Geochemical and Sedimentary Setting of the Ordovician in Central Hunan

Abstract

Due to the comprehensive tectonic transformation, the type of the basin of early Paleozoic in the central Hunan area has been a controversial issue. The Ordovician is an important transitional period for the basin type of the early Paleozoic in the south China. The widely distributed black siliceous rocks of the middle Ordorvician, with stable chemical properties, can well indicate the sedimentary and tectonic evolution of the basin. This paper synthesized the major and rare earth elements (REE) of cherts from Maduqiao (MDQ) section in Qidong county and Mianhuaping (MHP) section in Ningyuan county. Chert samples in MDQ section shows that the SiO₂ content ranges from 90.36 to 94.73%, with the average of 92.91%, AI₂O₃ content is in the range of 2.36~3.88%, the average is 2.73%, Fe₂O₃ content is from 0.61 to 1.21%, the average is 0.90%, Simples in MHP section showed SiO₂ values (89.40~91.90%, average 90.76%) , AI₂O₃ values (2.47*9~4.42%, averaged 3.43%) and Fe₂O₃ values (0.72~1.71%, average 1.12%) , which represent that the terrigenous clastic content in these chert samples is very low . The Al/(Al+Fe+Mn) ratios in MHP and MDQ is 0.54~0.83 and 0.65~0.79 respectively, which demonstrated that cherts in the two regions are biogenic and have little relation with hydrotherm. The Al/(Al+Fe) ratios in MHP and MDQ are 0.55~0.83 and 0.65~0.79, which means that the cherts have been mainly deposited in environment of the continental margin. Plotting diagrams of 100TFe₂O₃/SiO₂- AI₂O₃/(AI₂O₃+TFe₂O₃), TFe₂O₃/(100-SiO₂)- AI₂O₃/(100-SiO₂) and LaN/CeN- AI₂O₃/(AI₂O₃+TFe₂O₃), showes that
most of the cherts have deposited in the continental margin and a small amount deposited in the transition regime from the margin to pelagic environment. The concentration of the ΣREE in MHP (36.07 ~ 98.84) and MDQ (27.87~60.02), were far below the ΣREE concentration of the North American average shale (NASC), which suggested that the speed of cherts deposition in the shelf environment is higher. And (La/Yb) _N values were 0.72~1.30, and 1.05~1.30 and the enrichment of LREE was not obvious, mean that the sedimentation was not impacted by the terrestrial sources very much. And the negative to positive Ce anomalies (0.89~1.46) represented an anoxia and reducing environment. Combining with the previous research and chemical characteristics of the cherts in adjacent regions, this paper has concluded that: the south China ocean in the central Huanan was closed in the late Neoproterozoic and left a multi-island sea basin similar to the Mediterranean. Besides the early Cambrian, the basin was not very deep. Intracontinental down thrusting by the Cathysia at the beginning of the middle Ordovician, and the basin began to subsidy and transgress. Islands provided a small amount of clastics and obstructed circulation of water, making the sedimentary environment both with the characteristics of the continental margin and limited condition where the cherts deposited. After all, the whole area has been transformed to be fold belt in the late Silurian.