Neoproterozoic to Early Cambrian history of an active plate margin in the Teplá–Barrandian unit—a correlation of U–Pb isotopic-dilution-TIMS ages (Bohemia, Czech Republic)     

W. Drr  G. Zulauf  J. Fiala  W. Franke  Z. Vejnar 《Tectonophysics》2002,352(1-2)

The Teplá–Barrandian unit (TBU) of the Bohemian Massif shared a common geological history throughout the Neoproterozoic and Cambrian with the Avalonian–Cadomian terranes. The Neoproterozoic evolution of an active plate margin in the Teplá–Barrandian is similar to Avalonian rocks in Newfoundland, whereas the Cambrian transtension and related calc-alkaline plutons are reminiscent of the Cadomian Ossa–Morena Zone and the Armorican Massif in western Europe. The Neoproterozoic evolution of the Teplá–Barrandian unit fits well with that of the Lausitz area (Saxothuringian unit), but is significantly distinct from the history of the Moravo–Silesian unit.The oldest volcanic activity in the Bohemian Massif is dated at 609+17/−19 Ma (U–Pb upper intercept). Subduction-related volcanic rocks have been dated from 585±7 to 568±3 Ma (lower intercept, rhyolite boulders), which pre-dates the age of sedimentation of the Cadomian flysch ( t chovice Group). Accretion, uplift and erosion of the volcanic arc is documented by the Neoproterozoic Dob í conglomerate of the upper part of the flysch. The intrusion age of 541+7/−8 Ma from the Zgorzelec granodiorite is interpreted as a minimum age of the Neoproterozoic sequence. The Neoproterozoic crust was tilted and subsequently early Cambrian intrusions dated at 522±2 Ma (T ovice granite), 524±3 Ma (V epadly granodiorite), 523±3 Ma (Smr ovice tonalite), 523±1 Ma (Smr ovice gabbro) and 524±0.8 Ma (Orlovice gabbro) were emplaced into transtensive shear zones.  相似文献   

Continental crust and lithospheric mantle interaction beneath North China: isotopic evidence from granulite xenoliths in Hannuoba, Sino-Korean craton   总被引：40，自引：0，他引：40  

Xinhua Zhou  Min Sun  Guohui Zhang  Shaohai Chen 《Lithos》2002,62(3-4):111-124

Mafic granulite and pyroxenite xenoliths from Cenozoic alkaline basalts at Hannuoba, Hebei Province, North China have been selected for a systematic geochemical and Sr–Nd–Pb isotopic study, which provides a unique opportunity to explore nature of the lower crust and the interaction between the continental crust and lithospheric mantle beneath an Archean craton. The major, compatible and incompatible elements and radiogenic isotopes of these xenoliths suggest great chemical heterogeneity of the lower crust beneath the Hannuoba region. Petrological and geochemical evidences indicate a clear cumulate origin, and most likely, they are related to basaltic underplating in different geological episodes. However, the Sr–Nd–Pb isotopic compositions of the xenoliths reveal a profound enriched source signature (EM I) with some influence of EM II, which implies that some portion of pre-existing, old metasomatized subcontinental lithospheric mantle could have played an important role in their genesis. It is suggested that the interaction between continental crust and subcontinental mantle as manifested by basaltic underplating would be closely related to regional tectonic episodes and geodynamic processes in the deep part of subcontinental lithospheric mantle.  相似文献   

Paleomagnetic dating of Alleghanian orogenesis and mineralisation in the Mascot–Jefferson City zinc district of East Tennessee, USA     

D. T. A. Symons  K. K. Stratakos 《Tectonophysics》2002,348(1-3)

The Mascot–Jefferson City (M-JC) Mississippi Valley-type (MVT) deposits are in the Valley and Ridge province of the Appalachian orogen in East Tennessee. They have been a major source of zinc for the USA but their age is uncertain and thus their genesis controversial. About 10 specimens from each of 37 sites have been analysed paleomagnetically using alternating field and thermal step demagnetisation methods and saturation isothermal remanence methods. The sites sample limestones, dolostones, breccia clasts and sphalerite–dolomite MVT mineralisation from mines in the Lower Ordovician Kingsport and Mascot formations of the Knox Group. The characteristic remanent magnetisation (ChRM) is carried by magnetite in the limestones, by both magnetite and pyrrhotite in the dolostones and by pyrrhotite preferentially to magnetite in the mineralisation. Mineralized sites have a more intense ChRM than non-mineralised, indicating that the mineralising and magnetisation event are coeval. Paleomagnetic breccia tests on clasts at the three sites are negative, indicating that their ChRM is post-depositional remagnetisation, and a paleomagnetic fold test is negative, indicating that the ChRM is a remagnetisation, and a post-dates peak Alleghanian deformation. The unit mean ChRM direction for the: (a) limestones gives a paleopole at 129°E, 12°N (d_p=18°, d_m=26°, N=3), indicating diagenesis formed a secondary chemical remanent magnetisation during the Late Ordovician–Early Silurian; (b) dolomitic limestones and dolostone host rocks gives a paleopole at 125.3°E, 31.9°N (d_p=5.3°, d_m=9.4°, N=7), recording regional dolomitisation at 334±14 Ma (1σ); and (c) MVT mineralisation gives a paleopole at 128.7°E, 34.0°N (d_p=2.4°, d_m=4.4°, N=25), showing that it acquired its primary chemical remanence at 316±8 Ma (1σ). The mineralisation is interpreted to have formed from hydrothermal fluid flow, either gravity or tectonically driven, after peak Alleghanian deformation in eastern Tennessee with regional dolomitisation of the host rocks occurring as part of a continuum during the 20 Ma prior to and during peak deformation.  相似文献   

