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2024, Volume 30,  Issue 3

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2024, 30(3)
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2024, 30(3): 1-2.
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Tectonic System & Structural Geology
The Early Cretaceous extensional deformation in the southeastern Beishan Range, central Asia: Constrains from 2D seismic reflection profile interpretation and apatite fission track thermochronology
LIU Kui, CHEN Xuanhua, WANG Derun, GU Wenpei, SHAO Zhaogang, ZHANG Yiping
2024, 30(3): 377-393. doi: 10.12090/j.issn.1006-6616.2023151
Abstract (309) HTML (56) PDF (23430KB)(61)
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  Objective  The Beishan Range occupies a key position in Central Asia. This study aims to deepen the understanding of the timing, intracontinental deformation processes, and their dynamic mechanisms in the Late Mesozoic on the southern margin of the Central Asian Orogenic Belt (CAOB).   Methods  We conducted detailed analyses of the Early Cretaceous extensional and earlier compressional structures in the southeastern Beishan Range through field geological observations, interpretation of 2D reflection seismic profiles, and apatite fission track thermochronology.   Conclusion  Field observations show that Lower–Middle Jurassic strata have been strongly deformed by numerous thrusts and folds. 2D seismic reflection profiles reveal two NE- to NEE-striking normal faults. The Suosuojing fault is a SE-dipping low-angle listric normal fault, while the Wudaoming fault is a NW-dipping high-angle normal fault. These normal faults cut through the early-formed fold-thrust system, indicating a transition from contraction to extension. The Suosuojing and Wudaoming faults, respectively, define the northwestern and southeastern boundaries of the Early Cretaceous Zongkouzi basin. The Zongkouzi basin exhibits a graben geometry, with Lower Cretaceous strata displaying typical growth-strata relationships, suggesting that the normal faults were active during the late Early Cretaceous. Thermal history modeling results from apatite fission track data indicate that the footwall of the Suosuojing fault experienced rapid cooling between 132 and 110 Ma. This rapid cooling phase was closely related to the footwall exhumation during the normal slip of the Suosuojing fault. We argue that the Late Mesozoic intracontinental contraction–extension transition in the southeastern Beishan Range likely occurred between ~133 Ma and ~129 Ma in the late Early Cretaceous. The collapse of the thickened crust and coupled mantle upwelling triggered the Early Cretaceous extensional deformation in the southern CAOB.
Crustal Stress & Tectonic Stress Field
Estimation of the rock mechanics and in-situ stress parameters of carbonate reservoirs using array sonic logging: A case study of Shunbei No.4 block
LIU Jun, HUANG Chao, ZHOU Lei, CHEN Qun, ZHANG Shenglong
2024, 30(3): 394-407. doi: 10.12090/j.issn.1006-6616.2023110
Abstract (360) HTML (48) PDF (1685KB)(45)
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  Objective  Geomechanical analysis plays a crucial role in exploring and developing oil and gas reservoirs. However, the study of in-situ stress in the Yijianfang–Yingshan formations of the Middle to Lower Ordovician in the Shunbei oilfield has lagged behind. This paper compares and analyzes the rock mechanics and in-situ stress characteristics of different types of carbonate reservoirs in the Yijianfang-Yingshan formations of the Lower Paleozoic Ordovician in the Shunbei No. 4 block, and explores the control effect of micro-grain structures on the in-situ stress and rock mechanics parameters of carbonate reservoirs. It aims to provide fundamental geological data for the evaluating of ultra-deep carbonate reservoirs in the Shunbei area.  Methods  This study, based on the spring combination model, determined the rock mechanics and in-situ stress characteristics through rock mechanics experiments and array acoustic logging, and characterized the micro-pore structure of the limestone using casting thin sections and X-ray μ-computed tomography analysis.  Results  The results indicate that the Young's modulus of the Yijianfang–Yingshan formations ranges from 50 to 89 GPa, the compressive strength from 99 to 136 MPa, and the Poisson's ratio from 0.25 to 0.32. The maximum horizontal principal stress of the formations ranges from 200 to 225 MPa, while the minimum horizontal principal stress ranges from 125 to 160 MPa.  Conclusion  Significant differences in rock mechanics parameters and in-situ stress exist among different types of carbonate reservoirs in the Yijianfang–Yingshan formations. From Type I to Type III and non-reservoir carbonate rocks, the Young's modulus, compressive strength, and maximum horizontal principal stress increase dramatically, while the Poisson's ratio and minimum horizontal principal stress show little change. Compared to micritic limestone, sandy–bioclastic limestone has larger calcite particles and reduced particle cohesion, resulting in decreased compressive strength and Young's modulus. Some calcite particles appear sub-rounded or rounded with higher three-dimensional particle sphericity, also leading to weaker intergranular engagement and further reduction in compressive strength and Young's modulus. The presence of numerous pores and fractures in sandy-bioclastic limestone facilitates frictional sliding and potential failure along microfractures or particle interfaces, contributing to lower compressive strength and Young's modulus as well. In addition, micritic limestones in the Yijianfang–Yingshan formations are affected by hydrothermal activity, with some dolomite being replaced by quartz, which increases the compressive strength and Young's modulus. The macroscopic mechanical characteristics and rock mechanics parameters of the rock are constrained by the micro-grain structures. The low-value zones of the Young's modulus, compressive strength, and maximum horizontal principal stress in the Yijianfang–Yingshan formations are identified as the advantageous reservoir development zones in the Shunbei No. 4 block.
