Geological Journal of China Universities

HUGE INTRACONTINENTAL SUBDUCTION ZONE AT SOUTH MARGIN OF NORTH CHINA BLOCK AND PRESENT 3－D LITHOSPHERIC FRAMEWORK OF THE QINLING OROGENIC BELT

Zhang Guo－wei1，Meng Qing－ren2，Liu Shao－feng2，Yao An－ping1

1997, 3(2): 129-143.

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The northern boundary of the Qinling orogen is demonstrated by many pieces of evidence to be a huge intracontinental subduction zone where the southern North China Block underthrust beneath the Qinling from north to south and extended down below the Moho. This intracontinenta1 subduction zone is wel1 revealed by seismic reflection profiles across the eastern Qinling in western Henan provinee. North-directed thrust systems were developed in the middle and upper crust in association with the subduction zone, extending over 1000 km along their tectonic strike from Baoji eastward to Tanlu fault and probably connecting with the thrust system on the northern edge of the Qinling orogen to the west. The intracontinental subduction zone formed during the Cretaceous and then controlled the Mesozoic-Cenozoic tectonic evolution of the region between North China B1ock and the Qinling. Present three-dimensional lithospheric framework of the Qinling can be described as a flyover-type structure with three rheologically different layers, as revealed by geological, geochemical and geophysical studies. The upper layer, from 0-20 km, is actually a zone with brittle, viscoelastic and ductile deformation, and featured by antecedent E-W -striking lineament. The middle layer from 20-80 km is a zone typfied by flat rheological layering, including the Moho. The lower layer below the depth of 60-80 km is in adjusting upper mantle and characterized by N-S striking geophysical anomaly zones. The N-S-striking structures in lower layer, resulted from active adjustment of upper mantle materials, pass upwards to the antecedent E-W -striking lineaments in the upper layer through a middle flat rheological zone, showing a crosscutting relationship of structures in upper and lower tectonic layers of the Qinling lithosphere, so-called flyover-type 3-D framework. As its northern boundary, this intracontinental subduction zone is a basic component of present lithospheric framework of the Qinling orogen. It formed in the same dynamic regime as other Mesozoic-Cenozoic structures. Active deep mantle dynamic processes in the Qinling region are controlled by the interacting between three adjacent major plates, i.e., north-moving Indian plate, west-subducted Pacific plate and south-moving Siberian plate. The mantle activation exerted strong influence on the upper crust, resulting in rapid vertical uplift, lateral accretion, and modification of old structures. The present flyover-type 3-D framework of the Qinling is built up by joint action of tectonic processes of different crust-lithospheric layers and represents one type of orogenic structures preserved through long-term geological evolution.

STRUCTURAL FEATURE OF BASEMENT IN THE ORDOS BASIN AND ITS CONTROL TO PALEOZOIC GAS

Jia Jin-dou1, He Guo-qi1, Li Mao-song1, Zhou Ding-chao2, Zhang Lin-xiang2

1997, 3(2): 144-153.

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The Ordos basin, one of the large hydrocarbon bearing basins, is located in the west part of North China. There are many big basement faults making up a complex pattern with two uplifts lying on the top of the basement. The complex and heterogeneous basement structure controls obviously the overlying sedimentary lithology and its formation, and influence indirectly the generation, migration and accumulation of hydrocarbon. The south slope of the northern Yimeng Uplift and the slope region around the central uplift are favorable for seeking Paleozoic natural gas. Several large northeastern ductile shearing belts may influence the hydrocarbon distribution greatly and much attention should be devoted to the oil and gas exploration.

MAFIC DYKE EMPLACEM ENT MECHANISM AND ACCOMPANYING DEFORMATION

Shi Huo-sheng1, J.D.Hoek2, C.J.L.Wilsen2

1997, 3(2): 154-161.

