Geological Journal of China Universities

ON THE MESO-NEOPROTEROZOIC JIANGNAN ISLAND ARC: ITS KINEMATICS AND DYNAMICS

Guo Lingzhi1, Lu Huafu1, Shi Yangshen1, Ma Ruish1, Sun Yan1, Shu Liangsbu1, Jia Dong1, Zhang Qinglong1, J. Charvet2, M. Faure2

1996, 2(1): 1-13.

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The present authors rose the idea of Meso-Neoproterozoic Jiangnan island arc caused by a northwestward subduction of the paleo-South-China oceanic plate in 1970s. Since that the studies on the Jiangnan island arc has become a hot point of tectonic research oil east Asia in 1980s and early 1990s. Represented by the dismembered ophiolite suite having Sm-Nd ages of 1295Ma and 1734Ma and the uppermost sedimentary age of 988Ma of ‘‘Chencai Group” to the south of Jiangshan Shaoxing fault zone, the oceanic crust of paleo-South-China plate suhducted northwestward under the southeast margin of the Yangtze plate, resulting in the formation of the Anhui-Zhejiang-Jiangxi Meso-Neoproterozoic island arc (Huiyu island arc), northeastern segment of the Jiangnan island arc. The island arc mobilized in the age range of 980-1200Ma, which implied that the oceanic crust suhduction had lasted until 980Ma. The Fuchuan ophiohte to the north of the island arc represented the oceanic crust of a marginal sea, which ranged from 935Ma to 1024Ma roughly coincided with island arc in time. In fact, the northeast Jiangxi ophiolite belt (930～1150Ma) rending in northeast is nor located on the Jiangshan-Shaoxing fault zone but tends to be extended to the Fuchuan ophiolite, southern Anhui. Not only the northeast Jiangxi ophiolite is identified as the marginal sea type by petrochemistry and REE analysis, hut also the matrix graywacke of ophiolite blocks is proved as the back arc basin sediment. Therefore, taking the Jiangshan-Shaoxing fault zone as the subducdon zone, Anhui-Zhejiang Jiangxi volcanic island arc located to the north, and the Zhangshudun-Fuchuan as the back arc basin to the north of the arc in turn, a trench arc back arc basin system with the northward/northwestward polarity of plate subduction is reasonably disposed in spatial relationship. This is the typical model of the Jiangnan Meso-Neoproterozoic trench-arc-back arc basin system. An ape are collision event between the Chencai arc and Anhui-Zhejiang Jiangxi arc occurred during 980～770 Ma, which resulted in the intensive deformation of island arc rocks, the formation of collision type granites in the Anhui-Zhejiang-Jiangxi arc-and the deposits of late Mesoproterozoic, Stenian-Early Neoproterozoic, Tonian flysch and molasse which over laid the arc rock assemblage unconformably (Shengong movement)and of the S／man which overlaid the Tonian strata unconformably (Xuefeng movement). The collision also resulted in the collapse of the Zhangshudun-Fuchuan back arc basin, proved by the blueschists with the K Ar age of 866 14Ma. The oceanic crust subducted northward and that finally led to the processes of orogeny of back are continent-arc collision with a series of thrusts in the southeastward tectonic transportation direction. On the southern flank of the back arc basin, there possibly existed the southward subduction which related with the formation of a short life local volcanic rock assemblage (Shangshu formation of Tonian). The Late Neoproterozoic early Paleozoic sinistral strike slip faulting and the Late Paleozoic—early Mesezoic southward thrusting and gravitational nappe occurred in the southern flank of the Anhui-Zhejiang-Jiangxi paleo-island arc. The later deformation processes involve Carboniferous Permian radiolaria siliceous shales into the melange along northeast Jiangxi fault zone, the remains of Zhangshudun-Fuchuan marginal sea. The komatiite in the Jiangnan Meso-Neoproterozoic trench-arc back arc system implies that the processes of Isu collision model displayed there, which occurred in the situation of high heat flow. The Shengong movement in eastern Zhejiang occurred around 980Ma implied the collision between the Chencai arc and Anhui-Zhejiang-Jiangxi arc. This a arc collision event lasted until 770Ma, which is the early stage of Sinian. The Xuefeng movement recorded the late effect of the collision event. The time of unconformity surfaces vary from place to place, because the spatial and temporal variation of the collision events. That implies that the crust movement represented by the unconformities would exhibit diachronism and the correlation of unconformities is qualitative only. The northeast segment of Jiangnan Meso-Neoprorerozoic island arc collision orogenic belt is divided into following tectonic facies: Jiangsban-Sbaoxing fore-arc suture zone, Chencai mobilized basement, Anhui-Zhejiang-Jiangxi volcanic arc as part of overriding plate, Zhangshudun-Fuchuan back are suture zone.

