河北宣化姜家寨铁矿床串岭沟组底部碎屑锆石LA-MC-ICP-MS U-Pb年龄及其地质意义

宣龙式铁矿是我国北方最重要的沉积型铁矿床。华北克拉通长城系串岭沟组底部砂页岩是宣龙式铁矿床的赋存层位,对该地层的年代学研究有助于深入完善长城系地层年代框架、认识区域成岩成矿过程并反演克拉通的演化历史。本文对河北宣化姜家寨铁矿床串岭沟组底部铁矿体顶板砂页岩中碎屑锆石进行了LA-MC-ICP-MS锆石U-Pb年代学研究,获得了三组主要的峰值年龄,加权平均年龄分别为1774.1±7.9Ma、1849.0±7.8Ma和2453.0±7.8Ma。揭示出华北克拉通中部带经历了三期较为重要的地质构造、岩浆作用和变质作用事件。在串岭沟组下部砂岩中我们获得了4颗较年轻的岩浆碎屑锆石,年龄为1657.4～1694.4Ma,代表了串岭沟组底部形成的时间下限,制约了姜家寨宣龙式铁矿床的形成时代不早于1657Ma。对比研究得出,1774.1Ma的峰值年龄数据与华北克拉通内18亿年后广泛发育的基性岩墙群的形成时代一致,代表了华北克拉通的在拼合后的抬升事件时间。在1.8～1.6Ga华北克拉通拉张期间形成了大庙式等钒钛磁铁矿-磷灰石铁矿(1720Ma左右)。基于时间和区域的一致性,我们推断,遭受抬升剥蚀的富铁基性岩墙群不仅是串岭沟组的物源之一,极有可能也是宣龙式铁矿床中铁质的主要物源之一。

U-Pb dating study of detrital zircons from the Chuanlinggou Formation in Jiangjiazhai iron deposit, North China Craton and its geological significances

Chuanlinggou Formation of the Changcheng System is the major occurrence layer of the Xuanlong type iron deposits in North China Craton (NCC). In-depth study of this formation will improve the stratigraphic framework, enhance the understanding of petrogenesis and metallogenesis, and reveal the evolution of the NCC. Detrital zircon LA-MC-ICP-MS U-Pb dating were performed on the sandshale from the lower part of the Chuanlinggou Formation in Jiangjiazhai iron deposit, the representative of the Xuanlong type iron deposits. Three peak ages were gained, with the weighted average ages 1774.1±7.9Ma, 1849.0±7.8Ma and 2453.0±7.8Ma respectively. These indicate that the NCC had undergone three significant tectonic-magmatic-metamorphic events. In addition 4 younger ages of magmatic zircons been obtained, and the age range is 1657.4~1694.4Ma, which marks the lower limit age of the Chuanlinggou Formation, and constrain the geochronology of iron mineralization in Jiangjiazhai would not be older than 1657Ma. The peak age 1774.1Ma is in accordance with the age range of the widespread mafic dyke swarms that represent the initial rift event of NCC from the Columbia Supercontinent. The Damiao type (vanadium-titanium magnetite-apatite) iron deposits (~1720Ma), formed in an extensional tectonic regime period, have a closely relationship with anorthosite-norite (gabbro), which have the similar origin with the mafic dyke. Based on the accordant geochronology and regional feature, we infer that the mafic dyke swarms, which underwent uplift and erosion and been the sediment source of Chuanlinggou Formation, might be one of the significant iron sources of Xuanlong type deposits.