Earth Science Frontiers

Crustal-scale plate interactions beneath the dominant domain in the India-Eurasia collision zone—a tectonogeophysical study

GUO Xiaoyu, LUO Xucong, GAO Rui, XU Xiao, LU Zhanwu, HUANG Xingfu, LI Wenhui, LI Chunsen

2023, 30(2): 1-17.
DOI: 10.13745/j.esf.sf.2022.11.7

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Since the Early-Cenozoic onset of the Indian plate subduction beneath the Eurasian plate along the Yarlung-Zangbo suture zone (YZSZ), the subduction process has gained wide interests among geologists. However, crustal-scale vertical interactions between the two plates beneath the dominant domain in the collision zone remains unclear owing partly to the lack of high-resolution datasets, which has severely limited the understanding of the crustal thickening mechanism of the dominant collision domain and its deep geodynamic processes. In this study, fine structural interpretation of two deep seismic reflection profiles—180 km and 100 km long cutting through the middle and eastern part of the YZSZ, respectively—revealed the crustal-scale lateral and vertical contact relationships between the subducting Indian plate and the overriding Lhasa terrane. (1) Laterally, the Indian lower crust subducts northward, with limited subduction front beneath the southern margin of the Lhasa terrane (SLT) which is shown as non-reflective crust, while the Central Lhasa terrane (CLT) is north-dipping. (2) Vertically, the Indian lower crust undergoes subduction, while crustal duplexing occurs in the middle-upper crust. Nearly three quarters of the SLT crust are non-reflective crust, while the rest, the SLT upper crust, is south-dipping. The CLT crust can be divided into two domains: north-dipping lower crust and concave-downward upper crust. Differential vertical zonation is observed in all three tectonic units. (3) The upper crust of the dominant collision domain has a consistent deformation pattern, where a sequence of break-backward imbricate structures is present. This break-backward imbricate system can be traced from the Luobadui-Mila fault of the northern edge of SLT, beyond YZSZ, to the northern edge of the North Himalaya dome belt. Combing with the previous findings based on coincident magnetotelluric data on the southward migration of high conductive barrier of SLT thrusting along the main Himalayan into the northern Himalayas, we believe the episodic magmatism in the Tethyan Domain beneath SLT generated juvenile crust that is prone to anomalous thickening. Meanwhile, during India-Eurasia plate interaction, mantle-sourced magmatism in SLT—generated from northward subduction of the Neo-Tethyan oceanic slab and subsequent collision between the India and Eurasia plates—caused southward thermal migration, which induced anatexis in the northern Himalayas and weakened the crustal strength of the region. The ongoing crustal-scale duplexing therefore leads to antiformal stacking and causes crustal thickening. Rapid exhumation of the North Himalayan dome by the increasing antiformal stacking, meanwhile, exerts sudden northward compression to the overlying Tethyan Himalayan sequence, which eventually creates fault-propagation folds following a break-backward sequence in the upper crust through the whole dominant collision domain. Overall, vertical and lateral tectonic interactions within the dominant collision domain in the India-Eurasia collision zone played an important role in producing such anomalous thick crust, but the break-backward imbricates system in the upper crust lowered topographic relief in the dominant collision domain as well.

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The uplift and exhumation processes in the Qiangtang terrane of Central Tibet since the Cretaceous

BI Wenjun, ZHANG Jiawei, LI Yalin, DENG Yuzhen

2023, 30(2): 18-34.
DOI: 10.13745/j.esf.sf.2022.11.50

Abstract ( 396 )

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The Tibetan Plateau uplift resulted in regional and global climate changes; however, the geomorphic evolution of the Tibetan Plateau is still in dispute. As a significant part of the Tibetan Plateau, the Qiangtang terrane plays an important role in understanding the geomorphic evolution of Central Tibet. The published structural deformation and low-temperature thermochronological data showed that the uplift and exhumation of the Qiangtang terrane experienced three main phases: Early Cretaceous-Paleocene (120-65 Ma), Eocene (55-35 Ma), and Post-Oligocene (<30-0 Ma). During the Early Cretaceous-Paleocene, the Lhasa-Qiangtang collision and the tectonic load of the Central Qiangtang terrane led to outward-propagating deformation and exhumation originated from the Central Qiangtang terrane. During the Eocene, northward and southward subductions of the Lhasa and Songpan-Ganzi terranes, respectively, driven by continued convergence between the Indian and Asian plates, resulted in intensive deformation and exhumation of the Southern Qiangtang terrane and the northern edge of the North Qiangtang terrane, but little erosion and deformation in the Central Qiangtang terrane and central zone of the North Qiangtang terrane. The Qiangtang terrane had attained an elevation of 3-4 km by ~35 Ma. Since the Oligocene structural deformation in the Qiangtang terrane has ceased, where surface exhumation may be related to N-S trending normal fault activity in the region.

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Late-Triassic stratigraphic redefinition of and structural deformation in the Tethys Himalayan Belt in Gyaca area, Tibet

TANG Yu, WANG Genhou, HAN Fanglin, LI Dian, LIANG Xiao, FENG Yipeng, ZHANG Li, WANG Zhuosheng, HAN Ning

2023, 30(2): 35-56.
DOI: 10.13745/j.esf.sf.2022.8.55

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The Late-Triassic middle-low metamorphosed rock assemblage exposed in the Gyaca area of the northeastern Tethyan Himalaya, southern Tibet, including the Langjexue Group, had been regarded as a set of abyssal-bathyal flysch strata. This study reveals the Upper-Triassic strata are bounded by a steep shear zone (SSB) where obvious differences in composition, structural deformation and metamorphism between north and south are observed. The northern SSB is characterized by accretionary complex with a “matrix + blocks” structure, with little fossil preserved to reveal its primary sedimentary structure. The “blocks” are mainly composed of meta-sandstone, limestone, marble, green-schist, basaltic schist, metasdiabase, actinolite schist and garnet mica schist, while the “matrix” consists of sandy slate and sericite quartz phyllite (or phyllonite). Strata of the southern SSB chiefly consists of medium-fine grained felsic sandstones, mudstone and epimetamorphic slate. Sandstone layers display flute and groove casts, load casts, parallel lamination and graded bedding, while siltstone or mudstone beds preserve plant stem fossils and fodichnia. Structural analysis shows the Gyaca accretionary complex undergoes two stages of deformation (D₁ and D₂). D₁ is caused by bedding shearing in the top-to-south direction, which results in intensive penetrative foliation (S₁) and new-born felsic quartzose veins; and D₂ is characterized by middle-high dip angle with southward inclination-replaced foliation (S₂), consistent with north-south contraction deformation with top-to-south movement accompanied by syntectonic intermediate-basic veins. Meanwhile, under north-south compression stress, strata on the south side of SSB develop imbricated fold-thrust structure. Zircon U-Pb isotopic chronology reveals the “matrix” of the Gyaca accretionary complex and the southern Langjiexue strata are Latin to Norian (242-220 Ma), while the “blocks” include Late Triassic, Late Jurassic (146 Ma) and Early Cretaceous (144 Ma), showing multi-age characteristics. Besides, the timing of D₂ is constrained to be 56 Ma by ages of syntectonic dikes, representing early collision tectonics in the Early Eocene, which further, indirectly, indicates D₁ might have occurred during initial to early collision of India-Asia, i.e., in the Paleocene-Eocene. Research on deformation structures and tectonic evolution suggests the two stages of deformation in the Gyaca accretionary complex occur in responses, respectively, to subduction-accretion and early collision between the passive continental margin of India and Asian active margin. That is, the accretionary complex is formed by subduction of the passive continental margin of India, and the fold-thrust structure is induced by contraction of India and Asia. The positive flower-like ‘Gyaca accretionary complex-Langjiexue fold-thrust zone’ structure might represent thickening of the India crustal and uplift of the Himalaya orogenic belt. The development of south-south-east (SEE) lineations within SSB indicates its strike-slip tectonic characteristics, which might suggest diagonally downward collision between India and Asia in the Late Eocene.

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Moho geometry in the eastern North Himalayan tectonic belt: An example of the receiver function 3DCCP method

LI Chunsen, XU Xiao, XIANG Bo, GUO Xiaoyu, WU You, WU Jiajie, LUO Xucong, YU Jiahao, TONG Xiaofei, YUAN Zizhao, LIN Yanqi

2023, 30(2): 57-67.
DOI: 10.13745/j.esf.sf.2022.8.56

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The India-Eurasia collision zone remains uncertain with respect to deep geological contacts beneath the eastern North Himalayan tectonic belt. In order to confirm the specific Moho interface beneath the dominant collision zone, we obtained high-resolution 3D Moho geometry in the region based on data generated by the deployed short-period dense array and previously published broadband station, by tele-seismic P-wave 3DCCP stack method using the improved Moho picking algorithm. Together with previous 2DCCP profiles, tomography and magnetotelluric profiles, we obtain the following results: (1) the Moho depth increases from ~60 km beneath the Great Himalayas to ~70-75 km beneath the dominant collision zone (YZSZ, the Yarlung-Zangbo suture zone). (2) An 120 km-long east-west-oriented depth gradient of the Moho interface exists at about 28.9°N to the south of YZSZ, where appears opposite Moho dip direction constituting the depth gradient. (3) The Moho interface dipping to the north represents the subducting Indian crust and indicates no further extension of the Indian crust beyond YZSZ to the north. In a broader context, this Moho depth gradient to the south of YZSZ in the eastern North Himalayas is resulted from subduction resistance by the juvenile southern Lhasa terrane and the contemporaneous clockwise rotation of the Indian continent that is pulled by the ongoing subduction of the Indian oceanic crust to the east of the Eastern Himalayan Syntax.

