Table of Content

20 March 1998, Volume 4 Issue 1

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Article

THREE MAIN STYLES OF EARLY PRECAMBRIAN METAMORPHIC EVOLUTION IN NORTHERN SINO-KOREA CRATON AND THEIR GEODYNAMICS

Lu Liang-zhao, Xu Xue-chun, Dong Yong-sheng

1998, 4(1):  1-10. 

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The northern portion of Sino-Karea Craton consists of four independent blocks and each includs several metamorphic terrains or belts. Three styles of early Precambrian metamorphic evolution and their PTt-D paths have been distinguished in these terrains. The first style was found in Jning-Huaian Archaean high-grade terrain of Inner Mongolia. This district is characterized by widespread Al-rich pars-gneiss (Khondalite suite) and some hypersthene plagioclase gneisses of granulite facies. They were subjected to five episodes of tectonic deformation and six stages of metamorphism from 2.6Ga to 1.9Ga. The mineral paregenesis and transformation of Al-rich gneisses in the whole process of metamorphism show a trend of Ky-Sil+Grt+Bt+Kfs-- Crd士Opx-- And. The metamorphic PTt-D path of this terrain clockwise with T= 800℃ ～ 850℃ , P=0.90 GPa～ 1.00 GPa in its peak stage and a typical isothermal decompression(ITD)process in post-peak stage. It reflects double thickening of crust by geodynamic process such as A-subduction and tectonic stacking in a collisional belt, followed by tectonic thinning and uplifting in the later stage. The second style has been established in eastern Hebei-western Liaoning Archaean high-grade terrain, which is mainly composed of late Archaean grey gneiss (TTG intrusions) and other granitic rocks with Subordinate volcano-sedimentary supracrustal sequences and some layered basic intrusions. The minera1 evolution of basic rocks in granulite-facies belt is, characterized by assemblage of Opx+Cpx+Hb+Grt+pl in its peak metamorphism with T= 800~ 850℃ , P=0.70～ 0.80GPa, and extensive development of Grt corona around 0px or Cpx during its post-peak stage. All data give rise to a counter clockwise PTt-D path with a nearly isobaric cooling process at its post-peak stage. It indicates the tectonic environment of vertical crustal thickening by magmatic accretion. The third style of PTt-d path was constructed in eastern Liaoning-southern Jilin early Proterozoic metamorphic belt. It was a rifted ensialic depression within the Jiao-Liao Massif during the midddle stage of early Proterozoic with the formation of sedimentary sequences of shallow sea and continental shelf, by the end of early Proterozoic, they were suhjected to three deformational episodes and five stages of metamorphism with the development progradal zones and a typical thermal anticline. Its axis area is characterized by high amphibolite-facies metamorphism. The mineralogic evolution in its A1-rich rocks is And-- Sil+ Grt+Crd- Ky. Its PTt-D loop is also counter clockwise, but it is distinguished from the second style by rapid raising pressure in its early stage and succeeded by a nearly isobaric heating process and lower pressure(O.5O～ 0.60(3Pa)in peak stage. Such PTt-D path may be genetically related to the closing process of a rifted depression within a rather stable continental massif.

COMPOSITIONAL FEATURES AND PETROGENETIC IMPLICATION OF PLAGIOCLASES FROM TERTIARY VOLCANICS IN KING GEORGE ISLAND, ANTARCTICA

Xing Guang-fu1;2, Jin Qing-min3,Tao Kui-yuan2, Shen Wei-zhou1, Wang De-zi1

1998, 4(1):  11-19. 

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King George Island consists mainly of Tertiary volcanics which contain plentiful plagioclases. In the Admiralty Bay area(including Keller Peninsula, Ullman Spur and Point Hennequin)of central Island expose predominantly basaltic andesites and andesites, and in the Maxwell Bay area (including Barton and Weaver peninsulas)of southwestern Island. Occur mainly High-A1 basalts and basaltic andesites. Chemical analyses and electron microprobe analyses indicate that plagioclases are andesine-labradorite (An41.47～61.26)for the Admiralty Bay area and mostly bytownite and anorthite. (An 72.05～ 90.97)for the Maxwell Bay area, revealing an evident chemical correlation between plagioclases and their host volcanics. In addition, plagioclases and their specific host volcanics are also corresponding in ΣREE, La/Yb and δEu. On the other and,various plagioclses have relatively similar feature of transitional metallic elements(such as Cr, Co, Ni, Zn), probably implying that contents and differentiation pattern of these elements are chiefly controlled by crystal structure rather than chemistry of plagioclase. In a word, synthetic studies show dependence of plagioclase compositions on those of their host volcanics. Considering that both the Admiralty Bay and the Maxwell Bay areas exist evolutional series from high-Al basalts to dacites, but have different mineralogy, it is suggested that high-A1 basaltic magma in the Maxwell Bay area can be the parental magma of magmatic series in the Admiratly Bay area.

