2008 Vol. 14, No. 1

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METAMORPHIC COLLISIONAL MÉLANGE IN THE DABIE MOUNTAINS, EASTERN CHINA
XU Shu-tong, WU Wei-ping, LIU Yi-can, WANG Hua-ming
2008, 14(1): 1-21.
Abstract (373) HTML (199) PDF (1011KB)(9)
Abstract:
The Dabie Mountains are a collisional orogen between Sino-Korean and Yangtze continents. It is characterized by thin-skin tectonics as revealed by the seismic reflection profile. The rocks forming a detachment (thrust shear)zone between the two collisional continents are collisional mélanges. The detachment can only be defined by recognizing collisional mélanges in the Dabie Mountains because the overriding wedge has been completely removed by erosion and the ultrahigh-pressure (UHP)metamorphic belt exposed extensively. This is a necessary step to perform a geometric analysis of the Dabie Mountains. The Dabie collisional mélanges may be divided into two parts:the northern part consists of a banded gneiss-ultramafic rock assemblage and the southern part a mica plagiogneiss-eclogite assemblage. Both assemblages have mostly undergone UHP metamorphism. Their common features are the presence of "giant mortar texture" due to pervasive shearing and mixing of rocks from various tectonic settings. The so-called "Luotian dome" was a huge duplex anticline formed in the early stage of orogeny, which finally formed a dome formed by superimposed contraction in an east-west direction in the late stage of the orogeny.
RESEARCH ON IMPACT TECTONICS AND IMPACTITES:STATUS AND PROSPECTS
YOU Zhen-dong, LIU Rong
2008, 14(1): 22-36, 21.
Abstract (312) HTML (173) PDF (989KB)(26)
Abstract:
Transient (within 1 second)dynamic high pressures (ca tens of to up to one hundred GPa) and high temperatures (>1500℃)caused by the impact process will eventually result in the development of an astrobleme when a large meteorite impacts the earth's surface, and the surrounding rocks must undergo a special kind of metamorphism called impact metamorphism. The studies of impact tectonics include the following four topics:1. structure and morphology of the impact crater and its formation mechanism; 2. impact metamorphism and impact metamorphic rocks; 3. numerical simulation and experimental calculation of impact tectonics; and 4. economic benefits of impact tectonics. The studies of impact tectonics not only have academc significance but also have actual economic benefits. Vredefort-Witwatersrand in South Africa is one of the oldest impact tectonics and is also famous for its gold-uranium deposits. The Sudbury impact structure in Canada is famous for occurrence of a large syn-impact Cu-Ni deposit. Now oil and gas reservoirs related to post-impact tectonics are being explored for in a number of areas of North America and Central America. In addition, many impactites are used as building materials and many astroblemes become famous resort places, such as the Ries crater in Nordlingen, Germany. Much progress has been made in impact studies in China since the end of the 20th century. The Baisha crater found in Hainan in the early 1990s is known to the world and there are reports on possible craters such as Taihu in Jiangsu, Duolun in Inner Mongolia and Luoquanli near Xiuyan in Liaoning. However these structures remain to be further confirmed. The studies of global impact tectonics show that long-term stable cratons are favorable to discovering impact craters and many craters are hidden beneath Quaternary and older sediments. So satellite-borne photography and geophysical exploration will be effective in determining targets, and on that basis, through strengthening petrological and structural studies, impact craters may be confirmed.
OBSERVATION AND DETERMINATION OF THE NANO-SIZED PARTICLE LAYER IN ROCKS AND ITS GEOLOGICAL SIGNIFICANCE
SUN Yan, LU Xian-cai, SHU Liang-shu, LIU Hao
2008, 14(1): 37-44.
