首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
《China Geology》2023,6(1):50-60
The Lomagundi-Jatuli Event (LJE) refers to the significant positive carbon isotope excursion in seawater constituents that occurred immediately after the increase in atmospheric oxygen content during the Paleoproterozoic (2.22–2.06 Ga). The δ13C values of 46 dolostone samples collected from the Paleoproterozoic Yongjingshao Formation varied in the range of 0.05 ‰–4.95 ‰ (V-PDB; maximum: 4.95‰) in this study, which may be related to the multicellular eukaryotes in the Liangshan Formation in the Yimen Group. They are much higher than the δ13C values of marine carbonates (?1.16‰ on average). The δ13C values of other formations in the Paleoproterozoic Yimen Group are negative. The notable positive carbon isotope anomalies of the Yongjingshao Formation indicate the response to the LJE at the southwestern margin of the Yangtze Block, which is reported for the first time. Furthermore, they are comparable to the δ13C values of carbonates in the Dashiling Formation of the Hutuo Group in the Wutaishan area in the North China Craton, the Wuzhiling Formation of the Songshan Group in the Xiong’ er area, Henan Province, and the Dashiqiao Formation of the Liaohe Group in the Guanmenshan area, Liaoning Province. Therefore, it can be further concluded that the LJE is a global event. This study reveals that LJE occurred in Central Yunnan at 2.15–2.10 Ga, lasting for about 50 Ma. The macro-columnar, bean-shaped, and microfilament fossils and reticular ultramicrofossils of multicellular eukaryotes in this period were discovered in the Liangshan Formation of the Yimen Group. They are the direct cause for the LJE and are also the oldest paleontological fossils ever found. The major events successively occurring in the early stage of the Earth include the Great Oxygenation Event (first occurrence), the global Superior-type banded iron formations (BIFs), the Huronian glaciation, the Great Oxygenation Event (second occurrence), the explosion of multicellular eukaryotes, the positive carbon isotope excursion, and the global anoxic and selenium-rich sedimentary event. The authors think that the North China Craton and the Yangtze Craton were possibly in different tectonic locations of the same continental block during the Proterozoic.©2023 China Geology Editorial Office.  相似文献   

2.
The ca. 2.2–2.1 Ga Magondi Supergroup on the Zimbabwe Craton in Southern Africa is mainly composed of sedimentary rocks deposited in a rift basin/passive continental margin, which record a unique episode in carbon isotope perturbation called the Lomagundi–Jatuli Event (LJE). This study reports new U–Pb ages of detrital zircons from the Deweras and Lomagundi groups of the Magondi Supergroup, and of igneous zircons from underlying granitoids, to constrain the timing of the LJE and to identify the provenance of the Magondi Supergroup. Most analysed detrital zircon grains range in ages between ca. 2.9 and 2.6 Ga. Three ca. 2.3–2.2 Ga detrital zircons from sandstone of the Deweras Group, with the youngest 207Pb‐206Pb age of 2,216 ± 22 Ma, indicate the onset of LJE in the Zimbabwe Craton was almost simultaneous to that in Fennoscandia and the Superior Craton, supporting the global synchronicity of the LJE.  相似文献   

3.
The Qianfanling Mo deposit, located in Songxian County, western Henan province, China, is one of the newly discovered quartz-vein type Mo deposits in the East Qinling–Dabie orogenic belt. The deposit consists of molybdenite in quartz veins and disseminated molybdenite in the wall rocks. The alteration types of the wall rocks include silicification, K-feldspar alteration, pyritization, carbonatization, sericitization, epidotization and chloritization. On the basis of field evidence and petrographic analysis, three stages of hydrothermal mineralization could be distinguished: (1) pyrite–barite–quartz stage; (2) molybdenite–quartz stage; (3) quartz–calcite stage.Two types of fluid inclusions, including CO2-bearing fluid inclusions and water-rich fluid inclusions, have been recognized in quartz. Homogenization temperatures of fluid inclusions vary from 133 °C to 397 °C. Salinity ranges from 1.57 to 31.61 wt.% NaCl eq. There are a large number of daughter mineral-CO2-bearing inclusions, which is the result of fluid immiscibility. The ore-forming fluids are medium–high temperature, low to moderate salinity H2O–NaCl–CO2 system. The δ34S values of pyrite, molybdenite, and barite range from − 9.3‰ to − 7.3‰, − 9.7‰ to − 7.3‰ and 5.9‰ to 6.8‰, respectively. The δ18O values of quartz range from 9.8‰ to 11.1‰, with corresponding δ18Ofluid values of 1.3‰ to 4.3‰, and δ18D values of fluid inclusions of between − 81‰ and − 64‰. The δ13CV-PDB values of fluid inclusions in quartz and calcite have ranges of − 6.7‰ to − 2.9‰ and − 5.7‰ to − 1.8‰, respectively. Sulfur, hydrogen, oxygen and carbon isotope compositions show that the sulfur and ore-forming fluids derived from a deep-seated igneous source. During the peak collisional period between the North China Craton and the Yangtze Craton, the ore-forming fluids that derived from a deep igneous source extracted base and precious metals and flowed upwards through the channels that formed during tectonism. Fluid immiscibility and volatile exsolution led to the crystallization of molybdenite and other minerals, and the formation of economic orebodies in the Qianfanling Mo deposit.  相似文献   

