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1.
《International Geology Review》2012,54(16):2044-2064
The Neoproterozoic succession in the Aksu region of northwestern China forms an unconformable boundary with the lower Precambrian Aksu basement group and consists of the Qiaoenbrak, Yuermeinak, Sugetbrak, and Chigebrak Formations. The two lowermost units include distinct glaciogenic diamictites that indicate distinct episodes of glaciation. In this study, we report the LA-ICP-MS U–Pb ages of detrital zircons and geochemical data from the lower Neoproterozoic strata. The age of the detrital zircon constrains the maximum depositional age to between 769 ± 10 and 727 ± 8 Ma for the Qiaoenbrak diamictites, which are associated with the Kaigas glaciation that occurred during the early Cryogenian period. The youngest detrital zircon age of 719 ± 9 Ma corresponds to the maximum depositional age of the Yuermeinak diamictites, which are associated with the Sturtian glaciation. The detrital zircons from the lower Neoproterozoic strata in the Aksu area indicated four peak ages of 2484, 1948, 861, and 647–581 Ma, which are consistent with the major tectonic episodes in the Tarim Block. The peak age of 2484 Ma represents an Archaean basement, which participated in the worldwide continental nuclei growth event from the late Neoarchaean to the early Palaeoproterozoic. The peak age of 1948 Ma may be associated with the assembly of the Columbia supercontinent, and the 861 and 647–581 Ma are likely associated with the break-up of the Rodinia supercontinent. The combination of geological and geochemical characteristics between the Qiaoenbrak Formation and Aksu Group indicates that the Qiaoenbrak Formation may be penecontemporaneous with the Aksu Group in an active continental margin tectonic setting. Following the break-up of the Rodinia supercontinent, the margin of the Aksu evolved into a passive margin and the Yuermeinak and Sugetbrak Formations were deposited.  相似文献   

2.
The Witputs section of the Gariep Belt (S Namibia) comprises a sequence of clastic and chemical sediments, which have been interpreted as representative of a Late Neoproterozoic global or near-global ice age event, and recent biostratigraphic work in the upper rocks of the Witputs suggest a Late Ediacaran age. To further characterise this sequence and provide additional age constraints, a detailed sedimentological and detrital zircon study has been carried out. The petrographic, sedimentological and geochemical characteristics of the Witputs diamictite determined in this study are homogenous and indicative of debris flow or palaeo-valley infill sediments, deposited in an oxic environment with no glaciogenic evidence. This homogeneity is also reflected in the detrital zircon age spectra with most ages falling between 1.0 and 1.3?Ga, representing the local geology, with the youngest grain at 1030.2?±?10.9 (2??) (n?=?92?<10% discordance), despite the fact that mid and Late Neoproterozoic volcanic activity is known in the local region. The overlying carbonate rocks, often considered to be ??cap carbonates??, show high Mn (up to 60% MnO), with base metal precipitation (Zn, V, Co), and are recrystallised. Their ??13CVPDB isotope ratios are homogeneous at around-3. Major and trace element ratios reach values which indicate that C?CO isotopes may be disturbed and might not reflect primary global seawater composition, thus questioning their use for global correlation and comparison with composite chemostratigraphic curves. The contact to the overlying Late Ediacaran Sanddrif Member is not exposed, and the rocks dip in a different direction than the underlying carbonate rocks. The c. 40-m-thick section is characterised by rapid lithology changes including shales, calcareous sandstones and wackes, fine-grained conglomerates and rare clean quartz-rich sandstones, all of which have strikingly similar detrital zircon populations, and the youngest zircon is dated at 1082.8?±?10?Ma (2?? errors, from 72 grains with <10% discordance). Acritarchs earlier found in the Sanddrif Member, however, indicate a post-570?Ma depositional age. If the diamictites are glacio-marine deposits, then an interesting conclusion is that the clastic sediments can display a very immature geochemical signature, indicating a localised provenance, with derivation purely from the local basement rocks, which is also reflected in the detrital zircon populations. However, we would hesitate to assign a glacial origin to the deposits as no glacial indicators, other than a diamictitic texture, were observed. Clearly, far more work on the detailed mapping and sedimentology of the Neoproterozoic Gariep Belt deposits is required, particularly as many are currently used for global correlation. Age constraints derived from extensive detrital zircon work can only constrain the deposits as being post 1.03?Ma with the detritus being purely locally derived.  相似文献   

