2000 Vol. 6, No. 3

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A SEMINAR ON ANOMALOUS PRESSURE IN THE CRUST
CHEN Qing-xuan, REN Xi-fei, LU Gu-xian, DONG Shen-bao, LIU Rui-xun, CHEN Jing, WANG Fang-zheng, REN Ji-shun, LI Ting-dong, LIU Dun-yi, ZHAI Yu-sheng, ZHANG Bing-xi, SHAO Li-qin
2000, 6(3): 1-13.
Abstract (255) PDF (426KB)(9)
Abstract:
Papers collected in this volume are focusing on anomalous pressures present in the crust. Some theoretical considerations and calculations of pressures at certain depths are proposed.
The pressure-depth relation can generally be obtained from a certain density-depth distributaion in the region. It is known from stress measurements that the horizontal stress is almost always great than the vertical stress. This indicates that tectonic stress may play some important role in the stress state.
Furthermore,some other additional stresses may also be generated by facies changes,metamorphism,crystallization in the crust. How the UHPM rocks now exposed at the surface were formed is open to discussion. Lu et at (1988) proposed a method for determination of pressure at some depth.
ON THE DEPTH OF THE FORMATION OF COESITEBEARING ECLOGITE OF DABIE UHPM ZONE,CENTRAL CHINA
LU Gu-xian, CHEN Jing, DING Ti-ping, LI Xiao-bo, LIU Rui-xun
2000, 6(3): 15-25.
Abstract (161) PDF (418KB)(9)
Abstract:
The plastic deformation of garnet in coesite-bearing eclogite,quartz eclogite and garnet amphibolite of the ultrahigh-pressure metamorphic (UHPM) complex in Yingshan County in the Dabie Mt. is studied by the transmission electron microscope (TEM). It is shown than the garnets have undergone ductile deformation during eclogite and amphibolite metamorphic facies. Microstructures of the garnet vary greatly from coesite-bearing eclogite to quartz eclogite and garnet amphibolite.
The 3-D tectonic principal stresses and the hydrostatic pressure with a coupled tectonic stress(Ps) are reconstructed by differential stress and strain ratio (α) of garnet in minor coesite-bearing eclogite,and whereby the gravity and thickness of overlying rocks are determined. The formation depth of ≥ 32.09~32.11 km of the coesite-bearing eclogite in Yingshan County in the Dabie UHPM zone is obtained from PG(P-PS) where P is the pressure where the transformation of coesite phase took place,and PS=(σ1+σ2+σ3)/3. P is known to be 2.8 GPa by Qz-coes geobarometer. This result (ca 32km depth) is much smaller than 100km depth of eclogite in Dabie UHPM zone as achived by others by the method of weight/special weight (W/SW). Strong tectonism is thought to have made a great contribution to the formation of eclogite in the Dabie UHPM zome.
METAMORPHISM OF DABIE MOUNTAINS AND ITS RELATIONSHIP WITH THE COLLISIONAL OROGENY
WEI Chun-jing, CHEN Bin, ZHANG Cui-guang, CHEN Jing
2000, 6(3): 25-38.
