Table of Content

20 December 1995, Volume 1 Issue 2

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Article

A GENERAL REVIEW ON THE RECENT STUDIES OF GRANITE

Dong Shenbao

1995, 1(2):  1-12. 

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Inspired by the experimental studies and overall geochemical investigations in granites in association with the detailed works in some granite batholiths, the recent studies of granite have been advancing toward the elucidation of the ultimate reasoning of the origin of the granite in connection with the geological context through dynamic aspect. The experimental and thermal constraints on some granitic magmas of crustal sources and on the deep-seated parentage related to subduction revealed that: H2O-saturated liquid of granitic composition is a normal consequence of regional metamorphism; a wide range of granitoid magmas (including tonalite) can be generated by combinations of granitic liquid with suspended crystals of the refractory residual group; batholiths largely of granite and granodiorite could be produced by crustal anatexis, whereas those dominated by tonalites the heat and material transported from mantle in addition to crustal materials;and the granitoids are not primary magmas from mantle or subducted oceanic crust. The source and amount of H2O available have a direct effect on the initial melting temperature, the subsequent chemical composition of the magma and its crystallization history. The replacement textures.which play an important role in the formation of granitic melt, are directly related to the H2O available in the melt. They can be distinguished as the migmatic and magmatic replacement textural series corresponding to the initial stage of the granitic melt through ultrametamorphism and the sub-solidus crystallization in the late magmatic stage, respectively. Owing to the advancement of the geochemical works in studying granites, there has been controversy between the restite and mixing hypotheses-a recurrent theme as anatexis held by migmatists versus syntexis devoted by magmatists. The restite hypothesis rests on the geochemical and mineralogical characteristics of granitoids, which appear as the products of ultrametamorphism in melts residuum. The nature of the source material can be deduced in first approximation as I- and S-type granites from igneous and sedimentary sources, respectively. Based on type localities such as synplutonic dykes granite batholith and some mafic microgranular enclaves within granites, the mixing hypothesis argued that the mixing processes induced by the injection of coeval basaltic magma into the pre-existing granitic magma, which has been melted with the aid of the rising heat of basaltic magma itself, are important in categorizing the granite suites. It seems, however, from field occurrence, that the mixing may represent but secondary processes, despite their importance in some areas. Granite types, imaging their sources, also have various processes during their formation. Their ensemble is directly related to the overall geological Context, a manifestation of the thermal perturbation initiating the granitic melt. Under this heading, Pitcher’s classification named as granite typology is cited and discussed.

THE TECTONIC CONSTRAINT ON THE LATE MESOZOIC A-TYPE GRANITOIDS IN EASTERN CHINA

Wang Dezi1, Zhao Guangtao2, Qiu jiansheng1

1995, 1(2):  13-21. 

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A giant Late Mesozoic A -type granitoid zone occurred in east China can be divided into five sub-zones of two episodes. The products of the first episode (135～ 100M a) include lower Yangzi zone, Northern Jiangsu-Jiaonan zone, Shanhaiguan zone and Nianzishan zone, and those of the second one (100～ 70M a) are located at the Southeastern coast of China named Fujian-Zhejiang zone. They exhibit not only various rock assemblages but also obvious regional change in their emplacement patterns of the intrusives for the different zones and episodes. The study indicates that the A-type granitoids of the first episode were produced in the continent-continent collision transpression orogenic zone in the eastern Asia active continental margin where wide shearing tectonics was well developed (160～100M a), and emplaced in the places where the extensional jogs of P-shears and the collapse in the junction of P,T, and R shear zones were developed, which was generated by the intensive transtension in the pressure-shear stress field setting. The A-type granitoids of the second episode were produced in the collisional zone of the accretional terrane in the oblique subduction continental margin of southeast China, and related to the back-arc extension resulted from the Kula plate subducted beneath the Euroasian plate (100～70M a). The magmas emplaced in the areas of collapse which was generated by the differential uplifting and depression of the crust. Moreover, the extensional degree is possibly one of the important factor that constrained the formation of the A-type granitoids and affected the characteristics of the magma nature and emplacement.

A STUDY ON THE FUSION CRUST OF METEORITE

Wang Kui-ren, Hong Ji-an

1995, 1(2):  22-28. 

