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, Available online , doi: 10.31035/cg2022019
Abstract:
To accelerate the achievement of China’s carbon neutrality goal and to study the factors affecting the geologic CO2 storage in the Ordos Basin, China’s National Key R&D Programs propose to select the Chang 6 oil reservoir of the Yanchang Formation in the Ordos Basin as the target reservoir to conduct the geologic carbon capture and storage (CCS) of 100000 t per year. By applying the basic theories of disciplines such as seepage mechanics, multiphase fluid mechanics, and computational fluid mechanics and quantifying the amounts of CO2 captured in gas and dissolved forms, this study investigated the effects of seven factors that influence the CO2 storage capacity of reservoirs, namely reservoir porosity, horizontal permeability, temperature, formation stress, the ratio of vertical to horizontal permeability, capillary pressure, and residual gas saturation. The results show that the sensitivity of the factors affecting the gas capture capacity of CO2 decreases in the order of formation stress, temperature, residual gas saturation, horizontal permeability, and porosity. Meanwhile, the sensitivity of the factors affecting the dissolution capture capacity of CO2 decreases in the order of formation stress, residual gas saturation, temperature, horizontal permeability, and porosity. The sensitivity of the influencing factors can serve as the basis for carrying out a reasonable assessment of sites for future CO2 storage areas and for optimizing the design of existing CO2 storage areas. The sensitivity analysis of the influencing factors will provide basic data and technical support for implementing geologic CO2 storage and will assist in improving geologic CO2 storage technologies to achieve China’s carbon neutralization goal.
To accelerate the achievement of China’s carbon neutrality goal and to study the factors affecting the geologic CO2 storage in the Ordos Basin, China’s National Key R&D Programs propose to select the Chang 6 oil reservoir of the Yanchang Formation in the Ordos Basin as the target reservoir to conduct the geologic carbon capture and storage (CCS) of 100000 t per year. By applying the basic theories of disciplines such as seepage mechanics, multiphase fluid mechanics, and computational fluid mechanics and quantifying the amounts of CO2 captured in gas and dissolved forms, this study investigated the effects of seven factors that influence the CO2 storage capacity of reservoirs, namely reservoir porosity, horizontal permeability, temperature, formation stress, the ratio of vertical to horizontal permeability, capillary pressure, and residual gas saturation. The results show that the sensitivity of the factors affecting the gas capture capacity of CO2 decreases in the order of formation stress, temperature, residual gas saturation, horizontal permeability, and porosity. Meanwhile, the sensitivity of the factors affecting the dissolution capture capacity of CO2 decreases in the order of formation stress, residual gas saturation, temperature, horizontal permeability, and porosity. The sensitivity of the influencing factors can serve as the basis for carrying out a reasonable assessment of sites for future CO2 storage areas and for optimizing the design of existing CO2 storage areas. The sensitivity analysis of the influencing factors will provide basic data and technical support for implementing geologic CO2 storage and will assist in improving geologic CO2 storage technologies to achieve China’s carbon neutralization goal.
, Available online , doi: 10.31035/cg2022025
Abstract:
To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19, extensive geophysical logging data and cores were analyzed to assess the reservoir properties. Sediment porosities were estimated from density, neutron, and nuclear magnetic resonance (NMR) logs. Both the resistivity and NMR logs were used to calculate gas hydrate saturations, the Simandoux model was employed to eliminate the effects of high clay content determined based on the ECS and core data. The density porosity was closely in agreement with the core-derived porosity, and the neutron porosity was higher while the NMR porosity was lower than the density porosity of sediments without hydrates. The resistivity log has higher vertical resolution than the NMR log and thus is more favorable for assessing gas hydrate saturation with strong heterogeneity. For the gas hydrate reservoirs at GMGS3-W19, the porosity, gas hydrate saturation and free gas saturation was 52.7%, 42.7% and 10%, on average, respectively. The various logs provide different methods for the comprehensive evaluation of hydrate reservoir, which supports the selection of candidate site for gas hydrate production testing.
To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19, extensive geophysical logging data and cores were analyzed to assess the reservoir properties. Sediment porosities were estimated from density, neutron, and nuclear magnetic resonance (NMR) logs. Both the resistivity and NMR logs were used to calculate gas hydrate saturations, the Simandoux model was employed to eliminate the effects of high clay content determined based on the ECS and core data. The density porosity was closely in agreement with the core-derived porosity, and the neutron porosity was higher while the NMR porosity was lower than the density porosity of sediments without hydrates. The resistivity log has higher vertical resolution than the NMR log and thus is more favorable for assessing gas hydrate saturation with strong heterogeneity. For the gas hydrate reservoirs at GMGS3-W19, the porosity, gas hydrate saturation and free gas saturation was 52.7%, 42.7% and 10%, on average, respectively. The various logs provide different methods for the comprehensive evaluation of hydrate reservoir, which supports the selection of candidate site for gas hydrate production testing.
