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1.
The Lower Cambrian on the Yangtze Platform in South China (internationally equivalent to the Nerreneuvian and 2nd series of the Cambrian) is valuable for understanding the early evolution of life, the global biogeochemical circles, and the major changes of the ocean. In particular, a precise radiometric calibration of the Lower Cambrian of this region is a critical realm in its multidisciplinary studies. A SHRIMP U-Pb geochronology study of the K-bentonite in the topmost Laobao Formation at the Pingyin section, Guizhou, South China yielded an age of 536±5 Ma (MSWD = 0.75), suggesting that the K-bentonite here can be correlated with the intensely studied K-bentonite within the middle Zhongyicun Member (Bed 5) of the Zhujiaqing Formation at the Meishucun section in Yunnan. Such a regional correlation result reveals that the Bed 5 K-bentonite also extends into deeper-water environments rather than being deposited merely in the shallow platform interior of the Yangtze Platform. The age of the K-bentonite at the Pingyin section also implies that its overlying polymetallic Ni-Mo layer should be younger than 536±5 Ma. Hence the previous placement of the Precambrian/Cambrian boundary at this layer is inappropriate. Combined with the results of stratigraphic correlations, it is suggested that the K-bentonites in the middle Zhongyicun Member of the Zhujiaqing Formation (or the equivalents) and the base of the Shiyantou Formation (or its equivalents), together with the polymetallic Ni-Mo layer, serve as three important marker beds. Their self-consistent radiometric ages have established an improved geochronologic framework for the Lower Cambrian in South China.  相似文献   

2.
More than 20 K-bentonite beds were discovered from the Wufeng Formation and the lowest Longmaxi Formation in two sections, both adjacent to the Ordovician-Silurian (O-S) boundary and located in Tongzi, Guizhou Province and Yichang, Hubei Province, some 500 km apart from each other in South China. This indicates that many volcanic eruptions occurred near the southeast margin of the Yangtze Platform between the latest Ordovician and the earliest Silurian. Mainly through biostratigraphic and sequence stratigraphic studies, it was found that almost all these far-apart K-bentonite beds may be correlated with each other. This is the first time that a succession of volcanic ash deposits with high potential of correlation was discovered within a strata interval on the main platforms of China. Therefore, these K-bentonite beds may afford excellent event-marker beds helpful to high-resolution research in integrated stratigraphy as well as other lines of research on the O-S boundary in South China.  相似文献   

3.
Determination of the age of the Precambrian-Cambrian boundary is critical in understanding early evolution of life on Earth. SIMS U-Pb zircon analyses of the Bed 5 tuff layer of the Meishucun section were carried out closely following the guidance of cathodoluminescence images, and the majority of analyses were conducted on the oscillatory zircon grains. Thirteen measurements yield a highly reliable Concordia U-Pb age of 536.7 ± 3.9 Ma for the Bed 5 horizon. A grand mean of 206Pb/238U age of 535.2± 1.7 Ma (...  相似文献   

4.
High initial parent/daughter element ratios and a unique dual decay scheme make UPb zicron ages more precise and reliable than most isotopic ages, and thus inherently superior for time-scale calibration. Employing improved techniques to the conventional method of UPb dating, we have analyzed microgram-size (2–12 × 10−8 g) zircon fractions from biostratigraphically controlled volcanic ashes and dated key Paleozoic time-markers with a precision better than 1% (±2Ma). Four of the stratotype samples from Britain for which fission-track ages [1] were previously reported have yielded improved ages of:438.7 ± 2.0Ma for the lower Silurian zone of Coronograptus cyphus from Llandovery strata at Dob's Linn, southern Scotland;457.5 ± 2.2 Ma for a Middle Ordovician Caradoc (Longvillian) ash near Bala, North Wales, and;465.7 ± 2.1and464.6 ± 1.8 Ma for the Didymograptus artus Zone and the type Didymograptus Murchisoni Zone, respectively, of the Llanvirn Series at Arenig Fawr and Abereiddi Bay, Wales. Another sample from the zone of Dicellograptus anceps (P. pacificus Subzone) of the Ashgill Series at Dob's Linn has been dated at445.7 ± 2.4Ma, suggesting placement of the Ordovician-Silurian time boundary at approximately 441 Ma. A sixth bentonite from Caradocian age strata of North America (Spechts Ferry Shale, Decorah Formation, Missouri) is453.7 ± 1.8Ma old, indicating that the Rocklandian Stage of the Mohawkian Series is only slightly younger than the Longvillian Stage of the Caradoc Series in Britain.  相似文献   

