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P-T-t Path of Mafic Granulite Metamorphism in Northern Tibet and Its Geodynamical Implications

Acta Geologica Sinica(English Edition) 2004年第1期78卷 155-165页

作者： HUDaogong WUZhenhan JIANGWan YEPeisheng instituteofgeomechanics ChineseAcademyofGeologicalSciencesBeijing100081

Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10 km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early metamorphic assemblage (M1) is preserved only in the granulites and represented by plagioclase+hornblende inclusions within the cores of garnet porphyroblasts. The peak assemblage (M2) consists of garnet+clinopyroxene+hornblende+plagioclase in the mafic granulites. The peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of hornblende+plagioclase symplectites surrounding embayed garnet porphyroblasts, and decompression-cooling (M4) is represented by minerals of hornblende+plagioclase recrystallized during mylonization. The peak (M2) P-T conditions of garnet+ clinopyroxene+plagioclase+hornblende were estimated at 769-905℃ and 0.86-1.02 GPa based on the geothermometers and geobarometers. The P-T conditions of plagioclase+hornblende symplectites (M3) were estimated at 720-800℃ and 0.55-0.68 GPa, and recrystallized hornblende+plagioclase (M4) at 594-708℃ and 0.26-0.47 GPa. It is impossible to estimate the P-T conditions of the early metamorphic assemblage (M1) because of the absence of modal minerals. The combination of petrographic textures, metamorphic reaction history, thermobarometric data and corresponding isotopic ages defines a clockwise near-isothermal decompression metamorphic path, suggesting that the mafic granulites had undergone initial crustal thickening, subsequent exhumation, and cooling and retrogression. This tectonothermal path is considered to record two major phases of collision which resulted in both the assemblage of Gondwanaland during the Pan-African orogeny at 531 Ma and the collision of the Qiangtang and Lhasa Terranes at 174 Ma, respectively.

关键词： continental collision mafic granulite P-T-t path Gondwanaland Qiangtang and Lhasa Terranes Tibet

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Mangshan Loess in Central China and the Paleomonsoon Variations since the Last Interglaciation

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Acta Geologica Sinica(English Edition) 2004年第3期78卷 813-819页

作者： JIANGFuchu WANGShubing ZHAOZhizhong FUJianli instituteofgeomechanics ChineseAcademyofGeologicalSciencesBeijing100081

The Mangshan Yuan is a loess platform on the southern bank of the Yellow River, which is located in northwestern Zhengzhou of Henan Province, China. The typical Zhaoxiayu section of the Mangshan Yuan preserves stratigraphical loess units above S10 with a total thickness of 172.1 m, which includes 15.7 m of the last interglacial paleosol S1, 77.3 m of the last glacial loess L1 that consist of 41.6 m of the late stade L1LL1, 13.2 m of the interstade L1SS1 and 22.5 m of the early stade L1LL2. Based on the age marking points by correlating magnetic susceptibility of the section with the SPECMAP curve, the timescale of the section was constructed, and the average accumulation rate and the resolution of each loess strata over the S2 were subsequently calculated using the susceptibility age model. The results indicate that strata units developed in the glacial, interglacial stages, stadial and interstadial show substantial differences in grain size, average accumulation rate and time resolution ub the Zhaoxiayu section. Specifically, the average accumulation rate of the loess L1LL1 is 3.45 mm/a, whereas that of paleosol S1 is only 0.28 mm/a. Based on the high-resolution records of magnetic susceptibility and >45μm fraction percentage of the loess-paleosol, the summer and winter monsoon variations as well as their interrelations since the last interglaciation have been discussed, which were correlated with the SPECMAP and the GRIP climate records.