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The Eyre Peninsula, South Australia, makes up the southern part of the Gawler Craton. It contains a major portion of Archean to mid-Proterozoic crystalline basement province of the Gawler Craton. It is a region which, except minor, localized movements, has been a reasonably stable platformal area since it was stabilized before 1423 Ma. The north-eastern parts of the craton are unconformably overlain by Late proterozoic and Cambrian platform sediments deposited on the Suart Shelf. The Adelaide Geosyncline, which now forms the Delamarian Fold Belt, provides the north-eastern boundary for the Gawler Craton and the Suart shelf. The eastern boundary developed between 800-490 Ma and is actually delineated by the Torrens Hinge Zone. The Gawler Craton has been subjected to substantial extension 2000 Ma. This formed a sedimentary basin into which to Hutchison Group was deposited. The Kimban Orogeny (1840- 1550 Ma) was associated with intrusive magmatism and deformed the new intrusives, the Hutchison Group and the underlying basement. The Paleoproterozoic Kimban mobile belt was exposed in the study area and divides the area into two contrasting crustal terrains juxtaposed by the Kalinjala Shear Zone. The western terrain consists of the Hutchison Group meta-sediments (1964- 1845Ma), meta-basics and mete-volcanics, and the Moody Granitoid Suite (1740- 1700Ma). The Hutchison Group unconformably over1ies the older Sleaford Complex which was affected by the Sleafordian Orogeny (2600- 2400Ma) as well as the Kimban Orogeny. The eastern terrain essentially consists of a batholith, here referred to as the Lincoln Batholith, composed of the Donington Granitoid Suite (185O-1843 Ma), mafic dykes that syn-plutonic with the Donington Granitoid Suite, and the Colbret Granitoid Suite. The Lincoln Batholith is intruded by the Tournefort mafic dyke suite. The mafic dykes and the mylonite in Jussieu Peninsula are mainly exposed along the coastal section. Although they are deformed, boudinaged and recrystallized, their development in an environment of transtensional and transpressional tectonic settings can still be recognized. By using transfer geometry, we can explain the propagation and intrusion of the mafic dykes, the deformation along the mafic dyke boundaries, and localization of deformation that formed the ductile mylonite zone.

METAMORPHISM EVOLUTION OF THE LATE ARCHEAN COLLISION OROGENIC BELT IN WUTAISHAN

Liu Zhi-hong1, Wang An-jian2, Li Xiao-feng3

1997, 3(2): 162-170.

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The late Archean collision orogenic belt in Wutaishan is situated in the central part of the North China Plate. It is separated from the high grade gray gneiss terrane at north Hengshan by dutile shear zones (DDZ)distributed over the north side of Hutuo River Cenozoic fault depression, and fro m the high grade terrane at southwest Fuping by Longquanguan DDZ, to form a wedge-shape terrane opening towards southwest. Based on rock association metamorphic deformation, magmatism and geochronological data, the collision orogenic belt can be subdivided into three tectonic units: the south tectonic section (STS) (fore-arc melange zone), the central tectonic section (CTS) (paleo-island arc), the north tectonic section (NTS) (back-arc melange zone). The boundaries between them are giant DDZ. The rocks in the NTS and STS are metamorphosed to amphibolite facies, whereas those in the CTS to greenschist facies. The differences of metamorphic degrees in the three sections are due to their tectonic environments. Studies on the mineral assemblages and metamorphic reactions indicate similar PTt paths of orogeny for these three tectonic sections. The temperatures and pressures of the metamorphism increased apparently in the three tectonic sections from early to peak stage (STS: ≤ 500℃, 0.4GPa--589～670℃, 0.86GPa: CTS: 375～400℃, ＜0.5GPa--500～ 560℃, ≤0.5GPa; NTS: 500℃, ≤0.4GPa--569～663℃, 0.98～ 1. 36GPa), which mainly related to the crustal subduction of the fore-arc and back-arc melange zones in the island-arc-continent, continent-continent collisions. The isothermal decompression of the metamorphism from peak to late stage (STS: 589～ 670℃, 0.86GP--560～650℃, 0.2～ O.3GPa; CTS: 500～ 560℃, 0.5GPa--400～ 445, 0.1～ 0.4GP; NTS: 569～ 663℃, 0.98～ l.36GPa--560～ 650℃, 0.2～ 0.3GPa) was related to the crustal detachment which was resulted from the extensive longitudinal extension.Tectonism provided the dynamic basis for metamorphism.

ENCLAVES IN THE TYPICAL MINING DISTRICTS OF TONGLING, ANHUI AND THEIR IMPLICATION TO THE PROCESS OF MAGMATISM-METALLOGENY

Du Yang-song, Li Xue-jun

1997, 3(2): 171-182.

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Two types of enclaves occur in the magmatic plutons in Tongling area, Anhui. The first type is residuals of metamorphic rocks of high amphibolite facies. The other one is magmatic rocks of monzonitic-dioritic compositions. A detailed petrologic and minera1ogic study and a forming condition calculation have been carried out on the two types of enclaves, on the basis of the study and the calculation, a systematical discussion has been made on the relation of the enclaves to their hosts, the material source of the magmatic rocks and associated mineral deposits, and the mechanism of magmatism-metallogeny. This leads to propose a new metallogenetic model for stratahound skarn type ore deposits associated with the magmatic rocks of the syntectic type. The new model can be simply summarized as partial melting of old metamorphic basement rocks in depth, followed by accumulating, differentiating and emplacing of magmas to form the deep-level and shallow-leve1 magma chambers, then the mixing of different composition magmas associated with their crypto-explosion, migration of the gas hearing ore fluid and precipitating of metals in the fluid within the magmas.

