三峡地区新元古代莲沱组底部凝灰岩锆石SHRIMP U-Pb年代学及其地质意义

湖北三峡地区发育一套新元古代地层,其可能记录了地球系统的一系列剧变,如莲沱组可能记录了与Rodinia超大陆的裂解以及南华冰期起始时间等相关的地质信息。然而,莲沱组的沉积年龄及区域地层对比却仍存在较大争议,限制了我们对这些地质信息的理解。本文从花鸡坡剖面莲沱组底部凝灰岩层获得两组SHRIMP U-Pb锆石年龄(810±7)、(763±10)Ma,前者可能为莲沱组下伏黄陵花岗岩剥蚀后再沉积的结果,而后者应代表莲沱组底部沉积开始的年龄。结合前人在三峡地区对莲沱组地层年代学的研究,认为莲沱组沉积年龄为780~714Ma,区域上可与滇中、川西澄江组/开建桥组及湘北的板溪群、黔东南的下江群及桂北的丹洲群进行对比,但其缺失下江系早期—中早期(820~790 Ma)沉积地层,因此只相当于下江群、丹洲群的中上部地层。

A SHRIMP U-Pb Zircon Geochronology of a Tuff Bed from the Bottom of Liantuo Formation in the Three Gorges Area and Its Geological Implications

The Mid-late Neoproterozoic strata in the Three Gorges area may record a series drastic changes on the earth system.Although the Liantuo Formation may experience the breakup of the Rodinia and the start age of the Nanhuan glaciation,its age and regional stratigraphic correlation are controversial.In this study,we obtained two SHRIMP U-Pb age of (810 ±± 7) Ma and (763± 10) Ma from a tuff bed in the bottom of the Liantuo Formation.The older one may represent the resedimentation of the Huangling granites.Together with the reported ages,we suggest that the Liantuo Formation in the Three Gorges area was deposited during 780-714 Ma.The regional stratigraphic correlation suggests that the Liantuo Formation is related to Chengjing/Kaijianqiao Formation and the Banxi,Xiajiang and Danzhou Group.Whereas,due to the lack of the early stage deposition (820-790 Ma),the Liantuo Formation can be related only to the mid-upper part of the Xiajiang Group and Danzhou Group.