2006 Vol. 17, No. 3
An attempt is made to confirm the existence of the two discrete UHP and HP metamorphic belts in the central orogenic belt in China. Detailed geological mapping and structural and petrological analyses of the Kanfenggou(看丰沟) and Xiangfanggou (香坊沟) slices exposed in the eastern Qinling(秦岭) orogen indicate that they experienced ultrahigh pressure and high pressure metamorphism, respectively. The former, situated in northern Qinling, contains a large volume of fine-grained coesite and quartz pseudomorphs after coesite-and microdiamond-bearing eclogite lenses, whereas the latter, located in southern Qinling, preserves the relicts of a high pressure metamorphic mineral assemblage. Based on extensive fieldwork together with compilations at the scale of the orogenic belt, and a comparison of Pb isotopic compositions between the UHP metamorphic rocks from Kanfenggou slice and the Dabie UHP metamorphic belt, we propose that there are at least two discrete ultrahigh pressure metamorphic belts with different ages and tectonic evolution within the central orogenic belt in China. The first is the South Altun-North Qaidam-North Qinling ultrahigh pressure metamorphic belt of Early Paleozoic age (～ 500- 400 Ma). The Kanfenggou ultrahigh pressure slab is located at its eastern segment. The second is the well constrained Dabie (大别)-Sulu (苏鲁) ultrahigh/high pressure metamorphic belt of Triassic age(～ 250 - 220 Ma). The Xiangfanggou high pressure metamorphic slab is a westward extension of the Dabie-Sulu ultrahigh/high pressure metamorphic belt. The Pb isotopic compositions of the UHP metamorphic rocks from Kanfenggou UHP fragment in East Qinling are different from those of the UHP rocks in Dabie UHP metamorphic belt, but are consistent with those of the rocks from the Qinling rock group and Erlangping(二郎坪) rock group. The East QinlingUHP metamorphic belt does not appear to link with the Dabie-Sulu UHP metamorphic belt. These twoultrahigh metamorphic belts are separated by the suture-like Shangnan(商南)-Danfeng (丹凤) fault system or Guishan (龟山)-Meishan (梅山) fault, and a series of fault-bounded tectonic slices with different ages and rock assemblages. The Early Paleozoic ultrahigh metamorphic belt towards the east cannot be linked to the Triassic Dabie-Sulu ultra-high/high pressure metamorphic belt, and so does not form a huge ultrahigh pressure metamorphic belt extending more than 4 000 km in the central orogenic belt. In addition, the South Altun-North Qaidam-North Qinling ultrahigh metamorphic belt probablyrepresents an intercontinental deep subduction/colli-sion belt between the Sino-Korean and Yangtze cratons, occurring during the Early Paleozoic. The Dabie-Sulu ultrahigh/high pressure belt may have been formed by an intracontinental deep subduction/collision process occurring in the northern part of the Yangtze craton during the Triassic. It is suggested that the two phase crustal subduction/collision processes, with an interval of ～ 200- 300 Ma, responsible for the formation of the twoultrahigh/high pressure metamorphic belts occurred along spatially different belts within the central orogenic belt, China.
Both the macroscopic feature and the sequence-stratigraphic position of the molar-tooth structure developed in the third member of the Gaoyuzhuang(高于庄) Formation at the Jixian(蓟县) Section in Tianjin(天津) can provide some useful information about its origin and can reveal some problems to be further researched in the future. The Mesoproterozoic Gaoyuzhuang Formation is a set of～ 1 600 m thick carbonate strata. This formation can be divided into four members. The first member is mainly made up of stromatolitic dolomites;the second is marked by a set of manganese dolomites;the third ismainly composed of lamina limestoneswith the development of molar-tooth strcutures;the fourth is a set of stromatolitic-lithoherm dolomites. According to lithofacies and its succession, several types of meter-scale cycles can be discerned in the Gaoyuzhuang Formation:the L-M type, the subtidal type and the peritidal type. There is a regularly vertical stacking pattern for meter-scale cycles in the third-order sequence. Therefore, the Mesoproterozoic GaoyuzhuangFormation can be divided into 13 third-order sequences(SQ 1 to SQ 13 ) and can further be grouped into 4 second-order sequences. The third member is marked by lamina limestones and can be grouped into three third-order sequences(SQ 9 to SQ 11 ). The molar-tooth structure is developed in the middle part of the third sequence, i. e. SQ 11 , in the third member. Several features of this kind of molar-tooth structure reflect some features of carbonate sedimentation in the Precambrian, such as the particular configuration, abundant organic matter, and easy silication. Stromatolites are chiefly formed in a shallow tidal-flat environment;lamina are mainly formed in the shallow ramp and molar-tooth structures are mainly generated in a relatively more deep-water environment from the middle to the deep ramp. Therefore, similar to stromatolite and lamina, the molartooth structure might alsobe a kind of bio-sedimentation structure. This suggestion is based on macroscopic observation and the sedimentary-facies analysis of the molar-tooth structures from the sequence-stratigraphic position. These features of Precambrian sedimentation also reveal the problem of Precambrian carbonate sedimentation. With more detailed study, a more practical solution for these problems may be obtained in the future.
