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Zircon U-Pb dating on tuff bed from Gaoyuzhuang Formation in Yanqing, Beijing: Further constraints on the new subdivision of the Mesoproterozoic stratigraphy in the northern North China Craton.

Authors:
  • Tianjin Institute of Geology and Mineral Resources

Abstract and Figures

A tuff bed in the upper horizon of Gaoyuzhuang Formation has boon nowly disoovorod in Yanqing County, Boijing. Two almost identical precise zircon U-Pb ages, 1559 ± 12Ma ( SHRIMP) and 1560 ± 5Ma ( LA-MC-ICPMS) , have been obtained for the tuff bed, which indicates that the Gaoyuzhuang Formation in the northern North China Craton was formed in the early stage of Mesoproterozoic Calymmian Period (1600 ∼ 1400Ma). Combining with the available zircon U-Pb age data ( 1622 ∼ 1625Ma) for the volcanics from the upper part of Dahongyn Formaiton, the lower boundary age of Gaoyuzhuang Formation could be accurately positioned at !600Ma. Moreover, with the newly determined SHRIMP U-Pb age ( ∼ 1440Ma) of the K-bentonite in Tiding Formation, the authors thereiore propose again that the basal boundary of Gaoyuzhuang Formation should be taken as the lower boundary of Mesoproterozoic Jixian System. The voluminous carbonate succession from Gaoyuzhuang Formation to Tieling Formation with time span of !600Ma to 1400Ma comprises the Jixian System, almost identically comparable with the international Mesoproterozoic Calymmian System. The new achievement in dating on the tuff of the Gaoyuzhuang Formation provides strong age constraints on the Meso- to Neoproterozoic stratigraphic subdivision and other relevant important scientific issues in the northern North China Craton.
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... A geochronological framework of the Paleo-to Mesoproterozoic strata in the Yanshan Basin has been constructed (Fig. 1D). Zircon age data have been reported from multiple intervals in the Paleo-to Mesoproterozoic, these ages constrain the Jixian Group into the early Mesoproterozoic (1600-1400, equivalent to the Calymmian period (Gao et al., 2008;Li et al., 2010b;Su et al., 2010;Tian et al., 2015). Volcanic tuffs in the Third Member of Gaoyuzhuang Formation at Yanqing County, Beijing give the zircon U-Pb ages of 1560 ± 5 Ma (LA-MC-ICPMS) and 1559 ± 12 Ma (SIMS) (Li et al., 2010b). ...
... Zircon age data have been reported from multiple intervals in the Paleo-to Mesoproterozoic, these ages constrain the Jixian Group into the early Mesoproterozoic (1600-1400, equivalent to the Calymmian period (Gao et al., 2008;Li et al., 2010b;Su et al., 2010;Tian et al., 2015). Volcanic tuffs in the Third Member of Gaoyuzhuang Formation at Yanqing County, Beijing give the zircon U-Pb ages of 1560 ± 5 Ma (LA-MC-ICPMS) and 1559 ± 12 Ma (SIMS) (Li et al., 2010b). Combined with the SHRIMP zircon U-Pb age of 1625 ± 6 Ma for the trachyte from the underlying Dahongyu Formation, Changcheng Group (Lu and Li, 1991), and the SHRIMP zircon U-Pb age of 1483 ± 13 Ma for the bentonite beds from the overlying Wumishan Formation , the deposition time limit of the Gaoyuzhuang Formation was constrained to between 1600 and 1550 Ma (Li et al., 2010b;Tian et al., 2015), representing the earliest Mesoproterozoic marine sedimentary record in the NCC. ...
... Volcanic tuffs in the Third Member of Gaoyuzhuang Formation at Yanqing County, Beijing give the zircon U-Pb ages of 1560 ± 5 Ma (LA-MC-ICPMS) and 1559 ± 12 Ma (SIMS) (Li et al., 2010b). Combined with the SHRIMP zircon U-Pb age of 1625 ± 6 Ma for the trachyte from the underlying Dahongyu Formation, Changcheng Group (Lu and Li, 1991), and the SHRIMP zircon U-Pb age of 1483 ± 13 Ma for the bentonite beds from the overlying Wumishan Formation , the deposition time limit of the Gaoyuzhuang Formation was constrained to between 1600 and 1550 Ma (Li et al., 2010b;Tian et al., 2015), representing the earliest Mesoproterozoic marine sedimentary record in the NCC. ...