Uniform postglacial slip-rate along the central 600 km of the Kunlun Fault (Tibet), from ²⁶Al, ¹⁰Be, and ¹⁴C dating of riser offsets, and climatic origin of the regional morphology     

Jerome Van Der Woerd  Paul Tapponnier  Frederick J. Ryerson  Anne-Sophie Meriaux  Bertrand Meyer  Yves Gaudemer  Robert C. Finkel  Marc W. Caffee  Zhao Guoguang  Xu Zhiqin 《Geophysical Journal International》2002,148(3):356-388

  相似文献   

Mixing, sediment accumulation and focusing using 210Pb and 137Cs     

San Miguel  J.P. Bolívar  R. García-Tenorio 《Journal of Paleolimnology》2003,29(1):1-11

The Portil lagoon is a natural freshwater reservoir located at the southwest of Spain, near the coast. In its surroundings an important tourist complex has been developed since the earlier 1970s. This has resulted in increased loads of nutrients, specially during summer months. In order to evaluate the impact from humans on the lagoon caused by anthropogenic activities, we have determined vertical profiles of excess ²¹⁰Pb and ¹³⁷Cs in a sediment core taken from its bottom. Vertical profile of excess ²¹⁰Pb seems to indicate that the sediment core was affected by large-scale mixing processes. Nevertheless this possibility was ruled out based on the shape of the ¹³⁷Cs vertical distribution after modelling the ¹³⁷Cs profile considering large-scale mixing. The chronology developed from ²¹⁰Pb and ¹³⁷Cs has allowed us to calculate the temporal evolution of the mass sedimentation rate during the last century. This mass sedimentation rate increased in 1973: from 0.08(2) g cm^-2 a^-1 in the period 1900-1973 to 0.17(5) g cm^-2 a^-1 in the period 1973-1995. This increase could be related to land-movements in the drainage area and to permitted sewage inputs into the lagoon, both with its origin in the growing/running of the surrounding tourist complex.  相似文献   

Late Pleistocene–Holocene hydrologic changes in the interfluve areas of the central Ganga Plain, India     

Pradeep Srivastava  Indra Bir Singh  Shikha Sharma  Uma Kant Shukla  Ashok Kumar Singhvi   《Geomorphology》2003,54(3-4):279-292

Abandoned channel belts, ponds and point bar deposits of palaeochannels in the interfluve regions of the central Ganga Plain suggest changes in the morphohydrologic conditions during the Latest Pleistocene–Holocene. Stratigraphy of these ponds comprises channel sand at the base overlain by shell-bearing clayey silt. The contact of the two facies marks the phase when channels converted into standing water bodies. Point bar deposits of some palaeochannels are overlain by oxidised aeolian sand, indicating that the channel abandonment possibly occurred due to the desiccation and aridity in the region.Optically stimulated luminescence (OSL) chronometry of the pond sediments suggests that the deposition of the basal channel sand started before 13 ka and continued up to 8 ka. The ponds formed around 8–6 ka when the channel activity ceased. Evidence from the point bar deposits also indicates that the fluvial activity in the region ended sometime during 7–5 ka. This was followed by aeolian aggradation. The present study thus suggests that the hydrologic conditions in the Gangetic plains, i.e. initiation of channels and their abandonment, formation of microgeomorphologic features such as ponds and their eventual siltation, were controlled largely by climatic changes (i.e. monsoon changes) supported by tectonic activity. For the past 2 ka, increasing human and related agricultural activity has substantially accentuated the natural siltation rate of ponds.  相似文献   

Luminescence chronology of incision and channel pattern changes in the River Ganga, India     

Pradeep Srivastava  Maneesh Sharma  Ashok K. Singhvi 《Geomorphology》2003,51(4):259-268