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In the present paper, the fusion crust of Boxian meteorite is analysed by several methods. Based on the chemical composition and compared with the whole meteorite, it can be found that the redox state of Fe in the fusion crust has changed: the amount of metal iron and FeS decreases, while that of Fe increases, and new phases wustite and magnetite are formed. The phase analysis by X-ray powder diffraction shows that all the olivine in the fusion crust is forsterite compared to chrysolite in the internal meteorite because of phase transformation and phase rebuilding at high tempratures. In the meantime. the cell volume of the three dominant minerals in the fusion crust is larger than that inside the meteorite. The crystallographic parameters of all these three minerals decrease in one direction, and increase or keep constant in the other two directions. All these are concordant with one-direction stress. Due to the shock pressure, the minerals suffered one-dimensional crystal compression. The infrared spectrum pattern of the fusion crust reveals the occurrence of diaplectic glass, which indicates that the sh0ck pressure is higher than 53.0 GPa. The SEM micro-images of the fusion crust give a perfect section, which shows that the crust can he divided into three layers from the surface inwards: glassy layer, proous layer and transitional layer. The outer layer-s made up entirely of glass, about 50m thick, and it is a real fusion crust. The mid-layer is porous, about 210~m thick. The internal layer, about 250μm thick, is similar to that inside the meteorite except the grain sizes of the minerals. In these three layers, the grain size is of micrometer scale, unlike that in the internal meteorite. The absence of chondrules indicates that the fusion crust suffered breaking action under high shock pressure. The occurrence of the real fusion glass indicates that melting did take place in the fusion crust due to high temperature. The pores are formed due to extracting action. In summary the fusion crust has been modified by the following processes: oxidation, shock, breaking, melting and extraction. All these results can give guides to the research of some aerospace materials.

EARLY PALAEOZOIC SEDIM ENTATION AND TECTONIC EVOLUTION IN QINLING

Mei Zhichao, Cui Zhilin, Meng Qingren, Qu Hongjun

1995, 1(2):  29-36. 

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The Qinlng orogenic belt has a complex evolutionary history. During early Palaeozioic time, the Southern Qinling was a passive continental margin of the Yangtze Plate, whereas the Northern Qinling became an active continental margin with the northward subduction of oceanic crust to the south. The belt underwent two great transgressive-regressive events. The first event occurred in the Cambrian and up to the Early Arenig, and the second one took place in the Late Arenig up to the Silurian. Thick platform carbonares in Zhenan Xichuan of the Southern Qinling and the North China Craton, and widespread pelagic and hemipelagic sediments composed of radiolarian cherts, siliceous and carbonaceous slates in oceanic or rift basins, suggesting that the Qinling area was in general in a spreading stage during the Early Palaeozoic Era. The uplift and denudation in the southern margin of the North China Craton during the Middle Arching and the appearance of a deep-water rift basin in the southwestern margin of the North China Craton in the Caradoc might be related to plate subduction. From the Late Ordovician to the Silurian, a large-scale regression occurred in this area. In the wide area, south of the Shangdan suture zone, the tectonic deformation and foreland basin deposits related to Caledonia movement have not been found and the Silurian sediments are still characterized by quartz sandstone with high maturity. Thus, we consider that the collision orogenesis did not occurred in this time,and this regression was probably related to the global sea—level descending.

A DISCUSSION ON THE ORIGIN OF ORE FLUID OF THE YINSHAN POLYMETALLIC DEPOSIT, JIANGXI, CHINA

Hua Renmin, Wu Peihong, Chen Kerong

1995, 1(2):  37-44. 

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On the basis of geological and geochemical characteristics of the Yinshan polymetallic deposit, Jiangxi province, and from the view point of water-rock interaction and related isotope exchange, the present paper queries the validity of the conclusion that magmatic water was the main origin of ore-forming fluid in Yinshan deposit, which was proposed by some previous studies. Oxygen isotope compositions of altered rocks and fluid were determined for reverse calculation of water-rock isotope exchange. It shows that the ore fluid of Yinshan deposit was originated from meteoric water instead of magmatic one. This fluid was the product of interaction between the down going meteoric water and phyllitic wall rocks which had higher a δ18O values in deeper portion at higher temperature (≥ 400℃) and lower W／R ratio (≤ 0.05). As a result, the δ18O value of the fluid had been greatly enhanced and could reach the value similar to mgmatic water, which is easy to lead to the conclusion of a magmatic origin. As a matter of fact, the small-some subvolcanic magmatism in Yinshan area mainly served as a heat source but not a water source for the large-size mineralization.

FEATURES OF As, Sb, Hg, TI AND THEIR MINERAL ASSEMBLAGES OF SOME DISSEMINATED GOLD DEPOSITS IN GUIZH0U AND GUANGXI PROVINCES

Qian Handong, Chen Wu, Hu Yong

1995, 1(2):  45-52. 