5.
The Permian-Triassic Boundary Stratigraphic Set (PTBST), characteristic of the GSSP section of Meishan and widespread in marine Permian-Triassic Boundary (PTB) sequences of South China, is used to trace and recognize the PTB in a continental sequence at Chahe (Beds 66f―68c). Diversified Permian plant fossils extended to the PTBST, and a few relicts survived above that level. Sporomorphs are dominated by fern spores of Permian nature below the PTBST, above which they are replaced by gymnosperm pollen of Triassic aspect. In the nearby Zhejue Section, the continental PTBST is charac- terized by the fungal 'spike' recorded in many places throughout the world. The boundary claybeds (66f and 68a,c) of the PTBST are composed of mixed illite-montmorillonite layers analogous with those at Meishan. They contain volcanogenic minerals such as β quartz and zircon. U/Pb dating of the upper claybed gives ages of 247.5 and 252.6 Ma for Beds 68a and 68c respectively, averaging 250 Ma. In con- trast to the situation in Xinjiang and South Africa, the sediment sequence of the Permian-Triassic tran- sition in the Chahe section (Beds 56―80) become finer upward. Shallowing and coarsening upward is not, therefore, characteristic of the Permian-Triassic transition everywhere. The occurrence of relicts of the Gigantopteris Flora in the Kayitou Fm. indicates that, unlike most marine biota, relicts of this pa- leophytic flora survived into the earliest Triassic. It is concluded that Bed 67 at Chahe corresponds to Bed 27 at Meishan, and that the PTB should be put within the 60-cm-thick Bed 67b④, now put at its base tentatively. This is the most accurate correlation of the PTB in continental facies with that in the marine GSSP.  相似文献   

6.
The Induan sequence in the West Pingdingshan Section, Chaohu, Anhui Province, displays a series of superimposed mudstone/limestone cycles. The lithological character of the cycles, combined with power spectral and wavelet analysis of magnetic susceptibility readings, reveals 12 short eccentricity and 56 precession Milankovitch cycles - obliquity cycles are not apparent. The uniformity of cycle thicknesses indicates a stable depositional setting making this section ideal to perform various geo-logical studies. Accordingly, the Induan Stage is estimated to have lasted 1.1 Ma, and the depositional rate for this part of the section is about 3.7 cm/ka. This places the Induan-Olenekian boundary in the West Pingdingshan Section at about 251.5 Ma based on an age of 252.6 Ma for the Permian-Triassic boundary.  相似文献   

7.
The petrographic characteristics of Jingshan "migmatitic granite" and the occurrence of the magmatic zircons indicate that the granite was formed by normal crystallization of felsic melts. All zircons in the granite have inherited cores and fine-scale oscillatory zoning rims of magmatic origin. It is realized that the granite was formed at 160.2±1.3 Ma through dating magmatic zircons. The generation of the granitic magma could be related to the lithospheric mantle and/or lower crust delamination after the ultrahigh pressure metamorphism (UHPM) in Triassic. Most inherited zircons yield the ages of 217.1±6.6 Ma, which is consistent with the peak UHPM in the Dabie-Sulu orogenic belt. Some of the inherited zircons (433-722 Ma) constitute a discordia line with the upper intercept age of 850+85/-68 Ma and a lower intercept age of 261+100/-140 Ma. These ages imply that the granite could be derived from the partial melting of the crustal materials of the South China Block that was intensively superimposed by t  相似文献   

8.
The structural mapping and section study indicate that the “greenstone belts” in the southern to central parts of Hengshan were intensively sheared and transposed mafic dyke swarm, which originally intruded into the Neoarchean grey gneiss and high-pressure granulite terrain (HPGT). The HPGT is characterized by flat-dipping structures, to the south it became steep and was cut by the Dianmen mafic dyke swarm. After high-pressure granulite-facies metamorphic event, the mafic dyke swarm occurred, and was associated with the extensional setting and reworked by the late strike-slip shearing. The zircon age dating proves that the Dianmen mafic dyke swarm was emplaced during the period between 2499±4 Ma and 2512±3 Ma, followed by late tectonothermal reworking. The Dianmen mafic dyke swarm further documents the extensional episode in the central to northern parts of North China Craton (NCC), providing the important constraint for the limit between Archean and Proterozoic and correlation between NCC and other cratonic blocks of the world.  相似文献   