Abstract (269) PDF (604KB)(13)
Abstract:
The Dabie orogenic belt can be divided into 5 meta-tectonic units from south to north,i.e.,the northern Yangtze blueschist belt,Susong metamorphic complex,southern Dabie collisional complex,northern Dabie metamorphic complex and northern Huaiyang metamorphic belt. The rocks of these different units have undergone three types of metamorphism:(1) The ultrahigh pressure type is repressented by the coesite (and/or diamond)-bearing eclogites and only found in the southern Dabie collisional complex,the matamorphic PT path of which implies an oceanic B-type subduction.(2) The high pressure type occurs in the blueschist belt,Susong complex and metasediments and some gneisses in the southern Dabie complex and is probably related with the continental A-tpe subduction of Yangtze Plate.The increase of metamorphic grades from south to north suggests the increase of depth in this direction of subduction. (3) The medium-low pressure metamorphism widespread in the Dabie Mountains may be related to the thermal relaxation in thickened crustal regions caused by large-scale collision,with the metamorphic grade being as high as amphibolite facies (locally to granulite facies) in the northern Dabie complex,and declining southwards and northwards, to epidote amphibolite facies and greenschist facies. The boundaries between different facies are consistent with the strike-slip faults and decollement faults of a late stage. This late stage of meium-low metamorphism has modified to different extents the high and ultrahigh pressure metamorphic rocks of early stages. The southern Dabie collisional complex comprises rocks at least of these three metamorphic types having different metamorphic histories or PT paths. These three type of metamorphism together with the spatial distribution of metamorphic facies belts in Dabie Moutains give the orogenic evolution through oceanic B-type subduction-continental A-type subduction-thrusting arising from continent-continent collision to decollement due to uplifiting.
ISOTOPIC CONSTRAINTS ON THE FORMATION DEPTH OF ULTRAHIGH PRESSURE METAMORPHIC ROCKS IN THE DABIE REGION
DING Ti-ping
2000, 6(3): 39-44.
Abstract (187) PDF (270KB)(8)
Abstract:
The formation depth of metamorphic rocks in the Dabie ultra-high pressure metamorphic (UHPM) zone is a question attrating attention of geologists in the world. It has some bearing not only on our understanding of formation mechanism and evolution precesses of collision orogenic belt,but also on studies on earth's interior and geodynamic processes.
In this study, the isotopic data of metamorphic rocks in the Dabie UHPM zone are analyzed and discussed to give constraints on the formation depth of in the Dabie UHPM zone.
The εsr of eclogite in the Dabie UHPM zone varies from 18 to 42,and εNd from -6.1 to -17,showing remarkable isotopic disequilibrium. The oxygen isotope studies indicate that the protoliths of these UHPM rocks have experienced oxygen isotope exchange with meteoric water(or sea water) depleted in 18O before metamorphism and remained unchanged in the processes of metamorphism. Except for one from the Bixiling,all samples of eclogite from Dabie UHPM zone have the 3He/4He ratios falling within the range between 0.79×10-7 and 9.35×10-7,indicating an important contribution of He from continental crust. All studies of Sr,Nd,O and He isotopes indicate that the UHPM rocks have retained the isotopic characteristics of the original surface rocks. No significant infulence of mantle derived materials has been found in them.
Some researchers attributed the above isotopic characterisitics, to rapid subduction and exhumation that no isotopic exchange between the two would take place,with no remains of mantle materials in these UHPM rocks. However,this assumption is not justified with pressent knowledge. It was estimated that the whole process of UHPM has spanned a time of at least 15Ma. During such a long period,it seems impossible that no isotopic exchange between mantle materials and original surface rock would occur at a depth of ≥ 100km where the temperature would rise to ≥ 700℃.Conversely,the isotopic characteristics will be well explained by assuming that the UHPM occurred in the crust instead in the upper mantle.
CONSIDERATIONS ON DETERMINATION OF THE DEPTH OF GEOLOGICAL PROCESSES
LIU Rui-xun, LU Gu-xian
2000, 6(3): 45-49.
Abstract (137) PDF (208KB)(11)
Abstract:
The method sofar adopted to determine the depth where geological processes occur is to divide the pressure by specific weight (density) of rocks. This method is correct only when the rocks are assumed to be hydrosatic. Actually,rocks from the shallow crust to the deep mantle,are not ideally hydrosatic,and the viscosity coefficient of them is known to increase gradually with depth.Also,stress measurements show that horizontal compressive stress is usually greater than vertical stress at some depth,indicating the important role of tectonic force in the stress state of rocks. It is worthy of note that Lu et al(1998)proposed a method for measuring the depth where some geological processes occurred.
