鄂西走马地区南华纪大塘坡间冰期古气候研究

新元古代后期地球经历了至少两次全球规模的冰期,分别为Sturtian冰期和Marinoan冰期。鄂西走马地区南华系古城组与南沱组分别属于Sturtian和Marinoan冰期沉积,大塘坡组代表间冰期沉积。本文研究了走马地区ZK701钻孔岩芯大塘坡组细碎屑岩样品元素地球化学特征,计算了化学蚀变指数(CIA)、化学风化作用指数(CIW)、n(K)/n(Na)、n(Mg)/n(Ca)、Rb/Sr等古气候代用指标,探讨了研究区源区南华纪间冰期的古气候演化过程,结果显示:源区古城冰期晚期气候寒冷干燥(两件样品CIA值分别为57.1和58.1),大塘坡间冰期早期气候依然寒冷(CIA值介于56.5~64.6,均值59.8),大塘坡中晚期气候恢复温暖湿润(CIA值介于69.8~78.8,均值75.5);CIW、n(K)/n(Na)、n(Mg)/n(Ca)、Rb/Sr值等指标反映的古气候演化过程与CIA值反映一致。此外,对该钻孔大塘坡组下部的凝灰岩进行了LA-ICP-MS锆石U-Pb同位素年龄测定,获得了658.1±2.6 Ma(MSWD=1.5,n=28)的n(206Pb)/n(238U)加权平均年龄,结合前人对扬子板块大塘坡组底部凝灰岩的同位素年代学研究成果认为,大塘坡间冰期早期(663~658 Ma),扬子板块上南华盆地内海冰虽已消融、但古陆物源区气候依然寒冷。该时期扬子板块"源、汇"两区气候条件可能不同步,其原因涉及深层次的岩石圈、水圈、大气圈和生物圈的耦合关系。

Study on the paleoclimate during the Datangpo interglacial stage of the Nanhua Period in the Zouma area, western Hubei Province

Objectives:The Gucheng Formation and the Nantuo Formation of Nanhua System(≈Cryogenian System)in Western Hubei belong to Sturtian and Marinoan glacial deposits respectively,and the Datangpo Formation represents interglacial deposits.In this paper,the evolution of paleoclimate in the Datangpo interglacial period was studied,and the zircon U-Pb age of the tuff in the lower part of the Datangpo Formation was used to restrict the time of paleoclimate warming,and to provide the basis for the establishment of the time framework of the glaciation.Methods:In this paper,the main and trace elements of the fine clastic rock samples from the Datangpo Formation of the ZK701 drillcore in the Zouma area were measured,and the paleoclimate proxy indexes,such as CIA,CIW,n(K)/n(Na),n(Mg)/n(Ca),Rb/Sr,were calculated.Zircon U-Pb age of tuff was determined by LA-ICP-MS.Results:The CIA values of the two samples in the upper part of the Gucheng Formation were 57.1 and 58.1,respectively.The CIA values of the lower part of the Datangpo Formation were 56.5~64.6,with an average of 59.8,and the CIA values of the upper part were 69.8~78.8,with an average of 75.5.The evolution process of CIW,n(K)/n(Na),n(Mg)/n(Ca),Rb/Sr was consistent with the CIA.The n(206Pb)/n(238U)weighted average age of zircon in DTPN1 tuff sample was 658.1±2.6 Ma(MSWD=1.5,n=28).Conclusions:The climate of the provenance area in the Yangtze Plate was cold and dry during the late Gucheng glacial period.The climate in the early Datangpo interglacial stage was still cold,and the climate of the middle and late Datangpo stage was warm and humid.The paleoclimate warming time of the Datangpo interglacial was about 658 Ma,and the early Datangpo period in the Yangtze Plate(663~658 Ma)may still belong to the end of Sturtian.