3.
华南扬子陆块成冰纪冰川作用的启动时限及其全球对比*   总被引：1，自引：1，他引：0  
江口冰期和南沱冰期是华南地区引人注目的2次成冰纪冰川事件,但其确切启动时间及其全球对比关系仍未有定论。为此,对桂北地区成冰系(南华系)长安组底部和南沱组底部冰成杂砾岩开展了碎屑锆石U-Pb年代学研究。长安组碎屑锆石U-Pb年龄集中分布于958—717 Ma,显著峰值为720、753、805及848 Ma,最年轻一组 206Pb/238U 年龄的加权平均值为719.6±6.1 Ma,可解释为长安组最大沉积年龄;南沱组碎屑锆石U-Pb 年龄集中分布于987—649 Ma,显著峰值为650、720、753、779、803、823及848 Ma,最年轻一组 206Pb/238U 年龄的加权平均值为649.3±6.2 Ma,可解释为南沱组最大沉积年龄。结合已发表的相关年龄数据可知,江口冰期很可能启动于ca.715 Ma,与塔里木、阿拉伯—努比亚、劳伦等陆块的Sturtian冰川作用高度同步;南沱冰期的启动应晚于650 Ma,与西伯利亚、澳大利亚、劳伦等陆块的Marinoan冰川作用基本同步。另外,碎屑锆石U-Pb年龄谱与CL图像显示,长安组和南沱组的物质来源主要为下伏新元古界岩浆—沉积记录,揭示出冰川对下伏地层的强烈刨蚀作用和华南新元古代幕式构造岩浆热事件。扬子陆块成冰纪冰川刨蚀作用可能与Rodinia 超大陆“裂离”有关的强烈伸展活动存在联系,并可能持续至Marinoan 冰期结束。  相似文献   

4.
新元古代冰期事件记录了“雪球地球”事件重要的地质信息。塔里木盆地周缘新元古代冰碛岩地层露头发育,是研究新元古代冰期事件的理想基地。由于发育多套新元古代火山岩,盆地东北缘库鲁克塔格地区新元古代冰碛岩地层时代已获得较多年代学数据约束;但盆地周缘其他地区新元古代冰碛岩地层公开报道年代学数据较少,不能准确限定其沉积时代,导致冰期事件对比存在争论。为此,本文选择塔里木盆地研究程度较低的西南缘叶城地区新元古代冰碛岩地层,开展岩石学、同位素年代学、岩石地球化学等研究,明确其冰期沉积特征,约束其沉积时代,开展冰期事件对比,讨论古气候风化条件等。南华系波龙组和雨塘组冰碛岩地层具有较低的化学蚀变指数(CIA),分别代表新元古代2次寒冷的冰川气候记录。冰川沉积及其相邻层位的碎屑锆石U-Pb年代学数据显示,波龙冰期的起始年龄晚于(710±13) Ma,与全球Sturtian冰期对应;雨塘冰期的起始年龄不会早于(656±18) Ma,其结束年龄可被南华系顶界年龄635 Ma或上覆震旦系库尔卡克组碎屑锆石年龄(634±9) Ma限定,与全球Marinoan冰期对应。  相似文献   

