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华夏地块基底变质岩同位素年龄数据评述

沈渭洲   

  1. 南京大学地球科学系,南京 210093
  • 收稿日期:2006-12-20 修回日期:2006-12-20 出版日期:2006-12-20 发布日期:2006-12-20

Comment on the Isotopic Age Data of Basement Metamorphic Rocks in Cathysia Block

Shen Wei-zhou   

  1. Department of Earth Sciences, Nanjing University, Nanjing, 210093,China
  • Received:2006-12-20 Revised:2006-12-20 Online:2006-12-20 Published:2006-12-20

摘要: 根据基底正变质岩原岩的同位素年龄、地壳岩石中继承锆石U-Pb年龄和变质沉积岩的Nd模式年龄数据, 华夏地块存在一个主要由古元古代和中元古代地壳组成的变质基底. 我们在使用文献中报道的Sm-Nd等时线年龄数据时要慎重, 必须根据同源、同时、封闭和具有合适母子体比值的等时线判别原则对其合理性进行鉴别的基础上才能确定取舍. 继承锆石U-Pb年龄和Nd模式年龄都大于基底变质岩的原岩形成年龄, 因而它们不能代表基底的地层年龄. 对于继承锆石U-Pb年龄和Nd模式年龄反映的华夏地块广大区域内存在太古代地壳再循环组分, 不能笼统认为来自遥远的华北地块而排除华夏地块本身存在太古代地壳的可能性. 华夏地块是否存在太古代地质体, 应引起我们高度重视, 值得进行进一步研究和确定

Abstract: Data of isotope dating for the prolith of orthometamorphic rocks and for the relic zircons of crustal rocks and data of Nd isotope model age for metasedimentary rocks suggest that in the Cathysia Block there is a metamorphic basement mainly consisting of Paleoproterozoic and Mesoproterozoic crust rocks. Cautions have to be taken when citing Sm-Nd isochrons reported in early literatures in China. Reevaluation has to be made to these isochron by isochron criteria that samples of an isochron must have the same origin, the same formation time, closed isotope system since rock formation and large enough variation in parent/daughter ratio values. Since U-Pb ages of relic zircons and Nd isotope model ages of metasedimentary rocks are greater than ages of original rock of the metamorphic rocks, the formers do not reflect deposition age of strata in basement. The U-Pb ages of relic zircons and Nd isotope model ages of metasedimentary rocks suggest that there were recycled Archean crust components in many regions within the Cathysia Block. These recycled Archean crust components can not be regarded as exclusively transported from the remote North China Block. It is worth while to pay more attention on and to study the issue that whether there is Archean terrain in the Cathysia Block.