Table of Content

20 April 2021, Volume 27 Issue 2

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Water Content in Ertan Basalts: Implications for the Regional Distribution of Water in the Mantle Sources of Emeishan Large Igneous Province

SUN Hao, XIA Qunke, LIU Jia, WANG Zizhen, BI Yao

2021, 27(2):  121-132.  DOI: 10.16108/j.issn1006-7493.2019108

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Recent research on the water content and mantle potential temperature of komatiites and picrites has found that the genesis of both Archaean and Phanerozoic large igneous provinces is related to hydrous mantle plume. The Late Permian Emeishan large igneous provinces (ELIP) is located in the western margin of the Yangtze craton in SW China, which is also considered as the effect of mantle plume. ELIP was divided into western zone, middle zone and eastern zone based on the work of geology and geochemistry in previous study. The previous study on the water contents for the picrites and basalts from Dali and Binchuan demonstrated that the water content in the mantle source in the western zone is higher than 2500×10^-6. However, the water content of source in other zones is still not clear, which makes it difficult to understand the role of water in the genesis of ELIP. Here, we estimated the water content of the mantle source of the high-Ti basalts in the lower part of Ertan sesstion, in the middle zone of ELIP, by the mothed of clinopyroxene phenocrysts. The result shows that the H2O content of clinopyroxene phenocrysts is 76×10^-6-424×10^-6, and the H2O content of the corresponding melt is 3.01 wt%. Considering the effect of fractional crystallization of minerals, the H2O content of primary magma is estimated to be 2.71±0.95 wt%, which is slightly lower than that of Dali picrites, and similar to that of Binchuan picrites. The estimated H₂O content in mantle source of Ertan high-Ti basalts is 1357×10^-6, which is lower than that of Dali and Binchuan picrites, but significantly higher than that of mid-ocean ridge basalts and ocean island basalts. The high H2O content of basalts and picrites in different areas in ELIP thus indicates that water plays an important role in both the genesis and the evolution of ELIP.



Geological Features and Geochronology of the Late Paleozoic Migmatites from the Harlik Range and Tectonic Significance

NI Xinghua, WANG Bo, LIU Jiashuo

2021, 27(2):  133-148.  DOI: 10.16108/j.issn1006-7493.2020005

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The Harlik Range is located in the northeastern part of the Tianshan Orogenic Belt and resulted from subduction of the Paleo-Asian Ocean and arc-continent or arc-arc accretionary orogeny. The Harlik metamorphic belt occurs at the southern foot of the Harlik Range and comprises high-grade metamorphic rocks and migmatites. Until now, the timing and genesis of migmatites are poorly studied. In this study, we conducted field structural investigations and LA-ICP-MS zircon U-Pb dating for the migmatites. The results show that the migmatites were likely derived from partial melting of high-grade meta-sedimentary rocks of the Harlik metamorphic belt. Numerous leucocratic dykes and in-source leucosome were deformed in various degrees and yielded consistent zircon U-Pb ages of 332-330 Ma. Considering the syn-kinematic characteristics of the in-source leucosome, it can be proposed that the migmatization and deformation of the Harlik metamorphic belt occurred in ca. 330 Ma. Combining the previous studies on the metamorphic rocks and nearby plutonic rocks and taking the regional tectonic evolution into account, we suggest that the Harlik area probably underwent a post-orogenic setting since 330 Ma. The migmatites were likely formed by decompression partial melting and occurred as part of the basement of the Harlik arc, exhumation and uplifting of which might be related to the emplacement of the coeval post-collisional plutons.

Constraints of Lithium on the Petrogenesis and Mineralization of Granite in Xianghualing Area, Hunan Province

QIN Lixi, RAO Can, LIN Xiaoqing, WU Runqiu, WANG Qi

2021, 27(2):  149-162.  DOI: 10.16108/j.issn1006-7493.2020007

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Lithium (Li), one of strategic key metals, is mainly concentrated and crystallized in granitic rocks during magmatic stage. Owing to its incompatibility and volatile properties, Li has an important restriction on the petrogenesis and mineralization of granite. In this paper, the petrography, major-trace elements and mineralogy of Laiziling and Jianfengling granites in Xianghualing area (Hunan province), were systematically studied by means of electron microprobe analysis and LA-ICP-MS analysis. The results show: (1) during the crystallization differentiation of granitic magmas, the Li content increases gradually, which greatly reduces melt viscosity and increases crystallization temperature range, and granitic rocks were thus fully differentiated, resulted in the vertical zonation of granite; (2) in the granites Li contents have positive relationships with rare-metals’contents, and Li has synergistic mineralization with Ta, Nb and Sn etc.; (3) mica group minerals evolved to Li-rich phase (mainly zinnwaldite), with the crystallization of zinnwaldite, Nb, Ta and Sn etc. are successively precipitated, leading to decreasing contents of Ta and Nb in late mica. Although H2O, F, etc. in the melt have great influence on the properties and crystallization differentiation of granitic magma, but they are not enough to cause granite to be vertical zonation.


