Table of Content

20 September 1999, Volume 5 Issue 3

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Article

CRUST MANTLE INTERACTION AND THE GENESIS 0F GRANHTE: A CASE STUDY 0F C0ASTAL AREA 0F S0UTHEASTERN CHINA

XU Xi-sheng, ZHOU Xin-min, WANG De-zi

1999, 5(3):  241-250. 

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Granitoids are the main component of the continental crust, and the key problem for understanding the structure，composition and evolution of the continental lithosphere．Late Mesozoic calc-alkaline magmatism is wel1 developed in the coastal area of southeastern China, and is similar to the magmatism developed in the Basin and Range of North American． During the previous studies on the granites in the coastal area of southeastern China，however, 1ittle attention has been paid to the temporally and spa1ial1y coexisted A—type granite，gabbroic stocks or dikes, as well as the bimodal volcanic rocks (basalt-rhyolite composite lavas). The authors have been studying the granites in the coastal area for many years and noticed recently that this region experienced extensive crust-mantle interaction at the crust-mantle boundary, and underplating was the important process which involved in the chemical and thermal contributions from the mantle to the crust. The petrography, geochemistry and isotope studies for the gabbroic granulite from Qilin basaltic pipe in the same region，and integration with the regional geophysical, lava indicate that gabbroic granulites were formed by underplating of basaltic magmas at the bottom of the crust at ca 112 Ma．This is consistent with the extensive granitic magmatism in the Late Mesozoic time along the coasta1 area of southeastern China. Therefore，underplating plays important part in the evolution of the continental crust of southeastern China． Most of the granit formed in the earlier stage of Late Mesozoic time in southeastern China are S-type granites. They might be generated from the partial melting of the deeper crust material under the compressive tectonic setting. While the extensive I-type and A-type granitic magmatism in the coastal area of southeast China is closely related with back—arc extension，lithosphere thinning and asthenosphere upwelling，which may have been induced by the earlier stage subduction of the Pacific plate towords the Eurasian continent. In fact，the late Mesozoic magmatisms along the coastal area have an essentially extension related bimodal character，and apparently，the genesis of granities in the crustal area of southeastern China is directly related with the basaltic magma underphlating. The source age of 1O0～110 Ma of southeast China may represent the transition age of tectono-magmatism rom” compression-crust thicking-continental crust re-melting” to “extension-lithosphere thinning-bimodal magmatism”.

DIAGNETIC ALBITIZAT10N IN THE MESOZ0IC AND CEN0Z0IC RESERVOIR SANDST0NES FROM KUCHE DEPRESSION, N0RTH TARIM, XINJIANG, CHINA

YE Ying, SHEN Zhong-yue, ZHEN Li-bo, WANG Huai-zhao, FANG Da-jun

1999, 5(3):  251-259. 

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The Mesozoic and Cenozoic in Kuche Depression is predominantly a set of terrestrial detrital formation. In the reservoir sandstones in the area, albite is one of the widely developed authgenic minerals. Micro-structural evidence shows that diagenetic albite occur as euhedral crystals in the pores, and “places feldspar and other detritus along cleavage and micro-cracks. EPMA analysis indicates that same of the alteration product has a chemical composition transitiona1 between the pure albite and the replaced detrital feldspar. For example, composition of replaced alkali feldspar is between microcline and albite, and An content of altered plagioclase decrease continuously. This has been regarded as a direct evidence for fluid replacement, because the feldspar crystallized from solution or magma usually has a fixed chemical composition. On the basis of mineral assemblage observed by SEM , EPMA and XRD , there exist three kinds of albitization: (1)Ab replaces Kf; (2)Ab replaces the assemblage of Kf, Kaol and Q, forming iliite simultaneously; (3)Ab replaces Pl and Q. In reactions(2)and(3), the volume of resultants much less than that of thereactants. And in reaction (1), dissolution of Kf is more apparent than precipitation of Ah, so that the albitization is in favor of development of secondary porosity and improvement of the physical properties of the reservoir. Aagaard et.al.(1990) found that in eodiagenetic zone(2 km)albite makes up 3 to 5％ of the total volume of the rock, while in mesodiagenetic zone(4-5km)it is about 9％ This means that 30～50％ of detrital Kf is replaced by albite. In Kuehe Depression, albite distribution is also correlated to burial depth. XRD analyzing result shows that in the reservoir sandstone below 4000m , the albite content increases with increasing depth, but in the shallower levels, no such relation is found, Taking into account of the distribution tendency of kaolinite. XRD results show that in the reservoir sandstones above the burial depth of 3000 m. 38.9％ samples were found with XRD detectable kaolinite, but in the sandstones below the depth of 4000 m. only 11 5％ samples contain kaolinite. Both of them suffered no surface weathering, with no significant difference in source materials and sedimentation environment. It seems that more kaolinite was consumed in the deeper reservoirs during diagenetic process. It can be concluded that albitization described by reactions (1)to (3)is dependent on geothermal gradient, and mainly takes place in 9O℃～l10℃ thermal girdle. This is in consistent with the view point of Morad et. a1. (1990) and Saigal et a1. (1988)made on the reservoir sandstones in Norwegian North Sea oil field. In North Tarim, most of the proved oil and gas accumulations are concentrated within the burial depth of 4000 to 5500m , corresponding well to the geothermal zone of albitization. At present we can riot say that this means any causal relationship between them , but there is no doubt that albitization contributes a lot for the development of secondary porosity which in turn improves the physical properties of the reservoir.