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The early Mesoproterozoic (1.6–1.4 Ga) is a critical stage for oxygenation of Earth’s surface and early eukaryote evolution. However, the relationship between both remains uncertain. Here, we provide new carbonate associated sulfate (CAS) sulfur isotopic compositions (δ34SCAS) and trace elements from the 1.6 to 1.55 billion-year old Gaoyuzhuang Formation in multiple sections within the Yanshan Basin, North China, to investigate the temporal and spatial variation of ocean redox conditions. New and collected δ34SCAS data from five sections (Pingquan, Kuancheng, Qianxi, Jixian and Gan’gou sections) reveal similar sulfur isotopic excursions, but the differences in absolute values indicate spatial heterogeneity of sulfur isotopic compositions, which may reflect marine low-sulfate concentrations and oceanic redox tratification. The most elevated δ34SCAS values appeared at the top of the Second Member of the Gaoyuzhuang Formation, followed by distinct negative excursions at the bottom of the Third Member, suggesting a dynamic sulfur cycle and seawater redox status during this period. Combined with previous reported carbon and sulfur isotopes and some redox proxies (e.g., Ce anomalies, manganese-rich deposit, I/(Mg + Ca) and Fe speciation), we suggest that at least two possible oxygenation events may have occurred in the Gaoyuzhuang Formation: (1) the bottom of the Second Member and (2) the bottom or middle part of the Third Member. Different sections show asynchronous timing of these events, indicating that the redox status was spatial heterogenous within the Yanshan Basin. The spatial heterogeneity may be controlled by facies and paleogeographic locations. Considering that decimeter-scale multicellular eukaryotes are preserved in the middle part of the Third Member, we argue that oxic bottom water condition may be a critical factor for early eukaryote evolution.
... U-Pb zircon dating of tuff beds in the lower and upper horizons of Member III (Figure 2) yields ages of 1,577 ± 12 Ma (Tian et al., 2015) and 1,560 ± 5 Ma (H. Li et al., 2010), respectively. ...
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Marine carbonate fabrics are intrinsically related to ocean chemistry, physical processes and biological activity. Molar tooth structure (MTS), a globally distributed structure in Proterozoic carbonate sediments, has been widely studied for more than a century; yet its connections with physical and biological processes remain unclear. Using multiple techniques, we studied ∼1.57 Ga MTS and identified a connection between its occurrence and increased marine oxygenation. In our samples, the matrix surrounding MTS is typically dominated by carbonate mud with early diagenetic dolomite crystals. High I/(Ca + Mg) ratios (up to 4.1 μmol/mol) and negative Ce anomalies (∼0.8) detected in the matrix are consistent with the oxidative removal of inhibitors such as Fe²⁺ and Mn²⁺ in the water‐column that permitted carbonate “whiting” mud precipitation stimulated by cyanobacterial photosynthesis. This cohesive but not rigid seafloor carbonate mud was a prerequisite for synsedimentary MTS crack formation. Systematically higher carbon isotope (δ¹³C) values in MTS microspars, relative to host sediment, support origination of the cracks by methane degassing in the organic‐rich carbonate mud. Low, but non‐zero, I/(Ca + Mg) values of the MTS microspar suggest that the precipitation of the microspar that filled the MTS cracks was triggered by oxidative removal of residual Fe²⁺ and Mn²⁺ in porewater through mixing with overlying oxygenated seawater. We therefore propose that MTS formed under moderately oxygenated conditions and that its sporadic occurrence prior to ∼1.2 Ga reflects episodes of pulsed marine oxygenation in an overall anoxic setting.
