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1.
塔里木板块上元古界火山岩SHRIMP定年及其对新元古代冰期时代的制约 总被引：16，自引：7，他引：9
新疆库鲁克塔格地区贝义西组顶部火山岩锆石的SHRIMP定年结果为732±7Ma,它表明贝义西冰期的上限。考虑该组杂砾岩之下火山岩已有的定年结果,贝义西冰期的时限为740~732Ma。综合上元古界三层火山岩SHRIMP定年结果,可以明确地将库鲁克塔格地区上元古界四个含杂砾岩的组限定在三个时间段内,即740Ma到732Ma的贝义西组,732Ma到615Ma之间的阿勒通沟组和特瑞爱肯组,以及615Ma到542Ma之间的汉格尔乔克组,这些年龄段代表了库鲁克塔格地区新元古代各冰期的时代范围。与冰期有关的同位素年代学资料分析表明,贝义西冰期可与Kaigas冰期对比; 阿勒通沟冰期和特瑞爱肯冰期可能与Sturtian冰期和Elatina冰期对比; 而汉格尔乔克冰期和Gaskiers冰期可以对比。 相似文献
2.
皖南新元古代两次冰期事件 总被引：13，自引：4，他引：13
新元古代的冰期事件一直是地质界研究的热点之一。包括中国在内 ,世界许多地区新元古代地层中普遍发育有一至两层冰碛岩 ,有的地区甚至可以见到三层冰碛岩 ,在冰碛岩之上往往有碳酸盐岩盖层 ( Cap Carbonate)。通过对皖南休宁新元古代冰碛岩的岩石地层学 ,以及碳、氧稳定同位素化学地层学的研究 ,证实了休宁新元古代地层存在两期冰川的记录 ,并通过与国内外同时代典型地层剖面的对比 ,认为休宁蓝田剖面的两层冰碛岩可能分别相当于 Sturtian冰期和 Marinoan冰期的沉积 ,其时代分别约为 710— 73 0 Ma,5 90— 60 0 Ma 相似文献
3.
Fan Feng Ping Guan Wenhui Liu Wei Zhang Peixian Liu Xing Jian Ling Fu 《Arabian Journal of Geosciences》2016,9(18):715
The Snowball Earth hypothesis postulates that the whole earth was covered by ice sheets for millions of years in the Neoproterozoic. In Tarim Basin, there are four intervals of glacial deposits recorded in the Quruqtagh area during the Neoproterozoic. In chronological order, they are Bayisi, Altungol, Tereeken, and Hankalchough glaciations. In this study, we analyzed carbon, oxygen, and strontium isotopes; whole-rock geochemistry; and acid-insoluble residua of the ～5-m-thick Altungol cap dolostone (ten samples) in Quruqtagh unit of the Tarim basin. The geochemical data of whole rock are used to explain the ocean environmental changes, while the geochemical data of acid-insoluble residua reflect the changes of the continental environment. The δ13CPDB of cap dolostone varies from ?2.5 to ?1 ‰, which is likely due to the influence of organics. From the bottom to the top of the Altungol cap dolostones, the chemical index of alteration of acid-insoluble residua rises from 72 to 77, but the index of chemical variation drops from ～3 to ～1. Ni, Zn, Cu, and Y contents of acid-insoluble residua are enriched in 2.5 m. The values of iron maintain at a high level in all of the whole-rock samples (>5.15 %), but in the samples of acid-insoluble residua, the values of iron are higher than in the whole rock (19～60 %) below 1.5 m. The variations in chemical composition were probably associated with the changes of continental weathering. The geochemistry suggests that ocean and continent environmental variations had taken place after the Altungol glaciation, including the destruction of ocean stratification and the changes of chemical weathering. During this period, the source of this area had changed. At first, the weathering residues of the parent rocks were exhausted by meltwater. Then, the vigorous weathering of freshly exposed continental crust provided a new source. 相似文献
4.
新元古代冰期及其年代 总被引：7，自引：0，他引：7
新元古代在全球范围内出现了几期冰期事件,称之为“雪球地球”事件。这种剧烈的环境变化带来此后地球上生命演化的一次飞跃。“雪球地球”事件的核心是全球冰期的同时性,需要同位素地质年代学的证据。新元古代末期两次主要的冰期事件是Marinoan冰期和Sturtian冰期,其中Marinoan冰期结束于635Ma;Sturtian冰期可能发生在710～720Ma,已发表的年龄数据限定它在670Ma之前结束。Marinoan冰期后的Gaskiers冰期发生在580～590Ma。对华南的古城、铁丝坳、长安组、江口组等进行进一步精确定年,将对限定Sturtian冰期持续时间和Cryogenian、南华系的下限年龄具有重要意义。 相似文献
5.