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What forces are driving the Indian subcontinent to drift northward?

LIANG Guanghe, YANG Weiran

2023, 30(2): 68-80.
DOI: 10.13745/j.esf.sf.2022.11.5

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It is widely recognized that the Indian continental plate splits and drifts from Gondwana in the southern hemisphere to its current location, but the driving force behind such movement has been under debate ever since the theory of continental drift was put forward. Quantitative estimation of the driving force may help to resolve the issue. We collected two deep reflection seismic exploration profiles in the passive continental margin basin area of the southern Indian subcontinent. We interpreted the data structurally, estimated the dip angle of Moho surface in detail, and obtained the magnitude of crustal gravity slip shear force which was used to explain the dynamic mechanism of the Indian plate movement. The results show that the Indian continental plate can produce enough gravity slip force on the inclining interface formed by mantle upwelling to drive the Indian subcontinent to drift northward. Hence, a “mantle upwelling and gravity slip” dual-drive continental-drift model is proposed. That is, continental plate can drift by relying on continuous mantle thermal upwelling and gravity slip force. This model can reasonably explain the continental fragments in the Indian Ocean and the genetic mechanism of left rotation in the northward drift of the Indian continent. The gravity-slip driving mechanism provides a new dynamic model for plate motion and more accurate constraints for understanding the driving force behind plate motion.

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Distribution characteristics and formation mechanism of sediment waves on the continental margin of the northern South China Sea—a literature research

LIU Xiaolei, LI Weijia, LU Yang, LI Xingyu, ZHANG Shuyu, YU Heyu

2023, 30(2): 81-95.
DOI: 10.13745/j.esf.sf.2022.2.67

Abstract ( 442 )

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Studying the formation process of deep-sea sediment waves is of great significance for the reconstruction of paleoclimate, paleocean and paleotectonics, and it provides strong support for the development of marine oil and gas hydrate resources. The northern continental margin of the South China Sea is a focal area for such study because of its complex tectonic sedimentary system, unique topography and rich mineral resources. This paper systematically summarizes the recent progress in the relevant studies. Combing the results on the key regions of sediment-wave development in the South China Sea, this paper analyzes the development depth, slope, wavelength, wave height and wave peak characteristics of sediment waves from the sea areas of southeastern Hainan to southwestern Taiwan, and discusses the genetic mechanism and main controlling factors of sediment waves in different regions. It is found the Taiwan orogeny and the deep water of the North Pacific have a strong impact on the sediment waves in the southwestern Taiwan sea area, while the Pearl River Estuary sea area is mostly influenced by internal waves. It is also found the sediment waves in the South China Sea provide superior geological condition for gas hydrate accumulation. But most of the existing studies only consider a single influencing factor and lack of knowledge of the development status of sediment waves in the southern South China Sea, as in-situ seabed observation methods are also lacking. Therefore, future research should pay attention to multi-factor dynamic processes in sediment-wave formation, deepen the research on sediment-wave development in the southern South China Sea, and strengthen the research on seabed geophysical exploration technology and in-situ comprehensive long-term observation technology, so as to provide a scientific basis for in-depth understanding of the formation and evolutionary process of deep-sea sediment waves and enhance the safety of marine development projects.

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A dissolution porosity increase model for sandstone reservoir in the Yanchang Formation in central and southern Ordos Basin—model building and model applications

LIU Zhen, ZHU Maolin, PAN Gaofeng, XIA Lu, LU Chaojin, LIU Mingjie, LIU Jingjing, HOU Yingjie

2023, 30(2): 96-108.
DOI: 10.13745/j.esf.sf.2022.2.73

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Dissolution as a constructive diagenetic process plays an important role in expanding reservoir storage capacity and improving reservoir quality of sandstone reservoirs. However, in the existing studies, the influence of dissolution on sandstone porosity is mostly characterized from a qualitative perspective, whilst the amount of dissolution porosity increase in sandstone reservoirs and the effect of dissolution on reservoir quality improvement are not quantitatively characterized. In this paper, based on the mud log and wireline logging data, combined with sequence stratigraphy, burial history and thermal history data, the porosity characteristics and genesis of sandstone of the Yanchang Formation in central and southern Ordos Basin are analyzed by using the basic principle of chemical reaction dynamics, and a dissolution porosity increase model for sandstone is established. Using this model porosity evolution of the Yanchang sandstone is investigated via simulation. The main results are: (1) The present porosity profile for the Yanchang sandstone can be divided into two parts: “normal trend” section and “dissolution porosity increase” section; and, according to the initial horizon and morphological characteristics of secondary pore formation, the sandstone porosity profile can be divided into four types: bimodal I and II, and unimodal I and II. (2) The present Yanchang sandstone mainly experiences two types of porosity changes: porosity decrease in shallow or deep strata by mechanical compaction or by compaction and cementation, and porosity increase by organic acid-induced dissolution of feldspar and other minerals. (3) The dissolution porosity increase model calculates dissolution porosity increase as follows: ① Before the sandstone acidizing condition is reached in the strata, the amount of dissolution porosity increase is zero. ② As the strata condition moves inside the window of acidification, the accumulative dissolution porosity increase is a function of the present secondary porosity increase and time. ③ As the strata continue to be deeply buried post acidification, dissolution porosity does not change. That is, the model defines a piecewise function to calculate dissolution porosity increase. (4) Porosity evolution modeling allows quantitative characterization of dissolution porosity increase in the Yanchang sandstone. It reveals secondary porosity increases of 5.6%, 3% and 6.5%, respectively, in three sandstone reservoirs in the study area, namely, the Chang-8 sandstone of well Honghe-1, Zhenjing area in the south; the Chang-6 sandstone of well Dan-40, Ansai area in the east; and the Chang-4+5 sandstone of well Feng-12, Jiyuan area in the north. The model established in this paper can be used to quantitatively evaluate the amount of secondary porosity increase due to dissolution within the window of acidification. And for the first time quantitative simulation of sandstone dissolution and dissolution porosity increase in sandstone is demonstrated, which is of great significance for quantifying the effect of dissolution on improving reservoir quality of sandstone reservoirs.

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Deep-time sea-level change curve recovery: A recovery method based on stratigraphic inverse modeling

SHEN Luyin, PAN Renfang, DUAN Taizhong, LIU Yanfeng, LI Meng, LIAN Peiqing, HUANG Yuan, ZHANG Demin

2023, 30(2): 109-121.
DOI: 10.13745/j.esf.sf.2022.2.64

Abstract ( 227 )

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The commonly used sea-level curve recovery methods are mainly based on the cyclic analysis and isotopic measurement of stratigraphic sedimentation. These methods are greatly affected negatively by the incompleteness of stratigraphic data, resulting in unreasonable recovery results. In view of this, a global sea-level curve recovery method based on stratigraphic sedimentary inversion is proposed. Based on the fact that the global sea-level cycle has good corresponding relationships with astronomical and sedimentary cycles as multi-scale, multi-level cycles, this method uses trigonometric Fourier series expansion to transform the sea-level change curve, which can greatly smooth out the sea-level curve in stratigraphic sedimentary inversion. Only seven parameters are needed in the new method to characterize sea-level changes in the Haq sea-level curve, demonstrating its applicability. The global sea-level curve of the Changxing Period in the Yuanba area is obtained by stratigraphic sedimentary simulation, which yields more information on the sea-level change, and the recovered sea-level change is more reasonable compared to the Haq sea-level curve and the sea-level change curve of Fischer for the same period. Thus, Fourier series-functionalized sea-level change curve is useful not only in quantification and functionalization of sea-level change, but also in stratigraphic sedimentary inversion to obtain sea-level change curve. Compared with the traditional sea-level change recovery method, this new idea of measuring sea-level change has higher level of quantification, lower requirements for input data, and better applicability and wider application prospect. The application prospect is also discussed.

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Sequence stratigraphic framework and sedimentary evolution of the Cretaceous in southern Iraq

HAN Haiying, GUO Rui, WANG Jun, QIN Guosheng, SUN Xiaowei, YU Yichang, SU Haiyang, GAO Yang

2023, 30(2): 122-138.
DOI: 10.13745/j.esf.sf.2022.2.61

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The Cretaceous is an important exploration and development target in southern Iraq. However, a lack of systematic sequence stratigraphic study among major oilfields has greatly restricted fine planning for the oilfield development and production. Therefore, it has great theoretical and practical significance to investigate the sequence stratigraphy and sedimentary systems in the study area. Here, guided by sequence stratigraphy and sedimentology theory, and based on the core data, mud logs, well logs and seismic data, the sequence boundaries are identified and sequences are delineated; meanwhile the evolutionary characteristics of the sedimentary system are analyzed. According to the results, the Cretaceous in southern Iraq is mainly composed of skeletal limestone, and the sequence interface types include unconformity, exposed surface, lithology abrupt surface and sedimentary facies transition surface types. The Cretaceous is divided into 2 first-level sequences (AP8, AP9), 6 second-level sequences (SS1-SS6) and 22 third-level sequences (SQ1-SQ22); the duration of each 3^rd sequence varies from 2-4 Ma. In the Lower Cretaceous, SS1 develops, from south to north, ramp-to-basin sedimentary system, and sediments evolve upward from basinal mudstone to inner ramp grainstone; sediments of the lower SS2 gradually evolve, SW-NE, from delta sandstone to deep-water shelf carbonates, while sediments of the upper part evolve from basinal mudstone to shallow-water shelf carbonates; and in SS3, sediments of the lower part evolve, SW-NE, from braided river coarse sandstone to shallow sea fine sandstone, while marine carbonate rock develops in the upper part. In the Upper Cretaceous, SS4 develops, from west to east, upward shallowing depositional successions of deep shelf-shallow water carbonate ramp-platform sedimentary systems; while fine-grained carbonate ramp sedimentary system, dominated by planktonic foraminifera limestone, develops in SS5; and in SS6, shallowing upward carbonate ramp sedimentary system develops in three stages, and shelf dolomite develops on the uppermost part. Carbonate ramp shoals, platform marginal shoals and two-stage sandstone deposits are the most favorable exploration and development targets in this area. The tectonic movement plays an important role in controlling the development and distribution of the inner ramps and marginal reefs and shoals.

