Yang Jiedong

Variations in Sr and C isotopes and Ce anomalies in successions from China: evidence for the oxygenation of Neoproterozoic seawater?

Abstract Samples of marine carbonates, phosphatic fossils, phosphorites and manganese deposits were systematically collected from the Neoproterozoic Sinian System in the Yangtze Region, South China, and data of C and Sr isotopes and Ce anom values were obtained and investigated. These data, combined with Nd isotopic data (Yang et al. 1997a, Chem. Geol. 135, 127–133; Yang et al. 1997b, Chinese Sci. Bull. 42, 2072–2075), provide a detailed record of isotopes and Ce anomaly variations in Chinese seawater during the Sinian and early Cambrian periods, and show significant changes: δ 13 C: −4∼+6‰; ϵ Nd (T): −4.5∼−7.6; 87 Sr/ 86 Sr: 0.7077∼0.7087; Ce anom : +0.20∼−0.65 , respectively. The changes exceed or match the largest in seawater throughout the Phanerozoic. The generally high values of δ 13 C during the Sinian may reflect an increase in the C org burial rate. It, in turn, may be attributed to high productivity at the surface of the ocean and a rise in the sedimentary rate, which resulted from enhanced tectonism and a global temperature rise. A trend of declining Ce anom values may also reflect an increase in organic productivity at the ocean surface and a rise in O 2 concentration in the atmosphere and the upper oceans. The most important point revealed by the Nd and Sr isotopic studies is that the strongest and most widespread tectonism took place towards the end of the Proterozoic. The tectonism caused a paleogeographic reordering, rapid rise in the global sedimentary rate and great changes in ocean chemistry, and ultimately led to the variations in C, Sr and Nd isotopes and Ce anomaly in the Sinian seawater.

Variations in chemical compositions of the eolian dust in Chinese Loess Plateau over the past 2.5 Ma and chemical weathering in the Asian inland

Major and trace elements as well as strontium isotopic composition have been analyzed on the acid-insoluble (AI) phase of the loess-paleosol sequence from Luochuan, Shaanxi Province, China. Results show that the chemical composition of AI phase of loess and paleosols is distinctive to the average composition of upper continental crust (UCC), characterized by depletion of mobile elements Na, Ca and Sr. The distribution pattern of elements in AI phase reveals that initial dust, derived from a vast area of Asian inland, has suffered from Na- and Ca-removed chemical weathering compared to UCC. Some geochemical parameters (such as CIA values, Na/K, Rb/Sr and87Sr/86Sr ratios) display a regular variation and evolution, reflecting that the chemical weathering in the source region of loess deposits has decreased gradually since 2.5 Ma with the general increase of global ice volume. This coincidence reflects that the aridity of Asian inland since the Quaternary is a possible regional response to the global climate change.

Nd isotopic variations of Chinese seawater during Neoproterozoic through Cambrian

Abstract Samples of sedimentary phosphatic rocks and manganese deposits were collected from the Neoproterozoic Sinian System and the Lower Cambrian Series in the Yangtze region, China. The results from SmNd isotopic determinations show a considerable change in Nd isotopic composition of the Chinese seawater during the Neoproterozoic Period, which was similar to that of the Panthalassa ocean, buth with smaller swing. The e Nd ( T ) values of the Chinese seawater were about −4.5 in the Nantuo ice age (corresponding to the Varanger ice age), falling within the range of the Panthalassa ocean, then gradually decreased and separated from the Panthalassa ocean after the Nantuo ice age, and finally reached the lowest point (about −8.0) at the beginning of the Early Cambrian, obviously distinct from the Panthalassa ocean (−20 to −10). As one might infer, the Chinese seawater was not co-oceanic with the Panthalassa ocean in terms of Keto and Jacobsen (1988) and belonged to another major ocean as a result from reconstruction of the global palaeoceanographic pattern during the Sinian (Vendian) through the Cambrian.

Sm—Nd isotopic age of Precambrian—Cambrian boundary in China

By the new method of Sm—Nd isotopic dating on phosphatic small skeletal fossils and collo-phanite minerals, the Zhongyicun Member of the earliest Cambrian Meishucun Stage at Meishucun in Yunnan, southern China, has been dated at 562.8 ± 7.9 Ma and 562.1 ± 5.7 Ma. Another Sm—Nd age, 570.3 ± 17.1 Ma, has been obtained with samples from the Zhongyicun Member in Yunnan and its stratigraphic equivalents in Sichuan and Xinjiang. These data tend to suggest that the best age estimate of the Precambrian—Cambrian boundary is very likely within the range of 560–570 Ma. As biophosphates and sedimentary phosphates are widely distributed in sequences of the Precambrian—Cambrian transition, the Sm-Nd isotopic method is recommended as an effective approach for precise dating of the initial Cambrian boundary.

