浙西松木坞群的解体——同位素定年的证据

THE BREAKUP OF THE SONGMUWU GROUP IN WESTERN ZHE JIANG: EVIDENCE FROM ISOTOPE DATING

松木坞群是浙西前震旦纪重要地层单元,其上部主要为玄武岩-流纹岩组合,中、下部主要为灰紫色、灰绿色滨海砂屑岩。现对其上部酸性火山岩中的单颗粒锆石测得U-Pb同位素年龄值827Ma和809Ma,由此确定它相当于杭州—萧山地区的上墅组火山-沉积岩单元。中、下部的砂屑岩组合相当于虹赤村组。根据岩石地层单元清理原则,建议放弃使用松木坞群名称,而将晚元古代江南造山带中这一岩石地层单元统称为上墅组和虹赤村组。

The Songmuwu Group is an important pre-Sinian stratigraphic unit in western Zhejiang. Its upper part is mainly composed of the basalt-rhyolite association whereas the middle and lower parts consist mainly of grayish purple and grayish green. The U-Pb dating of single-grain zircon from acid volcanics in its upper part yields ages of 827 Ma and 809 Ma. Thus it is determined that the upper part is equivalent to the volcanic - sedimentary unit of the Shangshu Formation in the Hangzhou-Xiaoshan area. The arenite association in the middle and lower parts should be the equivalent of the Hongchicun Formation based on the comparison of lithologic associations. In order to simplify the names of the stratigraphic units in the Proterozoic Jiangnan orogen it is suggested that the name of the Songmuwu Group should be abolished and that this lithostratigraphic unit be called the Shangshu and Hongchicun Formations.