... The age of the Gaoyuzhuang Formation is well constrained by zircon U-Pb ages of 1,577 ± 12 Ma and 1,560 ± 5 Ma from tuffaceous beds in the lower and upper Member III, respectively (Li et al., 2010;Tian et al., 2015). The oxygenation event reported by Shang et al. (2019) was estimated to be between ∼1,580 and ∼1,560 Ma in age, with its peak at ∼1.57 ...
... This locates the Mesoproterozoic-Paleoproterozoic boundary to between the Gaoyuzhuang Formation and the underlying Dahongyu Formation. A zircon U-Pb age of 1560 ± 5 Ma from the upper Gaoyuzhuang Formation (Li et al., 2010) constrains the age of the Gaoyuzhuang Formation to the range of 1600-1500 Ma. ...
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It is hypothesized that the Mesoproterozoic was characterized by a prevailing warm climate and a high atmospheric pCO2 level. However, quantitative constraints on Mesoproterozoic atmospheric pCO2 are scarce. Here, we report high-resolution organic and inorganic carbon isotope records from sediment samples aged ∼1.6 Ga, collected from Hebei Province in North China. The δ¹³Corg values of these Mesoproterozoic samples vary from −28.4‰ to −34.3‰, and the δ¹³Ccarbonate values range from −2.1‰ to 0.4‰. The paired carbonate and organic carbon isotope records can be used as a proxy to quantify the atmospheric CO2 concentration during the Mesoproterozoic. According to the calibration between the photosynthetic isotopic effect and atmospheric CO2, atmospheric CO2 concentrations are estimated to be within the range of 910–18,800 ppmv, ∼2–50 times greater than the present atmospheric level. The results support the previous hypothesis which suggests that high levels of atmospheric CO2 prevailed in the Early Mesoproterozoic. High atmospheric pCO2 levels could have played an important role in compensating for the possible lower solar luminosity and maintaining the temperature of the Mesoproterozoic Earth's surface.
... Some tuff beds have been discovered in the Mb3 of the Gaoyuzhuang Fm. Li et al. (2010b) obtained two identical zircon U-Pb ages of 1559 ± 12 Ma and 1560 ± 5 Ma from a tuff bed in the top horizon of the Mb3 of the Gaoyuzhuang Fm. Subsequently, Tian et al. (2015) reported a zircon U-Pb age of 1577 ± 12 Ma from a tuff bed in the lower horizon of the Mb3 of the Gaoyuzhuang Fm. ...
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Recent studies of the upper Gaoyuzhuang Formation (Fm) (ca. 1600–1540 Ma) in north China have suggested that Earth underwent an oxygenation event associated with a negative carbon isotope excursion and this event may have led to the expansion of eukaryotic life. However, the duration of this negative δ¹³Ccarb excursion, which is crucial to understand its origin, remains uncertain. Here, we report a floating astronomical timescale based on Milankovitch cycles extracted from the carbonate-dominated strata of the Member 3 of the Gaoyuzhuang Fm in north China, in which the oxygenation event is recorded. High-resolution magnetic susceptibility, anhysteretic remnant magnetization and saturation isothermal remnant magnetization series were obtained from the stratigraphic sections in the Jixian and Yanqing regions. Rock magnetic experiments suggest that the magnetic parameters are dominated by the detrital materials whose flux was likely controlled by orbitally forced climatic changes. Our results for the first time reveal well-constrained eccentricity, obliquity, and precession cycles from the Gaoyuzhuang Formation and demonstrate that the ca. 1560 Ma negative δ¹³Ccarb excursion and associated transient oxygenation event indicated by I/(Ca + Mg) had durations of ~1.7 Myr and ~ 6.2 Myr, respectively. The ca. 1560 Ma δ¹³Ccarb excursion is comparable with the short-term Neoproterozoic excursions in respect of duration, rather than with the long-term ones such as the ca. 574–567 Ma Shuram excursion. The similarity may indicate a common mechanism in origin of the short-term excursions in the Mesoproterozoic and Neoproterozoic time, which deserve testing by more investigations.