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
6.
B. G. Pokrovsky N. M. Chumakov V. A. Melezhik M. I. Bujakaite 《Lithology and Mineral Resources》2010,45(6):577-592
The characteristic feature of many Upper Neoproterozoic glacial sequences is their “cap carbonates” (CC) resting without visible
unconformity upon glaciogenic diamictites. Such an unusual association, peculiar structures and textures, and negative δ13C values (approximately −4 ± 2‰) that are atypical of marine carbonates provoked long debates about the nature of these carbonates,
which play an important role in the Snowball Earth hypothesis. According to this hypothesis, the Earth was entirely covered
by ice during large-scale glaciations, and CC accumulation was related to the global change in geochemical processes. In this
work, we discuss data on the chemical and isotopic (C, O, Sr) compositions of CCs, which overlie glacial sediments of the
Nichatka and Bol’shoi Patom formations accumulated in different parts of the Neoproterozoic Patom paleobasin (Central Siberia).
High concentrations of Fe (up to 6400 ppm), Mn (2320 ppm), and radiogenic Sr (87Sr/86Sr0 up to 0.7172) established in CCs indicates a strong influence of the continental flow. Extraordinary Snowball Earth conditions
are not necessary for the accumulation of these rocks, geochemical and sedimentological properties of which may be explained
by the discharge of thawing waters into partly or completely isolated near-glacier basin, their intermittent freezing, and/or
washout of “frozen” carbonates from the surface of thawing glaciers. The peculiar thin-laminated texture of CC may be related
to seasonal processes of climatic cycles. They were accumulated in the course of general (relatively long-term) depletion
of the atmosphere and hydrosphere in 13C, which has nothing to do with the CC formation as a specific type of carbonate sediments. Amplitude and duration of the
negative δ13C excursion in carbonates associated with the Lower Vendian glacial sediments (665–635 Ma) are appreciably lower than the
negative anomaly in rocks of the Zhuya Group that likely correspond to the Shuram-Vonoka Event (∼560−580 Ma ago), which probably
marks the crucial point in the Precambrian deglaciation: mass destabilization of methane hydrates and degradation of the Early
Vendian psychrosphere in oceans. 相似文献
7.
The Neoproterozoic Earth witnessed major global glaciation events with significant impact on paleoclimate and life evolution. The Tarim Craton in China preserves the records of four glaciation events during the Neoproterozoic which were correlated with the global glaciations, the nature and impact of these with respect to Neoproterozoic paleoclimatic–paleogeographic reconstructions remain unresolved. Here we report the discovery of a suite of source rocks from northeastern Tarim in which the strata formed during 655–635 Ma, corresponding to the interglacial period between the Sturtian and Marinoan diamictites. These source rocks are dominated by black shales and mudstones of up to 300 m thickness, and are characterized by high content of organic matter with TOC (total organic carbon) of 0.46%–3.5% (average 1.64%), vitrinite reflectance Ro of 1.28%–1.60%, and kerogen carbon isotope δ13C value between −28.58‰ and −31.89‰. Biomarker compounds indicate that the organic matter in these saprolite source rocks are made up of microorganisms such as algae and bacteria. The Pr and Ph values indicate a weak reducing–oxidizing environment, and most values of CIA (Chemical Index of Alteration) are >68, suggesting an interglacial temperate paleoclimate. The La/Th–Hf and Co/Th–La/Sc relationship suggests that the provenance of these rocks is mainly mixed felsic/mafic rocks. In the Tarim basin, these source rocks comprise an area of up to 90,000 km2 within Cryogenian rifts as inferred from seismic reflection profiles. Based on zircon UPb ages of volcanic rocks underlying the shale units, it is inferred the source rock formed during the temperate Sturtian glaciation events with subsequent extensive biotic recovery and high productivity. 相似文献
8.