, Available online , doi: 10.31035/cg2022023
Abstract:
This study focused on the quantitative analysis of the petrophysical parameters in characterizing the reservoir properties of the Srikail gas field using multi-scale wireline logs. Petrophysical parameters (shale volume, porosity, water saturation and hydrocarbon saturation) were estimated from the combination of gamma ray log, resistivity log, density log and neutron log for three hydrocarbon (gas)-bearing zones at well#3. At the first time, log records at 0.1 m and 0.2 m intervals were read for this study. Result showed the average shale volume is 21.07%, 53.67% and 51.71% for zone-1, zone-2 and zone-3, respectively. For these zones, the estimated average porosity was 35.89%, 29.83% and 28.76%, respectively. The average water saturation of 31.54%, 16.83% and 23.39% and average hydrocarbon saturation of 68.46%, 83.17% and 76.61% were calculated for zone-1, zone-2 and zone-3, respectively. Thus zone-2 is regarded the most productive zone of well#3. It was found that the values of some parameters (porosity, hydrocarbon saturation and permeability) are higher than the existing results. Therefore, this study confirmed that the log reading at minute/close interval provides better quantitive values of the reservoir’s petrophysical properties. It is expected that this result will contribute to the national gas field development program in future.
This study focused on the quantitative analysis of the petrophysical parameters in characterizing the reservoir properties of the Srikail gas field using multi-scale wireline logs. Petrophysical parameters (shale volume, porosity, water saturation and hydrocarbon saturation) were estimated from the combination of gamma ray log, resistivity log, density log and neutron log for three hydrocarbon (gas)-bearing zones at well#3. At the first time, log records at 0.1 m and 0.2 m intervals were read for this study. Result showed the average shale volume is 21.07%, 53.67% and 51.71% for zone-1, zone-2 and zone-3, respectively. For these zones, the estimated average porosity was 35.89%, 29.83% and 28.76%, respectively. The average water saturation of 31.54%, 16.83% and 23.39% and average hydrocarbon saturation of 68.46%, 83.17% and 76.61% were calculated for zone-1, zone-2 and zone-3, respectively. Thus zone-2 is regarded the most productive zone of well#3. It was found that the values of some parameters (porosity, hydrocarbon saturation and permeability) are higher than the existing results. Therefore, this study confirmed that the log reading at minute/close interval provides better quantitive values of the reservoir’s petrophysical properties. It is expected that this result will contribute to the national gas field development program in future.
, Available online , doi: 10.31035/cg2022016
Abstract:
This study identified two palynological assemblages, namely Bayanhuasporites-Cycadopites-Protoconiferus and Cicatricosisporites-Cedripites-Perinopollenites, in the Tongbomiao Formation in the Hongqi Sag in the Hailar Basin, Inner Mongolia, China for the first time. The former is distributed in the lower part of the Tongbomiao Formation and is characterized by abundant gymnosperm pollen and diverse fern spores. Among them, the gymnosperm pollen is dominated by Paleoconifer (4.98%–31.62%) and Cycadopite (8.55%–25.23%) pollen grains and also includes other pollen grains such as Classopollis, Parcisporites, Erlianpollis, Callialasporites, and Jiaohepollis. The fern spores in the former palynological assemblage contain Bayanhuasporite (0–8.96%), Granulatisporites (0.93%–6.97%), and some important Cretaceous genera, such as Cicatricosisporites, Concavissimisporites, Densoisporites, Hsuisporites, Foraminisporis, and Leptolepidites. The Cicatricosisporites-Cedripites-Perinopollenites palynological assemblage is distributed in the upper part of the Tongbomiao Formation. Gymnosperm (77.30%), Pinaceae (31.9%), and Paleoconiferus (19.02%) pollen predominate this palynological assemblage, and Quadraeculina, Erlianpollis, and Jiaohepollis pollen are also common in this assemblage. The fern spores in this palynological assemblage include abundant Cicatricosisporites (4.29%). Besides, Concavissimisporites, Aequitriradites, and Leptolepidites are also common in this palynological assemblage. No angiosperm pollen has been found in both palynological assemblages. The identification of both palynological assemblages provides important evidence for the biostratigraphic correlation between the Hailar Basin and its adjacent areas. It also enables the reconstructions of the Berriasian-Valanginian (Early Cretaceous) vegetation and the paleoclimate on the eastern Mongolian Plateau during 141–132 Ma. The results of this study show that the Berriasian-Valanginian vegetation in the Hailar Basin mainly included temperate coniferous forests mixed with occasional broad-leaved forests and that this basin had warm temperate paleoclimate during the Berriasian-Valanginian and was much warmer than today.