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Tectono-depositional environment and prototype basin evolution in the Ordos Basin during the Early Permian

ZHAI Yonghe, HE Dengfa, KAI Baize

2023, 30(2): 139-153.
DOI: 10.13745/j.esf.sf.2022.2.70

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Major breakthroughs have been made in tight-gas sandstone exploration in the Lower Permian of the Ordos Basin with the discovery of a series of large tight-gas fields in recent years. Studying the tectono-depositional environment and prototype basins of this period is fundamental to understanding the evolutionary process of the Ordos Basin area, and is helpful for advancing oil and gas exploration in this region. Using the latest borehole and outcrop data and combining previous research results, we reconstructed the regional tectono-depositional environment in the Early Permian. Furthermore, we explored the properties and evolution of the prototype basin.The tectono-depositional environment of the Early Permian is characterized by active continental margins, impacted by subduction of the Paleo-Asian Ocean in the northern margin and Mianlue Ocean in the southern margin. The inherited Taiyuan Formation developed on the basis of the paleo-terrain of the Late Carboniferous, and the base plane indicates an upward paleogeography in this period dominated by a broad tidal flat and lagoon environment, with small tide-dominated delta formed in the north and south of the basin. The Shanxi period was the key period of sea-land transition in the Ordos Basin as seawater quickly retreated from the southeastern part of the basin. During the early depositional stage, the Shanxi Formation, influenced by regional compression, formed sea-land transitional system dominated by delta system, with residual tidal flat-open platform in the eastern part of the basin; in the late stage, base plane first rose and then fell, and the residual marine facies receded southward. After deposition of the Shanxi Formation, continental sedimentation began. The basin was characterized by stable subsidence in the Early Permian, with stable cratonic properties. Thus the Early Permian basin had the characteristics of an intra-cratonic depression. Peripheral tectonic environment, differential subsidence of lake basins, and development of paleo-uplift/depression had a significant impact on the evolution of sedimentary filling during this period in controlling the expansion of the deposition center to the east of the basin.

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Reconstruction of the Yangtze, Australian and Indian plates in the Late Neoproterozoic (750-540 Ma) using paleomagnetic constraints

LIU Leixin, LI Jianghai, MA Changming

2023, 30(2): 154-162.
DOI: 10.13745/j.esf.sf.2021.11.3

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As an important part of the Rodinia supercontinent in the Neoproterozoic, the position of the Yangtze Plate has always been a hot research topic. In order to discuss the position of the Yangtze Plate in the Late Neoproterozoic (750-540 Ma), the relative positions of the Yangtze, Australian and Indian plates in the Late Neoproterozoic (750-540 Ma) are studied by combining the previously published paleomagnetic data and using the paleomagnetic research method. Based on the stratigraphic correlation, zircon dating and other evidences, the Yangtze Plate is placed in the present-day northern margin of the Indian Plate and the northwestern margin of the Australian Plate. Based on the analysis of the kinematic characteristics of Yangtze, India and Australia, it is considered that the Yangtze Plate moves to the high latitude area during the breakup of the Rodinia supercontinent. During the period of 750-635 Ma, the Yangtze Plate is in the middle-high latitude areas, and it starts to move rapidly toward the low latitude area at 635 Ma. Although Yangtze is placed on the northern rim of India, the two plates are not thought to be connected, until after their collision at around 570 Ma and both join Gondwana.

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Discovery of Early Cretaceous metamorphic basic rock and plagioclase amphibolite in Yalaxiangbo, Tibet and its geological significance

ZHAO Xiaoyan, YANG Zhusen, YANG Yang, CAO Yu, FAN Jianbiao, ZHAO Miao

2023, 30(2): 163-182.
DOI: 10.13745/j.esf.sf.2022.2.63

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Cretaceous metamorphic basic rocks have been discovered for the first time in the Yalaxiangbo area of the Tethyan Himalayan belt. In order to study their petrogenesis, magmatic source and metamorphism process, this paper reports the petrogeochemistry, zircon U-Pb geochronology and electron microprobe composition of Yalaxiangbo metamorphic basic rocks and plagioclase amphibolite. The metamorphic basic rocks have a zircon U-Pb isochron age of (127.97±0.47) Ma, indicating their formation in the early Cretaceous. They can be divided into two types: weak deformation (Group 1) and strong deformation (Group 2). Group 1 basic rocks have low contents of SiO₂ (41.82%-48.23%), Na₂O (0.48%-3.06%) and K₂O (0.05%-0.12%), and relatively high contents of Al₂O₃ (11.31%-13.88%), CaO (8.87%-15.36%) and MgO (4.62%-7.47%). Compared to Group 1, Group 2 rocks have significantly higher Al₂O₃ (19.65%-22.46%) and lower MgO (3.26%-3.5%) contents. In plagioclase amphibolite, SiO₂ content ranges from 47.41% to 48.53%, while the contents of Na₂O (3.41%-3.84%) and K₂O (0.56%-0.6%) are low, and the contents of Al₂O₃ (13.98%-14.24%), CaO (10.88%-11.40%) and MgO (6.12%-6.15%) are relatively high. The metamorphic basic rock and plagioclase amphibolite belong to basalts with the characteristics of mid-ocean ridge basalt (MORB). Group 1 rocks and plagioclase amphibolite show gentle REE patterns, which are similar to normal mid-ocean ridge basalts (N-MORB), while Group 2 rocks are characterized by enriched mid-ocean ridge basalts (E-MORB). The contribution of crustal components of metamorphic basic rocks is very small, while plagioclase amphibolite shows obvious contamination of crustal materials. The Yalaxiangbo metamorphic basics represent a retrograde metamorphic evolutionary process through stages of garnet and epidote-amphibolite facies metamorphism. Metamorphic basic rocks of Yalaxiangbo, with both N-MORB and E-MORB characteristics, can be well correlated with the known basic rocks of the Comei giant igneous province in terms of their geochemical properties, which suggest they share similar magmatic source and geodynamic background and formed in the extensional setting related to the splitting of great India from Australia controlled by mantle plume activity.

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Genetic mineralogy of Late Cretaceous intermediate intrusive rocks in the eastern segment of the Gangdese Belt, southern Tibet—construction of a trans-crustal magma system

WANG Wenlu, LI Xiaowei, ZHANG Zeming, TIAN Zuolin, LI Zengsheng, SUN Yuqin, LIU Qiang, DING Huixia, HAO Zhaoge

2023, 30(2): 183-214.
DOI: 10.13745/j.esf.sf.2022.2.83

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The trans-crustal magmatic system provides a new perspective for understanding the genesis of mineral and inter-mineral disequilibrium before consolidation. The rock-forming minerals may have experienced growth, erosion or fluid transformation in the magma chamber at different crustal levels. How to trace and sort out the evolutionary history of different rock-forming minerals has become a key scientific issue for understanding the genesis of magmatic rocks. We focus on the clinopyroxene-bearing quartz diorite and biotite diorite in the Lilong area on the southern margin of the Gangdese arc in Tibet to conduct a systematic morphological study of the main rock-forming minerals (clinopyroxene, amphibole, biotite, plagioclase, etc.), and to constrain the key elements of the trans-crustal magmatic system. According to the total REE (ΣREE) content of each mineral and the contact relationships between different minerals, clinopyroxene and amphibole in clinopyroxene-bearing quartz diorite are divided into type Ⅰ clinopyroxene (high ΣREE), type Ⅱ clinopyroxene (low ΣREE), type ⅠA amphibole (high ΣREE) and type ⅠB amphibole (low ΣREE). Amphibole in biotite diorite is classified as type Ⅱ amphibole. Thermobarometric and oxybarometric calculations yield the following results: type Ⅰ clinopyroxene: temperature 1159-1175 ℃, pressure 6.37-7.99 kbar, water content 2.5%-3.4%; type Ⅱ clinopyroxene: temperature 1180-1181 ℃, pressure 4.82-5.24 kbar, water content 2.4%-2.8%; Type ⅠA amphibole: temperature 806-854 ℃, pressure 3.20-4.34 kbar, water content 7.5%-7.9%; type ⅠB amphibole: temperature 776-848 ℃, pressure 1.96-3.86 kbar, water content 6.9%-8.6%; type Ⅱ amphibole: temperature 783-857 ℃, pressure 2.46-3.27 kbar, water content 6.5%-9.4%. The above results suggest that clinopyroxene and amphibole originate from different magmatic chambers, as the former has a higher temperature and pressure but lower water content than the latter. Based on the compositions of mineral melts at equilibria, it is inferred that clinopyroxene is derived from evolved basaltic-andesite magma, while amphibole melts, with lower Mg^# value, from rhyolitic magmas. In this study, the “antecrysts” of clinopyroxene and amphibole are identified, which suggest that the magmatic process was completed in an open magmatic system, and experienced different stages of magmatic recharge or mixing events. This study also finds that intermediate magmas can be produced via mixing of antecrysts of different sources with andesitic to rhyolitic melts.

