2018年, 第21卷, 第3期　
刊出日期：2018-09-25

  

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  Geochronology, geochemistry and geodynamics implications of Middle Triassic garnet-bearing muscovite monzogranite in central Jilin Province

  JIAO Ji, JIA Haiming, PEI Fuping, ZHOU Hao, ZHOU Zhongbiao, ZHANG Ying, XU Wenliang

  Global Geology. 2018, 21(3): 151-165. https://doi.org/10.3969/j.issn.1673-9736.2018.03.01

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  Zircon LA-ICP-MS U-Pb geochronology and geochemical analyses are undertaken for the garnet-bearing muscovite monzogranite in Yitong area, central Jilin Province. The formation age and petrogenesis of the granite are determined, and the regional tectonic background is discussed. Zircons from the granite are euhedral-subhedral in shape, and display fine-scale oscillatory growth zoning, indicating a magmatic origin. LA-ICP-MS zircon U-Pb dating result indicates that the garnet-bearing muscovite monzogranites formed in the middle Triassic (243 Ma). The garnet-bearing muscovite monzogranites have high SiO₂ (SiO₂=74.1%-76.49%), rich aluminum (Al₂O₃=14.47%-14.68%) and alkali (Na₂O+K₂O=4.31%-7.9%), low Fe₂O₃T and MgO (MgO=0.1%-0.13%, Fe₂O₃T=0.46%-1.02%). The ratio of CaO/Na₂O is between 0.17-0.21. The garnet-bearing muscovite monzogranites in Yitong region are relatively enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and depleted of heavy rare earth elements (HREEs) and high field strength elements (HFSEs). They are characterized by high Sr and Ba, poor Rb and Y, and negative Eu abnormally (δEu=0.48-0.62), with Rb/Sr<1. The ε_Hf(t) values and T_DM2 of zircons range from +6.10 to +8.00 and from 725 Ma to 814 Ma, respectively. The above features indicate that garnet-bearing muscovite monzogranites in Yitong area were derived from partial melting of metasedimentary rock. These granites have high Sr/Y ratios, which suggest they formed in a thickened continental crust. Combined with the coeval granitic rocks in central Jilin Province, we suggested that a significant collisional and thickening event took place during the Middle Triassic.
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  Petrogenesis of Early Cretaceous hornblende gabbro in Khanka Massif: evidence from geochronology and geochemistry

  WEI Chunxia, WEI Xu, ZHU Weigang, XU Wenliang

  Global Geology. 2018, 21(3): 166-176. https://doi.org/10.3969/j.issn.1673-9736.2018.03.02

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  The authors report zircon U-Pb geochronological, whole-rock geochemical and zircon Lu-Hf isotope data for the hornblende gabbro within the Khanka Massif, with the aim of constraining its formation time and petrogenesis. The zircon U-Pb dating shows that ²⁰⁶Pb/²³⁸Pb ages of zircons from the hornblende gabbro range from 120 to 129 Ma, yielding a weighted mean age of 123±2 Ma, i.e., the Early Cretaceous. The hornblende gabbro has SiO₂ of 44.77%-46.58% and belongs to the tholeiitic series on FeO^t/MgO-SiO₂ diagram. It displays a right-inclined REE pattern with (La/Yb)_N ratios of 3.44 to 4.42. The trace element spidergram shows that they are enriched in large ion lithophile elements (LILE) such as Rb, Th, U, K and Pb, and depleted in high field strength elements (HFSE) such as Nb, Ta, Ti and P, indicating an affinity to arc igneous rocks. The ε_Hf(t) values of zircons vary from -2.6 to +3.9 and Hf model ages (T_DM1) range from 622 to 883 Ma. These geochemical characteristics indicate that primary magma of the hornblende gabbro could be derived from partial melting of young mantle material accreted during the Neoproterozoic. Combined with the Early Cretaceous igneous rock assemblages in NE Asia. It is concluded that the hornblende gabbro formed in an active continental margin related to the westward subduction of the Paleo-Pacific Plate beneath the Khanka Massif.
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  Re-Os isotopic dating and geological significance of molybdenite from Xintian Cu-Mo polymetallic deposit in Yanbian, Jilin

