Geochemical Characteristics of Wuyang Siliceous Rocks in the Southern Margin of North China Craton and its Constraint on the Formation Environment of BIF of Tieshanmiao Formation
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Abstract:
Precambrian banded iron formation (BIF) is one of the most important mineral resources in China, mostly abundant in the North China Craton (NCC) with relatively less common in South China. Since the BIF and siliceous rocks both originated from chemical deposition, the syngenetic BIF and Siliceous rocks can help evaluate their environment of formation. We examine here the mineralogy and geochemistry of siliceous rocks associated with the Tieshanmiao Formation BIF, aiming to decipher the conditions of formation of both BIF and Siliceous rocks in the Wuyang area in the NCC. Analysis of the geochemical characteristics of whole rock shows that the SiO 2 content of the siliceous rock ranges from 90.11% to 94.85% and is relatively high overall. Trace element contents of Ba and U are also high, the Ba/Sr ratio ranges from 3.89 to 25.28 and the U/Th ratio ranges from 0.09 to 0.20. Finally, the ΣREE value of rare earth elements ranges from 57.03 ppm to 152.59 ppm, and these indexes all indicate that siliceous rock resulted from hydrothermal deposition. Plots of Al 2 O 3 ‐SiO 2 , SiO 2 /(K 2 O+Na 2 O)‐MnO 2 /TiO 2 and Mn‐10×(Cu+Co+Ni)‐Fe in discrimination diagrams also verify this interpretation. However, both the MgO content, ranging from 0.16 to 0.32, and the Fe/Ti ratio, ranging from 2.50 to 9.72, suggest that terrigenous material was added during the depositional process. Major and trace element parameters of siliceous rock, such as the Al/(A1+Fe+Mn) ratio (from 0.81 to 0.93), MnO/TiO 2 (from 0.00 to 0.17), Al/(Al+Fe) (from 0.82 to 0.93), Sc/Th ratio (from 0.21 to 0.50), U/Th (from 0.09 to 0.20), (La/Yb) N (from 0.83 to 3.04), and the (La/Ce) N (from 0.01 to 0.02) all imply that the siliceous rock formed in a continental margin. In addition, the Sr/Ba ratio from 0.08 to 0.26, the δ Ce value from 0.31 to 0.90, and the δ Eu value from 0.14 to 0.58, all indicate that the siliceous rock was formed at a relatively deeper water depth and under weak hydrodynamic conditions. Siliceous rock and BIF formed in the same geological setting, with the SiO 2 /(K 2 O+Na 2 O) ratio of siliceous rock ranging from 28.61 to 47.43, the SiO 2 /Al 2 O 3 ratio from 16.53 to 32.37, and the SiO 2 /MgO ratio from 287.28 to 592.81, which are all in agreement with chemical deposition associated with volcanic eruptions. The Al 2 O 3 /TiO 2 ratio from 37.82 to 50.30 indicates that the magma source of siliceous rock was of slightly intermediate composition. During the Late Archean in the Wuyang area, the high concentration and high purity SiO 2 quickly precipitated from hydrothermal fluids to finally result in the accumulation of siliceous rock in a marginal sea, while the input corresponding to iron formation components was deposited to form iron formation layers, and limestone was only the product formed during the deposition intervals of siliceous rock and iron formations. In this study, the synsedimentary siliceous rocks of BIF act as a new way to provide direct evidence to understand the formation environment of BIF due to its high geochemical stability.Keywords:
Banded iron formation
Terrigenous sediment
Rare-earth element
Trace element
Similarities between Proterozoic (~ 1.8-2.5 Gyr) and Archean (> 2.5 Gyr) banded iron-formations are probably more significant than their differences. The contrasts largely reflect differences in the tectonic settings of Proterozoic and Archean terrains. Archean banded iron-formations are not as thick nor laterally as extensive as the major Proterozoic iron-formations. Nevertheless, some Archean iron-formations have strike lengths of over 150-200 km and may have been quite extensive prior to the deformation that has affected most Archean terrains. Stratigraphic sequences in which iron-formations occur are highly variable and indicate that iron-formations formed in many depositional environments. Sedimentary textures in the iron-formations are dominated either by granules and oolites or laminations (including microbanding) reflecting differences in their physical conditions of deposition. Granular and oolitic textures are abundant in only three Proterozoic depositional basins and most Precambrian iron-formations are laminated. Despite differences in associated lithologies and sedimentary textures Precambrian iron-formations have similar bulk compositions and mineral assemblages, implying that the chemical conditions of iron-formation deposition were similar through much of the Precambrian. The formation of banded iron-formation appears not to have reached a maximum around 1.8-2.0 Gyr but to have been an important process over a long period in the Precambrian.