PRELIMINARY STUDY ON METALLOGENETIC SERIES OF NONFERROUS METAL DEPOSITS IN YOUJIANG FOLD BELT

Hua Ren-min, Zhu Jin-chu, Zhao Yi-ying, Zhou Jian-ping Wu Yan-yu, Chen Xiao-dong

1997, 3(2): 183-191.

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The Youjiang fold belt is located in southwestern China, and is geotectonically a portion of the South China Caledonian Fold System, which was generally formed by alternating tension and compression from mid-Proterozoic to early Devonian. Initiated in the late stage of early Devonian period, a northwest trending rifting took place in Youjiang area. Deep faults and consequent grabens and horsts were developed in the bottom of this rift, resulting in structural complex of this area. This particular background offered good conditions for the formation of many ore deposits, especially those of Sn, Cu, Ag, Sb etc. Several large to superlarge deposits have been found. Although different deposits have different characteristics, they can be classified into two major metallogenetic series. The first series is Sn-Cu-(Pb Zn)-Ag series related to granitoids, which can be genetical1y attributed into the crust-remelting type, or transformation type. In Youjiang fold belt, tin is the dominant metal, which differs from the other parts of South China Caledonian Fold System, such as southern Jiangxi, where tungsten is more dominant. Another feature of these deposits is that they are mostly polymetallic deposits. Therefore the zoning of elements is commonly developed, which display from Cu-Sn to Pb-Zn to Ag. Based on the spatial relation between ore deposits and related granitic rocks, this series can be divided into two sub-series. One is the Cu-Sn sub-series of which the deposits have close spatial relation with granites. The other is the Ag sub-series, of which relationship between ore bodies and granites is more obscur. This lead to different explanations on the ore genesis, such as the sedimentary exhalation hypothesis. However, there are still much evidence showing that further or deeper emplaced granitic plutons are the key control of ore formation. The second series is the strata bound Sb deposits of sedimentation-reworking type. They mainly occur in Devonian strata. The most favourable lithological assemblage for Sb mineralization is an ore-bearing siliceous bed, combined with an overlying shale and an underlying carbonate bed. No granitic rocks can be found in the mining and vicinity areas. The Sb mineralization is closely related with structural deformation. Geological evidence and age dating show that they are epigenetic. Thus the term reworking is used for their genesis. The mineralization took place in the Yanshanian period,when fluid circulation of large extent generated by deep fracturing desolved and transported Sb to form ore bodies.

BIOMETALLOGENESIS OF Pb-Zn-Ag POLYMETALLIC DEPOSIT OF QIXIASHAN IN NANJING

Xie Sha-cheng, Yin Hong-fu

1997, 3(2): 192-201.

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Biological pre-enrichment of metals in the Pb-Zn-Ag polymetallic deposit of Qixiashan in Nanjing has been previously discussed. As biometallogenesis is a developing process together with geological processes, this paper mainly deals with migration and reduction by organic matter and organic fluid derived from bacteria and algae. The mineralization organisms and organic matter absorbed in ores in the deposit are systematically investigated, which reveals that they are al1 derived from bacteria and algae. Combining non-destructive analyses such as infrared and ultraviolet fluorescence microspectroscopy with destructive ones such as GC-MS, the authors focus on the hydrocarbon inclusions developed in the deposit. Series of biomarkers in the ore-forming fluid are identified, indicating their algal and bacterial sources. The basic analyses include the characteristics of organic matter in metallogenic fluid, the relationship among metallogenic elements and the distribution of metals in organic matter. Biometallogenesis other than pre-enrichment is thereby clarified to form an organism-organic matter-fluid (OOF) metallogenetic system.

GEOCHEMICAL CHARACTERISTICS OF UPPER PALAEOZOIC VOLCANIC ROCKS AND THEIR TECTONIC SETTINGS IN THE SANTANGHU BASIN, XINGJIANG

Lin Ke-xiang, Li Yi-bin, Gong Wen-ping, Yu Hui-long

1997, 3(2): 202-211.

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The geochemical analysis indicates that the Early Carboniferous basalts in the Santanghu basin are characterized by higher Al203, lower FeO, Fe: 03 and TiO2, and by high concentrations of large-ion lithophile element (LIL) and LREE, showing the characteristics of the calc-alkaline series. In addition, results of discriminant analysis show that the Santanghu area was in an island arc setting during Early Carboniferous. The petrochemical characteristics and REE and trace element abundance show that the andesites of Early Permian belong to high potassium orogenic andesites of calc-alkaline series, comparable with that of the Andes-type active continental margin, indicating that the Santanghu area was in the active continental margin of Siberia plate during Early Permian time.

CONCEPTUAL MODEL OF THE REHAI (HOT SEA) GEOTHERMAL FIELD IN TENGCHONG, YUNNAN PROVINCE, CHINA

Liao Zhi-jie1, Yin Zhen-wu2, Jia Xi-yi3, Lu Wei-xin4

1997, 3(2): 212-221.