THE ORIGIN AND EVOLUTION FOR THE ORE — FORMING FLUIDS OF TONGCHANG PORPHYRY COPPER DEPOSIT, DEXIN: OXYGEN IS0TOPIC C0NSTRAINS OF GRAN0DIORITES

Guo Xinsheng1, Ji Kejian2, Hung Yaosheng3, Chen Jiangfeng1

1999, 5(3):  260-268. 

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Tongchang corphyry copper deposit, together with the Zhushahong and Fujiawu porphyry deposits, makes up Dexin copper field-the largest copper field in China．It is located in the northeast part of Jiangxi province, China．The ore bodies are mainly found in the contact zone between the Proterozoic Jiulin group and granodiorite porphyry body．This paper focuses on the oxygen isotope composition and variation pattern of the granodiorites in vertical direction, and their relation with the alteration of granodiorite in Tongchang porphyry copper deposit Dexin. The result show that the δ180 values of granodiorite, whole rock; vary from 6.4‰ to 10.7‰, and the feldspar separates have theδ18O value of 6.5‰ ～ 8.9％. There exits certain variation pattern for the oxygen isotope composition of the granodiorite whole rocks and feldspar, separates, namely, theδ18O values of the whale rocks and feldspar separates decrease with the depth from the surface to the deeper levels Through oxygen isotope exchange calculation between meteoric water andgranodiotite using Taylor’s model, it can be inferred that the meteoric water with a value of -8.5‰ circulated in the wall rocks of granodiorite firstly, and then continued to flow from deeper levels to the surface in mining area. Because of different interaction temperatures and δ18O values of the interacted water at the shallower and deeper positions, the granodiorite whole rocks have different variation patterns: the altered rocks of the shallower levels usually, have higherδ18O values, while the lowerδ18O values of the grannodiorites arc found in the deeper positions. The oxygen Isotope compositions and their variation pattern of quartz samples are different from those of whole rocks and feldspar separates. Theirδ18O values(8.4-9.8‰ )are not related to depth, but related to the extent of alteration. The quartz samples of intensely altered grannodiorties have the highest δ18O values The increment of quartz during alteration of feldspars is responsible for their oxygen isotope composition change. It can be inferred that the ore-forming fluids of Tongchang porphyry copper deposits were derived from the evolved meteoric water.

STUDY ON THE CHR0N0GENESIS RELAT1ON OF MINERALlZAT10N AND DEF0RM ATION By MAGNETIC FABRIC ANALYSIS

Chen Bai-lin

1999, 5(3):  269-274. 

