Geoscience

Tectonic Evolution of Songnan-Baodao Sag in Qiongdongnan Basin and Its Relationship with Oil-gas Reservoir

WANG Kai, WANG Genhou, JIA Qingjun, ZHANG Xiao

2023, 37(02): 245-258. DOI: 10.19657/j.geoscience.1000-8527.2022.057

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The Songnan-Baodao depression is an important exploration area with great oil-gas exploration and development potential in the Qiongdongnan Basin (after the Yacheng-Lingshui Depression), and its tectonic evolution, tectonic style and prototype basin restoration are of great significance to understand the oil and gas accumulation regularities. Based on seismic data interpretation, we used 2DMove software to analyze the tectonic evolution period and the spread regularities of tectonic patterns of the Songnan-Baodao depression, and restored the characteristics of Paleogene prototype basins. The results show that the tectonic-sedimentary evolution of the Songnan-Baodao depression can be divided into six stages in three periods (pre-rift, syn-rift and post-rift), and the structural styles comprise three major categories and 14 sub-categories of simultaneous deformation styles, and simultaneous style and multistage style assemblages. Eight dustpan-like fault deposits were recovered in the local Paleogene prototype basins, with sediment thickness of 3,000-6,000 m. Preservation status of the three types of Paleogene prototype basins in the strong, medium and weak transformation areas were identified, and the local erosion thickness exceeds 5,000 m. The oil and gas transport characteristics of the Songnan-Baodao depression are characterized by source proximal, high maturity and late accumulation, and the Songtao bulge, Lingshui low bulge and Songnan bulge at the periphery represent both the oil-gas transport directional zones and effective traps of buried-hill and nappe structure, stratigraphic lithology, and inverted anticline. accumulation match the large-scale hydrocarbon generation period, which are favorable for oil and gas exploration.

Dynamic Model Analysis of Formation and Evolution of the South China Sea

LIU Chen, LI Jianghai, WANG Zhichen

2023, 37(02): 259-269. DOI: 10.19657/j.geoscience.1000-8527.2022.033

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The South China Sea is located in the convergence zone of three tectonic plates. Due to the wide distribution and complex geodynamic background, its genesis and dynamic model are still controversial. Based on geological data, gravity, and magnetic data from the South China Sea, the structural characteristics of the South China Sea are analyzed in plane and profile. Accordingly, numerical simulation shows that the weaker lower crust has decoupled from the crust-mantle, and the rift has migrated to one side. Due to the insufficient magmatic heat supply, the upwelled mantle has cooled and stagnated. The new magma supply may have jumped to the opposite direction and formed a new spreading center. The weak rheological lower crust and high spreading rate may have together controlled the southward ridge migration. We analyze and summarize the existing dynamic models of the South China Sea. Based on tectonic interpretation and numerical simulation, we concluded that the South China Sea formation was mainly affected by three factors: the collision between the India-Australia and Eurasia plates, the subduction drag of the Proto-South China Sea, and the upwelling of the deep mantle. The tectonic stress field in the South China Sea was controlled by the Indo-Australia-Eurasia collision and the subduction and drag of the Proto-South China Sea, and the magma migration was controlled by the deep mantle upwelling.

Deformation Characteristics of Ultra-deep Glide Faults in the Halahatang Area and Their Petroleum Geological Significance

ZUO Liang, NENG Yuan, HUANG Shaoyin, LUO Caiming, CHEN Shi, ZHU Tie, WANG Chuan, LU Chengmei