4.
Carbon isotope compositions of both sedimentary carbonate and organic matter can be used as key proxies of the global carbon cycle and of its evolution through time,as long as they are acquired from waters where the dissolved inorganic carbon(DIC)is in isotope equilibrium with the atmospheric CO2.However,in shallow water platforms and epeiric settings,the influence of local to regional parameters on carbon cycling may lead to DIG isotope variations unrelated to the global carbon cycle.This may be especially true for the terminal Neoproterozoic,when Gondwana assembly isolated waters masses from the global ocean,and extreme positive and negative carbon isotope excursions are recorded,potentially decoupled from global signals.To improve our understanding on the type of information recorded by these excursions,we investigate the pairedδ^13Ccarb andδ^13Corg evolution for an increasingly restricted late Ediacaran-Cambrian foreland system in the West Gondwana interior:the basal Bambui Group.This succession represents a 1~(st)-order sedimentary sequence and records two majorδ^13Ccarb excursions in its two lowermost lower-rank sequences.The basal cap carbonate interval at the base of the first sequence,deposited when the basin was connected to the ocean,hosts antithetical negative and positive excursions forδ^13Ccarb andδ^13Corg,respectively,resulting inΔ^13C values lower than 25‰.From the top of the basal sequence upwards,an extremely positiveδ^13Ccarb excursion is coupled toδ^13Corg,reaching values of+14‰and-14‰,respectively.This positive excursion represents a remarkable basin-wide carbon isotope feature of the Bambui Group that occurs with only minor changes inΔ^13C values,suggesting change in the DIC isotope composition.We argue that this regional isotopic excursion is related to a disconnection between the intrabasinal and the global carbon cycles.This extreme carbon isotope excursion may have been a product of a disequilibria between the basin DIC and atmospheric CO2 induced by an active methanogenesis,favored by the basin restriction.The drawdown of sulfate reservoir by microbial sulfate reduction in a poorly ventilated and dominantly anoxic basin would have triggered methanogenesis and ultimately methane escape to the atmosphere,resulting in a^13C-enriched DIC influenced by methanogenic CO2.Isolated basins in the interior of the Gondwana supercontinent may have represented a significant source of methane inputs to the atmosphere,potentially affecting both the global carbon cycle and the climate.  相似文献   

5.
Multicellular animals first appeared on the earth during the Ediacaran period. However, the relationship between the abrupt biological evolution and environmental changes is still ambiguous. In order to examine seawater temperature and the carbon cycle through the Ediacaran, we analyzed the carbon and oxygen isotope compositions of carbonate rocks from drill cores from the Three Gorges area, South China. Importantly, the core samples include the Nantuo tillite, corresponding to the Marinoan glaciation, through the Doushantuo to the lower Dengying Fms. in ascending order.The δ13C profile displays five positive and five negative anomalies (PI-1 to 5 and NI-1 to 5), and the oxygen isotopes display very high absolute values around 0‰ with the highest at + 1.83‰. The combined δ18O and δ13C chemostratigraphies display both positive and negative correlations between the δ18O and δ13C values. The occurrence of the negative correlations supports the preservation of primary δ18O and δ13C values.The sample NI-4 has a negative correlation of the δ18O and δ13C excursions. The correlation supports a primary signature for both δ18O and δ13C variations. The positive δ18O excursion, accompanied by evidence of a eustatic sea-level fall, provides direct evidence for global cooling in the mid-Ediacaran; the 580 Ma Gaskiers Glaciation is a potential candidate for this global cooling event. The negative δ13C excursion was possibly caused by an increase in remineralization of dissolved organic carbon (DOC) due to enhanced continental weathering during the glaciation.Sample NI-5 is characterized by very low δ13C values, down to ? 10‰, corresponding to the Shuram-Wonoka-Pertatataka Excursion. The cause of the δ13C negative excursion is still not clear. However, a ubiquitous occurrence in excursions worldwide, and the lower δ13C values in deeper sections favor the enhancement of remineralization and respiration rather than secondary alteration, a restricted sea environment and lithification in coastal areas.  相似文献   