Energy Resources Geology
Seismic facies and sedimentary distribution characteristics of the Upper Carboniferous in the Ounan Depression, eastern Qaidam Basin
CHEN Cheng, WANG Li, ZHAO Weiyong, TANG Jianchao, WEI Xiaojie, FANG Xinxin, ZHANG Hao
2024, 30(3): 408-418. doi: 10.12090/j.issn.1006-6616.2023077
Abstract (148) HTML (25) PDF (4714KB)(9)
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  Objective  The Carboniferous system of the Ounan Depression in the eastern Qaidam Basin is a key area for oil and gas exploration in new regions and strata. Drilling and outcrops reveal significant differences in the sedimentary characteristics of the Upper Carboniferous strata across the plane. Previous research, mainly based on outcrop and drilling data, has focused on longitudinal sedimentary evolution and the overall sedimentary background, leaving the distribution characteristics of sedimentary facies unclear. This uncertainty affects the deployment of oil and gas exploration in the Upper Carboniferous of the eastern Qaidam Basin.   Methods  The article focuses on the Upper Carboniferous strata of the Ounan Depression. It predicts the distribution characteristics of sedimentary facies under the guidance of sedimentary model theory using a limited amount of drilling data and 2D seismic data. Utilizing 2D seismic data from the Ounan Depression, it also conducts a detailed study of the internal structure and external geometric features of seismic reflections using seismic facies identification technology.   Results  The comprehensive study identifies six types of seismic facies within the Upper Carboniferous of the Ounan Depression: parallel sheet, sub-parallel sheet, mounded, wedge-shaped, oblique foreset, and chaotic. A planar assemblage map of the seismic facies was developed. By integrating regional geological data, outcrops, well logs, and seismic data, and under the guidance of seismic sedimentology and continental shelf sedimentation theory, the relationship between seismic facies and sedimentary facies in the Upper Carboniferous of the Ounan Depression was established, and the distribution characteristics of sedimentary facies in the study area were predicted.   Conclusion  The results indicate significant differences between the seismic facies in the western and eastern regions of the Ounan Depression. Overall, the western part exhibits lower frequency and more continuous phases compared to the eastern region; the western region also has greater monolayer thickness and a more stable sedimentary environment. From east to west, the Ounan Depression develops a sequential fluvial-delta plain sedimentary system, delta front sedimentary system, and prodelta–deep water shelf sedimentary system.   Significance  The research findings provide a scientific basis for predicting favorable reservoir distribution areas and deploying exploration targets for Carboniferous oil and gas exploration in the eastern region of the Qaidam Basin.
