Geochronological study of Zircons from high-grade gneisses of Fuping Complex by LP-ICPMS technique

Abstract

Zircons from different kinds of gneisses of Fuping Complex were analyzed using Laser-Probe-Inductively Coupled Plasma Mass Spectrometry (LP-ICPMS). For relatively simple zircons, LP-ICPMS 207Pb- 206Pb ages are comparable with TIMS' results. In polymetamorphic rocks, through analyzing population of zircon grains, and different locations within single grains, the LP-ICPMS technique can identify several generations of zircon growth process. Except ~2.7Ga age for small volume, less deformed Hb-bearing gneisses, Fuping high-grade gneisses had experienced the dominant ~2.5Ga magmatic event and two generations of metamorphic events, i.e. ~2.2Ga and ~1.9Ga.

利用激光探针等离子体质谱方法研究了阜平杂岩中几种不同类型片麻岩中的锆石样品。实验结果显示, 对于较简单的锆石群, 其207Pb-206Pb 数据分布的峰值年龄给出与常规锆石铅的TIMS 方法一致的结果; 通过分析大量的锆石颗粒, 可以弥补T IMS 方法分析数据较少的缺陷, 从而给出更全面的年龄分布特征。对于具有多期混合的锆石, 以及具有继承铅、多期增生历史的锆石, 通过对一个样品中多个颗粒的分析以及对单个颗粒的不同位置的分析, 可区分出不同期次地质事件的信息, 从而为阜平杂岩的形成、变质演化历史提供了年代学依据。研究结果为阜平杂岩中大量发育的灰片麻岩给出约2. 5Ga 的岩浆活动年龄信息, 出露面积较小的角闪斜长片麻岩给出了约2. 7Ga 的年龄信息, 同时还给出了约2. 2Ga 和约1. 9Ga 的两期变质事件年龄信息。