2023, 37(02): 270-282. DOI: 10.19657/j.geoscience.1000-8527.2022.064

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Strike-slip faults in the Halahatang area control the development of carbonate reservoir and oil-gas enrichment. Influenced by multistage tectonic activities and stratigraphic lithological differences, the spatial structure of strike-slip faults in Halahatang area is diverse and the fault evolution is complex. The control mechanism of structural differences of strike-slip faults on oil and gas enrichment is still controversial. Based on the integrated research of high-precision three-dimensional seismic data and drilling data, the plan deformation pattern and layered and segmented model of strike-slip faults at Halahatang are established in this study. By analyzing the single-well productivity of oil and gas under the control of strike-slip faults, the controlling effect of structural deformation difference of strike-slip faults on oil and gas enrichment is elucidated. The results show that: ①Local strike-slip faults are featured by planar segmentation. The intersection of fault zones leads to the complex spatial fault structure and forms various combination styles. A single fault zone can be divided into three segments: tail, main displacement and overlapping zones, as well as the cutting parts between faults. There are nine plane styles, including feather, horsetail and goose split styles of the tail, and the linear and branching styles of the main displacement zone, The braided and soft-connection types are developed at the overlapping part of the faults, and the intersection and termination types at the cutting part. The deformation characteristics of strike-slip faults are conformable to the Riedel shear model and branch faults are developed around the main faults. The fault development is mainly connected growth, and the cutting part is accompanied by the regulatory deformation caused by successive fault sliding; ②Vertical layering deformation of strike-slip faults controlled the oil-gas migration and accumulation. Fault penetrating across the gypsum layer is the key to the oil and gas ascent. The transformation of Ordovician carbonate rocks controlled the reservoir scale and oil-gas connectivity. Tectonic activities in Silurian clastic rocks influenced the oil-gas filling and preservation; ③The oil-gas enrichment patterns at Halahatang is controlled by the fault pattern. The high-yield wells are mainly distributed in the horsetail and feather trunk branch intersection at the fault zone tail, the braided structural high of the overlapping section, and junctions of the main faults. The prediction results have been verified by ultra-deep wells with remarkable coincidence, and would have guiding significance for the deployment of ultra-deep wells.

Development Characteristics and Evolution Model of F_Ⅰ19 Fault in Fuman Oilfield, Tarim Basin

ZHANG Yintao, CHEN Shi, LIU Qiang, FENG Guang, XIE Zhou, LIANG Xinxin, LI Ting, SONG Xingguo, KANG Pengfei, PENG Zijun

2023, 37(02): 283-295. DOI: 10.19657/j.geoscience.1000-8527.2022.058

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At present, the Fuman oilfield is a hot area for oil and gas development of fault-controlled fracture-cave reservoirs in the Tarim basin. F_Ⅰ19 fault zone belongs to a current key reservoir-increasing area, but the development characteristics and formation mechanism of the faults are unclear. By using various seismic attributes, such as coherence, maximum likelihood and reservoir amplitude change rate, the newly acquired and processed three-dimensional seismic data are accurately interpreted, the development characteristics of fault profile are determined, and the spatial fault structural distribution is characterized. Based on the vertical stratigraphic displacement and the fault zone width, the fault activity intensity is inferred. Combined with the characteristics of fault deformation and the activity patterns of the peripheral orogenic belts, the fault evolution stages are analyzed and the fault development model is established. Superimposing the reservoir development characteristics and the fault distribution patterns, the fault controlling effect on the reservoir is preliminarily analyzed. The study shows that the fault has remarkable plane segmentation characteristics, which can be roughly divided into three segments (north, middle, and south) according to strike variation, and into the lower structural layer (below T∈₃), the middle structural layer (T∈₃-TO₃t) and the upper structural layer (above TO₃t) according to the fault deformation characteristics. The fault formation and evolution are multistage, and the F_Ⅰ19 fault evolution comprises roughly of three stages: Early Caledonian (late Middle Cambrian), Middle Caledonian (Ordovician) and Late Caledonian (Silurian), among which the Middle Caledonian stage is the most active stage. The reservoir plane is mainly distributed in the strike-slip fault zone, with major differences in the reservoir development strength along the F_Ⅰ19 fault strike. It weakens gradually from south to north, which is related to the faulting intensity. Affected by the multistage Middle Caledonian faulting, the Ordovician carbonate reservoir distribution is featured by vertical multi-layer distribution. Under the joint influence of Tabei and Tazhong fault systems, the local maximum principal stress in the Early Caledonian may have deflected counterclockwise from south to north, and gradually transferred from NNE to near NS. Two groups of X-type small-angle intersecting faults were developed, which established the local distribution pattern of F_Ⅰ19 fault.

Layered Deformation Characteristics, Formation Mechanism of Strike-slip Faults on the Western Margin of Mosuowan Uplift,Junggar Basin

