Identification of 3.5 Ga detrital zircons from Yangtze craton in south China and the implication for Archean crust evolution

摘要

The LA-ICP-MS U-Pb dating of hundreds of detrital zircon grains from the Sinian sandstones of Liantuo formation and tillites of Nantuo formation at Sanxia area in Yichang identified 3319—3508 Ma zircon grains. Their ~(207)Pb/~(206)Pb and ~(206)Pb/~(238)U ages show excellent agreement (concordia degree 99%—100% ). Their CL images exhibit well-developed oscillatory zoning and the Th/U ratios are within 0.46—0.76, implying that they are igneous zircons which formed during middle-early Archean. Thesezircons are the oldest ones discovered in Yangtze craton until now. However, the detrital zircons with ages older than 3. 3 Ga in the metamorphic rocks of Kongling group were not found by further investigation, which suggests the presence of crust olderthan high-grade metamorphic Kongling terrain in Yangtze craton.