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Origin and gold mineralization significance of Late Triassic syn-tectonic dykes in the Qingchengzi area, Liaodong Peninsula

DONG Xiaoyu, KONG Ruoyan, YAN Danping, QIU Liang, QIU Junting

2023, 30(2): 215-238.
DOI: 10.13745/j.esf.sf.2022.1.21

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The Qingchengzi area is an important gold ore concentration area in the Liaodong Peninsula where Mesozoic magmatism played a key role in gold mineralization. The widely developed Late Triassic granitic dykes, which are closely related to gold veins, are emplaced mainly along the NE-/EW-trending faults around 224-217 Ma and subsequently experienced extensional event. These dykes, as revealed by geochemical analysis, have high Sr (362.00×10^-6-1296.85×10^-6) and low Y (4.73×10^-6-12.30×10^-6) and Yb (0.49×10^-6-1.05×10^-6) contents and high Sr/Y ratios (38-118), showing the characteristics of high-Sr, low-Yb granitoid, while the structural discrimination diagram indicates they are likely syn-collisional granites. Sr-Nd isotopic composition shows that these dykes have high (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7078-0.7109) and low ε_Nd(t) values (-16.34--15.30) and T_DM2 ages of 2323-2237 Ma, indicating they are derived from the melting of thickened lower crust. Meanwhile geochemical analysis indicates a neighboring rock mass, Dixiongshan pluton (211 Ma), is formed in a post-collision extensional environment. Thus we suggest that these dykes might be formed during the tectonic transition from compression to extension in the late stage of continent-continent collision between the North and South China Blocks. In the Qingchengzi area, Late Triassic dykes and gold veins are closely related in emplacement time, space and genesis as they commonly develop in parallel in similar tectonic settings, and some dykes are also gold veins. Multi-stage gold mineralization in the Qingchengzi area includes the main mineralization period, resulted by the Late Triassic compressional tectonics, and the re-enrichment period, resulted by the Early Cretaceous tectono-magmatism. Therefore the widely developed Late Triassic dykes in the Qingchengzi area may be an important indicator for gold deposit prospecting.

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Discovery of volcanic rocks in the Pingchau Formation in Tungpingchau, Hong Kong UNESCO Global Geopark: Zircon U-Pb geochronology, geochemistry and geological implications

WANG Lulin, LIU Xiaohong, ZHANG Zhiguang

2023, 30(2): 239-258.
DOI: 10.13745/j.esf.sf.2022.2.87

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The age of the Pingchau Formation in Hong Kong UNESCO Global Geopark has been controversial due to lack of geochronological data. In this study a new volcanic interbed was discovered in the lower part of the middle section of the Pingchau Formation, and identified as rhyolitic vitric tuff by thin section analysis. To better understand the petrology and genesis of Pingchau volcanic rocks a detailed geochronology and geochemical study was conducted through zircon U-Pb dating and major and trace element analysis. LA-ICP-MS zircon U-Pb dating of zircons from two rock samples yielded Early Cretaceous age. These rock samples have high total alkalis (w(Na₂O+K₂O)) content (5.58%-9.45%), and show obvious light rare earth element fractionation; they are enriched in light rare earth elements and depleted in heavy rare earth elements, with appearance of Eu anomalies. The rock samples are enriched in large ionic lithophile elements (e.g., Rb, Ba), high field strength elements (e.g., Th, U, Pb, Zr), extremely depleted in Sr, and relatively depleted in Ta and Nb. Combined with previous studies on Late Mesozoic volcanic belt in southeastern China, we propose that volcanic rocks of the Pingchau Formation were likely originated from partial melting of crustal rocks in back-arc extensional setting induced by subduction of the Paleo-Pacific Plate, and formed by late-stage Early Cretaceous volcanic activity in Hong Kong in association with large-scale lithospheric extension, thinning and magmatism along the southeast coast of China. The petrogeochemical and isotopic chronology data in this paper are the latest on the Pingchau Formation, and our results provide valuable new information for better understanding Early Cretaceous volcanism and tectonic evolution in southeastern China.

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Emplacement and episodic denudation of basement granites from the southern Jiergalangtu Sag, Erlian Basin and its tectonic implications

GUO Zhixin, YANG Yongtai, REN Yi, WANG Zhengqing, FENG Zhigang, CHEN Liang, TANG Zhenping

2023, 30(2): 259-271.
DOI: 10.13745/j.esf.sf.2022.2.58

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The Erlian Basin is one of the most important oil, gas, coal, and uranium-bearing basins in northern China, and basement granites of the Erlian Basin are gaining importance as uranium source and petroleum reservoir. However, the emplacement time and denudation history of the basement granites are poorly studied. Here, based on newly obtained zircon U-Pb geochronological data and zircon/apatite fission track evidence, the diagenetic age and uplift history of basement granites of the southern Jiergalangtu Sag, Erlian Basin are analyzed. The diagenetic age is constrained to the Early Jurassic (ca. 175 Ma), and two cooling events are recognized. The two cooling events are refined to the Late Jurassic (ca. 154 Ma and ca. 146 Ma) and late Early Cretaceous (ca. 114 Ma), respectively, and considered to be related to the contractional deformation in the Erlian Basin. The present study tentatively attributes the formation of basement granites to the southward subduction of the Mongol-Okhotsk Ocean Block, Late Jurassic cooling to the closure of the eastern Mongol-Okhotsk Ocean, and late Early Cretaceous cooling to the collision of micro-block with East Asia.

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Crustal P-wave tomographic study of the Yishu fault zone

CUI Xin, SONG Xiuqing, SHI Yaolin, YIN Haitao, ZHU Yuanqing, DONG Tengchao

2023, 30(2): 272-281.
DOI: 10.13745/j.esf.sf.2022.2.62

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The Yishu Fault zone is an important fault zone in the eastern North China Craton and historically experienced a magnitude 8¹/₂ earthquake. To protect against future strong earthquakes and reduce earthquake damage, it is helpful to study the regional seismic velocity model so as to improve the accuracy of seismogram and rapid reporting of seismic intensity. In this study we collect digital seismic data recorded between 2008-2020 in the Yishu Fault zone, and preprocess the data using declustering and normal distribution travel-time selection techniques to obtain the local epicenter parameters and travel-time data; then by tomographic inversion using FMTOMO software we obtained three-dimensional seismic velocity model of the Yishu Fault zone. According to the 3D model, the velocity distribution features in the near surface layer are closely related to the geological structural units, as the fold belt and bedrock outcropping areas often present high velocity distribution while low velocity distribution is generally associated with the sag or sea area with thick soil layers. In the middle crustal layer, seismicity is closely related to the velocity distribution features, and the earthquake epicenters are often densely distributed in the transition zone between high and low velocities. At the Moho depth, velocity distribution in the Yishu belt shows segmental differences and differs on both sides of the belt, this could mean that the fault zone reaches the mantle. High velocity distribution is present in the middle and southern part of the Yishu belt, which may be related to the regional upwelling of mantle material, or may constitute the seismogenic background of strong earthquakes. In the Tancheng area, high velocity anomalies are observed from the earth’s surface to the top of the mantle, suggesting that high stress could accumulate in this area, and an earthquake of magnitude 8 or above might occur, but the recurrence period is considered very long.

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Shale oil and gas in the Mesozoic Basins, eastern Yanshan Orogenic Belt—exploration status and outlooks

MU Hansheng, XUE Xinyu, JIANG Zaixing

2023, 30(2): 282-295.
DOI: 10.13745/j.esf.sf.2022.2.72

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Continental basins in China possess abundant shale oil and gas resources and are an important strategic successor area for increasing oil and gas reserves and production in the future. The Mesozoic basin group in the eastern section of the Yanshan Orogenic Belt (western Liaoning) has rich yet unexplored shale oil and gas resources, therefore, petroleum prospecting in this area has become inevitable. Through field exploration and comparisons of basin’s geological conditions with that of typical domestic and foreign shale oil and gas basins, combined with comprehensive analysis of geochemical parameters for sedimentary systems and source rocks of the area, the shale oil and gas resource potential of the Mesozoic basin group is evaluated. The results show that (1) the Beipiao Formation of the Lower Jurassic and the Jiufotang Formation of the Lower Cretaceous are the main source rocks, with the extensive developments of thick and dark shales. (2) The source rocks are dominated by type II1/II2 kerogen; their total organic carbon (TOC) content is 1%-4% on average, and vitrinite reflectance (Ro), an indicator of source rock thermal maturity, is generally higher than 0.6%, indicating good petroleum generation potential. (3) The two sets of source rocks are both rich in volcanic clastic components, with high content of brittle minerals, and have good transformability. In view of the problems in shale oil and gas exploration in continental petroliferous basins, the multi-source genetic mechanism and oil and gas enrichment types of carbonate fine-grained sedimentary rocks, an important carrier of shale oil and gas, are first discussed, which suggests that volcanic hydrothermal fluids and mixed-source “sweet spot” are the main enrichment types in the study area; then the enrichment effect of volcano-lake interaction on organic matter is discussed, which indicates the volcanic ash input enhances the enrichment efficiency of organic matter. Meanwhile, based on previous exploration experience, it is proposed that the prototype basin restoration technology and the “space-earth-drilling” trinity technology are the key to solving many problems in the oil and gas exploration of complex orogenic belts (especially densely planted mountainous areas).