5.
ABSTRACT There is much debate regarding the intensity and geographic extent of glaciation during the Neoproterozoic, particularly in response to recent geochemical work suggesting that the Neoproterozoic earth was at times ice covered from equator to poles (the ‘Snowball Earth’ hypothesis). A detailed sedimentological analysis of the Neoproterozoic Smalfjord Formation of northern Norway was conducted in order to determine the extent and intensity of glacial influence on sedimentation. In the Tarmfjorden area, the Smalfjord Formation consists of a stacked succession of diamictites interbedded with fine‐grained laminated mudstones containing rare outsized clasts. Diamictites and interbedded mudstones are interpreted as the product of subaqueous mass flows generated along the basin margin. In the Varangerfjorden area, chaotically interbedded diamictites, conglomerates and sandstones are overlain by a thick succession of stacked sandstone beds; onediamictite unit at Bigganjargga overlies a striated pavement. The Varangerfjorden outcrops appear to record deposition on a subaqueous debris apron. Although diamictites contain rare striated and faceted clasts, suggesting a glacial sediment source, their origin as subaqueous mass flows prevents the interpretation of ice mass form or distribution. Rare lonestones may be associated with floating ice in the basin, which may be of glacial or seasonal origin. Glacial ice may have contributed poorly sorted glacial debris to the basin margin, either directly or through fluvioglacial systems, but there is no evidence of direct deposition by ice at Varangerfjorden or Tarmfjorden. The overall fining‐upward trend identified in the Smalfjord Formation and overlying Nyborg Formation is consistent with depositional models of rift basin settings. This fining‐upward trend, the predominance of mass flow facies including breccias associated with scarps and the evidence for extensional tectonic activity in the region suggest that tectonic activity may have played an important role in the development of this Neoproterozoic succession. The Smalfjord Formation at Tarmfjorden and Varangerfjorden does not exhibit sedimentological characteristics consistent with severe glacial conditions suggested by the snowball Earth hypothesis.  相似文献   

6.
扬子东南缘两界河组的岩性以岩屑砂岩和石英砂岩为主,代表了长安冰期和古城冰期之间的间冰期沉积,其沉积时限的厘定对认识华南Sturtian冰期地层的时空分布特征具有重要意义.对黔东地区两界河组碎屑锆石进行了系统的形态学和U-Pb年代学研究,大多数锆石为典型的岩浆锆石,锆石U-Pb年龄主要分布于740~900 Ma,另有少量古元古代和太古宙年龄,主要峰值为~760 Ma、~780 Ma、~800 Ma、~820 Ma和880~900 Ma.在两界河组底部获得最年轻的单颗粒锆石年龄为708±15 Ma,在上部获得最年轻的单颗粒锆石年龄为703±22 Ma,结合区域上相当地层渫水河组的顶部年龄（~690 Ma）,认为黔东地区两界河组的沉积时代应在708~690 Ma之间.两界河组碎屑锆石U-Pb年龄谱记录了扬子陆块新元古代幕式岩浆事件及早期地壳演化的信息,结合锆石形态认为其物质来源可能包括下伏新元古代岩浆岩及沉积地层、扬子西北缘和西南缘的基底岩石.研究区两界河组底部碎屑锆石年龄约束了江口间冰期沉积晚于~708 Ma,考虑到南华纪早期地层分布在一定程度上受控于盆地构造活动,不排除长安冰期沉积物在黔东地区局部存在的可能性.   相似文献   

7.
《Gondwana Research》2003,6(1):65-77
A sedimentary succession included in the lower section of the Playa Hermosa Formation from the Playa Verde Basin, Uruguay, is described. Two facies associations, one mainly coarse- to medium-grained and other one fine-grained, have been defined (FA I-II). In the first one, breccias, conglomerates, sandstones and minor mudstones were deposited in a subaqueous depositional setting (proximal) suggesting slope instability and resedimentation processes. The second one contains diamictites, rhythmites, sandstones and mudstones and presents abundant evidence of soft-deformation, also interpreted to be deposited in a subaqueous environment (distal). Dropstones, clast layers, diamictites, rhythmites and varve-like deposits are interpreted as ice rafting processes generated during a glacial episode. This glacial-related succession constitutes the first record from the Varanger glaciation at the Río de la Plata Craton of the late Neoproterozoic age and also represents one of the oldest sedimentary records after the collision of the Río de la Plata and Kalahari Cratons. A combined interaction of extensional faulting and glaciation in a tectonically active basin with locally high subsidence rates, resulted in high rates of sedimentation and resedimentation processes. As a whole, the sedimentary succession sets a relevant datum to be used in future paleogeographic reconstructions of the Vendian glacial record in southern South America.  相似文献   