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A systematic study on characteristics of As, Sb, Hg, T1 and their assemblages of some sediment-hosted disseminated gold deposits in southwestern Guizhou and northern Guangxi has been carried out based on the field geological investigation and experimental mineralogy． The gold deposits involved are situated in the southwestern margin of the Yangzi Paraplateform and bordered oil the Southern China fold belt which is controlled structurally and stratigraphically. They Occur mainly in the Upper Permian and Middle-lower Triassic siltstone, claystone, sandstone-black shale and marl which are important ore-bearing formations of polymetals for they have rather high contents of Au, As, Sb, Hg and TI. The wall rock alteration related to gold mineralization is generally silicification, pyritization, arsenopyritization and argillic alteration. Among them the silicification is very common in these areas. The gold deposits are usually composed of pyrite, marcasite, arsenopyrite．stibnite, realgar, opiment, cinnabar, quartz, hydromica group, carbonates etc.. It is suggested that pyrite and arsenopyrite are the most im portant Au-bearing minerals in all these gold deposits．The majority of Au in the ores are present in visible grains (usually less than 0.0n mm ). It is obvious that the gold mineralization in these areas is accompanied a special indicative element assmeblage and definite zonally distributed mineral intergrowths. Enrichment of gold is promoted by multistaged mineralization. Five types of mineral paragenesis can be classified according to the features of geochemical elements and mineral associations in these gold deposits: 1) native gold-stibnite-pyrite; 2) native gold-cinnabar-stibnite-pyrite; 3) native gold-arsenopyrite-realgar-orpiment-stibnite; 4) native gold-cinnabar-realgar-orpiment-stibnite-pyrite; 5) native gold realgar – orplment – cinnaba r- arsenopyrite-TI minerals. The genetic interpretation of the As, Sb, Hg, TI element associations with Au is also discussed in this study based on the results of experimental mineralogy. The stable phase relations of the pseudoternary As2S3-HgS-TI2S system and the ternary Au-TI-S system are successfully used to estimate the formation conditions of the element associations and mineral assemblages of the gold deposits. It is noteworthy that the tectonic activity of the faults also play an important role in the mineralization process of these low-temperature, low-pressure disseminated gold deposits. Both macroscopic and microscopic studies show that gold mineralization usually presents in strongly alternated tectonic belts in which not only pyrite and arsenopyrite but also orpiment, realgar, stibnite or cinnabar and TI-beating minerals can be observed． It is suggested that the characteristics of the indicative element associations, such as As, Sb, Hg and TI, and the corresponding mineral assemblages are of typomorphic significance for searching and evaluating the disseminated gold deposits．

THE TECTONIC CHARACTERISTICS AND EVOLUTION OF THE QINLING-DABIE HIGH-PRESSURE AND ULTRAHIGH—PRESSURE METAMORPHIC BELT, CENTRAL CHINA

Yang Weiran, You Zhendong, Han Yujing, Zhang Zeming, Wei Bize, Liu Rong

1995, 1(2):  53-64. 

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The Qinling-Dabie high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic belt is one of the major constituents in the quasilatiludina1 global mobile belt of Mediterraean which has a long mobilization history. The HP-UHP rocks are developed in folded belts of various ages． In the Qinling-Dabie HP and UHP metamorphic belt the two HP and UHP metamorphic terranes, namely，the Qinling and North Hubei-Tongbai-Dabie，are subdivided by the delineation of the convergent suture of Dangfeng-Shangnan-Xinyang-Tongcheng fault. For the former, Qinling terrane, there are two HP belts in the south and north respectively. For the latter it is further subdivided into blueschist belt，HP／LT eclogite belt, UHP eclogite belt，HP／MT eclogite belt and eclogite-poor belt. All of them ace roughly symmetric in spatial distribution． The HP and UHP metamorhpic rocks are greatly diversified in various regions. They were even formed in different ages. Generally speaking, there are five stages of deformation, namely. HP-UHP stage (D1), HP-UHP metamorphic stage (D2), the formation of regional foliation stage(D3), ductile shear deformation stage(D4 ),uplift and exhumation stage(D5), comprising a complete deformational sequence． Based on the study of tectono-isotope chronology, five age groups have been obtained. The first group about 2000Ma is probably the protolith age of HP and UHP metamorphic rocks: the 2nd group of Jiningian (700~ 1000Ma) is the formation stage of HP metamorphic rocks；the 3rd group of Caledonian (400~ 540M a) represents the main period formation and metamorphic peak stage of HP-UHP metamorphic rocks the 4th group of Hercynian (271~ 326Ma)represents the retrogressive metamorphism of eclogites; the 5th group of Indosinion (205~ 243Ma)is another age of formation of HP metamorhpic rocks. Therefore, we can identify four tectonic evolution stages: Meso-Neoproterozoic stage, early Paleozoic stage, late Paleozeic to Middle Triassic, and the post-Triassic stage.

