五台地区高凡群形成时代新证据:锆石SHRIMP U-Pb定年

Formation age of the Gaofan Group in Wutai area:New evidence from SHRIMP U-Pb zircon dating

五台地区高凡群对于华北克拉通早前寒武纪变质地层层序建立具有重要意义。本文对高凡群磨河组开展了地球化学和锆石年代学研究。变质沉积岩稀土总量为(132.78~231.84)×10^(-6)(样品W1927的稀土总量为1016.55×10^(-6)),轻重稀土分离弱((La/Yb)_(N)=4.5~12.5),具明显负铕异常(δEu=0.37~0.62),稀土模式与太古宙后泥砂质碎屑沉积岩十分类似。一个白云石英片岩样品(D004)的变质原岩中存在大量2.7 Ga、2.54~2.5 Ga、2.3 Ga和2.18 Ga碎屑锆石,最年轻锆石可能来自同时代火山岩。一个含黄铁矿白云片岩样品(W1927)的最年轻锆石年龄为2194 Ma,被认为代表了岩石形成时代。结合前人研究,可把高凡群形成时代限定在<2176 Ma和2350 Ma之间,碎屑物质主要来自恒山、五台、阜平、云中山、吕梁地区新太古代—古元古代变质基底或更远的地区。

The Gaofan Group in Wutai area is of great significance for the sequence establishment of Early Precambrian metamorphic strata in the North China Craton.This paper carried out the geochemical analysis and SHRIMP U-Pb zircon dating of the Mohe Formation of the Gaofan Group.The metamorphic sedimentary rocks have a total REE(rare earth elements)content of(132.78-231.84)×10^(-6)(except for sample W1927,which has a total REE of 1016.55×10^(-6)),with weak enrichment of LREE(light REE)relative to HREE(heavy REE)((La/Yb)_(N) between 4.5-12.5)and obvious negative Eu anomaly(δEu between 0.37-0.62),and are similar in REE composition to post-Archean argillaceous rocks.A muscovite-quartz schist sample(D004)has a large number of detrital zircons aged around 2.7 Ga,2.5 Ga,2.54 Ga,2.3 Ga and 2.18 Ga,with the youngest grains derived probably from contemporary volcanic rocks.A pyrite-bearing muscovite schist sample(W1927)has the youngest zircon age of 2194 Ma,which is considered to represent the deposition time of the rock.Combined with previous studies,it is concluded that the formation age of the Gaofan Group can be constrained between less than 2176 Ma and 2350 Ma,and the detrital materials are mainly derived from Neoarchean Paleoproterozoic metamorphic basement in Hengshan,Wutai,Fuping,Yunzhongshan,Luliang areas or further afield.