8.
The assessment of detrital zircon age records is a key method in basin analysis, but it is prone to several biases that may compromise accurate sedimentary provenance investigations. High to ultrahigh temperature (HT-UHT) metamorphism (especially if T > 850 °C) is herein presented as a natural cause of bias in provenance studies based on U-Pb detrital zircon ages, since zircon from rocks submitted to these extreme and often prolonged conditions frequently yield protracted, apparently concordant, geochronological records. Such age spreading can result from disturbance of the primary U-Pb zircon system, likewise from (re)crystallization processes during multiple and/or prolonged metamorphic events. In this contribution, available geochronological data on Archean, Neoproterozoic and Palaeozoic HT-UHT metamorphic rocks, acquired by different techniques (SIMS and LA-ICP-MS) and showing distinct compositions, are reassessed to demonstrate HT-UHT metamorphism may result in modes and age distributions of unclear geological meaning. As a consequence, it may induce misinterpretations on U-Pb detrital zircon provenance analyses, particularly in sedimentary rocks metamorphosed under such extreme temperature conditions. To evaluate the presence of HT-UHT metamorphism-related bias in the detrital zircon record, we suggest a workflow for data acquisition and interpretation, combining a multi-proxy approach with: (i) in situ U-Pb dating coupled with Hf analyses to retrieve the isotopic composition of the sources, and (ii) the integration of a petrochronological investigation to typify fingerprints of the HT-UHT metamorphic event. The proposed workflow is validated in the investigation of one theoretical and one natural example allowing a better characterization of the sedimentary sources, maximum depositional ages, and the tectonic setting of the basin. Our workflow allows to the appraisal of biases imposed by HT-UHT metamorphism and resulting disturbances in the U-Pb detrital zircon record, particularly for sedimentary rocks that underwent HT-UHT metamorphism and, finally, suggests ways to overcome these issues.  相似文献   

9.
The Mount Cook area in the Southern Alps of New Zealand is heavily glacierized with numerous peaks over 3000 m a.s.l. feeding several large valley glaciers. The region is subject to rapid tectonic uplift and heavy precipitation (up to 15 m per year). This paper describes the clast roundness, clast shape and textural characteristics associated with five glaciers (Fox, Franz Josef, Hooker, Mueller and Tasman) in terms of inputs to the glacier system, transport by the glaciers and reworking following glacial deposition. Inputs include rockfall, alluvial fan and avalanche material delivered to the surface of valley glaciers. Basal debris, where observed at the terminus of two glaciers, consists mainly of incorporated fluvial material. Following deposition, reworking is mainly by subglacial and proglacial streams. The dominant facies are (i) boulder gravel with mainly angular clasts on the steep slopes above the glaciers, (ii) sandy boulder gravel, with mainly angular and subangular clasts, forming lateral and end moraines, and (iii) sandy boulder/cobble gravel with mainly subrounded clasts, and sand, which represent glacially transported sediment reworked by braided rivers. Diamicton is rare in the contemporary glacial environment. Since most sediment associated with glaciers in the Southern Alps lacks unambiguous indications of glacial transport, interpretation of similar sediments in the geological record should not necessarily exclude the involvement of glacial processes.  相似文献   

