华北克拉通西部鄂尔多斯地块长城系碎屑锆石LA-ICP-MS U-Pb测年及意义

LA-ICP-MS U-Pb Dating of Detrital Zircons from Changcheng System in Ordos Block,Western North China Craton and Its Implications

自1.85 Ga西部陆块与东部陆块沿中部造山带碰撞拼合形成统一的华北克拉通之后,中—新元古代克拉通经历了广泛的陆内拉伸,形成了熊耳裂陷槽、燕辽裂陷槽、渣尔泰—白云鄂博—化德裂陷槽及东缘裂谷系。前人根据钻井资料及区域地层对比认为鄂尔多斯地块变质基底之上沉积了中元古代长城系—蓟县系。其中,长城系下部主要为石英岩夹板岩,上部为粉砂质板岩、硅质板岩及含燧石条带白云质灰岩与石英砂岩;蓟县系以白云岩为主,夹少量砂岩、页岩。本文对采自鄂尔多斯地块6口钻井中的长城系进行了碎屑锆石年代学研究。结果表明,鄂尔多斯地块长城系浅变质沉积岩的碎屑锆石年龄组成了1.60 Ga、1.85 Ga、1.95 Ga、2.35 Ga和2.50 Ga等峰值。与华北克拉通内部中—新元古代沉积岩碎屑锆石年龄峰值对比结果表明,长城系沉积岩的碎屑物质来自华北克拉通内部。结合区域钻井资料及前人研究成果,推断中元古代鄂尔多斯地块北缘及东缘为隆起剥蚀区,西南部为沉积区,其西南缘为被动大陆边缘,与北秦岭地体之间被宽坪洋分隔。

The formation of the North China Craton （NCC）was resulted from the collision of the Western and Eastern blocks along the Trans-North China Orogen at ～1 .85 Ga.Then the NCC experienced intracontinental extension during Meso- to Neoproterozoic,which resulted in the formation of the Xiong’er rift,Yanliao rift, Zhaertai—Bayan Obo—Huade rift and Eastern margin rift.According to analysis of drilling data and strata correlation,some researchers suggested that Mesoproterozoic Changcheng—Jixian systems developed in the Ordos block.The Changcheng System is composed of quartz interlayered with slate in the lower part and silty slate,silicon slate,chert bend bearing dolomitic limestone,quartz sandstone in the upper part.The Jixian System is dominated by dolomite,and interlayered with sandstone and shale stone.Detrital zircon dating of the Changcheng System from 6 drillings has been carried out by this paper.The detrital zircons of the sedimentary rock samples from the Changcheng Systems show 4 prominent n（^207 Pb）/n（^206 Pb）age peaks of 1 .6 Ga,1 .85 Ga,1 .95 Ga,2.35 Ga and 2.5 Ga,which is similar to the age distributions of detrital zircons of Meso- to Neoproterozoic in the NCC,and indicate similar provenance.According to this,we conclude that the detritus of the Changcheng System were sourced from the inner NCC.Accompanied with the drilling data and materials obtained by other researchers,we suggested that the north and east margin of the Ordos block was uplift erosion area during Mesoproterozoic,and south-western part was sedimentary area,whereas the southwest margin was a passive continental margin and separated with the North Qinling terrain by the Kuanping ocean.