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The magnetic fabric is a susceptibility anisotropy of the rocks. Magnetic fabric analysis, as a method of geological researches, is widely u 10 study structure deformation. According to its features of thermal demagnetization and thermal homogenization, the author determined the chronogenesis relation of the mineralization and the deformation by means of comparative ana1ysis between the characteristics of the magnetic fabric and the deformation of the rocks. A negative correlation has been found between the gold abundance and susceptibility anisotropy (P) in the mineralized and unmineralized mylonite from Qifengcha altered mylonite type gold deposit, North Beijing, i.e. the higher the Au—abundance is, the lower the susceptibility anisotropy (P).The results of this s1udy suggest that the time of mineralization is later than that of deformation of the mylonite. This is important for the study of mineralizing mechanism of this type gold deposit. There is also a negative correlation between the content of Pb + Zn and the susceptibility anisotropy (P value) of the samples from the section of the ore, bearing structural gone in the Zongshuban vein Pb-Zn mega-deposit, south of Hunan, indicating that the deposit is formed by lateralization of ore-beaning hydrothermal solution and that the deformation is weaker after the mineralization. Meanwhile, the P values of the altered rock and ore-rock are higher (1.O441)than that of the granite-porphyry (1.0146), and the orientation of the minimum susceptibility axis of the altered rock and ore-rock(northeast)is different to that of the granite-porphyry(southeast). There also suggest that the period of the emplacement of granite-porphyry-veins with weak deformation is tater than that of mineralization, concordant to its genesis of magmatic hydrothermal deposit controlled by ear1y Yanshanian granite and the fault or fractures drawn from the data of gao1ogy, geochemistry and isotopic geochronology.

EVOLUTION OF JIYANG DEPRESSION AND ITS TECT0NIC IMPLICATIONS

Zong Guohong1, Xiao Huanqin1, Li Changbo11, Shi Yangshen2 Wang Liangshu2

1999, 5(3):  275-282. 

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The Meso-Cenozoic Jiyang depression is an assemblage of the negative inversion basin, dextral transtensional basin and active rift with four evolutionary phases. 1. The intraplate thrust orogenic phase (from the Triassic to Middle Jurassic): represented by the development of five NW-striking compressive belts, which were mainly reverse faults. 2. The negative inversion phase (from the Late Jurassic to Early Eocene, i.e., 149～45Ma): the negative inversion of the previous reverse faults, leading to the development of SN-striking high- dip normal faulting. Therefore, the sediments mainly deposited in the NW –trending half-grabens. 3. The dextral transtensional phase (from the Late Eocene to Oligocens, i.e., (45～24.6Ma): development of the NE-striking dextral transtensional faults, the WNW-trending sinistral transtensional fault, the secondary ENE-striking dextral transtensional fault and the EW- striking normal fault. These faults controlled the sedimentation in the basin and the previous faults became inactive. Consequently, the dextral transtensional basin overlapped the negative in version basin. 4. The active rifting phase (from the Miocene to Holecene, i.e., (24. 6Ma～present): the previous differential block faulting was replaced by the regional depression. The evolution of major faults in the Jiyang depression could be used to interpret the shearkinematics history of the Tan. Lu fault zone in the Meso-Cenozoic. The results of this study demonstrated that the Jiyang depression underwent dextral shear during the Triassic, sinistral shear from the Late Jurassic to the Early Eocene, dextral shear from the Middle Eocene to the Oligocene, an d weak dextral transpression from the Miocene to the Holocene.

METHOD OF COMMON DEFORMATION FOR ANALYSIS OF SUPPORTING STRUCTURE IN DEEP EXCAVATION

Zhang Yang-song， Luo Guo-yu

1999, 5(3):  283-289. 

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Based on the study of the advantages and shortcomings of the general elastic foundation beam method for deep excavation, the authors proposed a computational model and analysis procedure of common deformation method considering interaction between supporting structure and soil layers. The common deformation method comprises the following analysis steps: (1) Calculating of the initial static pressure; (2) Defining the horizontal foundation reacting force coefficients representing soil around the wall, stiffness of the wall, and the struts or anchors; (3) Calculating of the incremental displacement; (4) Recalculation of the earth pressure by adding the initial static earth pressures to the changes in earth pressure, which is derived from the multiplication of incremental displacement by reacting force coefficient; (5) Comparing the earth pressure with soil strength limitation criteria i.e., the convention al active or passive limit. If any strength is infringed, a set of correction forces is calculated, which then will be used to restore the earth pressures consistent with the strength criteria and plastic deformation within the soil. (6) Replacing the forces used in the step (1) new forces. Repeat the step (3) to (5)until convergence is achieved. (7) Calculating the total displacements, earth pressures, strut forces, wall shear stresses and bending moments. A comparison of the results of two computation methods has demonstrated that the horizontal foundation reacting force coefficient, which is same as in the general elastic foundation beam method, could be used in the common deformation method. Different methods (constant or “m” method) of selecting the horizontal foundation reacting force coefficient may have important influence on the results of computation. Our study suggested that the common deformation method is useful in the practice, and more reasonable than the general elastic foundation beam method.