This study identified two palynological assemblages, namely Bayanhuasporites-Cycadopites-Protoconiferus and Cicatricosisporites-Cedripites-Perinopollenites, in the Tongbomiao Formation in the Hongqi Sag in the Hailar Basin, Inner Mongolia, China for the first time. The former is distributed in the lower part of the Tongbomiao Formation and is characterized by abundant gymnosperm pollen and diverse fern spores. Among them, the gymnosperm pollen is dominated by Paleoconifer (4.98%–31.62%) and Cycadopite (8.55%–25.23%) pollen grains and also includes other pollen grains such as Classopollis, Parcisporites, Erlianpollis, Callialasporites, and Jiaohepollis. The fern spores in the former palynological assemblage contain Bayanhuasporite (0–8.96%), Granulatisporites (0.93%–6.97%), and some important Cretaceous genera, such as Cicatricosisporites, Concavissimisporites, Densoisporites, Hsuisporites, Foraminisporis, and Leptolepidites. The Cicatricosisporites-Cedripites-Perinopollenites palynological assemblage is distributed in the upper part of the Tongbomiao Formation. Gymnosperm (77.30%), Pinaceae (31.9%), and Paleoconiferus (19.02%) pollen predominate this palynological assemblage, and Quadraeculina, Erlianpollis, and Jiaohepollis pollen are also common in this assemblage. The fern spores in this palynological assemblage include abundant Cicatricosisporites (4.29%). Besides, Concavissimisporites, Aequitriradites, and Leptolepidites are also common in this palynological assemblage. No angiosperm pollen has been found in both palynological assemblages. The identification of both palynological assemblages provides important evidence for the biostratigraphic correlation between the Hailar Basin and its adjacent areas. It also enables the reconstructions of the Berriasian-Valanginian (Early Cretaceous) vegetation and the paleoclimate on the eastern Mongolian Plateau during 141–132 Ma. The results of this study show that the Berriasian-Valanginian vegetation in the Hailar Basin mainly included temperate coniferous forests mixed with occasional broad-leaved forests and that this basin had warm temperate paleoclimate during the Berriasian-Valanginian and was much warmer than today.
, Available online , doi: 10.31035/cg2022021
Abstract:
The existing genetic models of the South China Sea (SCS) include an extrusion model of the Indochina Peninsula, a back-arc extension model, and a subduction and dragging model of the Proto-South China Sea (PSCS). However, none of these models has been universally accepted because they do not fully match a large number of geological phenomena and facts. By examining the regional tectonics and integrating them with measured data for the SCS, in this study, a back-arc spreading-sinistral shear model is proposed. It is suggested that the SCS is a back-arc basin formed by northward subduction of the PSCS and its formation was triggered by left-lateral strike-slip motion due to the northward drift of the Philippine Sea Plate. The left-lateral strike-slip fault on the western margin caused by the Indo-Eurasian collision changed the direction of the Southwest Sub-basin’s spreading axis from nearly E–W to NE–SW, and subduction retreat caused the spreading ridge to jump southward. This study summarizes the evolution of the SCS and adjacent regions since the Late Mesozoic.
The existing genetic models of the South China Sea (SCS) include an extrusion model of the Indochina Peninsula, a back-arc extension model, and a subduction and dragging model of the Proto-South China Sea (PSCS). However, none of these models has been universally accepted because they do not fully match a large number of geological phenomena and facts. By examining the regional tectonics and integrating them with measured data for the SCS, in this study, a back-arc spreading-sinistral shear model is proposed. It is suggested that the SCS is a back-arc basin formed by northward subduction of the PSCS and its formation was triggered by left-lateral strike-slip motion due to the northward drift of the Philippine Sea Plate. The left-lateral strike-slip fault on the western margin caused by the Indo-Eurasian collision changed the direction of the Southwest Sub-basin’s spreading axis from nearly E–W to NE–SW, and subduction retreat caused the spreading ridge to jump southward. This study summarizes the evolution of the SCS and adjacent regions since the Late Mesozoic.
, Available online , doi: 10.31035/cg2022009
Abstract:
Determining the main controlling factors of earthquake-triggered geohazards is a prerequisite for studying earthquake geohazards and post-disaster emergency response. By studying these factors, the geomorphic and geological factors controlling the nature, condition, and distribution of earthquake-induced geohazards can be analyzed. Such insights facilitate earthquake disaster prediction and emergency response planning. The authors combined field investigations and spatial data analysis to examine geohazards induced by seismic events, examining 10 earthquakes including the Wenchuan, Yushu, Lushan events, to elucidate the main control factors of seismic geohazard. The authors observed that seismic geohazard occurrence is usually affected by many factors, among which active nature of the seismogenic fault, seismic peak ground acceleration (PGA), topographic slope and geomorphic height differences, and distance from the fault zone and river system are the most important. Compared with strike-slip earthquakes, thrust earthquakes induce more high-altitude and high-speed remote landslides, which can cause great harm. Slopes of 0–40° are prone to secondary seismic geohazards, which are mainly concentrated 0–6 km from the river system. Secondary geohazards are not only related to seismogenic fault but also influenced by the associated faults in the earthquake area. The maximum seismic PGA and secondary seismic geohazard number are positively correlated, and the horizontal and vertical ground motions play leading and promoting roles in secondary geohazard formation, respectively. Through our research, the spatial distribution of seismic geohazards is predicted, providing a basis for the formulation of emergency response plans following disasters.