Abstract (150) HTML (130) PDF (344KB)(8)
Abstract:
Nano-sized particles are extremely tiny (≤ 100 nm)and homogeneous in size, consist of polybasic materials and have multiple functions. The nano-sized particle layer of shear slip plane in rocks was firstly discovered in granitic mylonite of the San Andreas Fault Zone, United States, and later this phenomenon was found in different rock types in some areas both at home and abroad. Under actions of shear slip, nano-sized particles with roundness and sphericity can become alien ones, and those alien particles may form layered textures and various structural types. There are different views about the genesis of nano-sized particle layers in rocks, and we incline to the view that shear friction plays a dominant role and that the nano-sized particle layer may be called a frictional-viscous thin layer. The mechanical, physical and chemical effects of nano-sized particles are multiple and novel and the study of nano-sized particle layers of rocks has great theoretical and practical significance without doubt. At present the study has just started both at home and abroad.
MAGNETIC FABRICS OF CENOZOIC SEDIMENTARY ROCKS IN THE WESTERN QAIDAM BASIN AND THEIR STRUCTURAL SIGNIFICANCE
LI Li, JIANG Rong-bao, QI Wan-xiu, WU Yi-ping, YANG Yi, LI Xue-zhi, CHEN Zheng-le, CHEN Xuan-hua, WANG Xiao-feng, REN Xiao-juan
2008, 14(1): 45-56.
Abstract (166) HTML (92) PDF (500KB)(9)
Abstract:
Magnetic fabric analysis of Cenozoic sedimentary rocks in the vicinity of Shizigou, western Qaidam basin, shows that the magnetic fabrics are characterized by well-developed magnetic foliation, undeveloped magnetic lineation and a flattened magnetic susceptibility ellipsoid, and the P value symbolizing the degree of magnetic susceptibility anisotropy is low, suggesting that ductile deformation is relatively weak. The stress state reflected by the rock magnetic fabric is mainly manifested by NE compression, which is consistent with the arrangement of the anticlines with axes running NW. The magnetic fabrics in the area mostly have the features of primary sedimentary fabrics, with the magnetic foliations largely reflecting the bedding of sedimentary rocks and also recording the signs of NE compression. According to the relation between the magnetic fabrics of rocks and bedding of strata, the Shizigou anticline with two asymmetric limbs in the western Qaidam basin has the nature of a faultpropagation fold and its formation was due to the southwestward obduction of the Huatugou fault underneath.
FRACTAL ANALYSIS OF THE TEMPORAL-SPATIAL DISTRIBUTION OF CRETACEOUS MINERAL DEPOSITS IN CHINA
SHEN Wei
2008, 14(1): 57-64.
Abstract (163) HTML (104) PDF (264KB)(9)
Abstract:
The concepts of fractals are used in the Earth sciences to depict the self-similar characteristics of geological variables and geologic bodies. The fractal model often provides powerful tools for characterizing the basic spatial distribution structure of geological variables and bodies. Many geological phenomena are scale invariant. Examples include frequency-size distributions of rock fragments, faults, earthquakes, volcanic eruption, mineral deposits and oil fields. Cretaceous ore deposits in China are la rgely distributed in six important mineral deposit-concentration regions, i. e. Northeast China, PaleoYinshan-Yanshan, Paleo-Qinling-Dabieshan, Yangtze platform and Southeast coastal region; whereas few deposits of Cretaceous age are known in Northwest China and Southwest China. Some important deposits have been discovered in eastern China. The distribution of Cretaceous deposits in China has the following fractal characteristics:in terms of time, they were concentrated in the Early Cretaceous, and in terms of space, they are mainly located in Northeast and Southeast China. Take for example Cretaceous gold deposits of China, the authors constructed a fractal model of the spatial distribution of gold deposits and predicted the number of gold deposits. The method is applicable not only to gold deposits but also to other ore deposits; so it has universal significance.
METALLOGENIC MODEL OF THE XIANGSHAN URANIUM ORE FIELD, JIANGXI PROVINCE
SHAO Fei, CHEN Xiao-ming, XU Heng-li, HUANG Hui-ming, TANG Xiang-sheng, ZOU Mao-qing, HE Xiao-mei, LI Mei
2008, 14(1): 65-73.