6.
The Nanhuan manganese deposits in the southeastern Yangtze Platform occur in the black shale series in the lower part of the Datangpo Formation. In order to constrain the genesis of the deposits, a detailed study was undertaken that involved field observations, major and trace element analyses, organic carbon analyses, and isotope analyses (C, O, S). The major findings are as follows. (1) The ore-bearing rock series, morphology of the ore bodies, and characteristics of ores in several deposits are similar. The ore minerals are rhodochrosite and manganocalcite. The gangue minerals are mainly quartz, feldspar, dolomite, and illite. Minor apatite and bastnaesite occur in the manganese ores. (2) The ores are enriched in Ca and Mg, whereas they are depleted in Si, Al, K, and Ti compared to wall rocks. The ores normalized to average Post-Archean Australian shale (PAAS) are enriched in Co, Mo, and Sr. The chondrite-normalized rare earth element (REE) patterns for ores and wall rocks are between those of typical hydrogenous and hydrothermal type manganese deposits. Additionally, the ores have positive Ce anomalies with an average Ce/Ce* of 1.23 and positive Eu anomalies with an average Eu/Eu* of 1.18 (normalized to PAAS). (3) The average content of organic carbon is 2.21% in the samples, and the average organic carbon isotopic value (δ13CV-PDB) is − 33.44‰. The average inorganic carbon isotopic value (δ13CV-PDB) of carbonates in Gucheng is − 3.07‰, while the values are similar in the other deposits with an average of − 8.36‰. The oxygen isotopic compositions (δ18OV-PDB) are similar in different deposits with an average of − 7.72‰. (4) The sulfur isotopic values (δ34SV-CDT) of pyrite are very high and range from + 37.9‰ to + 62.6‰ (average of 52.7‰), which suggests that the pyrite was formed in restricted basins where sulfate replenishment was limited. The sulfate concentrations in the restricted basins were extremely low and enriched in δ34S, which resulted in the very high δ34S values for the pyrite that formed in the manganese deposits. Therefore, a terrigenous weathering origin for manganese can be excluded; otherwise, the sulfate would have been introduced into the basins together with terrigenous manganese, which would have decreased the δ34S values of pyrites. The manganese, which originated from hydrothermal processes, was enriched in the restricted and anoxic basins, and then, it was oxidized to manganese oxyhydroxide in the overlying oxic waters whereby the products precipitated into the sediments. The manganese oxyhydroxide in the sediment was then reduced to Mn2 + and released to the pore waters during the process of diagenesis. Some organic carbon was oxidized to CO32 , which made the depletion of 13C in manganese carbonates. Therefore, we suggest that the Nanhuan manganese deposits are hydrothermal–sedimentary/diagenetic type deposits.  相似文献   

7.
The 2.33–2.06 Ga positive δ13Ccarb excursion, associated with environmental change and the breakup of the Kenorland or Superia supercontinent, is called the Lomagundi or Jatulian Event or Great Oxidation Event, and has been reported in many Early Precambrian cratons, but not yet in the Sino-Korean craton. The Guanmenshan Formation of the Liaohe Group occurs in the northeastern part of the Sino-Korean craton. δ13Ccarb and δ18O values in 42 samples from this formation range from 3.5–5.9‰ (V-PDB), and 15.4–24.8‰ (V-SMOW), respectively, showing a clear positive δ13Ccarb excursion that characterizes the Lomagundi Event. Thirty-five of the 42 samples with less hydrothermal alteration have higher δ13Ccarb and δ18Ocarb values than the other 7 samples obviously affected by fluid flow, confirming that it was fluid flow that reduced the δ13Ccarb and δ18Ocarb values. This positive δ13Ccarb excursion places deposition of the Guanmenshan Formation within the age range of 2.33–2.06 Ga.  相似文献   

8.
《Gondwana Research》2014,25(3-4):1276-1282
Concentrations of total organic matter (TOC), carbon isotopic compositions of carbonate and organic matter (δ13Ccarb, δ13Corg), and sulfur isotopic compositions of carbonate associated sulfate (δ34Ssulfate) across the Guadalupian–Lopingian (G–L) boundary were analyzed from identical samples of Tieqiao section, Laibin, Guangxi province, South China. The δ13Ccarb values show a positive excursion from − 0.45‰ to the peak of 3.80‰ in the Laibin limestone member of the Maokou Formation, followed by a drastic drop to − 2.60‰ in the lowest Heshan formation, then returned to about 1.58‰. Similar to the trends of the δ13Ccarb values, Δ13Ccarb–org values also show a positive excursion followed by a sharp negative shift. The onset of a major negative carbon isotope excursion postdates the end Guadalupian extinction that indicates subsequent severe disturbance of the ocean–atmosphere carbon cycle. The first biostratigraphic δ34Ssulfate values during the G–L transition exhibit a remarkable fluctuation: a dramatic negative shift followed by a rapid positive shift, ranging from 36.88‰ to − 37.41‰. These sulfate isotopic records suggest that the ocean during the G–L transition was strongly stratified, forming an unstable chemocline separating oxic shallow water from anoxic/euxinic deep water. Chemocline excursions, together with subsequent rapid transgression and oceanic anoxia, were likely responsible for the massive diversity decline of the G–L biotic crisis.  相似文献   

