鄂尔多斯盆地长城系碎屑锆石SHRIMP U-Pb定年和Hf同位素研究

The Changcheng System in Ordos Basin: Detrital zircon SHRIMP U-Pb dating and Hf isotope analysis

沉积盖层的碎屑锆石内部结构和年龄谱是了解碎屑沉积岩形成环境和物源区的重要研究对象。本文报道了鄂尔多斯盆地3口钻井中的4个长城系(变质)沉积岩样品的碎屑锆石SHRIMP U-Pb定年和Hf同位素分析结果。结果表明,主年龄峰值为~1. 8Ga,另外,在~2. 5Ga存在一个次年龄峰值,最年轻碎屑锆石年龄为~1. 6Ga,但存在大的误差。锆石的Hf同位素组成存在很大变化,εHf(t)值变化范围为-10. 5^+6. 7。结合前人研究,可得出如下结论:(1)鄂尔多斯盆地长城系底界沉积时代小于1. 7Ga,与华北克拉通东部的类似;(2)鄂尔多斯盆地长城系碎屑沉积物来自其北部古元古代孔兹岩系和/或鄂尔多斯变质基底本身;(3)鄂尔多斯盆地长城系与华北克拉通东部地区长城系形成于类似的构造环境,如大陆裂谷或坳拉槽。

The Changcheng System was widely developed in an extensional tectonic setting in the North China Craton after the formation of the Xiong ’er Group in the earliest Mesoproterozoic. The internal texture and age histogram of detrital zircons from sedimentary rocks are important for the understanding upon their formation backgrounds and provenance. This paper reports SHRIMP U-Pb ages and Hf isotope analysis of detrital zircons from four( meta-) sedimentary rock samples of the Changcheng System from three cores in the Ordos Basin,western China. According to their internal textures,two types of zircons can be divided,namely magmatic and metamorphic in origin. The detrital zircons have a major age peak at about 1. 8 Ga and a minor age peak at ~ 2. 5 Ga. The youngest detrital zircon is ~ 1. 6 Ga in age but with a large error. Zircons have different Hf isotopic compositions,with εHf( t) values ranging from-10. 5 to + 6. 7. Combined with previous work,the main conclusions can be drawn as follows:( 1) the bottom boundary age of the Changcheng System in Ordos Basin is younger than 1. 7 Ga,probably the same as that of the Changcheng System in eastern North China Craton;( 2) the detrital sediments of the Changcheng System in Ordos Basin are mainly derived from the Paleoproterozoic Khondalite Belt in the north and/or the Ordos metamorphic basement itself. The zircons with magmatic textures have εHf( t) values of-6. 8 ^+ 5. 2,suggesting that mantle addition to continental crust or mantle-continent mixture occur in the source region;( 3) the Changcheng System in the Ordos Basin and eastern North China Craton were formed in similar tectonic settings,such as continental rift or aulacogen.