THE FINITE ELEMENT METHOD FOR MODELINC 2-D MT FIELD ON A GEOELECTRICAL MODEL WITH CONTINUOUS VARIATION OF CONDUCTIVITY WITHIN EACH BLOCK

XU Shizhe, Yu Tao, Li Yugao, Liu Bin

1995, 1(2):  65-73. 

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The finite element method is used for modeling 2-D MT field on geoelectrical model with continuous variation 0f conductivity within each block. In the first part of this paper, the calculating formulae of coefficient matrixes in the FEM are given，which can be used for programming. The results of calculation for some 2-D models and the analysis of accuracy of calculation are discussed in the second part of this paper．

STRESS FIELD OF THE EASTERN AREA OF THE YANGTZE GORGE RESERVOIR AND THE SHIZIKOU GRAVITATIONAL GLIDING SYSTEM

Song Lihao, Yang Sennan, Wu shuren, Ren Jianye

1995, 1(2):  74-81. 

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Systematic Study on the regional Stress field in the eastern area of the Yangtze Gorge Reservoir and the local stress for the Shizikou gravitational gliding system has been carried out by stereographic and statistical analyses on numerous conjugate joints, slickensides, styolites and micostructures including fractures with their veinlets, pressure solution seams and tectonic lenses. The regional principal compressive stress (σ1 )is determined to be horizontal with two stress groups directed S-N and NEE75°－SWW250°respectively. Based upon quantitative analysis on regional paleostress field by rock rupture criterion combined with e-twin ratio and dislocation density methods, the differential stress during S-N compressive deformation is estimated at 200Mpa, whereas that during NEE75°-SWW250°compression at 80-120MPa. The cross-cutting relationships between conjugate joints, fault striae, microstructures and regional faults and folds indicate that the S-N-and NEE75°-SWW250°oriented compressions were active during the main stages of the Yanshanian and the Himalayan movements respectively. It is shown by finite-element simulation that the Yanshanian differential stress in the southern area has higher strength than that in the northern section In contrast, no significant spacial variations in differential stress have been found for the Himalayan compression. Stereographic and statistical analyses on minor structures inside the Shizikou gravitational gliding system reveal that the stress field of that system is generally characterized by front-edge compression and hack-edge extension with gently dipping σ3 towards NE SW and moderately dipping to nearly vertical σ1 at the back edge, and gently dipping σ1 towards NE-SW and moderately dipping to nearly verticalσ3 at the front edge. The strss field of the Shizikou gravitational gliding system is defined to be the local one superimposed on the regional field yielded by the main stage of the Yanshanian movement．

THE GEOLOGICAL INTERPRETATION OF THE FORMING AND EV0LUTION OF LOP NUR, NW CHINA

Guo Zhaojie, Zhang Zhicheng

1995, 1(2):  82-87. 

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The main rivers in the Tarim basin flow towards the Lop Nur area, the lowest part of the basin. This phenomenon is resulted from the special geological background around Lop Nur. Because of the Himalaya collision orogenic event, there are Kuluktag dextral strike-slip fault zones on the northeast margin and Altyn sinistral strike-slip fault zones on the southeast margin of the Tarim basin. There are a few subsequent faults in both above-mentioned fault zones. Resulting from the sinistral displacement of the Ruoqiang fault(RF)and the dextral displacement of the Kongquehe fault(KF), there is an E- W extensional tectonic region setting in the Lop Nur area. The Lop Nur lake sag is a half- graben resulted from this extension. The distribution of rivers around Lop Nur is controlled by that of the fault structures. The later activity of the Tarim river dextral strike- slip fault(TF)and the Qiemo sinistral strike- slip fault (QF)resulted in the Luobuzhuang- Algan fault depression. The forming mechanism of the Algan sag is similar to that of the Lo p Nut fault depression. The generation of the Algan fault depression caused the diversion of some rivers in the northeast Tarim basin. For example, the Tarim river is now flowing through the Algan sag and then towards the Lop Nur lake. The diversion of rivers caused the changes of environment in this area. Therfore, the special structural background is one of the factors controlling the distribution of drainage system and development of Lop Nur.