  YIN Yue, CUI Bo, WANG Li, WANG Linlin, HUO Liang, LI Liang, DUAN Hongchang

  Global Geology. 2018, 21(3): 177-185. https://doi.org/10.3969/j.issn.1673-9736.2018.03.03

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  Xintian polymetallic deposit is a hydrothermal vein type Cu-Mo deposit. Analysis on Re-Os isotopic dating for the molybdenite from this deposit is undertaken to determine metallogenic epoch and dynamics setting of the deposit. Re-Os isotopic dating for the molybdenite yields model ages ranging from 123.9±1.9 Ma to 124.7±1.8 Ma, with a weighted mean age of 123.4±0.81 Ma, and an isochron age of 127±16 Ma, MSWD=0.25, indicating that the metallization was at late Yanshanian. The content of Re in molybdenite is (330.1±4.0-367.9±3.1)×10^-6, which demonstrates mantle derived metallogenic sources. Integrating the regional tectonic evolution and the metallogenic characteristics of other contemporaneous Cu-Mo deposits in the region, it is suggested that the Cu-Mo mineralization of this deposit occurred in an active epicontinental environment under lithospheric extension resulted from the subduction of the Izanagi plate to the Paleo-Asia continental margin at late Yanshanian. The Mantle wedge was partially melted with large amount of metallogenic elements dissolved in it. Under the favorable conditions of crust-mantle interaction, large-scale magmatic activity turns out to be the important mechanism of the mineralization of many Cu-Mo deposits in the Xiaoxingan Range-Zhangguangcai Mountain metallogenic belt, including the Xintian Cu-Mo polymetallic deposit.
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  Using satellite magnetic data to divide geological tectonic units of South China Sea

  ZHU Yixian, LI Tonglin, ZHANG Rongzhe, SHI Huiyan, CHEN Hanbo, LI Gen

  Global Geology. 2018, 21(3): 186-193,208. https://doi.org/10.3969/j.issn.1673-9736.2018.03.04

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  The South China Sea locates at the convergence center of the Eurasian Plate, the Pacific Plate and the Indo-Australia plate. The Cenozoic seafloor spreading in the South China Sea Basin is an important part of the tectonic evolution of the South China Sea that records information of the continental margin tectonic history and its impact on regional geologic evolution. Magnetic data contains abundant geological structure information from the surface to deep. This paper reports magnetic data of the South China Sea. Through the conventional processing of these magnetic data, we report general results on the regional magnetic anomalies, such as the upward continuation graph, the polar magnetic anomaly map and the magnetic anomaly partition map. The magnetic anomaly field in the South China Sea is divided into eight areas, of which the characteristics are explained, and the tectonic evolution of the South China Sea is preliminarily discussed.
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  A new approach of extracting vegetation points from urban airborne LiDAR data

  CUI Shaochen, YANG Yuanxin, YANG Guodong, ZHANG Xuqing

  Global Geology. 2018, 21(3): 194-202. https://doi.org/10.3969/j.issn.1673-9736.2018.03.05

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  Urban vegetation has been an important indicator for the evaluation of eco-cities, which is of great significance to promote eco-city construction. We study and discuss the commonly used urban vegetation extraction methods. The extraction of vegetation points in this study is completed through mathematical statistics, mean-square error, successive differences and iterative algorithm which are based on the analysis of different spatial morphological characteristics in urban point clouds. Linyi, a city of Shandong Province in China, is selected as the study area to test this method and the result shows that the proposed method has a strong practicality in urban vegetation point cloud extraction. Only 3D coordinate properties of the LiDAR point clouds are used in this method and it does not require additional information, for instance, return intensity, which makes the method more applicable and operable.
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  Analysis and evaluation of band-limited ray tracing method

  WANG Rui, SUN Jianguo

  Global Geology. 2018, 21(3): 203-208. https://doi.org/10.3969/j.issn.1673-9736.2018.03.06

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  From the analysis and the contrast of band-limited ray tracing method based on Kirchhoff integral, it was found that the method performs many approximations in the derivation process and omits the derivative term of the phase factor in the Kirchhoff integral. Numerical calculations and analysis show that the omission of correlation terms in Kirchhoff integrals has a non-negligible effect on the calculation results, and the different terms in the Kirchhoff integral have different effects on different frequency ranges. The method can be applied to velocity models containing complex interfaces without changing the velocity model and can be extended to elastic waves and continuous media.