Quantitative evaluation of maximum operating pressure and storage capacity for gas-top sandstone reservoir-type gas storage
HU Caiyun, LI Cong, YANG Zhibin, JIA Qian, SUN Yanchun, LI Chaofeng, SUN Junchang, YANG Yuehui, SUN Dongsheng
2024, 30(3): 419-426. doi: 10.12090/j.issn.1006-6616.2023075
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  Objective  The maximum operating pressure for underground gas storage facilities designed for oil and gas reservoirs, both constructed and under construction in China, is currently set at the original formation pressure. There have yet to be successful cases of overpressure operation, which significantly impacts the economic benefits of converting depleted oil and gas reservoirs into underground gas storage facilities. This article aims to evaluate the maximum operating pressure and storage capacity of the Nanpu 1-29 gas storage facility from the perspective of the ultimate bearing capacity of cap layers and faults, with the goal of effectively enhancing the construction benefits of the facility.  Methods  The evaluation of the maximum operating pressure for the Nanpu 1-29 gas storage facility in eastern Hebei is based on the minimum principal stress measured in situ in the mining wells. Different effective porosity calculation methods are employed to quantitatively evaluate the effective storage capacity of gas and oil reservoirs, as well as the incremental capacity after pressure boosting operation, based on their development differences.  Results  The evaluation of the maximum operating pressure for the Nanpu 1-29 gas storage facility indicates that the minimum principal stress of the cap layers determined by the in-situ measurements in the mining wells is 34.00 MPa. Based on the tensile failure criteria determined by the minimum principal stress, the maximum operating pressure for the tensile failure of the cap layer is 27.20 MPa. Combined with the maximum safe injection pressure corresponding to shear failure of the cap layer (30.60 MPa) and the maximum safe injection pressure corresponding to unstable slip of the fault (27.60 MPa), the final maximum operating pressure for the Nanpu 1-29 gas storage facility is determined to be 27.20 MPa. Based on the effective storage capacity calculation model, considering factors such as the water content of the gas reservoir, residual water and edge porosity as well as the coefficient of influence, the efficiency of gas-driven fluid, and the utilization rate of oil-containing space, the maximum operating pressure increased from the original formation pressure of 22.50 MPa to 27.20 MPa. The practical storage capacity of the gas storage facility increased from 15.46×108 m3 to 18.14×108 m3, an increase of approximately 17.3%.  Conclusion  (1) The construction of gas storage facilities can be re-evaluated for the maximum operating pressure based on the minimum principal stress measured in situ in the mining wells, and overpressure design can be conducted under appropriate conditions. (2) Overpressure design can effectively increase storage capacity and improve the economic benefits of reservoir construction.  Significance   The research results have a certain reference value for the quantitative evaluation of the maximum operating pressure and storage capacity of other underground gas storage facilities, and are expected to significantly improve the economic benefits of overpressure-designed reservoir-type gas storage facilities in China.
Ore Field Structure & Mineralization
Discussion on the ore-controlling factors in the Longlin–Xilin Sb–Au mining district of western Guangxi, South China
LIU Xiaohu, WANG Xinyu, XIAO Changhao, ZHANG Wengao, LIU Xiangchong, YU Pingping, MAO Cheng’an, FU Wei
2024, 30(3): 427-442. doi: 10.12090/j.issn.1006-6616.2023120
Abstract (239) HTML (33) PDF (5438KB)(33)
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  Objective  Sb deposits are characterized by simple mineral assemblage. The ore-forming ages, sources of ore-forming materials, and genesis of Sb deposits are controversial owing to the absence of suitable minerals for analysis. Sb resources in the South China Sb metallogenic region account for over 83% of the national total, with the Dian–Qian–Gui Sb belt in the southwest being an significant component of this region.   Methods  Taking the Longlin–Xilin Sb–Au mining district of western Guixi in the central part of the Dian–Qian–Gui Sb belt as an example, this paper systematically summarizes the ore-bearing strata, lithology of ore-bearing wall rocks, ore-bearing structures, and the coexistence relationship of Au and Sb deposits in 86 ore deposits (points) in the area. Combined with the geological characteristics of three typical deposits (Maxiong, Longtan, and Mahao) and the spatiotemporal distribution of Jurassic felsic intrusions, the inherent connection between Sb mineralization and clastic rocks and felsic intrusions was explored.   