9.
《Gondwana Research》2014,25(3):1045-1056
A remarkable increase of the animal genera and a subsequent mass extinction in the late Early Cambrian are well known as the “Cambrian explosion” and the “Botomian–Toyonian crisis.” A composite global curve of the carbon isotope ratios for inorganic carbon (δ13Ccarb) shows multiple fluctuations during the evolution events, and it indicates significant changes of the oceanic carbon cycle at that time. This study reveals a new continuous isotopic chemostratigraphy for inorganic carbon (δ13Ccarb) from the bottom of the Shipai to the base of the Shilongdong formations in Three Gorges area, South China. This section covers the Canglangpuian to the Longwangmiaoian stages in the Lower Cambrian. The δ13Ccarb variation exhibits three negative excursions: a remarkably negative excursion down to ca. − 12‰ in the middle Canglangpuian stage, a negative excursion to ca. − 1.0‰ in the upper Canglangpuian stage, and a negative excursion to ca. − 1.0‰ in the Longwangmiaoian stage, respectively. The largest negative δ13Ccarb excursion and a positive excursion before the excursion are definitely consistent with the δ13Ccarb negative shift (AECE) during the mass extinction and the δ13Ccarb positive values (MICE) during the increase of animal genera, respectively. However, the minimum values of the negative shifts among South China, Siberia, and Canada sections are different from each other. The positive δ13Ccarb excursion at the bottom of the Canglangpuian stage indicates that primary productivities and organic carbon burial were enhanced. A sea level rise in the Qiongzhusian to bottom of the Canglangpuian stages in South China corresponds to the Sinsk transgression event in Siberia and Canada. A eutrophication due to higher continental weathering during the transgression after the long-term retrogression enhanced the high primary production and consequently promoted the significant increase of animal diversity.On the other hand, deposition of laminated black shales without bioturbation signatures and a decline of trilobite diversity are observed during the negative δ13Ccarb excursion in the Canglangpuian stage, indicating that the shallow water environment became anoxic at that time. The negative δ13Ccarb shift indicates an influx of abundant 12CO2 due to oxidation of organic carbons in seawater. The difference of the minimum values among sections implies the local difference in size of the organic carbon reservoirs and extent of the degradation of the carbons. The largest δ13C anomaly in South China suggests the presence of the largest OCPs due to higher activity of primary production and high degree of oxidation of the OCPs because of higher activity of animals. The coincidence of the timing of the negative δ13C excursions in the Canglangpuian stage among the sections indicates a global event, and suggests that the onset was caused by increase of oxygen contents of seawater and atmosphere. Abundant oxygen yielded by the increased primary productivity in the Atdabanian and the Qiongzhusian stages caused onset of the oxidation of OCP, and possibly led to the shallow water anoxia and the mass extinction of benthic animals in the Botomian and the Canglangpuian stage.  相似文献   

10.
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 655635 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   

11.
Isotopic characteristics of river sediments on the Tibetan Plateau   总被引:1,自引:0,他引:1  
We systematically collected 40 modern clastic sediment samples from rivers in different tectonic units of the Tibetan Plateau and measured their Sr–Nd isotopic compositions. The isotopic characteristics provide insight into the controversial paleo-tectonic affinity of terranes of the Tibetan Plateau and the provenance of Songpan–Ganzi flysch complex. The Qilian Terrane and Himalaya Terrane have more negative εNd(0) values (from ? 14.3 to ? 11.8 and from ? 20.64 to ? 13.26, respectively) and high 87Sr/86Sr values (from 0.719674 to 0.738818 and from 0.721020 to 0.824959, respectively), reflecting old and mature continental crust origin of these two terranes. The southern Lhasa Terrane is more radiogenic in εNd(0) values (from ? 8.82 to ? 3.8) and low in 87Sr/86Sr values (from 0.711504 to 0.719489), implying the combined impact of the Neo-Tethys mantle and Himalaya old continental crust. Sr–Nd isotopic compositions of the Qilian Terrane are similar to those in the Yangtze Craton, indicating that the Qilian Terrane was probably separated from the Yangtze Craton. Sr–Nd isotopic characteristics of the Songpan–Ganzi Terrane are similar to the Yangtze Craton and are remarkably different to those in the North China Craton, eastern Kunlun–Qaidam and the central Qiangtang metamorphic belt, implying that the widely distributed flysch complex of the Songpan–Ganzi Terrane was sourced from the Yangtze Craton.  相似文献   