Geological Features and Prospecting Indicators of Martabe Epithermal Au-Ag Deposit, Sumatra, Indonesia

ZHANG Haikun, HU Peng, JIANG Junsheng, CHENG Xiang, ZHAN Mingguo, PAN Luozhong, DAI Yu, PAN Beihong

2021, 27(2):  163-171.  DOI: 10.16108/j.issn1006-7493.2020001

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The Martabe deposit, located in the North Sumatra, Indonesia, is a world-class high-sulfidation epithermal Au-Ag deposit discovered in recent years. Based on a comprehensive review of previous knowledge of this deposit, this paper present a summary of its metallogenic geological features, aiming to enrich the cases of epithermal deposits and guide exploration of this type of deposits. The Martabe deposit has similar alteration zonation with typical high-sulfidation epithermal deposit, with zones such as silica, advanced argillic, argillic, and prophylitic zones from the core to the margin. Zircon U-Pb dating on ore-bearing dacite yielded an age of (3.8±0.5)Ma, while two ages of (3.3±0.1) Ma and (2.1±0.1)Ma were obtained from Ar-Ar age determination on

hydrothermal alunite, indicating a brief interval between diagenesis and mineralization and two stages of advanced argillic alteration. Mineralization was associated with the interaction between magma-sourced fluid and meteoric water and further intensified by weathering and erosion following ore-forming. Martabe deposit shares characteristics with the classical high-sulfidation epithermal deposits, displaying prospecting signals in the aspects of tectonic setting, lithology, alteration, and geochemical explorations.


Mineralogical Study of Iron-rich Elongated Concretions in Holence Silt Layer in Taihu Lake Region

ZUO Shuhao, XIE Zhidong

2021, 27(2):  172-182.  DOI: 10.16108/j.issn1006-7493.2019093

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The paper mainly focuses on the original upright occurrence of the elongated (rod-shaped) concretions in one specific silt layer, distribution, morphology, internal colloidal structure and mineral assemblages of the iron-rich elongated concretions in Taihu Lake area. The main purpose of the paper is to discuss the formation mechanisms of the iron-rich elongated concretions. In horizontal,
the rod-shaped concretions are widely distributed in one specific silty layer in Taihu Lake. Vertically, the rod-shaped concretions are found at different heights, from the bottom of Tai lake to the farm fields on the hillside, and in the grooves near the foothills. Fresh rodshaped concretions only can be seen in the landmark silty layer with a rigid gray for the outer surface and the color of the natural broken surface is gray. The original occurrence of the rod-shaped concretions is upright inserted in the landmark silty layer, with the head down and the tail upward, and the heads of elongated concretions have hemispherical shape and curved threads lines. While, another kind of concretions found in the disturbed layer have short columns with smooth outer surface due to moving wearing, and the color appears grayish brown or yellowish brown due to weathering. The length of the rod-shaped concretions range from 1 to 80 cm, some is short and some is long. Most of short ones are broken sections of the long one. The diameters of the elongated concretions range from half cm to 2 cm. Most of rod-shaped concretions have weathering crusts with a thickness about 1 to 2 mm. The outer surface of the rod-shaped concretions has unique spiral patterns. Major inclusions in the concretions are angular sharp-edged quartz grains with poor roundness and poor separation surrounding by fine matrix of cement of the concretions. Minors inclusions are rock debris or dusts aggregates. The main cements are siderite euhedral crystals with size of several micrometers. In addition, many iron-rich spherulites adhere to outer surface of the concretions, and part of elongated concretions consisting with iron-rich spherules. The porosity of the elongated concretions is less than 20%, and they are much denser than most concretions formed in surface depth. The original upright occurrence of the rod-shaped concretions indicate that it is not the residue product after leaching the underlying hard loess layer. Also, the upright occurrence infers that the age of the elongated concretions are not older than the age of the specific silty layer which is dated as ~7 ka B. P. There are two possible origins of iron-rich concretions. One is lake deposition hypothesis, that concretions grow and form in the porous space of mudlayer

in lake bottom；the other is airburst fallout hypothesis, that elongated concretions form in airburst plume and fall down and insert
into lake mud layer. We think airburst fallout hypothesis is more reasonable, because the fallout materials of airburst plume can  explain the unique morphology and internal characteristics of the elongated concretions.


