【作者】 江博明； 张宗清；

【Author】 Borming Jahn~*and Zhang Zongqing~(**) *CAESS,Universite de Rennes,Institut de Geologie Campus de Beaulieu 35042 RENNES Cedex,France.**Institute of Geology,Chinese Academy of Geological Sciences,Baiwanzhuang Road,Beijing,China.

【机构】 法国Armorican基底构造研究中心,雷恩大学地质研究所； 中国地质科学院地质研究所；

【摘要】 <正>一、引言中国北部广泛分布着太古代岩石,这些岩石构成了中朝准地台的主要组成部分。位于河北省东部的太古代岩层属迁西群,长期以来它一直被认为是中国最古老的地层。迁西群主要由基性－酸性麻粒岩组成,角闪岩相岩石也有产出。虽然在1981年以前已获得了许多同位素年龄数据,但是,还没有得到任何肯定的论据以确认迁西群是属于早太古代的。传统的看法是,在一个多次遭受变质作用的地区,变质程度越高的岩石年龄越老。然而, 我们新近获得的 Rb-Sr 和 Sm-Nd 同位素分析结果表明,迁西群麻粒岩的年龄不属于早太古代(≥3．5AE。1AE=10亿年,即≥3500Ma),而是比较年轻的(～2500Ma,Ma 为百万更多还原

【Abstract】 The granulitic gneisses and their metromorphic products of the Qianxi Group from eastern Hebei Province,China,have been investigated for their isotope and trace element geochemistry.A consistent age of about 2.5 AE had been obtained by the Rb-Sr and Sm-Nd whole-rock isochron methods in the present study and by the zircon U-Pb method （Pidgeon,1980）.With the consideration of initial isotopic ratios (I_Srand I_Nd) and elemental distribution patterns,particularly REE and K/Rb and Rb/Sr ratios,we conclude that this age of about 2.5 AE represents the time of granulite facies metamorphism,which must have followed closely the primary emplacement of their protoliths.Pre- vious claims for early Archean ages（≥3.5AE） are not substantiated by the present analyses.The granulitic gneisses have also recorded in their mineral isotope systematics an important thermal event at about 1.7 AE,roughly corresponding to the period of widespread Luliang Orogeny （Ma and Wu, 1981）or Chungtiao Movement （Huang,1978）.更多还原