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Diagenesis of Carboniferous volcanic rocks in Wulungu area and its implications for the volcanic reservoir development

GU Wenlong, NIU Huapeng, ZHANG Guanlong, WANG Shengzhu, YU Hongzhou, JIAO Xiaoqin, XIONG Zhengrong, ZHOU Jian

2023, 30(2): 296-305.
DOI: 10.13745/j.esf.sf.2022.2.65

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The Carboniferous of the Junggar Basin has broad prospects for oil and gas exploration, but the exploration result has not been ideal. Through core observation, thin section identification as well as cathodoluminescence spectroscopy, scanning electron microscope and micron CT investigations, this paper focuses on the diagenesis of volcanic reservoir in the Wulungu area, Junggar Basin and its implications for the volcanic reservoir development. Our study shows that the diagenetic process in the study area can be divided into four stages—condensation consolidation, post magmatic hydrothermal, weathering denudation-leaching and burial stages, involving six main processes—condensation consolidation, compaction, dissolution, filling cementation, alteration and metasomatism. Condensation consolidation and dissolution can form primary and dissolution pores and dissolution fractures, playing a key role in improving reservoir quality. Compaction, filling cementation, alteration and metasomatism, on the other hand, greatly reduce primary reservoir space. Quantitative micron CT analysis and logging interpretation results show that the reservoir quality of the Upper Carboniferous is slightly better than that of the Lower Carboniferous, but they all show strong heterogeneity.

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Investigation of CO₂ flooding considering the effect of confinement on phase behavior

ZHANG Yuan, ZHANG Min, LIU Renjing, CHEN Junjie

2023, 30(2): 306-315.
DOI: 10.13745/j.esf.sf.2022.2.80

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CO₂ injection can effectively improve oil recovery in tight oil reservoirs. However, the micro-nano pores in tight formations lead to confinement, such as adsorption or fluid-pore wall interaction; whilst the conventional theory of phase equilibrium can not well describe the interaction between CO₂ and hydrocarbons. To address this problem, this paper first proposes a phase equilibrium model considering micro-nano confinement. The model modifies the Peng-Robinson Equation of State (PR-EOS) by introducing adsorption effect and fluid-pore wall interaction parameters. The relative deviations of dimensionless critical temperature and pressure of fluid components are evaluated respectively. The calculated results show good agreements with the experimental data, validating the proposed model. Then, fluid phase behavior under reservoir temperature and pressure is evaluated. Results show that micro-nano confinement reduces the bubble point pressure, increases the dissolved gas/oil ratio and formation volume factor, and reduces oil viscosity and interfacial tension. In addition, results of light hydrocarbon extraction and CO₂ diffusion indicate that the micro-nano confinement effect reduces the extraction coefficient of light hydrocarbon and enhances CO₂ diffusion, which facilitate CO₂-crude oil contacts and improve oil recovery. The proposed model can accurately predict the phase behavior of the CO₂-multicomponent mixture in tight formations, and provide strong theoretical support for the application of CO₂-EOR in tight oil reservoirs.

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High-temperature characteristics and geodynamic background at depth of geothermal anomaly areas in eastern China

ZHANG Jian, FANG Gui, HE Yubei

2023, 30(2): 316-332.
DOI: 10.13745/j.esf.sf.2022.1.20

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There are many geothermal resource areas in eastern China, such as the Dongbei (northeastern China), Huabei-Huadong (northern and eastern China) and Southeast Hilly Coast geothermal anomaly areas. In order to rationally exploit and efficiently utilize geothermal resources, it is necessary to study the geothermal distribution and geodynamic background at depth of these geothermal anomaly areas. Different geothermal anomaly areas in eastern China have different geothermal geological conditions and heat source genesis. The Dongbei geothermal anomaly area—with geothermal resources mainly distributed in the Changbai Mountain, Songliao Basin, Erlian Basin and Yilan-Yitong fault graben—is an active area of volcanos, rifts and deep faults, and its deep heat-source and high-temperature anomaly are mainly related to magmatism caused by the western Pacific subduction. The Huabei-Huadong geothermal anomaly area—where geothermal resources are mainly distributed in the Bohai Bay Basin, South Huabei Basin Group, Linyi-Tancheng section of the Tanlu fault zone as well as Subei Basin—is a development area of rift valleys, active deep faults and karst thermal reservoirs; the deep heat-source and high-temperature anomaly in this area are mainly related to basin extension, fault depression and deep fault sliding caused by eastward compression of the Qinghai-Tibet Plateau and subduction-retreat of the Pacific plate. The Southeast Hilly Coast geothermal anomaly area—where geothermal resources are mainly distributed in central and western Guangdong, central and southern Jiangxi and northeast Fujian—is an area with volcanic belts and rock masses that have high radioactive element contents, and its deep heat-source and high-temperature anomalies are mainly related to magmatism caused by lithospheric tension thinning in the South China Sea. According to our calculation, in areas with heat flow greater than 80 mW/m2, ground temperatures at 10 km depth can range between 260-360 ℃ in Dongbei, 270-400 ℃ in Huabei-Huadong, and 260-340 ℃ in Southeast Hilly Coast area. When the Curie temperature is taken as 475 ℃, the Curie depths can range between 14.7-23.7 km in Dongbei, 18.4-26.5 km in Huabei-Huadong, and 16.9-24.9 km in Southeast Hilly Coast area. Through comprehensive analysis we found that there are three types of high-quality geothermal resource areas in eastern China, with different thermal structures: 1) high shallow-ground temperature, deep Curie depth and shallow mantle heat-source in the Changbai Mountain and Dongying area; 2) high shallow-ground temperature, deep Curie depth and deep mantle heat-source in the central Songliao Basin and northeastern Bohai Bay Basin; and 3) low shallow-ground temperature, shallow Curie depth and shallow mantle heat-source in the area between Quanzhou and Zhangzhou.

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Formation and iron oxidation mechanisms of BIFs: Research progress review and outlook

HUANG Liuqin, LI Linxin, JIANG Hongchen

2023, 30(2): 333-346.
DOI: 10.13745/j.esf.sf.2022.2.81

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Banded-iron formations (BIFs), typically consisted of interbanded iron- and silica-rich layers, are unique iron-rich sediments precipitated by extensive Fe(II) oxidation in the anoxic-hypoxic ocean during the Early Precambrian Era (mainly 3.8-1.8 Ga). The most abundant iron-containing minerals in BIFs are magnetite and hematite. BIFs provide the largest iron source to form giant iron ores that are of great economic value. Furthermore, as BIFs are closely related to the two billion-year co-evolution of life and earth environments, they are unique geological records for studying the important evolutionary events (e.g., the rise of atmospheric O₂) on the early Earth. However, few modern BIF analogues have been discovered due to the uniqueness of the Early Precambrian paleomarine environment and sedimentary conditions. Thus, there are still many unsolved mysteries about the origin of BIFs, despite intensive research in the past century. Among them, the most critical scientific question is how the dissolved Fe(II) was oxidized to form specific mineral assemblages in the anoxic paleo-ocean. In this review, the basic information on BIF types, material compositions and sources, as well as sedimentary conditions are summarized, followed by the proposed mechanisms of Fe(II) oxidation and their inherent problems from the perspective of special sedimentary environment of BIFs. Also discussed are the contribution of microbial-mediated nitrogen biogeochemical cycle to Fe(II) oxidation and the formation of BIFs and related research progress. Lastly, an outlook on the genetic study of BIFs is discussed.

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Genesis and geological significance of hydrothermal Pb-Zn orebodies in the Xiarihamu mining area, East Kunlun Mountains, China