SUBM ARINE EXHALATIVE SEDIMENTAION AND MINERALIZATION OF THE SILICALITE FORM ATION IN WESTERN QINLING, CHINA

Liu Jia-jun1, Zheng Ming-hua2, Liu Jian-ming3, Lin Li2, Zhou Yu-feng2, Gu Xue-xiang2, Zhang Bin2

1998, 4(1):  20-33. 

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The Cambrian silicalite formation,composed of bedded chert and slate, is the host rock of Laerma and Qiongmo gold deposits in western Qinling. The presence of the typical chert offers an important evidence to evaluate the possible submarine exhalative system and its controls over the formation of gold deposits. The ore bearing chert is characterized by massive, banded, laminated, porous spherulitic and pseudobrecciated structures which changes regularly in space. The thickness of a chert bed generally varies from 30 to 200 meters. It contains principally SiO2 (95.30% on average), with FeO, Fe2O3 and organic carbon greater than 1.0%. The ratios of Al/(Al+ Fe+Mn) in all chert samples are lower than 0.3 (0.153 on average). The chert is enriched in most trace elements, which is characterized by not only the basic and ultrabasic elements group (e.g. Ag, Pt, Pd, Cu, Cr)but also the acid elements group ( e.g. W, Mo, U ). Rare-earth are characterized by a low total contents (ranging between 3.29× 10^-6 and 100×10^-6), negative Ce anormaly and a gradually increasing NASC-normalized value with increasing atomic number δ18O Values of chert mainly vary from l7.6O‰ to 23.24‰ (19.17‰ On average), suggesting that the tempreature for deposition was from 70℃ to 118℃, δ30Si values of the chert in the area range mainly from +0.4‰ to +0.8‰. All the geological and geochemical characteristics show that the chert is a product of submarine exhalative sedimentation. During the submarine exhalative sedimentary period, the silicalite formation and mineralization of some elements were all formed.

TYPOM ORPHIC CHARACTERISTICS OF CARBONATE MINERALS FROM BAYAN OBO, INNER MONGOLIA, CHINA

Yang Xue-ming1;2, Yang Xiao-yong1;2, Zhang Pei-shan3, M. J. Le Bas4, R. N. Wilson4

1998, 4(1):  34-42. 

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Carbonate rocks from Bayan Obo, Inner Mongolia, can be divided into sedimentary limestone and dolostone (H8a), coarse-grained dolomite marble(H8c) fine-grained dolomite marble(H8f)and carbonatite dykes(D)based on their textures and occurrences. H8, which shows very weak metamorphism and deformation is distributed to the north of the Kuangou fault zone, occurring together with quartz sandstone, sandstone and shale to form the Bayan Obo Group of middle Proterzoic age of normal sedimentary origin. In contrast to H8a, however, H8c and H8f are distributed to the south of the Kuangou fault zone and have distinctive petrologicl features, both of which are strongly deformed and sheared and metamorphosed to dolomite marble. H8 which hosts ores is mainly consisted of dolomite, apatite, magnetite, and pyroclore etc. These minerals show strong lineation which is the result of numerous clusters of euhedral magnetite and apatite drawn out into granular schlieren. The lineation resulted from shearing stretching and deformation is interpreted to be related to the regional thrusting and folding H8, is close to the ores and is composed of dolomite, magnetite, monazite, bastnaesite and parisite etc. The fine-grained monazites are lined to fil1 fractures in dolomites. Similar to H8c, the minerals in H8f also show strong lineation. Electronic microprobe is used to analyze the chemica1 compositions of carbonate minerals from these rocks at Bayan Obo. Results show that the main rock-forming minerals-dolomite or ankerite in ore-hosted rock from giant Bayan Obo REE-Nb-Fe ore deposit are similar to those from carbonatite dykes(D), which occur 0 to 3.5 km northeast to the Bayan Obo deposit, and cut the Archean granitic gneiss and migmatite of the Wutai Group and the Proterozoic detrita1 rocks of the Bayan Obo Group. A1l these minerals contain MnO higher than 0.50 wt％ and SrO greater than 0.15wt, which are identical to the typical signatures of the mantle derived carbonatite elsewhere in the world. However, they differ completely in chemica1 compositions from calcite and dolomite, with MnO and SrO content 1ess than 0.1 wt％, in typical sedimentary 1imestone and dolostone (H8a) north to the Kuangou fault. The variation of MnO and SrO content in dolomite from ore-hosted dolomite can be interpreted by fractional crystallization of a carhonatltic magma origining from the mantle, which results in REE enrichment in the evolved residual magma. MnO and SrO content in carbonate minerals can be used as an important typomorphic characteristics to identify the genesis of minerals. It is suggested that the value of MnO and SrO content in carbonate minerals higher than 0.15 wt％ is an indicator of crbonatite from sedimentary carbonate rocks.