TIAN Anqi, CHEN Shi, YU Yixin, XIU Jinlei, JIN Feng

2023, 37(02): 296-306. DOI: 10.19657/j.geoscience.1000-8527.2022.035

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Many strike-slip faults are developed around the Mosuowan uplift in Junggar basin, with complex deformation characteristics. Due to the limit seismic data quality, the time division of fault activity is different, with the formation mechanism unknown. Taking the Zhengshacun area on the western margin of Mosuowan uplift as an example, we studied the geometric morphology and kinematic characteristics of strike-slip faults at Zhengshacun. The results show that the strike-slip faults at Zhengshacun have layered differential deformation, which is mainly divided into three layers. The deep structural layer is Permian, which develops two strike-slip fault groups in near N-S and near E-W directions. The E-W-trending faults cut the nearly N-S sinistral faults, and the section is mainly in negative flower structure style. The middle structural layer of Triassic-Jurassic Badaowan Formation only developed near E-W trending faults, forming horsetail, flat and overlapping structural styles on the plane, and the section developed positive flower structure. The fault strike of Jurassic Sangonghe Formation in the shallow structural layer is near E-W directed, which is linearly distributed on the plane, and the section is dominated by flexural deformation. The faults in the three structural layers are superimposed and developed to form vertical linear, Y-shaped and flower-shaped structural styles, in which the near N-S directed faults are dextral strike-slip and the near E-W directed ones are sinistral strike-slip. Evolution of the Zhengshacun fault are divided into three stages. In the Late Permian, the Junggar block rotates counterclockwise to form a near N-S strike-slip fault, showing a negative flower structure. In the early Jurassic, the NE-SW compressive stress of Tianshan formed a near E-W strike-slip fault, showing a positive flower structure. In the Late Jurassic, a few near E-W faults are inherited and developed.

Cenozoic Tectonic Difference and Its Formation Mechanism of the Tiantai Slope in the Xihu Sag

QI Peng, GUO Gang, CUI Min, WANG Xin, LI Feng, LI Linzhi

2023, 37(02): 307-315. DOI: 10.19657/j.geoscience.1000-8527.2022.081

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The Tiantai slope is located in the southern section of the western slope of the Xihu Sag. After 35 years of exploration, only four wells have been drilled and no commercial discoveries have been made, and the poorly-known geological conditions have restricted the local oil and gas exploration. Based on three-dimensional (3D) seismic data interpretation and comprehensive geological analysis, the Cenozoic tectonic style of the different sections of the Tiantai slope, and its geometrical features and formation mechanism are systematically summarized. From north to south, the Tiantai slope is characterized by a complex slope in the northern section (controlled by two-way faults), a complex gentle slope in the middle section (controlled by antithetic faults), and a simple gentle slope in the southern section (controlled by a synthetic fault). The regional structural differences of the Tiantai slope are closely related to its Cenozoic tectonic evolution. From Paleocene to Eocene, the differential extension may have controlled the different degrees of fault development and structural styles in the northern, middle and southern sections of the Tiantai slope. A special arc-shaped fault development zone may have formed between the northern and middle sections.

Characteristics of Fault System and Its Influence on Hydrocarbon Accumulation in Gaoyou Sag, Subei Basin

TANG Xu, YU Yixin, YU Wenquan, TANG Haiqing, WANG Xiaoyan

2023, 37(02): 316-327. DOI: 10.19657/j.geoscience.1000-8527.2022.039

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The Gaoyou sag in Subei Basin is rich in oil and gas resources. The types and distribution of discovered oil and gas reservoirs are closely related to the fault development characteristics and evolution. In this paper, the basic characteristics, types, and active periods of faults in the Gaoyou sag are studied with high-precision 3D continuous seismic data, and the fault systems are further divided to elucidate the controlling effect of fault systems on hydrocarbon accumulation. The results show that there are four types of fault systems in the sag: Late Cretaceous-Paleocene normal fault system, Eocene Dainan normal fault system, Eocene Sanduo transtensional fault system and long-term active fault system. There are three periods of intensive fault activity: the Late Cretaceous-Paleocene dispersed deformation under the NW-SE extensional stress field, forming the ENE-NE extensional faults. The concentrated deformation occurred in the NW-SE extensional stress field in the Eocene and formed the NE-directed extensional faults. During the Eocene Sanduo stage, extensional deformation occurred under the near N-S extensional stress field, forming late tensional faults near EW direction. Long-term active faults are characterized by multiphase active faults. The long-term active fault system carries oil and gas vertically. The normal fault systems of Upper Cretaceous-Paleocene and Eocene Dainan formation have good lateral sealing conditions, and facilitated the formation of various fault trap types. The Eocene Sanduo normal fault system may have destroyed cap traps and was not conducive to oil and gas preservation. The rolling anticline trap and transverse anticline in the hanging-wall of the long-term active fault in the deep depression zone. The fault block trap in the footwall of the fault between the sub-points of the long-term active fault in the fault step zone, and the tilted fault block trap in the eastern slope of the slope zone are favorable targets for oil-gas migration and accumulation.