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We present rare earth elements (REE) and Mo isotope data from the middle Gaoyuzhuang Formation in North China in order to contribute to the debate on the Mesoproterozoic marine redox state. Sequential leaching of the Gaoyuzhuang calcareous shales exhibits distinct enrichment patterns of trace metals between carbonate and authigenic phases, which are likely caused by element redistribution during organic matter degradation. Collectively, our results and literature data of the Gaoyuzhuang Formation show stratigraphically coupled variations in Ce anomalies and Mo stable isotope signatures. These geochemical shifts can be interpreted as a response to progressive oxygenation in regional and global settings, respectively. However, the oxygenation magnitudes expressed by Ce anomalies (Ce/Ce* = ∼0.8) and Mo isotopes (δ⁹⁸Mo = 2.33‰) are markedly different. Rather than prevalent ocean oxygenation, we suggest that the modern-like seawater Mo isotope values represent accelerated input of reactive Fe from volcanism-induced weathering and subsequent burial of light Mo isotopes into the sediments. Furthermore, the documented transition coincides with the occurrence of macroscopic fossils, supporting a causal relationship between volcanism, environmental change, and biological evolution.
Chapter
The Precambrian sedimentary strata in the Yanliao Faulted-Depression Zone (YFDZ) on the North China Craton is the most-developed Meso-Neoproterozoic sequences in China. The sequences consist of the Pt21 Changchengian, Pt22 Jixianian, Pt23x Xiamaling Formation and Pt31 Qingbaikouan, which are traditionally subdivided into 12 formations and 43 members. This chapter aims to review the history and the state of art of the studies on the stratigraphic sequences in aspects covering the tempo-spatial distribution, lithostratigraphic correlation, geochronology and palaeontology. Some important aspects have been discussed in detail. As the oldest unmetamorphosed sedimentary sequence in China, the Changchengian (1670–1600 Ma) shows a regional micro-angle unconformity with the underlying Dahongyu Formation of Jixianian (1600–1400 Ma) with a local conformity at the Dahongyu depocenter. Therefore, both Changchengian and Jixianian should be referred to a set of basically continuous sedimentary strata in the Jixian stratotype section. The Changchengian may be attributed to the aulacogen clastic deposition in the early stage of the YFDZ related to early breakup of the Supercontinent Columbia.
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Predominantly anoxic conditions with a heterogeneous stratified water column are widely accepted for the Mesoproterozoic ocean. However, transient and episodic oxygenation during the Mesoproterozoic has been recently hypothesized, which may have been an important trigger for eukaryote evolution. In this study, we analyzed the Ce anomalies of the Gaoyuzhuang and Hongshuizhuang formations in North China, to investigate the Earth’s surface environment during the Mesoproterozoic. Our data of negative Ce anomalies in the Gaoyuzhuang and Hongshuizhuang formations, reaching 0.73 and 0.84, respectively, suggest transient oxygenations of the surface environment. We applied a CeN/CeN* depth model to quantify these transient oxygenations and the results suggest that the atmospheric oxygen level would have increased by 0.01–0.03% present atmospheric level (PAL), 0.02–0.05% PAL, and 0.1–0.4% PAL, with the respective initial levels of 0.05% PAL, 0.1% PAL, and 1% PAL, in the Gaoyuzhuang and Hongshuizhuang formations. Our results suggest that although oxygen levels in the Mesoproterozoic were low, transient oxygenation may have occurred in the surface ocean, which would have provided vital niches for the evolution of eukaryotes.
