论板溪群与板溪蛇绿混杂岩

THE BANXI GROUP AND THE BANXI OPHIOLITE MELANGE

华南大地构造问题已经引起了国内外地质界的普遍关注,板溪群的问题是其中的关键。本文从地层、构造、同位素年龄和古生物等方面对板溪群及有关问题做了讨论,认为按多重地层划分原则定义的板溪群是一套正常的沉积地层;原带状大“板溪群”中含蛇绿岩的变质岩系是前震旦纪的产物;板溪(中生代)蛇绿混杂岩的概念难以成立。

The Banxi Group, a significant stratigraphic unit in south China, has attracted wide attention of geologists in the world since Kenuth J. Hsu and his colleagues put forward their interpretation of the Banxi (Mesozoic) Ophiolite Melange with their Mesozoic collision tectonic model for South China in 1980's. After a field study and literature examination back to 1936 when it was named, the authors believe that it is reasonable to redefine the Banxi Group as a lithostratigraphic unit composed of the Madiyi and Wuqiangxi Formations in the Xuefeng-Wuling Mountains area. Hsu's interpretation is based on the large 'Banxi Group'suggested in 1960's and 1970's, which occurs in a belt form in the Jiangnan stratigraphic province. The Banxi Group and Hsu's 'Banxi Melange/Group' are different in conception. The former is only a part of the latter. Some questions arise consequently: Is all the large'Banxi Group' a melange? Which part of it is melange? And was the Melange formed during the Mesozoic collision as prosposed in Hsu's interpretation?The Banxi Group redefined according to the principle of multiclassification is a normal set of sedimentary rocks deposited on passive continental margins. The lower subgroup of the large 'Banxi Group' is a metamorphic rock series with exotic ophiolite blocks. Further geological study is required to call the subgroup as an ophiolite melange. But it is clear that the metamorphic rocks were formed before the Sinian: 1 ) they are overlian unconformably by the Sinian; 2 ) chrono- logical and paleaontologic data tell us that they were formed in the Precambrianj and 3 ) according to the present authors' investigations on the tectonic windows and klippen given by Hsu et al, none of them can be taken as the evidence for Hsu's Mesozoic collision tectonic model for South China and the Banxi (Mesozoic) Ophiolite Melange. There is no Mesozoic melange in the 'large Banxi Group' and even in South China. Although a melange can be called a group or complex according to multiclassification, the term 'Banxi (Ophiolite) Melange' is not suitabale because: 1 ) it is earlier to name the lithostratigraphic unit composed of the Madiyi and Wuqiangxi Formations as the Banxi Group; 2 ) the Banxi Group and the metamorphic rocks with exolitic ophiolite blocks in the large'Banxi Group' are different stratigraphic bodies; 3 ) the Banxi Group is a normal sedimentary sequence in Banxi Town where it was named; and Hsu's Banxi Ophiolite Melange has a wrong age concept of Mesozoic collision between the Yangtze and Huanan blocks and orogeny.