12.
A large number of Paleoproterozoic borate deposits are hosted by the lower units of a volcanic-sedimentary sequence in Liaoning Province, northeastern China, and are a major source of boron in China. The ore-bearing wall rocks in the deposits are serpentinized ultrabasic rocks and carbonates, with layered leptynites, leptites, amphibolites, and migmatites adjacent to the ore. Both the borate ores and country rocks contain tourmaline, although the country rocks have much lower abundances of the mineral. Based on in situ boron isotope measurements using laser ablation–multi-collector–inductively coupled plasma–mass spectrometry (LA–MC–ICP–MS), boron isotope data show that: (1) δ11B values of borate ores range from + 6.8‰ to + 13.9‰ (mean + 10.8‰); (2) tourmalines from the borate ores have δ11B values from + 9.5‰ to + 12.7‰; and (3) the wall rocks within the borate ores yield slightly lower δ11B values ranging from + 5.7‰ to + 7.6‰, and those outside the deposits from − 9.9‰ to − 5.9‰. Positive δ11B values in borates as well as in tourmalines inside the mining area indicate that boron in these Paleoproterozoic borate deposits was derived from marine evaporites. δ34SV-CDT (where V-CDT is Vienna Canyon Diablo Troilite) values of borate ores, serpentinized marbles, and anhydrites range from + 16.1‰ to + 24.7‰, whereas δ13CV-PDB (where V-PDB is Vienna Pee Dee Belemnite) values of marbles range from + 3.2‰ to + 5.9‰. These isotopic characteristics are interpreted to reflect formation in a marine evaporative environment. LA–MC–ICP–MS zircon weighted207Pb/206Pb ages of leptite and serpentinized olivine basalt from the hanging wall of the borate deposits are 2139 ± 13 Ma and 2130 ± 19 Ma, respectively. Therefore, the (~ 2.2 Ga) borate deposits may have originated from marine evaporative boron-bearing sediments, which were interbedded within bimodal volcanic rocks during the early stages of development of the Liaoji rift.  相似文献   

13.
The Upper Triassic Xujiahe Formation in the Sichuan Basin, SW China consists of a series of coal measures. The first, third and fifth members of this formation are dominated by gas prone dark mudstones and coals. The mudstones contain Type II and III kerogens with average organic carbon contents around 1.96%. These source rocks are mature in the central Sichuan and highly mature in the western Sichuan Basin, characterized by gas generation with subordinate amounts of light oil or condensate oils. The source rocks are intercalated with the sandstone dominated second, fourth and sixth members of the Xujiahe Formation, thus leading to three separate self contained petroleum systems in the region. The proven gas reserves in the Xujiahe Formation are only less than that of the Triassic Feixianguan Formation and the Xujiahe Formation has the second largest gas field (Guang’an gas field) in the basin. Gases derived from the Xujiahe Formation coals generally show a normal stable carbon isotopic trend for C1–C4 n-alkanes, with the highest δ13C2 values among the nine gas pay zones in the basin (?20.7‰ to ?28.3‰), and δ13C1 values as low as ?43.0‰ in the central Sichuan. Gas accumulations with an oil leg have also been found in the eastern and southern Sichuan where the thickness of the Xujiahe Formation is significantly reduced. Gases in these accumulations tend to show low δ13C2 values (?30.0‰ to ?36.3‰), characteristic of oil prone source rocks.  相似文献   