CHEN Xin, WANG Hui, MAO Jingwen, YU Miao, QIAO Jianfeng, WANG Zhian

2023, 30(2): 347-369.
DOI: 10.13745/j.esf.sf.2022.2.76

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The Xiarihamu deposit is a giant magmatic Ni-Co sulfide deposit discovered recently in the East Kunlun metallogenic belt. With the advancement of deep, peripheral ore prospecting, however, some distinct hydrothermal Pb-Zn-(Cu, Ag) orebodies—which are obviously different from the magmatic sulfide deposit—have been found at the Xiarihamu mining area, with nearly 100,000 tons of proven reserves. In order to further reveal their genesis and production background, a systematic study of the mineral alteration characteristics and metallogenic chronology of the newly discovered hydrothermal orebodies is carried out. The hydrothermal orebodies are mainly stratified in the marble/gneiss interlayer structural belt of the Paleoproterozoic Baishahe Formation of the Jinshuikou Group. The main metallic minerals are galena and sphalerite, followed by chalcopyrite, and the gangue minerals are pyroxene, garnet, epidote, chlorite, titanite, apatite, quartz, calcite, etc. According to EPMA analysis, the pyroxene is mainly composed of the diopside-hedenbergite series, with the end component mainly Di_65.44-79.01Hd_18.55-30.78Jo_1.63-0.51; the garnet belongs to the andradite-spessartine series, with the end component mainly Gro_24.93-92.17And_3.29-9.95(Spe + Alm)_1.68-67.29; and the epidote and chlorite in the skarn are both characterized by high Mn contents. Based on the above results it is considered that the hydrothermal orebodies are of skarn type. The trace element composition of the hydrothermal titanite from the skarn is similar to that of typical hydrothermal titanite according to LA-ICP-MS analysis. It has relatively low ΣREE, is enriched in LREE, with small LREE/HREE fractionation, and has a Th/U ratio of 0.23-1.97 (averaging 0.94). By LA-ICP-MS in-situ U-Pb dating the ²⁰⁶Pb/²³⁸U weighted mean age (after ²⁰⁷Pb correction) of the hydrothermal titanite is (413.3 ± 3.6) Ma (n = 23, MSWD = 0.96), and for the hydrothermal apatite it is (414 ± 13) Ma (n = 17, MSWD = 1.03). These results indicate that mineralization of the skarn orebodies occurred in the Early Devonian, which is consistent with the metallogenic age (within the error range) of the magmatic Ni-Co sulfide ore body. Combined with the regional metallogenic geological conditions, this paper considers that there are two temporospatially related metallogenic systems-magmatic and hydrothermal systems-developed in the Xiarihamu mining area. Briefly, under the extensional background the upwelling of the mantle-derived magma results in the partial melting of rocks in the middle-upper crust to form a felsic magmatism chamber; the melts then rise along the extensional structure zone and differentiate to produce the ore-forming fluids which then react with Paleoproterozoic marble or calcareous siltstone to form the Pb-Zn-(Cu, Ag) orebodies. This paper provides direct metallogenic and chronological evidences for the existence of Devonian Pb-Zn skarn deposits in the East Kunlun metallogenic belt. It is of theoretical and practical significance, therefore, to consider the coexistence of magmatic and hydrothermal ore-forming systems in the region and carry out a comprehensive prospecting evaluation accordingly.

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REE enrichment process in the Bayan Obo Fe-REE-Nb deposit: Genetic and mineralogical evidence

SHEN Junfeng, YAN Guoying, ZHANG Mengmeng, WANG Zhaojing, XU Kexin, MENG Wenxiang

2023, 30(2): 370-383.
DOI: 10.13745/j.esf.sf.2022.10.40

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The enrichment mechanism of rare earth element (REE) in the Bayan Obo deposit is of global interest, and it is of great importance to define the REE mineralization process for high-quality development and utilization of REE resource. According to scanning electron microscope (SEM) observation of samples from the main orebody, two main REE minerals, bastnaesite and monazite, occur commonly as veinlets formed by filling localized micro fissures of host minerals. Of note, small and globular-shaped bastnaesite and monazite are wrapped by dolomite or occur on the grain boundaries of dolomite outside the veinlets of REE-bearing minerals; occasionally they also appear within the veinlets. The above observations indicate that REE is initially hosted by dolomitic carbonate magma, then migrates to the shallow crust, and eventually assumes its occurrence state as solidification of carbonate magma occurs. The globular droplet-like crystals also might be attributed to strong liquid immiscibility in carbonate magma during solidification. Furthermore, Nb-Cr-rich monazite can be observed in the REE-bearing mineral veinlets, also seen is Si-rich monazite from the early stage wrapped around by Si-poor monazite, suggesting that the enrichment of bastnaesite and monazite is closely associated with magma at depth. In addition, most of bastnaesite and monazite are characterized by subhedral-anhedral crystals apart from a few globular ones in the veinlets. Dolomite commonly undergoes dissolution and is wrapped by bastnaesite, monazite and fluorite. Especially, abundant fluorite and apatite are enriched together with bastnaesite and monazite veinlets, and globular-shaped monazite wrapped by apatite can be observed. These features indicate that bastnaesite, monazite and dolomite crystalized from early-stage magmatism are strongly modified by late-stage F-P-rich hydrothermal fluids—that is, the formation of bastnaesite and monazite veinlets is also closely linked to hydrothermal fluids. The above mineralogical phenomenon shows that REE enrichment in the Bayan Obo deposit underwent metallogeny at least twice: REEs are firstly carried by carbonate magma and migrate to the shallow crust where magmatic immiscibility leads to the initial enrichment; then F-P-rich hydrothermal fluids cause re-mobilization and cyclic migration of REEs, resulting in further enrichment.

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Discriminating characteristics of hydrothermal fluids using epidote mineral chemistry and strontium isotopes: A case study of the Duotoushan Fe-Cu deposit, eastern Tianshan

ZHANG Weifeng, CHEN Huayong, DENG Xin, JIN Xinbiao, LIU Shuzhan, TAN Juanjuan

2023, 30(2): 384-400.
DOI: 10.13745/j.esf.sf.2021.10.4

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Epidote, a common Ca-Al-bearing silicate mineral in many types of hydrothermal deposits, usually has high concentrations of trace elements, thus its mineral chemistry as well as Sr isotopic characteristics can reveal the physico-chemical conditions and sources of the ore-forming fluid. In this contribution we present a study on the epidote-group minerals precipitated during Fe mineralization at the Duotoushan deposit. With high Al-coordination numbers (1.95-2.24), and enriched in Fe³⁺ but depleted in Fe²⁺ and rare earth elements (REE), the studied mineral grains are identified as the epidote end member. The mineral grains are further characterized by MREE enrichment and LREE depletion with moderate negative Eu anomalies, suggesting the complexation of hard acids at neutral to basic pH may have caused the REE fractionation. Correlation analysis indicates REEs are mainly incorporated into the Duotoushan epidote through isomorphous substitution. At Duotoushan, the epidote-group minerals coexist with quartz, titanite and magnetite, exhibit negative Ce anomalies (0.64-0.91), and have high U/Th ratios, indicating they are formed under relatively high oxygen fugacity. Since the in-situ (⁸⁷Sr/⁸⁶Sr)_i ratios (0.70462-0.70495) lie between the Bailingshan granitic complex and hosted rocks or Carboniferous seawater, we propose that the initial fluids are magmatic in origin and external basinal fluid may have added into the hydrothermal system during water-rock interaction. Furthermore, our study indicates that epidote is a robust indicator for tracing fluid evolution in magmatic hydrothermal deposits.

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Growth heterogeneity and zonation characteristics of emeralds from Davdar, Xinjiang

CUI Di, WU Qiong, LIAO Zongting, QI Lijian, ZHOU Zhengyu, ZHANG Lingmin, ZHONG Qian, LIU Yiceng, LI Ling

2023, 30(2): 401-414.
DOI: 10.13745/j.esf.sf.2021.11.9

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The chemical composition, microstructure and element-mapping characteristics of the color-zoned emeralds from Davdar, Xinjiang are investigated using gem microscope, polarized microscope, electron microprobe (EPMA) and X-ray diffraction techniques. The microstructure reveals reliable evidence of periodic growth, which shows the emerald core is fragmented and corroded and the rim is relatively complete. X-ray diffraction data indicate that the Davdar emeralds can be assigned to “octahedral” beryl, with Al substitutions mainly by mafic ions, and alkali ions, mainly Na⁺, occupying the beryl channels for charge compensation. Chemical composition analysis shows the alkali metal contents decrease from the core to the rim in both new and old mine samples. The alkalinity of new mine samples is low (alkali metal <1%), while old mine samples have medium or high alkalinities (alkali metal between 1%-2% or > 2%). The growth zoning types include multiple-color or colorless core-green rim types, and Cr and V are the dominant coloring elements; and chemical heterogeneity of the growth microzones reflects the variable contributions from coloring ions and alkali and mafic ions.

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Relationship between the large scale fenitization and REE mineralization on the border between Zhejiang and Fujian provinces: A review of recent research progress

CHEN Shizhong, ZHOU Yan, XING Guangfu, XU Mincheng, FAN Feipeng, XI Wanwan, ZHU Xiaoting, GUO Weimin

2023, 30(2): 415-425.
DOI: 10.13745/j.esf.sf.2022.2.79

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Fenitization is considered to be a typical post magmatic alkaline metasomatic process usually associated with REE mineralization. The typical super large REE deposits related to fenitization include the Bayan Obo REE deposit in Inner Mongolia and the Maoniuping REE deposit in western Sichuan as respectively the first and second largest LREE deposits in China. The Tieshan metasomatic rock (TMR), located on the northeastern side of the Zhenghe-Dapu fault zone on the border between Zhejiang and Fujian Provinces, is the largest potassic metasomatite in southeastern China. Its prospective rare earth oxide (REO) resource is expected to reach ~62 million tonnes, with porphyry copper and apatite deposits showing prospects for critical metals such as REEs. The widely occurring fenites in TMR are closely related to the mineralization of P, REE, Ti and Ga. The fenite belt is about 1-2 km long and hundreds of meters wide, where ~40 REE-rich apatite orebodies were discovered. The fenites are composed of feldspar, pyroxene, amphibole, biotite and melanite, etc. Among them, potassic feldspars occur in four modes: (1) in facial alkali alteration or in alkali alteration with melanites, (2) in light or (3) dark alkali feldspar veins, or (4) in association with black garnet. Pyroxenes has three occurrence modes: in breccias or feldspar, or in association with amphiboles. Aegirine pyroxene displays obvious girdle structure under the microscope, with a light-green core and dark green crust, indicating there are higher diopside components at the core and higher aegirine content at the edge. The fenite was formed in the second magmatic stage of the Yanshanian Movement (157-156 Ma) and probably related to the intrusion of the Tongpeng’an potassic feldspar granite to the east, as REEs were enriched during the magmatic-hydrothermal activity. In the third stage of the Yanshannian Movement (118-114 Ma), the Tieshan metasomatic rock was altered by alkaline hydrothermal fluid from the porphyries and polymetallic mineralization occurred.