Advances in the Study of Biogeochemical Cycles of Phosphorus

ZHOU Qiang, JIANG Yunbin, HAO Jihua, JI Junfeng, LI Wei

2021, 27(2):  183-199.  DOI: 10.16108/j.issn1006-7493.2020002

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Phosphorus is an essential element for life and an important limiting factor for food production. The biogeochemical cycles of phosphorus not only regulate marine primary production, but also has an impact on the global climate system; it also determines the formation and distribution of phosphate resources and affects the continuity of life on earth. The current theory of “earth system science” integrates the subsystems of atmosphere, hydrosphere, lithosphere (crust and upper mantle) and biosphere, providing a broader view for studying the global phosphorus cycles. Based on the existing research and combined with the theory of “earth system science”, the following important understandings of the biogeochemical cycles of phosphorus has been obtained: The evolution of phosphorus in geological history determines the current cycle pattern of phosphorus on a global scale (terrestrial ecosystems and marine ecosystems); Human industrial and agricultural activities, as an important geological agent, has changed the biogeochemical cycles of phosphorus, resulted in resource crisis of phosphate depletion and environmental problem of eutrophication of water bodies; The key to solve the resource crisis problem of phosphorus shortage and environmental pollution problem of phosphorus surplus lies in regulating the biogeochemical cycle process that cause these problems. 


Investigation of the Feasibility of Appling Low-density Geochemical Mapping in Tropical Rainforest Areas: A Case Study of Padang and Bengkulu Area in Sumatra, Indonesia

CHENG Xiang, HU Peng, ZHAN Mingguo, PAN Luozhong, DAI Yu, PAN Beihong

2021, 27(2):  200-210.  DOI: 10.16108/j.issn1006-7493.2020006

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Whether geochemical mapping at different scales could obtain stable and traceable geochemical patterns and whether it could truly reflect the background values of different elements are important for testing the feasibility of the selected geochemical mapping methods. The tropical rainforest of Padang-Bengkulu area in Sumatra, Indonesia was selected for this study, and the results from geochemical mapping at the scales of 1: 1000000 (1 sample/100 km2) and 1: 250000 (1 sample/4 km2) were compared in this area. Their background values, medians, and geochemical patterns obtained are very consistent. The geochemical provinces are very similar and reproducible by different sampling densities, and the concentration centers are overlapped, which indicates that the low-density geochemical mapping can obtain stable and traceable geochemical patterns and that this method is applicable in tropical rainforest areas. The geochemical provinces delineated by the low-density geochemical mapping cover the large ore deposits and granite bodies, and truly reflect the distribution of different elements in the study area. Geochemical mapping at a larger scale could be implemented in these geochemical provinces and many local anomalies could be delineated for exploration, which thereby reduces risks in exploration and investment.

Wettability and Heterogeneity Impact on DNAPL’s Migration: Pore-scale Model and Mechanism Analysis

PENG Jinxiao, JIANG Jianguo, WU Jichun

2021, 27(2):  211-217.  DOI: 10.16108/j.issn1006-7493.2019095

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Dense Non-aqueous Phase Liquids (DNAPLs) are common pollutants in groundwater. Understanding the migration mechanisms is of great significance. To explain the effect of wettability of porous media on the flow rate and residual saturation of DNAPL in the experiment, we simulated the DNAPL migration via solving the basic fluid mechanics equations at pore scale and reproduced the
behaviors similar to the experimental results. The results show that the multi-interface model can better explain the migration mechanism and reproduce a phenomenon similar to the laboratory scale. The falling speed is affected by both advancing interface and rear interface while the residual saturation is mainly determined by the rear interface. In addition, we demonstrate that the effect of wettability on motion works through the heterogeneity of porous media.