10.
The origin of the Luoquan Formation which occurs along the southern margin of the North China Blockhas long been argued. Based on recent work. the Formation is considered as a glacial sedimentary sequencepartially reworked by sediment gravity flow. The major evidence for the glacigene of Luoquan Formationdiamictites is as follows: 1, a striated and polished pavement with various features resulting from glacialabrasion and plucking, such as crescentic gouge, crescentic fracture, streamlined form and glaciated step; 2.unsorted diamictites with striated clast. faceted clast and iron-shaped stone formed by glaciation; 3. rhythmitewith dropstones; 4. a glacial sedimentary sequence bearing advance-retreat cycles; and 5. wide distribution ofthe diamictites. Glacial deposits can be distinguished from sediment gravity flow deposits by the features men-tioned above. Some characteristics of sediment gravity flow existing in the Luoquan Formation diamictites in-dicate that glacial deposits might have been partially reworked by sediment gravity flow. Therefore, this papersuggests that the Luoquan Formation diamictite is a result of a glacial event rather than a mud flow deposit.The primary tillites are the principal contribution of the Luoquan Formation, while sediment gravity flow de-posits are the redeposited diamictites and should be termed as glacigenic sediment gravity flow deposits.  相似文献   

11.
Jiri Slama 《地学学报》2016,28(6):394-401
Rare late Neoproterozoic (Cryogenian and Ediacaran) detrital zircons are detected in sedimentary sequences of Ediacaran to Carboniferous age in SW Scandinavia. New data on five samples of clastic metasediment corroborate their presence. Three distinct sources are proposed for late Neoproterozoic zircons based on new and literature data: (1) rift‐related magmatism along the W margin of Baltica connected with the opening of Iapetus in the Neoproterozoic, (2) the Neoproterozoic to Cambrian Timanian orogeny at the NE margin of Baltica and (3) Cadomian terranes incorporated into the Variscan orogen accreted from the south to Laurussia during the Devonian and Carboniferous. The appearance of late Neoproterozoic detritus, which is otherwise exotic to Baltica, is therefore bound to tectonic processes active at the margins of the palaeocontinent. In a broader perspective, rare detrital zircon populations, when confirmed as genuine, are extremely valuable provenance indicators.  相似文献   

12.
王超  何世平  时超  于浦生 《地质通报》2012,31(8):1233-1243
博查特塔格组分布于塔里木地块西南缘的铁克里克地区,前人认为其主要由一套海相碳酸盐岩组成,底部为含砾粗砂岩,并将其时代划为蓟县纪。笔者对来自博查特塔格组中的条带状大理岩和含砾粗砂岩中的碎屑锆石样品进行了LA-ICP-MSU-Th-Pb同位素分析。分析结果显示,含砾粗砂岩中的碎屑锆石频谱图具有1953、2038、2163~2182、2225~2243、2281~2343Ma等多个年龄峰值,2281~2343Ma年龄值为主要峰值,其中最小谐和年龄1953Ma代表了该粗砂岩最早的沉积时代;条带状大理岩中的碎屑锆石年龄峰值主体介于772~856Ma之间,最小谐和年龄值为772Ma,代表了该大理岩最早的沉积时代。分析结果说明,原划为蓟县系的博查特塔格组由不同时代的物质组成,该地层单元需要解体。博查特塔格组的岩石组合应为中上部的碳酸盐岩夹细碎屑岩建造,该地层最早的沉积时代为772Ma,形成时代为南华纪。  相似文献   

13.
Detrital zircons are important proxies for crustal provenance and have been widely used in tracing source characteristics and continental reconstructions. Southern Peninsular India constituted the central segment of the late Neoproterozoic supercontinent Gondwana and is composed of crustal blocks ranging in age from Mesoarchean to late Neoproterozoic–Cambrian. Here we investigate detrital zircon grains from a suite of quartzites accreted along the southern part of the Madurai Block. Our LA-ICPMS U-Pb dating reveals multiple populations of magmatic zircons, among which the oldest group ranges in age from Mesoarchean to Paleoproterozoic (ca. 2980–1670 Ma, with peaks at 2900–2800 Ma, 2700–2600 Ma, 2500–2300 Ma, 2100–2000 Ma). Zircons in two samples show magmatic zircons with dominantly Neoproterozoic (950–550 Ma) ages. The metamorphic zircons from the quartzites define ages in the range of 580–500 Ma, correlating with the timing of metamorphism reported from the adjacent Trivandrum Block as well as from other adjacent crustal fragments within the Gondwana assembly. The zircon trace element data are mostly characterized by LREE depletion and HREE enrichment, positive Ce, Sm anomalies and negative Eu, Pr, Nd anomalies. The Mesoarchean to Neoproterozoic age range and the contrasting petrogenetic features as indicated from zircon chemistry suggest that the detritus were sourced from multiple provenances involving a range of lithologies of varying ages. Since the exposed basement of the southern Madurai Block is largely composed of Neoproterozoic orthogneisses, the data presented in our study indicate derivation of the detritus from distal source regions implying an open ocean environment. Samples carrying exclusive Neoproterozoic detrital zircon population in the absence of older zircons suggest proximal sources in the southern Madurai Block. Our results suggest that a branch of the Mozambique ocean might have separated the southern Madurai Block to the north and the Nagercoil Block to the south, with the metasediments of the khondalite belt in Trivandrum Block marking the zone of ocean closure, part of which were accreted onto the southern Madurai Block during the collisional amalgamation of the Gondwana supercontinent in latest Neoproterozoic–Cambrian.  相似文献   