Structural Geometric and Kinematic Characteristics of Grein Depression in the West African Rift System

PANG Sichen, LI Chuanxin, MAO Fengjun, WANG Tao, YUAN Shengqiang, HE Songgao, SHI Chenyi

2023, 37(02): 328-339. DOI: 10.19657/j.geoscience.1000-8527.2022.085

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The West African rift system is made up of a series of Mesozoic-Cenozoic rift basins, and the Grein depression is located at the northern end of the system. Elucidating the structural geometry and kinematic characteristics of the depression is of great significance in understanding the evolution of the West African rift system.Based on the latest seismic data and the fine-scale interpretation of regional seismic profiles,the structural geometry and kinematic processes are revealed in the Grein depression. We also discussed the dynamic mechanism in combination with regional background analysis. The conclusions are as follows: (1)The Grein depression is a half-garden with gentle slope, which is characterized with fault in the east and over lapping in the west. There are fault terraces in the north and subsidence in the south;(2)The Cretaceous-Quaternary regional tectonic evolution includes two rifting phases, which can be divided into 5 stages from the Early Cretaceous,i.e.Early Cretaceous rifting(I), Late Cretaceous depression(II), End Cretaceous structural inversion(III), Paleogene rifting(IV) and Neogene and Quaternary depression(V); (3)In the Santonian (Late Cretaceous), the African-Arabian and Eurasian plates began to converge, which resulted in the Santonian and Maastrichtian intensive tectonic events in the African plate. The Maastrichtian tectonic event was clear in the Grein depression. During this period, the depression underwent tectonic inversion and the northwestern side was uplifted, resulting in the denudation of the Upper Cretaceous strata.

Discovery and Geological Significance of Bayanengger Active Fault in the Southern Hetao Basin

LI Chenglu, ZHANG Xujiao, WU Fadong, YE Peisheng, LU Jing

2023, 37(02): 340-352. DOI: 10.19657/j.geoscience.1000-8527.2022.082

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The study of active faults in the southern margin of Hetao Basin can reveal the structural and geomorphological evolution of the northwestern margin of Ordos Block. Recently, we found a 15 km long ENE-trending active fault in the eastern Dengkou County, Inner Mongolia. Displacement of channels and geomorphic features suggest that the Bayanengger Fault (BEF) is a normal fault with dextral strike-slip component. The kinematic characteristics and active periods of the BEF have been identified through field work, surface wave seismic exploration, and OSL dating. The results indicate that the Yellow River tributaries were displaced dextrally, with an average horizontal displacement of 179 m since the Late Pleistocene. The normal motion with dextral strike-slip component between (118.98±20.96) ka and (76.97±13.5) ka may have formed fault scarps on the planation surface of the Ordos Plateau and high terraces of the Yellow River. The latest event of the BEF occurred in (38.20±2.3) -30 ka, which was dominated by vertical offset. The latest geomorphic surface faulted by the BEF is T3 of the Yellow River tributary. The slip rate was 0.337 mm/a. The BEF activity likely caused the exposure of the Oligocene Wulanbulage Formation on the planation surface of Ordos Plateau. It also led to the rapid incision of T3 in the southwest margin of Hetao Basin, which played an important role in the regional geomorphological evolution.

Geochemical Characteristics,Tectonic Setting, and Mineralization of Magmatic Rocks in Gudui Area, Southern Tibet

LOU Yuanlin, CHENG Ming, TANG Yao, ZHANG Chaoming, LAN Jingzhou, YUAN Yongsheng, YANG Tao

2023, 37(02): 353-374. DOI: 10.19657/j.geoscience.1000-8527.2022.062

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Many Au-Sb-Pb-Zn polymetallic deposits are developed in Gudui area of southern Tibet, which is located in the North Himalayan metallogenic belt. Magmatism in southern Tibet was likely intense in response to the India-Asia collision, and Mesozoic-Cenozoic magmatic rocks are widely exposed in Gudui area. Based on field investigation, we focused on the Indosinian-Yanshanian and Himalayan tectono-magmatic activities, and described the spatial distribution characteristics and age of local magmatic rocks. We analyzed the characteristics of major, trace, and rare earth elements of typical intrusive and volcanic rocks, and discussed the tectonic setting and petrogenesis of these magmatic rocks. We concluded that the magmatism at Gudui area provided the initial heat source and drove the transport of ore-forming material. Upwelling of the mantle-derived hydrothermal fluids and the intrusion of the intermediate-felsic sub-volcanic rocks may have caused dynamic metasomatism and concentrated various ore-forming elements into the ore fluid, and then migrated and precipitated them along near EW-trending major faults and near NS-trending secondary faults to form the ore deposits. This study provides a new idea for exploring the relationship between the tectono-magmatic evolution and mineralization at Gudui area, which may also be applicable for the whole southern Tibet. Moreover, we provide fundamental geological data to establish new and accurate metallogenic models and ore prospecting patterns for the region.

