论文部分内容阅读
对条带状铁建造(BIF)进行精确的年代学约束,具有非常重要的科学意义,可以获得BIF的时间分布趋势,反映对应时代的大气和海洋环境,准确建立与重大地质事件的对应关系,为铁建造的成因提供依据。本文在综合前人资料的基础上,重新绘制了全球BIF形成时代的分布图,结果表明BIF在3.6~1.6 Ga连续分布,在2.8~2.7 Ga、2.5~2.4 Ga、1.9~1.8 Ga的3个区间范围内形成高峰;Algoma型BIF主要集中于中-晚太古代,而Superior型BIF主要出现在古元古代。本文重点对前人关于BIF年代学的测试方法及其应用进行了介绍和评述,结果表明目前锆石U-Pb测年法依然为主要手段,而Sm-Nd等时线法和Re-Os等时线法尽管有效,但直接测定BIF条带的Sm-Nd等时线和磁铁矿的Re-Os等时线年龄仍存在较大问题,但可用来反映物质的来源以及后期构造热事件的改造等问题,而采取BIF相关单元的年龄来间接代表BIF的形成年龄是目前较为成熟的方法,其中测定铁建造火山岩夹层锆石U-Pb年龄是国际上测定BIF时代常用的手段。
Precise chronological constraints on BIF are of great scientific significance. They can obtain the temporal distribution trend of BIF, reflect the atmospheric and marine environment corresponding to the current era, accurately establish the corresponding relationship with major geological events, Provide the basis for the cause of iron construction. Based on the previous data, the paper redraws the distribution of the BIF era in the world. The results show that the BIF distributes continuously in the range of 3.6-1.6 Ga at 3 levels ranging from 2.8 to 2.7 Ga, 2.5 to 2.4 Ga and 1.9 to 1.8 Ga The peaks of Algoma-type BIF mainly concentrated in the mid-late Archean, and the Superior-type BIF mainly appeared in the Paleoproterozoic. This paper focuses on the predecessors on the BIF chronology test methods and their applications were introduced and reviewed, the results show that the current zircon U-Pb dating is still the main means, and Sm-Nd isochronism and Re-Os, etc. Although the time-line method is effective, the direct determination of the Sm-Nd isochron and the Re-Os isochron age of the magnetite are still problematic, but they can be used to reflect the source of the matter and the later tectonic thermal events It is a mature method to indirectly represent the formation age of BIF by adopting the age of BIF related units. The determination of zircon U-Pb ages of iron-built volcanic intercalations is a commonly used measure in the international BIF era.