THE MAXIMUM SINISTRAL STRIKE-SLIP AND ITS FORM ING AGE OF TANCHENG-LUJIANG FAULT ZONE

Wan Tianfeng, Zhu Hong

1996, 2(1): 14-27.

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Using the estimating method of earth crust deformation velocity from the displacement of the opposite walls of Tan-Lu fault zone, the maxim um sinistral strike-slip of the fault zone is suggested as 390km. According to the displacement of south marginal fault of Sino-Korean block, the maximum sinistral strike-slip of Tan Lu fault zone is estimated to be 430km. By the paleomagnetic method, the strike slip is about 300-400km. The forming age of maximum sinistral strike-slip occurred in Middle-Late Triassic. In Jurassic, the Tan-Lu fault was a reverse one. The strike slip in Cretaceous was more than 100km, and that in Eogene was unclear. In the Neogene Early Pleistocene, about 50km of sinistral displacement occurred along the Tan-Lu fault zone. From 0.73Ma on (Neotectonic stage), the dextral strike slip is less than 100 m.

ON THE ACTION OF GRAVITY AND TECTONIC STRESS IN EARTH CRUST

Lü Guxian1, Liu Ruixun2

1996, 2(1): 28-37.

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This paper argues against the know ledge that hydrostatic pressure is equal to the gravity value of the overlying rocks in studying a dynamic state of certain underground site, and discusses the influence of tectonic stress on hydrostatic pressure and its application. The research of solid mechanics and data from deep drillings indicated that gravity and tectonic stress in middle and shallow crust are directional external forces, whereas hydrostatic pressure is a force of non-directionless, so they possess different physical meanings. Gravity is not quantitatively equal to hydrostatic pressure either. During 10^8 a –10^6 a of stress are laxation, the directional force on solid rock of the crust produces a deviatoric stress field. The authors suggest that stress field T of the crust is a combination or superposition of hydrostatic pressure P with differential stress , and the total hydrostatic pressure P at any point in the crust comprises two parts, one is spherical stress tensor PR caused by the gravity and the other is spherical stress tensor P caused by tectonic stress, therefore P is not only from the gravity of overlying rocks. The results obtained by a finite element simulation indicate that the additiona1 hydrostatic pressures borne by rocks decrease gradually from the compressive zone (Psc) to the shear zone (Pssh)and to the tensile zone (Pst), i.e. Psc> Pssh >Pst; and the difference of the tectonic additional hydrostatic pressure between these deform ed zones trends to increase, following the increase in absolute value and or difference of external forces in different directions. So the tectonic additional hydrostatic pressure is a major factor causing the heterogeneity of 1ocal stress field in the same depth of the crust. The above mentioned research has a wide range of application: 1) to set up a mode1 of unequal hydrostatic pressures in same depth of the crust; 2) to establish the method of measurement of metallogenetic depth corrected by structure, i.e., firstly minute tectonic added hydrostatic pressure P from general hydrostatic pressure P, then measure and calculate the depth data of overlying rocks; 3) to get into a Flew research field of tectonic physicochemistry; 4) reconsider the depth of eclogite in superhtgh pressure metamorphic zone,. such as the knowledge that the diamond and coesite in Dabie tectonic-metamorphic zone are probably the products of the inner crust; and 5) to introduce some new ideas about experiments of chemical kinetics.

TYPES OF CHEMICAL ACTIVITY AND METAMORPHIC EQUILIBRIUM SYSTEMS OF THE ELEMENT Si IN FAULT ZONES

Sun Yan, Wan Ling

1996, 2(1): 38-47.

