放射虫硅质岩对华南古地理的启示

放射虫硅质岩在中国南方及邻区广泛分布,有重要的古地理意义。按其时空分布,可分为南区及北区和东带。南区包括滇西、滇东南、桂西和桂南,主要为晚古生代至中生代早期放射虫硅质岩。滇西硅质岩带南延至泰国和马来西亚,代表古特提斯主支。滇东南、桂西和桂南的硅质岩则指示古特提斯多岛洋的分支海盆。北区包括两广中北部、湘赣中南部和长江中下游一带,主要为二叠纪中晚期放射虫硅质岩,标志当时扬子台缘与古特提斯洋连通的深水盆地。东带沿南海和东海外侧的岛弧分布,从菲律宾北巴拉望经琉球到西南日本内带,主要为中二叠世至晚侏罗世放射虫硅质岩,与古地磁证据一起指示了该期间华南南方一个低纬度的远洋盆地,可称为“古南中国海”。它的张开可能是中晚二叠世云开地体和中国东南部其他地方造山事件的原因,它的随后发展对华南东部三叠纪和侏罗纪古地理演化也有重大影响。晚侏罗世太平洋伊泽奈崎板块的迅速北移,使“古南中国海"俯冲消减,导致东南沿海大规模的钙碱性岩浆活动。

IMPLICATIONS OF RADIOLARIAN CHERT FOR THE PALAEOGEOGRAPHY OF SOUTH CHINA

Radiolarian chert,as an important environmental indicator, is widespread in South China and adje-cent areas. Based on its temporal and spacial distribution, two regions and one belt can be recognized. The southern region includes western and southeastern Yunnan as well as western and southern Guangxi. The Late Palaeozoic and Early Mesozoic radiolarian chert sequences occur there. Of which the chert belt in western Yunnan extends southward into Thailand and Malaysiya Peninsula, representing the main trend of the Palaeo-Tethys. The chert sequences in southeastern Yunnan as well as western and southern Guangxi indicate some branch marine basins of the Palaeo-Tethys. The northern region includes the central-north Guangxi and Guangdong, middle south Hunan and Jiangxi,as well as the middle and lower Yangtze areas. There the Middle-Upper Permian radiolarian cherts indicate the existence of deep basins along the Yangtze platform margin during that period and the seaway linked with the Palaeo-Tethys Ocean. The east belt appeared in the island arcs outside of the South China Sea and East China Sea,from the northern Bal-awan through the Ryukyu Islands to the inner belt of southwest Japan. There the Middle Permian to Upper Jurassic radiolarian chert sequences together with the palaeomagnetic evidences indicate a low-latitude pelagic marine basin located to the south of South China during that time. Then the marine basin can be called "Palaeo-South China Sea". The opening of the Palaeo-South China Sea might be the cause of the middle-upper Permian orogeny happened in the Yunkai terrane and other places in southeast China. Its later development also resulted in Triassic and Jurassic palaeogeographic changes in southeast China. The fast northward movement of the Izanagi plate in the Pacific during the Late Jurassic caused the under-thrusting and subducting of the Palaeo-South China Sea and resulted in the extensive calc-alkaline mag-matism along the coast area of southeast China.