R. I. F. Trindade E. Font M. S. D'Agrella-Filho A. C. R. Nogueira C. Riccomini 《地学学报》2003,15(6):441-446
Palaeomagnetic study of the carbonates that ubiquitously cap glacial deposits may constrain the latitudinal extent of Neoproterozoic glaciations and the duration of the greenhouse recovery. We present the first palaeomagnetic data on the Neoproterozoic cap carbonates covering the Amazon craton, which are folded along the Paraguay Belt. Samples collected at deformed beds along the Paraguay Belt present a single‐polarity secondary magnetization acquired by the end of the Brasiliano orogeny (540–520 Ma). In the cratonic area, a dual‐polarity component was isolated in dolostones at the base of the sequence. The presence of a stratabound reversal stratigraphy along with high unblocking temperatures strongly suggest that this magnetization is primary. This result implies a low palaeolatitude (22+6/?5°) for the Amazon block just after deposition of Puga diamictites. In addition, the presence of multiple reversals across the first 20 m of the cap carbonate sequence suggests that their sedimentation must have spanned hundreds of thousands of years at least. 相似文献
9.
A global-scale glaciation occurred at about 600 Ma ago. As a result, the Earth became the “Snowball Earth“. The glaciation came to the end abruptly when atmospheric carbon dioxide increased to such an extent as to be about 350 times the modem level because of subaerial volcanic degassing. The rapid termination of glaciation would have led to warming of the Snowball Earth and extreme greenhouse conditions would have been created. The transfer of atmospheric carbon dioxide to oceans would give rise to the rapid precipitation of calcium carbonate in warm surface seawaters, thus forming the cap carbonate rocks as observed worldwide today. Regionally persisting, thin layers of carbonate rocks directly and ubiquitously overlie Proterozoic glacial deposits almost on every continent, and are commonly referred to as cap carbonates. Their unusual litho- fabrics, stratigraphically abrupt basal and upper-level contacts and strongly negative carbonate isotopic signatures ( δ ^13Cearb. values range from -7.0‰ —0‰) suggest a chemical oceanographic origin, the details of which remain unknown. It is proposed that these enigmatic deposits are related to the destabilization of gaseous hydrate in terrestrial permafrost following rapid postglacial warming and flooding of widely exposed continental shelves and internal basins. The authors carried out studies on the geochemistry, sedimentology and palacontology of the Sinian cap carbonates in Guizhou and Hunan provinces, including the occurrence of cap carbonates of unusual fabrics, strongly negative carbon isotopic signatures, and a lot of bitumen nodules.From the results it is suggested that the cap carbonates were formed from solid methane seepage, and it is in agreement with Kennedy‘‘ s viewpoint (2001) that the cap carbonates resulted from the rapid precipitation of calcium carbonate in response to solid methane seepage. 相似文献
10.
中国东南部新元古代冰碛岩地层分布于扬子板块的皖南、浙北、浙西和赣东地区,其中皖南休宁、歙县、浙北富阳、浙西建德及开化等地保存有两套冰碛岩和含锰碳酸盐岩地层,自下而上四分,而非以往认为的三分,分别记录了冰期-解冻期-冰期-解冻期四个阶段的沉积,这两个冰期分别为古城冰期和南沱冰期,相当于国外的司图特冰期(Sturtian glaciation)和马林诺冰期(Marinoan glaciation)。浙西江山和赣东广丰等地则只保存了一套冰碛岩和含锰碳酸盐岩地层,记录了冰期-解冻期两个阶段的沉积,地质年代为南沱冰期。由上可见,南沱冰期比古城冰期规模更大,冰碛沉积的范围更广泛,对铁、锰、硅及磷的富集成矿有更加重要的意义。 相似文献
11.
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
12.
Isotopic compositions of C, O, and Sr in carbonates, as well as Rb-Sr systems in the silicate material from Upper Precambrian
and Lower Cambrian rocks exposed by the Chapa River in the northern Yenisei Ridge, are studied. The Late Precambrian part
of the section includes the following formations (from the bottom to top): Lopatinskaya (hereafter, Lopatino), Vandadykskaya
(hereafter, Vandadyk) or Kar’ernaya, Chivida, Suvorovskaya (hereafter, Suvorovo), Pod”emskaya (hereafter, Podyom), and Nemchanka.