14.
In order to examine the causal relationships between the carbon cycle in a shallow euphotic zone and the environmental changes in a relatively deep disphotic zone at the end-Guadalupian (Middle Permian), isotopic compositions of carbonate carbon (δ13Ccarb) of the Guadalupian–Lopingian (Upper Permian) rocks were analyzed in the Chaotian section in northern Sichuan, South China. By analyzing exceptionally fresh drill core samples, a continuous chemostratigraphic record was newly obtained. The ca. 65 m-thick analyzed carbonate rocks at Chaotian comprise three stratigraphic units, i.e., the Limestone Unit of the Guadalupian Maokou Formation, the Mudstone Unit of the Maokou Formation, and the lower part of the Wuchiapingian (Lower Lopingian) Wujiaping Formation, in ascending order. The Limestone Unit of the Maokou Formation is characterized by almost constant δ13Ccarb values of ca. +4‰ followed by an abrupt drop for 7‰ to −3‰ in the topmost part of the unit. In the Mudstone Unit of the Maokou Formation, the δ13Ccarb values are rather constant around +2‰, although distinct three isotopic negative excursions for 3‰ from ca. +2 to −1‰ occurred in the upper part of the unit. In the lower part of the Wujiaping Formation, the δ13Ccarb values monotonously increase for 5‰ from ca. 0 to +5‰. The present data newly demonstrated four isotopic negative excursions in the topmost part of the Maokou Formation in the Capitanian (Late Guadalupian) at Chaotian. It is noteworthy that these negative excursions are in accordance with the emergence of an oxygen-depleted condition on the relatively deep disphotic slope/basin on the basis of litho- and bio-facies characteristics. They suggest multiple upwelling of oxygen-depleted waters with dissolved inorganic carbon of relatively low carbon isotope values along the continental margin, from the deeper disphotic slope/basin to the shallower euphotic shelf, slightly before the end-Guadalupian extinction. Although the negative excursions at Chaotian are apparently correlated with the previously proposed large negative excursion in the middle Capitanian in South China, the age difference according to the biostratigraphic constraints clearly exclude this interpretation. The isotopic negative excursions at Chaotian are unique and no similar isotopic signal in the same period has been reported elsewhere. The multiple upwelling of oxygen-depleted waters onto the euphotic shelf may have represented local phenomena that occurred solely around northwestern South China.  相似文献   

15.
The sedimentary record of carbonate carbon isotopes (δ13Ccarb) provides one of the best methods for correlating marine strata and understanding the long‐term evolution of the global carbon cycle. This work focuses on the Late Ordovician Guttenberg isotopic carbon excursion, a ca 2·5‰ positive δ13Ccarb excursion that is found in strata globally. Substantial variability in the apparent magnitude and stratigraphic morphology of the Guttenberg excursion at different localities has hampered high‐resolution correlations and led to divergent reconstructions of ocean chemistry and the biogeochemical carbon cycle. This work investigates the magnitude, spatial scale and sources of isotopic variability of the Guttenberg excursion in two sections from Missouri, USA. Centimetre‐scale isotope transects revealed variations in δ13Ccarb and δ18Ocarb greater than 2‰ across individual beds. Linear δ13Ccarb to δ18Ocarb mixing lines, together with petrographic and elemental abundance data, demonstrate that much of the isotopic scatter in single beds is due to mixing of isotopically distinct components. These patterns facilitated objective sample screening to determine the ‘least‐altered’ data. A δ18Ocarb filter based on empirical δ18Ocarb values of well‐preserved carbonate mudstones allowed further sample discrimination. The resulting ‘least‐altered’ δ13Ccarb profile improves the understanding of regional as well as continental‐scale stratigraphic relations in this interval. Correlations with other Laurentian sections strongly suggest that: (i) small‐scale variability in Guttenberg excursion δ13Ccarb values may result in part from local diagenetic overprinting; (ii) peak‐Guttenberg excursion δ13Ccarb values of the Midcontinent are not distinct from their Taconic equivalents; and (iii) no primary continental‐scale spatial gradient in δ13Ccarb (for example, arising from chemically distinct ‘aquafacies’) is required during Guttenberg excursion‐time. This study demonstrates the importance of detailed petrographic and geochemical screening of samples to be used for δ13Ccarb chemostratigraphy and for enhancing understanding of epeiric ocean chemistry.  相似文献   

16.
The Eastern Qinling Molybdenum Belt (EQMB) in central China represents the largest Mo province in the world. Located at the southern margin of the North China Craton, this Mo province is largely associated with Mesozoic granitoids. Here we report Re–Os isochron age of 1761 ± 33 Ma (2σ, MSWD = 2.9) from molybdenite in the recently discovered Zhaiwa porphyry Mo–Cu system within the EQMB. This age coincides with the peak eruption period (1.78−1.75 Ga) of the Xiong'er Group volcanic rocks which unconformably overlie the Taihua Supergroup and are interpreted to have developed on a continental arc. The Re concentrations (1 to 43 ppm) of the Zhaiwa molybdenites are similar to those of typical porphyry Cu–Mo–Au systems developed on continental arcs. The δ34S values of the Zhaiwa ores (2.7–7.3‰, average 5.2‰) suggest that the sulfur was mainly sourced from a magmatic system. The sulfides from the ores yield Sri average of 0.70533, similar to that of the Xiong'er Group (0.70547), albeit higher than that of the Kuanping Group (0.70334) and lower than the Taihua Supergroup (0.70763), suggesting a mixed source. The average εNd(1.76 Ga) of the sulfides from the ores (0.9) falls between the averages of the Taihua Supergroup (ca. − 6.8) and the Kuanping Group (ca. 7.4). The Pb isotope ratios of the ores are similar to, but slightly higher than those of the Taihua Supergroup, reflecting preferential leaching of radiogenic Pb from the Taihua Supergroup by magmatic fluids. Integrated data and interpretations constrain the Zhaiwa Mo–Cu deposit to be a porphyry mineral system developed within a ca. 1.76 Ga continental arc related to oceanic plate subduction beneath the southern margin of the North China Craton. A correlation between tectonic settings and metallogenic types clearly indicate that the Xiong'er Group was formed in a volcanic arc, confirming Paleo-Mesoproterozoic continental outgrowth along the margins of the Columbia supercontinent. The isotopic characterization of the Zhaiwa deposit provides new insights into the enigma of the widespread distribution of large and super-large Mo deposits of EQMB along the southern margin of the North China Craton.  相似文献   