Determining the main controlling factors of earthquake-triggered geohazards is a prerequisite for studying earthquake geohazards and post-disaster emergency response. By studying these factors, the geomorphic and geological factors controlling the nature, condition, and distribution of earthquake-induced geohazards can be analyzed. Such insights facilitate earthquake disaster prediction and emergency response planning. The authors combined field investigations and spatial data analysis to examine geohazards induced by seismic events, examining 10 earthquakes including the Wenchuan, Yushu, Lushan events, to elucidate the main control factors of seismic geohazard. The authors observed that seismic geohazard occurrence is usually affected by many factors, among which active nature of the seismogenic fault, seismic peak ground acceleration (PGA), topographic slope and geomorphic height differences, and distance from the fault zone and river system are the most important. Compared with strike-slip earthquakes, thrust earthquakes induce more high-altitude and high-speed remote landslides, which can cause great harm. Slopes of 0–40° are prone to secondary seismic geohazards, which are mainly concentrated 0–6 km from the river system. Secondary geohazards are not only related to seismogenic fault but also influenced by the associated faults in the earthquake area. The maximum seismic PGA and secondary seismic geohazard number are positively correlated, and the horizontal and vertical ground motions play leading and promoting roles in secondary geohazard formation, respectively. Through our research, the spatial distribution of seismic geohazards is predicted, providing a basis for the formulation of emergency response plans following disasters.
, Available online , doi: 10.31035/cg2022005
Abstract:
By the end of 2020, 83 silver deposits (or ore occurrences), including four super-large-scale deposits, nine large large-scale deposits, 33 medium-scale deposits and 37 small-scale deposits or ore occurrences, have been proved. The amount of silver metal exceeds 86000 t with average grade of 100 g/t, which makes Daxing’anling region one of the the most important silver ore belt in China. However, the metallogenic characteristics and metallogenesis need to be clarified. The silver deposits in the study area are classified into three main types, which are magmatic hydrothermal vein type, continental volcano-subvolcanic type and skarn type, respectively. The super large deposits include the Shuangjianzishan deposit (silver metal amount of 15214 t with average grade of 138 g/t), the Baiyinchagandongshan deposit (silver metal amount of 9446 t with average grade of 187 g/t), the Huaobaote deposit (silver metal amount of 6852 t with average grade of 170 g/t), and the Fuxingtun deposit (silver metal amount of 5240 t with average grade of 196 g/t). The silver deposits are mainly distributed in the central and south of the Daxing’anling area, and mainly formed in the Yanshanian period. The silver polymetallic deposits in the Daxinganling area are significantly controlled by regional faults and the junction zone of volcanic rock basins and their margins. The north-east trending deep faults are the most important ore-controlling structures in this area. The distribution of silver polymetallic deposits along the main faults is obvious, and the intersection area of multiple groups of faults often form important mine catchments. The Permian is the most important ore-bearing formation in this area, but some important silver polymetallic deposits occur in Mesozoic volcanic basins or pre-Mesozoic strata. The magmatic rocks related to mineralization are mainly intermediate acidic or acidic intrusions, intermediate acidic lavas, pyroclastic rocks, and small intrusions of ultra-shallow or shallow facies of the Yanshanian Period. The mineralization element combination is mainly determined by the elemental geochemical background of surrounding rocks or source layers. In addition, the type of deposit, the distance from the mineralization center, and the degree of differentiation of ore-forming rock mass are also important influence factors. The article analyzes the prospecting prospects of each silver deposit type in the study area, discusses the relationship between mineralization center and deep prospecting, and proposes that porphyry silver deposits should be paid attention to. In the prospecting and exploration of silver deposits, comprehensive evaluation and multi-target prospecting need to be strengthened because silver can coexist or be associated with a variety of metals.