9.
The Nanling Mountains lying in the southern part of South China are an economically important gran-ite-related multi-metallogenic province. The Nanling Mountains granites can be described as: temporally spanning from Caledonian to Yanshanian and spatially distributed as three EW trending zones: the north one in Zhuguangshan-Qingzhangshan, the middle one in Dadongshan-Guidong, and the south one in Fogang-Xinfengjiang with two neighboring zones’ midline having an interval of ca. latitude …  相似文献   

10.
The Late Permian to Early Triassic transition represents one of the most important Phanerozoic mass extinction episodes. The cause of this event is still in debate between catastrophic and gradual mechanisms. This study uses the U-Pb method on zircons from the uppermost Permian/lowermost Triassic clay deposits at Chahe (Guizhou Province, SW China) to examine time constraints for this event. The results of both this and previous studies show that the ages of Bed 68a and 68c (the upper clay bed of the terrestrial Permian-Triassic boundary (PTB)) respectively are 252.6±2.8 and 247.5±2.8 Ma. This age (within the margin of error) almost accords with the upper clay bed (Bed 28) age of Meishan and the eruption age of Tunguss Basalt, and is so far the most accurate age obtained from terrestrial PTB. The claystone of Bed 68 was formed in the earliest Triassic. The biotic crisis occurred at nearly the same time in terrestrial and marine environments during Permian-Triassic interval; however the extinction patterns and processes are different. The extinction pattern of the terrestrial plants shows a major decline at the PTB after long-term evolution, followed by a retarded extinction of the relicts in the earliest Triassic.  相似文献   

11.
Jianshangou Bed of the Yixian Formation in West Liaoning,China   总被引：4，自引：0，他引：4  
The Jianshangou fossil locality was first found byChang Zhenglu and his colleagures in 1962, now thearea of Jianshangou, Sihetun and Huangbanjigou be-comes famous all over the world for its yielding feath-ered dinosaurs, Confuciusornis and Archaefructus. Thename of Jianshangou Bed was proposed by the firstauthor of present paper as an informal suggesting unitduring the research on Jehol fossils of western Liaon-ing in 1963 and 1964. The unit was named then asJianshan Bed[1], indicating th…  相似文献   

12.
Neoproterozoic igneous and metamorphic complexes occur as tectonic domes in the Longmen Mountains of the western margin of the Yangtze Block, and are important in reconstructing the Rodinian supercontinent and constraining the timing and mechanism of tectonic denudational processes. The Pengguan dome consists of granitic intrusions and metamorphic rocks of the Huangshuihe Group and is tectonically overlain by ductilly deformed Sinian to Paleozoic strata. The plutonic intrusions consist of granites with abundant amphibolite enclaves. New LA-ICP-MS zircon U-Pb dating yielded an emplacement age of 809±3 Ma and a protolith age of 844±6 Ma for the granite. The granitic rocks have geochemical signatures typical of A-type granites, indicating their formation under an extensional environment, by melting of newly formed tonalite-trondhjemite-granodiorite (TTG) rocks. A detachment fault, characterized by variable ductile shear deformation of S-C fabric and ESE-ward kinematics, separates the Pengguan dome from the Sinian-Paleozoic cover. 40Ar/39Ar dating of muscovite from the mylonite in the detachment fault of the dome demonstrates that ductile deformation occurred at ~160 Ma. This study indicates the existence of a Neoproterozoic magmatic arc-basin system, which was denudated by a Jurassic middle crustal ductile channel flow along the Longmenshan thrust belt.  相似文献   

