Table of Content

20 December 2002, Volume 8 Issue 4

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Article

Isotope Geodynamics of the Subduction and Exhumation of Continental Plate

ZHENG Yong-Fei, FU Bin, GONG Bing

2002, 8(4):  365-379. 

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The Dabie-Sulu Orogen has been recognized as an ultrahigh-pressure metamorphic terrane in central-eastern China that was produced at Triassic by collision between the Yangtze and North China Plates. Stable isotope studies were carried out for ultrahigh-pressure metamorphic rocks from the Dabie-Sulu Orogen in east-central China.The results show not only 1oca1 oxygen isotope anomaly with the extremely low δ18Oof -10‰ in coesite-bearing eclogite but also heterogeneous oxygen isotope ratios with δ18O varying from -10‰ to + 10‰.The former indicates that eclogite protolith underwent meteoric-hydrothermal alteration before UHP metamorphism and the subducted plate has the nature of continental crust,whereas the latter suggests rapid subduction of the Yangtze Plate otherwise oxygen isotope homogenization would be attained.Preservation of the negativeδ18O anomaly in the eclogite indicates very limited crust-mantle interaction during UHP metamorphism at mantle depths.As a result,a short period of 5 t0 10Ma is reckoned when the plate containirig the eclogite precursor was subducted into mantle depths greater than 200km , so that no oxygen isotope reequilibration was achieved between the 18O depleted eclogite and the hot mantle.Oxygen isotope geothermometry yields concordant temperatures of 650 to 750℃ for different mineral pairs that have a large difference in oxygen diffusivity,and the isotope temperatures are in agreement with the petrological temperatures of 600 to 800℃ from cation exchange thermometry. Thus oxygen isotope equilibrium among the minerals has been frozen in at eclogite-facies conditions, and no significant retrograde exchange took place during eclogite exhumation.This points to a process of rapid decompression or cooling subsequent to the UHP metamorphism ,corresponding to rapid ascent of the UHP metamorphosed s1ab.These isotope results provide geochemical constraints on the geodynamics of the Yangtze continenta1 plate,and the bulk period for this cycling is estimated to be about 10 to 20Ma.An ice-frying mode1 is therefore advanced for the rapid subduction,the short UHP metamorphism , and the rapid exhumation of the Yangtze Plate.

Geochemical Characteristics and Compl ex,Luding County,Genesis of the Qiaotou Basic Western Yangtze Block

SHEN Wei-zhou, GAO Jian-feng, XU Shi-jin , ZHOU Guo-qing

2002, 8(4):  380-389. 

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he K-Ca Age of the Eclogite from Bixiling,Dabie M ountains：A New Evidence of the Excess Argon in Phengite

PENG Zi-cheng1 2, WANG Jun-xin1, LI Shu-guang1, LIU Gui-jian1, ZHU Xue-yun1, ZHANG Xun1

2002, 8(4):  390-398. 

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40K is subjected to branched decay to form 40Ar by electron capture and 40Ca by beta (β-)decay.In general cases,the 40K-40Ar or 39Ar-40Ar and 40K-40Ca ages are consistent in the error range.In order to test the measuring system ,the K and Ca contents and their isotopic compositions in the potassium feldspar(W03116),pellet(W7200),biotite(ZBH-25),hornblende(ZBJ) national standards,and the Tachiti secondary international standard have been determined and their results are certified values in the error range.The 40K-40Ca isochron was made for 30 data of 40Ca／42Ca and 40K／42Ca in phengite, omphacite and garnet in eclogite from Bixiling,Dabie Mountains and showed a slope of S=0.1208±0.0423.corresponding to a 40K-40Ca age of 228±75Ma. This is close to the Rb-Sr age of 212 M a and Sm-Nd age of 239Ma in the eror range.However.the 39Ar-40Ar isochron ages of phengite and omphacite are 807.1± 32.6 Ma and 456.7± 27.2 Ma, respectively,which are obviously older than that of 40K-40Ca age.It provides a direct evidence for phengite and omphacite with excess argon due to both isotopic systems of K-Ar an d K -Ca belonging to the branched decays. In addition.the primary value(40 Ca／40Ca)i of the K-Ca isochron is equal to 151.424±0.013.and corresponds to Ca +27.Because the mantle rocks have Ca=0,it is certified that the original rocks of the eclogite have the continental character. This provides a new criterion that the ultrahigh-pressure metamorphic rocks at Bixiling in Dabie Mountains were formed during the process of subduction of the continental materials to the upper mantle and their exhumation back to the continent.