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Typomorphic characteristics of magnetite and prediction of deep iron-rich orebody in the Bayan Obo ore deposit

XU Zhihao, YAN Guoying, YANG Zongfeng, WANG Zhaojing, SHEN Junfeng, ZHANG Mengmeng, LI Peipei, XU Kexin

2023, 30(2): 426-439.
DOI: 10.13745/j.esf.sf.2022.10.45

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The Bayan Obo rare earth-niobium-iron ore deposit contains a large amount of iron resources, among which magnetite has been widely studied as an ore mineral. Typomorphic characteristics of magnetite can be used to explore the genesis, metallogenic regularity and deep prospecting of the ore deposit, however, they have not been commonly studied in the Bayan Obo ore deposit. In this paper, the total iron data of 22 exploration lines were used, and mineral micro-zone composition testing was performed on 45 samples from 10 exploration lines covering 600 × 600 meters in area and 800 meters in depth, using energy dispersive spectroscopy (EDS) and electron probe (EPMA) methods. The Bayan Obo magnetite can be divided into magmatic and hydrothermal magnetites, indicating both are formed or modified in the ore deposit. Combined with thermometric data, it can be seen that wherever magnetites are enriched, the local total iron content is also higher, and iron-rich orebodies are easy to form at formation temperatures ranging between 350-650℃. Under such observation, deep iron-rich orebodies are likely to form in the eastern direction of Line 12 and 13 and may be connected to the eastern deep ore deposit, which is worthy of further research and exploitation.

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Quantitative determinations of the dispersion pattern and geological significance of geochemical anomalies in Biguo area, Jiaodong Terrane

ZHU Pingping, LIU Yue, CHENG Qiuming

2023, 30(2): 440-446.
DOI: 10.13745/j.esf.sf.2022.2.68

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The Jiaodong Terrane is famous for the development of a large number of Mesozoic gold deposits. However, the genesis of the gold deposits is still controversial, where a focus of debate is whether the formation of gold deposits is controlled by the shallow structure or by the deep geodynamic background, i.e., retraction of the Pacific plate after its westward subduction. In order to further quantitatively describe the plate movement and abnormal geological events during the formation of the gold deposits, this paper, based on previous work, uses the 1∶50,000 geochemical exploration data of Biguo area, Jiaodong to extract and decompose the Au anomaly data. Singularity analysis, quantile regression and Fry analysis were performed in sequence to quantitatively characterize Au geochemical anomalies and determine the anomaly dispersion pattern. The results show that Au enrichment is in three directions: N-E (or S-N), E-W and N-W, which may correspond to westward subduction and slab rollback of the Pacific plate in the Mesozoic, northward movement of the Yangtze plate in the Cenozoic, and large scale N-W/S-W strike-slip faulting and structural superposition in the Tanlu fault zone in the Middle Cenozoic, respectively. This may indicate that the general pattern of Au enrichment and the formation of gold deposits in Jiaodong may have been controlled by the deep geological process. This study identified geochemical anomalies associated with the known Au mineral deposits in the area, which is useful not only for guiding Au exploration but also for providing surface geochemical patterns reflecting influence of plate tectonics in the Jiaodong Terrane.

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Carbon sequestration assessment methods at home and abroad for terrestrial ecosystems: Research progress in achieving carbon neutrality

XIE Lijun, BAI Zhongke, YANG Boyu, CHEN Meijing, FU Shuai, MAO Yanchao

2023, 30(2): 447-462.
DOI: 10.13745/j.esf.sf.2022.2.78

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The purpose of this paper is to differentiate the characteristics of different mainstream carbon sink accounting methods developed in the past 30 years for the four main terrestrial ecosystems—forest, grassland, farmland, and wetland—in order to accurately assess the current status of the carbon sink capacity and future carbon sink potential of terrestrial ecosystems. Through literature review, comparative analysis, and induction we show that 1) the existing carbon sink accounting methods are mainly based on formula model calculations, and are affected by sample size, measurement and parameter errors, inconsistent data sources, and model universality when analyzing the spatial pattern of vegetation carbon sinks, which lead to great uncertainty in the calculated result; thus, China urgently needs to establish an unified standard terrestrial ecosystem carbon sink monitoring system. 2) Accurate carbon sink calculation for terrestrial ecosystems requires reliable data sources, reasonable calculation methods and models, multi-channel cross-validation, appropriate error analysis, and interpolation corrections. 3) Methods such as site survey, model construction, micrometeorology, and remote sensing monitoring should be effectively integrated to solve the problem of multi-scale coupling. Future research should be based on a large number of measured data, explore differentiated carbon sink accounting methods according to ecosystem types, regions, and climates, and establish a full-scale, generalized carbon sink calculation model to provide reference for a carbon sink accounting standard for terrestrial ecosystems in China.

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Methane clumped isotopes: Research progress and application in carbon cycling in Earth surface systems

WANG Xinchu, LIU Congqiang, LI Siliang, XU Sheng, DING Hu, PANG Zhiyong, SHUAI Yanhua

2023, 30(2): 463-478.
DOI: 10.13745/j.esf.sf.2022.2.74

Abstract ( 585 )

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Clumped isotopes are a frontier field of geochemical research today. Basic researches on the technical principles of clumped isotope techniques, indicative value of clumped isotopes, clumped isotope calibration, etc. have yielded preliminary results; whereas applications of clumped isotopes are scarce in the study of Earth surface systems as well as in geography, environmental sciences and ecology. In Earth surface systems, carbon cycling has far-reaching implications, and for methane (CH₄), an important part of the natural carbon cycle, revealing its source and geochemical process is of great importance. The introduction of methane clumped isotope method can provide an effective tool as likely it can be used to reveal carbon cycling at different scales in time and space. This article summarizes the theoretical basis of clumped isotopes and the current test analysis results, with focuses on the research status and theoretical models of methane clumped isotopes as well as the factors influencing isotopic disequilibrium, and discusses typical applications of clumped isotopes in studying methanogenesis and CH₄ transformation in natural gas, atmosphere and water. Finally, test result comparisons between different laboratories, sample collection under natural isotopic abundances, and discussion on quasi-equilibrium process are put forward to provide a reference for future application of methane clumped isotopes and others, such as carbon dioxide clumped isotopes, in studying carbon cycling in Earth surface systems.

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Research progress in microbial-mediated sulfur cycles in geothermal habitats: Insights into biological processes on the early Earth

MA Li, XIE Yihao, WU Geng, JIANG Hongchen

2023, 30(2): 479-494.
DOI: 10.13745/j.esf.sf.2022.2.82

Abstract ( 367 )

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Geothermal habitats are characterized by high temperature and hypoxia similar to the conditions of early Earth. Therefore, thermophiles in modern geothermal habitats can be used to study the co-evolution of life and environment on the early Earth. The sulfur cycle is one of main biogeochemical processes on the early Earth, thus studying microbial-mediated sulfur cycle and its coupling with other elements in geothermal habitats can deepen our understanding of the biogeochemical sulfur cycle and its environmental effects on the early Earth. This review summarizes the characteristics of microbial diversity in the sulfur cycle and their influencing factors in geothermal habitats, as well as the coupling of the sulfur cycle with the iron, methane, and nitrogen cycles. Combined with Archean geochemical conditions and stratigraphic isotope record, this review also discusses the possible environmental effects of the sulfur cycle on the early Earth. Finally, an outlook on future research on microbial-mediated co-evolution of the sulfur cycle and environment on the early Earth is discussed.

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Temporal and spatial distribution characteristics of nitrogen and phosphorous in the water of Qionghai Lake, Sichuan Province and their response to ecological environment

SHI Yao, ZHANG Lei, QIN Yanwen, MA Yingqun, YANG Chenchen, LIU Zhichao, ZHU Huailin

2023, 30(2): 495-505.
DOI: 10.13745/j.esf.sf.2022.2.86

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The aim of this article was to explore the characteristics of nitrogen and phosphorus pollution in the Qionghai Lake. We analyzed the temporal and spatial distribution characteristics of TN, $NH 4 +$ -N and TP concentrations of water samples from 11 lake sampling sites in 2020. The relationships between nitrogen/phosphorus and chlorophyll-a concentrations were discussed, and the eutrophication status of the Qionghai Lake was evaluated in order to strengthen the prevention and control of lake eutrophication and ensure the safety of the lake water source. The results showed that TN concentrations ranged from 0.315-0.500 mg·L^-1, with a mean value of 0.399 mg·L^-1, and $NH 4 +$ -N concentrations ranged from 0.063-0.171 mg·L^-1, with a mean value of 0.098 mg·L^-1, both meeting the standard for grade Ⅱ surface water of lake and reservoir; while TP concentrations also met the standard, with a range of 0.019-0.035 mg·L^-1 and a mean value of 0.023 mg·L^-1. Both nitrogen and phosphorus concentrations in the Qionghai Lake show temporal and spatial differences: TN, $NH 4 +$ -N and TP concentrations varied significantly with time and were higher in dry season than in wet season; they have similar spatial distribution patterns, with high concentration areas mainly observed at station 1 (Haihekou) and 10 (Gaojian Bay) in the northwestern part of the lake, and were all higher in the northwest than in the east of the lake. The average TN/TP ratio was higher in the dry season, characterized by phosphorous limitation, than in the wet season, characterized by nitrogen and phosphorus limitation. TN, TP and $NH 4 +$ -N concentrations positively correlated with Chla concentration, indicating nitrogen and phosphorous could promote the algae growth. Evaluation of eutrophication in the Qionghai Lake showed that the average trophic status index (TSI) value for the lake was 38.011; and the study area was in a mesotrophic state as a whole, with a higher TSI value for the northwest and a lower TSI value for the east.

