Zircon U-Pb age of Xuanlong-type iron deposits in North China Craton and its constraints on genesis

Xuanlong-type iron deposit is the most important sedimentary-type iron ores in North China,which was deposited during the Mid-Proterozoic era(1800~800 Ma). Here we report the results of zircon U-Pb dating for the Xuanlong-type iron deposit. In this study,we have found abundant detrital zircons in the iron ores in the Dalingbu area,Hebei Province,and we have reported the LA-ICP-MS U-Pb ages of these detrital zircons. In addition,we also dating the granite veins which intruded into the Chuanlinggou Formation in Pangjiabu area. Combing previous studies,two peak detrital zircons ages can be observed,i.e.,the ages of 1873 Ma and 2530 Ma,respectively. These ages effectively record ca.1850 Ma and 2500 Ma tectonic thermal events of North China Craton,which indicated that the iron ores may share a similar zircon sources with the surrounding rocks in Xuanlong area. However,their sources are slightly different from that of the Chuanlinggou Formation in the Ming Tombs District,Beijing. Therefore,we speculate that Xuanlong-type iron ores may be the product of North China Craton in response to the breakup of the Columbia supercontinent. The U-Pb age of zircons from the granite vein was 202.3±1.4 Ma(n=27,MSWD=0.96),which suggests that the occurrence of Indosinian magmatic activity in the studied area. Meanwhile,this indicates that the formation of magnetite ores may not be related to the Yanshanian magmatism,and its genesis needs further study.