河北平山小觉地区阜平群变质作用和深熔作用同位素年代研究的初步报道

Preliminary Study on the Isotopic Age of Metamorphism and Anatexis of the Fuping Group of the Environs of Xiaojue, Pingshan County, Hebei Province

河北平山小觉地区早前寒武纪阜平群的部分岩石,在经历了区域变质高峰之后,又遭受了差异性深熔作用的改造。笔者采用单颗粒锆石U-Pb稀释法测年技术,对变质岩及深熔作用改造形成的岩石,进行了同位素年代研究。认为不一致线上交点数值即为相应岩石所经受的变质作用或深熔作用的年龄。测定结果说明小觉及其以东地区三种岩石所经历的区域变质作用的年龄,不小于2.55±0.04 Ga,小觉附近深熔岩石(花岗闪长质岩石)的深熔年龄为2.47±0.09Ga。因而变质作用和深熔作用都可划归阜平期。小觉西南下康家峪两块标本的测定结果,提供了该地区阜平群岩石在2.06～2.08 Ga经历了古元古代时期重熔作用改造的信息。

Part of the metamorphic rocks of the Early Precambrian Fuping Group of the study area have been subjected to differential anatexis shortly after the climax of regional metamorphism. Isotopic age studies of the zircons from both the metamorphic rocks and their anatectic derivatives by the single crystal U-Pb method have been carried out by the authors. The upper intercept age of the U-Pb concordia has been interpreted as the age of metamorphism or anatexis of the samples concerned. The results of the isotopic determination of zircons from the samples of three different rock types collected near Xiaojue and in the area further east (Fig. 1) give the age of regional metamorphism not younger than 2. 55±0. 04 Ga, and those of the anatectic rock of gran-odioritic composition collected just east of Xiaojue, value of 2. 47±0.09 Ga for the age of anatexis. Thus both the metamorphism and anatexis are of the Neo-Archaean Fupingian movement. And the results for two samples from Xiakangiayu, southeast of Xiaojue give an approximate age around 2. 06-2. 08 Ga for another episode of anatexis in the Paleo-Proterozoic time.