Chemical and isotopic characteristics of gabbroic xenoliths from Hannuoba, China

Nine pieces of gabbroic xenoliths from Hannuoba were examined for their major and trace elements and Nd, Sr and Pb isotopes. The results show that the gabbroic xenoliths are of more mafic basaltic composition. Their abundances show narrow variations in major elements. The trace element contents are highly variable in contrast with those of host basalts and lherzolite xenoliths. The gabbroic xenoliths are rich in Nd (0.51159–0.51249), Sr (0.70491–0.70768) and low in radiogenic Pb(16.283–17.046, 15.191–15.381 and 36.999-37.476), significantly different from basalts and lherzolites in isotopic space. The calculated Nd and Pb model ages are about 3.0–3.5 Ga. The rocks have relatively low equilibriumT(~850°C) andP (0.8–0.9 GPa). They could be interpreted to be the product of upper mantle melting at the boundary between the lower crust and the upper mantle. Their chemical and isotopic variations can be ascribed to different degrees of melting, segregation and long-term evolution.

Nd isotopic composition and material source of pre- and post-Sinian sedimentary rocks in Xiushui area, Jiangxi Province

Reported in this paper are the Nd isotopic compositions of the pre-Sinian and Sinian-Cambrian sedimentary rocks in the Xiushui area, Jiangxi Province. Significant differences are noticed between them in their Nd isotopic compositions. As for the pre-Sinian lightly metamorphozed sedimentary rocks,143Nd/144Nd=0.512000 ∼ 0.512214, ɛNd (T) = −2.57 ∼ 3.91, and TDM=1563 ∼ 1859 Ma;143Nd144Nd=0.511710 ∼ 0.511902, ɛNd(T)= −8.04 ∼ −9.99, and TDM= 1833 ∼ 2426Ma are suggested for the Sinian-Cambrian sedimentary rocks. These differences would reflect the diversity of material source for the sedimentary rocks deposited before and after the Sinian period. Mantle material appears to have been involved in the formation of the pre-Sinian sedimentary rocks while the post-Sinian sedimentary rocks are composed mainly of recycled detritus from the continental crust.Reported in this paper are the Nd isotopic compositions of the pre-Sinian and Sinian-Cambrian sedimentary rocks in the Xiushui area, Jiangxi Province. Significant differences are noticed between them in their Nd isotopic compositions. As for the pre-Sinian lightly metamorphozed sedimentary rocks,143Nd/144Nd=0.512000 ∼ 0.512214, ɛNd (T) = −2.57 ∼ 3.91, and TDM=1563 ∼ 1859 Ma;143Nd144Nd=0.511710 ∼ 0.511902, ɛNd(T)= −8.04 ∼ −9.99, and TDM= 1833 ∼ 2426Ma are suggested for the Sinian-Cambrian sedimentary rocks. These differences would reflect the diversity of material source for the sedimentary rocks deposited before and after the Sinian period. Mantle material appears to have been involved in the formation of the pre-Sinian sedimentary rocks while the post-Sinian sedimentary rocks are composed mainly of recycled detritus from the continental crust.

Isotope and trace element geochemistry of Fuchuan ophiolite suite, Anhui, China

The Fuchuan ophiolite suite in Shexian County, Anhui Province, was formed in the Middle-Late Proterozoic. It is characterized by varying Nd [∈Nd (T) =0.7−3.8], Sr [∈Sr(T) = 30.7−53.9] and O(δ18O=3.2−11.0%.) and low ratios of Nd/La (<0.8), Ti/Y (<350) and Ti/V (<30). These characteristics, in combination with geological features, indicate that the ophiolite suite was formed in the axial part of the back-arc basin of the Jiangnan ancient island arc at the southeastern margin of the Yangtze Plate. The varying ∈Nd (T) was caused by the contamination of the underlying inmature sialic crust during the formation of the ophiolite and variations in ∈Sr(T) and δ18 O may have resulted from hydrothermal alteration by seawater during or shortly after its formation.