STUDY ON THE OCCURRENCE OF MICROGRAINED GOLD IN MINERALS BY APPLICATION OF PROTON AND NUCLEAR MICROPROBES AS EXEM PLIFIED BY SHAXI PORPHYRY C0PPER AND GOLD DEPOSIT, ANHUI

Yang Xiao-yong, Wang Kui-ren, Dai Xiao-ping, Yang Xue-ming, Sun Li-guang

1998, 4(1):  43-48. 

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Shaxi Porphyry copper deposit, located in the central Anhui Province, eastern China, is one of the important discoveries in the middle—lower Yantse reaches in mid-1980s’. Gold mineralization also widely exists in the deposit. However, the occurrence of gold is not fully investigated and remains unknown at present. In this paper-gold occurrence has been studied by application of proton and nuclear microprobes in PIXE system. It shows that the gold in the porphyry copper deposit mainly occurs as micro-grained native gold with μm grade, and the host minerals of gold an mainly pyrite and chalcopyrite. The results provide the theoretical evidence for the synthetical utilization of gold in porphyry copper deposits and a new method for the study of gold occurrence in other similar ore deposits.

ON THE KALPIN TRANSPRESSION TECTONICS OF NORTHWEST TARIM

Lu Hua-fu, Jia Dong, Cai Dong-sheng, Wa Shiming, Chen Chu-ming, Shi Yang-shen, Guo Ling-zhi

1998, 4(1):  49-58. 

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Located on the northwest margin of the Tarim basin, the Kalpin Ranges are totally composed of thrust sheets, which comprise from Cambrian to Permian stata generally dipping to north. The Cambrian strata at the basal part of the sheets thrust over the Permian strata of the top part of further south thrust sheets. Therefore the verging direction of the thrusting is southward obviously. These thrust sheets arrange parallelly with the length of 30 to 150 km for a single sheet. The valleys between two sheets are usually filled with present alluvial fans. The paleogene and early Neogene which sometimes appear above the Permian strata are the top of the thrust sheet. These Tertiary sediments lie on the Permian parallelly or with angles only a few degrees, suggesting that the thrusts emplaced after or during Miocene deposition. The Miocene Kanchun formation exhibits non-marine sandstones, mudstones and gradually grows into the Pliocene Kuqa formation exhibits massif sandstones and conglomerates. The sedimentary features of these two formations as uniform mollase deposits indicate the uplifting and the thrusting during the Neogene time. There is no Mesozoic in Kalpin area except minor Cretaceous strata in the west, most of Kalpin Ranges, suggesting that the Kalpin area uplifted during Mesozoic and exhibited as a plateau untill Neogone. The Tianshan mountains reacted since Miocene and started the mollase deposition and the thrusting in Kalpin area. In the Kalpin Ranges there are six rows of thrust sheets, but in some places only two. They were aranged straight or curved forming special small orocline pattern in the map view. Using the strata polarity of the thrust sheet, considering the long itudinal strike ship faulting along the thrust sheets, the mystical structure pattern of Kalpin Range is solved clearly. In Kalpin Ranges, there are two longitudinal strike slip faults F1 and F2, two strike slip faults bend contortion (A)and (C)and one strike slip fault propagation contortion (B) of asymmetric type structurtes of strika slip fault related contortion. ENE trending Aheqi fault zone starts from Kalakuaili, passing Heiyingshan, Wushi County, Aheqi town, to north of Wuqa, then extends into Kirghizstan. As a boundary fault zone it separates the Mesozoic and Cenozoic strata and structures of Tarim basin from those of south Tianshan mountains. It is northwest boundary fault zone of Tarim basin. In the point view of the stress field and deformation mechanism, Aheqi fault zone is a broad fault zone, and the structures of Kalpin are involved into the Aheqi fault system. The strike of Aheqi fault zone is N60°E. According to slip line field caused by the India-Tibetan collision,the Aheqi fault zone falls into the position of sinistral strike slip line. Closely located to the south of Aheqi fault zone, the Kalpin Range faults get their E-W strike in their western part. Therefore the Kalpin Range faults fall in the transpression or restraining fault bend. The transpression resulted in the southward thrusting in Kalpin Ranges. The amount of sinistral strike slip (304 km) of Aheqi fault zone closely coincides with the 300 km slip (Tapponnier et a1. 1986) of Altyn fault.