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Formation and transformation of condensate water inside rocks: Insight into source of rock moisture affecting weathering

OUYANG Kaigao, JIANG Xiaowei, MA Ce, YAN Hongbin, REN Jianguang, FAN Yao, ZHANG Runping, FU Qianfang, LI Xu, WAN Li

2023, 30(2): 506-513.
DOI: 10.13745/j.esf.sf.2022.2.15

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Water plays an important role in controlling rock weathering, but there is still limited understanding of the source of rock moisture in rocks not directly exposed to sunshine and rainfall. In this study, a cave wall of the Yungang Grottoes in northern China is selected as an example for rock moisture measurement using the frequency domain reflection (FDR) method. Using the hourly monitoring data on the apparent rock water content at the rockwall between May 2020 to May 2021, different mechanisms controlling rock moisture fluctuations in summer and winter are identified. From early June to early October, the change of atmospheric humidity is the direct controlling factor for the increase and decrease of the rock moisture at the rockwall. Specifically, when atmospheric humidity gradient points to the interior of the mountain, water vapor migrates into the porous rock and condenses into liquid water, which increases rock moisture; inversely, liquid water within the rock evaporates thus decreases rock moisture. From middle October to early June, although the rock is relatively dry, the apparent rock water content of the rockwall surface layer still increases significantly in mid-December via cryosuction-induced water migration. This indicates that there is weakly bound water in the rockwall which can migrate and promote chemical weathering. By identifying an important moisture source in stone heritages, this study deepens the understanding of the rock weathering mechanism, and provides a decision-making basis for the conservation of stone heritages.

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Soil chromium in Shantou City, Guangdong Province: Spatial distribution characteristics, source apportionment and influencing factors

JIANG Xingchao, XU Jing, LI Ruyi, JIA Yifan, YANG Pan, LUO Jie

2023, 30(2): 514-525.
DOI: 10.13745/j.esf.sf.2022.2.77

Abstract ( 509 )

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Shantou City, one of the special economic zones of Guangdong Province, is the economic and political center of Chaoshan area, with high level of industrialization and urbanization. In the past, soil environmental quality researches in Guangdong were mainly concentrated in the Pearl River Delta, while they lacked systemic investigation on the distribution and source apportionment of environmental soil elements in the Han River Delta. This study sought to assess the soil chromium (Cr) pollution status of Shantou City to aid prevention and control of Cr pollution in the area. According to the standard grid, 511 surface soil samples (0-20 cm) and 138 deep soil samples (>15 cm) were collected to determine the soil Cr contents. The Cr pollution status of surface soil was analyzed by the enrichment factor method, and the source of soil Cr was identified using PAC analysis; through Kriging interpolation and ANOVA, the spatial distribution characteristics of soil Cr and the influencing factors were explored. According to the results, the geometric mean content of Cr in the surface soil was 32.40 mg·kg^-1, which is lower than the Cr background value for Shantou City (39.52 mg·kg^-1) and much lower than the national Cr risk control standard (300 mg·kg^-1, 6.5<pH≤7.5). Most of the soil samples showed no (66.34% samples) or slight (34.66% samples) Cr accumulation, with no obvious Cr contamination. The areas with high soil Cr values were mainly in the northern Chaonan District and northern and southwestern Chanyang District, and the lowest value areas were in the eastern Chaonan District and the coastal area of southeastern Haojiang District. PAC and Pearson’s correlation analysis results indicate the source of soil Cr in Shantou City is a combination of human activities and natural sources, mainly influenced by the soil parent material, with relatively high Cr in Triassic sandstone and low Cr in cretaceous granite. Cr content in paddy soil was higher than other soil types and relatively low in coastal sand soil, but there were no obvious differences between different land-use types; and human activities had no significant influence on soil Cr. The soil Cr background value obtained by this study laid a foundation for subsequent researches. Meanwhile by integrating multiple indicators this study accurately determined the source apportionment of soil Cr and analyzed its influencing factors, which provides an important reference for assessing Cr pollution and formulating the corresponding prevention measures.

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Geochemical characteristics of aquifer sediments and their influence on fluoride enrichment in groundwater in the Hualong-Xunhua basin

XING Shiping, WU Ping, HU Xueda, GUO Huaming, ZHAO Zhen, YUAN Youjing

2023, 30(2): 526-538.
DOI: 10.13745/j.esf.sf.2022.9.10

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Natural occurrence of high-fluoride groundwater in arid and semi-arid areas seriously threatens the safety of drinking water. The chemical characteristics and formation processes of high-fluoride groundwater have been well studied, but the characteristics of high-fluoride aquifer sediments and their influence on the enrichment of groundwater fluoride remain unclear. This paper investigated the hydrogeochemical characteristics of high fluoride groundwater and the elemental compositions and water-soluble components of high-fluoride aquifer sediments to reveal how sediment geochemical characteristics affect fluoride enrichment in groundwater in the Yellow River Valley of the Hualong-Xunhua basin. The results showed that groundwater fluoride concentrations ranged from 0.51 to 3.78 mg/L (average 1.38 mg/L) and tended to increase with depth. Na⁺ was the major cation in the high fluoride groundwater and positively correlated with F^-. Total fluoride contents of sediments ranged from 460 to 1030 mg/kg, and water-soluble F^- contents ranged from 0.53 to 19.9 mg/kg. In groundwater at below fluorite saturation, a positive correlation between F^- and Ca²⁺ indicates that fluorite continues to dissolve and provides F^- to groundwater, and positive correlations between F^- and Si and F^- and Al illustrate that weathering of fluoride-containing silicate also promotes fluoride enrichment in groundwater. Positive correlations between F^- and pH, F^- and $HCO 3 -$ , and F^- and Na⁺/Ca²⁺ in both water-soluble extract of sediment and groundwater indicate that desorption and cation exchange are important hydrogeochemical processes in the formation of high-fluoride groundwater. Positive correlations between water-soluble F^- and total Fe/Mn in sediments suggest that desorbed F^- originated from Fe/Mn oxide-bound fluoride. Additionally, groundwater salinity had a positive impact on fluoride enrichment in groundwater. The results of this study are of great significance for revealing the genetic mechanisms of high fluoride groundwater.

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Characteristics of petroleum hydrocarbon-polluted karst groundwater environment: A case study of groundwater source in Dawu, Zibo City, northern China

GUO Yongli, XIAO Qiong, ZHANG Cheng, WU Qing

2023, 30(2): 539-547.
DOI: 10.13745/j.esf.sf.2022.1.23

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Karst aquifers are the main source of water in northern China. Here, the Dawu karst aquifer, located in Zibo City, Shandong Province, China, is selected to study the environmental characteristics of groundwater pollution due to petroleum hydrocarbons contamination, which would provide scientific guidance for sustainable utilization of karst groundwater resources and lay the foundation for studying the evolution of karst groundwater environment. Groundwater level, temperature, pH, Eh, DO, $NO 3 -$ , $SO 4 2 -$ , $HCO 3 -$ , Cl^-, trichloromethane, 1,2-dichloropropane, 1,2,3-trichloropropane, tetrachloroethylene, trichloroethylene and ΣVOCs are selected as the hydrogeochemical indicators. Isotope technique, multivariable statistical method, graphical method and other methods are used to analyze the interrelationships among the hydrogeochemical indicators, in order to reveal the pollution processes and their influencing factors. Hydrodynamic field and environmental conditions in the aquifer are suitable for biodegradation of petroleum hydrocarbons. The occurrences of aerobic and anaerobic respiration without methanogenic activities are demonstrated by hydrogeochemical indicators and isotopes. Although ΣVOC is the small part of DOC in the groundwater system, petroleum hydrocarbons are one of the most important factors influencing groundwater environment contributing 33.93% groundwater DIC on average. Thus, a comprehensive evaluation of the chemical characteristics of petroleum hydrocarbons-polluted groundwater can help to better understand the mechanisms influencing groundwater environment.

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The clustering feature of time-space distributions of coal mine disasters and earthquake activities in China—an in-depth investigation

CHEN Bo

2023, 30(2): 548-560.
DOI: 10.13745/j.esf.sf.2022.2.69

Abstract ( 253 )

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The previous research on the time-space distribution features of coal mine disasters and earthquakes is a preliminary exploration on the basis of natural earthquakes. With microearthquakes added and time-space analysis methods improved, this paper obtains more comprehensive time-space distribution features. On the whole, the basic knowledge of the clustering feature of coal mine disasters and earthquakes remains unchanged. The data show that the number of clustering events increases significantly while the spatial range of clustering events decreases sharply, which, according to analysis, is primarily due to the close proximity between the high frequency microearthquakes and mining accident areas. Furthermore, the spatial range and duration of stress disturbance correspond to the degrees of spatial and temporal clustering, and this finding is of great significance to the scientific research and mining production. Based on the clustering research methods, a temporal threshold of 7 days and a spatial threshold of 27 km is obtained, meaning once a stress disturbance event occurs in the mining area, stress disturbance events may occur within 7 days and a 27 km radius. Still, typical cases show that the range of coal mine disasters and macroearthquakes can reach more than 100 km. The complete data of clustering events are available online (https://pan.baidu.com/s/1sjnCEh3). The time-space thresholds obtained here are for reference only.

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