Genesis and Geological Significance of Late Cretaceous Intermediate Intrusions in Sangye, Tibet

DENG Ke, WANG Jingui, DONG Yujie, HE Linwu, YUAN Renhua, ZHANG Zeguo, CHEN Shouguan, XIN Tang

2023, 37(02): 375-389. DOI: 10.19657/j.geoscience.1000-8527.2022.027

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The Gangdese magmatic belt is a huge tectonic-magmatic belt, which is nearly E-W directed along the Yarlung-Zangbo suture zone. Mesozoic magmatism in the southern Gangdese experienced four stages: 205-152 Ma, 109-80 Ma, 65-41 Ma and 33-13 Ma, all of which were the products of a compression environment. However, the triggering mechanism has been controversial. Focus of controversy lies in the subduction mode of the Neotethys and the Bangong-Nujiang Ocean. In order to explain the genesis and tectonic environment of Mesozoic intermediate-acid magmatic rocks in the southern margin of Gangdese belt, we analyzed the quartz monzonite in Sangye area (from Zhanang County, Tibet) and discussed the magmatic origin, petrogenesis and tectonic environment through geochronology, whole-rock geochemistry and Lu-Hf isotopes. LA-ICP-MS zircon U-Pb dating shows that the intrusion was formed in the Late Cretaceous (91-88 Ma). The rocks are metaluminous and high-K calc-alkaline. They are enriched in LILEs and LREEs, depleted in HFSEs and HREEs, with no obvious negative Eu anomaly. ε_Hf(t) value is +10.6 to +14.2. Quartz monzonite magma was likely formed in the transition zone between the plagioclase and garnet stability fields. The magma may have experienced the fractionation of amphibole, apatite and biotite. The typical trace element ratios indicate that the magma was originated from partial melting of the young subducted oceanic crust and the addition of minor juvenile mantle-derived materials, which is I-type granite. We inferred that it was formed in the subduction-related island arc setting, indicating that the Late Cretaceous southern margin of Gangdese belt was influenced by the north-dipping subduction of the Neotethys.

Geochemistry, Petrogenesis and Tectonic Setting of the Middle Jurassic Taxi Plutons in Heilongjiang

YANG Wenpeng, LI Chenglu, YANG Yuanjiang, FU Anzong, ZHENG Bo, ZHOU Tengfei, ZHAO Ruijun

2023, 37(02): 390-403. DOI: 10.19657/j.geoscience.1000-8527.2021.158

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To enrich the understanding on the Mesozoic tectonic setting of the eastern Xingmeng orogenic belt,a study of the petrology,geochronology and geochemistry of the quartz monzonite,quartz monzodiorite and quartz syenite of Taxi plutons in the Hengenshan-Nenjiang-Heihe tectonic belt were reported.LA-ICP-MS zircon U-Pb dating yielded Middle Jurassic age (167.2±1.3 Ma) for the quartz monzonite. Intrusive rocks in Taxi can be divided into high-Sr and low-Sr ones: the early quartz monzodiorite is high-Sr, while the late quartz monzonite and quartz syenite are low-Sr. Our results indicate that the high-Sr and low-Sr intrusive rocks have rather different features of major element, trace element and rare earth element. All these data suggest that they show transition features from I-type to A-type granites, and should be defined as I- and A-type composite pluton. The emplacement was likely related to the subduction and closure of the Mongol-Okhotsk Ocean and the Farallon plate beneath Eurasia and Mesozoic intracontinental extension. The emplacement occurred under the tectonic background from crustal compression/thickening to extension, causing asthenospheric mantle upwelling and the mantle-derived basaltic magma underplating. This may have triggered partial melting of the lower crust and mantle, forming granitic magma. The presence of composite pluton is confirmed at Taxi, indicating that the crust in the region may have transitioned from compression and thickening to extension in the Middle Jurassic, which constrains the closure of the eastern Mongol-Okhotsk Ocean.