FORMATION AND EVOLUTION OF THE XORKOL BASIN AND ITS RELATION TO THE ALTUN TAGH FAULT

Gun Zhao-jie, Zhang Zhi-cheng, Wang Jian-jun

1998, 4(1):  59-63. 

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The Altun Tagh fault, a famous huge sinistral strike-slip fault in NW China, is composed mainly of the Altun master shear and several secondary fracture. The relationship between master and secondary structures may he explained by a simple shear model. It is one of the typical strike-slip pull-apart basins which formed in the setting of releasing splay area in Altun Tagh. The Xorkol basin began to extend in Pliocene and its extensional width is now about 40km. The extending rate of the Xorkol basin is about 0.8cm/a. Based on the same rate, the displacement of the Altun master shear is estimated 320km from later Eocene.

BASIC CHARACTERISTICS OF EXTENSION STRUCTURE IN NORTHERN NANSHA ISLANDS, CHINA, AND ITS DYNAMICAL IMPLICATIONS

Liu Hai-ling1, Yang Shu-kang2, Zhou Di2, Zhang Yi-xiang2, Guo Lingzhi1, Sun Yan1, Shu Liangshu1

1998, 4(1):  64-72. 

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Based on a study of geometric and kinematic characteristics of the extensional structure in the east and west sections of northern Nansha block, the basic models and the dynamics of the Cenozoic extension in South China Sea (SCS) area are discussed in this paper. It is proposed that the evolution of extension includes a pure-shear mod el in late Cretaceous-early Oligocene and a simple-shear model after mid-Oligocene in the east section. The pure-shear extension was initiated by the delamination of the subducted slabs of paleo-Pacific plate. It yielded a number of half-grabens which distributed symmetrically on both sides of the southwest basin (SWB) of SCS. And the upper crust and lithospheric mantle on both sides of SW B had undergone uniform extensional deformation. The simple-shear extension, which was caused by further upwarping and partial melting of the hot asthenospheric mantle and by the collision of Indian plate with Eurasia plate in Tibet Plateau, resulted in asymmetrical extensional deformation Oil both northern and southern margin of central basin of SCS. In the northern margin, there are several different plane assemblage types of half-grabens, including series-type, parallel-type, en echelon-type, face-to-face-type and back-to-back-type. But there is only one kind of plane normal faults in the eastern section. This is different from the western section which separated from the eastern section by the Zhongnan fault. In t northern and southern margin of eastern SCS, the extensional deformations of the crusts were evidently nonuniform with that of the lithospheric mantle under them.

DISCUSSI0N 0N THE EXTENSIONAL DYNAMIC SETTING IN THE SOUTH BORDER OF JUNGGAR BASIN, XINJIANG, CHINA

Li Zhong-quan1, Zhang Shou-ting1, Chen Geng-sheng

1998, 4(1):  73-78. 

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The tectonic dynamic setting in the south border of Junggar Basin has been generally considered as a Mesozoic- Cenozoic compressive basin. In the discussion of tectonic evolution and structural deformation, only compressive regime and compressive structure were considered, whereas the extensional situation and structure were ignored. In this paper, based on field survey al analysis of former materials, it proposed that the tectonic dynamic regime of the south border of Junggar Basin should be extensional setting during Mesozoic and even a long period of Cenozoic era-it did not become a compressive setting until late Cenozoic era. Six aspects about this viewpoint are demonstrated. Finally, combining with the peak period of oil-generating, it is found that extensional structures (fault block, syndepositional anticlines)were important for capturing early stage oil and gas, and the Mesozoic extensional structures and later inversion structures should be considered attentively in future study of gas- oil structural traps.

RECONSTRUCTION OFGEOTHERMAL HISTORY AND SIMULATION OF HYDROCARBONGENERATION FROM PALEOZOIC TO MESOZOIC IN NORTHERN JIANGSU AREA

Sun Zhen, Xu Shou-li, Wang Liang-shu, Li Cheng, Guo Sui-ping

1998, 4(1):  79-84. 