COMPREHENSIVE STUDY ON GEOLOGICAL IENVIR0NMENT QUALITY OF RELOCAT10N CITY SITES IN RESERVOIR AREA OF THE THREE GORGES PROJECT

Li Tie-feng1, Pan Mao1, Sun Wei-dong2

1999, 5(3):  290-297. 

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The Three Gorges project is attracting worldwide attention. The migrating population of the Three Gorges reservoir is about 1.2 million which is rare in the present world. However, the reservoir area of the Three Gorges project lies in a medium-low mountains, hills and gorges with complicated geological conditions and limited capacity of geological environment. Various geological hazards happen seriously in this area. As the growth of population and development of 1ocal economy, many problems of environmental geology will be worse. Lacking of land resources and water resource, intensifying of landslide and rock collapse, caving and reconstruction of reservoir bank, and karst collapse are the primary problems in construction relocation cities. Based on the practical studies on the transfer and reconstruction of cities and towns in the Three Gorges reservoir area, a comprehensive evaluation indexes system is established, which includes individual geologic body quality, the synthetical disaster of city geological environment, water pollution level, natural, resources superiority, the economic loss-profit degree of geological environment and the carrying capacity of cities. With the method of duster analysis, the geological environment quality of nine new city sites is analyzed and compared comprehensively. Three different geologica1 environment grades can be divided among the new city sites. Comparatively, the geological environment quality of new city sites in Zigui and Yunyang is better than that in Fengjie, Badong and Zhongxian. The quality of the new city sites in Wushan, Wanxian, Fengdu and Fuling is the worst. The grades of the gological environment quality reflect the coordination between the city sites environment and the human activity. The synthetical environment quality of the new city sites is affected by many factors, in which individual geologic body quality, the synthetical disaster of city geological environment and natural resources superiority are crucia1. During constructions and development of the new cities in the reservoir area, the advantages of environment conditions should be used fully and the land resources should be used rationally. The monitor net of geological hazards in the reservoir area should be established and strengthened. The loss cause by geological hazards should be minimized. Under the guiding of geological engineering theory, construction of the new cities must act in accordance with the prevention and control of geological hazards. The human-activity-induced geological hazards must be avoided as far as possible.

THE CONTENT VARIATION OF HARMFUL ELEMENTS IN SOILAND VEGETABLES IN FIELD AMELIORATED WITH FLYASH AND THEIR ENVIRONM ENTAL SIGNIFICANCE

Tan Qu-lin1, Zhai Jian-ping1, Zhou Wei-ke2 Qian Qin1

1999, 5(3):  298-305. 

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How to deal with the coal fly ash drained from the power plants is a crucial problem related to the sustained development of the whole society. Soil amelioration with fly ash is an important approach to comprehensive utilization of the latter However，fly ash contains harmful elements such as Cd, Hg, As, Cr, Pb, Ni, U, Th. How do these elements affect soil and vegetables? Is there an enrichment of harmful elements? The experiment of vegetable and soil amelioration with fly ash from the Second Power Plant in Nanjing has been performed. The applied amount of fly ash was of four levels: 0t/hm2, 15t/hm2, 30t/hm2 and 45t/hm2. The contents of harmful elements of 38 samples, including fly ash rumples, soil samp1es applied with different amount of fly ash and vegetable samples grown in the soil were analyzed. The results show that the contents of harmful elements in the soil and vegetable are lower than the lever stipulated by the state standards about the soil and vegetables. There does not exist a positive correlation between the contents of the harmful dements and the applied amount of fly ash. Consequently, these vegetables are eatable as common vegetables. It is safe to use the fly ash drained from the Second Power Plant in Nanjing into the soil with an amount in the range of present experiment, namely not higher than 45t/hm2

HYDROCHEMICAL IDENTIFICATION OF BIODEGRADATION IN THE GROUNDWATER

Chen Yun-dao1, Liu Jian-li2, Wu Jian-feng2 Zhu Xue-yu2

1999, 5(3):  306-311. 