Geochronology and Petrogeochemical Characteristics of I-type Granites in Xinlitun Area, Northern Greater Hinggan Mountains and Their Geological Significance

ZHANG Guobin, KONG Jingui, WU Zijie, FENG Yue, HE Yunlong, CHEN Xingkai

2023, 37(02): 404-418. DOI: 10.19657/j.geoscience.1000-8527.2022.092

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There are voluminous Phanerozoic granites in the Greater Hinggan Mountains. In this paper, zircon U-Pb geochronology and geochemistry of the Xinlitun granites in the northern Greater Hinggan Mountains have been studied. We compared the geochemical characteristics of these Late Mesozoic granites, and discussed their formation age, genesis and geological significance. Zircon LA-ICP-MS U-Pb dating indicates that the medium-fine-grained monzogranite and medium-fine-grained granodiorite were both formed in the Early Jurassic, at (187.2±2.3) Ma and (192.4±1.6) Ma, respectively. Whole-rock geochemical results show that these granites are Si-rich, high in alkalis (K-rich), and contain low Mg, P, Ti, and ^TFeO/MgO. The rocks are enriched in light rare earth elements and large ion lithophile elements (e.g., Rb, Ba, Th, K), but depleted in heavy rare earth elements and high-field strength elements (e.g., Ta, Nb, P, Ti). No distinct negative Eu anomalies have been identified. The contents of P₂O₅ and Al₂O₃ decrease with increasing SiO₂. Collectively, we suggested that these granites are moderately differentiated I-type. The petrological and geochemical features indicate that their primary magmas were originated from a mixed crust-mantle source. Considering the regional tectonic background, we suggested that these Early Jurassic granites were formed in a syn-collision volcanic arc setting, closely related to the southward subduction of Mongolia-Okhotsk Ocean.

Geochemistry and Tectonic Significance of the Weijing Pluton in Airgin Sum Area, Inner Mongolia

LIU Jinbao, ZHU Luoting, LI Longxue, HOU Qingye

2023, 37(02): 419-432. DOI: 10.19657/j.geoscience.1000-8527.2021.148

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Airgin Sum area is located in north of the Paleozoic Solonker-Xar Moron suture zone, and is a key region for studying the Xing-Meng orogenic belt. Many previous studies were focused on the Paleozoic tectonic evolution, magmatic rocks and sedimentary rocks. However, few studies were dedicated to the Early Mesozoic magmatic rocks. In this study, the Weijing pluton was selected for detailed petrological, geochemical and geochronological analyses. The results show that the Weijing moyite was formed in the Early Cretaceous (144.7 Ma). The moyite is subalkaline metaluminous, high-K calc alkaline, rich in Si and alkali, relatively rich in Fe but poor in Mg, Ca, and Al, showing A-type granite geochemical characteristics. The low (⁸⁷Sr/⁸⁶Sr)_ibut, high ε_Nd(t) and ε_Hf(t) values of the Weijing moyite indicate that it was derived mainly from the juvenile accreted crust from the depleted mantle. The Weijing moyite was likely formed in an intraplate extensional environment.

Mesoarchean Crustal Accretion of the Yangtze Block: Evidence from the Granodioritic Gneiss in the Southern Huangling Anticline

WANG Qing, JIANG Xingfu

2023, 37(02): 433-442. DOI: 10.19657/j.geoscience.1000-8527.2021.051

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Granodioritic gneiss in the southern Huangling anticline of the Yangtze block is an important component of the Mesoarchean tonalite-trondhjemite-granodiorite (TTG) gneisses of the block. Geochemical results show that the granodioritic gneiss possesses high contents of Al₂O₃, Na₂O, Sr, and low values of Rb and Ta, which is classified as the high-Al TTG rock series. Moreover, geochemical features of the gneiss are consistent with adakite affinity. In combination with the enrichment of the HFSE (Nb, Ta, and Hf) and depletion of LILE (Th, Pb, and U) on the primitive mantle normalized diagrams of the granodioritic gneiss, indicating that the gneiss could be generated from partial melting of the basaltic oceanic crust during subduction process. Based on the documented 2.9-3.0 Ga TTG gneisses and arc-affinity amphibolite in the northern Huangling anticline, it is proposed that the 2.9-3.0 Ga TTG gneisses in the Huangling anticline represent an event of continental crustal growth of the Yangtze block by horizontal crustal accretion of the micro-continent or small-scale plate tectonics during the subduction process.