THE RESEARCH ON QUANTITATIVE PREDICTION OF FRACTURE IN QIULING OIL FIELD，TUHA BASIN

Tong Hengmao1, Qian Xianglin2

1995, 1(2):  88-94. 

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As oil exploration and technique development progressed rapidly, fractured reservoirs play more and more important role in oil and gas production, and the knowledge about fracture permeability of liquid becomes more clear. Fractured reservoir is very complex and very difficult to be evaluated. There are many obstacles in the quantitative prediction of fractured reservoir, mainly because of highly anisotropic distribution, complicated origin, and multiple-staged development of the fractures. According to the theory of brittle fracturing and progressive deformation of rocks, this paper indicates the possibility of quantitative prediction of fracture development theoretically. Based on outcrop and core fracture data, taking that the fracture density in rocks is determined by the strain as the premise, after structure curvature analysis, theoretica1 numerical calculation of elastic strain energy, and analysis of fault-fracture relationship, the fracture density is estimated quantitatively in Qiuling oil field. The result of prediction shows that the fracture density in the west and central parts is m ore developed than that in the east, north and south flanks in Qiuling oil field. The predicted value of fracture density almost coincides with the measured value in well No.24 and 25. Furthermore, the reliability of the prediction result is evaluated preliminarily.

EXPERIMENTAL STUDY ON SIMULATING HYDROCARBON-GENERATION OF COCCOLITHOPHORIDAES BY HEATING I. Generation Rate and Property of Hydrocarbons, Characteristic of Alkanes and Alkenes

Song yitao, Li Shuqing

1995, 1(2):  95-106. 

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Heating experiments to simulate hydrocarbon generation have been made for two kinds of Coccolithophoridaes which belong to the same species but live in different water-temperature environments, and the mechanism of hydrocarbon generation of Coccolithophoridaes has been discussed. It shows that there are many kinds of crude-oil, hydrocarbons in Coccoljthophoridaes, and the distribution of n-alkanes and isoprenoid alkanes generated by heating is similar to that of oil-source rocks containing Coccolithophoridaes, Experiments on temperature dependence of the, generation of Coccolithophoridaes indicate that the generation of a large quantity of chloroform and the essential change in hydrocarbon composition take place at 200- 300 C. The product of this stage corresponds to the oil-source rocks containing coccolithophoridaes with an Rc range of O.35～0.55％, as is in the case of low-maturity crude oil. This study a1so discusses the isoprenoid in the products of heating experiments on coccolithophoridaes, such as phytenes and C20-, C22- Camtane. A compound considered as C22- Dimethyl-Cyclahexane has been discovered in the experiments.

GEOCHEMICAL VARIATION ACROSS CAMBRIAN- ORDOVICIAN BOUNDARY IN THE HULE SECTION, ANHUI PROVINCE

Fang Yiting, Shen Weizhou, Ni Qisheng, Liu Yan, Lin Yuping

1995, 1(2):  107-113. 

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The Cambian-Ordovician boundary section in Hule is 0ne of the well-established section of this stratigraphic sequence. Fossils from bed 5 to bed 12 are dominated by the graptolites. Two graptolite zones were recognized. They are the Staurograptus zone in the lower and the Anisograptus-Clonograptus zone in the upper. No fossil graptolites have been found from bed l to bed 4 except of trilobite Charchaqia? sp. Accordingly, the Cambrian-0rdovician boundary is placed on the base of Bed 5. However, only minor negative excursion of δ13C displays across the Cambrian-Ordovician boundary though the trilobites dominated shelly fauna was replaced by the graptolites dominated pelagic fauna around this leve1. Dramatic changes in value ofδ13C, δ18O and REE appear across the boundary between two lithostratigraphic units, i.e.the TangJiaqiao and the Xiyangshan Formations. These conspicuous geochemical changes are likely coincident with shifting from anoxic to oxide environments following rapid falling of sea-leve1.

LATE PERMIAN LAMELIBRANCHIA FROM SOUTHEASTERN MARGIN OF TARIM BASIN WITH A DISCUSSION ON THEIR AGES

Fu Junhui

1995, 1(2):  114-119. 

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The Lamellibranchia fossils collected from the Pusige Formation in Aqike, Loupu Coounty of the Southeastern Margin of the Tarim Basin have been systematically studied in this paper, with a thorough discussion on fossil associations and their ages. According to the feature of fossil associations, lithology and sedimentary characteristics as well as section structure the Pusige Formation is referred to the Late Permian.