北京密云地区放马峪条带状铁建造的形成与变质时代:锆石SHRIMP U-Pb定年

摘要

This paper reports zircon SHRIMP U-Pb ages for magnetite-garnet-plagioclase gneiss from host rocks in the Fangmayu iron deposit,Miyun of Beijing.Zircon 207pb/206pb ages of the gneiss we mainly obtained are (1 817 ± 13) Ma and (2 456 ±6) Ma,respectively.Based on the cathodoluminescence images of zircons,together with the petrography of rocks,the ages represent two stages of metamorphism of the banded magnetic quartzite,corresponding to similar ages of two large-scale metamorphic events (namely ca.2.44 Ga and ca.1.82 Ga) reported from the eastern block of North China Craton.Then,we also found some owning oscillatory zoning zircons with 207 pb/206pb age of (2 529 ± 8) Ma,which represent the age of the detrital zircon from the magnetite-garnet-plagioclase gneiss.It also indirectly constrains that the banded iron formation was formed between (2 456 ± 6) to (2 529 ± 8) Ma,most likely at the end of Neoarchean by combining previous research data.The results also provide further detailed information for research of early tectonic evolution of North China Craton and iron deposits (BIF) in China.%通过对北京密云地区放马峪条带状铁矿矿石围岩中含磁铁矿石榴子石斜长片麻岩进行SHRIMP锆石U-Pb测年研究,主要获得3组锆石207 pb/206Pb年龄,分别为(1 817±13) Ma、(2 456 ±6) Ma和(2 529±8)Ma.结合其岩相学、阴极发光图像分析,其中具有变质成因的锆石年龄分别为(1 817±13) Ma和(2 456±6)Ma,代表了该区条带状铁建造的两期变质时代;而另外1组年龄为(2 529±8)Ma的锆石,具岩浆锆石韵律环带特征,代表了该含磁铁矿石榴子石斜长片麻岩中碎屑锆石的年龄,从而间接限定了该区条带状铁建造的形成时代为(2 456±6) Ma到(2 529±8) Ma之间.结合前人研究资料,认为其可能形成于新太古代晚期.这些成果为该区乃至整个华北地区前寒武纪BIF型铁矿以及晚太古代至古元古代的构造演化提供了进一步详实的年代学资料.