华北克拉通宣龙式铁矿锆石U-Pb年龄及其对矿床成因的制约

华北克拉通宣龙式铁矿是中国北方最重要的沉积型铁矿类型,形成于元古宙中期(1800~800Ma)。河北大岭堡地区鲕状赤铁矿石发育大量碎屑锆石,对这些碎屑锆石及庞家堡地区侵入串岭沟组的花岗岩脉进行了LA-ICP-MS锆石U-Pb年龄分析。鲕状赤铁矿石碎屑锆石获得了2组主要的峰值年龄,分别为1873Ma和2530Ma,记录了华北克拉通约1850Ma和约2500Ma两次构造热事件,结合前人研究,表明其与围岩具有基本一致的碎屑锆石源区,与北京十三陵地区串岭沟组源区略有差异,推测宣龙式铁矿可能为华北克拉通响应Columbia超大陆裂解的产物。花岗岩脉锆石U-Pb年龄为202.3±1.4Ma(n=27,MSWD=0.96),表明区内发育印支期岩浆侵入活动,暗示磁铁矿石可能并非前人认为的燕山期的产物,其成因还需要进一步深入研究。

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.