17.
The Hadamengou-Liubagou Au-Mo deposit is the largest gold deposit in Inner Mongolia of North China. It is hosted by amphibolite to granulite facies metamorphic rocks of the Archean Wulashan Group. To the west and north of the deposit, there occur three alkaline intrusions, including the Devonian-Carboniferous Dahuabei granitoid batholith, the Triassic Shadegai granite and the Xishadegai porphyritic granite with molybdenum mineralization. Over one hundred subparallel, sheet-like ore veins are confined to the nearly EW-trending faults in the deposit. They typically dip 40° to 80° to the south, with strike lengths from hundreds to thousands of meters. Wall rock alterations include potassic, phyllic, and propylitic alteration. Four distinct mineralization stages were identified at the deposit, including K-feldspar-quartz-molybdenite stage (I), quartz-pyrite-epidote/chlorite stage (II), quartz-polymetallic sulfide-gold stage (III), and carbonate-sulfate-quartz stage (IV). Gold precipitated mainly during stage III, while Mo mineralization occurred predominantly in stage I. The δDH2O and δ18OH2O values of the ore-forming fluids range from −125‰ to −62‰ and from 1.4‰ to 7.5‰, respectively, indicating that the fluids were dominated by magmatic water with a minor contribution of meteoric water. The δ13CPDB and δ18OSMOW values of hydrothermal carbonate minerals vary from −10.3‰ to −3.2‰ and from 3.7‰ to 15.3‰, respectively, suggesting a magmatic carbon origin. The δ34SCDT values of sulfides from the ores vary from −21.7‰ to 5.4‰ and are typically negative (mostly −20‰ to 0‰). The wide variation of the δ34SCDT values, the relatively uniform δ13C values of carbonates (typically −5.5‰ to −3.2‰), as well as the common association of barite with sulfides suggest that the minerals were precipitated under relatively high fo2 conditions, probably in a magmatic fluid with δ34SƩS  0‰. The Re-Os isotopic dating on molybdenite from Hadamengou yielded a weighted average age of 381.6 ± 4.3 Ma, indicating that the Mo mineralization occurred in Late Devonian. Collectively, previous 40Ar-39Ar and Re-Os isotopic dates roughly outlined two ranges of mineralizing events of 382–323 Ma and 240–218 Ma that correspond to the Variscan and the Indosinian epochs, respectively. The Variscan event is approximately consistent with the Mo mineralization at Hadamengou-Liubagou and the emplacement of the Dahuabei Batholith, whereas the Indosinian event roughly corresponds to the possible peak Au mineralization of the Hadamengou-Liubagou deposit, as well as the magmatic activity and associated Mo mineralization at Xishadegai and Shadegai. Geologic, petrographic and isotopic evidence presented in this study suggest that both gold and molybdenum mineralization at Hadamengou-Liubagou is of magmatic hydrothermal origin. The molybdenum mineralization is suggested to be associated with the magmatic activity during the southward subduction of the Paleo-Asian Ocean beneath the North China Craton (NCC) in Late Devonian. The gold mineralization is most probably related to the magma-derived hydrothermal fluids during the post-collisional extension in Triassic, after the final suturing between the Siberian and NCC in Late Permian.  相似文献   