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The Dawu Well Field is one of the largest karst-fractured groundwater sources in the east of zibo City, Shandong Province and It is the most important resource for the urban water supply in Zibo City. However, data from 35～ 56 monitoring wells in the Well Field indicated that groundwater was seriously polluted by petroleum products. Maximum concentration of organic contaminants up to 77.34 mg/1 was detected in the water supply wells located at the Liuhang-Hougao zone in the went part of Dawu Well Filed. A series of environmental investigations during the recent years showed that serious leaking of petroleum contaminant occurred in the subsurface of petrochemical factory areas in the upriver of Dawn Wel1 Field forming a major source of groundwater contamination by petroleum hydrocarbons. Since the Hougao and Liuhang zones are seriously polluted, hydrochemical environment has changed greatly in the past ten years (1987 to 1996). Concentrations of nitrate and sulfate in the groundwater oontaminated by pe trochemicals were reduced from 29.04 to <0.4mg/l and 113.59 to 51.23 mg/l respectively. On the other hand, concentrations of bicarbonate and ammoniumion, were increased from 298.79 to 426.84 mg/l and < 0.04 to 0.43 mg/1. Dissolved oxygen was almost not detected. Comparing with the zones seriously polluted, concentrations of nitrate and sulfate in the ground water of Xixia wells located in the east of Dawa Wel1 Field were increased from 15.75 to 21.71mg/1, and 38.90 to 55.23mg/l in the past ten years. And concentration of bicarbonate changed from 237.97 to 215.81 mg/l. These changes of hydrochemical characteristics suggested that some microbial processes have graded organic materials in the groundwater leading to a decrease in the concentrations of electric acceptors and an increase in concentration of bicarbonate near the zones seriously polluted. These microbial processes included biodegradations with oxygen, nitrate and sulfate ere. However, the hydrogeochemical conditions of the Hougao and Liuhong aquifers are characterized by absences of nitrate and dissolved oxygen and the low concentration of sulfate. The subsurface environmental conditions would reduce the rate of biodegradation beeaL1se of the absence of the electric acceptors. Therefore, the results of this research have demonstrated that： 1. The microbial processes could be evaluated through the hydrochemical data of groundwater. 2. Biorestoration method to renovate aquifers contaminated with petroleum productions in Zibo city is feasible. 3. To enhance the activities of micro-organisms in the subsurface water, increasing the concentrations of electric acceptors, such as nitrate, probably is a effective biorestoration method.

DISTRIBUTION GE0-TEMPERATURE IN DONGYING DEPRESSION，SHENGLI OIL AND GAS FIELD, NORTH CHINA BASIN

Xiong Zhen1, Wang liang-shu2, Li Cheng2, Shi Xiao-bin2, Guo Sui-ping2, Wang Jie3

1999, 5(3):  312-321. 

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Geo-temperature is a decisive factor for the formation of oi1 and gas. Distribution of geotemperature is a geophysical field changing with time and boundary conditions. Present geothermal field is the result of the origin and evolution of sedimentary basin, and a basic constraint for the paleo-geothermal field modeling and inversion. The thermal resistance method and bottom hole temperatures are used to determine heat flow and subsurface temperature. The dispersive model of sedimentary rock thermal conductivity is applied to determine the in situ thermal conductivity of Tertiary strata in Dongying Depression, which ranges from 1.6w/mK to 2.4W/mK for Minghuazhen Formation (Nm), from 1.6W/m K to 3.3mW/mK for Guantao Formation(Ng) and from 2.5W/mK to 3.1mW/mK for Dongying Formation (Ed). It has been found that in situ thermal conductivity in Dongying Depression is mainly related to sandy content and depth of the Tertiary sedimentary strata. Based on the results of above calculation and the measured temperature data, temperatures on different interface, and some particular depths are calculated at the 170 temperature measured wells using thermal resistance method. The temperature changes from 2212 to 5512 on the top surface of Minghuazhen Formation (Nm), and from 41℃ to 72℃ at its bottom surface. Temperatures on the bottom surface of Guantao Formation(Ng) are from 53℃ to 86℃. On the bottom surface of Dongying Formation(Ed), geo-temperatures are from 50 to 104℃. In Dongying Depression, geo-temperature ranges from 4912 to 8012 at the depth of lO00m, from 72℃ to 106℃ at the depth of 2000m , from 89℃ to 146℃ at the depth of 3000m, from 103℃ to 186℃ at the depth of 4000m. from 119℃ to 222℃ at the depth of 5000m The geo-temperature distribution at the same depth is closely related to the heat flow.