Abstract (291) HTML (209) PDF (357KB)(18)
Abstract:
The authors studied the regional geological setting and metallogenic characteristics of the Xiangshan ore field and analyzed the sources of ore substances and ore fluids and channels for transport of ore substances, and on that basis, they constructed a uranium metallogenic model for the Xiangshan ore field. Uranium metallogenesis of the Xiangshan ore field took place in a specific temporal-spatial domain controlled by the regional geological setting; regional uranium-rich strata provided substances for metallogenesis; and ore fluids derived from magmatic water, mixed meteoric water, magma and postmagmatic hydrothermal fluids were carriers of uranium transport. The uranium metallogenic model emphasizes that the formation process of volcanic rocks was a process of ore substance concentration. The evolution of the volcanic postmagmatic hydrothermal ore system gave birth to ore-forming processes with a time span of 50 Ma in the Xiangshan volcanic basin. The coupling of metallogenic mechanisms such as temperature decline, concentration and mixing of fluids promoted uranium precipitation and ore formation.
IGNEOUS SERIES AND EPITHERMAL PORPHYRY Cu-Au-Ag MINERALIZATION SYSTEM IN THE ZIJINGSHAN ORE FIELD, FUJIAN PROVINCE
HUANG Ren-sheng
2008, 14(1): 74-86.
Abstract (161) HTML (86) PDF (500KB)(13)
Abstract:
The rocks in the Zijingshang ore field are from the surface to depth as follows:Lower Cretaceous intermediate-acid volcanic rocks, subvolcanic dacite-porphyry, hypabyssal granodioriteporphyry and middle-and deep-level porphyritic granodiorite, forming an intermediate-acid volcanicintrusive sequence. Strong alteration-mineralization occur around the Zijinshan volcanic edifice, forming high-sulfur epithermal Cu-Au deposits, low-sulfur epithermal Ag-Au deposits and porphyry Cu (Mo) deposits. In the ore field, various types of Cu-Au-Ag deposit have close temporal-spatial and material source relations. They evolved continuously in terms of time and space and are products of consanguineous ore-bearing intermediate-acid magmas in the same background of mineralization but in different stages of evolution. The occurrence of different types of ore deposit in diverse geological positions depended on the physico-chemical properties and temporal-spatial migration of ore fluids, thus forming an epithermal porphyry Cu-Au-Ag mineralization system related to intermediate-acid subvolcanic rocks and porphyry.
GIS-BASED ANALYSIS OF GEOMORPHOLOGICAL FACTORS FOR LANDSLIDE HAZARDS
GUO Fang-fang, YANG Nong, ZHANG Yue-qiao, MENG Hui, YE Bao-ying
2008, 14(1): 87-96.
Abstract (265) HTML (113) PDF (1013KB)(12)
Abstract:
Based on SRTM-DEM data and using the ArcGIS platform, the authors have extracted and calculated the elevations, relief amplitudes and slopes of 2319 landslide sites on the eastern margin and the Qinghai-Tibet Plateau and in the Sichuan basin. The correlation between the geomorphological factors and landslide distribution has been preliminarily established. The landslides in the study area occur at sites with specific geomorphological factors and are mainly distributed along the active faults and river valleys. The elevations at which the landslides occur mostly range between 400 and 800 m and between 1400 and 2000 m. The landslides occurring in the two ranges account for 40% and 28% of the total in the study area. The relief amplitudes of the landslides are mainly in the range of 300-600m, and the lan dslides occurring in this relief amplitude range account for 48.68% of the total. The topographic slopes mainly range from 10° to 25°, and the landslides occurring in this slope range accounts for 44.70% of the total. The geomorphological types at these sites are mainly deeply incised V-shaped valleys and mountainous river valleys. In addition, active faults play an important role in controlling the distribution and occurrence of the landslides.