Experimental Investigation on the Interaction Between Calcite and Silica-bearing Fluid: Implications for the Formation of Silicified Carbonate Reservoir

YANG Yuanxian, CHEN Qianglu, QIU Ye, YOU Donghua, WANG Xiaolin

2021, 27(2):  218-228.  DOI: 10.16108/j.issn1006-7493.2020017

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Recent oil and gas exploration shows that silica-bearing hydrothermal fluid is an important acidic fluid in carbonate sequences. Knowledge on the interactions between silica-bearing fluid and carbonate rock is critical to understand the origin of the silicified carbonate reservoir and the prediction of reservoir distribution. In this study, experimental investigation was carried out on the interaction between calcite and silica-bearing fluid at temperatures ranging from 200 to 375℃ by using fused silica capillaries and
hydrothermal reactor as the reaction chambers. In situ Raman spectroscopy was used to describe the process of the reaction. Besides, Scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS) was used to observe the morphology and to identify the composition of the quenched solids. Firstly, the temperature condition of the decarbonization reaction between silica-bearing fluid and calcite is revealed. Calcite reacts with silica-bearing fluids at temperatures above 275℃ to form CO₂, and the solid phase is non-wollastonite calcium silicate. The detailed structure of this calcium silicate needs further investigation. This result indicates that the dissolved silica itself cannot react with limestone at the reservoir temperatures. Secondly, the high salinity and the CO₂-bearing nature of the silica-bearing fluid are the important factors causing limestone dissolution. Finally, the presence of CO₂ can promote the precipitation of siliceous component, including quartz. Based on the above experiments, the formation of the silicified carbonate reservoir in the Shuntuoguole area of the Tarim basin is proposed, integrated with the previous studies. The silica-bearing hydrothermal fluid migrates upward along the deep and large faults, passing through the Sinian-Lower Ordovician dolomite layer, where the siliceous components will react with the dolomite to form magnesium-rich silicate and CO₂. CO₂ is an important acidic component, which is conducive to the dissolution of the shallow carbonate and the preservation of pores. Decrease in fluid temperature and pressure, and the presence of CO₂ result in the precipitation of quartz, forming large amounts of intercrystalline pores.

3-Dimensional Simulation of Land Subsidence with Consideration of Dewatering and Foundation Pit Excavation

MA Xinyu, WANG Jun, ZHOU Xin, ZHANG Yun, XU Yongfu

2021, 27(2):  229-239.  DOI: 10.16108/j.issn1006-7493.2020008

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Cut and cover method is a common method in tunnel construction, but it inevitably cause land subsidence. To protect urrounding environment and buildings from severe adverse effects, land subsidence is a key factor to assure that the construction supporting design is reasonable. In the process of the construction, the main factors that can cause disturbance are dewatering, excavation and supporting. Previous studies were mostly focused on the influence of one single factor. To better understand actual cases, these three factors should be jointly taken into consideration and a three-dimensional simulation involving the whole construction process is needed. Based on the construction of a tunnel under a lake in Wuxi, two three-dimensional numerical models were built with ABAQUS software in this paper, and each model was designed to represent one section with a specific excavation and supporting plan. The models were employed to simulate land subsidence and soil displacement caused by dewatering and foundation pit excavation. The simulation can represent the construction of cut-off wall, retaining wall, pile foundation, dewatering, excavation, and supports. The simulated results have also been examined by the measurement data from the tunnel construction. The results show that (1) with the increasing distance to the foundation pit, the displacement of soil changes from uplift to subsidence and then gradually decreases. The proportions resulting from excavation and dewatering to the final settlement are 30%-40% and 60%-70%, respectively. (2) the ground settlement of the section supported by bored piles is greater than that of the step-slope excavation section with the same excavaing depth. (3) pile foundations are helpful for controlling ground subsidence.




Influence of Subducting Plate Dynamic Properties to Flat-slab Subduction by Numerical Modeling

ZHU Zhiyuan, WU Benjun

2021, 27(2):  240-248.  DOI: 10.16108/j.issn1006-7493.2019110

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This particular phenomenon of flat-slab subduction mainly occurs in South American, which is closely related to the structural geological phenomena such as earthquakes and volcanoes in this area. However, mechanics of flat-slab subduction is not yet
well understood. By numerical modeling, we investigate influences of subducting plate dynamic properties to the geometry of flat-slab subduction. The model results show that subducting plate thickness and density contrast between slab and mantle have great impacts to flat-slab formation. The appropriate slab thickness (around 70 km) facilitate slab flattening, while thicker slab is difficult to bend and hesitates to flatten. The smaller the density anomaly of subducting plate, the easier slab flattens and the longer flab-slab length. When density anomaly is very large, no flat-slab is predicted. In addition, strong plate promotes flat-slab formation. Flat-slab length increases with plate viscosity increasing. We also find that trench rollback velocity decreases during the slab flattening episode. Our reference model REF_MODEL has comparable flat-slab geometry with central Chile subduction, which provides the insight of flat subduction formation in this area.