侵入梵净山群白岗岩锆石U-Pb年龄及白岗岩底砾岩对下江群沉积的制约

SHRIMP U-Pb dating of intrusive alaskite in the Fanjigshan Group and alaskite basal conglomerates: constraints on the deposition of the Xiajiang Group

下江群代表了新元古代末期的新裂谷沉积的起点,确定下江群的时代对新元古代构造运动有着重要的地质意义。贵州印江县地区有一套新元古代白岗岩侵入到梵净山群并被下江群覆盖,在下江群底砾岩中发现大量未经搬运的白岗岩底砾岩砾石。因此,笔者对梵净山地区的白岗岩进行锆石U-Pb年代测定,获得锆石U-Pb年龄(835±5)Ma。该年龄对下江群沉积启动时间和下覆梵净山群的沉积上限均有时代的制约。结合下江群甲路组斑脱岩锆石U-Pb年龄(814±6 Ma)和考虑到武陵运动的年限,下江群沉积年代不应老于白岗岩底砾岩的年代和武陵运动的界面。

The deposition of the Xiajiang Group represents a new rift in late Neoproterozoic.The determination of the age of the Xiajiang Group is of great importance for the study of the Neoproterozoic tectonic movement.A suite of Neoproterozoic alaskite intruded into the Fanjiangshan Group and was covered by the Xiajiang Group,which developed large quantities of alaskite basal conglomerate.However,the alaskite zircon U-Pb age of 835±5 Ma will constrain the starting time of the deposition of the Xiajiang Group and the upper limit of the deposition of Fanjiangshan Group.Combined with the bentonite zircon U-Pb age of 814±6 Ma of the Jialu Formation in Xiajiang Group and considering the age of the Wuling Movement,the authors hold that the deposition age of the Xiajiang Group is not older than the age of the alaskite basal conglomerates.