Sm-Nd and Rb-Sr Isotopic Ages of Shaba Granulite from Western Sichuan Province and Their Geological Significance

XU Shi-jin, YU Hang-bo, WANG Ru-cheng, LU Huai-peng, FANG Zhong

2002, 8(4):  399-406. 

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Using the Sm-Nd and Rb-Sr mineral isochron methods. we have dated the Shaba granulite,which is one of the oldest basement rocks in the western margin of the Yangtze Plate. The Sm-Nd mineral isochron of 1 140 ± 1 10Ma represented the time of granulite facies metamorphism during the later Middle-Proterozoic era, when the Grenville-age Orogeny was developing around the world.The Rb-Sr mineral isochron of 218± 1 1Ma might be a thermal event age disturbed by the Late Permian -Triassic intrusion mass of. stratified gabbro. Its depleted mantle Nd model age of 1 1 58Ma and εNd= 5.5 showed that protolith of the Shaba granulite was experienced rapid granulite facies metamorphism after petrogenesis of the protolith. Based on above-mentioned dating results and the closure temperatures of Nd isotope in hypersthene and 40Ar／39Ar age of hypersthene, the authors suggest that the lower-crustal rocks in western margin of Yangtze Plate, including Shaba granulite, were slowly uplifted after 1140Ma.

Garnet and Quartz Exsolutions in Clinopyroxenes of Mafic Rock Xenoliths from Xuzhou-Northern Anhui Province

WANG Qing-hai1, XU Wen-liang1, WANG Dong-yan1, LIN Jing-qian1, LIU Xiao-chun2

2002, 8(4):  407-415. 

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There occurs an Early Jurassic intrusive complex belt in Xuzhou-northern Anhui province which is distributed in NE direction parallel to Tan-Lu Fault.It is mainly composed of diorite and monzodiorite.The 40At／39Ar plateau ages of hornblendes from diorite and monzodiorite are in the range of 189～191Ma.The eclogite,garnet-pyroxenite and clinopyroxenite xenoliths are found in this intrusive complex.Abundant exsolved quartz rods and exsolution lamellae of garnet are identified within or between clinopyroxenes of eclogite, garnet-clinopyroxenite and clinopyroxenite xenoliths under microscope.The core of the omphacite,in which quartz rods occur,is relatively poor in SiO2,Al2O3,CaO,but rich in FeO and MgO than its rim.There is also a systematic compositional change within the clinopyroxenes containing exsolved garnets：the core is obviously rich in A12O3,Na20 and TiO2,and the rim in contact with exsolved garnet is rich in Mgo,SiO2 and CaO.The garnets exsolved within clinopyroxene are similar to those exsolved between clinopyroxenes in mineral chemistry,which indicates their exsolution origin.A spectacular microstructure of oriented quartz rods within clinopyroxene implies preexistence of silica-oversaturated omphacite stabile at ultrahigh pressure(≥ 25kbar)conditions.The existence of exsolved garnet within single clinopyroxene crystal indicates that decrease of temperature and pressure would be a major cause of exsolution. Therefore, we conclude that most exsolution microstructures in the xenoliths may have been resulted from the decomposition of very high pressure precursor phases(clinopyroxene)due to decrease of pressure and temperature(especially pressure).The mineral assemblage and petrography of the xenoliths show that the xenoliths were subjected to eclogite-facies metamorphism and amphibolite-facies retrogresive metamorphism ,which is consistent with mineral exsolution due to decrease of presure.The discovery of high-pressure xenoliths and mineral exsolution will be helpful for us to understand the subduction and exhumation history of Dabie-Sulu UHP metamorphic belt.

Clinopyroxene in the Basalt from Okinawa Trough and Its Petrological Significance

TAN Qing-quan, CHEN Xiao-ming

2002, 8(4):  416-422. 