Results  (1) Statistics and field works show that the most favorable ore-bearing stratum in the Longlin–Xilin mining district is the Lower Devonian Yujian Formation (D1y) , followed by the Lower Triassic Luolou Formation (T1Ll) and the Middle Triassic Banna Formation (T2b). The lithologies most conducive to mineralization are carbonaceous shale, pyrite-rich sandstone, and siltstone. The Sb content in these strata or lithologies is tens or even hundreds of times higher than the crustal abundance, which has the potential for Sb mineralization. (2) Within the NWW–SEE trending Nongsang–Shijiazhai fault zone in the Longlin area, the middle and late Jurassic felsic intrusions, which have consistent spatiotemporal occurrences with Sb and Au deposits, can directly contribute to antimony mineralization (as sources of Sb and S) and indirectly influence it (as a heat source), both favoring the formation of antimony deposits. (3) Statistical results show that Sb, Au, and Sb–Au deposits account for 48%, 46%, and 6% in the Longlin–Xilin district, respectively. This suggests that the ore-forming fluids for Sb and Au in the study area may originate from different sources. We also can not rule out the possibility that Sb and Au deposits derive from the same fluid. In the latter case, the precipitation of stibnite consumes H2S in the ore-forming fluid, destabilizing the Au complex in the solution and resulting in localized Au precipitation. This competition between Sb and Au in the fluid for H2S leads to a negative correlation in the grades of Sb and Au in coexisting deposits. (4) The study area experienced NS-striking compression in the Indosinian period, followed by the NW–SE shortening in the middle–late Jurassic. The intersection of NWW–SEE and NE–SW faults is the favorable ore-bearing space. The NWW–SEE faults displayed strike-slip movement in response to the NW–SE shortening, whereas the NE–SW faults exhibited transpression. Consequently, the NE–SW faults are less conducive to Sb mineralization compared to the NWW–SEE faults. The distribution direction of the NWW–SEE Douhuang–Xilin fault aligns with the axial direction of the main folds in the area, with most fault planes trending northward, displaying horizontal scratches, silicification, and extensional characteristics. The intersection of the Dohuang–Xilin fault and the NE–SW fracture exhibits significant Sb anomalies.  Conclusion  Based on the above studies, the promising areas we propose for Sb prospecting in Longlin–Xilin mining district are (1) Black shale and pyrite-rich siltstones of the Yujiang Formation in the core of the Xinzhou anticline as the key strata; (2) The periphery of the concealed intrusions within the NWW–SEE Nongsang–Shijaizhai fault (Longlin County) and the intersection area of the NWW–SEE Douhuang–Xilin fault and the NE–SW fault as the favorable areas. [ Significance ] The findings provide new insights into the genesis and metallogenic regularities of Sb–Au deposits in the study area, enriching the theoretical understanding of Au mineralization processes.
Geo-hazards & Engineering Geology
Analysis of the development characteristics of co-seismic geological hazards and their controlling factors in the Maerkang MS 6.0 earthquake swarm, Sichuan, on June 10, 2022
SUN Dong, QIN Liang, MENG Minghui, YANG Tao, ZHANG Xu, HU Xiao
2024, 30(3): 443-461. doi: 10.12090/j.issn.1006-6616.2023038
Abstract (324) HTML (64) PDF (12347KB)(26)
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  Objective  This study aims to reveal the distribution patterns and characteristics of co-seismic geological hazards in earthquake swarms, clarify the differences in induced geological hazards by different types of earthquakes, further understand the seismic risks around and within the Bayan Har Block, and provide efficient guidance for the prediction and prevention of secondary geological hazards induced by earthquakes.   Methods  We take the co-seismic geological hazards of the Maerkang MS 6.0 earthquake swarm in 2022 as the research focus. Through systematic data and results analysis on earthquake swarm sequence, regional tectonic environment, regional crustal deformation, and post-earthquake short-term geological hazard, the regional and deep structural environment of the Maerkang MS 6.0 earthquake swarm and the main controlling factors of co-seismic geological hazards are revealed.   Results  The results show that the Maerkang MS 6.0 earthquake swarm is a deep-seated sticky-slip earthquake that occurred on a secondary fault in the active strong earthquake zone within the Bayan Har block, a region with extremely strong peripheral boundary activity. The earthquakes with similar magnitudes may be the result of ruptures of secondary faults of the Songgang fault and the successive ruptures of the barriers between them. The earthquake has resulted in 83 newly discovered geological hazard risks, exacerbating deformation in 106 existing hazard spots and triggering multiple high-altitude landslides and a series of fractured mountain slopes. The areas of extreme, high, and medium geological hazard risk in Caodeng Town, the epicenter area after the earthquake, account for 1.62%, 4.80%, and 12.37%, respectively. The occurrence of secondary geological hazards following earthquakes exhibits a positive correlation with the earthquake magnitude, with the number increasing linearly as the magnitude rises.   Conclusion  The significant differences in the GPS horizontal velocity field and vertical velocity field on both sides of the Darlag–Songgang–Fubianhe fault zone are key factors contributing to the activity of this fault and triggering the recent earthquake.The main controlling factors of co-seismic geological hazards induced by earthquakes are, from primary to secondary, the ruptured fault and its associated faults, seismic magnitude and energy attenuation, terrain slope and altitude difference, and rock mass structure and density of structural surfaces.  Significance  This study predicts a high risk of future strong earthquakes in the intersection area of the Songgang fault, which triggered this earthquake, and the Longriba active fault. The surrounding area of the seismogenic fault and its associated faults are at a high risk of geological hazards during earthquakes. The findings provides a reference for predicting and controlling the risk of co-seismic geological hazards in this area.