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Northern Jiangsu basin is an extentional basin formed in late cretaceous. Its basement, the strata from Z to K2, experienced three main tectonic stages: Paleozoic passive continental margin, late Paleozoic to early Mesozoic stable subsidence and Mesozoic thrusting. The carbonate strata of Paleozoic and Mesozoic are quite good source rocks for hydrocarbon. Reconstruction of paleo-geotemperature and maturation degree will provide geat value for oil and gas exploration in this area. In this paper, the authors reconstruct the paleo-getempeature and model the hydrocarbon maturation history of Paleozoic. Mesozoic source rocks during the stages of basement evolution and Neocene basin-formation. The calculated maturation degree is in accordance with the practical measuring values of part strata, some new information about source rock maturation is provided for further evaluation of oil-gas prospect in northern Jiangsu area with complicated tectonic evolution.

ON ACCOMMODATION DIAGRAM OF DEVELOPING MECHANICS OF TERRIGENOUS SEQUENCE IN THE STEEP SLOPE ZONE OF BIYANG FAULT-DEPRESSION

liu Shou-quan

1998, 4(1):  85-94. 

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In Biyang fault-depressed lacustrine basin, the tectonic settlement of basement presented a nonlinear or disconnected function and an anti-hinge type. Due to the joint effect of lacustrine level change and basement structural subsidence the accommodation change in steep slope zone of Biyang fault-depressed lacustrine basin only has three possibilities：1. decreasing at southern and northern rims but increasing in central zone; 2. invariant at southern rim but increasing in central and northern rim zones; 3. increasing at both southern and northern rims and central zone. In the light of the three cases, the sedimentation-response characteristics to the accommodation variation at different structural positions of the steep slope zone in fault-depressed lacustrine basin is analyzed respectively at different recharge rate of sediment-source. Taking Shuanghe-Zhaoao area in Biyang fault-depression as an example in this paper, accommodation mechanism to select reference well is discussed. Using well B207 as a reference well, the terrigenous sequences of the upper member of Eh2 are divided, and the isochronous stratigraphic framework model is set up.

EARLY-MIDDLE TRIASSIC SEQUENCE STRATIGRAPHY AND SEDIMENTARY GEOCHEMISTRY AT THE FACIES TRANSITION REGION IN GUIZHOU,CHINA

Yang Rui-dong

1998, 4(1):  95-102. 

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Based on field investigation of exposed sequence stratigraphy and referred Triassic sea-level changes in South Chins, sequences of two 3rd order and seven 4th order of Lower and Middle Triassic are established in this paper. In addition, the Triassic sequence stratigraphy framework of Guizhou is constructed，which is the first detailed sequence division of Triassic in Guishou and is important to further pa1eogeography and facies studies. This paper also studies the geochemistry and distribution of trace elements in the Triassic strata. The relationship between trace elements and sequence stratigraphy is discussed. It is suggested that the enrichment of Pb，Ba，Mn，Si，Ga and Ni was caused by hydrotherma1, activities accompanied with intercontinental tensile subsidence in the passive continental margin.

PRELIMINARY INVESTIGATION OF BANTANG THERMAL SPRING, ANHUI

Wang Guo-qiang

1998, 4(1):  103-108. 

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Through the study of geological, geohydrological survey and geothermal, geophysical measurement of Bantang Area, a preliminary knowledge of geohydrological condition for underground storage of thermal water and the water-controI1ing structure of this area are obtained.

GEOTECHNICAL PROBLEM IN FOUNDATION PIT EXCAVATION OF SKYSCRAPER IN THE FINE SILT BED OF NANJING DISTRICT

Zheng Qing-huai, Yon Chang-hong, He Chang-ming

1998, 4(1):  109-114. 

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The ancient stream channel of Qinhuai river is characterized by its binary structure of sand and clay，and is also art important aquifer．Based on the analysis of its particular engineering geology, hydrogeology and surrounding conditions, this paper discussed the geotechnical problem in the foundation pit excavation of skyscrapers and its settlement, which can be used in the design and construction of deep foundation in this areas．

ABOUT THE HYPOTHESIS OF MELTING IN-SITU AND THE OTHERS: REPLY TO THE COMMENTS BY Wei Chunjing

Chen Guo-neng

1998, 4(1):  115-117. 

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DISCUSSION ON THE PAPER TITLED BY “PONDERING OVER SOME PROBLEMS IN THE STUDY OF CONTINENTAL GEOLOGY”

Du Yang-song

1998, 4(1):  118-120. 

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