14.
Detrital zircon U-Pb geochronology combined with Hf isotopic and trace element data from metasedimentary rocks of the Aracuai Belt in southeastern Brazil provide evidence for break-up of the Congo-Sao Francisco Craton. The U-Pb age spectra of detrital zircons from metasediments of the Rio Doce Group(RDG) range from 900-650 Ma and define a maximum depositional age of ca. 650 Ma. Zircon trace element and whole rock data constrain an oceanic island arc as source for the deposition setting of the protoliths to the metasediments. Zircon ε_(Hf)(t) values from these rocks are positive between +1 and +15, supporting previous evidence of a Neoproterozoic extensional phase and oceanic crust formation in a precursor basin to the Aracuai Belt. Recrystallization of detrital zircon at ca. 630 Ma is compatible with a regional metamorphic event associated with terrane accretion to the Paleoproterozoic basement after transition from an extensional to a convergent regime. The juvenile nature, age spectra and trace element composition recorded in detrital zircons of metasediments from the Aracuai Belt correspond with zircons from metasedimentary rocks and oceanic crust remnants of other orogenic belts to its south. This suggests that rifting and oceanic crust formation of the entire orogenic system, the so-called Mantiqueira Province, was contemporaneous, most likely related to the opening of a large ocean. It further indicates that the cratonic blocks involved in the orogenic evolution of the Mantiqueira Province were spatially connected as early as 900 Ma.  相似文献   

15.
The Neoproterozoic Era includes some of the most largest ice ages in the geological history. The exact number of glaciations is unknown, though there were at least two events of global glaciation. Neoproterozoic glacial deposits in the Kuruktag Mountain, Xinjiang, western China have proven that there had occurred three discrete Neoproterozoic glaciations. Diamictite units occurred in the Bayisi, Tereeken, and Hankalchough formations, carbonate units were recognized among the diamictites and immediately overlied the Bayisi, Tereeken and Hankalchough diamictites. Carbonates at the top of the Bayisi Formation are characterized by the dolo-sility stones with negative δ13C values ranging from -4.10‰ to -8.17‰ (PDB), comparable to the Sturtian cap carbonates that overlie the Sturtian glacial deposits from other Neoproterozoic sequences. Carbonates overlying the Tereeken Formation are characterized by the pinkish cap dolostones (ca. 10 m thick) with negative δ13C values ranging from -2.58‰ to -4.77‰ (PDB), comparable to the Marinoan cap carbonates. The cap is also characterized by tepee-like structures, barite precipitates and pseudomorphous aragonite crystal fan limestones. Carbonates at the top of the Hankalchough Formation are characterized by subaerial exposure crust (vadose pisolite structure, calcareous crust structure) dolostones with negative δ13C values ranging from -4.56‰ to -11.45‰ (PDB) and the calcareous crust dolostones, implying that the Hankalchough cap carbonates differ from either the Sturtian or Marinoan cap carbonates in sedimentary environment and carbon isotopic composition. In addition, it is suggested the Hankalchough glaciation belongs to a terrestrial glaciation and it is the third largest glaciation during the Neoproterozoic period on the Tarim platform.  相似文献   