关键词： loess magnetic susceptibility grain size paleomonsoon Mangshan Henan Province

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Thrusting of the North Lhasa Block in the Tibetan Plateau

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Acta Geologica Sinica(English Edition) 2004年第1期78卷 246-259页

作者： WuZhenhan HuDaogong YePeisheng ZhaoXun LiuQisheng instituteofgeomechanics ChineseAcademyofGeologicalSciencesBeijing100081 ChineseAcademyofGeologicalSciences Beijing100037

A huge thrust system, the North Lhasa Thrust (NLT), was discovered in the northern Lhasa block of the Tibetan Plateau based on geological mapping of the Damxung region and its vicinity, the Deqen-Lunpola traverse and the Amdo-Bam Co profile. The NLT consists of the Dongqiao-Lunpola thrust (DLT), the west Namco thrust (WNT) and the south Damxung thrust (SDT) and ductile shear zones, ophiolite slices and folds extending in a WNW direction. Major thrust faults of the NLT seem to merge into a single deep-seated detachment of the upper-crust and totally displaced southward as far as 100-120 km. Chronological analyses with 39Ar-40Ar of plagioclase and hornblende, Rb-Sr isochron of minerals and fission-tracks of apatite from mylonite within the WNT yield ages of 174-173 Ma, 109 Ma and 44 Ma, showing 3 periods of thrusting in the north Lhasa block caused by subduction of the Tethys oceanic plate and the India-Eurasia continental collision respectively.

关键词： thrusting mylonite chronological dating north Lhasa block Tibetan Plateau

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Quaternary Geology and Faulting in the Damxung-Yangbajain Basin

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Acta Geologica Sinica(English Edition) 2004年第1期78卷 273-282页

作者： WUZhonghai ZHAOXitao WUZhenhan JIANGWan HUDaogong ZHOUChunjing instituteofgeomechanics ChineseAcademyofGeologicalSciencesBeijing100081 InstituteofGeologyandGeophysics ChineseAcademyofSciencesBeijing100029

The detailed geological mapping, conducted in the Damxung-Yangbajain basin, shows that there are many types of deposits formed since the Pliocene. The oldest sediments are formed during the Pliocene. The most prominent sediments are three sets of moraines and fluvioglacial deposits. The ESR, U-series and OSL dates indicate they are formed about 700-500 ka B.P., 250-125 ka B.P. and 75-12 ka B.P. respectively and indicate that there are three glacial periods since the mid-Pleistocene in the Nyainqentanglha Range. Along the southeast side of the Nyainqentanglha Range, the main southeast dipping fault zone which bounds the Damxung-Yangbajain Graben on its western edge was mapped. The fault zone consists of three secondary fault zones and their initiation ages that the fault zones became active gradually decrease southeastward. Prominent faulting occurred in about 700-500 ka B.P., 350-220 ka B.P., -140 ka B.P. and 70-50 ka B.P. since the mid-Pleistocene. The height of fault scarps which offset the sediments formed since the mid-Pleistocene suggest that the vertical slip rates change between 0.4 -2 mm/a and the cumulative average vertical movement at rates of 1.1±0.3 mm/a during the Quaternary period and the Holocene vertical throw rate is 1.4±0.6 mm/a along the fault zones on the western side of the Damxung-Yangbajain Graben.

关键词： Qinghai-Tibet plateau Nyainqentanglha north-south trending graben active fault glaciation

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Origin, Age and Significance of Pseudotachylites from the Eastern Dabieshan Orogenic Belt, China

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Acta Geologica Sinica(English Edition) 2004年第1期78卷 52-60页

作者： LIUJianmin DONGShuwen ZHANGJiasheng LIUXiaochun CHENBailin instituteofgeomechanics CAGSBeijing100081 InstituteofGeology ChineseSeismologicalBureauBeijing100029