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Based on the data of chemical analyses of the fractured rocks within cross sections of a few tens of fault zones in eastern China, there are obviously successive change pattern for the content of element Si, and from various different presents including the metamorphism, component composition, micro-structure, zonal differentiation, and tectonic background etc., the tectonogeochemical behaviour of the element Si can be divided into a low grade metamorphism type (abbreviated as L type) and a very low grade metamorphism type(VL type). In the former, the metamorphic temperature is more than 200-250℃, and the metamorphic facies matches with the greenschist. But in the latter, the metamorphic temperature is less than 200℃, which coincides the zeolite metamorphic facies. As a matter of fact the complex change of element Si in fault zones is closely corresponding to fieldspars in practical examples. Obviously multi-period features of the dynamic differentiation can result in the multi-times of metamorphic differentiation and chemical differentiation.Thus many kinds of dynamic metamorphic equilibrium systems of the element Si could be formed such as SiO2-K2O-Al2O3-MgO-MnO and so on. Finally the significance of the discussion mentioned above to the geological research of resources and environment has been briefly described. On the one hand, most of mylonites hosting gold ores are some siliceous mylonites, and they were distributed in many provinces in the eastern area of China. On the other hand, the silicified rock can weld fault zone to prevent the sliding slope, and these phenomena are often discovered in the Lower Yangtzi area.

EVOLUTION OF THE VOLCANIC TYPE PASSIVE CONTINENTAL MARGIN AND THE UNDERPLATING OF MAGMATIC ACTIVITY DURING EXTENSION

Zhang Chen

1996, 2(1): 48-57.

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This paper expounds the latest advance of research on continental margin evolution and discusses the model of pure shearing and simple shearing on continental lithosphere extension and new conceptions of paired passive continental margin. The deep tectonic setting of formation and evolution on the volcanic passive continental margin and the underplating of magmatic activity during extension are discussed. The significance of extended transitional crust on the research of intercontinenta1 foldbelt evolution is emphasized.

TECTONIC CHARACTERISTICS OF THE SHUANGXIWU GROUP AND ITS GEOLOGICAL SIGNIFICANCE IN NORTHERN ZHEJIANG PROVINCE

Zhu Guoqing, Chen Zijun, Yang Shufeng, Chen Hanlin

1996, 2(1): 58-64.

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Zhangcun region is tectonically located in the west side of the northeast end of the Jiangshao fracture zone, with a series of pre-Sinian formations appearing. For a long time, the tectonics in this region is considered commonly simple. After field exploration and study, the authors draw a conclusion, that, with multiplate orogenics acting on this region, the tectonics may be complex. The so-call“Zhangcun Formation” and “Beiwu Formation”have many similar characteristics, and may be resulted by volcanic orogeny. The structure and metamorphism of tile “Shuangxiwu Group” are different from that of the Luojiamen Formation”, “Hongchi Formation”, etc.. A11 of these indicate that after the “Shuangxiwu Group” was formed, it underwent a strong tectonic orogeny, namely, the “Shengong movement”, and a tightly closed linear recumbent or reclined fold with an even axial plane was caused. Afterwards, Zhangcun region has undergone subsequent tectonic movement, and thus formed a superimposed fold.

INTERPRETATION FOR WELL LOGGING DATA FROM LOW POROSITY AND PERMEABILITY RESERVOIRS——Exemplified by Porosity and Permeability calculation for Upper Palaeozoic Reservoirs in the Central Gas Fields of the Erdos Basin

Liu Fenxia1, Cheng Qirong2, Yuan Haihan2, Zhang Qingguo1

1996, 2(1): 65-74.

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Based upon the interpretation for well logging data from the Upper Paleozoic Central Gas Fields of the Erdos Basin, it is suggested that the sandstones of the Permian Shihezhi and the Shanxi Groups can be distinguished mineralogical1y into quartz sandstone and lithic sandstone. The matrix parameters of these two sandstones are estimated to be Pmax= 2.65g／cm^3, △tmax=182μs／m and Pmax=2.7g／cm^5, △tmax=164μs／m respectively. Porosity values were calculated by both density-minimum acoustic wave porosity and empirical equation methods. The density minimum acoustic wave time difference porosity method with an equation of φ＝min(φμ,φs) appears to have yielded the best results. The calculations for permeability were performed by medium value of grain size, specific surface of rocks and bound water methods. The best results seems to have been given by the regressions between medium value of grain size, porosity and relative natural gamma values with the following equations: lgK=1.313+1.005lgφ+1.298lgMd, lgMd=-0.0736-0.02△GR, and △GR=(GR -GRmax )／(GRmax-GRmax) The calculations have been we1l verified by the practica1 data from 66 wells.