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Bsed on optical microscope and electron microprobe studies of clinopyroxenes in the basalt from Okinawa Through,we found three different generations of clinopyroxenes：phenocryst pyroxene,microphenocryst pyroxene and groundrnass pyroxene,with average size of 400μm ± ,30 μm ± and 5 μm ± ,respectively.There are obvious compositional differences among these three different pyroxene generations. From the core of phenocryst pyroxene, through microphenocryst pyroxene and the rim of phenocryst pyroxene,to groundmass pyroxene,the chemical composition evolved from Fe-rich to Mg and Ca-rich, which is opposite to the compositional evo1ution trend of pyroxene crystallized in closed system .The morphologic and compositional features reflect the magma mixing beatures of the basalt magma,and demonstrate the dynamic proeess of this mantle magma：The tholeiitic magma from the upper mantle was mixed by the magma carrying the Fe-rich pyroxene in the process moving up to the magma chamber：the mixed magma at magma chamber crystallized microlites and the rim of phenocryst；groundmass pyroxene was resulted from the quenching process of eruption. Consequently, clinopyroxenes in the Okinawa basalts keep well the records of the dynamic process of mantlemagma.

New Evidences for Natural Gas Source of Zhujiadun Gasfield

XU Zheng-long

2002, 8(4):  423-428. 

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Zhujiadun gasfield is located in Yancheng depression,Jiangsu Province of China. Former researchers believed that gases deposited in the E1-K2t sandstone beds came from the Paleozoic source-rocks,but liquid hydrocarbons in condensate associated with natural gas mainly came from sapropelic kerogen in the Taizhou Formation (K2t). Recent studies on maturity correlation of gases and source-rocks, anatomizing oil-gas mixing, the Paleozoic source-rock outcrop of depression bottom have suggested that high iso-heptane value(5～8)is characteristic for over maturity of the natural gas.Experiments also indicate that the iso-heptane values from the Triassic and the Paleozoic source-rocks areas high as the natural gases,and that iso-heptane values from the Early Tertiary and the Late Cretaceous source-rocks are much louser than the natural gases.Therefore,it is provided again that natural gases originated in the Paleozoic source-rocks. Based on carbon isotopic comparison of ethane and propane in natural gas and pyrolysis gas from source-rocks of various age,it also is suggested that limestone of the Qixia Formation of the Permian is likely a source-rock of the natura1 gas.The distribution of norma1 chain hydrocarbon analyzied recently has“two peaks”structure,but that analyzed two years ago only has one peak for same condensate sample associated with natural gas.The two peaks appeared because frontal peak became louser as the dissipater of light hydrocarbon actually reflect mixing of two source component.The latter peak would be crude oil came from sapropelic kerogen in the Taizhou.Formation(K2t).It is important that a feculence part of condensate has lighter carbon isotopic composition(δ13C-30.5‰)and shows sapropelic feature,but the natural gas has heavier carbon isotopic composition(δ13C2-27‰,δ13C3-25.6‰ )and shows humic feature. According to ananlysis of some hole data,outcrop under the Yancheng depression bottom is southern half of a large anticline and from north to south the formation age gradually becomes yanger.Limestone of the Qixia Formation in the Permian locates southern side of Yancheng depression bottom . The natural gas from source-rocks passed fault No.2 up to pool,and／or sidespin into reservoir from southern side of fault,and fault No.3 plays good side envelop.

Analysis of Effective Generation Center of Coal-Formed Gas in South Slope of Huimin Sag

CHEN Qing-chun1 2, CHEN Yong-hong2, WANG Xiu-peng 2

2002, 8(4):  429-436. 

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Coal beds occur in a wide area and developed with big thickness in south slope of Huimin Sag.These coal beds are sediments of marine-terrestrial interface which deposited under environments of tidal flat and lagoon.It is shown by gas generation simulation experiments that the gas generation potential of the Taiyuan and Shanxi Formation are 150mL/g and 1 10mL/g organic matter respectively.The source rock of coal-formed gas has a very complex thermal evolution process.The second gas generation occurred from the Early Tertiary to the Quaternary, with a threshold tempreture of 135℃ and corresponding depth of 4000m..The effective gas generation center is located in eastern of the Jiyang Graben and Quti Horst.The gas generation intensity is about 20 ×10 8m3／km2 to 40 ×10 8m3／km. Therefore,eastern parts of Quti Horst is the main target of exploration.

A Quantitative Study on Reservoir Heterogeneities of the 2rd Member of Shahejie Formation in Tuo 28 Block in Shengtuo Oilfield

ZHOU Jian-lin1, LIU Jing2, YANG Shao-chun3

2002, 8(4):  437-445. 