PREDICTING FRACTURED RESERVOIRS IN BURIAL HILL POOL WITH SEISMIC METHODS：A CASE STUDY IN S0UTHERN BOHAI BASIN

Yang Feng-li1, Zhou Zu-yi1, Zhang Shan-wen2, Oyang Yong-ling3

1999, 5(3):  322-327. 

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Fractured reservoir is of vital importance in oil and gas exploration However, because of their non-isotropic characters and 1ateral variations, the identification and prediction of fractured reservoirs are often difficult. Currently, most seismic approaches are still in a trial stage. In this paper, we will describe two seismic approaches that are applicable in solving the problem, i.e. coherence analysis and synthetic runic log technologies. The essential of coherence analysis is to as the corrdation or similarity between seismic traces. The distribution pattern of fractured reservoirs can be identified after the processing of 3D seismic data by coherence analysis. Based oil the sensitivity of the studied fractured reservoir, we further calculated, eight seismic parameters from 3D coherence date volumes and selected the minimum, seismic parameter from these parameters. The purpose of synthetic sonic logging is to convert seismic datas into bedded log sections by extracting the interval velocity information. Three critical technologies have been applied in this process based on synthetic seismogram’s scaling. They are frequency-domain deconvolution, phase correction and low-frequency modeling. The interval velocities that are consistent with the reservoir development section are extracted after the synthetic sonic data volume is obtained. These data are then presented as plane maps. Their trends reflect the distribution of fractured reservoirs. These two seismic approaches are applied to 3D seismic data volumes in the predication of fractured reservoirs that are developed in the Paleozoic carbonates and the Archaean granite-gneiss in southern Bohai basin. The predictions have been further tested by drilling exploration.

EPR STRUCTURAL EVOLUTION OF DEFORMED COALS AND ITS SIGNIFICANE IN STRUCTURAL GEOLOGY

Jiang Bo, Qin Yong

1999, 5(3):  328-333. 

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Electron paramagnetic resonance(EPR)experiment has been conducted to study 9 tectonic coal samples and 10 experimental coal samples deformed under high temperature and confining pressure．The results of this study demonstrated that the EPR struetura1 parameters, increasing with the maximum reflectance of vitrinite (Ro, max), are of a characteristic evolutionary pattern．In addition to the high temperature and confining pressure, high strain and high strain rate are beneficial for the increase of spin density(Ng)in medium and high coal ranks．Similarity in the evolution of peak width(△H)h been found between the sample of experimentally deformed and tectonic coals, suggesting that high temperature and confining pressure, as well as low strain rate are favorable for the decrease of H in its stage evolution. Evolution of EPR structural parameters of deformed coals is apparently affected by their original coal ranks. Directional pressure or tectonic stress is of particularly importance in controlling the increase of vitrinite anistropy, structural ordering and orientation of macromolecules of deformed coals. Therefore, Ro, max is also an external reflection of micro-structure evolution of deformed coals and reflects the features of strain environment and stress field. The vitrinite reflectance of tectonic coal is an important indicator for analysis of stress and strain in coalfield structures.

SOME THOUGHTS ON THE DEVELOPMENT OF MULTIMEADIA TITLES FOR HIGH EDUCATION: WITH THE EDITION OF “EARTH SCIENCES” AS AN EXAMPLE

Xu Shi-jin, Zhao Lian-ze, Lu Xian-cai

1999, 5(3):  334-339. 

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Based on the current condition of high education, the necessity and possibility of developing multimedia title as electronic textbooks are discussed, and the main characters of the new born textbooks is summarized. The key steps and main designing objectivs of multimedia titles, taking “Earth Sciences” as an example, are introduced, in which the integrity of knowledge and various forms of expression are stressed.