By the end of 2020, 83 silver deposits (or ore occurrences), including four super-large-scale deposits, nine large large-scale deposits, 33 medium-scale deposits and 37 small-scale deposits or ore occurrences, have been proved. The amount of silver metal exceeds 86000 t with average grade of 100 g/t, which makes Daxing’anling region one of the the most important silver ore belt in China. However, the metallogenic characteristics and metallogenesis need to be clarified. The silver deposits in the study area are classified into three main types, which are magmatic hydrothermal vein type, continental volcano-subvolcanic type and skarn type, respectively. The super large deposits include the Shuangjianzishan deposit (silver metal amount of 15214 t with average grade of 138 g/t), the Baiyinchagandongshan deposit (silver metal amount of 9446 t with average grade of 187 g/t), the Huaobaote deposit (silver metal amount of 6852 t with average grade of 170 g/t), and the Fuxingtun deposit (silver metal amount of 5240 t with average grade of 196 g/t). The silver deposits are mainly distributed in the central and south of the Daxing’anling area, and mainly formed in the Yanshanian period. The silver polymetallic deposits in the Daxinganling area are significantly controlled by regional faults and the junction zone of volcanic rock basins and their margins. The north-east trending deep faults are the most important ore-controlling structures in this area. The distribution of silver polymetallic deposits along the main faults is obvious, and the intersection area of multiple groups of faults often form important mine catchments. The Permian is the most important ore-bearing formation in this area, but some important silver polymetallic deposits occur in Mesozoic volcanic basins or pre-Mesozoic strata. The magmatic rocks related to mineralization are mainly intermediate acidic or acidic intrusions, intermediate acidic lavas, pyroclastic rocks, and small intrusions of ultra-shallow or shallow facies of the Yanshanian Period. The mineralization element combination is mainly determined by the elemental geochemical background of surrounding rocks or source layers. In addition, the type of deposit, the distance from the mineralization center, and the degree of differentiation of ore-forming rock mass are also important influence factors. The article analyzes the prospecting prospects of each silver deposit type in the study area, discusses the relationship between mineralization center and deep prospecting, and proposes that porphyry silver deposits should be paid attention to. In the prospecting and exploration of silver deposits, comprehensive evaluation and multi-target prospecting need to be strengthened because silver can coexist or be associated with a variety of metals.
, Available online , doi: 10.31035/cg2021079
Abstract:
The eastern Central Asian Orogenic Belt (CAOB) in NE China is a key area for investigating continental growth. However, the complexity of its Paleozoic geological history has meant that the tectonic development of this belt is not fully understood. NE China is composed of the Erguna and Jiamusi blocks in the northern and eastern parts and the Xing'an and Songliao–Xilinhot accretionary terranes in the central and southern parts. The Erguna and Jiamusi blocks have Precambrian basements with Siberia and Gondwana affinities, respectively. In contrast, the Xing'an and Songliao–Xilinhot accretionary terranes were formed via subduction and collision processes. These blocks and terranes were separated by the Xinlin–Xiguitu, Heilongjiang, Nenjiang, and Solonker oceans from north to south, and these oceans closed during the Cambrian (ca. 500 Ma), Late Silurian (ca. 420 Ma), early Late Carboniferous (ca. 320 Ma), and Late Permian to Middle Triassic (260–240 Ma), respectively, forming the Xinlin–Xiguitu, Mudanjiang–Yilan, Hegenshan–Heihe, Solonker–Linxi, and Changchun–Yanji suture zones. Two oceanic tectonic cycles took place in the eastern Paleo–Asian Ocean (PAO), namely, the Early Paleozoic cycle involving the Xinlin–Xiguitu and Heilongjiang oceans and the late Paleozoic cycle involving the Nenjiang–Solonker oceans. The Paleozoic tectonic pattern of the eastern CAOB generally shows structural features that trend east–west. The timing of accretion and collision events of the eastern CAOB during the Paleozoic youngs progressively from north to south. The branch ocean basins of the eastern PAO closed from west to east in a scissor-like manner. A bi-directional subduction regime dominated during the narrowing and closure process of the eastern PAO, which led to “soft collision” of tectonic units on each side, forming huge accretionary mountain belts in central Asia.
The eastern Central Asian Orogenic Belt (CAOB) in NE China is a key area for investigating continental growth. However, the complexity of its Paleozoic geological history has meant that the tectonic development of this belt is not fully understood. NE China is composed of the Erguna and Jiamusi blocks in the northern and eastern parts and the Xing'an and Songliao–Xilinhot accretionary terranes in the central and southern parts. The Erguna and Jiamusi blocks have Precambrian basements with Siberia and Gondwana affinities, respectively. In contrast, the Xing'an and Songliao–Xilinhot accretionary terranes were formed via subduction and collision processes. These blocks and terranes were separated by the Xinlin–Xiguitu, Heilongjiang, Nenjiang, and Solonker oceans from north to south, and these oceans closed during the Cambrian (ca. 500 Ma), Late Silurian (ca. 420 Ma), early Late Carboniferous (ca. 320 Ma), and Late Permian to Middle Triassic (260–240 Ma), respectively, forming the Xinlin–Xiguitu, Mudanjiang–Yilan, Hegenshan–Heihe, Solonker–Linxi, and Changchun–Yanji suture zones. Two oceanic tectonic cycles took place in the eastern Paleo–Asian Ocean (PAO), namely, the Early Paleozoic cycle involving the Xinlin–Xiguitu and Heilongjiang oceans and the late Paleozoic cycle involving the Nenjiang–Solonker oceans. The Paleozoic tectonic pattern of the eastern CAOB generally shows structural features that trend east–west. The timing of accretion and collision events of the eastern CAOB during the Paleozoic youngs progressively from north to south. The branch ocean basins of the eastern PAO closed from west to east in a scissor-like manner. A bi-directional subduction regime dominated during the narrowing and closure process of the eastern PAO, which led to “soft collision” of tectonic units on each side, forming huge accretionary mountain belts in central Asia.
