| 其他摘要 | The Early Cambrian black rock series in South China hosts many kinds of mineral deposits, such as Ni-Mo-PGE, V-U, Se, phosphorite, barite (witherite) and coal stone, etc., many kinds of toxic elements, such as As, Se, Hg, Gd, Pb, etc., and many kinds of fossils, such as small shelly fossils, sponge specula, arthropods, etc., implying that this is of importance to mineral resources exploration, environmental protection and evolution of Earth history. However, researches on these aspects is still in its infancy, lots of problems are far away from being unresolved, which is partly due to the lack of uniform stratigraphic division and precise dating of Early Cambrian strata in South China, and partly due to the lack of systematic sedimentary geochemistry study. In order to understand the paleo-environmental characteristics and evolution of Early Cambrian ocean thoroughly and to promote the establishment of a uniform stratigraphic frame, a black rock series profile of Early Cambrian named Zhongnancun, which was located at Yangtze Platform in Zunyi, Guizhou Province was selected for the investigation of sedimentary geology and geochemistry, and some findings can be put forward as following:
(1)The Zhongnancun black rock series profile composed of, in ascending order, phosphorites, pyrite-rich claystone, cracked layer, siliceous rock/shale, V-mineralized layer, silt-bearing black shale, Ni-Mo-mineralized layer, fossil-bearing black shale, black shale and silty black shale. And two obvious characteristics could be found: one is the change of sedimentary environment from the offshore at bottom to the inshore in the upper of the profile, the other is the relatively active geologic process in the lower part of the profile confirmed by the volcanogenic cracked layer and hydrothermal siliceous rock, as well as the three mineralized layers of sulfide.
(2)Phosphorite is mainly composed by francolite and authigenic quartz, with minor authigenic barite and V-bearing illite. The francolites yield so-called “old phosphorite” REE patterns, which contain higher REE concentration, pronounced negative Ce anomaly, enriched MREE but are depleted in HREE, and they also have higher sulfate sulfur isotope with the mean value of 38.1‰. Therefore, it can be proposed that phosphorite of this time could form at the water- sediment surface which is located within the oxygen minimum zone in the area of upwelling of a redox-stratified ocean, and absorbed seawater sulfate and REE during deposition and diagenesis. The sulfur isotopic composition of Early Cambrian seawater sulfate could be 36.6±1.6‰.
(3)As for the black (siliceous) shale, the concentrations of Ba, Ni, U, V, and Mo are relatively higher with good correlations between their element/Sc ratios and TOC/Sc ratio, while the organic carbon isotopic compositions are considerably lower with values of -34.3‰～-31.2‰, indicating that the Early Cambrian ocean could have been redox-stratified, exhibiting a higher level of productivity, and euxinic conditions. Upward along the profile, the element/Sc ratios decrease, 13Corg values increase, and the REE patterns of organic matter become seawater type, indicating that the level of paleo-productivity and the degree of anoxia drop gradually with time.
(4)Comparing to other rocks, some of the geochemical parameters like U/Sc ratio, Mo/Sc ratio,13Corg, REE patterns in organic matter at the sites of siliceous rock, cracked layer, V-mineralized layer, and Ni-Mo-mineralized layer show clear anomalies, which indicate that they relate to the hydrothermal solutions. Especially the anomalies at the site of Ni- Mo-mineralized layer could be attributed to a history regulation of ocean, therefore they can be used to subdivide the “pretrilobite series” in South China, while the anomalies at other site can only be used as symbol of stratigraphic correlation. |
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