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We present new detrital zircon U–Pb and Hf isotopic dating results from the late Mesoproterozoic to early Neoproterozoic sandstones exposed in the southern-eastern Liaoning Province, eastern North China craton (NCC). The Qiaotou Formation (Fm) in the Benxi and Dalian regions yield similar detrital zircon age patterns characterized by peaks around ∼1750 Ma, ∼1600 Ma, ∼1175 Ma and ∼1100 Ma. The Kangjia Fm in the Benxi region and the Changlingzi Fm in the Dalian region share similar detrital zircon age patterns, which in combination with their similarities in stratigraphic sequences and lithologic associations, suggest that they are correlative rock units. Based on the detrital zircon dating results and U–Pb ages of mafic sills intruding the strata, we suggest that the Qiaotou Fm is younger than ∼1077 Ma and older than ∼948 Ma, and the age of the Kangjia and Changlingzi formations is between ∼1111 Ma and ∼924 Ma. We synthesize the new data with detrital zircon age and Hf isotopic data reported in the region and other Meso-Neoproterozoic basins in the eastern NCC and suggest that Grenville-age (∼1300–1000 Ma) detrital zircons widely discovered in those strata might be transformed from other cratons. The possibility that those non-NCC provenance detrital zircons might be sourced from the Baltica, São Francisco–Congo and Laurentia cratons existed. But the former two solutions that the NCC directly connected to either of those cratons are contradictory to the global paleogeographic reconstruction context of supercontinent Rodinia. The Grenvillian and Sveconorwegian orogenic belts on Laurentia–Baltica conjoined continents are more likely the sediment provenance of non-NCC Grenville-age detrital zircon provenance in the Meso-Neoproterozoic basins in eastern NCC in the configuration context of the possible NCC–NW Laurentia conjugation in Rodinia.
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The large to middle-sized magmatic Ni–Cu sulfide deposits are spatially situated in the eastern portion of Eastern Tianshan, whereas those in the western portion have been poorly understood due to their quantity and scale. The Haibaotan Ni–Cu sulfide occurrence is newly discovered in the western portion of Eastern Tianshan, which provides an opportunity for understanding how its magmatic conduit system was generated and evolved. The Haibaotan mafic–ultramafic intrusion is dominated by peridotite, pyroxenite, olivine gabbro, and gabbro, and sulfide orebodies occur mainly within gabbro. Gabbro from the Haibaotan intrusion is dated at 315.5 ± 1.9 Ma, which suggests that the Haibaotan intrusion is generated in a subduction evironment. The age is much earlier than the emplacement of other mineralized mafic–ultramafic intrusions (~ 280 Ma) in Eastern Tianshan. High zircon εHf(t) values (+ 7.09 to + 18.44) indicate that the parental magmas of the Haibaotan intrusion have been derived from a metasomatized mantle source. Additionally, mafic–ultramafic rocks in the Haibaotan intrusion exhibit enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE), together with a depletion of Zr, Ti, and Nb, which is indicative of a subduction-modified mantle source. Core–rim variations of olivine forsterite (Fo) and Ni contents imply that the Haibaotan intrusion resulted from successive magma pulses, and the variable PGE tenors reflect that the sulfide droplets scavenged metals from multiple magma pulses. The low Se/S ratios of sulfide mineralization suggest that the addition of crustal sulfur triggered sulfide segregation in the shallow magma conduit system. The evolution history of the Haibaotan magma conduit system meets the requirements for generating a magmatic Ni–Cu deposit. This study highlights that the subduction environments are favorable for the formation of magmatic sulfide deposits in Eastern Tianshan.
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Single zircons from two Early Cambrian volcanic horizons have been analysed using the SHRIMP ion microprobe. Full details of the analytical procedures and data reduction are given. Zircons from tuff within the Lie de Vin Formation, near Tiout, Morocco, show little spread in U-Pb age and have a mean value of 521 +/- 7 Ma (2-sigma). Those from a bentonite within unit 5 of the Meishucun section near Kunming, southern China, show relatively dispersed U-Pb ages, revealing the presence of both detrital or xenocrystic grains as well as areas within grains that have lost radiogenic Pb. The main population has as mean age of 525 +/- 7 Ma, but a mean Pb-207/Pb-206 age of 539 +/- 34 Ma which is a maximum estimate for the bentonite age. These results conflict with previous Rb-Sr whole rock ages of c. 580 Ma for overlying Cambrian shales at Meishucun, and c. 570 Ma for Atdabanian shales from the E. Yangtse Gorges area.