, Available online , doi: 10.31035/cg2021067
Abstract:
The Neoproterozoic Sugetbrak Formation in the Aksu area, which is located at the northwest margin of Tarim Block, comprises mafic rocks and provides key records of the evolution of the Rodinia supercontinent. However, the genetic relationship among these mafic rocks exposed in different geographical sections are still unclear. In this study, the petrology, geochemistry, and Sr-Nd-Pb isotope geochemistry of the mafic rocks exposed in the Aksu-Wushi and Yuermeinark areas have been studied in some detail along three sections. The authors found that the mafic rocks in these three typical sections were mainly composed of pyroxene and plagioclase, containing a small amount of Fe-Ti oxides and with typical diabasic textures. All the mafic rocks in this region also showed similar geochemical compositions. They were characterised by high TiO2 contents (1.47%–3.59%) and low MgO (3.52%–7.88%), K2O (0.12%–1.21%). Large ionic lithophile elements (LILEs) (e.g., Rb, Sr, and Cs) were significantly depleted. Meanwhile, high field strength elements (HFSEs) were relatively enriched. In the samples, light rare earth elements (LREEs) were enriched, while heavy rare earth elements (HREEs) were depleted. Based on the Zr/Nb, Nb/Y, and Zr/TiO2 ratios, the Aksu mafic rocks belong to a series of sub-alkaline and alkaline transitional rocks. The mafic rocks along the three typical sections showed similar initial values of 87Sr/86Sr (ISr) (0.7052–0.7097) and εNd(t) (–0.70 to –5.35), while the Pb isotopic compositions with 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of 16.908–17.982, 15.487–15.721, 37.276–38.603, respectively. Most of the samples plot into the area near EM-Ⅰ, indicating that the magma of the mafic rocks might have derived from a relatively enriched mantle with some crustal materials involved. The geochemical element characteristics of most samples showed typical OIB-type geochemical characteristics indicating that the source region had received metasomatism of recycled materials. Combining with the regional geological background and geochemical data, we inferred that the mafic rocks of the Sugetbrak Formation in the Aksu area were formed in an intraplate rift environment. Summarily, based on our study, the mafic rocks of the Sugetbrak Formation in the Aksu area were derived from a common enriched mantle source, and they were product of a magmatic event during the rift development period caused by the breakup of the Rodinia supercontinent.
The Neoproterozoic Sugetbrak Formation in the Aksu area, which is located at the northwest margin of Tarim Block, comprises mafic rocks and provides key records of the evolution of the Rodinia supercontinent. However, the genetic relationship among these mafic rocks exposed in different geographical sections are still unclear. In this study, the petrology, geochemistry, and Sr-Nd-Pb isotope geochemistry of the mafic rocks exposed in the Aksu-Wushi and Yuermeinark areas have been studied in some detail along three sections. The authors found that the mafic rocks in these three typical sections were mainly composed of pyroxene and plagioclase, containing a small amount of Fe-Ti oxides and with typical diabasic textures. All the mafic rocks in this region also showed similar geochemical compositions. They were characterised by high TiO2 contents (1.47%–3.59%) and low MgO (3.52%–7.88%), K2O (0.12%–1.21%). Large ionic lithophile elements (LILEs) (e.g., Rb, Sr, and Cs) were significantly depleted. Meanwhile, high field strength elements (HFSEs) were relatively enriched. In the samples, light rare earth elements (LREEs) were enriched, while heavy rare earth elements (HREEs) were depleted. Based on the Zr/Nb, Nb/Y, and Zr/TiO2 ratios, the Aksu mafic rocks belong to a series of sub-alkaline and alkaline transitional rocks. The mafic rocks along the three typical sections showed similar initial values of 87Sr/86Sr (ISr) (0.7052–0.7097) and εNd(t) (–0.70 to –5.35), while the Pb isotopic compositions with 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of 16.908–17.982, 15.487–15.721, 37.276–38.603, respectively. Most of the samples plot into the area near EM-Ⅰ, indicating that the magma of the mafic rocks might have derived from a relatively enriched mantle with some crustal materials involved. The geochemical element characteristics of most samples showed typical OIB-type geochemical characteristics indicating that the source region had received metasomatism of recycled materials. Combining with the regional geological background and geochemical data, we inferred that the mafic rocks of the Sugetbrak Formation in the Aksu area were formed in an intraplate rift environment. Summarily, based on our study, the mafic rocks of the Sugetbrak Formation in the Aksu area were derived from a common enriched mantle source, and they were product of a magmatic event during the rift development period caused by the breakup of the Rodinia supercontinent.