They are characterized by alternation of horizons with anomalously high and low δ13C values (such alternation is typical of the ∼700–550 Ma interval). The lower, relatively thin (20 m), positive excursion
(δ13C up to 4.3‰) was established in dolomites from the lower subformation of the Vandadyk (Kar’ernaya) Formation (hereafter,
lower Vandadyk subformation). The upper positive excursion (δ13C = 2.2 ± 0.6‰) was recorded in the 3-km-thick Nemchanka Formation enriched in terrigenous rocks. The lower negative excursion
stands out as uniform, moderately low δ13C values (−2 ± 1‰) and significant thickness. It comprises the upper part of the Vandadyk Formation, as well as Chivida and
Podyom formations. The upper negative excursion is related to a thin (∼20 m) marker carbonate horizon of the upper Nemchanka
subformation, in which δ13C values fall down to −8.3‰. The lower part of the Lebyazhinskaya (hereafter, Lebyazhino) Formation, which overlies the Nemchanka
Formation, shows a step-by-step increase in δ13C from −2.2 to 2.5‰ typical of the Vendianto-Cambrian (Nemakit-Daldyn Horizon/Stage) transitional sequences. The absence of
relationships between the carbon and oxygen isotope compositions and other parameters of postsedimentary alterations suggests
that the excursions characterized above could form at the sedimentation stage and coincide in general with δ13C fluctuations in seawater. The value of 87Sr/86Sr = 0.7076−0.7078 in limestones of the Podyom Formation points to their early Ediacaran age. Values of 87Sr/86Sr = 0.70841 and 0.70845 in dolomites of the lower Lebyazhino subformation correspond to the Early Cambrian. The Rb-Sr systems
of the clay material from the Vandadyk and Chivida formations are approximated by a straight line, parameters of which correspond
to the age of 695 ± 20 Ma (87Sr/86Sr0 = 0.7200 ± 0.0013) and probably characterize the epigenetic stage of older sedimentary rocks, which were subjected to very
rapid exhumation and “polar” sulfuric acid weathering in the course of glacioeustatic regression. 相似文献
13.
M. A. Semikhatov A. B. Kuznetsov A. V. Maslov I. M. Gorokhov G. V. Ovchinnikova 《Stratigraphy and Geological Correlation》2009,17(6):574-601
The main objective of this work is the generalization of lithostratigraphic, biostratigraphic and isotopic-geochronological
data characterizing carbonate rocks from type succession of the broadly acknowledged chronostratigraphic subdivision of the
Lower Riphean, such as the Burzyan Group of the Southern Urals and its analogs. Using an original approach to investigation
of the Rb-Sr and Pb-Pb isotopic systems in carbonates and strict criteria of their retentivity, we studied the least altered
(“best”) samples of the Burzyan carbonates, which retain the 87Sr/86Sr ratio of the sedimentation environment. As long ago as 1550 ± 30 and 1430 ± 30 Ma, that ratio corresponded to 0.70460–0.70480
and 0.70456–0.70481. The results confirm the influx of the mantle material predominantly into the World Ocean of the Early
Riphean. The influence of meteoric diagenesis was likely responsible for local declines of δ18O in the Burzyan carbonates down to the values of −2.5 to −1.5‰ V-PDB. In the “best” samples, this parameter ranges from −0.7
to 0‰, which is consistent with the assumption that δ18O values (0 ± 1‰) characterized the stasis of the carbonate carbon isotopic composition in oceanic water 2.06–1.25 Ga ago.
C-isotopic data on carbonate from the Paleoproterozoic-Lower Riphean boundary formations of the Urals, India, North America
and Siberia suggest that the mentioned stasis ended by the commencement of the Early Riphean ca. 1.6–1.5 Ga ago. In the least
altered carbonates of the Early Riphean, the δ18O variation range corresponds to 4.0–4.5‰. 相似文献
14.
Grant M. Young 《Geological Journal》2013,48(1):1-21
Near the beginning and end of the Proterozoic Eon (2.5 Ga–542 Ma) the Earth went through dramatic climatic perturbations. The Palaeoproterozoic (Huronian) glaciations are best known from the Canadian Shield where there is evidence of at least three such episodes. Glacial deposits of comparable age are also known from Fennoscandia, South Africa and Western Australia. In the type area, the Huronian glacial deposits are preserved in an ancient rift system that preceded break‐up of the supercraton, Kenorland, whereas those in the southern hemisphere may have been deposited in a foreland basin setting. Detailed correlations between the two hemispheres must await more geochronological data. Following a long period (~1.5 Ga) with little evidence of glaciation, the climatic upheavals of the Neoproterozoic Era began. The two most widespread glacial events are known as the Sturtian and Marinoan. The Neoproterozoic glaciations also took place on a supercontinent (Rodinia). Some were accompanied by unexpected rock types such as dolomitic cap carbonates and iron formations, both of which show evidence of hydrothermal influence. Major influences on surface temperatures on Earth include solar luminosity (increasing throughout geological history) and the concentration of atmospheric greenhouse gases such as CO2 (generally diminishing with time). It is suggested that the two great Proterozoic climatic oscillation periods resulted from perturbations of the balance between these two variables, triggered by drawdown of atmospheric CO2 during intensive weathering of supercontinents. A weathering‐related negative feedback loop resulted in multiple glaciations with intervening warm periods. Climatic stability only returned after the supercontinent broke apart and reduced continental freeboard moderated continental weathering. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
15.
