滇中新元古代澄江组层型剖面锆石U-Pb年代学及其地质意义

摘要

澄江组是华南新元古代地层中的重要地层单元之一,其确切沉积时限对于建立和完善华南新元古代区域地层格架具有关键意义.对滇中澄江组层型剖面顶部的凝灰岩夹层进行高精度SHRIMP锆石U-Pb定年研究,获得3组有效的206Pb/238U加权平均年龄值,其分别为(819±14) Ma (MSWD =0.15)、(781±11) Ma (MSWD=0.24)和(725±11)Ma(MSWD=0.65).其中,最年轻的一组年龄值(725±11) Ma被解释为澄江组顶部凝灰岩夹层的形成时间,可以代表滇中澄江组的顶界年龄,从而进一步确认澄江组的沉积时限为800～725 Ma.结合相关研究资料,证实澄江组与开建桥组、莲沱组的沉积时限基本相当,三者与下冰期长安组不存在对比关系,而应与冰期前板溪群的上部进行对比.此外,推测两组年龄值较老的锆石可能是与新元古代罗迪尼亚(Rodinia)超大陆裂解有关的幕式岩浆活动的记录.%The Chengjiang Formation is one of the most important Neoproterozoic stratigraphic units in South China; therefore,its accurate depositional age is significant for establishing and completing the Neoproterozoic regional stratigraphic framework of South China.The tuff interbed from the topmost part of the stratotype section of the Chengjiang Formation in Central Yunnan was dated by SHRIMP zircon U-Pb dating.Three groups of weighted mean 206pb/238U ages were obtained,which are (819 ± 14) Ma (MSWD =0.15),(781 ± 11) Ma (MSWD =O.24) and (725 ± 11) Ma (MSWD =0.65),respectively.The youngest age ((725 ± 11) Ma) is interpreted as the formation time for the tuff interbed from the topmost part of the Chengjiang Formation,which can represent the top boundary age of the Chengjiang Formation in Central Yunnan.Thus,it is further confirmed that the Chengjiang Formation was deposited between 800 and 725 Ma.Combined with related research data,it is proved that the Chengjiang Formation is equivalent to the Kaijianqiao Formation and Liantuo Formation,and all the three formations should correlate with the upper part of the preglacial Banxi Group,rather than the glacial Chang'an Formation.In addition,it is also inferred that the both groups of zircons with older age might be records of the episodic magmatism related to Neoproterozoic break-up of the Rodinia supercontinent.