A METHOD FOR ESTIMATING THE DEPTH OF PETROGENESIS AND METALLOGENESIS
LU Gu-xian, LIU Rui-xun, WANG Fang-zheng, DING Ti-ping, LI Xiao-bo, CHEN Jing
2000, 6(3): 50-62.
Abstract (190) PDF (542KB)(22)
Abstract:
The method proposes to calculate the hydrostatic pressure by subtracting the tectonic component from the total pressure at a point in the crust. such a pressure is referred to as a tectonic-stress free pressure or a teetonic-stress corrected prossure. In this paper,the depth of the sold mineralization is calculated as 3.5km~2.5km,and the depth of the formation of the coesite-bearing eclogite,Dabie UHPM zone,China as 32km. Details of the calcuation are given in te text.
A PRESSURE MODEL FOR ULTRAHIGH-PRESSURE METAMORPHISM
HU Bao-qun, WANG Fang-zheng, XIAO Long
2000, 6(3): 63-68.
Abstract (184) PDF (226KB)(8)
Abstract:
Based on the study on the ultrahigh-pressure metamorphism and the calculation of pressure pervading in the crust,the present paper shows that the pressure causing ultrahigh-pressure metamorphism should include in addition to the load of surrounding rocks also the pressure arising from phase change,and tectonism. A model for ultrahigh-pressure metamorphism to is thus proposed. In this model the pressure-depth relation is not linear.
OVERPRESSURE SYSTEM IN YINGGEHAI BASIN AND ITS RELATIONSHIP WITH OIL/GAS POOLS
YIN Xiu-lan, LI Si-tian
2000, 6(3): 69-77.
Abstract (202) PDF (357KB)(7)
Abstract:
The Unique geological evolution of the Yinggehai basin resulted in more than three sets of thick muddy deposits. subsequent compaction of the sediments and differential discharge of fluids from the pores led to the existence of large scale overpressure systems. Formation of large amount of hydrocarbons and high heat flow reinforced the overpressure. Diapirism made the overpressure propagate upwards. As a result,the overpressure systems were developed in the well-dinpired central part of the basin,where the top of the system rises to a depth of 2000~3000m and slopes towards the margins. The episodic release of the overpressure may cause an upward migration of the oil and gas to shallow depth to accumulate into pools.
STUDY OF PLASTIC DEFORMATION OF GARNET AND CALCULATION OF FOSSIL DIFFERENTIAL STRESS
CHEN Jing
2000, 6(3): 78-82.
Abstract (140) PDF (202KB)(11)
Abstract:
Any thermal event that has occurred during and after the formation of rocks can be traced in the rocks themselves. How to search these traces has been the focus of geological research in our country for a long time,and it is also a problem that needs to be solved as soon as possible. In recent years,results of studies by transimission electron microscopy (TEM),have shown that garnets occurring in different types of metamorphic rocks have undergone different degrees of plastic deformation.An intensive study on the extent and the characteristics of the deformation will be very useful in disclosing the history of metamorphism and deformation that the rocks have suffered. By determing the dislocation density of garnets one can roughly calculate the ancient differential stress tha the rocks have experienced to provide data for a model of tectonic evolution of the area concerned.
DUAL METASOMATISM BETWEEN MAGNETITE AND PYRITE AND ITS IMPLICATIONS
XU He-ling, CHEN Ke-qiao, MA Xing-hua, SUN Zhi-ming
2000, 6(3): 83-89.
Abstract (259) PDF (258KB)(9)
Abstract:
There is convincing mineralogical evidence such as metasomatic pseudomorph and replacement remnants to support that dual replacements have taken place in the Early Paleozoic carbonate rocks form southern part of the Huabei basin. Two kinds of replacements that have proceeded in opposite directions in two different periods were found to occur even in the same grain. The dual opposite metasomatism between the ferromagnetic magnetite and the paramagnetic pyrite would produce noise on the palaeomagnetic data to distort the palaeomagnetic results of a given region,if passed unnoticed.

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