北大别片麻岩的锆石U-Pb年龄离子探针测定及其地质意义

本文对北大别西部的湖北英山万家老屋、罗田七道河尹家店和安徽金寨丁埠乡李集3个片麻岩样品中锆石进行了BSE显微结构分析,并在此基础上对锆石进行了离子探针U－Pb年龄测定。结果表明北大别存在两种类型的片麻岩,它们的原岩分别形成于晋宁期和燕山期,其中英山万家老屋片麻岩的形成年龄为794Ma,后期地质事件对其锆石U－Pb年龄造成的影响较小;七道河尹家店片麻岩的形成年龄为730Ma,并可能受到燕山期岩浆事件的影响,得到了140Ma左右的变质年龄;丁埠乡李集片麻岩的形成年龄为140Ma。结合已有的年代学结果,北大别地体广泛存在晋宁期的岩浆事件,表明其可能为扬子板块的北缘。

Zircon U-Pb Ion Probe Ages of Gneisses from the Northern Dabie Terrain and Their Geological Implications

Three gneiss samples were taken from Wanjialaowu at Yingshan and Yinjiadian at Qidaogou, Hubei Province, and Liji of Dingbuxiang, Anhui Province, in the western part of the northern Dabie Mountains. Backscattered electron (BSE) analysis of the microstructure of zircons from the three samples was performed, and on that basis zircon U-Pb ages were determined using the U-Pb ion probe. The results indicate that the protoliths of two types of gneiss in the study area were formed in the Jinningian and Yanshanian respectively. The protolith of the Wanjialaowu gneiss at Yingshan was formed at 794 Ma and its zircon U-Pb age was little affected by the late-stage geological event. The age of the Yinjiadia gneiss at Qidaohe is 730 Ma, and as it was probably affected by the Yanshanian magmatic event, a metamorphic age of about 140 Ma was obtained. The age of the Liji gneiss at Dingbuxiang is 140 Ma. From these ages combined with the available chronological results , it may be inferred that the Jinningian magmatic event occurred extensively in the northern Dabie terrain, which indicates that the area was likely to be the northern margin of the Yangtze plate.