NECTIC CHERT, A VERY PECULIAR TYPE OF SILICEOUS ROCK

Zhu Si-zhao1, Hiromi Na-gai2, Masao Kmetaka3, Wu Chun-yin1

1999, 5(3):  340-344. 

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NECTIC CHERT, A VERY PECULIAR TYPE OF SILICEOUS ROCK

Zhu Si-zhao1，Hiromi Nagai2，Masao Kametaka3, Wu Chun-yin1

1999, 5(3):  345-350. 

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There is a very peculiar type of bedded siliceous rock occurred in the Gufeng Formation of the Lower Permian in the Hushan of Nanjing, jiangsu Province and the Pingdiangshan, Anmenkou of the northern Chaohu, Anhui Province. The Gufeng Formation is dominated by a suite rock, usually grey of light yellow in color, is intercalated with bedded radiolarian chert or shales. It is characyerized by soft and porous nature, and looks like shle or diatomite at the surface outcrops. It can be seen under the microscope that the rock contains a large amount of rhombic and irregular cavities surrounded by siliceous cement consisting of microquartz. Sometimes small amount of calcite presents in the cements either as scattered crystals or as small irregular aggregates. Occasionally, residual calcareous bioclasts can also be found in the rock. Observation with SEM closed by fine anhedral micoquarrtz crystal. Only a few appear to be euhedral crystals. The chemical analysis date demonstrate that these porous siliceous rocks from the Gufeng Formation in northern Chaohu have a higher content of SiO2 (＞90%) and a very small amount of impurities. X-ray determination of the porous rock indicates that it consisted almost entitely of quartz. The preliminary study suggests that this is a siliceous rock originated by replacement of sedimentary carbonates, such as dolomitic limestone. The pores correspond to the casts of euhedral dolomite rhombs contained in a dolomitic limestone and at a later time dissolved out. Since the rock similar to the “chert nectique” (Cayeux,1929) in composition and feature of porosity, the authors proposed to name it nectic chert.

U－Pb ZIRCON AGES OF THE PYROXENITE-GABBRO INTRUSIONS IN DABIE MOUNTAINS AND THEIR GEOLOGICAL IMPLICATIONS

Li Shu-guang1, Hong Jian1, Li Huimin2, Jiang Laili3

1999, 5(3):  351-355. 

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Four U-Pb zircon ages ranging 123～130Ma for the Zhujiapu, Jiaoyan gabbro and Xiaohekou pyroxenite-gabbro intrusions in the Dabie Mountains suggest that these mafic-ultramafic intrusio11s are of the post-collisional origin and crystallized in the Cretaceous．In view of that the Cretaceous granitic dykes cut across the mafic-ultramafic intrusions, the large scale Cretaceous magmatic event in the Dabie Mountains was initiated by partial melting of the mantle in depth, which may be caused by upwelling of the mantle beneath the Dahie Mountains at that time．

DISCOVERY OF ECL0GITE IN XIAO QINLING, SHAN XI PROVINCE AND ITS TECTONIC SIGNIFICANCE

Zhang Chuan-lin1, Zhu Li-hua1, Zhang Yang-song1, Yang Zhi-hua2,

1999, 5(3):  356-360. 

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The ecoligte enclave was found for the first time in the group of the Neo-Arehean to Paleo-Proterozoic Taihua Group in Xiao Qinling, A detailed study has been conducted to investigate its outcropping characteristics, texture, structure and facieology. The EMP analysis demonstrated that the garnet composition is of 60～ 64％ Alm, 10～ l4％ Pyr and 18～ 19.5％ Spe．The percentage of the pyrope suggests that the eclogite in the Taihua Group is high-pressure metamorphic treks. The garnet enclaved in the amphibole has the highest pyrope content, which could reach up to 14％ and is similar to garnet in the eclogite in Dabieshan Mountain．The amphibole chemical composition is compatible with the amphibole compositions from granulite in the Taihna Group and appears to be high-pressure-type amphibole．Based on comparison of characteristics of the eclogite in North China craton, we discussed the forming environment and tectonic significance of the eclogite in Xiao Qinling.