16.
Ultramafic-hosted deposits of magnesite (MgCO3) have been studied on Margarita Island, Venezuela, to elucidate the source of carbon and conditions of formation for this
type of ore. Petrographic, mineralogic, and δ18O data indicate that magnesite precipitated on Margarita in near-surface environments at low P and T. δ13C ranges from −9 to −16‰ PDB within the magnesite and −8 to −10‰ PDB within some calcite and dolomite elsewhere on the island.
The isotopically light dolomite fills karst and the calcite occurs as stock-work veins which resemble the magnesite deposits.
These carbon isotopic ratios are consistent with a deep-seated source rather than an overlying source from a zone of surficial
weathering. However, there is not much enrichment of precious metals and no enrichment of heavy rare-earth elements, as would
be expected if the carbon had migrated upward as aqueous carbonate ions. The carbon probably has risen as a gaseous mixture
of CO2 and CH4 which partially dissolved in near-surface water before leaching cations and precipitating as magnesite and other carbonates.
The process probably is ongoing, given regional exhalation of carbonaceous gases. 相似文献
17.
Neoproterozoic cap carbonates: a critical appraisal of existing models and the plumeworld hypothesis 总被引：3，自引：0，他引：3
Graham A. Shields 《地学学报》2005,17(4):299-310
Evidence for glaciation during the mid-late Neoproterozoic is widespread on Earth, reflecting three or more ice ages between 730 Ma and 580 Ma. Of these, the late Neoproterozoic Marinoan glaciation of approximately 635 Ma stands out because of its ubiquitous association with a characteristic, microcrystalline cap dolostone that drapes glacially influenced rock units worldwide. The Marinoan glaciation is also peculiar in that evidence for low altitude glaciation at equatorial latitudes is compelling. Three models have been proposed linking abrupt deglaciation with this global carbonate precipitation event: (i) overturn of an anoxic deep ocean; (ii) catastrophically accelerated rates of chemical weathering because of supergreenhouse conditions following global glaciation (Snowball Earth Hypothesis); and (iii) massive release of carbonate alkalinity from destabilized methane clathrates. All three models invoke extreme alkalinity fluxes into seawater during deglaciation but none explains how such alkalinity excess from point sources could be distributed homogeneously around the globe. In addition, none explains the consistent sequence of precipitation events observed within cap carbonate successions, specifically: (i) the global blanketing of carbonate powder in shallow marine environments during deglaciation; (ii) widespread and disruptive precipitation of dolomite cement; followed by (iii) localized barite precipitation and seafloor cementation by aragonite. The conceptual model presented here proposes that low latitude deglaciation was so massive and abrupt that the resultant meltwater plume could extend worldwide, physically separating the surface and deep ocean reservoirs for ≥103 years. It is proposed that cap dolostones formed primarily by microbially mediated precipitation of carbonate whitings during algal blooms within this low salinity plumeworld rather than by abiotic precipitation from normal salinity seawater. Many of the disruption features that are characteristic of cap dolostones can be explained by microbially mediated, early diagenetic dolomitization and cementation. The re-initiation of whole ocean circulation degassed CO2 into the atmosphere in areas of upwelling, triggering localized, abiotic CaCO3 precipitation in the form of aragonite fans that overlie cap dolostones in NW Canada and Namibia. The highly oxygenated shallow marine environments of the glacial and post-glacial Neoproterozoic world provided consistently favourable conditions for the evolutionary development of animals and other oxygenophiles. 相似文献
18.