STUDY ON THE P-T PATH OF GRANULITES IN FUPING AREA, HEBEI PROVINCE

Liu Shuwen

1996, 2(1): 75-84.

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Fuping granulites distribute in Fuping gneiss complex, and correspond to the Suojiazhuang formation of the Fuping supergroup at Daliushu, Fangli and other regions in Fuping county. They occur lens and layered bodies in different scales. Granulites consist of garnet, clinopyroxen-orthopyroxene, amphibole, plagioclase and a few quartz. Depending on metamorphic textures and reactive relationships among minerals, the evolution of metamorphism can be divided into four stages: ① the pre peak stage which was represented by mineral inclusions of amphiboles. plagioclase and clinopyroxene in coarse garnet; ② the most principal mineral assemblage which contains coarse grain garnet, amphibole, clinopyroxene?plagioclase、a few hypersthene and quartz; ③ irregular fine grain assemblage of hypersthene, plagioclase and quartz at outer part of symplectic rims around porphyroblasts garnet is the post-peak mineral association; and ④ the fine grain amphibole, plagioelase and quartz at the inner part of the symplectic rim were form ed in late retrometamorphie stage. In orders, P-T conditions during the evolution of metamorphism are: 636℃ and 0.824Gpa at pre-peak, 51～833℃ and 0.854～1.085Gpa at peak. 670～740℃ and 0. 55～0.70Gpa at post-peak?and 665℃and 0.727Gpa at retrometarnorphic stage. They construct a clockwise P-T path?which suggest that this metamorphic terrain had experienced the geodynamic processes from early folding-crust thickening to Later tectonic uplifting of the Archean crust.

CHARACTERISTICS AND ORIGIN OF THE YANBEI PORPHYRY TIN DEPOSIT

Shen Weizhou, Wang Dezi, Liu Changshi Xiong Xiaolin, Lai Mingyuan

1996, 2(1): 85-91.

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The Yanbei porphyry tin deposit (Sn-F type) is characterized by enrichment of topaz of magmatic origin and thus differs from the world-famous Bolivian porphyry tin deposits (Sn-B type)which were rich in tourmaline. The ore hearing parent rocks are Mesozoic volcanic intrusive complex. They have high silica, peralumina, enriched incompatible elements weak Ce and intensive Eu negative anomalies and lower εNd values, high 205Pb／204Pb and higher (87Sr／86Sr), and δ18O values. These features indicate that the ore-bearing parent rocks were derived from the crustal materials and belong to the continental crust remelting type. The ore bodies were located at the contact zone of granite porphyry with rhyolite porphyry in great thickness and homogeneous mineralization. Mineralogical composition of the deposit is relatively complex, of which cassiterite. pyrite and chalcopyrite are dominant, sphalerite and galena are less important. The wall-rock alterations are intense in mining district and are roughly grouped into three zones in space: topazitization, chloritization and sertctltzatton from granite porphyry to fresh country rocks. The potassic zone which is typical for other porphyry type deposits is very weak in this ore district. The ore forming temperatures are mainly in the ranges of. 339-324℃, 280-228℃ and 180-129℃ .displaying polymodaI distribution feature. The ore forming solution has 6 0 value of 7. 8-4. 2‰ and δ18O value of-49‰ to -6l ‰ indicating that it consisted dominantly of magmatic water. Lead isotopic compositions of pyrite are similar to those of feldspar in volcanic intrusive complex and δ44S values (-0. 3‰-1. 5‰) of pyrite close to zero. These characteristics suggest that the ore forming materials were mainly derived from the magma. Therefore, the mineralization of Yanbei porphyry tin deposit is closely related to Mesozoic volcanic-intrusive complex in time, space and material; source.

THE DIAGENESIS AND MICROPORES OF MICRITES

Liang Baihe1, Wang Yinghua2 Huang Zhicheng3; Zhu Sulin4

1996, 2(1): 92-99.