LA-ICP-MS Monazite U(Th)-Pb Dating and Geological Applications

ZHANG Hongyu, YANG Liming, SU Li, SONG Shuguang, WANG Dachuan

2023, 37(02): 443-462. DOI: 10.19657/j.geoscience.1000-8527.2022.065

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Monazite [(LREE,Th)PO₄], widespread in most geological samples, has high Th and low common lead contents, and can thus be used for U(Th)-Pb dating. In this paper, monazite LA-ICP-MS U(Th)-Pb age determination method with monazite (44069) as external standard was established, using laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) of the Element Geochemistry Laboratory at the China University of Geosciences (Beijing). Monazite from the Bohemian Massif granulite were selected to analyze, aiming to address the mass fractionation of monazite U(Th)-Pb isotope system under different conditions, such as the integration time, denudation conditions (point denudation and line scanning), beam spot diameter (25 μm and 12 μm) and parameters of data processing software. Isotopic ratio and age results show that the analytical accuracy is the best under 25 μm vertical and 25 μm line scanning mode, and that the mass fractionation of Th-Pb isotope system is slightly higher than that of U-Pb isotope system. The mass fractionation between Th-Pb and U-Pb system of monazite, however, is significantly smaller than that of zircon. We suggest that the monazite ²⁰⁸Pb/²³²Th age is consistent with the ²⁰⁶Pb/²³⁸U age within error, which can be used as a new way to date geological bodies with high-U zircon. Based on the age results of monazite and zircon from metamorphic rocks of the Qingshuiquan granulite belt in East Kunlun, it is revealed that monazite geochronology can effectively constrain the metamorphic age, especially in sedimentary-metamorphic rocks that lack zircon. LA-ICP-MS monazite U(Th)-Pb dating method will be widely used in determining regional metamorphism, sedimentary diagenesis and S-type granite intrusion.

Thermoelectric Characteristics of Pyrite from the Fengning Ag Polymetallic Deposit, Hebei Province and Its Implications

LI Yujie, LI Shengrong, YUAN Maowen

2023, 37(02): 463-474. DOI: 10.19657/j.geoscience.1000-8527.2022.063

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The Fengning Ag polymetallic deposit is one of the most important Ag polymetallic deposits located in northern Hebei metallogenic area, north margin of North China Craton. In order to better explore the genesis of the deposit and make predictions for deep and peripheral mineralization of the mining area, the thermoelectric typomorphism of pyrite in different mineralization stages of the Niujuan Ag (Au) mining area of the Fengning Ag polymetallic deposit was studied. The results show that the pyrite thermoelectric coefficient ranges from 26-367 μV·℃^-1, mainly in the range of 200-330 μV·℃^-1. The pyrite thermoelectric conductivity type is all P type, and the average value of thermoelectric coefficient is 267 μV·℃^-1. The gradient of thermoelectric coefficient is 0.74 μV·℃^-1·m^-1 in the vertical space of about 80 m from 1,190 m depth to 1,110 m depth. In summary, only the shallow ore body has been disclosed in the Niujuan silver (gold) mining area, and the deep ore body has not been disclosed, and the erosion of the ore body is shallow, so there is a great prospect for deep ore prospecting. In addition, the pyrite formation temperature ranges from 147 ℃ to 281 ℃, mainly concentrated in 180 ℃ to 280 ℃, which is basically consistent with the variation range of fluid inclusion homogenization temperature measurement results, indicating that this deposit is a medium-low temperature deposit.

Comparision of Heavy Mineral Provenance Analysis Methods Based on SEM

CAO Yulu, ZENG Yuke, ZHANG Yuanyuan

2023, 37(02): 475-485. DOI: 10.19657/j.geoscience.1000-8527.2022.032

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Heavy minerals inherit the characteristics of source rocks and can reveal the source-sink connection between the sedimentary basin and the source area. With the development of scanning electron microscope and automatic mineral identification technology, the identification of heavy minerals has gradually changed from manual microscope identification to automatic quantitative analysis. In this paper, on the basis of traditional microscope identification, scanning electron microscopy-energy dispersive spectrometer/backscatter electron (SEM-EDS/BSE) and Tescan Integrated Mineral Analyzer (TIMA) are used to identify and count heavy minerals. We also compared and summarized the characteristics and applicability of different analysis methods. Besides, the Mesozoic provenance area in the northwestern margin of Junggar Basin is preliminarily discussed. The results show that both analysis methods based on SEM can determine and count various heavy minerals well: SEM-EDS/BSE can be more flexible in the statistics of altered minerals; TIMA can quickly and accurately obtain a variety of minerals information, but caution should be exercised in the statistics of altered minerals. According to the heavy mineral assemblage, ZTR index, and previous U-Pb age and paleocurrent data, it is believed that the Mesozoic provenance in the northwestern margin of Junggar Basin mainly came from the Zaire Mountain, and the Halaalat Mountain uplifted and began to provide provenance for Karamay and Bailishan areas during the Late Triassic to Early Jurassic.

