皖南赣北宁国组和胡乐组沉积环境的古盐度特征及其地质意义

The palaeosalinity and its geological signification of the depositional environment of the Ningkuo and Hulo Formations from South Anhui and North Jiangxi

通过对皖南赣北地区早、中奥陶世宁国组和胡乐组合笔石黑色岩系的相当硼含量和镁铝比值的研究，揭示该时期皖南赣北海盆古海水具有不断淡化的历史；宁国组下部沉积环境的古盐度比宁国组上部以及胡乐组沉积环境的古盐度高；古海水发生明显淡化的转折期是在Ｕ．ａｕｓｔｒｏｄｅｎｔａｔｕｓ带（Ｎｒ）与Ｄ．ｅｌｌｅａｓｅ带（Ｎ８ａ）的界线附近；结合笔石动物群组合面貌的变化，古盐度有可能是早、中奥陶世笔石动物群演替的外界环境因素之一。

The palaeosalinity is one of the most useful gauges to reveal the change of depositional environment. Although there are many other methods for studying palaeosalinity, in this paper 111 geochemical samples were selected from the Early-Middle Ordovician Ningkuo and Hulo Formations of South Anhui and North Jiangxi, we make use of the geochemical characteristics to f the 'equivalent boron'and MgO/Al2O3 ratios to study the palaeosalinity and its geological signification of the depositional environment.According to Walker et al. (1962, 1963), it was suggested that the partition coefficient for the element Boron in shales is variable and depends on composition and grain size of the illites, and the palaeosalinity can be computed from the boron and K,O concentration of illites so called 'equivalent boron'. Furthermore it has been widely accepted that the 'equivalent boron' values more than 400×10-6 indicate the rocks were deposited in a salinized environment, between 300 and 400 ×10-6 in a normal marine salinity environment, ranging from 200 to 300 ×10-6 in a brackish or fresh-water environment and less than 200×10-6 in a low salinity environment. In this paper 14 samples collected from Wuuing, Yushan and Hule of the study area have been selected to analyse the content of Boron and K2O of the illites. The 'equivalent Boron' values computed from these analyses of both boron and K2O concentration in illites are ranging from 85 to 252 ×10-6, and it would indicate that the depositional water of the Ningkuo and Hulo Formations in the study area was a brackish salinity or a low salinity environment. In addition it has also been noted that the 'equivalent boron' values of these samples in the low part of the Ningkuo Formation (under the boundary between Undulograptus austrodentatus Zone (N7) and Didymograptus ellesae Zone (N8)) seem greater than those in the upper part of the Ningkuo Formation (up the boundary between N7/N8) and the Hulo Formation, and would suggested that the depositional environment of the low part of the Ningkuo Formation has a higher palaeosalinity than that of the upper part of the Ningkuo and Hulo Formations. The MgO/Al2O3 ratio (m) varies in marine sediments in a consistent fashion that can be directly related to depositional environment, and be regarded as a useful criterion for determxination of environmental palaeosalinity (Zhang, 1988). Three depositional environment types (based on the criteria m = 100×MgO/Al2O3) can be identified: low salinity environment with m <1, brackish salinity environment with 1<m<10, and normal salinity environment with m > 10. 97 samples of the Ningkuo and Hulo Formations from the study area have been analysed for MgO and Al2O3, so it provides more evidences for us with 97 values of MgO/Al2O3 ratios (m).It is very clear that the m values of rocks between the low part and the upper part of the Ningkuo Formation and the Hulo Formation are greatly different. The m values of the rocks are all more than 10 under the boundary of N7/N8 and all less than 10 above the boundary of N7/N8, so that similarly with the analysis result of equivalent boron above, it would furtherly indicate that the palaeosalinity of the depositional environment of the rocks in the low part of the Ningkuo Formation was higher than that of the rocks in the upper part of the Ningkuo Formation and the Hulo Formation. For this reason, we suggest that the depositional environment of the south Anhui-north Jiangxi basin has a desalinate history of palaeosea-water in the Early and Middle Ordovician.Based on the study of the 'equivalent boron' and MgO/Al2O3, it is not only revealed that the palaeosea-water of the south Anhui-north Jiangxi basin has a desalinate history, and on the other hand, the boundary between U. austrodcntatus Zone and D. ellesae Zone can be recognized to be a important boundary since round the boundary the changes of palaeosalinity of the depositional environment between the low part and the upper part of the Ningkuo Formation were greatly obvious. The great change of palaeosalinity co