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Considerable progress has been made in the radiometric dating of Precambrian iron-formations, but the age of relatively few is known precisely. Early Precambrian or Archean deposits, older than 2,600 m. y., are closely associated with greenstone belts. Many, if not all, of these older iron-formations on the Canadian Shield were formed in the interval from 2,750 to 2,700 m.y. ago. Banded iron-formations in Western Australia may date back to 3,000 m. y. and some in South Africa, Venezuela, and the USSR may be well over 3,000 m. y. old.The greatest development of banded iron-formations occurred between 2,600 and 1,800 m. y. ago in the Middle Precambrian or Lower Proterozoic (Aphebian of the Geological Survey of Canada). The huge iron-formation of the Hamersley Range in Australia is dated between 2,200 and 2,000 m. y. A similar age is a reasonable estimate for Krivoy Rog, Ukrainian Shield. Rb-Sr isochron ages on metasedimentary rocks associated with iron-formations in the Lake Superior region and in eastern Canada range from 1,890 m. y. (Sokoman Iron Formation, Labrador Trough, Quebec) to 1,700 m. y. (Gunflint Iron Formation, Thunder Bay district, Ontario). The Rb-Sr ages have been considered to represent the time of deposition, but are here interpreted as secondary or metamorphic ages. In this interpretation a minimum age of 1,900 m. y. is indicated, but the available data do not preclude the possibility that the Middle Precambrian iron-formations on the Canadian Shield are approximately 2,100 m. y. old.The iron-formation of the Rapitan Group in the Snake River area, northwestern Canada has not been dated isotopically, but has been correlated with the Windermere System of Late Precambrian age.
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Abstract Banded iron formations (BIFs) in Archean cratons provide important “geologic barcodes” for the global correlation of Precambrian sedimentary records. Here we report the first finding of late Archean BIFs from the Yangtze Craton, one of largest Precambrian blocks in East Asia with an evolutionary history of over 3.3 Ga. The Yingshan iron deposit at the northeastern margin of the Yangtze Craton, displays typical features of BIF, including: (i) alternating Si-rich and Fe-rich bands at sub-mm to meter scales; (ii) high SiO 2 + Fe 2 O 3total contents (average 90.6 wt.%) and Fe/Ti ratios (average 489); (iii) relative enrichment of heavy rare earth elements and positive Eu anomalies (average 1.42); (iv) and sedimentary Fe isotope compositions (δ 56 Fe IRMM-014 as low as −0.36‰). The depositional age of the BIF is constrained at ~2464 ± 24 Ma based on U-Pb dating of zircon grains from a migmatite sample of a volcanic protolith that conformably overlied the Yingshan BIF. The BIF was intruded by Neoproterozoic (805.9 ± 4.7 Ma) granitoids that are unique in the Yangtze Craton but absent in the North China Craton to the north. The discovery of the Yingshan BIF provides new constraints for the tectonic evolution of the Yangtze Craton and has important implications in the reconstruction of Pre-Nuna/Columbia supercontinent configurations.
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The continental mass of Australia (Fig. 8.1) is made up of four great structural units, to which we may add a fifth represented in the fringing island arcs to the north and east. The oldest and most extensive unit is the Precambrian craton which comprises most of the central and western parts of the continent where Precambrian rocks are exposed beneath an intermittent Phanerozoic cover.
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Radiometric dating
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