Mesostructure and strength characteristics of granite under freeze-thaw cycles based on CT scanning
HOU Shengshan, HE Xiao, MENG Xiansen, CHEN Liang, FENG Zhen, LIU Mingxue, LI Ang, GUO Changbao, JI Feng
2024, 30(3): 462-472. doi: 10.12090/j.issn.1006-6616.2022126
Abstract (282) HTML (45) PDF (2124KB)(21)
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  Objective  With the rapid increase in construction projects in the western regions in recent years, the impact of seasonal freeze-thaw cycles in the high-altitude areas of western China has become more pronounced. Conducting research on the microscopic characteristics and strength degradation properties of rocks under freeze-thaw cycles is crucial for guiding engineering construction in these cold, high-altitude regions.   Methods  To study the influence of freeze-thaw cycles on rock structure and mechanical properties, we collected diorite samples from a tunnel in the Kangding area and examined the effects of freeze-thaw cycles on their microstructure and mechanical characteristics. Firstly, thin rock sections were observed under a polarizing microscope to obtain mineral compositions and microstructures. Then, CT scanning technology was used to scan the granite samples after freeze-thaw cycles, and the scanned layers were binarized using threshold segmentation. The scanning images of different layers were binarized using threshold segmentation, and high-resolution 3D data and images of the internal and external structures of the samples were obtained by stacking the binary image layers. Fractal theory was applied to calculate the box-counting dimension of the images and quantitatively assess their complexity. This analysis allowed us to examine the evolution and distribution characteristics of the internal structure of granite under freeze-thaw cycles.  Results  Under a polarizing microscope, the rock exhibits a block-like structure with a patchy, coarse-grained, and unequal-grained granite texture, with locally visible metasomatic worm structures. The main phenocryst minerals are alkaline feldspar. Other minerals range in size from 0.25 to 4.0 mm and primarily include quartz, plagioclase, and alkaline feldspar. Secondary minerals include biotite and epidote, while accessory minerals comprise apatite, zircon, and pyrite. Microscopically, the rock is identified as porphyritic, coarse-grained, and unequal-grained biotite diorite granite. Freeze-thaw cycles were applied to the granite samples in the laboratory to study the strength evolution and explore the relationship between structural evolution and strength. The results indicate that the freeze-thaw cycle effect leads to an overall increase in the internal porosity of the granite's microstructure, though the rock's permeability changes minimally, with an increase of only 0.003×10−3 μm2. The internal pore development is uneven, primarily due to the emergence of new micropores, causing changes in the overall structure of the sample. After freeze-thaw cycles, the complexity of the internal structure of the rock decreases, but the overall integrity remains good, with the fractal dimension staying at a high level. Fractal analysis shows that 20 freeze-thaw cycles do not cause significant changes in the structural complexity of granite. However, the overall mechanical properties of the sample decline, viscosity increases, and long-term strength shows significant attenuation, raising the strain threshold for entering the creep test stage.   Conclusion  When evaluating the safety of rocks with dense primary structures, considering only their structure may lead to deviations from the actual situation. It is essential to combine necessary strength indicators for a comprehensive evaluation. After undergoing freeze-thaw cycles, rocks tend to exhibit more significant deformation while maintaining lower strength. Therefore, appropriate treatments are required for construction in high-altitude areas. [ Significance ] This study provides a reference for applying fractal theory to the evolution of rock microstructure and the relationship between rock microstructure and strength evolution. It also offers valuable guidance for engineering construction in high-altitude and cold regions.