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In China, Palaeoproterozoic stromatolites are best represented by taxa from the Hutuo Group of the Wutai Mountains and the Changchengian Group of the Yanshan Range, which all belong to Palaeoproterozoic rocks with a time range from ∼2500 to 1600 Ma. From the present data, Palaeoproterozoic stromatolite assemblages are mainly characterized by the following features. 1.(1) The Palaeoproterozoic stromatolite succession contains many special elements such as forms of Pilbaria, Externia, Kussoidella, Confunda, Discorsia, Vertexa, Tibia, Dongyeella, Jiananella, Shugongsiella, Yaochicunia, Zhongtiaoshania, which so far have not been found in Meso- and Neoproterozoic rocks.2.(2) The assemblages of the Palaeoproterozoic stromatolites cannot be compared wich those of the Meso- and Neoproterozoic stromatolites in representative elements.3.(3) The microstructures of the Palaeoproterozoic stromatolites are dominated by banded and streaky types, but grumelous type, which is found frequently in Meso- to Neoproterozoic rocks, has been found only occasionally.4.(4) The stromatolite-building microorganisms of the Palaeoproterozoic stromatolites are dominated by quite thin filaments (<5 μm in width) and solitary coccoids, and display more primitive characteristics in their morphology and structure and are of a lower grade in evolution.Failure to distinguish between the palaeoproterozoic stromatolite succession and the Meso- to Neoproterozoic one, using tge above characteristics, can result in incorrect correlations in the study of Proterozoic stratigraphy.
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This paper gives a general outline of the Upper Precambrian (Sinian Suberathem) in China, including its geographic distribution, development, stratigraphic sequence and classification. The Sinian Suberathem, with a time range of 1950-615 Ma, constitutes the greater part of the Proterozoic. It is characterized by the development of stromatolites, appearance of eukaryotes and the presence of metazoa in its upper part. Its sedimentological characteristics are quite similar to those of the overlying Paleozoic.On the basis of the biological evolution, isotopic dating and stages of tectonic development, the Sinian Suberathem can be divided into four systems, in ascending order: the Changcheng (ca. 1950-1400 Ma), Jixian (ca. 1400-1000 Ma), Qingbaikou (ca. 100–800 Ma) and Sinian (ca. 800-615 Ma) Systems. The stratotype for the first three systems is located in Jixian County, and the type section for the Sinian System in the Yangzi Gorges area. The contact between the Sinian and Cambrian Systems is, in some places, a continuous transition, with abundant small shelly fauna above the Precambrian—Cambrian boundary, e.g. in the Yangzi Gorges, Yunnan and Sichuan. With regard to sedimentological characteristics, the rocks of the Sinian Suberathem may be subdivided into three main facies, such as: (1) carbonate facies of stable region; (2) volcanic—sedimentary facies of mobile zones; and (3), argillo-arenaceous—carbonate facies of transitional zones.Progress in the study on the succession of the Sinian Suberathem has been made in recent years in regard to palaeontology, stratigraphy, isotopic geochronology, magnetostratigraphy, and palaeoglaciology. They provide important evidence for the stratigraphic correlation of the Upper Precambrian of China and other continents.
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Precambrian rocks are widely distributed in China. The Precambrian is divided into two time units, i.e., the Archaean and Proterozoic Eon, each of these is separated into three chronological intervals, also with the status of eras, with the prefixes early, middle or late. The time boundary between the Archaean and Proterozoic Eon is placed at ∼ 2500 Ma.According to the present isotopic data, the proposed subdivision for the Archaean of China is two-fold. The age of the Fuping Group is younger than 2800–2900 Ma, and that of the Qianxi Group and the corresponding stratigraphic units of eastern Liaoning are older than 2800 Ma, so that 2800+ Ma is selected as the boundary between the early—middle and late Archaean.Based on the representative stratigraphic units, the Wutai and Huto Groups, and an intervening major unconformity formed by the Wutaiian orogeny at 2200–2300 Ma, the early Proterozoic is further divided into two periods, with a time demarcation at 2200+ Ma. A major episode of orogeny known as the “Luliangian Movement” occurred at the end of the early Proterozoic at ∼ 1900 Ma. This disturbance was very extensive and is, in a way, responsible for the difference in geological conditions between the lower and middle—upper Proterozoic in China. The boundary (1900 Ma) that relates to the Luliangian Movement is more important than the boundary corresponding to the age of 1600 Ma, which is recommended as the time boundary between Proterozoic I and II, so we propose to use 1900 Ma as the boundary between the early and middle Proterozoic in China.The time boundary between the middle Proterozoic, including the Changcheng System and the Jixian System, and the late Proterozoic, which is composed of the Qingbaikou and Sinian Systems, is ∼ 1000 Ma. The age for the boundary between Cambrian and Precambrian, based upon the recent isochron data, is inferred to be 610 Ma.