16.
Severe climate changes culminating in at least three major glacial events have been recognized in the Neoproterozoic sedimentary record from many parts of the world. Supportive to the global nature of these climatic shifts, a considerable amount of data have been acquired from deposits exposed in Pan-African orogenic belts in southwestern and western Africa. By comparison, published data from the Pan-African belts in Central Africa are scarce. We report here evidence of possibly two glacial events recorded in the Mintom Formation that is located on the margin of the Pan-African orogenic Yaoundé belt in South-East Cameroon.In the absence of reliable radiometric data, only maximum and minimum age limits of 640 and 580 Ma, respectively, can at present be applied to the Mintom Formation. The formation consists of two lithostratigraphic ensembles, each subdivided in two members (i.e., in ascending stratigraphic order the Kol, Métou, Momibolé, and Atog Adjap Members). The basal ensemble exhibits a typical glacial to post-glacial succession. It includes diamictites comprising cobbles and boulders in a massive argillaceous siltstone matrix, and laminated siltstones followed by, in sharp contact, a 2 m-thick massive dolostone that yielded negative δ13C values (<?3‰ V-PDB) similar to those reported for Marinoan cap carbonates elsewhere. However, uncertainty remains regarding the glacial influence on the siliciclastic facies because the diamictite is better explained as a mass-flow deposit, and diagnostic features such as dropstones have not been seen in the overlying siltstones. The Mintom Formation may thus provide an example of an unusual succession of non-glacial diamictite overlain by a truly glacial melt-related cap-carbonate.We also report the recent discovery of ice-striated pavements on the structural surface cut in the Mintom Formation, suggesting that glaciers developed after the latter had been deposited and deformed during the Pan-African orogeny. Striations, which consistently exhibit two principal orientations (N60 and N110), were identified in two different localities, in the west of the study area on siltstones of the Kol Member, and in the east on limestones of the Atog Adjap Member, respectively. N60-oriented striae indicate ice flow towards the WSW. Assigning an age to these features remains problematical because they were not found associated with glaciogenic deposits. Two hypotheses can equally be envisaged, i.e., either the striated surfaces are correlated: (1) to the Gaskiers (or Neoproterozoic post-Gaskiers) glaciation and represent the youngest Ediacaran glacial event documented in the southern Yaoundé belt; or (2) to the Late Ordovician Hirnantian (Saharan) glaciation, thereby providing new data about Hirnantian ice flows in Central Africa.  相似文献   

17.
胡蓉  李双庆  王伟  陈福坤 《地球科学》2016,41(10):1630-1654
扬子陆块新元古代地层发育完整并广泛分布,三峡地区南沱组冰碛岩是扬子北部南华纪的冰成杂砾岩,不整合地覆盖在新元古代黄陵花岗岩之上.南沱组冰碛岩成分复杂,分选性差,缺乏可对比的标志,其沉积时代和物源组成研究较少且存在争议.对南沱组冰碛岩开展锆石U-Pb定年工作,得到最年轻的锆石年龄为706±7 Ma,印证了南沱组沉积时代应不早于700 Ma的说法,并可与Marinoan冰期对应.所获得锆石U-Pb年龄谱峰值主要分为~3.0 Ga,2.5~2.3 Ga,2.05~1.95 Ga和900~700 Ma,反映了扬子陆块4次地壳再造事件,其中古元古代和太古代年龄与崆岭杂岩相似,900~800 Ma和800~700 Ma的锆石年龄与附近黄陵花岗岩和扬子北缘岩体一致.冰碛砂岩的微量元素和同位素组成分异明显,可能是扬子北缘岩体、黄陵花岗岩和崆岭杂岩不同程度混合的结果.南沱组冰碛岩中包裹的花岗质砾石表现出不同的微量元素特征,具有偏低的稀土含量和亏损的重稀土组成,结合其锆石U-Pb年龄和Sr-Nd同位素特征,这些花岗质砾石主体可能来自于附近黄陵花岗岩和崆岭岩基.南沱组冰碛岩的物质组成特征反映了新元古时期,扬子地区强烈的裂谷岩浆作用所导致的北高南低的地势环境.   相似文献   