Study on regional stress background and prevention of the rock burst accident on October 20th, 2018 in the Longyun Coal Industry area, Shandong, China
MENG Jing, ZHANG Peng, WANG Jiming, FENG Chengjun, FAN Yulu, QI Bangshen, SUN Mingqian
2024, 30(3): 473-486. doi: 10.12090/j.issn.1006-6616.2023094
Abstract (301) HTML (34) PDF (4480KB)(28)
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  Objective  The stability of underground chambers such as mine tunnels and transportation tunnels is closely related to the stress environment of the surrounding rock mass and the geological conditions of the area. Analyzing the relationship between deep-seated stress and factors such as the orientation and shape of underground chambers can help to proactively mitigate the risks associated with chamber excavation.   Methods  This study, set against the background of the rock burst accident on October 20th in the Longyun Coal Industry area in Shandong, reveals the current stress environment of the shallow crustal layers in western Shandong through in-situ stress measurement and monitoring work.   Results  According to the characteristics of the current ground stress field near the Longyun coal mining area, the study investigates the regional stress background that led to the rock burst accident and proposes corresponding prevention and control suggestions from the perspective of ground stress. The results indicate that the magnitude of the principal stress generally increases linearly with depth within the measurement range, with the maximum horizontal principal stress ranging from 3.48 to 20.76 MPa and a gradient of 0.0182 MPa/m with increasing depth, while the minimum horizontal principal stress ranges from 3.44 to 14.95 MPa with a gradient of 0.0130 MPa/m. The maximum horizontal principal stress azimuth in the area ranges from NE 43°to 89°, with an average azimuth of NE 75°. The tectonic action in the shallow crust is mainly horizontal, but with increasing depth, they gradually transition to vertical.   Conclusion  The triggering mechanism of the rock burst accident in the Longyun Coal Industry area on 20th October is primarily attributed to the vertical stress exceeding the horizontal principal stress, indicating a current extensional stress environment, especially when the tunnel orientation is parallel to the direction of maximum horizontal principal stress. It is suggested that the angle between the tunnel axis and the direction of maximum horizontal principal stress in the Longyun Coal Industry area should be between 60° and 90°, and that the tunnel roof can be designed as an arch-shaped roof to ensure the stability of the tunnel rock mass.
Fundamental Geology & Regional Geology
Metamorphism and geochronology of the spinel−cordierite granulite in the Mirror Peninsula, East Antarctica
LIU Xinshu, WANG Wei-(RZ), BAO Hong, GONG Tingnan, ZHAN Liqing, LIU Xiaochun, ZHAO Yue
2024, 30(3): 487-505. doi: 10.12090/j.issn.1006-6616.2023172
Abstract (322) HTML (49) PDF (5255KB)(39)
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  Objective  The Prydz Bay belt in East Antarctica recorded two significant tectono-thermal events, the Grenvillian event and the Pan-African event, which are considered to be closely related to the evolution of the Rodinia and Gondwana supercontinents. However, the geological history and the tectonic nature of the two events remain controversial.   Methods  Mineralogical and petrological analyses, phase equilibria modelling and zircon geochronology are combined to investigate the spinel−cordierite granulite from the Mirror Peninsula in order to better understand the tectono-thermal history of the Prydz Bay belt.   Results  The spinel−cordierite granulite contains different stages of mineral assemblages. The major stage of mineral assemblage involves cordierite, spinel, biotite, sillimanite, K-feldspar and minor garnet and ilmenite. The later stage of mineral assemblage is indicated by the emergence of magnetite as the increasing volumes of biotite and cordierite. Minor garnet and corundum are locally preserved, implying the mineral reaction ‘g+cor→sp+sill’ and more garnet and corundum in the peak stage. The garnet grains consist of 70%−72% almandine, 20%−22% pyrope, ~4% grossularite and ~4% spessartine. The XFe (Fe2+/(Fe2++Mg2+)) of representative garnet grains ranges from 0.77 to 0.80. The spinel exhibits an XFe range from 0.80 to 0.86. Different cordierite grains have similar compositions with Al of 3.89−3.93 a.p.f.u (atoms per formula unit) and XFe of 0.32−0.36. Biotite has high TiO2 (4.13%−5.23%) and Ti (0.23−0.30 a.p.f.u). K-feldspar grains consist of 78%−85% orthoclase, 15%−23% albite and ~1% anorthite. Based on the mineral compositions and phase equilibrium modelling, the pressure−temperature (PT) conditions of the major stage of mineral assemblage are constrained to 870−910 °C and 0.64−0.69 GPa, followed by later retrogression to 810−820°C and 0.49−0.53 GPa. A peak stage with higher PT conditions (T>910 ℃, P>0.69 GPa) can be inferred based on the relict peak minerals and characteristic mineral compositions (e.g. Ti in biotite). Zircon grains commonly show core-mantle-rim structures in cathodoluminescence (CL) images. The LA−ICP−MS zircon U−Pb dating analyses reveal a wide age range from 613±7 Ma to 877±9 Ma (except a maximum of 916±11 Ma) for the cores. The zircon bright rims yield a weighted mean age of 526±8 Ma with a wide range of Th/U (0.06−1.23), mostly higher than 0.1.   