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Study of reservoir heterogeneity plays an important role in the exploration and development stages of oilfield,especially in the later stage of development.With the method of fuzzy mathematics to calculate heterogeneous synthetic index and permeability heterogeneity parameter,we conducted a quantitative study to investigate the second member of the Shahejie Formation. Tuo 28 fault block of Shengtuo Oilfield. The results suggested that the heterogeneities of interlayer,internal and plane are strong in the reservior beds.However as the development of further waterflood ,the heterogeneity shows a trend of reducing but the range of reduction is small and the heterogeneous degree of each substratum and rhythmic unit is apparently different in various stages of development, Sedimentary microfacies controls the heterogeneous degree, diagenesis makes heterogeneity complicated and the deficient injection and production pattern increases the heterogeneity in evidence.The results of study have good effects in increasing production of well bed ,well pattern adjustment,new well drilling and bring great economic benefits.

Study on Mechanics Experiments of Multiphase Medium Coal Rocks

FU Xue-hai, QIN Yong, JIAN Bo, WANG Wen-feng

2002, 8(4):  446-452. 

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Experiments were conducted on natural,water-saturated and water-gas-saturated coal samples to compare their mechanics.The results indicate that elasticity modulus,compression strength and volume compress coefficient of natural coal samples are larger than those of water-saturated coal samples and the latter are larger than those of water-gas saturated coal samples .However,their poisson’s ratios are opposite,and strains of parallel layer direction are larger than that of vertical layer orientation.The swelling experiments demonstrated that coal volume swells when it adsorbs gas and/or water medium ,and the coal rock strength and mechanical energy decreases. It was pointed that coal matrixes will shrink continuously, the coal reservoir mechanical intensity will gradually increase, and the poisson’s ratio and the plane stress ponderance of the covering stratum gravity will decrease,and the coal reservoir permeability may be improved with the discharging of water and gas medium during the coal bed methane recovery in ground wells.

Stability Analysis of Subway in Nanjing Using Preferred Plane Theory

ZHUANG Qian-cheng1, LUO Guo-yu1, LI Xiao-zhao1, YAN San-bao2

2002, 8(4):  453-459. 

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Using preferred plane theory,this paper developed a new analytical method for the preferred planes which control the urban shallow tunnel stability,taking the subway in Nanjing as an example.The new analytical method categorizes the soils into different layers and determines the preferred layers for the stability analysis of buildings in the soils.The results of the study on Nanjing subway are：① the regional preferred faults of Nanjing are Nanjing-Shuhu fault and Dinghuaimen-Gulou fault which control the tunnels stability in Gulou and Xiaohongshan；②the most favorable and mechanically stable layer for Nanjing subway is Tertiary hard clay layer(Ⅲ1), the next is the Tertiary soft clay layer(III2)；the most unfavorable layer for subway is Quaternary soft clay layer(II2)and loose layer(II4)；③the elevation of bottom of Nanjing subway tunnels is suggested to be at -5 ~-15m..

A Multi-Factor Evaluation Model for Landslide Hazard in Qingyuan,Eastern China

ZHAO dian-hua, YANG Shu-feng, CHEN Han-lin, CHENG Xiao-gan, MA Zhi-jiang, SHEN Xiao-hua

2002, 8(4):  460-465. 

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In recent years,remote sensing technology and GIS (Geographical Information System)technologies are developing fast,widely used in the researches on landslide hazard evaluation and prediction.The authors conducted a detailed study to analyze the triggering factors that could cause the landslides in Qingyuan region,Zhejiang Province,and their relationships with GIS and TM-image technologies.A new evaluation model is established based on our analysis,and the simulation results are in accordance with the facts of the study area,i.e.81.25％ of the landslide sites are located in the dangerous zones of the evaluation map.Therefore, this model can be used to evaluate the landslide hazard in Qingyuan region,Zhejiang Province.

Study on the Transition of the Most Dangerous Slip Surface in Layered Rocky Slope

FENG Zhao-xiang1, LIN Min2 1, CHEN Zhi-jian1

2002, 8(4):  466-472. 

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In engineering practice,the safety of slope is described generally by the stability safety coefficient.Each coefficient is corresponding to specific sliding boundary. In order to evaluate the stability of slope rightly,the most dangerous slip surface must be exactly determined in advance. Factors such as excavation,load and reinforcement affect engineering slope stability greatly.They often cause the change in the most dangerous slip surface,make evaluation difficult and even 1ead to wrong conclusion.In this paper,based on engineering practice,factors caused the phenomena, including structure type of slope,resisting action of the possible slip surface,loading, excavation, hydrogeology condition,prestressed reinforcement and bulkhead support. And then take Jiangyin Bridge Project as an example,effects of loading and excavation on the dangerous slip surface are discussed.The FEM calculation results based on interference displacement showed that. Under the action of vertical load of 1.2×10 6kN ,the most dangerous pasive slip surface of south tower foundation slope moved 30 meters downward.Excavation made the most dangerous slip surface of the man made slope in south tower zone moved 6 meters downward. The FEM calculation of water inlet slope in Xiaolangdi Project showed that engineering excavation made the most dangerous slip surface moved from deep to surface.