Fluid origin and structural enhancement during mineralization of the Jinshan orogenic gold deposit, South China 总被引：3，自引：0，他引：3
The Jinshan orogenic gold deposit is a world-class deposit hosted by a ductile shear zone caused by a transpressional terrane
collision during Neoproterozoic time. Ore bodies at the deposit include laminated quartz veins and disseminated pyrite-bearing
mylonite. Most quartz veins in the shear zone, with and without gold mineralization, were boudinaged during progressive shear
deformation with three generations of boudinage structures produced at different stages of progressive deformation. Observations
of ore-controlling structures at various scales indicate syn-deformational mineralization. Fluid inclusions from pyrite intergrown
with auriferous quartz have 3He/4He ratios of 0.15–0.24 Ra and 40Ar/36Ar ratios 575–3,060. δ18Ofluid values calculated from quartz are 5.5–8.4‰, and δD values of fluid inclusions contained in quartz range between −61‰ and
−75‰. The δ13C values of ankerite range from −5.0‰ to −4.2‰, and ankerite δ18O values from 4.4‰ to 8.0‰. The noble gas and stable isotope data suggest a predominant crustal source of ore fluids with
less than 5% mantle component. Data also show that in situ fluids were generated locally by pervasive pressure solution, and
that widespread dissolution seams acted as pathways of fluid flow, migration, and precipitation. The in situ fluids and fluids
derived from deeper levels of the crust were focused by deformation and deformation structures at various scales through solution-dissolution
creep, crack-seal slip, and cyclic fault-valve mechanisms during progressively localized deformation and gold mineralization. 相似文献
19.
兰中伍 《沉积与特提斯地质》2023,43(1):180-187
华南南华系对应于国际上的成冰系,关于其沉积时限长期存在争议。近十余年来,通过高精度锆石U-Pb定年,已经基本敲定南华系的重要时间节点。长安组底界年龄被限定在ca. 717 Ma,莲沱组顶部沉积时间被限定在ca. 714 Ma。长安冰期中期暂时冰退的时间被限定在ca. 690 Ma,冰期终止时间限定在ca. 659 Ma。小行星撞击地球可能导致了长安冰期中期的暂时冰退,这期间形成的风暴沉积构造和丘状交错层理可以提供最直接的沉积学证据。结合世界其它地区报道的年龄,斯图特（长安）冰期的起止时间限定在了717～659 Ma。马力诺（南沱）冰期的启动时间被大致限定为649 Ma,终止时间被限定在ca. 635 Ma。结合世界其它地区年龄数据,马力诺冰期的启动时间可限定在649～639 Ma。随着后续工作的深入,马力诺冰期的启动时间范围应会被进一步缩小。华南南华系沉积时限的准确厘定对于理解全球成冰纪地质-生物-环境事件具有重要意义。马力诺冰期持续时间约14 Myr,远远小于长安冰期的持续时间（约58 Myr）。冰期末期大规模岩浆作用是导致这两次冰期持续时间不同的直接原因。通过模拟计算发现,扬子北缘... 相似文献
20.
Early carbonate cements in the Yanchang Formation sandstones are composed mainly of calcite with relatively heavier carbon isotope （their δ^18O values range from -0.3‰- -0.1‰） and lighter oxygen isotope （their δ^18O values range from -22.1‰- -19.5‰）. Generally, they are closely related to the direct precipitation of oversaturated calcium carbonate from alkaline lake water. This kind of cementation plays an important role in enhancing the anti-compaction ability of sandstones, preserving intragranular volume and providing the mass basis for later disso- lution caused by acidic fluid flow to produce secondary porosity. Ferriferous calcites are characterized by relatively light carbon isotope with δ^13C values ranging from -8.02‰ to -3.23‰, and lighter oxygen isotope with δ^18O values ranging from -22.9‰ to -19.7‰, which is obviously related to the decarboxylation of organic matter during the late period of early diagenesis to the early period of late diagenesis. As the mid-late diagenetic products, ferriferous cal- cites in the study area are considered as the characteristic authigenic minerals for indicating large-scaled hydrocarbon influx and migration within the clastic reservoir. The late ankerite is relatively heavy in carbon isotope with δ^13C values ranging from -1.92‰ to -0.84‰, and shows a wide range of variations in oxygen isotopic composition, with δ^18O values ranging from -20.5‰ to -12.6‰. They are believed to have nothing to do with decarboxylation, but the previously formed marine carbonate rock fragments may serve as the chief carbon source for their precipitation, and the alkaline diagenetic environment at the mid-late stage would promote this process. 相似文献