扬子西缘东川落雪地区元古宙地层中3890 Ma锆石的发现

The discovery of 3890Ma zircon in Proterozoic strata in the Dongchuan Luoxue area,western margin of the Yangtze plate

利用LA-ICP-MS U-Pb同位素测定技术,在扬子西缘滇中东川地区古元古界阿不都组流纹质玻屑熔结凝灰岩中发现了1颗冥古宙锆石,锆石具有明显的核-幔-边结构,其核部的207Pb/206Pb年龄为3890±47 Ma,此年龄被解释为岩浆结晶,Th/U比值为0.34;变质边的207Pb/206Pb年龄为3561±45~3616±55 Ma, Th/U比值为0.07~0.19。锆石的原位Hf同位素测定给出了176Hf/177Hf=0.280317~0.280451,εHf(t)=-4.6~0.6(平均值为-2.5),TDM1=3861~3980 Ma,TDM2=4036~4106 Ma(平均值为4059 Ma)。这颗锆石为扬子陆块目前发现的最老锆石,其Hf同位素及微量元素特征说明扬子陆块存在4.0 Ga的地壳增生作用及古老陆壳的再循环;结合前人同位素研究资料,说明扬子西缘东川落雪地区元古宙时同华北陆块可能具有亲缘性;本次的发现,为扬子陆块寻找冥古宙和始太古代物质实体提供重要依据,也为认识地球早期大陆演化过程提供了重要新信息。

Using the LA-ICP-MS U-Pb isotope measurement technique, a Hadean zircon was found in the rhyolitic vitreous sintered tuff of the Paleoproterozoic Abudu Formation in Dongchuan Luoxue area, western margin of the Yangtze plate. The zircon has an obvious core-mantle-edge structure. The 207Pb/206Pb age of the core is 3890±47 Ma. This age is interpreted as magmatic crystallization, and the Th/U ratio is 0.34. The 207Pb/206Pb age of the metamorphic edge is 3561±45~3616±55 Ma, Th/U ratio is 0.07~0.19. The in-situ Hf isotope measurement of zircon yields 176Hf/177Hf=0.280317~0.280451, εHf(t)=-4.6~0.6(average value is-2.5), TDM1=3861~3980 Ma, TDM2=4036~4106 Ma(average value 4059 Ma). This zircon is the oldest zircon found in the Yangtze block. Its Hf isotope and trace element characteristics indicate that there is 4.0 Ga crustal hyperplasia in the Yangtze block and recycling of the ancient continental crust. Combined with the data of previous isotope studies, it shows that the Proterozoic in the Dongchuan area on the western margin of the Yangtze may be related to the North China block during this period. This discovery provides an important basis for the Yangtze block to be investigated for the pseudo-continent and Archaean material entities, and also provides important new information for understanding the evolution process of the early continent.