河北阜平杂岩中阜平岩群浅粒岩锆石U-Pb-Hf同位素特征及其地质意义

Zircon U-Pb-Hf isotopic characteristics of leptite in the Fuping Group,Hebei Province, and its geological significance

华北克拉通中部阜平地区的阜平岩群是该区分布较广的太古宙地层,经历了角闪岩相-麻粒岩相变质,其时代限定对研究阜平杂岩的早期演化过程具有重要意义。本文利用LA-MC-ICP-MS(多接收激光剥蚀电感耦合等离子体质谱仪)对阜平岩群元坊岩组中的浅粒岩进行了锆石U-Pb-Hf同位素原位分析,获得两期变质锆石年龄分别为2 531±15 Ma和1 943±16 Ma,并根据碎屑锆石内部结构特征和年龄结果,认为核部年龄在2 549~2 500 Ma的碎屑锆石中最大207Pb/206Pb年龄2 549±4 Ma可以代表原岩的最大沉积年龄,初步限定元坊岩组浅粒岩原岩沉积时代为2 550~2 530 Ma。锆石Lu-Hf同位素结果中,176Lu/177Hf值为0. 000 289~0. 004 262,176Hf/177Hf值为0. 281 255~0. 281 791。176Hf/177Hfi值为0. 281 230~0. 281 623,εHf(t)值为-5. 86~13. 62,变化范围较大。单阶段和两阶段模式年龄分别为2 748~2 242 Ma和2 810~2 272 Ma。根据锆石U-Pb年龄和Hf同位素结果,提出元坊岩组浅粒岩物源区主要来自新太古代TTG质片麻岩,2. 8~2. 6 Ga为阜平地区强烈的地壳生长阶段,阜平地区2. 5 Ga和1. 95 Ga变质信息分别代表华北初步克拉通化和最终克拉通化过程中的变质作用。

The Fuping Group, which has undergone amphibolite to granulite facies metamorphism, is the most widely distributed Archean strata in Fuping area of the Central North China Craton. The age of the Group is vital for probing the evolutionary history of the Fuping Complex. In this paper, LA-MC-ICP-MS( laser ablation multicollector inductively coupled plasma mass spectrometry) was used to analyze the zircon U-Pb-Hf isotope of leptite in the Yuanfang Formation of the Fuping Group. The authors obtained two metamorphic zircon ages of 2 531 ± 15 Ma and 1 943 ± 16 Ma, respectively. Based on the internal structure of zircons and U-Pb ages, the authors chose the207 Pb/206 Pb age of 2 549 ± 4 Ma from the 2 549 Ma to 2 500 Ma detrital zircon domains as the maximal depositional age and confirmed the Yuanfang Formation between 2 550 Ma and 2 530 Ma. The zircon Lu-Hf isotopic analyses demonstrate that176 Lu/177 Hf and176 Hf/177 Hf are 0. 000 289 ~ 0. 004 262 and 0. 281 255 ~ 0. 281 791, respectively with176 Hf/177 Hfiand εHf( t) range being 0. 281 230 ~ 0. 281 623 and-5. 86 ~ 13. 62. The single-stage and twostage model ages are in the range of 2 748 ~ 2 242 Ma and 2 810 ~ 2 272 Ma, respectively. On such a basis, the authors infer that the source of the Yuanfang Formation was mainly from Neoarchean TTG gneisses in the Fuping Compex, and the intensive crustal growth in the Complex was concentrated from 2. 8 Ga to 2. 6 Ga. In addition, the authors infer that metarmorphic ages of 2. 5 Ga and 1. 95 Ga may represent metamorphic events related to the initial and finial cratonization of the North China Craton.