18.
通过对辽宁地区出露的新元古代殷屯组、永宁组调查研究取得了新认识,认为辽宁地区存在南华纪地层,以新元古代殷屯组、永宁组为代表,可划分出二次冰川活动（二个冰期和一次间冰期）的沉积,自下而上为殷屯冰期（第一次冰期）、八家子间冰期、河冰期（第二次冰期）.依据殷屯组和永宁组冰碛物组合特点及接触关系,并结合碎屑锆石U-Pb（LA-ICP-MS）同位素测年成果,推测辽宁南华系殷屯组、永宁组形成时代应小于1000 Ma,与南方南华纪形成的时间基本接近或略早（约850 Ma）.  相似文献   

19.
Ion-microprobe U–Pb analyses of 589 detrital zircon grains from 14 sandstones of the Alborz mountains, Zagros mountains, and central Iranian plateau provide an initial framework for understanding the Neoproterozoic to Cenozoic provenance history of Iran. The results place improved chronological constraints on the age of earliest sediment accumulation during Neoproterozoic–Cambrian time, the timing of the Mesozoic Iran–Eurasia collision and Cenozoic Arabia–Eurasia collision, and the contribution of various sediment sources of Gondwanan and Eurasian affinity during opening and closure of the Paleotethys and Neotethys oceans. The zircon age populations suggest that deposition of the extensive ~ 1 km-thick clastic sequence at the base of the cover succession commenced in latest Neoproterozoic and terminated by Middle Cambrian time. Comparison of the geochronological data with detrital zircon ages for northern Gondwana reveals that sediment principally derived from the East African orogen covered a vast region encompassing northern Africa and the Middle East. Although most previous studies propose a simple passive-margin setting for Paleozoic Iran, detrital zircon age spectra indicate Late Devonian–Early Permian and Cambrian–Ordovician magmatism. These data suggest that Iran was affiliated with Eurasian magmatic arcs or that rift-related magmatic activity during opening of Paleotethys and Neotethys was more pronounced than thought along the northern Gondwanan passive-margin. For a Triassic–Jurassic clastic overlap assemblage (Shemshak Formation) in the Alborz mountains, U–Pb zircon ages provide chronostratigraphic age control requiring collision of Iran with Eurasia by late Carnian–early Norian time (220–210 Ma). Finally, Cenozoic strata yield abundant zircons of Eocene age, consistent with derivation from arc magmatic rocks related to late-stage subduction and/or breakoff of the Neotethys slab. Together with the timing of foreland basin sedimentation in the Zagros, these detrital zircon ages help bracket the onset of the Arabia–Eurasia collision in Iran between middle Eocene and late Oligocene time.  相似文献   

20.
博查特塔格组分布于塔里木地块西南缘的铁克里克地区,前人认为其主要由一套海相碳酸盐岩组成,底部为含砾粗砂岩,并将其时代划为蓟县纪。笔者对来自博查特塔格组中的条带状大理岩和含砾粗砂岩中的碎屑锆石样品进行了LA-ICP-MSU-Th-Pb同位素分析。分析结果显示,含砾粗砂岩中的碎屑锆石频谱图具有1953、2038、2163-2182、2225-2243、2281-2343Ma等多个年龄峰值,2281-2343Ma年龄值为主要峰值,其中最小谐和年龄1953Ma代表了该粗砂岩最早的沉积时代;条带状大理岩中的碎屑锆石年龄峰值主体介于772-856Ma之间,最小谐和年龄值为772Ma,代表了该大理岩最早的沉积时代。分析结果说明,原划为蓟县系的博查特塔格组由不同时代的物质组成,该地层单元需要解体。博查特塔格组的岩石组合应为中上部的碳酸盐岩夹细碎屑岩建造,该地层最早的沉积时代为772Ma,形成时代为南华纪。  相似文献   

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