Recent field survey in the eastern Dabieshan Mountains has revealed extensive occurrences of pseudotachylite. The pseudotachylite tends to occur as simple veins and injected networks along the NE-SW-trending fracture zones or shear zones, which are parallel to the Tanlu fault zone and cut all the pre-Cretaceous geological bodies. The characteristics of both the microstructures gained by the optical microscope and SEM imaging and the geochemistry between the pseudotachylites and their host rocks show that the pseudotachylites were formed mainly by ultracataclasis of their wall rocks in which they occur. The bulk K-Ar ages of the pseudotachylites yielded a narrow range of 81 -93 Ma, and moreover the laser-probe 40Ar/39Ar dating of phengite overprinting on the pseudotachylite gave a weighted mean age of 78.9 Ma. These results show that the pseudotachylites from the eastern Dabieshan Mountains formed along the NE-SW-trending fault zone during the uplifting of the orogenic belt at 80-90 Ma, which places important constrains on the cooling and exhumation history of the Dabieshan Mountains during and after the late Cretaceous.

关键词： pseudotachylite origin microstructure geochemistry age the eastern Dabieshan Mountains

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Tectonic implication of stockwork microbreccias

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Chinese Science Bulletin 2005年第1期50卷 67-74页

作者： ZHANGJiasheng HUANGXiongnan LIUJianmin InstituteofGeology ChinaEarthquakeAdministrationBeijing100029China instituteofgeomechanics ChineseAcademyofGeologicalSciencesBeijing100081China

The stockwork microbreccia found in the Dabie and Helan Mountains bears no relation to either meteorite impact and cryptoexplosion, or fault displacement. It is con- trolled by tensional or transtensional fracture network in relatively hard felsic crystalline rocks, appearing as breccia with dark cryptocrystalline matrix. The kilometer-scale dis- tribution and complicated relation to fracture system of the stockwork microbreccias, randomly distributed autochtho- nous fragments of host rock, ultracataclasis without notable displacement in the cryptocrystalline matrix, and a probable solid-state amorphization deformation mechanism all suggest that large-scale, high-energy and rapid brittle fracturing event might have occurred in the rocks at relatively deep crustal level in the areas.

关键词：网状脉细角砾岩大别山脉隐晶质矩阵固态非晶化地壳运动

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PRELIMINARY STUDY ON THE AGE OF YUANMOU MAN BY PALAEOMAGNETIC TECHNIQUE

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Science in China,Ser.A 1977年第5期 645-664页

作者：李普钱方马醒华浦庆余邢历生鞠石强 instituteofgeomechanics ChineseAcademyofGeologicalSciences

Since the discovery of the fossil teeth of Yuanmou man (Homo erectus yuanmouensis) on May 1, 1965 from Yuanmou county, Yunnan Province, the Institute of Geomechanics, relying on the local poor and lower-middle peasants and co-operating with other units concerned, has been conducting intensive researches on the fossil man-bearing strata as well as Quaternary glaciation. Palaeomagnetic technique lately used in the study has for the first time shown that the Yuanmou man appeared around 1.7 million years ago, earlier than both Peking man and Lantien man. Hence the age formerly assigned to the earliest fossil man in China has to be increascd by more than a million years. Furthermore, as the age of Yuanmou formation is fixed at 1.5—3.1 million years, the length of Quaternary has to be extended by more than two million years. Of special interest is that under the Yuanmou formation there occur glacial traces which prove beyond doubt the existence of glaciation over 3 million years ago. This is a new discovery

关键词： PRELIMINARY STUDY ON THE AGE OF YUANMOU MAN BY PALAEOMAGNETIC TECHNIQUE 洲口

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Chronology and Sources of Mesozoic Intrusive Complexes in the Xuzhou-Huainan Region, Central China: Constraints from SHRIMP Zircon U-Pb Dating

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Acta Geologica Sinica(English Edition) 2004年第1期78卷 96-106页

作者： XUWenliang WANGQinghai LIUXiaochun WANGDongyan GUOJinghui CollegeofEarthSciences JilinUniversityChangchunJilin130061 instituteofgeomechanics ChineseAcademyofGeologicalSciencesBeijing100081 InstituteofGeologyandGeophysics ChineseAcademyofSciencesBeijing100029