From Global Glaciation to Snowball Earth：Recent Researches on the Neoproterozoic Glaciation Events

ZHANG Qi-rui, CHU Xue-lei, ZHANG Tong-gang, FENG Lian-jun

2002, 8(4):  473-481. 

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The glacial rocks of Neoproterozoic are quite different from those of the Phanerozoic and have attracted attention of many outstanding geoscientists of different fields. In the recent decades,especially in recent years,the ’Snowball Earth’hypothesis aroused even more interests on this topic.The Neoproterozoic glacial environments and the causes of the glaciations have now become a hot topic in earth science. The recent researches are characterized by the high comprehensiveness,which is related to studies on litho- ,hydro-,atmo- ,and the biosphere,and event to the astronomy. Recent studies are mainly focused on the low altitude and low latitude glaciation of the Neoproterozoic,the effect of the breakup of supercontinent Rodinia,the counter relation between the icehouse and the warmhouse effects,the possibility of the complete covering of the oceans by kilometer thick ice,causes of the formation of the cap carbonates,environmental significance of carbon,sulfur,and strontium isotopes ,form ation of ironstone within the glacial deposition,ages of the Neoproterozoic ice,numerical simulation of the snowball earth theory,and the relationships between the variation of glacial environments and the Cambrian bio-radiation. The present researches are aimed to a large extent to reveal the characteristics of the glacial environments. and the isotope geochemical and geochemica l methods for extracting important environme.ntal information are widely used. However, the traditional studies on paleo-geomagnetism and geochronology remain crucial in the reconstruction of the paleo-environments. The stratigraphy of the Nanhua Period in South China are attracting attention of more and more geoscientists all over the world and become an important region {or the investigation of relationship between the Neoproterozoic glaciation and the Cambrian Bioradiation.

Geochemical Evolution Pattern and Enrichment Regularities of the Inorganic Oil and Gas of Huangqiao,Jiangsu Province

HUO Ming-yuan

2002, 8(4):  482-488. 

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The latest study shows that Huangqiao area,Jiangsu Province is located at the concealed deep fault from Binhai to Tonglu.Trending in NS and developing along 120°E longitude.the fault extends more than 500km long and is 20～30km wide.With a transcurrent shear property, it almost vertically cuts the late Tertiary system down to the bottom of lithosphere,and provides a favorable path for the inorganic oil and gas from the depth to move up toward the surface. The CO2 from the CO2 gas field of Huangqiao was considered to be originated from the mantle(Dai Jinxing,1995；Xu Yongchang,1994；Chen Husheng,1999).The research results in this paper indicate that the CO2 of this gas field was not derived from the mantle,but resulted from oxidization of mantle CH4 on the way moving up toward the surface(-3000~0m).Like a quilt with cotton wad,the CO2 gas field of Huangqiao is considered to cover over the underneath inorganic oil and gas.A deep-seated oil and CH4 gas field with a large reserves of more than one billion tonnes should be concealed under the CO2 field of Huangqiao. The practica1 sense to build such a geochemical evolution pattern is that there should exist oi1 and gas fields below any CO2 gas fields,but with different depths and reserves.It is not only a typical pattern summarised from the evolution of oil and gas resources of Huangqiao area,Jiangsu Province,but also has a general significance in the global scope.The evolution stage of the upper crust in this pattern indicates that the inorganic CO2 gas field of Huangqiao area was not directly produced from the upper mantle,but was an inevitable result from oxidation of mantle CH4 under oxidizing condition.Therefore,a CH4 gas field or a condensated oil field,or both of them ,should be found below the CO2 gas field of Huangqiao.To search for the inorganic oil and gas deposits in Huangqiao area,the favourable site should be the space under the C02 gas field and at the depth between -5000m and – l0000m .This paper suggests a three-dimensional prospecting model for the inorganic oil and gas deposits in the unknown area,which is more economic and with higher efficiency.