河西走廊晚泥盆世地层中冥古宙碎屑锆石的发现

The discovery of Hadean detrital zircon in Late Devonian strata in Hexi Corridor,Northwest China

河西走廊地区晚泥盆统中宁组地层中,利用LA-ICP-MS法测年获得了3.9Ga和4.0Ga两颗碎屑锆石,其Th/U依次为1.01和0.58,均为岩浆锆石。两颗锆石稀土元素呈轻稀土(LREE)亏损、重稀土(HREE)富集,均具有Ce正异常,其中4.0Ga锆石具有Eu负异常,3.9Ga锆石无Eu负异常。利用锆石中49Ti的含量计算原岩岩浆温度分别为792±36℃(3.9Ga)和967±45℃(4.0Ga)。3.9Ga锆石获得原位Hf同位素结果,176Hf/177Hfi=0.280169,εHf(t)=-3.6,tDM=4139Ma,tDMC=4319Ma。这两颗>3.9Ga碎屑锆石为西北地区首次发现,其微量元素特征说明在冥古宙时地球上可能存在地壳;结合前人古生物和古地磁研究结果,说明河西走廊在晚泥盆世时同澳大利亚西北部可能具有亲缘性。

Two detrital zircons with the age of 3.9Ga and 4.0Ga have been dated from Late Devonian strata in Hexi Corridor by LA-ICP-MS method,They are both magmatic zircons and the Th/U ratios are 1.01 and 0.58 respectively,Both zircons are enriched in heavy REE（HREE） compared to light REE（LREE） and show a distinctive positive Ce,The 4.0Ga zircon is with negative Eu while the 3.9Ga zircon without negative Eu,Using the geothermometer-49Ti content of zircon,we calculated magmatic temperatures that were 792±36℃（3.9Ga） and 967±45℃（4.0Ga） respectively,The 3.9Ga zircon was analyzed for Hf-isotope：176Hf/177Hfi=0.280169,εHf（t）=-3.6,tDM=4139Ma,tDMC=4319Ma,It is the first report of the 3.9Ga zircons in Northwest China,The trace element characteristics of the zircons prove that the crust may exist on earth in the Hadean,Further,Combined with previous paleontological and paleomagnetic results,these results indicate that the Hexi Corridor might have the affinity with northwestern Australia in the Late Devonian.