Gemological and Chemical Composition Characteristics of Basalt-Hosted Rubies from Thailand

LI Enqi, ZHANG Yufei, XU Bo

2023, 37(02): 486-499. DOI: 10.19657/j.geoscience.1000-8527.2021.143

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We analyze the basalt-hosted ruby from Thailand to explore the gemological, chemical and spectral characteristics of ruby, by using gemological conventional instruments, EPMA, LA-ICP-MS, UV-Vis spectrophotometer, FTIR, Raman spectrometer and other testing methods. We then compare our data with the ruby from Myanmar and Mozambique, and identify the characteristics of different deposit origins. The results show that the Thai rubies have dark color, and they are mostly of medium-weak red fluorescence under UV-long wave and inert under UV-short wave. Internal inclusions are plentiful, with various forms of solid and fluid inclusions and twin-crystals. The rubies contain mainly Al₂O₃, with trace elements including Cr, Fe, Ti, Mg, V, Ga, Si, and Ni, and are characterized by high Fe, high Mg and low Ga contents. The UV-Vis absorption spectral data show Cr and Fe, and the Cr-related fluorescence peak is weak. The infrared spectral test result shows ruby. There are six vibration peaks in Raman spectrum, which display high coincidence with the standard corundum Raman spectrum. The ruby origin can be identified by the ruby appearance, fluorescence, solid inclusions, and fluid inclusions. Contents of Cr, Fe, Mg, Ga, V, and Ti have obvious origin inheritance. The UV-Vis spectra of rubies from different origins have slightly different absorption peaks.

Trilobites from the Cambrian Upper Changhia Formation in Northern Anhui

LEI Qianping

2023, 37(02): 500-511. DOI: 10.19657/j.geoscience.1000-8527.2021.146

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This study is the first comprehensive description of the trilobites from the upper part of the Changhia Formation (Cambrian, Miaolingian, Guzhangian) at Fenghuangshan in Xiaoxian (Anhui Province). The samples include 15 genera, 12 species, and 4 indet. species, of which 4 genera, 3 species and 1 indet. species belong to Agnostida, 2 genera and 2 species belong to Ptychopariida, 9 genera, and 7 species and 3 indet. species belong to Lichida. The separated cranidia, pygidia and librigena are well preserved in the limestone. The upper Changhia Formation in northern Anhui is equivalent to the Kushan Formation of Shandong Province, with several same trilobite species. Comparing with Shandong and other regions, the dominant trilobites of the Changhia Formation of the studied area are Pseudoblackwelderia lubrica and Wongia triangulata. The review of the genus Pseudoblackwelderia is also revised.

Characteristics and Assessment of Geoheritage of the Proposed Hualong National Geopark in Qinghai Province

WANG Chongge, LI Junlei, ZHANG Xujiao, YUAN Xiaoning, ZHANG Xiangge, WANG Yifan, WANG Kaiya, LIU Xinlan, RAO Haoshu, LIU Jiang, HOU Engang

2023, 37(02): 512-528. DOI: 10.19657/j.geoscience.1000-8527.2023.047

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Establishing national geopark is an effective way to protect geoheritage as well as promote ecological protection and high-quality development of the Yellow River Basin. Also, investigation and assessment of geoheritage are significant to the design and construction of geopark. Hualong county is located at the northeastern margin of Qinghai-Tibetan Plateau, and has rich Yellow River cultural relics since the river flows through the county for 168 km. But systematic research is still needed to verify if the geoheritage distributed in the county can meet the requirements of geopark. Based on the geoheritage investigation, this paper quantitatively assesses the value of geoheritage distributed in Hualong county by analytic hierarchy process (AHP),and the boundary of the proposed Hualong National Geopark is confirmed. 40 geoheritages are found in the geopark, covering 6 large categories, 9 categories and 20 sub-categories, including 2 world-class, 6 national-class geoheritages. Among them, Early Paleozoic suture zone on Mount Lajishan, as well as angular unconformity between the Xining Group and Hualong Complex, have significant scientific and global correlation value on the evolution of the Proto-Tethyan Ocean and the uplift of the Qinghai-Tibetan Plateau, separately. The result shows that Hualong county is well of various, characteristic, significant-value and well-preserved geoheritages, which have the resource natural endowment for establishing National Geopark and the potential to be listed as a UNESCO Global Geopark. This paper provides not only scientific evidence for the application and construction of Hualong National Geopark, but also references for creating the Yellow River Cultural Tourism Belt.

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