Xifeng(西峰) oilfield was recently found in the southwest of the Ordos basin. The oil source rocks are the Chang 7 Section of Yanchang(延长) Formation. In order to study the paleoclimate that controlled the formation of source rocks, a systemically palynological research on related beds in Yanchang Formation has been carried out. The core samples were analyzed with classical palynological techniques and the organic-walled sporomorphs from these samples were observed, identified and photographed under a light microscopy and a fluorescence microscopy. Abundant sporopollen were found in drilling cores from Chang 8 and Chang 7 sections, and two assemblages were distinguished:the Aratisporites-Punctatisporites assemblage and the Asseretospora-Walchiites assemblage. Their characteristics are similar to those of the assemblage of Tongchuan (铜川) Formation and the assemblage of Yanchang Formation in southeast Ordos basin, respectively. Their geological times are Ladinian of late Middle Triassic and Carnian of early Late Triassic, respectively. The correlation of palynoflora with their parent plants suggests the paleoclimate of eastern Gansu (甘肃) Province in the Middle and Late Triassic was warm and rainy with prosperous vegetation. The palynoflora indicated the area was in a temperate to subtropical zone then. Both the ecological types and differentiation degree curves of sporopollen indicated the period during Chang 8 and Chang 7 sections was warm and wet, and the phase accorded with large-scale lake transgression in Chang8 Section and the largest lake area in Chang7 Section. North China in Middle and Late Triassic was located in a warmand rainy, temperate and subtropical zone. The palynofloras in Chang 8 and Chang7 sections have the characteristics of North China flora, however they also indicate wetter and warmer climate due to their proximity to the large lakes. The period of Chang 8 to Chang 7 sections is the climax of the expansion of the lake, and the bloom of fresh algae during the period, which helped form the high-quality source rocks in Chang 7 Section.
The research on biofacies and its provincialization is of important significance not only for the increasingof precision of stratigraphic subdivision and correlation in South China, the reconstruction of ancient environment and paleogeography and even the guiding of oil and gas exploration, but also for the study of paleobiogeogrphy and sea level changes of southern China in Ordovician. On the basis of the studies of the ecologicalcharacteristics of Ordovician cephalopods fromSouth China, eighteen cephalopod biofacies are recognized and described:(1)Open platform Proterocameroceras biofacies;(2) Restricted platform Pseudoectenolites-Xiadongoceras biofacies; ( 3) Open platform Retroclitendoceras-Pararetroclitendoceras biofacies;(4)Open platformPronajaceras-Mamagouceras biofacies;(5) Shelf slope-basin Cyclostomiceras biofacies;(6) Open platform Cameroceras-Cyrtovaginoceras biofacies;(7) Open platform Coreanoceras-Manchuroceras biofacies;(8) Shelf slope-basin Kaipingoceras-Kyminoceras biofacies;(9) Inner shelf Bathmoceras-Protocycloceras biofacies;(10) Middle shelf Dideroceras-Ancistroceras biofacies;(11) Deep-water shelf Lituites-Cyclolituites biofacies;(12) Stagnant basin Lituites-Trilacinoceras biofacies;(13) Deep-water basin Paraendoceras-Sactorthoceras biofacies;(14) Deep-water shelf Sinoceras-Michelinoceras-Disoceras biofacies;(15) Deep-water shelf Beloitoceras-Jiangshanoceras biofacies;(16) Deep-water shelf -basin Eurasiaticoceras biofacies;(17)Shelf-slope Jiangxiceras-Yushanoceras biofacies;(18) Deep-water basin Michelinoceras biofacies. The cephalopods of these biofacies, their ecological characteristics, and livingconditions are elucidated in this article. The association law of cephalopod biofacies in time and space shows that there were three cephalopod biofacies provinces in South China during the Ordovician, i. e. , Yangtze biofacies province, East Guizhou(贵州)-West Hunan (湖南) biofacies province (mixed-type biofacies province), and Central Hunan-West Zhejiang(浙江) biofacies province. It is suggested that differentiation of cephalopod biofacies wasmainly controlled by sea level changes and tectonic evolution. The differentiation is obvious during lower sea level and not developed during high sea level.