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This article deals with the diagenesis and micropores of micrites in the Palaeo-zoic and Mesozoic marine carbonate sequences from the middle and lower Yangtze area. The micrites are characterized by simple original composition. fine grain, low porosity and permeability. They are not considered commonly as reservoir rocks. Our preliminary studies show that diagenetic processes such a pressure solution dissolution. penecontemporaneous dolomitization. late dedolomitization. and desilicifiation have contributed to the develop merit of micropores and even visible pores. The micropores formed during sedimentation and diagenesis can be distinguished into primary and secondary types, and the majority of microfissures are caused by dissolution and pressure solution. The anionnt of such pores and fissures increases with increasing intensity of constructive diagenesis. Although their practical significance to the reservoir rocks is not ye clear enough. the well development of the micropores and their connectivity could be considered as an indication for the possibility that these micrites wil1 become reservoir rocks. and hence they deserve further investigation.

A STUDY ON RARE EARTH ELEMENTS AND THE GENESIS OF LAM PROPHYRES IN LAOWANGZHAI GOLD DEPOSIT AREA, YUNNAN

Huang Zhilong1 Wang Liankui2 Zhu Chengming2

1996, 2(1): 100-111.

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he Laowangzhai superlarge type gold deposit of the northern Ailao shan structure zone. Yunnan Province, is a typical example where lamprophyres are temporally and spatially related to gold mineralization. The ages of lamprophyres are in the range of 22.7-49.0 Ma. They can be subdivided into minettes and kersantites according to mineral assemblage. Major elements show that rocks are alkalic series and potassic and potassium rich calc-alkaline lamprophyres. Chondrite-normalized transitional element patterns of lamprophyres are “W” shaped which suggests that the rocks be mantle derived. MORB normalized incompatible element patterns are “camelbump” shaped which are relatively enriched in large ion lithophile elements (LILE) and high field strength elements (HFSE), and have high 85Sr／86Sr (0.70694-0.70769) but low 143Nd／144Nd(0.812488-0.51 2493) ratios. All the characteristics mentioned above suggest that the source of lamprophyres in Laowangzhai gold deposits area he abnormal (rich) mantle. The REE parameters of lamprophyres are ZREE 130.19-357 75×10^-6, LREE (l5.80-342 41×10^-6, HREE 12.64-l6.4l×l0^-6, LREE／HREE 8.10-21.92, and the chondrite normalized REE patterns are LREE-rich types. According to the REE contents, all lamprophyres may be subdivided into two groups: the high -ΣREE group with ΣREE 236.84-357.75×10^-6, LREE 220.43-342.41×l0^-6, HREE l5.15 -16.4l×10^-16, LREE／HREE 13.43-21.93: and the low -ΣREE group which have ΣREE 130.19-177.84×10^-16, LREE l15.80-163.81×10^-16,HREE 12 64-l5.01×10^-6, and LREE/HREE 8.10-11.96. Two groups of rocks are similar in the REE patterns. Modelling calculation of representative samples YD -20 (high -ΣREE )and YLW –21 (1ow-ΣREE) shows that two groups of lamprophyres in Laowangzhai gold deposit area are the products of different partial melting degrees (high -ΣREE is 7.5± and low -ΣREE is 1 3± ) of a common REE rich metasomatic mantle. The geological and structural development histroy of western Yunnan. Sr and Nd isotopic compositions, the patterns of incompatible elements and linear programming calculation indicate that the fluid form ed by dehydration of submarine sediments which were rich in ALK. LREE and incompatible elements and carried to mantle wedge by subducting plate is the major factor giving rise to formation of metasomatic rich mantle in the area.

CARBON ISOTOPE RECORD OF SINIAN SEAWATER IN YANGTZE PLATRORM

Wang Zhongzhe1, Yang Jiedong2, Sun Weiguo1

1996, 2(1): 112-120.

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Carbon isotopic compositions of Sinian marine carbonates from the Yangtze Gorges sections near Yichang, South China, have been investigated. The δ13C values showed large variation during the Sinian. The marine carbonates have typical lowδ13C values of about -4‰ in the Nantuo Period. At the beginning of the Doushantuo Period the values increased rapidly, and reached +5～+6‰ in the early and middle time of the Doushantuo Period. The mass of 13Cvalues remained stably positive, ranging generally between +2.0‰ and +3.4‰. The large varations of carbon isotopic compositions of Sinian seawater are mainly due to violent growth and wide extending of acritarch and algae, rapid increasing of global sedimentary rate, and development of “oceanic anoxic event”, i.e. the existence of anoxic layer in deep seawater during that time.

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