, Available online , doi: 10.31035/cg2021065
Abstract:
The Jiama porphyry copper deposit in Tibet is one of the proven ultra-large-scale copper deposits in the Qinghai-Tibet Plateau at present, with the reserves of geological resources equivalent to nearly 20×106 t. However, it features wavy and steep terrain, leading to extremely difficult field operation and heavy interference. This study attempts to determine the effects of the tensor controlled-source audio-magnetotellurics (CSAMT) with high-power orthogonal signal sources (also referred to as the high-power tensor CSAMT) when it is applied to the deep geophysical exploration in plateaus with complex terrain and mining areas with strong interference. The test results show that the high current provided by the high-power tensor CSAMT not only greatly improved the signal-to-noise ratio but also guaranteed that effective signals were received in the case of a long transmitter-receiver distance. Meanwhile, the tensor data better described the anisotropy of deep geologic bodies. In addition, the tests also show that when the transmitting current reaches 60 A, it is still guaranteed that strong enough signals can be received in the case of the transmitter-receiver distance of about 25 km, sounding curves show no near field effect, and effective exploration depth can reach 3 km. The 2D inversion results are roughly consistent with drilling results, indicating that the high-power tensor CSAMT can be used to achieve nearly actual characteristics of underground electrical structures. Therefore, this method has great potential for application in deep geophysical exploration in plateaus and mining areas with complex terrain and strong interference, respectively. This study not only serves as important guidance on the prospecting in the Qinghai-Tibet Plateau but also can be used as positive references for deep mineral exploration in other areas.
The Jiama porphyry copper deposit in Tibet is one of the proven ultra-large-scale copper deposits in the Qinghai-Tibet Plateau at present, with the reserves of geological resources equivalent to nearly 20×106 t. However, it features wavy and steep terrain, leading to extremely difficult field operation and heavy interference. This study attempts to determine the effects of the tensor controlled-source audio-magnetotellurics (CSAMT) with high-power orthogonal signal sources (also referred to as the high-power tensor CSAMT) when it is applied to the deep geophysical exploration in plateaus with complex terrain and mining areas with strong interference. The test results show that the high current provided by the high-power tensor CSAMT not only greatly improved the signal-to-noise ratio but also guaranteed that effective signals were received in the case of a long transmitter-receiver distance. Meanwhile, the tensor data better described the anisotropy of deep geologic bodies. In addition, the tests also show that when the transmitting current reaches 60 A, it is still guaranteed that strong enough signals can be received in the case of the transmitter-receiver distance of about 25 km, sounding curves show no near field effect, and effective exploration depth can reach 3 km. The 2D inversion results are roughly consistent with drilling results, indicating that the high-power tensor CSAMT can be used to achieve nearly actual characteristics of underground electrical structures. Therefore, this method has great potential for application in deep geophysical exploration in plateaus and mining areas with complex terrain and strong interference, respectively. This study not only serves as important guidance on the prospecting in the Qinghai-Tibet Plateau but also can be used as positive references for deep mineral exploration in other areas.