Low temperature is the key for maintaining the wellbore stability of the gas hydrate well and for ensuring the safety in the well. In this article, the characteristics of drilling in the permafrost containing gas hydrates are analyzed initially. And later , the relative properties and the inhibitory mechanisms of the polyethylene glycol and the hydrate inhibitor of the drilling fluids ensuring drilling safety are discussed. On the basis of these, the effects of the molecular weight and the content of the polyethylene glycol , and its interaction with the salts on the performances of the four groups of drilling fluids under low temperature are mainly analyzed. The results of these analyses show that for the relative drilling fluid system , the 5 % content of polyethylene glycol with a molecular weight of 10 000 combined with 15 % NaCl+5 % KCl or 20 % NaCl is suitable.
For understanding the Mesozoic tectonics of Yanshan (燕山) belt, the authors took geological mapping in the belt. A large-scale thrust structure was identified in Yong’anpu(永安堡) area, in the western part of Suizhong(绥中) County, Liaoning(辽宁) Province during our recent mapping in the Yanshan belt. The hanging wall of the thrust was composed of Archean gneiss and the overlying Early Cretaceous Zhangjiakou (张家口) Formation;meanwhile, the strongly ductile deformed volcanic rocks of Zhangjiakou Formation comprised the footwall in Yong’anpu tectonic window. This discovery indicates the existence of stronglycontractional deformationin the Yanshan belt after the eruption of EarlyCretaceousZhangjiakouvolcanic rocks. On the basisof mappingand research, it is concluded that the published official geological mapshave failed toidentify the major structural features of the Yanshan belt.
U-Pb dating was conducted on different domains of zircons from metamorphosed leu- cosomes in Delingha(德令哈) complex, the lower basement rocks of the Olongbuluke (欧龙布鲁克) microcontinent, North Qaidam, in order toreview its complex tectonothermal history. The zircon core is comprised of highly-modified magmatic zircon relicts, the zircon mantle was produced in response to anatexis of a Late Protoproterozoic thermal event;age and isotopic composition of both the zircon core and the zircon mantle have been seriously disturbed due to the thermal event related with growth of the zircon overgrowth rim. The 207 Pb/ 206 Pb apparent age of the overgrowth rim was estimated to be ～ 1 030 Ma. This Late Mesoproterozoic thermal event hasbeen interpreted as aresponse to the global Rodinia supercontinent assembly event in the Olongbuluke microcontinent, Northwest China.