18.
This study examined the distributions and stable carbon isotopic compositions of saturated fatty acids (SaFAs) in one 300 cm long sedimentary profile, which was named as Site4B in Shenhu, northern South China Sea. The concentrations of total SaFAs in sediments ranged from 1.80 to 10.16 μg/g (μg FA/g dry sediment) and showed an even-over-odd predominance in the carbon chain of C12 to C32, mostly with n-C16 and n-C18 being the two major components. The short-chain fatty acids (ScFAs; n-C12 to n-C18) mainly from marine microorganisms had average δ13C values of −26.7‰ to −28.2‰, whereas some terrigenous-sourced long-chain fatty acids (LcFAs; n-C21 to n-C32) had average δ13C values of −29.6‰ to −34.1‰. The other LcFAs (n-C24 & n-C26  n-C28; average δ13C values are −26.1‰ to −28.0‰) as well as n-C19 and n-C20 SaFAs (average δ13C values are −29.1‰ and −29.3‰, respectively) showed a mixed signal of carbon isotope compositions.The relative bioproductivity calculation (marine vs. terrigenous) demonstrated that most of organic carbon accumulation throughout the sedimentary profile was contributed by marine organism. The high marine productivity in Shenhu, South China Sea may be related to the hydrocarbon seepage which evidenced by diapiric structures. Interestingly, there is a sever fluctuation of terrigenous inputs around the depth of 97 cm below the seafloor (bsf), probably resulting from the influence of the Dansgaard–Oeschger events and the Younger Dryas event as revealed by 14C age measurements.  相似文献   

19.
Zircons from 71 diverse rocks from the Qinling-Tongbai-Dabie-Sulu orogenic belt in east-central China and, for comparison, eight from adjoining areas in the South China and North China blocks, have been analyzed for in situ 18O/16O ratio and/or U–Pb age to further constrain the spatial distribution and genesis of Neoproterozoic low-δ 18O magmas, that is, δ 18O(zircon) ≤4 ‰ VSMOW. In many metaigneous rock samples from Tongbai-Dabie-Sulu, including high-pressure and ultrahigh-pressure eclogites and associated granitic orthogneisses, average δ 18O values for Neoproterozoic “igneous” zircon cores (i.e., 800–600 Ma) vary from ?0.9 to 6.9 ‰, and from ?9.9 to 6.8 ‰ for Triassic metamorphic rims (i.e., 245–200 Ma). The former extend to values lower than zircons in primitive magmas from the Earth’s mantle (ca. 5–6 ‰). The average Δ 18O (metamorphic zircon ? “igneous” zircon) values vary from ?11.6 to 0.9 ‰. The large volume of Neoproterozoic low-δ 18O igneous protoliths at Tongbai-Dabie-Sulu is matched only by the felsic volcanic rocks of the Snake River Plain hotspot track, which terminates at the Yellowstone Plateau. Hence, the low-δ 18O values at Tongbai-Dabie-Sulu are proposed to result from shallow subcaldera processes by comparison with Yellowstone, where repeated caldera-forming magmatism and hydrothermal alteration created similar low-δ 18O magmas. However, the possibility of involvement of meltwaters from local continental glaciations, rather than global Neoproterozoic glaciations, cannot be precluded. Our data indicate that Neoproterozoic low-δ 18O magmas that are either subduction- or rift-related are present locally along the western margin of the South China Block (e.g., Baoxing Complex). It appears that Neoproterozoic 18O-depletion events in the South China Block as the result of hydrothermal alteration and magmatism affected a much larger area than was previously recognized.  相似文献   

20.
The Honghuagou Au deposit is located in the Chifeng-Chaoyang region within the northern margin of the North China Craton. The auriferous quartz veins are mainly hosted in the mafic gneiss and migmatite of the Neoarchean Xiaotazigou Formation along NNW- and NE-striking faults, with pyrite as the predominant ore mineral. The gold mineralization process can be divided into two stages, involving stage I quartz-pyrite and stage II quartz-calcite-polymetallic sulfide. Three types of fluid inclusions (FIs) have been identified in the Honghuagou deposit, namely, carbonic inclusions, aqueous‑carbonic inclusions, and aqueous inclusions. Quartz of stage I contains all types of FIs, whereas only aqueous inclusions are evident in stage II veins. The FIs of stages I and II yield homogenization temperatures of 275–340 °C and 240–290 °C with salinities of 3.4–10.7 wt% and 1.4–9.7 wt% NaCl eqv., respectively. The ore-forming fluids are characterized by medium temperature and low salinity, belonging to the H2O–NaCl–CO2 system. The δ18OH2O values of the ore fluids are between 2.1‰ and 5.9‰, within the range of enriched mantle-derived fluids in the North China Craton. The carbon isotope compositions of calcite (δ13CPDB = −4.4‰ to −4‰) are also similar to mantle carbon. He-Ar isotope data (3He/4He = 0.38–0.44 Ra; 40Ar/36Ar = 330–477) of fluid inclusions in pyrite indicate a mixed crustal and mantle source for the ore-forming fluids. Whereas, S-Pb isotope compositions of sulfides reveal that ore metals are principally derived from crustal rocks. On the basis of available geological and geochemical evidence, we suggest that the Honghuagou deposit is an orogenic gold deposit.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号