Formation is 0. 4 14. 3 rim with a wide distribution range and an average particle size of 6. 8 rim, while the distribution range of strawberry-shaped pyrite grain size in the source rock of the Enping Formation is 4. 9-15. 9 rim with an average particle size of 8. 7 rim; the overall environment is sub-oxidation. The primary productivity level of the semi-deep lacustrine source rocks is lower than that of the shallow lacustrine source rocks, and the distribution range of the strawberry-shaped pyrite grain size is narrow, ranging from 5. 6 to 8. 0 rim with an average particle size of 5. 8 rim, which is in a sulfide-sub oxidation environment with good preservation conditions for organic matter. The primary productivity level of terrigenous marine source rocks is relatively low, the preservation conditions of organic matter are good, and they receive terrigenous nutrient inputs, resulting in the enrichment of organic matter. The deltaic source rocks received a large amount of nutrients from terrigenous clastic inputs, resulting in high sediment accumulation efficiency and rapid burial. The average grain size of strawberry-shaped pyrite is > 7. 0 rim and is characterized by euhedral pyrite in an oxidized-strongly oxidized environment with poor preservation conditions for organic matter. With a large amount of source input and accumulation efficiency, it also forms a favorable area for organic matter enrichment.