Conclusion  Based on the results, a few conclusions can be drawn: (1) The spinel−cordierite granulite recorded medium−low pressure/high-ultrahigh temperature metamorphism with a clockwise P−T evolution path and high dT/dP. (2) The results of zircon geochronological analysis show that zircon cores mainly record U−Pb ages in the range of 800~600 Ma, younger than typical ages of Grenvillian events, which may reflect younger inherited zircon cores or significant isotopic resetting. (3) The age of ~530 Ma of zircon rims is interpreted to represent the post-peak cooling stage of the Pan-African tectono-thermal event. [Significance] This study examined the P−T conditions and the zircon ages of the spinel−cordierite granulite in the Mirror Peninsula. In combination with previous results, the P−T−t path constructed for the spinel−cordierite granulite provides new constraints on the evolution of the Prydz Bay belt during the Pan-African period.
Constraints for post-orogenic extension of the northern margin of the Qaidam Basin from the Late Silurian–Late Devonian igneous rocks in the Gahai–Nanshan area
GAO Wanli, WANG Zongxiu, WU Lin, JIANG Wan, QIAN Tao
2024, 30(3): 506-518. doi: 10.12090/j.issn.1006-6616.2023178
Abstract (165) HTML (24) PDF (4411KB)(16)
Abstract:
  Objective  The northern margin tectonic zone of the Qaidam Basin underwent a transition from oceanic subduction to continental subduction during the Early Paleozoic, resulting in the formation of the ultrahigh-pressure metamorphic zone known as the northern Qaidam tectonic zone (NQTZ). There has been a longstanding debate regarding the collapse time of the NQTZ. The Maoniushan Formation has long been regarded as a sign of the end of orogeny; however, recent studies show that the Maoniushan Formation has spanned a long time, and it is controversial when the orogenic belt began to collapse. This study aims to determine the age and genetic background of the Maoniushan Formation and to understand the tectonic transition process of the northern Qinghai–Tibet Plateau from the Proto-Tethys to the Paleo-Tethys.   Methods  Deep-earth samples, including volcanic and intrusive rocks, offer valuable insights into the activity of the crust during this period. In order to investigate the volcaniclastic rocks and granites in the Gahai–Nanshan area, located in the eastern section of the NQTZ, zircon LA–ICP–MS U–Pb chronology and Lu–Hf isotope methods were employed to explore the formation age of igneous rocks and the characteristics of source rocks.   Results  The zircon U–Pb chronology reveals that the volcaniclastic rocks of the Maoniushan Formation originated approximately 423 Ma. Furthermore, the intrusive granite was formed at 370 Ma, indicating that the volcanic eruption occurred during the Late Silurian, while the subsequent intrusion and crystallization of the granites occurred during the Late Devonian. The zircon Lu–Hf isotope data reveals that the εHf(t) values of the Late Silurian tuffs are concentrated within the range of –11.5 to –8.3, and the corresponding two-stage Hf model ages are primarily between 1945–2133 Ma. These results indicate that the volcanic rocks predominantly originated from partially melting ancient crustal materials. In contrast, the εHf(t) values of the Late Devonian intrusive granites exhibit a distribution within the range of 3.9–9.1, accompanied by two-stage Hf model ages primarily falling within the 792–1118 Ma range. The results suggest that the granites mainly resulted from partially melting Meso-Neoproterozoic crustal materials. Based on a comprehensive analysis of regional geological and petrological data, it is postulated that the Late Silurian–Early Devonian witnessed pronounced orogenesis resulting from continental deep subduction. This event led to substantial crustal thickening in the NQTZ, where the thickened crustal basement of the Oulongbruk experienced partial melting, ultimately giving rise to the volcanic rocks observed during this period. During the Late Devonian, delamination of the thickened crust facilitated the upwelling of the asthenosphere mantle, triggering regional crustal extension. The interaction between mantle material and crust results in the formation of granitic–volcanic rocks.   Conclusion  The Maoniushan Formation in the region encapsulates a significant period, making it unsuitable to represent the end of orogeny. Late Devonian igneous rocks indicate that the NQTZ entered a period of substantial crustal extension during this time. [ Significance ] The late Devonian igneous rocks of Maoniushan Formation regionally mark the end of orogeny and the beginning of the Paleo-Tethys tectonic domain.