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We introduce and propose zircon M257 as a future reference material for the determination of zircon U-Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS-determined, mean isotopic ratios (2s uncertainties) of 0.09100 +/- 0.00003 for (206)pb/(238)U and 0.7392 +/- 0.0003 for (207)pb/(235)U. Its (206)pb/(238)U age is 561.3 +/- 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U-Pb system is concordant within uncertainty of decay constants. Zircon M257 contains similar to 840 mu g g(-1) U (Th/U similar to 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit-cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the "Sri Lankan trends", with actinide concentrations, U-Pb age, and the calculated alpha fluence of 1.66 x 10(18) g(-1). This, and a (U+Th)/He age of 419 +/- 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc-silicate skarn.
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Zircon grains of magmatic origin from tuffite layers in the Xiamaling Formation at Zhaojiashan Village, Xuanhua area, Hebei Province, were used for zircon dating with a Sensitive High-Resolution Ion Microprobe (SHRIMP II), which gives a weighted mean 207Pb/206Pb age of 1366±9 Ma. It shows a very similar age (1368±12 Ma) as the zircon dating from the Xiamaling Formation in Western Hill, Beijing. This age proposes that the Xiamaling Formation in the North China plate should be of Mesoproterozoic, instead of Neoproterozoic based on K-Ar, Ar-Ar dating. The new zircon age also indicates the development of macrofossils algae from the Mesoproterozoic age.
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This paper reports new developments in in situ U–Pb zircon geochronology using 266 and 213 nm laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS).Standard spot ablation (spot diameters 40–80 μm) was employed, with no sampling strategies employed specifically to minimise elemental fractionation. Instead, He ablation gas and carefully replicated ablation conditions were employed to maintain constant ablation-related elemental fractionation of Pb and U between analyses. Combining these strategies with calibration on a new zircon standard (GJ-1) allows elemental fractionation and instrumental mass bias to be corrected efficiently, and accurate 206Pb/238U and 207Pb/235U ratios to be measured with short-term precision (2 r.s.d.) of 1.9% and 3.0%, respectively.Long-term precision (2 r.s.d.) of the technique (266 nm ablation), based on 355 analyses of the 91500 zircon (1065 Ma) standard over more than a year, was 3.8%, 4.0% and 1.4% for the 206Pb/238U, 207Pb/235U and 207Pb/206Pb ratios, respectively. Long-term precision (2 r.s.d.) for the 206Pb/238U, 207Pb/235U and 207Pb/206Pb ratios of the Mud Tank zircon (732 Ma) was 3.9%, 4.1% and 1.7%, respectively (359 analyses). Selective integration of time-resolved signals was used to minimise the effect of Pb loss and common Pb enrichments on the measured ages. The precision and accuracy of our data compare very favourably with those obtained using more involved procedures to correct or minimise ablation- and ICP-MS-induced biases.213 nm laser ablation produced comparable precision to 266 nm ablation using generally smaller spot sizes (40–50 vs. 60–80 μm), and offered significant advantages in terms of ablation duration and stability, particularly for small zircons (<60 μm). For the 91500 zircon, but not the Mud Tank zircon, 213 nm ablation also produced significantly older and more accurate Pb/U ages. This suggests that shorter wavelength ablation may have reduced a matrix-dependent elemental fractionation difference between sample and standard.The accuracy and precision of the technique for young zircons are demonstrated by analysis of three zircon populations ranging in age from 417 to 7 Ma. In each case, the zircons have yielded concordant ages or common Pb discordia which give concordia intercept ages that are in agreement with independently determined ages for the same samples. Application of Tera–Wasserburg diagrams [Earth Planet. Sci. Lett. 14 (1972) 281] was found to be the most useful approach to handling common Pb contributions that were not removed by selective integration of signals.