华夏地块基底变质岩同位素年龄数据评述

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

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。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,应引起高度重视,值得进行进一步研究和确定。

Data of isotope dating for the protoliths of orthometamorphic rocks and for the relic zircons of crustal rocks and data of Nd isotope model ages for metasedimentary rocks suggest that in the Cathaysia 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 literature in China. Reevaluation needs to be made to these isochrons by isochron criteria that samples of an isochron must have the same origin, the same formation time, a closed isotope system since rock formation and a 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 rocks 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 Cathaysia Block. These recycled Archean crust components can not be regarded as exclusively transported from the remote North China Block. It is worthwhile to pay more attention to study the issue whether there is a Archean terrain in the Cathysia Block.