Geological and evolutionary characteristics of the Gagarin Region on the far side of the Moon
SHAO Tianrui, HAN Kunying, JIN Ming, SHI Chenglong, PANG Jianfeng, DING Xiaozhong
2024, 30(3): 519-534. doi: 10.12090/j.issn.1006-6616.2023035
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Abstract:
  Objective  This study focuses on the Gagarin region on the far side of the Moon, aiming to reveal the geological characteristics, distribution features, and genesis of typical areas on the lunar far side. Additionally, it seeks to explore the regional geological evolution history of the Gagarin region.  Methods  The study primarily employs methods such as multi-source remote sensing data interpretation, regional geological mapping, and quantitative analysis of geological elements' quantity and distribution characteristics.  Results  (1) 656 impact craters were discovered in the study area, of which 552 have diameters greater than 20 kilometers. Approximately 71.5% of the Gagarin region is covered by ancient basins and their ejecta from the Aitken period. Based on comprehensive area and diameter data, the Aitken period is identified as the geological era with the largest proportion of large impact craters (diameter greater than 70 kilometers) and the largest average diameter. From the Aitken period to the Copernican period, the total area of impact craters in each geological era shows a decreasing trend from old to new. (2) In the study area, six parallel lunar grabens, 62 lobate scarps, one sinuous rille , 50 crater floor fractures, and 70 shallow faults were discovered. It also includes parts of the two longest inferred deep faults on the Moon, originating from the South Pole–Aitken tectonic zone and almost spanning the entire highland tectonic zone. According to Bouguer gravity anomalies and crustal thickness data, linear crustal thickness anomalies extending outward from the South Pole–Aitken basin reach the major basins on the near side of the Moon. (3) The Gagarin region is primarily located in the anorthositic highlands on the far side of the Moon. The rocks mainly consist of ferroan anorthosite (fa) suites, with some crater floors showing magnesium anorthosite (ma) suites. In the central and southern parts of the Gagarin region, low-titanium (TiO2 > 1.5 and < 4.5) and very low-titanium (TiO2 < 1.5) basalts are sparsely distributed on the floors of certain impact craters and basins. (4) For this study, we selected impact craters such as Aitken and Van der Graaf, with diameters ranging from 350 to 1400 m, for dating analysis. The results of crater size-frequency distribution dating indicate ages of 3.47 GA and 3.32 GA, respectively. (5) The quantitative statistics of impact craters and the dating results of basalt units indicate that the Aitkenian to Imbrian periods were active periods of external dynamic geological processes in the Gagarin region, while the Imbrian period was an active period of internal dynamic geological processes.   Conclusion  (1) The region’s longest and deepest faults are the result of the combined effects of the South Pole–Aitken impact event and internal and external stresses, including lunar thermal expansion. (2) The variations in the number and size of impact craters in the Gagarin region on the far side of the Moon are related to the evolution of the Earth–Moon system and the solar system. (3) Based on the quantitative statistical results of impact craters and the dating results of basalt units, this study elucidates the regional geological evolution history, and different stages of the geological processes in the Gagarin region were divided according to the active periods and stage characteristics of internal and external dynamic geological processes.   Significance  The study revealed the geological features of key areas on the far side of the moon, delving into the geological history of the Gagarin region and tentatively establishing a correlation between its geological traits and the lunar evolutionary history.