The research on the trace elements of Ordovician carbonates plays an important part in the whole work on reservoir in Tazhong(塔中) area. This paper systematically studies the characteristics and sedimentary settings of Ordovician dolomites in Tazhong area, Tarim basin, and debates their enrichment of mechanisms and different element existing patterns. The studymakesuse of ICP-MS analysis technology to test the strontium and manganese content of 109 samples from four wells in the Tazhong area, Tarim basin and strontium isotope composition tests have also been done on 25 samples fromwells Zhong-1 and Zhong-4 on VG354 solid isotope mass spectrograph. By means of analyzing contents of strontium and manganese elements, doing research on the strontium isotope composition characteristics from wells Zhong-1 and Zhong-4 and comparingthe results of strontiumanalysis to the global Ordovician marine carbonate and its evolution trend, in the combination of sedimentary facies characteristics of iso-lated wells in this area, we can come to the following cognitions:(1) The marine carbonate strontium isotope curve in the Tazhong area of Tarim basin is consistent to the global evolution trend which is overall descendingwith time, the direct reason of which is the evolution of paleogeographic environment.The Ordovician paleogeographic environment goes through restricted platform to open platform and then shallowmarine shelf in Tazhongarea, Tarim basin. Dolomitization is another subordinate reason and the inversion of fluid with high manganese can lead to heavy strontium;(2) The fact that the 87 Sr/ 86 Sr ratios of the UpperOrdovician in Tazhong area have an apparent mono-decline trend with the time going by,which is similar to the global strontium isotope ratio, is suggesting that the variation of the Upper Ordo- vician sea level is starting, the overall trend of which is the rising of the sea level;(3) Compared to the global seawater strontium ratios, the 87 Sr/ 86 Sr ratio of the Lower Ordovician is much high, and 87 Sr/ 86 Sr ratio of the Lower Ordovician is overall increasing with its maximum value reaching 0. 709 727, the reasons of which are dolomitization, evaporation of the supratidal and the reflux of high manganese haline, those reasons can lead to the increasing amount of the strontiumand the rising of the 87 Sr/ 86 Sr ratio;(4) The manganese element content changes little. Dolomitization has little impact on it;(5) Big variation occurs in the strontium values of Ordovician carbonate in the region of Tazhong. Apart from paleogeographic environment and petrography, the factors which can affect the values of strontium are the content of the terrestrial clastics and the diagenesis modification which may result in the further changing of the strontium values.
Quartz grains in contact with uranium-bearingminerals or fluids are characterized bynatural radiation-induced paramagnetic defects(e. g. , oxygen vacancy centers, silicon vacancy centers, and peroxy radicals), which are amenable to studyby electron paramagnetic resonance(EPR) spectroscopy.These natural radiation-induced paramagnetic defects, except for the oxygen vacancy centers, in quartz are concentrated in narrow bandspenetrated byαparticles:(1) in halosaround U-and Th-bearingmin-eral inclusions and(2) in outer rims or along fractures. The second type of occurrence provides information about uranium mineralization or remobilization (i. e. , sources of uranium, timing of mineralization or remobilization, pathways of uranium-bearing fluids). It can also be used toevaluate sedimentary basins for potential of uranium mineralization. In particular, the peroxy radicals are stable up to 800 ℃ and, therefore, are useful for evaluating metasedimentaryrocks(e. g. , Paleoproterozoic metasedimentary sequences in the central zone of the North China craton). EPRstudy of the Changcheng Series can focus on quartz fromthe sediment-basement unconformity and faults todetermine the presence and types of natural radiation-induced paramagnetic defects, with which to identify and prioritize uranium anomalies. Other potential applications of natural radiation-induced paramagnetic defects in quartz include uranium-bearing hydrocarbon deposits in sedimentary basins. For example, the Junggar, Ordos, and Tarim basins in northwestern China all contain important oil and natural gas fields and are well known for elevated uranium concentrations, including economic sandstone-hosted uraniumdeposits. Therefore,systematic studies on the distribution of natural radiation-induced paramagnetic defects in quartz from host sedimentary sequencesare expected toprovide information about the migration of oil and natural gas in those basins.
BaMgAl10O17:Mn2+ green phosphor was prepared by high temperature solid state reaction and the optimal chemical formula is Ba0.85MgMn0.15Al11.993B0.007O19. The influences of milling and ultrasonic - dispersal on the luminescent properties, granularity and appearance of the phosphor were investigated , and the green phosphor with fine grain and uniform dispersion to be adapted to plasma display panels was obtained. A Hitachi F - 4500 fluorescence spectrophotometer was used to measure the lu-minescent performance of the phosphor , and the vacuum ultra violet (VUV ) fluorescence spectroradio -metric system (Zhejiang University Sensing Instruments Co. , Ltd. ) was used to measure the luminescent performance under 147 nm excitation. A scanning electron microscope (SEM ) was used to identify the size and shape of the particles. X - ray diffraction (XRD ) was used to confirm its crystalline structure. According to this study , the optimal after - treatment processes were as follows: milling time 5 min , milling medium 15 % C2 H5OH , dispersal time 90 min and dispersal medium 15 % C2 H5 OH.