, Available online , doi: 10.31035/cg2021033
Abstract:
The A-type granites with highly positive εNd(t) values in the West Junggar, Central Asian Orogenic Belt (CAOB), have long been perceived as a group formed under the same tectonic and geodynamic setting, magmatic source, and petrogenetic model. Geological evidence shows that these granites occurred at two different tectonic units related to the southeastern subduction of Junggar oceanic plate: the Hongshan and Karamay granites emplaced in the southeast of West Junggar in the Baogutu continental arc; whereas the Akebasitao and Miaoergou granites formed in the accretionary prism. Here the authors present new bulk-rock geochemistry and Sr-Nd isotopes, zircon U-Pb ages and Hf-O isotopes data on these granites. The granites in the Baogutu continental arc and accretionary prism contain similar zircon εHf(t) values (+10.9 to +16.2) and bulk-rock geochemical characteristics (e.g., high SiO2 and K2O contents, enriched LILEs (except Sr), depleted Sr, Ta, and Ti, and negative anomalies in Ce and Eu). The Hongshan and Karamay granites in the Baogutu continental arc have older zircon U-Pb ages (range: 315 Ma to 305 Ma) and moderate 18O enrichments (δ18Ozircon=+6.41‰–+7.96‰); whereas the Akebasitao and Miaoergou granites in the accretionary prism have younger zircon U-Pb ages (range: 305 Ma to 301 Ma) with higher 18O enrichments (δ18Ozircon=+8.72‰–+9.89‰). The authors deduce that the elevated 18O enrichments of the Akebasitao and Miaoergou granites were probably inherited from low-temperature altered oceanic crusts. The Akebasitao and Miaoergou granites were originated from partial melting of low-temperature altered oceanic crusts with juvenile oceanic sediments below the accretionary prism. The Hongshan and Karamay granites were mainly derived from partial melting of basaltic juvenile lower crust with mixtures of potentially chemical weathered ancient crustal residues and mantle basaltic melt (induced by hot intruding mantle basaltic magma at the bottom of the Baogutu continental arc). On the other hand, the Miaoergou charnockite might be sourced from a deeper partial melting reservoir under the accretionary prism, consisting of the low-temperature altered oceanic crust, juvenile oceanic sediments, and mantle basaltic melt. These granites could be related to the asthenosphere's counterflow and upwelling, caused by the break-off and delamination of the subducted oceanic plate beneath the accretionary prism Baogutu continental arc in a post-collisional tectonic setting.
The A-type granites with highly positive εNd(t) values in the West Junggar, Central Asian Orogenic Belt (CAOB), have long been perceived as a group formed under the same tectonic and geodynamic setting, magmatic source, and petrogenetic model. Geological evidence shows that these granites occurred at two different tectonic units related to the southeastern subduction of Junggar oceanic plate: the Hongshan and Karamay granites emplaced in the southeast of West Junggar in the Baogutu continental arc; whereas the Akebasitao and Miaoergou granites formed in the accretionary prism. Here the authors present new bulk-rock geochemistry and Sr-Nd isotopes, zircon U-Pb ages and Hf-O isotopes data on these granites. The granites in the Baogutu continental arc and accretionary prism contain similar zircon εHf(t) values (+10.9 to +16.2) and bulk-rock geochemical characteristics (e.g., high SiO2 and K2O contents, enriched LILEs (except Sr), depleted Sr, Ta, and Ti, and negative anomalies in Ce and Eu). The Hongshan and Karamay granites in the Baogutu continental arc have older zircon U-Pb ages (range: 315 Ma to 305 Ma) and moderate 18O enrichments (δ18Ozircon=+6.41‰–+7.96‰); whereas the Akebasitao and Miaoergou granites in the accretionary prism have younger zircon U-Pb ages (range: 305 Ma to 301 Ma) with higher 18O enrichments (δ18Ozircon=+8.72‰–+9.89‰). The authors deduce that the elevated 18O enrichments of the Akebasitao and Miaoergou granites were probably inherited from low-temperature altered oceanic crusts. The Akebasitao and Miaoergou granites were originated from partial melting of low-temperature altered oceanic crusts with juvenile oceanic sediments below the accretionary prism. The Hongshan and Karamay granites were mainly derived from partial melting of basaltic juvenile lower crust with mixtures of potentially chemical weathered ancient crustal residues and mantle basaltic melt (induced by hot intruding mantle basaltic magma at the bottom of the Baogutu continental arc). On the other hand, the Miaoergou charnockite might be sourced from a deeper partial melting reservoir under the accretionary prism, consisting of the low-temperature altered oceanic crust, juvenile oceanic sediments, and mantle basaltic melt. These granites could be related to the asthenosphere's counterflow and upwelling, caused by the break-off and delamination of the subducted oceanic plate beneath the accretionary prism Baogutu continental arc in a post-collisional tectonic setting.
, Available online , doi: 10.31035/cg2021073
Abstract:
Based on the comprehensive study of geology and geophysics in African continent, three types of lithosphere (craton-type, orogenic-type and rift-type) can be identified. Considering lithosphere discontinuities as the boundary, two first-order tectonic units (mainly cratonic-type in the West and rift-type in the East) are proposed. Different types of lithosphere can be divided into secondary-order and third-order structural units, and the blocks within lithosphere can be further divided into fourth-order structural units. The geological history, the formation process and significance of different types of lithosphere in African continent are briefly discussed.
Based on the comprehensive study of geology and geophysics in African continent, three types of lithosphere (craton-type, orogenic-type and rift-type) can be identified. Considering lithosphere discontinuities as the boundary, two first-order tectonic units (mainly cratonic-type in the West and rift-type in the East) are proposed. Different types of lithosphere can be divided into secondary-order and third-order structural units, and the blocks within lithosphere can be further divided into fourth-order structural units. The geological history, the formation process and significance of different types of lithosphere in African continent are briefly discussed.
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