13.
Granitoid intrusives such as Saishitenshan, Tuanyushan, Aolaohe and Sanchagou occur widely in the western segment of North Qaidam. All these bodies trend NW, roughly parallel to the regional structure. Zircon SHRIMP dating for these granites show that they range in age from Ordovician to Permian; 465.4±3.5 Ma for Saishitenshan, 469.7±4.6 Ma and 443.5±3.6 Ma for Tuanyushan, 372.1±2.6 Ma for Aolaohe, and 271.2±1.5 Ma and 259.9±1.2 Ma for Sanchagou. Both the Tuanyshan and Aolaohe plutons record two distinct intrusive events. Geochemically, the early Paleozoic granites have an island arc or active continental margin affinity, and their protolith may have been Mesoproterozoic oceanic crust derived from depleted mantle. The protolith of the late Paleozoic granites may have been Mesoproterozoic lower crust from the root of an island arc with the magmas reflecting a mixture of mantle and crustal material.  相似文献   

14.
The trace element and rare earth element (REE) variations across the Ordovician-Silurian succession are presented from two outcrop sections on the Yangtze Platform: the Nanbazi section, Guizhou Province, deposited in a shallow platform interior setting, and the Wangjiawan section, Hubei Province, deposited in a deeper basinal environment. Geochemical analysis of closely spaced samples through three intervals, the Wufeng, Guanyinqiao and Longmaxi, revealed vast palaeoceanographic changes. Some geochemical proxies, including Th/U, Ni/Co, V/Cr, and V/(V+Ni) ratios, together with sedimentary facies and biotic data, indicate that an anoxic condition on the most of the Yangtze Platform during the Wufeng and Longmaxi intervals, but an oxic condition during the Guanyinqiao time. The shift of the anoxic to the oxic environment during the Guanyinqiao time coincided with a global sea-level lowstand, in parallel with the global glaciation. The Longmaxian anoxic environment was a result of a global sea-level rise, which may be synchronized with a mainly catastrophic event in the latest Ordovician. Although the two sections generally show similar variation patterns of trace and REE concentrations and some element ratios, a minor difference occurs between the Wangjiawan and Nanbazi sections, likely reflecting a difference in depositional setting during the accumulation. Such an oceanic oxygen-level variation may add a useful constraint to the current arguments on the cause and consequence of the latest Ordovician mass extinction. Supported by National Basic Research Program of China (Grant No. 2005CB422101)  相似文献   

15.
Located northwest of the Jiangshan-Shaoxing Fault Zone,western and northwestern Zhejiang and northeastern Jiangxi provinces experienced major changes in bio-and litho-facies and paleogeography through the Ordovician-Silurian transition (late Katian,Hirnantian,and early Rhuddanian),as manifested by stratigraphic,paleontologic and synecologic records.Three geographic units under consideration are South,Central,and North areas.The western margin of the South area was occupied by the Huaiyu Mountains,whereas th...  相似文献   

16.

Timing of the intermediate-basic igneous rocks developed in the area of Kuhai-A’nyêmaqên along the southern east Kunlun tectonic belt is a controversial issue. This paper presents new zircon SHRIMP U-Pb dating data for igneous zircons from the Kuhai gabbro and the Dur’ngoi diorite in the Kuhai-A’nyemaqen tectonic belt, which are 555±9 Ma and 493±6 Ma, respectively. The trace element geochemical features of the Kuhai gabbro and the Dur’ngoi diorite are similar to those of ocean island basalts (OIB) and island arc basalts (IAB), respectively. Thus, the Kuhai gabbro with the age of 555±9 Ma and OIB geochemical features is similar to the Yushigou oceanic ophiolite in the North Qilian orogen, whereas the Dur’ngoi diorite with the age of 493±6 Ma and IAB geochemical features is similar to the island arc volcanic rocks developed in the north Qaidam. The Late Neoproterozoic to Early Ordovician ophiolite complex in the area of Kuhai-A’nyêmaqên suggests that the southern margin of the “Qilian-Qaidam-Kunlun” archipelagic ocean in this period was located in the southern east Kunlun tectonic belt. Therefore, the southern east Kunlun tectonic belt in the early Paleozoic is not comparable to the Mianlüe tectonic belt in the Qinling orogenic belt.