SHRIMP zircon U-Pb dating in the Liguo and Jiagou intrusives indicates that they were formed at -130 Ma in the Early Cretaceous. Most inherited zircons in the Liguo intrusive were formed at 2509±43 Ma. Most inherited and detrital zircons in the Jiagou intrusive were formed at -2500 Ma, -2000 Ma and -1800 Ma. The SHRIMP zircon U-Pb dating in two gneiss xenoliths from the Jiagou intrusive yields the ages of 2461±22 Ma and 2508±15 Ma, respectively. The dating results from inherited and detrital zircons in the intrusives and the gneiss xenoliths imply that the magmas could be derived from the partial melting of the basement of the North China Block (NCB). The magmatism is strong and extensive in the periods from 115 to 132 Ma, which is of typical bimodal characteristics. It is suggested that the lithospheric thinning in the eastern North China Block reached its peak in 115-132 Ma.

关键词： Mesozoic intrusive complex zircon U-Pb dating source of magma Xuzhou-Huainana region China

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Methods for Calculation of Geogenetic Depth

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Journal of China University of Geosciences 2004年第2期15卷 145-151页

作者： LiuRuixun LuGuxian WangFangzheng WeiChangshan GuoChusun SchoolofEarthandSpaceSciences PekingUniversityBeijing100871China instituteofgeomechanics ChineseAcademyofGeologicalSciencesBeijing100081China FacultyofEarthSciences ChinaUniversityofGeosciencesWuhan430074China instituteofgeomechanics ChineseAcademyofGeologicalSciencesBeijing100081China

Some current methods for the calculation of the geogenetic depth are based on the hydrostatic model, it is induced that the depth in certain underground place is equal to the pressure divided by the specific weight of rock, on the assumption that the rock is hydrostatic and overlain by no other force but gravity. However, most of rock is in a deformation environment and non hydrostatic state, especially in an orogenic belt, so that the calculated depth may be exaggerated in comparison with the actual depth according to the hydrostatic formula. In the finite slight deformation and elastic model, the relative actual depth value from the 3 axis strain data was obtained with the measurement of strain including that of superimposed tectonic forces but excluding that of time factor for the strain. If some data on the strain speed are obtained, the depth would be more realistically calculated according to the rheological model because the geological body often experiences long term creep strains.

关键词： geogenetic depth ultrahigh pressure metamorphism hydrostatic model elastic dynamic model liquid-affecting fluid static model rheological dynamic model

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SHRIMP U-Pb zircon dating on Nyainqentanglha granite in central Lhasa block

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Chinese Science Bulletin 2004年第1期49卷 76-82页

作者： LIUQisheng WUZhenhan HUDaogong YEPeisheng JIANGWan WANGYanbin ZHANGHancheng instituteofgeomechanics InstituteofGeologyChineseAcademyofGeologicalSciencesBeijing100081China BeijingSHRIMPCenter InstituteofGeologyChineseAcademyofGeologicalSciencesBeijing100037China ChinaUniversityofGeosciences Beijing100083China

The Nyainqentanglha granite, a batholith with an area of 1500 km^2 in the central Lhasa block, comprises mainly medium-fine-grained biotite monzonitic granite and medium-coarse-grained biotite monzonitic granite. Their contact relationship suggests that the medlum-fine-grained biotite monzonitic granite was crystallized earlier than the medium-coarse-grained monzonitic granite. A SHRIMP U-Pb dating on zircons from representative samples of the batholith shows that the crystallization age of the medium-fine-grained biotite monzonitic granite is 18.3 Ma and that of the medium-coarse-grained monzonitic granite is 11.01 Ma. The emplacement and crystallization of the Nyainqentanglha granite, the youngest granite batholith ever known inside the Lhasa block, have a close relationship with the collisional orogeny, crustal thickening, plateau uplifting,E-W crustal extension and have a dynamic genetic relationship with the crustal partial melting.

关键词：锆石花岗岩拉萨青藏高原

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