The River Ganga in the central Gangetic plain shows the incision of 20 m of Late Quaternary sediments that form a vast upland terrace (T₂). The incised Ganga River Valley shows two terraces, namely the river valley (terrace-T₁) and the present-day flood plain (terrace-T₀). Terrace-T₁ shows the presence of meander scars, oxbow lakes, scroll plains, which suggests that a meandering river system prevailed in the past. The present-day river channel flows on terrace-T₀ and is braided, sensu stricto. It is thus inferred that the River Ganga experienced at least two phases of tectonic adjustments: (1) incision and (2) channel metamorphosis from meandering to braided.Optical dating of samples from three different terraces has bracketed the phase of incision to be <6 and 4 ka. Different ages of the top of terrace-T₂ show that this surface experienced differential erosion due to tectonic upwarping in the region, which also caused the river incision. River metamorphosis occurred some time during 4 and 0.5 ka.  相似文献   

长江中下游晚中生代中基性岩的铅同位素特征:富集地幔的证据   总被引：33，自引：3，他引：33  

闫峻  陈江峰  喻钢  钱卉  周泰禧 《高校地质学报》2003,9(2):195-206

长江中下游东段庐枞、怀宁、繁昌、铜陵和宁芜地区的中基性岩属于碱性系列 ,具有高的U ,Th含量和Th/Pb ,U/Pb比值 ,分别平均为 2 .82× 10 -6,9.5 6× 10 -6和 0 .6 35 ,0 .184。样品的初始铅同位素 (130Ma)组成为 :(2 0 6Pb/ 2 0 4Pb) i=17.6 5～ 18.6 0 ,(2 0 7Pb/ 2 0 4Pb) i=15 .4 2～ 15 .5 0 ,(2 0 8Pb/ 2 0 4Pb) i=37.6 7～ 38.0 4。形成中基性岩的原始岩浆来源于富集的岩石圈地幔 ,具有EMⅠ和EMⅡ ,且以EMⅡ为主的特征。和长江中下游西段黄石地区以及大别地块西南部玄武岩的比较表明 ,长江中下游地区岩石圈地幔高的Th/Pb和U/Pb比值可能和俯冲板片析出流体的交代有关。晚中生代时期 ,华北板块岩石圈地幔以EMⅠ特征为主 ,华南板块岩石圈地幔以EMⅡ特征为主 ,岩石圈地幔性质的区域性分布与印支期扬子板块深俯冲事件密切相关。虽然中国东部新生代玄武岩因岩石圈大规模减薄表现了亏损特征 ,但残留的富集岩石圈地幔在中国东部新生代玄武岩的Pb同位素中仍有所反映  相似文献   

渤海湾老狼坨子海岸带^14C,^137Cs、^210Pb测年与现代沉积速率的加速趋势   总被引：10，自引：5，他引：10  

王宏 姜义李建芬  夏威岚M．VanStrydonck S．L．Goodbred 康慧 《地质通报》2003,22(9):658-664

通过对渤海湾西岸L剖面中同一关键层的碳酸钙和原生腹足类壳债权的^14C测年对比研究，确定了晚全新世的2800cal BP的层位及沉积速率0.045cm/a;8个样柱的^137Cs和^210Pbex强度、蓄积量揭示了距今约120年来堤后盐沼的平均沉积速率约为0.35cm/a，而面向开放海湾的潮坪上部则达到约2-3cm/a。研究区近1个世纪以来沉积速率的加速趋势，是渤海湾西岸泥质海岸带的特征之一。  相似文献   

~(40)Ar/~(39)Ar Dating of Deformation Events and Reconstruction of Exhumation of Ultrahigh-Pressure Metamorphic Rocks in Donghai, East China   总被引：3，自引：0，他引：3  

LI Jinyi  YANG Tiannan  CHEN Wen  ZHANG Sihong Institute of Geology  Chinese Academy of Geological Sciences  Beijing E-mail: jyli@cags.net.cn 《《地质学报》英文版》2003,77(2)

Recent investigations reveal that the ultrahigh-pressure metamorphic (UHPM) rocks in the Donghai region of East China underwent ductile and transitional ductile-brittle structural events during their exhumation. The earlier ductile deformation took place under the condition of amphibolite facies and the later transitional ductile-brittle deformation under the condition of greenschist facies. The hanging walls moved southeastward during both of these two events. The 40Ar/39Ar dating of muscovites from muscovite-plagioclase schists in the Haizhou phosphorous mine, which are structurally overlain by UHPM rocks, yields a plateau age of 218.0±2.9 Ma and isochron age of 219.8Ma, indicating that the earlier event of the ampibolite-facies deformation probably took place about 220 Ma ago. The 40Ar/39Ar dating of oriented amphiboles parallel to the movement direction of the hanging wall on a decollement plane yields a plateau age of 213.1±0.3 Ma and isochron age of 213.4±4.1 Ma, probably representing the age o  相似文献