  相似文献   

17.
The Bashikaogong-Shimierbulake granitoid complex is about 30 km long and 2―6 km wide, with an area of 140 km2, located at the north margin of the Bashikaogong Basin in the north Altun terrain. It intruded into schist, metapelite and metatuff of Precambrian ages. This granitoid complex consists of darkish quartz diorite, grey granite, pink granite and pegmatite. Geochemically, the quartz diorite has I-type granite affinity and belongs to Calc-alkaline sereies, and the other gran- ites have S-type affinity and to high-K calc-alkaline series. Zircon SHRIMP U-Pb dating shows that the quartz diorite has a bigger age than those of other granites, which is 481.6±5.6 Ma for quartz diorite, 437.0±3.0 Ma―433.1±3.4 Ma for grey granite and 443±11 Ma―434.6±1.6 Ma for pink granite, re- spectively. Combined with regional geology, we think that the quartz diorite formed in tectonic envi- ronment related to oceanic crust subduction and the granites in post-collision.  相似文献   

18.
Baogutu copper deposit in Western Junggar area is a mesoscopic porphyry deposit found in recent years. Study on its geochronology will help further understand ore genesis and regional ore-forming pattern. A series of small quartz-diorite and granodiorite stocks outcrop at Baogutu area, numbered I―X according to their size. A detailed exploration on Number V stock confirmed it as a mesoscopic scale copper deposit, and various exploration work has been carried out on other stocks with ore-forming evidence. Th...  相似文献   

19.
The Qinling Mountains in central China are the joint orogenic zone between the Sino-Korean (or North China) and the Yangtze craton blocks. The age and genesis of the Danfeng mafic volcanics in the north of the Shangzhou-Danfeng fault zone, i.e. the main suture zone in the Qinling orogenic belt, have been controverted for a long time because their age is closely related to the converged time of two blocks. The ages and the geochemical data of the Heihe pillow lavas for the Danfeng mafic volcanics in the Heihe River area in the Qinling orogen are reported in this paper. The obtained isochron age by the Sm-Nd isotopic data of the 13 whole-rock samples for the mafic pillow lavas is 963±130 (2σ) Ma, corresponding to INd = 0.51173±16 (2σ),ɛ Nd(T)= +6.6, MSWD0.57. However, the Rb-Sr isotopic analytical results for the same samples as the Sm-Nd whole-rock ones are disperse. For the Sm-Nd isotopic systems were interfered during the later geological functions, the Sm-Nd isochron age for the whole-rock sample (Q9511WR) and the mineral phenocrystal samples: amphiboles (Hb) and plagioclases (Plag) presents the better uncertainty, whereas isochron ages of 930 Ma and 437 Ma are given if the WR-Plag and WR-Hb are calculated respectively, and their Rb-Sr isochron age is 268±47(2σ) Ma, Isr = 0.70475±11 (2σ), MSWD0.96. The major and trace elements for the lavas show that they were formed in the quasi-N-MORB setting.  相似文献   

20.
We present new high-precision 40Ar/39Ar ages on feldspar and biotite separates to establish the age, duration and extent of the larger Siberian Traps volcanic province. Samples include basalts and gabbros from Noril'sk, the Lower Tunguska area on the Siberian craton, the Taimyr Peninsula, the Kuznetsk Basin, Vorkuta in the Polar Urals, and from Chelyabinsk in the southern Urals. Most of the ages, except for those from Chelyabinsk, are indistinguishable from those found at Noril'sk. Cessation of activity at Noril'sk is constrained by a 40Ar/39Ar age of 250.3 ± 1.1 Ma for the uppermost Kumginsky Suite.The new 40Ar/39Ar data confirm that the bulk of Siberian volcanism occurred at 250 Ma during a period of less than 2 Ma, extending over an area of up to 5 million km2. The resolution of the data allows us to confidently conclude that the main stage of volcanism either immediately predates, or is synchronous with, the end-Permian mass extinction, further strengthening an association between volcanism and the end-Permian crisis. A sanidine age of 249.25 ± 0.14 Ma from Bed 28 tuff at the global section and stratotype at Meishan, China, allows us to bracket the P–Tr boundary to 0.58 ± 0.21 myr, and enables a direct comparison between the 40Ar/39Ar age of the Traps and the Permo–Triassic boundary section.Younger ages (243 Ma) obtained for basalts from Chelyabinsk indicate that volcanism in at least the southern part of the province continued into the Triassic.  相似文献   

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