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The Rehai (Hot Sea) geothermal field at Tengchong County of Yunnan Province is the largest one in mainland China. Based on the results of geological, geochemical and geophysical reconnaissance surveys since 1973, a conceptual model of the Rehai field can be built. Volcanism started in Tengchong in the Upper Miocene/Pliocene, reached a peak about 1 Ma ago, and lasted during the whole Pleistocene. The effect of this long period of volcanic activity was to produce the Rehai geothermal field. The surface manifestations of the Rehai field include boiling springs, fumaroles and steaming ground, with temperature of 96℃. The springs discharge sodium-chloride-bicarbonate waters with TDS of 2g/kg. Based on micro-earthquake and magneto-telluric studies, it is inferred that a magmatic heat occurs beneath the Rehai field. It appears to be a cooling intrusion below about 6-7 km depth. The anoma1ously high 3He/4He ratio of the Rehai field confirms the existence of degassing upper mantle rocks at crustal 1evels. The texture of the thermal field has been surveyed by reflection seismic method. The cap rock of the Rehai field is composed of Miocene stratium of about 300m thick, and the reservoir rock is the low velocity layer within the Precamberian Gaoligongshan group about 1500m below surface. According to some temperature data in shallow drillholes, the thermal gradient is as high as to 100℃/lOOm. Because of the lack of deep drillhole in the field, reservoir temperature in the range of 230℃ to 275℃ can be estimated by cation geothermometers. The temperature at top of cooling magma pocket is estimated to be about 670℃ by higher conductive heat flow of 288 mW/m2 measured in the low-permeability Miocene rocks at Liuhuangtang-huangguaqing.

METHOD FOR EXPANSIVE SOIL CLASSIFICATION BASED ON THE ARTIFICIAL NEURAL NETWORK EXEMPLIFIED BY NING-LIAN HIGHWAY

Du Yan-jun

1997, 3(2): 222-225.

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Previous quantitative methods for classifying expansive soil such as “Fuzzy” and “Grey Cluster Analysis” have certain shortcomings．Both of them rely on artificial factors and thus may influent the true results of the soil classification．A new method of Artificia1 Neura1 Network (ANN ) model presented in the paper can well solve the difficulties and it has good objectivity and excellent disturbance resistance．The model is applied for classifying soil samples of D section in Ning-Lian high-grade highway．The results are well in accord with those by “Fuzzy” and Grey Cluster Analysis” methods．The ANN method has been approved available and reasonable in practice and may become an effective way on determining expansive soil’s properties quantitatively．

EVOLUTION OF ASYMMETRY OF LUNAR CRUST

Li Jin-bao

1997, 3(2): 226-235.

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The thickness of the lunar crust facing the earth is 40-60km, where most maria exist. Whereas that of the other side is up to 150km. This makes the asymmetry of the moon. The present paper put forward an explanation for the formation of this asymmetry. In the early stage, there was solid lunar core surrounded by magma “ocean”, and the lunar crust was formed by gravitational differential and cooling consolidation. Owing to the synchronous rotation with the earth, one side of the moon always faces the earth. The solid lunar core moved towards the earth by earth’s gravitational field, and as a result, the light substances on the lunar surface floated to the back side of the moon. This evolution mechanism make the lunar surface thinner in the near-earth side while thicker in the back side. Through theoretical inducement, the paper gives a formula to calculate the distance of lunar core moing toward the earth as: r=2Md×R3/My(D2-R2) Where Md is the mass of the earth, My is the mass of the moon, Ris the radius of the moon, and D is the distance from moon to earth. According to this theory, it is concluded that the distance between the moon and the earth was 162.700km when their synchronous rotation happened.

REPLY TO “COMMENTS ON Spirisosphaerospora pacifica AND Miniactinomyces chinensis IN PELAGIC MANGANESE NODULES”

Lin Cheng-yi1，Zhang Fu-sheng1，Bian Li-zeng2，Chen Jian-lin3， Shen Hua-ti3, HanXi-qiu3

1997, 3(2): 236-240.

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Whether Miniactinomyces chinensis and Spirisosphaerospora pacifica in the pelagic manganese nodules from the East Pacific Ocean are ultra-microbes is discussed in detail. Firstly, they were identified by authors as ultra microfossils instead of ultra-microbes based on their morphology, structure and elemental composition. Secondly, the complete microbiological structures including hyphae, spores, sporangia and colonies can be distinguished obviously in the TEM images. The size proportion between them is appropriate to each other. In addition, the TEM images also revealed that the laminae in manganese stromatolites were formed as a result of rhythmic growth of these ultra-microbes, which are similar to the laminae in strand stromatolites. The minimum size of known microbes can not be considered as a criterion for negating the newly-discovered ultra-microfossils. The term, namely ultra-microfossil, is discussed as wel1.

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